Laterally grown InxGa1−xAs nanowires (NWs) are promising candidates for radio frequency and quantum computing applications, which, however, can require atomic scale surface and interface control. This is challenging to obtain, not least due to ambient air exposure between fabrication steps, which induces surface oxidation. The geometric and electronic surface structures of InxGa1−xAs NWs and contacts, which were grown directly in a planar configuration, exposed to air, and then subsequently cleaned using atomic hydrogen, are studied using low-temperature scanning tunneling microscopy and spectroscopy (STM/S). Atomically flat facets with a root mean square roughness of 0.12 nm and the InGaAs (001) 4 × 2 surface reconstruction are observed on the top facet of the NWs and the contacts. STS shows a surface bandgap variation of 30 meV from the middle to the end of the NWs, which is attributed to a compositional variation of the In/Ga element concentration. The well-defined facets and small bandgap variations found after area selective growth and atomic hydrogen cleaning are a good starting point for achieving high-quality interfaces during further processing.

III–V semiconductor nanowires (NWs) have demonstrated significant promise for applications in (opto)electronics and now quantum computing.1–6 In particular, the surfaces of the NWs play a significant role in defining their function due to the large surface-to-bulk ratio. This can result in Fermi level pinning, recombination, changes in conductivity, as well as carrier scattering at the surfaces. Recently, selectively grown lateral NWs have gained significant interest. This so-called template-assisted selective epitaxy (TASE)7,8 or selective area growth (SAG)9–11 is especially promising for applications such as metal–oxide–semiconductor field-effect transistors (MOSFET) for radio frequency applications9,12,13 and for quantum transport in lateral NWs for Majorana-based semiconductor/superconductor qubits.1–5 

In this work, we focus on metal−organic vapor phase epitaxy (MOVPE) laterally grown InxGa1−xAs NWs, relevant for both radio frequency and quantum computing applications.2,9 MOVPE is the preferred technique for commercial synthesis of III–Vs. However, after the MOVPE growth of the NWs, the sample will be exposed to air as it is moved to other deposition systems, for example, to apply superconducting contacts and perform lithography involved in device fabrication. Therefore, the surface will oxidize, potentially introducing defects in the heterojunction contacts in the system, which likely reduces the performance in particular, important for quantum computing devices. For this reason, Molecular Beam Epitaxy (MBE) has until now been the dominant method for growing semiconductor/superconductor quantum devices, where contact formation by, e.g., Al deposition, can be integrated in the same vacuum chamber.8,14 For MOVPE-grown devices, one would need to remove the surface oxide while still maintaining highly perfected surfaces and not ruining the morphology. Atomic hydrogen cleaning has achieved this goal for several binary compound NW surfaces.15–18 However, it has not been explored if this strategy works for ternary NWs and wires grown using TASE/SAG. It is a non-trivial question as the surface structure and morphology have been found to vary depending on III-V material and nanostructuring,19 and in some cases, it has been difficult to obtain crystalline surfaces.20 Another open question for laterally grown ternary NWs is the material composition along and across the wire, which can potentially vary, inducing gradients in the surface bandgap. Detecting such local surface changes in lateral NWs is a challenge compared to free-standing NWs, which can more easily be peeled off and investigated.

Scanning tunneling microscopy (STM) and spectroscopy (STS) are unique techniques for mapping both precise topography and electronic properties down to the atom scale on surfaces. STM/S studies of NWs have become more common.15–18,21–23 While a number of STM studies exist on nanowire {110} and {11‐20} side facets,15,17,18,24–28 {001} facets of laterally grown III–V semiconductor NWs have not been studied by STM yet. In particular, there are only a few studies found for NW devices.21–23 

The InxGa1−xAs NW devices were selectively, laterally grown on a semi-insulating InP:Fe (001) substrate by MOVPE as described previously9 and in the supplementary material. Schematic illustrations of the device are shown in Fig. 1. Scanning electron microscopy (SEM) was performed using a FEI Nova NanoLab™ 600 SEM. In-air atomic force microscopy (AFM) was performed using a JPK AFM in tapping mode. STM/S was performed in ultra-high vacuum using a low temperature closed-cycle SIGMA Surface Science Infinity STM running at 9 K. Measurement details are found in the supplementary material. A commercial hydrogen cracker (MBE Komponenten) was operated at 1700 °C with a hydrogen pressure of 5 × 10−6 mbar in the UHV chamber.

FIG. 1.

