Si and Ge thin films & heterostructures for multifunctional devices
This is the research area on which we are working on for a decade and the results are considered as significant breakthroughs for future nanodevice applications. In this context, we are mainly interested in developing pseudomorphic strained Si-Ge alloy heterostructures for high mobility MOSFETs to low dimensional quantum structures. Fabrication of Ge quantum dots, Ge-Sn alloy and strained Ge epilayers have been done to achieve light emission from an indirect bandgap semiconductor, which will be useful to realize CMOS compatible optical sources in future. A present project focuses on Ge quantum dots/Si nanowire heterostructure based FET arrays for detection in Infrared and THz wavelengths. Si nanostructures such as Si nanocrystals, Si nanocones (black Si templates), Si nanowires and their heterojunctions with other semiconductors and 2D materials are utilized for fabrication of photodetectors, solar cells and LEDs. Ultra-thin Si membranes have been fabricated for mechanically flexible optoelectronic applications. Ge nanocrystals are also used for a floating gate and resistive switching memory devices.
Black Si for photonic devices
Large area Si wafer is etched to produce black-Si (b-Si). Ultralow reflective (<1.5% in the visible range) b-Si is produced by metal-assisted chemical-etching technique. The heterojunction devices with other lower dimensional materials like MOS2 and CZTS nanocrystals have high responsivity and efficiency. Inks of the lower dimensional materials can be coated by spin-coating method. The CZTS-NCs/b-Si heterojunction photovoltaic devices have shown efficiency >5 % and wideband photodetection. A light emitting diode have also been reported where heterojunction of n-CdS film on vertically standing nanoporous Si nanocones exhibit intense white light emission.
-Singh, S et al. “Solution-Processed Black-Si/Cu2ZnSnS4 Nanocrystal Heterojunctions for Self-Powered Broadband Photodetectors and Photovoltaic Devices” in ACSaem (2021) DOI: 10.1021/acsaem.1c00448
-Sarkar, A et al., “Geometry Controlled White Light Emission and Extraction in CdS/Black-Si Conical Heterojunctions” in ACSaem (2018), DOI: 10.1021/acsaelm.8b00001
Tunable electroluminescence from Si nanocrystals
Tunable electroluminescence properties of size-controlled Si nanocrystals embedded in silicon rich oxide films are demonstrated at room temperature, using an active light emitting layer in the metal oxide semiconductor device structure. The carrier transport and light emission mechanism have been studied in detail through current–voltage characteristics and ultrafast transient spectroscopy, respectively. The origin of electroluminescence and the size-tunable emission peak have been analyzed and attributed to the radiative recombination of carriers within Si nanocrystal quantum wells.
-Sarkar, A et al. “Size-tunable electroluminescence characteristics of quantum confined Si nanocrystals embedded in Si-rich oxide matrix” in APL (2020) DOI: 10.1063/5.0001840
Emission properties of MBE grown Ge microdisks
We have reported the optical characteristics of relatively large sized (~7.0–8.0 μm) but low aspect ratio Ge microdisks grown on a virtual Si0.5Ge0.5 substrate using molecular beam epitaxy (MBE) following the Stranski–Krastanov growth mechanism. P-I-N diodes with an intrinsic layer containing Ge microdisks have been fabricated to study their emission and photoresponse characteristics at an optical communication wavelength of ~1550 nm for Si CMOS compatible on-chip optical communications.
-Singh, S et al., “Superior optical (λ ~ 1550 nm) emission and detection characteristics of Ge microdisks grown on virtual Si0.5Ge0.5/Si substrates using molecular beam epitaxy” in Nanotechnology (2019), DOI: 10.1088/1361-6528/ab5abe
A unique light trapping mechanism associated with nano-conical textured black Si templates has been utilized to achieve improved photoresponse MoS2-QD/Si heterojunction device. The combination of MoS2 QDs and black Si has resulted in a broader spectral response with enhanced optical absorption in the nano-conical heterojunction devices. Finite element based optical simulation results revealed the superiority of MoS2 QDs/Si nano-conical heterojunctions due to improved light trapping.
-Sarkar, A et al., “Photoresponse characteristics of MoS2 QDs/Si nanocone heterojunctions utilizing geometry controlled light trapping mechanism in black Si” in Nanotechnology (2019), DOI: 10.1088/1361-6528/ab3c9f
Ge quantum dots grown by MBE
Explanation to the individual Ge QD charging phenomena and current transport is reported, which is very important to understand the Ge/Si nano devices. It is fundamentally important to understand the nanoscale electronic properties of a single quantum dot (QD) contrary to an ensemble of QDs. The study contains characteristics of self-assembled Ge QDs, which was grown on Si substrates by solid source molecular beam epitaxy driven by the Stranski-Krastanov method.
-Singha, R. K et al. “Surface potential, charging and local current transport of individual Ge quantum dots grown by molecular beam epitaxy” in Appl. Surf. Sci. (2017) DOI: 10.1016/j.apsusc.2017.02.212
Photovoltaic device on black-Si
We show that a significant enhancement of solar cell efficiency can be achieved in cells fabricated on black Si made using inductively coupled plasma–reactive ion etching (ICP-RIE). The ICP-RIE-fabricated black Si results in an array of vertically oriented defect-free Si nanocones exhibiting an average reflectance ≤2% over most of the relevant solar spectral range. The fabricated n-CdS/p-Si heterojunction exhibits promising power conversion efficiency close to 3%, up from a mere efficient 0.15% for a similar cell fabricated on a planar Si.
-Katiyar, A. K et al. “Enhancement of Efficiency of a Solar Cell Fabricated on Black Si Made by Inductively Coupled Plasma–Reactive Ion Etching Process: A Case Study of a n-CdS/p-Si Heterojunction Cell” in ACSami (2015), DOI: 10.1021/acsami.5b04978
