Tensile-strained Germanium Research Articles

Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (ε-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1–xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 ± 2 ns and low surface recombination velocity of 21.3 ± 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1–xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the ε-Ge epilayer and InxGa1–xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.

Multivalley Electron Conduction at the Indirect-Direct Crossover Point in Highly Tensile-Strained Germanium

For applications such as low-power tunneling transistors and memory, lasers, qubits, and spintronics, there is interest in using strain to enhance carrier mobility in semiconductors. This experimental and theoretical study investigates transport in highly strained germanium, in which multivalley electron conduction emerges. Surprisingly, all conduction occurs via the L valley in Ge for biaxial tensile strain below 1.6%, while above the contribution from the \ensuremath{\Gamma} valley increases, with negligible contributions from the underlying strain template.

Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure

Strain and band gap engineered epitaxial germanium (ε-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of an InGaAs layer to simultaneously provide tensile strain in Ge (0.7–1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth), and loss mechanisms that dominate in the ε-Ge QW laser structure were calculated using first-principles-based 30-band k·p electronic structure theory, at injected carrier concentrations from 3 × 1018 to 9 × 1019 cm–3. The higher strain in the ε-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a Jth of 300 A/cm2 can be reduced to <10 A/cm2 by increasing strain from 0.2% to 1.96% in ε-Ge lasing media. The measured room-temperature photoluminescence spectroscopy demonstrated direct band gap optical emission, from the conduction band at the Γ-valley to heavy-hole (0.6609 eV) from 1.6% tensile-strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k·p theory. The detailed plan-view transmission electron microscopic (TEM) analysis of 0.7% and 1.2% tensile-strained ε-Ge/InGaAs structures exhibited well-controlled dislocations within each ε-Ge layer. The measured dislocation density is below 4 × 106 cm–2 for the 1.2% ε-Ge layer, which is an upper bound, suggesting the superior ε-Ge material quality. Structural analysis of the experimentally realistic 1.95% biaxially strained In0.28Ga0.72As/13 nm ε-Ge/In0.28Ga0.72As QW structure demonstrated a strained Ge/In0.28Ga0.72As heterointerface with minimal relaxation using X-ray and cross-sectional TEM analysis. Therefore, our monolithic integration of a strained Ge QW laser structure on GaAs and ultimately the transfer of the process to the Si substrate via an InGa(Al)As/III–V buffer architecture would provide a significant step toward photonic technology based on strained Ge on a Si platform.

Self-organized 〈1 0 0〉 direction growth of germanium film on insulator obtained by high speed continuous wave laser annealing

To realize next-generation electronics, germanium crystal growth on insulating substrates is attracting attention due to its high carrier mobility, and further improvement of carrier mobility is anticipated by applying tensile strain to germanium. We have previously realized highly tensile-strained germanium films on insulating substrates using a high speed continuous wave laser annealing process. This letter reports how we characterized the grown films using electron diffraction backscatter measurements and discovered an interesting self-organized 〈100〉 direction type of growth that results from scanning the annealing laser at high speed. Dominant {110} oriented films were obtained in this way. This phenomenon is discussed below from the viewpoint of surface energy.

Germanium microlasers on metallic pedestals

Strain engineering is a powerful approach in micro- and optoelectronics to enhance carrier mobility, tune the bandgap of heterostructures, or break lattice symmetry for nonlinear optics. The dielectric stressors and bonding interfaces used for strain engineering in photonics can however limit thermal dissipation and the maximum operation temperature of devices. We demonstrate a new approach for enhanced thermal dissipation with stressor layers by combining metals and dielectrics. The method is applied to the germanium semiconductor. All-around tensile-strained germanium microdisks have been fabricated with metallic pedestals. The transferred tensile strain leads to a germanium thin film with a direct bandgap. Under continuous wave optical pumping, the emission of the whispering gallery modes is characterized by a threshold and an abrupt linewidth narrowing by a factor larger than 2. The occurrence of stimulated emission is corroborated by modeling of the optical gain. This demonstrates lasing with pure germanium microdisks.

