Properties Of InAs Quantum Dots Research Articles

Influence of GaAsBi Matrix on Optical and Structural Properties of InAs Quantum Dots

InAs/GaAsBi dot-in-well structures were fabricated using gas-source molecular beam epitaxy and investigated for its optical and structural properties. GaAsBi-strained buffer layer and strain reduction layer are both effective to extend the photoluminescence (PL) emission wavelength of InAs quantum dot (QD). In addition, a remarkable PL intensity enhancement is also obtained compared with low-temperature-grown GaAs-capped InAs QD sample. The GaAsBi matrix also preserves the shape of InAs QDs and leads to increase the activation energy for nonradiative recombination process at low temperature. Lower density and larger size of InAs QDs are obtained on the GaAsBi surface compared with the QDs grown on GaAs surface.

Optical study of high index substrate effect in multilayer InAs/GaAs quantum dot solar cells

We present a study of the optical properties of InAs quantum dot (QD) solar cells (SC) grown by molecular beam epitaxy on GaAs(11N)A orientation substrates (N = 4, 5). Optical properties of InAs/GaAs (11N)A QD SCs were characterized by spectroscopic ellipsometry (SE), photoluminescence (PL) and photocurrent (PC) measurements. From SE data, optical constants are calculated and the different transition energies for the two SCs are calculated and identified in the energy range 1–6 eV. The surface and volume energy loss functions are calculated and show different dependences on the incident photon energy for the two InAs/GaAs QD SCs in the energy interval 1–6 eV. PL measurements revealed good optical properties of the InAs QD SC grown on GaAs(115)A compared to that grown on GaAs(114)A orientation substrate. Two additional peaks are observed at 1.17 and 1.34 eV in the PL spectrum of the SC grown on the (115)A direction and are attributed to the transitions between excited states in the InAs QDs. Photocurrent measurements show that above GaAs bandgap, higher photocurrent is measured for the sample grown on the (114)A orientation. This was attributed to the multiple exciton generation in InAs QDs since in this sample the built-in field is strong enough to accelerate photocarriers generated in GaAs.

Structure, strain, and composition profiling of InAs/GaAs(211)B quantum dot superlattices

The morphology, nanostructure, and strain properties of InAs quantum dots (QDs) grown on GaAs(211)B, uncapped or buried, are explored by transmission electron microscopy and related quantitative techniques. Besides the built-in piezoelectric field, other differences of (211) growth compared to (100)-oriented growth are discussed in terms of the (211) surface non-singularity, leading to anisotropic shape of the QDs and local chemical inhomogeneity of the wetting layer. The shape of the uncapped QDs was precisely defined as truncated pyramidal, elongated along the 〈111〉 direction, and bounded by the {110}, {100}, and {213} facets. Local strain measurements showed that large surface QDs were almost unstrained due to plastic relaxation, exhibiting small residual elastic strain at the interface that gradually diminished toward their apex. Conversely, buried QDs were pseudomorphically grown on GaAs. By postulating a plane stress state, we have established a systematic increase of the local strain from the base toward the apex region of the QDs. Using Vegard's law, their chemical composition profiles were calculated, revealing an indium content gradient along the growth direction and compositional variants among different QDs. Photoluminescence measurements showed variations in emission energy between the QDs and consistency with a graded In-content, which complied with the quantitative strain analysis.

Fabrication and optical properties of type‐II InP/InAs nanowire/quantum‐dot heterostructures

The growth and optical properties of InAs quantum dots on a pure zinc blende InP nanowire are investigated. The quantum dots are formed in Stranski–Krastanov mode and exhibit pure zinc blende crystal structure. A substantial blueshift of the dots peak with a cube‐root dependence on the excitation power is observed, suggesting a type‐II band alignment. The peak position of dots initially red‐shifts and then blue‐shifts with increasing temperature, which is attributed to the carrier redistribution among the quantum dots. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

The size-dependent electronic and optical properties of InAs quantum dots

Energy levels, direct energy band gap, refractive index and high-frequency dielectric constant of zinc-blende InAs spherical quantum dots and their size dependence have been investigated. The calculations are based on a pseudopotential approach. As a result of the quantum confinement effect, the predicted direct energy band gap is found to be significantly augmented relative to the bulk value, whereas the obtained high-frequency dielectric constant is shown to be significantly reduced.

