Growth of extremely uniform layers by rotating substrate holder with
molecular beam epitaxy for applications to electrooptic and microwave
devices
A. Y. Cho and K. Y. Cheng
Citation: Appl. Phys. Lett. 38, 360 (1981); doi: 10.1063/1.92377
View online: http://dx.doi.org/10.1063/1.92377
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Published by the American Institute of Physics.
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Growth of extremely uniform layers by rotating substrate holder with
molecular beam epitaxy for applications to electro-optic and microwave
devices
A. Y. Cho and K. Y. Cheng a)
Bell Laboratories, Murray Hill, New Jersey 07974
(Received 20 September 19S0; accepted for publication 12 December 19S0)
Integrated optics and integrated microwave circuits require extremely uniform epitaxial layer
thicknesses, composition profiles, and doping profiles. With a sample rotating mechanism,
molecular beam epitaxy can for the first time prepare GaAs and AI, Gal _ , As layers with
thickness variation ofless than 1% over a lateral dimension of 5 cm. The variation of AlAs mole
fraction ofthe AIo'3Gao'7As over a to-cm 2 area was less than 0.4%. The variation of the "pinchoff" voltage for a field effect transistor structure was less than 1.4% over a to-cm 2 wafer. These
results represent the most uniform epitaxial layers ever prepared with any crystal growth
technology.
PACS numbers: S1.15.Ef, 73.60.Fw, 72.S0.Ey, S1.10.Bk
The development of microelectronics depends in large
measure on advancement in the preparation of thin epitaxial
semiconducting layers. As the device becomes more miniaturized, the requirements for the film material become more
stringent. Many devices require abrupt doping profiles, 1-3
precise film thicknesses,4-7 and excellent surface morphology in order to be compatible with fine-line lithography. The
prerequisite for the development of integrated optics is the
availability of extremely uniform multilayered structures so
that the mesa-etching process can be performed in a controllable manner. Similarly, for the fabrication of integrated microwave circuits, one requires extreme uniformity in doping
profiles in addition to the uniformity in layer thickness so
that all the identical devices will perform identically with the
same biasing voltage. Even for discrete devices, uniformity is
extremely important for the yield in fabrication.
In the past, although molecular beam epitaxy (MBE)
(Ref. S) can prepare epitaxial layers in atomic dimensions,
the uniformity of the layer over a large area was limited by
the inherent intensity profile of the molecular beam at the
substrate surface. The profile may follow a cosine distribution or a modified intensity9 depending on the effusion cell
geometry. We have overcome this difficulty, and this work
describes the excellent uniformity of the growth of GaAs
and AI, Gal _ • As epitaxial layers with thickness variations
less than 1% over a wafer 5 cm in diameter using a rotating
substrate holder in the MBE system. 10
Figure 1 shows the top view of the MBE system. The
substrate holder can accommodate wafers 5 em in diameter,
and it can rotate continuously at a speed offrom 0.1 to 5 rpm.
The results reported in this letter were obtained with a rotating speed of 2 rpm. All the effusion cells are mounted between 5 and 32° from the horizontal plane. They have apertures 2.5 cm in diameter and are located 12 cm from the
substrate. A liquid-nitrogen-cooled shroud is used to enclose
the entire growth area in order to minimize the residual wa-
ter vapor and carbon-containing gases in the vacuum chamber during epitaxy.
Three separate experiments were performed to evaluate
the uniformity of the epitaxial layer: The first measured the
cleaved cross section of the GaAs and AI, Gal _ ,As layers
with a Normarski interference microscope after they were
stained in a solution composed ofHN0 3:H 20 = 1:3. This
measurement will determine the uniformity in growth rates.
The second measured the wavelength of the photoluminescent peaks of AI, Gal _ x As layers across the entire wafer in
order to determine the uniformity in chemical composition.
The third measured the distribution of the carrier concentration and the pinch-off voltage of a thin n-type layer grown on
a semi-insulating substrate. This will determine the uniformity of impurity incorporation and the feasiblity for application to integrated circuits.
