Density Of Nonradiative Recombination Centers Research Articles

The Influence Mechanism of Quantum Well Growth and Annealing Temperature on In Migration and Stress Modulation Behavior.

This study explores the effects of growth temperature of InGaN/GaN quantum well (QW) layers on indium migration, structural quality, and luminescence properties. It is found that within a specific range, the growth temperature can control the efficiency of In incorporation into QWs and strain energy accumulated in the QW structure, modulating the luminescence efficiency. Temperature-dependent photoluminescence (TDPL) measurements revealed a more pronounced localized state effect in QW samples grown at higher temperatures. Moreover, a too high annealing temperature will enhance indium migration, leading to an increased density of non-radiative recombination centers and a more pronounced quantum-confined Stark effect (QCSE), thereby reducing luminescence intensity. These findings highlight the critical role of thermal management in optimizing the performance of InGaN/GaN MQWs in LEDs and other photoelectronic devices.

Editors’ Choice—Structural, Electrical, and Luminescent Properties of Orthorhombic κ-Ga2O3 Grown by Epitaxial Lateral Overgrowth

The properties of orthorhombic κ-Ga2O3 films grown by Epitaxial Lateral Overgrowth (ELOG) were studied by Scanning Transmission Electron Microscopy (STEM), X-ray diffraction, capacitance-voltage profiling, Microcathodoluminescence (MCL) spectroscopy and imaging. ELOG mask was formed by deposition of SiO2 stripes on TiO2 buffer prepared on basal plane sapphire, with the stripes going along the [110] direction of sapphire. κ-Ga2O3 ELOG growth was performed using Halide Vapor Phase Epitaxy (HVPE), with ELOG wing of the structure formed by lateral overgrowth over the 20 μm-wide SiO2 stripes, while growth in between the stripes proceeded initially by vertical growth in the 5-μm-wide windows. TEM analysis showed that the material in the windows comprised 120o rotational nanodomains typical of κ-Ga2O3, while, in the wing regions, the material was single-domain monocrystalline. The films were conducting, with the net donor density close to 1013 cm−3. The data suggested the material in the windows have much higher resistance than in the wings. MCL spectra and imaging revealed much higher density of nonradiative recombination centers in the windows than in the wings.

Luminescent iron disilicide film growth by metal–organic chemical vapor deposition

Semiconducting iron disilicide (β-FeSi2) films were epitaxially grown on the Ag-layer pre-coated Si(111) substrates with an initial β-FeSi2 layer by the metal–organic chemical vapor deposition method. These β-FeSi2 films had (101)-preferred orientations, and their crystal quality was improved as the growth temperature increased from 893 to 1093 K. The photoluminescence (PL) intensity of the (101)-oriented β-FeSi2 films grown at 973 K was larger than those of β-FeSi2 films at the other deposition temperatures, which indicates the decrease of the density of nonradiative recombination centers in β-FeSi2. A clear A-band emission originated from β-FeSi2 was observed for these films up to 285 K. This pronounced PL intensity enhancement from β-FeSi2 is attributed not only to the crystallinity evaluated by XRD measurement but also to the decrease in density of thermal equilibrium Si vacancy in β-FeSi2.

Optical and structural properties of GeSn/SiGeSn multiple quantum wells for infrared optoelectronics

We have investigated the correlation between structural and defect properties, and the optical performance of GeSn/SiGeSn multi-quantum well (MQW) structures as a function of the number of QWs. Our results show that significant diffusion and intermixing occur at the GeSn/SiGeSn interfaces during the sample growth. The intermixing is more pronounced at larger depths below the surface where the Sn and Si composition profile in the QWs layers undergoes a transition from a block-shaped to a compositionally graded Λ-shaped profile. Simultaneously, the peak Sn concentration in the QWs reduces with depth up to 2.0 ± 0.2 at.% while the integrated photoluminescence (PL) is significantly degraded. The reduced PL emission from both the GeSn buffer and GeSn QWs is attributed to the increasing density of non-radiative recombination centers in the MQWs due to alloy disordering due to diffusion. The use of PL from the GeSn buffer layer as a reference is shown to be important to analyze PL from SiGeSn/GeSn QWs and more generally from (Si)GeSn materials.

Defect characterization of {101¯3} GaN by electron microscopy

Advances in obtaining untwinned (101¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (101¯3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (101¯3) has the potential for higher radiative efficiency than established semi-polar orientations.

