S. F. LeBoeuf

Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes

We have studied the electrical characteristics and optical properties of GaN/InGaN multiple quantum well (MQW) light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition. It appears that there is an essential link between material quality and the mechanism of current transport through the wide-bandgap p-n junction. Tunneling behavior dominates throughout all injection regimes in a device with a high density of defects in the space-charge region, which act as deep-level carrier traps. However, in a high-quality LED diode, temperature-dependent diffusion-recombination current has been identified with an ideality factor of 1.6 at moderate biases. Light output has been found to follow a power law, i.e., L /spl prop/ I/sup m/ in both devices. In the high-quality LED, nonradiative recombination centers are saturated at current densities as low as 1.4 /spl times/ 10/sup -2/ A/cm/sup 2/. This low saturation level indicates that the defects in GaN, especially the high density of edge dislocations, are generally optically inactive.

Defect generation in InGaN/GaN light-emitting diodes under forward and reverse electrical stresses

Abstract Electrical and optical degradations of GaN/InGaN single-quantum-well light-emitting diodes (LEDs) under high-injection current (150 A/cm2) and reverse-bias (−20 V) stresses were investigated. A substantial increase in the tunneling components of both forward and reverse currents was observed in the devices subjected to reverse biases. However, the stressed LEDs exhibited minimal degradation of optical characteristics. For devices subjected to high forward currents, a monotonic decrease in light intensities with stress time, accompanied by an increase of forward leakage current, was observed in the low-injection region, but a positive stress effect was found on the light output measured at high currents. These degradation behaviors can be explained by slow generation of point defects in the LEDs via different mechanisms, i.e., thermally induced defect formation in the InGaN active region in the devices subjected to high-injection currents, and destructive microstructual changes as a result of impact ionization in the cladding layer in the devices under high reverse-bias stress.

Temperature-dependent emission intensity and energy shift in InGaN/GaN multiple-quantum-well light-emitting diodes

Temperature-dependent electroluminescence(EL) of InGaN/GaN multiple-quantum-welllight-emitting diodes(LEDs) has been investigated to illustrate the role of localization effects in carrier capture and recombination. The devices have identical structure but with varying indium content in the active region. A large redshift of the emission peak with decreasing temperature is observed in the UV and blue LEDs over the temperature range of 77–200 K, accompanying a pronounced decrease of EL intensity. This redshift reflects carrier relaxation into lower energy localized states and the change in carrier recombination dynamics at low temperatures. In contrast, the peak energy of the green LEDs exhibits a smaller temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. Based on a rate equation analysis, we find that the densities of the localized states in the green LEDs are more than two orders of magnitude higher than that in the UVLED.

Blue and near-ultraviolet light-emitting diodes on free-standing GaN substrates

Blue and near-ultraviolet (UV) InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with peak emission at 465 nm and 405 nm, respectively, were grown on GaN and sapphire substrates. The densities of surface and bulk defects in the homoepitaxially grown LEDs were substantially reduced, leading to a decrease in reverse currents by more than six orders of magnitude. At a typical operating current of 20 mA, the internal quantum efficiency of the UV LED on GaN was twice as high compared to the UV LED on sapphire, whereas the performance of the blue LEDs was found to be comparable. This suggests that the high-density dislocations are of greater influence on the light emission of the UV LEDs due to less In-related localization effects. At high injection currents, both the blue and UV LEDs on GaN exhibited much higher output power than the LEDs on sapphire as a result of improved heat dissipation and current spreading.

Growth and characterization of GaN PiN rectifiers on free-standing GaN

GaN PiN rectifiers with high structural quality were grown on free-standing GaN substrates using metalorganic chemical vapor deposition. The lattice mismatch between the substrate and the epitaxial GaN was found to be ∼1×10−4. The full width at half maximum of the (0002) rocking curve was 79arcs compared to 230arcs for similar materials grown on sapphire. The incorporation of C, H, and O impurities in the homoepitaxial drift layer was reduced by a factor of 2–4. The rectifiers on GaN demonstrated rectification to −265V, which represents a 1.6× improvement over the rectifiers on sapphire and corresponds to a critical electric field ∼2.7MV∕cm. The homoepitaxial rectifiers also showed two orders of magnitude lower reverse leakage and a smaller negative temperature coefficient for breakdown voltage, consistent with a reduced defect density in the drift region.GaN PiN rectifiers with high structural quality were grown on free-standing GaN substrates using metalorganic chemical vapor deposition. The lattice mismatch between the substrate and the epitaxial GaN was found to be ∼1×10−4. The full width at half maximum of the (0002) rocking curve was 79arcs compared to 230arcs for similar materials grown on sapphire. The incorporation of C, H, and O impurities in the homoepitaxial drift layer was reduced by a factor of 2–4. The rectifiers on GaN demonstrated rectification to −265V, which represents a 1.6× improvement over the rectifiers on sapphire and corresponds to a critical electric field ∼2.7MV∕cm. The homoepitaxial rectifiers also showed two orders of magnitude lower reverse leakage and a smaller negative temperature coefficient for breakdown voltage, consistent with a reduced defect density in the drift region.

