Morphology Transition of Te-Doped InAs Nanowire on InP(111)B Grown Using MOCVD Method

Abstract

In this paper, we reported changes in the growth morphology of n+InAs nanowires (NWs) doped with Te which were selectively grown on nano-hole patterned InP(111)B substrates using an MOCVD method. While the vertical growth of InAs NWs in the <111> direction was extremely suppressed, their lateral growth was enhanced when the diethyl-tellurium (DETe) flow rate was increased as they grew. Moreover, the sidewall planes evolved from ($1\overline{1}0$) (90° against the (111) plane) to a reverse-tapered morphology, which had a 62° slope against the InP (111)B plane, when the Te flow rate and growth time were increased. This indicates that the surfactant effect of adsorbed Te atoms on InAs changes the relative growth rate between (111) and ($1\overline{1}0$) due to the increase in surface free energy in the growth plane.

1. Introduction

Nanowires (NWs) have attracted attention due to their applications in several fields, such as in high-sensitivity sensors and electronic and optical devices [1,2,3,4,5,6,7]. This is because semiconductor NWs have been considered as a building block [8]. The potential of III-V semiconductors is being studied in terms of their advantage of having higher electron mobility than Si [9,10,11]. Particularly, InAs, which is a III-V semiconductor, has a narrow bandgap (E_g ~ 0.35 eV) and has a large electron mobility of 40,000 cm² V⁻¹ s⁻¹. These properties make it suitable for use in quantum wells, quantum dots, and Tunnel Field-Effect Transistors (TFETs).

Growing InAs NWs using Metal Organic Chemical Vapor Deposition (MOCVD) can typically be achieved in two ways: the vapor–liquid–solid (VLS) mode and the vapor–solid (VS) mode. The VLS mode is a typical method used to grow NWs with Au droplets, but it has the disadvantage of degrading their characteristics due to the addition of impurities from the Au droplets [12,13]. On the other hand, the VS mode enables the growth of NWs without the use of any droplets that generate impurities.

Unlike bulk III-V materials which only consist of a cubic Zinc Blende (ZB) phase, III-V NWs have a mixture of the ZB and the hexagonal Wurtzite (WZ) phase, which is known as a polytypism. In addition, twins and stacking faults in III-V NWs have also been reported [14,15]. As differences in the energy band exist between WZ and ZB in the VS mode, polytypism affects electrical and optical characteristics [16]. Moreover, twins and stacking faults in NWs are reported to have the same results [17,18,19]. A degradation in charge transport can be counteracted by doping NWs. It has been proven that the use of in situ-doped InAs nanowires is an effective way to achieve high doping levels and to attain junction abruptness in nanowires, which are important characteristics in tunnel diodes and TFETs [20,21].

Materials used for the n-type doping of InAs are Si (group IV material), S, Se, and Te (group VI materials). As Si displays amphoteric properties, it can act as a p-type or an n-type dopant depending on the substrate orientation and growth conditions [22]. On the other hand, group VI materials act as effective n-type dopants without amphoteric behavior. Group VI elements are used as n-type dopants due to donor impurities. Tellurium was previously accumulated by displacing As atoms from an As-stabilized growth surface and forming a stable surface compound in GaAs [23]. This means that Te was bonded to Indium by displacing the As atoms. The advantages of using Te (a group VI material) as a dopant are that it does not have amphoteric properties, it has higher doping levels, and it has a lower diffusion coefficient [23,24,25,26]. However, doping can affect the nucleation and crystal structure of NWs [27].

In general, NWs have different lateral and vertical crystal orientations and surface reconstructions. These differences create different doping incorporation and growth behaviors in NWs. For instance, previous studies have shown that the lateral growth of NWs increased with Si doping [26,28]. It has been shown that the growth characteristics of Te-doped InAs NWs grown on Si(111) via Molecular Beam Epitaxy (MBE) change from being doped [8]. Surfactants (i.e., Sb and Te) are assumed to trigger lateral growth and a decrease in the vertical growth of NWs due to a decrease in the diffusion length of adatoms on the sidewall [8,22,29,30,31]. Through the surface energy approach, it has been shown that NWs have various growth facets with different surface energies related to crystal growth rates. The effects of surfactants modify both surface diffusion and surface energy via surfactant adhesion [30,32,33,34]. The growth morphology of NWs can be affected by surfactants because of the orientation of various vertical and horizontal crystal planes.

