Indium tin oxide spreading layers for AlGaInP visible LEDs

Abstract

In this paper the effectiveness of indium tin oxide (ITO) is assessed as a current spreading layer (CSL) for AlGaInP visible light emitting diodes (LEDs). A range of device structures has been fabricated in order to test the CSL characteristics. As a basis for this comparison test devices have been fabricated both with and without any ITO spreading layers and for comparison with an alternative GaP spreading layer. These measurements confirm that ITO is an effective alternative to the GaP structure with greater potential for applications in low-cost LED arrays.

Theoretical and experimental analysis of current spreading in AlGaInP light emitting diodes

“…This has been done for the devices reported here. Figure 5 shows a typical result of the data obtained by Morgan et al 4 The conclusion drawn from this data is that light ceases to be emitted when the potential falls below a critical experimental value V c that is close to the turn-on voltage V 0 of the idealized diode characteristic assumed in the modeling. Above this voltage, to a very good approximation the light intensity is L͑r͒ ϰ V͑r͒ − V c , in accordance with the data of Fig.…”

“…A lossy transmission line model proposed by Morgan et al 4 for calculating current spreading has been quantified by Morgan and Porch 9 for circular device geometries. The analysis can be extended to account for finite die size, as will be shown below, from which the radial dependence of both the radial spreading current density J s ͑r͒ and the current density J i ͑r͒ injected into the pn junction outside of the contact ͑when r Ͼ r 0 ͒ can be calculated.…”

“…1,2 A typical LED structure consists of an active region sandwiched between two carrier-confining layers, thus forming a double-heterojunction structure. [3][4][5] Issues relating to external quantum efficiency arise in such devices since current is injected via an opaque metal contact, so that light generated in the active region below the contact is shadowed by the contact itself. This problem is further exacerbated by the low conductivity of the top p-type cladding layer of the LED ͑Refs.…”

“…Morgan et al 4 described measurements of an exponential decay in light emission intensity radially outwards to the perimeter of the device, with the rate of decay increasing with increasing sheet resistance. A simple, infinite transmission line model was developed to characterize current spreading layers such as those grown onto AlGaInP surface emitting LEDs.…”

“…Assuming that the injected current is proportional to the emission intensity, good agreement between this model and the experimental data for ITO material was obtained. 4 This analysis has been extended so that the current spreading through current spreading layers of finite radial size can be calculated 9 and will be developed in Sec. II ͑be-low͒ in order to carry out a detailed comparison with recent experimental work.…”