PEALD-Grown Crystalline AlN Films on Si (100) with Sharp Interface and Good Uniformity
© The Author(s). 2017
Received: 31 December 2016
Accepted: 5 April 2017
Published: 18 April 2017
Aluminum nitride (AlN) thin films were deposited on Si (100) substrates by using plasma-enhanced atomic layer deposition method (PEALD). Optimal PEALD parameters for AlN deposition were investigated. Under saturated deposition conditions, the clearly resolved fringes are observed from X-ray reflectivity (XRR) measurements, showing the perfectly smooth interface between the AlN film and Si (100). It is consistent with high-resolution image of the sharp interface analyzed by transmission electron microscope (TEM). The highly uniform thickness throughout the 2-inch size AlN film with blue covered surface was determined by spectroscopic ellipsometry (SE). Grazing incident X-ray diffraction (GIXRD) patterns indicate that the AlN films are polycrystalline with wurtzite structure and have a tendency to form (002) preferential orientation with increasing of the thickness. The obtained AlN films could open up a new approach of research in the use of AlN as the template to support gallium nitride (GaN) growth on silicon substrates.
With a direct wide bandgap of 6.2 eV , high resistivity and resistance of breakdown voltage, and good thermal conductivity and stability , aluminum nitride (AlN) is suitable for various applications, such as photodetectors, ultraviolet light-emitting diodes, complementary metal-oxide-semiconductor (CMOS), and solar cells. As we know, low-temperature prepared AlN was used as a critical buffer layer for the growth of epitaxial gallium nitride (GaN) layers on sapphire substrates [3–5], which contributed to the development of GaN electronic and optoelectronic devices. Since large and high-quality silicon wafers are readily available at relatively low cost, AlN films grown on silicon substrates are highly desirable and have the potential to develop GaN electronic and optoelectronic devices on silicon substrates in future. Recently, ultrathin AlN films deposited at low temperatures were widely applied for passivation layers on high electron mobility transistors (HEMTs) by controlling their thickness at atomic level [6–11]. Therefore, great efforts have been carried out for fabricating high-quality AlN growth at low temperature. It is well known that plasma-enhanced atomic layer deposition (PEALD) is a low-temperature growth method based on self-limiting growth mechanism, which can deposit highly uniform and conformal angstrom-scale thin films. In the literatures, Alevli et al.  fabricated polycrystalline AlN films using PEALD, and the polar (002)-preferred orientation appeared with increasing the temperature up to 400 °C. Ozgit et al.  obtained (100)-oriented polycrystalline AlN films on Si (100) substrates. Epitaxial growth of (002)-oriented crystalline AlN films on GaN and sapphire were achieved [14, 15]. However, high-quality (002)-preferred orientation AlN films on silicon substrates have not been realized at low temperature up to now.
In this work, we have deposited polycrystalline hexagonal AlN films with (002) preferential orientation on Si (100) substrates at temperature as low as 300 °C. Interface between the AlN film and Si (100) has been investigated. AlN films with sharp interface and good uniformity are obtained.
Deposition conditions of the AlN films by PEALD
Gas line temperature
Flow rate of carrier gas (UHP Ar)
Flow rate of N precursor
RF plasma frequency
After deposition, the thickness and the optical constants of AlN films were measured by spectroscopic ellipsometer (SE) in the energy range of 1.5–4.5 eV at incidence angle of 70°. X-ray reflectivity (XRR) with a PANalytical system X-ray reflectometry was used to study the interface between the films and substrates. The crystallinity of the as-deposited AlN was analyzed by grazing incidence X-ray diffraction (GIXRD) measurement. The thickness, uniformity, and interface of the as-deposited AlN films were further characterized by transmission electron microscope (TEM).
Results and Discussion
The extracted thickness of the 2-inch size AlN film at different points
where η and d are the non-uniformity and thickness of the film, respectively, the non-uniformity η of around 1% is calculated, suggesting that the AlN nucleation on Si (100) is highly uniform. Deposition of large-size uniform AlN films by PEALD at low temperatures broadens application of AlN in the areas that require uniform growth at low temperature with thickness controlled at the atomic level.
In summary, polycrystalline hexagonal AlN films with sharp interface and good uniformity have been deposited on Si (100) at 300 °C by PEALD. Increasing the thickness of AlN films promotes crystallization in (002) orientation. AlN films exhibit a high transparency in the visible region of the spectrum, which can be utilized in solar photovoltaic technology. The achieved AlN films are not only potential buffer layer materials for GaN growth but also promising materials for applications in other microelectronic and optoelectronic devices.
This work was supported by the National Nature Science Foundation of China (Grant Nos. 61274134, 51402064), USTB Start-up Program (Grant No. 06105033), Beijing Innovation and Research Base (Grant No. Z161100005016095), Fundamental Research Funds for the Central Universities (Grant No. FRF-UM-15-032, 06400071), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2015387). Authors would like to acknowledge Yuanjun Song from USTB for TEM measurements and thank Yangfeng Li from Institute of Physics CAS for the XRR measurements. Authors would also like to acknowledge Zhengwei Chen from Saga University for his useful comments and suggestions.
SJL designed and performed the experiments, analyzed the data, and drafted the manuscript. XHZ and MZP supervised this study. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
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