Low Temperature Thermal Atomic Layer Deposition of Aluminum Nitride Using Hydrazine As the Nitrogen Source

materials

Article Low Temperature Thermal Atomic Layer Deposition of Aluminum Nitride Using Hydrazine as the Nitrogen Source

Yong Chan Jung 1, Su Min Hwang 1, Dan N. Le 1, Aswin L. N. Kondusamy 1, Jaidah Mohan 1, Sang Woo Kim 1,2, Jin Hyun Kim 1,2, Antonio T. Lucero 1, Arul Ravichandran 1 , Harrison Sejoon Kim 1 , Si Joon Kim 3 , Rino Choi 2, Jinho Ahn 4, Daniel Alvarez 5, Jeﬀ Spiegelman 5 and Jiyoung Kim 1,* 1 Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, USA; [email protected] (Y.C.J.); [email protected] (S.M.H.); [email protected] (D.N.L.); [email protected] (A.L.N.K.); [email protected] (J.M.); [email protected] (S.W.K.); [email protected] (J.H.K.); [email protected] (A.T.L.); [email protected] (A.R.); [email protected] (H.S.K.) 2 Department of Materials Science and Engineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon 22212, Korea; [email protected] 3 Department of Electrical and Electronics Engineering, Kangwon National University, 1 Gangwondaehakgil, Chuncheon, Gangwon-do 24341, Korea; [email protected] 4 Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Korea; [email protected] 5 RASIRC Inc., 7815 Silverton Avenue, San Diego, CA 92126, USA; [email protected] (D.A.); [email protected] (J.S.) * Correspondence: [email protected]

Received: 20 June 2020; Accepted: 28 July 2020; Published: 31 July 2020

Abstract: Aluminum nitride (AlN) thin films were grown using thermal atomic layer deposition in the temperature range of 175–350 ◦C. The thin films were deposited using trimethyl aluminum (TMA) and hydrazine (N2H4) as a metal precursor and nitrogen source, respectively. Highly reactive N2H4, compared to its conventionally used counterpart, ammonia (NH3), provides a higher growth per cycle (GPC), which is approximately 2.3 times higher at a deposition temperature of 300 ◦C and, also exhibits a low impurity concentration in as-deposited films. Low temperature AlN films deposited at 225 ◦C with a capping layer had an Al to N composition ratio of 1:1.1, a close to ideal composition ratio, with a low oxygen content (7.5%) while exhibiting a GPC of 0.16 nm/cycle. We suggest that N2H4 as a replacement for NH3 is a good alternative due to its stringent thermal budget.

Keywords: atomic layer deposition (ALD); aluminum nitride; hydrazine; trimethyl aluminum (TMA)

1. Introduction Aluminum nitride (AlN) is one of the promising materials for electronic and optoelectronic devices due to its wide band gap structure (6.2 eV), high thermal conductivity (2.85 W/cm K at 300 K), · melting point (2750 ◦C), and large critical electric ﬁeld (12 MV/cm) [1–3]. Additionally, using a highly thermal conductive material like AlN as a thermal spreader can result in enhanced thermal dissipation, which is highly beneﬁcial in scaled devices [4–6]. These nitride deposition processes should be compatible with the thermal budget of back-end-of-line (BEOL) processes in conventional complementary metal-oxide-semiconductor (CMOS) fabrication. A lower deposition temperature (<300 ◦C) is preferred and conformality over high-aspect ratio structures, commonly found in novel,

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Materials 2020, 13, x FOR PEER REVIEW 2 of 11 complex device structures, is also desirable. Hence, the atomic layer deposition (ALD) technique is a forerunnercomplex device among structures, other deposition is also desirable. techniques Hence, that meets the atomic these speciﬁcations layer deposition while (ALD) providing technique excellent is a thicknessforerunner controllability. among other Plasma-enhanced deposition techniques ALD (PEALD) that meets provides these the specifications plasma radicals while required providing to pushexcellent the boundaries thickness controllability. of ALD reactions Plasma-enhanced towards a lower temperatureALD (PEALD) but provides sensitive substrates the plasma can radicals suﬀer fromrequired plasma-induced to push the damage boundaries [7]. PEALD of ALD has reactions a relatively towards poor conformal a lower temperaturedeposition on but complicated sensitive 3Dsubstrates nano-structures can suffer compared from plasma-induced to thermal ALD. damage [7]. PEALD has a relatively poor conformal depositionCurrently, on complicated AlN ALD using3D nano-structures ammonia (NH compared3) and trimethyl to thermal aluminum ALD. (TMA) results in an incompleteCurrently, reaction AlN at temperaturesALD using ammonia below 300 (NH◦C[83)]. and High-temperature trimethyl aluminum ALD above (TMA) 450 ◦ resultsC is required in an inincomplete order to achieve reaction a at vigorous temperatures reaction below with 300 methyl °C [8].groups High-temperature (–CH3) and the ALD complete above removal 450 °C of is by-productsrequired in order in the to ALD achieve process a vigorous using NHreaction3 and with TMA methyl [8]. One groups way (–CH of circumventing3) and the complete this issue removal is by introducingof by-products a more in the reactive ALD process nitrogen using source NH than3 and NH TMA3. From [8]. thisOne perspective,way of circumventing hydrazine (Nthis2H issue4) can is beby usedintroducing as a replacement a more reactive for NH nitrogen3 as the N–Nsource bond than in NH N23H. From4 (~167 this kJ/ mol)perspective, is weak hydrazine when compared (N2H4) tocan the be N–H used bondas a replacement (~386 kJ/mol) for in NH NH3 as3 [ the9]. TheN–N molecular bond in N structure2H4 (~167 ofkJ/mol) N2H4 isis weak as shown when in compared Figure1 usingto the aN–H ball-stick bond model.(~386 kJ/mol) Safety in was NH a3 key[9]. The concern molecular while structure handling of N 2NH24H,4 but is as the shown newly in available Figure 1 ultra-highusing a ball-stick purity anhydrousmodel. Safety N2 Hwas4 source a key concern is compliant while with handling the safety N2H standard4, but the requirementsnewly available and ultra- has alsohigh demonstrated purity anhydrous the deposition N2H4 source of metalis compliant nitrides with at low thetemperatures safety standard [10 requirements–12]. and has also demonstrated the deposition of metal nitrides at low temperatures [10–12].

