Exciton Ionization Research Articles

Exciton dynamics studied via internal THz transitions

AbstractWe employ a novel, ultrafast terahertz probe to investigate the dynamical interplay of optically‐induced excitons and unbound electron–hole pairs in GaAs quantum wells. Resonant creation of heavy‐hole excitons induces a new low‐energy oscillator linked to transitions between the internal exciton degrees of freedom. The time‐resolved terahertz optical conductivity is found to be a probe well suited for studies of fundamental processes such as formation, relaxation and ionization of excitons.

Modulation of the electronic properties of GaN films by surface acoustic waves

We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by the SAW strain field.

Dynamical quantum interference and its controllability in semiconductors irradiated by an intense terahertz laser

The ionization of excitons by an intense terahertz (THz) laser, in nature, results from the dynamic quantum interference between the discrete state and the THz–photon sidebands of the continuum. As the acceleration of quasimomentum states hinders the simultaneous conservation of the momentum and energy, an intense THz field can block the momentum transfer in the ionization process, and thus suppress the ionization (dephasing) rate. Considering the fact that momentum transfer is essential in various scattering processes, an intense THz field is expected to control and even suppress other dephasing channels. This idea is demonstrated by a seminal example, i.e., the dephasing due to the conventional Fano resonance associated with the subband structures in low-dimensional semiconductors.

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

Femtosecond transient transmission pump–probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100–250 femtoseconds was observed by the time-resolved pump–probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.

Electric-field-induced impact ionization of excitons in GaN and GaN/AlGaN quantum wells

Impact ionization of exciton states in epitaxial GaN films and GaN/AlGaN quantum-well structures was studied. The study was done using an optical method based on the observation of exciton photoluminescence quenching under application of an electric field. It was established that electron scattering on impurities dominates over that from acoustic phonons in electron relaxation in energy and momentum. The mean free path of the hot electrons was estimated. The hot-electron mean free path in GaN/AlGaN quantum wells was found to be an order of magnitude larger than that in epitaxial GaN films, which is due to the electron scattering probability being lower in the two-dimensional case.

Two-dimensional exciton with spatially-separated carriers in coupled quantum wells in external magnetic field

Two-dimensional exciton with spatially-separated electron (e) and hole (h) in coupled quantum wells in transverse magnetic field is studied. Wide range of parameters—distance between wells d and magnetic field H—is considered. Dispersion laws E i ( P) for different values d and H are presented. Exciton energy spectra and wave functions are calculated using numerical diagonalization of the Hamiltonian on different bases. Exciton energy spectrum for a weak fixed H transforms with growth of magnetic momentum P from Coulomb-like regime to strong magnetic field regime. The possibility of ionization of two-dimensional spatially-separated exciton moving in transverse magnetic fields is discussed.

Impact ionization of excitons in an electric field in GaN

The impact ionization of excitonic states is studied in epitaxial GaN filmsand GaN/AlGaN quantum well structures. The investigation is carried out byoptical methods involving the observation of quenching of the excitonphotoluminescence under an applied electric field. It is found that impurityscattering rules the momentum and energy relaxation, rather than the acousticphonon scattering. The effective mean free path of hot electrons is estimated.In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to bean order of magnitude larger than those of GaN films due to the decrease inscattering probability of the electron in the two-dimensional case.

Temperature dependence of electric-field induced photoluminescence from an InGaN-based light-emitting diode

Temperature dependence of reverse-biased photoluminescence has been investigated for understanding the radiative recombination mechanism in an InGaN single-quantum-well light-emitting diode. It is found that the applied-voltage dependence of luminescence intensities is strongly affected by temperature from 17 to 100 K, and a dramatic decrease in the luminescence intensity is observed over 100 K. The model of a field ionization of excitons cannot explain this dramatic decrease in the luminescence intensity. It is therefore suggested that the free-carrier recombination process becomes dominant over 100 K. Two emission components are found on the condition of reverse bias. The lower-energy component becomes strongly dependent on reverse-bias voltage with increasing temperature, and fully disappears under the applied voltage of only −2 V at 100 K.

