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In this paper, we theoretically investigate the linear and nonlinear optical activity of polymers based on poly(ortho-anisidine) and polyaniline using density functional theory. The PBE0 exchange correlation functional is used to predict their dipole moment, mean polarizability, polarizability anisotropy, and the static first hyperpolarizability. The polymers show high hyper–Rayleigh scattering first hyperpolarizability, which make them suitable for nonlinear optical devices. The relationship between the first hyperpolarizability and the energy gap is also taken into account. [ABSTRACT FROM AUTHOR]

This study entails two parts: The first reports on the synthesis of a novel N-(2,4-dinitrobenzylidene)-3-chlorobenzenamine Schiff base and its analysis by ultraviolet–visible (UV–vis) spectroscopy, 1H and 13C nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy. The UV–vis measurements showed absorption between 264 and 304 nm. The experimental optical gap energy was obtained using the Tauc method, and was estimated to be around 3.1 eV. On the other hand, we calculated optical gap energy using the M06-2X level, which gave a value of 3.20 eV, proving to be very close to the experimental one. The second part of this study focused on the dipole moment, dynamic linear and dynamic nonlinear optical (NLO) properties of this new compound. Theoretical calculations using density functional theory (DFT) were used to optimize the geometry, and to calculate this compound's dynamic polarizability and dynamic first-order hyperpolarizability. For the polarizability, we studied the mean polarizability α and the polarizability anisotropy Δ α . To elucidate the correlation between the molecular architecture of this Schiff base and the dynamic first hyperpolarizability, the dynamic behavior of the hyper-Rayleigh scattering (HRS) first hyperpolarizability β HRS , the dynamic electric field-induced second harmonic generation (EFISHG) β / / , and the depolarization ratios (DR) were calculated and analyzed in detail. Moreover, a correlation of β HRS with the optical band gap and between the β HRS and β / / was found. Based on these results, we can conclude that the synthesized Schiff base is a good candidate for the design of organic NLO materials. [ABSTRACT FROM AUTHOR]

Formation of low-resistance ohmic contacts to p-GaP is important for development of high-efficiency AlGaInP light-emitting diodes (LEDs), which emit light from red to yellow-green and have a wide variety of applications such as traffic light lamps, automobile tail lamps, and in biotherapy. The current flow behavior can be understood by investigating the effect of the Schottky barrier height (SBH; Φ) on the work function of metals (Φ). In this work, SBHs and their dependence on Φ at (001) p-GaP surfaces were investigated. With increasing temperature, the SBH increased, while the ideality factor decreased. This behavior is explained by means of a thermionic field-emission (TFE) model. The SBH and ideal factor ranged from 0.805 eV to 0.852 eV and from 1.18 to 1.50, respectively, for different Schottky metals. The S-parameter (dΦ/dΦ) was estimated to be 0.025, with this approximately zero value implying that the surface Fermi level is virtually perfectly pinned at the surface states at ~0.85 eV above the valence-band edge. [ABSTRACT FROM AUTHOR]

We present gas discharge phenomena in argon and air media using a gallium phosphide (GaP) semiconductor and metal electrodes. The system has a large-diameter ( D) semiconductor and a microscaled adjustable interelectrode gap ( d). Both theoretical and experimental findings are discussed for a direct-current (dc) electric field ( E) applied to this structure with parallel-plate geometry. As one of the main parameters, the pressure p takes an adjustable value from 0.26 kPa to 101 kPa. After collection of experimental data, a new theoretical formula is developed to estimate the minimal breakdown point of the system as a function of p and d. It is proven that the minimal breakdown point in the semiconductor and metal electrode system differs dramatically from that in metal and metal electrode systems. In addition, the surface charge density σ and spatial electron distribution n are calculated theoretically. Current-voltage characteristics (CVCs) demonstrate that there exist certain negative differential resistance (NDR) regions for small interelectrode separations (i.e., d = 50 μm) and low and moderate pressures between 3.7 kPa and 13 kPa in Ar medium. From the difference of currents in CVCs, the bifurcation of the discharge current is clarified for an applied voltage U. Since the current differences in NDRs have various values from 1 μA to 7.24 μA for different pressures, the GaP semiconductor plasma structure can be used in microwave diode systems due to its clear NDR region. [ABSTRACT FROM AUTHOR]

