Growth of Highly Doped P-Type Znte Films by Pulsed Laser ablation in Molecular Nitrogen
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highest obtained for ZnTe by any method thus far. 8 PLA differs from many other film growth methods in that growth occurs from a highly nonequilibrium pulsed flux of energetic atoms, ions, and electrons. Spectroscopic measurements carried out during PLA of ZnTe in N2 do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in PLA ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. All but one of the few previous attempts to deliberately dope laser ablated Il-VI films involved substitutions on the column-II site, using ablation targets that were pressed-powder mixtures containing the dopant atom. Shen and Kwok grew (001)-oriented zinc blend CdS films (Eg = 2.50 eV) on (001) GaAs and InP substrates by adding Li 3 N or In powders, respectively, to CdS3 targets, and obtained electron and hole concentrations in the 1019-1020 cm-3 and -1017 cmranges, respectively. 10 Similarly, Compaan and co-workers 1 1,12 grew polycrystalline p-ZnTe films on glass substrates by mixing metallic Cu and ZnTe powders in a pressed target. A hole concentration in the 1018-1019 cm-3 range was estimated. However, both of these efforts required dopant-atom concentrations in the targets that far exceeded the resulting carrier concentrations, indicating that most dopant atoms either were not incorporated or were electrically inactive. Dubowski and co-workers grew epitaxial CdTe (Eg = 1.49 eV) and CdlpxMnxTe films by PLA, as well as dilute magnetic semiconductor superlattice and quantum well structures. 13- 16 However, no doping of either material was achieved. 17 Nevertheless, their results are encouraging in demonstrating PLA-grown heterostructures comparable in structural quality to those grown by MBE. 13,14,18 EXPERIMENTAL A conventional PLA system was used together with a pulsed KrF (248 nm) excimer laser. 19 Cylindrical lenses brought the laser beam to a horizontal line focus (-9mm x -0.75 mm) at an energy density Ed - 0.66 J/cm 2 along the radius of a 25.4 mm-diam ZnTe target.20 The target was rotated at -8 rpm and the laser beam was switched alternately from one side of the target to the other to prevent cones from forming, thereby maintaining a smooth target surface and a uniform deposition rate. 19 This resulted in ZnTe films that were free of particulates. 21 The ultra-high purity (six nines) N2 dopant gas was introduced through a mass-flow controller (MFC). In some experiments Ar gas, controlled by a second MFC, was introduced to permit variation of the ablation beam's kinetic energy (via collisions with ambient gas molecules) independent of the N2 dopant-gas partial pressure. The total chamber pressure was controlled by a capacitance manometer and a throttle valve. Single-crystal semi-insulating (001) GaAs substrates were solvent-cleaned and sulfur-passivated as described elsewhere. 22 The substrate heater face wa
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