Photoconductivity of Si/Ge/SiO<Subscript>x</Subscript> and Si/Ge/Si structures with germanium quantum dots

A nonmonotonic dependence of the lateral photoconductivity (PC) on the interband light intensity is observed in Si/Ge/Si and Si/Ge/SiO_x structures with self-organized germanium quantum dots (QDs): in addition to a stepped increase in PC, a stepped decrease in PC is also observed. The effect of temperature and drive field on these features of the PC for both types of structures with a maximum nominal thickness of the Ge layer (N_Ge) is studied. The results obtained are discussed in the context of percolation theory for nonequilibrium carriers localized in different regions of the structure: electrons in the silicon matrix and holes in QDs.     

Negative interband photoconductivity in Ge/Si heterostructures with quantum dots of the second type

It was found that irradiating an array of Ge nanoclusters in n-Si with light that induced interband transitions gave rise to negative photoconductivity. This result was explained by localization of equilibrium electrons at the Si/Ge interface in the potential of the nonequilibrium holes trapped on deep states in Ge islands.     

Interband photoconductivity of Ge/Si structures with self-organized quantum rings

At low temperatures a lateral photoconductivity (PC) of Ge/Si (1 0 0) self-organized quantum rings (QRs) structures as a function of interband light intensity has been investigated for different values of lateral voltage and temperature. In contrast to self-organized Ge/Si quantum dots (QDs) structures (grown at the same conditions) where the stepped PC was registered, for QRs structures essential smoothing of PC steps was observed. Such behavior is determined by decreasing of strain potential around QRs in conductive Si matrix due to a transfer of Ge atoms from the center of QDs to its periphery accompanied by Ge/Si intermixing.     

Spatial separation of electrons in Ge/Si(001) heterostructures with quantum dots

It is shown experimentally that the excitation of interband optical transitions in arrays of Ge/n-Si(001) quantum dots leads to a decrease in the concentration of electrons in the conduction band. The phenomenon observed is due to the formation of negatively charged exciton complexes in Ge islands and represents the first experimental confirmation of the spatial separation of electrons in the silicon matrix surrounding the islands.     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In_xGa_1−xN QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.     

Multicomponent Electron–Hole Liquid in Si/SiGe Quantum Wells

The density functional theory is used to calculate the energy of an electron–hole liquid in Si/Si_1–xGe_x/Si quantum wells. Three one-dimensional nonlinear Schrödinger equations for electrons and light and heavy holes are solved numerically. It is shown that, in shallow quantum wells (small x), both light and heavy holes exist in the electron–hole liquid. Upon an increase in the Ge content, a transition to a state with one type of holes occurs, with the equilibrium density of electron–hole pairs decreasing by more than a factor of 2.     

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.     

Ge/Si quantum dots in external electric and magnetic fields

Electric field-induced splitting of the lines of exciton optical transitions into two peaks is observed for Ge/Si structures with quantum dots (QDs). With increasing field, one of the peaks is displaced to higher optical transition energies (blue shift), whereas the other peack is shifted to lower energies (red shift). The results are explained in terms of the formation of electron-hole dipoles of two types differing in the direction of the dipole moment; these dipoles arise due to the localization of one electron at the apex of the Ge pyramid and of the other electron under the base of the pyramid. By using the tight-binding method, the principal values of the g factor for the hole states in Ge/Si quantum dots are determined. It is shown that the g factor is strongly anisotropic, with the anisotropy becoming smaller with decreasing QD size. The physical reason for the dependence of the g factor on quantum-dot size is the fact that the contributions from the states with different angular-momentum projections to the total wave function change with the QD size. Calculations show that, with decreasing QD size, the contribution from heavy-hole states with the angular-momentum projections ±3/2 decreases, while the contributions from light-hole states and from states of the spin-split-off band with the angular-momentum projections ±1/2 increase.     

Surface integral determination of built-in electric fields and analysis of exciton binding energies in nitride-based quantum dots

We present a simple analytical approach to calculate the built-in strain-induced and spontaneous piezoelectric fields in nitride-based quantum dots (QDs) and then apply the method to describe the variation of exciton, biexciton and charged exciton energy with dot size in GaN/AlN QDs. We first present the piezoelectric potential in terms of a surface integral over the QD surface, and confirm that, due to the strong built-in electric field, the electrons are localised near the QD top and the holes are localised in the wetting layer just below the dot. The strong localisation and smaller dielectric constant results in much larger Coulomb interactions in GaN/AlN QDs than in typical InAs/GaAs QDs, with the interaction between two electrons, J_ee, or two holes, J_hh, being about a factor of three larger. The electron–hole recombination energy is always blue shifted in the charged excitons, X⁻ and X⁺, and the biexciton, and the blue shift increases with increasing dot height. We conclude that spectroscopic studies of the excitonic complexes should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

Stepwise dependence of the photoconductivity of Si/Ge structures with quantum dots on the interband illumination intensity

It was found that a stepwise increase in the interband light intensity causes an increase in the low-temperature lateral photoconductivity of a Si/Ge structure containing six layers of germanium quantum dots in a silicon host. As was previously observed in structures with a single layer of quantum dots, strengthening of the driving field results in the step positions shifting to lower light intensities. This effect was also found to take place under a dark driving field. The results are discussed in terms of the percolation theory of nonequilibrium electrons localized in the states between quantum dots.     

