PHOTOLUMINESCENCE AND KINETICS OF ZINC OXIDE
DOPED WITH RARE EARTHS
A Thesis Presented to
The Faculty of the
Fritz J. and Dolores H. Russ College of Engineering and Technology
Ohio University
In Partial Fulfillment
of the Requirement for the Degree
Master of Science
By
Bhavnesh Patel
August, 1998
\h
11,
iii
Acknowledgments
I sincerely thank Dr. Henryk Lozykowski, Dr. Voula Georgopoulos, Dr. Costas Vassiliadis
and Dr. Martin Kordesch of my thesis committee, for reviewing my work.
I am greatly indebted to Dr. Lozykowski for having introduced me to the world of optics and
providing me with the necessary means to carry out the research that follows, and also for
guiding and inspiring me to pursue this research. My humble gratitude to Dr. Georgopoulos
for guiding me in a lot of different ways though out my MS.
My co-workers in the lab W. Jadwisienczak and K. Cao deserve special thanks for helping
me out whenever I got stuck or needed teamwork. Wojtek, especially deserves my heartfelt
gratitude for sharing so much knowledge with me and inspiring me to work harder.
iv
Table of Contents
LIST OF TABLES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
VI
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. vii
CHAPTER
PAGE
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
1
1.1
Review of work on ZnO in the past. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.2
About Zinc oxide
.........
4
1.3
The research undertaken
...............................
6
2
Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3
Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
3.1
Photoluminescence (PL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
3.1.1
Setup for Photoluminescence (PL) . . . . . . . . . . . . . . . . . . . . . ..
11
3.1.2
Alternative setup for PL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16
1
4
3.2
Photoluminescence Excitation (PLE) . . . . . . . . . . . . . . . . . . . . . . . . . .
18
3.3
Photoluminescence Kinetics
19
Results and Analysis
.........
22
4.1
Zinc oxide doped with Dysprosium and Lithium (ZnO:Dy,Li). . . . . . .. 22
4.2
Zinc oxide doped with Erbium and Lithium (ZnO:Ey,Li). . . . . . . . . . .. 32
v
5
4.3
Zinc oxide doped with Neodymium and Lithium (ZnO:Nd,Li). . . . . . .. 42
4.4
Zinc oxide doped with Thulium and Lithium (ZnO:Tm,Li). . . . . . . . ..
49
4.5
Zinc oxide doped with Ytterbium and Lithium (ZnO:Yb,Li). . . . . . . ..
55
Conclusions and Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
59
REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61
vi
List of Tables
1
Sample Preparation
2
Summary of Rise and Decay times
'. . . . . . . . . . . . . . . ..
7
60
vii
List of Figures
1
Band Structure of ZnO showing the conduction and valence bands. . . . . . . ..
4
2
Sample Preparation Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3
Typical Photoluminescence measurement setup. . . . . . . . . . . . . . . . . . . . . . ..
12
4
Monochromator and Spectrograph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
5
Typical Photoluminescence Excitation measurement setup of equipment . . . . .. 17
6
Setup for kinetics measurement .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
20
7
Energy levels of Dy3+ ion in ZnO and assigned transitions . . . . . . . . . . . . . . ..
23
8
Top: PL of ZnO:Dy,Li excitation 325 nm at different temperatures
Bottom: PL + PLE of ZnO:Dy, Li for peak 579 nm . . . . . . . . . . . . . . . . . . . . ..
9
25
Top: PL of ZnO:Dy,Li excitation 457.9 nm at different temperatures
Bottom: PL of Zn O:Dy,Li excitation 476.5 nm at different temperatures. . . . . .. 26
10
Top: PL of ZnO:Dy,Li excitation 488 nm at different temperatures
Bottom: PL of ZnO:Dy,Li excitation 496.5 nm at different temperatures. ..... 27
11
Top: PLE of ZnO:Dy,Li for peak at 580 nm at different temperatures
Bottom: PL + PLE of ZnO:Dy, Li for peak 580 nm . . . . . . . . . . . . . . . . . . . . ..
12
28
Photoluminescence kinetics of ZnO:Dy, Li for peak 580 nm, excitation
at 476.5 nm (10K) fitting curve rise: double exponential rise
decay : single exponential decay .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
30
Top: Excitation Intensity dependance of ZnO:Dy, Li for peak 580 nm
Bottom: Variation of peak at 580 nm with excitation power . ..
31
viii
14
Energy levels of Er 3+ ion in ZnO and assigned transitions. . . . . . . . . . . . . . . .. 32
15
Top: PL of ZnO:Er,Li excitation 325 nrn at different temperatures
Bottom: PL + PLE of ZnO:Er, Li for peak 555 nrn . . . ..
16
Top: PL of ZnO:Er,Li excitation 457.9 nrn at different temperatures
Bottom: PL of ZnO:Er,Li excitation 476.5 nrn at different temperatures
17
37
Top: PL of ZnO:Er,Li excitation 501.7 nrn at different temperatures
Bottom: PL of ZnO:Er,Li excitation 514.5 nm at different temperatures
19
36
Top: PL of ZnO:Er,Li excitation 488 nrn at different temperatures
Bottom: PL of ZnO:Er,Li excitation 496.5 nrn at different temperatures
18
34
38
Top: PLE of ZnO:Er,Li of peak at 555 nm at different temperatures
Bottom: PL + PLE of ZnO:Er,Li of peak at 555 nm . . . . . . . . . . . . . . . . . . . . . .. 39
20
Photoluminescence kinetics of ZnO:Er, Li of peak at 560 nm
excitation at 514.5 nrn (10K) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40
21
Top: Excitation Intensity dependance of ZnO:Er, Li excitation at 457.9 nrn
Bottom: Variation of Intensity of peak at 557 nrn with excitation power. . . . . .. 41
22
Energy levels of Nd 3+ ion and the assigned transitions. . . . . . . . . . . . . . . . . . . ..
42
23
PL of ZnO:Nd,Li excitation 325 nrn at 10K and 300K. . . . . . . . . . . . . . . . . . ..
44
24
Top: PL of ZnO:Nd,Li excitation at 488,496.5,514.5 nrn (10K)
Bottom: PL of ZnO:Nd,Li excitation at 488 nrn at 10K (resolution 0.2 nm) ..... 45
25
Top: PL + PLE of ZnO:Nd,Li for peak at 904 nrn PL excitation 488 nrn (10K)
Bottom: PL + PLE of Zn O:Nd,Li for peak at 904 nrn PL excitation 325 nrn (10K)
................................................................
46
ix
26
Photoluminescence kinetics of ZnO:Nd,Li for peak at 899.3 nm
excitation at 488 nm (10K) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
27
47
Top: Excitation Intensity Dependance of ZnO:Nd, Li
Bottom: Variation of the peak intensity with excitation power for peak at 925 nm
................................................................. 48
28
Energy levels of Tm 3+ ion in ZnO and assigned transitions . . . . . . . . . . . . . . . .. 49
29
Top: PL of ZnO:Tm,Li excitation at 325 nm (10K, 300K)
Bottom: PL of ZnO:Tm,Li excitation at 457.9 nm and 476.5 nm (10K)
30
Top: PL + PLE of ZnO:Tm,Li PL excitation at 325 nm, PLE for peak at 800 nm
Bottom: PL of ZnO:Tm,Li PL exc at 476.5 nm, PLE for peak at 800 nm . . . . ..
31
52
Photoluminescence kinetics of ZnO:Tm,Li monitored at peak 801.2 nm
excitation at 488 nm (10K) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
51
53
Top: Excitation Intensity Dependance of ZnO:Tm, Li excited by 488 nm (15K)
Bottom: Variation of peak 804 nm with excitation power. . . .. . . . . . . . . . . . . .. 54
33
Energy levels in Yb3+ ion in ZnO and the assigned transition. . . . . . . . . . . . . .. 55
34
Top: PL of ZnO:Yb,Li excitation at 325 nm (10K, 300K)
Bottom: PL + PLE of ZnO:Yb,Li PLE monitored at 1002 nm PL exc 514.5 nm
................................................................. 57
35
Photoluminescence kinetics of ZnO:Yb,Li monitored at 979.3 nm
excitation at 514.5 nm (10K) . . . . . . . . . . . ..
58
1
Chapter 1
Introduction
There is a lot of research going on currently on semiconductors doped with rare earth
elements. Rare earth (RE) doped semiconductors have long been the topic of research owing
to their prominent and desirable optical and magnetic properties. Typically trivalent rare
earth elements have very stable emissions, due to the 4-f electrons which are deeply buried
and hence well shielded from the outer shells. This property of the rare earth elements make
it possible, to incorporate them into various hosts with different lattice and still preserve the
typical rare earth emissions. The sharpness of many lines in the emission spectra of RE
doped semiconductor enables us, in principle to investigate interactions in a solid by optical
means with a degree of accuracy similar to that usually possible with free atoms or ions.
Light emission due to radiative transitions in the partly occupied 4f shell of rare earth ions,
which is found in the form of many narrow bands throughout the visible and near infra-red
spectra, form the basis of a great number of applications, such as solid state lasers and
phosphors. For example, the red fluorescence ofeuropium is used for color television screens
and the 1.06Jlm laser line of neodymium used in various hosts, are very commonplace. It is
evident from numerous studies that the 4f-4f transitions within RE doped IT-VI
semiconductors can be very efficiently excited.
2
1.1 Review of work on ZnO in the past
In the fifties and sixties research on RE-doped II-VI compounds was mainly stimulated by
efforts toward developing efficient phosphor materials (typically for Cathode Ray Tube
applications) and, laser materials to a certain extent. A new impetus came in the middle of
seventies, from the activities aiming at multicolored electroluminescence displays, and these
have initiated the extensive work being carried out in this field up to the present time. Rather
conclusive experimental investigations involving EPR and high resolution optical
spectroscopy were started in the sixties, when RE-doped single crystals became available.
