External Differential Quantum Efficiency Research Articles

Development of High‐Reflectivity and Antireflection Dielectric Multilayer Mirrors for AlGaN‐Based Ultraviolet‐B Laser Diodes and their Device Applications

Fabrication techniques for high‐reflectivity (HR) and antireflection (AR) dielectric multilayer mirrors for AlGaN‐based ultraviolet‐B (UV‐B) laser diodes are developed. After depositing several dielectric materials and evaluating their complex refractive indices via ellipsometry, it is determined that SiO2 as a low‐refractive‐index material and Ta2O5 as a high‐refractive‐index material are appropriate material combinations in the UV‐B region at a light wavelength of ≈300 nm due to their low extinction coefficients and large refractive index difference. Based on these results, HR mirror with a reflectance of >99% in the UV‐B region at a center wavelength of 310 nm and an AR mirror with a reflectance of ≈8% in the same wavelength range are demonstrated; a mirror with reflectance that is almost equal to the designed value is demonstrated. Furthermore, these mirrors are coated on the respective edge surfaces of the UV‐B laser diodes. A comparison of the characteristics of the same device before and after edge coating reveals a reduction in the threshold current density of laser oscillation, whereas, simultaneously, an increase in slope efficiency and external differential quantum efficiency is observed. The improvement of these device characteristics, estimated from the above reflectance values, is confirmed to be almost theoretically explainable.

Effect of Ridge Width on the Lasing Characteristics of Triangular and Rectangular InAs/In0.53Ga0.47As Quantum Well Lasers

The lasing characteristics of InP-based InAs/In0.53Ga0.47As quantum well (QW) lasers with different ridge widths are investigated. Two groups of lasers are grown for comparison, one with active triangular QW regions and the other with rectangular QW regions. Their output powers, characteristic temperatures (T0), external differential quantum efficiencies (ηd) and junction temperatures (Tj) are analyzed and compared. The parameter of ridge width is found to play an important role in the performance of the lasers. In triangular QW lasers, by broadening the ridge width from 8 to 12 μm, output power and ηd of the lasers are decreased for the temperature range of 100–320 K due to heating effect. But by broadening the ridge width from 8 to 100 μm in rectangular QW lasers, output power has about 3.5 time increase at 100 K and ηd also has a little increase for temperatures from 100 to 180 K due to much larger emission area and much faster heat dissipation. Tj, the real temperature of the active region, is also found to have accelerated increase at high injection current and heat sink temperature. Besides, compared to the rectangular QW laser of the same ridge width, the improved thermal performance of triangular QW laser is also demonstrated.

Impact of the ridge etching‐depth on GaSb‐based laser diodes

We have fabricated from the same epitaxial wafer series of GaSb-based Fabry–Pérot laser diodes emitting near 2.3 μm with different ridge etching-depths. The analysis of the device performances allows quantifying the detrimental impact of deep ridge etching. The threshold current density is increased, whereas the external differential quantum efficiency is reduced, due to a reduced internal quantum efficiency and higher optical losses.

Analysis of carrier injection efficiency of AlGaN UV-B laser diodes based on the relationship between threshold current density and cavity length

The carrier injection efficiency η i of recently developed UV-B laser diodes (LDs) is estimated on the basis of fundamental calculations and measurements. A general procedure to estimate η i based on the relationship between the differential external quantum efficiency and the cavity length proves to be inadequate because of the large variation. To estimate η i without relying on the unstable output power, we analyze in detail how the threshold current density relates to the cavity length. By applying a light confinement factor of 3.5%, an internal loss of 10 cm−1, a current density of 0.56 kA cm−2 in the emission layers at zero gain, and a reflectivity of the mirror facet of 0.16, we estimate η i ≈ 3.5% for UV-B LDs. This low η i in UV-B LDs is due to unbalanced injection between electron and hole currents, which leads to electron overflow to the p-GaN side, as indicated by a simulation.