The structural, electronic and optical properties of the GaAsP ternary alloys together with their binary GaP and GaAs compounds were investigated in the zinc-blende (ZB) phase using the density functional theory. The lattice constant of the GaAs compound decreases while its bulk modulus increases when the doping concentration of the P dopant is increased. In addition, both parameters (lattice constant and bulk modulus) show small deviations from the linear concentration dependence. The energy band gap of the GaAs compound is of the direct nature, which increases with the increase in the P dopant concentration, whereas at higher P dopant concentration, the band gap shifts from direct to indirect character. On the other hand, the hydrostatic pressure has a significant effect on the band structure of the investigated compounds where the binary GaAs compound changes from a direct band gap semiconductor to an indirect band gap semiconductor at P ≥ 5 GPa. Furthermore, the pressure-dependence of the optical properties of the GaAs, GaP and GaAsP alloy were also investigated, where the calculated zero frequency refractive index and the dielectric function are also compared with the experimental results as well as with different empirical models. [ABSTRACT FROM AUTHOR]

We report layered growth of GaInP on GaP substrates using single-step liquid phase epitaxy (LPE) with a Sn-based melt when the lattice mismatch is greater than 0.4 % ( x < 0.95). Compositional control was observed by (1) varying the cooling rate and (2) changing the melt-back temperature at the beginning of the growth. Possible growth mechanisms are proposed to explain the principles of both approaches of compositional control. Smooth epilayers have been observed. High resolution x-ray diffraction was used to characterize the composition of the epilayers, and room temperature photoluminescence was reported for one of the samples with the composition of x = 0.11. Plan-view TEM measurements revealed threading dislocation densities on the order of 10cm in the upper regions of the GaInP epilayers. In contrast, when using In-based melts, LPE of GaInP on GaP (100) substrates exhibited island growth at large misfits, whereas edge growth dominated when using GaP (111B) substrates under equivalent growth conditions. [ABSTRACT FROM AUTHOR]

We report on the characterization of gettered p-type GaP substrates for application in high-efficiency multijunction solar cells. A commercial zinc-doped GaP substrate was divided, with one piece soaked in a phosphorus-saturated gallium-aluminum melt at 975°C. Low-temperature continuous-wave photoluminescence indicated a significant decrease in deep-level impurity peaks due to oxygen and zinc-oxygen complexes after gettering in the phosphorus-saturated gallium-aluminum melt. To illustrate what effect this has on minority-carrier diffusion lengths, Au/GaP Schottky solar cells were fabricated on the substrates, and the spectral response of each was examined. A marked increase in response across all wavelengths on the gettered sample indicates an increase in minority-carrier diffusion lengths. To ensure these results were not simply due to an increase in the depletion region width resulting from a change in carrier density, C- V profiling was performed and found only a small change in carrier concentration of the gettered sample. [ABSTRACT FROM AUTHOR]

The results of luminescence studies conducted on the same samples of GaP over a 40-year period are discussed. The results strongly imply that periodic ordering of impurities improved the overall optical and mechanical properties of the material over time. Forty years after preparation, “hot” luminescence spectra in gallium phosphide (GaP) are similar to those for nanocrystals. The aged pure and N-doped crystals exhibit stimulated emission at 300 K. The aged GaP:N:Sm at room temperature generates bright green or yellow and red tunable luminescence. These results correlate with Raman light scattering and microhardness data obtained from the same crystals. [ABSTRACT FROM AUTHOR]

Evolution of luminescence is reported from GaP crystals that were grown over 40 years ago. This is the longest running cycle of experiments to study the temporal effects of crystal lattice and impurity ordering. The results clearly indicate impurity ordering and the formation of a new type of crystal lattice in which periodically disposed impurities modify and improve properties of the crystal. A high-density exciton system bound to a nitrogen impurity superlattice and GaP:N, GaP:N:Sm nanocrystals distributed in transparent fluorine-containing polymers will be used as the base elements for a new generation of optoelectronic devices. [ABSTRACT FROM AUTHOR]

