金红石型TiO_2中四种点缺陷态研究

利用第一性原理计算方法研究了金红石型TiO_2中四种缺陷的电子态.这四种缺陷包括氧空位(O_v)、钛空位(Ti_V)、钛间隙(Ti_S)以及氧空位O_v与钛间隙态Ti_S共存态.氧空位的存在导致禁带内施主缺陷能级较浅,而深施主能级与Ti间隙态有关.预测了氧空位更倾向于与钛间隙结合,主要通过钛间隙态的3d电子部分转移到近邻近氧空位的部分形成O_V-Ti_S对缺陷.具有O_v、Ti_S或O_V-Ti_S缺陷的体系都出现间隙态,促进体系出现红外吸收.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.     

Nature of native defects in ZnO

This study revealed the nature of native defects and their roles in ZnO through positron annihilation and optical transmission measurements. It showed oxygen vacancies are the origin for the shift in the optical absorption band that causes the red or orange coloration. It also revealed experimental evidence that the donor nature of oxygen vacancy is approximately 0.7 eV. In addition, this work showed the Zn interstitial was not the donor in the as-grown ZnO and supported recent calculations that predicted hydrogen in an oxygen vacancy forms multicenter bonds and acts as a shallow donor.     

Mutual passivation of donors and isovalent nitrogen in GaAs

We study the mutual passivation of shallow donor and isovalent N in GaAs. We find that all the donor impurities, SiGa, GeGa, SAs, and SeAs, bind to N in GaAs:N, which has a large N-induced band-gap reduction relative to GaAs. For a group-IV impurity such as Si, the formation of the nearest-neighbor SiGa-NAs defect complex creates a deep donor level below the conduction band minimum (CBM). The coupling between this defect level with the CBM pushes the CBM upwards, thus restoring the GaAs band gap; the lowering of the defect level relative to the isolated SiGa shallow donor level is responsible for the increased electrical resistivity. Therefore, Si and N mutually passivate each other's electrical and optical activities in GaAs. For a group-VI shallow donor such as S, the binding between SAs and NAsdoes not form a direct bond; therefore, no mutual passivation exists in the GaAs:(S+N) system.     

Li 掺杂ZnO薄膜的导电和发光特性

通过将含有原子数分数为2%锂的Zn-Li合金薄膜和金属锌薄膜在500℃氮气氛中退火2h,然后在700℃氧气氛下退火1h的方法分别制备出ZnO:Li和ZnO薄膜。Hall效应测量表明,其导电类型分别为p型和n型。通过He-Cd激光器的325nm线激发,测量了样品室温和低温(12K)光致发光光谱,并根据ZnO:Li薄膜的低温发光光谱特征,计算出Li相关受主能级位于价带顶137meV处。     

Shallow donors in semiconductor nanoparticles: limit of the effective mass approximation

The spatial distribution of the electronic wave function of a shallow, interstitial Li donor in a ZnO semiconductor nanocrystal has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by electron nuclear double resonance spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles.     

Effect of interstitial hydrogen impurities on ferroelectric polarization in PbTiO3

We investigate the electronic and atomic structure of interstitial hydrogen impurities in PbTiO3 and investigate their effect on ferroelectric polarization through first-principles total-energy calculations. Interstitial H in PbTiO3 is found to bond to oxygen and to act as a shallow donor impurity. At equilibrium, H does not much affect nearby Ti-O bonds and the H-O dipole increases the polarization. The barrier for reversing the defect dipole can give rise to imprint (extra stabilization of one of the two polarization modes).     

分子束外延高Mg掺杂GaN的发光特性

采用分子束外延技术在蓝宝石衬底上制备Ｍｇ掺杂的立方相ｐ－ＧａＮ,并对其不同温度下的光致发光光谱进行了研究．实验观察到高Ｍｇ掺杂ＧａＮ中施主受主对发光的反常温度行为．理论分析表明,高Ｍｇ掺杂ＧａＮ中施主受主对的发光受到陷阱与受主间竞争俘获非平衡空穴过程和空穴隧穿输运过程的影响．     

Role of point defects on conductivity, magnetism and optical properties in In2O3

In order to assess the effects of point defects including transition metal doping on its electronic structure, the self-consistent band structure of the transparent oxide In₂O₃ (in the Ia₃ structure) has been calculated with oxygen vacancies, oxygen and indium interstitial atoms and several transition metal dopants using density functional theory based first principles calculations. An oxygen vacancy alone does not act as a strong native donor but when combined with interstitial indium and (substitutional) transition metal doping, shallow donor levels close to the conduction band are formed. Spin polarized calculations show measurable magnetism in some of the transition metal doped systems while the dielectric functions indicate whether such systems remain transparent among other things.     

Direct kinetic correlation of carriers and ferromagnetism in Co2+: ZnO

The hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (Co2+: ZnO) is mediated by charge carriers was tested by controlled introduction and removal of the shallow donor interstitial zinc. Using oriented epitaxial Co2+: ZnO films grown by chemical vapor deposition, kinetics measurements demonstrate a direct correlation between the oxidative quenching of ferromagnetism and the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of Co2+: ZnO and, in the process, unambiguously reveal this ferromagnetism to be dependent upon charge carriers. The distinction between defect-bound and free carriers in Co2+: ZnO is discussed.     

