Showing papers by "Keisuke Ohdaira published in 2012"

Polycrystalline silicon films with nanometer-sized dense fine grains formed by flash-lamp-induced crystallization.

Abstract: Flash lamp annealing (FLA) with millisecond-order pulse duration can crystallize microm-order-thick a-Si films on glass substrates through explosive crystallization (EC), and flash-lamp-crystallized (FLC) poly-Si films consist of densely-packed nanometer-sized fine grains. We investigate the impact of the hydrogen concentration and the defect density of precursor a-Si films on crystallization mechanism and the microstructures of FLC poly-Si films, by comparing chemical-vapor-deposited (CVD) and sputtered precursor a-Si films. Transmission electron microscopy (TEM) observation reveals that FLC poly-Si films with similar periodic microstructures are formed by the FLA of the two kinds of precursor films, meaning no significant influence of hydrogen atoms and defect density on crystallization mechanism. This high flexibility of the properties of precursor a-Si films would contribute to a wide process window to reproducibly form FLC poly-Si films with the particular periodic microstructures.

Distribution of Phosphorus Atoms and Carrier Concentrations in Single-Crystal Silicon Doped by Catalytically Generated Phosphorous Radicals

Abstract: A phosphorus (P)-doped ultrathin n+ layer is formed on crystalline silicon (c-Si) using radicals generated by the catalytic cracking reaction of phosphine (PH3) gas with a heated catalyzer. The carrier concentration and the depth distributions of P atoms are investigated by Hall effect and secondary ion mass spectrometry (SIMS), respectively. The surface of the p-type c-Si substrate is converted to n-type c-Si by this doping even at a substrate temperature of 20 °C, when the tungsten (W) catalyzer is heated at 1300 °C. SIMS measurements demonstrate that P atoms exist on the c-Si surface. However, the distributions of P atoms obtained by SIMS do not change, even for the increase in substrate temperature from 80 to 350 °C or the increase in radical exposure time from 60 to 3600 s. Although the sheet carrier concentration increased with the substrate temperature, the sheet carrier concentration increased only slightly with the radical exposure time. It is revealed that the doping mechanism does not appear to be the same as that of the thermal diffusion, but that the reaction of the P-related species with Si atoms on the surface plays a key role for this radical doping.

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

Abstract: The flash lamp annealing (FLA) of electron-beam- (EB-) evaporated amorphous silicon (a-Si) films results in the formation of polycrystalline Si (poly-Si) films with at least a few µm long grains stretching along lateral crystallization directions. Unlike the case of using chemical-vapor-deposited (CVD) hydrogenated a-Si films as precursors, no peeling of Si films occurs even in the absence of Cr adhesion layers. Such a flash-lamp-induced crystallization occurs also in doped EB-evaporated a-Si films as in the case of undoped films. The p+/p-/n+ stacked structure is sufficiently kept even after crystallization, although the profiles of dopants are slightly modified. This fact clearly indicates that the crystallization observed is not based on liquid-phase epitaxy (LPE) after the complete melting of the whole a-Si precursor during millisecond-order treatment but through LPE-based explosive crystallization (EC), self-catalytic lateral crystallization driven by the release of latent heat. The formation of poly-Si films with large grains and the sufficient preservation of dopant profiles would lead to the utilization of the poly-Si films formed for solar cell devices.

Scanning transmission electron microscope analysis of amorphous-Si insertion layers prepared by catalytic chemical vapor deposition, causing low surface recombination velocities on crystalline silicon wafers

Abstract: Microstructures of stacked silicon-nitride/amorphous-silicon/crystalline-silicon (SiNx/a-Si/c-Si) layers prepared by catalytic chemical vapor deposition were investigated with scanning transmission electron microscopy to clarify the origin of the sensitive dependence of surface recombination velocities (SRVs) of the stacked structure on the thickness of the a-Si layer. Stacked structures with a-Si layers with thicknesses greater than 10 nm exhibit long effective carrier lifetimes, while those with thin a-Si layers have very short effective carrier lifetimes. A remarkably close correlation was found between the dependence of interface structures on the thicknesses of a-Si layers and the SRVs. In samples with a-Si layers less than 10 nm thick, significant damage occurred in c-Si wafers close to the interfaces, while those near a-Si layers larger than 10 nm remained nearly defect-free during observations over long periods. The observation of stacked structures without an SiNx layer, along with energy dispers...

Effect of Radical-Doped n+ Back Surface Field Layers on the Effective Minority Carrier Lifetimes of Crystalline Silicon with Amorphous Silicon Passivation Layers Deposited by Catalytic Chemical Vapor Deposition

Abstract: To reduce surface recombination at an amorphous silicon (a-Si)/crystalline silicon (c-Si) interface in heterojunction solar cells, a thin phosphorus-doped back surface field (BSF) layer is applied to c-Si. Thin BSF layers are doped at temperatures lower than 350 °C by radical doping. The reduction in the surface recombination velocity of n-type c-Si is investigated by comparing the effective minority carrier lifetimes of c-Si samples with and without doping. Using radical-doped BSF layers, the effective minority carrier lifetimes of the samples with the thin intrinsic a-Si passivation layers increase significantly. The change in effective minority carrier lifetime under the BSF layer doping condition is also investigated. An effective minority carrier lifetime of 1.6 ms is observed in the radical-doped sample with the 6-nm-thick intrinsic a-Si passivation layer. The high carrier concentration of the radical-doped BSF layers can also decrease the contact resistivity to a metal electrode. Therefore, the radical-doped BSF layers can be utilized for passivation and ohmic contact formation on the back surface of the heterojunction solar cells.

