Shallow acceptors in Ge/GeSi strained multilayer heterostructures with quantum wells

The impurity photoconductivity spectra of Ge/Ge_1−x Si_x strained heterostructures with quantum wells are investigated. It is established that the built-in deformation in quantum-size Ge layers substantially changes the spectrum of shallow acceptors, shifting it into the long-wavelength region of the far-IR range. In strong magnetic fields the photoconductivity lines are observed to split and shift as a function of the field. This makes it possible to carry out a classification of the transitions. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 194–198 (25 January 1997)     

Shallow acceptors in Ge/GeSi multi-quantum well heterostructures

The experimental spectra of residual shallow acceptors in strained quantum well Ge/GeSi heterostructures are interpreted on the basis of a new theoretical approach taking into account both confinement and strain effects. It is shown that the main lines in the spectra of undoped samples with narrow quantum wells result from the photoexcitation of the on-edge acceptors that have binding energy two times less than on-center acceptors.     

Intersubband cyclotron resonance of holes in strained Ge/GeSi(111) heterostructures with germanium wide quantum wells and cyclotron resonance of 1<Emphasis Type="Italic">L</Emphasis> electrons in GeSi layers

The submillimeter (?ω=0.5–5 meV) magnetoabsorption spectra of strained Ge/Ge_1?xSi_x(111) multilayer heterostructures with thick Ge layers (d_Ge=300–850 Å, d_GeSi≈200 Å, x≈0.1) are investigated at T=4.2 K upon band-gap optical excitation. It is revealed that the absorption spectra contain cyclotron resonance lines of 1L electrons localized in GeSi solid solution layers (unlike the previously studied structures with thin Ge layers as quantum wells for 3L electrons). The absorption spectra of the samples with thick Ge layers (d_Ge=800–850 Å) exhibit cyclotron resonance lines of holes due to transitions from the lower Landau levels in the first quantum-well subband to the Landau levels belonging to the third and fifth higher subbands.     

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.     

Thermoelectric transport in strained Si and Si/Ge heterostructures

The anisotropic thermoelectric transport properties of bulk silicon strained in the [111]-direction were studied by detailed first-principles calculations focusing on a possible enhancement of the power factor. Electron and hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high-temperature and high-doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of a [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole doping was drastically suppressed due to quantum-well effects, while under electron doping an enhanced power factor was found. For this, we state figures of merit of ZT?=?0.2 and 1.4 at T?=?300?and 900?K for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features.     

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.     

Forming Dislocation Pairs in the Ge/GeSi/Si(001) Heterostructure

Physics of the Solid State - In the Ge/LTGe/GeSi/Si(001) heterostructures, the GeSi buffer layer remains pseudomorphic in a certain range of the heterostructure parameters and growth regimes, while...     

Ge/Si(111)与Si/Ge(111)体系的生长特性与表面再构研究

本文利用RHEED和AES对Ge/Si(111)和Si/Ge(111)体系的生长特性与表面再构进行了研究。由此提出了其生长模式,并讨论了应力对生长特性、界面特性和表面再构的作用。     

Binding of electron states in multilayer strained Ge/Si heterostructures with type-II quantum dots

Mechanical strains in a multilayer Ge/Si(001) heterostructure with vertically aligned Ge nanoclusters (quantum dots) are calculated using an interatomic potential based on the Keating valence-force-field model. It is found that the nonuniform spatial elastic strain distribution in this medium gives rise to a three-dimensional potential well for electrons in the strained Si layers near Ge nanoclusters. The depth of the potential well reaches 100 meV, and its spatial dimensions are determined by the diameter of the Ge nanoclusters. For a structure consisting of four Ge islands 23 nm in diameter arranged one above another, the electron binding energies in this well and the spatial electron density distribution are determined. The ground state has an s-like symmetry and is characterized by an electron binding energy of ～95 and ～60 meV for the elemental composition of Ge in the nanoclusters c = 1 and c = 0.7, respectively. The existence of bound electron states in the conduction band of strained Si must lead to a relaxation of the selection rules that determine the low efficiency of the radiative recombination in indirect-gap semiconductors. This explains the high value of the oscillator strength observed for the interband transitions in multilayer Ge/Si(001) structures with vertical correlation of the arrangement of Ge nanoclusters.     

Superstructure phase transitions in Ge on Si (111) at the initial stage of epitaxial growth

The surface phase transitions 7 × 7 → 5 × 5 for two-dimensional and c2 × 8 →7 × 7 and 7 × 7 → c2 × 8 for three-dimensional Ge islands epitaxially grown on Si(111) were found experimentally using scan- ning tunneling microscopy. The first two transitions are associated with an increase in the level of misfit strains, while the last is related to a decrease in the stress level during plastic relaxation.     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.     

Sn/Ge(111) surface charge-density-wave phase transition

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1 / 3 monolayer of Sn on Ge(111) in both the room-temperature (sqrt[3]xsqrt[3] )R30 degrees phase and the low-temperature ( 3x3) charge-density-wave phase. The results reveal a gap opening around the ( 3x3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting. A highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the ( 3x3) symmetry.     

Strain-driven mesoscopic reconstruction of the As/Ge(111) surface

Periodic arrays of large hexagonal tiles (up to 170 A in size) are observed on As/Ge(111) surfaces. First-principles total energy calculations combined with scanning tunneling microscopy reveal a (5-7-5)-ringed structure for the trenches that separate the tiles. We find that trenches form via an exothermic process. The calculated equilibrium trench spacing of approximately 104 A agrees with experiment. Comparison between first-principles calculations and continuum elasticity theory suggests that the observed mesoscopic reconstruction is driven entirely by long-range surface strain relaxation.     

Hole effective mass in strained Si (111)

The directional, averaged, and density-of-states effective masses of holes have been calculated for strained Si/(111)Si1-xGex. The results for the directional effective mass show that the effect of strain makes the constant energy surface of heavy holes more obvious warping than that in relaxed Si. The [111] and [110] directional effective masses of heavy holes decrease significantly under strain. It is found that the averaged effective mass of heavy holes decreases with increasing Ge fraction, while that o...     

Low temperature disordered phase of alpha-Pb/Ge(111)

A new structural phase transition has been observed at low temperature for the one-third of a monolayer (alpha phase) of Pb on Ge(111) using a variable-temperature scanning tunneling microscope. The well-known (sqrt[3] x sqrt[3])R30 degrees to (3 x 3) transition is accompanied by a new structural phase transition from (3 x 3) to a disordered phase at approximately 76 K. The formation of this "glasslike" phase is a consequence of competing interactions on different length scales.     

Ge/Pb/Si(111)生长中Ge原子沿团簇边缘扩散的三维Monte-Carlo模拟

用动态Monte-Carlo方法对Ge在单层表面活性剂Pb覆盖的Si(111)表面上沿团簇边缘扩散进行三维模拟.重点讨论Ge原子是否沿团簇边缘扩散,沿边缘扩散时的最大扩散步数及最近邻原子数对三维生长的影响,并计算薄膜表面粗糙度研究三维生长模式.模拟表明Ge沿团簇边缘扩散的行为对薄膜生长模式的影响很大,同时讨论了ES势对三维生长模式的影响.