pubs.acs.org/NanoLett
Confined States of Individual Type-II GaSb/GaAs Quantum Rings Studied by Cross-Sectional Scanning Tunneling Spectroscopy Rainer Timm,*,†,§ Holger Eisele,† Andrea Lenz,† Lena Ivanova,† Vivien Vossebu¨rger,†,| Till Warming,† Dieter Bimberg,† Ian Farrer,‡ David A. Ritchie,‡ and Mario Da¨hne† Institut fu¨r Festko¨rperphysik, Technische Universita¨t Berlin, 10623 Berlin, Germany, and Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom ABSTRACT Combined cross-sectional scanning tunneling microscopy and spectroscopy results reveal the interplay between the atomic structure of ring-shaped GaSb quantum dots in GaAs and the corresponding electronic properties. Hole confinement energies between 0.2 and 0.3 eV and a type-II conduction band offset of 0.1 eV are directly obtained from the data. Additionally, the hole occupancy of quantum dot states and spatially separated Coulomb-bound electron states are observed in the tunneling spectra. KEYWORDS GaSb, quantum rings, quantum dots, type-II band offset, scanning tunneling spectroscopy
G
chemical vapor deposition (MOCVD), the transition from a 2D quantum well over very small islands to small and compact but optically active QDs was observed.8,17 By using molecular beam epitaxy (MBE), more GaSb material could be accumulated during growth, although this was accompanied by strong group-V exchange effects at the growth surface and significant antimony segregation.9,18 Furthermore, capped GaSb/GaAs QDs grown by MBE under various growth conditions were found to exhibit a ring shape.19 Here we reveal the interplay between the atomic structure and local electronic properties of GaSb/GaAs QDs by combining XSTM with cross-sectional scanning tunneling spectroscopy (XSTS). In this way, we obtain the energy of the confined hole states as well as the conduction band (CB) offset of individual ring-shaped GaSb/GaAs quantum dots grown by MBE. The hole occupancy of GaSb QD states and Coulomb-bound electron states within the surrounding GaAs are directly observed in the XSTS spectra, which will be discussed in the context of type-II band alignment and tipinduced band bending. The investigated GaSb/GaAs QDs were grown by solidsource MBE using As4 and Sb4 at a growth rate of 0.7 monolayer (ML)/s.9,10 Growth temperatures were 600 °C for GaAs and 515 °C for the GaSb layer and the subsequent 10 nm of GaAs. After a 1-min-long growth interruption (GI) during which the GaAs(001) surface had already been exposed to Sb4sthe so-called soaking step, during which substantial Sb-for-As exchange processes take place at the growth surface9-11s2 ML of GaSb was directly deposited, followed by a GI of 15 s under an Sb background and another 15 s without Sb but with increasing As pressure. The GaSb layer is embedded in undoped GaAs and AlGaAs buffer
aSb quantum dots (QDs) in GaAs show a staggered type-II band alignment with large hole confinement energies and a spatial separation between holes being confined within the QD and electrons being only weakly Coulomb-bound in the surrounding GaAs matrix.1 These properties make GaSb/GaAs QDs not only an interesting system for fundamental physics2 but also very promising for charge storage devices3,4 or in photovoltaics.5 However, the exact band alignment and the distribution of the GaSb/ GaAs QD states are still under discussion6,7 and depend strongly on the strain1,2 and on the chemical composition of the most often alloyed GaAsSb/GaAs nanostructures.4,8,9 Confinement energies of more than 0.8 eV have been calculated for pure GaSb/GaAs QDs,2,4 whereas experimental results obtained by photoluminescence (PL)1,10-14 or deeplevel transient spectroscopy3,15 typically reach only about half the predicted value, probably because the atomic structure and the chemical composition of the experimentally realized QDs do not match the model assumptions.4 Recently, we showed the first atomically resolved structural results on GaSb/GaAs QDs obtained using crosssectional scanning tunneling microscopy (XSTM) and could unravel the onset of QD formation.16 For metal-organic * To whom correspondence physik.tu-berlin.de. †
Technische Universita¨t Berlin.
