Since the advent of graphene, two-dimensional (2D) materials have generated interest as promising building blocks of next generation ultrathin electronics and optoelectronics.^[1–6] Despite the various advantages presented by graphene, the zero-bandgap nature of pristine graphene greatly limits its applications in logic devices due to its high standby power consumption.^[3,5,7,8] However, with a finite density of states, the Fermi level of graphene can be readily modified by a gate potential, making graphene an ideal candidate for contact to other 2D materials, which is the basis for fabricating graphene based optoelectronic devices.^[9–16] Black phosphorus (BP), as a single elemental (P) layered material, has a thickness-dependent bandgap ranging from 0.3 eV to 1.2 eV with mechanical flexibility,^[17–22] high anisotropic charge carrier mobilities,^[23–25] and ambient stability via encapsulation or surface decoration,^[26–30] all of which make it a promising candidate for applications in future electronics.^[23,31–36]

Extensive investigation on BP electronic devices indicates that the device performance, such as the hole mobility and ON state current, is strongly relied on the metal-BP contact.^[37] Due to the lack of controllable and sustainable substitutional doping techniques to lower the contact resistance, the common way is to choose contact metals with appropriate work function to inject proper types of carriers into the bands of BP.^[38] However, a finite Schottky barrier height is always present at such contacts.^[37,38] It is even more challenging when the thickness of BP is smaller than the depletion and transfer lengths since the conventional concepts of the alignment of the Fermi level with the bands of the BP channel break down.^[39] Here we investigate the intrinsic transport behaviors in graphene/BP van der Waals heterojunction devices under different carrier densities and temperatures. At high carrier densities (−40 V ≤ V_bg ≤ 0 V or 20 V ≤ V_bg ≤ 40 V), tunneling through the Schottky barrier at the graphene/BP dominates at low temperatures. With temperature increasing, the Schottky barrier at the interface is vanishing, and the channel current starts to decrease with increasing temperature, behaving like a metal. On the other hand, at low carrier density or in the OFF state (V_bg = 10 V), thermal emission over the Schottky barrier at the interface dominates the carriers transport. These results demonstrate that graphene is a good contact electrode for BP devices.

The fabrication process of graphene/BP heterostructures is shown in Fig. 1(a). Firstly, monolayer graphene was grown on Cu foil by the chemical vapor deposition method and then transferred to a 300 nm SiO₂/p+ doped Si substrate (I in Fig. 1(a)).^[40] After identifying the graphene by optical microscope and Raman spectrum, the sample was transferred into a glove box filled with inert Ar gas. A graphene/BP van der Waals heterojunction was formed by stacking the mechanically exfoliated few-layered BP flakes (∼ 10 nm) onto the graphene flake on the SiO₂ substrate (II in Fig. 1(a)). To protect the BP from degradation, a layer of PMMA 495 (A2) was spin coated before taken it out for contact fabrication. An additional thin layer of PMMA 495 (A5) was spin-coated as an electron-beam resist.^[41] Reactive ion etching was performed to define the structure of the heterojunction device after electron beam lithography (III in Fig. 1(a)). Finally, a second electron-beam lithography process was employed to define the Ni/Au (5/60 nm) electrical contacts (IV in Fig. 1(a)). To minimize the degradation of BP, all electrical measurements were conducted in a four-probe STM with a vacuum level of 10⁻¹⁰ mbar. A schematic and a representative optical image of the graphene/BP heterojunction device are shown in Fig. 1(b) and the inset of Fig. 1(c), respectively. The thickness of the BP flake (∼ 10 nm) was determined by atomic force microscopy, as shown in Fig. 1(c). The Raman spectrum of the graphene/BP heterojunction region (marked by a star in Fig. 1(c)) is shown in Fig. 1(d). The observed Raman-active modes of both BP and graphene are consistent with those in previous reports, which demonstrates the high quality of the graphene/BP heterojunction.^[42]

Fig. 1.

