Amane Shikanai

BIAXIAL STRAIN DEPENDENCE OF EXCITON RESONANCE ENERGIES IN WURTZITE GAN

We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.

Effects of biaxial strain on exciton resonance energies of hexagonal GaN heteroepitaxial layers

Exciton resonance energies of hexagonal (h‐) GaN(0001) epilayers were determined by a combination of high‐resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger‐Kohn type Hamiltonian for the valence bands under the in‐plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h‐GaN was suggested.

Luminescence spectra from InGaN multiquantum wells heavily doped with Si

A systematic study has been carried out on emission spectra of heavily Si-doped InGaN multiquantum wells with different degree of potential fluctuation of InGaN in the lateral plane by the use of the various excitation sources. It is demonstrated that the quantum-confined Stark effect due to the piezoelectric field plays no serious role in optical spectra under appropriate doping conditions and, then, the degree of potential fluctuation of InGaN alloys is clearly reflected in spontaneous emission spectra.

Comparison of Optical Properties of GaN/AlGaN and InGaN/AlGaN Single Quantum Wells

Static, field-modulated and time-resolved spectroscopies were carried out to compare the electronic states between GaN/AlGaN binary and InGaN/AlGaN ternary single quantum wells (SQWs). The internal field that exists across the quantum well (QW) naturally induces a quantum-confined Stark effect (QCSE), namely the redshift of the QW resonance energy and decrease of the electron-hole wavefunction overlap. The GaN/AlGaN SQW exhibited a weak emission due to QCSE. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character was maintained for thin QWs with the well width nearly the same as the bulk free exciton Bohr radius even under an electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited a bright luminescence peak in spite of an effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence (CL) mapping method. The lateral interval of each light-emitting area was estimated from the spatial resolution of the CL mapping to be smaller than 60 nm. Such local potential minima is considered to be formed due to the presence of a structure similar to quantum-disks [M. Sugawara: Phys. Rev. B 51 (1995) 10743]. Carriers generated in the InGaN QWs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.

A pump and probe study of photoinduced internal field screening dynamics in an AlGaN/GaN single-quantum-well structure

Photogenerated carrier dynamics in an AlGaN/GaN single quantum well has been studied using a conventional degenerate pump and probe technique at room temperature. Photoinduced absorption at the exciton resonance has been observed. It is explained by the absorption coefficient change, through the quantum-confined Stark effect and the quantum-confined Franz–Keldish effect, caused by the photoinduced internal electric-field screening. In comparison with biased GaAs multiple quantum wells, a slower time evolution of differential transmission signals has been also found. Its origin is attributed to the longer carrier sweep-out time due to the potential profile of the sample in conjunction with the longer carrier recombination time.

No spin polarization of carriers in InGaN

Abstract We report the spin polarization of carriers photoexcited in bulk InGaN by circularly polarized femtosecond optical pulses. No spin polarization is observed in the picosecond time region using spin-dependent pump and probe absorption measurements with a time resolution of 0.35 ps. This is in contrast to the existence of spin polarization in GaAs quantum wells or in InGaAs quantum wells which have a spin relaxation time in the picosecond time region. The unique band structure of InGaN, which has weak spin–orbit interaction, and an in-plane potential fluctuation due to the compositional inhomogeneity of In explains the lack of spin polarization.

Pulse area control of exciton rabi oscillation in InAs/GaAs single quantum dot

We investigated the optical properties of an exciton and a charged exciton in an InAs/GaAs single quantum dot (QD) with truncated pyramidal shape by microspectroscopy, and clarified the difference of sub-band structure between the exciton and the charged exciton in the same single QD. We observed the exciton population of the excited states by monitoring the luminescence of the ground state exciton and succeeded in the experimental demonstration of Rabi oscillation of the exciton and the charged exciton. The transition dipole moments estimated from experimental results in a pure InAs QD are 32 and 40 D for the charged exciton and exciton, respectively, which were comparable to those in InGaAs QD.

Observation of Bonding States in Single Pair of Coupled Quantum Dots Using Microspectroscopy

The electronic structures of InAs/GaAs single and coupled quantum dots have been studied using photoluminescence (PL) and photoluminescence excitation (PLE) spectra obtained by microspectroscopy. We observed two peaks located at the band-edge energy region of the PL spectrum of single quantum dots, which originated from exciton ground states and resonance states associated with one longitudinal optical (1LO) phonon. There was no peak in the energy area between the two peaks in the single quantum dot spectrum (This area is referred to as the zero-absorption region). However, in the case of coupled quantum dots, we found new peaks in the zero-absorption region in PL and PLE spectra that are due to coupled states of coupled dots. Moreover, we found that the energy difference between the bonding and anti-bonding states depends on dot size. These results can provide the first step towards realizing quantum logic gate devices using coupled quantum dots with scalability.