Peter Micah Sandvik

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.

Demonstration of ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes

We report ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes. An external quantum efficiency of 83% (187 mA/W) at 278 nm, corresponding to unity gain after reach-through was achieved. A gain higher than 1000 was demonstrated without edge breakdown.

Impact Ionization Coefficients in 4H-SiC

Photomultiplication measurements using 244- and 325-nm excitation have been undertaken on a series of thick 4H-SiC avalanche diodes. With avalanche widths of between 2.7 and 6 mum and the ability to measure multiplication as low as 1.001, a much wider electric field range has been covered than reported to date. The results show that the hole ionization coefficient (beta) can be obtained with a high degree of accuracy down to electric fields as low as ~0.9 MV/cm. The value of electron ionization coefficient (alpha) has been determined from mixed carrier multiplication characteristics, and the beta/alpha ratio is found to increase significantly with decreasing electric fields. Ionization coefficients are parameterized over the electric field range from 0.9 to 5 MV/cm, enabling the multiplication and breakdown characteristics of 4H-SiC to be predicted accurately.

Electrical and Optical Modeling of 4H-SiC Avalanche Photodiodes

Optimal physical models and material parameters for 4H-SiC avalanche photodiodes (APDs) were studied using a two-dimensional device simulation tool. In the models, we took account of temperature-dependent impact ionization and absorption coefficient as a function of wavelength. The absorption coefficient spectra derived in this work exhibited a rapid increase below ~300 nm, which can be qualitatively incorporated into indirect and direct band transition models. The simulated characteristics were in good agreement with the measured characteristics.

Electrical characterization of 4H–SiC avalanche photodiodes containing threading edge and screw dislocations

The reverse voltage current characteristics and electroluminescence of small area 4H–SiC avalanche photodiodes were investigated and correlated with the presence of threading screw and edge dislocations. Localized electroluminescence was observed at threading dislocations at voltages close to breakdown whereas diodes without any extended defects exhibited uniform light emission in the active area. Diodes containing either edge or screw dislocations were found to have excess leakage currents and breakdown prematurely compared to diodes without dislocations.

Deep UV photon-counting detectors and applications

Photon counting detectors are used in many diverse applications and are well-suited to situations in which a weak signal is present in a relatively benign background. Examples of successful system applications of photon-counting detectors include ladar, bio-aerosol detection, communication, and low-light imaging. A variety of practical photon-counting detectors have been developed employing materials and technologies that cover the waveband from deep ultraviolet (UV) to the near-infrared. However, until recently, photoemissive detectors (photomultiplier tubes (PMTs) and their variants) were the only viable technology for photon-counting in the deep UV region of the spectrum. While PMTs exhibit extremely low dark count rates and large active area, they have other characteristics which make them unsuitable for certain applications. The characteristics and performance limitations of PMTs that prevent their use in some applications include bandwidth limitations, high bias voltages, sensitivity to magnetic fields, low quantum efficiency, large volume and high cost. Recently, DARPA has initiated a program called Deep UV Avalanche Photodiode (DUVAP) to develop semiconductor alternatives to PMTs for use in the deep UV. The higher quantum efficiency of Geiger-mode avalanche photodiode (GM-APD) detectors and the ability to fabricate arrays of individually-addressable detectors will open up new applications in the deep UV. In this paper, we discuss the system design trades that must be considered in order to successfully replace low-dark count, large-area PMTs with high-dark count, small-area GM-APD detectors. We also discuss applications that will be enabled by the successful development of deep UV GM-APD arrays, and we present preliminary performance data for recently fabricated silicon carbide GM-APD arrays.

Solar-Blind 4H-SiC Single-Photon Avalanche Diode Operating in Geiger Mode

A solar blind 4H-SiC single photon avalanche diode (SPAD) with a sharp cutoff at a wavelength of 280 nm is reported. The SPAD with separate absorption and multiplication layers was designed for operation in Geiger mode. A thin film optical filter deposited on a sapphire window of the device package provided sensitivity in the wavelength range between 240 and 280 nm with a very high solar photon rejection ratio. An estimated dark current of 0.4 pA (0.75 nA/cm2) at a gain of 1000 was measured on a device with an effective mesa diameter of 260 mum. A single photon detection efficiency of 9.4% and a dark count probability of 4 times 10-4 were demonstrated at a wavelength of 266 nm for the same device.

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.

Temperature Dependent Characteristics of Nonreach-Through 4H-SiC Separate Absorption and Multiplication APDs for UV Detection

Silicon carbide (SiC) separate absorption multiplication region avalanche photodiodes (SAM-APDs) for UV detection in harsh environment applications were designed and fabricated. The devices were intentionally designed to operate under nonreach-through conditions in order to eliminate field-induced leakage current. The gain of 2500 and quantum efficiency of ~45% at room temperature were achieved at the wavelength of 290-300 nm for a packaged device with an active area of 1 x 1 mm2. The temperature dependency of the current-voltage characteristics and responsivity was examined in the temperature range from room temperature to 230degC.