Time-resolved Differential Transmission Measurements Research Articles

Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots.

Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, whereas holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation. Additionally, holes can be trapped at the core/shell interface, leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.

Probing the free-carrier absorption in multi-layer black phosphorus

We study the carrier relaxation dynamics in thin black phosphorus (bP) using time-resolved differential transmission measurements. The inter-band and intra-band transitions, relaxation, and carrier recombination lifetimes are revealed by tuning the mid-infrared probe wavelength above and below the bandgap of black phosphorus. When the probe energy exceeds the bandgap, Pauli blocked inter-band transitions are observed. The differential transmission signal changes sign from positive to negative when the probe energy is below the bandgap, due to the absence of inter-band transitions and enhancement in the free-carrier absorption. The minority carrier lifetime and radiative recombination coefficient are estimated to be 1.3 ns and 5.9 × 10−10 cm3/s, respectively. The overall recombination lifetime of bP is limited by radiative recombination for excess carrier densities larger than 5 × 1019 cm−3.

All-optical measurement of vertical charge carrier transport in mid-wave infrared InAs/GaSb type-II superlattices

Time-resolved differential transmission measurements were used to investigate vertical charge carrier transport in mid-wave infrared InAs/GaSb type-II superlattices (T2SLs). By optically generating excess carriers near one end of the mid-wave T2SL and measuring the transit time to a thin, lower-bandgap T2SL at the other end, the time-of-flight of vertically diffusing carriers was measured. Through investigation of both unintentionally doped and p-type T2SLs, the vertical hole and electron diffusion coefficients were measured to be 0.04 ± 0.03 cm2/s and 4.7 ± 0.5 cm2/s, corresponding to vertical mobilities of 6 ± 5 cm2/Vs and 700 ± 80 cm2/Vs, respectively, at a temperature of 77 K.

Exciton fine structure splitting in dot-in-a-well structures

A range of dot-in-a-well structures designed for long-wavelength (>1.3μm) emission is studied in polarization- and time-resolved differential transmission measurements. Quantum beats observed in differential transmission are employed to measure the fine structure splitting (EFS) of the bright exciton states. A strong dependence of EFS on In content in the InGaAs well surrounding the dots is observed. Large magnitudes of EFS up to 87μeV are found.

Out-of-well carrier screening in a strainedInxGa1−xN/GaNmultiple quantum well structure

Steady state and time-resolved differential transmission spectroscopy in a strained In xGa12xN/GaN multiple quantum well structure embedded within a p-i-n structure has been studied to identify the dominant screening mechanisms and measure the carrier sweep-out time. The results are consistent with a long-range out-of-well carrier screening where the photoexcited carriers escape the wells and separate, creating a space-charge field that induces an increase of the in-well field and shifts the excitonic transition to lower energies. A redshift of excitonic resonance under photoexcitation suggests that the direction of in-well field is opposite to that in the barriers due to the piezoelectric polarization in this strained In xGa12xN well. Compared with GaAs multiple quantum wells, a long carrier sweep-out time of 650 ps was observed in this structure. DOI: 10.1103/PhysRevB.68.233304 PACS number~s!: 78.67.De, 71.35.2y, 78.47.1p Time-resolved differential transmission measurements on GaAs multiple quantum wells ~MQW’s! embedded within a p-i-n structure have shown that the photoexcited electronhole pairs in the quantum well ~QW! under the influence of the electric field escape the well and travel in opposite directions to produce a space-charge field which opposes the built-in field. 1‐ 4 This out-of-well carrier screening mechanism, through the quantum-confined Stark effect ~QCSE!, shifts the excitonic resonance and modifies the absorption of the QW. Measurements of the change in the absorption as a function of time delay and wavelength provide a unique way to determine the dynamics of perpendicular carrier transport and carrier sweep-out times in p-i(MQW)-n structures. It is well known that hexagonal wurtzite III-nitride semiconductors have pronounced piezoelectric constant. 5,6 For InxGa12xN/GaN QW’s, the strength of the in-well electric field generated by spontaneous polarization and piezoelectric polarization can be as large as several MV/cm. Several groups have attributed the blueshift of the photoluminescence peak energy in undoped InxGa12xN/GaN QW’s with increasing carrier injection to the reduction of the QCSE due to in-well carrier screening. 7,8 Furthermore, built-in electric fields due to the p-n junction are present in these p-i(MQW)-n structures. Therefore, it is of interest to determine if the dominant carrier screening mechanism is in-well or long range out-of-well in such structures. A clear understanding of these carrier dynamics can provide great value for the design and optimization of InxGa12xN based emitters as well as for the optimization of material growth conditions. More importantly, such a study is expected to determine the carrier sweep-out times and provide insight into the potential profile in the strained InxGa12xN/GaN MQW’s. In particular, the sweep-out rate, a key property that limits the response time of nitride based p-i-n photodetectors, can be determined. In this Brief Report, we report experimental results on steady state and time-resolved differential transmission measurements of an InxGa12xN/GaN p-i(MQW)-n structure at room temperature. The differential transmission spectra are used to identify the dominant screening mechanisms and simultaneously to estimate the spatial band potential profile. A redshift of excitonic resonance immediately following pulsed excitation and a carrier sweep-out time of 650 ps were observed, indicating that photoexcited carriers sweep out of the well and screen the entire MQW structure. The increase of the in-well field under photoexcitation indicates that the direction of the in-well field is opposite to that in the barriers due to the piezoelectric polarization in the strained InxGa12xN wells. The sample described in this work was grown using metalorganic vapor phase epitaxy. Initially, a 4 mm thick GaN:Si layer was deposited on a c-face sapphire substrate. This layer was followed by the growth of ten periods of 7 nm thick GaN barriers and 2.5 nm thick In0.2Ga0.8N wells. The last QW was capped with a GaN barrier region followed by a 0.2 mm thick GaN:Mg p-contact layer. In order to determine the magnitude of the field distribution, we solved the Poisson equation for a p-i(MQW)-n structure within the depletion approximation. Neglecting the screening effects of background carriers, the net electric field within the wells ~barriers! Ew (Eb )i s

