Band Tailing Effect Research Articles

Observation of a blue shift in the optical response at the fundamental band gap in Ga1−xMnxAs

We report the observation of a sharp band-edge response in spectrally resolved differential reflectivity experiments on GaMnAs, in contrast to linear optical experiments in which large band-tail effects are known to dominate. The differential reflectivity response exhibits a blue shift relative to results in GaAs and LT-GaAs, consistent with the valence-band model of ferromagnetism. Our results demonstrate the utility of nonlinear optical techniques for studying the electronic structure of III-Mn-V diluted magnetic semiconductors.

Band tailing effects in neon-implanted GaN

High resolution x-ray diffraction and optical transmission measurements were performed on neon-implanted metal-organic chemical-vapor deposition-grown GaN thin films, successively annealed at 800 and 1000 °C, to study the effect of implantation. Several subsidiary peaks on lower angle side of the main GaN peak were observed in high resolution x-ray diffraction pattern recorded after implantation. The peaks are ascribed to the expansion of the lattice in the implanted part of the samples. Optical transmission measurements revealed a consistent increase in band tailing and redshift in optical band gap of the implanted samples with increasing dose. The band tailing is proposed to be due to expansion of the lattice caused by the production of point defects in the implanted samples. A correlation between optical and structural properties of the implanted samples was studied using results obtained from dose-dependent variation in interplanar lattice spacing, optical band gap, and Urbach energy measurements.

Formation of germanates on germanium by chemical vapor treatment

The surface of a single crystal Germanium wafer was transformed to fluoride and oxide crystals upon exposure to a vapor of HF and HNO 3 chemical mixture. Ellipsometry, X-ray, SEM and photoluminescence were used to investigate the physical properties of the resultant surface structure. The analysis indicates that the transformation results in a polycrystalline hexagonal ammonium fluogermanates and a hexagonal α-Germanium oxide clusters with a preferential crystal growth orientation in <101> direction. The fluogermanates grow particularly around the germanium oxide clusters as evidenced by electron dispersive spectroscopy profiling. Local vibrational mode analysis confirm the presence of N–H and Ge–F vibrational modes of NH 4 + and GeF 6 − ions. The vibrational modes at around 840 cm − 1 is significative of GeO x stretching bands originating from the partial coverage surface oxide formed together with the fluogermanates and clusters on the Germanium. Electronic band structure as probed by ellipsometry is typical of Ge and any discrepancy was associated with disorder induced band tailing effects originating possibly from the effect of oxide clustering.

Transformation of germanium to fluogermanates

The surface of a single crystal Germanium wafer was transformed to crystals of germanium fluorides and oxides upon exposure to a vapor of HF and HNO3 chemical mixture. Structure analysis indicate that the transformation results in a germanate polycrystalline layer consisting of germanium oxide and ammonium fluogermanate with a preferential crystal growth orientation in <101> direction. Local vibrational mode analysis confirms the presence of N-H and Ge-F vibrational modes in addition to Ge-O stretching modes. Energy dispersive studies reveal the presence of hexagonal {\alpha}-phase GeO2 crystal clusters and ammonium fluogermanates around these clusters in addition to a surface oxide layer. Electronic band structure as probed by ellipsometry has been associated with the germanium oxide crystals and disorder induced band tailing effects at the interface of the germanate layer and the bulk Ge wafer. The acid vapor exposure causes Ge surface to emit a yellow photoluminescence at room temperature.

3D Monte Carlo analysis of potential fluctuations under high electron concentrations

Potential fluctuations and hot electron effects associated with the Coulomb interaction among electrons are investigated with the self-consistent 3D MC simulations under the thermal equilibrium condition. It is demonstrated that the present simulations could successfully simulate the collective plasma waves owing to careful optimizations of simulation parameters. As a result, the potential fluctuations due to the plasma waves greatly disturb the local density of states and induce the band-tailing effects, which lead to the electron kinetic energy much larger than that of room temperature, whose magnitude coincides well with the previous results obtained from the 2D MC simulations.

