Hexagonal crystal phase SiGe has recently been shown to be an efficient direct bandgap light emitter [1]. This accomplishment directly raises the question whether light emission from hex-SiGe quantum wells and quantum dots is also feasible. Recent theoretical work [2] predicts a type I band alignment between hex-Ge wells and hex-Si0.25Ge0.75 barriers. To demonstrate hex-SiGe multishells, we employ the crystal transfer approach, in which the crystal structure is copied from a template, which is a wurtzite GaAs nanowire. The Si1-xGex then forms a shell around the GaAs core nanowire. In this case, the core was overgrown with multiple Ge shells separated by Si1-xGex shells. A cross-section in the middle of the nanowire is shown in Fig. 1a. A structure of alternating Ge (light) and Si1-xGex (dark) rings is observed, with the GaAs core at the center. 

The objective of this study is to show light emission from group IV multishells. It is important to realize that this is not possible for cubic Ge multishells, since unstrained cubic Ge is an indirect bandgap semiconductor. Efficient light emission is expected to become possible for direct bandgap hexagonal Ge/SiGe multishells. We observe strong PL (Fig. 1b) above the bandgap of unstrained hex-Ge, located at 0.35 eV, but below the bandgap of hex-Si0.1Ge0.9 at 0.45 eV [1]. We attribute the PL to the approximately 1% compressively strained Ge layers within the hex-Ge/SiGe nanowire shells. The room temperature PL-efficiency at an excitation density of 3.5 kW/cm2 is around 10% of the PL-efficiency at low temperature, indicating a similar internal radiative efficiency to pure hex-Ge nanowire shells [1]. The observation of efficient light emission proves that the Ge multishell nanowires at least partly possess the hexagonal crystal phase. Although we cannot yet conclude about the carrier confinement effects in these relative thick Ge/SiGe multishells, our results provide strong indications for the feasibility of efficient light emission from a group IV multiple quantum well structures. 

References

[1] E. M. T. Fadaly et al., Nature, 2020, vol. 580, no. 7802, pp. 205–209.

[2] A. Belabbes, S. Botti, F. Bechstedt, Phys. Rev. B, 2022, 106, 085303.