Aluminum-induced Crystallization Research Articles

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.

The effects of silicon substrate thickness and annealing temperature on surface coverage for aluminum-induced crystallization of germanium films

This work investigates polycrystalline germanium films for photovoltaic applications that were developed using aluminum-induced crystallization on silicon substrates coated with 500 nm of thermally grown silicon dioxide. Five silicon substrate thicknesses were investigated between 200 and 525 µm and the samples were annealed for 50 hours at either 365 °C, 375 °C, or 385 °C. When annealed, the amorphous germanium films deposited on the 200 µm silicon substrate developed into discrete monocrystalline germanium islands with an average grain size of 10.76 µm, but poor overall surface coverage between 50% and 59%. The germanium films deposited on silicon substrates with thickness between 250 and 375 µm and annealed below 385 °C revealed the highest and most uniform polycrystalline germanium surface coverage between 80% and 97% and demonstrated ~300 nm grain size. The 525 µm silicon substrate exhibited dense island formation at 385 °C as well as high residual aluminum content, and cracks in the film at lower temperatures.

Effect of annealing time on aluminum-induced crystallization of silicon suboxide thin films

Polycrystalline silicon (poly-Si) thin films were obtained by aluminium induced crystallisation of amorphous silicon suboxide (a-SiOx, x = 0.22) via annealing of a-SiO0.22/Al bilayer structures at 550 °C for 4 - 30 h. The a-SiO0.22/Al thickness ratio was approximately 1. According optical microscopy measurements, the crystallized fraction reached the saturation value of 85% after annealing for 20 h. The further increase in the annealing time didn’t lead to an increase in this value. X-ray diffraction measurements revealed that the formed poly-Si had a strong Si (111) preferred orientation.

Aluminum-induced crystallization of silicon suboxide thin films

Polycrystalline silicon (poly-Si) thin films are fabricated by aluminum-induced crystallization (AIC) of amorphous silicon suboxide (a-SiOx, x = 0.22) at 550 °C for 20 h. AIC of a-SiO0.22 via thermal annealing of a-SiO0.22/Al bilayer structures leads to layer exchange with the formation of poly-Si films with (111) preferential orientation and high-crystalline quality-coated with a layer of a mixture of Al, Si, and O atoms with inclusions of silicon nanocrystallites. The initial a-SiO0.22/Al thickness ratio approximately equal to 1 provides the formation of a discontinuous poly-Si film with a crystallized fraction of 85%, as shown by optical microscopy.

Revisiting the interface sensitive selective crystallization in HWCVD a-Si:H/Al bilayer system

In the kinetics of Aluminum Induced Crystallization (AIC) process, the sequence of deposition plays a significant role; specifically, for the hydrogenated amorphous silicon (a-Si:H) films deposited by Hot Wire Chemical Vapour Deposition (HWCVD). We show that while the layer sequence of substrate/a-Si:H/Al crystallizes at a relatively lower temperature of 300 °C in 5 min (relatively faster rate), the layer sequence of substrate/Al/a-Si:H takes about 1 h at 450 °C for crystallization. We have carried out extensive Raman spectroscopy, X-ray diffraction, focused ion beam (FIB) imaging, field emission scanning electron microscopy (FESEM) and glow discharge optical emission spectroscopy (GDOES) to first establish the difference in the kinetics of crystallization in these two layer sequences and then to explain the reasons for the same. We put forward a hypothesis followed by a supportive experimental evidence, using FTIR spectroscopy as a tool, that the high hydrogen concentration at the a-Si:H/Al interface slows down the kinetics in AIC.

Nano-crystalline thin films fabricated by Si-Al co-sputtering and metal induced crystallization for photovoltaic applications

Fabrication of nano-crystalline thin film solar cell by co-sputtering of silicon and aluminum, and metal induced crystallization is reported in this paper. After a brief introduction on magnetron sputtering and aluminum-induced crystallization techniques, it discusses the drawbacks of “aluminum induced layer exchange” concept as the common method for fabricating poly-Si thin films; and proposes Si-Al co-sputtering as a simple alternative fabrication method. Then a three-steps process is introduced for the fabrication of “Al electrode/n-Si wafer/p-Si thin film/Al electrode” solar cell. Material and electrical characterizations of the fabricated p-Si thin films are done by Raman spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and Hall Effect measurement to study the crystallization ratio, grain size, morphology, carrier density and mobility of the prepared films, respectively. The best fabricated samples show a crystallization ratio of 88% and the largest grain size of 68 nm. The hole concentration in the films is in the order of 1018 to 1019 cm−3 with a maximum mobility value of 17 cm2/V s. Then, photovoltaic properties of the fabricated solar cells are determined by under dark and illumination J-V curves, and external quantum efficiency measurements. Dark J-V curve of the best sample represents an ideality factor of 1.39, rectification ratio of 1993 at ±1 V and reverse saturation current density of 62.1 nA/cm2 which confirms the formation of a good p-n junction. The illuminated J-V curve under AM1.5G conditions exhibits a photovoltaic conversion efficiency of 6.02% for the best sample, with a short circuit current density of 24.9 mA/cm2 and open circuit voltage of 456 mV. The external quantum efficiency of this sample reaches to more than 59% at the wavelength of 700 nm. Finally, a simple model is proposed to describe the formation of nano-crystalline Si thin film, deposited by Si-Al co-sputtering, during the thermal annealing.

