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Heteroleptic cyclopentadienyl-amidinate precursors for ALD of Y, Pr, Gd and Dy oxide thin films Sanni Seppälä, Jaakko Niinistö, Timothee Blanquart, Mikko Kaipio, Kenichiro Mizohata, Jyrki Räisänen, Clement Lansalot-Matras, Wontae Noh, Mikko Ritala, and Markku Leskelä Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01869 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 12, 2016

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Chemistry of Materials

Heteroleptic cyclopentadienyl-amidinate precursors for ALD of Y, Pr, Gd and Dy oxide thin films Sanni Seppäläa*, Jaakko Niinistöa, Timothee Blanquarta, Mikko Kaipioa, Kenichiro Mizohatab, Jyrki Räisänenb, Clement Lansalot-Matrasc, Wontae Nohc, Mikko Ritalaa, Markku Leskeläa Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland a

b

Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland

C

AirLiquide Laboratories Korea, Yonsei Engineering Research Park, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea

ABSTRACT: Thin films of rare earth (RE) oxides (Y2O3, PrOx, Gd2O3 and Dy2O3) were deposited by atomic layer deposition from liquid heteroleptic RE(iPrCp)2(iPr-amd) precursors with either water or ozone as the oxygen source. Film thickness, crystallinity, morphology and composition were studied. Saturation was achieved with Gd2O3 when O3 was used as the oxygen source at 225 oC and with Y2O3 with both oxygen sources at as high temperature as 350 oC. The growth rates were 0.90-1.3 Å/cycle for these processes. PrOx was challenging to deposit with both oxygen sources but with long, 20 s purges after the water pulses uniform films could be deposited. However, saturation was not achieved. With Dy 2O3 uniform films could be deposited and the Dy( iPrCp)2(iPramd)/O3 process was close to saturation at 300 oC. The different oxygen sources had an effect on the crystallinity and impurity contents of the films in all the studied processes. Whether ozone or water was better choice for oxygen source depended on the deposited metal oxide material.

INTRODUCTION Rare earth (RE) oxide thin films are versatile materials that have shown potential in many applications. Due to the large band gap (4-6 eV),1,2 high dielectric constants (12-27)2 and high band offsets to silicon3, they are considered as potential dielectric materials in microelectronics. Due to the hygroscopicity of the rare earth oxides, ternary compounds where RE oxides are combined for example with HfO2 , Al2O3 or ZrO2 are also studied.2,4,5 High mobility semiconductors Ge and GaAs are lacking stable native oxides that could reach to the same level as Si/SiO2 systems. Rare earth oxides have been studied in the passivation of these materials with good results.4,6,7 Rare earth doped rare earth oxides such as Eu-doped Y2O3 and Tb or Eu-doped Gd2O3 are well known phosphor materials that can be used for example in flat panel displays. Compared to powders, thin luminescent films have good adhesion to the substrate, uniform properties across the covered area and have no outgassing problems.8 Another area of interest for RE oxide thin films is solid oxide fuel cells where they have been used as electrolyte materials 9,10 and buffer layers11.

Atomic layer deposition (ALD) is a thin film deposition method based on alternating precursor pulses. The film growth in ALD is self-limiting and the film thickness can easily be controlled by the number of ALD cycles applied. With ALD, highly conformal, uniform over large substrates and pinhole free films can be produced which is an important feature in many thin film applications, especially in microelectronics.12 ALD is an attractive method also for doped films because of the controllability of the dopant concentration. ALD has been used to deposit RE (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) oxide thin films from a variety of precursors. For example, β-diketonate, 13 cyclopentadienyl14,15 and amidinate16 type precursors have been applied. However, there is still need for development of ALDtype processes with self-limiting characteristics and higher growth rates for many RE oxides. The most commonly applied precursors in the ALD of RE oxides is RE(thd)x (thd=2,2,6,6-tetramethyl-3,5-heptanedione) but ALD processes with thd-precursors have low growth rates and ozone is required as an oxygen source. For example, Y(thd) 3, Dy(thd)3

