Physical mechanism of oxygen diffusion in the formation of Ga2O3 Ohmic contacts

Su-Yu Xu(徐宿雨), Miao Yu(于淼),?, Dong-Yang Yuan(袁東陽), Bo Peng(彭博),Lei Yuan(元磊), Yu-Ming Zhang(張玉明), and Ren-Xu Jia(賈仁需),?

1Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices,School of Microelectronics,Xidian University,Xi’an 710071,China

2The 13th Research Institute China Electronics Technology Group Corporation,Shijiazhuang 050051,China

Keywords: Ga2O3,Ohmic contacts,oxygen diffusion,density functional theory

1.Introduction

Gallium oxide (Ga2O3) is an ultra-wide bandgap semiconductor material that has garnered significant attention as a candidate for next-generation power electronics and optoelectronic devices.[1–10]It is characterized by its excellent chemical and thermal stability,high visible light transmission rate,and Si-compatible preparation process that enables mass production.Nevertheless, the development of Ga2O3-based devices is still in its infancy, and numerous challenges, including the formation of good Ohmic contacts, persist.[11–16]The formation of Ohmic contacts between metal electrodes and semiconductors is a fundamental prerequisite for any semiconductor device to operate successfully.The magnitude of the contact resistance between the metal and semiconductor is a crucial factor that influences the performance of the device, particularly its power, noise frequency, and thermal stability.However,the precise mechanism governing the formation of Ohmic contacts between metals and Ga2O3remains incompletely understood.Prior research has demonstrated that the Au-covered Ti(Ti/Au)electrode, annealed at 400°C,exhibits the lowest Ohmic contact resistance with Ga2O3, and thus is the most commonly used approach to achieve Ohmic contact with gallium oxide.[17–21]Although the rapid annealing of Ti/Au electrodes is known to result in interdiffusion at the interface between the electrode and Ga2O3,the exact role played by the diffusion of various elements in the formation of Ohmic contacts in Ga2O3is not yet clear.[22]As oxygen diffusion is the most pronounced interfacial diffusion,studying the contribution of oxygen element diffusion to the formation of Ohmic contacts is critical for gallium oxide.

This study investigates the contribution of oxygen atom diffusion to the formation of Ohmic contacts in Ga2O3.Specifically, we prepared titanium/gold electrodes on a single crystal gallium oxide substrate and evaluated their currentvoltage characteristics, energy dispersive x-ray elemental mapping spectrum,and electron energy loss spectroscopy.We constructed interface models of gallium oxide and titanium and calculated their charge density, partial density of state,planar electrostatic potential energy,and current–voltage characteristics.Our results reveal that oxygen atoms at the Ga2O3interface diffuse into the titanium layer,where they oxidize the Ti and leave oxygen vacancies near the interface.The presence of these vacancies lowers the interfacial potential barrier, enhances the overlap degree of electron cloud at the interface,and ultimately facilitates the flow of electrons across the interfacial potential barrier, enabling Ohmic properties.These findings provide valuable insights into the underlying mechanisms governing the formation of Ohmic contacts and underscore the importance of considering oxygen atom diffusion in the design of Ga2O3-based electronic devices.

2.Methods

The Ti/Au electrodes were deposited on a single crystal commercial Ga2O3substrate with a (ˉ201) phase using direct current magnetron sputtering.The Ti electrode was sputtered at a power of 50 W for 1200 s, while the Au electrode was sputtered at a power of 20 W for 3600 s.In this work, the thickness of Ti is about 50 nm and the thickness of Au is about 110 nm.Subsequently, the samples underwent rapid thermal annealing(RTA)for 120 s under a nitrogen atmosphere at an annealing temperature of 470°C, in accordance with established protocols.[22]The current–voltage(I–V)characteristics were measured using a cascade probe table fitted with an Agilent B1500A.The structural and elemental features of the electrodes and the interface were characterized using Thermo Scientific Tecnai F20 transmission electron microscopy.In this study,all calculations were performed using density functional theory with the linear combination of atomic orbitals(LCAO)method, as implemented in the Quantum ATK package.The MGGA-TB09 exchange–correlation functional was employed to describe the exchange–correlation energy between electrons, while the PseudoDojo pseudopotential was used to model the electron–ion interaction potential.[23,24]Brillouin zone integration in reciprocal space was carried out using a grid density ofk-points of 12×2×1, and the cut-off energy was set to 550 eV.The convergence criteria for energy,force,stress, and displacement during the calculations were set at 5×10?6eV, 0.03 eV/?A, 0.02 GPa, and 5×10?4?A, respectively,to ensure high accuracy and reliability of the results.

