Sublimation Molecular-beam Epitaxy Research Articles

On the Influence of Pulsed Gamma-Neutron Irradiation on the Morphology of Self-Assembled GeSi/Si(001) Nanoislands

The influence of pulsed gamma-neutron irradiation on the surface morphology of GeSi/Si(001) heterostructures with surface self-assembled GeSi nanoislands grown by the combined sublimation molecular-beam epitaxy of Si and the low-pressure vapor-phase epitaxy of Ge is experimentally studied. Gamma-neutron irradiation is found to cause a change in the shapes of self-assembled GeSi nanoislands, in particular, a reduction in the aspect ratio of the nanoislands. The effect is related to elastic-strain relaxation in the GeSi nanoislands under their exposure to elastic waves generated in the course of gamma-neutron irradiation.

Conditions of growth of high-quality relaxed Si1–x Ge x layers with a high Ge content by the vapor-phase decomposition of monogermane on a sublimating Si hot wire

The conditions of the epitaxial growth of high-quality relaxed Si1–x Ge x layers by the combined method of the sublimation molecular-beam epitaxy and vapor-phase decomposition of monogermane on a hot wire are considered. The combined growth procedure proposed provides a means for growing Si1–x Ge x layers with a thickness of up to 2 µm and larger. At reduced growth temperatures (T S = 325–350°C), the procedure allows the growth of Si1–x Ge x layers with a small surface roughness (rms ≈ 2 nm) and a low density of threading dislocations. The photoluminescence intensity of Si1–x Ge x :Er layers is significantly (more than five times) higher than the photoluminescence intensity of layers produced under standard growth conditions (T S ≈ 500°C) and possess an external quantum efficiency estimated at a level of ~0.4%.

Photodiodes based on self-assembled GeSi/Si(001) nanoisland arrays grown by the combined sublimation molecular-beam epitaxy of silicon and vapor-phase epitaxy of germanium

We investigate the photosensitivity spectra of photodiodes based on Si p-i-n structures with single-layered and multilayer self-assembled GeSi/Si(001) nanoisland arrays in the i region, which are grown using a technique combining Si molecular-beam epitaxy and Ge vapor-phase epitaxy, in dependence on the temperature, diode bias, and GeSi nanoisland parameters. We show that the temperature and field dependences of the diode photosensitivity in the spectral range of the interband optical absorption in GeSi nanoislands are determined by the ratio between the rate of emission of photoexcited holes from the nanoislands and the rate of the recombination of excess carriers in them. We demonstrate the possibility of determination of the hole recombination lifetime in GeSi nanoislands from the temperature and field dependences of the photosensitivity.

Specific features of the nonradiative relaxation of Er3+ ions in epitaxial Si structures

The specific features of the nonradiative relaxation of Er3+ ions in Si:Er layers grown by sublimation molecular-beam epitaxy (SMBE) are studied. In Si:Er/Si diode structures containing precipitation-type emitting centers, a resonance photoresponse at the wavelength λ ≈ 1.5 μm is observed, which is indicative of the nonradiative relaxation of Er3+ ions via the energy back-transfer mechanism. Saturation of the erbium-related photocurrent is for the first time observed at high temperatures. This allows estimation of the concentration of Er centers that undergo relaxation via the above-mentioned back-transfer mechanism (N 0 ≈ 5 × 1016 cm−3). In terms of order of magnitude, the estimated concentration N 0 corresponds to the concentration of optically active Er ions upon excitation of the Si:Er layers by means of the recombination mechanism. The features of the nonradiative relaxation of Er3+ ions in Si:Er/Si structures with different types of emitting centers are analyzed.

Auger de-excitation of different Er centers in Si:Er layers grown with sublimation molecular beam epitaxy

The Auger de-excitation of erbium ions in sublimation molecular beam epitaxy (SMBE) grown Si:Er/Si layers differing in the type of Er centers is investigated. In layers with the precipitate-like emitting centers (Tgr ∼ 500 °C) the Auger de-excitation of Er3+ ions depends on the ionization of donors with the ionization energy Ei ∼ 38 meV and a concentration of about 1 · 1016 cm−3, whereas in layers with the Er-O1 centers (Tgr ∼ 400 °C) it is determined by the ionization of donors having a lower ionization energy Ei ∼ 20 meV and a higher concentration of ∼2 · 1017 cm−3. It is shown that the compensation ratio of SMBE grown n-Si:Er layers with the precipitate-like emitting centers reaches 10% and more. The nature of the compensating impurity has not been identified.