The effects of substrate pre-annealing on deep level density in Mg-doped GaP liquid-phase epitaxy (LPE) layers were investigated by photocapacitance measurement. With annealing under optimum phosphorus-vapor pressure, concentration of deep donor at EC — 1.9–2.1 eV increased in undoped GaP substrate. Deep level densities in Mg-doped layers were also affected by pre-annealing of the substrates. Densities of dominant deep levels at EV + 0.85 eV and EV + 1.5 eV were an order of magnitude reduced and, in contrast, the level at EC − 2.1 eV in Mg-doped layer increased with long pre-annealing. This level at EC − 2.1 eV is supposed to be involved with P-rich-type nonstoichiometric defects, such as phosphorus interstitial atoms diffused from the surface of the substrate. [ABSTRACT FROM AUTHOR]

Photocapacitance (PHCAP) measurements have been carried out on GaP crystals grown by the liquid-encapsulated Czochralski (LEC) method with heat treatment under various phosphorus-vapor pressures at different temperatures. Electron traps of EC−1.1 eV, EC−1.6 eV, EC−1.9 eV, and a hole trap of EV+2.26 eV are mainly detected. The phosphorus-vapor pressure dependence of the EC−1.9 eV trap density and their diffusion behavior indicate that they are interstitial phosphorus atoms. The densities of both EC−1.1 eV and EC−1.6 eV traps are strongly dependent on the shallow impurity concentrations. Moreover, the density of EC−1.1 eV traps increases with increasing phosphorus-vapor pressure. From these results, we suggest that EC−1.1 eV traps are the complexes of shallow donors and antisite phosphorus atoms. Deep-level densities in GaP crystals after annealing at 860°C or 960°C for 60 min are decreased almost one order of magnitude lower than those in untreated substrate crystals, which should have occurred via out-diffusion of interstitial phosphorus atoms. However, such an effect is not prominent for 800°C treatment for 60 min. [ABSTRACT FROM AUTHOR]

Bromineion-beam-assisted etching produces smooth vertical sidewalls in GaAs, GaP, InP, AlSb, and GaSb as well as in the usual alloys formed from these materials. Care must be taken, however, during etching to match the specific material system with an appropriate substrate etch temperature. For example, vertical walls were obtained using substrate temperatures in the range of 150 to 200°C with InP, 80 to 140°C with GaAs and GaP, and below 30°C with AlSb and GaSb. GaN has also been etched with the technique. Our etching experience and the vapor pressure data for bromine with group III and group V elements lead us to believe that all of the various technologically important III-V binaries, ternaries, and quaternaries can be etched. Etch rates of most of the materials can be varied from several nm/min to 0.16 µm/min through the bromine flow rate, Ar+ ion beam density and energy, and the substrate temperature. Bromine ion-beam-assisted etching also appears to have an advantage over chlorine ion-beam-assisted etching in many situations, in that substrate temperature ranges can be found for which vertical sidewalls are maintained while etching through layered structures composed of various alloys of the materials. Here we present results obtained from etching a number of III-V binaries, alloys, and heterostructures. [ABSTRACT FROM AUTHOR]

Orientation dependent etching of photolithographically patterned $$(\bar 1\bar 1\bar 1)$$ GaP was investigated using solutions of HCl:CH3COOH:H2O2. The pattern was prepared using standard ultraviolet lithography and was a two-dimensional grid with an 18 µm repeat, consisting of 15 µm squares separated by 3 µm spaces. The mask sides were aligned along the $$[0\bar 11]$$ and $$[\bar 211]$$ directions. Under appropriate etching conditions, high quality arrays of pyramids were prepared. These pyramids were defined by $$(\bar 100)$$ , $$(0\bar 10)$$ and $$(00\bar 1)$$ facets. It was shown that the etching process depended on the degree of solution aging after initial mixing. For a freshly prepared solution, the etching rate showed an inverse dependence on time. For short etching times (below 5 min), an intermediate etching profile was followed, while for long times (greater than 5 min) etching was kinetically controlled. We demonstrated that controlled etching at extremely low rates (0.1–0.5 µm/min) is feasible with this new approach. [ABSTRACT FROM AUTHOR]

The growth of homoepitaxial GaP layers using Te-doped GaP as source material has been obtained by the so-called closed space vapor transport technique. The photoluminescence study shows that these layers, when grown under optimized thermodynamical conditions, have both a large luminescence efficiency and the same optical quality as the ones obtained by liquid phase epitaxy. The variation of the luminescence properties with the conditions of growth has been investigated. Both electron paramagnetic resonance and deep level transient spectroscopy detect the presence of deep levels that are not observed in liquid phase epitaxy materials. [ABSTRACT FROM AUTHOR]

