We fabricated high standoff voltage (450 V) Schottky rectifiers on hydride vapor phase epitaxy grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate overlapping a SiO2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at a current density of 100 A/cm2 and a saturation current density of 10^–5 A/cm2 at a reverse bias of 100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown in GaN to be (2.2 ± 0.7) × 10^6 V/cm. This value for the critical field is a lower limit since most of the devices exhibited abrupt and premature breakdown associated with corner and edge effects

Bandić, Z. Z.

Bridger, P. M.

McGill, T. C.

Phanse, V. M.

Piquette, E. C.

Redwing, J. M.

Vaudo, R. P.

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 9 1 MARCH 1999High voltage 450 V GaN Schottky rectifiers
Z. Z. Bandic´,a) P. M. Bridger, E. C. Piquette, and T. C. McGill
Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125
R. P. Vaudo, V. M. Phanse, and J. M. Redwing
Epitronics, 7 Commerce Drive, Danbury, Connecticut 06810
~Received 31 August 1998; accepted for publication 23 December 1998!
We fabricated high standoff voltage ~450 V! Schottky rectifiers on hydride vapor phase epitaxy
grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including
lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate
overlapping a SiO2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at
a current density of 100 A/cm2 and a saturation current density of 1025 A/cm2 at a reverse bias of
100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown
in GaN to be (2.260.7)3106 V/cm. This value for the critical field is a lower limit since most of
the devices exhibited abrupt and premature breakdown associated with corner and edge effects.
© 1999 American Institute of Physics. @S0003-6951~99!02409-2#Wide band gap materials, primarily SiC and GaN, have
recently attracted a lot of interest for applications in high
power and high temperature electronics. Although the pro-
cessing technology for SiC is more mature, GaN offers sev-
eral advantages. First, there are various device possibilities
using GaN/AlGaN heterojunctions which are not available in
the SiC system. Second, the availability of cheap and effi-
cient hydride vapor phase epitaxy ~HVPE! growth technol-
ogy achieving growth rates in excess of 100 mm/h, have
produced thick, high quality GaN layers on sapphire.1 Third,
by using AlGaN layers, one can take advantage of a larger
band gap to achieve higher critical electric fields than in GaN
alone.
In this study, we focus on the fabrication of high voltage,
GaN based Schottky rectifiers and the measurement of the
critical field for electric breakdown. The critical field for
electric breakdown is one of the most significant parameters
in the design and performance of high power devices. It di-
rectly influences the required thickness of the standoff region
in the Schottky rectifier and bipolar devices, such as the thy-
ristor. Since the thickness of the standoff region sets the
resistivity of the device, it will determine power dissipation
and maximum current density of the device.2,3 In previous
studies, Schottky diodes have been fabricated on GaN using
a variety of elemental metals including Pd and Pt,4,5, Au, Cr,
and Ni,6,7, and Mo and W.8 More details on the metal-GaN
contact technology can be found in Ref. 9.
In this work, Schottky rectifiers were fabricated on 8–10
mm thick GaN layers grown by HVPE on sapphire, where
the electron concentration changes with the distance from the
GaN/sapphire interface. We carried out conductivity and
Hall measurements on a series of HVPE GaN films of vary-
ing thickness ranging from 0.07 to 9.2 mm, and fitted the
data to a two layer model.10–13 From the model, we con-
cluded that the GaN films consisted of a low conductivity,
low electron concentration, 8–10 mm thick top layer on a
a!Electronic mail: zzb@ssdp.caltech.edu1260003-6951/99/74(9)/1266/3/$15.00
Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tvery thin (,100 nm), highly conductive, high electron con-
centration bottom layer. The electron concentrations and mo-
bilities in the thin interface layer and thick upper layer were
231020 cm23 and 35 cm2/Vs, and 231016 cm23 and
265 cm2/Vs, respectively. These values correspond to con-
ductivities of 1120 and 0.85 S cm21 for the interface layer
and upper layer, indicating that the interface layer is approxi-
mately three orders of magnitude more conductive.
Capacitance–voltage (C – V) measurements performed on
the 9.2 mm thick samples determined that the net donor con-
centration in the top few microns of the film was (261)
31016 cm23, consistent with the two layer model. A cross
sectional transmission electron microscopy ~XTEM! study
on our HVPE samples ~grown under similar conditions! was
presented in Ref. 14, where it was concluded that the region
adjoining the interface was highly disordered and included a
large number of subgrain boundaries, stacking faults and
prismatic plane faults. Previous XTEM studies on GaN
samples grown by HVPE indicated the presence of a similar
100–200 nm thick, highly defective interface layer.13,15 Sec-
ondary ion mass spectroscopy studies done on our samples
revealed large oxygen impurity concentrations near the GaN/
sapphire interface, which also can account for the observed
high electron concentration. Therefore, we conclude that the
high electron concentration and low electron mobility at the
GaN/sapphire interface are a combination of both the ob-
served high defect density and high concentration of impuri-
ties ~oxygen!.
We tested several device and contact geometries includ-
ing lateral, mesa, and Schottky metal field plate devices as
shown in Fig. 1. Prior to metal deposition, the GaN surfaces
were cleaned with organic solvents, dipped in HF:H2O
~1:10!, rinsed in de-ionized water, and blown dry with nitro-
gen gas. Following cleaning, gold ~1500 Å! was sputtered in
a chamber with a background pressure of 231028 Torr and
patterned to produce Schottky contacts. Ti/Al/Ni/Au ~150
Å/1500 Å/100 Å/1000 Å! was then sputtered to produce
ohmic contacts as deposited. On some devices, large area Au6 © 1999 American Institute of Physics
o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1267Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Bandic´ et al.metallization was used as a low resistivity contact instead.
The mesa edge termination was produced by chemically as-
sisted ion beam etching, using Xe ions accelerated with 1000
V, and 25 sccm of Cl2 flow.2,3 For the metal field plate de-
vices, SiO2 was sputtered using SiO2 targets and 10 sccm of
O2 flow, and then patterned.
The current–voltage (I – V) measurements were taken
with a Keithley 237 high voltage source measure unit and
with an HP 4156A precision semiconductor parameter ana-
lyzer. The Keithley 237 was used for high voltage measure-
ments while the HP 4156A was used to determine the
Schottky barrier height and for I – V measurements up to 100
V. I – V characteristics for the fabricated devices are shown
in Figs. 2 and 3. The reverse breakdown voltages observed
were in the range between 250 and 450 V and exceeded 450
V in several devices. We associate this variation in the de-
vice breakdown voltage with nonuniformities in the electron
concentration. The Au Schottky barrier heights obtained
from the I – V measurements varied between 1.1 and 0.8 eV.
The ideality factor of the diodes ranged between 1.6 and 4.
In the case of the lateral diode, the ON-state voltage was 5 V
for a current density of 100 A/cm2. Large ON-state voltages
are a consequence of both the large ohmic contact resistances
and the low electron concentration of the upper layer. The
series resistance of the device is significantly affected by the
presence of the highly conductive interface layer. This layer
‘‘shorts’’ the conduction path, so that the series resistance
consists mainly of the ohmic contact resistance, the resis-
tance of the 8–10 mm thick layer between ohmic contact and
the highly conductive layer, and the resistance of the 8–10
mm thick layer between the highly conductive layer and the
Schottky contact. Without the highly conductive interface
FIG. 1. Different device geometries for GaN Schottky rectifiers. The thick-
nesses of the layers of different electron concentration are not drawn to
