AlxInyGa1-x-yN Ultraviolet Light-Emitting Triodes
Efficiency droop, heretofore observed in ultraviolet lightemitting diodes, can be overcome.
Light-emitting triodes (LETs) have been the focus of a program of research with a goal of increasing the quantum efficiencies of ultraviolet-lightemitting devices implemented in the AlxInyGa1-x-yN (0 ≤ x+y ≤ 1) material system. Heretofore, AlxInyGa1-x-yN-based light-emitting diodes (LEDs) have exhibited "efficiency droop" — decreasing quantum efficiency with increasing injection current. LETs can be used as means of studying and overcoming efficiency droop.
Figure 1 depicts relevant aspects of two alternative structures typical of AlxInyGa1- x-yN-based ultraviolet LEDs. In one structure, an electron blocking layer (EBL) is inserted between an electron-acceptortype (p-type) cladding layer and a multiple- quantum-well (MQW) active region. The EBL serves to prevent the overflow of electrons from (and thereby confine electrons to) the active region. If the EBL is heavily p-doped, then it does not impede the injection of holes into the active region. However, depending upon the chemical composition of the specific LED, heavy p-doping of the EBL is not possible, in which case, in addition to confining electrons to the active region, the EBL also constitutes a potential barrier for holes, hindering injection of holes into the active region and thereby limiting the internal quantum efficiency.
In the other alternative structure, instead of an EBL, there is a p-type (Mg-doped) AlxGa1-xN/GaN superlattice. In such a superlattice, the transport of charge carriers is enhanced in lateral directions [directions in the superlattice planes (directions in planes perpendicular to the page)]. However, transport of charge carriers through the p-n junction (along the right/left direction in the page), is less efficient than along the lateral directions because most of the holes ionized from the acceptors are localized inside the quantum wells, which are clad by potential barriers as high as 0.1 to 0.4 V. Like the potential barrier in the EBL in the structure described above, the potential barriers in this structure hinder the desired injection of holes into the active region. Efficient injection of holes into the active region of an LED is necessary for high radiative efficiency. In inefficient injection of holes into the active region, internal quantum efficiency is reduced because electrons diffuse through the active region into the p-type confinement layer, wherein non-radiative electron/ hole recombination is likely.
The main difference between an LED and a corresponding LET of a type considered in the present research is that whereas the LED has one anode, the LET has two anodes (see Figure 2). The anodes can be biased at different potentials, causing a current to flow between the anodes (in a lateral direction). The electric field between the differentially biased anodes accelerates holes to higher energies, thereby increasing their probabilities of overcoming the EBL potential barrier or of quantum tunneling through the EBL or superlattice potential barrier(s), thereby, further, increasing the injection of holes into the active region and correspondingly increasing the internal quantum efficiency.
It has been proposed to use an LET to investigate the relationship between the hole injection efficiency and the efficiency droop: Computational simulations have led to the tentative conclusion that in a case of low efficiency of injection of holes into the active MQW region, the overflow of electrons into the p-type cladding layer is the mechanism responsible for the efficiency droop. Therefore, the LET, in which the hole injection efficiency can be controlled by controlling the bias potential on a second anode, is expected to be a means of confirming the above-mentioned tentative conclusion concerning the cause of the efficiency droop.
This work was done by E. Fred Schubert and Jong Kyu Kim of Rensselaer Polytechnic Institute, for the Army Research Laboratory.
This Brief includes a Technical Support Package (TSP).
AlxInyGa1-x-yN Ultraviolet Light-Emitting Triodes
(reference ARL-0025) is currently available for download from the TSP library.
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