The sample preparation is similar to that described earlier. 5 The epitaxial growth was carried out at 580 T after a
a'On leave from Chung Cheng Institute ofTechno!ogy, Taiwan. Republic of
China.
FI G. 1. Schematic of the MBE system viewed from the top. The rotating
sample holder has a variable speed from 0.1 to 5 rpm.
360
Appl. Phys. Lett. 38 (5).1 March 1981
HEED GUN
LIQUID NITROGEN
COOLED SHROUDS
MAIN SHUTTER
/
ROTATING SUBSTRATE
HOLDER
1
SAMPLE
EXCHANGE
LOAD LDCK
EFFUSION
CELL
SHUTTERS
FLUORESCENT
SCREEN
0003-6951/81/050360-03$00.50
TO VARIABLE
SPEED MOTOR
AND SUBSTRATE
HE ATER SUPPLY
© 1981 American Institute of Physics
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360
flash desorption of the surface oxide at 620 ·C. The Ga effusion cell was heated to lO60·C to give a growth rate of 2.25
,umlh. The As effusion cell was heated to 350 ·C to permit
growth under an As-stabalized condition, II and the As background pressure in the growth chamber during epitaxy was 6
X 10- 8 Torr. In the case of growing Alx Gal _ xAs, the additional Al effusion cell was heated to 1140 ·C for a growth rate
of 3.22 ,u/h of Alo.3Gao'7As.
The thickness distribution across a 5-cm wafer showed
a variation oflarger than 8% when the substrate was not
rotated and the variation reduced to less than 1% when the
substrate was rotated with a speed of2 rpm. Figure 2 shows a
series of photoluminescent peaks of an Alo.3Gao'7As layer on
a 4-cm wafer. Peaks labeled (a)--(e) correspond to the positions where the photoluminescence was taken on the wafer
as indicated in the insert. It is seen here that the variation of
the peak wavelengths is less than 20 A over the whole wafer.
This may be converted to variation of the AlAs mole fractions of 0.004 of the Alo'3Gao'7As layer over a 4-cm distance
on the wafer. We believe this is the most uniform
Alx Gal _ x As layer ever reported.
Another critical evaluation of the uniformity is to study
the impurity incorporation characteristics. An undoped
Ao.3Gao'7As 2-,um-thick layer was first grown as a buffer
layer on a Cr-doped semi-insulating substrate followed by
the growth of a Sn-doped n-type GaAs 6ooo-A-thick (metallurgic thickness). 12 Schottky-barrier diodes were formed
over the whole wafer by evaporating Au dots through a mask
with opening areas of 4 X 1O-~ cm -2. This permits the
assessment of the net donor concentration, variation with
distance both in the film growth direction and the lateral
direction on the wafer, and the variation in pinch-off voltage.
The pinch-off voltage referred to in this letter is defined as
the voltage required to deplete the ionized donors to a value
one order of magnitude below the doping level when measured with a doping profiler. 13
The depletion width I may be expressed as,14
0
1---4 em~
30.64
30.53
30.59
3021
30.30
X!l~)= 6900
6905
6902
6920
6916
j
j
j
I
X(%)=
,..
f-
<ii
1.0
j
z
~t=
z_
7~
f-
z,..
tJ ~
0.5
UlII:
~~
:ifl1
=>..:
-'afa:r
"-
(0)
(b)
(e)
WAVELENGTH, X(
(d)
(e)
A)
FIG. 2. Five room-temperature photoluminescent spectra (aHe) of
Alo.)Ga.,.,As taken at various locations on the lO-cm2 wafer. The variation
of the peak wavelength is 20 A, which corresponds to a variation of the AlAs
mole fraction of 0.4%.
