Structural investigation of the 2D to 3D transition in stacked submonolayer InAs nanostructures

Roca R.C., Kamiya I.

We report the precise and broadband tuning of the photoluminescence (PL) from a stacked submonolayer (SML) InAs nanostructure across the 2D to 3D transition. We have recently reported the growth of stacked SML InAs nanostructures by molecular beam epitaxy, which leads to the formation of two distinct types of SML nanostructures: 2D islands and 3D structures. In contrast to the well-investigated transition in Stranski–Krastanov (SK) growth of InAs nanostructures, the transition in SML growth is still largely unexplored. Here, the properties of three- and four-stack SML InAs nanostructures are investigated by PL measurements and are interpreted in the context of the transition. At the transition, a characteristic change in the PL is observed, while the controllability of the PL is maintained across the transition. Furthermore, control of the transition itself is possible by changing the SML stack configuration. A brief comparison with the SK transition is also discussed.

Tsuji T., Ozaki N., Yamauchi S., Onoue K., Watanabe E., Ohsato H., Ikeda N., Sugimoto Y., Childs D.T., Hogg R.A.

Abstract In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT.

Roca R.C., Kamiya I.

The cover page shows schematic illustrations (left) and topographic measurement images (right) of the two forms of molecular beam epitaxy (MBE)-grown stacked submonolayer InAs nanostructures: 2D islands (top) and 3D structures (bottom). The significant change in topography is brought about by the transition from 2D growth regime to 3D growth. The topography of the 2D islands is marked by monolayer-high steps forming a terrace-like pattern, whereas that of the 3D structures is dominated by few-nanometer-high mounds surrounded by sunken plateaus. For further details, see article number 2000349 by Ronel Christian Roca and Itaru Kamiya. - - The article is part of the Special Section on “Compound Semiconductors”, comprising 12 articles, guest edited by Mattias Hammar, Anders Hallén and Sebastian Lourdudoss (see the Guest Editorial, article number 2100012).

Maidaniuk Y., Kumar R., Mazur Y.I., Kuchuk A.V., Benamara M., Lytvyn P.M., Salamo G.J.

A nondestructive approach is described that is applicable for studying the In-segregation phenomena in ultra-thin In(Ga)As/GaAs nanostructures grown by molecular beam epitaxy. The proposed method utilizes only the experimental photoluminescence (PL) spectroscopy data and the effective bandgap simulation of specially designed ultra-thin In(Ga)As/GaAs nanostructures. On the example of InAs and In0.25Ga0.75As quantum wells with thicknesses of 1 monolayer (ML) and 4 MLs, respectively, a good correlation for the In segregation coefficient obtained from the proposed method and STEM (scanning transmission electron microscope) Z-contrast cross section imaging is demonstrated. However, PL has a significant advantage over STEM for being a nondestructive, reliable, and rapid technique for measuring multiple samples or large areas. Furthermore, tuning of In segregation in ultra-thin In(Ga)As/GaAs nanostructures, as well as the possibility of modifying and controlling the In depth-distribution profile by the change of growth temperature or the thickness of the low-temperature GaAs capping layer, are additionally demonstrated. A detailed analysis of indium segregation allows the design and precise growth of ultra-thin In(Ga)As/GaAs nanostructures for lasers, solar cells, and infrared photodetectors.

Norman J.C., Mirin R.P., Bowers J.E.

We describe the initial efforts to use molecular beam epitaxy to grow InAs quantum dots on GaAs via the Stranski–Krastanov transition and then discuss the initial efforts to use these quantum dots to demonstrate quantum dot lasers. We discuss the developments in quantum dot lasers over the past 20 years and the future prospects for these lasers for scientific and commercial applications.

Roca R.C., Kamiya I.

Abstract The correlation between the structure, measured by atomic force microscopy (AFM), and luminescence, measured by photoluminescence (PL), of InAs submonolayer stacked (SMLS) nanostructures near the 2D to 3D transition is investigated. Topographic measurements using AFM reveal a significant change in the structure of uncapped InAs SMLS samples occurs under certain conditions. This structural change is attributed to the transition from 2D to 3D growth. Optical measurements by PL of corresponding capped SMLS samples showed a significant change in the luminescence properties, in the form of significant redshift and linewidth broadening, also occurs at the same conditions where the structural change occurred. Therefore, the data in the present work establishes a strong correlation between the structural and luminescence properties of InAs SMLS nanostructures. Furthermore, the results demonstrate that two forms of InAs SMLS, stacked 2D islands and 3D structures, possess distinct properties in terms of both structure and luminescence.

