Study and Analysis of Electrical Behavior for ZnTe-InSb Heterojunction.

1. INDIAN INSTITUTE OF TECHNOLOGY, PATNA
Winter Internship 2022
Project Topic – “Study and Analysis of Electrical behavior for ZnTe/InSb
heterojunction"
Under the Supervision of
Dr. Saurabh Kumar Pandey (IIT Patna, Electrical Engineering)
Under the Guidance of
Iraban Mukherjee (IIEST, Shibpur)
Applicant’s Details
NAME – SUPRATIK MONDAL
ENROLLMENT NUMBER – 510719009
COLLEGE NAME – Indian Institute of Engineering Science and Technology,
Shibpur
DEPARTMENT – Electronics and Telecommunication Engineering
YEAR – 4th Year

2. Study and Analysis of Electrical Behavior for ZnTe/InSb
Heterojunction
Abstract – Heterojunctions have acquired a great
attention in the semiconductor device domain from the
past years, due to many of its special characteristics and its
great efficiency in several applications. In this report we
have simulated a novel group II-chalcogenide binary alloy-
based single heterojunction device by taking Indium
Antimonide (InSb) as the substrate layer and Zinc
Telluride (ZnTe) as the epitaxial layer. Thorough analysis
has been performed to investigate the electrical behavior
of this proposed device. Additionally, the effect of epitaxial
layer thickness and epitaxial doping on critical parameters
– forward junction current and junction electron density,
reverse junction current and junction electron density are
studied and simulated to optimize the device performance.
Index Terms— Heterojunction, group II-chalcogenide, Thermionic
Emission, group II-VI, group III-V, simulation
I. INTRODUCTION
Heterojunctions have proved to be an efficient tool for many
electronics and optoelectronics applications such as Solar-cells,
Laser diodes, Heterostructure Field Effect Transistors (HFETs),
Light Emitting Diodes (LEDs) and other high Frequency
electronic devices [1]. Group III-V binary and ternary
compounds are famous for their high spontaneous and
piezoelectric properties, which has a contribution in current and
electron channel formation in many heterostructure devices [2].
So, group III-V compounds are extensively studied [3-5].
Aluminum Gallium Nitride (AlxGa1-xN) and Gallium Nitride
(GaN) heterostructure (AlxGa1-xN/GaN) being a wurtzite
material are widely famous all over the technical arena due to
its very low lattice mismatch, high electron mobility, wide band
gap and high spontaneous piezoelectric properties. Several
aspects of AlGaN/GaN based (HFETs) and heterojunctions are
extensively researched and have been published in several
publications [6-10]. Beside group III-Nitride, recent works are
now focused on group II-chalcogenide heterostructures and its
applications because of its several properties like – wide range
of band gap and good band gap engineering, high piezo-electric
constants, high electron mobility, etc. Group II-chalcogenide
based HFETs, laser diodes and other devices are also reported
in recent works [11-14]. But before analyzing a device with new
composition, its current-voltage and carrier density modeling
and simulation is necessary, which may judge the parameters
limitations in that particular device. Recently, Zinc Telluride
(ZnTe), a group II-VI alloy and Indium Antimonide (InSb), a
group III-V alloy have proved to be an efficient material in
electronics material domain due some of their special features.
In this paper a novel group II-VI/III-V based heterojunction
device of p-type InSb and n+-type ZnTe (p-type InSb/n+-type
ZnTe) is simulated. Mainly the critical parameters of the device
such as - forward junction current and junction electron density,
reverse junction current and electron density are simulated and
compared with that of other pre-existing device structures.
