Takayuki Sota

Spontaneous emission of localized excitons in InGaN single and multiquantum well structures

Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum‐confinement Stark effect and band filling of the localized states by excitons.

Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...

Luminescences from localized states in InGaN epilayers

Optical spectra of the bulk three-dimensional InGaN alloys were measured using the commercially available light-emitting diode devices and their wafers. The emission from undoped InxGa1−xN(x<0.1) was assigned to the recombination of excitons localized at the potential minima originating from the large compositional fluctuation. The emission from heavily impurity-doped InGaN was also pointed out related to the localized states.

First-principles study on electronic and elastic properties of BN, AlN, and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at Γ point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1–D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in c...

Polarized Raman spectra in GaN

We have measured polarized Raman spectra in a 2.0 mu m GaN epitaxial layer of high quality, grown on a sapphire substrate. All symmetry-allowed optical phonons in GaN have been assigned as follows: A1(LO), 735 cm-1; A1(TO), 533 cm-1; E1(LO), 743 cm-1; E1(TO), 561 cm-1; E2, 144 and 569 cm-1. Using the Lyddane-Sachs-Teller relation, the static dielectric constants of GaN for the ordinary and extraordinary directions have been estimated as epsilon perpendicular to 0=9.28 and E/sub //0/=10.1. We have also observed quasi-LO phonons in GaN. A brief discussion on these will be given.

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.

Correlation between the photoluminescence lifetime and defect density in bulk and epitaxial ZnO

Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

Exciton localization in InGaN quantum well devices

Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm...

Emission mechanisms of bulk GaN and InGaN quantum wells prepared by lateral epitaxial overgrowth

The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs.

Excitonic emissions from hexagonal GaN epitaxial layers

Excitonic photoluminescence (PL) peaks from hexagonal GaN epilayers were investigated making a connection with the analysis of the photoreflectance spectra. Free exciton emissions associated with transitions from the conduction (Γ7c) band to the A (Γ9v) and B (Γ7uv) valence bands are dominant above 100 K. Values of the full widths at half maximum of them were smaller than the thermal energy kBT up to room temperature, which suggests the dominance of excitons in the PL spectra.