InN量子点的液滴外延及物性表征

由于1. 55μm波段广泛应用于通信领域,为了探索不同生长温度对InN量子点的形貌影响,并且实现自组装InN量子点在1. 55μm通信波段的发光,对InN量子点的液滴外延及物性进行了相关研究。首先利用射频等离子体辅助分子束外延(PA-MBE)技术在GaN模板上,采用液滴外延方法在3种温度下生长了InN量子点结构。生长过程中靠反射高能电子衍射(RHEED)对样品进行原位监控。原子力显微镜(AFM)表征结果表明随着生长温度升高,量子点尺寸变大,密度减小。在生长温度350℃和400℃下,观测到了量子点;当温度高于450℃时,未观测到InN量子点。当生长温度为400℃时,量子点形貌最好,密度为6×10~8/cm~2,对400℃下生长的InN量子点进行了变温PL测试,成功得到InN量子点在1. 55μm波段附近的光致发光,并且随着测试温度的升高,量子点的发光峰位发生了先红移后蓝移最后又红移的S型曲线变化,这种量子点有望在未来应用于量子通信领域。     

MOCVD生长的InN薄膜中In分凝现象

利用金属有机物化学气相淀积(MOCVD)方法不同生长条件下在c面蓝宝石衬底上制备了InN薄膜,通过不同的物理表征手段研究了InN薄膜的物理性质,结果表明:合适的生长温度可以抑制InN薄膜表面分凝现象。研究认为较低的生长温度使作为N源的NH3分解率较低,In—N的成键可能性小,导致In在表面聚集出现分凝;而较高生长温度时,InN薄膜中In—N键能较小,易发生断裂,反应活性较强的N和H原子逸离薄膜表面,In滞留在薄膜表面也导致In分凝现象的出现。相对于表面分凝的样品,未出现表面分凝的样品的薄膜晶体质量和表面形貌也得到了提高。同时,通过Raman散射谱研究了晶格振动E2声子模的应力效应。     

MOCVD生长的InGaN薄膜中InN分凝的研究

利用X射线衍射(XRD)技术测量了MOCVD生长的InGaN薄膜中的InN分凝量.利用Vegard定理和XRD 2θ扫描测得实验的InGaN薄膜的In组分为0.1～0.34.通过测量XRD摇摆曲线的InN(0002)和InGaN(0002)的积分强度之比测得InN在InGaN中的含量为0.0684%～2.6396%.根据XRD理论,计算出InN和InGaN的理论衍射强度.InN含量在所有样品中均小于3%,这表明样品的相分离度比较低.还发现InN在InGaN薄膜中的含量与氮气载气流量和反应室气压明显相关.     

GaxIn1-xN输运性质的温度特性的数值模拟

利用多粒子Monte Carlo方法对含位错GaxIn1-x材料的输运性质进行了研究,计算表明材料的漂移速度随温度升高而降低,InN的漂移速度的温度特性优于GaN.GaN和InN的迁移率在相同温度范围内随温度的变化趋势不同,同时位错密度对材料迁移率的温度特性影响较大.     

氮气流量对磁控溅射InN薄膜特性的影响

采用射频磁控溅射法在蓝宝石衬底上制备了InN薄膜. 研究了N2流量对InN薄膜的晶体结构、表面形貌、光学和电学特性的影响. X射线衍射(XRD)测试结果显示,InN呈六方纤锌矿结构,具有明显(002)择优取向;SEM与AFM图像显示InN薄膜均匀致密,低N2流量下随流量增加,表面逐渐趋于光滑平整,过高的N2流量使薄膜生长方式发生改变;通过检测薄膜吸收特性,利用线性外推法计算禁带宽度为1.81~1.96 eV;电学测试结果表明,制备的薄膜样品均呈现n型导电特性,且迁移率较低,最大为12.2 cm2/v∙s;载流子浓度较高,保持在1021 cm-3数量级;电阻率较小,范围是0.202~0.33 mΩ∙cm.     

