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Single photon avalanche diodes (SPADs) are key enabling technologies for a wide range of applications in the near-infrared wavelength range. Recently, AlAs0.56 Sb0.44 (hereafter AlAsSb) lattice-matched to InP has been demonstrated for extremely low excess noise avalanche photodiodes (APDs) due to its large disparity between electron and hole ionization coefficients ( $\alpha $ and $\beta $ respectively). The $\alpha /\beta $ ratio also plays a role in Geiger mode operation as it affects the avalanche breakdown probability and hence detection efficiency. In this work, we theoretically investigate the performance of AlAsSb based SPADs. The probability of breakdown for electron-initiated Geiger mode operation increases more sharply with multiplication region width due to progressively more dissimilar ionization coefficients. In comparison with other common avalanche materials, such as InAlAs, InP and Si, our result also suggests that SPADs based on AlAsSb have a sharper breakdown probability than the other three materials under similar low overbias ratio. The calculated breakdown probability of 0.81 in AlAsSb is 0.18 and 0.28 higher than that of InAlAs/Si and InP respectively at 5% overbias ratio and with avalanche region width of 1500 nm.

Fast, sensitive avalanche photodiodes (APDs) are required for applications such as high-speed data communications and light detection and ranging (LIDAR) systems. Unfortunately, the InP and InAlAs used as the gain material in these APDs have similar electron and hole impact ionization coefficients (α and β, respectively) at high electric fields, giving rise to relatively high excess noise and limiting their sensitivity and gain bandwidth product1. Here, we report extremely low excess noise in an AlAs0.56Sb0.44 lattice matched to InP. A deduced β/α ratio as low as 0.005 with an avalanche region of 1,550 nm is close to the theoretical minimum and is significantly smaller than that of silicon, with modelling suggesting that vertically illuminated APDs with a sensitivity of −25.7 dBm at a bit error rate of 1 × 10−12 at 25 Gb s−1 and 1,550 nm can be realized. These findings could yield a new breed of high-performance receivers for applications in networking and sensing.

We propose and experimentally demonstrate a high-efficiency single-pixel imaging (SPI) scheme by integrating time-correlated single-photon counting (TCSPC) with time-division multiplexing to acquire full-color images at extremely low light level. This SPI scheme uses a digital micromirror device to modulate a sequence of laser pulses with preset delays to achieve three-color structured illumination, then employs a photomultiplier tube into the TCSPC module to achieve photon-counting detection. By exploiting the time-resolved capabilities of TCSPC, we demodulate the spectrum-image-encoded signals, and then reconstruct high-quality full-color images in a single-round of measurement. Based on this scheme, the strategies such as single-step measurement, high-speed projection, and undersampling can further improve the imaging efficiency., Comment: 5 pages, 4 figures

This paper presents the electron and hole avalanche multiplication and excess noise characteristics based on bulk AlAs0.56Sb0.44 p-i-n and n-i-p homojunction diodes lattice matched to InP, with nominal avalanche region thicknesses of 0.6 -1.5 μm. From these, the bulk electron and hole impact ionization coefficients (α and β respectively), have been determined over an electric field range of 220-1250 kV/cm for α and from 360-1250 kV/cm for β for the first time. Excess noise characteristics suggest an β/α ratio as low as 0.005 for an avalanche region of 1.5 μm in this material, close to the theoretical minimum and significantly lower than AlInAs, InP, or even silicon. This material can be easily integrated with InGaAs for networking and sensing applications, with modeling suggesting that a sensitivity of -32.1 dBm at a bit-error rate (BER) of 1×10-12 at 10 Gb/s at 1550 nm can be realized. This sensitivity can be improved even further by optimizing the dark currents and by using a lower noise transimpedance amplifier.

Topological insulators, such as Bi2Te3, have been confirmed to exhibit plasmon radiation over the entire visible spectral range. Herein, we fabricate bullseye nanoemitters, consisting of a central disk and concentric gratings, on the Bi2Te3 nanoflake. Due to the existence of edge plasmon modes, Bi2Te3 bullseye nanostructures are possible to converge light towards the central disk. Taking advantage of the excellent spatial resolution of cathodoluminescence (CL) characterization, it has been observed that plasmonic behaviors depend on the excitation location. A stronger plasmonic intensity and a wider CL spectral linewidth can be obtained at the edge of the central disk. In order to further improve the focusing ability, a cylindrical Pt nanostructure has been deposited on the central disk. Additionally, the finite element simulation indicates that the electric-field enhancement originates from the coupling process between the plasmonic emission from the Bi2Te3 bullseye and the Pt nanostructure. Finally, we find that enhancement efficiency depends on the thickness of the Pt nanostructure.

