This paper presents a novel heterostructure with multilayer Stranski-Krastanov (SK) quantum dots (QDs) heterogeneously coupled to Submonolayer (SML) QDs that shows better performance in terms of both optical and material characteristics. The 2.5 ML InAs/GaAs multilayer SK QDs are grown on the 6 stack 0.3 ML InAs SML QDs using a Molecular Beam Epitaxy system. Multilayer includes single, bi, tri, penta, hepta and ten-layer SK QDs and a different growth mechanism is adapted to maintain the dot size similar in each layer irrespective of the residual strain from the bottom layers. Photoluminescence (PL) and high resolution X-ray Diffraction (HRXRD) experiments are done in order to analyze the optical and material characteristics of these grown heterostructures. PL results show that the ground state peak wavelength of SK QD for all the heterostructures is at ~1035 nm, which confirms a uniform dot size for all heterostructures. However, the sample with ten SK QD layers coupled to SML QD has the highest luminescence intensity, lowest full width half maxima (FWHM: ~50 meV), and highest activation energy (~397 meV). A peak at ~947 nm in the PL spectra confirms the presence of SML QDs and the tunneling of carriers from ground state of SML QD to excited state of SK QD is observed. HRXRD results show that the compressive strain experienced by the QDs is reducing with increase in the number of SK QD layers, which depicts ten SK QD layers coupled to SML QD heterostructure as the best even in terms of material characteristics.
Over the last decade InAs Stranski–Krastanov (S-K) quantum dots (QDs) grown on GaAs substrate have been widely explored for optoelectronic devices. In the recent past, the uncapped surface QDs are getting much attention for sensing application with adequate sensitivity to lower molar concentration of contaminants. Though non-uniform size distribution is an inherent property of the self-assembled S-K growth process, sensitivity of the surface QDs is significantly affected by this. Here, we have grown the surface QDs upon buried QD layer with a low barrier layer thickness to reduce the non-uniformity. In general, the 2D to 3D transition of the InAs QDs occurs only above 1.7 monolayer (ML). However, the 3D transition may be possible even at a lower monolayer coverage with the residual strain induced from the underlying QDs. In this study, particularly we have grown InAs surface QD layer at 1.6 ML coverage above the 2.7 ML buried QD layer with 8 nm thick GaAs spacer. The impact of vertically induced strain of the underneath InAs QD layer on the growth of surface dots has been investigated. The morphology of surface QDs is observed through Atomic force microscopy (AFM), which indicates the formation of uniform QDs with lower defects. The low temperature photoluminescence (PL) spectroscopy provides the evidence of the wave function overlap between the buried and surface QDs.
This study presents a novel approach to enhance the photoluminescence and minimize strain in selfassembled bilayer InAs quantum dot (QDs). To obtain this approach, multi-level digital alloy capping layer (DACL) is introduced in the growth of the ternary InGaAs capping layer having different material composition, instead of conventional thick layer. A single thick 4 nm InGaAs capping on the InAs QD layers (Sample A1) is divided into four equal parts each having 1 nm thicknesses. The composition of indium (In) in strain reducing layer (SRL) in growth direction is varied from 45% to 15% for both first layer and second layer QDs (Sample D1). The experimental low temperature ground state emission wavelength for the sample A1 and sample D1 was 1096 and 1167 nm respectively. The biaxial and hydrostatic strain in growth direction was theoretically computed and compared for both analog and digital samples. The computed hydrostatic and biaxial strain in sample D1 is improved by 7.19% and 6.79% respectively, when compared to that of analog sample A1. The improved hydrostatic strain provides the better carrier confinement. The improved biaxial strain offers more band splitting between heavy-hole and light-hole band in valence band. This decreases the ground state band gap and thus offers a red shift in photoluminescence (PL). The experimental PL for both samples were also validated by simulating both heterostructures. The sample with DACL growth mode provided better crystalline quality, enhanced quantum yield and lesser defects.
Properties of self-assembled III-V quantum dot (QD) heterostructures for optoelectronic devices mainly rely on growth parameters and also on substrate used. The research community mainly preferred GaAs substrate instead of Si substrate for optoelectronics. However, the low cost and abundance of Si impels the researchers and industrialists to use Si for the commercial application using SixGe1-x graded layer and Migration Enhanced Epitaxy (MEE) layer. Here we have studied the optical and structural study of Stranski–Krastanov (S-K) InAs quantum dots grown on Ge substrate with 6° offcut toward the (110) plane (Sample A) without MEE layer, which may be easy to integrate on Si. Starting from the thick GaAs buffer layer, AlAs/GaAs super-lattice buffer layers followed by three consecutive layers of 2.7 ML S-K InAs QDs with 50 nm GaAs capping were grown. Another sample (B) with the same heterostructure was grown on GaAs substrate for comparison. Low temperature photoluminescence (PL) for the sample (A) is blue-shifted as compared to sample B, which might be due to smaller dots formation. The bi-modal dot size distribution of the sample A and sample B was confirmed from the power dependent PL. In the low temperature PL spectrum, full width half maxima (FWHM) of the sample A is very close to that of the sample B. Rocking curve obtained from high resolution X-ray diffraction (HRXRD) for the sample A, shows Ge substrate peak and GaAs peak from the GaAs layer grown on the Ge substrate. The strain calculated from the HRXRD rocking curve for the sample A and sample B is -4.12x10-3 and -2.0x10-3 respectively. Strain value indicates crystalline quality of sample A is good and comparable to the same in sample B, grown on the GaAs substrate. The optical properties for the sample A can be enhanced further via monolayer coverage of the dots, capping material, capping thickness and ex-situ annealing techniques.
