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Evolution of Inas Quantum Dots and Wetting Layer on Gaas (001): Peculiar Photoluminescence Near Onset of Quantum Dot Formation

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Evolution of Inas Quantum Dots and Wetting Layer on Gaas (001): Peculiar Photoluminescence Near Onset of Quantum Dot Formation Evolution of InAs quantum dots and wetting layer on GaAs (001): Peculiar photoluminescence near onset of quantum dot formation Cite as: J. Appl. Phys. 127, 065306 (2020); https://doi.org/10.1063/1.5139400 Submitted: 19 November 2019 . Accepted: 25 January 2020 . Published Online: 11 February 2020 Rahul Kumar , Yurii Maidaniuk, Samir K. Saha, Yuriy I. Mazur , and Gregory J. Salamo ARTICLES YOU MAY BE INTERESTED IN Advanced Thermoelectrics Journal of Applied Physics 127, 060401 (2020); https://doi.org/10.1063/1.5144998 Mid-infrared electroluminescence from type-II In(Ga)Sb quantum dots Applied Physics Letters 116, 061103 (2020); https://doi.org/10.1063/1.5134808 Excitation intensity and thickness dependent emission mechanism from an ultrathin InAs layer in GaAs matrix Journal of Applied Physics 124, 235303 (2018); https://doi.org/10.1063/1.5053412 J. Appl. Phys. 127, 065306 (2020); https://doi.org/10.1063/1.5139400 127, 065306 © 2020 Author(s). Journal of Applied Physics ARTICLE scitation.org/journal/jap Evolution of InAs quantum dots and wetting layer on GaAs (001): Peculiar photoluminescence near onset of quantum dot formation Cite as: J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 Submitted: 19 November 2019 · Accepted: 25 January 2020 · View Online Export Citation CrossMark Published Online: 11 February 2020 Rahul Kumar,1,2,a) Yurii Maidaniuk,1 Samir K. Saha,1 Yuriy I. Mazur,1 and Gregory J. Salamo1,2 AFFILIATIONS 1Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA 2Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA a)Author to whom correspondence should be addressed: [email protected] ABSTRACT 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 forma- tion, 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. Published under license by AIP Publishing. https://doi.org/10.1063/1.5139400 INTRODUCTION photon sources.7,8 To achieve a low InAs QD density (QDD) on a GaAs substrate, various methods have been utilized, such as Self-organized quantum dots (QDs) are a topic of continuing 9,10 interest since they have broad potential for applications and droplet epitaxy at low temperatures, in which metal droplets – provide an opportunity to study fundamental physics.1 3 While first form on the substrate and then are crystalized using arsenic several methods for the growth of QDs have been investigated, vapor. Another growth method is based on intentionally creating a – temperature gradient across the substrate surface, which produces a growth by self-assembly via the Stransky Krastanov (SK) method is 9 perhaps the most utilized. As an example of this method, consider different critical thickness across the sample. Yet, another tech- the growth of InAs on GaAs, which has about a 7% lattice mis- nique utilizes the natural flux inhomogeneity and corresponding match between them. In this case, island growth of InAs on a GaAs growth rate across the substrate surface to produce a variation in 2 substrate takes place layer-by-layer, until a critical thickness is thickness across the surface. In this work, to achieve a varying QD reached, for which strain relaxation can pay for the added surface density across the substrate, we cause a flux inhomogeneity across energy in forming a dotlike structure on the surface. Using this the substrate by tilting the growth stage from its optimum growth growth method, self-assembled QD size, density, shape, etc., can be position. By this method, we achieve a varying QD density from controlled and tuned by varying the growth parameters and sub- the order of 1010 QDs/cm2 down to regions on the surface where – strate orientation to suit specific applications.4 6 For example, a low we observe that no QDs have been formed [Fig. 1(a)]. density of QDs can be selected for QD based single electron tran- We achieve this low density on a GaAs on a nominal singular sistors, sources for entangled photons, and for fabricating single GaAs (001) substrate “mounded” surface where the mounds of