Highly Microcrystalline Phosphorous-doped Si:H Very Thin Films Deposited by HF-PECVD
Résumé
Finely tuning crystallinity and doping of Si, Ge and SiGe thin films is a keypoint into obtaining high quality devices for above-IC near infrared SiGe-based image sensors. This study focuses on the structural and electrical properties of HF-PECVD deposited microcrystalline silicon (µc-Si:H) contact layers. µc-Si:H typically exhibits an amorphous incubation layer which can span up to 100 nm before nucleation of the crystalline phase. In previous work done on SiGe-based P-I-N sensors the n and p type layers typically have a thickness of the order of a few tens of nanometers. It is thus of great interest to ensure doped layers exhibit an incubation layer as thin as possible, independently from the doping level, as electrical and optical properties of amorphous and microcrystalline silicon differ significantly. In this work, phosphorus doped µc-Si:H was deposited by capacitive coupled plasma HF-PECVD (13,56 MHz) from a SiH4 H2 PH3 gas mixture with varying phosphine level. Two sets of deposition conditions were investigated, i.e. « low power » and « high power » conditions. For each of these conditions, thick samples (> 100 nm) were deposited for gas flow ratios ? = PH3/SiH4 between 0 and 1 % and samples of variable thickness were deposited for a ratio ? = 0.1% . The influence of phosphine concentration in the gas mixture on dopant concentration, active dopant concentration and crystallinity was studied by SIMS, Hall effect measurement and Raman spectroscopy respectively. High doping level were attained, reaching up to 1.6x10^20 cm-3. Topography and surface defects were investigated by AFM and SEM while thickness was measured by spectroscopic ellipsometry. Deposited films show « low power » samples reach a high crystallinity for films as thin as 40 nm whereas « high power » samples transition to highly crystalline films only between 40 and 80 nm despite exhibiting clear signs of early crystallite nucleation. These « low power » conditions also contribute to a better incorporation of active phosphorus in the thin film. For both set of deposition parameters the rise of blister-like surface defects due to compressive stress in P doped µc-Si:H shows there is a thickness limit for the n-type layer above which the films no longer sticks to the substrate.