D. J. Norris

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

High-performance nMOSFETs using a novel strained Si/SiGe CMOS architecture

Performance enhancements of up to 170% in drain current, maximum transconductance, and field-effect mobility are presented for nMOSFETs fabricated with strained-Si channels compared with identically processed bulk Si MOSFETs. A novel layer structure comprising Si/Si/sub 0.7/Ge/sub 0.3/ on an Si/sub 0.85/Ge/sub 0.15/ virtual substrate (VS) offers improved performance advantages and a strain-compensated structure. A high thermal budget process produces devices having excellent on/off-state drain-current characteristics, transconductance, and subthreshold characteristics. The virtual substrate does not require chemical-mechanical polishing and the same performance enhancement is achieved with and without a titanium salicide process.

Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system.

Picosecond intersubband dynamics in p-Si/SiGe quantum-well emitter structures

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multiquantum-well structures in the far-infrared (FIR) regime, ℏω<ℏωLO, utilizing the Dutch free electron laser, (entitled FELIX—free electron laser for infrared radiation). The calculated scattering rates for optic and acoustic phonon, and alloy scattering have been included in a rate equation model of the transient FIR intersubband absorption, and show excellent agreement with our degenerate pump-probe spectroscopy measurements where, after an initial rise time determined by the resolution of our measurement, we determine a decay time of ∼10 ps. This is found to be approximately constant in the temperature range from 4 to 100 K, in good agreement with the predictions of alloy scattering in the Si0.7Ge0.3 wells.

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

Si-based electroluminescence at THz frequencies

Experimental results of electroluminescence in the terahertz gap, at 6 THz (or 40 μm) from Si/SiGe multi quantum well structures, grown by a commercial chemical vapour deposition system are presented. Theoretical simulations were used to design the heterolayer structure and to explain the emission and absorption features. Electrical and materials characterisation is also presented to demonstrate the quality of the heterolayers.

Si/SiGe quantum-cascade emitters for terahertz applications

The quantum cascade laser provides one possible method of realizing high efficiency light emission from indirect band gap materials such as silicon. Strain-symmetrized Si/SiGe samples designed to investigate the intersubband properties of quantum wells are examined. Electroluminescence data from Si/SiGe quantum-cascade staircases demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions are presented. In surface-normal emission only light-hole to heavy-hole electroluminescence is observed at ( wavelength) as predicted by theory. Modulation-doped SiGe quantum well samples are also investigated to determine the underlying physics in the system. Results of picosecond time resolved studies of the dynamics of the intersubband transitions using a free electron laser are presented which show approximately constant relaxation times of below .

Evaluation of strained Si/SiGe material for high performance CMOS

The enhanced electrical performance of dual quantum well strained Si/SiGe n-channel MOSFETs has been investigated as a function of SiGe material quality. The higher electron mobility in strained Si compared with bulk Si has been translated into performance gains in terms of device transconductance and on-state drain current exceeding 120% compared with simultaneously fabricated Si controls. Increased performance was demonstrated for a wide range of gate lengths and operating conditions. Trade-offs between optimum device design and SiGe material quality have been investigated. The greatest performance enhancements are achieved through device fabrication on SiGe virtual substrate material grown by low-pressure chemical vapour deposition (LPCVD) at high temperature. Improved surface morphology, defect density and gate oxide quality are found to be the dominating factors in the enhanced performance of the devices compared with strained Si/SiGe MOSFETs fabricated on LPCVD material grown at low temperature. However, even degraded SiGe material arising from low temperature LPCVD growth resulted in strained Si/SiGe n-channel MOSFETs exhibiting significant improvements in device operation compared with conventional Si MOSFETs. The performance advantages offered by strained Si/SiGe devices fabricated on material grown at both low and high temperatures exceed that of a typical Si CMOS technology generation.

Indium segregation in (111)B GaAs-InxGa1-xAs quantum wells determined by transmission electron microscopy

We have used energy-filtered transmission electron microscopy combined with low-temperature photoluminescence to study the effects of indium segregation within (111)B oriented GaAs-InGaAs single quantum wells. The microscopy provides an accurate measure of the relative indium profile whilst the photoluminescence allows the determination of the absolute concentrations of indium. A thermodynamic model of the segregation process reproduced the main features of the distribution of indium in the quantum wells. We have also studied a single quantum well grown immediately following the deposition of an InAs monolayer, intended to compensate for the loss of indium by segregation. Both modelling and measurement show that the effect is merely to broaden significantly the resulting quantum well.

Similarity of Stranski-Krastanow growth of Ge/Si and SiGe/Si (001)

This study investigates the onset of islanding (Stranski-Krastanow transition) in strained pure germanium (Ge) and dilute silicon-germanium (SiGe) alloy layers grown by chemical vapour deposition on Si(001) substrates. Integration of compositional profiles is compared to a novel method for quantification of X-ray maps acquired in cross-sectional scanning transmission electron microscopy, together with simulations of surface segregation of Ge. We show that Si1−xGex alloys for germanium concentrations xu2009≤u20090.27 grow two-dimensionally and stay flat up to considerable layer thicknesses, while layers with concentrations in the range 0.28u2009<u2009xu2009≤u20091 form islands after deposition of ∼3.0/x monolayers (=quarter unit cells in the diamond lattice, ML). The uncertainty in the amount of deposited material for pure Ge is ±(0.2–0.3) ML. Modelling shows that of the amount of germanium deposited, 0.7 ML segregate towards the free surface so that only ∼2.3/x ML are directly incorporated in the layer within a few nanometres, i...