Urbach Tail Slope Research Articles

Inherent photoluminescence Stokes shift in GaAs

The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 μm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter.

Optical absorption spectrum measurement of films at a low temperature using photothermal bending spectroscopy

The optical absorption spectrum of a semiconductor film was obtained at a low temperature using an optical cantilever method called photothermal bending spectroscopy. The film, clamped on one side in a cryostat, was uniformly cooled with nitrogen gas. The product of the measurable optical absorption coefficient and the thickness of the film was better than 1×10−3 at 110K. We demonstrated that the 1.74eV peak in the optical absorption spectrum of a C60 film could not be observed at room temperature, but could be observed at 110K due to a sharpening of the Urbach tail slope.

Morphology, optical properties and single-electron spectrum of ‘detector-quality’ polycrystalline diamond layers, prepared by MW CVD

Room-temperature spectral dependencies α( hν) of coefficient of optical absorption of free-standing polycrystalline diamond layers grown by MW CVD technique have been studied experimentally within photon energy hν range from 4.5 to 5.7 eV. The 〈100〉-oriented samples with thickness from 2 to 20 μm and 〈110〉-oriented ones with thickness from 3 to 12 μm were grown at the invariable (for each sets) technological regimes, but different deposition times. Pyramidal morphology of the layers and almost linear enlargements in the average spatial extents of the pyramids at the film thickness increment are observed. In particular, an average transverse size of the bases rises approximately from 0.4 to 4.0 μm for the 〈100〉-oriented samples and from 0.8 to 3.1 μm for the 〈110〉-oriented ones. Considerable lowering in the subgap optical absorption coefficient and characteristic energy of Urbach tail slope E U (it drops from 615 ± 15 to 299 ± 4 meV) are detected when the thickness of the diamond samples rises, while the optical gap width E G remains almost constant and close to 5.45 eV for all studied layers. Such abnormal influences of the thickness and morphology of the layers on their α( hν) spectra and E U quantities are attributed to significant effect of average sizes of polycrystals (pyramidal grains) on the density N( E) of electron states within and just beyond the band gap of the material. Roles of morphological characteristics of polycrystalline CVD diamond layers in formation of features of their α( hν) and N( E) spectrums have been interpreted within frameworks of a semi-empirical adiabatic Generalized Skettrup Model.

The long-range ordering, electron spectrum, and properties of amorphous silicon films – II. Defect states and optical parameters

Abstract In the first part of this article, the semi-empirical generalized Skettrup model (GSM) was presented and used to simulate the effect of a spatial extent of long-range potential fluctuations (LRPF) on the parameters of the single-electron spectrum N ( E ) of non-homogeneous amorphous silicon (a-Si:H) films. Here, the GSM is applied to the modelling of optical parameters and structural defects (dangling bonds) of such films. The spectral dependence of the optical absorption coefficient, simulated by N ( E ) convolution, was found to contain the Tauc, Urbach and ‘defect’ sub-ranges. Changes in temperature and spatial extent of the LRPF provide almost linear dependencies of the optical gap versus the Urbach tail slope in the model. Simulated parameters of the a-Si:H films prepared by both radio frequency sputtering (RFS) and glow discharge (GD) decomposition of silane showed good agreement with existing experimental data. The optical gap and Urbach tail slope energies under the GSM model were found to be typical of ‘device quality’ a-Si:H films when the LRPF spatial extent was of the order of 1 μm. Differences in the properties of a-Si and a-Si:H films, specific features of the GD and RFS a-Si:H materials, as well as the fundamental nature of the ‘hydrogen dilution’ regime were also interpreted within the framework of the GSM.

A Study of Electronic Defects in Hydrogenated Amorphous Silicon Prepared by the Expanding Thermal Plasma Technique

AbstractThe electronic properties of amorphous silicon films prepared by the expanding thermal plasma technique have been studied using steady-state and transient photoconductivity measurements. It is found that films deposited at a substrate temperature of 400°C have a conduction band tail slope of 29 meV, deep defect density of order 3×1016 cm-3, an Urbach tail slope of 65 meV, defect absorption of 5-10 cm-1, and a mobility-lifetime product of 1.3×10-7 cm2 V-1. Aslight increase in defect density and reduction in mobility-lifetime product is observed on moderate light-soaking. The overall optoelectronic quality is somewhat poorer than commercial PECVD material, but there is scope for improvement as deposition conditions are further optimised.

