E. Placzek-Popko

A deep acceptor defect responsible for the yellow luminescence in GaN and AlGaN

In the present study, the electrical and optical properties of deep defects in p-i-n GaN junction and AlGaN/GaN heterojunction are investigated by means of the deep level transient spectroscopy (DLTS), Laplace DLTS, and electroluminescence (EL) techniques. We demonstrate that in both structures the yellow luminescence (YL) is a dominant band in the EL spectra recorded at room temperature. We correlate the YL band with the minority DLTS peaks observed at about 370 K. A gallium vacancy-related defect seems to be a probable candidate as to the origin of the defect. Another dominant majority peak observed in the DLTS studies was concluded to be linked with a donor-like defect in the upper half of the bandgap. The origin of the defect is discussed.

Thermally stimulated current in high resistivity Cd0.85Mn0.15Te doped with indium

Charge carrier traps in Cd0.85Mn0.15Te doped with indium were studied using thermally stimulated current measurements. The investigations were performed in temperatures ranging from 100to300K. Four peaks in the current spectrum were identified. From the initial rise method and the best fit of the spectrum to the theoretical model, the activation energies and the relaxation parameters for the corresponding traps were determined.

Si/ZnO nanorods/Ag/AZO structures as promising photovoltaic plasmonic cells

The test structures for photovoltaic (PV) applications based on zinc oxide nanorods (NRs) that were grown using a low-temperature hydrothermal method on p-type silicon substrates (100) covered with Ag nanoparticles (NPs) were studied. The NPs of three different diameters, i.e., 5–10 nm, 20-30 nm, and 50–60 nm, were deposited using a sputtering method. The morphology and crystallinity of the structures were confirmed by scanning electron microscopy and Raman spectroscopy. It was found that the nanorods have a hexagonal wurtzite structure. An analysis of the Raman and photoluminescence spectra permitted the identification of the surface modes at 476 cm−1 and 561 cm−1. The presence of these modes is evidence of nanorods oriented along the wurtzite c-axis. The NRs with Ag NPs were covered with a ZnO:Al (AZO) layer that was grown using the low-temperature atomic layer deposition technique. The AZO layer served as a transparent ohmic contact to the ZnO nanorods. The applicability of the AZO layer for this purpo...

Photovoltaic properties of ZnO nanorods/p-type Si heterojunction structures

Summary Selected properties of photovoltaic (PV) structures based on n-type zinc oxide nanorods grown by a low temperature hydrothermal method on p-type silicon substrates (100) are investigated. PV structures were covered with thin films of Al doped ZnO grown by atomic layer deposition acting as transparent electrodes. The investigated PV structures differ in terms of the shapes and densities of their nanorods. The best response is observed for the structure containing closely-spaced nanorods, which show light conversion efficiency of 3.6%.

Stretched-exponential photoionization of the metastable defects in gallium doped Cd0.99Mn0.01Te: Statistical origins of the short-time power-law in response data

The subject of the present study is the low temperature nonexponential transients of photoconductivity build-up in gallium doped Cd0.99Mn0.01Te semiconducting mixed crystals possessing metastable defects, so called DX centers. The phototransients were analyzed in terms of two approaches. The first one was the two-exponential fitting that is usually applied to explain the persistent photoeffect build-up in materials with DX centers. The second, implemented in the above-mentioned semiconductors, was the stochastic model of relaxation leading to the stretched-exponential result. The latter fitting was found to be more appropriate for it justifies the short-time power-law exhibited by the phototransient response. According to the stochastic approach this behavior results from a heavy-tailed distribution of photoionized DX centers. The distribution can have its origin in different local arrangements.

Deep hole traps in Be-doped Al0.5Ga0.5As layers grown by molecular beam epitaxy

Deep hole traps in p-type Al0.5Ga0.5As grown by molecular beam epitaxy have been studied by the deep-level transient-spectroscopy method applied to samples with a Schottky diode configuration. Five hole traps, labeled as H0 to H4, were found. For traps H1, H3, and H4 the activation energies for emission were ET1=0.14 eV, ET3=0.40 eV, and ET4=0.46 eV, respectively. Hole emission from trap H2 was dependent on the external electric field. The emission rate obeyed the Poole–Frenkel relation. When extrapolated to zero electric field, the thermal activation energy for hole emission was ET2,0=0.37 eV. Capture cross sections for traps H1 and H4 were thermally activated with energy barriers EB1=0.04 eV and EB4=0.18 eV, respectively.

