![]() Unlike bulk optical phonons, the mixing between available phonon frequencies of AlN and GaN induces an optical phonon mode that possesses a high propagation velocity at the heterointerface. To enhance the heat dissipation efficiency, a double heterostructure consisting of AlN/GaN/AlN is introduced to exploit properties of the interface mode optical phonons. In this work, we propose a novel phonon engineering technique that portends applications related to reducing the maximum temperature of the localized hotspot in GaN-based devices. This mismatch between the optical phonon generation rate and the relaxation rate into acoustic phonons results in a large accumulation of optical phonons in a localized region at the channel and eventually causes the electronic properties to degrade 10, 11. The process is known as the Ridley process 8 and the decay time is reported to be ~5 ps which is much longer than the electron interaction time of ~9 fs 9. To remove the excessive heat that is generated in the electron channel by the collision of hot electrons and the lattice (at the rate of ~10 THz) 7, the non-equilibrium optical phonons (whose propagation velocity is close to zero) must decay into acoustic phonons. As in common semiconductors, heat in GaN is carried mainly by the long wavelength acoustic phonons 6. The drawback associated with these high energy optical phonons is their short interaction time with electrons compared to the long decay time into acoustic phonons. Taking a closer look into the material, the large mismatch between the cation and anion masses causes a large splitting between the energies of the optical and acoustic phonon branches which raises the energy of optical phonons 5. The electron velocity saturation occurs with the onset of emission of these optical phonons and therefore their energy roughly determines the electron saturation velocity according to v 0 ≈ 1/2, where m is the effective electron mass. The spontaneous and piezoelectric polarization fields of this heterostructure allow the GaN layer to form a high-density electron channel through which electrons can flow with high saturation velocity (2.5 × 10 7 cm/s) this is partly due to the optical phonons with high energy ( ħω LO = 92 meV) in GaN 4. In particular, AlGaN/GaN high electron mobility transistors (HEMTs) are among the most promising devices for high-power applications 3. GaN-based semiconductors are of great interest in the electronics and optoelectronics communities because they possess large electronic bandgaps (3.4 eV) suitable for fabricating semiconductor lasers with wavelengths in blue and ultraviolet 1 as well as electronic devices designed to tolerate high electric fields (3.3 MV/cm) and elevated operating temperatures (700 ☌) 2. This suggests that the high group velocity interface mode optical phonons can be exploited to remove heat more effectively and reduce junction temperatures in GaN-based heterostructures. By adjusting the GaN thickness in the double heterostructure, the average group velocity can be engineered to become larger than the velocity of acoustic phonons at a specific electron energy. At the onset of interface phonon emission, the average group velocity is small due to the large contribution of interface and confined mode phonons with close-to-zero group velocity, but eventually increases up to larger values than the bulk GaN acoustic phonon velocity along the wurtzite crystal c-axis (8 nm/ps). The dispersion relation of the interface phonons shows a convergence to the resonant phonon frequencies 577.8 and 832.3 cm −1 with a steep slope around the zone center indicating a large group velocity. The formalism describing the interface and confined mode optical phonon dispersion relation, electron–phonon scattering rates, and average group velocity of emitted optical phonons are developed and numerically calculated. Here we present a detailed theoretical analysis of the interaction between electrons and optical phonons of interface and confined modes in a wurtzite AlN/GaN/AlN quantum well heterostructure based on the uniaxial dielectric continuum model.
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