Band Tail Region Research Articles

Theoretical study of leaky-mode resonant gratings for improving the absorption efficiency of the uncooled mid-infrared photodetectors

In this work, we studied a new theoretical approach to enhance the PbSe-based uncooled photodetector's performance in the mid-infrared (mid-IR) wavelength region. A one-dimensional grating layer was proposed to be implemented and inserted under the PbSe photosensitive layer. Due to the leaky-mode resonance tuning mechanism, this grating layer could manipulate mid-IR light-and-matter interactions in a unique way. It not only prevents the light transmission loss from the backside of the device but also tunes the destructive interference in the reflectance spectra, which consequently maximize the light absorption in the weak-absorbing energy band-tail region. By combining these two capabilities, it is possible to improve detector's sensitivity significantly. With this goal, several grating parameters including grating period, fill factor, and grating thickness were systematically investigated and optimized by using the rigorous coupled wave analysis method. Through the optimization, a 33% broadband absorption enhancement was achieved by using a Si (n = 3.489)/SiO2 grating on a soda-lime glass (n = 1.45) substrate with the listed parameters: grating period = 2.4 μm, fill factor = 0.49, and thickness = 1.22 μm. Apparently, this simple and effective method could practically advance the uncooled mid-IR PbSe detector's performance. But more importantly, this photonic-design concept can be used in and impact many other light-matter interaction related research fields.

Electronically active defects in the Cu2ZnSn(Se,S)4 alloys as revealed by transient photocapacitance spectroscopy

Transient photocapacitance (TPC) spectra were obtained on a series of Cu2ZnSn(Se,S)4 absorber devices with varying Se:S ratios, providing bandgaps (Eg) between 1 eV and 1.5 eV. Efficiencies varied between 8.3% and 9.3% for devices with Eg ≤ 1.2 eV and were near 6.5% for devices with Eg ≥ 1.4 eV. The TPC spectra revealed a band-tail region with Urbach energies at or below 18 meV for the first group, but in the 25-30 meV range for the higher band-gap samples. A deeper defect band centered near 0.8 eV was also observed in most samples. We identified a correlation between the Urbach energies and the voltage deficit in these devices.

Structure of the band tails in amorphous selenium

Transient photocurrent measurements on evaporated a-Se layers indicate thepresence of two sets of discrete traps in the band tail region. The shallower traps, atEV+0.20 eVand EC−0.28 eV, are found to be electrically neutral, while the deeper ones atEV+0.38 eV andEC−0.53 eV are related to thecharged negative-U centres of a-Se. The density of the discrete traps is of the same order of magnitude as thedisorder-induced background density in the valence and conduction band tails, preventingthe characterization of the a-Se tail-state densities by a simple functional form.

Temperature, thickness, and interfacial composition effects on the absorption properties of (Hg,Cd)Te epilayers grown by liquid-phase epitaxy on CdZnTe

Transmission spectra of liquid-phase epitaxy (LPE) Hg1-xCdxTe with Cd mole fractions in the range of 0.23<x<0.30 have been obtained as a function of temperature and thickness. The results are described using a model consisting of exponential (Urbach) absorption in the band tail region and band-to-band absorption, predicted by a Kane k p model, for the above band gap region. Modifications to the Urbach and transmission expressions are found to be necessary to properly describe the shape, temperature, and thickness dependence of the spectra. A known composition gradient was found to be sufficient to describe the spectra obtained for thin (<20-µm) samples.

Detailed study of above bandgap optical absorption in HgCdTe

HgCdTe remains the material of choice for high-performance infrared (IR) detectors due to its tunable direct bandgap energy corresponding to the IR spectral region, and the advancement of HgCdTe materials growth and processing technologies. Accurate knowledge of the HgCdTe optical absorption coefficient is important for IR detector design, layer screening, and device analysis. The spectral response for IR detectors is dependent on optical absorption above the bandgap energy, where much of the study of absorption coefficient in HgCdTe has focused on the bandtail region. In this work, the optical absorption coefficient was studied by theoretical bandstructure calculations and experimental measurements on HgCdTe layers using techniques of IR spectroscopic ellipsometry and IR transmission. The theoretical and experimental results suggest that the absorption coefficient between 600 cm−1 and 5,000 cm−1 is related to energy relative to bandgap with a fractional exponent between 0.6 and 1, rather than the previously used expressions relating to a parabolic or hyperbolic bandstructure. The fitting parameters for Hg1-xCdxTe with x=0.22–0.60 are presented to develop a model for the optical absorption coefficient spectra. The calculated detector spectral response using the new and previously reported absorption coefficient models suggests that next generation IR detectors employing multilayer structures with graded compositional profiles will likely benefit from this new model.

