Al,Al/C and Al/Si implantations in 6H-SiC

Multiple-energy Al implantations were performed with and without C or Si coimplantations into 6H-SiC epitaxial layers and bulk substrates at 850°C. The C and Si co-implantations were used as an attempt to improve Al acceptor activation in SiC. The implanted material was annealed at 1500, 1600, and 1650°C for 45 min. The Al implants are thermally stable at all annealing temperatures and Rutherford backscattering via channeling spectra indicated good lattice quality in the annealed Al-implanted material. A net hole concentration of 8 × 10¹⁸ cm⁻³ was measured at room temperature in the layers implanted with Al and annealed at 1600°C. The C or Si co-implantations did not yield improvement in Al acceptor activation. The co-implants resulted in a relatively poor crystal quality due to more lattice damage compared to Al implantation alone. The out-diffusion of Al at the surface is more for 5Si co-implantation compared to Al implant alone, where 5Si means a Si/Al dose ratio of 5.     

Effect of annealing temperature on 1.5 μm photoluminescence from Er-lmplanted 6H-SiC

The effect of post-implantation anneal on erbium-doped 6H-SiC has been investigated. 6H-SiC has been implanted with 330 keV Er⁺ at a dose of 1 × 1013 /cm2. Er depth profiles were obtained by secondary ion mass spectrometry (SIMS). The as-implanted Er-profile had a peak concentration of∼1.3 × 10¹⁸/cm³ at a depth of 770Å. The samples were annealed in Ar at temperatures from 1200 to 1900°C. The photoluminescence intensity integrated over the 1.5 to 1.6 μm region is essentially independent of annealing temperature from 1400 to 1900°C. Reduced, but still significant PL intensity, was measured from the sample annealed at 1200°C. The approximate diffusivity of Er in 6H SiC was calculated from the SIMS profiles, yielding values from 4.5 × 10⁻¹⁶ cm²/s at 1200°C to 5.5 × 10⁻¹⁵ cm²/s at 1900°C.     

Phosphorus implantation into 4H-silicon carbide

Sheet resistances in nitrogen- and phosphorus-implanted 4H-SiC are measured to assess the time and temperature dependencies of this variable. In 4H-SiC implanted with 3 × 10¹⁵ cm^?2 nitrogen ions to a depth of 2800 Å, the minimum sheet resistance observed is 534 Ω/□. The minimum sheet resistance in 4H-SiC implanted with 4 × 10¹⁵ cm^?2 phosphorus ions to a depth of 4000 Å is 51 Ω/□, a record low value for any implanted element into any polytype of SiC. Time-independent sheet resistances are observed following anneals at 1700°C for nitrogen and phosphorus samples. Lower temperature anneals produce sheet resistances which decrease monotonically with increasing time of anneal. Overall, sheet resistances from phosphorus-implanted 4H-SiC are an order of magnitude below those measured from nitrogen implanted samples. The response of phosphorus to low-temperature annealing is significant, and sheet resistances below 500 Ω/□ are achieved at 1200°C. Activation of phosphorus is attempted in an oxidizing atmosphere with and without prior argon annealing. A three-hour gate oxidation in wet O₂ at 1150°C, followed by a 30 min argon anneal, produced a sheet resistance of 1081 Ω/□. Oxidation after argon annealing caused sheet resistances to increase by about 20% compared to samples subjected solely to argon annealing. It is also found that oxide growth rates are much higher over phosphorus implanted than over unimplanted 4H-SiC. Reasons for the disparity in sheet resistances between nitrogen and phosphorus implants, and for the difference in oxide growth rates are suggested.     

Al, B, and Ga ion-implantation doping of SiC

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.     

