Internal Differential Quantum Efficiency Research Articles

Рабочие характеристики полупроводниковых лазеров на квантовых ямах в зависимости от ширины волноводной области

Operating characteristics of semiconductor quantum well (QW) lasers are theoretically studied in terms of the thickness of the waveguide region [optical confinement layer (OCL)]. We calculate the maximum modal gain, optical confinement factor (in QW, OCL, and cladding layers), threshold current density, electron and hole densities (in QW and OCL), internal optical loss (in QW, OCL, and cladding layers), internal differential quantum efficiency, stimulated and spontaneous recombination currents, and output optical power of the laser as functions of the OCL thickness. It is shown that up to the pump current density 50 kA/cm2 the output power of the considered lasers depends only slightly on the OCL thickness in the range of thicknesses 1.5–2.8 m. This result is important for designing high brightness lasers as broadened waveguides are used in such lasers to attain low beam divergence. At high pump current densities, the output power is shown to have a maximum as a function of the OCL thickness.

Quantum dot lasers with asymmetric barrier layers: Close-to-ideal threshold and power characteristics

A theory of static (threshold and power) characteristics of novel diode lasers – quantum dot (QD) lasers with asymmetric barrier layers (ABLs) – is developed. The barrier layers are asymmetric in that they have considerably different heights for the carriers of opposite signs. The ABL located on the electron- (hole-) injecting side of the structure provides a low barrier (ideally no barrier) for electrons (holes) [so that it does not prevent electrons (holes) from easily approaching the active region] and a high barrier for holes (electrons) [so that holes (electrons) injected from the opposite side of the structure do not overcome it]. The use of ABLs should thus ideally prevent the simultaneous presence of electrons and holes (and hence parasitic electron – hole recombination) outside the QDs. It is shown in this work that in such a case of total suppression of parasitic recombination, the QD lasers with ABLs offer close-to-ideal performance: the threshold current density is below 10 A cm−2 at any temperature, the absolute value of the characteristic temperature is above 1000 K (which manifests a virtually temperature-independent operation), the internal differential quantum efficiency is practically unity, and the light – current characteristic is linear at any pump current.

Impact of thermal annealing on internal device parameters of GaAs 0.965 Bi 0.035 /GaAs 0.75 P 0.25 quantum well lasers

The impact of post-growth thermal annealing on the internal device parameters such as internal loss ( α i ), internal differential quantum efficiency (η 0 d ) and modal material gain (Γ g 0J ) of a single-quantum well (QW) laser diodes employing GaAs 0.965 Bi 0.035/ GaAs 0.75 P 0.25 active regions and emitting near λ ~980 nm was investigated. Parameter extraction from a conventional cavity length analysis indicates that the internal loss remains unchanged while internal differential quantum efficiencies degrade as the annealing time increases. Also, the variation of the modal material gain with annealing correlates to the corresponding change in the photoluminescence intensity. Comparisons between single- and double-QW devices indicate that the relatively high internal loss originates from the QW active region.

Two-Valued Characteristics in Semiconductor Quantum Well Lasers

Due to internal optical absorption loss, which varies with carrier density in the optical confinement region, the operating characteristics can be two valued in semiconductor quantum well lasers. Particularly, there can be two lasing thresholds and two branches in the light–current characteristic. While the internal differential quantum efficiency for the first (conventional) branch of the light–current characteristic is less than 1 and decreases with increasing pump current, that for the second (unconventional) branch is greater than 1 and nonmonotonous.

Theory of the power characteristics of quantum-well lasers with asymmetric barrier layers: Inclusion of asymmetry in electron- and hole-state filling

The power characteristics of quantum-well lasers with asymmetric barrier layers, which represent a novel type of injection laser, are calculated on the basis of an extended model taking into account asymmetry in the filling of electron and hole states. The electron–hole asymmetry is shown to have no significant effect on the characteristics of these lasers. Even in the presence of intermediate layers (located between the quantum well and each of the two asymmetric barrier layers), where parasitic electron–hole recombination does occur, the internal differential quantum efficiency of such a laser exhibits only a weak dependence on the pump current and remains close to unity; therefore, the light–current characteristic remains linear up to high pumping levels.

Dependence of the electron capture velocity on the quantum-well depth in semiconductor lasers

Three laser structures with a single strained InGaAs quantum well of different depths and a GaAs or Al0.1Ga0.9As waveguide region are grown by metal-organic chemical vapor deposition. Using the experimentally determined values of the threshold current density and internal differential quantum efficiency, the velocity of electron capture into the quantum well is calculated for each of these structures. It is found that the capture velocity into deep quantum wells is significantly lower than into shallow ones.

