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.     

The effects of incomplete annealing on the temperature dependence of sheet resistance and gage factor in aluminum and phosphorus implanted silicon on sapphire

The temperature coefficient of resistivity (TCR) of ion implanted silicon can be significantly reduced by partially annealing the crystal damage produced during implantation. The extent to which this method can be used to temperature compensate the resistivity and the gage factor has been determined for 300 ohm-cm silicon on sapphire implanted with either 100 keV Al²⁷ or P³¹ ions. The implantations were made at room temperature parallel to the 〈100〉 axis and in four fluences ranging from 1 × 10¹³cm^?2 to 1·25 × 10¹⁵ cm^?2. Sheet resistance, Hall coefficient, and effective mobility were measured from ?150°C to 150°C for various anneal temperatures. It was possible to obtain very low temperature dependences of sheet resistance at 300°K for all dopant fluences by appropriate partial annealing. On samples having the lowest temperature dependence of sheet resistance, the gage factor was measured from ?75°C to 75°C. The measurements were made along the 〈100〉 direction for phosphorus doped samples, and along the 〈110〉 direction for aluminum doped samples for all four fluences. The gage factor and its temperature dependence for these crystal orientations are not drastically affected by the crystal damage. These results are interpreted in terms of a model previously developed to explain the effect of electron damage on the temperature dependence of the resistivity and the piezoresistance of silicon.     

Ir Diffusion Barriers in Ni/Au Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-Type CuCrO<Subscript>2</Subscript>

The use of Ir diffusion barriers in Ni/Au-based Ohmic contacts to p-type CuCrO₂ layers was investigated. A specific contact resistance of ~5 × 10⁻⁴ Ω cm² was achieved after annealing at 500°C for the Ir-containing contacts, and the contacts were rectifying for lower anneal temperatures. In this case, the contact resistance was basically independent of the measurement temperature, indicating that tunneling is the dominant transport mechanism in the contacts. The morphology for the Ir-containing contacts was still smooth at 500°C although Auger electron spectroscopy depth profiling showed that some of the nickel had diffused to the surface and had oxidized. Contacts annealed at 800°C showed that some copper and most of the nickel had diffused to the surface and oxidized. The presence of the Ir diffusion barrier does increase the thermal stability of the contacts by ∼200°C compared to conventional Ni/Au contacts. By contrast, the use of other materials such as TaN, ZrN, and W₂B₅ as the diffusion barrier led to poorer thermal stability, with the contact resistance increasing sharply above 400°C.     

Surface roughening in ion implanted 4H-silicon carbide

Silicon carbide (SiC) devices have the potential to yield new components with functional capabilities that far exceed components based on silicon devices. Selective doping of SiC by ion implantation is an important fabrication technology that must be completely understood if SiC devices are to achieve their potential. One major problem with ion implantation into SiC is the surface roughening that results from annealing SiC at the high temperatures which are needed to activate implanted acceptor ions, boron or aluminum. This paper examines the causes and possible solutions to surface roughening of implanted and annealed 4H-SiC. Samples consisting of n-type epilayers (5 × 10¹⁵ cm⁻³, 4 μm thick) on 4H-SiC substrates were implanted with B or Al to a total dose of 4 × 10¹⁴ cm⁻² or 2 × 10¹⁵ cm⁻², respectively. Roughness measurements were made using atomic force microscopy. From the variation of root mean square (rms) roughness with annealing temperature, apparent activation energies for roughening following implantation with Al and B were 1.1 and 2.2 eV, respectively, when annealed in argon. Time-dependent activation and surface morphology analyses show a sublinear dependence of implant activation on time; activation percentages after a 5 min anneal following boron implantation are about a factor of two less than after a 40 min anneal. The rms surface roughness remained relatively constant with time for anneals in argon at 1750°C. Roughness values at this temperature were approximately 8.0 nm. Annealing experiments performed in different ambients demonstrated the benefits of using silane to maintain good surface morphology. Roughnesses were 1.0 nm (rms) when boron or aluminum implants were annealed in silane at 1700°C, but were about 8 and 11 nm for B and Al, respectively, when annealed in argon at the same 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.     

