Showing papers by "Nils Weimann published in 2002"

High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy

Abstract: We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (∼20 μm) GaN templates are characterized by low threading dislocation densities (∼5×108 cm−2) and by room temperature resistivities of ∼108 Ω cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x⩽0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns⩽2×1012 cm−2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ∼2×1...

SiGe differential transimpedance amplifier with 50 GHz bandwidth

Abstract: InP and SiGe technologies are both attractive for design of circuits operating at 40 GB/s and beyond. In this paper, we describe a fully differential SiGe transimpedance amplifier (TIA) suitable for differential phase-shift keying applications. The TIA exhibits 49 dB-/spl Omega/ transimpedance, greater than 50-GHz bandwidth, and input-referred current noise less than 30 pA//spl radic/Hz. For comparison, we have also developed a similar TIA in an InP double-heterostructure bipolar transistor technology. The InP TIA had 48 dB-/spl Omega/ transimpedance and 49-GHz bandwidth.

Dislocation and morphology control during molecular-beam epitaxy of AlGaN/GaN heterostructures directly on sapphire substrates

Abstract: We report on the growth and transport characteristics of high-density (∼1013 cm−2) two-dimensional electron gases confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on sapphire substrates. For structures consisting of a 25 nm Al0.30Ga0.70N barrier deposited on a 2 μm insulating GaN buffer, room-temperature mobilities averaging 1400 cm2/V s at a sheet charge density of 1.0×1013 cm−2 are consistently achieved. Central to our approach is a sequence of two Ga/N ratios during the growth of the insulating GaN buffer layer. The two-step buffer layer allows us to simultaneously optimize the reduction of threading dislocations and surface morphology. Our measured sheet resistivities as low as 350Ω/□ compare favorably with those achieved on sapphire or SiC by any growth method. Representative current–voltage characteristics of high-electron-mobility transistors fabricated from this material are presented.

Thin-film resistor fabrication for InP technology applications

Abstract: In this study we evaluated both NiCr and TaN thin-film resistor material for use with our InP technology. Thermal stability, sensitivity to oxidation, temperature coefficients, and patterning techniques were compared for the two materials. The film stoichiometry was determined using Rutherford backscattering and Auger electron spectroscopy analysis. Electron-beam evaporation of NiCr (80:20) resulted in films that were rich in Cr (30%), due to a higher vapor pressure of Cr. Thus, it was preferentially evaporated from the source. The TaN, on the other hand, was stoichiometric to within a few atomic %. This was due to better composition control with sputter deposition as opposed to electron (e)-beam evaporation from an alloy source. The NiCr showed a 5% increase in Rs when exposed to O2 plasma, and was stable for anneal temperatures of up to 300 °C. Alternatively, the TaN was stable for O2 plasma exposure, but had an increase in Rs of 5% or 10% when annealed at 300 °C in N2 or air ambients, respectively. Fro...

Submicron AlGaN/GaN HEMTs with very high drain current density grown by plasma-assisted MBE on 6H-SiC

Abstract: High electron mobility transistors (HEMTs) were fabricated from AlGaN/-GaN layers grown by plasma-assisted molecular beam epitaxy on semi-insulating 6H-SiC substrates. Room-temperature Hall effect measurements yielded a polarization-induced 2DEG sheet charge of 1.3/spl middot/10/sup 13/ cm/sup -2/ and a low-field mobility of 1300 cm/sup 2//V/spl middot/s. Submicron gates were defined with electron beam lithography using an optimized two-layer resist scheme. HEMT devices repeatedly yielded drain current densities up to 1798 mA/mm, and a maximum transconductance of 193 mS/mm. This is the highest drain current density in any AlGaN-GaN HEMT structure delivering significant microwave power reported thus far. Small-signal testing of 50-/spl mu/m wide devices revealed a current gain cutoff frequency f/sub T/ of 52 GHz, and a maximum frequency of oscillation f/sub max/ of 109 GHz. Output power densities of 5 W/mm at 2 GHz, and 4.9 W/mm at 7 GHz were recorded from 200-/spl mu/m wide unpassivated HEMTs with a load-pull setup under optimum matching conditions in class A device operation.

AlGaN/GaN HEMTs grown by molecular beam epitaxy on sapphire, SiC, and HVPE GaN templates

Abstract: Molecular beam epitaxy of GaN and related alloys is becoming a rival to the more established metalorganic vapor phase epitaxy. Excellent control of impurity, interface abruptness, and in situ monitoring of the growth are driving the increase in quality of MBE epilayers. We have developed nucleation schemes with plasma-assisted MBE on three types of substrates, consisting of sapphire, semi-insulating (SI-) SiC, and HVPE SI-GaN templates on sapphire. While sapphire and SI-SiC are established substrates for the growth of AlGaN/GaN HEMT epilayers, HVPE GaN templates may provide a path to low-cost large-diameter substrates for electronic devices. We compare device results of HEMTs fabricated on these substrates. As a metric for device performance, the saturated RF power output in class A operation is measured at 2 GHz. We achieved a saturated power density of 2.2 W/mm from HEMTs on sapphire, 1.1 W/mm from HEMTs on HVPE GaN templates on sapphire, and 6.3 W/mm. from HEMTs on semi-insulating 6H-SiC substrates. The difference in output power can be attributed to self-heating due to insufficient thermal conductivity of the sapphire substrate, and to trapping in the compensation-doped HVPE template.

Method for growing layers of group III-nitride semiconductor having electrically passivated threading defects

Abstract: One method includes epitaxially growing a layer of group III-nitride semiconductor under growth conditions that cause a growth surface to be rough. The method also includes performing an epitaxial growth of a second layer of group III-nitride semiconductor on the first layer under growth conditions that cause the growth surface to become smooth. The two-step growth produces a lower density of threading defects. Another method includes epitaxially growing a layer of group III-nitride semiconductor on a lattice-mismatched crystalline substrate and then, chemically treating a growth surface of the layer to selectively electrically passivate defects that thread the layer.

Prime Quasientropy and Quasichaos

Abstract: We construct a prime symmetry relation for integers that is equivalent to Goldbach's conjecture and show that numerical computations of this prime symmetry property strongly resemble a chaotic sequence We define and examine the notions of global and local prime quasientropies Finally, we employ the fact that the prime number sequence satisfies the property of deterministic randomness to consider its utility for the field of quantum computation

AlGaN/GaN HEMTs grown by MBE on semi-insulating HVPE GaN templates

Abstract: Summary form only given. Attempts to circumvent the low thermal conductivity of sapphire substrates by including in a device structure a thick (15 /spl mu/m) GaN buffer grown by HVPE between the sapphire substrate and the MBE device layer stack. The heat generated in the device close to the surface of the structure is spread laterally in the HVPE buffer, increasing the effective thermal area of the device, and lowering the thermal impedance of the HEMT device. In contrast to SiC, the sapphire substrate technology is mature and affordable. Principally, both MBE and HVPE growth may be scaled up in area to grow on readily available large diameter sapphire substrates.