Ultrathin InAs/GaAs quantum wells : electronic structure, excitonic effects and carrier capture

In this thesis a comprehensive experimental and theoretical study of the electronic structure, excitonic effects, and the carrier capture in ultrathin InAs layers embedded in a GaAs matrix is presented. Ultrathin InAs layers have recently attracted strong interest from both the application and fundamental point of view. Based on their unique optical properties, such as a strong photoluminescence line below the GaAs bandgap, they represent a novel concept for optoelectronic devices with an operation wavelength widely tuneable around 980 nm but employing the well-established GaAs process technology. Evidently, for an efficient device optimalization reliable data about the band alignment are absolutely necessary and a fundamental understanding of the optical properties and the carrier and exciton dynamics is desirable. In turn, the tremendous fundamental interest in monolayer thick InAs layers itself is motivated by the fact that they represent the ideal case of a two-dimensional direct semiconductor system as the intermediate state between the usual quantum well and isolated isoelectronic impurities. Moreover, since the growth of InAs on GaAs is governed by the large lattice mismatch, InAs layers of a thickness below 5 Å are the only possibility to study the band alignment at the highly strained InAs/GaAs heterointerface. However, according to the contradictory results in literature concerning effective masses, confinement energies, and localization of the confined states related to the InAs layer, or the dimensionality and binding energies of the bound excitons, there seemed to be a problem in the understanding of their basic properties. This culminated e.g. in a debate about the existence of a localized lh state and the validity of elasticity theory in ultrathin highly strained layers. Beyond that, experimental data for the capture times and a detailed knowledge about the capture mechanism of photogenerated carriers or the relaxation of excitons, which are important to explain the strong photoluminescence from ultrathin InAs layer, were simply lacking. For the calculation of the electronic structure in pseudomorphically grown ultrathin InAs layers a new model was developed, which is based on the concept of band offsets but which does not have the general limitations of the quantum well model. By describing the confinement potential of the InAs with a d-potential it is naturally considered that the wavefunctions of the confined states extend entirely in the GaAs barrier. As a consequence the effective masses in the direction of quantization (m z m e * ( ) = 0.0665´ 0, mhh z m * ( ) = 0.3774 ´ 0 , mlh z m * ( ) = 0.0905´ 0) as well as in the plane of the InAs layer (x, y-direction), which were derived from the two-band Luttinger-Hamiltonian in spherical approximation, are solely determined by the barrier material. Due to the confinement a reversal in the heavy- and light-hole in-plane effective masses is observed (mhh x y m * ( , ) = 0.155´ 0, mlh x y m * ( , ) = 0.363´ 0 ). By using the d-potential approach and employing the controlled coupling between two ultrathin InAs layers of identical thickness separated by a GaAs barrier of different widths a novel spectroscopic technique for the determination of band offsets was developed. The band offsets were extracted independently from the photoluminescence excitation measurements of the coupling induced splitting between the symmetric and antisymmetric states, exploiting the large difference between the electron and heavyhole effective masses in growth direction. The major advantage of this method is that it is explicitly sensitive to the band offset ratio and that it eliminates substantial errors due to excitonic effects. The band offsets and the band offset ratio for the highly strained InAs/GaAs heterointerface determined in this work (DEc=535 meV, DEhh=385 meV, DElh=225 meV, Eg=0.6 eV, dEhh-lh=160 meV, Qc=0.58) agree for the first time with elasticity theory and theoretical predictions from LDA calculations. In turn, the good agreement between the experimental and theoretical values demonstrates that continuum elasticity theory and the concept of band offsets are suitable approaches even for InAs layers of monolayer thickness. Moreover, the magnitude of the light-hole band offset unambiguously provides a localized light-hole state at cryogenic temperatures. Excitonic effects in monolayer thick InAs layers, which dominate their optical properties, were investigated both experimentally and theoretically. Due to the strong confinement potential, the InAs layer binds heavy-hole (hh) and light-hole (lh) excitons. These excitons are GaAs-like because the wavefunctions of the confined states of the InAs layer extend entirely into the GaAs barrier. The exciton binding energies, determined by PLE and temperature dependent PL measurements, were found to be 10 meV for the hh exciton and 5.5 meV for the lh exciton. With the exciton binding energies and the in-plane effective masses known, the dimensionality of the excitons was clarified. The hh excitons were found to be almost