Nowadays parallel to the efforts related to electroluminescence displays, basic research
concentrated on phenomena occurring in RE-doped polycrystalline and crystalline samples
are more common. Some of the more recent work on ZnO follows.
Reboul [1] of the University of Pennsylvania had reviewed luminescence from pure ZnO as
early as 1953 as his Ph.D thesis. He found that the sintering temperature has a dramatic effect
on the 390nm peak as well as on the green bump, as the temperature is increased the
fluorescence from both the 390 nm peak and the 550nm bump decrease. Reference to his
work can be found in [2], which also has many other references to work done on ZnO until
the late fifties.
In 1970 Schirmer and Zwigel [3] investigated the yellow luminescence of zinc oxide single
crystals and the effect of doping with different Li isotopes (Li 6 and Li 7 ) . They found that the
yellow luminescence (around 550-590nm) decays very slowly (30 minutes) and that it is due
to a recombination of an electron with the neutral Li acceptor.
3
Pierce and Hengehold, of the Wright Patterson Air Force Base, investigated the influence of
different co-activators on ZnO luminescence such as Li, Na, Nand P at different
implantation energies in 1976. [4]
Garcia, Remon and Piqueras researched ZnO doped with Bi and Mn in 1987, and observed
that the blue green PL of ZnO is affected by co-doping with Bi and Mn. [5]
The work by Kossanyi et aI, in early 1990, investigated the PL of semiconducting ZnO
containing rare earths and as reference [6] shows they did not observe all the transitions as
observed in our work for Er 3+ and Nd 3+. Further work by the same authors can be found in
reference [7] on ZnO doped with nickel, cobalt, and neodymium, in reference [8] for the
impurities H0 3+ and Sm3+ , in reference [9] on Sm 3+ and Eu 3+, and their other publications
[10] to [13]. A recent publication [13, (1997)] of their investigations does not include data
for exciting ZnO above the bandgap nor any temperature, intensity or wavelength
dependance.
Hayashi and co-workers found in 1995, that in particular, for the increase of the Eu 3+
luminescence intensity Li co-doping is not effective but stoichiometric control of ZnO is
essential. [14]
Vanheusden
and
fellow
workers
investigated
the
mechanisms
behind
green
Photoluminescence in ZnO phosphor powders in 1996. They found that the broad band green
emission (540-560nm) intensity is strongly influenced by free carrier depletion at the particle
surface. [15, 16]. The detailed effects of different co-activators on the II-VI semiconductors
doped with rare earths can be found in the paper by Simon Larach [17].
4
1.2 Zinc oxide
ZnO is a good host for incorporating rare earth elements due to a number of different
reasons.
First some of its properties can be markedly changed by introducing rare earths. Secondly
it lends itself readily to investigation, since it is one of the simplest crystalline compounds
and can be prepared easily.
ZnO crystal has a wurtzite lattice, consisting of two interpenetrating hexagonal close packed
lattices, one containing the anions (0--), the other the cations (Zn'").
5
4
r1
3
~
e>
~
2
1
rs
0
r6
-1
-2
k =(0,0,0)
Figure 1
Band Structure of ZnO showing the conduction and valence bands
5
When Zn and 0 combine Zn loses two valence electrons to 0, thus eventually due to loss
of an outer shell the Zn atom shrinks in size from 1.33
A to 0.74 A,
increases in size due to addition of an outer shell from 0.64
A to
1.4
while the 0 atom
A.
[2]. The wide
disparity in size between the zinc and oxygen atoms leaves relatively large open spaces thus
enabling incorporation of foreign atoms.
The band gap of ZnO is 3.2 eV at room temperature and 3.44 eV at 4K [2]. Reference [31]
is a comprehensive information guide on ZnO properties.
According to D. Walsh, ref [32], the absorption band for ZnO is at the band edge (3.25 eV)
and the emission band is centered at 2.11 eV corresponding to 580nm, the blue-green band.
ZnO also features some radiation-less transitions at the crossover between valence and
conduction bands (0.05 eV). Walsh assumes that the luminescing center is influenced by 6
nearest neighbor atoms (3 Zn and 3 0). Yoshikawa and Adachi, ref [33] found that polarised
light does not have much difference in the PL of ZnO. Both light polarised E II c and E-lC
show radiative recombination occurring around 3.4 eV (360nm). Pierce and Hengehold
have shown the broad band luminescence from ZnO doped with Li. [4] The electronic
structure of ZnO (0001) was studied by angle resolved spectroscopy by Girard et al. [34].
Thus we conclude that the blue green bump (540-560nm), the 735nm bump and the radiative
recombination peak at 365nm are all characteristic of ZnO.
Varistors, transparent conductive electrodes in display devices, silicon solar cells, energy
efficient windows, SAW and NO devices are only some of the potential applications
ofZnO.
6
1.3 The research undertaken
Our research concentrates on investigating the optical characteristics of ZnO (zinc oxide)
doped with RE 3+ ions, typically Dy (Dysprosium), Er (Erbium), Nd (Neodymium), Tm
(Thulium) and Yb (Ytterbium). Additional inspiration for the study of such luminescence
is the fact that ZnO is one of the most suitable hexagonal substrates with the best matching
lattice constant for GaN growth. The band gap energy of ZnO is 3.24 eV, and that of GaN
is 3.39 eV at 300K. The heterojunction between n-type ZnO and p-type GaN can play an
important role in new light emitting devices. The results are very promising since we
obtained, very strong photoluminescence emission even at room temperature from the above
mentioned rare earths co-doped with ZnO and with Li (Lithium) as a co-activator, which
other workers [13] in the field failed to produce.
In the following chapters, sample preparation, experimental setup and at the end the results
and conclusions from the experiments are discussed.
7
Chapter 2
Sample Preparation
Powders of ZnO, Li 2C03 and the rare earth compounds were carefully weighed and mixed.
The table below describes composition and other information about the samples.
TABLE 1 Sample Preparation
1
Name: Composition, Weight(g), % by mole, Company, purity
Annealing
Conditions
ZnO:Dy
ZnO
9.295g Zn:93.7%
+DY203 O.128g Dy:O.6%
+Li 2C03 O.244g Li :5.78%
5 hrs at 1000°C in air
Cerac Inc., 99.995% pure
Cerac Inc., 99.999%
Speciality Products, 99.999%
ZnO:Er
ZnO O.929g
Zn:93.62%
Cerac Inc., 99.995% pure
J M COl, 99.9%
+ Er(N03)3.5 H 20 O.0305g Er:O.6%
+ Li 2C03 O.0244g Li:5.78% Speciality Products, 99.999%
5 hrs at 1000°C in air
ZnO:Nd
ZnO
6g
Zn:98.4%
Cerac Inc., 99.995%
+ Nd(N03)3.5 H 20
O.25867g Nd:O.8% J M Co, 99.9%
+ Li 2C03 O.02179g Li:O.8% Speciality Products, 99.999%
5 hrs at 1180°C in air
ZnO:Tm
ZnO
6g
Zn:98.4%
Cerac Inc., 99.995%
+ Tm(N03)3.5 H 20
O.27319g Tm:O.8% J M Co, 99.9%
+ Li 2C03 O.02179g Li:O.8% Speciality Products, 99.999%
5 hrs at 1180°C in air
ZnO:Yb
ZnO
6g
Zn:98.4%
Cerac Inc., 99.995%
+ Yb(N03)3.5-6 H 20 O.2698g Yb:O.8% J M Co, 99.9%
+ Li 2C03 O.02179g Li:O.8% Speciality Products, 99.999%
5 hrs at 1180°C in air
Johnson Matthey Company
8
STEP 1
Powders of ZnO, Li co-activator compound,
and rare earth compound are mixed
by hand in an ethanol solution.
J
1
STEP 2
~
The thoroughly mixed solution is then
dried on a hot plate for 5 minutes
=:J
C
1
D
STEP 3
The semi-dry mixture is then pressed
under a mechanical press (5000 Ibs)
to form tablets of -1 em dia and 1 mm thick
D
Mechanical Press
1
STEP 4
The tablets are annealed in a
Electric furnace in air
Electric furnace (1200 DC max)
Applied Test Systems Inc.
Figure 2
Sample Preparation steps
Note: Pellets of ZnO with the different impurities, were obtained by carrying out these steps
9
As described above we carefully weigh the amounts of powders of different compounds of
stated purity (Table 1). The powders are mixed with a few drops of ethanol to form a
thoroughly mixed uniform semi solid/semi liquid mixture. This mixture is compressed using
a mechanical press (5000 lbs) to form tablets, about 10mm in diameter and 1-2 mm thick.
Now the tablet is ready to be annealed in a furnace. Annealing was carried out at 10000 e in
air for the various amounts of time as shown in Table 1, for individual samples, in an
Applied Test Systems furnace. The zonal characteristics of the furnace allow uniform
temperature throughout the surface area of the sample (typically less than 1 crrr'). The
dependance of PL on sintering conditions have been investigated for crystalline ZnO by
Reboul [1] and also by Pierce and Hengehold [4]. A good collection of references and a
brief description of ZnO can be found in reference [2] (pgs. 25 and 40-43). Once annealed
the ZnO re-crystallizes and RE 3+ ions are incorporated into the ZnO lattice.
10
Chapter 3
Experimental
Experiments undertaken during the course of this thesis are primarily Photoluminescence
(PL), and kinetics of PL. In depth investigation of how the PL emission varies as a function
of temperature, excitation wavelength and excitation intensity were performed.
Photoluminescence excitation (PLE) of the samples was observed both above and below the
ZnO band gap. The data obtained from the above mentioned experiments when correlated
with the energy band structure of the compound is pivotal in predicting the exact processes
going on within the material. Knowledge of the intrinsic energy levels and the changes in
the band structure due to incorporation of RE 3+ help us predict the behavior of the material,
which in turn is essential to use the material for light emitting devices.