scale. ~a! Lateral Schottky rectifier. ~b! Mesa device offers lower resistivity
since ohmic contacts are deposited closer to the high conductivity interface
layer. ~c! Lateral Schottky rectifier processed with Schottky metal field plate
overlapping a SiO2 layer.
FIG. 2. I – V curve taken with Keithley 237 high voltage source measure
unit for the high voltage rectifier displaying large standoff voltage.
Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tlayer, the device series resistance would be determined by
the distance between the ohmic and Schottky contact. To
reduce the series resistance we etched 5 mm of the 8–10 mm
top layer and deposited ohmic contacts closer to the more
conductive interface layer @Fig. 1~b!#. This reduced the ON-
state voltage from 5 to 4.2 V at a forward current density of
100 A/cm2. However, as can be observed from Fig. 3, the
reverse leakage current increased to approximately
1022 A/cm2, probably due to the etch damage of the mesa
walls.
The electric field crowding at the edges and corners of
the Schottky contact will reduce the effective barrier height
and increase the reverse leakage current. This can also lead
to premature breakdown due to nonuniform current spread-
ing and excessive heating of the device, as can be observed
in Fig. 4. To improve the Schottky contacts, we fabricated
metal field plate contacts using SiO2 as an insulator.16 The
edge of the Schottky contact overlaps sputtered SiO2,
thereby reducing electric field crowding at edges and
corners.16 In this case, as can be observed from Fig. 3, mea-
sured saturation current at a reverse bias of 100 V was ap-
proximately 1025 A/cm2, an improvement of two orders of
magnitude when compared to the saturation current density
of diodes with lateral contacts.
From the experimental results, we find the critical field
for electric breakdown to be (2.260.7)3106 V/cm by using
the formula for the punch-through diode,16 VPT5ECRW
2qNdW2/2e0er , and substituting measured values. This is
FIG. 3. I – V curves obtained with the HP 4156A precision semiconductor
parameter analyzer for the three device geometries discussed above. In all
three data sets the Schottky contacts were squares with 100 mm sides.
FIG. 4. Scanning electron microscope ~SEM! photo of the Schottky contact
of a diode after breakdown. The melted Au at its edges and corners indicates
premature corner and edge breakdown.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1268 Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Bandic´ et al.in agreement with recent experimental and theoretical
predictions17,18 Since our devices suffered from premature
corner and edge breakdown ~Fig. 4!, and since device geom-
etries were not fully optimized, we conclude that this value
of critical field is only a lower limit.
It is of considerable interest to theoretically estimate and
optimize the required thickness and doping of the GaN
standoff layer in the Schottky rectifier. Smaller required
thickness and larger doping will both contribute to smaller
resistance and power dissipation. We calculated the reverse
breakdown voltage of a uniformly doped GaN Schottky rec-
tifier, by solving: *0
xsca@E(x)#dx51, where xsc is the width
of the space-charged layer. Ionization coefficients a for elec-
trons ~n! and holes ~p! in GaN were obtained from Ref. 18 by
fitting calculated values of a at large values of electric field,
and are: a5an5ap52.93108 exp@232.7(MV/cm)/E# .
The results of the calculation are shown in Fig. 5. For ex-
ample, 5 kV Schottky rectifiers can be fabricated using 18
mm thick GaN layers with a doping concentration of ap-
proximately 1.531016 cm23. At the point of reverse break-
down, the electric field is 53106 V/cm, which we can accept
as the theoretical maximum value of the critical field. How-
ever, the particular device geometry will affect the value of
the critical field for electric breakdown.
One of the most important consequences of this study is
the experimental demonstration of a high critical field for
electric breakdown in GaN. A high critical field indicates
feasability of GaN as a material for a variety of unipolar and
bipolar devices.2,3 Furthermore, assuming that the critical
field for electric breakdown approximately scales as the
square of the band gap, even higher critical fields can be
expected for AlGaN. The lack of a suitable conductive GaN
substrate which can provide large area back ohmic contact
and therefore uniform current spreading, can be circum-
FIG. 5. The calculation of the reverse breakdown voltage of uniformly
doped GaN Schottky rectifier based on Ref. 18. For a chosen breakdown
voltage one can find corresponding doping concentration ~solid line!, which
further gives required thickness ~dotted line!. For example, 5 kV Schottky
rectifiers can be fabricated using 18 mm thick GaN layers with a doping
concentration of approximately 1.531016 cm23.Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tvented by using highly doped layers underneath the active
standoff layer. Although ON-state voltages demonstrated in
this work are rather high, our estimates based on the Rich-
ardson equation show that the ON-state voltage for a current
density of 100 A/cm2 can be as low as 1–2 V. However, it is
necessary to improve Schottky contact edge terminations, re-
duce ohmic contact resistances and achieve more precise
control over doping.
In conclusion, we have fabricated Schottky rectifiers on
HVPE-grown GaN which had a standoff voltage of 450 V, a
minimum saturation current density of 1025 A/cm2 at reverse
bias of 100 V, and 4.2 V ON-state voltage at a forward
current density of 100 A/cm2. We have also demonstrated a
critical field for electric breakdown in GaN of (2.260.7)
3106 V/cm which approaches the theoretical estimate. This
value is only a lower limit since the reverse breakdown volt-
age was limited by premature edge breakdown. Theoretical
calculations indicated the possibility of 18 mm thick nitride
devices with a 5 kV standoff voltage.
This work was supported by DARPA and monitored by
the ONR under Grant No. N00014-92-J-1845, and was also
supported by DARPA/EPRI and monitored by the ONR un-
der Grant. No. MDA972-98-1-0006.
1 N. R. Perkins, M. N. Horton, Z. Z. Bandic´, T. C. McGill, and T. F. Kuech,
Mater. Res. Soc. Symp. Proc. 395, 243 ~1996!.
2 Z. Z. Bandic´, P. M. Bridger, E. C. Piquette, T. F. Kuech, and T. C.
McGill, Mater. Res. Soc. Symp. Proc. 483, 399 ~1998!.
3 Z. Z. Bandic´, E. C. Piquette, P. M. Bridger, R. A. Beach, T. F. Kuech, and
T. C. McGill, Solid-State Electron. 42, 2289 ~1998!.
4 L. Wang, M. I. Nathan, T. H. Lim, M. A. Khan, and Q. Chen, Appl. Phys.
Lett. 68, 1267 ~1996!.
5 S. N. Mohammad, Z. Fan, A. E. Botchkarev, W. Kim, O. Aktas, A. Sal-
vador, and H. Morkoc, Electron. Lett. 32, 598 ~1996!.
6 E. V. Kalinina, N. I. Kuznetsov, V. A. Dmitriev, K. G. Irvine, and C. H.
Carter, Jr., J. Electron. Mater. 25, 831 ~1998!.
7 P. Hacke, T. Detchprohm, K. Hiramatsu, and N. Sawaki, Appl. Phys. Lett.
63, 2676 ~1993!.
8 E. C. Piquette, Z. Z. Bandic´, and T. C. McGill, Mater. Res. Soc. Symp.
Proc. 482, 1089 ~1998!.
9 Q. Z. Liu and S. S. Lau, Solid-State Electron. 42, 677 ~1998! and refer-
ences cited therein.
10 R. P. Vaudo, V. M. Phanse, M. C. Cattrel, and J. M. Redwing, 2nd Inter-
national Conference on Nitride Semiconductors, Tokushima, Japan,
25–31 October 1997.
11 D. C. Look, Electrical Characterization of GaAs Materials and Devices
~Wiley, New York, 1989!.
12 D. C. Look and R. J. Molnar, Appl. Phys. Lett. 70, 3377 ~1997!.
13 W. Go¨tz, J. Walker, L. T. Romano, N. M. Johnson, and R. J. Molnar,
Mater. Res. Soc. Symp. Proc. 449, 525 ~1997!.
14 Y. Golan, X. H. Wu, J. S. Speck, R. P. Vaudo, and V. M. Phanse, Appl.
Phys. Lett. 73, 3090 ~1998!.
15 W. Go¨tz, L. T. Romano, B. S. Krusor, N. M. Johnson, and R. J. Molnar,
Appl. Phys. Lett. 69, 242 ~1996!.
16 B. J. Baliga, Power Semiconductor Devices ~ITP P, Boston, 1996!, p. 76.
17 N. Dyakonova, A. Dickens, M. Shur, R. Gaska, and J. W. Yang, Appl.
Phys. Lett. 72, 2562 ~1998!
18 J. Kolnik, I. H. Ogusman, K. F. Brennan, R. Wang, and P. P. Ruden, J.
Appl. Phys. 82, 726 ~1997!.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


High voltage (450 V) GaN Schottky rectifiers

https://authors.library.caltech.edu/2460/1/BANapl99.pdf

High voltage (450 V) GaN Schottky rectifiers

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main