The Band Alignment And Mid-infrared Luminescence Properties Of â…£-â…¥Semiconductor Heterostructures

IV-VI compounds, including PbSe, PbTe and PbS, are narrow and direct bandgap semiconductors, which have been widely used in the fabrication of mid-infrared opto-electronic devices. Such devices are simply structured, high optoelectronic efficiency and low cost, which make them potential candidates for commercial applications. This thesis studies on the physical properties of Ⅳ-Ⅵ semiconductor heterostructures, including two themes. One is the study of band aglinments of Ⅳ-Ⅵ heterostructures by synchrotron radiation photoelectron spectroscopy (SRPES). The heterostructures studied in this work are formed by IV-VI semiconductors and other semiconductors, including CdTe, Ge, and ZnO. The other is the study of mid-infrared luminescence properties of Ⅳ-Ⅵ heterostructures by photoluminescence (PL) spectroscopy. The surface plasmon coupling effects in mid-infrared region are studied in this theme. The main innovations are as follows:1. The energy band structure with type-I alignment at the interface of PbTe/CdTe (111) heterostructure is determined by SRPES. The valence band offset and conduction band offset are acquired to be0.09eV and1.19eV, respectively. The results are in agreement with the theoretical prediction.2. PbTe films were grown on Ge (100) substrates using a molecular beam epitaxy system. The band alignment of PbTe/Ge (100) heterostructure was studied by SRPES. A valence band offset of△EV=0.07eV, and a conduction band offset of△EC=0.27eV are concluded. The conduction band offset is consistent with Anderson’s law.3. The band alignments of ZnO/PbSe heterostructures were studied by SRPES, including ZnO/PbSe film and ZnO/PbSe nanocrystal (NC) heterostructures. A type-Ⅰ band alignment with the valence band offset of△EV=0.73eV and the conduction band offset of△EC=2.36eV is concluded for ZnO/PbSe film heterojunctions. The band structures of ZnO/PbSe NC depend on the size of PbSe NC. The critical size of PbSe NC is4.8nm. Type Ⅱ alignment is formed at the interface of ZnO/PbSe NC when the size of PbSe NC is smaller than4.8nm. Type I alignment is formed at the interface when the size of PbSe NC is larger than4.8nm.4. The luminescence properties of CdTe/PbTe (111) heterostrtucture have been studied. Intense PL enhancement was observed in the CdTe/PbTe heterostructure, which is attributed to the coupling of localized surface plasmons (SPs) and mid-infrared photons. Hall effect measurement indicated that electron gas is localized at the interface of the CdTe/PbTe heterostructure. The electron gas can act as a supporting layer for SPs. And the coupling of mid-infrared photon and localized SP modes increases the radiative efficiency of dipoles in the PbTe layer, and thus enhances luminescence intensity.5. The luminescence properties of Ag/PbTe heterostructures have been studied. Great enhancement of PL intensity has been found in the Ag/PbTe heterostructure. An SP related model has been proposed to interpret the observed phenomena. In a Ag/PbTe heterostructure, mid-infrared photons can couple into propagating SP modes through surface modulations on the PbTe epilayer. The PL intensity enhancement caused by SPs-photons coupling depends on two factors:1. The enhancement factor of SP.2. The coupling coefficient between SPs and photons. Propagating SP modes have smaller enhanement factor compared to the localized SP modes, but have much larger coupling coefficient since the smaller difference between mid-infrared photons and propagating SPs. Besides, in a Ag/PbTe heterostructure, the totally reflected photons in the PbTe layer have much higher efficiency coupling to propagating SP modes than the emitting photons, which means that the totally reflected photons will couple into SP modes, enhance radiative recombination efficiency of dipoles in the PbTe layer, and thus enhance the emitted photon.