Luminescent properties of MBE-grown Si:Er/SOI structures

Here we report on luminescent properties of multilayer Si:Er structures grown by sublimation molecular-beam epitaxy on “silicon-on-insulator” substrates. We demonstrate formation in such structures of a unique erbium-related center Er-1. This optical complex stands out among other known erbium-related centers in silicon for its record narrow luminescent line (<10μeV) and largest absorption cross-section and, therefore, provides the best conditions to achieve practically significant amplification and stimulated emission in erbium-doped silicon structures.

Electroluminescence at a wavelength of 1.54 μm in Si:Er/Si structures consisting of a number of p-n junctions

A method of connecting several p +-n junctions in the same Si:Er/Si structure is demonstrated; this method makes it possible to increase the electroluminescence intensity at a wavelength of 1.54 μm. The structures have been grown by sublimation molecular-beam epitaxy.

On the nature of electroluminescence at 1.5 μm in the breakdown mode of reverse-biased Er-doped silicon p-n-junction structures grown by sublimation molecular beam epitaxy

Electroluminescence features in the wavelength range of 0.9–1.65 μm were experimentally studied in the breakdown mode of reverse biased Si/Si:Er/Si p-n-junction structures grown by sublimation molecular-beam epitaxy. Based on the results of this study, a new physical model is proposed, in which radiative transitions in the near-infrared region are excited by recombination of electrons arriving at corresponding energy levels in the Si:Er layer due to their tunneling from the valence band of the p+-layer in the electric field of the reverse biased p-n-junction. The model proposed is in qualitative agreement with main available experimental results.

Dependence of the concentration of ionized donors on epitaxy temperature for Si:Er/Si layers grown by sublimation molecular-beam epitaxy

The dependence of the concentrations of the Er impurity and ionized donors on the epitaxy temperature has been studied before and after annealing of Si:Er/Si layers grown by sublimation molecular-beam epitaxy. n-Si:Er layers have been grown in the temperature range 400–800°C and annealed in hydrogen atmosphere at a temperature of 800°C for 30 min. The possible nature of the donor centers is discussed.

Electroluminescence at a wavelength of 1.5 μm in Si:Er/Si diode structures doped with Al, Ga, and B acceptors

Si:Er layers in diode structures were doped with Al, Ga, or B during growth by sublimation molecular-beam epitaxy. As a result, a sharp increase in the electroluminescence intensity at a wavelength of 1.5 μm was observed in diodes with thick bases (as large as 0.8 μm).

Confocal Raman microscopy of self-assembled GeSi/Si(001) Islands

The method of confocal Raman microscopy is used for the first time to study the spatial distribution of elemental composition and elastic strains in self-assembled GexSi1 − x/Si(001) islands grown by the method of sublimation molecular-beam epitaxy in the GeH4 ambient. The lines related to vibrational modes Si-Si, Ge-Ge, and Si-Ge are identified in the Raman spectra measured in the regions with dimensions <100 nm on the surface of the samples. The spatial distribution of the Ge atomic fraction x in the GexSi1 − x alloy and of the elastic strain ɛ (averaged in depth over the island layer) have been calculated from the maps of the Raman shifts of the corresponding lines over the sample surface. The dependences of x and ɛ on the islands’ growth temperature and on the nominal thickness of the deposited Ge layer have been studied.

Sublimation molecular beam epitaxy of silicon-based structures

The capabilities of various methods for fabricating silicon-based single-crystal structures are analyzed. The features and advantages of sublimation molecular-beam epitaxy are discussed.

Si:Er/Si diode structures for observing room-temperature electroluminescence at a wavelength of 1.54 μm

Si:Er/Si diode structures for observing room-temperature electroluminescence at a wavelength of 1.5 μm are analyzed. The structures were grown by sublimation molecular-beam epitaxy. Some ways to increase the electroluminescence intensity are discussed.

10.1007/s11453-008-2003-z

Temperature dependences of the concentration and electron Hall mobility in Si:Er/Sr epitaxial layers grown at T = 600°C and annealed at 700 or 900°C have been investigated. The layers were grown by sublimation molecular-beam epitaxy in vacuum (∼10−5 Pa). The energy levels of Er-related donor centers are located 0.21–0.27 eV below the bottom of the conduction band of Si. In the range 80–300 K, the electron Hall mobility in unannealed Si:Er epitaxial layers was lower than that in Czochralski-grown single crystals by a factor of 3–10. After annealing the layers, the fraction of electron scattering from Er donor centers significantly decreases.

