Properties of Al₂O₃ Thin Films Grown by PE-ALD at Low Temperature Using H₂O and O₂ Plasma Oxidants

Abstract

Al₂O₃ layers with thicknesses in the 25–120 nm range were deposited by plasma enhanced atomic layer deposition at 70 °C. Trimethylaluminum was used as organometallic precursor, O₂ and H₂O as oxidant agents and Ar as a purge gas. The deposition cycle consisted of 50 ms TMA pulse/10 s purge time/6 s of plasma oxidation at 200 W/10 s purge time. The optical constants and thicknesses of the grown layers were determined by spectroscopic ellipsometry, while the roughness was measured by atomic force microscopy, giving RMS values in the 0.29–0.32 nm range for films deposited under different conditions and having different thicknesses. High transmittance, ~90%, was measured by UV–Vis spectroscopy. X-ray photoelectron spectroscopy revealed that, with both types of oxidants, the obtained films are close to stoichiometric composition and, with high purity, no carbon was detected. Electrical characterization showed good insulating properties of both types of films, though the H₂O oxidant leads to better I-V characteristics.

1. Introduction

Aluminum oxide (Al₂O₃) is a promising material for various optoelectronic applications due to its optical, chemical and electrical properties. It has been used as gate dielectric in transparent thin film transistors [1,2], diffusion barrier for gases [3], for encapsulation of photovoltaic devices [4], surface coverage of electrodes and photoelectrodes for electrochemical energy storage systems [5] and passivation against corrosion [6]. Moreover, α-Al₂O₃ has been studied as a radiation resistant material, important for application in components and as coatings in fusion technology [7]. Al₂O₃ thin films have been deposited by sputtering [8], pulsed laser deposition (PLD) [9], e-beam evaporation [10], sol-gel [11] and atomic layer deposition (ALD) [12,13,14]. ALD offers several advantages compared to the other techniques, such as precise thickness control, high uniformity and conformality of the deposited films over large areas, and low roughness [15]. Thin Al₂O₃ films with very good dielectric properties have been prepared at low temperatures, ≤100 °C, by the ALD technique [16,17,18,19,20,21]. However, when operating in this temperature range, the technique has some disadvantages, such as low growth rates per cycle (GPC), high consumption of organometallic precursor and long deposition times. Recently, plasma enhanced atomic layer deposition (PE-ALD) has been proposed in order to overcome the limitations and extend the capabilities of the conventional ALD processes. An advantage of the PE-ALD technique is that plasma excitation during the reactant exposure step creates reactive species, such as electrons, ions and radicals, which determine new chemical reactions that can be controlled by the plasma process parameters. In this way, a greater flexibility and control of the properties of the deposited films may be achieved [15].

The conventional oxidant source for thermal ALD growth of Al₂O₃ is water. However, for preparation of PE-ALD Al₂O_3, oxygen has also been used as the oxidant source. In this work, we present results for the electrical, optical and morphological properties of Al₂O₃ thin films grown at low temperatures by PE-ALD using two types of oxidizing plasma, H₂O and O₂. The deposited films are intended to be used as gate dielectrics in thin film transistors fabricated on flexible substrates. High-quality Al₂O₃ films deposited at low temperatures with high GPC are of great importance for the new era of flexible electronics. The obtained results are compared with published data for Al₂O₃ films deposited by thermal ALD.

2. Materials and Methods

2.1. Thin Film Deposition

Aluminum oxide thin films were deposited on p-Si wafers (100), quartz and ITO/glass substrates in a Beneq TFS 200 reactor with a capacitively coupled plasma unit having a 13.56 MHz r.f. power source. The depositions were carried out in direct plasma mode under the following conditions: temperature of 70 °C, power of 200 W, base pressure of 0.014 mbar and working pressure of 1.54 mbar. The deposition temperature was measured by a thermocouple positioned at the sample holder and was monitored during the entire deposition process. The growth initiated when the substrate reached thermal equilibrium with the sample holder. Two types of oxidants were used, H₂O and O₂. The deposition parameters were optimized to achieve saturation in each reaction step. An optimized ALD cycle was found to consist of 50 ms TMA/10 s Ar purge/6 s O₂ or H₂O plasma/10 s Ar purge. With this recipe, saturation was achieved for minimum processing time. Films with thicknesses in the 50–110 nm range were deposited using the optimized recipe on silicon and quartz substrates (

Table 1. Al₂O₃ thin films with various thicknesses deposited on Si and quartz.

Sample	Cycles	GPC (Å/Cycle)	Thickness (nm)
H2O Plasma	300	~1.7	50
	470	~1.7	80
	650	~1.7	110
O2 Plasma	300	~1.6	50
	470	~1.6	75
	680	~1.6	100

) for determination of GPC.

2.2. Thin Film Characterization

UV–Vis transmittance spectra of films deposited on quartz were measured in the 400–800 nm range using Shimadzu UV-2600 (Kyoto, Japan). The thickness and optical constants of Al₂O₃ layers deposited on silicon were determined by ellipsometric measurements in the 240–1000 nm range using the J.A. Woollam M-2000U instrument (Lincoln, NE, USA). Excellent fit of the ellipsometric data was achieved using the Cauchy dispersion equation assuming zero extinction coefficient. The chemical composition, elemental concentration and oxygen binding states were determined by X-ray photoelectron spectroscopy (XPS) using the Escalab 250 Xi Thermofisher instrument (Waltham, MA, USA)with a monochromatic Al Kα X-ray source (1486.68 eV). Survey scans were acquired using an analyzer pass energy of 200 eV with a step of 1 eV. For high-resolution scans, a pass energy of 20 eV and a resolution of 0.1 eV were used. Spectra acquisition was carried out on Al₂O₃ films before and after sputtering the surface with Ar ions for 10 s to remove environmental carbon from the surface. Atomic force microscopy (AFM) was used to analyze the surface morphology of the deposited films (MFP3D-SA Asylum Research instrument) (Santa Barbara, CA, USA). Electrical characterization was carried out using Keithley 4200 Semiconductor Characterization System (Cleveland, OH, USA). Two types of samples were electrically characterized on Si and ITO/glass substrates. Layers with thickness of ~100 nm were grown on (100) p-Si with resistivity of 1.5–2.5 Ω cm using both oxidants. Au top contacts with thickness of ~500 nm and area of 100 × 100 μm² were thermally evaporated through a mask. An Al layer with the same thickness was used as a back contact. Al₂O₃ layers with thicknesses of ~25 nm were deposited on ITO/glass substrates using H₂O as an oxidant. Sputtered Al, ~200 nm thick, was used for top contact. Capacitors with areas of 25 × 25, 50 × 50 and 100 × 100 μm² were defined by photolithography.

3. Results and Discussion

3.1. Thin Films Growth

The GPC determined from depositions consisting of 100 cycles were ~1.6 Å/cycle for H₂O and O₂ plasma. Figure 1 shows the GPC saturation curves as a function of the TMA pulse time for H₂O and O₂ oxidants. However, as it is shown in Table 1, for layers deposited with H₂O plasma having thickness ≥50 nm, the GPC increased to 1.7 Å/cycle.

The results in Figure 1 and Table 1 show that a very competitive GPC at 70 °C was achieved considering that 1.57 Å/cycle is the highest value that has been reported for PE-ALD [22], even at the higher temperature of 90 °C. The obtained result is a good indicator for the precursor dosing efficiency, taking into account that the dosing time here is four times shorter than that in the work of Zhu et al. [22] and is comparable with times used in high temperature, >200 °C, depositions [23,24]. At low temperatures, <100 °C [22,25,26], the use of larger precursor dosing is common. Thus, the proposed process has the advantage to drastically reduce the waste of TMA. Furthermore, the obtained growth rate allows a higher throughput in fabrication of flexible devices that require low-temperature processing.

