In recent years, AlxGa1-xN semiconductor alloys, with a direct bandgap tunable between 3.4eV and 6.2eV, become the most suitable materials for the fabrication of UV detectors. In this paper we describe the fabrication and characteristics of an UV 64×1 focal plane array (FPA) based on front illuminated GaN p-i-n photodiodes. The diode structure consists of a base n-type layer of GaN followed by unintentionally doped and p-type layers deposited by metal organic chemical vapor deposition on GaN buffered sapphire substrate. Standard photolithographic, Ar+ ion beam etching, SiO2 passivation and metallization procedures were employed to fabricate the devices. I-V, responsivity and spectral response were tested. The linear photodiode array was indirectly hybridized to a silicon readout integrated circuit (ROIC) chip. The ROIC chip consists of capacitor feedback transimpedance amplifier (CTIA) input circuits, correlated double sampling (CDS) circuits, shift registers etc. The 64×1 UV linear FPA was packaged into a 28-pin chip carrier. The response ununiformity is 1.86%. The mean detectivity is about 2.0×109cmHz1/2W-1.
The contact resistivity of Ni/Au contact on p-type GaN was drastically decreased through the surface treatments in sequence using alcohol-based HCl and KOH solution. The surface oxide on p-type GaN formed during epitaxial growth was removed in the alcohol-based HCl and KOH solution, The O 1s and C 1s core-level peaks in the x-ray photoemission spectra showed that the alcohol-based HCl treatment was more effective in removing of the surface oxide layer. Compared to the KOH solution treated sample, the alcohol-based HCl-treated sample showed a Ga 2p core-level peak which was shifted toward the valence-band edge by 0.3 eV, indicating that the surface Fermi level was shifted toward the valence-band edge. These results suggest that the surface barrier height for hole injection from Ni/Au metal to p-type GaN be lowered by the surface treatment, which results in a drastic reduction in specific contact resistance.