James R. Grandusky

High Output Power from 260 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Improved Thermal Performance

This letter reports on the improved performance of a pseudomorphic ultraviolet light-emitting diode (LED). At 100 mA input current, 9.2 mW of quasi-CW output power was measured in a calibrated integrating sphere. The addition of a heat sink, required for CW and higher power operation, introduced a numerical aperture of 0.86, and 72 mW was measured in pulsed mode at 1.7 A, indicating that the total output power exceeds 100 mW when corrected by the coupling factor. The high characteristic temperature of 983 K was instrumental in achieving these record output powers for an LED with wavelength shorter than 265 nm.

270 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Over 60 mW Continuous Wave Output Power

In this letter, the achievement of over 60 mW output power from pseudomorphic ultraviolet light-emitting diodes in continuous wave operation is reported. Die thinning and encapsulation improved the photon extraction efficiency to over 15%. Improved thermal management and a high characteristic temperature resulted in a low thermal rolloff up to 300 mA injection current with an output power of 67 mW, an external quantum efficiency (EQE) of 4.9%, and a wall plug efficiency (WPE) of 2.5% for a single-chip device emitting at 271 nm in continuous wave operation.

Development of strain reduced GaN on Si "111… by substrate engineering

We report on a novel scheme of substrate engineering to obtain high-quality GaN layers on Si substrates. Ion implantation of an AlN∕Si substrate is performed to create a defective layer that partially isolates the III-nitride layer and the Si substrate and helps to reduce the strain in the film. Raman spectroscopy shows a substantial decrease in in-plane strain in GaN films grown on nitrogen implanted substrates. This is confirmed by the enhancement of the E2 (TO) phonon frequency from 564 to 567cm−1 corresponding to 84% stress reduction and substantial decrease in crack density for a 2-μm-thick GaN film. GaN films grown on implanted AlN∕Si substrate have better optical properties and smoother surface morphology as compared to nonimplanted AlN∕Si substrate.

Properties of Mid-Ultraviolet Light Emitting Diodes Fabricated from Pseudomorphic Layers on Bulk Aluminum Nitride Substrates

High quality bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1-xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid-ultraviolet light emitting diodes with peak wavelengths in the range of 240–260 nm. The low dislocation density of the pseudomorphic quantum wells resulted in improved performance over previously published data. The output powers of the on-wafer measurements were greater than 5 mW in continuous wave operation, and 16 mW in pulsed operation. This was achieved utilizing single die (with an active area of 1×10-3 cm2).

Mechanism of large area dislocation defect reduction in GaN layers on AlN∕Si (111) by substrate engineering

A reduction of edge dislocations in the GaN layer on Si substrate by almost an order of magnitude to 8.0×107∕cm2 and reduction in screw dislocations by a factor greater than 4 are achieved for the film grown on the Si (111) substrate engineered to have a polycrystalline defective layer at the AlN∕Si interface. The formation of a polycrystalline defective layer at the AlN∕Si interface by N+ ion implantation provides substrate conditions that result in a heteroepitaxial GaN film with much improved surface morphology and better crystal quality as compared to the film grown directly on AlN∕Si. A mechanism of dislocation defect reduction in the epitaxial film is given based on the detailed study of AlN∕Si interfaces as well as the evolution of the AlN buffer layer in the context of this substrate engineering technique, which shows partial decoupling of the III-nitride layers from the substrate to be responsible for the improved characteristics.

Strain dependent facet stabilization in selective-area heteroepitaxial growth of GaN nanostructures

We report on the selective-area heteroepitaxy and facet evolution of submicron GaN islands on GaN-sapphire, AlN-sapphire, and bare sapphire substrates. It is shown that strain due to the lattice mismatch between GaN and the underlying substrate has a significant influence on the final morphology and faceting of submicron islands. Under identical metalorganic chemical vapor deposition growth parameters, islands with low or no mismatch strain exhibit pyramidal morphologies, while highly strained islands evolve into prismatic shapes. Furthermore, islands grown with relatively low compressive mismatch strain yield more uniform arrays of pyramids as compared to the nonstrained, homoepitaxially grown crystals. It is proposed that the strain dependency of Ehrlich-Schwoebel barriers across different crystallographic planes could potentially account for the observed morphologies during selective area growth of GaN islands.

AlGaN based tunable hyperspectral detector

The application of III-nitrides to the development of a tunable hyperspectral detector is reported. The device consists of a triangular step barrier provided by a heterostructure of AlN∕AlxGa1−xN∕GaN. The structure is carefully designed to avoid relaxation of strained layers to prevent further introduction of defects and cracking due to the large tensile strain between different layers of the device. This structure is envisioned for tunable detection of ultraviolet through infrared wavelengths. The particular device structure reported here is expected to span detection energies from ∼1to2eV and from 3.4to5.4eV. The adjustable height of the triangular barrier with applied bias voltage provides tunability of the detected wavelength. The results from a first generation device are reported.

High-power pseudomorphic mid-ultraviolet light-emitting diodes with improved efficiency and lifetime

Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L50 lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.

Optimization of the active region of InGaN∕GaN 405 nm light emitting diodes using statistical design of experiments for determination of interaction effects

Output performance of InGaN based violet light emitting diode structures emitting at 405 nm was optimized using the statistical design of experiments (DOE) approach. Two separate DOEs were utilized to optimize the active region. The variables studied included the gallium flow rate, indium flow rate, temperature, well and barrier growth times, NH3 flow rate, and the silicon doping of the barrier while holding all other parameters and layers constant. Photoluminescence (PL) measurements were analyzed for wavelength, intensity, and full width at half maximum (FWHM) for each sample in both DOEs while electroluminescence measurements were completed for the samples from the second DOE and analyzed based on optical output power. Statistically valid transfer functions were obtained for each response based on the variables investigated. An overall improvement of 7% in the intensity with a reduction of 20% in the FWHM of the 405 nm PL band was obtained based on the starting point of the first DOE, while an improvem...

III-Nitride Heterostructure Layered Tunnel Barriers For a Tunable Hyperspectral Detector

We report on the fabrication and characterization of Ill-nitride layered tunnel barriers with applications for a new type of tunable hyperspectral imaging detector with intrinsically hyperspectral pixels. This would enable each pixel to be individually tunable in real-time through a range of wavelengths, with the number and width of spectral channels being dynamically adjustable. Shape-engineered electron barriers fabricated from III-nitride heterostructures allow barrier height to be varied by application of a voltage. A spectroscopy of photon wavelength is enabled via the collection of photoexcited electrons across this barrier. The device is envisioned for tunable detection of ultraviolet through infrared wavelengths.