Photoluminescence modulation by high-frequency lateral electric fields in quantum wells

We investigate the modulation of the excitonic photoluminescence (PL) of GaAs quantum wells by high-frequency (frf) lateral electric fields. Under these fields, the PL becomes modulated in the form of pulses with repetition frequency of 2 frf. The periodic PL modulation is attributed to the time-dependent ionization of photogenerated excitons under the lateral electric field. The exciton ionization mechanism is proposed to be the impact ionization with electrons accelerated by the electric fields with a threshold field for ionization of about 15 V/cm. The different transport properties of electrons and holes are found to play a role in the exciton ionization process.

Excitonic photoluminescence quenching by impact ionization of excitons and donors inGaAs/Al0.35Ga0.65Asquantum wells with an in-plane electric field

We present a detailed experimental study on photoluminescence quenching due to exciton and donor impact ionization by accelerated electrons under an in-plane nanosecond duration electric field created in $\mathrm{GaAs}/{\mathrm{Al}}_{0.35}{\mathrm{Ga}}_{0.65}\mathrm{As}$ quantum wells. From the photoluminescence transients measured by the time-correlated single-photon counting technique, we have determined the experimental conditions under which donor impact ionization can have an influence on quenching of the excitonic photoluminescence. The coefficient of two-dimensional exciton impact ionization has been estimated; its dependences on the applied electric field, lattice temperature, and width of the quantum wells are given.

Energy transfer of excitons between quantum wells separated by a wide barrier

We present a microscopic theory of the excitonic Stokes and anti-Stokes energy-transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch (\ensuremath{\Delta}) at low temperatures (T). Several important intrinsic energy-transfer mechanisms have been examined, including dipolar coupling, real and virtual photon-exchange coupling, and over-barrier ionization of the excitons via exciton-exciton Auger processes. The transfer rate is calculated as a function of T and the center-to-center distance d between the wells. The rates depend sensitively on T for plane-wave excitons. For localized excitons, the rates depend on T only through the T dependence of the exciton localization radius. For Stokes energy transfer, the dominant energy transfer occurs through a photon-exchange interaction, which enables the excitons from the higher-energy wells to decay into free electrons and holes in the lower-energy wells. The rate has a slow dependence on d, yielding reasonable agreement with recent data from ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells. The dipolar rate is about an order of magnitude smaller for large d (e.g., $d=175\AA{})$ with a stronger range dependence proportional to ${d}^{\ensuremath{-}4}.$ However, the latter can be comparable to the radiative rate for small d (e.g., $d<~80\AA{}).$ For anti-Stokes transfer through exchange-type (e.g., dipolar and photon-exchange) interactions, we show that thermal activation proportional to $\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}{/k}_{B}T)$ is essential for the transfer, contradicting a recent nonactivated result based on the F\"orster-Dexter's spectral-overlap theory. Phonon-assisted transfer yields a negligibly small rate. On the other hand, energy transfer through over-barrier ionization of excitons via Auger processes yields a significantly larger nonactivated rate which is independent of d. The result is compared with recent data.

Ultrafast dynamics of the photo-induced magneto-optical Kerr effect in CdTe at room temperature

A time-resolved, all-optical technique based on the photo-induced magneto-optical Kerr effect is applied to the spectral study of the sub-picosecond, room temperature spin dynamics in CdTe between 1.44 and 1.63 eV. Two relaxation processes with respective decay times of up to 200 fs and 2.5 ps are distinguished. While the longer one is attributed to electron-spin relaxation, several processes such as the redistribution of electrons in the conduction band or exciton ionization near the band gap are believed to take place on the timescale of the faster one. A change of the spectral dependence of the Kerr effect from a paramagneticlike to a diamagneticlike regime during this faster relaxation is observed and discussed.

Spatially-Indirect Exciton in Coupled Quantum Wells or Coupled Quantum Dots in Magnetic Field

Two-dimensional excitons with spatially-separated carriers in coupled quantum wells (CQW) and also coupled quantum dots (CQD) in external magnetic field are considered. Energy spectrum dependences on the controlling parameters of the problem (interwell or interdot separation d, magnetic field H and confining potential steepness) were obtained both analytically and numerically. The possibility of ionization of two-dimensional spatially-separated excitons moving in transverse magnetic field is discussed.

Time-resolved magnetospectroscopy ofInxGa1−xAs/GaAsV-shaped quantum wires

We present a continuous-wave and time-resolved magnetoluminescence investigation of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ V-shaped quantum wires (QWR's) with In content $x=0.1,$ 0.15, $0.2.$ The diamagnetic shifts and the field dependence (up to 8 T) of the recombination lifetime show a competition between exciton and free-carrier processes. In QWR's with low In content $(x=0.1)$ the observed increase of the photoluminescence decay time with the magnetic field is explained by a theoretical calculation of free-carrier recombination. In-rich QWR's exhibit instead a decrease of the decay time at low temperature, which is characteristic of the exciton recombination, and an increase at high temperatures when thermal ionization of excitons occurs. Our results indicate that the extent to which exciton or free-carrier recombination predominates depends on the temperature and on the strength of the built-in piezoelectric field.

High-field franz-keldysh effect and exciton ionization in semiconductor quantum wires

We investigate the Franz-Keldysh effect and exciton ionization in semiconductor quantum wires. Absorption spectra are calculated near the band gap by solution of the low-density semiconductor Bloch equations in real space. The Sommerfeld factor and field-induced tunnel ionization of the exciton significantly affect the continuum portion of the absorption spectrum and remove the well known divergence problem associated with the 1D density of states at all field strengths. For reasonable electric field strengths substantial and tunable absorption oscillations appear above the band gap. Moreover, for very large fields, transparency can be achieved in the continuum for certain spectral positions.

Recombination dynamics of carriers in an InGaN/AlGaN single-quantum-well light-emitting diode under reverse-bias voltages

The radiative recombination process of the blue emission band in an InGaN single-quantum-well light-emitting diode has extensively been investigated by means of the dependence of an external electric field on photoluminescence and time-resolved photoluminescence spectra. Two emission (higher and lower) components separated by about 40 meV are found in the emission band on the condition of reverse bias at 77 K. It is also found that the luminescence intensity decreases dramatically with increasing reverse-bias voltage at room temperature. The model based on field ionization of excitons cannot explain the present experimental phenomena. It is, therefore, suggested that the free-carrier recombination process is dominant at room temperature.

Motional Narrowing in the Spin Relaxation of Free Excitons?

The signature of motional narrowing in the spin relaxation of excitons is the inverse proportionality between the spin relaxation time τs and the momentum relaxation time T2. We find such a relation in ZnSe quantum wells over a range of temperatures reaching from 8 up to 200 K. For low temperatures T < 60 K where the excitonic T2 is determined by scattering with acoustic phonons motional narrowing might be the reason for the increase of spin lifetime. But for higher temperatures where exciton ionization by optical phonons dominates we attribute the effect to a suppression of electron–hole exchange interaction due to the reduced spatial overlap of electron and hole wave functions.

Diminished photoluminescence polarization due to exciton ionization in strained Si1−xGex/Si(001) quantum wells

The photoluminescence (PL) of strained Si1−xGex/Si(001) quantum wells was measured in an edge-emitting configuration. The no-phonon PL was found to be polarized largely along the quantum-well plane at low temperature, whereas such in-plane polarization was diminished as the sample temperature was increased. Exciton dissociation occurring at higher temperature is responsible for the diminution of PL polarization, which is consistent with an analogous reduction of in-plane polarization due to field ionization of excitons.

Relation between spin and momentum relaxation in [formula omitted] quantum wells

First, we measure the coherence time of optically oriented carrier spins in wide gap quantum wells and observe an increase with temperature from less than 10 ps at 20 K to 500 ps at 200 K. Second, we measure the temperature dependence of the momentum relaxation time by four wave mixing and absorption experiments. The spin relaxation time is approximately proportional to the inverse of the momentum relaxation time. In the low-temperature regime, the strong increase of spin lifetime can be caused by motional narrowing due to the faster scattering of excitons with increasing temperature. In the high-temperature regime, quenching of the electron-hole spin flip dephasing due to an increasing ionization of excitons with increasing temperatures can prolongate spin lifetime.

Properties of Quantum Well Excitons in GaN/AlGaN and InGaN/GaN/AlGaN UV, Blue, Green, and Amber Light Emitting Diode Structures

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron–hole (e–h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e–h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.