3.2. Optical Properties

Figure 2 shows transmittance spectra of ~50 nm thick Al₂O₃ films deposited on quartz. The transparency of both oxides was high, ~90%, which are typical results for Al₂O₃ thin films [9,20,27]. However, the transmittance of the layer deposited with O₂ plasma was slightly higher than that of the H₂O plasma layer.

Figure 3 shows the refractive index (n) dependence on wavelength (λ) determined from ellipsometric measurements of films deposited with H₂O and O₂ plasmas. The Cauchy dispersion equation was used to fit the experimental data for the ellipsometric parameters Psi (Ψ) and Delta (Δ). The fit quality was estimated by the value of mean square error (MSE). Dependencies n vs. λ close to each other were obtained for both types of oxidants with values of n slightly higher for films prepared with H₂O plasma. The values of n at 632.8 nm and the determined layer thicknesses are presented in

Table 2. Refractive index values at λ = 632.8 nm.

Sample	Thickness (nm)	MSE	n (at λ = 632.8 nm)
H2O-Plasma ~50 nm H2O-Plasma ~75 nm H2O-Plasma ~100 nm O2-Plasma ~50 nm O2-Plasma ~75 nm O2-Plasma ~100 nm	59.8 89.7 120.6 52.0 81.5 112.4	4.9 3.7 3.9 3.2 3.4 4.1	1.63 1.63 1.64 1.63 1.63 1.63

. The obtained results are in agreement with those reported in the literature (

Table 3. Refractive index values reported in the literature.

Techniques	Thickness (nm)	n	Year/Ref
ALD	200–400	1.64	2009/[12]
ALD	210–380	1.64	2010/[27]
PE-ALD	10–50	1.62–1.66	2011/[28]
HIPIMS	240–1030	1.59–1.77	2012/[29]
Pulsed sputtering	1140–1960	1.65–1.66
Annealing of Al	300–350	1.66–1.67
ALD	50	1.64	2013/[26]
ALD	50–100	1.65	2016/[13]
PE-ALD	50–100	1.63–1.64	This work

) [12,13,26,27,28,29].

3.3. Compositional Analysis

The chemical composition of both types of oxides was determined by XPS spectroscopy. Only one contribution was observed in the Al 2p spectrum of both oxides, while the O 1s spectra were deconvoluted into two contributions using a Voigt peak shape and a Shirley–Sherwood background (Figure 4). The contribution with greater area and lower energy is positioned at 531.18 eV and 531.11 eV for H₂O and O₂ plasma oxides, respectively. This peak is associated with the Al-O bond [19,26,30]. The contribution at higher energy, 532.70 eV for H₂O and 532.55 eV for O₂ plasma oxides, is attributed to the Al-OH bond [19,26,30]. The relative atomic contents were evaluated using the peak areas and the instrument sensitivity factors, 0.56 for Al 2p and 2.88 for O 1s. The ratio between total O and Al amount, in Al₂O₃ and Al (OH)₃, and Al (Total O/Al) was found to be 1.48 and 1.50 for H₂O and O₂ oxidants, respectively (

Table 4. Concentration ratios of Al₂O₃ films obtained with the two types of plasma oxidants.

Sample	Total O/Al	OAl-O/AlAl-O	OH/Al
H2O plasma O2 plasma	1.48 1.50	1.39 1.36	0.09 0.14

). However, the O/Al ratio corresponding to O in Al₂O₃ was calculated to be 1.39 and 1.36 for H₂O and O₂ plasma, indicating slightly off-stoichiometric oxides for both oxidants. Furthermore, the number of OH groups in the oxide grown with H₂O plasma was smaller than that in the O₂ plasma oxide (Table 4). This result is in contrast with what would be expected since the number of hydroxyl radicals in H₂O plasma is higher. A greater reactivity of the H₂O plasma compared to O₂ plasma could be a possible explanation, in agreement with the results in [26]. Haeberle et al. [26] reported that, at temperatures below 80 °C, oxygen radicals are less effective in oxidizing organometallic precursors of aluminum in PE-ALD processes. The XPS results revealed high purity of the obtained Al₂O₃ films, and no carbon was detected.