3.Results and discussion

In this study, we fabricated Ti/Au electrodes on a single crystal Ga2O3substrate oriented along the(ˉ201)direction.The electrical behavior of the device before and after rapid thermal annealing (RTA) was evaluated usingI–Vmeasurements,as illustrated in Fig.1(a).TheI–Vcurve prior to RTA exhibited distinct Schottky characteristics,while theI–Vcurve following RTA displayed clear Ohmic behavior, indicating a significant change in the quality of the interface.To determine the specific contact resistance of the RTA-treated sample,we employed the transmission line model(TLM)method.[25]The electrode structure utilized for this measurement is depicted in Fig.1(b).By varying the electrode spacing, we obtained theI–Vcharacteristics as shown in Fig.1(c), from which we extracted the specific contact resistanceρcand plotted it against electrode spacing,as shown in Fig.1(d).The specific contact resistance of the RTA-treated sample was found to be 2.21×10?5?·cm2.These results reveal that the initial Ti/Ga2O3contact is Schottky in nature, and that the RTA treatment is critical in transforming it to an Ohmic contact.The change in the electrical behavior of the interface is likely attributed to the reduction of the barrier height at the Ti/Ga2O3junction caused by the thermal treatment.

Fig.1.(a) Comparison of I–V characteristics for Ti/Au electrodes with and without rapid annealing; (b) electrode structure for contact resistance measurement using TLM method; (c) effect of electrode spacing on I–V characteristics; (d) fitting curve for extracting contact resistance of Ti/Au electrodes.

Figure 2 displays the x-ray photoelectron spectroscopy(XPS)spectrum of the interface between Ti and Ga2O3in the unannealed sample.The binding energies of all peaks are calibrated to the C 1s peak located at 284.8 eV.The spectrum exhibits distinct peaks of Ga at approximately 1116 eV, O at approximately 530 eV, and Ti at approximately 460 eV.The XPS spectrum of Ti at the interface is presented in the inset,revealing a double-peak spacing of 5.7 eV between the Ti 2p1/2and Ti 2p3/2orbital peaks.This indicates that Ti in its elemental state is present at the interface of the unannealed sample,and no reactions have occurred at the interface.[26]

Fig.2.The XPS analysis of the interface between unannealed Ti and Ga2O3,with insert showing the Ti XPS spectrum at the interface.

Fig.3.The EDX elemental mapping spectrum of the Ti/Au–Ga2O3 interface after rapid thermal annealing.

The energy-dispersive x-ray (EDX) element mapping spectrum of the annealed sample is displayed in Fig.3.The spectrum demonstrates the distribution of O, Ti, Ga, and Au elements.The deep blue area indicates the distribution of O elements, the light blue area indicates the distribution of Ti elements, the purple area indicates the distribution of Ga elements,and the orange area indicates the distribution of Au elements.It is evident that a substantial amount of oxygen exists in the Ti metal layer at the interface.To further examine the valence state of Ti at the interface,we conducted electron energy loss spectroscopy(EELS)on the sample after rapid thermal annealing and thoroughly analyzed the high-energy loss region of the EELS.[27,28]

The EELS results are illustrated in Fig.4,where the three peak values marked as A,B,and C correspond to energy values of 458.3 eV, 460.7 eV, and 463.3 eV, respectively.These results demonstrate that the Ti at the interface is in the Ti3+state after rapid annealing.[29]

Fig.4.The electron energy loss spectroscopy of Ti at the Ti/Au interface after rapid annealing.

