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The microstructure and strain characteristics of self-assembled InAs/GaAs quantum dots (QDs) were studied by using transmission electron microscopy. Compressive strain was induced to uncapped QDs from GaAs substrate and the misfit strain largely increased after the deposition of GaAs cap layer. Tensile strain outside QD was extended along the vertical growth direction; up to 15 nm above the wetting layer. Vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The QDs with a lens-shaped morphology were formed in the early stage of growth, and their apex was flattened by the out-diffusion of In atoms upon GaAs capping. However, aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers.

The magnetic and structural properties of Fe ion-implanted GaN was investigated by various measurements. XRD results did not show any peaks associated with second phase formation. The magnetization curve at 5 K showed ferromagnetic behavior for 900 °C-annealed sample. In zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, the irreversibility and a cusp-like behavior of the ZFC curve were observed for 900 °C-annealed sample. These behaviors are typically observed in superparamagnetic or spin glass phase. While the temperature dependence magnetization of 800 °C-annealed sample showed non-Brillouin-like curve and it is not exhibited ferromagnetic hysteresis at 5 K. In XPS measurement, the coexistence of metallic Fe (Fe 0 ) and Fe–N bond (Fe 2+ and Fe 3+ ) for Fe 2p core level spectra is observed in as-implanted sample. But 700–900 °C-annealed samples showed only Fe–N bond (Fe 2+ and Fe 3+ ) spectra. For Ga 3d core level spectra only Ga–N bonds showed for as implanted with 700–900 °C-annealed samples. From XPS results, it could be explained that magnetic property of our films originated from FeN structures.

We present terahertz (THz) generation and detection characteristics of InGaAs with periodically-positioned InAlAs insertion layers (InGaAs PPIL). A THz transmitter with the InGaAs PPIL showed three times higher than that of an InGaAs epilayer without the insertion layer at the current signal for THz generation properties. Also, the detection properties of a THz receiver with the InGaAs PPIL showed over twenty-five times higher than that of a simple InGaAs epilayer. Also current signals of the InGaAs PPIL was improved with increasing the number of the InAlAs insertion layers.

Recently, optical fiber coupled terahertz (THz) transceiver module has been developed. The THz transceiver chip in the module has twin dipole antennas which are independently aligned by famto-second laser beam using polarization maintaining (PM) optical fiber. The measured THz pulse is 1 nA peak-to-peak amplitude, 1,000:1 signal-to-noise ratio, and 2.0 THz bandwidth when the THz pulse is reflected by metal surface.

We report structural and electrical properties of catalyst-free Si-doped InAs nanowires (NWs) formed on Si(111) substrates. The average diameter of Si-doped InAs NWs was almost similar to that of undoped NWs with a slight increase in height. In the previous works, the shape and size of InAs NWs formed on metallic catalysts or patterned structures were significantly changed by introducing dopants. Even though the external shape and size of the Si-doped NWs in this work were not changed, crystal structures inside the NWs were significantly changed. For the undoped InAs NWs, both zincblende (ZB) and wurzite (WZ) structures were observed in transmission-electron microscope images, where the portion of WZ structure was estimated to be more than 30%. However, only ZB was observed with an increase in stacking fault (SF) for the Si-doped NWs. The undoped and Si-doped InAs NWs were used as channels of four-point electrical measurements with Al/Ni electrodes to investigate electrical properties. The resistivity calculated from the current-voltage curve of a Si-doped InAs NW showed 1.32 × 10−3 Ωcm, which was dramatically decreased from 10.14 × 10−3 Ωcm for the undoped InAs NW. A relatively low resistivity of catalyst-free Si-doped InAs NWs was achieved without significant change in structural dimensions.

