Chemical and Electronic Structure of Surfaces and Interfaces in Compound Semiconductors

摘要

The interface formation between two different materials is important in applications for optoelectronic devices. Often, the success or performance of these devices is dependent on the formation of these heterojunctions. In this work, the surface and interfaces in such materials for optoelectronic devices are investigated by a suite of X-ray analytical techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), and X-ray emission (XES) spectroscopies to provide novel insight.For the group III-nitrides (e.g., AlxGa1-xN) used in many light emitting devices, a significant challenge exists to form an Ohmic contact. The electron affinities and band gaps of GaN and AlN are different, and thus it is difficult to find one contact scheme compatible for the entire AlxGa1-xN system. Contact schemes are empirically derived such that they result in optimal electrical properties, and thus this work focuses on providing a deeper understanding of the empirically derived contact-schemes. For the ndoped alloys, the presence of VN was identified at the V-AlxGa1-xN interface after contact formation. The amount of VN present varied for n-GaN and n-AlN, and was indicative of the VN dependency of the n-AlxGa1-xN composition. These findings provide detailed insight into the contact formation of (Al,Ga)N-based devices and the performance of V-based contacts.Next generation thin film solar cells based on CdS/Cu(In,Ga)Se2 and CdTe/CdS heterojunctions, which are expected to replace the current Si-based technologies within a decade, are constantly driven to improve their device efficiencies. However, to optimize the entire device, the interfaces and layers within such a device must be understood. The interface formation between high-efficiency Cu(In,Ga)Se2 absorbers and CdS buffer layer was followed, and the findings suggest the presence of a S-containing interlayer between Cu(In,Ga)Se2 and CdS. For CdTe/CdS solar cells, post-absorber deposition processing (CdCl2 activation and back contact treatment) is necessary. The findings demonstrate that the CdCl2 activation drives the sulfur atoms from the CdS layer towards the back contact. While both of the processing steps influence the morphology of the back contact, the spectroscopic results suggest that the CdCl2 activation has a larger impact on the surface and interface composition involved in CdTe solar cells.The surface and interface structure are complex in these optoelectronic devices, and they are expected to influence the electrical properties (and thus performance) of the final device. The goal of this dissertation is to provide new insight and physical explanations which could aid in future optimization and designs of heterojunctions.