In view of a study on spin-polarized photodiodes, the helicity-dependent photocurrent (I) in a Fe/-AlO x /p-GaAs Schottky diode is measured at room temperature by illuminating a circularly polarized light beam ( = 785 nm) either horizontally on the cleaved sidewall or at an oblique angle on the top metal surface. The plane of incidence is fixed to be parallel to the magnetization vector of the in-plane magnetized Fe electrode. The conversion efficiency F, which is a relative value of I with respect to the total photocurrent I ph , is determined to be 1.0  10 3 and 1.2  10 2 for sidewall illumination and oblique-angle illumination, respectively. Experimental data are compared with the results of a model calculation consisting of drift-diffusion and Julliere spin-dependent tunneling transports, from which two conclusions are obtained: the model accounts fairly well for the experimental data without introducing the annihilation of spin-polarized carriers at the -AlO x /p-GaAs interface for the oblique-angle illumination, but the model does not fully explain the relatively low F in terms of the surface recombination at the cleaved sidewall in the case of sidewall illumination. Microscopic damage to the tunneling barrier at the cleaved edge would be one possible cause of the reduced F.

We demonstrate arbitrary helicity control of circularly polarized light (CPL) emitted at room temperature from the cleaved side-facet of a lateral-type spin-polarized lightemitting diode (spin-LED) with two ferromagnetic electrodes in an anti-parallel magnetization configuration. Driving alternate currents through the two electrodes results in polarization switching of CPL with frequencies up to 100 kHz. Furthermore, tuning the current density ratio in the two electrodes enables manipulation of the degree of circular polarization. These results demonstrate arbitrary electrical control of polarization with high speed, which is required for the practical use of lateral-type spin-LEDs as monolithic CPL light sources.(97/100 words) a) Electronic mail: nishizawa.n.ab@m.titech.ac.jp and b) mail: munekata.h.aa@m.titech.ac.jp. 2Circularly polarized light (CPL) has unique applicability in various fields beyond the application area of linearly polarized light (LPL). Circularity-related difference in photo-response to right-and left-handed CPL have been used in chemical analyses and for separation of optical isomers [1,2], while differences in reflectivity of optical orthogonal components have been utilized in ellipsometry [3]. Three-dimensional displays based on right-and left-handed CPL [4] accommodate head tilt and prevent simulator sickness due to crosstalk between helicities, which owes to the rotational symmetry of CPL [5]. In addition to these existing technologies, numerous potential applications based on CPL are proposed and being developed. Entanglement of photon polarization including LPL and CPL can be utilized in quantum cryptographic communication techniques [6,7]. Because CPL can maintain its polarization longer than LPL when it undergoes many-body scattering on large (larger than the wavelength) particles such as cell nuclei, it can be used as a probe for bio-tissues, [8,9]. The realization and development of these applications requires a light source with the following characteristics: high polarization emission at room temperature (RT), monolithic device construction without the use of another excitation light source and external electric/magnetic fields, and high-speed and arbitrary electrical controllability of polarization, as well as compactness for achieving a high integration.In addition to optical filters consisting of a linear polarizer (LP) and a quarterwave plate (QWP) with a conventional light source, various CPL emitters have been studied, including organic light-emitting diodes (OLED) with chiral polymers [10], chiral photonic crystal (metamaterials) devices [11,12], chiral light-emitting transistors [13], spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) [14 -17], and spinpolarized light-emitting diodes (spin-LEDs) [18,19]. All these devices have their

The correlation between the structure, measured by atomic force microscopy (AFM), and luminescence, measured by photoluminescence (PL), of InAs submonolayer stacked (SMLS) nanostructures near the 2D to 3D transition is investigated. Topographic measurements using AFM reveal a significant change in the structure of uncapped InAs SMLS samples occurs under certain conditions. This structural change is attributed to the transition from 2D to 3D growth. Optical measurements by PL of corresponding capped SMLS samples showed a significant change in the luminescence properties, in the form of significant redshift and linewidth broadening, also occurs at the same conditions where the structural change occurred. Therefore, the data in the present work establishes a strong correlation between the structural and luminescence properties of InAs SMLS nanostructures. Furthermore, the results demonstrate that two forms of InAs SMLS, stacked 2D islands and 3D structures, possess distinct properties in terms of both structure and luminescence.

