M. Gutierrez

Solid solution strengthening in GaSb/GaAs: A mode to reduce the TD density through Be-doping

The need for a low bandgap semiconductor on a GaAs substrate for thermophotovoltaic applications has motivated research on GaSb alloys, in particular, the control of plastic relaxation of its active layer. Although interfacial misfit arrays offer a possibility of growing strain-free GaSb-based devices on GaAs substrates, a high density of threading dislocations is normally observed. Here, we present the effects of the combined influence of Be dopants and low growth temperature on the threading dislocation density observed by Transmission Electron Microscopy. The Be-related hardening mechanism, occurring at island coalescence, is shown to prevent dislocations to glide and hence reduce the threading dislocation density in these structures. The threading density in the doped GaSb layers reaches the values of seven times less than those observed in undoped samples, which confirms the proposed Be-related hardening mechanism.

An approach to the formation mechanism of the composition fluctuation in GaInNAs quantum wells

This work is focused on the study by transmission electron microscopy of the composition inhomogeneity of GaInNAs quantum wells. A variation of the contrast along the quantum well is reported which is related to phase separation in the alloy. This variation becomes more intense on increasing the growth temperature. From the intensity profiles taken from the images, we have calculated the energy of activation for surface diffusion in the alloy. The results suggest that the diffusion of indium controls the process of formation of phase separation. With regard to the thermodynamics of the process, we have extended the spinodal decomposition model of Ipatova to the quaternary alloy GaInNAs. Our calculations have shown that composition fluctuations of indium are stimulated by the introduction of nitrogen into the ternary alloy GaInAs. It is proposed that the observed compositional inhomogeneity of GaInNAs is composed mainly of a fluctuation of the indium content but secondly that N bonds preferentially to Ga-rich regions in the alloy.

Control of the Alumina Microstructure to Reduce Gate Leaks in Diamond MOSFETs

In contrast to Si technology, amorphous alumina cannot act as a barrier for a carrier at diamond MOSFET gates due to their comparable bandgap. Indeed, gate leaks are generally observed in diamond/alumina gates. A control of the alumina crystallinity and its lattice matching to diamond is here demonstrated to avoid such leaks. Transmission electron microscopy analysis shows that high temperature atomic layer deposition, followed by annealing, generates monocrystalline reconstruction of the gate layer with an optimum lattice orientation with respect to the underneath diamond lattice. Despite the generation of γ-alumina, such lattice control is shown to prohibit the carrier transfer at interfaces and across the oxide.

Silicon (001) Heteroepitaxy on 3C-SiC(001)/Si(001) Seed

We report for the first time the successful heteroepitaxial growth of Si(100) oriented layer on top of a 3C-SiC(001) seed. By using a post-growth modification of the 3C-SiC surface (pulse insertion of precursors during cooling), it led to a change in Si nucleation, favoring squared (100) islands instead of elongated (110) ones. Without this surface modification step, the Si layers grown on 3C-SiC were always polycrystalline with a mixture of (110) and (100) orientations. Using such Si(100) layer grown on top of 3C-SiC(100), a (100) oriented 3C-SiC single crystalline layer was successfully grown on top, fabricating thus for the first time a fully (100) oriented multilayer heterostructure made of Si(substrate)/SiC/Si/SiC.

GaSb and GaSb/AlSb Superlattice Buffer Layers for High-Quality Photodiodes Grown on Commercial GaAs and Si Substrates

The objective of this work is the integration of InGaAs/GaSb/GaAs heterostructures, with high indium content, on GaAs and Si commercial wafers. The design of an interfacial misfit dislocation array, either on GaAs or Si substrates, allowed growth of strain-free devices. The growth of purposely designed superlattices with their active region free of extended defects on both GaAs and Si substrates is demonstrated. Transmission electron microscopy technique is used for the structural characterization and plastic relaxation study. In the first case, on GaAs substrates, the presence of dopants was demonstrated to reduce several times the threading dislocation density through a strain-hardening mechanism avoiding dislocation interactions, while in the second case, on Si substrates, similar reduction of dislocation interactions is obtained using an AlSb/GaSb superlattice. The latter is shown to redistribute spatially the interfacial misfit dislocation array to reduce dislocation interactions.

