Janos Olajos

Electroluminescence at room temperature of a SinGem strained-layer superlattice

We report for the first time on room temperature electroluminescence in the region 1.3–1.7 μm from a strain‐adjusted Si6Ge4 superlattice. These results, together with photoluminescence, short‐circuit photocurrent spectroscopy, and voltage‐intensity and current‐intensity measurements indicate that the observed electroluminescence consists of two emission bands which are believed to be caused by defect and interband recombination processes.

Fabrication and properties of epitaxially stabilized Ge / α-Sn heterostructures on Ge(001)

We have investigated the influence of the growth parameters during molecular beam epitaxy on the realizibility of diamond crystal structure Ge / α-Sn alloys and superlattices on Ge(001) substrates. The segregation behaviour of Sn during Ge overgrowth has been studied. We find that for growth temperatures higher than 300°C the incorporation rates are less than 0.005 ML-1. The low-energy electron diffraction data of a series of Ge0.9Sn0.1 films deposited at substrate temperatures in the range of 185 to 275°C indicate a transition to amorphous growth for thicknesses beyond 20 A. Single-crystal GenSnm superlattices with α-Sn layer thicknesses m of 1 and 2 monolayers and periodicities n + m between 10 and 22 monolayers have been fabricated by an unconventional molecular beam epitaxy technique which involves large substrate temperature modulations during growth. Structural characterization of the samples by means of transmission electron microscopy. Raman spectroscopy and X-ray diffraction exhibits distinct superlattice effects. The downward shift of the fundamental energy gap of the superlattices with increasing Sn content, as extracted from absorption measurents with a Fourier transform spectrometer, is in excellent agreement with theoretical values obtained from pseudopotential band structure calculations. The films were found to be stable against phase transition up to temperatures of 430–465°C, depending on the average Sn content.

Impurities in Semiconductors

A brief outline is presented on recent developments in defect characterization and identification in semiconductors which have been made possible by the application of methods other than junction space charge techniques (JSCT). Chalcogens and several transition metals in silicon are used as examples in order to show how important parameters and properties of defects can be revealed by using spectroscopic methods. One of the methods, namely photothermal ionization spectroscopy is discussed in more detail. Si/Ge is taken as an example to show how JSCT can be used for the study of low-dimensional structures.

Room‐temperature photoluminescence of GemSinGem structures

Photoluminescence of pseudomorphic Ge wells grown by conventional molecular beam epitaxy on Si substrate is studied. The samples consist of p‐type doped Gem–Sin–Gem structures embedded in a Si1−xGex alloy. The luminescence lines shift to lower energy with increasing m, the observed band gap agrees with subband calculation based on an effective mass approximation. The temperature stability of the luminescence depends on m. In the case of m=4 the luminescence persists up to room temperature with only small reduction in intensity. The activation energies determined from the exponential drop of luminescence intensity agree with band discontinuities in the sample structures.

Photo- and electroluminescence in short-period Si/Ge superlattice structures

Interband optical transitions have been studied in a variety of short-period Si/Ge superlattice structures by means of photocurrent spectroscopy, infrared absorption, photo- and electroluminescence. Furthermore, the bandgap photoluminescence from strain-adjusted SimGen (m=9, 6, 3; n=6, 4, 2) superlattices was studied under applied hydrostatic pressure. The strain adjustment was achieved by a thick, step-graded Si1-xGex buffer layer resulting in an improved quality of the superlattice with respect to dislocation density. The hydrostatic pressure dependence was modelled using an approach based on deformation potentials and effective-mass theory. In samples annealed at 500 degrees C and higher, a systematic shift of the bandgap was observed which is discussed in terms of a process involving interdiffusion of the Si and Ge atoms. Bandgap-related electroluminescence was observed in mesa diodes at room temperature, whereas the photoluminescence disappeared at about 40 K. The electroluminescence from samples based on different buffer-layer concepts is compared. Apart from the strain-symmetrized Si/Ge superlattices, another structure that has been proposed to act as an efficient, light-emitting device in the Si-based systems is an ultrathin Ge layer (1-2 monolayers) embedded in bulk Si. We report on the electroluminescence spectra at various temperatures from a sample based on this concept, namely a layer sequence consisting of two periods of Si17Ge2 grown pseudomorphically on an n+ Si substrate. A very intensive, well resolved electroluminescence was obtained at 55 K from the QW.

