Refractory metal-based ohmic contacts on β-Ga₂O₃ using TiW

The recent developments in the fabrication of electronic devices based on the semiconductor β-Ga₂O₃ have demonstrated the high potential of this material to be used in next-generation power electronics applications.^1–3 Due to the ultra-wide bandgap of 4.8 eV, a high breakdown strength of 8 MV/cm is expected, which could pave the way for power devices with even higher breakdown voltages and efficiencies than are possible with the SiC and GaN counterparts.^4,5 Current research activities using β-Ga₂O₃ are preferentially focusing on the development of Schottky barrier diodes^6–8 and field-effect transistors.^9–13 Promising performances have been achieved so far with electronic devices reaching a peak field strength as high as 5.5 MV/cm¹⁴ and breakdown voltages up to 8.03 kV.¹⁵ Apart from the expected high breakdown strength, β-Ga₂O₃, like any other wide bandgap semiconducting material, offers the possibility to be used in high-temperature electronic applications, such as in the field of automotive or aerospace engineering.^16–18 However, the predominately used ohmic contact system Ti/Au for β-Ga₂O₃, which is commonly annealed around 500 °C for contact formation, shows severe degradation when exposed to temperatures beyond 500 °C.^19,20 To overcome this issue, this paper analyzes the ohmic contact properties of the refractory metal alloy TiW (10:90 wt. %) on β-Ga₂O₃ for potential high-temperature applications. Although it has recently been shown that pure tungsten forms Schottky contacts to β-Ga₂O₃,^21,22 we demonstrate that by the addition of 10% titanium, reliable ohmic contacts to β-Ga₂O₃ can be realized. Optimum contact formation is achieved at annealing temperatures of 700 °C, emphasizing the high-temperature stability of this metallization system. However, significant degradation is identified beyond this temperature due to changes in the TiW/β-Ga₂O₃ interface quality.

In order to analyze the impact of the doping concentration of the semiconducting β-Ga₂O₃ on the contact resistance, two types of structures, named structure A and structure B, were fabricated, as shown in Fig. 1. For both structures, the experiments started with the homoepitaxial growth of a Si-doped semiconducting β-Ga₂O₃ layer by metal organic chemical vapor deposition (MOCVD) on Mg-doped semi-insulating β-Ga₂O₃ (100) substrates.^23,24 The substrates with dimensions of 10 × 10 × 0.5 mm³ were prepared with a miscut of 4° from 2 in. diameter bulk β-Ga₂O₃ single crystals obtained by the Czochralski method at the Leibniz-Institute for Crystal Growth.^25–27 Triethylgallium (TEGa) was used as the gallium precursor, pure O₂ as the oxygen source, tetraethyl-orthosilicate (TEOS) as the Si-dopant, and high purity Ar as the carrier gas. A growth temperature of 825 °C and a chamber pressure of 25 mbar were used, which resulted in a growth rate of about 3.6 nm/min. The molar flows of the used gases were adjusted to realize epitaxial Ga₂O₃ layers with a doping concentration N_D of around 1 × 10¹⁸ cm⁻³ and a charge carrier mobility μ of 106 cm²/Vs for both structure types. In addition to this layer, a 30 nm thin heavily doped n⁺⁺-layer with a doping concentration of around 1 × 10¹⁹ cm⁻³ and a charge carrier mobility of 33 cm²/Vs was deposited subsequently atop in the case of structure B. The total thickness of the epitaxial layers for both structure types was set to 200 nm. Doping concentration and mobility values of the respective epitaxial layers were determined separately for each epitaxial layer by Hall measurements on reference test samples.