Selenium implantation in GaAs

The dependence of carrier concentration and mobility profiles on the dose of 400 keV Se ions implanted into Cr-doped semi-insulating GaAs, and on the annealing temperature has been studied for doses ranging from 3 × 10¹²/cm² to 2 × 10¹⁵/cm² and for annealing temperatures between 800 and 1000°C. Sputtered aluminum oxy-nitride and silicon nitride films were used as encapsulants for protection of the implanted surface during annealing treatments. The carrier profiles exhibited deep tails for implantations along both random and {110} planar directions. It was found that annealing temperatures of 900°C or above were necessary to obtain high carrier density and mobility values for implantation doses above 1 × 10¹⁴/cm². Samples encapsulated with aluminum oxy-nitride films exhibited 3 to 4 times higher carrier concentration values and also slightly higher mobility values than those encapsulated with silicon nitride films. The maximum carrier concentration obtained was about 4 × 10¹⁸/cm³ with aluminum oxy-nitride films as the encapsulant.     

Beryllium and sulfur ion-implanted profiles in gaas

Atomic profiles of ion-implanted Be and S in GaAs have been measured as a function of implant fluence and annealing temperature. Concentration versus depth profiles were ob-tained by means of secondary ion mass spectrometry (SIMS) techniques. Pyrolytically deposited and sputter-coated Si0₂ and Si₃N₄ films were used as encapsulants for the 500 to 900° annealing study. Semi-insulating GaAs was implanted with 200 keV³⁴S⁺ to fluences of 1 × 10¹⁴ and 5₂× 10¹⁴/cm², and 100 keV⁹Be⁺ in the 1 × 10¹³ to 1 × 10¹⁵/cm² fluence range. The S profiles did not change significantly after annealing at 800°C, although there was some skewing after annealing above 600°C. In contrast, the Be profiles showed significant changes and a decrease in the peak concentration for the ≥ 5 × 10^T4/cm² implants after a 700°C anneal. After a 800°C anneal the Be profile was essentially flat with a monotonic decrease from the surface into the implanted re-gion and a 900°C anneal caused a further decrease in the Be concentration. Profiles of Be implants of ≤ 1 × 10¹⁴/cm² did not change significantly after annealing indicating that the higher fluence cases were related to solubility effects. This work supported by the Naval Electronic Systems Command and the Office of Naval Research.     

Comparison of Solid-State Microwave Annealing with Conventional Furnace Annealing of Ion-Implanted SiC

Rapid solid-state microwave annealing was performed for the first time on N⁺-, Al⁺-, and B⁺-implanted SiC, and the results were compared with the conventional furnace annealing. For microwave annealing, temperatures up to 2,000 °C were attained with heating rates exceeding 600 °C/s. An 1,850 °C/35 s microwave anneal yielded a root-mean-square (RMS) surface roughness of 2 nm, which is lower than the 6 nm obtained for 1,500 °C/15 min conventional furnace annealing. For the Al implants, a minimum room-temperature sheet resistance (R _s ) of 7 kΩ/□ was measured upon microwave annealing. For the microwave annealing, Rutherford backscattering (RBS) measurements indicated a better structural quality, and secondary-ion-mass-spectrometry (SIMS) boron implant depth profiles showed reduced boron redistribution compared to the corresponding results of the furnace annealing.     

Si-implantation activation annealing of GaN up to 1400°C

Implant activation annealing of Si-implanted GaN is reported for temperatures from 1100 to 1400°C. Free electron concentrations up to 3.5×10²⁰ cm⁻³ are estimated at the peak of the implanted profile with Hall mobilities of ∼60 cm²/Vs for annealing at 1300°C for 30 s with an AIN encapsulant layer. This mobility is comparable to epitaxial GaN doped at a similarly high level. For annealing at ≥1300°C, the sample must be encapsulated with AIN to prevent decomposition of the GaN layer. Channeling Rutherford backscattering demonstrates the partial removal of the implant damage after a 1400°C anneal with a minimum channeling yield of 12.6% compared to 38.6% for the as-implanted spectrum. Scanning electron microscope images show evidence of decomposition of unencapsulated GaN after a 1300°C anneal and complete sublimation after 1400°C. The use of AIN encapsulation and annealing at temperatures of ∼1300°C will allow the formation of selective areas of highly doped GaN to reduce the contact and access resistance in GaN-based transistors and thyristors.     

