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

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.

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 0.8 μm 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 (𝐼_Pd) can be expressed as [15, 16] 𝐼_Pd=𝐴^∗⋅𝐴⋅𝑇²⋅𝑒^{−𝑞{𝜙𝐵−𝑞/4𝜋𝜀𝑆ℰ𝑚}/𝑘𝑇}, (1) where ℰ_𝑚=(2𝑞𝑁_𝐷/𝜀^′_𝑆)(𝑉_𝑛+𝜙_𝐵+(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, 𝜙_𝐵 = 0.83 eV, 𝑁_𝑑 = 8 × 10¹⁶ cm⁻³, 𝜀^′_𝑆 = 10.8, 𝜀_𝑆 = 12.9, 𝑉_𝑛 = 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 𝐼_Pd 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 𝐼_MS designed in consideration of the thermionic emission over the metal-semiconductor barrier; the second is the 𝐼_MIS 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 𝐸_𝑚 from (1), 𝐼_MS can be obtained [15, 16] as follows: 𝐼_MS=𝐴^∗⋅𝐴_Pd⋅𝑇²⋅𝑒^{𝑞{𝜙𝐵−[(𝑞3𝑁𝑑/8𝜋2(𝜀′𝑆)2𝜀𝑆)(𝜙𝐵−𝑉𝑛−(𝑘𝑇/𝑞)−𝑉)]0.25}/𝑘𝑇}, (2) where 𝐴_Pd≈ 2.90 × 10⁻⁴ cm² is the effective Pd-contact area. 𝜙_𝐵 = 0.81 eV is not the same as 𝐼_Pd because MMO MSM diodes do not fabricate simultaneously with Pd MSM diodes. Other parameters are the same as 𝐼_Pd. Particularly, 𝐴_Pd is given by 𝐴_Pd=3/𝐷_Pd^2/33/𝐷_Pd^2/3+1/𝐷_ox^2/3⋅𝐴, (3) where 𝐷_Pd = 12.023 g/cm³ and 𝐷_ox = 2.648 g/cm³ are the density of Pd and the density of SiO₂, respectively. Figure 3(a) shows the 𝐼-𝑉 curve with 𝑉^0.25. A curve of ln 𝐼 against 𝑉^0.25 represents a straight line from 𝑉^0.25 = 0 V to 0.58 V, meaning that 𝐼_MS is dominant from 𝑉 = 0 V to 0.12 V.

(a)

(b)

Figure 3 

Current of Pd-mixture-GaAs MSM diodes as a function of (a) 𝑉0.25 with 𝐼MS and (b) 𝑉0.5 with 𝐼MIS for comparison.

In the discussion of thermionic emission over the insulator-semiconductor barrier on 𝐼_MIS, we obtain [15] 𝐼_MIS=𝐴^∗⋅𝐴_ox⋅𝑇²⋅𝑒^{−𝑞{𝜙𝐵−𝑞𝑉/4𝜋𝜀𝑖𝑑}/𝑘𝑇}, (4) where 𝑑 = 30 nm, 𝜀_𝑖 = 3.7, and 𝐴_ox≈ 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 𝐼_MS. Particularly, 𝐴_Pd is given by 𝐴_Pd=1/𝐷_ox^2/33/𝐷_Pd^2/3+1/𝐷_ox^2/3⋅𝐴. (5)

Figure 3(b) shows the 𝐼-𝑉 curve with 𝑉^0.5. A curve of ln 𝐼_MMO is proportional to 𝑉^0.5 from 𝑉^0.5 = 1.2 V to 1.9 V, meaning that 𝐼_MIS 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 𝐸_𝑖 at the surface crosses over the Fermi level 𝐸_𝐹. 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 (𝐼_RB) is proportional to 𝑞𝑉/𝜂𝑘𝑇. 𝐼_RB can be expressed as [15] 𝐼_RB=𝐼_RBS⋅𝑒^{𝑞𝑉/𝜂𝑘𝑇}, (6) where 𝐼_RBS = 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 𝐼_MMO against 𝑉, representing a straight line when the applied voltage is greater than 4 V. Figure 5(b) shows the summation of 𝐼_MS, 𝐼_MIS, and 𝐼_RB as a dot symbol. The results of simulation and experiment match each other.

Figure 4 

Band diagram under a larger applied voltage.

(a)

(b)

Figure 5 

Current as a function of applied voltage for Pd-mixture-GaAs MSM diodes with (a) 𝐼RB and (b) simulation result for comparison.

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.

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.