Aluminum-induced Crystallization Research Articles

Effect of annealing temperature on the piezo-resistivity in crystalline silicon formed by aluminum-induced crystallization

Effect of annealing temperature on the piezo-resistivity in crystalline silicon formed by aluminum-induced crystallization

Thermokinetic Study of Aluminum-Induced Crystallization of a-Si: The Effect of Al Layer Thickness.

The effect of the aluminum layer on the kinetics and mechanism of aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) in (Al/a-Si)n multilayered films was studied using a complex of in situ methods (simultaneous thermal analysis, transmission electron microscopy, electron diffraction, and four-point probe resistance measurement) and ex situ methods (X-ray diffraction and optical microscopy). An increase in the thickness of the aluminum layer from 10 to 80 nm was found to result in a decrease in the value of the apparent activation energy Ea of silicon crystallization from 137 to 117 kJ/mol (as estimated by the Kissinger method) as well as an increase in the crystallization heat from 12.3 to 16.0 kJ/(mol Si). The detailed kinetic analysis showed that the change in the thickness of an individual Al layer could lead to a qualitative change in the mechanism of aluminum-induced silicon crystallization: with the thickness of Al ≤ 20 nm. The process followed two parallel routes described by the n-th order reaction equation with autocatalysis (Cn-X) and the Avrami-Erofeev equation (An): with an increase in the thickness of Al ≥ 40 nm, the process occurred in two consecutive steps. The first one can be described by the n-th order reaction equation with autocatalysis (Cn-X), and the second one can be described by the n-th order reaction equation (Fn). The change in the mechanism of amorphous silicon crystallization was assumed to be due to the influence of the degree of Al defects at the initial state on the kinetics of the crystallization process.

Temperature dependent aluminum induced crystallization of amorphous germanium thin films

In this work, we present the sequential growth of metal–semiconductor bilayer structure on HF-treated textured glass substrates by thermal evaporation method. The process of aluminum-induced crystallization of amorphous germanium thin films was critically observed under heat treatment. Temperature assisted structural modifications triggered the nucleation phenomenon for controlled grain growth which transformed the nature of Ge-thin film from amorphous-to-polycrystalline. Interestingly, the electrical resistivity also decreased by subsequent annealing which promoted the conduction mechanism. Optical measurements indicated a dwindling in band tail states and related structural defects, leading to better crystallinity.

Electric-field-enhanced aluminum-induced crystallization of amorphous silicon thin film using decreasing stepwise current method

Electric-field-enhanced aluminum-induced crystallization (AIC) was applied to the crystallization of amorphous Si (a-Si) using a stepwise current method with two to four decreasing steps. In AIC, Si diffuses into an Al layer, which generates a layer exchange from an a-Si/Al layer to an Al/polycrystalline Si (p-Si) layer. This increases the electrical resistivity of the Al/p-Si layer. The stepwise decreasing current was an attempt to overcome the limitations of electric-field-enhanced AIC using a constant current. The samples were fabricated by depositing a 200 nm-thick a-Si layer and sputtering a 300 nm-thick Al layer onto an Eagle XG glass substrate. In-situ resistance and reflectivity were measured to monitor the AIC mechanism and layer exchange. The reflectivity measurements were compared with thin-film optics calculations on the a-Si/Al and Al/p-Si layers to indicate the growth of p-Si. The temperature variations during the heating process were supported by a numerical analysis. The crystallinity of produced polycrystalline silicon (p-Si) was verified by a Raman peak at around 519–520 cm−1. The stepwise current supply increased the crystallization time and improved the crystallinity of the produced p-Si compared to the constant current method.

Influence of Temperature of Silicon Dioxide Powder Annealing on the Aluminum-Induced Crystallization of Polycrystalline Silicon

Influence of Temperature of Silicon Dioxide Powder Annealing on the Aluminum-Induced Crystallization of Polycrystalline Silicon

Influence of current density on constant current electric field enhanced aluminum induced crystallization of amorphous silicon thin films

In this study, electric field-enhanced aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) thin films was investigated considering various current densities. Constant electric currents of 4, 5, and 6 A were applied to assess the Joule heating characteristics during specific durations (40, 15.5, and 9 s). The samples were prepared by depositing a 200-nm thick a-Si layer on a glass substrate and sputtering a 300-nm thick aluminum (Al) layer onto the a-Si layer. During the AIC process, layer exchange occured via the diffusion of silicon (Si) into the Al layer. This phenomenon was verified by the in-situ reflectivity, which was in excellent agreement with thin-film optics calculations. During the annealing, processing temperatures were lower than 470 °C, which is less than the Al-Si eutectic temperature of 577 °C. The Raman peak observed near a wavenumber of 519 cm−1 revealed the formation of polycrystalline silicon films through Joule heating processes. Field-effect scanning electron microscopy images were captured and analyzed to investigate the surface morphology of the samples, specifically focusing on grain growth.

