High-quality Polycrystalline Silicon Research Articles

Nanosecond laser-induced crystallization of SiOx/Au bilayers in air and vacuum

High-quality polycrystalline silicon films on low-cost and low-temperature substrates have attracted much attention as promising materials for high-speed thin-film transistors and thin-film solar cells fabrication. To obtain poly-Si films on low temperature substrates, several concepts have been proposed. Usually the amorphous material undergoes crystallization which can be achieved by various methods including solid-phase crystallization, metal-induced crystallization or liquid-phase crystallization. In this work, we tried to combine the advantages of metal-induced crystallization and liquid-phase crystallization. To achieve this we explored the nanosecond laser crystallization of a bilayer structure consisting of Au and SiO0.1 layers with thicknesses of 30 nm and 130 nm, respectively. The study reveals that when exposed to 532 nm wavelength radiation leads to its destruction due to rupture. On the other hand, when subjected to 1064 nm wavelength radiation, no similar material behavior is observed, and the measured modification threshold is 0.15 J/cm2, representing a 40 % reduction compared to SiO0.1 film without gold. It is demonstrated that at laser fluences of 0.35 J/cm2 and higher, the treatedsurfaceinair becomes enriched with silicon dioxidenanoporous coating, attributed to the return of evaporation products to the target surface. Theoretical modeling, assuming thermal evaporation of the coating, suggests that the undesirable nanoporous layer formation can be avoided.

Solution-Doped Polysilicon Passivating Contacts for Silicon Solar Cells.

In this work, we present a simple and efficient solution-doping process for preparing high-quality polycrystalline silicon (poly-Si)-based passivating contacts. Commercial phosphorus or boron-doping solutions are spin-coated on crystalline silicon (c-Si) wafers that feature SiO2/poly-Si layers; the doping process is then activated by thermal annealing at high temperatures in a nitrogen atmosphere. With optimized n- and p-type solution doping and thermal annealing, n- and p-type poly-Si passivating contacts featuring simultaneously a low contact recombination parameter (J0c) of 2.4 and 12 fA/cm2 and a low contact resistivity (ρc) of 29 and 20 mΩ·cm2 are achieved, respectively. Taking advantage of the single-sided nature of these solution-doping processes, c-Si solar cells with poly-Si passivating contacts of opposite polarity on the respective wafer surfaces are fabricated using a simple coannealing process, achieving the best power conversion efficiency (PCE) of 18.5% on a planar substrate. Overall, the solution-doping method is demonstrated to be a simple and promising alternative to gas/ion implantation doping for poly-Si passivating-contact manufacturing.

Solid phase epitaxial thickening of boron and phosphorus doped polycrystalline silicon thin films formed by aluminium induced crystallization technique on glass substrate

Aluminium induced crystallization (AIC) technique can be used to form the high-quality and large-grained polycrystalline silicon (poly-Si) thin films, which are with the thickness of ~200 nm and used as a seed layer, on silicon nitride coated glass substrate. Thanks to aluminium metal in AIC process, the natural doping of AIC thin films is p+ type (~2 × 1018 cm−3). On the other hand, recombination of carriers can be controlled by partial doping through the defects that may have advantages to improve the thin film quality by the overdoping induced passivation. In this study, boron (B) and phosphorus (P) doped AIC seed layers were thicken to ~2 μm by solid phase epitaxy (SPE) technique at 800 °C for 3 h under nitrogen flow in a tube furnace. During the crystallization annealing, exodiffusion of dopants was formed through the SPE film from the AIC seed layer. Optical microscope and electron back scattering diffraction technique (EBSD) were used to analyse the structural quality of the Si films. The poly-Si layer with an average grain size value of ~32 μm was formed by AIC + SPE technique for P doped samples while EBSD analysis gave no results for B doped samples due to the quite deterioration on the surface of the films. AIC + SPE films were analyzed in terms of structural properties by using micro-Raman Spectroscopy and X-ray diffraction systems. The results showed that the crystallinity of compressive stress formed AIC + SPE films reached up to 98.55%. Additionally, the Raman analysis pointed out that no temperature-induced stress were generated in the AIC + SPE films while compressive stress was induced by increasing the annealing duration for doped AIC film. For all samples, the preferred orientation was 〈100〉, and the crystallite size up to 44.4 nm was formed by phosphorus doping of AIC films. The doping efficiency was determined by time-of-flight secondary ion mass spectroscopy for doped samples. A graded n+n doping profile was obtained by exo-diffusion of phosphorus from the overdoped seed layer during the epitaxial thickening while boron doping of SPE film has failed with exo-diffusion of boron from AIC seed layer into SPE film. Finally, high-quality n+n type poly-Si films were fabricated on glass substrate by using AIC + SPE technique.

