Effect of In/Al ratios on structural and optical properties of InAlN films grown on Si(100) by RF-MOMBE
© Chen et al.; licensee Springer. 2014
Received: 3 March 2014
Accepted: 17 April 2014
Published: 1 May 2014
In x Al1-xN films were deposited on Si(100) substrate using metal-organic molecular beam epitaxy. We investigated the effect of the trimethylindium/trimethylaluminum (TMIn/TMAl) flow ratios on the structural, morphological, and optical properties of In x Al1-xN films. Surface morphologies and microstructure of the In x Al1-xN films were measured by atomic force microscopy, scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. Optical properties of all films were evaluated using an ultraviolet/visible/infrared (UV/Vis/IR) reflection spectrophotometer. XRD and TEM results indicated that In x Al1-xN films were preferentially oriented in the c-axis direction. Besides, the growth rates of In x Al1-xN films were measured at around 0.6 μm/h in average. Reflection spectrum shows that the optical absorption of the In x Al1-xN films redshifts with an increase in the In composition.
Recently, InAlN film is a highly attractive III-nitride semiconductor with numerous potential applications because InAlN has band gap energy in the range from 6.2 eV for AlN to 0.7 eV for InN. Therefore, InAlN alloys are attractive for possible applications in light-emitting diode (LEDs) and high-efficiency multijunction tandem solar cell in the wide spectral range from ultraviolet to infrared[1–3]. In addition, compared with Ga(In, Al)N, InAlN has not been so intensively investigated because the growth of InAlN suffers from the difficulty of phase separation due to large immiscibility, optimum growth temperatures, lattice constant, bonding energy, and difference of thermal stability between InN and AlN. Moreover, few studies have been performed because InAlN has an unstable region concerning miscibility. Therefore, it was very difficult to grow high-quality InAlN since there were many variables in the growth condition.
Previous studies of InAlN growth on an AlN buffer layer show that it has improved the crystallinity of the InAlN films and prevented oxygen diffusion from the substrate. Besides, the growth of the InAlN film in all composition regions has been realized with the molecular beam epitaxy (MBE) growth method, while it was reported that In-rich InAlN with an In content >32% grown by metal-organic vapor phase epitaxy (MOVPE) showed the phase separation. Also, Houchin et al. indicated that the film quality of InAlN was degraded with increasing Al content. However, phase separation is not observed for the films obtained in their study. Kariya et al. conclude that lattice matching is important in order to grow high-quality InAlN with a smooth surface morphology. Especially, Guo and coworkers fabricated the first single-crystal Al x In1-xN films with x being from 0 to 0.14 in the low-Al composition regime using MOVPE. On the other hand, Sadler et al. indicated that trimethylindium flux was increased; the indium incorporation initially increased but then leveled off; and for further increases, the amount of indium on the surface as droplets increases significantly. Various growth techniques have been used for growth of InAlN films, such as radio-frequency molecular beam epitaxy (RF-MBE), metal-organic chemical vapor deposition (MOCVD), pulse laser deposition (PLD), and magnetron sputtering.
On the other hand, silicon is a very promising material for growth of III-nitride materials, with its good thermal conductivity which is especially interesting for electronic applications and also for low-cost light-emitting diode (LED) applications. Also, very few studies indicated that In-rich InAlN films were grown on Si substrate using radio-frequency metal-organic molecular beam epitaxy (RF-MOMBE), although InAlN films often were grown by MOCVD and MBE methods. Compared with the MOCVD method, the RF-MOMBE technique generally has the advantage of a low growth temperature for obtaining epitaxial nitride films[19, 20]. Also, our previous study indicated that the RF-MOMBE growth temperature for InN-related alloys was lower than the MOCVD growth temperature.
In this paper, the InAlN films were grown on Si(100) by RF-MOMBE with various trimethylindium/trimethylaluminum (TMIn/TMAl) flow ratios. Structural properties and surface morphology are characterized by high-resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Optical properties of all InAlN films were also investigated by an ultraviolet/visible/infrared (UV/Vis/IR) reflection spectrophotometer with integrating sphere.
