Nitrogen doping of Te-based II–VI compounds during growth by molecular beam epitaxy

Abstract

We present a detailed study of p-type doping of Te-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, and CdZnTe) during molecular beam epitaxy, using nitrogen atoms produced by a direct current glow plasma source. Characterization includes capacitance–voltage, Hall effect, low temperature reflectivity and luminescence, double crystal x-ray diffraction (XRD), nuclear reaction analysis and secondary ion mass spectroscopy. Doping introduces shallow hydrogenic acceptors NTe, whose ionization energy was determined. For ZnTe, doping as high as p≈1020 cm−3 can be obtained when ≈1.5×1020 cm−3 nitrogen atoms are incorporated in the layer. This doping level decreases considerably for CdZnTe and ZnMgTe alloys as the Zn content decreases. The highest concentration obtained for CdTe is p≈1018 cm−3. For a CdTe layer with a doping level p≈1017 cm−3 the hole mobility is μp=235 cm2/V s at 65 K. In ZnTe, incorporation of nitrogen atoms in NTe configurations induces a noticeable change of the lattice parameter, a change which was not observed in CdTe:N layers. The XRD pattern of ZnTe:N/ZnTe pseudosuperlattices allows measurement of the Zn–N bond distance dZn–N=2.16±0.05 Å. Study of the doping efficiency as a function of the growth conditions indicates that the compensation mechanism is related to the formation of nitrogen interstitial defects or complex defects involving metal vacancies, but no deep center has been detected in luminescence. A comparative study of various doped telluride materials indicates that presence of Zn atoms strongly enhances the solubility limit of NTe, hence the key role of Zn in the doping efficiency. Results are discussed on the basis of geometric and thermodynamic considerations. The first argument is that the Zn–Te distance is the closest fit to the metal–N bond distance, and this minimizes the elastic contribution to the energy of formation of the NTe acceptor. As a corollary, the doping efficiency decreases when the alloy lattice parameter increases. The second argument compares the formation enthalpies ΔHF of II3N2 nitride (II being Zn, Cd, or Mg) to that of the IITe compound.