Broadband control of emission wavelength of InAs/GaAs quantum dots by GaAs capping temperature

We investigated the effects of the GaAs capping temperature on the morphological and photoluminescence (PL) properties of InAs quantum dots (QDs) on GaAs(001). The broadband tuning of the emission wavelength from 1.1 to 1.3 μm was achieved at room temperature by only adjusting the GaAs capping temperature. As the capping temperature was decreased, the QD shrinkage due to In desorption and In-Ga intermixing during the capping process was suppressed. This led to QDs with a high aspect ratio, and resultantly, the emission wavelength shifted toward the longer-wavelength side. In addition, the linearly polarized PL intensity elucidated anisotropic characteristics reflecting the shape anisotropy of the embedded QDs, in which a marked change in polarization anisotropy occurred at capping temperatures lower than 460 °C.

Thermally activated inter-dots carriers' transfer in InAs QDs with InGaAs underlying layer: Origin and dependence on the post-growth intermixing

This paper reports on experimental and theoretical investigation of atyical temperature-dependent photoluminescence properties of InAs quantum dots in close proximity to InGaAs strain-relief underlying quantum well. The impact of a post-growth intermixing process on these properties has been studied. For the as-grown sample, the maximum of the emission band follows a sigmoidal function while the photoluminescence linewidth mimics a V-shape function as the temperature increases, from 11 to 300 K. These behaviors are attributed to thermally activated carrier transfer mechanisms within the inhomogenious distribution of quantum dots. These atypical behaviors are found to disappear progressively with the degree of intermixing and consequent narrowing of the dot size dispersion. The experimental results have been interpreted in the frame of the localized states ensemble model revealing that the large dots size distribution is the main origin of the observed anomalies. Furthermore, the calculations show that the quantum well continuum states act as a transit channel for the redistribution of thermally activated carriers.

Theoretical analyses of the elastic and electronic properties of InAs QDs and QD-in-WELL structures grown on GaAs high index substrates

We report a theoretical study of the wetting layer thickness, InxGa(1−x)As quantum-well thickness and Indium composition effects on the physical properties of InAs quantum dots (QDs) and quantum dots-in-well (QD-in-WELL) grown on GaAs high index substrates. Finite element method is used to calculate the strain, piezoelectric field distributions and the electronic structure. Coulomb interaction has been taken as a perturbation in the interband transition energy. The ground–state transition energy is influenced by the wetting layer thickness (WL) and the substrate orientation; however, it is not affected by the InGaAs quantum-well thickness. We have found that the tensile strain at the interface is the main factor responsible for the difficulty of self-assembled InAs QDs formation on GaAs(111) substrate. On the other hand, the stability of the relaxed strain into QD–IN–WELL depended on the Indium composition and the quantum-well thickness as well as the orientation substrate. The appropriate Indium composition in the InGaAs quantum-well is found to be 0.3 for the QD–IN–WELL grown on GaAs(111) and 0.2 for the QD–IN–WELL grown on GaAs(119). This work can be helpful to controlling the wavelength of QD–IN–WELL grown on high index substrates by changing the In composition or the quantum well-thickness.

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In0.15Ga0.85As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In0.15Ga0.85As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.

The Role of Groove Periodicity in the Formation of Site-Controlled Quantum Dot Chains.

