R. Bartlome

Low-temperature plasma-deposited silicon epitaxial films: Growth and properties

Low-temperature (≤200 °C) epitaxial growth yields precise thickness, doping, and thermal-budget control, which enables advanced-design semiconductor devices. In this paper, we use plasma-enhanced chemical vapor deposition to grow homo-epitaxial layers and study the different growth modes on crystalline silicon substrates. In particular, we determine the conditions leading to epitaxial growth in light of a model that depends only on the silane concentration in the plasma and the mean free path length of surface adatoms. For such growth, we show that the presence of a persistent defective interface layer between the crystalline silicon substrate and the epitaxial layer stems not only from the growth conditions but also from unintentional contamination of the reactor. Based on our findings, we determine the plasma conditions to grow high-quality bulk epitaxial films and propose a two-step growth process to obtain device-grade material.

Practical silicon deposition rules derived from silane monitoring during plasma-enhanced chemical vapor deposition

We clarify the difference between the SiH4 consumption efficiency η and the SiH4 depletion fraction D, as measured in the pumping line and the actual reactor of an industrial plasma-enhanced chemical vapor deposition system. In the absence of significant polysilane and powder formation, η is proportional to the film growth rate. Above a certain powder formation threshold, any additional amount of SiH4 consumed translates into increased powder formation rather than into a faster growing Si film. In order to discuss a zero-dimensional analytical model and a two-dimensional numerical model, we measure η as a function of the radio frequency (RF) power density coupled into the plasma, the total gas flow rate, the input SiH4 concentration, and the reactor pressure. The adjunction of a small trimethylboron flow rate increases η and reduces the formation of powder, while the adjunction of a small disilane flow rate decreases η and favors the formation of powder. Unlike η, D is a location-dependent quantity. It is...

Optical emission spectroscopy to diagnose powder formation in SiH4-H2 discharges

Silane and hydrogen discharges are widely used for the deposition of silicon thin film solar cells in large area plasma-enhanced chemical vapor deposition reactors. In the case of microcrystalline silicon thin film solar cells, it is of crucial importance to increase the deposition rate in order to reduce the manufacturing costs. This can be performed by using high silane concentration, and usually high RF power and high pressure, all favorable to powder formation in the discharge that generally reduces the deposition rate as well as the deposited material quality. This work presents a study of powder formation using time-resolved optical emission spectroscopy. It is shown that this technique is suitable to detect different regimes in powder formation ranging from powder free discharge to discharge producing large dust particles. Intermediate powder formation regimes include the formation of small silicon clusters at plasma ignition as well as cycle of powder growth and ejection out of the discharge, and both are observable by this low-cost and experimentally simple technique.

High-efficiency silicon heterojunction solar cells: From physics to production lines

Silicon heterojunction technology (Si-HJT) consists of thin amorphous silicon layers on monocrystalline silicon wafers and allows for photovoltaic solar cells with energy-conversion efficiencies above 20 %, also at industrial-production level. This article reports how this may be achieved. First, we focus on the surface-passivation mechanism of intrinsic and doped amorphous silicon films in such solar cells, enabling record-high values for the open-circuit voltage. Next, the industrial upscaling in large-area reactors of such film deposition is discussed, including the fabrication of solar cells with energy-conversion efficiencies as high as 21%.

High rate deposition of microcrystalline silicon with silicon oxide doped layers: Highlighting the competing roles of both intrinsic and extrinsinc defects on the cells performances

Hydrogenated microcrystalline silicon (μc-Si:H) has become a material of increasing interest these last years mainly for its use in cost-effective production of tandem and triple junction thin film silicon based solar cells. Lately, the use of novel doped silicon oxide (SiOx) layers were shown to be very promising for increasing the solar cells efficiency [1,2]. We present in this study a detailed analysis on the possible reasons behind this significant increase of electrical performances. Complete solar cells were developed in an industrial type reactor with their intrinsic layer (i-layer) deposited at a high growth rate of 1 nm/s by VHF-PECVD. Different i-layer material quality and substrate roughness were systematically evaluated during this investigation. We demonstrate conversion efficiency increase of up to 29% when both these p-type and n-type doped SiOx layers are used instead of the regular microcrystalline ones, while keeping the bulk of intrinsic material unchanged and efficiencies over 8% are achieved for a wider range of plasma parameters and substrate roughness. Extensive material analysis is presented hereafter to understand the physical origins for the improvements observed. XRD, Raman and FTIR spectroscopy, intrinsic stress, FTPS and SIMS measurements were done along with SEM images of the solar cells. It is found that devices with very different efficiencies can lead to similar FTIR and FTPS spectrum. We show that the integration of doped SiOx layers reduces to some extent the influence of porous regions, i.e. microcracks, on the electrical properties of the solar cells, and the possible physical reasons for this improvement are discussed. The development of these extrinsic defects, not detected by FTPS and FTIR, is becoming especially detrimental on highly textured substrates, required for increased light trapping. This highlights the fundamental nature difference of intrinsic and extrinsic defects which can both drive the cells performances.

Laser-based plasma diagnostics for PECVD of silicon thin films

We present two laser systems to monitor plasma conditions in a plasma-enhanced chemical vapor deposition chamber. The first optical system is a high-resolution quantum cascade laser-based infrared absorption spectrometer designed to measure the input silane depletion fraction (dissociation efficiency) and to determine the amorphous-to-microcrystalline silicon transition regime. The second optical system is a compact and low-cost laser light scattering device designed to detect the formation of powder particles. In the absence of such particles, the silane depletion fraction provides an insitu measurement of the film growth rate.