Sensitivity analysis of ion implanted silicon wafers after rapid thermal annealing

In this study, we have investigated sensitivities of the ion implanted silicon wafers processed by rapid thermal annealing (RTA), which can reveal the variation of sheet resistance as a function of annealing temperature as well as implantation parameters. All the wafers were sequentially implanted by the arsenic or phosphorous implantations at 40, 80, and 100 keV with the dose level of 10¹⁴ to 2 × 10¹⁶ ions/cm². Rapid thermal annealing was carried out for 10 s by the infrared irradiation at a temperature between 850 and 1150°C in the nitrogen ambient. The activated wafer was characterized by the measurements of the sheet resistance and its uniformity mapping. The values of sensitivities are determined from the curve fitting of the experimental data to the fitting equation of correlation between the sheet resistance and process variables. From the sensitivity values and the deviation of sheet resistance, the optimum process conditions minimizing the effects of straggle in process parameters are obtained. As a result, a strong dependence of the sensitivity on the process variables, especially annealing temperatures and dose levels is also found. From the sensitivity analysis of the 10 s RTA process, the optimum values for the implant dose and annealing temperature are found to be in the range of 10¹⁶ ions/cm² and 1050-1100°C, respectively. The sensitivity analysis of sheet resistance will provide valuable data for accurate activation process, offering a guideline for dose monitoring and calibration of ion implantation process.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.     

Hot-implantation of nitrogen donors into p- type α-SiC and characterization of n+-p junction

N⁺ implantation into p-type a-SiC (6H-SiC, 4H-SiC) epilayers at elevated temperatures was investigated and compared with implantation at room temperature (RT). When the implant dose exceeded 4 × 10₁₅ cm₋₂, a complete amorphous layer was formed in RT implantation and severe damage remained even after post implantation annealing at 1500°C. By employing hot implantation at 500~800°C, the formation of a complete amorphous layer was suppressed and the residual damage after annealing was significantly reduced. For implant doses higher than 10¹⁵ cm⁻², the sheet resistance of implanted layers was much reduced by hot implantation. The lowest sheet resistance of 542Ω/ was obtained by implantation at 500 ~ 800°C with a 4 × 10¹⁵ cm⁻² dose. Characterization of n⁺-p junctions fabricated by N⁺ implantation into p-type epilayers was carried out in detail. The net doping concentration in the region close to the junction showed a linearly graded profile. The forward current was clearly divided into two components of diffusion and recombination. A high breakdown voltage of 615 ∼ 810V, that is almost an ideal value, was obtained, even if the implant dose exceeded 10¹⁵ cm⁻². By employing hot implantation at 800°C, the reverse leakage current was significantly reduced.     

The annealing of 1 MeV implantations of boron in silicon

Buried layers of boron in silicon have been made by 1 MeV implantations up to a dose of 10¹³ cm^?2. The annealing of the implantation damage has been studied with Van der Pauw and Hall measurements. It is concluded that lattice damage reduces the mobility only for annealing temperatures below 600°C. The average mobilities measured after annealing at temperatures above 600°C correspond accurately to the values calculated from the most recent literature data, based on scattering by the lattice and by the active impurities. Complete activation was obtained after 60 min annealing at 700°C.     

Characterization of TiN/TiSi2 bilayer for application to ULSI

The properties of TiN/TiSi₂ bilayer formed by rapid thermal annealing (RTA) in an NH₃ ambient after the titanium film is deposited on the silicon substrate is investigated. It is found that the formation of TiN/TiSi₂ bilayer depends on the RTA temperature and a competitive reaction for the TiN/TiSi₂ bilayer occurs at 600°C. Both the TiN and TiSi₂ layers represent titanium-rich films at 600°C anneal. The TiN layer has a stable structure at 700°C anneal while the TiSi₂ layer has C₄₉ and C₅₄ phase. Both the TiN and TiSi₂ layers have stable structures and stoichiometries at 800°C anneal. When the TiN/TiSi₂ bilayer is formed, the redistribution of boron atoms within the TiSi₂ layer gets active as the anneal temperature is increased. According to secondary ion mass spectroscopy analysis, boron atoms pile up within the TiN layer and at the TiSi₂−Si interface. The electrical properties for n⁺ and p⁺ contacts are investigated. The n⁺ contact resistance increases slightly with increasing annealing temperature but the p⁺ contact resistance decreases. The leakage current indicates degradation of the contact at high annealing temperature for both n⁺ and p⁺ junctions.     

Alloyed tin-gold ohmic contacts to n-type indium phosphide

The formation of alloyd ohmic contacts on n-InP using sequentially deposited Sn plus Au films was investigated. The specific contact resistance for metallizations with a Sn content of 5 at. % was determined for annealing temperatures between 250 and 500°C. The minimum specific contact resistance, r_c = (1.8±0.9) × 10^?6 ohm-cm² occurred for a narrow range of annealing temperatures between 380 and 410°C on substrates with n = 3 × 10¹⁸/cm³. For annealing temperatures 350°C the contacts were non-Ohmic and above 420°C the resistance increased dramatically. Contact morphology and metallurgy were studied by optical and scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy and Rutherford backscattering. Films annealed above 320°C contained several phases, mainly Au₄In, AuSn and polycrystalline InP. The contacts annealed at temperatures above 410°C were composed predominantly of the single phase Au₃In₂.     

Low-temperature processing of antimony-implanted silicon

It is shown for the first time that antimony-implanted silicon produces the highest electrical activation (90%) with low resistivity (<200 ohms/square) following low-temperature processing. Thus, annealing at 650°C produces the best results for antimony, whereas for arsenic, it is necessary to anneal at temperatures above 1000°C to get optimum results. Silicon was implanted with antimony at 12 keV and 40 keV and doses of 8.5×10¹⁴ cm⁻² and 4×10¹⁴ cm⁻², respectively, and arsenic at equivalent energies and doses. The electrical data from both implants are compared in order to identify the process conditions require to obtain optimum results. It is demonstrated that annealing below 800°C produces electrical profiles with no measurable diffusion of the antimony, but higher temperature anneals produce significant diffusional broadening.     

Sheet resistance variations of phosphorus implanted silicon at elevated temperatures

Silicon has been implanted to high doses (2 × 10¹⁵–2 × 10¹⁶ ions cm^?2) with 40 keVP⁺ at constant temperatures in the range 20–300°C. The sheet resistance following implantation is shown to have a break point at 167°C in its temperature variation characteristic. It is suggested that sheet resistance variations result from changes in the depth of the amorphous layer formed during implantation. A dose rate effect has also been observed. The behaviour correlates with the variations which have been observed on colour-banded wafers. It is also demonstrated that small temperature rises during implantation can result in significant variations in the sheet resistance, even after high temperature annealing.     

Halogen lamp rapid thermal annealing of Si- and Be-implanted In0.53Ga0.47As

Halogen lamp rapid thermal annealing was used to activate 100 keV Si and 50 keV Be implants in In_0.53Ga_0.47As for doses ranging between 5 × 10¹²−4 × 10¹⁴ cm⁻². Anneals were performed at different temperatures and time durations. Close to one hundred percent activation was obtained for the 4.1 × 10¹³ cm⁻² Si-implant, using an 850° C/5 s anneal. Si in-diffusion was not observed for the rapid thermal annealing temperatures and times used in this study. For the 5 × 10¹³ cm⁻² Be-implant, a maximum activation of 56% was measured. Be-implant depth profiles matched closely with gaussian profiles predicted by LSS theory for the 800° C/5 s anneals. Peak carrier concentrations of 1.7 × 10¹⁹ and 4 × 10¹⁸ cm⁻³ were achieved for the 4 × 10¹⁴ cm⁻² Si and Be implants, respectively. For comparison, furnace anneals were also performed for all doses.     

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.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².