Investigation of Electron Effective Mass in AlGaN/GaN Heterostructures by THz Spectroscopy of Drude Conductivity

Abstract

Terahertz time domain spectroscopy (TDS) of the two-dimensional (2-D) electrons in various commercial AlGaN/GaN heterostructures was performed in the frequency range of 0.1–2.0 THz at selected temperatures of 80 and 300 K. Experimental transmission spectra were analyzed using analytical model derived for the thin conductive layer on dielectric substrate assuming a high-frequency Drude response of electrons. Two-dimensional electron mobility and density values found from the best fit parameters were compared with the results of independent Hall experiment demonstrating good agreement within uncertainty range of 10%. Thermal renormalization of the electron effective mass was revealed in all investigated samples manifesting as the increase of up to 1.45 ± 0.13 fold of nominal value obtained at cryogenic temperatures. Fast method based on measurement of the ratio of transmission spectra at two temperatures (80 and 300 K) was proposed to monitor the renormalization phenomenon in the samples with thin Drude conductivity layer where carrier density is relatively independent on temperature.