Methodology to Improve Strain Measurement in III–V Semiconductors Materials

Geometric phase analysis (GPA), a fast and simple Fourier space method for strain analysis, can give useful information on accumulated strain and defect propagation in multiple layers of InAs/GaAs quantum dot (QD) materials. In this work, GPA has been applied to both high-resolution transmission electron microscopy (HRTEM) and high angular annular dark field scanning transmission electron microscopy (HAADF STEM) images. Strain maps determined from these images have been compared to each other, in order to analyze and assess the GPA technique in terms of accuracy. Ways to improve STEM data to get more reliable results are discussed

Electronic properties of reduced molybdenum oxides

The electronic properties of MoO3 and reduced molybdenum oxide phases are studied by density functional theory (DFT) alongside characterization of mixed phase MoOx films. Molybdenum oxide is utilized in compositions ranging from MoO3 to MoO2 with several intermediary phases. With increasing degree of reduction, the lattice collapses and the layered MoO3 structure is lost. This affects the electronic and optical properties, which range from the wide band gap semiconductor MoO3 to metallic MoO2. DFT is used to determine the stability of the most relevant molybdenum oxide phases, in comparison to oxygen vacancies in the layered MoO3 lattice. The non-layered phases are more stable than the layered MoO3 structure for all oxygen stoichiometries of MoOx studied where 2 ≤ x < 3. Reduction and lattice collapse leads to strong changes in the electronic density of states, especially the filling of the Mo 4d states. The DFT predictions are compared to experimental studies of molybdenum oxide films within the same range of oxygen stoichiometries. We find that whilst MoO2 is easily distinguished from MoO3, intermediate phases and phase mixtures have similar electronic structures. The effect of the different band structures is seen in the electrical conductivity and optical transmittance of the films. Insight into the oxide phase stability ranges and mixtures is not only important for understanding molybdenum oxide films for optoelectronic applications, but is also relevant to other transition metal oxides, such as WO3, which exist in analogous forms

Compositional and structural properties of pulsed laser-deposited ZnS:Cr films

We present the properties of Cr-doped zinc sulfide (ZnS:Cr) films deposited on Si(100) by pulsed laser deposition. The films are studied for solar cell applications, and to obtain a high absorption, a high Cr content (2.0–5.0 at.%) is used. It is determined by energy-dispersive X-ray spectroscopy that Cr is relatively uniformly distributed, and that local Cr increases correspond to Zn decreases. The results indicate that most Cr atoms substitute Zn sites. Consistently, electron energy loss and X-ray photoelectron spectroscopy showed that the films contain mainly Cr2+ ions. Structural analysis showed that the films are polycrystalline and textured. The films with ~4 % Cr are mainly grown along the hexagonal [001] direction in wurtzite phase. The average lateral grain size decreases with increasing Cr content, and at a given Cr content, increases with increasing growth temperature

Optical emission spectroscopy in pulsed laser deposition of silicon

The generation of a homogeneous plasma plume is necessary for the pulsed laser deposition of thin films. In this work, we investigate the effects of nanosecond-duration laser pulses to ablate polycrystalline Si targets in vacuum (<10-4 Pa) at room temperature. The laser wavelength covers the range from ultra-violet to infrared by using a KrF (248 nm, 25 ns) and a Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) laser. The films were deposited at laser fluences from 1 to 6 J/cm2 and characterized by atomic force microscopy and spectroscopic ellipsometry. Time-integrated optical emission spectra were obtained for excited neutrals and ionized Si species in the plasma produced between 0.5 and 11 J/cm2. The relation between the ionized species and film properties were discussed

Laser ablation and growth of Si and Ge

In this work, we investigated the laser ablation and deposition of Si and Ge at room temperature in vacuum by employing nanosecond lasers of 248 nm, 355 nm, 532 nm and 1064 nm. Time-integrated optical emission spectra were obtained for neutrals and ionized Ge and Si species in the plasma at laser fluences from 0.5 to 11 J/cm2. The deposited films were characterized by using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Amorphous Si and Ge films, micron-sized crystalline droplets and nano-sized particles were deposited. The results suggested that ionized species in the plasma promote the process of subsurface implantation for both Si and Ge films while large droplets were produced from the superheated and melted layer of the target. The dependence of the properties of the materials on laser wavelength and fluence were discussed

Molecular beam and pulsed laser deposition of ZnS:Cr for intermediate band solar cells

We have investigated the structural and optical properties of Cr-doped ZnS (ZnS:Cr) thin films (0–7.5 at.% Cr) for use in intermediate band solar cells. The films were grown on Si(100) in molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) equipments. Introducing Cr into ZnS resulted in Cr related subbandgap absorption, but also reduced the grain size. The sub-bandgap absorption increased with increasing Cr content, and with increasing growth temperature, but did not depend on the growth method. In contrast, the crystallinity depended strongly on the growth method, and smoother and highly textured films were obtained by PLD. The data indicate that stacking faults are present in all films

Quantitative strain analysis of InAs/GaAs quantum dot materials

Geometric phase analysis has been applied to high resolution aberration corrected (scanning) transmission electron microscopy images of InAs/GaAs quantum dot (QD) materials. We show quantitatively how the lattice mismatch induced strain varies on the atomic scale and tetragonally distorts the lattice in a wide region that extends several nm into the GaAs spacer layer below and above the QDs. Finally, we show how V-shaped dislocations originating at the QD/GaAs interface efficiently remove most of the lattice mismatch induced tetragonal distortions in and around the QD

Band-edge modification and mid-infrared absorption of co-deposited FexZn1-xS thin films

The bandgap of iron-doped ZnS has been reported by others to change significantly under the addition of a few atomic percent of iron, which would have significant implications for solar energy. Here, thin films of FexZn1-xS with x = 0 to 0.24 were made by co-deposition of Fe and ZnS using thermal evaporation. In contrast to results on nanoparticles and electrodeposited materials, all co-deposited films had optical properties consistent with a direct bandgap of ~3-3.5 eV. The absorption peak at 2.7 µm from substitutional Fe2+ in the ZnS films was well isolated up to concentrations of over 2% (~1021cm−3), despite the small crystallite size, suggesting the films may have applications as mid-infrared saturable absorbers. Increasing dopant concentration resulted in band edge softening. Density functional calculations are presented and are consistent with our observations of the Fe:ZnS films, demonstrating spin-polarized midgap states and additional states at the band edge

Nanosecond laser ablation and deposition of silicon

Nanosecond-pulsed KrF (248 nm, 25 ns) and Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) lasers were used to ablate a polycrystalline Si target in a background pressure of < 10(-4) Pa. Si films were deposited on Si and GaAs substrates at room temperature. The surface morphology of the films was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Round droplets from 20 nm to 5 mu m were detected on the deposited films. Raman Spectroscopy indicated that the micron-sized droplets were crystalline and the films were amorphous. The dependence of the properties of the films on laser wavelengths and fluence is discussed