Transparency of Semi-Insulating, n-Type, and p-Type Ammonothermal GaN Substrates in the Near-Infrared, Mid-Infrared, and THz Spectral Range

Robert Kucharski,Robert Kudrawiec,Monika Welna,Marcin Motyka,Czesław Skierbiszewski,Marcin Zajac,Łukasz Janicki

doi:10.3390/cryst7070187

Abstract

GaN substrates grown by the ammonothermal method are analyzed by Fast Fourier Transformation Spectroscopy in order to study the impact of doping (both n- and p-type) on their transparency in the near-infrared, mid-infrared, and terahertz spectral range. It is shown that the introduction of dopants causes a decrease in transparency of GaN substrates in a broad spectral range which is attributed to absorption on free carriers (n-type samples) or dopant ionization (p-type samples). In the mid-infrared the transparency cut-off, which for a semi-insulating GaN is at ~7 µm due to an absorption on a second harmonic of optical phonons, shifts towards shorter wavelengths due to an absorption on free carriers up to ~1 µm at n ~ 1020 cm−3 doping level. Moreover, a semi-insulating GaN crystal shows good transparency in the 1–10 THz range, while for n-and p-type crystal, the transparency in this spectral region is significantly quenched below 1%. In addition, it is shown that in the visible spectral region n-type GaN substrates with a carrier concentration below 1018 cm−3 are highly transparent with the absorption coefficient below 3 cm−1 at 450 nm, a satisfactory condition for light emitting diodes and laser diodes operating in this spectral range.

Highlights

A choice of growth substrate for III-N based technology is directed most often by the cost effectiveness of the solution combined with a final quality of the device
In this paper we analyze the optical transparency of SI and doped ammonothermal GaN substrates in the near-infrared, mid-infrared, and terahertz spectral range
Summarizing this analysis, it is clearly visible that the transparency of n-type GaN substrates in the near and mid infrared region is controlled by the electron concentration and the wavelength cut-off is very well described by the plasma frequency for n > 1 × 1018 cm−3

Summary

Introduction

A choice of growth substrate for III-N based technology is directed most often by the cost effectiveness of the solution combined with a final quality of the device. While the transparency window of SI GaN is 0.36–7 μm, doping can significantly affect it in the infrared and at longer wavelengths, and in the visible range. This is unfavorable from the optoelectronics point of view where a reabsorption of the light emitted from an active part of the device (i.e., a quantum well) can occur in the doped passive part (e.g., a contact layer). In this paper we analyze the optical transparency of SI and doped ammonothermal GaN substrates in the near-infrared, mid-infrared, and terahertz spectral range. We show how the absorption coefficient is affected by the carrier concentration in the visible and near-infrared region where GaN crystals are quite transparent

Materials

Results

Conclusions