High Quality Non-Alloyed Pt Ohmic Contacts to p -Type GaN Using Two-Step Surface Treatment
- PDF / 74,296 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 72 Downloads / 197 Views
nt leads to a decrease in the specific contact resistance up to 7.1×10–3 Ωcm2. Kim et al.[10] used HNO3:HCl (1:3) to modify surface conditions and showed that for Pd/Au ohmic contacts to p-GaN, the surface modification results in a specific contact resistance of 4.1×10–4 Ωcm2. They attributed the low resistance to the removal of a native oxide layer that inhibits hole transport from the metal to p-GaN. In this paper, we report on the formation of low resistance Pt contacts to p-GaN by two-step surface treatment technique using buffered oxide etch (BOE) and ammonium sulfide [(NH4)2Sx]. It is shown that specific contact resistances and Schottky barrier heights depend sensitively on the surface-treated conditions. In addition, the electronic transport mechanisms for the surface-treated Pt contacts are described and discussed. EXPERIMENTAL PROCEDURE Metalorganic chemical vapor deposition (Emcore DGaN125TM) was used to grow 1-µm-thick p-GaN:Mg (na = 1.5-3×1017 cm–3) on (0001) sapphire substrates. The GaN layer was ultrasonically degreased in trichloroethylene, acetone, methanol, and ethanol, and rinsed in deionised (DI) water for 5 min. Prior to the fabrication of TLM patterns, mesa structures were patterned by inductively coupled plasma etching (Oxford Plasma 100) using Cl2/Ar/H2. The first-step surface treatment was performed after the mesa etching process. The mesa-patterned layers were chemically treated by three different conditions: (i) not-treated (termed here ‘A-treated’); (ii) ultrasonically boiled in BOE solution for 10 min (‘B’); (iii) first ultrasonically boiled in BOE for 10 min and then boiled in (NH4)2Sx for 10 min (‘C’). After the first-step treatment, TLM patterns were defined by photolithographic technique. The size of the pads was 100×200 µm2 and the spacing between the pads was 5, 10, 15, 20, 25, and 35 µm. After the TLM patterning, the second-step treatment was performed. All the TLM-patterned layers were dipped into BOE for 30 s. Metallisation patterns were defined using lift-off technique. The samples were then rinsed in DI water, blown dry by N2, and immediately loaded into an electron beam evaporation chamber (PLS 500). The thickness of the Pt films was 25 nm. Currentvoltage (I-V) data were measured at room temperature using a parameter analyzer (HP 4155A) and Schottky barrier heights (SBHs, φb) were calculated using the I-V method. Electronic transport mechanisms were investigated by I-V-T data. X-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to investigate the variously treated surfaces of p-GaN. RESULTS AND DISCUSSION Figure 1 shows the I-V characteristics of Pt contacts on the various surfacetreated p-GaN. The A-treated Pt contact reveals nonlinear I-V behaviour. However, the Band C-treated contacts show near linear and linear characteristics, respectively. Specific contact resistances (Rsc) were determined from a plot of the measured resistances versus the spacings between the TLM pads. The least square method was used to fit a straight
F99W10.4
line
Data Loading...
