Kenneth A. Jones

Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

Low-resistance Ti/Al/Ti/Au multilayer ohmic contact to n-GaN

A metallization scheme has been developed for obtaining low ohmic contacts to n-GaN with a low contact resistance. The metal contact is a Ti/Al/Ti/Au composite with layers that are respectively 30, 100, 30, and 30 nm thick. Contacts with a specific contact resistivity ρs, as low as 6.0×10−7 Ω cm2 for a doping level of 1.40×1020 cm−3 were obtained after annealing the sample for 30 s at 750 °C in a rapid thermal annealer. The Ti placed on top of the traditional Ti/Al contact appears to have the advantage of tying up the excess Al; therefore it does not form a mottled contact. Some of the additional Ti–Al intermetallic alloys that are formed also have beneficial effects. The Ti–Au layer forms a robust upper portion of the composite, which enables the contacts to have high-temperature applications.

Deposition factors and band gap of zinc-blende AlN

Successful deposition of zinc-blende AlN films with thickness up to 1000 A was performed with plasma source molecular beam epitaxy. The films were epitaxial to the Si(001) substrate. The formation of a thin 3C-SiC layer on the Si(001) surface is one of the important factors for the formation of zinc-blende AlN. Evidence for the presence of 3C-SiC is provided by an Auger electron spectroscopy depth profile and a high-resolution transmission electron microscopy plot profile. Spectroscopic ellipsometry was used to determine the optical constants of zinc-blende AlN in the range from 1.85 to 6.5 eV. The extinction coefficient data indicates that zinc-blende AlN is an indirect semiconductor with a band gap of ∼5.34 eV.

1000-V, 30-A 4H-SiC BJTs with high current gain

This paper presents the development of 1000 V, 30A bipolar junction transistor (BJT) with high dc current gain in 4H-SiC. BJT devices with an active area of 3/spl times/3 mm/sup 2/ showed a forward on-current of 30 A, which corresponds to a current density of 333 A/cm/sup 2/, at a forward voltage drop of 2 V. A common-emitter current gain of 40, along with a low specific on-resistance of 6.0m/spl Omega//spl middot/cm/sup 2/ was observed at room temperature. These results show significant improvement over state-of-the-art. High temperature current-voltage characteristics were also performed on the large-area bipolar junction transistor device. A collector current of 10A is observed at V/sub CE/=2 V and I/sub B/=600 mA at 225/spl deg/C. The on-resistance increases to 22.5 m/spl Omega//spl middot/cm/sup 2/ at higher temperatures, while the dc current gain decreases to 30 at 275/spl deg/C. A sharp avalanche behavior was observed at a collector voltage of 1000 V. Inductive switching measurements at room temperature with a power supply voltage of 500 V show fast switching with a turn-off time of about 60 ns and a turn-on time of 32 ns, which is a result of the low resistance in the base.

Shallow ohmic contact to both n- and p-GaAs

A shallow Pd/Ge/Ti/Pt/ohmic contact for both n‐ and p‐GaAs has been investigated. The contacts were rapid thermally annealed in N2 for 15 s at temperatures from 350 to 550 °C. The lowest average specific contact resistances were 4.7×10−7 and 6.4×10−7 Ω cm2 for the n‐ and p‐GaAs, respectively, when the n‐GaAs was doped with Si to 2×1018 cm−3 and the p‐GaAs was doped with carbon to 5×1019 cm−3. Electrical measurements and Auger depth profiles showed that the contacts were stable as they remained ohmic after an anneal at 300 °C for 20 h for both n‐ and p‐GaAs. The p contact is more stable than the n contact at the higher temperatures where there is more As outdiffusion as determined by Auger depth profiles. Transmission electron microscopy showed that the interfaces between the p‐GaAs and the contacts were smooth for both as‐grown and annealed samples, and no oxides were detected.

Electrical characteristics of AlxGa1−xN Schottky diodes prepared by a two-step surface treatment

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...

Annealing effects on heavily carbon‐doped GaAs

The hole concentrations and lattice mismatch with the GaAs substrate of heavily carbon‐doped epilayers (4.7×1019 and 9.8×1019 cm−3) were increased and the mobilities were decreased as compared with the as‐grown samples by rapid thermal annealing silicon nitride capped samples at temperatures from 500 to 900 °C. However, for the more heavily doped sample, the hole concentration, mobility, and lattice mismatch decreased with increasing annealing temperature for annealing temperatures higher than 700 °C, but the hole concentration and lattice mismatch were still larger than those of the as‐grown samples. Secondary ion mass spectroscopy results showed that annealing produced no change in the C concentration or distribution, but the hydrogen concentration decreased. Cross‐sectional transmission electron microscopy indicated that no mismatch dislocations formed at the interface.

High-transparency Ni/Au bilayer contacts to n-type GaN

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. ...

Pt/Ti/Ge/Pd ohmic contacts to GaAs: A structural, chemical, and electrical investigation

Interface morphology, phase composition, and elemental diffusion of Pt/Ti/Ge/Pd ohmic contacts to both n+‐ and p+‐GaAs have been investigated as a function of annealing temperature. Structural and chemical results were correlated with specific contact resistances (ρc) measured for each thermal treatment in an attempt to determine how the indiffusion of contact metal and the out‐diffusion of Ga and As affect specific contact resistance, and how effective Ti is as a diffusion barrier. Annealing at 450 °C yielded the lowest ρc, ∼6.4×10−7 Ω cm2. The interface was observed to be smooth and abrupt, and there was little As out‐diffusion and Pt indiffusion. Two interface phases were detected; a primary phase, PdGe, and a secondary, Ga rich Pd–Ga–As ternary phase. The presence of this ternary phase is believed to be critical to contact formation on n‐type GaAs. The Ti and Pt layers remained pristine. Annealing at 550 °C resulted in a slightly higher ρc,∼2.1×10−6Ω cm2. There was significant elemental diffusion with...

InN-based Ohmic contacts to InAlN

The ability to form low resistance Ohmic contacts to InAlN using refractory metallization on In-containing contact layers has been investigated. The crystal quality as measured by surface roughness and x-ray diffraction was found to be a determining factor in the contact resistance which could be obtained using WSix contacts. InN growth conditions which resulted in poorer structural quality, such as the use of GaAs rather than sapphire substrates, low nitrogen flows, and low growth temperatures, resulted in contact resistances of 10−4–10−3 Ω cm2, as compared to ∼10−5 cm2 for InAlN alone. Using optimized conditions, contact resistances to InAlN of ∼3.5×10−6 Ω cm2 were obtained.