G. Kelner

Fabrication and characterization of GaN FETs

Abstract The current status of GaN-based FET technology and performance is reviewed. The fabrication details and the dc and microwave characteristics of GaN MESFETs that utilize Si-doped channels on semi-insulating buffer layers are presented. MESFETs with a 0.8 μm gate have exhibited an f T and f max of 6 and 14 GHz, respectively. These devices have excellent pinchoff characteristics and a source-drain breakdown voltage of over 85 V. A high-field current-collapse phenomenon is observed in these MESFETs in the absence of light. The characteristics of this current collapse as a function of temperature, illuminating wavelength, and time are described. A model describing the current collapse in terms of hot electron injection into the buffer layer is presented.

H, He, and N implant isolation of n‐type GaN

The effect of ion‐implantation‐induced damage on the resistivity of n‐type GaN has been investigated. H, He, and N ions were studied. The resistivity as a function of temperature, implant concentration, and post‐implant annealing temperature has been examined. Helium implantation produced material with an as‐implanted resistivity of 1010 Ω‐cm. He‐implanted material remained highly resistive after an 800 °C furnace anneal. The damage associated with H implantation had a significant anneal stage at 250 °C and the details of the as‐implanted resistivity were sample dependent. N implants had to be annealed at 400 °C to optimize the resulting resistivity but were then thermally stable to over 800 °C. The 300 °C resistivity of thermally stabilized He‐ and N‐ implanted layers was 104 Ω‐cm, whereas for H‐implanted layers the 300 °C resistivity was less than 10 Ω‐cm.

Al and B ion‐implantations in 6H‐ and 3C‐SiC

Low (keV) and high (MeV) energy Al and B implants were performed into n‐type 6H‐ and 3C‐SiC at both room temperature and 850 °C. The material was annealed at 1100, 1200, or 1400 °C for 10 min and characterized by secondary ion mass spectrometry, Rutherford backscattering (RBS), photoluminescence, Hall and capacitance‐voltage measurement techniques. For both Al and B implants, the implant species was gettered at 0.7 Rp (where Rp is the projected range) in samples implanted at 850 °C and annealed at 1400 °C. In the samples that were amorphized by the room temperature implantation, a distinct damage peak remained in the RBS spectrum even after 1400 °C annealing. For the samples implanted at 850 °C, which were not amorphized, the damage peak disappeared after 1400 °C annealing. P‐type conduction is observed only in samples implanted by Al at 850 °C and annealed at 1400 °C in Ar, with 1% dopant electrical activation.

Phosphorus and boron implantation in 6H–SiC

Phosphorus and boron ion implantations were performed at various energies in the 50 keV–4 MeV range. Range statistics of P+ and B+ were established by analyzing the as-implanted secondary ion mass spectrometry depth profiles. Anneals were conducted in the temperature range of 1400–1700 °C using either a conventional resistive heating ceramic processing furnace or a microwave annealing station. The P implant was found to be stable at any annealing temperature investigated, but the B redistributed during the annealing process. The implant damage is effectively annealed as indicated by Rutherford backscattering measurements. For the 250 keV/1.2×1015 cm−2 P implant, annealed at 1600 °C for 15 min, the measured donor activation at room temperature is 34% with a sheet resistance of 4.8×102 Ω/□. The p-type conduction could not be measured for the B implants.

PN junction formation in 6HSiC by acceptor implantation into n-type substrate

Abstract A1 and B implantations were performed into n-type 6H-bulk SiC and epitaxial layers at both room temperature and 850°C. Annealings were performed in the temperature range of 1100–1650°C in a SiC crucible. For single-energy implants, the implant gettered to the 0.7Rp location for annealing temperatures ≥1400°C. For the 850°C implanted samples the RBS yield in the annealed material is comparable to the yield in the as-grown material, indicating a good lattice recovery. A maximum activation of 18% for Al-implanted samples was observed. PN junction diodes were made using Al-implanted material.

Material and n-p junction properties of N-, P-, and N/P-implanted SiC

Elevated temperature (ET) multiple energy N, P, and N/P implantations were performed into p-type 6H-SiC epitaxial layers. For comparison, room temperature (RT) N and P implantations were also performed. In the N/P coimplanted material a sheet resistance of 2.1×102 Ω/□ was measured, which is lower compared to the values measured in N or P implanted material of the same net donor dose. The RT P implantation resulted in heavy lattice damage and consequently low P electrical activation, even after 1600 °C annealing. After annealing the Rutherford backscattering yield either coincided or came close to the virgin level for ET implantations and RT N implantation, whereas for RT P implantation the yield was high, indicating the presence of high residual damage. Vertical n-p junction diodes were made by selective area ET N, P, and N/P implantations and RT N and P implantations using a 2.5 μm thick SiO2 layer as an implant mask. The diodes were characterized by capacitance–voltage and variable temperature current–v...

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.

Nitrogen and aluminum implantation in high resistivity silicon carbide

In this article, the results on N and Al implantations into undoped high-resistance and vanadium doped semi-insulating bulk 6H-SiC are reported for the first time. The N implants were performed at 700 °C and the Al implants at 800 °C to create n- and p-type layers, respectively. For comparison, implants were performed into epitaxial layers at the above temperatures and, for N, also at room temperature. The implanted/annealed material was characterized by van der Pauw Hall, secondary ion mass spectrometry, and Rutherford backscattering (RBS) measurements. After annealing, the room temperature N implantation gave similar electrical and RBS results as the 700 °C implantation for a total implant dose of 8×1014 cm−2 which corresponds to a volume concentration of 2×1019 cm−3. The Al implant redistributed in the bulk crystals during annealing, resulting in a shoulder formation at the tail of the implant profile. Lower implant activation was obtained in V-doped material compared to the undoped bulk and epitaxial ...

Compensation implants in 6H–SiC

In this work, we have performed Si and C isoelectronic implantations in n-type and vanadium (V) implantations in p-type 6H–SiC to obtain highly resistive regions. The compensation is achieved by the lattice damage created by the Si and C implantations and the chemically active nature of the V implant. For the Si and C implantations, the as-implanted resistivity initially increased with increasing implant fluence due to the introduction of compensating levels caused by the lattice damage, then decreased at higher fluences due to hopping conduction of the trapped carriers. The resistivity of the Si and C implanted material has been measured after isochronal heat treatments over the temperature range of 400–1000 °C. The maximum resistivity values measured for Si and C implanted and heat treated material were ∼1012 Ω cm. For the 700 °C V implantation in p-type SiC, resistivities of >1012 Ω cm were measured after 1500 or 1600 °C annealing to activate the V implant. Redistribution of the V implant is observed a...

Elevated temperature nitrogen implants in 6H-SiC

Elevated temperature (700°C) N ion implantations were performed into 6H-SiC in the energy range of 50 keV-4 MeV. By analyzing the as-implanted depth distributions, the range statistics of the N+ in 6H-SiC have been established over this energy range. Annealing at 1500 and 1600°C for 15 min resulted in Rutherford backscattering spectrometry scattering yields at the virgin crystal level, indicating a good recovery of the crystalline quality of the material without any redistribution of the dopant. A maximum electron concentration of 2 × 1019 cm−3, at room temperature, has been measured even for high-dose implants. The p-n junction diodes made by N ion implantation into a p-type substrate have a forward turn-on voltage of 2.2 V, an ideality factor of 1.90, and a reverse breakdown voltage of 125 V with nA range leakage current for -10 V bias at room temperature. By probing many devices on the same substrate we found uniform forward and reverse characteristics across the crystal.