Inside view

Abstract

Gallium arsenide finds use in a great number of electronics applications, and one of these is antireflection components in optoelectronic devices. Professor Douglas Hall of the University Of Notre Dame in the US tells us a bit more about his group's latest work in this field. Yuan Tian, Jinyang Li, and Doug Hall outside the Notre Dame Nanofabrication Facility, where the processing and some of the characterisation work was performed Gallium arsenide (GaAs) is widely used for optoelectronic devices such as photodetectors, solar cells, and red through amber LEDs. Uncoated GaAs has a reflectance of ∼33-47% across the visible spectrum. All of these devices can benefit from an antireflection layer on the surface to improve light collection or extraction efficiency. Many researchers (dating back to Henry Minden at General Electric in 1962) have tried a wide variety of methods to form a useful, insulating dielectric layer on the III-V compound semiconductor GaAs, akin to silicon dioxide grown on silicon. However, similar thermal oxidation processes do not readily work for GaAs because the material decomposes (i.e., Ga and As atoms “dissociate”) at the temperatures over 800°C typically used with silicon. We have recently discovered a means to directly oxidise GaAs at the relatively low temperature of 420°C. In this Letter, we show that the GaAs oxide films formed have a suitable refractive index and sufficiently smooth interfaces to form a useful antireflection (AR) layer on GaAs. The thermal oxidation methods previously reported for GaAs have typically used temperatures in the 510-700°C range and often resulted in low quality films with poor adhesion and rough surfaces and interfaces due to extensive pitting caused by dissociation. Most prior work doesn't report surface or interface roughness for these reasons. To form a high-performance AR layer based on thin-film interference effects, very smooth top and bottom oxide interfaces are absolutely essential so that two highly specular reflections are able to interfere destructively (when a relative phase shift of 180° is obtained for a given thickness and refractive index). In addition to our reported atomic force microscopy measurements of roughness, the close match between modelled and measured reflectance for the GaAs oxide AR layers realised in this work demonstrates the excellent interface quality of the oxides obtained through our process. When conventional wet-thermal oxidation of aluminum gallium arsenide (AlGaAs) was discovered by John Dallesasse et al. at the University of Illinois in 1990, it was found to be widely applicable only to alloys containing higher Al/Ga ratios, given the much greater chemical reactivity of Al. Around 2001, we found that oxidation rates of lower Al content alloys could be significantly enhanced (an order of magnitude or more) by mixing dilute amounts of oxygen into the water vapor process gas. In subsequent studies, we have learned that even Al-free alloys like GaAs and InGaAs can be oxidised, but the gas mixture must be carefully optimised and precisely controlled within a quite narrow “sweet spot”, a range of ∼0.1-0.2% oxygen flow rates relative to our nitrogen carrier gas. If too high or too low, or if small amounts of air leak into the system, surface quality and uniformity degrade, or no oxide grows at all. Even in 1962, Minden mixed water vapor with oxygen in the first reported study on GaAs oxidation, but concluded that it had no effect for temperatures under 800°C! This indicates that finding the proper O2/H2O ratio is critically important, and not just any mixture of these gases will do. We have elucidated the “why and how” details of the chemistry involved in some of our group's earlier papers on this subject. Optical microscope image of three oxidised GaAs samples on top of an unoxidised GaAs wafer We have recently been working to further enhance several aspects of the process control in our III-V compound semiconductor oxidation furnace, especially to more precisely regulate and monitor the optimal mixed gas ratios. Our wider research interests include applications of these native oxides to improve optical waveguides for semiconductor lasers and photonic integration, and we are currently actively exploring the extension of our process to telecommunications-wavelength InP-based materials and devices. As much as oxides of high-Al content alloys have found very widespread use for current apertures in commercial vertical cavity surface emitting lasers over the past 10-20 years, we would like to see the enhanced performance of other optoelectronic and photonic devices through the incorporation of oxidised low-Al content, Al-free, and InP-based alloys enabled by the mixed-gas oxidation process demonstrated in this work.