Impact of Nitrogen Concentration and Post‐Deposition Annealing on Electrical Properties of AlON/Etched N‐GaN MOS Capacitors

Electrical and material properties of plasma‐enhanced atomic layer deposited (PE‐ALD) AlON on dry‐etched n‐type GaN substrates are investigated for nitrogen concentration ranging from 1.5% to 7.1%. Firstly, an increase in flat‐band voltage (VFB) and its hysteresis with increasing nitrogen concentration is reported. The increase in VFB is associated with the nitrogen content whereas the increase in hysteresis to the presence of impurities (hydroxyl groups and carbon‐related impurities). Alongside the nitrogen concentration, the impact of different post‐deposition annealing (PDA) temperatures is studied (400–800 °C). Stable AlON layers and interfaces with etched GaN substrates are reported with slight gallium oxide growth or gallium diffusion towards the dielectric layer. Finally, with increasing PDA temperature, an increase in VFB and a significant reduction of both VFB hysteresis and interface state density (Dit) are observed, notably at the measuring temperature of 150 °C. These results present a promising pathway toward more reliable and stable normally‐OFF GaN‐based MOS‐channel high electron mobility transistors (MOSc‐HEMTs).


Introduction
Lateral GaN-based power devices have gained interest thanks to GaN's physical properties (wide bandgap, high breakdown voltage, high electron saturation velocity).Moreover, with the AlGaN/GaN heterojunction, high electron mobility transistors (HEMTs) can be fabricated due to a 2D electron gas (2DEG) with high mobility forming at the heterojunction.HEMTs allow high switching frequencies and low ON resistance (R ON ) but have a negative threshold voltage (V TH ). [1]The latter introduces safety issues and complicates the driver system design.HEMTs with positive V TH , called normally-OFF, are hence highly desirable.
Different solutions to obtain a normally-OFF behavior exist such as the p-GaN gate, [2] the fluorinated gate, [3] and recently the double quantum well (DQW) gate structure. [4]Fully recessed Metal-Insulator-Semiconductor (MIS) gate GaN HEMTs, also called Metal-Oxide-Semiconductor-channel-HEMTs (MOSc-HEMTs), are also a promising solution to achieve normal OFF behavior.Compared to the p-GaN gate topology, this approach benefits from a reduced leakage current and a higher voltage swing. [5]Moreover, contrary to the partially recessed Metal-Oxide-Semiconductor (MOS) gate, the full recess of the AlGaN barrier reduces V TH variation. [6]ielectric deposition on GaN is mandatory to fabricate the MOS gate since growing thermal Ga 2 O 3 is difficult. [7]The commonly studied dielectrics are SiO 2 , [8] SiN x , [9][10][11] and Al 2 O 3 , [9,12,13] but the latter stands out thanks to its high permittivity (≈8 to 10) and high band gap (≈7 eV).However, the Al 2 O 3 /etched GaN gate stack suffers from V TH instability due to Al 2 O 3 native defects [14,15] and deteriorated interface from the necessary AlGaN barrier etching, consequently jeopardizing the implementation of MOSc-HEMTs.][18][19] After dielectric deposition, post-deposition anneal (PDA) improves the dielectric/etched GaN gate stack with reduced V TH hysteresis and increased V TH . [12,20]Nonetheless, the low thermal stability of Al 2 O 3 deposited on GaN limits the use of high-temperature PDA. [21,22]here has been a growing interest in aluminum oxynitride (AlO x N y ) for improved dielectric/GaN stack.[25][26][27][28][29][30][31][32] The thermal stability for AlON deposited on AlGaN is also reported to increase up to 800 °C. [24]Moreover, similarly to Al 2 O 3 , annealing was reported to increase V FB and reduce its hysteresis for annealing under forming gas and O 2 . [23,26]If some reports discuss the benefits of using AlON on MOSc-HEMT after a high temperature, [29,30] few studies present a systematic analysis of the electrical properties of the AlON/etched GaN stack for different nitrogen concentrations and after different PDA temperatures.
We have recently reported the indirect growth of plasmaenhanced atomic layer deposited (PE-ALD) AlON on etched GaN substrate with controlled nitrogen concentration, [33] with the O 2 plasma oxidation step expected to lower the presence of impurities. [34,35]This work reports the impact of different nitrogen concentrations and PDA temperatures for PE-ALD AlON/etched n-GaN gate stack through electrical measurements at 25 and 150 °C alongside physical-chemical measurements.