For the electrical characterization of the TiW ohmic contacts, circular transfer length measurement (TLM) structures were fabricated by starting with a surface preparation of the β-Ga₂O₃ samples in BCl₃-Ar-plasma. Here, the samples were transferred into a reactive ion etch (RIE) system in order to etch ∼10 nm of Ga₂O₃ within the ohmic contact region. Previously, it has been reported that a short RIE treatment of Ga₂O₃ prior to the metal contact deposition leads to the generation of high-density surface defects, such as oxygen vacancies that act as donors enhancing the charge carrier injection.^28,29 Subsequently, TiW contacts with a thickness of 150 nm were deposited by an RF-sputter process in a Leybold Z590 using a Ti:W (10:90 wt. %) target with 99.99% purity. The sputtering process was performed at an RF power of 160 W, an Ar-flow of 15 sscm, and a chamber pressure of 0.5 Pa. Afterward, the metal layer was structured using photolithography and SF₆-based dry etch processing. The circular TLM structures featured a pad diameter of 200 µm and electrode spacings of 4, 8, 12, 16, 20, and 24 µm. Finally, the samples were subjected to rapid thermal annealing for 60 s in a nitrogen atmosphere at temperatures ranging from 400 to 900 °C.

Sheet and contact resistances were extracted from the TLM structures at room temperature using the four-point probe method with measuring currents between −100 and 100 mA. The surface topography of the as-deposited and annealed TiW layers was investigated by atomic force microscopy (AFM) in tapping mode using a Bruker Dimension Edge AFM system. Scanning Transmission Electron Microscope [(S)TEM] investigations were performed with a FEI Titan 80-300 transmission microscope operated at an acceleration voltage of 300 kV. The microscope is equipped with a high brilliance X-FEG emitter and a CS corrector for imaging. Energy-dispersive x-ray spectroscopy (EDX) measurements were performed using a JEOL JEM2200FS operated at an acceleration voltage of 200 kV. TEM samples were prepared in cross-sectional geometry by conventional plan parallel polishing down to a thickness of about 5 µm. For achieving electron transparency, the samples were further thinned by ion-milling in a Gatan PIPS, with an acceleration voltage of 3.5 kV. As a cleaning step, the acceleration voltage was stepwise reduced down to 0.2 kV.

Figure 2 shows the I–V measurements of circular TLM patterns with an electrode spacing of 4 µm for structures A and B, which were annealed at different temperatures. It can be seen that structure A shows significant non-ohmic properties for all annealing temperatures with non-linear curve progressions. A similar trend can be observed for samples based on structure B, which have been annealed at temperatures below 700 °C. However, a distinct linear correlation between current and voltage is observed at an annealing temperature of 700 °C, emphasizing ohmic contact properties, which disappears again at annealing temperatures above 700 °C. In order to extract the values for the contact resistance R_C and the specific contact resistivity ρ_C of the TiW on β-Ga₂O₃ for all the annealed samples based on structures A and B, the total resistance as a function of the electrode spacing is plotted. It should be noted that the extraction of the contact resistance for all curves was done at low voltages where the curves still show almost linear behavior, avoiding underestimation of the resistance values. Although the TLM method is usually only applied to contacts that show full ohmic properties, this approach allowed a qualitative comparison of the contact properties of the TiW contacts annealed at different temperatures. A comparison of the specific contact resistivity as a function of the annealing temperature for both structures is shown in Fig. 3. Here, it can be seen that in the case of structure A, the value for ρ_C stays almost constant at around 10⁻³ Ω cm² for annealing temperatures between 400 and 700 °C and slightly goes down to around 2 × 10⁻⁴ Ω cm² after annealing above 700 °C. The latter value equals a contact resistance of 8 Ω mm for each contact, which is unacceptably high for use in efficient electronic devices, since it already contributes a high percentage of the total on-resistance of recent β-Ga₂O₃ power transistors.^12,13 Structure B, in turn, shows a steady decrease in the specific contact resistivity with increasing annealing temperature and reaches a minimum value of 1.5 × 10⁻⁵ Ω cm² at 700 °C. However, at higher annealing temperatures above 700 °C, the contact resistivity increases again to values similar to those of structure A. The TLM plot of structure B, which was annealed at 700 °C, showing the lowest specific contact resistivity, is presented in Fig. 4, revealing a contact resistance of 1.6 Ω mm for each contact. In addition to R_C and ρ_C, the TLM plot also allows us to extract the sheet resistance R_S, which did not change with the annealing temperature for both structures and had a value of 2.2 and 1.6 kΩ/sq for structures A and B, respectively. As shown in the previously reported investigations, the optimum specific contact resistance of Ti/Au contacts to β-Ga₂O₃ is typically in the range of 10⁻⁶ to 10⁻⁵ Ω cm², which is usually achieved by highly n-type doping of the β-Ga₂O₃ contact region prior to metal deposition either using Si-implantation^12,30,31 or epitaxial regrowth.^14,32 Up to now, the lowest contact resistance on β-Ga₂O₃with a value of 8.3 × 10⁻⁷ Ω cm²was achieved by using the latter approach.³⁶ Benchmarking our optimum contact resistance value with the data from the literature emphasizes the good ohmic contact properties of TiW to β-Ga₂O₃ annealed at 700 °C.

