Low-temperature materials and thin-film transistors for electronics on flexible substrates

The deposition processes and electronic properties of thin-film semiconductors and insulators based on silicon in relation to the fabrication of electronic devices on flexible plastic substrates are considered. The films of amorphous hydrogenated silicon (a-Si:H), nanocrystalline silicon (nc-Si), and amorphous silicon nitride (a-SiN_x), and also thin-film transistors are fabricated at comparatively low temperatures (120°C, 75°C) using existing commercial plasma-chemical equipment. The parameters of thin-film transistors based on a-Si:H and fabricated at the aforementioned relatively low temperatures are compatible with those of high-temperature analogues.     

热丝CVD法沉积超薄α-Si∶H钝化膜

采用热丝化学气相沉积法在n型直拉单晶硅圆片表面双面沉积厚度为10 nm的本征非晶硅(α-Si∶H)薄膜.利用光谱型椭偏测试仪和准稳态光电导法研究热丝电流、H2体积流量和热丝与衬底之间的距离对α-Si∶H薄膜结构和钝化效果的影响.结果表明,热丝电流为21.5～23.5 A时,钝化后硅片的少子寿命随着热丝电流的增加呈现先增加后降低的趋势,热丝电流为23.0A时,钝化效果最好;H2体积流量为5～ 20 cm3/min时,少子寿命随着H2体积流量的增加呈现先增加后降低的规律,体积流量为15 cm3/min时,钝化效果最好;热丝与衬底间距为4～5 cm时,随着间距的增加,薄膜的结构由晶化向非晶化转变,在间距为4.5 cm时硅片的钝化效果达到最优.     

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.     

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.     

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40 MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiO_x:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiO_x:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density J_sc. The increased J_sc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680 nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.     

Composition and optical properties of amorphous <Emphasis Type="Italic">a</Emphasis>-SiO<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>:H films with silicon nanoclusters

The phase composition and optical properties of hydrogenated amorphous films of silicon suboxide (a-SiO_x:H) with silicon nanoclusters are studied. Ultrasoft X-ray emission spectroscopy show that silicon- suboxide films with various oxidation states and various amorphous silicon-cluster contents can be grown using dc discharge modulation. In films with an ncl-Si content of ～50%, the optical-absorption edge is observed, whose extrapolation yields an optical band gap estimate of ～3.2–3.3 eV. In the visible region, rather intense photoluminescence bands are observed, whose peak positions indicate the formation of silicon nanoclusters 2.5–4.7 nm in size in these films, depending on the film composition.     

Gamma-irradiation-induced metastable states of undoped amorphous hydrogenated silicon

The conductivity of intrinsic amorphous hydrogenated silicon (a-Si:H) becomes higher upon gamma irradiation. This effect is due to an increase in the number of metastable D ⁺ states in the mobility gap. At the same time, the conductivity of extrinsic (undoped) irradiated a-Si:H decreases. Most likely, gamma irradiation creates hydrogen-containing complexes in this material. The results obtained are discussed in comparison with the known data for B-or P-doped a-Si:H.     

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.     

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.     

X-ray spectroscopic study of electronic structure of amorphous silicon and silicyne

X-ray and ultrasoft X-ray spectroscopy have both been applied to study SiK _β and SiL ₂₃ emission spectra of crystalline silicon (c-Si), amorphous hydrogenated silicon (a-Si:H), and silicyne (a new allotropic linear-chain form of silicon). SiL ₂₃ spectra of silicyne show three peaks instead of two observed in crystalline and amorphous silicon. The third peak lies in the high-energy range at 95.7 eV, its intensity constituting ～75% of that of the main peak. An additional peak is also observed in the short-wavelength part of the SiK _β spectrum. Such a significant difference in shape between the X-ray spectra of amorphous silicon and silicyne is attributed to the existence of a pronounced π component in the chemical bonds between silicon atoms in silicyne.     

Photoinduced conductivity change in erbium-doped amorphous hydrogenated silicon films

Changes in the dark conductivity of erbium-doped amorphous hydrogenated silicon (a-Si:H(Er)) films after their preliminary illumination at room temperature have been studied. The effect of a compensating boron impurity on the photoinduced change in the conductivity of a-Si:H(Er) films is analyzed. It is established that the magnitude and the sign of the change in conductivity depend on the duration of illumination and position of the Fermi level in the mobility gap. Possible mechanisms leading to a photoinduced change in the conductivity of a-Si:H(Er) films are discussed.     

Improvement of hydrogenated amorphous silicon germanium thin film solar cells by different p-type contact layer

In this study, we report an appreciably increased efficiency from 6% up to 9.1% of hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells by using a combination of different p-doped window layers, such as boron doped hydrogenated amorphous silicon (p-a-Si:H), amorphous silicon oxide (p-a-SiO_x:H), microcrystalline silicon (p-µc-Si:H), and microcrystalline silicon oxide (p-µc-SiO_x:H). Optoelectronic properties and the role of these p-layers in the enhancement of a-SiGe:H cell efficiency were also examined and discussed. An improvement of 1.62 mA/cm² in the short-circuit current density (J_sc) is attributed to the higher band gap of p-type silicon oxide layers. In addition, an increase in open-circuit voltage (V_oc) by 150 mV and fill factor (FF) by 6.93% is ascribed to significantly improved front TCO/p-layer interface contact.     

