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This paper reports the structural optimization and the temperature-dependent electrical characterization of GaAs single-junction solar cells (SJSCs) based on the p+-n-n+ junction structure. First, the effects of the p+-Al0.9Ga0.1As window layer were investigated using illuminated current density-voltage (LJ−V) and photoreflectance measurements to determine the optimal structure of the SJSC. In addition, the thickness and the doping concentration of different layers, such as the p+-GaAs emitter and the n-GaAs base layers, were optimized, leading to the maximum efficiency (η = 21.53%). These results show that the device performance can be improved further when compared to the world record (η = 28.8%). Second, the capacitance-voltage (C−V) and the dark J−V characteristics of a GaAs SJSC with the optimal structure were investigated over the temperature range from 100 to 300 K. As the temperature was increased, both the junction capacitance at zero voltage (Cj0) and the carrier concentration in the active region (Nd) increased while the depletion width (W) and the built-in potential (Vbi) decreased due to purely thermal effects. Among the SC parameters obtained from the dark J−V curves, the ideality factor (n) decreased from 4.38 to 1.70 and the reverse saturation current density (J0) increased from 6.05 × 10 −12 to 4.26 × 10 −10 A/cm2 with increasing temperature. These values at 300 K were similar to those simulated theoretically and reported previously for high-quality GaAs junction diodes.

This study investigated the effects of the number of stacking layers (S) on the optical properties of InAs/InGaAs sub-monolayer quantum dot (SML-QD) infrared photodetectors by photoluminescence spectroscopy. As S was increased from two to six, the room temperature PL emission energies were redshifted remarkably (~84 meV) and the full width at half maximum was increased by approximately 10 meV due to a change in the size distribution of SML-QDs. Furthermore, the excitation intensity-dependent PL spectra of the SML-QD (S = 6) showed improved PL integrated intensity caused by a change in the QD distribution and density. With increasing S, the carrier thermal activation energies (Ea) for the InAs QD and InGaAs quantum well (QW) increased by approximately 27 and 8 meV, respectively, due to the QD size effect. These results suggest that the mini-band structure of multiple SML-QDs formed with a very thin spacer thickness (~1 nm) could be tuned by controlling S.

Compared with solid scintillators, liquid scintillators have limited capability in dosimetry and radiography due to their relatively low light yields. Here, we report a new generation of highly efficient and low-cost liquid scintillators constructed by surface hybridisation of colloidal metal halide perovskite CsPbA3 (A: Cl, Br, I) nanocrystals (NCs) with organic molecules (2,5-diphenyloxazole). The hybrid liquid scintillators, compared to state-of-the-art CsI and Gd2O2S, demonstrate markedly highly competitive radioluminescence quantum yields under X-ray irradiation typically employed in diagnosis and treatment. Experimental and theoretical analyses suggest that the enhanced quantum yield is associated with X-ray photon-induced charge transfer from the organic molecules to the NCs. High-resolution X-ray imaging is demonstrated using a hybrid CsPbBr3 NC-based liquid scintillator. The novel X-ray scintillation mechanism in our hybrid scintillators could be extended to enhance the quantum yield of various types of scintillators, enabling low-dose radiation detection in various fields, including fundamental science and imaging., Optics: Hybrid liquid scintillator with enhanced light output A hybrid liquid scintillator could be used in low-dose radiation detectors for use in imaging applications and scientific research. Highly sensitive X-ray detection is increasingly being used in industrial and military applications and for fundamental scientific research. Although liquid scintillators are more resilient to damage from exposure to intense radiation than crystalline or plastic scintillators, they have relatively low density and low radioluminescence quantum yields, both of which are critical for achieving high resolution and contrast in X-ray imaging. Now, a team of Korean researchers led by Hyunsik Im from Dongguk University has created a liquid scintillator detector made from colloidal perovskite metal halide nanocrystals and organic molecules with a significantly enhanced quantum yield. The novel device could see use in a wide range of X-ray technologies that require high-performance detectors and imagers.