Photoelectric properties of the GeSi/Si heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH4 medium

The photosensitivity (PS) spectra of the GeSi/Si(001) heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH4 medium have been studied by photo-emf spectroscopy at the semiconductor/electrolyte junction (PSE) and by photo-emf and photocurrent spectroscopy of Schottky barriers including the temperature dependence of the PS spectra in the temperature range of 10–300 K. The bands related to the phonon assisted and phonon-less interband of spatially indirect optical transitions in the GeSi nanoclusters have been observed in the PS spectra even at 300 K. The scatter effect of the GeSi nanoclusters in size and/or in composition on the PS spectrum’s edge shape has been theoretically considered. The emission theory of the photoexcited carriers from the quantum wells of the GeSi/Si nanoclusters built-in to a Schottky barrier (p-tn junction, semiconductor/electrolyte junction) has been developed.

Optical properties of Si/Si:Er multi-nanolayer structures grown by SMBE method

Among Er-doped crystalline Si materials, Si/Si:Er multi-nanolayer structures grown by sublimation molecular beam epitaxy (SMBE) technique present extraordinary optical properties, with a narrow linewidth of the Er-related emission at 1.5 μm. Based on spectral analysis, the splitting of the ground state of Er3+ ions and the presence of only a single type of Er-related optical center, labeled Er-1, have been conclusively established. In this contribution, we briefly summarize some of the unique optical properties of Si/Si:Er multi-nanolayer structures: preferential formation of the Er-1 center and its microscopic structure, level of optical activity of Er, and relations with a donor level as well as with oxygen co-doping. Then we report on the most recent results on optical gain in Si/Si:Er multi-nanolayers. From variable stripe-length (VSL) and shifted excitation spot (SES) experiments, we estimated maximum gain cross section of Er-1 in the multi-nanolayer structure at about σ~10−17 cm2. We also evaluate the magnitude of induced absorption by free carriers, and show that it precludes observation of the net optical gain.

High-resolution spectroscopy and time-resolved study of electroluminescence of Er-1 center in silicon

Abstract High-resolution and time-resolved measurements on the electroluminescence from Er-doped silicon diode structures with Er-1 center, grown with sublimation molecular beam epitaxy, have been performed within the temperature interval 30–120 K. We find emission lines with full width down to 0.2 cm −1 (25 μeV) at 30 K, the narrowest lines ever observed in Si:Er electroluminescence spectra, and excitation cross-section of 4×1 −15 cm 2 . Auger-deexcitation of Er 3+ ions with the activation energy of 16 meV was found to be the only deexcitation process in these structures and no ‘back-transfer’ deexcitation was observed. Due to ultra narrow emission lines and a high excitation cross-section such diode structures are promising for realization of an electrically pumped silicon-based laser.

Extremely sharp electroluminescence from Er-doped silicon

Electroluminescence from Er-doped silicon diode structures, grown with sublimation molecular beam epitaxy, has been investigated with high spectral resolution down to 0.05 cm−1. In structures optimized for preferential formation of the Er-1 center we find emission lines with full width down to 0.2 cm−1 (25 µeV) at 30 K—the narrowest lines ever observed in Si:Er electroluminescence spectra. Due to ultra-narrow emission lines and a high excitation cross-section of 4 × 10−15 cm2, such diode structures are promising for realization of an electrically pumped silicon-based laser.

Growth of n-Si layers by molecular-beam epitaxy on the substrates heavily doped with boron

Epitaxial n-Si layers doped with phosphorus or erbium have been grown by sublimation molecularbeam epitaxy at 500°C on heavily boron-doped p+-type substrates with resistivity ρ = 0.005 Ω cm. Distribution profiles of the B, Er, and O impurity concentrations in the samples were determined by secondary-ion mass spectrometry. A thermal annealing of the substrate in vacuum at 1300°C for 10 min and growth at a very low substrate temperature made it possible to obtain an extremely abrupt profile for doping impurities at the layer-substrate interface. This method for growth of n-p+ junctions considerably improves their electrical and luminescent characteristics.

Photoluminescence of self-forming nanoclusters grown in GeSi/Si heterostructures by sublimating molecular beam epitaxy in GeH4 medium

Morphology and photoluminescence spectra of GeSi/Si(001) with self-forming nanoclusters obtained by the sublimation molecular-beam epitaxy in GeH4 medium under various process conditions are studied. It is found that the clusters are formed by the Stransky-Krastanov growth followed by the important coalescence processes. Doubling of the photoluminescence lines from the nanoclusters is caused by the radiative recombination; the progressing blueshift of the lines, by the Si diffusion from the substrate into the clusters. The conditions ensuring the growth of uniform nanocluster arrays exhibiting photoluminescence at room temperature are determined.