GaN Layers Grown by HVPE on P-type 6H-SiC Substrates
The HVPE method is a promising technique for deposition of thick GaN single
crystal layers. The main advantages of this method are high growth rate and low
cost. Recent progress in HVPE growth shows that high quality GaN layers can be
obtained [1] [2] [3]. The crystalline, electrical and optical properties
of these films are comparable with the best characteristics reported in the
literature for GaN produced by MOCVD [4] [5] and MBE [6] [7].
Besides, HVPE allows one to grow high quality GaN layers on SiC substrates
without any buffer layer [1]. This feature is important for producing
electronic devices with vertical geometry.
The direct deposition of GaN on SiC also provides one the opportunity to create
devices based on GaN/SiC heterojunctions (e.g. heterojunction bipolar
transistor) [8] [9]. Recently p-GaN
on n-SiC
has been successful grown by MBE and junction characteristics have been
achieved. [10].
One of the obstacles in GaN device fabrication is the absence of a suitable
ohmic contact to p-GaN.
To overcome this problem, the inverse GaN device structure i.e.
pSiC (substrate)/pGaN/.../nGaN may be used. The best ohmic contact to
p-GaN
[11] has a resistivity about 3 orders of magnitude higher than that for n-GaN
[11] [12] and about 2 orders of magnitude higher than that for p-SiC
[13]. A large area p-contact
to SiC will contribute negligibly to the overall device series resistance.
However to the best of our knowledge there are no reports on growth GaN on
p-type
SiC substrates.
In this paper we report on properties of n-GaN
layers grown by HVPE directly on p-type
SiC substrates. Preliminary result on n-GaN/p-SiC
heterojunction are presented.
GaN n-type
layers were grown by HVPE on p-type
6H-SiC
manufactured by Cree Research, Inc. The Nd-Na
concentration in the wafers was about 1x1018 cm-3.
The layers were deposited on the on-axis (0001)Si face of the substrates. A
hot-wall open flow reactor was employed. HCl was reacted with liquid Ga to form
GaCl gas which was transported to the growth zone where it was reacted with
NH3 resulting in GaN deposition on the SiC substrate. The thickness
of the GaN layers ranged from 0.5 to 2 µm. The GaN growth rate was
controlled from 0.02 to 0.4 µm/min
Mesa-structures (150 µm diameter) were formed by reactive ion etching [14]
using dichlor-difluoromethane (CCl2F2). Two kind of metal
compositions were used as ohmic contacts to n-GaN.
They were Ti/Ni contacts annealed at 1000°C [11] and as deposited Al contacts
(for GaN films with Nd-Na>1018
cm-3).
Both of them were used as a mask for GaN plasma etching. Al or In were used as
a backside contact to SiC substrate.
The layers were characterized using x-ray
diffraction. Capacitance- voltage measurements were performed employing a
Hg-probe.
Luminescent properties of the GaN layers were also examined. Current-voltage
and capacitance-voltage characteristics of mesa-structures were investigated.
Electron beam induced current (EBIC) and back scattered electron (BSE)
techniques were used to study GaN/SiC heterojunction.
The GaN layers was a smooth over the entire wafer. The full width at half
maximum (FWHM) of double crystal x-ray
-scan
rocking curves (RC) for (0002) GaN reflection ranged from 120 to
700 arcsec (Figure 1). It was found that the FWHM of RC's for
-scan
are much larger than FWHM for ,2
-scan.
This feature of the x-ray
RC is similar to RC characteristic obtained for GaN grown on n-6H-SiC
substrates [l]. It may be proposed that in both cases the dislocation
distribution in the epitaxial layers is between an uniform distribution and a
block-mosaic structure where blocks of the material having low dislocation
density are separated from one another by dislocation boundaries. It was found
that there are rather large residual thermal strains in GaN layers. If the
thickness of the layer exceeds ~ 1.5 µm cracks formation is observed.
