Michael W. Dashiell

Room temperature operation of epitaxially grown Si/Si0.5Ge0.5/Si resonant interband tunneling diodes

Resonant interband tunneling diodes on silicon substrates are demonstrated using a Si/Si0.5Ge0.5/Si heterostructure grown by low temperature molecular beam epitaxy which utilized both a central intrinsic spacer and δ-doped injectors. A low substrate temperature of 370 °C was used during growth to ensure a high level of dopant incorporation. A B δ-doping spike lowered the barrier for holes to populate the quantum well at the valence band discontinuity, and an Sb δ-doping reduces the doping requirement of the n-type bulk Si by producing a deep n+ well. Samples studied from the as-grown wafers showed no evidence of negative differential resistance (NDR). The effect of postgrowth rapid thermal annealing temperature was studied on tunnel diode properties. Samples which underwent heat treatment at 700 and 800 °C for 1 min, in contrast, exhibited NDR behavior. The peak-to-valley current ratio (PVCR) and peak current density of the tunnel diodes were found to depend strongly on δ-doping placement and on the annea...

Quaternary InGaAsSb Thermophotovoltaic Diodes

InxGa1-xAsySb1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of nTPV = 19.7% and PD = 0.58 W/cm2, respectively, for a radiator temperature of Tradiator = 950 degC, diode temperature of Tdiode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are nTPV = 28% and PD = 0.85 W/cm2 at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (VOC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode VOC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that VOC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects

Current-voltage characteristics of high current density silicon Esaki diodes grown by molecular beam epitaxy and the influence of thermal annealing

We present the characteristics of uniformly doped silicon Esaki tunnel diodes grown by low temperature molecular beam epitaxy (T/sub growth/=275/spl deg/C) using in situ boron and phosphorus doping. The effects of ex situ thermal annealing are presented for temperatures between 640 and 800/spl deg/C. A maximum peak to valley current ratio (PVCR) of 1.47 was obtained at the optimum annealing temperature of 680/spl deg/C for 1 min. Peak and valley (excess) currents decreased more than two orders of magnitude as annealing temperatures and times were increased with rates empirically determined to have thermal activation energies of 2.2 and 2.4 eV respectively. The decrease in current density is attributed to widening of the tunneling barrier due to the diffusion of phosphorus and boron. A peak current density of 47 kA/cm/sup 2/ (PVCR=1.3) was achieved and is the highest reported current density for a Si-based Esaki diode (grown by either epitaxy or by alloying). The temperature dependence of the current voltage characteristics of a Si Esaki diode in the range from 4.2 to 325 K indicated that both the peak current and the excess current are dominated by quantum mechanical tunneling rather than by recombination. The temperature dependence of the peak and valley currents is due to the band gap dependence of the tunneling probability.

CARBON INCORPORATION IN SI1-YCY ALLOYS GROWN BY MOLECULAR BEAM EPITAXY USING A SINGLE SILICON-GRAPHITE SOURCE

Pseudomorphic Si1−yCy alloys on silicon (100) were grown by molecular beam epitaxy using a single effusion source of silicon contained in a graphite crucible, producing carbon concentrations of y=0.008. The behavior of carbon incorporation using this source was studied as a function of growth temperature using x-ray diffraction and infrared spectroscopy, and was compared to previous studies, where Si1−yCy was grown from separate silicon and graphite sources. An increased energy barrier for the surface diffusion of carbon was observed using the single silicon–graphite source. An infrared absorption mode near 725 cm−1, observed for growth temperatures up to 700 °C, was attributed to a transitional phase between the loss of substitutional carbon and the formation of silicon carbide precipitates.

THERMALLY OXIDIZED ALN THIN FILMS FOR DEVICE INSULATORS

The structural, optical, and electronic properties of an insulating material prepared by the thermal oxidation of AlN thin films on Si have been studied by a number of different experimental techniques. The thermal oxidation at 1100 °C of reactively sputtered AlN films on Si wafers was found to result in the formation of an oxide with a relative Al to O concentration near Al2O3 with small amounts of incorporated N. The structure of the AlO:N oxide could be varied between amorphous and polycrystalline, depending on the preparation conditions, and the oxide surface was found to be approximately three time smoother than the as-sputtered AlN films. Metal–oxide–silicon capacitors had an oxide charge density of about 1011 cm−2, capacitance–voltage characteristics similar to pure SiO2, and a dielectric constant of 12.4. Infrared measurements yielded a refractive index of 3.9. These results indicate that thermally oxidized AlN films show promise as insulating structures for many integrated circuit applications, p...

Influence of underlying interlevel dielectric films on extrusion formation in aluminum interconnects

Knowledge of the mechanical properties of interlevel dielectric films and their impact on submicron interconnect reliability is becoming more and more important as critical dimensions in ultralarge scale integrated circuits are scaled down. For example, lateral aluminum (Al) extrusions into spaces between metal lines, which become more of a concern as the pitches shrink, appear to depend partially on properties of SiO2 underlayers. In this article nanoindentation, wafer curvature, and infrared absorbance techniques have been used to study the mechanical properties of several common interlevel dielectric SiO2 films such as undoped silica glass using a silane (SiH4) precursor, undoped silica glass using a tetraethylorthosilicate precursor, phosphosilicate glass deposited by plasma-enhanced chemical vapor deposition and borophosphosilicate glass (BPSG) deposited by subatmosphere chemical vapor deposition. The elastic modulus E and hardness H of the as-deposited and densified SiO2 layers are measured by nanoi...

The effect of composition on the thermal stability of Si1−x−yGexCy/Si heterostructures

The thermal stability of molecular beam epitaxy grown Si1−x−yGexCy/Si heterostructures (0⩽x<0.30, y∼0.008) was studied using infrared absorption spectroscopy. The local vibrational mode of C in Si and Si1−x−yGex was used to quantify the loss of C atoms from substitutional sites with high temperature annealing. The activation energy (Ea=4.9 eV) for the loss of substitutional C achieved a maximum for the strain compensated alloy (x∼0.1). An additional increase of Ge content resulted in a rapid decrease in Ea, which was found to be 3.4 eV for x∼0.27. The nonmonotonic behavior of Ea on Ge content is explained by the effect of the interface strain between the epitaxial layer and Si substrate.

1.3 μm photoresponsivity in Si-based Ge1−xCx photodiodes

Ge1−xCx/Si heterostructure photodiodes with nominal carbon percentages (0⩽x⩽0.02), which exceed the solubility limit, were grown by solid source molecular beam epitaxy on n-type (100) Si substrates. The p-Ge1−xCx/n-Si photodiodes were fabricated and tested. The p-Ge1−xCx/n-Si junction exhibits diode rectification with a reverse saturation current of about 10 pA/μm2 at −1 V and high reverse breakdown voltage, up to −80 V. A significant reduction in diode reverse leakage current was observed by adding C to Ge, but these effects saturated with more C. Photoresponsivity was observed from these Si-based p-Ge1−xCx/n-Si photodiodes at a wavelength of ⩾1.3 μm, compatible with fiber optic wavelengths. External quantum efficiency of these thin surface-normal photodetectors was measured up to 2.2%, which decreased as the carbon percentage was increased.

Microwave properties of silicon junction tunnel diodes grown by molecular beam epitaxy

The bias dependence of the single-port microwave reflection gain of 15 /spl mu/m-diameter Si Esaki tunnel diodes, grown by molecular beam epitaxy, was studied as a function of frequency. A simple equivalent circuit accurately modeled the data and yielded the forward-bias junction capacitance, which cannot be obtained by conventional low frequency capacitance-voltage techniques. The diodes were highly-doped step p-i-n junctions and exhibited a peak current density of 16 kA/cm/sup 2/. The microwave reflection gain and cut-off frequency were 12 dB land 1.6 GHz, respectively, with a speed index (slew rate) of 7.1 V/ns.

Precipitation of β-SiC in Si1−yCy alloys

The infrared modes of annealed Si1−yCy alloys were studied experimentally and theoretically. The alloys were grown on Si(100) substrates by solid-source molecular beam epitaxy and were characterized by Fourier transform infrared spectroscopy. At annealing temperatures above 850 °C, the localized vibrational mode of substitutional C around 605 cm−1 diminished in intensity while another mode due to incoherent silicon carbide precipitates appeared at 810 cm−1. For lower processing temperatures, a peak around 725 cm−1 has been tentatively attributed to a C-rich phase, which is a precursor to SiC precipitation. Theoretical calculations based on the anharmonic Keating model predict that small (1 nm) 3C–SiC coherent precipitates may actually produce a mode at 725 cm−1. This mode occurs if the bonds gradually vary in length between the C-rich region and the host lattice. On the other hand, if the bonds are abruptly distorted at the edges of the precipitate, it becomes elastically isolated from the host lattice, a...