The effect of argon implantation on the conductivity of boron implanted silicon

Silicon wafers have been implanted with boron (3 × 10¹⁴ or 1 × 10¹⁵ ions cm^?2) and with argon (up to 1 × 10¹⁵ ions cm^?2). The energies were chosen to approximately superimpose the two impurity distributions. After the boron and argon implantations the sheet resistance of each wafer was measured following annealing in nitrogen at temperatures in the range 400–1050°C. The highest dose argon implantation produced an increase in sheet resistance which persisted throughout the entire temperature range. Lower argon doses produced a reduction in sheet resistance for anneal temperatures between 550 and 800°C. The magnitude of the reduction is a function of the boron and argon doses and of the anneal temperatures. The greatest reduction, observed after a 600°C anneal, was by a factor of 5.8. Above 800°C the low dose argon did not affect the sheet resistance.The observed reduction in sheet resistance is expected to lead to an improvement in metal to p-type silicon contacts. A particular application is in the contacts to resistors in fast bipolar logic circuits. As high electrical activity can be obtained at moderate annealing temperatures with combined boron and argon implantations, these implantations can be carried out at a late stage in an integrated circuit process schedule without the danger of additional movement of existing junctions.     

A technique to reduce the contact resistance to 4H-silicon carbide using germanium implantation

The effects of implanted Ge on the resistance of nickel-metal contacts to n-type and p-type 4H-SiC are reported. The Ge was implanted with an energy of 346 keV and a dose of 1.7×10¹⁶ cm⁻², and the wafer was annealed up to 1700°C for 30 min. Contact resistance measurements using the transfer length method (TLM) were performed on etched mesas of n-type and p-type 4H-SiC, with and without the Ge. For the annealed-Ni metal contacts, the Ge lowered the specific contact resistivity from 5.3×10⁻⁴ Ωcm² to 6.0×10⁻⁵ Ωcm² for n-type SiC and from 1.2×10⁻³ Ωcm² to 8.3×10⁻⁵ Ωcm² for p-type SiC. For the as-deposited (unannealed) Ni, the Ge produced ohmic contacts, whereas the contacts without Ge were rectifying. These results suggest that the addition of Ge can be an important process step to reduce the contact resistance for SiC-device applications.     

Improved oxidation procedures for reduced SiO2/SiC defects

A significant reduction in the effective oxide charge and interface state densities in oxides grown on p-type 6H-SiC has been obtained by lowering the oxidation temperature of SiC to 1050°C. Further improvements are obtained by following the oxidation with an even lower temperature re-oxidation anneal. This anneal dramatically improves the electrical properties of the Si/SiC interface, and substantially lowers the interface state density. The net oxide charge density on p-type 6H-SiC is also lowered significantly, but remains quite high, at 1.0 × 10¹² cm^-2. The interface state densities of 1.0 × 10¹¹ cnr⁻²/eV are approaching acceptable MOS device levels. The breakdown fields of the oxides are also substantially improved by both the lower oxidation temperature and re-oxidation anneal. Using a low temperature oxidation followed by a re-oxidation anneal for MOSFETs results in a room temperature mobility of 72 cm²/V-s, the highest channel mobility reported for SiC MOSFETs to date.     

Boron implantation and epitaxial regrowth studies of 6H SiC

Implantation of B has been performed into an epitaxially grown layer of 6H SiC, at two different B concentrations, 2×10¹⁶ cm⁻³ and 2×10¹⁸ cm⁻³. Subsequently, an epitaxial layer was regrown on the B implanted layer. The samples were investigated by transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). In the highly B-doped layers plate-like defects were found, associated with large strain fields, and an increased B concentration. These defects were stable at the originally implanted region during regrowth and at anneal temperatures up to 1700°C. In the samples implanted with the lower B concentration, no crystal defects could be detected by TEM. No threading dislocations or other defects were observed in the regrown epitaxial layer, which shows the possibility to grow a layer with high crystalline quality on B implanted 6H SiC. By SIMS, it was found that B piles up at the interface to the regrown layer, which could be explained by enhanced diffusion from an increased concentration of point defects created by implantation damage in the region. B is also spread out into the original crystal and in the regrown layer at a concentration of below 2×10¹⁶ cm⁻³, with a diffusion constant estimated to 1.3×10⁻¹² cm²s⁻¹. This diffusion is most probably not driven by implantation damage, but by intrinsic defects in the grown crystal. Our investigation shows that the combination of implantation and subsequent regrowth techniques could be used in SiC for building advanced device structures, with the crystal quality in the regrown layer not being deteriorated by crystal defects in the implanted region. A device process using B implantation and subsequent regrowth could on the other hand be limited by the diffusion of B.     