In order to investigate the impact of the annealing temperature on the surface morphology of the sputtered TiW on the β-Ga₂O₃ epitaxial substrates, AFM measurements were carried out on samples with as-deposited TiW layers as well as on samples annealed at 700 and 900 °C. The results of these measurements are presented in Fig. 5 and indicate smooth TiW layers right after deposition with a root-mean-square (rms) roughness of 0.8 nm, which is only slightly higher than the rms roughness value of the β-Ga₂O₃ epitaxial layer of 0.2 nm. After annealing at 700 °C, the surface roughness slightly increases to an rms value of 1.7 nm, which still indicates smooth surfaces of the TiW. However, annealing at 900 °C results in the formation of layers with increased surface roughness up to an rms roughness value of 3.6 nm. The height distribution presented in Fig. 5(d) illustrates these observations, showing a two-times higher rms roughness for samples annealed at 700 °C as well as a four-times higher rms roughness for samples annealed at 900 °C compared to the as-deposited layers. In addition, the deteriorated surface morphology at the higher annealing temperature is reflected by a distinct broadening of the height distribution. This significant change in the surface morphology between 700 and 900 °C could be an indicator that the interface quality between the TiW and the β-Ga₂O₃ is reduced, explaining the observed increase in the specific contact resistivity for structure B. In order to verify this assumption, cross-sections of structure B annealed at 700 °C and 900 °C were prepared and analyzed using TEM.

