Determination of Free Electron Density in Sequentially Doped In x Ga 1-x As by Raman Spectroscopy

The advent and exponential growth of mobile computing has spurred greater emphasis on the adoption of III–V compound semiconductors in device architectures. The introduction of high charge carrier densities within In_xGa_1-xAs and the development of metrologies to quantitate the extent of doping have thus emerged as an urgent imperative. As an amphoteric dopant, Si begins to occupy anionic sites at high concentrations, thereby limiting the maximum carrier density that can be obtained upon Si doping of III–V semiconductors. Here, we present Raman results on sequentially doped In_0.53Ga_0.47As wherein sulfur monolayer doping is used to introduce additional carrier density to Si-doped samples. The sequential doping of Si and S allows for high carrier concentrations of up to 1.3 × 10¹⁹ cm⁻³ to be achieved without damaging the III–V lattice. The coupling of the plasmon in the doped samples to the longitudinal optic phonons allows Raman spectroscopy to serve as an excellent probe of the extent of dopant activation, charge carrier density, and the surface depletion region. In particular, the energy position of a high-frequency coupled mode (HFCM) that is detected above 400 cm⁻¹ is used to extract the free electron density in these samples. The extracted free electron densities are well correlated with measured sheet resistance values and the carrier densities deduced from Hall measurements.