[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The temperature dependences of the junction electric fields and photovoltage have been investigated for a GaAs p+-i-n+ structure by using photoreflectance (PR) spectroscopy. The electric field strength was examined through three types of Franz-Keldysh oscillation (FKO) analyses; then, the photovoltage was evaluated with respect to temperature in the range from 30 to 300 K. From the PR results, we observed two electric fields that are estimated to originate from two regions of FKOs in undoped GaAs and from the space charge region in highly-doped GaAs. The electric field under illumination decreased with decreasing temperature while the photovoltage obtained from the electric field increased. We also demonstrate that PR spectroscopy is a good method for investigating the photovoltaic effect in solar-cell structures.
No preview · Article · Sep 2015 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: In this work, to investigate the optical and the electrical properties of GaAs solar cells, we adopted optical/electrical biased electroreflectance spectroscopy (ER) and current-voltage (J-V) measurements. The p-n junction’s electric fields were evaluated through an analysis of the Franz-Keldysh oscillations (FKOs) in the ER spectra. By comparing the electrical and the optical biased ER and optical biased J-V results, we calculated the carrier densities contributing to the photovoltage (σ
) and the photocurrent (σ
J). We confirmed that the σpv could affect the field screening for the p-n junction’s electric field. With increasing number of incident photons (σ
), the ratio of σ
J was almost constant. From these results, for a concentrated photovoltaic device, we propose that the photovoltaic effect can affect the solar cell’s efficiency in the relatively high excitation intensity region.
No preview · Article · Aug 2015 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: The electroreflectance (ER) and current–voltage (J–V) of InAs/InGaAs dots in a well (DWELL) solar cell (SC) were measured to examine the optical and electrical properties. To investigate the carrier capturing and escaping effects in the quantum dot (QD) states the above and below optical biases of the GaAs band gap were used. In the reverse bias region of the J–V curve, the tunneling effect in the QD states was observed at low temperature. The ideality factors (n) were calculated from the J–V curves taken from various optical bias intensities (Iex). The changes in the ideality factor (n) and short circuit current (JSC) were attributed mainly to carrier capture at low temperature, whereas the carrier escaping effect was dominant at room temperature. ER measurements revealed a decrease in the junction electric field (FJ) due to the photovoltaic effect, which was independent of the optical bias source at the same temperature. At low temperature, the reduction of photovoltaic effect could be explained by the enhancement carrier capturing effect due to the strong carrier confinement in QDs.
No preview · Article · Jul 2015 · Current Applied Physics
[Show abstract][Hide abstract] ABSTRACT: 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
[Show abstract][Hide abstract] ABSTRACT: The temperature and the excitation-intensity dependences of the junction electric fields in the GaAs p-i-n solar cell structure have been investigated by using photoreflectance (PR) spectroscopy. In the p-i-n solar cell structure, two different electric fields are observed. The fast Fourier transform (FFT) analysis implies that the two electric fields can be assigned to the p-i and the i-n interfaces. The strengths of the electric fields at the p-i and the i-n interfaces are 38 and 44 kV/cm, respectively. The electric fields gradually increase due to the temperature-dependent photovoltage effect with increasing sample temperature. With increasing excitation intensity, the electric field at the p-i interface gradually decreases due to the photovoltage effect caused by carrier screening while that at the i-n interface is insensitive to the light’s intensity. This abnormal behavior can be explained by the anisotropy carrier dynamics at the p-i and the i-n interfaces., The relation between the open-circuit voltage (V
) and the ideality factor in concentration photovoltaic (CPV) devices is discussed.
No preview · Article · Feb 2015 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Dec 2014 · Biomedical Optics Express
[Show abstract][Hide abstract] ABSTRACT: 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-InGaAsP 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 (GD-WELLLDs). 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.
No preview · Article · Dec 2014 · Journal of Nanoscience and Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Aug 2014 · Biomedical Optics Express
[Show abstract][Hide abstract] ABSTRACT: A triple-band mid-/far-infrared (MIR/FIR) photodetector tunable by polarity is demonstrated by asymmetric quantum dot-in-double-well (DdWELL) structure that exhibits unique photoresponse (PR) transitions. In contrast to the MIR2 band with no dependence, the two MIR1/FIR PR bands are blue/red-shifted by the bias voltage, and the MIR2-FIR dual-band spectrum changes to a single-band feature due to the polarity. A four-level energy band model is proposed for the transition scheme, and the electric field dependence of the FIR band numerically calculated by a simplified DdWELL structure is in good agreement with the experimental PR spectra.
No preview · Article · May 2014 · Current Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We report on morphological, optical, and lasing characteristics of InAs quantum dots (QDs) embedded in an In0.69Ga0.31As0.67P0.33 quantum well (having a bandgap energy corresponding to a wavelength of 1.35 μm (1.35Q-InGaAsP)), which formed a dot-in-a-well (DWELL) structure. This DWELL was further sandwiched in In0.85Ga0.15As0.32P0.68 layers (1.15 μm, 1.15Q-InGaAsP). A 2 monolayer-thick GaAs layer was simultaneously introduced right below the InAs QD layer in the DWELL structure (GDWELL). The emission wavelength of the InAs GDWELL was 1490 nm, which was slightly shorter than that of the InAs QDs embedded only in 1.15Q-InGaAsP layers. To evaluate the effects of the GDWELL structure on lasing characteristics, gain-guided broad-area (BA) and index-guided ridge-waveguide (RW) laser diodes (LDs) were fabricated. The BA-LDs with the InAs QDs embedded only in 1.15Q-InGaAsP layers did not show the lasing at room temperature (RT) even in pulsed mode. For the GDWELL structure, however, the lasing emissions from both the BA-LDs and RW-LDs were successfully achieved at RT in continuous-wave mode.
No preview · Article · Apr 2014 · Journal of Crystal Growth
[Show abstract][Hide abstract] ABSTRACT: We investigated the optical and the electrical properties of GaAs solar cells (SCs) by using photoreflectance (PR) spectroscopy and current-voltage (J-V) measurements. The electric fields (F-j) in p-n junctions were evaluated through an analysis of the Franz-Keldysh oscillations (FKOs) of the PR spectra. From the excitation light intensity (I-ex) dependence of the PR, we found that the photovoltaic effect resulted in a reduction of the F-j. The F-j was gradually reduced by up to 37% from the dark built-in electric field when the I-ex was increased (I-ex = 2.3 similar to 181 mW/cm(2)). We evaluated the ideality factor (n) of the SC via PR spectroscopy, and the results matched the results of the J-V measurements well. From these results, we demonstrate that the F-j and the n of the SCs can be evaluated by using PR spectroscopy.
No preview · Article · Mar 2014 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: We investigated the optical and the electrical properties of InAs quantum-dot solar cells (QDSCs) with various InAs deposition thicknesses ( θ) via photoluminescence (PL), spectral response (SR), and current density-voltage (J-V) measurements. We fabricated three QDSCs with thicknesses of 2.0, 2.5, and 3.0 monolayers (MLs). Our measurements revealed the effects of the QD size on the spectral response, the conversion efficiency ( η) and the device parameters. The QDSCs had a maximum η of 17% for θ = 2.0 ML under AM1.5G conditions. The change of device parameters in various QDSCs could be explained by the effects of the balance between enhanced carrier production from the QD layers and carrier trapping/re-capturing by strain-induced defect/QD states.
No preview · Article · Feb 2014 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: We report on the effect of confinement barriers on the performance of InAs/InGaAs sub-monolayer quantum dot infrared photodetectors. Two samples with different AlxGa1-xAs barrier compositions (x = 0.07 for sample A and x = 0.20 for sample B) were grown with four-stacks of sub-monolayer quantum dot. Sample A had a peak response at ~7.8 μm, whereas sample B demonstrated three peaks at ~3.5, ~5, and ~7.0 μm with the intensity of the peaks strongly dependent on the applied bias. At 77 K, sample A and B had a detectivity of 1.2 × 1011 cm.Hz1/2/W (Vb = −0.4 V bias) and 5.4 × 1011 cm.Hz1/2/W (Vb = −1.5 V bias), respectively.
Full-text · Article · Feb 2014 · Optical Materials Express
[Show abstract][Hide abstract] ABSTRACT: We have studied photovoltaic characteristics of single-junction GaAs solar cells with/without an anti-reflective coating (ARC) illuminated by low-density concentration (
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We report on the evolution of the surface plasmon (SP) and waveguide mode (WM) as the core thickness is varied in InGaAs slab waveguides covered by metallic sub-wavelength slit arrays. By comparing transmission spectrum in the near-infrared region with numerical simulations, transmission dips were assigned to resonant excitations of either SPs or WMs. As the core thickness was smaller than the SP penetration depth, the resonance energy exhibits a blue shift, which scales with the field intrusion into the substrate region. For the core thickness of 400 nm supporting both the WM and SP, effective refractive index of the SP is almost constant due to the field decay within the InGaAs region, which is different to the case of the WM.
No preview · Article · Jul 2013 · Applied Physics B
[Show abstract][Hide abstract] ABSTRACT: The structural and optical properties of InAs quantum dots (QDs) embedded in an In0.85Ga0.15As0.32P0.68 matrix with the emission wavelength of 1.10 µm (1.1Q-InGaAsP) on InP were modified by adopting an In0.69Ga0.31As0.67P0.33 (1.35 µm, 1.35Q-InGaAsP) quantum-well (QW) structure (dot-in-a-well, DWELL) and by inserting an additional GaAs layer right below the QDs. The spatial QD density with the DWELL structure and the GaAs insertion layer was reduced by 53% from that for the simple InAs QDs with only a 1.1Q-InGaAsP barrier (reference sample). However, the photoluminescence intensity was improved by more than 2.5 times compared to that of the reference sample. These results are discussed in terms of the increase in the capture probability of carriers due to the 1.35Q-InGaAsP QW and the relatively strong localization of carriers according to variations in the wetting layer and the distances among QDs.
No preview · Article · May 2013 · Journal- Korean Physical Society
[Show abstract][Hide abstract] ABSTRACT: In order to investigate an influence of carrier trap by quantum dots (QDs) on the solar parameters, in this study, the solar cells with InAs/GaAs QD active layers are fabricated, and their characteristics are investigated and compared with those of a GaAs matrix solar cell (MSC). Two different types of QD structures, the Stranski-Krastanow (SK) QD and the quasi-monolayer (QML) QD, have been introduced for the QD solar cells, and the parameters (open-circuit voltage (), short-cirucuit current (), fill factor (FF), conversion efficiency (CE)) are determined from the current-voltage characteristic curves under a standard solar illumination (AM1.5). In SK-QSC, while FF of 80.0% is similar to that of MSC (80.3%), and are reduced by 0.03 V and , respectively. CE is lowered by 2.6% as results of reduced and , which is due to a carrier trap into QDs. Though another alternative structure of QML-QD to be expected to relieve the carrier trap have been firstly tried for QSC in this study, it shows negative results contrary to our expectations.
[Show abstract][Hide abstract] ABSTRACT: 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 (∼13%) and quantum wells (∼2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence.
No preview · Article · Jan 2013 · Applied Physics Letters