Challenges towards the simulation of GaN-based LEDs beyond the semiclassical framework

Goano, Michele; Bertazzi, Francesco; Zhou, Xiangyu; Mandurrino, M.; Dominici, Stefano; Vallone, Marco; Ghione, Giovanni; Tibaldi, Alberto; Calciati, Marco; Debernardi, Pierluigi; Dolcini, Fabrizio; Rossi, Fausto; Verzellesi, Giovanni; Meneghini, Matteo; Trivellin, Nicola; Santi, Carlo De; Zanoni, Enrico; Bellotti, E.

doi:10.1117/12.2216489

Cited by 8 publications

(6 citation statements)

References 74 publications

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“…While considerable uncertainty generally surrounds the quantitative description of optical and electrical properties of III-N QWs (see, e.g., refs. [18][19][20]), such uncertainties do not significantly affect the main conclusions of this paper due to the comparative nature of the simulations and since our results mainly focus on the charge transport in the bulk layers. For the surface recombination at the edges of the DHJ active region we employ the simple surface recombination model based on ref.…”

mentioning

confidence: 99%

Elimination of Lateral Resistance and Current Crowding in Large‐Area LEDs by Composition Grading and Diffusion‐Driven Charge Transport

Kivisaari

Kim

Suihkonen

et al. 2017

Adv Elect Materials

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from the resistance of the CSL, and they also reduce the effective area of the ohmic contacts and further aggravate the droop.To mitigate the effects of the resistive losses, Hurni et al. recently showed that increasing the thickness of the n-type CSL leads to significantly less pronounced current crowding and improves the wall-plug efficiency of LEDs at large currents. [8] However, this approach still necessitates using expensive bulk GaN substrates and a part of the active region (AR) needs to be cut out to deposit n-type contacts, exposing the edges of the AR to surface recombination. The basic structure of Hurni's low resistance LEDs as well as essentially all conventional LEDs relies on a double heterojunction (DHJ) design, where electrons and holes are injected to the AR from nand p-doped regions located on the opposite sides of the AR. This sets evident limitations to designing the structures so that the contacts do not substantially decrease the total AR area or increase the free surface area susceptible to surface recombination. To avoid some of the limitations of conventional DHJ structures related to device scaling, resistive losses and electrical excitation in general, diffusion-driven charge transport (DDCT) was very recently introduced as an alternative way to electrically excite the AR of LEDs. [9][10][11][12] In DDCT, the pn junction is partly separated from the AR, and in contrast with conventional solutions it relies on transporting electrons and holes to the AR from the same side by bipolar diffusion, enabling additional degrees of freedom for designing new devices.For practical reasons, the previous works on DDCT have mainly focused on structures containing vertically formed pn-homojunctions, which entail potential barriers and lead to suboptimal device performance. In this paper, however, we show that the electrical inefficiencies observed earlier can be fully eliminated by adapting the DDCT concept to realize laterally doped heterojunction (LHJ) structures (see Figure 1), and that the SAG techniques needed to realize such LHJ devices do not compromise the internal quantum efficiency of the AR. Furthermore, our results show that the LHJ structures can be further engineered using appropriate material composition gradings, that can reduce the resistive heating of the devices even below the limit set by ideal conventional DHJ devices.The fundamental difference between the DHJ and LHJ structures is the placement and geometry of the doped p-and n-GaN charge injection regions, which are expected to mainly affect Gallium nitride based light-emitting diodes (LEDs) are presently fundamentally transforming the lighting industry, but limitations in the materials and fabrication methods of LEDs introduce substantial challenges to their future development. Among the remaining key bottlenecks of GaN LEDs are the resistive losses and current crowding that strongly increase the heat generation at high powers. In this work the authors show how a new design paradigm based on diffusion-driven charge transport (...

