Self-assembled Ge Quantum Dots Research Articles

Tuning the Luminescence Response of an Air-Hole Photonic Crystal Slab Using Etching Depth Variation.

Detailed studies of the luminescent properties of the Si-based 2D photonic crystal (PhC) slabs with air holes of various depths are reported. Ge self-assembled quantum dots served as an internal light source. It was obtained that changing the air hole depth is a powerful tool which allows tuning of the optical properties of the PhC. It was shown that increasing the depth of the holes in the PhC has complex influences on its overall photoluminescence (PL) response due to the simultaneous influences of counteracting factors. As a result, the maximal increase in the PL signal of more than two orders of magnitude was obtained for some intermediate, but not full, depth of the PhC's air holes. It was demonstrated that it is possible to engineer the PhC band structure in such a way as to construct specific states, namely bound states in continuum (BIC), with specially designed dispersion curves being relatively flat. In this case, such states manifest themselves as sharp peaks in the PL spectra, and have high Q-factors which are larger than those of radiative modes and other BIC modes without such a flat dispersion characteristic.

Highly Tensile-Strained Self-Assembled Ge Quantum Dots on InP Substrates for Integrated Light Sources

Highly tensile-strained Ge quantum dots (TS-Ge-QDs) emitting structures with different size were successfully grown on InP substrates by molecular beam epitaxy. Dislocation-free TS-Ge-QDs were observed by transmission electron microscopy. Finite element modeling indicates a maximum tensile strain of 4.5% in the Ge QDs, which is much larger than the required strain to achieve direct band gap conversion of Ge based on theoretical prediction. Photoluminescence (PL) from a direct band-gap-like transition of TS-Ge-QDs with a peak energy of 0.796 eV was achieved and confirmed by the etch depth-dependent PL, temperature-dependent PL, and excitation-power-dependent PL. In addition, a strong defect-related peak of 1 eV was observed at room temperature. The band structure of the TS-Ge-QDs emitting structures was calculated to support the experimental results of PL spectra. Achieving PL from direct band-gap-like transitions of TS-Ge-QDs provides encouraging evidence of this promising highly tensile strained semiconductor-nanostructure-based platform for future photonics applications such as integrated light sources.

Mid-infrared optical resonances in quantum dot photodetectors coupled with metallic gratings with different aperture diameters

The paper is devoted to optical testing of mid-infrared Ge/Si photodetectors obtained by stacking of self-assembled Ge quantum dots in multilayer structures, which are near-field coupled to the adjacent nanoplasmonic arrays of subwavelength holes in metallic films. It is shown that photocurrent and near-field spectra consist of several sets of peaks, which are attributted to surface plasmon waves, localized surface plasmon modes or diffractive Rayleigh anomaly depending on the hole diameter and the angle of incidence θ. We find that for small holes the greatest contribution to the photocurrent enhancement is due to the excitation of the surface plasmon-polariton waves for all θ. As the hole diameter is increased and becomes comparable with the array periodicity, the normal-incident photoresponse improvement is provided by the Rayleigh anomaly. With the increase of incident angle, the photocurrent enhancement is supposed to arise from coupling of the localized shape resonance and propagating plasmon modes.

In-Situ Annealing and Hydrogen Irradiation of Defect-Enhanced Germanium Quantum Dot Light Sources on Silicon

While light-emitting nanostructures composed of group-IV materials fulfil the mandatory compatibility with CMOS-fabrication methods, factors such as the structural stability of the nanostructures upon thermal annealing, and the ensuing photoluminescence (PL) emission properties, are of key relevance. In addition, the possibility of improving the PL efficiency by suitable post-growth treatments, such as hydrogen irradiation, is important too. We address these issues for self-assembled Ge quantum dots (QDs) that are co-implanted with Ge ions during their epitaxial growth. The presence of defects introduced by the impinging Ge ions results in pronounced PL-emission at telecom wavelengths up to room temperature (RT) and above. This approach allows us to overcome the severe limitations of light generation in the indirect-band-gap group-IV materials. By performing in-situ annealing, we demonstrate a high PL-stability of the defect-enhanced QD (DEQD) system against thermal treatment up to 600 °C for at least 2 h, even though the Ge QDs are structurally affected by Si/Ge intermixing via bulk diffusion. The latter, in turn, allows for emission tuning of the DEQDs over the entire telecom wavelength range from 1.3 µm to 1.55 µm. Two quenching mechanisms for light-emission are discussed; first, luminescence quenching at high PL recording temperatures, associated with the thermal escape of holes to the surrounding wetting layer; and second, annealing-induced PL-quenching at annealing temperatures >650 °C, which is associated with a migration of the defect complex out of the QD. We show that low-energy ex-situ proton irradiation into the Si matrix further improves the light emission properties of the DEQDs, whereas proton irradiation-related optically active G-centers do not affect the room temperature luminescence properties of DEQDs.

