ALBERT

Notes: Double-crystal and triple-axis x-ray diffractometry was used to characterize in detail the strain and composition of short period Si6Ge4, Si8Ge8, Si9Ge6, and Si17Ge2 strained-layer superlattices (SLSs), grown by molecular-beam epitaxy. Nominally strain-symmetrized superlattices, intended to be free standing from underlying buffer layers and the substrate, grown on rather thin (20 nm thick) SiGe alloy buffers with constant Ge content (Si6Ge4 and Si8Ge8) are compared to those grown on 1.3-μm-thick step-graded SiGe alloy buffers (Si6Ge4 and Si9Ge6). Due to the much higher instrumental resolution offered by triple-axis diffractometry (Δ2aitch-theta=12 arcsec) buffer and SLS peaks are clearly separated from each other, which overlap in corresponding double-crystal-diffractometry measurements (Δ2aitch-theta in the range of 180 arcsec to 2°). The lattice constants parallel and perpendicular to the [001] growth direction are determined independently from each other and thus precise strain data of the buffers and the SLS constituting layers were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points (RELPs). The only fitting parameter in a dynamical simulation of conventional rocking curves is then the relative thickness ratio of the Si and Ge layers in the superlattice. The strain relaxation process and the principle of tailoring the strain status in the electronically active layers are shown to be different in structures with single-step and step-graded buffers. For these superlattice samples the RELPs originating from the zeroth-order SLS reflection show significant mosaic broadening (full width at half-maximum of 1100–1300 arcsec perpendicular to the ω/2aitch-theta scan direction in reciprocal space). In contrast, the corresponding RELPs from a pseudomorphic SLS with a much higher average Si content (Si17Ge2), but consisting just of 10 periods, which was grown directly on a Si buffer layer deposited on (001)-oriented Si substrate, are not broadened (full width at half-maximum of 14 arcsec). Besides the determination of strain and composition, examples for the interpretation of diffuse x-ray scattering are given, conveniently measured by reciprocal space mapping.

Notes: We discuss the Raman spectra of SinGem short-period superlattices both theoretically and experimentally. A lattice dynamical model is used to generate spectra which are compared to experimental results. This comparison is useful for determining superlattice periods and detecting interface roughness.

Notes: Ultrathin strained layer SinGem superlattices (SLS) with periods of a few atomic monolayers (n-ML Si, m-ML Ge; n+m≤20) have been grown by molecular-beam epitaxy. On a 〈100〉-oriented, n+ -doped silicon substrate a buffer with a Si1−yBGeyB alloy layer has been grown applying the concept of strain symmetrisation for the subsequent SLS layer. The thickness of the SLS layer was chosen to be around 200 nm. The buffer and SLS layer have been n doped by incorporation of an antimony adlayer deposited prior to growth. A thin (20 nm), p+ -doped (B, 6×1019 cm−3 ) Si1-ycGeyc contact layer was grown on top of the SLS layer. We have fabricated mesa photodiodes (AM=1.3×10−5 cm2) by standard semiconductor processing techniques and characterized them by current-voltage (I-V) and capacitance-voltage (C-V) measurements. We compare the Sb-dopant profiles of the buffer and the SLS layer measured by secondary-ion mass spectroscopy and by C-V analysis. Due to depletion of the Sb adlayer the SinGem SLS samples exhibit a declining Sb-dopant profile in the growth direction where the integral concentration of the SLS layer is found to depend on the n/m ratio and is up to 60% lower than a reference Si homodiode grown under the same conditions. The Si0.6Ge0.4 alloy sample shows a 2–3 times lower Sb concentration than the corresponding superlattice samples with the same integral Ge content (Si6Ge4 and Si12Ge8 SLS). We conclude that the Sb incorporation in a SLS can be used as a probe for the quality of the SLS growth.

Notes: (100) silicon molecular beam epitaxy films with etch pit densities below 103 cm−2 and χmin values of 3.3–3.9% were grown at very low temperatures (Ts =250–350 °C). Although dopant activation is significantly below unity at n=1018 Sb atoms/cm3 Hall mobilities of homogeneously Sb-doped samples (Ts =250 °C, 300 °C) are found to match reasonably bulk values. δ doping at a monolayer Sb deposition shows a dopant activation of 0.45–0.81 with no detectable broadening at Ts =200 °C.

