Takehiko Tawara

Low-threshold lasing of InGaN vertical-cavity surface-emitting lasers with dielectric distributed Bragg reflectors

Lasing action is achieved in InGaN vertical-cavity surface-emitting lasers (VCSELs) with dielectric distributed Bragg reflectors (DBRs). We fabricated III-nitride VCSELs by removing a SiC substrate from a III-nitride cavity with a dry etching technique and then wafer bonding the cavity and SiO2/ZrO2 DBRs. These VCSELs have a high quality factor of 460 and a spontaneous emission factor of 10−2. We observed lasing at a wavelength of 401 nm at room temperature with optical pumping. This lasing action was demonstrated at a low threshold of 5.1 mJ/cm2 by using a high-quality crystalline cavity and quantum-well layers without surface roughening or cracking.

Synthesis of GaAs nanowires with very small diameters and their optical properties with the radial quantum-confinement effect

We report the synthesis and optical properties of GaAs nanowires with very small diameters. We grew the GaAs nanowires by using size-selective gold particles with nominal diameters of 5, 10, 20, 40, and 60 nm. The diameter-controlled nanowires enable us to observe blueshifts of the free exitononic emission peak from individual nanowires with decreasing gold-particle size due to the two-dimensional radial quantum-confinement effect. We also analyze the absorption and emission polarization anisotropies of these bare GaAs quantum nanowires.

Growth and luminescence properties of self-organized ZnSe quantum dots

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1×1010 cm−2. Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution.

High drain current density and reduced gate leakage current in channel-doped AlGaN∕GaN heterostructure field-effect transistors with Al2O3∕Si3N4 gate insulator

Channel-doped AlGaN∕GaN heterostructure field-effect transistors (HFETs) with metal-insulator-semiconductor (MIS) structures have been fabricated to obtain the high drain current density and reduced gate leakage current. A thin bilayer dielectric of Al2O3(4nm)∕Si3N4(1nm) was used as the gate insulator, to simultaneously take advantage of the high-quality interface between Si3N4 and AlGaN, and high resistivity and a high dielectric constant of Al2O3. A MIS HFET with a gate length of 1.5μm has exhibited a record high drain current density of 1.87A∕mm at a gate voltage (Vg) of +3V, which is ascribed to a high applicable Vg and a very high two-dimensional electron gas (2DEG) density of 2.6×1013cm−2 in the doped channel. The gate leakage current was reduced by two or three orders of magnitude, compared with that in normal HFETs without a gate insulator. The transconductance (gm) was 168mS∕mm, which is high in the category of the MIS structure. Channel-doped MIS HFETs fabricated have thus been proved to exhibit...

STABILITY OF CDSE AND ZNSE DOTS SELF-ORGANIZED ON SEMICONDUCTOR SURFACES

Several monolayers (ML) of CdSe were deposited on (001) GaAs surfaces to study the stability of the CdSe films. The CdSe film with the 2 ML thickness showed atomically flat surfaces just after the growth. However, in three days after the growth, self-organization into dots at room temperature was clearly observed. This unexpected self-organization of dots observed at room temperature from the once coherently-grown CdSe film will be closely correlated to the enhancement of the heterointerface diffusion observed in this combination of CdSe and GaAs. This correlation between the stability of the dots and the heterointerface diffusion was examined in the common cation case of ZnSe/ZnS, which is known to show low interface diffusion. Self-organization of ZnSe dots was observed with an atomic force microscope on (001) ZnS surfaces. The ZnSe dots were stable as expected and did not show instability such as observed for the CdSe dots on GaAs or on ZnSe.

Fabrication of an InGaN multiple-quantum-well laser diode featuring high reflectivity semiconductor/air distributed Bragg reflectors

High-quality deeply-etched GaN-based semiconductor/air distributed Bragg reflectors (DBRs) have been fabricated. A 50% reduction relative to the value for reflectors of Fabry–Perot type in the threshold pump intensity was realized by the introduction of such DBRs at the ends of the laser cavity. The reflectivity of the grating was evaluated between 44% and 62%. Finite-difference time-domain simulation had earlier been used to obtain a design for the semiconductor/air DBRs, which provides high values for reflectivity despite a significant inclination from the vertical of the sidewalls of the structure.

Photoluminescence lifetime and potential fluctuation in wurtzite Zn1−xCdxO alloy films

Carrier recombination dynamics in wurtzite Zn1−xCdxO alloy films has been studied by time-resolved photoluminescence (PL) to evaluate the potential fluctuation. Typical PL lifetime τ2 in Zn1−xCdxO is around 200 ps and gradually increases with a Cd content of up to 0.19. At a Cd content over 0.3, τ2 becomes roughly 50 ns. The degree of potential fluctuation E0 is increased from 9 to 157 meV with an increment in Cd content of 0.19. In contrast, at a Cd content over 0.3, E0 decreases to 35 meV with a Cd content of 0.55. This suggests that the potential fluctuation in Zn1−xCdxO with a high Cd content is greatly improved, which is qualitatively supported by the Zimmermann’s model [R. Zimmermann, J. Cryst. Growth 101, 346 (1990)].

Charged exciton emission at 1.3μm from single InAs quantum dots grown by metalorganic chemical vapor deposition

We have studied the emission properties of self-organized InAs quantum dots (QDs) grown in an InGaAs quantum well by metalorganic chemical vapor deposition. Low-temperature photoluminescence spectroscopy shows emission from single QDs around 1300nm; we clearly observe the formation of neutral and charged exciton and biexciton states, and we obtain a biexciton binding energy of 3.1meV. The dots exhibit an s-p shell splitting of approximately 100meV, indicating strong confinement.

Cavity mode emission in weakly coupled quantum dot - cavity systems

We study the origin of bright leaky-cavity mode emission and its influence on photon statistics in weakly coupled quantum dot - semiconductor cavity systems, which consist of a planar photonic-crystal and several quantum dots. We present experimental measurements that show that when the system is excited above the barrier energy, then bright cavity mode emissions with nonzero detuning are dominated by radiative recombinations of deep-level defects in the barrier layers. Under this excitation condition, the second-order photon autocorrelation measurements reveal that the cavity mode emission at nonzero detuning exhibits classical photon-statistics, while the bare exciton emission shows a clear partial anti-bunching. As we enter a Purcell factor enhancement regime, signaling a clear cavity-exciton coupling, the relative weight of the background recombination contribution to the cavity emission decreases. Consequently, the anti-bunching behavior is more significant than the bare exciton case - indicating that the photon statistics becomes more non-classical. These measurements are qualitatively explained using a medium-dependent master equation model that accounts for several excitons and a leaky cavity mode.

Dot-Height Dependence of Photoluminescence from ZnO Quantum Dots

We report on the optical properties of ZnO nanodots and how they are influenced by the height of the dot configuration. The nanodots were grown on SiO2/Si substrates by remote-plasma enhanced metalorganic chemical vapor deposition (RPE-MOCVD). The dot configuration was regulated by controlling the growth time. The photoluminescence (PL) spectra of nanodots having average dot height of 2.5, 3.5, and 4.5 nm grown on SiO2/Si substrates were measured at 16 K. A blue shift in the excitonic emission was observed for shorter dots due to a larger quantum confinement effect. The fractional-dimensional space model was used to estimate a dimensionality of 2.35 for ZnO nano dots with an average dot height of 2.5 nm.