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.     

量子点光电子器件及其研究进展

作为未来新型光电子器件的有源区材料,采用能带工程形成的各种半导体量子点,以其所具有的许多独特光电特性而日益显示出潜在的重要应用。着重评述了量子点激光器、量子点红外光探测器和量子点单光子发射器件在近3至5年内取得的最新进展,并对存在的问题进行了分析和讨论。最后,提出了进一步改善器件性能的几种可能途径。     

Emerging technologies in Si active photonics

Silicon photonics for synergistic electronic–photonic integration has achieved remarkable progress in the past two decades. Active photonic devices, including lasers, modulators, and photodetectors, are the key challenges for Si photonics to meet the requirement of high bandwidth and low power consumption in photonic datalinks. Here we review recent efforts and progress in high-performance active photonic devices on Si, focusing on emerging technologies beyond conventional foundry-ready Si photonics devices. For emerging laser sources, we will discuss recent progress towards efficient monolithic Ge lasers, mid-infrared GeSn lasers, and high-performance InAs quantum dot lasers on Si for data center applications in the near future. We will then review novel modulator materials and devices beyond the free carrier plasma dispersion effect in Si, including GeSi and graphene electro-absorption modulators and plasmonic-organic electro-optical modulators, to achieve ultralow power and high speed modulation. Finally, we discuss emerging photodetectors beyond epitaxial Ge p–i–n photodiodes, including GeSn mid-infrared photodetectors, all-Si plasmonic Schottky infrared photodetectors, and Si quanta image sensors for non-avalanche, low noise single photon detection and photon counting. These emerging technologies, though still under development, could make a significant impact on the future of large-scale electronicSilicon photonics for synergistic electronic-photonic integration has achieved remarkable progress in the past two decades. Active photonic devices, including lasers, modulators, and photodetectors, are the key challenges for Si photonics to meet the requirement of high bandwidth and low power consumption in photonic datalinks. Here we review recent efforts and progress in high-performance active photonic devices on Si, focusing on emerging technologies beyond conventional foundry-ready Si photonics devices. For emerging laser sources, we will discuss recent progress towards efficient monolithic Ge lasers, mid-infrared GeSn lasers, and high-performance InAs quantum dot lasers on Si for data center applications in the near future. We will then review novel modulator materials and devices beyond the free carrier plasma dispersion effect in Si, including GeSi and graphene electro-absorption modulators and plasmonic-organic electro–optical modulators, to achieve ultralow power and high speed modulation. Finally, we discuss emerging photodetectors beyond epitaxial Ge p–i–n photodiodes, including GeSn mid-infrared photodetectors, all-Si plasmonic Schottky infrared photodetectors, and Si quanta image sensors for non-avalanche, low noise single photon detection and photon counting. These emerging technologies, though still under development, could make a significant impact on the future of large-scale electronic–photonic integration with performance inaccessible from conventional Si photonics technologies-photonic integration with performance inaccessible from conventional Si photonics technologies.     

半导体量子点的器件应用

半导体量子点的生长和性质已成为当今研究的热点。以Ｓ－Ｋ生长模式已成功地生长了无缺陷的半导体量子点。由于其较强的量子效应,量子点结构的光电器件和电子器件有望比量子阱结构的器件具有更好的性能。介绍了量子点在光电器件中的一些应用,包括单电子晶体管、激光器和红外探测器等。     

硅基量子点器件

硅基量子点器件由于其独特的性能以及和硅集成电路相容的特点成为研究的重点。量子点中电子和空穴强的量子限制作用使其表现出一些新颖的物理性能,从而在微电子和光电子器件方面有着重要的应用价值。本文介绍了硅基量子点器件的一些应用,包括单电子晶体管、红外探测器、发光二极管。     

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.     

利用ArF准分子激光退火获得小锗量子点的研究

使用ArF准分子激光脉冲对UHV/CVD条件下生长的Ge量子点进行退火处理,获得了底宽为20～25nm的光致量子点(LIQD),远小于退火前的量子点大小.LIQD的密度约为6×1010cm-2.分析表明,在ArF准分子激光脉冲作用下,退火样品只有表面扩散,并没有体扩散.激光脉冲对表面Ge原子的扩散控制导致了Ge量子点形貌发生了巨大的改变.该方法为获得高密度小尺寸的Ge量子点提供了新的途径.采用原子力显微镜对光致量子点的表面形貌进行了研究.     

利用ArF准分子激光退火获得小锗量子点的研究

使用ArF准分子激光脉冲对UHV/CVD条件下生长的Ge量子点进行退火处理,获得了底宽为20～25nm的光致量子点(LIQD),远小于退火前的量子点大小.LIQD的密度约为6×1010cm-2.分析表明,在ArF准分子激光脉冲作用下,退火样品只有表面扩散,并没有体扩散.激光脉冲对表面Ge原子的扩散控制导致了Ge量子点形貌发生了巨大的改变.该方法为获得高密度小尺寸的Ge量子点提供了新的途径.采用原子力显微镜对光致量子点的表面形貌进行了研究.     

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.     

