† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 61306006).
In this paper, high temperature direct current (DC) performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25 °C to 200 °C. At a gate voltage of −8 V (far from Dirac point), the drain-source current decreases obviously with increasing temperature, but it has little change at a gate bias of +8 V (near Dirac point). The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies. Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures. The devices exhibit almost unchanged DC performances after high temperature measurements at 200 °C for 5 hours in air ambience, demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices.
Graphene has attracted significant attention as a potential emerging semiconductor material for future electronics due to its excellent physical and electrical properties, such as the outstanding carrier mobility, high current density, large mechanical strength, excellent thermal conductivity, etc.[1–3] Over the past ten years, a number of advances in graphene based devices have been made. A intrinsic cutoff frequency (fT) of 427 GHz with 67-nm gate length graphene field-effect transistor (GFET) has been reported for peeled graphene. Recently, 100-nm gate length epitaxial graphene transistor fabricated on SiC substrate has exhibited an intrinsic fmax of 137 GHz. Several other significant efforts have also been made to improve the device performance and develop new graphene-based applications,[6–9] such as graphene nanoribbons, terahertz devices, photodetectors, and other graphene-based devices and circuits. In practical applications, however, graphene devices demand not only good operation performances but also thermal stabilities. Thermal effects, including self-heating generated during device operation and ambient temperature variations, will strongly affect the characteristics of devices, leading to degradations of device performances. Therefore it is very important to understand the operation mechanisms of graphene devices at different temperatures.
Many groups have reported on the temperature dependent characteristics of graphene devices at relatively low temperatures, typically ranging from ∼1.5 K to 380 K.[10–12] For example, Bolotin et al. investigated the temperature-dependent transport in suspended graphene in a temperature range 5 K < T < 240 K. Feng et al. studied the temperature-dependent electrical properties of GFETs in ambient atmosphere with temperature rising from 300 K to 380 K. These early studies gave an insight into the understanding of the basic physical and electronic properties of graphene. In many practical applications, devices often work under different kinds of high temperature conditions, such as the temperature above 100 °C. It is important to investigate the behaviors of graphene devices under high temperature. But few reports on high temperature performances of graphene devices can be found in the literature.
In this work, bilayer epitaxial GFETs are fabricated on 4H–SiC substrate. The DC performances of graphene devices are studied at the temperatures ranging from 25 °C to 200 °C. The operation mechanisms and thermal stabilities of GFETs under high temperature are investigated.
Epitaxial graphene layers were synthesized on 10 mm × 10 mm semi-insulating Si-face 4H–SiC (0001) substrates. Prior to the graphene growth, the substrates were hydrogen etched at 1650 °C to form a regularly stepped surface. Monolayer graphene was grown on the SiC substrate in Ar ambient at 1650 °C. Then, the as-grown graphene was annealed in molecular hydrogen atmosphere at 900 °C, which can result in the formation of the bilayer quasi-free-standing epitaxial graphene (QFSEG). More details of the growth can be found in our early work.[13,14] The as-grown bilayer epitaxial graphene shows p-type doping with carrier density ns = 1.41 × 1013 cm−2, Hall mobility μ = 3110 cm2/V·s, and sheet resistance Rsheet = 142.3 Ω/□. In this work, bilayer QFSEG was used to fabricate the graphene devices.
An improved device process is used to fabricate GFETs, in which an Au metal film was pre-deposited on graphene surface as a protection layer to avoid undesired damage and contamination to graphene during processing. A similar process flow can be found in our previous work[5,15] and is summarized as follows. First, a 20-nm Au metal film was deposited on the surface of bilayer epitaxial graphene on SiC substrate. Channel region was then patterned by optical lithography and Au film outside the channel was removed by KI-I2 etchant solution. Electrical isolation was achieved by exposure of the graphene surface to oxygen plasma. The metal electrodes (Ti/Au, 10 nm/250 nm) were evaporated at the top of the Au film followed by a lift-off process. The exposed Au layer between metal electrodes was wet etched away to form the valid source/drain separations automatically. The devices were equipped in an electronbeam evaporator to deposit 2-nm Al nucleation layer. Then, 20-nm Al2O3 film was deposited on the surface of device via the Atomic layer deposition (ALD) at 250 °C. Rapid thermal annealing at 400 °C for 2 min in nitrogen was conducted to improve the quality of the Al2O3 film. Finally a 250-nm Al top-gate electrode was thermally evaporated and a lift-off process was performed.
Raman spectrum of bilayer epitaxial graphene on SiC substrate is shown in Fig.