II. NUMERICAL ANALYSIS
This simulation has been performed in COMSOL Multiphysics
computing software. Here, ZnTe (group II-VI) is the epitaxial
layer with InSb (group III-V) as the substrate layer. The
proposed device structure is shown in Fig. 1. ZnTe is a wide
band gap (WBG) whereas InSb is a narrow band gap (NBG)
material. Also, the electron affinity of ZnTe is less than that of
InSb. Due to this band gap discontinuity and difference in
electron affinity, electrons are confined in a quantum notch,
formed at the junction (fig. 2). These confined electrons are
mainly drifted through the junction due to the application of
electric field and thus generates a high current density. As the
electron mobility in InSb is very high, it also enhances the drift
current flowing through it. As the lattice mismatch of the
structure is quite high (> 4%), the dimensions are scaled down
to nano scale. Here, the ZnTe Epitaxial layer plays a vital role
in the contribution of junction current and junction electrons. It
is because, as the epitaxial layer is doped as n-type, it will act
as an electron supplier to the p-type substrate layer, which will
then diffuse and drift (drift dominates) due to the lateral applied
electric field. The details about current transportation of this
device have been discussed subsequently. Thus, this computing
software is applied to do a 1-D simulation of the ZnTe/InSb
proposed heterojunction device and also to study the effect of
epitaxial thickness and epitaxial doping on the critical
parameters. For simulation, the device cross sectional area is
taken to be 4x10-12 m2. In COMSOL, several models have been
considered for simulation purposes, such as - complete impurity
ionization model, the Shockley-Read-Hall Recombination
(SRH) model, low field constant mobility model are taken into
account. Additionally, for the junction current, the Thermionic
Emission (TE) model is considered at the heterojunction as it is

3. a significant model for current transport in heterojunction [15].
According to the TE model, the current transport equation
which the software will solve is given by [1]:
𝐽 ≅ {
𝑞
𝐽𝑠 exp (𝑞𝑉/𝜂𝑘𝑇), 𝑉 ≫
𝑘𝑇
:𝐹𝑜𝑟𝑤𝑎𝑟𝑑 𝐵𝑖𝑎𝑠
𝑞
−𝐽𝑠, 𝑉 ≪ −
𝑘𝑇
: 𝑅𝑒𝑣𝑒𝑟𝑠𝑒 𝐵𝑖𝑎𝑠
kT
Where, Js = A*T2 exp (-
ɸb
) --- (I)
In these equations, ‘Js’ is the Saturation current density, ‘q’
represents the charge of electron, ‘V’ represents the applied bias
voltage, ‘’ represents the ideality factor, ‘k’ represents the
Boltzmann Constant, ‘T’ is the ambient temperature, ‘A*’ is
Richardson's Constant and ‘Φ’ is the Schottky barrier potential.
From equation (I) it is evident that the reverse saturation current
density (Js) depends on the Schottky barrier potential (Φ).
Whenever the carrier particles don’t cross any Schottky barrier
on its path, then this TE model will be invalid, Drift and
Diffusion Transport (DDT) model will then dominate the
current transport process.
Here, ZnTe is given a voltage (Va) by keeping InSb layer
ground, to make the whole device forward or reverse bias. The
epitaxial thickness is denoted by te and epitaxial doping by Ne
(n-type). The substrate layer thickness denoted by ts and
substrate doping by Ns (p-type). Some material properties of
ZnTe and InSb are shown in table 1 [16-21].
Fig.1 Device architecture
Fig. 2 - Conduction Energy Band diagram at Zero Bias.
.
Table – 1 Material Properties
Sl.
No.
