Steve Arthur

High-power and reliable operation of vertical light-emitting diodes on bulk GaN

InGaN∕GaN light-emitting diodes (LEDs) with lateral and vertical geometries have been fabricated on free-standing GaN substrates. Current spreading was significantly enhanced in the vertical LED, leading to a reduced series resistance of 7Ω compared to 12.2 and 14.2Ω for the lateral LEDs on GaN and sapphire, respectively. As a result, the light output and power conversion efficiency of the vertical LED on GaN were greatly improved at high injection currents. The vertical LED was subjected to a stress test at 400mA and showed minimal degradation of optical power, whereas the same stress resulted in the destruction of the lateral LED on sapphire due to increased current crowding and self-heating. However, lateral LEDs on sapphire with optimized current spreading exhibited excellent reliability, indicating the presence of a high density of dislocations (∼109cm−2) in the heteroepitaxial device does not accelerate LED degradation at current densities up to 700A∕cm2.

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.

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.

Structure of stacking faults formed during the forward bias of 4H-SiC p-i-n diodes

Using site-specific plan-view transmission electron microscopy (TEM) and light emission imaging, we have identified stacking faults formed during forward biasing of 4H-SiC p-i-n diodes. These stacking faults (SFs) are bounded by Shockley partial dislocations and are formed by shear strain rather than by the condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during p-i-n diode operation, suggesting the presence of a complex inhomogeneous strain field in the 4H-SiC layer.

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.

Influence of defects on electrical and optical characteristics of GaN/InGaN-based light-emitting diodes

The microstructural, electrical and optical properties of GaN/InGaN light emitting diodes (LEDs) with various material quality grown on sapphire have been studied. Burgers vector analyses showed that edge and mixed dislocations were the most common dislocations in these samples. In defective devices, a large number of surface pits and V-defects were present, which were found to be largely associated with mixed or screw dislocations. Tunneling behavior dominated throughout all injection regimes in these devices. The I-V characteristics at the moderate forward biases can be described by I = I0 exp (eV/E), where the energy parameter E has a temperature-independent value in the range of 70 -110 meV. Deep level states-associated emission has been observed, which is direct evidence of carrier tunneling to these states. Light output was found to be approximately current-squared dependent even at high currents, indicating nonradiative recombination through deep-lying states in the space-charge region. In contrast, dislocation bending was observed in a high quality device, which substantially reduced the density of the mixed and screw dislocations reaching the active layer. The defect-assisted tunneling was substantially suppressed in this LED device. Both forward and reverse I-V characteristics showed high temperature sensitivity, and current transport was diffusion-recombination limited. Light output of the LED became linear with the forward current at a current density as low as 1.4x10-2 A/cm2, where the nonradiative recombination centers in the InGaN active region were essentially saturated. This low saturation level suggests optical inactivity of the edge dislocations in this LED.

100 Amp, 1000 Volt Class 4H-Silicon Carbide MOSFET Modules

The development of large area, up to 70m/1kV (0.45cm x 0.45cm) 4H-SiC vertical DMOSFETs is presented. DC and switching characteristics of high-current, 100Amp All-SiC power switching modules are demonstrated using 0.45cm x 0.225cm DMOSFET die and commercial Schottky diodes. The switching performance from room temperature up to T=200°C of the All-SiC modules is presented, with as much as ten times lower losses than co-fabricated Si-based modules using commercial IGBTs.

950 Volt 4H-SiC MOSFETs: DC and Transient Performance and Gate Oxide Reliability

SiC vertical MOSFETs were fabricated and characterized to achieve a blocking voltage of 950 Volts and a specific on-resistance of 8.4 mW-cm2. Extrapolations of time-dependent dielectric breakdown measurements versus applied electric field indicate that the gate oxide mean-time to failure is approximately 105 hours at 250°C.

Partial Dislocations and Stacking Faults in 4H-SiC PiN Diodes

Using plan-view transmission electron microscopy (TEM), we have identified stacking faults (SFs) in 4H-SiC PiN diodes subjected to both light and heavy electrical bias. Our observations suggest that the widely expanded SFs seen after heavy bias are faulted dislocation loops that have expanded in response to strain of the 4H-SiC film, while faulted screw or 60° threading dislocations do not give rise to widely expanded SFs. Theoretical calculations show that the expansion of SFs depends on the Peach-Koehler (PK) forces on the partial dislocations bounding the SFs, indicating that strain plays a critical role in SF expansion.

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...