H.S. Gamble

Toward Silicon-Based Lasers for Terahertz Sources

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

Electrical characterization of SOI substrates incorporating WSi/sub x/ ground planes

Silicon-on-insulator (SOI) substrates incorporating tungsten silicide ground planes (GPs) have been shown to offer the lowest reported crosstalk figure of merit for application in mixed signal integrated circuits. The inclusion of the silicide layer in the structure may lead to stress or defects in the overlying SOI layers and resultant degradation of device performance. It is therefore essential to establish the quality of the silicon on the GPSOI substrate. MOS capacitor structures have been employed in this paper to characterize these GPSOI substrates for the first time. High quality MOS capacitor characteristics have been achieved with minority carrier lifetime of /spl sim/0.8 ms. These results show that the substrate is suitable for device manufacture with no degradation in the silicon due to stress or metallic contamination resulting from the inclusion of the underlying silicide layer.

In search of a Si/SiGe THz quantum cascade laser

While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at /spl sim/3 THz (/spl sim/100 /spl mu/m) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies.

Towards a Si/SiGe Quantum Cascade Laser for Terahertz Applications

While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at similar to 3 THz (similar to 100 mu m) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies. The effects of high doping levels in Si/SiGe quantum cascade structures is also investigated with impurity emission demonstrated rather than intersubband emission for the highest doping levels used in the cascade active regions.

Characterization of CVD tungsten deposited by silane reduction

The deposition of tungsten by silane reduction of WF6 was investigated to determine the effect of the deposition chemistry on the layer properties. The influence of the deposition chemistry on the titanium adhesion layer was also investigated. To perform a direct comparison of the effect of the deposition parameters on the layer properties layers of equal thickness were deposited. In order to do this the deposition rates first had to be established experimentally. When the SiH4WF6 ratio was maintained constant at 1 and the deposition temperature increased the resistivity of the layers decreases, and the roughness increased significantly. When the temperature was maintained constant at 450 °C and the SiH4WF6 ratio was varied, it was found that the resistivity remained constant until the tungsten transformed to beta phase tungsten. At this transformation point the stress of the deposited layer and the roughness were seen to decrease significantly. It was found when the correct chemistry was applied at deposition temperatures up to 400 °C the initial reaction between the WF6 and the Ti could be eliminated or reduced.

Silicon quantum cascade lasers for THz sources

This paper covers some recent advances in the search for a silicon quantum cascade laser. These include intersubband lifetime measurements, growth of high quality structures on buried suicide layers, and demonstration of THz electroluminescence.

Terahertz emission and absorption characteristics of silicon containing boron and phosphorous impurity dopants and the effect of temperature

The emission and absorption characteristics of boron-doped and phosphorous-doped silicon at terahertz frequencies are investigated. Different doping concentrations are considered and individual terahertz optical transitions are identified. The effect of temperature is considered.

Novel materials for the semiconductor industry, deposited using a rapid thermal chemical vapour deposition system

Abstract This paper describes a custom-built rapid thermal chemical vapour deposition system developed to allow the growth of silicon and silicon carbide at low pressure and with minimal thermal budget. The reactor uses tungsten-halogen lamps to heat the single process wafer to a maximum temperature of 1080°C with a maximum rate of 250°C/s. A microwave magnetron provides gas plasma chemistry capability. Silicon layers with low oxygen contamination are produced at around 720°C and 0.07 mbar using silane. Silicon carbide is deposited at around 970°C and 10 mbar using silane/propane chemistry. Both processes incorporate a CF4 plasma clean of the wafer surface prior to deposition. Bipolar transistors produced with a silicon carbide emitter gave higher gains than standard reference devices.

Silicon germanium quantum cascade heterostructures for infrared emission

This work demonstrates that there are no fundamental obstacles to the realization of a Si/SiGe quantum cascade laser. This study also reports a number of significant achievements which indicate that the technology is now sufficient, or can be developed to a sufficient level, to attain a working device.

Low-temperature plasma enhanced chemical vapor deposition of tungsten and tungsten nitride

Tungsten and tungsten nitride layers have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Tungsten layers deposited at low deposition temperatures T∼150 °C using this method showed good uniformity over dielectric and silicon substrate areas. As the deposition temperature decreased, the silicon consumed during the deposition reaction decreased, at T∼150 °C no silicon consumption was measurable. PECVD tungsten nitride layers were deposited directly on oxidized silicon substrates with no requirement for a nucleation layer. As the NH3 flow rate was increased, whilst maintaining all other parameters constant, deposited layers were found to change from metal tungsten to tungsten-rich amorphous layer to W2N. The resistivity of the layers was found to be high compared to published literature for higher-temperature deposited layers. The high resistivity is attributed to the incorporation of fluorine into the layer at low deposition temperatures. A deposition process was established for smooth amorphous tungsten-rich WxN layers at 150 °C.