‡
University of Cambridge.
should
be
addressed.
E-mail:
timm@
§
Current address: Nanometer Structure Consortium, Lund University, 22362 Lund, Sweden. | Current address: Material Sciences Center, Philipps-Universita¨t Marburg, 35032 Marburg, Germany. Received for review: 05/24/2010 Published on Web: 09/23/2010
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central opening as well as within the first several nanometers of GaAs above the QDs, many single Sb atoms can be identified as bright spots in the XSTM images, indicating strong Sb segregation during the capping of GaSb/GaAs nanostructures.9,14 The ring bodies of the QDs consist of intermixed GaAsSb material with a stoichiometry that varies for different QDs from 30 to 80% GaSb content. In spite of the small volume and intermixed composition, photoluminescence (PL) results give clear evidence of QD confinement. At a reference sample, which was grown under the same growth conditions but on an undoped GaAs substrate with only a thin cap layer and no AlGaAs barriers, PL spectra were obtained at low temperature (Figure 1d). The signal at 1.49 eV can be attributed to bulk GaAs, where the bandgap with a literature value of 1.52 eV gets slightly reduced by the exciton binding energy or by some defects, whereas the peaks obtained at 1.38 and 1.11 eV correspond to the GaSb wetting layer and the GaSb QDs, respectively. It should be noted that this QD peak lies perfectly within the small energy window between 1.08 and 1.14 eV, which covers most published PL data on GaSb/GaAs QDs.1,11-13,15 The broadness of the QD peak with a full width of halfmaximum of 0.14 eV indicates that the confinement energies of individual QDs vary significantly. Much more detailed information about the electronic properties of individual QDs, even in correlation to their atomic structure, can be obtained from spectroscopy data.21-24 Figure 2 shows XSTS point spectra acquired at distinct positions at and around a typical example of a ring-shaped GaSb/GaAs QD, as marked in the XSTM image in Figure 2a. The resulting I - V curves, obtained using the variable gap mode, are plotted semilogarithmically in Figure 2b for the GaSb QD and averaged for the different positions in the GaAs matrix. In the spectrum representing the GaAs matrix (blue curve), a steep, smooth increase in the current with increasing absolute value of the voltage can be seen, clearly showing the GaAs band gap. The spectrum taken at the GaSb QD (red curve) shows an apparently much smaller band gap: at negative sample voltages, corresponding to tunneling out of the filled states of the valence band (VB), the current at the GaSb QD is significantly larger. This increased current can be explained by the energetically much higher VB maximum of GaSb compared to that of GaAs, resulting in large hole confinement energies. For larger positive voltages, the GaSb I - V curve is similar to that of GaAs, indicating a similar probability for tunneling into empty CB states. At smaller positive voltages, however, the current at the GaSb QD is significantly increased, as marked in Figure 2b by the red arrow. This additional current contribution, which looks similar to dopant-induced currents25 although the QD region of this sample is undoped, is due to the hole occupancy of GaSb QD states, as will be discussed further below. Although the I - V spectra provide qualitative trends for the GaAs matrix material and the GaSb QD, more quantitative results can be obtained from the differential conductivity
FIGURE 1. (a, b) Atomically resolved filled-state XSTM images of ringshaped GaSb/GaAs QDs obtained at a sample voltage of VS ) -2.1 V. (c) Sketch of the cleaved QD shape. (d) PL spectrum of a reference sample.
layers and sandwiched between a highly n-doped GaAs substrate and a p-doped GaAs cap layer. The sample was cleaved in ultrahigh vacuum, resulting in a clean (110) surface, and analyzed at room temperature using a home-built XSTM setup with an RHK Technology SPM 1000 control unit. Electrochemically etched tungsten tips that had been cleaned in situ by electron bombardment were employed. Topographical XSTM images were obtained in constant current mode at tunneling currents of 40-80 pA. For XSTS point spectra, the acquisition of the XSTM images was interrupted at specific predefined sample positions, where I - V and (dI/dV) - V spectra were recorded. For the latter ones, showing the differential conductivity, a lock-in amplifier was used with modulation amplitudes of ∼50 mV and frequencies of 10-25 kHz. To increase the dynamic range of the tunneling current measurement near the semiconductor band gap, the tip-sample distance was decreased with decreasing absolute value of the bias voltage by 0.2-0.4 nm/V, according to the variable gap mode.20 XSTM images of typical ring-shaped GaSb/GaAs QDs are shown in Figure 1 together with a sketch of the cleaved QD structure. The QDs have an average base length of 19 ( 6 nm and a height of 1.3 ( 0.4 nm, corresponding to a very flat outer shape. They are characterized by a ring shape with clear central openings of different extensions.19 In the © 2010 American Chemical Society
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FIGURE 3. Schematic diagram of the tunneling conditions between the tip and the cleavage surface of the GaSb/GaAs QD sample, considering tip-induced band bending. Energies of the band edges (EVB, ECB), confined hole (Eh) and electron (Eel) states, and the Fermi energies of the sample (EF,S) and tip (EF,T) are indicated. The situation for a negative sample voltage is shown in a-c on the left-hand side, corresponding to the gray-shaded box A in Figures 2b,c and 5b: in the GaAs matrix (a), the CB minimum near the surface is shifted close to the Fermi energy. Electron tunneling out of the GaAs VB can occur. At the GaSb QD (b), the current is additionally increased by electron tunneling out of confined QD states within the GaAs band gap. These states are tailing slightly into the surrounding GaAs (c), where they weakly increase the tunneling current. The situation for a small positive sample voltage is shown in d-f on the righthand side, corresponding to box B in Figures 2b,c and 5b: in the GaAs matrix (d), the VB maximum near the surface is shifted close to the Fermi energy. No tunneling current can be measured. At the GaSb QD (e), hole occupancy of the confined QD states occurs, leading to a small current induced by electrons tunneling from the tip into the QD hole states. The hole occupancy also induces Coulomb-bound electron states at the GaAs surrounding the QD (f). These states, which are located slightly below the CB minimum of bulk GaAs, enable an additional small tunneling current.
FIGURE 2. (a) XSTM image acquired at VS ) -2.1 V, (b) I - V spectra, and (c) (dI/dV)/(I/V) - V spectra of an individual representative GaSb/ GaAs QD. The positions of the point spectra in b and c are marked in a. Red curves correspond to a position at the GaSb QD, and blue curves show the averaged data of positions in the GaAs matrix. Note that in b the current is increased at small absolute voltages because of the variable gap mode. In c, the CB and VB edges are also indicated. Gray boxes indicate two significant voltage regions, labeled A and B. The tunneling conditions for these two regions are illustrated in Figure 3.