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

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 ordered array of Ge(Si) quantum dots on the textured surface of a substrate followed by the formation of a photonic crystal on this array. In the other approach, the photonic crystal itself serves as a template for the ordered growth of quantum dots. It is shown that, by varying the diameter of holes of photonic crystals in the second approach, it is possible to implement the growth of quantum dots in two modes, in which quantum dots are formed inside or outside the holes of the photonic crystal. For structures with ordered quantum dots incorporated into a photonic crystal, an increase in the photoluminescence signal intensity is detected at room temperature in the spectral range 0.9–1.2 eV. This increase is attributed to the interaction of emission from the structure with radiation modes of the photonic crystal.

Role of surface passivation on visible and infrared emission of Ge quantum dots formed by dewetting

The dual action of oxide-related defects in the visible and infrared emission of germanium (Ge) self-assembled quantum dots (QDs) is discussed. The Ge particles were fabricated by solid-state dewetting on a thin layer of \({\hbox {SiO}}_{2}\). Subsequent surface passivation by amorphous silicon was carried out for several samples. All samples were encapsulated by \({\hbox {SiO}}_{2}\). Atomic force microscopy analysis indicates a linear relationship between the size of QDs and the initial thickness of the amorphous Ge films. The crystallization of the QDs was evidenced by transmission electron microscopy and Raman spectroscopy. Photoluminescence measurements show that the main visible emission is blue-green centred around 520 nm. The luminescence attributed to the radiative recombination of quantum-confined excitons is only observed when the surface is in-situ passivated prior to the deposition of the oxide matrix. The results of this work are helpful for optimizing the performance of the optoelectronic devices based on the infrared emission of Ge nanocrystals.

Упорядоченные массивы квантовых точек Ge(Si), встроенные в двумерные фотонные кристаллы

Two approaches to the integration of ordered Ge(Si) self-assembled quantum dots (QDs) into two-dimensional photonic crystals (PhC) are considered in the paper. The first approach involves the synthesis on the structured silicon surface of the ordered Ge(Si) QD array, on which PhC is then formed. In the second approach, the PhC itself serves as the template for the ordered QD growth. It is shown that varying the diameter of PhC holes in the framework of the second approach, two growth modes of QDs can be realized, when they are formed inside or outside the PhC holes. For structures with ordered QDs built into PhC, an increase in the intensity of the photoluminescence signal was detected at room temperature in the spectral range of 0.9-1.2 eV, which is associated with the interaction of the emission with the radiation modes of PhC.

Toward precise site-controlling of self-assembled Ge quantum dots on Si microdisks

A feasible route is developed toward precise site-controlling of quantum dots (QDs) at the microdisk periphery, where most microdisk cavity modes are located. The preferential growth of self-assembled Ge QDs at the periphery of Si microdisks is discovered. Moreover, both the height and linear density of Ge QDs can be controlled by tuning the amount of deposited Ge and the microdisk size. The inherent mechanisms of these unique features are discussed, taking into account both the growth kinetics and thermodynamics. By growing Ge on the innovative Si microdisks with small protrusions at the disk periphery, the positioning of Ge QDs at the periphery can be exactly predetermined. Such a precise site-controlling of Ge QDs at the periphery enables the location of the QD right at the field antinodes of the cavity mode of the Si microdisk, thereby achieving spatial matching between QD and cavity mode. These results open a promising door to realize the semiconductor QD-microdisk systems with both spectral and spatial matching between QDs and microdisk cavity modes, which will be the promising candidates for exploring the fundamental features of cavity quantum electrodynamics and the innovative optoelectronic devices based on strong light–matter interaction.

