Growth Of Amorphous Silicon Films Research Articles

Low-Temperature Growth of Amorphous Silicon Films and Direct Fabrication of Solar Cells on Flexible Polyimide and Photo-Paper Substrates

Direct deposition of hydrogenated amorphous silicon (a-Si:H) thin films and fabrication of solar cells on polyimide (PI) and photo-paper (PP) substrates using a rf-plasma-enhanced chemical vapor deposition technique is reported. Intrinsic amorphous silicon films were deposited on PI and PP substrates by varying the substrate temperature (Ts) over 70–150°C to optimize the deposition parameters for best quality films. The films deposited on both PI and PP substrates at a temperature as low as 70°C showed a photosensitivity (σph/σd) of nearly 4 orders of magnitude which increased to 5–6 orders of magnitude when the substrate temperature was increased to 130–150°C. The increase in σph/σd is due to the presence of a few nanometer-sized crystallites embedded in the film. Solar cells (n-i-p) were fabricated directly on PI, PP and Corning 1737 glass (Corning) at 150°C for different thicknesses of an intrinsic amorphous silicon layer (i-layer). With the increase in i-layer thickness from 330 nm to 700 nm, the solar cell efficiency was found to increase from 3.81% to 5.02% on the Corning substrate whereas on the flexible PI substrate an increase from 3.38% to 4.38% was observed. On the other hand, in the case of cells on PP, the i-layer thickness was varied from 200 nm to 700 nm and the best cell efficiency 1.54% was obtained for the 200-nm-thick i-layer. The fabrication of a-Si (n-i-p) solar cells on photo-paper is presented for the first time.

Lateral Grain Growth of Amorphous Silicon Films With Wide Thickness Range by Blue Laser Annealing and Application to High Performance Poly-Si TFTs

We report high performance p-type poly-Si thin-film transistors (TFTs) achieved by lateral grain growth of amorphous silicon (a-Si) by using a continuous-wave blue diode laser of wavelength 445 nm. The blue laser beam is efficiently absorbed into the a-Si film, such that full melting and lateral crystallization is achieved in all thicknesses investigated, 50-200 nm. TFTs fabricated with 75-, 100-, and 125-nm-thick poly-Si films laterally grown by the blue laser annealing exhibited field-effect mobility of 108 ± 7, 104 ± 9, and 134 ± 12 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V · s, and subthreshold swing of 210±51, 191±16, and 193±53 mV/decade, respectively, and high ON/OFF current ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> .

비정질 규소막의 공정조건이 HSG-Si 형성에 미치는 영향

본 논문은 비정질 규소막 성장의 공정 조건이 저장 전극의 표면에 HSG-Si를 형성할 때 미치는 영향을 조사하였다. 그 결과 비정질 규소막의 인 농도가 <TEX>$5.5{\pm}0.1E19atoms/cm^3$</TEX> 이상이면 HSG-Si가 제대로 형성되지 않는 인 농도 의존성을 나타내었다. 또한 HSG 두께가 <TEX>$500{\AA}$</TEX> 이상에서는 전극과 전극을 단락시키는 비트 불량을 유발하기 때문에 비정질 규소막의 인농도는 <TEX>$4.5E19atoms/cm^3$</TEX>, HSG 임계 두께는 <TEX>$450{\AA}$</TEX>이 최적 조건임을 확인하였다. In this paper, the processing conditions of the amorphous silicon film growth were investigated the effect in forming the HSG-Si on the surface of the storage electrode. As a result, when the amorphous silicon film phosphorus concentration is greater than <TEX>$5.5{\pm}0.1</TEX><TEX>E19atoms/cm^3$</TEX>, HSG-Si is not formed correctly and showed the concentration dependency of HSG formation. Also, the optimum condition of the phosphorus concentration for amorphous silicon and HSG thickness are <TEX>$4.5E19atoms/cm^3$</TEX> and <TEX>$450{\AA}$</TEX>, respectively, because of the HSG thickness over the <TEX>$500{\AA}$</TEX> create to bit failure according to a short of the electrodes and the electrode.

Features of the growth of amorphous silicon thin films synthesized by magnetron sputtering

Features of the growth of amorphous silicon films synthesized by DC magnetron sputtering are studied. The effect of the working-gas pressure on the film-surface roughness and growth rate is investigated. It is shown that the film-growth rate decreases with increasing gas pressure in the working chamber and film-surface roughness changes in proportion to the pressure variation. The obtained experimental dependences can be used in modifying the technique for growing films with specified characteristics.

Ultrasmooth growth of amorphous silicon films through ion-induced long-range surface correlations

Ultrasmooth amorphous silicon films with a constant roughness below 0.2 nm were produced for film thickness up to ∼1 μm by magnetron sputtering under negative voltage substrate biasing (100–400 V). In contrast, under unbiased conditions the roughness of the resulting mounded films increased linearly with growth time due to shadowing effects. A detailed analysis of the amorphous film growth dynamics proves that the bias-induced ultrasmoothness is produced by a downhill mass transport process that leads to an extreme surface leveling inducing surface height correlations up to lateral distances close to 0.5 μm.

