Zhengxin Liu

Reduction of iron diffusion in silicon during the epitaxial growth of β-FeSi2 films by use of thin template buffer layers

We fabricated continuous highly (110)/(101)-oriented β-FeSi2 films on Si (111) substrates by the facing-target sputtering method. An epitaxial thin β-FeSi2 template buffer layer preformed on the silicon substrate was found to be essential in the epitaxial growth of thick β-FeSi2 films. It was proved that the template reduced the iron diffusion into the silicon substrate during thick β-FeSi2 film fabrication. Even though the annealing was performed at high temperature (880 °C) for a long duration (10 h), iron diffusion was effectively hindered by the template. By introducing this template buffer layer, an abrupt interface without appreciable defects between the β-FeSi2 film and the silicon substrate formed. The mechanism for the reduction of iron diffusion by the template buffer layer is discussed.

Seeding method with silicon powder for the formation of silicon spheres in the drop method

Silicon spheres with a size distribution around 1.0 mm diameter, which are applicable to spherical solar cells, were formed by dropping molten silicon through a nozzle in a free-fall tube, namely, the drop method. Here we show a seeding technique for the formation of silicon spheres. In this technique, pure silicon powders with a size distribution of 1−75 μm were ejected to the molten silicon droplets at a selected part of the free-fall tube using argon carrier gas. It was considered that the attached silicon powders on the droplets worked as nuclei and stimulated the solidification to occur at low undercooling from one place. Characterizations with scanning electron microscope, carrier lifetime, and photoluminescence measurements demonstrated that the crystallinity of silicon spheres were significant improved by the seeding method. The undercooling of molten silicon droplets at solidification was speculated to decrease from ∼250 °C to below 50 °C by seeding power ejection. This resulted in an increase of...

β-FeSi 2 as a Kankyo (environmentally friendly) semiconductor for solar cells in the space application

β-FeSi2 defined as a Kankyo (Environmentally Friendly) semiconductor is regarded as one of the 3-rd generation semiconductors after Si and GaAs. Versatile features about β-FeSi2 are, i) high optical absorption coefficient (>105cm-1), ii) chemical stability at temperatures as high as 937°C, iii) high thermoelectric power (Seebeck coefficient of k ~ 10-4/K), iv) a direct energy band-gap of 0.85 eV, corresponding to 1.5μm of quartz optical fiber communication, v) lattice constant nearly well-matched to Si substrate, vi) high resistance against the humidity, chemical attacks and oxidization. Using β-FeSi2 films, one can fabricate various devices such as Si photosensors, solar cells and thermoelectric generators that can be integrated basically on Si-LSI circuits. β-FeSi2 has high resistance against the exposition of cosmic rays and radioactive rays owing to the large electron-empty space existing in the electron cloud pertinent to β-FeSi2. Further, the specific gravity of β-FeSi2 (4.93) is placed between Si (2.33) and GaAs ((5.33). These features together with the aforementioned high optical absorption coefficient are ideal for the fabrication of solar cells to be used in the space. To demonstrate fascinating capabilities of β-FeSi2, one has to prepare high quality β-FeSi2 films. We in this report summarize the current status of β-FeSi2 film preparation technologies. Modified MBE and facing-target sputtering (FTS) methods are principally discussed. High quality β-FeSi2 films have been formed on Si substrates by these methods. Preliminary structures of n-β-FeSi2 /p-Si and p-β-FeSi2 /n-Si solar cells indicated an energy conversion efficiency of 3.7%, implying that β-FeSi2 is practically a promising semiconductor for a photovoltaic device.

Formation of total-dose-radiation hardened materials by sequential oxygen and nitrogen implantation and multi-step annealing

Separation by implantation of oxygen and nitrogen (SIMON) silicon-on-insulator (SOI) materials were fabricated by sequential oxygen and nitrogen implantation with annealing after each implantation. Analyses of SIMS, XTEM and HRTEM were performed. The results show that superior buried insulating multi-layers were well formed and the possible mechanism is discussed. The remarkable total-dose irradiation tolerance of SIMON materials was confirmed by few shifts of drain leakage current-gate source voltage (I-V) curves of PMOS transistors fabricated on SIMON materials before and after irradiation.

Improved passivation effect at the amorphous/crystalline silicon interface due to ultrathin SiOx layers pre-formed in chemical solutions

To improve the passivation effect at a-Si:H/c-Si interface in heterojunction (HJ) solar cells, ultrathin SiOx layers with a thickness of approximately 2 nm were pre-formed on c-Si surfaces in chemical solutions. It was demonstrated that the SiOx layers pre-formed in hot de-ionized water and hydrochloric acid solutions improve effective carrier lifetime, and it is further enhanced through a post annealing process. When the thin SiOx layers were applied to HJ solar cells, increase in both Voc and Jsc was achieved, implying the improved interface quality for these HJ solar cells, as compared with the reference without the SiOx layer.