Rapid electric field-enhanced crystallization of amorphous silicon thin films with an aluminum layer

A rapid aluminum induced crystallization (AIC) method of amorphous silicon (a-Si) thin film was suggested. A constant electrical current is supplied to an aluminum (Al) layer deposited on the a-Si thin film and anneals the Al/Si stack fast by Joule-heating. The Joule heating is presumed to expedite a-Si thin film crystallization. To investigate the crystallization mechanism, an in-situ temperature and reflectivity measurement was adopted during the process. The maximum crystallization temperature was 744.15 K, which was found to be similar to the typical furnace AIC process temperature and less than the Al/Si eutectic temperature of 850 K. The measured and calculated surface reflectivity demonstrated the rapid Al induced layer exchange behavior between Si and Al layers. The entire process was accomplished for only a few dozens of seconds. This rapid layer exchange is caused by sudden increase of temperature-dependent Al diffusion coefficient in a Si layer and additional electric field enhancement. The Raman peak of 519 cm−1 verified the formation of polycrystalline silicon (p-Si) after the Al etching process. Scanning electron microscopy images showed that the resulting p-Si structure was porous with an average pore size of around 1.2 μm.

Controlled autocrystallization in magnetron co-sputtered Si–Al films

Results of investigations on Al and Si segregation effects in μm-thick magnetron co-deposited Si–Al films are reported. Three characteristic samples of Si–Al films were studied differing by the surface resistance (high, low and intermediate). It was found that increase of Al content in the film results in its segregation to a separate crystalline phase, which is accompanied by emergence of c-Si phase, fraction of which is also increased with Al content. It was demonstrated that silicon crystallization process is governed by redistribution of components in the depth of the film, which can be attributed to the known aluminum-induced crystallization (AIC) process. This redistribution of Al and Si crystalline phases in amorphous matrix can be utilized to tailor the electrophysical properties of the resulting coatings which for applications in perspective mm-band RF devices components.

Fabrication of BaSi2 homojunction diodes on Nb-doped TiO2 coated glass substrates by aluminum-induced crystallization and two-step evaporation method

BaSi2 homojunction diodes on Nb-doped TiO2 (TiO2:Nb) coated glass substrates were fabricated using aluminum-induced crystallization (AIC) and a two-step evaporation method. From Raman scattering spectra, the growth of BaSi2 on TiO2:Nb was confirmed when the thickness of poly-Si grown by AIC (AIC-Si) was more than 150 nm. The partial formation of BaSi2 diodes was confirmed from the samples prepared at temperature during AIC T AIC = 475 °C–525 °C. The long-wavelength edge of photoresponsivity of the diodes was located around 950 nm, which corresponds to the bandgap of BaSi2 of 1.3 eV, suggesting that this photocurrent is derived from BaSi2 thin films. At T AIC = 500 °C, the maximum value of photoresponsivity was obtained. Since the largest grains in AIC-Si were also obtained at T AIC = 500 °C, these results suggest that larger grain of AIC-Si leads to the improvement of the quality of BaSi2 thin films themselves and the performance of BaSi2 diodes.

P-type poly-Si/SiO contact by aluminium-induced crystallization of amorphous silicon