Hydrogen shallow donors in Co-doped ZnO showing room-temperature ferromagnetism

Room-temperature ferromagnetism has been identified in sol–gel-prepared Co-doped ZnO (ZnCoO), hydrogen being incorporated by a linear polymer polyvinyl pyrrolidone. The magnetic order was investigated by the X-ray photoemission and magnetization measurements in view of the interstitial hydrogen being coupled with Co²⁺. The electron paramagnetic resonance measurements revealed that hydrogens not contributing to the magnetic order occupy the shallow donor levels in the hydrogenated ZnCoO system.     

CuI晶体缺陷态电子结构及其施主-受主对发光机理的研究

运用相对论密度泛函理论和嵌入分子团簇方法，模拟计算了具有γ相CuI晶体的本征缺陷态电子结构.结果表明，四面体间隙Cu和Cu空位最有可能在禁带中引入浅施主和受主能级，从而形成施主-受主对(DAP)，产生420—430 nm的DAP复合发光. 关键词： γ-CuI晶体 密度泛函理论 电子结构 发光机理     

Theory of the electron and nuclear spin coherence times of shallow donor spin qubits in isotopically and chemically purified zinc oxide

In this article, I present a theoretical study of the electron and nuclear spin coherence times of shallow donor spin qubits in zinc oxide (ZnO) at low temperature. The influence of different spin-phonon processes as well as different spin-spin processes on the spin coherence time of shallow donors in ZnO is considered, both in the case of an electron spin qubit and in the case of a nuclear spin qubit encoded on a shallow donor. It is estimated that the electron spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of microseconds, limited by the nuclear spectral diffusion process. The electron spin coherence time of an isolated indium shallow donor can be extended to few milliseconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the electron spin coherence time of an isolated indium shallow donor is only limited by a spin-lattice decoherence process. It is also estimated that the nuclear spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of milliseconds, limited by the nuclear spectral diffusion process. The nuclear spin coherence time of an isolated indium shallow donor can be extended to few seconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the nuclear spin coherence time of an isolated indium shallow donor is only limited by the cross relaxation decoherence process. This study thus shows the great potential of electron and nuclear spin qubits encoded on shallow donors in isotopically and chemically purified quasi-intrinsic ZnO for the implementation of quantum processor and/or quantum memories.     

Long Lifetime State of Shallow Donor Centres for Silicon Based THz Sources

Impurity centres in high purity silicon with ionization energies 38.32 and 40.09meV are observed by photothermal ionization spectroscopy measurements. Their typical shallow donor characteristics are approved by good agreement with theoretic evaluation under varying magnetic fields. The 2p0 state lifetime of one donor centres are more than 3 times longer than that of phosphorus in the same silicon sample. Compared to phosphorus which has been already successfully exploited to produce silicon-based THz radiations, this shallow donor centre has a pronounced enhancement on lifetime and quality factor of population inversion level. Hence, silicon with this donor centre would be a potential excellent candidate to develop improved silicon-based THz sources.     

Shallow donor states induced by in-diffused Cu in ZnO: a combined HREELS and hybrid DFT study

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnO(0001) surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.     

Influence of hydrostatic pressure on the native point defects in wurtzite ZnO: Ab initio calculation

The formation energies and transition energy levels of native point defects in wurtzite ZnO under applied hydrostatic pressure are calculated using the first-principle band-structure methods. We find that the pressure coefficient of the (2+/0) level for oxygen vacancy is larger than that of the (2+/1+) level for zinc interstitial, which demonstrates that the donor level of oxygen vacancy is deeper than that of zinc interstitial, therefore the latter is the more probable electron resource in native n-type ZnO. And the significantly different pressure dependence of the transition levels between them can be used to determine the origin of the green luminescence center in ZnO. Zinc octahedral interstitial and oxygen tetrahedral interstitial configurations became the dominant defects under 5 GPa at their favorable growth conditions, respectively. The formation of defects under applied pressure is the result of fine interplay between internal strains, charges on the defects and applied external pressures.     

Energy levels from lattice defects in AgGaS2

The luminescence of single crystals of AgGaS₂ from shallow centers is studied. As-grown crystals and crystals annealed in S, Ga, or in vacuum, were used. We show that the S vacancy causes a donor level at 50 meV (probably due to a charge state of V_S) and that the cation vacancy (perhaps Ag) introduces an acceptor level at 110 meV. Two other levels are found: a donor level at 30 meV and an acceptor level at 200 meV. Their origin is unknown.     

The binding energy of a shallow donor in type-II quantum wells

The binding energy of a shallow donor in type-II quantum wells was calculated by a variational method, using a single parameter. A type-II AlAs/GaAs single quantum well was chosen to calculate the binding energy of a shallow donor as a function of the donor position and the barrier width.     

氮化镓纳米粒子的制备及光致发光研究

采用直接氮化法制备出尺寸不同的GaN纳米粒子,分别利用X射线衍射仪（XRD）、扫描电子显微镜（SEM）和光致发光（PL）谱测试手段对所制样品进行表征和发光特性的研究。结果表明：所制备的两种GaN纳米粒子直径分别为100 nm和300 nm左右。在GaN纳米粒子的PL谱中,中心在357 nm的发射源于本征发光,中心在385 nm的发射带源于浅施主能级到价带的辐射复合,中心在560 nm左右的发射带源于浅施主能级到深受主能级间的施主-受主对辐射发光。     

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the ²⁹Si and ¹³C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the ²⁹Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T₁. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T₁ and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.