This PDF file contains the front matter associated with SPIE Proceedings Volume 10929, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

GaAs/AlGaAs multiple quantum wells (MQWs) on Ge substrate are grown by molecular beam epitaxy and their properties are compared with MQWs on GaAs substrate. The scheme of the growth includes GaAs deposition by migration enhanced epitaxy (MEE) at low-temperature and followed by growth of thin GaAs layer at high temperature and annealed during growth. This mechanism reduced the anti-phase domain disorder and blocked the dislocations. An interesting phenomenon of increase in integrated PL intensity of wells with higher activation energy at higher temperatures is observed and is correlated with simultaneous quenching of PL intensity in wells with lower activation energies. TEM images confirm lack of dislocations in QW layers. X-ray diffraction measurements confirm very good structural. In conclusion, multiple quantum wells grown on Ge substrate have properties comparable to those grown on GaAs substrate.

Nanowires containing quantum structures (dots or wires) offer direct integration of light emitters with ultra-small optical cavities. Applications include nanoscale lasers and single photon sources with high extraction efficiencies. These structures have high surface to volume ratios so surface passivation is a significant consideration, requiring a careful choice of materials. We report the growth and structural and optical characterisation of GaAsP nanowires containing either GaAs quantum dots (QDs) or quantum wells (QWs). This novel common group-III system offers a number of advantages in comparison to previously studied systems, including deep confinement potentials, reduced surface recombination and lack of an absorbing core when fabricating laser structures. Structures are grown by molecular beam epitaxy (MBE) on Si substrates allowing potential direction integration with drive electronics. TEM studies reveal high quality, defect free structures but a complex GaAsP structure with regions having different P compositions, suggesting the formation of QD and quantum wire regions within the nanowire. Evidence for GaAsP related-QDs is seen in low temperature micro-PL spectra. Nanowires containing radial QWs exhibit highly efficient QW carrier capture and a large activation energy for temperature dependent PL, consistent with strong carrier confinement. A laser structure containing three radial GaAs QWs exhibits a very low pumping threshold at low temperatures. Initial studies of nanowires containing GaAs QDs show excellent structural and optical properties. Our results indicate the potential of the GaAs/GaAsP system for improved lasers and single photon sources.

While the Stranski–Krastanov (SK) growth of InAs three-dimensional (3D) islands has been extensively investigated on GaAs(001), the SK growth mode is not observed on GaAs (110) and (111), the other low index surfaces of GaAs. This is unfortunate as these surfaces have symmetries (Cs and C3v, respectively) which differ from that of the (001) surface, and owing to their low energy they are often present in self-assembled nanostructures such as nanowires. Here we show that using a Bi surfactant to modify surface properties can profoundly influence epitaxial growth on GaAs (110) and (111)A surfaces. On GaAs(110), the Bi surfactant alters the fundamental growth mode of InAs from 2D layer growth to a 3D SK mode. Furthermore, a morphological phase transition can be induced “on-demand” in static strained 2D InAs(110) layers by exposing them to Bi, resulting in a rapid rearrangement of the InAs layer into 3D islands. Small (110) QDs are coherently strained to the substrate. These islands are optically active and exhibit emission that is linearly polarized, showing perspective for polarized single-photon emitters. On GaAs(111)A, GaAs buffer layer growth under a Bi flux results in ultra-smooth surfaces, which are free from the typically-observed morphological defects. Similar to the (110) case, exposing 2D InAs/GaAs(111)A layers to Bi induces the InAs 3D island self-assembly. These findings illustrate how surface-energy-modifying surfactants open the door to QDs synthesis on new substrates and with new materials.

While on GaAs(001) three-dimensional (3D) growth of InAs quantum-dots (QD) is observed on GaAs surfaces, such as the (110), the InAs deposition typically results in a two-dimensional (2D) growth and the misfit relaxes plastically. {110} facets often form the sidewalls in self-assembled GaAs nanowires. And the growth of 3D nanostructures like quantum dots on these sidewalls is of high interest for light emitting devices [1]. Recent investigations show that the presence of surface Bismuth (Bi) induces 3D growth on GaAs(110) by modifying the surface energy [2]. Furthermore, Bi exposure on already grown 2D InAs layers causes a morphological phase transition, resulting in a rapid re-organization of the 2D layer into 3D nanostructures. These so-called 3D islands have optical properties of quantum dots and open the possibility to generate linearly polarized single photons [1]. We investigate these 3D InAs islands grown on GaAs(110) substrate using cross-sectional scanning tunnelling microscopy (XSTM). For this purpose, we cleaved the sample containing capped InAs/GaAs(110) layers with four different growth conditions and Bi contents in ultra-high vacuum perpendicular to the [110] growth direction. Atomically resolved XSTM images allow for the characterization of the geometric structures in terms of size, density, and atomic composition depending on the presence of Bi. Furthermore, we carry out stoichiometric analysis of the chemical composition by analyzing the variation of local lattice parameter [3]. Thus, we are able to contribute to better understanding of the growth mechanisms of InAs/GaAs(110) with Bi in order to improve the properties for device applications. References: [1] P. Corfdir et al. Phys. Rev. B 96, 045435 (2017) [2] R. B. Lewis et al. Nano Lett,17,4255-4260 (2017) [3] A. Lenz et al. J. Vac. Sci. Technol. B 29, 04D104 (2011)

Resonant tunneling diodes (RTDs) are next-generation candidates for core THz generation technologies, with proven quasi-optical tunable emission capability, with centre frequencies of 0.1 - 1.98 THz at cca. 1mW, when coupled into a suitable monolithically integrated antenna. For this purpose, the strained InGaAs/AlAs/InP material system is approaching technological maturity, with its offering of high electron mobility, suitable conduction band offsets, and very low resistance contacts. However, the epitaxially thin layers used for RTDs, realise devices with current densities in excess of 10 mAμm^-2 and electric fields approaching that of the breakdown of the material. As a high current density is a traditional indicator of performance for these oscillators, it is now increasingly important to grow crystalline layers with near-atomic perfection. In previous work, we showed how the inclusion of a nominally identical, un-doped electrically neutral copy of the RTD double barrier - quantum well (QW) system, leads to the observance of a type-II QW emission in addition to the type-I emission from the active region QW. This could be used to establish the quasi-bound elastic energy, whose level is directly correlated to the peak voltage of the N-shape I-V characteristic. Here we extend this approach with the addition of high-resolution X-ray diffractometry and low-temperature photoluminescence spectroscopy. Through a step-by-step process of curve fitting, comparing to simulation and results, we can comment on the quality and thickness of the ternary InGaAs alloy interfaces surrounding the AlAs barriers. These findings are confirmed with scanning transmission electron microscopy

In this work we show successful metalorganic vapor phase epitaxy (MOVPE) of an AlN/AlGaN distributed Bragg reflector (DBR) that is wavelength matched to GaN quantum dots (QDs) in an AlGaN lambda cavity on top. Full insight into the growth of these structures enables the epitaxy of resonant cavity deep UV single photon emitters. The DBR was grown on an AlN/sapphire template. In order to obtain a high reflectivity as well as a sufficiently large stopband width, the refractive index contrast needs to be maximized. Additionally, the absorption of QD emission in the high gallium containing layer needs to be minimized. A compromise was found for nominal Al-concentration of 70 % in the AlGaN layers. The resulting DBR splits up into self-organized AlN/Al(X)Ga(1-X)N/Al(Y)Ga(1-Y)N trilayers, which add up to desired lambda/2-periods. Therefore, the stopband at 272 nm with a width of 6 nm shows a maximum reflectivity of 99.7 %. GaN QDs were obtained by growth of GaN on AlGaN for 10 s with a V/III-ratio of 30 followed by a growth interruption of 30 s. The QDs exhibit sharp emission lines with a FWHM down to 1 meV in µ-PL measurements. The main intensity of the QD ensemble emission is in the range of 250 nm to 275 nm. Finally, spatially resolved low temperature CL measurements show resonant DBR-enhanced GaN QD emission at 271 nm showing successful wavelength match between a AlN/AlGaN deep UV DBR and GaN QDs in an AlGaN lambda-cavity on top.

