Study of Defects in Hydrogenated Amorphous Silicon by Constant Photocurrent Method and Positron Annihilation
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Study of Defects in Hydrogenated Amorphous Silicon by Constant Photocurrent Method and Positron Annihilation A.Amaral 1,2, G. Lavareda 1, C. Nunes de Carvalho 1, P. Brogueira 2, P. M. Gordo 3, V. S. Subrahmanyam3, C. Lopes Gil 3, M.. Duarte Naia 4, A. P. de Lima 3. 1 Centro de Física Molecular, Complexo Interdisciplinar I, IST/UTL, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. 2 Departamento de Física, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa Codex, Portugal. 3 ICEMS, Departamento de Física, Universidade de Coimbra, Rua Larga da Universidade, 3004-516 Coimbra, Portugal. 4 Secção de Física da U.T.A.D., Quinta de Prados, Apartado 202, 5001-911 Vila Real, Portugal.
ABSTRACT The influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by rf-PECVD, is studied by constant photocurrent method, CPM, and by slow positron beam spectroscopy, respectively. Deep defect density, NDD, remains approximately constant at 10-16 cm-3, typical of device quality material, for PD in the range 7 - 20 mW.cm-3 calculated from CPM. NDD increases roughly one order of magnitude for both low and for high power densities. Positron annihilation spectroscopy reveals the presence of two kinds of vacancy type defects in the films: large vacancy clusters or voids for PD ~ 7 mW.cm-3 and small vacancy type defects as PD increases to ~30 mW.cm-3. The relative abundance of the structural defects is shown to be controlled by the power density used during the deposition without affect the electronic deep defect density, significantly. INTRODUCTION A great amount of fundamental research has been devoted to hydrogenated amorphous silicon (a-Si:H) since this cheaply produced material became a promising candidate to be used in a large variety of applications, such as solar cells and thin-film transistors (TFT´s) Among a wide variety of deposition techniques used to produce a-Si:H films, the rf PECVD is the most commonly used for industrial production of electronic grade material. The electrical and optical properties of the a-Si:H material are controlled by the localized states in the mobility gap. The principal defect state in the mobility gap of aSi:H is the dangling bond. The dangling bonds are most likely associated to small multivacancy complexes (divacancies, trivacancies, etc.) within the disordered tetrahedrally coordinated network. In this work, a-Si:H electronic deep defects density was studied by CPM measurements. Positron annihilation techniques were used to A22.5.1
investigate the defects structure, particularly vacancy type defects produced during the film growth in order to contribute to the understanding of the correlation among growth conditions and optoelectronic properties of the films. EXPERIMENTAL DETAILS Undoped a-Si:H films were deposited by magnetically confined rf-PECVD (13.56 MHz) technique simultaneously on two different substrates: (1) alkali free 45 glass, for optical transmission and parallel transport measurements; (2) double-side polished Si, for po
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