Millisecond-Range Electron Spin Memory in Singly-Charged InP Quantum Dots

Capture, relaxation and recombination in quantum dots

We demonstrated millisecond-range spin memory of resident electrons in an ensemble of singly negatively charged InP quantum dots under a small longitudinal magnetic field of 0.1 T, at T>5 K. For this we used a pump-probe photoluminescence (PL) technique to measure degree of circular polarization of the probe PL as a function of pump-probe delay (τ). Effect of spin orientation imparted to the resident electrons by the pump pulse was evident

The propagation of spin-polarized currents from a magnetic to a nonmagnetic material is fundamental to spintronics and, furthermore, has become a topic of great interest for basic research. The techniques of electrical spin injection and detection in bulk metals and thin metal films are reviewed, along with recent studies of spin injection between a ferromagnetic metal and a two-dimensional electron gas (2DEG). Electric current crossing the interface between a ferromagnet (F) and nonmagnetic metal (N) is also a current of magnetization, JM, and provides a source for spin diffusion in N. When the spin relaxation rate 1/T2 in N is relatively slow, a nonequilibrium population of polarized spins accumulates in a region characterized by the interfacial area and the spin diffusion length δs. The spin accumulation, equivalently called nonequilibrium magnetization M̃, can be detected by application of a transverse external magnetic field which causes a Hanle effect. Spin–orbit effects are the dominant spin relaxation mechanism in nonmagnetic metals. In a noncentrosymmetric 2DEG, unusual spin–orbit effects cause a spin splitting of the electron energy band. A bias current applied to such a 2DEG can generate nonequilibrium spin magnetization, which can be detected by a ferromagnetic electrode. Preliminary experiments on the injection and detection of spin polarized current at a F–2DEG are relevant to the realization of a spin injected field effect transistor (FET).

Multiple layers of GaN/AlN quantum dot (QD) ensembles were grown by the Stranski–Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperature, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing the vertically stacked QDs leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film. We have selectively modified the thermal stress in the QD layers by etching a cross-hatched pattern into the as-grown sample using inductively coupled Cl2/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width from ∼2 to 10 µm and orientation along [11−20] and [1−100] can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing e–h pair excitation for the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼46 K). A theoretical modelling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the and effective-mass methods for calculations of the e–h wavefunctions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during e–h pair excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing e–h pair excitation density at T = 300 K.

