| Both Ⅲ-Ⅴ nitride semiconductor material (including AlN, GaN, InN and their alloys) represented by GaN and Ⅱ-Ⅵ compound semiconductor such as ZnO, ZnSe and ZnTe, have obtained a rapid development in the application of optoelectronic devices.Ⅲ-Ⅴ nitride can be used in high temperature, acid, alkali and adiation environment due to the excellent optoelectronic properties and chemical stability. They are direct band gap material and cover a large band gap range (from InN of0.7to AlN of6.28eV). As a result, they have a wide range of applications in blue, green and ultraviolet optoelectronic device. As a broad band gap semiconductor ZnTe is widely used in green devices and photovoltaic devices. It can serve as an excellent material for a green LED and the back contact layer CdTe solar cells. Meanwhile, because of excellent phase matching characteristics, ZnTe has become the most common and appropriate material of terahertz emitter and receiver. In recent years, the theoretical research and practical application of ZnTe-related materials develop very quickly. However, both the preparation of ZnTe epitaxial material and the application of the related devices exist many difficulties such as low optical performance, short service life and so on. All these problems are mainly caused by the quality of ZnTe material, such as difficult to obtain n-type ZnTe of good quality and reduce lattice and thermal mismatch between substrate and epilayer. Therefore, it is an important issue to explore the growth technology of high quality ZnTe and ZnTe-based heterostructure and seek characterization means of the structural quality on semiconductor optoelectronic field.In this paper, photoluminescence is used as the main measurement means with other measurement methods for the supplementary to investigate structural and optical properties of ZnT and GaN, such as X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy.ZnTe epilayers were grown by the home-built MOVPE at atmospheric pressure on (100) GaAs substrates with different growth temperatures. DMZn and DETe were used as source materials and H2as carrier gas. Polished (100) GaAs substrates were chemically cleaned, degreased in organic solvents, and then etched in a solution composed of H2SO4+H2O2+H2O (5:1:1) for about20s at60℃followed by deionized water rinse. In order to remove the native oxide on the surface of GaAs substrate, it was cleaned by thermal annealing at580℃for30min before the growth. The growth system was evacuated to the order of10-7Torr with a turbomolecular pump before the source materials and the carrier gas were bled into the chamber. The transport rates of DMZn and DETe were kept at15μmol/min. The substrate temperature changed from390to440℃. To discuss the substrate temperature dependence, the thickness of all ZnTe epilayers in our experiment was controlled at around10μmby changing growth time.Morphology, structural and optical properties of epitaxial layers were studied. For PL measurements, the samples were mounted in a closed-cycle He cryostate and the temperature was controlled from6to300K. The442nm line of a He-Cd laser was used as an excitation light source with the spot size of～250μm, and the excitation power changed from0.1μW to30mW. The PL results show that the substrate temperature affects both crystalline quality and optical properties of of ZnTe heteroepitaxial layer. The PL spectra of ZnTe epitaxial layer grown at between390and440℃are dominated by sharp excitonic emission at around2.371eV. Meanwhile, Y line, DAP emission peak and OBE transition are absent in all ZnTe epilayers. It indicates ZnTe epilayers have a good optical quality. However, by comparing the spectra of the epilayers, it is found that a too low (390℃) or too high (440℃) substrate temperature deteriorates epilayer quality. This is because the substrate temperature, especially the too low substrate temperature (390℃), may lead to the formation of defects or the inclusion of impurities easily. Moreover, the too high substrate temperature (440℃) also results in the diffusion of the As atom into the epilayer and the inclusion of impurities easily. The As atoms are found to mainly distribute near the epilayer/substrate interface. In contrast, a moderate substrate temperature (around420℃) is considered to be suitable to obtain high-quality ZnTe epilayer due to its narrowest linewidths of the excitonic emission peaks and strongest emission intensity among the epilayers. Further, XRC measurement results are consistent with PL measurement results. It indicates PL measurements can be used to characterize the quality of the epitaxial layer. The energy of PL peaks changes with the variation of excitation power and epitaxial layer thickness, which manifests that the stress in ZnTe epilayer reduces