Materials Science Forum Vols. 527-529

Abstract: In this work ion-implanted p+/n diodes have been used as minimum ionizing particle (MIP) detectors. The diode structure is based on a 0.45 $m deep, NA = 4×1019 cm-3 doped p+ anode, ion implanted in an n-type epilayer with thickness equal to 55 $m and nominal donor doping ND = 2×1014 cm-3. The diode breakdown voltages were above 1000V. At 1000V reverse bias the diode leakage current was of the order of 1 nA. The punch through depletion voltage was nearing the range 220-250 V. The charge collection efficiency to minimum ionizing particle was investigated by a 90Sr β source. The pulse height spectrum was measured as a function of the reverse voltage in the range 0-605 V. At each bias point the signal was stable and reproducible, showing the absence of polarization effects. At 220 V the collected charge was 2970 e- and saturated at 3150 e- near 350 V. At the moment, this is the highest collected charge for SiC detectors. At bias voltages over 100V the spectrum was found to consist of two peaks clearly separated. Around 250 V the signal saturates, in agreement with CV results.

Abstract: The effect of irradiation with protons, electrons, neutrons, x-ray radiation and gamma-ray photons as well as with different ions on properties of starting SiC material and devices based on it was studied. The rectifying properties of the diode structures, which degraded as a result of irradiation with high energy particles, were recovered at higher operation temperatures. The transistor structure SiC-based detectors were realized with the signal amplification by a factor of tens under irradiation. The energy resolution of 0.34 %, commensurable with Si-detectors, has been achieved for SiC detectors and is correct for all classes of short range ions. The maximum signal amplitude corresponds, in SiC, to a mean electron-hole pair creation energy of 7.7 eV.

Abstract: A full modeling of deceleration of α-particles in SiC is carried out using a Monte-Carlo method. The distribution of energy losses in nuclear elastic collisions is calculated. The spectrum has a characteristic asymmetric form and the line width at half peak maximum is 4.62 keV. The final form of a spectral line is obtained by convolution with a Gaussian peak, and including the contribution of ionization fluctuations and noise. The resulting value of the line width was 8.75 keV (at a noise dispersion of detector and equipment of 1.7 keV). The resolution of detectors reached in practice is twice the lowest calculated value. It is shown that charge losses during transport of nonequilibrium carriers through the volume of the detector are insignificant, while a resolution divergence may result from a non-optimized “entrance window”.

Abstract: Silicon carbide (SiC) has become the substrate of choice for III-N epilayers applied to electronic devices due to the lack of a native III-N substrate. This is particularly true for high power applications, since the thermal conductivity of the substrate enhances device performance. Although the GaN lattice match is slightly better for SiC than for sapphire, the dislocation densities that result are still very high (generally in the high 108 cm-2 range) and often deleterious to device performance. Screw-component dislocations are especially critical since they serve as leakage paths in vertically conducting III-N devices. In this paper efforts to reduce the extended defect density in III-N films grown on SiC will be reviewed. Details on recent efforts to use step-free SiC mesa surfaces arrayed on commercial 4HSiC substrates will then be highlighted showing dramatic reductions in extended defect densities and the virtual elimination of critical defects for vertically conducting devices. In these experiments, SiC surfaces that are homoepitaxially grown step-free or of very low step density have been used as growth templates for thin (<3 μm) GaN films deposited on a novel 1000 Å AlN nucleation layer characterized by a total dislocation density two orders of magnitude lower than the previous state-of-the-art, and with no evidence of screw-component dislocations.

Abstract: Cubic GaN, AlxGa1-xN/GaN and InyGa1-yN/GaN multiple quantum well (MQW) layers were grown by plasma assisted molecular beam epitaxy on 200 &m thick free standing 3C-SiC substrates. The influence of the surface roughness of the 3C-SiC substrates and the influence of metal coverage during growth are discussed. Optimum growth conditions of c-III nitrides exist, when a one monolayer Ga coverage is formed at the growing surface. The improvement of the structural properties of cubic III-nitride layers and multilayers grown on 3C-SiC substrates is demonstrated by 1 μm thick c-GaN layers with a minimum x-ray rocking curve width of 16 arcmin, and by c-AlGaN/GaN and c-InGaN/GaN MQWs which showed up to five satellite peaks in X-ray diffraction, respectively.

Abstract: In order to improve the crystal quality of MBE-grown GaN layers we employed a high temperature growth process and enhanced the lateral overgrowth. The grain size of the GaN layer was enlarged up to 2 "m in diameter. Significant improvement in the XRD characteristics was found, and the FWHM value of the asymmetric (10-12) XRD ω-scan peak became less than 400 arcsec when the layer thickness was 3 "m. Further, to planarise the surface, the low temperature gallium-rich growth process was employed and the large grooves between the grains vanished.

Abstract: The benefits of depositing AlN-SiC alloy transition layers on SiC substrates before the seeded growth of bulk AlN crystals were determined. The presence of the AlN-SiC alloy layer helped to suppress the SiC decomposition by providing vapor sources of silicon and carbon. It enabled a higher growth temperature, and hence a higher growth rate. In addition, cracks in the final AlN crystals can be decreased because of the intermediate lattice constants and thermal expansion coefficient of AlN-SiC alloy. AlN-SiC alloys were first grown on off-axis SiC substrates by the sublimation-recondensation method. Then pure AlN crystals were grown upon those. For comparison, AlN crystals were directly grown on SiC substrates under similar conditions. X-ray diffraction (XRD) confirmed the formation of a pure single crystalline AlN layer upon the AlN-SiC alloy on SiC substrate. The presence of an AlN-SiC transition layer effectively inhibited the appearance of cracks in the resultant AlN crystals. X-ray topography (XRT) demonstrated that the thick AlN layer effectively released the strain present.

Abstract: AlN is considered as the most suitable substrate material for further development of high quality and high performance nitride-based micro- and opto-electronics. AlN ingots are often grown on SiC seeds. To solve the formation of cracks due to the difference in lattice parameters between seed and crystal we chose to “adapt” the lattice mismatch by a buffer layer of the (AlN)x(SiC)1-x solid solution. This paper gives some inputs on the growth of AlN and the solid solution by the sublimation technique, in terms of materials compatibility, hetero- and homo-epitaxial growth of AlN and on the preparation of crack-free solid solution single crystals.

Abstract: Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on in situ-formed SiN and TiN porous network templates. The room temperature carrier decay time of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN (1.73 ns). The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably with the use of SiN and TiN layers, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity related nonradiative centers are the main parameters affecting the lifetime.

Abstract: Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.