Lubomír Grmela

Morphological features in aluminum nitride epilayers prepared by magnetron sputtering

Abstract The aim of this study is to characterize the surface topography of aluminum nitride (AlN) epilayers prepared by magnetron sputtering using the surface statistical parameters, according to ISO 25178-2:2012. To understand the effect of temperature on the epilayer structure, the surface topography was investigated through atomic force microscopy (AFM). AFM data and analysis of surface statistical parameters indicated the dependence of morphology of the epilayers on their growth conditions. The surface statistical parameters provide important information about surface texture and are useful for manufacturers in developing AlN thin films with improved surface characteristics. These results are also important for understanding the nanoscale phenomena at the contacts between rough surfaces, such as the area of contact, the interfacial separation, and the adhesive and frictional properties.

Diode I-U Curve Fitting with Lambert W Function

Application of the Lambert W function in diode forward i-u curve fitting for the purpose of diode parameters determination is described. CFTOOL of Matlab7 is utilized. Fitting and its results are discussed. Semiconductor PN junction solar cells are used as the diode samples. Brief Lambert W function appendix is attached

Detection and Localization of Defects in Monocrystalline Silicon Solar Cell

Near-surface defects in solar cell wafer have undesirable influence upon device properties, as its efficiency and lifetime. When reverse-bias voltage is applied to the wafer, a magnitude of electric signals from defects can be measured electronically, but the localization of defects is difficult using classical optical far-field methods. Therefore, the paper introduces a novel combination of electric and optical methods showing promise of being useful in detection and localization of defects with resolution of 250 nm using near-field nondestructive characterization techniques. The results of mapped topography, local surface reflection, and local light to electric energy conversion measurement in areas with small defects strongly support the development and further evaluation of the technique.

Investigation of excess 1/f noise in CdTe single crystals

Experimental studies of noise characteristics of CdTe crystals, prepared using the travelling heater method, have been carried out. Three types of basic material were used: low-ohmic n-type with n = 5 ? 1015 cm?3, semi-insulating n-type with n = 1.5 ? 109 cm?3 and low-ohmic p-type with holes concentration p = 7 ? 1014 cm?3. The noise measurements show that the dominant noise is 1/fn noise with parameter n in range from 0.9 to 1.5 and often very close to 1. The experimental value of 1/f noise is always much higher than the theoretical value which corresponds to the total value of free carriers in the sample. The voltage noise spectral density of 1/f noise depends on the quantity of free carriers in the sample. Free carriers are distributed uniformly throughout the homogenous part of the sample. But within the depleted region there is very low concentration of free carriers which increases exponentially in the direction from the contact area to the homogenous part of the sample. Analysis shows that the excess 1/f noise is caused by the low carrier concentration within the depleted region at the metal?semiconductor junction.

Near-field measurement of ZnS:Mn nanocrystal and bulk thin-film electroluminescent devices.

A near‐field study of the electro‐optical phenomena and aging characteristics of nanostructured and bulk ZnS:Mn alternating‐current thin‐film electro‐optical devices is presented. ZnS:Mn nanocrystals embedded in the glass matrices as well as ZnS:Mn thin‐film phosphors contain four different concentrations of Mn (from 0.05 to 1.0 mol%). The activator impurity in the phosphor influences the spectral properties and, to a large extent, the temporal properties of optical emission and an aging process of the devices. Therefore, a local photoluminescence and electroluminescence investigation using a scanning near‐field optical microscope technique is provided and the aging characteristics of ZnS:Mn nanocrystal structure also presented.

Hybrid STM/R-SNOM with novel probe

Abstract A reflection-mode scanning near-field optical microscope joined with scanning tunneling microscope having a common revolving detection head and mounted on a classical optical microscope stage is proposed. This arrangement combines advantages of both near-field and far-field imageries and is able to display the images of surfaces with superresolution beyond the diffraction limit. Its function is characterized by the wide range of resolution and/or by the possibility to study different kinds of surfaces with nanometer-scale accuracy. A novel probe is used in the optical stage of the microscope. During its fabrication the light power of an Ar–Kr laser coupled to the single-mode fiber allows a local heating of a buffered HF solution in the vicinity of the extremity of fiber. By tuning the output power of the laser, the flux of energy at the very tip of the fiber varies, and different cone angles of the probe could be obtained. This process is faster than the classical chemical etching. The etching speed, and shapes of the fiber versus temperature are therefore discussed. A comparison of the two different images obtained with a STM and R-SNOM parts of the microscope is also presented.