(Invited) High-Performance Top-Gate and Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors Fabricated Using Continuous-Wave-Laser Lateral Crystallization on a Glass Substrate

A high-quality polycrystalline silicon (poly-Si) film is a technology for improving the performance of low-temperature (LT) poly-Si thin-film transistors (TFTs). We have already developed continuous-wave-laser lateral crystallization (CLC) using a diode-pumped solid-state laser to improve the crystalline quality of the thin poly-Si film. In this paper, we report on two approaches for improving the performance of LT poly-Si TFTs, which are based on a thin CLC poly-Si film and are fabricated at 550 ºC on a glass substrate. The first approach is a combination of a CLC poly-Si film and sputtered HfO2 as the gate dielectric. The other approach is a combination of a CLC poly-Si film and a double-gate structure for realizing four-terminal LT poly-Si TFTs. The former exhibited an enhancement in the on-current that was four times greater than a device with a silicon dioxide gate stack. In the latter, high controllability of the threshold voltage was confirmed.

Role of Al2O3 intermediate layer for improving the quality of polycrystalline-silicon film in inverted aluminum-induced layer exchange

A thin Al2O3 intermediate layer prepared by atomic layer deposition was introduced into inverted aluminum-induced layer exchange (inverted-ALILE) to form high-quality polycrystalline silicon (poly-Si) thin layer. It was demonstrated that the continuity and quality of poly-Si were obviously improved by the Al2O3 layer. The fraction of (100)-oriented crystals reached 93%, and the average grain size of 28μm with uniform surface morphology and low defect density were achieved at the optimal Al2O3 thickness of 4nm. It was also found that an a-AlOx layer always existed at the poly-Si/Al interface after inverted-ALILE process, which is independent on the original surface states. The results suggested that the thin poly-Si layer would be a promising epitaxial template for Si based thin film solar cells.

Photoluminescence study of polycrystalline silicon thin films prepared by liquid and solid phase crystallization

Polycrystalline silicon thin films were prepared by depositing amorphous or microcrystalline silicon layers onto glass substrates and subsequent crystallization via solid or liquid phase crystallization approaches. Differences in layer morphology and quality were characterized using low temperature photoluminescence (PL) spectroscopy and electron backscatter diffraction spectroscopy. The evaluation of spectral and spatially resolved PL response was used to conclude about the relative density of defects in these layers. Liquid phase crystallization using a line shaped electron beam proves to be the most promising technique in order to obtain large-grained high quality polycrystalline silicon thin films.

Preparation of high quality polycrystalline silicon thin films by aluminum induced crystallization

In this paper, high-quality polycrystalline silicon (poly-Si) thin films on glass substrates are formed by Aluminum-induced crystallization (AIC). In AIC processes, bi-layer structures of amorphous silicon (a-Si) / Al are transformed into ones of (Al+ residual Si)/ poly-Si after simply annealing at 500°C in vacuum furnace. After Al chemical etchings, it isobserved that the obtained structures are poly-Si thinfilms on glasses with some amount of residual Si as“ islands”scattered on theirsurfaces. The number of these “Si islands” remarkedly reduced by choosing an appropriate thickness ratio of pre-annealled Al and Si layers that prepared by magnetron dc sputtering. In this study, at initial Al/a-Si thickness ratio of 110/230 nm, the high-quality poly-Si thin films are formed with very few“Si islands” on the surfaces after AIC processes. Theobtained smooth surfaces are not appearing “dendritic” in optical transmission microscopy (OTM ) images, have large grain size of tens of nanometers in SEM images and have average surface roughness of about 2.8 nm in AFM images. In addition, XRD Ө -2Ө measurements show a strong Si (111) peak at the 2Ө angle of 28.5°, presenting good crystalline phases. The films also reveal good p-type electrical conductivityin that their high carrier concentration and mobility in Hall effect measurements are 1018 cm-3 and 48 cm2/Vs, respectively.

