Heavily doped N-polar GaN with a mirror-like surface achieved via Ge doping

Abstract

We used metal organic chemical vapor deposition (MOCVD) to grow germane (GeH₄) doped N-polar GaN on sapphire substrates with misorientations of 2° and 3° off A-plane (2A and 3A). At a GeH₄ doping flow rate of 156 nmol min⁻¹, a carrier concentration of 1.46 × 10²⁰ cm⁻³ was achieved, and the surface morphology and crystal quality show almost no deterioration compared with undoped N-polar GaN (RMS = 0.821 nm and electron mobility μ = 89 cm² V⁻¹ s⁻¹). As the GeH₄ flow rate increases, the doping concentration increases while the mobility decreases, and the surface morphology and crystal quality are less affected. However, if an excessively high doping flow rate is applied, the crystal quality will deteriorate, and pits will appear on the surface. The results of Raman spectroscopy show that as the doping flow rate increases, the compressive stress first increases and then decreases. We compared the GeH₄ doping of N-polar, Ga-polar, and non-polar GaN. At the same n++ doping level, only the N-polar GaN can maintain excellent crystal quality and surface undulation, which has important application potential for devices requiring heavily doped layers in the future.