Effects of crossed states on photoluminescence excitation spectroscopy of InAs quantum dots
© Shih et al; licensee Springer. 2011
Received: 26 January 2011
Accepted: 2 June 2011
Published: 2 June 2011
In this report, the influence of the intrinsic transitions between bound-to-delocalized states (crossed states or quasicontinuous density of electron-hole states) on photoluminescence excitation (PLE) spectra of InAs quantum dots (QDs) was investigated. The InAs QDs were different in size, shape, and number of bound states. Results from the PLE spectroscopy at low temperature and under a high magnetic field (up to 14 T) were compared. Our findings show that the profile of the PLE resonances associated with the bound transitions disintegrated and broadened. This was attributed to the coupling of the localized QD excited states to the crossed states and scattering of longitudinal acoustical (LA) phonons. The degree of spectral linewidth broadening was larger for the excited state in smaller QDs because of the higher crossed joint density of states and scattering rate.
Self-assembled semiconductor nanostructures with three-dimensional carrier confinement provide the ultimate quantum system with discrete energy levels that can be tailored and controlled to tune the electrical and optical properties of these nanostructures. In particular, InAs on GaAs (001) self-assembled quantum structures is one of the most-studied systems. Quantum dots (QDs) provide another approach in making lasers, photodetectors, and memory devices as well as finding applications in quantum computing. Quantum dot lasers are predicted to have a high efficiency, low threshold current densities, and low temperature dependence of the threshold current [1–3]. The use of QDs may offer possibilities for low-power nonlinear devices. Therefore, the understanding of optical properties in these nanostructures is of extreme relevance for device applications to be a realistic prospect.
Theoretically, carriers confined in QDs show an atomic-like energy spectrum, characterized by discrete low-lying confined states, followed by spatially delocalized states associated to the InAs wetting layer and to the GaAs cap layer. The current perspective and analysis of the controversies regarding the phonon bottleneck in semiconductor QDs have been discussed by Prezhdo . However, this simple picture fails when the actual dots are probed using advanced local probe techniques that provide excellent spatial and spectral resolutions. These techniques enable us to study only a few dots or even one dot. Near-field photoluminescence excitation (PLE) spectra of single quantum dots display 2D-like continuum states and a number of sharp lines between a large zero-absorption region and the 2D wetting layer edge . The carriers were also found to relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons. Limitations of the isolated artificial atom picture of an InAs QD were investigated in Ref. . This study showed that the continuum background in the up-converted photoluminescence signal is possibly related to the wetting layer. Microphotoluminescence excitation spectra for neutral excitons revealed a continuum-like tail and a number of sharp resonances above the detection energy . Temperature-dependent PL studies of an ensemble of self-assembled (In, Ga)As QDs provide insight into the nature of the continuous states between the wetting layer and QDs . However, in contrast to other findings, PLE results of single InGaAs dot experiments by Hawrylak and co-workers showed spectra free of any continuum background and sharp emission lines .
Broadening of the QD bound states due to the influence of the couplings to the longitudinal optical phonons was discussed by Verzelen et al. , which revealed an effect of the dot environment to the dot eigenstates. Quantum kinetics of carrier relaxation in self-assembled QDs was investigated by taking into account the influence of the energetically nearby continuum of wetting layer states . The interaction of the discrete QD excited states with a quasicontinuum of states was investigated in Ref. , in which a correlation between the acoustic phonon broadening efficiency and the background intensity in the PLE spectra of single InAs QD was found. The continuous background feature experimentally extends downward very deeply in energy, which makes it difficult to associate the fluctuations of the WL near the dot.
More recently, another source of intrinsic broadening for the dot bound states was demonstrated theoretically. This was stimulated during the interband optical excitation when electron-hole pairs are photo-generated in the dots, and is related to the existence of transitions involving one bound state and one delocalized state near the dots . The broadening of the excited QD levels and interband absorption background are attributed to these cross transitions, which are inherent to the joint nature of the valence-to-conduction density of states in QDs.
This paper demonstrated that by engineering the QD size and shape, modification of the joint density of the cross transitions and changing the bound excited-state energies with respect to the continuum density of electron-hole states, which extend far below the wetting layer edge, can be made. The PLE spectroscopy of the self-assembled InAs QDs at low temperature and high magnetic field was reported. The resonances in the PLE spectra associated with the QD bound-state transitions were confirmed further by scanning the magnetic field. By evaluating the lineshape change of the PLE resonances, the coupling strength between the bound states and the cross transitions in different QDs was determined.
The conventional PL spectra were obtained directly using an argon ion laser as the excitation source. PLE measurements for the above samples were recorded with a continuous wave (CW) tunable Ti:sapphire laser pumped with a DPSS laser as light source and detected using a 0.18-m double spectrometer equipped with a TE-cooled InGaAs photodetector. The samples were mounted in a closed-cycle helium dewar for low-temperature measurement. However, for the optical measurements under low temperature and high magnetic field, the sample was placed in a sample holder with N-grease at the bottom of an insert equipped with a fiber probe. The insert was placed in a dilution refrigerator and cooled down to 1.4 K. Laser output from the CW Ti:sapphire laser was delivered into the refrigerator using a fiber. The PLE signals collected through the same fiber were dispersed with a 0.55-m spectrometer and detected with a TE-cooled InGaAs photodetector at different magnetic field intensities.
Results and discussion
The homogeneous linewidth Γ(T) of excitons is usually described as the sum of a static broadening Γ 0 including radiative broadening and a temperature-dependent one, which accounts for acoustic and optical phonon broadening . Efficient coupling to acoustic phonons exists not only for QD ground states but also for excited states. For low temperature, in which the interaction with acoustic phonon is dominant, linewidth broadening is written as [12, 13]: Γ(T) = Γ 0 + αT, where α accounts for the acoustic phonon broadening efficiency. The acoustic phonon broadening efficiency α increased with the normalized background intensity .
In conclusion, the magnetic field dependence of the PL and PLE spectra were measured for MBE-grown QD nanostructures with different sizes and shapes. The interband optical properties of QDs were affected by the crossed electron-hole levels. Additionally, The broadening of discrete (resonant) interband optical absorption is attributed to the combined effects of the crossed electron-hole levels and low energetic LA phonon scattering.
This work was supported by the National Science Council of Republic of China under contract No. NSC 96-2112-M-009-024-MY3 and the MOE ATU program.
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