We report on the fabrication, through photolithography techniques, and the detailed characterization, through direct transmission measurements, of a periodic system composed of five layers of photolithographically aligned micrometer-sized Ag split-ring resonators (SRRs). The measured transmission spectra for propagation perpendicular to the SRRs plane show a gap around 6 THz for one of the two possible polarizations of the incident electric field; this indicates the existence of a magnetic resonance, which is verified by detailed theoretical analysis. To our knowledge this is the first time that a system of more than one layer of micrometer-sized SRRs has been fabricated. The measured optical spectra of the Ag microstructure are in very good agreement with the corresponding theoretical calculations.

No abstract

Gate and drain‐lag effects are studied in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTs grown on sapphire. Electron trapping on the surface states between the gate and the drain forming the net negative charge up‐to ∼2 × 1013 cm–2 is found to be responsible for the gate‐lag effect in the (GaN)/InAlN/GaN HEMTs. If the polarization charge at the device surface is decreased by GaN capping, then density of the trapped charge is not changed, however the electron de‐trapping process becomes faster. The drain‐lag effect is caused by electron injection and trapping in the source‐gate area reaching ∼1 × 1013 cm–2 of the trapped charge in the steady state. In the studied voltage range the InAlN/AlN/GaN HEMT is shown to be gate‐lag‐free suggesting that this parasitic transient can be avoided if thin AlN is used in the epi‐layer growth sequence. It is assumed that this breakthrough quality relates to the decreased or even reverted electric field in the InAlN layer if AlN is inserted. Surface states need not to be generated in this case. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nitridation of Al2O3 (0001) substrate surfaces by radio-frequency nitrogen plasma has been investigated. A 1.5-nm-thick surface nitride layer occurred for 100 min nitridation at high substrate temperature, while the nitridation appeared to be limited to a surface atomic plane at low temperature. In-plane lattice constant relaxation was observed in both cases. A high nitridation temperature resulted into a Ga face and a low temperature to N-face polarity of overgrown GaN films. However, low temperature nitridation and an AlN buffer layer also produced a Ga-face polarity. The results are consistent with low formation energy of AlN/sapphire films with Ga-face polarity.