two-dimensional. In contrast, the Summary 149 lh excitons turned out to be almost three-dimensional, reflecting the weak confinement of the lh state. The question arising over whether ultrathin InAs layers at low temperatures provide a bound lh exciton, was unambiguously answered in cw and timeresolved resonant excitation experiments. Under selective excitation of the lh exciton transition a sharp emission line was discovered, which emerges close to the hh exciton photoluminescence line exactly one GaAs LO phonon below the excitation energy. By studying the transient behaviour of the sharp line it was found that its origin continuously transforms from resonant luminescence for excitation on resonance into doubly resonant Raman scattering for off resonant excitation. Relaxation of hot excitons as a third possible mechanism to account for the sharp line, could be ruled out by additional measurements of the hh exciton lifetime. Since in the regime of doubly resonant Raman scattering the observation of the sharp line requires the existence of both a bound hh and lh exciton state, the dramatic decrease of the intensity of the sharp line due to a temperature rise from 4 K to 18 K directly proves the existence of a well bound lh exciton. For the special case of excitation on resonance, a polarization analysis of the sharp line revealed a negligible exciton-phonon interaction by deformation potential coupling. Based on this fact, our measurements of the lh exciton lifetime yield a lh exciton dephasing time of 40 ps. This relatively long dephasing time provides strong evidence that short-range potential fluctuations due to local strain fields, or the growth of InAs islands with a lateral extension smaller than the 120Å lh exciton Bohrradius, is absent. This is in excellent agreement with the results of the structural characterization by X-ray diffraction, which shows that the InAs layers are coherently strained and that more than 80 % of the deposited InAs is confined in one atomic plane. In order to explain the giant photoluminescence from ultrathin InAs layers a detailed experimental study of the carrier capture was performed. To our knowledge, these are the first experimental data for capture times ever reported for quantum well structures thinner than 25 Å. The capture times were determined by picosecond time-resolved twowavelength pump-probe phototransmission. Since this method utilizes two synchronously pumped pulse lasers operating at different wavelengths, it allows a direct spectral control over the initial and final states of the capture and relaxation process. Due to the high sensitivity of this time-resolved modulation technique the low excitation regime becomes accessible. This range is of particular interest, because in the absence of carrier-carrier scattering the capture process is exclusively governed by interaction with optical and acoustical phonons. Around the lh exciton transition the capture time was found to be 20 ps, which is fast for such a thin layer whose confinement potential is localized within one lattice constant. However, according to the quantum mechanical interpretation of the carrier capture, which firstly was applied to the trapping of electrons by Brum, Bastard and Blom, this efficient capture is explained by the large barrier penetration depth of the confined lh state, and subsequently by its large overlap with resonant barrier states, in excellent agreement with the results for the band alignment. In contrast to that, the capture time significantly increases from 22 ps to 55 ps towards higher photon energies within the hh exciton transition. Since in monolayer thick InAs layers the energy separation between hh and lh states amounts to one GaAs LO phonon, this feature demonstrates that a direct capture of the holes by the hh state is suppressed, but that the capture occurs in a two-step process with the lh level as intermediate capture state. After the capture by the lh state, the holes almost instantaneously cool down to the hh state under LO phonon emission when the energy separation between an occupied lh state and an unoccupied hh state is larger than the phonon threshold. However, when the energy separation is smaller than 36 meV, the cooling from the lh states occurs via acoustic phonons and the capture time increases. The efficient capture by the lh state in combination with the two-step capture mechanism satisfactory explains the strong photoluminescence from ultrathin InAs layers. Moreover, the capture time of 20 ps is in excellent agreement with the observed photoluminescence intensity ratio between the GaAs exciton and the hh exciton related to the InAs layer. Finally, in order to investigate whether time-resolved phototransmission indeed represents the evolution of the population in the InAs layer, a model for the excitonic absorption of a two-dimensional system in the presence of photogenerated carriers was developed. By considering both phase-space-filling and exciton screening, it has been shown for the first time that the rise and fall times in transient phototransmission only represent the capture and recombination times for excitation densities below 3´108 cm-2.