Abstract
This work studies the effect of four different types of buffer layers on the structural and optical properties of InGaN layers grown on Si(111) substrates and their correlation with electrical characteristics. The vertical electrical conduction of n-InGaN/buffer-layer/p-Si heterostructures, with In composition near 46%, which theoretically produces an alignment of the bands, is analyzed. Droplet elimination by radical-beam irradiation was successfully applied to grow high quality InGaN films on Si substrates for the first time. Among several buffer choices, an AlN buffer layer with a thickness above 24 nm improves the structural and optical quality of the InGaN epilayer while keeping a top to bottom ohmic behavior. These results will allow fabricating double-junction InGaN/Si solar cells without the need of tunnel junctions between the two sub-cells, therefore simplifying the device design.
Highlights
Due to the inadequacy of a single solar cell to absorb light over the full solar spectrum, a stack of multiple sub-cells have been proposed and studied intensively during the last few decades.1 Most of these efforts have focused on material systems such as Ge, InP, GaAs and GaSb
The buffer layers employed in this study were the following: i) AlN layers grown by molecular beam epitaxy (MBE) at 860 ◦C with nominal thickness of 6, 24, 42 and 84 nm. ii) A 6 nm thick In0.10Al0.90N layer grown by MBE at 550 ◦C. iii) RF-sputtered In0.39Al0.61N layers grown at room temperature (RT) with nominal thickness of 5 and 20 nm. iv) Intentionally nitridated Si at 860 ◦C to generate a 2-3 nm thick β-SixNy layer
The structural and chemical composition of a 42 nm thick AlN buffer layer was analyzed by plan-view TEM and energy-loss spectroscopy (EELS) respectively
Summary
Due to the inadequacy of a single solar cell to absorb light over the full solar spectrum, a stack of multiple sub-cells (multijunction) have been proposed and studied intensively during the last few decades. Most of these efforts have focused on material systems such as Ge, InP, GaAs and GaSb. Due to the inadequacy of a single solar cell to absorb light over the full solar spectrum, a stack of multiple sub-cells (multijunction) have been proposed and studied intensively during the last few decades.. Due to the inadequacy of a single solar cell to absorb light over the full solar spectrum, a stack of multiple sub-cells (multijunction) have been proposed and studied intensively during the last few decades.1 Most of these efforts have focused on material systems such as Ge, InP, GaAs and GaSb. not so much work has been reported on the use of III-Nitrides alloys to produce this type of multijunction solar cells. For the double-junction case, a theoretically calculated maximum efficiency of 39% can be achieved with two sub-cells with a bandgap combination of 1.74 and 1.13 eV respectively; values that can be obtained by using an InGaN-based sub-cell together with a Si one. The integration of InGaN alloys with the mature Si photovoltaic (PV) technology would yield high efficiency solar cells at a reasonable cost
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