Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_x Zn_(1-x) O Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg x Zn 1-x O cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg x Zn 1 x O QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L 3.5 nm, but the interband emission wavelength has a red-shift when L 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

溶胶-凝胶法制备MgxZn1-xO薄膜的结构及光学特性

用溶胶-凝胶法制备了一系列的Mg_xZn_1-xO(0≤x≤0.3)薄膜,并用X射线衍射(XRD)和光致发光(PL)研究了不同的退火温度和Mg的掺杂含量对Mg_xZn_1-xO薄膜的结构和光学性质的影响.研究表明:Mg_xZn_1-xO薄膜为单相六角纤锌矿结构,并且具有沿c轴的择优取向;发现了中间热处理温度为350℃的Mg_xZn_1-xO薄膜退火温度的转折点为700℃,低于这个温度时随退火温度的升高,(002)衍射峰强度增强且掺Mg的薄膜既有紫外光又有绿光发射;800℃退火时,薄膜的(002)衍射峰强度减小,出现了(100)和(101)衍射峰,且掺Mg的薄膜只有紫外发光峰.不同的掺杂浓度对于发光也有影响,低于700℃退火时,ZnO薄膜只出现紫外发光峰,掺Mg的薄膜却出现了紫外和绿光两个发光峰.     

第一性原理的广义梯度近似+U方法的纤锌矿Zn1-xMgxO极化特性与Zn0.75Mg0.25O/ZnO 界面能带偏差研究

在纤锌矿结构Zn_1-xMg_xO/ZnO异质结构中发现了高迁移率的二维电子气(2DEG), 2DEG 的产生很可能是由于界面上存在不连续极化, 而且2DEG通常也被认为是由极化电荷产生的结果. 为了探索2DEG的形成机理及其产生的根源, 研究Zn_1-xMg_xO合金的极化特性与ZnO/Zn_1-xMg_xO超晶格的能带排列是非常必要的. 基于第一性原理广义梯度近似+U方法研究了Zn_1-xMg_xO合金的自发极化随Mg组分x的变化关系, 其中极化特性的计算采用Berry-phase方法. 由于ZnO与Zn_1-xMg_xO 面内晶格参数大小相当, ZnO 与Zn_1-xMg_xO 的界面匹配度优良, 所以ZnO/Zn_1-xMg_xO 超晶格模型较容易建立. 计算了Mg_0.25Zn_0.75O/ZnO超晶格静电势的面内平均及其沿着Z(0001)方向上的宏观平均. (5+3)Mg_0.25Zn_0.75O/ZnO超晶格拥有较大的尺寸, 确保远离界面的Mg_0.25Zn_0.75O与ZnO区域与块体计算情况一致. 除此之外, 基于宏观平均为能量参考, 计算得到Mg_0.25Zn_0.75O/ZnO超晶格界面处价带偏差为0.26 eV, 并且导带偏差与价带偏差的比值处于合理区间, 这与近来实验上报道的结果相符. 除了ZnO在[0001]方向上产生自发极化外, 由于在ZnO中引入Mg杂质会产生应变应力, 导致Mg_xZn_1-xO层产生额外的极化值. 这样必然会在Mg_0.25Zn_0.75O/Zn界面处产生非连续极化现象, 促使单极性电荷在界面处积累, 从而在Mg_0.25Zn_0.75O/Zn超晶格中产生内在电场. 此外, 计算了Mg_0.25Zn_0.75O/ZnO超晶格的能带排列, 由于价带偏差Δ E_V=0.26 eV与导带偏差ΔE_C=0.33 eV, 表明能带遵循I型排列. Mg_0.25Zn_0.75O/ZnO 的这种能带排列方式足以让电子与空穴在势阱中产生禁闭作用. 2DEG在电子学与光电子学领域都有重要应用, 本文的研究结果将对Mg_0.25Zn_0.75O/ZnO 界面2DEG的设计与优化中起到重要作用, 并且可以作为研究其他Mg组分的Mg_xZn_1-xO/ZnO超晶格界面电子气特性的参考依据.     

MgZnO薄膜及其量子阱和超晶格的发光特性

MgO和ZnO形成合金Mg_xZn_1-xO的带隙可以在3.3～7.9eV之间变化,在制备紫外波段光电器件方面有着广阔的应用前景.由ZnO和MgZnO交替沉积而成的ZnO/Mg_xZn_1-xO量子阱和超晶格在激光器、光探测器和其他光电器件方面也有潜在的应用价值.回顾最近几年对MgZnO薄膜材料发光特性的研究进展,介绍在不同衬底上用不同方法制备MgZnO合金薄膜的制备技术、发光特性以及发光特性与薄膜中Mg含量的关系;综述近年来在ZnO/Mg_xZn_1-xO超晶格、量子阱研究上的成果,特别介绍了ZnO/Mg_xZn_1-xO对超晶格、量子阱的发光特性、发光机理以及发光特性与势垒层镁含量、器件温度的关系.     