In this work, we explored aluminium-induced crystallization (AIC) of amorphous Si (a-Si) as a low-temperature alternative to make p-type poly-Si/SiOx contacts for Si solar cells. In this approach, a stack of Al and a-Si deposited on a SiOx-passivated Si wafer surface is annealed at temperatures below 577 °C (the eutectic temperature of Al–Si mixture), which induces the exchange of Al and Si layer positions in the stack and the simultaneous crystallization of a-Si to Al-doped p-type poly-Si, potentially leading to the formation of an Al/poly-Si/SiOx passivating contact. The Al-doped AIC poly-Si has low carrier concentration, and thus to compensate for this, we used highly B-doped a-Si for AIC and could achieve a tenfold increase in the carrier concentration. We found that the typical thickness of 1.5 nm of the passivating SiOx layer is too low for this AIC-based approach, as Al reduces SiOx during the process and degrades its passivation quality. By starting with a thicker SiOx layer, we could demonstrate that AIC poly-Si/SiOx contacts with low contact recombination is achievable. However, the contact characteristic remains non-ohmic. This can be improved by extended annealing after AIC, during which Al diffuses into the Si substrate. The Al diffusion, however, damages the SiOx layer and in turn severely increases contact recombination. We conclude that with AIC poly-Si/SiOx contacts the process window to achieve low contact resistivity and low contact recombination seems to be very narrow, if existing at all.

Effect of annealing temperature on the kinetics of aluminum-induced crystallization of amorphous silicon suboxide thin films

In this work, the kinetics of aluminum-induced crystallization (AIC) of non-stoichiometric silicon oxide a-SiO0.25 was investigated for the case of annealing at temperatures of 370, 385 and 400oC, as a result of which thin films of polycrystalline silicon were obtained. It is shown that, at low annealing temperatures, the surface morphology of the crystalline material is represented by dendritic structures corresponding to the growth model with diffusion-limited aggregation. In addition, with increasing annealing temperature, the nucleation density increases from 3 to 53 mm-2. From the Arrhenius plot, the activation energy of the a-SiO0.25 AIC was obtained for the first time and appeared to be 3.7± 0.4 eV. Keywords: aluminum-induced crystallization, silicon suboxide thin films, polycrystalline silicon, activation energy.

Layer exchange during aluminum-induced crystallization of silicon suboxide thin films

Aluminum-induced crystallization of a-SiO1.8 in the layer exchange mode was carried out. Annealing of a “SiO2 substrate/Al/a-SiO2 membrane/a-SiO1.8” stacked structure at 550 °C led to the formation of a continuous polycrystalline Si (poly-Si) thin film on the substrate, which is characteristic of the layer exchange process. However, the upper layer formed in the process had structural features that were not previously observed for the pure and slightly oxidized amorphous silicon as a Si-containing thin-film precursor. The high oxygen content in the system led to the non-uniform transformation of a-SiO1.8 layer, namely, to the formation of Al oxide islands of submicron size nearby the interface with poly-Si thin film as a result of the complete oxidation of the Al layer, whereas the other part of the upper layer was still unchanged. In addition, the embedding of Al into the SiO2 substrate accompanied by its oxidation was observed.

Влияние температуры отжига на кинетику процесса алюминий-индуцированной кристаллизации тонких пленок аморфного субоксида кремния

In this work, the kinetics of aluminum-induced crystallization (AIC) of non-stoichiometric silicon oxide a-SiO0.25 was investigated for annealing temperatures of 370, 385 and 400 °C, as a result of which thin films of polycrystalline silicon were obtained. It is shown that for low annealing temperatures, the surface morphology of the crystalline material is represented by dendric structures corresponding to the growth model with diffusion-limited aggregation. In addition, with an increase in the annealing temperature, the nucleation density increases from 3 to 53 mm–2. From the Arrhenius plot, the activation energy of the AIC process of a-SiO0.25 was obtained for the first time, which was 3.7±0.4 eV.

Piezoresistive pressure sensor using nanocrystalline silicon thin film on flexible substrate

A flexible pressure sensor fabricated at processing temperatures as low as 300°C using piezoresistive elements of nanocrystalline silicon (nc-Si) on flexible polyimide substrate is reported. Aluminium induced crystallization (AIC) of Hot Wire Chemical Vapour deposited a-Si:H films is used to obtain the nc-Si films with almost 100 % crystalline fraction as revealed by Raman Spectroscopy, and with a measured piezoresistive gauge factor of 41±3. A novel processing method is adopted, wherein the photoresist itself acts as a mask for the transfer of the pattern on the polyimide diaphragm. This enables bypassing the micromachining step, which otherwise is unavoidable for patterning the sensing elements. A full Wheatstone bridge configuration of nc-Si piezoresistors is realized with acceptable linear response of the output signal for an applied pressure range up to 50 psi and the sensitivity is found to be 183.10±3.48 mV/mA/MPa. A novel pressure jig is also designed with the capability to characterize four sensors simultaneously with an arrangement to apply hydrostatic pressure of compressed air.