Structural and optical properties of InAs quantum dot (QD) chains formed in etched GaAs grooves having different periods from 200 to 2000 nm in [010] orientation are reported. The site-controlled QDs were fabricated by molecular beam epitaxy on soft UV-nanoimprint lithography-patterned GaAs(001) surfaces. Increasing the groove periods decreases the overall QD density but increases the QD size and the linear density along the groove direction. The effect of the increased QD size with larger periods is reflected in ensemble photoluminescence measurements as redshift of the QD emission. Furthermore, we demonstrate the photoluminescence emission from single QD chains.

Enhancing optical characteristics of InAs/InGaAsSb quantum dot structures with long-excited state emission at 1.31 μm.

In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs(1-x)Sb(x) (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.

Effects of InAlAs strain reducing layer on the photoluminescence properties of InAs quantum dots embedded in InGaAs/GaAs quantum wells

Two kinds of self-assembled quantum dots (QDs) embedded within InGaAs/GaAs quantum wells were grown by molecular beam epitaxy: one was capped with an InAlAs strain reducing (SR) layer, while the other was not. Their emission dynamics was investigated by time-resolved and temperature dependent (TD) photoluminescence (PL) measurements. A significant redshift can be observed in the emission peak position of InAs QDs with thin InAlAs SR cap layer, which results from SR effects. Different behaviors of the integrated PL intensity for the samples with or without InAlAs layer may be ascribed to the reduced carrier transition at higher temperature for the higher energy barrier of the InAlAs layer, and the TD mode of carrier migration. The PL decay time of quantum dots grown with InAlAs layer was much longer than that without the layer, which implies that the InAlAs layer with higher energy barrier may enhance the quantum restriction of carriers in InAs QDs. These observations are discussed from the viewpoint of strain compensation and potential barrier variation with SR layers. Our experiments also demonstrate that the main mode of carrier migration is quantum tunneling effect at lower temperature, while it is quantum transition at higher temperature. The results demonstrate the importance of InAlAs SR layer for the optical quality of InAs QDs.

Morphological and temperature-dependent optical properties of InAs quantum dots on GaAs nanowires with different InAs coverage

We report a study on the morphological and temperature-dependent optical properties of InAs quantum dots on GaAs nanowires with different InAs coverage. We find that the size, density, and distribution of quantum dots strongly depend on the InAs coverage. At higher coverage, the quantum dots exhibit a longer peak wavelength and broader linewidth at low temperature, suggesting a larger size and increased size fluctuations. Particularly, a great difference in the linewidth dependence on temperature for different InAs coverage is observed, corresponding to a different result of competition between electron-phonon scattering and thermal penetration of carriers between neighboring quantum dots.

Controlled nucleation and optical properties of InAs quantum dots grown on faceted GaAs microstructures

AbstractWe present a study of the site‐controlled nucleation of InAs quantum dots on faceted GaAs microstructures grown by migration enhanced epitaxy and demonstrate single quantum dot emission. Well defined GaAs templates with facets mostly belonging to the crystal plane families {01n }, {11n } and (001) are realized. Subsequent InAs nucleation occurs on the plane with the highest growth rate, and allows a precise positioning of optical active InAs quantum dots. The site‐controlled quantum dots reveal excitonic and biexcitonic transitions with narrow linewidths. The results are promising for the realization of single photon sources from site‐selected InAs QDs on faceted GaAs microstructures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influences of dot-in-a-well structure and GaAs insertion layer on InP-based InAs quantum dots

The structural and optical properties of InAs quantum dots (QDs) embedded in an In0.85Ga0.15As0.32P0.68 matrix with the emission wavelength of 1.10 µm (1.1Q-InGaAsP) on InP were modified by adopting an In0.69Ga0.31As0.67P0.33 (1.35 µm, 1.35Q-InGaAsP) quantum-well (QW) structure (dot-in-a-well, DWELL) and by inserting an additional GaAs layer right below the QDs. The spatial QD density with the DWELL structure and the GaAs insertion layer was reduced by 53% from that for the simple InAs QDs with only a 1.1Q-InGaAsP barrier (reference sample). However, the photoluminescence intensity was improved by more than 2.5 times compared to that of the reference sample. These results are discussed in terms of the increase in the capture probability of carriers due to the 1.35Q-InGaAsP QW and the relatively strong localization of carriers according to variations in the wetting layer and the distances among QDs.

