Synthesis and optical properties of II-O-VI highly mismatched alloys
Journal of Applied Physics
We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdO x Te 1-x thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O + -implantation in CdTe followed by pulsed laser melting. Quaternary Cd 0.6 Mn 0.4 O x Te 1-x and Zn 0.88 Mn 0.12 O x Te 1-x with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be
... due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E_) and upper (E + ) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, singlejunction solar cells. PACS numbers: 71.20.Nr; 78.66.Hf; 61.72.Vv; 61.80.Ba improved donor activation efficiency of the group VI donors [13,14] and the mutual passivation of the group IV donors and the nitrogen [15-18]. Although similar or even more pronounced effects are also expected in II-O-VI HMAs , much less work has been done on these materials because of the difficulties in the synthesis of the alloys with large enough O content [19,20]. Recently we have reported on the successful synthesis of Cd 1-y Mn y O x Te 1-x alloys by oxygen implantation into Cd 1-y Mn y Te crystals followed by rapid thermal annealing (RTA) . We observed a large decrease in the band gap for crystals with y > 0.02 due to the incorporation of O. Using the band anticrossing model (BAC) we estimated that the substitutional O content (i.e., x) in the Cd 0.8 Mn 0.2 O x Te 1-x alloys formed by O + - implantation is ~0.0013 and 0.0024 for 2.7 and 5.4% of implanted O, respectively. This approach is very effective for incorporating impurities to levels well above the solubility limit. In addition to GaN x As 1-x , synthesis of diluted ferromagnetic Ga 1-x Mn x As with Curie temperature as high as 80K using the PLM process has recently been demonstrated  . In this paper we report our systematic investigation of the synthesis of II-O x VI 1-x layers using O ion implantation followed by pulsed laser melting in a large variety of II-VI single crystal substrates, including CdTe, CdMnTe, ZnTe and ZnMnTe. EXPERIMENTAL Multiple energy implantation with 90 and 30 keV O ions was carried out on various II-VI single crystals to form ~0.2 µm thick layers with relatively constant initial O concentrations, corresponding to O mole fraction of 0.01 to 0.04 (or 1 to 4%). The O +implanted layers on the crystals were pulsed-laser melted in air using a KrF laser (λ= 248 nm) with a FWHM pulse duration ~38 ns. After passing through a multi-prism homogenizer, the energy fluence at the sample ranged between 20 and 300 mJ/cm 2 . Some of the samples were rapid thermally annealed after the PLM process at temperatures between 300 and 600°C for 10 seconds (RTA) in flowing N 2 . The band gap of the synthesized layers was measured using photomodulated reflectance (PR) spectroscopy at room temperature. Radiation from a 300W halogen tungsten lamp dispersed by a 0.5m monochromator was focused on the samples as a probe beam. A chopped HeCd laser beam (λ=442 nm or 325nm) provided the photomodulation. PR signals were detected by a Si or Ge photodiode using a phasesensitive lock-in amplification system. The values of the band gap and the linewidth both the lower (E_) and upper (E + ) conduction subbands are clearly observed in these quaternary diluted II-VI oxides, in good agreement with the BAC model. The weak pressure dependence of the E_ transition confirms the more localized nature of this band. These alloys have a three band structure making them suitable for testing the theoretical predictions of highly-efficient multiband, single-junction photovoltaics.