Search

Atomic force microscopy (AFM) and power-dependent micro-photoluminescence (μ-PL) spectroscopy are used to study the structure and exciton energy states in InAs quantum dots (QDs) grown on an In0.35Ga0.65As template on GaAs (311)B. The In0.35Ga0.65As template, consisting of a two-dimensionally modulated layer of closely packed connected cells, has a remarkable effect on the optical properties of the InAs QDs. By comparing the emission spectra of the samples without and with InAs QDs and the work carried out by Gong et al. [J. Cryst. Growth 251 (2003) 150; Appl. Phys. Lett. 81 (2002) 3254] we conclude that the existence of the In0.35Ga0.65As template enhances the photo-absorption and therefore the exciton emission from the QDs due to efficient exciton transfer from the template into the QDs. Furthermore, the PL emission from the QDs clearly reveals four discrete energy levels, S, P, D, and F with increasing excitation power.

The reentrant spin glass behaviour was observed in half doping (La,Pr)1/2Ca1/2MnO3 manganites with a CE-type antiferromagnetic structure. It shows sequential multiple magnetic transitions from the paramagnetic to the ferromagnetic, antiferromagnetic, spin glass (SG) transitions, which reflects the complex magnetic interaction in the ground state of manganites. This can be explained by the competition interaction between the ferromagnetic and the CE-type antiferromagnetic matrix, and the disorder due to the tolerance factor t and A-cation size variance ?2. The results reveals the coexistence of ferromagnetic clusters and SG clusters in the background of the antiferromagnetic matrix in the ground state of half doping (Pr,La)1/2Ca1/2MnO3 systems.

In this paper the nucleation and growth processes of GaAs(001) molecular-beam epitaxy were studied by Monte Carlo simulation. The density of islands and the density of isolated Ga adatoms were obtained for different growth temperatures, and the island size distribution at low coverage, as well as the correlation function between atoms and its relation with the temperature were studied in detail.

We report the far-infrared photoconductivity spectroscopy (FIRPCS) of the high purity n-GaAs grown by molecular beam epitaxial (MBE) technique. The current-voltage (I-V) measurement and FIRPCS under different electric field strength have been performed on high purity n-GaAs at 4.2 K. Except for photothermal ionization process, a new far-infrared photoconductivity (FIRPC) mechanism is observed. This photo ionization process is assisted by impact ionization and depends on external electric field. This new mechanism originates from photo-field ionization.

The behaviour of doped impurity in poly (3-methyl thiophene) - P3MT has been studied by Raman, photoluminescence (PL) and photoreflectance (PR) spectroscopies. It is experimentally demonstrated that the behaviour of doped impurity in conducting polymer P3MT is different from that in conventional inorganic semiconductors. The doped impurity is located at the spaces between the molecular chains in P3MT film and the band gap of P3MT increases with doping density.

An empirical relation between the localization of impurity local modes, and the mass defect factor is obtained by calculating the local density of states (LDOSs) of the impurity and its surrounding atoms in a semiconductor. Based on this empirical relation, the calculation of the frequency of local mode by the mass defect model including the localization effect shows a good agreement between the theoretical and experimental results.

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.

The optimum photo-thermal ionization range near liquid helium temperature for the shallow impurities in n type high purity silicon single crystal was determined experimentally. At temperature within this range, the high resolution photo-thermal ionization spectra (PTIS) of this sample were measured. Positive PTIS responses from the majority residual impurity phosphor (P) and compensating impurity boron (B) were observed by simultaneously irradiating the sample with far infrared and band-edge light. Under action of applied external magnetic fields, the positive response from compensating impurity B changes to negative response. The posibility of these phenomena being the result of temperature effect was excluded, which revealed the deficiency of Darken model and supported the model of rapid recombination of minority carriers for PTIS response of compensating impurity in elemental semiconductors.

The results of high sensitivity photo-thermal ionization spectroscoic (PTIS) investigation of shallow acceptors in high purity p-type germanium sample with room temperature resistivity of about 50Ω·cm, at temperature above liquid helium by Fourier transform spectrometer and associated magneto-optical measurement systems, are reported. The optimum photo-thermal ionization temperature range for the shallow impurities in high purity germanium is determined experimentally. At temperatures within this range, the PTIS spectra of this sample were measured. Boron and aluminum are found to be the main shallow acceptors in the sample. The causes of line splits of spectra, rapid recombination of compensating impurity and random strain, are analyzed and discussed.

We have investigated two types of Zn1-xCdxSe quantum islands in CdSe/ZnSe heterostructure with different size and Cd composition using micro-photoluminescence and micro-Raman spectroscopy. The micro-photoluminescence spectra at 4.2 K indicate that the photoluminescence peaks of the islands have a large red-shift of 166 meV when the thickness of CdSe deposition layer increased from 1.8 ML to 2.3 ML, which can't be explained by considering the change of quantum confinement potential only. We also found two other important mechanisms which may lead to the large red-shift by comparing the micro-photoluminescence and micro-Raman spectra. First, the sheet density of large Zn1-xCdxSe islands which have lower exciton ground state energy will increase and the large islands will dominant gradually the photoluminescence properties of the samples with increasing thickness of the CdSe layer. Second, the increase of Cd composition in the Zn1-xCdxSe quantum islands contributes to the large red-shift as well. This is verified by resonant Raman scattering spectra of the samples.

We have investigated the temperature_dependent photoluminescence of two types of quantum islands in ultrathin CdSe/ZnSe layers. At low temperatures, excitons in these two types of quantum islands can transfer by tunneling. The hopping process of the localized excitons gradually becomes the dominant mechanism with increasing temperature. We found that the small islands are nano_scale clusters with quasi_zero_dimensional properties, which are different from those of the large islands. Experiments indicated that these two types of islands are optically correlated although their excitonic properties are different.

Using the material chip technology,large area photodiodes of n-on-p structure with different boron implantation dose are fabricated on the Hg1-xCdxTe film for mid-infrared wavelength region(x=0.291).Current-voltage characteristics of the photodiodes are measured at 77K and zero bias resistance-area products of different photodiodes are fitted from the data in the voltage range of -0.2—0.08V.The study indicated that the R0A products of different elements depended distinctly upon the implanted boron dose.A large R0 value has also obtained in another chip with x=0.2743.All the samples in this study are grown by Riber 32P molecular-beam epitaxy system and all the junctions-forming process is same to the standard planar technology but using a series of metallic masks during the boron ion implantation.

The photocurrent spectra characteristics of the 15μm very long wavelength GaAs/AlGaAs quantum well infrared photo detector(QWIP) have been investigated.The two photocurrent response peaks, which have a Gaussian lineshape due to the random fluctuation of the well width,are attributed to the transitions from the ground state E0 to the first excited state E１ and the third excited state E3 in the well,respectively.The transition peaks shift linearly to higher energy as the bias across the detector increases.Taking into account electrons exchange effect,we find that the theoretical calculation using the transfer matrix method agrees well with the experimental data.

We report the growth of Bi2Ti2O7 thin films on n type Si substrates by the chemical solution decomposition technique. Both the X-ray double-crystal diffraction and atomic force micro spectroscopy measurements are used to check the film properties. It is shown that the film is a multi crystal film dominated by the Bi2Ti2O7 phase. The C-V measurements are also performed on Au/Bi2Ti2O7/n-Si(100) MOS structure. It is revealed that both the fixed and mobile negative charges are contained in the film. The mobile negative charge results in the hysteresis loops on C-V curve.

We have studied the carrier concentration and the mobility in GaN expitaxial thin films deposited on sapphire substrate using infrared reflection spectroscopy. By theoretical calculation and fitting with the experimental IR reflection spectra for a series of Si-doped GaN epilayers and the sapphire substrate, we obtain the phonon vibrating parameters and plasmon frequency and damping constant in GaN. The carrier concentration and mobility have been deduced. The results show that the data for carrier concentration coincide with Hall measurement while the mobility is lower than Hall data by a factor of about 0.5. The variation of the LO phonon-plasmon coupling mode with doping level has been clearly observed. Raman measurement has been performed on the same series of samples, showing that the behavior of the LO phonon-plasmon coupling mode is similar to that in IR measurement.

This paper reports the molecular beam epitaxy growth of the GaAs single crystal film on the perovskite oxide SrTiO\-3(001) substrate. The photo modulated reflectance spectroscopy and the photoluminescence measurement show that the energy band gap and optical property of the GaAs single crystal film on the SrTiO\-3 substrate is similar to that of the GaAs bulk material and the broadening of band\|to\|band emission of the GaAs epitaxial film is 5 times that of the GaAs single crystal. PACC: 6855; 8110; 8115; 7125

The electron effective masses, scattering time and mobilities near the Fermi level for the lowest two subbands of the two-dimensional electron gases in a pseudomorphic In0.80Ga0.20As/In0.53Ga0.47As/In0.52Al0.48AsHEMT structure are determined by fitting the cyclotron resonance spectra. The significant enhancement of the electron effective masses, resulting from the conduction band nonparabolicity and the wave function penetration, is observed. The evident subband dependencies of the electron scattering time and mobilities dominated by the alloy scattering and ionized impurities scattering are also discussed.

