ZnO基稀磁半导体的制备与性质研究

本论文利用溶胶一凝胶法和水热法制备了不同离子（Eu~3＋，Sm~3＋，Mn~2＋，Fe~3＋，Co~2＋，Ni~2＋）作为磁性杂质的ZnO基稀磁半导体，并系统地研究了材料的薄膜、粉末和纳米结构的结晶特性、结构形态和光、电、磁性质。溶胶一凝胶法制备的薄膜的晶体为c轴取向生长的六方纤维锌矿结构。薄膜的取向生长受烧结气氛、烧结温度和掺杂离子浓度的影响，其中烧结气氛是影响薄膜取向生长的最直接、最显著因素。随着烧结气氛中氧含量的减小，薄膜的沿c轴生长的趋势加强。此外，烧结温度的提高也增强薄膜沿c轴生长的趋势，但掺杂离子浓度的增加却抑制薄膜的c轴取向生长特性。通过薄膜表面形态的研究发现，在空气中烧结的薄膜由立方晶粒构成，而在真空中烧结的样品则由不规则的片状晶粒组成。组成薄膜的多晶颗粒粒径小于10Onm，15层薄膜的膜厚为357-366nm。掺杂离子在薄膜中均匀分布，成膜过程不改变掺杂离子（Eu3+，Sm3＋，Mn2+，Fe3＋，Co2＋，Ni2＋）和基质离子（Zn2+和O2-）的价态。 不同Eu3+掺杂浓度的ZnO薄膜样品的吸收光谱的吸收边出现在363nm和368nm之间，对应半导体材料的禁带宽度Eg=3.42～3.40ev。由于Eu3＋改变了薄膜的表面性质，Zn1-xEux（0.005≤x≤0.15）薄膜在可见光区出现了一系列干涉带。Zn1-xTMxO薄膜的吸收光谱的吸收边位置出现在356nm-369nm，对应半导体的禁带宽度为3.34-3.46eV，在可见光区发现了Co2＋的电子的d-d跃迁引起的吸收带。随着掺杂浓度的增加，薄膜的透光率逐渐减小。Zn1-xCoxO薄膜在近紫外与可见光区的透光率都在60％以上，Zn1-xEuxO薄膜的透光率则高达90％。在Zn1-xEuxO薄膜的激发发射光谱中，以613nm作为监控波长，激发光谱除了检测到Eu3＋的7F→5D能级的吸收跃迁外，还检测到最大值位于378nm附近的ZnO的吸收带。以394nm为激发波长，发射光谱检测到Eu3+的5D0→7FJ（J＝1，2，3，4）跃迁。以zno的带隙能量378nm作为激发波长进行激发，检测到Eu3+的5D0→7F2跃迁，说明基质zno和E矿十之间存在能量交换。薄膜磁性测试在4-400K温度范围内进行，发现在此温度范围内Zn0.9Eu0.1O薄膜表现居里一外斯顺磁性；在低温区，存在磁性增强现象。zno．gCoo．IO薄膜在23oK以下表现为铁磁性，200K的M-H曲线显示薄膜的剩磁（Br）约为0．21em侧g，矫顽力（Hc）约为327Oe。但Zn0.9Mn0.1O，Zn0.9Ni0.1O，Zn0.9Co0.1O薄膜的磁性测试则显示在80K以上三种薄膜均表现为顺磁性。Zn0.9Eu0.1O薄膜的电阻呈现典型的半导体性质，在ZT的磁场下，薄膜在110K获得最大14.53％的磁阻率。Zn1-xTMxO薄膜的电阻也表现典型的半导体特性，实验研究了薄膜在不同掺杂离子浓度、外加磁场以及温度条件下的磁阻性质。粉末样品中磁性离子的掺杂浓度均小于薄膜样品。Co，Fe，Ni，Mn掺杂的Zn1-xTMxO粉末在80以上均为顺磁性。在Co2＋掺杂的粉末样品中没有发现类似于薄膜样品的铁磁性，说明DMS的磁性与制备条件关系密切。实验证明了利用sol-gel方法，Zno：TM稀磁半导体能够有效地组装在MCM-41和AAO的孔道内。ZnO:TM材料组装进在MCM-41孔道后，不改变孔道的六方结构但使孔径变小。随着组装次数的增加，MCM-41的孔径和孔容累进减小。组装在AAo模板孔道内的材料呈单分散纳米颗粒状态，颗粒粒径小于loonm。组装材料的磁性测试显示：组装在MCM-41内的Zn0.9Co0.1O材料在80K-30OK呈现超顺磁性。而Mn，Fe，Ni掺杂的Zno在此温度范围内表现顺磁性。组装在AAO内的ZnO:TM(TM=Mn，Fe，Co，Ni）材料在SOK-30OK温度范围内都呈现顺磁性。在水热法合成ZnO:A（A＝Bu，Sm，co）纳米粒子的过程中，发现反应温度、压力、时间和溶液浓度等因素只影响Znl．xCoxO纳米粒子的的产量，而溶液的酸度却影响产物的形貌。控制溶液的酸度，可以控制产物的形貌从粒状向棒状转变。当溶液的PH＝5时，在甲醇：水体系中可以水热合成规则的棒状ZnO：RE（RE=Eu，Sm）纳米晶。所得到的Zn0.98Co0.02O纳米晶在80K呈超顺磁行为，而ZnO：RE（RE＝Eu，Sm）纳米晶在80K则表现较弱的顺磁性。 实验通过控制水热条件，制备了一种新型结构的柠檬酸锌晶体。由于利用了水热反应的非平衡合成条件，所得到的晶体的层状结构不同于目前已知的所有柠檬酸配合物的离散型分子结构。单晶衍射结果表明：化合物是一个由八面体和一个非对称单元交替相连构成的二维层状结构。