Semiconductor quantum dots (QD) have been extensively applied in optical and optoelectronic devices because of their strong quantum confinement and bandgap tunability. Most research has focused on the design, material growth, and characterization of self-assembled QDs grown by Stranski- Krastanov (S-K) growth mode. As an alternative to S-K QDs, sub-monolayer (SML) QDs have recently attracted much attention due to their ultrahigh dot density, excellent size uniformity, and high crystal quality. These better material properties of SML QDs promise great application potential in optoelectronic devices such as infrared photodetectors and solar cells. In this review, we present and discuss the material and device characteristics of the infrared photodetectors and solar cells with InAs QDs grown by S-K and SML growth modes.

The effects of quantum dot (QD) size on the optical and electrical properties of InAs/GaAs QD solar cells (QDSCs) were investigated. QDSCs with varying InAs QD size were fabricated by controlling the total InAs deposition thickness ( θ ) from 0 to 3.0 mono-layers (ML). The optical and electrical properties of the QDSCs were investigated using photoluminescence (PL), time-resolved PL (TRPL), photoreflectance (PR) spectroscopy, capacitance-voltage ( C - V ), and current-voltage ( J-V ) measurements. The QD size effects on the p-n junction electric fields ( F pn ) and the efficiencies ( η ) of the QDSCs were revealed. The QDSCs had a maximum η of 21.17% for θ =2.0 ML (the efficiency is enhanced by 17.4% over the reference GaAs-SC) and minimized F pn (113 kV/cm) by an enhanced photovoltaic effect caused by improved carrier generation. We find that these optimal properties result from a balance between carrier generation and exhaustion processes through trapping and re-capturing by defects and relatively large QDs.

We investigated high-field terahertz (THz) responses and the nonlinear conductivities of n- and p-type GaAs thin films in the THz region. As the THz pulse intensity was increased to 59 μJ/cm 2 , the THz transmission of the n-type GaAs thin film was significantly increased, more than that of the p-type film. This result is correlated with the conductivity of the electronic system caused by scattering processes. Intervalley scattering was dominant in n-type GaAs, whereas hot–cold hole interactions and intervalence band transitions were the main factors reducing the conductivity in p-type GaAs. In addition, we extracted the frequency-domain conductivity of the n- and p-type GaAs thin films and used the Drude–Smith model to explain non-Drude-like behavior due to high-field excitation.

To improve the efficiency of InAs quantum dot solar cells (QDSCs), we propose a QDSC structure with sub-monolayer (SML) QDs. The optical and electrical properties of Stranski–Krastanow (SK) QDs and SML-QDs embedded in dot-in-a-well (DWELL) were investigated by photoluminescence and photoreflectance spectroscopy and illuminated J-V measurement. The SML-QDSC showed the improved probability of carriers thermally escaping from the QD states due to the small QD size. The suppression of carrier re-capturing enhanced the photovoltaic effect due to the enhanced carrier screening. The conversion efficiency of the SML-QDSC (η = 10.65%) was enhanced by about 12.58% compared to that of the SK-QDSC (η = 9.46%). The improved SC efficiency of the SML-QD is attributed to the suppressed carrier re-capturing and carrier trapping caused by the smaller QD size and lower defect density.

A two- to three-dimensional (2D to 3D) transition in InAs/GaAs sub-monolayer quantum dot (SML-QD) growth was investigated, and its evolution was traced. From the AFM results, the SML-QDs disappeared gradually and plateaus around each island formed with increasing GaAs capping layer thickness (tCL) from 1.5 ML to 4.5 ML. The SML-QDs become fully covered with a GaAs capping layer, leading to a planarization process caused by strain relaxation. In addition, when the individual InAs coverage was is insufficient to cause a 3D transition, similar plateaus formed when stacking with an increasing number of stacking layers (S) from 2 to 6. On the other hand, as the InAs coverage was increased from 0.3 to 0.5 ML, 3D dots with a comparable size to typical SK-QDs were observed beyond S = 4. This means that as the total InAs deposition thickness increases above a critical thickness (~2.0 ML), 2D-small islands transform spontaneously into 3D-pyramid QDs. Such structures then show similar optical characteristics to SK-QDs. Overall, the transition from 2D to 3D in InAs/GaAs SML-QD growth could be controlled by careful choice of the parameters tcl, θ, and S.

The original version of this article unfortunately contained a mistake. The spelling of the 6th author was incorrect. The correct name is Honggyun Kim.

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 ( I ex ). The changes in the ideality factor ( n ) and short circuit current ( J SC ) 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 ( F J ) 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.

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.

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 pv ) 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 ph ), the ratio of σ pv to 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.

A novel terahertz (THz) otoscope is designed and fabricated to help physicians to diagnose otitis media (OM) with both THz diagnostics and conventional optical diagnostics. The inclusion of indium tin oxide (ITO) glass in the THz otoscope allows physicians to diagnose OM with both THz and conventional optical diagnostics. To determine THz diagnostics for OM, we observed reflection signals from samples behind a thin dielectric film and found that the presence of water behind the membrane could be distinguished based on THz pulse shape. We verified the potential of this tool for diagnosing OM using mouse skin tissue and a human tympanic membrane samples prior to clinical application. The presence of water absorbed by the human membrane was easily distinguished based on differences in pulse shapes and peak-to-peak amplitudes of reflected THz pulses. The potential for early OM diagnosis using the THz otoscope was confirmed by alteration of THz pulse depending on water absorption level.

We design and optimize monolithic III–V GaInP/GaAs/Ge triple-junction (TJ) solar cells by using a commercial software Silvaco ATLAS simulator to obtain the maximum short-circuit current density J sc . The maximum J sc , which is a current matching value between the GaInP top and GaAs middle subcells, can be determined by varying the base thicknesses of the GaInP top and GaAs middle subcells. From the numerical simulation results, a matched maximum J sc value of 13.92 mA/cm2 is obtained at base thicknesses of 0.57 μm and 3 μm for the GaInP top and GaAs middle subcells, respectively, under 1-sun air mass 1.5 global spectrum illumination, leading to a high power conversion efficiency of 30.72%. The open-circuit voltage and the fill factor are 2.55 V and 86.55%, respectively. For the optimized cell structure, the external quantum efficiency and the photogeneration rate distributions are also investigated. To obtain efficient antireflection coatings (ARCs), we perform optical reflectance calculations by using a rigorous coupled-wave analysis method. For this, a silicon oxide/titanium oxide double-layer is used as an ARC on the TJ solar cell.

We report on morphological, optical, and lasing characteristics of InAs quantum dots (QDs) embedded in an In 0.69 Ga 0.31 As 0.67 P 0.33 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 In 0.85 Ga 0.15 As 0.32 P 0.68 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.

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 ∼ 181 mW/cm2). 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.

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.

We have studied photovoltaic characteristics of single-junction GaAs solar cells with/without an MgF₂/ZnS anti-reflective coating (ARC) illuminated by low-density concentration (＜10 suns). By the ARC deposition, the short-circuit current density (J SC ) and the fill factor (FF)are increased by 5 mA/cm² and 5% at a standard illumination (1 sun), respectively, and the resulted conversion efficiency is enhanced by 45%. In contrast with the cell with no ARC showing a rapid degradation with increasing concentration power, the efficiency of ARC-deposited cell remains almost constant as (17.7±0.3)% regardless of the concentration. It informs that ARC treatment is very effective in GaAs concentrator solar cells.

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.

본 연구에서는 양자점(quantum dot, QD)에서의 전하트랩이 태양전지의 특성에 미치는 영향을 조사하기 위하여, GaAs 모체 태양전지(MSC)의 활성층에 InAs/GaAs QD을 삽입한 p+-QD-n/n+ 태양전지(QSC)를 제작하여 그 특성을 비교 조사하였다. Stranski-Krastanow (SK)와 준단층(quasi-monolayer, QML)의 2종류 QD를 도입하였으며, 표준 태양광(AM1.5)에서 얻은 전류-전압 곡선으로부터 태양전지의 특성인자(개방전압(VOC), 단락전류(ISC), 충만도(FF), 변환효율(CE))를 결정하였다. SK-QSC의 FF값은 80.0%로 MSC의 값(80.3%)과 비슷한 반면, VOC와 JSC는 각각 0.03 V와 2.6 mA/cm2만큼 감소하였다. VOC 및 JSC 감소 결과로 CE는 2.6% 저하되었는데, QD에 의한 전하트랩이 주요 원인으로 지적되었다. 전하트랩을 완화시키기 위한 구조로서 QML-QD 기반 태양전지를 본 연구에서 처음 시도하였으나, 예측과는 달리 부정적 결과를 보였다.

We designed and fabricated a primary terahertz (THz) otoscope to help physicians to diagnose otitis media (OM) with both THz diagnostics and conventional optical diagnostics. We evaluated the potential of this equipment for diagnosing OM using mouse skin tissues and a human tympanic membrane samples prior to clinical application.

We developed an optical fiber-coupled compact terahertz (THz) transceiver module which uses photoconductive generator and detector driven by a mode-locked Ti:Sapphire laser. The cross section of the module has a diameter of 10 mm and a length of 18 mm, making it small enough to be used as an endoscope system. For a feasibility test, the transceiver module was used to measure the reflective THz signals from glandular stomach (GS) and fore stomach (FS) samples of rats. The THz reflection from the GS samples was 6 % greater than that of the FS sample, indicating that the adipose content of FS in the tissue is greater than that of GS in the tissue.

We designed and fabricated a novel terahertz (THz) otoscope to help physicians to diagnose otitis media (OM) with both THz diagnostics and conventional optical diagnostics. We verified the potential of this tool for diagnosing OM using mouse skin tissue and a human tympanic membrane samples prior to clinical application. (C) 2016 Optical Society of America

Ga droplets were totally removed from the top region of self-catalyzed GaAs nanowires (NWs) formed on Si(111), which was done by controlling the arsenic conditions during the growth period and the cooling process of the substrate temperature. From the spatial luminescence profiles of the GaAs NWs measured by using laser confocal scanning microscopy, the emission position from the NWs without Ga droplets was 896.4 nm. This was shorter than that with the Ga feature (912.7 nm). The difference in the emission wavelengths can be explained by the scattering probability of the photons from GaAs NWs in Ga droplets and by a modification in the energy level caused by the interface condition.

Electrical characteristics of p-GaSb are examined by applying the junction Hall effect measurement (JHEM) to homoepitaxially grown p-GaSb/n-GaSb layers, and analysis is made on the disagreement with those of an equivalent heteroepitial p-GaSb/semi-insulating (SI) GaAs. The photoluminescence spectra reveal that a large number of intrinsic defects are induced due to the lattice mismatch in p-GaSb/SI–GaAs, and the shallow donors of [Sb Ga ] involved in the electrical conduction was significantly reduced in p-GaSb/n-GaSb. This proves that the homoepitaxial p-GaSb is quite different from the heteroepitaxial one in the electrical and the optical characteristics, and that the JHEM approach is correct in determining the Hall parameters of the epitaxial layers whose high-resistive substrates are not available.

By using droplet epitaxy, we fabricated asymmetrically-coupled GaAs quantum dot molecules (AQDMs) and symmetrically-coupled GaAs concentric quantum-double rings (SCQRs). The geometrical coupling distances, dot-to-dot for the AQDMs and ring-to-ring for the SCQRs, were 30 nm and 28 nm, respectively. The photoluminescence (PL) measurement was performed with a femtosecond laser as an excitation source to investigate the structure-dependent optical properties of the AQDMs and the SCQRs as functions of the excitation intensity (corresponding to the injection carrier density). The excitation-power-dependent PL showed that the emission peak of the AQDMs was systemically red-shifted with increasing excitation intensity while the emission peak of the SCQRs was blue-shifted. The anomalous behavior in the shift of the emission peak for the AQDMs can be explained by using the inhomogeneous carrier distribution due to the geometrical asymmetry of the coupled QDs.

