LOW-TEMPERATURE DEPOSITION OF POLYCRYSTALLINE SILICON THIN-FILMS BY HOT-WIRE CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430 degrees C, The material properties have been optimized for photovo ltaic applications by varying the hydrogen dilution of the silane feed stock gas, the gas pressure and the wire temperature. The optimized ma terial has 95% crystalline volume fraction and an average grain size o f 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, e qual to crystalline silicon. An activation energy of 0.54 eV for the e lectrical transport confirmed the intrinsic nature of the films. The m aterial has a low dangling bond-defect density of similar to 10(17) cm (3). A photo conductivity of 1.9 x 10(-5) Ohm(-1) cm(-1) and a photore sponse (sigma(ph)/sigma(d)) of 1.4 x 10(2) were achieved. A high minor ity-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mo bility-lifetime (mu tau) product of 7.1 x 10(-7) cm(2) V-1 from steady -state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n-i-p cell made in the conf iguration n(+)-c-Si/i-poly-Si : H/p-mu c-Si : H/ITO yielded 3.15% effi ciency under 100 mW/cm(2) AM 1.5 illumination.