Symmetric Multicycle Rapid Thermal Annealing: Enhanced Activation of Implanted Dopants in GaN

Abstract

Selectively activated p-type regions are necessary for many electronic devices that require planar processing. The standard process of implanting p-type dopants, such as Mg, in GaN is notoriously more difficult than in other material systems, as the extremely high temperatures required to activate the implanted Mg also damage the GaN surface. In this research, a novel annealing technique is introduced for this purpose -symmetric multicycle rapid thermal annealing (SMRTA). It is shown that SMRTA is superior to the earlier developed multicycle rapid thermal annealing (MRTA) in terms of improvement of the crystalline quality of implanted GaN. The SMRTA technique was applied to Mg-implanted GaN to realize a rectifying junction. The annealing process detailed in this research will be a key enabling step for future GaN-based GaN and other III-nitride based semiconductors have received a great deal of attention from the research community due to favorable material properties which include a tunable direct bandgap, radiation hardness, and a favorable Baliga figure of merit compared to Si and SiC.1,2 The doping of p-type GaN during growth is challenging and has been the focus of numerous studies because it has many potential transformative applications in power electronics and optoelectronics.3 P-type dopant implantation and activation adds additional complexity to the synthesis of p-type GaN. The ability to implant and activate p-type species in GaN is a key enabling process for devices that require selective area doping. Device structures that benefit from the ability to selectively implant and activate p-type dopants in GaN include implanted guard rings for electric field spreading, implanted current blocking layers for current aperture vertical electron transistors, and implanted regions below contacts to lower contact resistance. 4,5 Unfortunately, the implantation and activation of Mg in GaN is difficult because of the high temperatures required (over 1300• C) for the activation anneal, 6 which are significantly higher than the decomposition temperature of GaN at atmospheric pressures (845 • C). 7 The decomposition of GaN occurs due to a loss of nitrogen and results in surface damage and the formation of N vacancies, which are compensating donors. 8To avoid GaN decomposition at the elevated annealing temperatures required for Mg activation, high pressure environments and non-equilibrium annealing conditions combined with capping layers have been investigated. 9,10 GaN can be successfully annealed at about 1500• C, but a 1.5 GPa overpressure of N 2 is necessary to avoid surface degradation.11 Such high gas pressures also require a complicated experimental setup and are not easily industrial scalable. 12A second, more scalable, alternative to the high pressure annealing of GaN relies on using non-equilibrium annealing conditions. For this type of annealing, a capping layer is used to prevent nitrogen loss from the GaN surface. Depending on the material, structure, and thickness of the capping layer, higher annealin...

Strain Recovery and Defect Characterization in Mg‐Implanted Homoepitaxial GaN on High‐Quality GaN Substrates

“…The total time at high temperatures can be extended to hours under these conditions, although it is recognized that there may be significant Mg diffusion during such long time anneals. As noted earlier, earlier studies based on Mg‐implanted GaN/sapphire structures possess a high intrinsic defect density (>10 8 cm −2 ) . Using high‐quality GaN substrates can promote homoepitaxial GaN film growth with a much lower intrinsic defect concentration (<10 6 cm −2 ) and is anticipated to further improve device performance as well as the ability to separate implant‐induced defects from those in the preexisting heteroepitaxial structure.…”

“…In any case, the Mg dopant activation requires annealing at a much higher temperature (≥1300 °C), but the critical issue is that the decomposition of GaN is significant at 840 °C at one atmosphere, which is well below the dopant activation annealing temperature. Annealing techniques have been developed to overcome this issue, including microwave‐based annealing and multicycle rapid thermal annealing (MRTA) . Both techniques utilize a fast heating source that can reach high temperatures while keeping the total anneal time short.…”

Review of the Recent Progress on GaN-Based Vertical Power Schottky Barrier Diodes (SBDs)

“…Besides, the extremely high temperatures required to activate the implanted Mg during the annealing process also damage the GaN surface [61,62]. Greenlee, J.D., et al [63] reported that the second annealing process followed by multicycle rapid thermal annealing (MRTA) process can compensate for the defects of the GaN surface and improve the crystalline quality of implanted p-GaN.…”

Materials and processing issues in vertical GaN power electronics

“…Mg) requires an annealing temperature over 1300°C [39], which causes the decomposition of GaN at atmospheric pressure. The decomposition brings about a loss of nitrogen, surface damage and the creation of nitrogen vacancies [33,40]. In order to prevent the loss of nitrogen loss at the GaN surface, it is desirable to deposit a high-quality capping layer before the annealing step, followed by a reliable removal process after annealing [41].…”

“…The MRTA process enabled the activation of implanted Mg with activation efficiency of over 8% and the demonstration of p-type conductivity [42]. Although the implantation-induced lattice damage can be restored by the proposed MRTA process, additional structural changes in the GaN sample were created because of the process with rapid heating and cooling cycles [33]. To overcome this issue, a modified MRTA process has been developed by introducing an conventional annealing step after the multi rapid heating and cooling cycles as shown in Fig.…”