Bastien Bonef

1550-nm InGaAsP multi-quantum-well structures selectively grown on v-groove-patterned SOI substrates

We report direct growth of 1550-nm InGaAsP multi-quantum-well (MQW) structures in densely packed, smooth, highly crystalline, and millimeter-long InP nanoridges grown by metalorganic chemical vapor deposition on silicon-on-insulator (SOI) substrates. Aspect-ratio-trapping and selective area growth techniques were combined with a two-step growth process to obtain good material quality as revealed by photoluminescence, scanning electronic microscopy, and high-resolution X-ray diffraction characterization. Transmission electron microscopy images revealed sharp MQW/InP interfaces as well as thickness variation of the MQW layer. This was confirmed by atom probe tomography analysis, which also suggests homogenous incorporation of the various III-V elements of the MQW structure. This approach is suitable for the integration of InP-based nanoridges in the SOI platform for new classes of ultra-compact, low-power, nano-electronic, and photonic devices for future tele- and data-communications applications.

Atomic arrangement at ZnTe/CdSe interfaces determined by high resolution scanning transmission electron microscopy and atom probe tomography

High resolution scanning transmission electron microscopy and atom probe tomography experiments reveal the presence of an intermediate layer at the interface between two binary compounds with no common atom, namely, ZnTe and CdSe for samples grown by Molecular Beam Epitaxy under standard conditions. This thin transition layer, of the order of 1 to 3 atomic planes, contains typically one monolayer of ZnSe. Even if it occurs at each interface, the direct interface, i.e., ZnTe on CdSe, is sharper than the reverse one, where the ZnSe layer is likely surrounded by alloyed layers. On the other hand, a CdTe-like interface was never observed. This interface knowledge is crucial to properly design superlattices for optoelectronic applications and to master band-gap engineering.

Nanometer scale composition study of MBE grown BGaN performed by atom probe tomography

Laser assisted atom probe tomography is used to characterize the alloy distribution in BGaN. The effect of the evaporation conditions applied on the atom probe specimens on the mass spectrum and the quantification of the III site atoms is first evaluated. The evolution of the Ga++/Ga+ charge state ratio is used to monitor the strength of the applied field. Experiments revealed that applying high electric fields on the specimen results in the loss of gallium atoms, leading to the over-estimation of boron concentration. Moreover, spatial analysis of the surface field revealed a significant loss of atoms at the center of the specimen where high fields are applied. A good agreement between X-ray diffraction and atom probe tomography concentration measurements is obtained when low fields are applied on the tip. A random distribution of boron in the BGaN layer grown by molecular beam epitaxy is obtained by performing accurate and site specific statistical distribution analysis.

Indium segregation in N-polar InGaN quantum wells evidenced by energy dispersive X-ray spectroscopy and atom probe tomography

Energy dispersive X-ray spectroscopy (EDX) in scanning transmission electron microscopy and atom probe tomography are used to characterize N-polar InGaN/GaN quantum wells at the nanometer scale. Both techniques first evidence the incorporation of indium in the initial stage of the barrier layer growth and its suppression by the introduction of H2 during the growth of the barrier layer. Accumulation of indium at step edges on the vicinal N-polar surface is also observed by both techniques with an accurate quantification obtained by atom probe tomography (APT) and its 3D reconstruction ability. The use of EDX allows for a very accurate interpretation of the APT results complementing the limitations of both techniques.

Atomic Scale Structural Characterization of Epitaxial (Cd,Cr)Te Magnetic Semiconductor

A detailed knowledge of the atomic structure of magnetic semiconductors is crucial to understanding their electronic and magnetic properties, which could enable spintronic applications. Energy-dispersive X-ray spectrometry (EDX) in the scanning transmission electron microscope and atom probe tomography (APT) experiments reveal the formation of Cr-rich regions in Cd1-x Cr x Te layers grown by molecular beam epitaxy. These Cr-rich regions occur on a length scale of 6-10 nm at a nominal Cr composition of x=0.034 and evolve toward an ellipsoidal shape oriented along directions at a composition of x=0.083. Statistical analysis of the APT reconstructed volume reveals that the Cr aggregation increases with the average Cr composition. The correlation with the magnetic properties of such (Cd,Cr)Te layers is discussed within the framework of strongly inhomogeneous materials. Finally, difficulties in accurately quantifying the Cr distribution in the CdTe matrix on an atomic scale by EDX and APT are discussed.