Gajjela R.S., Hendriks A.L., Alzeidan A., Cantalice T.F., Quivy A.A., Koenraad P.M.

Cross-sectional scanning tunneling microscopy (X-STM) was employed to characterize the InAs submonolayer quantum dots (SMLQDs) grown on top of a Si-doped GaAs(001) substrate in the presence of (2X4) and c(4X4) surface reconstructions. Multiple layers were grown under different conditions to study their effects on the formation, morphology and local composition of the SMLQDs. The morphological and compositional variations in SMLQDs were observed by both filled and emptystate imaging. A detailed analysis of indium segregation in the SMLQDs layers was described by fitting local indium concentration profile with a standard segregation model. A strong influence of arsenic flux over the formation of the SMLQDs and indium incorporation was observed and reported. We investigated the well-width fluctuations of the InGaAs quantum well (QW) in which SMLQDs were formed . The monolayer fluctuations of the well width were negligible compared to the more pronounced compositional fluctuations in all the layers. Keywords: Submonolayer quantum dots, Surface reconstruction, X-STM, Indium segregation

Owschimikow N., Herzog B., Lingnau B., Lüdge K., Lenz A., Eisele H., Dähne M., Niermann T., Lehmann M., Schliwa A., Strittmatter A., Pohl U.W.

The cycled depositionCycled deposition of small InAs islands into a GaAs matrix leads to the formation of a tailored rough quantum wellQuantum well containing densely spaced In-rich agglomerationsIn-rich agglomerations referred to as submonolayer quantum dots, which support an efficient excitonExciton formation. Carrier localization properties of the submonolayer structures are further enhanced by alloyingAlloying with antimonyAntimony. In this chapter we address the growth, the structure, and the optical and optoelectronic properties of alloyed and unalloyed InAs submonolayer quantum dots and devices based on these structures. Based on structural and optical characterization, we find densities of localization centers exceeding those of self-assembled quantum dots by an order of magnitude. Submonolayer quantum dots show quantum-dot like ultrafast carrier dynamics, while at the same time providing a significantly larger modal gainModal gain, which reaches values known for InGaAs quantum-well structures. We develop a numerical model for the density of states and relevant scattering channels in the submonolayer potential landscape. Alloyed and unalloyed submonolayer quantum dots differ predominantly in the degree of hole localization, which is dramatically increased by the addition of antimonyAntimony. We show that the alloyed submonolayer quantum dots support a heterodimensionalHeterodimensional confinementConfinement, from fully zero-dimensional to hetero-confinement with zero-dimensionally confined holes and electrons free in two dimensions.

Kumar R., Maidaniuk Y., Saha S.K., Mazur Y.I., Salamo G.J.

InAs quantum dots (QDs) have been grown on a GaAs (001) substrate in the subcritical region of InAs coverage for transition from a 2-dimensional (2D) to a 3-dimensional growth mode. Evolution of QDs and the corresponding wetting layer (WL) with InAs coverage has been investigated. Under specific growth conditions, quantum dot formation was observed only in samples where InAs coverage is more than 1.48 ML. The QD density increases sharply with InAs deposition initially but slows down with increased coverage. Photoluminescence (PL) shows the existence of a third peak, other than QD and WL peaks, at the low energy side of the WL peak, which is named the precursor peak. Evidence is presented supporting the theory that this peak is due to 2D InAs islands on a monolayer of InAs, which are small enough to localize excitons. Meanwhile, the WL peak is due to larger InAs islands under high compressive strain. During QD formation, the WL peak energy increases with the increase in InAs deposition. This is due to the sudden transfer of material from the bigger size of InAs islands to the QD. Our results show that the QD, WL, and precursor peaks coexist near the onset of QD formation. The power dependence of the three PL peaks is evident, which supports to our conclusion.

Das D., Saha J., Panda D., Tongbram B., Raut P.P., Ramavath R., Mondal S., Paul S., Chakrabarti S.

