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Co-implantation of Mg with N has been shown to improve p-type conductivity in Mg-implanted GaN. Achievement of p-type material still requires temperatures beyond the thermodynamic stability of GaN, however. In this study, we present results of implantation and anneal activation of GaN, co-implanted with Mg and N or Mg only by repeated, short thermal cycles of 1350 °C using a high-power gyrotron microwave source with a quasi-gaussian intensity profile. Spatial variations in optical and electrical properties of the resulting films are characterized by photoluminescence and diode I–V and C–V measurements. Resistive Mg/N co-implanted and annealed material shows dominant luminescence of the VN-related green luminescence (GL2) band at 2.37 eV and relatively lower intensity acceptor-related ultraviolet luminescence (UVL) at 3.27 eV. However, a material showing p–n diode behavior shows higher-intensity UVL luminescence and suppression of the GL2 band, permitting observation of the yellow luminescence (YL) present in the as-grown GaN. The YL is attributed to unintentionally introduced CN–ON complexes and is commonly observed in GaN grown by metalorganic chemical vapor deposition but is typically absent in implanted/annealed GaN. Co-implanted material is compared to material implanted only with Mg and annealed under the same conditions, which shows p-type activation, but contains persistent GL2 luminescence post-anneal and lowers maximum hole concentration.

Power diodes and transistors fabricated from the ultra-wide bandgap (UWBG) semiconductors, AlGaN, Ga2O3, and diamond using state-of-the-art materials are compared with those fabricated from GaN and SiC. Parameters used in the operation of the devices are discussed, methods for improving them are described, and the ability to, and importance of, improving them are assessed. Equations for the resistance of different elements of the device are also provided so one can assess their importance and make meaningful comparisons between the different types of transistors.

The advancement in potential selective placement of p-GaN regions places the gallium nitride (GaN) material system on the forefront of next generation power semiconductor devices. The ion implantation technique is commonly used in fabrication of semiconductor devices to achieve high conductivity regions with successful demonstration for n-GaN. This technique has shown little success for post-Mg-implanted activation anneal to achieve p-GaN [1], largely due to the formation of substantial point defects as a result of the implant process. To activate Mg and repair these defects, high annealing temperatures of > 900 °C are required. Considering that GaN dissociates at 840 °C at atmospheric pressure, higher temperature annealing should be performed under a high overpressure of nitrogen and in combination with a protective cap layer. We report on the results of using a novel Gyrotron annealing technique for Mg implant-activation. Mg implanted GaN layers have been annealed at temperatures and pressures as high as 1300 °C and 40 bar respectively with and without a protective cap. It is observed that Gyrotron annealing at 1100 ˚C for 30 seconds eliminates secondary GaN (0004) Bragg peaks, due to stress relaxation. In addition, skew-symmetric x-ray rocking curves show no stress induced by annealing in the GaN (10-12) peak with FWHM of 59’’, although the GaN (30-32) peak is observed to broaden. We will present an extensive array of characterizations detailing results from Gyrotron annealing, conventional RTA, and high temperature (1300 °C) RTA on similar Mg-implanted, homoepitaxially grown GaN samples. [1] F. J. Kub et al. Electron. Lett., 50, 3 (2014): 197–198

Here we present for the first time the use of gyrotron beam for selective annealing/activation of implanted Mg in bulk GaN films. Samples with 1 μm uGaN epi-Iayer grown by MOCVD on HVPE bulk n+GaN substrates were implanted with Mg (1 x 1019 cm−3) to a target depth of 500 nm. A systematic investigation of annealing temperature, N 2 overpressure, surface capping layer, and sample orientation to the beam are presented. Post-implantation, volumetric lattice deformation was recovered by performing continuous annealing at temperatures $>$ 1100 °C for 30 s. Both AIN capped and uncapped samples were studied. It is shown that the surface of uncapped GaN samples which were annealed at 1100 °C in 40 bar N 2 overpressure remained intact. Photoluminescence spectroscopy was performed and confirmed the incorporation of Mg in substitutional sites with this technique, along with formation of large density of nitrogen vacancy defects.

