(Invited) Narrow Linewidth, Highly Efficient, and Integrated Light Emitting Diodes Based on Ge Quantum Dots in Optical Microcavities
Authors: Xu Xuejun , Maruizumi Takuya , Shiraki Yasuhiro 
Published: February 2020

Ge self-assembled quantum dots (QDs) grown on Si are promising solution for Si-based light sources due to their emission wavelength in the telecommunication band, capability of electrical injection and CMOS process compatibility. However, as indirect band gap material, their light emission efficiency is still rather low. By embedding Ge QDs into optical microcavities with large Q-factor and small mode volume, the spontaneous emission rate can be enhanced due to the Purcell effect and the light extraction efficiency can also be improved due to the modified cavity mode fields. We have demonstrated strong and sharp resonant photoluminescence peaks from Ge QDs in photonic crystal (PhC) nanocavities and microdisk resonators [1]. For practical application, light emission by electrical injection is desirable. In this abstract, we will review some of our results on electrical injected light emitting diodes (LEDs) based on Ge QDs in microcavities.
Electrical injected LEDs were firstly demonstrated by integrating vertical PIN diodes with H3-type PhC cavities [2]. Room-temperature electroluminescence (EL) was successfully observed, but with relatively broad emission peaks. This is due to the large cavity loss induced by the electrical structure. By using lateral PIN diode structure, in which the P+ and N+ regions are located laterally on the PhC slab, while the cavity region is left intrinsic, we achieved intense EL peaks around 1.3 µm with Q-factor of about 260, corresponding to a linewidth of 5 nm [3]. After optimization of PhC nanocavities and electrical structures, the Q-factor of emission peaks increased to 800-1500. The required current for occurrence of resonant peaks was reduced by one order, to 50 µA [4]. The schematic diagram and EL spectra of the device are shown in Fig. 1(a) and 1(b), respectively. Moreover, at injected current of 3 mA, we obtained a measurable output power on the pW order. We also confirmed that the output power was limited by the fabrication process and could be increased further.
In order to realize monolithic integration with other photonic devices, waveguide-coupled LEDs are necessary. Waveguide-coupled microdisks containing Ge QDs were fabricated with vertical PIN diodes structures [5], as shown in Fig. 1(c). When there was current injection, light emission from the microdisk was coupled into the waveguide. Sharp resonant peaks around 1.55 µm with Q-factor over 5000 were obtained in the EL spectra collected through the waveguide, as shown in Fig. 1(d). The coupling between the waveguide and microdisk was confirmed by comparing the EL and optical transmission spectra along the waveguide. The LEDs thus can be easily integrated with other waveguide devices. Interestingly, the devices also can be operated as resonant-cavity enhanced photodetectors under reverse bias. Wavelength selective photo-response, with high responsivity and ultra-low dark current around 1.55 µm was also realized [6].
The light emission efficiency of Ge QDs themselves is fundamental to obtain higher intensity. Due to the type-II band lineup of Si/Ge heterostructure, only holes can be well confined in Ge QDs, and there is very weak confinement for electrons. By using n-type doping, extrinsic electrons can be provided and the radiative recombination is expected to be enhanced. In the talk, we will also present some results about light emission enhancement by phosphorus delta-doping between Ge QDs and Si spacers.
References:

X. Xu et al, IEEE J. Sel. Top. Quantum Electron. 18, 1830 (2012)

T. Tsuboi et al, Appl. Phys. Express 5, 052101 (2012)

X. Xu et al, Opt. Express 20, 14714 (2012)

X. Xu et al, Appl. Phys. Express 10, 102101 (2012)

X. Xu et al, JSAP Spring Meeting, 29p-B4-11 (2013)

X. Xu et al, CLEO Pacific Rim, WM1-4 (2013)