Fumitaro Ishikawa

Effects of growth interruption, As and Ga fluxes, and nitrogen plasma irradiation on the molecular beam epitaxial growth of GaAs/GaAsN core–shell nanowires on Si(111)

We investigate fundamental issues on the growth of GaAs/GaAsN core–shell heterostructure nanowires (NWs) by plasma-assisted molecular beam epitaxy. A Ga catalyst crystallizes during growth interruption at a high As pressure, and afterwards the growth dominantly progresses mainly increasing the NW diameter, thereby forming a wire shell. The shell diameter increases linearly depending on growth time and group III flux, similarly to the growth mechanism of planar layers. The lateral growth rate is 0.19 times lower than the growth rate of planar GaAs on a (100) substrate. At a substrate temperature 570 °C, nitrogen incorporation is inefficient in the shell layer. At a substrate temperature of 430 °C, the nitrogen is effectively introduced under continuous plasma irradiation during the growth of the GaAsN shell, resulting in the introduction of nitrogen within the shell estimated up to about 0.5%.

Origin of radiative recombination and manifestations of localization effects in GaAs/GaNAs core/shell nanowires

Radiative carrier recombination processes in GaAs/GaNAs core/shell nanowires grown by molecular beam epitaxy on a Si substrate are systematically investigated by employing micro-photoluminescence (μ-PL) and μ-PL excitation (μ-PLE) measurements complemented by time-resolved PL spectroscopy. At low temperatures, alloy disorder is found to cause localization of photo-excited carriers leading to predominance of optical transitions from localized excitons (LE). Some of the local fluctuations in N composition are suggested to lead to strongly localized three-dimensional confining potential equivalent to that for quantum dots, based on the observation of sharp and discrete PL lines within the LE contour. The localization effects are found to have minor influence on PL spectra at room temperature due to thermal activation of the localized excitons to extended states. Under these conditions, photo-excited carrier lifetime is found to be governed by non-radiative recombination via surface states which is somewhat suppressed upon N incorporation.

Suppression of non-radiative surface recombination by N incorporation in GaAs/GaNAs core/shell nanowires

III-V semiconductor nanowires (NWs) such as GaAs NWs form an interesting artificial materials system promising for applications in advanced optoelectronic and photonic devices, thanks to the advantages offered by the 1D architecture and the possibility to combine it with the main-stream silicon technology. Alloying of GaAs with nitrogen can further enhance performance and extend device functionality via band-structure and lattice engineering. However, due to a large surface-to-volume ratio, III-V NWs suffer from severe non-radiative carrier recombination at/near NWs surfaces that significantly degrades optical quality. Here we show that increasing nitrogen composition in novel GaAs/GaNAs core/shell NWs can strongly suppress the detrimental surface recombination. This conclusion is based on our experimental finding that lifetimes of photo-generated free excitons and free carriers increase with increasing N composition, as revealed from our time-resolved photoluminescence (PL) studies. This is accompanied by a sizable enhancement in the PL intensity of the GaAs/GaNAs core/shell NWs at room temperature. The observed N-induced suppression of the surface recombination is concluded to be a result of an N-induced modification of the surface states that are responsible for the nonradiative recombination. Our results, therefore, demonstrate the great potential of incorporating GaNAs in III-V NWs to achieve efficient nano-scale light emitters.

Indium distribution at the interfaces of (Ga,In)(N,As)∕GaAs quantum wells

The indium distribution across (Ga,In)(N,As) quantum wells is determined by using transmission electron microscopy techniques. Inside the quantum well, the indium distribution is well described by Muraki’s segregation model; however, it fails in reflecting the concentration at the interfaces. To describe them, we propose a sigmoidal law which defines the smooth variation of the indium concentration with the position and provides a systematic and quantitative characterization of the interfaces. The thermal stability of the interfaces and their interplay with segregation effects are discussed. A connection between the high thermal robustness of the interfaces and the inherent thermodynamic miscibility gap of the alloy is suggested.

Critical parameters for the molecular beam epitaxial growth of 1.55μm (Ga,In)(N,As) multiple quantum wells

The authors discuss the effect of substrate temperature and As beam equivalent pressure (BEP) on the molecular beam epitaxial growth of (Ga,In)(N,As) multiple quantum wells (MQWs). Transmission electron microscopy studies reveal that a low substrate temperature essentially prevents composition modulations. Secondary ion mass spectrometry results indicate that a low As BEP reduces the incorporation competition of group V elements. The low substrate temperature and low As BEP growth condition leads to (Ga,In)(N,As) MQWs containing more than 4% N preserving good structural and optical properties, and hence demonstrating 1.55μm photoluminescence emission at room temperature.