Figure 6(a) shows a low magnification STEM HAADF image of the contact structure of the 700 °C annealed sample in the cross-sectional view. The TiW is much brighter in comparison to the β-Ga₂O₃ layer, caused by the higher mean atomic number of the TiW compared to the β-Ga₂O₃. The surface structure of the metal contact is in good agreement with the AFM images shown in Fig. 5. Although the interface between the contact metal and the β-Ga₂O₃ layer appears to be completely sharp in this image, a small interlayer [Figs. 6(b) and 6(c)] with a thickness of about 3–5 nm can be observed at higher magnification. This interlayer appears dark in the STEM HAADF image [Fig. 6(b)] in contrast to the β-Ga₂O₃ layer as well as to the TiW contact, while no atomic pattern can be resolved. A similar interlayer has been previously described by Peterson et al. in the case of Ti/Au contacts, indicating the formation of a TiO_x interlayer.^33,34 To elaborate the atomic species involved in the formation of this interlayer, we performed STEM EDX measurements, as shown in Fig. 7. The STEM HAADF image shows the interface region between the TiW and the β-Ga₂O₃ layer of the 700 °C annealed sample. Figures 7(b)–7(e) represent the corresponding elemental distribution maps of this area as measured by STEM EDX. We included the main species in the specimen, namely, oxygen, tungsten, titanium, and gallium. The K_α line was chosen for the detection of oxygen, titanium, and gallium, while the L_α line was used for tungsten since the W-K line is far above the maximum energy of the EDX spectrum of 20 keV. First of all, it can be seen that the tungsten signal can only be detected in the contact region, indicating a high stability of tungsten. Another observation is that gallium can be found in the β-Ga₂O₃ layer as well as in the TiW contact. This is due to an out-diffusion of Ga atoms as previously described.³³ Interestingly, almost no Ga signal can be detected in the interface region (dark area in the STEM HAADF image), whereas oxygen can be partially found here as well as in the β-Ga₂O₃ layer. The strongest signal in the interface region is generated by titanium. This indicates that the intermediate phase is most likely based on TiO_x with very low gallium concentration as is also the case with Ti/Au contacts on β-Ga₂O₃. However, it should be noted that the formation of this interlayer with comparable dimensions using Ti/Au contacts was already observed at an annealing temperature of 470 °C.^33,34 Since titanium is randomly distributed within the TiW layer with a concentration of only 10 wt. %, we suspect that the limited availability of titanium at the TiW/β-Ga₂O₃ interface is the reason that higher temperatures are necessary to form a TiO_x interlayer with a comparable thickness. Another feature can be observed in the high-resolution TEM image [Fig. 6(c)] below this interlayer in the β-Ga₂O₃ layer. Line shape contrasts at the interface are appearing along the $2̄01$ lattice planes (marked by arrows). These defects are not present in as-grown homoepitaxial layers and are most likely formed due to a diffusion process of either Ga or O from the β-Ga₂O₃ layer into this interlayer. Moreover, EDX analysis indicates the diffusion of Ti downward into these defective regions forming TiO_x. This means that even at annealing temperatures of 700 °C, we observe the solid solution reaction of the present species, which has been also observed previously.³⁵ 

Figure 8(a) shows a low magnification STEM HAADF image of the interface of a β-Ga₂O₃ sample coated with TiW and annealed at 900 °C. One of the first noticeable features in contrast to the sample annealed at 700 °C is the formation of metal islands on top of the TiW contact layer, which is in good agreement with the rough surface observed by AFM. The second noticeable feature is a much more pronounced, dark but discontinuous line at the interface between the β-Ga₂O₃ and the TiW contact as compared to the sample annealed at 700 °C. High-resolution STEM HAADF images taken at different positions of this interface region are presented in Figs. 8(b) and 8(c), verifying an increase of the interlayer thickness up to 5–15 nm. In addition, the layer exhibits increased structural ordering. Figure 8(b) reveals a quadratic pattern of the projected atomic columns with an inter-atomic distance (white lines) of 0.32 nm. In contrast to this, only bright lines are seen in Fig. 8(c) with no sign of distinct atomic columns, indicating an orientation of the crystalline material, where the interatomic distance within the lines is smaller than the resolution limit of the imaging method. The distance between the white lines is again about 0.32 nm. These observations most likely correspond to an intermixed, polycrystalline, tetragonal, or cubic phase, forming rotational domains with respect to the β-Ga₂O₃ layer.

We also performed STEM EDX measurements on this sample, as shown in Fig. 9, with a STEM HAADF image showing the interface region between the TiW and the β-Ga₂O₃ layer as well as the corresponding elemental distribution maps of this area. The analysis reveals that the elemental distribution is almost identical to the sample annealed at 700 °C with the exception of a thicker interface region. Again, this region mostly consists of titanium and oxygen, indicating a titanium oxide intermediate phase. The main crystal modifications of titanium oxide are anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal). A comparison of these modifications with the experimental findings excludes anatase and brookite. The pattern observed in the high-resolution HAADF STEM images of Fig. 8(b) perfectly agrees with the rutile structure seen along the 〈001〉 projection. In addition, the measured lattice distances of 0.32 nm are in perfect agreement to the {110} planes of pure TiO₂ in the rutile modification.

