Mi-Hee Ji

Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers

Data and analysis are presented for the study of efficiency droop in visible III-nitride light-emitting diodes (LEDs) considering the effects of both electron spill-over out of active region and hole injection into the active region. Performance characteristics of blue LEDs with lattice-matched In0.18Al0.82N electron-blocking layers (EBLs) with different thicknesses were measured in order to exclude the effects of strain and doping efficiency of the EBL, and the quantum efficiencies were analyzed taking account of the electron spill-over current and the relative hole concentration. The results suggest that the highest efficiency in LEDs with a 15-nm In0.18Al0.82N EBL is due to relatively lower hole-blocking effect, hence higher hole injection than in LEDs with a 20-nm EBL, while providing a higher potential barrier for reduced electron spill-over than in LEDs with thinner EBLs. This study suggests that the EBL hole-blocking and electron-confinement effects should be considered in order to achieve higher l...

GaN/InGaN avalanche phototransistors

We report on III–nitride (III–N) avalanche phototransistor (APT) action by illuminating ultraviolet (UV) photons onto a GaN/InGaN npn heterojunction bipolar transistor in an open-base configuration. A high responsivity of >1 A/W was measured for the device operating at a collector-to-emitter voltage (VCE) of 68 A/W at VCE = 95 V. The InGaN APT demonstrates the feasibility of using III–N bipolar transistor structures for high-sensitivity UV photodetection applications.

Comparison of AlGaN p–i–n ultraviolet avalanche photodiodes grown on free-standing GaN and sapphire substrates

We compare the performance characteristics of Al0.05Ga0.95N UV avalanche photodiodes (APDs) grown on different substrates. UV-APDs grown on a free-standing GaN substrate show lower dark-current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapphire template. In addition, a stable avalanche gain higher than 5 × 105 and a significant increase in the responsivity of UV-APDs grown on a free-standing GaN substrate are observed. We believe that the high crystalline quality of Al0.05Ga0.95N UV-APDs grown on a free-standing GaN substrate with low dislocation density is responsible for the observed low leakage currents, high performance characteirstics, and reliability of the devices.

Direct periodic patterning of GaN-based light-emitting diodes by three-beam interference laser ablation

We report on the direct patterning of two-dimensional periodic structures in GaN-based light-emitting diodes (LEDs) through laser interference ablation for the fast and reliable fabrication of periodic micro- and nano-structures aimed at enhancing light output. Holes arranged in a two-dimensional hexagonal lattice array having an opening size of 500 nm, depth of 50 nm, and a periodicity of 1 μm were directly formed by three-beam laser interference without photolithography or electron-beam lithography processes. The laser-patterned LEDs exhibit an enhancement in light output power of 20% compared to conventional LEDs having a flat top surface without degradation of electrical and optical properties of the top p-GaN layer and the active region, respectively.

Temperature-Dependent Characteristics of GaN Homojunction Rectifiers

We report a homojunction gallium nitride (GaN) p-i-n rectifier fabricated on free-standing GaN substrates with the breakdown voltage 800 V and low specific ON-resistance (R_ONA). At 298 K, RONA is 0.28 mΩ-cm² at the current density (J) of 2.5 kA/cm² and the corresponding Baligas figure of merit is 2.5 GW/cm². At a given temperature, R_ONA values decrease with J due to conductivity modulation in the drift region. The ambipolar lifetime (τ_a) is also determined by open-circuit voltage decay measurement. The value for τ_a is 9.6 ns at 298 K and it monotonically increases to 22 ns at 448 K. The reverse I-V measurement reveals the reverse leakage current mechanism is mainly attributed to a field-assisted ionization process from deep-level centers in the space-charge region. The analysis of T-I-V curve yields the Poole-Frenkel coefficient (~3.1 × 10^-4 eV · V^-1/2 · cm^-1/2) and a deep-level trap (~0.7 eV) at zero bias.

