Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields

We have studied a 50/50\AA superlattice of GaAs/Al$_{0.21}$Ga$_{0.79}$As composition, modulation-doped with Si, to produce $n=1.4\times 10^{12}$ cm$^{-2}$ electrons per superlattice period. The modulation-doping was tailored to avoid the formation of Tamm states, and photoluminescence due to interband transitions from extended superlattice states was detected. By studying the effects of a quantizing magnetic field on the superlattice photoluminescence, the miniband energy width, the reduced effective mass of the electron-hole pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)

Cross-sectional scanning tunneling microscopy of InAs/GaAs(001) submonolayer quantum dots

Cross-sectional scanning tunneling microscopy (X-STM) was employed to characterize the InAs submonolayer quantum dots (SMLQDs) grown on top of a Si-doped GaAs(001) substrate in the presence of (2X4) and c(4X4) surface reconstructions. Multiple layers were grown under different conditions to study their effects on the formation, morphology and local composition of the SMLQDs. The morphological and compositional variations in SMLQDs were observed by both filled and emptystate imaging. A detailed analysis of indium segregation in the SMLQDs layers was described by fitting local indium concentration profile with a standard segregation model. A strong influence of arsenic flux over the formation of the SMLQDs and indium incorporation was observed and reported. We investigated the well-width fluctuations of the InGaAs quantum well (QW) in which SMLQDs were formed . The monolayer fluctuations of the well width were negligible compared to the more pronounced compositional fluctuations in all the layers. Keywords: Submonolayer quantum dots, Surface reconstruction, X-STM, Indium segregatio

Effect of As flux on InAs submonolayer quantum dot formation for infrared photodetectors

The performance of infrared photodetectors based on submonolayer quantum dots was investigated as a function of the arsenic flux. All the devices showed similar figures of merit and a very high specific detectivity above 1 × 1011 cm Hz1/2/W at 12 K, despite the fact that cross-sectional scanning tunneling microscopy images pointed out a strong reduction in the density of such nanostructures with decreasing arsenic flux. This contrast is a consequence of the small size and low In content of the submonolayer quantum dots that lead to a strong delocalization of the electrons wave function and, therefore, reduce the advantage of samples having a very high density of quantum dots. A simple strain model showed that the properties of these nanostructures are limited by the lack of vertical alignment of the small two-dimensional InAs islands resulting from the strong segregation of In atoms. We have proposed some ways to improve the growth of submonolayer quantum dots and believe that, after further optimization, such nanostructures might provide devices with superior performance.</p

Atomic-scale characterization of single and double layers of InAs and InAlAs Stranski-Krastanov quantum dots

We report a detailed structural characterization of single and double layers of InAs and InAlAs quantum dots (QDs) and their wetting layers (WLs) by atomic force microscopy (AFM) and cross-sectional scanning tunneling microscopy (X-STM). The X-STM analysis with atomic resolution showed that the InAlAs WL consists of two distinct layers: a bottom part where all the Al atoms of the InAlAs alloy settled, and a top part containing exclusively In and Ga atoms. The QDs formed from the InAlAs layer contains no Al atoms at all and lie on top of the Al-rich WL. In the double layers of QDs, the InAlAs QDs were used as a seed to influence the nucleation of the InAs QDs grown on top. A gradual decrease in the density of the top InAs QDs was observed in the AFM images with increasing thickness of the GaAs spacer. The X-STM images showed that both QDs layers were completely intermixed for a 2-nm-thick spacer, while effective strain-induced stacking of both types of QDs was observed for a GaAs spacer thickness of 4 nm. However, both QD layers were completely decoupled for a GaAs spacer thickness of 8 nm and could thus be treated as individual layers

Influence of the InAs Coverage on the Performance of Submonolayer-Quantum-Dot Infrared Photodetectors Grown with a (2×4) Surface Reconstruction

Two infrared photodetectors based on submonolayer quantum dots, having a different InAs coverage of 35% and 50%, were grown, processed and tested. The detector with the larger coverage yielded a specific detectivity of 1.13×10 11 cm Hz 1/2 W -1 at 12K, which is among the highest values reported in the literature for that kind of device

Quantum Hall ferromagnet in a parabolic well

We report the observation of an anomalous magnetoresistance peak in a tilted magnetic field corresponding to filling factor of 2 in several parabolic wells of different width. This phenomenon is due to the unpolarizedferromagnetic transitions in quantum Hall ferromagnets. The domain formation induced by the random impurity potential is responsible for this magnetoresistance peak. The shift of the peak position with the tilt angle is attributed to the magnetic-field dependence of the exchange-correlation energy across the transition