Lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots

Spectra of lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots, fabricated by molecular-beam epitaxy are studied. The photoresponse caused by optical transitions between hole levels of quantum dots and Si electronic states was observed in the energy range of 1.1–0.3 eV at T = 78 K. It was shown that the electronic states localized in the region of Si band bending near the Ge/Si interface mainly contribute to lateral photoconductivity. The use of the quantum box model for describing hole levels of quantum dots made it possible to understand the origin of peaks observed in the photoconductivity spectra. A detailed energy-level diagram of hole levels of quantum dots and optical transitions in Ge/Si structures with strained Ge quantum dots was constructed.     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Light-emitting tunneling nanostructures based on quantum dots in a Si and GaAs matrix

InGaAs/GaAs and Ge/Si light-emitting heterostructures with active regions consisting of a system of different-size nanoobjects, i.e., quantum dot layers, quantum wells, and a tunneling barrier are studied. The exchange of carriers preceding their radiative recombination is considered in the context of the tunneling interaction of nanoobjects. For the quantum well-InGaAs quantum dot layer system, an exciton tunneling mechanism is established. In such structures with a barrier thinner than 6 nm, anomalously fast carrier (exciton) transfer from the quantum well is observed. The role of the above-barrier resonance of states, which provides “instantaneous” injection into quantum dots, is considered. In Ge/Si structures, Ge quantum dots with heights comparable to the Ge/Si interface broadening are fabricated. The strong luminescence at a wavelength of 1.55 μm in such structures is explained not only by the high island-array density. The model is based on (i) an increase in the exciton oscillator strength due to the tunnel penetration of electrons into the quantum dot core at low temperatures (T < 60 K) and (ii) a redistribution of electronic states in the Δ₂-Δ₄ subbands as the temperature is increased to room temperature. Light-emitting diodes are fabricated based on both types of studied structures. Configuration versions of the active region are tested. It is shown that selective pumping of the injector and the tunnel transfer of “cold” carriers (excitons) are more efficient than their direct trapping by the nanoemitter.     

Lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots

The spectral dependences of the lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots are studied. The photoresponse of the Ge/Si structures with Ge nanoclusters is detected in the range 1.0–1.1 eV at T = 290 K, whereas the photocurrent in the single-crystal Si substrate is found to be markedly suppressed. This result can be attributed to the effect of elastic strains induced in the structure on the optical absorption of Si. At temperatures below 120 K, the heterostructures exhibit photosensitivity in the spectral range 0.4–1.1 eV, in which the Si single crystal is transparent. The photocurrent in this range is most likely due to the transitions of holes from the ground states localized in the quantum dots to the extended states of the valence band.     

The band structure and photoluminescence in a Ge0.8Si0.2/Ge0.1Si0.9 superlattice with vertically correlated quantum dots

The energy band diagram of the multilayered Ge_0.8Si_0.2/Ge_0.1Si_0.9 heterostructures with vertically correlated quantum dots is analyzed theoretically. With regard to fluctuations of the thickness layer in the columns of quantum dots and to the exciton-phonon coupling, it is shown that the electron states constitute a miniband. The hole wave functions remain localized in the quantum dots. The spectrum of optical transitions calculated for a 20-layered structure at room temperature is in good agreement with the experimental photoluminescence spectrum that involves an intense band at about 1.6 μm. From theoretical considerations and experimental measurements, specific evidence for the miniband in the superlattice is deduced; it is found that the overlap integrals of the wave functions of electrons and holes and the integrated intensity of the photoluminescence band of the Ge quantum dots are described by quadratic functions of the number of the structure periods.     

Study of phonons in self-organized multiple Ge quantum dots

Raman scattering measurements were carried out in a self-organized multi-layered Ge quantum dot sample, which was grown using solid-source molecular-beam epitaxy, and consisted of 25 periods of 20-Å-high Ge quantum dots sandwiched by 20-nm Si spacers. The Ge-Ge optical phonon mode was found at 298.2 cm^?1, which was tuned by the phonon confinement and strain effects. Acoustic phonons related to Ge quantum dots have also been demonstrated.     