XPS was also used to evaluate the oxide band gaps. The method is based on energy loss of high-energy electrons due to excitation of electrons from the valence band maximum to the conduction band minimum [31,32]. The O1s electrons were used as high-energy electrons for obtaining the band gap of the samples [33,34], since the onset of the scattered electrons does not overlap with the core level peaks due to the wide band gap of Al₂O₃. From Figure 5 values of 6.64 and 6.58 eV were determined for the oxides obtained with H₂O and O₂ plasma, close to those in [31].

3.4. Morphological Properties

Figure 6 shows 3-D AFM images of films deposited with both oxidants. The root mean square (RMS) values of the roughness of H₂O and O₂ plasma oxides were determined to be ~0.32 nm and ~0.29 nm, respectively, indicating that surface morphology of both oxides was similar. AFM results show that the deposition conditions used did not cause pulverization, sputtering or damage of the film during its growth.

3.5. Electrical Characteristics

Figure 7a shows the absolute value of current density vs. applied voltage (│J│-V) characteristics of capacitors with 100 nm oxides deposited on Si substrates with O₂ and H₂O plasmas. The asymmetry in the │J│-V dependences in Figure 7a is due to differences in the conduction (ΔE_C) and valence (ΔE_V) band offsets between Si and Al₂O₃, ΔE_C ~ 2.1–2.44 eV and ΔE_V ~ 2.95–3.24 Ev [35,36]. For example, the current flow through the O₂ plasma oxide started at electric fields of ~2 and −4 MV/cm at positive and negative bias voltages, while, for oxide obtained with H₂O plasma, these values were much higher, ~5 and −7.5 MV/cm. The current at lower electric fields (before the beginning of current flow) was ~15 pA (│J│ ~ 10⁻⁷ A/cm²), indicating good insulating properties of the Al₂O₃ films. The breakdown electric fields for oxygen and water plasma oxides were found to be ~3 and 6 MV/cm at positive bias voltages and approximately −5.8 and −9 MV/cm at negative bias voltages, respectively. Since the current through the films deposited with H₂O plasma starts at higher electric fields, i.e., the films are better insulators, this oxidant was selected for preparation of capacitors with 25 nm thick Al₂O₃ on ITO/glass substrates. Similar J–V characteristics as those for oxides deposited on Si wafer with H₂O plasma at V > 0 were measured (Figure 7b). The current flow through the Al₂O₃ started at ~4 MV/cm, independent of the capacitor area, while values between 4.8 and 5 MV/cm were obtained for the breakdown field. The slightly higher breakdown electric field, as well as the field at which conduction through oxide begins, in the case of capacitor on Si wafer, is most likely due to native SiO₂, formed by oxidation of the Si substrate in air before the Al₂O₃ deposition. The voltage drop on SiO₂ was not taken into account when the breakdown voltage was estimated. The dielectric constant (k) of oxides grown with H₂O plasma on ITO/glass substrate was determined by high-frequency capacitance–voltage (HF C–V) measurements at 100 kHz. Values for k between 6.3 and 7.6 were obtained (

Table 5. Dielectric constant of oxides grown with H₂O plasma on ITO/glass substrate determined from C–V measurements.

Capacitor Area	Dielectric Thickness	Capacitance (pF)	Dielectric Constant (κ)
25 × 25 μm2 50 × 50 μm2 100 × 100 μm2	25 nm	1.69 5.77 22.3	7.6 6.5 6.3

). It should be noted that the value for k determined from the 25 × 25 µm² capacitor is higher than the values obtained from the larger area capacitors, which is most likely because of some error in the measurement of its very small capacitance, ~1.7 pF.

The results for Al₂O₃ films grown with H₂O oxidant obtained in this work are compared with previously reported data in

Table 6. Comparison of this work with previously reported Al₂O₃ thin films.

Tech.	T (°C)	Oxidant Agent	TMA Time (s)	GPC (Å/Cycle)	Growth Rate Per Second (Å/s)	Dielectric Thickness (nm)	Dielectric Constant (κ)	Breakdown Electric Field (MV/cm)	Year/Ref.
PEALD	300	O2	0.025	~0.94	~5.2 × 10−2	~37.5	8.6	-	2021 [23]
PEALD-ALD	90	O2/H2O	0.2	0.84–1.28	-	-	-	-	2020 [25]
PEALD	250	O2	0.25	~1.37	~6.8 × 10−2	~40	9 ± 1	-	2020 [39]
ALD	200–300	H2O	0.1	-	-	3.5–4.2	6.2–6.9	4.7–6.5	2020 [37]
ALD	200–300	H2O	0.1	-	-	3.5–4.2	6.2–6.9	-	2019 [38]
PEALD	90	O2	0.2	1.07–1.57	-	~4	-	-	2018 [22]
PEALD	250	O2	0.06	~1.1	~3.6 × 10−2	27–28	5–6.1	-	2017 [24]
ALD	100–450	H2O	-	0.73–0.98	-	33–44	7–9	2–4	2016 [17]
ALD	85	H2O	-	0.80	~4.0 × 10−2	20	10 ± 3	~2.5	2015 [18]
AALD	25–50	H2O	-	0.62–0.97	-	20	-	0.75–1.8	2014 [19]
ALD	35	H2O	2	1.14	-	19	-	-	2014 [20]
ALD	80	H2O and O3	-	0.90–0.95	-	60	-	~1.3	2013 [21]
PEALD	70	H2O	0.05	~1.7	~7.8 × 10−2	~25	6.3–7.6	4.8–5	This work

. The dielectric constant was close to published values for films obtained by ALD and PE-ALD. However, the breakdown electric field was higher than that of ALD layers deposited at low temperatures [17,18,19,20,21] and close to the breakdown field of oxides deposited at temperatures above 200 °C [35,37]. Another advantage of the proposed process is the higher GPC. In addition, the TMA time was shorter than that of Al₂O₃ oxides deposited by ALD [20,37,38] and PE-ALD at low temperatures [22].

4. Conclusions

Al₂O₃ layers with thickness in the 25–120 nm range were prepared by PE-ALD at 70 °C using H₂O and O₂ oxidants. The GPC for layers deposited with H₂O and O₂ plasma and having thickness ≥50 nm was found to be 1.6 and 1.7 Å/cycle, respectively. The transmittance of the films prepared with O₂ plasma is slightly higher than that of the H₂O plasma films. The ellipsometric measurements gave a value for the refractive index of ~1.63 for both types of plasmas. Excellent fit of the raw ellipsometric data was obtained assuming zero extinction coefficient, confirming the high transparency of the films. XPS analysis revealed composition close to the stoichiometric one for both types of oxides. However, the oxygen participating in the Al–O bond was found to be slightly higher in films grown with water plasma. The estimated band gap values were 6.64 and 6.58 eV for layers deposited with H₂O and O₂ plasma, respectively. AFM revealed similar surface morphology for films deposited with both types of oxidants. The breakdown electric fields for oxygen and water plasma oxides were found to be ~3 and 6 MV/cm at positive bias voltages and approximately −5.8 and −9 MV/cm at negative bias voltages, respectively. The determined breakdown electric fields are higher than those of ALD layers deposited at low temperatures and are close to breakdown fields of oxides deposited at temperatures above 200 °C.

The excellent optical, morphological, compositional and electrical properties of Al₂O₃ layers prepared by PE-ALD at 70 °C make them a promising candidate for electronic and optoelectronic applications that require low-temperature processes.