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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. These Energy-Band Diagrams are typical of two alternative structures of AlxInyGa1-x-yN-based ultraviolet LEDs.
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.

Figure 2. A Light-Emitting Triode is similar to a light-emitting diode, except that the triode has two anodes. By biasing the two anodes to different potentials, one can increase the injection of holes from the p-type confinement region into the active region and reduce non-radiative electron/hole recombination in the p-type confinement region.
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.

Topics:
Computer simulation Electronics & Computers Light emitting diodes (LEDs) Research and development Simulation and modeling
This Brief includes a Technical Support Package (TSP).
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AlxInyGa1-x-yN Ultraviolet Light-Emitting Triodes

(reference ARL-0025) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the February, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 1).

    Read more articles from the archives here.

    Overview

    This document presents research on GaN-based light-emitting diodes (LEDs), particularly focusing on a novel device called the light-emitting triode (LET). The study addresses the critical challenge of efficiency droop in III-V nitride pn-junction devices, which is characterized by a decrease in efficiency with increasing injection current. This phenomenon is particularly problematic for deep UV LEDs, which have low internal quantum efficiency.

    The research highlights the role of carrier loss over the electron-blocking layer (EBL) as a significant factor contributing to efficiency droop. Through current transport simulations using the Crosslight APSYS program, the authors confirm that electron overflow into the p-type GaN cladding layer occurs due to low hole-injection efficiency into the multi-quantum well (MQW) active region. This loss of carriers is identified as a viable hypothesis for the physical origin of the efficiency droop.

    The LET is designed to enhance hole injection efficiency by utilizing a lateral electric field created between two anodes. This field provides additional energy to carriers, allowing them to overcome potential barriers and improving their injection into the active region. The study demonstrates that applying a negative bias to the second anode increases both the current and light output power, confirming the effectiveness of this approach.

    Theoretical calculations indicate that the lateral electric field can significantly raise the temperature of holes, enabling them to surmount barriers posed by superlattices or the EBL. The findings suggest that the LET can serve as an effective tool for investigating the relationship between hole-injection efficiency and efficiency droop, thereby contributing to the understanding and resolution of this major issue in LED technology.

    In conclusion, the research provides valuable insights into the mechanisms behind efficiency droop in GaN-based LEDs and proposes the LET as a promising solution for enhancing hole injection efficiency. This advancement is expected to facilitate the development of more efficient UV emitters, which are crucial for applications in chemical and biological detection, sterilization, and phosphor-based white LEDs. The document emphasizes the importance of addressing efficiency challenges to advance III-V emitter technology.

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