A key step to the development of future (Al)GaN-based power devices is the ability to form selective-area p-type regions, and a promising method of accomplishing this is through ion implantation of the dominant acceptor dopant, Mg. Ion implantation in GaN induces lattice damage and creates point defects, including vacancies and interstitials. To remove this damage, heal defects, and activate the implanted dopants requires temperatures >1000 °C, beyond the thermodynamic equilibrium stability of GaN [1]. Removal of point defects, especially donor-like nitrogen vacancies (VN) in the material can be accomplished through high temperature annealing and may be aided by co-implantation with N [2]. It is speculated that compensation of implant-induced VN may be enhanced during the annealing process by diffusion of implanted Ni to the corresponding site in the N sublattice, however evidence for such compensation through suppression of VN-related photoluminescence has not been reported in p-type material. Methods of high-temperature nonequilibrium annealing such as symmetric multicycle rapid thermal annealing (SMRTA), microwave annealing, and laser annealing have been reported to achieve p-type material, however activation of Mg-implanted material remains an area of active study. In this study, we report our latest results in obtaining p-type conductivity in implanted GaN by repeated, short-duration thermal cycles using a high-power (kW) gyrotron microwave source and systematic study of Mg implantation and Mg+N co-implantation into GaN and the resulting defect microstructure [3]. We examine the effect of co-implantation to depths of 250 nm and 500 nm, and Mg:N concentration ratios of 1 and 2 with [Mg]=1019 cm-3. Implanted MOCVD-grown u-GaN is capped with AlN and annealed for 2 overpressure of 3 MPa. The optical and electrical properties of the resulting films are characterized by temperature-dependent photoluminescence (PL) and I-V measurements of vertical p-i-n diodes. Highly resistive Mg+N co-implanted and annealed material shows dominant luminescence of the VN-related GL2 band at 2.37 eV and relatively lower-intensity acceptor-related UVL and UVL* bands (at 3.05–3.27 eV). However, implanted/annealed samples showing p-n diode behavior (Vth=3–3.1 V at room temperature) and hole concentrations in the range of 1015–1017 cm-3 show dominant/high UVL/UVL* bands and YL band centered at 2.30 eV. The YL is attributed to unintentionally introduced CN-ON complexes and is commonly observed in GaN grown by MOCVD but is typically not observed in implanted/annealed GaN. Additionally, samples implanted only with Mg and activated under the same annealing conditions show persistent dominance of the GL2 band in this range. This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli. [1] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018) [2] R. Tanaka et al. Japan. J. Appl. Phys., 59 , SGGD02 (2020) [3] V. Meyers et al. manuscript submitted for publication (May 2020) Figure 1

We demonstrate p-type activation of GaN doped by Mg ion implantation, and in situ during metalorganic chemical vapor deposition through sequential short-duration gyrotron microwave heating cycles at temperatures of 1200–1350 °C. GaN is implanted with 1019 cm−3 Mg ions, capped with AlN, and annealed under 3 MPa N2 overpressure in 5 s heating cycles for less than 60 s total using a high-power gyrotron microwave heating source. Through I–V characterization, photoluminescence spectroscopy, and Raman spectroscopy, we study the evolution of electrical properties, optically active point defects, and material strain in response to implantation and annealing. For Mg-implanted samples, increasing annealing temperature is characterized by an increase in the PL substitutional Mg-related peak (UVL) relative to the shallow-donor vacancy-related peak (GL2). Through comparison of implanted and in situ doped samples, it is demonstrated that the origin of compensating VN lies primarily in implantation rather than degradation from the annealing process. Transmission line measurements and diode I–V measurements show a sheet resistance of 1083 kΩ/□ and a hole concentration of 1.23 × 1015 cm−3, respectively, in the Mg-implanted material annealed at 1350 °C. We conclude that temperature-cycled gyrotron annealing at 1350 °C decreases implant-induced compensating point defects and activates Mg to obtain selective p-type conduction.

c-oriented GaN micropillars created from single crystals containing ∼103 or ∼106 dislocations/cm2 and a thick heteroepitaxially grown film containing ∼109 were compressed to study methods to accommodate strain during heteroepitaxial growth. The yield stress in the 103 samples was found to be the highest, and it was the lowest in the 109 samples. The 103 and 106 pillars often failed catastrophically but the 109 pillars almost never did. This was linked to the high stresses required to generate sufficient pyramidal dislocations to accommodate plastic strain and dislocation interactions, which precipitated axial fracture. Transmission electron microscopy analysis shows categorically that the first formed dislocations are ⅓ ⟨ 11 23 _ ⟩{1122} dislocations, and that a few ⅓ ⟨ 11 23 _ ⟩{0111} dislocations found were formed by a cross slip in the vicinity of where the former dislocations interacted. When compared with the similar stress patterns created in the heteroepitaxial growth of AlGaN films on GaN substrates, the analysis suggests that there is no pathway for creating basal plane dislocations during growth from the pyramidal dislocations, which require high applied stresses; the basal plane dislocations would provide relief for the mismatch strain while not penetrating the region where active devices are fabricated in the film. Rather, it will be necessary to find a method for creating shear stress in the basal plane during growth to form them directly.

An AlN annealing cap has been developed for GaN films grown on sapphire substrates that enables them to be annealed at temperatures as high as 1300 °C for times as long as 30 min or times as long as 120 min at a temperature of 1150 °C without creating a significant number of electrically active N vacancies or new thermal etch pits due to the preferential evaporation of N. However, the pits in the as-grown sample became larger and their sidewalls became steeper, and the flat areas of the film became a little rougher when the annealing time and/or temperature was larger. The films became more relaxed as indicated by a >10% reduction in the X-ray rocking curve peak widths and the creation of a more uniform convex bow. Both the net carrier concentration and electron mobility were reduced by the anneal suggesting that compensating point defects had been created. One possible explanation is that more C in these metal organic chemical vapor deposition (MOCVD) grown films occupied N sites where it is believed to be a deep acceptor.

