Direct Band Gap Of Ge Research Articles

Light emission from direct band gap germanium containing split-interstitial defects

The lack of useful and cost-efficient group-IV direct band gap light emitters still presents the main bottleneck for complementary metal-oxide semiconductor-compatible short-distance data transmission, single-photon emission, and sensing based on silicon photonics. Germanium, a group-IV element like Si, is already widely used in silicon fabs. While the energy band gap of Ge is intrinsically indirect, we predict that the insertion of Ge-Ge split-[110] interstitials into crystalline Ge can open up a direct band gap transmission path. Here, we calculate from first principles the band structure and optical emission properties of Ge, Sb, and Sn split-[110] interstitials in bulk and low-dimensional Ge at different doping concentrations. Two types of electronic states provide the light-emission enhancement below the direct band gap of Ge: a hybridized L-$\mathrm{\ensuremath{\Gamma}}$ state at the Brillouin zone center and a conduction band of $\mathrm{\ensuremath{\Delta}}$ band character that couples to a raised valence band along the $\mathrm{\ensuremath{\Gamma}}$-X direction. Majority carrier introduced to the system through doping can enhance light emission by saturation of nonradiative paths. Ge-Sn split interstitials in Ge shift the top of the valence band towards the $\mathrm{\ensuremath{\Gamma}}$-X direction and increase the $\mathrm{\ensuremath{\Gamma}}$ character of the L-$\mathrm{\ensuremath{\Gamma}}$ state, which results in a shift to longer emission wavelengths. Key spectral regions for datacom and sensing applications can be covered by applying quantum confinement in defect-enhanced Ge quantum dots for an emission wavelength shift from the midinfrared to the telecom regime.

(Invited) Theory of Silicon-Based Light Emitting

The indirect band gap preclude the silicon from making light-emitting devices, which is, however, a quest for exploration of large-scale integration of both electronic and photonic devices on a single microchip. Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory, based on occupied d orbitals in cations, for explaining why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. This theory also unravels that it is impossible to make allotropes of Si and compounds consisting by Si mixed together with other elements high efficient direct bandgap light emitting. Interface and quantum confinement in low-dimensional Si nanostructures are two knobs to relax the momentum conservation law of optical transition. Our theory confirms that the light emission efficiency of Si quantum dots increases exponentially as reducing dot size, but its highest reachable efficiently is still two orders of magnitude smaller than that of direct bandgap InP and GaAs. Fortunately, we discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. Furthermore, regarding the direct bandgap is only 140 meV above the indirect bandgap in Germanium (Ge), it is well established that a tiny (1-2%) extensile strain could render an indirect-to-direct bandgap transition and makes it direct bandgap. However, there is no feasible way to create such extensive strain on Ge since the lattice parameter of Ge is even 4.3% larger than Si. We propose insert external atoms into interstitial sites of Ge, which grows on Si substrate through SiGe buffer layer, to induce an effective tensile uniaxial strain on Ge, a new pathway toward direct band gap Ge. Because our proposed scheme is highly compatible with CMOS fabrication process, our presented new pathway would be a solution for on-chip lasers.

(Invited) Theory of Silicon-Based Light Emitting

Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory to explain why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. We excluded Si QDs as a candidate for on-chip lasers. We also discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. We further propose to insert external atoms into interstitial sites of Ge to induce an effective tensile uniaxial strain on Ge toward direct band gap Ge. This scheme is highly compatible with CMOS fabrication process.

Finite Element Stress Model of Direct Band Gap Ge Implementation Method Compatible with Si Process

As an indirect band gap semiconductor, germanium (Ge) can be transformed into a direct band gap semiconductor through some specific modified methods, stress, and alloying effect. Direct band gap-modified Ge semiconductors with a high carrier mobility and radiation recombination efficiency can be applied to optoelectronic devices, which can improve the luminous efficiency dramatically, and they also have the potential application advantages in realizing monolithic optoelectronic integration (MOEI) and become a research hotspot in material fields. Among the various implementations of Ge band gap-type conversion, the related methods that are compatible with the Si process are most promising. It is such a method to etch around the Ge epitaxial layer on the Si substrate and introduce the biaxial tensile stress by SiGe selective filling. However, the influence of the width of the epitaxial layer, Ge composition, and Ge mesa region width on strain distribution and intensity is not clear yet. Accordingly, a finite element stress model of the selective epitaxy-induced direct band gap Ge scheme is established to obtain the material physical and geometric parameters of the Si1−xGex growth region. The result of finite element simulation indicates when the Si1−xGex epitaxial layer is 150–250 nm wide and the Ge composition is 0.3∼0.5, Ge mesa with 20–40 nm in width can be transformed into direct band gap semiconductors in the depth of 0–6 nm. The theoretical results can provide an important theoretical basis for the realization of subsequent related processes.