The X-ray diffraction (Siemens D5000, Siemens Co., Munich, Germany) measurements were carried out in a θ-2θ coupled geometry using Cu-K α radiation to identify the presence of secondary phases or crystalline structures. The lattice parameters of In x Al1-xN films and the value of x were calculated by high-resolution X-ray diffraction (Bruker D8, Bruker Optik GmbH, Ettlingen, Germany). The diffraction angle 2θ was scanned from 20° to 40° at 0.005°/s. The surface and cross-sectional morphologies of the In x Al1-xN films were analyzed using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4300, Hitachi, Ltd., Chiyoda, Tokyo, Japan). The microstructure of the InAlN films was investigated in detail by TEM in cross-sectional configuration (TEM, Philips Tecnai 20 (FEI/Philips Electron Optics, Eindhoven, Netherlands) and JEOL 2010 F (JEOL Ltd., Akishima, Tokyo, Japan)). The In x Al1-xN film's composition was determined with HRXRD. The optical reflectance measurements were performed by using a UV/Vis/IR reflection spectrophotometer with integrating sphere (PerkinElmer Lambda 900, PerkinElmer, Waltham, MA, USA) from 200 to 2,000 nm.
Results and discussion
Vegard's law has been applied to determine the average In composition of the ternary alloy films via measurement of lattice parameters from HRXRD.
where ν(x) is Poisson's ratio defined as ν(x) = 2C13/C33; C13 and C33 are the elastic constants of the hexagonal III-nitrides. The material constants used in this study are a = 0.311 nm, c = 0.498 nm, C13 = 99 GPa, and C33 = 389 GPa for AlN; and a = 0.354 nm, c = 0.5706 nm, C13 = 121 GPa, and C33 = 182 GPa for InN. For In x Al1-xN ternary alloy, both lattice constants and Poisson's ratio v(x) are obtained by linear interpolation from the values of binaries. As a result, it can be concluded that the molar fraction of InN on a biaxially strained In x Al1-xN film is the only possible solution between 0 and 1 for the following third-order equation which presents x as a function only of two variables. The In composition (x) is accordingly to be calculated as x = 0.57 ± 1% (TMIn/TMAl, approximately 1.29), 0.64 ± 1% (TMIn/TMAl, approximately 1.4), 0.71 ± 1% (TMIn/TMAl, approximately 1.51), and 0.80 ± 1% (TMIn/TMAl, approximately 1.63) by Vegard's law.
The XRD pattern of an In content of <0.64 exhibits extremely weak and broad peaks, which indicates that the film is of poor quality due to structural defects. Also, the In0.64Al0.36 N film shows a polycrystalline structure, suggesting that the in-plane residual stress of the In0.64Al0.36 N film is almost relaxed after growth.
At above x = 0.71, the pattern indicates that the InAlN films are preferentially oriented in the c-axis direction. In addition, a weak shoulder peak (2θ, approximately 31.909°) was detected at the highest In content of approximately 0.71, indicating an intermediate layer between the film and the Si substrate. As can be seen in Figure 2b, the lattice parameters for c-axis and a-axis obtained from symmetric (0002) and asymmetric () diffractions of InAlN increased with the increase of In content. The results agree with the theoretical calculations and report of Guo et al..
The lattice parameter of the In0.57Al0.43 N film was calculated to be larger than the theoretical value, which may be caused by phase separation and/or lattice strain. The in-plane residual stress of all InAlN films is shown in the inset of Figure 2b. The residual stress was tensile at an In content of >71%. The compressive stresses occurred in the films deposited at an In content of <64%. When the In content is high (>71%), small tensile intrinsic stresses are observed. It has been proposed that one reason for the occurrence of tensile intrinsic stresses is the existence of numerous grain boundaries. Therefore, small tensile residual stresses were obtained at an In content of >71%, and large compressive stresses were obtained at In composition x = 0.57.
Highly c-axis-oriented In x Al1-xN films were grown on Si(100) by RF-MOMBE. From XRD results, In0.71Al0.29 N has the best crystalline quality among the In x Al1-xN samples for various values of the In composition fraction x studied here. However, the strain of all InAlN films has not been relaxed after growth. At an In content of <57%, the InAlN/Si(100) exhibits worse crystallinity which observed obviously large residual stress. The surface roughness of InAlN films increased with the increase of In composition. The corresponding reflection spectra of the In x Al1-xN films are observed at around 1.5 to 2.55 eV.
This work was supported by the National Science Council (NSC) of Taiwan under contract no. NSC 101-2221-E-009-050-MY3.
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