AlAs0.56Sb0.44 is a promising avalanche material which can be grown lattice-matched to InP and therefore use InGaAs as the absorption region in a Separate Absorption and Multiplication APD (SAM-APD). The electron and hole ionisation coefficients in this material are very dissimilar and our experiments show that this leads to AlAs0.56Sb0.44 having the lowest excess noise performance of any InP based material system (F = 2.2 at M = 40) reported to date. Simulations suggest that operation at 1550 nm and 25 GB s-1 with a sensitivity of -25.7 dBm is possible in a normal incidence SAM-APD.

Driven by tensile strain, GaAs quantum dots (QDs) self-assemble on In0.52Al0.48As(111)A surfaces lattice-matched to InP substrates. In this study, we show that the tensile-strained self-assembly process for these GaAs(111)A QDs unexpectedly deviates from the well-known Stranski-Krastanov (SK) growth mode. Traditionally, QDs formed via the SK growth mode form on top of a flat wetting layer (WL) whose thickness is fixed. The inability to tune WL thickness has inhibited researchers’ attempts to fully control QD-WL interactions in these hybrid 0D-2D quantum systems. In contrast, using microscopy, spectroscopy, and computational modeling, we demonstrate that for GaAs(111)A QDs, we can continually increase WL thickness with increasing GaAs deposition, even after the tensile-strained QDs (TSQDs) have begun to form. This anomalous SK behavior enables simultaneous tuning of both TSQD size and WL thickness. No such departure from the canonical SK growth regime has been reported previously. As such, we can now modify QD-WL interactions, with future benefits that include more precise control of TSQD band structure for infrared optoelectronics and quantum optics applications.

Tin selenide (SnSe) has recently attracted great attention as a new promising thermoelectric material. In this paper, a-axis inclined oriented SnSe thin films have been fabricated by pulsed laser deposition for the development of the transverse thermoelectric effect. Large open-circuit voltage signals along the film plane are all clearly observed when the film surface is illuminated by different lasers with the wavelength ranging from 532 to 2200 nm with no external bias or a built-in electric field, and the amplitude of the output voltage can be enhanced by tuning the inclination angle of the films or the applied temperature difference across the film thickness. Moreover, transverse open-circuit voltage signals are also detected when the film surface is irradiated by a thermal heater. This work demonstrates the novel potential application of SnSe for self-powered broad-band light detectors or thermal sensors.

Time-resolved photoluminescence (TRPL) has been implemented experimentally to measure the carrier lifetime of semiconductors for decades. For the characterization of nanowires, the rich information embedded in TRPL curves has not been fully interpreted and meaningfully mapped to the respective material properties. This is because their three-dimensional (3-D) geometries result in more complicated mechanisms of carrier recombination than those in thin films and analytical solutions cannot be found for those nanostructures. In this work, we extend the intrinsic power of TRPL by developing a full 3-D transient model, which accounts for different material properties and drift-diffusion, to simulate TRPL curves for nanowires. To show the capability of the model, we perform TRPL measurements on a set of GaAs nanowire arrays grown on silicon substrates and then fit the measured data by tuning various material properties, including carrier mobility, Shockley-Read-Hall recombination lifetime, and surface recombination velocity at the GaAs-Si heterointerface. From the resultant TRPL simulations, we numerically identify the lifetime characteristics of those material properties. In addition, we computationally map the spatial and temporal electron distributions in nanowire segments and reveal the underlying carrier dynamics. We believe this study provides a theoretical foundation for interpretation of TRPL measurements to unveil the complex carrier recombination mechanisms in 3-D nanostructured materials.

Recently, vertically oriented few-layer 2D materials prepared by chemical vapor deposition (CVD) have attracted much attention due to their attractive properties. However, the integration of these materials as heterojunctions in optoelectronic sensors has been rarely reported. In this paper, large-area, high crystalline quality, and vertically oriented few-layered MoS2 (V-MoS2) nanosheets were synthesized and transferred successfully onto a silicon substrate to form a V-MoS2/Si heterojunction photodetector. The photodetector exhibits high photoelectric performances in a wide broadband ranging from visible to near-infrared with a photoresponsivity of up to 908.2 mA W−1, and a detectivity of up to 1.889 × 1013 Jones. More importantly, an unprecedented response speed of (rise time ∼ 56 ns, fall time ∼ 825 ns) was found in the photodetector by time response measurements, which is the best result achieved so far in any other 2D-based photodetectors or phototransistors. These excellent performances can be ascribed to the strong light absorption and the quick longitudinal intralayer carrier transport speed of the V-MoS2 nanosheets as well as the good heterojunction formed between V-MoS2 and Si. This intriguing vertical oriented structure in combination with both ultrafast time response and ultrahigh detectivity in the V-MoS2/Si heterojunction provide great potential for application in optoelectronic devices.

