Milan Pophristic

Spatial characterization of doped SiC wafers by Raman spectroscopy

Raman spectroscopy has been used to investigate wafers of both 4H–SiC and 6H–SiC. The wafers studied were semi-insulating and n-type (nitrogen) doped with concentrations between 2.1×1018 and 1.2×1019 cm−3. Significant coupling of the A1 longitudinal optical (LO) phonon to the plasmon mode was observed. The position of this peak shows a direct correlation with the carrier concentration. Examination of the Raman spectra from different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. These data are compared with a resistivity map of the wafer. These results suggest that Raman spectroscopy of the LO phonon–plasmon mode can be used as a noninvasive, in situ diagnostic for SiC wafer production and substrate evaluation.

High performance AlGaN/GaN power switch with HfO2 insulation

High performance AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor was fabricated using HfO2 as the surface passivation and gate insulator. The gate and drain leakage currents are drastically reduced to tens of nanoamperes before breakdown. Without field plates, for 10 μm of gate-drain spacing, the off-state breakdown voltage is 1035 V with a specific on resistance of 0.9 mΩ cm2. In addition, there is no current slump observed from the pulse measurements. This is the best performance reported on GaN-based power-switching devices on sapphire up to now, which efficiently combines excellent device forward, reverse, and switching characteristics.

Time-resolved photoluminescence measurements of InGaN light-emitting diodes

We have used time-resolved photoluminescence (PL) to examine light-emitting diodes made of InGaN/GaN multiple quantum wells (MQWs) before the final stages of processing. The time-resolved photoluminescence from a dim MQW was quenched by nonradiative recombination centers. The PL kinetics from a bright MQW were not single exponential but stretched exponential, with the stretch parameter β=0.59±0.05. The emission lifetime varied with energy, within error β was independent of the emission energy. the stretched exponential kinetics are consistent with significant disorder in the material. We attribute the disorder to spatial fluctuations of the local indium concentration.

Time-resolved photoluminescence measurements of quantum dots in InGaN multiple quantum wells and light-emitting diodes

We have used time-resolved photoluminescence to examine InGaN/GaN multiple quantum wells (MQWs) and light-emitting diodes (LEDs) before the final stages of processing at room temperature. The photoluminescence kinetics are well described by a stretched exponential exp[−(t/τ)β], indicating significant disorder in the material. We attribute the disorder to nanoscale quantum dots of high local indium concentration. For the three MQWs examined, the stretching parameter β and the stretched exponential lifetime τ were found to vary with emission energy. The stretching parameter β for the emission peak of the three MQWs was observed to increase from 0.75 to 0.85 with apparently increasing indium phase segregation. A higher degree of indium phase segregation is consistent with more isolated quantum dots inside the two-dimensional quantum well. The time-resolved photoluminescence from a LED wafer, before the final stages of processing, suggests the importance of quantum dots of high indium concentration on the LED...

Time-resolved spectroscopy of InxGa1−xN/GaN multiple quantum wells at room temperature

We have measured the time-resolved photoluminescence (PL) from a series of InxGa1−xN/GaN (x=0.22) multiple quantum well structures at room temperature. Lifetimes longer than 1 ns (1.87±0.02 ns) were measured at room temperature. The emission lifetime was found to lengthen with increasing excitation power, this is attributed to the saturation of recombination centers. The PL decay kinetics were found to be quite sensitive to the emission wavelength. The energy dependence of the emission lifetime is attributed to nanoscale fluctuations in the indium concentration.

RAMAN MICROSCOPY OF LATERAL EPITAXIAL OVERGROWTH OF GAN ON SAPPHIRE

We have used confocal Raman microscopy to investigate lateral epitaxially overgrown (LEO) GaN on sapphire substrates. The one-phonon Raman spectra are consistent with pyramidal growth of the GaN before coalescence has occurred. The position and asymmetric line shape of the A1 longitudinal optical (LO) phonon demonstrate that the LEO GaN is doped. The dopant is most likely Si from the SiN mask used to produce the LEO GaN. The carrier concentration is estimated to be 1×1017 cm−3. We have also used Raman microscopy to spatially resolve the yellow emission from different regions of the LEO GaN.

High-conductivity n-AlGaN with high Al mole fraction grown by metalorganic vapor phase deposition

Highly-conductive and crack-free n-Al0.6Ga0.4N films with thickness up to 1 μm were achieved by using high-temperature AlN or AlGaN/AlN superlattice (SL) buffer layers. Room-temperature Hall measurements show the highest electron concentration of 3.5×1018 cm−3 with mobility of 25 cm2/V s. Electron mobility was increased from 25 to 35 cm2/V s by introducing the AlGaN/AlN SL buffer layer. Second ion mass spectroscopy indicates that there is high oxygen doping concentration in the film, and that the film resistivity decreases with increasing oxygen concentration from 1×1017 to ∼1×1019 cm−3.

Demonstration of Low-Leakage-Current Low-On-Resistance 600-V 5.5-A GaN/AlGaN HEMT

This letter demonstrates a high-voltage, high-current, and low-leakage-current GaN/AlGaN power HEMT with HfO₂ as the gate dielectric and passivation layer. The device is measured up to 600 V, and the maximum on-state drain current is higher than 5.5 A. Performance of small devices with HfO₂ and Si₃N₄ dielectrics is compared. The electric strength of gate dielectrics is measured for both HfO₂ and Si₃N₄. Devices with HfO₂ show better uniformity and lower leakage current than Si₃N₄ passivated devices. The 5.5-A HfO₂ devices demonstrate very low gate (41 nA/mm) and drain (430 nA/mm) leakage-current density and low on-resistance (6.2 Omegamiddotmm or 2.5 mOmegamiddotcm²).

Ultraviolet nitride LED fabrication for high-flux white LED

The requirements for maximizing the external quantum efficiency of UV nitride LEDs are discussed. It is shown that as the chip wavelength progressively decreases, nitride epi growth on a sapphire substrate becomes advantageous in terms of light extraction. The epilayer requirements for UV LEDs dictate the growth of n-AlGaN, with increasing Al contents, and the growth of UV-transparent p-GaN. It is shown that MOCVD growth in a Emcore D-180 or Ganzilla reactor is ideal for meeting the stringent epilayer requirements. Increasing light extraction efficiency and wall-plug efficiency also requires optimization of the reflecting P-contact. The relative merits of Al- and Ag-based reflecting contacts are discussed. Performance data for UV LEDs on sapphire, for drive currents up to 700 mA is shown. Finally, a practical high power UV-based white lamp is demonstrated.

600V GaN Schottky Barrier Power Devices for High Volume and Low Cost Applications

The first commercially viable high voltage (>600V) gallium nitride (GaN) Schottky barrier devices are reported. Though GaN does not have any “micropipe” defects, which commonly exists in SiC material, defects like dislocations due to lattice mismatch hamper the material development of GaN high power devices. Improvements in the nitride epitaxial film growth have led to significant reduction of conductive dislocations. Conductive Atomic Force Microscope (CAFM) analysis of conductive dislocations shows only on the order of 103 cm-2 density of conductive dislocations, which are believed to be responsible for the undesired leakage current. GaN diodes compare to SiC or Si devices demonstrate a significant advantage in the thermal resistance. The insulating properties of Sapphire substrates allow fabrication of the devices in TO220 packages with insulating frame and thermal resistance better than 1.8°C/W compare to 3°C/W of SiC or Si devices with insulating frame. Performance of GaN, SiC and Si devices in the switch mode power supplies is compared.