Search

It is highly challenging to grow high-quality gallium nitride (GaN) layers on silicon (Si) substrates due to the intrinsic mismatching of their structural and thermal properties. Aluminum nitride (AlN) interlayers have been used to induce a compressive strain to GaN layers during growth, which compensates for the tensile strain in these layers on Si substrates during cooling. In this study, we investigated the effect of the growth temperature and layer structure of the AlN interlayer to understand the relationship between surface flatness and relaxation ratio of the AlN interlayer and the compressive strain in the overlying GaN layer. Low-temperature (LT) growth enhanced lattice relaxation of the AlN interlayer, whereas the AlN surface was atomically flat at high temperature (HT). We also examined a two-step growth to combine the advantages of LT- and HT-AlN. This approach resulted in a surface with multiple flat regions separated by grooves, which had the largest compressive strain in the overlying GaN layer at the early stages of growth. At later stages, the strain was the largest on the HT-AlN interlayer. In both cases, the experimentally measured compressive strain exceeded simulated predictions. Finally, possible solutions for inducing a larger compressive strain in the GaN layer using interlayers were discussed. [ABSTRACT FROM AUTHOR]

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here, we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating in a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal–insulator–metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of −27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of ±30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance and the output impedance of the driver. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications. [ABSTRACT FROM AUTHOR]

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of "surface charging" on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the "extended" electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions. [ABSTRACT FROM AUTHOR]

Precise strain compensation for lattice-mismatched quantum wells is crucial for obtaining high performance devices such as quantum well solar cells. High-accuracy in situ curvature monitoring is a more efficient tool to adjust growth conditions for perfect strain balancing, and we have achieved curvature measurement during growth of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy. We have also developed the curvature calculation model taking into account of thermal expansion and lattice relaxation effects based on Stoney's equation. The measured periodical curvature behavior corresponds to the growth of compressive InGaAs well layers and tensile GaAsP barrier layers and fits perfectly with a theoretical curve assuming the structural parameters (thicknesses and atomic contents) obtained by x-ray diffraction analysis, confirming correctness of the developed calculation method. Considering the proper thermal expansion coefficients for InGaAs and GaAsP, we have obtained much accurate fitting results for measured curvature. [ABSTRACT FROM AUTHOR]

Electrically tunable active metasurfaces are attracting great interest as a possible means to realize two-dimensional pixelated surface-normal modulators for wide applications, such as high-speed imaging, optical interconnects, and photonic switching. In this work, we fabricate a metallic metasurface embedded with an electro-optic (EO) polymer and experimentally demonstrate the electrical tuning of its reflectance property. Unlike the previously demonstrated surface-normal modulators based on the EO polymer, we utilize the Fabry-Perot resonance of a metal-insulator-metal mode to trap the incident light inside a thin EO polymer layer to enhance the modulation efficiency. By applying voltage between the top and bottom layers of Au, we observe a clear spectral shift in the plasmonic resonance as well as a 5-MHz dynamic modulation at 1630-nm wavelength. [ABSTRACT FROM AUTHOR]

A multiple stepped quantum well (MSQW) structure, in which GaAs step layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed and improvements in sub-GaAs-bandgap QE have been demonstrated. We have studied the effect of indium diffusion on the optical properties in quantum well solar cell by measuring the absorptance and photoluminescence spectra. Theoretical calculations indicate that the transition energy level and the absorption magnitude are sensitive to indium diffusion. The MSQW structure with GaAs step layer is a feasible way to suppress indium diffusion and enhance the absorption. [ABSTRACT FROM AUTHOR]

A major challenge for multiple quantum well (MQW) solar cells is to extract sufficient photo-excited carriers to an external circuit through the MQW region under forward bias. The present study reports the effectiveness of compensation doping in the i-region, which includes MQWs, for more efficient transport of both electrons and holes. Unintentional p-type background doping occurs in GaAs by inevitable carbon incorporation during metal-organic vapor phase epitaxy, causing undesirable bending of the band lineup in the i-region of p-on-n devices. By cancelling this out by sulfur compensation doping to obtain a uniform electric field distribution, we achieved much a high carrier collection efficiency (CCE) >90% at the operating bias voltage regardless of the excitation wavelength, compared to < 50% without compensation doping. Consequently, cell performance was greatly improved, in particular showing an enhancement of the fill factor from 0.54 to 0.77, and degradation-free quantum efficiency within the GaAs absorption wavelength range. The photoluminescence (PL) intensity from the MQW increased as the CCE decreased at a large forward bias, and radiative recombination loss was significantly suppressed by compensation doping. Furthermore, time-resolved PL measurements indicated a much higher speed of carrier escape from the wells, showing a quicker PL decay time of 7 ns at 0.6 V, compared to 18-51 ns without compensation doping. [ABSTRACT FROM AUTHOR]

Intersubband absorption saturation at 1.57 μm wavelength was observed in a 400-μm long Si3N4-rib AlN-based waveguide with GaN/AlN multiple quantum wells (MQWs) fabricated by metalorganic vapor phase epitaxy (MOVPE). The self-saturation measurement was employed using a 1.56-μm short pulse laser which has a temporal width of 0.4 ps (full-width at half-maximum) and a repetition rate of 63 MHz. An intersubband absorption saturation by 5 dB was achieved using a pulse energy of 115 pJ. We have demonstrated the capability of MOVPE-grown GaN/AlN MQWs for intersubband optical devices operated at communication wavelength. [ABSTRACT FROM AUTHOR]

We have developed III-V-semiconductor-on-insulator (III-V-OI) structures on Si wafers with excellent bottom interfaces between In0.53Ga0.47As-OI channel layers and atomic-layer-deposited Al2O3 (ALD-Al2O3) buried oxides (BOXs). A surface activated bonding process and the sulfur passivation pretreatment have realized the excellent In0.53Ga0.47As-OI/ALD-Al2O3 BOX bottom interface properties. As a result, the III-V-OI n-channel metal-insulator-semiconductor field-effect transistors under the back-gate configuration showed the peak mobility of 1800 cm2/V s and the higher electron mobility than the Si universal one even in the high effective electric field range because of the reduction in the surface roughness and fixed charges. [ABSTRACT FROM AUTHOR]

The strain in low-temperature-grown GaN/AlN multiple quantum wells (MQWs) have been tailored by inserting an AlGaN interlayer between an AlN template and the MQWs, for the purpose of intersubband transition (ISBT) at shorter wavelength (1.52 μm) with smaller full-width at half-maximum (FWHM) (113 meV). The strain in GaN wells, ISBT wavelength and its FWHM were dependent on Al-content in the AlGaN interlayer. The compressive strain in GaN wells shifted ISBT to shorter wavelengths and narrowed absorption peaks. The interlayer with an appropriate Al-content has been proved to be mandatory for achieving strong and short-wavelength ISBT by metal organic vapor phase epitaxy-grown GaN/AlN MQWs. [ABSTRACT FROM AUTHOR]

AlN/GaN multiquantum wells (MQWs) were grown at different growth temperatures via a metal organic vapor phase epitaxy (MOVPE) system using a pulse injection method and their intersubband transition (ISBT) properties were investigated. Strong ISBT at 1.58 μm measured at room temperature was realized with MQWs grown at 770 °C and its absorption properties was the best reported in MOVPE system using GaN buffer layer. Clear blueshift of ISB absorption wavelength by lowering growth temperature was observed, which suggests that interdiffusion within MQWs was suppressed at lower growth temperatures. [ABSTRACT FROM AUTHOR]

An in situ surface passivation of InP(100) using H2S during metal organic vapor phase epitaxy has been characterized by x-ray photoemission spectroscopy and photoluminescence. X-ray photoelectron spectra indicate that the H2S-treated InP at 300 °C is free of P and In oxides even after exposure to air. The enhancement of photoluminescence intensity confirms that H2S passivation of an InP epilayer can reduce the surface defects. It is shown that H2S treatment results in In–S bonds, which dominate the sulfur-passivated InP surface, effectively suppressing interface oxidation during the subsequent ultrathin Al2O3 dielectric film growth. [ABSTRACT FROM AUTHOR]