Photoconductivity of Si/Ge/Si structures with 1.5 and 2 ML of Ge layer

A photoconductivity (PC) of Si/Ge/Si structures with narrow Ge layer [thickness's 1.5 and 2 monolayers (ML)] on interband light intensity has been investigated for the different values of lateral voltage U, and temperature T. In contrast to the Si/Ge structure with 2 ML, where only monotonous PC growth was registered, for the 1.5 ML structure a stepped and a fluctuated PC were observed. These PC features are explained by a percolation of photoexcited carriers via the localized states induced by one monolayer scale Si/Ge interface roughnesses.     

Interband Optical Transitions due to Donor Bound Excitons in Wurtzite InGaN Strained Coupled Quantum Dots： Strong Built-in Electric Field Effects

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field （BEF） in the wurtzite InGaN strained coupled quantum dots （QDs）, the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF, and the structure parameters of the QD system.     

Excitons and multi-exciton complexes bound to a two-dimensional hole layer at a silicon surface: the Kondo effect,the Coulomb blockade and a negative photoconductivity

The recombination radiation line of surface excitons and the recombination radiation line of multi-exciton complexes bound to a two-dimensional hole layer are observed in luminescence spectra of [100] silicon metal–oxide–semiconductor structures at low two-dimensional hole density. The circular polarization of these two lines in a transverse magnetic field is defined by the average electron spin. The hole spin contribution to the circular polarization is very small due to Kondo spin correlations of holes in the excitons and complexes and holes in the two-dimensional hole layer. The Coulomb blockade excludes a direct contribution of the complexes to a surface photoconductivity. Moreover, a significant negative photoconductivity of the two-dimensional holes is observed at high excitation levels, presumably as a result of the quantum scattering of the two-dimensional holes by the complexes. A shell model of surface multi-exciton complexes is introduced.     

Multiparticle states and the factors that complicate an experimental observation of the quantum coherence in the exciton gas of SiGe/Si quantum wells

The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si_0.905Ge_0.095/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si_0.905Ge_0.095 layer, a spatially indirect biexciton is the most favorable energy state of the electron–hole system at low temperatures. This biexciton is characterized by a lifetime of 1100 ns and a binding energy of 2.0–2.5 meV and consists of two holes localized in the SiGe layer and two electrons mainly localized in silicon. The formation of biexcitons is shown to cause low-temperature (5 K) luminescence spectra over a wide excitation density range and to suppress the formation of an exciton gas, in which quantum statistics effects are significant. The Bose statistics can only be experimentally observed for a biexciton gas at a temperature of 1 K or below because of the high degree of degeneracy of biexciton states (28) and a comparatively large effective mass (about 1.3m _e ). The heat energy at such temperatures is much lower than the measured energy of localization at potential fluctuations (about 1 meV). This feature leads to biexciton localization and fundamentally limits the possibility of observation of quantum coherence in the biexciton gas.     

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.     

Many-electron Coulomb correlations in hopping transport along layers of quantum dots

Experimental data are analyzed on the hopping transport of holes in two-dimensional layers of Ge/Si(001) quantum dots (QDs) under conditions of the long-range Coulomb interaction of charge carriers localized in QDs, when the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the parameters of hopping conduction significantly deviate from the predictions of the model of one-electron excitations in “Coulomb glasses.” Many-particle Coulomb correlations associated with the motion of holes localized in QDs play a decisive role in the processes of hopping charge transfer between QDs. These correlations lead to a substantial decrease in the Coulomb barriers for the tunneling of charge carriers.     

掺杂对多层Ge/Si(001)量子点光致发光的影响

利用超高真空化学气相沉积设备, 在Si (001) 衬底上外延生长了多个四层Ge/Si量子点样品. 通过原位掺杂的方法, 对不同样品中的Ge/Si量子点分别进行了未掺杂、磷掺杂和硼掺杂. 相比未掺杂的样品, 磷掺杂不影响Ge/Si量子点的表面形貌, 但可以有效增强其室温光致发光; 而硼掺杂会增强Ge/Si量子点的合并, 降低小尺寸Ge/Si量子点的密度, 但其光致发光会减弱. 磷掺杂增强Ge/Si量子点光致发光的原因是, 磷掺杂为Ge/Si量子点提供了更多参与辐射复合的电子. 关键词： Ge/Si量子点 磷掺杂 光致发光     

ZnS薄膜参数对有机/无机复合发光器件特性的影响

半导体量子点(QDs)具有发光效率高和发光波长可调等特点。采用胶体CdSe QDs作电致发光器件的有源材料,TPD(N,N′-biphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine)作空穴传输层,ZnS作电子传输层,研究了有机/无机复合发光器件ITO/TPD/CdSe QDs/ZnS/Ag的电致发光特性。TPD和CdSe QDs薄膜采用旋涂方法、ZnS薄膜采用磁控溅射方法沉积,器件表面平整。CdSe QDs的光致发光和电致发光谱峰位波长均位于～580 nm,属于量子点的带边激子发光。我们与以前的ITO/ZnS/CdSe QDs/ZnS/Ag发光器件结构进行了对比,发现新的器件结构的电致发光谱没有观察到QDs表面态的发光,而且新器件的发光强度是ITO/ZnS/CdSe QDs/ZnS/Ag结构的～10倍。发光效率的提高归因于碰撞激发与载流子注入两种发光机制并存的结果：一方面电子经过ZnS 层加速后,碰撞激发CdSe QDs发光;另一方面,空穴从TPD层注入CdSe QDs 与QDs中激发的电子复合发光。我们进一步研究了ZnS电子加速层厚度对发光特性的影响,选择ZnS薄膜的厚度分别是80,120 和160 nm,发现随着ZnS层厚度增大,器件启亮电压升高,EL强度增大,但是击穿电压降低。EL峰位随着ZnS厚度的减小发生明显蓝移,对上述实验现象进行了机理解释。