Impact of boron incorporation on properties of silicon solar cells employing p-type polycrystalline silicon grown by aluminum-induced crystallization

We report on our attempt to increase the carrier density in the p-type polycrystalline silicon (poly-Si) layers grown by aluminum-induced crystallization (AIC) employing a B-doped Si target. Hall measurement revealed that we could obtain a lower-resistance and heavily doped p-type continuous poly-Si thin layer formed by AIC using B-doped a-Si. According to our evaluation of tunnel oxide passivated contact (TOPCon) solar cells with AIC-grown poly-Si, the AIC-TOPCon solar cells fabricated at 570 °C using B-doped a-Si showed higher conversion efficiency of 13.5% than that of 12.8% when using nondoped a-Si. It is considered that the cell characteristics, particularly FF and Voc, were improved owing to the lower series resistance and higher carrier density since both Al and B were successfully incorporated into the AIC-grown poly-Si.

Aluminum Induced Crystallization of <italic>In-Situ</italic> Phosphorus Doped E-Beam Evaporated Silicon Films for High Gauge Factor Piezo-Resistors

A polysilicon film with high gauge factor formed at a low thermal budget of 450 °C is reported in this paper. The polysilicon was formed using in-situ phosphorus doped ultra-high vacuum e-beam evaporated amorphous silicon film as a precursor for aluminum induced crystallization (AIC). The crystallization, electrical and piezo-resistive properties of the resulting films for various phosphorus doping levels have been studied. The results show that the gauge factor of the AIC polysilicon film increases with phosphorous doping level of the precursor. The reduction in doping concentration and formation of large grains with average size of $8.77~\mu \text{m}$ have allowed a gauge factor of 77 to be at obtained with phosphorus doping of $2\times 10^{19}$ cm−3 in the precursor. This, to our knowledge, is the highest gauge factor reported for polysilicon and may lead to post-CMOS monolithic integration of high performance strain sensors.

Study on silicon crystallization with aluminum deposition temperature in the aluminum-induced crystallization process using silicon oxide

Aluminum-induced crystallization (AIC) is one process which increases silicon grain size at low temperatures. In this study, we analyzed the effect of silicon crystallization according to the aluminum deposition conditions in the AIC process using silicon oxide. The initial aluminum layer was analyzed using a field emission-scanning electron microscopy (FE-SEM) after cutting the samples with a focused-ion-beam (FIB). Through FE-SEM, we observed that the aluminum grain size of the original aluminum layer increased in proportion to the aluminum deposition temperature. However, not only aluminum grain size but also surface roughness and porosity of the initial aluminum layer were increased. The initial aluminum layer, according to the deposition temperature, significantly affected the crystallized silicon grain size. The silicon grain size was decreased from 16.97 μm to 7.81 μm according to the increase of the aluminum deposition temperature. This was because the Si diffusion area was increased by the increase of the aluminum surface roughness.

Heteroepitaxy of Highly Oriented GaN Films on Non‐Single Crystal Substrates Using a Si(111) Template Layer Formed by Aluminum‐Induced Crystallization

Growth of epitaxial III‐nitride (AlGaInN) films has long been confined to single crystal substrates which are crystallographically compatible with the hexagonal GaN (0001) surface. However, for lighting, display, and power electronics applications, growth on amorphous substrates such as fused quartz glass or on polycrystalline diamond is desirable. Several approaches to produce c‐plane oriented, textured GaN films on glass have been previously demonstrated, but are process‐intensive and limited in scalability. In this report, we demonstrate the heteroepitaxial growth of textured GaN films on fused quartz and other substrates using thin silicon film templates fabricated by aluminum‐induced crystallization (AIC). The AIC‐Si films have a uniform (>95%) Si (111) oriented surface, enabling well‐developed GaN‐on‐Si epitaxial growth processes to be adapted to new substrates. GaN films grown on fused quartz using metalorganic chemical vapor deposition (MOCVD) have uniformly c‐axis oriented grains, ≈40–50 μm in size, with random in‐plane orientations similar to those of the underlying AIC‐Si template layer. Threading dislocation densities of 8.5 ± 1.8 × 109 cm−2 within regions bounded by grain boundaries are comparable to GaN films grown on single crystal Si (111) substrates. Finally, highly oriented GaN growth on AIC‐Si is also achieved on both oxidized Si (001) substrates and polycrystalline diamond, demonstrating the general applicability of this approach.

