GaAs纳米线晶体结构及光学特性

采用分子束外延技术在N-型Si (111)衬底上利用自催化生长机制外延砷化镓(GaAs)纳米线,对生长的纳米线进行扫描电子显微镜测试,纳米线垂直度高,长度直径均匀度好.对纳米线进行光致发光(photoluminescence, PL)光谱测试,发现低温10 K下两个发光峰P1和P2分别位于1.493 eV和1.516 eV,推断可能是纤锌矿/闪锌矿(WZ/ZB)混相结构引起的发光以及激子复合引起的发光;随着温度升高,发现两峰出现红移,并通过Varshni公式拟合得到变温变化曲线.对纳米线进行变功率PL光谱测试,发现P1位置的峰位随功率增加而蓝移,而P2位置的峰位不变.通过拟合发现P1峰位与功率1/3次方成线性相关,判断可能是WZ/ZB混相结构引起的Ⅱ型发光;同时,对P2位置的峰位进行拟合,P2为激子复合发光.对纳米线进行拉曼光谱测试,从光谱图中发现GaAs WZ结构特有的E_2声子峰,因此证明生长出的纳米线为WZ/ZB混相结构,并通过高分辨透射电子显微镜更直观地观察到纳米线的混相结构.     

Optical properties of undoped,Be-doped,and Si-doped wurtzite-rich GaAs nanowires grown on Si substrates by molecular beam epitaxy

The photoluminescence (PL) of undoped, Si-doped, and Be-doped GaAs nanowires (NWs) grown on Si substrates by molecular beam epitaxy was investigated. PL peaks of the undoped and Be-doped NWs were observed at higher energies than the bandgap energy of GaAs bulk. According to X-ray diffraction analysis, the blue-shift is attributed to the wurtzite-rich GaAs NW structure. Impurity-related peaks were observed in the undoped NWs and the impurity was Si that diffused via interaction with the adatoms on the Si surface during the growth. A slight bandgap narrowing of the Be-doped GaAs NWs was observed from their PL spectra. The Si-doped NWs showed a very broad PL peak due to a larger density of Si-related defects originating from the heavy doping level. The dependence of the PL peaks of the NWs on temperature was also investigated.     

Raman spectra and structural peculiarities of GaAs nanowires

The Raman scattering spectra of nanowire samples in the region of fundamental modes (50–320 cm^?1) are studied to analyze the structural peculiarities of GaAs nanowires (nanowhiskers) grown on a Si (111) substrate. Factor analysis of the experimental data array consisting of 55 spectra, each of which contains 300 points, is carried out. It is found that the number of linearly independent contributions in these data arrays amounts to only two spectra. The form of each linearly independent contribution is close to zincblende (ZB) and wurtzite (WZ) spectra. It is established that ZB and WZ phases exist in nanowhiskers. Comparison with the calculated frequencies of the normal vibrations of hexagonal GaAs polytypes makes it possible to assume that the spectra of crystal nanowhiskers are combinations of ZB and WZ spectra with good accuracy and agree with the scheme of phonon branch folding of the ZB Brillouin zone.     

Photoluminescence investigation of Be-doped Npn AlGaAs/GaAs heterojunction bipolar transistor structures

Highly complex Npn AlGaAs/GaAs single heterojunction bipolar transistor (HBT) layers with Be-doped base were investigated by photoluminescence (PL) spectroscopy. Room temperature PL shows only a broad peak of GaAs due to thermalization; 15 K PL shows five peaks. The peak at 1.481 eV is from a p-type GaAs base, that at 1.517 eV is from a low-doped GaAs layer and that at 1.55 eV is from a high-doped GaAs collector. The that at 1.849 eV is due to bound exciton recombination in an AlGaAs emitter, and that at 1.828 eV is due to the acceptor-related transition from the AlGaAs layer. The integrated intensity ratio of these two peaks can be used to investigate the Be outdiffusion behavior, thus optimizing the growth conditions of base. The DC current gain of the HBT structure with different growth conditions was found to be in good agreement with the PL results.     

Bright blue emission from Te-doped ZnS nanowires

Optical properties of Te-doped ZnS (ZnS:Te) nanowires (NWs) synthesized by a thermal chemical vapor deposition method were investigated by cathodeluminescence and photoluminescence (PL) measurements. ZnS:Te NWs exhibit the blue emission with the maximum peak at ∼440 nm at room temperature. We calculated Te-induced states on the valence band and conduction band in ZnS bulk crystal compared with PL peaks of ZnS:Te NWs. Temperature-dependent PL indicated that the activation energy of electron confined in ZnS:Te NWs is 85 meV. Blue light-emitting dot matrix displays were also fabricated using ZnS:Te NWs. This result suggested that ZnS:Te NWs could be applied as a blue-color-emitter on display devices.     