We report the influences of a dot-in-a-well structure with a thin GaAs layer and the thickness of a waveguide (WG) on the lasing characteristics of InAs quantum dots (QDs) based on InP. The QD laser diodes (QDLDs) consist of seven-stacked InAs QDs separated by a 10 nm-thick InGaAsP (1.15 μm, 1.15Q-InGaAsP) layer, which is further sandwiched by a 800 nm-thick 1.15Q-lnGaAsP WG (reference QDLD). For comparison, the InAs QDs were inserted into the InGaAsP (1.35 μm, 1.35Q-InGaAsP) quantum well embedded in the 1.15Q-InGaAsP matrix at the active layer. And a 2 monolayer (ML)-thick GaAs layer was additionally introduced right before the QD layer (GDWELL-LDs). Lasing emission from the reference QDLD with only the 1.15Q-InGaAsP structure was not observed at room temperature (RT). However, the lasing emission from the GDWELL-LDs was clearly observed at the wavelength of 1.46 μm at RT under continuous-wave (CW) mode. The threshold current density of the GDWELL-LD with the 800 nm-thick InGaAsP WG was measured to be 830 A/cm2, which was lower than that of the GDWELL-LD with the 200 nm-thick WG (900 A/cm2). Also, the slope efficiency of the GDWELL-LD was significantly improved with increasing thickness of the InGaAsP WG.

We have investigated the feasibility of THz time-domain reflectometry for the discrimination of human early gastric cancer (EGC) from the normal gastric region. Eight fresh EGC tissues, which were resected by endoscopic submucosal dissection, were studied. Of them, six lesions were well discriminated on THz images and the regions well correlated with tumor regions on pathologically mapped images. Four THz parameters could be suggested for quantitative discrimination of EGCs. (C) 2015 Optical Society of America

Current–voltage and low frequency excess electrical noise characteristics of two different—Schottky diode and n-i-n diode—GaAs structures embedded with self-assembled In(Ga)As quantum dots are reported. We find the growth of quantum dots induces defects not only near the quantum dot but also extended to quite a distance toward the growth direction. In Schottky diode structure, comparing with the reference sample without the quantum dot layer, the current dependence of the low frequency noise spectral density indicated that the noise is from the generated interface states with the density increasing towards the band tail. Also the crystal quality of the Schottky diode including the quantum dot layer, deduced from the Hooge parameter, was slightly worse than that of the reference sample. For n-i-n diode structure, the current–voltage relation was linear, and a quadratic current dependence of the noise spectral density was observed. The Hooge parameter for the n-i-n structure was determined to be on the order of unity indicating the general degradation of the structure.

2-㎛ thick GaN epilayer was prepared, and 80 KeV Co? ions with a dose of 3 × 1016 ㎝ ?2 were implanted into GaN at 350 ℃. The implanted samples were post annealed at 700 ℃. We have investigated the magnetic and structural properties of Co ion-implanted GaN by various measurements. HRXRD results did not show any peaks associated with second phase formation and only the diffraction from the GaN layer and substrate structure were observed. SIMS profiles of Co implanted into GaN before and after annealing at 700 ℃ have shown a projected range of ~390 A with 7.4% concentration and that there is little movement in Co. AFM measurement shows the form of surface craters for 700 ℃-annealed samples. The magnetization curve and temperature dependence of magnetization taken in zero-field-cooling (ZFC) and field-cooling (FC) conditions showed the features of superparamagnetic system in film. XPS measurement showed the metallic Co 2p core levels spectra for 700 ℃-annealed samples. From this, it could be explained that magnetic property of our films originated from Co magnetic clusters.

We report some comparative results on the normal-incidence device characteristics accomplished with a couple of self-assembled InAs/GaAs quantum-dot infrared photodetectors (QDIPs) doped in InAs QDs and GaAs barriers. The peak values of responsivity and detectivity for the barrier-doped device are 650 mA/W and 3.2 × 108 cm Hz1/2/W (18 K) at λp ≅ 5 μm, respectively, which are approximately two and ten times higher than those for the QD-doped one. In addition, while there is no spectral response over 6 μm in the QD-doped structure, a strong photoresponse is extended up to around 10 μm in the barrier-doped one. Although the direct doping in InAs QDs is effective for blocking the dark current, the doping in GaAs barriers has better device performance of QDIP.