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and Gd(thd)3 with ozone result in growth rates of 0.23, 0.31 and 0.30 Å/cycle within the ALD window which is the temperature range of constant growth rate.13,17,18 The use of ozone can lead to too strong oxidation of the substrate, silicon in particular. Cyclopentadienyl- (Cp) and alkoxide-based precursors show higher growth rates and reactivity towards water but thermal stability of these precursors vary. Especially La and Pr Cp-precursors have low thermal stabilities and self-decomposition plays a significant role in the growth processes destroying the ALD-type self-limiting growth mode.19 Praseodymium oxide has been particularly difficult to deposit saturatively by ALD. Pr precursors that have been tested besides the Cp-type precursors include Pr(thd)3 with ozone20 and Pr[N(SiMe3)2]3 and Pr(mmp)3 (mmp = 1-methoxy2-methyl-2-propanol) with water.21,22 None of these processes are saturative. Besides Gd(thd)3, other Gd precursors used in ALD are guanidinate-based Gd(DPDMG)3 (DPDMG = N,Ndiisopropyl-2-dimethylamido-guanidinato),23 Gd(mmp)322 and precursors with cyclopentadienyl ligands 18,19. Recently, heteroleptic precursors have drawn increasing interest.24 With heteroleptic precursors the best properties of different types of ligands can possibly be combined in order to tailor precursors with high thermal stability and reactivity. The asymmetry of the heteroleptic molecules also helps the precursors to remain in liquid form, which impacts the reproducibility of the precursor evaporation rate. 25 This is important for reproducible precursor delivery and also allows the use of liquid injection methods, if desired. In this work, we report ALD of RE (Y, Pr, Gd and Dy) oxide films from liquid heteroleptic cyclopentadienyl-amidinate precursors, RE bis-isopropylcyclopentadienyl-diisopropylacetamidinate (RE(iPrCp)2(iPr-amd)) (Figure 1) with either water or ozone as the oxygen source. The main focus is on Pr and Gd precursors and Y and Dy precursors are examined for comparison. EXPERIMENTAL ALD process for RE oxide films. All films were deposited with an F-120 hot-wall flow-type ALD-reactor (ASM Microchemistry Ltd) using liquid RE(iPrCp)2(iPr-amd) precursors (Air Liquide) with water or ozone. The synthesis of the precursors was similar to the synthesis of the Y(iPrCp)2(iPr-amd) described earlier26. The thermogravimetric data of the precursors is shown in Figure S1. The pressure in the chamber was in the order of 5 mbar. Nitrogen (AGA 99.999% H 2O ≤ 3 ppm, O2 ≤ 3 ppm) was used as the carrier and purging gas. Precursors were evaporated inside the reactor at a temperature of 117 oC for Gd(iPrCp)2(iPr-amd), 125 oC for Pr(iPrCp)2(iPr-amd), and 135 oC for Y(iPrCp)2(iPr-amd) and Dy(iPrCp)2(iPr-amd). Ozone with concentration of 100 g/m 3

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was generated from O2 (AGA 99.999%) in an ozone generator (Wedeco Modular 4 HC). Si(100) wafers were used as substrates. Characterization of deposited films. Films were characterized for thickness, crystallinity, morphology, and composition. Film thicknesses were determined from reflectance spectra measured within wavelengths of 370-1100 nm with a Hitachi U-2000 spectrophotometer by modelling the spectra.27 Some films were annealed at 700 oC in air atmosphere for one hour. Crystallinity and phase of the films were determined by grazing incidence X-ray diffraction (GIXRD) with a PANalytical X´Pert Pro MPD X-ray diffractometer with Cu Kα (λ=1.5419 Å) radiation. Film morphologies were studied with scanning electron microscopy with Hitachi S-4800 field emission scanning electron microscope (FESEM). Film composition was measured with time-of-flight elastic recoil detection analysis (TOF-ERDA) at a detection angle of 40o by 40 MeV 79Br7+ ions obtained from a 5MV EGP-10-II tandem accelerator.

