ALBERT

Notes: It will be proven that if a gauge-invariant Lagrangian density having the local form L=L(gij;Ai;Ai,j) is such that its Euler–Lagrange equations Ei(L)=0 have the same set of solutions as Ei(L0)=0, where L0=g1/2FijFij, then L and cL0 are equivalent for same constant c, i.e., Ei(L)=Ei(cL0). From a previous result it follows that L=cL0+D+eg1/2, where D is a divergence and e is a constant.

Notes: The correlation between the structural (average size and density) and optoelectronic properties [band gap and photoluminescence (PL)] of Si nanocrystals embedded in SiO2 is among the essential factors in understanding their emission mechanism. This correlation has been difficult to establish in the past due to the lack of reliable methods for measuring the size distribution of nanocrystals from electron microscopy, mainly because of the insufficient contrast between Si and SiO2. With this aim, we have recently developed a successful method for imaging Si nanocrystals in SiO2 matrices. This is done by using high-resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Then, by varying the time of annealing in a large time scale we have been able to track the nucleation, pure growth, and ripening stages of the nanocrystal population. The nucleation and pure growth stages are almost completed after a few minutes of annealing time at 1100 °C in N2 and afterward the ensemble undergoes an asymptotic ripening process. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing at 1100 °C. Forming gas postannealing considerably enhances the PL intensity but only for samples annealed previously in less time than that needed for PL saturation. The effects of forming gas are reversible and do not modify the spectral shape of the PL emission. The PL intensity shows at all times an inverse correlation with the amount of Pb paramagnetic centers at the Si–SiO2 nanocrystal–matrix interfaces, which have been measured by electron spin resonance. Consequently, the Pb centers or other centers associated with them are interfacial nonradiative channels for recombination and the emission yield largely depends on the interface passivation. We have correlated as well the average size of the nanocrystals with their optical band gap and PL emission energy. The band gap and emission energy shift to the blue as the nanocrystal size shrinks, in agreement with models based on quantum confinement. As a main result, we have found that the Stokes shift is independent of the average size of nanocrystals and has a constant value of 0.26±0.03 eV, which is almost twice the energy of the Si–O vibration. This finding suggests that among the possible channels for radiative recombination, the dominant one for Si nanocrystals embedded in SiO2 is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration. © 2002 American Institute of Physics.

Notes: We have carried out a detailed study of Franz–Keldysh oscillations observed in the photoreflectance spectra of molecular beam epitaxy grown GaAs/Si/GaAs and AlAs/Si/AlAs heterostructures with a Si nominal thickness of two monolayers. The oscillations in the photoreflectance spectra were due to internal electric fields generated by graded p-n junctions created by Si diffusion. The data were analyzed employing the asymptotic Franz–Keldysh theory. It is concluded that different contributions from degenerate heavy and light hole bands, to transitions around the Γ point of the Brillouin zone, must be expected for different heterostructures depending upon the particular characteristics of the internal electric fields present in the sample.

Notes: We report the growth mechanism of GaAs on vicinal Si(110) substrates by molecular beam epitaxy, and investigate the effects of off-angle and -direction on the crystalline quality of grown films. The quality was improved with the increase of the off-angle and strongly influenced by the off-direction. Off-direction toward the [001] was better than that toward the [11¯0] for obtaining high-quality films. The cause is related to the structure and density of the steps. We infer from reflection high-energy electron diffraction results that GaAs films on vicinal Si(110) with off-angles above 2° toward the [001] direction grew up to a thickness of 3 nm in step-flow-like mode. The spotty reflection high-energy electron diffraction patterns characteristic of three-dimensional growth in GaAs/Si(100) were not observed at a thickness above 3 nm. © 1994 American Institute of Physics.

Notes: Smooth GaAs layers were successfully grown by migration-enhanced epitaxy on exactly (110) oriented substrates. The surface of layers grown by conventional molecular beam epitaxy was completely covered with facets, whose density was higher than 106 cm−2. The facet density was reduced remarkably by three orders of magnitude using the migration-enhanced epitaxy method. Observing the intensity oscillations of the specular spot of reflection high-energy electron diffraction patterns, the growth mode and the migration characteristics of surface adatoms have been investigated.

Notes: The ground-state magnetic properties of clusters with N-atomic sites are studied by solving the single-band Hubbard Hamiltonian in the unrestricted Hartree–Fock approximation. Three-dimensional arrangements of spins are considered in the solution. Results for stability and magnetic order, as a function of the Coulomb interaction strength U/t, and number of electrons ν, are presented for all the clusters with N=3, 4, and for the set of N=13 fcc-, hcp-, icosahedral-, and bcc-like clusters. In most cases, for band fillings (ν) close to half-band |ν/N−1|≤0.2−0.4, it is found that the antiferromagneticlike noncollinear spin solutions yield the smallest energy for U/t larger than a critical value (0〈(U/t)c≤6). ©1997 American Institute of Physics.

Notes: High-resolution x-ray diffractometry has been applied to the structural characterization of piezoelectric strained InGaAs/GaAs multiquantum well p-i-n diodes grown by molecular beam epitaxy on (111)B GaAs substrates. Reference samples simultaneously grown on (001) GaAs have been also characterized. Diodes with 3, 7, and 10 periods and different well to barrier thickness ratio have been studied. Symmetric and asymmetric reflections at various azimuths were measured and the scans were fitted with theoretical curves obtained through a dynamical simulation program developed in our lab. The comparison between experimental and simulated profiles has enabled us to determine the main structural parameters of the samples. High-resolution x-ray diffractometry provided accurate data about period and capping layer thicknesses, indium content in the wells and state of relaxation, information which cannot be always obtained in (111)B samples from other characterization techniques such as photoluminescence or photocurrent spectroscopies. © 1996 American Institute of Physics.

Notes: ZnSe films with thickness between 800 and 7500 Å were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples. © 1998 American Institute of Physics.

Notes: Fully strained single-crystal Ge1−xSnx alloys (x≤0.22) deposited on Ge(001)2×1 by low-temperature molecular beam epitaxy have been studied by Raman scattering. The results are characterized by a Ge–Ge longitudinal optical (LO) phonon line, which shifts to lower frequencies with increasing x. Samples capped with a 200-Å-thick Ge layer exhibit a second Ge–Ge LO phonon line whose position remains close to that expected from bulk Ge. For all samples, capped and uncapped, the frequency shift ΔωGeSn of the Ge–Ge LO phonon line from the Ge1−xSnx layer, with respect to the position for bulk Ge, is linear with the Sn fraction x (ΔωGeSn=−76.8x cm−1) over the entire composition range. Using the elastic constants, the Grüneisen parameter, and the shear phonon deformation parameter for Ge, we calculate the contribution of compressive strain to the total frequency shift to be Δωstrain=63.8x cm−1. Thus, the LO phonon shift in Ge1−xSnx due to substitutional-Sn-induced bond stretching in fully relaxed alloys is estimated to be Δωbond =−140.6x cm−1. © 1998 American Institute of Physics.

Notes: We report on the preparation of photoluminescent porous GaAs by the application of high voltage spark discharges in atmospheres of pure oxygen, pure nitrogen, and in mixed N2:O2 ratios of 4:1 and 1:4. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. Some differences are observed in the initial PL spectra of the spp-GaAs according to the atmosphere of preparation under similar sparking and time conditions. The PL consists of two dominant bands, a yellow-green band between ∼2.2 and 2.6 eV and a blue-violet band, centered at 3.1 eV. Comparison with Raman and PL results from As2O3 and As2O5 indicates that the PL in the spp-GaAs is produced by the formation of these compounds by exposure to oxygen during the preparation. This is reinforced by energy dispersive x-ray spectroscopy measurements that indicate that the spp-GaAs is always oxidized, even when prepared under a nitrogen flow. The blue-UV emission at 3.1 eV suggests that we cannot rule out confinement as a contributing mechanism for this PL. Raman spectra indicate that for samples prepared in pure nitrogen, the resulting material consists of amorphous As and GaAs, and the cubic form of As2O3, arsenolite. PL and Raman indicate that there exists an increasing degree of amorphization of the resultant material with the introduction of nitrogen in the preparation atmosphere. © 2000 American Institute of Physics.