M. López-López

Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates with and without GaAs buffer layers

ZnSe films with thickness between 800 and 7500 A 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.

GaAs surface oxide desorption by annealing in ultra high vacuum

Abstract We have studied the desorption mechanism of Ga- and As-oxides on GaAs (100) by subjecting the substrates to two thermal processes in ultra high vacuum (UHV) conditions. The first process was an outgassing at 350°C, and the second process consisted of an annealing at 530°C. The pressure variations in the UHV chambers recorded during both thermal treatments showed a behavior related to the removal of As- and Ga-oxides from the substrate surface. The thermally treated GaAs (100) substrates were analyzed by in situ reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and ex situ atomic force microscopy (AFM). AFM images clearly showed the presence of surface pits on the GaAs (100) samples exposed to the high-temperature oxide desorption process. The pits have a density of the order of 10 9 /cm 2 , and some are as deep as 120 A. We explain the pits formation mechanism in terms of chemical reactions of the surface oxides with the elements of the substrate.

Interdiffusion of Indium in piezoelectric InGaAs∕GaAs quantum wells grown by molecular beam epitaxy on (11n) substrates

Pseudomorphic In0.2Ga0.8As∕GaAs quantum wells (QWs) were grown by molecular beam epitaxy on GaAs substrates oriented along (11n) directions, with n=1,2,3,4. The optical and structural properties of the heterostructures were studied by photoluminescence (PL), photoreflectance (PR) spectroscopy, and atomic force microscopy measurements. The energy transitions in the QWs have two contributions, a blueshift due to the compressive strain, and a redshift due to the quantum confined Stark effect produced by the piezoelectric field present in the QWs. A variational approach was employed to calculate the QWs ground energy transitions employing an ideal potential well with sharp interfaces. The theoretical energy transitions were fitted to the PL peaks energy to obtain the electric fields in the InGaAs QWs. The obtained electric fields show discrepancies with theoretical piezoelectric fields calculated from the strain present in the QWs. In order to overcome these discrepancies, we propose to include interdiffusion...

Study of internal electric fields in AlGaAs/GaAs two-dimensional electron gas heterostructures

Abstract Modulation-doped GaAs/AlGaAs heterostructures have been studied by photoreflectance spectroscopy. The spectra at room temperature show Franz–Keldysh oscillations associated to the substrate–buffer layer interface. The built-in electric field magnitude calculated from these oscillations is related with the two-dimensional electron gas (2DEG) mobility. In addition we observed two signals associated to the GaAs capping layer and to the 2DEG, respectively.

Study of structural properties of cubic InN films on GaAs(001) substrates by molecular beam epitaxy and migration enhanced epitaxy

InN epitaxial films with cubic phase were grown by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on GaAs(001) substrates employing two methods: migration-enhanced epitaxy (MEE) and conventional MBE technique. The films were synthesized at different growth temperatures ranging from 490 to 550 °C, and different In beam fluxes (BEPIn) ranging from 5.9 × 10−7 to 9.7 × 10−7 Torr. We found the optimum conditions for the nucleation of the cubic phase of the InN using a buffer composed of several thin layers, according to reflection high-energy electron diffraction (RHEED) patterns. Crystallographic analysis by high resolution X-ray diffraction (HR-XRD) and RHEED confirmed the growth of c-InN by the two methods. We achieved with the MEE method a higher crystal quality and higher cubic phase purity. The ratio of cubic to hexagonal components in InN films was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) and hexagonal (101¯1) planes measured by X-ray reciprocal spa...

Synthesis of GaAs quantum dots on Si-layers on AlGaAs films grown on GaAs(100)

Abstract In this work we report a novel method for obtaining GaAs quantum dots (QDs) by molecular beam epitaxy (MBE) on an AlGaAs underlying film. We propose to use a Si monolayer (ML) grown on AlGaAs in order to induce a three-dimensional nucleation during the GaAs overgrowth. We observe that when 2.1 MLs of GaAs are grown on 1 ML of Si, the GaAs is self-assembled in islands. Islands of 15-A height and a lateral size of 200 A were clearly observed. 77 K photoluminescence and photoreflectance spectra of capped GaAs dots showed an additional signal at ∼1.88 eV, presumably related to the islands emission, thereby verifying an efficient quantum confinement.