Room-Temperature Ferromagnetism in Co-Doped In2O3 Nanocrystals

Co-doped In2O3 nanocrystals showing room-temperature ferromagnetism have been successfully prepared by a simple sol-gel synthesis route. The sample displays it clear ferromagnetism behavior above 300 K. Phase and structure analyses reveal that the nanocrystals are crystallized with Co ions substituted for In ions in the In2O3 matrix, and no trace of secondary phases or clusters is detected. The experimental results are explained theoretically by first-principles calculations based on density functional theory, which indicate that the native ferromagnetic behavior of Co-doped In2O3 could be mainly ascribed to the strong d-d coupling of the magnetic ions.

First principles study of N-N split interstitial in GaN nanowires

Atomic and electronic properties of N-N split interstitial in GaN nanowires have been investigated using first principles calculations. The formation energy calculations show that the N-N interstitial favors substituting an N atom at the surface of the nanowires. The interstitial induces localized states in the band gap of GaN nanowires.

Charge Separation in Wurtzite/Zinc-Blende Heterojunction GaN Nanowires

The electronic properties of wurtzite/zinc-blende (WZ/ZB) heterojunction GaN are investigated using first-principles methods. A small component of ZB stacking formed along the growth direction in the WZ GaN nanowires does not show a significant effect on the electronic property, whereas a charge separation of electrons and holes occurs along the directions perpendicular to the growth direction in the ZB stacking. The later case provides an efficient way to separate the charge through controlling crystal structure. These results have significant implications for most state of the art excitonic solar cells and the tuning region in tunable laser diodes.

Ultrafast Kerr rotations and zero-field dephasing time of electron spins in InAs/GaAs quantum disks

Time-resolved Kerr rotation (TRKR) measurements based on pump-probe arrangement were carried out at 5 K on the monolayer fluctuation induced InAs/GaAs quantum disks grown on GaAs substrate without external magnetic field. The lineshape of TRKR signals shows an unusual dependence on the excitation wavelength, especially antisymmetric step-shaped structures appearing when the excitation wavelength was resonantly scanned over the heavy- and light-hole subbands. Moreover, these step structures possess an almost identical decay time of similar to 40 Ps which is believed to be the characteristic spin dephasing time of electrons in the extremely narrow InAs/GaAs quantum disks.

BAND-STRUCTURE OF MG1-XZNXSYSE1-Y

The empirical pseudopotential method within the virtual crystal approximation is used to calculate the band structure of Mg1-xZnySySe1-y, which has recently been proved to be a potential semiconductor material for optoelectronic device applications in the blue spectral region. It is shown that MgZnSSe can be a direct or an indirect semiconductor depending on the alloy composition. Electron and hole effective masses are calculated for different compositions. Polynomial approximations are obtained for both the energy gap and the effective mass as functions of alloy composition at the GAMMA valley. This information will be useful for the future design of blue wavelength optoelectronic devices as well as for assessment of their properties.

EMPIRICAL PSEUDOPOTENTIAL BAND-STRUCTURE CALCULATION FOR ZN1-XCDXSYSE1-Y QUATERNARY ALLOY

The band structure of the Zn1-xCdxSySe1-y quaternary alloy is calculated using the empirical pseudopotential method and the virtual crystal approximation. The alloy is found to be a direct-gap semiconductor for all x and y composition. Polynomial approximation is obtained for the energy gap as a function of the composition x and y. Electron and hole effective masses are also calculated along various symmetry axes for different compositions and the results agree fairly well with available experimental values.

