Deep-level impurities, semiconductor

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... 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

The passivation of deep level defects and shallow impurities in semiconductors by hydrogen has been studied extensively in recent years (Pearton et al., 1987, 1989 Haller, 1989). For Si in most cases, complexing with hydrogen eliminates the electrical activity of a defect.Once passivated, the... 

The donor electron level, cd, which may be derived in the same way that the orbital electron level in atoms is derived, is usually located close to the conduction band edge level, ec, in the band gap (ec - Ed = 0.041 eV for P in Si). Similarly, the acceptor level, Ea, is located close to the valence band edge level, ev, in the band gap (ea - Ev = 0.057 eV for B in Si). Figure 2-15 shows the energy diagram for donor and acceptor levels in semiconductors. The localized electron levels dose to the band edge may be called shallow levels, while the localized electron levels away from the band edges, assodated for instance with lattice defects, are called deep levels. Since the donor and acceptor levels are localized at impurity atoms and lattice defects, electrons and holes captured in these levels are not allowed to move in the crystal unless they are freed from these initial levels into the conduction and valence bands. 

A. Mandelis, A. Budiman and M. Vargas, Photothermal Deep-Level Transient Spectroscopy of Impurities and Defects in Semiconductors... 

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. 

Efficient recombination occurs in direct-gap semiconductors. The recombination probability is much lower in indirect-gap semiconductors because a phonon is required to satisfy momentum conservation. The radiative efficiency of indirect-gap semiconductors can be increased by isoelectronic impurities, e. g. N in GaP. Isoelectronic impurities can form an optically active deep level that is localized in real space (small Ax) but, as a result of the uncertainty relation, delocalized in k space (large Ak), so that recombination via the impurity satisfies momentum conservation. 

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. 

Unfortunately, diamond itself, though intensively studied, has been found to be less suitable for device applications. Although it can be doped by boron to make it a p-type semiconductor it is almost impossible to obtain the n-type counterpart for a p-n junction. Diamond cannot be doped by the most likely candidate, nitrogen, as a substitutional impurity. Instead nitrogen moves off its lattice site causing a dangling bond on one of its carbon neighbors which in turn results in a deep level [9-12]. 

Deep energy level impurities Doping impurities or other impurities whose energy level lies toward the center of the bandgap. Important for carrier recombination in indirect gap semiconductors. 

The Hall effect provides a measure of the net carrier concentration of the dopants. Depending on the depth of the dopants, the activation of the impurity can be very much reduced. For example, Mg in GaN forms a level at 250 meV above the valence band, and the percentage of activation of the magnesium atoms at room temperature is about 1%. DLTS provides a measure of the deep states within the bandgap of the semiconductor. However, it only provides the activation energy and the impurity concentration, and it does not give the exact nature of the impurity concerned. Implantation experiments are required to correlate known impurities with the energy levels measured by DLTS. 

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