SiGe Heterostructures Research Articles

Lateral quantum dots in Si∕SiGe realized by a Schottky split-gate technique

Lateral quantum dots are formed in the two-dimensional electron gases of a high-mobility Si∕SiGe heterostructures by means of split Schottky gates. Palladium gates, defined by e-beam lithography and lift-off, show Schottky barriers with very well controlled leakage currents. At low temperatures we observe Coulomb-blockade and stability diamonds on lateral quantum dots containing a total charge of about 25 electrons. The experiments demonstrate that, in contrast to recent reports, Schottky gates are a feasible approach for the fabrication and integration of single electron transistors in the strained Si∕SiGe heterosystem.

Raman investigation of strain in Si∕SiGe heterostructures: Precise determination of the strain-shift coefficient of Si bands

Raman scattering experiments were carried out on Si∕SiGe heterostructures. The strain in both the top Si layer, and the Si1−xGex buffer layers with various Ge compositions was evaluated using several excitation sources, together with x-ray diffraction and secondary ion mass spectrometry. The strain-shift coefficient, which is a necessary quantity to evaluate the strain by Raman spectroscopy, was precisely determined. The dependence of the Si–Si band frequency on the Ge composition in the SiGe alloy was also examined. We found that the strained top-Si layers with a thickness below 25nm experience coherent growth on Si1−xGex buffer layers with composition x<0.35.

Features of the Shubnikov–de Haas oscillations of the conductivity of a high-mobility two-dimensional hole gas in a SiGe∕Ge∕SiGe quantum well

The Shubnikov–de Haas oscillations in a two-dimensional hole gas in a quantum well of pure germanium in a SiGe∕Ge∕SiGe heterostructure with a hole concentration pH=5.68×1011cm2 and mobility μ=4.68×104cm2V−1s−1 are investigated in magnetic fields up to 15 T at temperatures from 40 mK to 4 K. The observed deviation from the known relation describing the conductivity oscillations in the Shubnikov–de Haas effect are explained by additional broadening of the Landau levels due to the existence of a nonuniform distribution of the concentration of charge carriers, and, accordingly, of their energy, in the plane of the two-dimensional gas. It is assumed that the latter is due to natural atomic-step variations of the well width. The effective hole mass (m*=0.112m0) is determined from the temperature dependence of the oscillation amplitude, and its dependence on magnetic field is used to determine the quantum scattering time and the value of the carrier concentration fluctuations.

Phonon-induced breakdown of negative bend resistance in an asymmetric Si∕SiGe cross junction

An asymmetric nanoscale cross junction is fabricated from a high-mobility Si∕SiGe heterostructure. At T=4.2K, the four-terminal current-voltage characteristics reveal a polarity-dependent breakdown of the negative bend resistance. The breakdown is accompanied by negative differential conductance found in the two-terminal current-voltage characteristics of the orthogonal current leads. We attribute this behavior to phonon emission by hot electrons. From gate-voltage-dependent measurements, we determine a phonon threshold of 19meV.

Thickness Dependence of Strain Field Distribution in SiGe Relaxed Buffer Layers

The buffer thickness dependence of strain field distribution was investigated in SiGe heterostructures by micro-Raman spectroscopy. Crosshatch-like strain fluctuations were clearly observed in strained-Si and SiGe buffer layers, and the fluctuation wavelength was found to increase almost linearly with increasing buffer thickness. It was also found that SiGe homoepitaxial growth on planarized SiGe buffer layers gave rise to crosshatch roughness on the surface with almost the same morphology as the strain distribution, indicating that the strain fluctuation caused the roughness formation associated with growth kinetics. The strain fluctuation still remained on the buffer layer thicker than 7 µm, which should be taken into consideration for device applications.

Fabrication and characterization of electrostatic Si∕SiGe quantum dots with an integrated read-out channel

A nontraditional fabrication technique is used to produce quantum dots with read-out channels in silicon/silicon–germanium two-dimensional electron gases. The technique utilizes Schottky gates, placed on the sides of a shallow etched quantum dot, to control the electronic transport process. An adjacent quantum point contact gate is integrated to the side gates to define a read-out channel, and thus allow for noninvasive detection of the electronic occupation of the quantum dot. Reproducible and stable Coulomb oscillations and the corresponding jumps in the read-out channel resistance are observed at low temperatures. The fabricated dot combined with the read-out channel represents a step toward the spin-based quantum bit in Si∕SiGe heterostructures.

