Journal of Physics D: Applied Physics

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Optical characterization of type-I to type-II bandalignment transition in GaAs/Al x Ga1−x Asquantum rings grown by droplet epitaxyTo cite this article: Linlin Su et al 2017 J. Phys. D: Appl. Phys. 50 32LT01

 

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Journal of Physics D: Applied Physics

L Su et al

Printed in the UK

32LT01

JPAPBE

© 2017 IOP Publishing Ltd

50

J. Phys. D: Appl. Phys.

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10.1088/1361-6463/aa7b04

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Journal of Physics D: Applied Physics

1. Introduction

Semiconductor nanostructures grown via droplet epitaxy (DE) have recently attracted much attention, as the DE growth mode enables flexible control of the geometry, dimension, density, and position of nanostructures, subsequently allowing for the tailoring of both their optical and electrical properties

[1–5]. Versatile well-defined nanostructures of high quality have been achieved by DE growth, including quantum dots (QDs), QD-pairs, QD-clusters, quantum holes, and quantum rings (QRs) [6–10].

The DE growth was initially proposed for the strain-free GaAs/AlGaAs material system [2]. To date, many GaAs/AlGaAs nanostructures, in particular QDs and QRs with

Optical characterization of type-I to type-II band alignment transition in GaAs/AlxGa1−xAs quantum rings grown by droplet epitaxy

Linlin Su1, Ying Wang1, Qinglin Guo1, Xiaowei Li1, Shufang Wang1, Guangsheng Fu1, Yuriy I Mazur2, Morgan E Ware2, Gregory J Salamo2, Baolai Liang3 and Diana L Huffaker3

1 College of Physics Science and Technology, Hebei University, Baoding 071002, People’s Republic of China2 Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, AR 72701, United States of America3 California NanoSystem Institute, University of California, Los Angeles, CA 90095, United States of America

E-mail: hbuwangying@126.com and liangbaolai@gmail.com

Received 22 March 2017, revised 7 June 2017Accepted for publication 22 June 2017Published 20 July 2017

AbstractOptical properties of GaAs/AlxGa1−xAs quantum rings (QRs) grown on GaAs (1 0 0) by droplet epitaxy have been investigated as a function of the Al-composition in the AlxGa1−xAs barrier. A transition from type-I to type-II band alignment is observed for the QRs via photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. While x ⩽ 0.45, the QR PL spectra show a blue-shift and an increasing intensity with increasing Al-composition, revealing the enhancement of quantum confinement in the QRs with type-I band alignment. While x ⩾ 0.60, the characteristic large blue-shift with excitation intensity and the much longer lifetime indicate the realization of a type-II band alignment. Due to the height fluctuation of QR structures grown by droplet epitaxy mode, it is not the large blue-shift of emission energy, but the long lifetime that becomes the more important feature to identify the type-II band alignment.

Keywords: quantum rings, droplet epitaxy, band alignment, photoluminescence, carrier lifetime

(Some figures may appear in colour only in the online journal)

Letter

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https://doi.org/10.1088/1361-6463/aa7b04J. Phys. D: Appl. Phys. 50 (2017) 32LT01 (7pp)

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controllable size and geometry, have been obtained via DE. For example, Watanabe, et  al achieved type-II GaAs/Al0.85Ga0.15As QDs grown by DE, which have a long lifetime of >10 ns [8]. Jiang et  al obtained multiple stacked GaAs/AlGaAs QD nanostructures for photodetectors and photovol-taics by vertically correlated DE growth [5, 10]. Mano et al reported that the geometry transition from dot to ring can be implemented by tailoring the substrate temperature and changing the As4 flux, which changes the balance between crystallization at the core and at the edge of the droplets [3]. This kind of GaAs/AlGaAs quantum structures [11–17] have been widely investigated due to the characteristics of persis-tent current and Aharonov–Bohm effect [18–20].

The GaAs/AlGaAs QD and QR structures grown by DE can also obtain a type-II band alignment with holes confined in the GaAs quantum structures but electrons confined in the AlGaAs barrier, which has promising advantages for opto-electronic device applications. For example, the absence of dot–dot coupling through a wetting layer prevents the estab-lishment of common quasi-equilibrium in the QD ensemble, resulting in a negligible temperature-dependent property so that the PL emission signal is clearly observed even at room temperature [21]. The negligible temperature dependence is technologically ideal for applications in both lasers and mem-ories. In this paper we have studied optical properties of GaAs QRs in AlxGa1−xAs barriers with varied Al-composition. The type-I to type-II band alignment transition is investigated for the QRs by both photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements.

2. Experiments

The samples were grown by DE on semi-insulating GaAs (1 0 0) substrates in a solid source molecular beam epitaxy (MBE) reactor. First, a 150 nm GaAs buffer and a 100 nm AlxGa1−xAs (x = 0.20, 0.25, 0.35, 0.45, 0.60, 0.65, and 0.70) layer were grown at 580 °C. Then, the substrate temper ature was low-ered to 500 °C and the As2 flux was turned off. After the MBE chamber background pressure was pumped to <8 × 10−10 Torr, Ga atoms equivalent to form six-mono layer (ML) GaAs were deposited to obtained Ga droplets, followed by a 60 s growth interruption with no As2 flux. After that, the surface was exposed to the As2 molecular beam (As2 beam equivalent pressure = 2 × 10−6 Torr) to turn the Ga droplets into well-defined GaAs QRs. The GaAs QR layer was initially capped by 10 nm AlxGa1−xAs at 500 °C and then 90 nm AlxGa1−xAs at 580 °C. Finally, a 3 nm GaAs capping layer was grown to protect the sample surface. For morph ology study, several uncapped GaAs QR samples were also grown using the same conditions mentioned above. Atomic force microscope (AFM) measurements were implemented immediately after removing these samples from the MBE growth chamber.

The capped QR samples were characterized by PL and TRPL measurements to investigate the optical properties of GaAs QRs. The samples were mounted in a closed-cycle cry-ostat with temperature variable from 10 K to 300 K. For PL measurements, the samples were excited by a continuous-wave

semiconductor laser with a wavelength of 532 nm. The PL signal was detected by a liquid nitrogen cooled CCD detec-tor array attached to an Acton spectrometer. For TRPL measurement, the QR samples were excited by a NKT super- continuum laser and the TRPL signals were measured by a PicoHarp 300 time-correlated-single-photon-counting (TCSPC) system with the overall system resolution of ~50 ps.

3. Results and discussion

Figures 1(a) and (b) show 2 µm × 2 µm AFM images of the uncapped GaAs QRs grown on a reference GaAs surface and an Al0.65Ga0.35As surface, respectively. In order to clearly see the morphologic features, each inset presents a 3D projection of individual QRs. Figures 1(c) and (d) give the cross-sectional height profile and height fluctuation from a representative QR, as indicated by the green lines in the inset plots. It can be seen that, after the growth, every Ga droplet has turned into a hybrid nanostructure with a hole in the center with a ring-like structure surrounded it. The formation of such deep holes during DE growth has been attributed to the nano drill effect under Gallium-rich conditions [22]. The DE growth dynamic for QRs is complicated, and it combines several processes, including droplet ripening, As desorption, droplet crystalli-zation, GaAs diffusion, and Al–Ga intermixing [23, 24]. We suppose that the formation of GaAs QRs is mainly due to the migration of Ga atoms during the crystallization of the Ga droplets under a suitable temperature and arsenic flux [3].

The QRs grown on the GaAs surface have an average top diameter of ~37 nm, an outside diameter of ~63 nm, an areal density of ~4.1 × 109 cm−2, and an average height of ~1.25 nm. Similarly, the QRs grown on an Al0.65Ga0.35As sur-face have an average top diameter of ~36 nm, an average out-side diameter of ~61 nm, an areal density of ~4.0 × 109 cm−2, and an average height of 1.21 nm. In the present study, the diameter and height of QRs grown on the AlxGa1−xAs sur-faces with different Al-composition are found to be very sim-ilar. An important characteristic of all the rings grown, shown by the plots in figure 1 is that each individual QR is not in a perfectly formed toroidal shape, but has fluctuations in height around the ring. Figures  1(c) and (d) show that the height fluctuation of the individual QR grown on GaAs surface and Al0.65Ga0.35As surface to be ~0.52 nm and ~0.85 nm, respec-tively. As will be demonstrated through PL measurements, this QR height fluctuation can lead to the variation of the quantum confined energy levels and subsequently a unique state filling effect for the QRs. Furthermore, no large incoher-ent islands or threading dislocations are observed on the sur-face, indicating good crystal quality for both the AlxGa1−xAs barrier layer and the GaAs QRs. Also, large terraces concen-tric with each GaAs/Al0.65Ga0.35As QR are observed (figure 1(b)), which are similar to the result of Al droplets on a GaAs surface. Due to the small distance between neighboring QRs, some of the outer terraces overlap resulting in an elliptical shape. However, these terraces are not found from the QRs grown on the GaAs surface. It has been demonstrated that the finite terraces are the result of reduced mobility of the

J. Phys. D: Appl. Phys. 50 (2017) 32LT01

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Ga adatoms on AlGaAs surface during the As crystallization process [23–25].

The PL spectra of the GaAs/AlxGa1−xAs QRs meas-ured with a low laser excitation intensity of 0.03 W cm−2 at T = 10 K are plotted in figure 2(a). For convenience each spec-trum is normalized and shifted up. The PL peak wavelengths, which are extracted in figure 2(b), are strongly dependent on the Al-composition. With increasing Al-composition from x = 0.20 to x = 0.70, the PL peak wavelength monotonically shifts from 759.7 nm (1.631 eV) to 693.3 nm (1.788 eV). As shown in figure 2(c), the PL integrated intensity also increases monotonically with increasing Al-composition from x = 0.20 until it reaches a maximum at x = 0.45. As the height of QRs has almost no change with the variation of Al-composition, the increased PL intensity and blue-shift are attributed to the enlarged quantum-confinement due to the higher AlGaAs bar-rier potential as shown in figure 2(d). With further increase in the Al-composition, the PL intensity decreases. Here we assume that the GaAs QRs have the confined ground energy level of electrons above the X-valley in the AlGaAs barrier as illustrated in figure 2(e). This results in the electrons relax-ing into the AlGaAs barrier, while the holes are still confined inside the GaAs QRs, ultimately realizing a type-II band

alignment, giving a much lower optical emission intensity from the QR structures [26, 27].

Figures 3(a) and (b) show the laser excitation intensity dependent PL spectra of the QRs at 10 K from samples with x = 0.2 and x = 0.65, respectively. Correspondingly, the PL integrated intensity and peak energy are extracted as functions of the laser excitation intensity in figures 3(c) and (d), respec-tively. The integrated PL intensity (IPL) is generally described by Ipl = ηPα, where P is the laser excitation intensity, and α and η are fitting coefficients. By fitting the experiment data, we obtain the coefficient α to be approximately 1.65 and 1.46 for these two samples, indicating that in addition to exciton recombination, there is a significant contribution from defects recombination and carrier transfer to the optical emission [28]. Additionally, for the GaAs/Al0.2Ga0.8As QRs in figure  3(a), the ground state peak is observed at ~760 nm (1.631 eV) under low excitation intensity, and an excited state evolves at ~731 nm (1.696 eV) as the excitation intensity increases above 30 W cm−2.

As shown in figure 3(d), the ground state PL peak from the GaAs/Al0.2Ga0.8As QRs exhibits a blue-shift of 13 meV when the excitation intensity increases from 0.003 W cm−2 to 3300 W cm−2. Very interesting, this blue-shift starts from

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Figure 1. The 2 µm × 2 µm AFM images show GaAs QRs grown on a (a) GaAs surface and an (b) Al0.65Ga0.35As surface. The cross-section profile and height fluctuation of individual QR grown on (c) GaAs surface and (d) Al0.65Ga0.35As surface.

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very low laser excitation intensity. The PL spectra of GaAs QRs on samples with x = 0.25 and 0.35 have similar charac-teristics, although their PL spectra are not shown here. The GaAs/Al0.2Ga0.8As QRs, which have type-I band alignment, generally should not have a band-bending-caused blue-shift for PL emission with increasing excitation density. As we mentioned earlier, this blue-shift feature is likely due to

the band filling effect caused by the height fluctuations of the QRs. These fluctuations lead to modulations in the con-finement potential. At low temperatures the local potential minimum traps excitons preferentially at very low excitation powers. With increasing laser intensity, these lowest energy levels begin to saturate, therefore increasing the probability of recombination from higher energy levels, resulting in the observed blue-shift for PL emission from the type-I GaAs/AlxGa1−xAs QRs.

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Figure 2. (a) Normalized PL spectra of GaAs/AlxGa1−xAs QRs measured at T = 10 K with the laser excitation intensity of 0.03 W cm−2, (b) PL peak wavelength as a function of the Al-composition, x, for the AlxGa1−xAs barrier, (c) PL integrated intensity as a function of x, (d) as x increases, the schematic of band alignment changes for an AlxGa1−xAs barrier with a direct bandgap (x < 0.45) and (e) band alignment transition from type-I to type-II for an AlxGa1−xAs barrier with an indirect bandgap (x > 0.45). In (d) and (e) solid lines represent the local Γ valleys while dotted lines represent the local X valleys.

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In figure 3(b) the PL spectra of GaAs/Al0.65Ga0.35As QRs also exhibits a blue-shift from 707 nm (1.753 eV) to 702 nm (1.767 eV) as the laser intensity increases from 0.003 W cm−2 to 3300 W cm−2. A large blue-shift is a signature fea-ture for GaAs/AlxGa1−xAs nanostructures with a type-II band alignment and it is generally interpreted by the concept of band-bending caused by the build-up of a static electric field [29, 30]. However, we already observe that the GaAs/Al0.2Ga0.8As QRs with a type-I band alignment also exhibit a similar blue-shift due to band-filling effect. Therefore, the blue-shift for GaAs/Al0.65Ga0.35As QRs is possible of a type-II band alignment effect or a band-filling effect due to QR height fluctuation, or a combination of both. In other words, the blue-shift is not an effective characteristic any more for judging a type-II band alignment for the GaAs/AlxGa1−xAs QR structures grown by DE.

Another feature for GaAs/AlxGa1−xAs nanostructures of type-II band alignment is the longer carrier lifetime due to the spatial separation of carriers. As a consequence of the small wave function overlap between electrons and holes, the prob-ability of electron–hole recombination for type-II structures reduces strongly in comparison with their type-I counterparts. Both the low carrier recombination probability and the strong hole confinement induces a long carrier lifetime for GaAs/AlxGa1−xAs nanostructures with the type-II band alignment. To further support the transition from type-I to type-II for GaAs/AlxGa1−xAs QRs with different Al-composition, the PL decay behavior is detected at the peak energy for each sam-ple with a laser excitation intensity of 3.2 W cm−2. As shown in figure 4(a), the PL decay curve of GaAs/Al0.2Ga0.8As QRs displays a mono-exponential decay behavior with a lifetime of 0.299 ns, which is typical for GaAs/AlGaAs quantum struc-tures of type-I band alignment. Similar behavior is observed for the x = 0.25, 0.35, 0.45 QR samples, with estimated lifetimes of 0.330 ns, 0.366 ns, and 0.428 ns, respectively. However, for QR samples with Al-composition of, x = 0.6 and 0.7, the decay curves are fitted well by double-exponential decay with a long lifetime of about 5.317 ns, and 6.742 ns, respectively. These are more than one order of magnitude longer than the lifetime for QRs samples with Al-composition x ⩽ 0.45.

Figure 4(b) shows the initial stage of the PL decay curves. The rise time for each GaAs/AlxGa1−xAs QR sample is depicted in the inset. Clearly, the rise time increases with increasing the Al-composition. The PL rise time generally reflects the carrier generation and relaxation dynamics inside the quantum con-fined nanostructures. For the QR samples (x = 0.20, x = 0.25, and x = 0.35) with type-I band alignment, the rise time is measured to be ~75 ps, which is approximately the resolution limit of the system. However, the rise time of the QR samples (x = 0.60, 0.65 and x = 0.70) with type-II band alignment increases to between ~140 and ~200 ps, which is much longer than that of the three QR samples with type-I band alignment. The much longer rise time is also attributed to the separa-tion of the electrons and holes in the type-II structures. In our TRPL measurements, the electron and hole pairs are mainly created in the AlGaAs barrier initially, then the electrons relax into the X-valley potential minimum of AlGaAs barrier, while the holes are captured by QRs and subsequently relax to the

ground states. The TRPL data in figure 4 indicate that, in our type-II QR structures, the electrons are relaxed to potential minimum in ~200 ps, but subsequently recombine in a time-scale of several nanoseconds.

The inset plot in figure  4(b) indicates that when the Al-composition is lower, the change of rise time is slower. A sudden increase occurs as the Al-composition becomes greater than 45%. According to theoretical calculations and experimental observations [31], the crossover point where AlxGa1−xAs changes from direct bandgap to indirect bandgap happens at x ≌ 45%. The clear increase of the rise time for GaAs/Al0.45Ga0.65As QRs indicates that the AlGaAs barrier probably has become indirect, but the QRs are approaching the transition point between a type-I and type-II band align-ment. In other words, in order to realize the type-II band align-ment for the QRs, it still requires that the QR dimensions are small enough to push up the quantum confined energy levels in the QRs to above the Χ-valley of the AlxGa1−xAs, or fur-ther increase the Al-composition of the AlxGa1−xAs to push up the quantum confined energy levels in the QR to above the Χ-valley of the AlxGa1−xAs barrier. The QRs with barriers

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Figure 4. (a) The PL decay curves at 10 K for GaAs/AlxGa1−xAs QRs. (b) The early time of the same PL decay curves, and the inset shows the rise time as a function of Al composition. The PL decay curves are detected at the PL peak position with the laser excitation intensity of 3.21 W cm−2 at 10 K.

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having Al-composition of x = 0.60 demonstrate luminescent properties such as very long lifetime and low intensity emis-sion which indicates that the crossover between the electron ground energy level of QRs and the Χ-valley of AlGaAs bar-rier has already occurred. In consideration of the PL and TRPL measurement, we believe that the transition point from type-I and type-II band alignment for QRs is close to x = 0.60.

In the PL and TRPL measurements, a large blue-shift is observed for QR emission for all samples with increasing excitation intensity, regardless of the band alignment of the QRs. This large blue-shift correlates with different mech-anisms. It is possibly due to a type-II band alignment, a band-filling effect due to QR height fluctuations, or a combination of both. Therefore, we conclude that the large blue-shift of the PL peak is not a sufficient criteria for judging a GaAs/AlGaAs structures to have a type-II band alignment or not. On the con-trary, TRPL measurements become a more useful tool to iden-tify the band alignment. A much longer lifetime becomes an important feature to evaluate the type-II band alignment. This is due to the special dynamics of the DE growth mode, which results in height fluctuations even within individual QR struc-tures. Corresponding to the height fluctuation inside the QR structures, we believe that there are strong exciton localization effects for the QRs grown by DE growth mode.

4. Conclusions

We examine the optical characteristics of GaAs/AlxGa1−xAs QRs grown by DE. The PL and TRPL results show a trans ition from type-I to type-II band alignment through varying the Al-composition of the barrier layer from x = 0.20 to 0.70. The Al-composition corresponding to the transition is identified to be between x = 0.45 and x = 0.60. For x ⩽ 0.45, the blue-shift of the PL spectra and the increasing emission intensity with increasing Al-composition correlates with the enhancement of quantum-confinement inside the GaAs QRs due to the rising of the AlGaAs barrier potential. While, for x ⩾ 0.60, the car-rier relaxation time is measured to be on the order of ~200 ps and the carrier recombination time is measured to be ~6 ns for the type-II GaAs/AlGaAs QRs. Both are much longer than their GaAs/AlGaAs QR counterparts with type-I band align-ment. A large blue-shift is observed for QR emission for all samples with increasing excitation laser intensity, regardless of the band alignment. This large blue-shift is correlated with different mechanisms. Due to the height fluctuation of QRs growth by DE, it is not a large blue-shift, but the characteristic PL lifetimes which become a more useful tool to identify the type-II band alignment.

Acknowledgment

The authors acknowledge the financial support by the ‘Hebei Province 100-Talents Program’ (Grant # E2013100013) of People’s Republic of China and the CNSI/HP seeding fund-ing (449041-HD-79740). This research is also supported at the University of Arkansas by the National Science Founda-tion of the US (EPSCoR Grant # OIA-1457888).

ORCID

Baolai Liang https://orcid.org/0000-0002-2192-9340

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