3.1
Photoluminescence (PL)
Photoluminescence is luminescence due to absorption of light. It provides a non-destructive
technique to determine optically active impurities in semiconductors. Identification of
impurities is easy with PL, usually all the impurities that recombine radiatively can be
detected [5] and [6]. In our case the intentionally added RE 3+ ions luminesce very strongly
within the host ZnO. The setup used to obtain PL is shown in Figure 2. The sample is placed
on a cold finger in an optical cryostat and cooled to temperatures around 9K.
11
Low temperature measurements are necessary to obtain the fullest spectroscopic information
by minimizing thermally activated non-radiative recombination processes and thermal line
broadening. The thermal distribution of carriers excited into a band, contributes a width of
approximately kT/2 to an emission line originating from that band. The thermal energy kT/2
is only 1.8 meV at T=4.2 K. As observed in ZnO at temperatures below 10K the thermal
noise in the PL spectrum tends to subside enough to allow examining the features of the PL
spectra.
3.1.1 Setup for Photoluminescence (PL)
In the typical setup shown in Figure 3, the He-Cd (Helium Cadmium) laser emits 325nm
(UV) light. An interference filter is used to remove plasma lines of the laser. The beam is
then directed via mirrors and focused by a quartz lens onto the sample surface mounted on
the cold finger in the optical cryostat. The luminescence emitted from the sample is collected
by a quartz lens and fed to the monochromator, after passing through filter 'd' which cuts off
all luminescence below 345 nm, thus cutting the laser line. The optical cryostat is a very
versatile equipment. It is pumped to below 10-6 Torr using the vacuum system (Pfeiffer
Vacuum TCP 015) and once it reaches <10-6 Torr, which is made sure using the ionization
gauge control (Veeco Model RGS-7), the valve is closed and the cryostat is cooled down to
9K.
T~e
cooling system is a closed circuit liquid Helium system by CTI Cryogenics (Model
22C), the cooling temperature can be controlled by the Palm Beach model 4075, thermocontroller. The monochromator separates polychromatic light it receives
monochromatic light of individual wavelength.
into
,
\I
\~
d
b
'iSA Instrument ,Inc.
ISA Instruments SA, Inc.
/~--
a/
f}
b
I
c
~--8---
1_
I-
Laser He-Cd
Model 4240PS LICONIX
a: mirrors
b:quartzlenses
c: interference filter ORI EL
d: LPF WG345 ORIEL
Optlcal Cryostat
with samples mounted on a cold finger
e . .---
Palm Beach 4075
Thermometer/
Contol.
Pfeiffer Vacuum TCP 01
Vaccum System
I
I
-B-8----------i---~~-~~~~~~~~--~-Monochromator/Spectrograph
HR320-SMCXOO
Controller of Stepper
Motor
,/
II
II
1\
I \ I
I \ Ii
I
\ I
I
\ I
I I
\ I
I I
\ I
1-1_----_
-,...
Detector
TE/CCD-512-TKBM/1 NISAR
Princeton Instruments, Inc.
Veeco RGS-7
lonizationrsauqe Control
'--
Ie
H
n
CTI Cryogenics 22C
Compressor
Figure 3 Typical Photoluminescence measurement setup of equipment
Note: The path of light travel and the equipment interconnections are shown
Computer PC
~D:
D.
-
I
Princeton Instruments
ST-130S
I
Controller
Photoluminescence Experiment Setup
~
tv
13
Figure 4 depicts the basic principle of monochromator and spectrograph. Light enters the
entrance slit and is collected by the collimating mirror. Collimated light strikes the grating
and is dispersed into individual wavelengths. Each wavelength leaves the grating at a
different angle and is re-imaged at the exit slit by the focusing mirror. As each wavelength
images at a different horizontal position, only the wavelength at the slit opening is allowed
to exit the monochromator. Varying the width of the entrance and exit slits allows more or
fewer wavelengths to exit the system. Rotating the diffraction grating scans wavelengths
across the exit slit opening. Monochromatic light thus obtained is used to illuminate the
sample. In PL setup, the wavelengths scanned across the exit slit opening are detected using
a Hamamatsu R928 photomultiplier tube (PMT) and measured for intensity at individual
wavelengths.
A spectrograph is essentially a monochromator, except that in place of the exit slit, an array
of detectors such as a CCD or array of PIN diodes is positioned. Individual wavelengths
focused at different horizontal positions along the exit port of the spectrograph are detected
simultaneously by the CCD system.
In the setup shown in Figure 3, the latter configuration (spectrograph) is used. The signal
from CCD is sent to a computer via a controller to be recorded as ASCII data. The computer
also controls the stepper motor that moves the grating of the spectrograph.
For different excitation wavelengths we used the 12W CW Argon ion laser (Laser lonics
Model 1400). This laser is capable of lasing at the following wavelengths: 457.9,476.5,
488.0,496.5, 501.7 and 514.5nm.
14
Collimating
Mirror
Focusing
Mirror
Entrance Slit
Grating
Exit Slit
ochromat
Simplified Mon
angemen
or Optical Arr
t
Collimating
Mirror
Focusing
Mirror
Entrance Slit
Grating
Exit Port with
C eD Array
ctrograph
Simplified Spe
Figure 4
Monochromat
gemen
Optical Arran
t
graph
or and Spectro
rinci
e underlying p
th
in
la
p
ex
y
rl
travel to clea
graph
s path of light
w
o
sh
r and a spectro
re
o
gu
at
fi
m
ro
he
T
ch
o
e:
n
ot
o
N
m
operation o f a
ple o f
I
I
I
I
I
I
I
I
I
I
15
Thus for exciting ZnO below the band gap the Ar" laser is ideal. For excitation above band
gap the He-Cd laser's 325nm emission is used. For investigating the wavelength dependance
of the PL, we excite the sample using different excitation wavelengths emitted by the Ar and
He-Cd lasers and record the PL response.
The CCD system used for the experiments is TE/CCD-512SF (Princeton Instruments Inc.).
It is a back illuminated system, the advantages of back illuminated system over the front
illuminated version are : Superior quantum efficiency, typically for our system we have
average QE (quantum efficiency) of >60% over the range 500 to 900 nm (as opposed to 510% in the front illuminated case). This factor alone justifies the greater cost of the system.
Another advantage is the natural UV response that comes with the system. The pixel format
is 512 x 512 which is enough for the applications we used it for, typically for measuring the
excitation intensity dependance.
The CCD though is a slow scan system, is much faster than the scanning monochromator
with the PMT configuration, but the trade off is resolution in wavelength.
Temperature dependance was measured by varying the temperature of the cryostat by the
thermo-controller. The cryogenic closed circuit cooling system is capable of temperatures
up to 9K. Data for temperatures from 9K to 300K has been obtained.
Intensity dependance of the PL was investigated by varying the power of excitation laser line
reaching the sample. This was achieved by passing the beam through gray filters (Melles
Griot) of different transmittances. For power variation from 2 mw/mnr' to 100 m'W/mrrr'
the PL variations were recorded.
16
3.1.2 Alternative setup for PL
In order to have greater resolution we used a PMT as a detector, in lieu of CCD camera
(Fig. 3) for PL measurements that mandated higher resolution. The Hamamatsu PMT is far
more sensitive than the CCD and when coupled with the scanning monochromator, the setup
can deliver resolution of 0.04 nm and can detect upto single photons. Two different PMTs
were used for optimizing performance over the whole spectrum from UV (350nm) to near
infra red (1000 nm). Model R928, which has a pretty flat response curve for the region 300
to 800 nrn is a Hamamatsu side-on PMT (Detector Sb-Na-K-Cs). For wavelengths >900nm
we used the R316 head-on model which can detect wavelengths up to 1000 nm. The response
curves of the R316 is not ideally flat over the entire region scanned and hence mandates
the curve multiplication of response and the spectra to eliminate the non linearity. Also
compensation is needed for the shift in wavelength due to monochromator.
Calibrating the monochromator using the sharp emission lines from a Xenon lamp, allows
to estimate the compensation for the shift in wavelength, if any. The sharp peaks observed
are compared with the standard spectrum for a xenon lamp and the shift in wavelength, if
any, is incorporated into the final data.
DIGIKROM CM 112.
j
i
I
o
.
- olD:
,
Data
~o:
,
o
Computer PC
~e
.
J_~
~~~~ I
CVI Instruments
DIGIKROM DV242
Double
Monochramator
~
~ ~ ()
I
I
~
"
e
~_~
Oriel Corp.
Tungsten Halogen
lamp
Pfeiffer Vacuum
Model TCP 015
Vaccum System
Palm Beach 4075
ThermoControlier
L - lenses
M - mirrors
F - long pass filter WG 345 (Oriel)
___ ~
Water
Filter
Optical Cryostat
with cold finger
__=_
e
H
Figure 5 Typical Photoluminescence Excitation measurement setup
of equipment
Note: The path of light travel and the equipment connections are shown
Stanford Research SR-400
Gated Photon Counter
DATA
CVllnstru ments
Hand held controller
Preamplifier
Model VT120A
Hamamatsu R92
PMT
,---------11
Double
Monochramator
~~
rM[-:
Photoluminescence Excitation Experiment Setup
~
'-J
18
3.2
Photoluminescence Excitation (PLE) Spectra
Photoluminescence excitation is a technique wherein, we excite the sample with different
excitation wavelengths above and below the band gap (387 nm in our case, since E g of ZnD
is 3.24 eV), while observing the luminescence response at a particular wavelength, that
corresponds to a peak of interest in the PL. PLE investigates the variation of the PL structure
both above and below the band gap for an observed peak in PL. Figure 5 shows the PLE
setup used. Here the optical cryostat setup is the same as in PL (3.1.1). Instead of the laser
light here we excite the sample with light from a Tungsten Halogen lamp (Photon
Technology International) which emits multi-wavelength light. Light from the lamp is
collected via the lens and filtered through a water filter to remove IR from the spectrum and
then is fed to the double monochromator (CVI Instruments DV242) which precisely selects
the excitation wavelength and delivers it, to the cryostat via mirrors and lenses. A double
monochromator consists oftwo identical monochromators connected, to provide an extended
optical path which minimizes stray light. The emission from the samples is collected by a
lens and directed into the second monochromator (CVI Instruments CMl12). The single
wavelength light coming out of the monochromator is detected by a PMT (R928). The signal
output of PMT is then fed to a electrical pre-amplifier. The pre amplified signal is coupled
to a gated photon counter (Stanford Research SR-400). Data from this photon counter is fed
via GPill connection to a computer which stores the spectrum as ASCII data. The scanning
in this case is controlled via a apm connection, by a PC which is connected to the hand held
controller of the monochromator CM112 (grating 1200 grImm, 300nm blaze).
