Artificially layered heteropolytypic structures based on SiC polytypes: molecular beam epitaxy, characterization and properties

We have studied intrinsic and impurity related defects in silicon carbide (SiC) epilayers grown with fast epitaxy using chemical vapor deposition in a vertical hot-wall reactor. Using capacitance transient techniques, we have detected low concentrations of electron traps (denoted as Z1/2, EH6/7 and titanium) and hole traps (denoted as HS1 and shallow boron) in the n-type 4H–SiC epilayers. The concentration of intrinsic defects (Z1/2, EH6/7, and HS1 centers) increases with increasing growth temperature. The incorporation of shallow boron (B) decreases at higher growth temperatures, whereas the titanium (Ti) concentration is not sensitive to the growth temperature. The concentration of shallow B and Ti increases with increasing C/Si ratio. The concentration of the EH6/7 and the HS1 centers however, decreases with increasing C/Si ratio. We have also tested graphite susceptors with TaC or SiC coating and observed that the purity of the susceptor material plays a critical role in reducing the background impurity incorporation. The correlation with the carrier lifetime of these epilayers indicates that the EH6/7 and the Z1/2 centers may be the lifetime limiting defects in the investigated epilayers.

Electrically active defects in n-type 4H-silicon carbide grown in a vertical hot-wall reactor

By thermal oxidation of 4H-SiC at 1150–1300 °C, the Z1/2 and EH6/7 concentrations can be reduced to below 1×1011 cm-3. By the oxidation, however, a high concentration of HK0 center (EV + 0.78 eV) is generated. Additional annealing in Ar at 1550 °C results in elimination of the HK0 center. Thus, all the major deep levels can be eliminated by the two-step thermal treatment. Based on the depth profiles of deep levels, a model for the defect generation and elimination is proposed. The carrier lifetime in 4H-SiC epilayers has been improved from 0.64 (as-grown) to 4.52 µs by this method.

https://iopscience.iop.org/article/10.1143/APEX.2.091101/pdf

Elimination of the Major Deep Levels in n- and p-Type 4H-SiC by Two-Step Thermal Treatment

The present stage of development for the AlN-GaN-InN class of III-V nitrides is reviewed, with emphasis on the electronic properties of the materials. We also briefly cover the most important types of device structures presently under study, as well as application areas for III-nitride devices now foreseen. A brief overview of the main growth techniques is presented, since a further development of the growth of these materials is indeed the key towards a desired substantial improvement in material quality. This is needed both for the establishment of definite data for physical parameters, as well as for the proper functioning of most devices. The basic band structure as well as excitonic properties are briefly discussed. It is pointed out that these materials are in most cases excitonic at room temperature, for bulk as well as for quantum structures. Doping and properties of donors and acceptors are highlighted, here, in particular, the parameters for shallow acceptors, such as their binding energies, are far from well known. There is no complete picture of the identification of characteristic optical spectra (such as bound exciton spectra) with specific donors and acceptors. The properties of defects, in particular dislocations which are very abundant in the heteroepitaxial material studied today, are discussed. Dislocations may not themselves be severe recombination centers, and the degradation of active devices via dislocation climb seems to be absent in III-nitrides. The progress in the area of heterostructures and quantum wells based on the AlGaN/GaN and the InGaN/GaN systems is reviewed, in relation to the present strong development of related devices. An interesting observation is the importance of the piezoelectric effect, due to a strong built-in strain in most structures. Application areas cover a very broad range in optoelectronics, but also high frequency power devices, with applications for example in satellite communication systems.

Iii-v nitrides : important future electronic materials

With the advancement in molecular biology, several metabolic and physiological processes have been elucidated at molecular levels discovering the involvement of different genes. Since the advent of plant transformation 33 years ago, use of plant transformation techniques sparked an interest in fundamental and applied research leading to the development of biological and physical methods of foreign DNA delivery into 130 plant species. Modern molecular biology tools have developed rich gene sources which are waiting to be transformed into plant species. But unavailability of efficient transformation methods is a major hurdle to expedite the delivery of these genes into plants. Ever-expanding available gene pools in the era of third generation transgenic plants stressing the delivery of multiple genes for different traits; development and application of new transformation methods is the big need of the time to meet the future challenges for plant improvement. In recent years, silicon-carbide whiskers have proven valuable and effective alternative in which silicon carbide fibers are mixed with plant cells and plasmid DNA, followed by vortexing/oscillation. Cell penetration appears to occur thus whiskers function as numerous fine needles, facilitating DNA entry into cells during the mixing process. This technique is simple, easy and an inexpensive transformation method to deliver the DNA into monocot and dicot plant species. Whiskers, cells and plasmid DNA are combined in a small tube and mixed on a vortex or oscillating mixer. In this chapter we will discuss the use of silicon carbide fibers/whiskers to transform and produce different transgenic plants. This chapter will help the reader to know about emerging applications of silicon carbide and other fibers in the delivery of foreign DNA into plants, and critical parameters affecting DNA delivery efficiency will also be discussed.

Properties and Applications of Silicon Carbide

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 A appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 A peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At T=2 K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapolation of the temperature dependence of the free-to-bound peak to T=0 K, after correction for the temperature dependence of the exciton energy gap, leads to the value EA(B)−EX=628±1 meV, where EA(B) is the ionization energy of the deep boron center and EX is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.

Photoluminescence and transport studies of boron in 4H SiC

Cathodoluminescence of AlN–GaN short period superlattice films was measured at 6 K, 77 K, and room temperature. The superlattice films were deposited using a switched atomic layer metalorganic chemical vapor deposition process onto a buffer layer of either AlN or GaN, which was deposited on basal plane sapphire substrates. The individual AlN and GaN layers of the superlattice films ranged in thickness from 2.6 to 20.8 A. The cathodoluminescence of these samples was measured at several electron acceleration voltages to allow depth profiling of the samples. This allows the region of the sample (superlattice film, buffer layer, and substrate) from which the spectral features originate to be determined. A spectral peak in the ultraviolet region above the 3.5 eV band gap of GaN has been observed in all the superlattice samples studied to date. Our results indicate that the location of this peak is determined by quantum confinement in the GaN layers.

L. L. Clemen

Papers

Photoluminescence and transport studies of boron in 4H SiC

Cathodoluminescence of AlN–GaN short period superlattices