Nearly all the air-water purification/polymer curing systems and bio-medical instruments require 250-300 nm wavelength ultraviolet light for which mercury lamps are primarily used. As a potential replacement for these hazardous mercury lamps, several global research teams are developing AlGaN based Deep Ultraviolet (DUV) light emitting diodes (LEDs) and DUV LED Lamps and Laser Diodes over Sapphire and AlN substrates. In this paper, we review the current research focus and the latest device results. In addition to the current results we also discuss a new quasipseudomorphic device design approach. This approach which is much easier to integrate in a commercial production setting was successfully used to demonstrate UVC devices on Sapphire substrates with performance levels equal to or better than the conventional relaxed device designs.
We are developing micro chemical sensor nodes that can be used for real time, remote detection and early warning of
chemical agent threats. The chemical sensors in our sensor nodes utilize GaN HEMTs (High Electron Mobility
Transistors) fabricated with catalytically active transition metal gate electrodes. The GaN HEMT chemical sensors
exhibit high sensitivity and selectivity toward chemical agent simulants such as DECNP (Diethyl cyano phosphonate),
and this is the first time that chemical agent simulants have been detected with GaN micro sensors. Response time of the
GaN HEMT sensor to a chemical species is within a second, and the maximum electronic response speed of the sensor is
~3 GHz. A prototype micro chemical sensor node has been constructed with the GaN sensor, a micro controller, and an
RF link. The RF sensor node is operated with a single 3V Li battery, dissipating 15 mW during the RF transmission with
5 dBm output power. The microcontroller allows the operation of the RF sensor nodes with a duty cycle down to 1 %,
extending lifetime of the RF sensor nodes over 47 days. Designed to transmit RF signals only at the exposures to
chemical agents and produce collective responses to a chemical agent via a sensorweb, the GaN micro chemical sensor
nodes seem to be promising for chemical agent beacons.
GaN / Al1-xGaxN-based hetero-structures have demonstrated a versatility in RF electronic applications which
is practically unmatched by any other material system. There are many device structures under consideration
for use in RF and Power amplifiers, suitable for both commercial and military applications.
In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC
substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN
epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth
was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled
showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating
SiC substrates.
Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM
measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance
mapping, optical reflectance, and high-resolution X-ray diffraction.
Several of the wafers were fabricated into HEMT devices, and the results on these devices are also
presented.
Deep UV LEDs emitting at on 280 nm with powers as high as 1 mW at 20 mA have been reported recently. These devices have mesa size of 100 μm x 100 μm to avoid current crowding due to the high Al-composition of the AlxGa1-xN buffer layers. Small mesa size results in pump current density of 200 A/cm2 for a device current of 20 mA. Small area of p-contact also leads to higher operating voltage and higher thermal impedance for the flip-chip packaged device. These factors limit the device lifetime for 50 % power reduction to only a few hundred hours. From temperature and bias dependent power degradation measurements we found the output power to decay with two characteristic time constants indicating two degradation mechanisms. The faster time constant is bias dependent and temperature independent. The slower time constant varies exponentially with junction temperature having the activation energy of 0.27 eV at 200 A/cm2 pump current density. For the devices with high thermal impedance this degradation mechanism controls the long term power degradation. To increase the device area for better reliability we used the interconnected micro-pixel device design with 10x10 array of 22 μm in diameter pixels. This design allowed for the four-fold increase of the junction area and thereby led to improved reliability performance with the operation life-time for 50 % power reduction of about 1000 hours. In this paper we will present the details of the reliability measurements and use the experimental results to determine possible degradation mechanisms.
In this paper we will present our recent work aimed at developing deep ultraviolet light-emitting diodes with emission from 250-280 nm. These devices were fabricated using AlGaN multiple quantum wells that were deposited on basal plane sapphire substrates using low-pressure MOCVD. Innovative MEMOCVD grown AlN buffer layers and AlN/AlGaN superalttices were also employed in the device structures to manage strain and allow the deposition of thick AlGaN layers, which was necessary to reduce the lateral spread resistance. Devices with square, multifinger and micro-LED geometries were fabricated and flip-chip mounted on AlN carriers for improved thermal management and light extraction. We have now succeeded in obtaining devices at 275 and 280 nm with cw powers in excess of 1.5 mW and pulsed powers well over 20 mW. Recently we have also succeeded in obtaining nearly milliwatt powers using an innovative micro-LED design at 250 nm. Now, for the first time, we also present dc operation of micro-pixel design deep UV LED.
