Optical Phonon Sidebands of Electronic Intersubband Absorption in Strongly Polar Semiconductor Heterostructures

Abstract

We present the first evidence for a distinct optical phonon progression in the linear and nonlinear intersubband absorption spectra of electrons in a GaN=Al 0:8 Ga 0:2 N heterostructure. Femtosecond twocolor pump-probe experiments in the midinfrared reveal spectral holes on different vibronic transitions separated by the LO-phonon frequency. These features wash out with a decay time of 80 fs due to spectral diffusion. The remaining nonlinear transmission changes decay with a time constant of 380 fs. All results observed are described by the independent boson model.The optical line shapes of electronic transitions in condensed matter reflect the ultrafast dynamics of the elementary excitations to which the electrons are coupled. Of particular interest for a broad range of phenomena is the coupling between electrons and nuclear motions, i.e., local vibrational modes and/or phonons.(i) Coupling to underdamped nuclear motions gives rise to spectrally distinct sidebands in electronic spectra [1]. In molecules, such an interaction with intramolecular modes underlies the vibronic structure of electronic absorption and emission bands, which are additionally broadened by dephasing and spectral diffusion processes originating from a coupling to (overdamped) motions of the fluctuating surroundings [2,3]. In crystalline solids with much more delocalized electronic wave functions and phonon modes, phonon sidebands of electronic transitions have been observed mainly for interband spectra of impurities [1], for quantum-dot-like structures [4][5][6][7][8], and through coherent phonon oscillations modulating the optical reflectivity [9,10].(ii) Electron-phonon coupling strongly influences the nonequilibrium dynamics of electrons, both in the quantum-kinetic [11] and in the incoherent scattering regimes [12]. Such phenomena occur on femtosecond to picosecond time scales and have been investigated extensively by ultrafast nonlinear spectroscopy.So far, the majority of investigations of electron-phonon coupling focused on interband transitions in solids. The influence of electron-phonon coupling on intraband transitions was mainly studied in the context of polaron physics [13,14]. Intraband transitions provide direct access to the nonequilibrium dynamics of electrons. Theoretical studies suggest a prominent role of electron-phonon coupling for the line shape of intraband (free-carrier) absorption and emission. So far, clear experimental signatures of electron-phonon coupling are missing for the line shape of intersubband (IS) transitions (dipole-allowed transitions between quantized conduction subbands [15]), although first theoretical studies predict such polaronic signatures [16].Because of their strong electron-phonon coupling, nanostructures made from strongly polar materials like group-III nitrides are promising systems for the occurrence of IS phonon sidebands. Because they have large conduction band discontinuities and thus allow IS transitions at short wavelengths, GaN=AlGaN multiple quantum wells have received much interes...

Microwave noise technique for measurement of hot-electron energy relaxation time and hot-phonon lifetime

“…This difference is expected to result from the dependence of the lifetime on the electron density. Figure 7 compares the lifetime obtained for GaN / AlGaN / GaN / AlN / GaN (hexagon) with those reported earlier for GaN (stars) [3] and 2DEG channels (closed diamond, closed square, open circle) [1,4,18]. Different techniques lead to similar results if the electron density per unit volume is approximately the same.…”

“…Dependence of hot-phonon lifetime on electron density for GaN bulk and 2DEG channels. Open symbols correspond to timeresolved laser experiments: Raman scattering for bulk GaN (stars) [3] and intersubband absorbtion for AlGaN / GaN (circle) [18]. Closed symbols are for microwave noise measurements of 2DEG channels: AlGaN / GaN (square) [4], AlGaN / AlN / GaN (diamond) [1], and GaN / AlGaN / GaN / AlN / GaN (hexagon, present paper).…”

Fluctuation technique for investigation of ultrafast processes

“…The fluctuation technique has yielded the first experimental result, 350 fs [19], on the hot-phonon lifetime measured in a 2D channel with a degenerate electron gas of interest for high-power microwave HEMTs. Two years later, the value of 380 fs is obtained for a similar 2D channel from the absorption experiment [20]. These lifetime values, 350 fs and 380 fs, measured for AlGaN / GaN structures with 2D channels located in GaN, are close to the Raman lifetime value of 350 fs obtained for bulk GaN under extremely high pump intensity [15].…”

“…3, closed circle [19]) was confirmed two years later by the time-resolved LOphonon-assisted inter-sub-band infrared-absorption experiment carried out for a similar AlGaN / GaN channel (black down-triangle [20]). The obtained values nearly coincide (closed circle and black down-triangle).…”

“…The alternative techniques for hot-phonon lifetime measurement are based on hot-electron fluctuations [19] and time-resolved hot-phonon-assisted inter-subband absorption [20]. The fluctuation technique has yielded the first experimental result, 350 fs [19], on the hot-phonon lifetime measured in a 2D channel with a degenerate electron gas of interest for high-power microwave HEMTs.…”