Fabrication of large-grained thin polycrystalline silicon films on foreign substrates by titanium-assisted metal-induced layer exchange

Metal-induced layer exchange (MILE) is a well-known method to grow large-grained high quality polycrystalline silicon on foreign substrates. We have modified the commonly used layer stack by an additional titanium interfacial layer (substrate/metal/titanium/oxide/amorphous silicon). The resulting layer exchange process is called titanium-assisted metal-induced layer exchange (Ti.MILE). For the investigated metals, Al (Ti.ALILE) and Ag (Ti.AgILE), the additional Ti layer does not affect the overall layer exchange process but results in a strong enlargement of the grains in the resulting polycrystalline silicon layer up to 250 μm. We have investigated the influence of the titanium interfacial layer on the process dynamics and grain growth. Furthermore, the structural and optical properties of the resulting polycrystalline silicon layer are investigated by means of different analysis methods.

Fabrication of Poly Seed Layer for Silicon Based Photovoltaics by Inversed Aluminum-Induced Crystallization

The formation of high-quality polycrystalline silicon (poly-Si) on relatively low cost substrate has been an important issue in the development of thin film solar cells. Poly-Si seed layers were fabricated by an inverse aluminum-induced crystallization (I-AIC) process and the properties of the resulting layer were characterized. The I-AIC process has an advantage of being able to continue the epitaxial growth without an Al layer removing process. An amorphous Si precursor layer was deposited on Corning glass substrates by RF magnetron sputtering system with Ar plasma. Then, Al thin film was deposited by thermal evaporation. An <TEX>$SiO_2$</TEX> diffusion barrier layer was formed between Si and Al layers to control the surface orientation of seed layer. The crystallinity of the poly-Si seed layer was analyzed by Raman spectroscopy and x-ray diffraction (XRD). The grain size and orientation of the poly-Si seed layer were determined by electron back scattering diffraction (EBSD) method. The prepared poly-Si seed layer showed high volume fraction of crystalline Si and <100> orientation. The diffusion barrier layer and processing temperature significantly affected the grain size and orientation of the poly Si seed layer. The shorter oxidation time and lower processing temperature led to a better orientation of the poly-Si seed layer. This study presents the formation mechanism of a poly seed layer by inverse aluminum-induced crystallization.

Study on High Quality Polycrystalline Silicon p/n Films Fabrication through Recrystallization by RTP

Aluminum films were evaporated on quartz substrates, and then an npp+ structure was formed by a p/n junction deposited on the aluminum film. The Raman, XRD and SEM were used to analyze the crystallization and components of the samples before and after annealed. The results showed that they were compact and almost 100% in crystallization after annealing and carrier motility also increase. The mechanism of the rapid thermal process is also proposed, its superior effect mainly contributes to the photoelectron induced diffusion process, in which the short wavelength of the tungsten plays an important role.

Fabrication of High-Quality Polycrystalline Silicon Film by Crystallization of Amorphous Silicon Film Using AlCl3 Vapor for Thin Film Transistors

Development of high quality polycrystalline silicon film is in strong demand for active matrix organic light emitting displays. In this study, AlCl3 vapor was applied to enhance the crystallization of amorphous silicon film. The crystallization of amorphous Si showed that round-shaped Si grains were nucleated from amorphous Si film and they impinged each other to form polyhedral-shaped grains. The grain size in the crystallized Si film was very large and the surface roughness was very small. The field-effect hole mobility and subthreshold swing of the p-channel thin film transistor were 94 cm2/V s and 0.4 V/decade, respectively. The high performance of this thin film transistor is attributed to the large grain size, high crystallinity, and smooth surface of the poly-Si film crystallized in AlCl3 vapor.