缓冲层对量子阱二能级系统中电子子带间跃迁光吸收的影响

由于ZnO缓冲层对纤锌矿ZnO/Mg_xZn_(1-x)O有限深单量子阱结构左垒的限制作用,导致阱和右垒的尺寸、Mg组分值等因素将影响系统中形成二能级.本文考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法数值求解Schr?dinger方程,获得电子的本征能级和波函数,探讨ZnO缓冲层对此类量子阱形成二能级系统的尺寸效应及三元混晶效应的影响;利用费米黄金法则探讨缓冲层、左垒、阱及右垒宽度和三元混晶效应对此类量子阱电子子带间跃迁光吸收的影响.计算结果显示:对于加入ZnO缓冲层的ZnO/Mg_xZn_(1-x)O有限深单量子阱二能级系统,左垒宽度临界值会随着阱宽和Mg组分值的增大而逐渐减小,随着右垒宽度和缓冲层厚度的增大而逐渐增大;量子阱中电子子带间跃迁光吸收峰会随着左垒、右垒尺寸以及Mg组分的增大发生蓝移,随着阱宽增大而发生红移.本文所得结果可为改善异质结器件的光电性能提供理论指导.     

ZnO单晶和BeZnO合金的生长及其紫外探测器研究

用分子束外延(MBE)的方法在c面蓝宝石衬底上生长出了高质量的ZnO单晶薄膜和BexZn1-xO合金薄膜。X射线光电子能谱(XPS)测试结果表明,合金材料中Be元素的摩尔分数分别为1.8%、4.9%、8.0%和15.3%。在此基础上制备了ZnO基和BexZn1-xO基的金属-半导体-金属(MSM)结构紫外探测器。ZnO单晶探测器的响应波长为375nm,在1V电压下,350nm处的光响应度高达43A/W,光电流和暗电流之比达到105量级。在BexZn1-xO基紫外探测器中,其截止响应波长随着合金中Be含量的增加逐渐蓝移,其中Be0.153-Zn0.847O合金探测器的截止响应波长为366nm,紫外波段和可见波段的光电流之比达到2~3个数量级,具有良好的信噪比。此外,提出了氧气等离子体表面处理降低探测器暗电流的方法,并使ZnO单晶探测器的暗电流降低了4个数量级。     

Influence of Mg content on optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the structural parameters (the dot height, the barrier thickness between the coupled wurtzite ZnO QDs and Mg content x in the barrier layers) is calculated in detail. The results elucidate that Mg content have a significant influence on the exciton state and optical properties of ZnO coupled QDs. When Mg content x increases, the strong built-in electric field increases and leads to the redshift of the effective band gap of the Mg_xZn_1−xO layer. These theoretical results are useful for design and application of some important photoelectronic devices constructed by using ZnO strained QDs.     

m面蓝宝石上外延（110）取向立方MgZnO薄膜及其日盲紫外探测器件研究

由于在日盲紫外探测方面的应用前景,具有合适带隙的MgZnO合金半导体薄膜受到越来越多的关注。获得具有择优取向的单一相MgZnO对提升MgZnO基日盲紫外探测器性能至关重要。本文利用低压金属有机化学气相沉积（LP-MOCVD）方法在m面蓝宝石衬底上制备了一系列不同组分的Mg_xZn_1-xO薄膜。光学和结构特性测试结果表明：Zn摩尔分数达到55%的Mg_0.45Zn_0.55O薄膜依然是单一立方相,其光学带隙可以达到4.7 eV。立方岩盐结构MgZnO与m面蓝宝石衬底的外延结构关系为（110）_MgZnO‖（1010）_sapphire、[001]_MgZnO‖[1210]_sapphire和[110]_MgZnO‖[0001]_sapphire。唯一确定的面内取向有利于薄膜晶体质量的提高。基于（110）取向立方相Mg_0.45Zn_0.55O薄膜制备金属-半导体-金属（MSM）结构器件,获得了光响应峰在260 nm、光响应截止波长278 nm的日盲紫外探测器。     

利用脉冲激光沉积法制备高Mg掺杂的六方相MgZnO薄膜

选用Mg0.2Zn0.8O陶瓷靶,利用脉冲激光沉积(PLD)法,在单晶Si(100)和石英衬底上生长了一系列MgZnO薄膜(MZO)。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱(EDS)和紫外可见光透射光谱(UV-Vis)等实验手段,研究了在不同工作压强下生长的薄膜样品的晶体结构、微观形貌和光学性能的变化。结果表明:所有的薄膜样品都是单一的ZnO六方相,禁带宽度随生长压强的升高而增加,变化范围在3.83～4.05eV之间,最短吸收边接近300nm。     

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically.The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included.Numerical calculations are performed using a variational procedure within the single band effective mass approximation.Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions.The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed.The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton.It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased,and that the exciton binding energy,the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field.The obtained results are useful for the design of some opto-photoelectronic devices.     

