High performance GZO/p-Si heterojunction diodes fabricated by reactive co-sputtering of Zn and GaAs through the control of GZO layer thickness

Abstract

The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn–GaAs target is investigated. GZO films were deposited at 375 °C with 0.5% GaAs area coverage of Zn target and 5% O₂ in sputtering atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy show that c-axis orientation of crystallites, Ga/Zn ratio and oxygen related defects depend substantially on the thickness of films. The 200–350 nm thick GZO films display low carrier concentration ∼10¹⁷ cm⁻³, which increases to >10²⁰ cm⁻³ for thicker films. The diodes fabricated with >500 nm thick GZO layers display non-rectifying behaviour, while those fabricated with 200–350 nm thick GZO layers display nearly ideal rectification with diode factors of 1.5–2.5, along with, turn-on voltage ∼1 V, reverse saturation current ∼10⁻⁵ A, barrier height ∼0.4 eV and series resistance ∼200 Ω. The drastically improved diode performance is attributed to small Ga/Zn ratio (∼0.01) and extremely low dopant activation (∼0.3%), owing to diffusion and non-substitutional incorporation of Ga in thin GZO layers, which cause self-adjustment of doping concentration. These factors, together with c-axis orientation and chemisorbed oxygen at grain boundaries, facilitate ideal diode characteristics, not reported earlier for GZO/p-Si heterojunctions.