Interfacial-Mixing and Band Engineering Induced by Annealing of CdS and a-Ga2O3 n–n-Type Thin-Film Heterojunction and Its Impact on Carrier Dynamics for High-Performance Solar-Blind Photodetection

Heterojunctions of dissimilar materials are increasingly being used in optoelectronics for their superior properties. However, the heart of the heterojunction─its interface─and its impact on the device performance are seldom studied in detail. Herein, we report on the band alignment modification of heterojunction formed between amorphous Ga2O3 and CdS, two intrinsically n-type materials, with high optical absorbance but different band gaps. The resultant heterostructure-based devices remain solar-blind and outperform the singular bare photodetectors. To further improve upon device performance, the heterostructure is subjected to a moderate annealing of 300 °C. The annealed heterojunction device shows a reduction in dark current by more than 1 order of magnitude along with an enhanced photocurrent. The response time of the devices reduces from 1.35 s/2.87 s (rise/fall time) to about 0.38 s/0.75 s upon annealing. To study this change in the device performance between the pristine and the annealed interface, the two heterojunctions are compared using X-ray photoelectron spectroscopy depth profiling, and results show that the pristine heterostructure has a sharp interface whereas upon annealing, it leads to a sort of diffuse interface. This produces a reduced valence band offset, resulting in a change in the band alignment from type II to type I. The carrier dynamics across the two interfaces therefore changes and is further validated using Kelvin probe force microscopy. This study reveals how the change at the interface by mere annealing can lead to a huge alteration in the band alignment and thus, the carrier dynamics, thereby completely altering the ultimate device performance.