GaAs LO Phonon Research Articles

Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.

The effect of temperature on a double-barrier generator of terahertz coherent phonons

Abstract In this work we study the generation of coherent beams of terahertz acoustic phonons in GaAs-Al x Ga1− x As double barrier heretostruture (DBH). In this device, for a given external potential, the electrons in the excited level relax in the well emitting GaAs LO phonons. Due to anharmonicity, the LO phonon decays into a pair of phonons LO and TA. The TA phonons form an intense coherent beam in the [111] direction. The Keldysh non-equilibrium formalism is used to calculate the electronic current through the double barrier at finite temperatures. The system is described by a tight-binding Hamiltonian that includes the electrons, the phonons and the electron-phonon interaction. In our results, we analyze the behavior of the TA phonon emission rates (saser) when the temperature is increased.

Relaxation of hot electrons in a degenerate two-dimensional electron system: Transition to one-dimensional scattering

The energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three-terminal device. We observe a transition from two dimensions at zero magnetic field to quasi--one-dimensional scattering of the hot electrons in a strong magnetic field. In the two-dimensional case electron-electron scattering is the dominant relaxation mechanism, while the emission of optical phonons becomes more and more important as the magnetic field is increased. The observation of up to 11 optical phonons emitted per hot electron allows us to determine the onset energy of LO phonons in GaAs at cryogenic temperatures with a high precision, $\eph=36.0\pm0.1\,$meV. Numerical calculations of electron-electron scattering and the emission of optical phonons underline our interpretation in terms of a transition to one-dimensional dynamics.

Molecular Self-Assembly at Bare Semiconductor Surfaces: Investigation of the Chemical and Electronic Properties of the Alkanethiolate−GaAs(001) Interface

High-resolution X-ray photoelectron spectroscopy (HRXPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and Raman scattering have been used to characterize the bonding and electronic properties of the interfaces formed by ambient-temperature, solution self-assembly of octadecanethiol and dodecanethiol under oxygen-free conditions on GaAs(001) surfaces. The combination of HRXPS and ToF-SIMS shows that both monolayers form direct S−[GaAs] attachment to the bare substrate with dominant As−S bonding and some fraction of Ga−S bonding, but the densely packed octadecanethiolate self-assembled monolayers (SAMs) are significantly more effective in protecting the interface from oxide regrowth compared to the less dense dodecanethiolate SAMs. Raman scattering measurements of the GaAs LO phonon modes indicate that the formation of these direct, oxide-free S−[GaAs] interfaces does not induce any significant changes in the GaAs surface electronic states, whereas control experiments with inorganic sulfide t...

Electron-phonon interaction in individual strain-freeGaAs∕Al0.3Ga0.7Asquantum dots

We report accurate measurements of the excitonic ground state transitions in individual, strain free, $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum dots. We determine the spectral width and the energy of the zero-phonon line as a function of the temperature for a series of quantum dots with different sizes. In particular, the thermal broadening is well reproduced by a thermally activated process having a single activation energy ($36\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, corresponding to the GaAs LO phonon energy) independently of the dot size. Similarly, the energy of the zero-phonon line follows the GaAs gap temperature dependence irrespective of the dot size. Our findings demonstrate that (a) the exciton decoherence in quantum dots cannot be attributed to inelastic electron-phonon scattering but rather to pure dephasing processes driven by GaAs-LO phonons and (b) there is no quantum size effect on the excitonic energy renormalization with the temperature.

Cyclotron resonance of an interacting polaron gas in a quantum well: Magnetoplasmon-phonon mixing

Cyclotron-resonance (CR) spectra of a gas of interacting polarons confined in a GaAs/AlAs quantum well are theoretically investigated taking into account the magnetoplasmon-phonon mixing and band nonparabolicity. Contributions of different magnetoplasmon-phonon modes to the total magnetopolaron coupling strength are investigated as a function of the electron density. It is confirmed theoretically, that the resonant magnetopolaron coupling in a high-density GaAs/AlAs quantum well occurs near the GaAs TO-phonon frequency rather than near the GaAs LO-phonon frequency. Calculated CR spectra are in agreement with recent experimental data.

Filling-factor–dependent oscillations of the coupled intersubband plasmon-LO phonon mode in a tunneling coupled-bilayer system

We have investigated an ${\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ tunneling coupled-bilayer heterostructure with magnetotransport and far-infrared spectroscopy. With a front gate the charge density could be tuned. A grating coupler was prepared to investigate the collective excitations. We observed an intersubband plasmon which is coupled to the GaAs LO phonon. In a magnetic field applied perpendicular to the two-dimensional electron gas plane, the mode exhibits a strong filling-factor--dependent oscillation of the resonance energy.

