Christophe Avis

With the discovery of amorphous oxide semiconductors, a new era of electronics opened. Indium gallium zinc oxide (IGZO) overcame the problems of amorphous and poly-silicon by reaching mobilities of ~10 cm²/Vs and demonstrating thin-film transistors (TFTs) are easy to manufacture on transparent and flexible substrates. However, mobilities over 30 cm²/Vs have been difficult to reach and other materials have been introduced. Recently, polycrystalline In₂O₃ has demonstrated breakthroughs in the field. In₂O₃ TFTs have attracted attention because of their high mobility of over 100 cm²/Vs, which has been achieved multiple times, and because of their use in scaled devices with channel lengths down to 10 nm for high integration in back-end-of-the-line (BEOL) applications and others. The present review focuses first on the material properties with the understanding of the bandgap value, the importance of the position of the charge neutrality level (CNL), the doping effect of various atoms (Zr, Ge, Mo, Ti, Sn, or H) on the carrier concentration, the optical properties, the effective mass, and the mobility. We introduce the effects of the non-parabolicity of the conduction band and how to assess them. We also introduce ways to evaluate the CNL position (usually at ~E_C + 0.4 eV). Then, we describe TFTs’ general properties and parameters, like the field effect mobility, the subthreshold swing, the measurements necessary to assess the TFT stability through positive and negative bias temperature stress, and the negative bias illumination stress (NBIS), to finally introduce In₂O₃ TFTs. Then, we will introduce vacuum and non-vacuum processes like spin-coating and liquid metal printing. We will introduce the various dopants and their applications, from mobility and crystal size improvements with H to NBIS improvements with lanthanides. We will also discuss the importance of device engineering, introducing how to choose the passivation layer, the source and drain, the gate insulator, the substrate, but also the possibility of advanced engineering by introducing the use of dual gate and 2 DEG devices on the mobility improvement. Finally, we will introduce the recent breakthroughs where In₂O₃ TFTs are integrated in neuromorphic applications and 3D integration.

We have fabricated amorphous tin oxide (a-SnO_x) thin-film transistors (TFTs) with Al₂O₃ gate insulator from deep eutectic solvents (DESs). DESs were formed using the chloride derivates of each precursor (SnCl₂, or AlCl₃) mixed with urea. The DESs were then used as precursors for the semiconductor and dielectric. Our target was to form extremely thin semiconductor film, and a sufficient high capacitance insulator. We characterized the physical and chemical properties of the DES-derived thin films by X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). We could evaluate that the highest content of metal–oxygen bonds was from the DES condition SnCl₂–urea = 1:3. At a low 300 °C budget temperature, we could fabricate a 3.2 nm thick a-SnO_x layer and 30 nm thick Al₂O₃, from which the TFT demonstrated a mobility of 80 ± 17 cm²/Vs, threshold voltage of −0.29 ± 0.06 V, and subthreshold swing of 88 ± 11 mV/dec. The proposed process is adequate with the back-end of the line (BEOL) process, but it is also eco-friendly because of the use of DESs.

Amorphous tin oxide (a‐SnO_x) is a potential transparent oxide semiconductor candidate for future large‐area electronic applications. The thin‐film transistor (TFT) mobilities reach ≈100 cm² Vs⁻¹, a mobility higher than other multiple cation‐based oxide semiconductor TFTs. Few optical properties have been reported so far and therefore both the effect of visible light and negative bias illumination stress (NBIS) on a‐SnO_x TFT performances, known to dramatically impact oxide semiconductor‐based TFTs, have been investigated. The variation of density of states (DOS) due to NBIS by device simulation is analyzed, and a fourfold increase of the donor‐like states and a decrease in the band edge DOS from 2.3 to 2.0 × 10¹⁹ cm⁻³ eV⁻¹ are showed. The evaluation of the effect of neutral, singly, and doubly ionized oxygen vacancies by density functional theory using 95 atoms reveals not only states in the gap of SnO₂, but also variations in the electron density, and modifications in the crystal parameters compared to a structure without an oxygen vacancy. Material and device simulation analysis reveal that the oxygen vacancies have a dramatical impact on the DOS in the gap of SnO₂ and can explain the NBIS phenomenon observed in a‐SnO_x TFT.

We studied the impact of NF₃ plasma treatment on the HfO₂ gate insulator of amorphous tin oxide (a-SnO_x) thin-film transistors (TFTs). The plasma treatment was for 0, 10, or 30 s. The HfO₂ insulator demonstrated a slightly higher breakdown voltage, whereas the capacitance value remained almost constant (~150 nF/cm²). The linear mobility slightly increased from ~30 to ~35 cm²/Vs when the treatment time increased from 0 to 10 s, whereas a 30 s-treated TFT demonstrated a decreased mobility of ~15 cm²/Vs. The subthreshold swing and the threshold voltage remained in the 100–120 mV/dec. range and near 0 V, respectively. The hysteresis dramatically decreased from ~0.5 V to 0 V when a 10 s treatment was applied, and the 10 s-treated TFT demonstrated the best stability under high current stress (HCS) of 100 μA. The analysis of the tin oxide thin film crystallinity and oxygen environment demonstrated that the a-SnO_x remained amorphous, whereas more metal–oxygen bonds were formed with a 10 s NF₃ plasma treatment. We also demonstrate that the density of states (DOS) significantly decreased in the 10 s-treated TFT compared to the other conditions. The stability under HCS was attributed to the HfO₂/a-SnO_x interface quality.

