Raman scattering of Zn doped CuGaS2 layers grown by vapor phase epitaxy

Raman spectra for non-site-selectively and site-selectively Zn-doped CuGaS₂ layers grown by vapor phase epitaxy (VPE) were investigated. Although an appearance of characteristic Raman line(s) related with the doped Zn atom was not seen, an enhancement of the Raman intensity ratio of the highest LO mode to the A₁ mode (I_LO/I_A1) was observed. The site-selectively Zn-doped layers with p-type conductivity exhibited larger I_LO/I_A1 ratio compared to those with n-type conductivity. The observed correlation between the I_LO/I_A1 ratio and the peak energy of the photoluminescence characteristic for Zn-doped p-type samples (L emission) suggests that the enhancement of I_LO/I_A1 is due to the increase of Zn atom substituting Ga site (Zn_Ga) which is acting as an acceptor.     

Ferromagnetism above room temperature in bulk sintered gallium phosphide doped with manganese

Evidence for ferromagnetism in bulk sintered gallium phosphide (GaP) doped with 3% manganese, having a Curie temperature of 600 K considerably higher than previous observations, is obtained using ferromagnetic resonance (FMR) and AC magnetization measurements. The field position and line width of the resonance showed a strong temperature dependence characteristic of FMR spectra. A non-resonant derivative signal centered at zero field was also observed starting at 600 K further confirming high temperature ferromagnetism. AC magnetization measurements also show the existence of ferromagnetism at high temperature.     

Optical and electrical characterization of cadmium selenide doped with cobalt

The optical and electrical properties of Co²⁺ ions in CdSe have been investigated. Absorption, photoluminescence, electron paramagnetic resonance, and Hall measurements were used to characterize a cobalt-doped (1×10¹⁹ cm⁻³) single crystal. Infrared absorption and emission spectra associated with transitions between the ⁴A₂(F) ground state and the ⁴T₁(F) and ⁴T₂(F) excited states are described. At 10 K, spin-orbit splittings result in three structured absorption bands associated with the ⁴A₂(F) to ⁴T₁(F) transition having zero-phonon lines at 4926, 5101, and 5724 cm⁻¹. The ⁴A₂(F) to ⁴T₂(F) transition shows two zero-phonon lines at 2874 and 3286 cm⁻¹, also accompanied by vibronic structure. Intrinsic lattice modes explain most of the sharp-line structure observed at low temperature, except for a subset of peaks where local modes (25-30 cm⁻¹) are invoked. Using below-band-gap light, selective excitation allows detection of the ⁴T₁(F) to ⁴A₂(F) recombination at liquid-helium temperatures. The activation of free carriers in our n-type material containing shallow donors is affected by the presence of cobalt, and we find the Co^+/++ level to be about 34 meV below the conduction band of CdSe.     

Incorporation of AsSe centers in ZnSe far from equilibrium

An experimental technique employing nuclear transmutation to probe the effects of arsenic doping in ZnSe is presented. ZnSe epitaxial layers are grown from elemental Zn and elemental Se that contains the ⁷⁵Se isotope. Arsenic dopants are incorporated in ZnSe through the decay of ⁷⁵Se to ⁷⁵As which proceeds via electron capture. The 118.5 day half-life of the decay allows ZnSe epitaxial layers to be deposited prior to significant arsenic formation so that As_Se dopants are incorporated after all crystal growth processes are complete. Also, the temperature at the time of dopant incorporation is under investigator control. Because the decay process produces nuclear recoils that are far too small to displace the arsenic dopants from their substitutional selenium sites, no post-growth thermal annealing is required. This experimental technique permits a limiting case of far-from-equilibrium doping, and the long half-life of ⁷⁵Se results in the gradual incorporation of As_Se so that time-resolved studies can be performed. In the experimental work reported here, As_Se centers are introduced in ZnSe homoepitaxial layers in concentrations greater than 1.5×10¹⁸ cm⁻³. Time-resolved low temperature photoluminescence is employed to observe the effects of As_Se doping of ZnSe.     

Photoluminescence of CdGeP2 and (Cd,Mn)GeP2

Photoluminescence of CdGeP₂ (112) single crystal and CdGeP₂ epitaxial film grown on GaAs (001) substrate have been studied and their spectral similarity found. Spectral bands associated with donor/acceptor transitions peak at close energies for both substances and all are lower than the energy gap of the chalcopyrite crystal.On the other hand, the growth of (Cd,Mn)GeP₂ ferromagnetic layer on CdGeP₂ (112) single crystal was performed to make it possible observation of PL from both the ferromagnetic layer and substrate. The green laser excitation (514, 532 nm) produces a proper photoluminescence similar to that in the undoped CdGeP₂ crystal and film. An extra emission from the ferromagnetic-nonmagnetic heterojunction occurs to extend up to photon energies exceeding E_g of the host semiconductor. The short wavelength photoluminescence is to be due to (Cd,Mn)GeP₂ dilute magnetic semiconductor (DMS). This fact states that Mn-doped II-IV-V₂ chalcopyrites are closer to II-VI DMS than to another group III-V DMS, where the heavy Mn-doping suppresses photoluminescence at all. Features of the observed short wavelength emission are discussing based on the temperature and spectral analyses.     