The resonant subband-Landau-level coupling(RSLC) between the N=1 Landau level of the lowest subband and the N＝0 Landau level of the subbands with quantum index up to i＝5 of two-dimensional electron gases in GaAs/AlGaAs heterojunction was observed under relatively low magnetic field.Several intersubband energies were accurately determined. The experimental results were in good agreement with the self-consistent calculations including the depolarization shift and exciton-like effect corrections.The quantum index dependence of the level splitting due to the RSLC was discussed.

Photoconductivity (PC) spectra of impurities in n type GaAs under magnetic field are presented. The resonant interaction between high excited states of bound electrons and LO phonons under relatively low magnetic field was observed. The magnetic field dependency of PC response of resonant polaron and the physical origin of the PC peak stuck on the energy position of LO phonon are discussed.

A Series of samples of ultra thin AlGaAs films grown on GaAs(100) substrate by MBE are studied by photomodulated reflectance (PR)spectroscopy in ultra high vacuum chamber.More than one peak are observed for samples with thickness less than 35nm,but only one peak is observed for 100nm thick sample,The position and width of these peaks change with sample thickness.The experimental results are well explained by step potential model.

The 2p± and 3p± excited lines of P donors in Si0.92Ge0.08 epitaxial layer have been observed using photothermal ionization spectroscopy.The ionization energy of P in Si0.92Ge0.08 is 45.51meV according to effective mass approach.The spectral lines of P in Si1-xGex compared with those in pure Si apparently broaden due to alloy disorder effect.

The room-temperature photoluminescence (PL) spectrum of the modulation-doped strained In0.60Ga0.40As/In0.52Al0.48As multiple quantum wells (MQW) is reported. In addition to two strong PL peaks related to the recombination between electrons in the first (n = 1), second ( n = 2) subbands and heavy holes in the first ( n = 1) subband, a weak PL shoulder related to the recombination between n = 1 electrons and n = 1 light holes is obsevered at low temperatures. We demonstrate that the main mechanism of the decrease in radiative QW recombination efficiency is due to the carrier trapping on the misfit dislocations, based on the temperature and excitation power dependence of the PL peak intensity and energy and the steady-state PL model.

The calculation method of quantum chemistry has been extended to describe the lattice instabilities and the nature of the lattice instabilities of hexahalometallates. Some interesting calculation results have been obtained and compared with the exper-imental results obtained by Raman and Brillouin scattering.

We report direct experimental determination of the wavefunction mixture and composition for hybridized Zeeman states of P in Si. The intensity evolution of the Zeeman transitions nearby anti-crossing region for the isolated P donors in high pure Si bas been measured quantitatively by use of high sensitive and high resolution photothermal ionization spectroscopy (PTIS); The wavefunction compositions and their evolutions upon magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation under the frame of effective mass approach has been performed to estimate theoretically the wavefunction composition. The results are campare with experimental ones.

The transmission spectra of p-PbTe have been measured by the strip-line technique with different far infrared laser wavelengths at different anhgles of the direction of maqnetic field relative to the crystal axis at low temperature of 1.5 k. A strong influence of mode coupling on strip-line transmission spectra has been observed and analyzed. The final results extracted from experimental spectra by numerical simulation are in good agreement with the six band k·p model calculation.

Using diamond-anvel cell high pressure equipment, the phase transition of Cd1-xMnxSe under high pressure has been measured by optical absorption and X-ray diffraction methods, It has been found that the phase transition is irreversible. Under common pressure, Cd1-xMnxSe has wurzite structure, but it transforms to Nad structure under high pressure. When the pressure is removed, Cd1-xMnxSe becomes zinc blend structure. In addition, the relation between the pressure of phase transition and component x is also obtained.

The far infrared reflection spectra of Hg1-xMnx Te with different composition in the temperature region of 4.2-300 K are reported. It is demonstrated that there are a gap mode as well as a local mode associated with impurity Mn in HgTe. The gap mode has a frequency of 97 cm-1. The theoretical analysis of the gap mode has been proceeded.

The far infrared reflectances of the high-Tc superconductor YBa2Cu3O9−δ(Tc=90k) are measured in the frequency range 25–150 cm−1 and the temperature range 42k-200k. The reflectance curve with 42k rapidly drops near 125 cm−1, whereas the curves with T>90k go smoothly. It indicates such superconductors have a energy gap 2Δ=15.5 mev, and the ratio 2Δ/kTc=2.0 is close to that of superconductors La-Ba-Cu-O and La-Sr-Cu-O, but well smaller than the BCS value 3.5.

We report here the acoustic local mode and TA band mode for the lattice vibration of the mixed crystals Cd1-xMnxTe as observed by very far infrared spectra. The occurence of the acoustic local mode and the weakly optical activity of TA mode induced by crystalline-mixing are discussed in light of the defect lattice dynamics.