BeZnCdSe quantum-well ridge-waveguide laser diodes under low threshold room-temperature continuous-wave operation

Low threshold current ridge-waveguide BeZnCdSe quantum-well laser diodes (LDs) have been developed by completely etching away the top p-type BeMgZnSe/ZnSe:N short-period superlattice cladding layer, which can suppress the leakage current that flows laterally outside of the electrode. The waveguide LDs are covered with a thick SiO2 layer and planarized with chemical-mechanical polishing and a reactive ion etching process. Room-temperature lasing under continuous-wave condition is achieved with the laser cavity formed by the cleaved waveguide facets coated with high-reflectivity dielectric films. For a 4 μm-wide green LD lasing around a wavelength of 535 nm, threshold current and voltage of 7.07 mA and 7.89 V are achieved for a cavity length of 300 μm, and the internal differential quantum efficiency, internal absorption loss, gain constant, and nominal transparency current density are estimated to be 27%, 4.09 cm−1, 29.92 (cm × μm)/kA and 6.35 kA/(cm2 × μm), respectively. This compact device can realize a significantly improved performance with much lower threshold power consumption, which would benefit the potential application for ZnSe-based green LDs as light sources in full-color display and projector devices installed in consumer products such as pocket projectors.

Method for determination of capture velocity of charge carriers into quantum well in semiconductor laser

A simple method for the determination of the capture velocity of charge carriers from a three-dimensional (3D) region (waveguide region) into a 2D region (quantum well) is proposed. The method is based on measurement of the threshold current density and internal differential quantum efficiency in a semiconductor laser structure. The method also allows determining the 2D carrier density in a quantum well, which is otherwise not easy to measure in a multilayer laser structure.

Calculation of output characteristics of semiconductor quantum-well lasers with account for both electrons and holes

Using an extended theoretical model, which includes the rate equations for both electrons and holes, we have studied the output characteristics of semiconductor quantum-well lasers. We have found non-trivial dependences of electron and hole concentrations in the waveguide region of the laser on the capture velocities of both types of carriers from the waveguide region into the quantum well. We have obtained the dependences of the internal differential quantum efficiency and optical output power of the laser on the capture velocities of electrons and holes. An increase in the capture velocities has been shown to result in suppression of parasitic recombination in the waveguide region and therefore in a substantial increase in the quantum efficiency and output power.

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.

Measurements of Cavity-Length-Dependent Internal Differential Quantum Efficiency and Internal Optical Loss in Laser Diodes

We measured the cavity length dependence of the internal differential quantum efficiency ηint and the internal optical loss αint in 1500-nm-wavelength laser diodes (LDs). By evaluating αint directly from gain/absorption spectra for various injection current densities and measuring the external differential quantum efficiency ηext, we obtained the ηint and αint values of all the LDs with different cavity lengths. The obtained ηint and αint respectively showed strong and weak cavity length dependences, and were both very different from those derived by the widely used method of plotting ηext-1 against the cavity length.

High-Performance Bound-to-Continuum Quantum-Cascade Lasers for Broad-Gain Applications

Based on the bound-to-continuum active region design, we shall present a high performance continuous-wave (CW) quantum-cascade laser. In contrast to high performance lasers based on a two-phonon resonance transition and a narrow linewidth (< 165 cm-1), the device presented here exhibits a spontaneous emission full-width at half-maximum as large as 295 cm-1. Thus, such devices are very suitable for broadband tuning. At 30degC, it shows a maximum output power and slope efficiency of 188 mW and 500 mW/A, as well as a threshold current density of only 1.79 kA/cm2. Furthermore, at this temperature, the device demonstrates an internal differential quantum efficiency of 71% and a wall plug efficiency of 2.0%. The maximum CW operation temperature reached is 110degC. A thermal resistance of 4.3 K/W was attained by epi-down mounting on diamond submounts. The waveguide losses of 14 cm-1 are explained by intersubband absorption in addition to free-carrier absorption.

