Experimentally-Calibrated Modeling of Molecular Beam Epitaxy Selective Area Regrowth

Abstract

Abstract Body: A molecular beam epitaxy (MBE) approach to selective area epitaxy (SAE) of III-V semiconductors could enable the seamless integration of metals, dielectrics, and high-quality crystalline semiconductors. This technique could advance novel optoelectronic structures, such as high-contrast photonics, site-controlled quantum emitters, stacked pixel detectors, and photonic integrated circuits. Using patterned dielectrics and metals to define crystal seeding regions, SAE can be performed to embed microstructures into crystalline semiconductors1. While SAE by metal organic chemical vapor deposition has been widely successful due to its high material deposition selectivity, a SAE MBE method could enable further advances through its high layer precision and access to non-equilibrium growth conditions1,2. SAE is difficult to achieve with conventional (continuous) MBE growth due to III-V polycrystalline nucleation on the mask, even at growth temperatures as high as 700°C. As a result, Allegretti et al. developed periodic supply epitaxy (PSE), a method to inhibit polycrystalline deposition by cycling group III deposition under a constant group V flux2,3. SAE with PSE was achieved using high growth temperatures and low growth rates3-4. This mitigates nucleation of poly-GaAs through a combination of increased Ga adatom diffusion to seeding windows and desorption of remaining Ga adatoms3-4, enabling high-quality planar encapsulation of the mask1. While an all-MBE approach has been demonstrated to achieve selective growth of features ~2µm in width and ~300nm in height, applications requiring larger features are limited by nucleation of polycrystalline semiconductor on the amorphous surface due to poor deposition selectivity and low adatom surface diffusion1,3. We present experimental results of GaAs growth on SiO2 films that demonstrate decreased poly-GaAs formation at higher temperatures and lower As4/Ga flux ratios, enabling SAE of larger patterned features. However, this improvement in selectivity has yet to be explained quantitatively. In order to expand the applicability of all-MBE SAE, we developed a PSE model that (1) identifies selective growth regimes to clarify their underlying mechanisms and (2) allows optimized SAE growth conditions to be determined for a given SiO2 pattern. A numerical 1D model was developed to describe PSE selectivity by fitting adsorption, desorption, and diffusion constants to GaAs growth on SiO2 films at 600°C with a growth rate of 0.7 Å/s and 65x As4/Ga ratio5-8. Two growth regimes have been identified: desorption- and diffusion-limited. The desorption-limited regime depends only on thermal desorption from the mask surface to achieve selectivity and as a result, is feature-independent. This model has identified the desorption-based selectivity limit to be an 18.7% PSE duty cycle and was demonstrated by observing no polycrystal formation after growth of 100nm GaAs with 10% PSE duty cycle on an SiO2 film. The remainder of the selective growth space is diffusion-limited and sufficient III adatom diffusion off the mask is required to achieve selectivity. Experiments to fully investigate the influence of growth temperature and V/III ratio on selectivity are underway and will be reported at the conference. [1] D. J. Ironside et al., ACS J. Crystal Growth and Design (2019). [2] A. M. Skipper et al., 2019 MRS Electronic Materials Conf. [3] F. E. Allegretti et al., Journal of Crystal Growth (1995). [4] S.C. Lee et al. Journal of Applied Physics (2002). [5] S. Shankar. Diffusion in 1D and 2D, MATLAB. Retrieved April 11, 2020. [6] Aseev et al. Nano. Lett. (2019). [7] S. C. Lee et al. Cryst. Growth Des. 2016. [8] E.M. Gibson et al. Appl. Phys. Lett. (1990). This research was partly done at the Texas Nanofabrication Facility supported by NSF grant NNCI-2025227 and was supported by Lockheed Martin and NSF through the UT CDCM: an NSF MRSEC under Cooperative Agreement No. DMR-1720595, as well as CCF-1838435 and DMR-1839175.