Aluminum oxide (Al2O3) amorphous structure with short-range order and long-range disorder has presented promising applications in optical and optoelectronic devices. In this paper, the Al2O3 films with different thickness were prepared by atomic layer deposition (ALD) technology at 200°C in order to achieve amorphous structure. X-ray diffraction (XRD) and energy dispersive spectrum (EDS) results indicated that the Al2O3 films were amorphous structure and stable O/Al ratio. The surface topography investigated by atomic force microscopy (AFM) showed that the samples were smooth and crack-free. Spectroscopic ellipsometer (SE) measurements were operated to investigate the effect of thickness on the structure and optical properties of films with Tauc-Lorentz model. It is found that the band gap exhibits a steady value ~2.3 eV by the UV-VIS transmittance method, but the T-L model was ~3.0 eV. The refractive index and extinction coefficient are related to the variation of thickness and the samples surface quality of amorphous network structure in the thin films. The outstanding optoelectronic properties and facile fabrication of Al2O3 films amorphous structure can be extended to other similar oxides, which could display wide applications in various engineering and industrial fields.
Aluminum oxide (Al2O3) is a technologically promising material in optics, machinery, batteries, and microelectronics applications because of its advantages like high dielectric constant, excellent stability, favorable thermal conductivity, high hardness, and low refractive index [
In this paper, the effect of structure evolution on the optical properties of amorphous Al2O3 films deposited by ALD has been examined. By means of X-Ray Diffraction and Scanning Electron Microscopy-Energy Dispersive Spectrometer, the atomic configuration, the surface morphology, and the element composition were characterized. Furthermore, the optical properties of samples were analyzed by spectroscopic ellipsometer (SE) in spectral range [300 nm, 800 nm]. The ellipsometer spectra were fitted by Tauc-Lorentz dispersion model. The ultraviolet and visible transmission spectrum were used to confirm the band gap evolution and support the SE results. This investigation enables a better understanding of the structural and optical properties of amorphous Al2O3 films, also providing a new reference for other similar materials.
A single crystal silicon substrate was used as the substrate for Al2O3 films growth, with 4 inches’ N type,
In the experiment, Al2O3 films were deposited on Si and glass substrates by ALD (TFS-200, Bene, Finland). Trimethylaluminum (TMA; (Al(CH3)3)) and H2O were used as the precursor of metallic aluminum and oxygen ions. Simultaneously, high purity N2 (99.999%) was used as the carrier and purge gas. To ensure the vacuum of the reaction chamber, the pressure differences about 3 Torr between internal and external chamber were controlled. The reacting temperature remained constant at 200°C. The Al2O3 films were deposited on silicon wafer and glass for 100, 200, 300, 400, and 500 cycles, respectively. TMA and H2O were alternately entrained in the N2 carrier flow using gas switching valves. The N2 carrier gas pressure was 2.0 Torr. Each ALD cycle was as follows: 0.2 s to pulse TMA, 0.2 s to pulse H2O as the second reactant, 2 s and 3 s to purging the chamber with high purity N2 in 1st and 2st. Total time of a deposition cycle is 5.4 s.
The film crystal structure was examined by X-ray diffraction (XRD, X’Pert PRO, PANalytical B.V., Netherlands). The compositions were evaluated by Scanning Electron Microscopy-Energy Dispersive Spectrometer (SEM-EDS, Zeiss 1450VP Scanning Electron Microscope). The roughness and surface topography were characterized by atomic force microscopy (AFM; Bruker Dimension Icon VT-1000, Santa Barbara, CA, USA). The optical properties of Al2O3 films were investigated by spectroscopic ellipsometer (SE, SENTECH SE850 UV/VIS/NIR, spectroscopic ellipsometer, Germany), of which the wavelength ranges from 300 to 800 nm at the incident angle of 65°. Besides, the ultraviolet-visible spectra were examined with ultraviolet and visible spectrophotometer (UV, SHIMADZU UV-1700 ultraviolet and visible spectrophotometer) which can extract the band gap evolution to support the SE results.
When a beam of light enters a membrane at a certain angle, the thickness and optical parameters of the film can be determined by the polarization state variations of reflected light and transmitted light.
The ellipsometric parameters
In this case, the refractive index
The intensity of XRD is sensitive to the atomic arrangement of material, so it is capable of analyzing crystal structure. Figure
XRD patterns of Si (100) and all films of different cycles grown on Si (100) substrates.
In ALD, the monolayer deposition and the alternating precursor pulse can guarantee an excellent film deposition quality and uniform rate [
ALD process schematic.
The surface chemistry reaction for ALD-Al2O3 can be expressed as
The chemical composition of the films was evaluated using SEM-EDS. As shown in Table
EDS data of different cycles Al2O3 film on silicon wafer (oxygen, aluminum, and silicon (%); the total = 100%).
ALD cycles | Elements | Wt.% | At. (%) | | O/Al ratio |
---|---|---|---|---|---|
100 | O | 2.3 | 3.97 | 11.62 | —— |
Al | 0 | 0 | |||
Si | 97.7 | 96.03 | |||
| |||||
200 | O | 4.05 | 6.90 | 19.63 | 4.964029 |
Al | 1.38 | 1.39 | |||
Si | 94.57 | 91.71 | |||
| |||||
300 | O | 5.85 | 9.82 | 30.95 | 5.223404 |
Al | 1.89 | 1.88 | |||
Si | 92.27 | 88.30 | |||
| |||||
400 | O | 7.97 | 13.18 | 39.05 | 4.592334 |
Al | 2.93 | 2.87 | |||
Si | 89.1 | 83.95 | |||
| |||||
500 | O | 11.23 | 18.15 | 50.40 | 5.385757 |
Al | 3.52 | 3.37 | |||
Si | 85.25 | 78.48 |
The film surface quality has a direct influence on the optical and electrical performance of device, such as the optical scattering and electrical contact. Therefore, a precise control of surface morphology during film growth process is very crucial to device performance. In this work, AFM is used to characterize the surface structure of the amorphous Al2O3 films. Figure
AFM micrographs of deposited Al2O3 films with different thicknesses. (a) 100 cycles. (b) 200 cycles. (c) 300 cycles. (d) 400 cycles. (e) 500 cycles. (f) Trend chart of roughness for the five samples. RMS represents root mean square roughness.
The surface morphologies of all samples are smooth and crack-free with similar root mean square (RMS) surface roughness values, which indicates that Al2O3 films were well fabricated. In order to study the cause of different growth rates of those of 50 cycles, we also studied the growth rate of films with ALD cycles below 50. The ALD surface reactions are very efficient, and Steps 1 and 3 are also known as the half reaction with the self-limitation and complementarity, while the self-limiting reaction is determined by the saturation adsorption of the group and the number of surface activated groups. Each group has a certain size, and the bonding location is blocked by adjacent chemical group, so the remaining groups can only be adsorbed in the next cycle. The number of surface activation groups determines the number of molecules per cycle. The Al2O3 films growth occurs during alternating exposures to TMA and H2O. The main driver for the efficient reactions is the formation of a very strong Al-O bond. The films tend to be smooth and pinhole-free because the randomness of the precursor flux and the self-limiting of the surface reactions. The reaction dose, deposition temperature, and so forth have little or no effect on the reaction. Thus, the growth rate is constant in the ALD process, which is insensitive to changes in process parameters, and the deposited films have excellent thickness uniformity and three-dimensional conformity.
Spectroscopic ellipsometer (SE) is an important thin film measurement technology for its nondestructive and sensitive advantages [
Spectroscopic ellipsometer schematic of Al2O3 film micronanosystem.
The ellipsometry spectra of Al2O3 thin film with different ALD cycles have been obtained using the above theoretical model. Since the related parameters of all samples are extracted from
Structural and electronic parameters extracted from SE analysis of the five different samples.
ALD cycle | | | | | | | | |
---|---|---|---|---|---|---|---|---|
100 | 1.66 | 118.24 | 644.70 | 74.33 | 3.08 | 11.62 | 0.08 | 0.68 |
200 | 1.39 | 156.83 | 393.56 | 70.30 | 2.99 | 19.63 | 0.25 | 0.67 |
300 | 1.22 | 72.65 | 39.07 | 25.23 | 2.98 | 30.95 | 0.89 | 0.58 |
400 | 0.59 | 334.78 | 430.62 | 92.86 | 2.92 | 39.05 | 0.15 | 0.53 |
500 | 1.11 | 207.70 | 295.86 | 59.76 | 2.92 | 50.40 | 0.21 | 0.73 |
Al2O3 films ellipsometry spectra of fit data (continued curves) and measured data (gauge points).
SE measurement shows that the fundamental structural parameter, film thickness, increases regularly with the increase of ALD cycles, corresponding to a growth rate of ~0.1 nm/cycle. The relationship between thickness and cycle can be described by a linear fit function
Trend chart of thickness for the five different cycle samples. The inset is SEM image of 1000 cycles ALD-Al2O3 film, where
The optical band gap
Optical transmittance spectra by UV-VIS transmittance measurement. The inset shows the optical band gaps from the plots of
According to the Tauc theory [
The optical band gap changes with the ALD cycles.
Another important structural parameter extracted from SE analysis is the surface roughness
RMS roughness
The refractive index
(a) Refractive index
The absorption coefficient also can be calculated from analysis through the relationship
The electronic response of material depends on its dielectric function, which reflects the interaction between the electrons and the applied field in materials. Figure
The dielectric functions of Al2O3 films with different thickness samples by the T-L model: real part (a) and imaginary part (b).
In summary, the amorphous ALD-Al2O3 films with various thicknesses are successfully fabricated at 200°C. The XRD and EDS measurements show that the films were amorphous structure and stable O/Al ratio. The AFM indicates that the surface morphologies of all samples are smooth and crack-free with similar surface roughness values. The SE analysis reveals that the film thickness increases regularly with the increase of ALD cycles, corresponding to a growth rate of ~0.1 nm/cycle. Further analysis on SE and UV-VIS transmittance spectrum shows that the band gap
Atomic layer deposition
Energy dispersive spectrum
X-ray diffraction
Atomic force microscopy
Spectroscopic
Aluminum oxide
Trimethylaluminum
Tauc-Lorentz
Forouhi-Bloomer
Root mean square
Root mean square error.
No potential conflicts of interest were reported by the authors.
This work is supported by the National Natural Science Foundation of China (Grants nos. 51605449, 51422510, 51675493, and 61471326), Shanxi Province Applied Fundamental Research Program (201601D021064), the National High Technology Research and Development (863) Program of China (Grant no. 2015AA042601), and Shanxi “1331 Project” Key Subject Construction (1331KSC).