Solid-State Electronics 47 (2003) 1487–1490

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Comparison of Pt/GaN and Pt/4H-SiC gas sensors

Jihyun Kim a, B.P. Gila b, C.R. Abernathy b, G.Y. Chung c,F. Ren a,*, S.J. Pearton b

a Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USAb Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA

c Sterling Semiconductor, Tampa, FL 33619, USA

Received 19 September 2002; received in revised form 2 November 2002; accepted 2 December 2002

Abstract

The characteristics of Pt/GaN and Pt/4H-SiC Schottky diodes as gas sensors were measured as a function of

temperature and ambient. Both types of diode rectifiers show rapid (<1 s) changes in forward current upon introduction

of different gases (N2, air, H2, CF4) into the ambient. The diodes can be operated at large forward currents, leading to

large signal sizes for switching from one gas ambient to another. For GaN, a shift of �0.2 V at 25 �C and �0.7 V at 150

�C was obtained at a fixed forward current for switching from N2 to 10% H2 in N2. For SiC, under similar conditions,

shift of 1.34 V at 25 �C was obtained at a fixed forward current of 0.2 A for switching from N2 to 10% H2 in N2. The

signal size increases with increasing measurement temperature due to more efficient cracking of the gas molecules. Both

types of devices appear well suited to combustion control and leak detection.

� 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

Increasing regulations on the release of gases or other

chemicals into the environment have led to the increased

attention on development of advanced sensors. There is

a strong interest in GaN and SiC-based gas sensors for

applications including fuel leak detection in automobiles

and aircraft, fire detectors, exhaust diagnosis and emis-

sions from industrial processes [1–14]. These materials

are capable of operating at much higher temperatures

than many of the conventional semiconductors such as

Si because of their large bandgap (3.4 eV for GaN, 3.26

eV for the 4H-SiC polytype vs. 1.1 eV for Si) [15]. Simple

Schottky diode or field-effect transistor structures fab-

ricated in SiC are sensitive to a number of gases, in-

cluding hydrogen and hydrocarbons [1,7]. The sensing

mechanism is thought to be creation of a polarized layer

on the semiconductor surface by hydrogen atoms dif-

* Corresponding author. Tel.: +1-352-392-4757; fax: +1-352-

392-9513.

E-mail address: ren@che.ufl.edu (F. Ren).

0038-1101/03/$ - see front matter � 2003 Elsevier Science Ltd. All ri

doi:10.1016/S0038-1101(02)00495-1

fusing through the metal contact [14]. One additional

attractive attribute of SiC is the fact that gas sensors

based on this material could be integrated with high-

temperature electronic devices on the same chip. For

similar reasons there has also been recent interest in the

development of GaN-based gas and liquid monitors [16–

20]. In this case there are additional effects due to the

presence of piezoelectric and piezoresistive properties.

The GaN devices could be integrated with UV sensors in

the same chip.

In this paper we report on characteristics of Pt/GaN

and Pt/4H-SiC Schottky diodes exposed to different

gases. The device structures are essentially identical to

those employed for power rectifiers [21] and emphasizes

how similar structures could be used for both gas sens-

ing and high power electronics applications in both

materials systems.

2. Experimental

The gas sensing experiments were performed in a

tube-furnace that contained electrical feedthroughs

ghts reserved.

Fig. 1. Schematic of test set-up.

1488 J. Kim et al. / Solid-State Electronics 47 (2003) 1487–1490

connected to either an HP4145 parameter analyzer or an

I–V measurement system (Fig. 1). Measurements were

performed at either 25 or 150 �C, with flowing gas

ambients of N2, 10% H2 in N2, air or CF4.

Approximately 6 lm of n-GaN was grown on sap-

phire substrates by metal organic chemical vapor de-

position. Ohmic contacts was formed by lift-off of Ti/Al/

Pt/Au, annealed at 500 �C. The Pt(150 �AA) Schottky

contacts were formed by lift-off.

For SiC, the starting substrates were nþ (n � 1019

cm�3) 4H-SiC. Approximately 10 lm of undoped (n �2� 1015 cm�3) was grown on these substrates by vapor

phase epitaxy. A full-area back contact of e-beam

evaporated Ni(2000 �AA) was annealed at 970 �C for 3 min

to produce a low resistance (1:5� 10�6 Xcm2) Ohmic

contact. A front-side rectifying contact of e-beam evap-

Fig. 2. Schematic of Pt/GaN and Pt/4H-SiC gas sensors.

orated Pt(150 �AA thick) was patterned by lift-off. The

contact diameter was generally fixed at 80 lm, althoughmuch larger devices were also fabricated (0:5� 0:5 cm2).