Formation of shallow acceptors in ZnO doped by lithium with the addition of nitrogen

We performed first-principle total-energy calculations to investigate the mechanism for the realization of high quality p-type ZnO codoped with lithium and nitrogen. We find that the higher hole concentrations measured in the codoped ZnO is related to decreased ionization energy of acceptors and reduction of compensations. The dual acceptor N_O-Li_Zn complex proposed in experiments is unstable. While in the (Li_I-N_O)-Li_Zn complex, where acceptor Li_Zn binds to the passivated (Li_I-N_O) complex is stable and acts as a single acceptor. The activation energy of this complex is about 60 meV lower than that of Li_Zn in Li-monodoped ZnO. The formation of inactive (Li_I-N_O) complexes creates a fully occupied impurity band just above the valence band maximum of ZnO. Thus Li atoms binding to this complex is activated by the electrons from the complex state rather than from the host states, accounting for decreased activation energy. Besides, Li_I⁺ and N_O⁻ bind tightly through the Coulomb interaction. Such binding will suppress the amount of compensating donor Li_I and limit the compensation for the desired acceptor Li_Zn.     

Ionic pseudopotential for semiconductors without cut-off parameter for core-repulsion with application to Si

An ionic pseudopotential for semiconductors is proposed, which consists of a set of continuous exponential functions. Introduced damping and amplitude parameters into the pseudopotential are to be treated as adjustable. The most important features of the proposed pseudopotential is that (1) it has no sharp cut-off parameter for the core-repulsion and (2) it is continuous and has continuous derivatives to arbitrary order. The proposed pseudopotential is applied to Si and the adjustable parameters are determined so as to be consistent with the Si crystal empirical pseudopotential of high quality by taking a valence electron dielectric screening effect into account. The effectiveness of the proposed ionic pseudopotential is discussed by (1) comparing the calculated ionic energy levels of Si with experiments, (2) checking the consistency between the ionic and crystal pseudopotentials for Si, and so on.     

Admittance spectroscopy of polycrystalline and epitaxially grown CuGaSe2

Using either single crystalline, epitaxially grown p-type CuGaSe₂ (CGSe) films in Schottky diodes or polycrystalline p-CuGaSe₂/n-CdS single-junction solar cells, we employed thermal admittance spectroscopy (TAS) to gain insight into the electronic transport mechanisms of CGSe. In both types of devices, the capacitance decreases about 50% to its geometrical value in a frequency dependent step between 250 and 150 K. For the Schottky diodes, this capacitance step reflects the response of the shallowest acceptors whose energy level is located 150 meV above the valence band. In the solar cells, a comparable response occurs but is superposed by carrier freeze-out outside the space-charge region.     

Spontaneous organisation of ZnS nanoparticles into monocrystalline nanorods with highly enhanced dopant-related emission

A natural self-assembly process of semiconductor nanoparticles leading to the formation of doped, monocrystalline nanorods with highly enhanced dopant-related luminescence properties is reported. ∼4 nm sized, polycrystalline ZnS nanoparticles of zinc-blende (cubic) structure, doped with Cu⁺-Al³⁺ or Mn²⁺ have been aggregated in the aqueous solution and grown into nanorods of length ∼400 nm and aspect ratio ∼12. Transmission electron microscopic (TEM) images indicate crystal growth mechanisms involving both Ostwald-ripening and particle-to-particle oriented-attachment. Sulphur-sulphur catenation is proposed for the covalent-linkage between the attached particles. The nanorods exhibit self-assembly mediated quenching of the lattice defect-related emission accompanied by multifold enhancement in the dopant-related emission. This study demonstrates that the collective behavior of an ensemble of bare nanoparticles, under natural conditions, can lead to the formation of functionalized (doped) nanorods with enhanced luminescence properties.     

Epitaxial Cu(In,Ga)S2 thin film solar cells

Epitaxial layers of the quaternary compound Cu(In,Ga)S₂ and the ternary compound CuInS₂ were grown on Si(111) substrates via Molecular Beam Epitaxy. The layers were investigated for their morphological and structural properties using Rutherford backscattering spectroscopy, atomic force microscopy, reflection high-energy electron diffraction and X-Ray diffraction. Furthermore, complete solar cell devices were processed from these layers and their photovoltaic properties were investigated by means of I(U)-curves under illumination. Thus, efficiencies up to η=3.2% were achieved. The comparatively low performance of the solar cell devices is attributed to certain heterogeneities of the samples as a result of the growth process.     

