Abstract
The application of GeSn is extended to semiconductor lasers thanks to its band engineering via Sn composition and strain manipulation. As one of the strain engineering methods, thermal annealing, however, is not yet being widely adopted by the majority due to the thermal instability it induces. The thermal stability of GeSn is highly sensitive to initial material conditions, consequently thorough investigations are still demanded with different purposes. A detailed investigation on the thermal annealing effects of thick GeSn layers with a nominal 8% Sn grown on Ge-buffered Si (001) substrate by molecular beam epitaxy is presented here. Atomic force microscopy and high-resolution x-ray diffraction were used to trace the change of GeSn surface morphology and the strain relaxation after annealing. It is confirmed that the tetragonal compressive strain in GeSn, which is a proven detriment to the realisation of direct-bandgap material, can be relaxed by 90% while improving crystal quality, e.g. reduced surface roughness by appropriate annealing conditions. These findings reveal the potential of annealed GeSn to serve as a much thinner (750 nm), better lattice-matched to GeSn active layer and highly strain-relaxed platform to grow GeSn on compared to the thick Ge or the compositional-graded (Si)GeSn buffer layers, which are complicated and time-consuming in growth procedures and also securing an easier approach.
Highlights
Introduction due to theSn-induced conduction band mixing effect [1] while a direct-to-indirect bandgap crossover is shown to happen with around 8% Sn [2]
GeSn, which is a proven detriment to the realisation of direct-bandgap material, can be relaxed by 90% while improving crystal quality, e.g. reduced surface roughness by appropriate annealing conditions. These findings reveal the potential of annealed GeSn to serve as a much thinner (750 nm), better lattice-matched to GeSn active layer and highly strain-relaxed platform to grow GeSn on compared to the thick Ge or the compositionalgraded (Si)GeSn buffer layers, which are complicated and time-consuming in growth procedures and securing an easier approach
Inspired by the benefits of thermal annealing in strain relaxation and crystal quality improvement, here we present investigations on ex-situ thermal annealing of 500 nm GeSn grown on 250 nm Ge based on such as applying graded (Si) (100) substrate by MBE
Summary
As the goal is to grow a high-crystal-quality relaxed buffer for GeSn optoelectronic devices, pre-relaxed GeSn layers were chosen to provide better thermal stability and a Sn composition around GeSn bandgap indirect-to-direct crossover was selected to ensure better lattice match. 500 nm. 130 °C was the target growth temperature, due to the heating from the source effusion cells, there was an unintended temperature fluctuation between 130 and 150 °C during growth.The Ge buffer growth was performed following our previously optimised growth conditions [26]. The samples were cleaved into identical pieces and annealed at temperatures between 300 and 600 °C at step intervals of 50 °C for 5 minutes each in nitrogen ambient conditions in a Solaris 150 Rapid Thermal. AFM was performed to probe surface morphology. The results of HRXRD (004) symmetric scan and reciprocal space maps (RSMs) were analysed and compared for lattice parameters and strain relaxation calculation. HRXRD investigation was performed in room temperature with a Jordan Valley Bede D1 x-ray diffractometer equipped with a. RSMs were generated by collecting omega-2theta scans at different omega offsets (0.2oin total, with a step of 0.02o)
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