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.     

On the process of hole trapping in Ge/Si heterostructures with Ge quantum dots

Admittance spectroscopy is used to determine the cross sections and energy levels of holes in Ge/Si heterostructures with Ge quantum dots. The structures are grown by molecular-beam epitaxy. It is established that, in layers of quantum dots produced at low growth temperatures T _g ≤ 450°C, the capture cross section for hole trapping into quantum dots exponentially increases with increasing hole binding energy (the Meyer-Neldel rule), with the same characteristic energy ～25 eV independent of T _g . It is shown that the Meyer-Neldel rule is violated in structures grown at higher temperatures or in samples treated in hydrogen plasma. In the case of nanoclusters synthesized at low temperatures, the experimental results suggest that charge-carrier trapping into Ge quantum dots proceeds via the electron-phonon mechanism with the participation of structural defects.     

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.     

Photoresistance of Si/Ge/Si structures with germanium quantum dots

An exponential decrease in the resistance of a Si/Ge/Si structure containing germanium quantum dots with an increase in the band-to-band optical excitation intensity is observed at 4.2 K. Two different exponential regions in the dependence of structure resistance on the optical excitation intensity are observed in elastically strained structures, but only one such region is observed in unstrained structures. The experimental results obtained are explained within the model of the hopping conduction of nonequilibrium electrons, which are localized at and between quantum dots in the strained structures, but are localized only between quantum dots in the unstrained structures.     

Ge/Si量子点的控制生长

采用离子束溅射技术,在生长了Si缓冲层的硅晶片上制备了一系列Ge量子点样品.借助原子力显微镜(AFM)和Raman光谱等测试手段研究了Ge/Si量子点生长密度、尺寸及排列均匀性的演变规律.结果表明,改变Si缓冲层厚度及其生长方式,可以有效控制量子点的尺寸、均匀性和密度.随缓冲层厚度增大,量子点密度先增大后减小,停顿生长有利于提高缓冲层结晶性,从而提高量子点的密度,可以达到1.9×1010 cm-2.还研究了Si缓冲层在Ge量子点生长过程中的作用,并提出了量子点的生长模型.     

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.     

Ge/Si Self-Assembled Quantum Dots and Their Optoelectronic Device Applications

In recent years, quantum dots have been successfully grown by self-assembling processes. For optoelectronic device applications, the quantum-dot structures have advantages such as reduced phonon scattering, longer carrier lifetime, and lower detector noise due to low-dimensional confinement effect. Comparing to traditional optoelectronic III-V and other materials, self-assembled Ge quantum dots grown on Si substrates have a potential to be monolithically integrated with advanced Si-based technology. In this paper, we describe the growth of self-assembled, guided Ge quantum dots, and Ge quantum-dot superlattices on Si. For dot growth, issues such as growth conditions and their effects on the dot morphology are reviewed. Then vertical correlation and dot morphology evolution are addressed in relation to the critical thickness of Ge quantum-dot superlattices. In addition, we also discuss the quantum-dot p-i-p photodetectors (QDIPs) and n-i-n photodetectors for mid-infrared applications, and the quantum-dot p-i-n photodetectors for 1.3-1.55 mum for communications applications. The wavelength of SiGe p-i-p QDIP can be tuned by the size as grown by various patterning methods. Photoresponse is demonstrated for an n-i-n structure in both the mid-infrared and far-infrared wavelength ranges. The p-i-n diodes exhibit low dark current and high quantum efficiency. The characteristics of fabricated light-emitting diode (LED) devices are also discussed, and room-temperature electroluminescence is observed for Ge quantum-dot LED. The results indicate that Ge dot materials are potentially applicable for mid-infrared (8-12 mum) detectors as well as fiber-optic (1.3-1.55 mum) communications.     

Structures with vertically stacked Ge/Si quantum dots for logical operations

Ge/Si structures with vertically stacked quantum dots are simulated to implement the basic elements of a quantum computer for operation with electron spin states. Elastic-strain fields are simulated using the conjugate gradient method and an atomistic model based on the Keating potential. Calculations are performed in the cluster approximation using clusters containing about three million atoms belonging to 150 coordination spheres. The spatial distributions of the strain energy density and electron potential energy are calculated for different valleys forming the bottom of the silicon conduction band. It is shown that the development of multilayer structures with vertically stacked quantum dots makes it possible to fabricate deep potential wells for electrons with vertical tunnel coupling.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.     

