TECHNOLOGY LED DROOP
Moving to a material with a wider bandgap for the electron blocker promises to improve the confinement of this carrier, but recent studies show that a switch to wider-bandgap Alx
Ga1-x not that effective. A more promising replacement is In0.18 Al0.82 N. It
combines the opportunity for lattice-matching with GaN with a wider energy bandgap than AlGaN and a larger conduction-band offset. What’s more, strained – and especially in-plane compressive- strained – Inx
Al1-x N electron-blocking layers offer
unique features for visible LEDs. They change strain in the blocking layer, which can offset the interface charges induced by spontaneous polarization between InAlN and GaN in the barrier of the active region, and also mitigate the quantum-confined Stark effect in multiple quantum wells.
When we replaced the ‘standard’ AlGaN electron- blocking layer with In0.18
Al0.82 N, we found that this
reduced the droop in the LED (see Figure 1). In the absence of an electron-blocking layer, there is a rapid efficiency droop: It is 69 percent, when defined as the decline between peak quantum efficiency and efficiency at a current density of 360 A cm-2
. In comparison, the LED with an
N electron-blocking layer has a droop of 30 percent, while that with InAlN has 18 percent droop. However, although that last figure represents an improvement, droop is still significant. That might be because other mechanisms besides electron spill-over contribute to efficiency droop, or that the suppression of electron spill-over is not complete, even with a wider-bandgap In0.18
Al0.2 Ga0.8 Al0.82 electron-blocking layer.
Adding an electron-blocking layer can actually be a double-edged sword. While it creates a barrier that stops electrons from leaking out of the device, it also can form a potential barrier for the injection of holes from a p-type layer (refer to the inset of Figure 2). Especially at low current densities, this barrier may limit hole injection, leading to lower device efficiency.
The In0.18 Al0.82 N layer is perfect for studying
electron confinement and hole injection simultaneously. All that is needed is to alter its thickness (see Figure 2). That’s not the case for AlGaN, because changes in aluminium richness don’t just change the barrier height of this blocking layer – they also influence p-type doping efficiency and strain.
Measurements of light output at different current densities with this series of LEDs provided an insight into device behaviour. Below current densities of 300 A cm-2
, an LED without an
electron-blocking layer produced a higher quantum efficiency than a similar device with a 5 nm-thick electron-blocking layer. Meanwhile, the variant with a 20 nm-thick electron-blocking layer
N
These results show that the injection of holes into the active region, plus their transportation through it, play a major role in the efficiency droop and peak quantum efficiency. To mitigate droop, every well within an active region must be populated with a uniform distribution of electrons and holes with the same concentration. Changing
To gain an insight into what is really happening in these devices, we extended the widely used ABC model, which features rate equations and efficiencies for various recombination paths, to include carrier spill-over and hole-injection effects. This led us to carry out the first ever theoretical and experimental study to determine the electron spill-over and the hole-blocking contributions to efficiency droop and limitations in peak quantum efficiency.
Modelling efforts revealed that, as expected, more spill-over leads to higher droop, and it also showed that it produced a small decrease in the peak quantum efficiency, which occurred at a lower current density (see Figure 3 (a), (b) and (c)). Hole blocking levels also impact droop, but not as significantly as electron spill-over (see Figure 3 (d), (e) and (f)). However, the current density that the peak efficiency occurs at is found to be more dependent on hole injection than electron over-spill.
N is
generally emitted less light than the device with a 15 nm-thick electron-blocking layer. Such results are impossible to explain when only considering the electron blocking effect of the InAlN layer.
Figure 3. Calculated QE versus injection current densities of LEDs for various spill-over currents ((a), (b), and (c)) and hole/electron concentration ratios ((d), (e), and (f)). The QE curves for the LEDs with high spill-over current show more pronounced droop. For the effects of hole injection, the peak QE depending on hole concentration seems to be more pronounced than that depending on spill-over. (Reprinted with permission from Appl. Phys. Lett. 101 161110 (2012). Copyright 2012 American Institute of Physics.)
October 2013
www.compoundsemiconductor.net 51
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