Non-equilibrium quantum well populations and active region inhomogeneity in polar and nonpolar III-nitride light emitters (original) (raw)
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Inhomogeneous injection in III-nitride light emitters with deep multiple quantum wells
Journal of Computational Electronics, 2015
Excessive depth of optically active quantum wells (QWs) and related increase in QW population capacity is one of the main causes of inhomogeneous carrier injection and unequal QW populations in multiple-quantum-well (MQW) III-nitride light emitters operating in the visible range. In turn, uneven distribution of injected carriers across the device's active region creates imbalance between confined electron and hole QW populations and supports large residual QW charges, especially in marginally located n-side and p-side QWs. QW charges are strongly non-equilibrium as determined by dynamic balance between carrier capture and recombination rates, with the later being progressively faster in excessively deep QWs.
Physics and Simulation of Optoelectronic Devices XX, 2012
III-nitride visible light emitters employ deep QWs and feature strong disparity of electron and hole transport in diode structures. As a result, multi-QW active regions of such devices suffer from inhomogeneous carrier injection, large residual charges of active QWs, and overall active region electrical non-uniformity which unfavorably affects the emitter efficiency. In this work, we show that electron and hole populations of deep optically active III-nitride QWs are highly imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems. Non-equilibrium QW populations are self-consistently determined by carrier injection and light generation processes in active QWs. In turn, QW residual charges impose strong feedback on the active region electrical uniformity. Our selfconsistent modeling of QW radiative characteristics and multi-QW carrier transport in diode structures relates the effects of non-equilibrium QW populations, inhomogeneous QW injection and residual QW charges to the structure internal efficiency. Comparative modeling of polar and nonpolar diodes shows that in both types of structures the nonequilibrium effects tend to decrease the QW operational electron populations; this trend benefits the active region electrical uniformity. For device simulation, we use COMSOL-based Optoelectronic Device Modeling Software (ODMS) developed at Ostendo Technologies Inc.
Modeling of III-Nitride Multiple-Quantum-Well Light-Emitting Structures
IEEE Journal of Selected Topics in Quantum Electronics, 2000
Spatial inhomogeneity of carrier injection across the multiple quantum well (MQW) active region of a semiconductor light emitter can impose severe limitations on the device efficiency. In III-nitride based devices, the large disparity of electron and hole transport and the excessive depth of active QWs trigger the process of inhomogeneous QW injection which is further aggravated by strong dependence of QW radiative characteristics on the QW injection conditions due to (i) intra-QW screening of polarization fields in polar and semipolar materials, (ii) phase-space filling effect in lowest QW subbands at higher levels of carrier injection, and (iii) exceedingly nonequilibrium character of the electron and hole populations in deep QWs. All these tendencies become more pronounced in longer-wavelength emitters. The residual QW charges provide strong feedback to the QW injection conditions thus requiring a high level of self-consistency between the active region transport simulation and the QW emission modeling.
COMSOL-based Ostendo's Opto-electronic Device Modeling Software (ODMS) has been updated to include effects of non-equilibrium QW populations in semiconductor light-emitting and laser diodes. III-nitride light emitters with different levels of polarity have been compared as an illustrative example of ODMS performance. Modeling proved that high intra-QW recombination rates in III-nitride light emitters make the QW populations strongly non-equilibrium and vulnerable to inhomogeneous injection in multiple-QW devices. QW popu-lations are further affected by disparate electron and hole transport across the active region.
Inhomogeneous injection in polar and nonpolar III-nitride light-emitters
Solid-State Electronics, 2010
Despite the absence of polarization-induced potential barriers, the nonpolar III-nitride multiple-quantum well (MQW) structures are shown to suffer from strongly inhomogeneous population of active QWs dominated by the QW closest to the N-side of the diode structure. This situation is the opposite of polar structures, where the extreme P-side QW always prevails in optical emission. Inhomogeneity of QW populations in nonpolar structures is supported by QW residual charges and becomes stronger in structures with deeper QWs. Indium incorporation into waveguide and/or barrier layers improves the uniformity of QW injection.
Journal of Applied Physics, 2010
Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well ͑QW͒ light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW ͑MQW͒ active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.
physica status solidi (a), 2014
The results of experimental investigation of forward currentvoltage characteristics of InGaN/GaN multiple quantum well light-emitting diodes are presented. A new model for explaining the complex current dependence on voltage is proposed. The model is based on the assumption of space charge limited current, and ballistic overflow of electrons through the multiple quantum well region. It is shown that electrons are captured in the shallow traps while transferring through the active region. The results of measurements indicate that the activation energy of traps decreases with a temperature decrease, which corresponds to the theory of hopping in exponential band tails.
Charge transport in non-polar and semi-polar III-V nitride heterostructures
Semiconductor Science and Technology, 2012
Compared to the intense research focus on the optical properties, the transport properties in non-polar and semi-polar III-nitride semiconductors remain relatively unexplored to date. The purpose of this paper is to discuss charge-transport properties in non-polar and semi-polar orientations of GaN in a comparative fashion to what is known for transport in polar orientations. A comprehensive approach is adopted, starting from an investigation of the differences in the electronic bandstructure along different polar orientations of GaN. The polarization fields along various orientations are then discussed, followed by the low-field electron and hole mobilities. A number of scattering mechanisms that are specific to non-polar and semi-polar GaN heterostructures are identified, and their effects are evaluated. Many of these scattering mechanisms originate due to the coupling of polarization with disorder and defects in various incarnations depending on the crystal orientation. The effect of polarization orientation on carrier injection into quantum-well light-emitting diodes is discussed. This paper ends with a discussion of orientation-dependent high-field charge-transport properties including velocity saturation, instabilities and tunneling transport. Possible open problems and opportunities are also discussed.
physica status solidi (a), 2017
The performance of nitride-based light emitting diodes is determined by carrier transport through multi-quantum-well structures. These structures divide the device into spatial regions of high carrier density, such as n-GaN/p-GaN contacts and InGaN quantum wells, separated by barriers with low carrier density. Wells and barriers are coupled to each other via tunneling and thermionic emission. Understanding of the quantum mechanics-dominated carrier flow is critical to the design and optimization of light-emitting diodes (LEDs). In this work a multi-scale quantum transport model, which treats high densities regions as local charge reservoirs, where each reservoir serves as carrier injector/receptor to the next/previous reservoir is presented. Each region is coupled to its neighbors through coherent quantum transport. The nonequilibrium Green's function (NEGF) formalism is used to compute the dynamics (states) and the kinetics (filling of states) of the entire device. Electrons are represented in multi-band tight-binding Hamiltonians. The I-V characteristics produced from this model agree quantitatively with experimental data. Carrier temperatures are found to be about 60 K above room temperature and the quantum well closest to the p-side emits the most light, in agreement with experiments. Auger recombination is identified to be a much more significant contributor to the LED efficiency droop than carrier leakage.