C.K. Tan

A Chemical Sensor Based on AlGaN

There is a particular interest in the development of wide band gap semiconductor gas sensor because of their potential for high temperature operation and the ability to integrate them with power or microwave electrodes or with UV solar-blind detector and emitters fabricated in the same materials. AlGaN based devices are attractive for gas sensing in automotive exhausts and flow-gas, because of strong spontaneous polarization of AlGaN (free carrier concentration profiles inside this material that is very sensitive to any manipulation of surface change). In this report, we characterized the Ni/AlGaN/Sapphire Schottky barriers as hydrogen gas sensor at temperature range of 25°C to 500°C. A change in forward current was obtained in response to a change in ambient from pure N2 to 2% H2/ 98% N2, higher than the change in forward current obtained in Ni/GaN or Ni/Si Schottky diodes measured under the same conditions. The sensor response time was independent on the rate of mass transport of gas into the test chamber, while at high temperature, dissociation of gas controlled by the diffusion of atomic hydrogen through the metal/AlGaN surface, increased the sensor response time.

The Study of Thermal Treatment on Electrical Properties at Cr/p-GaN

The barrier height of as-deposited Cr contacts was found to be ΦB= 0.87eV with the ideality of η=1.51 and remained almost unchanged after further annealing at 500 oC for 5 minutes. The barrier height of diodes were increased drastically after annealed at 600 oC where ΦB=1.01eV with η=1.69. Upon annealing at 700 oC for 5 minutes, the ΦB decreased to 0.61eV and the Cr diodes were degraded for higher temperature in this annealing duration. The ΦB remained in 0.80- 0.90eV respectively at 800 oC for 2 minutes and 900-1000 oC for 1 minute.

Pinning Fermi Level of p-GaN due to Three Different (Zr, Ti, and Cr) Metal Contact

The Current-Voltage-Temperature (I-V-T) characteristics of single layer deposition, consisting of Zr, Ti, or Cr/p-GaN Schottky diodes were determined in the temperature range 27- 100oC. Sputtering method was used for deposition of these metals on p-GaN. Analysis of the measured characteristics at room temperature allows the determination of the electrical parameters, the saturation current Io and the ideality factorη. The barrier heights and effective Richardson coefficients were determined through activation energy plot. It was found that pinning of Fermi level occurred for these metal contacts on p-GaN with the carrier concentration of 5.6x 1017 cm-3, where the Schottky barrier heights of Zr, Ti, or Cr/p-GaN are determined to be in the same range (~0.87eV).

Multi layer metallization scheme (Ni/Pd/Ag) ohmic contact on p-type GaN

In this work, we report on the characteristics of the first ever work using a multilayer Ni/Pd/Ag contacts on p-GaN with the carrier concentration of 8 10/sup 17/ cm/sup -3/. The electrical behavior and thermal stability at different annealing temperatures (400 C-800 C) with annealing time of 5 minutes and 10 minutes were investigated. Specific contact resistivity using transmission line method (TLM) and current voltage (I-V) measurements were investigated. The minimum specific contact resistivity of 2.8 10/sup -1/ /spl Omega/-cm/sup 2/ was achieved after annealing at 400 C for 5 minutes. As-deposited samples exhibited near linear characteristics. Changes in the surface morphology of the contacts were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The agglomeration of the samples increased with the annealing temperatures.

Effect of Thermal Treatment for Pd and PdSi Schottky Contacts on p-GaN

Pd Schottky diode exhibited stable rectifying behavior up to 500°C for 35 minutes in sequential annealing; with the Schottky barrier heights (SBHs), ΦB (I-V) of 0.6-0.7eV with the leakage current (LC) of 20 A at -5V. With the same range of SBHs, PdSi diodes were stable up to 500°C for 5 minutes with the LC of 0.182mA at -5V. The electrical characteristics obtained in this study are also compared with those obtained for Pd and PdSi Schottky diodes on p-GaN.

Response Mechanism of Pd-GaN Schottky Barriers Comparative to Pd-Si Gas Sensors

In this paper we report on the characteristics of Pd/GaN and Pd/Si Schottky barriers exposed to different gases at various temperature range from 25°C to 500°C. The characteristics of Pd/GaN and Pd/Si Schottky barriers as gas sensors were measured as a function of temperature and ambient. Both types of sensor show changes in forward current upon introduction of different gases (N2, air, H2) into the ambient. The devices can be operated at large forward current, leading to large signal size for current at short response time for switching from one gas ambient to another such as N2 to H2 (2%) in N2. The signal size increases with the increase in measurement temperature due to more efficient cracking of the gas molecules. Both types of devices appear well suited to combustion control and leak detection.

A methane sensitive Ni/n-GaN Schottky barrier sensor

In this work, the forward current of Ni/GaN Schottky barrier is found to increase significantly upon Ar, 5%CH/sub 4//90%Ar, 10%CH/sub 4//90%Ar and 15%CH/sub 4//85%Ar ambient. Analysis of the current-voltage characteristics as a function of temperature showed strong correlations Schottky barrier height (SBH) which is related to the work function of Ni and electron affinity of GaN. The resistance of Ni/n-GaN sensor decreased with operating temperatures or concentration of methane were increased. The Ni/ n-GaN samples in Ar ambient exhibited a higher resistance than the CH4 within the temperature range used in this study.

High-temperature Pt Schottky barrier gas sensor on p-type GaN

In this report, Pt Schottky barriers to p-type GaN are described. The response of the sensors to argon (Ar) and 10 percent methane (CH/sub 4/) in Ar at temperatures from 50-500/spl deg/C and the thermal stability of the sensors have been investigated by long term annealing at 500/spl deg/C in nitrogen. When exposed to changes in ambient, changes in the surface potential lead to large changes in channel current. The results indicated that Pt sensors on p-type GaN for Ar and 10 percent CH/sub 4/ in Ar detection with this temperature range. Sensor response has been characterized by current-voltage measurements, revealing it ability to detect 10 percent CH/sub 4/ in Ar from 50-500/spl deg/C and Ar from 150-500/spl deg/C. This is roughly double the detection sensitivity of comparable GaN Schottky gas sensors tested under the same conditions, confirming that the Pt/p-type GaN based sensors has advantages for applications requiring the ability to detect combustion gases even at room temperature.

Investigations of surface roughness of GaN based gas sensor using atomic force microscope

In this paper, the properties of GaN based gas sensor with different thickness of platinum (Pt), prepared by sputtering method and annealed in air at 600/spl deg/C were investigated. AFM micrography showed that the GaN and Pt films are grain-like, and the grain size increases after annealing upon gas exposure. The interaction between the Pt clusters and the GaN grains was investigated. Sensors fabricated with the Pt films exhibited a significant increase in their sensitivity and selectivity towards H/sub 2/ detection when the thickness of Pt films is increased. Changes in the surface roughness for the different thickness of Pt/GaN due to specific adsorptions are presented. Possible applications and improvements of this technique are discussed.