Study of AlGaAs/GaAs quantum wells overgrown on in situ Cl2-etched GaAs substrates

We have studied the properties of in situ Cl2-etched GaAs surfaces and overgrown quantum well (QW) structures as a function of the etching temperature. From reflection high-energy electron diffraction, atomic force microscopy, and Auger electron spectroscopy analysis on Cl2-etched surfaces we found that low etching temperatures (∼50 °C) results in Ga-rich rough surfaces, because at these temperatures the desorption rate of Ga chlorides is much slower than that for As chlorides. At high etching temperatures (∼200 °C) both Ga and As chlorides can be removed, resulting in a more stoichiometric etching which yields a smooth GaAs surface. The optical properties of QW structures overgrown on in situ etched surfaces were compared to those of an ex situ processed sample. For the ex situ processed sample we found degraded photoluminescence (PL) characteristics and a high amount of impurities. The PL properties improved, and the amount of impurities sharply decreased in the in situ sample Cl2 etched at 200 °C. Phot...

Study of the optical and structural properties of GaN films grown on Si substrates with a SiC layer

Abstract GaN films were grown on Si(100) substrates by molecular beam epitaxy employing an RF activated N-plasma source. The substrates were coated with a thin SiC layer to reduce the reaction of N with Si. The substrate temperature was set at 750 °C, and the flux of Ga atoms was varied by changing the Ga-Knudsen cell temperature (TGa) from 950 to 1100 °C. The effects of the different growth conditions on the optical and structural characteristics of the films were studied by X-ray diffraction, atomic force microscopy, photoluminescence and photoreflectance spectroscopy. The results show that for TGa=950 °C, the films presented a very poor crystal quality with a mixture of hexagonal (α) and cubic (β) GaN phases. By increasing TGa the crystal quality improved. The films presented predominately the β-GaN phase for an optimal temperature TGa of 1050 °C.

Molecular Beam Epitaxial Growth of ZnSe Layers on GaAs and Si Substrates

We present a study of the molecular beam epitaxial (MBE) growth of ZnSe layers on GaAs and Si substrates. For the growth on GaAs substrates we investigated the effects of introducing buffer layers of AlxGa1—xAs and InxGa1—xAs. The characterization by reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL) showed that best ZnSe crystal quality was obtained on buffer layers of AlxGa1—xAs and InxGa1—xAs with x = 0.01. Moreover, an analysis by secondary ion mass spectroscopy (SIMS) revealed that the use of AlGaAs buffer layers effectively suppresses the Ga segregation onto the ZnSe layer surfaces. On the other hand, for the growth of ZnSe on Si substrates, we achieved a significant improvement in the crystal quality of ZnSe by irradiating the Si substrates with a plasma of nitrogen prior to the MBE growth.

NEAR BAND-EDGE OPTICAL PROPERTIES OF GAAS AT INTERFACES OF ZNSE/GAAS/GAAS BY PHASE SELECTION IN PHOTOREFLECTANCE

GaAs at the ZnSe/GaAs and GaAs/GaAs interfaces of ZnSe/GaAs/GaAs heterostructures is studied by phase selective photoreflectance (PR) spectroscopy. Four samples with ZnSe layers of various thickness were examined. We unambiguously determined the origin of two different features observed in the PR spectra by combining in phase and out of phase measurements, with PR measurements employing excitation lasers with different wavelengths. These two features are found to originate at different regions of the heterostructure. One contributing transition is a bulk-like signal, resembling that of bare GaAs, which originates in a region that encompasses the buffer layer/substrate GaAs homointerface. A second contributing signal is attributed to a strained region adjacent to the ZnSe/GaAs heterointerface. Both this second signal and the bulk-like signal show Franz–Keldysh oscillations that allow us to determine the electric field strength at the ZnSe/GaAs and GaAs/GaAs interfaces. It is found that the electric field s...