Silicon Doping Superlattices Research Articles

Evidence for terahertz acoustic phonon parametric oscillator based on acousto-optic degenerate four-wave mixing in a silicon doping superlattice

We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength grating was used to couple the THz laser radiation into the DSL. Superconducting granular aluminum bolometric detection, coupled with Si:B piezophonon spectroscopy, revealed excitation of THz coherent compressional and shear waves, along the <111> direction only. The Bragg scattering condition for distributed feedback, and the energy conservation requirement for the D4WM process, were both verified. A THz MAPPO could provide a testbed for studies of non-classical acoustic phonon fields.

Evidence of Longitudinal Acoustic Phonon Generation in Si Doping Superlattices by Ge Prism-Coupled THz Laser Radiation

We report on the direct excitation of 246 GHz longitudinal acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation of the same frequency. A longitudinally polarized evanescent laser light field is coupled to the superlattice through a germanium prism providing total internal reflection at the superlattice interface. The ballistic phonon signal is detected by a superconducting aluminum bolometer. The sample is immersed in low-temperature liquid helium.

Superlattice ultrasonic generation

We report the first experimental evidence for the resonant excitation of coherent high-frequency acoustic phonons in semiconducting doping superstructures by far-infrared laser radiation. After a grating-coupled delta-doped silicon doping superlattice is illuminated with ~1 kW/mm2 nanosecond-pulsed 246 GHz laser radiation, a delayed nanosecond pulse is detected by a superconducting bolometer at a time corresponding to the appropriate time-of-flight for ballistic longitudinal acoustic phonons across the (100) silicon substrate. The absorbed phonon power density in the microbolometer is observed to be ~10 μW/mm2, in agreement with theory. The phonon pulse duration also matches the laser pulse duration. The absence of any delayed transverse acoustic phonon signal by the superconducting bolometer is particularly striking and implies there is little or no incoherent phonon generation occurring in the process.

Progress on coherent generation of terahertz acoustic phonons by resonant absorption of nanosecond-pulsed far-infrared laser radiation in silicon doping superlattices

We report progress on experiments to generate and detect coherent terahertz acoustic phonons. Coherent phonons are generated1 in a silicon-doping superlattice by the absorption of grating-coupled high peak-power cavity-dumped far-infared laser radiation2. The superlattice period is chosen to match the phonon wavelength at the excitation frequency of the laser radiation. The phonons propagate across the Si:B substrate and are detected by a novel superconducting granular aluminium/palladium bilayer microbolometer3 with sub-nanosecond resolution. The phonon spectrum is obtained by piezo-phonon spectroscopy4 via the boron impurities in the silicon substrate. (1) P. Ruden and G.H. Dohler, "Anistropy Effects and Optical Excitation of Acoustic Phonons in n-i-p-i Doping Superlattices", Solid State Commun. 45 (1), 23 (1983). (2) T.E. Wilson, "A High-Power NH3 Laser Pumped in a Three-Mirror CO2 Laser Cavity with Optically-Switched Cavity-Dumping", International Journal of Infrared and Millimeter Waves 14 (2), 303 (1993). (3) T E Wilson, "Fabrication and characterization of granular aluminum/palladium bilayer microbolometer", Meas. Sci. Technol. 18 N53-N59 (2007). (4) S. Roshko and W. Dietsche, "Phonon Spectroscopy in High Magnetic Fields: The B+ Center in Si", Solid State Comm. 98(5), 453 (1996).

Ion mass influence on transient enhanced diffusion and boron clustering in silicon: Deviation from the “+1” model

Boron in silicon doping superlattices is used to trace native point defect behavior during a 790 °C, 15 min anneal following a 200 keV, 1×1013/cm2 Pb+ or 40 keV 1×1013/cm2 Si+ implant. These nonamorphizing implants lead to transient enhanced diffusion and clustering of the boron doping spikes. The enhancement in B diffusion scales sublinearly with mass of the ion implant. Clustering of the boron occurs deeper and more extensively in the Pb+-implanted sample due to greater mass of the ion. Measurement of the number of interstitials bound by extended defects after an 800 °C/10 s rapid thermal anneal confirm that the Pb+ implant has “+4.5” of the implant dose bound by extended defects, compared to “+0.6” in the Si+ implant for the same anneal. Both of these results indicate that the “+1” model is not valid for heavy mass ion implants.

