Nonradiative Defects Research Articles

Contact light-emitting diodes based on vertical ZnO nanorods

We report vertical contact light-emitting diodes (VCLEDs), that are based on heterojunctions formed by using the point contacts of n-ZnO nanorods (NRs) to the p-type semiconductor substrate and that are fabricated using a new approach to the formation of LEDs (Appl. Phys. Lett. 98, 093110 (2011)). A p-type GaN film grown on a sapphire substrate was used to form n-ZnO NRs/p-GaN VCLEDs on a large area of about 4 cm2. The VCLEDs emitted a pure blue electroluminescence with high efficiency. Electroluminescence at 470 nm, which is visible to the naked eye, started at small current of about 50 μA and is attributed to the good optical properties of the structurallyperfect heterojunctions in the point contacts. The VCLED configuration allows the creation of ZnO/p-GaN nano-LEDs of high density and high-quality with a greatly reduced concentration of nonradiative defects in the active regions. The VCLEDs showed the high brightness of light required for active matrix displays and general solid-state lighting.

Rapid Optical Degradation of GaN-Based Light-Emitting Diodes by a Current-Crowding-Induced Self-Accelerating Thermal Process

GaN-based light-emitting diodes (LEDs) rapidly degraded by a current-crowding-induced self-accelerating thermal process were investigated. The LEDs stressed at a dc current of 200 mA and a temperature of 100 °C exhibited a rapid degradation rate of 2.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , while those stressed at 100 mA showed a relatively slow degradation rate of 3.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , in which the degradation rate corresponds to a slope of optical output power versus time plots. The corresponding degradation kinetics were also different depending on the stress current, i.e., the generation of nonradiative recombination defects was mainly responsible for the degradation of the 100-mA stressed LEDs, while multiple degradation kinetics including a drastic increase of defects, contact failure, and chip detachment from the lead frame were responsible for the rapid optical degradation of the 200-mA stressed LEDs. The origin of the multiple degradation kinetics could be the current-crowding effect, which causes local heating of the device and hence a self-accelerating thermal process. The rapidly degraded LEDs consistently showed a very large junction temperature from 36 °C (pristine) to 74 °C (failed). These results show that the LEDs should be avoided when current crowding occurs under harsh conditions in reliable lifetime testing and/or for practical use.

Hydrogen plasma induced modification of photoluminescence from a-SiNx:H thin films

Low temperature (250–350 °C) hydrogen plasma annealing (HPA) treatments have been performed on amorphous hydrogenated silicon nitride (a-SiNx:H) thin films having a range of compositions and subsequent modification of photoluminescence (PL) is investigated. The PL spectral shape and peak positions for the as deposited films could be tuned with composition and excitation energies. HPA induced modification of PL of these films is found to depend on the N/Si ratio (x). Upon HPA, the PL spectra show an emergence of a red emission band for x ≤ 1, whereas an overall increase of intensity without change in the spectral shape is observed for x &amp;gt; 1. The emission observed in the Si rich films is attributed to nanoscale a-Si:H inclusions. The enhancement is maximum for off-stoichiometric films (x ∼ 1) and decreases as the compositions of a-Si (x = 0) and a-Si3N4 (x = 1.33) are approached, implying high density of non-radiative defects around x = 1. The diffusion of hydrogen in these films is also analyzed by Elastic Recoil Detection Analysis technique.

Properties of the main Mg-related acceptors in GaN from optical and structural studies

The luminescent properties of Mg-doped GaN have recently received particular attention, e.g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional MgGa acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (&amp;lt;1018 cm−3), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27 eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 ± 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the MgGa acceptor.

Origin of visible and near-infrared photoluminescence from chemically etched Si nanowires decorated with arbitrarily shaped Si nanocrystals

Arrays of vertically aligned single crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) have been fabricated by a silver assisted wet chemical etching method. Scanning electron microscopy and transmission electron microscopy are performed to measure the dimensions of the Si NWs as well as the Si NCs. A strong broad band and tunable visible (2.2 eV) to near-infrared (1.5 eV) photoluminescence (PL) is observed from these Si NWs at room temperature (RT). Our studies reveal that the Si NCs are primarily responsible for the 1.5–2.2 eV emission depending on the cross-sectional area of the Si NCs, while the large diameter Si/SiOx NWs yield distinct NIR PL consisting of peaks at 1.07, 1.10 and 1.12 eV. The latter NIR peaks are attributed to TO/LO phonon assisted radiative recombination of free carriers condensed in the electron–hole plasma in etched Si NWs observed at RT for the first time. Since the shape of the Si NCs is arbitrary, an analytical model is proposed to correlate the measured PL peak position with the cross-sectional area (A) of the Si NCs, and the bandgap (Eg) of nanostructured Si varies as Eg = Eg (bulk) + 3.58 A−0.52. Low temperature PL studies reveal the contribution of non-radiative defects in the evolution of PL spectra at different temperatures. The enhancement of PL intensity and red-shift of the PL peak at low temperatures are explained based on the interplay of radiative and non-radiative recombinations at the Si NCs and Si/SiOx interface. Time resolved PL studies reveal bi-exponential decay with size correlated lifetimes in the range of a few microseconds. Our results help to resolve a long standing debate on the origin of visible–NIR PL from Si NWs and allow quantitative analysis of PL from arbitrarily shaped Si NCs.