dI/dV. To become independent of the varied tip-sample distance, the data were normalized by the absolute conductivity I/V, which needed to be broadened to avoid divergence at the band gap.20 This broadening is performed by convoluting the measured conductivity by a Gaussian distribution according to
I/V )
1 2∆V
e-(V′-V) /∆V ∫-∞∞ I(V′) V′ 2
2
dV′
the bandgap and smoothly reaching zero because of a broadening of about 0.1 eV due to thermal and lock-in effects. From the extrapolated linear onsets of the curve, the band edges can be located at about -0.56 eV for the VB and +1.08 eV for the CB, as indicated in Figure 2c. The slightly enlarged apparent band gap of 1.64 eV compared to the literature value of 1.42 eV can be explained by moderate tip-induced band bending.25 For the GaSb QD (red curve), in contrast, a smaller and slightly shifted apparent band gap containing additional states can be seen. On the VB side (negative voltages), the normalized differential conductivity shows an onset for already much smaller negative voltages, being located about 0.23 eV above the GaAs VB edge. This energy difference is the hole confinement energy of this individual QD, which is imaged in Figure 2a. Therefore, it is possible to link the
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with a voltage broadening of ∆V ) 1.5 V, similar to the method suggested by Feenstra et al.20 The resulting value for the normalized differential conductivity (dI/dV)/(I/V) is a measure of the local density of states of the sample.26 Figure 2c shows normalized differential conductivity spectra for the GaSb quantum dot and the GaAs matrix. At the GaAs matrix material (blue curve), a clear bandgap without any electronic states can be observed, with the signal at the VB and CB decreasing monotonically toward © 2010 American Chemical Society
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atomically resolved structure of an individual QD directly to its specific confinement energy, without averaging over an ensemble. On the CB side, for positive voltages above ∼1 V, the normalized differential conductivity at the GaSb QD is smaller than that of the surrounding GaAs matrix material, and from the energy shift between both spectra, a type-II CB offset of about 0.09 eV is derived. It should be noted that, in principle, the measured spectra do not correspond to complete QDs but only to that part of the original QD that has not been cleaved away. This will, however, lead only to small spectral deviations because the main electronic confinement is in the [001] direction as a result of the flatness of the QDs, and the energy states are thus mostly unaffected by the cleavage process. Apart from the VB and CB states, an additional contribution of states is observed in the GaSb QD spectrum between about 0.5 and 1.1 eV (i.e., below the GaSb CB minimum and also within the GaAs bandgap), as indicated by the red arrows in Figure 2b,c. To understand this contribution, the interplay between the type-II band alignment and tipinduced band bending needs to be considered. Calculated values of this band bending and its effect on the XSTM image contrast are discussed in ref 27, whereas here a qualitative discussion is sufficient. The corresponding tunneling conditions are illustrated in Figure 3 for voltage intervals of around -0.9 to -0.6 V (region A) and around +0.7 to +1.0 V (region B). These two regions are also indicated in Figure 2b,c by gray-shaded boxes. For negative sample voltages (region A) corresponding to tunneling out of the filled VB states, the electric field of the tunneling tip causes a downward bending of both CB and VB near the tip, leading to the Fermi energy being closer to the GaAs CB minimum (Figure 3a-c). This band bending leads to an enlarged appearance of the GaAs band gap in the spectra of Figure 2b,c. Additionally, at GaAs only a small tunneling current is established for voltages near the VB maximum (Figure 3a). This tunneling current is significantly increased at GaSb, where the confined QD states also contribute, as shown schematically in Figure 3b. For positive sample voltages representing empty-state tunneling, tip-induced band bending is upward, which results in a Fermi energy being slightly above the GaAs VB maximum (Figure 3d-f). If the voltage is sufficiently small (region B), no tunneling occurs at the GaAs matrix (Figure 3d). At the GaSb QD, however, because of the large VB offset, the highest confined states lie above the Fermi energy, as sketched in Figure 3e. In the terminology of tip-induced band bending, this corresponds to an inversion condition, implying electron tunneling from the tip into empty electron states of the VB. From the QD perspective, the same situation can actually be better described by the hole occupancy of the confined GaSb QD states. This hole occupancy then leads to a small contribution of electrons tunneling from the tip into the confined QD hole states. This additional contribution, occurring © 2010 American Chemical Society
FIGURE 4. Schematic band alignment and confined states of a GaSb/ GaAs QD at the cleavage surface of the sample for (a) negative and (b) positive sample voltages. At negative sample voltages (region A), the sample Fermi energy is close to the GaSb CB as a result of tipinduced band bending. Thus, the QD remains uncharged.27 At positive sample voltages (region B), the sample Fermi energy is close to the GaSb VB and even below the first confined QD states. This leads to hole occupancy and positive charging of the QD, which causes additional band bending and induces Coulomb-bound electron states in the surrounding GaAs.27
at small positive sample voltages, can well be seen in the spectra of Figure 2 as marked by the red arrows. (Please note the semilogarithmic scale of Figure 2b.) Besides the tunneling conditions for the GaAs bulk material (Figure 3a,d) and for the GaSb QD (Figure 3b,e), the situation for the GaAs material directly surrounding the QD also needs to be considered (Figure 3c,f). Here, the dominant contributions to the tunneling current are again those from filled states of the bulk GaAs VB at negative voltages and those in empty states of the bulk GaAs CB at positive voltages. However, the type-II band alignment at the GaSb/ GaAs interface, as sketched in Figure 4, leads to additional contributions. At a negative voltage (region A) and in close proximity to the QD, the current is slightly increased by electrons tunneling out of QD states tailing into the GaAs matrix (Figures 3c and 4a). At positive voltages (region B), the hole occupancy of confined GaSb QD states gives rise to positive charging of the QD and thus to a weak confinement of Coulomb-bound electron states in the GaAs surrounding the QD, which is the specific property of type-II QDs. These confined electron states lie slightly below the CB minimum of the GaAs far away from the QD, as marked in green in Figure 3f, and typically extend several nanometers from the GaSb/GaAs interface into the GaAs matrix,27 as shown in Figure 4b. Accordingly, these states should be visible in the XSTS spectra as an additional contribution at small positive voltages slightly below the onset of tunneling into the GaAs bulk CB. Indeed, all of the discussed contributions can clearly be found in the experimental XSTS data shown in Figure 5. Here, at another representative ring-shaped QD, the positions of the point spectra were chosen with special emphasis on the direct surroundings of the QD, as shown in Figure 5a. Spectra were taken directly at the GaSb QD ring body (red positions), at the GaAs far away from the QD (blue positions), and at the GaAs directly around the QD (green positions). In the latter case, no significant difference could be found between spectra taken below, above, or at the 3975
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bulk GaAs data (green arrow in Figure 5b). This shift is related to the confinement energy of the GaAs electron states bound to the charged GaSb QD, as sketched in Figures 3f and 4b. Because this contribution can be seen only in spectra taken at the GaAs within a few nanometers around the QD, it directly demonstrates the spatial charge separation of this type-II nanostructure. Finally, the XSTS results are compared with the PL transition energies of the QDs, which should be similar to the band gap reduced by the hole confinement energy. The rather broad QD peak in the low-temperature PL spectrum, shown in Figure 1d, is centered at 1.11 eV, with a full width of half maximum of 0.14 eV. This corresponds to a VB offset of 0.40 ( 0.07 eV at 7 K, which is reduced to 0.37 ( 0.07 eV when normalized to room temperature. The broadness of the QD peak indicates a large size fluctuation, which agrees with the XSTM results showing an average base length of 19 ( 6 nm and a height of 1.3 ( 0.4 nm of the ring-shaped QDs. Additionally, the QD PL peak is asymmetric with a larger shoulder on the high-energy side, indicating that a considerable number of QDs with a 0.3 or even a 0.2 eV VB offset exist within the sample. The XSTS data of the two representative ring-shaped QDs presented here show room-temperature VB offsets of 0.23 ( 0.04 and 0.3 ( 0.1 eV, respectively. These values lie well within the energy range covered by the broad QD PL peak. It should be noted that the measured VB offsets of the cleaved QDs should be only slightly smaller than the actual values of the complete QDs because the main electronic confinement in the [001] direction is mostly unaffected by the cleavage process, as discussed above. The strain fields in and around a QD, which in principle strongly influence the piezoelectric fields and therewith the confined states,28 are different on a partly relaxed cleavage surface and within a capped QD. Because of the very small height of the ringshaped QDs studied here, however, the resulting effect on the QD confinement energies should be small. Also, the effect of tip-induced band bending should in principle mostly affect the apparent total band gap but not the relative energies of features within the VB or CB. Especially for negative sample voltages, which are applied to measure the VB offset, the tip-induced band bending has about the same value for the GaAs matrix and for the GaSb nanostructures.27 From a deep-level transient spectroscopy study, Geller et al.3 obtained a hole localization energy of 450 eV for GaSb/ GaAs QDs grown by MOCVD. Our recent XSTM results on MOCVD-grown GaSb QDs, using similar growth conditions although depositing less material than in the literature,3 showed capped QDs of a small size but with a rather compact shape and hardly any Sb segregation into the capping layer,8,17 in strong contrast to the MBE-grown flat ring-shaped structures studied in this work. This difference in the atomic structure may result in the significantly reduced hole localization energies obtained here.
FIGURE 5. (a) XSTM image of another representative ring-shaped GaSb/ GaAs QD, acquired at VS ) -2.1 V. Point spectra have been taken at the GaSb ring body (positions marked in red), at the GaAs matrix far away from the QD (marked in blue), and directly above or beneath the ring-shaped QD or at its GaAs-filled interior (marked in green). (dI/dV)/(I/V) - V spectra are averaged according to their spatial position and are shown in b. Contributions induced by the hole occupancy of GaSb QD states and by Coulomb-bound electron states in the GaAs CB are indicated by a red and a green arrow, respectively. Gray boxes indicate the same two significant voltage regions A and B as in Figure 2b,c. The tunneling conditions for these two regions are illustrated in Figure 3.
GaAs-filled interior of the ring-shaped QD. Thus, spectra taken at the same kind of position with respect to the QD could be averaged, as shown for the normalized differential conductivity spectra in Figure 5b. On the VB side (negative voltages, region A), a monotonic decrease in the local density of states toward the band gap can be seen for all three curves. The noise level of these spectra is higher than that in Figure 2c; therefore, it is hardly possible to mark the exact VB and CB onsets. Nevertheless, by comparing the spectra taken at the GaAs matrix (blue curve) and at the GaSb QD (red curve), a QD confinement energy of about 0.3 eV can be estimated for the QD in Figure 5. The green curve, taken at the GaAs close to the QD, lies between the blue and the red ones because it shows the GaAs VB states plus a small additional contribution from the tailing QD states. For positive voltages larger than about 1.2 V, the GaSb QD (red curve) exhibits a smaller normalized differential conductivity than the GaAs matrix (blue curve), again underlining the type-II CB offset. Between about 0.5 and 1.0 V (region B), the hole occupancy of GaSb QD states can be seen by the rise of the red curve within the GaAs band gap (red arrow in Figure 5b), as described above. Additionally, at small positive voltages close to the GaAs CB onset, the spectrum taken at GaAs near the QD (green curve) is shifted by about 0.1 eV toward smaller energies compared with the © 2010 American Chemical Society
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(8)
In conclusion, we could directly measure the hole confinement energies of individual GaSb/GaAs QDs, amounting to values of between 0.2 and 0.3 eV. Furthermore, the XSTS data impressively demonstrate the type-II band alignment, directly showing a type-II CB offset of 0.1 eV and revealing a hole occupancy of GaSb QD states as well as spatially separated Coulomb-bound electron states in the surrounding GaAs. The measured hole confinement energies are in good agreement with PL results of a reference sample and are comparable to typical literature PL data1,11,15 but are significantly smaller than the values that are needed and predicted for storage applications.2,4 However, the simultaneously acquired XSTM images of the corresponding QDs reveal a flat ring-shaped structure with a small GaSb volume, which well explains the small confinement energies. Thus, the GaSb/GaAs system has a large potential for storage devices provided that larger and more compact QDs can be grown than those yet known from XSTM data.17,19
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Acknowledgment. This work was supported by project nos. Da 408/8 and Da 408/13 as well as Sonderforschungsbereiche 296 and 787 of the Deutsche Forschungsgemeinschaft (German Research Foundation).
(19) (20) (21)
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