Schottky-barrier thin-film transistors based on HfO2-capped InSe plasma of β- Ga2O3: Changes in electrical properties and deep trap spectra

Indium selenide (InSe) is an emerging two-dimensional semiconductor and a promising candidate for next generation thin ﬁlm transistors (TFTs). Here, we report on Schottky barrier TFTs (SB-TFTs) in which a 0.9-nm-thick HfO 2 dielectric layer encapsulates an InSe nanosheet, thus protecting the InSe-channel from the environment and reducing the Schottky-contact resistance through a dielectric dipole effect. These devices exhibit a low saturation source-drain voltage V sat < 2V and current densities of up to J ¼ 2mA/mm, well suited for low-power elec-tronics. We present a detailed analysis of this type of transistor using the Y-function method from which we obtain accurate estimates of the contact resistance and ﬁeld-effect mobility.

An InSe van der Waals (vdW) crystal is a novel III-VI twodimensional (2D) semiconductor within the large family of 2D materials, which includes graphene, transition-metal chalcogenides, black phosphorous, and many others. 1 It has relatively low mass conduction band electrons and high electron mobility at room temperature even in atomically thin films, 2,3 which is the highest among that of 2D vdW semiconductors. In addition, this 2D material has a bandgap energy that increases markedly with the decreasing layer thickness down to a single layer from the infrared to the ultraviolet range. [2][3][4][5][6][7][8] These properties make InSe an ideal material candidate for several electronic and optoelectronic devices, such as high-frequency transistors and photodetectors. 7,[9][10][11][12][13][14][15] However, there are still several technological challenges to address. For example, a high contact resistance between a metal and a 2D layer can arise from the pinning of the Fermi energy 16 caused by interface defects created during the exposure of the layers to air or lithography-induced doping. 9 On the other hand, the contact resistance can be modified by the insertion of an intermediate ultrathin dielectric layer, such as h-BN, Ta 2 O 5 , and TiO 2 . [17][18][19] The Schottky barrier height at the metal-2D layer interface is effectively reduced by the high-k dielectric layer through a dielectric dipole effect. 20 Compared to conventional thin-film transistors (TFTs) with ohmic contacts, TFTs with source and drain Schottky contacts, called Schottky-barrier TFTs (SB-TFTs), can offer a number of advantages including a low saturation voltage, V sat , and thus a low power consumption desirable for several applications, such as wearable and portable electronics. 21 In this work, we report on the fabrication and electrical properties of SB-TFTs based on InSe. In these devices, a 0.9-nm-thick HfO 2 layer forms an InSe-HfO 2 -Ti/Au Schottky contact and acts as the high-k screening dielectric layer [ Fig. 1(a)]. These devices exhibit a low saturation voltage (V sat < 2 V) and a relatively large current density (J ¼ 2 lA/lm). We estimate the field effect-mobility (l 0 ¼ 83.7 cm 2 /V s) and contact resistance (R c ¼ 200 kX lm) of the SB-TFT using the Yfunction method (YFM). The value of l is higher than that extracted from the standard linear transfer approach (l ¼ 42.2 cm 2 /V s), which significantly underestimates l 4,16 due to the contribution of the Schottky contact resistance. The YFM also offers an effective method to extract the contact resistance compared to transfer length methods that are difficult to apply to 2D materials due to their small in-plane area. [22][23][24][25] Figure 1(a) shows the key steps in the fabrication of the InSe SB-TFT. A heavily p-doped Si substrate was used as the bottom-gate electrode, and a 100-nm-thick thermally grown SiO 2 layer was used as the dielectric layer. To obtain a clean SiO 2 surface, the substrate was successively immersed in acetone and ethanol and hence cleaned in ultrasonic bath with de-ionized water for 3 min. Finally, the substrate was exposed to oxygen plasma for 3 min and subjected to rapid thermal annealing (RTA) at 990 C for 10 min in an oxygen atmosphere. 26 InSe flakes were exfoliated from a Bridgman-grown InSe crystal onto a Si substrate. A selected InSe flake was dry transferred onto the substrate. This was followed by the deposition of a 0.9-nm-thick HfO 2 film (6 cycles) using atomic-layer deposition (ALD, kemicro TALD-150A) at 150 C. Compared to other dielectric materials, such as Al 2 O 3 and SiO 2 , HfO 2 has a much higher dielectric constant. This enables a stronger interface dipole effect and thereby a more significant reduction of the Schottky barrier to ensure a lower contact resistance and a more effective gate voltage modulation. 18,27 Ti/Au (20 nm/ 50 nm) source and drain electrodes were formed by electron-beam evaporation. A channel of length L ¼ 10 lm and width W ¼ 30 lm was defined by electron-beam lithography. Figure 1(b) shows an optical image of the device under monochromatic illumination. An InSe TFT with ohmic contacts was also fabricated using a shadow mask with a channel width and length of W ¼ L ¼ 60 lm on a p-doped Si wafer (back gate) with a 300 nm thick thermally grown SiO 2 layer. Compared to the lithography technique, the shadow mask avoids the unintentional doping due to contamination as it does not make use of photoresists/developers and requires only a very short processing time. The electrical characteristics of the devices were measured using a Keysight B2902A source/measurement unit.
The thickness of the InSe nanosheet is 50 nm, as measured by atomic-force microscopy (AFM) [ Fig. 1(c)]. The Raman spectrum of the HfO 2 -capped InSe [ Fig. 1(d)] reveals the A 1 1g , E 1 2g , and A 2 1g Raman active vibrational modes characteristics of pristine InSe, 12,15 suggesting that no significant contamination or distortion of the InSe lattice has been induced by the HfO 2 capping layer. Figure 2 shows the output current-voltage (I D -V D ) characteristics of the InSe SB-TFT [ Fig. 2(a)] and InSe TFT [ Fig. 2(b)]. Compared to the TFT with ohmic contacts, the SB-TFT shows a much lower saturation drain voltage (V sat ) at all applied gate voltages V G . In a standard TFT with ohmic source and drain contacts, the saturation voltage V sat where VG is the gate voltage, Vth is the threshold voltage, n s is the dielectric constant of the semiconductor, q is the element charge, N A is the doping density, w B is the Schottky barrier height at the source/ drain contact, and C 0 is the capacitance per area of the dielectric layer.
For a separate TFT with ohmic source and drain contacts, we estimate that V th ¼ À1.70 V. This gives values of V sat in agreement with the experimental values. Our data indicate that the behavior of the SB-TFT is qualitatively different from that of the TFT: the SB-TFT maintains a much lower saturation voltage at all applied gate voltages, in agreement with previous reports on different material systems. [28][29][30] Figure 2(c) shows the transfer (I D -V G ) and the transconductance (g m -V G ) curves at V D ¼ 0.1 V for the SB-TFT. The transfer curve demonstrates the n-type conductivity of the InSe channel. The g m drops when the gate voltage, V G , is increased above 4 V, as shown in Fig.2(c), indicating the existence of a contact resistance (R c ). 31 In the low field mode, 28 the transistor can be described as the series of a contact resistance and a traditional ideal TFT, as shown in the inset of Fig. 3(a).
According to the standard linear transfer model for an ideal TFT and for V D ( V G -V th , the mobility can be expressed as 32 where V th is the threshold voltage, C ox ¼ e o e r /d is the gate capacitance per unit area, e 0 is the vacuum permittivity, e r ¼ 3.9 is the dielectric constant of SiO 2 , and d ¼ 100 nm is the thickness of SiO 2 . If the  contact resistance of the SB-TFT is neglected, using Eq. (1), we estimate l ¼ 42.2 cm 2 /V s. The total resistance (R total ), which is the sum of R c and the channel resistance (R ch ), is R total ¼ V D /I D ¼ 24.63 kX at V G ¼ 4 V. As discussed below, this model significantly underestimates the value of the mobility. Figure 3 describes the operation mechanism of the InSe SB-TFT [28][29][30]33 for increasing values of the drain voltage, V D1 , V D2 , and V D3 . Under a positive V D , the source Schottky contact is reversebiased, the drain Schottky contact is forward-biased, and I D is limited by the reverse current of the source Schottky contact. When 0 < V D1 ( (V G ÀV th ), the depletion region is thin and acts as a source contact resistance. Thus, the device operates approximately as a series of R c and the resistance of a standard TFT [inset of Fig. 3(a)]. When V D increases to V D2 and becomes comparable to (V G -V th ), the depletion region expands [ Fig. 3(b)] and R c increases, thus dominating the I D -V D characteristics. When V D3 ! V G -V th > 0, the depletion envelope reaches the interface with the SiO 2 layer, thus pinching off the channel [ Fig. 3(c)]. Hence, I D reaches a saturation value that is independent of V D . The resistance of the thin 0.9-nm-thick HfO 2 layer can be neglected compared to the resistance of the depletion region of the Schottky junction. Thus, when V G À V th > 0, as long as the channel is more conductive than the region beneath the source, the drain current is largely limited by the Schottky barrier at the source.
