Remarkably High Hole Mobility Metal-Oxide Thin-Film Transistors

High performance p-type thin-film transistor (p-TFT) was realized by a simple process of reactive sputtering from a tin (Sn) target under oxygen ambient, where remarkably high field-effect mobility (μFE) of 7.6 cm2/Vs, 140 mV/dec subthreshold slope, and 3 × 104 on-current/off-current were measured. In sharp contrast, the SnO formed by direct sputtering from a SnO target showed much degraded μFE, because of the limited low process temperature of SnO and sputtering damage. From the first principle quantum-mechanical calculation, the high hole μFE of SnO p-TFT is due to its considerably unique merit of the small effective mass and single hole band without the heavy hole band. The high performance p-TFTs are the enabling technology for future ultra-low-power complementary-logic circuits on display and three-dimensional brain-mimicking integrated circuits.


Figure 1(a) and (b)
show the transistor's drain-source current versus drain-source voltage (I DS -V DS ), |I DS | versus gate-source voltage (|I DS |-V GS ) and μ FE -V GS characteristics of the HfO 2 /SnO x p-TFTs, where the SnO x was formed by reactive sputter from a Sn target. Good device performance was reached at a low V DS of −1.2 V that is vital to lower the switching power of CV DS 2 f/2, where C and f are the capacitance and operation frequency, respectively. Besides, high hole μ FE of 7.6 cm 2 /Vs, a SS of 140 mV/dec, and an I ON /I OFF of 3 × 10 4 were obtained. The device mobility is among the best reported p-TFTs in literature 19,20 . It is important to notice that the device performance is highly related to oxygen content. The μ FE was degraded by an order of magnitude at higher O 2 /Ar ratio, where the degraded mobility is related to the increasing SnO 2 content inside the SnO.
The reactive sputtering from a Sn target is the crucial technique to reach high hole mobility. Figure 2(a) and (b) show the device characteristics of HfO 2 /SnO x p-TFTs, where the SnO x was formed by directly sputtering from a SnO target. A low hole μ FE of 0.83 cm 2 /Vs, a poor SS of 430 mV/dec, and a small I ON /I OFF of 1.2 × 10 3 were measured. Even poor μ FE value was measured at annealing temperature higher than 200 °C. The high temperature is needed to anneal out the sputtering damage by energetic ions. But the annealing temperature higher than 200 °C cannot be applied to SnO device, because the SnO will translate to low mobility Sn 3 O 4 and SnO 2 at high temperatures 24,25 .
We have further performed the material analysis to understand the large device performance difference between sputtering from the Sn and SnO targets. device. From the cross-sectional transmission electron microscopy (TEM), the SnO x active layer on HfO 2 has a thickness of 12 nm. The microscopic structure of SnO x was analyzed by X-ray diffraction (XRD) as shown in Fig. 3(b). For SnO x formed by reactive sputtering from a Sn target, a mixture of major tetragonal α-SnO phase and small amount of β-Sn phase is observed that was caused by the incomplete Sn oxidation 26,27 . In contrast, only a pure α-SnO phase was found from the SnO target. The atomic composition of SnO x in p-TFT are further characterized by X-ray photoelectron spectroscopy (XPS) in Fig. 3(c). The de-convoluted spectra in both cases show a major Sn 2+ peak with tiny Sn 4+ and Sn° peaks, although the later ones are smaller for directly sputtered SnO than those from reactive sputtering of a Sn target. Therefore, the SnO x by sputtering from a SnO target gives better material quality. Nevertheless, the μ FE is significantly lower than that from the reactive Sn target. The potential reason may be related to the sputter damage from the SnO target, which is difficult to be detected by XRD and XPS analysis. Unfortunately such damage cannot be annealed out because of the limited process temperature of SnO, which can react as scattering centers to low the mobility. The other possibility to reach high mobility may be related to the multi-phonon assisted tunneling 28 via small amount of metallic Sn in SnO. This is also associated with the lower off-current in reactive sputtered SnO device than that formed by sputtering from the SnO target. Further theoretical analysis will be required to understand the role of metallic Sn inside SnO. Nevertheless, the metallic Sn is difficult to form by sputtering from the SnO target.
It is crucial to notice that the measured hole μ FE is the highest value among oxide semiconductors. We further perform the first principle quantum-mechanical calculations on SnO and the other potential candidate of Cu 2 O (Figures S1 and S2). The structures of both SnO and Cu 2 O semiconductors were obtained using local density approximation plus U (LDA + U) method with appropriate U p and U d value. The good accuracy is supported by the calculated band structure of Cu 2 O; a direct 2.1 eV bandgap and cubic structure were obtained, agreeing well with experiments 14,29,30 . Both light hole and heavy hole bands were found in Cu 2 O that are typical for most major semiconductors of Si, Ge, GaAs, InP, InAs etc. Besides, the density of state (DOS) of heavy hole band is considerably higher than that of light hole band to cause the low hole mobility. In sharp contrast, the SnO only has a single hole band, leading to the high hole μ FE . The calculated DOS of SnO and Cu 2 O are further shown in Fig. 4. The Cu 2 O has much higher DOS than SnO due to its heavy hole bands. For Cu 2 O, the d-orbital holes have complex intra-atomic hybridization between d and s, p states that lowers the hole mobility. This is also applied for most oxide semiconductors to result in a low hole mobility [31][32][33] . In sharp contrast, the Sn 5s orbital is occupied and exhibits a s-p coupling with the O 2p ligand orbitals, unlike the p-d interaction of the d 10 Cu 2 O. The delocalized character of the 5s states leads to a strong valence band dispersion and small hole effective masses in SnO ( Figure S3). The SnO has much smaller hole effective mass than Cu 2 O and other major semiconductors of Si, Ge, GaAs, InP, InGaAs etc. This is the extremely unique merit of SnO p-type transistor to reach high hole mobility. Table 1 compares the device performance of various TFTs. For p-TFTs, the SnO device has good hole μ FE , SS, and I ON /I OFF , which is supported from the small effective mass and single band without heavy hole band. The low V D operation is important to lower AC power consumption.
In conclusion, record high hole mobility of SnO p-TFT was realized. The superb device performance, simple process, and low-cost material make SnO the excellent candidate for next generation ultra-low power display devices and 3D brain-mimicking IC 10,13 .  The gate length and width were 50 and 500 μm, respectively. The electrical characteristics of the fabricated devices were measured using an HP4155B parameter analyzer and a probe station. The SnO film was analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The XPS spectra were measured with a PHI 5000 VersaProbe system (ULVAC-PHI, Chigasaki) using a microfocused (100 µm, 25 W) Al X-ray beam. Cross section TEM images of devices were obtained from high