Burnham, R. D.

Collins, R. T.

Lambe, J.

McGill, T. C.

English

Caltech Authors - Main

Summary Abstract: Inelastic tunneling characteristics of AIAs/GaAs 
heterojunction barriers 
A. T. Collins, J. Lambe, and T. C. McGill 
California Institute of Technology, Pasadena, California 91125 
R. D. Burnham 
Xerox Corporation, Palo Alto, California 94304 
!Received 20 October 1983; accepted 8 February 1984) 
PACS numbers: 73.40.Lq, 73.40.Gk, 71.38. + i 
Several studies of current transport perpendicular to 
Al.., Ga1 _..,As barriers in GaAs/ Al.., Ga1 _..,As double he-
terojunctions have recently been reported. •-3 The currents 
obtained in these measurements were attributed to ther-
mionic emission over the offsest barrier between the GaAs 
and Al.., Ga1 _..,As and tunneling through the barrier. Chang 
et al. looked at the tunneling behavior of two AI.., Ga1 _..,As 
barriers separated by a GaAs quantum well.4 In this study 
we report the first observation of inelastic tunneling in elec-
tronic transport perpendicular to a single AlAs barrier sand-
wiched between two GaAs layers. Temperature dependent 
I-V and second derivative (d 2I /dV 2) measurements were 
made on AlAs/GaAs double heterojunctions. Reproducible 
structure was seen in the second derivative spectrum indicat-
ing the presence of inelastic tunneling currents. A more de-
tailed account of these measurements and results is given in 
Ref. 5. 
The samples used in this study were grown by a metalor-
ganic chemical vapor deposition (MOCVD) technique. An 
epitaxial layer of GaAs doped with Se was grown on a degen-
erately doped GaAs substrate. A thin AlAs layer was then 
grown on top of the GaAs. The thickness of the AlAs layer 
varied between 60 and 250 A from wafer to wafer. A final Se-
doped GaAs layer was then grown on the AlAs. The dopings 
in the GaAs layers were typically 1-5X 1018 cm- 3• The 
AlAs was either doped n-type with Se at approximately 
1 X 1018 em - 3 or doped p-type with Mg at about 1 X 1018 
em - 3• Mesas were defined on the samples using convention-
allithography. (Au, Ge)-Ni contacts were fabricated on the 
mesas, and the resultant devices were mounted on standard 
transistor headers and wire bonded to provide contacts. 
Temperature dependent I-V and second derivative spectra 
were taken on the samples from room temperature to 4.2 K. 
Second derivative spectra were taken using a second har-
monic detection system as described by Lambe. 6 
First, the results of measurements made on samples with 
Mg-doped AlAs barriers will be described. The I-V curves 
for these p-type barriers exhibited increasing resistance with 
decreasing temperature until approximately 77 K at which 
point the curves maintained the same shape down to 4.2 K. 
The resistivity of the samples increased by many orders of 
magnitude as the width of the AlAs barrier was increased 
from 60 to 250 A. For the smaller widths the increase in 
zero-bias resistivity was approximately exponential with 
width. 
A second derivative spectrum taken from a sample with a 
60 A, p-type AlAs barrier is given in Fig. 1. Two structures 
are present in both forward and reverse bias. The first, la-
beled a, is at approximately the optical phonon frequencies 
of GaAs. It is attributed to an electronic density-of-states 
effect caused by electron-phonon coupling in the GaAs lay-
ers. Similar effects have previously been observed in GaAs 
Schottky barriers. 7 The peak labeled b occurs at SO me V and 
corresponds to the LO phonon frequency of AlAs. This peak 
is probably caused by the inelastic excitation of optical phon-
ons in the AlAs layer by tunneling electrons. This is the first 
observation of these effects in the AlAs/GaAs system. 
I-V curves for theSe-doped, n-type AlAs layers also dis-
played increasing resistance with decreasing temperature, 
but, in contrast with the p-type layers, the resistance contin-
ued to increase until approximately 30 K. Furthermore, the 
sample resistivity increased by less than an order of magni-
c 
:::1 
>. 
.... 
0 
.... 
.0 
.... 
0 
...... IN N > 
"'0 "'0 
-200 
-GaAs I AlAs I GaAs-- p- Type AlAs 
-100 0 100 200 
V (mV) 
FIG. 1. Second derivative spectrum at 4.2 K for ap-type AlAs layer sand-
wiched between two n-type GaAs layers. The layer thickness was approxi-
mately 60 A. The structure labeled a is attributed to an electronic density-