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Fig. 1. (color online ) (a) Schematic image of the device fabrication process. (b) Schematic image of the graphene/BP heterojunction device. (c) An AFM image of the heterojunction device. Section analysis shows the thickness of the BP flake is ∼ 10 nm. The inset is an optical microscope image of the device (scale bar inset is 10 μm). (d) Raman spectrum collected at the heterojunction region as indicated (red star) in (c).

Output characteristics of the graphene device as a function of back gate voltage V_bg from −20 V to 20 V at a step of 5 V with source-drain bias voltage V_ds from −100 mV to 100 mV are shown in Fig. 2(a). The highly linear behavior at different V_bg suggests a good contact between graphene and the Ni/Au electrode. Output characteristics of the pure BP (graphene/BP heterojunction) device are shown in Figs. 2(b) and 2(c) (Figs. 2(d) and 2(e)). A linear behavior can be found for both pure BP (Fig. 2(b)) and graphene/BP heterojunction (Fig. 2(d)) devices at negative gate voltages. A higher drain current delivered in the heterojunction device indicates that graphene is a better contact electrode for holes transport. While for electrons transport (at positive gate voltage bias), apparent non-linearity is observed for the pure BP device (Fig. 2(c)), suggesting that the Schottky barrier is formed at the Ni/BP interface for electron transport. By contrast, the graphene/BP heterojunction device (Fig. 2(e)) delivers a larger drain current and shows a near linear behavior, suggesting that compared to Ni, graphene is also a better contact electrode for electrons transport in BP. This can be further confirmed by the transfer curves shown in Fig. 2(f). Both BP and graphene/BP devices show ambipolar transport behavior, and the graphene/BP heterojunction has enhanced electron transport behavior. The higher drain current in the graphene device ensures that the ambipolar transport in the graphene/BP heterojunction is dominated by the graphene/BP interface rather than the Ni/graphene one. A schematic of the energy band diagram of the heterojunction device at different regimes of V_bg is shown in Fig. 2(g). When V_bg = 10 V, the Fermi level of graphene is located at the Dirac point and a Schottky barrier forms at the interface between graphene and BP. This Schottky barrier is thinned and lowered when V_bg ≫ 10 V with positive polarity or V_bg ≪ 10 V with negative polarity due to heavy doping by electrons or holes. It is worth noting that the current minimum occurs at V_bg ∼ 20 V rather than V_bg ∼ 10 V, suggesting that V_bg = 20 V is the flat band voltage of BP.

Fig. 2.

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Fig. 2. (color online) (a) Output curves of the graphene device with Vds ranging from −100 mV to 100 mV at different Vbg from −50 V to 50 V at a step of 10 V. (b) and (c) Output curves of the BP device with Vds ranging from −1 V to 1 V at Vbg from −20 V to 0 V and from 10 V to 20 V at a step of 5 V, respectively. (d) and (e) Output curves of the graphene/BP heterojunction device with Vds ranging from −1 V to 1 V at Vbg from −40 V to 0 V and from 10 V to 50 V at a step of 10 V, respectively. (f) Transfer curves of the graphene device (black),the graphene/BP heterojunction device (red), and the BP device (blue) with Vds = 10 mV. (g) Schematic of the energy band diagram of the heterojunction device at different Vbg regimes.

To probe the intrinsic charge transport behavior in graphene/BP heterojunctions, variable-temperature measurement of the transfer characteristics was performed. Figure 3(a) shows the representative transfer curves of the heterojunction device with temperature increasing from 100 K to 300 K at a step of 20 K. The ON-state current I_on (V_bg = −50 V) shows a weak temperature dependence, while the OFF-state current I_off is decreased by nearly three orders of magnitude with decreasing temperature, resulting in a greatly increased on/off current ratio for both holes and electrons. As shown in Fig. 3(b), I_on/I_off of the device increases with the decreasing temperature; and it increases from 540 to 115000 for holes conduction (black) and from 18.3 to 3700 for electrons conduction (red) when decreasing the temperature from 300 K to 100 K. These results indicate that the thermally activated mechanism dominates the charge transport in the OFF state at low temperatures.^[43] It is found that the gate voltage at which the current reaches its minimum (V_off) also changes with temperature from V_bg ∼ 10 V at 100 K to V_bg ∼ 20 V at 300 K, as shown in Fig. 3(c), indicating that the charge transport behavior in the heterojunction depends not only on the temperature but also on the charge carrier density. When the temperature is decreased to 100 K, few charge carriers have enough energy to overcome the Schottky barrier at the graphene/BP interface. With the temperature increasing, more charge carriers are thermally excited to overcome the barrier, and V_bg corresponding to the current minimum shifts to the flat band voltage of BP.

