xGex Heterostructures Research Articles

The thermovoltaic effect in variband solid solution Si1–x Ge x (0 ≤ x ≤ 1)

The thermovoltaic effect in films of variband solid solution Si1–xGex (0 ≤ x ≤ 1) has been observed for the first time. The samples comprised n-Si–p-Si1–xGex (0 ≤ x ≤ 1) heterostructures grown by liquid phase epitaxy. An electromotive force within 0.05–0.3 mV and a current of 0.0025–0.0035 μA appeared on heating samples in a temperature range from 40 to 250°C.

Work function bowing in Si1−xGex heterostructures: Ab initio results

A systematic theoretical study of the work function behavior for Si1−xGex heterostructures over the whole composition range, from Si (x = 0) to Ge (x = 1), is presented. Our results, obtained through Density Functional Theory calculations and in good agreement with experimental evidences, show that increasing the Ge content lowers the work function value. We find that in order to exactly reproduce this behaviour in relation to the work function of pure Ge and Si systems and their concentrations, a deviation from the linear Vegard's rule is necessary. However, the calculated bowing parameter is very small, thus making the simple linear interpolation a valid approximation to obtain the work function of complex SiGe alloys.

Work Function Measurement of Silicon Germanium Heterostructures Combining Kelvin Force Microscopy and X-ray Photoelectron Emission Microscopy

Work function in Si1–xGex heterostructures with Ge content in the 6% to 49% range was studied with high energy resolution by combining Kelvin force microscopy and X-ray photoelectron emission microscopy. Although the two methods are based on distinct physical mechanisms, we show that both techniques give the same work function differences between each Si1–xGex layer, as small as 20 meV. To detect such small work function differences, we put in evidence the necessity of preparing the Si1–xGex sample surface with polishing, HF etching and Ar+ sputtering. Such surface preparation allows, in principle, to reduce the deleterious influence of surface states, coming for instance from carbon atoms or native oxide, on quantitative work function extraction. We show in this paper that even after such a sample surface preparation, a strong band bending can be present, which causes a contrast inversion on the surface of the material and yields an artificially lower surface work function with respect to theoretical val...

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si1 − xGex heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si1 − xGex crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.

Features of electronic transport in relaxed Si/Si1−XGeX heterostructures with high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si1−xGex heterostructures with two-dimensional electron channel (ne≥1012cm−2) in an elastically strained silicon layer of nanometer thickness have been studied. The detailed calculation of the potential and of the electrons distribution in layers of the structure was carried out to understand the observed phenomena. The dependence of the tunneling transparency of the barrier separating the 2D and 3D transport channels in the structure, was studied as a function of the doping level, the degree of blurring boundaries, layer thickness, degree of relaxation of elastic stresses in the layers of the structure. Tunnel characteristics of the barrier between the layers were manifested by the appearance of a tunneling component in the current–voltage characteristics of real structures. Instabilities, manifested during the magnetotransport measurements using both weak and strong magnetic fields are explained by the transitions of charge carriers from the two-dimensional into three-dimensional state, due to interlayer tunneling transitions of electrons.

Optical spin orientation in group-IV heterostructures

We investigate the electron spin polarization upon photoemission from different Si1−xGex heterostructures by means of Mott polarimetry. We demonstrate the possibility to lower the vacuum energy level below the bottom of the conduction band at the Γ point of the Brillouin zone in compressively strained Si1−xGex alloys and we show that the optimization of the stoichiometry of group-IV heterostructures leads to a spin polarization of the electrons in the conduction band up to P=72%±3%. Such a value is not only greater than those attainable in compressively strained pure Ge heterostructures, but it is also comparable to the typical electron spin polarization values of III-V semiconductor heterostructures.

Upper limit of two-dimensional electron density in enhancement-mode Si/SiGe heterostructure field-effect transistors

In this paper, we present our study of the maximum electron density, nmax, accessible via low-temperature transport experiments in enhancement-mode Si/Si1−xGex heterostructure field-effect transistors. Experimentally, we find that nmax is much higher than the value obtained from self-consistent Schrödinger-Poisson simulations and that nmax can be changed only by changing the Ge concentration in the Si1−xGex barrier layer, not by varying the barrier layer thickness. The discrepancy between experiments and simulations is explained by a non-thermal-equilibrium tunneling-limited model.

Si–Ge interdiffusion under oxidizing conditions in epitaxial SiGe heterostructures with high compressive stress

Si–Ge interdiffusion under oxidizing and inert conditions has been studied in epitaxial relaxed Si1−xGex/compressive Si1−yGey/relaxed Si1−xGex heterostructures. The interdiffusion was measured by secondary ion mass spectroscopy (SIMS) and studied using simulations. Within the SIMS accuracy, the measured Ge profiles show that oxidation has a small effect, if any, on the Si–Ge interdiffusion of these structures. These results suggest that oxidation does not accelerate Si–Ge interdiffusion significantly, which lessens process integration constraints for SiGe devices such as high mobility dual channel metal oxide semiconductor field effect transistors and heterostructure tunneling field effect transistors.

