High Injection Current Densities Research Articles

Fundamental transverse mode selection and self-stabilization in large optical cavity diode lasers under high injection current densities

It is shown that in high-power, large optical cavity laser diodes at high injection currents, the optical losses due to nonuniform carrier accumulation in the optical confinement layer can ensure the laser operation in the fundamental transverse mode. An experimental demonstration of switching from second order mode to fundamental mode in large optical cavity lasers with current and/or temperature increase is reported and explained, with the calculated values for the switching current and temperature in good agreement with the measurements. The results experimentally prove the nonuniform nature of carrier accumulation in the confinement layer and may aid laser design for optimizing the output.

Enhanced performance of GaN-based light-emitting diodes by using a p-InAlGaN/GaN superlattice as electron blocking layer

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) with various kinds of electron blocking layers (EBLs) are analyzed numerically. The results indicate that an enhanced hole injection efficiency and a reduced electron leakage could be achieved with the GaN-based LED where a p-InAlGaN/GaN superlattice (SL) was employed as EBL as compared with the conventional GaN-based LEDs using rectangular p-AlGaN EBL or p-AlGaN/GaN SL EBL. Moreover, it was found that the efficiency droop could be significantly improved at high injection current density for GaN-based LEDs with p-InAlGaN/GaN SL EBL.

16 kV, 1 cm<sup>2</sup>, 4H-SiC PiN Diodes for Advanced High-Power and High-Temperature Applications

In this work, we report our recently developed 16 kV, 1 cm2, 4H-SiC PiN diode results. The SiC PiN diode was built on a 120 µm, 2×1014/cm3 doped n-type SiC drift layer with a device active area of 0.5175 cm2. Forward conduction of the PiN diode was characterized at temperatures from 20°C to 200°C. At high injection-current density (JF) of 350 ~ 400 A/cm2, the differential on-resistance (RON,diff) of the SiC PiN diode decreased from 6.08 mΩ·cm2 at 20°C to 5.12 mΩ·cm2 at 200°C, resulting in a very small average temperature coefficient of –5.33 µΩ·cm2/°C, while the forward voltage drop (VF) at 100 A/cm2 reduced from 4.77 V at 20°C to 4.17 V at 200°C. This is due to an increasing high-level carrier lifetime with an increase in temperature, resulting in reduced forward voltage drop. We also observed lower RON,diff at higher injection-current densities, suggesting that a higher carrier lifetime is needed in this lightly doped n-type SiC thick epi-layer in order to achieve full conductivity modulation. The anode to cathode reverse blocking leakage current was measured as 0.9 µA at 16 kV at room temperature.

15 kV, Large Area (1 cm<sup>2</sup>), 4H-SiC p-Type Gate Turn-Off Thyristors

In this paper, we report our recently developed 1 cm2, 15 kV SiC p-GTO with an extremely low differential on-resistance (RON,diff) of 4.08 mΩ•cm2 at a high injection-current density (JAK) of 600 ~ 710 A/cm2. The 15 kV SiC p-GTO was built on a 120 μm, 2×1014/cm3 doped p-type SiC drift layer with a device active area of 0.521 cm2. Forward conduction of the 15 kV SiC p-GTO was characterized at 20°C and 200°C. Over this temperature range, the RON,diff at JAK of 600 ~ 710 A/cm2 decreased from 4.08 mΩ•cm2 at 20°C to 3.45 mΩ•cm2 at JAK of 600 ~ 680 A/cm2 at 200°C. The gate to cathode blocking voltage (VGK) was measured using a customized high-voltage test set-up. The leakage current at a VGK of 15 kV were measured 0.25 µA and 0.41 µA at 20°C and 200°C respectively.

Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes

The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (μLEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller μLEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller μLEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densities.

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

AbstractIt is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–104 times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.

