Baxter Moody

Deep-Ultraviolet Light-Emitting Diodes Fabricated on AlN Substrates Prepared by Hydride Vapor Phase Epitaxy

AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were fabricated on AlN substrates. The AlN substrates were prepared by growing thick hydride vapor phase epitaxy (HVPE)-AlN layers on bulk AlN substrates prepared by physical vapor transport (PVT). After growing an LED structure, the PVT-AlN substrates were removed by mechanical polishing. This process allowed the fabrication of DUV-LEDs on HVPE-AlN substrates with high crystalline quality and DUV optical transparency. The DUV-LEDs exhibited a single emission peaking at 268 nm through the HVPE-AlN substrates. The output power as high as 28 mW was obtained at an injection current of 250 mA.

Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

On the origin of the 265 nm absorption band in AlN bulk crystals

Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.

Lasing and longitudinal cavity modes in photo-pumped deep ultraviolet AlGaN heterostructures

To unambiguously distinguish lasing from super luminescence, key elements of lasing such as longitudinal cavity modes with narrow line-width, polarized emission, and elliptically shaped far-field pattern, need to be demonstrated at the same time. Here, we show transverse electric polarized lasing at 280.8 nm and 263.9 nm for AlGaN based multi-quantum-wells and double heterojunction structures fabricated on single crystalline AlN substrates. An elliptically shaped far-field pattern was recorded when pumped above threshold. With cavities shorter than 200 μm, well-defined, equally spaced longitudinal modes with line widths as narrow as 0.014 nm were observed. The low threshold pumping density of 84 kW/cm2 suggests that the electrically pumped sub-300 nm ultraviolet laser diodes are imminent.

Performance and Reliability of Deep-Ultraviolet Light-Emitting Diodes Fabricated on AlN Substrates Prepared by Hydride Vapor Phase Epitaxy

The reliability and output power of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) fabricated on AlN substrates prepared by hydride vapor phase epitaxy are reported. TEM analysis revealed that dislocation density in LED layers, except the p-GaN layer, was below 106 cm-2. DUV-LEDs emitting at 261 nm exhibited an output power of 10.8 mW at 150 mA. The lifetime of these LEDs was estimated to be over 10,000 h for cw operation at 50 mA. No significant acceleration of output power decay at higher operation currents was observed. The estimated lifetime at the operation current of 150 mA was over 5,000 h.

Preparation of a Freestanding AlN Substrate from a Thick AlN Layer Grown by Hydride Vapor Phase Epitaxy on a Bulk AlN Substrate Prepared by Physical Vapor Transport

The structural and optical quality of a freestanding AlN substrate prepared from a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a bulk (0001)AlN substrate prepared by physical vapor transport (PVT) were investigated. The prepared HVPE-AlN substrate was crack- and stress-free. High-resolution X-ray diffraction ω-rocking curves of symmetric (0002) and skew-symmetric (1011) reflections had small full widths at half maximum (FWHMs) of 31 and 32 arcsec, respectively. Deep-ultraviolet optical transparency of the HVPE-AlN substrate was higher than that of the PVT-AlN substrate, which was related to lower concentrations of C, O impurities, and Al vacancy.

Development of green, yellow, and amber light emitting diodes using InGaN multiple quantum well structures

The authors present optical and electrical data for long wavelength (573–601nm) InGaN∕GaN multiple quantum well light emitting diodes (LEDs) grown by metal organic chemical vapor deposition. These results are achieved by optimizing the active layer growth temperature and the quantum well width. Also, the p-GaN is grown at low temperature to avoid the disintegration of the InGaN quantum wells with high InN content. A redshift is observed for both the green and yellow LEDs upon decreasing the injection current at low current regime. In the case of the yellow LED, this shift is enough to push emission into the amber (601nm).

Strain-induced piezoelectric field effects on light emission energy and intensity from AlInGaN/InGaN quantum wells

We report on the effects of the piezoelectric field and well width on the transition energy and intensity for InGaN quantum well structures with GaN or AlInGaN quaternary barriers. It was found that the emission energy of compressively strained GaN/In0.08Ga0.92N quantum wells exhibits a strong well width dependence not accounted for by quantum confinement subband energy shifting alone. However, for unstrained quantum well layers with quaternary barriers, no emission energy dependence on width was observed due to the elimination of the piezoelectric field, which was measured to be at least 0.6 MV/cm for the strained quantum wells. Furthermore, the unstrained quantum wells demonstrated a higher intensity than their strained counterparts for all quantum well widths investigated. The current data will help clarify the origin of emission in InGaN quantum wells.

Vacancy compensation and related donor-acceptor pair recombination in bulk AlN

A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.

Effects of tensile, compressive, and zero strain on localized states in AlInGaN/InGaN quantum-well structures

The recombination dynamics of optical transitions as well as strain effects in AlInGaN/In0.08Ga0.92N quantum wells (QWs) were studied. QW emission energy, photoluminescence decay behavior, photoluminescence emission line shape, and nonradiative recombination behavior were found to be strong functions of strain as well as localization. The degree of carrier localization was inferred by modeling several aspects of optical behavior obtained from variable temperature time-resolved photoluminescence experiments. According to the modeling results, the degree of localization was found to be a minimum for unstrained QWs and increased as either tensile or compressive strain increased, indicating that InGaN QW microstructure is a function of the lattice-mismatch-induced strain experienced during deposition.