N. L. Rowell

Intense photoluminescence between 1.3 and 1.8 μm from strained Si1−xGex alloys

Intense photoluminescence (PL) from strained, epitaxial Si1−xGex alloys grown by molecular beam epitaxy is reported with measured internal quantum efficiencies up to 31% from random alloy layers, single buried strained layers, and multiple quantum wells. Samples deposited at ∼400 °C exhibited low PL intensity, whereas annealing at ∼600 °C enhanced the intensity by as much as two orders of magnitude. This anneal treatment was found to be optimal for removal of grown‐in defect complexes without creating a significant density of misfit dislocations. PL peak energies at 4.2 K varied from 620 to 990 meV for Ge fractions from 0.53 to 0.06, respectively. Efficient PL was due to exciton accumulation in the strained Si1−xGex layers of single and multiple quantum wells, where the band gap was locally reduced. Optical transitions associated with the PL occurred without phonon assistance.

Ge dots and nanostructures grown epitaxially on Si

We review recent progress in the growth and characterization of Si1−xGex islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si1−xGex coverage, and the effect of growth parameters or post-growth annealing on the shape of islands and dots. We outline some of the structural, vibrational, and optical properties of Si1−xGex islands and review recent advances in the determination of their composition and strain distribution. In particular, we present an analytical electron transmission microscopy study of the Ge spatial distribution in Ge dots and Si /Si1−xGex island superlattices grown by molecular beam epitaxy and ultra-high vacuum chemical vapour deposition. We describe the use of undulated Si1−xGex island superlattices for infrared detection at telecommunication wavelengths. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and, in particular, control their size, density, and spatial distribution. As examples, we show how C pre-deposition on Si(001) can influence nucleation and growth of Ge islands and how low temperature Si homo-epitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

Luminescence origins in molecular beam epitaxial Si1−xGex

Interstitial‐type features smaller than ∼1.5 nm and in areal densities up to 7×108 cm−2 have been identified as the origin of a broad photoluminescence (PL) band from thick, fully strained layers of Si1−xGex alloys grown by molecular beam epitaxy. The strong PL band was predominant when the alloy layer thickness was greater than 4–10 nm, depending on x and the growth temperature. Thinner alloy layers exhibited phonon‐resolved transitions originating from shallow dopant bound excitons, similar to bulk material but shifted in energy due to strain and hole quantum confinement.

High quantum efficiency photoluminescence from localized excitons in Si1−xGex

We report a new photoluminescence process in epitaxial Si1−xGex layers grown on Si by rapid thermal chemical vapor deposition which we attribute to the recombination of excitons localized at random alloy fluctuations. This luminescence is characterized by saturation at very low excitation densities (≂100 μW cm−2), very long decay times (≳1 ms), and high quantum efficiency at low excitation. We have directly measured an external photoluminescence quantum efficiency of 11.5±2%.

Electroluminescence and photoluminescence from Si1-xGex alloys

Electroluminescence has been observed from Si1−xGex/Si p‐n heterostructures grown by molecular beam epitaxy and fabricated into mesa diodes. The luminescence from each sample was observed at temperatures up to 80 K with diodes forward biased at current densities up to 50 A/cm2. For x=0.18 and x=0.25, broad (∼80 meV) electroluminescence peaks were observed at 890 and 860 meV, respectively. These energies as well as the peak shapes and quantum efficiencies (∼1%) were the same as those from corresponding photoluminescence spectra.

Photoluminescence studies of Si(100) doped with low-energy (100-1000 eV) B+ ions during molecular beam epitaxy

Temperature‐dependent photoluminescence (PL) measurements have been used to characterize 5‐μm‐thick Si(100) epitaxial layers doped in situ during molecular beam epitaxial growth with low‐energy (100, 500, and 1000 eV) 11 B+ ions at growth temperatures of 500, 650, and 800 °C. Moderate doping (NB ∼1017 cm−3) yielded PL features comprised of both sharp and broad peaks in the boron bound exciton (B‐BE) region. At 4.2 K a broad B‐BE feature near 1086 meV dominated, although the sharp transverse optical phonon‐assisted B‐BE peak (B1TO ) at 1092.5 meV was resolvable for NB<1017 cm−3. Increasing the PL sample temperature above 4.2 K caused a rapid decay of the broad B‐BE peak intensity, thus permitting comparison of B1TO intensity for a range of ion energies and growth temperatures. At 10 K, a bulk‐like spectrum containing a sharp B1TO peak with weaker multiexciton peaks B2TO and B3TO was observed for the film growth at the highest temperature and lowest ion energy (800 °C and 100 eV). However, the intensity of ...

Exciton luminescence in Si1−xGex/Si heterostructures grown by molecular beam epitaxy

Coherent Si1−xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low‐temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon‐resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 A, depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109 cm−2) of strain perturbations (local lattice dilation ∼15 A in diameter) observed in plan‐view TEM. Thinner alloy layers exhibited phonon‐resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time‐resolved PL decay, together with the power and temperature dependence of luminescence inten...

Growth and characterization of SiGe atomic layer superlattices

Abstract The MBE growth of ultrathin (Si m Ge n ) p atomic layer superlattices (ALSs) consisting of p periods of alternating m and n monolayers of silicon and germanium is reported. Various structures were prepared on (100) Si, (100) Ge and on a 20-nm Si 0.5 Ge 0.5 buffer on (100) Si. The physical properties of the ALSs were investigated by Raman scattering (RS), double-crystal X-ray diffraction, transmission electron microscopy and photoluminescence (PL). The samples were found to have accurate stoichiometry and thicknesses. Substrate-dependent variations in the growth morphology were observed. The microstructures showed some waviness on silicon or germanium substrates and were heavily defected when an alloy buffer was introduced. Phonon peaks due to folding of acoustic modes were seen in the frequency range 20–200 cm −1 . The optical modes in RS were calculated using a linear chain model and were compared with the experimental spectra. Good fits were obtained after introducing interdiffusion in the model by adjusting the mass of the interfacial atoms. The PL investigation revealed features related to defects in silicon. However, no strong luminescent features that could be ascribed to a direct band-gap transition were observed.

Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and∕or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal.

Suppression of interfacial reaction for HfO2 on silicon by pre-CF4 plasma treatment

In this letter, the effects of pre-CF4 plasma treatment on Si for sputtered HfO2 gate dielectrics are investigated. The significant fluorine was incorporated at the HfO2∕Si substrate interface for a sample with the CF4 plasma pretreatment. The Hf silicide was suppressed and Hf–F bonding was observed for the CF4 plasma pretreated sample. Compared with the as-deposited sample, the effective oxide thickness was much reduced for the pre-CF4 plasma treated sample due to the elimination of the interfacial layer between HfO2 and Si substrate. These improved characteristics of the HfO2 gate dielectrics can be explained in terms of the fluorine atoms blocking oxygen diffusion through the HfO2 film into the Si substrate.