J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-1 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap 〈110〉 directions, have been used. A 0.5 μm homoepitaxial GaAs buffer layer has been grown at 580 °C after the substrate native oxide is desorbed. This is followed by two periods of InAs QDs (separated by 100 nm GaAs spacer) grown at 460 °C and with a 0.005 ML/s growth rate. For the InAs deposition, the substrate is not rotated and the growth stage is tilted by 5° from its optimum position. The initial 10 nm of the GaAs spacer was grown at the same temperature of the QD growth (460 °C), while the remaining 90 nm was grown at 580 °C. The top InAs QD layer was kept uncapped for topographical measurements using an atomic force microscope (AFM). The back- FIG. 1. (a) Grown QDs have gradients in their density across the wafer. (b) ground As4 pressure during the entire growth was held at − Schematic of the grown structures. 3×10 6 Torr. After each layer of InAs QD growth, 60 s of postgrowth annealing was performed. The growth temperature was quenched after postgrowth annealing of the 2nd QD layer. RHEED was used GaAs are elongated along the [1−10] direction.11,12 Our objective throughout the growth for real time monitoring of the growth  is to explore the evolution of the topographical and optical prop- surface. For example, a well-defined 2 4 RHEED pattern was  erties of InAs QDs and the corresponding wetting layer (WL) on observed during GaAs growth at 580 °C, while a clear C (4 4) the GaAs surface. For this investigation, InAs coverage has been pattern was observed during GaAs growth at 460 °C. kept below but close to the critical coverage needed for the 2D to Figure 2 shows the typical RHEED pattern before and after 3D transition of InAs [as detected by reflection high energy postgrowth annealing. The RHEED pattern at the end of InAs electron deflection (RHEED)]. Similar studies of QD formation in deposition shows only the primary streaks. It does not show any the critical region of the InAs coverage have been reported13,14 by superstructure reflection or any signature of dot formation. others and explained in terms of the metastability of the WL.13,15 Keeping the sample at the same temperature of InAs deposition However, the formation of InAs QDs and the correlation to the under As4 flow, however, changes the morphology significantly. evolution of InAs WL are not clear.16 In fact, to our knowledge, After postgrowth annealing, the RHEED pattern transitions to a there is no complete and detailed description of the evolution bulk diffraction pattern, indicating 3D dot formation. This shows of the QD and its correlation to the evolution of the WL at the material transfer from metastable InAs WL to form QD during the critical 2D to 3D transition region of InAs coverage.17 In this postgrowth annealing of InAs. paper, we report on precisely this correlation. In contrast to After removing the sample from the growth chamber, surface the concept of a constant WL thickness with InAs coverage at and morphology studies were performed using a Veeco Nanoscope after QD formation, we observe large shifts in the photolumines- AFM. Meanwhile, photoluminescence (PL) spectra were measured cence (PL) peak energy from the WL even after the onset of for InAs QDs within the GaAs matrix using a continuous-wave QD formation. diode-pumped solid-state 532 nm excitation laser and a TE-cooled silicon photodetector. Low-temperature PL measurements were performed in a closed-cycle He cryostat at 10 K. EXPERIMENTAL PROCEDURE Samples studied in this work have two QD layers, separated RESULTS AND DISCUSSION by a 100 nm GaAs spacer layer. A schematic diagram of the grown samples is shown in Fig. 1(b). Samples are named based on the Using AFM, we captured dot formation at different deposi- InAs coverage; e.g., a sample where 1.52 ML InAs was deposited tion thicknesses of InAs. For example, AFM topographical images has been named S1.52. of the samples with varying InAs coverage are shown in Fig. 3. All samples were grown using a solid source, RIBER32 molecu- Thesesurfacesshowmoundsofheight1–2 nm elongated along lar beam epitaxy (MBE) ultrahigh vacuum (UHV) chamber equipped the [110] direction as expected.11,12 No dot formation was with an arsenic valve cracker source.
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