Low temperature buffer offers the possibility to produce large-area gallium-nitride substrates at lower cost
In short: Gallium nitride (GaN) devices are often grown on foreign substrates such as sapphire or silicon carbide, which give rise to problems with defects and cracking. We have developed a technique to manufacture freestanding native GaN substrates. The technique uses a thin so-called buffer layer of GaN, grown on sapphire at low temperature (~600 oC). On this buffer layer, thick GaN is grown at standard temperature. Layers thicker than one millimeter spontaneously self-separate from the sapphire substrate. This low cost process has importance for the development of ultra-high brightness light emitting diodes.
Gallium nitride (GaN) is a semiconductor commonly used in energy-saving light emitting diodes and blue laser diodes, and it has been described as “the most important semiconductor since silicon”. By alloying it with aluminum or indium, its properties can be tuned in a wide range. For instance, the wavelength of the emitted light can be tuned from ultraviolet, across the visible spectrum, to the infrared. Thus, this unique characteristic makes it interesting for applications in optoelectronic, high-power and high-frequency devices. Because GaN offers very high breakdown voltages, high electron mobility, and saturation velocity it is also an ideal candidate for high-power and high-temperature microwave applications.
GaN devices are often grown on foreign substrates such as sapphire or silicon carbide, which give rise to stressed layers with high density of defects and problems with cracking due to the mismatch of the atom-to-atom distance and large difference in thermal expansion coefficient between the substrate and the GaN layer. Due to the large defect density, the operating lifetime and the light-emitting efficiency drops with higher current, so for more demanding devices such as visible or ultraviolet laser diodes, high brightness light emitting diodes for illumination, and possibly for high performance high power transistors, elaborative growth techniques or native GaN substrates are necessary. However, these fabrication techniques are expensive since it requires several complicated process steps. Thus, there is a need to further develop substrates for manufacturing of high performance GaN based devices.
Here we study an alternative fabrication technique to manufacture freestanding GaN substrates of two-inch diameter. The technique uses a thin so-called buffer layer of GaN grown on sapphire at low temperature (~600 oC). On this buffer layer, thick GaN is grown at standard temperature (~1000 oC) with a method known as halide vapour phase expitaxy. In this process, galliumchloride and ammonia is reacting to form GaN. The quality of the resulting GaN crystal is analyzed by X-ray diffraction, where a beam of X-rays is reflected off the surface of the crystal and produces a pattern of intensity peaks. High crystal quality with good atom regularity produces narrow peaks and vice versa.
We optimize the quality of the GaN layer with respect to the buffer thickness and the gas flows used during the growth of the buffer layer. It is found that peak width of the X-ray reflection is minimized using a buffer thickness about 100-300 nanometer. We also observed that the ratio between the gas flows of the galliumchloride and ammonia strongly affectes the crystalline quality.
For layers thicker than one millimeter, the GaN was spontaneous self-separated from the sapphire substrate and by utilizing this process, thick free freestanding two-inch GaN wafers were manufactured.
By using the manufacturing technique proposed here, several complicated process steps can be avoided and, consequently, the cost of producing GaN wafers is reduced. A lower manufacturing cost is necessary in order to ultilize the full potential of GaN technology and to develop ultra-high brightness light emitting diodes and laser diodes for applications such as general lightning, picoprojectors, water purifiers, medical applications, and power amplifiers for radars, base-stations and microwave ovens.
Additional details of the research can be found in Journal of Crystal Growth 366, 61 (2013).
Carl Hemmingsson and Galia Pozina
- Swedish Research Council (VR).
- Swedish Energy Agency.
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Last updated: 02/09/14