Relation between Erbium Photoluminescence and Dangling-Bond Defects in a-Si:H
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Relation between Erbium Photoluminescence and Dangling-Bond Defects in a-Si:H Minoru Kumeda, Hiroshi Itoh, Norio Shitakata, and Tatsuo Shimizu Department of Electrical and Electronic Engineering, Kanazawa University, Kanazawa 920-8667, Japan ABSTRACT
In order to study a role of Si dangling bonds in the Er photoluminescence mechanism in a-Si:H, we prepared Er-doped a-Si:H films with various densities of Si dangling bonds using rf magnetron sputtering method. The density of Si dangling bonds was changed by three ways: (1) change of Ar-to-H2 gas ratio in the sputtering ambient, (2) light soaking and (3) annealing. The intensity of the Er photoluminescence at 1540 nm tends to decrease with increasing the density of Si dangling bonds in the first two cases. In the case of annealing, the intensity of the Er photoluminescence first increases with a decrease in the density of Si dangling bonds due to the structural relaxation up to 250 °C and continues to increase up to 350 °C even though the density of Si dangling bonds increases due to hydrogen evolution. This increase in the photoluminescence intensity is probably caused by a change of the environment of Er ions. INTRODUCTION
Er-doped a-Si:H films have drawn a wide attention recently for the photoluminescence (PL) at 1540 nm for the application in the optical fiber communication. a-Si:H films are known to be superior to crystalline Si as the Er-PL host materials because of a higher PL intensity at room temperature [1]. The mechanism of the PL at 1540 nm in Er-doped a-Si:H is not so clear, although a model of the energy transfer from the photoexcited electrons in the conduction band to Er ions via Si dangling bonds (DBs) has been proposed[2]. In this model, Si DBs have an essentially important role to activate Er ions. The intensity of the Er PL is known to increase with an increase in the Er content up to 1020 cm-3 [3], but the density of Si DBs in a-Si:H is usually 1016 - 1017 cm-3. These facts suggest that the increase in the density of Si DBs might be useful for increasing the intensity of the Er PL. However, the recombination of photoexcited carriers is known to occur mainly at Si DB sites[4]. Therefore, it is interesting to study a role of Si DB for the Er PL in a-Si:H with various amounts of the
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density of Si DBs. In the present work, the relation between the intensity of the Er PL and the density of Si DBs was studied in a-Si:H films with various amounts of the density of Si DBs prepared by rf magnetron sputtering. EXPERIMENTAL
Er-doped a-Si:H films are prepared by rf magnetron sputtering method. The films were deposited on substrates of silica glass for ESR and optical measurements, highly resistive Si crystals for IR measurements and aluminium for PL measurements. The rf power was 200 W and the substrate temperature was 200 °C. The film thicknesses were around 0.7 microns. The incorporation of a small amount of Er in the film was attained by two step sputtering: First we prepared thick a-Si films doped with Er by putting Er chips on the targe
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