Emission Mechanism of High Al-content AlGaN Multiple Quantum Wells

紫外LED的发光功率和效率还远不能令人们满意,波长短于300 nm的深紫外LED的发光效率普遍较低。厘清高Al组分Al Ga N多量子阱结构的发光机制将有利于探索改善深紫外LED的发光效率的新途径、新方法。为此,本文通过金属有机气相外延技术外延生长了表面平整、界面清晰可辨且陡峭的高Al组分AlGa N多量子阱结构材料,并对其进行变温光致发光谱测试,结合数值计算,深入探讨了Al Ga N量子阱的发光机制。研究表明,量子阱中具有很强的局域化效应,其发光和局域激子的跳跃息息相关,而发光的猝灭则与局域激子的解局域以及位错引起的非辐射复合有关。The quantum efficiency of deep UV light emitting diodes( LED) drops dramatically with the increasing of Al content. Understanding the emission mechanism of high Al-content Al Ga N multiple quantum wells( MQW) is the one of the most important objects for improving the quantum efficiency of deep UV LED. In this work,high Al-content Al Ga N MQW structure with atomically flat hetero-interfaces was grown and characterized by photoluminescence( PL) measurements at different temperatures. The results indicate that there is a strong exciton-localization effect in the MQW structure and the emission is closely related to the hopping of the excitons. Due to the exciton delocalization and nonradiative recombination at defects,the PL intensity is strongly quenched at high temperatures.“973”国家重点基础研究发展计划(2012CB619300);; “863”国家高技术研究发展计划(2014AA032608);; 国家自然科学基金(U1405253;61227009;11204254;11404271);; 福建省自然科学基金(2015J01028)资助项

Phase Segregation of ZnO/ZnMgO Superlattice Affected by Ⅱ-Ⅵ Ratio

在zn1-X Mg X O中,X=0.4~0.6仍为一个岩盐矿和纤锌矿共存的结构,影响了其晶格质量。本文利用等离子体辅助的分子束外延设备在C面蓝宝石衬底上外延生长了znO/znMgO超晶格,并改变其生长过程中的Ⅱ-Ⅵ比,利用原子力显微镜、X射线衍射、透射谱和X射线光电子能谱对样品进行了表征分析。发现在较低氧分压下制备的样品结构以岩盐矿为主导,而在较高氧分压下两相共存并以纤锌矿为主。这种相分离现象与裂解氧原子的密度有关。ZnMgO alloy allows for tunable optoelectronic devices.However,the compositional range between ZnO and MgO is interrupted by a crystalline miscibility gap where the wurtzite crystal structure of ZnO is structurally incompatible with the rocksalt structure of MgO.In this article,ten periods of ZnO / ZnMgO superlattice were produced by plasma-assistant molecular beam epitaxy on cplane sapphire substrate with different oxygen condition.It is found that the sample grown at lower oxygen flow and radio-frenquency( RF) plasma power tends to form rocksalt phase.With the increase of oxygen flow and RF plasma power,wurtzite phase tends to dominate and phase segregation is enhanced.The phase transform affected by the oxygen atoms density is reasoned by the formation enthalpies of ZnO and MgO.国家自然科学基金(61076084)资助项

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films

The Structure Properties of Mg_xZn_(1-x)O

采用第一性原理计算模拟了不同组分的MgxZn1-xO半导体混晶的晶格常数、总能、结构,以及禁带宽度的变化。计算结果显示,随着Mg组分的增加,晶格常数逐渐减小,晶体逐渐偏离纤锌矿结构。对各种不同的Mg原子排列情况进行比较认为,MgxZn1-xO的结构随组分x的增大,发生从纤锌矿到岩盐矿的结构相变的可能性高于发生相分离。另一方面,禁带宽度随组分增大主要由价带顶的移动所致。进一步分析Mg原子各种电子态对价带的影响表明,Mg对价带顶附近能带的贡献依次来自p、d、s态电子。随着组分x的增加,p态电子在价带顶附近的密度明显提高,说明sp轨道杂化不但对晶体的几何结构产生影响,而且对其电子结构也起重要作用。A first principles calculation is used to simulate the changes of the lattice constant,band gap,total energy and structure of MgZnO semiconductor alloys with the different MgO mole fractions.The calculated results show that the lattice constant diminishes and the crystal structure departures gradually from the wurtzite type as the MgO mole fraction increases.By comparison with the different positions of the Mg atom,it is suggested that the structure of Mg_xZn_ 1-xO alloy will change and the possibility of the phase transition will be higher than that of the phase separation as the MgO mole fraction increases.On the other hand,when the MgO mole fraction is increased,the band gap enlarges,which is mainly attributable to the shift of the top of the valence band away from the Fermi level.By construing the influence of all kinds of electronic states in Mg atom on the valence band,it shows that the contribution of Mg to the energy band near the top of the valence band takes turn from its p,d,s states.The density of the p states near the top of the valence band enhances prominently when the MgO mole fraction is increased.It indicates that the sp hybridization not only affects the structure of the crystal but also plays an important role for its electronic structures.国家自然科学基金(60376015,90206030,60336020,10134030);; 国家“973”计划(001CB610505);; 福建省科技项目(2004H054,E0410007)资助项目~

