Electric-field strength, polarization dipole, and multi-interface band offset
in piezoelectric Ga
1 x
In
x
N/GaN quantum-well structures
C. Wetzel*
High Tech Research Center, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
T. Takeuchi, H. Amano, and I. Akasaki
High Tech Research Center and Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi,
Tempaku-ku, Nagoya 468-8502, Japan
Received 17 June 1999; revised manuscript received 7 September 1999
The piezoelectric properties of Ga
1-x
In
x
N/GaN multiple quantum well structures are analyzed in two sets of
samples covering the composition range of 0 x 0.2 and well widths 23 ÅL
z
130 Å. In photoreflection
spectroscopy we observe Franz-Keldysh oscillations near the barrier band-gap energy and directly derive huge
electric field values in the range of 0.23–0.90 MV/cm. The field scales with composition and strain. The onset
of Franz-Keldysh oscillations marks a three-dimensional critical point that tunes with the electric field and well
width. It is found to correspond to a direct interband transition between continuum states controlled in energy
by the polarization dipole, i.e., the product of the polarization field and well width. By variation of the
composition alone the level can be tuned over a large energy range from 3.15 to 3.37 eV. This correspondence
provides a direct means to accurately determine the properties of such polarization controlled systems.
I. INTRODUCTION
The emergence of high-quality heterostructures
1,2
of
wurtzite group-III nitride semiconductors reveals new means
of electronic band-structure control. The uniaxial nature of
this compound system together with partly ionic bonding
conditions give rise to large polarization and piezoelectric
effects
3,4
in thin-film heterostructures typically grown along
the unique c-axis z. From an observation of the quantum
confined Stark effect in the bias voltage dependence of the
luminescence in Ga
1 -x
In
x
N/GaN quantum wells QW’s we
recently concluded the presence of very large electric fields
within the pseudomorphically strained well layers.
5
In thin
strained Ga
1 -x
In
x
N/GaN films we moreover identified
Franz-Keldysh oscillations
6
FKO’s and directly determined
field strengths up to 1.1 MV/cm ( x =0.18).
7
FKO’s have
also been identified in Al
y
Ga
1 -y
N/GaN layers.
8
These large
field values should also reflect in the electronic level scheme
of the QW heterostructures. Here, we report the observation
of a three dimensional critical point in the joint density of
states DOS which is controlled in energy over a wide range
of 225 meV by means of piezoelectric heterostructure
design.
9
We employ photoreflectance PR spectroscopy on a
large set of Ga
1 -x
In
x
N multiple quantum well MQW struc-
tures with variable composition x and variable well width
L
z
. We derive field values in the QW’s and determine the
multi interface bandoffsets and the associated piezoelectric
dipoles.
II. EXPERIMENT
A set of nine pseudomorphic Ga
1 -x
In
x
N/GaN MQW
structures A , B ,... I with variable composition was grown
by metal organic vapor phase epitaxy on 0001 sapphire
using the technique of low-temperature deposited AlN buffer
layers.
5,10
On top of a 2- m GaN epilayer five sequences of
L
z
=30-Å Ga
1 -x
In
x
N QW’s embedded in L
b
=60-Å GaN
barriers were grown with x in the range of 0 x 0.2. From
a dynamical x-ray rocking analysis the composition of three
samples was determined to be x =0.12 sample E ), 0.15 ( F ),
and 0.18 ( H ).
1
A second set of four Ga
1 -x
In
x
N/GaN MQW
samples with variable well width was grown using a similar
growth process and low-temperature deposited GaN buffer
layers. On top of a 2- m GaN epilayer n = 9,6,5,3 se-
quences of L
z
= 23,34,47,70 Å Ga
1 -x
In
x
N QW’s of com-
position x = 0.13,0.13,0.13,0.11 were embedded in L
b
=2 L
z
GaN barriers, respectively. A further control structure
of 4 wells with L
z
=L
b
=130 Å, x =0.06 was grown in a
process similar to the latter. For all samples the well and
barrier regions are undoped at residual donor concentrations
of about 10
17
cm
-3
.
Excellent compositional homogeneity of the material on
the length scale 1 r 50 m was assessed by spatially re-
solved micro photoluminescence PL at excitation power
densities of 1 mW/ m
2
.
7
The full width of half maximum
variation of the peak energy was typically 20 meV. PR
was measured using a Xe white light source and above bar-
rier band gap excitation by an 325 nm, 40 mW HeCd laser
for photomodulation. A mechanical chopper at 1.4 kHz and
lock-in technique was employed for the detection. To form
the PR signal the ac component was normalized to the dc
part. Due to the excitation geometry thickness interference
fringes did not occur in the ac part nor in the dc part.
6
All
experiments were performed at room temperature.
III. COMPOSITION DEPENDENCE
PR spectra over a selected energy range of the full com-
position set are presented in Fig. 1. Spectra are arranged in
the sequence of the PL peak energy see labels in Fig. 1
11
which is a measure also for the composition x and the in-
PHYSICAL REVIEW B 15 JANUARY 2000-I VOLUME 61, NUMBER 3
PRB 61 0163-1829/2000/613/21595/$15.00 2159 ©2000 The American Physical Society