Increase In Band Gap Energy Research Articles

Increase in In 0.32Ga 0.68P band-gap energy due to the electron capturing of DX centers

Anomalous increase of band gap energy was observed in the temperature-dependent photoluminescence spectra of S-, Se- and Te-doped In 0.32Ga 0.68P. This blue shift could be explained by the change of many body effect of the conduction electrons due to the electron capture at the DX centers. The ratio of the electron concentration captured at the DX centers to the free electron concentration could be determined empirically from the values of the observed band gap energy shift. The ratios were 0.40, 0.12 and 0.15 for S-, Se- and Te-doped In 0.32Ga 0.68P samples with dopant concentration of mild 10 17 cm −3, respectively.

Electroluminescence in conducting polymers based on poly(phenylene ethynylene)

Abstruct Electroluminescence (EL) in various conducting polymers (CP) based on poly(phenylene ethynylene) (ROPPE) which have C-C triple bond in their main chains were studied. EL in Al/CP/ITO structure utilizing copolymer based on ROPPE and pyridine as CP was blue-green and stronger than that in same structure utilizing well-known poly(dialkoxy-p-phenylene vinylene) (ROPPV). However, weaker red EL was observed in same structure utilizing copolymer based on ROPPE and anthracene. These result suggests that the increased band gap energy and improve exciton confinement efficiency, due to shortened conjugate length, were realized by introduction of C-C triple bonds in main chain of conducting polymers such as ROPPV, while these effects of C-C triple bonds are suppressed by introduction of electron-rich moiety such as anthracene which should increase effective conjugation length. Electrochemical studies on these copolymers also confirmed this interpretation.

Visible and Ultraviolet Photoluminescence from Cu–III–VI2 Chalcopyrite Semiconductors Grown by Metalorganic Vapor Phase Epitaxy

We optimized the overall process of heteroepitaxial growth of wide-gap Cu–III–VI2 chalcopyrite semiconductors by using the low-pressure metalorganic vapor phase epitaxy technique, and obtained very high quality epilayers of them. All end-point compounds, namely CuGaSe2, CuGaS2, CuAlSe2 and CuAlS2, exhibited predominant free and bound excitonic photoluminescence (PL) peaks at low temperature. The color of the emission varied from red to ultraviolet. The room temperature (RT) PL spectra exhibited predominant near-band-edge emission except for that for CuAlSe2. A noticeable excitonic feature was found in the PL spectra for CuAlS2 and CuGaS2 even at RT. Time-resolved and excitation intensity-dependent PL measurements revealed the existence of free-to-bound (FB) and donor-acceptor pair recombination emission centers in some undoped and impurity-doped compounds and alloys. The ionization energy of the recombination centers increased with increasing band gap energy of the matrix, reflecting an increase in the hole effective mass. The potential of heteroepitaxial layers of chalcopyrite semiconductors as new light-emitting materials was demonstrated.

Thermal spike model applied to the irradiated yttrium iron garnet: Mean diffusion length of the energy deposited on the electrons

Single crystals of yttrium iron garnet Y 3Fe 5O 12 have been irradiated with 270 MeV 52Te, 760 MeV 86Kr and 1040 MeV 208Pb ions. The extent of the induced damage is determined using Rutherford backscattering spectrometry (RBS) in channeling geometry with a 2 MeV 4He beam. Taking into account these new results and the previous ones, the thermal spike model was applied to calculate track radii versus electronic stopping power for different beam energies using only one fitting parameter, λ the mean diffusion length of the energy deposited on the electrons by the slowing down of a swift heavy ion in the garnet. As the latent heat of fusion is unknown for yttrium garnet, the calculation was first performed by neglecting it. A λ value of (6.3±0.3) nm is extracted by fitting all the different results taking into account a velocity effect. Such a λ value is the high limit of the mean diffusion length since it decreases if the latent heat of fusion is not neglected. This value of λ is in agreement with the previous determinations made in the same conditions for resistant insulators, showing that λ should decrease when the band-gap energy increases.

X-ray-diffraction study of size-dependent strain in quantum-wire structures.

We report a synchrotron x-ray-diffraction study of the strain field in embedded ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs (001) quantum wires of widths 50--250 nm. Our results show a size-dependent orthorhombic lattice deformation in the wires and a linearly strained interfacial region near the wire sidewalls. The measured strain is responsible for an unusual band-gap energy increase that is several times larger than the quantum confinement effect, indicating that strain effects contribute significantly to band-edge energies in this and other quantum structures. \textcopyright{} 1996 The American Physical Society.

Photoionization and luminescence properties of Bi 3+ in In1 − xLuxBO3 solid solutions

The luminescence properties of the In 1 − x Lu x BO 3: Bi 3+ (0⩽ x⩽1) solid solutions were studied. In the concentration range 0 < x⩽0.30 both the emission of Bi 3+ ions and the emission due to an annihilation of the impurity-trapped excitons are observed. The impurity-trapped exciton band shifts towards higher energies with increasing x. At x ⪢ 0.30 only emission of Bi 3+ centers is observed. This behavior is correlated with increasing band gap energy of the solid solutions and can be described in terms of configuration coordinate curve diagrams.

Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film

Spectroelectrochemical techniques may be used to determine the absolute energy of the valence and conduction band edges of a transparent nanocrystalline semiconductor electrode. Such determinations have been made for ZnO (wurtzite) and TiO2 (anatase) electrodes constituted from nanocrystallites possessing average radii close to, and substantially larger than, the radius of a bound exciton in the corresponding bulk semiconductor. Electrodes constituted from crystallites whose radii are close to that of a bound exciton exhibit an onset for band gap absorption that is significantly blue-shifted. Those constituted from crystallites whose radii are substantially larger than that of a bound exciton exhibit an absorption onset characteristic of the bulk material. Knowing the absolute energies of band edges, the observed increase in band gap energy for electrodes constituted from confined nanocrystallites may be partitioned between the conduction and valence bands. A subsequent analysis permits determination of t...

Wide band gap MgZnSSe grown on (001) GaAs by molecular beam epitaxy

We report molecular beam epitaxial study of wide band gap (≳2.9 eV at room temperature) MgZnSSe on (001) oriented GaAs using ZnS, Mg, Zn, and Se sources. Although the growth is under group II rich condition, the compositions of S and Mg in the MgyZn1−ySxSe1−x are linear functions of flux ratios, PZnS/PSe and PMg/PZnS, up to 35%, respectively. Mirrorlike surface and low defect density (5×104 cm−2) MgZnSSe with band gap close to 3.1 eV can be achieved. Composition modulation, tweedlike contrasts and strain contrasts in the MgZnSSe are observed from transmission electron microscope analysis. For the first time, a miscibility gap at high S and Mg compositions is reported. Nitrogen-free radicals are used as the p-type dopant for the doping study. For the MgZnSSe with room-temperature band gap energy higher than 2.9 eV, the net acceptor concentration decreases as the band gap energy increases.

Epitaxial growth of p-type ZnMgSSe

N-doped p-type ZnMgSSe was grown by molecular beam epitaxy. Nitrogen was induced by electron cyclotron resonance plasma. The maximum net acceptor concentration (NA−ND) and the activation energy of the nitrogen acceptor (EN) depend on the band-gap energy of ZnMgSSe. With increasing band-gap energy, the maximum NA-ND is decreased and EN is increased. The maximum NA-ND and the EN of ZnMgSSe with a band-gap energy of 3.05 eV at 77 K are 2.5×1016 cm−3 and 140 meV, respectively.

Scanning tunneling microscopy of porous silicon surfaces

The scanning tunneling microscope (STM) was used to examine several porous silicon based surfaces. First are samples of photoluminescent porous silicon made from single crystal wafers of 13.5 Ω cm p− (100) and 0.002 Ω cm p+ (100) silicon. These were observed in an ambient He environment. The development of the porous structure in these wafers due to different etching stages is investigated with the STM. Measurements show that the vertical surface roughness increases with etching while the lateral dimensions of the structures present remain almost constant in agreement with known etching models. The porous silicon surface density of states (DOS) was also measured using scanning tunneling spectroscopy. DOS measurements of porous silicon formed on both p− and p+ substrates do not show the expected increase in band gap energy when compared with unetched crystalline silicon. However, these measurements do show that while the unetched silicon surface remains p type, porous silicon surfaces fabricated from p− Si behaves like an n-type material whereas those formed from p+ Si remain p type after etching.

Magnetic field dependence of selective pair luminescence in GaAs

The shift in the final state emission energy, resulting from selective pair luminescence in an applied magnetic field in GaAs, is very small. Any shift that is observed is toward lower energy. The relatively constant emission energy in the presence of an applied magnetic field is realized because the increase in band-gap energy, which would shift the emission toward higher energies, is offset by an increase in the donor binding energy which shifts the emission toward lower energies. The acceptor binding energy is essentially constant with applied field while the change in the Coulomb term, due to a change in the pair separation in a magnetic field, is quite small. As a result, the same acceptor excited states are identified in the presence of an applied magnetic field, as in zero field for the same pump energy. The results of this experiment are limited to applied fields ≤36 kG.

Diffusion and electronic states of hydrogen in n-type GaAs and n-type Ga1-xAlxAs.

Deuterium diffusion experiments have been performed in silicon-doped GaAs and ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As (x\ensuremath{\le}0.30). In GaAs, the hydrogen diffusion profile closely fits a complementary error function. In ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As alloys with x\ensuremath{\ge}0.055, the profiles present a plateau followed by a sharp decrease. To interpret this difference we propose that in n-type GaAs:Si and ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As:Si, hydrogen behaves as a deep acceptor with a level ${\mathit{H}}^{\mathrm{\ensuremath{-}}/0}$ slightly resonant in the conduction band of GaAs and emerging as a localized state for x&gt;0.07 as the band-gap energy increases. Then, for x&gt;0.07, the ${\mathit{H}}^{\mathrm{\ensuremath{-}}}$ species become dominant and are trapped on the positively charged donors during diffusion.

