Interfacial abruptness study in Au Catalyzed Si1-xGex/Si/Si1-xGex Heterostructure Nanowires Grown by VLS-CVD method P. Periwal1, N. Sibirev2, G. Patriarche3, V. Brouzet1, F. Bassani1, B. Salem1, V. Dubrovskii2, T. Baron1 1 Laboratoire des Technologies de la Microelectronique (LTM) UMR 5129 CNRS-‐UJF, CEA Grenoble, 17 Rue des Martyrs , 38054 Grenoble, France 2 IOFFE
Physical Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia 3Laboratoire de Photonique et de Nanostructures (LPN-‐CNRS), Route de Nozay, 91460 Marcoussis, France
INTRODUCTION
As MOSFETs are scaled down, power dissipation remains challenge for nanoelectronics devices. To circumvent it, alternative devices such as tunnel \ield effect transistors are potential candidates, where the carriers are injected by quantum band to band tunneling mechanism. In this context, axial nanowire heterointerfaces with well-‐controlled interfacial abruptness offer an ideal structure. In this poster, we present the effect of tuning Ge concentration in a Si1-‐xGex part of the nanowire on the Si/Si1-‐xGex and Si1-‐xGex/Si interfacial abruptness in axial Si-‐SiGe heterostructure nanowires grown by the Au-‐catalyzed vapor-‐liquid-‐solid method. The two heterointerfaces are always asymmetric irrespective of the Ge concentration or nanowire diameter. For a \ixed diameter, the value of interface abruptness decreases with increasing the Ge content for the Si/Si1-‐xGex interface but shows no strong Ge dependence at the Si1-‐xGex/Si interface where it shows a linear correlation with the nanowire diameter. To rationalize these \indings, a kinetic model for the layer-‐by-‐layer growth of nanowire heterostructures from a ternary Au-‐Ge-‐Si alloy is established which predicts a discrepancy in Ge concentration in the layer and the catalyst droplet. The Ge concentration in each layer is predicted to be dependent on the composition of the preceding layer. The most abrupt heterointerface (~5 nm) is achieved by growing Si1-‐xGex with x=0.85 on Si in a 25 nm diameter nanowire. Finally, we consider the in\luence of strain induced by the lattice mismatch on the Si/ Si0.15Ge0.85 nanowire heterostructure and show the absence of mis\it dislocations.
VLS GROWTH OF NANOWIRES
GROWTH CONDITION FOR GOOD MORPHOLOGY Pressure Change
Temperature Change
SiGe
600 to 450 °C
Si
SiGe
Growth Interruption
4.5 T to 1.5 T
Si
Process \low of VLS method
SiGe
SiGe
CVD Equipment
VLS Process involves 3 steps: 1. Adsorption of gases 0.3 µm 2. Decomposition of precursors 3. Supersaturation & Nucleation STEM Image
2 min growth interruption at heterointerface produces high density of straight NWs having good morphology (T=450°C, P= 4.5 T)
EFFECT OF TUNING Ge COMPOSITION & DIAMETER ON INTERFACIAL ABRUPTNESS AT HETEROINTERFACES SiGe
SiGe/Si
Ge concentration in Si1-‐xGex part NW diameter/nm 30±2% 60±5% 80±5%
Si/SiGe
Si
SiGe HAADF-‐STEM Image
HAADF-‐STEM intensity pro\ile
35±5 nm Si/SiGe SiGe/Si 55±5 nm Si/SiGe SiGe/Si 110±10 nm Si/SiGe SiGe/Si
20 nm 50 nm 30 nm 60 nm 30 nm 75 nm
10 nm 40 nm 16 nm 55 nm 27 nm 65 nm
5 nm 40 nm 11 nm 50 nm 25 nm 70 nm
Ø Two heterointerfaces are asymmetric Ø SiGe/Si is broader while Si/SiGe is sharper Ø At high Ge composition & small NW diameter, sharp interface (~5 nm ) is seen.
LAYER-‐BY-‐LAYER KINETIC MODEL Postulates Ø
Diameter dependence of interfacial abruptness: Slope is ê with an é in Ge composition.
Ø
Discrepancy in the layer & catalyst composition: Composition of each layer is dependent on the composition of the preceding layer
Layer-‐by-‐layer model Ø Linear dependence of interfacial abruptness with diameter
Abrupt Si/SiGe heterointerface at high Ge content & small diameter
BEST RESULTS
STRAIN ANALYSIS BY GPA on Si/Si0.15Ge0.85 RXY
Abrupt Si/Si0.15Ge0.85 interface
Experimental data on the interface abruptness of Si/ SiGe (cross symbols) and SiGe/Si (circle symbols) NW heterostructures at different Ge concentrations and NW radii, \itted by the model equations (lines).
⇒ No plastic relaxation, only elastic. ⇒ Nanowires relaxes at the edges. ⇒ Small deformation is seen
DXY
ACKNOWLEDGEMENTS
The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement NANOFUNCTION n°257375, the French Research National Agency for NAHDEVI project (ANR-11-IS09-0008). The authors want to thank P. Gentile from CEA-INAC for fruitful discussions.