Red AIGaAs light-emitting diodes - new indicator and display products containing red light-emitting diode - aluminum gallium arsenide

Hewlett-Packard Journal, August, 1988 by Frank M. Steranka, Dennis C. DeFevere, Michael D. Camras, Chin-Wang Tu, David K. McElfresh, Serge Rudaz, L., Louis W. Cook, Wayne L. Snyder

Red AlGaAs Light-Emitting Diodes

UNTIL RECENTLY, all commercially availabe visible LEDs were homostructures. That is, they consisted of pn junctions formed in one type of material (see Fig. 1). Over the past few years, new types of LEDs made of several layers of materials having different bandgaps (heterostructures) have appeared on the market. Heterostructure LEDs have several advantages over the standard homostructures, and LEDs made with them offer significant improvements in light output efficiency. The increase in efficiency is the result of the single-sided injection and reduced internal absorption that heterostructures can provide.

In homostructures under forward blas, electrons are injected into the p-type material and holes are injected into the n-type material. Some fraction of these minority carriers then recombines with the majority carriers on the p and n sides of the junction and emits the near-bandgap light characteristic of the LED. The radiative efficiency on the p and n sides is usually quite different and more light could be generated if minority-carrier injection into the less-radiatively-efficient material could be eliminated.

One way of achieving this is to have the pn junction occur at the interface between two materials of different bandgap. One can change the bandgap by changing the alloy composition in the AlGaAs system. This is done by changing the ratio of aluminum to gallium in the compound. The energy diagram for such a situation is shown in Fig. 2b with the homojunction case in Fig. 2a for comparison. The discontinuity in the valance band adds much more to the hole potential barrier than the conduction band discontinuity adds to the electron potential barrier. The band configuration depicted in Fig. 2b is that of a single heterostructure (SH) device, and it effectively eliminates hole injection into the wide-gap n-type material under forward bias and thus provides single-sided injection. The second advantage of this structure is that the wide-gap material is transparent to the light generated on the narrow-gap side of the junction. Hence, there is much less reabsorption inside the material than in the homostructure case.

The reabsorption can be further reduced by putting a second wide-gap layer on the other side of the narrow-gap layer as shown in Fig. 2C. This is called a double heterostructure (DH) device. The second heterointerface prevents the injected electrons from diffusing out of the narrow-gap active layer and gives the light that is generated a better chance to escape. A problem with making such devices, however, is that the wide-gap and narrow-gap materials must have nearly the same lattice constant to avoid dislocations at the material interfaces, which severely reduce the light-generation efficiency. In the AlGaAs system, the lattice constant changes very little with alloy composition, and this makes it a near-ideal system for making such devices. It has been used extensively for the last 15 years or so to fabricate high-efficiency infared LEDs and low-threshold lasers.

The AlGaAs system can also be used to fabricate efficient diodes that emit in the red portion of the spectrum. However, it is quite difficult to grow the high-aluminum-content layers that are required. Liquid phase epitaxy (LPE) is the only technology that can provide material quality comparable to that of the infared devices. It has taken many years of development to create LPE reactors capable of growing large volumes of high-quality multilayer devices. Many of these problems have now been solved and devices of this type are now available. This paper provides an overview of the different types of AlGaAs devices that are available and compares their performance to that of the other red LED technologies.

Types of Red AlGaAs LEDs

There are three versions of commercially available red AlGaAs LEDs and schematics of all three are shown in Fig. 3. The device shown in Fig. 3a is an SH LED and consists of at least two layers of AlGaAs on GaAs substrate. The first layer is the active layer made of AL.sub.35.Ga.sub.35.As. The top layer is a "window" layer made of AlGaAs with an Al mole fraction greater than *0.6. The light generated in the active layer can escape out the top and sides of the epitaxial material, but virtually all of the light that hits the substrate is absorbed there.

The SH device is the easiest to grow because it has the fewest layers and good thickness control is not essential. It is also the dimmest type of red AlGaAs chip that is available and is only marginally brighter than the best nitrogen-doped GaAsP (GaAsP:N) devices that will be described in the next section. Since it is the simplest to fabricate, it has been commercially available the longest.

A DH chip is shown in Fig. 3b. It consists of at least three AlGaAs layers on a GaAs substrate. The first layer is a wide-gap injecting layer and the second is a *2-[mu]m-thick active layer. Above the active layer is a thick wide-gap confining layer. As is the case for the SH device, most of the internally generated light in the DH chip depicted in Fig. 3b is absorbed by the substrate, so it has been labeled DH-AS for double heterostructure absorbing substrate. However, it is still twice as bright as the SH version because of reduced absorption in the epitaxial material.