We present an effective approach to calculating the low-frequency part of the spectrum of uniaxially patterned periodic structures. In this approach we ignore to zeroth-order the Bragg scattering by crystalline planes but include local field effects in first order perturbation theory. Bragg reflections are shown to be important only near points of symmetry-induced spectral degeneracy, where they can be taken into account by the degenerate perturbation theory. We apply this approach to waveguiding by thin patterned slabs embedded in a homogeneous medium. This results in an effective medium approximation, similar to the Maxwell Garnet theory but modified for the local field corrections specific to 2D geometry. Slab spectra are well described by a single frequency-independent parameter, which we call the guiding power. Simple analytic formulae are presented for both TM and TE polarizations. Comparing these formulae with similar expressions for homogeneous uniaxial slabs of same thickness, we derive the principal values of the effective homogeneous permittivity that provides identical waveguiding. We also discuss the extinction of waves due to the Rayleigh-like scattering on lattice imperfections in the slab. The TE waves that are normally better confined are scattered out more effciently, in part because of the higher scattering cross-section and in part because the better confinement leads to higher exposure of TE waves to lattice imperfections in the slab.
Polar semiconductor surfaces with planes parallel to the surface similar to GaAs (001) surface which is formed by either Ga or As atomic layers, are technologically important. This type of surface can be prepared by molecular beam epitaxy (MBE). Among different orientations the (001) surface is the one that is used widely in optoelectronic devices. Despite of intensive investigation of GaAs (001) surface its structural and electronic properties are under debate up to now. A number of the first principle total energy investigations of GaAs (001) surface have been reported. However, the theoretical studies of the polar semiconductor surface encounter many difficulties. For the surface simulation a slab model is usually used. In this case several problems emerged and the first one is a charge transfer between two surfaces of the slab. Relative position and dispersion of the surface state bands can be changed by the effect of electrostatic potential due to charge transfer. Another problem is the interaction of two surface states with each other through the slab. In order to exclude the manifestations of its interaction, the usage of a thick slab is necessary. Different models were developed for calculations of GaAs (001) surface. The main goal of the present report is to provide the mechanism of Cs and Sb adsorption on the GaAs (100) surface and the electron structure (ES) of the activation layer during the oxygen coadsorption. The influence of the substrate composition on the work function is also investigated.
Full-potential linearized augmented plane wae (FLAPW) method is used to investigate absorption of K, Na, and Cs on GaAs (110) and (001) -- oriented surfaces. The layer-resolved densities of states, electron energy spectrum and valence charge-densities are analyzed. Coverage dependence of the work function is obtained and is found to be in satisfactory agreement with experiments. The role of oxygen adsorption in GaAs activation to negative electron affinity state is discussed.
A full-potential linearized augmented plane wave (FLAPW) method is used to investigate (MeAs)1(GaAs)n (001) multi-layer structures wiht Me=Al, In, Cs as well as GaAs (001) and (110) surfaces. The layer-resolved densities of states, electron energy spectrum and valence charge-densities are analyzed. The adsorption of cesium is studied on both GaAs surfaces. The coverage dependence of the work function is obtained and is found to be in satisfactory agreement with experiments.
Integrated polarization and quantum yield spectra of the electrons, photoemitted from strained AlInGaAs layer, capped by heavily doped thin GaAs quantum well layer, as well as high resolution energy distriubtion curves and a polarization versus energy distribution curves are studied as a function fo light excitation energy and power. The polarization P of up to 83% in conjunction with quantum yield Y=0.5 at T=130K have been measured. At high excitation intensity decrease of Y and P is observed which is attributed to the surface photovoltage effects, i.e. decrease of the negative electron affinity, narrowing of the band-bending region and an increase of the hole concentration in the quatnum well layer. The narrow-band quantum well is shown to provide high effective negative electron afffinity values with no harm to electron polarization.
Spin-polarized electron kinetics is studied by time-resolved polarized photoemission with picosecond resolution. The response time of strained layer photocathodes is found to be in a range of a few picosecond offering an ultrafast response and high spin-polarization of emitted electrons. The studies of the sub-picosecond spin dynamics are facilitated in high-intensity excitation regime when the length of the emission pulse is enlarged due to the dispersion of acceleration time and Coulomb repulsion of the electrons in their flight in the vacuum.
Time resolved photoemission of highly spin-polarized electrons from thin strained and unstrained GaAsxP1-x films of various thicknesses has been investigated. An upper limit for the response time of a photocathode has been found to be 1 ps for layer thicknesses less than 150 nm. We show that the electron depolarization during the electron extraction to the surface band bending region can be as low as 2% while the losses in the band bending region can contribute to 4% spin relaxation.
The parameters of spin-polarized electron photocathode based on strained layer GaAs0.95P0.05/GaAs0.7P0.3 structure have been improved on the base of X-ray, Raman and polarized photoluminescence studies of such structure. The polarization maximum value 86% in conjunction with Y equals 0.16% makes such cathodes one of the best no matter where.
Yuri Mamaev, Arsen Subashiev, Yuri Bolkhovityanov, Aleksandr Toropov, Askhat Bakharov, Mikhail Revenko, Alexander Gilinskii, Yuri Yashin, Anton Ambrajei, Alexander Rochansky
We report the results on polarized electron emission from a new strained InGaAlAs/AlGaAs superlattices with an enlarged band gap, modulation doping and an optimized last GaAs layer. The yield and polarization measurements show that this structure delivers more than 0.1% quantum yield at the maximum of the polarization spectrum, the maximum polarization degree being equal to 80% at room temperature. These values present a 10-fold improvement in the quantum yield over that of the previously designed InAlGaAs-AlGaAs superlattices at the same vacuum conditions.
Photoemission from a semiconductor with negative electron affinity surface is investigated theoretically for the case of local excitation regime and Gaussian distribution of the light intensity over a sample surface. While the maximum emission current is an exponential function of the negative electron affinity value, the inhomogeneous intensity distribution results in smoothed dependence of the emission current on the light intensity in the region of maximum current. The relaxation processes to the stationary emission and the cathode restoring time are found to be less sensitive to light non-homogeneity due to photovoltage level over the surface.
We report the results on polarized electron emission from a new strained wide-gap AlxInyGa1-x-yAs/AlzGa1- zAs SL with tunable position of polarization maximum. These SL's were optimized to have a minimal conduction-band offset which comes from the band line-up between the semiconductor layers of the SL. The In layer content was chosen to give minimal conduction-band offset with large strain splitting of the V-band. Simultaneous changing of Al content in both SL layers provides variation of the structure band gap. We demonstrate that tuning of the SL to the excitation energy can be achieved without loss of the electron polarization. The polarization of up to 84% was measured at room temperature.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.