technology LEDs
Our efforts reveal that increases in the aluminium composition and thickness of the EBL can block electrons more effectively. But there’s a catch - these measures impede hole transport, and the upshot is a degradation in LED efficiency
Improvements in this key figure of merit are possible by optimising the thickness, doping density and aluminium composition of the EBL. Exposing the perfect combination by experimental efforts would be very costly and time- consuming, and a modelling-based effort has far more appeal. Such an approach is possible using numerical tools that we have developed at Crosslight Software, which is located in Burnaby, British Columbia, Canada. Our software, which has been used by many academic and industrial researchers to design the active region and passive layers of LEDs, determines the carrier transport and optical generation in these devices using multi-dimensional finite-element analysis. All salient features for nitride semiconductors are catered for, including polarization charge at heterojunctions and the influence of different crystal orientations on QW properties.
The core of the LED – the multi-quantum well, EBL and contact layer – is usually designed with one-dimensional simulations; two- and three-dimensional simulations tend to be employed for uncovering and understanding issues related to packaging, such as thermal effects, current spreading and optical extraction. We have simulated LED performance for a range of devices with different AlGaN EBLs. These efforts reveal that increases in the aluminium composition and the thickness of the EBL can block electrons more effectively. But there’s a catch – these measures impede
Figure 3.Voltage and current relations for a p-side up and p-side down LED device
hole transport, and the upshot is a degradation in LED efficiency. One theoretical solution to this problem is to ramp up the p-type doping density of EBL, a step that effectively increases the barrier for electrons and lowers that for holes. This is impractical, however, because it is difficult to obtain very high p-doping concentrations in GaN-based materials. The energy level for the commonly used acceptor dopant, magnesium, is very deep – at room temperature only about 1 percent is ionized and contributing to the hole density. Adding substantial amounts of magnesium into the structure is very difficult, and even if this were possible, it would degrade the device.
Debdeep Jena and co-workers from the University of Notre Dame, Indiana, have uncovered a possible way to overcome this hole doping issue. They have shown that an AlGaN layer with a properly graded aluminium composition can induce hole doping due to the intrinsic polarization of these materials. This technique promises to circumvent many of the difficulties associated with magnesium doping because polarization-induced hole doping is not thermally activated.
Figure 2.A novel,p-side down nitride LED that promises higher output powers and a lower forward
voltage.An EBL layer with aluminium composition graded from 10 to 15 percent induces hole density in this
layer.Note the direction of carrier injection is reversed,which helps quantum-wells to capture electrons and generate photons
26
www.compoundsemiconductor.net November/December 2011
Inverting the epi Traditionally, LEDs are fabricated by growing an n-doped region on top of the substrate and adding QWs, an EBL and a p-doped region (see Figure 1). This configuration has a major downside: The electric field that stems from interface polarization charges between the quantum barrier and the well sucks the carriers out of this region, hampering efficient carrier capture in the well and leading to a high leakage current. We have shown that a novel, p-side down LED architecture
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