Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With rotor
Patent
1987-11-25
1989-06-13
Tokar, Michael J.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With rotor
324158R, 324158P, 324 58B, G01R 3126
Patent
active
048395889
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for the examination of electrically active impurities (deep levels) of semiconductor materials and to a measuring arrangement for carrying out the method.
BACKGROUND OF THE INVENTION
Electrically active impurities such as foreign atoms, native defects of the crystal or the complexes thereof can all exert a significant influence on the electrical and optical properties of semiconductor materials as well as of structures and devices made of such materials, therefore the examination of such impurities forms an indispensable method both of the research of semiconductor materials and of the quality control of manufacturing processes of active elements in the field of microelectronics.
For investigating semiconductor materials and controlling active elements the sensitivity limit for detecting electrically active impurities should be at least 10.sup.10 atoms/cm.sup.3. At present such a high sensitivity can be obtained by measuring a particular single process only. In such a process a space charge layer is provided in the semiconductor under test which can be realized by depositing a suitable metal layer (i.e. forming a Schottky diode), by forming a p-n junction or by establishing a MOS structure, etc. In case of a reverse bias the space charge layer has insulating properties i.e. it does not comprise free carriers. Those portion of electrically active defects which fall within the space charge layer can be either in an electrically active or neutral state. In the process in question the active defects are filled with free charge carriers and the subsequent thermal emission recovery process is examined.
In a known examination method of this process the structure is electrically short-circuited and cooled down from room temperature to the temperature of liquid nitrogen, and in this latter temperature a reverse bias is applied to the sample. The electrically active defects remain saturated with free charge carriers that correspond to a non-equilibrum thermal state. The time constant of the recovery process to the thermal equilibrum is electronvolt units
In low temperature this time constant can be even some years long. It follows from equation (1) that the time constant decreases exponentially with increasing sample temperatures, and the thermal emission takes place when the characteristic temperature associated with the electrically active defects has been reached. The free charge carriers released during such emission can be detected by means of conventional current measurement or by detecting the changes of the capacitance of the sample. Corresponding known experimental methods are as follows: `Photoelectronic Materials and Devices` Ed. S. Larach pp. 100-139, 1965 (D. Van Nostard Comp.). Carabelles at al: Solid-St Communication 6, 167, 1968).
Another widely used way of examining the thermal emission process is represented by the transient measurement technique. In such technique the sample under test is reverse biased at predetermined constant temperature and short circuited during perodically repeated short intervals. During the short-circuiting intervals the defects are filled with free charge carriers and during re-establishment of the reverse bias a thermal emission recovery process is started which has the characteristic time constant defined by the equation (1).
The filling of defects can be made not only in an electrical way by short-circuiting the sample but also by optical excitation, by an electron beam or by means of other kinds of ionizing radiation. The excitation should be, however, periodically repetitive.
The thermal emission can be detected from the transient changes in the capacitance or current of the sample under test, see e.g. R. Williams, J. Appl. Phys. 37, 3411 (1966).
The automatic detection and evaluation of the transients have been solved by means of Deep Level Transient Spectroscopy (DLTS) technique. Such a method is described e.g. by Miller et. al: Rev. os Sci. Instrum 48, pp. 237-239, 1977 or in Hungarian Pat. No. 181.136.
REFERENCES:
patent: 4169244 (1979-09-01), Plows
patent: 4563642 (1986-01-01), Munakata
patent: 4686463 (1987-08-01), Logan
patent: 4704576 (1987-11-01), Tributsch
patent: 4727319 (1988-02-01), Shahriary
Ferenczi Gyorgy
Jantsch Wolfgang
Magyar Tudomanyos Akademia Muszaki Fizikai Kutato Intezet
Tokar Michael J.
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