Electrical and optical activation studies of Si-implanted GaN

Comprehensive and systematic electrical and optical activation studies of Si-implanted GaN were made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10¹³ cm^?2 to 5×10¹⁵ cm^?2 at room temperature. The samples were proximity-cap annealed from 1050°C to 1350°C with a 500-Å-thick AlN cap in a nitrogen environment. The optimum anneal temperature for high dose implanted samples is approximately 1350°C, exhibiting nearly 100% electrical activation efficiency. For low dose (≤5×10¹⁴ cm^?2) samples, the electrical activation efficiencies continue to increase with an anneal temperature through 1350°C. Consistent with the electrical results, the photoluminescence (PL) measurements show excellent implantation damage recovery after annealing the samples at 1350°C for 20 sec, exhibiting a sharp neutral-donor-bound exciton peak along with a sharp donor-acceptor pair peak. The mobilities increase with anneal temperature, and the highest mobility obtained is 250 cm²/Vs. The results also indicate that the AlN cap protected the implanted GaN layer during high-temperature annealing without creating significant anneal-induced damage.     

High-Dose Phosphorus-Implanted 4H-SiC: Microwave and Conventional Post-Implantation Annealing at Temperatures ≥1700°C

Semi-insulating 4H-SiC ⟨0001⟩ wafers have been phosphorus ion implanted at 500°C to obtain phosphorus box depth profiles with dopant concentration from 5 × 10¹⁹ cm⁻³ to 8 × 10²⁰ cm⁻³. These samples have been annealed by microwave and conventional inductively heated systems in the temperature range 1700°C to 2050°C. Resistivity, Hall electron density, and Hall mobility of the phosphorus-implanted and annealed 4H-SiC layers have been measured in the temperature range from room temperature to 450°C. The high-resolution x-ray diffraction and rocking curve of both virgin and processed 4H-SiC samples have been analyzed to obtain the sample crystal quality up to about 3 μm depth from the wafer surface. For both increasing implanted phosphorus concentration and increasing post-implantation annealing temperature the implanted material resistivity decreases to an asymptotic value of about 1.5 × 10⁻³ Ω cm. Increasing the implanted phosphorus concentration and post-implantation annealing temperature beyond 4 × 10²⁰ cm⁻³ and 2000°C, respectively, does not bring any apparent benefit with respect to the minimum obtainable resistivity. Sheet resistance and sheet electron density increase with increasing measurement temperature. Electron density saturates at 1.5 × 10²⁰ cm⁻³ for implanted phosphorus plateau values ≥4 × 10²⁰ cm⁻³, irrespective of the post-implantation annealing method. Implantation produces an increase of the lattice parameter in the bulk 4H-SiC underneath the phosphorus-implanted layer. Microwave and conventional annealing produce a further increase of the lattice parameter in such a depth region and an equivalent recovered lattice in the phosphorus-implanted layers.     

Effect of Si or Al interface layers on the properties of Ta and Mo contacts to p-type SiC

