The influence of local surroundings of Er atoms on the kinetics of decay of Er photoluminescence in amorphous hydrogenated silicon

Kinetics of the decay of photoluminescence of Er impurity in the films of amorphous hydrogenated Si a-Si:H〈Er〉 was studied for the first time. The films were obtained either by cosputtering of Si and Er targets with the use of the technology of dc silane decomposition in a magnetic field (MASD) or by radio-frequency decomposition of silane. In the second case, an Er(TMHD)₃ polymer powder was used as the source of Er. It is shown that, at room temperature, the a-Si:H〈Er〉 films obtained by the MASD method feature the characteristic times of Er photoluminescence decay equal to 10–15 μs, which is 20 times smaller than in the case of Er-doped crystalline Si (c-Si〈Er, O〉) as measured at liquid-nitrogen temperature. For the a-Si:H〈Er〉 films obtained by radio-frequency decomposition of silane, the decay times of Er photoluminescence amount to 2 μs. The difference in the photoluminescence decay times is related to dissimilarities in the local surroundings of Er atoms in the a-Si:H〈Er〉 films obtained by different methods. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 1, 2000, pp. 90–92. Original Russian Text Copyright ? 2000 by Terukov, Kudoyarova, Kon’kov, Konstantinova, Kamenev, Timoshenko.     

Photoelectric phenomena in (μc xa1−x )-Si:H/c-Si heterostructures

Heterostructures (μc _xa_1−x )-Si:H/c-Si with a various volume fraction x of microcrystalline and amorphous Si phases were obtained by plasmochemical deposition. The fraction variation was achieved by changing the silane content in H₂. The steady-state current-voltage characteristics and spectral dependences of photosensitivity of the structures obtained were investigated. The latter dependences were recorded by exposing the samples to the natural and linearly polarized light in relation to the phase composition of thin (d ₁ ≅ 0.6–0.8 μm) films of microcrystalline and amorphous Si. The photovoltaic effect and induced photopleochroism of the structures obtained were detected. The prospects of using a new type of heterojunction in photoconverters of natural and linearly polarized light were assessed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 11, 2001, pp. 1316–1319. Original Russian Text Copyright ? 2001 by Mell, Nikolaev, V. Rud’, Yu. Rud’, Terukov.     

Lithium intercalation into amorphous silicon thin films

Electrochemical lithium intercalation into a-Si:H thin films grown on stainless steel substrates at temperatures of 100 and 250°C was studied. The intercalation capacity of films grown at 250°C is ～1750 and ～500 mA h/g in the first and hundredth cycles. The lithium diffusivity in films is ～ 10^?12 cm²/s.     

Effect of thermal treatment on structure and properties of a-Si:H films obtained by cyclic deposition

The effect of thermal treatment in a vacuum on the structure and properties of amorphous hydrogenated silicon (a-Si:H) films obtained by cyclic deposition with intermediate annealing in hydrogen plasma was studied. a-Si:H films deposited under optimal conditions are characterized by the nonuniform distribution of the nanocrystalline phase (<1 vol %) across the film thickness and have the optical gap E _g=1.85 eV, the activation energy of conductivity E _a =0.91 eV, and a high photosensitivity (σ_ph/σ_d≈10⁷ under illumination of 100 mW/cm² in the visible spectral range). Transmission electron microscopy studies demonstrated that thermal treatment in a vacuum leads to blurring of the initial layered structure of a-Si:H films and to a somewhat higher amount of nanocrystalline inclusions in the amorphous phase matrix. Thermal treatment above 350°C causes a dramatic increase in the dark conductivity, as well as the resulting decrease in the photosensitivity, of a-Si:H films.     

