Hard X-ray photoelectron spectroscopy probes deeper into InGaN epilayers

Indium gallium nitride (InGaN), a III-nitride semiconductor material, has various modern electronic and optoelectronic applications including in LEDs and solar photovoltaic cells. However, InGaN rich in indium (In) is difficult to make with high crystalline quality due to an extremely high nitrogen vapor pressure.

The crystalline quality of In-rich InGaN epilayers improved with the development of a method called radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), but the accumulation of surface electrons prevents the exploration of their intrinsic bulk properties. To this end, a new study published in the Journal of Applied Physics used angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-ray photoelectron spectroscopy (SX-PES) to probe In_0.7Ga_0.3N epilayers.

Although the chemical states and electronic structures of InGaN epilayers have been investigated by SX-PES, using HX-PES with a synchrotron radiation source has the benefits of both high resolution and increased probing depths. The authors employed both techniques to determine the magnesium doping effects on the surface and bulk electronic structures of In_0.7Ga_0.3N epilayers.

They compared three samples with different magnesium concentrations, including an unintentionally doped sample, grown by RF-MBE. All showed a large downward energy band bending as a result of the surface electron accumulation layer that formed, although it became larger for the magnesium-doped samples. Also, the valence band energy’s maximum only in the bulk tended to shift towards the Fermi energy level, whereas at the surface this maximum could not shift because of the existence of the surface electron layer.

The authors concluded that the downward energy band bending must occur due to the coexistence of a surface electron accumulation layer and a magnesium-doped p-layer formed in the bulk. As the magnesium concentration increased, the energy peak splitting occurred in the HX-PES spectra under the condition sensitive to the bulk. Thus, they discovered, the energy band formed the anomalous downward bending structure with a singular point due to the generation of the depleted region.

Source: “Surface and bulk electronic structures of unintentionally and Mg doped In_0.7Ga_0.3N epilayer by hard X-ray photoelectron spectroscopy,” by Masataka Imura, Shunsuke Tsuda, Hiroyuki Takeda, Takahiro Nagata, Ryan G. Banal, Hideki Yoshikawa, AnLi Yang, Yoshiyuki Yamashita, Keisuke Kobayashi, Yasuo Koide, Tomohiro Yamaguchi, Masamitsu Kaneko, Nao Uematsu, Ke Wang, Tsutomu Araki, and Yasushi Nanishi, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5016574 .