This research presents a coherent structured illumination single-pixel imaging scheme to image objects with complex amplitudes. By utilizing a phase-only spatial light modulator for phase modulation, we can efficiently generate the Hadamard basis structured light and the reference light that interfere with each other to form the coherent structured illumination. Using the 4-step phase-shifting, the spectrum of the object is acquired by detecting the zero-frequency component of the object light with a single-pixel photodetector. The desired complex-amplitude image can be further retrieved by applying an inverse Hadamard transform. The proposed scheme is experimentally demonstrated by imaging two etched glass objects, a dragonfly wing, and a resolution test chart. Benefiting from the phase modulation, this scheme has a high efficiency, a high imaging quality, a high spatial resolution, and a simple and stable configuration to obtain both the phase and amplitude information of the target object. The proposed scheme provides a promising complex-amplitude imaging modality with single-pixel detection. Thus it might find broad applications in optical metrology and biomedical science.

We have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In 1−xGa xAs quantum dot (QD) nanostructures on (111)A surfaces. During molecular beam epitaxy, In 1−xGa xAs islands form spontaneously on InAs(111)A when the Ga content x≥50%. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In 1−xGa xAs coverage, In 1−xGa xAs composition, and substrate temperature. We calculated the conduction and valence band energy values for these QDs in an InAs matrix. This work supports the efforts to establish InAs(111)A as a platform for future incorporation with other (111)-oriented materials from the 6.1 A family of semiconductors.

Digital alloy Al0.85Ga0.15As0.56Sb0.44 and random alloy Al0.79In0.21As0.74Sb0.26 avalanche photodiodes with a 1-μm multiplication layer exhibit low excess noise under 543-nm laser illumination comparable to Si avalanche photodiodes. This has motivated a study of the optical characteristics of these materials. The absorption coefficients and complex refractive indices were extracted via variable-angle spectroscopic ellipsometry. Features of three semiconductor layer fitting approaches are compared, and a Kramers–Kronig-consistent basis spline function was chosen due to its flexibility and accuracy of approximating optical constants of quaternary materials. The external quantum efficiency has been calculated based on the extracted absorption coefficients and is shown to agree well with the measured external quantum efficiency.

Ternary I-III-VI (such as CuInSe2 and AgInS2 ) quantum dots (QDs) have been increasingly studied in the field of photoelectric conversion applications. Here, we prepare a relatively less toxic Cu–In–Sn–Se (CISSe) QD by an organic high-temperature hot injection method, and this material is subsequently applied as a functional sensitizer to produce QD-sensitized solar cells (QDSSCs). Because of the effect of Sn doping and ZnS passivation , the TiO 2/CISSe/ZnS photoanode obtains the greatest composite resistance (985.5 Ω·cm2) and highest electron lifetime (120.23 ms) among the experimentally compared materials, showing the further inhibition of charge recombination when compared with bare Cu–In–Se solar cells while effectively enhancing the collection efficiency of photogenerated electrons. Using CISSe/ZnS QDs as a sensitizer, the power conversion efficiency of the QDSSC reaches 6.7%, of which Voc, Jsc, and FF reach 0.559 V, 22.93 mA/cm2, and 0.52, respectively. It provides a new possibility for the development of QDSSCs.

Optical response of the indium arsenide (InAs)/gallium arsenide (GaAs) quantum dots (QDs) with a bimodal distribution is investigated through varying an initial GaAs capping layer between 35 and 18 monolayers. Photoluminescence (PL) measurements confirm that a thinner initial capping layer can reduce the QD dimensions and also modify the population ratio between the large QDs and the small QDs. Therefore, PL quenching related to QD dimension and carrier transfer between the bimodal QD populations has been affected. Manipulation of the initial GaAs capping layer provides a feasible approach to tailor the formation and optical performance of InAs QDs for optoelectronic device applications.

By combining the photorefractive spatial soliton waveguide of a Ce:SBN crystal with a coherent 4-f system we are able to manipulate the spatial frequencies of an input optical image to perform edge-enhancement and direct component enhancement operations. Theoretical analysis of this optical image processor is presented to interpret the experimental observations. This work provides an approach for optical image processing by using photorefractive spatial solitons.

An antimony sulfide (Sb2S3) semiconductor is appealing as a promising light absorber due to its suitable bandgap (1.5–1.7 eV), ‘one dimensional’ crystal structure and non-toxic constituents. In this work, the orientation preferences of the Sb2S3 thin films grown on the CdS layer and bare glass substrates were compared by SEM and XRD measurements. The growth direction of (Sb4S6)n ribbons was found to be substrate-dependent, and the growth mechanism was discussed. Thin film solar cells in the configuration of glass/(SnO2:F) FTO/CdS/Sb2S3/Au were fabricated and the highest conversion efficiency reached 3.01%. The defects and recombination losses in the thin film solar cells were investigated by voltage-dependent quantum efficiency and time-resolved photoluminescence measurements. The CdS/Sb2S3 (n–p) heterojunction solar cells showed a little decrease by about 0.2% in conversion efficiency to 2.84% after three month storage in ambient air without encapsulation.

