Measurements of localized-state density in chemically vapour deposited amorphous silicon by the field-effect method

New techniques to determined the localized-state density in chemically vapour deposited amorphous silicon over a wide energy range by the field-effect method are presented. The density is measured for the first time as 10¹⁸ ～ 10²⁰ cm^?3 eV^?1 in the range ±0.4～±0.5 eV around the Fermi level. From the results it is suggested that the silicon atom network of CVD a-Si has a similar randomness to that of glow discharge a-Si, providing a density of tail states of about 10¹⁹ ～ 10²⁰ cm^?3 eV^?1 and native defect states of about 10¹⁸ cm^?3 eV^?1.     

Influence of temperature on the microcrystalline structure of thermally evaporated Sb2S3 thin films

Thin films of antimony trisulfide (Sb₂S₃) were prepared by thermal evaporation under vacuum (p=5×10^–5 torr) on glass substrates maintained at various temperatures between 293 K and 523 K. Their microstructural properties have obtained by transmission electron microscopy (TEM). The electron diffraction analysis showed the occurrence of amorphous to polycrystalline transition in the films deposited at higher temperature of substrates (523 K). The polycrystalline thin films were found to have an orthorhombic structure. The interplanar distances and unit‐cell parameters were determined by high‐resolution transmission electron microscopy (HRTEM) and compared with the standard values for Sb₂S₃. The surface morphology of Sb₂S₃ thin films was investigated by scanning electron microscopy (SEM). The optical transmission spectra at normal incidence of Sb₂S₃ thin films have been measured in the spectral range of 400–1400 nm. The analysis of the absorption spectra revealed indirect energy gaps, characterizing of amorphous films, while the polycrystalline films exhibited direct energy gap. From the photon energy dependence of absorption coefficient, the optical band gap energy, E_g, were calculated for each thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin films

Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (T_sb = 30°, 400°C). X‐ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 – 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (E_g^opt) 1.56 eV; where as the films formed at 400°C were found to have a E_g^opt of 1.92 eV. The increase in the value of E_g^opt with T_sb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current ‐Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Refractive index,band gap and oscillator parameters of amorphous GaSe thin films

GaSe thin films are obtained by evaporating GaSe crystals onto ultrasonically cleaned glass substrates kept at room temperature under a pressure of ∼10^–5 Torr. The X‐ray analysis revealed that these films are of amorphous nature. The reflectance and transmittance of the films are measured in the incident photon energy range of 1.1–3.0 eV. The absorption coefficient spectral analysis revealed the existence of long and wide band tails of the localized states in the low absorption region. The band tails width is calculated to be 0.42 eV. The analysis of the absorption coefficient in the high absorption region revealed an indirect forbidden band gap of 1.93 eV. The transmittance analysis in the incidence photon wavelength range of 500–1100 nm allowed the determination of refractive index as function of wave length. The refractive index–wavelength variation leads to the determination of dispersion and oscillator energies as 31.23 and 3.90 eV, respectively. The static refractive index and static dielectric constant were also calculated as a result of the later data and found to be 9.0 and 3.0, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Device grade hydrogenated polymorphous silicon deposited at high rates

Hydrogenated polymorphous silicon (pm-Si:H) thin films have been deposited by plasma-enhanced chemical vapor deposition at high rate (8–10 Å/s), and a set of complementary techniques have been used to study transport, localized state distribution, and optical properties of these films, as well as the stability of these properties during light-soaking. We demonstrate that these high deposition rate pm-Si:H films have outstanding electronic properties, with, for example, ambipolar diffusion length (L_d) values up to 290 nm, and density of states at the Fermi level well below 10¹⁵ cm^?3 eV^?1. Consistent with these material studies, results on pm-Si:H PIN modules show no dependence of their initial efficiency on the increase of the deposition rate from 1 to 10 Å/s. Although there is some degradation after light-soaking, the electronic quality of the films is better than for degraded standard hydrogenated amorphous silicon (values of L_d up to 200 nm). This result is reflected in the light-soaked device characteristics.     

Electrical conduction in amorphous WO3 films

Measurements are reported on the dc and ac conductivity and dielectric constant for vacuum-deposited films of tungsten trioxide. The behaviour is consistent with that expected for hopping conduction in an amorphous solid containing long-range potential centres associated with oxygen vacancies. A new method of calculating the density of states gives N(E) ～ 5.0 × 10²⁰ eV^?1 cm^?3.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

High field effects in amorphous germanium

Measurements of high field current have been made on amorphous Ge (a-Ge) films over the temperature range from 100 to 300 K. Non-ohmic conduction in a-Ge occurs at electric fields greater than 1–2 × 10⁴ V/cm. Field dependence of the conductivity has been explained in terms of the enhanced emission probability of carriers from the screened coulombic trap centers. Assuming the optical dielectric constant for a-Ge films, the screening length of the trap centers and the density of states at the Fermi level are estimated to be 12 Å and ～6.1 × 10²⁰ cm^?3 eV^?1, respectively.     

The validity of the double layer model on the amorphous semiconductor surface including the bulk localized state within the gap

Under the two assumptions that the origin of surface states may be different from that of bulk localized states within the gap and the density of surface states is sufficiently high, the validity of the double layer model on the amorphous semiconductor surface is investigated in comparison with the case of a crystal. It is suggested that the criteria concerning the double layer should be determined by the relative value of the surface states to that of bulk localized states. The existence of the double layer can be confirmed when the bulk localized state density n(E_f) is smaller than 10¹⁹ cm^?3 eV^?1. When n(E_f) is high at about 10²⁰ cm^?3 eV^?1, surface states cannot be distinguished from the localized states within the gap. This double layer model is strongly supported by the results of previous experiments by others who have measured the dependence of the Schottky barrier height on the work function of metal and the dependence of the surface potential on the preparation conditions of a-SiH.     

Study of amorphous Ge–SiO2–P+Si tunnel junctions

A new method to determine ac conductivity of amorphous Ge using Al-amorphous Ge–SiO₂–P⁺Si tunnel junctions is presented. Frequency dependence of ac conductivity is found to satisfy the power law in the frequency range between 1 and 50 kHz and the density of localized states at the Fermi level is estimated to be ～ 1.7 × 10²⁰ cm^?3 eV^?1 which decreases to ～ 4.5 × 10¹⁹ cm^?3 eV^?1 after annealing at 175°C.Temperature dependence of tunneling conductance of Al-amorphous Ge–SiO₂–P⁺Si junctions is appreciable only near zero bias. Zero bias conductance of the junctions obeys the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law of Mott; the density of localized states obtained from the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law is one order of magnitude smaller than that obtained by ac conductivity measurements, being insensitive to annealing. This behavior of the tunnel junctions differes in many respects from those of Al–Al₂O₃-amorphous Ge tunnel juntions.     

Amorphous silicon-electrolyte interface

Amorphous silicon-electrolyte (AS-E) interface has been studied to determine the density of localized states in evaporated amorphous silicon films. Due to the large density of such states in amorphous semiconductors, very large fields are required to cause changes in the space-charge layer. AS-E system presents a more convenient system to meet this need for large fields than the usual MOS structure. A study of cpacitance as a function of bias shows that the density of localized states around E_F is of the order of 10¹⁸/cm³ · eV and that it is fairly uniform in the midgap region.     

Electrical,structural and optical properties of amorphous carbon

The planar and transverse electrical resistivity of amorphous carbon (a-C) films getter-sputtered at low temperature (77–95 K) is well-fitted by the expression $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ The exponent T₀ being approximately the same in both cases (≈ 7 × 10⁷ K) suggests that the amorphous films are isotropic. Films thinner than 600 Å display a two-dimensional hopping conductivity from which one deduces a density of states N(E_F) at the Fermi level of 10¹⁸ eV^?1 cm^?3 and a radius of the localized wave functions (a) of 12 Å. Tunneling experiments and optical absorption measurements are consistent with a pseudogap of approximately 0.8 eV. Electron diffraction experiments indicate that a-C films consist of a mixture of diamond and graphite bonds; this fact taken in the light of the other experiments would suggest that the graphite bonds act as the localized conduction states.     

Effect of substrate temperature on polycrystalline Cd0.9Zn0.1Te thin films studied by Raman scattering spectroscopy

Cd_0.9Zn_0.1Te thin films were prepared by vacuum evaporation onto well‐cleaned glass substrates maintained at 300, 373 and 473 K. X‐ray diffraction studies revealed that the films have zinc blende structure with preferential (111) orientation. Raman peak of the room temperature deposited film appeared at 140.30 cm^‐1 and 159.65 cm^‐1 were for the transverse optic (TO) and longitudinal optic (LO) phonons respectively. The XRD patterns of the higher substrate temperature deposited films exhibited an improvement in the crystallinity of the films. The Raman peak intensity increases and the FWHM decreases for the films deposited at higher substrate temperature. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Influence of thermoannealing on crystallinity and optical properties of Sb2S3 thin films

Sb₂S₃ thin films are obtained by evaporating of Sb₂S₃ powder onto glass substrates maintained at room temperature under pressure of 2×10^‐5 torr. The composition of the thin films was determined by energy dispersive analysis of X‐ray (EDAX). The effect of thermal annealing in vacuum on the structural properties was studied using X‐ray diffraction (XRD) technique and scanning electron microscopy (SEM). The as‐deposition films were amorphous, while the annealed films have an orthorhombic polycrystalline structure. The optical constants of as‐deposited and annealed Sb₂S₃ thin films were obtained from the analysis of the experimental recorded transmission spectral data over the wavelength range 400‐1400 nm. The transmittance analysis allowed the determination of refractive index as function of wavelength. It was found that the refractive dispersion data obeyed the single oscillator model, from which the dispersion parameters (oscillator energy, E₀, dispersion energy, E_d) were determined. The static refractive index n(0), static dielectric constant, ε_∞, and optical band gap energy, E_g, were also calculated using the values of dispersion parameters. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of annealing on the conduction mechanism in amorphous silicon

This paper reports the effect of annealing on conduction mechanisms in amorphous silicon films. High field transport is mainly by the Poole-Frenkel mechanism in unannealed films of amorphous silicon. As they are annealed, the conduction meachnism changes from Poole-Frenkel to space charge conduction. The films are fully annealed at 120°C for 15 h and any more annealing for longer periods does not change the I–V characteristics. From the measured values of threshold voltage at which space charge conduction sets in, the density of localized states around the Fermi level is calculated to be about 10¹⁷–10¹⁸/eV · cm³ which agrees very well with our results from other experimental techniques.     

Influence of substrate temperature on the properties of vacuum evaporated InSb films

Indium Antimonide (InSb) thin films were grown onto well cleaned glass substrates at different substrate temperatures (303, 373 and 473 K) by vacuum evaporation. The elemental composition of the deposited InSb film was found to be 52.9% (In) and 47.1% (Sb). X‐ray diffraction studies confirm the polycrystallinity of the films and the films show preferential orientation along the (111) plane. The particle size (D), dislocation density (δ) and strain (ε) were evaluated. The particle size increases with the increase of substrate temperature, which was found to be in the range from 22.36 to 32.59 nm. In Laser Raman study, the presence of longitudinal mode (LO) confirms that the deposited films were having the crystalline nature. Raman peak located at 191.26 cm^–1 shift towards the lower frequencies and narrows with increase in deposition temperature. This indicates that the crystallinity is improved in the films deposited at higher substrate temperatures. Hall measurements indicate that the films were p‐type, having carrier concentration ≅10¹⁶ cm^–3 and mobility (4–7.7) ×10³ cm²/Vs. It is observed that the carrier concentration (N) decreases and the Hall mobility (μ) increases with the increase of substrate temperature. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

KTiOAsO4 Thin Films Prepared by Pulsed Laser Deposition

A pulsed Nd:YAG Laser was used to evaporate solid targets of KTiOAsO₄ (KTA) at power densities of 0.6 to 2.0x10⁹ W/cm². KTA thin films were deposited on glass, Si (100). After proper annealing treatment, single phase, (orthorh-ombic) polycrystalline KTA thin films were obtained. Some propitious of these thin films were also examined.     

Study of the surface state density and potential in MIS diode Schottky using the surface photovoltage method

ABSTRACT

The effects of surface preparation and illumination on electric parameters of Au/GaN/GaAs Schottky diode were investigated. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses of GaN layers (0.7 – 2.2 nm). In order to study the electrical characteristics under illumination, we use an He-Ne laser of 632 nm wavelength. The I(V) current- voltage, the surface photovltage SPV measurement were plotted and analysed taking into consideration the influence of charge exchange between a continuum of the surface states and the semiconductor. The barrier height Ф_Bn, the serial resistance R_s and the ideality factor n are respectively equal to 0.66 eV, 1980 Ω, 2.75 under dark and to 0.65 eV, 1160 Ω, 2.74 under illumination for simple 1 (GaN theckness of 0.7 nm). The interface states density N_ss in the gap and the excess of concentration δn are determined by fitting the experimental curves of the surface photovltage SPV with the theoretical ones and are equal to 4.5×10¹² eV^?1 cm^?2, 5×10⁷ cm^?3, respectively, for sample 1 and 3.5×10¹² eV^?1 cm^?2, 7×10⁸ cm^?3 for sample 2 (GaN theckness of 2 nm). The results confirm that the surface photovoltage is an efficient method for optical and electrical characterizations.     

Microstructure and optical properties of (Pb,Ca)TiO3 films grown on Si(1 0 0) substrates by a simple sol-gel process

Amorphous and polycrystalline (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films deposited on an Si(1 0 0) substrate have been prepared by a simple sol-gel process. The microstructure and surface morphologies of the thin films have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The polycrystalline PCT film on the Si(1 0 0) substrate has a tetragonal perovskite structure with grain size from 60 to 110 nm. AFM reveals smooth surfaces and root mean square (rms) roughness of 0.17 and 4.4 nm for amorphous and polycrystalline films, respectively. The refractive index n and extinction coefficient k of the amorphous and polycrystalline thin films was obtained by spectroscopic ellipsometry as a function of the photon energy in the range from 2.0 to 5.4 eV. The maximum n and direct bandgap energies of amorphous and polycrystalline thin films were 2.66 and 4.11 eV, 2.64 and 3.84 eV, respectively.