Concentration Of Recombination Centers Research Articles

Improved midgap recombination lifetimes in GaN crystals grown by the low-pressure acidic ammonothermal method

To investigate the carrier recombination processes in GaN crystals grown by the low-pressure acidic ammonothermal (LPAAT) method, the photoluminescence (PL) spectra and PL lifetimes of LPAAT GaN crystals grown on acidic ammonothermal (AAT) GaN seed crystals were correlated with the growth polarity and species/concentration of point defects. The PL spectra of LPAAT GaN grown toward the (0001¯) direction (−c region), which provided the highest growth rate, exhibited a predominant near-band edge (NBE) emission. Neither bandgap narrowing nor Burstein–Moss shifts due to high concentration residual impurities were observed in the NBE emissions, indicating higher purity than the previously reported AAT GaN crystals. In addition, strain-induced energy shift or energy broadening of excitonic emission peaks was not observed, indicating excellent crystal coherency. Because of the reduced concentration of midgap recombination centers, a record-long room-temperature PL lifetime for the NBE emission of ammonothermal GaN (40 ps) was obtained from the −c region. Meanwhile, the PL spectra also exhibited the yellow and blue luminescence bands originating from particular deep-state radiative recombination centers. The major vacancy-type defects acting as midgap recombination centers are identified as vacancy complexes comprising a Ga vacancy (VGa) and a few N vacancies (VN), namely, VGa(VN)n buried by H and/or O, where n is an integer. Further reduction of such defect complexes will allow less compensated stable carrier concentration in the LPAAT GaN crystals.

Annealing effects on Cu(In,Ga)Se2 solar cells irradiated by high-fluence proton beam

Radiation tolerance of Cu(In,Ga)Se2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 1016 MeVneq cm−2, however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

The effect of copper and iridium input atoms on the recombination properties of silicon

In monocrystalline silicon, which is easily condensed with copper and iridium, the recombination time is more stable compared to conventional samples. If the concentration of recombination centers in silicon is less than the concentration of adhesion levels, the residence time of non-basic charge carriers increases, and its values are determined by the concentration of adhesion levels. Compared to conventional silicon samples that were legalized with copper and iridium, a large concentration of copper and iridium was observed in neutron-legalized samples. Сoncentration maxima are observed in the DLTS spectra after thermal diffusion of samples of n-Si<P,R,Cu>, n-Si<P,P,Ir> at 1150 oC. This was followed by the study of the structure, size and distribution of silicon mixing defects, which was legitimized by copper and iridium, using ATM.

Dislocation Filter Based on LT-GaAs Layers for Monolithic GaAs/Si Integration.

The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 106 cm-2, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (>108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.

Impact of a defect trapping layer on the reliability of 1.3 μm quantum dot laser diodes grown on silicon

In this paper, we studied the impact of a defect trapping layer on the degradation kinetics of 1.3 μm quantum dot laser diodes (QD LDs) for silicon photonics (SiPh). The purpose of the analysis is to prove how the inclusion of trapping layers (TLs) for misfit dislocations can improve the performance and reliability of QD LDs. To this aim we stressed two kinds of devices, that only differ from the inclusion of TLs. By submitting the samples to current step-stress, we tested the devices from mild to highly accelerated conditions. Our results demonstrate that a) the addition of TLs shifts the roll-off of the optical characteristics to higher current densities; b) in devices with TLs, excited state (ES) emission is reached at higher current densities, compared to standard devices; c) in terms of reliability, the addition of TLs results in a slower degradation of the threshold current, to a lower concentration of misfit dislocations in the active region; d) degradation is ascribed to the lowering of the injection efficiency, driven by the escape of carriers out of the active region, which causes an increase in the concentration of non-radiative recombination centers.

Small Atom Doping: A Synergistic Strategy to Reduce SnZn Recombination Center Concentration in Cu2ZnSnSe4

