Rashba spin splitting of the minibands of coupled InAs/GaAs pyramid quantum dots

The Rashba spin splitting of the minibands of coupled InAs/GaAs pyramid quantum dots is investigated using the k center dot p method and valence force field model. The Rashba splitting of the two dimensional miniband in the lateral directions is found due to the structure inversion asymmetry in the vertical direction while the miniband in the vertical direction has no Rashba spin splitting. As the space between dots increases, the Rashba coefficients decrease and the conduction-band effective mass increases. This Rashba spin splitting of the minibands will significantly affect the spin transport properties between quantum dots. (C) 2008 American Institute of Physics.

Preliminary design of a tensile-strained p-type Si/SiGe quantum well infrared photodetector

Considering tensile-strained p-type Si/Si1-yGey quantum wells grown on a relaxed Si1-xGex ( 0 0 1) virtual substrate ( y < x), the hole subband structure and the effective masses of the first bound hole state in the quantum wells are calculated by using the 6 x 6 k center dot p method. Designs for tensile-strained p-type quantum well infrared photodetectors ( QWIPs) based on the bound-to-quasi-bound transitions are discussed, which are expected to retain the ability of coupling normally incident infrared radiation without any grating couplers, have lower dark current than n-type QWIPs and also have a larger absorption coefficient and better transport characteristics than normal unstrained or compressive-strained p-type QWIPs.

Highly anisotropic and electric field tunable Zeeman splittings in Mn-doped CdS nanowires

The electronic structure, Zeeman splitting, and g factor of Mn-doped CdS nanowires are studied using the k center dot p method and the mean field model. It is found that the Zeeman splittings of the hole ground states can be highly anisotropic, and so can their g factors. The hole ground states vary a lot with the radius. For thin wire, g(z) (g factor when B is along the z direction or the wire direction) is a little smaller than g(x). For thick wire, g(z) is mcuh larger than g(x) at small magnetic field, and the anisotropic factor g(z)/g(x) decreases as B increases. A small transverse electric field can change the Zeeman splitting dramatically, so tune the g(x) from nearly 0 to 70, in thick wire. The anisotropic factor decreases rapidly as the electric field increases. On the other hand, the Zeeman splittings of the electron ground states are always isotropic.

Room-temperature highly anisotropic ferromagnetism and uniaxial spin amplification in (In,Mn)As quantum dots

The hole-mediated ferromagnetism in (In,Mn)As quantum dots is investigated using the k center dot p method and the mean field model. It is found that the (In,Mn)As quantum dot can be ferromagnetic at room temperature when there is one hole in the dot. For the spherical quantum dots, the Curie temperature decreases as the diameter increases, and increases as the effective composition of magnetic ions increases. It is interesting to find that the (In,Mn)As oblate quantum dot has highly anisotropic Zeeman splitting and ferromagnetism due to the spin-orbit coupling effect, which can be used as an uniaxial spin amplifier. (c) 2008 American Institute of Physics.

Scanning electron microscopy observation of in-device InAs/AlAs quantum dots by selective etching of capping layers

Self-assembled InAs/AlAs quantum dots embedded in a resonant tunneling diode device structure are grown by molecular beam epitaxy. Through the selective etching in a C6H8O7 center dot H2O-K3C6H5O7 center dot H2O-H2O2 buffer solution, 310 nm GaAs capping layers are removed and the InAs/AlAs quantum dots are observed by field-emission scanning electron microscopy. It is shown that as-fabricated quantum dots have a diameter of several tens of nanometers and a density of 10(10) cm(-2) order. The images taken by this means are comparable or slightly better than those of transmission electron microscopy. The undercut of the InAs/AlAs layer near the edges of mesas is detected and that verifies the reliability of the quantum dot images. The inhomogeneous oxidation of the upper AlAs barrier in H2O2 is also observed. By comparing the morphologies of the mesa edge adjacent regions and the rest areas of the sample, it is concluded that the physicochemical reaction introduced in this letter is diffusion limited.

Hole-mediated electric field tunable high Curie temperature in Mn-doped wurtzite ZnO nanowires

The hole-mediated Curie temperature in Mn-doped wurtzite ZnO nanowires is investigated using the k center dot p method and mean field model. The Curie temperature T-C as a function of the hole density has many peaks for small Mn concentration (x(eff)) due to the density of states of one-dimensional quantum wires. The peaks of T-C are merged by the carriers' thermal distribution when x(eff) is large. High Curie temperature T-C > 400 K is found in (Zn,Mn)O nanowires. A transverse electric field changes the Curie temperature a lot. (Zn,Mn)O nanowires can be tuned from ferromagnetic to paramagnetic by a transverse electric field at room temperature. (c) 2007 American Institute of Physics.

InAs/GaSb superlattices for photodetection in short wavelength infrared range

The first report of a short wavelength infrared detector based on type II InAs/GaSb superlattices is presented. Very short period superlattices containing InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The photoluminescence showed a cut-off wavelength at 2.1 mu m at 10 K and 2.6 mu m at 300 K. Room-temperature optical transmittance spectra shows obvious absorption in InAs (2ML)/GaSb (8ML) SL in the range of 450-680 meV, i.e. 1.8-2.7 mu m. The cut-off wavelength moved from 2.3 mu m to 2.6 mu m with temperature rising from 77 K to 300 K in photoresponse spectra. The blackbody response R-v exponentially decreased as a function of 1/T in two temperature sections (130-200 K and 230-300 K). The blackbody detectivity D-bb(center dot) was beyond 1 x 10(8) cmHz(1/2)/W at room temperature. (C) 2009 Elsevier B.V. All rights reserved.