We present a study of the real-time monitoring of the homoepitaxial growth of GaP, InP, and the growth of InP/GaP and GaP/Si(001) heterostructures, combining single wavelength p-polarized reflectance (PRS), reflectance-difference spectroscopy (RDS), and laser light scattering (LLS) during pulsed chemical beam epitaxy with tertiarybutylphosphine, triethylgallium, and trimethylindium sources. The growth rate and the bulk optical properties are revealed by PRS with submonolayer resolution over 1000A of film growth. The surface topography is monitored by LLS providing additional information on the evolution of the surface roughness as well as the nucleation/growth mechanism. The optical surface anisotropy, which is related to surface reconstruction and/or surface morphology, is monitored by RDS and compared with the results of PRS and LLS. The results are discussed with respect to the deposition kinetics, in particular as a function of the V:III flux ratio. The pulsed supply of chemical precursors causes a periodic alteration of the surface composition, which is observed as correlated periodic changes in the RD and PR signals, confirming the high sensitivity of both methods to surface chemistry. [ABSTRACT FROM AUTHOR]

Defects in LEC GaP have been analyzed quantitatively using an ESR method. Transition-metal impurities and native defects were studied in as-grown crystals. The antisite defect, P, and the isolated gallium vacancy, V, were emphasized since no other method has given definite, quantitative information on these defects. P antisites are present in some Cr-doped and some Zn-dopeii samples in concentrations high enough to affect their electrical properties. V was not detected in as-grown crystals indicating that V concentrations are smaller than the antisite concentrations at least in p-type material. Typical ESR detection., limits for transition-metal impurities are in the middle 10 cm range. The method is especially sensitive to Fe traces which were observed in all the samples studied. [ABSTRACT FROM AUTHOR]

Vapor-phase etching of (111), (111), (110) GaAs and (111), (111) GaP wafer in H + HBr or H + PBr gas mixtures has been conducted. The results of two cases, with liquid-forming metal (gold) layers or without coatings, are compared. Etching rates as a function of temperature in the range of 600‡ to 950‡ C were measured in Arrhenius coordinates. For both materials and for different gas environments, low-, medium- and high-temperature regions are distinguished, the activation energies in the low-temperature region (below about 650‡c) being quite different for the two cases. The kinetic results are correlated with morphological changes arising from temperature variations. Some conclusions are made about the mechanisms of chemical vaporization in various temperature regions. The vaporization mechanisms have much in common with those for A - B compounds. [ABSTRACT FROM AUTHOR]

The degradation of light output with operating time has been studied for nitrogen doped GaP light emitting diodes fabricated from vapor phase epitaxial material. The degradation is found to saturate at a non-zero value of efficiency. The process is characterized in terms of a degradation rate constant and the saturation value of efficiency. The rate is found to be a strong function of current density during operation and to a lesser degree materials parameters such as dislocation density. The saturation value appears to be independent of these parameters. The degradation is specifically associated with a decrease in the minority carrier lifetime in the p-side of these diffused LEDs. A model for the generation of non-radiative recombination centers which describes the degradation process quantitatively is presented. [ABSTRACT FROM AUTHOR]

A combination of optical microscopy and transmission electron microscopy has been used to provide direct evidence that pits formed by the action of a solution of 3 HF:1 HO on epitaxial and substrate GaP material close to the (001 ) orientation are associated with dislocations. The formation of different pit shapes can be correlated with previous work on etch pits in GaP, namely the formation of D and S pits. Grooves have been produced on certain crystallographic orientations which are associated with certain types of stacking faults. It is now possible to use this etch for a direct count of the dislocation density in (001) GaP and to indicate the extent of stacking faults. [ABSTRACT FROM AUTHOR]