Effect of oligomers on the growth of amorphous silicon films in a PECVD reactor

A plasma-chemical model of processes in a PECVD reactor is constructed that is an extension of the earlier model and takes into account the formation of oligomers Si n H m ( n ≤ 5). The corresponding scheme of chemical reactions is developed, and simulation of film growth is carried out. It is found that Si 2 H 5 and Si 3 H 7 components strongly influence the film growth. It is of interest to obtain more reliable experimental data dra- matizing these effects.

Use of Tritium in the Study of Defects in Amorphous Silicon

Hydrogen is known to strongly affect the physical properties of amorphous semiconductors. Indeed hydrogen is introduced during the growth of amorphous silicon films, used in active matrix displays and solar cells, to passivate silicon dangling bonds and to relax the lattice thereby reducing the density of states in the energy gap by several orders of magnitude and giving rise to device grade material. Ideally, hydrogenated amorphous silicon (a-Si:H) is a continuous covalently bonded random network of silicon-silicon and silicon-hydrogen atoms, with the predominant nearest neighbour environment similar to that of crystalline silicon. a-Si:H typically contains about 10 atomic percent hydrogen.Tritium can readily substitute for hydrogen in a-Si:H without altering the physicochemical properties of the material. Tritium decay leads to a change in the local bond structure of the material as helium detaches from bonds leaving behind dangling bonds. The decay rate of tritium and therefore the rate of dangling bond formation is determined by the half-life of tritium. Hence, tritium provides a unique avenue to dynamically study the effect of dangling bonds on the density of states in the energy gap and therefore on the optoelectronic properties of a-Si:H. Tritiated hydrogenated amorphous silicon (a-Si:H:T) was deposited using mixtures of tritium and silane gases in a dc saddle-field glow-discharge deposition system. The amount of tritium in the films was controlled by adjusting the relative flow of tritium and silane gases into the deposition chamber.Photoluminescence, isothermal capacitive transient spectroscopy and constant photocurrent spectroscopy were used to measure defect concentration as a function of time in the films. The defect concentration was found to increase between 1 and 2 orders of magnitude, in about 300 hours. Thermal annealing decreased the defect concentration. It was found that tritium permits a study of the change in the density of defect states due to dangling bond formation in a-Si:H without the uncertainties introduced by the use of multiple samples.

Surface-diffusion-controlled incorporation of nanosized voids during hydrogenated amorphous silicon film growth

The incorporation of nanosized voids during hydrogenated amorphous silicon film growth is studied by measurements of the film mass density and the hydrogen present at the void surfaces. The observed dependence of the density of nanosized voids on the growth flux and substrate temperature is explained in terms of a surface-diffusion-controlled void incorporation model. From this analysis, an activation energy for surface diffusion in the range of 0.77–1.05eV has been found, a value which is in agreement with the activation energy obtained from the analysis of the surface roughness evolution during growth in a previous study.

Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

Low Tempeature Lateral Crystallization of Amorphous Silicon on Glass

AbstractLow temperature lateral growth of amorphous silicon films has been achieved on thin flexible glasses using ultra-violet assisted metal-induced crystallization technique. 125μm ordinary glass substrate is sputter-coated with 1500Å chromium and a 1000Å SiN layer, respectively. 1000Å Si film was deposited using e-beam evaporation at a temperature of 350°C. Equally spaced dots of nickel pads with 140μm separation were used as seed of crystallization of a-Si layer. Crystallinity of the samples was studied using XRD, SEM and optical microscopy. Some crystallographic etchants were used to develop the crystal orientations for SEM analysis. Based on this study, a lateral growth rate of 2μm/hr is obtained at a temperature of 380°C. Activation energy of 1.4 to 1.5eV is extracted for this UV-assisted MILC process.

Effect of Ar+ Ion Bombardment During Hydrogenated Amorphous Silicon Film Growth in Plasma Chemical Vapor Deposition System

We have developed the double tubed coaxial line type microwave plasma chemical vapor deposition (MPCVD) system to fabricate hydrogenated amorphous silicon (a-Si:H) films. We have studied the influence of Ar+ ion bombardment during a-Si:H film growth and clarified that the ion bombardment causes film surface heating effect and ion implanting effect. It is not sufficient to discuss only whether films are of good quality or not, when the ion bombardment energy is increased. In this study, we show that the effect of ion bombardment can be separated into the film surface heating effect and the ion implanting effect and discuss the influence of each effect on the film properties. We also show that the film surface temperature can be expressed as a function of the sheath voltage. It is clarified that a film with low dangling bond density can be fabricated at low temperatures if there is no ion bombardment.