Impact of Nitrogen Concentration
AlON with different nitrogen concentrations ranging from 1.5 to 7.1 N% were studied with planar MOS capacitors (MOSCAPs) together with a reference Al 2 O 3 (H 2 O-based ALD) (Figure 1).In this section, the impact of the different nitrogen concentrations in AlON was investigated without PDA.Figure 2 shows the C-V and G m / −V characteristics measured at 25 and 150 °C for the reference Al 2 O 3 and the AlON samples with different nitrogen concentrations.For AlON, C-V characteristics are shifted positively with higher nitrogen concentration as previously reported, [23,25] but the shift does not go beyond Al 2 O 3 for both measuring temperatures.A double slope at AlON's C-V characteristics is also observed at 25 °C but disappears at 150 °C.This slope seems to be related to interface states since conductance peaks are observed close to the slope.By increasing the temperature, deeper interface states can respond to the C-V measurement while their emission time is reduced according to the Shockley-Read-Hall (SRH) model, thus leading to the disappearance of the slope.All studied samples present clockwise hysteresis.It increases at 150 °C compared to 25 °C due to electron trapping in the oxide and at the interface being enhanced by the elevated temperature.Among all samples, Al 2 O 3 has the highest hysteresis for 150 °C.
From the C-V characteristics, the V FB and ΔV FB are extracted and shown in Figure 3.As mentioned before for AlON, the increased V FB with higher nitrogen concentration is noticed for both measuring temperatures.Hysteresis seems to be slightly impacted by nitrogen content as ΔV FB increases for a nitrogen concentration higher than 1.5% at both 25 and 150 °C.However, no clear trend with nitrogen concentration can be observed.Nonetheless, at both measuring temperatures, the lower hysteresis of AlON samples compared to Al 2 O 3 seems to indicate that nitrogen incorporation in AlON indeed reduces electron trapping, especially for the higher measuring temperature of 150 °C.The lower electron trapping for AlON could be explained by the reduction of oxygen vacancies (V O ) [28] and/or the increase of conduction band off-set between AlON and GaN, [30] although AlON's band gap is reported to be slightly smaller than Al 2 O 3 . [24,27]n terms of interface quality, the interface state density (D it ) at both measuring temperatures is extracted from the conductance peaks using the methodology by Vandendaele et al. [36] From Equation ( 1), with a frequency of 1 kHz and at both measuring temperatures of 25 and 150 °C, the extracted defect energy levels from the conduction band are around 0.49 and 0.71 eV, respectively.
where k is the Boltzmann constant, T the temperature,  t the thermal velocity,  s the electron capture cross-section, N C the effective density of states in GaN's conduction band, and f the frequency. s was assigned to 1 × 10 −14 cm −2 ,  t to 2.6 × 10 7 cm.s−1 , and to 3.1 × 10 7 cm.s−1 for 25 and 150 °C, respectively.Although the GaN surface is etched, the results summarized in Table 1 show an overall better interface for the AlON/GaN interface, as previously observed, [29,30] specifically when nitrogen concentration is reduced, whereas the interface states density for 7.1 N% AlON is close to Al 2 O 3 .As such, the increase in nitrogen concentration induces a small degradation of the AlON/GaN interface.Therefore, when compared to Al 2 O 3 , the insertion of nitrogen at a concentration lower than 5% seems to improve the quality of both the dielectric and the interface with reduced hysteresis and interface states.However, within the AlON samples, the increase in nitrogen concentration leads to deterioration.This is contrary to the reported literature, where an increase in nitrogen incorporation leads to lower electron trapping. [27]To further understand the impact of nitrogen on the V FB hysteresis and D it , ToF-SIMS analyses were carried out on the AlN reference sample, 2.6, 4.9, and 7.1 N% AlON samples.As shown in Figure 4, ToF-SIMS analyses reveal with higher nitrogen content an increase in hydroxyl groups and carbon impurities.The deteriorated hysteresis and D it with increasing nitrogen concentration could be related to the  increase of hydroxyl groups and carbon impurities, as previously reported. [22,37,38]The increase of these impurities seems not to be directly related to the nitrogen concentration but mostly to the deposition technique, especially for carbon impurities.In order to obtain lower nitrogen concentration, thinner AlN layers have to be oxidized, meaning that fewer carbon impurities from AlN deposition remain in the AlON layer.As for hydroxyl groups, no particular explanation was found for their increase with respect to the nitrogen concentration.Thus, in comparison with Al 2 O 3 , the introduction of nitrogen for all concentrations is more advantageous specifically for high operating temperatures.However, within AlON samples, lower nitrogen concentrations have lower electron trapping due to the lower presence of impurities.