Theoretical investigations for zinc blende-wurtzite polytypism in GaAs layers at Au/GaAs(1 1 1) interfaces

The zinc blende (ZB)-wurtzite (W) polytypism of GaAs layers at the Au/GaAs(1 1 1)B interfaces is investigated based on total-energy electronic-structure calculations within density functional theory. The calculations for the abrupt interfaces including a GaAs top layer with ZB and W stacking sequences reveal that the ZB sequence is energetically favorable, but the energy of W sequence with an interstitial Au atom at the top GaAs layer of the interface is lower than that of ZB sequence. This is because electrons accumulate around the interstitial region due to the hybridization between Au-6s and As-4p orbitals, resulting in the reduction of Ga-As bond charges. As a result, the relative stability at the top GaAs layer is determined by the electrostatic energy due to ionic charges. The results imply that the stabilization of W sequence at the Au catalyst-semiconductor interfaces as well as that on the nanowire faces are the origins for the appearance of W segments in NWs, qualitatively consistent with experiments.     

First principles study of structural stability,electronic structure and mechanical properties of ReN and TcN

The crystal structure, structural stability, electronic and mechanical properties of ReN and TcN are investigated using first principles calculations. We have considered five different crystal structures: NaCl, zinc blende (ZB), NiAs, tungsten carbide (WC) and wurtzite (WZ). Among these ZB phase is found to be the lowest energy phase for ReN and TcN at normal pressure. Pressure induced structural phase transitions from ZB to WZ phase at 214 GPa in ReN and ZB to NiAs phase at 171 GPa in TcN are predicted. The electronic structure reveals that both ReN and TcN are metallic in nature. The computed elastic constants indicate that both the nitrides are mechanically stable. As ReN in NiAs phase has high bulk and shear moduli and low Poisson's ratio, it is found to be a potential ultra incompressible super hard material.     

Low temperature photoluminescence study of Ga As defect states

Low temperature(77 K)photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality.Several defect-related luminescence peaks have been observed,including 1.452 eV,1.476 eV,1.326 eV peaks deriving from 78 meV Ga_(As) antisite defects,and 1.372 eV,1.289 eV peaks resulting from As vacancy related defects.Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states.The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.     

Ab initio study of the stability and electronic properties of wurtzite and zinc-blende BeS nanowires

In this work we study the structural stability and electronic properties of the Beryllium sulfide nanowires (NWs) in zinc-blende (ZB) and wurtzite (WZ) phases (with triangle and hexagonal cross sections), using first principle calculations within the plane-wave pseudopotential method. A phenomenological model is used to explain the role of dangling bonds in the stability of the NWs. In contrast to the bulk phase, the ZB-NWs with diameters less than 133.3 Å are found to be less favorable over the WZ-NWs, in which the surface dangling bonds (DBs) on the NW facets play an important role to stabilize the NWs. Furthermore, both ZB- and WZ-NWs are predicted to be semiconductor and the values of the band gaps are dependent on the surface DBs as well as the size and shape of the NWs. Finally, we obtain atom projected density of states (PDOSs) by calculating the localized density of states on the surface atoms, as well as on the core and edge atoms.     

Self-consistent model of nanowire growth and crystal structure with regard to the adatom diffusion

A self-consistent model of growth and structure of semiconductor nanowires is proposed. The crystal phase of group III–V semiconductor nanowires is studied. The critical radius of the transition from the hexagonal wurtzite (WZ) structure to the cubic structure of zinc blende (ZB) type is calculated as a function of parameters of the system of materials and the gaseous medium supersaturation. The model presented here is applicable to both gas-phase and molecular beam epitaxies and allows one to calculate the probability of formation of the WZ and ZB phases under various deposition conditions.     

Critical diameters and temperature domains for MBE growth of III–V nanowires on lattice mismatched substrates

We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au‐assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation‐free III–V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and properties of self‐catalyzed (In,Mn)As nanowires

Mn‐assisted molecular beam epitaxy is used for the growth of (In,Mn)As nanowires (NWs) on GaAs(111)B. The transmission electron microscopy measurements revealed that despite the relatively high growth temperature regime this technique can be used to obtain (In,Mn)As NWs with high crystalline quality without any crystal defects, such as dislocations, stacking faults or precipitates inside the investigated NWs or on their side‐walls, although the growth processes of NWs were accompanied by the formation of MnAs precipitates between the NWs at the interface of the wetting layer. The results obtained are of importance for the realization of new spintronic nanostructured materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effects of thermal annealing on the interband transitions of single and vertically stacked InAs/GaAs self-assembled quantum dots

Photoluminescence (PL) measurements have been carried out to investigate the annealing effects in one-period and three-periods of InAs/GaAs self-assembled quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. After annealing, the PL spectra for the annealed InAs/GaAs QDs showed dramatic blue shifts and significant linewidth narrowing of the PL peaks compared with the as-grown samples. The variations in the PL peak position and the full width at half-maximum of the PL peak are attributed to changes in the composition of the InAs QDs resulting from the interdiffusion between the InAs QDs and the GaAs barrier and to the size homogeneity of the QDs. These results indicate that the optical properties and the crystal qualities of InAs/GaAs QDs are dramatically changed by thermal treatment.     