Raman and resonant Raman studies of GaAs1−xNx

Recent developments in Raman and resonant Raman studies of GaAs1−xNx are discussed with respect to the nitrogen localized vibrational mode and the abnormal behaviour of this material. The normalized Raman intensity of nitrogen LVM with respect to that of GaAs–LO phonon as well as ωLVM exhibits a linear dependence on the nitrogen concentration for x ≤ 0.03, providing a useful calibration to determine the nitrogen composition in the ternary alloy GaAs1−xNx. The Raman intensity and linewidth resonances of the LO(Γ) phonon provide strong evidence for the formation of nitrogen-induced resonance states above the conduction-band minimum. Various phonons at the L and X zone boundaries not only emerge as distinct and sharp Raman features for excitations near the E+ transition but also exhibit the same intensity resonance enhancement as observed for the zone-centre phonons, LO(Γ) and TO(Γ), which provides strong evidence of significant L and X components in the wave function of the nitrogen-induced E+ state in GaAs1−xNx.

Composition dependence of Raman intensity of the nitrogen localized vibrational mode in GaAs1−xNx

We have investigated the nitrogen-localized vibrational mode (LVM) Raman intensity and its frequency (ωLVM) as a function of nitrogen concentration in GaAs1−xNx (x⩽0.04) at 300 K. The normalized Raman intensity of nitrogen LVM with respect to that of GaAs–LO phonon as well as ωLVM exhibits a remarkable linear dependence on the nitrogen concentration for x⩽0.03. This provides excellent calibration to determine nitrogen composition not only in the ternary alloy GaAs1−xNx but also in the quaternaries such as Ga1−yInyAs1−xNx.

Resonant quenching of photoluminescence in In xGa 1− xAs/Al yGa 1− yAs/GaAs self assembled quantum dots

We investigated a large series of self-assembled InAs and In x Ga 1− x As quantum dot (QD) structures by means of photoluminescence (PL) and PL excitation (PLE) techniques. A pronounced dip of the QD PLE spectra just below the GaAs absorption edge has been observed in all the investigated samples, denoting a resonant quenching of the QD PL intensity. PL spectra with excitation in such spectral region show, beside the QD PL band, a new extrinsic band at 1.356 eV with strong GaAs–LO phonon replicas. The PLE spectrum of this extrinsic band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and the defects associated with the 1.356 eV emission. The chemical etching of the QD and of the surrounding barrier layers does not eliminate the extrinsic PL band, which is, therefore, attributed to a native defect of the GaAs buffer. The spectral position and shape of the 1.356 eV band allows us to associate it to a complex of Ga vacancy with deep acceptor states in the GaAs layers. We conclude that the resonant PL quenching of QDs arises from the competition in the carrier capture between the QDs and deep defects in the GaAs layers.

Magneto-optics on single quantum dots

We report about magneto-optical experiments on single self-assembled InGaAs quantum dots. In the first part of our contribution we concentrate on phonon assisted absorption studied by photoluminescence excitation spectroscopy. We observe phonon assisted absorption via InGaAs and GaAs LO phonons, as well as sequential phonon assisted biexciton generation followed by sequential biexciton decay. The magnetic field dependence of the observed phonon resonances exhibits distinct differences between exciton and biexciton lines, which are caused by spin conservation and decay statistics. In the second part, we report about controlled single electron charging of a single quantum dot investigated by magneto-photoluminescence spectroscopy on electric field tunable structures. We observe the emission lines of neutral, single, and double charged exciton states for different bias conditions. The application of high magnetic fields results in fully resolved Zeeman splittings and diamagnetic shifts. The upper Zeeman component of the single charged exciton is found to be quenched at higher electric fields. This behavior is explained in terms of an enhanced tunneling probability of the triplet versus the singlet configuration.

Optical and magneto-optical investigations on single-quantum dots

We report about optical experiments on single self-assembled quantum dots. In power-dependent low-temperature magneto-photoluminescence experiments we have analysed the emission spectra of single dots for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to a renormalization of the emission lines in the region of the s- and p-shell of the dot. Photoluminescence excitation spectroscopy further allows us to explore the absorption properties of a single-quantum dot. Both interband absorption and comparably strong phonon-assisted absorption via InGaAs and GaAs LO phonons are observed, as well as sequential phonon-assisted biexciton generation followed by sequential biexciton decay. Working towards applications of single quantum dots, local charge injection/extraction into/from dots is performed on special p–i–n structures via a STM-tip. STM-induced luminescence from a single-dot results in single-line emission from the quantum dot ground state under the condition of low injection currents. Reverse operation of such a single quantum dot LED allows in addition for spectrally resolved photocurrent experiments. Characteristic sharp peaks in the STM tip current vs. excitation energy are attributed to resonant quantum dot interband absorption processes.

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.

