David Sands

Effect of deposition process on the thin‐film ZnS/p‐Si interface

Thin films of ZnS have been deposited on p‐Si by evaporation, radio‐frequency sputtering, and magnetron sputtering to form metal‐insulator‐semiconductor structures. The 1 MHz admittance‐voltage characteristics of each have been compared for a qualitative study of the ZnS/p‐Si interface. It is shown that radio‐frequency sputtering results in Fermi‐level pinning, which is ascribed to the presence of silicon dangling bonds caused by radiation damage. It is argued that this damage is advantageous for ZnS/p‐Si electroluminescent diodes.

The relative contributions of recombination and tunnelling at interface states to the a.c. conductance of metal-insulator-semiconductor diodes

Abstract The effect on the a.c. conductivity of MIS structures of localized states at the interface between the insulator and semiconductor has been described previously by either Shockley-Read recombination or tunnelling effects. Both of these models are examined here using MIS diodes fabricated from films of semi-insulating silicon doped with oxygen (SIPOS) on p -type Si. The tunnelling model as presently understood does not explain the frequency dispersion seen in the conductance but the inclusion of an extra Shockley-Read term corrects this. Nevertheless we conclude that, although the “extended” tunnelling model provides a better physical understanding of the a.c. conductance, it is more convenient for computational purposes to use the statistical model.

The contribution of bulk states to the ac conductance of metal‐insulator‐semiconductor diodes

Measurements of the density of interface states in metal‐insulator‐semiconductor diodes by conductance techniques have revealed two resonances in the equivalent parallel conductance. Interface states are responsible for one, but the other is due to bulk states in the silicon crossing the Fermi energy in the depletion region. This latter contribution is found to resonate at a fixed frequency independent of the bias applied to the diode, unlike the interface state response which varies in the normal way. From the maximum in the equivalent parallel conductance we have calculated the density of bulk states to be 1014 cm−3 and identified sodium as the likely impurity responsible.

The density of localized states at the semi‐insulating polycrystalline and single‐crystal silicon interface

The density of localized states at the interface between as‐deposited 51 at.u2009% semi‐insulating polycrystalline silicon and silicon has been measured using the conductance technique. Aluminum‐gated metal‐insulator‐semiconductor structures were made on n‐n+ and p‐p+ epilayered silicon and full details of capacitance‐voltage and conductance‐voltage characteristics are given. The interface state response is shown to fit the statistical model and comparisons with the Si‐SiO2 interface were made. The density of states is found to be between ∼6×1010 cm−2u2009eV−1 near midgap rising to ∼6×1011 cm−2u2009eV−1 at the band edges.

COMPOSITION AND STRUCTURE OF SEMI-INSULATING POLYCRYSTALLINE SILICON THIN-FILMS

Abstract The structural properties of semi-insulating polycrystalline silicon (SIPOS) are reviewed. The models derived from physical examinations have been used as the basis for physical models of electrical conduction, and two possibilities have emerged. Either the material is viewed as a continuous network of amorphous silicon with oxide regions interspersed or, vice versa, an oxide network with silicon regions interspersed. In order to establish which model is correct, we have examined SIPOS films containing between 7 and 51 at.% oxygen. Rutherford backscattering and X-ray photoemission are the main tools employed. Both as-deposited films, and films annealed such that changes in crystallinity are observed, have been examined and the following conclusions drawn: SIPOS containing less than about 30 at.% oxygen is mainly amorphous silicon with oxide islands and crystallization of the silicon is evident after annealing at 900°C; SIPOS containing more than 38 at.% oxygen is mainly amorphous silicon oxide co...

Conductance measurements on p‐Si/SiO2 metal‐oxide‐semiconductor capacitors

Conductance measurements have been performed on p‐Si/SiO2 metal‐oxide‐semiconductor capacitors fabricated by thermal oxidation of the silicon in three different reactors under different conditions in three different commercial systems. In only one case (system 3) did we find the very broad conductance curves usually associated with p‐Si/SiO2 junctions, the others from systems 1 and 2 exhibiting normal interface state response. However, in the sample from system 2, the response of bulk states inside the depletion region was found to distort the Gp/ω spectra, while the sample from system 1 showed no such response until annealing in H2/Ar gas at 500u2009°C reduced the interface state density to such a level that the bulk state responses were visible. We argue that the presence of bulk states probably explains the very broad Gp/ω curves observed by us, and further propose that some such mechanism accounts for the historical difficulties encountered with the conductance technique on p‐Si/SiO2 capacitors.

Electronic states at the interface between thin films of ZnS and crystalline p‐silicon

Thin films of zinc sulfide have been evaporated onto p−‐p+ silicon substrates and metal‐insulator‐semiconductor diodes fabricated. Capacitance‐voltage characteristics have been analyzed qualitatively and quantitative measurement of the density of interface states has been made using the conductance technique developed by Nicollian and Goetzberger [Bell Syst. Tech. J. 46, 1055 (1967)]. Similarities between this interface and that between sputtered zinc sulfide are noted. A model is developed to explain the high densities of interface states observed (≥1012 cm−2u2009eV−1) and the suitability of these films for use in electroluminescent devices is discussed.

Growth of wide band gap polycrystalline semi‐insulating polycrystalline silicon

Semi‐insulating polycrystalline silicon (SIPOS) films have been grown by low‐pressure chemical vapor deposition at 590u2009°C rather than the conventional 650u2009°C. Three compositions have been achieved; 6.1 at.u2009% of oxygen, 17.7 at.u2009% oxygen, and 26.4 at.u2009% oxygen with 15.3 at.u2009% hydrogen. Films of the first two compositions are amorphous as grown and have properties very similar to known properties of SIPOS. In the last composition, wherein hydrogen is incorporated, the film is polycrystalline as grown with an optical band gap of 2.1 eV, and it has some properties similar to more conventional SIPOS containing 51 at.u2009% oxygen—49 at.u2009% silicon which has been annealed at 900u2009°C. Upon implantation with a dose of 2×1016 P atoms/cm2 followed by annealing at 1150u2009°C, conductivities of 5×102 (Ωu2009cm)−1 are possible.

Heterojunctions of radio-frequency-sputtered thin films of ZnS and p-Si

Metal/zinc sulphide/p-silicon diodes have been fabricated by radio-frequency sputtering in argon of sintered ZnS targets onto silicon substrates in a stainless steel chamber. The films are found to be non-stoichiometric with zinc in excess by up to 6%. The silicon surface, however, is rich in sulphur and both zinc and sulphur diffuse into the substrate during annealing giving rise to trapping states inside the silicon depletion region. The interface itself is dominated by a discrete level of states near the mid-gap which pin the Fermi level such that the silicon surface is inverted. These are believed to arise from dangling silicon bonds created during the sputtering process.

Changes in the SIPOS/Si interface induced by annealing at 900°C

Abstract The interface regions at heterojunctions between thin films of SIPOS (semi-insulating polycrystalline silicon doped with 51 atomic% oxygen) and both n- and p-Si substrates have been examined by capacitance-voltage, conductance-voltage and d.c. current-voltage techniques. Following annealing at 900°C deep depletion is observed in both n- and p-type diodes, although absent in unannealed films of SIPOS. The occurence of deep depletion is shown to arise from an increase in the electrical conductivity of the SIPOS films created by annealing. Annealing also increases the density of interface states at the Si/SIPOS heterojunction. Also observed are discrete states originating at the interface between SIPOS and n-Si; these states being exhibited as a stretch-out of the capacitance-voltage characteristics. In conclusion evidence from these and related optical measurements indicate the barrier height between Si and annealed films of SIPOS is of the order of a few kT. Concomitantly the barrier height between thin Al films and SIPOS is of the oder of 0.8 eV. Consequently annealing of SIPOS films is essential for their operation in heterojunction-bipolar transistors.