A quantum-well model and the optical absorption edge in structurally nonuniform a-Si:H-based alloys

This paper describes studies of the microstructure and optical absorption edge of films of a-Si: H and a-SiN_x: H alloys (x=0.0–0.72), obtained by decomposing gas mixtures in an rf glow discharge. For the a-Si: H films, the gas mixture was monosilane diluted by hydrogen, for the a-SiN_x: H alloys it was SiH₄+NH₃. Structurally nonuniform films with “device-quality” optoelectronic characteristics were obtained when the rates of growth were increased (to 8 Å/s). Atomic force microscopy and infrared absorption spectroscopy are used to identify a characteristic feature of the microstructures of these films: the presence of islets ～500 Å in diameter, whose boundaries are formed by clusters of hydrogen atoms (in the case of a-Si: H) or of hydrogen and nitrogen (in the case of a-SiN_x: H). In this case the optical band gap of a-Si: H (a-SiN_x: H) is determined by the concentration of SiH (SiN) bonds in the interior of the islets and is not sensitive to changes in the content of hydrogen (nitrogen) at their boundaries. This result is explained by a quantum-well model which takes into account the characteristic sizes of the microstructures formed by hydrogen or nitrogen atoms.     

Study of the correlation properties of the surface structure of <Emphasis Type="Italic">nc</Emphasis>-Si/<Emphasis Type="Italic">a</Emphasis>-Si:H films with different fractions of the crystalline phase

The correlation properties of the structure of nc-Si/a-Si:H films with different volume fractions of the crystalline phase are studied using 2D detrended fluctuation analysis. Study of the surface relief of experimental samples showed that with increasing in volume fraction of the crystalline phase in the nc-Si/a-Si:H films, the size and number of nanoclusters on their surface grow. The size of Si nanocrystals in the a-Si:H matrix (6–8 nm) indicates the formation of coarse nanoclusters due to the self-organization of Si nanocrystals in groups under laser radiation. According to 2D detrended fluctuation analysis data, the number of correlation vectors (harmonic components) in the nc-Si/a-Si:H film structure increased with an increase in the nanocrystal fraction in the films.     

Phase transformations initiated in thin layers of amorphous silicon by nanosecond excimer laser pulses

The reflection of a probing beam with a wavelength λ=0.63 μm from a silicon surface layer (a-Si) amorphized by ion implantation was detected during its melting and solidification initiated by excimer ArF laser radiation. When the irradiation energy is below the epitaxial threshold, a single event of a-Si layer melting leads to the formation of single nanocrystals separated from one another in the amorphous matrix rather than to the appearance of polycrystalline material. The presence of nanocrystals makes possible the formation of polycrystalline Si from a melt under exposure to a second laser pulse and allows the intermediate crystallization of Si in a laser-induced sequence of phase transitions. The data obtained are compared to the results of studying the phase transitions initiated in similar experimental conditions in thin layers of hydrogenated a-Si layers on glass substrates. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 5, 2003, pp. 622–628. Original Russian Text Copyright ? 2003 by Ivlev, Gatskevich.     

Special features of photoelectric properties of nanostructured films of hydrogenated silicon

Nanostructured Si films differing in hydrogen content, in the forms of Si-H bonds, and in certain characteristics of Si inclusions in an amorphous matrix (volume fraction, size, and structure) were studied. The behavior common to all the studied films, i.e., an increase in the defect density and nonmonotonic enhancement of photoconductivity at the “red wing” of the spectral characteristic compared to a-Si:H, was assessed. At the same time, there are films with either enhanced or reduced photoconductivity compared to a-Si:H.     

Effect of substrate material on the rate of growth and the optical parameters of a-C:H layers

a-C:H layers were grown by dc magnetron reactive sputtering of a graphite target in Ar + H₂ plasma. Ellipsometric measurements were carried out and analyzed at a wavelength of 6328Å for three sets of a-C:H layers with different thicknesses (different sputtering times) on silicon, fused quartz, and glass-ceramic substrates. It was shown that the substrate material had a substantial effect upon a-C:H growth: a-C:H layers on Si substrates were uniform up to ～ 7000Å thickness; for thin layers (<1000 Å) the growth rate was greater on quartz than on Si; the refractive index values of a-C:H were slightly different on quartz and Si substrates (1.60–1.65 and 1.65–1.72, respectively); a-C:H layers on glass-ceramic substrates were not uniform and had variable refractive index.     

Nanostructured a-Si:H films obtained by silane decomposition in a magnetron chamber

Nanostructured a-Si:H films grown by the MASD method at various deposition temperatures (T _s =300–390°C) were studied. Among these films, those “on the verge of crystallinity” are of particular interest, because they tend to crystallize. In addition, although their electron-transport parameters are slightly inferior to those of conventional device-grade a-Si:H, they are preferable because of the higher stability of their photoconductivity under exposure to light.     