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb3Fe5O12, the exchange effective field can be expressed as H-eff = -lambda center dot center dot center dot M-Fe = -lambda chi(eff)center dot center dot center dot H-e = -gamma center dot center dot center dot H-e where gamma is named as the exchange field parameter and H-e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter gamma, the properties of exchange interaction in ytterbium iron garnet (Yb3Fe5O12) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter gamma is the function of the temperatures under different external magnetic fields, and gamma = a+b center dot center dot center dot T+c center dot center dot center dot T-2, where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb3Fe5O12 are revealed. Also, excellent fits to the available experiments are obtained. (C) 2009 Elsevier B.V. All rights reserved.

Temperature dependent spectral response characteristic of III-V compound tandem cell

The GaInP/GaAs/Ge triple-junction tandem cells with a conversion efficiency of 27.1% were fabricated using metalorganic chemical vapor deposition (MOCVD) technique. Temperature dependence of the spectral response measurements of the GaInP/GaAs/Ge tandem cell was performed by a quantum efficiency system at temperatures ranging from 25A degrees C to 160A degrees C. The red-shift phenomena of the absorption limit for all subcells were observed with increasing temperature, which is dued to the energy gap narrowing with temperature. The short-circuit current densities (J (sc)) of GaInP, GaAs and Ge subcells at room temperature calculated based on the spectral response data were 12.9, 13.7 and 17 mA/cm(2), respectively. The temperature coefficient of J (sc) for the tandem cell was determined to be 8.9 mu A/(cm(2) center dot A degrees C), and the corresponding temperature coefficient of the open-circuit voltage deduced from the series-connected model was -6.27 mV/A degrees C.

Anisotropic Zeeman splitting and Stark shift of In1-yMnyAs1-xNx oblate quantum dots

The electronic structure, Zeeman splitting, and Stark shift of In1-yMnyAs1-xNx oblate quantum dots are studied using the ten-band k center dot p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level. (C) 2007 American Institute of Physics.

Highly anisotropic Zeeman splittings of wurtzite Cd1-xMnxSe quantum dots

The electronic structure and Zeeman splittings of wurtzite Cd1-xMnxSe quantum spheres are studied using the k center dot p method and mean-field model. It is interesting to find that the Zeeman splittings of some hole states in quantum spheres are highly anisotropic due to the spin-orbit coupling and wurtzite crystal structure. The anisotropy of the Zeeman splittings of hole ground states in large dots is large, while that in small dot is small because the hole ground states vary with radius. An external electrical field can change the Zeeman splitting significantly, and tune the g factor from nearly 0 to about 100.

Energy band design for Si/SiGe quantum cascade laser

This paper introduces in detail the working principle of Si/SiGe Quantum cascade laser(QCL). Appropriate parameters are used to calculate the hole subband structure of Si/Si1-xGex quantum well using a six-band k center dot p method. The dispersion relation and energy band for different layer thickness and compositions are investigated. Meanwhile, the energy separations between hole subbands in Si/Si1-xGex/Si quantum wells are also analyzed. Finally the calculated results are used for the Si/SiGe QCL design, which will be beneficial to the structure optimization of Si/SiGe QCL.

Electric field tunable electron g factor and high asymmetrical Stark effect in InAs1-xNx quantum dots

The electronic structure, electron g factor, and Stark effect of InAs1-xNx quantum dots are studied by using the ten-band k center dot p model. It is found that the g factor can be tuned to be zero by the shape and size of quantum dots, nitrogen (N) doping, and the electric field. The N doping has two effects on the g factor: the direct effect increases the g factor and the indirect effect decreases it. The Stark effect in quantum ellipsoids is high asymmetrical and the asymmetry factor may be 319. (c) 2007 American Institute of Physics.

mc-Si : H/c-Si solar cell prepared by PECVD

Hetero-junction solar cells with an me-Si: H window layer were achieved. The open voltage is increased while short current is decreased with increasing the mc-Si:H layer's thickness of emitter layer. The highest of V-oc of 597 mV has obtained. When fixed the thickness of 30 nm, changing the N type from amorphous silicon layer to micro-crystalline layer, the efficiency of the hetero-junction solar cells is increased. Although the hydrogen etching before deposition enables the c-Si substrates to become rough by AFM images, it enhances the formation of epitaxial-like micro-crystalline silicon and better parameters of solar cell can be obtained by implying this process. The best result of efficiency is 13.86% with the V-oc of 549.8 mV, J(sc) of 32.19 mA center dot cm(-2) and the cell's area of 1 cm(2).

Nonlinear Rashba model and spin relaxation in quantum wells

We find that the Rashba spin splitting is intrinsically a nonlinear function of the momentum, and the linear Rashba model may overestimate it significantly, especially in narrow-gap semiconductors. A nonlinear Rashba model is proposed, which is in good agreement with the numerical results from the eight-band k center dot p theory. Using this model, we find pronounced suppression of the D'yakonov-Perel' spin relaxation rate at large electron densities, and a nonmonotonic dependence of the resonance peak position of the electron spin lifetime on the electron density in [111]-oriented quantum wells, both in qualitative disagreement with the predictions of the linear Rashba model.