We present our results on the role of Si or Al interface layers on the structure and electrical properties of tantalum and molybdenum contacts to p-type 6H-SiC. Thin films of Ta or Mo were deposited on p-type SiC with and without p-doped Si or Al interface layers. The Ta/p-SiC, Ta/p-Si/p-SiC, Ta/Al/p-SiC, Mo/p-SiC, and Mo/Al/p-SiC structures were annealed at high temperatures up to 1200°C using the rapid thermal annealing process, in Ar-H₂ or N₂-H₂ ambient. X-ray diffraction analysis showed TaSi₂ in both Ta/p-SiC and Ta/p-Si/p-SiC structures annealed in Ar-H₂ ambient. For the N₂-H₂ ambient anneal tantalum nitride (TaN) was formed in Ta/p-SiC and Ta/Al/p-SiC, and TaN plus TaSi₂ in Ta/p-Si/p-SiC. While there was evidence of interaction between Mo and Si or Al no intermetallic phases were observed. Electrical measurements revealed that both TaN in Ta/p-SiC and TaN + TaSi₂ in Ta/p-Si/p-SiC structures made ohmic contacts, with specific contact resistances of about 2.13 × 10⁻³ and 1.47 × 10⁻¹ Ω-cm², respectively. The specific contact resistance for Ta/Al and Mo/Al layers on p-SiC decreases with increasing temperature and varies with anneal ambient. The values calculated for Ta/Al/p-SiC and Mo/Al/p-SiC were about 4.22 × 10⁻⁴ at 1100°C and 4.5 × 10⁻⁵ Ω-cm² at 1200°C, respectively. The heavy surface doping provided by Al in Ta/Al/p-SiC and Mo/Al/p-SiC is responsible for the low specific contact resistance.     

Electrical and optical characterization studies of lower dose Si-implanted AlxGa1−xN

Electrical and optical activation studies of lower dose Si-implanted Al_xGa_1?xN (x=0.14 and 0.24) have been made systematically as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10¹³ cm^?2 to 1×10¹⁴ cm^?2 at room temperature. The samples were proximity cap annealed from 1,100°C to 1,350°C with a 500-Å-thick AlN cap in a nitrogen environment. Nearly 100% electrical activation efficiency was obtained for Al_0.24Ga_0.76N implanted with a dose of 1 × 10¹⁴ cm^?2 after annealing at an optimum temperature around 1,300°C, whereas for lower dose (≤5×10¹³ cm^?2) implanted Al_0.24Ga_0.76N samples, the electrical activation efficiencies continue to increase with anneal temperature up through 1,350°C. Seventy-six percent electrical activation efficiency was obtained for Al_0.14Ga_0.86N implanted with a dose of 1 × 10¹⁴ cm^?2 at an optimum anneal temperature of around 1,250°C. The highest mobilities obtained were 89 cm²/Vs and 76 cm²/Vs for the Al_0.14Ga_0.86N and Al_0.24Ga_0.76N, respectively. Consistent with the electrical results, the photoluminescence (PL) intensity of the donor-bound exciton peak increases as the anneal temperature increases from 1,100°C to 1,250°C, indicating an increased implantation damage recovery with anneal temperature.     

Aluminum and boron ion implantations into 6H-SiC epilayers

Aluminum and boron ion implantations into n-type 6H-SiC epilayers have been systematically investigated. Redistribution of implanted atoms during high-temperature annealing at 1500°C is negligibly small. The critical implant dose for amorphization is estimated to be 1 × 10¹⁵ cm⁻² for Al⁺ implantation and 5 × 10¹⁵ cm^-2 for B⁺ implantation. By Al⁺ implantation followed with 1500°C-annealing, p-type layers with a sheet resistance of 22 kΩ/^— can be obtained. B⁺ implantation results in the formation of highly resistive layers, which may be attributed to the deep B acceptor level.     

