Intrinsic, n- and p-Doped a-Si:H Thin Films Grown by DC Magnetron Sputtering with Doped Targets
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PVD also allows modification of the growth conditions far from thermodynamic equilibrium. This work investigates the possibilities to fabricate device quality intrinsic, n- and p-type a-Si:H by magnetron sputtering with the intention of making photo diodes for digital cameras. The n- and p-type doping is obtained by sputtering from P- and B-doped targets, respectively. Doping by co-sputtering from composite Si/B 4C targets was also explored. A comparison to device quality PECVD samples from Xerox PARC was made. EXPERIMENTAL DETAILS All films were grown in a planar magnetron sputtering system. The system consists of two 3 inch magnetrons placed in the top lid of a cylindrical chamber. An electrically isolated plate is placed between the magnetrons to avoid cross-contamination. There are shutters to both the magnetrons and the rotating substrate table. The samples were loaded through a load-lock. For the co-sputtered films a 2 inch magnetron was used.
31 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
Before the films were grown, the system was evacuated to 5 x 10-7 Torr, using a turbomolecular pump and a titanium sublimation pump (TSP). Then the substrates were heated to 250'C, by a boron-nitride covered graphite heater placed in the substrate table. Thereafter, H 2 was introduced into the chamber to the desired partial pressure, and followed by 1.5 mTorr Ar, as measured by a capacitance manometer. The target current was kept constant at 0.2 A, giving a target voltage that varied between 450-500 V depending on target and H2 pressure. The substrates were kept at a floating potential. During the deposition the TSP was operated in a continuous mode, to obtain more uniform hydrogen pumping speed. The deposition rate of the a-Si:H films was 33-100A/imin depending on the hydrogen partial pressure. A polycrystalline target of purity 99.999% was used for the intrinsic films. The partial pressure of the 99.9996% pure hydrogen gas was varied between 0.4 - 0.7 mTorr. B- and P-doped targets of hot-pressed silicon with I at % B or P, respectively, were used
for the doped films. The co-sputtered film was grown using a pure silicon target with a sector replaced by a piece of B4C. The hydrogen partial pressure for both the doped series was varied in the range of 0.5 - 6 mTorr. The hydrogen concentration depth profiles were determined by nuclear reaction analysis (NRA) utilizing the 1H(15N, XY)'12C nuclear resonance reaction using the resonance at 6.385 MeV. The B- and P-concentration depth profiles were determined by secondary-ion mass-spectroscopy (SIMS) using calibrated standards as a reference. Film thickness values were measured both by a Jeol JSM-T220A scanning electron microscope, a Dektak 3030 profilometer and Rutherford back scattering spectroscopy. The surface structure was analyzed by a Nanoscope MIIa atomic force microscope system, from Digital Instruments, operated in tapping mode using silicon cantilevers. The scanned area was typically 1 JLm2 from which the surface root-mean-square (rms) rough
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