On the size distribution in three-dimensional quantum-dot crystals

The size distribution function of nanodots in artificial three-dimensional (Si)Ge/Si and In(Ga)As/GaAs quantum dot crystals grown using templates with perfect periodicity is calculated. Pyramidal nanodots were modeled by cone-shaped clusters for which the Thomson formula was derived, which is necessary to determine the growth (dissolution) rate of clusters during Ostwald ripening. A comparison of the calculated curve with experimental histograms shows that the size distribution itself is formed during Ostwald ripening and is caused by features of Ge and InAs quantum-dot formation on preliminarily textured Si and GaAs substrates.     

Impact of Local Strain From Selective Epitaxial Germanium With Thin Si/SiGe Buffer on High-Performance p-i-n Photodetectors With a Low Thermal Budget

This letter reports on the impact of selective epitaxial germanium, specifically its local strain effects, on high-performance p-i-n photodetectors for near-infrared applications. By combining a thin compliant Si epitaxial layer (~6 nm) with SiGe buffer (10-15 nm), we demonstrated a high-quality Ge film (~150 nm) prepared by two-step growth. Without using high-temperature cyclic anneal, Ge films with smooth surface (root mean square = ~ 0.67 nm) and low dislocation density (4 x 10⁶ cm^-2) have been produced. The Si buffer locally enhances the tensile strain (epsiv = 0.63%) in Ge while slightly suppressing the dark current by half to 0.12 muA (with circular ring area = 1230 mum² and spacing = 2 mum). A lateral p-i-n Ge photodetector has been demonstrated with enhanced photoresponse of ~190 mA/W at 1520 nm and a 3-dB bandwidth of 5.2 GHz at 1 V.     

Novel Quantum Dot Gate FETs and Nonvolatile Memories Using Lattice-Matched II–VI Gate Insulators

This paper presents the successful use of ZnS/ZnMgS and other II–VI layers (lattice-matched or pseudomorphic) as high-k gate dielectrics in the fabrication of quantum dot (QD) gate Si field-effect transistors (FETs) and nonvolatile memory structures. Quantum dot gate FETs and nonvolatile memories have been fabricated in two basic configurations: (1) monodispersed cladded Ge nanocrystals (e.g., GeO _x -cladded-Ge quantum dots) site-specifically self-assembled over the lattice-matched ZnMgS gate insulator in the channel region, and (2) ZnTe-ZnMgTe quantum dots formed by self-organization, using metalorganic chemical vapor-phase deposition (MOCVD), on ZnS-ZnMgS gate insulator layers grown epitaxially on Si substrates. Self-assembled GeO _x -cladded Ge QD gate FETs, exhibiting three-state behavior, are also described. Preliminary results on InGaAs-on-InP FETs, using ZnMgSeTe/ZnSe gate insulator layers, are presented.     

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.     

Growth of ternary InAlP and InGaP self-assembled quantum dots by metalorganic chemical vapor deposition

We report the characteristics of ternary InAlP and InGaP self-assembled quantum dots grown by metalorganic chemical vapor deposition. The structural and optical properties of these ternary quantum dots are compared with the characteristics of binary InP quantum dots grown under similar conditions. Because these ternary quantum dots have different bandgaps, strain, and composition compared to binary InP quantum dots, the ternary quantum-dot optical and physical properties are markedly different. The quantum-dot structures are grown uncapped (exposed QDs) and capped (embedded QDs) and characterized by atomic force microscopy (AFM) and photoluminescence (PL). InAlP quantum dots have higher densities and smaller sizes and InGaP quantum dots have smaller densities, as compared with InP quantum dots grown under similar conditions. Also, a random and broad size distribution is observed for InGaP quantum dots and the luminescence from InGaP dots is broader than for InP quantum dots.     

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.     

Hybrid GaAs/AlGaAs Nanowire—Quantum dot System for Single Photon Sources

III–V nanowires, or a combination of the nanowires with quantum dots, are promising building blocks for future optoelectronic devices, in particular, single-photon emitters, lasers and photodetectors. In this work we present results of molecular beam epitaxial growth of combined nanostructures containing GaAs quantum dots inside AlGaAs nanowires on a silicon substrate showing a new way to combine quantum devices with Si technology.     

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.     

A high efficient 820 nm MOS Ge quantum dot photodetector

A Ge quantum dot photodetector has been demonstrated using a metal-oxide-semiconductor (MOS) tunneling structure. The oxide film was grown by liquid phase deposition (LPD) at 50/spl deg/C. The photodetector with five-period Ge quantum dot has responsivity of 130, 0.16, and 0.08 mA/W at wavelengths of 820 nm, 1300 nm, and 1550 nm, respectively. The device with 20-period Ge quantum dot shows responsivity of 600 mA/W at the wavelength of 850 nm. The room temperature dark current density is as low as 0.06 mA/cm/sup 2/. The high performance of the photodetectors at 820 nm makes it feasible to integrate electrooptical devices into Si chips for short-range optical communication.     

1.3-μm CW lasing characteristics of self-assembled InGaAs-GaAsquantum dots

This paper presents the lasing properties and their temperature dependence for 1.3-μm semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-μm emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-μm ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25°C with a realistic cavity length of 300 μm and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm^-1 from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm^-1, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T₀ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T₀ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T₀ with a low threshold current (~1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-μm quantum-dot lasers     

Carrier Dynamics of Quantum-Dot, Quantum-Dash, and Quantum-Well Semiconductor Optical Amplifiers Operating at 1.55 μm

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (0-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In-GaAsP-InP quantum well (2-D), all of which operate near 1.55-mum wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer.