Femtosecond intersubband dynamics of electrons in AlGaN/GaN-based high-electron-mobility transistors

Abstract

Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pump-probe experiments with 50 fs mid-infrared pulses. Two-colour pump-probe measurements show an instantaneous transmission increase for all spectral positions of the probe, which demonstrates that homogeneous broadening is an important contribution to the total linewidth in this material. We observe the maximum of the induced transmission change around 5 µm. This large Stokes shift might be caused by the extremely large electron-LO-phonon scattering rate.Optical intersubband excitations in semiconductor nanostructures provide direct insight into the ultrafast dynamics of coherent optical polarizations and nonequilibrium carriers and play a key role for novel devices such as quantum cascade lasers. Because of the huge conduction band discontinuity and thus the possibility of fabricating quantum cascade lasers at short wavelengths, nanostructures made of group-III nitrides have recently attracted considerable interest. In particular, intersubband absorption in GaN/AlGaN multiple quantum wells (MQWs) was observed in the spectral range up to the communication wavelength λ ≈ 1.55 µm [1]. The first time-resolved experiments on MQWs (intersubband absorption at λ = 4.5 µm) gave a rough upper limit of 150 fs for intersubband relaxation [2]. Two-dimensional electron subbands can also be formed in the inversion layer of an AlGaN/GaN-based high-electronmobility transistor (HEMT). Recently, e1-e2 intersubband absorption was observed for the first time in AlGaN/GaNbased HEMTs [3] showing a broad absorption centred at λ = 4 µm [ figure 1(a)]. This HEMT (GS1282 in [3]) was grown on a c-face sapphire substrate using MBE. The layer sequence is 20 nm pure AlN, 1.2 µm nominally undoped GaN, 13 nm undoped Al 0.8 Ga 0.2 N, and finally 2 nm undoped GaN as a contact layer. For reflection a gold layer was sputtered onto the contact layer. In this sample, a large piezoelectric field at the heterointerface is present due to the lattice mismatch and the hexagonal crystal structure. The piezoelectric field together with the heterointerface results in a deep triangular well.Here, we present the first time-resolved study of the two-dimensional electron plasma in this Al 0.8 Ga 0.2 N/GaNbased HEMT, using direct intersubband excitation and probing of electrons via the e1-e2 intersubband transition. The intersubband absorption spectrum at a temperature of 300 K is plotted in figure 1(a). The broad absorption band is due to transitions of electrons from the n = 1 to the n = 2 subband in the triangular potential well.In our time-resolved experiments, electrons are excited from the e1 to the e2 subband by 50 fs mid-infrared pulses resonant with the optical IS transition. Pump and probe pulses at 1 kHz repetition rate are derived from the output of an optical parametric amplifier by difference frequency mixing in a 1 mm thick GaSe crystal [4]. After interaction with the sample, the probe pulses are spectrally dispersed in a monochromator (3 meV reso...

Intersubband Transition-Based Processes and Devices in AlN/GaN-Based Heterostructures

“…Nevertheless, recent experiments with AlN/GaN-based superlattice structures have revealed very interesting physical properties, which have eventually resulted in high-speed optoelectronic devices for the 1.3/1.55 "m wavelength region. That this statement is actually more than just a hypothesis has been confirmed already by several experiments which demonstrated intersubband scattering times on the order of hundreds of femtoseconds [21], [22]. This paper is organized as follows.…”

Photodetectors based on intersubband transitions using III-nitride superlattice structures

“…Additionally, thanks to their extremely strong electronlongitudinal optical phonon scattering effects, III-nitrides have the potential to outperform existing semiconductor-based telecommunication devices in terms of speed. That this statement is actually more than just a hypothesis, has been confirmed already by several experiments which demonstrated intersubband scattering times of the order of hundreds of femtoseconds [25,26].…”