Figure 1. Ball stick model of hydrazine (N H ) with N atoms and H atoms. Figure 1. Ball stick model of hydrazine (N22H44) with N atoms and H atoms. Abdulagatov et al. recently demonstrated AlN deposition by thermal ALD using tris(diethylamido) Abdulagatov et al. recently demonstrated AlN deposition by thermal ALD using aluminum (III) (TDEAA) and hydrazine in the deposition temperature range from 150 to 280 C[13]. tris(diethylamido)aluminum (III) (TDEAA) and hydrazine in the deposition temperature range◦ from Growth rates of 1.23, 1.16, and 1.72 Å/cycle were reported at 150, 200, and 280 C, respectively. 150 to 280 °C [13]. Growth rates of 1.23, 1.16, and 1.72 Å/cycle were reported at 150,◦ 200, and 280 °C, The higher growth rate observed at 280 C was mainly attributed to the organo-metallic precursor respectively. The higher growth rate observed◦ at 280 °C was mainly attributed to the organo-metallic decomposition and hence a chemical vapor deposition (CVD) reaction mechanism was suspected for precursor decomposition and hence a chemical vapor deposition (CVD) reaction mechanism was observing such a high growth rate. Additionally, it was also demonstrated that the impurity, such as suspected for observing such a high growth rate. Additionally, it was also demonstrated that the carbon and oxygen content, in nitride film deposited using hydrazine was comparable or lower when impurity, such as carbon and oxygen content, in nitride film deposited using hydrazine was compared to films deposited using NH3 [13,14]. comparable or lower when compared to films deposited using NH3 [13,14]. Previous studies revealed that the growth rate of AlN is less than 0.04 nm/cycle by thermal ALD Previous studies revealed that the growth rate of AlN is less than 0.04 nm/cycle by thermal ALD at temperatures below 400 ◦C using TMA and NH3 [13]. Furthermore, TMA starts to decompose at temperatures below 400 °C using TMA and NH3 [13]. Furthermore, TMA starts to decompose at at higher temperatures (above 377 C) [15] and reduces the film quality of AlN [16,17]. In order to higher temperatures (above 377 °C)◦ [15] and reduces the film quality of AlN [16,17]. In order to deposit high-quality AlN with a reasonable growth rate at low temperatures, it is essential to adopt deposit high-quality AlN with a reasonable growth rate at low temperatures, it is essential to adopt highly reactive precursors, such as hydrazine, into the ALD process. In this paper, we successfully highly reactive precursors, such as hydrazine, into the ALD process. In this paper, we successfully demonstrate AlN films deposition by thermal ALD at low temperatures. Ultra-pure anhydrous N2H4 demonstrate AlN films deposition by thermal ALD at low temperatures. Ultra-pure anhydrous N2H4 and TMA were used to deposit AlN thin films in the temperature range from 175 to 350 C, with the and TMA were used to deposit AlN thin films in the temperature range from 175 to 350 ◦°C, with the feasibility of AlN deposition using an Al ALD precursor of TMA. As a comparison, the growth rate and feasibility of AlN deposition using an Al ALD precursor of TMA. As a comparison, the growth rate surface roughness of AlN films deposited by thermal ALD using TMA and NH3 are also presented. and surface roughness of AlN films deposited by thermal ALD using TMA and NH3 are also presented.

2. Materials and Methods

2.1. Film Deposition

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ALD AlN was deposited using TMA and N2H4 as the Al precursor and nitrogen source, 2.respectively. Materials andThe Methodsfilms were deposited using home-built ALD system with a hollow-cathode plasma source (Meaglow Ltd., Thunder Bay, Canada) provision to generate plasma. This ALD system has 2.1.been Film used Deposition for deposition of various nitride films using thermal ALD or PEALD processes [18–21]. The stainless-steel chamber wall was heated to ~120 °C and the precursor delivery lines were ALD AlN was deposited using TMA and N2H4 as the Al precursor and nitrogen source, respectively.maintained at The 90 films°C to wereavoid deposited condensation. using The home-built precursors ALD were system maintained with a hollow-cathode at room temperature. plasma sourceFor the (Meaglow ALD process, Ltd., p-type Thunder Si Bay,(100) Canada)substrates provision (Silicon toValley generate Microelectronics, plasma. This Santa ALD systemClara, CA, has beenUSA) usedwith fora resistivity deposition of 3–10 of various Ω·cm were nitride dipped films in using 100:1 thermaldiluted HF ALD solution or PEALD to remove processes native [18 oxide.–21]. After blowing with N2, the substrates were directly transferred to the process chamber. During this The stainless-steel chamber wall was heated to ~120 ◦C and the precursor delivery lines were maintained ex situ process, re-oxidation of the silicon surface was negligible to cause interdiffusion of oxygen at 90 ◦C to avoid condensation. The precursors were maintained at room temperature. For the ALD into AlN films [22]. After loading the substrates, the chamber was pumped down to 10−6 Torr using process, p-type Si (100) substrates (Silicon Valley Microelectronics, Santa Clara, CA, USA) with a a turbomolecular pump to reduce adventitious contaminants introduced during the substrate resistivity of 3–10 Ω cm were dipped in 100:1 diluted HF solution to remove native oxide. After blowing transfer. The process· pressure was maintained at 0.5 Torr with continuous flow of Ar carrier gas. The with N2, the substrates were directly transferred to the process chamber. During this ex situ process, representative time sequence of one cycle of the ALD process condition was set to be: TMA pulse (0.1 re-oxidation of the silicon surface was negligible to cause interdiffusion of oxygen into AlN films [22]. s)–Ar purge (15 s)–N2H4 pulse (0.1 s)–Ar purge (120 s), as shown6 in Figure 2. The deposition After loading the substrates, the chamber was pumped down to 10− Torr using a turbomolecular pump temperature was varied between 175 and 350 °C. For comparison, all samples in this study were to reduce adventitious contaminants introduced during the substrate transfer. The process pressure deposited using 100 cycles. was maintained at 0.5 Torr with continuous flow of Ar carrier gas. The representative time sequence of In the case of material characterization of AlN grown at 225 °C, 4 nm-thick silicon nitride (SiNx) one cycle of the ALD process condition was set to be: TMA pulse (0.1 s)–Ar purge (15 s)–N2H4 pulse was deposited as a capping layer in order to prevent the surface oxidation in air. The SiNx capping (0.1 s)–Ar purge (120 s), as shown in Figure2. The deposition temperature was varied between 175 layer was subsequently deposited at 410 °C using hexachlorodisilane (Si2Cl6) and N2H4 in the same and 350 ◦C. For comparison, all samples in this study were deposited using 100 cycles. chamber without breaking the vacuum.

