Determination of the parameters of deep levels in semiconductors using nonstationary spectroscopy and low-frequency noise spectroscopy

Abstract

A development of the activation-drift model is obtained. This enables the procedure for determining the parameters of deep levels in semiconductors using the data of nonstationary spectroscopy and low-frequency noise spectroscopy to be made more accurate and increases the reproducibility and reliability of the results of measurements.Deep levels in semiconductors, localised in the forbidden-energy gap, have a consideable effect on the properties of materials and on the devices made from them. Hence, the problem of investigating the parameters of deep levels produced by different types of structural defects, and also of the physical processes which occur when they are recharged is of particular importance. Nonstationary (relaxational) deep-level spectroscopy (NDLS, including DLTS) and low-frequency noise spectroscopy are promising methods of determining the parameters of deep levels: the ionization energy AE and the density N r Capacitive NDLS involves measuring the relaxation of the capacitance when a reverse bias U is applied to the semiconductor structure with an asymmetrical p-n-junction or a Schottky barrier. A characteristic feature is the requirement for a fairly high depleting bias U on the barrier structure, which must ensure that mobile carriers, emitted from the deep level, are removed from the space-charge region. Temperature scanning enables the maxima of the NDLS signal to be recorded with relaxation times t that are characteristic for each type of deep center (trap) and which are identical with the specified length of the measuring reverse-bias pulse (the NDLS "window").To investigate the parameters of deep levels by the low-frequency noise spectroscopy method in barrier structures, the spectrum or temperature dependence of the spectral power density of the low-frequency noise is used as the response function.To determine the parameters of deep levels, the maxima of the temperature dependences of the spectral power density or characteristic inflection frequencies (changes in slope) of the low-frequency noise spectra are used when ~bx = 1, where ~t, is the angular frequency corresponding to the inflection or maximum and x is the relaxation time of the process.To describe relaxation processes having an activation nature, an expression of the following form is often used:where x o is a factor which has different physical meanings in different models, AE is the ionization energy, k is the Boltzmann's constant, and T is the absolute temperature. The exponential factor in (1) (the Boltzmann exponent) represents the emission (generation) parameters of the carriers. The slope of the Arrhenius curves Ig~ = ~p(T-l), constructed from the experimental results of investigations of the NDLS curves or the temperature dependences of the spectral power density of the low-frequency noise, enables the ionization energy of a deep center z~ in (1) to be determined.Many theoretical and experimental publications on NDLS and low-frequency noise spectroscopy are based on an approach that is common to all these methods, deve...