Growth of InN thin films on ZnO substrate by plasma-assisted molecular beam epitaxy

We have studied the microstructure property of InN epitaxial films grown on ZnO substrate by plasma-assisted molecular beam epitaxy. We found that the In₂O₃ compound was produced on ZnO substrate and many pits were formed on the InN films when InN was directly grown on ZnO substrate with the N/In flux ratio less than 40. We demonstrated that the quality of InN film was significantly improved when the In₂O₃ layer was used as a buffer to prevent the reaction between In and the ZnO substrate.     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

Preparation and characterization of scandia and Re doped tungsten matrix impregnated cathode

The paper describes the preparation and emission property of scandia and Re doped tungsten matrix impregnated cathode. By an easy and reproducible way, solid-liquid doping combined with two-step reduction, powders of tungsten particles covered with scandium oxide were obtained. Compared with scandia mixed tungsten powders prepared by mechanically mixing, scandia and rhenium doped tungsten powders had smaller particle size, for example, scandia (3 wt%) and Re (5 wt%) doped tungsten powders had the average size of about 50 nm in diameter. Based on this kind of powder, scandia and Re doped tungsten matrix with the sub-micrometer sized tungsten grains and a more uniform distribution of Sc₂O₃ were obtained in this paper. Scandia and Re doped tungsten matrix impregnated cathode had shown excellent emission property and good emission uniformity. The space charge limited current densities of more than 58A/cm² at 900 °C_b could be obtained and the work function of this cathode was as low as 1.18 eV.     

Experimental confirmation by galvanomagnetic methods of a complex transport model in In0.53Ga0.47As layers deposited by MBE on SI-InP

At low temperatures In_0.53Ga_0.47As samples show an increase of carrier concentration, which can be explained in terms of a two carriers transport model. This type of problem exists since the beginning of the semiconductor era, dating back to monocrystalline germanium.We propose that in all the investigated layers, there are X atoms or charged dislocations in the region of the first monolayers, which are built in during epitaxial growth. The layers were intentionally undoped. They form an impurity band in which low mobility carriers dominate over the localised electron scattering due to the s-d exchange interaction. These carriers do not freeze out at liquid helium temperature and give rise to two transport media for electrons; a conduction band at higher temperatures and an impurity band at lower temperatures. The electron which fall down onto the previously ionised X atoms, then move by thermally activated hopping. We show that the two carriers model for In_0.53Ga_0.47As epitaxial layers are confirmed by the carrier concentration-temperature, carrier concentration-magnetic field, resistivity-magnetic field behaviour, and also by YKA theory also. The differences between the two transport models are so distinctive that observed phenomena may exist. This paper presents experimental results, which constitute comprehensive evidence for the complicated structure of the semiconductor epitaxial layers on the sample of n-type In_0.53Ga_0.47As/InP layer with n=2.2×10¹⁵/cm³.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Crystal growth and the electronic structure of Tl3PbCl5

Total and partial densities of states of constituent atoms of two tetragonal phases of Tl₃PbCl₅ (space groups P4₁2₁2 and P4₁) have been calculated using the full potential linearized augmented plane wave (FP-LAPW) method and Korringa-Kohn-Rostoker method within coherent potential approximation (KKR-CPA). The results obtained reveal the similarity of occupations of the valence band and the conduction band in the both tetragonal phases of Tl₃PbCl₅. The FP-LAPW and KKR-CPA data indicate that the valence band of Tl₃PbCl₅ is dominated by contributions of the Cl 3p-like states, which contribute mainly to the top and the central portion of the valence band with also significant contributions throughout the whole valence-band region. Further, the bottom of the valence band of Tl₃PbCl₅ is composed mainly of the Tl 6s-like states, while the bottom of the conduction band is dominated by contributions of the empty Pb 6p-like states. The KKR-CPA results allow to assume that the width of the valence band increases somewhat while band gap, E_g, decreases when changing the crystal structure from P4₁2₁2 to P4₁. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion-irradiated surfaces of a Tl₃PbCl₅ monocrystal grown by the Bridgman-Stockbarger method have been measured.     

Electronic structure and transport of Ca3Co2O6 and Ca3CoNiO6

We have calculated the band structure of Ca₃Co₂O₆ and Ca₃CoNiO₆ by using the self-consistent full-potential linearized augmented plane-wave method within density function theory and the generalized gradient approximation for the exchange and correlation potential. The spin-orbit interaction is incorporated in the calculations using a second variational procedure. The relation of these band structure calculations to thermoelectric transport is discussed. The results illustrate that transport is highly anisotropic with much larger mobility in the a-b plane than out of the a-b plane, and the introduction of Ni in Ca₃Co₂O₆ alters its electronic structure and its thermoelectric transport properties.     

Optical characterization of CuIn5S8 crystals by ellipsometry measurements

Optical properties of CuIn₅S₈ crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2–6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple–DiDomenico and Spitzer–Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn₅S₈ crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis.