Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity

Photoadsorption and photodesorption for GaN

The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UVillumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaNsurface. In contrast, the upward band bending gradually decreases in vacuum under UVillumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration

Temperature-dependent Kelvin probe measurements of band bending in p-type GaN

The band bending in a Mg-doped, p-type GaN film grown by hydride vapor phase epitaxy was studied at various temperatures. At 295 K, the band bending in dark was calculated to be approximately −1.5 eV. However, when the sample was heated to 600 K for 1 h in dark before performing a measurement at 295 K, the calculated value of band bending in dark became about −2.0 eV. These results are explained by the fact that increasing the sample temperature exponentially increases the rate at which the band bending restores and allows for a more accurate value of band bending to be measured

Yellow luminescence in ZnO layers grown on sapphire

We conducted a detailed study of the yellow luminescence (YL) band that has a maximum of 2.19eV at 10K in undoped and N-doped ZnO layers grown on sapphire substrates. Important characteristics of this band and the related defect are established. The YL band is attributed to a transition between a shallow donor and an acceptor with an energy level ∼0.4eV above the valence band. Quenching of the YL intensity with activation energies of 85meV and 0.4eV is observed at temperatures above 100 and 320K, respectively. The YL band is possibly due to a defect complex that may include a Znvacancy

Determination of the absolute internal quantum efficiency of photoluminescence in GaN co-doped with Si and Zn

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the ZnGa acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE above 90% was calculated for several samples having different concentrations of Zn. Accurate and reliable values of the IQE were obtained by using several approaches based on rate equations. The concentrations of the ZnGa acceptors and free electrons were also estimated from the photoluminescence measurements

Transient photovoltage in GaN as measured by atomic force microscope tip

We studied restoration of the band bending at the surface of undoped GaN layers after illumination with above-bandgap light. The photovoltage saturated with illumination at about 0.2–0.3 eV at room temperature, although the upward band bending for GaN in the dark is of the order of 1 eV. We attribute the photovoltage effect to charging of the surface states, the density of which is estimated at about 10^12 cm^−2. Restoration of the barrier after a light pulse is simulated by a phenomenological model whereby the acceptorlike surface states are emptied of electrons under illumination and filled back in dark due to thermionic transfer of free electrons from the bulk to the surface states over the near-surface barrier. Photoinduced desorption of oxygen also affects the value of the photovoltage if the illumination is prolonged

Review of the New World genus Nanium Townes, 1967 (Hymenoptera: Ichneumonidae: Ctenopelmatinae), with two new species from the Neotropical region

Nanium Townes, 1967 is a small New World parasitoid wasp genus in the subfamily Ctenopelmatinae Forster, 1869 (Hymenoptera: Ichneumonidae). Previously, it comprised five species: one from North America and four from Costa Rica. The current study reviews the Neotropical species of the genus, and includes descriptions of two new species, N. medianum Reshchikov & Saaksjarvi sp. nov. from Ecuador and N. atitlanensis Reshchikov & Saaksjarvi sp. nov. from Guatemala. A key to the species is provided

Evaluating the effect of various types of disinfectants on bacterial biofilms

Biofilm formation on equipment surfaces is a potential food safety hazard, providing increased resistance and persistence of pathogens and spoilage microorganisms in food production environments. The issue of preventing the biofilm formation is extremely important, since a wide range of disinfectants does not always provide the proper effect. The article discusses the antimicrobial effectiveness of disinfectants with various active ingredients (based on active chlorine, peracetic acid and quaternary ammonium compounds (QAC) with enzymatic substances) on binary biofilms. The objects of the study were the strains of pathogenic and opportunistic microorganisms isolated from abiotic surfaces of food production environments and food products. Different effects of disinfectants on biofilms formed by bacteria have been established. Disinfectant based on peracetic acid and chlorine had the greatest effect on binary biofilms of Brochothrix thermosphacta/Salmonella spp. and Staphylococcus equorum/Salmonella spp. The greatest antimicrobial effect on biofilm of Listeria monocytogenes 12/Pseudomonas azotoformans 6 was shown by a chlorine-based disinfectant. Disinfectants based on chlorine and QAC with enzymatic substances were most effective against the binary biofilm of L. monocytogenes 12/Salmonella spp. 14. However, none of the disinfectants had absolute antimicrobial effectiveness against the studied binary biofilms. Biofilm-forming microorganisms have shown resistance to the recommended concentrations of disinfectants. Therefore, currently, it is extremely important to revise approaches to hygiene at enterprises by finding working concentrations of new antimicrobial agents and new procedure that are effective for destroying biofilms

Evaluation of approaches to increase the effectiveness of various disinfectants against biofilm communities of different ages

Disinfectants are used as the main agents against microorganisms circulating on the surfaces of food enterprises. However, the adaptive ability of microorganisms to form biofilms complicates the process of surface cleaning and reduces the effectiveness of disinfectants. Modern disinfectants act against freely circulating microflora, but it is known that they are not always effective against biofilms. The purpose of this study was to investigate effective disinfectant compositions with bactericidal effect on binary bacterial biofilms of different ages. The article describes the effects of disinfectants based on chlorine, peracetic acid and quaternary ammonium compounds with enzymes in concentrations recommended by the manufacturer and increased several times on Salmonella 38, Brochothrix thermosphacta 2726 and Staphylococcus equorum 2736 planktonic cultures and binary biofilms. Binary biofilms of different ages (2 and 9 days old) were exposed to disinfectants with various active ingredients in combination with adjuvants, i. e. hydrogen peroxide 6% and various concentrations of isopropyl alcohol (30%). All products in concentrations recommended by the manufacturer did not have a disinfectant effect against the studied biofilm cultures. As a result of the work, it was found that the most effective disinfectants against multispecies biofilms were quaternary ammonium compounds in combination with enzymes and chlorine in combination with isopropyl alcohol (30%). The results obtained allow to expand knowledge about effective methods for controlling biofilms.Disinfectants are used as the main agents against microorganisms circulating on the surfaces of food enterprises. However, the adaptive ability of microorganisms to form biofilms complicates the process of surface cleaning and reduces the effectiveness of disinfectants. Modern disinfectants act against freely circulating microflora, but it is known that they are not always effective against biofilms. The purpose of this study was to investigate effective disinfectant compositions with bactericidal effect on binary bacterial biofilms of different ages. The article describes the effects of disinfectants based on chlorine, peracetic acid and quaternary ammonium compounds with enzymes in concentrations recommended by the manufacturer and increased several times on Salmonella 38, Brochothrix thermosphacta 2726 and Staphylococcus equorum 2736 planktonic cultures and binary biofilms. Binary biofilms of different ages (2 and 9 days old) were exposed to disinfectants with various active ingredients in combination with adjuvants, i. e. hydrogen peroxide 6% and various concentrations of isopropyl alcohol (30%). All products in concentrations recommended by the manufacturer did not have a disinfectant effect against the studied biofilm cultures. As a result of the work, it was found that the most effective disinfectants against multispecies biofilms were quaternary ammonium compounds in combination with enzymes and chlorine in combination with isopropyl alcohol (30%). The results obtained allow to expand knowledge about effective methods for controlling biofilms