Molecular beam epitaxial In2Te3 electronic devices

Recently, there has been increasing interest in the preparation and characterization of group III-VI chalcogenide semiconductor material-based electronic and optoelectronic device applications^{1,2,3,4,5,6,7,8,9,10,11}. Among these materials, In₂Te₃ stands out as a narrow-band gap semiconductor with diverse applications in superconductors, optoelectronics, and electronic devices. For these applications, high-quality In₂Te₃ thin films are necessary. However, achieving high-quality In₂Te₃ films remains challenging despite the use of methods such as chemical vapor deposition (CVD) and pulsed laser deposition (PLD)^12,13. The variable stoichiometries of indium telluride (e.g., In₂Te₃, InTe, In₄Te₃, and In₇Te₁₀) complicate composition control^11,12,14,15, posing significant obstacles to the preparation of high-quality In₂Te₃ films.

Here, molecular beam epitaxy (MBE) was employed to achieve precise control over the composition and phase, enabling the production of high-quality In₂Te₃ thin films. In this study, we report on the fabrication and characterization of MBE-grown In₂Te₃ field-effect transistors (FETs) and Schottky diodes.

In₂Te₃ growth using MBE

In₂Te₃ thin films were grown on hexagonal boron nitride (h-BN) substrates using a custom-built MBE (Fig. S1) system for the growth of (In_xGa_1-x)₂(Se_yTe_1-y)₃ materials. The chamber base pressure of the MBE system was low at 10^–9 Torr. The h-BN layers were mechanically exfoliated onto a Si wafer covered with 300 nm-thick SiO₂ for substrate preparation. High-purity indium and tellurium fluxes were evaporated by Knudsen cells. The Te/In beam flux ratio was 14–16 for the growth. Before the 1st step of growth, a thermal cleaning process was conducted for 20 min at 500 °C under ultrahigh vacuum (UHV) conditions before the growth of In₂Te₃. First, the 1st step of growth was performed at 270 °C for 4 min. After this, the indium Knudsen cell shutter was closed, and the substrate temperature gradually increased to 570 °C at a rate of 10 °C per minute with tellurium flux. Further, the second step of growth was conducted at 570 °C for 9 min, after which the indium shutter was closed. The substrate temperature was subsequently decreased to room temperature at a rate of 10 °C per minute with tellurium flux. Finally, the substrate shutter was closed at 500 °C.

Optical characterization

A micro-Raman system was used to measure samples in ambient air at room temperature utilizing an ~150 μW, 532 nm laser. To acquire the Raman spectra, an accumulation time of 20 s was used.

Surface morphological and structural characterization

To study the surface morphology of In₂Te₃ films on h-BN substrates, field-emission scanning electron microscopy (SEM) (ZEISS-SUPRA, SIGMA, MERLIN COMPACT) and atomic force microscopy (AFM) (NX-10) were used. A 200 kV CS-corrected monochromatic transmission electron microscope (TEM) (Themis Z) was used for high-resolution scanning transmission electron microscopy (HR-STEM) imaging.

Device fabrication

Initially, h-BN layers were placed on a SiO₂/Si wafer, which acted as the back gate dielectric material and electrode, respectively. The MBE method was subsequently used to grow In₂Te₃ on h-BN substrates. Using an e-beam evaporator, an array of aligner markers (Ti/Au 10/30 nm) were deposited. For the ohmic contacts and Schottky contacts, Pd/Au 10/30 nm and Ti/Au 10/30 nm materials were deposited using an e-beam evaporator. The negative e-beam resist ma-N 2405 (microresist technology) layer was spin-coated (1000/4000 rpm 10/60 s). For patterning the ma-N 2405 layer, e-beam lithography was used. Unwanted In₂Te₃ films were then etched using reactive ion etching with Cl₂ gas. The electrical measurements were carried out in a Keithley 4200 semiconductor analyzer with 2601 A and 2400 source meters.

To study growth behavior, the surface morphology of the In₂Te₃ thin films were investigated at various growth temperatures, which ranged from 420 °C to 570 °C (Fig. 1a). Facets were absent at a lower growth temperature of 420 °C but became discernible as the temperature increased. The domain sizes expand from several tens of nanometers (at 470 °C) to several hundreds of nanometers (at 520 °C), suggesting that higher temperatures lead to larger terraces and smoother surfaces. However, no growth was observed at 570 °C, which can make uniform thin film formation difficult for electrical applications, primarily due to the lack of dangling bonds on h-BN, as previously reported¹⁶.

a SEM images of one-step-grown films at different growth temperatures of 420, 470, 520, and 570 °C, respectively. b SEM images of two-step-grown films with initial deposition at 270 °C and second-step growth at 550, 570, and 590 °C, respectively. c AFM image of In₂Te₃ grown on h-BN. AFM line profile obtained across the red line in the main figure shown in the inset in c. The Scale bar for the inset is 200 nm.

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A two-step growth strategy was adopted to address this issue, enhancing the number of nucleation sites at lower temperatures before proceeding to higher temperatures for full-coverage film growth. Initially, at a low temperature, the first growth step aimed to ensure nearly complete surface coverage. Following this, while the substrate temperature was gradually increased, the films were annealed under tellurium flux for several minutes. This approach effectively addresses the challenges posed by the inert nature of h-BN at high temperatures, increasing the number of nucleation sites and preventing film texturing. The effectiveness of this two-step growth approach is depicted in the surface morphologies of the films grown at various second step temperatures, as illustrated in Fig. 1b. Since the films that nucleated below this temperature maintained island structures even after high-temperature annealing, 270 °C was chosen as the growth temperature for the first step. For the second step, higher temperatures of 550 °C and 570 °C led to a flat and uniform surface morphology. However, at temperatures of approximately 590 °C, the surface coverage decreased, likely due to the re-evaporation of the In₂Te₃ films. Based on these observations, the optimal temperatures for the first and second steps were established at 270 °C and 550–570 °C, respectively.

AFM was employed to obtain atomic-scale surface information on the In₂Te₃ films. AFM analysis revealed smooth terraces spanning an area of over 200 nm, indicative of desirable growth behavior that produces atomically smooth films (Fig. 1c). Additionally, the AFM image displayed well-defined crystal facets in each domain, with a root mean square (RMS) roughness of 1.4 nm.

The atomic arrangement and interfacial quality of In₂Te₃/h-BN were analyzed using HR-STEM. Cross-sectional HR-STEM imaging revealed a thickness of In₂Te₃ ~ 12.2 ± 0.2 nm (the inset of Fig. 2a) and a clean interface between In₂Te₃ and h-BN, free of any interfacial layers, cracks, or dislocations, with In₂Te₃ exhibiting well-aligned growth directions parallel to the h-BN substrate (Fig. 2a). The composition at the interface was determined using TEM-energy dispersive X-ray spectroscopy (EDS) mapping, revealing atomic percentages of 40.2% indium and 59.8% tellurium, which is consistent with the stoichiometric composition of In₂Te₃, as depicted in Fig. 2b. Further stoichiometric characterization was conducted using Raman spectroscopy. As detailed in Fig. S1, three prominent peaks were identified at 104, 125, and 142 cm⁻¹. These peaks are characteristic of In₂Te₃ crystal symmetry and specifically correspond to the A_1g and E_g vibrational modes, respectively. These peaks highly correspond to In₂Te₃. These results are consistent with those in Fig. 2b. This precise control of composition underscores the benefits of using MBE to maintain a clean interface.

a Cross-sectional HR-STEM image of the In₂Te₃/h-BN heterointerface. The inset in a shows the low-magnification STEM image in the main figure. b Elemental mapping of In and Te using STEM–EDS. Scale bar for the inset, 1 nm.

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As illustrated in Fig. 3, these In₂Te₃ thin films grown on h-BN were utilized for electronic device fabrication. Initially, Pd/Au bilayer contacts were deposited on In₂Te₃/h-BN using e-beam evaporation to form ohmic contacts. The channel was subsequently defined through reactive ion etching (RIE) with chlorine gas, employing a negative tone resist.

Schematic representation of the fabrication process and cross-sectional schematic of the back-gated In₂Te₃ FETs.

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The electrical characteristics of the back-gated In₂Te₃ field-effect transistors (FETs) were evaluated by measuring the gate bias (V_gs)-dependent current (I_ds) curve. Typically, the back-gated In₂Te₃ FETs are on, and as V_gs increases, I_ds decreases. The threshold voltage is identified as V_gs = −10 V, at which point the device is turned off (Fig. 4a). The output characteristics demonstrate that the back-gated In₂Te₃ FETs function as typical p-channel FETs. Unlike conventional FETs, no I_ds saturation is observed, suggesting that traditional channel pinch-off and channel length modulation do not occur. The current gradually decreases as the gate voltage increases according to the V_gs–I_ds curves for a V_ds of 1 V (Fig. 4a). These transfer characteristics indicate typical p-channel FET behavior and operation in depletion mode. As V_gs increases, the current decreases drastically to approximately 10⁻¹⁰ A above −16 V, which is the threshold voltage (V_th) of the back-gated In₂Te₃ FETs. A high I_max/I_min ratio of 10⁵ was observed.

a Output curve, b transfer curve, and (c) benchmarking p-channel transistors based on group III-VI chalcogenides (field-effect mobility versus on/off ratio).

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Another important indicator of transistor performance is transconductance (g_m = dI_ds/dV_gs), which peaks at maximum values close to the subthreshold region at 28 nS µm⁻¹ when V_ds is set to 1 V. The peak field-effect mobility, determined from the transconductance in the linear regime (V_ds = 1 V), was measured to be 6.07 cm² V⁻¹ s⁻¹, which is higher than that in previous works. The subthreshold swing was 6 V dec⁻¹, and the gate leakage current was 10⁻¹⁰ A (Fig. 4b). Compared with other p-channel transistors fabricated using various methods and materials from the group III-VI chalcogenide family, the MBE-grown In₂Te₃ from the current study, demonstrates enhanced performance, making it more suitable for switching devices (Fig. 4c).

Furthermore, In₂Te₃ Schottky diodes were fabricated for potential applications in logic circuits alongside FETs. To evaluate their electrical properties, the I–V characteristics of the Ti/In₂Te₃ Schottky diodes were measured, which formed a metal/semiconductor Schottky junction with a titanium (Ti) electrode (Fig. 5a). The I–V characteristic curve of the Ti/In₂Te₃ Schottky diodes shows a turn-on voltage of 1.34 V and a breakdown voltage of −3 V (Fig. 5b). The observed rectifying behavior is attributed to the formation of a Schottky contact between the low work function of titanium (4.33 eV) and the p-type semiconductor behavior of In₂Te₃. Typical I–V characteristics of Schottky diodes can be described by the following equation:

where, k_B is the Boltzmann constant, n is the ideality factor, T is the absolute temperature, and I_s is the saturation current of the diodes. From the I–V curves, the rectification ratio, ideality factor, and Schottky barrier height values were measured to be 514, 26.7, and 0.68 eV, respectively. These results demonstrated enhanced performance compared with previously reported group III-VI chalcogenide-based Schottky diodes with lower reverse-bias leakage currents^{16,17,18,19,20,21}, as shown in Table 1. This performance suggests that the diode can reliably function under reverse current flow, making it suitable for applications that demand minimal leakage, such as high-frequency device applications.

a Cross-sectional schematic of Ti/In₂Te₃ Schottky diodes. b Typical I–V characteristic curve of Ti/In₂Te₃ Schottky diodes. The inset shows the log I vs. V plot. The I–V characteristic curve shows rectifying behavior.

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We have successfully fabricated p-channel FETs and Schottky diode based on MBE-grown In₂Te_3. The two-step growth strategy employed resulted in full-coverage, atomically smooth In₂Te₃ films. In₂Te₃ films grown on h-BN substrates were shown to have atomically clean interfaces by employing MBE for precise control over the composition and phase. The electrical characteristics of the MBE-grown In₂Te₃ FETs, including enhanced mobility, reduced subthreshold swing, and a highly improved on/off ratio, were significantly improved. Additionally, the I–V characteristics of the Ti/In₂Te₃ Schottky diodes showed a low saturation current and higher barrier height than those of previous In₂Te₃-based diodes. The high on-off ratio of the p-channel FETs and the low saturation current of the Schottky diodes increase the versatility and potential for group III-VI chalcogenide-based integrated circuits. These characteristics make MBE-grown In₂Te₃ FETs and Schottky materials useful in logic devices. We believe that this approach for preparing high-quality group III-VI chalcogenide thin films is promising for next-generation electronics.