Hong-Lie Shen

Formation of four new shallow emissions in Mn+ ion‐implanted GaAs grown by molecular beam epitaxy having extremely low concentration of background impurities

Mn+ ions were implanted into ultrapure GaAs layers grown by molecular beam epitaxy. 2 K photoluminescence revealed that in addition to the well‐established Mn‐related deep acceptor emission at ∼1.41 eV, four new shallow emissions denoted by (Mn°, X), ‘‘G’’, ‘‘H’’, and (D, A)2 are formed in the near band edge when the Mn concentration [Mn] exceeds 3×1016 cm−3. Both ‘‘G’’ and ‘‘H’’ exhibit no energy shift with growing [Mn] up to 1×1019 cm−3. In contrast, for shallow acceptor‐ (such as C) doped GaAs with extremely low background concentrations of donor impurities, a series of [g‐g]‐like energy levels, which present strong energy shifting with increasing acceptor concentration, are universally formed. These results show that pairs between deep Mn acceptors do not produce such [g‐g] like energy levels.

Surface modification and characterization of functional oxide ceramics using CO2 laser

Abstract Surface of powder-sintered oxides such as superconductor and ferroelectric barium titanate ceramics were recrystallized using scanned CO 2 laser irradiation. Conventionally, the ceramic is sintered at high temperature, to increase adhesive forces between powdered ceramic grain. During such process, however, many micro-pores and -cracks are produced owing to large shrinkage of fine aggregate grains into the surface layer. Superconductor surface does not allow the flow of superconductor current uniformly and the flow is concentrated on the surface. Thus properties of superconductor are so sensitive to their surface condition that some substantial modifications of the material are necessary. To clarify this issues, both sample of the powder-sintered superconductor YBa x Sr 2− x Cu 3 O 7− y and the BaTiO 3 ceramic film were modified by laser irradiation to recrystallize only the surface layer. Their microstructural features in the surface, the characteristic of superconducting and ferroelectric have been investigated. Thus laser-scanning process appears to be suited to generating large grains with preferred orientation by changing of irradiation condition.

Characterization of Ca+ ion‐implanted GaAs by photoluminescence

Optical properties of Ca+ ion‐implanted GaAs were characterized by photoluminescence measurements at 2 K. Four new emissions denoted by (Ca0,X), gCa, SM, and (e,Ca) were found to be produced by Ca incorporation into GaAs. By changing the Ca dopant concentration and excitation intensity, it was revealed that the (Ca0, X) emission is due to excitons bound to a neutral shallow Ca acceptor, and SM probably originates from excitons bound to a Ca impurity in a deep energy level. In addition, in an impure GaAs substrate, Ca could also form a shallow emission gCa, despite the fact that the g emission for well‐known shallow acceptors such as C, Be, and Mg was usually found only in ultrapure GaAs. It was demonstrated that Ca is a shallow acceptor impurity in GaAs with a binding energy of 28.4 meV.

Magnetoresistance effect in La-A-Mn-O (A = Ca, Ba) thin films synthesized by metal-organic decomposition

Polycrystalline La(0.67)A(0.33)MnO(z) (A=Ca, Ba) thin films were successfully synthesized on quartz substrates by metal-organic decomposition. Some of the La-0.67(Ca0.5Ba0.5)(0.33)MnOz films were deposited on quartz with a SrTiO3 template layer (LCBMT). The porous structure of the films was observed by atomic force microscopy. It was found that all the films showed a broad peak effect of resistance under the zero field, over a temperature range of 77-302 K. Under a 10 kOe field, however, only La0.67Ca0.33MnOz films revealed a broad magnetoresistance (MR) peak. An enhanced MR effect was exhibited at low temperatures in all films, which was related to the porous structure of the films. LCBMT presented larger MR values below 180 K, than the La-0.67(Ca0.5Ba0.5)(0.33)MnOz films without a template layer. The low-field MR effect was observed in all films at 77 K, but it weakened with increasing temperature, and disappeared near room temperature, which was attributed to the offset effect of the thermal hopping of carriers on the tunneling MR at high temperature

Ferromagnetic Co/Ni/Fe and Co/Fe multilayer films prepared by a three-filtered-MEVVA-ion-source-deposition system

Abstract This paper describes the preparation of ferromagnetic [Co(136 A)/Ni(25 A)/Fe(48 A)]16 and [Co(136 A)/Fe(48 A)]16 multilayer films with high magnetic moments by using a three-source magnetically filtered metal vapor vacuum arc deposition system. The variation of microstructure and magnetic properties of the films during a vacuum annealing process were monitored by Rutherford backscattering spectroscopy, X-ray diffraction and vibrating sample magnetometer experiments. It was found that both the saturation magnetization (σs) and coercivity (Hc) of these multilayer films were enhanced gradually by the interdiffusion and alloying effect within the multilayers of these films caused by vacuum annealing. The Hc of the Co/Ni/Fe film was found to be always lower than that of the Co/Fe film under the same thermal annealing conditions, which implied that the thin nickel interlayers played a magnetically softening role in the multilayer system. The σs value of the Co/Fe film was a little higher than that of the Co/Ni/Fe film, and the σs values of these two films gradually became close to each other after annealing.

Characteristics of La–Ba–Mn–O thin films grown by metal-organic decomposition

Abstract La 0.67 Ba 0.33 MnO z thin films were grown by means of metal-organic decomposition (MOD) on quartz substrates. Polycrystalline structure of the films with [202] preferred orientation was shown by X-ray diffraction spectrum. Atomic force microscope images revealed that the films were smoothly grown on the substrates and structural defects such as porosity were displayed in the films. The chemical states of the elements obtained from X-ray photoemission spectra were discussed in detail and oxygen deficiency was observed. Furthermore, the magnetic property and magnetotransport behavior of the films were investigated. The films showed a low ferromagnetic transition temperature ( T c ) of about 265 K due to the oxygen deficiency. A smooth metal–insulator transition peaking at a temperature far below T c presented in the temperature dependence of resistivity under zero field. No magnetoresistance (MR) peak but an enhanced MR effect at low temperature was observed under 10 kOe field. The above magnetotransport behavior was ascribed to the structural defects such as grain boundaries and porosity in the films.

Optical and structural properties of β-FeSi 2 layers on Si fabricated by triple 56 Fe ion implantations

Optical and structural properties of β-FeSi 2 layers fabricated by triple 56 Fe + ion implantations into Si and by subsequent annealing up to 930 °C were studied as a function of Fe dose, annealing temperature and its duration. In photoluminescence measurements at 2 K, not only two dominant emissions peaked at 0.799∼0.806 eV and 0.836∼0.843 eV but also sharp emissions peaked at 0.808 eV and 0.874 eV were observed after annealing at 900∼915 °C for 1 min ∼ 2 hours. Room temperature optical absorption measurements revealed the allowed direct energy gap of 0.850∼0.878 eV as well as a high absorption coefficient in the order of 10 5 cm -1 near the absorption edge. X-ray diffraction analysis showed that the phase transformation from β to α results from the annealing at 930 °C for 2 hours. The composition, thickness and crystalline characteristic of both the as-implanted and the annealed layers were determined by Rutherford Backscattering Spectrometry analysis.

Photoluminescence study of Sr+ ion‐implanted GaAs

Low‐temperature photoluminescence experiments were carried out for Sr+ ion‐implanted GaAs substrates as a function of Sr concentration and excitation intensity. It was found that the Sr impurity produces four emissions denoted by (Sr°, X), ‘‘G1’’, ‘‘G2’’, and ‘‘H’’ in the near band emission region in GaAs grown by molecular beam epitaxy. ‘‘G1’’ emission presents a red shift with increasing Sr concentration, while ‘‘G2’’ does not shift. In a Sr+ ion‐implanted impure GaAs substrate grown by the liquid‐encapsulated Czochralski method, all four of the above emissions were missing. Instead there appeared two other emissions, SM1 and SM2, which are considered to be related to two deep acceptor levels of Sr in GaAs. All results indicate that the Sr impurity produces both shallow and deep energy levels in GaAs and a small amount of the residual impurity will quench the emissions related to shallow energy levels.

Shallow acceptor behavior of calcium in GaAs

The shallow acceptor characteristics of calcium in GaAs were investigated by photoluminescence spectroscopy and Hall measurement. Calcium impurity was introduced into GaAs substrate by ion-implantation. The Ca concentration was ranged from 3×10 16 to 1×10 20 cm -3 . In 2K PL spectra, a transition from conduction band to shallow Ca acceptor (e, Ca) was found at ∼1.491eV. A binding energy of 28.4meV for Ca was obtained. The emission g C a , which is usually strongly related to shallow impurity like C in GaAs, presents in all Ca + implanted GaAs samples and its intensity increases with increasing Ca concentration, confirming that Ca is a shallow impurity in GaAs. Temperature dependence PL spectra showed that 1.491eV band in Ca + ion-implanted GaAs exists at temperature as high as 40K while in virgin GaAs, the 1.491eV band quenches at temperature below 20K, supporting our (e, Ca) assignment. Hall measurement revealed that all the samples are p-type conduction and the maximum sheet hole concentration is 4×10 12 cm -2 . All the results indicate that Ca is a shallow acceptor impurity in GaAs.