Optical Pumped Gesn Waveguides Room Temperature Amplified Spontaneous Emission Observation
Authors: Li Zairui , Mathews Jay , Zhao Yun , Gallagher James Dennis , Lombardo David , Agha Imad , Kouvetakis John , Menendez Jose 
Published: February 2020

The successful epitaxial growth of Ge-on-Si has opened a new gateway for Si-based laser research. Ge is not a direct band gap material, however it possible to induce Ge towards direct band emission due to the small band gap energy differences between the direct and indirect. Recently lasing cavities have been successfully engineered by introducing tensile strain to the Ge lattice or through Sn alloying. The two methods have similar effects, lowering both direct and indirect band gaps of Ge with different rate. For a critical or higher Sn content, the direct band can become the lowest gap. This research is motivated by GeSn alloys with high potential to overcome many difficulties such as integration scale limits, excessive cost, extreme operating condition, etc in terms of engineering Si-based lasers. In this work, we explore the properties of optical emission from optically-pumped waveguides fabricated from GeSn alloys with Sn concentrations below the direct band gap transition.

The fabrication process of the GeSn waveguides starts with growing undoped Ge buffer layers on Si (100) substrates using gas source molecular epitaxy with Ge4H10 and the wafers were annealed in situ at 650 °C for 3 min. After aqueous HF solution Ge virtual substrates cleaning, flows 5% Ge2H6 in H2 at 30 mTorr for 5 min for additional surface cleaning. Then the mixtures of Ge3H8, SnD4, and P(GeH3)3 (resulting in a concentration of 4.4% Sn in this research study) grows n-type doped GeSn films around 285-335 °C. On the GeSn surface, strip waveguides with width 3 μm and length 4 mm were BCl3 reactive ion etched by using standard UV contact photolithography. With plasma-enhanced chemical vapor deposition, a thickness of 180nm SiO2 was deposited for surface passivation after downright Ge buffer layer to Si interface etching process.

For the experimental procedures, the waveguide with both end mirror polished, and one end is Al deposited to form the cavity. The other open end is aligned with a lens fiber as the collection prosses of the output. A 976 nm laser beam is shaped into a line and focused onto the top surface of the waveguide. The signal is collected by an optical fiber and sent through a grating spectrometer with an extended InGaAs detector for spectrum analysis or direct into the detector for power measurements. For spectral analysis each data points are generated by wavelength corresponded grating reflection angle and its amplitude within still pump level and sets of data are measured at different pump levels. The power curve measurements signal amplitude is mapped directly from different pump levels. Room temperature is maintained by using active cooling for all pumping operations.

At 4.4% Sn, the material still has indirect bandgap as the lowest band and direct band energy is slightly above. This results in emission curves due to the indirect band gap and the direct band gap at lower output power from the spectrum analysis. It also shows that with increasing the pump level, the ratio between the emission power of the two processes increases and the two emission peaks separate further as the pump power is increased. Meanwhile the emission power from our waveguide versus pump power shows an exponential dependence, indicating the population inversion and optical gain at targeted spectrum reign. In this research the amplified spontaneous emission and its spectrum from optically pumped GeSn waveguide on Si at room temperature has been successfully observed. GeSn with 4.4% Sn content provides promising results as a gain material at mid-IR range.