A. Osinsky

High-temperature performance of AlGaN/GaN HFETs on SiC substrates

The performance results AlGaN-GaN Heterostructure Field Effect Transistors (HFETs) grown on SiC substrates are reported. The maximum transconductance of these devices was 142 mS/mm and the source-drain current was as high as 0.95 A/mm. The maximum dissipated DC power at room temperature was 0.6 MW/cm/sup 2/, which is more than three times higher than that in similar devices grown on sapphire. This high thermal breakdown threshold was achieved primarily due to the effective heat sink through the SiC substrate. These devices demonstrated stable performance at elevated temperatures up to 250/spl deg/C. The source-drain current saturation was observed up to 300/spl deg/C. The leakage current in the below threshold regime was temperature-activated with an activation energy of 0.38 eV.

Self-heating in high-power AlGaN-GaN HFETs

We compare self-heating effects in AlGaN-GaN heterostructure field effect transistors (HFETs) grown on sapphire and SiC substrates. Heat dissipation strongly affects the device characteristics soon after the application of the source-drain voltage (in less than 10/sup -7/ s). Our results show that in HFETs with the total epilayer thickness less than 1.5 /spl mu/m, the thermal impedance, /spl Theta/ is primarily determined by the substrate material and not by the material of the active layer. For our devices grown on 6H-SiC substrates, we measured /spl Theta/ of approximately 2/spl deg/C/spl middot/mm/W, which was more than an order of magnitude smaller than /spl Theta/=25/spl deg/C mm/W measured for similar AlGaN/GaN HFETs grown on sapphire. Our results demonstrate that AlGaN-GaN HFETs grown on SiC substrates combine advantages of superior electron transport properties in AlGaN/GaN heterostructures with excellent thermal properties of SiC, which should make these devices suitable for high-power electronic applications.

Electron transport in AlGaN–GaN heterostructures grown on 6H–SiC substrates

We investigated two-dimensional electron transport in doped AlGaN–GaN heterostructures (with the electron sheet concentration ns≈1013 cm−2) grown on conducting 6H–SiC substrates in the temperature range T=0.3–300 K. The electron mobility in AlGaN–GaN heterostructures grown on SiC was higher than in those on sapphire substrates, especially at cryogenic temperatures. The highest measured Hall mobility at room temperature was μH=2019 cm2/V s. At low temperatures, the electron mobility increased approximately five times and saturated below 10 K at μH=10250 cm2/V s. The experimental results are compared with the electron mobility calculations accounting for various electron scattering mechanisms.We investigated two-dimensional electron transport in doped AlGaN–GaN heterostructures (with the electron sheet concentration ns≈1013 cm−2) grown on conducting 6H–SiC substrates in the temperature range T=0.3–300 K. The electron mobility in AlGaN–GaN heterostructures grown on SiC was higher than in those on sapphire substrates, especially at cryogenic temperatures. The highest measured Hall mobility at room temperature was μH=2019 cm2/V s. At low temperatures, the electron mobility increased approximately five times and saturated below 10 K at μH=10250 cm2/V s. The experimental results are compared with the electron mobility calculations accounting for various electron scattering mechanisms.

MgZnO/AlGaN heterostructure light-emitting diodes

We report on p–n junction light-emitting diodes fabricated from MgZnO∕ZnO∕AlGaN∕GaN triple heterostructures. Energy band diagrams of the light-emitting diode structure incorporating piezoelectric and spontaneous polarization fields were simulated, revealing a strong hole confinement near the n‐ZnO∕p‐AlGaN interface with a hole sheet density as large as 1.82×1013cm−2 for strained structures. The measured current–voltage (IV) characteristics of the triple heterostructure p–n junctions have rectifying characteristics with a turn-on voltage of ∼3.2V. Electron-beam-induced current measurements confirmed the presence of a p–n junction located at the n‐ZnO∕p‐AlGaN interface. Strong optical emission was observed at ∼390nm as expected for excitonic optical transitions in these structures. Experimental spectral dependence of the photocurrent confirmed the excitonic origin of the optical transition at 390nm. Light emission was measured up to 650K, providing additional confirmation of the excitonic nature of the opti...

Schottky barrier photodetectors based on AlGaN

We report solar-blind AlxGa1−xN photovoltaic detectors with cutoff wavelengths as short as 290 nm. Mesa geometry devices of different active areas are fabricated and characterized for spectral responsitivity, speed, and noise performance. The responsivity of the devices near the cutoff wavelength is 0.07 A/W. The detector noise is found to be 1/f limited, with a noise equivalent power of 6.6×10−9 W over the total response bandwidth of 100 kHz.

Electron beam induced current measurements of minority carrier diffusion length in gallium nitride

Minority carrier diffusion length in epitaxial GaN layers was measured as a function of majority carrier concentration and temperature. The diffusion length of holes in n‐type GaN is found to decrease from 3.4 to 1.2 μm in the doping range of 5×1015–2×1018 cm−3. The experimental results can be fitted by assuming the Einstein relation and by the experimental dependence of hole mobilities on carrier concentration. The low injection carrier lifetime of ∼15 ns, used in the fit, is largely independent of the doping level. The diffusion length, measured for ∼5×1015 and 2×1018 cm−3 dopant concentrations, shows an increase with increasing temperature, characterized by an activation energy Ea of ∼90 meV, independent of the impurity concentration.

Electrical detection of immobilized proteins with ungated AlGaN∕GaN high-electron-mobility Transistors

Ungated AlGaN∕GaN high-electron-mobility transistor (HEMT) structures were functionalized in the gate region with aminopropyl silane. This serves as a binding layer to the AlGaN surface for attachment of fluorescent biological probes. Fluorescence microscopy shows that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affinity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.

High quality GaN–InGaN heterostructures grown on (111) silicon substrates

We report on the low pressure metal organic chemical vapor deposition of single crystal, wurtzitic layers of GaN and GaN/InGaN heterostructures on (111) GaAs/Si composite substrates. The structural, optical, and electrical properties of the epitaxial layers are evaluated using x‐ray diffraction, transmission electron microscopy, photoluminescence, and measurements of minority carrier diffusion length. These measurements demonstrate high quality of GaN grown on the composite substrate.

Cleaved cavity optically pumped InGaN–GaN laser grown on spinel substrates

We report an optically pumped multiple‐quantum‐well laser of InGaN–GaN grown on cubic, (111)‐oriented spinel substrates. The laser cavity is formed by cleaving. Atomic force microscopy shows that the cleaved GaN and spinel facets are of similar flatness. The onset of lasing is clearly demonstrated by the saturation of spontaneous emission, abrupt line narrowing, and the highly polarized light output. A lasing threshold power of 140 kW/cm2 is measured in a 400‐μm‐long cavity at 150 K.

Low-resistance ohmic contacts to p-type GaN

The specific contact resistance of two types of ohmic contacts to p-type GaN is analyzed. First, an ohmic contact formed by a metal electrode deposited on a highly doped p-type GaN layer. Second, an ohmic contact formed by a metal electrode deposited on a thin GaN layer with an internal electric field caused by polarization effects. It is shown that contacts mediated by polarization effects can result, for typical materials parameters, in low contact resistances comparable or better than contacts mediated by dopant-induced surface fields. A type of contact is proposed and demonstrated. These contacts employ polarization charges to enhance tunneling transport as well as high doping. Experimental results on Ni contacts to p-type AlxGa1−xN/GaN doped superlattices are presented. The contacts have linear current–voltage characteristics with contact resistances of 9.3×10−4 Ω cm2, as inferred from linear transmission-line method measurements. The influence of annealing at temperatures ranging from 400 to 500 °C ...