Uniform and Reliable GaN p-i-n Ultraviolet Avalanche Photodiode Arrays

GaN p-i-n ultraviolet avalanche photodiodes (UV-APDs) were fabricated from epitaxial structures grown on low-dislocation-density free-standing GaN substrates to form 4 × 4 UV-APD arrays with a device size of 75 × 75 μm². The devices in the UV-APD array showed a uniform and reliable distribution of breakdown voltage (V_BR) and leakage current density. The average V_BR of the 16 devices in one of the UV-APD arrays was 96±0.6 V, and the average dark current density (J_{R_Dark}) and photocurrent density (J_{R_Photo}) were measured to be (6.5±1.8)×10^-7 and (5.7±1.1)×10^-6 A/cm² at the reverse bias voltage of V_R = 48 V (50% of the average onset point of V_BR), respectively. The reliable device performance was confirmed by performing multiple reverse bias I-V scans for the selected devices in the UV-APD array. We also observed the significantly enhanced spectral responsivity from the 142 to 5485 mA/W due to the strong carrier impact ionization at high reverse bias.

Development of III-N UVAPDs for ultraviolet sensor applications

High-resolution imaging in ultraviolet (UV) bands has many applications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. In past work, UV avalanche photodiodes (APDs) have been reported as discrete devices demonstrating gain. The next frontier is to develop UVAPD arrays with high gain to demonstrate highresolution imaging. We will discuss a model that can predict sensor performance in the UV band using APDs with various gain and other parameters for a desired UV band of interest. Signal-to-noise ratios (SNRs) can be modeled from illuminated targets at various distances with high resolution under standard atmospheric conditions in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APDs. We will present recent data on the GaN-based APDs for their gain, detector response, dark current noise, and 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APDs in wide-bandgap semiconductors. The paper will also discuss the state of the art in UVAPDs and the future directions for small unit cell size and gain in the APDs.

Improved Hole Transport by

Studied is the effect of indium (In) mole fraction in p-InxGa1-xN:Mg layers with 0 ≤ xIn ≤ 0.035 on hole injection and transport behaviors in InGaN/GaN multiple quantum wells (MQWs) using dual-wavelength and triple-wavelength active regions. Electro-optical characteristics of light-emitting diodes containing p-layers with different In content and with silicon doping in selected QW barriers (QWBs) are compared to evaluate hole transport in the active region. The results show that enhanced hole transport and corresponding more uniform distribution of holes across the MQW region are achieved by increasing xIn in the p-InxGa1-xN:Mg layer, possibly due to modification in energy of holes by a potential barrier between the p-InGaN and GaN QWB.

{\rm p}\hbox{-}{\rm In}_{x}{\rm Ga}_{1-x}{\rm N}

In this study, various thermal characterization techniques and multi-physics modeling were applied to understand the thermal characteristics of GaN vertical and quasi-vertical power diodes. Optical thermography techniques typically used for lateral GaN device temperature assessment including infrared thermography, thermoreflectance thermal imaging, and Raman thermometry were applied to GaN p-i-n diodes to determine if each technique is capable of providing insight into the thermal characteristics of vertical devices. Of these techniques, thermoreflectance thermal imaging and nanoparticle assisted Raman thermometry proved to yield accurate results and are the preferred methods of thermal characterization of vertical GaN diodes. Along with this, steady state and transient thermoreflectance measurements were performed on vertical and quasi-vertical GaN p-i-n diodes employing GaN and Sapphire substrates, respectively. Electro-thermal modeling was performed to validate measurement results and to demonstrate th...

Layer in Multiple Quantum Wells of Visible LEDs

In this study, pioneering research was performed on GaN p-i-n diodes for the first ever assessment of surface temperature distribution by incorporating the use of infrared (IR) thermography, thermoreflectance thermal imaging, Raman thermometry, and thermal simulations. Each technique was advanced in order to obtain self-consistent results with higher accuracy. A two-temperature emissivity calibration procedure was utilized for IR measurements to acquire a temperature map of the p-contact metallization. Higher spatial resolution thermoreflectance thermal imaging was performed with a diverse range of illumination wavelengths including 470 nm and 530 nm. To confirm the results of thermoreflectance, TiO2 thermal nanoprobes were deposited on the device surface which enabled Raman thermometry to be performed on the p-contact metallization. Coherence of the techniques was then validated through thermal modeling. The results suggest that IR thermography, when using the two-temperature emissivity correction procedure, gives reasonable results at high power dissipating conditions. Thermoreflectance and nanopowder assisted Raman thermometry are viable options for GaN vertical device temperature assessment. However, results from Raman thermometry possess relatively large uncertainties and thermoreflectance measurements require multiple illumination wavelengths to ensure the validity of the measured temperatures that are derived from the thermoreflectance calibration coefficient.