126. INTERMEDIATE BAND SOLAR CELL MATERIAL GANASP

Department: Electrical & Computer Engineering
Faculty Advisor(s): Charles Tu

Primary Student
Name: Yanjin Kuang
Email: yakuang@ucsd.edu
Phone: 858-534-3014
Grad Year: 2013

Abstract
The incorporation of small amount of N (~1-2%) into GaAsP host splits the matrix conduction band into two subbands, namely E+ and E-, due to the strong interaction between the N localized states and the matrix extended states. The E- band, acting as an intermediate band between the valence and E+ bands, is a stepping stone for sub-band gap, low-energy photons to excite carriers across the gap via a two-step process. The E- and E+ band energy of this mixed-anion dilute nitride GaNAsP can be tuned according to the solar spectrum in favor of better photoabsorption by varying the group V composition in the alloy. This makes it a promising candidate for next generation photovoltaic devices. We have studied the growth and optical properties of GaNAsP grown by GSMBE. GaNAsP with [N]~1-2% and [As]~50% was grown on a GaP(100) substrate on top of a GaAsP metamorphic buffer layer. Considering the difficulty of group-V content determination with three group V elements, we developed an in situ method (by Reflection High Energy Electron Diffraction) to determine the As and P content and used X-ray reciprocal space mapping to determine the N content after growth. After growth, the As and P compositions were also profiled by Rutherford backscattering spectrometry (RBS), while [N] was characterized with nuclear reaction analysis (NRA). The RBS and NRA results show that the substitutional N fraction was larger than 0.85. Furthermore the As contents were overestimated in the in situ method due to an ignorance of thermal desorption of As in the in situ model and the overestimation of N followed. Nevertheless, the in situ and ex situ combined method serves as a good preliminary composition control for mixed anion arsenide and phosphide. The photoreflectance experiment shows the existence and the energy level of the E+ band, in addition to the E- band energy level, which is consistent with photoluminescence measurement. The extra photon absorption as a result of the available electron transition from the valence band to the E+ band due to the conduction band splitting was observed by photoabsorption measurement. These two optical results together are a strong demonstration of the proposed merit of GaNAsP.

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