Properties of III-V/Si heterojunction fabricated by HVPE
Abstract: Silicon is a promising material and is used for a wide range of applications in the electronics industry because of the high quality surface passivation given by the native oxide layer SiO2e. However, Si is not an ideal candidate for optoelectronic applications due to its indirect bandgap of 1.1eV, which limits the use of silicon for light emitting devices. III-V semiconductor materials have relatively higher electron mobility when compared to Si, they also have a direct bandgap which makes them more suitable for the fabrication of devices for electronic and optical applications. There are different III-V semiconductor materials such as GaN, InP, GaAs, GaP which can be used in the fabrication of optoelectronic devices. The limitation is that these compound semiconductors are costly because of their scarce availability in nature hence, it is economically expensive to fabricate devices using III-V compound semiconductors. This issue of economic feasibility can be resolved by integrating III-V and Si to fabricate devices with better electronic and optical properties and reasonable cost. Although it’s a reasonable argument to fabricate devices using III-V and Si. There is also a trade-off between enhanced electronic properties and the defects that are induced at the interface due to lattice mismatch and when the density of these defects is higher at the interface then the device performance degrades significantly. III-V compound semiconductor materials like GaAs and InP have lattice mismatch of 4% and 8% respectively with silicon, which would not be ideal as this will induce a lot of defects at the interface. GaP, on the other hand, has a lattice mismatch of 0.4% with silicon which will result in less defects. In this project, the research is focussed on studying the properties of three different III-V/Si heterojunctions i.e. GaAs/Si, GaP/Si, and GaAsP/Si. GaAs has direct bandgap and higher electron mobility with respect to Si, GaP on the other hand has a low lattice mismatch with Si which will induce less defects. GaAsP is a ternary compound with tunnable properties that are useful to fabricate III-V/Si heterojunction with desired requirements. A low-temperature epitaxial buffer layer growth is used for the fabrication of these heterojunctions, which was performed by using cost effective hydride vapour phase epitaxy (HVPE) technique. The focus was also on studying the optical properties of Selective Area Growth (SAG) of III-V/Si structures grown on (100) and (111) Si substrates by HVPE. Various characterization tools were used for analysing the morphology and optical properties of these heterojunctions. The morphological study was performed by using the scanning electron microscope (SEM) and Atomic force microscope (AFM). The optical properties of the sample were analysed by using photoluminescence (PL) and Raman spectroscopy. The dependence of morphology of III-V/Si heterostructure on Si substrate orientation was observed in both planar growth and SAG. The planar GaAs/Si(100) sample reveals meandering lines with step heights of 4-8nm, these could be associated with threading dislocations. However, planar GaAs/Si(111) sample revealed a comparatively smoother surface with high density of pits. In case of SAG the growth along the (100) plane was found to be more lateral than vertical, whereas the growth along the (111) plane was both lateral and vertical with pillar-like structures. Optical analysis suggests that the crystalline quality of SAG GaAs/Si(111) was better than the SAG GaAs/Si(100) sample. The morphology was also dependent on the growth temperatures as at low temperature the islands coalesce rapidly due to the decrease in critical radius for nucleation of III-V compound semiconductors. A further optimization in the growth process might be required as there might be a possibility that growth in SAG GaAs/Si(100) and SAG GaP/Si(100) samples have occurred through (111) facets. These findings can be used in future for device applications, such as solar cell, multi-junction PV, etc.
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