Effect of parameters of Ge(Si)/Si(001) self-assembled islands on their electroluminescence at room temperature

The electroluminescence (EL) of multilayered p-i-n structures with the self-assembled Ge(Si)/Si(001) islands are investigated. It is found that the structures with islands grown at 600°C have the highest intensity of the electroluminescence signal at room temperature in the wavelength range of 1.3–1.55 μm. The annealing of structures with the Ge(Si) islands leads to an increase in the EL-signal intensity at low temperatures and hampers the temperature stability of this signal, which is related to the additional Si diffusion into islands during annealing. The found considerable increase in the electroluminescence-signal intensity with the thickness of the separating Si layer is associated with a decrease in the elastic stresses in the structure with an increase in this layer’s thickness. The highest EL quantum efficiency in the wavelength range of 1.3–1.55 μm obtained in investigated structures amounted to 0.01% at room temperature.     

Effect of growth temperature on photoluminescence of self-assembled Ge(Si) islands confined between strained Si layers

The effect of growth temperature on photoluminescence is studied for structures with Ge(Si) islands grown on relaxed SiGe/Si(001) buffer layers and confined between strained Si layers. It is shown that, with decreasing growth temperature in the range from 700 to 630°C, the photoluminescence peak associated with the islands shifts to lower energies, which is due to the increase in Ge content in the islands and to suppression of degradation of the strained Si layers. The experimentally observed shift of the photoluminescence peak to higher energies with decreasing temperature from 630 to 600°C is attributed to the change in the type of the islands from domelike to hutlike in this temperature range. This change is accompanied by an abrupt decrease in the average height of the islands. The larger width of the photoluminescence peak produced by the hut islands in comparison with the width of the peak produced by the domelike islands is interpreted as a result of a wider size dispersion of the hutlike islands.     

Effect of silicon spacer thickness on the electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) islands

The results of investigation of the electroluminescence of multilayer p-i-n structures with Ge(Si)/Si(001) self-assembled islands are presented. The nonmonotonic dependence of the room-temperature intensity of the electroluminescence signal from islands on the Si spacer thickness is revealed. The highest electroluminescence signal intensity is observed for structures with a Si spacer thickness of 15?C20 nm. The significant decrease detected in the electroluminescence signal from the islands in structures with thick Si spacers (>20 nm) is explained by the formation of defect regions in them. The observed decrease in the electroluminescence signal in structures with thin Si layers is associated with a decrease in the Ge fraction in the islands in these structures, which is caused by enhanced Si diffusion into islands with increasing elastic strains in the structure.     

Effect of tensile-strained Si layer on photoluminescence of Ge(Si) self-assembled islands grown on relaxed SiGe/Si(001) buffer layers

The results of studying the photoluminescence of the structures with Ge(Si) self-assembled islands embedded into tensile-strained Si layer are reported. The structures were grown on smooth relaxed Si_{1 ? x} Ge_x/Si(001) (x = 0.2–0.3) buffer layers. The photoluminescence peak found in the photoluminescence spectra of the studied structures is related to the indirect (in real space) optical transition between the holes localized in the Ge(Si) islands and electrons localized in the tensile-strained Si layers under and above an island. It is shown that one can efficiently control the position of the photoluminescence peak for a specified type of structure by varying the thickness of the strained Si layers. It is found that, at 77 K, the intensity of the photoluminescence signal from the heterostructures with Ge(Si) self-assembled islands contained between the tensile-strained Si layers exceeds by an order of magnitude the intensity of the photoluminescence signal from the GeSi structures with islands formed on the Si(001) substrates.     

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.     

Time-resolved photoluminescence from self-assembled Ge(Si) islands in multilayer SiGe/Si and SiGe/SOI structures

The results of a study of the spectral and temporal characteristics of the photoluminescence (PL) from multilayer structures with self-assembled Ge(Si) islands grown on silicon and “silicon-on-insulator” substrates in relation to temperature and the excitation-light wavelength are presented. A substantial increase in island-related PL intensity is observed for structures with Ge(Si) islands grown on silicon substrates upon an increase in temperature from 4 to 70 K. This increase is due to the diffusion of nonequilibrium carriers from the silicon substrate into the active layer with the islands. In this case, a slow component with a characteristic time of ～100 ns appears in the PL rise kinetics. At the same time, no slow component in the PL rise kinetics and no rise in the PL intensity with increasing temperature are observed for structures grown on “silicon-on-insulator” substrates, in which the active layer with the islands is insulated from the silicon substrate. It is found that absorption of the excitation light in the islands and SiGe wetting layers mainly contributes to the excitation of the PL signal from the islands under sub-bandgap optical pump conditions.     