19
Another GPIB connection between the handheld controller and the monochromator, allows
controlled scanning of the wavelengths in the monochromator. The DV242 monochromator
has three different options of gratings (1200, 300gr/mm, 300/600nm blaze). To obtain the
PLE temperature dependance we vary the temperature of the cryostat using the thermocontroller, from 9K to 300K, and measure the PLE spectra at various temperatures.
3.3
Photoluminescence kinetics
Photoluminescence kinetics measurements involve exciting the sample by a square pulse of
excitation light and measuring the emitted response from the sample. The rise and decay of
the
PL
emitted are important factors in determining the fast and slow energy transfer
processes occurring within the material.
Figure 6 shows the setup for measurement of PL kinetics. Light modulation is accomplished
by using a Intra Action acousto-optic (Ala) modulator. A crystal (Tellurium dioxide Te02)
is used for the interaction medium. A lithium Niobate piezoelectric transducer generates the
RF frequency acoustic wave which travels inside the crystal. When an RF frequency acoustic
wave propagates inside an optically transparent medium, a periodic change in the refractive
index occurs due to the compressions and rarefactions of the sound wave. This periodic
variation produces a Bragg grating capable of diffracting a laser beam.
The pulse generator generates pulse of desired width with rise and decay times. Typically for
a pulse of 2.5 ms duration rise and decay times are <2 ns. This analog signal is fed to the
signal processor (Intra Action ME-2001).
,
I
I
I
DID:
IntraAction ME-2001
I
Compre ssor
CTI Cryogenics 22C
;
Pfeiffer Vacuum
Model TCP 015
Vaccum System
Palm Beach 4075
,
ThermoController
Model 1400 Laser lonics Inc
Laser Argon ion
I
Photodetector ,
Deflecte1 I
Newport 1815-C
beam
II
II Undetlected
II beam
II
VIDEO IN RF OUT
Signal Processor
Compu ter PC
f
INewport 1815-
Newport 883-UV
\:o:/-' Power Head
'CD
:
I
I
I
I
I
Figure 6
Setup for kinetics measurement
Note: The path of light travel and the equipment interconnections are
shown
Stanford Research Inc.
Model SR535
Pulse Generator
DyscStart
EG&G ORTEC T914
-a
I
Optical Cryostat
with cold finger
==:-~-~~
----Lf)-----~- __ -L-'F
I'~
/:::::::.:..
r--..-..
-.. -..-..-.. I
(.':::::::::'0----~
0
.
-
DIGIKROM CM 112
Multichannel Scaler
Preamplifier
Model VT120A
,
Double
Monochramator
Kinetics Experiment Setup
N
o
21
It provides the RF drive power for the AlO modulator at 200MHz. Laser light is modulated
by a AlO modulator and then guided onto the sample on the cold finger of the cryostat. The
resulting emission from the sample is collected by a lens, filtered to remove the laser line,
and aimed into the entrance slit of the monochromator. The monochromator disperses the
light received, singles out the wavelength of interest, and delivers it to the exit slit where the
PMT is attached. The PMT generates an electrical signal depending on the amount of light
detected. This signal is pre amplified electrically and fed to a Multi Channel Scaler (EG&G
Ortec MCS T914). The MCS gives a 'START' pulse, this and the pulse generated by the
Pulse generator have synchronous clocks. The data extracted by MCS is then fed to a
computer where it is stored as a ASCII file. The resulting PL rise and fall characteristics are
then analyzed using various softwares optimized for curve fitting applications (TableCurve,
Origin) which can fit the rise and decay curves to single or multi exponential equations, thus
describing the rise and decay times.
Variations of the PL kinetics experiment include, kinetics at different peaks in the PL, and
kinetics of the same peak excited by different wavelengths.
The former variation involved changing the wavelength of interest by scanning the
monochromator grating to the new wavelength, which corresponds to a different peak in the
PL spectrum.
In the latter variation pulses of same time duration but different wavelengths (from the Ar
ion laser) were used.
22
Chapter 4
Results and Analysis
In this chapter the data obtained from the experiments is segregated with respect to the
impurities and hence each section deals with an individual rare earth ion as an impurity in
host ZnO.
4.1 Zinc oxide doped with Dysprosium and Lithium (ZnO:Dy,Li)
Chapter 2 provides detailed information on sample composition.
Luminescence from Dy3+ incorporated into a variety of hosts has been demonstrated by quite
a few workers in the field. Chase et al [18] in the late sixties investigated the luminescence
of various rare earths doped into ZnS. Lozykowski and Szczurek investigated rare earths
doped with ZnSe thin films as host. [19] Dotsenko and Efryushina proposed the static
energy transfer in YA1 3B4 0 12
:
Dy3+. [20]
Garcia et aI, in 1985 researched the charge transfer excitation of rare earth emission in
CaGaA2S 4 [21]. Recently Kossanyi and co workers have also published data on ZnO doped
with Dy 3+ Er 3+ Ho 3+ Nd 3+ Sm 3+ and Tm 3+. [13]
Fig. 7 shows the energy levels of the 4f shell. The emissions observed in the PL and PLE
spectra have been assigned to the probable transitions.
23
E
co
~
2)
E
C
,....
~
IE
LO
~
(f)u
0
~
><
~
15
""-'"
"C
C
co
EE
E E
cc
,...,.....
ffi
o)~
~
[O~
-- ---- --- --- -- ------
~
Q)
l!i
10
•
5
o
Figure 7
•
r
6~5/2
Energy levels of Dy3+ ion in ZnO and assigned transitions
As opposed to ZnSe:Dy which does not show all of the transitions observed here [19], we
see that in addition to the 570, 660, 740nm group of emission lines, we here discover the
presence of 489.51,504.7 nrn lines in the PL (see Fig. 8 and subsequent PL spectras for
ZnO:Dy). Authors of ref [13] who worked on similar experiments failed to observe the
above mentioned lines.
24
From Figure 8 we infer that all the peaks vary in intensity with temperature although there
is no shift in wavelength. Except for the 648 nm line all other groups decrease in intensity
with increase in temperature while it is vice-versa for the 648 nm line.
Also of interest is the PLE of the sample presented in the bottom half of Figure 8. Here we
observe the marked difference in the spectrum below and above the band gap
(E, =3.24 eV
~
387.67 nm). The peak 428.3 nm has been assigned to the 4115/ 2 level and the
450.55 nm line to the 4F9/ 21evel [13]. As seen from Figure 8 when excited above band gap
the lines from Dy3+ are superimposed on the host emission bump centered around 530 nm.
The emission due to Dy3+ (373, 504, 579, 748nm) tend to be overshadowed by the blue green
ZnO bump at higher temperatures.
Figures 9 and 10 report the PL spectras excited with different excitation wavelengths and at
different temperatures. Unlike the spectrum for above band gap excitation here we observe
that the intensity of 579 nm peak increases with increase in temperature. As was expected
of ZnO the green bump decreases with temperature increase. (See ref [2] pg 42) Varying the
excitation wavelength we find that there gradually appears a hump centered around 580 nrn
as we move towards lower energy excitation ( from 457.9 to 496.5 nm). The group of
emission lines around 725 nm and those at 750 nm increase dramatically in intensity as we
decrease the excitation energy.
Figure 11 depicts the variation of PLE with respect to temperature, in general the
luminescence diminishes as temperature increases. The bottom plot of Fig. 11 depicts the
PLE with a PL spectra excited below band gap.
25
579.18
H...ofmQDy @325nm
400
450
500
550
600
650
700
750
Wavelength (nn1
PL + PLE blOD,t
~
~~
~~
------5.1
PL Oaser@32Snm, SDK)
~
~
~
450
500
~
600
650
~
~
Wavelength (nni
Figure 8
Top: PL of ZnO:Dy,Li excitation 325nm at different temperatures
Transitions in PL: 373.8 : 6P3/2 .... 6H
489.5 : 4F9/2 .... 6H 15/2' 504.7 : 4F9/2 .... 6H
15/2,
13/2,
579.1 : 4P9/2 .... 6H 13/2' 748.5 : 4P9/2 .... 6H
Note: 648 nm .... laser 2nd order
9/2
Botto m: PL + PLE of ZnO:Dy,Li for peak 579 nm
Assignments in PLE spectr um: 428.3 : 41 .... 6H
450.5 : 4F9/2 .... 6H 15/2
15/2
15/2,
26
55-r-----"7"
578.68 ;':;~--__,..--------
-----_.
PL ZnQDylaser@457nm
718.82
45
40
15
10
5
Wavelength (nn}
579.08
PL lnQD)' @ 476nm
,....
'l:t
M
,....
