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We investigate the influence of the AlGaN barrier thickness in Al0.14Ga0.86N/GaN heterostructures on both the electrical properties of the heterostructure itself as well as on high electron mobility transistors fabricated on these structures. With increasing barrier thickness, we observe decreasing sheet resistances, transconductances, and threshold voltages. The observed changes are well-described by modelling. We demonstrate that an increase in the barrier thickness in AlGaN/GaN heterostructures results in an increase in available high-frequency input power swing before turn-on of the Schottky gate. The device long-term stability under direct current stress is not affected by the increase in barrier thickness as shown by on-wafer reliability tests at 150 °C base plate temperature. These results pave the way towards AlGaN/GaN transistors offering high robustness under extreme mismatch conditions as well as excellent high-frequency power performance. [ABSTRACT FROM AUTHOR]

AlGaN/GaN-based high electron mobility transistor heterostructures with different GaN cap layer thicknesses were grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy. The influence of the different growth methods on the structure of the GaN caps was examined by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Due to the high surface sensitivity of these methods, several effects, such as residual Al incorporation into the GaN cap during growth, etching of the GaN cap during cool-down, and preferential etching of Ga out of the AlGaN barrier could be evaluated. It is demonstrated that these effects play a major role for MOVPE-grown structures. [ABSTRACT FROM AUTHOR]

Electrical properties of AlxGa1-xN/GaN heterostructures with an Al content below 15% and carrier concentrations as low as 1.0 × 1012 cm-2 were investigated by Hall effect measurements and capacitance-voltage profiling. The nominally undoped GaN capped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The threshold voltage of transistor devices follows the trend already found for high Al-containing structures, which are described by a model indicating a surface potential independent of Al content. Photoreflectance spectroscopy confirms the results for as-grown heterostructures. The Hall effect measured on the as-grown samples, however, shows a stronger decrease in carrier concentration than expected from the effect of polarization and constant surface potential. In contrast, Hall effect data determined on samples with Ni Schottky contacts and capacitance-voltage profiling on as-grown samples yield the expected behavior, with surface potentials of 0.86 eV and 0.94 eV, respectively. The inconsistency is eliminated by describing the results of the Hall effect on as-grown samples by a two-carrier model. Self-consistent Schrödinger-Poisson calculations support these considerations if we take into account a transition range at the AlxGa1-xN/GaN interface. [ABSTRACT FROM AUTHOR]

The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped AlxGa1-xN/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×1012 to 1.2×1013 cm-2. A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-content is the same for samples grown with different techniques. Thus, the interplay between epitaxy and process technology defines the threshold voltage. [ABSTRACT FROM AUTHOR]

We analyzed by high-resolution x-ray diffraction experiments the strain status and the chemical composition in wurtzite In[sub χ]Ga[sub 1-χ]N/GaN and Al[sub χ]Ga[sub 1-χ]N/GaN multiple quantum wells (MQW) grown on (0001) SiC substrates by plasma-assisted molecular beam epitaxy. In order to evaluate the lattice deformation in the wurtzite heterostructures, we derived a very general expression of the x-ray incidence parameter which relates the unit cell size to the experimental diffraction peaks separation. All the strain and rotation tensors components of the heterostructures could be determined, which is essential to determine the correct chemical composition. We found a coherent interface between the GaN buffer and the Al[sub χ]Ga[sub 1-χ]N/GaN superlattice, while a partial relaxation of the In[sub χ]Ga[sub 1-χ]N/GaN superlattice with respect to the GaN buffer layer was revealed. The Al and In mole fraction of Al[sub χ]Ga[sub 1-χ]N and In[sub χ]Ga[sub 1-χ]N SL layers were determined by using Vegard's rule and simulation of the experimental x-ray patterns. We found a pronounced In segregation if the MQWs were grown under metal-stable flux condition; in contrast no appreciable segregation effect was observed under the nitrogen-stable flux condition. [ABSTRACT FROM AUTHOR]

We report on recent results from our GaN transistor and circuit technology. Epitaxial growth can be performed on either SiC or Si substrates in order to provide high-quality AlGaN/GaN heterostructures. These heterostructures are then utilized in order to realize transistors and integrated circuits ranging from high-voltage transistors for voltage conversion in efficient power switches, L/S-band power bars and X-band MMICs for next-generation communication systems, and finally W-band MMICs for radar applications. The technology is characterized by state-of-the-art performance, good uniformity and high yield as well as excellent long-term stability. In combination with the space compatibility we believe that this technology is ideal for space. X-band MMICs from Fraunhofer IAF are scheduled to have the first in-orbit demonstration of European GaN within the Proba-V mission which is planned to be launched in spring 2013. [ABSTRACT FROM AUTHOR]

The influence of the Al content on the mobility of the two-dimensional electron gas (2DEG) in GaN/AlxGa1-xN/GaN heterostructures is studied by employing the ensemble Monte Carlo method. Using two interface polarization charge models, we calculate the room temperature low-field mobility at different Al compositions of the barrier layer ranging from 8% up to 35%. All relevant scattering mechanisms are considered to provide a quantitative description of the measured mobilities. We show that 2DEG transport in the heterostructures is mostly affected by dislocation scattering for all Al contents examined. The role of alloy scattering and interface roughness scattering is also discussed. [ABSTRACT FROM AUTHOR]

Discusses the essential strategies for the growth of (aluminum, gallium) nitride/gallium nitride (GaN) and GaN/(indium, gallium) nitride heterostructures on silicon carbide (0001) by both plasma-assisted and reactive molecular-beam epitaxy. Substrate preparation, nucleation and growth conditions; High structural, morphological, optical and electrical quality.