Ag3PO4(111)清洁面的原子弛豫对其体系能量及电子结构的影响

采用基于密度泛函理论的第一性原理GGA+U方法,研究了Ag3PO4(111)面的原子弛豫对其体系能量及电子结构的影响.发现Ag3PO4(111)面原子弛豫后,表面两个三配位的Ag原子均向里移动超过0.06 nm,而表面次层的O原子则均向外移动0.004 nm,导致弛豫后暴露在最表面的是O原子.而且弛豫使表面原子的核外电荷向表面内部发生了一定程度转移,表面键的杂化作用使共价左右增强,表面键长缩短.计算出的能带结构和态密度显示出原子弛豫窄化了价带宽度0.15 eV,增加了带隙宽度0.26 eV.同时未弛豫结构中存在于价带顶和导带底的两个表面峰在弛豫后完全消失,弛豫使Ag3PO4表面从金属特征向半导体特征转变.     

应力条件下GaN电子结构及光学性质研究

采用密度泛函理论框架下的第一性原理平面波计算方法,系统研究了不同应力作用下GaN的电子结构和光学性质,对比分析了外压调制对GaN的能带结构、态密度和光学性质变化的影响。计算结果表明:随着外应力的逐渐增加,Ga—N的键长逐渐变小,布局数逐渐增加,共价性明显增强,离子性减弱。电子结构计算结果显示导带向高能方向漂移,而整个价带向低能方向漂移,禁带带宽明显被展宽,Ga原子的3d态电子与N原子的2p态电子的杂化程度增强。光学性质计算结果揭示了在没有应力的作用下,在1.6 eV附近开始出现吸收边。随着外应力的逐渐增大,GaN的复介电函数和吸收谱向高能方向漂移,光谱发生了明显的蓝移现象,进而提高了光电转换效率。     

闪锌矿GaN(001)表面的电子结构

采用混合基表示的第一原理赝势方法,计算了闪锌矿结构的GaN(001)(1×1)干净表面的电子结构.分析了得到的各原子分态密度、面电荷密度分布以及表面能带结构等性质,比较了GaN(001)的Ga端表面和N端表面两种情况.结果显示,闪锌矿GaN(001)的Ga端表面比N端表面更稳定,这两种(1×1)表面都是金属特性.此外,还讨论了次表面层原子的性质.     

氮掺杂对4H—SiC电子结构的影响

采用广义梯度近似的密度泛函理论方法计算了4H—SiC的本征态的电子结构以及4H—SiC材料N掺杂后的电子结构,计算结果表明：与本征态相比,N掺杂后的4H—SiC的导带与价带均向低能端移动,导带移动幅度大于价带,使得掺杂后的禁带宽度小于本征态的禁带宽度：导带底进入N的2s态和2p态,但它们所占比重小,掺杂浓度变化对导带底...     

基于第一性原理的Ga-Eu共掺ZnO光电性质的研究

采用基于密度泛函理论的第一性原理方法,分析了Ga-Eu共掺杂ZnO(GEZO)结构的能带结构、态密度、马利肯布居分布以及光学性质。结果表明,计算得到的晶格常数及带隙与实验值一致。Ga,Eu的掺入贡献了导电载流子,使体系的电导率增强。费米能级进入导带,呈现n型导电。从态密度中可知费米能级处出现了由Eu的4f态引入的杂质带,Ga也在导带底处贡献了4p和4s态。原子和键的平均马利肯布居分布表明,Ga,Eu原子的掺入增强了键的离子性。光学性质方面,Ga,Eu的掺入使得介电函数实部和虚部的峰位向低能区转移,吸收率和反射率在可见光区均有提高。     

闪锌矿GaN(001)表面的电子结构

采用混合基表示的第一原理赝势方法 ,计算了闪锌矿结构的 Ga N(0 0 1) (1× 1)干净表面的电子结构 .分析了得到的各原子分态密度、面电荷密度分布以及表面能带结构等性质 ,比较了 Ga N(0 0 1)的 Ga端表面和 N端表面两种情况 .结果显示 ,闪锌矿 Ga N(0 0 1)的 Ga端表面比 N端表面更稳定 ,这两种 (1× 1)表面都是金属特性 .此外 ,还讨论了次表面层原子的性质     

半导体纳米颗粒载流子的超快弛豫过程

分析了纳米颗粒的能级结构,建立了载流子弛豫的简化模型,运用数值模拟方法讨论了激发密度、表面态密度及俘获态电子的弛豫率对弛豫过程的影响。讨论结果表明,激发密度的增大及表面态的减少都会导致表面态上电子的饱和,使导带上出现电子的积累,导带电子寿命增大;深俘获态电子的弛豫是影响材料响应速度的主要因素。最后应用此模型对近红外泵浦探测实验的结果进行分析,表明模型可望在实验结果分析上得到应用。     

GaInAs／AlInAs调制掺杂结构材料的光学性质研究

研究了ＧａＩｎＡｓ／ＡｌＩｎＡｓｎ型调制掺杂结构样品的光致发光及其激发光谱。当空穴态被局域化后．二维电子气的发光线形反映了导带二维态密度的填充效应：导带两个子带填充电子。发光强度则表明，导带第二子带电子波函数在空间上更扩展，与空间分离的空穴产生发光复合的几率较大。激发光谱提供了样品中异质结结构直接带边附近光吸收过程的信息。     

含空位的二维GaN电子结构和光学性质的第一性原理研究

基于密度泛函理论,计算了二维GaN及其Ga、N空位体系的电子结构和光学性质,通过形成能的计算分析了空位缺陷体系的稳定性,然后进一步计算了各体系的电子结构,分析并讨论了空位缺陷对吸收光谱的影响。计算结果表明:Ga-N空位体系形成能最小,该结构最容易形成;Ga空位体系产生的缺陷能级使二维GaN呈现p型半导体特性,反之N空位缺陷呈现n型半导体特性,缺陷能级的出现有利于提高二维GaN电子迁移率以及光响应能力;各空位体系的吸收光谱均发生红移,其吸收系数在低能区域均大于本征二维GaN,这说明Ga、N空位的产生可以提升二维GaN对可见光的吸收能力。     

p型InGaN/GaN超晶格N面GaN基LED的光电特性

随着氮(N)面GaN材料生长技术的发展,基于N面GaN衬底的高亮度发光二极管(LED)的研究具有重要的科学意义.研究了具有高发光功率的N面GaN基蓝光LED的新型结构设计,通过在N面LED的电子阻挡层和多量子阱有源层之间插入p型InGaN/GaN超晶格来提高有源层中的载流子注入效率.为了对比N面GaN基LED优异的器件性能,同时设计了具有相同结构的Ga面LED.通过对两种LED结构的电致发光特性、有源层中能带图、电场和载流子浓度分布进行比较可以发现,N面LED在输出功率和载流子注入效率上比Ga面LED有明显的提升,从而表明N面GaN基LED具有潜在的应用前景.     

Effects of surface treatment using aqua regia solution on the change of surface band bending of p-type GaN

Effects of surface treatment on the change of band bending at the surface of p-type GaN were studied using synchrotron radiation photoemission spectroscopy, and the results were used to interpret the reduction of contact resistivity by the surface treatment. The contact resistivity on p-type GaN decreased from (5.1±1.2)×10⁻¹ to (9.3±3.5)×10⁻⁵Ω cm² by the surface treatment using aqua regia prior to Pt deposition. Surface band bending was reduced by 0.58 eV and 0.87 eV after the surface treatments by HCl and aqua regia solutions, respectively. The atomic ratio of Ga/N decreased as the photoelectron detection angle was decreased, indicating that the surface oxide was mainly composed of Ga and O, GaO_x, formed during high-temperature annealing for the generation of holes, and Ga vacancies, V_Ga, were produced below the GaO_x layer. Consequently, the aqua regia treatment plays a role in removing GaO_x formed on p-type GaN, leading to the shift of the Fermi level toward the energy levels of V_Ga located near the valence band edge. This causes the decrease of barrier height for the transport of holes, resulting in the good ohmic contacts to p-type GaN.     

氮化镓注镁(Mg:GaN)的光致发光

利用低压MOCVD在蓝宝石衬底上外延生长了GaN,用离子注入法掺入Mg杂质,退火后,进行光致发光测量,观察到显著的蓝光发射和黄带发射.光谱分析给出了与注入Mg离子相关的GaN禁带中能级的精细结构,其中: 间位Mg(Mgi)能级(导带下170meV)到替位Mg(MgGa)受主能级(价带上250meV)的跃迁产生了415nm发光峰; 该能级到价带上390meV能级的跃迁,以及带有紧邻N空位的替位Mg(MgGaVN)能级(导带下310meV)到 MgGa受主能级的跃迁,均产生了438nm发光峰.另外,退火使GaN晶格结构部分恢复,再现了黄带发射.     

GaN/AlGaN超晶格透射电镜分析

用MOCVD法生长了120周期的GaN/Al0.14Ga0.86N超晶格，激光剥离技术被有效地用于GaN/AlGaN超晶格截面透射电镜样品的制作. 用透射电子显微镜观察到了清晰的超晶格及晶胞周期结构，电子衍射也表明生长的超晶格样品质量较好. 在透射电镜图中看到了Al, Ga原子的聚居点，这些聚居点达到应力临界时将可能成为新缺陷的起点. 同时观察到GaN缓冲层中的线缺陷大多呈现弯曲的弧形，这是外延生长导致的.     