Band alignment tuning of InAs quantum dots with a thin AlGaAsSb capping layer

We investigate the optical properties of InAs quantum dots (QDs) capped with a thin AlxGa1−xAsSb layer. As evidenced from power-dependent and time-resolved photoluminescence (PL) measurements, the GaAsSb-capped QDs with type-II band alignment can be changed to type-I by adding Al into the GaAsSb capping layer. The evolution of band alignment with the Al content in the AlGaAsSb capping layer has also been confirmed by theoretical calculations based on 8-band k⋅p model. The PL thermal stability and the room temperature PL efficiency are also improved by AlGaAsSb capping. We demonstrate that using the quaternary AlGaAsSb can take the advantages of GaAsSb capping layer on the InAs QDs while retaining a type-I band alignment for applications in long-wavelength light emitters.

As 차단 시간 변화에 의한 InAs 양자점의 광학적 특성

Arsenic interruption growth (AIG)법을 이용하여 GaAs 기판에 성장한 InAs 양자점(quantum dots, QDs)의 광학적 특성을 PL (photoluminescence)과 time-resolved PL을 이용하여 분석하였다. AIG법은 InAs 양자점 성장 동안 In 공급은 계속 유지하면서 셔텨(shutter)를 이용해서 As 공급과 차단을 조절하는 방법이다. 본 연구에서는 As 공급과 차단을 1초(S1), 2초(S2), 또는 3초(S3) 동안 반복하여 성장한 InAs QDs과 As 차단 없이 성장한 기준시료(S0)를 사용하였다. AIG법으로 성장한 시료들의 PL 세기는 기준시료보다 모두 강하게 나타나고, As 차단 시간에 따라 PL 피크는 적색이동(redshifted) 또는 청색이동 (blueshifted)하여 나타났다. 기준시료 S0의 PL 피크와 비교하였을 때 S1의 PL 피크의 적색이동은 양자점 평균 길이가 S0보다 증가하였기 때문이며, S3의 청색이동은 양자점 평균 길이가 S0보다 감소하였기 때문이다. AIG법으로 성장한 QDs 시료들의 PL 세기의 증가는 cluster의 감소, 양자점 밀도의 증가, 균일도의 향상, 종횡비(aspect ratio) 향상으로 설명된다. 온도에 따른 PL 세기와 PL 피크 에너지, PL 소멸 시간과 발광 파장에 따른 PL 소멸 시간을 측정하였다. As 공급과 차단을 2초로 하였을 때 cluster는 전혀 나타나지 않았고 양자점의 밀도는 증가하였으며 균일도와 종횡비도 향상되었다. 또한 S2는 가장 강한 PL 세기와 가장 긴 소멸 시간을 나타내었다. 이러한 결과는 AIG법을 이용하여 InAs 양자점의 크기, 조밀도, 균일도, 종횡비 등을 조절하여 원하는 파장대의 양자점을 성장할 수 있으며 발광 특성도 향상시킬 수 있음을 확인하였다. The optical properties of InAs quantum dots (QDs) grown on GaAs substrates grown by molecular beam epitaxy have been studied using photoluminescence (PL) and time-resolved PL measurements. InAs QDs were grown using an arsenic interruption growth (AIG) technique, in which the As flux was periodically interrupted by a closed As shutter during InAs QDs growth. In this study, the shutter of As source was periodically opened and closed for 1 (S1), 2 (S2), or 3 s (S3). For comparison, an InAs QD sample (S0) without As interruption was grown in a pure GaAs matrix for 20 s. The PL intensity of InAs QD samples grown by AIG technique is stronger than that of the reference sample (S0). While the PL peaks of S1 and S2 are redshifted compared to that of S0, the PL peak of S3 is blueshifted from that of S0. The increase of the PL intensity for the InAs QDs grown by AIG technique can be explained by the reduced InAs clusters, the increased QD density, the improved QD uniformity, and the improved aspect ratio (height/length). The redshift (blueshift) of the PL peak for S1 (S3) compared with that for S0 is attributed to the increase (decrease) in the QD average length compared to the average length of S0. The PL intensity, PL peak position, and PL decay time have been investigated as functions of temperature and emission wavelength. S2 shows no InAs clusters, the increased InAs QD density, the improved QD uniformity, and the improved QD aspect ratio. S2 also shows the strongest PL intensity and the longest PL decay time. These results indicate that the size (shape), density, and uniformity of InAs QDs can be controlled by using AIG technique. Therefore the emission wavelength and luminescence properties of InAs/GaAs QDs can also be controlled.

Realization of Stranski–Krastanow InAs quantum dots on nanowire-based InGaAs nanoshells

We investigate the formation and optical properties of InAs quantum dots on an InGaAs nanosubstrate. We find that Stranski–Krastanow InAs quantum dots are hardly formed on Au-catalyzed InGaAs nanowires due to the phase separation as well as stacking faults. High-quality Stranski–Krastanow InAs quantum dots are realized on a pure zinc blende InGaAs shell radially grown on a GaAs nanowire core. The quantum dots have a large size and regular shape, residing on a wetting layer of several nanometers. For optical characterization, we fabricate a “dot-in-well” structure by capping the quantum dots with InGaAs/GaAs double layers. Photoluminescence from the quantum dots is observed at 77 K, with a peak wavelength of 954 nm, which is distinctly redshifted compared with that of InAs quantum dots directly grown on GaAs nanowires. This work shows the potential of growing Stranski–Krastanow QDs on more types of NWs and obtaining longer wavelengths for wider applications.

InAs/GaAs 양자점의 발광특성에 대한 InGaAs 캡층의 영향

The optical properties of InAs quantum dots (QDs) grown on a GaAs substrates by migration enhanced molecular beam epitaxy method have been investigated by using photoluminescence (PL) and time-resolved PL measurements. The luminescence properties of InAs/GaAs QDs have been studied as functions of temperature, excitation laser power, and emission wavelength. The PL peak of InAs QDs capped with layer (QD2) measured at 10 K is redshifted about 80 nm compared with that of InAs QDs with no InGaAs layer (QD1). This redshift of QD2 is attributed to the increase in dot size due to the diffusion of In from the InGaAs capping layer. The PL decay times of QD1 and QD2 at 10 K are 1.12 and 1.00 ns taken at the PL peak of 1,117 and 1,197 nm, respectively. The reduced decay time of QD2 can be explained by the improved carrier confinement and enhanced wave function overlap due to increased QD size. The PL decay times for both QD1 and QD2 are independent on the emission wavelength, indicating the uniformity of dot size.

1300 nm Wavelength InAs Quantum Dot Photodetector Grown on Silicon

The optical and electrical properties of InAs quantum dots epitaxially grown on a silicon substrate have been investigated to evaluate their potential as both photodiodes and avalanche photodiodes (APDs) operating at a wavelength of 1300 nm. A peak responsivity of 5 mA/W was observed at 1280 nm, with an absorption tail extending beyond 1300 nm, while the dark currents were two orders of magnitude lower than those reported for Ge on Si photodiodes. The diodes exhibited avalanche breakdown at 22 V reverse bias which is probably dominated by impact ionisation occurring in the GaAs and AlGaAs barrier layers. A red shift in the absorption peak of 61.2 meV was measured when the reverse bias was increased from 0 to 22 V, which we attributed to the quantum confined stark effect. This shift also leads to an increase in the responsivity at a fixed wavelength as the bias is increased, yielding a maximum increase in responsivity by a factor of 140 at the wavelength of 1365 nm, illustrating the potential for such a structure to be used as an optical modulator.