Hot-phonon temperature and lifetime in a biased Al x Ga 1 Àx NÕGaN channel estimated from noise analysis A. Matulionis,* J. Liberis, I. Matulioniene ˙ , and M. Ramonas Semiconductor Physics Institute, A. Gos ˇtauto 11, Vilnius 2600, Lithuania L. F. Eastman, J. R. Shealy, V. Tilak, and A. Vertiatchikh Cornell University, 425 Philips Hall, Ithaca, New York 14853, USA Received 21 January 2003; revised manuscript received 13 May 2003; published 31 July 2003 The short-time-domain gated radiometric microwave noise technique is developed for the investigation of hot phonons in a two-dimensional electron gas channel subjected to a strong electric field applied in the plane of electron confinement. Nominally undoped pseudomorphic Al 0.15 Ga 0.85 N/GaN channels are considered in the field range where LO-phonon emission by hot electrons and hot LO-phonon disintegration are mainly respon- sible for the energy dissipation. At a 5 kV/cm electric field, the equivalent temperature of the emitted LO phonons reaches 590 and 460 K at 80 and 293 K ambient temperatures, respectively. The electrons and emitted LO phonons form a nonequilibrium electron–LO-phonon subsystem characterized by a temperature different from that of the remaining phonons. The LO-phonon lifetime for their disintegration into the acoustic and other phonons is 350100 fs; the lifetime is almost independent of the hot-phonon and ambient temperatures. The deduced value of the LO-phonon lifetime is used as an input parameter for Monte Carlo simulation of the hot-phonon effect on the two-dimensional electron transport in the biased channel, and a reasonable agreement with the experimental current-voltage characteristics is obtained. DOI: 10.1103/PhysRevB.68.0353XX PACS numbers: 63.20.Kr, 72.20.Ht, 72.70.+m I. INTRODUCTION Nitride two-dimensional electron gas 2DEG channels are the most promising for high-power microwave applications. 1 Under standard conditions of operation, the electrons are heated by an electric field and an efficient dis- sipation of the excess energy is of primary importance for channel performance. 2 Different experimental techniques 3–6 have been used to study hot-electron energy relaxation in nitride 2DEG channels. At a high bias, the main electron energy relaxation mechanism includes emission of longitudi- nal optical LO phonons, their disintegration into acoustic and other phonons, and energy transfer towards the heat sink located at some distance from the channel. The disintegration of the emitted LO phonons is known to be a bottleneck for the energy relaxation. 7 As a result, the LO-phonon distribu- tion in the channel is strongly displaced from thermal equilibrium. 8 The estimated equivalent temperature of the emitted LO phonons—the hot-phonon temperature—is higher than the ‘‘lattice’’ temperature of the rest phonons. 9 Of course, the lattice temperature in the channel also exceeds that of the remote heat sink held at ambient temperature. 10 Finally, since the 2DEG density is high, the electrons have their own hot-electron temperature. 11 The Monte Carlo technique was applied to simulate hot- electron effects in AlGaN/GaN. 8,12 The calculated hot- electron energy relaxation time was found to saturate at high electric fields, and the saturation value depended on the LO- phonon lifetime with respect to the nonequilibrium LO- phonon disintegration into other phonon modes. 8 The results of the simulation with hot-phonons taken into account were in a reasonable agreement with the experimental data. 4 An experimental high-field value of 550 fs was reported for the AlN/GaN 2DEG channel. 9 Hot phonons also manifested themselves during time- resolved Raman light scattering 7,13 and laser-excited–probe 14 experiments carried out on bulk GaN samples. For bulk GaN layers, the LO-phonon lifetime was estimated to decrease from 5 ps at 25 K down to values below 3 ps at 300 K. 13 These values are essentially longer than the high-field values of the hot-electron energy relaxation time determined for AlGaN/GaN and AlN/GaN 2DEG channels. 4,8,9,11 Conse- quently, the LO-phonon lifetime estimated for GaN bulk samples from the Raman light scattering data 13 are not ap- plicable for 2DEG channels located in GaN. On the other hand, to our knowledge, time-resolved Raman light scatter- ing experiments on hot phonons have not been carried out on nitride 2DEG channels. This paper aims to demonstrate that the hot-phonon lifetime in AlGaN/GaN 2DEG channels can be extracted from microwave noise experiments. II. SAMPLES Two-electrode samples for noise temperature measure- ments were prepared from a nominally undoped Al 0.15 Ga 0.85 N/GaN heterostructure with a 2DEG channel. Ohmic Ti/Al/Ti/Au electrodes were formed at 1100 K. The results will be presented for samples where the channel length L =12  m and the electrode width w =120  m. The heterostructure consisted of a 1- m-thick GaN buffer layer on a 150- m Al 2 O 3 substrate; the buffer layer was overgrown with a pseudomorphic 25-nm layer of Al 0.15 Ga 0.85 N and protected with a 33-nm layer of Si 3 N 4 for more details see Refs. 15 and 16. The conductive channel was located in the GaN layer close to the AlGaN/GaN inter- face. A degenerate 2DEG was induced by spontaneous polar- ization and piezoelectric fields. The electron sheet density was estimated from Hall effect measurements: n 2D =5 PHYSICAL REVIEW B 68, 035338 2003 0163-1829/2003/683/0353387/$20.00 ©2003 The American Physical Society 68 035338-1