of-states effect caused by electron-optical phonon coupling in the GaAs. 
The peak labeled b is caused by the inelastic excitation of optical phonons in 
the AlAs barrier. 
201 J. Vac. Sci. Technol. B 2 (2), Apr.~une 1984 0734-211X/84/020201-G2$01.00 @) 1984 American Vacuum Society 201 
202 Collins et 111. : Summary Abstract: Inelastic tunneling characteristics 202 
VI 
c: 
::::J 
>-
..... 
0 
..... 
.0 
..... 
0 
-~N N > 
""0 ""0 
-GaAs I AlAs I GaAs-- n-Type AlAs 
200 
FiG. 2. Second derivative spectrum at 4.2 K for an n-type AlAs layer sand-
wiched between two n-type GaAs layers. 
tude as the AlAs layer thicknesses increased from 60 to 250 
A. Changes in slope were visible in the I-V curves. To study 
these, second derivative spectra were taken at 4.2 K. 
Five peaks were visible in both the forward and reverse 
bias second derivative spectra of the n-type barrier samples 
(Fig. 2). They occurred at approximately 23, 50, 72, 125, and 
145 meV. There was also a strong zero-bias anomaly. These 
peaks did not match those seen in the p-type barriers. 
A definitive explanation of the results obtained on sam-
ples with n-type barriers has not been found, but some possi-
ble explanations can be proposed. The fact that the I-V 
curves for the n-type barriers froze out at 30 K suggests that 
the current flow at 4.2 K is due to tunneling processes. If 
transport in the n-type AlAs samples is the result of tunnel-
ing, the actual tunneling barrier seen by the electrons must 
not be increasing as the physical width of the AlAs layer is 
increased. Since doping in the AlAs layer is the critical factor 
in determining the type of I-V curve and second derivative 
J. Vac. Sci. Technol. 8, Vol. 2, No.2, Apr.-June 1984 
spectrum obtained from a sample, one possible explanation 
is that the doping in the n-type AlAs layer is actually larger 
than anticipated, and band bending in the AlAs results in the 
formation of two back-to-hack AIAs/GaAs tunneling bar-
riers with a connecting layer of AlAs. The tunneling resis-
tance of such a structure would be independent of thickness. 
The dopings given above were estimated from the Se flow 
rate and may be too low. Then, the peaks in the second deri-
vative spectrum may arise from the excitation of characteris-
tic phonons in the AlAs/GaAs system by tunneling elec-
trons (possibly wave vector-conserving phonons resulting 
from electrons moving from direct to indirect conduction 
band minima). A voltage drop would occur across each bar-
rier causing the voltages at which the phonon associated 
peaks occur to differ from the actual phonon energies. Other 
explanations for these results include the possibility that 
more impurities or defects are incorporated into then-type 
layers than into the p-type layer, giving rise to impurity con-
duction in the barrier. This would not have an exponential 
dependence on the width of the barrier. The peaks in the 
second derivative spectrum could indicate the onset of impu-
rity-assisted tunneling or the excitation of vibrational modes 
of the impurities. Work is currently underway to test these 
models of current transport in the n-type barrier samples. 
The authors wish to acknowledge D. L. Smith, C. Mail-
hiot, T. L. Paoli, and W. Streifer for valuable discussions and 
are grateful to H. Chung, R. D. Yingling, Jr., F. Endicott, M. 
Bernstein, M. Mosby, J. Tramontana, J. Walker, A. Ali-
monda, G. L. Harnage!, and R. Ritter for technical assis-
tance with this work. This study was supported in part by the 
Office of Naval Research under Contract No. N00014-82-
K-0556. 
1D. Delagebeaudeuf, P. Delescluse, P. Etienne, J. Massies, M. Laviron, J. 
Chaplart, and T. Linh, Electron. Lett. 18, 85 (1982}. 
2A. C. Gossard, W. Brown, C. L. Allyn, and W. Wiegmann, J. Vac. Sci. 
Technol. 20, 694 (1982}. 
3P. M. Solomon and T. W. Hickmott, Appl. Phys. Lett. 42, 821 (1983). 
4L. L. Chang, L. Esaki, and R. Tsu, Appl. Phys. Lett. 24, 593 (1974). 
'R. T. Collins, J. Lambe, T. C. McGill, and R. D. Burnham, Appl. Phys. 
Lett. 44, 532 (1984). 
0 ]. Lambe and R. C. Jaklevic, Phys. Rev. 165, 821 (1968}. 
7J. W. Conley and G. D. Mahan, Phys. Rev. 161,681 (1967}. 


Inelastic tunneling characteristics of AIAs/GaAs heterojunction barriers

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Inelastic tunneling characteristics of AIAs/GaAs heterojunction barriers

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