Morphology, microstructure, and doping behaviour: A comparison between different deposition methods for poly‐Si/SiO_x passivating contacts

Abstract

Crystallographic structures, optoelectronic properties, and nanoscale surface morphologies of ex situ phosphorus-doped polycrystalline silicon (poly-Si)/SiO x passivating contacts, formed by different deposition methods (sputtering, plasmaenhanced chemical vapour deposition [PECVD], and low-pressure chemical vapour deposition [LPCVD]), are investigated and compared. Across all these deposition technologies, we noted the same trend: higher diffusion temperatures yield films that are more crystalline but that have rougher surface morphologies due to bigger surface crystal grains. Also, the recrystallization process of the as-deposited Si films starts from the SiO x interface, rather than from the film surface and bulk. However, there are some distinct differences among these technologies. First, the LPCVD method yields the lowest deposition rate, roughest surfaces, and smallest degree of crystallinity on finished poly-Si films. In contrast, the PECVD method has the highest deposition rate and smoothest surfaces for both as-deposited Si and annealed poly-Si films. Second, as-deposited sputtered and PECVD Si films contain only an amorphous phase, whereas as-deposited LPCVD films already has some crystalline phase. Third, the LPCVD phosphorus in-diffusion into the substrate depends strongly on the initial film thickness, whereas for the other two methods, it is weakly dependent on thickness. Finally, the passivation quality of every poly-Si film type has different responses to the film thickness and diffusion temperature, suggesting that the ex situ doping optimization should be performed independently.

Boron Spin-On Doping for Poly-Si/SiOx Passivating Contacts

“…The doping of poly-Si passivating contacts can be categorized into two approaches: in situ and ex situ doping. The former approach utilizes the deposition process to incorporate dopants into Si films at the same time, followed by an annealing step. − Disadvantages of this approach are the complexity in controlling various parameters of the deposition and annealing and the inclusion of dangerous and toxic gases ( e.g. , phosphine and boron tribromide) which require additional safety measures.…”

Process–Structure–Properties Relationships of Passivating, Electron‐Selective Contacts Formed by Atmospheric Pressure Chemical Vapor Deposition of Phosphorus‐Doped Polysilicon

“…The deposition temperatures and PH 3 ratios listed in Table 1 were used to deposit the P‐doped Si films yielding deposition rates of around 11–14 nm s −1 , significantly higher than the PECVD and LPCVD processes. [ 23 ] Figure a–c shows the X‐ray diffraction (XRD) spectra of the APCVD P‐doped Si films in as‐deposited and annealed conditions.…”

“…[13,15,22] Also, the deposition rate in the APCVD process is significantly higher than the deposition rate in LPCVD and PECVD. [23] Moreover, in situ phosphorus (P) doped silicon (Si) films are grown with this APCVD process and therefore excludes the additional ex situ doping steps used in many other works.…”

Comparing the Gettering Effect of Heavily Doped Polysilicon Films and Its Implications for Tunnel Oxide‐Passivated Contact Solar Cells

“…It was reported that the grain size in the poly‐Si films increases with increasing annealing temperatures, [ 58 ] and the PECVD poly‐Si films generally have a smaller grain size than the LPCVD poly‐Si films due to the lower deposition temperatures. [ 59 ] Therefore, in the PECVD in situ P‐doped poly‐Si films, phosphorus may segregate and then precipitate at grain boundaries. [ 60 ] In terms of the immobile oxygen species, the oxygen concentration is expected to be negligible due to the lack of PSG layers.…”