Growth and characterization of oriented CdSe nanobelts and nanorods

Oriented CdSe nanobelts and nanorods were grown successfully on GaAs and Si substrates by metal organic chemical vapor deposition. The thickness of Au film coated on the substrate plays an important role in determining the orientation, size, and density of these one-dimensional CdSe nanostructures. Preferred orientation was observed for nanostructures grown on the GaAs substrate coated with thick Au film, but not for the nanostructures grown on the Si substrate. Photoluminescence, transmission electron microscope, and X-ray diffraction measurements show that the CdSe nanostructures could have either wurtzite or zinc-blende structures, and there are more nanostructures with wurtzite structure than with zinc-blende structure.     

Electron diffraction on GaAs nanowhiskers grown on Si(100) and Si(111) substrates by molecular-beam epitaxy

The crystal structure of GaAs nanowhiskers grown by molecular-beam epitaxy on Si(111) and Si(100) substrates is investigated using reflection high-energy electron diffraction (RHEED). It is revealed that, in both cases, the electron diffraction images contain a combination (superposition) of systems of reflections characteristic of the hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) phases of the GaAs compound. The growth on the Si(111) substrates leads to the formation of nanowhiskers with hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) structures with one and two orientations, respectively. In the case of the Si(100) substrates, the grown array contains GaAs nanowhiskers that have a cubic structure with five different orientations and a hexagonal structure with eight orientations in the (110) planes of the substrate. The formation of the two-phase crystal structure in nanowhiskers is explained by the wurtzite—sphalerite phase transitions and/or twinning of crystallites.     

Nanoscale spatial phase modulation of GaAs growth in V-grooved trenches on Si(001) substrate

This letter reports the nanoscale spatial phase modulation of Ga As growth in V-grooved trenches fabricated on a Si(001) substrate by metal–organic vapor-phase epitaxy. Two hexagonal Ga As regions with high density of stacking faults parallel to Si {111} surfaces are observed. A strain-relieved and defect-free cubic phase Ga As was achieved above these highly defective regions. High-resolution transmission electron microscopy and fast Fourier transforms analysis were performed to characterize these regions of Ga As/Si interface. We also discussed the strain relaxation mechanism and phase structure modulation of Ga As selectively grown on this artificially manipulated surface.     

Wurtzite and zinc-blende CdSe based core/shell semiconductor nanocrystals: Structure, morphology and photoluminescence

We report comparative study of core/shell nanocrystals based on wurtzite and novel zinc-blende CdSe core. Both wurtzite and zinc-blende CdSe are coated with CdS shell or CdS/ZnS multishell under identical synthetic parameters. Crystal structure analysis finds that CdS shell is wurtzite on either wurtzite or zinc-blende CdSe cores. Morphology and photoluminescence studies exhibit that for zinc-blende CdSe based samples, the shell growth is in fine epitaxy and the obtained core/shell nanocrystals show high quantum yield both before and after surface modification process; while wurtzite CdSe based samples have irregular shape indicating inhomogeneous shell growth, and are with lower quantum yield. Furthermore, in the photoluminescence spectra exited with UV radiation, wurtzite CdSe based samples show side peaks of independently nucleated nanocrystals from the shell material; while samples with zinc-blende CdSe cores are potent in restricting these byproducts, which may attribute to the highly effective arrestment of precursor ions onto the zinc-blende CdSe surface. These features manifest that zinc-blende CdSe is more talented than conventional wurtzite CdSe in achieving core/shell nanocrystals with higher qualities.     

Strain relaxation in epitaxial GaAs/Si (0 0 1) nanostructures

Abstract

Crystal defects, present in ~100 nm GaAs nanocrystals grown by metal organic vapour phase epitaxy on top of (0 0 1)-oriented Si nanotips (with a tip opening 50–90 nm), have been studied by means of high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The role of 60° perfect, 30° and 90° Shockley partial misfit dislocations (MDs) in the plastic strain relaxation of GaAs on Si is discussed. Formation conditions of stair-rod dislocations and coherent twin boundaries in the GaAs nanocrystals are explained. Also, although stacking faults are commonly observed, we show here that synthesis of GaAs nanocrystals with a minimum number of these defects is possible. On the other hand, from the number of MDs, we have to conclude that the GaAs nanoparticles are fully relaxed plastically, such that for the present tip sizes no substrate compliance can be observed.     