Multilayer strain-coupled P-i-P quantum dot infrared photodetectors (QDIPs) with different configurations are studied. Photoluminescence (PL) and photoluminescence excitation (PLE) measurements are carried out to investigate the improvement in the optical performance of these proposed devices. The samples are grown with a different growth strategy to minimize the dot size dispersion compared to the conventional QDIPs. Also, the effect of In0.15Ga0.85As strain reducing layer (SRL) in the proposed samples are analyzed. We report a monomodal PL spectrum and reduction of 28 meV in full-width half maximum (FWHM) of the ground state (GS) peak for the proposed structure in comparison with the conventional one. The monomodal behavior of the structures is confirmed by mapping deconvoluted PL peaks and PLE results. The GS peak of the conventional QDIP is observed at 1.2 eV, whereas the same for the proposed sample is at 1.18 eV. Further redshift in the peak position is achieved (1.14 eV) through the introduction of SRL, which also has a lesser FWHM than the conventional sample. A difference of 69 meV and 73 meV between GS and the first excited state (ES1) peak is observed in the PLE spectra of the conventional and proposed structure, respectively. However, two resolved excited state peaks (ES1 and ES2) are visible in the case of SRL-incorporated structure, which are 69.6 meV and 138 meV away from the GS peak. The proposed QD heterostructures with applied growth strategy and P-i-P configurations are expected to perform better at higher temperatures along with improved absorption efficiency.
Self-assembled III-V compound semiconductor quantum dots (QDs) on silicon (Si) substrate is much explored topic for optoelectronic devices. Here, we have investigated the optical and structural behavior of InAs QDs grown on (001)- oriented Si substrate. The heterostructure has been grown without Si-Ge buffer/graded layer and without Migration Enhanced Epitaxy layer which might reduce the anti-phase domain and dislocation propagation towards the active region. The heterostructure grown on Si (sample A) consists of a thick GaAs buffer layer which was followed by AlAs/GaAs super-lattice buffer layer and three consecutive layers of 2.7 ML InAs QDs with 50 nm GaAs capping. A heterostructure with similar active layers was grown on GaAs substrate (sample B). Samples were characterized using photoluminescence (PL) and high resolution X-ray diffraction (HRXRD) measurements. Sample A exhibited blue shifted PL peak as compared to sample B, which might be due to the formation of smaller dots. Moreover, from the power-dependent PL analysis, a multimodal and bimodal dot size distribution was observed in sample A and B respectively. HRXRD measurements showed the poor crystalline quality in sample A as compared to sample B. However, PL of sample A exhibited a higher intensity in comparison to sample B. In addition, sample A provided higher activation energy of 290 meV, whereas it was 198 meV in case of sample B. This indicates better confinement of charge carriers, which might improve the device performance. The optoelectronic performances could be enhanced by further optimizing this growth strategy through optimizing the dot layer periodicity, capping material, and capping thickness.
Submonolayer (SML) quantum dots (QDs) have higher confinement than conventional Stranski- Krastanov (SK) QDs. Moreover, hole-transport based QD infrared photodetectors (QDIPs) are anticipated to perform better at a higher temperature than its counterparts (electron-transport based devices). Effects of different stacking configuration and monolayer (ML) coverage of InAs SML QDs in In0.15Ga0.85As matrix are studied here for the development of high temperature operable, hole-transport based QDIPs. We increased the number of dot layers in the matrix as 4, 6 and 8. The monolayer coverage is varied from 0.3 ML to 0.5 ML. Radiative recombination is captured by photoluminescence (PL) and PL excitation (PLE) to observe the energy states of the grown heterostructures. The PL results in case of 0.3ML QDs show a gradual red shift in the ground state (GS) emission when we stack more dot layers in the matrix (1.334 eV, 1.269 eV, and 1.244 eV). Increase in dot size is suspected as the reason behind this change. A decrease in the difference between GS and first excited state (ES1) confirms the enlargement of dots for these samples. However, the PL (multimodal) peak position with maximum intensity changes more interestingly (1.195 eV, 1.154 eV and 1.188 eV) for 0.5 ML QDs with the increase in stacking. This variation is expected to be associated with the relaxation of dots via out diffusion of In atoms from the dot.