Figure 1. Chemical structure of the RE(iPrCp)2(iPr-amd) precursors. RESULTS AND DISCUSSION Gd2O3 films. In the case of Gd2O3, uniform films were deposited with both oxygen sources. Constant growth rate was observed at deposition temperatures between 275 and 325 oC when ozone was used as the oxygen source (Figure 1a). Above 325 oC growth rate increases probably due to precursor decomposition. Saturation was achieved at the deposition temperature of 200 oC with a growth rate of 0.4 Å/cycle and at 225 oC with a growth rate of 0.9 Å/cycle. At 250 oC slight increase in the growth rate was observed with long precursor pulse lengths (Figure 1b). With water, neither ALD window nor saturative characteristics were detected. However, the film thickness followed linearly the cycle number at 200 oC (Figure 1c). At 250 oC the dependence was not linear. The growth rate was 1.16 Å/cycle


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with low cycle numbers and increased to 1.38 Å/cycle when the films got thicker. This could be due to the changes in the crystallinity of the films: the phase of the films changes from mainly cubic Gd2O3 with 300 cycles to mainly monoclinic with 800 and 1500 cycles (Figure S2). Earlier it has been observed that the growth rate of ALD TiO2 thin films changes with film thickness as the structure of the film changes from amorphous to anatase.28 Saturation tests were done at 200 and 225 o C (Figure 1d). At 225 oC the growth rate rose from 1.0 Å/cycle with 1.0 s pulse to 1.4 Å/cycle with 2.0 s pulse and no saturation was achieved. At 200 oC the growth rate varied between 0.65 and 0.95 Å/cycle when the precursor pulses were 0.5-2.0 s with 1.5 s purge times after the water pulses. Due to the hygroscopicity of rare earth oxides, also very long, 20 s purge times were studied. With long purge time the growth rates were lower and the increase in growth rate with Gd precursor pulse time was reduced slightly. However, saturation was not achieved. The hygroscopicity of the RE oxides and its effect on the growth rates will be discussed in more detail in the following section on PrO x deposition.

GIXRD measurements showed that the films deposited at temperatures below 300 oC with ozone were amorphous, at 300 oC the phase was cubic Gd2O3 (ICDD-012-0797) and at 350 o C a mixture of cubic and monoclinic Gd2O3 (ICDD-042-1465) (Figure 2a). Annealing at 700 oC crystallized the amorphous film deposited at 200 oC to the cubic Gd2O3. With water, the films deposited at 200 oC were cubic Gd2O3 and at higher deposition temperatures mixtures of cubic and monoclinic Gd2O3 (Figure 2b). Cubic Gd2O3 phase has been reported previously for ALD films deposited with Gd(thd) 3, Gd(DPDMG)3 and Gd(MeCp)3.18,23 Gd2O3 can appear either in cubic or monoclinic form in normal conditions. 1

temperature and at 300 oC it was 3.3 at-% (Figure S3a). The carbon and hydrogen contents were reduced by annealing at 700 oC in air and for example the film deposited at 250 oC had 1.7 at-% of carbon after annealing. The Gd:O ratio of the films was very low, only 0.3 in a film deposited at 200 oC and 0.47 at 300 oC The stoichiometric value is 0.67. After annealing the film deposited at 250 oC had a ratio of 0.56. Because of the excess of oxygen, the impurities are most likely carbonate and hydroxyl species. At higher temperatures, during either the deposition or annealing, carbonate groups decompose and thus the carbon contamination decreases. High carbon content and excess oxygen were also reported in the Gd(thd)3/O3 process at low deposition temperatures.18 The films deposited with water as the oxygen source had significantly lower carbon content (around 1 at-%) than the films deposited with ozone. Hydrogen content was around 10 at-% in the films deposited at 200 and 225 oC and slightly lower, 6.5 at-% in the film deposited at 300 oC (Figure S4a). Because with water crystalline films were deposited already at 200 oC, the high carbon content of the films deposited with ozone may explain their amorphous phase at low deposition temperatures. A film deposited at 225 oC with water had the best Gd:O ratio of the as-deposited films, 0.64 (Table 1).

