Crystalline Silicon Solar Cells

Atmospheric-Pressure Plasma-Enhanced Spatial ALD of SiO2 Studied by Gas-Phase Infrared and Optical Emission Spectroscopy

An atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) process for SiO2 using bisdiethylaminosilane (BDEAS, SiH2[NEt2]2) and O2 plasma is reported along with an investigation of its underlying growth mechanism. Within the temperature range of 100-250 °C, the process demonstrates self-limiting growth with a growth per cycle (GPC) between 0.12 and 0.14 nm and SiO2 films exhibiting material properties on par with those reported for low-pressure PEALD. Gas-phase infrared spectroscopy on the reactant exhaust gases and optical emission spectroscopy (OES) on the plasma region are used to identify the species that are involved in the ALD process. Based on the identified species, we propose a reaction mechanism where BDEAS molecules adsorb on -OH surface sites through the exchange of one of the amine ligands upon desorption of diethylamine (DEA). The remaining amine ligand is removed through combustion reactions activated by the O2 plasma species leading to the release of H2O, CO2, and CO in addition to products such as N2O, NO2, and CH-containing species. These volatile species can undergo further gas-phase reactions in the plasma as indicated by the observation of OH*, CN*, and NH* excited fragments in OES. Furthermore, the infrared analysis of the precursor exhaust gas indicated the release of CO2 during precursor adsorption. Moreover, this analysis has allowed the quantification of the precursor depletion yielding values between 10 and 50% depending on the processing parameters. Besides providing insights into the chemistry of atmospheric-pressure PE-s-ALD of SiO2, our results demonstrate that infrared spectroscopy performed on exhaust gases is a valuable approach to quantify relevant process parameters, which can ultimately help evaluate and improve process performance.

Laser ablation of silicon with THz bursts of femtosecond pulses

In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range.

Ethylammonium Lead Iodide Formation in MAPbI3 Precursor Solutions by DMF Decomposition and Organic Cation Exchange Reaction

Extra peaks have constantly been observed in the X-ray diffraction measurement for the CH3NH3PbI3 film. Such mysteries have now been uncovered in this paper, in which powder X-ray diffraction, in situ X-ray diffraction, and scanning electron microscopy measurements were conducted, and these peaks were attributed to the ethylammonium lead iodide (CH3CH2NH3PbI3/EAPbI3). It was found that the formation of EAPbI3 was triggered by the breakdown of N, N-dimethylformamide (DMF), which was adopted as the solvent in the preparation of the precursor solutions. EAPbI3 was generated by the organic cation exchange reaction in the subsequent annealing process. A simple solution for this problem is proposed in this paper as well, which would hopefully help the community to eradicate this impurity.