Parametric optimum design of a photon-enhanced thermionic solar cell

National Natural Science Foundation [11175148]; 973 Program, People's Republic of China [2012CB619301]A photon-enhanced thermionic solar cell (PETSC) is a new type of solar energy conversion device that simultaneously uses light and heat from the sun. In this paper, a PETSC model is briefly described, which includes the thermal radiation between the cathode and the anode and the heat losses from the two electrode plates to the environment. Analytic expressions are derived for the power output and overall efficiency of the PETSC. Energy balance equations are used to calculate the operating temperatures of the cathode and anode electrodes for given values of the band gap energy and electron affinity of the cathode material. The performance characteristics of the PETSC are determined by numerical calculation. The maximum efficiency of the PETSC is clearly larger than that of traditional thermionic devices or solar cells. The optimal values of the band gap energy and electron affinity of the cathode material at the maximum efficiency are determined, and consequently, the operating temperatures of the cathode and anode electrodes can be optimally designed. The results obtained in this study may provide guidance for the appropriate selection of cathode materials and the optimum design of practical PETSC devices. (C) 2013 Published by Elsevier B.V

Symmetrically abrupt GaN/AlGaN superlattices by alternative interface-interruption scheme

973 program [2012CB619301, 2011CB925600]; "863" program [2011AA03A111]; FRFCU [2012121011, 2011121042]; National Natural Science Foundation; Science & technology program of Fujian of China; Science & technology program of Xiamen of ChinaWe report an alternative interruption scheme to effectively improve the abruptness of GaN/AlGaN superlattices by minimizing the asymmetric feature of different types of heterointerfaces. It is found by x-ray diffraction that the interface abruptness is degraded and the GaN thickness is reduced with the interruption time increasing. Detailed investigation with scanning transmission electron microscopy demonstrates that the Al diffusion and the interface etching effect at the GaN/AlGaN interface are the critical reasons leading to the interfacial asymmetry. An alternative interface-interruption scheme is then proposed to enhance the abruptness of the superlattice interfaces, and consequently, the emission efficiency can also be significantly enhanced

Strain State of MOCVD InN/GaN Quantum Wells

采用MOCVd外延生长11周期Inn/gAn量子阱结构样品,原子力显微镜表面形貌结果显示实现了台阶流动生长模式.通过高分辨率X射线衍射与掠入射X射线反射谱技术获得了阱层与垒层的实际厚度.从(102)非对称衍射面与(002)对称衍射面的倒异空间图,确认了Inn阱层处于与gAn共格生长的完全应变状态,获得了gAn缓冲层的晶体质量信息及其C轴与A轴晶格常数,确认外延层因受衬底热失配的影响处于压应变状态.Step flow growth has been observed on the surface morphology of eleven periods InN /GaN quantum well structure grown by MOCVD.The high-resolution x-ray diffraction results have been combined with reflectivity data to evaluate the layer thickness of InN and GaN.The asymmetric(102) reflection and symmetric(002) reflection reciprocal space map show that a-axis lengths of both InGaN well layers and GaN templates are identical.It is confirmed that InN / GaN MQWs structure of the sample is grown coherently on GaN templates.The c-axis and a-axis lengths of GaN buffer layer show that it is compressed,due to the thermal strain which is originated from the difference in thermal expansion coefficients between GaN epilayers and sapphire substrates during cooling from the growth temperature to room temperature.福建省教育厅科技项目(JA10249);厦门市科技计划高校创新项目(3502Z20093039