Minority carrier trap measurements in schottky barriers on N-type LPE GaAs

Photocapacitance transient technique has been applied to Schottky semitransparent barriers as an alternative method, instead of the classical capacitance voltage transient method in p+-n junctions for detecting minority carrier traps. In such a way and, under certain conditions, it is possible to detect and measure both majority and minority traps in Schottky barriers. The method applies well to Schottky barriers in LPE GaAs. A hole trap at 0.57 eV above valence band has been found in reasonable agreement with results in n-p+ junctions in which n-layer was grown by LPE. REVUE DE PHYSIQUE APPLIQUÉE TOME 12, DÉCEMBRE 1977, PAGE Classification Physics Abstracts 73 . 40 N In the past five years, considerable effort has been put forth in determining deep impurity states in semiconductors. In a recent paper, Sah [1] gives a very complete and interesting review of the different methods. Transient capacitance [2] has been proved to be the more reliable and useful method, although certain refinements have been introduced in order to extend the range in energies to be determined [3, 4]. In n-type material, majority carrier deep traps are generally determined in Schottky barriers while it is necessary to perform p+-n junctions for measuring minority carrier traps [5]. In this paper, an alternative method which permits determination, under certain conditions, of majority and minority carrier traps has been developed. Under strong illumination across semitransparent Schottky barriers the trapping state can be inverted and a hole trap can be observed. Assume a Schottky barrier on an n-type material with a deep center in the upper half of the gap. Simply by applying a negative voltage, a capacitance transient can be seen and from this, information can be obtained about energy level and concentration of the trap. However, if the deep center yields near or below the gap center, it is probable that applying a negative bias no transient can be observed because the electrons remain attached to the trap, or in other words, the time constant is extremely large. This is because ep > en, (hole trap) and en should be very small or negligible. Let us go on to consider a semitransparent Schottky barrier on n-type material in which hole emission rate is much higher than the electron emission rate ep » en r (see Fig. la). Providing carrier injection from illumination across the Schottky barrier, electrons will move towards the ohmic contact while holes travel towards the metallic barrier being trapped by the deep centers which become neutral. Thus, capacitance is increased. On the other hand, the electrons which move towards the ohmic contact are accumulated at the end of the depletion layer causing it to narrow and thus a capacitance increase. When light is switched oif, deep traps émit holes and the level and concentration of the deep center can be obtained from the corresponding transient decay, in the assumption that all traps have been filled. If the light intensity is strong enough, all traps become empty and the remaining electron-hole pairs created by the light cause an additional increase of the capacitance, that is, the excess electrons accumulate at the end near the ohmic contact, while excess holes remain near the barrier and the depletion layer becomes narrower (see Fig. 1 b). z Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197700120120181900