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

Micro Characteristics of InGaN/GaN Quantum Wells

采用VASP程序包模拟计算InGaN量子阱的能带,精细展示了量子阱实空间能带结构。计算结果表明,In原子所在区域出现局域束缚态,导带底与价带顶的简并能级发生分裂,同时量子阱沿垂直结面方向存在分立的能级。此外,针对影响能带的In组分波动、能带弯曲等问题进行探讨,以准确描述其电子行为,从而深入系统地了解InGaN/GaN量子阱的电学光学等特性。InGaN/GaN quantum well exhibits a series of unusual optical properties.However,the physics of quantum well on the micro scale is subjected to large uncertainties.Little is known on the discontinuities of energy band structure and local states on the atomic level in the case of different In atoms distribution in the InxGa1-xN layers caused by In compositional fluctuation and phase separation,which intensively alters the optical and electronic pro-perties.Using the efficient and accurate total energy and molecular-dynamics package VASP which is based on the density functional method,we performed the first principles calculations on InGaN/GaN quantum wells.The calcu-lated results exhibit discrete bands around the conduction band minimum and the valence band maximum that vary with the In atom distributions.Moreover,the local states with the discrete bands are likely to appear the quantum confined Stark effect in the InGaN/GaN interface enhanced by the polarization field in the crystal.The control and further understanding of those micro characteristics may lead to the improved performance of InGaN QWs.国家重点基础研究发展规划(001CB610505);; 国家自然科学基金(90206030,60376015,60336020,10134030);; 福建省自然科学基金(E0410007,2004H054)资助项目~

Electronic Structures and Their Correlation with Optical Properties for TiN_x System

【中文文摘】本文使用LMTO-ASA能带方法和超原胞方法计算了Ti-N系统中三个稳定相(α-Ti,ε-Ti2N和δ-TiN)的电子结构,然后通过将这三个稳定相作为样本并借助于统计超原胞方法,本文全面计算了TiNx系统的电子结构。在这基础上,我们又进一步计算了TiNx系统的联合态密度,并利用所得结果初步探讨了TiNx系统的光学性质。通过与金的联合态密度的比较,我们得到了当组分x在0.4至0.6范围内,TiNx镀膜表观上可呈现金黄色。 【英文文摘】By means of LMTO-ASA method and the supercell approach, we have calculated the electronic structures of the three stable phases in Ti-N system, i.e., α-Ti, ε-Ti2N and δ-TiN. Then, we chose these stable phases as the samples of the Ti-N system and used the statistical supercell method to calculate the electronic structures of TiNx system. Furthermore, the joint density of states for TiNx system have been calculated and the results have been utilized to investigate the optical properties of TiNx system. In co...福建省自然科学基金(F99031)资助项

Growth Mechanism of Strained AlN Surfaces

Aln半导体通常在异质衬底上外延,生长过程中往往承受较大的晶格失配应力.基于第一性原理,研究了应变Aln表面生长基元,如Al和n原子以及Al-nn团簇的微观生长动力学.与非应变Aln比较显示,n原子形成焓有所提升,难以形成稳定吸附;而Al原子则由活跃转为稳定吸附.通过比较不同团簇表明,Al-n3团簇结合能远低于Al-n、Aln2和Al-n4团簇,最为稳定.进一步计算不同生长氛围下形成焓差异的结果显示,应变Al-n3团簇形成焓略有提升,但总体仍呈现稳定吸附的特性.基于各生长基元在不同生长氛围下应变Aln的动力学特性差异,设计了分层生长步骤.生长初期通过生长源的切换,分别提供有利于n原子和Al原子扩散的氛围,以形成均匀分布的晶核;再通过同时供给n和Al源形成Al-n3团簇,利用其跨度大、吸附稳定等特性,促进表层晶核间的跨越生长,以形成连续的二维外延层,为在大失配异质衬底上外延致密、平整、优质的Aln薄膜提供了新的方案.AlN,emerging as a promising semiconductor compound for ultraviolet optoelectronics is commonly heteroexpitaxially grown on sapphire suffering from large lattice mismatch.In this work,we study the growth mechanism of the growth units including Al,N,Al-N and Al-N cluster on strained AlN surface based on first principle simulations.Compared with unstrained AlN,the formation entropy(FE)of N atoms is increased,which is hard to stably absorb on the surface,while active Al atoms turn to be stable on the surface.By evaluating the binding energy,the energy value of Al-N3 cluster is found to be lower than that of Al-N,Al-N2 and AlN4,indicative of the most likely cluster structure among clusters as the growth unit.The calculation of the FE of Al-N3 shows that Al-N3 cluster turns to be the most stable growth units although its FE is slightly increased under different growth conditions.On account of the growth mechanism,we design hierarchical growth steps.At the initial stage,the favorable atmosphere is provided for N and Al atoms,respectively,in order to form a uniform nucleation.The simultaneous supply of N and Al sources yields Al-N3 cluster with large span and stable absorption,which facilitates the growth across the nucleuses so as to continuous 2Depilayer.Our present design gives a general theoretical background and a new guideline to stimulate experimental fabrication of AlGaN films with compact structure,atomic smoothness,and high quality.国家重点基础研究发展计划(973计划)(2012CB619300); 国家高技术研究发展计划(863计划)(2014AA032608); 国家自然科学基金(11404271); 福建省教育厅科技项目B类(JB13150

Si被注入Gd后的磁性及其整流特性的研究

采用离子束技术,在n型硅基片中注入稀土元素钆,制备了磁性-非磁性p-n结.磁性层Gd_xSi_(1-x)表现出优良的磁学性能,高居里温度,高原子磁矩(利用RKKY模型可以得到解释),低矫顽力,并保持着半导体的属性,磁性-非磁性p-n结具有整流特性,但没有观察到明显的磁电阻效应

离子束外延生长(Ga,Mn,As)的退火研究

利用俄歇电子能谱(AES)和X射线衍射(XRD)分析了室温条件下离子束外延生长Ga、Mn、As样品,在不同的温度条件下进行退火后组分和元素分布的变化.结果表明退火有助于样品内部元素的均匀分布,温度为400 ℃会导致MnO2和Ga5.2Mn的结晶