Pressure-sensitive liquid phase epitaxy of highly-doped n-type SiGe crystals for thermoelectric applications

Hung-Wei Li,Chih-Wei Chang

doi:10.1038/s41598-019-39786-y

Abstract

Based on recent works, the most desirable high-temperature thermoelectric material would be highly-doped Si1−xGex crystals or films with sufficiently high Ge concentrations so that simultaneous enhancing the power factor and wave-engineering of phonons could be possible on the ballistic thermal conductor. However, available thin film deposition methods such as metal organic chemical vapor deposition, electron-beam evaporation, or sputtering are unable to produce highly-doped SiGe single crystals or thick films of high quality. To fabricate the desired material, we here employ liquid phase epitaxy to make highly-doped (up to 2% GaP doping) SiGe crystals with minimized concentration variations on Si (111) and (100) substrates. We find that growing Si1−xGex (x = 0.05~0.25) crystals from Ga solvents at relatively high vacuum pressure (0.1 torr) displays significant deviations from previous calculated phase diagram. Moreover, doping GaP into SiGe is found to affect the solubility of the system but not the resulting Ge concentration. We thus plot a new pressure-dependent phase diagram. We further demonstrate that the new pressure-induced liquid phase epitaxy technique can yield Si1−xGex crystals of much higher Ge concentrations (x > 0.8) than those grown by the conventional method.

Highlights

Recent experimental discoveries of room temperature ballistic thermal conduction have opened possibilities for engineering wave properties of phonons[1,2]
Because the dominant low-frequency phonons carrying out the heat conduction are estimated to have wavelengths longer than 10 nm, which is much shorter than the diameters (~200 nm) of SiGe nanowires, the observed ballistic thermal conduction is suggested to be a bulk property of SiGe1
We firstly employ energy dispersive X-ray spectroscopy (EDS) to investigate concentration profiles along the thickness of SiGe crystals/films grown by different Liquid phase epitaxy (LPE) methods and cooling rates

Summary

Introduction

Recent experimental discoveries of room temperature ballistic thermal conduction have opened possibilities for engineering wave properties of phonons[1,2]. Because the dominant low-frequency phonons carrying out the heat conduction are estimated to have wavelengths longer than 10 nm, which is much shorter than the diameters (~200 nm) of SiGe nanowires, the observed ballistic thermal conduction is suggested to be a bulk property of SiGe1. Www.nature.com/scientificreports dependent time-domain thermal reflectance measurements on bulk Si0.4Ge0.6 suggest pronounced contributions from ballistic phonons[13]. These property indicates that large area applications based on the novel ballistic thermal conduction could be realized in SiGe crystals or thick films of microscale thickness. To experimentally investigate their ballistic thermal conduction, SiGe crystals or films of micrometer thick will be needed To further realize their thermoelectric applications, highly-doped SiGe films are required. The subtleties of increased carrier concentrations and reduced thermal conductivity have not been fully understood, these works indicate the possibilities for further engineering σ and κ in SiGe

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Results