High quality Al2O3/(100) oxygen-terminated diamond interface for MOSFETs fabrication

In this letter, we report on the improvement of gate controlled Al2O3/(100) boron doped (B-doped) oxygen-terminated diamond (O-diamond) Metal Oxide Semiconductor Capacitors using 40 nm thick Al2O3 deposited by Atomic Layer Deposition at 380 °C and then annealed at 500 °C in vacuum conditions. The high quality of Al2O3 and an Al2O3/diamond interface is verified thanks to electrical measurements and Transmission Electron Microscopy (TEM) measurements. A density of interface states lower than 1012 eV−1 cm−2 is measured from the flat-band regime to the depletion regime. The shift of the flat-band voltage and the leakage current through the oxide are significantly reduced in good agreement with the mono-crystalline character of the Al2O3 layer revealed by TEM.In this letter, we report on the improvement of gate controlled Al2O3/(100) boron doped (B-doped) oxygen-terminated diamond (O-diamond) Metal Oxide Semiconductor Capacitors using 40 nm thick Al2O3 deposited by Atomic Layer Deposition at 380 °C and then annealed at 500 °C in vacuum conditions. The high quality of Al2O3 and an Al2O3/diamond interface is verified thanks to electrical measurements and Transmission Electron Microscopy (TEM) measurements. A density of interface states lower than 1012 eV−1 cm−2 is measured from the flat-band regime to the depletion regime. The shift of the flat-band voltage and the leakage current through the oxide are significantly reduced in good agreement with the mono-crystalline character of the Al2O3 layer revealed by TEM.

Influence of structure and defects on the performance of dot-in-well laser structures

Recent progress in the development of 1.3 mm InAs/InGaAs/GaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, consequence of the finite dot density and carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of different growth parameters which strongly modify the InAs QDs structure such as temperature and thickness of barrier layers and thickness and composition of the well. Analysis by Transmission Electron Microscopy (TEM), Photoluminescence (PL) and atomic force microscopy (AFM) have identified the presence of defects arising from the complex interaction of QDs, which propagate through the structure into the upper regions being the primary cause of the poor electronic device characteristics. The use of optimized growth has allowed, however, the fabrication of a defect free five layer-stacked structure with record low threshold current density.

Growth, Fabrication, and Operating Characteristics of Ultra-Low Threshold 1.31μm Quantum Dot Lasers

In order to achieve QDs operating at 1.31 μm, ultra low growth rates and atomic layer epitaxy have been employed. However, due to the low density of such long wavelength QDs such laser diodes suffer from gain saturation, resulting in high threshold current density (Jth) and poor temperature stability. Through careful optimization, the DWELL method can overcome these problems. Here, we describe advances made in the growth of multiple layer DWELL structures to obtain world leading QD laser performance at 1.31 μm.

Relaxation study of AlGaAs cladding layers in InGaAs/GaAs (111)B lasers designed for 1.0–1.1 μm operation

InGaAs/GaAs-based lasers require thick AlGaAs cladding layers to provide optical confinement. Although the lattice mismatch between GaAs and AlGaAs is very low, relaxation may occur due to the thickness requirement for an AlGaAs waveguide of the order of microns. We have studied the relaxation of InGaAs/GaAs lasers with AlGaAs waveguides grown on GaAs (111)B substrates. We have observed by transmission electron microscopy (TEM) that certain AlGaAs layers show a high density of threading dislocations (TDs), whilst other AlGaAs layers remain essentially dislocation free. To explain the experimental results a model based on dislocation multiplication has been developed. TDs in the AlGaAs cladding layers are observed when the critical layer thickness (CLT) for dislocation multiplication has been overcome. Consequently, a design rule based on a modified CLT model for AlGaAs/GaAs (111)B is proposed.