Impact of PDA
After post-deposition annealing, Figure 5 shows the C-V and G m / characteristics for different nitrogen content in AlON after a PDA at 800 °C.The Al 2 O 3 characteristics were not measured due to the large leakage current from crystallization after an 800 °C PDA.The presence of C-V characteristics for AlON confirms the higher thermal stability of AlON over Al 2 O 3 thanks to the introduction of nitrogen in aluminum oxide.Moreover, the positive shift of C-V curves with higher nitrogen concentration is also noticed after PDA, and at both measuring temperatures.Hysteresis reduces with the PDA temperature, as previously reported for a PDA under forming gas or O 2 . [23,26]Concerning the interface, conductance peaks are reduced for the different nitrogen concentrations and at both measuring temperatures, leading to the disappearance of the slope observed for the AlON samples without PDA.The oxide capacitance and permittivity at 800 °C are represented in Table 2.The permittivity is between 8.7 to 10.5, in the range of reported permittivity for Al 2 O 3 . [39,40]he extracted V FB and hysteresis at both measuring temperatures and for different PDA temperatures are shown in Figure 6.While nitrogen incorporation increases the V FB , PDA also increases   AlON. [26]In our case, PDA increases and stabilizes  7).At the defect energy level of 0.49 eV, little difference is noticed between the AlON samples.But for an 800 °C PDA, a lower interface state density is found for 1.5 N% AlON.The same observation is made at 0.72 eV where D it is lower for 1.5 and 2.6 N% AlON.Hence, with a lower nitrogen concentration and a PDA temperature higher than 600 °C, a better interface is obtained for the AlON/etched GaN stack.
In order to verify the nitrogen content and distribution in the AlON after PDA, HAXPES, and ToF-SIMS were performed using the 4.9 N% AlON sample for its intermediary nitrogen content.As shown in Table 3, a stable nitrogen concentration is found for the different PDA temperatures, highlighting stable nitrogen incorporation within AlON even after 800 °C PDA, similar to AlON deposited by Nozaki et al.After 800 °C. [27]The difference in nitrogen content from the expected value of 4.9N% can be associated with the uncertainty of the fitting process used to take away the N-Ga contribution from the GaN substrate.
GIXRD analyses of annealed AlON samples at 900 °C (see Figure S4, Supporting Information) also confirm the stability of the dielectric layer with no sign of crystallization for a PDA at 900 °C.
Regarding the interfacial gallium oxide, AR-HAXPES for 1.5N% AlON samples shown in Figure 8 reveals a slight increase  in oxidation at the AlON/GaN interface with increasing PDA temperature.However, the thickness remains under 1 nm.The observed overall stability of AlON with high-temperature annealing is consistent with a previous report of AlON on AlGaN, [24] in our case both the AlON layer and the interface proven to be stable for high-temperature PDA above 600 °C on etched GaN surface.This absence of significant growth of interfacial gallium oxide correlates to the stable and improved interface discussed previously.
Finally, in Figure 9, the ToF-SIMS analyses of the AlON/etched GaN stack are represented.The AlN − profiles confirm the absence of a degraded layer or inhomogeneous nitrogen distribution with increasing PDA temperatures.Concerning the interface stability, ToF-SIMS analyses of GaO − profiles also confirm minor growth of GaO x or gallium diffusion with increasing PDA temperature.The observed reduction of hydroxyl groups with increasing PDA temperature could partially explain the reduction of hysteresis with PDA temperature. [37,22]Possible other defects such as border traps in the dielectric layer [20,41] could also be reduced by the PDA process.However, the carbon impurities do not reduce with PDA.This means that they are well incorporated within the AlON layer and cannot explain the ΔV FB reduction with the PDA temperature.Assuming that the carbon content for the other nitrogen concentration has the same behavior, this assumption could explain the difference observed between the different AlON samples even after PDA.

