Carrier Dynamics Studies of Thick GaN Grown by HVPE
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ABSTRACT
We present a comparison between optical properties of two samples grown by hydride vapor phase epitaxy, one directly on Sapphire substrate (non-ELO) and one epitaxial lateral overgrown (ELO) on SiO2 patterned Sapphire substrate. The ELO material shows an improvement of the UV emission and slight decrease of the yellow emission. The band edge emission is red shifted due to the relaxation of the compressive strain. In spite of the increase in the UV emission, the lifetime of the excitons in the ELO material is more than twice lower than non-ELO material. We attribute this to screening effects of the background electron concentration.
INTRODUCTION
GaN and related alloys are currently among the most extensively studied semiconductor materials for a variety of applications [1] from blue-green and UV light emitting diodes to high power/high temperature transistors. Recently, significant progress has been reported in growing GaN films by hydride vapor phase epitaxy (HVPE) [2]. Excellent optical quality of the material [3] combined with high growth rates make GaN films grown by HVPE very attractive for use as a substrate for GaN epilayers, or even asgrown devices [4]. Epitaxial lateral overgrowth (ELO) on patterned substrates is of particular interest because of the realization of the longest lifetime GaN based laser diode [5]. The leakage current of p-n junctions grown on ELO material on Sapphire substrates has been shown to reduce by three orders of magnitude [6]. Time resolved photoluminescence (TRPL) is a powerful experimental tool to study new materials because the temporal information combined with spectral data help determine the dynamics of carriers involved in optical processes. In this paper we report on a comparison of optical properties of two samples grown by HVPE, one directly on Sapphire substrate and one epitaxial lateral overgrown on SiO2 patterned Sapphire substrate.
EXPERIMENTAL DETAILS
For ELO sample, a 200 nm thick SiO2 film was used to mask the substrate. Stripe arrays with openings of 2 µm and spacing of 40 µm were formed by standard lift-off technique. Both samples (referred to as ELO and non-ELO) were grown on c-plane
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Sapphire substrates. The growth technique employs a chloride-transport HVPE vertical reactor, with growth rate ~ 21 µm/h at 1050 °C, which results in unintentionally doped GaN. The thickness of the non-ELO sample is 63 µm, and room temperature (RT) Hall effect measurements give a carrier concentration of ~ 6x1016 cm-3 and a mobility of 823 cm2/Vs. A SEM picture of ELO sample is presented in figure 1. The sample exhibits a VPDOOFXUYDWXUH 3HDNWR9DOOH\a
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thickness of the sample is 58 µm, and RT Hall effect measurements show a background electron concentration of ~ 2x1018 cm-3 and mobility of 220 cm2/Vs.
Figure1. SEM image of epitaxial lateral overgrown (ELO) sample.
The experimental set-up consists of a frequency-doubled picosecond Ti:Sapphire laser, with a pulse width of 1.5 ps and excitation wavelengt
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