Deep traps and photo-electric properties of p-Si/MgO/n-Zn1−xMgxO heterojunction

In the paper, the photoluminescence (PL) measurements, current–voltage–temperature (I-V-T) measurements, space charge techniques (C-V and deep level transient spectroscopy (DLTS)), and photocurrent spectral characteristics have been applied to investigate defects in p-Si/MgO/ n-Zn1−xMgxO heterojunction (HJ). The HJ structure was grown on p-type Si (111) substrate with resistivity equal to 0.1 Ω cm by the plasma-assisted molecular beam epitaxy technique. A radio-frequency cell was used for the generation of oxygen plasma. PL spectrum let us determine the Mg content ∼10%. Besides the excitonic Zn0.9Mg0.1O line, the PL spectrum also contains green and yellow emission bands indicating the presence of defect states in the investigated structures. I-V measurements reveal the rectifying properties of the HJ and the current thermally activated with a trap with the activation energy equal to 0.42 eV. DLTS studies yield the majority trap of the activation energy 0.42 eV, confirming the result obtained from the I-V ...

Deep traps in n-type GaN epilayers grown by plasma assisted molecular beam epitaxy

In this study, we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77 K–350 K. Undoped GaN layers were grown using the plasma-assisted molecular beam epitaxy technique on commercial GaN/sapphire templates. The quality of the epilayers was studied by micro-Raman spectroscopy (μ-RS) which proved the hexagonal phase and good crystallinity of GaN epilayers as well as a slight strain. The photoluminescence spectrum confirmed a high crystal quality by intense excitonic emission but it also exhibited a blue emission band of low intensity. DLTS signal spectra revealed the presence of four majority traps: two high-temperature and two low-temperature peaks. Using the Laplace DLTS method and Arrhenius plots, the apparent activation energy and capture cross sections were obtained. For two high-temperature majority traps, they were equal to E1 = 0.65 eV, σ1 = 8.2 × 10−16 cm2 and E2 = 0.58 eV, σ2 = 2.6 × 10−15 cm2 wher...

Laser irradiation effects on the CdTe/ZnTe quantum dot structure studied by Raman and AFM spectroscopy

Micro-Raman spectroscopy has been applied to investigate the impact of laser irradiation on semiconducting CdTe/ZnTe quantum dots (QDs) structures. A reference sample (without dots) was also studied for comparison. Both samples were grown by molecular beam epitaxy technique on the p-type GaAs substrate. The Raman spectra have been recorded for different time of a laser exposure and for various laser powers. The spectra for both samples exhibit peak related to the localized longitudinal (LO) ZnTe phonon of a wavenumber equal to 210 cm−1. For the QD sample, a broad band corresponding to the LO CdTe phonon related to the QD-layer appears at a wavenumber of 160 cm−1. With increasing time of a laser beam exposure and laser power, the spectra get dominated by tellurium—related peaks appearing at wavenumbers around 120 cm−1 and 140 cm−1. Simultaneously, the ZnTe surface undergoes rising damage, with the formation of Te aggregates at the pinhole edge as reveal atomic force microscopy observations. Local temperatu...

Structural and optical characterization of GaN nanowires

Optical properties of GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on Si(111) substrates were studied with the use of micro-Raman spectroscopy and photoluminescence. Two types of NWs, hereafter labeled as A and B, grown with different values of Ga flux were studied. Morphology of the samples was probed by high resolution scanning electron microscopy. It was found that large Ga flux has led to a partial coalescence of nanowires in sample A. Reduction of Ga flux during growth of sample B resulted in an ensemble of separated nanowires. Micro-Raman and photoluminescence spectra were taken under illumination of 325 nm He-Cd laser line. Micro-Raman data reveal hexagonal phase of GaN NWs as well as a slight strain of Si substrate. Photoluminescence data yield that ensembles of separated NWs in sample B are defect free, whereas the spectra for coalesced wires in sample A exhibit both broad yellow luminescence band and defect-related band centered around 3.36 eV. Moreover, it was found that ...