A Current Transport Mechanism on the Surface of Pd-SiO 2 Mixture for Metal-Semiconductor-Metal GaAs Diodes

This paper presents a current transport mechanism of Pdmetal-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 themetal-semiconductor barrier andover the insulator-semiconductor barrier is considered on the carrier transport of amixed 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.


Device Structure and Fabrication
The process started with mesa isolation.HCl was used to remove the native oxide on the 0.8 m n-GaAs layer with 8 × 10 16 cm −3 doping concentration after a device mesa.Two multifinger Schottky electrodes forming a metalsemiconductor-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.

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 imageforce lowering, the current of Pd MSM diodes ( Pd ) can be expressed as [15,16] where .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  Pd 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 metalsemiconductor 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  MS designed in consideration of the thermionic emission over the metal-semiconductor barrier; the second is the  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 SiO 2 deposited upon the semiconductor layer.To discuss the thermionic emission over the metalsemiconductor barrier, substituting for   from (1),  MS can be obtained [15,16] as follows: where  Pd ≈ 2.90 × 10 −4 cm 2 is the effective Pd-contact area.  = 0.81 eV is not the same as  Pd because MMO MSM diodes do not fabricate simultaneously with Pd MSM diodes.Other parameters are the same as  Pd .Particularly,  Pd is given by where  Pd = 12.023 g/cm 3 and  ox = 2.648 g/cm 3 are the density of Pd and the density of SiO 2 , respectively.Figure 3(a) shows the - curve with  0.25 .A curve of ln  against  0.25 represents a straight line from  0.25 = 0 V to 0.58 V, meaning that  MS is dominant from  = 0 V to 0.12 V.
In the discussion of thermionic emission over the insulator-semiconductor barrier on  MIS , we obtain [15] where  = 30 nm,   = 3.7, and  ox ≈ 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  MS .Particularly,  Pd is given by Figure 3(b) shows the - curve with  0.5 .A curve of ln  MMO is proportional to  0.5 from  0.5 = 1.2 V to 1.9 V, meaning that  MIS 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 ( RB ) is proportional to /. RB can be expressed as [15] where  RBS = 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  MMO against , representing a straight line when the applied voltage is greater than 4 V.

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.

Figure 1 :Figure 2 :
Figure 1: Schematic diagram of the MSM diodes with two multifinger Schottky contacts.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Current of Pd-mixture-GaAs MSM diodes as a function of (a)  0.25 with  MS and (b)  0.5 with  MIS for comparison.
Figure 5(b)   shows the summation of  MS ,  MIS , and  RB as a dot symbol.The results of simulation and experiment match each other.