Structural, Optoelectronic and Electrical Properties of GaAs Microcrystals Grown from (001) Si Nano-areas

Abstract

An innovative approach is being investigated to develop III–V compounds on silicon (Si) substrates with the purpose to offer a technological alternative for the development of high efficiency solar cells ( ∼ 30 %). Until now, germanium (Ge) substrate has been the privileged material for the development of III–V multi-junctions (MJ) solar cells mainly dedicated to space applications. Ge offers several advantages, namely the lattice matching to Si and its use as a bottom cell in the MJ. However, the main drawback remains the cost of Ge substrates, which makes it inappropriate for terrestrial photovoltaic (PV) applications. New routes for high efficiency MJ solar cells are expected through the significant improvements of the selective area epitaxy (Li et al., J Appl Phys 103:106102, 2008; Deura et al., J Cryst Growth 310:4768–4771, 2008; Hsu et al., Appl Phys Lett 99:133115, 2011) allowing defect free III–V compounds to be grown on Si substrates patterned with dielectric films. In this work, Si nanoscale areas opened through a SiO₂ layer ( < 1 nm) formed on (001) Si have been used to grow GaAs microcrystals by chemical beam epitaxy (CBE) in the temperature range 550–600 ^∘C (Renard et al., Appl Phys Lett 102:191915, 2013). Structural, optoelectronic and electrical properties of GaAs microcrystals have been analyzed at room temperature by micro-Raman, photoluminescence and conductive probe atomic force microscopy (CP-AFM). The fine structure of crystals (facet orientations, crystal defects) has also been investigated by transmission electron microscopy (TEM). Linear polarized Raman spectroscopy performed on multiple microcrystals shows exclusively the TO mode which is typically expected for (110) GaAs plane orientations and/or heavily n-type Si-doped GaAs (Zardo et al., Phys Rev B 80:245324, 2009). TEM confirms that all facets are {110}, but unintentionally Si doping cannot be excluded. Indeed, PL measure-ments point out a red shift for the microcrystals for which nucleation seeds were created by silane exposure. CP-AFM imaging of GaAs microcrystals performed at + 1 and − 1 V, respectively, points out a current rectification behavior confirmed by local I–V measure-ments (Fig. 37.1). These results can be interpreted as a sign of the presence of a p-n junction, which agrees well with the p-type doping of Si substrates used in this study (1–5 Ωcm) and the unintentionally n-type doping of GaAs microcrystals suggested by PL measurements (Pavesi and Henini, Microelectron J 28:717–726, 1997).