APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 13 28 SEPTEMBER 1998

Schottky barrier engineering in III–V nitrides via the piezoelectric effect

E. T. Yu,a) X. Z. Dang, L. S. Yu, D. Qiao, P. M. Asbeck, and S. S. Lau
Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla,
California 92093-0407
G. J. Sullivan
Rockwell International Science Center, Thousand Oaks, California 91358
K. S. Boutros and J. M. Redwing
Epitronics/ATMI, Phoenix, Arizona 85027-2726
~Received 6 April 1998; accepted for publication 25 July 1998!
A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on
the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/Alx Ga12x N
barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess
significantly larger effective barrier heights than those for Alx Ga12x N, and the influence of
composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure
optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement
of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and
reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.
© 1998 American Institute of Physics. @S0003-6951~98!03939-4#

III–V nitride heterostructure field-effect transistors procedures and conditions have been described elsewhere.10
~HFETs! have emerged as highly attractive candidates for Schottky diodes were fabricated using evaporated Ti/Al an-
high-voltage, high-power operation at microwave nealed at 650–750 °C to form large-area ohmic contacts to
frequencies.1–5 A number of recent studies have demon- the HFET layers, and Ni to form Schottky contacts consist-
strated that the piezoelectric effect plays a key role in gov- ing typically of 320 mm diam dots. Capacitance–voltage
erning carrier distributions and other electronic properties in (C – V) profiling was used to determine sheet carrier concen-
HFETs and other III–V nitride heterostructure devices.6–9 In trations in these structures, and photoresponse measurements
particular, the piezoelectric effect is largely responsible for were used to determine effective Schottky barrier heights.11
the extraordinarily high sheet carrier densities that can be The influence of these parameters on device properties was
achieved in the channel of a GaN/Alx Ga12x N HFET.7,8 confirmed in current–voltage (I – V) measurements per-
In this letter, we describe the design, experimental char- formed on Schottky diodes.
acterization, and analysis of GaN/Alx Ga12x N HFET struc- Figure 1 shows schematic diagrams of the epitaxial layer
tures in which the piezoelectric effect is employed to achieve structure, energy-band-edge profile, and electrostatic charge
distributions for a conventional GaN/Alx Ga12x N HFET
a marked enhancement of the effective Schottky barrier
structure and for a structure incorporating a two-layer
height. Specifically, a two-layer GaN/Alx Ga12x N barrier is
GaN/Alx Ga12x N barrier. As shown in Fig. 1~b!, incorpora-
employed, within which the piezoelectrically induced polar-
tion of a GaN layer at the top of the heterostructure increases
ization charge acts to increase the barrier height for gate
the effective Schottky barrier height by allowing the negative
leakage current in the HFET. This can be accomplished with
piezoelectric charge at the top of the Alx Ga12x N layer to be
no increase in the barrier thickness, and consequently little if positioned within the HFET barrier structure. This approach
any change in gate capacitance, and with only a minor im- is analogous to the use of a thin p 1 layer at or near the
pact on carrier concentration in the channel. It is anticipated metal–semiconductor interface of an n-type Schottky diode
that this will allow gate currents to be significantly reduced to increase the effective barrier height electrostatically.12 In a
in nitride HFETs with little penalty exacted in channel con-
ductance, transconductance, and other device properties.
TABLE I. Schottky barrier structures and corresponding sheet carrier con-
The epitaxial structures used in these experiments were
centrations and effective Schottky barrier heights measured by C – V profil-
grown on c-plane ~0001! sapphire substrates by low-pressure ing and photoresponse, respectively.
metalorganic vapor phase epitaxy ~MOVPE!. For all
samples, a 3 mm GaN buffer layer was deposited initially, Sample No. Barrier structure n s (cm22) f eff
B ~V!

followed by various GaN/Alx Ga12x N structures constituting 1 250 Å Al0.15Ga0.85N 2.831012 1.2960.05
the barrier in an HFET structure. A series of several 2 150 Å GaN/250 Å Al0.15Ga0.85N 1.831012 1.4160.05
GaN/Alx Ga12x N heterostructures, enumerated in Table I, 3 300 Å Al0.25Ga0.75N 5.031012 1.5260.05
was grown for these studies. Details of the epitaxial growth 4 75 Å GaN/225 Å Al0.25Ga0.75N 4.531012 1.8960.05
5 300 Å Al0.30Ga0.70N 5.531012 1.5660.05
6 75 Å GaN/225 Å Al0.30Ga0.70N 5.131012 1.8360.05
a!
Electronic mail: ety@ece.ucsd.edu

0003-6951/98/73(13)/1880/3/$15.00 1880 © 1998 American Institute of Physics

Appl. Phys. Lett., Vol. 73, No. 13, 28 September 1998 Yu et al. 1881

III–V nitride HFET, epitaxial growth and compositional structure shown in Fig. 1~b! may be used to obtain analytic
control allow the magnitude and position of the charge expressions for the effective barrier height, f effB , and the
within the barrier to be controlled very precisely. sheet carrier concentration, n s , in the two-dimensional elec-
A straightforward electrostatic analysis of the epitaxial tron gas ~2DEG! at the lower GaN/Alx Ga12x N interface. The
layer structure and corresponding charge distribution for the sheet carrier concentration given by such an analysis is

n s5
1
e
S s pz 2 S D
e AlGaN
d AlGaN
~ f GaN
B 1E F /e2V ! 1
eN d d AlGaN
2
1

11 ~ e AlGaN / e GaN!~ d GaN /d AlGaN!

S D
e AlGaN
e GaN
eN d d GaN
, D ~1!

where s pz is the piezoelectrically induced polarization culations shown here are for V50. While clearly approxi-
charge density, e GaN and e AlGaN are the dielectric constants mate, Eqs. ~1! and ~2! provide a sound fundamental basis for
of GaN and Alx Ga12x N, respectively, f GaN B is the GaN design and analysis of nitride Schottky barrier structures in
Schottky barrier height, E F is the Fermi level ~relative to the our studies.
GaN conduction-band edge! at the lower GaN/Alx Ga12x N Figure 2~a! shows f effB as a function of d GaN for the
interface, d GaN and d AlGaN are the thicknesses of the GaN and structure shown in Fig. 1~b! with a 250 Å Al0.15Ga0.85N
Alx Ga12x N layers in the HFET barrier structure, N d is the layer, for various background dopant concentrations in the
background dopant concentration in the Alx Ga12x N layer, Al0.15Ga0.85N layer, calculated using Eqs. ~1! and ~2!. Also
and V is the bias voltage applied to the Schottky contact. We shown are experimentally measured effective barrier heights
assume a value for s pz of 2.531013x Ale/cm2. 7,8 The back- for samples 1 and 2, taken from Table I. A clear enhance-
ground dopant concentration in GaN is assumed to be negli- ment of approximately 0.1 V in the effective barrier height is
gibly small in comparison to the other charge densities observed for sample 2, and calculations of f eff
B and n s using
present. The effective barrier height shown in Fig. 1~b! is 17 18 23
values for N d of 5310 – 1310 cm are in close agree-
given by ment with the experimentally measured values. The results
1 ed GaN shown in Fig. 2~a! indicate that a low background dopant
f eff
B 5 DE c 1 f B 2V1
GaN
~ n s 2N d d AlGaN! , ~2! concentration in the Alx Ga12x N layer is required to achieve
e e GaN
the maximum increase in f eff B over that for a simple
where DE c is the conduction-band offset between GaN and Alx Ga12x N barrier.
Alx Ga12x N. Equation ~2! implies that for a simple To achieve optimum performance in a nitride HFET, one
Alx Ga12x N barrier, the Schottky barrier height is given by would ideally wish to maximize both the effective barrier
f GaN
B 1DE c ; this is consistent with direct measurements of height and the sheet carrier concentration in the channel
Alx Ga12x N Schottky barrier heights.11 Furthermore, f eff
B is a while maintaining a fixed barrier thickness and, conse-
function of applied bias voltage; the measurements and cal- quently, a nearly constant effective gate capacitance. The

FIG. 2. ~a! Effective Schottky barrier height f eff

B as a function of GaN layer
thickness for a Schottky barrier structure consisting of 250 Å Al0.15Ga0.85N
FIG. 1. Schematic diagram of epitaxial layer structure, band-edge energies, with a GaN cap. Lines represent calculated values; circles are measurements
and electrostatic charge distributions for ~a! a conventional GaN/Alx Ga12x N of f eff
B . ~b! f B for Schottky barrier structures consisting of GaN on
eff

HFET structure and ~b! an HFET structure incorporating a GaN/Alx Ga12x N Alx Ga12x N with a total barrier thickness of 300 Å. Circles and squares are
two-layer barrier. measured values for x Al50.25 and x Al50.30, respectively.
1882 Appl. Phys. Lett., Vol. 73, No. 13, 28 September 1998 Yu et al.