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

In this thesis the optical properties of self-assembled III-V semiconductor nanostructures are investigated by analyzing the photoluminescence (PL) with and without an external magnetic field. The studied PL is a result of the recombination of electrons and holes confined in the nanostructures. In this thesis we directly correlate the size, shape, composition and topology of the nanostructures to their magnetic and optical properties and study the influence of an electrical contact in the close proximity of these nanostructures. One of the most interesting nanostructures are quantum dots (QDs), which are able to confine charge carriers in all three dimensions. Both the exciton g-factor gex and the exciton diamagnetic coefficient ad determine the behavior of excitons confined in a QD in a magnetic field and provide further insight in the energy level structure of these nanostructures. The exciton Zeeman splitting is proportional to gex and is a result of the different spin configurations of the electron and hole, which constitute the exciton. The diamagnetic shift is a result of the additional confinement provided by the magnetic field B, and is proportional adB2, where ad scales with the lateral extend of the quantum dot. PL experiments on a large ensemble of InAs/GaAs QDs, as well as on individual InAs/GaAs QDs are performed. For these dots ad and gex have been determined. Importantly, there is a trend between gex and the emission energy E0: for larger emission energy a more negative value of the exciton g-factor is observed. From the power and temperature dependence of the ensemble PL it is shown that the sample consists of QDs with different height, where the highest dots correspond to the smallest emission energies. Moreover, QDs with larger ad have a more positive g-factor. From this it is inferred that quantum dots with an overall larger size have a less negative value of gex. By implementing the QDs in a Schottky device in close proximity to an electrical contact it is possible to tune the charged state of the exciton in the dot. We show that charge-tunable InAs/GaAs quantum dots also allow to study many-body interactions between a localized QD state and a Fermi sea of electrons. As a result of these many-body interactions new optical transitions are observed in the PL spectra. The same chargetunable quantum dots are also investigated in a magnetic field. Instead of the positive diamagnetic shift observed for the majority of the quantum dots, two different types of negative diamagnetic shift are reported. The shallow character of our quantum dots causes a negative quadratic diamagnetic shift for the highly negatively charged exciton complexes in line with predictions for shallow quantum dots. The second type of negative diamagnetic shift is observed even for the neutral exciton and is strongly linearly dependent on the magnetic field. We also investigated quantum dots grown from different semiconductor materials. InAs/InP quantum dots are studied using AFM, X-STM, macro-PL and micro-PL. Macro PL and X-STM measurements show that the studied InAs/InP dots have a multimodal height distribution. Single quantum dot luminescence, carried out on a large number of dots, shows a strong correlation between exciton g-factor, diamagnetic coefficient and emission energy. In fact, the strong dependence of gex on the emission energy results in a sign change of the exciton g-factor. In correspondence with what we found on the InAs/GaAs QDs, we find that dots having a smaller overall size will have a more negative gex as compared to quantum dots of larger overall size. We also show that for several quantum dots the exciton g-factor is quenched. A theoretical model is used to calculate the effect of the quantum dot size on the g-factor. The model is both qualitatively and quantitatively in good agreement with the experimental obtained results. Nanostructures of a different topology are self-assembled InAs/GaAs quantum rings (QRs). Using magnetization measurements on a large ensemble of quantum rings we demonstrate the presence of the Aharonov-Bohm (AB) effect in these nanostructures. Moreover, a model based on the X-STM measurements on these nanostructures reproduces the magnetic field position of this oscillation. The optical properties of these QRs are investigated for magnetic fields up to 30T for a large ensemble of quantum rings and for individual rings. Although the excitonic AB effect is suppressed in these nanostructures, the ring character of our nanostructures gives rise to non-equidistant energy level splittings and into a magnetic field induced splitting of each excited state into two states in the ensemble PL. This is different to what has been observed in measurements on quantum dots. The calculations based on the same model show a qualitative agreement with the experimental data, and allow us to identify the different PL peaks. Furthermore, analyzing exciton lines with high resolution revealed an anomalous quadruplet splitting, which is different as the one reported for InAs/GaAs quantum dots. Although the origin of this splitting is still unclear, it is most likely related to the singlet to triplet transition of singly charged excitons. Finally the magneto-luminescence experiments on type II self-assembled InP/GaAs quantum dots are discussed. For these dots the hole is located outside the quantum dot, whereas the electron is confined inside the quantum dot creating a type II exciton. The magneto-luminescence of the ensemble of dots did not resolve any oscillatory behavior related to the AB effect in contrast to experiments reported on the same sample. Decisive magneto-luminescence measurements on individual dots did also not reveal any AB related phenomena within the experimental resolution of 40 µeV. These results show the necessity to study subtle magneto-luminescence properties of nanostructures on an individual dot level, rather than on a large ensemble of dots.

This letter presents a technique for the investigation of the fine structure and spin properties of quantum dot (QD) ensembles, allowing measurement of QD parameters previously accessible only from studies of individual QDs. We show how ∼μeV splittings can be deduced from information contained in the shape of the ensemble polarization spectra and demonstrate the effectiveness of this technique by measuring Zeeman splittings, g-factors, and sensitivity to QD fine structure effects.

We have developed a theory of a transient intraband light absorption by semiconductor quantum dots. This absorption plays an important role in the two-pulse pump-probe method, which enables determining the energy relaxation rates of electron-hole excited states. We have considered all possible schemes of this process wherein the carrier frequency of optical pump pulses is close to the resonance with the interband transition of the quantum-dot electronic subsystem, while the carrier frequency of probe pulses is resonant to the intraband transition. For ensembles of identical and size-distributed quantum dots, the probe pulse energy absorption induced by the pump pulse is analyzed in relation to the delay time between the pulses. We have found that, under certain conditions, this dependence can be described by a single, two, or three exponentials. The exponents of the exponentials are proportional to the energy relaxation rates of electron-hole excited states.