Lateral NW device schematics: (a) Top view illustration of the NW sample design and material. x = 0.63 for the composition of the contact material, while x is higher in the NW material. (b) Cross-sectional schematic of the NWs and contacts. The dashed lines show where the NWs end.

FIG. 1.

Lateral NW device schematics: (a) Top view illustration of the NW sample design and material. x = 0.63 for the composition of the contact material, while x is higher in the NW material. (b) Cross-sectional schematic of the NWs and contacts. The dashed lines show where the NWs end.

Close modal

In order to confirm the in-air overall surface topography of the device, SEM and AFM imaging was performed [see the supplementary material, Figs. S1(a) and S1(b)]. This shows that the structures even after air exposure and, thus, formation of a 1–2 nm thick native oxide film29 are regular and flat. After AFM measurements, the sample was inserted into the STM UHV and hydrogen cleaning was used to remove the surface oxides. The sample was held at 420 °C during hydrogen cleaning for 50 min; afterward, the hydrogen cracker was turned off, and subsequently, the sample temperature was reduced to room temperature. The complete cleaning process consisted of 2 cleaning cycles of 50 min each. This has previously been sufficient to clean InAs and GaAs NWs.15–17,21–23 After the cleaning, the sample was inserted into the LT-STM where a temperature of 10 K was reached in about 15 min.

Initially, STM imaging was performed on the contacts to verify that the surface of the sample was clean. Then, the tip was navigated to the NW device region by a procedure described in the supplementary material. In the device region, the area with a regular pattern of parallel NWs separated by trenches could be recognized as seen in Fig. 2(a). The image compares well with the AFM images [Fig. S1(b)], and thus, the structure appears intact after the cleaning procedure and all the NWs appear similar in morphology. To study the NWs in more detail, we zoom in as shown in Fig. 2(b). The height profile taken at the same area [Fig. 2(c)] indicates that the NWs have a flat top facet and two side facets at an angle of ∼45°. The measured height of the NW in the image is 10 nm. This is a little less than the expected height of 13 nm, which can be attributed to the width of the tip, which prevents it from scanning the bottom of the trenches between the wires. Still, we can get clear images of the top and side facets of the NWs. The flatness of the NW top facet is highlighted in Fig. 2(d), with additional height profiles given in Fig. S2 of the supplementary material. The central ∼20 nm wide area of the facet has islands (∼5 nm wide and ∼0.5 nm high). The height of the islands is comparable to the lattice constant of InGaAs. The surface roughness of this central part of the NW top facet, as indicated by the dashed purple rectangle in Fig. 2(d), has a root mean square (RMS) value of 0.12 nm. These values confirm a flat surface with few major defects and a height variation corresponding to one to two atomic layers. This is consistent with previous studies of NWs after hydrogen cleaning.19,30 This low level of roughness is a good starting point for developing good semiconductor/superconductor interfaces and can be further carefully improved by optimization of the cleaning procedure.17 

FIG. 2.

STM topography images of the lateral NWs: (a) A 700 × 700 nm2 STM image shows the geometry of contacts and NWs (aligned vertically in this image). The 200 × 200 nm2 large area marked by the blue square is shown by a zoom-in image in (b). (c) A height profile taken across a NW from image (b) indicates a flat top facet and steep side facets of the NW. Green dots mark the positions where line spectroscopy data were obtained. (d) The 30 × 30 nm2 STM image, acquired on a single NW as indicated by the red arrow in (c), shows the flatness of the NW top facet.

FIG. 2.

STM topography images of the lateral NWs: (a) A 700 × 700 nm2 STM image shows the geometry of contacts and NWs (aligned vertically in this image). The 200 × 200 nm2 large area marked by the blue square is shown by a zoom-in image in (b). (c) A height profile taken across a NW from image (b) indicates a flat top facet and steep side facets of the NW. Green dots mark the positions where line spectroscopy data were obtained. (d) The 30 × 30 nm2 STM image, acquired on a single NW as indicated by the red arrow in (c), shows the flatness of the NW top facet.

Close modal

We can further zoom in to the top facet to investigate the atomic-scale surface structure of the InxGa1−xAs NW. Figure 3(a) shows periodic stripes and disordered areas, the latter likely due to random variations in the ternary alloying31 or remaining Ga-rich oxide clusters (most stable types of oxides). The periodic stripes indicate atomically ordered regions on top of the NW. The height profile shown in Fig. 3(b) reveals a distance between neighboring stripes of ∼1.7 nm and a height of ∼0.15 nm. A similar surface structure has been observed on the clean contact a few micrometers away from the NWs [indicated by the light blue dot in Fig. S1(a)], as is shown in Figs. 3(c) and 3(d), with a distance between the peaks of about 1.5 nm and a height of 0.13–0.17 nm. The observed surface pattern can be interpreted as a (4 × 2) surface reconstruction, which is common for GaAs or InAs (001) surfaces.32–36 This surface reconstruction is another indication that it is possible to obtain well-ordered NW surfaces after atomic hydrogen cleaning. The (001) surface direction of the NW top facet is unique for laterally grown NWs, while vertically grown III–V Zincblende NWs typically have {110} or {111}A/B twin plane facets.15–17,19 The angle between the NW (001) top facet and its side facets amounts to ∼45°, indicating that the side facets consist of {011} surface planes.

FIG. 3.

Surface reconstruction: (a) 15 × 15 nm2 STM image zoomed in from Fig. 2(b) at the location of the blue square shown in the inset. (b) Height profile along the blue arrow shown in (a). (c) 25 × 25 nm2 STM image taken on the contact. (d) Height profile along the blue arrow shown in (c). STM images were taken with sample biases of −3.0 V (a) and −5.5 V (c) and tunneling currents of 80 pA (a) and 50 pA (c).

FIG. 3.

Surface reconstruction: (a) 15 × 15 nm2 STM image zoomed in from Fig. 2(b) at the location of the blue square shown in the inset. (b) Height profile along the blue arrow shown in (a). (c) 25 × 25 nm2 STM image taken on the contact. (d) Height profile along the blue arrow shown in (c). STM images were taken with sample biases of −3.0 V (a) and −5.5 V (c) and tunneling currents of 80 pA (a) and 50 pA (c).

Close modal

In order to investigate the possible surface band structure variation along the NWs, STS point spectra were acquired at three positions on the NW. These positions are separated by about 75 nm, reaching from the middle of the NW toward its end, as shown in the inset of Fig. 4. Reference spectra were recorded on the contact, which is denoted position 4. From these four different places, we show spectra of the normalized differential conductance (dI/dV)/(I/V¯), which is proportional to the local density of states around the Fermi level.30 Each of the spectra shown is an average over 15 individual measurements. More measurement details are given in the supplementary material.

FIG. 4.

Bandgap variations along the NW: Averaged (dI/dV)/(I/V¯) spectra at positions 1–4, as indicated by blue squares in the inset. Dashed black lines show a linear fit of the VB (left) and CB (right) onset.

FIG. 4.

Bandgap variations along the NW: Averaged (dI/dV)/(I/V¯) spectra at positions 1–4, as indicated by blue squares in the inset. Dashed black lines show a linear fit of the VB (left) and CB (right) onset.

Close modal

In all spectra, the central area with none or very few states signifies the bandgap at the surface, while the steep increase in states for both negative and positive sample bias can be interpreted as the surface valence band (VB) and conduction band (CB) onsets, respectively. The slopes of these band onsets are fitted linearly, as shown in Fig. 4, and from the intersection of these lines with y = 0, the band edges can be determined, as described in Refs. 30 and 37. The absolute values of the obtained band onsets might be influenced by electronic effects such as tip-induced band-bending and by the presence of surface states, but the precision of the band edge determination from the experimental data and, thus, the relative error between band onsets measured at different positions is ∼3 meV as discussed in the supplementary material. From Fig. 4, an increase in the apparent bandgap from the NW center to the contact can be seen. The blue spectra recorded at position 1 (middle of the NW) show the VB edge at −210 meV and the CB edge at 200 meV, resulting in a surface bandgap of 410 meV. The VB edge remains constant for all three positions on the NW, while bandgap changes are due to the shifting of the CB edge. The fitted CB edges of positions 2 and 3 are at 210 meV and 230 meV. This results in a bandgap increase from the middle to the end of the wire with values of 410, 420, and 440 meV (position 1,2,3). On the contact (position 4), both the VB edge at −230 meV and the CB edge at 260 meV are shifted to larger values, resulting in a bandgap of 490 meV. In order to rule out the fact that the bandgap change is caused by tip changes, the first and the last dI/dV-V spectra of the entire STS series are recorded at the same area and found to be similar [Fig. S3(c) of the supplementary material].

As the variation in the bandgap is robust on top of the wire and no significant change in surface morphology is observed along the wire, the most likely explanation is a variation of the Ga/In concentration at least in the surface region. Along the NWs, the In to Ga composition ratio especially at the surface can vary, as diffusion along the wire might occur during both growth and annealing steps. Optical characterization of similarly grown lateral devices showed a higher In content in the NWs as compared to the contacts.9 Accordingly, the In concentration in the NWs might be expected to increase from the end to the middle, leading to a narrowing of the bandgap in the middle and a significantly larger bandgap at the contacts. This is in qualitative agreement with the STS observations.

As a baseline for discussing the surface bandgap change in more detail, we calculated “bulk” bandgap values depending on the In/Ga concentration in a NW as shown in the supplementary material, Fig. S4. The calculation is based on 8-band k × p theory assuming a 50 nm wide, 14 nm high, and infinitely long fully strained NW placed along the [100] direction on InP (001), as shown in the inset of supplementary material Fig. S4. The absolute values of the apparent bandgap found by STS along the NW and at the contact would indicate nearly pure InAs material at the middle of the NW and a Ga content of only about 12% at the contact material, which is far below the expected values. Tip-induced band bending (TIBB) can affect the measured bandgap, but this would result in larger experimental values, in contrast to what is observed (see the supplementary material for more discussions on this). Instead, the too low apparent bandgap can be related to the presence of surface states, giving rise to additional states at the band edges and then shifting the apparent bandgap to smaller values. A similar effect of the bandgap reduction has been observed in a previous STS study on an In0.53Ga0.47As (001) surface38 and found to be caused by the (4 × 2) surface reconstruction. Even in that study, a pure InAs (001)–(4 × 2) surface was measured, and the change in the surface bandgap between the InAs (001) and the In0.53Ga0.47As (001) samples, as observed in the STS spectra, was of approximately the same size as the change in the bulk bandgap between both materials.38 Additional studies of defects on the InxGa1−xAs (001) surface39,40 indicated a local perturbation of the electronic surface structure by defects, but a consistent average behavior was observed. Thus, there are several indications that a gradient in the bulk bandgap will be reflected in the surface bandgap for the present surface. In addition, defects that would give rise to mid-gap states can be ruled out.38,39 The k × p bulk bandgap calculations show that the increase in the Ga content mainly affects the CB edge, where the energy shift is about three times larger than that at the VB edge. This is consistent with the experimental observation that the surface VB onset is almost constant along the wire, while the surface CB onset changes. By quantitatively comparing the experimentally observed increase in the bandgap by 30 meV along the NW with the k × p calculation, we find that this corresponds to an increase in the Ga content by about 5% from the center to the ends of the wire. Compared to the values observed at the contact and assuming that this follows the nominal values of 63% In, the wires have an In content of 73%–78%. However, it must be stressed that STS measurements probe the surface region, and thus, it could be that the compositional variation is only present in the near surface region. Still, the present study shows that variations in the bandgap are small in the important surface region, and previous STS work on the surface38,39 indicates that a significant change in the bulk bandgap would be reflected in the STS as well, unless some complex In interdiffusion occurs during H cleaning.

In addition to the observed change in the bandgap along the NW long axis, we also evaluate changes in the surface electronic structure across the NW. The surface band edge values measured across the NW top facet are very stable, varying less than 50 meV between individual (non-averaged) spectra. However, we find an increase in the bandgap from the top facet to the side facet of the NW by nearly 100 meV, which we attribute to the absence of reconstruction-induced surface states on the {011} facets and to the possible influence of the tip shape. More details are found in the supplementary material and Fig. S5, including a calculation of the tip-induced band bending as a function of tip shape and surface state density.

In summary, STM and STS measurements have been performed on SAG lateral NW devices, including atomically resolved characterization of the surface structure and an analysis of the local variation of the surface bandgap. After native oxide removal by atomic hydrogen cleaning, the surface is found to be flat with an RMS roughness of only 0.12 nm. A (4 × 2) surface reconstruction is observed both on the contacts and on the (001) top facet of the InxGa1−xAs NW. A gradual increase in the surface bandgap by 30 ± 3 meV is found from the middle of the InxGa1−xAs NW toward its ends, which is consistent with a decrease in the In content by ∼5%, at least in the near surface region. This study demonstrates that a high surface quality and moderate elemental variations can be obtained after SAG and hydrogen cleaning, which opens a path toward further improved surface quality with atomic scale perfection.

See the supplementary material for details on sample preparation, AFM/STM imaging, navigation of the STM to the devices, and further details on STS measurements and their interpretation.

This work was supported by the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material