Electronic Structure Changes Due to Crystal Phase Switching at the

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Electronic Structure Changes due to Crystal Phase Switching at the Atomic Scale Limit Johan Valentin Knutsson, Sebastian Lehmann, Martin Hjort, Edvin Lundgren, Kimberly A. Dick, Rainer Timm, and Anders Mikkelsen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05873 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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Electronic Structure Changes due to Crystal Phase Switching at the Atomic Scale Limit Johan Valentin Knutsson1, Sebastian Lehmann1, Martin Hjort2, Edvin Lundgren1, Kimberly A. Dick1,3, Rainer Timm1 and Anders Mikkelsen1* 1

Department of Physics & NanoLund, Lund University, P.O. Box 118, 22 100 Lund, Sweden 2

Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 3

Center for Analysis and Synthesis, Lund University, P.O. Box 124, 221 00 Lund, Sweden

e-mail: [email protected]

Keywords: nanowire, electronic structure, crystal phase, InAs, wurtzite, STM/S, zinc blende

Abstract

The perfect switching between crystal phases with different electronic structure in III-V nanowires allows for the design of superstructures with quantum wells only a single atomic layer wide. However, it has only been indirectly inferred how the electronic structure will vary down to the smallest possible crystal segments. We use low-temperature scanning tunneling microscopy and spectroscopy to directly probe the electronic structure of Zinc blende (Zb) segments in Wurtzite (Wz) InAs nanowires with atomic-scale precision. We find that the major features in the band structure change abruptly down to a single atomic layer level. Distinct Zb electronic structure signatures are observed on both the conduction and valence band sides for the smallest possible Zb segment: a single InAs bilayer. We find evidence of confined states in

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the region of both single and double bilayer Zb segments indicative of the formation of crystal segment quantum wells due to the smaller band gap of Zb as compared to Wz. In contrast to the internal electronic structure of the nanowire, surface states located in the band gap were found to be only weakly influenced by the presence of the smallest Zb segments. Our findings directly demonstrate the feasibility of crystal phase switching for the ultimate limit of atomistic band structure engineering of quantum confined structures. Further, it indicates that band gap values obtained for the bulk are reasonable to use even for the smallest crystal segments. However, we also find that the suppression of surface and interface states could be necessary in the use of this effect for engineering of future electronic devices.

Quantum dots (QDs) and quantum wells (QWs) are entirely or partially quantized systems with tunable optical and electronic properties which can be scaled down to operate at the single photon and electron level.1, 2 Reproducible fabrication of QD and QW structures in larger superlattice or 2D arrays with atomic precision are central for nanophotonic and quantum information technologies1-5 as well as for fundamental studies of quantum effects.6-9 Traditionally such structures are formed via heterostructure interfaces between materials with different band gaps.10-14 However, their perfection is limited by the difficulty to control the material composition at the atomic level,10, 12-15 which unfortunately are determining factors for the physical properties of QDs and QWs as they define the shape of the potential well.11, 12, 16, 17

The actual shape and height of the potential barriers at these interfaces is not easy to predict

but can determine fundamental behavior such as metallic versus semiconducting.18

Recent studies have shown that perfect atomically sharp material interfaces can be achieved if the crystal structure is changed instead of the material composition.16, 19, 20 It is then possible to


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create QWs and QDs with true monolayer precision if the constituent crystal structures have different band gaps, as is the case for Wurtzite (Wz) and Zinc blende (Zb) crystal phases in IIIV materials.21-24 Many applications greatly benefit if, for instance, the QW width could be reduced to the atomic level, being only a few or even one single monolayer wide.6, 25-30 III-V nanowires (NWs) exhibit controlled crystal phase switching between Zb and Wz and can, therefore, be used to template QD and QW growth to create an array of quantum structures, all within a single NW.16, 17, 31, 32 Recently, controlled Zb-Wz switching have moved towards single atomic layer precision, thereby reaching the ultimate level of control.16,

19, 20, 33

Ideally,

introducing single monolayer crystal segments (referred to as a bilayer in III-V systems, consisting of one atomic layer of the group III and V atomic species, respectively) of either polytype in a NW would form the smallest possible quantum confined structures.21-24 However, then the question arises as to how the electronic structure responds to crystal phase changes at the atomic level to form a potential well.

Several studies exist that investigate the effects of crystal phase mixing on the electronic properties of NWs.16, 33-37 While the effect of barrier shapes and other electronic effects needs to be taken into account to explain the results,38 any knowledge of the electronic structure spatial variation is indirect. A few theoretical works exist that tries to predict how the electronic structure changes spatially over a crystal phase boundary at the single bilayer level but the results are inconclusive, varying from atomically abrupt to non-local effects spanning over several bilayers.38-40 The lack of experimental studies of individual few-bilayer crystal phase QWs in NWs is due to the difficulty of using conventional methods such as contact electrical measurements and photoluminescence measurements due to the extreme precision in NW growth, and sample preparation required.16 As a result, despite its technological impact,3-9, 25-30 it is currently unclear to what extent the depth and shape of the energy potential are retained when downscaling to the atomic level in crystal phase quantum structures.


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These issues can potentially be resolved for the III-V systems using Scanning Tunneling Microscopy and Spectroscopy (STM/S), as has been done previously for embedded compositional QW and QD superstructures of many different kinds.41-43 The studies use that the {110}-type surface easily cleaves making it accessible to STM (so-called XSTM). Importantly, this surface is unreconstructed and non-polar and will reflect the bulk electronic structure as intrinsic surface states are outside the band gap region. The same has been found to be true for many NW facets,21 such as the {11-20}-type of Wz, and we can expect to gain similar information using STM on these as is done in the present study.

While the surface structure itself does not introduce states in the band gap, imperfections at the surface such as steps or adsorbates might do so. However, as long as these effects can be distinguished it is possible to separate them from “bulk” phenomena and study how they are affected by the electronic structure changes due to the crystal phase switching. It is presently unclear to what degree such crystal phase QWs are affected by surface states, and vice versa.44 This is a critical question as surface or interface states are likely present due to the inherent curvature of a nanoscale objects such as the NWs and will influence both optical and electronic properties.45, 46

In this study, we directly show that the thinnest possible crystal phase QW (corresponding to a variation in the stacking of a single III-V bilayer in the NW growth direction) maintains both the depth and the well-defined shape needed to qualify as a QW. We find that step induced surface states on the non-polar NW facets are virtually unaffected by the single bilayer crystal phase variation and only little by a double bilayer. To accomplish this, we use STM/S, operated at 5K, which is capable of probing the local electronic structure with atomic resolution.6, 47 The well-defined crystalline surfaces of the InAs NWs that we study are produced in-vacuum using atomic Hydrogen treatments,48 a procedure that affects the overall NW morphology and


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structure very little. The investigated structures were created by intentionally varying the crystal structure in Wz InAs NWs during growth such that small Zb segments would form, ranging from single to several tens of bilayers in length. For single bilayer Zb segments we found distinct signatures characteristic for the Zb crystal phase in both the conduction and valence band sides in addition to an atomically sharp transition in the crystal phase dependent electronic structure at the QW boundaries. Also, for single and double bilayer Zb segments indications were found that a size-dependent confinement of states was present within the QWs. Finally, we demonstrate that the {110}/{11-20}-type facets of Zb/Wz exhibit some delocalized surface states at low temperatures which make it possible to perform STM imaging at energies which correspond to the band gap region. Our findings provide a proof of concept for atomistic band structure engineering, a potential research field which could revolutionize transistor,26 thermoelectric,27 and photovoltaic-technologies.28, 29

RESULTS AND DISCUSSION

STM/S investigations into the electronic properties of the Zb and Wz crystal phases require accurate information on the crystal structure variations and general morphology of the NWs investigated. These properties were carefully mapped out using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and STM (Figure 1). The SEM image (Figure 1A) shows a typical NW, having well-defined and smooth sidewall facets except for the top part where the sidewall facets are modulated. As seen in the dark-field TEM image of Figure 1B, the modulated area of the NW correlates with the intentionally grown Zb segments. The dark contrast and bright contrast in the TEM image corresponds to Zb and Wz, respectively, as shown by high-resolution TEM images as well as selective area electron diffraction patterns provided in Figure S1 of the supporting information. The STM image (Figure 1C) reveals that the Wz parts of the NW consist of flat and well-defined {10-10}-type and {11-20}-type facets


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having a 30° inclination relative to each other. The corresponding facets for the Zb inclusions were {110}-type and {111}A/B-type, respectively. A model of the NWs is shown in Figure 1D. The A- and B-type {111} facets are inclined downwards and upwards, respectively, relative to the {10-10}-type facets, resulting in the modulation of the sidewall facets as observed in the SEM image. The {111}A/B-type facets are not suitable for electronic structure STM/S investigation as they are polar in addition to being inclined and reconstructed.49 All STM/S measurements in this study were instead performed at {110}-type and {11-20}-type facets for Zb and Wz, respectively. These surfaces are suitable for STM/S investigation as they both are unreconstructed and in-plane relative to each other. As a result, the tip can be moved unperturbed from one crystal phase to the other. This is illustrated by the STM images and atomic models of a single and double bilayer Zb segment in a Wz NW, manifesting as a short {110} segment surrounded by {11-20} facets, which are shown in Figure 1E-F and 1G-1H, respectively.


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Figure 1. Based on the NW morphology, the {110}- and {11-20}-type facets were found to be the most suitable surfaces for studying crystal phase defects in the NWs with STM/S. (A) A 30° tilted SEM micrograph of the NW sample. The scale bar represents 200 nm. (B) A conventional dark-field TEM image of the top part of the NW, with Zb having darker contrast. Scale bar is 100 nm. (C) STM image, differentiated for clarity, of a Wz section of a NW with two small Zb segments. Note that {111}A- and {111}B-type facets are inclined downwards and upwards, respectively, Ubias=-2.7 V, Iset=50 pA. (D) Model illustrating the geometry relationship for the various side facets present on the NWs. (E) STM image and (F) model of a Wz {11-20}-type surface with a single bilayer Zb segment, Ubias=-1.7 V, Iset=50 pA. (G) STM image and (H) model of a Wz {11-20}-type surface with a double bilayer Zb segment, Ubias=-1.9 V, Iset=150 pA. The green and red crosses represent the expected position of As and In atoms, respectively, as based on the atomic models. Ubias, Uset, and Iset are discussed and defined in the methods section. High-resolution TEM images, as well as selective area electron diffraction patterns of the Wz and Zb segments, can be seen in Figure S1 of the supporting information.

When probed with STS, the electronic structure of large (i.e., at least 100 nm) Wz and Zb segments revealed identifiable band gaps (at the Γ-point of the Brillouin zone) with VB and CB onsets at approximately -0.5 eV and 0 eV, respectively. In Figure 2 we present STS data (in the form that is proportional to the local density of states (LDOS)), as described in the methods section, obtained on the Wz and Zb segments of the NWs, respectively. Details on the STS measurements can be found in the method section. Each plotted spectrum is an average of 3 individual spectra obtained in the same area, using the same set-point currents and voltages. Inherently the Fermi level is at 0 V (used as a reference for all further discussion), negative and positive sample biases are associated with tunneling from the filled states of the valence band (VB) and into the empty states of the conduction band (CB), respectively. Spectra of Wz and Zb InAs segments were obtained after establishing atomic resolution and making sure that no


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tip changes occurred when moving from Wz to Zb areas. Tip changes are identifiable in both STM images and STS spectra as significant and rapid changes in the tunneling current.

The precision when determining band edge positions using STS is typically in the range of 311 meV at 5K.50 We found a standard deviation of ±6 meV when extracting band edges by assuming a linear increase in LDOS close to the band edges and taking temperature into account via Gaussian broadening in accordance to ref. 50. An example of such a fit can be seen in the inset of Figure 2, in which the valence band edge of Zb {110} has been fitted to the STS data.

Figure 2. Differences in electronic structure between Wz and Zb are seen and quantified using STS. STS data obtained on {11-20}- and {110}-type facets of Wz and Zb, Uset=-1.2 V,


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Iset=200 pA. Each spectrum in the figure is an average of 3 individually obtained spectra. The Fermi level, EF, is indicated by a dashed line. Band onsets can be directly compared as no tip changes occurred in between measurements. The states observed in the band gap stems from step-induced surface states, as will be discussed later. The upper inset shows a zoomed in version of the Wz spectra where the gap state is more clearly visible. The lower inset shows a fitted band edge (red line) of the Zb valence band, with an estimated error margin of ±6 meV as marked by the green and black lines.

Absolute values of the Wz and Zb band gaps could be extracted after correcting for Tip-Induced Band Bending (TIBB). For Wz we found the VB and CB onsets at -519 meV and +47 meV respectively, resulting in a band gap of 566 meV. For Zb, we observe a VB onset at -414 meV and a CB onset at +46 meV. As expected, the resulting band gap of 460 meV for Zb is larger than the accepted value of 417 meV for Zb InAs at 5 K.23, 51, 52 This is a result of TIBB commonly giving rise to an enlarged apparent band gap.53, 54 The large dimensions of our NWs, i.e., diameters of approximately 80 nm and crystal segments of approximately 50 nm length, should rule out quantization effects as an explanation for the increase in band gap.55, 56 As discussed in more detail in the method section, TIBB can be modeled for a hyperbolically shaped probe tip in the vicinity of a semiconductor using a 3D Poisson solver. During STS acquisition we systematically probed adjacent Zb and Wz segments, taking great care that the physical properties of the STM tip remained constant during and in between the measurements. This methodology ensures that we can use the known value of the Zb band gap to evaluate the parameters that determine the TIBB and then apply these parameters for correcting the observed value of the Wz band gap, thereby revealing the exact difference between Wz and Zb band gap values.21, 22


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Table 1. Values of the total band gap (BG) as well as valence (EV) and conduction band (EC) onsets as obtained experimentally (exp.) as well as after TIBB-correction (TIBB).

{110} [meV] EV exp. EV TIBB EC exp. EC TIBB BG exp. BG TIBB

-414 -412 46 5 460 417

{11-20} [meV] -519 -496 47 8 566 504

Single Bilayer Double Bilayer Zb [meV] Zb [meV] -450 -440 43 4 493 444

-439 -431 42 3 481 434

After TIBB correction, we found that the band gap difference between Zb and Wz for InAs was 87 meV, with a Wz band gap of 504 meV, see table 1 for specifics. The band gap difference between Wz and Zb is in good agreement with both theoretically calculated values23, 24, 57-60 (that vary between 40 and 100 meV) and low-temperature experimental values as determined by optical or electrical measurements (that vary between 40 and 130 meV).17, 38, 51, 61-64 Note that although we used the same doping concentration for both Wz and Zb in our TIBB modeling, using a higher doping concentration for Wz, as suggested by some studies,34 only had a minor effect on the resulting band gap size and alignment. When doubling the doping concentration for Wz compared to Zb a band gap increase in the order of 10 meV was found, located exclusively on the CB side.

Our results show a flat band alignment between Wz and Zb at the CB side which is a result of Fermi level pinning. The full difference in band gap can instead be found as a band offset on the VB side, see Table 1. Such an alignment contradicts the prediction of a type II staggered band alignment between Wz and Zb23, 57, 58 but is consistent with a surface pinning of both materials due to n-type defects, effectively masking the CB alignment of the interface of the NW interior.21,

65, 66

This effect is supported by previous reports that NWs grown under

conditions analogous to ours exhibit n-type conductivity.21, 34


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Figure 3. STM/S shows distinct differences in the electronic structure between a single bilayer Zb segment and the surrounding Wz matrix; characteristics typical of their bulk counterparts are present in all spectra. (A) Atomically resolved STM image of a {11-20}-type Wz NW facet with a single bilayer Zb segment, Ubias=-400 meV, Iset=50 pA. The crosses denote spatial positions of STS probe events. (B) STS data obtained from positions as marked in (A): on the single bilayer Zb segment (red) as well as 1, 2, and 3 nm away (green, yellow, and blue, respectively). The spectra shown in (B) are averaged from probe events of equal distance to the single bilayer Zb segment, no tip changes could be observed between spectroscopy events. The dashed arrows indicate areas where clear and characteristic differences are found between the single Zb bilayer spectrum and the Wz spectra. (C) STM height profile plots across the single bilayer Zb segment in the [-1-1-1] direction (as indicated by the green line in (A)) at varying biases, Ubias. The line-scans are shifted along the y-axis for clarity. Black arrows show the position of the single bilayer Zb segment.


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STS measurements on smaller Zb segments that we now discuss, strongly indicate that the two criteria for Quantum confinement are fulfilled for even the smallest possible Zb segment, a single bilayer. (i) The electronic structure of the crystal segments must retain the band gap differences of their bulk counterparts and (ii) there must be an atomically sharp transition of the electronic structure between that of the bilayer and the surrounding crystal matrix. Figure 3A shows an atomically resolved STM image of a single bilayer Zb segment in a Wz matrix, obtained at a {11-20}-type facet of a NW. The crosses denote positions at which single point STS was performed (presented in Figure 3B); directly at the single bilayer Zb segment as well as c.a. 1, 2, and 3 nm (corresponding to roughly 2, 4 and 6 bilayers, respectively) away from the Zb segment. As seen in Figure 3B, the three Wz spectra are more or less identical to each other, independent of the distance to the single bilayer Zb segment which in turn is different in a few significant ways. From this, we conclude that the effect of the Zb segment on the electronic structure of the surrounding Wz surface is local, to within at least 1 nm. Several consistently observed differences exist between the Wz spectra and the Zb segment spectra. At approximately +0.9 eV a plateau in the LDOS can be seen for the Zb spectra, whereas the LDOS of the spectra obtained at Wz continues to increase at this energy. A feature at -0.6 eV shows up in the Wz but not for the single bilayer Zb segment (or for “bulk” Zb, see also Figure 2). Also, two peaks show up in the LDOS of the Zb spectra at -1.0 eV and -0.7 eV which is not present in the STS of the Wz surface. We have previously reported similar characteristic differences between Wz and Zb at +0.9 eV for InAs NWs.21 It was concluded that the rapid increase in LDOS for the Wz at +0.9 eV was attributed to the Γ8C band which has an onset at +0.9 eV relative the CB edge and should not be present in Zb,23 leading to the observed plateau in LDOS at that energy. It can thus be concluded that differences in the STS data from the single bilayer Zb segment and the Wz are found to be very similar to those of their bulk counterparts. The potential impact of both the use and study of quantum size effects of such atomically thin and


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abrupt QW structures that can be reproducibly made in a bottom-up procedure16, 19, 33 has been demonstrated in many studies.3-9, 26-30

STM images obtained at various energies further support the presence of an atomically abrupt QW structure in the single bilayer Zb segment. As a consequence of such a QW structure, a noticeable increase of the tunneling probability should be seen for the single bilayer segment in STM images at energies corresponding to the confined states above the Wz VB edge.47 We thus expect an energy-dependent change in relative corrugation of atoms in the bilayer Zb segment compared to the Wz surface when imaging in constant current mode (with the Zb segment appearing brighter). This effect was investigated by obtaining a series of STM images with energies varying from -1.0 eV to +1.0 eV. It should be noted that the tunneling was possible here even at voltages corresponding to energies slightly above the VB edge due to the existence of surface states, as will be discussed below. A line profile (indicated in Figure 3A) was obtained for each image, as shown in Figure 3C. For most energies, a height difference of 5-10 pm is found between the atoms in the single bilayer Zb segment and the surrounding Wz segments. This baseline offset is explained by an outward relaxation of the Zb bilayer as a way to reduce strain.22 However, for energies between -0.65 eV and 0 eV a significant increase in corrugation is observed for the atoms in the Zb segment. The maximum height difference is found to be 60 pm for energies between 0.35-0.45 eV, consistent with a position in the Wz band gap close to the Zb VB edge. This effect is also seen in the STM image of Figure 3A, obtained at -0.4 eV, as an atomically sharp difference, in contrast, is present between the single bilayer Zb segment and the atoms of the Wz surface. We have previously published STM-based studies of InAs NWs at room temperature where no intensity differences could be observed for few-bilayer Zb segments and the surrounding Wz segments.21, 49, 67 However, in none of these studies, we were able to image at biases as low as -0.7 V where this effect starts to appear.


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The difference in band edge alignment between a very thin Zb segment and the surrounding Wz are qualitatively similar to the difference found for larger Zb and Wz segments, but with small variations of the valence band position which can be explained as due to quantum confinement effects. After correcting for TIBB, the CB edge is obtained at +4 meV for the Wz segment and at +8 meV for the single bilayer Zb segment, again showing nearly flat-band conditions. The VB onset of the Wz segment is obtained at -496 meV, which is the same value as found before, while the VB region of the single bilayer Zb segment shows a ground state energy of -440 meV. This value is 56 meV above the VB edge of the surrounding Wz segment, but 28 meV below the VB edge of a larger Zb segment, and it is reproducibly observed in many spectra. We also performed corresponding measurements at a double bilayer Zb segment, where the ground state energy was found at -431 meV, which is 19 meV below the Zb VB edge. Since the Zb insertions in the Wz NWs can be viewed as QWs of different thickness, our results demonstrate the appearance of quantum confinement in both the single and double bilayer Zb segments. The observed CB and VB energies, as summarized in Figure 4, are consistent with a Zb/Wz QW structure with a depth of 84 meV and a confined ground state located 28 (19) meV below the Zb VB edge or 56 (65) meV above the Wz VB edge for a single Zb bilayer (double Zb bilayer). These numbers are similar to the behavior of nanometer-sized crystal phase QWs in GaAs NWs, as observed by photoluminescence.16, 68 The presence of quantum confinement effects in the single bilayer Zb segment further corroborates our previous statement that it indeed acts as a QW.


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Figure 4. Energy diagram showing VB and CB edges obtained experimentally, but modified to account for TIBB, for the Wz (black) and Zb (red) segments of varying sizes: (A) at least 100 nm long Zb segment, (B) double bilayer Zb segment, and (C) single bilayer Zb segment. The enlarged band gap in (B) and (C) is interpreted as due to confined ground state levels within the QW, marked by horizontal purple lines.


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Figure 5. STS at different positions of the NW reveals that the surface states found in the band gap are not affected by single bilayer stacking defects. STS spectra of the band gap region obtained on {11-20}-/{110}-type facets from two different series of STS measurements ((A) and (B)) in which no tip changes occurred between the recorded spectra. The dashed red line denotes the Fermi level (EF). Band edge fits are shown in red with a ±6 meV error margin as indicated by dark blue margins of the line. The band onsets can be directly compared between spectra in (A) and (B), respectively where the same tip was used in the two cases. (A) STS measurement series (green) was obtained at a Wz and a single bilayer Zb segment, top and bottom respectively. (B) STS measurements (blue) was obtained at a Wz segment, a Zb>100 nm


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segment, and a two bilayer Zb segment. Uset=-1.2 V and Iset=200 pA were used for both measurement series.

STM imaging at energies corresponding to the band gap was made possible due to the presence of weak surface states at the InAs surfaces at 5K, in both Wz and Zb. In contrast to the electronic structure in the VB and CB regions, these states appear not to be affected by the presence of a single Zb bilayer as seen in Figure 5A, comparing the band gap region at and off the single Zb bilayer. However, we observe some differences when we compare spectra from double Zb segments and from positions on Wz and large Zb segments that are several nm apart, still using the same tip, see Figure 5B. Changes in the tip also affect the appearance of these band gap states to some extent. From the spectra it is evident that, independently on the surface, the LDOS is non-zero in main parts of the band gap region and that at least one distinct feature is present, centralized between -150 to -200 meV. These observations are not convincingly explained by the presence of 2D electron gases on the surface68 or tip-induced quantum dots69, 70

which have been used previously to explain similar surface states on bulk InAs.71 The small

amount of band bending (43 meV for Zb) observed in our spectroscopy in relation to the position of the main band gap states (located at -150 to -200 meV) excludes tip-induced quantum dots as an explanation since the potential well formed by the band bending is too shallow to accommodate said states. Also, the upward band bending at zero applied bias due to tip and sample work function differences, strongly suggested by our TIBB modeling, in combination with the Fermi level position (located within the band gap) rules out the existence of the 2D electron gas at the surface as the source for the states.

STM/S data suggests that the surface states are likely to be delocalized states originating from atomic steps on the NW surface. The most common surface defects on the {11-20} surfaces are steps in directions, i.e., with step edges parallel to the [-1-1-1] growth direction. In


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Figure 6A and 6B, we show 3D renders of STM images of such steps on the {11-20} Wz surface, obtained at +700 meV and -300 meV, respectively. A distinct difference in step morphology can be seen in the two images. At +700 meV the step edge is distinct and very sharp in contrast to the less defined and slope-like step edge seen in the image obtained at -300 meV, signifying the presence of surface states below the step edge for small negative energies. A series of line-scans normal to the step edge, at a position illustrated by the black lines in Figure 6A and 6B, was obtained at several different energies and is presented in Figure 6C. From the line-scans, it becomes evident that a high LDOS is present below the step edge and extends several nanometers away from it in energy ranges from approximately -450 to +300 meV, indicative of a step induced surface state present within that energy range. This energy interval correlates quite well with positions of the band gap states observed in the spectroscopy, see Figure 5. Also, the surface states appear to be only weakly localized to the step edge, still being present at least 5 nm away. This degree of delocalization suggests that for moderate to high step densities (as in our case) surface states would be present across the whole NW facet and should thus be seen in STS, more or less independent of the probing position.


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Figure 6. STM images and line-scans at various energies reveal the presence of surface states below step edges. (A, B) 3D rendering of STM images obtained on Wz {11-20} with a step edge along the [-1-1-1]/[000-1] direction, with Iset=50 pA and (A) Uset=+700 meV and (B) Uset=300 meV. Image size is 12x12 nm2. (C) Line-scans normal to the step edge, as denoted by the black lines in (A) and (B), from images obtained at various tip-sample biases, Ubias. The linescans are shifted along the y-axis for clarity reasons. The high LDOS just below the step edge for energies between +300 to -450 meV is indicative of a step induced surface state within that energy region.


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CONCLUSIONS

The atomic scale crystal phase QWs has important, both detrimental and positive implications, for NW functionality3-9, 26-30 but their depth and shape has not previously been directly probed when downscaling to the atomic level. In this study we have experimentally demonstrated that the smallest possible Zb segment, a single bilayer, fulfill the two criteria necessary for downscaling of NW crystal phase QWs to the atomic level; (i) the electronic structure of the crystal segments is similar to their bulk counterparts and (ii) there is an atomically sharp transition of the electronic structure between that of the segments and the surrounding crystal matrix.

Specifically, single and double bilayer Zb segments in metal-organic vapor phase epitaxy (MOVPE) grown Wz InAs NWs were investigated using atomically resolved STM/S at 5K. Due to significant similarities in the STS spectra, we concluded that the electronic structure of Zb is relatively unperturbed when downscaling even to a single bilayer. Also, STS and STM results together show that the shift in electronic structure between the single bilayer Zb segment and the surrounding Wz matrix is atomically sharp (see Figure 3). It should be pointed out that although our experimentally obtained band edges were modified to account for TIBB (so that an absolute energy scale could be used), the trends on which our conclusions are based is present also in the unmodified data. There is no reason to believe that these results are confined to InAs. An interesting aspect of our work is then that the many physical effects even at the single bilayer level can likely be predicted using the calculated bulk values for crystal phases also in other IIIV systems.

Observed differences in band onset between larger Zb segments and single bilayer/double bilayer Zb segments indicate the presence of confinement effects within the single and double


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bilayer QW structures, as seen in Figure 4. These reproducible STS results are consistent with other STM studies of material heterostructure QW structures at the few-bilayer level.47 Nonetheless, further investigation into the exact nature of these confined states would be of significant interest as they are the basis of the QW function. Similarly, investigations into other material systems as well as other types of crystal phase switching(i.e., Wz QWs in Zb or more exotic stacking order such as 4H or 6H) could further enhance our understand of the electronic structure of these quantum sized systems.

Our studies also show that surface states are not confined by the presence of the smallest crystal phase segments. These additional electronic states will then spread out as a layer on top of the QWs which could lead to a diminishing of their localizing effect on the optical or electronic properties of the NW. It is thus conceivable that the surface states shown in Figure 5 and 6 needs to be suppressed or at least factored in for both fundamental and applied systems. Our data show that the surface states are step induced suggesting that further research into minimizing the step density of NW surfaces is important to improve device performance. This level of control over step formation is not an unreasonable task, as several studies have already shown how large and uniform surface morphologies can be obtained.67, 72, 73

We have imaged how crystal engineering at the single layer level is equivalent to robust atomistic band structure engineering, an essential feature when designing quantum confinement structures with high precision and quality.3-9 Our results validate and provide important information towards using III-V NWs as a template for producing large volumes of atomically precise QW structures. This would open up the wider use of such QWs to trap charges for enhancing the performance of NW based applications such as solar cells,74 quantum memories,30 and transistors.26


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METHODS

Nanowire growth and preparation

InAs NWs with mainly Wz crystal structure and an average nominal diameter of approximately 80 nm were grown in an AIXTRON 200/4 reactor by MOVPE seeded by Au aerosol particles. Zb inclusions were varying from 3-20 nm in length, separated by 35-50 nm of Wz, and were intentionally incorporated towards the top end of the NWs. Several smaller Zb segments, only being a single or few bilayers wide, were also accidentally included in the NWs. The crystal structure was varied by controlling the III/V-precursor flow as described in ref. 75.

The NWs were transferred onto an n-doped epi-ready InAs (111)B substrate by mechanical break-off22 and loaded into ultrahigh vacuum (UHV). Removal of native oxides by exposing the sample to hydrogen radicals at temperatures of 380 ℃ was done in accordance with our previous studies of InAs NWs,22, 67, 76 with the exception that deuterium was used instead of hydrogen, which was unavailable at the time of the experiments.

STM and STS measurements

The oxide-free NWs were investigated using a CreaTec STM operating at 5K under UHV (p < 10−9 mbar) at the Center for Functional Nanomaterials at the Brookhaven National Laboratory USA. The STM was operated in a constant current mode using custom built control electronics and software called “gnome X scanning microscopy” (GXSM),77 and the set-point current and sample bias will be denoted by Iset and Ubias, respectively. Electrochemically etched Tungsten (W)-tips, cleaning in UHV by electron bombardment was used for all experiments.


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A lock-in amplifier was used for the STS measurements to simultaneously record I-V and (dI/dV)-V signals. A modulation amplitude of Vmod=40 mV and a modulation frequency of fmod=1220 Hz were used for the lock-in. The (dI/dV) signal obtained in this fashion was normalized by the total conductance (I/V), which in turn had been broadened by an exponential function, with a width of 0.5 V, as explained in ref. 78. The final curve showing (dI/dV)/(I/V)V depicts a quantity that is proportional to the local density of states (LDOS). The set-point voltage for the presented STS spectra will be referred to as Uset.

TIBB correction

TIBB gives rise to an enlargement of the observed band gap in STS. We corrected our experimentally determined band gap values by modeling the TIBB for a hyperbolically shaped probe tip in the vicinity of a semiconductor using a 3D Poisson solver.50, 79, 80 We found that the STS spectrum obtained at Zb {110} could be fitted using physically reasonable assumptions: a tip-sample separation of 0.8 nm, a tip curvature of 10 nm with a 1 nm micro tip, a contact potential (tip-sample work function difference) of 0.4 eV, a donor concentration of 2e18 cm-3, a conduction band effective mass of 0.0635m0, and heavy and light hole effective masses of 0.41m0 and 0.026m0, respectively. The surface states located within the band gap are also accounted for by assuming a Gaussian distribution of surface states centered at -100 meV relative to the Fermi level with a FWHM of 200 meV, having a state density of 5e13 cm-2 and a charge neutrality level of 0.5 eV (relative to the VB edge).81

Although we find the above-given parameters to be the most physically reasonable, slight changes to the parameters would also result in a successful fit of the Zb {110} spectra. For example, if a change in contact potential on the order of ±0.1 eV was made, a fit could still be found if the tip-sample separation and tip curvature was assumed to be 0.8 – 1.2 nm and 5 – 30


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nm, respectively. Although a tip curvature larger than 10 nm is far from unreasonable, it is improbable that we have a tip-sample separation larger than 1 nm due to the low set-point voltage of -1.2 V, and (relatively) large current of 200 pA used for the spectra. For any larger changes in the contact potential, unrealistic settings had to be used, such as assuming a degenerate surface (which is not the case since the Fermi level is located within the band gap in all spectra) or a NW doping concentration of 1021 cm-3. The sample surface state energy and concentration had a small, almost negligible, effect on the TIBB calculations.

None the less, we would like to emphasize that irrespective of the fitting parameters used (within the reasonable limits mentioned above), the results (and conclusion) will be the same. The relative differences between band gaps and onsets of the Wz, Zb and single bilayer Zb segments will remain the same, to within few meV. Only the position of the Fermi level and small changes to the band gaps values are found when using different values than what is stated above.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS

This work was performed within NanoLund at Lund University, and was supported by the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), the Swedish energy agency, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, and the European Research Council under the European Union’s Seventh Framework


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Programme Grant Agreement No. 259141. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors want to thank Dr. Percy Zahl from the Center for Functional Nanostructures for substantial experimental support.

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ACS Nano

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TOC figure


Electronic Structure Changes Due to Crystal Phase Switching at the

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