Optical study of diffusion limitation in MBE growth of SiGe quantum wells

We report on detailed studies of the bandgap of Si/SixGe1-x quantum well structures grown on (001) Si by molecular beam epitaxy. Photocurrent and photoluminescence spectroscopy are used to determine the bandgap of the SiGe alloy up to x=0.67. We found that interdiffusion of the SiGe layers limited the maximum Ge content in the alloy layers at a high growth temperature (720 degrees C). At a lower growth temperature (500 degrees C) diffusion is negligible. This is verified by p-i-n structures and p-type modulation-doped quantum wells. In the modulation-doped samples the bandgap could be reduced to 1.5 mu m while still showing intense bandgap related photoluminescence. As well as an alloy-related onset the p-i-n diodes reveal a low-energy threshold, which is defect related. Low growth temperatures lead to defects located in the SiGe layers. Raising the number of quantum wells and Ge content up to almost critical thickness we found a maximum external responsivity of 4*10-4 A W-1 in normal incidence for mesa-type pin photodiodes.

Line spectrum of the interstitial iron donor in silicon

Photothermal ionization spectroscopy and infrared transmission measurements have been carried out on iron‐doped silicon. A series of sharp lines in the range from 6100 to 6400 cm−1 was observed with both techniques. Photoionization cross‐section spectra were determined by photothermal ionization spectroscopy and photoelectron paramagnetic resonance, and it is concluded that the lines originate from electronic transitions to excited shallow donor states at the interstitial iron impurity in the neutral charge state Fe0i. The lines and the Fe0i ‐related electron paramagnetic resonance signal annealed out together at approximately 170 °C. The line spectra are analyzed in terms of three overlapping donor series and the origins of these are discussed.

Electronic structure and optical properties of short-period α-SnnGem superlattices

Short period α-Sn/Ge strained layer superlattices have been prepared on Ge(001) substrates by low temperature molecular beam epitaxy. We have achieved almost defect free and thermally stable single crystalline structures. Photourrent measurements in a series of Sn1Gem(m>10) superlattices reveal a shift of the fundamental energy gap to smaller energies with decreasing Ge layer thickness m, in good agreement with band structure calculations. A direct fundamental energy gap and large direct band gap absorption is predicted for a slightly increased lateral lattice constant in α-Sn/Ge superlattices.

Transition Metal Excited States in Silicon

Recent absorption and photoconductivity studies of deep transition-metal impurities in silicon are discussed, with emphasis on optical transitions from the deep ground state to shallow Coulomb excited states. The P 3/2 line spectra of the deep Au and Pt acceptors closely resemble those of group II acceptors in silicon, whereas the P 1/2 lines show resonance effects due to interaction with the valence band continuum. Behavior under uniaxial stress is compatible with D 2d or C 2y point-group symmetry for the Au and Pt acceptors. A line spectrum in g-dopes Si can be attributed to excitations to shallow donor states since the phononassisted Fano resonances involve characteristic inter-valley phonons. Both the Ag donor spectrum and the corresponding Au spectrum are dominated by excited s-state transitions. Thus, the traditional fingerprint of a donor in silicon, i.e. the effective-mass like p-state series, is missing or at best observed weakly

A photoluminescence study of selenium-diffused silicon

Two optical bands due to short-time diffusion of Se in Si have been studied by a photoluminescence technique. The bands show similarities to the two Se-related bands reported previously but have shifted in energy. The Se-related bands reported here are only observed after short-time diffusion and, e.g., they are observed after 9 min but not for 18 min diffusion at 1100 °C. One of the bands consists of three sharp zero-phonon lines whereas the other one only shows broad structures. A thermalization study indicates that the sharp lines are due to spin-singlet and -triplet initial states similarly to the Se and S bands studied previously. The spin-triplet state appears to be split into three components which is interpreted to be due to a near axial symmetry crystal field. Luminescence from two of the states is directly observed as zero-phonon lines whereas only a phonon replica of the third one was detected. The excitation spectrum of the neutral isolated Se double donor was observed by photoconductivity measurements which establish that substitutional Se is present in detectable amounts in the samples even for the 9 min samples. It is speculated that the photoluminescence bands are related and/or are precursors to the previously reported bands that are observed after considerably longer diffusion times.