Structural, optical and electrical properties of high energy irradiated Cl-VPE grown gallium nitride

The effect of 80 MeV Si (fluences of 10 11, 10 12, 10 13 and 10 14 cm −2) on the structural and optical properties of chloride vapour phase epitaxy (Cl-VPE) grown unintentionally doped n-type gallium nitride (GaN) epitaxial layers have been studied by Raman scattering, high-resolution XRD, photoluminescence (PL), UV absorption and Hall measurement techniques. The behaviour of Raman shift for E 2(high) and A 1(LO) modes of GaN layers have been discussed. New peaks have been observed at the fluences 10 13 and 10 14 cm −2. Band tailing effect has been observed in the optical absorption spectra of GaN samples. Broadening of the PL emission peak were observed with increasing fluence concentration, which could be attributed to the band tailing effect which plays an important role in determining the optical properties of Si irradiated GaN layers. It is found that the irradiation induces lattice-disordering, decrease in the electron mobility and carrier concentration compared to as-grown samples.

Critical point transitions of wurtzite indium nitride

The optical transmission, photoluminescence, and reflection spectra have been measured on a high-quality wurtzite indium nitride (InN) single crystal in the range of 0.5–20.0 eV. The fundamental bandgap of intrinsic InN has been extracted by taking into account the Burstein–Moss shift, bandgap renormalization and Urbach band tail effects, and found to be very close to the recent strongly re-established value of ∼1.2 eV. With the aid of Adachi's dielectric function model for the vacuum ultraviolet reflection spectra and the empirical pseudopotential method approach for the electron band-structure, we are able to identify up to nine electronic transitions, showing clear picture for the critical point transitions in InN. The temperature dependence of these interband transitions has also been revealed.

Photoluminescence of n-type CdGeAs2

Samples of n-type CdGeAs2 were produced by intentional doping with indium, selenium, or telluriumimpurities. A near-edge photoluminescence (PL) band from heavily In-dopedCdGeAs2 samples shifts to higher energy and becomes broader with increasing electronconcentration. The observed shifts in peak energies are compared to predictions fordonor–acceptor pair and free-to-bound (electron–acceptor) recombinations including bandfilling, band tailing, and band gap shrinkage effects due to the high doping levels. Forn>2 × 1018 cm−3, the free-to-bound PL transition related to a shallow 120 meV acceptor level isdominant. A lower energy PL band due to deep acceptors and normally seenfor p-type samples is the only emission observed from less n-type samples(n∼1016–1017 cm−3) doped with indium, selenium, or tellurium impurities. Transitions involvingthe deep acceptor level are not present in the PL for heavily In-dopedCdGeAs2 crystals, which suggests that the deep acceptor may be a Cd vacancy.

Effect of Si ion irradiation on polycrystalline CdS thin film grown from novel photochemical deposition technique

CdS thin films have been deposited from aqueous solution by photochemical reactions. The solution contains Cd(CH 3COO) 2 and Na 2S 2O 3, and pH is controlled in an acidic region by adding H 2SO 4. The solution is illuminated with light from a high-pressure mercury-arc lamp. CdS thin films are formed on a glass substrate by the heterogeneous nucleation and the deposited thin films have been subjected to high-energy Si ion irradiations. Si ion irradiation has been performed with an energy of 80 MeV at fluences of 1×10 11, 1×10 12, 1×10 13 and 1×10 14 ions/cm 2 using tandem pelletron accelerator. The irradiation-induced changes in CdS thin films are studied using XRD, Raman spectroscopy and photoluminescence. Broadening of the PL emission peak were observed with increasing irradiation fluence, which could be attributed to the band tailing effect of the Si ion irradiation. The lattice disorder takes place at high Si ion fluences.

On the electron energy spectrum in heavily doped non-parabolic semiconductors

An attempt is made to present a simple theoretical analysis of the energy-wave vector dispersion relation of the conduction electrons in heavily doped non-parabolic semiconductors forming band tails. We observe that the complex energy spectrum in doped small-gap materials whose unperturbed conduction band is described by the three band model of Kane is due to the interaction of the impurity atoms in the tail with the spin-orbit splitting constant of the valence band ( Δ), For band-gap ( E g)< Δ the imaginary part predominates which tails in to the conduction band. For the opposite inequality the real part comes in to play which tails in to the split-off band. In the absence of the band tailing effect, the imaginary part of the complex energy spectrum vanishes and the same is also true for doped two-band Kane-type and parabolic energy bands respectively. The present formulation helps us in investigating the Boltzmann transport equation dependent transport properties of degenerate semiconductors and are expected to agree better with experiments. The well-known results of unperturbed three and two band models of Kane together with wide-gap parabolic energy bands have been obtained as special cases of our generalized analysis under certain limiting conditions.