High-Quality Polycrystalline Silicon Film Crystallized from Amorphous Silicon Film using NiCl2 Vapor

The development of high-quality polycrystalline silicon film is of interest for active-matrix organic light-emitting displays. Here, NiCl2 vapor was applied for the first time to enhance the crystallization of amorphous silicon film. The crystallization of amorphous Si showed that round-shaped Si grains grow and become impinged to form polyhedral-shaped grains with diameters of 10 to 25 μm. It was found that the growth of large grains was possible via the merging of fine needle grains with the same directional growth. The crystallized film showed a high degree of crystallinity and a very smooth surface with a roughness of 0.53 nm. The field-effect hole mobility and subthreshold swing of the p-channel thin-film transistor fabricated using the poly-Si film were 113 cm2/ and 0.4 V/decade, respectively. The high performance of the thin-film transistor is attributed to the large grain size, the high crystallinity, the low Ni contamination and the smooth surface of the poly-Si film crystallized in NiCl2 vapor.

Development of polycrystalline silicon films on flexible metallic substrates by aluminium induced crystallization

Thin film silicon solar cells on low cost foreign substrates could be attractive for highly efficient and low cost production of photovoltaic electricity. An attempt has been made to synthesise high-quality continuous polycrystalline silicon (pc-Si) layers on flexible metallic substrates using aluminium induced crystallization (AIC) for the first time. Amorphous silicon films deposited by ECR-PECVD were crystallized on diffusion barrier coated metallic substrates at lower temperatures (<577°C). The crystallization was studied using Raman as well as UV reflectance spectroscopy. The as-grown AIC pc-Si films were found to be continuous and densely packed without amorphous phase. The migration of impurities from the substrate to the pc-Si films and the conformability of the barrier layer with the substrate and pc-Si films were studied systematically in terms of chemical and stress level analysis, which are the important aspects to be considered when metallic foils are used as substrates. It was observed that the barrier layer also serves as a buffer layer to minimise the stress level enormously in the AIC grown pc-Si layer, though the supporting material has a thermal expansion coefficient of higher order at higher annealing temperatures. The present investigation proves the possibility to grow better-quality polycrystalline silicon films on flexible metallic foils and further demonstrates the steps that need to be considered to improve the quality of AIC pc-Si films as well as the strength of the barrier layer.

Preparation of high quality polycrystalline silicon thin films by aluminum-induced crystallization

High quality polycrystalline silicon (poly-Si) thin films without Si islands were prepared by using aluminum-induced crystallization on glass substrates. Al and amorphous silicon films were deposited by vacuum thermal evaporation and radio frequency magnetron sputtering, respectively. The samples were annealed at 500 °C for 7 h and then Al was removed by wet etching. Scanning electron microscopy shows that there are two layers in the thin films. After the upper layer was peeled off, the lower poly-Si thin film was found to be of high crystalline quality. It presented a Raman peak at 521 cm − 1 with full width at a half maximum of 5.23 cm − 1 , which is similar to c-Si wafer.

Highly Uniform Polycrystalline Silicon Thin Films Fabricated by Metal Plate Energy-Assisted Agent Method

In this paper, we proposed a novel technique to crystallize amorphous silicon films. A crystallized silicon film shows smooth surface and good crystallinity. Crystallization was conducted by a mechanism utilizing an energy-assisted agent. In this technique, an amorphous silicon film was in conjunction with a metal plate as a sample. Then, this sample was subjected to a rapid thermal annealing process. A uniform and high-quality polycrystalline silicon (poly-Si) can be obtained through an efficient energy transfer between the metal plate and the semiconductor thin film. As a result, the average size of grains obtained was 0.4 µm and the surface roughness was only about 0.34 nm. The result of X-ray diffraction analysis shows a preferential crystal orientation of (111). On the other hand, we conducted an in-depth electron spectroscopy for chemical analysis and secondary ion mass spectrometry measurement on the crystallized film. It was proved that the content of metal atoms within the crystallized film can be neglected.

