Hot-Wire CVD Poly-Silicon Films for Thin Film Devices

Abstract

Solar cell using profiled poly-Si:H by HWCVD as i-layer in the configuration SS/n-µSi:H(PECVD)/i-poly-Si:H(HWCVD)/p-µc-Si:H(PECVD)/ITO showed 3.7% efficiency. A current of 23.6 mA/cm2 was generated in only 1.5 µm thick poly-Si:H i-layer grown at ∼5Å/s. TFTs made with the poly-Si:H films (grown at ≥ 9Å/s) exhibited remarkable stability to long duration of 23 hours of gate bias stress of ∼lMV/cm. A saturation mobility of 1.5 cm2/Vs for the TFT has been achieved. Films made at low hydrogen dilution (Poly2) showed device quality (purely intrinsic nature, ambipolar diffusion length of 568 nm, only (220) oriented growth and low ESR defect density of <1017/cm3with complete absence of signal due to conduction electrons) but with an incubation phase of amorphous initial growth, whereas the films made at high hydrogen dilution (Polyl) had a polycrystalline initial growth, though with higher defect density, incorporated oxygen and randomly oriented grains. Poly2 films are compact and hydrogen bonding is at compact Si-H sites manifested as 2000 cm−1IR vibration and high temperature hydrogen evolution peak. Exchange interaction of spins and spin pairing are observed while increasing defects in such a compact structure. A new approach has been used to integrate these two regimes of growth to make profiled poly-Si:H layers. The new layers show good electronic properties as well as complete elimination of incubation phase.

Structural defects caused by a rough substrate and their influence on the performance of hydrogenated nano-crystalline silicon n–i–p solar cells

“…Using a highly crystalline thin nc-Si:H layer, or a ''seed layer'', normally deposited with a high hydrogen dilution ratio [15], the density of nucleation sites can be greatly enhanced, and therefore the diameter of the silicon columns (the ''pencils'') can be significantly decreased. Based on this understanding, Bailat et al [14] proposed a nuclei density model to generally describe the R H [defined as H 2 /(H 2 +SiH 4 ) gas flow ratio] dependence of cell V oc , in which the importance of the changes in the average amorphous volume fraction in this heterophase region (where the cones appear) is stressed.…”

Studies on grain boundaries in nanocrystalline silicon grown by hot-wire CVD

“…These facts lead to a different set of suitable technologic parameters to obtain device-quality material. In particular, contrary to what happens in Plasma Enhanced CVD (PECVD), where dense nc-Si:H is obtained at substrate temperatures close to 250ºC [1], moderately high substrate temperatures (>400ºC) seem to be required in HWCVD to achieve dense material [2]. This need of a significantly higher deposition temperature in HWCVD than in PECVD limits the type of substrate and solar cell structures available.…”

Nanocystalline silicon solar cells

“…In the initial development phase of nc-Si cells [11], materials with very high crystallinity were considered to be ideal for the devices. In 1998, the author proposed that nc-Si can be made in different regimes, and two specific regimes of growth had been identified as important for device performance [23]. These two types were named as Poly1 and Poly2.…”