Aluminum-induced Crystallization Research Articles

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.

The effect of high concentration of phosphorus in aluminum-induced crystallization of amorphous silicon films

Polycrystalline silicon (poly-Si) film prepared by aluminum-induced crystallization (AIC) of highly phosphorus (P)-doped amorphous silicon has been proposed for suppressing Si islands and voids. Si islands on the poly-Si surface and voids at the interface between the poly-Si and the glass are caused by Si atoms' out-diffusion toward the top surface during the AIC process, which also degrades the electrical characteristics of the film. High-P dopants can form P precipitations and retard the out-diffusion of Si atoms, hence suppressing Si islands and voids. Also, P precipitations can enhance the crystallization rate and grain growth. P dopants act as donors and compensate for the p-type poly-Si film, which is formed by Al doping during the AIC process. The hole concentration of undoped poly-Si is reduced by approximately three orders to 7×1012cm−3. Such near-neutral poly-Si films could be applied as intrinsic (i) layers in p-i-n structures to explore solar cell applications. Also, the enhanced quality of P-doped AIC-Si increases the mobility by five times owing to reduced scattering and trap density.

Fabrication of Si(111) crystalline thin film on graphene by aluminum-induced crystallization

We report the fabrication of a Si(111) crystalline thin film on graphene by the aluminum-induced crystallization (AIC) process. The AIC process of Si(111) on graphene is shown to be enhanced compared to that on an amorphous SiO2 substrate, resulting in a more homogeneous Si(111) thin film structure as revealed by X-ray diffraction and atomic force microscopy measurements. Raman measurements confirm that the graphene is intact throughout the process, retaining its characteristic phonon spectrum without any appearance of the D peak. A red-shift of Raman peaks, which is more pronounced for the 2D peak, is observed in graphene after the crystallization process. It is found to correlate with the red-shift of the Si Raman peak, suggesting an epitaxial relationship between graphene and the adsorbed AIC Si(111) film with both the graphene and Si under tensile strain.

Crystallized Si layer properties of novel aluminum-induced crystallization

Aluminum-induced crystallization (AIC) is usually used for making silicon seed layer. In this paper, we investigated the AIC process varied with different diffusion barrier materials. The barrier materials were native Al oxide, directly deposited Al2O3, and SiO2 in AIC process. The effects of these diffusion barrier materials were analyzed by using electron backscatter diffraction (EBSD), Raman spectroscopy, field emission scanning electron microscope (FE-SEM). The results showed that the case of native Al oxide showed diffusion-limited aggregation which usually resulted in polycrystalline structure. On the other hand, the case of SiO2 layer showed kinetic-limited aggregation, which generally resulted in the mono-crystalline structure. Therefore, we introduced the novel AIC process with SiO2 layers.

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.

Aluminum-Induced Crystallization of Hydrogenated Amorphous Silicon Thin Films With Assistance of Electric Field for Solar Photovoltaic Applications

In this work, the aluminum-induced crystallization of hydrogenated amorphous silicon (a-Si:H) has been investigated. Specifically, constant temperature and voltage were applied on a-Si:H films covered with a very thin layer of pure aluminum. Each sample was subjected to annealing for 15 minutes while connected to an external electrical power. Nine different treatments of annealing temperatures and voltages were included in this study. The levels of applied voltages were 0 V, 2 V, or 20 V, and the levels of annealing temperatures were 200 $^{\circ}{\hbox{C}}$ , 250 $^{\circ}{\hbox{C}}$ , and 300 $^{\circ}{\hbox{C}}$ . The electrical, morphological, and structural properties of the treated a-Si:H layer were studied. The results of energy dispersive X-ray spectroscopy showed a diffusion of aluminum atoms inside the a-Si:H layer. While, X-ray diffraction revealed that the crystallization depends on both the annealing temperature and the external voltage. Moreover, the obtained results showed that the electrical resistivity decreases considerably, the carrier concentration deceases slowly, while the Hall mobility increases sharply with increasing the annealing temperature and the applied voltage.

Formation of a hybrid pn junction via p-type aluminium induced crystallized polycrystalline silicon on hydrothermally grown n-type zinc oxide nanowires

p-Type silicon coated zinc oxide (ZnO) nanowire heterojunction was fabricated using a combination of aluminium induced crystallization (AIC) and hydrothermal growth. The p-type AIC Si/n-type ZnO nanowires stacked layers were extensively characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), UV–vis spectroscopy, XRD, photoluminescence emission spectroscopy and electrical measurements. Photovoltaic measurements indicate that the device is light-sensitive and maybe of potential in sensor applications.

Nanoforest of black silicon fabricated by AIC and RIE method

A nanoforest of black silicon was prepared using a method combining aluminum-induced crystallization (AIC) and reactive-ion etching (RIE). During the AIC period, a rough layer, incorporating AlSix intermetallics, was formed. This layer acted as an etch mask for the RIE step. X-ray photoelectron spectroscopic (XPS) measurements were performed to further analyze the formation of black silicon. Compared to Si3N4 and polysilicon, the black silicon exhibits a high degree of infrared absorption for wavelengths of about 3 to 5μm and 8 to 14μm, respectively. In addition, black platinum overlaid on the base of black silicon further improves the infrared absorption. This nanostructure forests of black silicon could be incorporated into the fabrication process for gas detectors, photovoltaic devices, and imaging applications to improve the infrared absorption characteristics.

Effect of Al vacuum annealing prior to a-Si deposition on aluminum-induced crystallization

Aluminum-induced crystallization (AIC) experiments were carried out in order to investigate the influence of vacuum annealing of the Al layer at 500 °C prior to deposition of e-beam evaporated amorphous silicon (a-Si). A control sample set using the identical deposition sequences but without the Al vacuum anneal was also produced as reference. Analysis revealed that after vacuum annealing, the Al grain size increases significantly. The surface Al-oxide layer thickness is reduced, however, the Al-oxide to metallic Al ratio is increased in this layer. Following a-Si deposition, the a-Si/Al/SiNx/glass stacked samples were annealed in N2 atmosphere at temperatures between 420 and 450 °C. It was seen that the crystal growth rate in the samples with vacuum-annealed Al is significantly reduced compared to the control samples, due to the reduction of Al grain boundary density.

Structural properties of a-Si films and their effect on aluminum induced crystallization

In this paper, we report the influence of the structural properties of amorphous silicon (a-Si) on its subsequent crystallization behavior via the aluminum induced crystallization (AIC) method. Two distinct a-Si deposition techniques, electron beam evaporation and plasma enhanced chemical vapor deposition (PECVD), are compared for their effect on the overall AIC kinetics as well as the properties of the final poly-crystalline (poly-Si) silicon film. Raman and FTIR spectroscopy results indicate that the PECVD grown a-Si films has higher intermediate-range order, which is enhanced for increased hydrogen dilution during deposition. With increasing intermediate-range order of the a-Si, the rate of AIC is diminished, leading larger poly-Si grain size.

Crystallization of Si Templates of Controlled Shape, Size, and Orientation: Toward Micro- and Nanosubstrates

Fiber-textured polycrystalline silicon thin films have demonstrated their interest as seed layers for the epitaxial growth of high-quality materials on substrates such as glass or plastics. In the present work, we report a comprehensive study of the aluminum-induced crystallization (AIC) of isolated Si domains. Our study not only demonstrates the fabrication by AIC of shape- and size-controlled Si monocrystals at the nanoscale but also allows for the prediction of minimal annealing conditions for complete crystallization of such structures. These ultrathin [111]-oriented monocrystals are promising candidates as selected-area epitaxy substrates with both an easy control on the morphology of these structures and a low-temperature fabrication process.

Consideration of a Local Aluminum-Induced Crystallization Process Guided along the µ-Holes Fabricated with fs Laser Pulses

Consideration of a Local Aluminum-Induced Crystallization Process Guided along the µ-Holes Fabricated with fs Laser Pulses

Growth of silicon quantum dots by oxidation of the silicon nanocrystals embedded within silicon carbide matrix

A moderately low temperature (≤800 °C) thermal processing technique has been described for the growth of the silicon quantum dots (Si-QD) within microcrystalline silicon carbide (μc-SiC:H) dielectric thin films deposited by plasma enhanced chemical vapour deposition (PECVD) process. The nanocrystalline silicon grains (nc-Si) present in the as deposited films were initially enhanced by aluminium induced crystallization (AIC) method in vacuum at a temperature of Tv = 525 °C. The samples were then stepwise annealed at different temperatures Ta in air ambient. Analysis of the films by FTIR and XPS reveal a rearrangement of the μc-SiC:H network has taken place with a significant surface oxidation of the nc-Si domains upon annealing in air. The nc-Si grain size (DXRD) as calculated from the XRD peak widths using Scherrer formula was found to decrease from 7 nm to 4 nm with increase in Ta from 250 °C to 800 °C. A core shell like structure with the nc-Si as the core and the surface oxide layer as the shell can clearly describe the situation. The results indicate that with the increase of the annealing temperature in air the oxide shell layer becomes thicker and the nc-Si cores become smaller until their size reduced to the order of the Si-QDs. Quantum confinement effect due to the SiO covered nc-Si grains of size about 4 nm resulted in a photoluminescence peak due to the Si QDs with peak energy at 1.8 eV.

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.

