The structural and magnetic properties of SmFeO 3 with B site substitution of non-magnetic atom Al are investigated. The x-ray diffraction patterns show that SmFe (1−x) Al x O 3 remains an orthorhombic structure within the whole doping range, and the unit-cell volume decreases monotonically with the increase of doped Al concentration. Besides, the octahedral tilting distortions of FeO 6 are found to be alleviated while the tolerance factor increases. However, the relationship between the lattice parameters and Al concentration is observed to deviate from Vegard's rule, and this may be caused by magnetostriction effects. For the doping content values in a range 0 ≤ x ≤ 0.6, the ferromagnetism, antiferromagnetism, and paramagnetism are observed to occur continuously. Moreover, the magnetization and the spin reorientation temperature (T k ) decrease monotonically as Al content value increases. With the doping content values being x = 0.8 and 1.0, these compounds only show paramagnetic behavior.
As an acceptor dopant with a solid:liquid distribution coefficient ks<1, iron is an example of an impurity which can be used in modest amounts to ensure that an adequate fraction of EL2 midgap defects are ionized along the length of a melt-grown GaAs crystal, as desired for semi-insulating behavior. The results of such deliberate doping with iron (when NFe is in the mid-1015 cm−3 range) are reported for crystals grown by both the liquid encapsulated Czochralski and the vertical gradient freeze methods. Except in the very tail region of such crystals (when NFe≳NEL2 and high resistivity p-type behavior results), GaAs with this modest iron modification to the compensation balance behaves with quite ordinary semi-insulating properties. The iron acceptors are then all ionized, and are optically ‘‘invisible.’’
The high-pressure behaviors of SmFeO 3 are investigated by angle-dispersive synchrotron X-ray powder diffraction under a pressure of up to 40.3 GPa at room temperature. The crystal structure of SmFeO 3 remains stable at up to the highest pressure. The different pressure coefficients of the normalized axial compressibility are obtained to be β a = 0.
Optical and electrical properties are described for bulk GaAs, grown from a melt doped with iron to create FeGa deep acceptors in a sufficient amount (exceeding the EL2 defect concentration) to make high-resistivity p-type rather than semi-insulating material. Both iron photoionization and EL2+ photoneutralization contribute to the near-infrared optical absorption. This made it possible to deduce the concentrations (NAi and NAn) of ionized and lattice-neutral iron, and the ratio (NAi/NAn). Temperature dependent measurements of dc electrical transport yielded quantities such as the free hole density, and hence the Fermi energy, for the 290–420 K range. This information combined with (NAi/NAn) led to a determination of the iron acceptor’s free energy εA(T): about 0.46 eV above the valence band at 300 K, and ∼40 meV closer at 420 K. The temperature dependence of εA for iron is shown to differ from εv, εc, midgap, or the free energy for CrGa acceptors in GaAs.
Attainment of semi-insulating status when an ‘‘undoped’’ GaAs crystal is grown from the melt requires a delicate balance among concentrations of ‘‘unintentional’’ donor and acceptor impurities, and defects, notably the EL2 midgap donor. In qualifying and improving material for device uses, defect identification and characterization is important. The compensation balance is analyzed in this paper for various ‘‘undoped’’ crystals, relying largely on Hall data over the 290–430 K range, coupled with optical absorption measurements of carbon and EL2. The temperature-dependent data, converted into Fermi energy and into EL2 ionized fraction, provide a clearer picture than just room-temperature measurements as to whether EL2 controls the Fermi energy (giving the desired semi-insulating behavior), or whether a shallower defect species is in control.
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