In the present article, we are introducing a novel heterogeneously coupled InAs Submonolayer (SML) on Stranski-Krastanov (SK) quantum dot (QD) heterostructure with reduced strain accumulation. Theoretical comparison manifests the superiority of this hybrid SML-SK QDs over bilayer SK QDs. Photoluminescence and photoluminescence excitation spectroscopy are employed to characterize the electronic interaction and carrier tunneling in between this hybrid quantum dot assembly. Growth rate is optimized at 0.1 ML/sec. To tune the coupling, the barrier layer thickness is varied from 5 nm to 10 nm. Even up to the highest barrier thickness, no signature of SML peak is obtained in the PL response. But, PLE depicts the presence of SML peak, which is partially overlapped with the SK 2nd excited peak. The sample with 7.5 nm GaAs barrier shows perfect resonance between SML ground state and SK 2nd excited state. Based on experimental and analytical results, this SML on SK (SML-SK) configuration is compared with SK on SML (SK-SML) configuration; where SML-SK exhibits better inter dot electronic interaction than its reciprocal configuration. For ex situ tailoring of this heterogeneous electronic coupling, rapid thermal annealing has been done. It shows tunability of inter dot interaction with improved crystalline quality. Finally, we have shown a SML-SK quantum dot infrared photodetector with a broad spectral response (from SWIR to near LWIR).

Cantalice T.F., Alzeidan A., Urahata S.M., Quivy A.A.

Liu W., Yang T., Hsueh W., Chyi J., Huang T., Hsu M.

In this study, multistacked InAs submonolayer (SML) quantum dots (QDs) were sandwiched in an InGaAs/GaAsSb dot-in-a-double-well (DDwell) structure to enhance the crystal quality and optical properties of QDs. The photoluminescence (PL) intensity of the InAs SML QDs with the DDwell structure was 5.5 times higher than that of conventional InAs/GaAs SML QDs because of the reduced number of nonradiative recombination centers and the enhanced carrier hole confinement. The PL results of the DDwell structure exhibit two peaks that represent the carrier overflow from SML QDs to InGaAs quantum wells (QWs) and hence the radiative recombination in InGaAs QWs because of the shallow carrier confinement of SML QDs. Among the compared samples, the DDwell structure exhibited the highest activation energy of 101.8 meV. Furthermore, the carrier thermal escape was suppressed in these InAs SML QDs. High-resolution transmission electron microscopy revealed that the microstructures of the InAs SML QDs demonstrated larger dots for the DDwell structure, thus verifying that the emission wavelength elongated in the PL measurement. These improved optical properties of the InAs SML QDs with the DDwell structure were attributable to the improved crystal quality because of the use of Sb surfactants and additional volume for carrier recombination provided by the InGaAs quantum well. The DDwell structure can thus be applied in optoelectronic devices to obtain advanced performance.

Mukherjee S., Pradhan A., Mukherjee S., Maitra T., Sengupta S., Satpati B., Chakrabarti S., Nayak A., Bhunia S.

In this paper, we present a combined optical and structural study of GaAs-hosted InAs sub-monolayer QD (SML-QD) vertical multi-stacks. The main feature of this paper is to demonstrate the feasibility of sub-monolayer InAs QD with as low coverage as 0.4 ML which shows all the characteristics of QD excitonic emission, emitting in the NIR region (1.496 eV). This emission energy could be precisely tuned successfully by systematically controlling InAs coverage fraction in the range of 0.4–0.8 with corresponding emission in the range of 1.406–1.496 eV. The luminescence efficiency (4 K) exhibited an increasing trend with the decrease in InAs coverage. This paper elaborately discusses the interdependence of structure, strain and emission characteristics through a combined study of high resolution x-ray diffraction, Raman Scattering and Photoluminescence measurement. Strain-induced growth of the dots with different vertical size distribution (height~2.3–1.4 nm) have been explored which were found to depend strongly on the InAs coverage at low temperature. The varying size distribution of the dot ensembles lead to different degree of carrier confinement, capture and localization, as determined from the low temperature (4 K) PL spectra. Significant enhancement of carrier localization inside almost 2D-like exciton was achieved by reducing InAs coverage well below one monolayer. From the relative temperature dependent photoluminescence measurements, it has been shown that the coupling and relaxation pathways of photo-carriers through the SML-QD multi-structure can be controlled by adjusting the InAs coverage.

Alfieri C.G., Waldburger D., Nürnberg J., Golling M., Jaurigue L., Lüdge K., Keller U.

Optically pumped continuous-wave (cw) semiconductor disk lasers (SDLs) have an established commercial impact, and recent progress in ultrashort-pulse operation makes them attractive for applications in frequency metrology. There is, however, a tradeoff between femtosecond pulse lengths and average output power. Quantum-dot (QD) materials could potentially solve this problem---but not just any dots. The authors show that SDLs based on active submonolayer QDs produce record-high cw output power, but without stable mode locking, which is due to fundamental physical reasons.