It must be pointed out that the crystal quality of the epitaxial layer strongly
depends on the substrate quality (Figure 1). It was found that the crystal
quality of GaN films grown on n-type SiC at the same growth conditions is
better than those grown on p-type SiC. This fact may be associated with
difference in crystal quality of used SiC wafers. The FWHM of RC's of
n-type SiC wafers is less than that of p-type SiC wafers. Detailed study of
substrate crystal quality effect on GaN structural perfection will be published
elsewhere. [15]
The GaN layers were characterized by photoluminescence (PL) at 80 and
300 K. The PL experiments were performed using a pulsed nitrogen laser
with a peak power of 2 kW. The PL spectra (Figure 2,3) showed a sharp
ultraviolet peak centered at ~361 nm (80 K), with the FWHM of ~4 nm
(37 meV). At 300K the FWHM was ~8 nm (72 meV). Some samples
exhibited a strong blue luminescence band at 430-450 nm
which may be attributed to structural defects or autodoped impurities.
Nd-Na
concentration in the n-GaN
layers was measured to be the range from 2x1017 to
1x1019 cm-3.
It was found that the background concentration depends on crystal quality of
the GaN layer. We may assume that high donor concentration is caused by some
kind of structural defects in GaN layer.
The capacitance-voltage (C-V)
characteristics of the GaN/SiC pn heterojunctions measured at frequencies of
10 kHz and 1 MHz were linear when plotted in C-2.5-V
coordinates (Figure 4), which is typical for an anisotype heterojunctions with
high densities of interface states. The cut-off voltage was about 2.02 V at 1
MHz and about 2.08 V at 10 kHz. This value is close to the built-in potential
for n-GaN/p-6H-SiC
heterojunction estimated using Anderson's model to be 2.17 V [16].
The structures showed good rectifying characteristics for GaN/SiC pn
heterojunctions. (Figure 5) Breakdown voltages is ranged from 15 to 100 V. The
turn-on voltage for forward I-V
characteristic was ~2V which corresponds to build-in potential of the
heterojunction. (Figure 5a) It should be note that the value of cut-off voltage
determined from C-V
measurements is closed to the turn on voltage for forward I-V characteristics.
Detail study of electrical characteristics of the GaN/SiC heterojunction,
influence of interface on electrical properties and the theoretical calculation
for the bandgap diagram will be published [16].
A weak electroluminesence (EL) was detected in some samples at room
temperature. The EL was dark red when a forward voltage was applied and
white-blue under reverse bias.
The location of heterojunction inside the structures was determined by
simultaneous detection of EBIC and BSE signals by scanning electron probe on
the cleaved structure. Figure 6 shows BSE and EBIC signal profiles across an
GaN/SiC heterostructure. As known, EBIC curve maximum corresponds to the middle
of the space charge region. In the investigated structures a shift of EBIC
signal maximum from the GaN/SiC interface to SiC was observed. The shift value
was about 0.03 µm. This value is close to half of the space charge region
width at zero bias of 0.06 µm determined by C-V
measurements.
Gallium nitride films have been successfully grown on SiC by HVPE technique.
For the first time, p-type
6H-SiC
wafers were used as substrates for GaN deposition. The layers exhibit high
crystal quality as was determined by X-ray
diffraction. The FWHM for the -scan
rocking curve for (0002) GaN reflection was ~120 arcsec. The
photoluminescence spectra for these films were dominated by band edge emission.
The FWHM of the edge PL peak was about 37 meV at 80 K. The minimum
Nd-Na
concentration in undoped layers was 2x1017 cm-3.
Mesa-structures were formed by reactive ion etching. GaN/SiC pn
heterojunctions showed good rectifying characteristics. The value of cut-off
voltage determined from C-V
and I-V
measurements was about 2 V. This value is close to the built-in potential
estimated by Anderson's model for 6H-SiC/GaN
heterojunctions. EBIC study showed that the space charge region of the
heterojunction is shifted towards the SiC substrate.
The authors are grateful to K. V. Vassilevski, E. V. Kalinina, V. E. Sizov and M. V. Rastegaeva for a mesa-structures preparation. The special thanks to K. V. Vassilevski for useful discussions. This work was supported in part by Russian Foundation of Fundamental Researches (contract number 95-02-04148-a).
last updated October 28, 1997 5:28:12 PM.
© 1996-1997 The Materials Research Society
