1. No category

Solid-State Electronics 45 (2001) 405±410
Schottky recti®ers fabricated on free-standing GaN substrates
J.W. Johnson a, J.R. LaRoch a, F. Ren a,*, B.P. Gila b, M.E. Overberg b,
C.R. Abernathy b, J.-I. Chyi c, C.C. Chuo c, T.E. Nee c, C.M. Lee c, K.P. Lee b,
S.S. Park d, Y.J. Park d, S.J. Pearton b
a
c
Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USA
b
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA
Department of Electrical Science and Engineering, National Central University, Chung-Li 32054, Taiwan, ROC
d
Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea
Received 12 October 2000; received in revised form 15 December 2000; accepted 16 January 2001
Abstract
GaN Schottky recti®ers have been fabricated on free-standing substrates and on epi/substrate structures. Forward
turn-on voltages were as low as 3 V at 25°C. Reverse recovery was complete in <600 ns, with a characteristic time
constant of 163 ns. The temperature coecient for reverse breakdown voltage (VB ) was 2:5 0:6 V K 1 which is
much lower than for lateral recti®ers reported previously, where values up to 30 V K 1 were achieved. Reverse
currents increased with rectifying contact diameter and VB decreased with increasing contact size. The best on-state
resistance was 20.5 mX cm 2 for diodes with VB ˆ 450 V, producing a ®gure-of-merit …VB †2 =RON of 10
MW cm 2 . Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
There is tremendous current interest is developing
GaN electronics for applications requiring high powers
and high temperature [1±9]. Very high reverse breakdown voltages (VB ) have been achieved in lateral Schottky recti®ers fabricated on insulating AlGaN epilayers
on sapphire substrates (up to 9.7 kV) [10±14]. However a
concern is thermal management, given the poor thermal
conductivity of sapphire. In this regard, better choices
would be either SiC or GaN itself, since the latter has
approximately the same thermal conductivity as Si. In
addition, these electrically conductive substrates would
allow for fabrication of vertical geometry recti®ers capable of much higher current conduction than lateral
recti®ers fabricated on insulating substrates [15].
*
Corresponding author. Tel.: +352-392-4727; fax: +352-1923235.
E-mail address: [email protected]¯.edu (F. Ren).
In this paper, we report on the size- and temperaturedependent performance of Schottky recti®ers fabricated
on free-standing GaN substrates. The reverse recovery
time constant is determined to be 163 ns in diodes
switched from forward to reverse bias. The VB values are
found to generally decrease as the contact size increase,
which increases the probability of a having a dislocation
within the device active area. In addition, the reverse
current density at a given bias increases for larger contact diameters, for a similar reason.
2. Experimental
The 200 lm thick, free-standing GaN substrates were
grown by hydride vapor phase epitaxy (HVPE) on
sapphire substrates, then lifted-o€ by laser beam heating. To smooth the respective surfaces, both chemical±
mechanical polishing and dry etching (Ga face) were
employed [16]. The typical defect density was 105 cm 2
for the Ga face and 107 cm 2 for the N-face, with rootmean-square roughness of <1 nm for the Ga-terminated
0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 0 5 9 - 4
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J.W. Johnson et al. / Solid-State Electronics 45 (2001) 405±410
Fig. 1. AFM scans of N-face (top) and Ga face (bottom) of GaN substrate. (note di€erence in z-axis scale)
face and 10 nm for the N-terminated face. The atomic
force microscopy (AFM) scans are shown in Fig. 1. The
®nal, free-standing GaN template was 10 7 mm2 , as
shown in Fig. 2.
The di€erence in surface quality for the two faces was
also re¯ected in the X-ray di€raction (XRD) scans. Fig.
3 shows triple-axis omega and omega/two-theta scans
from the Ga face (top), with FWHM of 80700 (omega)
and 600 (omega/two-theta), while the corresponding
data for the N-face is shown in the bottom of the ®gure.
The FWHM for that side were 81800 (omega) and 1900
(omega/two theta).
On some of the substrates we grew 6.8 lm of undoped GaN by metal organic chemical vapor deposition
(MOCVD) at 1040°C using conventional precursors [9].
Fig. 4 shows photoluminescence (PL) spectra taken at 25
and 298 K from the Ga face of the bare substrate. Note
that band edge luminescence is only observed at low
temperatures. After growth of the 6.8 lm thick epilayer
on top of the substrate the room temperature PL spectrum is still dominated by the yellow band centered at
around 600 nm.
Recti®ers were fabricated by depositing full-area
back Ohmic contacts (Ti/Al, heated to 750°C for 20 s)
and front-side rectifying contacts (Pt/Au, with diameters
44±148 lm) patterned by lift-o€. The current±voltage (I±
V) characteristics were recorded on an HP 4145B parameter analyzer with samples held at 25±300°C.
J.W. Johnson et al. / Solid-State Electronics 45 (2001) 405±410
Fig. 2. Photograph of free-standing GaN substrate.
407
Fig. 4. PL spectra from GaN substrate at 25 and 298 K. The
inset shows the 298 K PL from the 6.8 lm thick epilayer grown
on the GaN substrate.
3. Results and discussion
Fig. 3. XRD scans from Ga face (top) and N-face (bottom) of
GaN substrate.
Fig. 5 (top) shows I±V characteristics from 75 lm
Schottky recti®ers fabricated either directly on the GaN
substrate, or on the epilayer on top of the substrate. The
VB is clearly larger in the latter case because of the lower
doping (5 1016 cm 3 ) in the epilayer relative to the
substrate (>1017 cm 3 ). The bottom of Fig. 5 shows the
reverse I±V characteristics for recti®ers fabricated on
the bare substrate, as a function of the contact diameter.
