Redshift Of Peak Energy Research Articles

Effects of ultra-low Al alloying In(Al)As layer on the formation and evolution of InAs/GaAs quantum dots

We have introduced ultra-low Al composition at the two-dimensional to three-dimensional transition stage of InAs/GaAs quantum dots (QDs) formation. Two main effects of AlAs on the QDs are revealed: one is to lower the nucleation barrier so as to reduce the critical nucleation thickness of QDs, which is demonstrated by a surface kinetic nucleation model. The other is to facilitate the In atoms migration from wetting layer (WL) to QDs, which holds some signatures such as both increased QDs density and size with increasing AlAs composition, as well as the peak energy red-shift of photoluminescence spectra. The enhanced In atoms migration from WL to QDs is further confirmed via photo-modulated reflectance experiments and energy band calculation, which both demonstrate the reduction of effective WL thickness after AlAs insertion. The observed effects of AlAs on QDs formation and growth evolution could be explained by the Al-alloying effects of InAs wetting layer.

High-optical-quality nanosphere lithographically formed InGaAs quantum dots using molecular beam epitaxy assisted GaAs mass transport and overgrowth

Optically active, highly uniform, cylindrical InGaAs quantum dot (QD) arrays have been fabricated using nanosphere lithography combined with Bromine ion-beam-assisted etching and molecular beam epitaxy (MBE)-assisted GaAs mass transport. Previously fabricated QD nanopillar arrays showed significant degradation of optical properties due to the etch damage. Here, a novel mass transport process in a Riber 3200 was performed to encapsulate the lithographically defined InGaAs disk QDs in a GaAs matrix, resulting in the passivation of the etch-damaged QD sidewall layer. Photoluminescence emission intensity following the mass transport process increased by a magnitude of 4–10 as compared to that from unprocessed nanopillar sample. In addition, a PL peak energy redshift was observed after mass transport, presumably due to the decrease in the lateral barrier potential from vacuum to GaAs, as well as the elimination of the depletion layer. Furthermore, the mass transport process in the high vacuum MBE environment enables GaAs overgrowth with few defects and dislocations following mass transport for surface planarization. PL emission intensity increased by an additional factor of 4 following GaAs overgrowth, bringing the QD intensity to 12 of that of the original single quantum well. Thus, the potential of the MBE-assisted mass transport process has been demonstrated to fabricate high optical quality InGaAs quantum dots encapsulated in a GaAs matrix for device applications.

Structure and photoluminescence features of nanocrystalline Si/SiO2 films produced by plasma chemical vapor deposition and post-treatment

Experimental results of the visible photoluminescence (PL) from nanocrystalline Si (nc-Si) embedded in a SiO2 matrix, prepared by plasma Chemical vapor deposition and a subsequent post-treatment, are reported here. Scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and Fourier transform infrared are used to characterize the morphology, crystallite size, and the composition and structure of nc-Si/SiO2 films. The visible PL can be finely tuned from 1.3 to 1.75 eV by changing annealing time and temperature. The effect of high temperature (870 °C) forming gas (FG) annealing on the visible PL can be divided into three stages. In the first stage, the visible PL blueshifts from 1.3 to 1.55 eV, and the PL intensity increases. In the second stage, the peak energy shows a small shift, and the PL intensity continues increasing. In the last stage, the peak energy blueshifts to ∼1.75 eV, but the PL intensity decreases. The visible PL shows a maximum intensity around 1.5±0.05 eV. For a PL obtained after a high temperature anneal, a subsequent low temperature FG annealing (400 °C) will lead to a redshift of peak energy and an increase in PL intensity. In particular, for a PL around 1.75 eV, a kinetic oscillation of the spectral shift and the PL intensity has been observed upon this annealing. Detailed analysis indicates that the most probable candidates for the visible PL are two oxygen thermal donor-like defect states (TDs) (Si-NL8 and Si-NL10) generated during annealing. The effect of annealing temperature and time on the spectral change and the kinetic oscillation of the spectral change can be explained by the formation and decay kinetics of these two oxygen TDs-like defect states. On the one hand, these experimental results verified the Si–O bond related origin for the visible PL in this system; on the other hand, they also pointed out that apart from the common features of Si–O related visible PL, the detailed configuration and composition of this PL center by different synthesis methods may be different and possess some features of their own.

Investigation of the Oxidation Effect of Porous Silicon During Electroluminescence by In Situ FTIR

In situ FTIR spectra of porous silicon are measured during electroluminescence (EL). It was found that oxidation process of porous silicon surface occurs during EL, which is the main reason of the EL intensity decrease and red shift of peak energy with time.

Migration enhanced epitaxy of CdSe islands on ZnSe and their optical and structural characterization

Formation of self-assembling CdSe islands on ZnSe has been achieved using Migration Enhanced Epitaxy (MEE). Atomic Force Microscopy (AFM) images of uncapped samples show spherical islands with a diameter-to-height ratio of 4:1, which is not yet an unambiguous proof for CdSe quantum-dots. Transmission Electron Microscopy (TEM) images from samples with a ZnSe caplayer show an interrupted Quantum Well (QW) with pronounced thickness fluctuations when the CdSe exceeds its critical thickness. Compared to bulk CdSe, the Photoluminescence (PL) peak is blue-shifted by about 0.5 eV. PL Excitation (PLE) experiments indicate that the interrupted layer consists of CdSe islands embedded in Zn1−xCdxSe with a composition gradient. Temperature dependence of the integrated PL-intensity exhibits thermal quenching with an activation energy of about 100 meV. The red-shift of peak-energy with temperature is considerably larger than the decrease of the bandgap of bulk CdSe.

Possibility of coherent light emission from excitons in crystalline GaTe.

Free exciton photoluminescence (PL) spectra of crystalline GaTe has been studied as a function of continuous wave (cw) argon laser excitation intensity at T=6 K. When the excitation intensities are low (P4 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), the integrated PL intensities of free excitons for both singlet and triplet states increase approximately linearly with increasing excitation intensity. From about 4 to 9 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, the peak energy redshift, the linewidth broadening, and the sharp increase of the integrated PL intensity of the exciton peaks are observed. These results are explained tentatively as large coherent emissions from excitons of GaTe. \textcopyright{} 1996 The American Physical Society.

Optical and structural properties of spontaneously formed long-range compositional modulation in (111)A and (111)B AlGaAs

We have studied the spontaneously formed long-range Al-rich and Ga-rich AlxGa1−xAs/AlyGa1−yAs superlattices in both (111)A and (111)B AlGaAs during epitaxial growth. The spontaneously formed compositional modulation has been investigated by cross-sectional transmission electron microscopy in AlxGa1−xAs (x=0.3–0.4) grown on (111)A and (111)B GaAs substrates at 600–700 °C. In contrast, similar superstructures are not observed in layers simultaneously grown on (100) substrates. The observed structural compositional modulation is closely related to the large peak energy redshift and peak intensity enhancement in 15 K photoluminescence (PL) of samples grown on both (111)A and (111)B orientations to those on (100). The respective redshifts of PL peak energy for (111)A and (111)B Al0.27Ga0.73As to that of (100) are 137 and 45 meV, while the PL integrated intensity enhancement to (100) amounts to 200 and 2000 times, respectively. The effect of compositional modulation is reduced with the increasing growth temperatures as shown in the considerably less compositionally modulated (111)A and (111)B Al0.40Ga0.60As grown at 700 °C.