Ariane L. Beck

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.

Recent advances in avalanche photodiodes

Until the early 2000s, the avalanche photodiode (APD) was widely deployed in high-performance optical receivers that operated up to 10 Gb/s. In subsequent years, the use of APDs for high-capacity systems declined as a result of their limited gain bandwidth, the transition to coherent detection, and the development of high-efficiency modulation techniques. Recently, the rapid growth of optical-fiber communications systems that utilize baud rates up to 25 Gb/s represented by a 100-Gb/s Ethernet has led to a resurgence of research on APDs and the development of low-noise APDs with enhanced gain bandwidth. This paper presents a brief review of APD fundamentals and describes some of the recent advances.

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.

Demonstration of ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes

We report ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes. An external quantum efficiency of 83% (187 mA/W) at 278 nm, corresponding to unity gain after reach-through was achieved. A gain higher than 1000 was demonstrated without edge breakdown.

Study of reverse dark current in 4H-SiC avalanche photodiodes

Temperature-dependent current-voltage (I-V) measurements have been used to determine the reverse dark current mechanisms in 4H-SiC avalanche photodiodes (APDs). A pn junction vertical mesa structure, passivated with SiO/sub 2/ grown by plasma enhanced chemical vapor deposition, exhibits predominate leakage current along the mesa sidewall. Similar APDs, passivated by thermal oxide, exhibit lower dark current before breakdown; however, when the temperature is higher than 146/spl deg/C, an anomalous dark current, which increases rapidly with temperature, is observed. This current component appears to be eliminated by the removal of the thermal oxide. Near breakdown, tunneling is the dominant dark current mechanism for these pn devices. APDs fabricated from a pp/sup -/n structure show reduced tunneling current. At room temperature, the dark current at 95% of breakdown voltage is 140 fA (1.8 nA/cm/sup 2/) for a 100-/spl mu/m diameter APD. At a gain of 1000, the dark current is 35 pA (0.44 /spl mu/A/cm/sup 2/).

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.

21-GHz-Bandwidth Germanium-on-Silicon Photodiode Using Thin SiGe Buffer Layers

Backside-illuminated germanium photodiodes fabricated on silicon substrate with two Si xGe1-x buffer layers are reported. At 1.3 mum, the responsivity was 0.62 A/W for reverse bias greater than 0.1 V. The 3-dB bandwidth was 21.5 GHz at 10-V reverse bias, achieving a bandwidth-efficiency product of 12.6 GHz

Performance of Low-Dark-Current 4H-SiC Avalanche Photodiodes With Thin Multiplication Layer

The authors report on the fabrication and performance of low-dark-current 4H-SiC avalanche photodiodes with a thin 180-nm-thick p - multiplication layer. At a photocurrent gain M of 1000, the dark current of a 100-mum-diameter device was 35 pA (0.44 muA/cm2). The peak unity-gain responsivity was 100 mA/W (external quantum efficiency=46%) at lambda=268 nm, and at high gain, a responsivity greater than 107 A/W was achieved. The excess noise factor corresponds to k=0.12. Time-domain pulse measurements indicate an RC-limited unity-gain bandwidth of 300 MHz

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.

Edge breakdown in 4H-SiC avalanche photodiodes

We report suppression of edge breakdown in mesa-structure SiC avalanche photodiodes (APDs) by employing a 10/spl deg/ sidewall bevel. These devices exhibit low dark currents, <10 pA for a 160-/spl mu/m-diameter device, at the onset of avalanche gain. Two-dimensional raster scans of both beveled and nonbeveled devices, fabricated from the same wafer, show the photocurrent as a function of position and illustrate the spatial properties of avalanche gain in SiC APDs.