The generally accepted notions about the formation mechanisms for germanium islands with nanometer-scale sizes in a Ge-on-Si system are reviewed on the basis of analysis of recent publications. The...

Room-temperature infrared photoconductivity in a single layer of Ge self-assembled quantum dots incorporated into silicon p-i-n diode is reported. An in-plane polarized photocurrent response with two bias controlled maxima at 2.9 μm and at 1.7 μm wavelength has been observed. The two kinds of absorption driven by reverse bias in opposite ways are ascribed to the intraband hole bound-to-continuum transition and to the interband excitonic transition.

A self-assembled Ge/Si quantum dot interlevel infrared photodetector operating at room temperature and at normal incidence is demonstrated. The spectral response exhibits two peaks in the 58–82 and 132–147 meV energy regions with full width at half maximum linewidths as narrow as 25 meV. The two photocurrent maxima are ascribed to transitions from the hole ground state to the excited states in the dots. The peak detectivity and responsive quantum efficiency are ${\text{1.7×10}}^8$ ${\mathrm{cm Hz}}^{\text{1/2}}/\mathrm{W}$ and 0.1% for the transition from the ground state to the first excited state and ${\text{7×10}}^7$ ${\mathrm{cm Hz}}^{\text{1/2}}/\mathrm{W}$ and 0.08% for the transition from the ground state to the second excited state. At large dc bias, a redshift in the transition energies is observed. We argue that the resonance shifts are due to suppression of the depolarization field effect, representing the experimental manifestation of dynamic screening associated with collective electron–electron interaction in the dots.

An array of $3\times {10}^7$ Ge self-assembled quantum dots is embedded into the active channel of a Si metal-oxide-field-effect transistor. Conductance oscillations with a gate voltage resulting from a successive loading of holes into the dots are observed. Based on measurements of the temperature dependence of the conductance maxima, the charge-transfer mechanism in the channel is identified as being due to variable-range hopping between the dots, with the typical hopping energy determined by interdot Coulomb interaction. The characteristic spatial dimension of the hole wave functions as well as the charging energies of the dots are determined from the conductance data. The effect of the proximity of a bulk conductor on hopping transport is studied. We find that putting a metal plane close to the dot layer causes a crossover from Efros-Shklovskii variable-range hopping conductance to two-dimensional Mott behavior as the temperature is reduced. At the crossover temperature the hopping activation energy is observed to fall off. The metal plane is shown not to affect the conductance of samples which show Mott hopping. In the Efros-Shklovskii hopping regime, the conductance prefactor was found to be $\simeq e^2/h,$ and the conductance to scale with the temperature. In the fully screened limit, the universal behavior of the prefactor is destroyed, and it begins to depend on the localization length. The experimental results are explained by a screening of long-range Coulomb potentials, and provide evidence for strong electron-electron interaction between dots in the absence of screening.

Space-charge spectroscopy was employed to study electronic structure of a stack of four layers of Ge quantum dots (QD’s) coherently embedded in an $n$-type Si(001) matrix. Evidence for an electron confinement in Si in the vicinity of neutral Ge dots was found. From the temperature- and frequency-dependent measurements the electron binding energy was determined to be $\sim 50\mathrm{meV}$. Existence of localized electronic states is explained by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge dots. To support experimental results we performed numerical analysis of three-dimensional strain distribution and electronic structure of the sample under investigation. The strain distribution was found in terms of atomic positions using a valence-force-field model with a Keating interatomic potential. The electronic energy levels were calculated by solving a three-dimensional effective mass Schrödinger equation. The carrier confinement potential in this equation is modified by the strain distribution. The calculated confined eigenenergies agree with our experimental values deduced from the admittance spectroscopy. The data obtained in this work may serve as a guideline to interpret efficient photo- and electroluminescence as well as enhanced quantum efficiency of photodetectors and solar cells with Ge QD’s stacked in a multilayer structure.