# A Current Transport Mechanism on the Surface of Pd-SiO2 Mixture for Metal-Semiconductor-Metal GaAs Diodes

Authors: Shih-Wei Tan; Shih-Wen Lai
Journal: Advances in Materials Science and Engineering
 (2013)

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## 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 SiO2 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 SiO2 (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.


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## Body

## 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 (I-V) characteristics in this paper [14] represent the diode current of the sensor operated in N2. The reason that causes the two-step I-V curve is interesting.


## 2. Device Structure and Fabrication

The process started with mesa isolation. HCl was used to remove the native oxide on the 0.8μm n-GaAs layer with 8 × 1016 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 SiO2 with a weight ratio of 3. The area of the multifinger electrode was A≈ 8   × 10−4 cm2. 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.Figure 1
Schematic diagram of the MSM diodes with two multifinger Schottky contacts.


## 3. Results Discussion

I-V characteristics of MSM diode with and without a mixture of Pd and SiO2 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 I-V curve is the current of MMO MSM diodes. Obviously, the I-V curve is the same as the published paper [14]; therefore, to deposit a 30 nm mixture of Pd and SiO2 upon the GaAs layer is repeatable.Figure 2
Current-voltage characteristics for Pd-mixture-GaAs and Pd-GaAs MSM diodes. Inset diagram is the schematic view of mixture of Pd and SiO2 deposited upon the semiconductor layer.To consider the Schottky emission process and the image-force lowering, the current of Pd MSM diodes (IPd) can be expressed as [15, 16]
(1)IPd=A*·A·T2·e-q{ϕB-q/4πεSℰm}/kT,
where ℰm=(2qND/εS′)(Vn+ϕB+(kT/q)+V), A* = 9.6 A/k-cm2, A≈ 8 × 10−4 cm2, T = 300 K, q = 1.6 × 10−19 C, k = 1.38 × 10−23 J/K, ϕB = 0.83 eV, Nd = 8 × 1016 cm−3, εS′ = 10.8, εS = 12.9, Vn = 0.05 V, and V = 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 IPd as a dot symbol. The results of simulation and experiment match each other. However, the content of the Pd and SiO2 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 IMS designed in consideration of the thermionic emission over the metal-semiconductor barrier; the second is the IMIS 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 SiO2 deposited upon the semiconductor layer. To discuss the thermionic emission over the metal-semiconductor barrier, substituting for Em from (1), IMS can be obtained [15, 16] as follows:
(2)IMS=A*·APd·T2·eq{ϕB-[(q3Nd/8π2(εS′)2εS)(ϕB-Vn-(kT/q)-V)]0.25}/kT,
where APd≈ 2.90 × 10−4 cm2 is the effective Pd-contact area. ϕB = 0.81 eV is not the same as IPd because MMO MSM diodes do not fabricate simultaneously with Pd MSM diodes. Other parameters are the same as IPd. Particularly, APd is given by
(3)APd=(3/DPd)2/3(3/DPd)2/3+(1/Dox)2/3·A,
where DPd = 12.023 g/cm3 and Dox = 2.648 g/cm3 are the density of Pd and the density of SiO2, respectively. Figure 3(a) shows the I-V curve with V0.25. A curve of ln I against V0.25 represents a straight line from V0.25 = 0 V to 0.58 V, meaning that IMS is dominant from V = 0 V to 0.12 V.Current of Pd-mixture-GaAs MSM diodes as a function of (a)V0.25 with IMS and (b) V0.5 with IMIS for comparison.
(a)
(b)In the discussion of thermionic emission over the insulator-semiconductor barrier onIMIS, we obtain [15]
(4)IMIS=A*·Aox·T2·e-q{ϕB-qV/4πεid}/kT,
where d = 30 nm, εi = 3.7, and Aox≈ 5.10 × 10−4 cm2 are the thickness of mixture, the permittivity of mixture, and the effective oxide-contact area. Other parameters are the same as IMS. Particularly, APd is given by
(5)APd=(1/Dox)2/3(3/DPd)2/3+(1/Dox)2/3·A.Figure3(b) shows the I-V curve with V0.5. A curve of ln IMMO is proportional to V0.5 from V0.5 = 1.2 V to 1.9 V, meaning that IMIS is dominant from V = 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 Ei at the surface crosses over the Fermi level EF. 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 (IRB) is proportional to qV/ηkT. IRB can be expressed as [15]
(6)IRB=IRBS·eqV/ηkT,
where IRBS = 4.81 × 10−16 A is the saturation current of recombination and η = 8.1 is the ideality factor. Figure 5(a) shows the plot of ln IMMO against V, representing a straight line when the applied voltage is greater than 4 V. Figure 5(b) shows the summation of IMS, IMIS, and IRB as a dot symbol. The results of simulation and experiment match each other.Figure 4
Band diagram under a larger applied voltage.Current as a function of applied voltage for Pd-mixture-GaAs MSM diodes with (a)IRB and (b) simulation result for comparison.
(a)
(b)


## 4. Conclusions

This study examined the current transport mechanism for Pd MSM GaAs diodes with a new Schottky contact material formed by intentionally mixing SiO2 into a Pd metal. A mechanism concept has successfully explained the influence of the mixed SiO2 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.



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*Source: 531573-2013-06-16.xml*