High power deep ultraviolet (UV) light emitting diodes (LEDs) are good candidates for solid-state lighting, bio-chemical detection and short-range communications. In this paper we describe the progress from our and other research groups towards fabricating 340-280 nm LEDs. In past we have reported on deep UV LEDs on sapphire substrates with active region comprised of pulsed atomic layer epitaxy (PALE) deposited quaternary AlInGaN and ternary AlGaN multiple quantum wells (MQWs). These studies indicated a key role played by current crowding (thermal effects), active region design (polarization effects) and the base material quality (active region defects originating from buffer AlGaN layers) in controlling the emitted powers. Now using a unique AlGaN/AlN superlattice to control strain we have deposited Si-doped high Al-content n+-AlGaN layers over sapphire with thickness in excess of 2 μm. These layers and a new active layer design have yielded high power deep UV LEDs with emission wavelength from 280-340 nm. For 325 nm emission devices powers as high as 10 mW for 1 A pulsed pump current and 1 mW for 100 mA dc pump current were measured. For 280 nm emissions a power of 0.47 mW for 260 mA dc and 3 mW for 1 A of pulsed pump current was measured. For III-N deep UV LEDs, these values to date represent devices with the highest powers for the shortest wavelengths. In this paper we present the details of material and device fabrication and characterization. Initial data on device life-tests are also presented.
Generation-recombination (GR) noise in GaN and AlGaN thin films, GaN based Metal Semiconductor Field Effect Transistors (MESFETs), Heterostructure Field Effect Transistors (HFETs) and Schottky diode photodetectors was investigated. AlGaN thin films, AlGaN/GaN HFETs and Schottky barrier Al0.4Ga0.6N diodes exhibited GR noise with activation energies of 0.8 - 1 eV. AlGaN/GaN HFETs also presented GR noise with activation energies of 1 - 3 meV and 0.24 eV at cryogenic temperatures. No such noise was observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostructure Field Effect Transistors (DHFETs). We conclude that the local levels responsible for the observed noise in HFETs and DHFETs could be located in AlGaN barrier layers.
We review two complementary approaches to the development of white light solid-state sources. The first approach, which involves polychromatic LED modules, is targeted at advanced optimization of spectral power distribution in order to establish an optimal trade-off between luminous efficacy and color rendering. We apply a stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored primary LEDs. We present the results on optimized spectra for all-semiconductor lamps composed of four primary LEDs with the line widths typical of present AlGaInP and AlInGaN technologies. We point out the problem of the lack of efficient yellow-green (570 nm) emitters required for polychromatic lamps with four and more primary LEDs. The second approach is based on the development of AlInGaN-based UV emitters that can be tailored to directly excite different phosphors without sensitizers. AlInGaN materials system demonstrated potential for making UV LEDs with a high power and short wavelengths required for such applications. This has been achieved by using Strain Energy Band Engineering (SEBE) and Pulsed Atomic Epitaxy (PALE) techniques. SEBE relies on quaternary AlGaInN compounds for controlling strain and band offset and for producing UV emitters with improved device performance. PALE allows us to incorporate the required significant amount of indium (few percent) in AlGaN, since it can be performed at lower growth temperatures required for In incorporation. Further improvements in materials quality of AlInGaN layers with a high molar fraction of Al will be achieved by using bulk AlN substrates.
Wide energy gap and strong piezoelectric effects in A1GaN-based materials are very attractive for the development of visible-ultraviolet spectral range optoelectronic devices, such as optical waveguides and light modulators. In this paper, we report on the experimental studies ofthe acousto-optical diffraction in GaN-based layered structures grown by low-pressure MOCVD over sapphire substrates. We present the extracted values of the acoustooptic figures of merit and effective photoelastic constants for red (633 nm) and blue (442 nm) wavelengths. Our results demonstrate the potential of GaN-based structures for the development ofblue-ultraviolet acousto-optical devices.
Strain Energy Band Engineering of Group III-N heterostructures should allow us to prevent defect formation at the heterointerfaces ad to reduce the built-in electric field in the quantum wells. The strain, caused by lattice mismatch, may be decreased by incorporation of In into AlGaN. To monitor structural perfection of the quaternary compound AlInGaN and to evaluate electronic potential profile, we employed optical methods: reflectivity, site- selectively excited photoluminescence, photoluminescence excitation and time-resolved luminescence. AlGaN with the molar fraction of Al of 9% and two samples with the lattice mismatch reduced by partial substitution of Al by 1% and 2% of In were investigated. In AlGaN, the luminescence excited resonantly with the exciton position is red shifted. The photoluminescence excitation spectra indicate that the mobility edge is above the optical band gap, and the localization vanishes. These results show that the incorporation of approximately equals 2% indium into AlGaN leads to the disappearance of the band tail states and smoothing of the potential profile.
Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 angstroms/50 angstroms GaN/AlxGa1-xN (x approximately 0.07) and 45 angstroms/45 angstroms InxGa1-xN/GaN (x approximately 0.15) multiple quantum well structures. Microdisks, approximately 9 micrometers in diameter and regularly spaced every 50 micrometers , were formed by ion beam etch process. Individual disk was pumped from 10 K to 300 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical properties of these microdisks have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. From cw PL emission spectra, optical modes corresponding to (1) the radial mode type with a spacing of 49 - 51 meV (both TE and TM) and (2) the Whispering gallery mode with a spacing of 15 - 16 meV were observed in the GaN-based microdisk cavities. The spacings of these modes are consistent with theoretical calculation. The implications of our results to III-Nitride microdisk lasers are discussed.
Unique optical and electronic properties of the InGaN/GaN/AlGaN material system open up numerous opportunities for visible-blind optoelectronic devices. GaN based optoelectronic devices include InGaN-AlGaN light emitting diodes (LEDs), GaN photoconductive, Schottky barrier, and p-n junction ultraviolet detectors, and optoelectronic AlGaN-GaN heterostructure field effect transistors. These devices have a high sensitivity and a large gain-bandwidth product and can be integrated with GaN/AlGaN field effect transistors which have already demonstrated an operation at microwave frequencies. GaN and related materials (which include AlN, InN, and AlGaN and InGaN solid state solutions) span the range from visible to UV. Since these are direct gap materials, they are better suited for optoelectronic applications than SiC polytypes. In this paper, we review our recent results on GaN based optoelectronic devices, which were obtained using a spinel substrate. GaN films grown on sapphire are rotated by 30 degrees with respect to the sapphire substrate. This makes it practically impossible to cleave parallel surfaces needed for GaN-based lasers, which have been using vertical cavity surface emission laser (VCSEL) design. The spinel (MgAl2O4) cubic (111) substrates have a common cleave direction with the grown GaN epilayer. These substrates have a smaller lattice mismatch (approximately 9%) with GaN, and we recently demonstrated that GaN layers deposited over these substrates have a similar or better quality compared to GaN layers grown on sapphire.
We describe the deposition and characterization of single layers, homo-heterojunctions and superlattices of the AlxGa1-xN and InxGa1-xN material systems. Measurements are discussed indicating of 2D electron gas at heterojunction interfaces. Several photonic devices such as UV detectors, quarter wave reflector stacks, light emitting diodes and optically pumped lasing cavities are also described.
We describe optoelectronic effects in GaN/AlGaN Heterostructure Field Effect Transistors (HFETs) and Heterostructure Insulated Gate Field Effect Transistors (HIGFETs). GaN/AlGaN HFETs operate as visible blind photodetectors with responsivities as high as several thousand A/W for wavelengths from 200 to 365 nanometers. GaN/AlGaN HIGFETs exhibit light- sensitive long term current-voltage characteristic collapse after an application of a high drain- to-source bias. This collapse is removed by illumination with light with certain wavelengths. We suggest that this collapse is a consequence of hot electron trapping in the AlN barrier layer near the drain edge of the gate.
We have fabricated photoconductive and photovoltaic ultraviolet sensors from GaN single layers and pn-junctions. These sensors exhibit a sharp long wavelength cut-off in responsivity at the bandgap (365 nm). The active layers (GaN) were deposited using low pressure MOCVD. The p-type doping was accomplished using Mg as the dopant. Photoconductive, and schottky barrier detectors were then fabricated using photolithography, reactive ion etching and contact metallizations. These processing techniques were developed specific to the A1xGa1-xN material system. We will discuss growth, fabrication and characterization details for these various device types. The measured values of device parameters will be contrasted with those estimated from active layer material characterization.
With the emergence of fiber based links operating at 13 microns the need for high speed fiber compatible intensity modulators has increased significantly. We report on the development of a fiber pigtailed GaAs/AlGai. As based integrated optics Mach-Zehnder intensity modulator with speeds in excess of 3 GHz. 1.
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