L. F. Eastman

Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs

Surface passivation of undoped AlGaN/CaN HEMTs reduces or eliminates the surface effects responsible for limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2/spl times/125/spl times/0.5 /spl mu/m AlGaN/GaN sapphire based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 /spl mu/m gate length HEMTs on the same wafer. Finally, 4 GHz power sweep data for a 2/spl times/75/spl times/0.4 /spl mu/m AlGaN/GaN HEMT on sapphire processed using the Si/sub 3/N/sub 4/ passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMTs.

Scattering of electrons at threading dislocations in GaN

A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.

TRANSIENT ELECTRON TRANSPORT IN WURTZITE GAN, INN, AND ALN

Transient electron transport and velocity overshoot in wurtzite GaN, InN, and AlN are examined and compared with that which occurs in GaAs. For all materials, we find that electron velocity overshoot only occurs when the electric field is increased to a value above a certain critical field, unique to each material. This critical field is strongly dependent on the material, about 4 kV/cm for the case of GaAs but much higher for the III–nitride semiconductors: 140 kV/cm for GaN, 65 kV/cm for InN, and 450 kV/cm for AlN. We find that InN exhibits the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distances when compared with GaN and AlN. Finally, using a one-dimensional energy–momentum balance approach, a simple model is used to estimate the cutoff frequency performance of nitride based heterojunction field effect transistors (HFETs) and a comparison is made to recently fabricated AlGaN/GaN HFETs.

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.

The role of dislocation scattering in n-type GaN films

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy doped n type with Si to the levels of 1015–1020 cm−3 was investigated. The room temperature electron mobility versus carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model of scattering by charged dislocations. The mechanism of this scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. It was found that in the low carrier concentration region (<1017 cm−3), the electron mobility is thermally activated with an activation energy half of that of carrier concentration. This is in agreement with the prediction of the dislocation model.

Surface charge accumulation of InN films grown by molecular-beam epitaxy

A series of thin InN films down to 10 nm in thickness were prepared by molecular-beam epitaxy on either AlN or GaN buffers under optimized growth conditions. By extrapolating the fitted curve of sheet carrier density versus film thickness to zero film thickness, a strong excess sheet charge was derived, which must come from either the surface or the interface between InN and its buffer layer. Since metal contacts, including Ti, Al, Ni, and a Hg probe, can always form an ohmic contact on InN without any annealing, it is determined that at least part of the excess charge is surface charge, which was also confirmed by capacitance–voltage measurements.

Effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy

The effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy (MBE) is studied. Using an AlN buffer layer can significantly improve the structural and electrical properties of InN. With increasing thickness of the AlN buffer layer, the Hall mobility of InN will monotonically increase while the electron carrier concentration decreases. The surface morphology of the film also improves. A Hall mobility of more than 800 cm2/V s with a carrier concentration of 2–3×1018 cm−3 at room temperature can be routinely obtained on ∼0.1 μm InN film. More importantly, it is found that under optimum growth conditions, by using an AlN buffer layer, InN films with comparable quality can be achieved by the conventional MBE technique compared to InN grown by migration-enhanced epitaxy. Increasing InN thickness also increases Hall mobility.

Electron transport in wurtzite indium nitride

We present the velocity-field characteristics of wurtzite indium nitride, determined using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high peak drift velocity at room temperature, 4.3×107 cm/s, at a doping concentration of 1.0×1017 cm−3. We also demonstrate that the saturation drift velocity of indium nitride, 2.5×107 cm/s, is comparable to that of gallium nitride, and much larger than that of gallium arsenide. Our results suggest that the transport characteristics of indium nitride are superior to those of gallium nitride and gallium arsenide, over a wide range of temperatures, from 150 to 500 K, and doping concentrations, up to 1.0×1019 cm−3. Hence, indium nitride has considerable potential for device applications.