Investigation of mechanisms of vacancy generation in silicon in the presence of a TiSi2 film

We have determined the perturbance in the silicon vacancy concentration induced by the presence of TiSi2 films. Antimony in silicon doping superlattices was employed as a vacancy detector. Under all conditions studied (deposited titanium thickness 4–312 nm, 800–850 °C, 15–600 min), we always observe a relative vacancy supersaturation on the order of 1.5. Two mechanisms of vacancy injection during titanium silicidation were studied: (1) stress compensation; by varying the thickness of the deposited films and annealing for 60 min at 850 °C, a range of stresses was induced in the substrate via the coefficient of thermal expansion mismatch between the film and substrate. The observed vacancy supersaturation was independent of film thickness, indicating that stress compensation is not a mechanism of vacancy generation for titanium disilicide; (2) volume contraction; annealing for 15, 60, and 600 min at 800 °C after identical 30-nm-thick titanium films were deposited allowed the time variation of the vacancy supersaturation to be studied. While the vacancy supersaturation decayed slightly with time, its time dependence is incompatible with a large “pulse” of vacancies injected during the silicidation reaction. This indicates that volume contraction at the growing film interface is not a mechanism for vacancy generation. The thicker TiSi2 films (&amp;gt;22 nm) and those annealed for ⩽60 min were continuous in their coverage of the substrate as observed by transmission electron microscopy, while the thinner films and those annealed for longer times had islanded. However, there was no relationship between film coverage and vacancy behavior in the substrate, or was there any relation between deposition method (evaporation versus sputtering) and vacancy behavior.

A novel amorphous silicon doping superlattice device with double switching characteristics for multiple-valued logic applications

An amorphous silicon doping superlattice device with different period lengths is developed, providing a double switching characteristic that has been demonstrated to yield multiple stable states for multiple-valued logic applications. Unlike those of conventional switching devices, the switching characteristics of the present device are caused by avalanche multiplication and a barrier-lowering effect. A tristate memory cell using this device is proposed and discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Novel amorphous silicon doping superlattice device with bidirectional S-shaped negative differential characteristics

A novel amorphous silicon doping superlattice device with bidirectional S-shaped negative-differential characteristics has been fabricated and investigated experimentally. The occurrence of the S-shaped switching phenomenon is caused by the potential redistribution due to the avalanche multiplication process. The experimental results are demonstrated and the electronic transport theory is proposed in this letter.

Small signal ac analysis of a silicon doping superlattice under reverse bias

A simple linear electrical circuit model is developed to study the ac characteristics of a doping superlattice with selective contacts. Small signal ac measurements at different frequencies were carried out on silicon doping superlattices under reverse bias. Parameters such as capacitance, doping profile, ac conductance, and leakage resistance of a doping superlattice calculated from the model are shown to agree well with the experiment.

Theoretical calculation of the properties of two-dimensional holes in doping superlattices

Two-dimensional properties, in particular, the density-of-states masses of holes in silicon and germanium doping superlattices, are determined by using self-consistent calculations and the effective mass approximation. Physical parameters of the doping superlattice such as the doping level of the n-type layers and the thickness and doping level of the p-type layers are varied to determine their effects on the hole mass characteristics as a function of hole concentration. Very high doping levels of the p-type layers of the superlattice are used to avoid the problem of the freezing out of the hole carriers at absolute zero temperature. It is determined that the doping level of the p-type layers is the leading parameter that affects the hole mass characteristics.