Mechanism and control of crack generation in glass substrates during crystallization of a-Si Films by flash lamp annealing

Abstract: We investigate the impact of the materials of glass substrates on crack formation during flash lamp annealing (FLA) of 4.5 μm-thick precursor amorphous silicon (a-Si) films for the formation of polycrystalline Si (poly-Si) films. The use of soda lime glass substrates, with the largest thermal expansion coefficient ( α ) and the lowest glass transition temperature ( T g ) in glass materials attempted in this study, results in the serious formation of cracks on and inside the glass substrates. Cracks are also seen on the surface of quartz glass substrates, which have much smaller α and higher T g , after FLA. Furthermore, flash-lamp-crystallized (FLC) poly-Si films have linearly-connected low-crystallinity regions only when quartz glass substrates are used. These facts indicate that the expansion of Si films induces cracks in quartz glass substrates, while the expansion of the upper part of glass is the cause of the crack formation in glass substrates with large α. The generation of cracks is most significantly suppressed when we use alkali-free glass substrates, with a moderate α and a relatively high T g , which will contribute to the realization of high-quality poly-Si films and high-performance solar cells.

62.2: Pixel Controlling Substrate Fabricated by Embedding Millions of Silicon IC Chips in Plastic Substrate and Self‐Aligned Metal Interconnection among Such IC Chips

Abstract: This paper presents a novel method to fabricate pixel controlling substrates for LCD and OLED. Instead of a-Si TFTs, pixels are controlled by millions of Si IC chips embedded in a plastic substrate and metal lines connecting IC chips are formed in a self-aligned manner by using functional metal ink.

A Novel Method for Low-Resistivity Metal-Interconnection by Using Metallic Functional Liquids and Catalytically Generated Hydrogen Atoms

Abstract: A novel method to make low-resistivity metal lines in assembled silicon (Si) integrated circuit (IC) chips or other semiconductor chips with high-speed and low-cost is demonstrated. In the method, functional silver (Ag)-liquid (Ag-ink) which contains Ag nanoparticles (NPs) in organic solution is used to draw metal-lines in trenches formed on a plastic substrate by imprint technology. Surface energy of trenches is modified by exposing the substrate to ultra-violet (UV) light with the purpose of concentrating the functional Ag-ink into trenches by capillary effect in order to connect with electrodes of Si chips. The resistivity of such metal-lines can be lowered to 4×10−6 Ωcm by exposing the Ag metal-lines to hydrogen (H) atoms generated by catalytic cracking reaction with a heated tungsten catalyzer. X-ray photoelectron spectroscopy (XPS) proves that H atoms can remove organic compounds surrounding Ag NPs, resulting in the low-temperature sintering of NPs as confirmed by scanning electron microscopy (SEM). The method is promising for low-cost fabricating of IC cards or other electronic devices utilizing assemble of many semiconductor chips.

Low-temperature Phosphorus Doping To Silicon Using Phosphorus-related Radicals

Abstract: Phosphorus (P) doped ultra thin n+-layer is formed on crystalline silicon (c-Si) at low substrate temperatures of 80–350 °C using radicals generated by the catalytic reaction of phosphine (PH3) with a tungsten catalyzer heated at 1300 °C. The sheet carrier concentration obtained by Hall effect is in the range between 3×1012cm−2 and 8×1012cm−2. The distribution of P atoms obtained by secondary ion mass spectrometry (SIMS) indicates that P atoms locate within the depth of 4 nm from surface and the profile has almost the same distribution independent of any doping conditions such as substrate temperature or radical exposure time. The sheet carrier concentration is 1.15–2.12% of the amount of P atoms incorporated through the radical doping. The ratio of activated donors increases with substrate temperature during the radical doping, suggesting that P-related species bonded on the c-Si surface require thermal energy for their activation. Using the n+-layer formed by radical doping, the reduction of surface recombination velocity for n-type c-Si wafer is attempted. The effective minority carrier lifetime of the n-type c-Si sample covered with 6-nm-thick intrinsic amorphous Si (i-a-Si) layers on both side increases from 32 μs to1576 μs by the radical doping of P atoms to n-type c-Si surface, suggesting that the radical doping can be utilized for the formation of passivation layers on a-Si/ n-c-Si hetero-interface.

Formation of polycrystalline silicon films with μm-order-long grains through liquid-phase explosive crystallization by flash lamp annealing

Abstract: Flash lamp annealing (FLA) is a short-time annealing technique which can heat and crystallize μm-order-thick amorphous silicon (a-Si) films without inducing serious thermal damage onto whole glass substrates thanks to its proper fluence on the order of several tens of J/cm2 and millisecond-order duration. The FLA of a-Si films leads to lateral explosive crystallization (EC), driven by the release of latent heat, with a lateral crystallization speed on the order of m/s. When we use electron-beam- (EB-) evaporated a-Si films as precursors, EC only through liquid-phase epitaxy (LPE) occurs, resulting in the formation of polycrystalline Si (poly-Si) films with μm-order-long grains stretching along lateral crystallization directions. We see no remarkable difference in the microstructure of poly-Si films on various glass substrates, and this crystallization can take place also when doped EB-evaporated a-Si films are used. We have confirmed the simultaneous crystallization of pn-stacked a-Si films by FLA, and according to the data of scanning spread resistance microscopy (SSRM), the diffusion of dopants, B and P, can be sufficiently suppressed to a level at which the pn-stacked poly-Si films can be used as solar cells. The advantages of rapid crystallization and the formation of large grains would contribute to the establishment of the low-cost fabrication process of thin-film poly-Si solar cells.