L. Papadimitriou

Performance of thin-film transistors on polysilicon films grown by low-pressure chemical vapor deposition at various pressures

Defect properties of undoped low-pressure chemical-vapor-deposited (LPCVD) polysilicon films have been investigated by capacitance techniques on a simple metal-oxide-semiconductor (MOS) capacitor structure. The results show that the effective density of bulk and interface trap states is almost independent of the deposition pressure. After reducing the polysilicon film thickness by etching, although the grain size decreases due to the columnar mode of growth at low pressures, the trap states density reduces significantly. This finding could be explained by the hypothesis that, during the growth of the material, impurities are segregated at the film surface by fast diffusion through the grain boundaries. The transport properties of 0.5- mu m-thick polysilicon films deposited at a pressure ranging from 100 to 0.5 mtorr were evaluated from measurements on thin-film transistors (TFTs). The results demonstrate that at high pressures the grain boundaries and at low pressures the polysilicon-SiO/sub 2/ interface roughness scattering are the main factors in determining the transistor performance. >

Heterojunction solar cells on cuprous oxide

Abstract Heterojunction solar cells were made on Cu2O by sputtering In2O3, SnO2, CdO and mixtures of CdO and SnO2 and by thermal evaporation of ZnSe. Photovoltaic effects were observed in most cases. High series resistances were an indication of chemical reactions at the interface. Auger surface analysis showed evidence of copper metal at the interface, arising from chemical reactions. For a CdO/Cu2O junction formed at room temperature, no copper metal was found at the interface. The CdO/Cu2O cells showed V oc = 0.4 V and I sc = 2 mA cm −2 . The properties appear to be dominated by a multiple-step tunneling at the junction, presumably due to dislocations and impurities in the Cu2O samples as prepared.

Trap centers in cuprous oxide

Abstract The effect of annealing temperature on the trap density in Cu 2 O is investigated. From space charge limited current (SCLC), Hall effect and DLTS measurements it is demonstrated that there is a trap-band distribution with the Fermi level lying within the trap-band. The maximum of this trap-band distribution is just above the Fermi level. A theoretical model is introduced to explain this trap-band distribution. Annealing of the samples in air at 400–500°C for 5 h gives the lowest total trap concentration and the lowest resistivity of the material. The above annealing decreases the total number of traps by a factor of 3 compared to the as grown samples. The resistivity decreases also by at least one order of magnitude.

Resistivity dependence of the minority carrier diffusion length in in single crystals of Cu2O

Mesure de la longueur de diffusion des porteurs minoritaires dans Cu 2 O type p pour des resistivites comprises entre 200 et 1500 ω. cm. La mesure est basee sur lillumination dune diode Schottky Cu/Cu 2 O et sur la mesure du photocourant en fonction du coefficient dabsorption

High-temperature phases of YBa2Cu3O6+x related to the superconducting transition

Influence du degre doxygenation sur la transition supraconductrice. Si la densite de lacunes doxygene est elevee, elles se condensent en boucles de dislocation qui empechent la phase quadratique de se transformer en phase orthorhombique. Si la densite de lacunes doxygene est faible, la phase haute temperature devient instable et se transforme en structure orthorhombique a 370°C, qui est susceptible de transition supraconductrice

Conduction in n+‐i‐n+ thin‐film polycrystalline/silicon devices in relation to the film deposition conditions

The electrical conduction in n+‐i‐n+ thin‐film polycrystalline/silicon devices, in relation to the deposition conditions of the low‐pressure chemically vapor deposited film, is investigated. Transmission electron microscopy study showed the following: (i) By decreasing the growth pressure from 180 to 80 mTorr, the mean grain size increases by a factor of 3. (ii) In the material grown at 180 mTorr, there is a mean dilatation for the d111 lattice plane amounting to 2.7%, while in the material grown at 80 mTorr, the systematic dilatation previously observed is absent. The current‐voltage characteristics show a linear behavior in a voltage region and a nonlinear behavior at higher voltages attributed to Joule heating within the sample. The energy gap of the material grown at 80 mTorr is 1.12 eV, while for the material grown at 180 mTorr, it decreases to 0.96 eV. The shrinkage of the energy gap could be due to the high density of ‘‘tail states’’ close to the conduction or valence band.

Electrical properties of polysilicon n+-i-p junctions

The current-voltage (I-V) characteristics, the frequency and temperature dependence of the capacitance and deep-level transient spectroscopy measurements in n+-i-p polysilicon junctions are investigated. All the polysilicon junctions underwent the same fabrication process except for the ion-implantation step. From the analysis of the experimental results information about the structure and defects of the undoped polysilicon material are obtained. At a low injection level of electrons the diode current is controlled by the distribution in energy grain boundary states and the structure of the deep centres change, after application of high (10 mA) injection level of electrons, to the single trap 0.182 eV.

Photovoltaic properties of sputtered n-CdO films on p-Cu2O

The heterojunction CdO/Cu2O was prepared by sputtering CdO films on Cu2O at room temperature. The optical transmission of CdO films at a thickness of ≈1000 A was ≈85%. The analysis of the I–V characteristics at different temperatures gave information of the carrier transport mechanism at the junction. The forward and the reverse dark current is dominated by a tunneling process. Measurements of the open-circuit photovoltage resulted in a curve which indicates a sensitivity range from 2.25 to 2.48 eV, similar to that found in Cu2O MIS solar cells.

Electrical and low frequency noise properties of Gd and GdCo silicide contacts on n-type Si

Gd and GdCo layers were evaporated onto n-type silicon and silicides were formed by in situ annealing at 400 and 700xa0°C. The electrical properties of the resulting Schottky diodes were investigated by current-voltage, capacitance-voltage and low frequency noise measurements. The Gd/Si contacts show an ohmic behaviour for both annealing temperatures, while the GdCo/Si contacts show a rectifying behaviour with a high (~0.65xa0eV) and a low (~0.52xa0eV) Schottky barrier height for the annealing temperatures of 400 and 700xa0°C, respectively. It was found that Co retards the Gd-Si reaction and reduces the density of the donor-type point defects generated within the silicon substrate during the Gd silicidation process.

Deep trap levels in cuprous oxide

Abstract The effect of the annealing temperature and doping with cadmium on the deep trap levels in Cu 2 O is considered here. From photoluminescence and C - V (admittance spectroscopy) measurements it is demonstrated that in the energy gap of the undoped material there are deep traps at the energy levels 0.72, 1.19 and 1.33 eV from the valence band edge. Annealing of the samples in air at 500°C for 5 h decreases the concentration of the traps 1.19 and 1.33 eV, by a factor of 50. The origin of these two trap levels is singly and doubly ionized oxygen vacancies which are partially filled with the annealing. Doping with Cd decreases the trap concentration of these two trap levels by a factor of 7 and annealing of the doped material at 500°C in air removes completely these two trap levels. However, in the Cd-doped material a new trap appears at the energy level ∼0.90 eV from the valence band edge.