The structure and thermal stability of self-assembled InAs/GaAs quantum dots (QDs) grown by atomic layer epitaxy (ALE) and molecular beam epitaxy (MBE) QDs were studied using high-resolution electron microscopy with in situ heating experiment capabilities. The ALE QDs were larger and more regular shaped than MBE QDs because of the higher diffusivity of In compared with that of InAs in MBE growth. The QDs were found to form a lens-shaped structure with side facets in the early stage of growth. Upon capping with a GaAs layer, however, the QD apexes flattened because of the diffusion of In and As from the QDs. The structural behavior of QDs at elevated temperatures was observed directly on the atomic scale by in situ heating experiments within TEM. The in situ high-resolution electron microscopy revealed that the uncapped ALE and MBE QDs remained stable up to 580 °C. However, above 600 °C, the QDs collapsed due to the diffusion and evaporation of In and As from the QDs.

We have studied the influence of an AlGaN insertion layer in a GaN quantum well on the light emission from a strained GaN/AlGaN multiple-quantum well system. The structural properties of GaN/AlGaN multiquantum wells of the same composition were studied by using the reciprocal space mapping and simulation techniques of high-resolution x-ray diffraction. The lattice constants along the in-plane direction determined from the (1 0 5) reciprocal space maps were a = 3.161 A for normal multiquantum wells and a = 3.152 A for interlayer multiquantum wells. The spatial localization of the quantum well emission was unambiguously determined by monochromatic cathodoluminescence measurements. This observation indicates that the blue emission near 2.96 eV in the interlayer multiquantum wells originates from the MQW region, rather than from a deep level in the GaN buffer layer. These transitions could be a quantum confined Stark effect due to the piezoelectric field caused by the strain between the well and barrier.

The magnetic properties of the epitaxial Mg and Mn co-doped GaN films with low Mg and Mn concentrations grown by plasma-enhanced molecular beam epitay (PEMBE) are reported. Surface morphology was smoother as Mn effusion cell temperature increased. Room-temperature ferromagnetic ordering was observed. The saturation magnetization (Ms) of the films increased as Mn effusion cell temperature (TMn) increased. Saturation magnetization change was not observed with the variation of Mg effusion cell temperature, and this supports that Mg incorporation in the films is not related to ferromagnetic ordering in the films. The samples exhibit n-type conductivity with the room-temperature ferromagnetism, and the films’ characteristics are determined by Mn flux at fixed growth temperature.

We present low temperature photoluminescence investigations of the exciton ground state of In 0.14 Ga 0.86 As/GaAs quantum wells (QW) in the presence of pulsed magnetic fields up to 50 T. The exciton in-plane reduced mass and the heavy-hole in-plane mass are determined from the best fit of theoretical calculations to the magnetic field dependence of PL peaks. When the QW thickness decreases, their masses increases due to valence-band mixing effect.

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

InAs epilayers with thicknesses of 400, 500, 750, and 1500 nm were grown on GaAs by molecular beam epitaxy and their properties were investigated by reflection high-energy electron diffraction and photoluminescence (PL). For all InAs epilayers, the PL peak position measured at 10 K is blue-shifted from that of bulk InAs, which could be largely due to the residual strain in the epilayer. InAs epilayers have PL peaks red-shifted to lower energy as the thickness of InAs layer is increased. This is the first observation on PL of sub-μm thick InAs epilayers grown on GaAs substrate. While the PL peak position of 400 nm thick InAs layer is linearly blue-shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of thicker InAs epilayers are gradually blue-shifted and then, saturated above a power of 75 mW.

