ALBERT

Notes: The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as-prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≈690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggests that the source of the large blue shift of the visible emission compared to the bulk Si band gap energy is likely to be due to quantum confinement in the nanometer size Si clusters. The electron-hole recombination process, on the other hand, remains unclear.

Notes: Best-case evaluations are made for potential optoelectronic applications of erbium-doped silicon (EDS). The objective is to find the upper limit of performance when EDS is used as light-emitting diodes, amplifiers/modulators, and lasers. Every effort is made to use intrinsic parameters whose values are determined by physics rather than by factors such as material quality and processing quality. Consequently, the result is expected to be overly optimistic, and should be regarded as a feasibility study only. It is shown that Er-doped Si is not suitable for light-emitting-diode applications because of the low emitted power (microwatts). The intensity amplifiers/modulators made of Er-doped Si can only be expected to provide a very modest gain (〈6 cm−1). For laser applications, the threshold population inversion can be achieved in principle (assuming proper design and processing of the laser structure); however, a very efficient pumping mechanism is necessary for the laser to provide reasonable power output (of the order of mW/facet). Finally, a view on the direction of future research in this field is presented. Since the rare-earth ion luminescence is known to be fairly independent of the host materials, the results obtained from this study are expected to be applicable to most of the other rare-earth-doped semiconductors.

Notes: A procedure for the fabrication of two-dimensional carrier (electron and hole) gases in modulation doped GeSi/Si heterostructures is presented. The best 4.2 K mobilities measured for the two-dimensional electron and hole gases are 180 000 cm2/V s and 18 000 cm2/V s, respectively. Recently, two-dimensional hole gases with mobilities as high as 55 000 cm2/V s have been obtained. The carrier gases are fabricated on top of relaxed, compositionally graded GexSi1−x buffer layers with low threading dislocation densities (≈106 cm−2). Experimental evidence indicates that the function of the graded buffer is to promote dislocation propagation while suppressing nucleation. A comparative analysis is carried out for two dimensional electron gases in GeSi/Si/GeSi and in AlGaAs/GaAs structures. Although molecular beam epitaxy is used to grow the samples, the principle discussed here is independent of growth technique.

Notes: Photoluminescence (PL) spectroscopy, cathodoluminescence (CL) spectroscopy and imaging, and preferential defect etching and optical microscopy have been used to characterize compositionally graded Si1−xGex alloy layers grown by molecular beam epitaxy. Si1−xGex capping layers grown on the compositionally graded layers have low threading dislocation densities, and both PL and low-beam energy CL spectra show bound exciton luminescence features identical with those observed in bulk Si1−xGex alloys and relatively weak dislocation related D-band features. Increasing the beam energy increases the relative strength of the D bands in the CL spectra, indicating that they are associated with the misfit dislocations in the compositionally graded layer. This has been confirmed by combined chemical etching and PL spectroscopy measurements. The misfit dislocations can be observed by monochromatic CL imaging at a high-beam energy using a narrow band pass filter centerd on the D4 band.

Notes: A detailed study of the electrical and defect properties of ion-implanted erbium in silicon shows that erbium doping introduces donor states. The concentration of erbium related donors as a function of implant dose saturates at 4×1016 cm−3 at a peak implanted Er-ion concentration of (4–7)×1017 cm−3. The defect levels related to erbium in silicon are characterized by deep level transient spectroscopy and identified as E(0.09), E(0.06), E(0.14), E(0.18), E(0.27), E(0.31), E(0.32), and E(0.48). The dependence of the photoluminescence on annealing temperature for float zone and for Czochralski-grown silicon show that oxygen and lattice defects can enhance the luminescence at 1.54 μm from the erbium. Temperature-dependent capacitance-voltage profiling shows donor emission steps when the Fermi level crosses EC − ET = 0.06 eV and EC − ET = 0.16 eV.

Notes: The effect of impurity coimplantation in MeV erbium-implanted silicon is studied. A significant increase in the intensity of the 1.54-μm Er3+ emission was observed for different coimplants. This study shows that the Er3+ emission is observed if erbium can form an impurity complex in silicon. The influence of these impurities on the Er3+ photoluminescence spectrum is demonstrated. Furthermore we show the first room-temperature photoluminescence spectrum of erbium in crystalline silicon.

