Number Of Nonradiative Recombination Centers Research Articles

Improved internal quantum efficiencies of far-UVC AlGaN/AlN quantum wells by the use of semipolar r-planes

We investigate the internal quantum efficiencies (IQEs) of AlGaN/AlN quantum wells (QWs) on (0001) c- and semipolar (11¯02) r-planes in the far-ultraviolet C (far-UVC) region using time-integrated photoluminescence and time-resolved PL spectroscopies. Stronger emissions from r-QWs are observed, especially at shorter wavelengths, indicating that the r-QWs exhibit higher IQEs than the c-QWs. Analyses of the experimental results suggest that the main reason for the higher IQEs of r-QWs in the far-UVC region is shorter radiative lifetimes and an increase in a slow decay component, which might be related to a reduction in the number of nonradiative recombination centers. These findings indicate the superiority of the semipolar r-plane for fabricating QWs that emit in the far-UVC region.

Na induction effects for J–V properties of Cu2SnS3 (CTS) solar cells and fabrication of a CTS solar cell over-5.2% efficiency

The effects of Na on Cu2SnS3 (CTS) solar cells were investigated by focusing on the electrical properties obtained using electrochemical impedance spectroscopy and Hall measurements to clarify which factors and regions are strongly affected by sulfurization with a NaF layer. The increase in the depletion layer width (xdep) suggested that the number of non-radiative recombination centers decreased at the p–n interface of the CTS solar cell owing to the sulfurization process with the NaF layer. Thus, the increase in Jsc indicated that the carrier collection width widened as xdep increased. Furthermore, Voc increased because the CTS lifetime was lengthened. The performance of the CTS solar cells was improved by these factors. Moreover, as a result of an investigation of the dark and light J–V properties, suppression of the crossover behavior was accomplished by sulfurization with the NaF layer. This technique improved the CTS/Mo and CdS/CTS interfaces owing to the Na content, leading to increases in all desirable solar cell properties. Therefore, the induction of Na from a soda-lime glass substrate as well as sulfurization with a NaF layer is required to obtain highly efficient CTS solar cells. As a result of this study, a CTS solar cell with an efficiency higher than 5.24% was achieved.

Effects of Ag on the carrier lifetime and efficiency of (Cu1−x Ag x )2SnS3 solar cells

The effects of Ag in (Cu1–x Ag x )2SnS3 (CATS) were investigated in terms of photoluminescence (PL) lifetime, carrier concentration, and solar cell performance with varying Ag/(Cu + Ag) ratios, x. The PL lifetime of CATS solar cells had a maximum value when x was within 7%. A similar tendency was observed for other parameters of the CATS thin films and solar cells. One of the reasons for this is that the number of non-radiative recombination centers in the CATS layer is suppressed by the sulfurization process with Ag. Therefore, even if the Ag content changes slightly in the 0%–11% range, the crystal quality is changed in CATS, resulting in a significant effect on the properties of solar cells. These investigations on the characterization and device physics can be applied to improve the efficiency of CATS solar cells.

Temperature‐Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells

Temperature‐dependent transport of photoexcited charge carriers through a nominally undoped, c‐plane GaN layer toward buried InGaN quantum wells is investigated by continuous‐wave and time‐resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low‐temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN‐based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.

Improved optical properties of InAs submonolayer quantum dots in GaAsSb/InGaAs double-well structure

In this study, multistacked InAs submonolayer (SML) quantum dots (QDs) were sandwiched in an InGaAs/GaAsSb dot-in-a-double-well (DDwell) structure to enhance the crystal quality and optical properties of QDs. The photoluminescence (PL) intensity of the InAs SML QDs with the DDwell structure was 5.5 times higher than that of conventional InAs/GaAs SML QDs because of the reduced number of nonradiative recombination centers and the enhanced carrier hole confinement. The PL results of the DDwell structure exhibit two peaks that represent the carrier overflow from SML QDs to InGaAs quantum wells (QWs) and hence the radiative recombination in InGaAs QWs because of the shallow carrier confinement of SML QDs. Among the compared samples, the DDwell structure exhibited the highest activation energy of 101.8 meV. Furthermore, the carrier thermal escape was suppressed in these InAs SML QDs. High-resolution transmission electron microscopy revealed that the microstructures of the InAs SML QDs demonstrated larger dots for the DDwell structure, thus verifying that the emission wavelength elongated in the PL measurement. These improved optical properties of the InAs SML QDs with the DDwell structure were attributable to the improved crystal quality because of the use of Sb surfactants and additional volume for carrier recombination provided by the InGaAs quantum well. The DDwell structure can thus be applied in optoelectronic devices to obtain advanced performance.