Properties InSb ZnTe
1 Lattice Structure ZB ZB
2 Lattice constant, a (A0) 6.47 6.10
3 Band Gap, Eg (eV) 0.17 2.25
4 Electron affinity, χ (eV) 4.59 3.53
5 Relative Permittivity, εr 16.8 10.1
6 Electron effective mass, me
*
(kg)
1.27x10-32 1.27x10-31
7
*
Holes effective mass, mh
(kg)
3.91x10-31 5.46x10-31
8 Conduction Band Density of
States, Nc0 (1/m3)
4.15x1022 1.31x1024
9 Valence Band Density of
States, Nv0 (1/m3)
7.06x1024 1.16x1025
10 Electron mobility, µe0
(cm2/V.s)
7.7x104 1000
11 Hole mobility, µh0 (cm2/V. s) 850 100
12 Electron Recombination
lifetime, τe0 (ns)
10 10
13 Hole Recombination
lifetime, τh0 (ns)
10 10
III. RESULTS AND DISCUSSIONS
A. Device Validation: -
In this work a binary II-chalcogenide alloy of Indium
Antimonide (InSb) of thickness 8 nm is taken as a substrate over
which another binary II-chalcogenide alloy of Zinc Telluride
(ZnTe) is grown by Molecular Beam Epitaxy (MBE) technique,
to form the epitaxial layer of the ZnTe/InSb heterojunction.
The epitaxial layer thickness is varied within a limit judged by
the critical thickness (tc) of the epitaxial layer and lattice
mismatch parameter (Δ) between the epitaxial and substrate

4. layer. This is done because the epitaxial thickness plays a key
role in determining the critical parameters. Here, the epitaxial
layer thickness is kept below the critical thickness to avoid the
defects and dislocations formation in the epitaxy. The
expression for lattice mismatch and critical thickness is given
in the following equations:
𝑎𝑒
Δ=
|𝑎𝑠−𝑎𝑒|
x 100% (II)
c
t =
𝑎𝑠
2∆
(III)
Where, as and ae are substrate layer and epitaxial layer lattice
constants respectively. The simulation results of this
heterojunction are performed by keeping the temperature (T)
constant equals to 300 K (room temperature). At this
temperature, ZnTe and InSb are easily available in Zinc-Blende
(ZB) structure. By taking the value of lattice constant for InSb
equals to 6.479 A0 and that of ZnTe as 6.1037 A0, the lattice
mismatch is determined to be 6.148 % and tc as 5.268 nm which
judge the boundary for variation of tc.
Fig. 3 shows the forward and reverse bias J-V characteristics of
the proposed device. Here, epitaxial layer is 4 nm, substrate
layer is 8 nm epitaxial and substrate doping are 1018 atoms/c.c
and 1015 atoms/c.c respectively. This plot is also compared with
an existing structure of Al0.2Ga0.8As/GaAs (experimentally)
reported in [15]. It is quite evident from fig. 5 that the nature of
the simulation and experimental curve matches. Thus, we can
say that our device is valid and its ready to investigate.
Fig. 4 - Critical parameters Vs epitaxial thickness (te). Box markers represents
forward bias and triangular markers represents reverse bias condition. Blue
line is for ‘|Jf|’ and red line is for ‘n’.
Fig. 3 – J-V curve of Al0.2Ga0.8As/GaAs (experimentally) and ZnTe/InSb
(simulated) heterostructures. Rectangle markers - forward bias sweep, circular
markers - reverse bias sweep
B. Case-I: Effect of epitaxial thickness on critical
parameters:
In this study, ts is taken as 8 nm, Ne as 1018 atoms/c.c, Ns as 1016
atoms/cc, magnitude of forward and reverse voltage as 1.8 V
and te is varied from 1 nm to 5 nm (less than tc). The effect of
epitaxial thickness is studied and shown in fig. 6.
The first and second plot in fig. 4 shows the variation of
magnitude of forward current density (|Jf|) and forward bias
junction electron density (n) respectively with te. In the case of
zero-bias, there was a notch at the junction due to band gap
discontinuity. As forward bias is applied, the fermi level of
epitaxial layer will go upward and that of substrate layer will go
downward, eliminating the notch structure from the
heterojunction (fig. 5). As te increases, the conduction band
minima of substrate layer become flatter, reducing the majority
carrier from the junction by draining them off. So, as the
junction electron density decreases, the forward bias current
density also decreases.