High quality of IWO films prepared at room temperature by reactive plasma deposition for photovoltaic devices

High-quality tungsten-doped indium oxide (IWO) films are deposited on glass substrates at room temperature by the reactive plasma deposition (RPD) process under different oxygen/argon (O2/Ar) ratios. It is revealed that the O2/Ar ratio plays an important role in obtaining high conductivity without compromising the optical transmission of the films. The effect of the annealing temperature on the structure, electrical and optical properties of IWO thin films is investigated. The as-deposited film is crystalline and then re-crystallizes by postannealing. In this work, the IWO film with the O2/Ar ratio of 14% annealed at 220 °C exhibits the best electrical conductivity, with a lowest resistivity of 3.34 × 10−4 Ω cm and a highest mobility of 77.8 cm2 V−1 s−1, and which has the average transmittance of 85.50% (visible region) and 94.21% (near-infrared region). These optical and electrical characteristics of IWO films make them suitable for a-Si/C–Si heterojunction solar cell applications.

Oxygen-atmosphere heat treatment in spin-on doping process for improving the performance of crystalline silicon solar cells

Spin-on doping of phosphorus has been investigated and applied for the emitter fabrication of crystalline Si solar cells.Heat treatment in oxygen atmosphere at relatively low temperature of 550 ° C prior to phosphorus diffusion is proved effective for improving solar cell performance, showing a conversion efficiency enhancement of more than 0.2% absolute. Internal quantum efficiency measurements show obvious enhancements at both short and long-wavelength regions. Secondary ion mass spectroscopy and Infrared absorption analysis reveal reduced C impurities after the heat treatment, possibly caused by burning the organic residues in the coated dopant source layer.

Influence of Si∕Fe ratio in multilayer structures on crystalline growth of β-FeSi2 thin film on Si substrate

The deposited Si∕Fe ratio has been found to have a significant influence on crystalline growth of β-FeSi2 film on Si substrate. Stoichiometry is always satisfied by interdiffusion of Si, under both Si-lean and Si-rich conditions. However, Si diffusion from the substrate into the deposited layer, which compensates for deficient Si, induces an undulated interface, as well as Fe and Si vacancies. On the other hand, excess Si is driven to the β-FeSi2∕Si interface, which results in a lamellar structure with a large number of small grains. Fe and Si vacancies are significantly reduced by excess Si. These results suggest that precise control of the Si∕Fe ratio is essential for good crystallinity and fewer vacancies.

Boron doping for p-type β-FeSi2 films by sputtering method

High quality epitaxial β-FeSi2 thin films prepared by alternate Fe/Si multilayers stacking were doped for p-type by co-sputtering of silicon and boron, in which elemental boron chips were placed on silicon target. The starting β-FeSi2 films before doping were n-type with residual electron concentration of about 2 ×1017 cm-3 and mobility of about 200 cm2/Vs. After doping with boron, β-FeSi2 films showed the same epitaxial crystallinity with continuous structure as that of non-doped one. Doping level of p-type β-FeSi2 films with net hole concentration from 3 ×1017 to 1 ×1019 cm-3 and mobility from 100 to 20 cm2/Vs were successfully achieved. Desired net hole concentration was obtained by varying the area ratio of boron chips on silicon target.

Evolution of a Native Oxide Layer at the a-Si:H/c-Si Interface and Its Influence on a Silicon Heterojunction Solar Cell

The interface microstructure of a silicon heterojunction (SHJ) solar cell was investigated. We found an ultrathin native oxide layer (NOL) with a thickness of several angstroms was formed on the crystalline silicon (c-Si) surface in a very short time (∼30 s) after being etched by HF solution. Although the NOL had a loose structure with defects that are detrimental for surface passivation, it acted as a barrier to restrain the epitaxial growth of hydrogenated amorphous silicon (a-Si:H) during the plasma-enhanced chemical vapor deposition (PECVD). The microstructure change of the NOL during the PECVD deposition of a-Si:H layers with different conditions and under different H2 plasma treatments were systemically investigated in detail. When a brief H2 plasma was applied to treat the a-Si:H layer after the PECVD deposition, interstitial oxygen and small-size SiO2 precipitates were transformed to hydrogenated amorphous silicon suboxide alloy (a-SiO(x):H, x ∼ 1.5). In the meantime, the interface defect density was reduced by about 50%, and the parameters of the SHJ solar cell were improved due to the post H2 plasma treatment.