Strain-Induced Enhancement of Electroluminescence from Highly Strained Germanium Light-Emitting Diodes

The full exploration of Si-based photonic integrated circuits is limited by the lack of an efficient light source that is compatible with the complementary metal–oxide–semiconductor process. Highly strained germanium (Ge) is a promising solution, as its band structure can be fundamentally altered by introducing tensile strain. However, the main challenge lies in the incorporation of an electrical structure while maintaining high strain with uniform distribution in the active region. Here we present highly strained Ge LEDs driven by lateral p–i–n junctions and report the strain-induced enhancement of electroluminescence (EL) from Ge. Raman characterization shows that 1.76% strain along the ⟨100⟩ direction with relatively uniform strain distribution is achieved. The observed strain-induced red-shifts of EL spectra agree well with the theoretical prediction, revealing that the direct band gap of Ge can be tuned in the range of 0.785 eV (1580 nm) to 0.658 eV (1885 nm). This work offers a pathway toward a stra...

Silicon-Germanium Stressors for Germanium Photonic Devices on Silicon

Ge epitaxial layers on Si have been widely studied in Si photonics for active photonic devices such as photodetectors operating in the communication wavelengths (1.3–1.6 µm). The control in the direct bandgap of Ge or the fundamental optical absorption edge is important for the wavelengths of device operation [1]. One of the promising approaches is to apply a stress to Ge, and SiNx has been examined as an external stressor [2,3]. In order to change the absorption edge by several 10 nm, a uniaxial stress applied to Ge is required more than 0.5 GPa [2]. This indicates that the built-in stress in the SiNx stressor should be even larger (> 1 GPa). However, such a highly stressed SiNx is not easily deposited. In this work, an alternative stressor of strained SiGe epilayer is investigated. An advantage lies in the amount of stress, which is controlled by the alloy composition; e.g., a tensile stress in Si0.2Ge0.8 on Ge is as large as 1.4 GPa. Figures 1(a) and 1(b) schematically show the concepts to apply compressive and tensile stresses, respectively, to Ge mesa structures on Si. As in Fig. 1(a), a stressor of SiGe epitaxial overlayer on Ge possesses a built-in tensile stress due to the lattice mismatch, as long as the SiGe layer is pseudomorphically grown. For narrow mesa structures, the tensile stress in SiGe is partially relaxed by an elastic deformation, inducing a compressive stress in the underlying Ge mesa. This results in an increase of direct bandgap, i.e., a blue shift in the absorption edge. With increasing the thickness of SiGe as well as the Si composition, a larger stress is induced in Ge, although the critical layer thickness of SiGe should be taken into account to prevent the strain relaxation due to the generation of dislocations. In turn, a reduction in the direct bandgap, i.e., a red shift in the absorption edge is induced when a compressive SiGe overlayer is formed on Ge. As in Fig. 1(b), such a compressive SiGe is obtained when a relaxed Si layer is inserted between the SiGe overlayer and the Ge mesa structure. The Si layer can be as thin as 10 nm because of the critical thickness on Ge as small as a few nm. In order to experimentally examine the applicability of this concept, two types of epitaxial SiGe overlayers were prepared on trapezoidal Ge mesa structures running in the [110] direction, which were selectively grown on Si (001) wafers with SiO2 masks by ultrahigh-vacuum chemical vapor deposition. The Ge mesa was composed of a (001) top surface and (113) facet sidewalls. The height/thickness was 500 nm, and the base width was typically 5 µm. For the built-in tensile stress in Fig. 1(a), a 40-nm-thick Si0.2Ge0.8 layer was grown on the Ge mesa. On the other hand, for the built-in compressive stress in Fig. 1(b), a 10-nm-thick relaxed Si layer was grown on the Ge mesa, followed by a growth of 40-nm-thick Si0.8Ge0.2. X-ray diffraction measurements revealed that strained SiGe overlayers were successfully formed in both cases. As a reference, a Ge mesa was prepared with a 20-nm-thick relaxed Si overlayer. Photoluminescence (PL) spectra were measured in order to examine the change in the bandgap. Typical results are shown in Fig. 1(c). The PL peak positions were successfully shifted, depending on the stress in SiGe; a blue shift was observed for the tensile SiGe overlayer, while a red shift was observed for the compressive SiGe overlayer. Although the shift was as small as 10 nm in the present study, further shift would be realized by optimizing the thickness and the alloy composition of SiGe. [1] Y. Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003). [2] R. Kuroyanagi et al., Opt. Express 21, 18553 (2013). [3] G. Capellini et al., J. Appl. Phys. 113, 013513 (2013). Figure 1

