Physics of ultra-pure germanium

Abstract

Abstract A broad review of the physics of point defects, i.e. electrically active and neutral impurities and impurity complexes is given. Basic material science which is crucial for understanding the physics is summarized in an introductory section. It is followed by the detailed description of the novel measurement techniques—photothermal ionization spectroscopy, high-Q electron paramagnetic resonance and deep level spectroscopy. These spectroscopic techniques have had a profound impact on the investigations of ultra-pure germanium. The major part of this work deals with the physics of point defects in ultra-pure germanium. The high purity of this semiconductor allows the undisturbed observation of highly excited bound states of shallow donors and acceptors. Many new previously unknown acceptor and donor centres have been discovered in ultra-pure germanium. The nature of several of these centres can be understood in terms of a complex consisting of two impurities—a light interstitial atom (for example, hydrogen or lithium) tunnelling between four identical real space positions in the vicinity of a substitutional impurity (for example, carbon, silicon, oxygen or copper), or a defect (for example, divacancy). The motion of the tunnelling impurity is shown to influence the electronic structure of the complex leading to a symmetry of higher order than tetrahedral. Experimental results are presented which are fully consistent with the tunnelling model. Besides the review of recently published results, new findings on the solubility of substitional copper in ultra-pure crystals grown under a variety of conditions and on the two donors lithium D(Li) and lithium-oxygen D(Li, O) are presented.