L()
40
:i'
~
.~
~
.1:
tt
2)
J
_.,,-,,---1
500
720
700
840
Wavelength (nni
Figure 9 Top: PL of Zn O:Dy, Li excitation 457.9 nm at different tempe
ratures
Bottom: PL of ZnO:D y,Li excitation 476.5 nm at different temperatures
Transitions: 579.1 : 4P9/2 ~ 6H 1312, 667: 4P9/2 ~ 6H , 753: 4P9/2
~ 6H /
l 112
Note: In both of these plots the left portion was plotted on a differe 9 2
nt scale both in
wavelength and intensity to show the 580 nrn group of peaks distinctly.
27
PL lnODj [email protected]
Wavelength (nrrj
581.48
PL ZnQO)' [email protected]
'S
cd
"-'
~
en
c:
Q)
"E
~
SOK
1S0K
Figure 10 Top: PL of ZnO:Dy,Li excitation 488 nm at different temperatures
Bottom: PL of ZnO:Dy,Li excitation 496.5 nm at different temperatures
Transitions: 579.1: 4P9/2 -+ 6H 13/2 , 753: 4P9/2 -+ 6H9/2
28
387.67
350
PLE2hQDyof peak500rm
425
400
375
475
450
Wavelength (nni
Pl + PlE ZnQDy
579.18
~
~
~
~
~
~
~
~
~
~
~
'Nivelength (nm)
atures
for peak at 580 nrn at different temper
,Li
:Dy
ZnO
of
PLE
:
Top
11
ure
Fig
/
~ 6H 15/2 , 450 .5: 4P912 ~ 6H 15 Z
Assignments for PL E: 428 .3: 41 15/2
peak at 580 nm
Bottom: PL + PLE of ZnO:Dy,Li for
/
667 : 4P9/Z -.. 6H 111Z' 753 : 4P9/ 2 ~ 6H9 2
/ ,
Transitions in PL : 579.1 : 4P9/2 ~ 6H 13 2
29
In Figure 12 is plotted the fitted data for the rise and decay obtained from the excitation of
sample with a laser pulse of duration 2.5 ms (rise and decay times < 5 ns), wavelength
476.5 nm at 10K. The data was fitted using curve fitting softwares ; Table Curve and Origin.
The rise time found was 0.33 ms ( f + h, slow + fast) and decay time of 0.3012 ms was
obtained.
Figure 13 delineates the Excitation Intensity dependance of the peak around 580 nm (Top),
and the bottom plot shows the Intensity versus the excitation power. As is evident for
excitation power up to 75 mW/mrrr' the dependance is pretty linear. There seems to be some
disturbance in the linearity as the power is increased above 75 mW/mm2 , but these might be
due to measurement error.
30
PL of ZnQ Dy monitored at 579nm
excited by 476.5 nm (10K)
0.8
Fitting Equations
Rise: y=e(1-exp(-x/f))+g(1-exp(-x/h)
Decay: y=a+b(exp(-(x-c)/d)) (x>2.49)
f =0.3149
h=0.01758
d=0.3012
~
::J
~
~
.~
cQ)
0.4
+oJ
C
...J
o,
0.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Time (ms)
Figure 12
Photoluminescence kinetics of ZnO:Dy,Li for peak at 580 nm,
(Transition 4P912 -. 6H 13/2 ) excitation at 476.5 nrn (10K)
fitting curve Rise: double exponential rise
Decay: single exponential decay
Note: Constants f, hand d indicate time in ms
5.0
5.5
31
PL (j ZnQDyexcitedby 457nm (3XlK)
to showh1ensity dependance
of 579nm peak
Excitation power (mW)
a: 113
b: 100
c: 96
d: 92
e: 89
f:
82
g: 75
h:
i:
j:
k:
I:
70
65
60
54
47
m: 40
n: 35
0: 30
p: 26
q: 21
r. 17
s: 12
t: 9
u: 5
v: 3
560
540
ax>
500
640
Wavelength (nrrt
•
PL of ZnODf excitedby 457nm (300<)
Excitation intensity dependance of peakat 579nm
•
10-
8::J
~ 6-
~
Q)
1:
~
•
• •
...-...
cd
"'--'"
4-
2-
••
••
••
•
•
• •
I
I
I
T
1
0
10
2)
3)
40
•
•
I
eo
•
•
••
•
I
I
I
I
00
70
00
00
I
I
100 110 120
Excitation Fbw er (nW/rrrrf)
Figure 13
Top: Excitation Intensity dependance of ZnO:Dy,Li for peak at 580 nrn
Transition: 579.1 : 4F9/2 --. 6H 13/ 2
Bottom: Variation of peak at 580 nrn with excitation power
32
4.2 Zinc Oxide doped with Erbium and Lithium (ZnO:Er, Li)
For composition of the sample refer Chapter 2, Table 1. Figure 14 below shows the energy
levels within Er 3+ 4f shell. In addition to the references mentioned at the beginning of this
chapter some additional references especially for Er 3+ are [22] and [23].
E
c
~
25
,........-...
'E
2)
2
er>0
0
4
,...
856nm
X
"-"""
~
"-
~
4
15
H1 1/2
5312
F9/2
41
9 /2
10
r
5
E
c
~
LO
E
c
E
c
~ ~
E
c:
t§
E
c
~
o
Figure 14
Energy levels of Er 3+ ion in ZnO and assigned transitions
33
The PL emission from the sample when excited below band gap (Figs. 15, 16 and 17) are
very sharp and strong. Four very distinct group of lines are readily identified:
536 nm
2Hll/2 -. 4115/2
554 nm
4S3/2 -. 4115/2
663 nrn
4F9/2 -. 4115/2
856 nrn
4S3/2 -. 4115/2
or 411112 -. 4113/2
These have been assigned to the respective transitions as shown in Fig. 14. The bump
centered at 800 was assigned to the
2p3/2 -. 4S 3/2 transition in ref [19]. In our case there
appears to be a shift in this bump in wavelength towards the left. This discrepancy might be
due to the different host. ( ZnO as opposed to ZnSe).
It is noteworthy that Kossanyi and co-workers did not find any emission around 530 nm
which we clearly see in PL spectra excited by 457.9 nm, also they did not attempt to obtain
any excitation dependance. Fig. 15 shows the PL spectra of the sample excited above band
gap and is different from that observed with excitation below band gap. As we decrease the
excitation energy the typical bump centered around 750 nm decreases gradually. The bottom
half of Fig. 15 depicts the PLE plotted together with PL for better viewing. The PLE peaks
are assigned to the levels in Er 3+ as follows: [ 13]
459 nm - 2H9/2 and 4P3/2 -. 4115/2
497 nrn - 4P7/2 -. 4115/2
527nm - 2H1112 -. 4115/2
The temperature dependance shows that the 557 nm peak is very strong at lower
temperatures and superimposes the bump at 10K. (Fig. 15)
34
PL ZnOEr laser@325nm
smoothedFFT2 pts
50K
250K
300K
500
400
600
700
800
Wavelength (nm)
527.78
PL + PLEZnOEr
857
eso
400
450
500
550
eco
650
700
750
fIX)
850
soo
Wavelength (nrrt
Figure 15 Top: PL of ZnO:Er, Li excitation at 325 nm at different temperatures
Transitions in PL: 554.2: 4S3/2 ~ 4115/2, 663.1 : 4F9/2 ~ 4115/2 ,
Bottom: PL + PLE of ZnO:Er, Li for peak at 555 nm
Assignments for PLE: 459: 2H9/2 and 4P3/2 ~ 41 15/2, 497 : 4P7/2 ~ 4115/2,
527 : 2H 1112 ~ 4115/2
35
Figure 16 reports the PL in response to two different excitation lines, 457.9 and 476.5 nm
of the Ar + laser. Typically when excited at 457.9 nm at 300K we see a shift in the 750 nm
bump. All the peaks reduce in intensity as temperature rises at all wavelengths of excitation.
The hump at 754 nm shifts left at room temperature which might suggest that it might be
related to ZnO rather than the rare earth since rare earth emissions tend to not shift so much.
The PL response to laser lines 488 and 496.5 nm are reported in Figure 17. As observed
before we see a shift in the 750 bump in the second half of Fig. 17, occurring at lOOK.
When excited by 501.7 nm line the emission lines at 850 nm diminish drastically. The
gradual reduce of the bump at 750 nm is very evident in Figure. 18.
The variation of PLE response with temperature is plotted in Figure 19. As is with all other
Er spectrums, increase in temperature decreases the intensity. The bottom half shows the PL
due to excitation at 457 nm along with the PLE for the peak 555 nm.
Figure 20 shows the rise and decay curves and the fitting curve for the peak 560 nm excited
by a laser pulse of 2.5 ms, 514.5 nm at 10K. The rise time is 0.274 ms and decay time
0.0959 ms.
Excitation Intensity Dependance is shown in Figure 21 for the peak 555 nm. The incident
power was varied from 4 to 80 mW Imm 2. The bottom half illustrates the variation of the
peak when plotted against excitation power. As is seen the relationship is very linear except
for some saturation around 75 mW/mm 2, after which it is again linear.
36
PLZnQErJaser@457rm
3
O-t----.-..,--.,,....----.------,r-----.------,..--......~---------P--;;::::l:WI**
650
500
700
7&)
Wavelength (nrn)
555.86
PLDlOEr laser@476nm
3
500
600
eso
700
7fJJ
Wavelength (nni
Figure 16
Top: PL of ZnO:Er, Li excitation at 457.9 nrn at different temperatures
Transitions: 536.6: 2H ll/2 -+ 4115/2 , 554.2: 48 3/2 -+ 4115/2 ,
663.1 : 4F9/2 -+ 4115/2 ,
856.1: 48 3/2 -+ 4113/ 2 or 4111/2 -+ 4115/2
Note: The 555 nrn peak in the top plot has been reduced for clarity
Bottom: PL of ZnO:Er, Li excitation at 476.5 nrn at different temperatures
Note: 754 nrn bump due to ZnO.