We have investigated basic properties of normal gastrointestinal (GI) tract tissues, including glandular stomach (GS), fore stomach (FS), large intestine (LI), small intestine (SI), and esophagus (ESO), from a rat model using terahertz (THz) reflection imaging and spectroscopy. The THz images collected from stratified squamous epithelia (SSE) of FS and ESO show a lower peak-to-peak value compared to those from columnar epithelia (CE) of GS, LI, or SI because the SSE contains less water than CE. The refractive index and absorption coefficient of FS were less than those of GS or LI, both having values similar to those of water. Additionally, we report internal reflection THz signals from ESO, although we were unable to determine the exact interface for this internal reflection. (C) 2014 Optical Society of America

We demonstrated that tumors in freshly excised whole brain tissue could be differentiated clearly from normal brain tissue using a reflection-type terahertz (THz) imaging system. THz binary images of brain tissues with tumors indicated that the tumor boundaries in the THz images corresponded well to those in visible images. Grey and white-matter regions were distinguishable owing to the different distribution of myelin in the brain tissue. THz images corresponded closely with magnetic resonance imaging (MRI) results. The MRI and hematoxylin and eosin-stained microscopic images were investigated to account for the intensity differences in the THz images for fresh and paraffin-embedded brain tissue. Our results indicated that the THz signals corresponded to the cell density when water was removed. Thus, THz imaging could be used as a tool for label-free and real-time imaging of brain tumors, which would be helpful for physicians to determine tumor margins during brain surgery. (C)2014 Optical Society of America

The inter-diffusion kinetics of group-III elements at the interface between self-assembled InAs quantum dots (QDs) and InAlGaAs barriers were investigated indirectly by post-growth annealing treatments and photoluminescence (PL) spectroscopy. The emission wavelength of the InAs/InAlGaAs QDs subjected to thermal annealing at 550 °C was 1444 nm at 10 K, which indicated a 57 nm red shift compared to the as-grown sample (1387 nm). The emission wavelength was blue-shifted with further increases in annealing temperature to 650 °C. Although there was a blue shift in the emission wavelength at an annealing temperature of 600 and 650 °C, the emission peak was still longer than that of the as-grown sample. These results were explained by the difference in inter-diffusion probability between group-III elements at the interface between the InAs QDs and InAlGaAs barrier.

The thermal process of the growth of GaN-based semiconductors was analysed for two home-made horizontal reactors. The reactors were designed to make the ammonia gas flow in the opposite direction to the main gas flow. For two horizontal reactors different in dimension, the low Reynolds numbers of Re = 2.94 and 4.15 were chosen for stable laminar flow and the Rayleigh numbers governing the heat convection were optimized to the values of Ra = 6.0 and 76.2, respectively. The qualities of GaN and InGaN films were characterized by Hall effect measurement, x-ray diffraction and photoluminescence and compared with respect to the reactor dependency.

At low temperature, ZnSe emits strongly the near-band-edge luminescence consisting of the excitonic emissions and defect-related ones. However, it was observed that these emission lines are strongly fatigued under ultraviolet irradiation. Based on various experiments, we propose a mechanism of photo-induced traps to explain the photoluminescence fatigue phenomenon. These traps are created or enhanced in ZnSe by irradiation of the ultraviolet light with photon energy much above the band gap energy of cubic ZnSe.

We have studied the codoping characteristics of In 0.1 Ga 0.9 N : ZnSi/GaN grown on sapphire (0 0 0 1) substrates. The films were grown in a horizontal MOVPE reactor at the reduced pressure of 300 Torr and 820°C. The FWHMs of the DCXRD for (0 0 0 2) diffraction and PL for band edge emission at room temperature from the undoped In 0.1 Ga 0.9 N are 11 arcmin and 80 meV. With the appropriate Zn only doping in undoped In 0.1 Ga 0.9 N, the emission peak is shifted from 410 nm which originates from the band edge emission to blue of 460 nm which resulted from the band to acceptor transition due to a deep acceptor level. Besides the shift of the emission peak, the yellow luminescence peak around 600 nm nearly disappeared by the Zn doping. With the optimum codoping of Si in In 0.1 Ga 0.9 N : Zn, the intensity of blue emission increases abruptly by decreasing the defect density playing an important role as a nonradiative center.

