Hydrogen-included plasma-assisted reactive sputtering for conductivity control of ultra-wide bandgap amorphous gallium oxide

Amorphous oxide semiconductors (AOSs) with wide band gaps have been attracting attention since their unique material properties including extremely large bandgaps, a high break-down electric field, and acceptable electrical conductivity. Among them, amorphous gallium oxide (a-Ga₂O_x) is a promising candidate material for solar blind UV detectors, sensors, and channel layers of thin film transistors (TFTs). ^1–3)

In general, cation/anion off-stoichiometry such as generation of oxygen vacancy and hydrogen doping are the only effective doping method for AOS, and hence carrier control of AOS has been performed mainly by controlling the amount of oxygen introduced during thin film deposition, by post-deposition treatment such as thermal annealing, and by hydrogen doping into AOS. ⁴⁾ However, AOS with ultrawide bandgaps including a-Ga₂O_x require high doping densities to enhance carrier density. ⁵⁾ The formation of a-Ga₂O_x thin films has been reported by pulse laser deposition (PLD), ^6–11) sputtering, ^12–19) CVD, ^20,21) atomic layer deposition, ^21–24) and solution processes. ^25–29) However, it has been very difficult to achieve semiconducting properties in a-Ga₂O_x with common doping methods.

In order to overcome these issues, deposition of a-Ga₂O_x thin films by plasma-assisted reactive sputter deposition system have been demonstrated as one of the efficient doping methods for AOS. Plasma-assisted reactive sputter deposition system consists of a magnetron sputter source surrounded by multiple low inductance antenna (LIA) modules, ^30–38) which independently controls the inductively coupled plasma (ICP) generated by the LIAs and the magnetron discharge, thereby enabling independent control of sputter particles and reactive particles via the control target voltage and plasma density.

In our previous study, the plasma properties of a plasma-assisted reactive sputter deposition system have been investigated. The results exhibit that the feasibility for the independent control of the sputtering flux and reactivity via the control of target voltage and plasma density. In a-Si:H film deposition using a silicon target, the crystallinity control of a-Si:H films due to the independent flux control of sputtered atoms and reactive species have been demonstrated. ³⁹⁾ Moreover, in the deposition of a-IGZO films with this system, resistivity control of a-IGZO films using plasma-assisted reactive sputter deposition system have been performed. ³⁹⁾ Furthermore, the fabrication of IGZO TFTs using the a-IGZO films deposited with this system have been demonstrated. The electrical properties of a-IGZO TFTs with unannealed a-IGZO channels deposited at temperatures lower than 150 °C using this system produced a-IGZO TFTs with field effect mobility values higher than 18 cm² V⁻¹ s⁻¹. ³⁹⁾ These results indicate that it is possible to precisely control the reactive species introduced into the film during film deposition.

In the present work, conductivity control of a-Ga₂O_x films by cation/anion off-stoichiometry such as oxygen vacancy formation and hydrogen doping have been achieved by hydrogen-included plasma-assisted reactive sputter deposition system and physical and electrical properties of a-Ga₂O_x films formed by this system have been investigated.

The deposition of a-Ga₂O_x films was performed by plasma-assisted reactive magnetron sputter deposition system, which consists of four internal-type LIA modules installed around a circular magnetron target in the top flange of a cylindrical chamber with an inner diameter of 300 mm and a height of 70 mm. ^39–43) These four internal LIA modules are coupled to an RF power generator driven at 13.56 MHz via a matching network. A sintered Ga₂O₃ target with a 101.6 mm diameter was used to deposit the a-Ga₂O_x films. The Ga₂O₃ target is coupled to an RF power generator driven at 13.56 MHz via a matching network.

The distance between target to substrate was set at 50 mm. During a-Ga₂O_x thin film deposition, an RF power of 500 W was applied to the internal LIA modules and an RF power of 100 W was applied to the magnetron sputter source. Prior to each deposition of a-Ga₂O_x films, the reactor was evacuated to a base pressure of less than 4 × 10⁻⁵ Pa with a turbomolecular pump. In this study, pure Ar and Ar + H₂ mixture gas were supplied to the reactor through an inlet located around the target holder. The working pressure was kept at 0.53 Pa during the formation of a-Ga₂O_x films.

Optical emission spectroscopy (OES) analysis was conducted using a high-resolution fiber optic spectrometer (Ocean Optics USB2000+). The electrical properties of the a-Ga₂O_x films were measured by a four-point probe method. The film density of a-Ga₂O_x channel layer was estimated from X-ray reflectivity (XRR) data acquired using an XRD system (Rigaku SmartLab). The thickness of the a-GaO_x films was measured with a stylus surface profiler (Tokyo Seimitsu SURFCOM 1400D). The deposition rate was estimated from the measured a-Ga₂O_x film thickness. The optical transmittance of a-Ga₂O_x films was measured using an UV–visible (UV–vis) spectrophotometer (Shimadzu UV-2450). Hall effect measurement with the van der Pauw configuration was performed.

Firstly, in order to obtain information on the products in plasma when H₂ is added to argon gas, OES measurements were performed. Figure 1 show the variation of optical emission of OH radical (308.9 nm) and H_α (656.3 nm) on H₂ flow ratio. With increasing H₂ flow ratio, both emission of OH radical and H_α increased almost linearly, but the gradient of H_α emission intensity against the H₂ flow ratio is larger than that of OH radical. This result indicates that changing the H₂ flow rate ratio may have a larger effect on the reduction and/or oxidation of a-Ga₂O_x.

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The increase in the OH emission intensity is attributed to the generation of OH radicals via the dissociation of H₂O molecules, based on the equilibrium between the supply of O atoms sputtered from the Ga₂O₃ target into the gas phase and the generation of H radicals from the dissociation of H₂ gas, and the recombination of these radicals at the chamber wall. ⁴⁴⁾

The formation of a-Ga₂O_x thin films by hydrogen-included plasma-assisted reactive sputtering was attempted. Firstly, the dependence of deposition rate on H₂ flow rate ratio H₂/(Ar + H₂) was examined. The results are shown in Fig. 2. The deposition rate was almost constant at 10 nm min⁻¹ even when the H₂ flow rate ratio was increased. In general magnetron sputtering, increasing the hydrogen partial pressure relative to argon gas tends to lower the film formation rate due to a decrease in plasma density. In this system, however, the rapid decrease in plasma density due to the addition of H₂ can be suppressed by superimposing ICP, which is considered to subsequently suppress the decrease in the deposition rate. The results exhibit that precise control of film quality is possible because the flux of active species can be varied at a constant deposition rate.

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The change in resistivity of a-Ga₂O_x thin films deposited by hydrogen-included plasma-assisted reactive sputtering was then investigated by changing the H₂ flow rate ratio H₂/(Ar + H₂). Figure 3 shows the variation in resistivity of a-Ga₂O_x thin films deposited by changing the H₂ flow rate ratio H₂/(Ar + H₂). With increasing H₂ flow rate ratio to 0.35%, the resistivity of a-Ga₂O_x thin films decreased from 1.0 × 10⁴ Ωcm for 0% to 4.4 × 10² Ωcm for 0.35% and then increased linearly to 4.9 × 10⁵ Ωcm for 1.5%, saturating at 10⁶ Ωcm above 3%. The Hall mobility and the electron density of a-Ga₂O_x thin films deposited for H₂ flow rate ratio of 0.35% was 5.28 cm² V⁻¹s⁻¹ and 6.52 × 10¹³ cm⁻³, which indicates that this film has semiconductor properties.

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The results indicate that the resistivity of a-Ga₂O_x thin films can be controlled from 10² to 10⁶ Ωcm by changing the H₂ flow rate ratio during film deposition. The results of the resistivity exhibited a peculiar behavior in response to changes in H₂ flow rate ratio. In AOSs, carriers are generally controlled by only cation/anion off-stoichiometry and hydrogen doping. ¹⁰⁾ Therefore, the density of a-Ga₂O_x thin films, which is a major structural factor in defect formation including voids and hydrogen incorporation, was measured by XRR. Figure 4 shows the variation of the film density of a-Ga₂O_x thin films on H₂ H₂ flow rate ratio H₂/(Ar + H₂). With increasing H₂ flow rate ratio, the film density decreased drastically from 5.36 g cm⁻³ for 0% to 5.26 g cm⁻³ for 0.5% and then slightly to 5.09 g cm⁻³ for 5%. The decrease in film density due to the decrease in hydrogen flow ratio is considered to be due to the decrease in plasma density caused by the addition of hydrogen to argon gas. In a-Ga₂O_x thin films fabricated by PLD, electrical conductivities are obtained only if the film density is >5.2 g cm⁻³, where the electrical conductivity increases exponentially with the increase in the film density. ¹⁰⁾ In our study, the results are almost consistent with previous studies, since the a-Ga₂O_x thin film with a film density as high as 5.25 g cm⁻³ has semiconducting properties with a reasonably low resistivity <10⁵ Ωcm. In the cases of a-IGZO thin films deposition, it is known that electron trapping caused by oxygen deficiency with void structures can be suppressed by increasing film density. ¹⁰⁾ Therefore, the reduced resistivity of a-Ga₂O_x thin films deposited with this system is considered to be derived from the formation of the high-density films.

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To investigate the origins of the resistivity changes of a-Ga₂O_x due to changes in H₂ flow rate ratio H₂/(Ar + H₂), the state of defects in the a-Ga₂O_x thin film was estimated from the optical properties. The optical properties such as optical bandgap (E_g), Fermi level (E_F) and Urbach energy (E_u) of each film were calculated from the UV–vis spectroscopy. The optical bandgap was obtained through the Tauc equation (α hν )² = A (hν−E_g) for direct bandgap, where α is the absorption coefficient, A is the constant slope parameter, hν is photon energy, and E_g is the optical bandgap energy. While the Fermi level (E_F) relative position was obtained by subtracting the E_g and valence band offset (E_F = E_g −E_V-offset). The Urbach energy (E_u) was obtained from the Urbach relation, α = A exp(hν/E_g), below the optical absorption edge.