Morphology, Phonon Confinement and Properties of a-Si:H Films

ABSTRACTIt is usually assumed that optical and electrical properties of amorphous semiconductors are determined by the short-range-order (SRO) parameters of atomic structure. The SRO parameters behavior is considered here as governed by morphology geometrical parameters (MGP) variations. Generalized Skettrup model is developed for a quantitative description of the MGP influence on the density of electron states distribution N(E). Common N(E) relation for the power, exponential and the ‘defect’ regions of the dependence is derived both for adiabatic (optical) and non-adiabatic (thermal) electron excitation processes. The simulated (by the N(E) convolution) spectral dependence of the opti-cal absorption coefficient contains the Tauc, Urbach and the ‘defect’ parts. The optical gap and the Urbach tail slope energies are typical for the ‘device quality’ a-Si:H films at the average morphology geometrical parameters values of order of 1 μm. Nearly linear the optical gap versus the Urbach tail slope energies dependence is obtained at the mor-phology geometrical parameters and temperature changes. Good agreement of experi-mental and the simulation results is achieved for a-Si:H films, prepared both by the RF sputtering and by the silane decomposition.

Microcrystalline Silicon Film Deposition from H 2 ‐ He ‐ SiH4 Using Remote Plasma Enhanced Chemical Vapor Deposition

Microcrystalline silicon (μc‐Si:H) films were deposited at 250°C from the reaction of with gas excited in an inductively couple remote plasma. The deposition rate, microstructure, and material properties of μc‐Si:H depended on the composition of hydrogen and helium in the plasma. Si films deposited on a hydrogen plasma showed columnar and microcrystalline structure with a grain size of about 200 Å or less, and had an optical bandgap of about 2.05 to 2.1 eV. The microstructure parameter in the film) and Urbach tail slope decreased with increasing fraction in the plasma gas. On the other hand, amorphous (a‐Si:H) film was deposited with higher deposition rate when pure He was used as plasma gas and the optical bandgap of this film was 2.18 eV. The surface of a‐Si:H film was smoother than that of μc‐Si:H.

Structural and bonding properties of amorphous silicon nitride films

Amorphous Si nitride ( a-SiN x :H) films were deposited at 300°C by plasma-enhanced chemical vapor deposition. It is demonstrated that a charge-transfer model combined with a modified random-bonding model describes well the changes of the bonding properties as a function of x. The oscillator strength of the SiH absorption is determined, and shifts of the SiH peak and the core level spectra are systematically explained. As three or more N atoms are bonded to a Si atom, it is suggested that NH bonds are favorably formed in the films, and that the Urbach tail slope increases through enhanced bond distortion. The stress and buffered HF etch rate for films with a near-stoichiometric composition were also investigated as functions of rf power and hydrogen dilution ratio.

Defect Properties of Cathode Deposited Glow Discharge Amorphous Silicon Germanium Alloys

ABSTRACTWe have characterized the deep defect densities and their energy distributions for a series of a-Si1-xGex:H alloys with large Ge content (0.57< × < 1.00) prepared by the cathode deposited glow discharge method. Our results indicate markedly superior properties for these samples. A small Urbach tail slope (about 45meV) was found for all samples in this alloy range. The defect densities were either obtained directly from the drive-level capacitance profiling or deduced from the sub-band-gap optical spectra. Both are substantially lower than the trend line determined from previous studies of a-Si1-xGex:H samples produced by conventional glow discharge and by Photo-CVD methods. However, the relation between the total defect densities and the optical spectra in the cathodic samples obeys the same defect formation model that has been used to successfully predict the defect densities in other types of a-Si1-xGex:H material.

Electronic mobility gap structure and deep defects in amorphous silicon-germanium alloys

Amorphous hydrogenated silicon-germanium alloys have been studied using a variety of junction-capacitance techniques to establish the dependence of the mobility gap electronic structure and the density of deep defects on the germanium content. The Urbach tail slope is observed to be nearly constant over the whole alloy range. The energy position of the dominant deep defect band near midgap is deduced and evidence for a shallower unoccupied defect band undergoing a large lattice relaxation is also observed. The total density of deep defects is found to increase exponentially with increasing germanium content and the details of this increase are shown to be consistent with a weak bond to dangling bond conversion model.

