Thermal Diffusion Boron Doping of Single Crystal Diamond
With the best overall electronic and thermal properties, single crystal diamond (SCD) is the extreme wide bandgap material that is expected to revolutionize power electronics and radio frequency electronics in the future. However, turning SCD into useful semiconductors requires overcoming doping challenges, as conventional substitutional doping techniques, such as thermal diffusion and ion implantation, are not easily applicable to SCD. Here we report a simple and easily accessible doping strategy demonstrating that electrically activated, substitutional doping in SCD without inducing graphitization transition or lattice damage can be readily realized with thermal diffusion at relatively low temperatures by using heavily doped Si nanomembranes as a unique dopant carrying medium. Atomistic simulations elucidate a vacancy exchange boron doping mechanism that occur at the bonded interface between Si and diamond. We further demonstrate selectively doped high voltage diodes and half wave rectifier circuits using such doped SCD. Our new doping strategy has established a reachable path toward using SCDs for future high voltage power conversion systems and for other novel diamond based electronic devices. The novel doping mechanism may find its critical use in other wide bandgap semiconductors. With the advent of various new renewable energy sources and the emerging need to deliver and convert energy more efficiently, power electronics have received unprecedented attention in recent years. For the last several decades, Si based power devices have played a dominant role in power conversion electronics. Wide bandgap semiconductor material based power electronics, such as those employing GaN and SiC, are expected to handle more power with higher efficiency than Si based ones. GaN exhibits higher saturation velocity than Si. However, the thermal conductivity of GaN is low for power conversion systems. Moreover, it is currently difficult to obtain a thick and high quality GaN layer. SiC has its own native substrate, but it has inferior performance matrices (e.g. Johnson's figure of merit) versus GaN. In comparison, diamond exhibits most of the critical material properties for power electronics, except for its small substrate size at present. Diamond has a wide bandgap, high critical electric field, high carrier mobility, high carrier saturation velocities and the highest thermal conductivity among all available semiconductor materials 1 3 Due to its superior electrical properties, the thickness of the highest quality diamond required to block an equivalent amount of voltage is approximately onefifth to one fourth the thicknesses of GaN or SiC. In particular, the superior thermal conductivity of diamond could greatly simplify the design of heat dissipation and hence simplify entire power electronics modules. Therefore, diamond is considered the best material candidate for power electronics in terms of power switching efficiency, reliability, and system volume and weight. However, besides the lack of large area single crystalline diamond (SCD) substrate, SCD is ultra stable and chemically inert to most reactive reagents, due to the strong s bonds formed between adjacent carbon atoms, making substitutional doping very difficult 4 Because of the doping difficulty, the majority of diamond based diodes reported to date are Schottky diodes 5 8 Ion implantation has been attempted to achieve substitutional doping of SCD 9,10 However, the ion implantation process needs to be carried out at elevated temperatures (400 to 600 o C) to prevent bulk phase transition graphitization 11 13 In addition, a very high annealing temperature (1450 o C) under a high vacuum ~10 6 mtorr) is required to potentially restore damaged lattices and to activate implanted dopants. During this annealing process and for high dose implantation in particular, surface graphitization still occurs, thereby creating additional unwanted processing complications for practical applications 9 11 The alternative approach to ion implantation is insitu doping during the epitaxial growth of diamond 2,3,14,15 However this in situ doping method has a number of intrinsic limitations for practical use (e.g. selective doping) due to the need for high temperature and for high density plasma during growth. For example, realizing uniform doping concentrations across a single diamond substrate using plasma enhanced epitaxial growth is rather challenging due to high plasma concentrations near the edges of diamond substrates. The small size of diamond as a substrate worsens the non uniformity doping problem of in situ doping. Using in situ doping also further affects film growth uniformity and crystal quality during plasma assisted epitaxial growth. These issues make selective doping of diamond film nearly impossible, and yet selective and uniform doping is very attractive and necessary for most device applications. Previously, a solid boron film was deposited on polycrystalline diamond in order to allow boron thermal diffusion doping 7 While a very high temperature (1600 o C) is needed in this process and the feasibility of this method in the context of SCD doping is unknown, diodes made of polycrystalline diamond always show very high leakage current. Here we demonstrate a very simple, yet viable method for selective boron doping in natural SCD (nSCD) via an easy thermal diffusion process at a much lower temperature than those used in any other doping methods. We use heavily doped silicon nanomembranes (SiNMs) which are now easily accessible, as our dopant carrying medium. Standalone SiNM, since its first appearance in 2005, has been extensively investigated 16 The processing of SiNM, including transferring, doping and bonding, can be readily carried out with conventional processing tools. The SiNM is typically released in thin single crystal silicon sheets, e.g. from a silicon oninsulator (SOI) wafer, with thicknesses ranging from a few nanometers up to microns. The thinness of SiNM brings in a number of electronic and mechanical properties that are not possessed by its rigid counterparts (i.e. bulk Si) These unique properties have been extensively harnessed in recent years. A summary of the detailed processing methods of nanomembranes, including their creation, manipulation, etc. and their significant applications can be found elsewhere 16 18 In this work, we first exploit the bonding advantages of SiNMs. In comparison to a regular/bulk Si substrate that is rigid and rather difficult to bond to diamond, the bonding force of SiNMs is much stronger than rigid Si due to its thin thickness. As described earlier 16 the bonding force increases exponentially with the reduction of Si thickness, which is a unique and important property of flexible SiNM. The ultra mechanical flexibility of SiNM also allows much more conformal and higher fidelity bonding and better surface roughness tolerance when bonding with diamond, both of which facilitate intimate contacts between SiNM and diamond (nearly 100% bonding yield without using any adhesives and no limitation on SiNM's size or shape) Also, due to the thinness of SiNM, we are able to exploit it for dopant carrying. SiNM, like bulk Si, can be heavily doped via conventional ion implantation and post implant thermal annealing without destroying its single crystal structure. Unlike bulk Si, SiNM can be easily heavily doped across its full thickness due to its thinness. A detailed description of the method to realize SiNM full thickness heavy doping can be found from recent publications 17 19 Heavily boron doped thin SiNM was bonded to diamond to serve as the dopant carrying medium for thermal diffusion in this work. As detailed in the Supplementary Information (SI, Fig. S1) the preparation and application of SiNM as a dopant carrying medium for diamond are relatively easy and straightforward. It should also be noted that heavily doped SOI wafer is also commercially available and that the doping process described here (in SI) can be considerably simplified if using such SOI wafer. Besides the process simplicity, the SiNM based thermal diffusion doping method leads to effective, uniform and selective heavy doping in SCD. No graphitization is induced even on the diamond surface as long as it is covered by SiNM. Without involving high temperature or high vacuum furnace, the doping process can be readily realized using conventional rapid thermal annealing (RTA) process, which is widely accessible in comparison to ion implantation and insitu doping methods. In addition, the doping method proves to be very effective in forming a shallowly doped p type layer in diamond, which is sufficient for most device applications, while keeping its single crystallinity intact. It is noted that realizing very shallow doping in many semiconductors, such as Si, has been very challenging until the recent successful demonstration by M. L. Hoarfrost et al 56