Fabrication of Nanocrystalline Silicon Thin Films Utilized for Optoelectronic Devices Prepared by Thermal Vacuum Evaporation.

Metal-induced crystallization of amorphous silicon is a promising technique for developing high-quality and cheap optoelectronic devices. Many attempts tried to enhance the crystal growth of polycrystalline silicon via aluminum-induced crystallization at different annealing times and temperatures. In this research, thin films of aluminum/silicon (Al/Si) and aluminum/silicon/tin (Al/Si/Sn) layers were fabricated using the thermal evaporation technique with a designed wire tungsten boat. MIC of a:Si was detected at annealing temperature of 500 °C using X-ray diffraction, Raman spectroscopy, and field emission scanning electron microscopy. The crystallinity of the films is enhanced by increasing the annealing time. In the three-layer thin films, MIC occurs because of the existence of both Al and Sn metals forming highly oriented (111) silicon. Nanocrystalline silicon with dimensions ranged from 5 to 300 nm is produced depending on the structure and time duration. Low surface reflection and the variation of the optical energy gap were detected using UV–vis spectroscopy. Higher conductivities of Al/Si/Sn films than Al/Si films were observed because of the presence of both metals. Highly rectifying ideal diode manufactured from Al/Si/Sn on the FTO layer annealed for 24 h indicates that this device has a great opportunity for the optoelectronic device applications.

Scalable fabrication of GaN on amorphous substrates via MOCVD on highly oriented silicon seed layers

Abstract The integration of gallium nitride with low-cost, transparent substrates such as glass and crystallographically incompatible substrates such as Si(0 0 1) has been long desired for III-N electronics. Here the authors demonstrate how GaN growth on uniformly 〈1 1 1〉 oriented silicon seed layers fabricated by aluminum-induced crystallization provide a straightforward method for realizing large area GaN growth on these substrate by adopting well-developed GaN on Si(1 1 1) growth processes. Critical factors for promoting high quality GaN growth are identified, and remaining steps necessary for realizing GaN devices are discussed.

Surface-orientation control of silicon thin films via aluminum-induced crystallization on monocrystalline cubic substrates

Abstract Silicon thin films with uniform Si(111) surface orientation (>95%) have been fabricated on amorphous substrates through aluminum-induced crystallization. However, Si(100) oriented films have not been fabricated with nearly the same degree of uniformity. For Al and Si thicknesses below 50 nm, Si nucleation has been previously proposed to initiate at the Al/substrate interface, suggesting that a cubic (100) oriented substrate could promote highly uniform Si(100) oriented thin film fabrication. Using two single crystalline (100) oriented substrates, strontium titanate and germanium, we demonstrate how substrate surface orientation and surface termination affect the crystallization and preferential orientation of silicon thin films fabricated by aluminum-induced crystallization.

Microstructure of GaAs thin films grown on glass using Ge seed layers fabricated by aluminium induced crystallization

We perform the growth of GaAs epilayers by molecular beam epitaxy (MBE) on Ge pseudo-substrates obtained by the aluminium induced crystallisation (AIC) of thin amorphous Ge layers deposited on silica. Despite the apparent uniformity of the AIC-Ge layer, large domains (more than 50 µm wide) previously thought to be monocrystalline are found to actually consist in smaller grains (500 to 1000 nm wide), separated by low angle grain boundaries. These defects are transferred during epitaxy to the GaAs layer and degrade the quality of the III–V material. In our growth conditions, the MBE results in the selective deposition of thin GaAs layers on Ge with respect to the silica support, but selectivity is progressively lost with increasing layer thickness.

Fabrication of polycrystalline silicon thin films from a-SiOx via the inverted aluminum-induced layer exchange process

A novel fabrication method of polycrystalline silicon (poly-Si) films by aluminum-induced crystallization of amorphous silicon suboxide (a-SiO0.2) material via the inverted aluminum-induced layer exchange (ALILE) mechanism on glass substrates is presented. The presence of oxygen in the system prevented the formation of columnar crystalline Si precipitates (hillocks) in the bottom layer of the structure even at a process temperature of 550 °C. At the same time, this layer, consisting of Al, Si, and O atoms, contained nanocrystalline Si inclusions and a layer of initial a-SiOx with a thickness of about 60 nm, which was called the “a-SiOx residual layer”. The poly-Si formed had a preferred (1 1 1) crystal orientation and an average crystallite size of 4.5 µm.

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.