CARNEGIE MELLON Infrared Diodes Using Sil-xGexFilms Grown by Ultra High Vacuum Chemical Vapor Deposition

Two types of diodes designed for infrared applications were fabricated from SiGe/Si films grown using ultra high vacuum chemical vapor deposition. The first diode described is a PIN type design, where the I region is comprised of undoped Sil_xGex/Si multiple quantum wells. Photoluminescence and defect etching were used to assess the quality of the quantum well films, and electroluminescence and photoconductivity measurements performed to evaluate the emission and detection properties of the diodes in the near infrared. The second diode makes use of the heterojunction internal photoemission principle for detection of IR radiation in the 3-10um range. The active region is a heavily doped p type SiGe layer, and the barrier for photoemission is the valence band splitting between Si and strained SiGe. Several experiments are described, with one experiment yielding devices for which photoresponse was measured. Introduction The growth of strained SixGel-x films on Si substrates has led to considerable research on the application of band gap engineering concepts using silicon based devices.. The SiGe/Si semiconductor system may prove commercially interesting by allowing the integration of more novel structures with standard silicon technology. In this paper I will discuss two types of diodes designed for infrared applications fabricated from SiGe/Si films grown using the ultra high vacuum chemical vapor deposition (UHVCVD) system at CMU. The general properties of the SiGe/Si system and the basic principles of growth of SiGe by UHVCVD are briefly introduced. The first diode described is a PIN type design, which was evaluated both as an emitter and a detector of IR radiation in the 1-1.5um range. The second diode makes use of the heterojunction internal photoemission (HIP) principle for detection of IR radiation in the 3-10um range. Background Because of the lattice mismatch between Ge and Si, there is a limit to the thickness (known as the critical thickness tc) to which strained SiGe layers can be grown. Figure l a shows critical thickness as a function of germanium fraction or lattice mismatch [1]. By using a low temperature growth technique such as UHVCVD, it is possible to achieve thicknesses beyond the equilibrium strain limit without significant formation of misfit dislocations. This metastable strain does, however, place a thermal restraint on subsequent processing steps. Figure lb shows the minimum band gap as a function of Ge fraction for various strain configurations [1]. The bottom curve describing strained SiGe on unstrained Si (100) is relevant to the diodes presented in this report. Solid source MBE, gas source MBE, and a variety of CVD techniques have been used to grow SiGe films. The UHVCVD growth system at CMU uses silane and germane as the reaction gasses for SiGe layers. By varying the flow ratio of germane to silane, alloys of various concentrations can be obtained. Only p type dopant was required for growth of the diodes in this study; diborane was the gas source used for boron doping. The details of the growth system are described in [2]. All growths in this study were performed at temperatures equal to 600 degrees C and pressures of roughly 1 mT. PIN Diode PIN diodes were fabricated from GexSil-x/Si multiple quantum wells of various germanium fractions and well thicknesses grown on n type silicon substrates. Figure 2a shows a cross section of the layers involved. A 300 angstrom silicon capping layer with boron concentration of 3 x 1018 comprises the p region; the quantum wells and buffer, which were not intentionally doped, make up the i region; the substrate (0.1-2 ohm cm) is the n region. Theory To understand how these devices work, I will first discuss a single quantum well with the assistance of figure 2b. The band alignment for strained SiGe on Si is type 1, with the difference in bandgap split approximately 90% in the valence band and 10% in the conduction band. This band alignment establishes a potential well in the valence band, and we can use the simple 1-D square well model to locate the first bound state. Since the well in the conduction band is small in comparison, we choose to ignore any quantum confinement effects in the conduction band. The energy difference between the bottom of the SiGe conduction band and the first bound state for holes is labeled E0. The no phonon energy Enp is equal to: Enp = E0 Eex Ebw where Eex represents the exciton binding energy and Ebw represents the energy binding the exciton to a center within the well. When electron and hole pairs are created within the well --for example, by bombarding the semiconductor with a laser (photoluminescence) or injecting a current of electrons and holes (electroluminescence)-these carriers recombine to produce infrared radiation at Enp. Since germanium and silicon are both indirect semiconductors, phonon assisted recombination of electrons and holes also occurs which produces IR radiation at energies slightly lower than Enp. During absorption, incident infrared photons create electron hole pairs within the semiconductor. To first order, the minimum detectable photon energy is Enp. By varying Ge fraction and well thickness appropriately, it is possible to tailor the value of Enp for specific applications. In order to achieve measurable emission and absorption, it is necessary to combine the effects of multiple quantum wells. To ensure that this quantum well stack remains strained, both the thickness and germanium fraction of an individual well, as well as the thickness and average germanium fraction of the entire stack, must not exceed the metastable strain limits given in figure 1. Figures 3a and 3b use band diagrams to depict the processes which occur in the PIN diodes to emit and detect IR radiation. In electroluminescence, the diodes are forward biased, and the injected holes and electrons recombine to emit light. In photodetection, incident photons generate electron/hole pairs, which are separated by the built in field. By shorting together the P and N regions, a current flows in the circuit which is proportional to the number of photons absorbed by the semiconductor.