Pulsed Laser Ablation Growth and Doping of Epitaxial Compound Semiconductor Films
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3 gaps and lattice constants for photovoltaic and optoelectronic device structures. (2) The kinetic and internal energies of species in the ablation beam may be useful to reduce epitaxial film growth temperatures by enhancing surface diffusion. (3) PLA's ability to deposit films in reactive (low-pressure ambient gas) environments is intriguing for growth of epitaxial column-III nitrides and their alloys for optoelectronic applications. 4 ,5 The energetic ablation beam also can induce reactions with adsorbed or ambient gases such as H2 S and N 2, to produce films whose composition 6 and/or doping and electrical properties 7 ,8 can be controlled by the ambient gas partial pressure. (4) Dopants also can be incorporated directly in the ablation target and transferred to a film. 9 ,10 (5) Pulsed deposition provides inherently "digital" precision in controlling film thickness, and can be carried out so that significantly sub-Angstrom amounts of material are deposited with each laser pulse, yet attractive film-growth rates can be maintained at laser repetition rates of -10-40 Hz. Combining these capabilities with the use of a multi-target carousel in principle makes possible the growth of epitaxial quantum well or superlattice structures. Multinary targets could be used to precisely match lattice constants or band edge offsets in adjacent epilayers and to increase the range of band gaps and lattice constants available. Thus, PLA may make possible the growth of multinary-based epitaxial structures with compositional and doping profiles that are not easily achieved by conventional growth methods 6 ,11 for which the maintenance and controlled variation of multiple independent elemental fluxes may be difficult. Toward this goal, several groups recently carried out experiments to determine whether PLA can be used to grow structurally high quality and highly doped epitaxial compound semiconductor films.4- 5.7 - 10 We initiated a systematic study by focusing on the closely related II-VI and I-III-VI families of materials. The 11-VI semiconductors with moderate to wide energy band gaps are interesting because the achievement of bipolar doping in them has been a formidable barrier using conventional growth methods. Hole (p-type) doping, in particular, has required highly reactive atomic species generated in non-equilibrium RF 12 or DC 13,14 plasmas. Doping of Il-VI films is important for optically active semiconductor devices ranging from diodes and lasers for displays and communications to photovoltaic energy conversion. Thus, the II-VIs provide a challenging binary and ternary materials field within which to evaluate PLD. The more complex I-III-VI chalcopyrite (ternary through pentenary) semiconductors are still relatively new and unexplored with regard to epitaxial growth. One of these materials, CuInl.xGaSe2, is the absorber in photovoltaic cells that currently hold the "world record" for solar energy conversion efficiency by a polycrystalline thin-film device. 15 , 16 However, the presence of grain boundaries, together with
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