T. N. Adam

Electrical conduction and dielectric breakdown in aluminum oxide insulators on silicon

Leakage currents and dielectric breakdown were studied in MIS capacitors of metal-aluminum oxide-silicon. The aluminum oxide was produced by thermally oxidizing AlN at 800-1160/spl deg/C under dry O/sub 2/ conditions. The AlN films were deposited by RF magnetron sputtering on p-type Si (100) substrates. Thermal oxidation produced Al/sub 2/O/sub 3/ with a thickness and structure that depended on the process time and temperature. The MIS capacitors exhibited the charge regimes of accumulation, depletion, and inversion on the Si semiconductor surface. The best electrical properties were obtained when all of the AlN was fully oxidized to Al/sub 2/O/sub 3/ with no residual AlN. The MIS flatband voltage was near 0 V, the net oxide trapped charge density, Q/sub 0x/, was less than 10/sup 11/ cm/sup -2/, and the interface trap density, D/sub it/, was less than 10/sup 11/ cm/sup -2/ eV/sup -1/, At an oxide electric field of 0.3 MV/cm, the leakage current density was less than 10/sup -7/ A cm/sup -2/, with a resistivity greater than 10/sup 12/ /spl Omega/-cm. The critical field for dielectric breakdown ranged from 4 to 5 MV/cm. The temperature dependence of the current versus electric field indicated that the conduction mechanism was Frenkel-Poole emission, which has the property that higher temperatures reduce the current. This may be important for the reliability of circuits operating under extreme conditions. The dielectric constant ranged from 3 to 9. The excellent electronic quality of aluminum oxide may be attractive for field effect transistor applications.

Electroluminescence at 7 terahertz from phosphorus donors in silicon

Terahertz (THz) emissions corresponding to intracenter transitions of phosphorus impurities in silicon have been observed up to 30K. Electrical pulses (250ns) with a repetition rate of 413Hz were used for excitation, and the peak power was calculated to be ∼20μW∕facet for a 190×120μm2 device with a peak pumping current of 400mA at 12K. THz emission intensity increased linearly with pumping current and quenched when the sample temperature was above 30K. The current–voltage characteristics suggested a conduction and excitation mechanism by injection of electrons from a Schottky barrier followed by impact ionization of the neutral impurities.

Infrared electroluminescence from GeSn heterojunction diodes grown by molecular beam epitaxy

Infrared electroluminescence was observed from GeSn/Ge p-n heterojunction diodes with 8% Sn, grown by molecular beam epitaxy. The GeSn layers were boron doped, compressively strained, and pseudomorphic on Ge substrates. Spectral measurements indicated an emission peak at 0.57u2009eV, about 50u2009meV wide, increasing in intensity with applied pulsed current, and with reducing device temperatures. The total integrated emitted power from a single edge facet was 54u2009μW at an applied peak current of 100u2009mA at 100u2009K. These results suggest that GeSn-based materials maybe useful for practical light emitting diodes operating in the infrared wavelength range near 2u2009μm.

The electrical properties of MIS capacitors with ALN gate dielectrics

Abstract We report on the characteristics of metal–insulator–semiconductor (MIS) capacitors with aluminum nitride (AlN) as the dielectric material. Using reactive magnetron sputtering, we deposited layers of AlN on 1–10xa0Ωxa0cm p-type (1xa00xa00) silicon wafers. The deposition rates were investigated as a function of sputter pressure, power, gas composition, and substrate temperature. On films deposited over a range of sputter parameters, X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) were performed indicating that optimal deposition conditions for best crystal quality and stoichiometry were a total pressure between 4 and 10xa0mT, a gas mixture of 85% nitrogen and 15% argon, and a substrate temperature ≈200°C. The films had a weak microcrystalline structure with the c -axis preferentially orientated parallel to the substrate normal. MIS capacitors were fabricated on silicon substrates with Ti/Au contacts. Current–voltage (IV) and capacitance–voltage (CV) measurements revealed breakdown fields of 4–12xa0MV/cm. Depending on the thickness, leakage current densities were between 10 −10 and 10 −3 xa0A/cm 2 at 1xa0V reverse bias, the interface charge density was ≤10 13 xa0cm 2 , and flat band voltages were from −10 to 2xa0V. The dielectric permittivity was between 4 and 11 for thick layers (≥100xa0A) and decreased to values between 2 and 6 for thicknesses below 100xa0A.

Terahertz electroluminescence from boron-doped silicon devices

Terahertz emission was observed from electrically pumped boron-doped p-type silicon structures at cryogenic temperatures. At a current of 1.5 A and temperature of 4.4 K, we achieved a pulsed peak power of 31 μW from a single mesa facet, integrated over three closely spaced spectral lines centered about 8.1 THz. The radiation was slightly transverse magnetically polarized with respect to the plane of the substrate and was still detectable at temperatures as high as 150 K. These findings suggest that moderate power THz sources can be fabricated without epitaxially grown quantum wells using techniques compatible with silicon integrated circuit technology.

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.

The design and fabrication of microdisk resonators for terahertz frequency operation

The design and fabrication of resonators and waveguides, operating at THz frequencies are reported. Resonance frequencies, mode confinement, quality factors, and stop-bands were calculated for resonators with and without photonic elements. The estimations show that very narrow modes can exist within the propagation bandgap of a photonic lattice. Microdisk devices were designed and fabricated for high-quality whispering-gallery modes centered around 10 THz. Combined with silicon-germanium quantum wells grown by molecular beam epitaxy, these resonators are promising candidates for silicon-based miniature far-infrared lasers.

Characteristics of THz waves and carrier scattering in boron-doped epitaxial Si and Si1−xGex films

The absorption and reflection characteristics of boron-doped silicon and silicon-germanium alloys have been investigated in the frequency range from 1.6 to 60 THz. The absorption increases with doping concentration, in agreement with free carrier effects, but saturates for wavelengths longer than about 20 μm. As compared to silicon, the attenuation increases with the Ge fraction in the alloy. Terahertz reflectance data has been analyzed to study the doping dependent plasma-edge frequency, which may play an important role for the design of emitters, detectors, and plasmon waveguides. The best fitting of the experimental data with Drude theory has been used to extract the hole scattering relaxation time in doped silicon. The results have been utilized to explain the doping-dependent attenuation characteristics of the THz radiation.

Fabrication of high-fill-factor photonic crystal devices on silicon-on-insulator substrates

Optimization of the photonic bandgap in finite-height photonic crystal (PhC) slab structures requires high-fill-factor lattices. We present a method for fabrication of high-fill-factor PhC devices in silicon-on-insulator (SOI) substrates using electron-beam lithography and high-aspect-ratio reactive-ion etching (RIE). We achieve 8:1 aspect-ratio PhC structures with 60-nm vertical membrane walls using a custom deep reactive-ion etching process in a conventional low-end RIE with patterned resist as the only etch mask. We present examples of various PhC devices fabricated using this method including a high-efficiency coupling structure for PhC waveguides.

Hot hole redistribution in impurity states of boron-doped silicon terahertz emitters

The relative intensities of emission peaks from boron-doped silicon terahertz sources have been measured under various pumping conditions. These data have been analyzed to determine the hole occupations in the excited states. As the pumping current increased, the hole concentrations increased approximately linearly. The hole population increased faster in the lower energy 1Γ8− state than in other excited states. At a fixed pumping current, the hole population decreased as temperature increased, but the decrease was slower for the 1Γ8− lower-energy state. These results suggest that to achieve terahertz emission at high temperatures it would be best to use dopants with transitions that have a strong oscillator strength from the lowest-energy excited state.