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Ultra-Thin Silicon/Germanium Superlattices

Objective

SimGen SLS (SimGen means m monolayers of Si and n monolayers Ge, with m+n <40 monolayers) were synthesised and characterised and a theoretical model constructed. It was investigated if the material can deliver both optoelectronic receivers and transmitters and what limitations these have.
New materials that make possible the integration of light emitting diodes (LED) and photodiodes with conventional integrated circuit (IC) technology were synthesized. These new optoelectronic materials consist of silicon/germanium strained layer superlattices (SLS), and are produced using molecular beam epitaxy (MBE). The properties of these heterostructures were predicted with theoretical models and measured using extensive characterization techniques.

Ultrathin layer silicon germanium SLS have been grown on silicon substrates and several SLS structures have been analyzed by cross sectional transmission electron microscopy (TEM) and X-ray diffraction (XRD) where the structural parameters as well as the growth quality in terms of interface sharpness, morphology and defect characterization was assessed. In the photoluminescence (PL) spectrum, strong signals have been found for 10 monolayer SLSs in the near infrared region (0.8 eV). By etching away the SLS it was clearly proved that the PL signal come from the superlattice.
Computer codes for calculating the electronic band structure and related properties as a function of period length and composition have been developed. The band gap and oscillator strengths for a number of different structures have been calculated. The oscillator strength and the spectral form of the quasi direct optical transitions for ideal infinite SLS structures as well as for non ideal finite structures have been calculated.
The photoreflectance technique was successfully tested for analysis of optical transitions and the growth quality of theSLS was drastically improved by using a predeposited antimony adlayer as surfactant during growth.
Finally by applying standard semiconductor processing techniques mesa and waveguide (ridge) diodes were fabricated.
APPROACH AND METHODS
-Ultra-thin SimGen superlattices are synthesised on silicon substrates by molecular beam epitaxial (MBE) growth at low temperatures (300-400 C). Strain adjustment by a thin Si1-yGey alloy buffer layer is used to obtain device meaningful thicknesses (0.25 micron) of the subsequent superlattice layer.
-The superlattice structure is then characterised by methods including transmission electron microscopy (TEM), X-ray diffraction (XRD), and Rutherford back-scattering (RBS). The structural properties of the SLS are its periodicity (the total of n+m monol ayers), composition (the specific values of n and m), and strain distribution. Raman spectroscopy performed on the ultrathin SimGen superlattices reveals information on the built-in strain, interface sharpness, composition and period of the SLS.
-Theoretical models are developed that predict the optical and electrical properties of the SLS. Calculations of the electronic bandstructure and related properties of the superlattice as a function of its composition will be performed, and the results u sed to synthesise superlattices with optimum performance for optical device applications.
-Simple optoelectronic receiver and transmitter type devices will be built to probe material quality for possible industrial applications and exploitation.
PROGRESS AND RESULTS
-Ultrathin layer SimGen SLS have been grown on silicon substrates with periods including 2 (Si1Ge1), 8 (Si4Ge4), 10 (Si6Ge4,Si5Ge5), 20 and 40 monolayers
-Several SLS structures have been analysed by cross-sectional TEM and XRD where the structural parameters as well as the growth quality in terms of interface sharpness, morphology and defect characterisation was assessed. Selected area diffraction in the TEM analysis reveals complementary information to the X-ray diffraction.
-In the photoluminescence spectrum, strong signals have been found for 10 monolayer SLS's in the near-infrared region (0.8 eV). By etching away the SLS it was clearly proved that the PL signals come from the superlattice.
-Computer codes for calculating the electronic bandstructure and related properties as a function of period length and composition have been developed. The band-gap and oscillator strengths for a number of different structures have been calculated. Optim um oscillator strength is predicted for a symmetrical 10 monolayer superlattice, Si5Ge5. By tuning the strain distribution through varying the buffer layer, the bandstructure and oscillator strength values have been calculated.
-The oscillator strength and the spectral form of the quasi-direct optical transitions for ideal infinite SLS structures as well as for non-ideal finite structures have been calculated. A quantitative and qualitative assessment of the finite structures h as been made and compared with experimental data.
-The photoreflectance technique was successfully tested for analysis of optical transitions. Rather strong signals at 2.2 eV < hw < 2.4 eV have been seen in a Si5Ge5 superlattice.
-The growth quality of the SLS, i.e interface sharpness and planarity, was drastically improved by using a predeposited Sb adlayer as surfactant during growth. By applying techniques like spontaneous incorporation from a predeposited adlayer (Sb), coevap oration from an elemental effusion cell (B) and doping by secondary implantation (DSI;Sb) we have successfully achieved growth of p-n doped diodes with abrupt transitions, a prerequisite for the fabrication of diodes.
-By applying standard semiconductor processing techniques mesa and waveguide (ridge) diodes were fabricated. Electroluminescence (EL) as well as photoluminescence (PL) in the near infrared regime has been observed from these diodes up to temperatures ofT=160K - the first reported observation of EL from Si/GeSLS diodes at elevated temperatures.
POTENTIAL
Tailoring the optical properties of the SimGen SLS will have a dramatic impact on today's silicon-based optoelectronics, because it makes possible the integration of opto-electronic devices such as LEDs or photodiodes with the highly complex and mature silicon IC technology. The main result of this Action is the ability to grow SimGen superlattice semiconductors with periodic arrangement of atomic monolayers with sufficient quality for future device applications. In addition, a quantitative theoretical understanding of the optical and electrical properties of the simple SimGenSLS should be achieved which serves as model system for other mismatched heterostructure and superlattice systems. The methods and techniques acquired can be exploited in other systems too.

Coordinator

Daimler-Benz AG
Address
Sedanstraße 10
89077 Ulm
Germany

Participants (3)

Technische Universität München
Germany
Address

85748 Garching Bei München
UNIVERSITY OF LUND
Sweden
Address
Ole Romers Vag, 1118
221 00 Lund
UNIVERSITY OF NEWCASTLE UPON TYNE
United Kingdom
Address

NE1 7RU Newcastle Upon Tyne