An Interpretation on Existence of Thermochromismin Poly-(3-alkylthiophene) and Absence in Poly(3-alkylfuran):A Theoretical Study of Conformation Defect in Polymer Chain

The conformation defect in polythiophene and polyfuran has been studied based on their optimized geometries using the semi-empirical tight-binding SCF-CO (self-consistent crystal orbital) method. It has been demonstrated that the total energy, the ionization potential and the band gap energy increase with increasing torsion between rings for both polymers. From the viewpoint of the total energy, however, polyfuran is difficult to have the conformation defect in comparison with polythiophene. The π-conjugation length along the polymer chain is largely decreased due to the existence of the conformation defect in polythiophene substituted with alkyl side chain induced by the steric hindrance at high temperature but not in substituted polyfuran.

Trends in the open circuit voltage of semiconductor/liquid interfaces: studies of n-aluminum gallium arsenide/acetonitrile-ferrocene+/0 and n-aluminum gallium arsenide/potassium hydroxide-selenide-/2- (aq) junctions

Trends in open-circuit voltage (V{sub oc}), short-circuit current density (J{sub sc}), and energy conversion efficiency have been determined for the n-type Al{sub x}Ga{sub 1{minus}x}As series of photoelectrodes (x = 0.0, 0.09, 0.16, 0.24, 0.31) in contact with CH{sub 3}CN-ferrocene{sup +/0} and KOH-Se{sup {minus}/2{minus}}(aq) electrolytes. V{sub oc} increased linearly with increases in bandgap energy (E{sub g}) of the n-Al{sub x}Ga{sub 1{minus}x}As alloy electrodes, with {Delta}V{sub oc}/{Delta}E{sub g} = 0.45 {plus minus} 0.04 V eV{sup {minus}1} in CH{sub 3}CN and 0.41 {plus minus} 0.09 V eV{sup {minus}1} in KOH-Se{sup {minus}/2{minus}}(aq) at a light intensity sufficient to provide J{sub sc} = 1.0 mA cm{sup {minus}2}. J{sub sc} values under solar-simulated illumination decreased monotonically with increasing bandgap energy. The relatively low value of {Delta}V{sub oc}/{Delta}E{sub g} implies decreases in optimal energy conversion efficiency as the mole fraction of Al in the Al{sub x}Ga{sub 1{minus}x}As alloy is increased. This is in contrast to the behavior of the n-GaAs{sub x}P{sub 1{minus}x} alloy series in the same electrolytes.

Preparation of hydrogenated amorphous Si-C alloy films and their properties

Hydrogenated amorphous Si 1− x C x films were prepared by glow discharge decomposition from a mixture of SiH 4 and C 2H 4 at substrate temperatures between room temperature and 260°C under a low r.f. power condition. The deposition rate, hydrogen content, optical band gap and photoluminescence were investigated. These films contain hydrogen atoms at concentrations of 40–70 at.%. The optical band gap and photoluminescence peak energy increase monotonically with x and also with decreasing substrate temperature. Bright bluish white photoluminescence was observed at room temperature in samples of composition Si 0.14C 0.86. The film structure and the emission mechanism are discussed.

Optical Characteristics of CdSe/ZnS Quantum Dot with Precursor Flow Rate Synthesized by using Microreactor

&lt;p&gt;High-quality colloidal CdSe/ZnS (core/shell) is synthesized using a continuous microreactor. The particle size of the synthesized quantum dots (QDs) is a function of the precursor flow rate; as the precursor flow rate increases, the size of the QDs decreases and the band gap energy increases. The photoluminescence properties are found to depend strongly on the flow rate of the CdSe precursor owing to the change in the core size. In addition, a gradual shift in the maximum luminescent wave (λ&lt;sub&gt;max&lt;/sub&gt;) to shorter wavelengths (blue shift) is found owing to the decrease in the QD size in accordance with the quantum confinement effect. The ZnS shell decreases the surface defect concentration of CdSe. It also lowers the thermal energy dissipation by increasing the concentration of recombination. Thus, a relatively high emission and quantum yield occur because of an increase in the optical energy emitted at equal concentration. In addition, the maximum quantum yield is derived for process conditions of 0.35 ml/min and is related to the optimum thickness of the shell material.&lt;/p&gt;

Preparation and Properties of Noncrystalline Zinc Oxide Films

Noncrystalline films of zinc oxide have been formed by reactively sputtering zinc metal onto a low-temperature substrate in an oxygen-argon atmosphere. The structural state of the films is determined by the temperature of the condensing substrate, and noncrystalline films formed at low substrate temperatures crystallize on heating to 75°–100°C in air or vacuum. A change in optical absorption corresponds to an increase in band gap energy for the crystalline to noncrystalline transition; this is interpreted as resulting from the change in dielectric constant occurring for the same transition. A large reduction in electrical conductivity observed for the noncrystalline material is believed to result mainly from reduced carrier concentration.