We use reflection high-energy electron diffraction (RHEED) oscillations to study the substrate temperature and In/Ga flux ratio dependencies of the growth of pseudomorphic InGaAs layers on GaAs. The growth of InGaAs on GaAs shows strongly damped oscillations which is dependent on the substrate temperature. The In mole fraction drops almost linearly with increasing substrate temperature above 580°C, with a dropping rate of 0.08–0.13%/°C which is dependent on In/Ga beam equivalent pressure (BEP) ratio. The In mole fraction drops almost linearly with decreasing substrate temperature below 500°C. The decrease below 500°C reported here is observed for the first time.

The structural and optical properties of Co ion-implanted GaN were investigated by various measurements. HRXRD, TEM, and SADP results did not show any peaks associated with second phase formation. XPS depth measurements showed the metallic Co 2p core levels for 700– 900 ∘ C annealed-samples. From XPS results, it could be explained that magnetic property of our films originated from Co or CoGa magnetic clusters. The PL spectra obtained at 10 K display a clear intensity modulation, indicating that the optical interference effect occurs in the layers.

We have investigated the effect of Si–Mg codoping on the yellow luminescence. The codoped GaN films with GaN buffer layers were grown by introducing various SiH 4 flow rates (2–10 sccm) and constant Cp 2 Mg flow rate (3 sccm) at the same time. All the codoped GaN epilayers measured by Hall effect were n-type, leading to electron concentration from 2×10 18 up to 5×10 19 cm −3 . From the photoluminescence spectra, the integrated intensity ratio of the yellow luminescence to the band-edge emission of the codoped GaN samples is lower than that of the GaN samples doped with the Si donors alone, regardless of the electron concentration and the measuring temperature. We interpret that the Ga vacancy ( V Ga ) is responsible for the yellow luminescence.

We have optically determined carrier effective masses for a series of InGaAs/GaAs quantum wells with 0 or 2 monolayer AlAs sandwiched between InGaAs and GaAs. Photoluminescence investigations of exciton ground state were carried in the presence of a magnetic field up to 50 T. The reduced mass of heavy-hole exciton was determined by theoretical fits to its diamagnetic shifts. The heavy-hole in-plane mass increased when ultra-thin AlAs layers were inserted at the both interfaces of a quantum well.

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.

A GaN epilayer was grown on Al/sub 2/O/sub 3/ substrate by metal-organic chemical vapor deposition, and Co/sup -/ ions with a dose of 3/spl times/10/sup 16/ cm/sup -2/ were implanted into GaN at 350/spl deg/C. The implanted samples were postannealed at 700/spl deg/C-900/spl deg/C to recrystallize the samples and to remove implantation damage. We have investigated the magnetic and structural properties of Co ion-implanted GaN by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, and X-ray photoelectron spectroscopy (XPS). XRD results did not show any peaks associated with the second phase formation, and only the diffraction from the GaN layer and substrate structure were observed. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions showed the features of superparamagnetic system in films annealed at 700/spl deg/C-900/spl deg/C. The magnetization curves at 5 K for samples annealed at 700/spl deg/C-900/spl deg/C exhibits ferromagnetic hysteresis loops, and the highest residual magnetization (M/sub R/) and coercivity (H/sub c/) of M/sub R/=1.5/spl times/10/sup -4/ emu/g and H/sub c/=107 Oe were found in the 800/spl deg/C annealed sample. XPS measurement showed the metallic Co 2p core levels and the metallic valence band spectra for as-implanted and 700/spl deg/C-900/spl deg/C annealed samples. From these, it could be explained that the magnetic property of our films originated from Co and CoGa magnetic clusters.

