On the use of micro-Raman and photoluminescence spectroscopy for the study of doping concentration in classic and two-dimensional semiconductors: the case of p-type silicon and molybdenite

In this work, we develop characterization techniques based on Raman and PL spectroscopy for the study of doping effects in p-type Si and MoS2. Doping monitoring is a critical factor in the proper fabrication of Si-based electronics, which inherently requires the fabrication of targeted doped regions at the nanoscale. We show that it is possible to measure the dopant concentration in p-type Si over a wide range (10^15 ÷ 10^20 cm−3) using laser excitation in the visible and near UV regions. We exploit the effect of doping-dependent Fano interference in the first-order Raman spectrum of Si. The Raman spectra of differently doped Si samples are analysed with unprecedented accuracy using an improved fitting method that employs a convolution of Fano and Gaussian functions to distinguish the physical and instrumental contributions of emission. Regardless of the excitation wavelength, the onephonon Si peak exhibits broadening, frequency-softening, and intensity decay as a function of doping concentration, which are accurately determined for reliable doping characterization. The experimental data are directly compared with the relevant literature data of highly doped Si, showing that the proposed analysis is consistent with the previous studies at high doping concentrations and complementary in the intermediate concentration range (10^15 ÷ 10^18 cm−3). The fitted peak broadening and frequency shift parameters show linear trends as a function of doping concentration, which are used to construct calibration curves for doping monitoring applications. Differential penetration of light of different wavelengths into Si is used to probe the subsurface of Si wafers at different depths, allowing nondestructive characterization of doping. The application of near-UV Raman analysis to small-angle beveled samples enables doping profiling. The doping profile of p-type Si is reproduced down to 100 nm with good doping sensitivity (10 ppm) in the (10^18 ÷10^20) cm−3 concentration range. Excellent vertical (10 nm) and lateral (1 μm) resolutions demonstrate that Raman profiling is an efficient alternative to the more onerous SIMS and SRP techniques. This work also reports on the study of the effects of thermal annealing of mechanically exfoliated single and multilayer samples of MoS2 deposited on a SiO2/Si substrate. The changes in Raman and PL responses due to annealing of the samples in the temperature range of (200 ÷ 300 °C) are analysed. Optical microscopy and AFM analyses of samples annealed up to 400 °C are also reported. We demonstrate the presence of a nano-confined water film at the interface between MoS2 and the substrate. The thickness of this water film can be reduced by annealing and restored by water baths. Then we directly demonstrate the sublimation of the bottom layer of MoS2 at the interface with SiO2 by annealing at the highest temperatures. Annealing processes in the temperature range of (200 ÷ 300) °C lead to an increase in the sulphur vacancy concentration at the basal planes of the flakes. Passivation of sulphur vacancies by H2O and O2 adsorption after air exposure leads to the deplation of free electrons in MoS2, which is reflected in PL and Raman responses. PL emission, due to radiative recombination of neutral excitons (A0 ) and charged excitons (A- ), show an increase in integrated intensity after annealing, demonstrating the direct correlation between the population of type A excitons and sulphur vacancy concentration. The relative weight of the A0 and A- components of the PL spectrum can be tuned by annealing in the temperature range (200 ÷ 300) °C, with A- dominating at the lower temperatures and A0 dominating at the higher temperatures. Mathematical models using A0 and A- intensities allow to quantitatively estimate the variation in sulphur concentration and the change in free electron density in annealed MoS2 compared to pristine exfoliated MoS2.