Fabrication of InGaAlAs MQW buried heterostructure lasers by narrow stripe selective MOVPE

Fabrication of InGaAlAs MQW buried heterostructure (BH) lasers by narrow stripe selective MOVPE is demonstrated in this paper. High quality InGaAlAs MQWs were first grown by narrow stripe selective MOVPE without any etching process and assessed by analysing the cross sections and PL spectrums of the InGaAlAs MQWs. Furthermore, BHs were fabricated for the InGaAlAs MQW lasers by a developed unselective regrowth method, instead of conventional selective regrowth. The InGaAlAs MQW BH lasers exhibit good device characteristics, with a high internal differential quantum efficiency of 85% and a low internal loss of 6.7 cm−1. Meanwhile, narrow divergence angles of the far field pattern are obtained for the fabricated lasers.

Engineering laser gain spectrum using electronic vertically coupled InAs-GaAs quantum dots

Continuous large-broad laser gain spectra near 1.3 /spl mu/m are obtained using an active region of electronic vertically coupled (EVC) InAs-GaAs quantum dots (QDs). A wide continuous electroluminescence spectrum, unlike that from conventional uncoupled InAs QD lasers, was obtained around 230 nm (below threshold) with a narrow lasing spectrum. An internal differential quantum efficiency as high as 90%, a maximum measured external differential efficiency of 73% for a stripe-length of L=1 mm, and a threshold current density for zero total optical loss as low as 7 A/cm/sup 2/ per QD layer were achieved.

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.

Intrinsic nonlinearity of the light–current characteristic of semiconductor lasers with a quantum-confined active region

We describe a mechanism of nonlinearity of the light–current characteristic common to heterostructure lasers with a reduced-dimensionality active region. It arises from (i) noninstantaneous carrier capture into the active region and (ii) nonlinear (in the carrier density) recombination rate outside the active region. Because of (i), the carrier density outside the active region rises with injection current above threshold, and because of (ii), 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 that holds true for quantum well, quantum wire, and quantum dot lasers.

Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum dot lasers

Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum-dot lasers are achieved by the application of an annealing and growth interruption step at 600 °C after the deposition of the dots. The transparency current is reduced to below 20 A/cm2 at room temperature. The internal differential quantum efficiency is increased from below 50% to above 90% by improvement of the barrier material and subsequent reduction of leakage current. A peak power of 3.7 W at 1140 nm lasing wavelength in pulsed operation at room temperature is demonstrated.

Ultrasmall and ultralow threshold GaInAsP-InP microdisk injection lasers: design, fabrication, lasing characteristics, and spontaneous emission factor

We have calculated lasing characteristics of current injection microdisk lasers of several microns in diameter, taking account of the scattering loss at center posts and the carrier diffusion effect. We found that the optimum width of the disk wing exposed to the air is 0.6-0.7 /spl mu/m and the minimum threshold current is nearly 10 /spl mu/A for the disk diameter of 2 /spl mu/m. The internal differential quantum efficiency can be 95% if the transparent carrier density is reduced to 7.5/spl times/10/sup 17/ cm/sup -3/ and the diffusion constant is increased to 8 cm/sup 2//s. In the experiment, we have obtained the room temperature continuous-wave operation of a GaInAsP-InP device of 3 /spl mu/m in diameter, for the first time, with a record low threshold of 150 /spl mu/A. This achievement was mainly owing to the reduction of the scattering loss at the disk edge, and hence the reduction of the threshold current density. The spontaneous emission factor was estimated to be 6/spl times/10/sup -3/. This value was much reduced by the large detuning of the lasing wavelength against the spontaneous emission peak. A larger value over 0.1, which is expected for such a small device, will be obtained by the wavelength tuning and the narrowing of the spontaneous emission spectrum.

High slope efficiency, “cascaded” midinfrared lasers with type I InAsSb quantum wells

Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers, &amp;gt;2 mW (∼80 K, 3.7 μm) and &amp;gt;0.1 mW (∼300 K, 4.3 μm). A 3.8–3.9 μm laser structure operated up to T=180 K. At 80 K, peak power &amp;gt;100 mW and a slope efficiency of 48% (4.8% per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency &amp;gt;1 and a loss coefficient ⩾100 cm−1.

High-speed modulation of strain-compensated InGaAs-GaAsP-InGaP multiple-quantum-well lasers

Small signal direct modulation characteristics of InGaAs-GaAsP-InGaP multiple quantum well ridge waveguide lasers (4.5×220 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) are described. The compressive strain of four InGaAs quantum wells is compensated by the tensile strain of GaAsP barriers. The lasers have a threshold current of 8 mA and an internal differential quantum efficiency of 80%. A 3-dB bandwidth of 25 GHz is obtained at 54 mA. It is found that the strain-compensated lasers have a K factor as low as 0.15 ns, implying a maximum 3-dB bandwidth of 59 GHz.