Carrier Injection Efficiency Research Articles

Polarization-matching and carrier confinement in III-nitride deep-ultraviolet light-emitting diodes

The polarization-induced quantum confined Stark effect has been recognized as a significant factor contributing to the Internal Quantum Efficiency (IQE) droop in light-emitting diodes (LEDs). This study focuses on the design of LEDs by investigating the InAlN/AlGaN interface. By incorporating InAlN quantum wells, a polarization-matched (PM) multi-quantum well (MQW) LED architecture was developed. While the flat conduction and valence bands on PM MQWs indicate an improved recombination rate, it is crucial to examine the impact on IQE, considering carrier confinement and injection efficiency influenced by the band offsets. This paper presents a numerical analysis comparing two LEDs emitting at 245 and 275 nm, respectively. The results demonstrate that the PM LED operating at 275 nm exhibits enhanced performance, benefiting from high probability density overlap. Conversely, the PM LED emitting at 245 nm demonstrates poor confinement, resulting in an overall low performance, regardless of polarization matching.

X-ray Investigation of CsPbI3:EuCl3 Infiltrated into Gig-Lox TiO2 Spongy Layers for Perovskite Solar Cells Applications.

In this study, we explore the potential of a blended material comprising CsPbI3:EuCl3 perovskite and Gig-Lox TiO2, a unique transparent spongy material known for its multi-branched porous structure, for application in solar cells. The inclusion of EuCl3 in CsPbI3 serves to stabilize the photoactive γ-phase with a bandgap of 1.75 eV, making it suitable for solar energy conversion in tandem solar cells. Our study applies X-ray-based techniques to investigate the structural properties and interfacial behavior within this blended material, in comparison with a reference perovskite layer deposited on glass. In addition, Spectroscopic ellipsometry is complemented with density functional theory calculations and photoluminescence measurements to elucidate the absorption and radiative emission properties of the blend. Notably, our findings reveal a significant quenching of photoluminescence within the blended material, underscoring the pivotal role of the distributed interfaces in facilitating efficient carrier injection from the CsPbI3:EuCl3 perovskite into the Gig-Lox TiO2 sponge. These findings pave the way for the application of the blend as an Electron Transport Layer (ETL) in semi-transparent perovskite solar cells for tandem and building integrated photovoltaics.

Study of eigen frequency In1-xGaxAsyP1-y 1.55 µm VCSEL with SiO2 /TiO2 dielectric Bragg reflector

In this work, we focused on investigating the eigen frequency and internal standing wave characteristics of a vertical surface cavity emitting laser operating at a wavelength of 1.55 µm. The design of the cavity involved determining the cavity length, selecting the material for the cavity spacer, and carefully placing the quantum wells within the cavity to achieve maximum overlap with the electric field. In our case, the choice of the dielectric Bragg mirror with SiO2/TiO2 layers helps in achieving high reflectivity and low optical losses. The quantum wells are strategically placed within the cavity to ensure maximum overlap with the electric field. This allows for efficient carrier injection and recombination, leading to laser emission. The specific composition of the quantum wells, In0.54Ga0.46As0.99P0.01 / In0.75Ga0.25As0.55P0.45, indicates the proportions of indium (In), gallium (Ga), arsenic (As), and phosphorus (P) in the material. These compositions are chosen to achieve the desired electronic band structure and emission wavelength. By studying the eigen frequency and internal standing waves in our designed laser cavity, our aim is to understand the resonant modes and behavior of light within the device. This knowledge is crucial for optimizing the laser's performance and improving its efficiency for various applications.

The effect of different AlGaN insertion layer thicknesses on the photoelectric properties of InGaN/AlGaN near UV light emitting diodes

In this paper, a near-ultraviolet LED structure was fabricated on a sapphire substrate by using metal–organic chemical vapor deposition with an undoped AlGaN insertion layer introduced between multiple quantum wells and electron blocking layers, and the effect of layer thickness on light-electric performance was investigated. Results of epitaxial structure characterization show that the introduction of an undoped AlGaN insertion layer almost does not affect the crystal quality and surface morphology of the LED epitaxial structure. Electroluminescence testing results indicate that as the insertion layer thickness increases, the device performance initially increases and then decreases. Furthermore, simulation analysis suggests that increasing the insertion layer thickness reduces the injection efficiency of carriers, lowers the radiative recombination rate and light emission performance of the active region's carriers. Additionally, the AlGaN insertion layer can effectively suppress the diffusion of Mg impurities, reduce the non-radiative recombination rate in the multiple quantum well active region, and thus improve the LED's light emission performance. Overall, under the competing effects of these two mechanisms, the optimal light emission performance for the near-ultraviolet LED is achieved when the insertion layer thickness is 1 nm.

