Terahertz (far-infrared) intersubband electroluminescence is reported in p-type Si/SiGe quantum wells and quantum cascade structures. Surface-normal emission (without the aid of a surface grating) from light hole - heavy hole intersubband transitions has been observed for the first time in a quantum cascade device. Edge-emission measurements have also been performed, which show emission from both heavy hole - heavy hole and light hole - heavy hole transitions, and have allowed demonstration of the polarisation dependence of the emitted power, according to the selection rules for the intersubband interactions. The electroluminescence is visible up to temperatures of ~150K, in the multiple quantum well structures, and >=77K in the quantum cascade structure.
We have measured avalanche multiplication and noise in Si p- i-n diodes with avalanche widths, w, of 0.12 micrometers , 0.18 micrometers and 0.32 micrometers , both for pure electron and mixed carrier injection. Multiplication and excess noise measurements were also performed with hole injection on a n+-i-p+ diode with w equals 0.84 micrometers . Pure electron initiated avalanche noise results were found to be almost indistinguishable in all three layers. The excess noise factor increases dramatically with increasing w when the injection is mixed.
Two sensor techniques and a data analysis algorithm have been investigated, to assess their suitability for in-situ process monitoring and integrated metrology in semiconductor manufacture. The first sensor is a prototype system which uses two laser beams of different wavelength to module the surface temperature of a wafer. The ratio of detected thermal radiances at the two wavelengths is ideally independent of wafer emissivity. Initial measurements on differently-processed silicon wafers in the temperature range 620 - 1000 degree(s)C show systematic errors of the order 10 degree(s)C. The second sensor is a spectroscopic ellipsometry system fully integrated onto a low pressure chemical vapor deposition reactor for monitoring and controlling growth of silicon-germanium (SiGe) alloys. Algorithms for determining alloy composition and film thicknesses during growth are discussed. The use of a sample-based particle filter based on Bayesian statistics for tracking a semiconductor process is described for the first time.
Long wavelength Si0.8Ge0.2/Si quantum well infrared photodetectors (QWIPs) grown by low pressure CVD have been fabricated both as discrete devices and integrated onto a CMOS readout circuit to produce a monolithic Si-based sensor circuit for detection of thermal radiation. The peak photoresponse of the detectors near 8 micrometer is dominated by transitions to unbound final states associated with the spin-orbit split-off valence band. These optical transitions are allowed by symmetry reduction in the quantum wells, which is also evident in the electrical properties. The electrical noise is nearly ideal for temperatures up to 70 K, with no excess low frequency flicker noise. The capture probability for photoexcited holes into the quantum wells is approximately 0.55 at low temperature. The external 500 K black body responsivities for both the discrete and the monolithically integrated QWIPs are approximately 1.8 mA W-1 at 1 V bias, corresponding to a single pass of the radiation through the detectors. There is no degradation of either the CMOS transistors or the QWIPS caused by the integration process to create the monolithic sensor circuit.
Recently, resonantly enhanced photoresponse in the > 7 micrometers range has been demonstrated for long wavelength SiGe/Si multi-quantum well infrared photodetectors using reflection from a thick buried SiO2 layer. The SiGe/Si detector structures were grown epitaxially on bond-and-etch- back silicon-on-insulator substrates, with the separation of the reflecting oxide and detector surface determining the wavelength of resonant detection. Difficulties were, however, encountered in producing the desired cavity width. In this paper we show the origin to be a thickness-dependent error in the pyrometer measurement of wafer temperature caused by interference in the cavity of radiation to which the pyrometer is sensitive. Judicious choice of substrate oxide thickness is shown to reduce the effect. In-situ real- time monitoring of epitaxial growth rate and thickness using spectroscopic ellipsometry (SE) is demonstrated to be a more flexible solution. Thickness dependent oscillations in the SE spectra allow accurate position of the MQW and end- pointing of the cavity width to give optimum resonant enhancement effect. Use of surface sensitive regions of the SE spectra also allow monitoring of the repeatability of the individual MQW periods. Detectors grown using SE exhibit superior peak responsivities within 0.1 micrometers of the design wavelength.
A vertical resonant cavity detector for thermal imaging in the 8 - 9 micrometers wavelength range has been demonstrated, using a p-SiGe/Si quantum well structure on a silicon-on-insulator substrate with a 2 micrometers thick buried oxide layer. The photoresponse spectrum shows peaks at wavelengths corresponding to standing waves in the cavity, confirming resonant detection. The measured responsivity at the main cavity resonance near 8.7 micrometers , with 2 V bias, is 10 mA/W. This is several times larger than the responsivity typically observed, at the same wavelength and bias, for comparable non-resonant detectors grown on attenuating p+-Si substrates. The resonant device uses the Si/SiO2 interface as the buried mirror. The reflectance of this interface is particularly high between 7 - 9 micrometers , due of the dispersion of the refractive index near the oxide phonon absorption band.
Spectroscopic ellipsometry has been used to determine composition and thicknesses of pseudo- morphic Si/Si1-xGex multilayer structures. The effects of composition, strain, and quantum confinement on dielectric functions have been determined. The technique has been used to assess multi-quantum well structures with well thicknesses down to 20 angstroms.
We report the results of calculations of the optical properties of strained InGaAs/InP quantum wells based on realistic band structure. Emphasis is placed on those features relevant to the operation of quantum well lasers. A k. p model including spin and strain is used to calculate the electronic states of the quantum well. The optical matrix elements as well as the dispersion of the conduction and valence subbands are obtained directly from the model and used to calculate gain and spontaneous emission spectra to study some aspects of intervalence band absorption and to model the behaviour of multiquantum well lasers. The results show that interband mixing causes substantial departures from the predictions of simple models including the failure of band edge selection rules for optical transitions. It is demonstrated how the combined effect of alloy composition spatial confinement and strain can can be used to influence the optical properties of quantum wells and improve the performance of lasers based on these structures.
Hybridised optoelectronic modulator arrays with applications for the optical
interconnection of VLSI circuits are described. The design criteria and
predicted performance of long-wavelength MQW devices are described and
progress towards the realisation of optically interfaced high performance
silicon circuits is reported.
Optoelectronic component arrays for use in the optical interconnection of VLSI circuits are described. The design of Stark-effect modulators and of detectors for 'flip-chip' hybridization onto integrated circuits are discussed, and preliminary results are presented.
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