Effects of Post Annealing Treatments on the Interfacial Chemical Properties and Band Alignment of AlN/Si Structure Prepared by Atomic Layer Deposition
The influences of annealing temperature in N2 atmosphere on interfacial chemical properties and band alignment of AlN/Si structure deposited by atomic layer deposition have been investigated based on x-ray photoelectron spectroscopy and spectroscopic ellipsometry. It is found that more oxygen incorporated into AlN film with the increasing annealing temperature, resulting from a little residual H2O in N2 atmosphere reacting with AlN film during the annealing treatment. Accordingly, the Si–N bonding at the interface gradually transforms to Si–O bonding with the increasing temperature due to the diffusion of oxygen from AlN film to the Si substrate. Specially, the Si–O–Al bonding state can be detected in the 900 °C-annealed sample. Furthermore, it is determined that the band gap and valence band offset increase with increasing annealing temperature.
KeywordsValence Band Atomic Layer Deposition Rapid Thermal Annealing Increase Annealing Temperature Valence Band Maximum
AlN is considered to be a promising semiconductor material because of its wide direct band gap, high piezoelectric response and small mismatches of lattice constant, and thermal expansion coefficient with Si substrate. It is these advantages of AlN material that has made it an attractive candidate material for numerous applications in various optoelectronic and microelectronic devices on Si substrate [1, 2, 3]. Bulk AlN has a high dielectric constant of 9, and the interface between AlN and Si is established to be thermally stable. Preparing AlN/Si metal-insulator-semiconductor (MIS) structures with acceptable qualities will thus open the path to co-integration of light emitting nitrides with silicon microelectronics [4, 5]. Silicon field-effect transistors using AlN as gate dielectric have been demonstrated by several studies with good performance [6, 7]. So far, many deposition methods have been pursued to obtain AlN films, such as molecular beam expitaxy (MBE) , metal-organic chemical vapor deposition (MOCVD) , and sputtering . Considering a promising deposition method for AlN film, atomic layer deposition (ALD) has its unique advantage in uniformity, conformality, and thermal budget. Besides these properties mentioned above, ALD is also an excellent deposition method to control the thickness of the AlN film at a single atom layer scale.
In addition, AlON is also a promising material as gate insulator with a dielectric constant as high as 13. Furthermore, it is reported that the AlON layer can provide a prevention of silicon diffusion to the gate insulator [11, 12]. Inserting AlON layer between HfO2 gate oxide and Si substrate can suppress the interlayer between AlON and Si resulting in a high quality channel layer [13, 14]. As is known, film properties of AlON can be tailored between those of pure aluminum nitride (AlN) and aluminum oxide (Al2O3), depending on different applications .
Experimentally, the as-deposited AlN thin films usually not only possess a little of oxygen, but also easily form AlON compounds with the subsequent high temperature thermal treatments [16, 17, 18]. However, the annealing temperature dependence on the interfacial chemical properties and band alignment of the ALD-AlN/Si structure has not been thoroughly investigated, which plays an important part in the electronic properties of the corresponding device. As a result, thermal ALD system is used to grow thin AlN film on Si substrate in this work. The effect of annealing temperature under N2 ambient on the interfacial chemical bonding states of the obtained AlN/Si structure have been investigated by x-ray photoelectron spectroscopy (XPS) in detail. Furthermore, the electronic energy band alignment of the heterojunction structure is also determined as a function of N2 annealing temperature.
Thin AlN film with 70 cycles (~6.5 nm) was deposited on p-type silicon wafer with crystal orientation of (100) and sheet resistivity of 2~4 Ω cm using atomic layer deposition (ALD) at 370 °C. The precursors trimethylaluminum (TMA) and NH3 have been utilized as Al and N sources, respectively. Prior to the growth of AlN films, the Si substrates were cleaned using the Radio Corporation of America (RCA) cleaning process and then dipped into a 2% hydrofluoric acid (HF) aqueous solution for 2 min to remove the native oxides. To investigate the effect of the high temperature annealing on the interfacial chemical properties of the AlN/Si structure, ex situ post-deposition rapid thermal annealing (RTA) was conducted under N2 ambient condition with a temperature range of 600–900 °C for 60 s. An ex-situ SPECS XPS system with a monochromatic Al Kα source (hv = 1486.6 eV) for the excitation of photoelectrons was utilized for data acquisition. The source power was 150 W (10 kV, 15 mA) and the analysis region was a round spot with a radius of 500 μm, an incident angle of 58o, and a takeoff angle of 90o. Broad band scans with a step of 0.5 eV and pass energy of 25 eV are performed twice in order to acquire the binding energy of specific elements. Charge neutralization was performed with an electron flood gun. Narrow scans with a step of 0.05 eV and pass energy of 25 eV are performed for 20 times for binding energy of specific elements. Furthermore, the valence band scans with a step of 0.05 eV and pass energy of 25 eV are performed for 30 times to obtain the valence band spectra. Charge correction is performed using the position of C 1 s spectra at 284.6 eV. Moreover, the XPS spectrometer energy scale was calibrated using Ag 3d 5/2, Cu 2p 3/2, and Au 4f 7/2 photoelectron lines located at 368.06, 932.47, and 83.78 eV, respectively. Spectral deconvolution was performed by Shirley background subtraction using a Voigt function convoluting the Gaussian and Lorentzian functions. Spectroscopy ellipsometry measurements were also performed using a commercial instrument (Sopra GES5E, SOPRA, Courbevoie, France) to obtain the band gap and thickness of AlN film where the incident angle was fixed at 75°, and the wavelength region from 190 to 900 nm was scanned with 5-nm steps.
Results and Discussion
In summary, the influences of annealing temperature in N2 atmosphere on interfacial chemical properties and band alignment of AlN/Si structure prepared by thermal atomic layer deposition have been investigated. It has been found that more oxygen incorporated into AlN film with the increasing annealing temperature, resulting from a little residual H2O existed in N2 atmosphere reacting with AlN film during the annealing process. The Si–N bonding at the AlN/Si interface gradually transforms to Si–O bonding with the increasing temperature, due to the diffusion of oxygen from AlN film to the Si substrate. The band gap and valence band offset increase with increasing annealing temperature. These results indicate that the modification of the band structure of the AlN/Si heterojunction can be realized by properly selecting the annealing temperature.
This work is supported by the National Natural Science Foundation of China (No. 51102048, 61376008, 61376119, and U1632121), State Key Laboratory of Luminescence and Applications (Grant No. SKLA-2016-16), and the Innovation Program of Shanghai Municipal Education Commission (14ZZ004).
LS and HLL designed the study. LS, HYC, and TW carried out the deposition along with its characterization (XPS, SE) and interpretation of characterization. XMJ, WJL, SJD, DXZ, AD, and DWZ participated in the analysis and discussion of the results obtained from the experiments. HLL and DWZ supervised this study. The manuscript was prepared by LS, and HLL helped with draft editing. All authors approved and have equal contribution in drafting the final manuscript.
The authors declare that they have no competing interests.
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