Contact quality analysis and noise sources determination of CdZnTe-based high-energy photon detectors

Experimental studies of the transport and noise characteristics of a cadmium-zinc-telluride (CdZnTe) crystal with symmetric gold contacts and a guard ring electrode have been carried out. The current–voltage (IV) characteristics and the noise spectral density were measured at room temperature in the dark. The sample with a disconnected guard ring electrode showed symmetric characteristics for both bias voltage polarities. The shape IV characteristics indicated the presence of carrier injection, leading to IV characteristics nonlinearity. The semiconductor surface has been identified as the main noise and leakage current source. After connecting the guard ring electrode, the leakage currents were suppressed by two orders and the noise spectral density decreased by five orders. In the case of the connected guard ring electrode, the asymmetry of IV characteristics was observed. The contact with worse rectifying properties had a higher contribution of noise to the detector system. Increasing bias voltage causes steeper detector additive noise growth when the guard ring electrode is disconnected.

Study of electric field distribution and low frequency noise of CdZnTe radiation detectors

Polarization phenomena in a metal-semiconductor-metal (M-S-M) structure of metallic Schottky contacts deposited on CdZnTe radiation detectors were studied. We evaluate the distribution of the electric field along the biased M-S-M structure by Pockels measurements. The results show that almost all the electric field is developed across the depletion layer of the reverse-biased contact. The noise measurements of the CdZnTe detectors studied show that the dominant noise is 1/fm noise. The 1/fm noise, with the parameter m close to one, is present at frequencies below 100 Hz and its bandwidth decreases in the course of the polarization process. At higher frequencies, we observed an increase of the m parameter to 2, which indicates a strengthened effect of the generation-recombination processes. In the frequency band of dominating 1/fm = 1 noise, the increase of magnitude of the noise spectral density was proportional to the power of 6, in relation to the current through the detector. This high value is explained as a result of a screening effect of the space charge buildup during the polarization.

Measurement of dynamic variations of polarized light in processed meat due to aging

The propagation of laser light in biological tissues is of growing importance in many medical and food applications. This problem is seriously studied in live science. The biological tissues consist of cells which dimensions are bigger than wavelength of visible light and display large compositional variations, inhomogeneities, and anisotropic structures. Therefore a Mie scattering of transmitted or backscattered light occurs and different polarization states arise. The changes of polarization state due to the multiple scattering of light in the biological cellular tissues also allow measure the freshness of processed victuals. The transmitted and backscattered laser light exhibits multiple scattering on the thin slice of sample. The phenomenon is different if the cellular tissues are living or dead. In the case of meat, there are temporal and dynamic changes not only as a result of chemical process, but also geometric deformations due to the water evaporation from intracellular and extracellular sites. The polarization measurement shows the changes in polarization orientation due to the muscle orientation and meat aging. Two types of measurements were provided: a) Measurement of polarized light reflected and twice transmitted forward and backward through the biological tissue samples - meat slice attached on sample holder mirror. b) Measurement of polarized light transmitted through the biological tissue sample. The relationship between polarization changes and meat freshness, and a dynamic temporal behavior of polarization states in the aged meat is reported.

Nanooptics of locally induced photocurrent in monocrystalline Si solar cells

This paper presents the results of our experimental study of high resolution map of induced photocurrent in monocrystalline silicon solar cells. Photovoltaic solar cells are evaluated by Near-field Optical Beam Induced photocurrent (NOBIC), as well as by Scanning Near-field Optical Microscope (SNOM) topography and reflection. The correlation between reflection and transport characteristic indicates possibility of this diagnostic tool. Therefore the SNOM and NOBIC represent the coupling of very useful methods to provide a non-destructive local characterization on silicon semiconductor solar cells.