Comprehensive characterization of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN

We report on the material, electrical, and optical properties of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN, with active layers of 1.5 and 4.0 μm thickness. We have modeled current transport in the 1.5 μm devices using thermionic field emission theory, and in the 4.0 μm devices using thermionic emission theory. We have obtained a good fit to the experimental data. Upon repeated field stressing of the 1.5 μm devices, there is a degradation in the current–voltage $\mathrm{(I–V)}$ characteristics that is trap related. We hypothesize that traps in the GaN are related to a combination of surface defects (possibly threading dislocations), and deep-level bulk states that are within a tunneling distance of the interface. A simple qualitative model is presented based on experimental results. For devices fabricated on wafers with very low background free electron concentrations, there is a characteristic “punch-through” voltage, which we attribute to the interaction of the depletion region with the underlying low-temperature buffer layer. We also report GaN metal–semiconductor–metal photodetectors with high quantum efficiencies $\text{(∼50\%)}$ in the absence of internal gain. These photodetectors have a flat responsivity above the band gap (measured at $\text{∼0.15\hspace{0.17em}}\mathrm{A/W}$ ) with a sharp, visible-blind cutoff at the band edge. There is no discernible responsivity for photons below the band-gap energy. We also obtained record low dark current of $\text{∼800\hspace{0.17em}}\mathrm{fA}$ at $\text{−10\hspace{0.17em}}\mathrm{V}$ reverse bias. The dark current and ultraviolet photoresponse $\mathrm{I–V}$ curves are very flat out to $V_R\text{>−25\hspace{0.17em}}\mathrm{V},$ and do not show evidence of trap-related degradation, or punch-through effects.