Directed Self-Assembly of Ge Quantum Dots Using Focused Si2+ Ion Beam Patterning

We show that templating a Si surface with a focused beam of Si2+ or Si+ ions can create suitable nucleation sites for the subsequent growth of self-assembled Ge quantum dots by chemical vapor deposition. To determine the mechanism of patterning we use atomic force microscopy to show that, similar to Ga+ patterning, the formation of a surface pit is required to enable control over Ge quantum dot locations. We find that relatively high implantation doses are required to achieve patterning, and these doses lead to amorphization of the substrate. We assess the degree to which the substrate crystallinity can be recovered by subsequent processing. Using in situ transmission electron microscopy heating experiments we find that recrystallization is possible at the growth temperature of the Ge quantum dots, but defects remain that follow the pattern of the initial implantation. We discuss the formation mechanism of the defects and the benefits of using Si ions for patterning both defects and quantum dots on Si substrates.

(Invited) CMOS-Compatible Germanium Light Sources

Integration of high efficiency light sources on a Si platform is desirable for realization of optoelectronic integrated circuits (OEICs). Ge is one of the most promising candidates toward realization of monolithic-integrated laser sources on a Si substrate. Ge has the narrower bandgap energy than Si, allowing us to extract light emissions from Ge with wavelength ranges corresponding to telecommunication band. Also the emitted light from Ge is transparent in Si, making the Ge the most proper light source for optical interconnection based on Si waveguides. In addition, since the Ge has been already involved in the commercial CMOS circuit, the Ge-based optical devices are considered to be fully compatible with CMOS processes, and thus integration of Ge devices on the Si platform is more cost effective than that of compound semiconductors. Whilst there are many advantages of Ge as light sources on the Si chip, one critical drawback is the fact that Ge is indirect band gap material like Si, which means that the light emission efficiency from Ge is naturally difficult. To overcome this problem, however, various types of Ge light emitting structures have been researched so far, from bulk to nano structures. For most cases, key mechanisms to enhance the light emission efficiency are based on band engineering by strain introduction and doping control. Here, we show recent progresses of our researches on strained GOI (Ge-on-Insulator), highly strained Ge microbridges and n-doped Ge QDs. By inducing tensile strain and n-type doping, energy band structure of Ge can be modified to a direct one, leading to significantly enhanced light emission. The tensile strain can be induced into a Ge directly grown on a Si (Ge-on-Si) due to thermal expansion mismatch between Si and Ge, making the Ge-on-Si a very attractive light emitter. Toward realization of low threshold laser on silicon chip, higher efficiency light emitting structures are highly required. To this end, we show Ge-on-Insulator structure can eliminate defective Ge, the source of non-radiative recombination, leading to enhancements of the light emission. In addition, as a resonator we fabricate Ge microdisks with circular Bragg gratings (CBGs) to enhance the reflectivity and obtain sharper resonant light emission. For Ge-on-Si, the amount of strain is limited up to ~0.2%, which is not sufficient for direct-gap transition. To further increase amounts of the tensile strain, fabrication of a micro-structure is effective as a relatively large elastic stress is locally induced into the micro-region. The strained GOI is suitable for fabrication of the micro-structure as the underlying buried oxide is easily selective-etched and a free-standing Ge can be obtained. We fabricated a GOI microbridge as a micro-structure containing uniaxial strain and obtained high efficiency room-temperature PL. With the tensile strain increase, the PL peaks are found to shift to lower energies, indicating the direct band gap decreases with the strain. At higher strain levels (> ~ 0.5%), two peaks appear in the spectra, implying the splitting the heavy-hole (HH) and light-hole (LH) valence bands. Furthermore, we have recently observed resonant light emission corresponding to the resonance in Fabry-Perot (FP) cavities formed transversely to the bridge direction, implying a possibility of combining the microbridge with optical resonators. We also numerically designed FP cavities by adding distributed Bragg reflectors (DBR) laterally to the microbridge and obtained improved Q-factors without modification of the microbridge structure and resultant strain loss. A multilayer of self-assembled Ge quantum dots (QDs) is another attractive light emitting structure. To date, strong electroluminescence has been obtained from micro cavity devices involving Ge QDs. For further improvements of the light emission efficiency, stronger confinements of both electrons and holes in QDs are essential, whereas the Ge QD can confine only holes due to its type II band diagram. To confine electrons near the Ge QDs, phosphorous delta-doping at the Ge dot interface was attempted and enhancement of the PL intensity was obtained with optimal doping densities. In conclusion, by controlling higher tensile strain and n-type doping, Ge-based light sources offer highly enhanced light emission, indicating Ge has high potential for monolithic integrated laser diodes on the Si platform. Further performance improvements can be made by optimization of resonant cavity structures, increase in the tensile strain and reduction of defects toward realization of low-threshold laser operations.