A study of Pd2Si films on silicon using Auger electron spectroscopy

Auger electron spectroscopy (AES) combined with in situ sputter etching is used to quantitatively evaluate the growth kinetics of thin films of Pd₂Si on 〈111〉 Si substrates. The growth of Pd₂Si is found to be diffusion limited and to be characterized by an activation energy of 1·4±0·2 eV in close agreement with previous results obtained on thicker Pd₂Si films. AES is also used to uniquely identify the silicide phase Pd₂Si. Measurement of the expansion occurring during Pd₂Si film formation indicates the Pd₂Si film is about 20% thicker than expected. Correlation of the AES results with careful electrical measurements of the Pd₂Si/Si interface reveals that the contact barrier energy φ_Bn decreases slightly with increasing thickness of the Pd₂Si film. A 3% decrease in φ_Bn was observed for complete conversion of 500 Å of Pd to Pd₂Si.     

Photovoltaic effect in a-Si:H/n-InSe heterostructures

Heterostructures in an a-Si:H/InSe system were grown by the deposition of a-Si:H films onto the surface (001) of InSe single-crystal wafers and also by deposition of pure indium films with their subsequent selenization, in which case InSe films were synthesized at the a-Si:H surface. The photovoltaic effect was observed and studied for both types of heterostructures. It was concluded that the heterostructures obtained may be used as wide-band photoconverters of radiation.     

Structure and properties of a-Si:H films grown by cyclic deposition

Amorphous hydrogenated silicon films obtained by cyclic deposition with intermediate annealing in hydrogen plasma were studied. a-Si:H films deposited under optimal conditions are photosensitive (photoconductivity to dark conductivity ratio σ_ph/σ_d is as high as 10⁷ under 20 mW cm^?2 illumination in the visible region of the spectrum) and have an optical gap (E _g ) and activation energy of conductivity (E _a ) of 1.85 and 0.91 eV, respectively. Electron microscopy studies revealed a clearly pronounced layered structure of a-Si:H films and the presence of nanocrystalline inclusions in the amorphous matrix.     

Silicon network in a-Si:H films containing ordered inclusions

a-Si:H films with inclusions of (SiH₂)_n clusters or Si nanocrystals have been grown by magnetron-assisted SiH₄ decomposition (dc-MASD). The films were characterized by the microstructural parameter R=0.7–1.0. Ultrasoft X-ray emission spectroscopy was applied to establish the effect of these inclusions on the increasing ordering of Si network. It is shown that, irrespective of the nature of the inclusions, their effect is strongest for films of intrinsic material deposited at high temperatures (up to 400°C).     

a-SiOx:H passivation layers for Cz-Si wafer deposited by hot wire chemical vapor deposition

In order to get the high photoelectric conversion efficiency a-Si:H/c-Si solar cells, high quality intrinsic hydrogenated passivation layer between the a-Si:H emitter layer and the c-Si wafer is necessary. In this work, hot wire chemical vapor deposition (HWCVD) is used to deposite intrinsic oxygen-doped hydrogenated amorphous silicon (a-SiO_x:H) and hydrogenated amorphous silicon (a-Si:H) films as the intrinsic passivation layer for a-Si:H/c-Si solar cells. The passivation effect of the films on the c-Si surface is shown by the effective lifetime of the samples that bifacial covered by the films with same deposition parameters, tested by QSSPC method. The imaginary part of dielectric constant (ε₂) and bonds structure of the layers are analyzed by Spectroscopic Ellipsometry(SE) and Fourier Transfom Infrared Spectroscopy(FTIR). It is concluded that: (1) HWCVD method can be used to make a-SiO_x:H films as the passivation layer for a-Si:H/c-Si cells and the oxidation of the filament can be overcome by optimizing the deposition parameters. In our experiments, the lowest surface recombination velocity of the c-Si wafer is 3.0 cm/s after a-SiO_x:H films passivation. (2) Oxygen-doping in the amorphous silicon layers can increase H content and the band-gap of films, similar as the phenomenon of the films deposited by PECVD.     

