M.M. Wong

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.

Selective area growth and characterization of AlGaN/GaN heterojunction bipolar transistors by metalorganic chemical vapor deposition

The selective area growth (SAG) and properties of AlGaN/GaN heterojunction bipolar transistors (HBTs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD) are described and analyzed. Transistors based on group III-nitride material are attractive for high-power and high-temperature applications. Much work has been focused on improving p-type material, as well as heterojunction interfaces. However, there have been very few reports on HBTs operating at room temperature, At this time, current gains for nitride-based HBTs have been limited to /spl sim/10. Selective area regrowth was applied to the growth of AlGaN/GaN HBTs to analyze its potential advantages as compared to more traditional growth techniques in order to realize improved electrical performance of the devices.

High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching

We have fabricated and investigated high-voltage GaN vertical Schottky-barrier rectifiers grown by metalorganic chemical vapor deposition. A mesageometry Schottky-barrier rectifier having a 5-μm-thick i region, and processed using reactive-ion etching, exhibited a reverse breakdown voltage of −450 V (at 10 mA/cm2) and an on-resistance of 23 mΩ cm2. For comparison, we have also applied wet chemical etching for the fabrication of mesageometry Schottky-barrier rectifiers. The 2-μm-thick i-region GaN mesa-Schottky rectifiers showed a breakdown voltage of −310 and −280 V for wet-etched and dry-etched devices, respectively, and an on-resistance of 8.2 and 6.4 mΩ cm2, respectively. These results indicate that the performance of the wet-etched rectifiers is comparable to or better than that of comparable dry-etched devices.

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.

Temperature dependent common emitter current gain and collector-emitter offset voltage study in AlGaN/GaN heterojunction bipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V/sub CEOFF/ at lower temperature is related to an increase of V/sub BE/ as the base bulk current is increased, or to the reduction of the ideality factor n/sub BE/.

Observation of long photoluminescence decay times for high-quality GaN grown by metalorganic chemical vapor deposition

GaN epitaxial layers with different crystalline quality grown on sapphire substrates by metalorganic chemical vapor deposition are investigated using time-resolved photoluminescence at 300 K. It is found that the time-dependent photoluminescence of low-quality GaN decays faster than that of the high-quality GaN films. The time constants for the dual-exponential decay of the photoluminescence are calculated to be 50 and 250 ps for high-quality undoped GaN and 30 ps for low-quality undoped GaN. For high-quality Si-doped GaN, time constants of 150 and 740 ps are extracted while corresponding time constants of 40 and 200 ps are measured for low-quality Si-doped GaN. We believe that the time constant of 740 ps measured for our high-quality Si-doped GaN is the longest ever reported for thin GaN/sapphire films.

Local conductivity and surface photovoltage variations due to magnesium segregation in p-type GaN

Conductive atomic force microscopy (C-AFM) and surface photovoltage (SPV) microscopy were used to investigate local electronic structure in p-type GaN. C-AFM imaging revealed locally reduced forward- and reverse-bias conductivity near threading dislocations. In addition, regions near threading dislocations demonstrated significantly enhanced surface photovoltage response when compared to regions away from dislocations. Analytical treatment of the surface photovoltage as a function of pertinent material properties indicated that reduced background dopant concentration is the most likely cause for the increased SPV. Both reduced conductivity and enhanced surface photovoltage are shown to be consistent with Mg segregation to dislocation cores that results in regions of locally decreased electrically active Mg concentration surrounding the dislocations.

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.

Optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures grown by metalorganic chemical vapor deposition

The optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures (SHs) grown by metalorganic chemical vapor deposition are investigated at low temperature using photoluminescence measurements. The formation of a two-dimensional electron gas at the heterojunction is verified by temperature-dependent Hall mobility and 300 K capacitance-voltage measurements. Radiative recombination is observed between the electrons in two-dimensional quantum states at the heterointerface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped AlGaN/GaN SHs. These peaks disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the photoluminescence results under different laser excitation intensity and lattice temperature are also described for undoped and modulation-doped AlGaN/GaN SHs with various Al compositions and growth interrupt times.