The below-bandgap photoluminescence (PL) from semi-insulating (s.i.) GaAs is investigated. It is found that various electronic states that give rise to nearinfrared (NIR) PL peaks are generated through processes that are standard for epitaxial growth of InAlGaAs structures on (001) GaAs substrates. Moreover, the PL signals from these states overlap with those from InAs quantum dots, wetting layers, and InGaAs structures, complicating the design of devices for telecommunications or intermediate band solar cells. The spatial positions of the various defects are also investigated by below-gap excitation and back-illuminated PL measurements. The possible identities of the defects are also presented. The present results provide guidelines for distinguishing the desired electronic states from thermal treatment-induced defect states.

We report the observation of below-GaAs-bandgap photoluminescence (PL) emission from semi-insulating GaAs substrates subjected to thermal annealing during the standard pre-MBE-growth processes. The below-GaAs-bandgap luminescence from defects were investigated using a combination of PL techniques including below-gap-excitation (BGE) and backside illuminated (BI) PL. Using BGE and BI PL, defects deep within the substrates were probed, and their spatial positions along the sample were analyzed. A PL peak at 1000 nm was observed after pre-bake annealing at 300°C, and further peaks at 905, 940 and 1150 nm were found after oxide desorption annealing at 600°C. These are attributed to the Ga-vacancy related defect, Ga-vacancy-complex defect, As-vacancy defect, and InGaAs states, respectively. This is the first report of the formation of such optically-active defects after annealing of GaAs at moderate temperature ranges (≤600°C), providing guidelines to distinguish desired electronic states for device applications from those that arise from defects which often confuse, and also degrade the device performances.

A lateral-type spin-photodiode having a refracting facet on a side edge of the device is proposed and demonstrated at room temperature. The light shed horizontally on the side of the device is refracted and introduced directly into a thin InGaAs active layer under the spin-detecting Fe contact in which spin-polarized carriers are generated and injected into the Fe contact through a crystalline AlO x tunnel barrier. Experiments have been carried out with a circular polarization spectrometry set up, through which helicity-dependent photocurrent component, I, is obtained with the conversion efficiency F  0.4 %, where F is the ratio between I and total photocurrent I ph . This value is the highest reported so far for pure lateral-type spin-photodiodes. It is discussed through analysis with a model consisting of drift-diffusion and quantum tunneling equations that a factor that limits the F value is unoccupied spin-polarized density-of-states of Fe in energy region into which spin-polarized electrons in a semiconductor are injected.

An overview on the submonolayer stacking (SMLS) growth, by molecular beam epitaxy, is given for the growth of InAs-based quantum dots (QDs) and quantum well islands (QWIs) on GaAs in comparison with Stranski–Krastanov (SK) growth. While the size, shape, and density control of QDs by the substrate temperature or source fluxes has already been demonstrated by SK, SMLS provides novel possibilities due to its higher degree of freedom to control. By SMLS, QDs can be grown with higher size/shape control, and QWIs with varied thickness in disk-like shapes. These structures can be free from a wetting layer, being isolated from each other “floating” in the matrix. More importantly, the induced strain field is tunable, allowing us the opportunity to perform simultaneous strain and bandgap engineering. Our recent results in the tuning of photoluminescence wavelength and the transition from two-dimensional to three-dimensional structures together with atomic force microscopy are shown.

A model for lateral-type refracting-facet spin-photodiodes based on ferromagnetic metal-insulator-semiconductor (FM-I-S) junctions is described. The model utilizes spin and charge drift-diffusion equations and spin-dependent tunneling equations which are simultaneously solved numerically in order to obtain a self-consistent solution. The model is used to analyze and optimize the refracting-facet spin-photodiode structure. The relation between the active layer thickness and the effective lifetime of photo-generated electrons is investigated. Results show that the optimum active layer thickness depends on both effective lifetimes of photo-generated electrons and spins. The influence of empty density-of-state of ferromangetic metals is also explored.