Based on the analyses and observations carried out in this study, we can draw the following conclusions. At annealing temperatures of 700 °C, an interdiffusion process of Ti from the TiW contact into the β-Ga₂O₃ layer takes place. Here, Ti starts to react and steals oxygen from β-Ga₂O₃ by partially substituting Ga atoms as has been previously observed with Ti/Au contacts on β-Ga₂O₃ at annealing temperatures of 470 °C.³³ The substituted Ga, in turn, starts to diffuse upward into the TiW contact. The 3–5 nm thin TiO_x layer features amorphous properties and most probably possesses a high amount of vacancies and/or Ga impurities, which improves charge carrier injection from the TiW contact into β-Ga₂O₃, resulting in a low contact resistance. At higher annealing temperatures beyond 700 °C, this TiO_x interlayer dramatically alters its properties. First, the thickness increases up to 15 nm, and second, the crystallographic structure changes from an amorphous to a polycrystalline state of tetragonal TiO₂ (rutile). In this regard, we can assume that the amount of electrically active impurities in this interlayer is significantly reduced, which in combination with the increased thickness leads to a higher contact resistance. Furthermore, it can be stated that a stable ohmic contact based on TiW can only be realized on highly doped n⁺⁺ β-Ga₂O₃ layers with doping concentrations of ≥1 × 10¹⁹ cm⁻³. In contrast to pure Ti/Au contacts to Ga₂O_3, we believe that a much higher annealing temperature of 700 °C is needed for the TiO_x interlayer formation in case of pure TiW contacts. Since the amount of titanium within the TiW alloy is relatively low and, thus, the availability of titanium at the TiW/β-Ga₂O₃ interface is limited, we suspect that higher temperatures are necessary to support titanium diffusion toward this interface, resulting in a sufficient accumulation for the TiO_x interlayer formation.

In conclusion, this work demonstrates the possibility of realizing ohmic contacts based on TiW on β-Ga₂O₃. Despite the known fact that pure W only forms Schottky contacts with β-Ga₂O₃, we were able to achieve low contact resistances down to 1.5 × 10⁻⁵ Ω cm² by the addition of 10% Ti. This emphasizes that Ti plays a dominant role in the contact formation. A high annealing temperature of 700 °C is needed in order to initiate the diffusion process of Ti toward the TiW/β-Ga₂O₃ interface where it starts to react with O, leading to the formation of a thin amorphous TiO_x interface layer. We suspect that existing vacancies, defect states, or Ga impurities in this interlayer improve the charge carrier transport mechanism, leading to the reduction of contact resistance. Upon annealing at higher temperatures, the contact resistance increases due to the increase of the interlayer thickness and a modification of the structural properties. The as-formed rutile TiO₂ features a lower defect density as well as a low concentration of Ga impurities, which hinders the charge carrier injection. The investigations prove that an ohmic contact to β-Ga₂O₃ can be realized using TiW, which is stable up to temperatures of 700 °C. This is more than 200 °C higher than is possible with the conventionally used Ti/Au metallization. Nevertheless, it should be noted that highly doped β-Ga₂O₃ is a mandatory requirement to achieve this.

The authors thank Z. Galazka from IKZ for providing β-Ga₂O₃ wafers for homoepitaxy. Furthermore, the authors would like to thank the colleagues in the process department at FBH for process technological support, in particular, A. Arnlind for AFM measurements as well as N. Thiele for RTA processing. H. Lawrenz from FBH is acknowledged for the image preparation of the EDX data. This work was performed in the framework of GraFOx, a Leibniz-Science Campus partially funded by the Leibniz Association. Furthermore, this work was funded by the Federal Ministry of Education and Research in Germany within the frame of the Joint Research Project OXIKON (Funding No. 03VP03711) as well as within the Research Project GoNext (Funding No. 16 ES1084K).