To account for the contact resistance of the SB-TFT, 32 we use the Y-function method and extract the intrinsic field-effect mobility l 0 and R c . 24,34 The YFM is based on the analysis of the I D -V D curve in the linear region. Since the contact resistance due to the Schottkybarrier (R c ) causes an additional voltage drop, the drain current is expressed as 24 where h 0 is the intrinsic mobility degradation factor due to remote phonon scattering and surface roughness. 35 For convenience, the mobility degradation coefficient h is introduced to replace h 0 and R c , In the low-field limit (V D ¼ 0.1 V) and V G À V th ) V D =2, Eq. (2) is therefore rewritten as and the transconductance g m ¼ dI D /dV G is expressed as For small h 0 , h can be approximated by Thus, the dependence of the Y-parameter on V G is described by 17,22,24,36 Figure 4(a) shows the dependence of Y on V G . From the slope of the linear fit of the YÀV G curve, we extract an intrinsic field-effect mobility l 0 ¼ 83.7 cm 2 /V s. This is about twice the value of the mobility (l ¼ 42.2 cm 2 /V s) obtained using the standard TFT model [Eq. (1)]. When the device operates in the linear regime (the low-field mode), h is expected to be independent of V G , 24 as also shown in Fig.  4(b). The value of R c obtained from Eq. (6) is 6.63 kX, as shown in Fig. 4(c). As a result, with the incorporation of HfO 2 , the specific contact resistance R c W of the InSe SB-TFT is 200 kXÁlm, which is lower than that of organic TFTs with Schottky contacts ($10 4 -10 5 kX lm) but higher than that reported in 2D multilayer TFTs with ohmic contacts ($1-10 kX lm). [37][38][39] The R c W value of the InSe TFT with ohmic contacts in Fig. 2(b) is 44 kX lm and is indeed lower than that of the InSe SB-TFT. The value of l extracted using the standard linear model [Eq. (1)] and that extracted using the YFM are very similar in this case, i.e., 48.9 and 51 cm 2 /V s, respectively.
In the strong accumulation regime, the dependence of l on the normalized contact resistance (R c /R total ) can be expressed as 24 where l FE0 $ l 0 is the contact-resistance-independent intrinsic field effect mobility. By using R c ¼ 6.63 kX, R total ¼ V D /I D ¼ 24.63 kX at V G ¼ 4 V, and Eq. (8), we find that l FE0 ¼ 80 cm 2 /V s, which agrees well with the value of l 0 ¼ 83.7 cm 2 /V s extracted from the YFM. Figure 4(d) also shows that the mobility l extracted by the standard TFT model assuming perfect ohmic source and drain contacts indeed significantly underestimates the channel mobility at large gate voltages.
In conclusion, we have reported Schottky barrier TFTs in which a 0.9-nm thick HfO 2 dielectric layer encapsulates an InSe nanosheet. These devices have a better performance than standard InSe-based TFTs, including a low saturation source-drain voltage (V sat < 2 V) and a relatively large current density (J ¼ 2 lA/lm). We have shown that an accurate analysis of this type of TFT requires the use of the Y-function model. The corrected standard TFT model taking into account the contact and channel resistance gives a channel mobility of 78.95 cm 2 /V s at room temperature. This agrees well with the value from the Y-function model (83.7 cm 2 /V s). Our results suggest that the Y-function method can be well applied to determine the contact resistance and intrinsic field-effect mobility of transistors with a source Schottky contact. In addition, the low saturation of the InSe SB-TFT has potential for low-power electronics.
See the supplementary material for the electronic characteristics of more InSe SB-TFTs.