Evolution of InAs quantum dots and wetting layer on GaAs (001): Peculiar photoluminescence near onset of 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

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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 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 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. Epiready, semi-insulating, observed for samples where InAs coverage was less than or equal quarter of 200 singular GaAs (001) ± 0.1° substrates, cleaved along to 1.48 ML. For 1.49 ML of InAs coverage, an ultralow QD

FIG. 2. RHEED pattern along [1−10] (a) after InAs deposition and (b) after postgrowth annealing.

J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-2 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap

FIG. 3. InAs coverage vs AFM topographical image of the uncapped QD layer. Sample of InAs coverage 1.47 ML has been taken on a 5 × 5 μm2 area, whereas other images are taken on a 2 × 2 μm2 area.

density was observed. Dots with average diameter and height, The QDD with InAs coverage is also plotted in Fig. 4.As 16 nm and 4 nm, respectively, along with tiny dots (average height expected, near the onset of dot formation, the QDD shows a rapid ∼1.2 nm) were observed for 1.49 ML InAs coverage. For 1.5 ML increase with very small additional InAs coverage. However, above InAs coverage, a more uniform QD size distribution is seen in the the onset of dot formation, for example, at 1.52 ML InAs coverage, AFM image. Meanwhile, for 1.52 ML InAs coverage, a broad dot the QDD increases very slowly with further InAs deposition; size distribution of dots was observed. In all cases, dot formation rather, the additional material appears as larger dots due to the shows a preference at the steps of the GaAs mounds as expected ripening and further growth of the dots. based on lower adatom chemical potential (thermodynamic Another telling characteristic of growth as a function of depo- explanation) or higher diffusion barrier for adatoms (kinetic sition is PL. The PL of the QD samples was measured at 10 K and – explanation).18 20 With a further increase in the InAs coverage, at an InAs coverage that was close to the onset of QD formation the spatial distribution of dots was observed to become more [Fig. 5(a)]. These samples show an asymmetric WL peak with an uniform. extended tail toward the low energy side. While this feature was

J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-3 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap

– peaks in between the QD and WL have been reported.23 25 In this case, the origin of the peak nearest the WL has been attributed to the InAs islands and called the precursor of the QD in the model of Priester and Lannoo26 These are self-assembled islands that are assumed to arise from some floating features,27 2D platelets,26 and quasi-3D islands.28 Here, we speculate that the PL peak on the low energy side of the WL peak is due to the presence of 2D islands of InAs, on a monolayer of InAs, which are small enough to localize excitons. Meanwhile, the WL peak is due to larger InAs islands with high compressive strain existing just before transition to QDs. To examine these two PL peaks further, consider the three low InAs coverage samples (S1.48, S1.49, and S.150). The PL double peaked spectra [Fig. 5(a)] are deconvoluted into two Gaussian com- ponents (in addition to the QD peak). Of these two PL peaks, the PL peak at the higher energy is named WL, while the second PL peak is named the precursor. The PL spectra of S1.52 and S1.56 are shown in Fig. 5(b). These PL results show a considerable shift in the transition energy of the QD, WL, and precursor peaks with FIG. 4. Evolution of QDD with InAs coverage. InAs coverage. For example, the QD, WL, and precursor peak positions, with InAs coverage, are shown in Fig. 6. The WL peak shows a decrease in the transition energy with an increase in InAs coverage except true for all InAs coverage up to sample S1.52, the asymmetry in near the onset of QD formation [shown by the dashed red circle in samples having InAs coverage close to the onset of QD formation Fig. 6(a)]. Near the onset of QD formation, the WL peak energy (S1.48, S1.49, and S1.50) was more pronounced. Interestingly, increases with an increase in InAs coverage. This can be under- sample S1.50 shows a clear signature of a 2nd peak near the WL stood by the sudden transfer of some material from the WL to the peak. PL peaks at the low energy side of the WL peak have been formation of QDs. This shift also reveals an important point: the reported21,22 by others and attributed to indium composition fluc- behavior of the PL from the WL, with increasing deposition, is at tuations in the WL, which resulted in 3D localization of excitons. least partially connected to the size and density of islands on the They named these features as natural QDs. In fact, even multiple surface. This is further supported by the fact that the features of

FIG. 5. (a) PL spectra of samples having 1.48, 1.49, and 1.5 ML InAs coverage and (b) PL spectra of S1.52 and S1.56.

J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-4 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap

FIG. 6. (a) WL transition energy with InAs coverage. The inset shows the WL2 peak with InAs coverage. (b) QD peak position with InAs coverage (the line connecting the symbols are just a guide to eyes).

