Optical Band Edge Research Articles

Photoresponse Studies of The Polarization Dependence of the CdGeAs2 Band Edge

AbstractMid-infrared photoresponse studies were performed on an oriented p-type cadmium germanium arsenide uniaxial crystal. The effects of optical polarization alignment, parallel and perpendicular to the c-axis of the crystal, were studied, as well as the effects of the transport electric-field direction. The measured optical band edge was 0.578 eV at 10 K for all polarization and bias configurations. This band gap energy is in good agreement with absorption and photoluminescence results for this sample. However, the photoresponse spectrum measured with unpolarized light at 10 K showed a much lower onset at 0.50 eV. This difference in the low temperature activation energy of the photoresponse is attributed to deep native defect levels near the band edge. These deep levels at times obscure the true band edge and can cause under estimates of the band gap energy. These results can explain the wide disparity in the reported CdGeAs2 band gap in the literature. In addition, the intensity of the photoresponse was found to be only slightly dependent on the optical polarization direction, but strongly dependent on the bias electric-field direction. The largest photoreponse was observed when the optical polarization was parallel to the c-axis and the bias electric field was perpendicular to the c-axis. The bias electric-field direction also had a significant effect upon the temperature dependence of the peak photoresponse intensity. The temperature dependence of the CdGeAs2 energy band gap was determined by empirical fitting.

Preparation and characterization of Bi 2S 3 thin films spray deposited from non-aqueous media

Semiconducting Bi 2S 3 thin films were prepared on glass substrates from non-aqueous media using spray pyrolysis. Characterization of the films was carried out using X-ray diffraction, scanning electron microscopy, optical absorption and dark resistivity techniques. These studies reveal that as-deposited films are polycrystalline, having an optical band edge (direct) of 1.92 eV. The electrical resistivity is of the order of 10 6 Ω cm.

Photoluminescence behavior of hydrogenated amorphous carbon

The photoluminescence (PL) of amorphous hydrogenated carbon (a-C:H) has been investigated as a function of optical gap and excitation energy. The PL spectra exhibit a red shift when the excitation energy falls below the optical gap. The bandwidth of the PL spectra increases with the excitation energy but depends very weakly on the gap. The results are discussed in terms of the electronic structure of a-C:H and the configuration of sp2 sites. It is proposed that PL arises by the geminate radiative recombination of the electron–hole pairs confined to the π states of the sp2 bonded clusters. The quantum efficiency is found not to decrease for excitation above the gap, indicating that the electron–hole pair remain bound, and suggesting that any mobility edges lie beyond the optical band edges. The PL efficiency is found to increase roughly exponentially with optical gap, displaying two regimes. At higher gap the efficiency depends just on the gap, while at lower gap the dependence may depend on the capture radius of nonradiative recombination centers.

Intercalation of hydrazines in lead iodide

Abstract Hydrazine and methylated hydrazines were intercalated into thin films and crystalline powders of lead iodide. Interplanar spacing increased with increasing methylation. Weight gain and X-ray data provide estimates of the guest orientation. All three guest molecules have their major axis approximately perpendicular to the layers of the host. The large optical band-edge shifts in this intercalation decreased with increasing methylation. Intercalation of 1,2-dimethylhydrazine produces the first evidence for staging in the heavy-metal iodide systems.

Sub-bandgap absorption of gallium nitride determined by Photothermal Deflection Spectroscopy

Photothermal Deflection Spectroscopy (PDS) is used to study the sub-bandgap absorption of hexagonal gallium nitride (GaN) in the energy range from 0.6 to 3.8 eV. Auto-, n- and p-doped GaN layers deposited by chemical vapour deposition from organometallic precursors (MOCVD) or electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) are measured to investigate the optical band edge, the sub-bandgap absorption and the absorption due to free carriers. PDS is also useful to study the long term and thermal stability of GaN. Changes in sub-bandgap absorption by heating the samples to 600 °C for several hours are presented.

Near-edge optical absorption in liquid iodine under high pressure.

Absorption coefficient measurements near the optical band edge of liquid iodine along the melting curve up to 1.52 GPa and 683 K are reported. An externally heated diamond anvil cell has been used to pressurize the sample. The spectra are dominated in the 4000--12000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range by exponential tails of the Urbach type with strongly temperature-dependent slopes and amplitudes. The possibility of connecting the evolution of this spectral feature with the conductivity jumps recently observed in liquid iodine around 3--4 GPa is discussed.

Temperature Dependence of the Absorption Band Gap Edge of GaN

ABSTRACTThe optical absorption near the fundamental absorption edge in GaN thin films grown on sapphire substrates is studied as a function of temperature. The absorption band edge was determined from either the energy position of the exciton line in samples grown by metalorganic chemical vapor deposition technique, or from the first derivative of the absorption spectra in samples grown by molecular beam epitaxy technique. The band edge energies determined in the temperature range of 13 – 300 K were fitted with Varshni empirical relationship: Eg(K) = Eg(0) – α T2/(T + θD) and with the expression: Eg(K) = Eg(0) – κ/[exp(θE/T) – 1]. It is found that Eg(0), α, θD, and θE to be sample-dependent, which suggests that defects and dislocations significantly affect the optical band edge in GaN.

