HALOGEN DOPING OF II-VI SEMICONDUCTORS DURING MOLECULAR-BEAM EPITAXY

Results on the halogen doping of CdTe, (CdMn)Te as well as (CdMg)Te th in films and quantum well structures are reported. The structures were grown by molecular beam epitaxy. The samples have been investigated b y Van der Pauw, photoconductivity, X-ray diffraction, XPS and SIMS mea surements. ZnCl2 and ZnBr2 have been used as dopant sources. Free carr ier concentrations at room temperature above 10(18) cm-3 can easily be achieved for CdTe for a wide range of Cd/Te flux ratios and substrate temperatures. In the temary alloys, the free carrier concentration de creases drastically with increasing x-values, despite a constant incor poration of the dopant species. In addition, persistent photoconductiv ity has been observed in n-type doped temary thin films at low tempera tures. The decrease of the free carrier concentration with x-value is common to other wide-gap ternary alloys, and the reason for it is disc ussed in the frame of DX-like deep donor impurities in ternary II-VI c ompounds. In first experiments on planar halogen doping of CdTe, a dop ing level of 5 X 10(18) cm-3 could be reached in the doped regions, th e highest value ever reported for CdTe. A clear influence of dopant in corporation on the structural quality of CdTe thin films has been seen even for dopant concentrations of as low as 10(18) cm-3. The FWHM of the rocking curves decreased by a factor of 2 with increasing dopant i ncorporation. SIMS as well as XPS mesurements demonstrate that the Cl/ Zn and Br/Zn ratio in the doped films is 2/1, but no chemical shift co rresponding to Zn-Cl or Zn-Br bonds could be detected. A model for the incorporation of the halogens is proposed on the basis of these resul ts.