Contactless topographic resistivity mapping is used to characterize SiC and Cd(Zn)Te wafer material. For locally inhomogeneous material, detailed analysis of the deformed charge transients allows the evaluation of the partial resistivity contributions.

Semi-insulating SiC substrates originating from different vendors are analysed with nondestructive techniques. Contactless resistivity mapping (COREMA) provides full wafer images of the resistivity ρ in the range 1x10 – 1x10 Ωcm. Based on this room temperature precharacterization particular spots on the wafer are selected for measurement of the temperature dependence ρ(T) to obtain the local carrier activation energy. We find that it varies widely and may be quite different even if ρ values are comparable. Hence both quantities must be measured to obtain a meaningful electrical analysis of semi-insulating SiC. For wafers with persistent conductivity at room temperature the heating curve shows a complex behaviour reflecting the heat-stimulated recombination of centers which are persistently ionized at room temperature.

The reliability and degradation mechanisms of 70 nm gate length metamorphic InAlAs/InGaAs HEMTs for low noise applications will be presented and discussed. Based on a 10% gmmax failure criterion, a median time to failure of 106 h and an activation energy of 1.3 eV in air were found. By comparing the electrical device characteristics before and after stress, gate sinking, ohmic contact degradation, and hot electron degradation were found to be the major failure mechanisms. The stress induced platinum diffusion into the semiconductor was quantified by cross-section TEM.

The long-term stability of AlGaAs/GaAs and InAlAs/InGaAs high electron mobility transistors (HEMTs), tested under high drain voltage and/or high temperature operation is reported. HEMTs with high In content in the active channel, alternatively fabricated on InP substrates and on GaAs substrates covered by a metamorphic buffer (MHEMT), are compared. Despite the high dislocation density in the buffer layer MHEMTs and InP based HEMTs exhibit comparable reliability. AlGaAs/GaAs HEMTs are more reliable than their InAlAs/InGaAs counterparts, especially when operated at high drain voltage. Failure mechanisms are thermally activated gate sinking, Ohmic contact degradation and hot electron induced degradation.

Semi-insulating silicon carbide single crystals have been grown using high temperature chemical vapor deposition without vanadium doping. The resistivity of standard and exploratory 2″ diameter substrates has been analysed topographically with 1 mm lateral resolution with an improved contactless resistivity mapping technique. Absolute resistivity values are ranging from below 1 × 105 to above 1 × 1012 Ω cm. The lateral homogeneity of state-of-the-art material is very satisfactory, whereas strong localized inhomogeneities in exploratory materials underscore the decisive advantage of a nondestructive and topographic resistivity evaluation in supporting the optimization of the crystal growth procedure.

The reliability of InAlAs/InGaAs HEMTs grown on GaAs substrate with metamorphic buffer (MHEMT) designed for power applications has been investigated by accelerated lifetime tests. MHEMTs manufactured before and after fabrication technology modifications, aiming at improvements of the stability of the gate contact and the ohmic resistance, are compared. By combining biased MHEMT tests with high temperature: storage tests of TLM structures fabricated on the same wafers, the effects of the process modifications can be evaluated separately. Due to the gate contact stabilization, the improved devices show almost no change of transconductance after a 300 h stress test at 4V bias and a channel temperature of 170degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.

Further evidence was found that the 0.8 eV luminescence band in GaAs is correlated to the EL6 defect as identified by photo induced current transient spectroscopy (PICTS). From luminescence topography arguments are put forward that the EL6 defect should not be the dominant recombination centre in GaAs. By the application of microwave detected PICTS it could be shown that the EL6 is definitely not a relevant recombination centre. Recombination takes place most probably via different, partially not jet identified centres.

The reliability of high power AlGaAs/InGaAs/GaAs PHEMTs has been investigated. The influences of high temperature and of elevated drain voltage have been studied separately, using long term operation at elevated temperature and a 23-hour drain-voltage step-stress at room temperature. Results pertaining to different combinations of gate length and gate-drain ledge are reported and discussed, indicating safe operation values and showing that a wider gate-drain recess increases the device reliability. (C) 2002 Elsevier Science Ltd. All rights reserved.

An innovative procedure to evaluate the electron mobility of semi -insulating GaAs and InP wafers is presented. It is based on a capacitive contactless technique previously developed for resistivity mapping (COREMA). The measurement is consecutively performed with and without a magnetic field. The correlation with Hall data is satisfactory, whereas the agreement with calculated drift mobility is excellent. The new technique is superior to the conventional procedure in regard of cost, speed, nondestructiveness, reproducibility and lateral resolution. A system design meeting the requirements of eventual routine industrial application is described.

We report on full wafer and small area photoluminescence topography investigations of VGF GaAs:Si wafers. The wavelength-specific images exhibit various correlations and anti-correlations. Intensity variations due to competitive radiative and non-radiative recombination processes are mainly due to stoichiometric fluctuations and can be distinguished from those generated by the variation of the silicon dopant concentration. X-ray transmission topograms allow to identify grown-in defects like precipitates and dislocations and to correlate these with the observed macro- and microscopic luminescence variation patterns.

The influence of the gate metal on the reliability of metamorphic InAlAs/InGaAs HEMTs with a gate length of 0.12 μm was studied by biased accelerated life tests in air and in nitrogen. By replacing the Ti-Pt-Au gate by Pt-Ti-Pt-Au we found that the life time at 220°C can be increased at least by a factor of two. An activation energy of 1.5 eV and a life time of 1.1×106 h at 125°C in air were derived for MHEMTs with Pt-Ti-Pt-Au gate.

Micro-Raman facet temperatures of high-power diode lasers with different waveguide architectures are compared. For regular operation conditions, the thermal behavior of ‘unaged’ arrays emitting in the 808-nm wavelength region with different architectures is similar, however, with an increased load thermal behaviors differ significantly and exhibit failure events at facet temperatures typically between 150 and 450 °C. From various experiments, among them facet temperature measurements for ultrahigh-power operation as well as by preparative failure analytics, we provide evidence that in arrays the front facets are significantly affected by device operation and influence the failure behavior of the whole high-power diode laser also in cases when the device failure is accompanied by dislocation creation inside the device.

The modification of the bulk resistivity of semi-insulating GaAs caused by incorporation of copper has been studied. Wafers cut from an ingot annealed, intentionally Cu contaminated LEC grown crystal were investigated optically and electrically. A characteristic variation pattern of the Cu related photoluminescence intensity, gradually decreasing towards the center, is observed. The resistivity exhibits a quite similar radial dependence. The results convincingly show that Cu is diffusing across macroscopic distances into GaAs and is responsible for the radial resistivity variation. The topographic correlation allows to quantify the Cu acceptor content.

The long term stability of 0.25 μm InAlAs/InGaAs HEMTs grown on GaAs substrate with metamorphic buffer (MM-HEMT) was investigated by biased accelerated life tests in air and in nitrogen. By defining a 10%-degradation failure criterion of transconductance we found a life time of 2×107 h at Tch=125°C and an activation energy of 1.8 eV in nitrogen atmosphere. The median life and the activation energy were found to be much smaller in air, which can be explained by passivation of the silicon donors with fluorine. The life time and the degradation of electrical parameters of MM-HEMTs in air and nitrogen is comparable to InP-based HEMTs.

By performing biased accelerated life tests and three dimensional temperature simulations the effect of drain voltage on reliability and channel temperature of pseudomorphic InAlAs/InGaAs HEMTs for low noise applications was investigated. At Vd=1 V excellent long term stability in nitrogen ambient was observed. Increasing the drain voltage to Vd=2 V at constant channel temperature leads to a faster degradation rate which is caused by field accelerated degradation mechanism, probably involving fluorine diffusion. The influence of gate width on channel temperature and reliability was found to be small.

The failure of high power InGaAlAs/AlGaAs double quantum well (DQW) lasers has been studied by plan-view photoluminescence (PL), cross sectional (002) dark field TEM and X-ray microanalysis. The lasers were operated at elevated driving currents beyond rollover in order to stimulate and analyse degradation processes encountered during accelerated lifetime testing. The damage consists of elliptical areas within which the original DQW structure was destroyed due to outdiffusion of Al into the confinement layers. The damaged areas started from the output mirror indicating that the device temperature was somewhat higher near the facet. The Ga vacancies governing the Al diffusion were likely produced by a mechanism of recombination enhanced defect reaction (REDR). The dislocations always present in the damaged areas are expected to be generated by the point defects produced by REDR as well as by localized thermal stresses.

The lateral homogeneity of the electrical resistivity ρ of semi-insulating GaAs substrates is measured with high resolution using contactless capacitive mapping. The improved technique is capable of imaging mesoscopic ρ fluctuations correlated with the cellular structure of the dislocation density. The results compare favorably with data obtained by point contact topography. A measurement and statistical analysis procedure is described that allows an individual evaluation of the macro- and mesocopic contributions to the total on-wafer ρ variation.

Accelerated life tests of 0.15 /spl mu/m gate length InAlAs-InGaAs HEMTs were performed under DC electrical stress at four temperatures in nitrogen. By defining a 10% transconductance degradation as the failure criterion, we found an activation energy of 1.8 eV and a projected lifetime of 5/spl times/10/sup 6/ hrs at 125/spl deg/ ambient temperature. The degradation was found to be much faster in air than in nitrogen. High temperature storage tests showed that our devices are not sensitive to hydrogen.