Atomic layer deposition, a method of excellent step coverage and conformal deposition, was used to deposit TiO₂thin films for the surface passivation and antireflection coating of silicon solar cells. TiO₂thin films deposited at different temperatures (200°C, 300°C, 400°C, and 500°C) on FZ n-type silicon wafers are in the thickness of 66.4 nm ± 1.1 nm and in the form of self-limiting growth. For the properties of surface passivation, Si surface is effectively passivated by the 200°C deposition TiO₂thin film. Its effective minority carrier lifetime, measured by the photoconductance decay method, is improved 133% at the injection level of cm⁻³. Depending on different deposition parameters and annealing processes, we can control the crystallinity of TiO₂and find low-temperature TiO₂phase (anatase) better passivation performance than the high-temperature one (rutile), which is consistent with the results of work function measured by Kelvin probe. In addition, TiO₂thin films on polished Si wafer serve as good ARC layers with refractive index between 2.13 and 2.44 at 632.8 nm. Weighted average reflectance at AM1.5G reduces more than half after the deposition of TiO₂. Finally, surface passivation and antireflection properties of TiO₂are stable after the cofire process of conventional crystalline Si solar cells.

Titanium oxide (TiO₂) films and TiO₂/SiN_x stacks have potential in surface passivation, anti-reflection coatings and carrier-selective contact layers for crystalline Si solar cells. A Si wafer, deposited with 8-nm-thick TiO₂ film by atomic layer deposition, has a surface recombination velocity as low as 14.93 cm/s at the injection level of 1.0 × 10¹⁵ cm⁻³. However, the performance of silicon surface passivation of the deposited TiO₂ film declines as its thickness increases, probably because of the stress effects, phase transformation, atomic hydrogen and thermal stability of amorphous TiO₂ films. For the characterization of 66-nm-thick TiO₂ film, the results of transmission electron microscopy show that the anatase TiO₂ crystallinity forms close to the surface of the Si. Secondary ion mass spectrometry shows the atomic hydrogen at the interface of TiO₂ and Si which serves for chemical passivation. The crystal size of anatase TiO₂ and the homogeneity of TiO₂ film can be deduced by the measurements of Raman spectroscopy and spectroscopic ellipsometry, respectively. For the passivating contacts of solar cells, in addition, a stack composed of 8-nm-thick TiO₂ film and a plasma-enhanced chemical-vapor-deposited 72-nm-thick SiN_x layer has been investigated. From the results of the measurement of the reflectivity and effective carrier lifetime, TiO₂/SiN_x stacks on Si wafers perform with low reflectivity and some degree of surface passivation for the Si wafer.

In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 1010 to 1.1 × 1011 cm-2 by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots.

The photocatalysts of immobilized TiO₂ film suffer from high carrier recombination loss when compared to its powder form. Although the TiO₂ with rutile-anatase mixed phases has higher carrier separation efficiency than those with pure anatase or rutile phase, the single junction of anatase/rutile cannot avoid the recombination of separated carriers at the interface. In this study, we propose a TiO₂/SnO₂/Ni multi-heterojunction structure which incorporates both Schottky contact and staggered band alignment to reduce the carrier recombination loss. The low carrier recombination rate of TiO₂ film in TiO₂/SnO₂/Ni multi-heterojunction structure was verified by its low photoluminescence intensity. The faster degradation of methylene blue for TiO₂/SnO₂/Ni multi-junctions than for the other fabricated structures, which means that the TiO₂ films grown on the SnO₂/Ni/Ti coated glass have a much higher photocatalytic activity than those grown on the blank glass, SnO₂-coated and Ni/Ti-coated glasses, demonstrated its higher performance of photogenerated carrier separation.