37
555.86
PLOlOEr laser@488 rm
500
650
700
7fIJ
Wavelength (nrr]
PLOlOEr l~er@496rm
10K
500
650
7&)
Wavelength (nrrt
Figure 17
Top: PL of ZnO:Er, Li excitation at 488.0 nm at different temperatures
Transitions: 536.6: 2H ll/2 4115/2 , 554.2: 4S312 4115/2 ,
663.1 : 4F9/ 2 41 15/ 2 ,
856.1: 4S 3/2 41 13/2 or 41 11/2 41 15/2
Bottom: PL of ZnO:Er, Li excitation at 496.5 nm at different temperatures
Note: 754 nm bump due to ZnO, 555nm peaks have been reduced for clarity
-+
-+
-+
-+
-+
38
555.86
PLZnQErl~er@501rm
Wavelength (nrrt
650
700
750
Wavelength (nnj
Figure 18
Top: PL of ZnO:Er, Li excitation at 501.7 nm at different temperatures
Transitions: 536.6: 2H l1 /2 ~ 41 15/2 ' 554.2: 4S 3/2 ~ 41 15/2 ,
663.1 : 4F9/2 ~ 41 15/2 ,
856.1: 4S 3/2 ~ 41 13/ 2 or 41 11/2 ~ 41 15/2
Bottom: PL of ZnO:Er, Li excitation at 514.5 nm at different temperatures
Note: 754 nm bump due to ZnO, 555 nm peak has been reduced in the
bottom plot to show the finer details of the whole spectrum
39
527.78
PLEat 560rm of ZhOEr
600
Wavelength (nnl
PL + PLEZhOEr
527.78
554.25
eso
400
450
500
sso
ax>
650
700
7fIJ
eco
8&>
sco
Wavelength (nni
Figure 19
Top: PLE of ZnO:Er, Li of peak at 555 nm at different temperatures
Assignments for PLE : 459 : 2H9/2 and 4F312 -. 41 15/2, 497: 4P7/2 -. 41 15/2,
527 : 2H 1112 -. 41 15/2
Bottom: PL + PLE of ZnO:Er, Li of peak at 555 nm
Transitions in PL: 554.2: 48 3/2 -. 41 15/2, 663.1 : 4P9/2 -. 41 15/2,
856.1 :48 3/2 -. 41 13/2 or 41 u/2 -. 41 15/2
40
PLof :mo:Er rronitorecJ at 560nm
excited by 514.5nm (10K)
0.8
Rtting Equations
Rise: y=e(1 ~xp(-xlf»+Q(1 ~xp(-x/h»
Decay: y=s+e(exp(-(x-c)/d» (x>2.495)
f=O.20578 h=O.0697 d=O.0959
......-..
::J
cti
'-"'"
~ 0.4
~
Q)
c
~
0.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
lirre (rrs)
Figure 20
Photoluminescence kinetics of ZnO:Er, Li of peak at 560 nm
( Transition 554.2 : 45 3/2 -. 41 15/2 ) excitation at 514.5 nm (10K)
Note: Constants f, hand d represent times in ms
4.5
5.0
41
PL of 2hOErexcited by 457rm (15K)
to shcNv Intensitydependa1ce
of 557rm peak
Excitation PCJNer(mW)
a: 80
b: 75
c: 70
d: 65
e: 60
f: 55
g: 50
h: 45
i:
j:
k:
I:
m:
n:
30
24
0:
7
40
33
18
12
p: 4
1.0-
•
PL of 2hOEr excited by 457rm (15K)
Excitationintensitydependance of peak at 557rm
0.8-
•
'5'
~
~
0.6-
•
~
l:
~
0.4-
•
• •
•
•
• •
•
•
•
•
0.2-
• •
0.0
0
I
I
I
I
I
I
I
I
10
20
3)
40
50
60
70
00
6ccitatioo R>w er (rrW/rTlTf)
Figure 21
Top: Excitation Intensity Dependance of ZnO:Er, Li excitation at 457.9 nm
Bottom: Variation of Intensity of peak at 557 nm
( Transition 554.2 : 4S 3/2 .... 41 15/2 ) with excitation power
42
4.3 Zinc oxide doped with Neodymium and Lithium (ZnO:Nd, Li)
The energy level diagram for Nd3+ ion is given below and the observed peaks assigned to
transitions. Fig. 22 marks the transitions in the Nd3+ ion, observed in the following PL and
PLE spectrums for ZnO:Nd.
2
24
°5/2
------------
Z2
2)
2
____________ 2
P1/2
K11
,Q
============ 4 G 11
,Q
2
18
4~13,Q
4 912
G7,Q
16
2
14
4F9 / 2
4F7 / 2
4Gs/2
.............
,.-
IE
('f)U
0
H1 1 / 2
~
2Hg/2
............
4F3 / 2
x 12
~ 10
L.
~
8
E
c
E
c
v
v
ffi ~
E
c::
E
c::
~
~
6
4
2
0
Figure 22
r
r
Energy levels in Nd 3+ ion and the assigned transitions.
43
Neodymium differs from Er and Dy, since it is very rich in energy levels as is very evident
by comparing Figs. 22, 21 and 7. Thus providing a host of possibilities for transitions. As
opposed to ref [13] we observed very sharp emission lines from the sample when excited at
488 nm at 10K centered around 904 nm. They tend to decrease in intensity with decrease
in excitation energy (higher wavelengths).
Although the PL below band gap is not that enthusing the PL due to excitation above band
gap is encouraging. Featured in Fig. 23 is the PL due to 325 excitation at 10K and 300K
We have assigned the peaks observed to the transitions in the energy level diagram. The
luminescence decreases with increase in temperature, which was the case with the sample
with Er. Fig. 24, bottom half, shows a spectrum of the finely resolved peaks of the 904 nm
group of lines. The spectrum was scanned with a resolution of 0.2 nm per step to show
intricate details.
PLE of the sample for the peak 904 nm is pictured in Figure 25 along with the PL. The peak
assignments are as follows :
528, 545 nm - 4G912
605 nm -
4GS12
-+
-+
4~/2 ,
4~12'
587 nm - 4G7/2
700 nm - 4F9/2
-+
4~/2 ,
-+
4~12
,
825 nm - 2H9/2
-+
4~/2
The rise and decay times for the kinetics characteristics measured were 305.67 J..lS and
305.67 J..lS respectively. The data in Fig. 26 was monitored at 899.3 nm and the sample
excited by a laser pulse 5 ms long, 488nm at 10K.
Excitation Intensity Dependance is represented in Fig. 27. The plot of intensity versus
excitation power shows the very linear nature of the increase in intensity of the peak at
925 nm. Also plotted is the Full Width Half Maximum for the peak at 925 nm.
44
PLznc>.Nd Iaser@325rrn
700
700
eco
850
Wavelength (nm)
Figure 23
PL of ZnO:Nd, Li excitation at 325 nrn at 10K and 300K
Transitions: 533.4: 2K 1312 ~ 4lg12, 592.4: 4GS/2 -+ 4~/2,
820 : 2H9/2 -+ 4~/2'
898: 4P3/2 ~ 4~12
1CXXJ
45
PLZnQt-«j @1OK
\l'k~
I
~
~rJ
~
~~ 1~.1
~
~
t-~fo
v
.
\~"".,~~V"V"',A _
~
~
~
~
~
~
Wavelength (nni
894.44
899.25
ZnQNdPL [email protected] rrn
(10KO.2rmstep)
903.45
900
Wavelength (nIT)
Figure 24
Top: PL of ZnO:Nd, Li excitation at 488.0,496.5,514.5 nm (10K)
Bottom: PL of ZnO:Nd, Li excitation at 488.0 nm at 10K (resolution 0.2 nm)
Transitions: 533.4: 2K13/ 2 ~ 419/2, 592.4: 4GS/2 ~ 41g/2,
820 : 2H9/2 ~ 44/2'
898: 4F3/2 ~ 4Ig12
46
PL + PLE .aD:~, 10K
904
605
587
PLE of peak 904nm, 10K
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
Wavelength (rrr]
~ 622
PL + PLE.aD:Nd at 10K
~\
IV\
! f\ PLiasri~
5931'
/
/~~
/
E
~
~
~
\
605
\
587
~
1
\
770
"~!t\
899
'I \
545
758
~
~
~
m
~
1
904
~~~
~
~
~
~
1~
Wavelength (nni
Figure 25
Top: PL + PLE of ZnO:Nd, Li for peak at 904 nm PL excitation 488.0nm
Bottom: PL + PLE of ZnO:Nd, Li for peak at 904 nm PL excitation 325 nm
Transitions: 533.4 : 2K1312 4~12, 592.4: 4GS/2 4~/2, 820 : 2H9/2 4~/2'
-+
898:
4F312 -+
-+
-+
4~12
Assignments for PLE: 528, 545 : 4G9/2 4lg/2 ,
605 : 4GS/2 419/2 , 700:
-+
-+
587:
4P9/2
-+
4G7/2
-+
4lg12 ,
4~/2 (Both at 10K)
47
PL of ZrlO:Nd monitored at 899.3nm
excitation by 488rm (10K)
Rtting Equations
Rise: y=e(1-exp(-xlf)) (x<4799.4)
Decay: y=a{exp(-xlb)}+c(exp(-xld)
0.8
f=305.01 b=305.67 d=304.8
0.0
o
4(XX)
lirre (flS)
Figure 26
Photoluminescence kinetics of ZnO:Nd, Li for peak at 899.3 nrn
(Transition: 4P3/2 41g12) excitation at 488 nrn (10K)
Note: Constants f, band d represent time in us
-+
48
Excitationpower (mW/rrm2 )
PL of ZnOf'-tjexcited by 488rm (15K)
to showirtensity cIeper'lcB'lce
of 9OO1m gnx.pof emssions
a: 105
b: 100
c: 93
d: 85
e: 77
70
f:
g: 63
h:
i:
j:
k:
I:
m:
n:
55
49
43
37
31
26
20
16
p: 10
q: 6
0:
r: 4
s: 2
t: 2.5
I
I
ga)
sco
WaVelength (rrr]
PL of lnQf'ti excitedby 488rm (15K)
Excitationintensitydependance d peakat 925rm
1.0
~
N
0.8
g
0.6
~
I•
A
~I
~
~
!
0.4
Ii
0.2
••
••
0.0
AA ...
A-
0
10
••
A
....
2)
• •
A
•
•
. ..
3)
...
40
•
•
•
.. ..
50
...
60
•
• •
•
.. .. ..
70
00
...
00
• •
A
100
.
110
Excitation R>vver (nW/rmf)
Figure 27
Top: Excitation Intensity Dependance of ZnO:Nd, Li
Bottom: Variation of the peak intensity with excitation power for
peak at 925 nm ( Transition: 4P3/2 -+ 4lg/2)
49
4.4 Zinc Oxide doped with Thulium and Lithium (ZnO:Tm, Li)
Thulium is a more widely investigated impurity among the rare earths we are analyzing. A
lot of good literature has been published on incorporating Tm in various hosts. References
[24] to [28] are a good source for information on Tm in various hosts. Fig. 28 shows the
transitions for the ZnO:Tm sample in the energy band structure of Tm 3+ ion.
24
20
16
,........-....
IE
U
M
0
~
><
~
12
E
c
'-""
~
~
~
E
E
c
~
~
c
~
8
3
Hs
4
o
r
r
3t\,
Tnf+-
Figure 28
Energy levels in Tm 3+ ion and the assigned transitions
50
Luminescence from Tm is rather bland as opposed the nicely varied structure of and Er
spectra. Fig. 29 illustrates PL from both above and below band gap excitation of the sample.
The observed peaks in the first spectrum are characterized by the transitions shown in Fig.
28. Below band gap excitation results in decreased green luminescence due to ZnO as is the
case with all the other impurities too. Also to be noted is the fact that the emission at 800 nm
here is so strong that to be able to decipher the remaining spectra we had to chop it off by a
factor of 40 (exc 476.5 nm).
Fig. 30 delineates the PLE spectra along with the PL spectrums. The peaks are assigned as
follows to the levels in Tm 3+:
Source: ref [13]
Rise and Decay characteristics of Tm sample showed that even a 5 ms pulse duration could
not completely saturate the rise. Rise and decay times are indicated in the plot Fig. 31 by
f, hand b, d respectively. Summary of rise and decay times for all the samples can be
referred to at the end of Chapter 5 (pg. 60).
Excitation intensity dependance for the peak at 804 nm was investigated and is sketched in
Fig. 32. As seen from the plot at the bottom except for some non linearity around 70 -90 mW
the plot is linear.
51
450
500
550
600
eso
700
7fIJ
Wavelength (rrr]
786.4
PLOlOTm (10K)
806
450
500
550
eco
eso
700
850
Wavelength (rrr]
Figure 29
Top: PL of ZnO:Tm, Li excitation at 325 nrn (10K, 300K)
Transi tions:
478 : IG4 -+ 3H6 ,
650 : IG4 -+ 3F4 ,
800 : 3G4 -+ 3H6
Bottom: PL of ZnO:Tm, Li excitation at 457.9 nrn and 476.5 nm (10K)
52
PL+PLEOlOTm
800.4
Pllaser@325nm, 10K
~
~
~
~
~
~
~
~
~
~
~
Wavelength (nni
PL + PLE ZnOTm
698.87
800.6
PlE of peak 800nm, 10K
651.4
664.2
~
~
~
~
~
~
~
~
~
~
~
Wavelength (nrr}
Top: PL + PLE of ZnO:Tm, Li PL exc at 325 nrn, PLE for peak at 800 nrn
Bottom: PL of ZnO:Tm, Li PL exc at 476.5 nrn, PLE for peak at 800 nrn
Transitions:
478 : IG4 3H6 ,
650 : IG4 -+ 3F4 ,
800 : 3G4 3H6
Assignments for PLE: 467,476: IG4 3H6 , 671: 3P2 3H6 , 698: 3P3 3H6
Figure 30
-+
-+
-+
-+
-+
53
PL of znOTm monitored at 801.2nm
excited by 488nm(10K)
0.30
Fitting Equations
Rise: y=e(1-exp(-x/f»+g(1-exp(-x/h»
Decay: y=a(exp(-(x-i)/b»+e(exp(-(x-i)/d» (x>50
f=370.52 h=2075.48 b=291.36 d=1253.15
...-...
::::s
cti
"-'"
~
~ 0.15
Q)
C
tr
0.00
4(0)
6CXX)
Wavelength (nm)
Figure 31
Photoluminescence kinetics of ZnO:Tm, Li monitored at peak 801.2 nrn
excitation at 488 nrn (10K) (transition: 3G4 -. 3H6)
Note: Constants f, h, band d indicate times in IlS
54
PL of ZrQTm
excitedby 488rm (15K)
to showIntensitydependcn:;e
of peak 804rm
ExcitationpaNel' (mW)
a: 100
b: 95
c:
90
d: 85
e: 79
f: 70
g: 65
h:
i:
j:
k:
58
51
44
38
I: 32.5
m: 28
n: 23
0: 19
p: 14
q: 11
r: 8
s: 5
t: 2.5
740
em
-nD
700
840
Wavelength (nni
1.0-
PL of ZnQTmexcitedby 488nm (15K)
Excitationintensitydependance of peak at 804rm
•
0.8-
~
•
0.6-
(J)
j
J;
~
•
• •
S'
~
• •
•
0.4-
0.2-
,
0
••
•
••
•
•
•
•
•
•
•
,
,
,
I
I
I
I
I
I
I
10
2)
:D
40
50
60
70
00
00
100
Bccitation Fbwer (rrW/rrrrf)
Figure 32 Top: Excitation Intensity Dependance of ZnO:Tm, Li excited by 488 nrn (15K)
Bottom: Variation of peak 804 nrn ( transition: 3G4 -+ 3H6) with
excitation power
55
4.5 Zinc oxide doped with Ytterbium and Lithium (ZnO: Yb, Li)
Relatively few references were found for investigating Yb 3+ ion's energy levels. The only
possible energy levels in Yb3+ are depicted in Figure 33 below.
14
12
2F /
S 2
10
....-..
~
IE
(t')u
0
,....
8
x
............
~
~
E
6
tE
c:
~
4
2
2
0
F7 / 2
Yb3+
Figure 33
Energy levels in Yb3+ ion and the assigned transition
56
PL of ZnO: Yb, Li is plotted in Figure 34. The excitation used was above band gap, 325 nm
laser line of the He-Cd laser. Two spectrums were recorded at different temperatures.
In the plot we find that the curve for 300K does show a few sharp emissions peaking at
651.31 nrn, 975.31 nm, which are yet to be investigated. The luminescence is decreases as
we increase the temperature which is generally true for ZnO as we have seen in out other
results. The detection system used (CCD) limits the measurement for wavelengths in regions
above 900 nm drastically. From the response curve for the TE-512SB model we find that it
has only 3% Quantum Efficiency at 1000 nm. Thus it is a limitation due to which further
investigation of the 1000 nm peak as observed by other authors was not possible. Ref [20]
The bottom plot in Figure 34 features the PLE monitored at 1002 nm and it shows a very
sharp peak at 503.29 nm which is still a mystery to us. Other peaks in the PLE spectrum were
at 964.9 and 984.99 nm.
The kinetics rise and decay of the PL were measured and are reported in Fig. 35. The data
plotted is from a experiment wherein a laser pulse, 5 ms long, of wavelength 514.5 nm,
excited the sample and the luminescence from the sample was monitored at 979.3 nm at
10K. The rise is fitted to a double exponential curve giving a fast rise time of 37.41 Jls and
slow rise time of 420.27 us, The decay curve fits ideally to a fast decay time of 183.46 us and
a slow decay time of 1019.73 us,
57
PL ZnQYblaser@325rm
~
\
/
I
I
...~---,."
360
I
~
I
/
-----
~.""'r..I"I""""""'-\
'v
\
920
6••
"""-l""
960
10K
--~
1000
\
\
/ /
400
\
300K
'> M.~ ~~~
-
""---.---.. ..
500
Em
700
ax>
ton
Wavelength (nni
PL + PLEZnQYb
503.29
C')
~
S"
~
~
co
~
C\I
0)
r-,
CD
"I't
ci
m
~
L()
PLE of peak 1002nm, 10K
~
~
~
~
~
~
~
~
~
~
1~
Wavelength (nIT)
Figure 34
Top: PL of ZnO:Yb Li, excitation at 325 nm (10K, 300K)
Bottom: PL + PLE of ZnO:Yb Li, PLE monito red at 1002 nm
PL exc 514.5 nm
Transi tion in PL spectr a: 1000 nm: 2F / -+ 2P7/2
s2
58
PL of znaVb roonitored at 979.3nm
excitation by 515nm (10K)
Rtting Equations
Rise: y=e(1-exp(-x/f»-+g(1-exp(-x/h»
Decay: y=a(exp(-(x-i)/b»+e(exp(-(x-i)/d»
(x>4800)
f=420.27 h=37.41 b=183.46 d=1019.73
o
4COO
€(XX)
10c00
lirre (JiS)
Figure 35
Photoluminescence kinetics of ZnO:Yb Li, monitored at 979.3 nm
excited by 514.5 nm (10K) (Transition: 1000 nm: 2PS/2 --. 2F7/2)
Note: Constants f, h, band d indicate time in us
59
Chapter 5
Conclusions and Summary
The research undertaken is up-to-date with the currently ongoing effort by scientists allover
the world to better understand the microscopic processes involved in semiconductors. Zinc
oxide's optical potential has been relatively less investigated till the present date, and this
thesis attempts to further the understanding of intricate processes within ZnO when doped
with rare earth elements.