Simulation of performance enhancement of GaN-based VCSELs by composition gradient InGaN last-quantum barrier

Electron leakage in the active region decreases the internal quantum efficiency and impedes the performance of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs). In this study, we propose a novel InGaN last-quantum barrier (LQB) structure with gradient indium (In) composition, and the device performance was simulated by the commercial software PICS3D. Compared with the device with conventional GaN LQB, the electron leakage is greatly reduced and the hole injection efficiency is also improved by the graded LQB structure. Consequently, the threshold current is reduced by 44%, and output power is increased by 392% in GaN-based VCSEL based on composition gradient InGaN LQB. The composition gradient InGaN can also allow us to increase the thickness of the LQB in epitaxy without degrading the carrier injection efficiency due to the reduced polarization in the LQB. The results of this study suggest that the composition gradient InGaN LQB is promising for the realization of high-performance GaN-based VCSELs.

Cooperative Passivation Exciton Quenching Triggers Surpassing 30% External Quantum Efficiency for a Deep‐Blue Organic Light‐Emitting Diode

AbstractOvercoming exciton quenching and improving electroluminescence (EL) performances continue to pose persistent challenges in the development of efficient deep blue thermally activated delayed fluorescence (TADF) emitters. Herein, an asymmetric multi‐donors strategy is employed that provides the potential of establishing multiple radiative transition channels for fluorescence emission and reverse intersystem crossing (RISC), while allowing flexible modulation of packing modes in single‐crystal state and optimization of morphology in films by modifying the substitution site of the donor units. As a result, intermolecular π–π interactions are effectively alleviated and a very smooth surface with a reduced defect density is simultaneously achieved, which can significantly reduce the density of triplet excitons, accelerate the charge transfer rate, and enhance carrier injection efficiency. Consequently, a super‐high photoluminescence quantum yield (PLQY) of 99% and fast kRISC of 1.1 × 105 s−1 are concurrently achieved for the proof‐of‐concept emitter 3,4‐Cz‐SF‐SFAC. The corresponding deep‐blue organic light‐emitting diode (OLED) demonstrates an exceptional external quantum efficiency (EQE) of 31.1%, accompanied by an emission peak at 457 nm, one of the preeminent TADF materials within the deep blue region, ranking among the most efficient OLEDs with an EL spectra below 460 nm on record.

Quaternary AlInGaN/AlInGaN Polarization-Engineered Green Lights-Emitting Diodes by Using Inverted Trapezoidal Quantum Barriers

In this paper, a structure design using grading quaternary AlInGaN as inverted trapezoidal multiple quantum barriers is shown. This paper proposes an inverted trapezoidal structure in quantum barriers to decelerate the transmission of the electron which will enhance efficient carrier injection into the quantum wells and get a more uniform distribution of carriers. The polarization-matched AlInGaN/AlInGaN structures have less polarization and increase the overlap of electron and hole wave function, effectively improving the internal quantum efficiency and wall-plug efficiency, and making the In composition adjustable. The results show that the Inverted trapezoidal polarization-matched AlInGaN structure significantly improves the phenomenon of “green gap” and “green droop.” As a result, the new structure mitigates the droop ratio from 25.5% to 7%, and the LED power is increased from 59.95 mW to 109.68 mW at 200 mA. Finally, the device exhibits a uniform current distribution and greater radiative recombination while reducing resistance.

Enhanced temperature stability of threshold current of InAs/GaAs quantum dot lasers by AlGaAs lateral potential barrier layers.

We have investigated the incorporation of an AlGaAs lateral potential barrier layer (LPBL) as a novel approach to improve the temperature stability of the threshold current in InAs/GaAs quantum dot (QD) lasers. This layer serves to increase the energy separation (ΔE) between the ground and excited states of the QD while maintaining efficient vertical carrier injection. Theoretical calculations confirm that the LPBL is effective in increasing ΔE. The LPBLs were successfully formed using the preferential growth properties of AlGaAs induced by the non-uniform distribution of strain effects on the QD surface during molecular beam epitaxy growth. To confirm the usefulness of the LPBLs, we fabricated an InAs/GaAs QD laser incorporating AlGaAs LPBLs, demonstrating that the threshold current at 150°C was significantly reduced by 48% compared to a QD laser without LPBLs. The temperature stabilization achieved by incorporating the LPBLs provides a promising way for establishing high reliability and low power operation of QD lasers in high-temperature environments.

Hole Injection Characteristics and Annealing Temperature Dependence for Organic Light‐Emitting Diodes Using Spontaneous Polarization

An efficient carrier injection at the electrode/organic semiconductor interface of organic light‐emitting diodes (OLEDs) is challenging. It is well‐known that an electric dipole at the electrode/semiconductor interface improves the carrier injection from the electrode because of a vacuum‐level shift. In this study, poly (vinylidene fluoride‐trifluoroethylene) [P(VDF/TrFE)] is used as a polar polymer with spontaneous polarization. P(VDF/TrFE) is spin‐coated on an indium tin oxide (ITO) electrode surface as a hole injection layer for OLEDs. The P(VDF/TrFE) layer enhances the hole injection from the ITO electrode by causing a vacuum‐level shift. As a result, the device driving voltage can be lowered. In addition, annealing temperature dependence is observed. To investigate the origin of the hole injection effect, the P(VDF/TrFE)‐layered ITO surface and the thin film structure of the P(VDF/TrFE) layer are measured. The P(VDF/TrFE) layers at different annealing temperatures show no difference in surface properties but show significant differences in the thin film structure. The hole injection of OLEDs is significantly enhanced because of the structural change in ultrathin P(VDF/TrFE) layer.

High gain hot-carrier WSe2 phototransistor with gate-tunable responsivity

Hot-carrier injection at semiconductor/metal interface shows great potentials in infrared photodetection. However, the photoresponsivity of hot-carrier photodetector with diode mode is still limited in the scale of 1 mA/W due to the low injection efficiency of hot carriers and the lack of gain. Here, we demonstrate a high gain hot-carrier WSe2 phototransistor with gate-tunable responsivity. In this device, plasmonic resonances is used to enhance the light absorption of gold nanodisk, which provide hot holes. The hot holes are trapped into WSe2 and recycled in WSe2 channel in lateral direction, which introduce high gain for photodetection. Experiment shows that the photoresponsivity of the device can be over 0.23 A/W with a gain of 270 at the wavelength of 1310 nm. More interestingly, the responsivity of the device can be tuned by gate, which can be used to encode synaptic weights of the sensor pixel.

Disentangling the Impact of Point Defect Density and Carrier Localization-Enhanced Auger Recombination on Efficiency Droop in (In,Ga)N/GaN Quantum Wells.

The internal quantum efficiency of (In,Ga)N/GaN quantum wells can surpass 90% for blue-emitting structures at moderate drive current densities but decreases significantly for longer emission wavelengths and at higher excitation rates. This latter effect is known as efficiency "droop" and limits the brightness of light-emitting diodes (LEDs) based on such quantum wells. Several mechanisms have been proposed to explain efficiency droop including Auger recombination, both intrinsic and defect-assisted, carrier escape, and the saturation of localized states. However, it remains unclear which of these mechanisms is most important because it has proven difficult to reconcile theoretical calculations of droop with measurements. Here, we first present experimental photoluminescence measurements extending over three orders of magnitude of excitation for three samples grown at different temperatures that indicate that droop behavior is not dependent on the point defect density in the quantum wells studied. Second, we use an atomistic tight-binding electronic structure model to calculate localization-enhanced radiative and Auger rates and show that both the corresponding carrier density-dependent internal quantum efficiency and the carrier density decay dynamics are in excellent agreement with our experimental measurements. Moreover, we show that point defect density, Auger recombination, and the effect of the polarization field on recombination rates only limit the peak internal quantum efficiency to about 70% in the resonantly excited green-emitting quantum wells studied. This suggests that factors external to the quantum wells, such as carrier injection efficiency and homogeneity, contribute appreciably to the significantly lower peak external quantum efficiency of green LEDs.

