Synthesis and optical properties of II-O-VI highly mismatched alloys

K. M. Yu, W. Walukiewicz, W. Shan, J. Wu, J. W. Beeman, M. A. Scarpulla, O. D. Dubon, P. Becla
2004 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
more » ... 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 [10], 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) [21]. 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 [27] . 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.
doi:10.1063/1.1713021 fatcat:joewnaipdzawzbld6rghbiolla