Thermally stimulated conductivity in a-Si:H doping superlattices

Hydrogenated amorphous silicon doping superlattices are utilized to obtain a high metastable concentration of spatially separated electrons and holes trapped in their respective band tails. Thermally stimulated conductivity (TSC) experiments show that the thermal energy required to excite carriers to the transport path decreases with increasing concentration of metastable carriers (N meta). This is in contrast to IR-photoconductivity data which show a fixed threshold for the excitation of band tail carriers to the mobility edge which does not depend on N meta. A model is proposed in which the differences between thermal and optical excitation energies are due to potential fluctuations created by the trapped carriers themselves.

Excess carrier lifetimes in the silicon doping superlattice

The excess carrier lifetimes in a silicon doping superlattice were investigated by measuring the decay of both the photovoltage and the photoconductance. The photovoltage decayed exponentially with a time constant of 1 s at liquid-nitrogen temperature. In addition, persistent photoconductivity extending over many hours was observed in the n-type layers.

Selective contacts for silicon-doping superlattices

Results are presented that show that selective n- and p-type contacts with low resistance have been formed on silicon-doping superlattices. The superlattices were grown by molecular-beam epitaxy through a silicon mask to create built-in blocking regions at the edges of the mesas. Contacts of Mg2Si for the n-side and PtSi for the p-side were formed along the edges of the mesa, resulting in a successful diode-like behavior. A surface etch to remove doping impurities from the region surrounding the superlattice mesa was required to maintain contact–substrate isolation.

Measurement and mechanism of free carrier recombination in a silicon doping superlattice

The carrier lifetime of a silicon doping superlattice can be estimated by taking the product of the junction capacitance and the effective small signal resistance of the nipi structure. Carrier lifetimes as long as 1.2 s have been observed. Traps do no influence the carrier lifetimes.

Enhancement of carrier lifetime in silicon-doping superlattices

Silicon-doping superlattices have been grown by molecular beam epitaxy by using a Knudsen cell with boron oxide for p doping and an ion beam doper with arsenic for n doping. Depositions were done through an etched silicon mask to create nini and pipi regions at the edges of mesas to which selective contacts were made with the use of AuSb and PtSi. Recombination lifetimes as long as 14 ms have been measured at 77 K by carrier injection from 503 nm light pulses.

Optical absorption in silicon doping superlattices

The optical absorption beyond the fundamental edge in moderate to large period silicon doping superlattices of various designs is studied theoretically. At 1.3 μm, the largest absorption coefficient found is 0.2 cm −1. Only moderate external biases are required to reach maximum absorption.

Exchange-correlation energy in the subbands of silicon doping superlattices

The energy subbands in three types (pn +p, pnpn, nipi) of Si and GaAs doping superlattices are calculated self-consistently including the exchange-correlation energy given by the density functional method. The results show that the exchange-correlation term is more important in Si than in GaAs in all three cases. For the same doping levels, layer thicknesses and electron concentrations, the shift in the lowest subband energy from the value given by the Hartree Approximation is 20–50% greater in Si than in GaAs.

Defect formation in amorphous silicon doping superlattice films

Doping “superlattice” amorphous silicon (a-Si:H) films have been studied using junction capacitance-voltage (C-V) and light induced electron spin resonance (LESR) measurements at a series of temperatures. Our samples vary between n-type doped and intrinsic material (nini…) on a distance scale of a few hundred Angstroms. By numerically modeling the C-V results of these measurements we can deduce the densities of both shallow and deep states in each of the constituent layers. We conclude that the deep defect density in such intrinsic regions is much greater than that in bulk intrinsic material. Further, we find evidence for distinct interface states at the doping boundaries with trapping energies which suggest they are dangling bond defects.

Tuning the effective bandgap of a silicon doping superlattice

In this paper, we examine the tunability of the effective bandgap and carrier concentration in a silicon-doping superlattice with narrow n+-doped layers and wide p-doped layers. Quantitative tunability parameters are defined and their variation with dopant concentration and temperature are calculated self-consistently in the effective mass and Hartree approximations. It is found that the tunabilities increase as doping is increased or as temperature is decreased.