Molecular beam epitaxial growth window for high-quality "Ga,In…"N,As… quantum wells for long wavelength emission

We grow high-quality (Ga,In)(N,As) quantum wells containing 36% In and 4.5% N by molecular beam epitaxy, with a low As pressure and low substrate temperature growth concept. A V/III beam equivalent pressure ratio of 5 and a substrate temperature of 375°C lead to highly regular ten-period multiple quantum well structures having abrupt interfaces and smooth surfaces. By varying the quantum well width from 4to8nm, we observe 1.34–1.6μm emission of narrow linewidth (⩽50meV) at room temperature after annealing. The large conduction band offset of 410meV estimated from calculations is beneficial for a material system considered for high temperature laser operation.

Selective synthesis of compound semiconductor/oxide composite nanowires.

Semiconductor/oxide composite nanowires (NWs) were synthesized by molecular beam epitaxial growth and subsequent wet oxidation. Nonselective and selective oxidation conditions applied to the GaAs/AlGaAs core-shell NWs grown on silicon substrates produced GaOx/AlGaOx and GaAs/AlGaOx NWs, respectively. The oxidized amorphous AlGaOx shell produced cathodoluminescence over a wide spectral range encompassing ultraviolet and visible wavelengths, possibly sourced from molecular species related to oxygen. The wire core was buried in the oxides when the diameter of the oxide shell increased, forming a planar structure. These composites are expected to pave the way to future electrical and optical functions for NWs.

Nitrogen δ-doping for band engineering of GaAs-related quantum structures

We study energy-band engineering with nitrogen delta (δ)-doping in GaAs-related quantum structures. A tight-binding calculation indicates that the band structure can be engineered by introducing the one-dimensional doping profile of nitrogen into GaAs. Using molecular beam epitaxy, we prepare δ-doped samples of AlGaAs/GaAs quantum wells and GaAs/δ-doped nitrogen superlattice structures at the growth temperature 560 °C. Photoluminescence obtained from the samples shows a clear redshift of the spectral peak positions dependent on the nitrogen coverage. The transition energies of the superlattice structures agree well with those obtained from photoreflectance, indicating the feasibility of band modification with a single or a multiple nitrogen δ-doped layer.

Nitrogen Gas Flow Driven Unintentional Incorporation of Al during the Growth of Dilute Nitride Semiconductor by Plasma-Assisted Molecular Beam Epitaxy

We report the unintentional incorporation of Al during the growth of molecular beam epitaxy using RF plasma source, driven by N2 gas flow. The concentrations of N, Al, O, and C within GaNAs/GaAs/AlAs structure are investigated by secondary ion mass spectrometry. In spite of the closed shutter of Al cell, we observe Al incorporation with a concentration up to 1×1018 cm-3 in GaNAs layer and characteristically in the bottom side GaAs. Its concentration is solely dependent on N2 gas flow rate. Remarkably, the operation of the RF plasma has no impact on that. C and O show their concentrations corresponding to the extrinsic Al. The complex interactions between those elements predict a possible origin of material deteriorations and difficulty for the precise doping control.

Metamorphic GaAs/GaAsBi Heterostructured Nanowires.

GaAs/GaAsBi coaxial multishell nanowires were grown by molecular beam epitaxy. Introducing Bi results in a characteristic nanowire surface morphology with strong roughening. Elemental mappings clearly show the formation of the GaAsBi shell with inhomogeneous Bi distributions within the layer surrounded by the outermost GaAs, having a strong structural disorder at the wire surface. The nanowire exhibits a predominantly ZB structure from the bottom to the middle part. The polytipic WZ structure creates denser twin defects in the upper part than in the bottom and middle parts of the nanowire. We observe room temperature cathodoluminescence from the GaAsBi nanowires with a broad spectral line shape between 1.1 and 1.5 eV, accompanied by multiple peaks. A distinct energy peak at 1.24 eV agrees well with the energy of the reduced GaAsBi alloy band gap by the introduction of 2% Bi. The existence of localized states energetically and spatially dispersed throughout the NW are indicated from the low temperature cathodoluminescence spectra and images, resulting in the observed luminescence spectra characterized by large line widths at low temperatures as well as by the appearance of multiple peaks at high temperatures and for high excitation powers.