C.J. Collins

GaN avalanche photodiodes

We report the electrical and optical characteristics of avalanche photodiodes fabricated in GaN grown by metalorganic chemical vapor deposition. The current–voltage characteristics indicate a multiplication of >25. Experiment indicates and simulation verifies that the magnitude of the electric field at the onset of avalanche gain is ⩾3 MV/cm. Small-area devices exhibit stable gain with no evidence of microplasmas.

Improved solar-blind detectivity using an AlxGa1−xN heterojunction p–i–n photodiode

We report the improved detectivity of AlxGa1−xN-based solar-blind p–i–n photodiodes with high zero-bias external quantum efficiency. The zero-bias external quantum efficiency was ∼42% at 269 nm, and increased to ∼46% at a reverse bias of −5 V. In addition, the photodiodes exhibited a low dark current density of 8.2×10−11 A/cm2 at a reverse bias of −5 V, which resulted in a large differential resistance. The high quantum efficiency and large differential resistance combine to yield a high detectivity of D*∼2.0×1014 cm Hz1/2 W−1. These results are attributed to the use of an Al0.6Ga0.4N window n region, which allows improved transmission to the absorption region, and to improved material quality.

Back illuminated AlGaN solar-blind photodetectors

We report the growth, fabrication, and characterization of AlxGa1−xN (0⩽x⩽0.60) heteroepitaxial back-illuminated solar-blind p-i-n photodiodes on (0001) sapphire substrates. The group III-nitride heteroepitaxial layers are grown by low-pressure metalorganic chemical vapor deposition on double polished sapphire substrates using various growth conditions. The back-illuminated devices exhibit very low dark current densities. Furthermore, they exhibit external quantum efficiencies up to 35% at the peak of the photoresponse (λ∼280 nm). Improvements were made to the growth technique in order to achieve crack-free Al0.4Ga0.6N active regions on a thick Al0.6Ga0.4N window layer and to obtain activated p-type Al0.4Ga0.6N layers.

Low dark current GaN avalanche photodiodes

We report the fabrication and characterization of GaN avalanche photodiodes grown on sapphire by metalorganic chemical vapor deposition. Current-voltage characteristics indicate a gain higher than 23. The photoresponse is independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of /spl sim/4 MV/cm. Near avalanche breakdown, the dark current of a 30-/spl mu/m diameter device is less than 100 pA. The breakdown shows a positive temperature coefficient of 0.03 V/K that is characteristic of avalanche breakdown.

Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN

A femtosecond time-resolved electroabsorption technique employing an AlGaN/GaN heterojunction p–i–n diode with a p-type AlGaN window layer and a semitransparent p contact has been used to measure the transient electron velocity overshoot in GaN. A peak transient electron velocity of 7.25×107 cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a negative differential resistivity region of the steady-state velocity-field characteristic in this high field range.

Selective regrowth of Al0.30Ga0.70N p–i–n photodiodes

We report on the device performance of selective-area regrown Al0.30Ga0.70N p–i–n photodiodes. Tensile strain, induced by the lattice mismatch between AlxGa1−xN and GaN, leads to cracking above the critical thickness in layers with high aluminum concentration. Selective-area regrown devices with ⩽70 μm diameters were fabricated without signs of cracking. These devices show low dark current densities with flat photoresponse and a forward turn-on current of ∼25 A/cm2 at 7 V. A quantum efficiency greater than 20% was achieved at zero bias with a peak wavelength of λ=315 nm. A differential resistance of R0=3.46×1014 Ω and a detectivity of D*=4.85×1013 cm Hz1/2 W−1 was demonstrated.

Improved ultraviolet quantum efficiency using a semitransparent recessed window AlGaN/GaN heterojunction p-i-n photodiode

We report on the improved quantum efficiency of GaN-based ultraviolet heterojunction photodiodes using a semitransparent recessed window device structure. At a reverse bias of −5 V the quantum efficiency was ∼57% at the band edge, and remained relatively flat down to ∼330 nm after which some absorption in the p-AlGaN layer became evident. The quantum efficiency only gradually declines after this point, remaining >20% at 280 nm. We attribute these results to avoidance of the optical dead space at the surface of GaN homojunction p-i-ns. The semitransparent p-AlGaN layer was comparatively resistive, causing an electric field crowding effect which resulted in a spatially nonuniform temporal behavior.

Direction-dependent band nonparabolicity effects on high-field transient electron transport in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p–i–n diode provide evidence of electron velocity overshoot at fields as low as ∼130 kV/cm for transport in the c-direction of wurtzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ∼300 kV/cm, this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass rather than intervalley transfer. Similar calculations of transport in the basal plane indicate that in this case, both a higher threshold field for velocity overshoot and a lower steady-state velocity at a given field are expected.

Improved device performance using a semi-transparent p-contact AlGaN/GaN heterojunction positive-intrinsic-negative photodiode

We report on the improved device performance of GaN-based ultraviolet heterojunction photodiodes using a semi-transparent p-contact device structure. At a reverse bias of 10 V, these photodiodes exhibit a low dark current density of 0.3 nA/cm2. The external quantum efficiency is 38% at the band edge, with only a slight decrease at the shorter wavelengths. The forward current is >10 mA at Vf=5 V. Fitting of the forward current–voltage data to the diode equation yields a very low series resistance (Rs=62Ω), which results in a very fast decay of the time response. The improved performance afforded by the thin, semi-transparent, p-contact layer is due to an increase in the uniformity of the lateral field distribution.

High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition

We report the growth, fabrication and characterization of high-quality AlGaN/GaN solar-blind p-i-n and MSM photodetectors by low-pressure metalorganic chemical vapor deposition (MOCVD). The epitaxial layers were grown on double-polished c-plane (0001) sapphire substrates to allow for back-side illumination. The p-i-n photodiode structures typically consist of a 0.7 μm thick Al 0.58 Ga 0.42 N window layer, graded to a 0.2 μm thick Al 0.47 Ga 0.53 N n layer, a 0.15 μm thick Al 0.39 Ga 0.61 N i layer, a 0.2 μm thick Al 0.47 Ga 0.53 N p layer, and capped with a 25 nm GaN:Mg contact layer. At a 0 V bias, the processed p-i-n devices exhibit a solar-blind photoresponse having a maximum responsivity of 0.058 A/W at 279 nm, corresponding to an external quantum efficiency of ∼26%, uncorrected for reflections, etc. The MSM devices typically consist of an AIGaN x ∼ 0.58 window layer, and an undoped AlGaN x ∼ 0.44 absorbing layer. The MSMs exhibit an external quantum efficiency as high as ∼47% at a bias of 15 V with a peak response at 262 nm.