(Invited) Strain-Engineered Low-Threshold Group IV Lasers for Photonic-Integrated Circuits

Photonic-integrated circuits (PICs) are a key enabler for the ultimate miniaturization of next-generation optical computers that can fit in the palm of your hand. However, the realization of fully functional PICs is currently limited by the absence of an efficient light source on silicon (Si), thus making a practical Si-compatible light source the Holy Grail. Especially, it has been thus far considered close to impossible to create a practical on-chip laser using Si-compatible group IV semiconductor materials (e.g. germanium (Ge)) owing to their indirect bandgap nature. Strain-engineered Ge material has recently garnered much attention for the dramatic reduction of the lasing threshold of group IV lasers because large tensile strain fundamentally alters the Ge’s bandstructure. Here, we present our recent research works on highly strained group IV materials on a Si chip for the realization of low-threshold on-chip lasers. In the first part, we will introduce our innovative strain engineering platforms that can create direct bandgap Ge nanowires by inducing >5% elastic tensile strain. In the second part, we will present the first experimental observation of low-threshold optically pumped lasing in highly strained Ge nanowires. The strain engineering of direct bandgap germanium-tin will also be described for the ultimate reduction of the lasing threshold.

Optical Properties of Tensilely Strained Ge Nanomembranes.

Group-IV semiconductors, which provide the leading materials platform of micro- electronics, are generally unsuitable for light emitting device applications because of their indirect- bandgap nature. This property currently limits the large-scale integration of electronic and photonic functionalities on Si chips. The introduction of tensile strain in Ge, which has the effect of lowering the direct conduction-band minimum relative to the indirect valleys, is a promising approach to address this challenge. Here we review recent work focused on the basic science and technology of mechanically stressed Ge nanomembranes, i.e., single-crystal sheets with thicknesses of a few tens of nanometers, which can sustain particularly large strain levels before the onset of plastic deformation. These nanomaterials have been employed to demonstrate large strain-enhanced photoluminescence, population inversion under optical pumping, and the formation of direct-bandgap Ge. Furthermore, Si-based photonic-crystal cavities have been developed that can be combined with these Ge nanomembranes without limiting their mechanical flexibility. These results highlight the potential of strained Ge as a CMOS-compatible laser material, and more in general the promise of nanomembrane strain engineering for novel device technologies.

Room temperature lasing unraveled by a strong resonance between gain and parasitic absorption in uniaxially strained germanium

A complementary metal-oxide semiconductor compatible on-chip light source is the holy grail of silicon photonics and has the potential to alleviate the key scaling issues arising due to electrical interconnects. Despite several theoretical predictions, a sustainable, room temperature laser from a group-IV material is yet to be demonstrated. In this work, we show that a particular loss mechanism, inter-valence-band absorption (IVBA), has been inadequately modeled until now and capturing its effect accurately as a function of strain is crucial to understanding light emission processes from uniaxially strained germanium (Ge). We present a detailed model of light emission in Ge that accurately models IVBA in the presence of strain and other factors such as polarization, doping, and carrier injection, thereby revising the road map toward a room temperature Ge laser. Strikingly, a special resonance between gain and loss mechanisms at 4%--5% $\ensuremath{\langle}100\ensuremath{\rangle}$ uniaxial strain is found resulting in a high net gain of more than $400\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ at room temperature. It is shown that achieving this resonance should be the goal of experimental work rather than pursuing a direct band gap Ge.

Band structure model of modified Ge for optical device application

Ge is an indirect bandgap semiconductor, which can be converted into a direct bandgap semiconductor by using the modification techniques. The carrier radiation recombination efficiency of modified Ge is high, which can be used in optical devices. The mobility of Ge semiconductor carriers is higher than that of Si semiconductor carriers, so Ge device can work fast and have good frequency characteristics in electronic device. In view of the application advantages of modified Ge semiconductors in both optical devices and electrical devices, it has been a potential material of monolithic optoelectronic integration. The Ge and GeSn as optoelectronic device materials have a great competitive advantage, but there is no mature Ge-based monolithic photoelectric integration. In order to realize Ge-based optical interconnection, the bandgap of luminous tube, detector and waveguide active layer material must satisfy the following sequence:Eg,waveguide Eg,luminoustube Eg,detector. Therefore, in order to achieve the same layer monolithic photoelectric integration, we must modulate the energy band structure of the active layer material of the device. Unfortunately, the literature in this area is lacking. The band structure is one of the theoretical foundations for the monolithic photoelectric integration of the modified Ge materials, but the work in this area is still inadequate. In this paper, this problem is investigated from three aspects. 1) Based on the generalized Hooke's law and the principle of deformation potential, a modified Ge bandgap type transformation model is established under different modification conditions, perfecting the theory of converting the indirect switching into direct band gap of Ge. 2) On the basis of establishing the strain tensor and deformation potential model, a modified Ge band E-k model is established, and the relevant conclusions can provide key parameters for LED and laser device simulation models. 3) Based on the theory of solid energy band, the bandgap width modulation scheme of the modified Ge under the uniaxial stress is proposed, which provides an important theoretical reference for realizing the Ge-based single-layer photoelectric integration. The results in this paper can provide an important theoretical basis for understanding the material physics of the modified Ge and designing the active layers of the light emitting devices in the Ge based optical interconnection.

Highly n-doped germanium-on-insulator microdisks with circular Bragg gratings.

We demonstrate germanium (Ge) microdisks surrounded by highly reflective circular Bragg gratings on highly n-doped germanium-on-insulator (GOI) substrate. The GOI substrate is fabricated by wafer bonding from Ge grown on Si substrate, and n-type doping concentration of 2.1×1019 cm-3 is achieved by phosphorus diffusion from a spin-on-dopant source. Very sharp Fabry-Perot resonant peaks with high contrast fringes and Q-factors up to 400 are observed near the direct band gap of Ge in photoluminescence spectra. The reflectivity of gratings are enhanced by a factor larger than 3 in a wide wavelength range from 1.57 to 1.82 µm, compared with that of Ge/SiO2 interfaces in normal microdisks without circular Bragg gratings. The surface emission intensity of the devices is found to be increased by the grating period. Our results indicate that GOI microdisk with circular Bragg grating is a promising optical resonator structure suitable for realizing low threshold, compact Ge lasers integrated on Si substrate.

Design of Dipole-Allowed Direct Band Gaps in Ge/Sn Core–Shell Nanowires

Owing to the indirect band gap nature, Ge exhibits poor optical properties, limiting its usage for optical devices. However, since the direct band gap of Ge is only higher by 0.14 eV than the indirect band gap, band gap engineering has drawn much attention to realize the direct band gap. Here, we report a strategy to design the direct band gap in Ge/Sn core–shell nanowires (NWs), based on first-principles calculations. For [111]-oriented NWs, we show that the direct band gaps can be tuned by controlling the diameter and the core-to-shell ratio. We find that the intrinsic strain induced by the lattice mismatch between Ge and Sn drives an indirect-to-direct band gap transition. Even for Ge/Sn core–shell NWs with intrinsically indirect band gaps, the direct band gaps can be achieved by applying an external tensile strain lower than the critical values for pure Ge NWs and bulk Ge. The optical transitions of the direct band gaps are all dipole-allowed, suggesting that [111]-oriented Ge/Sn core–shell NWs are pr...

(Invited) Electrical Properties of Group 4 Metal-Nitride/Ge Contacts and the Application to Ge Optoelectronic Devices

1) Introduction Ge is of great interest as a candidate for the channel material in CMOS devices. In order to integrate Ge-CMOS with current Si platforms, metal source/drain (S/D) MOSFETs are promising candidate because of simple structures and low-temperature processing. To realize high-performance, metal/Ge contacts with low electron barrier height (ΦBN) and low hole barrier height (ΦB P) are needed for n- and p-MOSFETs, respectively. However, one difficulty is the Fermi-level pinning (FLP) where the Fermi level is pinned near the valence band edge at the metal/Ge interface. Thus, the formation of a p-MOSFET is straight forward, but an n-MOSFET is difficult. We found that TiN/Ge contacts can alleviate the FLP position toward the conduction band edge [1-3]. By using this contact technique, optical devices with function of light emission and detection are possible [4], which are useful for optical communication. This is because the direct band gap of Ge is 0.80 eV, corresponding to a wavelength of 1.55 mm. In this presentation, from investigations on electrical properties and interfacial structures of group-4 metal-nitride/Ge contacts such as TiN/Ge, ZrN/Ge, and HfN/Ge, we show that an amorphous interlayer (a-IL) with N plays an important role in the FLP alleviation. We also present direct-bandgap room temperature electroluminescence as well as light detection using bulk-Ge diodes with a fin type lateral PtGe/Ge/TiN structure. 2) Electrical and structural properties of group-4 metal-nitride/Ge contacts Metal nitrides were directly deposited on (100) Ge by rf magnetron sputtering using a nitride target. Then, a 50-nm-thick Al film was deposited by evaporation, and the contact pattern was formed by liftoff technique. Finally, PMA was carried out at temperatures below 600°C. HAADF-STEM was used to clarify the interfacial structures. A TiN/Ge contact with 350°C-PMA showeda high ΦBP of 0.53 eV. In the HAADF-STEM images for samples with no-PMA and 350°C-PMA, an IL was present at the TiN/Ge interface. The IL thicknesses were 1.3 and 1.9 nm for the no-PMA and 350°C-PMA samples, respectively. The IL structure was determined to be amorphous (a). EDX for the no-PMA and 350°C-PMA samples revealed that the ILs consisted of Ge (90 at%)/Ti (10 at%) and Ge (76 at%)/Ti (24 at%), respectively. The N atoms in the a-IL could be also detected by the EDX. The a-IL changed to a crystalline phase after 600°C-PMA, where the TiN was in direct contact with the Ge crystal, resulting in pinned contact property. An a-IL with a thickness of 1.9 nm exists at an interface in a ZrN/Ge contact with 450°C-PMA, leading to strong FLP alleviation (ΦBP=0.56 eV). An a-IL in a HfN/Ge contact is thin (0.9 nm) even for 350°C-PMA samples, leading to small FLP alleviation (ΦBP=0.39 V). Thus, it was found that FLP alleviation for metal nitride/Ge contacts is strongly associated with existence of an a-IL with N. The extrinsic states associated with bonding defects at an a-IL/Ge interface cause the creation of an interfacial dipole. Considering the upward shift of the FLP position, the dipole should be positively charged at the Ge side of the interface. The dipole is likely caused by N-related defects near the a-IL/Ge interface. 3 ) Light e mi ssion and detection Ge devices We fabricated fin type lateral PtGe/Ge/TiN device using above contact techniques. The contacts were passivated by SiO2/GeO2 bilayer. The light emission with a wavelength of 1.55 mm could be observed when forward currents were flowed in the diode. The wavelength corresponded to direct energy gap of Ge (0.80 eV). This is because electron and hole were effectively injected from both contacts. At the wavelength of 1.55 μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44 A/W at a reverse bias voltage of -1 V. Acknowledgement This work was supported by (JSPS) KAKENHI (grant No. 25249035 & 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

Direct bandgap germanium-on-silicon inferred from 57% 〈100〉 uniaxial tensile strain [Invited

We report uniaxial tensile strains up to 5.7% along 〈100〉 in suspended germanium (Ge) wires on a silicon substrate, measured using Raman spectroscopy. This strain is sufficient to make Ge a direct bandgap semiconductor. Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states; however, recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized. We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers. These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.

Material Characterization of Ge1−x Sn x Alloys Grown by a Commercial CVD System for Optoelectronic Device Applications

High-quality compressive-strained Ge1−xSnx/Ge films have been deposited on Si(001) substrate using a mainstream commercial chemical vapor deposition reactor. The growth temperature was kept below 450°C to be compatible with Si complementary metal–oxide–semiconductor processes. Germanium tin (Ge1−xSnx) layers were grown with different Sn composition ranging from 0.9% to 7%. Material characterizations, such as secondary-ion mass spectrometry, Rutherford backscattering spectrometry, and x-ray diffraction analysis, show stable Sn incorporation in the Ge lattice. Comparison of the Sn mole fractions obtained using these methods shows that the bowing factor of 0.166 nm (in Vegard’s law) is in close agreement with other experimental data. High-resolution transmission electron microscopy and atomic force microscopy results show that the films have started to relax through the formation of misfit and threading dislocations. Raman spectroscopy, ellipsometry, and photoluminescence (PL) techniques are used to study the structural and optical properties of the films. Room-temperature PL of the films shows that 7% Sn incorporation in the Ge lattice results in a decrease in the direct bandgap of Ge from 0.8 eV to 0.56 eV.

Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Highly tensile-strained layers of Ge were grown via molecular beam epitaxy using step-graded InxGa1−xAs buffer layers on (100) GaAs. These layers have biaxial tensile-strain of up to 2.33%, have surface roughness of <1.1 nm, and are of high quality as seen with transmission electron microscopy. Low-temperature photoluminescence (PL) suggests the existence of direct-bandgap Ge when the strain is greater than 1.7%, and we see a greater than 100× increase in the PL intensity of the direct transition with 2.33% tensile-strain over the unstrained case. These results show promise for the use of tensile-strained Ge in optoelectronics monolithically integrated on Si.

Extended performance GeSn/Si(100) p-i-n photodetectors for full spectral range telecommunication applications

First-generation n-i-GeSn/p-Si(100) photodiode detectors with Ge0.98Sn0.02 active layers were fabricated under complementary metal oxide semiconductor compatible conditions. It is found that, even at this low Sn concentration, the detector quantum efficiencies are higher than those in comparable pure-Ge device designs processed at low temperature. Most significantly, the spectral range of the GeSn device responsivity is dramatically increased—to at least 1750 nm—well beyond the direct band gap of Ge (1550 nm). This allows coverage of all telecommunication bands using entirely group IV materials.

First-Principles Analysis of Indirect-to-Direct Band Gap Transition of Ge under Tensile Strain

Ge is a promising material not only for its high carrier mobility but also for its unique optical property. The optical transition properties of Ge under six types of tensile strain, which was hardly investigated using a conventional semi-empirical method, have been analyzed using first-principles calculation. It has been found that the direct band gap of Ge shrinks faster than the indirect band gap under five types of tensile strain. In particular, Ge becomes a direct-transition semiconductor under 2% (001) in-plane biaxial tensile strain. Therefore, the use of Ge and strain technology is an excellent approach to the construction of Si-compatible photonic devices and to the monolithic integration of such devices with Si electronics. Furthermore, it has been predicted that direct-transition tensile-strained Ge can be constructed using only Si and Ge by a finite-element method.

High performance, waveguide integrated Ge photodetectors

Photonic systems based on complementary metal oxide semiconductor (CMOS) technology require the integration of passive and active photonic devices. The integration of waveguides and photodetector is one of the most important technologies. We report a Ge p-i-n photodetector that is monolithically integrated with silicon oxynitride and silicon nitride waveguides. All processes and materials are CMOS compatible and can be implemented in the current integrated circuit process technology. The small size of the devices results in low absolute dark current. The waveguidecoupled Ge devices show high efficiency (~90%) over a wide range of wavelengths well beyond the direct band gap of Ge, resulting in a responsivity of 1.08 A/W for 1550 nm light. The device speed of 7.2 GHz at 1V reverse bias is strongly affected by the capacitance of the probe pads. The high-performance of the devices at low voltage ( </= 1V) facilitates the integration with CMOS circuits.

Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si

We analyze the optical gain of tensile-strained, n-type Ge material for Si-compatible laser applications. The band structure of unstrained Ge exhibits indirect conduction band valleys (L) lower than the direct valley (Gamma) by 136 meV. Adequate strain and n-type doping engineering can effectively provide population inversion in the direct bandgap of Ge. The tensile strain decreases the difference between the L valleys and the Gamma valley, while the extrinsic electrons from n-type doping fill the L valleys to the level of the Gamma valley to compensate for the remaining energy difference. Our modeling shows that with a combination of 0.25% tensile strain and an extrinsic electron density of 7.6x10(19)/cm(3) by n-type doping, a net material gain of ~400 cm(-1) can be obtained from the direct gap transition of Ge despite of the free carrier absorption loss. The threshold current density for lasing is estimated to be ~6kA cm(-2) for a typical edgeemitting double heterojunction structure. These results indicate that tensile strained n-type Ge is a good candidate for Si integrated lasers.