Resonant light emission from uniaxially tensile-strained Ge microbridges

A highly strained germanium microbridge is a promising platform for realizing monolithically integrated lasers on a silicon substrate. However, it remains challenging to combine it with optical resonators. Here, we have observed resonant light emission peaks with Q-factors of about 180 in room-temperature photoluminescence spectra from uniaxially tensile-strained germanium microbridges. These peaks are found to correspond to the resonance in Fabry–Perot (FP) cavities formed transversely to the uniaxial stress axis. On the basis of this phenomenon, we design a Fabry–Perot cavity by adding distributed Bragg reflectors (DBRs) laterally to the microbridge. With this design, the optical performance can be optimized without disturbing to the mechanical structure. A Q-factor as high as 1400 is obtained from numerical simulation. Moreover, we prove by theoretical analysis deduction and calculation that the lateral structure will not decrease the strain, unlike the on-pad DBR structure. The structure thus provides a promising solution for the realization of highly strained germanium lasers in the future.

Splitting of frequencies of optical phonons in tensile-strained germanium layers

Tensile-strained germanium films in Ge/GeSn/Si/GeSnSi multilayer heterostructures grown by molecularbeam epitaxy on Si(001) substrates are investigated by Raman spectroscopy. Biaxial tensile strains in the films reach 1.5%, which exceeds values previously obtained for this system. Splitting of frequencies of long-wavelength optical phonons is experimentally observed; i.e., the shift of the frequency of the singlet induced by biaxial tensile strains is larger than the shift of the frequency of the doublet in agreement with calculations. The strain-induced shift of Raman scattering peaks from two-phonon scattering in germanium is also detected.

(Invited) Direct Band Gap Germanium

Germanium is an indirect band gap material. This property is a significant disadvantage to realize an efficient laser emitter with this material. This feature can nonetheless be circumvented by strain engineering of the band structure. It is well known that tensile strain can reduce the energy difference between the conduction band valleys and ultimately lead to a band inversion. With a direct band gap material, one should expect to obtain optical gain with a significantly reduced threshold as compared to unstrained or weakly-strained Ge. Consequently, the integration of a group IV emitter on a silicon chip could become more realistic. In this presentation, we will show that direct band gap germanium can be obtained by transferring tensile strain with nitride stressor layers. We have experimentally determined when the cross-over from indirect to direct band gap occurs using temperature-dependent photoluminescence measurements. This cross-over was found to occur for a biaxial tensile strain of 1.67%, the theoretical values varying between 1.5 and 2% in the literature. We will show that this level of strain transfer is compatible with the fabrication of optical micro-resonators with germanium microdisks. Tensile-strained germanium microdisks with circular Bragg reflectors have been successfully fabricated. They exhibit high quality factors for the quasi-radial modes. These types of structures are ideal candidates to realize microlasers with pure germanium. We will discuss the latest progress in this direction.

Heterogeneously-Grown Tunable Tensile Strained Germanium on Silicon for Photonic Devices.

The growth, structural and optical properties, and energy band alignments of tensile-strained germanium (ε-Ge) epilayers heterogeneously integrated on silicon (Si) were demonstrated for the first time. The tunable ε-Ge thin films were achieved using a composite linearly graded InxGa1-xAs/GaAs buffer architecture grown via solid source molecular beam epitaxy. High-resolution X-ray diffraction and micro-Raman spectroscopic analysis confirmed a pseudomorphic ε-Ge epitaxy whereby the degree of strain varied as a function of the In(x)Ga(1-x)As buffer indium alloy composition. Sharp heterointerfaces between each ε-Ge epilayer and the respective In(x)Ga(1-x)As strain template were confirmed by detailed strain analysis using cross-sectional transmission electron microscopy. Low-temperature microphotoluminescence measurements confirmed both direct and indirect bandgap radiative recombination between the Γ and L valleys of Ge to the light-hole valence band, with L-lh bandgaps of 0.68 and 0.65 eV demonstrated for the 0.82 ± 0.06% and 1.11 ± 0.03% strained Ge on Si, respectively. Type-I band alignments and valence band offsets of 0.27 and 0.29 eV for the ε-Ge/In(0.11)Ga(0.89)As (0.82%) and ε-Ge/In(0.17)Ga(0.83)As (1.11%) heterointerfaces, respectively, show promise for ε-Ge carrier confinement in future nanoscale optoelectronic devices. Therefore, the successful heterogeneous integration of tunable tensile-strained Ge on Si paves the way for the design and implementation of novel Ge-based photonic devices on the Si technology platform.

