量子阱数量对InGaN/GaN激光二极管性能的影响

Characteristic temperature coefficients of the threshold current (T0) and the external quantum efficiency (T1), on highly-strained In0.35Ga0.65As- ((lambda) =1.17micrometers ) and In0.4Ga0.6As- ((lambda) =1.19micrometers ) QW lasers, are studied as simple functions of the temperature dependent of the physical parameters of the semiconductor lasers. Simple expressions of characteristics temperature coefficients of the threshold current (T0) and the external differential quantum efficiency (T1) are expressed as functions as physical parameters and their temperature dependencies. The parameters studied here include the threshold (Jth) and transparency (Jtr) current density, the carrier injection efficiency (Ninj) and external (Nd) differential quantum efficiency, the internal loss ((alpha) i), and the material gain parameter (G0). Simple expressions for T0 and T1 are derived in terms of the fundamental device parameters, indicating that the observed cavity length dependence is due to the length dependence of the modal threshold gain. The influence of monomolecular, radiative, and Auger recombination on the characteristic temperature of the transparency current density, Ttr, are analyzed. A measurement of Ttr, then allows one to determine the dominant recombination mechanisms in the QW. The characteristic temperature of the internal loss, T(alpha i), can be used to determine the influence of intervalence band absorption on the value of T0. The analysis done on the InGaAs QW lasers show that the simple expressions for T0 and T1 values gives consistent results between experiments and theory.

Simulation and optimization of deep violet InGaN double quantum well laser

Interest in Quantum Dot Lasers stem mainly from the low threshold current which can result due to quantization of energy levels and change in density of state function. In the most used lasers, separate confinement heterostructures, the nanoscale active region is ‘built into’ the waveguide region (optical confinement layer, OCL) based on a wide gap semiconductor material.The most important characteristic of quantum dot laser is the amount of light it emits as current is injected into the device. The dependence of laser power output on current, cavity length, and various other parameters has been estimated based on the rate equations model. Linear relation has been obtained between power and current and as the cavity length increases, the slope of the power–current characteristic decreases.The external quantum efficiency indicates the efficiency of a laser device in converting the injected electron–hole pairs (input electric charges) to the photons emitted from the device (output light). External differential quantum efficiency decreases linearly with increasing cavity length. As the internal loss increases, the slope of external differential quantum efficiency versus cavity length increases. The internal quantum efficiency is independent of the geometrical properties of the laser device, such as the cavity length or the stripe width. Internal quantum efficiency is one of the main figures of merit that should be used in assessing the quality of the semiconductor wafer from which the quantum dot laser is manufactured.One of the most important device characteristics of a laser diode is the efficiency of conversion of the input electric power into output optical power. Power conversion efficiency is found to increase with increasing drive current, gets its maximum value and after that it decreases slightly with increasing drive current. Power conversion efficiency also depends on different parameters like cavity length, internal efficiency, and threshold current.

InGaN-based light emitting diodes and multiple quantum wells designed to emit in the green spectral region exhibit, in general, lower internal quantum efficiencies than their blue-emitting counter parts, a phenomenon referred to as the “green gap.” One of the main differences between green-emitting and blue-emitting samples is that the quantum well growth temperature is lower for structures designed to emit at longer wavelengths, in order to reduce the effects of In desorption. In this paper, we report on the impact of the quantum well growth temperature on the optical properties of InGaN/GaN multiple quantum wells designed to emit at 460 nm and 530 nm. It was found that for both sets of samples increasing the temperature at which the InGaN quantum well was grown, while maintaining the same indium composition, led to an increase in the internal quantum efficiency measured at 300 K. These increases in internal quantum efficiency are shown to be due reductions in the non-radiative recombination rate which we attribute to reductions in point defect incorporation.

In order to study the luminescent efficiency of InGaAs quantum wells on Si via SiGe interlayers, identical In0.2Ga0.8As quantum well structures with GaAs and Al0.25Ga0.75As cladding layers were grown on several substrates by an atmospheric metalorganic vapor deposition system. The substrates used include GaAs, Si, Ge, and SiGe virtual substrates. The SiGe virtual substrates were graded from Si substrates to 100% Ge content. Because of the small lattice mismatch between GaAs and Ge (0.07%), high-quality GaAs-based thin films with threading dislocation densities <3×106 cm−2 were realized on these SiGe substrates. Quantitative cathodoluminescence was used to compare the luminescent efficiency of the quantum well structure on the different substrates and cross-sectional transmission electron microscopy was used to characterize dislocation densities. Our results show that the InGaAs quantum wells grown on the GaAs substrates have the highest luminescent efficiencies due to the lowest dislocation densities. Interestingly, InGaAs quantum wells grown on the SiGe virtual substrates outperform those on Ge substrates, both in terms of luminescent efficiency and dislocation density. This difference is attributed to the variation in thermal expansion coefficient (α) and its impact on defect structure during the process cycle. The SiGe virtual substrate has a smaller α compared to a Ge substrate because of the smaller α of the Si substrate, which helps minimize compressive strain in the quantum well layer during the temperature decrease from the growth temperature. Consequently, fewer misfit dislocations are created between the quantum well and cladding interfaces. These misfits can greatly affect the luminescent efficiency since they can act as recombination sites. In general, the efficiencies of the quantum wells on the SiGe and Ge substrates were affected only by higher misfit dislocation densities, whereas the quantum wells on the Si substrate had low efficiency due to high threading dislocation density.

