S. N. Lee

Electrical characteristics of contacts to thin film N-polar n-type GaN

The electrical characteristics of metallization contacts to a thin film N-polar n-type GaN layer fabricated by a laser lift-off process combined with a dry etching are investigated. It is shown that for Pt Schottky contacts, the Schottky barrier height of the N-polar GaN is 1.27eV, which is larger than that (1.23eV) of reference Ga-polar GaN. Ti∕Al Ohmic contacts to the N-polar GaN experience thermal degradation even at 400°C. Such annealing-induced degradation is explained in terms of the presence of the complex surface states of the N-polar GaN, which consists of impurities and process-induced donorlike and acceptorlike defects.

Carrier loss and luminescence degradation in green-light-emitting InGaN quantum wells with micron-scale indium clusters

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

Metallization contacts to nonpolar a-plane n-type GaN

We have investigated the electrical characteristics of metallization contacts to nonpolar a-plane and polar c-plane n-type GaN. Pd Schottky diodes and x-ray photoemission spectroscopy measurements show that the Schottky barrier height of the a-plane GaN is lower than that of the c-plane GaN by 0.24 and 0.30eV, respectively. Ti∕Al Ohmic contacts to the a-plane n-GaN produce lower contact resistivity than that of the c-plane samples when annealed at 500°C. However, Ti∕Al contacts to the c-plane and a-plane GaN show opposite electrical behavior when annealed at temperatures above 500°C, which is attributed to the absence of polarization-induced surface charges for a-plane GaN.

High efficiency GaN-based light-emitting diodes fabricated on dielectric mask-embedded structures

We report on the enhanced quantum efficiency of GaN-based light-emitting diodes (LEDs) fabricated on inverted hexagonal pyramid dielectric mask (IHPDM)-embedded structure. The ray-tracing calculation showed that the extraction efficiency of LEDs fabricated on IHPDM-embedded structure could be enhanced up to 56%. Compared to the reference, the n-GaN template grown on IHPDM-embedded structure also showed a reduction in the dislocation density by 57%, leading to an increase in photoluminescence intensity by 82%. The LED fabricated on IHPDM-embedded structure exhibited a reduction in the forward leakage current by one order of magnitude (<1.5 V) and an enhancement in the output power by 41%.

Light extraction enhancement of GaN-based light emitting diodes using MgF2/Al omnidirectional reflectors

We report on the enhancement of the light extraction of GaN-based light emitting diodes (LEDs) by using MgF2/Al omnidirectional reflectors (ODRs). The ODRs consisting of a quarter-wavelength-thick MgF2 having a refractive index of 1.39 and Al metal produce a high-angle-integrated reflectivity of 96.6%. To optimize the electrical injection and light reflection, the MgF2/Al ODRs are combined with Pd/Ag metallic reflectors using mesh configuration. Compared to reference LEDs, LEDs fabricated with the MgF2/Al ODRs show an enhanced output power by 23% and a slight increase in the forward voltage by 0.18 V, leading to the improvement in power efficiency by 17%.

Low resistance and transparent Ni–La solid solution/Au ohmic contacts to p-type GaN

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (Na=5×1017 cm−3). The as-deposited contact reveals nonlinear current–voltage (I–V) characteristics. However, the contacts show considerably improved I–V behavior when annealed at temperatures of 350–550 °C for 1 min in air ambient. For example, the specific contact resistance as low as 7.2×10−5 Ω cm2 is obtained from the samples annealed at 550 °C, whose transmittance is measured to be 82.5% at a wavelength of 470 nm. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the annealed contacts are described and discussed.

Electrical Characteristics of Metal Contacts to Laser-Irradiated N-Polar n-Type GaN

Electrical characteristics of metal contacts to a laser-irradiated thin-film N-polar n-type GaN surface are investigated. It is shown that the laser irradiation of the N-polar n-GaN results in a significant increase (as high as 3.0 times 1020 cm-3) in the electron concentration and a decrease in the Schottky barrier height by 0.05 eV (0.28 eV), as estimated using Pt-based Schottky diodes (X-ray photoemission spectroscopy). Ti/Al contacts to the laser-irradiated N-polar sample exhibit an excellent Ohmic behavior with thermal stability up to 600degC, indicating that the laser irradiation treatment can be a very promising technology in practical applications.

Comprehensive study of time-lapsed peak shift in InGaN quantum well structures: Discrimination of localization effect from internal field effect

Time-lapsed emission peak shift behaviors in blue-light-emitting InGaN multiple quantum well (MQW) laser diodes with different well widths are systematically investigated by means of excitation power-dependent, time-resolved optical analysis. By investigating the main emission peak shift as a function of both time evolution and excitation power density, the amount of time-lapsed emission peak shift can be differentiated by two contributions: the excitation power dependent and independent ones. The authors conclude that the power-dependent (power-independent) time-lapsed peak shift can be attributed to the internal electric-field (carrier localization) effect present in vertical growth (lateral in-plane) direction of InGaN MQW laser diode structures.

Carrier Transport Mechanism of Pd/Pt/Au Ohmic Contacts to p-GaN in InGaN Laser Diode

The dependence of contact resistivity on the carrier concentration for the non-alloyed Pd contacts on p-GaN and the temperature dependence measurements of sheet resistivity of p-GaN suggest that carrier transport at the interface between the contact and the p-GaN would be dominated by deep level defect band, rather than the valence band. Based on these results, in order to reduce the operating voltage of the InGaN laser diode (LD), we designed a p-GaN:Mg contact layer, where the ohmic metals are contacted, and optimized the p-GaN contact layer.