Development of Catalytic-CVD SiNx Passivation Process for AlGaN/GaN-on-Si HEMTs

We optimized a silicon nitride (SiNx) passivation process using a catalytic-chemical vapor deposition (Cat-CVD) system to suppress the current collapse phenomenon of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs). The optimized Cat-CVD SiNx film exhibited a high film density of 2.7 g/cm3 with a low wet etch rate (buffered oxide etchant (BOE) 10:1) of 2 nm/min and a breakdown field of 8.2 MV/cm. The AlGaN/GaN-on-Si HEMT fabricated by the optimized Cat-CVD SiNx passivation process, which had a gate length of 1.5 μm and a source-to-drain distance of 6 μm, exhibited the maximum drain current density of 670 mA/mm and the maximum transconductance of 162 mS/mm with negligible hysteresis. We found that the optimized SiNx film had positive charges, which were responsible for suppressing the current collapse phenomenon.


Introduction
AlGaN/GaN high electron mobility transistors (HEMTs) are promising candidates for microwave power amplification and power switching applications owing to their excellent characteristics, such as wide energy bandgap, as well as high breakdown field, mobility, and saturation velocity [1,2]. Although significant progress has been achieved in the past decades, surface trapping and its related current collapse phenomena remain critical for device performance and reliability. The current collapse phenomenon that degrades the dynamic characteristics of the device contributes significantly to performance instability and reliability of the device. The current collapse phenomenon can be mitigated by surface passivation along with a field plate where the dielectric passivation process is carefully optimized [3]. Among the various dielectric materials, including SiO 2 , silicon nitride (SiN x ), SiON, Al 2 O 3 , and AlN [4][5][6][7], SiN x films have been used frequently for the passivation layer of GaN FETs and have been reported to be effective for suppressing the current collapse phenomenon [8,9]. Various deposition systems have been utilized for SiN x film deposition, including plasma-enhanced chemical vapor deposition (PECVD), inductively coupled plasma CVD (ICP-CVD), and low-pressure CVD (LPCVD). These systems employ high-power plasma and/or a high deposition temperature causing plasma damage at the surface and thermal budget problems. Therefore, a catalytic CVD (Cat-CVD) process was proposed to solve these problems. The molecules of the source gases are decomposed in a vacuum chamber by catalytic cracking reaction using heated catalyzers without plasma reaction and then the cracked species are transported to the substrates to form a film [10][11][12][13].
In this study, we determined high-quality SiN x deposition process conditions using a Cat-CVD system. It was found that the optimized SiN x passivation process was very effective in reducing   To optimize the SiN x deposition process, the films were deposited on Si wafers using a Cat-CVD system with SiH 4 and NH 3 gas mixtures, during which the gas pressure, flow rate, and catalyzer temperature were varied as the process parameters. The deposition rate, refractive index, and buffered oxide etchant (BOE) etch rate of each film were measured to evaluate the film quality. Detailed process conditions and measurement results are summarized in Table 1. The deposition rate and refractive index of SiN x films obtained under different process conditions are plotted in Figure 2a,b, respectively. The deposition rate for the gas pressure of 2 Pa is <0.4 Å/s, which takes a significant period to deposit conventional passivation layers whose thicknesses are typically at least tens of nanometers. The sample with a refractive index of 1.9 was considered as a reference sample. The wet etch rate is a simple indicator to evaluate the dielectric film quality. It was reported that the wet etch rate had a strong function of the hydrogen content that is known to degrade the film quality and