We have studied the growth of In x Ga 1 − x N/GaN on (0 0 0 1) sapphire substrates. The films were grown in a horizontal MOVPE reactor at the reduced pressure of 100 Torr. In composition, x in In x Ga 1 − x N/GaN grown at 810°C determined by the DCXRD is 0.08 and 0.13 for 770°C. The FWHMs of the DCXRD for (0 0 0 2) diffraction from the In 0.08 Ga 0.92 N and In 0.13 Ga 0.87 N are 11 arcmin and 9 arcmin, respectively, whose difference is considered to be resulted from In dissociation. In addition, we confirmed that the thermal pit density of In 0.08 Ga 0.92 N was higher than that of In 0.13 Ga 0.87 N with the SEM observation. The carrier concentration and mobility of In 0.08 Ga 0.92 N are 3 × 10 18 /cm 3 and 47 cm 2 /V s, and those of In 0.13 Ga 0.87 N are 9 × 10 18 /cm 3 and 70 cm 2 /V s, respectively. In spite of the lower carrier concentration, the mobility of In 0.08 Ga 0.92 N is lower than that of In 0.13 Ga 0.87 N, which is considered to be due to the higher defect density of In 0.08 Ga 0.92 N than that of In 0.13 Ga 0.87 N as can be seen from the result of DCXRD. The FWHMs of band-edge emission peak measured by PL at room temperature were 110 meV at 395 nm for In 0.08 Ga 0.92 N and 120 meV at 410 nm for In 0.13 Ga 0.87 N. The In composition in InGaN increased as the growth temperature decreased, and In was still dissociated at 770°C. Some high-quality InGaN films were grown on GaN films compared with the recent results of others.

We have studied the effects of growth conditions of GaN buffer layer on the properties of GaN epilayer which was grown by metalorganic chemical vapor deposition technique. As nucleus size was increased by growth conditions of the GaN buffer layer, hillocks dominated surface morphology. The hillocks were detrimental to the electrical properties of GaN epilayer because they enhance the incorporation of electrically active impurities. From post-growth annealing, it was proposed that dislocations and/or carbon are the electron Hall mobility limiting compensators.

We have studied the Si-doping characteristics in GaN on (0 0 · 1) sapphire substrates. The films were grown by horizontal metalorganic chemical vapor deposition technique at 100 Torr. Prior to Si-doping, achievement of optimum growth conditions was preceded in order to insure doping controllability. For undoped film, background carrier concentration was as low as 4 × 10 17 cm −3 . 10 K-photoluminescence (PL) in undoped film was dominated by the luminescence from two sharp free-excitons and one bound-exciton. n-Type doping with SiH 4 was carried out and electron concentration of up to 2 × 10 19 cm −3 was achieved. As the electron concentration increased, the activation efficiency of Si donor also increased. In spite of the increased carrier concentration, Hall mobility in the doped films was increased by a factor of up to two (220 cm 2 /V s) compared to the undoped films. The mobility behavior is attributed to a transport mechanism involving defect band conduction and does not correlate with the line width of the X-ray rocking curve. In addition, as the doping concentration increased, yellow luminescence occurring at around 2.2 eV gradually dominated PL spectra at 10 K and room temperature. Our PL study suggests that gallium vacancy is responsible for the yellow luminescence.

We have fabricated a newly designed horizontal counter-flow reactor for growing high-quality III-V nitrides and characterized the GaN/sapphire(0 0 0 1) grown in it. The surface morphology of the film was featureless and smooth without any defects such as hillocks or truncated hexagonals. The measured background concentration and carrier mobility of the film 1.5 m thick are 4 × 1017/cm3 and 180 cm2/V s, respectively. The defect density measured by TEM is about 1 × 109/cm2 and the FWHM of DCX-ray curving is 336 arcsec, respectively. This crystallinity is similar to what was commonly obtained for GaN on sapphire until recently. The FWHM of the band-edge emission peak measured by PL at room temperature is typically around 14 and 4 meV for the main extonic peak(DBE) at 10 K. Except DBE at 3.490 eV, two minor structures are detected on the high-energy and low-energy shoulder of DBE at 3.498 eV(FE) and 3.483(ABE).