The morphology of the Gd2O3 films was studied with electron microscopy. The FESEM images (Figure 3) taken from the films deposited at 250 oC support the XRD data. There are no structural features visible on the smooth surface of the amorphous film deposited with ozone as the oxygen source whereas the surface of the crystalline film deposited with water as the oxygen source has elongated features with lengths of about 50 nm. Film compositions were measured with TOF-ERDA (Table 1). The Gd2O3 films deposited with ozone at low temperatures contained high amounts of hydrogen (9 at-% at 200 oC and 5 at-% at 250 oC) and carbon (15 at-% at 200 oC and 7 at-% at 250 o C). The carbon content decreased with increasing deposition



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Figure 2. a) Growth rates of Gd2O3 films at different temperatures deposited with Gd(iPrCp)2(iPr-amd) with either O3 or H2O. 1.0 s precursor pulses with 1.5 s purges, were used. b) Saturation of the Gd(iPrCp)2(iPr-amd)/O3 process at 200, 225 and 250 oC with 1.0 s O3 pulse and 1.5 s purge c) Gd2O3 film thickness as a function of the number of cycles in the Gd( iPrCp)2(iPr-amd)/H2O process at 200 and 250 oC. d) Growth rate of the Gd(iPrCp)2(iPr-amd)/H2O process with different Gd-precursor pulse lengths. Short (1.5 s) and long (20 s) purge after the water pulse at 200 oC and with the short purge at 225 oC were used.


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Figure 4. FESEM images of the Gd2O3 films deposited at 250 oC with a) ozone and b) water as the oxygen source.

Figure 3. GIXRD patterns of the films deposited with the a) Gd(iPrCp)2(iPr-amd)/O3 and b) Gd(iPrCp)2(iPr-amd)/H2O processes. Film thicknesses were between 60 and 84 nm.

PrOx films. Different from the Gd precursor, Pr(iPrCp)2(iPramd) and ozone deposited very non-uniform films and the thicknesses of the films were impossible to measure. Only a few films were deposited with this process. Strong thickness gradients have also been reported by Hansen et al. with the Pr(thd)3/O3 process.20 They explained that with the catalytic activity of praseodymium oxide due to the mixed oxidation states: ozone oxidizes Pr2O3 to PrO2 which oxidizes the next pulse of Pr precursor leading to higher growth rates and thickness gradients since the oxidation is strongest at the area closest to the precursor inlet and decreases gradually. With water the Pr2O3 films looked uniform and the growth rates were between 1.47-1.66 Å/cycle with 0.8 s Pr precursor and H2O pulses and 8.0 s purge time after the water pulse at deposition temperatures of 200, 250 and 300 oC (Figure 4a). Self-decomposition of the praseodymium precursor was observed at 250 oC. This was verified by supplying only the Pr-precursor for 2000 cycles without water pulses which resulted in a clearly visible film at the front edge of the substrate. According to Figure 4a, the decomposition rate is not very high however.



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Table 1. Elemental compositions of the Gd 2O3 films deposited at temperatures between 200 and 300 oC measured with TOF-ERDA. Oxygen source

Tdep (oC)

