R. T. Troeger

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.

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.

Terahertz emission from electrically pumped gallium doped silicon devices

Current pumped terahertz (THz) emitting devices have been fabricated from gallium doped silicon. The time resolved peak power was 12μW per facet at a peak pumping current of 400mA, and the emission was observed up to temperatures near 30K. The spectra occurred in two distinct series at 7.9–8.5THz, and at 13.2–13.8THz. The emission was attributed to the radiative transitions of holes from the split sublevels of the 1Γ8 excited state to the sublevels of the 1Γ8+ ground state and the 1Γ7+ ground state, yielding an energy separation of 22±0.07meV between the two ground states. These results indicated that emitters based on Ga impurity transitions open up a range of THz frequencies, and the properties of their spectra can improve the understanding of impurity level physics.

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.

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.

Cyclic deep reactive ion etching with mask replenishment

A multi-step reactive ion etching (MS-RIE) process for silicon was developed for the fabrication of deep anisotropic, closely packed structures with vertical sidewalls. This process used repeated cycles of etching and the replenishment of masking layers, similar to the Bosch process (Laermer and Schilp 1996 US Patent 5,498,312) [1] that is employed in specialized etching tools. The process described here, however, can be used on conventional RIE tools, and is based on the isotropic deposition of an etch-inhibiting polymer to protect sidewalls, its anisotropic removal from the bottom etch front, and a subsequent isotropic etch into deeper layers. A conventional parallel plate etcher without fast gas management, cryogenic substrate cooling, or inductively coupled plasma density enhancement, produced these steps. Each process step was optimized for the maximal etch rate, minimal mask erosion, deposition of the thinnest polymer required to protect the sidewalls, and was tailored for use with 2 µm thick photoresist as the initial mask layer. This cyclic RIE process was used to fabricate photonic devices with high aspect ratios of etched depths over 100 µm and etch widths near 1 µm.

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.

The design and operation of TeraHertz sources based on silicon germanium alloys

During the past few years, vigorous studies have begun on semiconductor devices that generate and detect frequencies from 0.3 - 10 TeraHertz (1000 30 /spl mu/m). Previous THz sources were based on electrical methods using transistor oscillators (to 0.5 THz), diode frequency multipliers (to 2.5 THz), and femtosecond optical pulse switches. Infrared emitters such as the Quantum Cascade Laser in the III-V semiconductors have been difficult to extend to THz frequencies due to reststrahlen absorption by polar phonons. In contrast, Si has lower absorption and devices may be able to operate over a broader THz range than the III-V semiconductors. This report describes the fabrication and characterization of THz sources based on three different design approaches: intersubband transitions in Silicon Germanium quantum wells, resonant state transitions in boron-doped strained SiGe quantum wells, and dopant impurity transitions in doped Si layers.