p型未掺杂富锌ZnO薄膜的形成和性能研究

以高纯ZnO为靶材,氩气为溅射气体,利用射频磁控溅射技术在石英衬底上生长出纤锌矿结构的富锌ZnO薄膜.薄膜沿(002)择优取向生长,厚约为1.2μm,呈现电绝缘特性.将溅射的ZnO薄膜在10-3Pa,510~1 000 K的温度范围等温退火1 h,室温Hall测量结果表明ZnO薄膜的导电性能经历了由绝缘—n型—p型—n型半导体的变化.XPS测试表明ZnO薄膜的Zn/O离子比随退火温度的升高而降低,但一直是富锌ZnO,说明未掺杂的富锌ZnO也可以形成p型导电.p型未掺杂富锌ZnO薄膜的形成可归因于VZn受主浓度可以克服VO和Zni本征施主的补偿效应.     

不同生长条件下ZnO薄膜电学性质的研究

ZnO薄膜中的高的背景电子浓度能够对p型掺杂形成补偿,从而对p型掺杂造成障碍,了解高背景电子浓度的来源有助于对p型掺杂的研究。本文采用分子束外延技术在不同真空度下在a面蓝宝石衬底上生长了一系列氧化锌薄膜,发现在低真空度下生长的样品的载流子浓度较高,为10¹⁹ cm^-3量级;而高真空度下生长的样品,其载流子浓度比低真空生长的样品显著降低,降低了3个数量级。在相同条件下生长的样品,通过不同的后处理手段进行处理后,其电子浓度未发生明显变化,说明氧空位等本征缺陷不是ZnO薄膜中电子的主要来源,高背景电子浓度应该与生长过程中非故意引入的杂质相关。通过低温光致发光表征,发现低真空度下生长的样品在低温下3.366 eV处有强的施主束缚激子发光峰,而高真空度下生长的样品的此发光峰显著变弱。由此,高电子浓度被归结为与生长过程中非故意引入的氢杂质相关。     

飞秒激光改性6H-碳化硅晶体表面光电导增益现象研究

半绝缘6H型碳化硅(6H-SiC)具有高电阻率性质,在可见光照射下进行光电导测量时,通常光生电流很小;然而经过飞秒激光辐照改性之后,发现在可见光波段的光电导有明显的增益.本文利用紫外-可见-近红外吸收谱、X射线光电子能谱和发光光谱测量分析了激光改性之后碳化硅样品的光谱吸收、发射和晶体元素比例变化情况.分析认为碳化硅光电导增益的原因是飞秒激光辐照过程改变了碳化硅表面的硅碳元素的原子浓度比,形成新的物质结构形式,从而导致了表面光电导性能的提高.     

Fe3O4/MgO(100)薄膜外场诱导电阻变化特性

采用激光分子束外延（L-MBE）方法,以MgO(100)为基底生长了Fe3O4单晶薄膜, 研究了Fe3O4/MgO(100)薄膜外场（温度、磁和激光场）诱导电阻变化特性。X射线衍射(XRD)分析表明Fe3O4薄膜是沿MgO(200)晶面外延生长的单晶薄膜;反射高能电子衍射（RHEED）强度振荡曲线分析表明Fe3O4薄膜表面平整,而且生长模式为2维层状生长;原子力显微镜（AFM）分析表明Fe3O4薄膜表面粗糙度为0.201 nm,说明薄膜表面达到原子级平整度。外场作用下Fe3O4薄膜的电阻测试表明:薄膜样品的电阻在120 K(Verwey转变温度)出现一峰值,略微下降后继续增大, 展现出半导体型的导电特性; 在激光作用下,整个测量温度范围内薄膜样品的电阻减小,样品展示出瞬间光电导的特性;从降温曲线可以看出, Verwey转变温度由无激光作用时的120 K上升到有激光作用时的140 K; 光致电阻变化率随着温度的降低而增大,这主要是由于激光作用导致电荷有序态的退局域化。     

