Influence of Surface Treatments on the Ohmic Contact Performance on the N-Face of Iron-Doped Semi-Insulating Freestanding GaN

To achieve superior electrical performance in vertical GaN-based devices on iron-doped semi-insulating gallium nitride (SI-GaN:Fe) substrates, a profound comprehension of the ohmic contact on the N-face of SI-GaN:Fe is imperative. The low carrier concentration and high bulk resistivity of SI-GaN:Fe, together with the complicated surface states of N-face, result in an excessively elevated specific contact resistance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho _{\text {C}}$ </tex-math></inline-formula>), posing a significant barrier to the realization of optimal ohmic contact. This study focuses on the surface treatments on the N-face of SI-GaN:Fe to reduce <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho _{\text {C}}$ </tex-math></inline-formula> of ohmic contact on it. Surface band bending (BB), surface roughness, and oxidation are all considered to investigate the influence of surface treatments on the ohmic contact performance on the N-face of SI-GaN:Fe. Among various treatments, samples subjected to inductively coupled plasma (ICP) dry etching followed by a wet etching in hydrochloric acid solution (HCl:H2O =1:2) demonstrated the most pronounced reduction in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho _{\text {C}}$ </tex-math></inline-formula>. This is attributed to the surface BB after ICP etching, facilitating electron transition from the semiconductor to the metal. In addition, the N-face SI-GaN:Fe has a strong adsorption activity for oxygen, while the HCl solution effectively removes the surface GaOx layer and improves surface morphology, which is crucial for achieving ohmic contact. This study provides valuable insights into the fundamental physics of GaN ohmic contacts, thus enhancing the potential applicability of SI-GaN:Fe in vertical GaN-based devices.