Weakly-Disordered Infrared Semiconductor Alloys By Photoluminescence Under Various Conditions

This work focuses on disorder effects on band-edge structures of Hg Cd Te and In Ga As, with photoluminescence(PL) which is sensitive to band-edges. Meanwhile,analysing the fine structures of band-edges for both materials.Infrared semiconducting materials are of critical importance. The mainstream infrared semiconducting materials belong to semiconductor alloys, on one hand rendering those materials the electronic structures related with ordered lattice structures, and on the other hand distinguishing them from normal semiconductors from the perspective of disorder effects on band-edges. Previously, most of the research interests are drawn on those electronic properties concerning ordered lattices; while the experimental studies on disorder effects are void. Experimental studies on the those weakly disordered materials may fulfil the gap of condensed matter researches in infrared spectral region, provide insights and accumulate data for engineering applications. Hg Cd Te and In Ga As are the most typical infrared semiconductor alloys,corresponding to infrared detection and emission respectively. Researches concerning disorder effects on both infrared materials are of great practical significance; By comparing those two distinct materials in the study, the generality of the analysis can be established, and boundaries of the perspectives setted.PL is a classical vehicle to characterize semiconductors. Due to the rapid intraband thermalization of carriers, PL is able to reveal information concerning semiconductor band-edges. According to the disorder theory of semiconductor, sub-stitutional disorder introduces band-tails into the gap, which overlapped with the analysing ability of PL. Therefore, most of the experimental results in this dissertation were obtained through PL measurements.Because of the strong disturbance of background environmental emission centering at 10 μm and severe reduction of infrared detector detectivity, previous studies on band-edge structures with PL were limited; In this work, modulated PL technique based on step-scan Fourier transform infrared spectrometer is applied to overcome those difficulties, by which affluent amount of data and information is collected.The combinations of PL with various physical conditions, especially strong magnetic field, provide additional dimensions for characterization, therefore physical pictures concerning band-edge structures of narrow gap semiconductor alloys are enriched.Subtopics are covered in the dissertation as below:(a) Weakly disordered alloy In Ga As is analysed through PL for understanding the effect of disorder on band-edge:(i) Extrinsic doped In Ga As sample is studied to exclude the possibility that S-shape PL energy evolution is from discrete band-edge structures, validating the ability of PL on characterizing the disordered aspects of In Ga As band-edge, and the transition of radiative SRH to non-radiative SRH process is suggested for complicated band-edges;(ii) In Ga As quantum well is used to eliminated the disturbance of composition and excitation non-uniformity along growth direction;(iii) By studying the PL evolution of In Ga As quantum well under various conditions, the contribution of band tail on dominant recombination channel is illustrated;(iv) Thereafter,increasing the complexity of alloy disorder by introducing Bi atoms into InGa As quantum well, the effects of Bi on decorating In Ga As band-edges are researched, and current theory of dilute bismide is compared with experiments.(b) After systematic analysis of In Ga As and related structures,two different PL line-shapes, i.e. the ones with resolvable fine band-edge structure and the one modulated by severe interference, are analysed, and the nature of interferencemodulated PL is assigned to localized states with low density. The two key factors for PL interference, i.e. clear interface and media transparency, are considered, and the clear interfaces are minor factors since they are nearly perfect under current fabrication procedures; the PL interference is correlated with excitation power, however, it’s confirmed that the interference pattern has no relevance with discrete fine band-edge structures. By demonstrating magnetic evolution of interference-modulated PL, it’s revealed that the interference pattern is a sign of distributed localized states in the gap-side of the band-edges, thus nailing down its band-tail nature. In the meanwhile,fine structures of discrete band-edges of Hg Cd Te under different fabrication conditions are resolved, and carrier freeze-out effects on PL are discussed.(c) The effects of strong band-tail of Hg Cd Te on Hg Cd Te/Cd Te heterostructures are studied, through which above-Hg Cd TE-bandgap PL components are discovered and analysed. By comparing several samples, the generality of the above-Hg Cd Te-bandgap PL in Hg Cd Te/Cd Te with disordered Hg Cd Te is established. By assessing the energy difference between normal Hg Cd Te PL energy and the high energy one, both valence and conduction band offset between Hg Cd Te and Cd Te is analyzed against Cd composition.