Process Optimization and Integration of HFO2 and HF-Silicates

ABSTRACTWe have established in-line characterization techniques for analyzing the bulk and interface-charge properties of dielectric films, for process optimization. Surface charge analysis (SCA) is used to determine the densities of interface states, fixed charge, and near-interface traps in ultra-thin dielectrics, and is useful for tracking the influence of post-deposition processing on interface-charge properties. Spectroscopic ellipsometry (SE) is used to obtain the absorption spectra in the conduction band-tail region. The intensity of an extra absorption peak inside the bandgap of HfO2 is clearly correlated with leakage current density and near-interface trap density. Based on the observed process dependencies, defects within the HfO2 films are likely to be oxygen vacancies. The relative scalability of HfO2 and Hf-silicate films of various compositions is examined using a figure of merit based on the direct-tunneling leakage current model. Pure HfO2 is expected to be more scalable than Hf-silicates. However, it is typically accompanied by an interfacial layer which significantly increases the equivalent oxide thickness (EOT). A 20% Hf silicate with relative permittivity of 11 or higher can be more scalable than HfO2 with an interfacial layer. Alternatively, an ultra-thin interfacial Si3N4 diffusion barrier can be used with HfO2, to allow for more aggressive EOT scaling. The dependencies of interface-charge properties and surface roughness on the nitride barrier formation process are presented.

Optical absorption properties of HgCdTe epilayers with uniform composition

Hg1−xCdxTe is an important material for high-performance infrared detection for a wide spectral range, from 1.7 µm to beyond 14 µm. An accurate understanding of the relationship between optical absorption and bandgap energy of this semiconductor alloy is needed for pre-process layer screening, detector design, and interpretation of detector performance. There is currently a disparity among the infrared detector community in relating the optical absorption properties to HgCdTe alloy composition and bandgap energy. This disagreement may stem from a misunderstanding of absorption properties, where existing models were developed decades ago using either bulk material or material with nonuniform composition. In this work, we have initiated an investigation of the optical absorption properties of HgCdTe with uniform composition grown by molecular-beam epitaxy (MBE) with in-situ compositional control via spectroscopic ellipsometry. The absorption properties show unique characteristics in the bandtail region, with insignificant temperature dependence. The absorption properties above the bandgap suggest a hyperbolic bandstructure as opposed to the common assumption of a parabolic bandstructure. The temperature dependence of the bandgap energy shows good agreement to the commonly used expression developed previously by Hansen et al.

Photoconductivity studies of n-type hydrogenated amorphous silicon and microcrystalline silicon

Photoconductivity techniques serve as useful tools for the characterization of amorphous and microcrystalline silicon. From the link between the majority carrier mobility–lifetime product from steady-state photoconductivity and the position of the Fermi level, useful insight can be gained when comparing sample properties. The temperature dependence of the minority carrier mobility–lifetime product implies that the band-tail region of the density-of-states (DOS) is steeper in microcrystalline silicon than in amorphous silicon. Transient and modulated photoconductivity determine the DOS in the upper half of the band gap, for which we find an exponential variation. We indicate that the Fermi level or quasi-Fermi level impose limitations on the DOS extraction from the measured data. In samples in which the Fermi level is shifted towards the conduction band, the DOS calculation then yields values that are too low.

Femtosecond transmission studies of n-type and p-type hydrogenated amorphous silicon pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on n-type and p-type hydrogenated amorphous silicon are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different materials are quite different, allowing the technique to clearly distinguish between them. It is shown that while recombination is dominant in intrinsic a-Si:H, trapping becomes important in the doped material. A model of the experimental results gives additional insight into the density of states in the band tail region.

Femtosecond transmission studies of a-Si:H, a-SiGe:H and a-SiC:H alloys pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on hydrogenated amorphous silicon alloys are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different alloys are similar, but the technique is able to clearly distinguish between them based on their alloy composition and bandgap. It is shown that the results can be modeled as a free-carrier absorption spike followed by a residual plateau caused by both recombination and intraband heating. This model is adapted to work in the band tail region.

Femtosecond pump-probe spectroscopy and time-resolved photoluminescence of anInxGa1−xN/GaNdouble heterostructure

We report a study of the carrier dynamics in an ${\mathrm{In}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$ thin film photoexcited well above the band gap using nondegenerate pump-probe spectroscopy and time-resolved photoluminescence (TRPL) for carrier densities ranging from ${10}^{17}$ to ${10}^{19} {\mathrm{cm}}^{\ensuremath{-}3}$ at 10 K. At carrier densities greater than $4\ifmmode\times\else\texttimes\fi{}{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3},$ optical gain occurs across the entire band tail region after $\ensuremath{\sim}2.5 \mathrm{ps}$ time delay, when the hot carriers completely fill these states. From TRPL measurements performed in the surface emission geometry, we observed stimulated emission (SE) with a $\ensuremath{\sim}28 \mathrm{ps}$ decay time. Since this SE has a threshold density of $1\ifmmode\times\else\texttimes\fi{}{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3},$ which is larger than the total density of localized states, and the SE spectra at early time delays are quite different from the spontaneous emission spectra, we attribute the SE to the recombination of an electron-hole plasma from renormalized band-to-band transitions.