Formation of Ohmic contacts between ferromagnetic Cr2Ge2Te6 and two-dimensional metals

As a ferromagnetic semiconductor, two-dimensional (2D) Cr2Ge2Te6 holds significant implications for electronic and spintronic devices. To achieve 2D electronics, it is essential to integrate Cr2Ge2Te6 with 2D electrodes to form Schottky-barrier-free Ohmic contacts with enhanced carrier injection efficiency. Herein, by using first-principles calculations based on density-functional theory, we systematically investigate the structural, energetic, electronic and magnetic properties of 2D heterojunctions by combining Cr2Ge2Te6 with a series of 2D metals, including graphene, ZrCl, NbS2, TaS2, TaSe2, Zn3C2, Hg3C2, and Zr2N. Results show that NbS2, TaS2, TaSe2, Zn3C2, Hg3C2, and Zr2N form Ohmic contacts with Cr2Ge2Te6, in contrast to graphene and ZrCl that exhibit a finite Schottky barrier. By examining the tunneling barriers and Fermi level shift, we reveal that the heterojunctions with Zn3C2 and Hg3C2 as electrodes exhibit advantages of both high electron injection efficiency and spin injection efficiency, for which an apparent decrease of the magnetic moment of Cr atoms in Cr2Ge2Te6 can be observed. These findings not only provide physical insights into the role of interfacial interaction in regulating the physical properties of 2D heterojunctions, but also pave way for the development of high-performance spintronic nanodevices for practical implementation.

Photoluminescence characterization of interlayer carrier injection from InGaAs quantum well to InGaAs surface quantum dots with respect to GaAs spacer thickness

This work exploits carrier injection hybrid structures in which carriers are injected into a layer of In0.4Ga0.6As surface quantum dots (SQDs) from an adjacent In0.15Ga0.85As quantum well (QW) as a function of spacer thickness from 10 nm down to 2.5 nm. Photoluminescence (PL) measurements verify that all such hybrid structures indeed have carriers collected into the QW and subsequently obtain an enhancement for PL intensity over that of the reference SQDs. The hybrid structure with the 2.5 nm spacer obtains the best carrier injection efficiency, due to the strongest coupling between the QW and the SQDs, while a thicker spacer results in less carrier injection from decreased quantum tunneling. However, the carrier injection is less efficient than expected. This is due to the fact that the QW confined energy states line up with the broad wetting layer (WL) energy states of SQDs of our test samples, leading to resonant carrier tunneling from the QW to the WL. Thus, there is significant carrier loss through tunneling into the WL of SQDs and then to surface states via nonradiative recombination. This characteristic must be considered in the design of surface sensitive detection devices using SQD injection structures.

Thickness dependence of magnetic and magnetotransport properties of textured Fe3O4 ferrimagnetic films on SiO2/n-Si(001)

The efficient injection of spin-polarized charge carriers from a ferro- or ferrimagnet into a semiconductor is an urgent problem. Half-metal magnetite (Fe3O4) is an attractive candidate for the role of a spin injector in spintronic devices. Here, the influence of the thickness of magnetite films on their electrical, magnetic, and magnetotransport properties is presented. Polycrystalline magnetite films with (311)-texture and thickness ranging from 15 to 250 nm are grown on the oxidized silicon surface by reactive iron deposition in the molecular oxygen atmosphere. The saturation magnetization is found to increase nonmonotonically with the film thickness and reaches its maximum value at 170 nm, which is close to that of a single crystal (∼480 emu/cm3). It is shown that for films with a thickness of more than 150 nm, there is a rapid decrease in resistivity by almost three times. The critical film thickness at which the coercive force decreases to the lowest value of ∼250 Oe is 100 nm. The magnetoresistance value reaches − 2.5 % already at a thickness of 50 nm and remains almost unchanged with a subsequent increase in the thickness of the textured film. Since, at a small thickness, textured films have higher magnitudes of conductivity and magnetization than conventional polycrystalline films, the textured-F3O4/SiO2/n-Si(001) junction with amorphous SiO2 barrier is promising for Si-based spintronic devices.