Capture of charge carriers and output power of a quantum well laser

The effect of noninstantaneous carrier capture by a nanoscale active region on the power characteristics of a semiconductor laser is studied. A laser structure based on a single quantum well is considered. It is shown that delayed carrier capture by the quantum well results in a decrease in the internal differential quantum efficiency and sublinearity of the light-current characteristic of the laser. The main parameter of the developed theoretical model is the velocity of carrier capture from the bulk (waveguide) region to the two-dimensional region (quantum well). The effect of the capture velocity on the dependence of the following laser characteristics on the pump current density is studied: the output optical power, internal quantum efficiency of stimulated emission, current of stimulated recombination in the quantum well, current of spontaneous recombination in the optical confinement layer, and carrier concentration in the optical confinement layer. A decrease in the carrier capture velocity results in a larger sublinearity of the light-current characteristic, which results from an increase in the injection current fraction expended to parasitic spontaneous recombination in the optical confinement layer and, hence, a decrease in the injection current fraction expended to stimulated recombination in the quantum well. A comparison of calculated and experimental light-current characteristics for a structure considered as an example shows that good agreement between them (up to a very high injection current density of 45 kA/cm2) is attained at a capture velocity of 2 × 106 cm/s. The results of this study can be used to optimize quantum well lasers for generating high optical powers.

Ultraviolet electroluminescence from hybrid inorganic/organic ZnO/GaN/poly(3-hexylthiophene) dual heterojunctions

Based on hybrid inorganic/organic n-ZnO nanorods/p-GaN thin film/poly(3-hexylthiophene)(P3HT) dual heterojunctions, the light emitting diode (LED) emits ultraviolet (UV) radiation (370 nm - 400 nm) and the whole visible light (400 nm -700 nm) at the low injection current density. Meanwhile, under the high injection current density, the UV radiation overwhelmingly dominates the room-temperature electroluminescence spectra, exponentially increases with the injection current density and possesses a narrow full width at half maximum less than 16 nm. Comparing electroluminescence with photoluminescence spectra, an enormously enhanced transition probability of the UV luminescence in the electroluminescence spectra was found. The P3HT layer plays an essential role in helping the UV emission from p-GaN material because of its hole-conductive characteristic as well as the band alignment with respect to p-GaN. With our new finding, the result shown here may pave a new route for the development of high brightness LEDs derived from hybrid inorganic/organic heterojuctions.

The charging stability of different silica glasses studied by measuring the secondary electron emission yield

This paper reports that the charging properties of lead silica, Suprasil silica and Infrasil silica are investigated by measuring the secondary electron emission (SEE) yield. At a primary electron beam energy of 25 keV, the intrinsic SEE yields measured at very low injection dose are 0.54, 0.29 and 0.35, respectively for lead silica, Suprasil and Infrasil silica glass. During the first e-beam irradiation at a high injection current density, the SEE yields of lead silica and Suprasil increase continuously and slowly from their initial values to a steady state. At the steady state, the SEE yields of lead silica and Suprasil are 0.94 and 0.93, respectively. In Infrasil, several charging and discharging processes are observed during the experiment. This shows that Infrasil does not reach its steady state. Two hours later, all samples are irradiated again in the same place as the first irradiation at a low current density and low dose. The SEE yields of lead silica, Suprasil and Infrasil are 0.69, 0.76 and 0.55, respectively. Twenty hours later, the values are 0.62, 0.64 and 0.33, respectively, for lead silica, Suprasil and Infrasil. These results show that Infrasil has poor charging stability. Comparatively, the charging stability of lead silica is better, and Suprasil has the best characteristics.

Broadband near-IR double quantum-well heterostructure superluminescent diodes

Superluminescent diodes (SLDs) based on double quantum-well (AlGa)As/GaAs heterostructures emitting between 800 and 900 nm are studied experimentally. These SLDs provide a high enough output power in the emission bandwidth more than 55 nm at much shorter active channel lengths than SLDs based on similar single quantum-well heterostructures. Despite the high injection current density, the service life of the SLDs exceeds 10000 hours.

