Fine Structure Splitting in the Optical Spectra of Single GaAs Quantum Dots

Abstract

We report a photoluminescence study of excitons localized by interface fluctuations in a narrow GaAs͞AlGaAs quantum well. This type of structure provides a valuable system for the optical study of quantum dots. By reducing the area of the sample studied down to the optical near-field regime, only a few dots are probed. With resonant excitation we measure the excited-state spectra of single quantum dots. Many of the spectral lines are linearly polarized with a fine structure splitting of 20 -50 meV. These optical properties are consistent with the characteristic asymmetry of the interface fluctuations. PACS numbers: 78.55.Cr, 71.35.Cc In this Letter we describe the polarization dependence of the optical spectra of single naturally formed GaAs quantum dots. Most previous optical studies of quantum dots (QDs) have probed large ensembles which have led to inhomogeneous broadening of the spectral features. However, recently several groups have shown that it is possible to study single QDs with photoluminescence (PL) either by reducing the size of the sample, [1] by cathodoluminescence [2,3], or by reducing the size of the laser spot on the sample through microscopic [4,5] or optical near-field techniques [6]. Here we use a similar technique whereby we combine high spatial and spectral resolution optics with excitation spectroscopy to study in detail the spectrum of a single QD [7]. With improved resolution we are able to resolve the spectral lines and to study the polarization dependence of the PL spectrum of an individual QD. We often find that the PL is linearly polarized along the (110) crystal axes and observe a fine structure splitting in each of the spectral lines. These results are analogous to the early days of atomic spectroscopy as improvements in techniques allowed the observation of fine structure splittings in the optical spectra. However, the physical phenomena responsible for the effects presented here are unique to the quantized condensed matter system.The QDs we have studied were formed naturally by interface steps in narrow quantum wells [4][5][6][7]. Specifically, the electrons and holes become localized into QDs in regions of the quantum well that are a monolayer wider than the surrounding region and, therefore, have a slightly smaller confinement energy. These well width fluctuations arise from monolayer-high islands at the interfaces which are randomly formed on the growth-interrupted surface by the migration of the cations to step edges. By interrupting the growth these islands can grow to diameters larger than the exciton Bohr diameter (20 nm). A scanning tunneling microscope image of a growth-interrupted GaAs surface grown under similar conditions as our quantum dot sample is shown in Fig. 1. Large monolayer-high islands of varying lateral sizes are evident, and the islands tend to be elongated along the [110] crystal axis. Thus we intuitively expect that the optical properties associated with the localized excitons will reflect this characteristic interface structure. In fact, as we will...

Internal Transitions of Charged Excitons in AlGaAs/GaAs Lateral Fluctuation Quantum Dots

“…A sample for the single dot experiments was fabricated from this wafer by depositing an Al mask on the surface and defining an array of indexed apertures with diameters between 25 microns and 200 nm by e-beam lithography. This approach has been described in detail elsewhere [7,8].…”

“…The excited states of excitons and trions in 2D systems have received rather little experimental attention (see, e.g., [8][9][10]), and there has been even less work in quantum dots [7,11,12]. In addition, the coupling of the CM motion to the relative motion for interface fluctuation dots has received little experimental at-tention (see e.g.…”

“…It is of particular interest to determine the states of charged excitons in this system because of interesting electron-electron correlations and possible applications of charged excitons in quantum dots as Q-bits [6]. There have already been several previous investigations of both neutral and charged excitons in lateral fluctuation quantum dots by photoluminescence (PL), magneto-photoluminescence (MPL) and photoluminescence excitation (PLE) (e.g., [7,8]). …”

“…In addition, the coupling of the CM motion to the relative motion for interface fluctuation dots has received little experimental at-tention (see e.g. [7]). Theoretical predictions of the effect of lateral confinement on internal transitions of X and X − are available [13], and experimental work on internal transitions has been carried out via optically detected resonance (ODR) spectroscopy in quasi-2D systems.…”

“…7 As quantum well samples grown by molecular beam epitaxy with lateral fluctuation quantum dots produced by growth interruption. Monolayer fluctuations of the quantum well width form lateral quantum dots for which confinement energies are less than Coulomb correlation energies.…”