Zihao Zhai

Growth of ideal amorphous carbon films at low temperature by e-beam evaporation

Amorphous carbon (a-C) films were prepared by e-beam evaporation on silicon substrates. The effects of substrate temperature between room temperature and 600 °C were mainly studied. Raman spectroscopy, scanning electron microscopy, atomic force microscopy, UV-Vis-NIR and Hall-effect measurements were used to characterize the structure, morphology, roughness, transmittance and conductivity of a-C films, respectively. The results indicated the films obtained by e-beam evaporation were all graphite-like carbon films. The films deposited at 200 °C with a thickness of 75 nm presented the best performance with ID/IG of 4.50, FWHMG of 115.9 cm−1, an optical gap of 0.6 eV, a roughness of 0.825 nm and an electrical resistivity of 3.60 × 10−3 Ω cm. As the thickness reduced to about 8 nm while remaining at the same substrate temperature, the films still exhibited good structural quality, continuity and conductivity. SEM images from Ge/a-C/Si stacks and Ge/Si stacks confirmed the necessity of a-C buffer layer for smooth Ge film growth. Raman analysis of Ge films indicated that the crystalline quality of Ge films obtained from Ge/Si stacks was improved by inserting an a-C layer. The mechanism of a-C films as buffer layers was further explored by X-ray diffraction. The results suggested that a-C films with good properties prepared at such low temperature may be used as buffer layers for growing high quality Ge or GaAs films on Si.

Effect of e-beam evaporated elemental metal stack precursors on the property of Cu(InGa)Se2 thin films through two-step process

Two-step process is considered to be more simple than co-evaporation method in Cu(InGa)Se2 (CIGS) thin films preparation process. However, research on CIGS thin films prepared by two-step process based on evaporation of elemental metals Cu, In and Ga is hardly reported. In this work, four types of metal stacks engineered as In/Ga/Cu/Ga/In (type A), Cu/Ga/In (type B), Cu/In/Ga (type C) and Cu/Ga/In/Cu (type D) were prepared and effects of metal stack precursors on properties of CIGS thin films formed by two-step process were studied. All types of precursors consisted of Cu–In, Cu–Ga and In phases. CIGS thin film from type A precursor showed poor compactness and relative high surface roughness due to the ordered defect compounds on the surface. Type B precursor exhibited a better compactness and crystal quality which led to an optimal structural property of films among all types of CIGS thin films. The reduction of Ga/(Ga + In) and the gaps appeared at the Mo/CIGS interface of CIGS thin film from type C precursor were explained by the re-evaporation of Ga element and compensation of In element during selenization. Grain size in CIGS thin film from type D is a little smaller than that from type B. Models of selenization process in different types of precursors were given. CIGS solar cell from type B exhibited the highest conversion efficiency of 9.2%.

Evolution of Structural and Electrical Properties of Carbon Films from Amorphous Carbon to Nanocrystalline Graphene on Quartz Glass by HFCVD

Direct growth of graphene films on glass is of great importance but has so far met with limited success. The noncatalytic property of glass results in the low decomposition ability of hydrocarbon precursors, especially at reduced temperatures (<1000 °C), and therefore amorphous carbon (a-C) films are more likely to be obtained. Here, we report the hydrogen influence on the structural and electrical properties of carbon films deposited on quartz glass at 850 °C by hot-filament chemical vapor deposition (HFCVD). The results revealed that the obtained a-C films were all graphitelike carbon films. Structural transition of the deposited films from a-C to nanocrystalline graphene was achieved by raising the hydrogen dilution ratios from 10 to over 80%. On the basis of systematic structural and chemical characterizations, a schematic process with three steps including sp2 chain aggregation, aromatic ring formation, and sp3 bond etching was proposed to interpret the structural evolution. The nanocrystalline graphene films grown on glass by HFCVD exhibited good electrical performance with a carrier mobility of 36.76 cm2/(V s) and a resistivity of 5.24 × 10-3 Ω cm over an area of 1 cm2. Temperature-dependent electrical characterizations revealed that the electronic transport in carbon films was dominated by defect, localized, and extended states, respectively, when increasing the temperature from 75 to 292 K. The nanocrystalline graphene films presented higher carrier mobility and lower carrier concentration than those of a-C films, which was mainly attributed to their smaller conductive activation energy. The present investigation provides an effective way for direct growth of graphene films on glass at reduced temperatures and also offers useful insights into the understanding of structural and electrical relationship between a-C and graphene.