Burnham, R. D.

Collins, R. T.

Lambe, J.

McGill, T. C.

English

Caltech Authors - Main

Summary Abstract: Elastic and inelastic tunneling characteristics of AlAs/ 
GaAs heterojunctions 
R. T. Collins, J. Lambe, and T. C. McGill 
California Institute of Technology, Pasadena, California 9 Il25 
R. D. Burnham 
Xerox Corporation, Palo Alto, California 94304 
(Received 29 February 1984; accepted 24 April 1984) 
PACS numbers: 73.40.Gk, 73.40.Lq 
We report the observation, at low temperatures and bias vol-
tages, of elastic and inelastic tunneling through AlAs bar-
riers sandwiched between two GaAs layers. Previous studies 
of electronic transport perpendicular to Alx Ga1 _ x As bar-
riers sandwiched between GaAs layers 1-4 have attributed 
currents passing through the barriers to thermionic emission 
at temperatures above 150 K or to tunneling through the 
Alx Ga1 _ x As barrier at low temperatures, but for bias vol-
tages which were comparable to the barrier height. Two of 
these studies noted that at low temperatures and low biases 
more current passed through the barriers than could be ac-
counted for by either of these processes, indicating that some 
form ofleakage may be dominating current transport at low 
temperatures. 1•2 In all of these studies the barriers were 300 
A thick or thicker. The measurements reported here were 
made on samples with barriers thinner than 300 A in an 
effort to enhance the tunneling current at low bias voltages 
relative to leakage. A more detailed account of most of these 
measurements is given in Ref. 5. 
The samples used in this study were grown by metalor-
ganic chemical vapor deposition (MOCVD) and by molecu-
lar beam epitaxy (MBE). In both cases an n-type epitaxial 
layer of GaAs was grown on a degenerately doped n-type 
GaAs substrate. A thin layer of AlAs was grown on top of 
this GaAs layer, followed by a final n-type layer ofGaAs. In 
the MOCVD samples the epitaxial GaAs was doped with Se 
at 1-5 X 1018 em - 3, and the AlAs layer was either doped n 
type with Se or p type with Mg at approximately 1 X 1018 
cm- 3 • TheAlAslayerswerefrom 50-300A thick. The MBE 
samples came from two sources. In both cases the epitaxial 
GaAs layers were doped with Si at 1 X 1018 em - 3, and the 
AlAs was unintentionally doped. The estimated AlAs layer 
thicknesses were from 50-150 A. Devices were made on the 
wafers by defining mesas and fabricating (Au,Ge)-Ni con-
tacts on the mesas. Temperature dependent current-voltage 
(I-V), first derivative (di !dV), and second derivative (d 2I I 
dV 2 ) spectra were taken on the devices from room tempera-
ture to 4.2 K. Zero-bias resistances were obtained from the 
slope of the I-V curves at ± 3 m V. 
Results of the measurements made on MOCVD grown 
samples will be discussed first. There was a large difference 
in the characteristics of the samples, depending upon 
whether the AlAs barrier was doped p type with Mg or n 
type with Se. The p-type barriers exhibited increasing resis-
tance as temperature was decreased to 77 K, at which point 
the I-V curve maintained the same shape down to 4.2 K. The 
zero-bias resistances of p-type barrier samples increased ra-
pidly with barrier width as is seen in Fig. I. Figure I also 
includes the results of a WKB calculation of the zero-bias 
resistances expected for various barrier widths. This calcula-
tion was based on the technique ofKurtin eta/. 6 Only contri-
butions to the tunneling probability from the AlAs r points 
were included. The band offsets and effective masses were 
taken from Casey and Panish. 7 Although this model is only 
presented to give an idea of the orders of magnitude expected 
for tunneling resistances, the agreement is reasonable for 
small barrier widths. As the thickness of the barrier is in-
creased, the measured zero-bias resistance does not increase 
at the rate predicted for tunneling, indicating that for thick 
AlAs layers, most of the low temperature current is due to 
some form of leakage. At smaller thicknesses the tunneling 
N 
E 
u 
~ 
<1J 
u 
c 
0 
~ 
(f) 
Q) 
0::: 
Q 
Cl' 
0 
_j 
106 
104 
102 
Zero- Bios Res1stance vs Barrier 
Width tor AIAs/GoAs Tunneling 
Structures 
+ 
• 
• 
- WKB Approximation 
• p- Type MOCVD Samples 
+ MBE Samples 
10- 4 L-. ___ .__ ___ .__ ___ .L...-___ ;____J 
0 50 100 150 200 
Barner W1dth (~) 
FIG. I. Zero-bias resistivity at 4.2 K for MOCVD samples with p-type bar-
riers and for MBE samples which had uniform resistivities between neigh-
boring devices. The solid line is a theoretical calculation of zero-bias resis-
tivity, using a WKB approximation to the tunneling probability. 
597 J. Vac. Sci. Technol. 8 2 (3), July-Sept. 1984 0734-211 X/84/030597-G2$01.00 @ 1984 American Vacuum Society 597 
sse Collins tit al : Summary Abstract: Elastic and Inelastic tunneling 598 
c 
:::> 
,_.I"' "' > 
""Cl ""Cl 
-200 
Second Denvot1ve Spectrum for on AIAs/GaAs 
Double Heterostructure at 4.2 K 
-GoAs ~lAs I GoAs- p- Type AlAs 
-100 0 100 
V (mV) 
200 
FIG. 2. Second derivative spectrum at 4.2 K for a p-type AlAs barrier sam-
ple grown by MOCVD. The layer thickness was approximately 50 A. The 
structure labeled Ia) is attributed to an electronic density-of-states effect 
caused by electron-optical phonon coupling in the GaAs. The peak labeled 
ib) is caused by the inelastic excitation oflongitudinal optical phonons in the 
AlAs barrier. 
current, which decreases exponentially with width, begins to 
dominate leakage. 
When first and second derivatives were taken of the /-V 
curves of samples with 50 A thick, p-type, AlAs layers, re-
producible structure was evident (Fig. 2). Structure [labeled 
(a)] is present at the GaAs longitudinal optical (LO) phonon 
energy (36 meV) in both forward and reverse bias in Fig. 2 
and is attributed to a density-of-states effect caused by elec-
tron-phonon coupling in the degenerate GaAs layers. Simi-
lar observations have been made in Schottky barrier tunnel-
ing in GaAs. 8 A peak, labeled (b), is also visible at 
approximately the AlAs LO phonon energy (50 meV) in the 
second derivative spectrum.9 This peak is associated with 
the inelastic excitation oflongitudinal optical phonons in the 
AlAs by the tunneling electrons. This is the first observation 
of these effects in the AlAs/GaAs system. These results were 
reproducible across a given wafer and were seen in two wa-
fers with barriers of about the same thickness. When the 
AlAs barrier width increased to 100 A or greater, no repro-
ducible structure was observed. 
When the MOCVD grown AlAs barriers were doped n 
type, the results were different. The zero-bias resistance 
changed by less than an order of magnitude as the AlAs layer 
width was increased from 60 to 250 A. Five peaks were pres-
ent in the second derivative spectra in both forward and re-
verse bias for all of the n-type barrier samples regardless of 
width. These peaks did not match those seen in the p-type 
barrier MOCVD samples. A definitive explanation for the 
J. Vac. Sci. Technol. B, Vol. 2, No.3, July-Sepl1984 
results of measurements on n-type barriers has not been 
found. A more complete discussion of the differences 
between n-and p-type barrier MOCVD samples and the pos-
sible explanations for these differences is given in Ref. 5. 
The zero-bias resistances for devices made from the two 
MBE grown samples with the thinnest AlAs layers (one 
from each source) are given in Fig. 1. These resistances are 
also consistent with the tunneling calculations, although 
from Fig. 1, it appears that either the MBE or MOCVD 
barrier widths may be in error by around 10 A. The I-V 
curves for devices made from these wafers showed approxi-
mately the same behavior with temperature as the p-type 
MOCVD barrier samples. Devices made on the two wafers 
for which resistances are given in Fig. 1 showed some fluctu-
ation from device to device in the zero-bias resistance. The 
remaining MBE samples studied had barrier widths of 100 A 
or greater. The zero-bias resistances of neighboring devices 
made on these wafers fluctuated by many orders of magni-
tude. The largest resistances measured for the MBE samples 
were two or more orders of magnitude larger than the resis-
tances of p-type MOCVD samples with the same barrier 
width. Much smaller fluctuations occurred in the resistivi-
ties of devices on the MOCVD grown wafers. The first and 
second derivative spectra for the MBE wafers did not show 
any reproducible structure, possibly because they had such 
large resistances that tunneling currents were beyond the 
detectable limits of our system. A reason for the large flunc-
tuations in device resistance for the thicker barrier MBE 
samples has not been found. 
Acknowledgments: The authors wish to acknowledge D. 
L. Smith, C. Mailhiot, R. S. Bauer, T. L. Paoli, and W. 
Streifer for valuable discussions and L. P. Erickson and G. 
W. Wicks for providing samples. We are also grateful to H. 
Chung, R. D. Yingling, Jr., F. Endicott, M. Bernstein, M. 
Mosby, J. Tramontana, J. Walker, A. Alimonday, G. L. 
Harnage!, and R. Ritter for technical assistance with this 
work. This work was supported in part by the Office of Na-
val Research under Contract No. N00014-82-K-OS56. 
'D. Delagebeaudeuf, P. Delescluse, P. Etienne, J. Massies, M. Laviron, J. 
Chaplart, and N. T. Linh, Electron Lett. 18, 85 (1982). 
'A. C. Gossard, W. Brown, C. L. Allyn, and W. Wiegmann, J. Vac. Sci. 
Techno!. 20,694 (1982). 
'P.M. Solomon, T. W. Hickmott, H. Morkoc, and R. Fischer, Appl. Phys. 
Lett. 42, 82111983). 
"'T. W. Hickmott, P. M. Solomon, R. Fischer, and H. Morkoc, Appl. Phys. 
Lett. 44, 90 (1984). 
'R. T. Collins, J. Lambe, T. C. McGill, and R. D. Burnham, Appl. Phys. 
Lett. 44, 532 ( 1984). 
"S. L. Kurtin, T. C. McGill, and C. A. Mead, Phys. Rev. B 3, 3368 (1971 ). 
7H. C. Casey, Jr. and M. B. Panish, Heterostructure Lasers Part A: Funda-
mental Principles (Academic, New York, 1978), p. 192. 
81. W. Conley and G. D. Mahan, Phys. Rev. 161, 68111967). 
9 M. Ilegems and G. L. Pearson, Phys. Rev. B 1, 1576 (1970). 


Elastic and inelastic tunneling characteristics of AIAs/GaAs heterojunctions

http://authors.library.caltech.edu/32210/1/COLjvstb84a.pdf

Elastic and inelastic tunneling characteristics of AIAs/GaAs heterojunctions

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main