Figure 2. Schematic of atomic layer deposition (ALD) cycle for deposition of AlN using trimethylaluminum Figure 2. Schematic of atomic layer deposition (ALD) cycle for deposition of AlN using (TMA) and hydrazine (N2H4). One cycle of the ALD process condition is: TMA pulse (0.1 s)–Ar purge trimethylaluminum (TMA) and hydrazine (N2H4). One cycle of the ALD process condition is: TMA (15 s)–N2H4 pulse (0.1 s)–Ar purge (120 s). pulse (0.1 s)–Ar purge (15 s)–N2H4 pulse (0.1 s)–Ar purge (120 s). In the case of material characterization of AlN grown at 225 ◦C, 4 nm-thick silicon nitride (SiNx) was2.2. Film deposited Characterization as a capping layer in order to prevent the surface oxidation in air. The SiNx capping layerThe was thickness subsequently and depositedrefractive atindex 410 ◦(R.I.)C using of hexachlorodisilaneAlN thin films were (Si 2measuredCl6) and N by2H spectroscopic4 in the same chamberellipsometry without (SE, M-2000DI, breaking the J.A. vacuum. Woolam, Lincoln, NE, USA) and the values were fit using the spectra 2.2.measured Film Characterization at 3 different angles (55°, 65°, and 75°). The chemical composition and bonding states of AlN thin films were characterized by X-ray photoelectron spectroscopy (XPS). XPS analysis was performedThe thickness using a PHI and VersaProbe refractive index II (ULVAC-PHI, (R.I.) of AlN Chigasaki, thin films Kanagawa, were measured Japan) byequipped spectroscopic with a ellipsometrymonochromatic (SE, Al M-2000DI, Kα X-ray J.A. source Woolam, (EPhoton Lincoln, = 1486.6 NE, eV). USA) To and remove the values surface were contaminants, fit using the spectraAr gas measuredcluster ion atbeam 3 di ff(GCIB)erent anglessputtering (55◦ ,with 65◦, a and beam 75◦ energy). The chemicalof 1 kV and composition the cluster and size bonding of 2500 atoms states ofwas AlN employed. thin films The were elemental characterized composition by X-ray of the photoelectron films was calculated spectroscopy based (XPS). on the XPS peak analysis area wasand performedatomic sensitivity using a PHIfactor VersaProbe [23]. Surface II (ULVAC-PHI, roughness of Chigasaki, AlN films Kanagawa, was determined Japan) equipped by atomic with force a monochromaticmicroscope (AFM) Al (Veeco Kα X-ray Multimode source (E V.Photon Non-contact= 1486.6 AFM, eV). To Veeco, remove Plainview, surface contaminants,NY, USA). Ar gas cluster ion beam (GCIB) sputtering with a beam energy of 1 kV and the cluster size of 2500 atoms was employed.3. Results and The Discussion elemental composition of the films was calculated based on the peak area and atomic

Materials 2020, 13, 3387 4 of 10 sensitivity factor [23]. Surface roughness of AlN ﬁlms was determined by atomic force microscope (AFM) (Veeco Multimode V. Non-contact AFM, Veeco, Plainview, NY, USA).

3. Results and Discussion Materials 2020, 13, x FOR PEER REVIEW 4 of 11 The ALD deposition of AlN using TMA and N2H4 followed by sequentially pulsing the reactants, followedThe ALD by flushing deposition out theof AlN reaction using by-products TMA and N using2H4 followed Ar between by sequentially the self-limited pulsing surface the reactants, reactions. followedTo get the by ALD flushing process out condition, the reaction self-limited by-products growth using per Ar cyclebetween (GPC) the characteristics self-limited surface were examinedreactions. Toby get increasing the ALD N process2H4 pulse condition, and purge self-limited time, as showngrowth inper Figure cycle3 (GPC)a,b. With characteristics the increased were N 2examinedH4 pulse byand increasing purge time, N2H the4 pulse growth and rate purge saturated time, as at ashown constant in Figure level as 3a,b. expected With the in the increased ideal ALD N2H process.4 pulse andAn increasepurge time, of purging the growth time rate (300 saturated s or longer) at mighta constant be inappropriate level as expected for the in ALDthe ideal process ALD due process. to the Anfeasibility, increase hence of purging we set thetime pulse (300 ands or purgelonger) time might of N be2H inappropriate4 for AlN deposition for the processALD process to 0.1 sdue and to 120 the s, feasibility,respectively. hence Meanwhile, we set the the pulse R.I. and of film purge at time wavelength of N2H4 offor 633 AlN nm deposition was slightly process larger to 0.1 (<0.1) s and as 120 the s,pulse respectively. and purge Meanwhile, time increased. the R.I. This of variation film at wavelength was not suffi ofcient 633 tonm argue was thatslightly the stoichiometrylarger (<0.1) as of the the pulsedeposited and purge films hastime changed. increased. In addition,This variation it was was confirmed not sufficient that the to AlNargue thin that films the depositedstoichiometry by the of thethermal deposited ALD processfilms has had changed. an amorphous In addition, nature, it was as confirmed confirmed that by XRD the AlN analysis. thin films The densitydeposited of theby 3 thethin thermal films deposited ALD process at 350 had◦C confirmedan amorphous through nature, X-ray as reflectometry confirmed by analysis XRD analysis. was 2.9 gThe/cm density, which of is theabout thin 10% films lower deposited value compared at 350 °C to confirmed the reference through value ofX-ray bulk reflectometry crystalline AlN analysis of 3.26 gwas/cm 32.9. The g/cm unit3, whichcell dimensions is about 10% of the lower hexagonal value compared structure ofto wurtzitethe reference AlN werevalue reported of bulk crystalline as a = 3.1151 AlN Å, of b 3.26= 3.1151 g/cm Å,3. Theand unit c = 4.9880 cell dimensions Å [24]. Therefore, of the hexagonal a relatively structure larger of GPC wurtzite is expected AlN were than reported when the as crystalline a = 3.1151Å, AlN b =is 3.1151Å,grown. and c = 4.9880Å [24]. Therefore, a relatively larger GPC is expected than when the crystalline AlN is grown.

Figure 3. Saturation curves for the growth-per-cycle (GPC) for (a) hydrazine pulse and (b) purge time. Figure 3. Saturation curves for the growth-per-cycle (GPC) for (a) hydrazine pulse and (b) purge time. The ALD process was performed at a substrate temperature of 300 C. The refractive index (R.I.) at The ALD process was performed at a substrate temperature of 300 °C.◦ The refractive index (R.I.) at 633 nm of the deposited ﬁlms was measured using spectroscopic ellipsometer after deposition. 633 nm of the deposited films was measured using spectroscopic ellipsometer after deposition.

Figure4 shows the temperature dependence of the film growth rate from 175 to 350 ◦C. To obtain Figure 4 shows the temperature dependence of the film growth rate from 175 to 350 °C. To obtain the GPC of each point in Figure4, the AlN films were deposited using 100 ALD cycles. The growth the GPC of each point in Figure 4, the AlN films were deposited using 100 ALD cycles. The growth rate increased linearly with increasing deposition temperature, where the GPC was 0.08, 0.16, 0.25, rate increased linearly with increasing deposition temperature, where the GPC was 0.08, 0.16, 0.25, and 0.32 nm/cycle at 175, 225, 300 and 350 ◦C, respectively. In an ideal ALD process, the constant GPC and 0.32 nm/cycle at 175, 225, 300 and 350 °C, respectively. In an ideal ALD process, the constant GPC can be achieved at temperatures high enough to avoid precursor condensation and satisfy perceptible can be achieved at temperatures high enough to avoid precursor condensation and satisfy perceptible reactivity between the precursor and substrate, but sufficiently low to prevent precursor decomposition reactivity between the precursor and substrate, but sufficiently low to prevent precursor and desorption of chemisorbed species from the surface [25,26]. Nevertheless, the GPC can still be decomposition and desorption of chemisorbed species from the surface [25,26]. Nevertheless, the varied with temperature while maintaining self-limiting growth due to the temperature dependence of GPC can still be varied with temperature while maintaining self-limiting growth due to the the reactive sites on the surface and the reaction mechanism of the precursor itself [26]. Our observation temperature dependence of the reactive sites on the surface and the reaction mechanism of the confirmed that co-adsorption of TMA and N2H4 was self-limiting by forming a monolayer at substrate precursor itself [26]. Our observation confirmed that co-adsorption of TMA and N2H4 was self- limiting by forming a monolayer at substrate temperature <350 °C, while above this temperature reaction was rapid and formed a thick film of AIN. Consequently, the CVD effect became significant and an increase of GPC could be observed [16,17] as deposition temperature is up to 350 °C. It is also worth noting that the ALD window of AlN using TMA is narrow in earlier studies [27,28]. Furthermore, the growth rate of thermal ALD-AlN films from TMA and NH3 was inconsistent in previous reports. For example, Tian et al. reported the growth rate of AlN films was 0.01 nm/cycle at 375 °C [27], while Kim et al. deposited AlN films in the temperature range from 265 to 335 °C with a

