• Energy Research

Low-temperature plasma enhanced atomic layer deposition (PE-ALD) was successfully used to grow silicon (Si) doped amorphous and microcrystalline gallium phosphide (GaP) layers onto p-type Si wafers for the fabrication of n-GaP/p-Si heterojunction solar cells. PE-ALD was realized at 380 °C with continuous H2 plasma discharge and the alternate use of phosphine and trimethylgallium as sources of P and Ga atoms, respectively. The layers were doped with silicon thanks to silane (SiH4) diluted in H2 that was introduced as a separated step. High SiH4 dilution in H2 (0.1%) allows us to deposit stoichiometric GaP layers. Hall measurements performed on the GaP:Si/p-Si structures reveal the presence of an n-type layer with a sheet electron density of 6–10 × 1013 cm−2 and an electron mobility of 13–25 cm2 V−1 s−1 at 300 K. This is associated with the formation of a strong inversion layer in the p-Si substrate due to strong band bending at the GaP/Si interface. GaP:Si/p-Si heterostructures exhibit a clear photovoltaic effect, with the performance being currently limited by the poor quality of the p-Si wafers and reflection losses at the GaP surface. This opens interesting perspectives for Si doped GaP deposited by PE-ALD for the fabrication of p-Si based heterojunction solar cells.

Boron phosphide (BP) thin films are potential selective contact layers for photovoltaic (PV) devices. However, lower deposition temperatures are preferred in the fabrication of many PV devices, such as solar cells, to reduce their cost. Here, boron phosphide layers were grown on silicon (100) substrates using the standard plasma-enhanced chemical vapor deposition (PECVD) and time-modulated PECVD methods with trimethylboron [B(CH3)3, TMB] and phosphine precursors. The effect of plasma power and Ar addition on structural properties and chemical composition is investigated, while material properties are analyzed by transmission electron microscopy. Chemical characterization by the electron diffraction x-ray spectroscopy method showed high carbon content in the BP layer. Electron energy loss spectroscopy demonstrated almost stoichiometric B and P (1:1) content. Raman spectroscopy of annealed samples showed an increase in carbon-related peaks, therefore indicating that annealing does not lead to the crystallization of boron phosphide. Thus, using TMB as a precursor of boron leads to carbon contamination in both standard and time-modulated modes. Optical emission spectroscopy showed that the low-temperature growth of BP without plasma (Ar or phosphine) assistance using TMB is impossible. We conclude that there is a need to investigate other boron precursors for boron phosphide low-temperature growth to avoid carbon contamination in BP films.

The paper presents the results of the study of the effect of plasma etching of n-type silicon spin-coated with polystyrene balls on bulk silicon properties using careful surface chemical treatments and passivation. Plasma etching was carried out under different substrate temperatures (up to -140 °C). Photoluminescence decay time imaging was used to evaluate effective carrier lifetime in silicon samples after dry etching and a-Si:H passivation. According to the results obtained, plasma etching of n-Si(100) spin-coated with polystyrene balls within the temperature up to -140 °C in gas mixture of SF6/O2, RF and ICP plasma power of 30 W and 1000 W, respectively leads to minority carrier lifetime decrease. The difference in the rate of carrier lifetime degradation with etching time was found.