Microcrystalline silicon growth in the presence of dopants: effect of high growth temperatures

Abstract

High growth rate poly- and microcrystalline silicon materials such as those made by hot-wire chemical vapor deposition (HWCVD) require doped layers that are not only resistant to high temperatures, but also resistant to high atomic hydrogen fluxes. This has been achieved in our case by Layer-by-Layer (LbL) deposition at high temperatures. Layer-by-Layer deposition was achieved by alternating either boron (p-type) or phosphorous (n-type) doped amorphous silicon and hydrogen plasma treatments by very high frequency chemical vapor deposition (VHF PECVD). The experiments revealed the following observations. (1) An optimum thickness per deposition cycle (total thickness/deposition cycle) of 1.4 nm/cycle is needed for crystallization (irrespective of dopants and deposition temperature). We found that for continuous wave deposition (CW) increased boron doping leads the growth regime to amorphous nature, whereas at the same doping condition we obtain microcrystalline films by LbL deposition if the aforesaid thickness per cycle is followed. (2) The etching rate during the hydrogen treatment decreases monotonously at increasing substrate temperature. The observation that films grown at 400 °C (where etching is negligible) are microcrystalline, implies that etching does not play an important role in nucleation. (3) A minimum thickness of the first layer is needed for sustaining growth in the LbL process. From the above studies we propose a hydrogen mediated nucleation process, which is not affected by dopants at the growing surface as in the case of continuous growth. The doping efficiencies in our LbL deposited layers are orders of magnitude higher than those in CW deposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for CW). The best high-temperature doped layers with a small thickness have properties as follows: LbL p-type μc-Si:H ( T s=350 °C, 29 nm): activation energy=0.11 eV and dark conductivity=0.1 Ω −1 cm −1; LbL n-type μc-Si:H ( T s=400 °C, 31 nm): activation energy = 0.056 eV and dark conductivity=2.7 Ω −1 cm −1. A test cell using an HWCVD deposited μc-Si:H i-layer on top of the high temperature LbL μc-Si:H n-layer in an n-i-p cell configuration on a stainless steel substrate without a back reflector showed a high open circuit voltage of 0.65 V and a fill factor of 0.68, proving the high doping efficiency and crystallinity of the n-layer as well as its resistance against high-temperature conditions.