Crystalline tin oxide has been investigated for industrial applications since the 1970s. Recently, the amorphous phase of tin oxide has been used in thin film transistors (TFTs) and has demonstrated high performance. For large area electronics, TFTs are well suited, but they are subject to various instabilities due to operating conditions, such as positive or negative bias stress PBS (NBS). Another instability is hysteresis, which can be detrimental in operating circuits. Understanding its origin can help fabricating more reliable TFTs. Here, we report an investigation on the origin of the hysteresis of solution-processed polycrystalline SnO₂ TFTs. We examined the effect of the carrier concentration in the SnO₂ channel region on the hysteresis by varying the curing temperature of the thin film from 200 to 350 °C. Stressing the TFTs characterized further the origin of the hysteresis, and holes trapped in the dielectric are understood to be the main source of the hysteresis. With TFTs showing the smallest hysteresis, we could fabricate inverters and ring oscillators.

We report the effect of the curing (T_curing) and annealing (T_anneal) temperatures on the structural, electrical, and optical properties of solution processed tin oxide. T_anneal was varied from 300 to 500 °C, and T_curing from 200 °C to T_anneal. All T_anneal lead to a polycrystalline phase, but the amorphous phase was observed at T_anneal = 300 °C and T_curing ranging from 250 to 300 °C. This could be explained by the melting point of the precursor (SnCl₂), occurring at 250 °C. The crystallinity can be effectively controlled by the annealing temperature, but the curing temperature dramatically affects the grain size. We can reach grain sizes from 5–10 nm (T_curing = 200 °C and T_anneal = 300 °C) to 30–50 nm (T_curing = 500 °C and T_anneal = 500 °C). At a fixed T_anneal, Hall mobilities, carrier concentration, and conductivity increased with the curing temperature. The Hall mobility was in the range of 1 to 9.4 cm²/Vs, the carrier concentration was 10¹⁸ to 10¹⁹ cm⁻³, and the conductivity could reach ~20 S/cm when the grain size was 30–50 nm. The optical transmittance, the optical bandgap, the refractive index, and the extinction coefficient were also analyzed and they show a correlation with the annealing process.

The limited choice of materials for large area electronics limits the expansion of applications. Polycrystalline silicon (poly-Si) and indium gallium zinc oxide (IGZO) lead to thin-film transistors (TFTs) with high field-effect mobilities (>10 cm²/Vs) and high current ON/OFF ratios (I_On/I_Off > ~10⁷). But they both require vacuum processing that needs high investments and maintenance costs. Also, IGZO is prone to the scarcity and price of Ga and In. Other oxide semiconductors require the use of at least two cations (commonly chosen among Ga, Sn, Zn, and In) in order to obtain the amorphous phase. To solve these problems, we demonstrated an amorphous oxide material made using one earth-abundant metal: amorphous tin oxide (a-SnO_x). Through XPS, AFM, optical analysis, and Hall effect, we determined that a-SnO_x is a transparent n-type oxide semiconductor, where the SnO₂ phase is predominant over the SnO phase. Used as the active material in TFTs having a bottom-gate, top-contact structure, a high field-effect mobility of ~100 cm²/Vs and an I_On/I_Off ratio of ~10⁸ were achieved. The stability under 1 h of negative positive gate bias stress revealed a Vth shift smaller than 1 V.

The effects of gallium doping into indium–zinc–tin oxide (IZTO) thin film transistors (TFTs) and Ar/O₂ plasma treatment on the performance of a‐IZTO TFT are reported. The Ga doping ratio is varied from 0 to 20%, and it is found that 10% gallium doping in a‐IZTO TFT results in a saturation mobility (µs_at) of 11.80 cm² V⁻¹ s⁻¹, a threshold voltage (V_th) of 0.17 V, subthreshold swing (SS) of 94 mV dec⁻¹, and on/off current ratio (I_on/I_off) of 1.21 × 10⁷. Additionally, the performance of 10% Ga‐doped IZTO TFT can be further improved by Ar/O₂ plasma treatment. It is found that 30 s plasma treatment gives the best TFT performances such as µs_at of 30.60 cm² V⁻¹ s⁻¹, V_th of 0.12 V, SS of 92 mV dec⁻¹, and I_on/I_off ratio of 7.90 × 10⁷. The bias‐stability of 10% Ga‐doped IZTO TFT is also improved by 30 s plasma treatment. The enhancement of the TFT performance appears to be due to the reduction in the oxygen vacancy and OH concentrations.

We report the effect of purification of ZrO _x precursor on the performance of solution processed amorphous indium-zinc-tin oxide thin-film transistors with a ZrO _x gate insulator, which is processed at the maximum temperature of 300 °C in air. By purification, the saturation mobility (μ _sat ) increases from 2.45 ± 0.83 to 15.42 ± 4.01 cm ² V ^-1 s ^-1 , subthreshold swing decreases from 141.44 ± 14.08 to 87.90 ± 11.05 mV/decade and drain current ON/OFF ratio increases from ~10 ⁷ to ~10 ⁹ . The leakage currents are remarkably reduced by using purified ZrO _x as a gate insulator. The improvement is mainly due to the reduced impurities and less oxygen vacancies in ZrO _x .