In light of the necessity to anneal GaN to activate implanted dopants, the effects of the annealing temperature and time, the quality of the hydride vapor phase epitaxy grown GaN film, the quality of the annealing cap, and the effects of the stresses generated by the difference in the coefficients of thermal expansion of the film and the substrate are examined topographically using atomic force microscopy, and electrical measurements are made on Schottky diodes fabricated on the annealed samples. The results show that thermal decomposition begins at threading edge dislocations that form polygonized small angle grain boundaries during the annealing process; donor defects, probably nitrogen vacancies, are formed near the surface; and the donors are created more quickly when the annealing temperature is higher, the annealing time is longer, and the thermal stresses on the annealing cap are greater. The results suggest that the maximum annealing temperature is ∼1300 °C, and at that annealing temperature, the annealing time should not exceed 4 min.In light of the necessity to anneal GaN to activate implanted dopants, the effects of the annealing temperature and time, the quality of the hydride vapor phase epitaxy grown GaN film, the quality of the annealing cap, and the effects of the stresses generated by the difference in the coefficients of thermal expansion of the film and the substrate are examined topographically using atomic force microscopy, and electrical measurements are made on Schottky diodes fabricated on the annealed samples. The results show that thermal decomposition begins at threading edge dislocations that form polygonized small angle grain boundaries during the annealing process; donor defects, probably nitrogen vacancies, are formed near the surface; and the donors are created more quickly when the annealing temperature is higher, the annealing time is longer, and the thermal stresses on the annealing cap are greater. The results suggest that the maximum annealing temperature is ∼1300 °C, and at that annealing temperature, the ...

Point defects in the GaN cause premature breakdown in GaN Schottky diodes at least to the same extent as dislocations do. The most likely cause is impact ionization of deep acceptors. A primary deep acceptor in MOCVD-grown GaN is C. Another is a Ga vacancy, which is likely to be the primary deep acceptor in HVPE-grown material. Lower net carrier concentrations, and therefore larger breakdown voltages, can be achieved in the HVPE material.

Hydride vapor phase epitaxy (HVPE) grown GaN was evaluated for high power Schottky diodes (SDs) because it contains much less carbon and grows much more rapidly than other typical growth methods. The results are encouraging for applications

With its wide bandgap and associated high critical field, GaN is a suitable material for high power electronics. Our group has identified carbon incorporation into the drift region as problematic for the development of GaN power Schottky diodes. The likely source of carbon is from the trimethylgallium source during metalorganic chemical vapor deposition (MOCVD) growth. Diodes grown at low pressure (100 Torr) have a high onresistance, high turn-on voltage and display a snap-on effect. These observations are independent of MOCVD growth system where similar results were obtained for diodes with films grown at 100 Torr at multiple institutions. Similar results were also observed for diodes with films grown on sapphire or freestanding hydride vapor phase epitaxy (HVPE) GaN substrates. Adjusting growth pressure, temperature and V/III ratio can reduce carbon incorporation into the device layers. Diodes with device layers grown at a growth pressure of 500 Torr show low turn-on voltages and a low specific onresistance with a figure of merit (Vb/Ron) of ~ 261 MW/cm. One way of circumventing the carbon issue is to fabricate vertical diodes directly on freestanding HVPE substrates. Ohmic contacts on the N-polar GaN surface consist of a Ti/Al/Ni/Au metal stack annealed at a temperature of 750 oC for 30 s. Circular Schottky metal contacts consisting of a non-annealed Ni/Au metal stack are made to the Ga-polar surface. Metal diameters range from 30 – 300 microns. Carrier concentrations measured via capacitance-voltage techniques range between 10 10 cm. Shown in Figure 1, such diodes were able to achieve breakdown voltages of ~ 900 V; however, the overall figure of merit for power devices is lowered due to a high specific on-resistance. This high specific onresistance stems from the low-doped substrate. This is confirmed by fabricating front-side diodes on UID freestanding HVPE substrates, in which diodes show a reduced specific on-resistance compared to the vertical case. Ideally, GaN power Schottky diodes consist of a low-doped (< 10 cm) epitaxial film for the drift region on a highly conductive substrate, thereby permitting high breakdown voltages and reducing the overall series resistance through the substrate. This talk will focus on such vertical devices. Device layers grown by MOCVD and HVPE on multiple substrates including freestanding HVPE GaN substrates, as well as truly bulk GaN substrates grown by the ammonothermal growth technique, will be discussed. Results will be compared to lateral devices fabricated from GaN device layers grown on sapphire, as well as the previously mentioned diodes fabricated directly on low-doped freestanding HVPE GaN substrates.

