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

Robert Kucharski ¹

,Łukasz Janicki ²,Marcin Zajac ¹

,Monika Welna ²

,Marcin Motyka ²

,Czesław Skierbiszewski ³

 andRobert Kudrawiec ^2,* 

¹

AMMONO S. A, Prusa 2, 00-493 Warsaw, Poland

²

Department of Experimental Physics, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

³

Institute of High Pressure Physics, Polish Academy of Science, Sokołowska 29/37, 01-142 Warsaw, Poland

http://www.mdpi.com/2073-4352/7/7/187

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 10¹⁸cm⁻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