The role of point defects in strain relaxation in epitaxially grown SiGe structures

Strain relaxation commonly occurring in SiGe heteroepitaxial systems by misfit dislocations is a very important process affecting the properties of electronic and optoelectronic devices. Point defects introduced into epitaxially grown heterostructures can facilitate the strain relaxation and can even initiate the process themselves, without introducing misfit dislocations. In the present paper, an analysis of possible effects of point defects on the strain relaxation in epitaxial SiGe heterostructures is done. A phenomenological model of atomic rearrangement stimulated by point defects and resulting in the strain relaxation is proposed. Experimental data supporting the model are discussed.

Application of deconvolution to boron depth profiling in SiGe heterostructures

The analysis of SiGe heterostructures by SIMS using Cs ion bombardment provides information for all relevant species such as B, C, O, Si, P, As and Ge in a single profile. However, the B depth profile is distorted due to Cs-enhanced chemical segregation. If deconvolution is successfully applied to the B profile, the deconvolved B profiles are shown to be equivalent to those measured using an oxygen beam for Si, SiGe and Si/SiGe/Si samples.

Strain field and related roughness formation in SiGe relaxed buffer layers

Strain distribution in SiGe heterostructures and its influence on the roughness formation of the overgrown structures were investigated. The crosshatch surface roughness was found to appear after the homoepitaxial regrowth of SiGe on the planarized SiGe buffer layers. The morphology of this roughness was almost equal to the in-plane strain distribution observed by Raman mapping measurement, which clearly demonstrated that the crosshatch roughness is formed by the strain distribution with the mechanism associated with the growth kinetics. Strain distribution wavelength was found to strongly depend on buffer thickness and strain fluctuation still existed on the very thick buffer layers. It can be said that the buffer thickness should be carefully taken into account for device applications.

XRD analysis of strained Ge–SiGe heterostructures on relaxed SiGe graded buffers grown by hybrid epitaxy on Si(0 0 1) substrates

Ge/Si 1− x Ge x inverted modulation doped heterostructures with Ge channel thickness of 16 and 20 nm were grown by a method of hybrid epitaxy followed by ex situ annealing at 650 °C for p-HMOS application. The thicker layers of the virtual substrate (6000 nm graded SiGe up to x = 0.6 and 1000 nm uniform composition with x = 0.6) were produced by ultrahigh vacuum chemical vapor deposition (UHV-CVD) while the thinner, Si(2 nm)–SiGe(20 nm)–Ge–SiGe(15 nm + 5 nm B-doped + 20 nm) active layers were grown by low temperature solid-source (LT-SS) MBE at T = 350 °C. As-grown and annealed samples were measured by X-ray diffraction (XRD). Reciprocal space maps (RSMs) allowed us to determine non-destructively the precise composition (∼1%) and strain of the Ge channel, along with similar information regarding the other layers that made up the whole structure. Layer thickness was determined with complementary high-resolution Rutherford backscattering (RBS) experiments.

Improved thermal stability and hole mobilities in a strained-Si/strained-Si 1− yGe y/strained-Si heterostructure grown on a relaxed Si 1− xGe x buffer

