TECHNOLOGY LED DROOP
Figure. 2. Light-current characteristics of LEDs with In0.18
Al0.82 N EBLs
of various thicknesses. Inset shows equilibrium electronic band diagrams. These curves suggest that both hole-blocking and electron-confinement effects of the EBL should be qualitatively considered when addressing peak efficiency and efficiency droop for LED operating at high current densities. (Reprinted with permission from Appl. Phys. Lett. 101 161110 (2012). Copyright 2012 American Institute of Physics.)
possible to come up with droop-busting designs without uncovering a universal, unquestionable explanation for this energy-sapping mechanism.
harmful for device performance – but droop is the price that you’ll have to pay for this.
The reasoning behind this view is that the surprisingly high level of radiative recombination in such defect-ridden structures is a result of indium-rich, quantum-dot-like, localized states in InGaN quantum wells. At low currents, these states screen detrimental effects from crystalline defects, leading to a high quantum efficiency. But as the current through the LED is cranked up, more carriers overflow from the localized ‘shelter’ states to recombine non-radiatively in dislocations, causing the device’s quantum efficiency to plummet.
Today, this explanation of droop has fallen out of favour. That’s partly because it can’t explain why LEDs with much lower dislocation densities, which are formed on free-standing GaN substrates, are plagued by droop. However, it is also because many other conjectures for droop are being offered, due to mechanisms such as: Auger recombination (including direct band-to-band and indirect defect- or phonon- assisted recombinations); electron spill-over out of the active region; inefficient hole injection and transport in the active region; and several other theories, which all have their champions.
If you look at the academic papers that detail these conjectures, you’ll find that the data presented in each set of theoretical studies and experiments is fairly logical, and it supports the proposed mechanism; however, the findings and claims are not consistent with one another, and in some cases they can even be contradictory. This reveals that there is yet to be a unified, watertight explanation detailing the dominant mechanisms responsible for droop. Instead, prejudice abounds, with conclusions drawn that may heavily depend on a pre-emptive model. This state of affairs may even hamper efforts to get to the bottom of droop: It might be governed by several of the proposed mechanisms, which are inter-related and coupled to one another.
Fathoming the cause of droop is critical for advancing the understanding device physics, and it is one route towards the development of droop- free LEDs. But it is not the only way: It is also
50
www.compoundsemiconductor.net October 2013
Droop and carrier dynamics If you peruse the academic literature, you’ll find that all the leading conjectures for the origin of droop are related to carrier dynamics. Droop has been blamed on electron leakage, which is related to unsatisfactory carrier confinement; it’s been claimed to stem from poor hole transport into the active region, which depends on the injection of carriers and their concentration in each well; and droop has been linked to Auger recombination, which heavily depends on carrier density, so is influenced by injection efficiencies and carrier concentrations. Hence, tracking and understanding the injection, distribution and concentration of carriers will help with efforts to identify the origin of droop and possibly uncover ways to combat this malady.
It is critical that efforts to try and combat efficiency droop do not neglect the absolute value for peak quantum efficiency. Droop tends to be characterized by comparing the peak efficiency to that found at a high current density. It is possible to diminish droop by sacrificing the peak quantum efficiency, but that approach is not the right one to take, because the goal is to learn how to take LEDs that are really efficient at low current densities and replicate that performance at really high current densities.
Our US research team, a partnership between Georgia Institute of Technology, Arizona State University and the University of Houston, has focused our efforts at combatting droop on engineering carrier dynamics via alternative layer structures. Our modifications do not involve adjustments to the multi-quantum well active region, because experiments by other groups suggest that improvements in droop brought about by this come at the expense of the peak quantum efficiency of the LEDs (or even at the expense of quantum efficiencies over a wide range of current densities). Instead, we investigated how changes to the electron-blocking layers could influence electron confinement and the injection and transport of holes in the active region. As we looked at various different designs, our strategy was to: confine electrons in the active region as much as possible; inject as many holes into the active region as possible; and distribute, as uniformly as possible, both carriers among the wells within the active region.
Our first modification was to adjust the electron- blocking layer so that it is better at confining this carrier in the active region. AlGaN is the standard material for making the electron-blocking layer, which is sandwiched between a p-type layer and the active region and reduces the number of electrons that spill out of the quantum wells.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142