Route to Commercialising GaN Growth on Diamond Based Heat Extracting Substrates

Lead Research Organisation: University of Bath
Department Name: Electronic and Electrical Engineering


The high breakdown voltage and large sheet carrier density of GaN based HEMTs provide major advantages for rf and microwave systems owing to their power handling capability. These advantages have also underpinned the emergence of GaN based components for low frequency power electronics. The latter is a major growth area as energy efficiency and sustainability become critical factors in the design of electrical systems. The overwhelming cause for reduced electrical power efficiency in active electronic components and systems is unwanted increases in operating temperature, which degrade power gain in amplifiers, the internal quantum efficiency of light emitting diodes and power conversion efficiency of diode lasers. As an example of the impact device heating on system efficiency, about 70% of the electrical power consumed by mobile telephone transmitter is wasted as heat owing to Joule heating in the electronics and consequent reductions in power gain of its constituent transistors.
The most effective way to limit the temperature rise of a semiconductor device is to introduce high-thermal conductivity heat spreading layers as close as possible to its active region, for example over the top of the device or growing the device structure on a thermally conducting substrate. Typically GaN based HEMTs of the type used in rf circuits and high power electronics are grown on SiC or increasingly Si wafers substrates. Whilst SiC is a better thermal conductor than Si, polycrystalline or crystalline diamond are far superior, better even than metals. Recently GaN HEMT grown on crystalline diamond substrates have been recently demonstrated. However, the small size (5 x 5 mm) of current crystalline diamond (PD) substrates and their high cost prohibit this ideal approach. Thus, a polycrystalline diamond substrate offers the best solution. The calculations show that the larger thermal conductivity of polycrystalline diamond could bring to power HEMT performance compatible or better than that on SiC or Si substrates.
To date, the most widely investigated method of exploiting PD substrates in GaN power HEMT technology has been to grow the III-Nitride layers on a Si substrate, then transfer the epitaxy to carrier substrate and finally bonding the device layers to the PD wafer. The procedure involves two wafer bonding steps, a process that requires minimal wafer bow if breakage is to be avoided, something that is difficult to achieve owing to the lattice mismatch between Si and III-Nitride materials. There is also a tendency for the final structure to delaminate and despite several years of development by companies like Group4Labs, SOITEC and Nitronex, commercial products are still not established.
To overcome these difficulties, an alternative approach has been developed by the Applicants in collaboration with Element 6. Briefly, this involves forming a composite structure comprising a thin layer of Si on a thicker layer of polycrystalline diamond, intimately contacted without wafer bonding. The upper Si surface is suitable for immediate III-Nitride growth. More information is given in section 3. One patent application has already been filed (world wide) and a second is in preparation; in both instances Bath has assigned its rights to Element 6.
Independently of Element 6 and other parties, Bath has developed methods for growing III-Nitride hetero-epitaxy on these complex Si/PD substrates. The results of applying these methods to realise high quality III-Nitride epitaxial layers on Si/PD substrates has recently been reported in ICNS9, critical details in the process were not disclosed and thus the opportunity exists to create an intellectual property portfolio covering the realisation of device grade III-Nitride epitaxial films on Si/PD heat extracting substrates to complement the very separate existing IP covering the manufacture of the latter. The new knowledge will be owned in its entirety with the University of Bath.

Planned Impact

Self-heating in semiconductor devices is a major problem causing a waste of energy and reduced reliability in electronic systems. This is a particular problem in power electronics due to the high currents and voltages involved. It is an equally severe problem in microwave and radio frequency (rf) systems as such mobile phone networks. For example, 70% of the electricity consumed by a mobile telephone base station is wasted as heat. The proposed Follow-on Project addresses this problem in electron devices and integrated circuits based on Gallium Nitride (GaN), a wide band gap semiconductor which is set to become the material of choice for a large number of low frequency and rf power applications. It aims to develop the semiconductor growth technology required to harness a new type of heat extracting substrate comprising a thin layer of Silicon backed by a much thicker layer of polycrystalline diamond. Of materials known to Mankind, only crystalline diamond has a higher thermal conductivity than polycrystalline diamond making the latter a robust and cost effective alternative. This new substrate technology is a result of collaboration between the Applicants and Element 6.
So far these substrates are untested as a suitable choice for the GaN microelectronics industry. This project addresses this supply chain issue. Briefly, The manufacturer of these substrates (Element 6) needs to know if the growth of device grade layers is commercially viable before investing in the infrastructure needed to upscale their facilities to manufacture 4-inch, 6-inch and possibly larger diameter substrates. Equally, potential downstream users of these substrates need the same knowledge before committing R&D resources to new products.
Stress in the composite silicon complicates, but does not prevent, the growth of high quality III-Nitride heteroepitaxial layers on composite Silicon-polycrystalline substrate. In their background work the Applicants have demonstrated the growth of thin high quality GaN epitaxial layers. They now seek to apply knowledge gained in other EPSRC grants (EP/D08022/1, EP/H049576/1 and EP/I012591/1) to advance the fabrication of GaN electronic devices on heat extracting substrates into a commercially viable technology.
Companies on the value chain will support with materials, technical inputs and advice in both the technical and commercial spaces to a total in-kind value of well over £50k. These companies are Element 6, IQE and International Rectifier. Other companies across the World would be customers of the arising technology. The markets for the resulting products are global, potentially expanding both high technology exports and employment in this and associated areas.
The planned outputs of any funded will have broader societal impact via improved energy efficiency, thus helping energy sustainability, power and control electronics that will enhance the safety and reliability of machinery and vehicles including aircraft (a large prospective market) and through expanding long term employment opportunities in technological industry. The research outputs will also be disseminated through organisations such as Silicon South West to assure greater awareness of GaN electronics and thus enhance further its economic take-up.


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