Analysis of optical gain threshold in n-doped and tensile-strained germanium heterostructure diodes

The optical emission of germanium-based luminescent and/or laser devices can be enhanced by tensile strain and n-type doping. In this work, we study by simulation the interplay between electrical transport and optical gain in highly n-doped and intrinsic germanium p-n heterostructure diodes under tensile strain. The effects of strain and doping on carrier mobilities and energy distribution are taken into account. Whereas the n-doping of Ge enhances the filling of the indirect L and Brillouin zone-center conduction band states, the n-doping also reduces the carrier injection efficiency, which is detrimental for the achievement of optical gain at reduced current densities. For applied biaxial strains larger than 1.25%, i.e., far before reaching the cross-over from indirect to direct band gap regime, undoped germanium exhibits a lower optical gain threshold as compared to doped germanium. We also show that the threshold current needed to reach transparency in germanium heterostructures has been significantly underestimated in the previous works.

High-performance Ge p-i-n photodetector on Si substrate

High-performance and tensile-strained germanium (Ge) p-i-n photodetector is demonstrated on Si substrate. The epitaxial Ge layers were prepared in an ultrahigh vacuum chemical vapor deposition (UHV-CVD) system using low temperature Ge buffer technique. The devices were fabricated by in situ doping and using Si as passivation layer between Ge and metal, which can improve the ohmic contact and realize the high doping. The results show that the dark current of the photodetector with diameter of 24 μm is about 2.5×10−7 μA at the bias voltage of −1 V, and the optical responsivity is 0.1 A/W at wavelength of 1.55 μm. The 3 dB bandwidth (BW) of 4 GHz is obtained for the photodetector with diameter of 24 μm at reverse bias voltage of 1 V. The long diffusion time of minority carrier in n-type Ge and the large contact resistance in metal/Ge contacts both affect the performance of Ge photodetectors.

Strain-Engineered Biaxial Tensile Epitaxial Germanium for High-Performance Ge/InGaAs Tunnel Field-Effect Transistors

The structural, morphological, and energy band alignment properties of biaxial tensile-strained germanium epilayers, grown in-situ on GaAs via a linearly graded InxGa1−xAs buffer architecture and utilizing dual chamber molecular beam epitaxy, were investigated. Precise control over the growth conditions yielded a tunable in-plane biaxial tensile strain within the Ge thin films that was modulated by the underlying InxGa1−xAs “virtual substrate” composition. In-plane tensile strains up to 1.94% were achieved without Ge relaxation for layer thicknesses of 15 to 30 nm. High-resolution x-ray diffraction supported the pseudomorphic nature of the Ge/InxGa1−xAs interface, indicating a quasi-ideal stress transfer to the Ge lattice. High-resolution transmission electron microscopy revealed defect-free Ge epitaxy and a sharp, coherent interface at the Ge/InxGa1−xAs heterojunction. Surface morphology characterization using atomic force microscopy exhibited symmetric, 2-D cross-hatch patterns with root mean square roughness less than 4.5 nm. X-ray photoelectron spectroscopic analysis revealed a positive, monotonic trend in band offsets for increasing tensile strain. The superior structural and band alignment properties of strain-engineered epitaxial Ge suggest that tensile-strained Ge/InxGa1−xAs heterostructures show great potential for future high-performance tunnel field-effect transistor architectures requiring flexible device design criteria while maintaining low power, energy-efficient device operation.

Tensile-strained germanium microdisk electroluminescence.

We report room temperature electroluminescence of tensile-strained germanium microdisks. The strain is transferred into the microdisks using silicon nitride stressors. Carrier injection is achieved with Schottky contacts on n-type doped germanium. We show that a biaxial tensile-strain up to 0.72% can be transferred by optimizing the carrier injection profile. The transferred strain is measured by the electroluminescence spectral red-shift and compared to finite element modeling. We discuss the impact of this strain level to achieve population inversion in germanium.

Observation of improved minority carrier lifetimes in high-quality Ge-on-insulator using time-resolved photoluminescence.