Hydrogen influence on the electrical and optical properties of ZnO thin films grown under different atmospheres

In this work we studied the changes of the electrical and optical properties after hydrogen plasma treatment of polycrystalline ZnO thin films grown under different atmosphere conditions. The obtained results show that the gas used during the growth process plays an important role in the way hydrogen is incorporated in the films. The hydrogen doping can produce radiative and non-radiative defects that reduce the UV emission in ZnO films grown in oxygen atmosphere but it passivates defects created when the films are grown in nitrogen atmosphere. Impedance spectroscopy measurements show that these effects are related to regions where hydrogen is mostly located, either at the grain cores or boundaries. We discuss how hydrogen strongly influences the initial semiconducting behavior of the ZnO thin films.

Temperature dependence of defect-related photoluminescence in III-V and II-VI semiconductors

Mechanisms of thermal quenching of photoluminescence (PL) related to defects in semiconductors are analyzed. We conclude that the Schön-Klasens (multi-center) mechanism of the thermal quenching of PL is much more common for defects in III–V and II–VI semiconductors as compared to the Seitz-Mott (one-center) mechanism. The temperature dependencies of PL are simulated with a phenomenological model. In its simplest version, three types of defects are included: a shallow donor, an acceptor responsible for the PL, and a nonradiative center that has the highest recombination efficiency. The case of abrupt and tunable thermal quenching of PL is considered in more detail. This phenomenon is predicted to occur in high-resistivity semiconductors. It is caused by a sudden redirection of the recombination flow from a radiative acceptor to a nonradiative defect.

Alloying Buffer Layers in Colloidal CdSe/ZnS Core/Shell Nanocrystals

When a ZnS shell is coated onto a CdSe core, some non-radiative defects are formed with the relaxation of the strain induced by the large lattice mismatch between CdSe and ZnS even though there are Zn0.5Cd0.5Se or ZnSe buffer layers, as indicated by the decrease of photoluminescent (PL) quantum yield and the reverse evolution of temperature-dependent time-resolved PL decay. X-Ray photoelectron spectroscopy analysis reveals that these defects are induced by the formation of an interfacial alloy during the epitaxy process. These defects could be significantly suppressed if the ZnxCd1–xSeyS1–y alloy buffer layer is artificially introduced.

Impact of p-GaN Thermal Damage and Barrier Composition on Semipolar Green Laser Diodes

Dark triangle defects (DTDs) are common nonradiative defects in semipolar <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(20\bar{2}1)$</tex></formula> oriented green quantum wells (QWs), commonly used in green laser diodes (LDs). We show that DTDs do not appear “as-grown,” and DTD size depends strongly on post-QW-growth annealing time and temperature. Using low temperature p-GaN, we prevent catastrophic QW damage and directly compare LDs with GaN and AlGaN containing barriers. The GaN barrier LD exhibited a lasing wavelength of 511 nm, reduced operating voltage, and the lowest threshold current density, likely due to enhanced optical confinement factor and the elimination of very low growth temperature AlGaN in the active region.

Controlling indium incorporation in InGaN barriers with dilute hydrogen flows

InxGa1−xN multiple quantum wells (MQWs) with InyGa1−yN barriers were grown by adding dilute hydrogen flows to the QW growth conditions in order to modify the barrier indium composition. With the H2 flow off, the indium concentration in the InxGa1−xN QWs were x=0.183, x=0.163, and x=0.096 for growth temperatures of 730, 750 and 770°C respective. Using these same QW growth conditions, the H2 flow was increased up to 3 SLM resulting in a gradual decrease in the indium concentration in the InyGa1−yN barriers. Kinetic analysis suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the InyGa1−yN barriers. For the MQW structures grown at 750°C, the photoluminescence (PL) wavelength blue-shifts from 477 to ~450nm as the indium in the InyGa1−yN barriers increases as expected from the reduced influence of the piezoelectric fields. While a corresponding increase of spontaneous emission from the increasing overlap of electron states within the QW is also expected, the PL intensity of the QW instead decreases. This conflicting expectation of increased spontaneous emission and the observation of decreased PL intensity suggest either decreases in the QW-barrier height or increases in non-radiative defects offset any efficiency gains obtained when InGaN barriers to reduce polarization fields within the QWs.