InAs WL spectra, which are responsible for the asymmetric low as expected due to the observed increase in the size of InAs QD energy tail of the WL peak, disappear as the transition of islands with coverage and lower density. For yet even higher InAs coverage, into QDs is completed. These observations are in contrast to the the PL from the WL peak disappears from the PL spectra, indicat- understanding of the thermodynamic modeling of the SK growth ing that the optically excited carriers in WL relax in QD. mode where WL thickness is expected to be constant during QD Our understanding of the evolution of the QDs, WL, and pre- formation.29 Moreover, the behavior of the PL peak on the low cursor are also supported by the PL dependence of the intensity of energy side of the WL peak contradicts earlier work speculating the exciting optical laser source. Excitation power (P) dependent that In and Ga intermixing was responsible for the PL shift to PL spectra of the S1.50 sample is shown in Fig. 7(a). The linewidth lower energy with increasing submonolayer growth of InAs.30 and transition energy of the QD peak increase with the increasing Material transfer during from the WL to QD has also been P. This can be attributed to a larger portion of filled QD states at reported in the literature and explained by WL step erosion or higher P. The blue shift of WL and precursor peaks with increasing transfer of floating InAs on the surface to QD.31,32 However, P can be attributed to band-filling of high energy localized centers neither WL step erosion nor floating InAs explain the presence of a resulting from the inhomogeneous distribution of the indium precursor peak. Additionally, step erosion has been reported over within WL, which induces a blue shift of the emission energy. the large deposition range of InAs (1.54 ML–2.4 ML),31 but the Inhomogeneous indium distribution and consequently natural QD blue shift in the WL peak is observed only at the onset of QD for- formation in the InAs WL have previously been reported.21,22 The mation, which is, at maximum, within the 0.03 ML deposition extent of the blue shift in the precursor peak is higher than the WL range of InAs. peak. This might be related to the size distribution of 2D islands The behavior of the precursor peak is also consistent with this where with an increase in the P smaller island also participates understanding [the inset of Fig. 6(a)]. Here again, there is a shift to in PL. Even at low P, these three peaks are present. This lower energy for the precursor peak with the increase in InAs result indicates that the precursor peak is not related to the excited coverage, indicating an increase in the size of the 2D islands with state of QD. The integrated PL intensity of these peaks can be an increase in InAs coverage. In this case, however, there is no described by I ¼ α Â Pγ, where I is the integrated PL intensity, P is sudden increase in the PL energy from the precursor at the the excitation power, and α and γ are fitting parameters, which transition to QDs. These precursors are isolated islands and behave describe the power law.33,34 We find the slope γ to be approxi- much like QDs (which will be shown when discussing power mately unity in the case of QD and the precursor, which is dependent PL data). These islands are not large or strained enough consistent with the contributing mechanism to PL spectra as corre- to transition to QDs as yet. The PL spectra of S1.52 and S1.56, lated electron hole recombination for both QD and precursor however, do suggest that further increase in the InAs coverage peaks. The higher value (γ . 1) for the WL peak is also consistent results in dominance of the PL from WL and the QDs as might be with less localized carriers or less correlated electron hole recombi- expected due to ripening of islands and QDs. In fact, the QD PL nation. Variation of FWHM of WL and the precursor peak with peak energy monotonically decreases with an increase in coverage P is negligible.

J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-5 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap

FIG. 7. (a). Power dependent PL of the sample having 1.5 ML InAs coverage, excitation power vs (b) intensity, (c) peak position, and (d) linewidth as FWHM of different peaks in the spectra of a 1.5 ML InAs coverage sample.

CONCLUSION The precursor peak shows no such trend with InAs deposition, which indicates that the size of these islands is too small to take Inconclusion,wereportedontheevolutionoftheInAsQD part in QD formation. The power dependent PL demonstrates and WL near the onset of QD formation, purposefully using a thelocalizednatureofcarriersfortheprecursorpeak,whichisin very low growth rate of InAs. At this low growth rate, we agreement with the assumed small size of islands, whereas the observed a third PL peak (precursor peak) in addition to the WL WL PL peak shows a lower localized nature of carriers consistent and QD peaks, located at the low energy side of the WL peak. with the assumed bigger size of InAs islands. The precursor is found to be due to the existence of InAs 2D islands on 1ML InAs, which are small in size and localize the ACKNOWLEDGMENTS electron–hole pair. Meanwhile, the WL is formed from bigger InAs islands, which are under a high compressive strain. The The authors acknowledge the financial support of the observed increase in the PL peak energy from the WL with Institute of Nanoscale Science and Engineering, University of increased InAs deposition is found to be due to the sudden trans- Arkansas, and the National Science Foundation (NSF) Grant No. fer of material from WL to QDs near the onset of QD formation. 1809054.

J. Appl. Phys. 127, 065306 (2020); doi: 10.1063/1.5139400 127, 065306-6 Published under license by AIP Publishing. Journal of Applied Physics ARTICLE scitation.org/journal/jap

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