**1. Introduction**

68 Solar Cells – New Aspects and Solutions

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A critical goal for photovoltaic energy conversion is the development of high-efficiency, low cost photovoltaic structures which can reach the thermodynamic limit of solar energy conversion. New concepts aim to make better use of the solar spectrum than conventional single-gap cells currently do. In multijunction solar cells based on III-V heterostructures, better spectrum utilization is obtained by stacking several solar cells. These cells have achieved the highest efficiency among all other solar cells and have the theoretical potential to achieve efficiencies equivalent to or exceeding all other approaches. Record conversion efficiencies of 40.7 % (King, 2008) and 41,1 (Guter at al., 2009) under concentrated light for triple- junction allows hoping for practical realization of gianed values of efficiency in more multiplejunction structures. The expectations will be met , if suitable novel materials for intermediate cascades are found, and these materials are grown of an appropriate quality. Models indicate that higher efficiency would be obtained for 4-junction cells where 1.0 eV band gap cell is added in series to proven InGaP/GaAs/Ge triple-junction structures. Dilute nitride alloys such as GaInAsN, GaAsSbN provide a powerful tool for engineering the band gap and lattice constant of III-V alloys, due to their unique properties. They are promising novel materials for 4- and 5-junction solar cells performance. They exhibit strong bowing parameters and hold great potential to extend the wavelength further to the infrared part of the spectrum.

The incorporation of small quantity of nitrogen into GaAs causes a dramatic reduction of the band gap (Weyeres et al., 1992), but it also deteriorates the crystalline and optoelectronic properties of the dilute nitride materials, including reduction of the photoluminescence intensity and lifetime, reduction of electron mobility and increase in the background carrier concentration. Technologically, the incorporation probability of nitrogen in GaAs is very small and strongly depends on the growth conditions. GaAsN- based alloys and heterostructures are primarily grown by metaloorganic vapor-phase epitaxy (MOVPE) (Kurtz et all, 2000; Johnston et all, 2005)) and molecular-beam epitaxy (MBE) (Kurtz et al. 2002; Krispin et al, 2002; Khan et al, 2007), but the material quality has been inferior to that of GaAs. A peak internal quantum efficiency of 70 % is obtained for the solar cells grown by MOCVD (Kurtz et al. 1999). Internal quantum values near to unit are reported for p-i-n

Dilute Nitride GaAsN and InGaAsN Layers Grown by Low-Temperature Liquid-Phase Epitaxy 71

The sum of the film surface energy and the interface energy must be less than the surface energy of the substrate in order for wetting to occur and then layer by layer growth is expected. The VW growth mode is to be expected for a no wetting epitaxial layer. If γ and γ<sup>0</sup> are the surface free energies of the layer and substrate, respectively, and γi is the interfacial free energy the change in the free energy Δγ associated with covering the substrate with

If minimum energy determinates the mode for nucleation and growth, the dominated mechanism will be two-dimensional for Δγ <0 and three-dimensional for Δγ>0. However, even in the case of a wetting epitaxial layer (Δγ <0 ), the existence of mismatch strain can cause islanding after the growth of a few monolayers. This is because the strain energy , increases linearly with the number of strained layers. At some thickness, γ+γi exceeds γ0 and the growth mode transforms from FM to SK resulting in 3D islands on the 2D wetting layer. Whereas it is clear that the VW growth mode is expected for a nonwetting epitaxial layer, the behavior of a wetting deposit is more complex and requires further consideration. Often the interfacial contribution in the limit of zero lattice mismatch and weak chemical interactions between the film and substrate at the interface can be neglected in comparison to the surface free energy (γi ≈ 0). In this case the growth mode is determined entirely by the surface free energies of the film and substrate

Instead of these three main growth modes additional growth modes and epitaxial growth mechanisms could be distinguished (Scheel, 2003): columnar growth, step flow mode, step

The structural quality of the layer and surface morphology strongly depend on the growth method and the main growth parameters: supersaturation, misorientation of the substrate

In the case of flat substrate, the supersaturation increases until surface nucleation of a new monolayer occurs and its growth cover the substrate, followed by the nucleation of the next monolayer. For compound of limited thermodynamic stability or with volatile constituents like GaAs, GaN, SiC the appearance of the growth mode is largely predetermined by the choice of the growth method due to the inherent high supersaturation in epitaxy from the

The FM growth mode in LPE can only be obtained at quasi-zero misfit as it is established from thermodynamic theory (Van der Merwe, 1979) and demonstrated by atomistic simulations using the Lennard–Jones potential (Grabow and Gilmer, 1988) and also at low supersaturation. At high supersaturation a high thermodynamic driving force leads to a high density of steps moving with large step velocities over the surface and causes step

The VW mode is typical of VPE. Due to the high supersaturation a large number of surface nuclei arise, which then spread and form three-dimensional islands, that finally coalesce to a compact layer. Continued growth of a layer initiated by the VW mode often shows columnar growth which is a common feature in epitaxy of GaN and diamond. (Hiramatsu *et al*., 1991). The SK mode has been demonstrated by MBE growth of InAs onto GaAs substrate

and the difference of lattice constants between substrate and the epitaxial layer.

vapor phase and adjustable low supersaturation in LPE.

Δγ = γ + γi - γ0 (2.1)

The growth modes in heteroepitaxy are defined based on thermodynamic models.

epitaxial layer is:

material.

bunching.

(Nabetani *et al*., 1994).

bunching, and screw-island growth.

GaInAsN cell grown by MBE (Ptak et al 2005), but photovoltages in this material are still low. Recently chemical-beam epitaxy (Nishimura et al., 2007; Yamaguchi et al, 2008; Oshita et al, 2011) has been developed in order to improve the quality of the grown layer, but today it remains a challenge to grow dilute nitride materials with photovoltaic (PV) quality.

In this chapter we present some results on thick GaAsN and InGaAsN layers, grown by lowtemperature Liquid-Phase Epitaxy (LPE). In the literature there are only a few works on dilute nitride GaAsN grown by LPE (Dhar et al., 2005; Milanova et al., 2009) and some data for InGaAsN (Vitanov et al., 2010).
