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

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~10 20 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 10 18 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.

High-Frequency and High-Power Performance of n-Type GaN Epilayers with Low Electron Density Grown on Native Substrate

“…The shape and slope of the shoulders of the two-phonon absorption band measured in transmission spectrum in this work are similar to those of GaN crystals reported in Ref. [ 35 ], demonstrating that the free-carrier density indeed should be below the level of ~4 × 10 17 cm −3 (smallest doping value reported in [ 35 ]).…”

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

“…Growth rates were even improved with Ag capsules, while Mo reduced the oxygen concentration in comparison to the source material and was thus proposed as possible oxygen getter. The transparency of bulk GaN can also be tuned by compensation of oxygen donors with Mg acceptors as recently examined for various spectral ranges . Although previous developments provided ammonothermal GaN substrates with low dislocation densities and reduced impurity concentrations, additional studies will be necessary to further decrease the overall quantity of point defects.…”

Electrical Properties of Bulk, Non-Polar, Semi-Insulating M-GaN Grown by the Ammonothermal Method

“…It is used for substrates or active elements of various III-N devices. [1][2][3][4][5][6][7][8][9][10][11] The diameter of the wafers is up to 2 inches, the dislocation density is ≤ 10 4 cm −2 , and the wafers can be either polar (0001) orientation or any desired nonpolar or semi-polar orientation when sliced from (0001) oriented boules (although in this case the effective area is lower). The material can be grown n-type with Si doping, p-type with Mg doping or semi-insulating (SI) when compensated with Mg and/or Mn.…”