Improvement of Ohmic contacts on Ga₂O₃ through use of ITO-interlayers Free

There is renewed interest in Ga₂O₃ because of its large bandgap, availability of large diameter, high quality crystals, and excellent electrical transport properties. The β-polytype of Ga₂O₃ in particular can be used to fabricate gas sensors, high power electronic devices such as rectifiers and transistors, and truly solar blind UV photodetectors.^1–22 Ga₂O₃ has a very high theoretical breakdown field (∼8 MV cm⁻¹),^1,3,4 making it a candidate for ultrahigh power electronics.^18–21 While its thermal conductivity is modest, thermal management techniques developed for GaN will mitigate some of these issues.^23–26 

Low resistance Ohmic contacts are a prerequisite for any device and should provide low contact resistance at moderate anneal temperatures. Additional contact resistance leads to slower device switching speeds as well as reliability issues due to local contact heating during current flow during device operation.^27–30 The usual approaches to improving contact resistance are to locally increase the doping under the contact through ion implantation or deliberate introduction of vacancy-related donors, or to add a low resistance interlayer. The lowest specific contact resistances reported on Ga₂O₃ are ∼5 × 10⁻⁶Ω cm⁻² for Ti/Au contacts on Si-implanted n-Ga₂O₃ epitaxial layers.²⁷ Sputtered indium tin oxide (ITO) on n-Ga₂O₃, followed by annealing at 900–1150 °C (Ref. 28) was also found to improve Ohmic contacts. Pt/ITO contacts on n-Ga₂O₃ showed superior Ohmic contacts to Pt/Ti and this was attributed to the formation of an interfacial layer with lower bandgap and higher doping concentration than the Ga₂O₃ alone.²⁹ The band alignment at the heterointerface is also critically important in determining the favorability of carrier transport.^30,31 Several authors have found that the presence of upward band bending in low conductivity Ga₂O₃ complicates achievement of Ohmic contacts.^32–37 The surface states present on the Ga₂O₃ are crucial to the type of contact formed with Au/Ti electrodes. If there are a high density of surface states present, carriers can tunnel through the barrier easily. As a result, a good Ohmic contact between Au/Ti and Ga₂O₃ will be formed.³⁸ If there are a low density of surface states on the Ga₂O₃ surface, it should be a Schottky contact.³⁹ 

ITO has excellent conductivity and can be deposited in thin film form by the usual semiconductor physical or vapor deposition methods.^40–47 It is widely used as transparent electrodes in both GaN-based light-emitting diodes⁴⁰ and liquid crystal displays.^41–47 It is therefore an attractive candidate for use in contact schemes to Ga₂O₃.

In this paper, we report on the efficacy of ITO interlayers in promoting Ohmic behavior in Ti/Au contacts on n-type Ga₂O₃ and measure the contact resistance as a function of postdeposition annealing temperature The minimum specific contact resistance of 6.3 × 10⁻⁵ Ω cm², was achieved after 600 °C annealing. The improvement in electron transport across the contact interface is consistent with the band alignment in the ITO/Ga₂O₃ heterostructure.

The starting wafers were bulk 2 in. diameter β-Ga₂O₃ single crystals (∼679 μm thick) with (−201) surface orientation (Tamura Corporation, Japan) grown by the edge defined film-fed growth method. Hall effect measurements show unintentional n-type doping of the samples with an electron concentration of ∼3 $×1017$ cm⁻³. On this doped wafer, Si+ implantation was performed at an energy of 30 keV with a dose of $1×1015$ cm⁻². The samples were then annealed at 950 °C to activate the implanted ions and increase conductivity in the implanted region.

Cl₂/Ar based plasmas were employed to define mesa of linear transmission line method (TLM) testers in a Plasma-Therm Versaline inductively coupled plasma (ICP) system. The 300 W (2 MHz) ICP power and 150 rf (13.56 MHz) chuck power resulted a dc self-bias of −150 V on the sample electrode. Ten nanometers of ITO was deposited by RF magnetron sputtering on the Si implanted β-Ga₂O₃ at room temperature using a 3-in. diameter composite ITO (In₂O₃/SnO₂ 90/10) target. The RF power was 125 W and the working pressure was $5×10−6$ Torr in a pure Ar ambient. The DC bias on the electrode under these conditions is in the range of 30–40 V. The contact metal pads for TLM testers were formed by standard lift-off of E-beam deposited Ti/Au (20 nm/80 nm) metallization. The sheet resistance of the ITO/Ti/Au was 420 mΩ/sq. as measured by four point probe. ITO was wet etched from between contact pads and using 1:1 HCl:DI water. The contact pads are square, with dimension of 100 × 100 μm and gaps are 5, 10, 15, 20, 25, and 30 μm. A SSI Solaris 150 rapid thermal annealing system was used to anneal the contact in the nitrogen ambient for 30 s at either 500 or 600 °C. The ramp-up rate was 45 °C s⁻¹, and the ramp-down rate was 32 °C s⁻¹. Figure 1 shows a schematic of the contact stack structure.

A Wentworth automated temperature control chuck was used to perform temperature dependent dc measurements, and chuck temperature was varied from room temperature to 150 °C. Current–voltage (I-V) characteristics were measured with an Agilent 4145B parameter analyzer using Be/Cu probe tips.

Figure 2(a) shows the I-V characteristics of Au/Ti/Ga₂O₃ contact stacks as a function of annealing temperature. Annealing at temperatures up to 500 °C did not lead to Ohmic behavior. This annealing temperature is not sufficient to displace Si atoms that were incorporated into the crystal structure after Si implantation and thermal activation at 950 °C annealing. By sharp contrast, Fig. 2(b) shows that the addition of the ITO interlayer significantly improved current flow into and through the substrate. Low temperature annealing further improved the Ohmic performance, with compliance of 100 mA reached first by the sample annealed at 600 °C.

The sheet resistance, specific contact resistance, and transfer resistance were calculated from TLM data.⁴⁸ A relatively thick titanium layer of 10-nm is used to serve as an adhesion layer for the Au layer on the top of the oxide layer. The Ti layer thickness was not optimized, since the emphasis of this paper is to demonstrate the contact with ITO interfacial layer reducing the annealing temperature and achieve reasonable contact resistivity. A sample output resistance versus gap size from ITO contact stack annealed at 500 °C is shown in Fig. 3, a strong linear dependence (r² = 0.997) is observed.

From data of this type, we were able to extract the sheet resistance, specific contact resistance, and transfer resistance as a function of annealing temperature. All of these measurements were performed at room temperature. Specific contact and transfer resistances all decreased sharply from as-deposited values with annealing, as shown in Fig. 4. The lowest temperature data point is unannealed, but was shown to demonstrate the need for annealing to improve contact performance. Samples at 300 °C and 400 °C without mesa definition had been tested prior to this study, but true Ohmic performance was not achieved at those temperatures. Sheet resistance slightly increased with annealing from the as-deposited state due to removal of ion bombardment damage from ITO sputtering. The lowest transfer resistance and specific contact resistance of 0.6 Ω mm and 6.3 × 10⁻⁵ Ω cm² were achieved after 600 °C annealing, respectively. Bae et al.⁴⁹ reported that at the interface between Ga₂O₃ and Ti/Au contact layers, annealing of the contact structures caused a decrease in oxygen concentration in the Ga₂O₃ and an increase in the Ti. The annealing will cause formation of titanium oxide at the interface.⁴⁹ There was also intermixing of the Ti and Au, with concomitant diffusion of Ga into the Au. It is difficult to form TiO and TiO₂ by reaction between Ti and β-Ga₂O₃ since the formation enthalpy (ΔH°_f) for Ga₂O₃ is −1089.1 kJ/mol,⁵⁰ much lower than that of TiO and TiO₂. However, Ti₂O₃ and Ti₃O₅ have very low ΔH°_f, −1520.9 and −2459.4 kJ/mol, respectively, which are much lower than that of Ga₂O_3.⁵⁰ After thermal annealing, Bae et al.⁴⁹ showed a high atomic percentage of oxygen in the titanium layer (∼65%), consistent with formation of the Ti₂O₃ and Ti₃O₅ phases. In our case, the ITO interlayer should delay formation of these compounds to higher temperatures.

