Heraklion, 2014 Si/SiC Nanowire Growth by using Al Catalyst Linsheng Liu

Heraklion, 2014 Outline Introduction - Motivation - Nanowire Growth Mechanism - MBE (general, advantages for NW growth) Plan - Measure the thickness of Al - Cluster study of Al film - Si Nanowire Growth study - Si converted to SiC Nanowire - Characterization

Heraklion, 2014 Problems Occur with Size Reduction 1.Expensive manufacturing cost --- High defect densities --- Complicated manufacturing techniques 2. Physical limitation 3. High power dissipation

Heraklion, 2014 One possible solution: Nanowire FET （b）（b） 1. Ultimate electrostatic control of channel(surround gate) 2. Potential for lower power dissipation than MOSFET 3. Nanowires allow the growth of axial heterostructures without the constraints of lattice mismatch. This provides flexibility to create heterostructures of a broad range of materials.

Heraklion, 2014 Interest in SiC NW FETs Combination of NWs and SiC properties High-temperature sensors Field emission displays (as cathodes) Biosensors High temperature (“operating voltage”) FETs SiC NWFETs with increased operation temperature can substantially increase packing density.

Heraklion, 2014 State of the art in SiC NW FETs (a) Shematic of a SiC NWFET; (b) I-V characteristic of SiC nanowire FET for different back gating from -40 to 40V. In the inset of (b) the ID-VG charateristic is presented. --Rogdakis K et al.IEEE Trans. Electron Devices, 2008, 55: 1970 Drawback: --The carrier concentration is too high due to unintentional doping and low crystalline quality of SiC NW, resulting in very low electron mobility (16cm 2 V -1 s -1 ). Possible Solution: Use relatively high cost methods(MBE or high purity CVD)

Heraklion, 2014 Our aim for growing SiC NWs 1.Grow Si NW by using Al catalyst; 2.Convert to SiC NW; 3.Characterization; 4.Applications: SiC NWFET, high temperature sensor, field-emission device; …

Heraklion, 2014 First Approach : conversion of MBE-grown Si NWs 750C800C850C EDX-STEM EELS-STEM HAADF micrograph of a section of nanowire of each sample. EDX line-scan is superimposed that indicates the presence of C and Si together in the perimeter of the sections. Semi-facetted sidewalls is show in octagonal shape in the directions (2 0 0) and (1-1 1).. EELS spectra in the core and the shell of the nanowire of each sample [1] E. Pippel, O. Lichtenberger and J. Voltersdorf, J. Matt. Sci. Lett., 19, 2059 (2000) [1]

Heraklion, 2014 #A 100 nm #B 100 nm #C First Approach : conversion of MBE-grown Si NWs-2 750C 800C850C

Heraklion, 2014 First Approach : conversion of MBE-grown Si NWs  750ºC of temperature seems to be enough to carburize regions of the Si NW using MBE system.  Carburization is demonstrated in a depth of around 20nm.  High temperatures seem to stimulate the generation of staking faults. Conclusions:

Heraklion, 2014 V80S IESL-FORTH Nanowires growth by using MBE 1) Epitaxial growth under ultra-high vacuum and high pure sources conditions, it is easy to obtain the clean surfaces, free of an oxide layer; 2) Reflection high-energy electron diffraction(RHEED) is a powerful in-situ characterization tool generally used in MBE, which follows the crystalline evolution of nanowires in real time during growth;

Heraklion, 2014 deposit Al on the substrate Create Al cluster(Solid) by heating Supply suitable precursors(Vapor) Al+precursor (Liquid) Supersaturation=> nanowire growth alternating precursors => controllable nanowire heterostructures nanowires can extend more than 20 micrometers in length Nanowire Growth Process Al Seed Si Substrate SEM images: David Kohen, et al. Phys. Status Solidi A, 2011(208):

Heraklion, 2014 Vapor Liquid Solid Growth Model The supersaturation of the metal-alloy catalyst is the main driving force for nanowire growth 1 – precursors direct impingement 2 – desorption from the “hut” 3 – diffusion from the sidewalls 4 – desorption from the sidewalls 5 – diffusion from the substrate to the sidewalls, 6 – diffusion from the sidewall to the drop 7 – surface nucleation Nucleation-mediated wire growth resulting in the vertical growth rate V.G. Dubrovskii and N.V. Sibirev, Phys. Rev. B 2008

Heraklion, 2014 Phase Diagram of Al-Si (b)TEM and SAED pattern --YEWU WANG, VOLKER SCHMIDT, STEPHAN SENZ AND ULRICH GOSELE, Epitaxial growth of silicon nanowires using an aluminium catalyst, Nature Nanotechnology, Vol1, December 2006, (a) SEM Cross sectional image; Advantage and Disadvantage: Si NW growth is under VSS growth condition. The advantage is low Si solubility in Al catalyst, if we grow Si-Ge heterostructure in this condition, the interface will be sharp. The disadvantage is the growth rate is very low.

Heraklion, 2014 Phase Diagram of Al-Si 2.SEM image of Al-catalysed Si nanowire [1]Joonho Bae, et al, Journal of Crystal Growth, 2008(310): [2]David Kohen, et al, Phys. Status Solidi A, 2011(208): SEM image of Al-catalysed Si nanowire

Heraklion, 2014 Outline Introduction - Motivation - Nanowire Growth Mechanism - MBE (general, advantages for NW growth) Plan - Measure the thickness of Al - Cluster study of Al film - Si Nanowire Growth study - Si converted to SiC Nanowire - Characterization

Heraklion, 2014 Choice of the catalyst Au Advantages: physical and chemical stability, low Eutectic temperature Drawbacks: act as a deep level trap decrease carrier mobility, lifetime, diffusion length…avoid in standard IC techniques Al Advantages: compatible with standard IC techniques, better optical properties Drawbacks: oxidize quickly, need very clean deposition system

Heraklion, 2014 Binary phase diagram of Au-Si alloy and Al-Si alloy

Heraklion, 2014 Al catalyst thickness Al: 1 to 10nm

Heraklion, 2014 Cluster study of Al Diameter of particles are less than 20 nm SEM Images of Au Clusters on GaAs

Heraklion, 2014 State of the art of Al mediated Si NW Growth (a) SEM Cross sectional image; (b)TEM and SAED pattern --YEWU WANG, VOLKER SCHMIDT, STEPHAN SENZ AND ULRICH GOSELE, Epitaxial growth of silicon nanowires using an aluminium catalyst, Nature Nanotechnology, Vol1, December 2006,  There is very few experimental result showing Si NWs grown by using Al with very low density defects.  Up to now, there is no group to use MBE grow Al mediated Si NW. NWs grown under ultra-high vacuum and high pure sources conditions in MBE will be helpful towards the aim of high quality NWs.

Heraklion, 2014