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Interconnections on the top surface of the chip are now limiting the lifetime of power modules. It is necessary to evaluate the die top interconnection technologies in order to help module manufacturers to find a solution to prolong the lifetime of power modules. In this review, the reliability performance of different die top interconnections, such as Al bond wires, Al-clad Cu bond wires and Cu bond wires are compared. The Cu wire bonding shows the best power cycling capability among the interconnection materials. The failure mechanism has been changed from the bond wire lift-off to the substrate fatigue. The substrate becomes the limitation of the lifetime of power modules.

We investigated the influence of the V/III ratio on the surface morphologies of N-polar GaN epilayers during the stage of high-temperature (HT) GaN growth. The epilayers were grown on 4°-offcut sapphire substrates by metal-organic vapor phase epitaxy. We observed prominent undulations on the GaN surface when a low V/III ratio was used, and ridged hillocks on the GaN surface when a high V/III ratio was used. With the V/III ratio increased from 128 to 2237, the density of ridged hillocks increased but the surface roughness was significantly reduced from 13.8 nm to 1.7 nm in 25 × 25 μm2 AFM images. The results differ from the existing reports in which a smoother N-polar GaN surface morphology was usually obtained with a lower V/III ratio. Non-uniform island formation in the initial growth of HT-GaN and enhanced step-bunching effect under a low V/III ratio were blamed for the large surface roughness of the N-polar GaN film. The increasing density of ridged hillocks on the N-polar surface grown under a high V/III ratio was attributed to the reduced diffusion rate of Ga adatoms due to excessive N. Completely hillock-free smooth N-polar GaN surfaces were achieved by further increasing the growth temperature of the GaN buffer layer.

Cleaved laser facets on free-standing InGaN laser diode membrane created by laser lift-off were fabricated and studied. Cleaved laser facets on GaN on sapphire have been compared with those on free-standing laser membrane. Atomic force microscopy and scanning electron microscopy results show that the cleaved laser facets on free-standing laser membrane are much smoother than those on sapphire. Transmission electron microscopy images show that no significant crystal quality degradation has been introduced during the laser lift-off process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The intermediate nucleation layer effects on the crystal structure of GaN epitaxial layers grown on GaAs (0 0 1) substrates by solid-sourer molecular beam epitaxy using RF-N-2 plasma as a nitrogen source were investigated. The crystal structure of GaN grown on (0 0 1) GaAs substrates was critically influenced by the nucleation layer, that is, mainly cubic GaN was grown directly on the GaAs substrate with the epitaxial relationship of GaN (0 0 1)//GaAs(0 0 1) and GaN[1 1 0]//GaAs[1 1 0], while hexagonal GaN was frown on a very thin AlAs intermediate layer with the epitaxial relationship of GaN(0 0 0 1)//GaAs(0 0 1) and GaN[1 1 (2) over bar 0]//GaAs[1 1 0]. X-ray diffraction and transmission-electron-microscope are used to analyze the crystal structure of the two kinds of epilayers. (C) 2001 Elsevier Science B.V. All rights reserved.

The photoluminescence (PL) properties of Si-doped cubic GaN with different carrier concentrations were investigated at room temperature. The epilayers were grown on GaAs (0 0 1) by radio-frequency molecular beam epitaxy. It was found that when the carrier concentration is increased fi om 5 x 10(15) to 2 x 10(18) cm(-3), the PL peak shifted towards low energy, from 3.246 to 3.227 eV, and the PL linewidth increased from 77.1 to 121 meV. The PL peak shift is explained by the bandgap narrowing effect due to the high doping concentration. The PL linewidth includes two parts: one is doping concentration independent, which is caused by the imperfection of samples and phonon scattering; the other is doping concentration dependent. We assign the second part to the broadening by the microscopic fluctuation of the doping concentration. The experimental measurements are in good agreement with the model. (C) 2001 Published by Elsevier Science B.V.

Hexagonal GaN film has been grown on (0 0 1) GaAs substrate with nitridized AlAs buffer layer. The (0 0 0 1) plane of the hexagonal GaN(h-GaN) is parallel to the (0 0 1) plane of the GaAs substrate. The nitridized AlAs/GaAs(0 0 1) material was analyzed by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The HRTEM observations show that hexagonal AlN(h-AlN) can nucleate on the AlAs surface after nitridation with its (0 0 0 1) plane parallel to the (0 0 1) plane of the AlAs. Some cubic AlN(c-AlN) micro grains were also observed. The close-packed atom planes of the c-AlN grains are parallel to the (0 0 1) planes of the AlAs. The AFM result shows that the surface morphology of the nitridized AlAs is rough with high-density dots. Such AlN layer can be taken as a prelayer for growing h-GaN on the (0 0 1) GaAs substrate.