Ion Motion Determines Multiphase Performance Dynamics of Perovskite LEDs

AbstractPerovskite light‐emitting diodes (PeLEDs) currently reach up to about 20% external quantum efficiency (EQE) and are becoming a promising technology for display and lighting applications. Still, many issues regarding their performance remain unresolved, particularly those related to stability, operation in non‐stationary regimes, and efficiency roll‐off at high current densities. Here, some of those issues in PeLEDs based on MAPbI3 perovskite are addressed. The authors analyze the electroluminescence (EL) and current dynamics after the first‐time voltage application and after application of sequences of voltage pulses, at different temperatures. Analysis of the results suggests that the complex dynamics observed on time scales from sub‐seconds to minutes and hours can be explained by the spatial redistribution of mobile species, most likely iodine interstitials, characterized by ≈175 meV activation energy. This redistribution alters the carrier injection, spatial electric field, and charge carrier density distributions as well as density of nonradiative recombination centers within the perovskite layer. Mathematical modeling of the ion motion and related processes enabled to reproduce the EL and current dynamics and to disentangle complex sequence of processes governing the PeLED operation.

Crystal quality degradation in MoTe2 monolayers by a thermal annealing and its suppression by hexagonal boron nitride encapsulation

Two dimensional transition metal dichalcogenides (TMDs) have attracted great research interests owing to a crossover to a direct gap upon reduction of the layer number down to a monolayer (1L) and exciton formation at room temperature based on extraordinarily large binding energy. In particular, a semiconducting molybdenum ditelluride (MoTe2) emits photons in the near-infrared region around 1.1 eV. Furthermore, it has a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a path to practical Si-based optoelectronic device applications. However, the 1L-MoTe2 crystal has some drawbacks such as surface oxidation within several days owing to its poor structural stability and crystal defect formation by a thermal annealing at a low temperature of 200 °C. It is reported that encapsulation by hexagonal boron nitride (hBN) prevents the oxidation of 1L-MoTe2 in air. There are few reports on effects of a thermal annealing on structural stability in hBN-encapsulated 1L-MoTe2. In this study, we investigate the effects of a thermal annealing on the crystal quality of hBN-encapsulated 1L-MoTe2 by a photoluminescence (PL) measurement at low temperatures. We demonstrate that hBN encapsulation is effective for suppression of crystal defect formation in 1L-MoTe2, improving thermal stability.Bulk hBN crystals were grown using a temperature-gradient method at a high pressure (4.0-5.5GPa) and high temperature (1500-1750 °C). Monolayer MoTe2 and multilayered hBN flakes were exfoliated from bulk crystals and were prepared on a polydimethylsiloxane (PDMS) sheet. We fabricated two types of sample structures on a 100-nm-thick SiO2/Si (100) substrate by a standard dry transfer technique using these flakes. The first sample comprised 1L-MoTe2 directly deposited on the SiO2/Si substrate. The second sample comprised 1L-MoTe2 encapsulated by hBN. The fabricated sample structures were thermally annealed at temperatures ranging from 100 to 200 °C for 15min in an atmosphere.Figures 1(a) and 1(b) show the PL spectra at 25K of the SiO2-supported and hBN-encapslated 1L-MoTe2, respectively. These PL spectra are reproduced using multi-Voigt functions to evaluate PL peak intensities in each spectrum. PL emissions attributed to neutral exciton (X0), charged exciton (T), and defect-bound excitons (XB1, XB2) are observed in both samples of SiO2-supported and hBN-encapsulated 1L-MoTe2. We identify these MoTe2 crystals as monolayers from the X0 PL peak position of 1.19 eV. Figure 1(c) shows the trion spectral weight (I T /I tot) plotted as a function of the annealing temperature for the SiO2-supported and hBN-encapsulated 1L-MoTe2. For both sample structures, the I T /I tot values decreases by the thermal annealing at 200℃, implying the decrease in excess carriers in 1L-MoTe2. The excess carriers should originate from impurities and residues adsorbed on the 1L-MoTe2. Thus, the thermal annealing makes it possible to reduce the impurities and residues, realizing high-quality sample structures. Figure 1(d) shows the PL intensity ratio (I XB1 + I XB2)/I X0 plotted as a function of the annealing temperature. Here, I XB1, I XB2, and I X0 are PL intensities of XB1, XB2, and X0 peaks, respectively. For the SiO2-supported 1L-MoTe2, the (I XB1 + I XB2)/I X0 value increases by increasing the annealing temperature while it is kept to be almost constant for the hBN-encapsulated 1L-MoTe2. It is reported that the XB1 and XB2 peaks are attributed to the formation of Te vacancies by a thermal annealing and adsorption of O2 and H2O molecules at the crystal defect sites, respectively. Therefore, an increase in the (I XB1 + I XB2)/I X0 value of the SiO2-supported samples should result from the crystal defect formation in 1L-MoTe2, indicating a large density of non-radiative recombination centers at 200 °C. In contrast, for the hBN-encapsulated samples, the Te vacancy formation and adsorption of O2 and H2O molecules at the crystal defect sites should be avoided owing to the top hBN acting as the capping layer to suppress Te atom desorption.In summary, we investigated the effect of a thermal annealing on a PL spectrum for SiO2-supported and hBN-encapsulated samples. It was found that the thermal annealing at 200 °C were able to remove residues and impurities on the 1L-MoTe2. For the SiO2-supported samples, an increase in the (I XB1 +I XB2 )/I X0 value was seen by the thermal annealing, indicating crystal defect formation due to desorption of Te atoms. In contrast, for the hBN-encapsulated samples, it was kept to be almost constant even after the thermal annealing at 200 °C. It was therefore concluded that the hBN encapsulation should improve the thermal stability in 1L-MoTe2. This work was partly supported by JSPS KAKENHI Grant Numbers JP25107003, JP25107004, JP15K21722, JP26248061, JP25107002, JP16H00982, and JP20H00354 and the CREST (JPMJCR15F3), JST. Figure 1

Defect incorporation in In-containing layers and quantum wells: experimental analysis via deep level profiling and optical spectroscopy

Recent studies demonstrated that the performance of InGaN/GaN quantum well (QW) light emitting diodes (LEDs) can be significantly improved through the insertion of an InGaN underlayer (UL). The current working hypothesis is that the presence of the UL reduces the density of non-radiative recombination centers (NRCs) in the QW itself: during the growth of the UL, surface defects are effectively buried in the UL, without propagating towards the QW region. Despite the importance of this hypothesis, the concentration profile of defects in the quantum wells of LEDs with and without the UL was never investigated in detail. This paper uses combined capacitance-voltage and steady-state photocapacitance measurements to experimentally identify the defects acting as NRCs and to extract a depth-profile of the traps, thus proving the incorporation upon indium-reaction. Specifically: (i) we demonstrate that LEDs without UL have a high density (9.2 × 1015 cm−3) of defects, compared to samples with UL (0.8 × 1015 cm−3); (ii) defects are located near midgap (E C-1.8 eV, corresponding to E i-E T ∼ 0.3 eV), thus acting as efficient NRCs; (iii) crucially, the density of defects has a peak within the QWs, indicating that traps are segregated at the first grown InGaN layers; (iv) we propose a model to calculate trap distribution in the QW, and we demonstrate a good correspondence with experimental data. These results provide unambiguous demonstration of the role of UL in limiting the propagation of defects towards the QWs, and the first experimental characterization of the properties of the related traps.

In-situ spectroscopic analysis of the recombination kinetics in UVB LEDs during their operation

The recombination kinetics in the quantum well (QW) active region of ultraviolet light emitting diodes have been measured in situ during device operation. Non-radiative recombination is enhanced with increasing operation time. Saturation effects allow us to estimate that the density of non-radiative recombination centers in the QW layers increases by ∼5 × 1015 cm−3 after operation. Corresponding defects are proposed to be present in the pristine device already. The recombination kinetics after optical excitation can be controlled by an additionally applied DC. The experimental approach allows the direct comparison and quantification of the effects of optically and electrically generated non-equilibrium carriers on the QW luminescence.

InGaN quantum wells with improved photoluminescence properties through strain-controlled modification of the InGaN underlayer

The effect of internal strain on the luminescence properties of an InGaN single quantum well (SQW) was investigated as a function of modification via an underlayer (UL). Single In0.25Ga0.75N QWs (λ = 520 nm) 3 nm thick were grown on various ULs on a sapphire substrate, where the two UL types included (1) a buffer layer onto which an InGaN layer with a very small amount of In was inserted and (2) a buffer layer grown using different carrier gases. The SQWs were then analyzed by temperature-dependent time-resolved photoluminescence, scanning electron microscopy and cathodoluminescence. The experimental results show that the density of non-radiative recombination centers and the level of potential fluctuation in the SQWs decrease with insertion of an InxGa1−xN UL possessing a quite low but sufficient indium content (x = 0.007). The density of non-radiative recombination centers in the SQW on the H2 carrier-grown UL, however, is large.

Thermionic emission of carriers in InGaN/(In)GaN multiple quantum wells

The room temperature photoluminescence (PL) intensity of InxGa1-xN/InyGa1-yN multiple quantum wells were correlated with the structural properties such as polarization strength and ground-state quantum levels for the triangular quantum wells. For x – y ≤ 0.2, the PL intensity increased with increasing the In-content of the well. In this region, we found that the PL intensity can be well described by the thermionic emission process with an activation energy corresponding to the energy difference between the valence band of barrier and the quantum level of holes in the well at the well/barrier interface. The pre-factor of the exponential component of effective barrier, reflecting the effective density of nonradiative recombination centers in the barriers, was 3.7 times larger for the samples with GaN barriers than that with InGaN barriers, indicating that the PL intensity is also influenced by the optical quality of barriers when the effective barrier height is relatively low.

