Structural Study of Ga(As)PN Layers for High-Efficiency Solar Cells on Silicon Substrates
Monolithically integrated III-V compound semiconductor layers on silicon substrates will enable semiconductor industry to fabricate new interesting device concepts in the near future. The device concepts that have received the most attention include CMOS compatible lasers that could be used in on-chip and chip-to-chip communication, n-channel high mobility transistors and silicon based high-efficiency solar cells. In order to realize these devices, many challenges existing in the polar-on-nonpolar epitaxy (e.g., anti-phase domains, stacking faults, lattice mismatch) needs to be overtaken. Despite the challenges however, it has recently been shown that an atomically smooth low defect density GaP buffer layer can be fabricated on silicon . Therefore, an increasing amount of attention has been addressed on the properties of gallium phosphide based dilute nitride materials (E.g., GaPN, GaAsPN). The benefit of these materials is that the nitrogen incorporation enables strain compensation and the energy band structure engineering.
We have studied the growth of GaP and Ga(As)PN layers on silicon and GaP substrates by metalorganic vapor phase epitaxy. Properties of the grown layers have been examined by Raman scattering, photoreflectance, photoluminescence, IV, atomic force microscopy, Rutherford backscattering and XRD studies[2-5]. We discuss the properties of the fabricated Ga(As)PN layers and focus on the characteristics of the layers that could be utilized to realize a high-efficiency solar cells on silicon substrates. The solar cell device concepts that have drawn our attention include n-GaP heterojunction emitters, GaAsP tandem cells and GaAsPN based intermediate band solar cells (IBSC).
In this work, we present the main experimental results of the fabricated layers. The results show that defects in Ga(As)PN layers on silicon most likely arising from the III-V/Si interface degrade the quality of the layers and that high-quality layers are realized on GaP substrates. Anti-phase domain and stacking fault/threading dislocation type defects have been observed by transverse scan analysis performed by high-resolution XRD setup. Photoreflectance results of Ga(As)PN layers show the conduction band splitting which can then possibly in the future be used to realize IBSC device on silicon substrate. At the moment, we are fabricating our first IBSC devices. In addition, IV measurements performed under 0.54 SUN show that semiconductor devices such as n-GaP/p-Si heterojunction solar cell are realized.
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