Gd at-%

O at-%

Gd:O ratio

H at-%

C at-%

H2O

200

33.7 ±0.7

55.3 ±1.1

0.61

9.8 ±0.8

1.1 ±0.6

H2O

225

34.7 ±0.4

54.6 ±0.7

0.64

9.4 ±0.6

1.1 ±0.1

H2O

300

32.0 ±0.3

59.8 ±1.3

0.53

7 ±2

1.7 ±0.2

O3

200

17.5 ±0.6

57.4 ±1.5

0.30

9 ±2

15.5 ±0.7

O3

225

23.2 ±0.6

61.5 ±1.4

0.38

4.8 ±0.7

10.5 ±0.6

O3

250

26.9 ±0.6

60.7 ±1.4

0.44

5.5 ±0.7

6.9 ±0.5

O3

300

27.9 ±0.3

59.8 ±1.5

0.47

9 ±3

3.3 ±0.4

Praseodymium oxide is a hygroscopic material and can trap water. Trapped water can then desorb during the purging period and the following Pr precursor pulse to react with Pr(iPrCp)2(iPr-amd).29 In order to study the effect of the purge time after the water pulse on the growth rate, the purge time was varied between 2.0 and 20 s at 200 oC. The pulse times for the Pr precursor and water were 0.8 s. With 2.0 s purge time after the water pulse the growth rate of the film was 2.0 Å/cycle. When the purge time was increased to 10 s the growth rate dropped to 1.05 Å/cycle and with 20 s purge the growth rate was very close to the one obtained with 10 s purge. The effect of the purge time on the growth rate was considerably larger in the case of Pr2O3 as compared to the Gd2O3. This can be explained by the direct correlation between the hygroscopicity and electronegativity of the rare earth oxides: the lower the electronegativity, the stronger the hygroscopicity of the rare earth element.30 Pr has lower electronegativity than Gd and hence stronger hygroscopicity. Saturation test was carried out at 200 oC and in order to minimize the possible excess water remaining in the chamber after the pulse, 20 s purge time for water was used. Saturation was not completely achieved even with this long purge time (Figure 4b) but the films were uniform in thickness. Although the films deposited with ozone were nonuniform, GIXRD was measured from the few samples deposited. The films were crystalline already at a deposition temperature of 200 oC which is 100 oC less than that needed for crystalline films in the Gd(iPrCp)2(iPr-amd)/O3 process.

Figure 5. a) Pr2O3 growth rates at 200-300 oC for Pr(iPrCp)2(iPramd)/H2O with 0.8 s pulses, 4.0 s purge after the Pr precursor pulse and 8.0 s purge after the H2O pulse. b) Pr2O3 growth rate vs. Pr precursor pulse length at a deposition temperature of 200 oC with 0.8 s water pulse and 20 s purge.


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The peaks were broad which made it hard to determine the phase. It could be cubic Pr6O11 (ICDD-042-1121) or cubic PrO2 (ICDD-024-1006) (Figure 5a). Ozone is strong enough oxidizer to oxidize the Pr3+ in the precursor to Pr4+. In another ALD process where ozone was used, Pr(thd) 3/O3, similar results were obtained as a mixture of Pr6O11 and PrO2 was reported at a deposition temperature of 300 oC.20 Films deposited with water as the oxygen source were cubic Pr2O3 (ICSD-647290) or a mixture of cubic and hexagonal Pr2O3 (ICSD-647302) depending on the deposition temperature (Figure 5b). The cubic phase was dominant at all temperatures. At 200 oC the strongest reflection of the cubic phase was (400) while at higher temperatures the strongest was (222). The phase content seems to be sensitive to the experimental parameters since the phase ratio and peak intensities changed also with the length of the purge time. Previously, amorphous films have been reported with the Pr[N(SiMe3)2]3/H2O process in the deposition temperature range of 200-400 oC21 and poorly crystallized films with the Pr(mmp)3/H2O process at 225 oC showing two peaks that could be assigned to Pr2O3.22 Although the films deposited with water as the oxygen source were crystalline, the structural features were so small that with SEM it was hard to see any from the surface of the films (Figure 6). There seems to be some roundish features which are very different compared to the Gd2O3 film deposited with water as the oxygen source (Figure 3b). TOF-ERDA was performed only on samples deposited with the Pr(iPrCp)2(iPr-amd)/H2O process (Table 2). The results showed the Pr to O ratio to be around 0.6 in the films deposited at 200 and 225 oC. The main impurities in the films were hydrogen and carbon. Also very low amounts of nitrogen (max 0.24 ± 0.10 at-%) were detected. The carbon content was around 4 at-% in all the studied samples but the hydrogen content varied more. The film deposited at 300 oC had almost 25 at-% of hydrogen and the Pr:O ratio was only 0.40. The amount of hydrogen was the highest on the surface and decreased clearly deeper in the film which may be due to a reaction with moisture after the deposition. In the middle of the film the hydrogen concentration was around 10 at-% (Figure S4b). Similar Pr:O ratios and low carbon levels have been reported for the Pr(mmp)3/H2O process but the hydrogen content was not studied.22

Figure 6. GIXRD patterns of the a) cubic PrOx films deposited with the Pr(iPrCp)2(iPr-amd)/O3 process and b) Pr2O3 films deposited with Pr(iPrCp)2(iPr-amd)/H2O process, film thicknesses 50140 nm. h=hexagonal and c=cubic phase.

Figure 7. FESEM image taken from a film deposited at 250 oC with Pr(iPrCp)2(iPr-amd)/H2O process.



