Formation of shallow boron emitters in crystalline silicon using flash lamp annealing: Role of excess silicon interstitials

Abstract

This thesis explores various processing and materials aspects of an oxide solar cell. A tandem structure consisting of a Silicon (Si) bottom cell, and a Cuprous Oxide (Cu 2 O)/Zinc Oxide (ZnO) top cell is suggested, and several materials challenges relevant for the design is addressed. Using Flash Lamp Annealing, shallow p-type emitters are produced in Si by indiffusion of Boron (B) from a surface source. The B diffusion is shown to be enhanced by a transient of Si interstitials originating from the surface coating. The n ++-side of a Si tunneling junction is realized by sputter depositing highly Phosphorus (P) doped Si thin films on quartz. The P activation and thin film conductivity are highest after prolonged, high temperature annealing, and thin film roughness is significantly reduced after a Reactive Ion Etch treatment of the quartz substrates. A rectifying Cu 2 O/Si heterojunction is demonstrated, and most of its depletion layer is located in Si showing that the acceptor concentration in the sputtered Cu 2 O is significantly higher than 1.1×10 17 cm −3. Epitaxial growth of Cu 2 O is realized on single crystal ZnO, and XRD measurements reveal that the Cu 2 O grows in two separate [111]-oriented domains which are rotated 180 • relative to each other. Further, a thin, detrimental layer of CuO is evidenced by TEM at the interface between the sputtered Cu 2 O and the ZnO single crystal. Homoepitaxial sputter growth of ZnO is investigated in some detail, and the growth is found to depend on sputter target power density, substrate polarity, and growth ambient. Specifically, c-axis growth is promoted when growing on the Zn-face of ZnO, while slower, in-plane growth is promoted during O-face growth and during growth in a Nitrogen ambient. Treating sputter grown Cu 2 O thin films by Rapid Thermal Annealing is demonstrated by XRD, PL and Hall effect measurements to be highly beneficial for the thin film structural quality. Finally, two band gap defects in Cu 2 O are revealed using Temperature Dependent Hall effect measurements, and the defect ionization energy values are found to be dependent on sample strain.

A review of thermal processing in the subsecond range: semiconductors and beyond

“…Moreover, Normann et al and Riise et al investigated the in-diffusion of phosphorus and boron using spin-on deposition of a phosphorus and boron source, respectively, on the surface of monocrystalline silicon followed by FLA, as a viable method of forming high-concentration shallow emitters for solar cells [101,102]. By varying both the energy density of a 20 ms flash in the range from 62 to 132 J cm −2 and the sample preheating, it was observed that FLA treatments can in-diffuse a high concentration of phosphorus atoms that become electrically active.…”

Advances in ultrashallow doping of silicon

“…Annealing technique is another hot topic in the research of ultra-shallow doping. The desire to maximize dopant activation with minimized diffusion in USJ fabrication has reignited the research interests in subsecond annealing techniques such as FLA [61][62][63][64] and PLA [62,[65][66][67][68][69] that were first proposed in late 1970s [70,71].…”