stability characteristics [14][15][16]. The wet etching tests were carried out at room temperature using a 10:1 buffered oxide etchant (BOE), and the results are plotted in Figure 3. The film with a refractive index of 2.12 exhibited the lowest wet etch rate of 2 nm/min, whereas the reference film exhibited a wet etch rate of 7.1 nm/min. The deposition rate and refractive index of SiNx films obtained under different process conditions are plotted in Figure 2a,b, respectively. The deposition rate for the gas pressure of 2 Pa is <0.4 Å/s, which takes a significant period to deposit conventional passivation layers whose thicknesses are typically at least tens of nanometers. The sample with a refractive index of 1.9 was considered as a reference sample. The wet etch rate is a simple indicator to evaluate the dielectric film quality. It was reported that the wet etch rate had a strong function of the hydrogen content that is known to degrade the film quality and stability characteristics [14][15][16]. The wet etching tests were carried out at room temperature using a 10:1 buffered oxide etchant (BOE), and the results are plotted in Figure 3. The film with a refractive index of 2.12 exhibited the lowest wet etch rate of 2 nm/min, whereas the reference film exhibited a wet etch rate of 7.1 nm/min.  For a detailed analysis of the films, four different films were selected, which had a refractive index of 1.90, 2.02, 2.12, and 2.43. The surface roughness and breakdown field of each sample were measured. Moreover, X-ray reflection (XRR) and Fourier-transform infrared (FT-IR) measurements were carried out to evaluate the density and hydrogen content of the films, respectively. Table 2 presents the surface roughness, breakdown field, film density, and hydrogen content of the selected films. The deposition rate and refractive index of SiNx films obtained under different process conditions are plotted in Figure 2a,b, respectively. The deposition rate for the gas pressure of 2 Pa is <0.4 Å/s, which takes a significant period to deposit conventional passivation layers whose thicknesses are typically at least tens of nanometers. The sample with a refractive index of 1.9 was considered as a reference sample. The wet etch rate is a simple indicator to evaluate the dielectric film quality. It was reported that the wet etch rate had a strong function of the hydrogen content that is known to degrade the film quality and stability characteristics [14][15][16]. The wet etching tests were carried out at room temperature using a 10:1 buffered oxide etchant (BOE), and the results are plotted in Figure 3. The film with a refractive index of 2.12 exhibited the lowest wet etch rate of 2 nm/min, whereas the reference film exhibited a wet etch rate of 7.1 nm/min.  For a detailed analysis of the films, four different films were selected, which had a refractive index of 1.90, 2.02, 2.12, and 2.43. The surface roughness and breakdown field of each sample were measured. Moreover, X-ray reflection (XRR) and Fourier-transform infrared (FT-IR) measurements were carried out to evaluate the density and hydrogen content of the films, respectively. Table 2 presents the surface roughness, breakdown field, film density, and hydrogen content of the selected films. For a detailed analysis of the films, four different films were selected, which had a refractive index of 1.90, 2.02, 2.12, and 2.43. The surface roughness and breakdown field of each sample were measured. Moreover, X-ray reflection (XRR) and Fourier-transform infrared (FT-IR) measurements were carried out to evaluate the density and hydrogen content of the films, respectively. Table 2 presents the surface roughness, breakdown field, film density, and hydrogen content of the selected films. The surface roughness of the film with a refractive index of 2.12 was 0.28 nm, which is significantly lower than that of the reference film with a refractive index of 1.90. The film density of the sample with a refractive