Photoelectric phenomena in a-Si:H/p-CuInSe2 heterostructures

The photosensitive a-Si:H/p-CuInSe₂ heterostructures were obtained for the first time by the deposition of hydrogenated amorphous silicon on polycrystalline p-CuInSe₂ substrates. The photoelectric properties of the new system were studied. The conclusion was drawn on the prospects of the application of this system for solar and linearly polarized radiation photoconvertors.     

The density of states in the mobility gap of amorphous hydrogenated silicon doped with erbium

The effect of doping films of amorphous hydrogenated silicon (a-Si:H) with erbium on the density of the states in the mobility gap is studied. The data obtained are compared with those for a-Si:H films doped with arsenic. The data on the density of the states in the lower and upper halves of the mobility gap are determined from measurements of the spectral dependences of the absorption coefficient and the temperature dependences of the constant and modulated components of the photoconductivity in films exposed to modulated light, respectively. It is shown that doping the a-Si:H films with erbium leads to an increase in the density of states both in the lower and upper halves of the mobility gap.     

Multi‐crystalline Si solar cells with very fast deposited (180 nm/min) passivating hot‐wire CVD silicon nitride as antireflection coating

Hot‐wire chemical vapor deposition (HWCVD) is a promising technique for very fast deposition of high quality thin films. We developed processing conditions for device‐ quality silicon nitride (a‐SiN_x:H) anti‐reflection coating (ARC) at high deposition rates of 3 nm/s. The HWCVD SiN_x layers were deposited on multicrystalline silicon (mc‐Si) solar cells provided by IMEC and ECN Solar Energy. Reference cells were provided with optimized parallel plate PECVD SiN_x and microwave PECVD SiN_x respectively. The application of HWCVD SiN_x on IMEC mc‐Si solar cells led to effective passivation, evidenced by a V_oc of 606 mV and consistent IQE curves. For further optimization, series were made with HW SiN_x (with different x) on mc‐Si solar cells from ECN Solar Energy. The best cell efficiencies were obtained for samples with a N/Si ratio of 1·2 and a high mass density of >2·9 g/cm³. The best solar cells reached an efficiency of 15·7%, which is similar to the best reference cell, made from neighboring wafers, with microwave PECVD SiN_x. The IQE measurements and high V_oc values for these cells with HW SiN_x demonstrate good bulk passivation. PC1D simulations confirm the excellent bulk‐ and surface‐passivation for HW SiN_x coatings. Interesting is the significantly higher blue response for the cells with HWCVD SiN_x when compared to the PECVD SiN_x reference cells. This difference in blue response is caused by lower light absorption of the HWCVD layers (compared to microwave CVD; ECN) and better surface passivation (compared to parallel plate PECVD; IMEC). The application of HW SiN_x as a passivating antireflection layer on mc‐Si solar cells leads to efficiencies comparable to those with optimized PECVD SiN_x coatings, although HWCVD is performed at a much higher deposition rate. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of the femtosecond laser treatment of hydrogenated amorphous silicon films on their structural, optical, and photoelectric properties

The effect of the femtosecond laser treatment of hydrogenated amorphous silicon (a-Si:H) films on their structural, optical, and photoelectric properties is studied. Under the experimental conditions applied in the study, laser treatment of the film with different radiation intensities induces structural changes that are nonuniform over the film surface. An increase in the radiation intensity yields an increase in the contribution of the nanocrystalline phase to the structure, averaged over the sample surface, as well as an increase in the conductance and photoconductance of the samples. At the same time, for all of the samples, the absorption spectrum obtained by the constant-photocurrent method has a shape typical for those of amorphous silicon. Obtained results indicate the possibility of a-Si:H films photoconductance increase by femtosecond pulse laser treatment.     

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40 MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiO_x:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E₀₄, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiO_x:H/n-μc-Si:H structure showed an enhancement in short-circuit current density J_sc from 13.32 to 13.60 mA/cm², and in conversion efficiency from 8.53% to 8.61%. The increased J_sc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660 nm, as demonstrated by the external quantum efficiency.     

Investigation of surface morphology of copper-modified amorphous carbon films

A study of the surface morphology of copper-modified amorphous hydrogenated carbon films a-C:H(Cu) by scanning tunnel microscopy (STM) is reported. An algorithm is presented for the digital analysis of STM images to obtain the size distribution function for the longitudinal component of the surface relief. a-C:H(Cu) films were deposited by magnetron co-sputtering of graphite and copper onto two types of substrates: (100) n-Si with a heavily doped surface layer, and Si covered with a chromium layer. A mesoscopic surface structure of crystalline silicon, a chromium layer, and a-C:H(Cu) film has been revealed. A correlation between the structural elements of the film and the substrate is considered and a conclusion is made that inherent grains with characteristic size of 6–8 nm are formed in the film.     

Formation of optically active centers in films of erbium-doped amorphous hydrated silicon

We have observed photoluminescence at 1.54 μm from a-Si:H films doped with erbium of various degrees of purity. It is shown that the additional introduction of oxygen activates the Er ions. The effect of silicides and defects in amorphous silicon a-Si:H films and in crystalline silicon c-Si is investigated. Fiz. Tekh. Poluprovodn. 33, 1260–1263 (October 1999)     

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.