Enrique Grünbaum

Epitaxial growth of metal single-crystal films☆

Abstract A review of the epitaxial growth of metals in high and ultra-high vacuum leading to single-crystal films is presented. The conditions for obtaining such films are described and tabulated for two groups of substrates: (i) metals, Si, Ge (strong interfacial bonding) and (ii) alkali halides and MgO cleavage faces (weak interfacial bonding). Information on the crystal defects and their relation with the growth parameters and annealing procedures is also given.

Nanoscale potential distribution across multiquantum well structures: Kelvin probe force microscopy and secondary electron imaging

Ultrahigh vacuum cross-sectional Kelvin probe force microscopy has been used to characterize In0.17GaAs∕GaAsP0.06 multiquantum well structures, together with secondary electron microscopy. Individual 8nm quantum wells were well resolved in both methods, and were found to be in a good agreement with numerical simulations of the work function profile. It is shown that the surface potential contrast in the Kelvin probe force microscopy measurements is greatly enhanced using deconvolution algorithms, and the reasons for the different contrast in the electron microscopy images are discussed.

2. An uhv system for in situ preparation and analysis of thin films and surfaces

Abstract A versatile uhv system was designed in order to enable the preparation of thin films of different materials and their analysis to be undertaken. It includes various sources for evaporation: a high-power electron gun, conventional crucibles and Knudsen cells. An airlock is available for fast transfer of prepared films and new substrates. By a special sample holder the thin films can be manipulated to different positions for structural analysis by reflection high energy electron diffraction (RHEED), and for chemical analysis by Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS).

Spectral cathodoluminescence of impurity phases in Y1Ba2Cu3O7−δ superconductors

Spectral cathodoluminescence (CL) of the Y1Ba2Cu3O7−δ high Tc superconducting pellet and its related components (Y2O3, BaCO3, CuO) was studied in the scanning electron microscope (SEM). Characteristic CL spectra of the Y1Ba2Cu3O7−δ component phases and the Y2BaCuO5 impurity phase were obtained. These spectra were used for detecting unfamiliar luminescent impurity grains in the Y1Ba2Cu3O7−δ pellets. The dispersive CL SEM image was compared with the morphological information.

Electrical charging of percolating samples in the scanning electron microscope

Electrical properties of indium percolating samples were investigated by the charging effect in the scanning electron microscope. The finite clusters and the infinite cluster were identified due to the difference in their charging images. The finite clusters exhibited a capacitor‐like behavior. An electric breakdown in a tree‐like structure was obtained in the indium insulating films.

Secondary Electron Emission Contrast of Quantum Wells in GaAs p-i-n Junctions

The secondary electron (SE) signal over a cleaved surface of GaAs p-i-n solar cells containing stacks of quantum wells (QWs) is analyzed by high-resolution scanning electron microscopy. The InGaAs QWs appear darker than the GaAsP barriers, which is attributed to the differences in electron affinity. This method is shown to be a powerful tool for profiling the conduction band minimum across junctions and interfaces with nanometer resolution. The intrinsic region is shown to be pinned to the Fermi level. Additional SE contrast mechanisms are discussed in relation to the dopant regions themselves as well as the AlGaAs window at the p-region. A novel method of in situ observation of the SE profile changes resulting from reverse biasing these structures shows that the built-in potential may be deduced. The obtained value of 0.7 eV is lower than the conventional bulk value due to surface effects.

Effect of substrate defects on the properties of high Tc superconducting thin films

We studied the influence of substrate defects on the morphological and transport properties of Y1Ba2Cu3O7−x high Tc superconducting thin films. The substrate defects were characterized by dispersive and nondispersive cathodoluminescence in the scanning electron microscope. The morphology of the films was studied by the secondary electrons and was locally compared with the substrate information. Defects in the substrate induced surface variations in the thin films. Current transport was correlated with the geometry of the substrate defects.

Single-crystal transformation of sphalerite CdS to cubic phaseCuxS

Abstract Thin films, 800–2000Athick, of single-crystal (100) sphalerite CdS, which were grown epitaxially on single-crystal (100) NaCl substrates and possessed a density of 1012–13 defects/cm2, were converted toCuxS by dipping them in a hot CuCl solution. Transmission electron microscopy of the converted samples revealed a newCuxS cubic phase of high quality (106–7 defects/cm2), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS toCuxS transformation were carried out by heating CdS films with CuCl layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The newCuxS cubic phase was found to be stable up to 330°C, even in co-existence with CdS.

The electric field and dopant distribution in p-i-n structures observed by ionisation potential (dopant contrast) microscopy in the HRSEM

The method of ionisation potential (dopant contrast) microscopy in the HRSEM is applied to the study of the electric field distribution in p-i-n structures used as quantum well solar cells. Our results show a secondary electron signal which varies between the different layers, being greatest in the p-type and smallest in the n-type regions respectively. The stacks of 8 nm wide quantum wells and their corresponding barriers are clearly distinguished in the intrinsic region of the devices. In-situ observation of reverse biased structures has been performed to determine the effect of bias on the potential distribution within the devices.

The epitaxial growth on CaF2 and BaF2 single-crystal films on a sapphire substrate

Abstract Single-crsytal CaF 2 and BaF 2 films are being used as intermediate substrates for the epitaxial growth of single-crystal semiconductors (Ge and GaAs) films. p-jununctions in these films serve as solar cells. By dissolving the CaF 2 or BaF 2 in water the films are separated from the expensive substrate, which thus may be re-used. The epitaxial growth of CaF 2 or BaF 2 films by evaporation in uhv (MBC) onto a sapphire (110) wafer was studied. The growth conditions, the crystalline and surface quality and the epitaxial orientation relationships were determined by in situ reflection high energy electron diffraction (RHEED).