Correlation between bond distortion and the band-tail electronic density of states in amorphous silicon: a tight-binding recursion study

We study the spatial structure of the electronic states in amorphous silicon using a tight-binding Hamiltonian and the recursion method. A realistic cluster consisting of 4096 atoms in a periodic cubic supercell, obtained using reverse Monte–Carlo methods, is used as a model of amorphous silicon [B.R. Djordjevic, M.F. Thorpe, F. Wooten, Phys. Rev. B 52 (1995) 5685]. The local density of states for each of the 16 384 orbitals is computed. Our aim is to provide some insight into the nature of the band tail states and we particularly address the following issues. We examine the relative contributions of the p-orbitals and the s-orbitals to the density of states in the region of the band tail and gap. We investigate the geometric nature of these states. Finally, we study the correlation between the structural distortions in the amorphous silicon cluster and the spatial distribution of the band tail and gap states. Our results show that the p-orbitals contribute a large fraction of the density of states in the valence band tail. This fraction decreases as the energy increases across the gap into the conduction band. We observe the expected change from extended states deep in the valence and conduction band to localized states in the band gap. An attempt was made to characterize this change in terms of the fraction of the total density of states required for a percolating nearest-neighbor path in the cluster as a function of the energy. No abrupt change was observed in this quantity for energies in the region of the band tails. Although there are 4096 atoms in the cluster, 90% of the density of states in the band tail and gap is localized on only 416 atoms, that is, we find a backbone of atoms in the cluster responsible for a large fraction of the states in this energy range. We also find a strong correlation between atoms contributing a high density of states in the band tail and gap region and atoms with a large bond angle distortion.

N+ion–target interactions in PPO polymer: A structural characterization

N+ ion beam induced effects on the spin coated amorphous poly(2,6-dimethyl phenylene oxide) (PPO) films in terms of chemical structure and electronic and vibrational properties were investigated using Fourier Transform Infrared spectroscopy (FTIR) and Ultraviolet–Visible (UV–VIS) spectroscopy. Both techniques revealed that the stability of PPO was very weak towards 100 keV N+ ions revealing the threshold fluence to be 1014 ions/cm2 for fragmentation of the polymer. FTIR analysis showed disappearance of all characteristic IR bands at a total fluence of 1014 ions/cm2 except for the band CC at 1608 cm−1 which was found to shift to a lower wave number along with an enhancement in the full width half maximum (FWHM) value with increasing fluence. A new bond appeared due to oxidation as a shoulder at 1680 cm−1 in FTIR spectra indicating the presence of CO type bond as a result of N+ implantation on PPO films. The optical band gap (Eg) deduced from absorption spectra, was observed to decrease from 4.4 to 0.5 eV with fluence. The implantation induced carbonaceous clusters, determined using Robertson’s formula for the optical band gap, were found to consist of ∼160 fused hexagonal aromatic rings at the maximum energy fluence. An enhanced absorption coefficient as a function of fluence indicated incorporation of either much larger concentration of charge carriers or their mobility than that of the pristine sample. Calculated band tail width from Urbach band tail region for the implanted samples pointed the band edge sharpness to be strongly dependent on fluence indicating an increased disorder with increasing fluence.

Resonant Raman scattering in GeS 2

Microscopic structures giving rise to band-tail states in GeS 2 glass have been studied using resonant Raman-scattering spectroscopy. When the photon energy of excitation light approaches the band-tail region from a lower energy, a Raman peak ascribable to vibrations of S-atom pairs becomes stronger. The result suggests that the S-atom pairs, which possess interaction between lone-pair electrons, form the band-tail states.

Determination of the Density of States in a-SiC:H from Transient Photoconductivity

AbstractIn order to fill the gap of little knowledge about their details, the density of states distributions (DOS) in the upper half of the band gap were determined for a series of wellcharacterised amorphous hydrogenated silicon carbide samples with Tauc gaps between 1.78 and 1.94 eV. A DOS spectroscopic technique, based on the Fourier transform of time-sampled transient photocurrents, allowed the DOS determination on an absolute scale for a wide energy range. The DOS increases in the band tail region with carbon content. It exhibits a minimum at about 0.5 eV which is followed by a defect structure at deeper energies, the density of which also increases with C-content. We find a decreasing time-dependent drift mobility for larger C-content consistent with the lower transit time-determined drift mobility in time-of-flight.