UV and solar-based photocatalytic degradation of organic pollutants from ceramics industrial wastewater by Fe-doped ZnS nanoparticles

UV and solar-based photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) as an organic contaminant in ceramics industry wastewater by ZnS and Fe-doped ZnS NPs was the focus of this research. Nanoparticles were prepared using a chemical precipitation process. The cubic, closed-packed structure of undoped ZnS and Fe-doped ZnS NPs was formed in spherical clusters, according to XRD and SEM investigations. According to optical studies, the optical band gaps of pure ZnS and Fe-doped ZnS nanoparticles are 3.35 and 2.51 eV, respectively, and Fe doping increased the number of carriers with high mobility, improved carrier separation and injection efficiency, and increased photocatalytic activity under UV or visible light. Doping of Fe increased the separation of photogenerated electrons and holes and facilitated charge transfer, according to electrochemical impedance spectroscopy investigations. Photocatalytic degradation studies revealed that in the present pure ZnS and Fe-doped ZnS nanoparticles, 100% treatment of 120 mL of 15 mg/L phenolic compound was obtained after 55- and 45-min UV-irradiation, respectively, and complete treatment was attained after 45 and 35-min solar light irradiation, respectively. Because of the synergistic effects of effective surface area, more effective photo-generated electron and hole separation efficiency, and enhanced electron transfer, Fe-doped ZnS demonstrated high photocatalytic degradation performance. The study of Fe-doped ZnS's practical photocatalytic treatment capability for removing 120 mL of 10 mg/L 2,4-DCP solution made from genuine ceramic industrial wastewater revealed Fe-doped ZnS's excellent photocatalytic destruction of 2,4-DCP from real industrial wastewater.

83‐2: Inorganic ion treatment of Cd‐free quantum dots and applications to QD‐LED with improved characteristics

Inorganic ion treatment for Cd‐free colloidal quantum dot (QD) layers improved their carrier injection efficiency, resulting in improved external quantum efficiency (EQE) and electroluminescence (EL) lifetime of the QD light‐emitting diode (QD‐LED). Inorganic ion treated QD layers patterned at 176 ppi by photolithography enabled emission of each primary color.

More comprehensive heterojunction mechanism: Enhanced PEC properties originated from novel ZnIn2S4/Cu2S heterojunction assisted by changed surface states

The more comprehensive investigation of ZnIn2S4-based heterojunction mechanism is beneficial for rationalizing the PEC performances enhancement mechanism and optimizing the application of heterojunction. Herein, Cu2S is for the first time synthesized on ZnIn2S4 as a novel co-catalyst through in-situ ion exchange techniques. The holes are extracted and stored in ZnIn2S4/Cu2S through enhanced surface states (ss) due to the decreased holes transfer resistance to ss and improved ss capacitance, which improve the carrier separation efficiencies with the effect of built-in electric filed. With the sequential deposition of NiFe-LDH, despite the inevitable carrier recombination is caused by the conflicting chemical potentials between Cu and Ni ions at the Cu2S/NiFe-LDH interface, the carrier injection efficiency is increased to 71.60% due to the exposure of more intermediate surface states (iss). The highest photocurrent density of 1.56 mA/cm2 at 1.23 V (vs. RHE) is achieved by ZnIn2S4/Cu2S/NiFe-LDH when influence of the interface carrier recombination is counteracted by the synergistic effect containing extended light absorption, the construction of ZnIn2S4/Cu2S heterojunction and changed ss, which is 10.4 times higher than that of pristine ZnIn2S4. The work provides a more comprehensive approach to investigate the mechanism for enhanced PEC performances of ZnIn2S4-based heterojunction.

Single‐Crystal Perovskite Light‐Emitting Diodes with External Quantum Efficiency of over 8% Enabled by Nonstoichiometric Composition Tuning

AbstractPerovskite single crystals (SCs) have been emerging as promising materials for electroluminescence (EL) device applications owing to their superior optoelectronic properties. However, the device performance of single‐crystal perovskite light‐emitting diodes (SC‐PeLEDs) is limited by the lack of effective defect management and energy level modulation. Here, a nonstoichiometric composition tuning (NCT) strategy for the construction of high‐performance SC‐PeLEDs is reported. The NCT strategy, finely tuning the MABr (MA+ = CH3NH3+) excess in the nonstoichiometric MAPbBr3 SCs to enhance the crystal quality, reduce the trap density, and elevate the energy level of the resultant perovskite SC, enables markedly decreased nonradiative recombination and more efficient carrier injection for the devices. In consequence, the optimized SC‐PeLEDs exhibit an ultrahigh peak luminance of 161 900 cd m−2 and a large external quantum efficiency (EQE) of up to 8.1%, representing the most efficient SC‐PeLED reported thus far. This strategy also shows a high universality in enhancing the EL performance of other lead bromide perovskite SC‐PeLEDs, endowing them with luminances and EQEs three times larger than those of the pristine devices. This work opens an avenue for the development of high‐performance EL devices based on perovskite SCs.

