P. Kung

Electroluminescence at 375nm from a ZnO∕GaN:Mg∕c-Al2O3 heterojunction light emitting diode

n-ZnO∕p-GaN:Mg heterojunction light emitting diode (LED) mesas were fabricated on c-Al2O3 substrates using pulsed laser deposition for the ZnO and metal organic chemical vapor deposition for the GaN:Mg. High crystal quality and good surface morphology were confirmed by x-ray diffraction and scanning electron microscopy. Room temperature (RT) photoluminescence (PL) showed an intense main peak at 375nm and a negligibly low green emission indicative of a near band edge excitonic emission from a ZnO layer with low dislocation/defect density. The LEDs showed I-V characteristics confirming a rectifying diode behavior and a RT electroluminescence (EL) peaked at about 375nm. A good correlation between the wavelength maxima for the EL and PL suggests that recombination occurs in the ZnO layer and that it may be excitonic in origin. This also indicates that there is significant hole injection from the GaN:Mg into the ZnO.

AlGaN ultraviolet photoconductors grown on sapphire

AlxGa1−xN (0≤x≤0.50) ultraviolet photoconductors with a minimum cutoff wavelength shorter than 260 nm have been fabricated and characterized. The AlGaN active layers were grown on (00⋅1) sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The spectral responsivity of the GaN detector at 360 nm is about 1 A/W biased at 8 V at room temperature. The carrier lifetime derived from the voltage‐dependent responsivity is 0.13–0.36 ms.

High quality AIN and GaN epilayers grown on (00⋅1) sapphire, (100), and (111) silicon substrates

The growth of high quality AlN and GaN thin films on basal plane sapphire, (100), and (111) silicon substrates is reported using low pressure metalorganic chemical vapor deposition. X‐ray rocking curve linewidths of about 100 and 30 arcsec were obtained for AlN and GaN on sapphire, respectively. Room‐temperature optical transmission and photoluminescence (of GaN) measurements confirmed the high quality of the films. The luminescence at 300 and 77 K of the GaN films grown on basal plane sapphire, (100), and (111) silicon was compared.

High-power 280 nm AlGaN light-emitting diodes based on an asymmetric single-quantum well

We demonstrate high-power AlGaN-based ultraviolet light-emitting diodes grown on sapphire with an emission wavelength of 280 nm using an asymmetric single-quantum-well active layer configuration on top of a high-quality AlGaN/AlN template layer. An output power of 1.8 mW at a pulsed current of 400 mA was achieved for a single 300 μm×300 μm diode. This device reached a high peak external quantum efficiency of 0.24% at 40 mA. An array of four diodes produced 6.5 mW at 880 mA of pulsed current.

High-speed, low-noise metal–semiconductor–metal ultraviolet photodetectors based on GaN

We present the fabrication and characterization of nonintentionally doped GaN and GaN:Mg Schottky metal–semiconductor–metal (MSM) photodetectors, grown on sapphire by metalorganic chemical vapor deposition. Low-leakage, Schottky contacts were made with Pt/Au. The devices are visible blind, with an ultraviolet/green contrast of about five orders of magnitude. The response times of the MSM devices were <10 ns and about 200 ns for GaN and GaN:Mg, respectively. The noise power spectral density remains below the background level of the system (10−24 A2/Hz) up to 5 V, for the undoped GaN MSM detector.

Solar-blind AlGaN photodiodes with very low cutoff wavelength

We report the fabrication and characterization of AlxGa1−xN photodiodes (x∼0.70) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The peak responsivity for −5 V bias is 0.11 A/W at 232 nm, corresponding to an internal quantum efficiency greater than 90%. The device response drops four orders of magnitude by 275 nm and remains at low response for the entire near-ultraviolet and visible spectrum. Improvements were made to the device design including a semitransparent Ni/Au contact layer and a GaN:Mg cap layer, which dramatically increased device response by enhancing the carrier collection efficiency.

HIGH-QUALITY VISIBLE-BLIND ALGAN P-I-N PHOTODIODES

We report the fabrication and characterization of AlxGa1−xN p-i-n photodiodes (0⩽x⩽0.15) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible rejection of six orders of magnitude with a cutoff wavelength that shifts from 365 to 338 nm. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances, the transient response becomes non-exponential, with a decay time longer than the RC constant. This behavior is justified by the strong frequency dependence of the device capacitance. By an admittance analysis, we conclude that speed is not limited by deep levels, but by substitutional Mg capture and emission time.

4.5 mW operation of AlGaN-based 267 nm deep-ultraviolet light-emitting diodes

We demonstrate 4.5 mW output power from AlGaN-based single quantum well ultraviolet light-emitting diodes at a very short wavelength of 267 nm in pulsed operation mode. The output power in continuous-wave mode reaches a value of 165 μW at an injected current of 435 mA. The measurements were done on arrays of four devices flip chip bonded to AlN submounts for thermal management.

Visible blind GaN p-i-n photodiodes

We present the growth and characterization of GaN p-i-n photodiodes with a very high degree of visible blindness. The thin films were grown by low-pressure metalorganic chemical vapor deposition. The room-temperature spectral response shows a high responsivity of 0.15 A/W up until 365 nm, above which the response decreases by six orders of magnitude. Current/voltage measurements supply us with a zero bias resistance of 1011 Ω. Lastly, the temporal response shows a rise and fall time of 2.5 μs measured at zero bias. This response time is limited by the measurement circuit.

AlxGa1−xN (0⩽x⩽1) ultraviolet photodetectors grown on sapphire by metal-organic chemical-vapor deposition

AlxGa1−xN (0⩽x⩽1) ultraviolet photoconductors with cutoff wavelengths from 365 to 200 nm have been fabricated and characterized. The maximum detectivity reached 5.5×108 cmHz1/2/W at a modulating frequency of 14 Hz. The effective majority carrier lifetime in AlxGa1−xN materials, derived from frequency-dependent photoconductivity measurements, has been estimated to be from 6 to 35 ms. The frequency-dependent noise spectrum shows that it is dominated by Johnson noise at high frequencies for low-Al-composition samples.