Robert F. Karlicek

Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy

We report the growth of InGaN/GaN multiple-quantum-well blue light-emitting diode (LED) structures on Si(111) using metalorganic vapor phase epitaxy. By using growth conditions optimized for sapphire substrates, a full width at half maximum (FWHM) (102) x-ray rocking curve of less than 600 arcsec and a room-temperature photoluminescence peak at 465 nm with a FWHM of 35 nm was obtained. Simple LEDs emitting bright electroluminescence between 450 and 480 nm with turn-on voltages at 5 V were demonstrated.

Microcavity effects in GaN epitaxial films and in Ag/GaN/sapphire structures

Luminescence spectra of GaN epitaxial layers grown on sapphire display a strong intensity modulation of the below-band gap transitions and on the low-energy side of the near-band gap transition. The intensity modulation is attributed to a microcavity formed by the semiconductor–air and semiconductor–substrate interface. The microcavity effect is enhanced by using metallic reflectors which increase the cavity finesse. It is shown that microcavity effects can be used to determine the refractive index of the microcavity active material. Using this method, the GaN refractive index is determined and expressed analytically by a Sellmeir fit.

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 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.

InGaN blue light-emitting diodes with optimized n-GaN layer

In the extensive research dedicated recently to metal- organic chemical vapor deposition (MOCVD)-grown high- efficiency GaN LED device design, a significant effort has been made to increase the conductivity of p-GaN layers, while n-GaN layers received relatively little attention. We demonstrated, both experimentally and theoretically, that the resistivity of n-GaN layers has a profound effect on blue InGaN LED performance. Optimization of n-GaN epitaxial layers allows the achievement of device series resistances below 15 Ohms and forward voltages as low as 2.9 Volts at 20 mA. We have also shown that contactless measurements of sheet resistivity of the entire LED epitaxial structure closely correlate with the ohmic resistance of the GaN layer measured in the fabricated devices. This provides an excellent non-destructive characterization tool for n-GaN optimization. Insufficient n-GaN conductivity is shown to trigger a distinct degradation mechanism by initiating current crowding in a localized device area. InGaN LED lamps with optimized n-GaN layers had a high external quantum efficiency and a good long-term reliability.

GaN and AlGaN metal–semiconductor–metal photodetectors

Abstract GaN based interdigital metal–semiconductor–metal (MSM) photodetectors have been successfully fabricated. The MSM structures were patterned on highly resistive GaN and the ternary compound, AlGaN. For the highly resistive GaN detector, the lowest dark current is ∼0.1 nA and the UV responsivity of the device was about 460 A W−1 at a DC bias of 5 V. The AlGaN with 24% Al exhibited larger gains of up to 106 A W−1 at 20 V, but at a very high dark current, 1 mA, and very long detector responses, greater than 60 s. The high gain in this device is not well understood. The preliminary measurements indicate that tunneling occurs at high electric fields since a negative temperature coefficient for the breakdown voltage was observed.

Reproducibility of GaN and InGaN films grown in a multi-wafer rotating-disc reactor

Abstract The recent surge of interest, and subsequent material refinement, in the Ill-nitrides for optoelectronic and power applications has led to the development of several high-quality commercial devices such as light emitting diodes and the recently announced laser diode. Most of these devices, and much of the GaN research, has been completed on single-wafer metal-organic chemical vapor deposition (MOCVD) systems. For low cost and high yield, production-scale equipment is necessary in order to fully develop this expanding market. EMCORE, as a manufacturer of production-scale MOCVD equipment, has had an on-going research effort in the development and refinement of deposition systems for the growth of GaN. EMCORE has demonstrated high-quality p-doping of GaN with resistivities as low as 0.1 Ω cm. EMCORE has also demonstrated the growth of high-quality InGaN alloys and heterostructures, including quantum wells. In this paper, we report on the reproducibility of these films on a run-to-run and wafer-wafer basis. The high degree of reproducibility observed for these films will make the mass production of GaN-based devices cost effective.

In-situ controls for MOVPE manufacturing

Abstract Much of the early research that established MOVPE was done with simple, single-wafer systems. Uniformity and reproducibility were not show-stoppers — developing the precursors, growth processes and device fabrication techniques were more important. This situation has changed so that now the emphasis is on lower cost devices opening new applications or replacing older technologies. In this paper we will examine the major factors that affect manufacturing costs in the high volume production of MOVPE-grown material for compound semiconductor devices, emphasizing the importance of reproducibility.

Time-resolved photoluminescence measurements of InGaN light-emitting diodes, films, and multiple quantum wells

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 (beta) equals 0.59 +/- 0.05. The emission lifetime varied with energy, within error (beta) was independent of the emission energy. The stretched exponential kinetics are consistent with significant disorder in the material. Related results for an InGaN film and InGaN/GaN MQWs are also reported. We attribute the disorder to fluctuations of the local indium concentration.

Growth and characterization of high-efficiency InGaN MQW blue and green LEDs from large-scale-production MOCVD reactors

As more advances are made in the performance of GaN-based devices, a trend toward the use of large scale MOCVD reactors for epitaxial growth of GaN-based device structures is clear. In this paper we describe the use of Emcores SpectraBlueTM reactor for large-scale manufacturing of Blue and Green LEDs. The high throughput growth of GaN based LEDs is demonstrated without compromising LED uniformity or overall performance. In-situ control of key parameters critical to the production of high quality LEDs, such as buffer layer growth is now feasible using in-situ reflectance spectroscopy. Film properties as well as LED device performance are discussed.