The third and fourth plot in fig. 4 shows the variation of
magnitude of reverse current density (|Jr|) and reverse bias
junction electron density (n) respectively with te. As reverse
bias is applied, the fermi level of epitaxial layer goes down and
that of substrate layer goes up, still forming a triangular
quantum notch at the junction (fig. 6). This notch holds the
electrons donated by the epitaxial material. As te increases, the
depth of the notch decreases, which further decreases the
electron confinement in the junction which in turn reduces the
reverse bias junction current density.

5. Fig. 5 - Conduction band energy in forward bias
C.
Fig. 6 - Conduction band energy in reverse bias
Case-II: Effect of epitaxial doping on critical
parameters: -
In this study we have taken te as 4 nm, ts as 8 nm, reverse and
forward bias magnitude as 1.8 V, Ns as 1016 atoms/c.c. We have
varied Ne from 1016 atoms/c.c to 1018 atoms/c.c.
Fig. 7 shows the variation of the critical parameters with
acceptor doping. As donor doping of epitaxial material
increases, epitaxial layer will supply more electrons to the
substrate layer, thus increasing the junction electron density,
which further increases the junction current density. Thus, in
forward and reverse bias, as Ne increases, the forward and
reverse bias junction electron density and magnitude of forward
and reverse current density increases. As it is observed that on
increasing Ne, the magnitude of forward and reverse current
increases, so, we can draw an interpretation that as Ne increase,
static ON and OFF resistance decreases.
It is also observed from fig. 4 and 7 and interpreted from case I
and II that, at a particular biasing voltage, the reverse bias
junction electron density is greater than forward bias junction
electron density. It is because in reverse bias, there is a quantum
notch present at the heterojunction, which is absent in forward
bias condition (fig. 5 and 6). Also, the forward bias current
density is greater than reverse bias current density because in
forward bias the electrons are travelling down-hill (not crossing
any Schottky potential barrier) and also the diffusion supports
drift, thus enhance the total current. But in reverse bias the
electrons have to cross a huge Schottky barrier, so they require
a sufficient amount of energy to cross that barrier. Also in
reverse bias, diffusion opposes the drift current, which also
contributes in reduction of the current.
Fig. 7 – Plot of critical parameters Vs acceptor doping (Na). Blue line is for
forward bias and red line is for reverse bias condition.
D. Comparative analysis: -
Table 2 shows a comparative analysis of our simulated
structure with different literatures studied till date.
Table – 2 comparative analysis
|Jf| (Vbias = 1.8
V)
Materials |Jr| (Vbias = 1.8
V)
Ref.
2.4x10-5
(A/cm2)
CdS/ZnTe 1.9x10-14
(A/cm2)
[22]
0.011 (A/cm2) ZnO/ZnTe 2.2x10-14
(A/cm2)
[23]
1.4x1012
(A/cm2)
n-Ge/p-
GaAs
1.1x106 (A/cm2) [24]
1.6x1012
(A/cm2)
ZnTe/InSb 1.535x10-4
(A/cm2)
This
work
IV. CONCLUSIONS
In summary, we simulated a novel group II-chalcogenide binary
alloy-based heterojunction device. We observed that on
increasing the epitaxial thickness, the forward and reverse bias

6. junction current density and junction electron density
decreases. It is also observed that when epitaxial doping
increases, due to the enhancement of the concentration of
majority charge carriers, the forward and reverse bias junction
current and junction electron density increases. Thus, from the
results it is interpreted that the optimal value of the epitaxial
thickness can be 4 nm, because less than that may cause scaling
issues and more than that will decrease the forward bias critical
parameters much. Thus, for a doping concentration of Ne = 1018
atoms /c.c and te = 4 nm, we are getting a practically realizable
values of forward bias current and electron density. Moreover,
it is highly predicted that this unprecedent device structure can
also be applied in building a group II-VI/III-V based HFET
because of high electron mobility in InSb (substrate layer) and
high junction electron density. Also, for its high junction
current density, it can also be applied in power applications.
V. ACKNOWLEDGEMENT
The authors of this paper would like to acknowledge the support
from COMSOL support team for providing materials for this
work.
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