The morphology degraded during annealing due to reactions of the metals, ITO and Ga₂O₃, as shown in the optical micrographs of Fig. 5. This shows contact stacks (a) as deposited, (b) annealed at 500 °C and (c) annealed at 600 °C. The annealing was carried out in N₂ for 30 s at each temperature. All of these structures have the ITO etched off between pads and the images were taken using the same brightness and focus conditions on the microscope. When ITO is annealed on the doped Ga₂O₃ without any metallization above, no bubbling is observed in the ITO layer. The bubbling may be due in part to the ITO layer diffusing oxygen into the metal contact pads; however, similar bubbling was noted by Bae et al.⁴⁹ in Ti/Au contacts on Ga₂O₃ flake, and was determined to be due to diffusion of oxygen into the contact. The ITO layer is only 10 nm, so likely a combined effect from the two layers causes the change in surface morphology. Using a diffusion barrier may prevent diffusion of gallium and oxygen atoms into the metal electrodes and prevent formation of TiO₂. Bae et al.⁴⁹ show transmission electron microscopy of pre- and postannealed contacts and depth dependent energy dispersive spectroscopy spectra, which show out-diffusion of gallium and oxygen atoms into the metallization. Optical microscopy images of their contacts as well note a bubbling, and degradation of the surface. Bae et al.⁴⁹ observed the characteristics of contacts annealed in N₂ and air and reported similar degradation of contact morphology after 700 °C annealing. Those contacts annealed in air do not demonstrate the same bubbling, as there is not a deficiency of oxygen atoms in the ambient during annealing.

Figure 6 shows the effect of measurement temperature on the Au/Ti/ITO/Ga₂O₃ stack annealed at 500 °C in N₂ for 30 s. The specific contact resistance decreased minimally with temperature, which is ideal for providing a consistent contact resistance in a device regardless of operating temperature. The thickness of 10 nm ITO was chosen based on the desire to reduce the conduction band difference between metal and Ga₂O₃. Thicker ITO would increase the vertical resistance, and thinner ITO may have pinholes to cover the Ga₂O₃. In our previous work, we identified rf-sputtered ITO/single crystal β-Ga₂O₃ heterostructures as having a Type I band alignment via x-ray photoelectron spectroscopy using the Kraut method.^51,52 The individual component bandgaps were determined by reflection electron energy loss spectroscopy as 4.6 eV for Ga₂O₃ and 3.5 eV for ITO. The valence and conduction band offset was determined to be −0.78 ± 0.30 eV and −0.32 ± 0.13 eV, respectively. Shown in Fig. 7, the conduction band steps-down across the heterointerface favors electron transport and makes it easier to achieve an Ohmic contact. The Fermi level of the metal is hypothesized to have a larger offset on the conduction band of Ga₂O₃ than the ITO. Introducing an ITO interlayer allows for reducing the conduction band discontinuity between Ti and Ga₂O₃. Ohmic behavior was observed for Ti/Au deposited on ITO.^53,54 The typical Schottky barrier heights of a variety of metallizations on Ga₂O₃ range from 0.9 to 1.45 eV.^{21,22,30,55–58} Use of a thin layer of ITO between a metal contact and the Ga₂O₃ is a promising approach for reducing contact resistance on Ga₂O₃-based power electronic devices and solar-blind photodetectors. Recent results on aluminum zinc oxide interlayers show similar results.⁵⁹ Yao et al.⁶⁰ concluded that metal work function was not a dominant factor in forming Ohmic contacts to Ga₂O₃, but that interfacial reactions were important.

The use of sputtered ITO interlayers in Ti/Au contacts to Ga₂O₃ improves electron transport across the heterointerface and enhances formation of an Ohmic contact. The band alignment of ITO on Ga₂O₃ is favorable for electron injection. For Au/Ti/ITO/Ga₂O₃ contact stacks, the minimum transfer resistance and specific contact resistance of 0.6 Ω mm and 6.3 × 10⁻⁵ Ω cm² were achieved 600 °C annealing, respectively. Without the ITO, similar anneals did not lead to linear current-voltage characteristics of Au/Ti/Ga₂O₃ contact layers. In Ref. 27, high annealing temperatures of 900–1100 °C were used for forming Ohmic contacts—by using the ITO interlayer moderate annealing temperatures can be used. The use of the ITO interlayers is a convenient method for improving Ohmic contact resistance on n-type Ga₂O₃.

The project or effort depicted was also sponsored by the Department of the Defense, Defense Threat Reduction Agency, HDTRA1-17-1-011, monitored by Jacob Calkins. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. Part of the work at Tamura was supported by “The research and development project for innovation technique of energy conservation” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors also thank Kohei Sasaki from Tamura Corporation for fruitful discussions.