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IEEE ELECTRON DEVICE LETTERS, VOL. 23, NO. 2, FEBRUARY 2002 79
Positive Flatband Voltage Shift in MOS Capacitorson n-Type GaN
K. Matocha, T. P. Chow, Senior Member, IEEE, and R. J. Gutmann
AbstractGaN MOS capacitors were fabricated using silicondioxide deposited by low-pressure chemical vapor deposition oxideat 900 C. The MOS capacitor flatband voltage shift versus tem-perature was used to determine a pyroelectric charge coefficient of3.7 109 q cm2 -K, corresponding to a pyroelectric voltage coeffi-cient of 7.0 104 V m-K.
Index TermsGallium nitride, MOS capacitors, polarization,pyroelectricity, pyroelectric devices.
B OTH the spontaneous and piezoelectric polarization[1][3] of the AlN/GaN system has been used in creatingHFETs with high sheet charge densities ( 10 cm ) [4][6].Also, a significant pyroelectric effect is present in GaN with
theoretical estimates of the pyroelectric voltage coefficient of
7 10 V m-K [7]. Experimentally, the pyroelectric voltage
coefficient for GaN was approximated as 10 V m-K [8],
with an experimental value of 8 10 V m-K measured for
AlN [9]. In this letter, the GaN pyroelectric charge coefficient
is determined from the change in flatband voltage ( ) of GaN
MOS capacitors as a function of temperature.
The GaN polarization charge is such that a negative polar-
ization charge arises on the Ga-face and a positive polarization
on the N-face [1]. In metalorganic chemical vapor deposited
(MOCVD) material, epilayers are typically Ga-face. For n-type
samples, charge equilibrium is graphically represented in Fig. 1.The charge is balanced near the Ga-face by a depletion layer
with compensating positively charged donors. Near the N-face,
an electron accumulation layer is formed to balance the posi-
tive polarization charges. As the temperature is increased, the
polarization charge increases, thus, increasing the front surface
depletion layer thickness, as well as increasing the electron
density in the accumulation layer, .
The flatband voltage of an MOS capacitor on n-type material
is given by:
(1)
where is the metal workfunction, the semiconductor
electron affinity, the semiconductor bandgap, the semi-
conductor bulk potential ( ), the oxide capacitance
Manuscript received October 15, 2001; revised November 19, 2001. Thiswork was supported in part by GE-CRD and the Center for Power ElectronicsSystems and National Science Foundation (Award EEC-9731677). The reviewof this letter was arranged by Editor D. Ueda.
The authors are with the Center for Integrated Electronics, Rensselaer Poly-technic Institute, Troy, NY, 12180 USA (e-mail: [email protected]).
Publisher Item Identifier S 0741-3106(02)01495-7.
Fig. 1. Schematic of charges in n-type GaN epilayer.
per unit area, the fixed oxide charge density, the inter-
face-trapped charge density and the negative polarization
charge density at the oxide-semiconductor interface.
The metal workfunction and the semiconductor electron
affinity and bandgap are expected to be only weakly dependent
on temperature and their variation with temperature is ignored.The bulk potential, , does change with temperature as the
Fermi level shifts toward the intrinsic level. After determining
the doping, the bulk potential can be calculated as a function
of temperature. The fixed charges, , result from a thin
nonstoichiometric layer in the oxide and are independent of
temperature. The interface-trapped charge, , can vary with
temperature as the Fermi level moves nearer the intrinsic level.
The net change in interface-trapped charge at flatband with
temperature is related to the shift in the bulk potential by:
(2)
With the small change in and a low interface-state density
near the Fermi level at flatband conditions, the effect of will
not significantly contribute to the change in flatband voltage
with temperature. Simplifying according to the above discus-
sion, the flatband voltage change with temperature is given by:
(3)
The 2.3 m thick GaN sample used in this study was epitax-
ially grown at 1050 C on a sapphire substrate using MOCVD
07413106/02$17.00 2002 IEEE
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MATOCHA et al.: POSITIVE FLATBAND VOLTAGE SHIFT IN MOS CAPACITORS 81
the AlGaN/GaN interface should not change significantly with
temperature. However, for the top of the AlGaN layer in the
HFET, the negative polarization charge increases with temper-
ature, thus reducing the two-dimensional electron gas (2DEG)
density and causing current slump.
In summary, the pyroelectric polarization of GaN was mea-
sured using MOS capacitors using a high-temperature oxide di-
electric. A pyroelectric charge coefficient of 3.7 10 q cm -Kwas measured corresponding to a pyroelectric voltage coeffi-
cient of 7.0 10 V m-K.
ACKNOWLEDGMENT
The authors would like to thank GaN epitaxial layer growth
by R. Wang, H. Lu, and I. Bhat. They would also liketo thank M.
Lazzeri and P. Gipp at General ElectricCorporate Research
and Development (GE-CRD) for the high-temperature oxide
deposition.
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