铁基超导体Li0.8Fe0.2ODFeSe的红外光谱研究

测量和研究了铁基超导体Li_0.8Fe_0.2ODFeSe单晶的红外光学响应,发现室温下光电导率谱不存在Drude分量,载流子具有非相干输运行为.随着温度降低,Drude分量形成并不断变窄,同时在相应的反射率谱上出现清晰的等离子体边,表明散射率急剧降低.在最低温度,观察到超导能隙形成导致的光谱变化,光电导率谱在160 cm^-1以下受到显著压制.对比FeSe单晶的光谱数据,发现整体的光电导率谱型很相似,但自由载流子的谱重更低,揭示出样品具有更低的载流子浓度.另外还观察到温度变化诱导的谱重由低频向高频区域转移的现象,表明其存在强关联效应.     

InAs/GaSb量子阱中太赫兹光电导特性

太赫兹技术由于具有重大的科学价值及应用前景而引起了广泛关注,其核心问题是性能优异的室温太赫兹辐射源和探测器研究.本文用半经典的玻尔兹曼方程方法研究了In As/Ga Sb量子阱系统中载流子对电磁场的响应,运用平衡方程方法求解玻尔兹曼方程得到了量子阱系统中的光电导,系统地研究了量子阱结构对光电导的影响,揭示了在该量子阱系统中光电导产生的物理机制.研究发现,量子阱结构主要通过调节载流子的能级、浓度和波函数的耦合影响光电导,对称性较好的量子阱结构(8 nm-8 nm)的光电导信号更强,其峰值落在太赫兹区(0.2 THz),并且在低温下器件的性能较好,温度升高则吸收峰略有降低,且光电导峰值发生红移.研究结果表明该量子阱系统可以用作室温太赫兹光电器件.     

Mg-doped ZnO layer to enhance electron transporting for PbS quantum dot solar cells

In this work, the effect of Mg doping on the performance of PbS quantum dot (QD) solar cells (QDSCs) is investigated. To elucidate that, PbS QDSCs with pristine ZnO and Mg-doped ZnO (ZMO) as electron transporting layers (ETLs) are fabricated, respectively. The current density-voltage (J-V) measurements are performed. The results show that the cell efficiency of the device with ZMO as an ETL is 9.46%, which increases about 75% compared to that of the pristine ZnO based device (5.41%). Enhanced short current density (J_sc) and fill factor (FF) are observed. It is demonstrated that Mg doping could passivate the surface defects and suppress the carrier recombination in ZnO ETL, thus resulting in larger bandgap and higher Fermi level (E_F). The strategy of Mg-doped ZnO ETL provides a promising way for pushing solar cell performance to a high level.     

Observation of inversion domain boundaries in Mg-doped AlGaN layers grown by metalorganic chemical vapor deposition

We have investigated the formation of inversion domain boundaries in Al_0.13Ga_0.87N layers grown on sapphire substrates by metalorganic chemical vapor deposition using transmission electron microscopy (TEM). We found that the shape of inversion domain boundaries strongly depends on Mg source flow rate in the Mg-doped Al_0.13Ga_0.87N layers. By increasing the Mg source flow rate, the shape of inversion domain boundaries is changed from the vertical shape to the horizontal shape. In addition, the change of polarity by the horizontal shape inversion domain boundary (IDB) resulted in the inverted rotation of pyramidal shape IDB within the Mg-doped Al_0.13Ga_0.87N layers.     