In this paper, the flexibility of long-wavelength Type-II InAs/GaSb superlattice (Ga-containing SL) is explored and investigated from the growth to the device performance. First, several samples with different SL period composition and thickness are grown by molecular beam epitaxy. Nearly strain-compensated SLs on GaSb exhibiting an energy band gap between 105 to 169 meV at 77K are obtained. Second, from electronic band structure calculation, material parameters are extracted and compared for the different grown SLs. Finally, two p-i-n device structures with different SL periods are grown and their electrical performance compared. Our investigation shows that an alternative SL design could potentially be used to improve the device performance of diffusion-limited devices for long-wavelength infrared detection., Comment: 8 Figures

This work investigates midwave infrared Type-II InAs/GaSb superlattice (SL) grown by molecular beam epitaxy on GaSb substrate. In order to compensate the natural tensile strain of the InAs layers, two different shutter sequences have been explored during the growth. The first one consists of growing an intentional InSb layer at both interfaces (namely GaSb-on-InAs and InAs-on-GaSb interfaces) by migration enhanced epitaxy while the second uses the antimony-for-arsenic exchange to promote an InSb-like interface at the GaSb-on-InAs interface. SLs obtained via both methods are compared in terms of structural, morphological and optical properties by means of high-resolution x-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. By using the second method, we obtained a nearly strain-compensated SL on GaSb with a full width at half maximum of 56 arcsec for the first-order SL satellite peak. Relatively smooth surface has been achieved with a root mean square value of about 0.19 nm on a 2 $\mu m$ x 2 $\mu m$ scan area. Finally, a p-i-n device structure having a cut-off wavelength of 5.15 $\mu m$ at 77K has been demonstrated with a dark-current level of $8.3 * 10^{-8} A/cm^2$ at -50 mV and a residual carrier concentration of $9.7 * 10^{14} cm^{-3}$, comparable to the state-of-the-art., Comment: 8 figures

The GaSb quantum dots (QDs) with type II band alignment have attracted great attention recently. They are predicted to be optimizing active region materials for achieving high efficient intermediate-band solar cells and for obtaining ultra-long storage time for memory cells. In this research, GaSb QDs sandwiched inside InAlAs matrix lattice-matched to InP (001) substrate have been obtained via droplet epitaxy. The droplet epitaxy enable us to achieve low density (~2.6 x 10^9/cm^2) and large size (average height ~6.5nm) for the QDs while the lattice mismatch between the GaSb and InAlAs matrix is only ~4%. PL measurements reveal a type-II band alignment for these GaSb/InAlAs/InP QDs. The PL peak energy of QDs shows a blue-shift of >100 meV when the laser intensity increases by six orders of magnitude. Time-resolved PL measurements further confirm the type-II band alignment for the QDs by showing a maximum carrier lifetime of ~4.5 ns. The abnormal dependence of peak energy of QD PL band on the temperature in together with the special PL decay curve indicate that these GaSb/InAlAs QDs likely have different physics mechanism from common GaSb/GaAs type-II QDs. This study provide useful information for understanding the band structure and carrier dynamics of the GaSb/InAlAs QDs grown on InP surface.

In this work, the electronic band structure of the InAs/GaSb superlattice (SL) is calculated using a commercial 8-band k⸳p solver and the electrical performance of longwave nBp device structure evaluated with Atlas from Silvaco software. By taking into account an InSb interface layer and the interface matrix (formulated by P.C. Klisptein), the model can predict the measured energy band gap of different InAs/GaSb SLs having different period composition and thickness (7/4, 10/4, 12/4, 14/4 and 14/7 SLs) within an error corresponding to the ±

Regarding a phase-only Fourier hologram, coherent illumination is required in its optical reconstruction. However, this requirement faces a challenge in lensless Fourier-transform correlated imaging with thermal sources. In this research, a correlated reconstruction scheme for the phase-only Fourier hologram with spatially incoherent illumination by utilizing intensity fluctuation correlation measurements is proposed. Numerical simulation and optical experiments were conducted to verify the feasibility of the correlated reconstruction scheme. Because the proposal requires neither a coherent light source nor a lens, holographic reconstruction can circumvent the limitation of the spatial coherence light source and the influence of lens aberrations.

The post deposition treatment of CsF (CsF-PDT) plays an important role in improving the performance of CIGS solar cells. However, the doping mechanism of Cs-PDT is controversial. To further clarify it, the material properties of the CIGS thin film and photovoltaic characteristics of the devices with different amount of CsF doping are investigated in this paper. It can be demonstrated that the surface morphology, chemical properties and electrical structure of the absorber after CsF-PDT are similar to those of samples after KF-PDT and RbF-PDT. Especially, analogous wide band gap compound CsInSe2 is formed at the surface of the absorber, which can significantly reduce the interfacial recombination between the buffer layer and absorber. However, the measurement results indicate that the distribution and role of Cs in the absorber are somewhat different from those previously reported. Firstly, the entry of Cs does not promote the reduction of Na, but makes more Na enter the absorber. Secondly, the Cs atoms entering the film not only locate at the grain boundary but also enter the grains. Therefore, they can passivate the defects both at the grain boundary and grain interior, improving the hole carrier concentration and minority carrier lifetime. Finally, when excess Cs atoms enter the grain, the lattice constants and texture of the film transform significantly. In this case, the new defects (40 meV and 420 meV) will be generated in the film, which deteriorates the device performance.

