Subash Krishnankutty

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

Back-illuminated GaN/AlGaN heterojunction photodiodes with high quantum efficiency and low noise

Back-illuminated GaN/AlGaN ultraviolet (UV) heterojunction photodiodes with high quantum efficiencies are demonstrated. Photovoltaic (zero bias) responsivity of 0.2 A/W at 355 nm was achieved. The improved efficiencies primarily arise from the use of AlGaN/GaN heterojunction in which photons are absorbed within the p-n junction thus eliminates carrier losses due to surface recombination and diffusion processes in previously reported homojunction devices. Very high dark impedance and large visible rejection ratio were obtained. These results indicate high quality GaN/AlGaN interface and efficient photocarrier collection in the photodiode.

Visible-Blind UV Digital Camera Based On a 32 × 32 Array of GaN/AlGaN p-i-n Photodiodes

A visible-blind UV camera based on a 32 × 32 array of backside-illuminated GaN/AlGaN p-i-n photodiodes has been successfully demonstrated. Each of the 1024 photodiodes in the array consists of a base n-type layer of AlGaN (~20%) onto which an undoped GaN layer followed by a p-type GaN layer is deposited by metallorganic vapor phase epitaxy. Double-side polished sapphire wafers are used as transparent substrates. Standard photolithographic, etching, and metallization procedures were employed to obtain fully-processed devices. The photodiode array was hybridized to a silicon readout integrated circuit using In bump bonds. Output from the UV camera was recorded at room temperature at a frame rate of 30 Hz. This new type of visible-blind digital camera is sensitive to radiation from 320 nm to 365 nm in the UV spectral region.

UV-Specific (320-365 nm) Digital Camera Based On a 128×128 Focal Plane Array of GaN/AlGaN p-i-n Photodiodes

An ultraviolet-specific (320-365 nm) digital camera based on a 128×128 array of backside-illuminated GaN/AlGaN p-i-n photodiodes has been successfully developed. The diode structure consists of a base n-type layer of AlGaN (~23% Al) followed by undoped and then p-type GaN layers deposited by metal organic vapor phase epitaxy. Double-side polished sapphire wafers serve as transparent substrates. Standard photolithographic, etching, and metallization procedures were employed to fabricate the devices. The fully-processed photodiode array was hybridized to a silicon readout integrated circuit (ROIC) using In bump bonds for electrical contact. The UV camera was operated at room temperature at frame rates ranging from 15 to 240 Hz. A variety of UV scenes were successfully recorded with this configuration.

A solar blind, hybrid III-nitride/silicon, ultraviolet avalanche photodiode

A novel, hybrid III-Nitride/Si, ultraviolet (UV) avalanche photodiode (APD) is proposed. The device combines the favorable short wavelength interband absorption properties of the direct bandgap III-Nitride material with the unique impact ionization characteristics of silicon. Solar blind response is achieved through optical isolation of the multiplication region of the device.

GaN/AlGaN UV photodiodes and phototransistors

Using MOCVD grown GaN and AlGaN alloys and heterostructures, we realized heterojunction UV photodiodes and phototransistors. We achieved photovoltaic internal quantum efficiency in excess of 90 percent and large UV-visible rejection ratio in the GaN/AlGaN PIN photodiodes. The results indicate high quality heterojunction interface and efficient carrier collection. We demonstrated an n-p-i-n GaN/AlGaN heterojunction bipolar phototransistor with gain in excess of 105, and 360 nm to 400 nm rejection ratio of 108. The phototransistor features a rapid electrical quenching of photoconductivity therefore can be operated without DC drift, an option not available in unipolar photoconductors. The electrical bandwidth of the phototransistor can be changed to accommodate particular applications by simply adjusting the repetition rate of the quenching pulses. The operation and evaluation of these devices are compared with alternative devices including SiC photodiode and GaN photoconductors.