Strain in the heterostructure plays a vital role in the characteristics of Quantum Dot (QD) based optoelectronic devices. Optimization of the number of dot layers to be strain-coupled is analyzed here to attain QD infrared photodetectors with higher efficiency. Heterostructures are grown in a molecular beam epitaxy (MBE) system with two (Bi), three (Tri), five (Penta) and seven (Hepta) strain-coupled QD layers, to observe the variation in the optical properties. The effect of thin In0.15Ga0.85As Strain Reducing Layer (SRL) over these coupled structures is also analyzed. Photoluminescence (PL) and Photoluminescence Excitation (PLE) spectroscopy are carried out on the grown structures. Low-temperature Power Dependent PL and PLE revealed the discrete energy states in the dots. The ground state (GS) peaks are found at 1.16 eV, 1.18 eV, 1.195 eV, and 1.194 eV for Bi-, Tri-, Penta-, and Hepta-layer structures. The corresponding peaks redshifted to 1.12 eV, 1.14 eV, 1.154 eV, and 1.152 eV, with the incorporation of 2 nm SRL. It is observed from the PLE results that the excited state peaks of Bi-to-Heptalayer structures are 68 meV, 70 meV, 74 meV, and 72 meV away from the GS peak. However, the differences obtained for the samples with In0.15Ga0.85As SRL are 59 meV, 66 meV, 68 meV, and 70 meV. It is seen that the GS PL peaks of Penta-layer samples with both kinds of structures have the highest intensity. The study shows the importance of strain-coupling and provides an optimum QD heterostructure for better device performance.
In this report, the performance of Quantum Dot Infrared Photodetector (QDIP) is examined in which the active layer consists of 10 layers of uncoupled InAs quantum dots (QDs) with quaternary In0.21Al0.21Ga0.58As capping. The optical, structural, and electrical properties of the QDIP is observed and compared with a sample in which the QDs are capped with binary GaAs layer. The observation of full width half maximum (FWHM) in the low-temperature photoluminescence (PL) of both sample shows a change in dot size distribution. Variation in the dot size distribution is also observed from the low temperature power dependent PL. Activation energy calculated from the temperature dependent PL indicates better carrier confinement in the structure with In0.21Al0.21Ga0.58As capped QDs. This can be explained by the formation of higher barrier potential. Stain introduced due to lattice mismatch in the heterostructure is calculated from the high resolution X-ray diffraction (HRXRD) Rocking curves, which shows a relatively low value of strain in the QDIP heterostructure with In0.21Al0.21Ga0.58As capping with respect to the QDIP with GaAs capping layer. A five order reduction in the dark current density is also observed form the QDIP with In0.21Al0.21Ga0.58As capping due to insertion of Al in the capping layer. The dark current obtained for the In0.21Al0.21Ga0.58As capped QDIP is 1.9E-5 A/cm2, whereas the same for the GaAs capped QDIP is 4.91 A/cm2. This attributes to the confinement enhancement in the prior QDIP heterostructure.
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.
The improvement in performance of In0.5Ga0.5As Quantum Dot Infrared Photodetector (QDIP) has been investigated by introducing Al in the In0.15Ga0.85As capping layer. The QDIPs are consist of ten uncoupled In0.5Ga0.5As dot layers with 3/47 nm Inx(Aly(~x))Ga1-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 (In0.21Al0.21Ga0.58As) capped QD structure with respect to the ternary (In0.15Ga0.85As) 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.
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 In0.15Ga0.85As/GaAs capped conventional uncoupled QDIP has been compared with QDIP having 3/12 nm In0.15Ga0.85As/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 Vbias = -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.
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 In0.5Ga0.5As QDIPs comprise ten vertically-stacked uncoupled quantum dot (QD) layers with In0.15Ga0.85As/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.
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.
In this work, we have proposed two ex-situ treatments to annihilate bulk defects and non-radiative surface states in MBE grown AlGaN nanowires on silicon substrate. Vertically aligned nanowires were grown by molecular beam epitaxy (Veeco Gen II) system, equipped with RF plasma source for incorporating nitrogen, at a substrate temperature of 8000C. The nanowires were structurally characterized by SEM images and an areal density of 3.8×109 nanowires-cm-2 was calculated. The peak emission wavelength was measured to be 343nm at 19K from photoluminescence (PL) measurements. The as-grown nanowires were implanted with H- ions at various energies and fluences. A 2-fold increase in PL intensity without any wavelength shift was found in sample with irradiated energy of 3 MeV and a fluency of 1×1012 ions-cm-2. X-Ray diffraction measurements on (002) diffraction peak give an FWHM of 1440 arc-sec for ionimplanted samples as compared to 1872 arc-sec for as-grown nanowires indicating an improvement in crystalline quality. The nanowires were further treated in UV ozone environment for 25 minutes and an increase of 1.6 times in PL intensity was found without any wavelength shift. Both recipes were combined to get an effective increase of 3 times in PL intensity indicating the annihilation of bulk defects and non-radiative surface states. Hence, these treatments can be used to reduce non-radiative states and can be used to increase in radiative efficiency of AlGaN nanowires for applications in UV LEDs.
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