Experimental demonstration of energy-transfer ratchet intermediate-band solar cell

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediate-band solar cells (IBSCs) under maximum sunlight concentration. However, an actual IBSC device with an efficiency larger than the Shockley-Queisser (SQ) limit has so far not been reported. The main difficulties lie in realizing an efficient sequential two-photon absorption (STPA) which requires a sufficiently long lifetime intermediate state or intermediate band. In this article, we propose the concept of a ratchet type IBSC, utilizing a long lifetime of rare-earth ion luminescence centers in Erbium-doped GaAs. The temperature dependent differential external quantum efficiency reveals a significant STPA contribution originating from the Er3+ luminescence center. All the results were modeled and interpreted by integrating the ratchet effect with up-conversion along with a density functional theory (DFT) simulation. Our work demonstrates that the long lifetime energy-transfer mechanism in Er3+ centers contributes directly to the formation of a ratchet type IB.

Nano-height cylindrical waveguide in GaN-based vertical-cavity surface-emitting lasers

We have proposed a novel nano-height cylindrical waveguide in blue GaN-based vertical-cavity surface-emitting lasers (VCSELs). The proposed 5 nm step height cylindrical waveguide using simple processes provides lateral optical confinement and excellent performance in terms of light output power (LOP) and wall plug efficiency (WPE). A fabricated multi-mode VCSEL with 7 μm emission diameter achieved a LOP of 23.7 mW, maximum external differential quantum efficiency of 43.6%, and WPE of 10% under continuous wave operation at 20 °C. Additionally, a single-mode VCSEL with 3.3 μm emission diameter exhibited LOP of more than 5 mW and WPE of 9.9%.

Optical Gain and Lasing Properties of InP/AlGaInP Quantum-Dot Laser Diode Emitting at 660 nm

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al 0.10 Ga 0.90 ) 0.52 In 0.48 P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm 2 , an internal optical loss value of 5 ± 2 cm -1 and a peak modal gain of 39.3 cm -1 corresponding to a material gain of approximately 2675 cm -1 per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm 2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm 2 for a single layer of QDs were also demonstrated.

Effect of Epitaxial-Structure Design and Growth Parameters on the Characteristics of Metamorphic Lasers of the 1.46-μm Optical Range Based on Quantum Dots Grown on GaAs Substrates

The characteristics of lasers of the 1.44–1.46-μm optical range grown on GaAs substrates using a metamorphic buffer are studied. The active region of the laser contains 10 rows of InAs/In0.4Ga0.6As/In0.2Ga0.8As quantum dots. It is shown that the use of special selective high-temperature annealing along with the application of short-period In0.2Ga0.8As/In0.2Al0.3Ga0.5As short-period superlattices makes it possible to substantially decrease the density of threading dislocations in the active region. A threshold current density of 1300 A cm–2, external differential quantum efficiency of 38%, and maximal pulsed-mode output power of 13 W are attained for a laser with a broad area 3 mm in length.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.

Enhancement of slope efficiency and output power in GaN-based vertical-cavity surface-emitting lasers with a SiO2-buried lateral index guide

We have achieved a high output power of 6 mW from a 441 nm GaN-based vertical-cavity surface-emitting laser (VCSEL) under continuous wave (CW) operation, by reducing both the internal loss and the reflectivity of the front cavity mirror. A preliminary analysis of the internal loss revealed an enormously high transverse radiation loss in a conventional GaN-based VCSEL without lateral optical confinement (LOC). Introducing an LOC structure enhanced the slope efficiency by a factor of 4.7, with a further improvement to a factor of 6.7 upon reducing the front mirror reflectivity. The result was a slope efficiency of 0.87 W/A and an external differential quantum efficiency of 32% under pulsed operation. A flip-chip-bonded VCSEL also exhibited a high slope efficiency of 0.64 W/A and an external differential quantum efficiency of 23% for the front-side output under CW operation. The reflectivity of the cavity mirror was adjusted by varying the number of AlInN/GaN distributed Bragg reflector pairs from 46 to 42, corresponding to reflectivity values from 99.8% to 99.5%. These results demonstrate that a combination of internal loss reduction and cavity mirror control is a very effective way of obtaining a high output GaN-based VCSEL.