GaN films were grown by metal organic chemical vapor deposition on TaC substrates that were created by pulsed laser deposition of TaC onto (0 0 0 1) SiC substrates at ∼1000 °C. This was done to determine if good quality TaC films could be grown, and if good quality GaN films could be grown on this closely lattice matched to GaN, conductive material. This was done by depositing the TaC on on-axis and 3° or 8° off-axis (0 0 0 1) SiC at temperatures ranging from 950 to 1200 °C, and examining them using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The GaN films were grown on as-deposited TaC films, and films annealed at 1200, 1400, or 1600 °C, and examined using the same techniques. The TaC films were polycrystalline with a slight (1 1 1) texture, and the grains were ∼200 nm in diameter. Films grown on-axis were found to be of higher quality than those grown on off-axis substrates, but the latter could be improved to a comparable quality by annealing them at 1200–1600 °C for 30 min. TaC films deposited at temperatures above 1000 °C were found to react with the SiC. GaN films could be deposited onto the TaC when the surface was nitrided with NH3 for 3 min at 1100 °C and the low temperature buffer layer was AlN. However, the GaN did not nucleate easily on the TaC film, and the crystallites did not have the desired (0 0 0 1) preferred orientation. They were ∼10 times larger than those typically seen in films grown on SiC or sapphire. Also the etch pit concentration in the GaN films grown on the TaC was more than 2 orders of magnitude less than it was for growth on the SiC.

The effect of KOH pretreatment for Au/Ni Schottky contacts to GaN is investigated using I-V and x-ray photoemission spectroscopy (XPS) analysis. The molten KOH pretreatment reduces the interface trap density from 1.0×1012 to 2×1011 cm−2 eV−1, improves the on-state performance, and increases the barrier height by 10%. XPS indicates that KOH improves the GaN Schottky diode performance by eliminating an oxide layer between the metal and the semiconductor, increasing the band bending through charge transfer, and improving the GaN stoichiometry at the surface. First principle simulations indicate that the nitrogen antisite and to a minor extent the gallium antisite are also possible constituents of this interfacial layer along with gallium and nitrogen vacancies. These antisite defects can be passivated by KOH.

The effectiveness of AlN/BN and graphite annealing caps for ion implanted SiC was examined morphologically, structurally, chemically, and electrically. The AlN/BN cap more effectively blocks the out-diffusion of silicon because it is essentially inert. Relatively small amounts of silicon from the SiC diffuse out into the graphite cap and react with it. This has the effect of lowering the Si vapor pressure so that it does not create blow holes in this relatively weak structure. The graphite cap can be removed easily with an oxygen plasma, while warm KOH has to be used to remove the nitride cap, and not all of it can be removed after an 1800°C anneal. The out-diffusion of silicon through the graphite cap is most severe at 1800°C, where it roughens the SiC surface and forms reaction products when the Si reacts with the graphite at a significant rate. At lower temperatures these reaction products form small particulates on the surface that are visible only at higher magnifications. In those regions where the graphite cap crystallized, isolated morphological damage on the SiC surface was also detected, appearing to be in the vicinity of the grain boundaries. Scratches on the substrate surface were not affected even at temperatures as high as 1800°C.

Double and triple crystal rocking curve and peak position maps are constructed for a 4HSiC wafer for the symmetric (0 0 0 8) reflection in the normal position, the same reflection for a sample rotated 90º, and an asymmetric (1 23 6) reflection for the wafer in the normal position. These measurements were corrected for the ‘wobble’ in the instrument by scanning a 4” (1 1 1) Si wafer and assuming that the Si wafer was perfect and attributing the variations in the measurements to instrumental error. The x-ray measurements are correlated with a cross polar image, etch pit density map, white beam transmission x-ray topograph, and a laser light scan.

4H-SiC samples implanted with 1020 Al were annealed at various temperatures with a BN/AlN or graphite cap, and there morphological, structural, and electrical properties are compared. No blow holes were observed in either cap. Some Si out-diffuses through the graphite cap which results in a rougher surface and a structurally modified region near the surface. The BN/AlN cap annealed at 1800°C cannot be readily removed, whereas the graphite cap can be removed easily after any annealing temperature. The sheet resistances for both types of samples were about the same.

4H-SiC PiN rectifiers with implanted anode and single-zone JTE were fabricated using AlN capped anneal. The surface damage during the high temperature activation anneal is significantly reduced by using AlN capped anneal. The forward drop of the PiN rectifiers at 100A/cm2 is 3.0V while the leakage current is less than 10-7A/cm2 up to 90% breakdown voltage at room temperature. With 6μm thick and 2×1016cm-3 doped drift layer, the PiN rectifiers can achieve near ideal breakdown voltage up to 1050V. Hole impact ionization rate was extracted and compared with previously reported results.

4H-SiC samples implanted at 600°C with 1020 cm-3 of B or B and C to a depth of ~0.5 μm, capped with (BN/AlN), and annealed at temperatures ranging from 1400°C – 1700°C were studied using variable temperature cathodoluminescence. New emission lines, which may be associated with stacking faults, were observed in the samples co-implanted with B and C, but not in the samples implanted only with B. For both the B and B and C co-implanted samples, the intensity of the line near 3.0 eV decreases with increasing annealing temperature, TA, and this line is not observed after annealing at 1700°C. The D1 defect related emission lines are observed in the luminescence spectra of all samples and their relative intensities seem to vary with the implantation-annealing schedule and excitation conditions.

In this paper we report recent advances in pulsed-laser-deposited AIN thin films for high-temperature capping of SiC, passivation of SiC-based devices, and fabrication of a piezoelectric MEMS/NEMS resonator on Pt-metallized SiO2/Si. The AlN films grown using the reactive laser ablation technique were found to be highly stoichiometric, dense with an optical band gap of 6.2 eV, and with a surface smoothness of less than 1 nm. A low-temperature buffer-layer approach was used to reduce the lattice and thermal mismatch strains. The dependence of the quality of AlN thin films and its characteristics as a function of processing parameters are discussed. Due to high crystallinity, near-perfect stoichiometry, and high packing density, pulsed-laser-deposited AlN thin films show a tendency to withstand high temperatures up to 1600°C, and which enables it to be used as an anneal capping layer for SiC wafers for removing ion-implantation damage and dopant activation. The laser-deposited AlN thin films show conformal coverage on SiC-based devices and exhibit an electrical break-down strength of 1.66 MV/cm up to 350°C when used as an insulator in Ni/AlN/SiC metal-insulator-semiconductor (MIS) devices. Pulsed laser deposition (PLD) AlN films grown on Pt/SiO2/Si (100) substrates for radio-frequency microelectrical and mechanical systems and nanoelectrical and mechanical systems (MEMS and NEMS) demonstrated resonators having high Q values ranging from 8,000 to 17,000 in the frequency range of 2.5–0.45 MHz. AlN thin films were characterized by x-ray diffraction, Rutherford backscattering spectrometry (in normal and oxygen resonance mode), atomic force microscopy, ultraviolet (UV)-visible spectroscopy, and scanning electron microscopy. Applications exploiting characteristics of high bandgap, high bond strength, excellent piezoelectric characteristics, extremely high chemical inertness, high electrical resistivity, high breakdown strength, and high thermal stability of the pulsed-laser-deposited thin films have been discussed in the context of emerging developments of SiC power devices, for high-temperature electronics, and for radio frequency (RF) MEMS.

Co-implanting Si into 4H–SiC with Al hinders the ability of the Al acceptors to activate electrically at the lower annealing temperatures, but for annealing temperatures>1600°C, the effect is much less, suggesting that the activation energy for incorporating Al as an acceptor no longer controls the rate-determining step in this process. The cathodoluminescence data indicate that Al acceptors are trapped out by the DI defect, and this effect is more pronounced for the higher annealing temperatures. The increase in χmin with the annealing temperature at the higher temperatures can be explained by the nucleation and growth of structural defects, and the transmission electron miscroscopy results show that these defects are stacking faults. The stacking faults can be described as being quantum dots of different polytypes or domain walls with a point-defect periodic structure between the faulted and unfaulted regions, and they could be the source for the peaks associated with the DI defect. Also, we observed th...

AlN deposited on 4H–SiC by pulsed laser deposition (PLD) did not noticeably deteriorate when it was annealed in an Ar atmosphere at 1500 °C for 30 min, but it contained numerous thermal etch pits when it was annealed at 1600 °C as determined by SEM and AFM, and it completely evaporated at the higher annealing temperatures thereby allowing the silicon to evaporate preferentially that produced a pitted SiC surface. When the AlN film was annealed in nitrogen for 30 min, the film formed a protective cover up to 1650 °C, and when a sapphire cover was employed, it protected the SiC surface up to 1700 °C. The AlN surface was altered when it was annealed in nitrogen by both the evaporation of N from the surface and reactions with the N in the atmosphere. This produced a rougher surface as determined by SEM and AFM that was composed of more randomly oriented crystallites as determined by X-ray and TEM analyses, but it still protected the SiC surface below. Thermodynamic calculations are in qualitative agreement with these analyses.

Near-ideal Schottky barrier contacts to n-type Al0.22Ga0.78N have been developed by a two-step surface treatment technique. Plasma etching of the AlxGa1−xN surface prior to Schottky metal deposition, combined with sequential chemical treatment of the etched surface, holds promise for developing high quality low-leakage Schottky contacts for low noise applications and for recessed gate high electron mobility transistors. In this work, the effect of postetch chemical treatment of the n-type Al0.22Ga0.78N surface on the performance of the Ni∕Au based Schottky contact has been investigated. Three different types of chemical treatment: viz, reactive ion etching, reactive ion etching plus dipping in hot aqua regia, and reactive ion etching plus dipping in hot KOH, are studied. Detailed current-voltage studies of three different surface treated diodes and a comparison with as-deposited diodes reveal significant improvement in the diode characteristics. The latter surface treatment yields Ni∕Au Schottky diodes wi...