Localization of holes in an InAs/GaAs quantum-dot molecule

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U_r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U_r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.     

Optical properties of germanium monolayers on silicon

Photoluminescence and Raman spectra of thin germanium layers grown on silicon at a low temperature (250°C) have been studied. In structures of this kind, in contrast to those grown at high temperatures, luminescence from quantum wells is observed at germanium layer thicknesses exceeding ～9 monolayers (ML). With the development of misfit dislocations, the luminescence lines of quantum wells are shifted to higher energies and transverse optical (TO) phonons involved in the luminescence are confined to a quasi-2D germanium layer. Introduction of an additional relaxed Si_0.95Ge_0.05 layer into the multilayer Ge/Si structure leads to a substantial rise in the intensity and narrowing of the luminescence line associated with quantum dots (to 24 meV), which points to their significant ordering.     

Optical properties of structures with ultradense arrays of Ge QDs in an Si matrix

The structural and optical properties of ultrathin Ge insertions in an Si matrix were studied. Transmission electron microscopy revealed the spontaneous formation of arrays of disk-shaped quantum dots (QDs) with a small lateral size (3–10 nm) at a nominal Ge insertion thicknesses, from submonolayer to nearly critical, for the transition to 3D growth by the Stranski-Krastanow mechanism. Optical study revealed type-I band alignment in these structures, which results from the strong contribution of the electron-hole Coulomb interaction overpowering the repulsion potential for an electron existing in the Ge conduction band. The small lateral size of QDs lifts the selection rule prohibiting indirect recombination in the inverse k space. At the same time, the high surface density of QDs (10¹²–10¹³ cm^?2) and the possibility of their stacking with the use of ultrathin Si spacers allows the obtainment of an ultrahigh volume density of QDs (up to 10¹⁹ cm^?3), which is necessary to achieve stimulated emission in Si. A sample with stacked QDs formed by 0.7-nm-thick Ge insertions exhibited a superlinear increase of the photoluminescence (PL) intensity, accompanied by narrowing of the PL line. The doping of Ge-Si structures with donors allows for a drastic increase in the PL intensity at high temperatures, which prevents depletion of the active region in weakly localized electrons.     

Multilayers of Ge nanocrystals embedded in Al2O3 matrix: Structural and electrical studies

In this paper, Ge/Al₂O₃ multilayer systems were grown by pulsed laser ablation. The grown samples were annealed at 900 °C to promote the formation of Ge nanocrystals. Rutherford backscattering spectroscopy and transmission electron microscopy confirmed the presence of a multilayer system. Grazing incidence small angles X-ray scattering technique demonstrates the formation of Ge nanoclusters formed between alumina layers. Room temperature I-V measurements showed weak carrier trapping in the system. This was explained by the leakage caused by Ge diffusion through the multilayer.     

Stark effect in vertically coupled quantum dots in InAs-GaAs heterostructures

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects.     

Photoinduced and equilibrium optical absorption in Ge/Si quantum dots

The results of studies of the optical absorption spectra in Ge/Si quantum dot structures in the mid-infrared region are reported. Two types of structures different in terms of the method used for quantum dot formation and in terms of barrier layer thickness are explored. The photoinduced absorption associated with the nonequilibrium population of hole states and optical absorption in structures doped to different levels are investigated. Specific features that are associated with occupation of the ground and excited states of quantum dots and exhibit a polarization dependence are observed. From the experimental data, the energy spectrum of holes is determined for structures of both types.     

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I ₁(T) and I ₂(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ～50 K yields only a slight decrease in the total integrated emission intensity of both bands I _PL(T) and the intensities of each of the two bands, I ₁(T) and I ₂(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I ₁(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I ₂(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.     