Hot-electron relaxation via optical phonon emissions inGaAs/AlxGa1-xAs quantumwell structures: dependence upon the alloy composition and barrierwidth

We present a systematic investigation of thedependence of the hot-electron-optical-phonon interactions onAl composition and barrier width in GaAs/AlxGa1-xAsMQW structures. Raman scattering measurementsat 15 K are presented for samples with different barrier widthsand Al composition. The optical phonon energies emitted by thephotoexcited electrons in quantum wells were also determined byusing hot-electron-neutral acceptor luminescence techniques. Itis shown that the relaxation of hot electrons in the quantumwells is dominated by the GaAs LO phonon emission for smallx, but by AlAs-like LO phonons for larger Al composition. Forsamples with larger barriers, the electrons in the GaAs layerrelax mostly through the AlAs-like optical phonon emission.However, in samples with smaller barriers, the relaxation ofhot electrons is dominated by the GaAs optical phononemission.

Coupled bloch-phonon oscillations in semiconductor superlattices

We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

Magnetophonon resonance in photoluminescence excitation spectra of magnetoexcitons inGaAs/Al0.3Ga0.7Assuperlattice

Strong increase in the intensity of the peaks of excited magneto-exciton (ME) states in the photoluminescence excitation (PLE) spectra recorded for the ground heavy-hole magneto-excitons (of the 1sHH type) has been found in a GaAs/AlGaAs superlattice in strong magnetic field B applied normal to the sample layers. While varying B the intensities of the PLE peaks have been measured as functions of energy separation $\Delta E$ between excited ME peaks and the ground state of the system. The resonance profiles have been found to have maxima at $\Delta E_{\rm max}$ close to the energy of the GaAs LO-phonon. However, the value of $\Delta E_{\rm max}$ depends on quantum numbers of the excited ME state. The revealed very low quantum efficiency of the investigated sample allows us to ascribe the observed resonance to the enhancement of the non-radiative magneto-exciton relaxation rate arising due to LO-phonon emission. The presented theoretical model, being in a good agreement with experimental observations, provides a method to extract 1sHH magneto-exciton ``in-plane" dispersion from the dependence of $\Delta E_{\rm max}$ on the excited ME state quantum numbers.

Raman and hot electron–neutral acceptor luminescence studies of electron–optical phonon interactions in GaAs/Al xGa 1− xAs quantum wells

Using two optical techniques, we have studied the hot electron–optical phonon interactions in GaAs/Al x Ga 1− x As multiple-quantum wells. Raman scattering measurements at 15 K are presented for the Al composition of x=0.3, 0.5, 0.7 and 1.0. The GaAs-like and AlAs-like phonon frequencies of the first-order modes are also measured as a function of Al composition. The optical phonon energies emitted by the photoexcited electrons in quantum wells are determined by using hot electron–neutral acceptor luminescence techniques. It is shown that the relaxation of hot electrons in the quantum wells is dominated by the GaAs LO phonon emission for small x, but by AlAs-like LO phonons for larger Al composition.

Carrier capture in ultrathin InAs/GaAs quantum wells

The carrier capture into ultrathin InAs layers embedded in a GaAs matrix has been investigated by time-resolved two-wavelength pump-probe phototransmission at 4.2 K. Using an InAs thickness of 1.2 monolayers, we observe switching of the carrier relaxation from optical to acoustic phonon emission. At the light-hole (lh) exciton transition we find a constant capture time of 20 ps. In contrast, the capture time decreases abruptly from 50 ps to 22 ps within the heavy-hole (hh) exciton transition as the energy separation between lh and hh states exceeds the threshold for GaAs LO phonon emission. The combination of both characteristics provides strong evidence for a two-step capture process of the holes. First the holes are captured by the weakly confined lh state and then they cool down to the hh state. We calculated the transient bleaching of the excitonic absorption considering both phase-space filling and exciton screening. The calculations show in agreement with the measurements that the phototransmission transients directly reflect the population of the confined InAs states only at excitation densities below 3×108 cm-2. At larger excitation densities, the phototransmission rise time becomes significantly smaller than the capture times whereas its decay time appears longer than the carrier lifetime.

Multi-exciton spectroscopy on a self-assembled InGaAs[formula omitted]GaAs quantum dot

We report about spatially resolved magneto-optical experiments on a self-assembled InGaAs quantum dot. Using electron beam lithograpy for patterning a metal shadow mask we can isolate a single dot. This allows us to study the optical response of a single dot as a function of excitation power and magnetic field. We investigate the influence of many body interaction in the emission spectra for different exciton occupation numbers of the dot. The diamagnetic/orbital shift as well as Zeeman splitting in a magnetic field can be fully resolved and are used to identify the observed emission lines. Further we report on absorption properties of the quantum dot as a function of magnetic field. We analyse in detail the phonon-assisted absorption process connected with the GaAs LO-phonon 36 meV above the single-exciton ground state.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation- δ-doped GaAs/Al 0.3Ga 0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E LO at B>27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E LO+( E 2− E 1), where E 2, and E 1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm −1 around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LL of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process.