Typically PL temperature dependance, excitation wavelength dependance, intensity
dependance, kinetics ofPL for ZnO doped with the rare earths in this thesis have been made
available for the first time due to this research. Some previously unobserved emission lines
have been detected.
The method of preparation of sample and specifically the annealing conditions have made
it possible for us to see the previously undetected data and hence represents an original work.
Summarily this thesis was fruitful in achieving the objective of the research work undertaken
and would prove to be helpful in furthering our insight of ZnO as an optically viable
semiconductor.
60
Future directions on the subject should be directed towards investigation of
Cathodoluminescence and Electroluminescence of the samples. Repeating the same research
for crystalline ZnO implanted with the rare earths as dopants is right now in progress at the
quantum electronics laboratory, by Dr. Lozykowski's Ph.D student, Wojtek Jadwisienczak.
The following table summarizes the kinetics rise and decay times of the photoluminescence
data for the samples investigated. Details regarding the kinetics experiment conditions can
be referred to in chapter 3.3.
TABLE 2 Summary of Rise and Decay times
Sample
Rise time (us)
Decay time (fl s)
ZnO:Dy, Li
Fast-17.58
Slow - 314.9 1
301.2
ZnO:Er, Li
Fast - 69.7
Slow - 205.78
95.9
ZnO:Nd, Li
305.01
ZnO:Tm,Li
Fast - 370.52
ZnO:Yb, Li
Fast - 37.41
Fast - 304.8
Slow - 305.67
Slow - 2075.48
Fast - 291.36
Slow - 1253.15
Slow - 420.27
Fast - 183.46
Slow - 1019.73
Note that the fast and slow times indicate that the curve was fitted with a double
exponential equation, columns that have only one time indicate that the curve fitting used
only single exponential equation.
61
REFERENCES
[1]
Reboul, "Some aspects of the luminescence of chemically pure zinc oxide", Ph.D
thesis. University of Pennsylvania. (1953)
[2]
H. E. Brown, Zinc oxide rediscovered. The New Jersey Zinc Company, New York,
pp. 42 (1957)
[3]
o. F. Schrimer and D. Zwingel, "The yellow luminescence of zinc oxide", Solid State
Communications, Vol. 8,
pp.1559
(1970)
[4]
B. J. Pierce and R. L. Hengehold, "Depth-resolved cathodoluminescence of ionimplanted layers in zinc oxide", Journal ofApplied Physics, Vol. 47, No.2,
pp.644 (1976)
[5]
J. A. Garcia, A. Remon and J. Piqueras, "Influence of Bi and Mn on the green
luminescence of ZnO ceramics", Journal ofApplied Physics, Vol. 62, No.7,
pp. 3058 (1987)
[6]
J. Kossanyi et aI, D. Oelkrug and U. Mammel, G. P. Kelly and F. Wilkinson,
"Photoluminescence of semiconducting zinc oxide containing rare earth ions as
impurities", Journal ofLuminescence, Vo1.46, pp.17 (1990)
[7]
J. Kossanyi et al, D. Oelkrug and U. Mammel, "Quenching of zinc oxide Photo
luminescence by d- and f- transition metal ions", Journal ofLuminescence, Vo1.46,
pp.329 (1990)
[8]
J. C. Ronfard-Haret, D. Kouyate and J. Kossanyi, "Evidence for direct impactexcitation of luminescent rare-earth centers (Ho 3+ and Sm3+) in semiconducting zinc
oxide", Solid State Communications, Vol.79, No.1, pp.85 (1991)
[9]
D. Kouyate, J. C. Ronfard-Haret and J. Kossanyi, "Electroluminescence of rareearths doped semiconducting zinc oxide electrodes", Journal ofLuminescence,
Vol. 55, pp.209 (1993)
[10]
S. Bachir, J. C. Ronfard-Haret, K. Azuma, D. Kouyate and J. Kossanyi, "Direct
impact excitation of thulium (ill) luminescence in polycrystalline ZnO:Tm3+
electrodes in contact with an aqueous electrolyte and attribution of the luminescence
spectrum", Chemical Physics Letters, Vol.213, pp.213 (1993)
62
[11]
S. Bachir, J. Kossanyi and J. C. Ronfard-Haret, "Electroluminescence of Ho 3+ ions
in a ZnO varistor-type structure", Solid State Communications, Vo1.89, No.IO,
pp.859 (1994)
[12]
S. Bachir, J. Kossanyi, C. Sandouly, P. Valat and J. C. Ronfard-Haret,
"Electroluminescence of Dy3+ and Sm 3+ ions in polycrystalline semiconducting zinc
oxide", Journal ofPhysics and Chemistry, Vol. 99, pp.5674 (1995)
[13]
S. Bachir, K. Azuma, J. Kossanyi, P. Valat and J. C. Ronfard-Haret,
"Photoluminescence of polycrystalline zinc oxide co-activated with trivalent rare
earth ions and lithium. Insertion of rare earth ions into zinc oxide", Journal of
Luminescence, Vol. 75, pp.35 (1997)
[14]
Y. Hayashi, H. Narahara, T. Uchida, T. Noguchi and S. Ibuki, "Photoluminescence
of Eu-doped ZnO phosphors", Japanese Journal ofApplied Physics, Vo1.34,
pp. 1878 (1995)
[15]
K.Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt and B. E.
Gnade, "Mechanisms behind green photoluminescence in ZnO phosphor powders",
Journal ofApplied Physics, Vol. 79, No.10, pp.7983 (1996)
[16]
K.Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. Caruso, M. J.
Hampden-Smith, T. T. Kodas, "Green photoluminescence efficiency and free -carrier
density in ZnO phosphor powders prepared by spray pyrolysis",
(1997)
Journal ofLuminescence, Vol. 75, pp. 11
[17]
S. Larach, "Group II-VI phosphors with rare earth activators", Proceedings of
International Conference on Luminescence, pp. 1549-1569 (1966)
[18]
E. W. Chase, R. T. Hepplewhite, D. C. Krupka and D. Kaiing, "Electroluminescence
of ZnS lumocen devices containing rare-earth and transition-metal fluorides",
Journal ofApplied Physics, Vol. 40, No.6, pp.2512 (1969)
[19]
H. J. Lozykowski and I. Szczurek, "Electroluminescence ofZnSe thin films doped
with rare-earth fluorides", Journal of Luminescence, Vol. 14, pp.389 (1976)
[20]
V. P. Dotsenko and N. P. Efryushina, "Static energy transfer in YAI3B4012:Ln3+",
Physics Status Solidi (a), Vol. 130, pp.199
(1992)
[21]
A. Garcia, F. Guillen and C. Fouassier, "Charge transfer excitation of the Nd 3+, Sm 3+,
Dy3+ ,Ho3+, Er 3+ and Tm 3+ emission in CaGa 2S 4", Journal ofLuminescence, Vol.33,
pp. 15
(1985)
63
[22]
R. Ramanowski, "Investigation of radiative and non-radiative transitions of Er 3+ in
BaLaGa 30 / ', Journal ofPhysics and Chemistry of solids, Vol.51, No.3,
pp. 229 (1990)
[23]
S. Larach, R. E. Shrader and P. N. Yocom, "Luminescence from erbium-activated
group II-VI compounds containing alkali metal compensators", Journal of
Electrochemical Society, Solid State Science, Vol. 116, No.4, pp.471 (1969)
[24]
B. M. Walsh, N. P. Barnes and B. Di Bartolo, "Branching ratios, cross sections and
radiative lifetimes of rare earth ions in solids: Application to Tm 3+ and Ho 3+ ions in
LiYF4 " , Journal ofApplied Physics, Vol.83, No.5, pp.2772
(1998)
[25]
N. N. Grigoryev, M. V. Fok and L. Yastrabik, "Spectroscopy of ZnS-Tm
luminescence centers", Proceedings of Lebedev Physics Institute, Academy of
Sciences of the USSR, Vol.175, Nova Science Publishers, pp.73
(1986)
[26]
B.T.Collins, J. Kane, M. Ling, R. T. Tuenge and S-S. Sun, "Preparation and
photoluminescence of thulium-activated zinc sulfide films", Journal of
Electrochemical Society, Vol.138, No.1, pp.3515
(1991)
[27]
N. Garnier, R. Moncorge, H. Manaa, E. Descroix, P. Laporte and Y. Guyot, "Excitedstate absorption of Tm 3+ doped single crystals at photon-avalanche wavelengths",
Journal ofApplied Physics, Vol.79, No.8, pp.4323
(1996)
[28]
H. Masui, "Luminescent thulium centers in ZnSe crystals doped with thulium and
lithium", Journal ofPhysics and Chemistry of Solids, Vol.33, pp. 1129 (1972)
[29]
K. Heinz, "Structural analysis of surfaces by LEED", Progress in Surface Sciences,
Vol. 27, pp.236 (1988)
[30]
D. K. Schroder, Semiconductor Material and Device Characterization, John
Wiley and Sons Inc., New York, pp.490
(1990)
[31]
L. I. Berger, Semiconductor Materials, CRC Press, Boca Raton, pp. 183 (1997)
[32]
D. Walsh, "Luminescence and crystal damage in ion implanted Cds and ZnO",
Solid State Electronics, Vol. 20, pp.813-815 (1977)
[33]
H. Yoshikawa and S. Adachi, "Optical constants of ZnO", Japanese Journal of
Applied Physics, Vol.36, pp.6237
(1997)
64
[34]
R. T. Girard, o. Tjernberg, G. Chiaia, S. Soderholm, U. o. Karlsson, C. Wigren, H.
Nylen and I. Lindau, "Electronic structure of ZnO (0001) studied by angle-resolved
photoelectron spectroscopy", Surface Science, Vol. 373, pp.409-417 (1997)