Effect of irradiation on the luminescence properties of low-dimensional SiGe/Si(001) heterostructures

This study is concerned with the effect of irradiation on the luminescence properties of low-dimensional Si/Ge heterostructures with different degrees of spatial localization of charge carriers. It is shown that the radiation stability of Si/Ge heterostructures is improved with increasing efficiency of localization of charge carriers in the structures. The spatial localization of charge carriers in the SiGe nanostructures decreases the probability of nonradiative recombination of charge carriers at radiation defects produced in the Si matrix. It is demonstrated that, among the structures explored in the study, the highest radiation stability of luminescence properties is inherent in the multilayered structures containing self-assembled Ge(Si) nanoislands, in which the most efficient spatial localization of charge carriers is attained. In this case, the localization is three- and two-dimensional, correspondingly, for holes in the islands and for electrons in the Si layers that separate neighboring layers containing the islands.     

Special features of the photoluminescence of self-assembled Ge(Si)/Si(001) islands grown on a strained Si1−x Gex layer

The effect of the predeposition of strained Si_{1 ? x} Ge_x layers (x ≤ 20%) on photoluminescence (PL) of self-assembled Ge(Si)/Si(001) islands is studied. A shift of the PL peak related to dome-shaped islands (domes) to lower energies, with respect to the PL peak related to pyramidal islands is observed; this shift is related to a much larger height of the domes compared to that of pyramids. It is found that, as the Ge content in the Si_{1 ? x} Ge_x layer (x) becomes higher than 0.1, two separate peaks appear in the broad PL band related to the islands; these peaks are attributed to the zero-phonon and phonon-assisted optical transitions in the islands. The appearance of these transitions is caused by a change of the TO-phonon type involved in radiative recombination: a TO_Ge-Ge phonon is replaced by a TO_Si-Ge phonon with a shorter wavelength.     

Elastic strain and composition of self-assembled GeSi nanoislands on Si(001)

The growth of self-assembled Ge islands on Si(001) surface and changes in the island structure parameters in the course of subsequent annealing were studied. Island structures possessing a small (～6%) scatter with respect to lateral dimensions and heights of the islands were obtained. The Raman spectra and X-ray diffraction data show evidence that silicon dissolves in the islands. The atomic fraction of Si in the resulting Si_xGe_1?x solid solution was determined and the elastic strain in the islands was measured. It was found that annealing of the heterostructures with islands is accompanied by increasing Si fraction in the islands, which leads to changes in the island shape and size.     

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     

Elastic strain and enhanced light emission in dry etched Si/Si1-xGex quantum dots

Quantum dots of 50 ~ 60 nm diameter fabricated from both Si/Si_1-xGe_x (x = 0.1 ~ 0.3) strained layer superlattices and a strain symmetried Si₉/Ge₆ superlattice were investigated by a combination of Raman scattering, photoluminescence, and electroluminescence spectroscopy. It was found that, in addition to an enhanced luminescence intensity of the dots by over two orders of magnitude and improved luminescence quenching temperature, all of the nanostructure dots have residual built-in elastic strains, which are of the order of ~50% of the values in corresponding pseudomorphic heterostructures. This result suggests a possible mechanism for explaining the huge enhancement of the optical efficiency in our luminescence measurements.     

化学共沉淀法合成YAG:Eu~(3+)红色荧光粉及其光致发光性质

采用化学共沉淀法合成YAG∶Eu3+红色荧光粉,利用XRD、荧光光谱和激光粒度仪等表征荧光粉晶体结构、光致发光、粒径分布等。结果表明,当煅烧温度为1000℃时,荧光粉YAG晶相趋于稳定,无中间相的形成,合成温度比传统高温固相法降低近500℃;随着煅烧温度升高,荧光粉光致发光峰强度增大,但峰值波长并不随煅烧温度升高而发生移动;前驱体中添加表面活性剂PEG后,荧光粉粒径为1.3μm左右,且粒径分布范围窄;助熔剂NaF能够显著提高荧光粉的光致发光强度,但过量则会出现浓度猝灭,其最佳添加量为4%。     