Figure 5 shows the α spectra of the a-Ga₂O_x thin films in variation of H₂ flow rate ratio H₂/(Ar + H₂). With increasing H₂ flow rate ratio, α of the a-Ga₂O_x thin films increased slightly to 1.5% and then greater absorption was observed. Figure 6 shows E_g and E_u of the a-Ga₂O_x thin films as a function of H₂ flow rate ratio H₂/(Ar + H₂), estimated using the results of UV–vis spectroscopy. With increasing H₂ flow rate ratio, E_g is almost constant at 4.10−3.94 eV until 0.5% and then decreased considerably to 2.53 eV for 5.0%. On the other hand, E_u monotonically increases from 319 meV for 0.35% to 1071 meV with increasing H₂ flow rate ratio. The E_u is a combination of the tail states of the conduction band (CB) and valence band (VB). This represents the degree of subgap disorder and defect distribution, which may be due to amorphous disorder and defective impurities and so on. The wide spread of optical absorption in the low hν region (<4.0 eV) indicates higher subgap disorder as seen in Fig. 5. It in turn indicates that the small E_g values in the high H₂ flow rate ratio region reflects the subgap disorder but not corresponds to the fundamental bandgap. The disorder of a-Ga₂O_x thin films tends to increase with increase H₂ flow rate ratio. Furthermore, in this case, the subgap disorder, which are generated in the a-Ga₂O_x thin film during deposition process, has due to a combination of, oxygen deficiency, hydrogen defect impurity and amorphous disorder. The defects generated in AOSs including a-IGZO and a-Ga₂O_x tend to create tail states in the fundamental bandgap and the energy width of VB tail states is larger than CB tail states. ^4,10) For these reasons, the E_u is considered to be a good representation of the VB tail state. Hence, additional acceptor states were created above the valence band maximum (VBM). These results indicate that a-Ga₂O_x thin film formation with a large E_g and high conductivity is realized at the same time for small H₂ flow rate ratio and small disorder.

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The role of hydrogen radical in the a-Ga₂O_x thin film deposition process can be considered as follows. The increase in hydrogen radical production due to the increased H₂ flow rate ratio as shown in the optical emission results of Fig. 2 promotes the formation of oxygen defects in the a-Ga₂O_x thin film by the reduction effect of hydrogen. Therefore, with increasing H₂ flow rate ratio from 0% to 0.35% the resistivity decreases due to the balance between the formation of oxygen defects and the passivation of H radicals and/or OH radicals in the defects. However, when the hydrogen flow ratio is further increased, the decrease in plasma density reduces the a-Ga₂O_x film density, which affects void formation caused by the film structure of the a-Ga₂O_x thin film during deposition. Furthermore, due to the increase in hydrogen radical production by the increased hydrogen flow rate ratio as shown in the optical emission results of Fig. 2, the introduction of a high concentration of hydrogen into the film is considered to cause the formation of the subgap disorders such as oxygen deficiency, hydrogen defect impurity and amorphous disorder as shown in the results of Fig. 6.

The plasma-assisted reactive sputtering system with addition to H₂ is a promising method to convert ultra-wide bandgap oxide materials into semiconductor materials.

Conductivity control of a-Ga₂O_x films by cation/anion off-stoichiometry such as oxygen deficiency formation and hydrogen doping have been achieved by plasma-assisted reactive sputter deposition system and physical and electrical properties of a-Ga₂O_x films formed by this system have been investigated. The change in resistivity of a-Ga₂O_x thin films deposited by hydrogen-included plasma-assisted reactive sputtering was then investigated by changing the H₂ flow rate ratio H₂/(Ar + H₂). The a-Ga₂O_x thin films with a film density and optical band gap energy as high as 5.2 g cm⁻³ and 4.8 eV has semiconducting properties with a resistivity as low as 10² Ωcm was demonstrated using plasma-assisted reactive sputtering system with addition to H₂. The Hall mobility and the electron density of a-Ga₂O_x thin films deposited for H₂ flow rate ratio of 0.35% was 5.28 cm² V⁻¹ s⁻¹ and 6.52 × 10¹³ cm⁻³, which indicates that this film has semiconductor properties. These results exhibit that the electrical resistivity of a-Ga₂O_x thin films can be controlled from 10² to 10⁵ Ωcm by appropriately controlling the amount of hydrogen introduced from the plasma. The plasma-assisted reactive sputtering system with addition to H₂ is a promising method to convert ultra-wide bandgap oxide materials into semiconductor materials.

This work was partly supported by The Project, Design and Engineering by Joint Inverse Innovation for Materials Architecture (DEJI²MA) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS KAKENHI Grant No. JP 21H01671). KI, TaK and ToK are supported also by MEXT Data Creation and Utilization Type Material Research and Development Project (Grant No. JPMXP1122683430). KI was performed under the Joint Usage/Research Center on Joining and Welding, JWRI, Osaka University, Japan. KT and YS are supported in part by the Collaborative Research Project of Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology.