Deep Levels in SiC

In developing a material for use in device applications, it has been found that deep energy levels, in the forbidden energy gap, play an important role. Deep levels can act as carrier recombination or trapping centres and affect the performance of electronic and opto-electronic devices. Deep levels have been a subject of investigation for over thirty years and several excellent reviews are available Grimmeiss [1], Neumar and Kosai [2], Milnes [3] and on the capacitance measurement techniques Lang [4], 

Deep levels can be described by the Shockley-Read-Hall recombination statistics [5]. However, for a large number of deep states, the capture cross section for one type of carrier is many times larger than that for the other carrier. The state, therefore, interacts principally with only one of the band edges and can be characterised as either an electron or a hole trap. Capacitance techniques, such as DLTS (Deep Level Transient Spectroscopy), are particularly convenient for the determination of trap type and concentration. If additional experimental information is present to allow charge state determination, then the states can be characterised as deep acceptors or donors. 

In this Datareview, we concentrate on deep levels measured by capacitance and admittance techniques those measured by other techniques are detailed in Datareview 4.1. For completeness, trap parameters for major defects and impurities obtained from all techniques are listed. Capacitance techniques have proven useful for the characterisation of deep states in semiconductor devices. In particular, states which are non-radiative can be analysed by this technique. If the state under study is one which principally determines the conductivity of the crystal, the techniques of admittance spectroscopy are used. The set-up for doing capacitance and admittance spectroscopy on SiC is identical to that used for other semiconductors with the exception of the necessity to operate the system at higher temperatures in order to access potentially deeper levels in the energy gap. The data are summarised in TABLE 1. 

There have been a few reports on radiation damage in SiC [7]. In this area, the effects of ions and electrons have been considered. If irradiation is performed, six deep states are produced in 6H-SiC. These states have been denoted E1-E4, Z, and Z2. After thermal annealing, only the two Z states remain. It should be noted that these are the same Z states observed in as-grown bulk material. It should also be noted that the defects reported are rather shallow in energy and there are no reports of semi-insulating material produced by radiation damage. 

The author could find only one study of 4H-SiC [17]. That study was performed on epitaxial 4H films which were doped by ion implantation with Al. It was found that these films exhibited two levels. The first, thought to be associated with Al, was at an energy of 0.26eV 

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In the last few years Schneider and co-workers have performed a number of experiments on various SiC polytypes which exhibit a characteristic infrared emission in the 1.3 to 1.5 pm spectral range [98]. They have assigned this emission band to vanadium impurities substituting the various silicon sites in the lattice. In their extensive work they found three charge states of vanadium which act as an electrically amphoteric deep level in SiC. They also suggest that vanadium may have an important role in the minority-carrier lifetime in SiC-based optoelectronic devices [98,99], Recently, trace amounts of vanadium impurities have been detected in 3C-SiC grown by the modified-Lely technique [100]. 

Impurity trace analyses have shown that transition metals such as titanium and vanadium are pervasive impurities in Lely and/or modified-Lely grown SiC polytypes [90]. Since transition metals are unstable and have multiple charge states they can be electrically active deep levels. 

The ionisation energies of the electronically active impurities have been determined primarily by photoluminescence techniques and Hall measurements. Ionisation energy levels of such impurities as nitrogen and some of the group III elements (aluminium, gallium, boron) in 3C-, 4H-, 6H- and 15R-SiC polytypes are compiled in TABLE 2. Nitrogen gives relatively shallow donor levels. In contrast, other p-type dopants have deep-level acceptor states. 

Most impurity and defect states in SiC can be considered as deep levels. Both capacitance and admittance spectroscopy provide data on these deep levels which can act as donor or acceptor traps. Bulk 6H-SiC contains intrinsic defects which are thermally stable, up to 1700 °C. In epitaxial films of 6H-SiC a deep acceptor level is seen in boron-implanted samples but not when other impurities are implanted. Other centres, acting as electron traps, are also seen in p-n junction and Schottky barrier structures. Irradiation of 6H-SiC produces 6 deep levels, reducing to 2 after annealing. Only limited studies have been carried out on the 3C-SiC polytype, in the form of epitaxial films on silicon substrates. No levels were seen in thick films but electron traps were seen in thin n-type films and a hole trap (structural defect) was found to be a mobility killer. Neutron irradiation produces defects most of which can be removed by annealing. Two levels were found in Al-implanted 4H-SiC. 

Acceptor levels with Al, B, and Ga in SiC are so deep that the activation rates at room temperature are very low. For example, in the case of Al, the activation energy of acceptor levels is around 160 meV, which is much larger than kT at room temperature, 25 meV, resulting in a low activation rate, as low as 0.01. Therefore, high levels of doping more than two to three orders of magnitude higher than the carrier density, is required. 

L Patrick, WJ Choyke. Photoluminescence of Ti in four SiC polytypes. Phys Rev B10 5091, 1974. J Schneider, HD Muller, K Maier, W Wilkenning, F Fuchs, A Domen, S Leibenzeder, R Stein. Infrared spectra and electron spin resonance of vanadium deep level impurities in silicon carbide. Appl Phys Lett 56 1184, 1990. 

The low-temperature (8K) PL spectra of the GaN films on both on- xis and vicinal 6H-SiC(0001)si substrates showed strong near band-edge emissioh at 357.4 nm (3.47 eV). The FWTIM values of these I2 bound exciton peaks were 4 meV. The spectrum from the GaN film on the vicinal substrate revealed a very weak peak centered at 545 nm (2.2 eV), commonly associated with deep-levels (DL) in the band gap. A more intense 2.2 eV peak was observed in the GaN film grown on-axis. 

To achieve fracture in UHTC materials, ManLabs researchers decided to notch all subsequent test specimens. Notches were 6 mm deep x 1.6 mm wide and parallel to the axis of the cylinder, extending inward from the outer surface along the entire length of the specimen. The main disadvantage of using notched specimens was the lack of adequate experimental and analytical data on the shape factor required to compare the experimental results with predicted properties as well as the inability to compare these results with those of previous evaluations. Their results showed that materials with SiC and carbon additions displayed somewhat higher steady state thermal stress resistance than the other compositions. Nonetheless, all the diboride compositions tested showed a level of thermal stress resistance considerably above any other ceramics they had tested. ... 

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