For our device, when VG = −8 V, a high carrier density regime is formed in the channel of graphene. Thermal activation of electron-hole puddles is exponentially suppressed at this time, and the scattering effect plays a major role and dominates the channel of graphene. Hence the IDS (σ·E) of GFET decreases obviously with increasing temperature, and 12.58% reduction of IDS is observed under 200 °C with VG = −8 V. The gate voltage is scanned from negative to positive voltage, and the number of carriers in the channel is reduced, and IDS shows a continuous drop with VG increasing. At a VG of +8 V, a relatively low carrier density regime is formed in the graphene channel and the thermal activation will dominate. It is needed to point out that to avoid the electrical breakdown of Al2O3 dielectric the bias applied to the gate does not exceed +8 V. The Dirac point is not reached in this measurement for our device. Dirac point is larger than +8 V, implying that the carrier density under VG of +8 V is not low enough and a pure semiconducting behavior cannot be present here. It is observed in Fig.
It is important to note that the maximum IDS of GFET is only reduced by 12.58% after the temperature increasing from 25 °C to 200 °C. This value of reduction in the GFET is significantly smaller than those of other traditional semiconductors (e.g., silicon, GaN, etc.) devices. It is reported that a very large reduction of about 61% for the maximum IDS is observed in GaN-based MOSFET after the temperature rising from 25 °C to 200 °C. To give an insight into the reasons for this small IDS reduction, the temperature-dependent scattering effect is used to investigate the operation mechanisms of GFETs. IDS can be determined using the following expression:
where σ is the conductance of graphene channel, E is the electric field between drain-source electrodes, ns is the carrier density, e is the charge of electron, and μ is the carrier mobility. For a fixed VDS, IDS is proportional to the product of ns and μ. In our previous study, the carrier density ns of bilayer QFSEG from Hall measurements (from 1.26 × 1013 cm−2 of 300 K to 1.28 × 1013 cm−2 of 500 K) showed a very slightly increase with temperature. The ns can be regarded as a constant which is independent of temperature. Then, IDS can be described by a simplified formula as IDS = A1·μT, where A1 is the fitting parameter and μT is the mobility at different temperatures. It is well known that the carrier mobility is inextricably linked to the scattering. Here, four different scattering sources are considered: 1) μc, Coulomb scattering; 2) μgr, acoustic phonon scattering of graphene; 3) μRP, remote optical phonon scattering from SiC substrate and gate dielectric of Al2O3; 4) μsr, short range scattering, which can be approximately calculated by the following formulas for bilayer graphene, respectively:[13,14]
where A and B are constants, T is the temperature, and nimp is the impurity density;
where h is the reduced Planck’s constant, ρs is the mass density of graphene (7.6 × 10−7 kg·m−2), νs is the LA phonon velocity (2.1 × 104 m·s−1), VF is the Fermi velocity (∼ 1 × 106 m·s1), DA is the experimentally obtained deformation potential (18 eV), and kB is the Boltzmann’s constant;
where CRP−SiC and CRP−Al2O3 are the coupling strength Eop is the phonon energy coupled to the charge carriers in graphene, the values of Eop−SiC, Eop1−Al2O3, and Eop2−Al2O3 are 116 meV, 55.01 meV, and 94.29 meV, respectively;
where S is a constant. The total carrier mobility μ can be regarded as the result of interaction among the four different scattering sources and is given by
The temperature dependences of the four kinds of scatterings are different. The strength of short range scattering depends on the quality of the graphene sample. The μsr is a constant and temperature independent. The μc shows a proportional relationship with temperature. Owing to the parabolic band structure of the bilayer graphene, the energy averaging of the Coulomb scattering time can result in the mobility increasing proportionally with temperature. The acoustic phonon scattering of graphene is inversely proportional to temperature, which results in the mobility decreasing with temperature. The carrier in the graphene will also be scattered by optical phonons of SiC substrates and Al2O3 dielectric, which results in the mobility decreasing with temperature. The competition result between Coulomb scattering and acoustic phonon scattering of graphene, and the remote phonon (RP) scattering of substrates/dielectric cause a relatively small decrease of μ at high temperature, which leads to the slight reduction of IDS in the GFETs. The above analysis indicates that the small decrease of μ at high temperature lies in the increase of Coulomb scattering mobility with temperature rising due to its parabolic band structure of bilayer graphene. The bilayer graphene transistors show some advantages in high temperature applications.
In order to further investigate the performance of GFET under high temperature, the thermal stability test is conducted in this work. The measurement is carried out under 200 °C in air ambience. The output characteristics of a GFET at different heating times are shown in Fig.
In this paper, high temperature DC performances of bilayer epitaxial graphene devices on SiC substrate are studied. When graphene channel has a high carrier density, the IDS decreases obviously with temperature increasing, but it has little change when graphene channel has a low carrier density. The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies. The competition result between Coulomb scattering and acoustic phonon scattering of graphene, and the remote phonon (RP) scattering of substrates/dielectric cause a relatively small decrease of μ at high temperature, which leads to the slight reduction of IDS in the GFETs. In addition, the device maintains almost unchanged DC performance under 200 °C in air ambience for 5 hours, demonstrating the high thermal stabilities for bilayer graphene devices. These results are meaningful and will promote the development of graphene-based devices for practical applications in the future electronics.