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 jkr@iitm.ac.in, J.K.Rath@uu.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, R=Silane/Germane) § Dilution Ar/H 2 gas or H 2 alone. Sample ID Gas Used Flow ratio R=(Silane/Germane) Plasma mode S1 Ar: H 2 : SiH 4 :GeH 4 25 sccm: 25 sccm: 4 sccm: 2 sccm R=2 CW S2 Ar: H 2 : SiH 4 : GeH 4 25 sccm: 25 sccm: 4 sccm: 4 sccm R=1 Pulse (250 msec) S3 Ar: H 2 : SiH 4 : GeH 4 25 sccm: 25 sccm: 4 sccm: 4 sccm R=1 CW S4 H 2 : SiH 4 : GeH 4 50 sccm: 4 sccm: 4 sccm R=1 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 R=2 R=1 R=1 R=1 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 y=520-70x 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 R(Silane/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 Ar+H 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. Ferry, J. Vac. Sci. Technol B 23, 1821 (2005). § B. J. Yan, L. Zhao, B. D. Zhao, J. W. Chen, G. H. Wang, H. W. Diao, Y . L. Mao, and W. J. Wang, Vacuum 89, 43 (2013). § A. Torres, M. Moreno, A. Kosarev, and A. Heredia, Journal of Non-Crystalline Solids 354, 2556 (2008). § S. C. Erwin, L. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy and D. J. Norris, Nature, vol. 436, p. 91, (2005) § Y . Maeda, Phys. Rev. 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