GaAs (1 1 1) epilayers grown by MBE on Ge (1 1 1): Twin reduction and polarity

We perform the growth of GaAs (111) epilayers on nominal Ge(111) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (111)A and homongeneous over the full area, as measured by transmission electron microscopy and high energy electron diffraction. This orientation conflicts with the common growth model for GaAs on Ge(111). Twinned domains are the main defects in our GaAs (111) epilayers. Using cathodoluminescence, we observe that some twin boundaries hold large number of non-radiative recombination centers. During growth, we find that only a narrow domain of As:Ga ratios lead to the growth of smooth and twin-free GaAs (111)A epilayers. At low As:Ga ratio, the surface is rough; while at high As:Ga ratio the epilayers present large densities of twinned domains.

Effect of laser fluence on structural transformations and photoluminescence quenching of Zinc Selenide nanoparticles thin films

Abstract
 We reported in this work the growth of ZnSe nanoparticles thin films deposited on glass substrates were synthesized by a pulsed laser deposition (PLD) method. The as obtained films were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV-VIS Spectrophotometer, and Photoluminescence (PL) spectra. X-ray diffraction study confirmed the transformation the cubic phase of ZnSe nanoparticles into hexagonal phase by increase the laser fluence from (4.77-5.97) J/cm2. The particle size variations were achieved by varying the laser fluence of prepared films. XRD studies and TEM images confirmed the nanometer size was found to lie in the range of 12-80 nm. A UV-VIS study was carried out to measure the band gap of the ZnSe nanoparicles thin films and it showed a blue shift with respect to the bulk value. The PL spectra at room temperature (300K) of the films showed the decrease of maximum values at at 522 nm( 2.379), 521 nm (2.3838) and 520 nm (2.3882 eV) for the laser fluence (4.77, 5.57 and 5.97 J/cm2), respectively. We assigned the variation due a larger number of non-radiative recombination centers appears in the films.

A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire

This paper reviews the progress of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs), mainly focusing in the work of the authors’ group. The background to the development of the current device structure on sapphire is described and the reason for using a (0001) sapphire with a miscut angle of 1.0° relative to the m-axis is clarified. Our LEDs incorporate uneven quantum wells (QWs) grown on an AlN template with dense macrosteps. Due to the low threading dislocation density of AlGaN and AlN templates of about 5 × 108/cm2, the number of nonradiative recombination centers is decreased. In addition, the uneven QW show high external quantum efficiency (EQE) and wall-plug efficiency, which are considered to be boosted by the increased internal quantum efficiency (IQE) by enhancing carrier localization adjacent to macrosteps. The achieved LED performance is considered to be sufficient for practical applications. The advantage of the uneven QW is discussed in terms of the EQE and IQE. A DUV-LED die with an output of over 100 mW at 280–300 nm is considered feasible by applying techniques including the encapsulation. In addition, the fundamental achievements of various groups are reviewed for the future improvements of AlGaN-based DUV-LEDs. Finally, the applications of DUV-LEDs are described from an industrial viewpoint. The demonstrations of W/cm2-class irradiation modules are shown for UV curing.

Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses

We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e.g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h~384 h and 768 h~1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.

Defect properties of InGaAsN layers grown as sub-monolayer digital alloys by molecular beam epitaxy

The defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Eg = 1.03 eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5 nm) between wide GaAsN (7–12 nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900 nm and 1200 nm thick (less 1 × 1015 cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900 nm. An increase in thickness to 1200 nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5 eV (NT = 8.4 × 1014 cm−3) and a shallow defect level at 0.20 eV. The last one leads to the appearance of additional doping, but its concentration is low (NT = 5 × 1014 cm−3) so it does not affect the photoelectric properties. However, further increase in thickness to 1600 nm, leads to significant growth of its concentration to (3–5) × 1015 cm−3, while the concentration of deep levels becomes 1.3 × 1015 cm−3. Therefore, additional free charge carriers appearing due to ionization of the shallow level change the band diagram from p-i-n to p-n junction at room temperature. It leads to a drop of the external quantum efficiency due to the effect of pulling electric field decrease in the p-n junction and an increased number of non-radiative recombination centers that negatively impact lifetimes in InGaAsN.

