Effects of post-deposition annealing ambient on band alignment of RF magnetron-sputtered Y2O3 film on gallium nitride

The effects of different post-deposition annealing ambients (oxygen, argon, forming gas (95% N2 + 5% H2), and nitrogen) on radio frequency magnetron-sputtered yttrium oxide (Y2O3) films on n-type gallium nitride (GaN) substrate were studied in this work. X-ray photoelectron spectroscopy was utilized to extract the bandgap of Y2O3 and interfacial layer as well as establishing the energy band alignment of Y2O3/interfacial layer/GaN structure. Three different structures of energy band alignment were obtained, and the change of band alignment influenced leakage current density-electrical breakdown field characteristics of the samples subjected to different post-deposition annealing ambients. Of these investigated samples, ability of the sample annealed in O2 ambient to withstand the highest electric breakdown field (approximately 6.6 MV/cm) at 10−6 A/cm2 was related to the largest conduction band offset of interfacial layer/GaN (3.77 eV) and barrier height (3.72 eV).


Background
Increasing concerns regarding the escalating demand of energy consumption throughout the world has triggered the needs of developing energy-efficient highpower and high-temperature metal-oxide-semiconductor (MOS)-based devices. It has been projected that gallium nitride (GaN) has the potential of conforming to the needs of these MOS-based devices due to its promising properties, which include wide bandgap (3.4 eV), large critical electric field (3 MV/cm), high electron mobility, as well as good thermal conductivity and stability [1][2][3][4][5][6]. The fabrication of a functional GaN-based MOS device requires a high-quality gate oxide that is capable of resisting a high transverse electric field [7,8]. Native oxide (Ga 2 O 3 ) of GaN [9][10][11][12][13] and a relatively lowdielectric-constant (k) SiN x O y [2] or SiO 2 [14][15][16][17][18][19] have been successfully grown and deposited, respectively, as gate oxides in GaN-based MOS devices. However, these gate oxides are not the preferred choices. The shortcoming encountered by the former gate is the slow growth gate, high oxidation temperature (>700°C), and high leakage current [12,13] while the latter gate with a relatively low k is unable to withstand the high electric field imposed on GaN [7,20,21]. Thereafter, numerous high-k gate oxides [3,[20][21][22][23][24][25][26][27][28] have been selected for investigation on GaN-based MOS devices. Recent exploration on the employment of radio frequency (RF) magnetron-sputtered Y 2 O 3 gate subjected to post-deposition annealing (PDA) from 200°C to 1,000°C for 30 min in argon ambient has revealed that the Y 2 O 3 gate annealed at 400°C has yielded the best current density-breakdown field (J-E) characteristic as well as the lowest effective oxide charge, interface trap density, and total interface trap density [25]. It is noticed that the acquired J-E characteristic for this sample is better than majority of the investigated gate oxide materials [25]. The ability of Y 2 O 3 /GaN MOS structure to be driven at a high E and low J is attributed to the fascinating properties possessed by Y 2 O 3 , such as high k value (k = 12 to 18), large bandgap (approximately 5.5 eV), and large conduction band offset (approximately 1.97 eV) [25,[29][30][31]. Despite that, the presence of oxygen-related defects, changes in compositional homogeneity of Y 2 O 3 , and formation of interfacial layer (IL) are of particular concern as either of these factors might alter the bandgap of Y 2 O 3 and band alignment of Y 2 O 3 with respect to the GaN, which would influence the J-E characteristic of the MOS structure. Li et al. has reported previously that J-E characteristic of the MOS structure is dependent on the thickness of IL, wherein interface quality of the atomic layer deposited HfO 2 on Si can be altered via the IL thickness [32]. In order to reduce oxygen-related defects and restore compositional homogeneity of Y 2 O 3 , it is essential to perform post-deposition annealing on the oxide [33]. Besides, the oxygen content near the Y 2 O 3 /GaN interface can be regulated by varying the post-deposition annealing ambient and eventually controlling the formation of IL. Therefore, engineering of the bandgap of Y 2 O 3 gate and band alignment of Y 2 O 3 with GaN through different PDA ambients is of technological importance. In this work, effects of different PDA ambients (oxygen (O 2 ), argon (Ar) [25], nitrogen (N 2 ), and forming gas (FG; 95% N 2 + 5% H 2 )) at 400°C for 30 min on the Y 2 O 3 /GaN structure in modifying the bandgap of Y 2 O 3 gate and band alignment of Y 2 O 3 /GaN are presented. A correlation on the bandgap of Y 2 O 3 gate and band alignment of Y 2 O 3 /GaN with regard to the J-E characteristics is also discussed in this paper.

