Nanocrystalline Ge synthesis by the chemical reduction of hydrothermally grown Si0.6Ge0.4O2

Previously, we have reported the use of high pressure oxidation techniques for the growth of compositionally congruent oxides from Si_1−xGe_x. We have now used this technique as part of a two-step process of oxidation at 25 MPa and 475°C followed by reduction in 0.1 MPa (1 atm) H₂ at 700-850δC for the synthesis of nanocrystalline Ge precipitates. Using transmission electron microscopy, we show that the proposed method produces a dispersion of fine (<10 nm) precipitates of Ge embedded in an SiO₂ matrix. The structure of the oxide prior to reduction with H₂ was investigated with Fourier transform infrared spectroscopy which reveals SiO₂, GeO₂, SiO, Si-O-H, and Ge-O-H bonding states in the glass. In this paper, we discuss the thermodynamics and kinetics of both the hydrothermal oxidation technique and the proposed Ge nanocrystalline synthesis process.     

Characterization of reduced pressure chemical vapor deposited Si0.8Ge0.2/Si multi-layers

We have investigated the Si_0.8Ge_0.2/Si multi-layer grown directly onto the Si (001) substrates using reduced pressure chemical vapor deposition. The thicknesses of the Si_0.8Ge_0.2/Si multi-layer were determined using transmission electron microscopy. From the results of energy-dispersive X-ray spectroscopy and X-ray diffraction analyses on the Si_0.8Ge_0.2/Si multi-layer, Ge composition in the Si_1?xGe_x layers was determined as ～20% and the value of residual strain ε of the Si_0.8Ge_0.2 layer is calculated to be 0.012. Three peaks are observed in Raman spectrum, which are located at approximately 514, 404, and 303 cm^?1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence spectrum originates from the radiative recombinations both from the Si substrate and the Si_0.8Ge_0.2/Si multi-layer. For the Si_0.8Ge_0.2/Si multi-layer, the transition peaks related to the quantum well region observed in the photocurrent spectrum were preliminarily assigned to e–hh and e–lh fundamental excitonic transitions.     

Modification of quantum dots in Ge/Si nanostructures by pulsed laser irradiation

The goal of this study was the development of a method for the modification of a quantum dot (QD) structure in Ge/Si nanostructures by pulsed laser irradiation. The Ge_xSi_1?x QD structures were analyzed using data furnished by Raman spectroscopy. Frequency-dependent admittance measurements were used to study the energy spectrum of holes in the Ge/Si heterostructures with Ge_xSi_1?x QDs before and after the laser treatment. The obtained experimental data show that laser treatment makes it possible to reduce the sheet density of QDs, modify their composition, and increase the average size. The most important result is that the QD parameters become more uniform after the treatment with nanosecond laser pulses. In a sample with ODs of 8-nm average lateral size (six monolayers of Ge), the scatter of energy levels in the QD array is reduced by half after the treatment with 10 laser pulses.     

Photoluminescence and photoreflectance study of Si/Si0.91Ge0.09 andSi9/Ge6 quantum dots

Nanometer-scale quantum dots based on a series of Si/Si_0.91Ge_0.09 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.     

Annealing studies of Be-implanted GaAs0.6P0.4

Differential resistivity and Hall effect measurements and secondary ion mass spectrometry (SIMS) are used to study the annealing behavior of Be-implanted GaAs_0.6P_0.4. Results similar to that previously reported for Be-implanted GaAs are observed, including outdiffusion of Be into the Si₃N₄ encapsulant during 900‡C annealing of high dose implants. Nearly all (85–100%) of the Be remaining after a 900‡C, 1/2 hr anneal is electrically active. However, the electrical activation at low annealing temperatures (600–700‡C) is much lower in GaAs_0.6P_0.4 than in GaAs. A substantial amount of diffusion is observed even for the low fluence Be implants in GaAs_0.6P_0.4 annealed at 900‡C, indicating a greater dependence of the diffusion on defect-related effects in the ternary. This work was supported by the Joint Services Electronics Program (U.S. Army, U.S. Navy, U.S. Air Force) under Contract DAAB-07-72-C-0259, by Monsanto Company, and by the Naval Electronic Systems Command. On leave at Cornell University, Department of Electrical Engineering, Ithaca, NY 14853.     

Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating

By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si_0.4Ge_0.6/Si(100) heterostructure. For the strained Si_0.5Ge_0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm² V⁻¹ s⁻¹ with a carrier concentration of 1.3×10¹⁸ cm⁻³ for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p⁺Si/nSi_0.5Ge_0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.     

DC and RF Characteristics of Si0.8Ge0.2 pMOSFETs: Enhanced Operation Speed and Low 1/f Noise

This paper reports on our investigation of DC and RF characteristics of p‐channel metal oxide semiconductor field effect transistors (pMOSFETs) with a compressively strained Si_0.8Ge_0.2 channel. Because of enhanced hole mobility in the Si_0.8Ge_0.2 buried layer, the Si_0.8Ge_0.2 pMOSFET showed improved DC and RF characteristics. We demonstrate that the 1/f noise in the Si_0.8Ge_0.2 pMOSFET was much lower than that in the all‐Si counterpart, regardless of gate‐oxide degradation by electrical stress. These results suggest that the Si_0.8Ge_0.2 pMOSFET is suitable for RF applications that require high speed and low 1/f noise.     

Low temperature heteroepitaxial growth of Si1−xGexon-Si by photo-enhanced ultra high vacuum chemical vapor deposition using Si2H6 and Ge2H6

Strained-layer Si_1×Ge_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 10¹⁵ photons/cm²·pulse. To fit the Si_1−xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 × 10⁻¹⁶ cm², which is 30 times larger than that of Si₂H₆ (3.4 × 10⁻¹⁸ cm2). For Si_1−xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1−xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1−xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1−xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.     

Synthesis and physical properties of Si(Ge)-Se-Te glasses

Ternary Si₁₅Ge(Ga)₅Te₈₀, Si_19.7Te_78.7Se_1.6, Si_19.2Te_76.8Se₄, and Ge₁₉Te₇₂Se₉ telluride glasses were synthesized. Electrical, acoustic, acoustooptical properties, and the dispersion of optical transmittance of these films were studied in a wide range of temperatures and frequencies. Comparative analysis of the results obtained is performed. Possible mechanisms of the observed phenomena are discussed. It is shown that Ge₁₉Te₇₂Se₉ alloy is quite competitive with Si₂₀Te₈₀ alloy for the fabrication of highly efficient acoustooptical devices with a wide range of applications in the middle IR spectral region (2–12 μm). __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 7, 2003, pp. 822–826. Original Russian Text Copyright ? 2003 by Kulakova, Melekh, Bakharev, Kudoyarova.     

Numerical analysis of SiGe heterojunction bipolar phototransistor based on virtual substrate

SiGe heterojunction bipolar phototransistors based on Si_1?yGe_y virtual substrate have been proposed in this paper. The current gain and external quantum efficiency of the heterojunction bipolar phototransistors are calculated by solving one-dimensional diffusion equations. The computed results show that external quantum efficiency increases with the increase of Ge content of Si_1?yGe_y virtual substrate and a high external quantum efficiency of 142 can be obtained in the heterojunction bipolar phototransistor with ten periods of Ge/Si_0.4Ge_0.6 multi-quantum wells on virtual substrate with Ge content of 0.6. The dependences of external quantum efficiency and current gain on structure parameters of the heterojunction bipolar phototransistors are investigated in detail. It is useful for design and optimization of the heterojunction bipolar phototransistors.     

Structural and electrical properties of the Ge x Si1−x /Si heterojunctions obtained by the method of direct bonding

The results of studying the structural and electrical properties of structures produced by the method of direct bonding of Ge _x Si_1?x and Si wafers are reported. The wafers were cut from the crystals grown by the Czochralski method. Continuity of the interface and the crystal-lattice defects were studied by X-ray methods using synchrotron radiation and by scanning electron microscopy. Measurements of the forward and reverse current-voltage characteristics of the p-Ge _x Si_1?x /n-Si diodes made it possible to assess the effect of the crystallattice defects on the electrical properties of heterojunctions. Satisfactory electrical parameters suggest that the technology of direct bonding is promising for the fabrication of large-area Ge _x Si_1?x /Si heterojunctions.     

Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15

Interfacial reactions and electrical properties of Hf/p-Si_0.85Ge_0.15 as a function of the annealing temperature were studied. Hf₃(Si_1−xGe)₂ and Hf(Si_1−xGe)₂ were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si_0.85Ge_0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si_1−xGe_x)₂ films. All the as-deposited and annealed Hf/p-Si_0.85Ge_0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10⁻⁵ ω cm² could be obtained for the Hf₃ (Si_1−xGe_x)₂ contacts to p-Si_0.85Ge_0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf₃(Si_1−xGe_x)₂ and an interfacial Ge-rich layer between the Hf₃(Si_1−xGe_x)₂ and unreacted Si_0.85Ge_0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si_1−xGe_x)₂ and SiC as well as the roughness of the Hf(Si_1−xGe_x)₂ films.     

The band structure and photoluminescence in a Ge0.8Si0.2/Ge0.1Si0.9 superlattice with vertically correlated quantum dots

The energy band diagram of the multilayered Ge_0.8Si_0.2/Ge_0.1Si_0.9 heterostructures with vertically correlated quantum dots is analyzed theoretically. With regard to fluctuations of the thickness layer in the columns of quantum dots and to the exciton-phonon coupling, it is shown that the electron states constitute a miniband. The hole wave functions remain localized in the quantum dots. The spectrum of optical transitions calculated for a 20-layered structure at room temperature is in good agreement with the experimental photoluminescence spectrum that involves an intense band at about 1.6 μm. From theoretical considerations and experimental measurements, specific evidence for the miniband in the superlattice is deduced; it is found that the overlap integrals of the wave functions of electrons and holes and the integrated intensity of the photoluminescence band of the Ge quantum dots are described by quadratic functions of the number of the structure periods.     

Si/Si1−x Gex epitaxial layers and superlattices. Growth and structural characteristics

Si, Ge, and Si_1−x Ge_x epitaxial layers and Si/Si_1−x Ge_x superlattices have been obtained on (100) and (111) silicon substrates by molecular-beam epitaxy. The growth processes and the structural characteristics and chemical composition of the structures were studied by x-ray diffraction and Auger spectroscopy. It is shown that under the experimental conditions for obtaining Si/Si_1−x Ge_x superlattices structurally perfect, strained superlattices with satellites up to ±5 orders can be obtained. Fiz. Tekh. Poluprovodn. 31, 922–925 (August 1997)     

Role of a Si0.95Ge0.05 epilayer cap on boron diffusion in silicon under inert and dry oxidizing ambient annealing

Silicon (Si) and Si with a 60 nm Si_0.95Ge_0.05 epilayer cap (Si_0.95Ge_0.05/Si) were implanted with 60 keV, 1×10¹³ cm⁻² boron (B) followed by annealing in nitrogen (N₂) or dry oxygen (O₂) in two different anneal conditions. B⁺implantation energy and dose were set such that the B peak is placed inside Si in Si_0.95Ge_0.05/Si samples and concentration independent B diffusion is achieved upon annealing. For samples annealed above 1075 °C, Ge diffusing from the Si_0.95Ge_0.05 epilayer cap in Si_0.95Ge_0.05/Si samples reached the B layer inside Si and resulted in retarded B diffusion compared to the Si samples. For annealing done at lower temperatures, diffusion of Ge from Si_0.95Ge_0.05 epilayer cap does not reach the B layer inside Si. Thus B diffusion profiles in the Si and Si_0.95Ge_0.05/Si samples appear to be similar. B diffusion in dry oxidizing ambient annealing of Si_0.95Ge_0.05/Si samples further depends on the nature of Si_0.95Ge_0.05 oxidation which is set by the duration and the thermal budget of the oxidizing anneal.     

