Kinetics of island formation at the interfaces of AlGaAs/GaAs/AlGaAs quantum wells upon growth interruption

doi:10.1016/0749-6036(87)90182-0

Superlattices and Microstructures

Volume 3, Issue 1, 1987, Pages 79-82

https://doi.org/10.1016/0749-6036(87)90182-0 Get rights and content

Abstract

Surface roughness disappears and formation of islands of monolayer height occurs at GaAs surfaces upon interruption of the growth of AlGaAs/GaAs/AlGaAs quantum wells (QW's) at the respective interfaces. Both processes are directly visualized by the changes of the shape of luminescence spectra of such QW's. A detailed theory of the shape of QW luminescence which is presented here for the first time allows a $quantitative determination$ of interface roughness and of island formation. The kinetics at and the properties of $GaAs AlGaAs$ and $AlGaAs GaAs$ interfaces are found to be completely inequivalent at the present growth conditions of 620° C substrate temperature and 0.28 nm/s growth rate.

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We present an electron tomography method for the comprehensive characterization of buried III-V semiconductor interfaces that is based on chemical-sensitive high-angle annular dark-field scanning transmission electron microscopy. For this purpose, an (Al,Ga)As/GaAs multi-layer system grown by molecular beam epitaxy is used as a case study. Isoconcentration surfaces are exploited to obtain topographic height maps of 120 nm × 120 nm area, revealing the interface morphology. By applying the height-height correlation function, we are able to determine important interface properties like root mean square roughness and lateral correlation length of various interfaces of the (Al,Ga)As/GaAs system characterized by different Al concentrations. Height-difference maps based on isosurfaces corresponding to 30% and 70% of the total compositional difference at the interfaces are used to create topographic maps of the interface width and to calculate an average interface width. This methodology proves differences in the properties of direct and inverted interfaces and allows the observation of interfacial anisotropies.
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Interface roughness is harmful to controlled optical transitions in quantum wells (QW), and high-mobility modulation doped heterostructures [2]. Many research groups studied AlGaAs/GaAs quantum wells by using photoluminescence (PL) spectroscopy [3–8], the results showed the electronic and optical properties of quantum wells are closely correlated to the structure of the interface; such as the quantum well width and interface roughness could influence the luminescence halfwidth for a QW. Some researchers utilize scanning tunneling microscope (STM) on cross sections of multilayer structures to obtain the atomic-scale view of AlGaAs/GaAs multilayers on (110) interface [9,10].
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We report the observation of relative saturation among excitonic emission peaks at different sets of interface islands in growth-interrupted asymmetric-coupled quantum well GaAs/Al_0.2Ga_0.8As structures. The saturation is due to sequential filling of excitonic states at different sets of interface islands. In contrast to free excitonic states, the small total area density of excitonic states at the interface islands makes their filling observable at much lower excitation levels. As a result of the relative saturation, an effective blue shift of the apparent excitonic emission peak at the islands, with a magnitude as large as ∼6.1 meV is observed when the excitation intensity increases from ∼1.6 to ∼215 W cm⁻². The highest intensity required to observe the effective blue shift is about two orders of magnitude lower than that needed to observe a similar effect in a free excitonic emission peak.
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Photoluminescence spectroscopy can advantageously be used to determine the smoothness of quantum well interfaces. In analyzing the spectra one should take into account a possible distribution of islands which will modify the line shape of exciton transitions. In the present paper, the roughness of a single GaAs/GaAlAs quantum well is described by an island structure of different thickness. Depending on the ratio of the lateral extension of the exciton to the area of islands, a distribution of exciton energies is obtained. In the case of large islands a peak structure, corresponding to discrete thickness parameters is found; whereas in the opposite case a Gaussian distribution is derived. The model for the probability distribution of exciton energies is used to calculate luminescence spectra.
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The dynamic interaction between free excitons (FE's) and bound excitons (BE's) in center-doped GaAs/AlGaAs quantum wells (QW's) has been studied by picosecond transient spectroscopy. For narrow QW's (50 Å) the localization of FE's due to interface related localized potentials is an important mechanism, dominating also the BE capture process for acceptors. A strong spatial correlation between localized excitons (LE's) and capture to BE's is observed under these conditions. For donors such a correlation is also seen (although weaker) for a well width of 100 Å. In donor-doped samples the FE and BE are thermalized already at 5 K, while the corresponding temperature for acceptors is about 30 K. The radiative FE lifetime at 5 K is about 350 ps in wells 50–150 Å wide, while the corresponding radiative lifetime for BE's varies between 350 ps and 500 ps in this range. The observed strong temperature dependence of FE and BE lifetimes is qualitatively explained by detailed balance arguments.

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Paper from the Göteborg conferenceKinetics of island formation at the interfaces of AlGaAs/GaAs/AlGaAs quantum wells upon growth interruption

Abstract

Solid State Comm.

J. Vac. Sci. Technol.

Appl. Phys. Lett.

J. Appl. Phys.

J. Vac. Sci. Technol.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Vac. Sci. Techn.

Paper from the Göteborg conference
Kinetics of island formation at the interfaces of AlGaAs/GaAs/AlGaAs quantum wells upon growth interruption