Deep levels importance

In addition, a number of other deep level impurities have been hydrogen passivated. They include nickel, cadmium, tellurium, zirconium, titanium, chromium, and cobalt (Pearton et al., 1987). Most of these studies have been qualitative, and important work remains to be done if the hydrogenation of these and most probably additional impurities, such as gold, palladium, platinum and iron, is to be fully understood. 

Deep levels generally have less effect on electronic conductivity but are important in other ways, and especially influence optical properties. 

The next step is the definition of deep. The choice of quantitative values will again involve some arbitrariness. However, a further complication is that there is at present no universally accepted qualitative criterion. For instance, from the point of view of energy level calculations it is often convenient to define deep states as noneffective-mass-like or as those with a localized potential (see, for example, Bassani and Pastori Parravicini, 1975 Jaros, 1980). However, the disadvantage of this definition of deep is that it includes many isoelectronic states that are very shallow on an energy scale. On the other hand, if one uses an energy criterion, should the states be deep with respect to some fraction of the band gap, with respect to kT, or with respect to some shallow levels In this chapter we shall adopt an energy criterion for deep, and we shall require that our states be deep enough to be important in recombination. The importance of deep levels in recombination under many conditions of practical interest was already realized in the early work of Hall... 

The shallow and deep levels play the important role in the sensitization process. Detailed research in this field has shown the presence of four local electron centers in the energetic spectrum of the sensitized PVC in the range 0.6-3.3 eV [63,64]. The density of localized states was of the order 1018 1019 cm-3. These can play essential role in spectral and chemical sensitization due to their influence on photogeneration, recombination and charge transfer processes. 

Systematic investigations on the dependence of the PPC properties on different growth conditions are still needed to elucidate the nature of the deep level defects which are responsible for PPC. Needless to say, the future development of GaN devices depends critically on the improvements in impurity doping, which would rely heavily on the full understanding of the physics of doped impurities. For many device applications, it is important to eliminate (or minimise) effects of deep level impurities through improved crystal growth processes and device designs. 

Most of the III-V nitride materials utilised in optoelectronic or electronic devices contain a high density of structural defects. At present, the relationship between these defects and electrically active deep levels is only speculative. To shed light on the role of structural defects and impurities in III-V nitrides, it is important to detect and characterise deep levels in these novel semiconductors. 

Hydrogen plays an important role in III-V semiconductors especially in the nitrides, passivating the electrical activity of shallow and deep level impurities. This is more important in GaN grown by metal-organic vapour phase epitaxy (MOVPE) than by molecular beam epitaxy (MBE). Using SIMS... 

Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

A few selected techniques that are representative of recent advances are described as examples of the much broader field of semiconductor electrical characterization. In particular, resistivity, carrier concentration, junction depth, generation/recombination lifetime, deep level transient spectroscopy and NOSFET mobility measurements are discussed. The importance of non-contacting methods is stressed and recent trends in this direction are outlined. This paper serves as an introduction to some of the following papers in this volume. 

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]. 

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],... 

As a final point, which illustrates the importance of surface processes to semiconductor behaviour, Neave et al. [346] and Kiinzel and Ploog [347] have shown that deep level incorporation during the growth of GaAs films from beams of Ga and arsenic is dependent on the arsenic species used. The deep levels are believed to be associated with intrinsic defects and films prepared from As4, in which a pairwise interaction is involved, contain a higher concentration of three specific deep centres than those prepared from As2 where only simple dissociative chemisorption occurs. 

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