(Invited) CMOS-Compatible Germanium Light Sources
Authors: Sawano Kentarou , Xu Xuejun , Maruizumi Takuya 
Published: February 2020

Integration of high efficiency light sources on a Si platform is desirable for realization of optoelectronic integrated circuits (OEICs). Ge is one of the most promising candidates toward realization of monolithic-integrated laser sources on a Si substrate. Ge has the narrower bandgap energy than Si, allowing us to extract light emissions from Ge with wavelength ranges corresponding to telecommunication band. Also the emitted light from Ge is transparent in Si, making the Ge the most proper light source for optical interconnection based on Si waveguides. In addition, since the Ge has been already involved in the commercial CMOS circuit, the Ge-based optical devices are considered to be fully compatible with CMOS processes, and thus integration of Ge devices on the Si platform is more cost effective than that of compound semiconductors.
Whilst there are many advantages of Ge as light sources on the Si chip, one critical drawback is the fact that Ge is indirect band gap material like Si, which means that the light emission efficiency from Ge is naturally difficult. To overcome this problem, however, various types of Ge light emitting structures have been researched so far, from bulk to nano structures. For most cases, key mechanisms to enhance the light emission efficiency are based on band engineering by strain introduction and doping control. Here, we show recent progresses of our researches on strained GOI (Ge-on-Insulator), highly strained Ge microbridges and n-doped Ge QDs.
By inducing tensile strain and n-type doping, energy band structure of Ge can be modified to a direct one, leading to significantly enhanced light emission. The tensile strain can be induced into a Ge directly grown on a Si (Ge-on-Si) due to thermal expansion mismatch between Si and Ge, making the Ge-on-Si a very attractive light emitter. Toward realization of low threshold laser on silicon chip, higher efficiency light emitting structures are highly required. To this end, we show Ge-on-Insulator structure can eliminate defective Ge, the source of non-radiative recombination, leading to enhancements of the light emission. In addition, as a resonator we fabricate Ge microdisks with circular Bragg gratings (CBGs) to enhance the reflectivity and obtain sharper resonant light emission.
For Ge-on-Si, the amount of strain is limited up to ~0.2%, which is not sufficient for direct-gap transition. To further increase amounts of the tensile strain, fabrication of a micro-structure is effective as a relatively large elastic stress is locally induced into the micro-region. The strained GOI is suitable for fabrication of the micro-structure as the underlying buried oxide is easily selective-etched and a free-standing Ge can be obtained. We fabricated a GOI microbridge as a micro-structure containing uniaxial strain and obtained high efficiency room-temperature PL. With the tensile strain increase, the PL peaks are found to shift to lower energies, indicating the direct band gap decreases with the strain. At higher strain levels (> ~ 0.5%), two peaks appear in the spectra, implying the splitting the heavy-hole (HH) and light-hole (LH) valence bands.
Furthermore, we have recently observed resonant light emission corresponding to the resonance in Fabry-Perot (FP) cavities formed transversely to the bridge direction, implying a possibility of combining the microbridge with optical resonators. We also numerically designed FP cavities by adding distributed Bragg reflectors (DBR) laterally to the microbridge and obtained improved Q-factors without modification of the microbridge structure and resultant strain loss.
A multilayer of self-assembled Ge quantum dots (QDs) is another attractive light emitting structure. To date, strong electroluminescence has been obtained from micro cavity devices involving Ge QDs. For further improvements of the light emission efficiency, stronger confinements of both electrons and holes in QDs are essential, whereas the Ge QD can confine only holes due to its type II band diagram. To confine electrons near the Ge QDs, phosphorous delta-doping at the Ge dot interface was attempted and enhancement of the PL intensity was obtained with optimal doping densities.
In conclusion, by controlling higher tensile strain and n-type doping, Ge-based light sources offer highly enhanced light emission, indicating Ge has high potential for monolithic integrated laser diodes on the Si platform. Further performance improvements can be made by optimization of resonant cavity structures, increase in the tensile strain and reduction of defects toward realization of low-threshold laser operations.