Fig. 3.

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Fig. 3. (color online) (a) Transfer curves of the heterojunction device at different temperatures from 100 K to 300 K at a step of 20 K under a constant bias voltage Vds = 100 mV. (b) Ion/Ioff of the heterojunction devices for electrons conduction (red circles) and holes conduction (black squares) at different temperatures. (c) The gate voltages for the OFF states (Voff) of the heterojunction devices at different temperatures. (d) and (e) Arrhenius plot of ln(Isat/T2) versus q/KBT at different back gate voltages from −40 V to 0 V and from 10 V to 40 V at a step of 10 V, respectively. (f) Extracted Schottky barrier height as a function of the gate voltage.

Based on the metal-semiconductor contact theory,^[44] there are two mechanisms for carrier injection from graphene to BP. One is the thermionic emission over the Schottky barrier with an exponential dependence on the temperature, which can be described as where A is the area of the Schottky junction, A* is the effective Richardson constant, q is the elementary charge, η is the ideality factor, K_B is the Boltzmann constant, and T is the temperature. When the heterojunction device is reversely biased, the current becomes insensitive to V_bias and can be obtained. From the Arrhenius plots of ln (I_sat/T²) versus q/K_BT at different V_bg (shown in Figs. 3(d) and 3(e)), the Schottky barrier height ϕ_b at the graphene/BP interface can be extracted, as shown in Fig. 3(f). At V_bg = 10 V, a Schottky barrier height of 67.3 meV is obtained. While at other gate voltages, the Schottky barrier is a negative value, suggesting that the thermionic emission model cannot account for the transport behavior at these gate voltages. Therefore, the other mechanism for carrier injection from graphene to BP, the field emission (tunneling) mechanism, is considered,

When the heterojunction device is heavily electron- or hole-doped (−40 V ≤ V_bg ≤ 0 V or 20 V ≤ V_bg ≤ 40 V), the drain current I_ds shows a non-monotonic dependence on the temperature, as shown in the plot of I_ds versus T² (Fig. 4(a) for −40 V ≤ V_bg ≤ 0 V and Fig. 4(c) for 20 V ≤ V_bg ≤ 40 V). At low temperatures, I_ds increases with the increasing temperature and linear fitting of the I_ds curves as a function of T² at different gate voltages can determine the temperature T_Tunnel below which tunneling transport dominates, as shown in Figs. 4(a) and 4(c). Figures 4(b) and 4(d) show T_Tunnel as a function of the gate voltage. T_Tunnel decreases with increasing carrier densities (holes or electrons), implying that the height or the width of the Schottky barrier decreases with increasing charge carrier densities. At higher temperatures, on the other hand, I_ds decreases with the increasing temperature, behaving like a metal.^[45]

Fig. 4.

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	Fig. 4. (color online) (a) Plots of Ids as a function of T2 at different Vbg from −40 V to 0 V at a step of 10 V. (b) Plots of TTunnel as a function of Vbg. (c) Plots of Ids as a function of T2 at Vbg from 20 V to 40 V at a step of 10 V. (d) Plots of TTunnel as a function of Vbg.

We have investigated the intrinsic charge transport behavior in the graphene/BP heterojunction. It is found that the transport behaviors of the heterojunction devices depend on the carrier density and temperature. At high carrier densities, tunneling through the Schottky barrier at the graphene/black phosphorus dominates at low temperatures. With temperature increasing, the Schottky barrier at the interface is vanishing, and the channel current starts to decrease with increasing temperature, behaving like a metal. Finally, at low carrier density or in the OFF state, thermal emission over the Schottky barrier at the interface dominates the carrier transport.