Optical properties of gallium nitride bulk crystals grown by chloride vapor phase epitaxy

The optical properties of bulk crystals of gallium nitride grown by chloride vapor-phase epitaxy are investigated. It is shown that these crystals exhibit exciton luminescence bands. Analysis of the energy positions of the band maxima imply certain conclusions about the presence or absence of mechanical stresses in the bulk crystals of GaN obtained. Analysis of the luminescence spectra also reveals that the temperature dependence of the width of the GaN band gap E _g in the temperature range T=6–600 K is well described by the expression E _g (T)=3.51−7.4×10⁻⁴ T ²(T+600)⁻¹ eV. It is estimated that values of the free electron concentration in these crystals do not exceed 10¹⁸ cm⁻³. The optical characteristics of the bulk GaN crystals are compared analytically with literature data on bulk crystals and epitaxial layers of GaN grown by various methods. Fiz. Tekh. Poluprovodn. 33, 1173–1178 (October 1999)     

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band‐edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)₂PbI₄ are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 10³ cm s^?1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low‐lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.     

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of In_xGa_1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk In_xGa_1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.     

Binding energy of exciton-impurity complexes in semiconductors with diamond and zinc blende structure

The binding energies of four complexes — exciton + charged impurity, exciton + neutral impurity — were calculated by a variational method in semiconductors with diamond and zinc blende structure taking into account the degeneracy of the valence-band edge. The numerical calculations were performed for exciton-impurity complexes in a series of II–VI, III–V, and IV–IV crystals. Fiz. Tekh. Poluprovodn. 32, 583–587 (May 1998)     

Luminescence study of ZnTe:Cr epilayers grown by molecular-beam epitaxy

Incorporation of Cr into ZnTe epilayers grown by molecular-beam epitaxy (MBE) is reported. Photoluminescence (PL) using both continuous wave (CW) and pulsed-excitation sources is used to characterize the radiative efficiency of doped layers in the infrared region. The Cr²⁺ ions produce a broad emission band peaking in the 2–3 μm range, which is of potential use in tunable-laser devices. The optimum Cr concentration for achieving bright, room-temperature infrared emission was found to be in the range from low- to mid-10¹⁸ cm⁻³. Temperature-dependent luminescence studies were performed to determine thermal-quenching activation energies. Using a pulsed-laser operating at 1.9 μm, an investigation of emission lifetimes was made. The emission-decay curves for the Cr²⁺ recombination in ZnTe:Cr films could be described by a single exponential and were nearly independent of temperature from 80 K to 300 K. A room-temperature lifetime of ∼2.5 μsec in a ZnTe:Cr layer with [Cr] ∼1.4 × 10¹⁸ cm⁻³ compares favorably with values reported for bulk ZnTe:Cr.     

Characterization of very high purity InAs grown using trimethylindium and tertiarybutylarsine

The growth of high purity InAs by metalorganic chemical vapor deposition is reported using tertiarybutylarsine and trimethylindiμm. Specular surfaces were obtained for bulk 5-10 μm thick InAs growth on GaAs substrates over a wide range of growth conditions by using a two-step growth method involving a low temperature nucleation layer of InAs. Structural characterization was performed using atomic force microscopy and x-ray diffractometry. The transport data are complicated by a competition between bulk conduction and conduction due to a surface accumulation layer with roughly 2–4 × 10¹² cm⁻² carriers. This is clearly demonstrated by the temperature dependent Hall data. Average Hall mobilities as high as 1.2 x 10⁵ cm²/Vs at 50K are observed in a 10 μm sample grown at 540°C. Field-dependent Hall measurements indicate that the fitted bulk mobility is much higher for this sample, approximately 1.8 × 10⁵ cm²/Vs. Samples grown on InAs substrates were measured using high resolution Fourier transform photoluminescence spectroscopy and reveal new excitonic and impurity band emissions in InAs including acceptor bound exciton “two hole transitions.” Two distinct shallow acceptor species of unknown chemical identity have been observed.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

Auger and radiative recombination of acceptor bound excitons in semiconductors

We report on a theoretical and experimental study of acceptor bound exciton recombination. We present calculations of phononless Auger and radiative recombination in direct and indirect band gap materials. We consider hydrogenic acceptors in the direct band gap material Hg_1−xCd_xTe in which the band gap can be varied by changing alloy composition. We present calculations of the Auger transition rate and no-phonon oscillator strengths for the common acceptors in Si and Ge. We have measured the bound exciton lifetimes and no-phonon oscillator strengths for the acceptors in Si and find reasonable agreement with the calculated values.     

