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Ballistic transport in InGaN-based LEDs: impact on efficiency
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Semicond. Sci. Technol. 26 014022 (http://iopscience.iop.org/0268-1242/26/1/014022) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
SEMICONDUCTOR SCIENCE AND TECHNOLOGY
doi:10.1088/0268-1242/26/1/014022
Semicond. Sci. Technol. 26 (2011) 014022 (12pp)
Ballistic transport in InGaN-based LEDs: impact on efficiency ¨ Ozg ¨ ur ¨ 1 , X Ni1 , X Li1 , J Lee1 , S Liu1 , S Okur1 , V Avrutin1 , U A Matulionis2 and H Morko¸c1 1 Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA 2 Semiconductor Physics Institute, Center for Physical Sciences and Technology, A. Goˇstauto 11, 01108 Vilnius, Lithuania
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Received 9 June 2010, in final form 22 September 2010 Published 29 November 2010 Online at stacks.iop.org/SST/26/014022 Abstract Heterojunction light-emitting diodes (LEDs) based on the InGaN/GaN system have improved considerably but still suffer from efficiency degradation at high injection levels which unless overcome would aggravate LED lighting. Although Auger recombination has been proposed as the genesis of the efficiency degradation, it appears that the premise of electron overflow and non-uniform distribution of carriers in the active region being the immediate impediment is gaining popularity. The lack of temperature sensitivity and sizeable impact of the barrier height provided by an electron blocking layer and the electron cooling layer prior to electron injection into the active region suggest that the new concept of hot electrons and ballistic/quasi-ballistic transport be invoked to account for the electron overflow. The electron overflow siphons off the electrons before they can participate in the recombination process. If the electrons are made to remain in the active region e.g. by cooling them prior to injection and/or blocking the overflow by an electron blocking layer, they would have to either recombine, radiatively or nonradiatively (e.g. Shockley–Read–Hall and Auger), or accumulate in the active region. The essence of the proposed overflow model is in good agreement with the experimental electroluminescence data obtained for m-plane and c-plane LEDs with/without electron blocking layers and with/without staircase electron injectors. (Some figures in this article are in colour only in the electronic version)
physical mechanisms behind this degradation in efficiency to be understood and the problem mitigated. A fitting comment is that none of the above would have come into being if it were not for the seminal pioneering work done on heterostructures [3–5]. The physical origin of the EL efficiency loss at high currents in InGaN LEDs is heretofore not clearly understood and controversial, and thus the topic is open to further investigations. Carrier loss through nonradiative Auger recombination at high injection currents has initially been proposed for the efficiency degradation [6–8]. The Auger recombination coefficient deduced from a fit of the rate equation to the experimental photoluminescence (PL) data in an earlier effort is 1.4–2.0 × 10−30 cm6 s−1 for quasibulk InGaN layers [6], but varies several orders of magnitude
1. Introduction Performance of InGaN-based light-emitting diodes (LEDs) has improved considerably to the point where they now penetrate outdoor general lighting applications and are poised to penetrate indoor lighting applications as well. Lighting by LEDs is advantageous in terms of energy savings and long operation lifetime. One pivotal issue surrounding the application of the InGaN LEDs for general lighting is the lack of retention of the electroluminescence (EL) efficiency at high injection currents [1]. This manifests itself as the external quantum efficiency (EQE) reaching a peak value at relatively small current densities, such as 50 A cm−2 or lower, followed by a monotonic decrease even under low duty cycle short pulsed current operation [2]. It is essential for the 0268-1242/11/014022+12$33.00
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Semicond. Sci. Technol. 26 (2011) 014022
among different reports, 10−27 –10−24 cm6 s−1 , the latter group of figures being some three orders of magnitude or more higher than the other reported values [6, 8–11]. Note that the Auger recombination coefficient decreases exponentially with the bandgap energy if the process involves transitions across the gap owing to a seminal contribution [12], which is supported by a fully microscopic many body model [13]. This suggests that the carrier losses due to the Auger effect in InGaN-based LEDs, particularly in those emitting at energies away from the resonant conduction band state situated 2.5 eV above the bottom of the conduction band [8], would not necessarily be dominant. There are also other experimental observations which are not consistent with the Auger recombination proposal. For example, in below-the-barrier resonant photo excitation experiments, in which the photons are absorbed only in the InGaN active region with ensuing generation of equal number of cool electrons and holes followed by either radiative or nonradiative recombination only in the same region, the internal quantum efficiency (IQE) degradation has not been noted at photocarrier generation rates comparable to, if not beyond, the electrical injection levels where the EL efficiency degrades [14, 15]. Rather the IQE increases with optical generation and in fact reaches over 95% for carrier generation rates of 2 × 1018 cm−3 , the density being dependent on the radiative recombination coefficient used. This would then suggest that the EL efficiency degradation is of electrical nature and that it is very likely to be related to carrier injection, transport and leakage processes. The above discussion narrows the choices to electron overflow as being responsible in structures that are employed at the moment. It has been observed that a substantial EL efficiency reduction (by four to five times) occurs when an electron blocking layer (EBL) is not employed, regardless of whether polar or non-polar surfaces of GaN are used [16]. In InGaN LEDs, while not being the entire reason at this juncture, relatively low hole injection (due to relatively low hole doping of p-GaN) and/or poor hole transport inside the active region (due to large hole effective mass if quantum wells constitute the active region) could exacerbate the electron overflow, as electrons need accompanying holes in the active region for recombination [15, 17]. Theoretical calculations also indicate that electron density in equilibrium with the lattice even well above the room temperature would not have a sufficient Boltzmann tail to surpass the barrier present for notable electron spillover [18]. This further narrows the discussion in that the non-equilibrium processes must be invoked to account for electron overflow. In this regard, this paper treats ballistic and quasi-ballistic electron transport across the InGaN active region as a substantial source for electron overflow and the associated EL efficiency loss, as those electrons escape the radiative recombination in the active region. Evidence for ballistic transport and the associated electron leakage has been obtained from the temperature-dependent characteristics of InGaN LEDs and laser diodes [19–21]; however, ballistic electron transport has so far been included in the analysis of electronic devices only [1, 22]. Moreover, we demonstrate that an InGaN staircase electron injector (SEI, with a step-like
increased In composition, each corresponding an energy step equal to or greater than an LO phonon energy) on the n-side of the active region reduces if not fully eliminates the ballistic and quasi-ballistic electron overflow. The SEI structure serves to cool the electrons and bring them into equilibrium with the lattice in the active region where their radiative recombination with holes takes place if the holes are present. Ultimately, though, holes must be present for radiative recombination with electrons, and increase of the hole density is naturally expected to improve the efficiency in LEDs featuring designs to curb the ballistic transport and thus the ensuing electron overflow. It should be noted that the concept of non-equilibrium hot electrons in the context of light emitters is nonconventional and represents a major departure from the proverbial treatments. One might then suggest that models, particularly the commercially available software packages used to model the LEDs under discussion, which are void of the treatment of hot electrons, would be off target. We should also point out that the hot electron transport discussed here would also apply to semiconductor injection lasers.
2. Experimental procedures The investigated LED structures were grown on m-plane (1 1¯ 0 0) GaN or c-plane (0 0 0 1) sapphire substrates in a vertical low-pressure metalorganic chemical vapor deposition (MOCVD) system. In the case of m-plane, the ∼500 μm thick m-plane freestanding GaN substrates, sliced from boules grown in the c-direction (produced at Kyma Technologies, Inc.) had a threading dislocation density of