Origin of the anomalous trends in band alignment of GaX/ZnGeX_2(X = N,P, As,Sb) heterojunctions

Utilizing first-principles band structure method, we studied the trends of electronic structures and band offsets of the common-anion heterojunctions GaX/ZnGeX_2(X = N, P, As, Sb). Here, ZnGeX_2 can be derived by atomic transmutation of two Ga atoms in GaX into one Zn atom and one Ge atom. The calculated results show that the valence band maximums(VBMs) of GaX are always lower in energy than that of ZnGeX_2, and the band offset decreases when the anion atomic number increases. The conduction band minimums(CBMs) of ZnGeX_2 are lower than that of GaX for X = P, As, and Sb, as expected. However, surprisingly, for ZnGeN2, its CBM is higher than GaN. We found that the coupling between anion p and cation d states plays a decisive role in determining the position of the valence band maximum, and the increased electronegativity of Ge relative to Ga explains the lower CBMs of ZnGeX_2 for X = P, As, and Sb. Meanwhile, due to the high ionicity, the strong coulomb interaction is the origin of the anomalous behavior for nitrides.     

High-energy photoemission spectroscopy of ferromagnetic Ga1−xMnxN

Here we report investigation of valence band electronic states of ferromagnetic Ga_0.96Mn_0.04N by bulk-sensitive X-ray photoemission, which is realized at high flux X-ray undulator beamline BL29XU of SPring-8, at photon energy of 5.95 keV. We have observed that Mn doping introduces a new structure in the band gap region near the top of the valence band, and also a broader structure in deeper valence band region. Basing upon the first principle calculation, these structures are assigned as Ga 4s originated states, which are raised by hybridization between 3d orbitals of Mn with GaN host orbitals. The present result evidences the second nearest Ga bonds are affected by that Mn–N bond formation, suggesting the long-range interaction of Mn in this host material.     

GaAs/Si异质结的界面性质

本文应用普适参数紧束缚方法计算了GaAs/Si异质中界面组成、键能、自然键长、力常数以及键的弛豫等性质.由键能和力常数推断As和Ga原子均能在界面处与Si原子成键,但Si-As键更强一些,考虑到Si和Ga原子在界面的互扩散作用,提出了界面层的可能生长机理.作者认为界面层应由SiAs和GaSi组成的膺合金Ga_xA(?)_(1-x)Si构成,根据价带极大值随组分x的变化,指出x的范围处在0.1～0.6之间.由键的弛豫效应预示x的最佳范围在0.1～0.3之间.由此设想,通过控制组分,有可能使由晶格畸变产生的界面应力减至比较小,从而为消除因晶格畸变产生的应力缺陷给出了理论依据.     

蓝色发光GaN纳米晶和晶体的XAFS研究

利用XSAFS（X－射线吸收精细结构）方法研究六方的纳为晶和晶体GaN在78K和300K温度下Ga原子的局域配位环境结构,对于第一近邻Ga－N配位,纳米晶GaN的平均增长R、配位数N、热无序度σT和结构无序度σs与晶体GaN的相近,分别为0．194nm、4．0、0．0052nm、0．0007nm;当温度从78K增加到300K,GaN样品中Ga－N配位的σT增加不多,小于0．0005nm,表明第一近邻Ga－N配位的共价键作用力较强,几乎不受温度和晶体状态的影响。对于第二近邻Ga-Ga配位,R为0．318nm,纳米晶GaN的σs（0．0057NM）比晶体GaN的（0．001nm )大0．0047nm;在78K和300K时,纳米晶GsA样品的Ga-Ga位位的σT分别为0．0053nm和0.0085nm,这一结果表明Ga-Ga配位的σT受温度变化产生很大影响,纳米晶GaN中Ga原子的局域配位环境与晶体GaN的差别主要表现在第二近邻Ga-Ga配位的σs相对较大,可能是由于纳米晶GaN内部缺陷及存在较多的表面不饱和配位原子所致。     

Electronic structures and transport properties of BN nanodot superlattices of armchair graphene nanoribbons

The electronic and transport properties of embedded boron nitride(BN) nanodot superlattices of armchair graphene nanoribbons are studied by first-principles calculations.The band structure of the graphene superlattice strongly depends on the geometric shape and size of the BN nanodot,as well as the concentration of nanodots.The conduction bands and valence bands near the Fermi level are nearly symmetric,which is induced by electron-hole symmetry.When B and N atoms in the graphene superlattices with a triangular BN nanodot are exchanged,the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry.In addition,the hybridization ofπorbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level.Our results also show a series of resonant peaks appearing in the conductance.This strongly depends on the distance of the two BN nanodots and on the shape of the BN nanodot. Controlling these parameters might allow the modulation of the electronic response of the systems.