The energy level structure as well as the exciton recombination and spin dynamics are studied in a dense ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots (QDs). The band alignment in the QDs is shown to have type-I, indirect character with the lowest electron state at the $X$ valleys of the conduction band and the top hole state in the $\mathrm{\ensuremath{\Gamma}}$ point of the valence band, so that indirect excitons are formed in the QDs. Time-resolved photoluminescence and magnetic-field-induced circular polarization allow us to distinguish electron states belonging to the QDs and the wetting layer. Suppression of the exciton migration within the QD ensemble and along the wetting layer in the magnetic field is found. A pronounced effect of applied microwave radiation on the recombination and spin polarization of the indirect excitons is observed in longitudinal magnetic fields. Optically detected magnetic resonance (ODMR) is detected in both the intensity and the circular polarization degree of the QD emission. The ODMR resonance corresponds to the $g$ factor of 1.97, associated with $X$-valley electrons. The spin relaxation time of the $X$-valley electrons is measured to be $600\ifmmode\pm\else\textpm\fi{}25$ ns.

In our experiments we superimpose a near-infrared excitation laser field with a mid infrared field on an InAs quantum dot ensemble or on a single quantum dot. In the latter case a cw infrared laser excitation and a glow-bar mid infrared field are superimposed in a confocal micro-photoluminescence configuration on a single quantum dot. By comparing the recombination spectra for the superimposed spectra with the spectra from the laser excitation we detect changes in the count rates for the different excitonic states. The decrease of the luminescence intensity in the lower states together with an increasing intensity in higher excited states suggests a direct electron transfer between these states due to absorption of mid infrared light. For the quantum dot ensembles we use interband pump – intraband probe-time domain spectroscopy to study the electron capture and relaxation dynamics within the ensemble. By tuning the femtosecond infrared pulses into resonance with transitions between confined quantum dot states and the wetting layer continuum we gain knowledge about the electron population in the quantum dots ground state and first excited states as a function of delay time between pump and probe pulse. Our experiments indicate the stepwise relaxation through the excited dot states to be the most efficient relaxation pathway into the dot ground state on a timescale between 1.5 ps and 4.7 ps depending on temperature and excitation density. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+). Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.

For a better understanding of the physical properties of semiconductor quantum dot ensembles, we have followed the behaviors of the transport and photoluminescence above, at, and below the percolation threshold of ensembles of Si quantum dots that are embedded in a $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. Our study revealed the roles of the interdot conduction, the single dot charging, and the connectivity in such systems. We conclude that while the first two determine the global transport, a connectivity dependent migration determines the coupling between the electrical and optical properties.

The dynamics of exciton hopping in an array of inhomogeneously broadened CdSe/ZnS quantum dot (QD) ensembles is examined by measuring time- and spectrally resolved fluorescence intensities. We have found a decrease in the fluorescence decay time as well as a dynamic redshift of the fluorescence spectrum originating from exciton transfer. Both show the characteristic temperature dependence reflecting the peculiar exciton dynamics in the QD ensemble. We propose a model of coupled QD arrays where inhomogeneous distribution and dark QDs that are related to a long-lasting off-state of blinking QDs are taken into account. Experimental results together with numerical calculations based on this model suggest that at low temperatures, an exciton transfers to a local low-energy site and tends to be trapped, whereas at high temperatures, thermally activated hopping of the exciton occurs repeatedly. Furthermore, we show that the decay time decrease of the QD array is attributable to exciton hopping to dark QDs.

Magnetization is induced in an ensemble of quantum dots, each charged with a single electron, when it is illuminated with a short circularly polarized light pulse that is resonant with the fundamental energy gap of the quantum dots. In this investigation, a quantum-mechanical model for the light-induced magnetization is presented. The phase of the magnetization precession as a function of the strength of the magnetic field in a Voigt geometry is in excellent agreement with experimental data measured on (In,Ga)As singly charged quantum dot ensembles. It is demonstrated that the precession of the hole in the trion plays a vital role because it determines the amplitude and phase of the magnetization precession. The model could also be easily extended to describe positively charged quantum dots. We also suggest that our theory, combined with measurements of the phase as a function of magnetic field, can be used as a technique to measure the resonant trion lifetime as a function of QD emission energy.

We describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6% lattice mismatch between GaAs and GaP in the Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6 × 1010 per cm−2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared with the samples with smaller QDs. The large QDs luminescence up to room temperature. We attribute the high energy emission to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots.