Two novel types of superlattice photodetectors were studied. The first was a superlattice photomultiplier and the second a photodetector based on the real space transfer mechanism. A summary of the results is presented

Chuang, S. L.

Coleman, J. J.

Hess, K.

Leburton, J. P.

English

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SEMIANNUAL REPORT FOR
Grant No. NAG 1-500
SEMICONDUCTOR SUPERLATTICE PhOTODETECTORS
January, 1985 to June 30, 1985
Submitted to
Dr. Charles E. Byvik
FED M/S 283
National Aeronautics and Space Administration
Langley Research Center, Hampton, VA 23665
r.j-Cry
r.
Prepared by
S. L. Chuang
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaing
Urbana, IL 61801
and
K. Hess, J. J. Coleman, and J. P. Leburton
Department of Electrical and Computer Engineering
and
Coordinated Science Laboratory
University of Illinois at Urbana-Champaign
Urbana, IL 61801
;NASA-CR-176249) SEK ONDOC'.TOR SUPEPLATTIC	 N86-1^494
PNO^_'ODETSCTORS Semiannual Report, Jan. - 3D
,Tun. 1985 (Illinois Univ. ,
nrbana-Champaign.) 23 p HC A l 2/.",F A01	 Onclas
CSCL 14B 63/35 15723
rrr^
SEMIANNUAL REPORT FOR
Grant No. NAG 1-500
SEMICONDUCTOR SUPERLATTICE PHOTODETECTORS
January, 1985 to June 30, 1985
Submitted to
Dr. Charles E. Byvik
FED M/S 283
National Aeronautics and Space Administration
Langley Research Center, Hampton, VA 23665
' k.-
Prepared by
S. L. Chuang
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaing
Urbana, IL 61801
and
K. Hess, J. J. Coleman, and J. P. Leburton
Department of Electrical and Computer Engineering
and
Coordinated Science Laboratory
University of Illinois at Urbana-Champaign
Urbana, IL 61801
t
4{
1
a;
ii
,w
TABLE OF CONTENTS
Page
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 	 1
1.	 Period.	 . . . . . . . . . . . . . . . . . . . . 	 l
_	 2.	 Reporting Date . . . . . . . . . . . . . . . . . . . . 	 1
" t
	 3.	 Technical Personnel . . . . . . . . . . . . . . . . . 	 1
II. TECHNICAL PROGRESS . . . . . . . . . . . . . . . . . . . .	 2
1. Superlattice Photomultiplier . . . . . . . . . . . . . 	 2
2. A Photodector Based on Real Space Transfer
	
sMechanism . . . . . . . . . . . . . . . . . . . . . . 	 4
:_ a	 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . .
	 8
FIGURES . . . . . . . . . . . . 	 . . . . . . . . . . . . . .	 10
r
3Tli
1
I.	 INTRODUCTION
The research grant NAG 1-500 entitled "Semiconductor Superlattice
Photodetectors" was awarded to the University -)f Illinois by NASA-Langley
Research Center on June 27, 1984. Dr. Charles E. Byvik of NASA is the
Technical Officer, and Mr. John F. Royall is the Crants Officer. The total
amount of funds received by the University is $62,801 to cover the period from
July 1, 1984 to June 30, 1985.
This report is the second semiannual report.
1.	 Period
January 1,	 1985 to June 30,	 1985.
2.	 Reporting Date
July 5,	 1985 #
3.	 Technical Personnel
S. L.	 Chuang: Assistant Professor of Electrical and Computer
Engineering and TRW Assistant Professor of Electrical
and Computer Engineering
J. J. Coleman: Professor of Electrical and Computer_ Engineering and
Research Professor of Coordinated Science Laboratory
K. Hess Professor of Electrical and Computer Engineering and
Research Professor of Coordinated Science Laboratory
J.	 P. Leburton: Assistant Professor of Electrical and Computer
Engineering and Research Assistant Professor or
Coordinated Science Laboratory
Peter J.
	
hares: Research Assistant of Electrical and Computer
Engineering
41
1
+^..^
2
II. TECdNICAL PROGRESS
We studied two novel types of superlattice photodetectors: (1) a
superlattice photomultiplier and (2) a photodetector based on the real-space
transfer mechanism. A summary of the results is given in this report.
1.	 Superlattice photomultiplier
The configuration of the device is shown in Figure 1. The optically
generated electrons are accelerated by the applied electric field and "impact
ionize" the electrons confined in the quantum wells. Thus it works as a solid
state photomultiplier. An outline of the theoretical formulation has been
given in our first Semiannual Report [ 1] and a detailed discussion will be
presented in a future publication [2].
We have calculated numerically the ionization rate in the computer. An
upper limit of the ionization rate <T> u is obtained easily by ignoring the
fact that the final momenta of the electrons need to be larger than a
threshold value. The exact numerical integration for the ionization rate
<1> takes into account the threshold value for the momenta and needs more
t
computer time. Both results have been calculated and shown in Figures 2 to 7
as functions of the incident electron energy E 0 . The necessary material para-
meters used here are taken from refs. 3 and 4.
The average ionization rate <t> involves the integration over the
incident energy EO and the distribution function. The function <
T
1 >
multiplied by exp( -E0 /kBTe ) is represented by the dashed line in Figure 2. One
sees that the main contribution arises from energies between the threshold
value and 0.6 eV where < t ^ I ) > exp(-EO/kBTe ) has decreased by about two orders
U
of magnitude from its peak value. If the electron temperature T  is
3increased, the slope of the dashed line (-1/kBTe ) will be flatter, and the
average ionization rate will increase.
The upper limit of the impact ionization rate as a function of the energy
EO is shown as the dotted line in Figure 2. This curve is much easier and
faster to evaluate since analytical expressions could be obtained with
a
approximations. They also can serve as a means of checking the numerical
accuracy of the exact numerical integrations.
In Figure 3, we increase the doping concentration in the quantum-well
region to 1.0 x 10 18 cm 3 . Since there are more electrons in the quantum well
than those for the previous case in Figure 2, the ionization rate is larger
than that in Figure 2. In Figure 4, the width L z of the quantum well is
decreased to 360A. The other parameters are the same as those in Figure 2.
The impact ionization starts at lower threshold. This is because the momentum
of the electron in the quantum-well region has the component n/L z which is
increased. It becomes easier to be ionized. In Figure 5, the mol, fraction x
of ARAB in the AtxGa1-xAs region is decreased to 0.20. The band edge
discontinuity AE  is then smaller. Again the impact ionization rate increases
and starts at E0 equal to zero. Thus, AL  provides enough momentum for impact
ionization immediately. Note that we have taken into account the difference
of the effective masses in different regions. The energy due to the momentum
of the electron in the direction parallel to the interface provides part of
the energy required for ionization.
In Figures 6 and 7, we reduce the temperature to T - 77°K, T  = 3b5°K.
We have x = 0.2 for Figure 6 and x = 0.25 for Figure 7. The dashed line
decreases much faster compared with the previous cases with T  = 1500°K as
expected.
z
4In Figure 8, the average ionization rate <t> is shown as the
solid line as a function of the doping concentration. The upper limit <t>u
is shown as the dotted line. It is clear that the average transition rate is
increased generally because more electrons reside in the quantum-well
available for ionization. More calculations on various quantum-well sizes are
necessary since the quantization effect will be more important if the well
size is smaller than 400A.
2.	 Photodetector based on the real-space transfer mechanism
Photodetectors play vital roles in optical communication, laser range
finding, active laser imaging and picosecond light pulse measurement [5-81.
Today, photodetectors operate over a wide range of wavelengths. In our case,
the detection wavelength of interest lies in the micron range (2 to 12
microns) which corresponds to a conduction band edge discontinuity AEC:
AEC a 85% * AEG - 1.06 x	 (0 < x < 0.45)
where x is the mole fraction of the AXAs in the AL Ga 1 _xAs region, and AEG is
the energy gap difference between the two regions ;31. Through bandgap engi-
neering, i.e., proper selection of the AtxGa I _ xAs alloy composition, the pho-
todetector may be tailored to achieve desired wavelengths. For example, for
an atomic fraction of x - 0.1, AE C - 0.106 eV, the corresponding wavelength
for absorption is approximately 12 microns. If x - 0.45, AE C - 0.477 eV, the
absorption wavelength is 2.6 microns.
A photodetector based on the real-space transfer mechanism is shown in
Figure 9 [9,101. Hess et al. [11,12) have investigated this real-space
)
transfer mechanism for its negative differential conductivity and its fast
CO
5switching and storage properties. We have examined its optical applications.
In particular, the optical free carrier absorption coefficient, which is
related to the gain, the fast response time, and the detectivity of the
heterostructure photodetector, has been investigated.
.	 The real -space transfer mechanism is the key to the operation of this
'e=
t
photodetector. The basic structure of this device consists of a series of
	
t	 alternating GaAs and AtxGal-xAs multilayers ( Fig. 9). However, other lattice-
	
=	 matched materials having dissimilar bandgap energies and electron mobilities
x
may be used. The A t Ga l-x As is doped n-type. Before the photodetector is
=9^ 
exposed to an optical signal, the electrons from the At Ga l-xAs layers spill
into and reside in the well regions. When there is an applied field parallel
to the AAxGa 1-xAs - GaAs interface, they conduct essentially in the GaAs
quantum-well regions. When separated from their parent donors by more than
200A, these electrons experience strongly reduced impurity scattering
[9,13,141. As a result of this and due to the characteristics of GaAs, these
electrons experience very high mobilities. On the other hand, the mobility is
low in the At Ga 1 _xAs layers due to the high doping concentration. When the
device is optical'	 lluminated with photons whose energies are sufficient to	 ,.
excite the electrons out of the G£As quantum well, the majority of the then-
mionically emitted electrons will then conduct in the low mobility AtxGal-xAs
layers. This will cause a change in the conductivity and therefore a change
in the current density to occur as the electrons depart the GaAs layers. This
change in current is a consequence of the decreased population of electrons in
the GaAs layers and the lower mobility of the electrons in the AtxGa1-xAs
	
{	 layers. Hence, a distinct optical signal may be detected.
k
	
L	 1
.3
6The optical absorption coefficient was calculated from the free carrier
absorption. In this research, we applied a second-order time-dependent per-
tubation theory since a phonon is considered in the photon absorption process
[151. Intra-subband optical transitions in which the carriers either (1)
absorb a photon and then emit or absorb a phonon or (2) emit a photon and
then emit or absorb a phonon, account for the absorption of electromagnetic
radiation. The optical frequencies n are lower than those which give rise to
direct interband transitions in semiconductors, i.e., hn < E  where E  is the
bandgap of the semiconductor. Since III-V compound semiconducting materials
and the AtXGa1-XAs-GaAs system in particular are weakly ionic, polar optical
phonon scattering plays a part in determining the free carrier absorption.
Consequently, we have calculated the free carrier absorption for the case when
polar optical phonon scattering of the free carriers is dominant. For the
interaction with the radiation field, we considered the case where the
radiation field is polarized in the plane of the layer. For the scattering of
the carriers by polar optical phonons, we considered only the interaction of
longitudinal optical phonons with the carriers. The interaction between the
carriers and the transverse optical phonons and the screening of the interac-
tion by the free carriers are assLaed to be negligible. We considered the
case where the confined carriers are coupled to bulk LO phonons and assumed
that the bulk phonons are totally unaffected by the existing quasi-two-
dimensional structure. Consequently, the electron-polar optical phonon
coupling may be obtained from Frohlich's model.
The computation of the free carrier absorption from an initial bound
state where the electron is is the quantum well to a final free state where
the electron is out of the well (see Figure 10) was performed in two parts.
14A-\
€
First, we considered the absorption in which the intermediate step was a free
state, as shown in Figure 10a. Next, as shown in Figure 10b, we considered
the absorption for the case of an intermediate bound state. The total absorp-
tion coefficient is the sum of the absorption coefficients of each of the
respective parts. Having formulated the absorption coefficient, we performed
the calculations
aT M aA + aA - aE - aE
where aT is the total absorption coefficient. The subscripts A and H refer to
the absorption and emission of photons, respectively. Similarly, the
superscripts a and a refer to the absorption and emission of phonons, respec-
#	 tively. For example, aA is due to the absorption of both a photon and a pho-
non, and aA is due to the absorption of a photon and the emission of a phonon,
etc.
We have completed the sophisticated theoretical formulations. Studies,
which will include the numerical calculations and the transition without the
phonons, will be done in the future. 	 l
_
^J
8REFERENCES
111 S. L. Chuang, K. Hess, J. J. Coleman and J. P. Leburton, Semiannual
report for July 1, 1984 to Decembe 31, 1984, Semiconductor Superlattice
Photodetectors, January 5, 1985.
121 S. L. Chuang and K. Hess, "Impact ionization across the band edge discon-
tinuity of quantum well heterostructures", (in preparation).
13J H. C. Casey, Jr., and M. B. Panish, Heterostructure Lasers, Part A:
Fundamental Principles, Academic Press, Inc., New York, 1978.
141 J. S. Blakemore, J. AEpl. Phys., 53, R123 (1982).
151 D. Botez and G. J. Herskowitz, "Components for optical communications
systems: a review," Proc. IEEE, 68, 689 (1980).
161 J. Conradi, F. P. Kapron and J. C. Dyment, "Fiber-optical transmission
between 0.8 and 1.4 ym," IEEE Trans. Electron Devices, ED-25, 180 (1978).
17J T. P. Lee, C. A. Hurrus, Jr., and A. G. Dentai, "InGaAs/InP p-i-n photo-
diodes for lightwave communications at the 0.95 - 1.65 um Wavelength,"
IEEE J. Quantum Electron; QE-17, 232 (1981).
181 U. Koren, et al., "Recent developments in monolithic integration of
InGaAsP/InP optoelectronic devices," IEEE J. Quantum Electron., QE-18,
1653 (198.1).
191 H. Shichijo, K. Hess and B. G. Streetman, "Real space electron transfer
by thermionic emission in GaAs-Ai GaAs heterostructures: analytical
model for large layer widths," Sofid l Sfate Electron., vol. 23, pp.
817-822, 1980.
[101 K. Hess, B. G. Streetman and H. Morkoc, "Negative resistance heterojunc-
tion devices", U.S. Patent No. 4257055, March 1981.
111J K. Hess, H. Morkoc, H. Shichijo, and B. G. Streetman, "Negative differen-
tial resistance through real-space electron transfer," Appl. Phys. Lett.,
vol. 35, pp. 469-471, 1979.
1121 M. Keever, K. Hess, and M. Ludowise, "Fast switching and storage in
GaAs-AR Ga AS heterojunction layers," IEEE Electron Devices Lett.,
vol. ED -3; -pp. 297-300, 1982.
[13] R. Dingle, H. L. Stormer, A. C. Gossard and W. Weigmann, Appl. Nhys.
Lett. 33, 665 (1978); L. Esaki and R. Tsu, IBM Research Report No.
KC-2418 (1969).
9(141 K. Hess, Appl. Phys. Lett., vol. 35, 484 (1979).
(151 Halina Adamiska and Harold W. Spector, "Free carrier absorption in quan-
tum well structure for polar optical phonon scattering", J. Appl. Phys.,
vol. 56, No. 4, 15 August 1984, pp. 1123-1127.
0
. kv
by
fir•
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l
inn,
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10
I A s	 ^•--+^
r-GaAs
ND
Figure 1.	 Geometrical configuration of the impact ionization across the
band-edge discontinuity in a quantum-well heterostructure.
tb•
11
IONIZATION
RATE (1/sec
1012
..........................................................
(^(
I 	 ( upper limit)
Eo) u
1010 	--	 I
I%% **%	 r(Eo)
108
`	
lr ( I
	
e-Eo/kBTe
\ E
^	 o
106 1 1
	
.2	 .4	 .6	 .8	 !0	 1.2 E
°(ev?
r
3
ENERGY OF INCIDENT ELECTRONS
Figure 2.	 The impact ionization rate '77 	
> as a function of the
0
incident hot electron energy E 	 solid curve. The
v
dashed curve, which is the ionization rate multiplied by
exp(-E0 /kBTe ), illustrates the trend of the integrand to obtain
the average ionization rate. The dotted line is the upper
limit of the impact ionization rate. The parameters used are
ND = 1.0 x 10 17 cm-3 , x = 0.25, L  = 400A, L = 1200 A,
*
T = 300°K, T  = 1500°K, m* = 0.073 m 0 , c10 = 0.0825 m0.
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NOW
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(b) When no light is injected, electrons ccnauct in Ga ys Iayeri,
where their mobility is lar,ye.
^ci .vlCh li4nt injection, electrons condi.c, Ln AZ Ga 	 iS i3Ver.^,x	 1-:{
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19
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f > =	 Final State
Figure 10. (a) Transition involving intermediate free state.
(b) Transition involving intermediate bound state.
Ir
1
41


Semiconductor superlattice photodetectors

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