Characterization of Ge Quantum Dot Optical Waveguides for High Speed Optical Modulators

Quantum Dot (QD) Optical Modulators can provide high speed modulation in low cost indirect bandgap materials. Si based optical modulators can be realized with the inclusion of self-assembled Ge QDs to provide low cost, high speed CMOS compatible optical devices. In this paper, we present the optical characterization of a novel Ge-QD Si-SiO2 based waveguide for use in as an optical modulator. Optical performance figures of merit are investigated including insertion loss (IL) measurements, and Wavelength dependent loss (WDL). We present a multimode waveguide fabricated with conventional CMOS processing. The waveguide provides 4.43dB/cm loss and individual discrete absorption regimes corresponding to the unique minibands produced by superlattice properties of the self-assembled Ge QDs in the IR regime. Absorption properties of the Ge QDs are demonstrated and verified against the QD superlattice bandgap model. Analysis and simulation is presented to qualitatively compare the QD bandgap energies with the reported optical properties. The QD functionalized structure demonstrates the fundamental optical principles of a QD waveguide, setting the foundation for a active modulation testing of this QD based optical modulator.

Plasmon polariton enhanced mid-infrared photodetectors based on Ge quantum dots in Si

Quantum dot based infrared (IR) photodetectors (QDIPs) have the potential to provide meaningful advances to the next generation of imaging systems due to their sensitivity to normal incidence radiation, large optical gain, low dark currents, and high operating temperature. SiGe-based QDIPs are of particular interest as they are compatible with silicon integration technology but suffer from the low absorption coefficient and hence small photoresponse in the mid-wavelength IR region. Here, we report on the plasmonic enhanced Ge/Si QDIPs with tailorable wavelength optical response and polarization selectivity. Ge/Si heterostructures with self-assembled Ge quantum dots are monolithically integrated with periodic two-dimensional arrays of subwavelength holes (2DHAs) perforated in gold films to convert the incident electromagnetic IR radiation into the surface plasmon polariton (SPP) waves. The resonant responsivity of the plasmonic detector at a wavelength of 5.4 μm shows an enhancement of up to thirty times over a narrow spectral bandwidth (FWHM = 0.3 μm), demonstrating the potentiality of this approach for the realization of high-performance Ge/Si QDIPs that require high spectral resolution. The possibility of the polarization-sensitive detection in Ge/Si QDIPs enhanced with a stretched-lattice 2DHA is reported. The excitation of SPP modes and the near-field components are investigated with the three-dimensional finite-element frequency-domain method. The role that plasmonic electric field plays in QDIP enhancement is discussed.

Mn-doped Ge self-assembled quantum dots via dewetting of thin films

In this study, we demonstrate an original elaboration route for producing a Mn-doped Ge self-assembled quantum dots on SiO2 thin layer for MOS structure. These magnetic quantum dots are elaborated using dewetting phenomenon at solid state by Ultra-High Vacuum (UHV) annealing at high temperature of an amorphous Ge:Mn (Mn: 40%) nanolayer deposed at very low temperature by high-precision Solid Source Molecular Beam Epitaxy on SiO2 thin film. The size of quantum dots is controlled with nanometer scale precision by varying the nominal thickness of amorphous film initially deposed. The magnetic properties of the quantum-dots layer have been investigated by superconducting quantum interference device (SQUID) magnetometry. Atomic force microscopy (AFM), x-ray energy dispersive spectroscopy (XEDS) and transmission electron microscopy (TEM) were used to examine the nanostructure of these materials. Obtained results indicate that GeMn QDs are crystalline, monodisperse and exhibit a ferromagnetic behavior with a Curie temperature (TC) above room temperature. They could be integrated into spintronic technology.

