J. Kolodzey

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.

Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films

The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

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...

Growth of germanium-carbon alloys on silicon substrates by molecular beam epitaxy

Metastable Ge1−yCy alloys were grown by molecular beam epitaxy as homogeneous solid solutions having a diamond lattice structure. The substrates were (100) oriented Si wafers and the growth temperature was 600 °C. We report on measurements of the composition, structure, lattice constant, and optical absorption of the alloy layers. In thick relaxed layers, C atomic fractions up to 0.03 were obtained with a corresponding band gap of 0.875 eV. These alloys offer new opportunities for fundamental studies, and for the development of silicon‐based heterostructure devices.

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 0.57 eV, about 50 meV wide, increasing in intensity with applied pulsed current, and with reducing device temperatures. The total integrated emitted power from a single edge facet was 54 μW at an applied peak current of 100 mA at 100 K. These results suggest that GeSn-based materials maybe useful for practical light emitting diodes operating in the infrared wavelength range near 2 μm.

Thermal conductivity reduction in GaAs-AlAs distributed Bragg reflectors

Self-heating of vertical-cavity laser diodes is strongly affected by the thermal conductivity of the distributed Bragg reflectors (DBRs). Binary GaAs-AlAs DBRs are expected to be good heat conductors, but investigations of GaAs-AlAs superlattices indicate strong interface scattering of phonons. For the first time, we present direct thermal conductivity measurements of separated GaAs-AlAs DBRs with a quarter-wave layer thickness of more than 100 nm (tuned to 1.55-/spl mu/m wavelength). Using an ac calorimetric method and finite element analysis, we measure about 50% thermal conductivity reduction compared to the average bulk value. GaAs-AlAs DBRs for shorter wavelengths are expected to show an even lower thermal conductivity.

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.

Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy

Single crystal epitaxial Ge1−xSnx alloys with atomic fractions of tin up to x = 0.145 were grown by solid source molecular beam epitaxy on Ge (001) substrates. The Ge1−xSnx alloys formed high quality, coherent, strained layers at growth temperatures below 250 °C, as shown by high resolution X-ray diffraction. The amount of Sn that was on lattice sites, as determined by Rutherford backscattering spectrometry channeling, was found to be above 90% substitutional in all alloys. The degree of strain and the dependence of the effective unstrained bulk lattice constant of Ge1−xSnx alloys versus the composition of Sn have been determined.

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.