In order to realize ZnSSe/GaAs heteroepitaxial layers with lower defect density, we have introduced molecular beam epitaxy growth by a three-step temperature correction which takes account of the cell radiation and surface emissivity effects superposed on the Fabry - Perot interference oscillation in the pyrometric temperature. The optimum correction rates of growth temperature are determined by the analysis of double-crystal x-ray rocking curves, and an empirical profile for the control temperature as a function of growth time is proposed for the optimum ZnSSe growth. While the etch pit density is in normal molecular beam epitaxy growth, an etch pit density of , which is the lowest value known so far, is realized in ZnSSe/GaAs(100) epilayers grown by using the linearly corrected profile.

The steady-state and time-resolved photoluminescence (PL) studies of ZnSxSe1−x epilayers on GaAs substrate grown by molecular beam epitaxy around the lattice matching composition (0 < x < 0.12) are discussed. We have investigated the PL decay dynamics of ZnSxSe1−x epilayers and found that the decay time of the ZnSxSe1−x epilayer with sulfur composition closely lattice-matched with the substrate is longer than that of any other sample. This finding is interpreted as indicating that the defects induced by lattice mismatch act as nonradiative recombination centers and consequently reduce the PL lifetimes of the epilayers. These studies suggest that the lattice mismatch has a strong correlation with PL lifetimes of the ZnSxSe1−x epilayers.

Microstructural observations on 20 periods of InAs quantum dots on a GaAs substrate grown by molecular beam epitaxy system were carried out by using high resolution transmission electron microscopy. The spherical cap-shaped InAs quantum dots were formed in a self-organized fashion, dot over dot, along the growth direction. However, two types of anomalities were found in the growth of these superlattice structures. One is the stoppage of quantum dot formation after 4 or 5 layers have been deposited. The morphology of the quantum dots was rather flat and faceted and a black and white contrast layer has appeared in the dot structure. The other type was a volcano-like defect which was grown vertically along the growth direction with a size of about 120 nm in diameter and about 400 nm in spacing. Inside the defect, black and white contrast layers have been formed along the [110] direction at the bottom of the epilayer and then changed to the [111] direction as the growth continued to the top layer.

The optical properties of (In,Ga)As/AlAs/GaAs quantum wells with two monolayer-wide AlAs interface layers sandwiched between (In,Ga)As wells and GaAs barriers were investigated by photoluminescence and photoreflectance spectroscopy. The inclusion of AlAs interface layers caused blue shifts in intersubband transitions associated with the exciton ground state confined in (In,Ga)As wells. Room temperature photoreflectance showed oscillatory features above the GaAs bandgap revealing the existence of numerous quasibound states in both the conduction and valence bands of GaAs. A fairly good agreement is obtained between the experimental results and the calculated intersubband transitions of quasibound states.

GaN epilayers have been grown on GaN nucleation layers as a function of the flow rate of TMG (trimetylgallium) decreases by deposition (MOCVD). The size of crystalline and amorphous GaN droplets became smaller as the flow rate of TMG (10.0, 8.0, 6.0, 4.0 SCCM, respectively) decreases during the growth of nucleation layers having the same thickness. The surface morphology and crystallinity of the epilayers characterized by DCXRD (double crystal X-ray diffractometer) depend on the TMG flow rate during growth of the GaN nucleation layers. The GaN epilayers grown on a nucleation layer at a rate of 8.0 SCCM exhibits the best crystallinity.

A novel buried photomixer for integrated photonic terahertz devices is proposed. The active region of the mesa-structure InGaAs photomixer is buried in an InP layer grown by metalorganic chemical vapor deposition (MOCVD) to improve heat dissipation, which is an important problem for terahertz photomixers. The proposed photomixer shows good thermal properties compared to a conventional planar-type photomixer. The MOCVD regrowth process indicates the possibility for THz photomixers to be integrated monolithically with conventional photonic devices.