Kim Y., Kim J.O., Lee S.J., Noh S.K.

Liu W., Yang T., Hsu M., Hsu K., Kondapavuluri B., Chyi J.

Huang T.-., Borrely T., Yang Y.-., Alzeidan A., Jacobsen G.M., Teodoro M.D., Quivy A.A., Goldman R.S.

We have investigated the origins of photoluminescence from quantum dot (QD) layers prepared by alternating depositions of sub-monolayers and a few monolayers of size-mismatched species, termed as sub-monolayer (SML) epitaxy, in comparison with their Stranski–Krastanov (SK) QD counterparts. Using measured nanostructure sizes and local In-compositions from local-electrode atom probe tomography as input into self-consistent Schrödinger–Poisson simulations, we compute the 3D confinement energies, probability densities, and photoluminescence (PL) spectra for both InAs/GaAs SML- and SK-QD layers. A comparison of the computed and measured PL spectra suggests one-dimensional electron confinement, with significant 3D hole localization in the SML-QD layers that contribute to their enhanced PL efficiency in comparison to their SK-QD counterparts.

Roca R.C., Kamiya I.

Zhao Z., Zhao H., Du X., Zhang X.

Seawater is one of the most abundant sources of hydrogen on Earth, so its decomposition for producing hydrogen has a significant advantage. However, because of the presence of high concentration of chloride ion (Cl−) in seawater, it has corrosive effect on the catalyst. Therefore, the exploration and construction of highly robust anticorrosive OER electrocatalyst were studied. Furthermore, this work describes Co9S8@Ni3S2/NF as a catalyst, which provides a current density of 100 mA cm−2 with a required overpotential of only 154 mV, with emphasis on their remarkable OER selectivity. The experimental analysis demonstrate that the promotion of Co9S8@Ni3S2/NF catalyst activity can be attributed to the construction of heterogeneous interface, the rapid transfer of charge and the exposure of more reaction centers. Density functional theory analysis demonstrates that the Co9S8 material presents greater water adsorption energy, indicating that water preferentially adsorbs on its surface. The introduction of the Ni3S2 electrode greatly promotes the synergistic effect of the Co9S8@Ni3S2/NF electrode. It is the synergistic catalysis of these two electrodes that enhances the catalytic activity and selectivity of the materials. The work furnishes a novel idea for the exploration of efficient, environmentally friendly and highly selective electrodes for seawater splitting.

Mohammadi H., Roca R.C., Zhang Y., Lee H., Ohshita Y., Iwata N., Kamiya I.

Passivation capping of molecular beam epitaxy (MBE)-grown InAs surface quantum dots (SQDs) is achieved by ex situ atomic layer deposition (ALD)-grown ZnO using diethylzinc (DEZ) as the zinc precursor, the main passivation agent, and oxygen plasma. Photoluminescence (PL) intensity is enhanced by 2-fold as the DEZ/ZnO passivation cap thickness reached 30 nm. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) show that contrary to conventional wet chemistry passivation or MBE capping, in which InAs SQDs can be damaged or be shrunken by intermixing of In and Ga, DEZ/ZnO passivation capping almost preserves the shape of the underlying SQDs. The cross-sectional analysis with transmission electron microscopy (TEM) reveals the formation of the ZnO grains on top of the sample accompanied by slight SQD height and width reduction, which are presumably related to the removal of the InAs native oxides. The DEZ “self-clean-up” passivation mechanism, where zinc precursor is responsible for reduction of the surface non-radiative recombination sites, is studied by X-ray photoelectron spectroscopy (XPS). The factors that control the DEZ “self-clean-up” efficiency such as the chain of chemical reactions, steric hindrance, decomposition activation energy, Gibbs reactivity, or Lewis acidity, are evaluated. The results are discussed in comparison with trimethylaluminum (TMA), a precursor used for Al2O3 deposition. We find that the “self-cleaning” by DEZ and TMA occurs through processes of different chemical nature.

Christian Roca R., Kamiya I.

The 2D to 3D growth transition of stacked submonolayer (SML) InAs/GaAs nanostructures as a function of the number of SML InAs stacks is investigated by atomic force microscopy (AFM) measurements. It is found that critical amount of InAs per cycle decreases as the number of stacks increases. These results are analyzed in context of the balance between the average In content and the total deposited thickness in the SML stack.