Growth and characterization of straight InAs/GaAs nanowire heterostructures on Si substrate

Vertical InAs/GaAs nanowire (NW) heterostructures with a straight InAs segment have been successfully fabricated on Si (111) substrate by using AlGaAs/GaAs buffer layers coupled with a composition grading InGaAs segment. Both the GaAs and InAs segments are not limited by the misfit strain induced critical diameter. The low growth rate of InAs NWs is attributed to the AlGaAs/GaAs buffer layers which dramatically decrease the adatom diffusion contribution to the InAs NW growth. The crystal structure of InAs NW can be tuned from zincblende to wurtzite by controlling its diameter as well as the length of GaAs NWs. This work helps to open up a road for the integration of high-quality III-V NW heterostructures with Si.     

纤锌矿GaN/AlxGa1-xN量子阱中极化子能量

采用LLP变分方法研究了纤锌矿GaN/Al_xGa_1-xN量子阱材料中极化子的能级,给出极化子基态能量、第一激发态能量和第一激发态到基态的跃迁能量与量子阱宽度和量子阱深度变化的函数关系。研究结果表明,极化子基态能量、第一激发态能量和跃迁能量随着阱宽L的增大而开始急剧减小,然后缓慢下降,最后接近于体材料GaN中的相应值。基态能量和第一激发态到基态的跃迁能量随着量子阱深度的增加而逐渐增加,窄阱时这一趋势更明显。纤锌矿氮化物量子阱中电子-声子相互作用对能量的贡献比较大,这一值(约40meV)远远大于闪锌矿(GaAs/Al_xGa_1-xAs)量子阱中相应的值(约3meV)。因此讨论GaN/Al_xGa_1-xN量子阱中电子态问题时应考虑电子-声子相互作用。     

STM study of Si(111)1 × 1-H surfaces prepared by in situ hydrogen exposure

We have used scanning tunneling microscopy and low-energy electron diffraction to study the preparation of hydrogen-terminated Si(111)1 × 1 surfaces by in situ atomic-hydrogen exposure of Si(111)7 × 7 surfaces. We find that exposure at sample temperatures of 350–480°C with a hydrogen dose above 1000 L results in the complete transformation of the 7 × 7 structure to the H-terminated 1 × 1 structure. The highest quality Si(111)1 × 1-H surfaces are obtained for doses of 5000 L hydrogen at a temperature around 380°C. These surfaces have less than 5% of a monolayer stacking faults, approximately 1% point-like defects, and less than 0.5% of contamination. For larger hydrogen doses the amount of stacking faults is further reduced, but the surfaces become rough due to the formation of holes in the first bulk double layer. A discussion of the temperature dependence of the removal of the stacking faults is presented as well as a discussion on the origin of the most frequently occurring point-like defects.     

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy ($V_{\rm N})$ in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum (VBM) and a singly occupied triple state below the conduction band minimum (CBM) for wurtzite AlN and above the CBM for zinc-blende AlN. So $V_{\rm N}$ acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level $E({3 + } \mathord{\left/ {\vphantom {{3 + } + }} \right. \kern-\nulldelimiterspace} + )$ with very low formation energy appears at 0.7 and 0.6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.     

Interfacial reactions of ultrahigh vacuum deposited ytterbium thin films on silicon

Epitaxial ytterbium silicide thin films were grown on (111)Si by ultrahigh vacuum deposition and subsequent thermal annealing. The epitaxial YbSi(2-x) thin films consist of various kinds of defects such as vacancies, stacking faults, and pinholes. The vacancies were ordered so as to relax the compressive stress in Si sublattice of YbSi(2-x) thin films. The vacancy ordering structure is of an out-of-step structure with higher vacancy concentration after higher temperature annealing so that the compressive stress was further relaxed. A high density of stacking faults was present in the epitaxial YbSi(2-x) thin films. The stacking faults were annihilated by high temperature annealing. Pinholes also formed in the epitaxial YbSi(2-x) thin films and could be avoided by appropriate fabrication process. The epitaxial YbSi(2-x) thin films were thermally stable up to 1000 degrees C.     

Cubic InN on -plane sapphire

InN has been grown directly on r-plane sapphire substrates by plasma-induced molecular beam epitaxy. X-ray diffraction and transmission electron microscopy investigations have shown that the InN layers consist of a predominant zinc-blende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase whose content increases with proceeding growth. The InN layer is defect rich with a high number of stacking faults and twins. As a consequence a very high residual doping of was estimated. The lattice constant for the zinc-blende phase of InN was found to be a=4.986 Å. The optical investigations were strongly affected by a high number of defects, but nevertheless indicated an absorption edge below 0.6 eV. For this unusual growth of the metastable cubic phase on a noncubic substrate an epitaxial relationship was proposed, where the metastable zinc-blende phase grows directly on the r-plane of sapphire.     

Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X‐ray diffraction

In GaAs nanowires grown along the cubic [111]_c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)_c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X‐ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18–25 nm. The measurements are performed with a nano‐focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)_c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% ± 0.02% larger than in the zinc blende arrangement.     

Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111)

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.     

Pressure-induced phase transition in wurtzite ZnS: An ab initio constant pressure study

We study the pressure-induced phase transition of wurtzite ZnS using a constant pressure ab initio technique. A first-order phase transition into a rocksalt state at 30–35 GPa is observed in the constant pressure simulation. We also investigate the stability of wurtzite (WZ) and zinc-blende (ZB) phases from energy–volume calculations and Gibbs free energies at zero temperature and find that both structures show nearly similar equations of state and transform into a rocksalt structure around 14 GPa, in agreement with experiments. Additionally, we examine the influence of pressure on the electronic structure of the wurtzite and zinc-blende ZnS crystals and find that their band gap energies exhibit similar tendency and increase with increasing pressure. The calculated pressure coefficients and deformation potential are found to be comparable with experiments.     

Influence of impurities on the surface structures and fault generation in homoepitaxial Si (111) films

In situ LEED studies of the homoepitaxial growth of Si(111) films by uhv sublimation, indicate a strong correlation between the type of surface structure generated and the metallic impurity content of the silicon substrates as estimated from minority carrier lifetimes. The development of the familiar Si (111)−7 × 7 structure is favored by the presence of lifetime-killing impurities in the substrate. Experiments where Fe is introduced on high lifetime substrates prior to annealing and film growth, suggest that this impurity species plays a role in the generation of the 7 × 7 surface structure. Electron microscopy reveals that homoepitaxial Si(111) layers are generally faulted, the number density of which increases progressively as the growth temperature is lowered and the deposition rate increased. Films deposited on high lifetime silicon contain substantially fewer stacking faults than those grown on low lifetime substrates. These results suggest that the faults originate at the substrate surface at microprecipitates consisting of fast diffusing, low solubility impurity species.     

Stacking faults in close-packed clusters

Ground state geometries of small hard sphere clusters were studied using two different type of contact interaction, a pair-potential and a many-atom interaction. Monte Carlo method in an FCC lattice with all possible (111) stacking faults was used to obtain the minimum energy geometries for clusters up to 59 atoms. Due to the surface energy, FCC packing is generally favoured as opposite to the HCP structure. However, in most cluster sizes the ground state obtained with the many-atom interaction has one or more stacking faults. The most symmetric geometry is usually not the ground state. Clusters with 59 and 100 atoms were studied due the possibility of a high symmetry cluster with stacking faults in all four directions. The size dependence of the total energy has similarities with that of the average moment of inertia. Received 6 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002     

Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in wurtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs (111)A, and GaAs (111)B oriented substrates by means of molecular beam epitaxy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon frequencies are in agreement with other studies and with lattice dynamic calculations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrates than in samples grown on GaAs (001) substrates. The observation of disorder-activated modes gives information about sample quality. Comparison of the spectra from different thickness epilayers shows that the GaN is more highly disordered close to the substrate, particularly for the (001) substrates. Received 16 July 1999     

掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

Understanding periodically twinned structure in nano-wires

Using first-principles calculations we have investigated the high stability of twinned nano-wires, which explains why the stacking faults always appear. Furthermore, we present a growth model to describe the formation mechanism of the stacking faults in the compound nano-wire with zinc-blende structure (e.g. SiC). And the model is confirmed by the numerical calculation based on the point charge approximation.     

Structural evolution of self‐assisted GaAs nanowires grown on Si(111)

GaAs nanowires are grown on Si(111) by self‐assisted molecular beam epitaxy, and the ratio between wurtzite and zinc‐blende phases is determined as function of nanowire length using asymmetric X‐ray diffraction. We show that under the applied growth conditions, nanowires grow in both phases during the initial stage of growth, whereas the zinc‐blende content increases with growth time and dominates in long nanowires. Compared to the zinc‐blende units, the vertical lattice parameter of the wurtzite segments is 0.7% larger, as measured by the positions of respective diffraction peaks. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)