Current-voltage characteristics of Si/Si1 − x Ge x heterodiodes fabricated by direct bonding

We have studied the current-voltage (I–U) characteristics of Si/Si1 − xGex (0.02 < x < 0.15) heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si1 − xGex wafers of the same orientation containing 2–15 at % Ge. An increase in the germanium concentration NGe in Si1 − xGex crystals is accompanied by a growth in the density of crystal lattice defects, which leads to a decrease in the minority carrier lifetime in the base of the heterodiode and an increase in the recombination component of the forward current and in the differential resistance (slope) of the I–U curve. However, for all samples with NGe ≤ 15 at %, the I–U curves of Si/Si1 − xGex heterodiodes are satisfactory in the entire range of current densities (1 mA/cm2–200 A/cm2). This result shows good prospects for using direct bonding technology in the fabrication of Si/Si1 − xGex heterostructures.

Effect of C incorporation on relaxation of SiGe/Si

Thin fully strained Si1−xGex/Si1−x−yGexCy/Si1−xGex heterostructures (x=0.2), with controlled C incorporation sites, were grown on Si substrates using ultrahigh vacuum chemical vapor deposition. Following the growth, layers were relaxed using rapid thermal annealing at 1000 °C for 30 s, and high degrees of relaxation of 65% and 59% were achieved with and without interstitial C, respectively. We show that the difference in cross hatch density and step height between two samples, which correspond to different misfit dislocation pileup behaviors, suggests controllability of SiGe relaxation via variation in C incorporation sites in the SiGeC layer.

Energy characteristics of boron impurity in Si∕Si1−xGex heterostructures with on-center and on-edge selective doping of quantum wells

It is shown experimentally that when an acceptor impurity is shifted from the center to the edge of the quantum wells in Si∕Si1−xGex heterostructures, the binding energy of the ground state of the impurity decreases and the radius of localization of the carriers increases.

Capacitively induced high mobility two-dimensional electron gas in undoped Si∕Si1−xGex heterostructures with atomic-layer-deposited dielectric

The authors demonstrate that a high mobility two-dimensional electron gas can be capacitively induced in an undoped Si∕Si1−xGex heterostructure using atomic-layer-deposited Al2O3 as the dielectric. The density is tuned up to 4.2×1011∕cm2, limited by the gate leakage current. The mobility increases with the density rapidly and reaches 5.5×104cm2∕Vs at the highest density. The observation of well developed quantum Hall states and two-dimensional metal-insulator transition shows that the devices are suitable for two-dimensional electron physics studies.

Photoconductivity spectra of Ge/Si heterostructures with Ge QDs

Spectral dependences of lateral photoconductivity ofSi1−xGex heterostructures with Ge nanoislands were investigated at 77 and 290 K. It issupposed that the photocurrent in the range 0.81–1.02 eV at 290 K is conditioned by anonequilibrium carrier generated by interband transitions in Ge nanoislands.Si1−xGex heterostructures with Ge nanoislands showed lateral photocurrent in the range from 0.32to 1.2 eV at 77 K. Such a photocurrent is explained by hole transitions from thelocalized states of light and heavy holes in Ge nanoislands into the delocalizedstates of the valence band. It was found that the valence bandgap offset of theheterojunction between the nanoislands and strained c-Si surrounding was 0.48 eV.

Hole tunnelling properties in resonant tunnelling diodes with Si/strained Si0.8Ge0.2 heterostructures grown on Si(1 0 0) by low-temperature ultraclean LPCVD

Hole resonant tunnelling diodes (RTDs) with Si/strained Si1–xGex heterostructures epitaxially grown on Si(1 0 0) were fabricated, and sharp current peaks have been reproducibly observed. From the quantum well width dependence of the current–voltage characteristics, at the peak voltage, heavy holes in the accumulation region resonantly tunnel through the estimated resonant states of a light hole and a heavy hole in a Si1–xGex quantum well. The top contact and mesa area dependence of the peak current shows that the tunnel current only flows under a top contact electrode, i.e. the leakage current at the sidewall is negligibly small. The resonant tunnelling current density reaches as high as 3.6 kA cm−2 at 0.19 V with 2 nm thick barriers. Moreover, the introduction of higher Ge fraction effectively suppresses the increase of the valley current and enables negative differential conductance at higher operation temperature.

