ALBERT

Notes: A computer control for a Sandercock-type multipath tandem Fabry–Perot interferometer is described, which offers many advantages over conventionally used analog control: The range of stability is increased due to active control of the laser light intensity and the mirror dither amplitude. The alignment is fully automated enabling start of a measurement within a minute after start of align, including optionally finding the optimum focus on the sample. The software control enables a programmable series of measurements with control of, e.g., the position and rotation of the sample, the angle of light incidence, the sample temperature, or the strength and direction of an applied magnetic field. Built-in fitting routines allow for a precise determination of frequency positions of excitation peaks combined with increased frequency accuracy due to a correction of a residual nonlinearity of the mirror stage drive. © 1999 American Institute of Physics.

Notes: The frequencies of long-wavelength spin wave modes are calculated for single and multilayer structures composed of ferromagnetic films with strong out-of-plane surface and interface anisotropies. The properties of these modes are examined from the point of view of possible Brillouin light-scattering studies and the evaluation of exchange constants and interface anisotropies. The frequencies of the collective modes on multilayer structures as well as their degree of localization to the surface of the stack are strongly affected by perpendicular anisotropies. This is demonstrated by studying the spectral density function obtained from an effective medium theory for semi-infinite superlattices.

Notes: Results are presented of new theoretical investigations in which spin waves in multilayered structures are calculated, properly including both magnetic interface anisotropies and exchange.1 The work is an extension of a recently introduced theory for the calculation of single-layer spin-wave frequencies by Rado and Hicken, including interface anisotropies and exchange.2 The calculations were carried out for two different types of multilayered structures, i.e., for multilayers consisting of alternating magnetic and nonmagnetic multilayers, and for all-magnetic multilayers. In both cases all layers were of the same thickness. The solutions were obtained by solving forthe Maxwell equations and the Landau–Lifshitz torque equation. The boundary conditions consist of the Maxwell boundary conditions, and the Rado–Weertman boundary condition3 in the case of magnetic/nonmagnetic multilayers. In the case of all-magnetic multilayers, modified Rado–Weertman boundary conditions were used.4,5 In the case of magnetic/nonmagnetic multilayers, both dipolar-type (Damon–Eshbach-type) modes and exchange-dominated modes are obtained. The dipolar modes are frequency split as a result of the dipolar coupling of single-layer spin waves across the nonmagnetic spacer layers. Theexchange modes show no frequency splitting apart from the crossing regimes with the dipolar modes. In the regime of small layer thicknesses, where the dipolar modes are well frequency separated from the exchange modes, the calculations reproduce results obtained by neglecting exchange contributions.6 In the limit of very small layer thicknesses (〈40 A(ring)), the influence of interface anisotropies manifests itself for the dipolar modes in a frequency increase, as well as in a reduced coupling across the nonmagnetic layers. In the crossing regimes of dipolar modes and exchange modes, the modes exchange their mode characteristics, leading to a pronounced frequency gap. The width of the gap ismostly determined by the amount of interface anisotropy. The calculation of an all-magnetic multilayered structure, consisting of three Fe layers with interleaving Ni layers, was carried in the limit of maximum exchange coupling across the interfaces.1 The results show frequency splittings both for dipolar modes and for exchange modes. In the limit of an infinite number of layers, the exchange modes will eventually form a band of exchange-dominated collective spin waves reminiscent of the band of collective dipolar spin waves in magnetic/nonmagnetic multilayers. The crossing regimes of dipolar modes with theexchange modes show frequency gaps close to those observed for magnetic/nonmagnetic multilayers.

Notes: We analyzed the intrinsic defects and the n-type-carrier concentration generated by nitrogen ion implantation in n-type GaN by deep-level-transient spectroscopy and by capacitance–voltage measurements, respectively. The samples were grown on sapphire by metalorganic vapor-phase epitaxy. Nitrogen implantation with different ion doses and postimplantation rapid-thermal annealing (RTA) were investigated. We observed a growing n-type-carrier concentration and increasing defect concentration with increasing nitrogen ion implantation doses. After RTA the concentration of free carriers and deep levels as found in the as-grown state are restored. We also address contrarily seeming results from measurements of sheet resistance after N implantation published recently. © 1996 American Institute of Physics.

Notes: In this article, a comprehensive treatment of the implantation-induced disordering (IID) of CuPtB-ordered GaInP with regard to microstructuring is presented. Ion implantation reduces the thermal stability of the crystal, so that disordering occurs at temperatures smaller than 800 °C for which the ordered phase normally is stable. It is shown that IID is mediated by implantation defects and can be described quantitatively by a model based on defect annealing. From the temperature dependence of the disordering process an activation energy of 2.15 eV has been evaluated, which is supposed to be the migration enthalpy of group III vacancies. Lateral order/disorder structures were achieved by masked implantation using high resolution electron beam lithography for the definition of wire and dot implantation masks down to 35 nm width. These structures were examined using photoluminescence and transmission electron microscopy. Both methods show that the spatial resolution is determined by implantation straggling, whereas defect diffusion can be neglected. This is also confirmed by extracting the defect diffusion length from the disordering model. © 1997 American Institute of Physics.

Notes: Room-temperature gain coupled distributed feedback (GC-DFB) lasers have been realized by implantation induced intermixing in the GaAs/AlGaAs material system. The implantation dose has been systematically varied to realize GC-DFB lasers with different gain coupling coefficients due to different band-gap modulation of the active quantum wells. It is demonstrated that a band-gap modulation of 7 meV is sufficient to achieve a high single mode yield at room temperature. The results are discussed on the basis of calculations with the optical matrix theory. © 1995 American Institute of Physics.

Notes: Quantum wires with widths down to 45 nm have been realized by implantation induced intermixing of a surface near GaAs/AlGaAs quantum well. A very steep lateral potential has been achieved together with extreme low damage in the wire regions. As a result the optical transitions in photoluminescence excitation spectroscopy could be observed for all wire widths. With decreasing wire width an increasing Stokes shift has been determined due to the increasing importance of fluctuations in wire dimensions. A weak wire width dependence of transitions near the former two-dimensional light-hole level was observed, which is attributed to the predicted reduced energy shift of states near this level.

Notes: Ion implantation damaging of ordered (Al)GaInP reduces the temperature threshold for thermal disordering from about 850 °C to temperatures below 800 °C, therefore, annealing at 800 °C leads to selective disordering of implanted regions. Dark-field transmission electron microscopy has been used to study the spatial resolution of implantation-induced disordering of CuPtB-ordered GaInP/AlGaInP heterostructures. Lateral order/disorder structures smaller than 200 nm have been realized using implantation masks defined by electron beam lithography. The lateral extension of disordering observed by transmission electron microscopy is in agreement with photoluminescence data and calculations of the lateral implantation straggling. © 1997 American Institute of Physics.