Methods
Prior to the deposition of 60-nm thick Y 2 O 3 films on the commercially purchased Si-doped (n-type) GaN epitaxial layers with thickness of 7 μm and doping concentration of 1 to 9 × 10 18 cm −3 grown on sapphire substrates, the wafer, which was diced into smaller pieces, were subjected to RCA cleaning. Subsequently, these samples were loaded into a vacuum chamber of RF magnetron sputtering system (Edwards A500, Edwards, Sanborn, NY, USA). A comprehensive description on the deposition process of Y 2 O 3 films has been reported elsewhere [29,30]. Then, PDA was performed in a horizontal tube furnace at 400°C in different ambients (O 2 , Ar, N 2 , and FG (95%N 2 + 5% H 2 )) for 30 min. The heating and cooling rate of approximately 10°C/min was used for the PDA process. After the PDA process, X-ray photoelectron spectroscopy (XPS) measurements were conducted on the samples at the Research Center for Surface and Materials Science, Auckland University, New Zealand, using Kratos Axis Ultra DLD (Shimadzu, Kyoto, Japan) equipped with a monochromatic Al-K α X-ray source (hv = 1486.69 eV). The spectra of the survey scan were obtained at a low pass energy of 160 eV with an energy resolution of 0.1 eV, and the photoelectron take-off angle was fixed at 0°with respect to the surface normal. Chemical depth profiling was performed by etching the samples using an Ar ion gun operated at 5 keV in order to identify the boundary of Y 2 O 3 and interfacial layer between the oxide and GaN. To further determine the bandgap of Y 2 O 3 and IL, a detailed scan of O 1s was first performed at the same pass energy of 20 eV with an energy resolution of 1.0 eV. The energy loss spectrum of O 1s would provide the bandgap of Y 2 O 3 and IL by taking into consideration the onset of a single particle excitation and band-to-band transition. Kraut's method was utilized in the extraction of the valence band offset of Y 2 O 3 and IL [34,35]. In order to fabricate MOS test structure, the Y 2 O 3 film was selectively etched using HF/H 2 O (1:1) solution. Next, a blanket of aluminum was evaporated on the Y 2 O 3 film using a thermal evaporator (AUTO 306, Edwards). Lastly, an array of Al gate electrode (area = 2.5 × 10 −3 cm 2 ) was defined using photolithography process. Figure 1 shows the fabricated Al/Y 2 O 3 /GaN-based MOS test structure. The currentvoltage characteristics of the samples were measured using a computer-controlled semiconductor parameter analyzer (Agilent 4156C, Agilent Technologies, Santa Clara, CA, USA).

