The macroscopic nonlinear pyroelectric polarization of wurtzite Al x Ga 1−x N, In x Ga 1−x N and Al x In 1−x N ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. Models of

We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds CeCu 2 Si 2 and CeNi 2 Ge 2 . 'Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticlequasiparticle scattering cross section. For CeNi 2 Ge 2 , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at T 1 1 K.

No abstract

The frequencies and dampings of the zone-center optical phonons E2 and A1(LO) in wurtzite-type GaN and AlN layers have been measured by Raman spectroscopy in the temperature range from 85 to 760 K. The GaN layer was grown by metalorganic vapor phase epitaxy and the AlN layer by molecular beam epitaxy both on sapphire substrate. The experimentally obtained frequencies and dampings are modeled by a theory taking into account the thermal expansion of the lattice, a symmetric decay of the optical phonons into two and three phonons of lower energy, and the strain in the layers induced by the different thermal expansion coefficients of layer and substrate. The results were used to determine the local temperature of a GaN pn diode in dependence on the applied voltage.

No abstract

Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B)

We present evidence of strong Shubnikov-de-Haas magnetoresistance oscillations in a polarizationdoped degenerate three-dimensional electron slab in an AlxGa1−xN semiconductor system. The degenerate free carriers are generated by a novel technique by grading a polar alloy semiconductor with spatially changing polarization. Analysis of the magnetotransport data enables us to extract an effective mass of m ⋆ = 0.19m0 and a quantum scattering time of τq = 0.3ps. Analysis of scattering processes helps us extract an alloy scattering parameter for the AlxGa1−xN material system to be V0 = 1.8eV .

No abstract