Temperature-dependent electroluminescence of AlGaN-based UV LEDs

The electrical and optical characteristics of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) (265-365 nm) at elevated temperatures (25/spl deg/C-175/spl deg/C) were investigated, and compared to those of InGaN-based visible LEDs (400-465 nm). Strong carrier localization and localized-state emission were retained in the InGaN LEDs up to 175/spl deg/C, leading to temperature-independent emission intensity at low-energy tails. The deep-UV LEDs, however, showed dominant band-edge emission, much smaller alloy broadening, and weaker localization effects. The optical power of the UV LEDs decreased much more rapidly with increasing temperature. The characteristic temperature was in the range of 31-73 K, and decreased with increasing Al content in the active region. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the UV LEDs and suggest that increasing carrier-confining potentials will provide a critical means to improve their radiative efficiencies.

Templated wide band-gap nanostructures

In this two-pronged work we report (a) a study of defect nucleation in three-dimensional confined nanoislands and (b) a surface-elasticity induced size effect in the optoelectronic properties of embedded and templated semiconducting nanostructures. Several key features in the design of nanostructure templates are analyzed and dislocation free contour maps are presented for combination of various lattice mismatches, substrates, and geometrical dimensions. Unlike the case for thin epitaxial films, it is found that for nanostructures, below a certain critical lateral dimension, dislocation free structures of any thickness can be grown. With regards to the optoelectronic properties of nanostructures, while size dependency due to quantum confinement and electrostatic interactions are well known, we show that an additional size-dependent strain is caused by the distinct elastic behavior of surfaces and interfaces at the nanoscopic scale compared to the macroscopic scale. This is in contrast to the usual way str...

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

Abstract The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.

Effects of plasma treatment on the Ohmic characteristics of Ti∕Al∕Ti∕Au contacts to n-AlGaN

The effects of surface treatment using Cl2∕BCl3 and Ar inductive coupled plasmas on the Ohmic characteristics of Ti∕Al∕Ti∕Au contacts to n-type AlxGa1−xN (x=0–0.5) were investigated. Plasma treatment significantly increased the surface conductivity of GaN and Al0.1Ga0.9N, leading to improved Ohmic behaviors of the contacts. However, it reduced the surface doping level in AlxGa1−xN (x⩾0.3) and degraded the contact properties. Following a 900–1000°C anneal, the Ti∕Al∕Ti∕Au contacts to AlxGa1−xN (x=0–0.3) became truly Ohmic, with specific contact resistances of (3–7)×10−5Ωcm2, whereas the contact to Al0.5Ga0.5N remained rectifying even without the plasma treatment. X-ray photoelectron spectroscopy measurements confirmed that the Fermi level moved toward the conduction band in GaN after the plasma treatment, but it was pinned by plasma-induced deep-level states in Al0.5Ga0.5N. This study emphasizes the need to mitigate plasma damage introduced during the mesa etch step for AlGaN-based deep-UV emitters and detectors.The effects of surface treatment using Cl2∕BCl3 and Ar inductive coupled plasmas on the Ohmic characteristics of Ti∕Al∕Ti∕Au contacts to n-type AlxGa1−xN (x=0–0.5) were investigated. Plasma treatment significantly increased the surface conductivity of GaN and Al0.1Ga0.9N, leading to improved Ohmic behaviors of the contacts. However, it reduced the surface doping level in AlxGa1−xN (x⩾0.3) and degraded the contact properties. Following a 900–1000°C anneal, the Ti∕Al∕Ti∕Au contacts to AlxGa1−xN (x=0–0.3) became truly Ohmic, with specific contact resistances of (3–7)×10−5Ωcm2, whereas the contact to Al0.5Ga0.5N remained rectifying even without the plasma treatment. X-ray photoelectron spectroscopy measurements confirmed that the Fermi level moved toward the conduction band in GaN after the plasma treatment, but it was pinned by plasma-induced deep-level states in Al0.5Ga0.5N. This study emphasizes the need to mitigate plasma damage introduced during the mesa etch step for AlGaN-based deep-UV emitters and det...

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.