index of 2.12 was 2.7 g/cm 3 , which was the highest among the samples investigated. The high film density suggested that the film contained relatively less impurities. Indeed, the film density had the same tendency as the BOE wet etch rate. Figure 4 shows the FT-IR absorption spectra of the selected films. The IR absorption spectra exhibited peaks at 830 cm −1 , 2100 cm −1 , and 3350 cm −1 , corresponding to Si-N, Si-H, and N-H bonding, respectively [15]. The relative hydrogen concentration in atomic % was determined from the bond concentrations calculated from the FT-IR spectra. The hydrogen content of the film with a refractive index of 2.12 was 6%, which was significantly lower than that of the reference sample.  The surface roughness of the film with a refractive index of 2.12 was 0.28 nm, which is significantly lower than that of the reference film with a refractive index of 1.90. The film density of the sample with a refractive index of 2.12 was 2.7 g/cm 3 , which was the highest among the samples investigated. The high film density suggested that the film contained relatively less impurities. Indeed, the film density had the same tendency as the BOE wet etch rate. Figure 4 shows the FT-IR absorption spectra of the selected films. The IR absorption spectra exhibited peaks at 830 cm −1 , 2100 cm −1 , and 3350 cm −1 , corresponding to Si-N, Si-H, and N-H bonding, respectively [15]. The relative hydrogen concentration in atomic % was determined from the bond concentrations calculated from the FT-IR spectra. The hydrogen content of the film with a refractive index of 2.12 was 6%, which was significantly lower than that of the reference sample. In order to investigate the chemical composition of the film, X-ray photoelectron spectroscopy (XPS) analysis was carried on the reference film with a refractive index of 1.90 and a film with a refractive index of 2.12. Spectra of the core levels of Si, N, O, and C were collected and analyzed. Figure 5 shows the chemical compositions of the two films. Si 2p and N 1s are the main species in the In order to investigate the chemical composition of the film, X-ray photoelectron spectroscopy (XPS) analysis was carried on the reference film with a refractive index of 1.90 and a film with a refractive index of 2.12. Spectra of the core levels of Si, N, O, and C were collected and analyzed.
Crystals 2020, 10, 842 5 of 13 Figure 5 shows the chemical compositions of the two films. Si 2p and N 1s are the main species in the SiN x films. From Figure 5f, the oxygen impurity concentration for the film with a refractive index of 2.12 was significantly lower than that for the reference film. The oxygen impurities were reduced from 11.5% to 4.7%. Furthermore, the carbon impurity concentration decreased from 1.5% to 0.9%. As indicated in Figure 5e, the N 1s peak for the film with a refractive index of 2.12 is ascribed to the Si-N, N-O, and Si-O-N bonds whose respective binding energies are 397.7 eV, 398.1 eV, and 397.6 eV. The atomic concentrations of Si-N, N-O, and Si-O bonds in the film with a refractive index of 2.12 were 88.63%, 5.8%, and 5.58% [17], whereas those for the reference film were 70.9%, 14.13%, and 14.97%, respectively. The O 1s and Si 2p peaks were in agreement with the peaks reported in References [17][18][19]. The total amounts of Si and N peak intensity of the optimized film were 46.74% and 47.51%, respectively, while those of the reference film were 41.99% and 45%, respectively. SiNx films. From Figure 5f, the oxygen impurity concentration for the film with a refractive index of 2.12 was significantly lower than that for the reference film. The oxygen impurities were reduced from 11.5% to 4.7%. Furthermore, the carbon impurity concentration decreased from 1.5% to 0.9%. As indicated in Figure 5e, the N 1s peak for the film with a refractive index of 2.12 is ascribed to the Si-N, N-O, and Si-O-N bonds whose respective binding energies are 397.7 eV, 398.1 eV, and 397.6 eV.
The atomic concentrations of Si-N, N-O, and Si-O bonds in the film with a refractive index of 2.12 were 88.63%, 5.8%, and 5.58% [17], whereas those for the reference film were 70.9%, 14.13%, and 14.97%, respectively. The O 1s and Si 2p peaks were in agreement with the peaks reported in References [17][18][19]. The total amounts of Si and N peak intensity of the optimized film were 46.74% and 47.51%, respectively, while those of the reference film were 41.99% and 45%, respectively. Based on film analysis, the deposition conditions for the film with a refractive index of 2.12 were determined to be optimum; a NH3/SiH4 flow ratio of 29, a gas pressure of 4 Pa, and a chuck temperature of 250 °C with a catalyzer temperature of 1650 °C. The deposition rate of the optimized SiNx passivation process was ~0.6 Å/s with a thickness variation of 1.16% across a 4-inch wafer. The optimized film exhibited a film density of 2.7 g/cm 3 , a BOE etch rate of ~2 nm/min, and a breakdown field of 8.2 MV/cm. As shown in Figure 6, a Rutherford Backscattering Spectrometry (RBS) analysis was carried out to investigate the stoichiometry of the film with a refractive index of 2.12 where a 115 nm thick film was used to obtain reliable analysis data. The stoichiometry of SiNx film defined by x = [N]/[Si] was 1.2, which was lower than the standard stoichiometric Si3N4 film of 1.33 [20,21]. The smaller x value indicates a Si-rich thin film compared to the stoichiometric Si3N4 film, which is associated with positive charges in the film. Based on film analysis, the deposition conditions for the film with a refractive index of 2.12 were determined to be optimum; a NH 3 /SiH 4 flow ratio of 29, a gas pressure of 4 Pa, and a chuck temperature of 250 • C with a catalyzer temperature of 1650 • C. The deposition rate of the optimized SiN x passivation process was~0.6 Å/s with a thickness variation of 1.16% across a 4-inch wafer. The optimized film exhibited a film density of 2.7 g/cm 3 , a BOE etch rate of~2 nm/min, and a breakdown field of 8.2 MV/cm. As shown in Figure 6, a Rutherford Backscattering Spectrometry (RBS) analysis was carried out to investigate the stoichiometry of the film with a refractive index of 2.12 where a 115 nm thick film was used to obtain reliable analysis data. The stoichiometry of SiN x film defined by x = [N]/[Si] was 1.2, which was lower than the standard stoichiometric Si 3 N 4 film of 1.33 [20,21]. The smaller x value indicates a Si-rich thin film compared to the stoichiometric Si 3 N 4 film, which is associated with positive charges in the film.

Device Fabrication
The epitaxial layers consisted of a 2 nm GaN capping layer, 20 nm AlGaN barrier layer, 300 nm undoped GaN layer, and 1.6 μm GaN buffer layer grown on a Si (111) substrate. After solvent cleaning, 40 nm thick SiNx film was deposited using a Cat-CVD system to protect the GaN surface during high-temperature ohmic annealing. Ohmic contacts with a source-to-drain distance of 6 μm were formed by low-damage recess etching using a BCl3/Cl2-based plasma process prior to Si/Ti/Al/Mo/Au (=5/20/80/35/50 nm) metallization [22]. The ohmic contacts were annealed by a rapid thermal annealing process at 800°C for 1 min in N2 ambient. Mesa isolation was then performed by a BCl3/Cl2 plasma etching process with a relatively higher plasma power. The SiNx protection layer was removed by an SF6-based low-damage plasma etching process in order to eliminate any surface damage during ohmic annealing. A new SiNx film with a thickness of 30 nm was deposited again as a passivation layer using the same Cat-CVD system. SiNx films with three different refractive index values (1.9, 2.12, and 2.43) were deposited for comparison. The deposition conditions for each refractive index value were identical to those shown in Table 1. Post-deposition annealing was conducted at 500°C for 10 min in N2 ambient. Finally, a gate foot length of 1.5 μm was defined by photolithography, under which the SiNx layer was etched by an SF6 plasma etching process. A Ni/Mo/Au (=40/15/400 nm) metal stack was used for the Schottky gate contact with a gate overhang length of 0.5 μm toward the source and drain contacts. Metal-insulator-semiconductor (MIS) gate devices with the same gate process on the SiNx passivation layer were fabricated without etching for analyzing the interface quality. The cross-sectional schematics of the Schottky and MIS gate HEMTs are presented in Figure 7.

