Energy relaxations of hot electrons in AlGaN/AlN/GaN heterostructures grown by MOCVD on sapphire and 6H-SiC substrates

¹ Department of Physics, Faculty of Science and Letter, Balıkesir University, Çağış Kampüsü, 10145 Balıkesir, Turkey
² Department of Physics, Faculty of Science and Letter, Gazi University, Teknikokullar, 06500 Ankara, Turkey
³ Department of Physics, Department of Electrical and Electronics Engineering, Nanotechnology Research Center-NANOTAM, Bilkent University, 06800 Ankara, Turkey

^a e-mail: sibelgokden@gmail.com

Received: 4 May 2011
Accepted: 23 May 2011
Published online: 18 August 2011

Abstract

In this work, we investigated the hot-electron dynamics of AlGaN/GaN HEMT structures grown by MOCVD on sapphire and SiC substrates at 80 K. High-speed current-voltage measurements and Hall measurements over the temperature range 27–300 K were used to study hot-electron dynamics. At low fields, drift velocity increases linearly, but deviates from the linearity toward high electric fields. Drift velocities are deduced as approximately 6.55 × 10⁶ and 6.60 × 10⁶ cm/s at an electric field of around E ~ 25 kV/cm for samples grown on sapphire and SiC, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, we used the so-called mobility comparison method with power balance equations. Although their low field carrier transport properties are similar as observed from Hall measurements, hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. We found that LO-phonon lifetimes are 0.50 ps and 0.32 ps for sapphire and SiC substrates, respectively. A long hot-phonon lifetime results in large non- equilibrium hot phonons. Non-equilibrium hot phonons slow energy relaxation and increase the momentum relaxation. The effective energy relaxation times at high fields are 24 and 65 ps for samples grown on sapphire and SiC substrates, respectively. They increase as the electron temperature decreases.