Exploring the evolution of mass density and thickness of N-doped Ge-rich GeSbTe during multistep crystallization.

Among phase change materials, Ge-rich GeSbTe alloys (GGST) are key alloys for the next generation of embedded phase change memories because of their good thermal stability, allowing their use for the automotive applications. Several studies have investigated GGST crystallization, which takes place in several stages, including phase separation in the amorphous material, the crystallization of the cubic Ge and GST phases before a complete crystallization for higher thermal budget. So far, however, no information is available on the possible changes in density and thickness of such alloys. This paper investigates such variations in density and thickness for a N-doped GGST layer (GGSTN) during isothermal annealing, following the four main stages of its multistep crystallization process. X-ray reflectivity (XRR) and X-ray diffraction were employed for analysis. The study reveals that density and thickness exhibit distinct changes during crystallization, with density increasing by approximately 9% during transition from amorphous to crystalline states. These changes are attributed to alterations in layer morphology, particularly at the Ge crystallization temperature and at the onset of GST crystal formation. Additionally, at high thermal budgets, discrepancies between XRR analysis methods suggest the formation of a thin, lower density layer near the top interface of the GGSTN layer. These results provide insights into the structural evolution of the GGSTN layer, which is crucial for phase change random access memory applications.

Influence of the Mn5Ge3/Ge ohmic-contact interface on the Seebeck coefficient of the Mn5Ge3/Ge bilayer.

Thermoelectricity is a well-known effect that can be used to convert heat energy into electrical energy. However, the yield of this conversion is still low compared to current photovoltaic technology. It is limited by the intrinsic properties of materials, leading to intensive materials science investigations for the design of efficient thermoelectric (TE) materials. Interface engineering was shown to be a valuable solution for improving materials' TE properties, supporting the development of multiphase TE materials. In particular, interfaces have been suggested to promote the increase of the Seebeck coefficient of materials without significantly impacting their electrical conductivity through the so-called energy filtering effect. This work aims at determining experimentally the effect of a metal/semiconductor interface exhibiting an ohmic character on the effective Seebeck coefficient of multiphase materials, focusing on the n-type Mn5Ge3/p-type Ge interface. This interface is shown not to contribute to carrier transport, but to contribute to carrier concentration filtering due to carrier injection or recombination. The Seebeck coefficient of the bi-phase material is shown to be dependent on the direction carriers are crossing the interface. The interface effect mainly results from a modification of charge carrier concentrations in the semiconductor.

Magnetic moment impact on spin-dependent Seebeck coefficient of ferromagnetic thin films.

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µI), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µI influence on the spin-dependent S of model ferromagnetic thin films, allowing µI thermal fluctuations, ordering, and density variation influence to be independently investigated. µI influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

Thickness Effect on the Solid-State Reaction of a Ni/GaAs System.

Ni thin films with different thicknesses were grown on a GaAs substrate using the magnetron sputtering technique followed by in situ X-ray diffraction (XRD) annealing in order to study the solid-state reaction between Ni and GaAs substrate. The thickness dependence on the formation of the intermetallic phases was investigated using in situ and ex situ XRD, pole figures, and atom probe tomography (APT). The results indicate that the 20 nm-thick Ni film exhibits an epitaxial relation with the GaAs substrate, which is (001) Ni//(001) GaAs and [111] Ni//[110] GaAs after deposition. Increasing the film's thickness results in a change of the Ni film's texture. This difference has an impact on the formation temperature of Ni3GaAs. This temperature decreases simultaneously with the thickness increase. This is due to the coherent/incoherent nature of the initial Ni/GaAs interface. The Ni3GaAs phase decomposes into the binary and ternary compounds xNiAs and Ni3-xGaAs1-x at about 400 °C. Similarly to Ni3GaAs, the decomposition temperature of the second phase also depends on the initial thickness of the Ni layer.

Coupling Secondary Ion Mass Spectrometry and Atom Probe Tomography for Atomic Diffusion and Segregation Measurements.

For a long time, secondary ion mass spectrometry (SIMS) was the only technique allowing impurity concentrations below 1 at% to be precisely measured in a sample with a depth resolution of few nanometers. For example, SIMS is the classical technique used in microelectronics to study dopant distribution in semiconductors and became, after radiotracers were forsaken, the principal tool used for atomic transport characterization (diffusion coefficient measurements). Due to the lack of other equivalent techniques, sometimes SIMS could be used erroneously, especially when the analyzed solute atoms formed clusters, or for interfacial concentration measurements (segregation coefficient measurements) for example. Today, concentration profiles measured by atom probe tomography (APT) can be compared to SIMS profiles and allow the accuracy of SIMS measurements to be better evaluated. However, APT measurements can also carry artifacts and limitations that can be investigated by SIMS. After a summary of SIMS and APT measurement advantages and disadvantages, the complementarity of these two techniques is discussed, particularly in the case of experiments aiming to measure diffusion and segregation coefficients.

Nanoporous Ge thin film production combining Ge sputtering and dopant implantation.

In this work a novel process allowing for the production of nanoporous Ge thin films is presented. This process uses the combination of two techniques: Ge sputtering on SiO2 and dopant ion implantation. The process entails four successive steps: (i) Ge sputtering on SiO2, (ii) implantation preannealing, (iii) high-dose dopant implantation, and (iv) implantation postannealing. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology of the Ge film at different process steps under different postannealing conditions. For the same postannealing conditions, the Ge film topology was shown to be similar for different implantation doses and different dopants. However, the film topology can be controlled by adjusting the postannealing conditions.

Si/Ge intermixing during Ge Stranski-Krastanov growth.

The Stranski-Krastanov growth of Ge islands on Si(001) has been widely studied. The morphology changes of Ge islands during growth, from nucleation to hut/island formation and growth, followed by hut-to-dome island transformation and dislocation nucleation of domes, have been well described, even at the atomic scale, using techniques such as scanning tunneling microscopy and transmission electron microscopy. Although it is known that these islands do not consist of pure Ge (due to Si/Ge intermixing), the composition of the Ge islands is not precisely known. In the present work, atom probe tomography was used to study the composition of buried dome islands at the atomic scale, in the three-dimensional space. The core of the island was shown to contain about 55 atom % Ge, while the Ge composition surrounding this core decreases rapidly in all directions in the islands to reach a Ge concentration of about 15 atom %. The Ge distribution in the islands follows a cylindrical symmetry and Ge segregation is observed only in the {113} facets of the islands. The Ge composition of the wetting layer is not homogeneous, varying from 5 to 30 atom %.