In this study, we demonstrate a unique method to fabricate quantum dots light emitting devices(QD-LEDs) using nanoporous templates. Zinc Oxide(ZnO) is spin-coated on ITO(Indium-Tin-Oxide) patterned glass as ETL(Electron-Transport-Layer). Cylinder-forming polystyrene-block-poly(2-vinyl pyridine) copolymer (S2VP) is spin-coated on the ETL layer. To induce phase separation, the thin films of block copolymers (BCPs) are solvent-annealed in vapor of toluene which is a good solvent for polystyrene blocks. As immersed in ethanol that is a selective solvent for P2VP blocks, the surface of BCP thin films is reconstructed and well-ordered nanopores are generated over the whole surface area. Then, the nanoporous templates are successfully fabricated on ETL layer by oxygen plasma treatment. The well-ordered nanopores on ETL can play a role as versatile templates to localize uniform spherical BCP micelles due to surface energy difference and topographical contrast during simple spin-coating process. In addition, well-ordered array of quantum dots(QDs) having hydrophobic surface is obtained by using ZnO layer. Interestingly, it is found that their population in a pore with diameter of about 40nm can be precisely controlled by adjusting the concentrations or the size of QDs as EML(Emissive-Layer). As the QDs have been selectively arranged in hexagonally packed nanopores with regular separation distance of about 80 nm, it is possible to generate extremely high density QDs arrays. Then, HTL(Hole-Transport-Layer), HIL(Hole-Injection-Layer), and Anode are deposited with thermal chemical vapor deposition. The QD-LED based on the hexagonal array of QDs may become one of key elements for the fabrication of a high resolution colour display.

We present a unique growth technique for molecular beam epitaxial growth of multi-layer InGaAs/GaAs quantum dots on Ge substrate. The optical and structural properties are compared with similar heterostructure grown on GaAs with the aim of achieving similar optical efficiency and structural homogeneity. An interesting phenomenon of increase in integrated photoluminescence (PL) intensity at high temperatures due to thermally assisted inter-dot carrier transfer is investigated using a coupled model and activation energy and quantum efficiency of dots is calculated for grown samples. The optical properties measured using steady state photoluminescence (PL) is found to quite similar to reference sample on GaAs substrate. Structural comparison performed using TEM shows good agreement between samples on Ge and GaAs substrate. H^- ion-implantation is done on as-grown samples which further enhances optical properties.

Strain-coupling in Quantum Dot Infrared Photodetector (QDIP) structures has been used as a strategy to achieve higher absorption efficiency and better device characteristics. Improvement in device characteristics due to the incorporation of strain-coupled QD layers has been divulged in this report. A 3/47 nm In_0.15Ga_0.85As/GaAs capped conventional uncoupled QDIP has been compared with QDIP having 3/12 nm In_0.15Ga_0.85As/GaAs capped strain-coupled QDs. The single pixel photodetector fabricated from the coupled structure has a lower dark current density (7.9×10^-4 A/cm²) compared to the uncoupled structure (12.17 A/cm²) at V_bias = -1 V and 300 K, which attributes a lower sensitivity to the thermalization effect in the former one. The strain-coupled QD heterostructure has photoluminescence peak at longer wavelength and lower full width at half maxima (24.86 nm), which indicates homogeneous dot size distribution. The surface chemical potential is less near the QDs due to the strain-relaxation. Hence, the lower lying dots forge the preferential nucleation sites for the upper QDs and it inhibits the inhomogeneous broadening occurs due to dot size fluctuation. The rocking-curve analysis from HRXRD measurement shows higher average strain in the strain-coupled QDIP (9.27×10^-4) compared to the uncoupled one (5.42×10^-4), which probably happens due to the accumulation of longitudinal strain from the lower QD layer towards the upped QD. The mid-infrared spectral response obtained from the strain-coupled QDIP has low spectral width.

