Jacques Dumont

Selective growth of CdTe on patterned CdTe∕Si(211)

The authors have studied selective growth of cadmium telluride on Si(211) by molecular beam epitaxy (MBE). Patterned substrates were produced by optical lithography of MBE-grown CdTe∕As∕Si(211). Photoemission microscopy was used as the main tool to study selective growth. This is very powerful because Si or SiO2 can be very easily distinguished from areas covered with even small amounts of CdTe due to contrast from work function differences. It was found that CdTe grows on CdTe without sticking on bare Si areas if the temperature is sufficiently high. Based on the analysis of the temperature dependence of the growth rate of CdTe, we suggest that different physisorption energies on Si and CdTe are the main cause of this selective growth.

ZnO(0001) surfaces probed by scanning tunneling spectroscopy: Evidence for an inhomogeneous electronic structure.

The stability of the polar Zn-terminated ZnO surface is probed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Surface states in the bandgap of ZnO are evidenced by STS and their presence is correlated with the local surface corrugation. Very defective surface regions are characterized by a bulk electronic structure showing a wide bandgap while nanometer-scale defect free regions exhibit a narrower bandgap and surface states. We also image atomically resolved (root 3 x root 3)R30 degrees reconstructions on the defect-free areas

Demixing processes in AgPd superlattices

The present scanning tunneling microscopy (STM) study describes the growth of silver-palladium heterostructures at room temperature, with ab initio simulations of ordered AgPd phases supporting the interpretation of STM images. First, the growth of Pd on an Ag(111) surface proceeds in a multilayer mode, leading to the formation of a columnar structure. Then, upon Ag deposition on this structure, Ag and Pd partially mix and form a two-dimensional AgPd alloy on top of the columns. Finally, an atomically flat Ag(111) surface is restored, and two-dimensional growth continues. An interpretation of this peculiar growth mode including interfacial alloying is proposed based on thermodynamic and kinetic arguments.

Formation of (Zn,Co)O by annealing of Co overlayers on ZnO

The effect of annealing of Co∕ZnO(0001) was studied by scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. At room temperature, Co forms islands on ZnO. Annealing up to 940K leads to coalescence of the islands. At 970K, Co diffuses into ZnO where it partially replaces Zn. A model of the Auger intensities, based on exponential attenuation with thickness and including correction for matrix effects, confirms this interpretation and suggests that the fraction of Zn replaced by Co is near 50% or higher.

Novel high thermal barrier layers for flexible CIGS solar cells on stainless steel substrates

For the fabrication of monolithically integrated flexible CIGS modules on stainless steel, individual photovoltaic cells must be insulated from the metal substrates by a barrier layer that can sustain high temperature treatment. In this work combination of sol-gel (organosilane-sol) and sputtered SiAlxOy thin layers (TDBL) were prepared on stainless steel substrates. At first, the deposition of organosilane-sol dielectric layers on the commercial AISI-316–2RB stainless steel (Rz = 500 nm, RMS= 56 nm) induces a planarization of the surfaces (RMS = 16.4nm, Rz =176nm). The leakage current in DC mode through the dielectric layers was measured by preparing metal-insulator-metal (MIM) junction that acts as capacitor. This method proposed here allowed us to quantify the quality the quality of our TDBL insulating layer and its lateral uniformity. Indeed, evaluating a ratio of the number of valid MIM capacitors to the number of tested MIM capacitors, a yield of ∼ 95% and 50% has been reached respectively with non annealed and annealed samples based sol-gel double layer. A yield of 100% has been reached with reinforced PCDP by sputtered SiAlxOy thin layer showing perfect electrical insulation. Since this yield is obtained on several samples, it can be extrapolated to any substrate size. Furthermore, according to Glow Discharge Optical Emission Spectroscopy (GDOES) measurements, these barrier layers exhibit excellent barrier properties against the diffusion of undesired atoms which could spoil the electronic and optical properties of CIGS based photovoltaic cells.

Thermal diffusion of Co into atomically flat ZnO(000-1) surfaces investigated by scanning probe microscopies and low energy electron diffraction

In this study, we established a cleaning procedure to obtain atomically flat (000-1) surfaces from pressurized melt grown ZnO. Ex situ chemical cleaning removes fluid layer and trapped contaminants from the surface. This was followed by cycles of sputtering and annealing in ultrahigh vacuum. It is critical that the ion energy be high enough to produce small and highly mobile molecular fragments. On the other hand, annealing must be done at moderate temperatures to avoid segregation of potassium to the surface. Secondly, the interaction of Co thin films on atomically flat ZnO(000-1) has been investigated by low energy electron diffraction, scanning tunneling microscopy, and Auger electron spectroscopy. A high density of islands nucleates at the earliest stages of the growth and a subsequent Volmer-Weber growth of these islands is observed. Upon annealing at 550°C, an atomically flat surface (Zn,Co)O(000-1) is restored due to the diffusion of the Co into the semiconductor.