Rapid thermal annealing of si implanted gaas

Rapid thermal annealing (RTA) technology offers potential advantages for GaAs MESFET device technology such as reducing dopant diffusion and minimizing the redistribution of background impurities. LEC semi-insulating GaAs substrates were implanted with Si at energies from 100 to 400 keV to doses from 1 × 10¹² to 1 × 10¹⁴/cm². The wafers were encapsulated with Si₃N₄ and then annealed at temperatures from 850-1000° C in a commercial RTA system. Wafers were also annealed using a conventional furnace cycle at 850° C to provide a comparison with the RTA wafers. These implanted layers were evaluated using capacitance-voltage and Hall effect measurements. In addition, FET’s were fabricated using selective implants that were annealed with either RTA or furnace cycles. The effects of anneal temperature and anneal time were determined. For a dose of 4 × 10¹²/cm² at 150 keV with anneal times of 5 seconds at 850, 900, 950 and 1000° C the activation steadily increased in the peak of the implant with overlapping profiles in the tail of the profiles, showing that no significant diffusion occurs. In addition, the same activation could be obtained by adjusting the anneal times. A plot of the equivalent anneal times versus 1/T gives an activation energy of 2.3 eV. At a higher dose of 3 × 10¹³ an activation energy of 1.7 eV was obtained. For a dose of 4 × 10¹² at 150 keV both the RTA and furnace annealing give similar activations with mobilities between 4700 and 5000 cm²/V-s. Mobilities decrease to 4000 at a dose of 1 × 10¹³ and to 2500 cm²/V-s at 1 × 10¹⁴/cm². At doses above 1 × 10¹³ the RTA cycles gave better activation than furnace annealed wafers. The MESFET parameters for both RTA and furnace annealed wafers were nearly identical. The average gain and noise figure at 8 GHz were 7.5 and 2.0, respectively, for packaged die from either RTA or furnace annealed materials.     

Optical and Magnetic Properties of ZnO:V Prepared by Ion Implantation

We report on the optical and magnetic properties of the magnetic semiconductor Zn(V)O fabricated by implantation of 195 keV ₅₁V⁺ ions into bulk ZnO:Al grown by a hydrothermal technique. Two sets of the samples, containing N _d –N _a ∼ 10¹⁵ cm⁻³ and 10¹⁸ cm⁻³, were implanted to doses of 1 × 10¹⁵ cm⁻², 3 × 10¹⁵ cm⁻², and 1 × 10¹⁶ cm⁻². The ion implantation was performed at 573 K. To remove irradiation-induced defects, the samples were annealed in air at 1073 K. Photoluminescence (PL) measurements of Zn(V)O films were carried out at temperatures from 10 K to 300 K. The effects of implantation dose and free carrier concentration on the magnetic properties of Zn(V)O were studied using a superconducting quantum interference device magnetometer. Ferromagnetism has been observed in annealed highly conductive samples implanted to 1 × 10¹⁶ cm⁻². The PL studies of ZnO bulk samples implanted with V⁺ have revealed that thermal annealing at 1073 K restores to a large extent the optical quality of the material. A new emission line centered at 3.307 eV has been found in the PL spectrum of the highly conductive samples implanted to the dose of 1 × 10¹⁶ cm⁻², which is most probably due to complexes involving V ions.     

Dopant activation and surface morphology of ion implanted 4H- and 6H-silicon carbide

The activation of ion-implanted B into 4H-SiC, and B, and Al into 6H-SiC is investigated. Complete activation of B implants into 4H-SiC is achieved by annealing at 1750°C for 40 min in an Ar environment. Significant activation (>10%) is not achieved unless the annealing temperature is 1600°C or greater. Sheet resistances of Al-implanted 6H-SiC annealed at 1800°C are 32.2 kΩ/□, indicating high activation of Al at this temperature. Annealing conditions which result in good acceptor activation are shown to be damaging to the surface of either 4H- or 6H-SiC. Atomic force microscopy and Nomarski differential interference contrast optical microscopy are applied to characterize the surfaces of these polytypes. Roughening of the surfaces is observed following annealing in Ar, with measured roughnesses as large as 10.1 nm for B-implanted 4H-SiC annealed at 1700°C for 40 min. Based on data obtained from these techniques, a model is proposed to describe the roughening phenomenon. The premise of the model is that SiC sublimation and mobile molecules enable the surface to reconfigure itself into an equilibrium form.     

