Composition dependent doping and transport properties of CuGaSe 2

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Composition dependent doping and transport properties of CuGaSe2

Susanne Siebentritt, Andreas Gerhard, Stephan Brehme, Martha Ch. Lux-Steiner Hahn-Meitner-Institut, Glienicker Str. 100, 14109 Berlin, Germany ABSTRACT Chalcopyrites are doped by intrinsic defects, therefore their doping behavior depends on their composition. The doping and transport properties of epitaxial CuGaSe2 layers prepared under varying Cu excess have been investigated by temperature dependent Hall effect and conductivity measurements. Two acceptors, 134 meV and 80 meV deep, and a high degree of compensation, increasing with decreasing Cu excess, are found. The temperature dependence of the mobility indicates scattering with phonons, demonstrating high quality material. Defect scattering dominates at lower temperatures for CuGaSe2 grown under moderate Cu excess. CuGaSe2 grown under little or no Cu excess shows transport in a defect band at lower temperatures. INTRODUCTION CuGaSe2 is a promising photovoltaic material, its applications include high open circuit voltage modules and tandem devices. As for all chalcopyrite semiconductors its electrical transport and doping properties are determined by intrinsic defects, like vacancies or antisites, which depend on the exact composition of the material. There is a fundamental difference between CuGaSe2 grown under Cu-rich conditions and CuGaSe2 grown under Ga-rich conditions. According to the phase diagram [1] Ga-rich chalcopyrite exists, whereas under Curich conditions the system shows two phases: stoichiometric chalcopyrite and a copper selenide phase. Nevertheless, the details of the defects of material grown under Cu-rich conditions depend on the amount of Cu excess during growth. A detailed photoluminescence (PL) study [2] of the composition dependence of defects has shown for material prepared under Cu excess two donor-acceptor pair transitions (DA), the relative intensity of which depending on the Cu excess, and for Ga-rich material broad luminescence peaks, indicating a high density of charged defects, i.e. a high degree of compensation. The temperature dependence of the PL spectra revealed the existence of two acceptors, 100 and 60 meV deep, and a shallow donor, 12 meV deep. This is in contradiction to most work in the literature (see e.g. [3-6]), where in analogy to CuInSe2 an 80100 meV deep defect is identified as a donor. For near stoichiometric material the DA luminescence of the shallower acceptor dominates while for films grown under high Cu excess the deeper acceptor dominates the PL spectrum. In this paper we present Hall effect and conductivity measurements in support of the existence of two acceptors with corresponding depths together with a study of the transport properties.

PREPARATION AND HALL EFFECT MEASUREMENTS Epitaxial CuGaSe2 layers with varying [Cu]/[Ga] ratio have been grown on semiinsulating GaAs(001) wafers by metalorganic vapor phase epitaxy (MOVPE) in an Aixtron AIX200 H4.4.1

horizontal reactor. Films grow epitaxially in [001] direction as seen from X-ray diffract

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