H. Presting

Ultrathin SimGen strained layer superlattices-a step towards Si optoelectronics

Ultrathin SimGen (m monolayers (ML) Si, n ML Ge) strained layer superlattices (SLS) have been grown by molecular beam epitaxy. The optical properties of these structures depend on the concept of band-structure engineering by Brillouin zone folding and strain adjustment of the SLS by a Si1-ybGeyb alloy buffer layer. The energies and the oscillator strengths of the bandgap and intersubband transitions have been studied theoretically for SimGen SLS with a variety of period lengths, particularly those of m+n=10. Various characterization tools such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, photoluminescence (PL) and photocapacitance measurements have been used to analyse growth quality, interface sharpness, morphology, strain distribution and optical properties of the superlattice experimentally. The PL data indicative of the quasidirect energy gap of the 10 ML strain-symmetrized SLS in the near-infrared spectral regime (h nu approximately 0.8 eV) are presented and discussed as well as complementary photocapacitance measurements on a p-n doped Si4Ge4 SLS diode. The fabrication of test mesa diodes from Si/Ge SLS structures is described. Finally, device applications offering the possibility of monolithic integration of superlattice devices with complex silicon-based electronic circuits are outlined.

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.

Self-assembled Ge-islands for photovoltaic applications

Abstract We have fabricated silicon solar cells with embedded germanium layers to form three-dimensional islands in the Stranski–Krastanov growth mode. The additional Ge-layers increase the infrared absorption in the base of the cell to achieve higher overall photocurrent and overcome the loss in open circuit voltage of the heterostructure. In an UHV-MBE chamber up to 75 layers of germanium, each about 8 monolayers thick, separated by Si-spacer layers (9– 16 nm ) were grown on each other using standard 10 Ω cm p-type Si-substrates. The density of islands in the layers was increased by the use of antimony as surfactant, thus densities >10 11 cm −2 were realized. The islands were covered by a 200 nm thick Si-layer (n-type) on top which is used as emitter of the cell. Photoluminescence measurements, AFM and TEM-microscopy were used to characterize the growth of Ge-islands under various growth conditions and post-thermal treatment. Photocurrent measurements exhibit a higher response of the fabricated solar cells in the infrared regime compared to standard Si-cells.

Near and mid infrared silicon/germanium based photodetection

Abstract Short-period silicon/germanium (Si m Ge n ) superlattice and SiGe quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on (100)Si substrates for near (1.3 μ m) and mid-infrared (3–5 μ m; 8–12 μ m) detection. For the near IR detection – suitable for fibre optical communication within a Si integrated circuit (IC) chip – short-period Si m Ge n superlattices with period lengths of several atomic monolayers (ML; e.g. m = n =5 ML; 1 ML~1.4 A) and 2–4 monolayers wide p-doped Ge quantum well layers separated by 20 MLs of Si and embedded in two 10-nm thick Si 1− x Ge x layers have been grown. In the superlattice structure the zone folding effect (U. Gnutzmann, K. Clausecker, Appl. Phys. 3 (1974) 9) has been predicted to produce strong interband transitions near 0.8 eV (≈1.3 μ m), in the latter one the sharp Si/Ge hetero-interfaces break the k-selection rules and strong localisation of electron and hole wave function favour a strong interband excitonic transition at 1.3 μ m. This results in a rather efficient room temperature photo- and electroluminescence and in sufficient absorption. An integrated waveguide/photodetector deposited on a SIMOX (Si substrate with separation by implantation of oxygen) substrate has been fabricated and an external quantum efficiency of 11% with an impulse response time of 400 ps has been observed. For the mid IR range (3–5 μ m) highly p-doped Si/SiGe quantum well detectors have been deposited on an undoped, double-sided polished Si substrate based on hetero-internal photoemission (HIP) over the Si/SiGe barrier. The absorption and photocurrent spectra have been measured from fabricated mesa detectors at 77 K. The photoresponse spectrum of the HIP detectors is shown to be widely tunable in the technological important wavelength band 3–5 μ m by choice of Ge-content, well thickness and doping level. Quantum efficiencies of ~1% at 4 μ m and 77 K have been achieved from SiGe HIP structures, dark currents as low as 10 −8 A/cm 2 can be obtained by modulation doping. Detectivity values of D * of 10 9 cm √Hz/W have been achieved, the quantum efficiency spectrum is considerably broader and up to a factor of 4 higher than Pt:Si at 4 μ m.