Improved performance of GaInN based deep green light emitting diodes through V‐defect reduction

AbstractThe strong correlation between V‐defect density in the active region and light output power of GaInN based green and deep green light emitting diodes (LEDs) has been evaluated. The formation of such defects is confirmed as the major cause for rough growth morphology, drift of well and barrier widths, emission linewidth broadening, large color shift with current density, and thus poor light emission performance. We have optimized the epitaxial processes for the active region and successfully eliminated such defects from the entire device structure. The electroluminescence intensities from these V‐defect‐free devices with dominant wavelengths ranging from 520 to 570 nm, improve by a factor of two under high injection current density up to 100 A/cm2. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and characterization of circular geometry InP/InGaAs double heterojunction bipolar transistors

An MOCVD-grown circular geometry InP/InGaAs double heterojunction bipolar transistor utilizing a novel δ+-layer composite collector structure was fabricated. A breakdown voltage of 12 V and an offset voltage of just 100 mV were achieved and high injection current densities over 3.2 × 105 A cm−2, which suggest no significant current blocking related to the wide bandgap InP collector layers. A cut-off frequency of 42 GHz was measured at a collector current density of 1.1 × 106 A cm−2.

Fabrication and characterization of circular geometry InGaP/GaAs double heterojunction bipolar transistors

An MOCVD-grown circular geometry InGaP/GaAs double heterojunction bipolar transistor with a novel δ+-layer composite collector structure was fabricated. A breakdown voltage of 13 V and an offset voltage of just 110 mV were achieved as well as high injection current densities over 3.2 × 105 A cm−2, which suggest no significant current blocking related to the wide bandgap InGaP collector layers. A cut-off frequency of 35 GHz was measured at a collector current density of 2.4 × 105 A cm−2.

Point Normal Metal-Superconductor (NS) Contact in Nonballistic Regime

We analyze the point NS contact conductivity taking into account the depression of superconductivity at high-injection current density and Andreev reflection at the adaptive NS boundary. The dependence of the excess current on the voltage, as well as conductivity of contact at arbitrary voltage is obtained.

Internal efficiency of semiconductor lasers with a quantum-confined active region

We discuss in detail a new mechanism of nonlinearity of the light-current characteristic (LCC) in heterostructure lasers with reduced-dimensionality active regions, such as quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs). It arises from: 1) noninstantaneous carrier capture into the quantum-confined active region and 2) nonlinear (in the carrier density) recombination rate outside the active region. Because of 1), the carrier density outside the active region rises with injection current, even above threshold, and because of 2), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency /spl eta//sub int/ that holds true for QD, QWR, and QW lasers. This expression directly relates the power and threshold characteristics. The key parameter, controlling /spl eta//sub int/ and limiting both the output power and the LCC linearity, is the ratio of the threshold values of the recombination current outside the active region to the carrier capture current into the active region. Analysis of the LCC shape is shown to provide a method for revealing the dominant recombination channel outside the active region. A critical dependence of the power characteristics on the laser structure parameters is revealed. While the new mechanism and our formal expressions describing it are universal, we illustrate it by detailed exemplary calculations specific to QD lasers. These calculations suggest a clear path for improvement of their power characteristics. In properly optimized QD lasers, the LCC is linear and the internal quantum efficiency is close to unity up to very high injection-current densities (15 kA/cm/sup 2/). Output powers in excess of 10 W at /spl eta//sub int/ higher than 95% are shown to be attainable in broad-area devices. Our results indicate that QD lasers may possess an advantage for high-power applications.

Approaches to Advanced Organic Light Emitting Diodes: Materials and Devices

ABSTRACTWe present our recent findings on the development of organic light emitting diodes (OLEDs). One of the keys to highly efficient phosphorescent emission in organic light-emitting diodes is to confine triplet excitons generated within the emitting layer. To confine triplet excitons, we employ perfluorinated phenylene dendrimers (C60F42) as a both hole- and exciton-block layer, and a hole-transport material (4,4',4”-tris(N-carbazolyl) triphenylamine [TCTA]) as a host for the phosphorescent dopant, Ir(ppy)3, in the emitting layer. The maximum external quantum efficiency reaches to 19.2%, and is over 15% even at high injection current densities of 10 to 20 mA/cm2, where the brightness of the device reaches to approximately 10,000 cd/m2.

