E. Cruz-Hernández

Molecular Beam Epitaxial Growth of GaAs on (631) Oriented Substrates

The homoepitaxy of GaAs on (631)-oriented substrates has been studied as a function of the growth temperature. We observed the spontaneous formation of a high density of large scale features on the surface. The hilly like features are elongated towards the [-5,9,3] direction. When the growth temperature was varied from 490 to 580°C the hillocks length exponentially increases from 1.8 to 4.3 µm, their height linearly increases from 35 to 50 nm, and the density exponentially decreases from 2.8×106 to 3×105 /cm2. The hillocks formation is discussed in terms of adatoms diffusion anisotropy, sticking properties at step edges, and Ehrlich–Schwoebel diffusion barriers.

Effect of surface states on the electrical properties of MBE grown modulation doped AlGaAs/GaAs

The influence of near surface structure termination and surface treatments on the surface electric fields and mobility of modulation doped AlGaAs/GaAs heterostructures (MDH) were investigated. The built-in and surface electric fields were evaluated by photoreflectance spectroscopy, and these values were utilized to simulate the conduction band bending of the MDH. When the capping layer of the MDH was changed, both the built in internal electric field and the surface electric field are decreased, while the electron mobility of the samples is increased. After passivated the surface samples with Si, the surface electric fields were also reduced. Finally, a (NH4)2Sx-based treatment of the surface was applied, the surface electric field is annulled, and the conduction band modeling showed an important redistribution of carriers in the films. The electron mobility of the passivated samples does not show any change, neither the internal electric fields, corroborating the close relationship that exist between the...

Optical and electrical properties of Si-doped GaAs films grown on (631)-oriented substrates

This work studies the Si-doping of GaAs on (631)-oriented substrates as a function of the As4-beam equivalent pressure (PAs). The electrical properties obtained by Hall effect measurements show that the mobility of the layers grown on (631)-substrates present changes related to carrier compensation processes and the Si-doping changes from p- to n-type when PAs is increased. The optical properties of the samples, as observed by photoluminescence (PL) spectroscopy, modified according to the electrical characteristics. For (631)-samples, when PAs is increased the energy of the maximum intensity PL peak redshifts in the p-type region but, after the threshold of the conduction type conversion, the peak blueshifts in the n-type region. The variation of the PL excitation intensity also shifts the emission energy of the samples as a consequence of the increasing recombination rate for close pairs in donor acceptor pair recombination. Photoluminescence as a function of temperature shows that the activation energy ...

Photoluminescence study of self-assembled GaAs quantum wires on (631)A-oriented GaAs substrates

The authors report a low-temperature photoluminescence (PL) study of multiple GaAs layers grown between AlAs(0.6 nm)/GaAs(0.6 nm) short-period superlattice barriers (SLBs) simultaneously grown on both GaAs(631)A and (100) substrates. Five GaAs-layers of different nominal thicknesses (LW, ranging from 12 to 2.4 nm) were grown by molecular beam epitaxy. By using (631)A-oriented substrates a self-organized and highly ordered corrugation is obtained in the growth of the GaAs layers, and at the end of the SLB growth, flat surfaces are found. Whereas, for the (100)-oriented sample, flat interfaces are confirmed after the growth of GaAs and SLB layers. By reducing LW below ∼3.6 nm in the (631) sample, strong quantum wire (QWR)-like confinement is achieved as deduced from polarized PL spectroscopy where polarization degrees as large as 0.43 are obtained. The PL emission energy of the (631)-QWRs is redshifted, as compared with the transitions of the (100)-oriented quantum wells, when LW is reduced. The authors exp...

Structural characterization of AlGaAs:Si/GaAs (631) heterostructures as a function of As pressure

AlGaAs:Si/GaAs heterostructures were grown on (631) and (100) GaAs substrates and studied as a function of the As cell beam equivalent pressure. High-resolution x-ray diffraction patterns showed that the highest quality AlGaAs epitaxial layers were grown at PAs = 1.9 × 10−5 for (100)- and PAs = 4 × 10−5 mbar for (631)-oriented substrates. Raman spectroscopy revealed higher crystalline quality for films grown on (631) oriented substrates. The GaAs- and AlAs-like modes of the AlGaAs(631) films exhibited increased intensity ratios between the transverse optical phonons and longitudinal optical phonons with increasing PAs, whereas the ratios were decreased for the (100) plane. This is in agreement with the selection rules for (631) and high-resolution x-ray diffraction observations. Anisotropy and surface corrugation of the AlGaAs(631) films also were characterized using atomic force microscopy and Raman spectroscopy.

