D. J. Lockwood

Optical properties of porous silicon

Abstract The optical properties of porous Si films produced by electrochemical dissolution of Si are reviewed. From measurements of the optical absorption spectra an inverse relationship between the optical gap energy and the average nanoparticle size has been obtained demonstrating the quantum confinement of electron-hole pairs in Si nanocrystallites. There is also good agreement with theoretical predictions for Si quantum dots. A number of different sources of photoluminescence in anodized Si have been tentatively identified, and at least the red emission appears to be principally due to confinement in Si nanocrystallites.

Ge dots and nanostructures grown epitaxially on Si

We review recent progress in the growth and characterization of Si1−xGex islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si1−xGex coverage, and the effect of growth parameters or post-growth annealing on the shape of islands and dots. We outline some of the structural, vibrational, and optical properties of Si1−xGex islands and review recent advances in the determination of their composition and strain distribution. In particular, we present an analytical electron transmission microscopy study of the Ge spatial distribution in Ge dots and Si /Si1−xGex island superlattices grown by molecular beam epitaxy and ultra-high vacuum chemical vapour deposition. We describe the use of undulated Si1−xGex island superlattices for infrared detection at telecommunication wavelengths. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and, in particular, control their size, density, and spatial distribution. As examples, we show how C pre-deposition on Si(001) can influence nucleation and growth of Ge islands and how low temperature Si homo-epitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

Strain in coherent-wave SiGe/Si superlattices

We report measurements of the Raman spectra of Si12xGex=SiO0:4 # x # 0:6 and xa 1U wavy superlattices characterized by the presence of a coherent undulated alloy-layer structure. The strain induced shifts of the alloy Si‐Si, Si‐Ge and Ge‐Ge mode frequencies in the wavy superlattices from the corresponding unstrained bulk mode frequencies are compared with those found in Si12xGex=Si planar superlattices. Overall, the Si‐Ge and Ge‐Ge mode frequencies as a function of x are found to be reduced with respect to those in the planar superlattices, indicating a reduction of average strain in the wavy superlattices compared with the built-in strain in the planar superlattices. For the Si‐Si mode, the mode frequency surprisingly behaves the opposite way. We propose that a compositional fluctuation in the alloy layer of the wavy superlattices, caused by the outward diffusion of Ge into the Si layer during growth, can explain this inconsistency in the experimental results. q 2000 Elsevier Science Ltd. All rights reserved.

Optical absorption evidence for quantum confinement effects in porous silicon

Abstract Measurements of the optical and structural characteristics of various porous Si samples have been correlated. Although the photoluminescence peak wavelength shows no correlation with the Si nanoparticle size, the optical absorption edge exhibits a strong inverse correlation that is in excellent agreement with theoretical predictions for the optical gap in Si spheres or quantum dots. This constitutes direct evidence for quantum confinement effects in porous Si.

An annealing study of strain relaxation and dislocation generation in Si1−xGex/Si heteroepitaxy

The generation of interface misfit dislocations, and the accompanying strain relaxation, in a molecular‐beam epitaxy grown 0.17 μm thick metastable Si0.82Ge0.18/Si(100) strained epilayer have been studied in detail as a function of rapid thermal annealing treatments over the 500–850 °C temperature range. Charge collection and transmission electron microscopy were used to determine the onset of relaxation by directly imaging misfit dislocations and to investigate the variation in dislocation density with increasing anneal temperature. The strain variation in the epilayer was carefully monitored using double‐crystal x‐ray diffraction and Raman spectroscopy, and the annealing induced changes in strain related to the electron microscopy observed density of interface misfit dislocations. The relative merit of each experimental technique is discussed in the light of these results. The generation of strain relieving dislocations was found to be an activated process, with an activation energy on the order of 1.5 ...

Visible light emitting SiSiO2 superlattices

Abstract Six-period superlattices of Si SiO 2 have been grown at room temperature using molecular beam epitaxy. With this mature technology, the ultra-thin (1–3 nm) Si layers were grown to atomic layer precision. These layers were separated by ∼1 nm thick SiO2 layers whose thickness was also well controlled by using a rate-limited oxidation process. The chemical and physical structures of the multilayers were characterized by cross-sectional TEM, X-ray diffraction, Raman spectroscopy, Auger sputter-profile, and X-ray photoelectron spectroscopy. The analysis showed that the Si layer is free of impurities and is amorphous, and that the SiO 2 Si interface is sharp (∼0.5 nm). Photoluminescence (PL) measurements were made at room temperature using 457.9 nm excitation. The PL peak occurred at wavelengths across the visible range for these multilayers. The peak energy position E was found to be related to the Si layer thickness d by E (eV) = 1.60+0.72d−2 in accordance with a quantum confinement mechanism and the bulk amorphous-Si band gap.

Helium bubbles in silicon: Structure and optical properties

Silicon samples were implanted with 20 keV He at various temperatures. The damage and the size of the He bubbles created during the implantation were measured with Raman spectroscopy and transmission electron microscopy. Room temperature implantation with 2.5×1017 He atoms/ cm2 produced an amorphized layer with a high density of small voids (∼5 nm). After annealing at 923 K the amorphous layer was recrystallized, but still contained extended defects. The He bubbles coalesced forming large bubbles in the implanted region. Implantation at 723 K left the Si essentially crystalline, but with a large number of defects. The He bubbles created at this temperature were larger than after room temperature implantation. Light emitting properties of this porous material are briefly discussed.

Photoluminescence and Raman scattering in three-dimensional Si/Si1−xGex nanostructures

We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.

PHONON-ASSISTED TUNNELING AND INTERFACE QUALITY IN NANOCRYSTALLINE SI/AMORPHOUS SIO2 SUPERLATTICES

We report on the interface quality and phonon-assisted tunneling in nanocrystalline Si (nc-Si)/amorphous SiO2 (a-SiO2) superlattices (SLs) prepared by magnetron sputtering and thermal crystallization of nanometer-thick a-Si layers. Phonon-assisted tunneling is observed in a bipolar nc-Si based structure, which confirms that the nc-Si/a-SiO2 junction is not only abrupt but also nearly defect free. The conclusion is supported by capacitance–voltage measurements from which the estimated interface defect density is found to be ∼1011 cm−2 for an eight-period SL. Such high quality interfaces hold considerable promise for the development of nc-Si SL quantum devices.

Thermal oxide on CdSe

X-ray photoelectron spectroscopy (XPS) has been used to characterize the thin thermal oxide film grown on single crystal CdSe(0001) and polycrystalline CdSe by exposure to O2 (dry air) at 350 °C. SeOx species, where x=2,3, are clearly identified by a 5 eV shift of the Se 3d3/2,5/2 peaks to higher binding energy. A very weak shift to lower binding energy is observed for the Cd peaks. The positions of the Cd and O peaks do not match those found for the known cadmium oxides, CdO and CdO2. Instead, it is proposed that the Cd bound oxygen atoms occupy substitutional Se sites. The presence of Cd bound oxygen can also be inferred from the intensities of the SeOx, Cd, and O peaks. Raman spectroscopy confirms the existence of O in Se substitutional sites. Angle-resolved XPS is used to determine the thickness of the oxide and the relative amount of SeOx and Cd bound oxygen. The XPS data are consistent with an 8–9 A thick oxide where ∼60% of the oxygen is bound to Se and ∼40 is bound to Cd. The data show that the ox...