Notes: Si p+–i–n+ junctions, as used in modern devices, are investigated for excess current contributions in their forward characteristics. The excess current is caused by tunneling through band-gap states. The current characteristics of epitaxially grown junctions are studied experimentally at a temperature range from −60 to 100 °C, thereby the temperature dependence of the excess current is described and explained by the ratio of built-in voltage divided by the tunneling reference voltage Vt0. The voltage dependence is investigated by the curvature coefficient, the value of which is described by a modified Chynoweth theory. [A. G. Chynoweth, W. L. Feldmann, and R. A. Logan, Phys. Rev. 121, 684 (1961)]. © 2002 American Institute of Physics.

Notes: A method for determination of surface segregation during molecular-beam epitaxy is proposed. At constant beam fluxes, the temperature is switched which causes a segregation-dependent doping profile. The segregation ratio is extracted from the integrated profile avoiding problems with limited depth resolution in the nanometer regime. The method was tested with boron segregation on (100) silicon (T=500–700 °C). A clear proof of the concentration dependence of the boron segregation is given. At 600 °C, the segregation ratio rs decreases from 55 to 12 nm when the doping level is increased from 3.5×1018 to 7.5×1019 cm−3. © 2000 American Institute of Physics.

Notes: Ultrametastable silicon-germanium (Si1−xGex) layers with a Ge content x in the range from about 20% to 27% were grown by Si-MBE at temperatures far below 550 °C (325–450 °C). The thicknesses of the layers (up to 500 nm) exceed the equilibrium thickness by a factor of up to 50. We observe in the as-grown samples without any annealing both the excitonic Si1−xGex band-edge luminescence and a broad alloy luminescence of unknown origin. The two peaks have an energy difference of ≈144 meV and shift linearly with the Ge content. The alloy band luminescence disappears when strain relaxation sets on upon annealing at around 600 °C.

Notes: Forward-bias current-voltage characteristics of molecular beam epitaxy grown Si p+-i-n+ junctions have been determined at room temperature. At small widths of the i zone (Li=5 and 10 nm) band-to-band tunneling with a maximum peak-to-valley ratio of two is observed. Up to Li=30 nm (trap assisted) forward-bias tunneling is apparent with saturation tunneling current densities somewhat lower than in p-n junctions at comparable widths of the space-charge region WSCR(0). For Li(approximately-greater-than)30 nm and Tg=500 °C growth temperature surface recombination dominates the low bias range. At Li=35 nm and Tg=325 °C, both surface and bulk recombination is observed. We found evidence that Si molecular beam epitaxy layers grown at this low temperature get an increasing density of crystalline defects with growing thickness.

Notes: Double crystal and triple axis x-ray diffractometry was used to characterize the structural properties of short period Si6Ge4 superlattices grown by molecular beam epitaxy on either a thin (20 nm) single step Si0.6Ge0.4 alloy buffer or on a thick step-graded Si1−xGex(0〈x〈0.4, 700 nm thick) buffer followed by a 550 nm Si0.6Ge0.4 layer. Reciprocal space maps around the (004) and (224) reciprocal lattice points yield direct information on the strain status of the buffer and superlattice layers. For the thick step-graded buffer indeed all layers with different Ge content are fully relaxed and thus the growth of an almost freestanding superlattice is possible.

Notes: Triple axis x-ray diffractometry was employed for the structural characterization of a 100 period Si9Ge6 superlattice grown by molecular beam epitaxy on a thick step-graded SiGe alloy buffer. From the distribution of diffusely scattered intensity around reciprocal lattice points the correlation function of the deformation field due to structural defects has been calculated using kinematical theory of x-ray diffraction. From the extension of the correlation function it turns out that on the average the entire superlattice (0.2 μm thick) scatters coherently along growth direction, whereas laterally the coherently scattering regions are extended only over about 40 nm.