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We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in micron-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission.

Epitaxial growth of Ge films on Si introduces an in-plane biaxial tensile strain, opening up Ge applications in photonic, as recently demonstrated by the room temperature lasing action in Ge-on-Si heterostructure. The optical access to the direct gap transitions remarkably provides the possibility of injecting spin-polarized carriers via absorption of circular-polarized light. Nevertheless the effects of tensile-strain on the spin properties of Ge-on-Si have still to be addressed. In this work we applied polarization-resolved photoluminescence (PL) to spectrally resolve the radiative recombination involving strain-split light and heavy hole bands. We found that at low temperature, the fundamental transition has a record-high circular polarization degree of 85%, despite an off-resonance excitation of 300 meV. We show that this very high value is due to the characteristic energy dependence of the optically induced electron spin population and their concomitant dynamics. Finally, our observation of a direct gap PL doublet clarifies that the light hole contribution to the optical spectrum can dominate the room temperature PL. Our results provide a step forward the investigation of the dynamics of non-equilibrium spin populations and of the evaluation of optical gain in group IV materials. Above all, our findings confirm Ge as a prominent candidate for the development of next-generation CMOS-compatible devices featuring spintronics and photonics functionalities.

This articleThis article will form part of a virtual special issue of the journal, presenting some highlights of the 12th Biennial Conference on High-Resolution X-ray Diffraction and Imaging (XTOP2014). reports the X-ray diffraction-based structural characterization of the α12 multilayer structure SiGe2Si2Ge2SiGe12 [d'Avezac, Luo, Chanier & Zunger (2012). Phys. Rev. Lett. 108, 027401], which is predicted to form a direct bandgap material. In particular, structural parameters of the superlattice such as thickness and composition as well as interface properties, are obtained. Moreover, it is found that Ge subsequently segregates into layers. These findings are used as input parameters for band structure calculations. It is shown that the direct bandgap properties depend very sensitively on deviations from the nominal structure, and only almost perfect structures can actually yield a direct bandgap. Photoluminescence emission possibly stemming from the superlattice structure is observed.

The possibility to reduce the thickness of the SiGe virtual substrate, required for the integration of Ge heterostructures on Si, without heavily affecting the crystal quality is becoming fundamental in several applications. In this work, we present 1 μm thick Si1-xGex buffers (with x > 0.7) having different designs which could be suitable for applications requiring a thin virtual substrate. The rationale is to reduce the lattice mismatch at the interface with the Si substrate by introducing composition steps and/or partial grading. The relatively low growth temperature (475 C) makes this approach appealing for complementary metal-oxide- semiconductor integration. For all the investigated designs, a reduction of the threading dislocation density compared to constant composition Si 1-xGex layers was observed. The best buffer in terms of defects reduction was used as a virtual substrate for the deposition of a Ge/SiGe multiple quantum well structure. Room temperature optical absorption and photoluminescence analysis performed on nominally identical quantum wells grown on both a thick graded virtual substrate and the selected thin buffer demonstrates a comparable optical quality, confirming the effectiveness of the proposed approach. © 2014 AIP Publishing LLC.

We performed spin-resolved photoluminescence measurements on Ge/SiGe multiple quantum wells with different well thickness and using different exciting power densities. The polarization of the direct emission strongly depends on the relative weight of electrons photoexcited from the light and the heavy hole subbands. The study of the polarization as a function of the exciting power highlights the role of the carrier-carrier interactions in determining spin depolarization. © 2013 AIP Publishing LLC.

The polarization of the direct gap emission from Ge/SiGe multiple quantum wells has been studied in the 4 K to 300 K temperature range, and in samples with different well thicknesses. Our results demonstrate that the polarization type and degree strongly depend on the excitation of electrons from the heavy hole and the light hole subbands, thus providing an effective degree of freedom to control the polarization of the direct interband emission. In addition, the analysis of the temperature dependence of the polarization degree highlights spin depolarization mechanisms.

Photoluminescence (PL) is one of the most widely diffused experimental techniques for the characterization of semiconductor nanostructures, in particular quantum wells (QWs), and for the study of their electronic properties. PL allows to study interesting intrinsic effects in QWs, like the exciton binding energy increase due to carrier localization, the homogeneous broadening of the exciton recombination line, the carrier–carrier interaction, and so on. In addition, the analysis of the radiative recombination spectra can help in the nanostructure characterization, providing information on the interface morphology and on the materials quality. The information that can be gained from PL spectra is critically summarized with the aim of providing a reference scheme for the characterization of nanostructures. The discussion is supported by a wide and detailed bibliography, in which basic textbooks, review articles, and research papers are included in order to provide the reader with up-to-date information on the application of PL to the study of optical and structural properties of semiconductor QWs. As a significant example, the PL characterization of Ge/SiGe QWs, an indirect band-gap model system whose optical properties are strongly influenced by transitions involving Γ Γ -type states, is presented in the last Section of the Chapter.

We present a detailed experimental study of the photoluminescence decay of direct Γ-Γ and indirect L-Γ transitions in compressively strained Ge/Si0.15Ge0.85 type I multiple quantum wells. The lifetime of the fundamental L-Γ indirect-gap related transition is in the 6 to 13 ns range at the lattice temperature of 14 K. These values are just one order of magnitude higher than those typical of type-I direct gap III-V quantum wells and are significantly shorter than those characteristic of type-II indirect gap SiGe/Si quantum wells. The measured decay times show a clear dependence on the quantum well width and lattice temperature. The decay of the Γ-Γ direct-gap related transition is dominated by the ultrafast electron scattering from Γ-type to L-type states of the conduction band.

We present photoluminescence (PL) and spin polarized photo-emission (SPPE) measurements performed on bulk Ge, and Ge/Si(100) epilayers. A set of bi-axially strained Ge epilayers featuring different strain levels has been deposited by low energy plasma enhanced CVD (LEPECVD) and characterized by high resolution X-ray diffraction (HR-XRD). SPPE data indicate that compressive strain effectively lifts the heavy holes - light holes degeneracy raising the polarization of injected electrons above the P=50% limit of bulk material. Circularly polarized light PL measurements performed on p-type bulk crystals confirm the suitability of Ge for spintronics application showing a robust electron spin inizialization at the direct gap of Ge and giving a lower bound value for the spin relaxation time of ? 230 fs. © 2011 SPIE.

The intricate relaxation dynamics in compressively strained Ge quantum wells with Ge-rich SiGe barriers is investigated systematically for various excitation conditions. Evidence of efficient ultrafast intersubband relaxation in competition with the intrasubband electron cooling is found in continuous wave photoluminescence spectra measured with different excitation wavelengths. These findings are corroborated by time-resolved pumpprobe absorption measurements yielding an intersubband relaxation time in the range of 100 fs.

We report on the observation of room temperature direct band gap photoluminescence in compressively strained-Ge multiple quantum wells with Ge-rich SiGe barriers. A detailed experimental study of the temperature dependence of the photoluminescence is carried out from 5 K up to room temperature. We find that the direct gap photoluminescence at room temperature is due to the thermal excitation of carriers from L-type to -type confined states. Room temperature photoluminescence shows that Ge/SiGe multiple quantum wells are promising candidates for efficient light emitting devices monolithically integrated on Si. © 2011 American Institute of Physics.

The epitaxial growth of Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) on Si(111) substrates is demonstrated. A 3 μm thick reverse, double-step virtual substrate with a final composition of Si 0.10Ge0.90 has been employed. High resolution XRD, TEM, AFM and defect etching analysis has been used for the study of the structural properties of the buffer and of the QWs. The QW stack is characterized by a threading dislocation density of about 3 × 107cm-2 and an interdiffusion layer at the well/barrier interface of 2.1 nm. The quantum confined energy levels of this system have been calculated using the k·p and effective mass approximation methods. The Ge/Si0.15Ge 0.85 MQWs have been characterized through absorption and photoluminescence measurements. The optical spectra have been compared with those of Ge/Si0.15Ge0.85 QWs grown on Si(001) through a thick graded virtual substrate. © 2014 AIP Publishing LLC.

A systematic study of carrier relaxation in Ge quantum wells is performed applying photoluminescence and pump-probe spectroscopy. Intersubband-relaxation on a 100 fs time-scale and the presence of a non-thermal carrier distribution are identified. © 2012 OSA.