The two-dimensional transition metal dichalcogenides (TMDCs) have been considered as promising candidates for developing a new generation of optoelectronic devices. Accordingly, investigations of exciton dynamics are of great importance for understanding the physics and the performance of devices based on TMDCs. Herein, after exposure to ambient environment for six months, monolayer tungsten disulfide (WS2) shows formation of localized states. Photoluminescence (PL) and time-resolved PL (TRPL) spectra demonstrate that these localized states have significant impacts on the exciton dynamics, including energy states filling, thermal activation and redistribution, and the decay behavior of excitons. These observations not only enrich the understanding for localized states and correlated exciton dynamics of aged monolayer WS2, but also reveal a possible approach to modulate the optical properties of TMDCs via the aging process.

With the development of nanoengineering and nanotechnology, numerous emerging materials and constructions are being presented in optoelectronics to challenge traditional photoelectric effects and detecting techniques, and are inspiring innovation and growth in photoelectric research fields. Recently, due to its unique working mechanism, the lateral photovoltaic effect (LPE) has been identified as an indispensable and effective method of studying the properties of novel materials and also shows promising application in position sensors and photodetection. This article will present a comprehensive review of the recent progress on the LPE in novel materials, including metal nanomaterials, oxide semiconductors, organic semiconductors, two-dimensional layered materials, and perovskites. Moreover, it is demonstrated that the LPE is also very sensitive to different constructions or external modulations and can be substantially tuned by changing the structure or morphology of materials, adding external fields, and utilizing plasmon resonance. This summary may provide a comprehensive picture regarding the recent achievements of the LPE in novel materials and external modulations. Finally, we also present a discussion of the challenges and potential trends of the LPE in the near future.

The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices.

A recently developed growth technique enables the self-assembly of defect-free quantum dots on (111) surfaces under large tensile strains. We demonstrate the use of this approach to synthesize germanium (Ge) quantum dots on In0.52Al0.48As(111)A with >3% residual tensile strain. We show that the size and areal density of the tensile-strained Ge quantum dots are readily tunable with growth conditions. We also present evidence for an unusual transition in the quantum dot growth mode from Stranski-Krastanov to Volmer-Weber as we adjust the substrate temperature. This work positions Ge quantum dots as a promising starting point for exploring the effects of tensile strain on Ge’s band structure.

In(Ga)As surface quantum dots (SQDs) are promising candidates for surface-sensitive detection applications. As such, studies of the carrier dynamics are indispensable when attempting to understand both the physics and the device performances based on such SQD structures. Here, lateral carrier transfer among neighboring quantum dots (QDs) has been investigated through spectroscopic measurements for high density InGaAs/GaAs SQDs in a hybrid nanostructure with similar buried QDs (BQDs). Photoluminescence (PL) and temporal-resolved PL (TRPL) spectra demonstrate that the SQDs exhibit intra-layer exciton transfer and also strong carrier transfer to the surface states at 10 K. This imparts different optical characteristics to the SQDs with regards to the band filling and emission-energy-dependent carrier lifetime. In addition, PL excitation (PLE) spectra reveal a wetting layer absorption feature for the SQDs, which provides a channel for thermally activated carriers to transfer among the SQDs and to ultimately escape from the SQD layer. These investigations enrich our understanding of carrier dynamics for InGaAs SQDs, which may help to develop novel surface sensitive sensors.

We investigate the optical properties of InGaAs surface quantum dots (SQDs) in a composite nanostructure with a layer of similarly grown buried quantum dots (BQDs) separated by a thick GaAs spacer, but with varied areal densities of SQDs controlled by using different growth temperatures. Such SQDs behave differently from the BQDs, depending on the surface morphology. Dedicated photoluminescence (PL) measurements for the SQDs grown at 505 °C reveal that the SQD emission follows different relaxation channels while exhibiting abnormal thermal quenching. The PL intensity ratio between the SQDs and BQDs demonstrates interplay between excitation intensity and temperature. These observations suggest a strong dependence on the surface for carrier dynamics of the SQDs, depending on the temperature and excitation intensity.

InAs quantum dots (QDs) sandwiched inside a GaAsSb matrix possess advantages for achieving telecom wavelength lasers and for developing high efficiency solar cells. In this work, optical properties of InAs quantum dots (QDs) capped by GaAs1-xSbx (x = 0.11 and 0.15) are comparatively investigated. The photoluminescence measurements reflect that the energy state filling, thermal activation, quenching, and lifetime of the carriers in InAs/GaAs0.89Sb0.11 QDs are different from those in the InAs/GaAs0.85Sb0.15 QDs. These differences are attributed to the band alignment transition from type-I to type-II resulting from the Sb-composition change from x = 0.11 to x = 0.15 in the GaAs1-xSbx capping layer. Therefore, the emission and quenching involve excited states for type-I InAs/GaAs0.85Sb0.15 QDs, but involve InAs QDs as well as the GaAs0.85Sb0.15 QW recombination for type-II InAs/GaAs0.85Sb0.15 QDs. So the luminescence reveals complex and distinct physics mechanisms for these two samples.

As part of a study on radiation effects in optoelectronic materials, we exposed a series of AIGaAs/GaAs double heterostructures grown by molecular beam epitaxy and metalorganic chemical vapor deposition to electron and proton radiation. The active regions of the test articles were either p-, n-, or unintentionally doped. Steady state and time resolved photoluminescence spectroscopy were used to characterize radiation induced changes to the band structure and carrier dynamics. The effects of electron radiation on low temperature photoluminescence spectra and room temperature carrier dynamics varied with dopant type and density. Steady-state photoluminescence reveals distinct proton exposure effects for p-type materials grown by molecular beam epitaxy compared to n-type structures. Both the steady state and time resolved results suggest that n-type materials are more radiation hard to the effects of 1 MeV electrons than p-type materials.

Recently, to sandwich InAs QDs with GaAs1-xSbx layers have attracted enormous attention. It is expected to achieve a transition from type-I to type-II band alignment at ~ x=12%. The type-II InAs/GaAs1-xSbx QDs are predicted to be one of the optimizing active region materials for achieving high efficient intermediate-band solar cells. We first investigate PL properties of InAs/GaAs1-xSbx QD structures of different Sb compositions (x=0, 0.15, and 0.25) in the GaAs1-xSbx capping layer. By capping the InAs QDs with GaAs0.85Sb0.15 and GaAs0.75Sb0.25 layer, we are able to gain type II QDs. These type II QDs exhibit a clear multi-peaks characteristic under increasing laser excitation intensity, which stems from different carriers recombination routes due to the combination of InAs QDs with GaAs1-xSbx capping layer. Time-resolved PL measurements further confirm our assignment for the mnulti-peaks in the PL spectra. We then study carrier coupling inside vertically aligned InAs/GaAs and InAs/GaAs1-xSbx QD pairs. The features of InAs/GaAs1-xSbx QD pairs are very different from the traditional InAs/GaAs QD-pairs. Again, clear multi-peaks characteristic are observed under stronng laser excitation intensity, which stems from different carriers recombination routes from the top InAs/GaAs1-xSbx QDs. Our investigations indicate that the optical behavior and carrier dynamics in type-II InAs/GaAs1-xSbx QDs and QD-pairs are much more complicated than the InAs/GaAs QDs counterparts. This study provide useful information for understanding the band structure and carrier dynamics of the InAs/GaAs1-xSbx QDs for high efficiency solar cell applications.

The optical properties have been investigated for the GaAs/Al0.2Ga0.8As quantum dot-ring hybrid nanostructures grown by droplet epitaxy, in which each nanostructure consists of four quantum dots (QDs) sitting on a distinct ring of GaAs. A blueshift and narrowing of the photoluminescence (PL) spectra along with the nonlinear decay of the time-resolved PL curves of the QDs have been observed. These abnormal PL behaviors are caused by the unique state filling effect correlated with the quantum dot-ring structure feature, which is strongly affected by carrier transfer from smaller dots to larger dots via the wetting ring in the GaAs/Al0.2Ga0.8As hybrid structure.

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

We investigate stacked structures of InAs/AlAsSb/InP quantum dots using temperature- and power-dependent photoluminescence. The band gap of InAs/AlAsSb QDs is 0.73 eV at room temperature, which is close to the ideal case for intermediate band solar cells. As the number of quantum dot layers is increased, the photoluminescence undergoes a blue-shift due to the effects of accumulated compressive strain. This PL red shift can be counteracted using thin layers of AlAs to compensate the strain. We also derive thermal activation energies for this exotic quantum dot system.

III-(As, Sb) alloys are building blocks for various advanced optoelectronic devices, but the growth of their ternary or quaternary materials are commonly limited by spontaneous formation of clusters and phase separations during alloying. Recently, digital alloy growth by molecular beam epitaxy has been widely adopted in preference to conventional random alloy growth because of the extra degree of control offered by the ordered alloying. In this article, we provide a comparative study of the optical characteristics of AlAsSb alloys grown lattice-matched to GaSb using both techniques. The sample grown by digital alloy technique showed stronger photoluminescence intensity, narrower peak linewidth, and larger carrier activation energy than the random alloy technique, indicating an improved optical quality with lower density of non-radiative recombination centers. In addition, a relatively long carrier lifetime was observed from the digital alloy sample, consistent with the results obtained from the photoluminescence study.

We report structural properties as well as electrical and optical behaviors of beryllium (Be)-doped InGaAsP lattice-matched to InP grown by gas source molecular beam epitaxy. P type layers present a high degree of compensation on the order of 1018 cm−3, and for Be densities below 9.5×1017 cm−3, they are found to be n type. Enhanced incorporation of oxygen during Be doping is observed by secondary ion mass spectroscopy. Be in forms of interstitial donors or donor-like Be-O complexes for cell temperatures below 800°C is proposed to account for such anomalous compensation behaviors. A constant photoluminescence energy of 0.98 eV without any Moss-Burstein shift for Be doping levels up to 1018 cm−3 along with increased emission intensity due to passivation effect of Be is also observed. An increasing number of minority carriers tend to relax via Be defect state-related Shockley-Read-Hall recombination with the increase of Be doping density.

High-quality InAs quantum dots (QDs) with nominal thicknesses of 5.0–8.0 monolayers were grown on a digital AlAs0.56Sb0.44 matrix lattice-matched to the InP(001) substrate. All QDs showed bimodal size distribution, and their optical properties were investigated by photoluminescence (PL) and time-resolved PL measurements. Power dependent PL exhibited a linear relationship between the peak energy and the cube root of the excitation power for both the small QD family (SQDF) and the large QD family (LQDF), which is attributed to the type-II transition. The PL intensity, peak energy, and carrier lifetime of SQDF and LQDF showed very sensitive at high temperature. Above 125 K, the PL intensity ratio increased continuously between LQDF and SQDF, the peak energy shifted anomalously in SQDF, and the longer carrier radiative lifetime (≥3.0 ns at 77 K) reduced rapidly in SQDF and slowly in LQDF. These results are ascribed to thermally activated carrier escape from SQDF into the wetting layer, which then relaxed into LQDF with low-localized energy states.

The end performance of semiconductor optoelectronic devices is largely determined by the carrier dynamics of the constituent base materials. When combined with full-scale numerical models, optical spectroscopy is capable of providing detailed information about carrier generation and dynamics that is essential to accurate analysis of empirical test structure studies, and to translating those results into predictions for device performance. We have applied time-resolved and steady-state luminescence techniques to a variety of III-V materials and reference structures in order to investigate the mechanisms controlling carrier dynamics and to develop diagnostic tools to provide actionable feedback to R and D efforts for improvement and optimization of material/device performance.

The type-I to type-II band alignment transition in InAlAsAs/AlGaAs/GaAs self-assembled quantum dots (QDs) is investigated when the Al-composition in QDs and barrier are changed. In particular, the In0.46Al0.54As/Ga0.46Al0.54As/GaAs QDs show unique optical properties. The PL peak energy has a blue-shift of >40 meV when the laser intensity increases by four orders of magnitude, indicating a type-II band alignment of the QDs. The formation of the type-II band alignment is explained by that the quantum-confinement effect pulls up the minimum electron energy level in the QDs and the Γ→X transition in the Ga0.46Al0.54As barrier. The time-resolved PL (TRPL) spectrum of QDs at peak wavelength exhibits a double-component decay behavior, suggesting the possibility of type-I and type-II band alignment coexistence in this QD sample. The continuum state of the QDs is also investigated. Emission associated with the continuum states of the QDs is directly observed in PL spectra. The PL excitation (PLE) and TRPL spectra reveal an efficient carrier relaxation from the AlGaAs barrier into the InAlAs QD ground state via the continuum states. The carrier recombination in the continuum states can compete with that in the QDs due to the long recombination lifetime in the type-II QDs. This feature of continuum state emission can not be observed for normal InGaAs/GaAs QDs with the type-I band structure.

Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched\ud InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature,\ud interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth\ud (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with\ud the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift\ud and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations\ud of this system which correlate with the experimental observations indicate that a thin QW must be more affected by\ud interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information\ud on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses.

The authors have established a robust set of growth conditions for homoepitaxy of high-quality InAs with a (111)A crystallographic orientation by molecular beam epitaxy (MBE). By tuning the substrate temperature, the authors obtain a transition from a 2D island growth mode to step-flow growth. Optimized MBE parameters ( substrate temperature = 500 ° C, growth rate = 0.12 ML / s, and V / III ratio ≥ 40) lead to the growth of extremely smooth InAs(111)A films, free from hillocks and other 3D surface imperfections. The authors see a correlation between InAs surface smoothness and optical quality, as measured by photoluminescence spectroscopy. This work establishes InAs(111)A as a platform for future research into other materials from the 6.1 A family of semiconductors grown with a (111) orientation.

Tuning of the photoluminescence emission over a wider range of optical telecommunication wavelength (1.38 μm–1.68 μm) has been achieved by employing a GaAs1 − xSbx capping layer to the strain coupled bilayer InAs quantum dot (QD) heterostructures. It is shown that the modulation of strain between the two dot layers through variation in Sb-content and thickness of the capping layer strongly influence the dot size. The band alignment transformation from type-I to type-II is observed for high Sb-content in the capping layers. In addition, the carrier lifetime is improved by a factor of three in the QD heterostructures having type-II band alignment. This, we believe, is of importance for optoelectronic device applications.

Interface property has been considered one of the most critical factors affecting the performance of semiconductor devices. In this work, we demonstrate an efficient surface passivation for the interface between Cu2ZnSnS4 (CZTS) and CdS buffer layer by using UV-ozone treatment at room temperature. The passivation led to a significant enhancement of short circuit current density (Jsc) of the device from 11.70 mA/cm2 to 18.34 mA/cm2 and thus efficiency of the CZTS solar cells from 3.18% to 5.55%. The study of surface chemistry has revealed that the UV-ozone exposure led to formation of a Sn–O rich surface on CZTS, which passivates the dangling bonds and forms an ultra-thin energy barrier layer at the interface of CZTS/CdS. The barrier is considered to be responsible for the reduction of non-radiative recombination loss in the solar cells as confirmed by photoluminescence (PL) measurement. The elongated lifetime of minority carriers in the CZTS solar cells by time-resolved PL has further verified the interface passivation effect induced by UV-ozone treatment. This work provides a fast, simple yet very effective approach for surface passivation of CZTS film to boost the performance of CZTS solar cells.

This work develops a (NH4)2S/Al2O3 passivation technique for photodiode-based GaSb/AlAsSb heterostructure. Surface-sulfurated GaSb/AlAsSb heterostructure mesas show a significant suppression of reversed-bias dark current by 4–5 orders of magnitude after they are further passivated by Al2O3 layers. So the mesa sidewalls treated with (NH4)2S/Al2O3 layers can effectively inhibit the shunt path of dark carriers. The activation energies for both bulk and surface components are extracted from temperature-dependent current–voltage characteristics, which suggest that the bulk characteristics remain unchanged, while Fermi-level pinning at surfaces is alleviated. Additionally, temperature coefficients of the breakdown voltage are extracted, confirming that the breakdown process is confined entirely in the large bandgap AlAsSb regions. This study shows that the implementation of (NH4)2S/Al2O3 passivation can lead to room temperature GaSb-based photodiodes and GaSb/AlAsSb-based avalanche photodiodes for highly efficient photodetection.

Demonstrated are antimony‐based (Sb‐based) separate absorption and multiplication avalanche photodiodes (SAM‐APDs) for X‐ray and gamma‐ray detection, which are composed of GaSb absorbers and large bandgap AlAsSb multiplication regions in order to enhance the probability of stopping high‐energy photons while drastically suppressing the minority carrier diffusion. Well‐defined X‐ray and gamma‐ray photopeaks are observed under exposure to 241Am radioactive sources, demonstrating the desirable energy‐sensitive detector performance. Spectroscopic characterizations show a significant improvement of measured energy resolution due to reduced high‐peak electric field in the absorbers and suppressed nonradiative recombination on surfaces. Additionally, the GaSb/AlAsSb SAM‐APDs clearly exhibit energy response linearity up to 59.5 keV with a minimum full‐width half‐maximum of 1.283 keV. A further analysis of the spectroscopic measurement suggests that the device performance is intrinsically limited by the noise from the readout electronics rather than that from the photodiodes. This study provides a first understanding of Sb‐based energy‐sensitive SAM‐APDs and paves the way to achieving efficient detection of high‐energy photons for X‐ray and gamma‐ray spectroscopy.

Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel devices by taking advantage of the strong band gap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions. In this article, we demonstrate a method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110)-oriented surfaces. As a model system, we show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on InAlAs(110) surfaces. The tensile strain reduces the band gap in GaAs SAQDs by ~40%, leading to robust type-I quantum confinement and photoluminescence at energies lower than that of bulk GaAs. This method can be extended to other zinc blende and diamond cubic materials to form novel optoelectronic devices based on tensile SAQDs.

Carrier transfer in vertically-coupled InAs/GaAs quantum dot (QD) pairs is investigated. Photoluminescence (PL) and PL excitation spectra measured at low temperature indicate that the PL peak intensity ratio between the emission from the two sets of QDs—i.e., the relative population of carriers between the two layers of QDs—changes with increasing excitation intensity. Temperature-dependent PL reveals unexpected non-monotonic variations in the peak wavelength and linewidth of the seed layer of QDs with temperature. The PL intensity ratio exhibits a “W” behavior with respect to the temperature due to the interplay between temperature and excitation intensity on the inter-layer carrier transfer.