Kesterite Cu2ZnSnS x Se4−x (CZTSSe) is among the most promising inorganic Earth‐abundant thin‐film photovoltaic technologies, although currently, the larger voltage deficit compared with more mature chalcogenide technologies is hampering solar‐to‐electricity conversion efficiency progress in these materials. Most of the latest reports agree on the CZTSSe defect structure as the main limitation for the open‐circuit voltage. Small atom doping is suggested as an interesting strategy to reduce the concentration of defects without affecting secondary phase formation. Herein, an innovative approach based on the introduction of LiAlH4 and its further decomposition during the selenization process of CZTSe precursors, as a pathway for hydrogen and lithium/alkali transient doping, is explored. This process shows a strong beneficial influence on the crystal growth and solar cell device performance, especially with a significant improvement in V oc and fill factor. A reduction of nonradiative recombination and a remarkable fourfold increase in the carrier lifetime correlating with the reduction of the open‐circuit voltage (V oc) deficit below 330 mV is demonstrated. A mechanism on how small atoms (Li and H) interact to reduce the concentration of SnZn recombination centers while keeping doping relatively unchanged is proposed, opening fundamental perspectives for the simple and universal transient doping of thin‐film chalcogenide compounds.

Concentric core-shell tracks and spectroscopic properties of SrTiO3 under intense electronic excitation

A deeper understanding of the intrinsic link between the irradiation-induced microstructures and the corresponding spectroscopic properties is becoming increasingly attractive for the research fields of materials science, physics, and information technology. In this work, the structural damage response, spectroscopic features, and associated physical mechanisms of SrTiO3 single crystals under swift heavy ion irradiation were comparatively analyzed using a combination of experimental and theoretical approaches. Corresponding to 0.11–5.00 MeV/u ion irradiation with electronic energy loss ranging from 4.0 to 29.3 keV/nm, the inelastic thermal spike calculations combined with molecular dynamics simulations are compared with the experimental observations, revealing the track fine structures (individual spherical defects with a disordered region, and discontinuous and continuous tracks consisting of an amorphous core and a disordered outer shell) and demonstrating the dominant effects of the deposition energy and lattice temperature on track damage formation and evolution; thus, two essential thresholds for defect formation of ∼0.60 eV/atom and amorphous region formation of ∼1.81 eV/atom were identified to better describe the concentric core-shell track structure. The enhanced nanohillock formation in the surface region is attributed to the combined action of kinetic and potential energy depositions, and the fluence dependence in regulating the hillock dimensions is also presented. The measured refractive index profiles along different crystal axes further indicate the anisotropy of irradiation-induced lattice expansion. With increasing ion fluence and damage level, the intrinsic bandgap gradually decreased, and the concentration of additional radiative recombination centers (1.81 eV, 2.31 eV, and 2.42 eV) accordingly increased in the SrTiO3 crystal, providing the possibility of regulating related defects to achieve tunable/selective photon emission for more critical technological applications.

Free-standing large, ultrathin germanium selenide van der Waals ribbons by combined vapor-liquid-solid growth and edge attachment.

Among group IV monochalcogenides, layered GeSe is of interest for its anisotropic properties, 1.3 eV direct band gap, ferroelectricity, high mobility, and excellent environmental stability. Electronic, optoelectronic and photovoltaic applications depend on the development of synthesis approaches that yield large quantities of crystalline flakes with controllable size and thickness. Here, we demonstrate the growth of single-crystalline GeSe nanoribbons by a vapor-liquid-solid process over Au catalyst on different substrates at low thermal budget. The nanoribbons crystallize in a layered structure, with ribbon axis along the armchair direction of the van der Waals layers. The ribbon morphology is determined by catalyst driven fast longitudinal growth accompanied by lateral expansion via edge-specific incorporation into the basal planes. This combined growth mechanism enables temperature controlled realization of ribbons with typical widths of up to 30 μm and lengths exceeding 100 μm, while maintaining sub-50 nm thickness. Nanoscale cathodoluminescence spectroscopy on individual GeSe nanoribbons demonstrates intense temperature-dependent band-edge emission up to room temperature, with fundamental bandgap and temperature coefficient of Eg(0) = 1.29 eV and α = 3.0 × 10-4 eV K-1, respectively, confirming high quality GeSe with low concentration of non-radiative recombination centers promising for optoelectronic applications including light emitters, photodetectors, and solar cells.

Влияние условий формирования кремниевых диодов на их обратные токи

In this work, a study was made of the influence of silicon diode manufacturing technology on the emergence of generation and recombination centers. The electrical characteristics of p-n junctions formed in different ways on n-type silicon substrates were compared: a) the p-type layer was created by the diffusion method; b) the p-type layer was formed by ion implantation into an epitaxial n-layer preliminarily grown on the substrate; c) two n- and p-type epitaxial layers were successively deposited on the substrate. It has been established that for diodes based on a double epitaxial layer, the direct and reverse current-voltage characteristics (CVC) are due to the diffusion mechanism, and the structures themselves have a low concentration of recombination centers. At the same time, in diodes based on the diffusion method and ion implantation, the CVCs are due to the generation-recombination mechanism. With reverse bias, electron-phonon processes play a significant role in the formation of the CVC, and with forward bias, carrier recombination in the region of the space charge of the p-n junction. The concentrations and energies of recombination centers have been determined.

