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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.

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.

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 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.

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 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 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.

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)

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 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.

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.

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.

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 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.

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.

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.

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 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.

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 detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

GaAs and InP based systems of InAs self-assembled quantum dots (QDs) have been widely applied to optical devices such as a laser diode and a semiconductor optical amplifier due to their discrete energy levels. Energy states of QDs and their densities are very important to apply QDs to optical and electrical devices and to control energy state of QDs. Despite a discrete energy spectrum was theoretically predicted, most measurements such as photo-luminescence (PL) and deep-level transient spectroscopy (DLTS) have shown the existence of only few levels [1]. In this study, we demonstrate energy level properties of InAs/GaAs self-assembled QDs by using DLTS with selective carrier filling and extracting which can be achieved by very small pulse amplitude and varying the biases. Moreover, changing of the energy levels (i.e. number of levels, location of level, and carrier trapping capacity) that is originated by variation of QD sizes in the InAs/GaAs QDs is presented also.

Reduction in the dark current and wavelength selective extraction of the photocarriers in a quantum dots in a well structure using resonant tunneling barriers has been demonstrated. The passband of the two resonant tunneling barriers has been designed to extract longwave and midwave peaks respectively. Substantial reduction in the dark currents and increase in the detectivity has been obtained.

We report on the performance of multi-stack quantum dots in a well (DWELL) detectors. Present-day QD detectors are limited by low responsivity and quantum efficiency (QE). This can be attributed to the low absorption efficiency of these structures due to the small number of QD stacks in the detector. In this paper we examine the effect of the number of stacks on the performance of the detector. In particular, we investigate the InAs/GaAs/AlGaAs D-DWELL (Dots-in-double-well) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling the growth of many more stacks in the detector. The purpose of the study detailed in this paper is to examine the effects of varying the number of stacks in the InAs/InGaAs/GaAs/AlGaAs D-DWELL detector, on its device performance. The numbers of stacks grown using solid source molecular beam epitaxy (MBE), were 15, 30, 40, 50, and 60. Once fabricated as single pixel devices, we carried-out a series of device measurements such as spectral response, dark current, total current, responsivity along with computing the photoconductive gain and the activation energies. The goal of these experiments is to not only study the single pixel detector performance with varying number of stacks in a D-DWELL structure, but to also understand the effect of the transport mechanism in these devices.

We demonstrate the 1.35‐μm emission at room temperature from wire‐like anisotropic InGaAs quantum dots (QDs) grown on GaAs (001) substrate by atomic layer epitaxy technique. The QD formation has been carried out in ranges of In composition of x=0.23–0.5 and equivalent thickness of teq=2.5–25 monolayers (MLs). As InGaAs coverage and growth temperature increase, the QD distribution gradually evolves from isolated dots to wire‐like structure aligned to a preferential direction of [1–10]. The photoluminescence emission of 1.35 μm and the full width half‐maximum (FWHM) of 25 meV have been achieved from InGaAs QD with x=0.5 and teq=7.2 MLs.

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.

We have investigated optical properties and device performance of sub-monolayer quantum dots infrared photodetector with confinement enhancing (CE) barrier and compared with conventional Stranski-Krastanov quantum dots with a similar design. This quantum dots-in-a-well structure with CE barrier enables higher quantum confinement and increased absorption efficiency due to stronger overlap of wavefunctions between the ground state and the excited state. Normal incidence photoresponse peak is obtained at 7.5 μm with a detectivity of 1.2 × 1011 cm Hz1/2 W−1 and responsivity of 0.5 A/W (77 K, 0.4 V, f/2 optics). Using photoluminescence and spectral response measurements, the bandstructure of the samples were deduced semi-empirically.

From the transmission electron microscope image, the seven-stacked InAs/InAlGaAs QDs on an InP substrate were spatially ordered instead of usual on-top vertical alignment. The increasing rate of the QD size became almost saturated by increasing the number of layers. The photoluminescence (PL) intensity for the seven-stacked InAs/InAlGaAs QDs was decreased up to 60K and remained almost stable at the temperature range from 60 to 220K. And then, the intensity was again drastically decreased with further increasing temperature. The emission peak was first red-shifted at the ratio of 0.446 meV/K from 20 to 60K. However, the degree of the red-shift in the emission peak from 60 to 220K was decreased at the negligibly small ratio of 0.028 meV/K. Above 220K, the emission peak was again significantly decreased at the ratio of 0.323 meV/K. While increasing the temperature, the carrier lifetimes obtained from the PL decay profiles for the seven-stacked QDs initially enhanced and then, almost stable at a certain tempe...

We report a systematic study of different transitions in quantum dots-in-a-well infrared photodetectors in order to optimize the signal to noise ratio of the detector. Bound to continuum transitions offer very high extraction probability for photoexcited electrons but poor absorption coefficient, while the bound to bound transitions have higher absorption but poorer extraction probability. Bound to quasibound transitions are optimum for intermediate values of electric fields with superior signal to noise ratio. The bound to quasibound device has the detectivity of 4×1011 cm Hz1/2 W−1 (3V, f/2 system) at 77 K and 7.4×108 cm Hz1/2 W−1 at 200 K, which is highest reported detectivity at 200 K for detector with long wave cutoff wavelength.

The authors report the InAs/InGaAs/GaAs/AlGaAs quantum dots-in-double-well (D-DWELL) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling the growth of many more stacks in the detector. The purpose of this study is to examine the effects of varying the number of stacks in the double DWELL detector on its device performance. The structures are grown by solid source molecular beam epitaxy on GaAs substrates. After fabrication of single pixel devices, a series of device measurements such as spectral response, dark current, total current, and responsivity were undertaken and the photoconductive gain and the activation energies were extracted. The goal of these experiments is not only to optimize the device performance by optimizing the number of stacks but also to investigate the transport properties as a function of the number of stacks.