Band gap bowing parameter in pseudomorphic Al x Ga1− x N/GaN high electron mobility transistor structures (original) (raw)
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Applied Physics Letters, 2004
The effects of the isoelectronic Al doping of epitaxial GaN films grown by metalorganic chemical vapor deposition on a ͑0001͒ Al 2 O 3 single crystal substrate were investigated. It was found that the threading screw and edge dislocation densities of the GaN film decreased to less than half of that of the undoped GaN film up to Al doping concentration of 0.45%. The in-plane and out-of-plane strains were simultaneously reduced with the decrease in dislocation density as a result of the solution hardening effect. Accordingly, the electron mobility of the 0.45% Al-doped GaN film (524 cm 2 /Vs) was greatly improved compared to that of the undoped GaN film (178 cm 2 /Vs). However, the threading dislocation densities and strains were increased at a 0.64% Al concentration, and the electron mobility decreased accordingly. Therefore, the improvement in the electron mobility by Al doping up to 0.45% is the result of a decrease in the threading dislocation density and not a decrease in the number of point defects ͑Ga-site vacancy͒ as suggested earlier ͓Lee et al., Appl. Phys. Lett. 83, 917 ͑2003͔͒. Al x Ga 1Ϫx N/GaN heterostructure field effect transistors have recently attracted a great deal of interest for their applications in areas such as high power, high temperature, and microwave devices, due to their high electric breakdown field, large band gap, high thermal stability and high saturation-electron-drift velocity. 1,2 However, it is still difficult to grow high-quality epitaxial GaN films because a high density of treading dislocations and deep levels are inevitably generated as a result of the large lattice mismatch ͑16%͒ and the difference in the thermal expansion coefficients between the GaN thin film and the sapphire (Al 2 O 3 ) substrate. Lee et al. reported that isoelectronic doping of a small concentration of Al ͑Ͻ1%͒ in GaN was effective in improving the device performance. 3 They attributed these improvements to the reduction in the number of point defects ͓Gasite vacancies (V Ga ) and V Ga complexes͔. 4 However, it is difficult to understand why Al doping reduced the V Ga during the film growth because V Ga , as a point defect, is not the V Ga until the GaN crystal is formed and the incorporation of Al atoms does not have any specific reason to fill out the V Ga during the film growth. Furthermore, how the strain state of the GaN films changes with increasing Al concentration has not been explored.
Optical investigations and strain effect in AlGaN/GaN epitaxial layers
Journal of Physics: Conference Series
Al x Ga 1-x N epilayers with x ranging from 0.20 to 0.50 have been grown on c-plane sapphire substrate by metal-organic chemical vapor deposition. The thickness, composition, strain and stress values of the AlGaN were determined by high resolution X-ray diffraction. The optical properties of the epilayers were studied using photoluminescence (PL) and reflectivity measurements. The effect of the stress on the bandgap can be explained by room temperature PL. The temperature dependent PL result shows the well-known "S-shape" behavior.
Japanese Journal of Applied Physics
We have addressed the existing ambiguity regarding the effect of tensile strain in the underlying GaN layer on Al x Ga 1−x N/GaN heterostructure properties. The bandgaps and band-offsets for Al x Ga 1−x N on strained GaN were first computed using density functional theory (DFT), in the generalized gradient approximations (GGA) and hybrid functional Gaussian-Perdew-Burke-Ernzerhof (Gau-PBE) regimes. We propose a simple model to relate the GGA and Gau-PBE bandgaps, which is used to determine the realistic bandgaps of strained AlGaN. The bandgaps and bandoffsets from the DFT calculations are then used to analytically calculate the two-dimensional electron gas density in an Al x Ga 1−x N/GaN heterointerface. Our bandstructure calculations show that it is not possible to induce significant change in band-offsets through strain in the GaN layer. The charge-density calculations indicate that such strain can, however, modulate the polarization charge and thereby enhance the 2DEG density at the AlGaN/GaN interface substantially.
