Carrier Concentration Dependencies of Magnetization & Transport in Ga1-xMnxAs1-yTey

We have investigated the transport and magnetization characteristics of Ga1-xMnxAs intentionally compensated with shallow Te donors. Using ion implantation followed by pulsed-laser melting, we vary the Te compensation and drive the system through a metal-insulator transition (MIT). This MIT is associated with enhanced low-temperature magnetization and an evolution from concave to convex temperature-dependent magnetization.Comment: 2 pages, 2 figures. To appear in the proceedings of the 27th International Conference on the Physics of Semiconductors (ICPS-27, Flagstaff, AZ, July 26-30, 2004

Electron backscatter diffraction and photoluminescence of sputtered CdTe thin films

Journal ArticleElectron backscatter diffraction (EBSD) has been used to characterize the grain size, grain boundary structure, and texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with film microstructure. It is known that twin boundaries may have different electrical properties than high-angle grain boundaries and in this work we have included the effects of twin boundaries. We found better correlation of solar cell device performance to the twin-corrected grain size than to the standard grain size. In addition, we have correlated the photoluminescence (PL) spectra with device performance and with the EBSD results. We find that sputtering at 18 mTorr yields the highest efficiency, largest twin-corrected grain size and the strongest PL

Determination of the infrared complex magnetoconductivity tensor in itinerant ferromagnets from Faraday and Kerr measurements

Journal ArticleWe present measurement and analysis techniques that allow the complete complex magnetoconductivity tensor to be determined from midinfrared (11-1.6 μm; 100-800 meV) measurements of the complex Faraday (θF) and Kerr (θK) angles. Since this approach involves measurement of the geometry (orientation axis and ellipticity of the polarization) of transmitted and reflected light, no absolute transmittance or reflectance measurements are required. Thick-film transmission and reflection equations are used to convert the complex θF and θK into the complex longitudinal conductivity σxx and the complex transverse (Hall) conductivity σxy. θF and θK are measured in a Ga1−xMnxAs and SrRuO3 films. The resulting σxx is compared to the values obtained from conventional transmittance and reflectance measurements, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films

Synthesis of GaNxAs1-x thin films by pulsed laser melting and rapid thermal annealing of N+-implanted GaAs

Journal ArticleWe present a systematic investigation on the formation of the highly mismatched alloy GaNxAs1-x using N1-implantation followed by a combination of pulsed laser melting and rapid thermal annealing. Thin films of GaNxAs1-x with x as high as 0.016 and an activation efficiency of the implanted N up to 50% have been synthesized with structural and optical properties comparable to films grown by epitaxial deposition techniques with similar substitutional N content. The effects of N1 implantation dose, laser energy fluence, and rapid thermal annealing temperature on the N incorporation as well as optical and structural properties of the GaNxAs1-x films are discussed

Fabrication of GaNxAs1-x quantum structures by focused ion beam patterning

Journal ArticleA novel approach to the fabrication of GaNxAs1-x quantum dots and wires via ion beam patterning is presented. Photomodulated reflectance spectra confirm that N can be released from the As sublattice of an MBE-grown GaNxAs1-x film by amorphization through ion implantation followed by regrowth upon rapid thermal annealing (RTA). Amorphization may be achieved with a focused ion beam (FIB), which is used to implant Ga ions in patterned lines such that annealing produces GaAs regions within a GaNxAs1-x film. The profiles of these amorphized lines are dependent upon the dose implanted, and the film reaches a damage threshold during RTA due to excess Ga. By altering the FIB implantation pattern, quantum dots or wires may be fabricated

Synthesis and properties of highly mismatched II-O-I alloys

Journal ArticleTernary and quaternary dilute II-VI oxides were synthesised using a highly nonequilibrium method: the combination of O ion implantation and pulsed-laser melting. CdOxTe12x thin films have been produced with x up to 0.015 and with the energy gap reduced by 0.15 eV. Optical transitions corresponding to both the lower (E-) and upper (E+) conduction sub-bands, resulting from the anticrossing interaction between the localised O states and the extended conduction states of the matrix, are clearly observed in quaternary Cd0.6Mn0.4OxTe1-x and Zn0.88Mn0.12OxTe1-x layers. These results have important implications for the existing theoretical models of the electronic structure of the highly mismatched alloys. In Zn1-xMnxTe, where the O level lies below the conduction band edge, it was demonstrated that incorporation of a small amount of oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the ZnMnTe matrix. The three absorption edges of this material (~0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for multiband, single-junction, high-efficiency photovoltaic devices

Enhanced nitrogen incorporation by pulsed laser annealing of GaNxAs1-x formed by N ion implantation

Journal ArticleWe demonstrate that pulsed laser annealing followed by rapid thermal annealing greatly enhances incorporation of substitutional N in N1-implanted GaAs. Films implanted to 1.8% N exhibit a fundamental band gap of 1.26 eV (a band gap reduction of 160 meV), corresponding to an N activation efficiency of 50%. The optical and crystalline quality of the synthesized film is comparable to GaNxAs1-x thin films of similar composition grown by epitaxial growth techniques. Compared to films produced by N1 implantation and rapid thermal annealing only, the introduction of pulsed laser annealing improves N incorporation by a factor of 5. Moreover, we find that the synthesized films are thermally stable up to an annealing temperature of 950 °C

Diluted II-VI oxide semiconductors with multiple band gaps

Journal ArticleWe report the realization of a new mult-band-gap semiconductor. Zn1-yMnyOxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn1-yMnyTe host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0:88Mn0:12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at ~1:77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn1-MnyOxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%

Carrier concentration dependencies of magnetization & transport in Ga1-xMnxAs1-yTey

Journal ArticleWe have investigated the transport and magnetization characteristics of Ga1-xMnxAs intentionally compensated with shallow Te donors. Using ion implantation followed by pulsed-laser melting, we vary the Te compensation and drive the system through a metal-insulator transition (MIT). This MIT is associated with enhanced low-temperature magnetization and an evolution from concave to convex temperature-dependent magnetization

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

Journal ArticleWe have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdOxTe12x thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O1-implantation in CdTe followed by pulsed laser melting. Quaternary Cd0.6Mn0.4OxTe12x and Zn0.88Mn0.12OxTe12x 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 (E2) and upper (E1) 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, single-junction solar cells