RAMAN PHONONS OF SILICON WIRES

Using the Keating model and the Raman polarizability of Alben et al., the phonon Raman spectra of silicon wires are calculated. With the calculation results, the Raman spectra of porous silicon of some published papers are analyzed. Until now different authors have had different views on the luminescence mechanism of porous silicon, which may mainly result from the fact that they had different samples.

GROWTH OF WIDE-BAND GAP IMMISCIBLE II-VI ALLOYS BY METALORGANIC VAPOR-PHASE EPITAXY

Although metalorganic vapor phase epitaxy (MOVPE) is generally regarded as a non-equillibrium process, it can be assumed that a chemical equilibrium is established at the vapor-solid interface in the diffusion limited region of growth rate. In this paper, an equilibrium model was proposed to calculate the relation between vapor and solid compositions for II-VI ternary alloys. Metastable alloys in the miscibility gap may not be obtained when the growth temperature is lower than the critical temperature of the system. The influence of growth temperature, reactor pressure, input VI/II ratio, and input composition of group VI reactants has been calculated for ZnSSe, ZnSeTe and ZnSTe. The results are compared with experimental data for the ZnSSe and ZnSTe systems.

EXCIMER-LASER DOPING OF SPIN-ON DOPANT IN SILICON

Solid films containing phosphorus impurities were formed on p-type silicon wafer surface by traditional spin-on of commercially available dopants. The doping process is accomplished by irradiating the sample with a 308 nm XeCl pulsed excimer laser. Shallow junctions with a high concentration of doped impurities were obtained. The measured impurity profile was ''box-like'', and is very suitable for use in VLSI devices. The characteristics of the doping profile against laser fluence (energy density) and number of laser pulses were studied. From these results, it is found that the sheet resistance decreases with the laser fluence above a certain threshold, but it saturates as the energy density is further increased. The junction depth increases with the number of pulses and the laser energy density. The results suggest that this simple spin-on dopant pre-deposition technique can be used to obtain a well controlled doping profile similar to the technique using chemical vapor in pulsed laser doping process.

FIELD-EFFECT ON THERMAL EMISSION FROM THE 0.40 EV ELECTRON LEVEL IN INGAP

Results are reported of electric-field dependence on thermal emission of electrons from the 0.40 eV level at various temperatures in InGaP by means of deep-level transient spectroscopy. The data are analyzed according to the Poole-Frankel emission from the potentials which are assumed to be Coulombic, square well, and Gaussian, respectively. The emission mte from this level is strongly field dependent. It is found that the Gaussian potential model is more reasonable to describe the phosphorus-vacancy-induced potential in InGaP than the Coulombic and square-well ones.

HIGH-RESOLUTION DLTS AND ITS APPLICATION TO LATTICE-MISMATCH-INDUCED DEEP LEVELS IN INGAP

A new method of differentiating the deep level transient spectroscopy (DLTS) signal is used to increase the resolution of conventional DLTS. Using this method, more than one single deep level with small differences in activation energy or capture cross section, which are often hard to determine by conventional DLTS, can be distinguished. A series of lattice-mismatched InxGa1-xP samples are measured by improved DLTS to determine accurately the activation energy of a lattice-mismatch-induced deep level. This level cannot be clearly determined using conventional DLTS because the two signals partly overlap each other. Both the signals are thought to originate from a phosophorus vacancy and lattice-mismatch-induced defect.

PHOTOREFLECTANCE SPECTROSCOPY OF SEMIINSULATING GAAS

Clear observations of photoreflectance (PR) spectra due to excitonic transitions in semi-insulating GaAs bulk materials are reported. The modulation mechanism is attributed to the electromodulation induced by the Dember effect. This study indicates that the PR spectroscopy provides an important method for characterizing the crystal quality of high-resistivity GaAs.

OXYGEN IN INXGA1-XASYP1-Y GROWN ON GAAS

The measurement of DLTS on the alloy InxGa1-xAsyP1-y (0 less-than-or-equal-to y less-than-or-equal-to 0.3; 0.5 greater-than-or-equal-to x greater-than-or-equal-to 0.35) shows a new signal, labeled as E2, with an activation energy of E(c) - 0.61 eV and the SIMS signals show a large number of oxygen. To clarify is further, the energy of the deep level E2 is quantitatively calculated by using Vogl's tight-binding theory and Hjalmarson's deep level theory. As a result, the deep A1-symmetric level associated with an oxygen on the anion site of InxGa1-xAsyP1-y locates deeply in the band gap. Thus, the level E2 is considered to be induced by the oxygen impurity.