Interfacial-Mixing and Band Engineering Induced by Annealing ofCdS and a‑Ga2O3 n−n-Type Thin-Film Heterojunction and Its Impacton Carrier Dynamics for High-Performance Solar-BlindPhotodetection

Abstract

Heterojunctions of dissimilar materials are increasingly being used inoptoelectronics 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 betweenamorphous Ga2O3 and CdS, two intrinsically n-type materials, with high opticalabsorbance but different band gaps. The resultant heterostructure-based devices remainsolar-blind and outperform the singular bare photodetectors. To further improve upondevice 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 1order of magnitude along with an enhanced photocurrent. The response time of thedevices 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 annealedinterface, the two heterojunctions are compared using X-ray photoelectron spectroscopydepth profiling, and results show that the pristine heterostructure has a sharp interface whereas upon annealing, it leads to a sort ofdiffuse interface. This produces a reduced valence band offset, resulting in a change in the band alignment from type II to type I. Thecarrier dynamics across the two interfaces therefore changes and is further validated using Kelvin probe force microscopy. This studyreveals how the change at the interface by mere annealing can lead to a huge alteration in the band alignment and thus, the carrierdynamics, thereby completely altering the ultimate device performance.KEYWORDS: heterojunction, deep UV C, band alignment, interfacial studies, KPFM■ INTRODUCTIONHeterojunction-based photodetectors (PDs) are gainingimmense popularity in the field of research nowadays. Theamalgamation of the superior properties of two completelydifferent materials resulting in an enhanced performance hasadded to the charm of using dissimilar materials forheterostructures.1 Solar-blind PDs (SBPDs) are a specialclass of PDs responding to wavelengths of light less than 280nm, thereby leading to novel applications in military and civilareas such as missile tracking, non-line of sight communica-tions, ozone hole monitoring, flame detection, etc.2 Beinglimited by the small band gap of silicon, research is beingdirected to utilization of materials with higher band gaps forthe fabrication of these SBPDs. Gallium oxide is a wide bandgap semiconductor which is being researched as a potentialalternative to conventional Si as the active layer for suchPDs.2,3 The intrinsic solar-blindness, high chemical andthermal stability, high breakdown voltage, and excellentradiation hardness are some properties of Ga2O3 with anexceptional charm.4,5 The absence of p-type doping in galliumoxide has led to unipolar devices being fabricated.6 To improveupon the existing performance, fabrication of heterostructuresof Ga2O3 with various materials of different dimensionality anddifferent optoelectronic properties are being pursued.7−10Among this vast repository of the literature for hetero-structures, it becomes imperative to study what actuallyhappens at the core of these heterojunctions�the interface.Gallium oxide has previously been heterointerfaced withvarious materials such as p-type traditional oxides (NiO, CuO,etc.)11,12 for bipolar devices, 2D materials (MoS2, PtSe2,etc.)13−15 for enhanced charge carrier extraction, or evenfunctionalized with materials like quantum dots and nano-Received: April 21, 2023Accepted: June 16, 2023Published: June 30, 2023Articlepubs.acs.org/acsaelm© 2023 American Chemical Society 3798https://doi.org/10.1021/acsaelm.3c00527ACS Appl. Electron. Mater. 2023, 5, 3798−3808