XPS study of cleaning procedures of ZnO (0001), (000-1), (10-10) surfaces
Kumarappan Kumara and Greg Hughesa, Enda McGlynnb and Mahua Biswasb
aSurface
Science Research Laboratory, School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
bSchool of Physical Sciences / National Centre for Plasma Science and Technology, Dublin City University, Dublin 9, Ireland .
Introduction
Experimental
Applications of ZnO Substrate
SAW devices
Unique properties of ZnO
Varistor
Piezoelectric
devices
Transparent
Conductors
Gas Sensor
Widebandgap of 3.4eV at 300K
Exciton binding energy of 60meV
Transparent conducting oxide (TCO)
Piezoelectric and Pryroelectric
(0001)-Zn face
X-ray Photoelectron Spectroscopy
Substrate for
UV Laser
Substrate for LED
UHV Chamber
VG photoelectron spectrometer
X-ray source: Mg-Kα (1253.6eV) and AlKα(1486.6eV)
Electron analyser : CLAM2 cylindrical
Pass energy of the analyser : 20eV
Energy resolution :1.0eV.
Pressure in analyser : 1 X10-9mbar
Radiation
detectors
Substrate for
Spintronic devices
Photomultiplier
Surface science of ZnO
Prism face
(000-1)-O face
An important field of research due to different chemical and physical
properties of its polar and non-polar surfaces.
The polar (0001) & (000-1) surfaces of ZnO are highly hydrophilic
and readily adsorb ambient contaminants consisting of hydroxide
species and carbon containing compounds.
The non-polar (10-10) surface is more chemically stable in ambient
conditions due to absence of electrostatic instabilities.
In-situ Argon sputter cleaning
Auger gun
(10-10)-
Sample
Applied voltage: 5KV
Sample current; 50µA
Duration: 1hour
Base pressure: 10-7mbar
Ex-situ chemical cleaning
Surface cleaning of single crystal surfaces enhances
1.Lattice matching,
2.Ohmic contact
3.Chemical stability
4.Surface conductivity
The development of surface cleaning procedures of ZnO surfaces is essential for homoepitaxial growth of films and hetero-epitaxial growth of III-nitrides device structures.
The three step 10 minute ultrasonic chemical cleaning process investigated involved the initial degrease in
acetone followed by a clean in dimethylsulfoxyde (DSMO) and a final clean with toluene (99.9%). After
each cleaning step the crystal was dried in flowing nitrogen.
Role of chemicals
Acetone (CH3COOCH3) = removes surface organic contamination layer
Motivation
DMSO(C2H6OS) = reduces number of clusters on surface and dissolve polar organic contaminants
The aim of this study was to systematically investigate
(1) the effectiveness of organic solvent based wet chemical cleaning
(2) argon ion sputtering physical cleaning procedures at removing the surface contamination
layer.
Toluene(C7H8[C6H5CH3) = water replant, avoids further surface hydration and dissolves non-polar organic
contaminants
Results and Discussion
Model of H, OH, H2O, Zn(OH)2 contamination on ZnO surface
-Zn
XPS core level spectra of carbon (C1s), Oxygen( O1s), Zinc( Zn2p3/2)
As received
Chemical cleaning
Argon cleaning
(0001)-Zn face
O-Zn
45000
Zn(OH)2
H2O
O
H
H2O
O
H
Zn(OH)2
-H
H2O
H
H
ZnO Zn(OH)2
300000
40000
30000
35000
25000
30000
250000
200000
20000
15000
Intensity
25000
Intensity
Intensity (a.u)
35000
OH H2O
Zn(OH)2
O
H
-O
20000
15000
10000
10000
5000
5000
150000
100000
50000
0
0
0
-5000
-5000
280
282
284
286
288
290
526
292
528
530
532
534
536
538
1018
1020
1022
Binding energy (eV)
Binding Energy (ev)
1024
1026
1028
Bindind energy (eV)
(000-1)-O face
(0001)
80000
(000-1)-O face
(10-10)-prism face
250000
70000
200000
30000
Intensity
Intensity
40000
40000
100000
Contamination analysis
50000
10000
(1-210)
150000
20000
20000
(0001)
(-1-100)
(1-100)
60000
50000
(-1-120)
(11-20)
80000
60000
Intensity
(0001)-Zn face
ZnO Zn(OH)2
O-Zn OH H2O
100000
(10-10)
(0001)
0
0
0
-20000
526
-10000
280
282
284
286
288
290
292
(0001) Zn-terminated surface
528
530
532
534
536
538
1018
Binding energy(eV)
Binding Energy (eV)
1020
1022
1024
1026
(000-1) O-terminated surface
99.9
1028
(10-10) mixed-terminated surface
90
Binding energy (eV)
95
99
20000
60000
40000
150000
OH
H2O
1
100000
10000
5
OH
C OH
75
50
25
OH
10
5
H2O
OH
20000
50000
0
1
H2O
0
280
282
284
286
288
290
292
526
528
530
532
Binding Eenergy (eV)
Surface Stochiometry analysis
Crystal
surface
(0001)-Zn
face
(000-1)-O
face
(10-10) –
mixed face
Cleaning
details
Surface
stochiomerty of
Zn/O
as received
0.38
Organic clean
0.68
Argon sputtered
1.74
as received
0.29
Organic clean
0.65
Argon sputtered
1.18
as received
0.80
Organic clean
0.83
Argon sputtered
1.97
534
536
OH
10
5
OH
OH
H2O
H2O
C
1
0.1
-20000
Binding Energy (eV)
25
OH
0
-10000
278
C
C
Zn(OH)2
200000
10
H2O
Atomic %
30000
80000
v
C
50
C
Zn(OH)2
Atomic %
100000
Intensity
40000
Intensity
Intensity
250000
Atomic %
25
Zn(OH)2
300000
120000
Zn(OH)2
50000
75
95
90
ZnO Zn(OH)2
Zn(OH)2
O-Zn OH H2O
C
C
50
90
Zn(OH)2
C
Zn(OH)2
75
(10-10)-prism face
1018
1020
1022
1024
Binding energy (ev)
1026
As received
Organic cleaning
Ar Sputtering
0.1
As received
Organic cleaning
Ar Sputtering
As received
Organic cleaning
Ar Sputtering
Conclusion
From XPS investigation of ZnO single crystal surfaces, the major elemental components of the contamination layer on the surface were C, OH, H2O,
Zn(OH)2 and Cl.
The ultrasonic chemical cleaning only reduces the carbon signal by 20- 30%, however it increases Zn(OH)2 concentration and leads to increased water
adsorption.
The insitu argon cleaning reduces the carbon by 90% and completes removes water molecules, however, it impacts on the ZnO stoichiometry leaving
the films oxygen deficient.
Comparing the relative reactivity of the three cleaned surfaces investigated, the Zn terminated polar surface is the most reactive while the non-polar
surface is the most chemically stable. This is assessed on terms of the recontamination of the cleaned surface during ambient exposure following the
clean and prior to insertion into vacuum.
These studies indicate that the ZnO surfaces become highly reactive after the removal of the native surface contamination layer and can become recontaminated even in ultrahigh vacuum
International Conference on Solid Films and Surfaces, Trinity College Dublin, Ireland., 29th June – 4th July 2008.