Germanium set to storm substrate market

17 Jun 2002

With the availability of gallium arsenide substrates in question, researchers are looking for an alternative substrate for growing optoelectronic devices. Belgian company Union Minière believes that germanium could supplant gallium arsenide in components such as high-efficiency LEDs. Johan van der Linden reports.

From Opto & Laser Europe May 2001

Wafer mapping graph (1)

"All optoelectronic components that are grown on GaAs can be grown on germanium with comparable performances," says Ingrid Moerman, project manager at the University of Gent's department of information technology in Belgium. Her group has been using germanium - a substrate usually associated with solar cells - to make optoelectronic components.

Germanium substrates are available for roughly half the price of 4 inch GaAs wafers. Although this translates to a few percent in an end-product such as an LED, it remains significant because price pressures in this field are massive. Both 6 and 8 inch germanium wafers should also be coming to market soon, making component production even more cost-effective. Germanium exhibits a higher thermal conductivity than GaAs, so hetero-epitaxial devices could potentially be driven towards a higher optical output regime without reliability or lifetime constraints. Cleaving germanium is, however, more difficult than GaAs, and special care has to be taken to obtain perfectly cleaved facets.

Wafer mapping graph (2)

Moerman's team is involved in a feasibility study funded by the world's largest supplier of germanium: Belgian firm Union Minière (UM). Paul Mijlemans, business development manager of the advanced materials group, estimates that, if germanium were to replace GaAs in all feasible optical applications, UM would see sales of germanium wafers for optoelectronics applications outgrow its sales for the solar-cell market. It is this huge potential market that led the company in 1998 to form a joint venture with US firm Emcore to explore new markets for this versatile material.

Polishing

Mijlemans said: "Until a few years ago, germanium wafers were almost exclusively deployed in solar cells for space applications. For many years our developers have focused on meeting the increasingly stringent specifications of the solar-cell market. This gave us little margin to explore new markets during that period."

During the first year of the joint venture with Emcore, the focus was on the use of germanium substrates in the production of high-brightness light-emitting diodes (LEDs), sensors, and other specialized electronics applications. Moerman's group at INTEC carried out an important part of the hetero-epitaxy development for LEDs.

4 inch wafers

The group came up with its first result in December 1998, when it demonstrated the first room-temperature continuous-wave laser grown on germanium substrates. "We were very surprised by the direct room-temperature, continuous-wave operation from the first growth run," said Moerman. This is because some lattice matching issues were expected to cause problems.

Resonant cavity LED

As germanium is a single element its lattice structure is non-polar. GaAs has a polar structure, and this can cause lattice mismatch problems when growing layers. However, Moerman's group has overcome these problems. "It's not too difficult. For solar-cell applications, we introduced an intermediate GaAs buffer layer. Autodoping effects and wafer bowing formed the more challenging problems for producing efficient light emitters at kick-off."Because bow induces a temperature gradient over the wafer during growth, it limits the homogeneity for some material systems. Despite these difficulties, the team has reported equivalent quantum efficiencies for both AlGaAs/Ge LEDs and InGaAs/ Ge laser diodes compared with devices with the same multiple-quantum-well structure grown on a GaAs wafer in the same run.

Glass-based lenses

"Although slightly higher threshold currents were observed, laser operation was readily obtained, without any thermal cycling, selective growth on small areas or epitaxial lateral overgrowth, which are commonly used techniques in other hetero-epitaxial systems," said Moerman.

The group has now turned its attention onto AlGaInP/Ge resonant-cavity LEDs emitting at visible wavelengths for applications that include traffic-lights, displays, scanners, printers, high-definition televisions and short-haul plastic optical fibre-based communications (box 1).

The drawback that germanium does have is its incompatibility with GaN production processes - germanium has a lower melting temperature than GaN. This means that blue and green LEDs based on germanium are not feasible unless a low-temperature process can be developed.

Despite this, Union Minière believes that the market is huge and is actively looking for partners to develop and market devices. The company is currently in discussion with several LED manufacturers, and Mijlemans believes that germanium-based LEDs could be on the market in commercial quantities by the beginning of next year.

Researchers at Gent University's department of information technology are using germanium in AlGaInP/Ge resonant-cavity LEDs (RC-LEDs). These emit at visible wavelengths for a variety of applications, including traffic-lights, displays, scanners, printers, high-definition televisions and short-haul plastic optical fibre-based communications. Plastic optical fibre exhibits a low absorption window near 650 nm, making it an ideal application for RC-LEDs

RC-LEDs show enhanced spontaneous emission and a narrow linewidth as well as a lower beam divergence compared to conventional LEDs. They employ an active region placed inside a Fabry-Perot cavity formed by stacked AlAs/AlGaAs distributed Bragg reflectors. Germanium is opaque in the visible wavelength region, so Ingrid Moerman's group used a top-emitting structure with a highly reflective bottom mirror.

"With a new mask design we were able to circumvent previously encountered problems, such as current injection non-uniformity, high serial resistance and poor reliability," said Moerman. For the latest processed devices, the external quantum efficiency increased to 5.2% at 4 mA and she observed an optical output power of nearly 2 mW at 20 mA.

Moerman added: "A possible limiting factor is germanium's narrower bandgap compared with that of GaAs. The latter is transparent from about 870 nm - quite important for pump devices working at 980 nm - whereas germanium remains opaque up to about 1875 nm." This, however, is only important for substrate-emitting devices. Germanium is also used in lenses for infrared optics, phosphors for the lighting industry and in the manufacture of optical fibre for establishing the radial refractive index gradient in a fibre preform.

For infrared optics, germanium has consistently outperformed lenses and windows made from other materials. Owing to their high price-tag, however, germanium lenses are only used for high-specification infrared imaging devices, mainly for the military.

To tackle this issue, Union Minière's subsidiary, Vertex in Rennes, France, has developed a manufacturing process for chalcogenide glasses containing up to 20% germanium. These offer high quality at low cost for high-volume commercial infrared applications, such as night-vision systems for the automotive industry. This cost reduction has been achieved by simplifying the shaping process and replacing it with a single-step moulding operation.

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