Aluminum-induced Crystallization Of Amorphous Silicon Research Articles

Aluminum induced crystallization of amorphous silicon thin films with assistance of electric field for solar photovoltaic applications

In this current research, the aluminum-induced crystallization of amorphous silicon (a-Si) was studied. Particularly, a-Si film covered with a very thin film of pure aluminum was exposed to a constant electrical voltage and high temperature. Every sample was subjected to annealing for 15min with the application of an external electrical voltage. Nine treatments of different annealing temperatures and voltages were involved in this research. The levels of annealing temperature were 250°C, 300°C, and 350°C, and the levels of external voltage were of 0V, 2V, or 20V. After the annealing, the electrical, structural, and morphological properties of the a-Si layer have been investigated. Energy dispersive X-ray spectroscopy proved that there is diffusion of aluminum atoms inside the a-Si layer. X-ray diffraction showed that the crystallization depends on both temperatures and external voltage. We also noted that the electrical resistivity decreased significantly with annealing temperature and external voltage increasing. Moreover, the Hall mobility increased sharply with the increasing of annealing temperature and applied voltage, while the carrier concentration deceased slowly with increasing of both of them.

Study of growing zinc oxide on polycrystalline silicon/glass substrate prepared by aluminum-induced crystallization of amorphous silicon

Zinc oxide (ZnO) was grown on a polycrystalline silicon (pc-Si)/glass substrate prepared by the aluminum-induced crystallization (AIC) of amorphous silicon. ZnO directly grown on the AIC-pc-Si exhibits many defects with a distribution unlike that of conventional ZnO; the defect density is highest near the conduction band and decreases near the valence band. The defects can be greatly suppressed by depositing an epitaxial silicon (Epi-Si) layer on the AIC-pc-Si. The resulting defect distribution is similar to that of ZnO grown on a crystalline Si (c-Si) substrate. X-ray diffraction shows that the epitaxial silicon layer can improve the crystallization of ZnO. However, the AIC-pc-Si and Epi-Si/AIC-pc-Si substrates exhibit rougher surfaces; consequently, the ZnO has a more random structure and lower density than that grown on a c-Si substrate. X-ray photoelectron spectroscopy demonstrates that as compared to the ZnO grown on the AIC-pc-Si, the defect ratio could be dramatically reduced from 0.70 to 0.56 by depositing an epitaxial silicon layer on AIC-pc-Si, and the defect ratio is about the same as that (0.55) for the ZnO grown on a c-Si substrate.

The role of H-plasma on aluminum-induced crystallization of amorphous silicon

We propose a technique to improve and accelerate aluminum-induced crystallization (AIC) by hydrogen plasma. Raman spectroscopy and secondary ion mass spectrometry of crystallized poly-silicon thin films show that hydrogen plasma radicals reduce the crystallization time of AIC. This technique shortens the annealing time from 10 to 4 h and increases the Hall mobility from 22 to 42 cm2/V s. The possible mechanism of AIC assisted by hydrogen radicals is also discussed.

Controlled aluminum-induced crystallization of an amorphous silicon thin film by using an oxide-layer diffusion barrier

Aluminum-induced crystallization (AIC) of amorphous silicon with an Al2O3 diffusion barrier was investigated for controlling Si crystallization and preventing layer exchange during the annealing process. An Al2O3 layer was deposited between the a-Si and the Al films (a-Si/Al2O3/Al/Glass) and was blasted with an air spray gun with alumina beads to form diffusion channels between the Si and the Al layers. During the annealing process, small grain SixAl seeds were formed at the channels. Then, the Al2O3 diffusion barrier was restructured to close the channels and prevent further diffusion of Al atoms into the a-Si layer. A polycrystalline Si film with (111), (220) and (311) crystallization peaks in the X-ray diffraction pattern was formed by annealing at 560 °C in a conventional furnace. That film showed a p-type semiconducting behavior with good crystallinity and a large grain size of up to 14.8 µm. No layer conversion occurred between the Si and the Al layers, which had been the fundamental obstacle to the applications in the crystallization of a-Si films by using the AIC method.

