Invited Paper PS+TF-MoA3
Medium Range Order (MRO) in "Amorphous (a)-Si(H)" Alloys in PV and TFT Devices with Intrinsic, B and P Doped a-Si(H) and a-Si,Ge(H) Layers: Reduction of Photo- and Stress-induced Defects by O-bonding
Monday, October 28, 2013, 2:40 pm, Room 102 B
| Session: |
Plasma Deposition |
| Presenter: |
G. Lucovsky, North Carolina State University |
| Authors: |
G. Lucovsky, North Carolina State University
D. Zeller, North Carolina State University
C. Cheng, North Carolina State University
Y. Zhang, North Carolina State University
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| Correspondent: |
Click to Email |
Intrinsic photo-absorbing regions, and B and P doped contacts comprised of hydrogenated amorphous silicon, a-Si1-xHx, with ~10 at.% or x ~0.1±0.02, are used in photovoltaic devices (PV), and thin film transistors (TFT’s). A-Si thin films, assumed to be free of H, are used as precursors for polycrystalline gate electrodes in microelectronics. Intrinsic, and p-type and n-type layers in multi-layer stacks that include "a-Si,Ge(H)" have been assumed to be "amorphous continuous random networks (CRN)", with limited short range order (SRO) extending to 1st nearest neighbor (NN) bond lengths, and 2nd N-N bond-angles. A-Si(H) films are not CRNs. They have medium range order (MRO) extending to self-organized and symmetry determined dihedral angles. MRO and formation of non-periodic organized nm-scale ordered regions with crystalline-Si symmetries is responsible for enabling properties in a-Si(H) devices. Intrinsic a-Si(H) thin films have been deposited by glow discharge (GD), remote plasma-enhanced chemical vapor deposition (RPECVD), and reactive magnetron sputtering (RMS). The concentrations of bonded-H are determined by deposition precursors and substrate temperatures. Two conditions are necessary for low Si dangling bond densities to ~0.5 to 1x1016 cm-3: (i) a bonded mono-hydride, Si-H, concentration of ~10 at.% H, and (ii) a deposition, and/or a post-deposition anneal at ~240°C to 300°C [1]. These combine to reduce strain-induced defects by introducing MRO as 1 nm-ordered clusters. Si L2,3 X-ray absorption spectroscopy (XAS) confirms MRO by yielding non-vanishing ligand-field splittings (DLF) of eg and t2g atomic d-states. The MRO basis states are symmetry-adapted linear combinations (SALC) of atomic states and form molecular orbital valence bands. MRO symmetry promotes a H-atom transfer reaction from a the Si-H bond at the apex of the MRO cluster into a Si-H-Si bonds at NN sites. This reaction establishes the low level of dangling bond defect sites. The same H-atom transfer is induced by sun-light absorption in PV devices. This increase dangling bond concentrations is the Staebler-Wronski effect (SWE). The local bonding arrangements of P and B dopant atoms are qualitatively different. If the bonding sites were the same as substitutional sites in c-Si, each of these dopants would be 4-fold coordinated. The incorporation of P is the same as in crystalline Si, 4-fold coordinated, and the ionization energy of the P+ site is small giving rise to a high doping efficiency, i.e., in electrons/P atom [2]. B is 3-fold coordinated, and p-orbitals perpendicular to the 3-fold coordinated bonding plane act as an electron acceptor creating hole transport.
Each of the preferred bonding arrangements for P- and B-atoms includes remote induction stabilized Si-H reducing Si-atom dangling bond densities. This accounts for a reduction of the E' center signal strength in electron spin resonance (ESR) measurements. Finally, the local bonding of 2-fold coordinated O-atoms, and 3-fold coordinated N-atoms have similar effects as 1-fold coordinated H-atoms, introducing new MRO local bonding arrangements. When combined with 1-fold coordinated NN Si-H bonds, the 2- and 3-fold local symmetries introduce coupled mode motions that stabilize unique MRO clusters by increasing their total binding energy. This stabilization provides a reduction in the Staebler-Wronski Effect photo-degradation in PV devices by process-controlled low densities (<1018 cm-3) of plasma processing incorporation of O- and N-atoms and coupled O-H/H-H bonds.
Similar reductions reduce electron trapping in TFTs.
1. G. Lucovsky and F.L. Galeener, J. of Non-Cryst. Solids 35 & 36 (1980) 1209.
2. G.N. Parsons, C. Wang and M.J. Williams, Appl. Phys. Lett. 56, 1985 (1990).
3. D.E. Steabler and C.R. Wronski, J. Appl. Phys. 51 (6), (1980) 3262.