Using different nucleation layers, we have grown cubic and hexagonal GaN epilayers on (001)-oriented GaAs substrates by radio-frequency plasma-assisted molecular beam epitaxy. First- and second-order Raman spectra are taken from these epilayers in back-scattering configurations at room temperature. For the first-order Raman spectra, transverse-optical (TO) phonons and longitudinal-optical (LO) phonons of cubic GaN are found to be at 533 and 739 cm(-1), as well as the E-2 and A(1)(LO) frequency of hexagonal GaN are found to be at 568 and 733 cm(-1), and the polarized spectra are in good agreement with Raman selection rules for cubic and hexagonal GaN, respectively. These results confirm the single crystalline nature of the samples. As for the second-order Raman spectra, the bands show a continuum, at about twice the energy of the LO phonon of cubic GaN and A(1)(LO) phonon of hexagonal GaN, which are nearly 10 times weaker than the first-order scattering. Two-phonon spectra are dominated by contributions due to longitudinal optical phonons. (C) 2000 Elsevier Science B.V. All rights reserved.

Two kinds of GaN samples were grown on GaAs(0 0 1) substrates. One is grown on nitridized GaAs surface, the other is grown on nitridized AlAs buffer GaAs substrate. X-ray diffraction and photoluminescence measurements find that the GaN sample directly grown on GaAs substrate is pure cubic phase and those grown on AlAs buffer is pure hexagonal phase. The present study shows that the phase of GaN samples grown on GaAs substrates can be controlled using different buffer layers.

High-quality cubic GaN epilayer on GaAs(0 0 1) was grown by molecular beam epitaxy equipped with radio frequency nitrogen source. The optimized growth condition is to grow two monolayers thick initial GaN at 600°C under As atmosphere. X-ray diffraction rocking curve shows that the full-width at half-maximum of cubic GaN(0 0 2) diffraction peak is 10 arcmin. The X-ray reciprocal space mapping is used to identify the secondary hexagonal phase and estimate their relative content.

High quality Lateral Epitaxial Overgrowth (LEO) GaN films on (0001) sapphire substrates were grown by a commercial MOCVD system (Thomas Swan Corp.). We have studied the etch-pits and threading dislocations in GaN films by wet etching of pits, HCl vapor etching of pits and Scanning Electron Microscope (SEM). SEM images of GaN films etched in HCl vapor and molten KOH gave us notably different etching pits densities in the different region of LEO GaN films, which confirmed HCl vapor etching could show three kinds of Threading Dislocations (TDs), while molten KOH wet etching only could show pure screw TDs. In region above window TDs density is high (about 3 × 108/cm2) and edge TDs are in the majority, while it is almost free of TDs in region above mask, and only in middle region above mask there are some TDs (including all three kinds) and also edge TDs are in the majority. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High quality epitaxial GaN films on (0001) sapphire substrates were grown by a commercial metal-organic chemical vapour deposition system. The, etch pits and threading dislocations in GaN films is studied by a scanning electron microscope (SEM) and a transmission-electron microscope (TEM). The SEM images of GaN films, etched in mixed acid solution (H3PO4:H2SO4 = 1:3) and molten KOH exhibit notably different, etching pit densities of 5×108/cm2 and 4×107/cm2, respectively, which probably indicate that more kinds of, etching pits were revealed when, etched in mixed acid solution (H3PO4:H2SO4 = 1:3). Cross section TEM of GaN films with different g vectors showed the portions of different threading dislocations. Theoretical calculation indicates that the lattice and thermal expansion coefficient mismatch may be the main origins of pure edge threading dislocations.

Laser lift-off (LLO) technology is successfully used to prepare GaN-based TEM cross-sectional specimens. Detailed procedures of the method to prepare the specimens are demonstrated. Large thin areas suitable for TEM analysis were obtained. TEM images of the resulting GaN interface are studied, and the changes in structural quality are confined to approximately the first 250 nm of the epilayer. Clear TEM images of the whole epilayer and the InGaN quantum wells and the HRTEM images of the superlattice layer are demonstrated, showing that LLO is a quick and ideal method to study the crystal structure of the epilayer, especially if only the upper layers are of interest.

The effects of dopants on the defects of GaN films were investigated by using different methods, such as wet, etching of pits, x-ray diffraction and photoluminescence (PL). Three kinds of the samples were prepared with different dopants, that is, nominally undoped, Si-doped and Mg-doped GaN films. It was found that the lowest density of the, etched pit was existed in the nominally undoped GaN, while the highest in the Mg-doped sample. The effects of the dopants on the, etching pits were discussed.