Electronic mobility gap structure and the nature of deep defects in amorphous silicon-germanium alloys grown by photo-CVD

Amorphous hydrogenated silicon-germanium alloys grown by photo-CVD have been studied using a variety of steady-state and transient junction-capacitance techniques. The dependence of the electronic mobility gap structure, the density of deep defects, and the carrier trapping properties on the germanium content has been investigated systematically. The Urbach tail slope is found to be nearly constant over the whole alloy range. The dominant defect band is found to track the midgap energy position, and the density of deep defects increases exponentially with increasing germanium content. These results are fully consistent with weak bond breaking theories and suggest that the quality of amorphous silicon-germanium alloys and a-Ge:H is inherently inferior to pure a-Si:H materials.

Chemical Vapor Deposition of Amorphous Silicon Using Tetrasilane

Hydrogenated amorphous-silicon films have been deposited by the plasma-free chemical-vapor-deposition method using tetrasilane (Si4H10) at temperatures as low as 350°C. The film deposited at 350°C and 9 Torr had hydrogen content as high as 15 at.%, optical bandgap of 1.78 eV, logarithmic ratio of photoconductivity (at a light intensity of 100 mW/cm2 under AM1 conditions) to dark conductivity of 3.2, activation energy of 0.78 eV and the Urbach tail slope as small as 56 meV. Thin-film transistors have been fabricated using the film deposited at 350°C. The electron mobility was 0.6 cm2/V s under as-deposited conditions.

Compositional trends of the glass-transition temperature, urbach-tail slope and optical gap in As 0.5Se 0.5− xTe x glasses

The dependence of the glass-transition temperature ( T g ), Urbach-tail slope (Δ), and optical gap ( E 03) on the chemical composition in As-Se-Te glasses has been studied. It was found that both T g and Δ are compositionally invariant, while E 03 increases with increasing selenium content. It is supposed that softening of the studied glasses is controlled by the strength of the As-As bonds, a fraction of which is compositionally invariant. Similarly the As-As bonds broken during glass formation are supposed to be responsible for frozen-in potential fluctuations giving rise to the Urbach tail. Hence the As-As bonds and broken As-As bonds control both T g and Δ, which were found interrelated Δ ≈ 1.3 K B T g . On the other hand, the optical gap is controlled by anion substitution, and the E 03( x) dependence can be calculated using the virtual crystal model (VCM). The bowing parameter calculated using VCM has been found to be γ = 0.54 eV, in good agreement with experiment.

Compositional invariance of the glass-transition temperature and Urbach-tail slope in Ge 20Sb 15− xBi xS 65 glasses

A compositionally invariant glass transition temperature ( T g ) and Urbach-tail slope (σ) in Ge 20Sb 15− x Bi x S 65 glasses were found. This is in agreement with physically based correlations between T g and mean coordination number and between T g and σ. Using reasonable assumptions, good agreement between experimental and calculated T g and σ values has been found.

High-temperature annealing behavior of μτ products of electrons and holes in a-Si:H

The mobility-lifetime products of electrons and holes [(μτ)e and (μτ)h] in undoped hydrogenated amorphous silicon samples have been studied by photoconductivity and ambipolar diffusion length measurements. The density of dangling bonds Nd in the samples is changed over a range of 3×1015–2×1018 cm−3 by annealing at high temperatures. Nd and the Urbach tail slope Eov have been determined by the constant photocurrent method. In addition, the optical gap, the activation energy of dark conductivity, and the exponent governing the intensity dependence of σpc have been measured. The results show that there is a correlation between Nd and Eov which is consistent with equilibrium theory. (μτ)e and (μτ)h change in quite different ways as Nd increases, namely, (μτ)e decreases as a linear function of the inverse of Nd. However, (μτ)h remains almost constant when Nd≤5×1016 cm−3, then decreases fast for higher Nd. The asymmetric dependence of transport properties of electrons and holes on Nd suggests that for electrons recombination through dangling bond states is dominant; but, for holes, recombination mainly proceeds through deep band tail states, especially when Nd is relatively low.

Potential fluctuations and density of gap states in amorphous semiconductors

The exponential-band-tail (Urbach-tail) problem for structurally disordered material is treated by a simple model in which quantum wells (used to model the effects of local disorder) trap states in the gap. The density of states in the gap is calculated and is shown to have approximately exponential behavior over a range of energy of physical interest. A direct relation is established between the Urbach-tail slope and the glass transition temperature. Agreement with experimentally determined Urbach-tail slopes is obtained for the Ge-Sn-Se glasses for reasonable parameters employed in the present model.