Kinetics of light-induced degradation in a-Si:H films investigated by computer simulation

In this work we investigated the stability of a-Si:H films under illumination and following recovery in darkness at different temperatures. The a-Si:H films were fabricated with 55 kHz PECVD and with conventional rf 13.56 MHz PECVD. We measured the steady-state photocurrent and the dark-current after switching off the light source as a function of time. We observed photocurrent degradation and the following recovery of the dark current. The kinetics of the photocurrent degradation as well as the dark-current recovery demonstrated stretched-exponential behavior. The results of these straightforward measurements in combination with computer simulation were used to determine the effect of light-induced degradation and thermal recovery on the density of states distribution in the band gap of a-Si:H. We have found that the photocurrent degradation and the corresponding increase in the total defect concentration have different kinetics. The different kinetics were also determined for the dark-current recovery and the corresponding decrease in the total defect concentration. The results point out that slow and fast types of defects in a-Si:H films control the kinetics of light-induced changes of the defect distribution in the band gap. A model is proposed that relates the origin of the fast and slow metastable defects with the distribution of Si-Si bond lengths.     

Properties of periodic α-Si:H/a-SiNx:H structures obtained by nitridization of amorphoussilicon layers

The kinetics of nitridization of a-Si:H layers, the properties of the structures that are formed and a-Si:H in them have been investigated. The changes occurring in the resistance of the a-Si:H layers in the course of nitridization are described in terms of the competition between doping, transport, and change in the thickness of the remaining a-Si:H layer. The experimental data on the band spectrum of superlattices with a-Si:H and a-SiN_x:H layer thicknesses ～35 Å and ～5 Å, respectively, are in agreement with calculations in a model of interacting quantum wells with m*=(0.36±0.1)m ₀. Comparison of the properties of superlattices obtained by deposition of successive layers and nitridization of the a-Si:H layers showed that the latter can have a higher “structural perfection.”     

Photoelectric phenomena in ZnO(ITO)/a-Si:H(n)/c-Si(p)/Al solar cells

Solar cells Al/ZnO/a-Si:H(n)/c-Si(p)/Al and Al/ITO/a-Si:H(n)/c-Si(p)/Al were fabricated on single-crystal Si substrates. The photoelectric properties of these solar cells were investigated under exposure to natural and linearly polarized radiation at T=300 K. The polarization photosensitivity of the solar cells, which emerges under the conditions of oblique incidence of linearly polarized radiation, was observed. The origin of the induced photopleochroism of solar cells with ZnO and ITO antireflection coatings is ascertained. The oscillations in the spectrum of the induced photopleochroism, which are associated with interference phenomena in oxide films, are observed. The results obtained point to the possibility of using the solar cells as selective photosensors. At the same time, polarization spectroscopy can ensure effective monitoring of the antireflection coatings of solar cells.     

The effect of external factors on photoelectric parameters of amorphous hydrogenated silicon in relation to the initial characteristics of the films

The effects of heat treatments in vacuum, implantation of Si⁺ ions, and exposure to light on photoelectric parameters of a-Si:H films were studied. It is shown that the initial characteristics of the films play the determining role in the crystallization effect and in the manifestation of the Staebler-Wronski effect.     

The influence of erbium on electrical and photoelectric properties of amorphous silicon produced by radio-frequency silane decomposition

Thermal evaporation of tris(2,2,6,6-tetramethyl-2,5-heptadionato) Er(III) inside the plasma gap was used to introduce erbium into amorphous hydrogenated silicon (a-Si:H) obtained by radio-frequency silane decomposition. The samples obtained had a pronounced layered structure due to exhaustion of the erbium source. The layer nearest to the substrate was enriched with erbium, oxygen, and carbon; gave rise to luminescence with a wavelength of 1.535 µm characteristic of ⁴ I _13/2 → ⁴ I _15/2 intra-atomic transitions of erbium; and contained a large number of defects. The top layer contained much fewer defects, was close to undoped a-Si:H in the photoelectric characteristics, and was responsible for photoconductivity in the samples obtained. The experimental data are analyzed in the context of the models for doping of a-Si:H with Er with the resulting emergence of n-type conduction and formation of heterojunction as the film grows.