低压-MOCVD方法制备CdS-ZnS应变多量子阱

本文报道了用低压(LP)-MOCVD方法制备CdS-ZnS应变多量子阱结构.通过X-射线衍射谱证实了所制备的样品具有比较好的多层结构,并通过77K下的光致发光(PL)光谱,观测到了大的垒层对电子的限制效应及由大的应变引起的阱层带边变化所导致的发光峰蓝移.     

不同晶向SrTiO3上外延GaAs薄膜的光谱研究

利用MBE生长技术,成功地在不同晶向SrTiO3(100)(111)(110)衬底上生长了GaAs薄膜,利用显微Raman和荧光光谱（PL）对此进行了研究。实验结果表明,在不同晶向SrTiO3上生长的GaAs薄膜有不同的晶向和应力状态。荧光光谱（PL）研究表明在SrTiO3(100)(111)晶面上生长的GaAs薄膜的PL峰发生明显的蓝移。研究表明在SrTiO3(110)面上生长的GaAs薄膜和体单晶基本上一致,有更好的光学质量。     

Phase equilibrium of Cd_(1-x)Zn_xS alloys studied by first-principles calculations and Monte Carlo simulations

The first-principles calculations based on density functional theory combined with cluster expansion techniques and Monte Carlo(MC) simulations were used to study the phase diagrams of both wurtzite(WZ) and zinc-blende(ZB)Cd_(1-x)Zn_xS alloys.All formation energies are positive for WZ and ZB Cd_(1-x)Zn_xS alloys,which means that the Cd_(1-x)Zn_xS alloys are unstable and have a tendency to phase separation.For WZ and ZB Cd_(1-x)Zn_xS alloys,the consolute temperatures are 655 K and 604 K,respectively,and they both have an asymmetric miscibility gap.We obtained the spatial distributions of Cd and Zn atoms in WZ and ZB Cd_(0.5)Zn_(0.5)S alloys at different temperatures by MC simulations.We found that both WZ and ZB phases of Cd_(0.5)Zn_(0.5)S alloy exhibit phase segregation of Cd and Zn atoms at low temperature,which is consistent with the phase diagrams.     

Structural transition behavior of ZnS nanotetrapods under high pressure

ZnS nanotetrapods synthesized via a solvothermal route have a octahedral core with a zincblende (ZB) structure and four hexprism-shaped arms consisting of alternately stacking ZB and wurtzite (WZ) phases, where the WZ phase has a higher volume percentage. In situ angular-dispersive X-ray diffraction (ADXRD) measurements were carried out to study the structural behavior of ZnS nanotetrapods under high pressure up to 41.3?GPa. The initial WZ structure exhibits a very high mechanical stability to ～11.3?GPa. Both the WZ and ZB structures transform to the rocksalt (RS) structure at ～15.4?GPa. The bulk moduli of the WZ (148.2?±?8.9?GPa) and RS (165.6?±?9.9?GPa) phases are both larger than the previously reported values. These phenomena are discussed based on the alternating epitaxial growth of the WZ and ZB phases in the arms of nanotetrapods. Our study suggests that the internal structure of nanomaterials could also greatly affect their stability and transition behavior.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Hole-mediated stabilization of cubic GaN

We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.     

Surface saturation control on the formation of wurtzite polytypes in zinc blende SiC nanofilms grown on Si-（100） substrates

We investigate the formations of wurtzite （WZ） SiC nano polytypes in zinc blende （ZB） SiC nanofilms hetero-grown on Si-（100） substrates via low pressure chemical vapor deposition （LPCVD） by adjusting the Si/C ratio of the introduced precursors. Through SEM, TEM, and Raman characterizations, we find that the nanofilms consist of discrete WZ SiC nano polytypes and ZB SiC polytypes composed of WZ polytypes （WZ ＋ ZB） and disordered ZB SiC polytypes, respectively, according to Si/C ratios of 0.5, 1.5, and 3. We attribute the WZ polytype formation to being due to a kinetic mechanism based on the Si/C surface saturation control.