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Advances in Materials Science and Engineering

Volume 2013 (2013), Article ID 531573, 4 pages

http://dx.doi.org/10.1155/2013/531573

## A Current Transport Mechanism on the Surface of Pd-SiO_{2} Mixture for Metal-Semiconductor-Metal GaAs Diodes

Department of Electrical Engineering, National Taiwan Ocean University, 2 Peining Road, Keelung 202, Taiwan

Received 30 October 2012; Accepted 15 May 2013

Academic Editor: Markku Leskela

Copyright © 2013 Shih-Wei Tan and Shih-Wen Lai. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents a current transport mechanism of Pd metal-semiconductor-metal (MSM) GaAs diodes with a Schottky contact material formed by intentionally mixing SiO_{2} into a Pd metal. The Schottky emission process, where the thermionic emission both over the metal-semiconductor barrier and over the insulator-semiconductor barrier is considered on the carrier transport of a mixed contact of Pd and SiO_{2} (MMO) MSM diodes, is analyzed. The image-force lowering is accounted for. In addition, with the applied voltage increased, the carrier recombination is thus considered. The simulation data are presented to explain the experimental results clearly.

#### 1. Introduction

Schottky contact of metal on semiconductor is essential to MESFETs, HEMTs, optical sensors, and gas sensors. Recently, hydrogen has been widely used in hydrogen-fueled vehicles, medical treatment, chemical industries, and semiconductor fabrication. However, hydrogen-containing gases are explosive. Therefore, developing of hydrogen sensors for real-time in situ detection is highly necessary. Previous studies have demonstrated numerous palladium and platinum-based hydrogen sensors [1–14]. Among these sensors, metal-semiconductor (MS) diodes have been addressed as one of the most promising devices [2–7]. Hydrogen sensors employing metal-oxide-semiconductor (MOS) diodes have also been extensively studied [8–10]. In addition, Chiu et al. [11–14] reported a new metal-semiconductor-metal (MSM) hydrogen sensor with two multifinger Schottky contacts. Unlike conventional MS and MOS diodes, a mixture of palladium and silicon dioxides (MMO) is deposited upon the semiconductor layer. Compared to commonly used MS and MOS sensors, the MMO MSM sensors achieve excellent performance of high sensitivity. However, the current-voltage (-) characteristics in this paper [14] represent the diode current of the sensor operated in N_{2}. The reason that causes the two-step - curve is interesting.

#### 2. Device Structure and Fabrication

The process started with mesa isolation. HCl was used to remove the native oxide on the 0.8 *μ*m n-GaAs layer with 8 × 10^{16} cm^{−3} doping concentration after a device mesa. Two multifinger Schottky electrodes forming a metal-semiconductor-metal (MSM) diode were implemented by thermally depositing a 30 nm mixture of Pd and SiO_{2} with a weight ratio of 3. The area of the multifinger electrode was 8 × 10^{−4} cm^{2}. Another MSM diode with a 30 nm Pd directly deposited upon the GaAs layer was also fabricated for comparison. Figure 1 shows the MSM diode with two multifinger Schottky contacts.

#### 3. Results Discussion

- characteristics of MSM diode with and without a mixture of Pd and SiO_{2} are shown in Figure 2. Because the quality of the epitaxial wafer and evaporative mixture is excellent and uniform, all curves are bidirectional and symmetrical. Unlike the lowest curve representing the current of Pd MSM diodes, the upper curve with the two-step - curve is the current of MMO MSM diodes. Obviously, the - curve is the same as the published paper [14]; therefore, to deposit a 30 nm mixture of Pd and SiO_{2} upon the GaAs layer is repeatable.

To consider the Schottky emission process and the image-force lowering, the current of Pd MSM diodes () can be expressed as [15, 16]
where , = 9.6 A/k-cm^{2}, 8 × 10^{−4} cm^{2}, = 300 K, = 1.6 × 10^{−19} C, = 1.38 × 10^{−23} J/K, = 0.83 eV, = 8 × 10^{16} cm^{−3}, = 10.8, = 12.9, = 0.05 V, and = 0 V to 5 V are the maximal electric field, the Richardson constant, contact area, absolute temperature, unit electronic charge, Boltzmann constant, barrier height, doping concentration, permittivity of GaAs near the Pd, permittivity of GaAs, Fermi potential from conduction-band edge, and applied voltage, respectively. Figure 2 shows the simulation of as a dot symbol. The results of simulation and experiment match each other. However, the content of the Pd and SiO_{2} in mixture is uniform, and the thermionic emission over the metal-semiconductor barrier and the insulator-semiconductor barrier is responsible for carrier transport. Therefore, the current of the MMO MSM diodes is discussed according to two components. The first is the designed in consideration of the thermionic emission over the metal-semiconductor barrier; the second is the designed in consideration of the thermionic emission over the insulator-semiconductor barrier. The inset of Figure 2 shows the schematic view of the mixture of Pd and SiO_{2} deposited upon the semiconductor layer. To discuss the thermionic emission over the metal-semiconductor barrier, substituting for from (1), can be obtained [15, 16] as follows:
where 2.90 × 10^{−4} cm^{2} is the effective Pd-contact area. = 0.81 eV is not the same as because MMO MSM diodes do not fabricate simultaneously with Pd MSM diodes. Other parameters are the same as . Particularly, is given by
where = 12.023 g/cm^{3} and = 2.648 g/cm^{3} are the density of Pd and the density of SiO_{2}, respectively. Figure 3(a) shows the - curve with . A curve of ln against represents a straight line from = 0 V to 0.58 V, meaning that is dominant from = 0 V to 0.12 V.

In the discussion of thermionic emission over the insulator-semiconductor barrier on , we obtain [15]
where = 30 nm, = 3.7, and 5.10 × 10^{−4} cm^{2} are the thickness of mixture, the permittivity of mixture, and the effective oxide-contact area. Other parameters are the same as . Particularly, is given by

Figure 3(b) shows the - curve with . A curve of ln is proportional to from = 1.2 V to 1.9 V, meaning that is dominant from = 1.4 V to 3.6 V. Furthermore, when a larger voltage is applied (>4 V), the bands bend even more downward so that the intrinsic level at the surface crosses over the Fermi level . Figure 4 shows the band diagram under a larger applied voltage. At this point, the number of holes (minority carriers) at the surface is larger than the number of the electrons (majority carrier), and thermionic emission of electrons is recombined by holes. The current () is proportional to . can be expressed as [15]
where = 4.81 × 10^{−16} A is the saturation current of recombination and = 8.1 is the ideality factor. Figure 5(a) shows the plot of ln against , representing a straight line when the applied voltage is greater than 4 V. Figure 5(b) shows the summation of , , and as a dot symbol. The results of simulation and experiment match each other.

#### 4. Conclusions

This study examined the current transport mechanism for Pd MSM GaAs diodes with a new Schottky contact material formed by intentionally mixing SiO_{2} into a Pd metal. A mechanism concept has successfully explained the influence of the mixed SiO_{2} on the current for MMO MSM diodes under the applied voltage. The effectively simulated data for the Schottky emission process, image-force lowering, and carrier recombination clearly explain the experimental results.

#### Acknowledgment

This work is financially supported by the National Taiwan Ocean University of the Republic of China under the contract nos. NTOU-100-014 and 101B290031.

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