Low threading dislocation density Ge deposited on Si (1 0 0) using RPCVD

Epitaxial Ge layer growth of low threading dislocation density (TDD) and low surface roughness on Si (1 0 0) surface is investigated using a single wafer reduced pressure chemical vapor deposition (RPCVD) system. Thin seed Ge layer is deposited at 300 °C at first to form two-dimensional Ge surface followed by thick Ge growth at 550 °C. Root mean square of roughness (RMS) of ∼0.45 nm is achieved. As-deposited Ge layers show high TDD of e.g. ∼4 × 10⁸ cm⁻² for a 4.7 μm thick Ge layer thickness. The TDD is decreasing with increasing Ge thickness. By applying a postannealing process at 800 °C, the TDD is decreased by one order of magnitude. By introducing several cycle of annealing during the Ge growth interrupting the Ge deposition, TDD as low as ∼7 × 10⁵ cm⁻² is achieved for 4.7 μm Ge thick layer. Surface roughness of the Ge sample with the cyclic annealing process is in the same level as without annealing process (RMS of ∼0.44 nm). The Ge layers are tensile strained as a result of a higher thermal expansion coefficient of Ge compared to Si in the cooling process down to room temperature. Enhanced Si diffusion was observed for annealed Ge samples. Direct band-to-band luminescence of the Ge layer grown on Si is demonstrated.     

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.     

Special features of the formation of Ge(Si) islands on the relaxed Si1−xGex/Si(001) buffer layers

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si_1?xGe_x/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.     

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

Semiconductors - The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence...     

Specific features of the mechanisms of excitation of erbium photoluminescence in epitaxial Si:Er/Si structures

The excitation spectra and kinetics of erbium photoluminescence and silicon interband photoluminescence in Si:Er/Si structures under conditions of high-intensity pulse optical excitation are studied. It is shown that, in the interband photoluminescence spectra of the Si:Er/Si structures, both the luminescence of free excitons and the emission associated with the electron-hole plasma can be observed, depending on the excitation power and wavelength. It is found that the formation of a peak in the erbium photoluminescence excitation spectra at high pumping powers correlates with the Mott transition from the exciton gas to the electron-hole plasma. It is demonstrated that, in the Si:Er/Si structures, the characteristic rise times of erbium photoluminescence substantially depend on the concentration of charge carriers.     

Si3N4无杂质空位诱导InGaAs/InP 量子阱结构带隙的蓝移

用光荧光谱和二次离子质谱的方法,研究了由Si3N4电介质薄膜引起的无杂质空位诱导InGaAs/InP多量子阱结构的带隙蓝移。实验中选用Si3N4作为电介质层,用以产生空位,并经快速热退火处理。实验结果表明,带隙蓝移同退火时间和退火温度有关,合理选用退火条件可以控制带隙的蓝移量。二次离子质谱分析表明,电介质盖层Si3N4和快速热退火导致量子阱中阱和垒之间互扩,这种互扩是导致带隙蓝移的主要原因。     

Comparative analysis of radiation effects on the electroluminescence of Si and SiGe/Si(001) heterostructures with self-assembled Islands

The effect of neutron radiation on the electroluminescence of the Si p-i-n diode containing a multilayered Ge/Si heterostructure with self-assembled nanoislands is studied. In comparison with bulk Si, the diodes containing Ge(Si) nanoislands exhibit a higher radiation hardness of the electroluminescence signal, which is attributed to spatial localization of charge carriers in the Ge/Si nanostructures. The spatial localization of charge carriers impedes their diffusion to radiation defects followed by nonradiative recombination at the defects. The results show the possibilities of using Ge/Si heterostructures with self-assembled nanoislands for the development of optoelectronic devices resistant to radiation.     

Electroluminescence from MIS silicon-based light emitters with arrays of self-assembled Ge(Si) nanoislands

We report on the electroluminescence from silicon-based metal–insulator–semiconductor (MIS) diodes with arrays of self-assembled Ge(Si) nanoislands. Aluminum oxide (Al₂O₃) is used as an insulator material in the MIS contact. Variations in the electroluminescence spectra caused by changing the metal work function are examined. The intense electroluminescence from Ge(Si) nanoislands localized at a distance of 50 nm from the insulator–semiconductor interface is observed at room temperature. The emission spectrum is found to be controlled by choosing the design of the semiconductor structure and the barrier height for injected carriers.