Operation-induced degradation mechanisms of 275-nm-band AlGaN-based deep-ultraviolet light-emitting diodes fabricated on a sapphire substrate

Degradation mechanisms of 275-nm-band AlxGa1-xN multiple quantum well deep-ultraviolet light-emitting diodes fabricated on a (0001) sapphire substrate were investigated under hard operation conditions with the current of 350 mA and the junction temperature of 105 °C. The optical output power (Po) initially decreased by about 20% within the operating time less than 102 h and then gradually decreased to about 60% by 484 h. For elucidating the causes for the initial and subsequent degradations, complementary electrical, time-resolved photoluminescence (TRPL), and impurity characterizations were carried out making a connection with the energy band profiles. Because the degradation of the wells was less significant than the Po reduction, the initial degradation is attributed essentially to the decrease in carrier injection efficiency (ηinjection), not in internal quantum efficiency of the wells, most likely due to depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs) in a Mg-doped p-type Al0.85Ga0.15N electron blocking layer. The principal cause for the subsequent Po reduction until 484 h is attributed to further decrease in ηinjection due to the appearance of certain current bypasses in addition to continuous depassivation of the NRCs in p-type AlxGa1-xN layers. According to our database on the species of vacancy-type defects acting as NRCs in GaN and AlN, which have been identified using the combination of positron annihilation and TRPL measurements, vacancy clusters comprised of a cation vacancy (VIII) and nitrogen vacancies (VN), such as VIIIVN2∼4, are the most suspicious origins of the NRCs in Mg-doped p-type AlxGa1-xN layers.

GeSn/Ge Multiquantum-Well Vertical-Cavity Surface-Emitting p-i-n Structures and Diode Emitters on a 200 mm Ge-on-Insulator Platform

An efficient monolithically integrated light source with complementary metal-oxide-semiconductor (CMOS) compatibility remains the missing component to enable Si photonics for various applications. In particular, vertical-cavity-surface-emitting (VCSE) light sources, such as resonant cavity light-emitting diodes (RCLEDs) and vertical-cavity surface-emitting lasers (VCSELs), are strong contenders due to their compact size, circular emission profile with low beam divergence, wafer-scale fabrication compatibility, high bandwidth, and high coupling efficiency to fiber optic cables. We report the first demonstration of 8-in. wafer-scale GeSn/Ge multiple-quantum-well VCSE p-i-n structures and diodes for laser or light-emitting diode (LED) applications in Si photonics by wafer bonding and layer transfer techniques, which are challenging for all-epitaxy routes. The wafer could be 12 in. (300 mm) since the MQW gain medium is grown on a 300 mm Si wafer, and the bonding is wafer-scale. Alternative dielectric layers (SiO2/SiN/SiO2), introduced by wafer bonding, under the emitting structure, serve as the bottom mirror for the vertical cavity. The Ge0.92Sn0.08/Ge MQW layer is utilized to improve the material quality and to confine injected carriers. As a result, more than 8× enhancement of light emission due to the vertical cavity resonance was demonstrated by photoluminescence spectroscopy. Besides, the spectral purity is enhanced by the single-longitudinal-mode cavity. The intensity of light emission is insensitive to the temperature range from 4 to 300 K and even becomes stronger at higher temperatures since the stronger resonant cavity effect at higher temperatures compensates for the drop in light emission efficiency caused by nonradiative centers. The vertical cavity effect on the light emission is further verified by reflectivity spectroscopy and optical simulations. A positive gain can be achieved as indicated by optical gain calculations. Room-temperature electroluminescence and an excellent carrier injection efficiency of the MQW VCSE diode are observed, showing its potential for electrically injected RCLEDs and VCSELs.