An Investigation of the Influence of Plasma Characteristics on the Electronic and Optical Properties of Device Quality a
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AN INVESTIGATION OF THE INFLUENCE OF PLASMA CHARACTERISTICS ON THE ELECTRONIC AND OPTICAL PROPERTIES OF DEVICE QUALITY a-Si:H GROWN BY ELECTRON CYCLOTRON RESONANCE PLASMA DEPOSITION RALPH D. KNOX,* V.L. DALAL* AND B. MORADI* Microelectronics Research Center, Iowa State University, Ames, IA 50011 Department of Electrical Engineering and Computer Engineering, Iowa State University, Ames, IA 50011 ABSTRACT The electronic and optical properties of device quality hydrogenated amorphous silicon (a-Si:H) films grown by electron cyclotron resonance (ECR) plasma deposition were studied together with in-situ plasma characteristics. Hydrogen and helium plasmas, excited by 50-250 watts of 2.45 GHz microwave power under ECR conditions, were used to decompose silane at 6 to 20 mtorr pressures during the deposition of a-Si:H films at a 297 C substrate temperature. Both the electron temperature and density, and ion flux are measured near the deposition surface using plane and cylindrical Langmuir probes. An attempt is made to correlate these plasma properties with the light and dark photoconductivity, optical gap, refractive index, and subband gap photoconductivity.
INTRODUCTION ECR microwave plasma deposition technology has generated much interest as an alternative to conventional glow discharge (GD) techniques. Because of the ECR condition, plasmas are sustained at low pressures (10-5 to 10-3 torr) with a high degree of ionization (>10%).' In addition, the ECR plasma apparatus can serve as a remote source, producing a plasma stream that is capable of decomposing process gases introduced near the substrate. This geometry has the advantage of producing low energy ions, typically under 10-20 eV, which reduces the ion bombardment damage at the growth surface. The purpose of this research investigation is to deposit a-Si:H films under a variety of plasma conditions and attempt to correlate sample properties with the plasma characteristics. Sample properties discussed here include dark, light and subband gap photoconductivity, refractive index, and optical band gap. Plasma properties include floating and plasma potentials, electron temperature and concentration, ion current density and deposition rate.
EXPERIMENTAL CONDITIONS AND PROCEDURE A Microscience ECR 908 plasma source was used to deposit all films. Figure 1 illustrates the important components of the ECR deposition system. 2.45 GHz microwave radiation passes through a quartz window into the bare stainless steel ECR source and excites a plasma in the presence of a static magnetic field (_875 Gauss). A three stub tuner located in front of the quartz window minimizes reflected microwave power. Two electromagnets, which are independently powered and spaced 7 cm apart, generate the magnetic field necessary to achieve ECR. The Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
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plasma gas (hydrogen or helium) enters the plasma source near the quartz window and silane enters the deposition chamber through a 24 cm diameter gas
distribution ring located at t
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