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Table 2. Elemental compositions of the PrOx films deposited at 200, 225 and 300 oC with H2O as oxygen source. Oxygen source

Tdep (oC)

Pr at-%

O at-%

Pr:O ratio

H at-%

C at-%

H2O

200

32.1 ±0.5

53.7 ±1.0

0.60

10.3 ±1.2

3.9 ±0.3

H2O

225

32.4 ±0.6

55.0 ±1.0

0.59

7.9 ±1.1

4.5 ±0.3

H2O

300

20.1 ±0.2

50.6 ±1.4

0.40

25 ±4

4.5 ±0.4

Y2O3 films. Y2O3 thin films were deposited at temperatures between 200 and 350 oC. Pulse length of 0.7 s was used for the Y precursor and 1.0 s for water and ozone. Purge times were 1.2 s after the Y(iPrCp)2(iPr-amd) pulse and 1.5 s after the water and ozone pulse. Temperature range with fully constant growth rate was not found for either process (Figure 7a). The rapid increase of the growth rate in the ozone process while increasing the temperature from 250 to 275 oC may be related to a change from amorphous to crystalline film structure. The dependence of the growth rate on the Y precursor pulse length was tested at 350 oC. H2O and O3 pulses were fixed to 1.0 s. With water the process was self-limiting with a growth rate around 1.3 Å/cycle when the Y precursor pulse length was 0.7 s and above. With ozone the saturation was achieved when the pulse length was 1.3 s and longer (Figure 7b). Saturation at this high deposition temperature has previously been reported with Y(thd)3 but the growth rate was considerably lower, 0.23 Å/cycle.17 Y(CpCH3)3 and YCp3 with water as the oxygen source resulted in the saturation at 300 oC with growth rates of 1.3 and 1.62 Å/cycle.14 Y(iPr2amd)3 and water resulted in a growth rate of 0.8 Å/cycle at 280 oC.16

ferred orientation has been reported to be (111) in the films deposited with the YCp3/H2O process while the Y(thd)3/O3 and Y(CpCH3)3/H2O processes had preferred (100) orientation with (400) as the strongest reflection at 200-350 oC.11,14 In the Y(iPr2amd)3/H2O process the (400) and (222) reflections were almost equally strong in the film deposited at 280 oC.13

The Y(iPrCp)2(iPr-amd) precursor has been studied previously at low temperatures with H2O by Lee et al.25 They reported saturation at 180-200 oC with a growth rate of 0.4 Å/cycle. Also Park et al. studied the Y(iPrCp)2(iPr-amd) precursor with H2O at temperatures between 250 and 450 oC.26 Their results are quite different compared to our results with the same precursor. They reported saturation at 350 oC with a growth rate of 0.6 Å/cycle which is half of the growth rate we report here. The difference might be due to the very short H2O pulse, 0.2 s that Park et al. used in their work. The films deposited with O3 were amorphous at a deposition temperature of 250 oC and below while at 275 and 300 oC the films were weakly crystalline and above 300 oC the phase of the films was cubic Y2O3 (Figure 8a). With water, films were crystalline already at the deposition temperature of 200 oC (Figure 8b). The phase of the films was cubic Y2O3 (ICDD-0411105). At 350 oC a couple of small peaks from monoclinic Y 2O3 (ICDD-047-1274) were also visible. The strongest reflection of the cubic phase was (222) in both processes. Previously, pre-

Figure 8. a) Y2O3 growth rates at different temperatures using Y(iPrCp)2(iPr-amd) with either H2O or O3 as oxygen source. b) Saturation of the processes at 350 oC. H2O and O3 pulse lengths were 1.0 s and purges 1.5 s.


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Figure 10. FESEM images of the Y2O3 films deposited at 350 oC with a) ozone and b) water as the oxygen source.

Figure 9. GIXRD patterns of the films deposited with a) Y(iPrCp)2(iPr-amd)/O3 and b) Y(iPrCp)2(iPr-amd)/H2O process.