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confidence: 99%

Elimination of Lateral Resistance and Current Crowding in Large‐Area LEDs by Composition Grading and Diffusion‐Driven Charge Transport

Kivisaari

Kim

Suihkonen

et al. 2017

Adv Elect Materials

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Section: Doi: 101002/aelm201600494mentioning

confidence: 97%

“…Group III nitride (III‐N) based light‐emitting diodes (LEDs) are transforming general lighting and display applications due to their very high efficiencies and favorable spectral properties . However, many of the microscopic details of the materials and of LED operation remain poorly understood, presenting an obvious bottleneck for further LED development and optimization . For example, recent measurements have shown that III‐N LEDs exhibit notable hot carrier distributions already close to the peak efficiency, suggesting that hot carriers might have a more profound effect on the device characteristics than previously anticipated.…”

Section: Introductionmentioning

confidence: 99%

“…[5] For example, recent measurements have shown that III-N LEDs exhibit notable hot carrier distributions already close to the peak efficiency, [6][7][8][9] suggesting that hot carriers might have a more profound effect on the device characteristics than previously anticipated.To date, conventional LED simulations have mainly relied on the drift-diffusion (DD) model (see, e.g., refs. [10-13]), which cannot genuinely account for hot carriers and generally also produces larger turnon voltages for MQW LEDs than observed experimentally.…”

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confidence: 97%

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On the Monte Carlo Description of Hot Carrier Effects and Device Characteristics of III‐N LEDs

Kivisaari

Sadi

et al. 2017

Adv Elect Materials

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of LED operation remain poorly understood, presenting an obvious bottleneck for further LED development and optimization. [5] For example, recent measurements have shown that III-N LEDs exhibit notable hot carrier distributions already close to the peak efficiency, [6][7][8][9] suggesting that hot carriers might have a more profound effect on the device characteristics than previously anticipated.To date, conventional LED simulations have mainly relied on the drift-diffusion (DD) model (see, e.g., refs. [10-13]), which cannot genuinely account for hot carriers and generally also produces larger turnon voltages for MQW LEDs than observed experimentally. [14] Many reasons have been suggested to explain this discrepancy, including charge transport through indium fluctuations, [15] V-shaped pits, [16] tunneling through traps [17] or hot carrier transport. [18] At present it is, however, unclear if the discrepancy is of a physical origin or if it arises simply because the very foundations of the widely adopted DD model make it incapable of predicting the most essential device-level characteristics of LEDs.The Monte Carlo (MC) method has been established as the standard tool to simulate hot-carrier effects in unipolar semiconductor devices such as transistors. Recently the MC method has also been expanded by Bertazzi et al. to simulate hot electron effects in GaN barriers of LEDs using full-band MC methods, [19] and ourselves, as we have recently developed a coupled Monte Carlo-drift-diffusion (MCDD) simulation method to investigate hot-electron effects in full III-N MQW LED structures. [20][21][22] More recently, we have also introduced a full MC model, [23] which can be used to carry out full MC simulations of both electrons and holes in a complete LED device. Nevertheless, the previous works have not yet provided a full analysis of the pertinent physics and differences between MC and DD. To provide more insight into the physics of MQW devices and to facilitate their further development, we now deploy our full MC simulator to answer the following questions:

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“…State‐of‐the‐art modeling approaches for carrier transport in LEDs are based on drift‐diffusion semiclassical models, which either heuristically patch in quantum effects or simply ignore them. Shortcomings of such rather inconsistent carrier transport models often manifest in the prediction of unrealistic turn‐on voltages.…”

Section: Introductionmentioning

confidence: 99%

Quantitative Multi‐Scale, Multi‐Physics Quantum Transport Modeling of GaN‐Based Light Emitting Diodes

Geng

Sarangapani

Wang

et al. 2017

Physica Status Solidi (a)

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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 non‐equilibrium 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.

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Challenges towards the simulation of GaN-based LEDs beyond the semiclassical framework

Cited by 8 publications

References 74 publications

Elimination of Lateral Resistance and Current Crowding in Large‐Area LEDs by Composition Grading and Diffusion‐Driven Charge Transport

Elimination of Lateral Resistance and Current Crowding in Large‐Area LEDs by Composition Grading and Diffusion‐Driven Charge Transport

On the Monte Carlo Description of Hot Carrier Effects and Device Characteristics of III‐N LEDs

Quantitative Multi‐Scale, Multi‐Physics Quantum Transport Modeling of GaN‐Based Light Emitting Diodes

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