Rsh, CL, EPR, RBS, and TEM measurements were made on samples implanted with Al, Al and C, or Al and Si to a depth of 0.3 μm with a concentration of 10 20 cm -3 and annealed with an AlN cap. Rsh measurements suggest that at the lower annealing temperatures the implanted C facilitates the chemical reactions for the incorporation of Al into a Si site and Si impedes it, but for annealing temperatures >1600°C the co-implants have little effect because the activation energy for these reactions is no longer the rate determining step. The CL measurements show that the peak near 3.0 eV associated with a free electron recombining with a hole bound to an Al acceptor decreases as the annealing temperature increases suggesting defects are trapping out the holes. This can explain the lower mobilities, lower electrical activation, and our inability to detect the EPR peak associated with AlSi in these heavily implanted samples. Our RBS and TEM measurements suggest that these defects are extended residual defects that nucleate and grow, as opposed to being annealed out, and that they could be stacking faults.

Lightly n-doped, 4H-SiC films implanted with 1020 cm−3 Al or co-implanted with 1020 cm−3 Al and 1020 cm−3 C were examined in the unannealed state or annealed at 1300, 1400, 1500, 1600 or 1650 °C after they had been coated with an AlN cap. For a given annealing temperature, TA, the sheet resistance, Rsh, is smaller in the co-implanted samples at the lower TA's, but not at the higher ones because the C catalyzes the chemical reactions for the incorporation of Al into a Si site, and at the higher TA's the activation energies for these reactions no longer control the reaction kinetics. The CL peak created by a free electron recombining with a hole bound to an Al acceptor decreases in intensity as TA increases because the Al acceptors can be trapped by the deep donors related to the DI defect that increase in number as TA increases. However, the process must be more complicated because the temperature dependence of Rsh implies that the activation energy for conduction is much less than that of the 0.35 eV hole trap. The excellent temperature stability of the DI defect suggests it is likely to be associated with defects that nucleate and grow such as dislocation loops and/or stacking faults. This is consistent with decreased ion beam channeling at the higher TA's, and the small anisotropy of the EPR peak associated with implant damage. The higher energy peak in the DI doublet increases in intensity relative to the lower energy peak as TA increases, and for the same TA the relative intensity of the higher energy peak is larger in the co-implanted samples. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A metallization technique has been developed for obtaining low resistance Ohmic contact to n-GaN. The metallization technique involves the deposition of a metal layer combination Ta/Ti/Ni/Au on an n-GaN epilayer. It is observed that annealing at 750 °C for 45 s leads to low contact resistivity. Corresponding to a doping level of 5×1017 cm−3, the contact resistivity of the contact ρS=5.0×10−6 Ω cm2. The physical mechanisms underlying the realization of low contact resistivity is investigated using current–voltage characteristics, x-ray diffraction, Auger electron spectroscopy, transmission electron microscopy, and energy dispersive x-ray spectrometry.

Planar n+–p and p+–n junction diodes, fabricated in 4H-SiC epitaxial layers using a double-implantation technology (a deep-range acceptor followed by a shallow-range donor implantation and vice versa), are characterized using capacitance deep-level transient spectroscopy (DLTS) to detect deep levels, which may influence device electrical performance. Either Al or B was used as the acceptor, while N or P was used as the donor, with all implants performed at 700 °C and annealed at 1600–1650 °C with an AlN protection cap. Different traps were observed for the various dopants, which are believed to be related to different impurity-defect complexes. A trap at ∼EV+0.51 eV was observed in nitrogen-implanted samples, while an acceptor trap at ∼EV+0.28 eV and a donor trap at ∼EC−0.42 eV were observed in Al-implanted samples. A prominent boron-related D-center trap at ∼EV+0.63 eV is seen in the DLTS spectra of B-implanted diodes. In diodes with implanted phosphorus, three traps at ∼EV+0.6 eV, EV+0.7 eV, and EV+0.92...

kV 4H-SiC JBS rectifiers were fabricated using AlN capped anneal and compared with those annealed conventionally in a furnace. The surface damage during the high temperature activation anneal is significantly reduced using AlN capped anneal. The forward drop of the JBS rectifiers is 1 kV was achieved.

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ω cm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800 °C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. ...