Integration of a 2-D Periodic Nanopattern Into Thin-Film Polycrystalline Silicon Solar Cells by Nanoimprint Lithography

The integration of two-dimensional (2D) periodic nanopattern defined by nanoimprint lithography and dry etching into aluminum induced crystallization (AIC) based polycrystalline silicon (Poly-Si) thin film solar cells is investigated experimentally. Compared to the unpatterned cell an increase of 6% in the light absorption has been achieved thanks to the nanopattern which, in turn, increased the short circuit current from 20.6 mA/cm2 to 23.8 mA/cm2. The efficiency, on the other hand, has limitedly increased from 6.4% to 6.7%. We show using the transfer length method (TLM) that the surface topography modification caused by the nanopattern has increased the sheet resistance of the antireflection coating (ARC) layer as well as the contact resistance between the ARC layer and the emitter front contacts. This, in turn, resulted in increased series resistance of the nanopatterned cell which has translated into a decreased fill factor, explaining the limited increase in efficiency.

Fabrication of high gauge factor piezoresistive nanocrystalline Si film using aluminum-induced crystallization of HWCVD deposited a-Si:H

Aluminum-induced crystallization (AIC) has been used to achieve device quality nc-Si thin films, at annealing temperatures ranging from 300 to 450 °C, exhibiting electronic properties favourable for piezoresistive microelectromechanical systems sensor application. The nc-Si films obtained by AIC at an annealing temperature of 400 °C showed hole concentration of the order of 1018 cm−3 and the Hall mobility was measured to be 13.5 cm2V−1s−1. The films were subjected to piezoresistive gauge factor measurement and a gauge factor of 43 was recorded in the longitudinal mode under a sample scheme of constant force over the entire sample area, which is the highest reported to date for thin film nc-Si.

High carrier mobility in orientation-controlled large-grain (≥50 μm) Ge directly formed on flexible plastic by nucleation-controlled gold-induced-crystallization

High-carrier-mobility semiconductors on flexible-plastic are essential to realize flexible electronics. For this purpose, electrical properties of orientation-controlled large-grain Ge crystals on flexible-plastic directly formed by nucleation-controlled gold-induced-crystallization (GIC) are examined, and compared with those obtained by aluminum-induced-crystallization (AIC). The Ge crystals show p-type conductions. Here, hole concentrations are 2.2 × 1017 and 5.8 × 1020 cm−3 for GIC-Ge and AIC-Ge, respectively, which are explained on the basis of the solubility of Au and Al in Ge. Thanks to the low hole concentration, GIC-Ge shows high hole mobility (160 cm2 V−1 s−1) compared with AIC-Ge (37 cm2 V−1 s−1). These demonstrate significant advantage of GIC to realize high-performance flexible-electronics.

Silicon–hydrogen bond effects on aluminum-induced crystallization of hydrogenated amorphous silicon films

The effects of hydrogen dilution on aluminum-induced crystallization (AIC) of hydrogenated amorphous silicon (a-Si:H) films have been studied. The Raman spectra showed that the short-range order (SRO) and the intermedium-range order (IRO) of the as-deposited a-Si films increased with the increase of the H2 dilution from 0% to 20%. The optical microscope (OM) and X-ray diffraction (XRD) observation revealed that, compared to the a-Si:H film deposited in pure Ar, the a-Si:H films deposited with H2 dilution in the range of 3–8% possessed a lower crystallization rate while the a-Si:H films deposited with high H2 dilution in the range of 15–20% possessed a faster crystallization rate. It was found that majority of the hydrogen existed in the form of monohydride (SiH) bond in the a-Si:H films with H2 dilution ratio of 3–8%, the bonding energy of which was higher than that of Si–Si bond, leading to a lower crystallization rate of a-Si:H films. While the dihydride (SiH2) bond became dominant in the a-Si:H films with high H2 dilution of 15–20%, the bonding energy of which was lower than that of Si–Si bond, thus accelerating the crystallization rate. Therefore, it was illustrated that not the hydrogen concentration but the form of silicon–hydrogen bond determined the AIC process of a-Si:H films.

Influence of precursor a-Si:H dehydrogenation on the aluminum induced crystallization process

Polycrystalline silicon (poly-Si) thin film grown on low cost substrates such as glass at low temperature is an attractive material for cost-effective solar cells. This work studied the influence of dehydrogenation of a-Si:H precursor on the crystallization behavior by aluminum induced crystallization (AIC) process below the eutectic temperature of 577 °C. The a-Si:H films were deposited by PECVD and aluminum was evaporated in a vacuum evaporation equipment, respectively. Some of the a-Si:H thin films were dehydrogenated in nitrogen atmosphere before aluminum evaporation. It was found that hydrogen content in a-Si:H drops to a stable value after annealing at 480 °C for 1 h. Micro-Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy and scanning electron microscopy measurements were used to investigate the structural characteristics of the a-Si:H and the prepared poly-Si thin films. Our results show that although the dehydrogenation increases AIC temperature and reduces AIC rate, it can improve microstructural quality of poly-Si thin films, leading to less tensile stress, higher crystallinity, smoother and more conformal surface morphology.

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.