P.-C. Lv

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.

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.

Compact electrically pumped nitrogen-doped 4H-SiC terahertz emitters operating up to 150 K

We report a new type of electrically pumped THz source that emits at 9 THz with a maximum operating temperature of 150 K. The mechanism is based on dopant transitions in the 4H-SiC. The two nonequivalent donor sites of nitrogen in SiC were used to give the device a relatively high operating temperature and emission power. At a pumping current of 4.7 A at 4 K, the integrated spectral output power was 0.18 mWatt from the top surface with an area of 4mm2. These results suggest that high-temperature operating THz devices can be fabricated from doped SiC.

Increasing the operating temperature of boron doped silicon terahertz electroluminescence devices

High power electroluminescence near 8THz was observed from boron doped silicon devices operating at heat sink temperatures up to 118K. This represents the highest emission temperature yet observed for silicon dopant-based terahertz devices, and is a significant increase from previous reports. This letter compares the temperature dependence of the emission mechanism to the dopant occupation function and describes an empirical model that fits the variation of output power with temperature, and that can guide the design of future terahertz devices.

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.

Xenon Difluoride Dry Etching of Si, SiGe Alloy and Ge

Xenon diflouride (XeF2) vapor has been known to be able to spontaneously etch Si isotropically at high rates up to 10 mum/min. This dry etching process does not require plasmas or catalysts, and thus causes little damage to the electronic properties. It is useful for releasing free standing structures by etching away Si sacrificial layers or for gate oxide failure analysis by etching away the backside Si. In this work, the etching of Si, SiGe alloys and Ge was studied and results were discussed. Both SiGe and Ge were found to be etched by XeF2 vapor, and at faster rates than Si

The effects of uniaxial compressive stress on the terahertz emission from phosphorus-doped silicon devices

The effects of uniaxial compressive stress on the terahertz electroluminescence from P-doped silicon devices have been studied. A shift by ∼0.5THz in the emission peaks of donor state transitions: 2p0→1s(E) and 3p+∕−→1s(E) has been observed for a stress of ∼0.1GPa along the [100] direction. Transitions from excited states to the strain split states of 1s(E) showed a pronounced polarization effect. Transitions involving the 1s(T1) ground state, however, showed no polarization effect. These results suggest that it may be possible to realize a tunable impurity-doped silicon terahertz emitter by externally applied stress.

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.