FESEM images were taken from the films deposited at 350 C with the Y(iPrCp)2(iPr-amd)/O3 and Y(iPrCp)2(iPr-amd)/H2O processes (Figure 9). Both films were crystalline according to XRD and some small features could be seen in the images. Similar to the Pr2O3 films, the surface features were small and roundish and hard to image. o

Composition was analysed from the films deposited at 300 C with either water or ozone as the oxygen source (Table 3). According to TOF-ERDA, there were hydrogen impurities and excess oxygen mainly on the surface of the films whereas carbon was distributed throughout the films (Figure S3b and S4c). The carbon content of the films was 5.6±0.5 at-% when ozone was used as the oxygen source and 3.7±0.4 at-% when water was used. The carbon contents are high compared to the Y(CpCH3)3 and YCp3 precursors with water (less than 1 at%). The hydrogen content was higher with water (13.5±3 at%) than with ozone (5.3±1.8 at-%). o

Dy2O3 films. Dy(iPrCp)2(iPr-amd) was studied with either O3 or H2O as the oxygen source at deposition temperatures between 200 and 350 oC. With water, ALD window was observed between 300 and 350 oC (Figure 10a). With ozone, there was some scattering in the growth rates at different temperatures and no clear ALD window was observed. Saturation was tested at a deposition temperature of 300 oC with water and at 300 oC and 325 oC with ozone. The water process had growth rates around 1.3 Å/cycle with 0.7 and 1.3 s dysprosium precursor pulse lengths but with longer pulse length the growth rate was already 1.9 Å/cycle. With ozone the growth rates with different pulse lengths at the deposition temperature of 300 oC were closer to each other, around 1.0 Å/cycle with 0.7 and 1.3 s pulse lengths and 1.2 Å/cycle with 1.9 s pulses. At the deposition temperature of 325 oC the growth rate increased with the Dy(iPrCp)2(iPr-amd) pulse length (Figure 10b). There are not many reports on Dy precursors used in ALD. Dy(DPDMG)3 has been used with H 2O and Dy(thd)3 with O3.23,13 In the case of Dy(DPDMG)3, ALD window was observed at 200-275 oC with a growth rate of 1.0 Å/cycle. Saturation was confirmed at 250 oC. Dy(thd)3 was tested only at 300 oC. Growth rate was around 0.3 Å/cycle but saturation was not studied. Previously, the Dy(iPrCp)2(iPramd) precursor has been reported to react with O2 plasm



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Table 3. Elemental compositions of the Y2O3 films deposited at 300 oC. Oxygen source

Tdep (oC)

Y at-%

O at-%

Y:O ratio

H at-%

C at-%

H2O

300

29.7 ±0.5

53 ±2

0.56

14 ±3

3.7 ±0.4

O3

300

28.9 ±0.5

60 ±2

0.48

5 ±2

5.6 ±0.5

but not with water.31 The saturated growth rate with O2 plasma was 0.3 Å/cycle at 180 oC. Unfortunately, the temperature where the Dy(iPrCp)2(iPr-amd)/H2O process was attempted was not mentioned. Similar to the Y2O3 process, the films deposited with ozone were amorphous up to a deposition temperature of 250 oC and weakly crystalline at 275 and 300 oC. Films deposited at 325 and 350 oC were cubic Dy2O3 (ICDD-022-0612) (Figure 11a).

Figure 11. a) Dy2O3 growth rates at different temperatures and b) saturation tests at 300 and 325 oC using Dy(iPrCp)2(iPr-amd) with either H2O or O3 as oxygen source. Pulse lengths for H2O and O3 were 1.0 s and purges 1.5 s.

As with the other precursors, crystalline films were deposited already at 200 oC when water was used as the oxygen source (Figure 11b). Cubic Dy2O3 was the main phase but at 300 and 350 oC small additional peaks were observed. These were attributed to the monoclinic Dy2O3. In both processes, the preferred orientation of the cubic phase was in the (111) direction with (222) as the strongest reflection. The cubic Dy2O3 phase was also reported in the Dy(DPDMG)3/H2O process.23 In the bulk form, it is the most stable phase at low temperatures.1

Figure 12. a) GIXRD patterns of the films deposited with a) Dy(iPrCp)2(iPr-amd)/O3 and b) Dy(iPrCp)2(iPr-amd)/H2O process. The main phase of the films was cubic Dy2O3.


