Isotopically Modified Semiconductor

Most elements present in conventional semiconductors consist of several stable isotopes (exceptions: Al, P, and I, which have only one stable isotope). Because of the availability of separated, nearly pure stable isotopes at affordable prices, a by-product of the fall of the Iron Curtain, many semiconductors can now be prepared with a tailormade isotopic composition which involves either pure isotopes or their mixtures in preestablished proportions. Many crystal preparation methods can be used, with the constraint that the amounts of separated isotopes required must fall within the available budget (isotopes cost more than €1000/gr). Budget strictures impose, in the case of condensed matter physicists, an upper limit of a few k€ for the procurement of a given isotope [1]. Astrophysicists [2] and high energy (e.g., neutrino) physicists [3] can allocate a few M€ to this endeavor. Technological applications may also justify large investments in separated isotopes [4]. Important advances in the physics of semiconductors have often resulted from the possibility of varying their physical properties over a wide range through external agents such as temperature, pressure or magnetic fields. In the past decade, many tetrahedral semiconductor crystals with different isotopic compositions have been grown. They add the average isotopic masses, as well as their fluctuations, as parameters to vary. Among the properties strongly affected by the mass fluctuations we mention the thermal conductivity [4,5]. The lattice parameters and their temperature dependence also vary weakly when changing the average isotopic mass [6]. As these examples show, the average isotopic masses affect directly and primarily the vibrational properties of crystals, including the phonon dispersion relations. Many of the extant isotopic crystals have been used to investigate in great detail such dispersion relations, in particular the effect on them of isotopic disorder [7] and anharmonicity [8]. The isotopic masses also influence electronic properties (e.g. those related to the electronic band structure) through the mechanism of electron-phonon interaction. The optical properties of isotopic crystals have yielded detailed information on electron-phonon interaction in semiconductors and led, in metals, to the discovery of the BCS mechanism of superconductivity. The principles and the most recent work on the dependence of electronic properties in semiconductors will be discussed in this talk which will include effects of average isotopic masses on electronic energy gaps [7] and also the recently discovered effect of isotopic mass disorder on the edge emission of silicon [9].