Properties of gallium-doped hydrogenated amorphous germanium.

The effects of adding small quantities of gallium atoms to hydrogenated amorphous germanium (a-Ge:H) on its dark-conductivity, band-gap, electronic density of states and the hydrogen bonding, were studied in detail by dark-conductivity, optical and infrared-transmission, and photothermal-deflection-spectroscopy measurements. Films of a-Ge:H having relative Ga atomic concentrations ranging between 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ and 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ were deposited by the cosputtering of solid Ge and Ga targets in a rf-plasma sputtering chamber. The Ga incorporation was found to produce significant changes in the conductivity in the whole studied concentration range. The sign of the thermopower signal reveals a change from n- to p-type conduction for Ga concentrations reaching a value of about (1--5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. No variations in the magnitude of the band gap were detected for Ga atomic concentrations lower than 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. For higher Ga contents a gradual narrowing of the band gap is observed, which is not correlated by modifications in the hydrogen content nor in the H bonding structure as deduced from the infrared transmission spectra. The present results show that Ga acts as a p-type dopant in a-Ge:H, with a doping efficiency comparable to that of boron in a-Ge:H.The position of the Fermi level with respect to the conduction path levels was determined from the conductivity results, assuming a common constant prefactor for the observed thermally activated dark conductivity for both n- and p-type samples. The total energy region swept by the Fermi level in the studied Ga concentration range is approximately 0.3 eV, which is about the same as that reported in the literature for B doping of a-Ge:H. A semiquantitative electronic density of states distribution for a-Ge:H and its evolution with Ga doping was deduced systematically by fitting an analytical model of the optical absorption data. A significant increase in the defect states density was found to develop for Ga contents higher than about 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. This effect seems to be correlated with the Fermi level approaching the valence band tail region. Conductivity prefactors for both electrons and holes in the 140--260 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range were deduced from the analysis of the present conductivity and absorption data, which are in fair agreement with values reported in the literature for hydrogenated amorphous silicon and theoretical estimates. Significant tendency for Ga segregation was observed for a-Ge:H films having Ga atomic concentrations approaching 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$.

Evaluation of GaAs/Al0.3Ga0.7As multiple-quantum-well waveguides for pulsed squeezed light generation.

We have measured the nonlinear refractive index ${\mathit{n}}_{2}$ and two-photon absorption coefficient \ensuremath{\beta} in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well waveguides in the band tail region about 45 meV below the n=1 heavy hole exciton absorption line. At 883 nm we find that \ensuremath{\beta}=1.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ cm ${\mathrm{W}}^{\mathrm{\ensuremath{-}}1}$ and that ${\mathit{n}}_{2}$\ensuremath{\approxeq}-1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{2}$ ${\mathrm{W}}^{\mathrm{\ensuremath{-}}1}$. The linear loss coefficient is measured to be 6.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at the same wavelength. Based on these values, we estimate that up to 4.5 dB pulsed squeezing is possible.

Density of bulk and surface states and static characteristics of amorphous silicon field-effect transistors

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Theory of the band-tail absorption saturation in polar semiconductors.

A theory is presented which allows one to calculate the absorption and its saturation in the band-tail region of polar semiconductors for elevated temperatures. The influence of the LO phonons and of the electron-hole plasma on the dynamically broadened band tail is considered. The problem is decomposed into the calculation of the broadening and the shift of the electron-hole pair states. Both the broadening and the shift are temperature and density dependent, although the shift of the 1s-exciton line depends mainly on the temperature alone. The calculation of the absorption saturation includes a self-consistent determination of the plasma density in terms of the intensity of the exciting laser light. The saturation of the absorption sets in after the light intensity exceeds a critical value which increases with decreasing frequency of the exciting laser light.

Numerical study of symmetry effects on localization in two dimensions.

Effects of symmetry on localization on two-dimensional square lattices are studied numerically. The inverse localization length is determined by the system-size dependence of the Thouless number in magnetic fields or in the presence of strong spin-orbit interactions. A finite-size-scaling method is also applied to the case of spin-orbit interactions. Extended states, present in each Landau level in strong magnetic fields or in the case of small randomness, are found to merge together with increasing randomness and disappear beyond a certain critical randomness. In weak magnetic fields, the field tends to reduce the localization near the band center, while the localization is enhanced in the band-tail region. Spin-orbit interactions cause effects similar to, but much stronger than, that due to a weak magnetic field. The critical randomness and exponent for a metal-insulator transition are determined in the presence of strong spin-orbit interactions.