In this study, we investigated changes in the growth morphology of Te-doped InAs NWs grown using MOCVD by varying the gas flow of diethyl-tellurium (DETe) and the growth time. The diameter and height of NWs were measured using a Scanning Electron Microscope (SEM). In addition to the lateral growth shown in previous studies, we observed that the addition of Te dopants affects the transition of sidewall formation in NWs. Scanning Transmission Electron Microscopy (STEM) measurement was used to identify reverse-tapering sidewalls in Te-doped InAs NWs.

2. Materials and Methods

A patterned InP(111)B substrate with a SiO₂ mask was prepared via E-beam lithography. An 80 nm diameter hole pattern with a 1 µm pitch was designed. Before the E-beam lithography process, 10 nm thick SiO₂ was deposited on the InP(111)B substrate via Plasma-Enhanced Chemical Vapor Deposition (PECVD). SiO₂ was selectively etched via wet etching with a 30:1 BOE solution after Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE). Te-doped InAs NWs were grown on patterned InP(111)B with the vapor–solid method using AIXTRON AIX 200/4 MOCVD. During the chamber heating stage (before the growth stage), PH₃ gas was flowed into the chamber from 400 °C to 630 °C to prevent thermal decomposition on the InP(111)B substrate. Diethyl-tellurium (DETe) was continuously flowed to grow the in situ-doped nanowires on seed NWs. The total gas flow rate into the chamber (which included the hydrogen (H₂) carrier gas) was fixed at a flow rate of 10.5 slm. The chamber pressure was maintained at 160 mbar throughout every growth stage. Trimethyl-Indium (TMIn) was fixed at 0.02 sccm, and AsH₃ was fixed at 10 sccm. The DETe flow rates were varied from 4.04 × 10⁻⁵ to 2.00 × 10⁻³ sccm. The Te doping concentration in the InAs(111) film grown with a DETe flow rate of 4.04 × 10⁻⁵ sccm was 4.31 × 10¹⁹ cm⁻³, which was measured using a Van der Paw Hall measurement. In order to investigate the morphology evolution, the growth time of Te-doped NWs was varied from 0 min to 3 min 50 s under a DETe flow rate of 2.00 × 10⁻³ sccm. The growth morphology and characteristics of NWs were defined via Scanning Electron Microscopy (SEM), Selective Area Electron Diffraction (SAED), and Transmission Electron Microscopy (TEM).

3. Results and Discussion

Figure 1 shows the tilted-view SEM images of InAs NW samples grown under the un-doped and Te-doping conditions. InAs NWs were grown with a 0.02 sccm TMIn source, 10 sccm of AsH₃ gas at 630 °C, and 160 mbar for 5.5 min. Un-doped InAs NWs were uniformly grown with a height of ~840 nm on nano-hole patterned InP(111)B, as shown in Figure 1a,b. The InAs NWs only grew in the vertical direction [111], with six sidewall planes <$1\overline{1}0$> without lateral growth, as shown in Figure 1b. It has been reported that the vertical growth of nanowires progresses layer by layer in such a way that nucleation proceeds on the (111)B plane and then spreads to other surfaces [35,36]. Thus, un-doped InAs NWs showed this vertical growth behavior while forming sidewalls, as shown in Figure 1b.

In contrast, adding DETe (the gas flow: 4.04 × 10⁻⁵ sccm) for Te doping under the same InAs growth condition as that shown in Figure 1a resulted in a different growth behavior. Figure 1c and 1d show the morphology of Te-doped InAs NWs. The nano-holes were filled with Te-doped InAs and then grew laterally on the SiO₂ surface without vertical growth on six sidewall planes, as shown in Figure 1c,d. However, when grown with the Te dopant without un-doped InAs seed NWs, they showed different growth behavior. Without InAs seed NWs (Figure 1d), the patterned hole was filled via InAs:Te nucleation and grew laterally rather than vertically. Although the nucleation of Te-doped InAs in the nano-hole pattern proceeded successfully, it seems that the InAs growth in the vertical direction <111> was remarkably suppressed due to Te dopants being adsorbed on the top surface of the nucleation in the nano-hole patterns. It was reported that the top surface of NWs had low-index planes, including the (111)B surface plane [37,38,39]. Therefore, we assumed that the nucleation in the SiO₂ hole pattern had low-index planes. We expected Te to affect the surface energy in areas other than the area where nucleation was performed due to the Te dopant. Therefore, it was difficult for growth to occur layer by layer, as was observed in the growth of un-doped InAs NWs. These results verify the abnormal growth morphology that is shown in Figure 1d.

In order to mitigate the effect of Te on nucleation, un-doped InAs seed NWs were used before we investigated Te-doped InAs NWs’ growth; the results are shown in Figure 2. Un-doped InAs NWs with seed layers were grown for 40 s under the same growth condition as that shown in Figure 1a,b. Seed NWs were 80 nm in diameter and 110 nm in height with hexagonal sidewalls, as shown in Figure 1b.

Figure 2a shows Te-doped InAs NWs grown on un-doped InAs seed NWs. Unlike the results shown in Figure 1c, Te-doped InAs NWs were formed with uniform NW shapes. Figure 2b shows a high-resolution SEM image of Figure 2a. Te-doped InAs NWs grown on un-doped InAs seed NWs had hexagonal sidewalls, as shown in Figure 1b. The height of the Te-doped InAs NWs on un-doped InAs seed NWs was 140 nm, and the diameter was 210 nm. The growth characteristics of Te-doped InAs NWs with or without InAs seed NWs were completely different, as shown in Figure 1d and Figure 2b. By comparing the dimensions displayed in Figure 1d and Figure 2b, we observed that the growth of Te-doped InAs NWs mainly proceeded in the lateral direction rather than the vertical direction. Figure 2c represents the cross-sectional schematics of the NWs’ shape evolution when Te-doped InAs NWs were grown on InAs seed NWs. After growing un-doped InAs seed NWs which grew vertically on the nano-hole pattern, Te-doped InAs NWs proceeded to preferentially grow in the lateral direction, unlike un-doped InAs NWs. The growth behavior of the increasing diameter and decreasing height of NWs was previously reported by research groups who used Te or Sb as the dopant element when they grew NWs [8,22,32]. From these results, we expected the Te dopant to affect not only the diffusion length but also the surface energy of InAs NWs. Therefore, we adopted un-doped InAs seed NWs for Te-doped InAs NWs to form the shape of NWs.

Varying the gas flow rate of DETe during Te-doped InAs NWs’ growth for 3 min resulted in dramatic morphological changes to the sidewalls, as shown in Figure 3a–d. Un-doped and Te-doped InAs NWs (shown in Figure 3a–d) were grown on seed InAs NWs. Figure 3a shows InAs NWs grown without DETe with a typical NW structure, with ~6 µm heights and ~130 nm diameters. Compared with the dimensions of seed InAs NWs (being 80 nm in diameter and 110 nm in height), un-doped NWs mainly grew in the vertical direction, accompanied by minimal lateral growth. In contrast, by adding DETe flow rates (4.04 × 10⁻⁵ sccm) during growth, Te-doped InAs NWs mainly grew in the lateral direction, while the sidewalls were maintained with the pre-formed ($1\overline{1}0$) sidewalls of seed InAs NWs, as shown in Figure 3b. Figure 2 also shows these results. Further increases in the gas flow rate of DETe to 4.04 × 10⁻⁴ sccm resulted in the symmetric morphology breaking and the plane orientation of the NWs’ sidewalls changing, as shown in Figure 3c. When the DETe gas flow reached 2.00 × 10⁻³ sccm (Figure 3d), the reverse-tapered sidewalls were formed with symmetric sidewalls. The plot in Figure 3e shows the variation in the height and diameter of NWs due to the application of different DETe flow rates. The height of InAs NWs rapidly decreased from 6.2 μm to 0.3 μm upon the addition of DETe. In contrast, the NWs’ diameters measured on the top surface increased. However, the gas flow of DETe (4.04 × 10⁻⁵ sccm) caused large variations in the diameter.

The growth of InAs NWs has been studied via the pitch distance dependence [39]. S. Hertenberger et al. suggested that the collection area (which is also called the adatom capture area) affected the growth of InAs NWs. The collection area is one of the important parameters in the growth of NWs [40]. This area is defined by the diffusion length of adatoms on SiO₂. The source of growth for Te-doped InAs NWs could be the diffusion of adatoms on SiO₂ patterns. This is thought to affect adatom diffusion on SiO₂ when the flow rate of DETe is increased. This caused Te-doped InAs NWs to grow ununiformly. Furthermore, those results imply that Te adatoms on the surface limit vertical growth and evolve new sidewalls with different orientations.

In the conventional NW growth mode, the vertical growth of NWs proceeds through adatom diffusion to the top surface through the sidewall surfaces [41,42]. This adatom diffusion leads to the idea that the growth rate of the top surface ($111$)B ($G{R}_{\left(111\right)B}$) is larger than the sidewall ($1\overline{1}0$) ($G{R}_{\left(1\overline{1}0\right)}$). When NWs reach the critical height, the lateral growth is triggered by the adsorption of adatoms on the sidewall [43]. Therefore, un-doped InAs NWs can reach the critical height without lateral growth. In contrast, in Te-doped InAs NWs, it was shown that the vertical growth is limited and the lateral growth begins in the initial stage, even if the height is under the critical height, as described in Figure 2 and Figure 3. A similar tendency of doped NWs’ lateral-growth-enhancing behavior was reported, using the results of Au-catalyzed or self-catalyzed Te-doped GaAs NWs [22] and MBE-grown Te-doped InAs NWs [8]. In articles regarding the surfactant effect of Sb and Te during NW growth, a morphology change in NWs was reported when the surfactant elements were supplied during NW growth, and this was explained by the decrease in the diffusion length of adatoms, which originated from adsorbed surfactant elements on the NWs’ sidewalls [8,22,31]. According to the kinetic simulation carried out by [30], Te increases both the adatom sticking coefficiency and the Ehrlich–Schwoebel barrier height. Consequently, it was shown that Te doping decreases the diffusion length of the adatom on the sidewalls. This creates different orientations for nucleation and growth on the sidewalls, which results in lateral-direction growth. Additionally, besides the enhanced lateral growth of Te-doped InAs NWs shown in Figure 3b, distinct growth behavior of the sidewalls was observed as the Te flow rate increased in our results, as shown in Figure 3c,d. The ($1\overline{1}0$) sidewalls evolved into reverse-tapered structures as the Te-doped InAs NWs were grown on the seed InAs NWs.

To understand how the reverse-tapered sidewalls evolved via Te doping, we varied the growth time from 0 to 4 min for Te-doped InAs NWs at a DETe flow rate of 2.00 × 10⁻³ sccm, which resulted in reverse-tapered sidewalls. The evolution of InAs NWs’ shape and dimensions over growth time is shown and plotted in Figure 4.

Seed InAs NWs grown for 1 min 40 s were ∼100 nm in height and ∼90 nm in diameter, as shown in Figure 4a. As the growth time of Te-doped InAs NWs increased, the diameter continuously increased from ∼90 nm at 0 min to ∼220 nm at 4 min. The height of Te-doped InAs NWs slightly increased from ∼100 nm at 0 min to ∼110 nm at 4 min. Accordingly, it was confirmed that mainly lateral growth occurred, while vertical growth was suppressed from the initial growth under the DETe gas flow condition of 2.00 × 10⁻³ sccm. Figure 4 shows the magnified tilted-view SEM images from each growth time. Note that the transition of the sidewalls was observed in Te-doped InAs NWs grown for 1 min, as shown in Figure 4b. It was confirmed that the sidewalls of the seed InAs NWs—shown in Figure 4a—were ($1\overline{1}0$) planes. However, the Te-doped InAs NWs which were grown for 1 min no longer maintained lateral growth in the ($1\overline{1}0$) sidewall direction from the beginning of growth and formed sidewall planes with different orientations, as shown in Figure 4b. As the growth time increased, newly formed sidewall planes were formed uniformly and reversely tapered in shape, as shown in Figure 4c,d. These evolutions mean that the InAs NWs’ growth rates in both the vertical [111] and lateral [$1\overline{1}0]$ directions decreased significantly during growth and consequently formed NWs with new growth planes.

Regarding the relationship between the growth rate and surface free energy, we may conclude that the surface free energy in the planes changed when Te atoms were adsorbed on them, and this resulted in a change in the growth rate with crystal orientations. When un-doped InAs NW grew on nano-hole patterned InP(111)B, they only grew in the (111)B direction, because the InAs NW (111) plane had a relatively higher surface energy than the ($1\overline{1}0$) planes. However, when the surfactant Te atom was adsorbed on the NW surface, lateral growth prevailed through the nucleation and growth of new sidewall planes on the ($1\overline{1}0$) planes, which were promoted due to the decrease in the In adatom diffusivity and increase in the surface free energy of sidewall ($1\overline{1}0$) planes [8,28,29,30,31]. When InAs nanostructures were grown with Bi surfactants on GaAs substrates, it was reported that InAs islands were induced on the NW sidewalls via the Stranski–Krastanov mode [44]. In addition, J. Massies et al. reported that Te acted as a reactive surfactant which bound strongly with neighboring atoms. This increased the surface free energy of the sidewall ($1\overline{1}0$) planes, which led to a new surface with lower surface free energy [45].

Evidence of InAs nucleation with different orientations on the ($1\overline{1}0$) planes is observable in the saw-tooth shape in Figure 4b when compared with Figure 4a. When explaining the lateral growth of NWs, it was reported that they grow due to nucleation in the sidewall. The top surface (111)B terminated with the As adatom plane. Layer by layer growth proceeded due to the characteristics of III-V semiconductors, meaning the surface did not terminate by one adatom in the case of the sidewall. This suggests that a Te dopant combined with an In adatom on a sidewall changes the surface free energy of sidewall ($1\overline{1}0$). From this observation, the InAs nuclei on the sidewall could form several low-index planes via Te doping which were different from the sidewall ($1\overline{1}0$) planes of seed InAs NWs. As the growth time increased, the sidewall direction was gradually unified and uniformly formed with reverse-tapered shapes, as shown in Figure 4c,d.

We investigated reverse-tapered InAs NWs corresponding with Figure 4d using cross-sectional STEM to identify the sidewall plane orientation. A cross-sectional STEM image for the sample is shown in Figure 5a. The angle between the InP(111)B substrate and the red dashed lined sidewall was measured to be $62 °$, and a six-fold symmetrical sidewall was present. In general, InAs NWs showed crystallographic polytypism in which two crystal phases of ZB and WZ coexisted [46]. Figure 5b also shows the polytypism diffraction. The plane was indexed as the ($1\overline{1}0\overline{1}$) plane, as shown in Figure 5b. Figure 5c shows the projection of the WZ crystal structure on the [$11\overline{2}0$] zone axis. Figure 5c displays the structure of WZ to help explain this, but we think that WZ and ZB are mixed in Te-doped InAs NWs. Regarding reverse-tapering sidewalls, note that the formation of reverse-tapered sidewalls is quite different from the normal lateral growth of NWs, which form either a tapering shape from the bottom to the top or equal growth on the top and bottom part of the sidewall. In our case, the reverse-tapered sidewalls in the Te-doped InAs NWs were widened from the bottom up without lateral growth from the bottom.

This was expected because of the several low-index planes on the ($1\overline{1}0$) planes of seed InAs NWs by the nucleation on the sidewall. When InAs NWs were grown on InP(111)B, In atoms and As atoms grew alternately on the As-exposed InP(111)B plane. The top surfaces of InAs NWs were also composed of the InAs(111)B surface, which was terminated with As atoms and had relatively large surface energy. Therefore, the vertical growth of InAs NWs in the (111) direction was faster than the growth in the sidewalls. In the case of the sidewalls ($1\overline{1}0$) of un-doped InAs seed NWs, it is difficult to say that the sidewall was terminated with an In adatom or As adatom compared to the (111)B plane due to the stacking of III-V semiconductors. Therefore, it was thought that the Te dopant was adsorbed to the sidewall to change the sidewall surface energy, and a surface with a low growth rate was formed in the process of growing the sidewall after nucleation. In Te-doped InAs NWs with reverse-tapered sidewalls, it was shown that the vertical growth was suppressed and formed new stable sidewall planes. Figure 5c shows that the reverse-tapering sidewalls were formed with the ($1\overline{1}0\overline{1}$) plane which was terminated with an As atom, as in the (111)B plane. Considering this, we could speculate that the plane terminated by As atoms had the lowest surface free energy via the Te surfactant. From this discussion, we could conclude that Te adatoms on As-terminated InAs planes lower the surface free energy, and therefore, the growth rate of As terminated in the [111] direction is decreased.

4. Conclusions

In this paper, Te-doped InAs NWs were grown on seed InAs NWs via MOCVD in the VS mode. The amount of Te that was doped in NWs was controlled by changing the DETe gas flow rate. The Te-doped InAs NWs were analyzed via SEM and STEM. Morphological changes in NWs that suppressed vertical growth and increased lateral growth were found according to the DETe gas flow rate. Additionally, STEM showed that the sidewall plane direction of the previously grown un-doped InAs NWs gradually changed from the ($1\overline{1}0$) plane to the ($1\overline{1}01$) planes and was symmetrical at the highest DETe ratio. This shows that surface free energy is affected by the surfactant effect of Te. As a result, it was shown that the sidewalls of Te-doped NWs grow in a reverse-tapering shape.