Jan Grym

Growth and spatially resolved luminescence of low dimensional structures in sintered ZnO

Sintering of ZnO pressed powder under Ar flow at temperatures between 1250 and 1300 °C leads to the formation of elongated microstructures and nanostructures, with different morphologies, on the sample surface. Rods and needles with cross-sectional dimensions ranging from tens of nanometres to several tens of microns and up to hundreds of microns in length are obtained. In an advanced stage of growth, nanoneedles are frequently arranged in bundles, forming the walls of tubes with different cross-sectional dimensions. In addition, microcombs and microfeathers consisting of well oriented nanoneedles are observed. Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to characterize the structures grown. The formation of the elongated structures causes spectral changes, in particular an enhancement of the green–orange luminescence. High CL emission from the internal surface of the tubes has been observed.

Transport properties of metal-semiconductor junctions on n-type InP prepared by electrophoretic deposition of Pt nanoparticles

Electrical properties of highly rectifying Pt/InP junctions fabricated by electrophoretic deposition of Pt nanoparticles are investigated at different temperatures by the measurement of current–voltage and capacitance–voltage characteristics. The forward I–V characteristics of the junction are described by thermionic emissions theory at low forward bias (3kT/q 0.2 V. The reverse I–V characteristics are analysed in the scope of the thermionic emission model in the presence of shunt resistance. Electrical characteristics of these diodes are sensitive to gas mixtures with a low hydrogen concentration and show an extremely fast response and recovery time.

Thermal stability study of semimetal graphite n-InP and n-GaN Schottky diodes

The electrical properties of highly rectifying semimetal-graphite Schottky contacts fabricated by printing colloid graphite on n-type InP and GaN are investigated as a function of annealing temperature by current–voltage and capacitance–voltage techniques. As-deposited Schottky diodes exhibit excellent current–voltage rectifying characteristics of 7.5 × 107 and 1.9 × 1011 with Schottky barrier height of 1.13 and 1.29 eV at room temperature for InP and GaN, respectively. The key aspect of this technique, compared with conventional vacuum evaporation, is low deposition energy process, leaving the surface undisturbed.

Graphite/CdMnTe Schottky diodes and their electrical characteristics

The first Schottky diodes based on n-CdMnTe crystals with pronounced rectifying properties are investigated. It is shown that the I–V characteristics of the diodes fabricated by printing colloidal graphite can be described by the Sah–Noyce–Shockley theory of generation–recombination in the space charge region. Exponential increase of forward current with voltage is limited by a relatively low barrier height at the graphite/CdMnTe contact (~ 0.4 eV) and a significant series resistance of the crystal bulk (~ 106 Ω at room temperature). Tunneling due to high concentration of uncompensated impurities in investigated Cd0.9Mn0.1Te crystals (7 × 1017 cm−3) does not allow increasing the reverse bias voltage to the values needed for the operation of x- and γ-ray detectors. High concentration of uncompensated donors is interpreted by the fact that a certain part of the Mn atoms does not substitute for Cd but plays a role of over-stoichiometric impurities. In the case of the presence of a thin intermediate insulator layer in the graphite/CdMnTe contact, a rapid increase in the current for both polarities of high voltage due to the Frenkel–Poole emission is observed. The obtained results shed light on the problems of technology of growing and post-processing CdMnTe crystals regarded as a prospective material for x- and γ-ray detectors.

Temperature-dependent properties of semimetal graphite-ZnO Schottky diodes

Highly rectifying semimetal graphite/ZnO Schottky diodes with a low-ideality-factor (1.08 at 300 K) were investigated by temperature-dependent current-voltage measurements. The current transport was dominated by thermionic emission between 300 and 420 K and the extracted barrier height followed the Schottky-Mott relation. A Richardson constant (A** = 0.272 A cm−2K−2) extracted from the Richardson plot shows nearly linear characteristics in the temperature range 300–420 K.

Hydrogen sensors based on electrophoretically deposited Pd nanoparticles onto InP

Electrophoretic deposition of palladium nanoparticles prepared by the reverse micelle technique onto InP substrates is addressed. We demonstrate that the substrate pre-deposition treatment and the deposition conditions can extensively influence the morphology of the deposited palladium nanoparticle films. Schottky diodes based on these films show notably high values of the barrier height and of the rectification ratio giving evidence of a small degree of the Fermi level pinning. Moreover, electrical characteristics of these diodes are exceptionally sensitive to the exposure to gas mixtures with small hydrogen content.

Schottky barriers based on metal nanoparticles deposited on InP epitaxial layers

Fabrication of high-quality Schottky barriers on InP epitaxial layers prepared by liquid-phase epitaxy from rare-earth treated melts is reported. The Schottky structures are based on metal nanoparticles and a graphite layer deposited from colloidal solutions onto epitaxial layers with varying carrier concentration. The structures have notably high values of the barrier height and of the rectification ratio giving evidence of a small degree of the Fermi-level pinning. Electrical characteristics of these diodes are shown to be extremely sensitive to the exposure of gas mixtures with small hydrogen content.

Role of Rare-Earth Elements in the Technology of III-V Semiconductors Prepared by Liquid Phase Epitaxy

First applications of rare-earth (RE) elements in semiconductor technology are rooted in radiation tolerance improvements of silicon solar cells and purification of GaP crystals. The idea was later adopted in the technology of germanium and compound semiconductors. Since the 1980’s, considerable attention has been directed towards REs applications in III-V compounds both for epitaxial films and bulk crystals (Zakharenkov et al., 1997). The uniqueness of REs arises from the fact that the lowest-energy electrons are not spatially the outermost electrons of the ion, and thus have a limited direct interaction with the ion’s environment. The shielding of the 4f electrons by the outer filled shells of 5p and 5s electrons prevents the 4f electrons from directly participating in bonding (Thiel et al., 2002). The RE ions maintain much of the character exhibited by a free ion. This non-bonding property of the 4f electrons is responsible for the well-known chemical similarity of different REs. Since transitions between the electronic states of the shielded 4f electrons give rise to spectrally narrow electronic transitions, materials containing REs exhibit unique optical properties. By careful selection of the appropriate ion, intense, narrow-band emission can be gained across much of the visible region and into the near-infrared (Kenyon, 2002). Inspired by the striking results accomplished in the field of optical amplifiers and lasers based on REdoped fibres (Simpson, 2001), substantial research activity has been recently carried out on RE-doped semiconductor materials for optoelectronics (Klik et al., 2001). In most cases, however, achieving effective doping of III-V compounds by REs during growth from the liquid phase has proven difficult; the high chemical reactivity and the low solid solubility are the main restrictions on introducing RE atoms into the crystal lattices (Kozanecki & Groetzschel, 1990). On the other hand, the enhanced chemical affinity of REs towards most species of the shallow impurities leads to the formation of insoluble aggregates in the melt. Under suitable growth conditions, these aggregates are rejected by the growth front and are not incorporated into the grown layer: gettering of impurities takes place. Especially Si and main group-six elements acting as shallow donors in III-V semiconductors are effectively gettered due to REs high affinity towards them (Wu et al., 1992). Removal of detrimental impurities is of vital importance in applications such as PIN 13

Preparation of InP-based semiconductor materials with low density of defects: effect of Nd, Ho and Tb addition

Abstract Specific features of some rare earth elements (REE=Nd, Ho or Tb) are exploited to improve structural, optical and electrical properties of InP-based layer compounds for applications in ionising radiation detector structures. InP layers were grown by liquid phase epitaxy on (100)-oriented single crystal InP substrates with individual REE addition to the melt. The dependence of the layer thickness, overall surface morphology and defect density on the growth conditions were monitored by employing optical and scanning electron microscopy. The evaluation of electrical properties was gained from C–V measurements performed with the mercury probe and from a free carrier concentration profile determined by the probe profiling method on bevelled samples. The low-temperature photoluminescence spectroscopy was used to study the changes in optical properties. Significant improvement of all the studied layer parameters with increasing amount of REE in the melt was observed up to certain critical value of REE concentration. The density of structural defects was reduced by more than a half order of magnitude, free carrier concentration was reduced effectively by up to four orders of magnitude, photoluminescence peaks were narrowed substantially and fine spectral features were resolved. The conductivity of layers prepared with Tb admixture changed from n to p type when Tb exceeded certain limit of concentration in the melt.

ZnO-Based Gas Sensors Prepared by EPD and Hydrothermal Growth

Arrays of vertically well aligned ZnO nanorods (NRs) were prepared on nanostructured ZnO films using a low temperature hydrothermal method. We propose the use of the low cost, environmentally friendly electrophoretic deposition technique (EPD) as seeding procedure, which allows the obtaining of homogeneous, well oriented nanostructured ZnO thin films. ZnO nanorod arrays were covered with graphite in order to prepare graphite/ZnO NRs junctions. These nanostructured junctions showed promising current-voltage rectifying characteristics and gas sensing properties at room temperature.