Notes: Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt-Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.

Notes: The effects of growth temperature, substrate offcut, and dislocation pileup formation on threading dislocation density (TDD) in compositionally graded SiGe buffers are explored. To investigate dislocation glide kinetics in these structures, a series of identical samples graded to 30% Ge were grown at temperatures between 650 and 900 °C on (001)-, (001) offcut 6° towards an in-plane 〈110〉-, and (001) offcut 6° towards an in-plane 〈100〉-oriented Si substrates. The field threading dislocation density (field TDD) in the on-axis samples varied exponentially with temperature, from 3.7×106 cm−2 at 650 °C to 9.3×104 cm−2 at 900 °C. The activation energy for dislocation glide in this series, calculated from the evolution of field TDD with growth temperature, was 1.38 eV, much lower than the expected value for this composition. This deviation indicates that strain accumulating during the grading process at low growth temperatures is forcing further dislocation nucleation, resulting in a deviation from pure glide-limited relaxation. The TDD of samples grown on offcut substrates exhibited a more complicated temperature dependence, likely because films grown on offcut substrates have an increased tendency towards saturation in dislocation reduction reactions at high temperature. Dislocation reduction processes were further explored by initiating compositional grading up to 15% Ge at 650 °C and continuing the grade to 30% Ge at 900 °C. The low temperature portion of this growth provided an excess concentration of threading dislocations which could subsequently be annihilated during the high temperature portion of the growth, enabling a comparison of reduction rates for different substrate offcuts. Combining these results with threading dislocation densities in a variety of other samples, a complete picture of strain relaxation kinetics in compositionally graded SiGe/Si emerges. Generally, strain relaxation in these structures is limited by dislocation glide, and threading dislocation densities are independent of final Ge content. However, we theorize that dislocation pileup formation inhibits the strain relaxation process and is therefore accompanied by a rise in field threading dislocation density. Based on these results, we now have a predictive model for TDD in compositionally graded SiGe/Si over a wide range of growth conditions. © 2001 American Institute of Physics.

Notes: Strain-relaxed, compositionally graded Ge0.3Si0.7/Si heterostructures grown by ultrahigh vacuum chemical vapor deposition at 650 °C are shown to display a consistent change from p-type to n-type conductivity as a function of rapid thermal annealing (RTA) temperature in the range 700–850 °C. Cross-sectional transmission electron microscopy, spreading resistance, and electron beam induced current (EBIC) studies eliminate the dislocations themselves as a possible source of this type conversion, by demonstrating that the spatially invariant hole concentration of 2×1014 cm−3 is not correlated to the dislocation density, which decreases from ∼108 cm−2 in the graded region to 7×105 cm−2 in the 30% Ge cap. To identify the source of type conversion, a systematic investigation was performed on 650 °C as-grown and annealed samples with deep-level transient spectroscopy (DLTS), photoluminescence (PL) and capacitance–temperature (C–T) measurements. DLTS measurements on as-grown samples reveal a complex spectrum of deep and shallow hole traps, while C–T studies reveal a prominent temperature dependence of the zero bias capacitance, indicating that the p-type background conductivity is associated with a high degree of compensation. Post-growth RTA at T≥800 °C eliminates this compensation, and yields background n-type films, consistent with the background n-type conductivity that is always observed in graded films grown at T(approximately-greater-than)800 °C in the same reactor. This change in conductivity type is accompanied by a strong increase in EBIC signal strength and a significant simplification of DLTS and PL spectra. These results are discussed in terms of dislocation interaction within the graded layers which generates nonequilibrium concentrations of intrinsic point defects that form extended complexes at growth temperatures ≤800 °C. These complexes are associated with thermally unstable acceptor-like defect states in the energy range H(0.05)–H(0.30) that convert low growth temperature, relaxed, graded GeSi/Si films from background n type to background p type. © 1996 American Institute of Physics.