Table 1 presents the elemental proportion distribution of the Ga2O3layer at the interface.The O-to-Ga ratio is close to 1:1, much less than 2:3.Since the preparation and structural characterization of the sample were carried out under anoxic conditions,the oxygen in the Ti layer is attributed to the diffusion of oxygen atoms in the Ga2O3caused by rapid annealing.The findings of the related tests support this hypothesis.The current–voltage testing results,when combined with the previously discussed results, indicate that rapid annealing changes the Ti/Au electrode and Ga2O3from a Schottky contact to an Ohmic contact.The diffusion of oxygen atoms at the interface can be regarded as the most prominent change caused by rapid annealing.This indicates that the diffusion of oxygen at the interface of Ga2O3may have a significant influence on the formation of Ohmic contact.

Table 1.Element ratio distribution at the Ti/Au–Ga2O3 interface after rapid annealing.

To investigate the impact of interfacial oxygen diffusion on the formation of Ohmic contact between Ti and gallium oxide, we selected the (ˉ201) crystal phase of gallium oxide and the (100) crystal phase of Ti to construct various nanodevice models with different contact interfaces.Prior to building the interface models,we optimized the Ga2O3bulk material,and the results are illustrated in Fig.5(a).The bulk material parameters,namelya=12.43 ?A,b=3.08 ?A,c=5.87 ?A,α=γ=90°,andβ=103.68°,respectively,were determined.Furthermore, the energy band structure of Ga2O3is depicted in Fig.5(b),revealing a quasi-direct band gap of 4.87 eV.Both the lattice constant and the band gap width are consistent with previously reported experimental and calculated values.[30–32]

After optimizing the lattice structure of Ga2O3, we used 5 layers of Ga2O3in the(ˉ201)direction and 5 layers of Ti in the(100)direction as metal electrodes to build a device model.The parameter of the device model isa=3.08 ?A,b=14.96 ?A,c= 37 ?A,α=β=γ= 90°.Our study involved the generation of three interface models to describe the interaction between Ga2O3and Ti.These models included a direct contact model, a low concentration oxygen diffusion model, and a high concentration oxygen diffusion model.The direct contact model (M-direct) describes the absence of oxygen atom diffusion into the Ti layer upon contact with Ga2O3.In contrast, the low concentration oxygen diffusion model (M-low)represents the diffusion of oxygen atoms at a low concentration into the Ti layer, while the high concentration oxygen diffusion model (M-high) represents the diffusion of oxygen atoms at a high concentration into the Ti layer.The interface model between Ti and Ga2O3in direct contact is depicted in Fig.6(a).Due to differences in lattice constants,the interface experiences a slight degree of lattice distortion,while remaining well-matched.Figures 6(b)and 6(c)illustrate the oxygen diffusion models.Oxygen atoms from Ga2O3diffuse into the Ti layer,leaving oxygen vacancies at the interface.Figure 6(b)represents oxygen diffusion at a low concentration, with one oxygen atom diffusing into the Ti layer.Figure 6(c)represents oxygen diffusion at a high concentration, with two oxygen atoms diffusing into the Ti layer.In M-low,the Ti atoms at the oxygen vacancies move towards adjacent Ti atoms and share an oxygen atom with them,thus pulling the oxygen atoms on the Ga2O3side towards the Ti layer.In contrast, M-high is characterized by a more significant lattice distortion at the interface due to the Ti atoms moving larger distances.Ti atoms at the vacant sites take oxygen atoms from the deeper layers of Ga2O3, leading to a more pronounced tendency towards mutual diffusion at the interface.The high reactivity of Ti with oxygen results in Ti being more reductive than Ga.[33]Rapid annealing tends to move oxygen atoms towards the Ti layer,leading to the diffusion of oxygen atoms at the interface.

