Formation of GeO2 under Graphene on Ge(001)/Si(001) Substrates Using Water Vapor.

The problem of graphene protection of Ge surfaces against oxidation is investigated. Raman, X-Ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements of graphene epitaxially grown on Ge(001)/Si(001) substrates are presented. It is shown that the penetration of water vapor through graphene defects on Gr/Ge(001)/Si(001) samples leads to the oxidation of germanium, forming GeO2. The presence of trigonal GeO2 under graphene was identified by Raman and XRD measurements. The oxidation of Ge leads to the formation of blisters under the graphene layer. It is suggested that oxidation of Ge is connected with the dissociation of water molecules and penetration of OH molecules or O to the Ge surface. It has also been found that the formation of blisters of GeO2 leads to a dramatic increase in the intensity of the graphene Raman spectrum. The increase in the Raman signal intensity is most likely due to the screening of graphene by GeO2 from the Ge(001) surface.

Resistivity contrast imaging in semiconductor structures using ultra-low energy scanning electron microscopy.

The damage-induced voltage alteration (DIVA) contrast mechanism in scanning electron microscope (SEM) has been studied in broad range of the primary electron beam energies, with a special emphasis on the ultra-low energy range. The SEM imaging contrast related to resistivity changes in the In(0.55)Al(0.45)P irradiated with He2+ ions of 600 keV was subjected to an analysis in a range of 10 keV down to 10 eV of primary electron energies. The problem of specimen charging in ultra-low energy range and its effect on the contrast in SEM images has been tackled for the first time. Contrary to expectations based on the classical total emission yield approach, the potentials formed at the highly resistive part of irradiated area led to dramatic increase in the intensity of registered signal for primary electron energies below E1, which can be explained as signal saturation due to potential on the specimen surface acting as repeller for primary electrons. Nevertheless, the experimental data presenting the influence of the beam energy on the potential formation on the surface of an insulating material under electron irradiation have been presented for the first time in ultra-low energy regime.

Growth of highly oriented MoS2via an intercalation process in the graphene/SiC(0001) system.

A method of growing highly oriented MoS2 is presented. First, a Mo film is deposited on a graphene/SiC(0001) substrate and the subsequent annealing of it at 750 °C leads to intercalation of Mo underneath the graphene layer, which is confirmed by secondary ion mass spectrometry (SIMS) measurements. Formation of highly oriented MoS2 layers is then achieved by sulfurization of the graphene/Mo/SiC system using H2S gas. X-ray diffraction reveals that the MoS2 layers are highly oriented and parallel to the underlying SiC substrate surface. Further SIMS experiments reveal that the intercalation process occurs via the atomic step edges of SiC and Mo and S atoms gradually diffuse along SiC atomic terraces leading to the creation of the MoS2 layer. This observation can be explained by a mechanism of highly oriented growth of MoS2: nucleation of the crystalline MoS2 phase occurs underneath the graphene planes covering the flat parts of SiC steps and Mo and S atoms create crystallization fronts moving along terraces.

Damage-induced voltage alteration (DIVA) contrast in SEM images of ion-irradiated semiconductors.

The ion-irradiation damage effects in semiconductors were directly visualized by means of scanning electron microscopy at low beam acceleration voltages (low-kV SEM). The Al0.55Ga0.45As (p-type and n-type) epitaxial layers grown over GaAs substrates were irradiated with energetic He+ ions with fluencies ranging from 8e12 to 8e13 cm-2 and studied in cross-sectional view after cleavage. Secondary electron images collected at low energy (0.5 - 1 keV) of primary electrons show strong contrast related to the local differences in the material resistivity resulting from the ion-induced damage. The main aim of the paper is to present, for the first time, the interpretation of the mechanism of SEM image contrast formation, which has been referred to as Damage-Induced Voltage Alteration (DIVA) contrast and is based on the charging effect in SEM. The unrivalled advantage of this imaging technique is the possibility of immediate two-dimensional visualization of damage effects without complex sample preparation.

Destructive role of oxygen in growth of molybdenum disulfide determined by secondary ion mass spectrometry.

The application of secondary ion mass spectrometry (SIMS) in investigation and comparison of molybdenum disulfide (MoS2) films grown on SiO2, Al2O3 and BN substrates is presented. SIMS measurements of the MoS2/substrate interface reveals oxygen out-diffusion from the substrates containing oxygen and the formation of an amorphous MoOS layer in addition to MoS2. The total area of MoS2 domains covering the substrate is directly related to the type of substrate. For SiO2, small triangular domains of MoS2 separated by amorphous MoOS material are observed. For Al2O3, the sizes of the MoS2 domains are drastically improved due to the higher stability of sapphire. For a BN substrate, SIMS measurements reveal a uniform MoS2 coverage over the whole 2-inch wafer. These results show the destructive role of oxygen released from substrates such as SiO2 or Al2O3 during the growth process of MoS2. The fast and cheap growth process on a non-oxide substrate allows large wafer-scale uniform molybdenum disulfide material to be obtained, which is promising for device fabrication.

A-Crater-within-a-Crater Approach for Secondary Ion Mass Spectrometry Evaluation of the Quality of Interfaces of Multilayer Devices.

Further development and optimization of modern optoelectronic devices requires fast and reliable procedures that may evaluate the quality of interfaces. For thick multilayer devices, mixing effect may significantly prevent proper interpretation of secondary ion mass spectrometry depth profiles especially if a region of interest is located far from the sample surface. In this work, we present how to overcome this problem with a so-called a-crater-within-a-crater approach. In this notion, a high energetic primary ion beam is used to rapidly remove most of the material forming a large crater. Then, the energy is significantly reduced and a new smaller crater is formed at the bottom of the previous one. Close to the region of interest, the impact energy is decreased to 150 eV and thus an interface can be analyzed with minimal mixing effect and thus its quality can be adequately assessed. Usefulness of this approach is tested on an epitaxial structure of a triple-junction solar cell and reliable information about the structure imperfection has been obtained: p and n dopants in the tunnel junction overlapped, deteriorating the operation of the device.