Semi-empirical tight binding method for calculating energy levels of hydrogenated silicon nanoclusters as a function of size

A semi-empirical tight binding model is used for calculating energy levels of spherical hydrogenated silicon nanoclusters in the form of Si_mH_n and two independent bases. The first basis is the nearest neighbor approximation of sp³s^⁎ orthogonal basis and the second basis is the third nearest neighbor approximation of sp³ nonorthogonal basis. Also, bulk properties of silicon crystal are used in the calculations. As we expected, obtained results show that the change in the size of nanoclusters has a great influence on their energy levels. So optical properties of these nanoclusters can be controlled externally. A comparative study on the calculated results and experimental results also reveals a good agreement between the two approaches.     

Tunable Assembly of sp3 Cross‐Linked 3D Graphene Monoliths: A First‐Principles Prediction

One of the biggest challenges in graphene applications is how one can fabricate 3D architectures comprising graphene sheets in which the resulting architectures have inherited graphene's excellent intrinsic properties but have overcome its shortcomings. Two series of 3D graphene monoliths (GMs) using zigzag or armchair graphene nanoribbons as building blocks and sp³ carbon chains as junction nodes are constructued, and calculations based on first principles are performed in order to predict their mechanical and electronic properties. The perfect match between sp² nanoribbons and sp³ linkers results in favorable energy and mechanical/dynamic stability. Owing to their tailored motifs, wine‐rack‐like pores, and rigid sp³ linkers, these GMs possess high surface areas, appreciable mechanical strength, and tunable band gaps. Negative linear compressibilities in a wide range are found for the zigzag GMs. By solving the problems of zero gap and dimensionality of graphene sheets simultaneously, these GMs offer a viable strategy towards many applications, e.g., microelectronic devices, energy storage, molecular sieves, sensitive pressure detectors, and telecommunication line systems.     

Laser‐Induced Direct Lithography for Patterning of Carbon with sp3 and sp2 Hybridization

A new method of laser‐induced lithography for direct writing of carbon on a glass surface is described, in which deposition occurs from a transparent precursor solution. At the glass–solution interface where the laser spot is focused, a micro‐explosion process takes place, leading to the deposition of pure carbon on the glass surface. Transmission electron microscopy (TEM) analysis shows two distinct co‐existing phases. The dominant one shows a mottled morphology with diffraction typical of cubic (sp³) diamond. The other region shows an ordered array of graphene sheets with diffraction pattern typical of sp²‐bonded carbon. The sp³ crystallites range in size from 9 to 30 Å and are scattered randomly throughout the sample. A UV Raman spectrum shows a broad band at the location of the expected diamond peak, together with a peak corresponding to the graphite region. We conclude that the patterned carbon is composed of a mixture of nanocrystalline sp³ and sp² carbon forms.     

Density functional theory study of tin and titanium dioxides: Structural and mechanical properties in the tetragonal rutile phase

Structural and mechanical properties in rutile (tetragonal) phases of SnO₂ and TiO₂ are investigated by performing first-principle density functional theory (DFT) calculations. Generalized Gradient Approximation (GGA) potentials of electronic exchange and correlation part parameterized by Perdew–Burke–Ernzerhof (PBE) are used. Second order elastic stiffness constants, bulk modulus, first-derivative of bulk modulus, and pressure behavior of these mechanical properties are studied up to pressure of 10 GPa. Structural properties and elastic constants of SnO₂ and TiO₂ calculated in this study are compatible with experimental and other available theoretical studies. Electronic band gap energies of these semiconductors are also calculated. As expected, the calculated values by standard DFT calculations are underestimated in comparison to experimental values.     

Optical spectra of microcrystals of the layered semiconductor PbI2 grown in glass matrices

The transmittance, luminescence, and Raman spectra in PbI₂ microcrystals of 2H polytype, grown in the voids of alkali-borosilicate glasses (void dimensions 4–30 nm), are investigated. Depending on the conditions of growth, the exciton states of the microcrystalline systems were found to shift toward higher energies by 35–60 meV relative to the free exciton energy in bulk single crystals as a result of the quantum-well effect. The resonant Raman spectra of PbI₂ microcrystals exhibit not only the conventional Raman component A _1g , but also components associated with the appearance of the E _u and A _2u optical phonons and the two-phonon processes. Fiz. Tekh. Poluprovodn. 32, 151–154 (February 1998)     

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N₂ or H₂ as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N₂ carrier gas show two additional broad peaks, which are ascribed to carbon sp² clusters related modes, spreading in the frequency range 1300–1600 cm^?1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ～520 cm^?1, considered as some defects related local vibrations, appears in the samples grown with N₂ as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp² clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H₂ as carrier gas, the bands related to carbon sp² clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm^?1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N₂ carrier gas, were only weakened.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

