Date Published: April 18, 2019
Publisher: Public Library of Science
Author(s): Ahmed Mohamed, Kihoon Park, Can Bayram, Mitra Dutta, Michael Stroscio, Chih-Chun Chien.
In GaN-based high electron mobility transistors (HEMTs), the fast emission of longitudinal optical (LO) phonons can result in the formation of hot spots near the gate region where high electric fields produce hot electrons. In this work, we investigate the probability of phonon emission as a function of electron energy for confined and interface (IF) phonon modes for wurtzite GaN/InGaN/GaN heterostructures. Hot electrons radiate optical phonons which decay, anharmonically, into acoustic phonons that are essentially heat carriers. Herein, phonon engineering concepts are introduced which facilitate thermal management through the production of polar optical phonons. Some of the electrons near a semiconductor gate which manifests a strong electric field, are accelerated and the resulting hot electrons will produce confined and interface modes when the electrons are incident on a suitably-placed quantum well. This paper focuses on the production of confined and interface phonons. It is shown that interface modes may be preferentially produced which lead to elongated, lower-temperature hot spots.
III-nitride semiconductors are technologically important materials and as a result of their large bandgap energies, they are suitable for the optoelectronic intersubband devices ranging from the ultraviolet to the near infrared . Nowadays, they are widely used, for instance in lighting applications, including GaN-based white light emitting diodes or in blue-ray players, which rely on GaN-based laser diodes. They can also endure high electric power densities and high breakdown voltages, which makes them materials of interest for high-power, high-frequency electronics applications [1,2].
Numerous studies have discussed the growth and the detection of lattice-matched GaN/InGaN QWs with well thicknesses as low as 1nm [20,21]. In this paper, it is shown that inserting a GaN/In0.15Ga0.85N/GaN quantum well in the hot spot region of a GaN HEMT, results in the emission of confined and interface phonons instead of bulk phonons as a result of emission from hot electrons, moreover, the phonons can be engineered to enhance the production of fast-moving interface phonons. Indeed, the IF and confined modes emission rates of GaN/In0.15Ga0.85N/GaN heterostructure with In0.15Ga0.85N thickness of d = 5, 4, 3, and 2 nm are numerically calculated and plotted in Fig 5. The thicknesses included in this paper allow us to grow GaN/InGaN QWs without misfit dislocations since they are below the critical thickness values . In Fig 5, the left curves denote the IF scattering rates and the right curves denote the confined scattering rates. The total emission rate is presented in black which encompasses the LO emission in magenta, the LO absorption in green and the TO emission in red whereas the TO absorption is disregarded since it has a value well below 1010 s-1. These rates include both symmetric and asymmetric phonon process. The emission of TO-like and LO-like modes causes the first and the second step-like features in the black curves respectively. In the d = 5 nm case, hot electrons produce less confined phonons in GaN/InGaN QWs compared to GaN/AlN , this is referred to the weak confinement of phonons due to the lower potential barrier of the former QW.
The frequency-dependent electron energy relaxation rates are calculated for GaN/InGaN/GaN quantum wells for different well widths. We have calculated the total scattering rate which includes both emission of IF and confined polar optical modes. It is shown that by quantum engineering of the inserted GaN/InGaN/GaN QW that the dominant phonon emission channel is that of interface phonon emission. Based on the large group velocity of the IF modes, as can be seen by comparing the IF phonon emission rates with confined phonon emission rates, a substantial fraction of the hot electron energy radiated as phonons goes into the IF phonon channel. Moreover, the IF propagate at high velocities compared to the bulk phonon velocities of the HEMT without the inserted QW. Accordingly, the anharmonically emitted heat-carrying acoustic phonons are generated in reduced-temperature, elongated hot spots. In summary, quantum engineering of IF phonon modes offers a means of thermal management of the hot spot temperature.