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temperature <350 ◦C, while above this temperature reaction was rapid and formed a thick film of AIN. Consequently, the CVD effect became significant and an increase of GPC could be observed [16,17] as deposition temperature is up to 350 ◦C. It is also worth noting that the ALD window of AlN using TMA is narrow in earlier studies [27,28]. Furthermore, the growth rate of thermal ALD-AlN films from TMA and NH3 was inconsistent in previous reports. For example, Tian et al. reported the growth rate Materialsof AlN 2020 films, 13 was, x FOR 0.01 PEER nm REVIEW/cycle at 375 ◦C[27], while Kim et al. deposited AlN films in the temperature 5 of 11 range from 265 to 335 ◦C with a growth rate of 0.02–0.16 nm/cycle [29]. Unfortunately, in the case of growthAlN deposited rate of 0.02–0.16 using NH nm/cycle3 in a thermal [29]. Unfortunately, ALD process, the in sub-angstromthe case of AlN (less deposited than 0.5 using Å) growth NH3 in rate a thermalwas too lowALD for process, accurate the comparison. sub-angstrom On the (less other than hand, 0.5 Å) the growth deposition rate rates was observed too low for in our accurate study comparison.using N2H4 wereOn the much other larger hand, than the those deposition reported rates earlier observed for thermal in our ALD study AlN using using N2H TMA4 were and much NH3 . largerTo suppress than those changes reported from equipmentearlier for thermal difference, ALD we AlN deposited using AlNTMA using and NH NH33. asTothe suppress nitrogen changes source fromin the equipment same ALD difference, reactor. As we a result, deposited it was AlN confirmed using NH that3 theas the deposition nitrogen rates source at 300 in ◦theCincreased same ALD by reactor.2.3 times As with a result, N2H 4it. was confirmed that the deposition rates at 300 °C increased by 2.3 times with N2H4.

Figure 4. Growth-per-cycle (GPC) of AlN thin films as a function of the deposition temperature Figure 4. Growth-per-cycle (GPC) of AlN thin films as a function of the deposition temperature and and refractive index (R.I.) at the wavelength of 644 nm with different deposition temperatures for refractive index (R.I.) at the wavelength of 644 nm with different deposition temperatures for trimethylaluminum (TMA) and hydrazine (N2H4) precursors. trimethylaluminum (TMA) and hydrazine (N2H4) precursors. The R.I. of the AlN film at wavelength of 633 nm was extracted from the SE. The SE data were The R.I. of the AlN film at wavelength of 633 nm was extracted from the SE. The SE data were fit using the Cauchy model, which is widely used for semiconductor materials [30]. The R.I. of the fit using the Cauchy model, which is widely used for semiconductor materials [30]. The R.I. of the AlN film increases with increasing deposition temperature except at 175 ◦C. It is suspected that the AlN film increases with increasing deposition temperature except at 175 °C. It is suspected that the higher R.I. at 175 ◦C can be attributed by hydrogen species inside the film. Due to the relatively low higher R.I. at 175 °C can be attributed by hydrogen species inside the film. Due to the relatively low temperature, the remaining N–H bonds from N2H4 are dominant after ligand exchange with TMA, temperature, the remaining N–H bonds from N2H4 are dominant after ligand exchange with TMA, resulting in the lower GPC results [31]. The R.I. of AlN film deposited at 350 ◦C is 1.98, which is close resulting in the lower GPC results [31]. The R.I. of AlN film deposited at 350 °C is 1.98, which is close to the reported values of high-quality AlN films [32–34]. to the reported values of high-quality AlN films [32–34]. XPS measurements were performed to investigate chemical bonding states of AlN thin films. XPS measurements were performed to investigate chemical bonding states of AlN thin films. Figure5 shows XPS analysis results of AlN films deposited for 100 ALD cycles at 225 and 300 ◦C. Figure 5 shows XPS analysis results of AlN films deposited for 100 ALD cycles at 225 and 300 °C. AlN films, when exposed to air, react with oxygen and water to form an aluminum oxide film [34]. AlN films, when exposed to air, react with oxygen and water to form an aluminum oxide film [34]. As mentioned in the experimental section, we deposited SiNx on AlN film grown at 225 ◦C to prevent As mentioned in the experimental section, we deposited SiNx on AlN film grown at 225 °C to prevent the ambient oxidation of the nitride film deposited below 225 ◦C. It was an inevitable choice to passivate the ambient oxidation of the nitride film deposited below 225 °C. It was an inevitable choice to with the SiNx film, because ex-situ chemical analysis was impossible owing to the oxidation being too passivate with the SiNx film, because ex-situ chemical analysis was impossible owing to the oxidation active in the air. The O 1s peak for the AlN films deposited at 225 ◦C with and without capping layer being too active in the air. The O 1s peak for the AlN films deposited at 225 °C with and without is depicted in Supplementary Materials (Figure S1). It showed that 4 nm-thick SiNx capping layer capping layer is depicted in Supplementary Materials (Figure S1). It showed that 4 nm-thick SiNx provides effective barrier to oxidation of AlN surface. Nevertheless, it was indicated that the peak at capping layer provides effective barrier to oxidation of AlN surface. Nevertheless, it was indicated 532.4 eV is assigned to Al–O bonds in the AlN films, and these oxygen impurities in both films were that the peak at 532.4 eV is assigned to Al–O bonds in the AlN films, and these oxygen impurities in both films were considered the natural characteristics of the AlN films [35–37]. It is worth noting that the capping layer should be thin enough to avoid significant signal attenuation from the sample and there should be no interfacial reaction with the sample [38–40]. Meanwhile AlN films deposited at 300 °C do not have any capping layer, which may result in higher O content in the films compared to the films deposited at 225 °C with a capping layer. It should be noted that the peak positions from the AlN film with the capping layer were calibrated using the Si 2p peak at 99.4 eV, which comes from the Si substrate. In the case of AlN films without capping, the peak positions were calibrated with Al 2p peak at 74.3 eV, the calibrated peaks from the AlN films aforementioned. All narrow scans were deconvoluted for more accurate analysis of the chemical bonding status, such as metal oxide

Materials 2020, 13, 3387 6 of 10 considered the natural characteristics of the AlN films [35–37]. It is worth noting that the capping layer should be thin enough to avoid significant signal attenuation from the sample and there should be no interfacial reaction with the sample [38–40]. Meanwhile AlN films deposited at 300 ◦C do not have any capping layer, which may result in higher O content in the films compared to the films deposited at 225 ◦C with a capping layer. It should be noted that the peak positions from the AlN film with the capping layer were calibrated using the Si 2p peak at 99.4 eV, which comes from the Si substrate. In the case of AlN films without capping, the peak positions were calibrated with Al 2p peak at 74.3 eV, the calibrated peaks from the AlN films aforementioned. All narrow scans were deconvoluted for more accurateMaterials 2020 analysis, 13, x FOR of the PEER chemical REVIEW bonding status, such as metal oxide and metal nitrides. Both the 6 of Al 11 2p and N 1s peaks were slightly asymmetric, indicating the presence of different bonding features associatedand metal with nitrides. nitrogen Both in the the Al AlN 2p and thin N films. 1s peaks were slightly asymmetric, indicating the presence of different bonding features associated with nitrogen in the AlN thin films.

Figure 5. High resolution X-ray photoelectron spectroscopy (XPS) (a) Al 2p, (b) N 1s, (c) O 1s, and (d)C Figure 5. High resolution X-ray photoelectron spectroscopy (XPS) (a) Al 2p, (b) N 1s, (c) O 1s, and (d) 1s spectra of AlN thin ﬁlm deposited at 225 and 300 ◦C. C 1s spectra of AlN thin film deposited at 225 and 300 °C.

Figure5a shows the comparison of the Al 2p peaks for the AlN films deposited at 225 and 300 ◦C. DeconvolutionFigure 5a of shows the Al the 2p comparison spectrum gives of the rise Al to two2p peaks peaks, for one the at AlN 74.3 eV,films which deposited corresponds at 225 toand Al 300 in Al–N,°C. Deconvolution and the other atof 75.2the Al eV, 2p which spectrum corresponds gives rise to theto two BE ofpeaks, Al–O one bonds. at 74.3 The eV, values which of thesecorresponds peaks areto comparableAl in Al–N, withand the the other previously at 75.2 reported eV, which spectra corresponds for AlN filmsto the [ 28BE,41 of,42 Al–O]. As bonds. shown The in Figure values5a, of mainlythese peaks Al–N are bonds comparable and ignorable with the Al–O previously bonds were reported observed. spectra In for the AlN same films way, [28,41,42]. the N 1s spectrumAs shown wasin Figure deconvoluted 5a, mainly with Al–N two peaksbonds as and described ignorable in FigureAl–O 5bondsb. It showed were observed. one main In peak the centeredsame way, at athe BE N of1s 397.4 spectrum eV, corresponds was deconvoluted to N in Al–N, with two and thepeaks other as described at 398.6 eV, in corresponds Figure 5b. It to showed the BE ofone unbounded main peak nitrogencentered [43 at]. a BE of 397.4 eV, corresponds to N in Al–N, and the other at 398.6 eV, corresponds to the BE ofThere unbounded is a concern nitrogen regarding [43]. the source of unbounded nitrogen observed in the film. We suggest There is a concern regarding the source of unbounded nitrogen observed in the film. We suggest that the unbounded nitrogen came from incompletely reacted N2H4 without breaking the N–N bond, whichthat the remains unbounded after nitrogen reaction came with TMAfrom incompletely due to low depositionreacted N2H temperature.4 without breaking This the hypothesis N–N bond, is which remains after reaction with TMA due to low deposition temperature. This hypothesis is also also supported by the decrease in GPC with increase in N2H4 purge time, as shown in Figure3b. Thesupported composition by the distribution decrease in of theGPC AlN with films increase was also in N analyzed.2H4 purge This time, elemental as shown analysis in Figure for the 3b. AlN The filmscomposition is described distribution in Table1 ofwhich the showsAlN films that was the surfacealso analyzed. composition This consistselemental of aluminum,analysis for nitrogen the AlN andfilms oxygen, is described as expected. in Table The1 which surface shows composition that the surface analysis composition of sputtered consists film of requires aluminum, attention nitrogen to and oxygen, as expected. The surface composition analysis of sputtered film requires attention to interpret due to the preferential Ar ion sputtering effect [42]. Due to the unbroken N–N bonds of N2H4, itinterpret is supposed due thatto the the preferential Al:N ratio isAr 1:1; ion however, sputtering the effect ratio [42]. increased Due to over the 1:1unbroken with higher N–N nitrogenbonds of N2H4, it is supposed that the Al:N ratio is 1:1; however, the ratio increased over 1:1 with higher concentration. The total [N]/[Al] ratio of the AlN film deposited at 225 and 300 ◦C was 1.1 and 1.2, respectively.nitrogen concentration. Although this The di fftotalerence [N]/[Al] is small, ratio N 1sof spectrathe AlN clearly film deposited indicates theat 225 AlN and film 300 deposited °C was at1.1 and 1.2, respectively. Although this difference is small, N 1s spectra clearly indicates the AlN film 225 ◦C shows higher unbounded nitrogen content. The decrement of [N]/[N–Al] ratio from 0.34 to 0.15deposited with increasing at 225 °C deposition shows higher temperature unbounded may nitrogen suggest that content. the films The depositeddecrement at of higher [N]/[N–Al] deposition ratio from 0.34 to 0.15 with increasing deposition temperature may suggest that the films deposited at higher deposition temperature have larger R.I., i.e., high density. Nevertheless, the AlN thin film deposited at 225 °C showed an Al:N ratio of 1:1.1, which meant that stoichiometric AlN was successfully deposited.

Table 1. Chemical composition of the AlN films deposited at 225 and 300 °C as determined by high resolution XPS analysis.

Deposition Temperature [Al] at.% [N] at.% [O] at.% [C] at.% [N]/[Al] [N]/[N–Al] 225 °C 44.8 47.7 7.5

Materials 2020, 13, 3387 7 of 10

temperature have larger R.I., i.e., high density. Nevertheless, the AlN thin ﬁlm deposited at 225 ◦C showed an Al:N ratio of 1:1.1, which meant that stoichiometric AlN was successfully deposited.

Table 1. Chemical composition of the AlN ﬁlms deposited at 225 and 300 ◦C as determined by high resolution XPS analysis.

Deposition Temperature [Al] at.% [N] at.% [O] at.% [C] at.% [N]/[Al] [N]/[N–Al] 1 225 ◦C 44.8 47.7 7.5

For more precise comparison of the growth rate, we conducted ALD of AlN using diﬀerent For more precise comparison of the growth rate, we conducted ALD of AlN using different techniques, using the same ALD reactor. As shown in Figure6, the growth rates of tALD N 2H4, PEALD techniques, using the same ALD reactor. As shown in Figure 6, the growth rates of tALD N2H4, NH3, and tALD NH3 were 0.16, 0.15, and 0.03 nm/cycle at 225 ◦C and 0.25, 0.24, and 0.11 nm/cycle PEALD NH3, and tALD NH3 were 0.16, 0.15, and 0.03 nm/cycle at 225 °C and 0.25, 0.24, and 0.11 at 300 ◦C, respectively. When using N2H4 as the nitrogen source, the growth rates were 5.3 and 2.3 nm/cycle at 300 °C, respectively. When using N2H4 as the nitrogen source, the growth rates were 5.3 times higher than when deposited by tALD using NH3 at 225 and 300 ◦C, respectively. In addition, and 2.3 times higher than when deposited by tALD using NH3 at 225 and 300 °C, respectively. In the growth rate of AlN deposited by tALD N2H4 was comparable with that of AlN deposited by addition, the growth rate of AlN deposited by tALD N2H4 was comparable with that of AlN deposited PEALD NH3. by PEALD NH3.

Figure 6. Comparison of growth per cycle (GPC) of AlN thin ﬁlms deposited using TMA and hydrazine

(NFigure2H4) and 6. Comparison ammonia (NH of3 ), growth as the metal per cycle precursor (GPC) and of nitrogen AlN thin source, films respectively. deposited using The AlN TMA ﬁlms and werehydrazine deposited (N2H by4) usingand ammonia thermal atomic(NH3), layeras the deposition metal precursor (tALD) and and nitrogen plasma-enhanced source, respectively. atomic layer The depositionAlN films (PEALD) were deposited technique. by Itusing is also thermal indicated atomic that GPClayer of deposition AlN thin ﬁlms(tALD) deposited and plasma-enhanced using TDEAA

andatomic N2H layer4, as thedeposition metal precursor (PEALD) and technique. nitrogen It source is also [indicated13]. that GPC of AlN thin films deposited using TDEAA and N2H4, as the metal precursor and nitrogen source [13]. Figure7 shows the surface morphology of AlN thin films grown by thermal ALD using two differentFigure nitrogen 7 shows sources, the surface N2H4 morphologyand NH3. The of AlN root-mean-square thin films grown (RMS) by roughnessthermal ALD of theusing films two weredifferent measured nitrogen by AFMsources, and N their2H4 and values NH3 were. The 0.64root-mean-square and 0.72 nm for (RMS) AlN roughness films deposited of the usingfilms were 100 cycles,measured using by N AFM2H4 and and NH their3 as values the nitrogen were 0.64 source, and 0.72 respectively. nm for AlN There films is nodeposited degradation using of100 surface cycles, roughnessusing N2H with4 and N2 H NH4 despite3 as the the nitrogen higher growth source, rate, respectively. which makes There N2H 4 isan no attractive degradation nitrogen of source surface inroughness semiconductor with N fabrication.2H4 despite the higher growth rate, which makes N2H4 an attractive nitrogen source in semiconductor fabrication.

Materials 2020, 13, 3387 8 of 10 Materials 2020, 13, x FOR PEER REVIEW 8 of 11

FigureFigure 7. Surface 7. Surface roughness roughness of of AlN AlN thin thin ﬁlmsfilms deposited using using (a) (hydrazinea) hydrazine (N2H (N4) and2H (4b)) andammonia (b) ammonia (NH3),(NH as the3), as nitrogen the nitrogen source. source. Root-mean-square Root-mean-square (RMS) (RMS) values values ofof AlNAlN thin films ﬁlms deposited deposited using using N2H4 N2H4 and NH3 were 0.64 and 0.72 nm, respectively. and NH3 were 0.64 and 0.72 nm, respectively.

4. Conclusions4. Conclusions DepositionDeposition of low-temperature of low-temperature AlN AlN throughthrough thermal thermal ALD ALD has hasbeen been demonstrated. demonstrated. Optical Opticaland and chemical characterization has been performed to get an accurate assessment of the quality of the chemical characterization has been performed to get an accurate assessment of the quality of the deposited AlN films. Thicknesses and R.I.s of the films were analyzed using SE. A growth rate of 0.16 depositednm/cycle AlN and films. R.I. of Thicknesses 1.74 was obtained and at R.I.s the very of the low films temperature were analyzedof 225 °C. A using high growth SE. A rate growth can rate of 0.16 nmbe/cycle achieved and compared R.I. of 1.74 to NH was3 as obtained the nitrogen at the source very due low to temperaturethe high reactivity of 225 of N◦2C.H4. A XPS high results growth rate can beshowed achieved an Al:N compared ratio of 1:1.1 to NH and3 fewas theimpurities nitrogen in the source films. due to the high reactivity of N2H4. XPS results showedIn summary, an Al:N stable ratio AlN of thin 1:1.1 films and were few successfully impurities grown in the while films. the material parameters were Incomparable summary, to stable those of AlN aluminum thin films nitride were films successfully deposited using grown NH3. In while addition, the materialthe rapid deposition parameters were rate at low temperatures demonstrates the potential of the nitrogen source to replace NH3. comparable to those of aluminum nitride films deposited using NH3. In addition, the rapid deposition rate atSupplementary low temperatures Materials: demonstrates The following the are potential available online of the at nitrogen www.mdpi.com/xxx/s1, source to replace Figure NH S1: High3. resolution O 1s XPS spectra of (a) AlN deposited at 225 °C w/ and w/o capping layer and (b) AlN deposited at Supplementary225 °C w/ capping Materials: layerThe and deposited following at are 300 available°C w/o capping online layer. at http: //www.mdpi.com/1996-1944/13/15/3387/s1, Figure S1:Author High Contributions: resolution O Conceptualization, 1s XPS spectra of Y.C.J.; (a) AlN methodology, deposited Y.C.J., at 225 S.M.H.,◦C w/ andand A.T.L.; w/o capping validation, layer Y.C.J., and (b) AlN depositedS.M.H., at 225 and◦ C H.S.K.; w/ capping formal analysis,layer and Y.C.J., deposited S.M.H. at A.L.N.K., 300 ◦C D.A., w/o capping and J.K.; investigation,layer. D.N.L., A.L.N.K., AuthorS.W.K., Contributions: J.H.K., D.A.,Conceptualization, and J.K.; resources, Y.C.J. Y.C.J.; and methodology, J.K.; data curation, Y.C.J., Y.C.J., S.M.H., D.N.L., and A.L.N.K., A.T.L.; andvalidation, A.R.; Y.C.J., S.M.H.,writing—original and H.S.K.; formal draft analysis, preparation, Y.C.J., Y.C.J.; S.M.H., writing—review A.L.N.K., D.A.,and editing, and J.K.; Y.C.J., investigation, J.M., A.R., H.S.K., D.N.L., S.J.K., A.L.N.K., R.C., S.W.K., J.H.K., D.A.,J.A., D.A., and J.K.; and J.K.; resources, visualization, Y.C.J. and Y.C.J.; J.K.; supervision, data curation, R.C., D.A., Y.C.J., J.S., D.N.L., and J.K.; A.L.N.K., project andadministration, A.R.; writing—original J.K.; draft preparation,funding acquisition, Y.C.J.; D.A., writing—review J.S., R.C., J.A., and J.K. editing, All authors Y.C.J., have J.M., read A.R., and H.S.K.,agreed to S.J.K., the published R.C., J.A., version D.A., of and J.K.; visualization,the manuscript. Y.C.J.; supervision, R.C., D.A., J.S., and J.K.; project administration, J.K.; funding acquisition, D.A., J.S., R.C., J.A., and J.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by RASIRC Inc. This work was also partially supported by Brain Funding:PoolThis Program work through was financially NRF by the supported Ministry of byScience RASIRC and ICT Inc. in ThisKorea work (No. 2019H1D3A2A01101691) was also partially supported and the by Brain Pool ProgramMOTIE (Ministry through of NRF Trade, by Industry, the Ministry and Energy) of Science in Korea and under ICT in the Korea Fostering (No. Global 2019H1D3A2A01101691) Talents for Innovative and the MOTIEGrowth (Ministry Program of Trade, (P0008750) Industry, supervised and by Energy) the Korea in Institute Korea underfor Advancement the Fostering of Technology Global Talents(KIAT). for Innovative GrowthAcknowledgments: Program (P0008750) The authors supervised would by like the to acknowledge Korea Institute RASIRC for Inc. Advancement for providingof the Technology ultrapure anhydrous (KIAT). Acknowledgments:hydrazine. The authors would like to acknowledge RASIRC Inc. for providing the ultrapure anhydrous hydrazine.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Slack, G.A.; Tanzilli, R.A.; Pohl, R.O.; Vandersande, J.W. The Intrinsic Thermal Conductivity of AIN. J. Phys. Chem. Solids 1987, 48, 641–647. [CrossRef] Materials 2020, 13, 3387 9 of 10