We investigate the charging dynamics in epitaxially grown InAs quantum dots (QDs) under resonant excitation with and without additional low-power above-band excitation. Time-resolved resonance fluorescence from a charged exciton (trion) transition is recorded as the above-band excitation is modulated on and off. The fluorescence intensity varies as the QD changes from charged to neutral and back due to the influence of the above-band excitation. We fit the transients of the decay of the time-resolved resonance fluorescence after the above-band excitation turns off with a model that represents the neutralization process. The time dependence of the transients indicate that Auger recombination of resonantly excited trions is largely responsible for neutralization of the charged state when the above-band excitation is off. Additionally, a stretched exponential component of the transient of the fluorescence decay indicates the QD is supplied with charges via carrier migration through a stochastic collection of weakly-binding sites, resulting in sub-diffusion-like dynamics.

Modulation doping or localization of carriers in the detector or solar cell structure is an interesting technique which has piqued the interest of researchers. In this study, we demonstrate the effect of modulation doping on InAs/GaAs p-i-p QDIP grown on semi-insulating GaAs substrate using MBE. The active region consists of 10 layers of 2.7 ML InAs quantum dots followed up with 60 nm GaAs capping layer. In the GaAs capping, a modulation p-doping of 3 nm was introduced at 7, 12 and 17 nm from the InAs dot layer thus forming sample A, B and C, respectively. The ground state emission peak at 19 K from photoluminescence (PL) spectroscopy was measured at 1055.5, 1057.5 and 1062 nm for sample A, B and C respectively. Activation energies calculated from temperature dependent PL spectra were 157.57, 167.18 and 146.63 meV for the respective samples. The fabricated single pixel detectors exhibited spectral response peak from 1 to 3.5 μm in short wave infrared (SWIR) region for all the samples. The spectral response peaks observed were at 2.01 and 2.43 μm for device A, at 1.83 μm for device B and at 1.77 μm for device C. Highest operating temperature obtained from device A, B and C were 100K, 150K and 200K, respectively. The peak responsivities observed at 100K were 0.503, 0.154 and 0.33 A/W for the device A, B and C, respectively. Optimizing the position of localized carriers introduced in the active region can achieve the tunability in detection peak.

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.

We observe that the wavefunction overlap of the carriers in type-IIb quantum dots can be controlled by magnetic doping and strongly depends on the excitation power density. We study epitaxially grown ZnTe/ZnSe and magnetically doped (Zn,Mn)Te/ZnSe quantum dots that show a fast and slow recombination channel at two different energies. The emission shift of the slow recombination is independent of excitation power density (P_ex), whereas the fast recombination channel exhibits a larger emission shift with increasing P_ex. This blue shift saturates for high P_ex, however at a lower P_ex and smaller maximum shift in the magnetic system compared to the nonmagnetic system. The emission wavelength immediately after pulsed excitation typically changes as a function of charge carrier density due to the spatially indirect nature of type-IIb quantum dots. As carriers increase, the confinement potential of the system is altered due to a Coulomb interaction. The magnetic system exhibits a limited change in the wavefunction overlap of the carriers for high excitation power densities which we attribute to magnetic interactions to the hole wavefunction inside the dot. This would allow for an external manipulation of the magnetic polaron binding energy through varying the excitation power density P_ex in quantum dots.