Analysis of ion beam induced damage and amorphization of 6H-SiC by raman scattering

Raman scattering analysis of damaged SiC layers obtained by 200 keV Ge⁺ ion implantation into 6H-SiC has been performed as a function of the implanted dose (up to 10¹⁵ cm⁻²) and annealing temperature (up to 1500°C). The results obtained show the presence of three different damage levels: low damage level (doses ≤3 × 10¹² cm⁻²), medium to high damage level (doses between 10¹³ and 10¹⁴ cm^-2), and formation of a continuous amorphous layer for doses higher than the amorphization threshold of 2–3 × 10¹⁴ cm⁻². Moreover, at doses of about 10¹⁴ cm⁻² (below the amorphization threshold) amorphous domains are already observed. The Raman spectra indicate the existence of structural differences between the amorphous phase at doses below and above the threshold. After annealing, there is a residual damage which cannot be removed even at the highest annealing temperature of 1500°C. Differences in residual damage between the samples implanted at doses of 10¹⁴ and 10¹⁵ cm^-2 and annealed at the highest temperatures are observed from the peaks in the 1000–1850 cm^-1 spectral region. Finally, annealing at the highest temperature is required to observe the complete disappearance of the amorphous bands.     

Hydrogen incorporation in boron-doped 6H-SiC CVD epilayers produced using site-competition epitaxy

We report on the initial investigations of using site-competition epitaxy to control boron incorporation in chemical vapor deposition (CVD) 6H-SiC epilayers. Also reported herein is the detection of hydrogen in boron-doped CVD SiC epilayers and hydrogen-passivation of the boron-acceptors. Results from low temperature photoluminescence (LTPL) spectroscopy indicate that the hydrogen content increased as the capacitance-voltage (C-V) measured net hole concentration increased. Secondary ion mass spectrometry (SIMS) analysis revealed that the boron and the hydrogen incorporation both increased as the Si/ C ratio was sequentially decreased within the CVD reactor during epilayer growth. Epilayers that were annealed at 1700°C in argon no longer exhibited hydrogen-related LTPL lines, and subsequent SIMS analysis confirmed the outdiffusion of hydrogen from the boron-doped SiC epilayers. The C-V measured net hole concentration increased more than threefold as a result of thel700°C anneal, which is consistent with hydrogen passivation of the boron-acceptors. However, boron related LTPL lines were not observed before or after the 1700°C anneal.     

Improved Ni ohmic contact on n-type 4H-SiC

This paper presents the structural, chemical and electronic properties of Al/Ni/ Al-layers evaporated on 4H silicon carbide and then annealed at 1000°C for 5 min. The structure was investigated before and after annealing by transmission electron spectroscopy from cross-sectional specimens. With x-ray photoelectron spectroscopy, both element distribution and bonding energies were followed during sputtering through the alloyed metal-semiconductor contact. Voids are found in both annealed Ni/4H-SiC and Al/Ni/Al/4H-SiC contact layers, though closer to the metal-semiconductor interface in the former case. The first aluminum-layer is believed to prevent voids to be formed at the interface and also to reduce the oxide on the semiconductor surface. The contact was found to be ohmic with a specific contact resistance ρ_c - 1.8 × 10⁻⁵ Ωcm² which is more than three times lower ρ_c than for the ordinary Ni/4H-SiC contact prepared in the same way.     

Nearly Perfect Electrical Activation Efficiencies from Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with High Aluminum Mole Fraction

Electrical activation studies of Al _x Ga_1−x N (x = 0.45 and 0.51) implanted with Si for n-type conductivity have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻² at room temperature. The samples were subsequently annealed from 1150°C to 1350°C for 20 min in a nitrogen environment. Nearly 100% electrical activation efficiency was successfully obtained for the Si-implanted Al_0.45Ga_0.55N samples after annealing at 1350°C for doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² and at 1200°C for a dose of 1 × 10¹⁵ cm⁻², and for the Al_0.51Ga_0.49N implanted with silicon doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² after annealing at 1300°C. The highest room-temperature mobility obtained was 61 cm²/V s and 55 cm²/V s for the low-dose implanted Al_0.45Ga_0.55N and Al_0.51Ga_0.49N, respectively, after annealing at 1350°C for 20 min. These results show unprecedented activation efficiencies for Al _x Ga_1−x N with high Al mole fractions and provide suitable annealing conditions for Al _x Ga_1−x N-based device applications.