Device Fabrication
The epitaxial layers consisted of a 2 nm GaN capping layer, 20 nm AlGaN barrier layer, 300 nm undoped GaN layer, and 1.6 µm GaN buffer layer grown on a Si (111) substrate. After solvent cleaning, 40 nm thick SiN x film was deposited using a Cat-CVD system to protect the GaN surface during high-temperature ohmic annealing. Ohmic contacts with a source-to-drain distance of 6 µm were formed by low-damage recess etching using a BCl 3 /Cl 2 -based plasma process prior to Si/Ti/Al/Mo/Au (=5/20/80/35/50 nm) metallization [22]. The ohmic contacts were annealed by a rapid thermal annealing process at 800 • C for 1 min in N 2 ambient. Mesa isolation was then performed by a BCl 3 /Cl 2 plasma etching process with a relatively higher plasma power. The SiN x protection layer was removed by an SF 6 -based low-damage plasma etching process in order to eliminate any surface damage during ohmic annealing. A new SiN x film with a thickness of 30 nm was deposited again as a passivation layer using the same Cat-CVD system. SiN x films with three different refractive index values (1.9, 2.12, and 2.43) were deposited for comparison. The deposition conditions for each refractive index value were identical to those shown in Table 1. Post-deposition annealing was conducted at 500 • C for 10 min in N 2 ambient. Finally, a gate foot length of 1.5 µm was defined by photolithography, under which the SiN x layer was etched by an SF 6 plasma etching process. A Ni/Mo/Au (=40/15/400 nm) metal stack was used for the Schottky gate contact with a gate overhang length of 0.5 µm toward the source and drain contacts. Metal-insulator-semiconductor (MIS) gate devices with the same gate process on the SiN x passivation layer were fabricated without etching for analyzing the interface quality. The cross-sectional schematics of the Schottky and MIS gate HEMTs are presented in Figure 7.

Device Fabrication
The epitaxial layers consisted of a 2 nm GaN capping layer, 20 nm AlGaN barrier layer, 300 nm undoped GaN layer, and 1.6 μm GaN buffer layer grown on a Si (111) substrate. After solvent cleaning, 40 nm thick SiNx film was deposited using a Cat-CVD system to protect the GaN surface during high-temperature ohmic annealing. Ohmic contacts with a source-to-drain distance of 6 μm were formed by low-damage recess etching using a BCl3/Cl2-based plasma process prior to Si/Ti/Al/Mo/Au (=5/20/80/35/50 nm) metallization [22]. The ohmic contacts were annealed by a rapid thermal annealing process at 800°C for 1 min in N2 ambient. Mesa isolation was then performed by a BCl3/Cl2 plasma etching process with a relatively higher plasma power. The SiNx protection layer was removed by an SF6-based low-damage plasma etching process in order to eliminate any surface damage during ohmic annealing. A new SiNx film with a thickness of 30 nm was deposited again as a passivation layer using the same Cat-CVD system. SiNx films with three different refractive index values (1.9, 2.12, and 2.43) were deposited for comparison. The deposition conditions for each refractive index value were identical to those shown in Table 1. Post-deposition annealing was conducted at 500°C for 10 min in N2 ambient. Finally, a gate foot length of 1.5 μm was defined by photolithography, under which the SiNx layer was etched by an SF6 plasma etching process. A Ni/Mo/Au (=40/15/400 nm) metal stack was used for the Schottky gate contact with a gate overhang length of 0.5 μm toward the source and drain contacts. Metal-insulator-semiconductor (MIS) gate devices with the same gate process on the SiNx passivation layer were fabricated without etching for analyzing the interface quality. The cross-sectional schematics of the Schottky and MIS gate HEMTs are presented in Figure 7.

Results
It was observed that the maximum drain current and transconductance were enhanced along with the off-state drain current reduction as the refractive index of the passivation layer increased. As revealed in Figure 8, the maximum drain current density and transconductance of the device for the optimized passivation process condition with a refractive index of 2.12 are 670 mA/mm and 162 mS/mm, respectively. Crystals 2020, 10, x FOR PEER REVIEW 7 of 14

Results
It was observed that the maximum drain current and transconductance were enhanced along with the off-state drain current reduction as the refractive index of the passivation layer increased. As revealed in Figure 8, the maximum drain current density and transconductance of the device for the optimized passivation process condition with a refractive index of 2.12 are 670 mA/mm and 162 mS/mm, respectively. The transfer and sub-threshold slope characteristics of MIS AlGaN/GaN-on-Si HEMTs with 30 nm SiNx layers with different refractive indices are compared in Figure 9. It should be noted that the device with a refractive index of 2.12 exhibited significant negative shift in threshold voltage with the lowest leakage current density, which is 1.3 × 10 −9 A/mm, along with a small sub-threshold slope of 105 mV/dec. The negative shift in threshold voltage can be explained by positive charges in the SiNx film in conjunction with the low H content. As compared in Table 2, the film with a refractive index of 2.12 had the lowest H content and the films with refractive indices of 1.90 and 2.43 had relatively higher H contents. The interface fixed charges and bulk charges of the film with a refractive index of 2.12 are discussed later with capacitance-voltage (C-V) measurements.  The transfer and sub-threshold slope characteristics of MIS AlGaN/GaN-on-Si HEMTs with 30 nm SiN x layers with different refractive indices are compared in Figure 9. It should be noted that the device with a refractive index of 2.12 exhibited significant negative shift in threshold voltage with the lowest leakage current density, which is 1.3 × 10 −9 A/mm, along with a small sub-threshold slope of 105 mV/dec. The negative shift in threshold voltage can be explained by positive charges in the SiN x film in conjunction with the low H content. As compared in Table 2, the film with a refractive index of 2.12 had the lowest H content and the films with refractive indices of 1.90 and 2.43 had relatively higher H contents. The interface fixed charges and bulk charges of the film with a refractive index of 2.12 are discussed later with capacitance-voltage (C-V) measurements.

Results
It was observed that the maximum drain current and transconductance were enhanced along with the off-state drain current reduction as the refractive index of the passivation layer increased. As revealed in Figure 8, the maximum drain current density and transconductance of the device for the optimized passivation process condition with a refractive index of 2.12 are 670 mA/mm and 162 mS/mm, respectively. The transfer and sub-threshold slope characteristics of MIS AlGaN/GaN-on-Si HEMTs with 30 nm SiNx layers with different refractive indices are compared in Figure 9. It should be noted that the device with a refractive index of 2.12 exhibited significant negative shift in threshold voltage with the lowest leakage current density, which is 1.3 × 10 −9 A/mm, along with a small sub-threshold slope of 105 mV/dec. The negative shift in threshold voltage can be explained by positive charges in the SiNx film in conjunction with the low H content. As compared in Table 2, the film with a refractive index of 2.12 had the lowest H content and the films with refractive indices of 1.90 and 2.43 had relatively higher H contents. The interface fixed charges and bulk charges of the film with a refractive index of 2.12 are discussed later with capacitance-voltage (C-V) measurements.  In order to investigate the current collapse phenomenon, the devices were measured under pulse mode operation with a pulse width of 200 ns and a pulse period of 1 ms where the quiescent bias conditions were varied by V GSQ = 0/−6 V and V DSQ = 0/10/20/30/40 V. We observed that the pulsed current-voltage characteristics were significantly improved by employing the optimized passivation process. The current collapse phenomenon was defined by the reduction of drain current density from a drain voltage of 5 V with V DSQ = 0 V to a drain voltage of 10 V with V DSQ = 40 V [8,9]. The current collapse Crystals 2020, 10, 842 8 of 13 reduction of the Schottky gate HEMT with the passivation film with a refractive index of 2.12 exhibited superior characteristics to others in Figure 10. The current collapse reduction was only 6.5%.