The general trend is for a decrease in VB with increasing
contact diameter. This re¯ects the higher probability for
having defects within the active area that lead to premature breakdown, as also observed for SiC devices
[17,18].
Similar data is shown in Fig. 6 (top) for recti®ers
fabricated on the epilayer grown on the substrates. The
some general trend is observed (Fig. 6, bottom), but the
overall magnitude of the VB values is higher. These
values are fairly typical of reported numbers for vertically depleting GaN diode recti®ers [13±15]. Note that
the data is too scattered to extract a quantitative relationship.
Fig. 7 shows the variation of reverse current at 300
V reverse voltage for recti®ers fabricated on the epi/
substrate structure, as a function of contact diameter.
Since the current scales with contact diameter, this
suggests that the origin of the current is surface related
at this bias. Once again, this is typical of most of the
GaN recti®ers reported to date and emphasizes the need
for edge termination methods to reduce the in¯uence of
defects around the contact periphery. Applications for
such recti®ers in power switching or power conditioning
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J.W. Johnson et al. / Solid-State Electronics 45 (2001) 405±410
Fig. 5. I±V characteristics from 75 lm devides fabricated on
both the bare substrate and the epi±substrate structure (top)
and reverse I±V characteristics from diodes of di€erent diameter fabricated on the bare substrate.
Fig. 6. Reverse I±V characteristics from diodes of di€erent diameter fabricated on the epi±substrate structure (top) and
variation of VB as function of diode diameter (bottom).
equipment will require very large area recti®ers (many
cm2 ), emphasizing the need to reduce the density of
active defects in the GaN material in order to keep the
reverse current to a manageable level.
Fig. 8 shows the temperature dependence of the reverse I±V characteristics of recti®ers with 54 lm rectifying contact diameter. The VB values show a negative
temperature coecient b, where
ecient should be positive for GaN, i.e. the VB should
increase with increasing temperature [12,14]. However
the presence of a high defect density leads to a negative
value [17].
The temperature dependence of the forward I±V
characteristics from 54 lm recti®ers fabricated on the
epi/substrate structure is shown in Fig. 9 (top). The
forward turn-on voltage (de®ned as the forward voltage
cm 2 ) increases
at which the current density is 100 A
with temperature. This is surprising, given that we might
expect more ecient electron emission over the barrier
at higher temperatures. The current conduction mechanisms in GaN Schottky recti®ers are still not understood
particularly well. The bottom of Fig. 9 shows that at low
forward bias, the ideality factor of these recti®ers is
1.25. This is far from ideal, but does indicate a rea-
VB ˆ VB0 ‰1 ‡ b…T
T0 †Š
where VB0 is the breakdown voltage at room temperature and T is the absolute sample temperature. We have
previously found that b is strongly dependent on recti®er
geometry and the defect density in the sample. From the
data of Fig. 8 we can estimate b ˆ 2:5 0:6 V K 1 for
the current recti®ers. In principle, the temperature co-
J.W. Johnson et al. / Solid-State Electronics 45 (2001) 405±410
409
Fig. 7. Variation of reverse current at a reverse bias of 300 V
for diodes of di€erent diameter fabricated on the epi/substrate
structure.
Fig. 9. Temperature dependence of forward I±V characteristics
in 75 lm diodes fabricated on the epi±substrate structure (top)
and forward I±V characteristic at 100°C of a 75 lm diode
fabricated on the same structure.
IR ˆ 6:93 10 2 exp… t=163 ns†
Fig. 8. Temperature dependence of reverse I±V characteristics
in 54 lm diodes fabricated on the epi±substrate structure.
sonably good contact. The best on-state resistance was
20.5 mX cm 2 for recti®ers with VB ˆ 450 V, producing a
®gure of merit (VB )2 /RON of 10 MW cm 2 .
A key feature of wide band gap recti®ers is their
expected ability to be switched from forward voltage to
reverse without the large current overshoots and long
switching times observed with Si diodes [1±4]. Fig. 10
(top) shows reverse recovery plot for a 75 lm recti®er
fabricated on the epi/substrate structure. The inset
shows the time dependence of the voltage during its
switching. The current recovers to zero within 600 ns,
and the data can be ®t to the curve
where IR is the reverse current and t is the time after
switching polarity of the voltage. The bottom of Fig. 10
shows the dependence of the reverse recovery on input
pulse amplitude, with initial positive biases of 10±25 V
and ®nal biases of 10 to 25 V. There is little signi®cant di€erence in the reverse recovery characteristics
under these conditions.
4. Summary and conclusions
GaN Schottky recti®ers were fabricated on freestanding substrates formed by HVPE growth. There are
some very promising features of the diode performance,
with excellent forward current densities compared to
previously reported lateral recti®ers and very good
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J.W. Johnson et al. / Solid-State Electronics 45 (2001) 405±410
Acknowledgements
The work at UF is partially supported by DARPAEPRI (D. Radack/B. Damsky) and NSF-DMR (9732865). The work at NCU is partially supported by the
National Science Council of ROC under contact no.
NSC 88-2215-E-008-012.
References
Fig. 10. Reverse recovery characteristics from 75 lm diodes
switched from ‡25 to 25 V (top) and e€ect of di€erent bias
conditions on the reverse recovery characteristics (bottom).
reverse recovery characteristics. The temperature coef®cient for VB is still negative, indicating the need for
further defect reduction in the material and continued
optimization of epigrowth on these substrates. The
function of recti®ers on GaN conducting substrates is a
very promising approach for application requiring high
forward currents.
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