GaN-based Avalanche Photodiodes Research Articles

Fast neutron irradiation effects on Si- and GaN-based avalanche photodiodes

We report the behaviour of the dark current and gain characteristics of Si- and GaN-based avalanche photodiodes (APDs) irradiated by fast neutrons. For Si-based APDs, the dark current increases with the increase of neutron fluence, indicating that the avalanche property has been seriously affected. The gain values of Si-based APD slightly increase after irradiation by a low neutron fluence of 1.0 × 1012 cm−2, while the device exhibits gain degradation as the fluence increases to 1.0 × 1013 cm−2 or even above at high reverse bias voltage, unlike the previous studies where only degradation was observed. The steep increase of dark current and gain when approaching the breakdown voltage after neutron irradiation indicate that the avalanche property is almost unaffected for GaN-based APDs. Furthermore, we infer that the optical absorption between acceptor state and conduction band in the p-type layer plays an important role in influencing the change of dark current and gain at high reverse bias voltage. The findings not only enrich the understanding of neutron irradiation effect on Si- and GaN-based APDs, but also experimentally prove that GaN-based APDs hold better radiation resistance than Si-based devices.

On the Scope of GaN-Based Avalanche Photodiodes for Various Ultraviolet-Based Applications

We present a review of GaN avalanche photodiodes. GaN-based avalanche photodiodes are of emerging interest to the device community. The review covers various important aspects of the device such as the design space, substrate choice, edge termination efficacy, and last, but not least, the physics behind the avalanche breakdown in GaN. The study comprehends the reported impact ionization coefficients and how they may affect the device performances. Finally various reported GaN APDs are summarized and compared. We conclude that hole-initiated GaN APDs on free-standing GaN substrates can offer unprecedented advantages as ultraviolet light detectors, due to their ultra-high responsivity and low dark current.

Dependence of dark current and photoresponse characteristics on polarization charge density for GaN-based avalanche photodiodes

GaN/AlGaN avalanche photodiodes (APD) are fabricated based on GaN/AlGaN materials of epitaxial growth. Dark current and photoresponse characteristics are shown experimentally and theoretically. The effects of polarization charge density (PCD) on the dark current of GaN/AlGaN APDs are investigated in detail. It is demonstrated that an increased PCD reduces the electric field intensity in the GaN i-region, and results in an increase in breakdown voltage and a decrease in dark current above 65 V reverse bias. However, it is indicated that increasing the PCD elevates the electric field intensity at the GaN/AlGaN interface, and thus induces an enhancement of the band-to-band tunnelling and trap-assisted tunnelling processes, and eventually an evident rise in the dark current below 50 V reverse bias. Additionally, the effects of PCD on the optical responsivity are studied in detail. Results show that positive polarization charge has little effect on the photoresponse of the device. However, the negative polarization charge significantly affects the photoresponse. A metal–oxide–semiconductor inversion layer model has been proposed to explain the effect of negative polarization charge on photoresponse.

Effect of Wasted Space on Device Characteristics of Nitride-Based Avalanche Photodiodes

On the basis of our recently proposed theory [Jpn. J. Appl. Phys. 49 (2010) 114101], we report the effect of wasted space on the multiplication gain and excess noise characteristics of GaN-based avalanche photodiodes. By means of the Monte Carlo method, it is shown that with the adoption of the revised impact ionization coefficients, the multiplication gain exhibits an obvious discrepancy compared with conventional theory, and agrees well with that reported for InP. Moreover, the size-dependent impact ionization coefficients affect the noise properties under submicron scale, leading to a new feature different from InP and clarifying the reported experimental data.

Effect of Wasted Space on Device Characteristics of Nitride-Based Avalanche Photodiodes

On the basis of our recently proposed theory [Jpn. J. Appl. Phys. 49 (2010) 114101], we report the effect of wasted space on the multiplication gain and excess noise characteristics of GaN-based avalanche photodiodes. By means of the Monte Carlo method, it is shown that with the adoption of the revised impact ionization coefficients, the multiplication gain exhibits an obvious discrepancy compared with conventional theory, and agrees well with that reported for InP. Moreover, the size-dependent impact ionization coefficients affect the noise properties under submicron scale, leading to a new feature different from InP and clarifying the reported experimental data.

Effects of the intrinsic layer width on the band-to-band tunneling current in p-i-n GaN-based avalanche photodiodes

Dark current is critical for GaN-based avalanche photodiodes because it significantly increases the noise current and limits the multiplication factor. It has been found that the band-to-band tunneling current is the dominant origin of the dark current for avalanche photodiodes at the onset of breakdown voltage. Experimentally, for GaN-based avalanche photodiodes with a thinner intrinsic layer, the dark current increases nearly exponentially with the applied voltage even at a lower bias voltage. In this paper, the intrinsic layer (i-layer) width of GaN-based avalanche photodiodes has been varied to study its effect on the band-to-band tunneling current. A widely used equation was used to calculate the band-to-band tunneling current of avalanche photodiodes with different i-layer widths (i-layer 0.1 µm, 0.2 µm and 0.4 µm). At −40 V, the band-to-band tunneling current significantly reduces by a magnitude of 10−15 A with an increase in the i-layer width from 0.1 µm to 0.2 µm, and a magnitude of 10−29 A with an increase in the i-layer width from 0.2 µm to 0.4 µm. Then, GaN-based avalanche photodiodes (i-layer 0.1 µm, 0.2 µm and 0.4 µm) with different-sized mesa were fabricated. Also, the measurement of dark current of all three different structures was performed, and their multiplication factors were given.

GaN ultraviolet avalanche photodiodes with optical gain greater than 1000 grown on GaN substrates by metal-organic chemical vapor deposition

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼30μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼2.8MV∕cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼0.05V∕K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ∼360nm, devices with mesa diameters of ∼50μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

Solar-blind AlxGa1−xN-based avalanche photodiodes

We report the Metalorganic Chemical Vapor Deposition (MOCVD) growth, fabrication, and characterization of solar blind AlxGa1−xN∕GaN-based avalanche photodiodes. The photocurrent voltage characteristics indicate a reproducible avalanche gain higher than 25 at a 72 V applied reverse bias. Under a 25 V reverse bias voltage, the 100 μm diameter devices had a maximum quantum efficiency of 55% and a peak responsivity of 0.11A∕W at 254 nm, and a NEP of 1.89x10−16 W∕Hz1∕2.