S. I. Molina

Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell

Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.

Growth of III-nitrides on Si(1 1 1) by molecular beam epitaxy: Doping, optical, and electrical properties

The growth of high-quality III-nitrides by plasma-assisted molecular beam epitaxy on Si(1 1 1) substrates is addressed. A combination of optimized AIN buffer layers and a two-step growth process leads to GaN layers of high crystal quality (8 arcmin X-ray diffraction full-width at half-maximum) and flat surfaces (57 A rms). Low-temperature luminescence spectra, dominated by excitonic emissions at 3.465 ± 0.002 eV, reveal the presence of a biaxial tensile strain of thermal origin. AlGaN layers, grown within the alloy range 0.10 < x < 0.76, have flat surfaces and exhibit strong excitonic luminescence. Si-doping of GaN and AlGaN produces n-type films reaching electron densities up to 2 × 10 19 and 8 × 10 19 cm -3 , respectively. From photoluminescence and Hall data analysis a Si-donor ionization energy between 50 and 60 meV is derived in GaN. The exciton bound to Si neutral donors at 3.445 eV redshifts while the c-axis lattice parameter decreases as the Si-doping increases, indicating an enhancement of the biaxial tensile strain in the film. This strain increase is a consequence of a strong reduction of the density of dislocations reaching the free surface, due to a particular grain size and orientation governed by the presence of Si donors. Be-doping is also achieved on GaN giving the shallowest acceptor activation energy reported so far, around 90-100 meV. However, there is a severe limitation of the Be incorporation on substitutional sites, leading to the formation of complex, deep defects.

Room temperature emission at 1.6μm from InGaAs quantum dots capped with GaAsSb

Room temperature photoluminescence at 1.6μm is demonstrated from InGaAs quantum dots capped with an 8nm GaAsSb quantum well. Results obtained from various sample structures are compared, including samples capped with GaAs. The observed redshift in GaAsSb capped samples is attributed to a type II band alignment and to a beneficial modification of growth kinetics during capping due to the presence of Sb. The sample structure is discussed on the basis of transmission electron microscopy results.

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems.

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Column-by-column compositional mapping by Z-contrast imaging.

A phenomenological method is developed to determine the composition of materials, with atomic column resolution, by analysis of integrated intensities of aberration-corrected Z-contrast scanning transmission electron microscopy images. The method is exemplified for InAs(x)P(1-x) alloys using epitaxial thin films with calibrated compositions as standards. Using this approach we have determined the composition of the two-dimensional wetting layer formed between self-assembled InAs quantum wires on InP(001) substrates.

The effect of Si doping on the defect structure of GaN/AlN/Si(111)

The effect of Si doping on the structural quality of wurtzite GaN layers grown by molecular beam epitaxy on AlN buffered (111) Si substrates is studied. The planar defect density in the grown GaN layer strongly increases with Si doping. The dislocation density at the free surface of GaN significantly decreases when Si doping overpasses a limit value. Si doping affects the misorientation of the subgrains that constitutes the mosaic structure of GaN. The increase of the planar defect density and out-plane misorientation angles of the GaN subgrains with Si doping explain the decrease of dislocations that reach the free surface of GaN. A redshift in the photoluminescence spectra together with a decrease in the c-axis lattice parameter as the Si doping increases point to an increase in the residual biaxial tensile strain in the GaN samples.

Strain relief in linearly graded composition buffer layers: A design scheme to grow dislocation‐free (<105 cm−2) and unstrained epilayers

The strain relaxation in linearly graded composition InGaAs layers grown on (001) GaAs substrates by molecular beam epitaxy is studied by transmission electron microscopy (TEM) and double crystal x‐ray diffraction (DCXRD). The dislocation distribution in these layers does not coincide with the predicted equilibrium dislocation distribution [J. Tersoff, Appl. Phys. Lett. 62, 693 (1993)]. The dislocation density in the dislocation‐rich layer thickness is slightly smaller than the equilibrium density. The thickness of the dislocation‐rich region is different in the [110] and [110] directions. A good correspondence exists between the TEM and DCXRD strain measurements. The dislocation distribution observed by TEM has made it possible to design a scheme to grow dislocation‐free and unstrained top layers on linearly graded composition buffer layers.

Carrier recombination effects in strain compensated quantum dot stacks embedded in solar cells

In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications.

Molecular beam epitaxy of GaBiAs on (311)B GaAs substrates

We report the growth by molecular beam epitaxy of GaBixAs1−x epilayers on (311)B GaAs substrates. We use high-resolution x-ray diffraction (HRXRD), transmission electron microscopy, and Z-contrast imaging to characterize the structural properties of the as-grown material. We find that the incorporation of Bi into the GaBiAs alloy, as determined by HRXRD, is sizably larger in the (311)B epilayers than in (001) epilayers, giving rise to reduced band-gap energies as obtained by optical transmission spectroscopy.