Fig.6.Optimization of Ga2O3/Ti interface model with varying oxygen diffusion concentrations.(M-direct: no oxygen diffusion,M-low: low oxygen diffusion concentration,M-high: high oxygen diffusion concentration).

Electron cloud density is a key electronic property that characterizes the spatial distribution of electrons in a material.It quantifies the probability of finding an electron at a given point in space and is a fundamental parameter for understanding the electronic structure of materials.[34]Figure 7 presents the electron cloud density distributions for three distinct systems, namely, M-direct, M-low, and M-high, which are shown in panels(a),(b),and(c),respectively.The yellow regions in the figure represent the areas with non-zero electron cloud density, while the equivalence surface of the electron cloud density is set to a constant value of 0.025e/Bohr3for all systems,thereby enabling a direct comparison of the electron cloud distributions across the different systems.Fluctuations in the electron cloud density at the interface have a significant impact on the barrier height.Our results reveal that M-low and M-high exhibit significantly higher electron density at the interface as compared to M-direct,indicating that there are more available electron orbitals near the interface that facilitate easier electron transport between the two materials.Specifically,the presence of oxygen atoms on the surface of Ga2O3entering the Ti layer contributes to an increase in the electron cloud density at the interface,resulting in the formation of new electron orbitals that enable electron transport between the interfaces.

Fig.7.Distribution of electron cloud density for Ga2O3/Ti interface models with varying oxygen diffusion concentrations.(a)M-direct: no oxygen diffusion,(b)M-low: low oxygen diffusion concentration,and(c)M-high:high oxygen diffusion concentration.

In order to conduct a thorough analysis of the impact of diffusing oxygen atoms at the interface on the electronic structure of the system during electron transport,we have performed calculations of the partial density of states(PDOS)at the interface.The PDOS results for M-direct, M-low, and M-high are presented in Figs.8(a), 8(b), and 8(c), respectively.Given that the conduction band electrons at the interface mainly stem from Ti-3d orbital electrons and the valence band electrons are primarily contributed by O-2p orbital electrons,we have focused our investigation on these two regions.The density of states for O-2p and Ti-3d electrons in the Mdirect, M-low, and M-high systems are depicted in Fig.8(d).For the M-direct system, the cut-off energy of O-2p orbital electrons is?5.30 eV, and that of Ti-3d orbital electrons is?3.23 eV, giving rise to a potential barrier of 2.07 eV.Analogously, the potential barrier between O-2p orbital electrons and Ti-3d orbital electrons for the M-low system is 1.80 eV,and that for the M-high system is 0.58 eV.The results demonstrate that the diffusion of oxygen atoms into the Ti layer at the interface leads to a reduction in the potential barrier for electron transport, and this reduction becomes more pronounced as the concentration of diffusing oxygen atoms increases.This phenomenon arises due to the creation of new electron-occupied states for O-2p orbital electrons in Ga2O3at higher energy levels,resulting from the oxygen vacancies that are formed by the oxygen atoms at the interface.Concurrently,the oxygen atoms that enter the Ti layer combine with Ti to generate new electron-occupied states for Ti-3d orbital electrons at lower energy levels, thereby decreasing the potential barrier, which promotes the formation of Ohmic properties.The diffusion of oxygen atoms at the interface is a promising strategy for effectively reducing the interfacial potential barrier of the Ti–Ga2O3interface.

Fig.8.Partial density of states (PDOS) at the Ga2O3/Ti interface with varying oxygen diffusion concentrations.(a) PDOS for M-direct, (b) PDOS for M-low,(c)PDOS for M-high,and(d)PDOS of O-2p orbital electrons and Ti-3d orbital electrons at the interface for M-direct,M-low,and M-high.