Hydrogen Modulation-Doped Structures to Improve Crystalline Fraction of Polycrystalline Silicon Films Prepared by Excimer Laser Annealing at Low Energy Densities

An excimer laser annealing (ELA) of amorphous silicon (a-Si) with a hydrogen modulation-doped layer (ELHMD) was investigated using a-Si films with various H concentrations and H distributions for forming high-quality polycrystalline silicon (poly-Si) films at a low energy irradiation (100 mJ/cm2). Poly-Si films with a high crystalline fraction of 80% are obtained by controlling the H concentration distribution and shot number for ELA. In addition, the film exfoliation caused by a H2 burst can be suppressed, and secondary grain growth can be induced using HMD a-Si films. It is considered that the nucleation is enhanced by the recombination energy of the H atoms around the Si–H2 bond during Si melting and that H desorption affects grain growth and film exfoliation.

High-Quality Polycrystalline Silicon Films with Minority Carrier Lifetimes over 5 µs Formed by Flash Lamp Annealing of Precursor Amorphous Silicon Films Prepared by Catalytic Chemical Vapor Deposition

Polycrystalline silicon (poly-Si) films with a thickness over 1 µm are formed by the flash lamp annealing (FLA), with a duration of less than 10 ms, of precursor amorphous silicon (a-Si) films prepared by catalytic chemical vapor deposition (Cat-CVD) on quartz substrates. Thicker a-Si films can be crystallized under lower lamp irradiance, which can be understood by considering both the total generated heat in the a-Si films and the thermal diffusion into the quartz substrates. Results of microwave photoconductivity decay (µ-PCD) measurement indicate that high-pressure water vapor annealing (HPWVA) is effective for improving the minority carrier lifetime of the poly-Si films. The film with a thickness of 1.5 µm crystallized under a high lamp irradiance shows minority carrier lifetimes over 5 µs, indicating the high feasibility of applying the crystalline film to high-efficiency thin-film solar cells.

Directional growth method to obtain high quality polycrystalline silicon from its melt

We propose a new concept for growing a polycrystalline silicon (poly-Si) ingot suitable for solar cells by the casting method. Usually, poly-Si ingots grown by casting method have many crystal grains with different orientations and different sizes, and have many grain boundaries because such polycrystalline structures have not been controlled during casting. In this study, we found that to induce a dendrite growth in the initial stage of directional growth was useful for controlling a polycrystalline structure such as grain orientation and grain size of each grain. In fact, we obtained a poly-Si ingot which had large-size crystal grains with same orientation to the growth direction by inducing dendrite growth along the crucible wall in the initial stage of growth. The solar cell property of such a structure-controlled poly-Si was higher than that of structure-uncontrolled poly-Si. This growth method is very promising in the field of the poly-Si solar cells.

Epitaxial thickening of AIC poly-Si seed layers on glass by solid phase epitaxy

A novel method for producing high-quality polycrystalline silicon (poly-Si) films on glass by means of solid phase epitaxy (SPE) of evaporated amorphous silicon on aluminium-induced crystallisation (AIC) poly-Si seed layers is introduced. Optical transmission microscope, Raman, UV reflectance spectroscopy and cross-sectional transmission electron microscope measurements show consistently that a transfer of the crystal properties of the AIC poly-Si seed layer into the crystallised amorphous silicon layer has been achieved. A 1-sun open-circuit voltage of 337 mV is realised with a hydrogenated SPE/AIC p–n junction device, which is a promising result considering the early stage of process development. The SPE/AIC method appears well suited for the fabrication of poly-Si thin-film solar cells on glass and, due to the high crystal quality and the much larger average grain size, could lead to improved energy conversion efficiencies compared to Si solar cells made by solid phase crystallisation.

Polycrystalline silicon thin films obtained by Ni-induced crystallization on glass substrate

Thin films of high-quality polycrystalline silicon (poly Si) on different substrates are of interest for manufacturing thin film transistors, solar cells, and image sensors. In this study, we present the results of an investigation of poly Si films on glass, formed by nickel-induced crystallization. The process is based on isothermal annealing in air at 550 and 560°C of co-sputtered Ni+Si films deposited on glass at different substrate temperatures, with and without the application of an electric field to the films. The poly Si films were investigated by Raman spectroscopy and Rutherford back-scattering. It was established that the substrate temperature and the Ni-concentration in the precursor amorphous Si+Ni films are important parameters for nickel-induced crystallization.