Formation of cationic silicon clusters in a remote silane plasma and their contribution to hydrogenated amorphous silicon film growth

The formation of cationic silicon clusters SinHm+ by means of ion–molecule reactions in a remote Ar–H2–SiH4 plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is based on SiH4 dissociation in a downstream region by a thermal plasma source created Ar–H2 plasma. The electron temperature, ion fluence, and most abundant ion emanating from this plasma source are studied as a function of H2 admixture in the source. The electron temperature obtained is in the range of 0.1–0.3 eV and is too low for electron induced ionization. The formation of silicon containing ions is therefore determined by charge transfer reactions between ions emanating from the plasma source and SiH4. While the ion fluence from the source decreases by about a factor of 40 when a considerable flow of H2 is admixed in the source, the flux of cationic silicon clusters towards the substrate depends only slightly on this H2 flow. This implies a strong dissociative recombination of silicon containing ions with electrons in the downstream region for low H2 flows and it causes the distribution of the cationic silicon clusters with respect to the silicon atoms present in the clusters to be rather independent of H2 admixture. The average cluster size increases, however, strongly with the SiH4 flow for constant plasma source properties. Moreover, it leads to a decrease of the ion beam radius and due to this, to an increase of the ion flux towards the substrate, which is positioned in the center of the beam. Assuming unity sticking probability the contribution of the cationic clusters to the total growth flux of the material is about 6% for the condition in which solar grade a-Si:H is deposited. Although the energy flux towards the film by ion bombardment is limited due to the low electron temperature, the clusters have a very compact structure and very low hydrogen content and can consequently have a considerable impact on film quality. The latter is discussed as well as possible implications for other (remote) SiH4 plasmas.

Molecular dynamics simulation of amorphous silicon with Tersoff potential

We have performed molecular dynamics simulation of amorphous silicon (a-Si) by rapid quenching, with empirical interatomic potential (Tersoff potential) as a first step toward hydrogenated amorphous silicon film growth simulations. The numerical results on structural properties showed fairly good agreement with experimental data or numerical results from ab-initio molecular dynamics in other groups. In our a-Si sample, the average coordination is 4.2 and approximately 18% of the atoms have a coordination number of 5. In addition, visualization of the a-Si sample by computer graphics made it possible to see that there are some microcavities in the sample, which was predicted in experimental data.

Bond Selectivity in Silicon Film Growth

Hydrogen atoms can selectively eliminate strained bonds that form during the growth of amorphous silicon films. By periodically interrupting the growth and exposing the grown material to hydrogen, the film composition can be varied continuously from a non-equilibrium amorphous structure to that of a crystalline solid. Furthermore, by tuning the hydrogen exposure it is possible to discriminate between Si-Si bonds formed on different substrates, thereby allowing substrate-selective growth. The evolution of the film structure during hydrogen exposure is directly observed by scanning tunneling microscopy, and a model describing the role of hydrogen is presented.

Elementary processes of surface reaction in amorphous silicon film growth

Surface reactions contributing to a-Si:H film growth both in thermal and in plasma chemical vapor deposition (CVD) were investigated by ab initio molecular orbital calculations. The conclusions obtained are as follows: (i) The primary activation process of a-Si:H growing surface is the silylene structure formation by hydrogen elimination. (ii) At low temperature, the subsequent reaction on the growing surface is the hydrogen migration from the adjacent SiH 2 group to the silylene structure and then the disilene structure is produced. Silyl radicals prepared in plasma reactors stick themselves to the disilene structures and contribute to a-Si:H film growth. (iii) At high temperature, rows of silylene radicals are produced on the growing surface. The silylene structures in the rows have a life time long enough to react with silane molecules in the CVD reactor. This reaction contributes to a-Si:H film growth in the thermal CVD. (iv) The conclusions (ii) and (iii) are the reason that higher surface temperature is required for a-Si:H film growth in the thermal CVD than in the plasma CVD.

Monte Carlo simulations of magnetron sputtering particle transport

Monte Carlo simulations of the particle transport process during dc magnetron sputter deposition were performed to determine the energy and angular distributions of the energetic deposition species. The model itself is quite general, and here we present the specific example of hydrogenated amorphous silicon film growth. This process involves the sputtering of a silicon target in an argon-plus-hydrogen plasma. The three-dimensional model incorporates fractal trim data for the distribution of Si energies and emission angles sputtered from the target surface. Modified ‘‘universal’’ interatomic potentials are used to determine the scattering processes during gas phase transport. Energy and angular distributions of the deposition flux reaching the substrate are calculated as a function of pressure from 0.01 to 5.5 mTorr. As the pressure increases we find that the average energy per deposited atom remains essentially constant, but the energy and angular distributions of the arrival flux change dramatically.

Mass spectrometric analysis during the vapor deposition of amorphous silicon

The gaseous species produced by the electron beam evaporation of silicon from conventional copper and carbon hearths were studied by utilizing a residual gas analyzer. At low evaporation rates about 13% of the silicon evaporates as molecules containing two to seven atoms. The percentage of clusters increases only slightly with rate, indicating that their heat of evaporation differs only slightly from that of the silicon atoms. Additional molecules, predominantly Si 2C, occur if vitreous carbon liners are used. The resulting carbon contamination of the amorphous silicon films (about 1.5 mol.%) was confirmed by electron microprobe analysis. A discussion is given concerning the influence of cluster deposition on the growth of amorphous silicon films.