The planar electrostatic potential was calculated for different interface models, and the results are presented in Fig.9(a).The differences in electrostatic potential between Ti and Ga2O3in M-direct, M-low, and M-high are 5.35 eV,4.9 eV,and 4 eV respectively.Upon contact,a notable potential difference was observed between the Ti and Ga2O3interfaces, with the Ga2O3side exhibiting a lower potential.This phenomenon indicates that free electrons from the Ga2O3migrated towards the Ti, generating a built-in electric field that demonstrated Schottky properties.Comparatively, the potential difference between M-low and M-high interfaces was significantly lower than that of M-direct.Furthermore,the greater the concentration of diffused oxygen atoms, the more pronounced the potential difference reduction, which was consistent with density of states calculations.The planar electrostatic potential is directly linked to the charge transferred at the interface.To determine this, Bader charges of M-direct,M-low, and M-high at the interface were calculated, and the results are presented in Fig.9(b).The Bader charge values of M-low,and M-high are 10.62e,8.75e,and 7.80erespectively.The charge transferred at the interface was significantly lower for M-low and M-high compared to M-direct.This was due to the bonding of oxygen atoms with Ti atoms in the Ti layer, which resulted in a reduction in the number of charges transferred at the interface,thereby lowering the potential difference at the interface.

Fig.9.Planar electrostatic potential and Bader charge at the interface between Ga2O3 and Ti for M-direct,M-low,and M-high models.

We employed a combined approach based on density functional theory and non-equilibrium Green’s function to investigate the current–voltage(I–V)characteristics of M-direct,M-low, and M-high systems.[35,36]The results, presented in Fig.10, indicate a distinct difference in theI–Vbehavior between the three systems.Specifically, theI–Vcurve of Mdirect exhibited a Schottky-like behavior with an order of magnitude smaller current compared to that of M-low and M-high.This observation strongly suggests the presence of a Schottky contact at the interface between Ti and Ga2O3in direct contact.In contrast,M-low and M-high displayed distinct Ohmic characteristics, with theI–Vcurve’s slope for M-low being lower than that of M-high, indicating a lower contact resistance and superior Ohmic properties.Our theoretical analysis further revealed that the observed interfacial characteristics can be attributed to the diffusion of oxygen atoms from the Ga2O3surface into the Ti layer.This process creates new electron orbitals at the interface, lowering the interfacial barrier, and increasing the probability of electron tunneling at the interface, resulting in the Ohmic behavior observed in M-low and M-high.Our findings highlight the importance of oxygen diffusion in tuning the interfacial properties of the Ti/Ga2O3system and provide new insights into the design of high-performance electronic devices.

Fig.10.First principles calculation of the current–voltage characteristics for Ga2O3/Ti interface models with varying oxygen diffusion concentrations.

4.Conclusion

In this investigation,we examined the critical role of oxygen diffusion in the development of Ohmic contact between Ti and Ga2O3.Specifically, we fabricated Ti/Au electrodes on single crystal Ga2O3substrates, and our results revealed that direct contact between Ti and Ga2O3is initially Schottky in nature,transitioning to Ohmic behavior after rapid thermal annealing(RTA).We further observed via energy-dispersive xray spectroscopy(EDX)and electron energy loss spectroscopy(EELS)the diffusion of oxygen atoms from Ga2O3into the titanium layer after RTA, which caused the Ti to oxidize to a+3-valence state.To gain deeper insights into the underlying mechanisms driving the formation of Ohmic contacts,we employed density functional theory to construct several interface models.Our analysis of charge density, partial density of state, planar electrostatic potential energy, andI–Vcharacteristic calculations showed that oxygen diffusion decreases the interface potential barrier and enhances electron tunneling through the interface potential barrier.These outcomes explain the observed Ohmic behavior of the Ti/Ga2O3system.The experimental findings were in good agreement with the calculated results.This study provides valuable insights into the mechanisms behind the formation of Ohmic contacts between Ti and Ga2O3, which could have implications for the development of high-performance electronic devices.Further studies could explore the impact of variations in processing parameters on the formation and properties of Ohmic contacts.

Acknowledgment

Projects supported by the National Natural Science Foundation of China (Grant Nos.61874084, 61974119, and U21A20501).