**1. Introduction**

Oxide electronics are a promising alternative to amorphous silicon (a-Si:H) and organic semiconductors to build reliable Thin Film Transistors (TFT) and more complex electronic circuits, addressing the challenges of flexible electronics and of the low cost and disposable electronics. In spite of the earlier work made during the 60s concerning the processing of n-type TFT [1,2], only forty five years later, with the work of Hosono [3], Wager [4], Carcia [5] and Fortunato [6], a significant worldwide interest materialised, especially for the active matrix for organic light emitting diodes (AMOLED) technology, exploiting their electronic properties, such as high saturation mobility, excellent uniformity and homogeneity, together with a high reliability associated with a low or room processing temperature [7].

However, there is no report on p-type oxide TFTs that are processed and cured at low temperatures with a performance similar to n- type, due to the low hole mobilities so far achieved in running stable and reliable devices [8]. The achievement of reliable p-type TFT, with performances similar to n-type TFT is of grea<sup>t</sup> importance for shaping electronics challenges towards the production of complementary

metal oxide semiconductors (CMOS), a key device for analogic and digital electronic systems, thanks to their low power consumption. This is a noticeable relevant CMOS property for low cost flexible electronics. To this end, we could use organic semiconductors, aiming to exploit the advantage that they can be processed at low temperatures.

Concerning organic p-type TFTs, most device performances on stability and mobility (<2 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup>1) are low [9–12], while the n-type organic TFT still exhibits low mobilities (≤1 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup>1) and requires a high absolute on voltages to switch it on [13,14].

An alternative to this is the inorganic oxide TFT, which is robust but in most cases requires high process temperatures. So far, most of the reported oxide TFTs are n-type, processed either on rigid or flexible substrates in which exists a consolidate set of results for films processed via physical or chemical methods [1–8,15,16]. For p-type, the transport due to holes is associated wirh oxygen *p* asymmetric orbitals, which severely limit the carrier mobility and therefore the TFT performances. In spite of Cu2O being a p-type oxide with mobility >100 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup><sup>1</sup> [17,18], the TFT based on these thin films or their compositions as Cu:NiO, exhibit mobilities and On-Off current ratios of <1.5 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup><sup>1</sup> and 10<sup>4</sup> respectively [17–21]. Other materials have been also reported, such as NiOx processed/annealed at 300 ◦C, exhibiting mobilities above 25 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup><sup>1</sup> [22].

Tin oxide has been studied as an alternative material to produce p-type oxides, with similar performances as those obtained in n-type oxides. The structure, morphology and ambipolar characteristic of these films are well known for oxides processed by reactive sputtering using metal targets and heat treated at 400 ◦C [23]. Indeed, it is known that SnO has an indirect band gap structure specifically controlled by the divalent tin (SnII), in a layered crystal structure [24,25] with major contributions from Sn 5*s* and O 2*p* orbitals near the valence band maximum (VBM) and Sn 5*p* orbitals towards the conduction band minimum (CBM). The p-type behaviour is mainly attributed to the Sn vacancy and the O interstitial where tin is in Sn2+ oxidation state [24,25]. The excess oxygen in the film transforms some cations in Sn3+ to maintain electrical neutrality. This process is considered to be Sn2+ capturing a hole and forming weak bonded holes, located inside the bandgap, near the top of the valence band as localized acceptor states [26,27]. This means that the final free carriers' behaviour of the films process is highly dependent on how oxygen is bonded and how it may compensate for defects.

Here, the contributions from Sn 5*s* states to VBM offer appreciable hole mobility in this material, without using a high processes temperature [28,29]. This leads also to the production of TFT with different geometry configurations [30] or using, besides metallic targets, ceramic ones on films grown by rf magnetron sputtering, heat treated at 400 ◦C [31].

In Table 1 we present the set of developments obtained concerning the performances of p-type SnO TFTs produced by Radio Frequency Magnetron Sputtering (RFMS) in the last 10 years [28,32–49]. There, we also present the architecture selected (SBG: staggered bottom-gate; STG: staggered top-gate; CBG: coplanar bottom-gate; CTG: coplanar top-gate; DG: double-gate), the process temperature, the oxygen partial pressure (Opp) and the type of dielectric used.


**Table 1.** Recent developments concerning the performances of p-type SnO TFTs processed by Radio Frequency Magnetron Sputtering (RFMS), using different type of device configurations, Opp, dielectrics and process temperatures.

\* Post deposition annealed at 200 ◦C; n.r.: not reported; RT = Room Temperature; ATO = Aluminum Titanium Oxide.

Overall, we notice that the only devices processed at room temperature using the RFMS technique are those developed by the present group [28,33]. Here, it is also relevant to mention that the presence of low oxygen partial pressure during the deposition process enables the production of more stable devices [35,48,49]. Apart from that, the configuration most used is the staggered bottom-gate, while the most common dielectric used is the silicon dioxide. Apart from that, most of the substrates used are rigid (glass or silicon wafer), except that referred to as CMOS devices integrating p-type TFT based on SnOx made on paper [33].

Besides stability and reproducibility issues, the data presented show that the device with the best mobility (5.53 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup>1, with Perovskite-Mediated Photogating [47]) does not correspond to the device with the highest On/Off (Ion/Ioff) ratio (5.2 × 106, using argon-plasma surface treatment [45]). Apart from that, most of the TFT studied does not work on the enhancement mode, as desired for application purposes.

Moreover, we noticed that the thickness of the channel layer, together with the state of the surface (degree of roughness and surface defects), determine the electrical characteristics presented by TFT and its stability.

In Table 2 we present the most significant data achieved in the last ten years concerning the production of SnOx p-type TFT using different processing techniques such as: Pulsed Laser Deposition (PLD); Electron-Beam Evaporation (EBE); Thermal Evaporation (TE); direct current magnetron sputtering (DCMS); PVD: Physical Vapor Deposition (PVD); Atomic Layer Deposition (ALD); Spin-Coating (SC). As in Table 1, the different type of device configurations are also shown (SBG: staggered bottom-gate; STG: staggered top-gate; CBG: coplanar bottom-gate; CTG: coplanar top-gate; DG: double-gate); Oxygen partial pressures (Opp); dielectrics and process temperatures used.


**Table 2.** Recent developments concerning the performances of p-type SnO TFTs processed by other physical and chemical process techniques, for different device configurations, Opp, dielectrics and process temperatures.

\* Post deposition annealed at 200 ◦C; n.r.: not reported.

Overall, the best p-type TFTs fabricated so far have been those processed by DCMS, exhibiting a mobility of 6.54 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup><sup>1</sup> and an On/Off ratio of 105, working in the depletion mode [30]. Moreover, the p-type TFT processed by PVD and using a STG configuration exhibit the highest recorded On/Off ratio (9.6 × 106) [56].

From the present state of the art, we saw that there are several parameters that impact on the electrical performance presented by p-type TFT SnOx based, most of them connected to the process parameters used, the structure of the films obtained, as well as the dielectric and the geometry configuration used.

In this paper, we report the fabrication of p-type SnOx TFTs deposited by RFMS technique at RT that are post-annealed up to 200 ◦C, turning the process compatible with the use of low-cost flexible substrates as paper [7]. In this study, we aim to better understand the role that the structure, surface finishing and oxygen play during the growing process of SnOx in order to define a process window that allows the production of reliable and high stable p-type TFT with high electronic performances, such as field effect mobility and On-Off- current ratios.

#### **2. Materials, Methods and Results**

## *2.1. Experimental Details*
