High resolution minority carrier transient spectroscopy of Si/SiGe/Si quantum wells

Abstract

We have developed a high resolution technique for examining minority carrier emission from defect states in semiconductors called Laplace minority carrier transient spectroscopy (LMCTS). The experiment uses the same analytical approach to the capacitance transient as Laplace deep level transient spectroscopy (LDLTS), but minority carriers are injected into the depletion region by application of a suitable light pulse. The combination of LDLTS and LMCTS means that detailed emission properties of closely spaced majority and minority carrier traps across the whole band gap can be now characterized. The technique has been used to study minority carrier traps in gas source molecular beam epitaxy-grown Si/Si0.86Ge0.14 strained quantum wells. Initially the technique was evaluated by comparing LMCTS of a hole trap associated with the gold–hydrogen complex in n-type silicon with LDLTS of the same trap in p-type silicon. Both techniques confirm that this level consists of two states, as previously suggested in the literature. LMCTS was then applied to an n-type multiquantum well Si/SiGe layer. We have been able to measure directly the emission rate of holes out of SiGe quantum wells using LMCTS. The emission rate exhibited only slight temperature dependence, in strong contrast to that of holes which are thermally emitted from isolated point defects. We show that in the particular case of LMCTS, a temperature invariant emission rate out of quantum wells is to be expected, and this is consistent with theoretical predictions.

Laplace-transform deep-level spectroscopy: The technique and its applications to the study of point defects in semiconductors

“…49 If the light has a photon energy near the band gap of the semiconductor being studied (hence the extinction depth is long) and the diffusion length is long, the carrier flux through the depletion region will consist almost entirely of minority carriers and so the analysis of the results becomes relatively simple and is known as minority carrier transient spectroscopy (MCTS). 50 These techniques have been used in Laplace DLTS to study minority carrier capture by gold and gold-hydrogen defects, 51,52 and also shallow electron traps in p-type SiGe and trapping in silicon Si/ SiGe/ Si quantum wells. 52 In this work the layers studied were grown by gas source molecular-beam epitaxy (MBE) and consisted of ten strained Si 0.855 Ge 0.145 / Si quantum wells grown on a Si substrate, with a Si buffer 100 nm thick.…”

“…50 These techniques have been used in Laplace DLTS to study minority carrier capture by gold and gold-hydrogen defects, 51,52 and also shallow electron traps in p-type SiGe and trapping in silicon Si/ SiGe/ Si quantum wells. 52 In this work the layers studied were grown by gas source molecular-beam epitaxy (MBE) and consisted of ten strained Si 0.855 Ge 0.145 / Si quantum wells grown on a Si substrate, with a Si buffer 100 nm thick. The well thickness was 5.7 nm, and the barrier thickness was 55 nm.…”

“…It can be seen that the emission rate is only slightly temperature dependent, a feature which is attributed to the tunneling component. 52 Such data are very difficult to extract from conventional DLTS because of the proximity in emission of other defects in typical SiGe structures.…”