Level Transient Spectroscopy

The development in 1974 of deep level transient spectroscopy (DLTS) by Lang and co-workers (Lang, 1974 Miller et ai, 1977 Lang, 1979) coupled the idea of the transient measurement method with the temperature scan- 

In the sections that follow, we shall first discuss the physics common to all thermal emission spectroscopic techniques, then examine several of the more popular techniques in detail with the major emphasis on DLTS. Since the application of DLTS to the study of deep levels in crystals has been reviewed extensively (Miller et al., 1977 Lang, 1979), this discussion will focus on the special problems involved in applying DLTS to materials that contain a large continuous distribution of gap states. We shall then outline the procedure used to obtain a quantitative density of states from the measurement, and finally discuss the experimental results. As in the previous sections, we shall end with a discussion of the limitations and unresolved issues associated with thermal transient measurements. 

Basic Concepts Analysis of the Dynamic Junction Problem 

The basic physics of thermal emission measurements performed on semiconductor junctions is illustrated in Fig. 32. This figure displays the evolu- 

As time progresses (see 1 = 1 ), region A will eventually disappear as the system loses its memory of the initial conditions. Finally (t — ), region B will disappear as the new equilibrium is attained. However, for larger reverse ambient bias, a region similar to B will persist at a characteristic quasi-Fermi energy near midgap (see the previous discussion in Section 2). 

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Experimentally, local vibrational modes associated witli a defect or impurity may appear in infra-red absorjrtion or Raman spectra. The defect centre may also give rise to new photoluminescence bands and otlier experimentally observable signature. Some defect-related energy levels may be visible by deep-level transient spectroscopy (DLTS) [23]. 

Figure 1 shows a deep level transient spectroscopy (DLTS) (Lang, 1974) spectrum from a Au-diffused, n-type Si sample before and after hydrogenation of 300°C for 2h (Pearton and Tavendale, 1982a). The well-known Au acceptor level (Ec - 0.54 eV) was passivated to depths > 10 pm under these conditions and was only partially regenerated by a subsequent... 

An argument against the defect mediated diffusion model is the same one used earlier that is, there are not enough defects as determined by ESR (Brodsky and Title, 1969, 1976) or Deep Level Transient Spectroscopy measurements (Johnson, 1983) to account for the motion of all of the bonded hydrogen in a-Si H. This objection is removed if the floating bonds are 104-106 times more mobile than the hydrogen atoms. However, such highly mobile defects would rapidly self-annihilate via the process. 

Bhattacharya, R. N. Balcioglu, A. Ramanathan, K. 2001. Deep-level transient spectroscopy (DLTS) of CdS/CuIni xGaxSe2-based solar cells prepared from electroplated and auto-plated precursors, and by physical vapor deposition. Thin Solid Films 384 65-68. 

Lang, D. V. 1974. Deep-level transient spectroscopy A new method to characterize traps in semiconductors. J. Appl. Phys. 45 3023-3032. 

Kuranouchi, S. Konagai, M. 1995. Characterization of ZnO/CdS/CuInSe2 thin-film solar cells by deep-level transient spectroscopy. Jpn. J. Appl. Phys. 34 2350-2351. 

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

The first technique is successfully used not only in the stndy of chemical reactions bnt also in electronic reaction kinetics in solids. It is necessary to note here the recently developed technique of deep-level transient spectroscopy (DLTS). 

Because of the previously mentioned reviews (Sah et al., 1970 Milnes, 1973 Grimmeiss, 1974, 1977, 1980 Sah, 1976 Miller et al., 1977 Lang, 1979 Martin, 1980), this section will be brief and mainly discuss techniques not previously reviewed. However, in view of the widespread use of deep level transient spectroscopy (DLTS) and the number of variations it has inspired, it is appropriate to describe briefly the technique. The reader is referred to the original article by Lang (1974b) and the review papers by Miller et al. (1977) and Lang (1979) for details. 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

We now discuss some of the experimental aspects of temperature spectroscopy. Lang (1974) called his original method deep level transient spectroscopy (DLTS), and he measured capacitance transients produced by voltage pulses in diodes made from conductive materials. However, in SI materials, this method is not feasible and an alternate method, involving current transients produced by light pulses in bulk material (or Schottky structures), was... 

A deeper insight into the lateral electrical homogeneity of the films, the limiting mechanisms of the Hall mobility, and the thermal activation energies of shallow and deep defect levels can be gained by temperature-dependent Hall and deep level transient spectroscopy (DLTS) measurements [57,59,60]. To give an example, the temperature dependence of the Hall mobility and... 

Table 7.5. Energetic positions below the conduction band edge (Ec) and densities of shallow (Hi, Alzn) and deep (I ll -E5) donor-like defect levels (traps) in ZnO identified by temperature-dependent Hall effect and deep level transient spectroscopy, respectively, in undoped PLD films and single crystals grown by seeded chemical vapor deposition (Eagle Picher), taken from H. von Wenckstern [57]...

Capacitance transient spectroscopy encompasses a powerful set of techniques to detect and characterise deep levels in semiconductors. The list of techniques applied for III-V nitrides includes deep level transient spectroscopy (DLTS) [1,2], double correlation DLTS (DDLTS) [3], isothermal capacitance transient spectroscopy (ICTS) [2], photoemission capacitance transient spectroscopy (ODLTS) [4] and optical ICTS (OICTS) [5], This Datareview presents the current status of deep level studies by capacitance transient techniques for III-V nitrides. A brief introduction to the techniques is given, followed by an example that demonstrates the application of DLTS and DDLTS for Si-doped... 

ODENDOR ODLTS ODMR OICTS OLCAO OMVPE OSC optically detected electron nuclear double resonance optical deep level transient spectroscopy optically detected magnetic resonance optical isothermal capacitance transient spectroscopy orthogonalised linear combination of atomic orbitals organo-metallic vapour phase epitaxy on-surface-cracking... 

Some of the metastable centers have been associated with impurities such as oxygen (Crandall, 1981) and carbon (Crandall et al., 1983). Deep-level transient spectroscopy (DLTS) has been used to identify a center associated with oxygen that has a characteristic activation energy of 1.0 eV, whereas... 

Volume 21, Part C, is concerned with electronic and transport properties, including investigative techniques employing field effect, capacitance and deep level transient spectroscopy, nuclear and optically detected magnetic resonance, and electron spin resonance. Parameters and phenomena considered include electron densities, carrier mobilities and diffusion lengths, densities of states, surface effects, and the Staebler-Wronski effect. 

The first of these approaches is used in the technique of deep level transient spectroscopy (DLTS), which is perhaps the most common experiment for measuring deep levels in crystalline semiconductors (Lang 1974). The DLTS experiment is the measurement of the transient capacitance of a Schottky contact to the sample and is... 

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