硅锗合金纳米粒子的合成与表征

Synthesis and Characterization of Silicon-Germanium alloy Nanoparticles in Nonthermal Capacitively-Coupled Plasmas Jatin Rath Md. Seraj Uddin Ansari, C Vijayan Indian Institute of Technology Madras, India jkriitm.ac.in, J.K.Rathuu.nl SNEC 13th 2019 International Photovoltaic Power Generation and Smart Energy Conference June 3-5, 2019, Shanghai, China Contents Introduction/Motivation Experimental Results and Discussion Conclusion References Introduction/Motivation SiGe alloy nanoparticles NPs are very promising candidates for solar cell applications. The size dependent optical and electronic properties and bandgap engineering due to quantum confinement in SiGe NPs make them more favorable for quantum dot based Tandem Solar Cells. However, these NPs suffer from Ge segregation and also face Ge diffusion from core to the shell to minimize the dangling bonds in case of hydrogen free nanoclusters. To tackle these challenges a good process control is needed during NPs’ growth. A nonthermal plasma CVD technique used in this work for synthesis, gives good monodispersed, freestanding gas phase NPs of device quality. Experimental Fig Schematic of VHF PECVD reactor. ref J. Phys. D Appl. Phys., vol. 48 ,p. 375201, 2015. Frequency 60MHz Power 150 W Pressure 0.8 mbar Gas flow ratio R i.e, RSilane/Germane Dilution Ar/H 2 gas or H 2 alone. Sample ID Gas Used Flow ratio RSilane/Germane Plasma mode S1 Ar H 2 SiH 4 GeH 4 25 sccm 25 sccm 4 sccm 2 sccm R2 CW S2 Ar H 2 SiH 4 GeH 4 25 sccm 25 sccm 4 sccm 4 sccm R1 Pulse 250 msec S3 Ar H 2 SiH 4 GeH 4 25 sccm 25 sccm 4 sccm 4 sccm R1 CW S4 H 2 SiH 4 GeH 4 50 sccm 4 sccm 4 sccm R1 CW SiGe NPs synthesis VHF PECVD reactor The four samples prepared at different conditions are given in table CW plasma Pulsed plasma Results and Discussion Raman Spectroscopy Si-Si 480 cm -1 Ge-Ge 275 cm -1 Si-Ge 390 cm -1 Si-Si 469 cm -1 Si-Ge 391 cm -1 Ge-Ge 285 cm -1 Si-Ge 390 cm -1 Ge-Ge 276 cm -1 Si-Si 474 cm -1 Si-Ge 389 cm -1 Si-Si 474 cm -1 Ge-Ge 280 cm -1 a b c d The existence of TO Si-Ge Raman peaks in spectra conform the SiGe NP formation. There is a shift in Si-Si Raman peak in these spectra, this is due to heavy Ge atom incorporation in Si lattice during alloy formation. Ar H 2 SiH 4 GeH 4 25 25 4 2 sccm Ar H 2 SiH 4 GeH 4 25 25 4 4 sccm Ar H 2 SiH 4 GeH 4 25 25 4 4 sccm H 2 SiH 4 GeH 4 50 4 4 sccm R2 R1 R1 R1 CW CW CW Pulse 250 msec Raman Spectrosopy Continue ref J. Appl. Phys. 114, 134310 2013 The Ge content in SiGe NPs from Raman spectra a The shift in Si-Si peak Due to Ge atom incorporation and not from the shift due to Nano-crystallite size The peak shift value is very nominal from nano-crystallite size y520-70x y Si-Si TO phonon Raman peak position x is the fraction of Ge in SiGe NP Sample ID S1 S2 S3 S4 Ge content 0.57 0.66 0.66 0.73 The Ge content of Si 1-x Ge x in different samples Sample ID Flow ratio R in reactor R in SiGe NPs S1 2 0.75 S2 1 0.51 S3 1 0.51 S4 1 0.37 Comparison of RSilane/Germane in CVD chamber and in SiGe NPs TEM image images are from sample S1. a b d c a Nicely dispersed NPs of size around 20nm quantum confinement b HRTEM image of NPs, shows lattice fringes in NPs proving the crystalline nature c Conforms crystalline nature from SAED image d Area EDS spectrum, shows the presence of Si and Ge in sample and the intensity ratio of Si and Ge peak also corresponds to the R. EDS Mapping from HAADF-STEM Image The EDS mapping image clearly shows that the Si and Ge atoms are randomly distributed in NPs and so it form a homogeneous alloy. The surface atom are getting oxidized and forming and oxide shell layer on NPs. Dark I-V Characteristics of SiGe using cAFM Dark I-V characteristics study for S4 sample in cAFM mode. Derivative of current is plotted w.r.t applied voltage. The band gap calculated for nanoparticle is E g 1.88 eV. Conclusion The SiGe NPs synthesized in VHF CVD reactor good process control and the particle size measured in TEM is in accordance with the quantum dots. Raman spectra shows higher incorporation of Ge in NPs with H 2 dilution than pure ArH 2 dilution. Si/Ge value in NPs is lower compared to R value Silane to Germane ratio CVD chamber. Si and Ge atoms in NPs are randomly distributed and it conforms the homogeneity of alloy NPs and show no sign of segregation of Ge in NPs. Acknowledgement Akshatha Mohan Poulios Ioannis R. Akis and D. K. 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