Optical differentiator based on rapid collapse of Wannier–Stark localization due to space charge field screening

An optical differentiator operation is demonstrated using a GaAs/AlAs short-period superlattice near an optical absorption band edge. This operation is clearly explained by a rapid collapse of Wannier–Stark localization due to electric field screening by photogenerated space charges, because the screening causes a positive feedback loop between restoration of blueshifted wavelength of absorption band edge to the red side and an increase in optical absorption which causes an additional field screening.

Photoacoustic Spectroscopy Measurement of C60 Thin Film

Photoacoustic spectroscopy of C60 thin film is presented. The optical band edge of solid C60 and the optical absorption coefficient at absorption edge are determined. A weak absorption shoulder at 1.70 eV is observed. While the thickness of C60 thin film is less than 1 μm, photoacoustic saturation at band edge is avoided.

Light Output and Radiation Damage in A YAIO3:Ce Crystal

ABSTRACTThe optical absorption, light output and effects of radiation damage have been studied ina 2.1 × 2.1 × 2.1 cm3 crystal of YA103:Ce. The light yield produces approximately 2867 photoelectrons per MeV on a photomultiplier tube with abialkali photocathode. The scintillation and light transmission properties of the crystalwere characterized before and after exposure to 1 Mrad of 60 Co gamma rays. Significant radiation induced optical absorption was observed near the optical band edge ofthe crystal, along with an extremely intense phosphorescence after irradiation. Partial spontaneous recovery from the damage occurred at room temperature, and it was possible to induce additional recovery by optical bleaching.

Spectroscopic ellipsometric measurements of the dielectric function of germanium dioxide films on crystal germanium

From spectroscopic ellipsometry measurements in the 1.5–5.7 eV photon energy range we determined the complex dielectric function of thermally grown germanium dioxide in the 1.0–6.3 eV range. A Kramers–Kronig consistent dispersion formula utilizing an exponential-shaped optical band edge was used in conjunction with both previously published far ultraviolet absorbance data for amorphous GeO2 and our spectra. These measurements show that ε2 for GeO2 can be regarded to be zero in the range of E<5.5 eV, which differs from a previous report. Using these new optical results for the investigation of oxide growth, we find that GeO2 grows via a parabolic growth law with a growth constant, kp=1.2×10−19 m2 s−1 at 550 °C.

Physical and Electrical Properties of Memory Quality PECVD Silicon Oxynitride

Memory quality silicon oxynitride has been deposited using plasma‐enhanced chemical vapor deposition (PECVD). The effects of oxygen on the physical properties of the films were studied using ellipsometry, ultraviolet spectrophotometry, and Fourier transform infrared spectroscopy. Silicon oxynitride films of various compositions were deposited by changing the amount of nitrous oxide in the reactant gas stream from 5 to 85 sccm for a constant value of . The refractive index of the films decreases from 1.8 to 1.55 and the deposition rate increases as nitrous oxide flow rate increases. The Si‒H bond concentration increases slightly for nitrous oxide flows of 5 to 9 sccm and then decreases for higher flows. The optical bandedge varies monotonically from 3.2 eV for silicon nitride (no ) to 5.25 eV for a nitrous oxide flow of 85 sccm. Total bonded hydrogen decreases with increasing nitrous oxide flow due to a reduction in Si‒H bond concentration. Multiple bonding configurations are observed for silicon and hydrogen at the higher nitrous oxide flows. A correlation is observed between Si‒H bond concentration and memory window size. Although the addition of oxygen to silicon nitride enhances endurance performance, high nitrous oxide flows increase the charge decay rate and thus degrade the memory performance.

Electronic structure of amorphous silicon carbide compounds

Abstract The electronic structure of a-Si1–x Cx, has been calculated for the whole range of C concentrations (0 ⩽x ⩽1). The atomic structure of the alloy has been modelled by an ideal continuous random network in which some Si-C atomic correlations have been considered. The electronic levels have been obtained from a nearest-neighbour tight-binding Hamiltonian that has been solved within the Bethe lattice and effective-medium approximations. The gap increase that follows C incorporation is correctly reproduced by our calculations up to x values close to 0·7. The employed structural model based on fourfold coordination of both Si and C atomic species loses its validity for higher C contents. Besides average densities of states, other electronic magnitudes of physical interest such as joint densities of states, spectral and optical gaps, valence and conduction band edges, and energy levels of defect states produced by Si and C dangling bonds have been obtained. We have extended the definition of the charge ...

