N. G. Weimann

GaN nanotip pyramids formed by anisotropic etching

We experimentally demonstrate the formation of GaN nanotip pyramids by selective and anisotropic etching of N-polar GaN in KOH solution. For samples grown with adjacent Ga- and N-polar regions on the same wafer, the KOH solution was found to selectively etch only the N-polar surface while leaving the Ga-polar surface intact. An aggregation of hexagonal pyramids with well defined {10 1 1} facets and very sharp tips with diameters less than ∼20 nm were formed. The density of the pyramids can be controlled by varying the KOH concentration, solution temperature or the etch duration. The GaN etching activation energy is estimated to be Ea≈0.587 eV. Dense GaN pyramids with sharp tips have applications in both electronic and photonic devices.

Second-harmonic generation in periodically poled GaN

We report the experimental demonstration of second-harmonic generation in periodically poled GaN by first-order quasiphase matching. The periodically poled structure was grown by plasma-assisted molecular-beam epitaxy. We observed about 9 μW of second-harmonic power from a fundamental input laser wavelength of 1658.6 nm with a normalized conversion efficiency of 12.76% W−1 cm−2. The ability to perform nonlinear wavelength conversion by periodic poling and the fact that GaN has a very wide window of transparency, pave the way for GaN to be used for nonlinear optical devices in telecommunications as well as a nonlinear light source for biochemical detection in the far infrared and deep ultraviolet.

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

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...

Compact InP-based HBT VCOs with a wide tuning range at W- and D-band

Compact monolithic integrated differential voltage-controlled oscillators (VCOs) operating in W-band were realized using InP-based heterojunction bipolar transistors (HBTs). The oscillators, with a total chip size of 0.6 by 0.35 mm/sup 2/, are based on a balanced Colpitts-type topology with a coplanar transmission-line resonator. By varying the voltage across the base-collector junction of the HBT in the current mirror and by changing the current in the VCO, the oscillation frequency can be tuned between 84 and 106 GHz. At 100 GHz, a differential voltage swing of 400 mV is obtained, which should be sufficient to drive 100 Gb/s digital logic. By combining the balanced outputs of a similar differential VCO in a push-push configuration, a compact source with close to -10 dBm output power and a tuning range between 138 and 150 GHz is obtained.

Impact of Si doping on radio frequency dispersion in unpassivated GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy

We report on the effect of Si doping on the transient behavior of unpassivated high-power GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy on 6H–SiC. The incorporation of Si into the heterostructure barrier is found to reduce the level of radio frequency dispersion as compared to undoped structures. In some devices which incorporate Si doping of the barrier, the pulsed and steady-state current–voltage characteristics coincide, and gate lag is found to be insignificant. More typically, ∼90% of the dc value of drain current is restored at 1 μs after pulsing the gate from pinch off to VGS=0 V. Significant gate lag is observed in devices that are not doped with Si. In the undoped structure, the drain current reaches only ∼70% of the dc value within 1 μs. The transient behavior in the two designs is attributed to the same defect state with activation energy of 0.22 eV. Dispersion reduction is correlated with an increase of gate leakage current in Si-doped devices.

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

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.

Unpassivated AlGaN-GaN HEMTs with minimal RF dispersion grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates

High electron mobility transistors (HEMTs) are fabricated from AlGaN-GaN heterostructures grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating 6H-SiC substrates. At a sheet charge density of 1.3 /spl times/ 10/sup 13/ cm/sup -2/, we have repeatedly obtained electron mobilities in excess of 1350 cm/sup 2//Vs. HEMT devices with a gate length of 1/spl mu/m, a gate width of 200 /spl mu/m, and a source-drain spacing of 5 /spl mu/m show a maximum drain current of 1.1 A/mm and a peak transconductance of 125 mS/mm. For unpassivated HEMTs, we measured a saturated power output of 8.2-W/mm continuous wave (cw) at 2 GHz with an associated gain of 11.2 dB and a power-added efficiency of 41%. The achievement of high-power operation without a surface passivation layer suggests that free surface may not be the dominant source of radio-frequency (RF) dispersion in these MBE-grown structures. This data may help discriminate between possible physical mechanisms of RF dispersion in AlGaN-GaN HEMTs grown by different techniques.

Monolithic InP Dual-Polarization and Dual-Quadrature Coherent Receiver

We realized a monolithic dual-polarization dual-quadrature coherent receiver with balanced detection on InP using a single epitaxial step. It monolithically integrates the polarization splitters, 90° hybrids, and balanced photodiodes in 4.1 mm2 . We demonstrate reception of 112-Gb/s polarization-division-multiplexed quadrature phase-shift keying with 17.3-dB optical signal-to-noise ratio at 10-3 bit-error rate.

An InGaAs/InP HBT differential transimpedance amplifier with 47 GHz bandwidth

In this paper, we describe an InGaAs/InP heterostructure bipolar transistor differential transimpedance amplifier with high bandwidth of 47 GHz and high gain of 56 dB-ohms.

Smooth and vertical-sidewall InP etching using Cl2/N2 inductively coupled plasma

Inductively coupled plasma (ICP) operated in the reactive-ion etching mode is used for mesa etching of InP using Cl2/N2 chemistry with a Ni metal mask. Etch rates of approximately 140 nm/min with very smooth and vertical sidewalls are obtained at a dc bias of 120 V. The effects of temperature, gas flow, chamber pressure, ICP source power, and substrate bias power on etch rate are studied; sidewall profile and surface morphology will be discussed.