Microcathodoluminescence spectra evolution for planar and nanopillar multiquantum-well GaN-based structures as a function of electron irradiation dose

Effects of low energy electron beam irradiation (LEEBI) of planar and nanopillar InGaN/GaN multiquantum well light emitting diode structures are discussed. The bands observed in microcathodoluminescence (MCL) spectra were attributed to recombination involving two types of InGaN quantum dots with lower (2.92 eV MCL band) and higher (2.75 eV) indium concentration. During the LEEBI treatment, the intensity of both MCL lines first decreased, presumably due to the introduction of radiation defects, then, after the dose of 0.2 C/cm2 increased, reached a maximum and then again decreased. At the same time, the peak energy showed a red shift at low irradiation doses and a blue shift at high doses. The results are explained by an interplay between the increasing density of nonradiative recombination defects and quantum dots during irradiation. The difference between the nanopillar and planar structures is attributed to a stronger impact of surface defects in nanopillars.

디지털 합금 InGaAlAs 다중 양자 우물의 열처리 온도에 따른 발광 특성

The effect of rapid thermal annealing (RTA) on the optical properties of digital-alloy InGaAlAs multiple quantum well (MQW) structures have been investigated by using photoluminescence (PL) and time-resolved PL measurements as a function of RTA temperature. The MQW samples were annealed from to for 30 s in a nitrogen atmosphere. The MQW sample annealed at exhibited the strongest PL intensity and the narrowest FWHM (Full width at half maximum), indicating the reduced nonradiative recombination centers and the improved interfaces between the wells and barriers. The MQW samples annealed at and showed the decreased PL intensities and blueshifted PL peaks compared to -annealed sample. The blueshift of PL peak with increasing RTA temperatures are ascribed to the increase of aluminum due to intermixing of gallium (Ga) and aluminum (Al) in the interfaces of InGaAs/InAlAs short-period superlattices. The decrease of PL intensity after annealing at and are attributed to the interface roughening and lateral composition modulation caused by the interdiffusion of Ga and Al and indium segregation, respectively. With increasing RTA temperature the PL decay becomes slower, indicating the decrease of nonradiative defect centers. The optical properties of digital-alloy InGaAlAs MQW structures can be improved significantly with optimum RTA conditions.

Variations in junction capacitance and doping activation associated with electrical stress of InGaN/GaN laser diodes

This paper presents a study of the effects of combined electrical and thermal stress tests on commercially-available InGaN-based blue laser diodes. By means of electrical and optical measurements, we have found that stress has two major consequences: (i) an initial decrease in threshold current, which is ascribed to the increase in the activation of p-type dopant, and (ii) a subsequent increase in threshold current, which – based on optical and electrical characterization – is ascribed to the generation/propagation of non-radiative defects within the active region of the devices.

Influence of boron doping and hydrogen passivation on recombination of photoexcited charge carriers in silicon nanocrystal/SiC multilayers

The influence of boron (B)-doping and remote plasma hydrogen passivation on the photoexcited charge carrier recombination in silicon nanocrystal/SiC multilayers was investigated in detail. The samples were prepared by high temperature annealing of amorphous (intrinsic and B-doped) Si1−xCx/SiC superlattices. The photoluminescence (PL) intensity of samples with B-doped silicon rich carbide layers was found to be up to two orders of magnitude larger and spectrally red shifted in comparison with that of the other samples. Hydrogen passivation leads to an additional increase in PL intensities. The PL decay can be described well by a mono-exponential function with a characteristic decay time of a few microseconds. This behavior agrees well with the picture of localized PL centers (surface states) together with the passivation of non-radiative defects by boron. The samples with B-doped SiC layers exhibit an additional PL band in the green spectral region that is quenched by hydrogen passivation. Its origin is attributed to defects due to suppression of crystallization of amorphous SiC layers as a result of B-doping. Measurement of ultrafast transient transmission allowed us to study the initial (picosecond) carrier dynamics. It was found to be dependent of pump intensity and interpreted in terms of multiparticle electron-hole recombination.

Effect of annealing temperature on optical and electrical properties of ZrO2–SnO2 nanocomposite thin films

ZrO2–SnO2 nanocomposite thin films were deposited onto quartz substrate by sol–gel dip-coating technique. Films were annealed at 500, 800 and 1,200 °C respectively. X-ray diffraction pattern showed a mixture of three phases: tetragonal ZrO2 and SnO2 and orthorhombic ZrSnO4. ZrSnO4 phase and grain size increased with annealing temperature. Fourier transform infra-red spectroscopy spectra indicated the reduction of –OH groups and increase in ZrO2–SnO2, by increasing the treatment temperature. Scanning electron microscopy observations showed nucleation and particle growth on the films. The electrical conductivity decreased with increase in annealing temperature. An average transmittance greater than 80 % (in UV–visible region) was observed for all the films. The optical constants of the films were calculated. A decrease in optical band gap from 4.79 to 4.59 eV was observed with increase in annealing temperature. Photoluminescence (PL) spectra revealed an emission peak at 424 nm which indicates the presence of oxygen vacancy in ZrSnO4. PL spectra of the films exhibited an increase in the emission intensity with increase in temperature which substantiates enhancement of ZrSnO4 phase and reduction in the non-radiative defects in the films. The nanocomposite modifies the structure of the individual metal oxides, accompanied by the crystallite size change and makes it ideal for gas sensor and optical applications.

Effect of starting properties and annealing on photocatalytic activity of ZnO nanoparticles

Eight commercial ZnO samples were investigated, as-received and after annealing in the flow of oxygen or forming gas. We found that ZnO nanoparticles with higher photoluminescence (PL) intensity (fewer nonradiative defects) exhibited a high photocatalytic activity. Samples exhibiting high PL intensity presented lower concentrations of surface adsorbates, which is consistent with lower native defect density in those samples. It was observed that annealing did not result in an improvement in photocatalytic activity of the samples with low defect density (high PL intensity). For the samples with low PL intensity, the same annealing condition could result in an improvement or worsening of photocatalytic activity, depending on the starting properties of the samples. Variable effects of annealing on samples with different properties indicate that the effect of a post-growth treatment cannot be generalized for all ZnO samples. The improvements in the photocatalytic activity were observed for a cationic dye methylene blue, while for an anionic dye acid red 27 annealing resulted in the changes in the amount of adsorbed dye, but no improvement in the photocatalytic activity for dye degradation.

Antibacterial activity of ZnO nanoparticles under ambient illumination — The effect of nanoparticle properties

ZnO nanomaterials exhibit prominent antibacterial activity even without UV illumination. Their antibacterial activity was attributed to different mechanisms, such as production of reactive oxygen species (ROS) and/or release of zinc ions in solution. Here we examined the relationship between the properties of eight different ZnO nanoparticles, their production of ROS, zinc ion release and antibacterial activity. We found that while the particles exhibited differences in antibacterial activity, there was no correlation with ROS production and zinc ion release. However, ROS production was inversely correlated with the concentration of nonradiative defects indicating that the recombination losses of photogenerated carriers can negatively affect ROS production.

Native Defects in ZnO: Effect on Dye Adsorption and Photocatalytic Degradation

We investigated the influence of native defects on the adsorption and photocatalytic degradation of anionic and cationic dyes for different ZnO nanoparticles. We found that there was no relationship between the dye adsorption onto ZnO nanoparticles and their photocatalytic activity. Fast photocatalytic degradation could be observed for samples having a low concentration of nonradiative defects (and thus low recombination losses of photogenerated carriers), regardless of the amount of dye adsorbed onto the surface. While the absorption of cationic dyes was not significantly affected by ZnO nanoparticle properties, dye adsorption of several anionic dyes was strongly affected by native defects in ZnO. The defects involved in the dye adsorption are likely shallow donor centers exhibiting an electron spin resonance peak at g ≈ 1.957, resulting in positively charged sites at the surface.

Strain-related optical properties of ZnO crystals due to nanoindentation on various surface orientations

Nanoindentations were performed on various crystallographic orientations of single crystal ZnO using a cono-spherical diamond tip with a radius of curvature of 260 nm. The crystal orientations were the (112¯0) a-plane, (101¯0) m-plane, and (0001) c-plane (Zn-face). The optical properties associated with nanoindentation have been investigated by cathodoluminescence. The load-displacement curves show that the c-plane is the most resistive to deformation, followed by the m-plane, and the a-plane. A large number of non-radiative defects are created directly below the indentation, regardless of the crystal orientation. Nanoindentation on the a- and m-plane crystals activates slip along the (0001) basal planes, creating a band of non-radiative defects as well as tensile strain along the basal planes. Compressive strain is observed perpendicularly to the basal planes due to an absence of easy-glide mechanisms in these directions. The nanoindentation on the c-plane crystal results in regions under tensile strain extending away from the indentation along the six-fold a-directions.

Tunable thermal quenching of photoluminescence in Mg-dopedp-type GaN

We have studied the thermal quenching of the ultraviolet luminescence band with a maximum at about 3.25 eV in $p$-type Mg-doped GaN. The characteristic temperature of the thermal quenching of photoluminescence (PL) gradually shifted to higher temperatures with increasing excitation intensity. This effect is explained by a population inversion of charge carriers at low temperatures, which suddenly converts into a quasiequilibrium population as the temperature increases above the characteristic value. Tunable quenching of PL has been observed only in some of the GaN:Mg samples. The absence of the tunable quenching of PL in another group of GaN:Mg samples is preliminarily attributed to different types of dominant nonradiative defects in the two groups of samples.