The energy difference between the lowest conduction band valleys is a fundamental semiconductor parameter affecting performance of electronic devices via intervalley electron scattering. Surprisingly, the intervalley energy (IVE) value in GaN is still disputed. Recent photoemission experiments showed that IVE is 0.90 – 0.95 eV, which is considerably smaller than the >2 eV values obtained by ab initio calculations.
One of the suitable techniques to measure IVE is time-resolved spectroscopy. Excitation wavelength dependent photoluminescence and pump-probe transients allow pinpointing the onset of the intervalley scattering by increase of the electron relaxation time towards the bottom of the conduction band. In this work, we apply this approach by performing differential transmission (DT) and reflection (DR) measurements on n-GaN crystal. In DR, ultraviolet (UV) pump creates electrons in the Γ valley at energies around the scattering threshold, and the onset energy is determined by the change of the electron relaxation time towards the bottom of the conduction band. However, IVE evaluated using this technique is affected by the poor knowledge of the valence band dispersion at large k values. This problem is circumvented in the DT measurements, in which only conduction band states are involved. The DT decay time spectrum provided the IVE value of 0.97 ± 0.02 eV, close to the photoemission data. Comparison of DT and DR intervalley scattering onsets allowed estimating the hole mass as 1.4m0. Modelling of the DR transients with rate equations produced intra-and intervalley electron - LO phonon scattering times of 30 and 15 fs, respectively.
Saulius Marcinkevicius, Tomas K. Uzdavinys, Humberto M. Foronda, Daniel E. Cohen, James S. Speck, and Claude Weisbuch, "Evaluation of intervalley energy of GaN conduction band by ultrafast pump-probe spectroscopy (Conference Presentation)," Proc. SPIE 10104, Gallium Nitride Materials and Devices XII, 101040L (Presented at SPIE OPTO: January 31, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2250622.5391659233001.
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