Superhydrophilic surface on Cu substrate to enhance lubricant retention

The loss of lubricant from the surfaces of critical machine parts often leads to friction and heat-related failures which can cause catastrophic damage to machinery. In this study, the effect of micro- and nano-texturing on lubricant retention was investigated. Micro-textures were produced by sandblasting Cu substrates, while nano-textures were created by aluminum-induced crystallization of amorphous silicon. The topography and the wettability of the textured surfaces were characterized by means of scanning electron microscopy and a video-based optical contact angle measuring system, respectively. The lubricant retention on the textured surfaces was investigated using a dynamic method. It was found that superhydrophilic surfaces created by the combined micro- and nano-texturing technique can significantly enhance the lubricant retention on Cu substrates.

Effects of Al film thickness and annealing temperature on the aluminum-induced crystallization of amorphous silicon and carrier mobility

Si crystallization arising in Al(20, 50, 150 nm)/amorphous silicon (a-Si) (200 nm)/glass specimens prepared by the aluminum-induced crystallization (AIC) technique and the rapid thermal annealing (RTA) process is investigated. Compared with specimens with amorphous aluminum (a-Al), specimens with micro-crystallized aluminum (μc-Al) underwent Si crystallization at either a lower annealing temperature or with a thinner Al film. Si crystallization started only when the compressive annealing stress ( σ an) of the composite film formed in the annealing process reached a critical value which is independent of the Al film thickness. The compressive stress in specimens increased with increasing annealing temperature and Al film thickness. With decreasing Al film thickness the Si crystallization density increased in proportion to the hillock density, but in inverse proportion to the average micro-crystallized silicon (μc-Si) grain size. The product value ( R ∗) of the mean area and the number of nano voids in a unit area increased with increasing difference between the stresses formed during and after annealing (Δ σ = σ f − σ an). For the Al(50 nm)/a-Si(200 nm)/glass specimens an increase in annealing temperature resulted in a decrease in R ∗ but an increase in the mean μc-Si size. For the Al(150 nm)/a-Si(200 nm)/glass specimens the behavior is exactly opposite to that for the Al(50 nm)/a-Si(200 nm)/glass specimens. For all 10 specimens an increase in R ∗ led to an increase in the tensile Δ σ and a decrease in the mean μc-Si size. The carrier mobility of a specimen decreased with increasing tensile Δ σ, irrespective of the Al film thickness. The mean μc-Si size and thus the carrier mobility increased significantly with increasing Al film thickness.

Effects of a Native Oxide Layer on the Carrier Mobility and Void Formation at Aluminum-Induced Crystallization of Amorphous Silicon

In the present study, the effects of the native oxide layer that forms between an amorphous Si (a-Si) layer and a microcrystalline Al layer in the coating processes on the crystallizations of a-Si, the microstructure at the mixing layer [ and microcrystalline Si at the a-Si layer], and the carrier mobility of the film produced by aluminum-induced crystallization are investigated. The effect of the film on Si crystallization was also investigated by varying the thickness of the Al film in the specimens without a native oxide layer. The absence of a native oxide layer at the interface of the and a-Si layers allowed Si crystallizations at a lower annealing temperature and a shorter annealing time, and increased the carrier mobility significantly. A sufficiently large film thickness was required for Si crystallization in the specimens without a native oxide layer; however, an excessive thickness deteriorated the crystallization behavior. The native oxide layer detected using X-ray photoelectron spectroscope was a barrier for the penetration of into the a-Si layer. Si crystallizations were often accompanied by voids of various sizes and densities. The carrier mobility generally decreased with increasing void size and density of voids.

Thin-film monocrystalline-silicon solar cells made by a seed layer approach on glass-ceramic substrates

Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. One way to create crystalline-silicon thin films on non-silicon substrates is to use the so-called “seed layer approach”, in which a thin crystalline-silicon seed layer is first created, followed by epitaxial thickening of this seed layer. In this paper, we present the first solar cell results obtained on 10-μm-thick monocrystalline-silicon (mono-Si) layers obtained by a seed layer approach on transparent glass-ceramic substrates. The seed layers were made using implant-induced separation and anodic bonding. These layers were then epitaxially thickened by thermal CVD. Simple solar cell structures without integrated light trapping features showed efficiencies of up to 7.5%. Compared to polycrystalline-silicon layers made by aluminum-induced crystallization of amorphous silicon and thermal CVD, the mono-Si layers have a much higher bulk diffusion lifetime.

A Reduced Mask-Count Technology for Complementary Polycrystalline Silicon Thin-Film Transistors With Self-Aligned Metal Electrodes

The inexpensive glass substrate for building conventional low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs) imposes a ceiling on the TFT processing temperature. This results in a reduced efficiency of dopant activation and a high source/drain series resistance. A technique based on aluminum-induced crystallization of amorphous silicon has been applied to fabricate TFTs with low-resistance self-aligned metal electrodes (SAMEs). While at least two masked implantation steps are typically used for constructing the doped source and drain regions of conventional n- and p-channel TFTs in a complementary metal-oxide-semiconductor circuit technology, it is currently demonstrated that complementary SAME poly-Si TFTs can be constructed using a combination of a masked and a blanket source and drain implantation steps. The decrease in mask count reduces process complexity and cost. Control of ion channeling is the enabling factor behind the successful demonstration of the technology.

Production of a superhydrophilic surface by aluminum-induced crystallization ofamorphous silicon

The wettability of glass substrates textured via aluminum-induced crystallization(AIC) of amorphous silicon (a-Si) was studied. It was found that a stablesuperhydrophilic surface could be produced by AIC of a-Si. This research suggeststhat densely distributed micron-sized silicon islands with nanoscale siliconspikes produced by the technique of AIC of a-Si are responsible for the stablesuperhydrophilicity. The study also shows that these superhydrophilic surfacescan be easily converted to superhydrophobic ones by means of coating withC4F8.

Surface-nano-texturing by aluminum-induced crystallization of amorphous silicon

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) has been studied extensively to produce large poly-Si grains for electronic and photovoltaic applications such as thin-film transistors, solar cells, and display panels. This paper reports a novel use of AIC of a-Si technique in nano-scale surface-texturing for tribological applications. Various nano-textured samples were fabricated by using AIC of a-Si technique employing different a-Si thickness and annealing conditions. The crystalline properties of these nano-textured surfaces (NTSs) were then characterized using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM), and atomic force microscope (AFM). The adhesion and friction properties of the NTSs and a smooth Si surface were studied by a Triboindentor. These characterizations show that the nano-textures on the NTSs were poly-silicon crystallites and the NTSs significantly reduced the adhesion and friction forces compared to the smooth surface. Both texture height and density were found to affect the adhesion and friction performances of the NTSs.

Self-Assembly of Si Nanoparticles Produced by Aluminum-Induced Crystallization of Amorphous Silicon Film

Aluminum-induced crystallization of amorphous silicon (a-Si) was used to rapidly grow aluminum-doped silicon nanowires (SiNWs) that are parallel to the substrate surfaces on which they are grown. The study reveals that both aluminum thickness and native silicon oxide between Al and a-Si play critical roles in the formation of the nanostmctures. The research results suggest that the rapid in-plane growth of these SiNWs can be attributed to self-assembly of nanosilicon islands, which is different from traditional SiNWs growth mechanisms such as vapor-liquid-solid.

A Novel Bonding Technique Using Metal-Induced Crystallization of Amorphous Silicon

AbstractThis study investigates the used of aluminum-induced crystallization of amorphous silicon is a potential bonding mechanism for a sandwich stack of films between two silicon substrates. Similar procedures using copper diffusion bonds have been in use, but these require temperatures as high as 400°C. Using the crystallization of amorphous silicon as the bonding mechanism has allowed the bonding temperature to be lowered by more than 100 K. Fracture experiments for a low-k material were conducted, and the results using amorphous silicon bonding was compared to epoxy bonding control experiments. Essentially identical results were obtained for the two bonding mechanisms. Low-temperature bonding techniques are of great interest to future progress in the microelectronics industry, and these results are promising advances.

Amorphous Silicon Thickness Effect on Formation of Silicon Nanostructures by Aluminum-Induced Crystallization of Amorphous Silicon

This paper reports the studies of amorphous silicon (a-Si) thickness on the formation of aligned silicon nanostructures (ASiNSs) produced by aluminum-induced crystallization (AIC) of plasma-enhanced chemical vapor deposited a-Si. By varying a-Si thickness, the authors not only show that the thickness of an a-Si film has a significant impact on the formation of the ASiNS but also demonstrate that self-assembly is one of mechanisms that control the growth of such ASiNSs. This paper, for the first time, demonstrates that one can use AIC of a-Si to grow very straight-shaped ASiNS.