Influence of formation conditions of silicon diodes on their reverse currents

In this work, a study was made of the influence of silicon diode manufacturing technology on the emergence of generation and recombination centers. The electrical characteristics of p-n junctions formed in different ways on n-type silicon substrates were compared: a) the p-type layer was created by the diffusion method; b) the p-type layer was formed by ion implantation into an epitaxial n-layer preliminarily grown on the substrate; c) two n- and p-type epitaxial layers were successively deposited on the substrate. It has been established that for diodes based on a double epitaxial layer, the direct and reverse current-voltage characteristics (CVC) are due to the diffusion mechanism, and the structures themselves have a low concentration of recombination centers. At the same time, in diodes based on the diffusion method and ion implantation, the CVCs are due to the generation-recombination mechanism. With reverse bias, electron-phonon processes play a significant role in the formation of the CVC, and with forward bias, carrier recombination in the region of the space charge of the p-n junction. The concentrations and energies of recombination centers have been determined. Keywords: forward and reverse current-voltage characteristic, p-n junction, diffusion, ion implantation, epitaxy, recombination centers, the Poole--Frenkel effect, electron-phonon interaction.

Improved minority carrier lifetime in p-type GaN segments prepared by vacancy-guided redistribution of Mg

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3×1018 cm−3) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 1017 cm−3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7×10−6 cm3 s−1. Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I.

Reduced nonradiative recombination rates in c-plane Al0.83In0.17N films grown on a nearly lattice-matched GaN substrate by metalorganic vapor phase epitaxy

A record-long room-temperature photoluminescence (PL) lifetime (τPLRT) of approximately 70 ps was obtained for the sub-bandgap 3.7 eV emission band of a 300-nm-thick c-plane Al0.83In0.17N epilayer for the use in cladding layers of an edge laser structure, which were grown by metalorganic vapor phase epitaxy on a low threading dislocation density nearly lattice-matched GaN substrate. The recorded τPLRT value was twice as long as previously reported ones, indicating half concentration of nonradiative recombination centers. Room-temperature spatially resolved cathodoluminescence intensity images for the 3.7 eV band revealed nearly zero carrier diffusion length, which is consistent with the fact that τPLRT of 70 ps is 1/35 of the near-band-edge emission of the GaN substrate (2.4 ns). As the PL decay curves for the 3.7 eV band were sufficiently fitted by the stretched exponential function, the emission likely originates from extended states such as impurities, point defects, and their complexes, as well as localized states of uneven potential profile.

Evolution of the electrical characteristics of Cu(In,Ga)Se2 devices with sodium content

The impact of sodium on the electrical properties of Cu(In,Ga)Se2 (CIGS) thin films and corresponding solar cells was investigated by preparing nearly alkali-free CIGS layers and doping them with different Na amounts via NaF post-deposition treatment (PDT) at temperatures between 110 and 400 °C. The mean Na concentrations in the CIGS layers ranged from 0.1 to 400 ppm. Sodium was found also in the grain interior even for the lowest PDT temperature. All samples were subjected to extensive electrical characterization: current–voltage, capacitance profiling, conductivity, steady-state, and transient capacitance spectroscopy. A continuous increase in open-circuit voltage VOC and fill factor FF, an accompanying increase in hole density and mobility, and a decrease in secondary barriers responsible for the distortion of current–voltage characteristics were observed with increasing sodium content. An abrupt change in defect spectra and a dominant transport mechanism was found for PDT temperatures T(PDT) of ≥150 °C. We attribute a further improvement in VOC observed above 150 °C PDT temperature to the reduced concentration of recombination centers with increased sodium content. An explanation of both gradual evolution and the abrupt change is proposed based on passivation of grain boundaries and interfaces by sodium.

