Font Size: a A A

Molecular Beam Epitaxial Growth And Mechanism Of Cadmium Telluride Single Crystalline Films On Oxide Substrate

Posted on:2017-09-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:K TangFull Text:PDF
GTID:1318330512457590Subject:Physical Electronics
Abstract/Summary:PDF Full Text Request
Cadmium telluride (CdTe) is one of the most important II-VI semiconductors, which has been widely used as the substrate material for HgCdTe-based infrared array sensors, the absorber material for nuclear radiation detectors, and for thin film photovoltaic (PV) devices, and so on. Recently, a breakthrough has been made in the conversion efficiency of CdTe thin film solar cell devices to up to 21.5% by First Solar, however, which is far less than 32% of the theorical efficiency. The low open-circuit voltage and fill factor which is limited by the short minority carrier lifetime is the critical problem for the low conversion efficiency. The main process that limits the minority carrier lifetime in CdTe is correlated to the non-radiative recombination at grain boundaries and/or the point defects, such as Te vacancy (VTe) and Cd vacancy (VCd). Previous works and theories indicated that the minority carrier lifetime was reported to be prolonged in Te-rich CdTe samples because of VTe is the main deep level defect. However, recent experiments and theories indicate that the minority carrier lifetime of the Cd-rich samples is superior to that of the Te-rich ones. And the crystalline quality of the CdTe material is also improved significantly for samples grown with a Cd-rich ambient. Therefore, it is still controversial whether the conversion efficiency of CdTe-based solar cells can be promoted in either a Cd-rich or Te-rich ambient. Consequently, it is essential to distinguish the defect structure of CdTe with Cd-rich and Te rich state and the correlation between recombination and minority carrier lifetime which is crucial for improving the conversion effeiciency of CdTe thin film solar cells.It is difficult to grow large size and defect-free single crystalline CdTe bulk because of some inherent shortcomings in CdTe, such as frangibility, self-compensation effect, antisite defects, vacancy defects and so on, which limit its commercial applications. Thus, CdTe films have been tried to grown on semiconductors, such as Si, Ge, GaAs substrates, which have obtained some breakthroughs to achieve devices application. Meanwhile, with the development of micro and multifunctional devices, the integration of semiconductor and oxide heterojunction have attracted considerable attentions in recent years. And semiconductors grown on SrTiO3 (STO) substrates are useful to realize the compatibility between the colossal magnetoresistance, high temperature superconductivity, multiferroic behaviors and semiconductor properties, which will expand the functions of the materials and provide more variances for designing the devices. Though the epitaxial mechanism of the heterojunction is still unclear, the perovskite oxides, such as STO, LaxSr1-xMnO3 and BaTiO3, have been succeeded to epitaxially grown on the Si and GaAs substrates. Similarly, the ?, III-V and II-VI semiconductors, such as Ge, InP, GaAs, ZnSe and so on, have also been grown on STO substrates.Based on the above consideration, we have grown CdTe epilayers (CTEL) with different stoichiometry on CZT substrates. On the basis, we tried to grown CTELs on different crystal orientational STO substrates and explored the growth mechanism of the CdTe/STO heterojunction. Main results have been obtained as follow:1. We explore and optimize the epitaxial technology in growning different stoichiometry CTELs on CZT by molecular beam epitaxy (MBE) and summarize the growth mechanism. The different defect states of CTELs can be controlled by Cd-rich or Te-rich atmosphere.2. Low-temperature photoluminescence (PL) spectra were used to study the defect levels of the CTELs with different stoichiometry, which viries the VTe is a shallow donor level. The longest minority carrier lifetime was determined in Cd-rich samples by time resolved photoluminescence (TRPL) spectra. And the dominant deep recombination centers in CdTe is Te antisite and Te interstitial which limited the minority carrier lifetime.3. CdTe single crystalline films have been succeeded to grown on STO (111) and STO (001) substrates by MBE, respectively. It is found that the mechanism of CdTe grown on the STO (111) substrate is traditional epitaxial way with large lattice mismatch which usually relaxes the stress by a thick stress layer, however, the mechanism of CdTe grown on the STO (001) substrate is "Compliant Epitaxy" which adjusts the mismatch by an ultra-thin interface layer.4. Low-temperature transmittance spectra and photoluminescence spectra were used to characterize the optical properties of CTELs grown on STO (111) and STO (001), respectively which illustrates the different growth mechanism of CTELs grown on STO (111) and STO (001) substrate.
Keywords/Search Tags:Single Crystalline CdTe Epilayer, Molecular Beam Epitaxy, "Compliant Epitaxy", Large Mismatch Substrate, Complex Interface
PDF Full Text Request
Related items