The SiC wafers implanted with Al were capped with AlN, C, or AlN and C and were annealed at temperatures as high as 1700°C to examine their ability to act as annealing caps. As shown previously, the AlN film was effective up to 1600°C, as it protected the SiC surface, did not react with it, and could be removed selectively by a KOH etch. However, it evaporated too rapidly at the higher temperatures. Although the C did not evaporate, it was not a more effective cap because it did not prevent the out-diffusion of Si and crystallized at 1700°C. The crystalline film had to be ion milled off, as it could not be removed in a plasma asher, as the C films annealed at the lower temperatures were. A combined AlN/C cap also was not an effective cap for the 1700°C anneal as the N or Al vapor blew holes in it, and the SiC surface was rougher after the dual cap was removed than it was after annealing at the lower temperatures.

Multiple-energy box profile elevated-temperature (700 °C) phosphorus ion implantations were performed into 4H–SiC in the doping range of 1×1017–1×1020 cm−3. The implanted material was annealed at 1500, 1600, or 1650 °C with an AIN encapsulant to prevent degradation of the SiC surface. Within this temperature range the sheet resistance does not change significantly for a given dose. The percentage of electrical activation of the P donors initially decreased with increasing implant dose for P-implant concentration up to 3×1019 cm−3 and then increased again at higher doses. For 1×1020 cm−3 P implant, a carrier concentration of 4×1019 cm−3 was measured at room temperature. In the 1017 cm−3 P doping concentration range substitutional activation greater than 85% was measured. Despite performing the implants at 700 °C, a significant amount of as-implanted damage was observed in the Rutherford backscattering (RBS) spectrum, even for 1018 cm−3 range P implantations. The RBS yield after annealing is near the virgin level for P concentrations up to 1×1019 cm−3, but above this concentration the RBS yield is above the virgin level, indicating a significant amount of residual lattice damage in the crystal.

Aseries of single energy Al, B, and Ga ion implants were performed in the energy range 50 keV to 4 MeV into 6H-SiC to characterize the implant depth profiles using secondary ion mass spectrometry (SIMS). From the implant depth profiles empirical formulae were developed to model the range statistics as functions of ion energy. Multiple energy implants were performed into 6H- and 4H-SiC and annealed with both AlN and graphite encapsulants to determine the ability of the encapsulants to protect the implants from out-diffusion and redistribution. Al and Ga were thermally stable, but B out-diffused even with AlN or graphite encapsulation. Electrical activation was determined by Hall and capacitance-voltage measurements. An acceptor substitutional concentration of 7×1016 cm−3 was achieved for 1×1017 cm−3 Al implantation.

AlN films deposited on SiC or sapphire substrates by pulsed laser deposition were annealed at 1200°C, 1400°C, and 1600°C for 30 min in an inert atmosphere to examine how their structure, surface morphology, and substrate-film interface are altered during high temperature thermal processing. Shifts in the x-ray rocking curve peaks suggest that annealing increases the film density or relaxes the films and reduces the c-axis Poisson compression. Scanning electron micrographs show that the AlN begins to noticeably evaporate at 1600°C, and the evaporation rate is higher for the films grown on sapphire because the as-deposited film contained more pinholes. Rutherford backscattering spectroscopy shows that the interface between the film and substrate improves with annealing temperature for SiC substrates, but the interface quality for the 1600°C anneal is poorer than it is for the 1400°C anneal when the substrate is sapphire. Transmission electron micrographs show that the as-deposited films on SiC contain many stacking faults, while those annealed at 1600°C have a columnar structure with slightly misoriented grains. The as-deposited films on sapphire have an incoherent interface, and voids are formed at the interface when the samples are annealed at 1600°C. Auger electron spectroscopy shows that virtually no intermixing occurs across the interface, and that the annealed films contain less oxygen than the as-grown films.

Aluminum nitride (AlN) has been used as an encapsulant for annealing nitrogen (N), arsenic (As), antimony (Sb), aluminum (Al), and boron (B) ion-implanted 6H-SiC. Atomic force microscopy has revealed that the AlN encapsulant prevents the formation of long grooves on the SiC surface that are observed if the AlN encapsulant is not used, for annealing cycles up to 1600 °C for 15 min. Secondary ion mass spectrometry measurements indicated that the AlN encapsulant is effective in preserving the As and Sb implants, but could not stop the loss of the B implants. Electrical characterization reveals activation of N, As, Sb, and Al implants when annealed with an AlN encapsulant comparable to the best activation achieved without AlN.

