The laser scanned along the direction of the width to form a seed for lateral crystal growth. The twin boundaries are labeled in green.
The nanotip on the left has a significantly smaller radius than the other two corners, leading to the growth of a large single crystal grain from this sharpest nanotip. Figures 5c,d show an example of nanotip induced nucleation [ 26 , 27 ]. When there is a high curvature surface, such as a nanotip on an a-GeSn micropattern e.
This nucleus at the nanotip will subsequently seed the lateral growth of GeSn across the entire micropattern, leading to a piece of single crystal material. Since the tip on the left has a much smaller radius than the other two, upon temperature ramping nucleation first starts at this nanotip. There are grains growing from the other two tips on the right hand side, too, but their sizes much smaller than the dominant single crystal grain.
A small amount of sporadic grains mostly form twin boundaries with the dominant single crystal grain.
This result proves that the growth of single crystal GeSn on amorphous dielectric layers can indeed be seeded by the high curvature nanotip on an a-GeSn micropattern. Additionally, the microtaper structure in Figures 5c,d also facilitates mode conversion and optical coupling with waveguides, as has been applied to waveguide-coupled photodetectors [ 3 , 33 ]. Using GeSn micropatterns and laser-seeded or nanotip-seeded crystallization, the Sn composition has been further increased to 14—15 at. To address this challenge, we have recently developed NICE growth approach [ 34 , 35 ].
This method, counter-intuitively, is exactly opposite to the conventional approach of synthesizing diamond cubic GeSn. Figure 6. Sn dewets on SiO 2 during the deposition, leading to the self-assembly of nanodots.
Laser Crystallization of Silicon - Fundamentals to Devices - Norbert H. Nickel - كتب Google
Compared to the pure Ge reference sample, the peak of crystallized GeSn is drastically shifted to lower diffraction angle indicating a larger lattice constant due to the significant Sn alloying [ Figure 6c , step 3 ]. From the peak shift in the XRD data, we estimate an average composition of 26—27 at.
The Sn-rich region at the bottom and Ge-rich cap at the top can be clearly observed. An average Sn composition of 26 at. As an approach to evaluate the optoelectronic quality of crystallized GeSnOI discussed in section Substrate-Independent GeSn Crystallization on Amorphous Insulators as well as to elucidate some fundamental questions about the GeSn band structure near the indirect-to-direct gap transition, we performed ultrafast pump-probe optical gain dynamic measurements on the 0.
This sample provides a composition close to the indirect-to-direct transition with high crystallinity and large grains on mm-scale so that grain boundaries have negligible effect on the pump-probe measurement. We aim at addressing two fundamental questions:. The threshold does not show significant decrease even at higher Sn compositions of 16—18 at.
Therefore, it is important to find out the key limiting factors of the lasing threshold. This issue is still controversial theoretically and experimentally, with the Sn content ranging from 6.
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The increase of the integrated photoluminescence PL intensity with the decrease of temperatures has been applied as an evidence to prove the direct fundamental bandgap of epitaxial Ge 1 — x Sn x , in contrast to the behavior of direct gap emission in Ge. However, it has been found recently that there are possible pitfalls associated with this criterion. For example, Pezzoli et al. In fact, it is well-known that even indirect gap semiconductors such as Si tends to show higher PL intensity at lower temperatures due to less non-radiative recombination.
On the other hand, demonstration of lasing can provide evidence for direct gap GeSn at low temperatures, but cannot directly prove that the same composition remains direct gap at room temperature since GeSn lasers operating at K are still not available. Thus, direct measurement of the carrier dynamics and optical gain lifetime is required to undoubtedly determine the directness of the fundamental bandgap of Ge 1 — x Sn x , especially at room temperature.
Femtosecond fs pump-probe studies provides an ideal approach to address these two fundamental questions. On the other hand, the optical gain lifetime measurement provides affirmative information about the nature of the bandgap indirect vs. A challenge for ultrafast pump-probe measurement of GeSn, though, is that most of the epitaxial GeSn layers are grown on Ge buffers, which can complicate the pump-probe carrier dynamics analysis due to carrier diffusion between GeSn and the Ge buffer as well as non-linear effects in Ge such as two-photon absorption TPA.
Direct crystallization of high crystallinity GeSnOI provides a possible approach to circumvent the complication of Ge buffers. The thin SiO 2 layer also prevents photo-generated carrier diffusion from GeSn into the Si substrate, offering a more accurate evaluation of the injected carrier density.
Therefore, ultrafast pump-probe measurements on our GeSnOI samples reveals important information about the optical gain and carrier dynamics inherent to GeSn. Our experiments are carried out using a setup similar to what has been described in [ 29 ] and [ 41 ].