应力和掺杂对Mg:GaN薄膜光致发光光谱影响的研究

对化学气相沉积(MOCVD)法在宝石衬底上生长的Mg掺杂GaN薄膜的表面及其GaN缓冲层的光致发光(PL)光谱进行了测量，用Raman光谱和x射线衍射(XRD)对GaN薄膜中的应力进行确定，通过PL光谱中的中性束缚激子跃迁能量的变化确定薄膜中应力的影响，从而研究Mg掺杂对p型GaN的DAP跃迁影响规律. 关键词： 光致发光 应力 Raman光谱     

射频等离子体辅助MBE生长GaN及Mg掺杂的光致发光

采用射频等离子体辅助分子束外延(RF plasma-assisted MBE)系统生长非故意掺杂GaN和p型GaN,并且通过室温和低温光致发光(PL)谱测试研究了材料的发光特性及与杂质态的关系,对于GaN外延层出现的黄带发光进行分析。结果表明,富Ga条件下生长的GaN材料特性要优于富N生长的材料;非故意掺杂的富Ga样品中出现的黄带发光(YL)与GaN中生成能最低的氮空位(V_N)缺陷有关;不同的Mg掺杂浓度对样品的PL特性有较大的影响;结合Hall效应测量结果,认为在Mg重掺杂的样品中出现的黄带发光,与GaN的自补偿效应以及重掺杂导致的晶体质量下降有关。     

InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600-850°C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750°C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In_0.1Ga_0.9N/i-In_0.1Ga_0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400nm.     

Influence of carrier gas H_2 flow rate on quality of p-type GaN epilayer grown and annealed at lower temperatures

In this work, we study the influence of carrier gas H_2 flow rate on the quality of p-type GaN grown and annealed at lower temperatures. It is found that the concentration of H atoms in Mg-doped GaN epilayer can effectively decrease with appropriately reducing the carrier gas H_2 flow rate, and a high-quality p-type GaN layer could be obtained at a comparatively low annealing temperature by reducing the carrier gas H_2 flow rate. Meanwhile, it is found that the intensity and wavelength of DAP peak are changed as the annealing temperature varies, which shows that the thermal annealing has a remarkable effect not only on the activation of acceptors but also on the compensation donors.     

Effect of excitation intensity and electric field on the photoconductivity relaxation in CdxHg1−x Te/GaAs polycrystalline layers

The photoconductivity relaxation and the stationary photoconductivity in the n-Cd_0.8Hg_0.2Te compensated polycrystalline layers at T=300 K have been investigated as a function of the light intensity and the strength of applied electric field E. It is demonstrated that, at low excitation intensities, the saturation of stationary photoconductivity and a decrease in the relaxation time with an increase in E are caused by the minority carrier extraction. The effect of minority carrier extraction is analyzed with due regard for the internal electric field of potential barriers in intergranular layers. It is assumed that the features of nonequilibrium-carrier recombination, which proceeds through several channels and depends on the excitation intensity and extraction electric field strength, can stem from the polycrystalline structure of the Cd_0.8Hg_0.2Te layers.     

Temperature dependence of persistent photo-conductivity due to DX centers in AlxGa1−xAs:Si

Conductivity and Hall effect measurements were performed on molecular beam epitaxy grown Al_xGa_1–xAs:Si samples, which show a large persistent photoconductivity effect. We observe one, two, and three minima in the temperature-dependent carrier concentration during the heating process after having first illuminated the samples (x=0.25, 0.30, and 0.37, respectively) at low temperature. We interpret this structure in terms of the existence of different types of large lattice relaxation DX centers.     

Photoconductivity of Se85−xTe15Hgx thin films

The photoconductive properties such as dark conductivity, steady state and transient characteristics of a-Se_85−xTe₁₅Hg_x thin films, prepared by thermal vacuum evaporation technique have been studied in the temperature range 312–380 K. Analysis of data shows that the activation energy of dark current is greater as compared to the activation energy of photocurrent. The activation energy increases at higher concentration of Hg which shows that the defect density of states decreases. Analysis of intensity dependent photoconductivity shows that the bimolecular recombination is predominant. The transient photoconductivity shows that the carrier lifetime decreases with the increase in Hg concentration and increases at higher concentration of Hg. This decrease is due to the transition trapping process. Further the photosensitivity and carrier lifetime increases at higher concentration of Hg which also confirms that the density of defect states decreases.