Quantum dots based infrared photodetectors (QDIPs) having intra-valence band transitions and holes as majority carriers have been explored in this work. Here, we are demonstrating the effect of modulation doping on p-i-p QDIP (InAs/GaAs) grown using molecular beam epitaxy (MBE). The active region of the detector consists of 10 layers of selfassembled InAs quantum dots (2.7 ML) capped with GaAs layers and embedded in between p-type (beryllium-doped) GaAs layers. The performance of InAs/GaAs p-i-p QDIP (device A) was compared with modulation doped InAs/GaAs QDIP (device B). In the case of device B, modulation doping with p-type GaAs was introduced after growing 7nm of GaAs capping. The ground state emission peak at 10 K from photoluminescence spectroscopy was measured at 1060.5 nm and 1055.5 nm with a thermal activation energy of 222.93 meV and 157.57 meV for sample A and B, respectively. The measured dark current density at 75 K was 0.448 and 1.012 A/cm² at -1 V for device A and B, respectively. Spectral response peak in short wave infrared region (1.5 to 2.5 μm) were observed from both devices but in the case of device B, the spectral peaks were visible in mid wave infrared regime as well. At 75 K, the peak responsivity value measured was 35.11 A/W (at -1.5 V) and 0.333 A/W (at -1.5 V bias) for device A and B, respectively. High temperature of operation upto 200 K was observed from Device A whereas Device B exhibited response up to 125 K. Modulation doping close to the InAs quantum dots deteriorates the device performance.

The InGaAs surface quantum dots grown on GaAs surface without a capping layer (surface quantum dots, SQDs) are expected to play an important role for sensor applications due to their special surface sensitive properties. In this research, we investigated the photoluminescence (PL) characteristics of such In0.35Ga0.65As/GaAs SQDs with a layer of buried InGaAs QDs (BQDs) as reference. The uncapped InGaAs SQDs are integrated into a hybrid nanostructure with SQDs and buried quantum dots (BQDs) spaced by a 70 nm GaAs layer. Due to this thick GaAs spacer, we assert there is no quantum coupling between the SQDs and BQDs so that each layer of QDs has independent emission. The PL spectra show that the SQD PL intensity is far less than BQDs at low temperature but exceeds BQDs at high temperature, indicating a possible carrier transfer between the SQDs and surface states. With increasing excitation intensity, the PL spectra show clearly broaden on the high energy side and a blueshift for both the SQDs and BQDs. Therefore, there is lateral carrier transfer among each layer of QDs due to their high areal density. The intra-layer carrier transfer among SQDs as well as the inter-layer carrier transfer between SQDs and surface states attribute to carriers dynamics that make the SQDs having optical performance very different from the BQDs.

A noticeable repositioning of the characteristic photoluminescence peaks of homogeneous colloidal quantum dot solutions has been observed upon producing heterogeneous combinations of previously synthesized quantum dots. Typically, the wavelength of an emission peak is described as a fermionic property solely dependent on the size and the chemical composition of the nanoparticles involved. However, the experimental observations indicate that quantum dot interactions produce surface energy variations that trigger the aforementioned repositioning of photoluminescence emission peaks, and could represent an alternative route for controlling other fermionic properties such as melting temperature, ferromagnetic properties, cohesive energy, activation energy of diffusion and vacancy formation energy. Therefore, the extensive characterization of quantum dot interactions for tailoring and controlling fermionic properties could enable the demonstration of novel nanomaterials with unique properties for a variety of optoelectronic, photovoltaic and biomedical applications.

OPTICAL PROPERTIES OF WATER BASED COLLOIDAL SUSPENSIONS OF GRAPHENE QUANTUM DOTS Melita Parlak1, Elif Kayaaslan2, Dilan Ugus2, Bukem Bilen1, Hakan Erturk2 1Bogazici University, Department of Physics, Istanbul, 34342, Turkey 2Bogazici University, Department of Mechanical Engineering, Istanbul, 34342, Turkey Graphene quantum dots (GQDs) and their colloidal suspensions have been attracting attention for various applications such as biomedical imaging or energy harvesting. Optical properties need to be calculated for quantum dots’ effective use in different applications. Spectral absorbance for dilute colloidal suspensions of graphene quantum dots in water is predicted theo- retically by several methods including Rayleigh theory, Rayleigh Debye Gans approximation for fractal aggregates (RDG-FA) and discrete dipole approx- imation (DDA) within visible range of light (450- 700 nm). Confinement effect is observed while shrinking from 3 dimensional bulk material to quan- tum dots, and when the size of GQD is smaller than 10 nm, confinement effect becomes remarkable.The sizes of the inspected colloidal suspensions of quantum dots in water are reported to be 5 nm in average. After calculating optical properties of graphene quantum dots by using these approximations, the predicted spectral absorbances are compared to the experimental results. Absorption spectra of colloidal suspensions of GQDs in water with concen- trations ranging from 10000 ppm to 80000 ppm are obtained by Avantes Spectrometer using halogen lamp illumination and AvaSoft software. The size distribution and therefore the aggregation level of the quantum dots are identified through comparisons of the predicted and the measured spectral absorbances.

This work demonstrates the synthesis of indium tin oxide nanocrystals (ITO NCs) in mesoporous silica glasses (MSG) via the impregnation and decomposition method. The original MSG have many uniformly distributed mesopores (5-40 nm) which can regulate and control the size and distribution of ITO NCs. Factors that affect the properties of ITO NCs including temperature and holding time have been mainly studied. Interestingly, the enhanced down-conversion fluorescence intensity of Er³⁺/Yb³⁺ ions due to the energy transfer from ITO NCs has also been observed. Our work can widen the applications of semiconductor oxides in optoelectronics with a facile way.

The present study details the synthesis and characterization of photoluminescent ZnO quantum dots (QDs) and their effect in the performance of in-house-fabricated solar cells. The colloidal ZnO quantum dots were synthetized in an ethanol-based solution, where the growing dynamics was controlled by the pH of the precursor solutions with fixed reaction times. The down-shifting effects of the colloidal quantum dots were characterized by absorption and photoluminescence spectra. Additionally, the crystallographic characterization was performed employing X-ray diffraction (XRD). Planar single-crystal silicon solar cells were fabricated, and their window side was spin-cast with different pH-tuned ZnO quantum dots dispersed in polymethylmethacrylate (PMMA). To evaluate the impact of the aforementioned ZnO QDs films on solar cells, the power conversion efficiency (PCE) values were obtained from the J-V curves generated in a solar simulator and the short-current density (Jsc) was corroborated by external quantum efficiency (EQE) measurements. The mentioned characterization techniques indicated average PCE improvements of 8.67% using pH 10 ZnO QDs + PMMA films and as high as 14.14 % when using pH 12 ZnO QDs + PMMA films. Finally, the reflectance characterization shows a promising anti-reflection effect of the ZnO QDs + PMMA films layers.

Colloidal quantum dots (CQDs) are attractive material for single-photon sources (SPSs) that can be utilized at room temperature. In this work, we performed simultaneous measurements of photoluminescence (PL) decay curve and second-order photon correlation, g⁽²⁾, on single CQDs to study the purity of their single photons, which is determined by the value of g⁽²⁾at the time origin. We demonstrate that the single-photon purity can be improved through time-gated g⁽²⁾ analysis, where a part of the PL signal is excluded by applying a temporal filter to the PL decay curve. We show that by using a superconducting nanowire single-photon detector, it is possible to observe highly-purified single photons that exhibited g⁽²⁾ at the time origin ranging from 0.01 to 0.02.

Quantum dot infrared photodetectors (QDIPs) with different dot materials have been investigated in this study to analyze the optical, structural and electrical behavior. The InAs and In_0.5Ga_0.5As QDIPs comprise ten vertically-stacked uncoupled quantum dot (QD) layers with In_0.15Ga_0.85As/GaAs capping, whereas the overgrowth percentage in both the dot materials has been kept similar (~59%). The InGaAs QDIP has a red shifted photoluminescence spectra compared to the InAs QDIP along with a lower full width at half maxima (FWHM) and higher activation energy. This attributes the formation of dots with larger size and higher vertical barrier potential in the InGaAs QDIP heterostructure. The lattice mismatch between the dot and its underlying/capping layer is less in the InGaAs QDs, which has been observed from the HRXRD rocking-curve analysis. The average strain obtained in the InGaAs QD is less compared to the InAs QD. Moreover, a reduced dark current density has been obtained in the InGaAs QDIP compared to the InAs QDIP at room temperature. Both the QDIPs have their spectral response in the mid-infrared range. However, the InGaAs QDIP has peaks with lower FWHM due to minimized dot size dispersion in the structure.