Preparation of poly-Si films by inverted AIC process on graphite substrate

Graphite/a-Si/Al laminated structures were deposited on graphite substrate using magnetron sputtering technology. The substrate temperature of amorphous silicon (a-Si) deposition and the Al/Si thickness ratio have strong influences on a-Si crystallization, various conditions were used to investigate the polycrystalline silicon (poly-Si) thin films grown by inverted aluminum-induced crystallization (AIC) for process optimization. In this paper, we established the AIC model to explain the effects of the two factors in the inverted AIC process. The poly-Si thin films were characterized By means of Scanning electron microscope (SEM), X-ray diffraction (XRD) and Raman spectroscopy (Raman), which showed that the samples with strong preferred (111) orientation and high crystallization quality that were favorable for epitaxially growing poly-Si thick film cells.

Study of SiOx thickness effects on aluminum-induced crystallization

The thickness effects of SiOx which was deposited as an intermediate layer between aluminum and silicon were studied on Aluminum-induced crystallization (AIC). The SiOx layer thickness varied from 2 nm to 20 nm and affected the crystallization process of the AIC. In the case of the thin SiOx layer, crystallized silicon morphology showed kinetic-limited aggregation. On the other hand, crystallized silicon processed with the thick SiOx layer showed diffusion-limited aggregation due to slow silicon diffusion velocity. Kinetic-limited aggregation showed large grain. The schematic crystallization model was used to describe the relationship between crystallization and grain size in this paper.

Effect of annealing conditions on polycrystalline silicon produced by the inverted aluminium-induced crystallization of amorphous silicon films on glass substrates

The effect of various annealing treatments on the structure properties of crystalline silicon (c-Si) produced by the inverted aluminium induced crystallization of amorphous silicon (a-Si) films was studied. The surface morphology and grain size of c-Si films were observed by optical microscope and scanning electron microscope. X-ray diffraction and Raman spectroscopy were used to study quantity of Si crystallization due to thermal annealing. Results showed that the c-Si with an average grain size of 54 nm in a (111) orientation was obtained by the thermal annealing at 350 °C for 1 h. Prolonged heat treatment improved Si crystallite quality and increased the average grain size.

Controlling silicon crystallization in aluminum-induced crystallization via substrate plasma treatment

The effect of reactive ion etching using chlorine or fluorine-based plasmas on aluminum-induced crystallization (AIC) of silicon on fused silica glass substrates was investigated with the goal of chemically modifying the substrate surface and thereby influencing the crystallization behavior. Chlorine etching of the glass prior to AIC resulted in six times faster silicon crystallization times and smaller grain sizes than films formed on untreated substrates while fluorine etching resulted in crystallization times double than those on untreated surfaces. The differences in crystallization behavior were attributed to changes in surface chemistry and surface energy of the glass as a result of the plasma treatment as supported by X-ray photoelectron spectroscopy and contact angle measurements. The different surface treatments were then combined with optical lithography to control the location of crystallization on the substrate surface to realize the production of patterned polycrystalline silicon films from initially continuous aluminum and silicon.

On the Discontinuity of Polycrystalline Silicon Thin Films Realized by Aluminum-Induced Crystallization of PECVD-Deposited Amorphous Si

Crystallization of glass/Aluminum (50, 100, 200 nm) /hydrogenated amorphous silicon (a-Si:H) (50, 100, 200 nm) samples by Aluminum-induced crystallization (AIC) is investigated in this article. After annealing and wet etching, we found that the continuity of the polycrystalline silicon (poly-Si) thin films was strongly dependent on the double layer thicknesses. Increasing the a-Si:H/Al layer thickness ratio would improve the film microcosmic continuity. However, too thick Si layer might cause convex or peeling off during annealing. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) are introduced to analyze the process of the peeling off. When the thickness ratio of a-Si:H/Al layer is around 1 to 1.5 and a-Si:H layer is less than 200 nm, the poly-Si film has a good continuity. Hall measurements are introduced to determine the electrical properties. Raman spectroscopy and X-ray diffraction (XRD) results show that the poly-Si film is completely crystallized and has a preferential (111) orientation.

Study of SiOx Thickness Effects on Aluminum-induced Crystallization

The aluminum-induced crystallization (AIC) used SiOx was studied layer as an intermediate layer of aluminum and silicon. SiOx layer thickness was varied from 2 nm to 20 nm, and affected crystallization process of the AIC. In the case of thin SiOx layer, crystallized silicon morphology showed kinetic-limited aggregation. On the other hand, crystallized silicon processed with thick SiOx layer showed diffusion-limited aggregation due to slow silicon diffusion velocity. The kinetic-limited aggregation showed large grain. The crystallized silicon grain size relatively increased at kinetic-limited aggregation. The schematic crystallization model was used to describe a relationship between crystallization and grain size in this paper.

Study of polycrystalline silicon obtained by aluminum-induced crystallization depending on process conditions

In this paper, we have decided to consider an alternative method of producing polycrystalline silicon and study change of its electrophysical characteristics depending on process parameters. As an alternative low-pressure chemical vapor deposition method appears aluminum-induced crystallization (AIC), which allows to obtain a polycrystalline silicon film is significantly larger grain size, thereby reducing contribution of grain boundaries. A comprehensive study of polycrystalline silicon was carried out using a variety of microscopic (OM, SEM) and spectroscopic (RAMAN, XPS) and diffraction (EBSD, XRD) analytic methods. We also considered possibility of self-doping in AIC, result of which was obtained polycrystalline silicon with different resistance. Additionally considered changes in temperature coefficient of resistance depending on technological parameters of AIC process.

Effects of the Si/Al layer thickness on the continuity, crystalline orientation and the growth kinetics of the poly-Si thin films formed by aluminum-induced crystallization

The joint impact of the Si/Al layer thickness on the growth kinetics, the crystalline orientation and the size of the poly-Si grains resulting from aluminum-induced crystallization process is analyzed. It is shown that the surface coverage of resulting poly-Si layers rapidly decreases together with annealing temperature and the Si/Al ratio. The surplus of a-Si over the Al needed to ensure continuity of the poly-Si thin film is in the range of 35%–50% for Al layers thicker than 225nm, but rapidly goes up to 200% as the thickness of the Al layer decreases below 50nm. It is demonstrated that the angular distribution of grain orientations is discrete and shifts towards the {111} direction as the Si/Al increases. It is reported that during an isothermal annealing, the nucleation of Si grains occurs in two steps. Finally, a simple model of the aluminum-induced crystallization process explaining the two-step nucleation is proposed.

Poly-crystalline thin-film by aluminum induced crystallization on aluminum nitride substrate

Thin-film polycrystalline silicon (pc-Si) on foreign (non-silicon) substrates has been researched by various research groups for the production of photovoltaic cells. High quality pc-Si deposition on foreign substrates with superior optical properties is considered to be the main hurdle in cell fabrication. Metal induced crystallization (MIC) is one of the renowned techniques used to produce this quality of material. In the current study, an aluminum induced crystallization (AIC) method was adopted to produce pc-Si thin-film on aluminum nitride (AlN) substrate by a seed layer approach. Aluminum and a-Si layer were deposited using an e-beam evaporator. Various annealing conditions were used in order to investigate the AIC grown pc-Si seed layers for process optimization. The effect of thermal annealing on grain size, defects preferentially crystallographic orientation of the grains were analyzed. Surface morphology was studied using an optical microscope. Poly-silicon film with a crystallinity fraction between 95-100% and an FWHM between 5-6 cm−1 is achievable at low temperatures and for short time intervals. A grain size of about 10 micron can be obtained at a low deposition rate on an AIN substrate. Similarly, Focused ion beam (FIB) also showed that at 425 °C sample B and at 400 °C sample A were fully crystallized. The crystalline quality of pc-Si was evaluated using μ-Raman spectroscopy as a function of annealed conditions and Grazing incidence X-ray diffraction (GIXRD) was used to determine the phase direction of the pc-Si layer. The current study implicates that a poly-silicon layer with good crystallographic orientation and crystallinity fraction is achievable on AIN substrate at low temperatures and short time frames.

Effects of glass substrate coated by different-content buffer layer on the quality of poly-Si thin films

In this work, polycrystalline silicon (poly-Si) thin films were fabricated by aluminum induced crystallization (AIC) technique. SiNx, deposited as a function of NH3/SiH4 ratio, and AZO (Al-doped ZnO) films on glass were used as a buffer layer between glass and Si film. The effect of buffer layer content on the crystallinity of poly-Si thin films was studied by Raman analysis which shows that fully crystallization without stress was achieved for all samples. Moreover, the preferred crystalline orientation and crystallite size of films were deduced by X-ray diffraction (XRD) analysis. The preferred orientation is <100> as independent from the buffer layer content while the crystallite sizes increase up to 48.5 nm by increasing the amount of SiH4. The electrical properties of the films were carried out by four point probe and current–voltage (I–V) analysis. Both techniques demonstrated that the resistivity of the SiNx-based samples is around 0.1 Ωcm. The grain size analysis was accomplished by electron back scattering diffraction (EBSD) measurements. The grain size up to 25 µm was achieved as observed from EBSD images. The results show that the fabrication parameters of SiNx and AZO buffer layers have the great effects on the crystallography of poly-Si films.