In order to investigate the current collapse phenomenon, the devices were measured under pulse mode operation with a pulse width of 200 ns and a pulse period of 1 ms where the quiescent bias conditions were varied by VGSQ = 0/−6 V and VDSQ = 0/10/20/30/40 V. We observed that the pulsed current-voltage characteristics were significantly improved by employing the optimized passivation process. The current collapse phenomenon was defined by the reduction of drain current density from a drain voltage of 5 V with VDSQ = 0 V to a drain voltage of 10 V with VDSQ = 40 V [8,9]. The current collapse reduction of the Schottky gate HEMT with the passivation film with a refractive index of 2.12 exhibited superior characteristics to others in Figure 10. The current collapse reduction was only 6.5%. Trapping effects at the surface states are responsible for the current collapse phenomenon of AlGaN/GaN HEMTs. In order to suppress the electron trapping effects, the passivation film must have positive charges to attract the trapped electrons [23][24][25][26][27]. The net charges of the SiNx film can be modified under the deposition conditions that alter the stoichiometry of SiNx. To investigate the fixed charges of the passivation films, the frequency-dependent C-V characteristics of MIS gate devices were measured from 1 kHz to 1 MHz, from which the flat-band voltage and hysteresis characteristics could be determined. The MIS gate device had a circular MIS gate with a diameter of 100 μm, which was surrounded by an ohmic electrode. In Figure 10a,c, the threshold voltage is negatively shifted with increasing refractive index [24,28,29], which indicates that a high-refractive-index film has more positive fixed charges in the MIS gate than the films with lower refractive indices. In comparison with the reference film with a refractive index of 1.90, the flat-band voltage of the device with the film having a refractive index of 2.12 was negatively shifted from −7.7 V to −13 V with smaller flat band voltage hysteresis (Figure 11d). The flat-band voltage hysteresis was reduced from 440 mV for a refractive index of 1.90 to 280 mV for a refractive index of 2.12. Trapping effects at the surface states are responsible for the current collapse phenomenon of AlGaN/GaN HEMTs. In order to suppress the electron trapping effects, the passivation film must have positive charges to attract the trapped electrons [23][24][25][26][27]. The net charges of the SiN x film can be modified under the deposition conditions that alter the stoichiometry of SiN x . To investigate the fixed charges of the passivation films, the frequency-dependent C-V characteristics of MIS gate devices were measured from 1 kHz to 1 MHz, from which the flat-band voltage and hysteresis characteristics could be determined. The MIS gate device had a circular MIS gate with a diameter of 100 µm, which was surrounded by an ohmic electrode. In Figure 10a,c, the threshold voltage is negatively shifted with increasing refractive index [24,28,29], which indicates that a high-refractive-index film has more positive fixed charges in the MIS gate than the films with lower refractive indices. In comparison with the reference film with a refractive index of 1.90, the flat-band voltage of the device with the film having a refractive index of 2.12 was negatively shifted from −7.7 V to −13 V with smaller flat band voltage hysteresis (Figure 11d). The flat-band voltage hysteresis was reduced from 440 mV for a refractive index of 1.90 to 280 mV for a refractive index of 2.12. In order to extract the fixed charge density of the optimized film, four samples were prepared with different SiNx thicknesses: 20, 30, 35, and 40 nm. It should be noted that the AlGaN barrier layer used for the experiments was 9 nm, which was relatively thinner than that used for device fabrication. The density of fixed charges at the SiNx on the surface was extracted by Vth-thickness dependency. A second derivative method [29] was used to obtain a flat-band voltage from the C-V characteristics as In order to extract the fixed charge density of the optimized film, four samples were prepared with different SiN x thicknesses: 20, 30, 35, and 40 nm. It should be noted that the AlGaN barrier layer used for the experiments was 9 nm, which was relatively thinner than that used for device fabrication. The density of fixed charges at the SiN x on the surface was extracted by V th -thickness dependency. A second derivative method [29] was used to obtain a flat-band voltage from the C-V characteristics as a function of SiN x thickness. The flat-band voltage was plotted against the normalized total cap thickness (t n = t n (SiN x ) + t n (AlGaN) = t sinx /ε sinx + t AlGaN /ε AlGaN ) [30].
where V FB is the flat-band voltage, W MS is the work function difference between the metal and semiconductor, Q int is the total charge, Q bulk is the bulk charge, and t ox is the dielectric thickness. The total charge densities as a function of the SiN x film thickness are indicated in Figure 14.        Assuming that the AlGaN polarization charge for an Al mole fraction of 0.2 and a barrier thickness of 9 nm was 1.5 × 10 12 cm −2 [32], the extracted interface fixed charge density and the bulk charge density were estimated to be 6.63 × 10 12 cm −2 and 3.3 × 10 18 cm −3 , respectively. It is suggested that the large amounts of positive interface and bulk charges of the SiN x passivation film were responsible for the significant negative shift of V th of the MIS HEMT as shown in Figure 9. The positive charges in the MIS gate acted as a virtual gate that required more negative voltage to deplete the 2DEG channel.
The interface trap density (D it ) for MIS gates was investigated using a conductance method [33]. The extracted D it characteristics are plotted in Figure 15. The lowest D it level was achieved by a refractive index of 2.12, which was 2 × 10 12 cm −2 eV −1 at a trap energy level of 0.43 eV. The low D it level can further explain the improved current collapse phenomenon observed in the Schottky device. The current collapse characteristics observed in this work are compared with those reported in previous studies [33][34][35][36][37][38] in Table 3. We suggest that the Cat-CVD SiN x passivation process developed in this work is very promising for suppressing the current collapse phenomenon of GaN devices.
The extracted Dit characteristics are plotted in Figure 15. The lowest Dit level was achieved by a refractive index of 2.12, which was 2 × 10 12 cm −2 eV −1 at a trap energy level of 0.43 eV. The low Dit level can further explain the improved current collapse phenomenon observed in the Schottky device. The current collapse characteristics observed in this work are compared with those reported in previous studies [33][34][35][36][37][38] in Table 3. We suggest that the Cat-CVD SiNx passivation process developed in this work is very promising for suppressing the current collapse phenomenon of GaN devices.

Conclusions
We have developed a high-quality SiNx film deposition process using a Cat-CVD system for the passivation layer of AlGaN/GaN HEMTs. A Cat-CVD SiNx dielectric layer has been optimized to reduce the trapping phenomenon originating from the surface states. Modification of the SiNx film stoichiometry can result in positive net charges that can effectively suppress the trapping phenomenon. The optimized passivation process conditions were a NH3/SiH4 gas flow ratio of 29, gas pressure of 4 Pa, and chuck temperature of 250 °C with a catalyzer temperature of 1650 °C, which resulted in a refractive index of 2.1. The device fabricated under the optimized passivation process conditions exhibited excellent electrical characteristics. The high film density and low interface trap  Table 3. Comparison of the current collapse characteristics.

Conclusions
We have developed a high-quality SiN x film deposition process using a Cat-CVD system for the passivation layer of AlGaN/GaN HEMTs. A Cat-CVD SiN x dielectric layer has been optimized to reduce the trapping phenomenon originating from the surface states. Modification of the SiN x film stoichiometry can result in positive net charges that can effectively suppress the trapping phenomenon. The optimized passivation process conditions were a NH 3 /SiH 4 gas flow ratio of 29, gas pressure of 4 Pa, and chuck temperature of 250 • C with a catalyzer temperature of 1650 • C, which resulted in a refractive index of 2.1. The device fabricated under the optimized passivation process conditions exhibited excellent electrical characteristics. The high film density and low interface trap density of the optimized passivation film were responsible for the improved current-voltage characteristics and caused the suppression of the current collapse phenomenon. Therefore, the Cat-CVD SiN x film is a promising passivation layer for AlGaN/GaN HEMTs.