Quantitative analysis on the oxygen diffusion in pyramidal textured surfaces of silicon and copper via transmission electron microscopy

In the process of microelectronic device fabrication, the oxygen atom plays a double role. It oxidizes Si producing the SiO2 layer. Moreover, it oxidizes the metal electrodes causing failures and enters the semiconductor acting as an impurity recombination center. This phenomenon is closely related to the oxygen diffusion in the material surface. Although a textured material surface is frequently used, the oxygen diffusion inside this surface is rarely studied. Studies are available only for planar material surfaces. In this work, monocrystalline Si slices with a pyramidal textured surface and a series of Cu films, which were deposited onto this substrate, were thermally oxidized. The oxygen diffusion in such textured surfaces was quantitatively analyzed via transmission electron microscopy. The oxygen distribution and the variation in the oxygen:cation atomic concentration ratio as a function of the oxide thickness were obtained. Thus, suboxide diffusion layers of SiOx and CuOx were found for the surfaces. In addition, the oxidation rate of the textured material surface was significantly reduced due to its lower oxygen partial pressure when compared to a planar surface. These results can be used to control the oxide thickness, to resist the electrode oxidation, and to reduce the concentration of oxygen recombination centers when using a textured material surface for optoelectronic devices.

Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se2 absorber materials

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium–copper divacancy complex (VSe–VCu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at Ev + 0.98 eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the VSe–VCu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the VSe–VCu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

A discrete core-shell-like micro-light-emitting diode array grown on sapphire nano-membranes

A discrete core-shell-like micro-light-emitting diode (micro-LED) array was grown on a 100 nm-thick sapphire nano-membrane array without harmful plasma etching for chip singulation. Due to proper design for the sapphire nano-membrane array, an array of multi-faceted micro-LEDs with size of 4 μm × 16 μm was grown. Threading dislocation density in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. Enhancements in internal quantum efficiency by 44% and 3.3 times higher photoluminescence intensity were also observed from it. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an attractive platform for micro-LEDs by using sapphire nano-membrane.

Investigating the Formation of Defects in Silicon during Gamma Irradiation

The parameters of recombination centers in silicon photodiodes are studied before and after gamma irradiation. The technique of investigation is deep-level recombination spectroscopy. It is shown that, after irradiation, both the forward-bias and reverse-bias currents through the p–n junction increase, which is explained by the growth in the concentration of recombination centers due to the formation of vacancies during irradiation.

Creation and Annihilation of Nonradiative Recombination Centers in Polycrystalline Metal Halide Perovskites by Alternating Electric Field and Light

AbstractMetal halide perovskites are promising optoelectronic materials. Their electronic properties however are rather unstable which is often assigned to ion migration. Ion migration can be readily influenced by an electric field (EF). Here, the response of photoluminescence (PL) of individual MAPbX3 (MA = CH3NH3, X = I, Br) sub‐micrometer‐sized polycrystals to EF is studied. Alternating EF with frequency higher than 10 Hz is found to reversibly quench PL. It is proposed that an alternating EF when applied together with light increases ion migration. This leads to a shift in the equilibrium between creation and annihilation of defects toward higher concentration of nonradiative recombination centers. The PL quenching is found to increase with increasing frequency of the field. This can be rationalized by the frequency dependence of the dielectric constant, leading to stronger internal fields for high modulation frequencies compared to, e.g., a constant EF with the same external amplitude. PL quenching and enhancement observed under constant EF are hypothesized to be due to a reconfiguration of already existing nonradiative recombination centers situated on grain boundaries. The control of perovskite PL by alternating EF reported here can find applications in optoelectronic devices.

On the Theory of a Photocurrent Burst in an Intrinsic Photoresistor upon Longitudinal and Transverse Illumination

In this paper, we analyze the intrinsic photoconductivity burst of semiconductors at an increase in the recombination center concentration upon uniform and non-uniform weak illumination along the electric field. An equation for the concentration distribution of nonequilibrium carriers at an arbitrary illumination profile along the electric field is derived without using the quasi-neutrality approximation. Based on this equation, the bursts in photoelectric gain of the transverse and longitudinal photoresistors should significantly differ from each other under any recombination conditions at current contacts due to the photo-induced space charge. If the carrier photogeneration is not uniform, then, unlike uniform photogeneration, the photoelectric gain burst depends on the polarity of the applied voltage. The quasi-neutrality approximation does not produce these results. An analytical expression for the maximum value of the electron photoelectric gain depending on the recombination center concentration is derived in the case of an exponential photogeneration profile and pulling-out contacts. The relationship that was found between the concentrations of nonequilibrium electrons and holes allows derivation of an analytical expression for the maximum value of the hole photoelectric gain.