Stimulated emission and ultrafast carrier relaxation in InGaN multiple quantum wells

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different QW In compositions x. SE threshold energy densities (Ith) increased with increasing x-dependent QW depth. Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of Ith on x. Carriers are captured from the barriers to the QWs in <1 ps, while carrier recombination rates increased with increasing x. For excitation above Ith, an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing Ith.

Observation of phonon bottleneck in quantum dot electronic relaxation.

Time-resolved differential transmission measurements of self-assembled In0.4Ga0.6As quantum dots clearly indicate a phonon bottleneck between the n = 2 and n = 1 electronic levels. The key to this observation is the generation of electrons in dots where there are no holes so that electron-hole scattering does not mask the bottleneck. We use a simple carrier capture model consisting of two capture configurations to explain the bottleneck signal and offer arguments to rule out other possible sources of the signal.

Ultrafast optical characterization of carrier capture times in InxGa1−xN multiple quantum wells

Subpicosecond wavelength-degenerate differential transmission optical spectroscopy was used to characterize the electron capture time in a 10-period InxGa1−xN multiple-quantum-well (MQW) structure. Photoluminescence and photoluminescence excitation spectroscopies demonstrated enhanced MQW emission for injection within ±50 meV of the barrier energy. Time-resolved differential transmission measurements for excitation in this region reveal efficient electron capture in the quantum wells with a time constant between 310 and 540 fs. A slower exponential relaxation, with strongly wavelength-dependent subnanosecond decay constants, is also observed.

Ultrafast optical response and inter-valley scattering in GaSb/AlSb quantum wells

We report femtosecond time-resolved differential transmission measurements of GaSb/AlSb quantum wells which reveal direct evidence of electron Γ–L intervalley scattering. Raman measurements of non-equilibrium phonons generated by intra-Γ electron relaxation yield a complementary picture of the carrier dynamics, confirming the measured intervalley scattering rate.

Rapid carrier relaxation inIn0.4Ga0.6As/GaAsquantum dots characterized by differential transmission spectroscopy

Carrier relaxation in self-organized ${\mathrm{In}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots is investigated by time-resolved differential transmission measurements. The dots have a base dimension of around 14 nm and a height of 7 nm, leading to an average energy separation of the ground and first excited electronic states much greater than the LO-phonon energy, so the phonon-mediated electron relaxation is expected to be slow. Our measurements indicate that, even at low carrier densities (less than one electron-hole pair per dot), the electron and hole relaxation time constants are 5.2 and 0.6 ps, respectively; this indicates a lack of any ``phonon bottleneck'' and is consistent with a model of electrons scattering from holes which can relax rapidly via phonon emission.

Inter- and intrasubband relaxation times in quantum wells measured by femtosecond time-resolved differential transmission

Time-resolved differential transmission measurements with pump and probe laser pulse durations of less than 100 fs were made on quantum wells with varying well widths. The results were analysed with a phenomenological model using detailed rate equations with five fitting parameters, and comparisons are made with quantum mechanical calculations for carrier relaxations. The times reported here are in good agreement with both theoretical calculations as well as some of the recent experimental results.