An etchant has been developed which produces etch pits with a well-defined crystallographic structure on {100} gallium phosphide surfaces. Scanning electron microscope (SEM) studies using both the emissive and cathodoluminescent (CL) modes of operation have established that there is a precise 1:1 correlation between etch pits and dark dots seen in the CL micrographs. Since the dark dots in the CL image are produced at the sites of emergent dislocations, it is therefore concluded that the etch pits are dislocation etch pits. The described etchant makes possible a rapid and easy assessment of the degree of crystalline perfection of {100} gallium phosphide epitaxial layers. [ABSTRACT FROM AUTHOR]

models which have been successful in explaining observations of other defect phenomena are used to identify the native nonradiative recombination center in GaP. It is concluded to be the complex V P V acting via the configuration coordinate mechanism. It is believed to form during the cooling of the crystals from the quasi-equilibrium distribution of defects present during growth. Recent direct lattice-imaging experiments in GaAs have observed the corresponding defect complex in a concentration close to that predicted. It is predicted that the defect in GaP will exhibit recombination-enhanced diffusion. [ABSTRACT FROM AUTHOR]

Photo-ionization (P-I) from deep states in semiconductors can be used to explore the band structure of the crystal as well as to give detailed information about the symmetry and other properties of the bound states themselves. When 2 electrons are bound to a defect the P-I spectrum at high photon energy reflects the initial and final state properties of both electrons. Only at low energies, where one electron is transferred into its 1-electron ground state does the P-I process of a 2-electron state resemble that of a 1-electron state. This fact offers a resolution of the two different interpretations of P-I from the 0 state in GaP as reported by Henry et al. from photo-capacitance studies and by Grimmeiss et al. from photo-conductivity. In the latter the 4 lowest enercry thresholds (E = 0.65 to 1.19 eV) correspond to '1-electron' P-I with the 0 center left in its neutral ground state. Only above about 1.4 eV do the '2-electron' processes appear. In photo-capacitance the '2-electron' processes dominate (the '1-electron' thresholds being too weak or too diffuse to be identified), and the spectrum is strongly modified by relaxation of the lattice around the 0 ion. [ABSTRACT FROM AUTHOR]

The effect of nitrogen doping on the 300°K quantum efficiency and luminescence spectra of VPE-grown gallium phosphide has been investigated. The range of nitrogen concentrations studied was 2 × 10 -1.5 × 10 cm{ on the corrected Lightowlers scale. Over this range, the external quantum efficiency of contactless dice at 40 amp/cm in air varied from 0.04% to 0.13%, increasing monotonically with increasing nitrogen concentration. The photoluminescence spectra from as-grown epi layers and the electroluminescence spectra of contacted dice in reflecting cups were compared, and were characterized by the parameters lumens/watt, radiometric center of gravity, and photometric center of gravity. The relationship of these parameters to each other and their variation with nitrogen concentration in GaP has been determined. The problem of filtering the spectra to obtain greener devices with acceptable brightness has been investi-gated using the transmission spectra of two specific filters. [ABSTRACT FROM AUTHOR]

The PH-HCl-Ga-H technique for VPE growth of GaP is described. The influence of various growth parameters, including substrate temperature, orientation, and PH flow rate on morphology and growth rate are described. For both VPE and LPE nitrogen doping is known to be a major factor in obtaining high green luminescence efficiency. The major emphasis of this paper is an examination of the effect of nitrogen concentration in the range less than 10 cm (using the Lightowlers correction factor) on the growth process and materials properties, such as defect structure, photoluminescence spectra (at 300 and 77K) and photoluminescence intensity and lifetime. The LED device performance (B/J and efficiency) is used as the final test of material quality. Nitrogen is found to be incorporated far in excess of the solubility limit, and the solid gas distribution coefficient for nitrogen is found to increase rapidly with decreasing temperature below 840°C . The optimum nitrogen concentration for high diode efficacy, photoluminescence intensity, and lifetime is found to be approximately 5 × 10 cm, where diodes fabricated by Zn diffusion into the VPE GaP have efficiencies at a current density of 10 A/cm of 0.1%, comparable to the state-of-the-art in the more widely used grown p-n junctions using LPE. [ABSTRACT FROM AUTHOR]

Several optical techniques useful for characterizing GaP LPE crystals grown for light emitting diode applications will be described. The non-exhaustive discussion is primarily in the context of diagnostics developed in studies of the near bandedge emission and the minority carrier lifetime in GaP in the temperature range 100°-300°K. Optical absorption and transient and steady state photoluminescence measurements performed with photon counting techniques are emphasized. The diagnostics described provide many of the parameters pertinent to the luminescent quality of GaP crystals. [ABSTRACT FROM AUTHOR]

Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and <211> directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called 'inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 10 cm, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 10 cm to 1 × 10 cm. [ABSTRACT FROM AUTHOR]