Conclusion
In this work, the impacts of different nitrogen concentrations in AlON and post-deposition annealing were investigated and compared to Al 2 O 3 using electrical and physical-chemical characterizations.
Without annealing, an increase of V FB with nitrogen content was observed, consistent with the literature.Hysteresis is well improved by the presence of nitrogen when compared to Al 2 O 3 , especially for a measuring temperature of 150 °C.The increase of electron trapping with nitrogen seems to originate from the presence of impurities related to the deposition technique.
With annealing, stable AlON layers and interfaces with etched GaN were observed for PDA temperatures up to 800 °C.When combined with low nitrogen content, V FB was increased and stabilized.The V FB hysteresis was strongly reduced by PDA for AlON and Al 2 O 3 samples thanks partly to the hydroxyl groups' reduction.In addition, AlON exhibited a lower hysteresis than Al 2 O 3 at 150 °C while having a lower D it , highlighting the superior electrical properties of AlON and its interface with etched GaN.Considering hysteresis and interface quality at both measuring temperatures, the more suitable nitrogen concentrations are around 1.5 and 2.6% with an annealing between 600 and 800 °C.

Experimental Section
Device Fabrication: 200 mm n-GaN buffer layers with (0001) orientation were grown on p-Si (111) substrates by metal-organic chemical vapor deposition (MOCVD).The n-type doping concentration was about 5 × 10 17 cm −3 .To simulate the fully recessed GaN HEMT's gate bottom, the substrate underwent dry-etching by inductively coupled plasma reactive ion etching (ICP-RIE) and atomic layer etching (ALE).Samples of 1.5 × 1.5 cm 2 were cleaved and cleaned with HCl for 3 min, rinsed with deionized water, and then dried with N 2 .15 nm of AlON with different nitrogen concentrations were deposited by PE-ALD in a supercycle approach, with the precursors for Al, N, and O being trimethylaluminium (TMA), N 2 plasma, and O 2 plasma, respectively.In this approach, for one AlON cycle, thin PE-ALD AlN layers were oxidized by an O 2 plasma for 15 s.The AlN deposition cycle consists of TMA injection for 0.05 s, Ar purge for 3 s, N 2 plasma for 15 s, and again Ar purge for 3 s.In order to control the nitrogen concentration, different AlN thicknesses were oxidized.The concentrations were fixed at 1.5%, 2.6%, 4.9%, and 7.1% in atomic concentration.15 nm of Al 2 O 3 was deposited by ALD for the reference sample, with TMA and H 2 O as precursors.The deposition temperature was 300 °C for both dielectrics.After deposition, PDA was performed under N 2 for 5 min at 400, 600, and 800 °C for both AlON and Al 2 O 3 samples.Circular MOSCAP structures were fabricated for electrical measurements with Ni/Au deposited by e-beam evaporation as in our previous work. [42]he diameter of the circular gate contact was 100 or 200 μm.
Electrical and Physical-Chemical Characterizations: Back-to-back C-V measurements were carried out at 1 kHz with a forward voltage sweep where C ox , k, T,  GaN ,  0 , and q are respectively the oxide capacitance, the Boltzmann constant, the temperature in Kelvin, the relative GaN permittivity, the vacuum permittivity, and the electron elementary charge.The nitrogen atomic concentration and the interfacial gallium oxide thickness were analyzed by hard X-ray photoelectron spectroscopy (HAXPES) and angle-resolved HAXPES (AR-HAXPES) respectively.Both HAXPES and AR-HAXPES analyses were performed using PHI Quantes equipped with a monochromatic Cr K (h = 5414.7 eV) X-ray source which allowed the analysis of buried interfaces.The photoelectron take-off angle for HAXPES analyses was 45°.Since the C1s spectrum intensity was strongly reduced with the Cr K source, the binding energy was calibrated with Al1s binding energy at 1562 eV. [43]Three contributions for the N1s spectrum were used: N-Ga at around 398 eV, O-N-Al at 399.7 eV, [23,30,44,45] and N-O at 403 eV. [23,27,30,45]For AR-HAXPES, the different analyzed angles were 42°, 50°, and 65°.The binding energy calibration for AR-HAXPES was performed using Ga2p 3/2′ s binding energy at 65°, which corresponded to the angle having the most signal from GaN's bulk (information depth of ≈5 nm for 65°against ≈0.38 nm for 42°in the GaN substrate).Three contributions were used: Ga-N at around ≈1117 to 1118 eV, Ga-O at ≈1117.7 to 1118.7 eV, and Ga-Ga at ≈1115.7 to 1116.3 eV.The energy shift between Ga-N and Ga-O was fixed at 0.7 eV [46] while the shift between Ga-N and Ga-Ga was fixed at −1.3 eV. [47]With AR-HAXPES, the interfacial GaO x thickness was calculated using Equation (3).ln where I Ga-O and I Ga-N are the measured Ga-O and Ga-N peaks' intensity respectively, I ∞ Ga−O and I ∞ Ga−N the Ga-O and Ga-N peaks' intensity in bulk respectively.The ratio I ∞ Ga−N ∕I ∞ Ga−O was experimentally extracted as 0.873. was the inelastic mean free path in Ga 2 O 3 assigned at 5.831 nm using the TPP-2 model [48] and  was the photoelectron take-off angle.The estimated GaO x thickness t GaOx was obtained by linear extrapolation of Equation (3) (see Figure S3, Supporting Information).Both HAXPES and AR-HAXPES spectra are represented in the Supporting Information (see Figures S1 and S2, Supporting Information).The nitrogen distribution and impurities were analyzed by ToF-SIMS analyses using a TOF-SIMS 5 with a 500 eV Cs + sputter gun and by detecting negative secondary ions, obtained from a Bi 3 + ions analytical beam, energized at 15 kV.The intensity was normalized with the bulk GaN − signal while the sputtering time was adjusted to start at the dielectric/GaN interface of each sample where the GaN − signal was reduced by 50% compared to the bulk signal.Specifically for ToF-SIMS, an AlN reference sample was deposited by PE-ALD using TMA and N 2 plasma.

