W. Winter

Pseudomorphic Si1−yCy and Si1−x−yGexCy alloy layers on Si

Abstract High-quality pseudomorphic Si 1− y C y and Si 1− x − y Ge x C y alloy layers with a carbon concentration up to 7% are prepared by solid-source molecular beam epitaxy. Near band-edge photoluminescence (PL) is observed from Si/Si 1− y C y multiple quantum well (MQW) structures. The bandgap in the pseudomorphic films is reduced by about 65 meV per percent C. The data from Si/Si 1− y C y MQWs indicate a type-I heterostructure with the band offset being mainly in the conduction band. In Si 1− x − y Ge x C y MQWs compressive strain caused by Ge is partially compensated by C alloying and the bandgap increases with y . PL measurements from closely spaced Si 1− y C y /Si 1− x Ge x layers show a lower transition energy than that of isolated Si 1− y C y and Si 1− x Ge x reference samples. This is attributed to spatially indirect PL transitions between the electrons confined in the Si 1− y C y layers and the heavy holes located in the Si 1− x Ge x layers. The PL is dominated by no-phonon recombination. Electrical properties of n-type doped thick Si 1− y C y layers and modulation doped p-type Si/Si 1− x − y Ge x C y quantum well structures are presented. No carrier capture by C or C-related defects is observed at room temperature. A significant mobility enhancement is measured for n-type doped strained Si 0.996 C 0.004 layers at temperatures below 180 K, which is attributed to the splitting of the Δ valleys in the conduction band. In a modulation doped p-type Si 0.49 Ge 0.49 C 0.02 QW we observe an improved hole mobility at room temperature and 77 K compared to a corresponding sample without C, which is a consequence of the reduced strain in the layer due to substitutional C.

Self-assembling quantum dots for optoelectronic devices on Si and GaAs

The formation of self-assembling quantum dots for the different strained material systems Ge/Si, InAs/GaAs and InP/GaInP in molecular beam epitaxy is discussed. Intense 1.55 and 1.3μm room temperature photoluminescence is achieved for stacked Ge dots in Si and single InAs dot layers in GaAs, respectively. Small InP/Ga0.52In0.48P quantum dots emit in the visible red spectrum. Strain-induced vertical alignment, size modification and interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for relatively large size stacked Ge and InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For extremely small and densely stacked InP and InAs dots the stronger confinement causes a red shift of the ground state emission. InP/Ga0.52In0.48P quantum dot injection lasers with coupled three-fold stacked InP dots are presented. Ground state lasing is observed at room temperature with a maximum output power up to 250 mW. The threshold current density at 300 K is jthr=2.3kA/cm2, and an external quantum efficiency of ηext=8.5% is measured for a 2 mm long laser cavity.

Spatially indirect radiative recombination of carriers localized in Si1−x−yGexCy/Si1−yCy double quantum well structure on Si substrates

Closely spaced pseudomorphic Si1−x−yGexCy and Si1−yCy quantum well (QW) layers grown by solid‐source molecular beam epitaxy on Si substrates are studied by photoluminescence (PL) spectroscopy at low temperature. In thin Si0.84Ge0.16/Si1−yCy double QWs, a no‐phonon PL line of enhanced intensity and a weaker Si‐like TO‐phonon replica line are observed at lower energy, compared to reference structures with isolated Si0.84Ge0.16 and Si1−yCy QWs. These PL lines are attributed to spatially indirect (type II) transitions of electrons and holes confined to the coupled Si1−yCy and Si1−xGex layers, respectively. The PL intensity depends strongly on layer width. It increases exponentially for decreasing QW widths and it decreases when thin Si layers are deposited in between the QWs. This behavior is well described considering the wave function overlap of carriers which strongly influences the rate of indirect optical transitions.

Band alignment in Si1−yCy/Si(001) heterostructures

Photoluminescence peak energy shifts under applied [110] and [100] uniaxial stress are interpreted within the framework of a multi-band Kohn–Luttinger model which takes into account the mixing of heavy, light, and spin-orbit split-off holes within the valence band. Experimental data are presented for 0.5%, 1%, and 1.7% Si1−yCy/Si samples which are best fitted with a conduction band offset of approximately 70%. At this value of the conduction band offset, we show that small amounts of space charge induced band bending are required to explain the experimentally observed results.

Fabrication and band alignment of pseudomorphic Si1 − yCy, Si1 − x − yGexCy and coupled Si1 − yCySi1 − x − yGexCy quantum well structures on Si substrates

The structural and photoluminescence (PL) properties of several types of pseudomorphic Si 1-x-y Ge x C y quantum well (QW) structures grown by solid-source molecular beam epitaxy on (0 0 1) Si substrates are described. Optimum Si 1-y C y growth takes place at a substrate temperature of about 550°C and a growth rate ≤ 1 A/s. Well-defined alloy layers with no defects or SiC precipitates are observed by transmission electron microscopy (TEM). Excitonic band edge related PL is emitted from Si 1-y C y /Si multiple QWs (MWQ). The band gap of strained Si 1-y C y is drastically reduced by about ΔE = - y x 6.5 eV. Reducing the width of Si 0.99 C 0.01 layers results in a PL blueshift up to 45 meV which is attributed to the strong (weak) quantum well confinement of Δ(2) valley electron (light hole) states. The band alignment in Si 1-y C y /Si QWs is basically explained by the strain-induced shift of levels due to C incorporation. In Si 1-x-y Ge x C y QWs, compressive strain caused by Ge is partially compensated by C and the band gap increases by ΔE = y x 2.4 eV. Si 1-y C y , as well as Si 1-x-y Ge x C y QWs give rise to spatially direct PL transitions. Closely spaced Si 1-y C y /Si 1-x Ge x double quantum wells (DQW) give rise to spatially indirect PL recombination of Δ(2) electrons confined in the Si 1-y C y layers and heavy holes localized in the Si 1-x Ge x layers. The no-phonon transitions and the integrated PL intensity from thin DQWs are strongly enhanced compared to SQWs. In Hall transport studies, Si 1-y C y and SiGeC alloys on Si reveal electron and hole mobilities which are well comparable to Si and SiGe or even improved. C alloying provides a significant extension of the possibilities in band structure engineering of Group-IV semiconductors.

Photoluminescence study of Si1−yCy/Si quantum well structures grown by molecular beam epitaxy

Low‐temperature photoluminescence (PL) spectroscopy is applied to investigate pseudomorphic Si1−yCy/Si quantum well structures grown by solid‐source molecular beam epitaxy on Si substrates. The influence of substrate temperature during growth, growth rate, C content, and layer width on PL spectra is studied. Distinct band‐edge related no‐phonon and Si‐like TO phonon replica PL lines are observed from samples grown at substrate temperatures of about Ts=500–600 °C. The band gap in strained Si1−yCy alloy layers on Si decreases strongly with increasing C content. Even a single Si0.988C0.012 layer of 60 A thickness shows clear PL of confined excitons. Si1−yCy layers of 11 A width reveal band‐edge PL for a C content up to about y=6.4%. A high layer quality is observed in transmission electron microscopy. The intrinsic and extrinsic Si1−yCy alloy properties, quantum confinement, and layer quality are discussed on the basis of the PL results.

Self-assembling InAs and InP quantum dots for optoelectronic devices

Abstract Stranski–Krastanov growth in molecular beam epitaxy allows the preparation of self assembling InAs and InP quantum dots on GaAs and Ga 0.52 In 0.48 P buffer layers, respectively. InAs dots in GaAs prepared by slow growth rates and low temperature overgrowth provide intense photoluminescence at the technologically important wavelength of 1.3 μm at room temperature. Strain induced vertical alignment, size modification and material interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for the slowly deposited stacked InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For very small densely stacked InP and InAs dots the reduced confinement shift causes a red shift of the ground state emission. The InP quantum dots show intense and narrow photoluminescence at room temperature in the visible red spectral range. First InP/Ga 0.52 In 0.48 P quantum dot injection lasers are prepared using threefold stacked InP dots. We observe lasing at room temperature in the wavelength range between 690–705 nm depending on the size of the stacked InP dots.

Growth and thermal stability of pseudomorphic Ge1−yCy/Ge superlattices on Ge(001)

High quality Ge/Ge1−yCy superlattices with nominal carbon contents of 1.2% and 2.1% were grown by molecular beam epitaxy on Ge(001). In transmission electron microscopy the layers are planar and perfectly pseudomorphic without any extended defects observable. The infrared absorption line at 529 cm−1 is attributed to the local vibrational mode of substitutional carbon in germanium. However, in contrast to Si1−yCy alloys where almost 100% of the C is substitutional under optimized growth conditions, x-ray diffraction measurements indicate that the efficiency of carbon incorporation onto substitutional sites is only about 30% for low temperature growth at TS=200 °C. It reduces further for higher growth temperatures to only about 10% at TS=300 °C. Post-growth annealing experiments indicate thermal stability up to 450 °C. Annealing at higher temperature results in a reduction of substitutional carbon content. As in the case of Si1−yCy alloys the built-in strain is relaxed by C diffusion and not by nucleation o...

Growth and characterization of Ge1 − yCy/Si superlattice structures on Si substrates

Abstract The growth of short-period Ge1 − yCy/Si superlattice structures by solid-source molecular beam epitaxy on (001) Si substrates is investigated. High quality strained Ge1 − yCy alloy layers with a typical C content of 5% and a thickness up up to 7 A are grown at a substrate temperature of about 250°C. Transmission electron microscopy shows a high interface quality which is well comparable to Ge/Si structures. No lattice defects are observed. X-ray diffraction reveals a strong strain compensation of about 45% in Ge0.954C0.046 layers as compared to pure Ge layers. The diffraction data agree with simulation results which are based on Vegards law and on linearly interpolated elastic moduli. The phonon modes observed by Raman spectroscopy indicate well defined Ge1 − yCy layers. After annealing at temperatures up to about 750°C the Ge1 − yCy layers show a slightly lower threshold for interdiffusion and an improved stability for lattice relaxation, compared to pure Ge layers.

Magnetic-field-induced transitions between minibands in GaAs/AlxGa1−x As superlattices

The transition from closed to open electron orbits in a magnetic field has been studied by polarized hot electron photoluminescence (HEPL) in superlattices with varied widths of electron minibands. The dependence of the HEPL depolarization on the miniband width is observed. The strong depolarization occurring when the kinetic energy of electrons exceeds the energy gap between the minibands is interpreted in terms of a magnetic-field-induced transition between the electron minibands (magnetic breakdown).