Electrical characteristics of diodes fabricated in selective Si/Si1−x Ge x epitaxial layers

Diodes have been fabricated in layers of Si_1−x Ge _x and silicon deposited selectively on patterned wafers, and the electrical characteristics of the diodes have been examined. For 50 nm thick Si_1−x Ge _x layers containing about 22% Ge, the forward characteristics of larger diodes are nearly ideal. However, the reverse leakage current is higher when the edges of the diode intersect the oxide defining the selectively deposited layers than when the diode edges are separated from this oxide. The diode characteristics are more ideal when the diode edges are aligned along the [100] directions than when aligned along the [110] directions. Higher-temperature hydrogen pre-treatments before epitaxial deposition can degrade the diode characteristics.     

Shallow acceptors in strained multiquantum-well Ge/Ge1−x Six heterostructures

Far infrared photoconductivity spectra due to excitation of shallow acceptors in strained multiquantum well Ge/Ge_1?x Si_x (x≈0.1) heterostuctures are investigated. It is shown that these spectra are shifted toward longer wavelengths in the far infrared region compared with those of bulk p-Ge, owing to “built-in” strain and size quantization, which lead to splitting of the light-and heavy-hole subbands in the Ge layers. Shallow acceptor spectra are calculated variationally for bulk germanium under uniaxial tension, which is “equivalent” to the strained Ge layers in the heterostructures. Although this method is only appropriate for wide quantum wells (d _Ge≈800 Å), the calculations are shown to qualitatively account for photoconductivity spectra obtained from narrower wells (d _Ge≈200 Å) as well.     

Research and Design of Ge0.6Si0.4／Si Strained—layer Superlattice Planar Optical Waveguide

Calculation shows that the refraction index of Ge0.6Si0.4/Si strained-layer superlattice n≈3.64,when Lw=9 nm and Lb=24 nm.An algorithm of numerical iteration for effective refraction index is emploted to obtain different effective refraction indexes at different thickness(L) .As a result, the thickness of Ge0.6Si0.4/Si strained-layer superlattice optical waveguide,L≤363 nm, can be determlned, which is very important for designing waveguide devices.An optical waveguidecan be madeinto a nanometer device by using Ge0.6Si0.4/Si strained-layer superlattice.     

Voltage-tunable dual-band infrared photodetectors with Si/SiGe metal-semiconductor-metal heterostructure

A simple and low-cost structure of voltage-tunable dual-band near-infrared photodetector (PD) has been proposed, in which the PDs were developed by using Si_0.8Ge_0.2/Si metal-semiconductor-metal (MSM) heterostructure. The Si_0.8Ge_0.2/Si layers were deposited by ultrahigh-vacuum chemical vapor deposition system and a transparent layer of indium-tin oxide (ITO) was used as a metal layer to enhance the entrance of photons. In this study, we found that only one band was detected with a peak wavelength located at 950 nm at zero applied bias. When bias was increased to 1 V, in contrast a dual-band was achieved, where two peak wavelengths were centered at 950- and 1150-nm. It is suggested that the two bands are the absorption of top-Si and bottom-Si_0.8Ge_0.2 layers, respectively. The spectra of Si bulk and Si_0.8Ge_0.2 layer were also measured to verify our results and relating mechanisms are explained here.     

Thermoelectric Performance of Sb- and La-Doped Mg2Si0.5Ge0.5

La _x Mg_2?x Si_0.49Ge_0.5Sb_0.01 compounds (x?=?0, 0.005, 0.01, 0.02) were synthesized by solid-state reaction followed by spark plasma sintering. The thermoelectric properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and ZT, of these compounds have been studied in the temperature range of 300?K to 823?K. The figure of merit of this n-type compound has been raised above unity at 823?K for the sample with x?=?0.01, a value 60% higher than that of Mg₂Si_0.49Ge_0.5Sb_0.01. The reduction of the thermal conductivity via increasing phonon scattering is considered as the main reason for the enhanced ZT. These observations demonstrate an opportunity to improve the thermoelectric performance of Mg₂Si_1?x Ge _x solid solutions.