The devices were wire-bonded to a test fixture using Ti/

Au bond-pads and Au wires for contact. A schematic of

the final device structures is shown in Fig. 2.

3. Results and discussion

Fig. 3 shows forward I–V characteristics from Pt/

GaN diodes for three different gases (N2, 10% H2 in N2

or CF4) at either 25 �C (top) or 150 �C (bottom). There

is a shift of �0.2 V at 25 �C and �0.7 V at 150 �C in the

voltage needed to maintain a forward current of 5 mA.

These changes in forward characteristics are easily large

enough for the devices to be effective and sensitive gas

sensors, as reported previously [16,18].

For SiC operated at high forward currents, the ad-

dition of 10% H2 to the ambient produces a shift of 1.34

V at a forward current of 0.2 A. At lower voltage and

current, the signal change was also easily discernable,

e.g. 47 mA at a fixed forward bias of 1.5 V or equiva-

lently, 67 mV at a fixed current of 6 mA. The relative

changes in reverse current were much smaller, with

typical magnitudes of a few lA at )50 V bias.

The SiC devices were also able to differentiate be-

tween various gases, as shown in Fig. 4(top). The use of

either air or CF4 as the ambient produced significant

increases in forward current. Since the H2, O2 and F2 in

these gases can affect the dipole layer at the Pt–SiC in-

terface because of their reactivity, the electric field under

the Pt gate is altered and produces the resulting change

in diode forward current.

The diodes were operated up to 150 �C for extended

periods without deterioration of the Pt contact, al-

Fig. 4. Forward I–V characteristics from SiC sensors in 10%

H2 in N2, CF4 or air ambients at 25 �C (top) and comparison of

response to N2 or H2/N2 at different temperatures (bottom).

Fig. 3. Low forward bias I–V characteristics from GaN sensor

in 10% H2 in N2, pure N2 or CF4 ambients at 25 �C (top) or 150

�C (bottom).

J. Kim et al. / Solid-State Electronics 47 (2003) 1487–1490 1489

though this is a significant concern for higher tempera-

tures due to the possibility of PtGa or PtSi2 formation.

For very high operating temperatures, it is desirable to

use either a metal–oxide–semiconductor (MOS) ap-

proach [4] or else employ more thermally stable metal-

lization such as W or WSix. Fig. 4(bottom) shows the

forward I–V characteristics from Pt/SiC devices in N2 or

10% H2 in N2 ambients at both 25 and 150 �C. Thechange in forward current upon changing the gas be-

came larger at higher temperatures due to the increased

dissociation efficiency of the gas molecules. The disso-

ciation can occur through a catalytic reaction with the Pt

gate, or through additional surface reactions on the

semiconductor [4].

Fig. 5 shows the time response of GaN (top) or SiC

(bottom) diodes at 150 �C upon switching the gas in-

troduced into the enclosure from N2 to 10% H2 in N2.

For SiC, note that the change in voltage required to

maintain a forward current of 30 mA is very rapid (<1

s), with a saturation occurring �4 s after the switch of

the gases. The diffusion of hydrogen through the Pt layer

is not the limiting factor in the time response of the di-

odes, but rather the mass transport of gas into the en-

closure as we have observed by altering the introduction

rate. Similarly, the initial recovery of the characteristics

after introduction of the initial N2 ambient is most likely

dominated by removal of hydrogen atoms from the Pt/

GaN or Pt/SiC interfaces. These results demonstrate the

ability of both GaN and SiC diodes to perform as rapid,

sensitive gas sensors over a broad range of temperature.

In summary, both Pt/GaN and Pt/4H-SiC diode

rectifiers of the type used for high power electronic ap-

plications are also shown to be effective gas sensors for a

range of gases, including air, H2 and CF4. The time re-

sponse of the diodes is limited by the gas mass flow

transport characteristics, with the intrinsic response due

to changes in the interfacial OH-dipole layer being very

rapid. Future work will focus on development of stable

Schottky metallization capable of withstanding extended

Fig. 5. Time response of GaN (top) and SiC (bottom) sensors

upon changing from pure N2 to 10% H2 in N2 ambient at 150

�C.

1490 J. Kim et al. / Solid-State Electronics 47 (2003) 1487–1490

periods at temperatures > 400 �C, so that the sensors canbe used for applications such as spaceflights or moni-

toring of manufacturing processes.

Acknowledgements

The work at UF was partially supported by NSF

(CTS 991173) and NASA (NAG10-316) monitored by

Dr. William Knott.

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