MOCVD growth of β-FeSi2 film on modified Si surface by silver and enhancement of luminescence

The improving of the crystal quality and pronounced enhancement of photoluminescence intensity from β-FeSi2 were achieved by pre-coating silver (Ag) layers on Si(1 0 0) substrates. A modified-Si surface, which formed during the heating step before β-FeSi2 deposition due to Ag diffusion, was reconstructed the surface into domain structure that minimize the strain caused by lattice matching of β-FeSi2 and Si. This mechanism led to the formation of β-FeSi2 film with very low levels of density of non-radiative recombination centers, and led to the pronounced enhancement of PL intensity.

Perovskites: Between the Grains

Defect reduction and device stabilization will continue to be two important thrusts toward efficient long-lasting polycrystalline perovskite solar cells. Recently in Chem, Zong et al. show that a complementary hydrophobic/hydrophilic triblock copolymer continuously incorporates into methylammonium lead tri-iodide grain boundaries. This improves solar cell stability and performance, resulting in a stabilized 19.4% power conversion efficiency, 92% of which is retained after 480 hr of continuous 1-sun illumination.

The ABC model of recombination reinterpreted: Impact on understanding carrier transport and efficiency droop in InGaN/GaN light emitting diodes

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region.

Optical Characterization of AlAsSb Digital Alloy and Random Alloy on GaSb

III-(As, Sb) alloys are building blocks for various advanced optoelectronic devices, but the growth of their ternary or quaternary materials are commonly limited by spontaneous formation of clusters and phase separations during alloying. Recently, digital alloy growth by molecular beam epitaxy has been widely adopted in preference to conventional random alloy growth because of the extra degree of control offered by the ordered alloying. In this article, we provide a comparative study of the optical characteristics of AlAsSb alloys grown lattice-matched to GaSb using both techniques. The sample grown by digital alloy technique showed stronger photoluminescence intensity, narrower peak linewidth, and larger carrier activation energy than the random alloy technique, indicating an improved optical quality with lower density of non-radiative recombination centers. In addition, a relatively long carrier lifetime was observed from the digital alloy sample, consistent with the results obtained from the photoluminescence study.

Influence of defects and indium distribution on emission properties of thick In-rich InGaN layers grown by the DERI technique

We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Photocarrier dynamics near V-shaped pits in InxGa1−xN/GaN multiple quantum wells

Space- and time-resolved photoluminescence (PL) has been employed to investigate correlations between the carrier dynamics and structural defects known as V-shaped pits in InGaN/GaN multiple quantum wells (MQWs). The pits exhibit much lower PL intensity compared to MQWs of the normal c-plane indicating a high density of nonradiative recombination centers in the pits. However, the PL peak wavelength, which is expected to experience a blueshift at the pits due to the stronger confinement effect and reduced quantum-confined Stark effect, do not show any spatial dependence that is correlated with the defects. This is ascribed to dominant ultrafast (<10ps) nonradiative recombination at the pits and additional diffusion into the c-plane leading to radiative recombination. By contrast, weak but clear correlations between the pits and PL decay time were observed on nanosecond timescales. This can be explained by a kinetic model that includes the nonradiative recombination and diffusion of carriers at the pits.

Correlation between structure and photoluminescence properties in InGaN epilayers with thicknesses below and above critical thickness

The layer strain and its relaxation effects on the photoluminescence (PL) of InGaN layers are studied using confocal microscopy. The relaxation imposed structural changes are studied by X-ray diffraction (XRD) reciprocal space mapping and atomic force microscopy. Initial layer relaxation generated misfit dislocations were observed by confocal microscopy as intersecting parallel lines of lower PL intensity. The splitting of the PL spectrum into several PL bands indicated an onset of changes in the layer structure, which were confirmed by XRD measurements. The PL bands were attributed to two sub-layers of the sample: A relaxed upper sub-layer and a strained sub-layer underneath. Bright spots, approximately 250 nm in diameter, were observed on the background of the inhomogeneous PL intensity distribution due to fluctuations of In content. The bright spots correspond to column-like structures with relaxed lattice, In content as in the initial strained layer, and lower density of nonradiative recombination centers than that in the surrounding background.

Correlation between carrier localization and efficiency droop in AlGaN epilayers

Photoluminescence studies of carrier dynamics in AlGaN epilayers with different degrees of carrier localization and densities of nonradiative recombination centers show that the prevailing droop mechanism in AlGaN epilayers with strong carrier localization and comparatively high density of nonradiative recombination centers is enhanced nonradiative recombination due to the carrier delocalization at elevated carrier density. The photoluminescence was investigated under quasi-steady-state excitation in the temperature range from 8 to 300 K. The results proved that the onset of this droop effect is below the threshold for the droop due to high-density effects in the epilayers, such as carrier heating, phase space filling, nonradiative Auger recombination, and stimulated emission.