Influence of stress on the properties of Ge nanocrystals in an SiO2 matrix

In this work, self-assembled Ge quantum dot (QD) formation in a dielectric matrix is explored. Of particular interest were their structural and optical properties, in order to understand the stress build-up in such a process and its impact on the material properties during processing. To this end, thin films consisting of (Ge + SiO2)/SiO2 multilayers grown by RF magnetron sputtering were deposited at room temperature. Annealing of such films at 873 K in inert N2 atmosphere produced, at the position of the Ge-rich SiO2 layers, a high lateral density (about 1012 cm−2) of Ge QDs with a good crystallinity. SiO2 spacer layers separated the adjacent Ge-rich layers, where the Ge QDs were formed with a diameter of about the size of the (Ge + SiO2) as-deposited layer thickness, and created a good vertical repeatability, confirmed by the appearance of a Bragg sheet in two-dimensional small-angle X-ray scattering patterns. The structural analysis, by wide-angle X-ray diffraction, grazing-incidence small-angle X-ray scattering and transmission electron microscopy, has shown that the described processing of the films induced large compressive stress on the formed QDs. Optical analysis by time-resolved photoluminescence (PL) revealed that the high density of crystalline Ge QDs embedded in the amorphous SiO2 matrix produced a strong luminescence in the visible part of the spectrum at 2–2.5 eV photon energy. It is shown that the decay dynamics in this energy range are very fast, and therefore the transitions that create such PL are attributed to matrix defects present in the shell surrounding the Ge QD surface (interface region with the matrix). The measured PL peak, though wide at its half-width, when analysed in consecutive short spectral segments showed the same decay dynamics, suggesting the same mechanism of relaxation.

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 (θC) of 0.25 and 0.5ML by lowering SPE temperature (TS) to 450°C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75ML even at 450°C. From the investigation of SPE temperature effect on Ge QD formation for θC of 0.75ML, SK-mode Ge QDs of about 10nm in diameter and of about 4.5×1011cm−2 in density were formed at TS≥400°C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at TS≥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.

Single-Mode Emission From Ge Quantum Dots in Photonic Crystal Nanobeam Cavity

A freestanding one-dimensional photonic crystal nanobeam cavity embedded with Ge self-assembled quantum dots is designed and fabricated on a silicon-on-insulator substrate. Only one photoluminescence peak is observed in the wavelength range of 1000 to 1600 nm at room temperature. The emission peak dominates the photoluminescence spectrum over an almost flat and weak background emission, which indicates single-mode emission is realized in the proposed light emitter.

Effects of annealing temperature on shape transformation and optical properties of germanium quantum dots**Project supported by Ibnu Sina Institute for Fundamental Science Study, Universiti Teknologi Malaysia through Vote Q.J130000.2526.02H94, O5 and Postdoctoral Research Grant.

The influences of thermal annealing on the structural and optical features of radio frequency (rf) magnetron sputtered self-assembled Ge quantum dots (QDs) on Si (100) are investigated. Preferentially oriented structures of Ge along the (220) and (111) directions together with peak shift and reduced strain (4.9% to 2.7%) due to post-annealing at 650 °C are discerned from x-ray differaction (XRD) measurement. Atomic force microscopy (AFM) images for both pre-annealed and post-annealed (650 °C) samples reveal pyramidal-shaped QDs (density ∼ 0.26× 1011 cm−2) and dome-shape morphologies with relatively high density ∼ 0.92 × 1011 cm−2, respectively. This shape transformation is attributed to the mechanism of inter-diffusion of Si in Ge interfacial intermixing and strain non-uniformity. The annealing temperature assisted QDs structural evolution is explained using the theory of nucleation and growth kinetics where free energy minimization plays a pivotal role. The observed red-shift ∼ 0.05 eV in addition to the narrowing of the photoluminescence peaks results from thermal annealing, and is related to the effect of quantum confinement. Furthermore, the appearance of a blue-violet emission peak is ascribed to the recombination of the localized electrons in the Ge-QDs/SiO2 or GeOx and holes in the ground state of Ge dots. Raman spectra of both samples exhibit an intense Ge–Ge optical phonon mode which shifts towards higher frequency compared with those of the bulk counterpart. An experimental Raman profile is fitted to the models of phonon confinement and size distribution combined with phonon confinement to estimate the mean dot sizes. A correlation between thermal annealing and modifications of the structural and optical behavior of Ge QDs is established. Tunable growth of Ge QDs with superior properties suitable for optoelectronic applications is demonstrated.

C–V AND AS STUDY OF SELF-ASSEMBLED GE ISLANDS IN SI P-N JUNCTION

The electrical properties of self-assembled Ge quantum dots (QDs) and Ge quantum wells (QWs) embedded in Si p-i-n diodes were studied using capacitance-voltage(C-V) measurements and admittance spectroscopy(AS). The investigated samples were grown on Si(001) substrates by an ultra high vacuum chemical vapour deposition (UHV-CVD) system. A linear increase of the thermal activation energy observed in voltage-dependent admittance spectroscopy(from 250meV up to 380meV) for the sample with quantum dots shows that the ensemble of Ge islands has a low, continuous averaged density of states. For the sample without Boron pretreatment we found two levels with activation energies about 132meV and 202meV which were about constant in some reverse bias region.