Results and discussion
Bandgap (E g ) values for Y 2 O 3 and IL are extracted from the onset of the respective energy loss spectrum of O 1s core level peaks. The determination of E g values for Y 2 O 3 and IL is done using a linear extrapolation method, wherein the segment of maximum negative slope is extrapolated to the background level [36]. Typical valence band photoelectron spectra of Y 2 O 3 and IL for the sample annealed in O 2 ambient are presented in Figure 2b. By means of linear extrapolation method, the valence band edges (E v ) of Y 2 O 3 and IL could be determined by extrapolating the maximum negative slope to the minimum horizontal baseline [36]. The acquired valence band offset (ΔE v ) values of    and Ga-O-N bonding in the region of IL. Sample subjected to PDA in O 2 ambient attains the largest E g (Y 2 O 3 ) and E g (IL) as well as the highest values of ΔE c (Y 2 O 3 / GaN) and ΔE c (IL/GaN). This is related to the supply of O 2 from the gas ambient during PDA, which has contributed to the reduction of oxygen-related defects in the Y 2 O 3 film and the improvement in the compositional homogeneity of the oxide film. The absence of O 2 supply during PDA in Ar (inert) and reducing ambient, such as FG and N 2 , may be the reason contributing to the attainment of lower E g (Y 2 O 3 ), E g (IL), ΔE c (Y 2 O 3 /GaN), and ΔE c (IL/GaN) values than the sample annealed in O 2 . The presence of N 2 in both FG and N 2 ambient has caused the formation of O 2 -deficient Y 2 O 3 film, wherein N atoms dissociated from N 2 gas may couple with the oxygen-related defects in the Y 2 O 3 film [30,38]. In addition, the presence of N 2 in both FG and N 2 ambient is also capable of performing nitridation process to diminish the tendency of O 2 dissociated from the Y 2 O 3 film during PDA to diffuse inward and react with the GaN substrate [30]. Thus, the interfacial layer formed in between the Y 2 O 3 /GaN structure for these samples could be O 2 deficient. Despite the fact that FG and N 2 ambient are capable of providing nitridation and coupling process, the percentage of N 2 in FG ambient (95% N 2 ) is lower than that in pure N 2 . Hence, PDA in N 2 ambient will enhance the nitridation process and coupling of N atoms with the oxygen-related defects in Y 2 O 3 , which leads to the formation of more O 2 -deficient Y 2 O 3 film and IL when compared with the sample annealed in FG ambient. This could be the reason leading to the attainment of the lowest E g (Y 2 O 3 ), E g (IL), ΔE c (Y 2 O 3 /GaN), and ΔE c (IL/GaN) values for the sample annealed in N 2 ambient.
A schematic illustration of the energy band alignment of the Y 2 O 3 /IL/GaN structure that had been subjected to different PDA ambients is presented in Figure 5. Three different energy band alignment structures were obtained due to the effect of PDA ambient. It is noticed that the conduction band edge of IL is higher than that of Y 2 O 3 for the sample annealed in O 2 ambient, but it is lower in samples annealed in Ar, FG, and N 2 ambient. This band alignment shift would influence the leakage current density-electrical field (J-E) characteristics of the samples ( Figure 6). The dielectric breakdown field (E B ) is defined as the electric field that causes a leakage current density of 10 −6 A/cm 2 , which is not related to a permanent oxide breakdown but representing a safe value for device operation [39]. Of all the investigated samples, the sample annealed in O 2 ambient demonstrates the lowest J and the highest E B (approximately 6.6 MV/cm) at J of 10 −6 A/cm 2 . This might be attributed to the attainment of the largest E g (Y 2 O 3 ) and E g (IL) as well as In order to determine whether the E B of the investigated samples is either dominated by the breakdown of IL, Y 2 O 3 , or a combination of both Y 2 O 3 and IL, Fowler-Nordheim (FN) tunneling model is employed to the extract barrier height (Φ B ) of Y 2 O 3 on GaN. FN tunneling mechanism is defined as tunneling of the injected charged carrier into the conduction band of the Y 2 O 3 gate oxide via passing through a triangular energy barrier [7,8,30]. This mechanism can be expressed as J FN = AE 2 exp(−B/E), where A = q 3 m o /8 (hmΦ B , B = 4(2 m) 1/2 Φ B 3/2 /(3qh/2), q is the electronic charge, m is the effective electron mass in the Y 2 O 3 (m = 0.1m o , where m o is the free electron mass), and h is Planck's constant [8,40]. In order to fit the obtained experimental data with the FN tunneling model, linear curve fitting method has been normally utilized [8,20,41]. Nevertheless, data transformation is needed in this method owing to the limited models that can be presented in linear forms. Hence, nonlinear curve fitting method is employed using Datafit version 9.0.59 to fit the acquired J-E results in this work with the FN tunneling model. It is believed that the extracted results using nonlinear curve fitting method is more accurate due to the utilization of actual data and the minimization of data transformation steps required in the linear curve fitting [42,43]. Figure 7 shows the J-E results for the samples annealed in O 2 , Ar, and FG ambient, which fitted well with FN tunneling model. The extracted Φ B values of these samples are presented in the Figure 4. The highest Φ B value attained by the sample annealed in O 2 ambient (3.72 eV) was higher than that of metal-organic decomposed CeO 2 (1.13 eV) spin-coated on n-type GaN substrate [20]. No Φ B value has been extracted for the sample annealed in N 2 ambient due to the low E B and high J of this sample, wherein the gate oxide breaks down prior to the FN tunneling mechanism.    and IL, this sample should be able to sustain a higher E B and a lower J than the samples annealed in Ar and FG ambient. Therefore, the E B of the sample annealed in N 2 ambient is most likely dominated by the breakdown of bulk Y 2 O 3 .

Conclusions
In conclusion, three different energy band alignment models of Y 2 O 3 /interfacial layer/GaN structure subjected to post-deposition annealing at 400°C in different ambients (O 2 , Ar, forming gas (95% N 2 + 5% H 2 ), and N 2 ) have been established using X-ray photoelectron spectroscopy. It was proven that the dielectric breakdown field (E B ) of the sample annealed in O 2 ambient was dominated by the breakdown of IL, while the E B of the samples annealed in Ar, FG, and N 2 ambient was dominated by the breakdown of bulk Y 2 O 3 . The sample annealed in O 2 ambient demonstrated the best leakage current density-breakdown field due to the attainment of the largest bandgap, the largest conduction band offset, and the highest barrier height value.