MgxZn1-xO合金制备及MgZnO/ZnO异质结构的光学性质

利用射频等离子体辅助的分子束外延(P-MBE)技术在c面的蓝宝石衬底上生长了具有不同Mg含量(0≤x≤0.28)的六方相MgZnO合金薄膜,研究了该系列样品Raman频移的幅度与合金组分的对应关系,为MgZnO合金中Mg含量的确定提供了新的方法。在此基础上选择具有合适带宽的MgZnO合金作为垒层,制备了MgZnO/ZnO量子阱结构。在较高的光激发密度下,观测到了发光强度随激发密度的超线性增加,并将之归因于激子-激子碰撞引起的超辐射过程。     

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.     

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.     

Hydrostatic pressure effects on exciton states in InAs/GaAs quantum dots

Based on the effective-mass approximation, the hydrostatic pressure effects on exciton states in InAs/GaAs self-assembled quantum dots (QDs) are studied by means of a variational method. Numerical results show that the exciton binding energy has a minimum with increasing dot height for any hydrostatic pressure. The interband emission energy increases when the hydrostatic pressure increases. In particular, we find that hydrostatic pressure has a remarkable effect on exciton states for small QD size. Our results are in agreement with experiment measurements.     

脉冲激光沉积制备MgxNi1-xO合金薄膜的表征及光学性质研究

应用脉冲激光沉积(PLD)技术,固定脉冲激光能量密度为5 J/cm2,调节薄膜生长基底温度为300~700 ℃,制备了系列Mg_xNi_1-xO合金薄膜。通过透射电子显微镜(TEM)、X射线光电子能谱(XPS)、原子力显微术(AFM)等表征分析手段,详细分析了薄膜的成分及组织,研究了退火处理对样品的影响。通过UV-Vis分光光度计研究了透射光谱,结合理论计算了光学带隙宽度。结果表明:薄膜由非晶及多晶构成,紫外吸收边约为290 nm, 接近日盲波段的上限; 衬底温度为500 ℃、激光脉冲能量密度为5 J/cm2时生长的薄膜在短波部分吸收强烈,而长波部分几乎不吸收,有利于紫外探测; 退火处理改善了样品表面质量, 但不能有效拓宽光学带隙。     

纤锌矿结构ZnO/Mg_xZn_(1-x)O量子阱中带间光吸收的尺寸效应和三元混晶效应

对于纤锌矿结构ZnO/MgxZn1-xO有限深单量子阱结构,考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法和自洽法数值求解Schr?dinger方程和Poisson方程,获得电子(空穴)的本征能级和本征波函数.进而,采用费米黄金法则讨论带间光吸收的尺寸效应和三元混晶效应.结果表明:三元混晶材料MgxZn1-xO中Mg组分的增加会增强垒层和阱层的内建电场强度,使得电子(空穴)平均位置靠近左(右)垒,导致带间跃迁吸收峰呈指数减小且发生蓝移;ZnO/MgxZn1-xO量子阱带间跃迁吸收峰随阱宽增大而减小,吸收峰发生红移.所得结果可为改善异质结构材料和器件的光电性能提供理论指导,以期获得实际应用所需的光学吸收频谱和波长.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Electric field effects on optical properties in zinc-blende InGaN/GaN quantum dot

The effect of electric field on exciton states and optical properties in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated theoretically in the framework of effective-mass envelop function theory. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy, interband transition energy, oscillator strength and linear optical susceptibility in InGaN/GaN QD. It is also found that the electric field effects on exciton states and optical properties are much more obvious in QD with large size. Moreover, the ground-state exciton binding energy and oscillator strength are more sensitive to the variation of indium composition in InGaN/GaN QD with small indium composition. Some numerical results are in agreement with the experimental measurements.     

Magnesium incorporation efficiencies in Mg_xZn_(1-x)O films on ZnO substrates grown by metalorganic chemical vapor deposition

We investigate the magnesium(Mg) incorporation efficiencies in Mg_xZn_(1-x)O films on c-plane Zn-face ZnO substrates by using metalorganic chemical vapor deposition(MOCVD) technique. In order to deposit high quality Mg_xZn_(1-x)O films,atomically smooth epi-ready surfaces of the hydrothermal grown ZnO substrates are achieved by thermal annealing in O_2 atmosphere and characterized by atomic force microscope(AFM). The AFM, scanning electron microscope(SEM),and x-ray diffraction(XRD) studies demonstrate that the Mg_xZn_(1-x)O films each have flat surface and hexagonal wurtzite structure without phase segregation at up to Mg content of 34.4%. The effects of the growth parameters including substrate temperature, reactor pressure and Ⅵ/Ⅱ ratio on Mg content in the films are investigated by XRD analysis based on Vegard's law, and confirmed by photo-luminescence spectra and x-ray photoelectron spectroscopy as well. It is indicated that high substrate temperature, low reactor pressure, and high Ⅵ/Ⅱratio are good for obtaining high Mg content.     

Exciton states in zinc-blende InGaN/GaN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende(ZB) InGaN/GaN quantum dot(QD) are investigated by means of a variational approach, considering finite band offsets. The ground-state exciton binding energy and the interband emission energy are investigated as functions of QD structural parameters in detail. Numerical results show clearly that both the QD size and In content of InGaN have a significant influence on the exciton states and interband optical transitions in the ZB InGaN/GaN QD.