Growth of coherent BGaN films using BBr3 gas as a boron source in plasma assisted molecular beam epitaxy

Incorporating boron into gallium nitride to make BxGa1-xN solid solutions would create an avenue for extreme alloys due to the fact that wurtzite phase BN has a larger band gap and smaller lattice parameters compared to GaN. In this paper, the authors report the growth of high crystal quality, random alloy BxGa1-xN thin films with x up to 3.04% grown on (0001) Ga-face GaN on sapphire substrates using plasma assisted molecular beam epitaxy and BBr3 gas as a B source. High resolution x-ray diffraction was used to measure both the c plane spacing and the strain state of the films. It was determined that the films were fully coherent to the GaN substrate. Elastic stress-strain relations and Vegards law were used to calculate the composition. Atom probe tomography was used to confirm that the BxGa1-xN films were random alloys. In addition to demonstrating a growth technique for high crystal quality BxGa1-xN thin films, this paper demonstrated the use of BBr3 as a novel B source in plasma assisted molecular be...

Strain compensated superlattices on m-plane gallium nitride by ammonia molecular beam epitaxy

The results of tensile strained AlN/GaN, AlGaN/GaN, and compressive strained InGaN/GaN superlattices (SLs) grown by Ammonia MBE (NH3-MBE) are presented. A combination of atom probe tomography and high-resolution X-ray diffraction confirms that periodic heterostructures of high crystallographic quality are achieved. Strain induced misfit dislocations (MDs), however, are revealed by cathodoluminescence (CL) of the strained AlN/GaN, AlGaN/GaN, and InGaN/GaN structures. MDs in the active region of a device are a severe problem as they act as non-radiative charge recombination centers, affecting the reliability and efficiency of the device. Strain compensated SL structures are subsequently developed, composed of alternating layers of tensile strained AlGaN and compressively strained InGaN. CL reveals the absence of MDs in such structures, demonstrating that strain compensation offers a viable route towards MD free active regions in III-Nitride SL based devices.

Dopant radial inhomogeneity in Mg-doped GaN nanowires

Using atom probe tomography, it is demonstrated that Mg doping of GaN nanowires grown by Molecular Beam Epitaxy results in a marked radial inhomogeneity, namely a higher Mg content in the periphery of the nanowires. This spatial inhomogeneity is attributed to a preferential incorporation of Mg through the m-plane sidewalls of nanowires and is related to the formation of a Mg-rich surface which is stabilized by hydrogen. This is further supported by Raman spectroscopy experiments which give evidence of Mg-H complexes in the doped nanowires. A Mg doping mechanism such as this, specific to nanowires, may lead to higher levels of Mg doping than in layers, boosting the potential interest of nanowires for light emitting diode applications.

High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

N-polar grown III-nitrides are very interesting materials for the fabrication of heterostructures devices such as transistors, photodetectors, solar cells or optoelectronic devices. In GaN/(In,Ga)N/GaN heterostructures with thin (In,Ga)N layers, polarization engineering allows to achieve interband tunneling. Thereby the tunneling probability is proportional to the indium concentration in the InGaN layers. Indium incorporation is higher for N-polar InGaN films than for the typically grown Ga-polar ones. N-polar III nitride films are often grown on misoriented substrates enabling the growth of smooth, high quality layers [1]. The crystal misorientation leads to the formation of surface steps, and misorientation angles of 4°-5° can result in up to 3-4 unit cell high steps. The growth of N-polar InGaN films is further complicated by the necessary reduced growth temperatures and the required absence of hydrogen in the growth ambient. In quantum well structures, however, hydrogen, which acts as surfactant and promotes the growth of smooth layers, can be introduced during GaN barrier growth, allowing the deposition of thick multiple quantum well (MQW) stacks. Ga-polar InGaN films have been extensively studied and are known for their local fluctuations in the indium composition [2]. Not much is known about the uniformity of N-polar InGaN layers.

Atom Probe Tomography Quantification of Alloy Fluctuations in (Al,In,Ga)N

As compared with ternary nitrides systems such as InGaN, AlGaN and InAlN, quaternary (Al,In,Ga)N benefit from a significant increase in design freedom. Indeed changes in the III site atomic fraction allows for an independent choice of either band-gap, lattice constant or polarization which is particularly interesting in the field of solid state lighting, radio frequency (RF), and power electronics. Controlling the homogeneity of the alloy is of great interest to understand the physical properties of (Al,In,Ga)N based devices [1,2]. Atom probe tomography (APT) has demonstrated its ability to evidence alloy fluctuations in ternary nitride [3]. However, detection artifacts in APT could lead to misinterpretation of the actual compositions [4,5]. Calibration experiments are required to find evaporation parameters which enable for an accurate quantification of all III site atoms in (Al,In,Ga)N.