多层Ge量子点的生长及其光学特性

用超高真空化学气相淀积系统在Si(1 0 0 )衬底上生长了多层Ge量子点.分别用TEM和AFM分析了埋层和最上层量子点的形貌和尺寸,研究了量子点层数和Si隔离层厚度对上层Ge量子点的形状和尺寸分布的影响.观察到样品的低温PL谱线有明显蓝移(87meV) ,Ge量子点的PL谱线的半高宽度(FWHM )为46meV ,说明采用UHV/CVD生长的多层量子点适合量子光电器件的应用     

Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (T_D) of 150–400 °C, and it was heat-treated in the MBE chamber at 650 °C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for T_D ≤250 °C, density of QDs increased with increasing T_D due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1 nm were formed in the highest density of 8.3×10¹¹ cm⁻² for T_D =250 °C. On the contrary, in the case of polycrystalline Ge film for T_D ≥300 °C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.     

Structural and optical properties of GaAs-based heterostructures with Ge and Ge/InGaAs quantum wells

GaAs-based heterostructures with Ge and Ge/InGaAs quantum wells are grown by laser-assisted sputtering. Structural and optical studies of the heterostructures are carried out. A broad photoluminescence line is observed in the wavelength range from 1300 to 1650 nm. The line corresponds to indirect transitions in the momentum space of the Ge quantum wells and to transitions between the In_0.28Ga_0.72As and Ge layers, indirect in coordinate space, but direct in momentum space.     

Determination of bulk trap parameters using thermal dielectric relaxation techniques

A new non-steady-state technique for determining the energy levels of traps through which electron-hole pairs are generated in the depletion-layer of the silicon of metal-oxide-silicon (MOS) capacitor structures has been applied to metal-nitride-oxide-silicon (MNOS) structures. The technique consists of cooling the device to low temperatures and biasing it into the deep-depletion mode. The temperature of the device is then raised at a constant rate and the resulting I_g-T characteristic exhibits a pronounced wide peak. The energy level of the traps involved in the generation process was obtained from the slope of the log_eI_g - 1/T plot of the leading edge of the I_g-T characteristic curve, and is found to be 0·55 eV.A cross-check on the trap levels involved in the generation process was made by performing the experiment at two heating rates, β₁ and β₂. From a knowledge of the corresponding temperatures at which the maxima of the two peaks in the I_g-T characteristics occur, the energy level of the traps was found to be 0·54 eV. Furthermore, having obtained the trap level, the so-called carrier lifetime, τ, was found to be approximately 4μ sec.Experiments are described which provide convincing evidence that the generation of electron-hole pairs occurs in the depletion-layer of the silicon rather than at the silicon-silicon interface.     

Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

Ge/Si waveguide photodiodes with built-in layers of Ge quantum dots for fiber-optic communication lines

The results of research aimed at the development of high-efficiency Ge/Si-based photodetectors for fiber-optic communication applications are reported. The photodetectors are designed as vertical p-i-n diodes on silicon-on-insulator substrates in combination with waveguide lateral geometry and contain Ge quantum-dot layers. The layer density of quantum dots is 1×10¹² cm^?2; the dot size in the plane of growth is ～8 nm. Unprecedentedly high quantum efficiency suitable for the range of telecommunication wavelengths is attained; specifically, in the waveguides illuminated from the end side, the efficiency was as high as 21 and 16% at 1.3 and 1.55 µm, respectively.     

Modification of quantum dots in Ge/Si nanostructures by pulsed laser irradiation

The goal of this study was the development of a method for the modification of a quantum dot (QD) structure in Ge/Si nanostructures by pulsed laser irradiation. The Ge_xSi_1?x QD structures were analyzed using data furnished by Raman spectroscopy. Frequency-dependent admittance measurements were used to study the energy spectrum of holes in the Ge/Si heterostructures with Ge_xSi_1?x QDs before and after the laser treatment. The obtained experimental data show that laser treatment makes it possible to reduce the sheet density of QDs, modify their composition, and increase the average size. The most important result is that the QD parameters become more uniform after the treatment with nanosecond laser pulses. In a sample with ODs of 8-nm average lateral size (six monolayers of Ge), the scatter of energy levels in the QD array is reduced by half after the treatment with 10 laser pulses.