Origin of huge photoluminescence efficiency improvement in InGaN/GaN multiple quantum wells with low-temperature GaN cap layer grown in N2/H2 mixture gas

The nominal internal quantum efficiency of InGaN/GaN multiple quantum wells significantly increases from 5.6 to 26.8%, as a low-temperature GaN cap layer is grown in N2/H2 mixture gas. Meanwhile, the room-temperature photoluminescence (PL) peak energy shows a merely 73 meV blue shift. On the basis of temperature-dependent PL characteristics analysis, the huge improvement in PL efficiency arises mainly from the “etching effect” of hydrogen, which reduces the defect density and indium segregation at the upper well/barrier interface, and consequently contributes to the decrease in the number of nonradiative recombination centers and the enhancement of carrier localization.

Effects of rapid thermal annealing on two-dimensional delocalized electronic states of the epitaxial N δ-doped layer in GaAs

We have conducted rapid thermal annealing (RTA) for improving the two-dimensional (2D) arrangement of electronic states in the epitaxial nitrogen (N) δ-doped layer in GaAs. RTA rearranged the N-pair configurations in the GaAs (001) plane and reduced the number of non-radiative recombination centers. Furthermore, a Landau shift, representing the 2D delocalized electronic states in the (001) plane, was observed at around zero magnetic field intensity in the Faraday configuration.

Green Emission of Terbium Doped Silicon Rich Silicon Oxide Films Obtained By Chemical Vapor Deposition

The effect of silicon concentration, annealing temperature and influence of post-growth hydrogen incorporation on terbium (Tb3+) luminescence was investigated for thin silicon rich silicon oxide films (SRSO). The structures were deposited by means of plasma enhanced chemical vapor deposition (ECR-PECVD). The structural properties of these films were investigated by Rutherford backscattering spectrometry, transmission electron microscopy, FTIR and Raman scattering. The optical properties were investigated by means of photoluminescence, cathodoluminescence and photoluminescence decay spectroscopy.In this poster, we present a detailed discussion of these issues and determine the optimal conditions of the SRSO films from the point of view of Tb3+ emission properties.An increase in the Si concentration in SRSO influences the emission properties in two ways. First of all, the refractive index of the film increases which enhances the oscillator strength of the optical transitions. On the other hand, the non-radiative recombination also increases, limiting the PL intensity. Therefore, from the point of view of emission efficiency, the optimal Si concentration in the film is about 35 at.% Si. Besides the Si concentration, another effect investigated in this work was the influence of the annealing temperature on Tb3+ emission. It was found that the PL intensity increases as a function of the annealing temperature. This effect was ascribed to the structural changes of the SRSO matrix. At early stages of the annealing (up to annealing temperature equal to 900oC) the most important factor enhancing the Tb3 emission intensity is a reduction in the number of non-radiative recombination centers. However, as shown by Raman measurements, starting from Ta =1100oC structural changes occur in the matrix (crystallization and formation of the Si nanocrystals) and the Tb environment significantly changes. Therefore, the further increase in the PL intensity obtained for high annealing temperatures (temperatures in the 1100–1200 oC range) can be ascribed to the increasing oscillator strength of the intra-4f transitions, induced by the change of the crystal field around the ions.

Near-band-edge photoluminescence from ZnO film: Negative thermal quenching and role of adsorbed oxygen

We studied the photoluminescence (PL) spectra in the near-band-edge region of undoped ZnO films as a function of temperature to determine the thermal quenching behavior in the emission intensity. The quenching of the PL intensity was found to change to an increasing intensity with increasing temperature in the temperature region from 40 to 60 K. On the other hand, a reduction of the vacuum in the same temperature region was observed and was attributed to desorption of oxygen molecules from the cryostat finger. Further investigation revealed that the increase in the PL intensity was caused by adsorption of oxygen on the surface of ZnO films resulting in a decrease in the number of non-radiative recombination centers.

Photoluminescence Study of Plasma-Induced Damage of GaInN Single Quantum Well

Plasma-induced damage (PID) due to Cl2/SiCl4/Ar plasma etching of the GaN capping layer (CAP)/GaInN single quantum well (SQW)/GaN structure was investigated by conventional photoluminescence (PL), transmission electron microscopy (TEM), and time-resolved and temperature-dependent photoluminescence (TRPL). SQW PL intensity remained constant initially, although plasma etching of the CAP layer proceeded, but when the etching thickness reached a certain amount (∼60 nm above the SQW), PL intensity started to decrease sharply. On the other hand, TEM observations show that the physical damage (structural damage) was limited to the topmost surface region. These findings can be explained by the results of TRPL studies, which revealed that there exist two different causes of PID. One is an increase in the number of nonradiative recombination centers, which mainly affects the PL intensity. The other is an increase in the quantum level fluctuation owing mainly to physical damage.

