Deep-level impurities

In order to remove tlie unwanted electrical activity associated witli deep-level impurities or defects, one can eitlier physically displace tlie defect away from tlie active region of tlie device (gettering) or force it to react witli anotlier impurity to remove (or at least change) its energy eigenvalues and tlierefore its electrical activity passivation). 

Hydrogen Passivation of Deep Level Impurities and Defects 372... 

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. 

It is well known that the electrical activity of many deep-level defects disappears when the crystal is exposed to atomic hydrogen (see Pearton et al., 1987 and Chapter 5 of this Volume). This has been attributed to complex formation with the hydrogen, and it is very common for transition-metal impurities. Unfortunately, very little theoretical work has been reported for these systems. The deactivation of second- and third-period deep-level impurities is better understood theoretically. The substitutional oxygen defect in silicon ( A center Watkins and Corbett, 1961 Corbett et al., 1961) can be deactivated by exposure to hydrogen. Recently, a theoretical study of the deactivation of substitutional sulfur through the formation of a hydrogen-sulfur pair has been reported (Yapsir etal., 1988). 

Spin Resonance of Deep Level Impurities in Germanium and Silicon. 

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. 

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... 

Optical techniques like photoluminescence (10) and infrared photothermal spectroscopy (11.) work well for the characterization of shallow level impurities, while electrical techniques work well for deep level impurities. There are a number of methods that have been used for electrical characterization. I will only discuss deep level transient spectroscopy (DLTS), however, because it has become the most popular and gives a fairly complete characterization. 

The lifetimes of the previous section depend on the concentration, capture cross-section and energy level of the impurities. Lifetime measurements, however, cannot easily be used for these determinations. DLTS is the technique most frequently used instead. It is based on the concept in Figure 5 (22). First consider the n-type Schottky barrier diode of Figure 5 (a) with no deep level impurities. A reverse bias -Vj creates a scr of width W. When the bias is reduced to zero, the scr is also reduced. The scr capacitance, being inversely proportional to W, is small and equal for cases A, C and D and large for case B. If the voltage is pulsed between zero and -V., the capacitance follows almost instantaneously and no time-dependent capacitance is observed. 

A different behavior is observed when deep level impurities are present, as indicated in Figure 5(b). This figure is a composite showing the device and its scr, but superimposed on it is the energy level corresponding to the impurity, N. It is merely shown for convenience. Initially, the device has Deen reverse biased for some time and is empty of electrons (A). A short pulse brings it to... 

Chattopadhyay, R Sanyal, S. 1995. Capacitance-voltage characteristics of Schottky barrier diode in the presence of deep-level impurities and series resistance. Applied Surface Science, 89 205-209. 

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. 

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