Modulation of the high mobility two-dimensional electrons in Si∕SiGe using atomic-layer-deposited gate dielectric

Metal-oxide-semiconductor field-effect-transistors using atomic-layer-deposited (ALD) Al2O3 as the gate dielectric are fabricated on the Si∕Si1−xGex heterostructures. The low-temperature carrier density of a two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.5×1011to4.5×1011cm−2, virtually without any gate leakage current. Magnetotransport data show the homogeneous depletion of 2DES under gate biases. The characteristic of vertical modulation using ALD dielectric is shown to be better than that using Schottky barrier or the SiO2 dielectric formed by plasma-enhanced chemical-vapor-deposition.

Growth of strained Si on high-quality relaxed Si1−xGex with an intermediate Si1−yCy layer

An intermediate Si1−yCy layer in the Si1−xGex film, replacing the conventionally graded buffer layer, was used to form the high-quality relaxed SiGe substrate. With the 700 nm thick Si0.8Ge0.2 overlayer, such a Si1−xGex∕Si1−yCy∕Si1−xGex (x=0.2, y−0.014) heterostructure has a threading dislocation density of 5.5×105cm−2 and a residual strain of only 2%. The surface roughness was measured to be about 4.2 nm. The long-range misfit dislocation array was formed mainly at the interface of top Si1−xGex and Si1−yCy. Strained-Si n-channel metal-oxide-semiconductor transistors with various buffer layers were fabricated and examined. Effective electron mobility for the strained-Si device with this substrate technology was found to be 95% higher than that of Si control device. The scheme for the formation of the relaxed Si1−xGex film serving as a virtual substrate shall be applicable to high-speed strained-Si devices.

Electron transport in Si/SiGe modulation-doped heterostructures using Monte Carlo simulation

The electron transport in two-dimensional gas formed in tensile-strained Si1−xGex/Si/Si1−xGex heterostructures is investigated using Monte Carlo simulation. First the electron mobility is studied in ungated modulation-doped structures. Calculation matches the experimental results very well over a wide range of electron densities. The mobility typically varies between 1100 cm2/V s in highly-doped structures and 2800 cm2/V s at low electron density. The mobility is shown to be significantly influenced by the thickness of the spacer layer separating the strained Si channel from the pulse-doped supply layers. Then the electron transport is investigated in a gated modulation-doped structure in which the contribution of parasitic paths is negligible. The mobility is shown to be higher than in comparable ungated structures and dependent on the gate voltage as a result of the electron density dependence of remote impurity screening.

Observation of the apparent metal–insulator transition of high-mobility two-dimensional electron system in a Si/Si1−xGex heterostructure

Field-effect transistors are fabricated from the Si/Si1−xGex heterostructures. The density of the two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.13×1011 to 4.24×1011 cm−2. The temperature dependence of the resistivity of the 2DES was measured and the apparent metal–insulator transition was observed. Its main features are similar to those reported in other semiconductor-based two-dimensional systems.

Finite Element Analysis of Elastic and Plastic Deformation during Growth of Si1−xGex/Si Heterostructures

AbstractThe elastic and plastic stress distribution in strained Si1−xGex heterostructure (x=0.2 and x=0.5), was investigated during the growth process using a nonlinear transient finite element modeling. The material plastic behavior is described by the von Mises yield criteria coupled with isotropic work hardening conditions. The calculated stress distribution during growth of the SiGe cap layer shows that the von Mises stress fluctuates strongly within the layers and at the interfaces. The surface of constant composition Si1−xGex layer is found under compressive stress in both cases. Within that layer the normal stress in the growth direction remains compressive for x=0.2, while it changes from compressive to tensile for x=0.5. In the graded layer the stress goes from tensile to compressive for x=0.5 and in the opposite way for x=0.2. High plastic deformation is observed in the layers, with the maximum von Mises plastic stress being higher for x=0.5 and localized in the SiGe graded region. The plastic strain vanishes monotonically up to 8 μ deep into the Si bulk substrate, in agreement with TEM images that revealed dislocation loops penetrating into the substrate. The time dependent analysis shows that elastic and plastic deformation appear almost instantaneously in the sublayers, while in the Si substrate it is delayed up to 300 s.

Diffusion of phosphorus in relaxed Si1−xGex films and strained Si/Si1−xGex heterostructures

Phosphorus diffusion has been studied in relaxed Si1−xGex samples (x=0.11 and 0.19) and strained Si/Si1−xGex/Si heterostructures (x=0.08, 0.13, and 0.18). The diffusivity of P is found to increase with increasing Ge content, while the influence of compressive strain results in a decrease in diffusivity as compared to that in relaxed material. The effect of strain is found to be equivalent to an apparent activation energy of −13 eV per unit strain, where the negative sign indicates that the P diffusion is mediated by interstitials in Si1−xGex (x&amp;lt;0.20). This conclusion is also supported by an experiment utilizing injection of Si self-interstitials, which results in an enhanced P diffusion in strained Si1−xGex. Further, P is found to segregate into Si across Si/Si1−xGex interfaces and the segregation coefficient increases with increasing Ge concentration.