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that the strain relaxation by forming defects occurs when the number of quantum wells (QWs) increases to 30. It results in the low luminescence intensity and external quantum efficiency (EQE) due to the poor crystal quality of MQW region and the high non-radiative recombination rate in MQW region. However, the EQE of 25-period InGaN/GaN MQW solar cell with relatively high crystal quality of MQW region does not increase at all wavelengths compared with 15-period one. This may be attributed to the lower carrier collection efficiency resulting from the lower electric field of i-MQW region.

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.

The built-in electric field in an InGaN quantum well and emission wavelength are numerically evaluated at various GaN barrier thicknesses in blue InGaN/GaN/Al2O3 LED structures. The effect of GaN barrier thickness on the internal quantum efficiency of these structures was studied experimentally by temperature- and excitation-power-dependent photoluminescence measurements. In LED structures with 3-nm-thick GaN barriers in the active region the internal quantum efficiency at high excitation levels higher than that in LED structures with thicker GaN barriers. The results of measurements indicate that in structures with 3-nm-thick GaN barriers, the Auger recombination rate is reduced.

Over the last two decades III-nitride optoelectronic devices have experienced an impressive evolution in terms of performance. However, their potential is far from being fully exploited. Although they offer bandgaps from the deep UV to the infrared spectral range, lasing is currently restricted to the 336?536 nm wavelength region. Several challenges still need to be overcome. For example, the thermal stability of the InGaN active region becomes an issue in long-wavelength (? > 500 nm) laser diodes: high indium content (In > 25%) quantum wells (QWs) have been shown to markedly degrade when kept above 900 °C. For this reason, the growth temperature of the subsequent p-type waveguide and cladding layers must be sufficiently low to avoid QW degradation, which is accompanied by a reduction in the internal quantum efficiency. Thanks to the high vacuum environment, molecular beam epitaxy (MBE) allows growing these layers at a much lower temperature compared to metalorganic vapor phase epitaxy (MOVPE), and could be used to reduce the thermal budget on the active region, further extending the wavelength range. Thus the aim of this thesis is the realization of long wavelength laser diodes by combining high efficiency active regions grown by MOVPE with p-doped layers grown by ammonia-MBE. Hence, the p-type doping by ammonia-MBE is first scrupulously studied and optimized on 500 nm thick (Al)GaN layers. In order to comprehensively address the physical processes governing the doping efficiency, the role of the growth temperature, the V-III ratio, and the growth rate is investigated. In addition, the Mg cell configuration resulting in the best doping profile control and reproducibility is studied as well. Electrochemical capacitance-voltage profiling is extensively used to study the compensation mechanisms. The optimization leads to the achievement of layers exhibiting low dopant compensation (below 5%) up to high doping levels and resistivity values as low as 0.4 Ohm­ cm. These layers are then implemented on top of active regions grown by MOVPE and lasing is obtained for the first time in hybrid structures exhibiting state-of-the-art electrical characteristics. The threshold voltage is as low as 4.3 V in an index guided structure with 800x2 ?m² ridge dimension designed for violet emission (? = 400 nm). These excellent current-voltage characteristics are achieved owing to the doping profile control in the cladding and contact layers which is confirmed by the low contact resistance value of 5E-4­ cm². Lasing is also demonstrated beyond 500 nm, keeping state-of-the-art electrical characteristics, despite the reduced doping in the cladding layer. These devices also exhibit values for internal optical losses as low as 6 cm?1. The growth of tunnel junctions is also attempted. Low specific resistance values are demonstrated in ammonia-MBE grown GaN tunnel homojunctions. The absence of hydrogen passivation and the reduced memory effects of MBE are considered to be crucial for the realization of these devices together with the stability of the doping at reduced growth temperatures. n-p-n structures featuring a tunnel junction in series with a p-n junction are then used to spread the current across light-emitting diodes. A device architecture featuring a buried tunnel junction to be used for current injection inmicro-LEDs or vertical cavity surface emitting lasers is also demonstrated and excellent current confinement properties are obtained.

A detailed study was made of the dependence of the threshold current density and the external differential quantum efficiency on the cavity end loss in single-heterojunction laser diodes. To eliminate possible errors introduced by studying lasers of varying lengths, this study relied on changes of the facet reflectivity by SiO coatings of appropriate thickness. The results are consistent with a linear relationship between the threshold current density and the gain coefficient at threshold. The values of the gain factor, absorption coefficient, and internal quantum efficiency at 300 and 77°K are in excellent agreement with some of the previous studies. However, for effective end losses greater than ∼50 cm−1 the differential quantum efficiency shows an anomalous effect by decreasing with increasing end loss instead of increasing as predicted by simple theory. This effect occurs at 77°K as well as 300°K and is tentatively attributed to internal trapping of the stimulated emission.

In this paper, we have analyzed the performance of laser diodes (LDs) with an undoped InxGa1−xN waveguide in association with variable In mole fractions varying from 0% to 6%. The InGaN waveguide significantly affected optical confinement and carrier dynamics in the LD. Laser output power, internal quantum efficiency (IQE), quantum well (QW) gain and optical confinement factor (OCF) are analyzed in correlation with In content in the waveguide. The OCF for TE1 mode at x = 0 is 0.0079, and it increases with x. The OCF for x = 0.06 is 0.0109. Quantum states in the QW reduced with In content, which reduced QW gain. However, improved OCF consequently improved the overall gain of the device. Peak IQE for x = 0 is ~0.28, which increased to ~0.683 for x = 0.06. Laser output power, slope efficiency and IQE improved with increasing In content. The threshold current density for x = 0 is 3.911 kA cm−2, which drops to 1.806 kA cm−2 for x = 0.06. Auger recombination, Shockley-Read-Hall recombination and electron leakage are reduced for high In content in the waveguide. Auger carrier loss remains nearly constant above the threshold, hence efficiency droop above the threshold current density is attributed primarily to leakage current density. The efficiency droop for x = 0 is 25.59%, which increased to 36.66% for x = 0.06 at 25 kA cm−2 current density. However, the IQE for x = 0 at 25 kA cm−2 is 0.202, which increased to 0.436 for x = 0.06 at 25 kA cm−2. Slope efficiency increases with In content; slope efficiency for x = 0.06 improves by 0.2126 W A−1. Laser output power is improved by 1.91 W at 25 kA cm−2 for x = 0.06.

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.

The dependence of electrical properties as a function of substrate orientation and growth conditions for Si-doped GaAs grown by molecular beam epitaxy has been studied. A phase diagram concerning conduction type as a function of growth temperature Tg and V/III flux ratio has been examined for GaAs(311)A, GaAs(111)A, and GaAs(331)A. The conduction type of Si-doped GaAs(311)A abruptly changes from p to n type with varying Tg and V/III flux ratio. Furthermore, it was found that the dependence of electrical properties on growth conditions for Si-doped GaAs(311)A is more like GaAs(331)A rather than (111)A. In addition, InGaAs/GaAs strained-layer quantum well lasers have been fabricated on GaAs(311)A substrates using only Si dopant. The p-n junction was formed by changing the V/III flux ratio without growth interruption using the Si doping characteristic in GaAs(311)A as an amphoteric dopant. High crystal quality was maintained even when the V/III flux ratio was changed, and an abrupt p-n junction with a sharp boundary was achieved. Lasing operation at room temperature has been demonstrated for the first time, and a threshold current density of 260 A/cm2 and an external differential quantum efficiency of 67% were obtained for 720-μm-long broad area lasers. The dependence of threshold current density and external differential quantum efficiency on a laser cavity length was also examined. A laser structure with high crystal quality and abrupt p-n junction exhibits an internal quantum efficiency of 90%, an internal optical loss of 6 cm−1, and a transparency current density of 80 A/cm2.

Currently all commercially available diode lasers are fabricated by cleaving dice from semiconductor wafers with the cleaved facets serving as the cavity mirrors. This practice is well suited to producing discrete devices but severely limits the degree to which these devices can be integrated into large scale monolithic systems. A method of producing laser mirrors, diffraction gratings, and other optical components directly in the surfaces of wafers is needed to integrate light sources with other optical elements and electronic devices. Experiments demonstrating the practicality of micromachining such structures with focused beams of ions from liquid metal sources are described. The micromachining was accomplished with a 20-keV Ga+ ion beam of 0.3-nA current focused to a 250-nm spot. Diode laser output mirrors were formed which show no increase in the lasing threshold and only a 20% decrease in external differential quantum efficiency when compared to cleaved mirror facets. Construction of coupled cavity single and multiple stripe diode lasers with the liquid metal focused-ion-beam micromachining technique is also described. Coupling grooves with submicron widths and depth-to-width aspect ratios of more than 10:1 have been achieved while maintaining the high optical quality of the groove walls.

To avoid the catastrophic optical mirror damage (COMD), the value of equivalent transverse spot size has increased to 0.83μm through decreasing optical confinement factor (Γ) for quantum well diode lasers, which further reduces the power density on the facet. The design of asymmetric 980 nm InGaAs/InGaAsP broad-waveguide diode lasers with current blocking layer is designed for high-power, single-transverse-mode operation experimentally, which prevents carrier leakage and increases electro-optical conversion efficiency. According to the calculation result, for the device with uncoated 100-μm-wide stripes, 2-mm-long lasers exhibit a threshold-current density of 290A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the maximum output power is 1.19W with operating current of 2.0 A. The corresponding internal loss coefficient and differential quantum efficiency are 1.6 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 63% respectively.