We report improved minority carrier lifetimes in n-type-doped and tensile-strained germanium by measuring direct bandgap photoluminescence from germanium-on-insulator substrates with various levels of defect density. We first describe a method to fabricate a high-quality germanium-on-insulator substrate by employing direct wafer bonding and chemical-mechanical polishing. Raman spectroscopy measurement was performed to assess the purity of the transferred layer on an insulator. Using time-resolved photoluminescence decay measurement, we observe that minority carrier lifetimes can be improved by over a factor of 3 as the defective top interface of our material stack is removed. Our high-quality germanium-on-insulator should be an ideal platform for high-performance, germanium-based photonic devices for on-chip optical interconnects.

Effect of increasing thickness on tensile-strained germanium grown on InGaAs buffer layers

We have investigated the optical properties of tensile-strained germanium grown on InGaAs buffer layers as a function of film thickness and buffer layer composition. We study the dependence of the photoluminescence as a function of the strain amplitude and degree of relaxation which are also monitored by X-ray diffraction and Raman spectroscopy. We show that 0.75% biaxially strained germanium can be obtained up to a thickness of 150 nm, a value sufficiently high to allow confinement of the spontaneous emission in a guiding structure. For large thicknesses (&amp;gt;200 nm) and large indium content in the buffer layer, a partial relaxation of the film is observed characterized by a large in-plane anisotropy of the germanium lattice. In this case, a difference of strain magnitude deduced either by microphotoluminescence spectra or by X-ray or Raman measurements is reported. We explain this difference by the sensitivity of microphotoluminescence to the local properties of the material. This study provides guidelines in order to achieve high optical quality and high biaxial tensile strain in Ge films with thicknesses compatible with optical waveguiding.

The optical property of tensile-strained n-type doped Ge

Tensile-strained germanium is one of the promsing materials for Si-based photonic devices due to its quasi-direct band and compatiblility with silicon technology. The band structure of tensile-strained germanium is investigated based on the theory of van de Walle deformed potential. The carrier distributions in the conduction bands at Γ and L vallies under the strain, and the n-type doping concentratoin in germanium are analyzed. Considering the competition between radiative recombinations at Γ and L vallies and Auger recombination, as well as dislocation induced non-radiative recombination, internal quantum efficiency and optical gain for direct band transition in n-type Ge are calculated. It is shown that 74.6% internal quantum efficiency can be obtained in the 1.5% tensile-strained n-type doped Ge under carrier injection and a strong optical gain is predicted, which is comparable to those of III-V materials.

Optical gain in single tensile-strained germanium photonic wire

We have investigated the optical properties of tensile-strained germanium photonic wires. The photonic wires patterned by electron beam lithography (50 μm long, 1 μm wide and 500 nm thick) are obtained by growing a n-doped germanium film on a GaAs substrate. Tensile strain is transferred in the germanium layer using a Si₃N₄ stressor. Tensile strain around 0.4% achieved by the technique corresponds to an optical recombination of tensile-strained germanium involving light hole band around 1690 nm at room temperature. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A 20% decrease of the spectral broadening is observed as the pump intensity is increased. All these features are signatures of optical gain. A 80 cm⁻¹ modal optical gain is derived from the variable strip length method. This value is accounted for by the calculated gain material value using a 30 band k · p formalism. These germanium wires represent potential building blocks for integration of nanoscale optical sources on silicon.

High quality tensile-strained n-doped germanium thin films grown on InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is directly observed by room temperature direct band gap photoluminescence.

Direct-bandgap luminescence at room-temperature from highly-strained Germanium nanocrystals

Efficient room-temperature luminescence at optical telecommunication wavelengths and originating from direct band-to-band recombination has been observed in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction, micro-Raman and optical-absorption spectroscopy measurements indicate high tensile-strains while combined photoluminescence spectroscopy, excitation-power evolution and time-resolved measurements suggest direct band-to-band recombination. Such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb light-emission properties, facile fabrication and compatibility with standard microelectronic processes.

Band structure and optical gain of tensile-strained germanium based on a 30 band k⋅p formalism

We have investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm−1 is predicted for a carrier density of 1×1018 cm−3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption losses. This gain should be sufficient to achieve lasing in these structures.