with increasing epilayer thickness.ZnTe homoepitaxial layers were prepared on (100) ZnTe substrate by MOVPE and the effect of reactor pressure on the optical properties of the epilayers were investigated. Experimental results show that reactor pressure have a significant effect on crystal quality and optical properties of the ZnTe homoepitaxial layer. The PL spectra of ZnTe epilayer grown at low reactor pressure (300Torr) were dominated by the free exciton recombination. The Y line, DAP emission peak and OBE deep-level transition were not observed in the spectra, showing that epitaxial layers are of good optical quality. For ZnTe epilayers grown at high reactor pressure (700Torr), the PL spectra were dominated by impurity-related emissions. The reactor pressure dependences of the PL spectrum and XRC of the ZnTe epilayer manifest that reducing reactor pressure results in the enhancement of the free excitonic emission relative to the impurity-related emissions as well as the decrease of XRC linewidth, indicating that the reducing reactor pressure can improve the PL property and crystallinity of ZnTe epilayer. The facts can be explained as that the adsorbed precursors have enough time to diffuse over the surface to find its proper position at low reactor pressure due to the low reaction rate of the precursors, thus facilitating recrystallization of the adsorbed precursors. At the same time, the growth process at low reactor pressure also results in the elimination of parasitic nucleations in the gas phase and the reduction of autodoping. In addition, we also compared the quality of ZnTe substrate and the homoepitaxial layer. The results show that ZnTe epitaxial layer grown at reactor pressure of700Torr has poorer quality compared with the substrate. However, the low reactor pressure (300Torr) is appropriate for the homoepitaxial growth with good crystal quality.GaN and related heterostructures have been studied in last decade due to their possible applications for optoelectronic devices performing in the spectral region from the blue to near-UV and in electronic devices such as high power, high temperature and high frequency transistor. Great efforts have recently been devoted to the understanding of its optical properties. However, the investigation on its fundamental band edge transitions, such as the band-to-band, exciton, and band-to-impurity transitions, are still not well understood due to the lack of high quality GaN crystals and epilayers. Therefore, it is vital to understand the fundamental properties of GaN and the design of optoelectronic devices through the preparation of high quality GaN related structure and the investigation of the luminescence mechanism.GaN epitaxial layers and AIN/GaN heterostructures were fabricated on c-plane sapphire substrates by MOVPE. We studied the properties of the sample using PL spectra, Raman spectroscopy and X-ray diffraction. According to power and temperature dependences of PL specrta of GaN epitaxial layer, we defined the origin of near band edge emission peaks. At low temperature, neutral donor bound exciton emission dominates the PL spectrum and free exciton fine structure can be observed, showing that GaN epitaxial layer has high crystal quality. Bound excitons release with increasing temperature, so its strength rapidly reduce. At the same time, the intensity of free exciton declines slowly than that of bound exciton due to larger binding energie. Above150K, free exciton dominated the PL spectra of GaN epilayer. We also estimate the stress in GaN epilayer and the results indicate that GaN epitaxial layer exists compressive stress derived from the thermal mismatch between the substrate and the epitaxial layer. Hall measurement results show that the2DEG density and Hall mobility of AIN/GaN heterostructure with3nm-AIN layer at room temperature are1013cm-2and1720cm2/V.s, respectively. At10K Hall mobility is as high as9500cm/V.s. For AIN/GaN heterostructure with6nm-AIN layer, although the2DEG density increases, the Hall mobility reduces significantly. This may be because the quality of AIN/GaN heterostructure deteriorates with increasing thickness of AlN layer. We also studied the effects of the thickness of AlN layer on optical properties of GaN layer. For AIN/GaN heterostructure with3nm-AIN layer, the PL spectrum is dominated by GaN band edge exciton emissions. However, AIN/GaN heterostructure with6nm-AIN layer, in addition to GaN band edge exciton emissions, strong DAP emission appears in the PL spectrum, showing the thickness of AlN layer affects the optical quality of GaN layer. The measurement results have an important significance to improve the design and performance of AIN/GaN-based electronic devices. |
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