Realization of p-type gallium nitride by magnesium ion implantation for vertical power devices
Yating ShiFangfang RenWeizong XuXuanhu ChenJiandong YeLi LiDong ZhouRong ZhangYoudou ZhengHark Hoe TanC. JagadishHai Lu
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Abstract Implementing selective-area p-type doping through ion implantation is the most attractive choice for the fabrication of GaN-based bipolar power and related devices. However, the low activation efficiency of magnesium (Mg) ions and the inevitable surface decomposition during high-temperature activation annealing process still limit the use of this technology for GaN-based devices. In this work, we demonstrate successful p-type doping of GaN using protective coatings during a Mg ion implantation and thermal activation process. The p-type conduction of GaN is evidenced by the positive Seebeck coefficient obtained during thermopower characterization. On this basis, a GaN p-i-n diode is fabricated, exhibiting distinct rectifying characteristics with a turn-on voltage of 3 V with an acceptable reverse breakdown voltage of 300 V. Electron beam induced current (EBIC) and electroluminescent (EL) results further confirm the formation of p-type region due to Mg ion implantation and subsequent thermal activation. This repeatable and uniform manufacturing process can be implemented in mass production of GaN devices for versatile power and optoelectronic applications.Keywords:
Dopant Activation
High temperature, very short time annealing techniques have been used to study dopant activation during and immediately after solid phase epitaxial regrowth of amorphous layers produced by ion implantation of As into Si. Short annealing timescales have revealed electrically inactive As tails, correlated with a region of implant-induced excess point defects, indicating the formation of stable dopant-interstitial complexes which are not removed during the timescales of these anneals.
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With the recent advances in low-energy ion implantation, the challenges for device manufacturers become how to anneal the implant damage and how to electrically activate implanted dopants. Current rapid thermal processes cause undesired dopant diffusion and have a low electrical activation limited by solid solubility. We report on a process that utilizes a pulsed, ultraviolet laser beam to anneal and activate low-energy implanted junctions. Junctions with depths shallower than 35 nm and sheet resistance smaller than 100 /spl Omega//sq are demonstrated. The results indicate an activated dopant concentration higher than 10/sup 21/ cm/sup -3/. The de-activation of the highly-activated dopants in a subsequent thermal process is also studied. The results suggest that boron junctions will not suffer from de-activation and severe dopant diffusion, and phosphorus may be the choice of n-type dopant because it de-activates less than arsenic. Finally, we will discuss advantages of this process to device performance, present the integration issues, and forecast challenges to the ion implantation community if this process is adopted by device manufacturers.
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The effectiveness of low-energy 5–10 keV As ion implantation for sub-0.1 µm metal-oxide-semiconductor field-effect-transistors (MOSFETs) has been investigated. When implantation energy is lowered to 5 keV at a dose of 1×10 14 cm -2 , the sheet resistance of the diffused layer increases steeply. The origin of the sheet resistance increase in 5–10 keV As ion implantation has been quantitatively studied paying attention to dopant loss. We found that 43% of implanted As remains in a 5 nm screen oxide when implantation energy is lowered to 5 keV. Moreover 50–70% of As in Si is lost by dopant pileup at the SiO 2 /Si interface during 850°C annealing. The pileup problem becomes more severe with junction depth reduction. By optimizing the implantation energy and the ion dose, both low sheet resistance and ultrashallow junction depth have been simultaneously achieved.
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Rapid annealing of boron implanted (100) silicon has been used to fabricate p+-n diodes. After an implant dose of 3×1015 ion cm−2 and a 1-s anneal at 1100 °C, a sheet resistance of 40 Ω/⧠ is obtained. The junction depth is 0.34 μm, measured by spreading resistance profiling. The leakage current at −5 V is 40 nA cm−2. Secondary ion mass spectrometry shows that the boron dopant diffuses rapidly (≂50 nm) during the first second of an anneal at 1100 °C. The residual implantation damage does not appear to have a deleterious effect on diode characteristics.
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Ion implantation's high selectivity played a very important role in forming active device region and low resistance ohmic contact for bipolar and CMOS transistors. However the process of annealing for dopant activation and repair, problems related to anomalous transient enhanced diffusion (TED) negate the benefits of ion implantation. The irregular dopant diffusion makes the realization of sharp and shallow junction devices difficult. It is therefore very important to model the TED in relation to the implantation energy before any realistic design of the transistor can be made. In this work, both experimental result from special structures with different implantation energies and process simulator SILVACO SILVACO is studied and used to model TED.
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Be ion implantation and annealing conditions were optimized to demonstrate an effective method for selective area p-type doping in InAs. Optimized implantation and annealing conditions were subsequently utilized to produce planar InAs diodes. The Be implanted planar diodes had a superior dynamic resistance-area product and comparable dark current with n-i-p InAs mesa diodes when operated at low temperatures.
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Ion-implantation studies of (CH)x films were carried out with ∼10-keV sodium ions. The temperature dependence of the sheet resistance of the implanted layer exhibited a thermal-activation-type behavior. Its activation energy decreased with increasing the ion dose. A long-term observation of the capacitance-voltage characteristics of an implanted p-n junction showed that a step junction was formed long after implantation as a result of the competition between defects annealing and dopants diffusion. These results ensure that low-energy ion implantation is a useful process for the n-type doping of (CH)x.
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Polycrystalline Si films on oxidized Si wafers have been subjected to a rapid thermal processing anneal prior to As ion implantation. After ion implantation the films are given another rapid thermal processing anneal to activate the As. The preimplant anneal causes the as-deposited grain size to increase by ∼ a factor of 10. These films have a 20–30% lower sheet resistance than films that were post-implant annealed only. The increase in grain size by the preimplant anneal reduces the grain boundary area and therefore, minimizes the amount of dopant in the grain boundary relative to the grain.
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