Figure 1 .
Figure 1.Cross-sectional schematic representation of the fabricated MOS capacitors with its process flow.

Figure 2 .Figure 3 .
Figure 2. C-V characteristics for Al 2 O 3 and AlON samples without annealing at the measuring temperatures of a) 25 and b) 150 °C (median of three measurements); G m / characteristics at c) 25 and d) 150 °C (single measurement).

Figure 5 .
Figure 5. C-V characteristics for AlON samples with 800 °C PDA at the measuring temperatures of a) 25 and b) 150 °C (median of three measurements); G m / characteristics at c) 25 and d) 150 °C (single measurement).

Figure 6 .Figure 7 .
Figure 6.For the different PDA temperatures: a) Extracted forward V FB at the measuring temperatures of 25 and 150 °C; b) Extracted V FB hysteresis at 25 and 150 °C.

Figure 9 .
Figure 9. Normalized ToF-SIMS analyses of a) AlN − , b) GaO − , and c) both CN − and OH − depth profiles for 4.9 N% AlON samples without PDA and with respectively 600 and 800 °C PDA.

Table 1 .
Interface states density extracted from G m / peaks at 1 kHz for both 25 and 150 °C.
FB as previously reported for PDA under O 2 on -Sputtering time from the interface (s)

Table 2 .
Oxide capacitance and permittivity for samples annealed at 800 °C (median of three measurements).1.5 N% AlON 2.6 N% AlON 4.9 N% AlON 7.1 N% AlON C OX [nF.cm −2 ] V FB throughout the different annealing temperatures, especially for nitrogen concentrations under 4.9%.On the other side, V FB for Al 2 O 3 is strongly reduced with increasing PDA temperature, with a V FB being lower than AlON for a 600 °C PDA.As mentioned, hysteresis for all samples reduces for both measuring temperatures.At 25 °C, ΔV FB reduction for Al 2 O 3 , 1.5 N%, and 2.6 N% AlON follows a similar trend and stays below 4.9 and 7.1 N% AlON samples.However, at 150 °C, a more pronounced hysteresis is observed for Al 2 O 3 , while AlON samples have a lower hysteresis.This observation further confirms the reduction of electron trapping by nitrogen incorporation.Interestingly, hysteresis for 1.5 and 2.6 N% are strongly reduced even at 150 °C, with a low hysteresis for both measuring temperatures.Concerning the AlON/GaN interface electrical properties, a reduction of D it with PDA temperature is observed for AlON samples.D it is only reduced for Al 2 O 3 after a 400 °C PDA (Figure

Table 3 .
The atomic percentage for N, Al, and O in 4.9 N% AlON by using quantification with HAXPES.