Optical characteristics of Ge doped ZnO compound

The compound of ZnO: Ge which can be used as a blue light emission material has been prepared by the solid state reaction method. We have discussed the optical properties of the compound by photoluminescence (PL) measurements. We discovered that Zn 2GeO 4 phase is formed by the heavy doping of Ge atoms, and the solid solubility limit of Ge in ZnO is about 0.7 mol%. From the observed PL spectra of the compound there is a blue shift up to 0.7 mol%, but in concentrations higher than 0.7 mol% a red shift is observed. We can explain the shift of PL peak by Burstein-Moss and the band tail effects.

A Simple Theoretical Analysis of the Effective Electron Mass in Heavily Doped III–V Semiconductors in the Presence of Band-Tails

We study the effective electron mass at the Fermi-level on the basis of a newly formulated electron energy spectrum for heavily doped non-parabolic III–V semiconductors forming band-tails, taking n-InSb as an example. The investigation on the mass includes both the presence and absence of band-tailing states, in parabolic and non-parabolic energy band structures, respectively. The effective mass exists in the forbidden region, which is impossible without band-tailing effects. The analyses have been carried out for the Fermi-energy and other entities, like screening length and impurity screening potential, respectively. The values of the effective mass with band-tailing states are found much less as compared with in the absence of band-tails. The well-known results for unperturbed two-band model of Kane have also been obtained from our generalized analysis under certain limiting conditions.

Study of band gap narrowing effect in n-GaAs for the application of far-infrared detection

The band gap shrinkage Δ E g of n-GaAs in relation with the doping concentration ( N) is still controversial in various theories and experiments. Most of the experimental results of Δ E g from photoluminescence have already been questioned to have an overestimation by a few tens of meV. We have carried out detailed temperature-dependent transmission experiments on an n-GaAs homojunction far-infrared (FIR) detector structure. By calculating the transmission spectra taking into account the band tail effect, we obtained a 1/3 power rule of Δ E g in n-GaAs by Δ E g=−3.60×10 −8 (eV cm) N 1/3. Our results support Harmon et al.'s experimental Δ E g results from the electrical measurements of np product in n-GaAs (Appl. Phys. Lett. 64 (1994) 502). The yielded Δ E g is employed to calculate the cut-off wavelength λ c and dark current of n-GaAs homojunction FIR detectors to investigate the potential application. We find that n-GaAs is more suitable for a long λ c detector design, and has a similar dark current behavior with that of Si and p-GaAs homojunction FIR detectors.

Spatial inhomogeneity investigation of QW emission in InGaN MQW LEDs

The luminescence properties of InGaN multiquantum well (MQW) light emitting diodes (LEDs) with various emission wavelengths were investigated using electroluminescence (EL) microscopy (ELM) and micro-EL (μ-EL) spectroscopy. Spatial inhomogeneity of the QW emission was observed as a function of the emission wavelength in spectrally resolved ELM images. The results show that the emission from diodes with shorter emission wavelengths exhibit less spatial inhomogeneity, while bright features were observed on the surface of samples with longer emission wavelengths. μ-EL spectra obtained from 5×5 μm 2 regions show an increase in the full width at half maximum (FWHM) of the spectra with increasing injection current and emission wavelength. With increasing emission wavelength, there is a shift in the main emission area from the p-contact area to the n-contact. The μ-EL spectra obtained from different bright spots in the ELM images demonstrate the variation of main emission peak position and FWHM of the EL spectra. The results suggest that the band-tailing effect due to small self-organised In-rich regions play a key role in the emission of the InGaN MQWs.