We report on the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors (SML-QDIP). SML-QDIPs are grown with the number of stacks varied from 2 to 6. From detailed radiometric characterization, it is determined that the sample with 4 SML stacks has the best performance. The s-to-p (s/p) polarized spectral response ratio of this device is measured to be 21.7%, which is significantly higher than conventional Stranski-Krastanov quantum dots (similar to 13%) and quantum wells (similar to 2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774383]

We report the direct observation of numerous above-barrier quasibound states of potential barriers, formed by a thick layer of GaAs sandwiched between ultrathin AlAs interface layers in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs strained quantum wells. Interband transitions between the conduction and the valence subband up to n=26 are clearly observed in the room-temperature photoreflectance spectra. Fairly good agreement is obtained between the peak positions and the calculated interband transitions of the quasibound states found in the GaAs barriers. \textcopyright{} 1996 The American Physical Society.

We have investigated optical properties and figures of merit of sub-monolayer quantum dots (SML-QD) infrared photodetector and compared them with conventional Stranski-Krastanov quantum dots (SK-QD) with a similar design. The purpose of this study is to examine the effects of varying the number of stacks(2,3,4,5 and 6) in SML-QD detector on its device performance The peak of photoluminescence (PL) spectra of SK-QD and SML-QDs are observed at 1.07eV and 1.24~1.35eV at room temperature, respectively. The PL peak of 2 and 3 stacks SML QD are very close to the GaAs band edge peak (1.42eV) and the full width at half maximum (FWHM) of all the SML-QD are much narrower than SK-QD. Normal incidence photoresponse peak of 4 stacks SML QDIP are obtained at 7.5mm with responsivity of 0.5 A/W and detectivity of 1.2×1011 cm.Hz1/2/W (77K, 0.4V, f/2 optics), which is much narrower than spectral response of SK QDIP possibly due to bound-to-bound transition.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We demonstrate continuous wave (CW) THz generation with a compact dual-mode laser diode (DML) and InGaAs-based photomixers (PMs). By tuning the output wavelengths of the DML, we achieved THz tuning more than 1 THz. We detected THz output from PM emitter using coherent detection scheme.

In the past few years, there has been increasing interest in surface plasmon-polaritons, as a result of the strong near-field enhancement of the electric fields at a metal-dielectric interface. Here we show the first demonstration of a monolithically integrated plasmonic focal plane array (FPA) in the mid-infrared region, using a metal with a two-dimensional hole array on top of an intersubband quantum-dots-in-a-well (DWELL) heterostructure FPA coupled to a read-out integrated circuit. Excellent infrared imagery was obtained with over a 160% increase in the ratio of the signal voltage (V(s)) to the noise voltage (V(n)) of the DWELL camera at the resonant wavelength of λ=6.1 μm. This demonstration paves the way for the development of a new generation of pixel-level spectropolarimetric imagers, which will enable bio-inspired (for example, colour vision) infrared sensors with enhanced detectivity (D) or higher operating temperatures.

The quantized electron energies in the triangular well of a Si-MOSFET subjected to strong magnetic fields at tilt angles B=(Bx, 0, Bz) are calculated. Approximate but analytic solutions are also provided with numerical solutions. It is shown that the role of the coupling Hamiltonian (∼Bx·Bz) is crucial in obtaining the correct magnetization, although its contribution to the total energy is minor. This coupling term is responsible for the oscillation of the magnetizations with respect to the chemical potential or the two-dimensional electron density. The latter is also confirmed in the case of a parabolic quantum well.

We demonstrate a continuous THz emission using a low-temperature-grown InGaAs photomixer and a widely tunable dual-wavelength erbium-doped fiber laser as an optical beat source. The wavelength spacing can be tuned from 2.3 nm to 8.8 nm which corresponds to the CW THz frequency from 0.3 to 1.1 THz.

We demonstrate novel detuned dual-mode laser for tunable continuous-wave terahertz generation with InGaAs-based photomixers. The beat frequency from this dual-mode multisection laser is continuously tuned from 0.30 to over 1.15 THz.

Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators. However, device development and characterization are commonly addressed with large front side illuminated single pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated. Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip) that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in width. This approach offers an advantage over conventional device characterization because it incorporates mesas that are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral range.