Abstract
Advanced solid-state devices, including lasers and modulators, require semiconductor heterostructures for nanoscale engineering of the electronic bandgap and refractive index. However, existing epitaxial growth methods are limited to fabrication of vertical heterostructures grown layer by layer. Here, we report the use of finite-element-method-based phase-field modelling with thermocapillary convection to investigate laser inscription of in-plane heterostructures within silicon-germanium films. The modelling is supported by experimental work using epitaxially-grown Si0.5Ge0.5 layers. The phase-field simulations reveal that various in-plane heterostructures with single or periodic interfaces can be fabricated by controlling phase segregation through modulation of the scan speed, power, and beam position. Optical simulations are used to demonstrate the potential for two devices: graded-index waveguides with Ge-rich (>70%) cores, and waveguide Bragg gratings with nanoscale periods (100–500 nm). Periodic heterostructure formation via sub-millisecond modulation of the laser parameters opens a route for post-growth fabrication of in-plane quantum wells and superlattices in semiconductor alloy films.
Highlights
Advanced solid-state devices, including lasers and modulators, require semiconductor heterostructures for nanoscale engineering of the electronic bandgap and refractive index
Similar manifestations of phase segregation resulting in quasi-regular in-plane heterostructures have been observed in semiconductor superlattices due to the strain-induced lateral compositional self-modulation[15], and directionally solidified alloys where banding occurs due to oscillations in the speed of the solidification front near the instability regime[16]
We begin our investigations of laser-driven heterostructure formation with some preliminary experimental work by applying laser processing on homogeneous 575 nm thick SiGe epilayers (50% Ge)
Summary
Laser writing of in-plane longitudinal SiGe heterostructures with tunable compositionally-graded profiles at various constant scan speeds. The Ge concentration profiles calculated by the phase-field simulations along different directions at the top surface and cross-section are given in Fig. 3a–c for the in-plane longitudinal SiGe heterostructures laser-written at different scan speeds. The effect of laser driven phase segregation on the dispersion of the Ge molar fraction can be clearly seen by calculating histograms of the composition within the entire solidified volume in the steady-state region for different scan speeds (Fig. 3d) These results complement our understanding of the formation of in-plane longitudinal heterostructures using laser processing. We performed 20% modulation (reduction) of the laser power P0 = 200 mW with a duty cycle of 1/3 at 61.0 kHz to obtain an in-plane transverse SiGe superlattice with a period of Λ = 402 nm (see Fig. 6c) In this case, we were able to obtain a greater change in the Ge molar fraction in the range of Δx = 0.55–0.78 mol, via the higher acceleration of the solidification boundary when the spot size suddenly shrinks. A waveguide Bragg grating with a high number of periods (>1000) and a longer total length (>300 μm) would be straightforward to fabricate to achieve better rejection ratios in the stop band, and could find ready use in integrated photonics applications
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