Fabrication of abrupt InGaP/GaAs heterointerfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. An optimized gas-switching sequence to fabricate heterointerface of InGaP on GaAs layer by MOVPE was previously developed in which the unstable top surface layer of GaAs is stabilized and the exchange of P and As between InGaP and GaAs layers is suppressed. In this study, the effect of this optimized gas-switching sequence was quantitatively evaluated by using scanning transmission electron microscopy (STEM). Changes in atomic composition from GaAs to InGaP at the interface at the atomic layer level were revealed by using Z-contrast method in STEM. Quantitative evaluation using the Z-contrast method confirmed that the abruptness of the GaAs/InGaP interface was improved by this optimized gas-switching sequence. [ABSTRACT FROM AUTHOR]

Propagation losses were measured for surface plasmon-polariton (SPP) modes at metal waveguides on semiconductor substrates. The waveguides are simple strips of Au or Al deposited on InP substrates or 300-nm-thick SiO[sub 2] film covering the InP substrates. We used a direct method that can clearly discriminate SPP modes in vidicon-camera images, thereby allowing quantitative measurements. The loss coefficients measured at a wavelength of 1.55 μm were, as predicted by theory, in the range of 8.5–17 dB/mm, which shows the waveguides are feasible for practical applications. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two-hundred-period high-quality AlN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy are studied using high-resolution x-ray diffraction, transmission electron microscopy, and optical transmission spectroscopy. Excellent interfaces of the MQWs are confirmed by these measurements. The strong intersubband absorption peak at a wavelength of 1.68 μm is achieved for AlN (1.6 nm)/GaN(1.7 nm) MQW. The full width at half-maximum of the absorption peak is estimated to be 27 meV. [ABSTRACT FROM AUTHOR]

We present a demonstration of a waveguide-type depleted optical thyristor laser diode with InGaAs/ InGaAsP multiple-quantum-well structure. The measured switching voltage and current are 4.63 V and 10 µA, respectively. The holding voltage and current are, respectively, 0.59 V and 20 µA. The lasing threshold currents at 25°C and 10°C are 111 mA and 72.5 mA, respectively. The lasing wavelength is centered at 1.561 µm at a bias current equal to 1.41 times threshold. [ABSTRACT FROM AUTHOR]

We describe a GaAlAs/GaAs distributed feedback (DFB) laser with double channel planar buried heterostructure operating at λ=0.88 μm. The device has been prepared using three-step liquid phase epitaxy. A cw threshold current as low as 12 mA at room temperature has been accomplished even with third-order diffraction gratings. A single longitudinal mode oscillation without mode hopping has been observed at any injection level over a temperature range of 60 K. These characteristics are indeed almost comparable to the well-developed quaternary counterparts, thus showing that the excellent spectral purity of DFB lasers will soon be available in the fields where shorter wavelengths are needed. [ABSTRACT FROM AUTHOR]

By loading periodic loss perturbation into a GaAlAs/GaAs distributed feedback laser, we have realized complete single longitudinal mode oscillation which is hardly disturbed by cleaved facet reflection. Our coupled-mode analysis explains this empirical result well. [ABSTRACT FROM AUTHOR]

A modulated stripe width structure (MSW) has been applied to a GaAlAs/GaAs distributed feedback buried heterostructure laser to obtain complete single longitudinal mode oscillation. An MSW device having a gradual modulation scheme and antireflecting films on the cleaved facets has been fabricated using reactive ion etching (RIE). Principal results include realization of a reactive ion etched second-order grating with the grooves as deep as 0.15 μm after regrowth and a modulated stripe having extremely fine definition made possible by RIE. The device had a spectrum which agreed markedly well with an analytical result. The effective functioning of the MSW structure has thereby been confirmed experimentally. [ABSTRACT FROM AUTHOR]

Presents a correction to the article 'Shortest Intersubband Transition Wavelength (1.68) Achieved in AiN/GaN Multiple Quantum Wells by Metalorganic Vapor Phase Epitaxy,' by Ichitaro Waki, Chaiyasit Kumtornkittikul, Yukihiro Shimogaki and Yoshiaki Nakano, published in the March 2004 issue of the periodical "Applied Physics Letters".

We report an organic solid-state distributed feedback laser consisting of an organic active layer and a Bragg grating without morphological change. The active layer consists of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran as a laser dye, tris(8hydroxyquinoline) aluminum as a host, and poly(methyl methacrylate) as a binder. The threshold and linewidth of the laser are 400 μJ/cm2 and 1.5 nm, respectively. With this laser, the Bragg grating exists out of the active layer and the grating surface is flat, which are very important for device fabrication and electric driving of the laser. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]