Organometallic Epitaxy of Extrinsic N-Type HgCdTe Using Trimethylindium

ABSTRACTIndium doping of mercury cadmium telluride, grown by organometallic epitaxy, has been accomplished using trimethylindium (TMIn) as the dopant source. Layers, grown by the Direct Alloy Growth (DAG) process, exhibited a linear doping vs TMIn partial pressure characteristic over the 5 × 1016 to 3 × 1018 cm−3 range. A maximum doping concentration of 5 × 1018 cm−3 was obtained in these layers.The optical band edge in these layers was observed to move to higher energy with increased doping. It is shown that this is caused by the Burstein-Moss shift of the bandedge with doping, and that the Cd fraction is independent of the doping concentration.

A simple general model for charge transfer in polymers

A general but simple model is described to account for charge transport which is applicable not only to insulating polymers but also to those that have been doped to higher levels of conductivity. Charge transport is considered to occur by tunnelling transitions between localized states where the reorganization energy is an essential part of the localization. Expressions are obtained for the transient, a.c. and d.c. conductivities which, in their temperature and time and frequency dependences, are in agreement with experimental results for a number of polymers. An important feature is that these dependences can be used to determine the energy distribution of the localized transport states. It is concluded that these states correspond to the Urbach states at the optical absorption band edge of the polymer. The doping role of metal electrodes is also discussed.

Band edge absorption in CdxHg1− xTe grown by metal-organic vapour phase epitaxy

The first absorption coefficient spectra determined in the region of the fundamental band edge of metal-organic vapour phase grown Cd x Hg 1− x Te (CMT) with x between 0.2 and 0.3 are presented. Analysis of these spectra shows that the optical band edge correlates with α e, between 1000–2000 cm −1. Using these spectra the form of the absorbance curve of uniform CMT can be generated, which when compared with experimental data can be used to obtain a qualitative assessment of the depth uniformity of epitaxial layers.

Photochemical vapor deposition of amorphous silica films using disilane and perfluorosilanes: Defect structures and deposition mechanism

Amorphous silicon dioxide films were deposited from oxygen, disilane (Si2H6), and perfluorosilanes (Si2F6 or SiF4) by photochemical vapor deposition using a deuterium lamp at a substrate temperature of as low as 200 °C. It was found that by mixing Si2F6, defects such as H and OH in the films were effectively removed with an enhancement in the growth rate and a slight doping of fluorine into the films, while SiF4 had no effect except a little doping of fluorine. The generation and extinction of defects including H, OH, and SiSi were investigated quantitatively by measuring infrared spectra and vacuum ultraviolet absorptions at the optical band edge. The model on the deposition process was proposed that photodissociated F-containing radicals eliminate H and activate the growing surface, resulting in enhancing the deposition rate.

Optical Band Edge Absorption in CuInSe2

AbstractOptical properties are studied of (100) oriented CuInSe2 (D) crystals near the absorption edge. While the edge absorption spectra in the range 80 to 370 K are found to follow Urbach's rule, there are two regions in α(ħω) differing in the temperature dependence of Urbach's parameter. The results obtained suggest that the optical band edge absorption in CuInSe2 is dominated by electron–phonon interaction. Linear optical dichroism of the edge absorption in chalcopyrite‐structure CuInSe2 is revealed and investigated. CuInSe2 is established to differ from the other chalcopyrite crystals in a negative linear dichroism near the absorption edge.

Nonequilibrium many-body theory of the optical band edge nonlinearities

The Keldysh-Schwinger nonequilibrium Green's function theory is used to derive on the level of a screened Hartree-Fock approximation the relevant equations which determine the optical properties of laser-excited semiconductors. The equation for the optical interband polarization is derived for four different cases: (1) For situations in which the resonantly excited carriers are in quasi-equilibrium and in an (a) spatially homogeneous, or (b) spatially inhomogeneous state. (2) For ultrashort pulse excitation in which the excitations are in a coherent (unrelaxed) state; (a) for nonresonant excitation of the exciton, and (b) for resonant band-to-band excitation. In case (1a) a Bethe-Salpeter equation in the screened ladder approximation results, in case (2b) we obtain a generalization of Stahl's coherent band edge equation. In case (2a) our result agrees with the polarization equation of Schmitt-Rink and Chemla and explains the optical Stark shift of the exciton, while in case (2b) our result describes the spectral hole-burning (light induced gap) with an unrelaxed electron-hole population.

Novel optical modulators and bistable devices using the self-electro-optic effect in semiconductor quantum wells

The novel electroabsorption seen in quantum well structures near the optical band edge (the quantum-confined Stark effect (QCSE)) is discussed. Its applications to optical modulators and self-electro-optic effect devices (SEEDs) are described. The SEEDs, which rely on simultaneous optical modulation (through the QCSE) and optical detection in the same structure, offer a new class of low-energy optoelectronic devices.