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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: matock@rpi.edu).

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.

REFERENCES

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