arXiv: Applied Physics, 2017
We have addressed the existing ambiguity regarding the effect of process-induced strain in the underlying GaN layer on AlGaN/GaN heterostructure properties. The bandgaps and offsets for AlGaN on strained GaN are first computed using a cubic interpolation scheme within an empirical tight-binding framework. These are then used to calculate the polarization charge and two-dimensional electron gas density. Our bandstructure calculations show that it is not possible to induce any significant change in band offsets through strain in the GaN layer. The charge-density calculations indicate that such strain can, however, modulate the polarization charge and thereby enhance the 2DEG density at the AlGaN/GaN hetero-interface substantially, by as much as 25% for low Al mole fraction.
Strain relaxation in AlN epitaxial layers grown on GaN single crystals
Journal of Crystal Growth, 1999
AlN layers were grown by molecular beam epitaxy on GaN single crystals (grown at high-hydrostatic pressure). The layers were examined in situ using RHEED, high-resolution X-ray re#ectivity and X-ray di!raction at grazing incidence conditions. It was found that layers grown on single-crystal GaN had very smooth surfaces (3}5 A s RMS roughness). A 100 A s thick AlN layer was almost strained, while a 1500 A s thick one was partially relaxed. The obtained results, which di!er from previously published for AlN epitaxy on GaN on sapphire, are the "rst concerning lattice relaxation of AlN grown on GaN single crystals. The measurements performed for the strained AlN layer enabled us to determine the Poisson ratio for this compound "0.20$0.02.
Effect of Graded Al x Ga 1- x N Layers on the Properties of GaN Grown on Patterned Si Substrates
Japanese Journal of Applied Physics, 2012
2.2-m-thick crack-free GaN films were grown on patterned Si substrates. The crack-free GaN films were obtained by patterning Si substrate and optimizing the graded Al x Ga 1Àx N layers. With the increase of the graded Al x Ga 1Àx N layer thickness, the GaN crystal quality improved as judged from the X-ray diffraction data. By applying multi-Al x Ga 1Àx N layers on the patterned Si substrate, a 31% reduction of tensile stress for the GaN film was obtained as measured by micro-Raman. For the AlGaN/GaN high electron mobility transistor grown on 1 Â 1 cm 2 larger patterns, the device exhibits maximum drain current density of 776 mA/mm and maximum transconductance of 101 mS/mm.
Journal of Applied Physics, 2010
The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped Al x Ga 1−x N / GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase ͑MOVPE͒ and plasma-assisted molecular beam epitaxial ͑PA-MBE͒ grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6ϫ 10 12 to 1.2ϫ 10 13 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 ͑0.26Ϯ 0.04͒ and ͑0.61Ϯ 0.10͒ 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.
AIP Advances, 2014
In this work, cluster tool (CT) Plasma Assisted Molecular Beam Epitaxy (PA-MBE) grown AlGaN/GaN heterostructure on c-plane (0 0 0 1) sapphire (Al2O3) were investigated by High Resolution X-ray Diffraction (HRXRD), Room Temperature Raman Spectroscopy (RTRS), and Room Temperature Photoluminescence (RTPL). The effects of strain and doping on GaN and AlGaN layers were investigated thoroughly. The out-of-plane (‘c’) and in-plane (‘a’) lattice parameters were measured from RTRS analysis and as well as reciprocal space mapping (RSM) from HRXRD scan of (002) and (105) plane. The in-plane (out-of plane) strain of the samples were found to be −2.5 × 10−3(1 × 10−3), and −1.7 × 10−3(2 × 10−3) in GaN layer and 5.1 × 10−3 (−3.3 × 10−3), and 8.8 × 10−3(−1.3 × 10−3) in AlGaN layer, respectively. In addition, the band structures of AlGaN/GaN interface were estimated by both theoretical (based on elastic theory) and experimental observations of the RTPL spectrum.