Irradiation damages in electron beam lithography

 The irradiation damages in the electron beam lithography(EBL)to Al-gate MOS capacitors in the ranges of 10—30keV and 10~(-6)—10~(-3)C·cm~(-2) and the effects of annealing on damages at low temperature(<500℃)are given.The research on damages caused by high electron energy(30keV) and ultra-high dosages(10~(-4)—10~(-3) C·cm~(-2))is important and useful to the EBL.The resolution can be improved by high electron energy.Both the EBL with vapor-development and without development are all operated at ultra-high dosages.After irradiations,the concentrations of inter- face states can increase by about one to two orders of magnitude and the flat-band voltages by about a few to more than ten volts.Under constant exposure dosages,the fiat-band voltages are independent of the changes of electron energies in certain energy ranges.Under constant electron energies the concentrations of interface states are independent of the changes of exposure dosages in certain dosage ranges.After annealing,the flat-band voltages can recover the values before the irradiations for energies and dosages in the ranges of 10—30keV and 1×10~(-6)—6×10~(-3)C·cm~(-2) respectively.The interface state concentrations due to the damages of ultra dosages can not be removed completely.     

Thermal effects on structure and photoluminescence properties of diamond-like carbon films prepared by pulsed laser deposition

With their unique properties such as high hardness ,lowfriction,chemical inertness and optical transparency,Diamond-like carbon(DLC) fil ms are usedina wide ar-ea of industrial applications ,as a protective coating insuch areas as optical windows ,magneti…     

Carbon structural transitions and ohmic contacts on 4H-SiC

The structural properties of sputtered carbon films on SiC are investigated using X-ray photoelectron spectroscopy (XPS) and Raman scattering. The as-deposited films are amorphous with an sp²/sp³ ratio of 1. The sp² carbon structures gradually increase with increasing temperatures and consist of amorphous aromatic-like carbon, polyene-like carbon, and nano-size graphite flakes. Schottky contacts on carbon/SiC are converted to ohmic contacts after annealing. The concentration of nano-graphitic flakes relative to the aromatic-like and polyene-like carbon increases nearly linearly with annealing temperature. Stacked graphitic structures are not observed. The specific contact resistivities are at 10⁻³–10⁻⁴Ωcm² on the carbon/SiC after annealing from 1050°C to 1350°C.     

Defect chemistry and intrinsic carrier concentration for Hg1−x Cdx Te(s) for x = 0.20, 0.40, and 1.0

The theoretical equations governing the crystal-vapor equilibrium for Hg_1−x Cd_x Te(s) are summarized for a model with doubly-ionized native defects and applied to the data for x = 0.20, 0.40, and 1. The basic equations (Eq. (1) or (19)) are shown to contain those used elsewhere as special cases. Allowing the intrinsic carrier concentration, n₁, to vary, but under constraints, to obtain an optimum fit and assuming a non-degenerate semiconductor, the hole con-centration-mercury pressure isotherms can be fitted satis-factorily and better than before with standard deviations between 22 and 24% for x = 0.20 and 0.40. A number of sets of model parameters give these fits, some of which give values as large as 10/cm at 500δC for the square root of the Schottky constant for ionized vacancies. In agreement with experiment, each of these parameter sets for x = 0.20 and 0.40 predicts a negligible dependence of the 77 and 192 K Hall mobilities upon equilibration temperature and predicts that metal saturation between 250 and 300δC with foreign donors in the 10¹⁵/cm³ range will result in elec-tron concentrations in the same range and dependent on saturation temperature to less than 26%. The enthalpy to create a “Hg-vacancy” and two holes is an invariant of the fits and is 2.00 ± 0.04 eV for x = 0.40 and 1.94 ± 0.02 eV for x = 0.20. Then n_i is calculated independently assuming a parabolic valence band and a Kane conduction band. With an exact density of states for the latter obtained here, the momentum matrix element, P = 8.5 × 10₋₈ eV-cm, the spin-orbit splitting, Δ = 1 eV, and E (x, T) from Hansen et al., the experimental values below 300 K for 0 ≤x = 0.3 and at high temperature for x = 1 are fitted well with m _h ^∗ /m = 0.70. With these independently calculated values for n_i the electron concentration-cadmium pressure isotherms for CdTe can be fitted to about 7% for either of two incon-sistent data sets assuming 2 × 10¹⁶ /cm³ foreign donors. However data for x = 0.40 can be fitted to only 42% and that for x = 0.20, which is degenerate, to only 28%.     

Electronic and optical properties of ordered porous germanium

The electronic band structure and dielectric function of ordered porous Ge are studied by means of a sp³s^* tight-binding supercell model, in which periodical pores are produced by removing columns of atoms along [0 0 1] direction from a crystalline Ge structure and the pore surfaces are passivated by hydrogen atoms. The tight-binding results are compared with ab-initio calculations performed in small supercell systems. Due to the existence of periodicity in these systems, all the electron states are delocalized. However, the results of both electronic band structure and dielectric function show clear quantum confinement effects.