The improvement in performance of In_0.5Ga_0.5As Quantum Dot Infrared Photodetector (QDIP) has been investigated by introducing Al in the In_0.15Ga_0.85As capping layer. The QDIPs are consist of ten uncoupled In_0.5Ga_0.5As dot layers with 3/47 nm In_x(Al_y(~x))Ga_1-x-yAs/GaAs capping. The monolayer coverage for both QDIPs is 6, which accommodate an overgrowth percentage of 59%. A FWHM (46.59 nm) and higher activation energy (267 meV) has been obtained for the ground state photoluminescence emission due to the incorporation of Al in the InGaAs capping layer. This indicates better carrier confinement and homogeneous dot size distribution in the quaternary (In_0.21Al_0.21Ga_0.58As) capped QD structure with respect to the ternary (In_0.15Ga_0.85As) capped QD. A six order reduction in dark current density has been obtained in the InGaAs QDIP due to the incorporation of Al in the capping layer. The narrow spectral width of 0.07 for the transition peak at 7.8 μm represents the homogeneous dot size distribution in the InAlGaAs/GaAs capped QDIP heterostructure.

In this study, we demonstrate the device performance of modulation doped InAs/GaAs p-i-p QDIP (device A) and the effect of thin 1 nm quaternary (InAlGaAs) capping on the same heterostructure (device B). The ground state emission peak at 9 K from photoluminescence spectroscopy was measured at 1055.1 nm and 1046.4 nm for sample A and B, respectively. The measured dark current densities at 75 K for an applied bias of -1 V were 1.079 A/cm⁻² and 0.038 A/cm⁻² for device A and B, respectively. The fabricated single pixel detectors from device A exhibited emission peak in short wave infrared regime whereas whereas device B exhibited a multicolour spectral response from short wave (SWIR) to mid wave infrared (MWIR) region. The measured spectral peaks at low temperature were at 2 and 2.39 μm for device A and at 2.013, 2.49, 3.49 and 4.36 μm with a dominant peak in SWIR region for device B. Both the devices exhibited spectral response peak up to 75K with a responsivity of 0.832 A/W for device A compared to that of 0.545 A/W for device B at -2.5V bias. Tunability in detection peak with improvement in device performance was achieved by incorporating additional quaternary capping.

Self-assembled InAs/GaAs quantum dot infrared photodetectors (QDIPs) have been proved suitable candidates for infrared photodetectors due to their excellent carrier confinement, normal-incidence absorption, reduced electron– phonon scattering and long excited carrier lifetime. This study investigates the effect of ternary (InGaAs) capping on InAs/GaAs p-i-p QDIP grown on semi-insulating GaAs substrate using Molecular Beam Epitaxy (MBE). The performance of InAs/GaAs QDIP (device A) is compared with InAs/InGaAs/GaAs QDIP (device B), in which ternary (InGaAs) capping of 6nm thickness was introduced just above the InAs quantum dot layer. The room temperature photoluminescense peak was observed at a wavelength of 1139.7nm and 1310.1nm for sample A and B, respectively. The activation energy was calculated to be 222.93 and 142.325 meV for sample A and B, respectively. From fabricated single pixel detectors, the dark current densities measured for an applied bias of -0.5V at 13 K were 0.5mA/cm² and 0.54A/cm² for device A and B, respectively. Devices exhibited spectral response peaks around 1.93μm and 1.52 μm for device A and device B, respectively. The measured peak can be attributed to the transition between ground state of hole to split off band. At 175K, the peak responsivity calculated was 5.8 A/W for GaAs capped device (A) as compared to 0.55 A/W for InGaAs capped device (B). Highest operating temperature exhibited by GaAs capped device A was 200K. Since the observed dark current densities was higher in comparison with device A, the highest operating temperature observed for device B was limited to 175K.