Wetting and crystallization at grain boundaries: Origin of aluminum-induced crystallization of amorphous silicon

It has been shown experimentally that the grain boundaries in aluminium in contact with amorphous silicon are the necessary agents for initiation of the crystallization of silicon upon annealing temperatures as low as 438K. Thermodynamic analysis has shown (i) that Si can “wet” the Al grain boundaries due to the favorable Si∕Al interface energy as compared to the Al grain-boundary energy and (ii) that Si at the Al grain boundaries can maintain its amorphous state up to a thickness of about 1.0nm. Beyond that thickness crystalline Si develops at the Al grain boundaries.

Silicon Nanowires by Aluminum-Induced Crystallization of Amorphous Silicon

This paper discusses controlled studies on the effect of stress on the formation of silicon nanowires (SiNWs) produced by aluminum-induced crystallization (AIC) of plasma-enhanced chemical vapor deposited amorphous silicon (a-Si:H). The impact of stress on the growth of SiNWs was completely decoupled from other factors such as a-Si:H and Al deposition parameters and rapid thermal annealing conditions. This study shows, that AIC of a-Si:H can be used to fabricate SiNWs through the combination of rapid thermal annealing and stress adjustment.

Thin-film polycrystalline-silicon solar cells on high-temperature glass based on aluminum-induced crystallization of amorphous silicon

AbstractEfficient thin-film polycrystalline-silicon (pc-Si) solar cells on foreign substrates could lower the price of photovoltaic electricity. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening at high temperatures seems a good way to obtain efficient pc-Si solar cells. Due to its transparency and low cost, glass is well suited as substrate for pc-Si cells. However, most glass substrates do not withstand temperatures around 1000°C that are needed for high-temperature epitaxial growth. In this paper we investigate the use of experimental transparent glass-ceramics with high strain-point temperatures as substrates for pc-Si solar cells. AIC seed layers made on these substrates showed in-plane grain sizes up to 16 μm. Columnar growth was observed on these seed layers during high-temperature epitaxy. First pc-Si solar cells made on glass-ceramic substrates in substrate configuration showed efficiencies up to 4.5%, fill factors up to 75% and open-circuit voltage (Voc) values up to 536 mV. This is the highest Voc reported for pc-Si solar cells on glass and the best cell efficiency reported for cells made by AIC on glass.

Aluminum-induced crystallization of amorphous silicon: Influence of temperature profiles

Annealing of amorphous silicon/aluminum layer stacks below the eutectic temperature leads to a layer exchange and concurrent crystallization of silicon. The resulting polycrystalline silicon layers are of great interest for large area thin film devices. This paper reports about the influence of annealing temperature profiles on the layer exchange process. It is shown that temperature profiles have pronounced influence on the nucleation process. The obtained results are helpful for a better understanding of the physics of the layer exchange process and for an improvement of the layer exchange process by allowing for a reduction of the process time with a simultaneous increase in grain size. It is shown in particular that the characteristic suppression of nucleation is caused by Si-depletion around the growing Si-grains.

Experimental study of aluminum-induced crystallization of amorphous silicon thin films

This work was an experimental study of the aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) for the fabrication of polycrystalline silicon film. The a-Si film was deposited on silicon wafer by low pressure chemical vapor deposition (LPCVD) technique. Aluminum was sputtered on to the a-Si film at different thicknesses. The samples were annealed for 3 h at different temperatures from 250 to 550 °C. The annealed silicon films were analyzed with emphasis on their crystallinity and morphology. Results showed that in the presence of aluminum, a-Si film started crystallization at a temperature as low as 250 °C. However, high crystallization rate would be achieved only when the annealing was done at temperatures higher than 350 °C. For practical applications, this temperature might well be the lower limit in AIC method for crystallization of silicon. The thickness of aluminum film was found to play a critical role that dictated the extent of crystallization and the preferred orientation of the resulting polycrystalline thin film.

Aluminum-induced crystallization of amorphous silicon: preparation effect on growth kinetics

The kinetics of Al-induced crystallization of amorphous silicon (a-Si) are studied by variation of the thickness of an Al-oxide layer between the initial Al and a-Si films. The results show that the thickness of this Al-oxide layer strongly determines the nucleation time and the number of Si-grains (nucleation) and has only little influence on the growth velocity of the grains. The diameter of the grains increases in good approximation linearly with time and is strongly influenced by the annealing temperature. The activation energies for the growth velocity v g and the nucleation time t n were determined to be 1.8 and 1.9 eV, respectively. The general behavior suggests that the process kinetics of the aluminum-induced layer exchange are determined by the silicon diffusion across the interface which can be altered by the thickness of the oxide layer.