Engineering topics
Publications and source records attributed to Skowronski, M..
The ability to model the resistive switching devices depends on input material parameters and is particularly sensitive to values of electrical conductivity of the functional material as a function of composition and temperature. Here we report the results of conductivity measurements of TaO x with 0 AC ) changing from 0.34 eV at x=2.36 to 0.016 eV at x=1.
While previously published experimental results have shown that the step-free (0 0 0 1) 4H-SiC mesa growth surface uniquely enables radical improvement of 3C-SiC and 2H-AlN/GaN heteroepitaxial film quality (greater than 100-fold reduction in extended defect densities), important aspects of the step-free mesa heterofilm growth processes and resulting electronic device benefits remain to be more fully elucidated. This paper reviews and updates recent ongoing studies of 3C-SiC and 2H-AlN/GaN heteroepilayers grown on top of 4H-SiC mesas. For both 3C-SiC and AlN/GaN films nucleated on 4H-SiC mesas rendered completely free of atomic-scale surface steps, TEM studies reveal that relaxation of heterofilm strain arising from in-plane film/substrate lattice constant mismatch occurs in a remarkably benign manner that avoids formation of threading dislocations in the heteroepilayer. In particular, relaxation appears to occur via nucleation and inward lateral glide of near-interfacial dislocation half-loops from the mesa sidewalls. Preliminary studies of homojunction diodes implemented in 3C-SiC and AlN/GaN heterolayers demonstrate improved electrical performance compared with much more defective heterofilms grown on neighbouring stepped 4H-SiC mesas. Recombination-enhanced dislocation motion known to degrade forward-biased 4H-SiC bipolar diodes has been completely absent from our initial studies of 3C-SiC diodes, including diodes implemented on defective 3C-SiC heterolayers grown on stepped 4H-SiC mesas.
A new type of GaAs is developed which exhibits inverted thermal conversion (ITC); i.e., it converts from conducting to semiinsulating upon annealing at about 850 C. In device fabrication, its low resistivity prior to high-temperature processing differentiates ITC GaAs from the standard semiinsulating GaAs. The ITC characteristics are obtained through control of the concentration of the midgap donor EL2 based on heat treatment and crystal-growth modification. Thus EL2 does not exist in the conducting state of ITC GaAs. Conversion to the semiinsulating state during 850 C annealing is caused by the formation of EL2.
A high-resolution optical study was carried out on GaAs crystals grown by horizontal Bridgman and liquid-encapsulated-Czochralski methods. An excellent correlation was found between the intensity of the 1.039-eV no-phonon line and the characteristic absorption of EL2, the major deep donor level in GaAs. A correlation was also found between the characteristic optical absorption of EL2 and its concentration as determined by junction capacitance measurements. The presence of EL0, another midgap level contained in heavily oxygen-doped crystals at concentration always less than those of EL2, had no effect on the optical spectra, but altered the capacitance measurements. Accordingly, an accurate calibration for the determination of EL2 by optical absorption was obtained from capacitance measurements on crystals containing only EL2; in this way the uncertainties introduced by other midgap levels were eliminated.
A dominant hole trap has been identified in p-type bulk GaAs employing deep level transient and photocapacitance spectroscopies. The trap is present at a concentration up to about 4 x 10 to the 16th per cu cm, and it has two charge states with energies 0.54 + or - 0.02 and 0.77 + or - 0.02 eV above the top of the valence band (at 77 K). From the upper level the trap can be photoexcited to a persistent metastable state just as the dominant midgap level, EL2. Impurity analysis and the photoionization characteristics rule out association of the trap with impurities Fe, Cu, or Mn. Taking into consideration theoretical results, it appears most likely that the two charge states of the trap are the single and double donor levels of the arsenic antisite As(Ga) defect.
It is found that the rate of the photoinduced transition of the GaAs midgap level EL 2 to its metastable state increases as its occupation increases. High-resolution optical spectra of this transition exhibit a sharp peak very similar to the no-phonon line of the intracenter absorption of the As antisite defect. These findings show that the transition to the metastable state is initiated from the ground state 1A1, and it is finalized via the excited state 1T2 of the neutral As antisite defect. They thus provide a new basis for the critical assessment of the EL 2 metastability models and further confirmation of the association of EL 2 with the isolated As antisite defect.
Oxygen has profound effects on the key electronic properties and point defects of GaAs crystals. Thus, when added in the growth system, it decreases the free electron concentration and enhances the concentration of deep donors in the resulting crystals. Both of these effects are highly beneficial for achieving semi-insulating material and have been utilized for that purpose. They have been attributed to the tendency of oxygen to getter silicon impurities during crystal growth. Only recently, it has been found that oxygen in GaAs introduces also a midgap level, ELO, with essentially the same activation energy as EL2 but with four times greater electron capture cross section. The present report reassesses the electrical and optical properties of the midgap levels in GaAs crystals grown by the horizontal Bridgman (HB) and the Czochralski-LEC techniques. Emphasis is placed on the identification of the specific effects of ELO.