Examination of tunnel junctions in the AlGaNÕGaN system: Consequences
of polarization charge
Madhusudan Singh
a)
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor,
Michigan 48109
Yifei Zhang
Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109
Jasprit Singh
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor,
Michigan 48109
Umesh Mishra
Department of Electrical and Computer Engineering, University of California, Santa Barbara,
California 93106
͑Received 25 May 2000; accepted for publication 24 July 2000͒
A strong piezoelectric effect and a large spontaneous polarization allow one to incorporate a large
electric field (Ͼ10
6
V/cm) and high sheet charge ( Ͼ10
13
cm
Ϫ2
) without doping in the AlGaN/GaN
heterostructure. Theoretical studies are done to examine how polarization effects can be exploited
to design metal–AlGaN/GaN tunnel junctions. We find that with a proper choice of AlGaN
thickness undoped junctions can be made with very high metal to two-dimensional electron gas
tunneling. Thus, a Schottky junction can be converted to a tunnel junction without doping. The
tunneling probabilities approach those produced in a system doped at ϳ4 ϫ10
19
cm
Ϫ3
. This work
suggests that very interesting tunnel junctions can be made from undoped AlGaN/GaN
heterostructures. © 2000 American Institute of Physics. ͓S0003-6951͑00͒05138-X͔
Group III-nitride semiconductors have attracted wide at-
tention recently in view of their application in high power
devices and optoelectronic devices with wavelengths ranging
from the red into the ultraviolet.
1–9
To understand the properties of the AlGaN/GaN struc-
ture it is important to note that there is a spontaneous polar-
ization present in the structures as a result of the cation and
anion positions in the lattice.
10
In heterostructures the differ-
ence between spontaneous polarization of two layers can be
used to create a high density of mobile carriers. In addition
to spontaneous polarization in heterostructures with strain
͑resulting from epitaxy͒ the piezoelectric effects for the ni-
tride system are so large that effective built-in fields of
ϳ10
6
V/cm can be produced near the interfaces.
11
This fea-
ture has been exploited to design AlGaN/GaN high electron
mobility transistors ͑HEMTs͒ with very high sheet charge
without doping. It has been found experimentally
12,13
that in
nominally undoped HEMT structures sheet charge densities
greater than 10
13
cm
Ϫ2
can be produced. Since the polariza-
tion charges effectively act as dopant charges, it is expected
that with a proper understanding, tunnel junctions can be
made from undoped metal–AlGaN/GaN structures. This let-
ter sheds light on this issue.
Consider the case where the effective substrate is GaN
and an Al
x
Ga
1Ϫx
N overlayer is grown coherently along the
͑0001͒ direction. The total polarization is found to have the
value
12
P
͑
x
͒
ϭP
pz
ϩP
sp
ϭ
͑
Ϫ3.2ϫxϪ1.9x
2
͒
ϫ10
Ϫ6
C/cm
2
Ϫ5.2ϫ10
Ϫ6
x C/cm
2
. ͑1͒
We see that in this system the effects arising from piezoelec-
tric effect and spontaneous polarization mismatch are com-
parable. The electric field associated with the polarization
given above is
F
͑
x
͒
ϭ
͑
Ϫ9.5xϪ2.1x
2
͒
MV/cm. ͑2͒
We see that the built-in field and sheet charge values are very
large. It is easy to produce fields around 10
6
V/cm and
charge density around 10
13
cm
Ϫ2
. In undoped AlGaN/GaN
field effect transistors such charge densities have been
seen.
13
To understand the tunneling problem we first need to use
a charge control model to examine the band profile. Our
charge control model first obtains the potential profile in a
HFET structure by solving the Schro
¨
dinger equation and
Poisson equation self-consistently.
The detailed formalism is described in Ref. 14. Once the
band profile is known we use the WKB method to obtain the
tunneling probability from the metal to the GaN channel.
In this letter we examine how the metal–semiconductor
junction, shown in Fig. 1͑a͒, behaves as far as tunneling of
carriers is concerned. There is no doping in the structure and
the two-dimensional charge is induced as a result of the fixed
polarization charge at the AlGaN/GaN interface. If we exam-
ine the density of the two-dimensional electron gas as a func-
tion of AlGaN barrier thickness, we see a behavior schemati-
a͒
Electronic mail: chhabra@eecs.umich.edu
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 12 18 SEPTEMBER 2000
18670003-6951/2000/77(12)/1867/3/$17.00 © 2000 American Institute of Physics