Properties of amorphous silicon-germanium films and devices deposited at higher growth rates

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Properties of amorphous silicon-germanium films and devices deposited at higher growth rates Yong Liu and Vikram L. Dalal Dept. of Electrical and Computer Engr. and Microelectronics Research Center, Iowa State University, Ames, Iowa 50011 ABSTRACT We report on the growth and properties of amorphous Silicon-Germanium [a-(Si,Ge):H] films and devices fabricated at growth rates of ~ 5 Å/sec using a remote ECR plasma growth process. The films and devices were made using mixtures of germane and silane along with dilution with hydrogen and helium. The addition of He to the gas mixture significantly increased the growth rates. It was found that hydrogen was always necessary in order to achieve the best film and device properties. Films and devices were made across the entire bandgap range, from a-Si to a-Ge. High ratios of photo/dark conductivity and low values of Urbach energy ( < 50 meV) indicate good film properties. The defect densities were measured using space charge limited current techniques. The defect densities were in the range of 1-2 x 10 16/cm 3 –eV, about 5 times higher than for a-Si:H. Electron mobility-lifetime products were measured and found to be in the range of 2-4 x 10-7 cm2/V, even for low gap materials (1.35 eV). Single and graded gap devices were fabricated in these materials. Device fill factors of ~ 70% were obtained in graded gap devices. INTRODUCTION a-(Si,Ge):H is an important electronic materials for solar cells and image sensors. Most of these devices and films are made at a relatively low rate, about 1Å/sec, using significant hydrogen dilution. It would be useful from manufacturing considerations to increase the growth rate of these films and devices. It is usually found that just increasing the power in a standard glow discharge reactor does not produce good film and device properties. Various other methods, such as VHF plasma, have been used to produce better films and devices at higher growth rates [1,2]. In this paper, we report on the use of Helium as a diluent gas to increase the growth rates of these alloy materials and devices in an ECR reactor. We have reported previously on the growth of a-(Si,Ge) films using a hydrogen diluted ECR system.[3,4]. In this paper, we will show that using a mixture of hydrogen and helium produces films and devices at higher growth rates, upto ~ 5 Å/sec with very good properties, similar to the properties reported earlier for films and devices grown at ~ 1 Å/sec. GROWTH TECHNIQUE The basic growth technique has been described previously.[3-5]. It consists of using a remote, reactive ECR plasma source to provide a beam of ions which react with silane and germane at the substrate to produce films. The ion flux and energy can be controlled by varying pressure and power. A low pressure environment produces a higher ion flux and higher ion energies and electron temperatures. We have shown previously that the deposition pressure has a critical influence on the materials and devices, when hydrogen alone is used as a diluent gas, A18.3.1 Downloaded from https://ww

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