2. Zhang, Z.; Gao, B.; Fang, Z.; Wang, X.; Tang, Y.; Sohn, J.; Wong, H.S.P.; Wong, S.S.; Lo, G.Q. All-Metal-Nitride RRAM Devices. IEEE Electron Device Lett. 2015, 36, 29–31. [CrossRef] 3. Xiong, C.; Pernice, W.H.P.; Sun, X.; Schuck, C.; Fong, K.Y.; Tang, H.X. Aluminum Nitride as a New Material for Chip-Scale Optomechanics and Nonlinear Optics. New J. Phys. 2012, 14, 095014. [CrossRef] 4. Kuo, P.K.; Auner, G.W.; Wu, Z.L. Microstructure and thermal conductivity of epitaxial AlN thin films. Thin Solid Films 1994, 253, 223–227. [CrossRef] 5. Park, M.-H.; Kim, S.-H. Thermal conductivity of AlN thin films deposited by RF magnetron sputtering. Mater. Sci. Semicond. Process. 2012, 15, 6–10. [CrossRef] 6. Xu, R.L.; Rojo, M.M.; Islam, S.M.; Sood, A.; Vareskic, B.; Katre, A.; Mingo, N.; Goodson, K.E.; Xing, H.G.; Jena, D.; et al. Thermal conductivity of crystalline AlN and the influence of atomic-scale defects. J. Appl. Phys. 2019, 126, 185105. [CrossRef] 7. Knoops, H.C.M.; Faraz, T.; Arts, K.; Kessels, W.M.M. Status and Prospects of Plasma-Assisted Atomic Layer Deposition. J. Vac. Sci. Technol. A 2019, 37, 030902. [CrossRef] 8. Puurunen, R.L.; Lindblad, M.; Rootc, A.; Krausea, A.O.I. Successive Reactions of Gaseous Trimethylaluminium and Ammonia on Porous Alumina. Phys. Chem. Chem. Phys. 2001, 3, 1093–1102. [CrossRef] 9. Huheey, J.E. Inorganic Chemistry, 3rd ed.; Harper and Row: New York, NY, USA, 1983; p. A30. 10. Alvarez, D.; Spiegelman, J.; Andachi, K.; Holmes, R.; Raynor, M.; Shimizu, H. Enabling Low Temperature Metal Nitride ALD Using Ultra-High Purity Hydrazine: ET/ID: Enabling Technologies and Innovative Devices. In Proceedings of the 2017 28th Annual SEMI Advanced Semiconductor Manufacturing Conference, Saratoga Springs, NY, USA, 15–18 May 2017; pp. 426–430. 11. Wolf, S.; Edmonds, M.; Sardashti, K.; Clemons, M.; Park, J.H.; Yoshida, N.; Dong, L.; Nemani, S.; Yieh, E.;

Holmes, R. Low-Temperature Amorphous Boron Nitride on Si0.7Ge0.3(001), Cu, and HOPG from Sequential Exposures of N2H4 and BCl3. Appl. Surf. Sci. 2018, 439, 689–696. [CrossRef] 12. Hwang, S.M.; Pena, L.F.; Kim, H.S.; Kondusamy, A.L.N.; Qin, Z.; Jung, Y.C.; Veyan, J.-F.; Alvarez, D.;

Spiegelman, J.; Kim, J. Vapor-phase Surface Cleaning of Electroplated Cu Films Using Anhydrous N2H4. ECS Trans. 2019, 92, 265–271. [CrossRef] 13. Abdulagatov, A.I.; Ramazanov, S.M.; Dallaev, R.S.; Murliev, E.K.; Palchaev, D.K.; Rabadanov, M.K.; Abdulagatov, I.M. Atomic Layer Deposition of Aluminum Nitride Using Tris(Diethylamido)Aluminum and Hydrazine or Ammonia. Russ. Microelectron. 2018, 47, 118–130. [CrossRef] 14. Abdulagatov, A.I.; Amashaev, R.R.; Ashurbekova, K.N.; Ashurbekova, K.N.; Rabadanov, M.K.; Abdulagatov, I.M. Atomic Layer Deposition of Aluminum Nitride and Oxynitride on Silicon Using Tris(dimethylamido)aluminum, Ammonia, and Water. Russ. J. Gen. Chem. 2018, 88, 1699–1706. [CrossRef] 15. Mayer, T.M.; Rogers, J.W.; Michalske, T.A. Mechanism of Nucleation and Atomic Layer Growth of AlN on Si. Chem. Mater. 1991, 3, 641–646. [CrossRef] 16. Liu, S.; Peng, M.; Hou, C.; He, Y.; Li, M.; Zheng, X. PEALD-Grown Crystalline AlN Films on Si(100) with Sharp Interface and Good Uniformity. Nanoscale Res. Lett. 2017, 12, 279. [CrossRef][PubMed] 17. Riihela, D.; Ritala, M.; Matero, R.; Leskela, M.; Jokinen, J.; Haussalo, P. Low Temperature Deposition of AlN Films by an Alternate Supply of Trimethyl Aluminum and Ammonia. Chem. Vap. Depos. 1996, 2, 277–283. [CrossRef] 18. Meng, X.; Kim, H.S.; Lucero, A.T.; Hwang, S.M.; Lee, J.S.; Byun, Y.C.; Kim, J.; Hwang, B.K.; Zhou, X.; Young, C. Hollow Cathode Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using Pentachlorodisilane. ACS Appl. Mater. Interfaces 2018, 10, 14116–14123. [CrossRef] 19. Meng, X.; Lee, J.; Ravichandran, A.; Byun, Y.C.; Lee, J.G.; Lucero, A.T.; Kim, S.J.; Ha, M.W.; Young, C.D.; Kim, J. Robust SiNx/GaN MIS-HEMTs With Crystalline Interfacial Layer Using Hollow Cathode PEALD. IEEE Electron Device Lett. 2018, 39, 1195–1198. [CrossRef] 20. Kim, H.S.; Meng, X.; Kim, S.J.; Lucero, A.T.; Cheng, L.; Byun, Y.-C.; Lee, J.S.; Hwang, S.M.; Kondusamy, A.L.N.; Wallace, R.M.; et al. Investigation of the Physical Properties of Plasma Enhanced Atomic Layer Deposited Silicon Nitride as Etch Stopper. ACS Appl. Mater. Interfaces 2018, 10, 44825–44833. [CrossRef] 21. Hwang, S.M.; Kondusamy, A.L.N.; Qin, Z.; Kim, H.S.; Meng, X.; Kim, J.; Hwang, B.K.; Zhou, X.; Telgenhoﬀ, M.; Young, J. Hollow Cathode Plasma (HCP) Enhanced Atomic Layer Deposition of Silicon Nitride (SiNx) Thin Films Using Pentachlorodisilane (PCDS). ECS Trans. 2019, 89, 63–69. [CrossRef] 22. Zazzera, L.A.; Moulder, J.F. XPS and SIMS Study of Anhydrous HF and UV/Ozone-Modiﬁed Silicon (100) Surfaces. J. Electrochem. Soc. 1989, 136, 484–491. [CrossRef] Materials 2020, 13, 3387 10 of 10