361
Appl. Phys. Lett., Vol. 38, No.5, 1 March 1981
17
10 , - - - - - - - - , - - - - - - - - , - - - - - - - - - , - - - - - - - - ,
~
I~I
:-4cm---
",-
'§
c:
z
Q
f-
~
f-
z
16
10
w
u
z
a
u
II:
W
Il:
II:
..:
u
1015
L -_ _ _ _ _ _
o
~
________
0.2
~
_ _ _ _ _ _ _ _L -______
OA
OS
~
08
DISTANCE FROM SURFACE, d (/Lm)
FIG. 3. Net donor concentration ND - NA asa function of depletion depth
for a Sn-doped GaAs layer on Cr-doped semi-insulating substrate. Five traces taken at different locations (aHe) on a lO-cm 2 substrate as shown in
the insert are superimposed on each other. The uniformity of the carrier
concentration and the layer thickness is illustrated in this figure.
1= [(2€/qn)(Vbi - V-kT/q)]1!2,
where € is the permittivity of the semiconductor, q is the
electron charge, n is the donor impurity, Vbi is the built-in
potential, and Vis the bias voltage. It is clear from the above
expression that the bias voltage used to pinch off the conducting layer is proportional to n1 2 • In other words, the distribution of the pinch-off voltage over the wafer is a measure
of the variation of the product of the impurity concentration
and the layer thickness squared. This parameter determines
the yield in fabrication of the field effect transistor, which is
one of the most versatile microwave devices because it can be
used in a circuit of a mixer, oscillator, low noise amplifier,
and high-power generator. Most recently, it is used in integrated circuits as a high-speed switching element. As we
mentioned earlier, uniformity becomes even more important
when integration of these devices on one chip is required.
Figure 3 shows five superimposed doping profiles measured over a 4-cm wafer. The reverse bias voltage required to
deplete the layer where the net donor concentration decreased from 5 X 10 16 to 5 X 10 15 cm - 3 was about 5 V. The
variation of this pinch-off voltage is from 4.96 to 5.03 V over
the entire wafer. This represents a variation in the n/ 2 of
about 1.4% over a 4-cm wafer. The pinch-off voltage distribution measured in this manner actually represents the upper limit of the variation in nl 2 because the performance of
the ohmic contacts used in this measurement may not be
identical. The ohmic contacts were formed by electric arching of five Au-2% Sn wires adjacent to the Schottky-barrier
dots on the surface of the substrate. The uniformity of the net
donor concentration over the wafer is illustrated by the coincidence of these traces in Fig. 3. The electrical thickness
defined earlier l2 as where the donor concentration decreased
by a factor of 10 when measured with a doping profiler is
4750 A. Results on this uniform layer suggest that the elecA. Y. Cho and K. Y. Cheng
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361
trical thickness of a layer doped to 5 X 10 16 cm - 3 is 1250 A
less than the metallurgical thickness.
In summary, we have achieved the growth of extremely
uniform GaAs and Alx Gal _ xAs epitaxial layers. The thickness uniformity over a 5-cm wafer is better than 1%. The
photoluminescent studies showed that the Alo. 3 Gao. 7As alloy composition is uniform to 0.004 AlAs mole fraction over
a 4-cm wafer. The variation of the pinch-off voltage which
represents the value of nl 2 of the epitaxial layer is only 1.4%
over a 4-cm wafer. Further improvement of these values is
expected when all the parameters are optimized. The performance reported here encourages the development of integrated optics and integrated microwave circuits.
We would like to acknowledge that implementation of
the design of the rotating sample holder resulted from discussions among one of us (A. Y. C.) and J. V. DiLorenzo and
J. C. M. Hwang of Bell Laboratories and C. Buisson of Riber/Division ofInstruments S. A.
1M. V. Schneider, R. A. Linke, and A. Y. Cho, Appl. Phys. Lett. 31, 219
(1977).
2T. Mimura, S. Hiyamizu, T. Fujii, and K. Nanbu, Jpn J. App!. Phys. 19,
L225 (1980).
3L. F. Eastman, R. Stall, D. Woodard, N. Dandekar, C. E. C. Wood, M. S.
Shur, and K. Board, Electron. Lett. IS, 524 (1980).
4A. Y. Cho, J. V. DiLorenzo, B. S. Hewitt, W. C. Niehaus, W. O. Scholosser, and C. Radice, J. Appl. Phys. 48,346 (1977)
5 A. Y. Cho, H. C. Casey, Jr., C. Radice, and P. W. Foy, Electron. Lett. 16,
49 (1980).
"H. Ohno, J. Barnard, C. E. C. Wood, and L. F. Eastman, IEEE Electron.
Lett. EDL.l, 154 (1980).
7W. T. Tsang, C. Weisbuch, R. C. Miller, and R. Dingle, App!. Phys. Lett.
35, 673 (1979).
MA. Y. Cho and J. R. Arthur, in Progress in Solid State Chemistry, Vol. 10,
Edited by G. Somorhai and J. McCaldin (Pergamon, New York, 1975), p.
xx.
9B. B. Dayton, in 1956 Vacuum Symposium Transactions, (Committee on
Vacuum Techniques, Boston, 1956), p. 5.
"'The system used in this work was model No. 2300 manufactured by
Riber.
!lA. Y. Cho, J. Appl. Phys. 41, 2074 (1971).
12A. Y. Cho, J. V. DiLorenzo, B. S. Hewitt, W. C. Niehaus, W. O.
Schlosser, and C. Radice, J. Appl. Phys. 48, 346 (1977).
"G. L. Miller, IEEE Trans. Electron Devices ED·19, 1103 (1972).
l4S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969), p.
370.
Photovoltaic detectors in SnS produced by Sb + ion implantation
D. Trbojevic, P. M. Nikolic, a) B. Perovic, and V. Cvekic a)
Laboratory for Atomic Physics, "Boris Kidric" Institute, POB 522, 11001 Belgrade, Yugoslavia
(Received 30 May 1980; accepted for publication 12 December 1980)
Np junction photovoltaic detectors in thep-type single-crystal SnS have been produced by Sb + ion
implantation. Current-voltage and capacitance· voltage characteristics have been measured at 77
and 295 K. At 77 K, diodes had zero-bias resistance area products of 1.5 X 103 [ } cm 2 • The
spectral response has been measured at 77 and 295 K. At 295 K, the peak responsivity was at a
wavelength ofO.76,um. At 77 K, the peak detectivity, which occurred at 0.62,um, was 1.3 X 1011
cm Hzl/2 W- I with a peak quantum efficiency of 44%.
PACS numbers: 72.40.
+ w, 61.70.Tm, 73.40.Lq, 72.80.Ey
In this letter, we present some results on photovoltaic
SnS detectors obtained by Sb + ion implantation. The p-type
single-crystals used in these experiments were grown by the
Bridgman method.
The SnS crystal is characterized by an orthorombic
double-layered structure (which can be described as a deformed NaCI structure) resulting in easy cleavage planes
(00 1). SnS is a semiconductor with an indirect energy gap of
"Faculty of Electrical Engineering, Belgrade University, Belgrade,
Yugoslavia
362
Appl. Phys. Lett. 38 (5), 1 March 1981
1.08 and 1.11 eV at 295 and 77 K, respectively. 1-5
The Bridgman samples were cleaved into slices
(15 X 10 X 1 mm) with a (00 1) surface plane. This was confirmed by x-ray diffraction. At 295 K, the value of specific
resistance was 0.23 [} cm. Hall-effect measurements with a
bar measuring 11 X 9 X 0.3 mm showed a p-type carrier concentration of about 5 X 10 17 cm- 3 at 295 K.
After mechanical polishing (using Al 2 0 3 powder 0.1
,urn), the samples were electrolytically etched. 6 Mechanical
copper implantation masks, 0.1 mm thick, were used. Apertures with a I-mm diameter were opened in the masks by
0003·6951/81/050362-03$00.50
© 1981 American Institute of Physics
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362