Effects of Proton Irradiation on Optical and Electrical Properties of Cu(In,Ga)Se2 Solar Cells

The optical and electrical properties of proton irradiated Cu(In,Ga)Se2 (CIGS) solar cells and the composed thin films such as transparent conducting oxide (TCO) and CIGS were investigated. The transmittance and resistivity of TCO window layers remained constant for a fluence of up to 3×1015 cm-2. For CIGS thin films, the photoluminescence (PL) peak intensity tended to decrease, possibly because the number of nonradiative recombination centers increased under proton irradiation. In addition, the PL spectra of irradiated CIGS solar cells suggested that the CIGS absorber layer and/or the depletion layer around the CdS/CIGS interface are more prone to degrade than other layers. These results constitute the first step in clarifying the degradation mechanism of CIGS solar cells.

Two Distinct Origins of Highly Localized Luminescent Centers within InGaN/GaN Quantum‐Well Light‐Emitting Diodes

AbstractThe high light‐output efficiencies of InxGa1‐xN quantum‐well (QW)‐based light‐emitting diodes (LEDs) even in presence of a large number of nonradiative recombination centers (such as dislocations) has been explained by localization of carriers in radiative potential traps, the origins of which still remain unclear. To provide insights on the highly efficient radiative traps, spectrally resolved photoluminescence (PL) microscopy has been performed on green‐light‐emitting In0.22Ga0.78N QW LEDs, by selectively generating carriers in the alloy layers. PL imaging shows the presence of numerous inhomogeneously distributed low‐band‐gap traps with diverse radiative intensities. PL spectroscopy of a statistically relevant number of individual traps reveals a clear bimodal distribution in terms of both band‐gap energies and radiative recombination efficiencies, indicating the presence of two distinct classes of carrier localization centers within the same QW sample. Disparity in their relative surface coverage and photoemission “blinking” characteristics suggests that the deep traps originate from local compositional fluctuations of indium within the alloy, while the shallow traps arise from nanometer‐scale thickness variations of the active layers. This is further supported by Poisson–Schrödinger self‐consistent calculations and implies that radiative traps formed due to both local indium content and interface‐morphology‐related heterogeneities can coexist within the same QW sample.

Optical and electrical characterization of In‐doped ZnMgO films grown by spray pyrolysis method

AbstractIn‐doped Zn0.9Mg0.1O (0 ∼ 5 mol%) films on glass substrate were successfully grown by a spray pyrolysis method at 500 °C. The c‐axis orientation became weak with increasing indium concentration. This indicated that In‐doping caused the degree of crystallinity of the Zn0.9Mg0.1O films to decrease. Indium atoms could be acted as a donor type impurity because electrical conduction types in the undoped and In‐doped films indicated all n‐types and In‐doping caused the resistivity to decrease and carrier concentration to increase. Furthermore, In‐doping caused the number of nonradiative recombination centers to increase because photoluminescence intensity decreased with increasing indium concentration. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and Annealing of ZnO Films Grown by Spray Pyrolysis

Undoped and In-doped ZnO films on glass substrate were successfully grown by spray pyrolysis at 500 °C. The samples were annealed under N2 atmosphere between 100 and 600 °C for 5 min. The lattice constants and grain sizes of both samples were unchanged with increasing annealing temperature. However, the photoluminescence intensity of the In-doped sample markedly decreased in comparison with that of the undoped sample with increasing annealing temperature. The carrier concentration increased with increasing annealing temperature. These findings indicated that the number of nonradiative recombination centers of the In-doped ZnO samples markedly increased.

Ion beam-induced luminescence and photoluminescence of 100 MeV Si 8+ ion irradiated kyanite single crystals

Ionoluminescence (IL) of kyanite single crystals during 100 MeV Si 8+ ion irradiation has been studied in the fluence range 1.87–7.50×10 11 ions/cm 2. Photoluminescence (PL) of similar dimensional crystals was recorded with same ions and energy in the fluence range 1×10 11–5×10 13 ions/cm 2 with an excitation of 442 nm He–Cd laser beam. A sharp IL and broad PL peaks at ∼689 and 706 nm were recorded. This is attributed to luminescence centers activated by Fe 2+ and Fe 3+ ions. It is observed that up to a given fluence, the IL and PL peak intensities increase with increase of Si 8+ ion fluence. The stability of the chemical species was studied on with and without irradiated samples by means of FT-IR spectroscopy. The results confirm that the O–Si–H type bonds covering on the surface of the sample. This layer might be acting as a protective layer and thereby reducing the number of non-radiative recombination centers.