Dharmendra Kumar Rai

SILICON QUANTUM DOTS GROWTH IN SiNx DIELECTRIC: A REVIEW

This paper reviews research works carried out on silicon quantum dots (Si-QDs) embedded in the silicon nitride (SiNx) dielectric matrix films with different fabrication techniques and different characteristics. The advantages of SiNx as a dielectric compared to silicon dioxide (SiO2) for Si-QDs from a device point of view are discussed. Various fabrication techniques along with different optimized deposition conditions are summarized. The typical results of structural characteristics of the films with Raman spectroscopy and Transmission Electron Microscopy (TEM) are discussed. The origin of photoluminescence (PL) from the films and the chemical compositional analysis such as X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Secondary Ion Mass Spectroscopy (SIMS) analysis of the films are also made available in brief. The charge conduction mechanism in the films with metal–insulator–semiconductor (MIS) structure, with their electrical characterization like capacitance–voltage (C–V) and current–voltage (I–V) measurements are presented.

Optical bandgap tuning of ICPCVD-made silicon nanocrystals for next generation photovoltaics

Superior optical properties of Si-Nanocrystals (Si-NCs) compared to bulk Si, particularly tunability of bandgap by controlling size, can be exploited for realizing next generation “all-Si” tandem solar cells [1]. In this study, we present optical bandgap tunability of Si-NCs made by Inductively Coupled Plasma Enhanced Chemical Vapor Deposition (ICPCVD) using SiO2/SiOx superlattice approach [2]. Deposition time of SiOx sublayer, and hence the related thickness (TSRO), was used as a variable parameter to realize Si-NCs of varying sizes. All multilayer (ML) samples were subsequently furnace annealed at 900°C, to allow for Si-NC formation. Formation of Si-NCs was confirmed by Raman spectroscopy. Transmittance, reflectance and absorptance spectra indicated gradual increase of bandgap with decreasing TSRO. Optical bandgap, ETauc, estimated using Tauc analysis showed an increase in optical bandgap from 1.45 eV to 2.5 eV, as TSRO was varied from 10 nm to 2 nm respectively.

Improvement in short circuit current of p-i-n solar cell with silicon quantum dot superlattice structure by optimizing SiN x thickness

Superlattice consisting of 10 alternate layers of a-Si with QDs and SiN_x are incorporated as i-layer in a p-i-n solar cell using HWCVD method. Superlattice with QDs showed absorption coefficient in the range of ~10⁵-10⁴ cm^-1. Calculated optical bandgap of the SL with QDs (~1.84 eV) is higher than the SL without QDs (~1.79 eV) and this is attributed to QCE. The cells with QDs showed I_SC= 1.806 μA and V_OC= 25 mV. Increase in I_SC of cells is attributed to increase in tunneling current due to decrease in SiN_x thickness. Low V_OC has been attributed to the thin SiN_x layers, the defects and the interface states which result in recombination.

Mechanism for formation of ultra thin SiNx on a-Si - XPS and FTIR studies

The mechanism of the formation of in-situ ultra thin silicon nitride (SiNx) layer (<; 2 nm) on amorphous silicon (a-Si) layer has been studied by Fourier transform infra-red spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The ultra-thin SiNx layers have been obtained through a hot-wire chemical vapor deposition (HWCVD) route. Approximately 1-4 nm thick films of SiNx were obtained on 12 nm and 40 nm thick a-Si at nitriding temperatures of 250 °C and 850 °C. FTIR of the ultra-thin SiNx layer reveals the radicals of decomposed (NH3) gas reacts with a-Si to form SiNx. Further, our XPS results corroborate this mechanism for the formation of SiNx and confirms the formation of ultra thin SiNx layers. This work will enable the development of Si quantum well (Si-QW) superlattice and Si quantum dot (Si-QD) superlattice structures based on a-Si for third generation (3G) solar photovoltaics.

Red emission from nano-silicon nitride multilayer films prepared using hot-wire chemical vapor deposition

Ultrathin silicon nitride multilayer films were prepared by varying silane/ammonia flow ratios 1/10 and 1/20 alternately in a silicon nitride hot-wire chemical vapor deposition chamber. Some films were annealed at 1000 °C for 1 h in an argon environment. Cross-sectional transmission electron microscopy shows ~6 nm silicon clusters formed in the annealed film. Strong visible red emission in the wavelength range 810–820 nm was detected from the films when excited with the 325 nm He–Cd laser excitation. A blueshift in the photoluminescence of the annealed film with respect to the as-deposited film is attributed to the silicon quantum confinement due to the presence of silicon clusters. Disappearance of the photoluminescence peak at 521 nm related to silicon dangling bonds in the annealed film in comparison with the as-deposited film was confirmed by the photoluminescence measurements.

Effect of hydrogen passivation on optical properties of a-Si/SiN X multilayered films with Si-QDs and without Si-QDs

The objective is to develop better understanding of the optical behavior of a-Si/SiNX multilayer with silicon quantum dots (Si-QDs) which can be used as light absorber material for Si-QD based solar cells. In this work, reflectance of a-Si, SiNX/a-Si, a-Si/SiNX and SiNX/a-Si/SiNX structured films with Si-QDs and without Si-QDs were examined. Hydrogen passivation (H-passivation) of a film with Si-QDs or without Si-QDs showed negligible effect on the optical property of the film. The films with Si-QDs before and after H-passivation showed low reflectance of light in the wavelength range of 200-600 nm compared to the films without Si-QDs. The enhancement in the absorption of light in the films is attributed to the Si-QDs and the quantum confinement effect (QCE).

Ultra Thin SiNX on a-Si In Situ Hot-Wire CVD by Decomposing NH3 Gas

The fabrication of ultra thin silicon nitride (SiNX) layer (< 2 nm) on amorphous silicon (a-Si) in-situ hot-wire CVD by decomposing ammonia (NH3) gas is reported. Approximately 1.5 nm thin SiNX is formed by nitridation of 40 nm thick a-Si for 10 min at substrate temperature of 250 °C. The amorphous phase of SiNX formed on a-Si and a-Si layer deposited on c-Si wafer is identified by Raman spectroscopy. The formation of ultra thin SiNX by nitridation of a-Si at 250 °C is confirmed by X-ray photoelectron spectroscopy (XPS) depth profile measurement of SiNX/a-Si structured film. The report indicates that the HWCVD method can be used for fabricating superlattice structures consisting of ultra thin SiNX layers (< 2 nm).

Co-relation between capacitance-voltage, conductance-voltage and photoconductive properties of the as-deposited and annealed a-Si/SiN x multilayer films prepared using hot-wire CVD

In this paper, the electrical properties of the as-deposited and the annealed a-Si/SiNx multilayer (ML) films are compared. The a-Si/SiNx ML films are prepared at 250°C using hot-wire chemical vapor deposition (HWCVD) technique. Silicon quantum dots (Si-QDs) are formed in the a-Si layers of ML films upon annealing at 850°C. Metal insulator semiconductor (MIS) structures containing the ML films are characterized with high frequency capacitance-voltage (C-V), conductance-voltage (G-V) and photo-current measurements. These characterizations show that the annealed ML films have high defects centers than the as-deposited ML films. A shift in the C-V plot and a peak shift in the G-V plot for the annealed ML film are due to the charge storage in the defects centers and the quantized states of Si-QDs. The defect centers introduced low photo-response for the annealed ML film (Si-QD structure) than the as-deposited ML films.