We have analyzed theoretically the electron scattering related to the interface roughness produced by threading dislocations in the channel of an AlGaN/GaN HEMT heterostructure. On the basis of the analysis, we predict the use of the pendeo-epitaxy for growing the transistor structure to improve the low-temperature and roomtemperature mobility of two-dimensional electrons by ∼80% and ∼28%, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This report covers work done for the Director's Research Initiative (DSI) on Gallium Nitride (GaN) High Power Electronics (HPE) in which GaN devices are assessed in comparison to those fabricated from silicon carbide (SiC). We show that for low power applications (less than 1500 V) GaN diodes should have a lower on-resistance, and therefore less loss, than their SiC counterparts because the critical breakdown field and electron mobility are larger. We expect this will also be true for HPE GaN high electron mobility transistors (HEMTs) compared to SiC metal-oxide-semiconductor field effect transistors (MOSFETs). Although a few GaN devices have been made that have properties that exceed those made from SiC, these devices cannot yet be manufactured. Our work suggests the dominant problem is contamination of the metal-organic chemical vapor deposition (MOCVD) films by the carbon in the trimethyl gallium (TMGa). We suggest a better alternative is to grow the films by hydride vapor phase epitaxy (HVPE), which requires that conducting GaN substrates be grown to reduce the on-resistance (RON-SP) for the back side diodes. We also show that dislocations appear to strongly affect diode properties such as the ideality factor, but their effect on the breakdown voltage appears to be a less significant problem than was previously thought.

With its wide bandgap and high critical field, GaN is a promising material for high power electronics. To date, most GaN films have been grown on foreign substrates such as sapphire or SiC. Lattice mismatch between the film and substrate leads to a large number of threading dislocations (∼109 −1010 cm−2). These defects are thought to lead to poor device performance such as premature breakdown. Device properties are generally improved by growth of low-doped (< 1016 cm−3) GaN layers on high conductivity freestanding hydride vapor phase epitaxy (HVPE) GaN substrates. However, these films still have a large number of dislocations ∼ 106 cm−2. Dislocations are randomly oriented in both hetero and homoepitaxial films leading to a wide variation of material quality and thus device performance across the wafer. Recently, a true bulk GaN substrate became available using an ammonothermal growth technique. These substrates have both a low resistivity and low threading dislocation density. Growth of low-doped films on these bulk substrates can potentially address the problems of uniformity and premature breakdown in GaN power Schottky diodes.

This report details work from the Director's Strategic Initiative (DSI) on Gallium Nitride (GaN) High Power Electronics in which GaN devices are assessed compared to those fabricated from silicon carbide (SiC). For low power applications (

The contact resistance, ρ C , was measured for the traditional Ti/Al/Ni/Au Ohmic contact for samples implanted with Si to >10 20 cm −3 and annealed at 1100, 1150, 1200, or 1250 °C for 2, 5 or 10 min using an AlN annealing cap. These results are compared with those for samples annealed in the same way, but were not implanted. The as-grown samples were doped to 3.56 × 10 17 or 6.67 × 10 16 cm −3 or were unintentionally (UI) doped. In almost all cases, ρ C for the implanted sample was lower, and a record low ρ C = 2.66 × 10 −8 Ω cm 2 was achieved for the more heavily doped implanted sample annealed at 1200 °C for 10 min. ρ C decreased with the doping concentration, and for the UI samples, Ohmic contacts could be made only if the samples were implanted. The surface roughness was also measured, and it was found for an as-grown with an RMS roughness of 0.303 nm, the roughness increased from 0.623 after an 1100 °C anneal to 3.197 nm after a 1250 °C anneal for the implanted samples annealed for 10 min, and it increased from 1.280 nm to 5.357 nm under the same conditions for the samples that were not implanted.

Gallium nitride is one of the materials of choice for high power, high frequency and high temperature applications. However, one key component of GaN transistors and diodes is the Schottky contact and it can create a weakness in the device when high leakage currents occur at small isolated regions containing dislocations. It has been reported that the electron affinity increases at certain defects reducing the Schottky barrier height.[1] The metals used currently as Schottky contacts perform poorly due to their limited range of work functions and pinning of the Fermi level by electronic states at the metal-semiconductor interface. Therefore, we are investigating the use of graphene containing inks produced by Vorbeck Materials for the formation of Schottky contacts. This material is expected to change its electronic characteristics such as the work function based on how it is functionalized. Therefore by coming up with the proper functionalization we may obtain a contact material with very low reverse leakage by increasing the material's work function. Other benefits that graphene inks would provide as electrical contacts are excellent electrical and thermal conductivity. Results of applying this contact material to silicon and GaN Schottky diodes will be discussed and compared.

Gallium nitride (GaN) shows great promise as a wide band gap (WBG) semiconductor used in high power RF devices, and it also has the potential to be used for high power, high temperature applications. To date doping by ion implantation, necessary for Schottky diode guard rings and selectively implanted ohmic contact regions, has not been utilized due to the fact that the nitrogen evaporates preferentially at the temperatures required to activate the implants. In this work we describe the properties of a 'dual' AlN annealing cap characterized both before and after the sample is annealed, as well as the surface of the GaN film after selectively etching the cap off. The films are characterized using scanning electron microscopy (SEM), energy dispersive x-ray (EDAX) analysis, atomic force microscopy (ATM), and transmission electron microscopy (TEM). The activation of the implants was studied using a Hall effect and van der Pauw system, and the contact resistance of samples annealed at different temperatures was determined using TLM measurements.