Some Transport Coefficients in Heavily Doped n-Type Silicon

In this paper we have derived explicit expressions for some of the important galvanomagnetic transport coefficients, viz, the drift mobility, the Hall coefficient, and the coefficient of transverse magnetoresistance in heavily doped n-type silicon taking into account the effect of band tails. These expressions are then used to calculate the transport coefficients for various donor concentrations from 1 x 10 18 to 2 x 10 20 cm -3 at 300 K. Our calculations show that taking into account the effect of conduction band tails results in lowering the values of the transport coefficients by as much as 30-50% than those obtained by neglecting the effect of band tails.

Current mechanisms in VLWIR Hg1−xCdxTe photodiodes

VLWIR (c∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance. Id−Vd curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range. At 30 K the detectors are limited by tunneling currents in reverse bias. Since the detectors are diffusion limited near zero bias down to 40 K, the R0Aimp versus temperature data represents the diffusion current performance of the detector as a function of temperature. The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy Ea optical. The activation energy, Ea electrical, obtained from the measured diffusion limited R0Aimp versus temperature data is not equal to the activation energy, Ea optical, obtained from the spectral response measurement for all x values measured. Ea electrical=*Ea optical, where ranges between 0.64 and 1.0 For cutoff wavelengths in the 9 m at 78 K, Ea electrical=Ea optical. Ea electrical=0.65* Ea optical have been measured forc=17 m at 78 K detectors. As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system. In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor. Such a picture could explain the observed behavior. Further probing experiments and modeling efforts will help clarify the physics of this behavior.

Evaluation of free-carrier concentration in Si +-implanted InP by means of photoluminescence

Optical techniques can provide nondestructive, contactless determinations of free-charge density in doped semiconductors. We discuss the use of photoluminescence spectra to evaluate the free-charge density of doped semiconductor layers obtained by ion-beam implantation. A photoluminescence line-shape model that includes the conduction band nonparabolicity, band-tailing and band-gap narrowing effects is presented. The model is applied to Si +-implanted InP samples with electron densities in the 5×10 16–5×10 18 cm −3 range.

Evaluation of n-type doping of 4H-SiC and n-/p-type doping of 6H-SiC using absorption measurements

A non-destructive, absorption measurement based optical method has been developed in order to determine doping type (n- or p-type), doping level and doping level distribution in 4H and 6H silicon carbide (SiC) wafers. The bandgap absorption has been calculated numerically taking into account band filling, band shrinkage and band tailing effects which are a function of donor and acceptor concentration N D and N A, respectively. The numerical results are compared with experimental data. A calibration plot of the doping dependence of the absorption of n-type 6H SiC is presented and the application for mapping of the SiC wafer doping level distribution is demonstrated.

Linear optical properties of a heavily Mg-doped Al0.09Ga0.91N epitaxial layer

The room-temperature absorption coefficient and ordinary refractive index for a ∼0.4-μm-thick p-type wurtzite Al0.09Ga0.91N epitaxial layer were determined via optical transmission measurements. The layer was grown by metal organic chemical vapor deposition and heavily doped (∼5×1019 cm−3) with Mg. Additional measurements of the refractive index by prism coupling to the layer confirmed the transmission results. The low-temperature AlN buffer layer altered the expected interference fringes of the transmission spectrum below the band-gap energy and had to be accounted for in the analysis. The absorption coefficient exhibited band-tail effects and had a reduced slope near band-gap energy as compared to undoped GaN. Using a detailed balance argument, the reduced slope was consistent with the lack of a peak in the continuous-wave photoluminescent emission.

The Magnitude of the Piezoelectric Effect in InGaN Quantum Wells

ABSTRACTA photoluminescence from a multiquantum GaN/InxGa1−xN/GaN well structure (x varies between 0.1 to 0.4) was investigated at various temperatures, pumping powers. While the temperature dependence of the peak position indicates normal band to band character of radiative recombination, the large pumping power induced “blue”shift of the peak position (up to 200 meV) can be observed. This kind of shift cannot be easily explained by the band tailing effect but is most likely the result of the screening of the strain-induced piezoelectric field. By evaluating the theoretical values of the piezoelectric filelds in the quantum well, we can show that in order to account for the experimental results we have to assume the partial relaxation of the strain.