ADVANCES IN MEASUREMENTS OF PHYSICAL PARAMETERS OF SEMICONDUCTOR LASERS

We present a summary of the advances in characterization techniques allowing comprehensive study of physical processes in semiconductor lasers. The studies of the electrical characteristics and optical emission below threshold allow to measure the optical gain, linewidth enhancement factor, transparency wavelength, optical loss and carrier life-time. Some other parameters, such as leakage current and wavelength chirp, can only be deduced from the above threshold measurements. Measurements of the carrier temperature and carrier heating in semiconductor lasers allow to obtain important information about the devices performance at high injection current densities. Taken together, all these measurements provide critical experimental feedback in the laser design process. They also furnish essential information to guide our understanding of the microscopic physical processes determining the laser performance and our efforts to simulate those processes.

Low‐Field Trap Generation Dependence on the Injection Current Density in Gate Insulators: How Valid Are Accelerated Hot Electron Measurements?

“Continuous” low gate insulator field (1 MV/cm) electron injection in insulated gate field effect transistors using a pulsed injection technique (PIT) was conducted in the dose range 1013 to 1017 e/cm2 over the range of current densities from 1.5 × 10−7 to 3.4 × 10−5 A/cm2. PIT enables independent control of insulator fields and injection current densities, while not causing optically induced shallow trap depopulation. As is generally the case, the threshold voltage shift, ΔVD varies monotonically with dose, and can be modeled using a defect generation power law requiring fewer adjustable parameters than is necessary using a first order trapping model. It was also found that for a given dose the injection current density has a profound effect on the observed magnitude of trap generation. Previously, the total dose and insulator field were thought to be the only determining factors in trap generation. Based on these results, it appears that when the intrinsic defect concentration(s) is to be determined, a very low current density (injection rate) should be used to minimize trap generation effects which would confuse the issue. It is reported also for the first time that the expected in‐use lifetime of the devices, calculated from the injection data, also exhibits a power law dependency on the injection current density. These results raise serious questions about the validity of aggressive injection techniques (such as avalanche injection and Fowler‐Nordheim approaches), of the existence of ultrasmall cross section electron traps based on such methods, and of aggressive accelerated aging conclusions, based on the extrapolation of high injection current density data to end‐of‐life threshold voltage shifts.

Anomalous electrical conduction in silicon resistors at low temperatures

Charge transport phenomena in highly compensated Si resistors have been investigated in the temperature range 4.2 - 300 K from measurements of the I - V static characteristics. The prebreakdown and breakdown regions of I - V characteristics, observed at liquid-helium temperatures, were simulated by a one-dimensional model including the high compensation effect as well as the impact ionization of the frozen-out shallow dopants by the hot electrons injected into the base. An impurity hopping conduction mechanism is considered for the prebreakdown regime at temperatures between 4.2 and 20 K. Negative differential resistance (S-type) phenomena are also observed on the I - V characteristics for high injection current densities and are discussed in terms of the inhomogeneous distribution of the electric field into the base at liquid-helium temperatures.

Static Current-Voltage Characteristics of Silicon n+-i-n+ Resistors at Liquid Helium Temperatures

The behaviour of the (I-V) characteristics is investigated in n + -i-n + highly compensated Si resistors at temperatures 4.2-45K. The conduction mechanisms are discussed in detail here. The prebreakdown and breakdown regions of I-V characteristics were simulated by a one-dimensional model including the evidence of impurity high compensation and freeze out effects as well as the shallow-dopants impact ionisation by the injected hot carriers into the base. Negative resistance (S-type) phenomena are also observed on the characteristics for high injection current densities.