Background
Photovoltaic (PV) cells convert the energy from the sun into useful electrical energy. Indium gallium nitride (InGaN) is a III-N type semiconductor material, meaning elements from group III are combined with nitrogen to produce a semiconductor, that is gaining ground in the PV market as a viable and tunable device. By varying the composition of the material, the band gap energy of the material (the energy level at which the material responds most efficiently to incoming light) can be shifted. Typically, the composition of such alloys is written as In(x)Ga(1-x)N, with x indicating the atomic percent portion of In in the alloy. The following provides a comprehensive background on the current literature available for the material, with links provided to the original documents where possible.
Abstract
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Construction
The typical construction for an In(x)Ga(1-x)N cell as grown on a silicon substrate is shown here in Figure 1. Another common substrate is glass. The oxide layer occurs naturally on all surfaces of the substrate. The amorphous GaN (a-GaN) layer is deposited to match the crystal lattices between the InGaN and SiO2 layers, as the mis-match introduces residual stresses into the crystal lattice, leading to distorted optical and electrical properties. The top layer of InGaN is the layer in which solar energy is converted to electrical energy.
InGaN Articles
“Complete compositional tunability of InGaN nanowires using a combinatorial approach.” T. Kuykendall, Philipp Ulrich, Shaul Aloni, Peidong Yang. Nature Vol. 6 (Oct 2007)
This article covers the first experiment in which the composition of In(x)Ga(1-x)N nanowires were varied in composition over the entire range of x = 0 to x = 1, that is, from pure GaN to pure InN. It demonstrates the tenability of the band gap of InGaN from the near infrared region to the near ultraviolet region. The article also discusses some of the challenges of growing these nanowires which include:
- Threading dislocations in both a-GaN and InGaN layers which lead to recombination centres, significantly reducing efficiencies within the cell.
- An improved technique over hydride vapour-phase epitaxy, which is only a viable method up to x = 0.2, due to the elevated levels of hydrogen which are known to interfere with the incorporation of In into the crystal lattice.
- Reduced effects of carbon contamination as observed in metal-organic chemical vapour deposition (MOCVD).
The procedure used involved a horizontal single-zone furnace divided into four temperature zones, which was used to evaporate the raw materials onto Si or sapphire substrates at unique compositions determined by the precursor mixing gradients of temperature and evaporation rates. Several tests were then performed on the resulting material, including X-ray diffraction (XRD) to verify composition, scanning electron microscopy (SEM) images were captured to determine crystal structure and transmission electron microscopy (TEM) to verify ordered crystal structure within nanowires. Optical analysis demonstrated the PL spectrum over which the cell responded (varied from 1.0 eV to 4.0 eV, or approximately 325 nm to 850 nm).
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| header 1 | header 2 | header 3 |
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| row 1, cell 1 | row 1, cell 2 | row 1, cell 3 |
| row 2, cell 1 | row 2, cell 2 | row 2, cell 3 |
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