Katsuhiro Uesugi

Reexamination of N composition dependence of coherently grown GaNAs band gap energy with high-resolution x-ray diffraction mapping measurements

GaNAs films grown on GaAs(001) substrates by metalorganic molecular beam epitaxy were studied by high-resolution x-ray diffraction (XRD) mapping measurements. The lattice constants of epitaxial films are usually estimated from symmetric and asymmetric XRD 2θ−θ measurements. In this study, it is pointed out that the consideration of the tilt angle between the GaAs(115) and GaNAs(115) planes caused by elastic deformation of the films is crucial to determine the lattice constants of the GaNAs films coherently grown on GaAs substrates. Mapping measurements of (115) XRD (2θ−θ)−Δω were performed for this purpose. The band gap energy of the films was determined by Fourier transform absorption spectroscopy measurements. The band gap energy bowing measured up to the N composition of 4.5% will be discussed by comparing with other measurements and theoretical calculations.

Temperature dependence of band gap energies of GaAsN alloys

The temperature dependence of band gap energies of GaAsN alloys was studied with absorption measurements. As the N concentration in GaAsN increased, the temperature dependence of the band gap energy was clearly reduced in comparison with that of GaAs. The redshift of the absorption edge in GaAsN for the temperature increase from 25 to 297 K was reduced to 60% of that of GaAs for the N concentration larger than ∼1%. The differential temperature coefficient of the energy gap at room temperature was also reduced to 70% of that of GaAs. The main factor for this reduced temperature dependence in GaAsN was attributed to the transition from band-like states to nitrogen-related localized states with detailed studies of the temperature-induced shift of the absorption edge.

Bandgap Energy of GaNAs Alloys Grown on (001) GaAs by Metalorganic Molecular Beam Epitaxy

GaNAs layers have been successfully grown on GaAs(001) substrates by metalorganic molecular beam epitaxy using monomethylhydrazine (MMHy) as a N source. The N composition in GaNAs increased with decreasing growth temperature and with the increasing MMHy precursor ratio in the group-V precursors. We obtained a N composition of 7.2% in GaNAs. With the increased N composition, the absorption spectra shifted to lower energy and the absorption coefficient increased by one order of magnitude. When the N composition in GaNAs is less than 1%, the measured bandgap energy is very close to the theoretical bandgap energy based on the dielectric model. However, for N composition larger than 1%, the bandgap energy deviated considerably from the dielectric model, and approached the theoretical bandgap energy based on the first-principles supercell models.

Growth and structural characterization of III-N-V semiconductor alloys

The growth of GaNAs and related alloys has been studied using metal–organic molecular beam epitaxy. The lattice mismatch of GaNAs layers grown on GaAs induces lattice relaxation, the details of which are investigated with a mapping measurement of x-ray diffraction. The experimental results are compared to the calculated value of the critical thickness for the generation of misfit dislocations. It is shown that the surface morphology changes with the nitrogen (N) incorporation and, especially, wire-like surfaces associated with lateral compositional modulation are observed in a certain range of N compositions and with a thickness of more than 1 μm. The effects of indium and selenium doping on the N incorporation in GaNAs alloys are also discussed.

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.

Epitaxial growth of zinc-blende ZnSe/MgS superlattices on (001) GaAs

We report the growth of zinc‐blende ZnSe/MgS superlattices (SLs) on GaAs (001) substrates. The SLs were grown with metalorganic vapor phase epitaxy by selecting appropriate precursors for Mg and S. MgS naturally forms rocksalt structures, but zinc‐blende MgS layers were grown. The lattice constant of MgS was estimated to be 5.59 A. X‐ray diffraction measurements show that the ZnSe/MgS SLs are grown coherently to the GaAs substrates up to the total thicknesses of ∼3000 A.

Self-Organized CdSe Quantum Dots on (100)ZnSe/GaAs Surfaces Grown by Metalorganic Molecular Beam Epitaxy

II–VI semiconductor low-dimensional structures, quantum dots, have been grown on GaAs substrates by metalorganic molecular beam epitaxy (MOMBE). Before the heteroepitaxial growth, atomically flat, As-stabilized GaAs surfaces were prepared by high-temperature As cleaning using tris-dimethylamino-arsenic (TDMAAs). CdSe thin films deposited on (100)ZnSe/GaAs surfaces have been investigated with atomic force microscopy (AFM) and were found to form three-dimensional islands with rather uniform size distribution. A large mismatch (~7%) of lattice constants between CdSe and ZnSe pseudomorphically grown on GaAs possibly results in the Stranski-Krastanov growth mode. CdSe quantum dots with a diameter of 97±11 nm were successfully formed at 350°C.

GaNAs as Strain Compensating Layer for 1.55 µm Light Emission from InAs Quantum Dots

GaNAs strain-compensating layers (SCLs) are applied to bury InAs quantum dots (QDs) grown on GaAs substrates. The main idea is the compensation of the compressive strain induced by InAs QDs with the tensile strain in the GaNAs SCLs to keep the total strain of the system minimum. The application of the GaNAs SCLs resulted in a systematic shift of photoluminescence (PL) peaks of the InAs QDs toward the longer wavelengths with the increase of the nitrogen (N) composition in GaNAs, and luminescence at a wavelength of 1.55 µm has been achieved from the InAs QDs for the N composition of 2.7% in the GaNAs SCL. This result is promising for the application of GaNAs SCL for InAs-QDs-based long-wavelength light sources for optical-fiber communication systems.

Highly conductive GaAsNSe alloys grown on GaAs and their nonalloyed ohmic properties

Doping properties of Se in GaAsN alloys grown on GaAs (001) substrates by metalorganic-molecular-beam epitaxy were studied. Ditertiarybutylselenide (DtBSe) precursor was used as a Se source. It was found that Se was incorporated into GaAs and GaAsN layers up to a considerable concentration of ∼15%. It was also suggested that the N concentrations in GaAsNSe layers were increased by the DtBSe supply. The GaAsNSe layers were heavily doped n type, and the maximum electron concentration was as high as ∼1×1020 cm−3. With the increase of the carrier concentrations, the resistivity of GaAsNSe dramatically decreased to 1.2×10−4 Ω cm. This made it possible to have ohmic contacts without thermal annealing, which indicates that GaAsNSe alloys are an attractive candidate for the formation of nonalloyed ohmic contacts.

Atomic force microscope lithography on carbonaceous films deposited by electron-beam irradiation

A nanometer-scale mask that can be used for selective growth or selective etching was obtained by two-stage patterning. In the first step, a microscopic patterning was performed using a scanning electron microscope (SEM). In the SEM the mask was deposited by decomposition of residual oil molecules in the vicinity of the semiconductor surface. In the second stage, patterning of the carbonaceous film was made using an atomic force microscope (AFM). Use of the AFM assures not only a precise alignment with the previous marks but also a better resolution. Applying an electric bias between the conductive tip and the substrate surface previously covered with the carbonaceous film gave the local modification of the film composition. Further etching resulted in the pattern formation on the semiconductor surface with a minimum linewidth of ∼22 nm.