Reduced nonthermal rollover of wide-well GaInN light-emitting diodes

Maier, M.; Köhler, K.; Kunzer, M.; Pletschen, W.; Wagner, J.

doi:10.1063/1.3073860

Cited by 68 publications

(41 citation statements)

References 14 publications

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“…. This is especially helpful for the design of thick quantum wells or double heterostructure devices to mitigate the effects of the efficiency droop [2]. Limited availability of large-scale freestanding nonpolar GaN wafers as well as their high price favours the heteroepitaxial growth on foreign substrates.…”

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confidence: 99%

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

Mauder

Reuters

Khoshroo

et al. 2010

Journal of Crystal Growth

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The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO 2 substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 µm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) planes appear for samples with a thickness of ~0.5 µm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [11][12][13][14][15][16][17][18][19][20] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ~10 20 cm -3 to ~10 19 cm -3 for film thicknesses of 0.2 µm and ~1 µm while the electron mobilities rise from ~20 to ~130 cm 2 /Vs. The mobilities are significantly higher in [0001] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening. . This is especially helpful for the design of thick quantum wells or double heterostructure devices to mitigate the effects of the efficiency droop [2]. Limited availability of large-scale freestanding nonpolar GaN wafers as well as their high price favours the heteroepitaxial growth on foreign substrates. (100) LiAlO 2 is an interesting option for this approach because of the very low lattice mismatch with m-plane (1-100) GaN. It amounts to -1.7 % and -0.3 % in [11][12][13][14][15][16][17][18][19][20] and [0001] direction of GaN, respectively, giving rise to anisotropic strain in the layer [3]. In addition to that, the material is produced by the comparably cheap Czochralski pulling method. A major disadvantage of this substrate is the thermal and chemical instability which demands special care on the epitaxial process and leads to highly n-type doped films, most likely related to oxygen incorporation [4]. The growth of high-quality m-plane GaN on LiAlO 2 was already reported using various techniques as molecular beam epitaxy (MBE) [5], hydride vapour phase epitaxy (HVPE) [6] and metal organic vapour phase epitaxy (MOVPE) [7]. However, only few reports contain detailed data on the anisotropic film properties [8]. In this contribution, we present an elaborate investigation on the development of the anisotropic crystal properties during MOVPE of m-plane GaN on LiAlO 2 .

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confidence: 99%

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

Mauder

Reuters

Khoshroo

et al. 2010

Journal of Crystal Growth

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“…Further details on epitaxy and LED structure can be found in Ref. [5]. In the following, we concentrate, without loss of generality, on LEDs grown on ULD templates due to space limitations.…”

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confidence: 99%

“…Gardner et al [4] reported a delayed onset of the efficiency droop for LEDs with a double-heterostructure or wide well (WW) active region instead of narrow QWs, leading to higher maximum QE reached at high driving currents. Recently, Maier et al [5] provided evidence that combining the WW active region concept with growth on low defect density substrates is a promising approach to overcome the efficiency reduction at high driving currents at least for near-UV (about 400 nm) LEDs. In this paper, we present a detailed study of the carrier density dependent QE and carrier recombination times of such LEDs, from which a model is derived based on the interplay between defect assisted recombination, depending on dislocation density, and strongly carrier density dependent loss processes.…”

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Towards a deeper understanding of the reduced efficiency droop in low defect‐density GaInN wide‐well LEDs

Maier

Passow

Kunzer

et al. 2010

Phys. Status Solidi (c)

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A series of 400 nm emitting GaInN/GaN single-well light-emitting diodes, grown on ultra-low dislocation density GaN templates with well widths varying between 3 and 18 nm, were investigated by pulsed and time resolved electroluminescence measurements using small signal modulation technique for the latter. A reduction of the efficiency droop at high current densities with increasing well width was observed. The highest overall external quantum efficiency was obtained for LEDs with 11 nm thick double-heterostructure (or wide well) active region at current densities above 70 A/cm2. Furthermore, carrier lifetime and volume carrier density in the wells were determined. A model based on the interplay between defect assisted recombination, depending on dislocation density, and strongly carrier density dependent loss processes is presented explaining the well-thickness dependence of the efficiency droop

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“…One established solution is the use of thick InGaN QWs. [8][9][10] However, because of rapidly decreasing material quality of thick indium-rich InGaN films, no improvement over state-of-the-art multi-QW (MQW) LEDs could be shown. Alternatively, the active-layer carrier density can be reduced by enabling MQW operation.…”

Section: Figure 2 Emission Power Versus Current Density For a Green-mentioning

confidence: 99%

Improving the high-current efficiency of LEDs

Laubsch¹

2009

SPIE Newsroom

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Understanding the origin of efficiency losses is key to developing the ultimate solid-state light source.Indium gallium nitride (InGaN) has great potential in LEDbased applications becasue of its ability to emit in the shortwavelength region, from near-UV to green. The emitting region is deposited as a quasi-2D quantum-well (QW) heterostructure composed of InGaN and gallium nitride. The past decade has seen tremendous progress in both epitaxial (or crystaline-layer) growth and InGaN LED design.We recently demonstrated a wall-plug efficiency (i.e., the conversion of electrical to optical power as measured from the line source to the resulting emission) of about 60% for a blue ThinGaN R LED. 1 The chip-level light-extraction efficiency of such devices has reached values in excess of 80%. However, the internal quantum efficiency for light generation by electron-hole carrier recombination remains lower. Even worse, the internal efficiency usually peaks around current densities considerably below the operating current and decreases monotonously towards higher currents, a phenomenon frequently called 'droop.' Understanding and reducing droop is crucial to reach the ultimate efficiency of InGaN-based LEDs. Various mechanisms that may cause this effect have been suggested, including carrier escape, 2 losses due to dislocations, 3 and the Auger effect. 4,5 In our work, we validate that the efficiency drop at high current is QW internal. We compare temperature-and excitationpower-density-dependent resonant photoluminescence (PL) to electroluminescence (EL) using a green-emitting InGaN single-QW (SQW) LED structure. We measure PL and EL on the same ThinGaN chip. For PL excitation, a 405nm laser optically excites electron-hole pairs well below the band-gap energy of the GaN barrier. 6 This enables us to reliably isolate QW-internal losses from losses due to parasitic carrier recombination outside the QW. Our experimental data strongly supports QW-internal loss. Figure 1. Internal efficiency of a green-emitting single-quantum-well (SQW) LED measured by electroluminescence (EL, empty boxes) compared to that measured in a resonant-photoluminescence (PL) experiment (solid boxes). Both experiments were performed at 300 (black) and 4K (red). Carrier generation and recombination in EL and PL are shown schematically in the conduction-and valence-band (CB/VB) diagrams in the insets. a.u.: Arbitrary units.All significant trends of the EL efficiency are followed perfectly by the resonant-PL efficiency (see Figure 1).We can reproduce the dependence of the internal efficiency on current density, J, using a simple rate-equation model of the form J ∼ A × n + B × n 2 + C × n 3 , where n is the QW carrier density and A, B, and C are coefficients for nonradiative, radiative, and (nonradiative) Auger-like recombination, respectively. This model can describe the emission characteristics of greenemitting InGaN-based LED over a wide current-density range (see Figure 2). We thus identify a high-density QW-internal Auger-like process as the culpri...

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Reduced nonthermal rollover of wide-well GaInN light-emitting diodes

Cited by 68 publications

References 14 publications

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

Towards a deeper understanding of the reduced efficiency droop in low defect‐density GaInN wide‐well LEDs

Improving the high-current efficiency of LEDs

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