Measurement of Recombination Lifetimes in Gesn Alloys

Silicon photonics requires the development of Si-based lasers, modulators, waveguides, and detectors operating in the infrared beyond the band gap of silicon. A number of approaches to solving this problem have been pursued, including epitaxial growth of lower band gap materials on Si. The SiGe alloy system was explored for many years, but limitations in quality due the large lattice mismatch between Si and SiGe with high amounts of Ge, as well as the band gap (0.66 eV at its lowest for pure Ge) and its indirect nature, have prevented its widespread use. Integration of III-V materials such as InGaAs has also been tried extensively, but the problems of phase segregation and film quality have not been overcome.

The GeSn alloy system has been gaining attention in the photonics world due to its unique optical properties, and the growth of these alloys on Si platforms has jumpstarted efforts to employ this technology for the fabrication of CMOS-compatible infrared optoelectronics. These crystalline alloy films are grown directly on Si or Ge-buffered Si, have band gaps below that of Si or Ge, and are grown to thicknesses of up to 1 µm. Additionally, the band gap of GeSn should be direct for concentrations greater than about 10% Sn.

Much work has been done on the development of GeSn materials, and theoretical designs for lasers based on these alloys have been appearing in the literature frequently. However, the realization of complex optoelectronic devices like laser diodes requires materials with long recombination lifetimes to achieve population inversion. Thus, the study of recombination lifetimes in GeSn alloys is of paramount importance to the actualization of GeSn lasers. Unfortunately, the standard method of time-resolved photoluminescence (TRPL) is not well-suited to the study of GeSn alloys. For low Sn contents (<10% Sn), the band gap is indirect, so the intensity of optical emission is not high. Such a measurement would require single photon counting, but single photon detectors are not available at the emission wavelengths for the materials of interest. Conductivity-based methods should be more suitable, but the standard techniques of microwave photoconductive decay (µ-PCD) and RF-coupled photoconductive decay (RFPCD) suffer from low sensitivity in thin films or low temporal resolution.

In this work, we employ a new conductivity-based method for measurement of recombination lifetimes in thin film GeSn alloys. The method of transmission-modulated photoconductive decay (TMPCD) uses the transverse transmission of microwaves through a thin film, allowing for a very large interaction volume between the free carriers and the microwaves, thereby increasing the sensitivity for thin film materials. We will discuss this new method and show preliminary results on GeSn films with up to 7% Sn.