Traps level

The electrical current of a coplanar interdigilal gold/LPPP/gold device is space charge limited due to p-type charge earner traps localized in the bandgap [28]. This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels ,1 and ,", which can be calculated from the rise in current, /, below the peak temperature ... 

Table 1. Trap depth and density in LPPP T , temperature at peak current, ,rsc and t J 1A are the trap levels obtained from the TSC and PIA experiments respectively, N, and it, are the number of traps and the trap concentration, respectively...

In the case where there is one single trap level, E, is the energy difference between this level and the delocalized band edge, and a the ratio between the effective density of slates at the delocalized band edge and the concentration of traps. If traps are energy distributed, effective values of N, and a must be estimated. 

The value of tt is evaluated [177] as xt (2C i ), where C is the coefficient of electron capture by a trap is the local density of electrons at the instant the Fermi level intersects the trap level when a direct bias signal is applied to the barrier. 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

The net carrier concentration, shown in Fig. 7.8, was obtained at a frequency of 100 kHz. DLTS spectra were recorded using reverse- and forward-bias modes in the temperature range of 80-350 K. In the re verse-bias mode, the devices were reverse biased from -1.2V to -0.2V, with a pulse width of 1 ms. Two hole (majority-carrier) trap levels were found in all the devices. These levels were designated as Hi at I iv+0.26 and H2, for which an activation energy could not be resolved. Upon minority-carrier injection (forward-bias mode), DLTS showed two additional electron (minority-carrier) traps, which are labeled Ei (Ec-0.1eV) and E2 (Ec-0.83eV) in Table 7.1. The spectra were measured at an emission time of 465.2 s and the width of the... 

Devices Trap Level Energy Level (eV) Trap Concentration (cm 3)... 

Zeenath, N. A. Pillai, P. K. V. Bindu, K. Lakshmy, M. Vijakakumar, K. P. 2000. Study of trap levels by electrical techniques in p-type CuInSe2 thin films prepared using chemical bath deposition. J. Mater. Sci. 35 2619-2624. 

The surface-state model, in which the luminescent recombination occurs via surface states, was proposed to explain certain properties of the PL from PS, for example long decay times or sensitivity of the PL on chemical environment. In the frame of this model the long decay times are a consequence of trapping of free carriers in localized states a few hundred meV below the bandgap of the confined crystallite. The sensitivity of the PL to the chemical environment is interpreted as formation of a trap or change of a trap level by a molecule bonding to the surface of a PS crystallite. The surface-state model suffers from the fact that most known traps, e.g. the Pb center, quench the PL [Me9], while the kinds of surface state proposed to cause the PL could not be identified. 

Amorphous semiconductors are characterized by properties that are absent in their crystalline counterparts. On the one hand, they are unsuitable objects both for experimentalists and from the theoretical point of view on the other hand, they have widespread technical applications. Therefore, trap level spectroscopy in materials containing S, Se, and Te is necessary for further technical applications. Currently, there is no universal technique that probes the full spectrum of trapping levels in a mobility gap experimentalists use several complementary methods. This book is devoted to techniques that probe states in the mobility gap and the results of their use. 

Trap Level Spectroscopy in Amorphous Semiconductors. DOI 10.1016/B978-0-12-384715-7.00001-2... 

All other experimental TSR techniques used in trap level spectroscopy in semiconductors (insulators) are indirect methods for the determination of trapping parameters. The techniques involve the measurement of phenomena that are due to charge carriers emitted after thermal stimulation from the traps. 

During the TSR process, the concentration of holes and electrons is determined by the balance between thermal emission and recapture by traps and capture by recombination centers, hi principle, integration of corresponding equations yields ric(t,T) and p t,T) for both isothermal current transients (ICTs) or during irreversible thermal scans. Obviously, the trapping parameters hsted together with the capture rates of carriers in recombination centers determine these concentrations. Measurement of the current density J = exp(/in c + yUpP) will provide trap-spectroscopic information. The experimental techniques employed in an attempt to perform trap level spectroscopy on this basis are known as Isothermal Current Transients (ICTs) [6], TSC [7]. 

This experimental method, as well as the formal kinetics of the process, is closely related to trap level spectroscopy by thermally stimulated release of trap charge carriers. 

Thermally stimulated discharging of electrets provides spectroscopic information similar to trap level spectroscopy most importantly, their density and the activation energy required for the relaxation process to proceed. 

Observations of TSL have been reported as early as the seventeenth century, but Urbach [9] is generally credited with proposing it as a potentially useful experimental technique for trap level spectroscopy. However, only after the publication of the work of Randall and Wilkins [10] in 1945 did TSL receive much attention. First measurements of TSC and TSL were performed by Bube [11]. 

Let us now briefly consider the various steps in a typical TSL or TSC experiment in wide-band gap material. We choose electromagnetic radiation as a means of excitation. The interaction of this radiation with the solids leads to a number of electronic phenomena, many of which are not clearly understood. They include the production of new defects as well as filling of trap levels with electron and holes ... 

The principal goal of TSC trap level spectroscopy is to experimentally determine, by comparison of model glow curve with measured ones, the characteristic parameters that govern the nonisothermal relaxation kinetics of the solid. 

Analytic solntions for a(T) were reported by Simmons and Taylor [12] for the case that retrapping can be neglected in a thin sample at high electric fields. They considered the presence of several trap levels of density A and demonstrated the snperposi-tion of the individnal glow peaks when the thermal ionization energies of these levels are very close to each other (Fig. 1.3). 

The main attraction of TSC and TSL as experimental methods for the study of trapping levels in high-resistivity solids was, for many years, their apparent simplicity. The excited sample merely had to be placed onto a heater in front of an optical detector or, after attachment of two metallic contacts for voltage biasing, connected to a sensitive cnrrent meter. Work at low initial temperatures required the experiment to be performed inside a vacnnm chamber in an inert gas atmosphere. 

Figure 1.3 Examples of glow spectra resulting from several different discrete trap levels that are (a) widely separated, resulting in clearly resolved glow peaks (b) not sufficiently separated to be clearly resolved in the glow spectrum and (c) overlapping to produce a single, unresolved glow peak [12].

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