Hiroaki Ando

Red Shift of Photoluminescence and Absorption in Dilute GaAsN Alloy Layers

We present the first report on the optical properties of dilute GaAS1-xNx alloys (0<x<0.015). The layers have been grown by plasma-assisted metalorganic chemical vapor deposition (MOCVD). The grown layers show a systematic red shift of the band-edge luminescence with increasing N content. The assignement of the photoluminescence to band-edge transitions and not to isolated N-N pair emission is verified by the characteristics of the optical absorption.

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 &#181;m

Dark current, quantum efficiency, multiplication noise, and pulse response of germanium avalanche photodiodes with n+-p junction were studied to find an optimum structure. The dark current can be separated by graphical means into a leakage current component and a multiplied component which flows through the junction. The dark current components are also evaluated by using diodes with various diameters. The quantum efficiency and the multiplication noise are shown to be strongly affected by the n+ layer thickness. An n+ layer thickness optimized for signal-to-noise ratio is estimated from experimental and calculated results, using a figure of merit for avalanche photodiodes. The response waveform for mode-locked Nd:YAG laser shows a rise time of 100 ps and a half pulsewidth of less than 200 ps.

Photon-spin controlled lasing oscillation in surface-emitting lasers

We report on photon-spin controlled lasing oscillation in GaAs surface-emitting lasers at room temperature. We demonstrate experimentally that the partial electron-spin alignment, created by optically pumping the GaAs laser active media with circularly polarized pulses, drastically changes the polarization state of the lasing output, causing circularly polarized lasing emission. We discuss the laser polarization characteristics in relation to the measured electron-spin relaxation time.

SPIN RELAXATION OF EXCITONS IN ZERO-DIMENSIONAL INGAAS QUANTUM DISKS

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.

Tunneling Current in InGaAs and Optimum Design for InGaAs/InP Avalanche Photodiode

Breakdown voltage and dark current density in p+n and n+p In0.53Ga0.47As diodes are compared with theoretical values taking the backward tunneling current into account. Predominant origin of dark current in an InGaAs diode is attributed to the tunneling current. Using these results, optimum design of an InGaAs/InP avalanche photodiode (APD) to obtain low dark current, high multiplication gain, high quantum efficiency and fast response is also discussed.

Characteristics in InGaAs/InP avalanche photodiodes with separated absorption and multiplication regions

Improved characteristics of compound semiconductor avalanche photodiodes with separated absorption and multiplication regions (SAM) are discussed. Temperature dependences of dark current and breakdown voltage show that the tunneling current in the narrow energy gap layer can be suppressed in InGaAs/InP APDs with the SAM structure. Dark currents above punch-through voltages, at which the depletion layer reaches the InP-InGaAs heterointerface, are caused by the generation-recombination process in the InGaAs and at the heterointerface. Dark currents near breakdown depend on the n-layer thickness and are strongly affected by the electric field strength in the ternary layer. Tunneling currents are dominant in diodes with thin n-InP layers, while the generation-recombination processes in the InGaAs layers are dominant in those with a thick n-InP layer. The dark current was as low as7.8 \times 10^{4}A/cm2atM = 10when the interface electric field strength is reduced. A maximum multiplication factor of 60 was observed for the6 \times 10^{-7}A initial photocurrent. Rise time and full width at half maximum in a pulse response waveform were 100 and 136 ps, respectively, atM = 10.

RADIATIVE RECOMBINATION LIFETIME OF EXCITONS IN THIN QUANTUM BOXES

Exciton radiative recombination lifetime in a thin quantum box in the intermediate spatial dimension between the two-dimension and the zero-dimension is investigated by a theoretical analysis which rigorously treats the electron-hole Coulomb interaction. The higher exciton states as well as the ground exciton state are explicitly taken into account to estimate the temperature dependence of exciton recombination lifetime. We clarify how the temperature dependence of the recombination lifetime varies with a change in the quantum confinement dimension which can be controlled by the lateral width of a thin quantum box. We also discuss the effect of the exciton localization due to structural imperfection on the radiative recombination lifetime.

Nanometer‐scale imaging of potential profiles in optically excited n‐i‐p‐i heterostructure using Kelvin probe force microscopy

We report on measurements of the potential profile of a GaAs/AlGaAs n‐i‐p‐i multiple quantum well structure using a scanning Kelvin probe force microscope (KFM). Using this novel technique we directly measure with meV precision and sub‐100 nm spatial resolution the potential difference between n‐i‐p‐i layers with and without external optical excitation. The measured potential profiles, which have not been directly imaged previously, agree well with potential profiles calculated for optically excited n‐i‐p‐i structures, but modified by band bending effects at the surface.

Nonlinear absorption in n-i-p-i-MQW structures

Absorptive nonlinearity in a GaAs/AlGaAs n-i-p-i-MQW (multiple quantum well) structure consisting of alternating n-AlGaAs, i-GaAs/AlGaAs MQW, and p-AlGaAs layers is investigated. A change in the absorption coefficient of more than 4000/cm is obtained in the i-MQW layer with an extremely low excitation intensity on the order of 1 mW/cm/sup 2/. The figure of merit for absorptive nonlinearity, sigma /sub ch/, defined as the change in the absorption coefficient induced by excitation of an electron-hole pair per unit volume, is experimentally evaluated to be 7*10/sup -13/ cm/sup 2/, which is an order of magnitude larger than that for saturation of excitonic absorption in a conventional MQW structure. This experimental value agrees well with the theoretical estimation, which is calculated assuming an optical nonlinear process. >

NEAR-FIELD OPTICAL SPECTROSCOPY AND IMAGING OF SINGLE INGAAS/ALGAAS QUANTUM DOTS

We use near-field optical probing at low temperatures (T=5 K) to image and examine the linear and nonlinear luminescence properties of single InGaAs/AlGaAs quantum dots grown on (311)B oriented GaAs substrates. The high spatial resolution of near-field “nanoprobing,” which is typically 200 nm or less, makes the observation of single dots at different locations on the sample possible, even though the spatial density of quantum dots is on the order of 100/μm2. We observe narrow excitonic emission lines at low excitation powers and, with increasing excitation, we observe biexcitonic emission strongly shifted (3 meV) to the low-energy side of the exciton emission.