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FESEM images were taken from films deposited at 300 oC (Figure 12). The amorphous film deposited with ozone as the oxygen source does not show any structural features (Figure 12a). The film deposited with the Dy(iPrCp)2(iPr-amd)/H2O process was crystalline according to the XRD measurement and this can be verified from the FESEM image (Figure 12b). The surface features are approximately of the same size as in the Gd2O3 film and much larger than in the Y 2O3 and Pr2O3 films.

deposited with the two processes are about the same, the films deposited with ozone have lower crystallinity than the films deposited with water. Therefore, it is proposed that the carbon in the films is in different forms. With water there are most likely unreacted ligands but with the strong oxidizer like ozone carbonates are formed. Previously, Nieminen et al. have shown in the case of lanthanum oxide ALD that with ozone at low temperatures amorphous La2O2CO3 was formed instead of La2O3.32 CONCLUSIONS

Figure 13. FESEM images of a) amorphous Dy2O3 film deposited with ozone as the oxygen source and b) crystalline film deposited with water as the oxygen source at 300 oC.

In this work, we studied ALD of RE oxides using heteroleptic RE(iPrCp)2(iPr-amd) precursors for Y, Pr, Gd and Dy with either H2O or O3 as the oxygen source. The thermal stability of the precursors seems to decrease in the order Y>Dy>Gd>Pr, i.e. with increasing ionic radius. In general, the growth rates were higher with water than with ozone except in the deposition of PrOx. With water, crystalline films were deposited at considerably lower temperatures than with ozone due to the carbonate residues in the films deposited with ozone at low temperatures. Again, PrOx depositions made an exception in the crystallization temperature. Self-limiting ALD-type growth was confirmed in the Gd2O3 process with O3 as an oxygen source and in the Y 2O3 processes with both oxygen sources. The hygroscopicity of the rare earth oxide films was observed to clearly affect the depositions in the Pr(iPrCp)2(iPr-amd)/H2O process as the water uptake during the water pulses could not be fully eliminated even with 20 s purge time. Although the PrOx process was not completely self-limiting, the results with Pr(iPrCp)2(iPr-amd)/H2O show that this is the best ALD process reported for praseodymium oxide. In the case of Dy2O3, saturation was almost achieved at 300 oC and the films deposited with the Dy(iPrCp)2(iPramd)/O3 process were reasonably pure. There are not many ALD processes for Dy2O3 and saturation has only been achieved at 250 oC, and thus this new process is an attractive option for the deposition of Dy2O3 above 250 oC.

The results from the compositional analysis on the Dy 2O3 films showed low amounts of hydrogen (1.2±1 at-%) and carbon (1.0±0.2 at-%) in the film deposited at 300 oC with ozone as the reactant (Figure S3c)whereas the film deposited with water at the same temperature had over 12 at-% of hydrogen and 2.8 at-% of carbon (Table 4). The hydrogen concentration was higher on the surface, however, and decreased deeper in the film (Figure S4d). In the Dy(DPDMG)3/H2O process hydrogen content was not studied and carbon content around 2 at-% was detected.23 It is interesting to note that even if the carbon contents of the films



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Table 4. Elemental compositions of the Dy2O3 films deposited at 300 oC. Oxygen source

Tdep (oC)

Dy at-%

O at-%

Dy:O ratio

H at-%

C at-%

H2O

300

28.6 ±0.2

56.3 ±1.4

0.51

12 ±3

2.7 ±0.3

O3

300

28.0 ±0.3

67 ±2

0.40

1.2 ±1.0

1.0 ±0.2

ASSOCIATED CONTENT Supporting Information Thermogravimetric data of the precursors, XRD data of the Gd2O3 films deposited at 200 and 250 oC with water with different cycle numbers, TOF-ERDA depth profiles of the films deposited at 300 oC.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources This work was funded by the Finnish Centre of Excellence in Atomic Layer Deposition (Academy of Finland)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Funding from the Finnish Centre of Excellence in Atomic Layer Deposition (Academy of Finland) is gratefully acknowledged. Mr. Mikko Heikkilä is thanked for the help with the XRD measurements.

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