Suppressed power saturation due to optimized optical confinement in 9xx nm high-power diode lasers that use extreme double asymmetric vertical designs

Broad area lasers with novel extreme double asymmetric structure (EDAS) vertical designs featuring increased optical confinement in the quantum well, Γ, are shown to have improved temperature stability without compromising series resistance, internal efficiency or losses. Specifically, we present here vertical design considerations for the improved continuous wave (CW) performance of devices operating at 940 nm, based on systematically increasing Γ from 0.26% to 1.1%, and discuss the impact on power saturation mechanisms. The results indicate that key power saturation mechanisms at high temperatures originate in high threshold carrier densities, which arise in the quantum well at low Γ. The characteristic temperatures, T0 and T1, are determined under short pulse conditions and are used to clarify the thermal contribution to power limiting mechanisms. Although increased Γ reduces thermal power saturation, it is accompanied by increased optical absorption losses in the active region, which has a significant impact on the differential external quantum efficiency, . To quantify the impact of internal optical losses contributed by the quantum well, a resonator length-dependent simulation of is performed and compared to the experiment, which also allows the estimation of experimental values for the light absorption cross sections of electrons and holes inside the quantum well. Overall, the analysis enables vertical designs to be developed, for devices with maximized power conversion efficiency at high CW optical power and high temperatures, in a trade-off between absorption in the well and power saturation. The best balance to date is achieved in devices using EDAS designs with , which deliver efficiencies of 50% at 14 W optical output power at an elevated junction temperature of 105 °C.

Влияние конструкции эпитаксиальной структуры и параметров роста на характеристики метаморфных лазеров оптического диапазона 1.46 мкм на основе квантовых точек на положках GaAs

AbstractThe characteristics of lasers of the 1.44–1.46-μm optical range grown on GaAs substrates using a metamorphic buffer are studied. The active region of the laser contains 10 rows of InAs/In_0.4Ga_0.6As/In_0.2Ga_0.8As quantum dots. It is shown that the use of special selective high-temperature annealing along with the application of short-period In_0.2Ga_0.8As/In_0.2Al_0.3Ga_0.5As short-period superlattices makes it possible to substantially decrease the density of threading dislocations in the active region. A threshold current density of 1300 A cm^–2, external differential quantum efficiency of 38%, and maximal pulsed-mode output power of 13 W are attained for a laser with a broad area 3 mm in length.

Highly efficient red single transverse mode superluminescent diodes

Optimisation of the epitaxial growth of AlGaInP/GaInPAs nanoheterostructures and improvement of the technologies of active channel formation and p-contact deposition made it possible to considerably increase the external differential quantum efficiency (up to 0.5 mW mA−1), the catastrophic optical degradation threshold (up to 40 mW), and the spectral width (to FWHM exceeding 15 nm) of single transverse mode superluminescent diodes with the centre wavelength of about 675 nm. Lifetime tests demonstrated high reliability of these diodes at a cw output optical power up to 30 mW.

High-Efficiency Operation of Membrane Distributed-Reflector Lasers on Silicon Substrate

To advance on-chip optical interconnections, membrane distributed-reflector (DR) lasers with low threshold current and high-efficiency operation at one side output were realized. First, a membrane distributed Bragg reflector (DBR) laser with 80- μ m-long active section and 50- μ m-long DBR section was fabricated to clarify the DBR reflectivity. An external differential quantum efficiency of 35% for the output from the front facet was obtained, and the DBR reflectivity was estimated to be 75%. Next, a membrane DR laser with 61- μ m-long distributed feedback section and 50- μ m-long DBR section was fabricated. A threshold current of 0.48 mA, external differential quantum efficiency from the front side waveguide of 26%, and light output ratio from the front to the rear sides of 13 were obtained. The lasing spectrum showed a single-mode operation with a side-mode suppression-ratio (SMSR) of 40 dB. Finally, small-signal direct modulation was carried out and a modulation current efficiency factor of 7.9 GHz/mA1/2 and 7 GHz/mA1/2 were, respectively, obtained for the 30- μ m-long and 61- μ m-long devices.

High Efficiency Operation of GaInAsP/InP Membrane Distributed-Reflector Laser on Si

Record high power conversion efficiency operation of a GaInAsP/InP membrane distributed-reflector laser on silicon substrate was realized. In order to enhance the efficiency, both an improvement of external differential quantum efficiency and a reduction of differential resistance were achieved. As a result, an external differential quantum efficiency from the front facet of 36%, and a maximum power conversion efficiency of 14.6% were obtained for a device with a 40- $\mu \text{m}$ -long active region.

InP-based pseudomorphic InAs/InGaAs triangular quantum well lasers with bismuth surfactant.

An InP-based 2.1μm InAs/In0.53Ga0.47As triangular quantum well laser grown with Bi surfactant has shown improved performance in comparison to the device with the same structure but grown without Bi surfactant. Under continuous-wave driving operation, the output light power is increased from 32.6 to 37.5mW at the same injecting current of 850mA at 200K. The external differential and internal quantum efficiencies for the laser with Bi surfactant are 18.4% and 41%, respectively, which are correspondingly higher than 13.1% and 31% for the reference device. Furthermore, a decreased internal loss from 20.9 to 17.6 cm-1 for the Bi surfactant laser is also observed. These results suggest that Bi surfactant is promising for further enhancing performances of strained quantum well laser diodes.

Spectrally resolved nonlinearity and temperature dependence of perovskite solar cells

An accurate electrical device characterization of hybrid organic-inorganic halide perovskite solar cells (PSCs) is an important prerequisite for further improvement and industrial transfer of this promising photovoltaic technology. In this work, we study the nonlinearity of current versus irradiance as well as the temperature dependence of PSCs in a spectrally resolved manner by highly accurate differential external quantum efficiency (EQE) measurements. We investigate three different types of PSCs fabricated by different research groups. The nonlinearity of all samples is found to be spectrally invariant, which significantly simplifies spectral mismatch corrections. We demonstrate that misinterpretation of EQE measurements can result in a more than 10% relative error in efficiency measurements, if solar simulators are adjusted to photocurrents determined from differential EQEs. For obtaining an accurate integrated photocurrent from EQEs, we introduce a new, convenient approach that accounts for cell nonlinearities but avoids the time-consuming full analysis of spectrally resolved nonlinearity. Moreover, for the samples investigated here, it is shown that the differential EQE measured at 0.35 suns bias irradiance represents a reasonably good estimate of the actual EQE at 1 sun. Furthermore, we determine spectrally resolved temperature coefficients (TCs) and show how the band gap blue shift varies with perovskite absorber and temperature.

Introduction of AlInAs-oxide current-confinement structure into GaInAsP/SOI hybrid Fabry–Pérot laser

An AlInAs-oxide current-confinement structure was introduced into a III–V/Si-on-insulator (SOI) hybrid laser for realizing an efficient light source composed of III–V/Si hybrid photonic integrated circuits. Lasing of the hybrid Fabry–Pérot laser was confirmed after introduction of the confinement structure providing both optical and electrical confinement. We determined the lasing characteristics of the designed hybrid laser, which showed a low threshold current and high external differential quantum efficiency comparable to that of conventional lasers fabricated on III–V compound semiconductor wafers. Comparison of the photoluminescence spectra of the III–V/SOI hybrid wafers before and after the process proved that oxidation had no influence on the active layer. The threshold current and external differential quantum efficiency of the fabricated laser with a stripe width of 4.5 µm and cavity length of 500 µm were obtained as 50 mA and 11%/facet, respectively.

High-differential quantum efficiency operation of GaInAsP/InP membrane distributed-reflector laser on Si

The high-external differential quantum efficiency operation of a GaInAsP/InP membrane distributed-reflector laser bonded on a Si substrate was achieved by adopting a short cavity design and reducing the waveguide loss and differential resistance. A threshold current of 0.21 mA, an external differential quantum efficiency of 32% for the front-side output, and a power-conversion efficiency of 12% were obtained with a 32-µm distributed feedback section length, a 50-µm distributed-Bragg-reflector section, and a 0.8-µm stripe width. A side-mode suppression ratio of 41 dB was obtained at a bias current of 1 mA.

Over 1 W record-peak-power operation of a 338 nm AlGaN multiple-quantum-well laser diode on a GaN substrate

We have demonstrated the high-peak-power operation of an AlGaN-based ultraviolet laser diode (UV-LD) with a lasing wavelength of 338.6 nm. The UV-LD structure was fabricated on a bulk GaN(0001) substrate. The broad-area and vertical conductive structure of the UV-LD, whose ridge width and cavity length were 50 and 600 µm, respectively, was employed. The threshold current density and differential external quantum efficiency were estimated to be 38.9 kA/cm2 and 8.5%, respectively. The characteristic temperature of threshold current was estimated to be 119 K, and the temperature dependence of lasing wavelength was obtained to be 0.033 nm K−1. A peak power of over 1 W has been achieved in 338.6 nm under pulsed operation at room temperature, which is the highest peak power ever obtained for AlGaN-based UV-LDs.