K. Uesugi

Role of nitrogen in the reduced temperature dependence of band-gap energy in GaNAs

We have observed a significant reduction in the temperature dependence of the absorption-edge energy in GaNxAs1−x alloys with x<0.04. The effect has been analyzed in terms of the recently introduced band anticrossing model that considers a coupling of the temperature-independent localized states of substitutional nitrogen atoms and the temperature-dependent extended states of GaAs. The model explains very well the alloy composition and the temperature dependence of the absorption-edge energy. We also compare the parameters that determine the temperature dependence of the band-gap energies in GaNAs and GaInNAs alloys.

Photoluminescence study of InAs quantum dots embedded in GaNAs strain compensating layer grown by metalorganic-molecular-beam epitaxy

Self-assembled InAs quantum dots (QDs) embedded in GaN0.007As0.993 strain compensating layers have been grown by metalorganic-molecular-beam epitaxy on a GaAs (001) substrate with a high density of 1×1011 cm−2. The photoluminescence properties have been studied for two periods of InAs quantum dots layers embedded in GaN0.007As0.993 strain compensating layers. Four well-resolved excited-state peaks in the photoluminescence spectra have been observed from these highly packed InAs QDs embedded in the GaN0.007As0.993 strain compensating layers. This indicates that the InAs QDs are uniformly formed and that the excited states in QDs due to the quantum confinement effect are well defined. This is explained by tensile strain in GaNAs layers instead of the usual GaAs layers to relieve the compressive strain formed in InAs QDs to keep the total strain of the system at a minimum.

Improvement of InAs quantum-dot optical properties by strain compensation with GaNAs capping layers

Two kinds of self-assembled InAs quantum dots (QDs) grown on GaAs (001) substrates were studied. One is capped with GaAs layers and the other with GaNAs strain-compensating layers. Photoluminescence (PL) measurements on the two kinds of InAs QDs showed distinct dependence on the selection of the capping layers. The homogeneity and luminescence efficiency of the InAs QDs were much improved when the net strain was reduced with GaNAs layers. These results demonstrate the importance of net strain compensation for the improved optical quality of InAs QDs.

Semiconductor photonic dots: Visible wavelength-sized optical resonators

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160–300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots.

Strain effect on the N composition dependence of GaNAs bandgap energy grown on (0 0 1) GaAs by metalorganic molecular beam epitaxy

We study the N composition dependence of GaNAs bandgap energy up to 4.5%. The lattice structures and lattice strain of GaNAs films were characterized by the high-resolution X-ray diffraction (XRD) mapping measurements. By the asymmetric (1 1 5) XRD mapping measurements, the precise elastic deformation of the epitaxial films was determined. The measured bandgap of the coherently strained GaNAs is very close to the theoretical bandgap based on the dielectric model for N composition less than 1.5%. Above this value, it deviates considerably from the dielectric model calculation. The theoretical calculations based on the first-principle supercell calculations cannot explain the measured bandgap properties with the increase of the N composition. The influence of inhomogeneity in GaNAs films on the bandgap bowing will be also discussed.

1.55 μm emission from GaInNAs with indium-induced increase of N concentration

GaInNAs/GaAs multiple quantum well (MQW) structures were grown by metalorganic molecular-beam epitaxy (MOMBE). Increase of the N concentration in GaInNAs with increasing In concentration was observed. This trend of enhanced N incorporation for the higher In concentration made it possible to realize long-wavelength emission of 1.55 μm from a GaInNAs/GaAs MQW grown by MOMBE. This result is compared with the previous reports on the growth of GaInNAs alloys and the main factors which lead to the enhanced N incorporation are discussed.

MOVPE growth of ZnSe/ZnS distributed Bragg reflectors on GaAs (1 0 0) and (3 1 1)B substrates

Abstract High reflectivity ZnSe/ZnS distributed Bragg reflectors (DBRs) were grown on GaAs (1 0 0) and (3 1 1)B substrates by metalorganic vapor-phase epitaxy. The maximum reflectivities of ZnSe/ZnS DBRs grown with only 10 periods measured at room temperature were 91.2% at 479 nm for a (1 0 0) DBR and 94.5% at 468 nm for a (3 1 1)B DBR. These results indicate that II–VI semiconductors are useful for DBRs not only on GaAs (1 0 0) substrates but also on (3 1 1)B substrates.

Metalorganic molecular-beam epitaxy and characterization of GaAsNSe/GaAs superlattices emitting around 1.5-μm-wavelength region

GaAsNSe/GaAs superlattices (SLs) were grown on GaAs(001) substrates by metalorganic molecular-beam epitaxy. Strong photoluminesence (PL) emission around the 1.5-μm-wavelength region was observed from these SLs without thermal annealing, which suggests that high electron concentrations in GaAsNSe layers increase the radiative recombination rate, making the nonradiative recombination relatively unimportant. It is also demonstrated that (GaAsNSe/GaAs SLs)/(GaAsN/GaAsSb SLs)/GaAs heterostructures are effective to reduce the strain accumulation in the layers, which will also form effective separated confinement heterostructure. The temperature dependence of the PL peak intensity is drastically improved by combining the GaAsN/GaAsSb SLs with the GaAsNSe/GaAs SLs following this scheme. The PL peak intensity observed at 300 K was as large as 20% of that observed at 19 K. This improvement of the optical property will be attributed to the elimination of nonradiative defects by minimizing average strain in the samples.

Observation of reflection high-energy electron diffraction oscillation during metalorganic-molecular-beam epitaxy of AlAs and control of carbon incorporation

The in situ observation of reflection high-energy electron diffraction (RHEED) oscillations during the metalorganic-molecular-beam epitaxy deposition of AlAs and AlGaAs epitaxial layers is reported. In situ RHEED oscillations as well as atomic force microscopy measurements confirmed the layer-by-layer growth of the AlAs as well as the AlGaAs layers on GaAs substrates. RHEED oscillation was successfully applied to the precise control of the AlAs/GaAs superlattices and of the alloy compositions in the AlGaAs alloys. High-resolution x-ray diffraction and Hall effect measurements revealed the unintentional doping of carbon into the AlGaAs layers, but it was found that the increase in the V/III ratio is able to reduce the carbon incorporation.

p-type conductivity control of ZnSe with insertion of ZnTe:Li submonolayers in metalorganic molecular-beam epitaxy

p-type ZnSe with periodic insertion of ZnTe:Li submonolayers was grown by metalorganic molecular-beam epitaxy. The net acceptor concentration, NA–ND, up to 2×1017–5×1018 cm−3 was observed. However, under higher Li doping, reduction of the growth rate was observed and the growth direction of the islands on the surface changed from the [011] to [011] direction. This resulted in samples not uniformly doped along the growth direction especially for higher Li doping. This reduction of the growth rate was attributed to the adsorption of Li to the B steps, and this phenomenon was suppressed by a periodic Zn purge to the growing surface. By this method, we could keep the growth rate constant and succeeded in obtaining samples which are doped uniformly along the growth direction for higher Li doping. NA–ND up to 4.5×1017 cm−3 was measured in this way. The photoluminescence peaks were blueshifted and discrete peaks were observed with increasing Li doping. This interesting behavior was explained with a ZnTe quantum...