BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs

Abstract

InGaN-based visible LEDs find commercial applications for solid-state lighting and displays, but lattice mismatch limits the thickness of InGaN quantum wells that can be grown on GaN with high crystalline quality. Since narrower wells operate at a higher carrier density for a given current density, they increase the fraction of carriers lost to Auger recombination and lower the efficiency. The incorporation of boron, a smaller group-III element, into InGaN alloys is a promising method to eliminate the lattice mismatch and realize high-power, highefficiency visible LEDs with thick active regions. In this work we apply predictive calculations based on hybrid density functional theory to investigate the thermodynamic, structural, and electronic properties of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3 are better lattice matched to GaN compared to InGaN and, for indium fractions less than 0.2, nearly lattice matched. Deviations from Vegard's law appear as bowing of the in-plane lattice constant with respect to composition. Our thermodynamics calculations demonstrate that the solubility of boron is higher in InGaN than in pure GaN. Varying the Ga mole fraction while keeping the B:In ratio constant enables the adjustment of the (direct) gap in the 1.75-3.39 eV range, which covers the entire visible spectrum. Holes are strongly localized in non-bonded N 2p states caused by local bond planarization near boron atoms. Our results indicate that BInGaN alloys are promising for fabricating nitride heterostructures with thick active regions for high-power, high-efficiency LEDs.InGaN light-emitting diodes (LEDs) with an electricity-to-light conversion efficiency of ~39% provide large efficiency gains and cost reductions compared to incandescent and fluorescent light sources. 1 However, InGaN LEDs suffer from decreasing internal quantum efficiency at high currents (efficiency droop), particularly at longer wavelengths (green gap). The cause of the droop has been extensively studied, with Auger recombination identified as a major loss mechanism. 2 For equal electron and hole densities, the Auger recombination rate is equal to the carrier density cubed times a material-dependent Auger coefficient ! . Since the Auger coefficient is an intrinsic property of InGaN that does not depend strongly on composition, temperature, or strain, 3 the carrier density at a given current density must be lowered to reduce the Auger losses.Increasing the active-region volume is a straightforward approach to reduce the carrier density and hence the Auger loss, yet growth challenges limit its practicality. Devices using a single thick InGaN layer exhibit higher high-power efficiency than thinner quantum wells both for polar 4 and for semipolar 5 growth orientations. The thickness of InGaN active layers is however limited by the lattice mismatch with the underlying GaN layers, and the subsequent appearance of performance-degrading dislocations. On the other hand, the efficiency of multiple-quantum-well (MQW) structures is...

Boron influence on bandgap and photoluminescence in BGaN grown on AlN

“…The quaternary BInGaN alloys are also predicted to maintain bandgaps similar to InGaN at a given indium concentration, meanwhile reducing the lattice mismatch to GaN. 9 This gives the opportunity to overcome the problem of efficiency drop at longer wavelengths in InGaN LEDs. Ab initio calculations predict zero heterointerface polarization for BGaN/BAlN and BAlN/BGaN heterojunctions with a certain B content, 10 which allows for the elimination of quantum-confined Stark effect present in GaN-based devices.…”

Lattice-constant and band-gap tuning in wurtzite and zincblende BInGaN alloys

“…These quaternary alloys maintain an approximate lattice match to GaN while allowing for a band gap that is adjustable throughout the visible range. 9 Co-alloying has previously been demonstrated as a method to independently vary the lattice constant and band gap in other material systems. Chai et al found that the incorporation of Bi into In0.53Ga0.47As to form In0.53Ga0.47BixAs1-x increases the lattice constant and lowers the band gap for x up to 0.058.…”

BAlGaN alloys nearly lattice-matched to AlN for efficient UV LEDs

“…We have also previously shown that co-alloying produces BInGaN alloys that lattice match GaN with reduced band gaps. 12) Applying Vegard's law to BAlGaN alloys and using our calculated lattice constants of GaN (3.1927Å), AlN (3.1183Å), and w-BN (2.5516Å), we estimate that alloys with a composition of approximately (B 0.116 Ga 0.884 ) x Al 1-x N (B-to-Ga ratio of approximately 1:7.62) are lattice matched to AlN.…”

“…This behavior is the opposite of the trend seen in boron doping of InGaN alloys, in which the c lattice direction is more strongly affected by boron incorporation than the a lattice direction. 12) The exact cause of this behavior is unclear, but we speculate it could be due to anisotropic lattice distortions of the boron local bonding environments.…”

“…We have also previously shown that co-alloying produces BInGaN alloys that lattice match GaN with reduced band gaps. 12 Previous theoretical and experimental work on BAlGaN alloys is rather limited. Park and Ahn performed theoretical studies on wurtzite BAlGaN alloys for UV LEDs using multi-band effective mass theory.…”