Deep Level Transient Spectroscopy Sample Clauses

Deep Level Transient Spectroscopy. Deep levels may behave as carrier traps or as generation-recombination centers if they are energetically located near to mid-gap. As traps they can capture the free carriers reducing their effective density. It is clear that deep levels, although present in small quantities, can completely alter the electrical characteristics of a material. This allows them to be investigated using electrical measurements provided that the filling and emptying of the levels can be controlled. Deep-Level Transient Spectroscopy is a basic, yet powerful high-frequency transient measurement technique which can be used to observe and quantify various properties of traps in semiconductors [76]. It is a capacitance thermal scanning method and can indicate the location and concentration of a trap, in addition to facilitating the measurement of activation energy, concentration profile and carrier capture cross-sections for each trap. When a reverse bias is applied to a Schottky contact, the depletion width adjusts to its new position within the response time of the free electrons in the semiconductor lattice. However, after this immediate adjustment some of the full deep levels find themselves above the Fermi level where they slowly release their trapped charge at a characteristic emission rate:  Ea,n, p  en, p = σ n, pυth,n, p NC ,V exp−   k BT  Where σn is the capture cross section for electrons; υth is the thermal velocity of electrons within the lattice; Nc is the density of states in the conduction band; and Ea is the activation energy of the traps. For measurement purposes the transient may be produced in a controlled manner by applying a forward bias pulse to intentionally fill all the traps, and then returning to a quiescent reverse bias. The filling pulses are repeatedly applied whilst the temperature is ramped at a constant rate. Because deep level emission is thermally activated, the time constant of the transient will change with temperature. The usual method of measuring the capacitance transient is the boxcar technique, the transient is measured conventionally between two sampling points at times t1 and t2 (see Figure 3.1), the time period between the sampling points is referred to as the rate window.