Impact of annealing on surface morphology and photoluminescence of self-assembled Ge and Si quantum dots.

Controlled growth and characterization of germanium (Ge) and silicon (Si) nanostructure are the key issues for optoelectronic device fabrication. The role of post-annealing on the structural and optical properties of radio frequency (rf) magnetron sputtering grown of Ge and Si quantum dots (QDs) deposited on Si(100) substrate is studied. Atomic force microscopy confirmed the formation of Si and Ge QDs with estimated sizes lower than -17 nm and -14 nm respectively. The X-ray diffraction analysis revealed the formation of Si and Ge QDs accompanied by SiO2 with estimated sizes of -5 and -7 nm for post-annealed Si and pre-annealed Ge QDs respectively. The room temperature photoluminescence spectra for Ge and Si demonstrated an emission peak at 3.20 and 2.72 eV respectively, which are attributed to the electron and hole recombination within QDs. A shift in the PL peak is observed through annealing which is ascribable to the changes in size of QDs and quantum confinement effect. The thermal annealing at 600 degrees C is found to play an important role in controlling the shape, number density, root mean square (rms) roughness and the energy shift of the luminescence band for both Si and Ge QDs. The influence of annealing on growth morphology for Ge QDs is appeared to be stronger than Si.

(Invited) Narrow Linewidth, Highly Efficient, and Integrated Light Emitting Diodes Based on Ge Quantum Dots in Optical Microcavities

Ge self-assembled quantum dots (QDs) grown on Si are promising solution for Si-based light sources due to their emission wavelength in the telecommunication band, capability of electrical injection and CMOS process compatibility. However, as indirect band gap material, their light emission efficiency is still rather low. By embedding Ge QDs into optical microcavities with large Q-factor and small mode volume, the spontaneous emission rate can be enhanced due to the Purcell effect and the light extraction efficiency can also be improved due to the modified cavity mode fields. We have demonstrated strong and sharp resonant photoluminescence peaks from Ge QDs in photonic crystal (PhC) nanocavities and microdisk resonators [1]. For practical application, light emission by electrical injection is desirable. In this abstract, we will review some of our results on electrical injected light emitting diodes (LEDs) based on Ge QDs in microcavities. Electrical injected LEDs were firstly demonstrated by integrating vertical PIN diodes with H3-type PhC cavities [2]. Room-temperature electroluminescence (EL) was successfully observed, but with relatively broad emission peaks. This is due to the large cavity loss induced by the electrical structure. By using lateral PIN diode structure, in which the P+ and N+ regions are located laterally on the PhC slab, while the cavity region is left intrinsic, we achieved intense EL peaks around 1.3 µm with Q-factor of about 260, corresponding to a linewidth of 5 nm [3]. After optimization of PhC nanocavities and electrical structures, the Q-factor of emission peaks increased to 800-1500. The required current for occurrence of resonant peaks was reduced by one order, to 50 µA [4]. The schematic diagram and EL spectra of the device are shown in Fig. 1(a) and 1(b), respectively. Moreover, at injected current of 3 mA, we obtained a measurable output power on the pW order. We also confirmed that the output power was limited by the fabrication process and could be increased further. In order to realize monolithic integration with other photonic devices, waveguide-coupled LEDs are necessary. Waveguide-coupled microdisks containing Ge QDs were fabricated with vertical PIN diodes structures [5], as shown in Fig. 1(c). When there was current injection, light emission from the microdisk was coupled into the waveguide. Sharp resonant peaks around 1.55 µm with Q-factor over 5000 were obtained in the EL spectra collected through the waveguide, as shown in Fig. 1(d). The coupling between the waveguide and microdisk was confirmed by comparing the EL and optical transmission spectra along the waveguide. The LEDs thus can be easily integrated with other waveguide devices. Interestingly, the devices also can be operated as resonant-cavity enhanced photodetectors under reverse bias. Wavelength selective photo-response, with high responsivity and ultra-low dark current around 1.55 µm was also realized [6]. The light emission efficiency of Ge QDs themselves is fundamental to obtain higher intensity. Due to the type-II band lineup of Si/Ge heterostructure, only holes can be well confined in Ge QDs, and there is very weak confinement for electrons. By using n-type doping, extrinsic electrons can be provided and the radiative recombination is expected to be enhanced. In the talk, we will also present some results about light emission enhancement by phosphorus delta-doping between Ge QDs and Si spacers.