锗硅双层量子点的光电流特性

在分子束外延 (MBE)系统上用自组织方式生长了硅基双层锗量子点结构 ,并对样品进行光电流谱的测试。通过调节不同外加偏压来改变量子点中的费米能级位置 ,量子点中载流子所处束缚能级将随之发生变化 ,所得到的光电流谱的峰位也将因此而改变。由光电流谱得到的实验结果与常规的光致发光谱的结果相吻合。与单层锗量子点结构相比 ,双层结构的样品在光电特性上有着明显不同 :光电流谱中 ,在 0 .767e V及 0 .869e V处出现了两个峰 ,分别对应于载流子在不同的量子点层中的吸收。用这种结构的样品制成的红外光探测器能够同时对两种不同波长的光进行探测响应     

Self-assembling Ge(Si)/Si(100) quantum dots

The morphological evolution of self-assembled epitaxial quantum dots on Si(100) is reviewed. This intensely investigated material system continues to provide fundamental insight guiding the growth of nanostructured electronic materials. Self-assembled quantum dots are faceted, three-dimensional islands which grow atop a planar wetting layer. Pure Ge growth at higher substrate temperatures results in narrower island size distributions but activates additional strain-relief mechanisms which will alter the optical and electronic properties of the dots. Optical and electrical characterization has shown that electrons and holes are confined to different regions of the dot. This results in a spatially indirect, type II recombination mechanism. Emerging device applications which exploit properties of these nanoscale Ge islands are discussed.     

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.     

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     

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.     

Lateral photoconductivity in structures with Ge/Si quantum dots

The spectra of lateral photoconductivity and optical absorption caused by the intraband optical transitions of holes in Ge/Si quantum dots are studied at different lattice temperatures. Polarization-dependent spectral features related to the transitions of holes from the quantum dot (QD) ground state are revealed in the optical spectra. Temperature photoconductivity quenching caused by the reverse trapping of nonequilibrium free holes by the QD bound state is observed. The obtained experimental data make it possible to determine the height of the surface band bending at the QD heterointerface.     

Photoluminescence line width of self-assembled Ge(Si) islands arranged between strained Si layers

The effect of variations in the strained Si layer thicknesses, measurement temperature, and optical excitation power on the width of the photoluminescence line produced by self-assembled Ge(Si) nanoislands, which are grown on relaxed SiGe/Si(001) buffer layers and arranged between strained Si layers, is studied. It is shown that the width of the photoluminescence line related to the Ge(Si) islands can be decreased or increased by varying the thickness of strained Si layers lying above and under the islands. A decrease in the width of the photoluminescence line of the Ge(Si) islands to widths comparable with the width of the photoluminescence line of quantum dot (QD) structures based on direct-gap InAs/GaAs semiconductors is attained with consideration of diffusive smearing of the strained Si layer lying above the islands.     

Dependence of the Luminescence Properties of Ordered Groups of Ge(Si) Nanoislands on the Parameters of the Pit-Patterned Surface of a Silicon-on-Insulator Substrate

Semiconductors - This paper presents the results of studies of the luminescence properties of structures with Ge(Si) quantum dots (QDs), in which the pit-patterned surface of a silicon-on-insulator...     

Ordered Arrays of Ge(Si) Quantum Dots Incorporated into Two-Dimensional Photonic Crystals

Semiconductors - Two different approaches to the integration of self-assembled Ge(Si) quantum dots into two-dimensional photonic crystals are considered. One approach includes the synthesis of an...     

硼预淀积对自组织生长Ge量子点尺寸分布的影响

研究了以不同B2H6流量预淀积硼对UHV/CVD自组织生长Ge量子点尺寸分布的影响。在适当的生长条件下,得到了尺寸分布很窄的均匀Ge量子点,用AFM对量子点的形貌进行观察,Ge量子点尺寸的涨落小于±3%,量子点的水平尺寸和高度分别为60nm和10nm,密度为8×109cm-2。实验结果表明,通过预淀积硼表面处理,可以得到尺寸分布很窄的量子点,以满足量子点光电器件方面应用的要求。     

Observation and nucleation control of Ge nanoislands on Si(111) surfaces using scanning reflection electron microscopy

Using a high-resolution scanning reflection electron microscope with multifunctions, we investigate Ge nucleation processes on clean Si(111) surfaces and form Ge islands on them by controlling step arrangements of Si(111) surfaces and by using focused electron beam (EB)-induced surface reactions. It is found that three-dimensional (3D) Ge islands grow selectively at step band areas on the surfaces without growth of the islands at terrace areas. Three-dimensional Ge nanoislands are formed at given points by stimulating the Ge wetting layer using focused electron beams and scanning tunnelling microscopy. Ge nanoislands are also formed by depositing Ge on Si windows in ultrathin SiO2 films and subsequent annealing of the sample. The islands are formed only at the window positions. These results imply new methods for forming Ge quantum dots or nanostructures at given areas.