23. Moulder, J.F.; Stickle, W.F.; Sobol, P.E.; Bomben, K.D. Handbook of X-Ray Photoelectron Spectroscopy, 2nd ed.; Perkin-Elmer Corporation: Minnesota, MN, USA, 1992; pp. 252–253. 24. Paterson, W.G.; Onyszhuk, M. Co-ordination compounds of hydrazine Part 2.:The Interaction of Trimethylborane and Trimethylaluminum with Hydrazine. Can. J. Chem. 1961, 39, 2324–2329. [CrossRef] 25. George, S.M. Atomic layer deposition: An overview. Chem. Rev. 2010, 110, 111–131. [CrossRef][PubMed] 26. Sønsteby, H.H.; Yanguas-Gil, A.; Elam, J.W. Consistency and Reproducibility in Atomic Layer Deposition. J. Vac. Sci. Technol. A 2020, 38, 020804. [CrossRef] 27. Tian, L.; Ponton, S.; Benz, M.; Crisci, A.; Reboud, R.; Giusti, G.; Volpi, F.; Rapenne, L.; Vallee, C.; Pons, M.; et al. Aluminum Nitride Thin Films Deposited by Hydrogen Plasma Enhanced and Thermal Atomic Layer Deposition. Surf. Coat. Technol. 2018, 347, 181–190. [CrossRef] 28. Banerjee, S.; Aarnink, A.A.; Kruijs, R.; Kovalgin, A.Y.; Schmitz, J. PEALD AlN: Controlling growth and film crystallinity. Phys. Status Solidi C 2015, 12, 1036–1042. [CrossRef] 29. Kim, Y.; Kim, M.S.; Yun, H.J.; Ryu, S.Y.; Choi, B.J. Effect of Growth Temperature on AlN Thin Films Fabricated by Atomic Layer Deposition. Ceram. Int. 2018, 44, 17447–17452. [CrossRef] 30. Khoshman, J.M.; Kordesch, M.E. Spectroscopic ellipsometry characterization of amorphous aluminum nitride and indium nitride thin films. Phys. Status Solidi C 2005, 2, 2821–2827. [CrossRef] 31. Tzou, A.-J.; Chu, K.-H.; Lin, I.-F.; Østreng, E.; Fang, Y.-S.; Wu, X.-P.; Wu, B.-W.; Shen, C.-H.; Shieh, J.-M.; Yeh, W.-K.; et al. AlN Surface Passivation of GaN-Based High Electron Mobility Transistors by Plasma-Enhanced Atomic Layer Deposition. Nanoscale Res. Lett. 2017, 12, 315. [CrossRef] 32. Shih, H.-Y.; Lee, W.-H.; Kao, W.-C.; Chuang, Y.-C.; Lin, R.-M.; Lin, H.-C.; Shiojiri, M.; Chen, M.-J. Low-temperature Atomic Layer Epitaxy of AlN Ultrathin Films by Layer-by-Layer, in-situ Atomic Layer Annealing. Sci. Rep. 2018, 7, 39717. [CrossRef] 33. Brunner, D. Optical Constants of Epitaxial AlGaN Films and Their Temperature Dependence. J. Appl. Phys. 1997, 82, 5090–5096. [CrossRef]

34. Özgür, Ü.; Webb-Wood, G.; Everitt, H.O.; Yun, F.; Morkoç, H. Systematic Measurement of AlxGa1 xN − Refractive Indices. Appl. Phys. Lett. 2001, 79, 4103. [CrossRef] 35. Ozgit-Akgun, C.; Goldenberg, E.; Okyay, A.K.; Biyikli, N. Hollow Cathode Plasma-assisted Atomic Layer

Deposition of Crystalline AlN, GaN and AlxGa1 xN Thin Films at Low Temperatures. J. Mater. Chem. C − 2014, 2, 2123–2136. [CrossRef] 36. Rosenberger, L.; Baird, R.; McCullen, E.; Auner, G.; Shreve, G. XPS Analysis of Aluminum Nitride Films Deposited by Plasma Source Molecular Beam Epitaxy. Surf. Interface Anal. 2008, 40, 1254–1261. [CrossRef] 37. Zhang, J.; Zhang, Q.; Yang, H.; Wu, H.; Zhou, J.; Hu, L. Bipolar Resistive Switching Properties of AlN Films Deposited by Plasma-enhanced Atomic Layer Deposition. Appl. Surf. Sci. 2014, 315, 110–115. [CrossRef] 38. Greczynski, G.; Petrov, I.; Greene, J.E.; Hultman, L. Al Capping Layers for Nondestructive X-ray Photoelectron Spectroscopy Analyses of Transition-metal Nitride Thin Films. J. Vac. Sci. Technol. A 2015, 33, 05E101. [CrossRef] 39. Greczynski, G.; Primetzhofer, D.; Lu, J.; Hultman, L. Core-level Spectra and Binding Energies of Transition Metal Nitrides by Non-destructive X-ray Photoelectron Spectroscopy Through Capping Layers. Appl. Surf. Sci. 2017, 396, 347–358. [CrossRef] 40. Muneshwar, T.; Cadien, K. Comparing XPS on Bare and Capped ZrN ﬁlms Grown by Plasma Enhanced ALD: Eﬀect of Ambient Oxidation. Appl. Surf. Sci. 2018, 435, 367–376. [CrossRef] 41. Jokinen, J.; Haussalo, P.; Keinonen, J.; Ritala, M.; Riihela, D.; Leskela, M. Analysis of AlN Thin Films by Combining TOF-ERDA and NRB Techniques. Thin Solid Films 1996, 289, 159–165. [CrossRef] 42. Motamedi, P.; Cadien, K. XPS Analysis of AlN Thin Films Deposited by Plasma Enhanced Atomic Layer Deposition. Appl. Surf. Sci. 2014, 315, 104–109. [CrossRef] 43. Taborda, J.A.P.; Landazuri, H.R.; Londono, L.P.V. Correlation Between Optical, Morphological, and Compositional Properties of Aluminum Nitride Thin Films by Pulsed Laser Deposition. IEEE Sens. J. 2016, 16, 359–364. [CrossRef]

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