FIG. 3. Photocurrent per absorbed photon as a function of incident photon

energy for a 300 Å Al0.25Ga0.75N Schottky barrier structure and a 75 Å
GaN/225 Å Al0.25Ga0.75N barrier. A clear increase in effective Schottky
barrier height is evident. FIG. 4. Current–voltage characteristics for samples 3 and 4, demonstrating
clear reductions in forward-bias current and in reverse-bias leakage current
arising from the piezoelectric enhancement in Schottky barrier height for the
structure incorporating a GaN cap layer.
most effective mechanism for increasing f eff B and n s is to
increase the Al concentration in the Alx Ga12x N layer. For a tures in which the piezoelectric effect is exploited to achieve
fixed total barrier thickness, however, the widths of the GaN a large increase in effective Schottky barrier height com-
and Alx Ga12x N layers must be chosen to yield the optimum pared to that for a simple Alx Ga12x N barrier. The influence
combination of f effB and n s . Figure 2~b! shows f B calcu-
eff of Alx Ga12x N composition, GaN and Alx Ga12x N layer
lated for a GaN/Alx Ga12x N barrier structure with a total thicknesses, and doping has been assessed. An optimized
thickness of 300 Å, as a function of d GaN for x Al50.25 and structure yielded an increase in effective barrier height of
x Al50.30. As shown in the figure, f eff B reaches a maximum
0.37 V, with little penalty in sheet carrier concentration and
for d GaN5100– 125 Å and d AlGaN5175– 200 Å. However, barrier capacitance. I – V characteristics of Schottky diodes
from Eq. ~1! one sees that n s decreases with increasing d GaN , confirm that the enhanced barrier height leads to substantial
necessitating a tradeoff between f effB and n s in selecting an
reductions in forward-bias current and reverse-bias leakage,
optimum value of d GaN . suggesting that corresponding improvements in III–V nitride
To achieve a large increase in f eff
B without an excessive
HFET performance should ensue.
reduction in n s , 75 Å GaN/225 Å Alx Ga12x N Schottky bar-
The authors would like to acknowledge financial support
rier structures and 300 Å Alx Ga12x N control samples were
from BMDO ~Dr. Kepi Wu! monitored by USASMDC and
fabricated ~samples 3–6 in Table I!. Figure 2~b! shows mea-
contributions to sample growth by V. M. Phanse and R. P.
sured values of f eff
B for these structures, taken from Table I. Vaudo. One of the authors ~E.T.Y.! would like to acknowl-
A dramatic enhancement in f eff B is observed when the top edge financial support from the Alfred P. Sloan Foundation.
GaN layer is incorporated into the barrier structure; from
Table I we see that this is achieved with relatively little re- 1
M. A. Khan, Q. Chen, M. S. Shur, B. T. McDermott, J. A. Higgins, J.
duction in n s . Figure 3 shows photocurrent measured as a Burm, W. J. Schaff, and L. F. Eastman, IEEE Electron Device Lett. 17,
584 ~1996!.
function of incident photon energy for a 300 Å 2
A. Ozgur, W. Kim, Z. Fan, A. Botchkarev, A. Salvador, S. N. Moham-
Al0.25 Ga0.75 N and a 75 Å GaN/225 Å Al0.25 Ga0.75 N struc- mad, B. Sverdlov, and H. Morkoç, Electron. Lett. 31, 1389 ~1995!.
3
ture. For internal photoemission within the Schottky barrier Y. F. Wu, B. P. Keller, S. Keller, D. Kapolnek, P. Kozodoy, S. P. Den-
structure, the threshold for nonzero photoresponse provides a Baars, and U. K. Mishra, Appl. Phys. Lett. 69, 1438 ~1996!.
4
S. C. Binari, J. M. Redwing, G. Kelner, and W. Kruppa, Electron. Lett. 33,
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strate the large enhancement in Schottky barrier height cre- 5
Q. Chen, J. W. Yang, R. Gaska, M. A. Khan, M. S. Shur, G. J. Sullivan,
ated, via the piezoelectric effect, by the presence of the GaN A. L. Sailor, J. A. Higgins, A. T. Ping, and I. Adesida, IEEE Electron
cap layer. Figure 4 shows I – V characteristics for samples 3 Device Lett. 19, 44 ~1998!.
6
A. Bykhovski, B. Gelmont, and M. Shur, J. Appl. Phys. 74, 6734 ~1993!.
and 4. For the structure incorporating the 75 Å GaN cap 7
P. M. Asbeck, E. T. Yu, S. S. Lau, G. J. Sullivan, J. Van Hove, and J. M.
layer, there is a clear reduction in current at forward bias Redwing, Electron. Lett. 33, 1230 ~1997!.
8
voltages and a large suppression in reverse-bias leakage cur- E. T. Yu, G. J. Sullivan, P. M. Asbeck, C. D. Wang, D. Qiao, and S. S.
rent, both of which we interpret as consequences of the in- Lau, Appl. Phys. Lett. 71, 2794 ~1997!.
9
R. Gaska, J. W. Yang, A. D. Bykhovski, M. S. Shur, V. V. Kaminski, and
creased effective Schottky barrier height. These features in S. M. Soloviov, Appl. Phys. Lett. 72, 64 ~1998!.
the Schottky diode I – V characteristics should translate di- 10
J. M. Redwing, M. A. Tischler, J. S. Flynn, S. Elhamri, M. Ahoujja, R. S.
rectly into reduced gate leakage currents in III–V nitride Newrock, and W. C. Mitchel, Appl. Phys. Lett. 69, 963 ~1996!.
11
L. S. Yu, Q. J. Xing, D. Qiao, S. S. Lau, K. S. Boutros, and J. M. Redwing
HFETs.
~unpublished!.
In summary, we have proposed, experimentally demon- 12
S. M. Sze, Physics of Semiconductor Devices, 2nd ed. ~Wiley, New York,
strated, and analyzed GaN/Alx Ga12x N HFET barrier struc- 1981!, p. 293.