Room‐temperature electroluminescence from Si/Ge/Si1−xGex quantum‐well diodes grown by molecular‐beam epitaxy

Tunable room‐temperature electroluminescence, photocurrent, and photoluminescence in the near infrared (λ∼1.3 μm) has been observed from Ge/Si/Ge/Si1−xGex quantum‐well (QW) diodes grown by molecular‐beam epitaxy. The QWs are grown on a p+‐doped 〈100〉‐Si substrate and consist of two thin Ge wells separated by a thicker Si middle layer, and the whole structure is embedded by two Si0.85Ge0.15 alloy layers. Our theoretical analysis of the data suggests that the strength of the spectra is linked to states localized at the interface.

Photoluminescence studies of Si/Si1 − xGex quantum wells and SimGen superlattices

Abstract We report on photoluminescence studies of Si/Si1 − xGex quantum wells with systematically varied growth temperatures and well thicknesses. Well resolved band gap luminescence could be observed in quantum well structures grown at temperatures above 600 °C while for structures grown at lower temperatures defect-related lines dominate the luminescence spectra. We also present photoluminescence and electroluminescence studies for a strain-symmetrized Si5Ge5 superlattice. The photoluminescence observed below the band gap of the corresponding alloy is shown to be enhanced by growing the superlattice on a thick, single-step alloy buffer layer. Absorption measurements on the superlattice show an onset of the absorbance at an energetic position close to the observed photoluminescence. These findings provide strong evidence for band gap related photoluminescence and electroluminescence in a strain-symmetrized Si5Ge5 superlattice.

Characterization of short-period Sim Gen superlattices by high-resolution transmission electron microscopy and X-ray diffraction

Abstract High-resolution and analytical transmission electron microscopy as well as X-ray diffraction were used to characterize the structure of short-period strained-layer (Si m Ge n ) N superlattices ( m monolayers Si, n monolayers Ge, total number of periods N T = 300–500 °C) on different SiGe alloy buffer layers on Si(100) substrates. By a combination of these methods, detailed information can be obtained about periodicity, interface roughness on an atomic scale, strain and average composition of the superlattices. Superlattices of good morphology were grown, although defects were still present. Superlattices on thin buffers contained rather high defect-densities in general, whereas the defect-densities were much lower for superlattices grown on thick buffers, especially for those with composition gradients.

Strain adjustment in ultra thin Si/Ge superlattices

Abstract Ultra thin Si/Ge superlattices were grown by silicon MBE on an intermediate thin SiGe buffer layer on a silicon substrate. Strain adjustment is obtained by partly relaxing the buffer strain by a misfit dislocation network at the buffer-substrate interface. The virtual SiGe substrate composed of the silicon substrate and the buffer layer offers an in-plane lattice constant larger than the silicon lattice constant to the subsequent strained layer superlattice (SLS). Conditions for strain symmetrization were calculated taking into account the different elastic properties of silicon and germanium. Deviations from symmetrization in grown structures were analyzed by curvature measurements. The strong photoluminescence peak in Si6Ge4 (6 ML silicon, 6 ML germanium) was clearly assigned to the superlattice structure by comparison with the alloy sample of the same mean composition and strain adjustment.

Buffer concepts of ultrathin Sim Gen superlattices

Abstract Short-period Sim Gen strained-layer superlattices (SLS) were grown on (100) silicon substrate by molecular beam epitaxy. The SLSs had 145 repeat periods, each having a length of around 10 monolayers (ML) (total thickness 200 nm), and were grown on a Si1−ybGeyb alloy buffer layer serving as a strain symmetrizing substrate. Different buffer concepts for optimum growth quality and strain adjustment of the SLS were employed, and their influence on the photoluminescence spectrum of a Si6Ge4 SLS was investigated. The most recently grown buffer layer consisted of two parts: the first part had a rather thick alloy layer with a linear increase of the Ge content in the growth direction and was grown at a high temperature (Tg≈600 °C); the second part had a constant Ge concentration and was also grown at a high temperature (Tg = 500–600 °C). The shallow increase of the Ge content and the high growth temperature lead to full relaxation of each subsequent layer, with an incremental higher Ge concentration which results in a three to four orders of magnitude lower threading dislocation density in the buffer layer.

Photoluminescence and photoreflectance study of Si/Si 0.91 Ge 0.09 and Si 9 /Ge 6 quantum dots

Nanometer-scale quantum dots based on a series of Si/Si0.91Ge0.09 strained layer superlattices and a Si9/Ge6 strain-symmetrized superlattice were fabricated using electron beam lithography and reactive ion etching. They were investigated by photoluminescence and photoreflectance. It was found for the first time that the quantum efficiency of optical emission from the quantum well layers increased by over two orders of magnitude when the quantum dot sizes were reduced to ≤100 nm.