We investigate the effect of the quantum dot (QD) density on the thermal escape and the retrapping processes of carriers for unstrained GaAs/AlGaAs QDs through temperature-dependent photoluminescence measurements. We fabricated high-density GaAs QDs (8.4 × 1010/cm2, dot-dot distance ∼34 nm) on an Al0.3Ga0.7As/GaAs (111)A surface by using droplet epitaxy. The average lateral size and height of the GaAs QDs are 24 and 6 nm, respectively. Temperature-dependent photoluminescence (PL) studies show that high-density GaAs QDs undergo a sigmoidal-shape energy shift. The sigmoidal dependence of the PL peak energy can be explained by thermal escaping of carriers followed by re-trapping by QDs. Our analysis indicates that the re-trapping probability of thermally-escaped carriers increases with decreasing dot-to-dot distance (corresponding to an increase in the QD density).

The energy states of InAs/GaAs self-assembled quantum dots (QDs) were analyzed by comparing between two QD systems with different QD sizes. The electrical properties of the QD systems were investigated via capacitance-voltage measurements and capacitance transient spectroscopy (also known as deep-level transient spectroscopy) with selective carrier injection and extraction which can be achieved with very small pulse amplitude under bias variation. For the large QDs, several energy states were found with the use of selective carrier injection and extraction. The thermal-activation energies obtained from the capacitance transient spectra of the large QDs were distributed from 70 to 600 meV. This energy distribution was originated from the quantized states of the individual QDs and the size distribution of the QDs. The spectra of the small QDs showed a well-defined energy state of E(c) - 132 meV. From these results, it was estimated that two to four electrons fill a single QD under the proper measurement bias of 0.2 V pulse.

We analyze the current density - voltage (J - V) curve of a Cu(In,Ga) $Se_2$ (CIGS) thin-film solar cell measured at different irradiation power densities. For the solar-cell sample investigated in this study, the fill factor and power conversion efficiency decreased as the irradiation power density (IPD) increased in the range of 2 to 5 sun. Characteristic parameters of solar cell including the series resistance ( $r_s$ ), the shunt resistance ( $r_{sh}$ ), the photocurrent density ( $J_L$ ), the saturation current density ( $J_s$ ) of an ideal diode, and the coefficient ( $C_s$ ) of the diode current due to electron-hole recombination via ionized traps at the p-n interface are determined from a theoretical fit to the experimental data of the J - V curve using a two-diode model. As IPD increased, both $r_s$ and $r_{sh}$ decreased, but $C_s$ increased.

We report the influences of growth parameters on the characteristics of GaAs quantum rings (QRs) and quantum dots (QDs) formed on AlGaAs/GaAs by the droplet epitaxy (DE) method. After forming Ga droplets on the AlGaAs/GaAs surface, varying amounts of arsenic (As) flux were introduced to fabricate the GaAs quantum structures. By decreasing the As flux from 8 × 10 − 5 to 3 × 10 − 5 Torr, the shape of the GaAs quantum structures was changed from QDs to elongated QRs. With further decreasing As flux, the shape of the elongated QRs became symmetric. The formation characteristics of the GaAs QRs from the QDs with the amount of As flux were discussed in terms of migration behaviors of the gallium (Ga) atoms on the GaAs(001)- c (4 × 4) surface. The effects of the amount of Ga supply and the growth temperature for the deposition of Ga droplets on the formation of the GaAs quantum structures were also considered.

The use of resonant tunneling (RT) barriers in the design of quantum dots-in-a-well (DWELL) infrared photodetectors is reported. The design of RT barriers for a variety of goals has been discussed. For simple DWELL designs, we demonstrate 2-3 orders-of-magnitude reduction in the dark current, with significant increase in the specific detectivity (D *) of the device. Two RT barriers are designed to selectively extract midwave and longwave components of the spectral response. We also report the use of RT barriers on strain-optimized quantum dots-in-a-double-well (DDWELL) structures to achieve very low dark current levels with peak D * of 2.9 ×1010 cm· Hz1/2 /W for a longwave infrared detection. Ability to select a particular wavelength in the spectral response is demonstrated with DDWELL architectures as well.

The background carrier concentration is an important factor in the performance of detectors. We have investigated the effect of a variable background doping concentration in the absorber region of an nBn type-II InAs/GaSb superlattice infrared detector. The doping concentrations were 5 × 1016 cm-3 (non-intentionally doped, n.i.d.), 1.4 × 1017 cm-3, 4.0 × 1017 cm-3, and 3.5 × 1018 cm-3. The specific detectivity of the detectors was found to dramatically decrease as the doping concentration was increased. This was caused by an increase in dark current density which is attributed to the presence of a quantum well (QW) formed in the valence band. This QW allowed for enhanced electron-hole recombination at higher doping concentrations. Consequently the n.i.d. device demonstrated the best performance with a zero bias specific detectivity (D*) equal to 1.2 × 1011 Jones at 77 K. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum dot infrared photodetectors (QDIPs) have made significant progress after their early demonstration about a decade ago. We review the progress made by QDIP technology over the last few years and compare QDIPs with quantum well infrared photodetectors (QWIPs). It is shown that the performance of QDIPs has significantly improved using novel architectures such as dots-in-a-well designs, and large-format (1 K × 1 K) focal plane arrays have been realized. However, even though there are significant reports of performance parameters better than QWIPs from single-pixel devices, QDIP-based focal plane arrays are still a factor of 3–5 worse in terms of noise equivalent temperature difference. We discuss the reasons for the performance gap and the key scientific and technological challenges that need to be addressed to achieve the full potential of QD-based technology.

Infrared detectors based on quantum wells and quantum dots have attracted a lot of attention in the past few years. Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays, which are based on InAs quantum dots embedded in an InGaAs well having GaAs barriers. This focal plane array has successfully generated a two-color imagery in the mid-wave infrared (i.e. 3–5 μm) and the long-wave infrared (i.e. 8–12 μm) at a fixed bias voltage. Recently, the DWELL device has been further modified by embedding InAs quantum dots in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. This is expected to improve the operating temperature while maintaining a low dark current level. This paper examines 320 × 256 double DWELL based focal plane arrays that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit using Indium-bump (flip-chip) technology. The spectral tunability is quantified by examining images and determining the transmittance ratio (equivalent to the photocurrent ratio) between mid-wave and long-way infrared filter targets. Calculations were performed for a bias range from 0.3 to 1.0 V. The results demonstrate that the mid-wave transmittance dominates at these low bias voltages, and the transmittance ratio continuously varies over different applied biases. Additionally, radiometric characterization, including array uniformity and measured noise equivalent temperature difference for the double DWELL devices is computed and compared to the same results from the original first generation DWELL. Finally, higher temperature operation is explored. Overall, the double DWELL devices had lower noise equivalent temperature difference and higher uniformity, and worked at higher temperature (70 K and 80 K) than the first generation DWELL device.

The carrier trapping and emission processes of InAs self-assembled quantum dots (QDs) on GaAs substrates were measured and analyzed using capacitance–voltage techniques and deep-level transient spectroscopy (DLTS). We used different applied biases and filling pulse widths. This allowed the determination of the activation energies of defect/electronic states of the QDs within a range of 0.08–0.59 eV. These values represent the energy levels of the QDs with respect to the host matrix, showing that QDs have band-like interacting energy levels and that DLTS signals are largely affected by the electron density of states of QDs.

The quantum-confined Stark effect in intersublevel transitions present in quantum-dots-in-a-well (DWELL) detectors gives rise to a midIR spectral response that is dependent upon the detector's operational bias. The spectral responses resulting from different biases exhibit spectral shifts, albeit with significant spectral overlap. A postprocessing algorithm was developed by Sakoglu that exploited this bias-dependent spectral diversity to predict the continuous and arbitrary tunability of the DWELL detector within certain limits. This paper focuses on the experimental demonstration of the DWELL-based spectral tuning algorithm. It is shown experimentally that it is possible to reconstruct the spectral content of a target electronically without using any dispersive optical elements for tuning, thereby demonstrating a DWELL-based algorithmic spectrometer. The effects of dark current, detector temperature, and bias selection on the tuning capability are also investigated experimentally.

Energy levels of InAs/GaAs self-assembled quantum dot (QD) system were analyzed by capacitance–voltage (C –V) and deep level transient spectroscopy (DLTS) methods. The QD signals were partially separated by DLTS measurement with small bias changing. The activation energies of QD signals were varied from 66 meV to 610 meV by changing of 0.6 V applied bias, which energies are related to the confined energy levels of InAs QDs. Then, the ground states of InAs QDs were considered to be located at 0.61 eV below the conduction band edge of GaAs barrier. In addition, it showed that DLTS signal of QDs are largely affected by their density of energy state. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

150 주기의 InAs/GaSb (8/8-ML) 제2형 응력초격자 (SLS)를 활성층에 탑재한 초격자 적외선검출소자 (SLIP) 구조를 MBE 방법으로 성장하고, 직경 200 ㎛의 개구면을 가지는 SLIP 개별소자를 시험 제작하였다. 고분해능 투과전자현미경 (TEM) 이미지의 휘도분포와 X선회절 (XRD) 곡선의 위성피크의 분석 결과는 SLS 활성층은 균일한 층두께와 주기적 응력변형을 유지하는 급격한 계면의 초격자임을 입증하였다. 흑체복사 적외선 광원을 이용하여 측정한 입사파장 및 인가전압에 따른 반응도 (R)와 검출률 (D*)로부터, 차단파장은 ~5 ㎛이고 최대 R과 D* (λ=3.25 ㎛)는 각각 ~10³ ㎃/W (-0.6 V/13 K)와 ~10? ㎝.㎐ 1/2 /W (0 V/13 K)임을 보였다. 반응도의 온도의존성으로부터 분석한 활성화에너지 275 meV는 광반응 과정에 개입되어 있는 가전대 및 전도대 부준위 사이의 에너지 간격 (HH1-C)과 잘 일치하였다.

Sb에 기초한 응력 초격자 적외선검출소자의 구성 물질인 도핑하지 않은 기판 GaSb 결정과 GaSb/SI-GaAs 박막에 잔존하고 있는 진성결함 (intrinsic defect)을 비교 조사하였다. 상온 근처 (250 K)까지 광여기 발광 (PL)을 보이는 GaSb 결정에서의 발광 에너지의 온도의존성으로부터, 밴드갭 에너지에 관한 경험식인 Varshni 함수의 파라미터 (Eo, α, β)를 결정하였다. GaAs 기판 위에 성장된 이종 GaSb 박막에서는 GaSb의 주요 진성결함으로 알려져 있는 29 meV의 이온화 에너지를 가지는 위치반전 (antisite) Ga ([Ga Sb ]) 결함과 함께 위치반전 Sb ([Sb Ga ])와의 복합결함 ([Ga Sb -Sb Ga ])과 관련된 것으로 분석된 732/711 meV의 한 쌍의 깊은준위 (deep level)가 관측되었다. PL의 온도 및 여기출력 의존성을 분석하여, Sb-rich 상태에서 성장된 GaSb 박막에서는 잉여 Sb의 자발확산 (self-diffusion)에 의하여 치환된 위치전도 [Ga Sb ] 및 [Sb Ga ]가 결합하여 [Ga Sb -Sb Ga ]의 깊은준위를 형성하는 것으로 해석되었다.

The effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum dots (QDs) grown by molecular-beam epitaxy were studied using transmission electron microscopy. To investigate QD structure depending on spacer layer growth, first uncapped free-standing QDs were grown and their structure compared with that of multilayer stacked QDs. In addition, vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The uncapped QDs were found to form a lens-shaped structure with side facets. Upon capping with a GaAs spacer, the apex of nonaligned QDs flattened by In diffusion. However, the 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 results demonstrate the possibility of controlling QD structure by adjusting the thickness of spacer layers.

The frequency-dependent photoacoustic (PA) signals and phase spectra of GaAs samples with different doping densities are measured and analyzed. In particular, we find that the magnitude differences of the PA signal and phase between intrinsic GaAs and doped GaAs materials decrease linearly with the doping density in the frequency region of nonradiative bulk recombination. The results show that the PA spectroscopic technique can be used to measure doping density in bulk semiconductors with reasonable accuracy.