We performed Fourier transformed photoreflectance ͑PR͒ spectroscopy on GaAs/Ga 0.5 In 0.5 P heterojunction bipolar transistor wafers. The use of Fourier transformation of the PR spectrum resolves the signals coming from the emitter-base and base-collector interfaces. The evaluated interface electric fields were compared with the capacitance obtained from capacitance-voltage measurements. The result for the base-collector interface is consistent with the Poisson equation. On the other hand, the atomic ordering in the Ga 0.5 In 0.5 P emitter plays an important role in determining the characteristics of the emitter-base interface.

We observed unique light responsiveness in HR GaN leak current, in which the current was drastically reduced under LED illumination of 2.65 eV or lower photon energy. Free carrier excitation by the illumination and consequential electron capture into the electron traps would rationalize the phenomenon. The threshold energy in our measurement was well associated with the carbon related very deep electron trap reported elsewhere (P. B. Klein et al., Appl. Phys. Lett. 79, 3527 (2001) [3]). Observation of this phenomenon provides us a very simple way to characterize both the HR GaN resistivity and the concentration of electron traps in the GaN buffer simultaneously.1 Introduction GaN based high electron mobility transistor (GaN HEMT) has attracted significant attention in the compound semiconductor industry because of its supreme high power handling capability in microwave applications, especially for wireless mobile base stations [1]. Regarding the field effect transistor operations, high resistive (HR) GaN buffer layer plays an important role in the epitaxial layer structures because its quality would severely affect the breakdown voltage, pinch-off performance, isolation of the devices, and other DC and RF characteristics. The controversial current collapse or electron trap phenomenon in GaN HEMT [2], which is usually recognized as a surface state problem, has also been associated with the quality of underlying HR GaN buffer layers [3]. Achieving consistency between the high resistivity and low current collapse in HR GaN buffer layer would be therefore, one of the most important technical breakthroughs which realize prime GaN HEMT device performances. Thus the authors concentrated on characteristics of the HR GaN buffer layers, which were grown at varying process conditions, and found a unique relationship between the leak current and the photon energy of the illuminated light during the measurement. In this paper, the "odd" I-V characteristics of the leak current were described and then a model to rationalize this phenomenon will be proposed. The model would provide a very simple characterization method which quantitatively measures the current leakage and the degree of electron traps in HR GaN buffer layers simultaneously. The possible causes of the phenomenon will also be mentioned.

This work investigates eleetromigration (EM) in Al-l wt %Si, AI-l wt %Si~O.5 wt % Cu and AI-0.5 wt % eu films with TiN barrier metals. The EM resistance of the AI-O.5 wi % eu layered metallization was found to be higher than that of the AI-J wt % Si~O.5 wt % Cu layered metallization. The electromigration test results show that the reaction between Al films and underlying TiN layer degrades the electromigration performance of the Al-l wt % Si, Al-l wt % Si-O.5 wt % Cu films. The reaction kinetics between Al alloys and TiN layers was studied by transmission electron microscopy and by investigating the resistance rise mechanism. Si was found to enhance thin intermctallic compound formation between the Al alloys and TiN on samples annealed at 450°C. 143 J.

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