D.S. Katzer

Improved reliability of AlGaN-GaN HEMTs using an NH/sub 3/ plasma treatment prior to SiN passivation

A passivation method has been developed which reduces the degradation of AlGaN-GaN high electron mobility transistor (HEMT) electrical properties caused by extended dc bias or microwave power operation. The key aspect of this passivation technique is exposure to a low-power NH/sub 3/ plasma prior to SiN deposition. Devices fabricated with the NH/sub 3/ treatment prior to SiN passivation show minimal gate lag and current collapse after extended dc bias operation. In addition, the rate of degradation of the microwave power output while under continuous microwave operation is improved by at least 100 times as compared to SiN passivated HEMTs that were not treated with the NH/sub 3/ plasma.

Observation of vertical honeycomb structure in InAlN/GaN heterostructures due to lateral phase separation

The microstructure of InxAl1−xN∕GaN heterostructures (where x∼0.13–0.19), grown by molecular beam epitaxy, was investigated by transmission electron microscopy. Observations in the cross-section and plan-view geometries show evidence for lateral phase separation originating at the GaN surface that results in a vertical honeycomblike structure within the InAlN layers. The lateral dimensions of the honeycomb cells are ∼5–10nm. The vertical walls are In rich with a width of ∼1–2nm and align roughly perpendicular to ⟨112¯0⟩ and ⟨11¯00⟩ directions. The phase separation is attributed to random compositional fluctuations during the early stages of growth, possibly associated with misfit-strain relaxation.

Molecular beam epitaxy of InAlN∕GaN heterostructures for high electron mobility transistors

We describe the growth of InAlN∕GaN heterostructures by rf-plasma molecular beam epitaxy. Due to the weak In–N bond, the InAlN growth temperature must be below about 460°C for In to incorporate reliably into the film. Thus far, a thin AlN spacer layer has been required to form a low resistance two dimensional electron gas (2DEG) at the InAlN∕GaN interface. The thin AlN barrier is believed to reduce alloy scattering of carriers in the 2DEG. The best HEMT material with an InAlN barrier and a thin AlN spacer layer has a sheet resistance of 980Ω∕◻ with a sheet electron density of 1.96×1013cm−2.

Molecular beam epitaxy of beryllium-doped GaN buffer layers for AlGaN/GaN HEMTs

Group III-nitride semiconductors are promising materials for high-power microwave transistors. However, several materials issues remain to be solved. For example, conducting buffer or interfacial layers are a frequently observed problem in AlGaN/GaN HEMTs grown by both MOCVD and MBE. These conducting layers can cause poor pinch-off characteristics and poor inter-device isolation.

Effect of Al∕N ratio during nucleation layer growth on Hall mobility and buffer leakage of molecular-beam epitaxy grown AlGaN∕GaN heterostructures

AlGaN∕GaN high electron mobility transistor structures have been grown by plasma-assisted molecular beam epitaxy on semi-insulating 4H-SiC utilizing an AlN nucleation layer. The electron Hall mobility of these structures increases from 1050cm2∕Vs to greater than 1450cm2∕Vs when the Al∕N flux ratio during the growth of the nucleation layer is increased from 0.90 to 1.07. Buffer leakage currents increase abruptly by nearly three orders of magnitude when the Al∕N ratio increases from below to above unity. Transmission electron microscopy indicates that high buffer leakage is correlated with the presence of stacking faults in the nucleation layer and cubic phase GaN in the buffer, while low mobilities are correlated with high dislocation densities.

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

Atomic layer deposited Ta2O5 gate insulation for enhancing breakdown voltage of AlN/GaN high electron mobility transistors

AlN/GaN heterostructures with a 3.5 nm AlN cap have been grown by molecular beam epitaxy followed by a 6 nm thick atomic layer deposited Ta2O5 film. Transistors fabricated with 150 nm length gates showed drain current density of 1.37 A/mm, transconductance of 315 mS/mm, and sustained drain-source biases up to 96 V while in the off-state before destructive breakdown as a result of the Ta2O5 gate insulator. Terman’s method has been modified for the multijunction capacitor and allowed the measurement of interface state density (∼1013 cm−2 eV−1). Small-signal frequency performance of 75 and 115 GHz was obtained for ft and fmax, respectively.

Optimization of buffer layers for InGaAs/AlGaAs PIN optical modulators grown on GaAs substrates by molecular beam epitaxy

In this work we compare the effect of the buffer layer on the device quality and surface morphology of strained InGaAs/AlGaAs PIN multiple quantum well (MQW) modulators. We examine GaAs buffer layers and linearly graded InGaAs buffer layers. Our results indicate that for lower indium concentrations in the quantum wells (less than about 23%) better device performance and surface morphology are obtained by growing directly on GaAs. PIN MQWs with indium mole fractions higher than about 24% have better properties when a linearly graded buffer layer is used.

Demonstration of a microelectromechanical tunable asymmetric Fabry–Pérot quantum well modulator

Asymmetric Fabry–Perot multiple quantum well modulators can have much higher extinction ratios than are otherwise possible with surface normal devices. This performance comes at the price of very tight tolerances on the epitaxial growth and high sensitivity to temperature and angle of incidence. We demonstrate that a quantum well modulator with an integrated microelectromechanical mirror can remove these sensitivities.

Modulating retroreflector implementation of mil-std 1553 protocol with free-space optics

Abstract : A modulating retroreflector (MRR) is used for a free-space optics (FSO) implementation of the MILSTD-1553 protocol. A multiple quantum well (MQW) p-i-n structure is used for a single device that acts both as a modulator for transmitting data and as a photodiode for receiving data. A master node and two slave nodes with cats eye retroreflectors were designed using COTS optics. Two-way communication using the 1553 protocol is demonstrated at a separation of 3 meters, using widely available 980 nm pump lasers. The link was closed using only 15 mW of laser light. We have also demonstrated a coherent receiver to increase sensitivity and quadrature amplitude modulation (QAM) to enhance the data rate of a bandwidth-limited MRR.