Photoluminescence and photoreflectance studies of InAs self-assembled nanostructures on GaAs(631) substrates

The authors report the photoluminescence and photoreflectance characteristics of molecular beam epitaxy grown InAs nanostructures on GaAs (631)-oriented substrates. Prior to the InAs growth, self-assembled nanochannels on the GaAs buffer layer were formed, which later were used as templates for nanostructures formation. Different samples were prepared by varying the amount of InAs from 0.75 to 2 ML (monolayer), 50 nm of GaAs was grown on top as a capping layer. Low temperature photoluminescence spectroscopy showed intense optical transitions in the spectra, their energy position directly depends on the quantity of InAs deposited. The self-assembling of InAs quantum wires (QWRs) at the early stages of growth is suggested. Anisotropy effects in the InAs nanostructures were corroborated by polarized photoluminescence supporting the proposal of formation of QWRs. Photoreflectance spectroscopy at room temperature was also employed to characterize the samples. It is found that in addition to the band-gap energy...

Study of the homoepitaxial growth of GaAs on (631) oriented substrates

We have studied the GaAs growth on (631) oriented substrates by molecular beam epitaxy (MBE). Different samples were prepared by varying the growth temperature and the III/V equivalent pressure ratio. We observed by atomic force microscopy a high density of hilly like features elongated towards the [−5,9,3] direction formed during the MBE growth. The growth temperature dependence of the hillock length and width follows an Arrhenius-type behavior with activation energies of 1.4 and 0.5eV, respectively. The hillock formation is discussed in terms of adatom diffusion anisotropy and diffusion barriers. Employing photoreflectance spectroscopy we found a splitting of the GaAs band gap energy transition that increases with the hillock density.

Effects of growth temperature on the incorporation of nitrogen in GaNAs layers

Ternary III-N-V semiconductor alloys are interesting and complex materials. GaNAs is one such material that has been studied extensively; however, the accurate determination of the N content within this material in which the growth conditions significantly increases the amount of interstitial N has not yet been reported. To address this problem, GaNAs layers (100 nm) were prepared using molecular beam epitaxy at temperatures between 400 and 600 °C with a high nominal N concentration (3%). The N content was determined using high resolution x-ray diffraction (HRXRD), secondary ion mass spectrometry (SIMS), and low-temperature photoluminescence (PL). The N concentration determined using these techniques was compared. Additionally, the relationship between the growth temperature and N concentration is discussed. The incorporation of N into interstitial sites resulted in significant variations in the N content as estimated by SIMS, HRXRD, and PL.

Polarization and Excitation Dependence of Photoluminescence of InAs Quantum Wires and Dots Grown on GaAs(631)

We studied the optical properties of self-assembled InAs nanostructures grown by molecular beam epitaxy on GaAs substrates with (631) crystallographic orientation by photoluminescence (PL) spectroscopy. The growth of InAs proceeded on GaAs(631) buffer layers conformed by corrugated surfaces with a high degree of lateral periodicity (uniformly spaced nano grooves). The grooved surface is sustained even after the growth of InAs with thicknesses below 2 monolayers (ML), indicating the formation of InAs quantum wires. The one-dimensional confinement is corroborated by PL polarized along the [113] and [8,19,9] orthogonal directions. The calculated polarization degree, Πd, was 0.13 and 0.125 for InAs thicknesses of 1 and 1.5 ML, respectively. As the InAs thickness is increased, the InAs film reaches the critical thickness and quantum dots aligned along the grooved surface are formed, as observed by atomic force microscopy. Excitation-power-dependent luminescence is observed to a larger extent for low-energy PL lines in addition to a blue shift related to the band-filling effect.

Molecular beam epitaxy growth of AlGaAs on the (631)-oriented GaAs substrates

The authors report the molecular beam epitaxy growth and characterization of AlGaAs∕GaAs(631) heterostructures grown at different As4 molecular beam equivalent pressures. The reflection high-energy electron diffraction patterns taken along the [−120] azimuth showed that the twofold reconstruction commonly observed during the GaAs-buffer layer growth is preserved during the AlGaAs deposition. The 10K photoluminiscence (PL) characterization of the samples showed transitions related to the AlGaAs band edge, the incorporation of impurities, and deep centers. The temperature dependence of the band to band PL transition was fitted with the models developed by Varshni, Passler, and Vina. The intensity of the PL spectra drastically decreases as the As4 pressure is increased. Photoreflectance (PR) spectroscopy also showed the best crystal quality for the sample grown at low As pressure. The authors obtained the built-in internal electric field strength and the band-gap energy from an analysis of the PR spectra clo...