P-Type II-VI Compound Semiconductor Thin Films Grown by Pulsed Laser Deposition
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ABSTRACT In this paper the results on p-type ZnS, ZnSe, CdS and CdSe thin films grown by pulsed laser deposition will be discussed. These films were deposited on GaAs substrates. Li-doping has been shown to be effective in producing p-type II-VI thin films, while In-doping is excellent for ntype CdS and CdSe thin films. No post-annealing process was used. These preliminary results suggest a possible new approach through pulsed laser deposition to solve the doping problem of I-VI compound semiconductors.
INTRODUCTION II - VI compound semiconductors are well-known for their applications in a wide range of important optoelectronic devices, including detectors, light emitting diodes and solid state lasers operating in the visible spectral range. The major difficulty in fabricating optical devices from IIVI compound semiconductors is that they can only be doped to either p-type or n-type, but not both. Extensive efforts to produce p-type ZnS, ZnSe, CdS and CdSe or n-type ZnTe and CdTe have been ongoing for more than thirty years 1 -6 . Several explanations for these difficulties have been advanced: (1) the natural point defects (normally group VI vacancies), which act as donors (S and Se vacancies) or acceptors (Te vacancies) due to their ionic bonding (auto-compensation); (2) the low solid solubility of dopants; (3) dopants at interstitial sites act as compensating dopants (for example, Li and Na act as acceptors at Zn or Cd sites but donors at interstitial positions). Based on thermodynamics, it is believed that the maximum equilibrium carrier concentrations must be very low. A new solution to this problem is obtained by nonequilibrium and low temperature crystal growth techniques - such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) - where the growth is controlled by surface kinetics at a lower temperature. Recently, ZnSe was p-type doped with carrier concentrations in the range of 1017 - 1018 cm- 3 , and efficient blue light lasers were produced7 ,8 . High quality p-type ZnS, CdS and CdSe thin films have yet to be reported. Pulsed laser deposition (PLD) of II-VI compounds has been discussed previously 9 -13 . The focused pulsed laser beam produces such a rapid temperature rise (>1011 Ks-l) on the target that the stoichiometry of the target can be maintained in the growing film 9 . It provides means to reduce the group VI vacancies which generate the native compensation in II-VI compound thin films. High energy atom and ion species in the laser-induced plasma plume along with its pulsed behavior create high surface mobility and supersaturation enables the growth of high quality films at a relatively low substrate temperature 10 . This highly nonequilibrium pulsed process suggests that PLD may be an excellent approach to solve the long-standing II-VI compound doping problem. In this letter our results on p-type ZnS, ZnSe, CdS and CdSe thin films will be discussed. Li-doping has been shown to be effective in producing p-type II-VI thin films, while In91 Mat. Res. Soc. Symp. Proc. V
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