Origin of Black Color in Heavily Doped n‐Type GaN Crystal

In semiconductor materials, doping is used mainly for controlling the electrical properties. There have been attempts to grow low‐resistivity n‐type gallium nitride (GaN) crystals by doping oxygen, germanium, and silicon, because a low‐resistivity GaN substrate is required to reduce the power losses of optical and electrical devices. However, in those efforts, the crystal color turns black with the increase in the concentration of the n‐type additives, even though they are shallow donors. Herein, it is explained why heavily doped n‐type GaN crystals exhibit low transparency. From optical absorption profiles, the appearance of a band tail from the band edge to 1.5 eV is observed. Considering the band tail theory and our observations, it is concluded that Ga vacancy or Ga vacancy complexes behaving as acceptors induce the band tail and the black color. It is proposed that neutralizing the high charge of defects ensures that low‐colored GaN crystals with low resistivity can be obtained. Moreover, the fabrication of low‐resistivity wafers sliced from a large crystal with a laser produces inexpensive wafers and allows the spread of high‐efficiency GaN devices fabricated on low‐resistivity substrates for saving electric power.