Suvranta K. Tripathy

Stokes and anti-Stokes resonant Raman scatterings from biased GaN/AlN heterostructure

An electric field present in a GaN/AlN heterostructure can bring both the first-order and second-order Raman scattering processes into strong resonances. The resonant Stokes and anti-Stokes Raman scatterings result in the increase and decrease in nonequilibrium longitudinal-optical phonon temperatures, respectively. Moreover, the phonon temperature measured from the Raman scattering is increased with an applied electric field at a much higher rate than the lattice temperature due to the presence of field-induced nonequilibrium longitudinal-optical phonons.

Evidence of hot electrons generated from an AlN∕GaN high electron mobility transistor

We have observed that the temperature of the electrons drifting under a relatively high electric field in an AlN∕GaN-based high-electron-mobility transistor is significantly higher than the lattice temperature (i.e., the hot electrons are generated). These hot electrons are produced through the Frohlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field versus electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100fs.

Phonon-assisted ultraviolet anti-Stokes photoluminescence from GaN film grown on Si "111… substrate

Phonon-assisted anti-Stokes photoluminescence (ASPL) in the ultraviolet region has been observed in the GaN film grown on a Si (111) substrate. The ASPL peaks are observable only at sufficiently low temperatures. In addition, even if the photon energy is ≈318meV below the transition energy for bound excitons, the ASPL peaks can be still observed. Based on our analysis, the donor-acceptor pairs and bound excitons have played primary roles in the generation of ASPL. Upon the absorption of photons, the ionizations of the neutral donors and neutral acceptors are assisted by longitudinal-optical phonons.

Anti-Stokes photoluminescence from n-type free-standing GaN at room temperature based on competition between phonon-assisted and two-photon absorption

We have observed anti-Stokes photoluminescence from n-type free-standing GaN at room temperature. Such a process is caused by phonon-assisted absorption. When the excitation photon energy is sufficiently below the donor–acceptor transition energy, however, two-photon absorption becomes the dominant mechanism for anti-Stokes photoluminescence. By measuring the dependences of the photoluminescence spectra on temperature, excitation power and excitation photon energy, we have demonstrated that the donor–acceptor pair transition plays an important role in anti-Stokes photoluminescence. Our study could result in efficient laser cooling of semiconductors.

Investigation of hot electrons and hot phonons generated within an AlN/GaN high electron mobility transistor

We review our recent results obtained on an AlN/GaN-based high-electron-mobility transistor. The temperature of the electrons drifting under a relatively-high electric field is significantly higher than the lattice temperature (i.e., the hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. We have also measured the decay time constant for LO phonons to be about 4.2 ps. An electric field present in a GaN/AlN heterostructure can bring both the first-order and second-order Raman scattering processes into strong resonances. The resonant Stokes and anti-Stokes Raman scattering results in the increase and decrease of non-equilibrium longitudinal-optical phonon temperatures, respectively. Moreover, the phonon temperature measured from the Raman scattering is increased with an applied electric field at a much higher rate than the lattice temperature due to the presence of field-induced non-equilibrium longitudinal-optical phonons.

Observation of anti-Stokes fluorescence from GaN Film grown on Si (111) substrate

Phonon-assisted anti-Stokes fluorescence has been observed in GaN film grown on Si (111) substrate. The donor-acceptor pairs and bound excitons have played primary roles in the generation of anti-Stokes fluorescence.

Anti-Stokes photoluminescence from n-type free-standing GaN based on competing two-photon absorption and phonon-assisted absorption

Mechanisms for anti-Stokes photoluminescence observed at room temperature from n-type free-standing GaN have been attributed by us to the competition between two-photon absorption and phonon-assisted absorption.

Anti-Stokes photoluminescence in GaN single crystals and heterostructures

We review some of our recent results following the investigation of anti-Stokes photoluminescence (PL). Indeed, we have observed anti-Stokes photoluminescence from n-type free-standing GaN at room temperature. Such a process is induced by phonon-assisted absorption. When the excitation photon energy is sufficiently below the donor-acceptor transition energy, however, two-photon absorption becomes the dominant mechanism for anti-Stokes photoluminescence. By measuring the dependences of the photoluminescence spectra on temperature, excitation power, and excitation photon energy, we have demonstrated that donor-acceptor pair transition plays an important role in the generation of anti-Stokes photoluminescence. Our study could result in efficient laser cooling of semiconductors.

Resonant Raman scattering of coherent picosecond pulses by one and two longitudinal-optical phonons in GaN film grown on silicon (111) substrate

We have observed resonance-enhanced Stokes and anti-Stokes Raman scattering of coherent picosecond pulses by one as well as two longitudinal-optical phonons in GaN film grown on Si (111) substrate.

Hot and cold phonons induced by electric field and resonant Raman scattering in GaN/AlN triangular quantum well

We have evidenced hot and cold longitudinal-optical (LO) phonons induced by electric field and resonant Raman scattering in GaN/AlN triangular quantum well, probed by first-order and second-order resonant Raman scattering of 3-ps light pulses.