A dual channel heterostructure consisting of strained-Si/strained-Si 1− y Ge y on relaxed Si 1− x Ge x ( y > x), provides a platform for fabricating metal-oxide-semiconductor field-effect transistors (MOSFETs) with high hole mobilities ( μ eff) which depend directly on Ge concentration and strain in the strained-Si 1− y Ge y layer. Ge out-diffuses from the strained-Si 1− y Ge y layer into relaxed Si 1− x Ge x during high temperature processing, reducing peak Ge concentration and strain in the strained-Si 1− y Ge y layer and degrades hole μ eff in these dual channel heterostructures. A heterostructure consisting of strained-Si/strained-Si 1− y Ge y /strained-Si, referred to as a trilayer heterostructure, grown on relaxed Si 1− x Ge x has much reduced Ge out-flux from the strained-Si 1− y Ge y layer and retains higher μ eff after thermal processing. Improved hole μ eff over similar dual channel heterostructures is also observed in this heterostructure. This could be a result of preventing the hole wavefunction tunneling into the low μ eff relaxed Si 1− x Ge x layer due to the additional valence band offset provided by the underlying strained-Si layer. A diffusion coefficient has been formulated and implemented in a finite difference scheme for predicting the thermal budget of the strained SiGe heterostructures. It shows that the trilayer heterostructures have superior thermal budgets at higher Ge concentrations. Ring-shaped MOSFETs were fabricated on both platforms and subjected to various processing temperatures in order to compare the extent of μ eff reduction with thermal budget. Hole μ eff enhancements are retained to a much higher extent in a trilayer heterostructure after high temperature processing as compared to a dual channel heterostructure. The improved thermal stability and hole μ eff of a trilayer heterostructure makes it an ideal platform for fabricating high μ eff MOSFETs that can be processed over higher temperatures without significant losses in hole μ eff.

DC Performance of Deep Submicrometer Schottky-Gated n-Channel Si:SiGe HFETs at Low Temperatures

N-type Schottky-gated Si:SiGe heterostructure field-effect transistors with physical gate lengths between 70 and 450nm are characterized over a wide temperature range (T=10 K...300 K) for low electric fields. The room-temperature maximum low-field transconductance increases 61% to 440 mS/mm at T=10 K for the 70-nm device. The minimum subthreshold slope is 14...19 mV/dec at T=10 K. The off-state currents I/sub OFF/ are limited by parallel conduction at high temperatures and by the gate leakage current at low temperatures. Substrate leakage currents are found to be due to generation of carriers within the drain/substrate depletion layer and only make a minor contribution to I/sub OFF/. Operation of the devices at the lowest temperature is found to result in the occurrence of the floating-body kink effect, as a consequence of substrate freeze-out and subsequent self-biasing by impact ionization currents. Low temperature characteristics exhibit a nonlinear low-field drain current dependence on the drain voltage, due to the presence of parasitic Schottky source/drain contacts. An extraction method for access resistance consistent with this phenomenon is presented.

Spin-polarized resonant tunneling in double-barrier structures

Transparency of electron tunneling through double barriers of strained hetero-structures like InAs/GaSb/InAs/GaSb/InAs and Si/Si 0.75Ge 0.25/Si/Si 0.75Ge 0.25/Si has been calculated as a function of electron energy using the proposed model which includes the combined effects of Dresselhaus and in-plane Rashba spin–orbit interactions. Enhanced spin-polarized resonant tunneling in the double-barrier heterostructures due to Dresselhaus and Rashba spin–orbit coupling induced splitting of the resonant level is observed. Strong suppression of spin-down transmission in SiGe heterostructures is observed. This effect could be employed in the fabrication of spin filters.

저압화학증착을 이용한 실리콘-게르마늄 이종접합구조의 에피성장과 소자제작 기술 개발

Reduced pressure chemical vapor deposition technology has been used to study SiGe heterostructure epitaxy and device issues, including SiGe relaxed buffers, proper control of Ge component and crystalline defects, two dimensional delta doping, and their influence on electrical properties of devices. From experiments, 2D profiles of B and P presented FWHM of 5 nm and 20 nm, respectively, and doses in 5×10/sup 11/ ∼ 3×10/sup 14/ ㎝/sup -2/ range. The results could be employed to fabricate SiGe/Si heterostructure field effect transistors with both Schottky contact and MOS structure for gate electrodes. I-V characteristics of 2D P-doped HFETs revealed normal behavior except the detrimental effect of crystalline defects created at SiGe/Si interfaces due to stress relaxation. On the contrary, sharp B-doping technology resulted in significant improvement in DC performance by 20-30 ％ in transconductance and short channel effect of SiGe HMOS. High peak concentration and mobility in 2D-doped SiGe heterostructures accompanied by remarkable improvements of electrical property illustrate feasible use for nano-sale FETs and integrated circuits for radio frequency wireless communication in particular.

Numerical simulation of ballistic magnetoconductance and magnetic focusing in strainedSi–SiGe cavities

We present numerical simulations of the transport properties of a ballistic cavity connectedto two leads in the presence of a finite degree of decoherence. The cavity is obtainedby realizing a ‘quantum chicane’, i.e., by shifting a short section of a quantumwire defined by etching on a SiGe heterostructure. We compare our results withexperiments by Scappucci et al (2004 Trends in Nanotechnology (Segovia, Spain,Sept. 2004) unpublished) and show that the magnetoconductance features can bereproduced if we use a recently developed model that includes decoherence with aphenomenological statistical description. Otherwise, simulations based on completelycoherent transport would provide a much richer structure of magnetoconductance, that inexperiments is smoothed out by dephasing processes. In particular, we recoverexperimental features of the magnetoconductance such as magnetic focusing andShubnikov–de Haas oscillations. By computing the local partial density of states of thesystem at different values of the external field we recover the semiclassical orbits.

Shallow Acceptor Levels in Ge∕GeSi Heterostructures with Quantum Wells in a Magnetic Field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A+ centers.

Solid Phase Recrystallization and Strain Relaxation in Ion-Implanted Strained Si on SiGe Heterostructures

AbstractThe relaxation process of strained silicon films on silicon-rich relaxed SiGe alloys has been studied. Experimental structures were grown via Molecular Beam Epitaxy (MBE) growth techniques and contain a strained silicon capping layer approximately 50 nm thick. The relaxed SiGe alloy compositions range from 0 to 30 at.% germanium. A 12 keV Si+ implant at a dose of 1×1015 atoms/cm2 was used to generate an amorphous layer ∼30 nm thick, which was confined within the strained silicon capping layer. Upon annealing at 500 °C, it was found that the solid phase epitaxial regrowth process of the amorphous silicon breaks down for high strain levels and regrowth related defects were observed in the regrown layer. In addition, high-resolution X-Ray diffraction results indicate a reduction in strain for the silicon capping layer. This study addresses the critical strain regime necessary for the breakdown of solid phase epitaxial recrystallization in silicon.

감압화학증착법으로 성장된 실리콘-게르마늄 반도체 에피층에서 붕소의 이차원 도핑 특성

Reduced pressure chemical vapor deposition(RPCYD) technology has been investigated for the growth of SiGe epitaxial films with two dimensional in-situ doped boron impurities. The two dimensional <TEX>$\delta$</TEX>-doped impurities can supply high mobility carriers into the channel of SiGe heterostructure MOSFETs(HMOS). Process parameters including substrate temperature, flow rate of dopant gas, and structure of epitaxial layers presented significant influence on the shape of two dimensional dopant distribution. Weak bonds of germanium hydrides could promote high incorporation efficiency of boron atoms on film surface. Meanwhile the negligible diffusion coefficient in SiGe prohibits the dispersion of boron atoms: that is, very sharp, well defined two-dimensional doping could be obtained within a few atomic layers. Peak concentration and full-width-at-half-maximum of boron profiles in SiGe could be achieved in the range of 10<TEX>$^{18}$</TEX> -10<TEX>$^{20}$</TEX> cm<TEX>$^{-3}$</TEX> and below 5 nm, respectively. These experimental results suggest that the present method is particularly suitable for HMOS devices requiring a high-precision channel for superior performance in terms of operation speed and noise levels to the present conventional CMOS technology.

Magnetic field effects on spin relaxation in heterostructures

Effect of a magnetic field on electron spin relaxation in quantum wells is studied theoretically. We have shown that Larmor effect and the cyclotron motion of the carriers can either jointly suppress D'yakonov-Perel' spin relaxation or compensate each other. The spin relaxation rates tensor is derived for any given direction of the external field and arbitrary ratio of bulk and structural contributions to the spin splitting. Our results are applied to the experiments on electron spin resonance in SiGe heterostructures, and enable us to extract spin splitting value for such quantum wells.

Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: Effect of Ge concentration and biaxial stress

Si 1 − x Ge x ∕ Si 1 − y Ge y ∕ Si ( 100 ) heterostructures grown by molecular beam epitaxy were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that (i) for layers in epitaxy on (100)Si, B segregation decreases with increasing Ge concentration, i.e., with increased compressive stress; (ii) for unstressed layers, B segregation increases with Ge concentration; (iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that (i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration; (ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress; (iii) the volume of activation of B diffusion [ΔV=−kT(dlnD∕dP)] is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume.