Tellurium Doping Enhances Morphology, Crystal Structure, and Conductivity of Catalyst-Free InAs Nanowires on Silicon

Background

Nanowires (NWs) are increasingly regarded as a versatile building block for emerging technologies, from field‑effect transistors and photodetectors to solar cells [1, 2]. Their exceptional properties—high aspect ratio, ultra‑low power dissipation, and, for InAs, the absence of a Schottky barrier at metal interfaces—make them attractive for both electronic and quantum‑information applications [3–9]. InAs NWs exhibit high electron mobility, a low effective mass, a large g‑factor, and strong Rashba spin‑orbit coupling [10–14].

Conventional InAs NWs are grown via a vapor‑liquid‑solid (VLS) route using gold droplets as catalysts. Gold introduces unintentional impurities and hinders silicon‑based integration [2, 15, 16]. To overcome these limitations, we employ a catalyst‑free vapor‑solid (VS) method [20]. In this mode, both the cubic zinc‑blende (ZB) and hexagonal wurtzite (WZ) phases can coexist, with polytypism and defects such as rotational twins and stacking faults adversely affecting transport and optical properties [21–28].

Doping offers a pathway to enhance charge transport, but traditional III‑V doping strategies are not directly transferable to NWs due to their anisotropic growth facets and phase‑dependent surface reconstructions [29–31]. While silicon is a common n‑type dopant, it exhibits amphoteric behavior that can be problematic in NWs [34–37]. Tellurium, a group‑VI element, is an efficient n‑type dopant with low ionization energy, minimal diffusion, and no amphoteric activity [38–41]. Te has proven to be a robust dopant in both Au‑catalyzed and self‑catalyzed NWs, influencing morphology and crystal structure [42–44].

In this study, we systematically investigate Te doping in catalyst‑free InAs NWs, focusing on morphological changes, phase evolution, and electrical performance. Scanning electron microscopy (SEM), high‑resolution TEM (HR‑TEM), XRD, and two‑point electrical measurements reveal a consistent trend: Te incorporation increases the ZB segment ratio and improves conductivity.

Methods/Experimental

InAs NWs were grown in VS mode without any foreign catalyst on n‑type Si(111) substrates.

Substrate Preparation

Substrates were cleaned with HF and DI water, then treated with hydrogen peroxide for 45 s to form a few‑angstrom SiO₂ layer containing pinholes that act as nucleation centers [20]. After oxidation, samples were transferred to a load‑lock, heated to 200 °C for 45 min, and degassed at 400 °C for another 45 min.

Growth of the InAs Nanowires

Growth was performed at 475 °C for 1 h 20 min in an Omicron Pro 100 MBE chamber. An in‑growth rate of 0.1 µm h⁻¹ was used. Arsenic was supplied via an As cracker cell, with As₄ beam equivalent pressures (BEP) of 2.3 × 10^–5 Torr (series B) and 3.3 × 10^–5 Torr (series A). Tellurium was introduced through a GaTe source; the effusion cell temperature was varied from 401 °C to 562 °C, corresponding to carrier concentrations from 1 × 10¹⁵ cm⁻³ to 6 × 10¹⁹ cm⁻³ in calibration GaAs layers [38].

Device Processing

For two‑point contacts, NWs were mechanically transferred onto a pre‑patterned Si substrate with 200 nm SiO₂. After spin‑coating a three‑layer PMMA resist stack, e‑beam lithography defined the contact geometry. The contact area was passivated with diluted 3.5 % ammonium polysulfide at 60 °C for 30 min. Electrodes of 100 nm Ti and 40 nm Au were deposited by e‑beam evaporation.

The full list of samples examined by SEM, TEM, XRD, and electrical measurements is provided in Table 1. Letters A, B, and C denote series grown at different As partial pressures; a GaTe temperature of 0 °C indicates a closed shutter.

Results and Discussion

Morphology

SEM imaging shows that Te doping reduces wire length and increases diameter. Figure 1a displays series A (As‑BEP ≈ 3.3 × 10^–5 Torr) and Figure 1b shows series B (As‑BEP ≈ 2.3 × 10^–5 Torr). Up to a GaTe‑cell temperature of 500 °C, no clear trend emerges, but at 561 °C Te supply sharply increases diameter and shortens the wires. Series B exhibits a more pronounced length reduction at 401 °C, suggesting that lower As pressure amplifies Te’s impact on morphology.

Te’s large covalent radius likely acts as a surfactant, reducing indium adatom diffusivity and favoring radial over axial growth [48, 49]. Increasing the As pressure appears to mitigate this effect, offering a route to control morphological changes.

Crystal Structure

Using HR‑TEM and XRD, we quantified the ZB/(WZ + ZB) segment ratio. A stacking sequence of four bilayers defines a ZB segment (…ABCA…) or a WZ segment (…ABAB…). Segments lacking a full four‑bilayer sequence were classified as stacking faults or twins.

Figure 3 shows that the ZB/(WZ + ZB) ratio rises with higher GaTe‑cell temperatures in both series. At 447 °C, the ratio increases mainly due to a greater number of ZB segments, while at 500 °C WZ segments decrease and stacking faults rise slightly. Overall, Te incorporation promotes ZB formation, a trend more evident at higher As partial pressures (series A).

XRD φ‑scans confirm this behavior. The ZB (220) and WZ (10 1 1) reflections were isolated, and the intensity ratio I_ZB/(I_ZB + I_WZ) plotted in Figure 5b increases with Te doping, especially for series A. The weaker response in series B underscores the role of As in facilitating Te incorporation.

Electrical Measurements

Two‑point I–V curves show ohmic behavior consistent with InAs surface accumulation layers [6, 52]. Conductivity σ = A R L⁻¹ w was calculated using the hexagonal cross‑section formula A = 3√3 d_NW²/8, where d_NW and L_w were measured by SEM.

Figure 6e illustrates that conductivity increases by roughly an order of magnitude at the highest Te doping (GaTe‑cell 500 °C), reaching ≈ 80 S cm⁻¹ versus ≈ 8 S cm⁻¹ for undoped wires. The stronger conductivity gain in series A correlates with its higher ZB/(WZ + ZB) ratio. While crystal‑phase improvement alone can enhance conductivity by ~50 % [50, 53, 56], the tenfold increase observed here also reflects the elevated carrier density from Te donors.

Variability in conductivity arises from heterogeneous contact passivation, surface oxidation, and possible axial inhomogeneity of Te incorporation. Further studies, such as in‑situ Al₂O₃ passivation, are needed to reduce this spread.

Conclusion

We successfully grew Te‑doped catalyst‑free InAs nanowires on Si(111) via VS growth, demonstrating that Te modulates morphology, crystal phase, and electrical performance. Te increases the ZB/(WZ + ZB) ratio and boosts conductivity by an order of magnitude, particularly at higher arsenic partial pressures. These findings expand doping strategies for III‑V NWs, providing a pathway to mitigate defect‑induced transport degradation.