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The role of the interface potential on the effective mass of charge carriers is elucidated in this work. We develop a new theoretical formalism using a spatially dependent effective mass that is related to the magnitude of the interface potential. Using this formalism, we studied Ge quantum dots (QDs) formed by plasma enhanced chemical vapour deposition (PECVD) and co-sputtering (sputter). These samples allowed us to isolate important consequences arising from differences in the interface potential. We found that for a higher interface potential, as in the case of PECVD QDs, there is a larger reduction in the effective mass, which increases the confinement energy with respect to the sputter sample. We further understood the action of O interface states by comparing our results with Ge QDs grown by molecular beam epitaxy. It is found that the O states can suppress the influence of the interface potential. From our theoretical formalism, we determine the length scale over which the interface potential influences the effective mass. [ABSTRACT FROM AUTHOR]

We report a systematic study of time-resolved and power-dependent photoresponse in high-efficiency germanium quantum dot photodetectors (Ge-QD PDs), with internal quantum efficiencies greater than 100% over a broad wavelength, reverse bias, and incident power range. Turn-on and turn-off response times (τon and τoff) are shown to depend on series resistance, bias, optical power, and thickness (WQD) of the Ge-QD layer, with measured τoff values down to ∼40 ns. Two different photoconduction regimes are observed at low and high reverse bias, with a transition around -3 V. A transient current overshoot phenomenon is also observed, which depends on bias and illumination power. [ABSTRACT FROM AUTHOR]

The electrical activation and clustering of Ga implanted in crystalline Ge was investigated in the (0.3–1.2)×1021 Ga/cm3 concentration range. To this aim, Ge samples implanted with 50 keV gallium, and annealed at several temperatures up to 650 °C, have been subjected to a detailed structural and electrical characterization. The substrate was maintained at 77 K during implantation to avoid the formation of the honeycomb structure that occurs during implantation at room temperature of heavy ions at high fluence. Secondary ion mass spectrometry analyses indicated a negligible Ga diffusion and dopant loss during the thermal annealing. The carrier concentration in the recrystallized samples measured by Hall effect showed a maximum concentration of active Ga of ∼6.6×1020 Ga/cm3. A remarkable Ga deactivation occurred with increasing the annealing temperature from 450 to 650 °C although the sheet resistance did not change considerably in this temperature range. It turned out that the carrier concentration reduction is balanced by the enhancement of the hole mobility that exhibits a steep variation with the concentration of the ionized scattering centers in this range. A simple model is proposed to explain the experimental results taking into account the thermally activated Ga clustering. These studies, besides clarifying the mechanism of Ga deactivation in Ge, can be helpful for the realization of future generation devices based on Ge. [ABSTRACT FROM AUTHOR]

Experimental studies about electrical activation and clustering of B implanted in crystalline Ge (c-Ge) are reported. To this aim, we structurally and electrically investigated c-Ge samples implanted at different temperatures with B at 35 keV in the high-concentration dopant regime (0.67–25×1020 B/cm3). We elucidated that a high level of damage, in the form of amorphous pockets, favors the electrical activation of the dopant, and a complete activation was achieved for properly chosen implant conditions. We found, by joining channeling measurements with the electrical ones, that the reason for incomplete B activation is the formation of B-Ge complexes with a well-defined stoichiometry of 1:8. The thermal stability of the B-doped samples, up to 550 °C, was also investigated. The tested stability demonstrates that the B clustering, responsible of B inactivity, is characterized by high binding energies and higher thermal budgets are needed to make them to dissolve. These studies, besides clarify the physical mechanisms by which B dopes Ge, can be helpful for the realization of ultrashallow junctions for the future generation devices. [ABSTRACT FROM AUTHOR]

The localization of voids in thin Si1-xGex layers after He+ implantation and thermal annealing is reported. A Si/Si1-xGex multilayer grown onto (001) Si was implanted with He+ in the 10-30 keV range, with fluences from 7×1015 up to 1×1016 cm-2, and annealed at 800 °C for 1 h. Samples were analyzed by transmission electron microscopy, showing void formation only within the two layers containing Ge or at the film/substrate interface. Our results support the idea that the compressive strain in the Si1-xGex layers induces the nucleation of small cavities and the growth of voids by a mechanism where vacancies are stabilized by He. [ABSTRACT FROM AUTHOR]

SiGeO films have been produced by a sol-gel derived approach and by magnetron sputtering deposition. Post-thermal annealing of SiGeO films in forming gas or nitrogen atmosphere between 600 and 900 °C ensured the phase separation of the SiGeO films and synthesis and growth of Ge nanoclusters (NCs) embedded in SiO. Rutherford backscattering spectrometry analysis evidenced a similar Ge concentration (~12 %), but a different Ge out-diffusion after annealing between the two types of techniques with the formation of a pure SiO surface layer (~30 nm thick) in sol-gel samples. The thermal evolution of Ge NCs has been followed by transmission electron microscopy and Raman analysis. In both samples, Ge NCs form with similar size increase (from ~3 up to ~7 nm) and with a concomitant amorphous to crystalline transition in the 600-800 °C temperature range. Despite a similar Ge concentration, a significant lower NCs density is observed in sol-gel samples attributed to an incomplete precipitation of Ge, which probably remains still dispersed in the matrix. The optical absorption of Ge NCs has been measured by spectrophotometry analyses. Ge NCs produced by the sol-gel method evidence an optical band gap of around 2 eV, larger than that of NCs produced by sputtering (~1.5 eV). These data are presented and discussed also considering the promising implications of a low-cost sol-gel based technique towards the fabrication of light harvesting devices based on Ge nanostructures. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we investigate the spectral response of metal-oxide-semiconductor photodetectors based on Ge nanoclusters (NCs) embedded in a silicon dioxide (SiO2) matrix. The role of Ge NC size and density on the spectral response was evaluated by comparing the performance of PDs based on either densely packed arrays of 2nm-diameter NCs or a more sparse array of 8nm-diameter Ge NCs. Our Ge NC photodetectors exhibit a high spectral responsivity in the 500–1000nm range with internal quantum efficiency of ~700% at −10V, and with NC array parameters such as NC density and size playing a crucial role in the photoconductive gain and response time. We find that the configuration with a more dispersed array of NCs ensures a faster photoresponse, due to the larger fraction of electrically-active NCs and the partial suppression of recombination centers. The photoconduction mechanism, assisted by trapping of photo-generated holes in Ge NCs, is discussed for different excitation power and applied bias conditions. Our results provide guidelines for further optimization of high-efficiency Ge NC photodetectors. [Copyright &y& Elsevier]

Experimental studies on the damage produced in (100) Ge substrates by implantation of Ge ions at different energies (from 25 to 600 keV), fluences (from 2×10 to 4×10 cm) and temperature (room temperature, RT, or liquid-nitrogen temperature, LNT) have been performed by using the Rutherford backscattering spectrometry technique. We demonstrated that the higher damage rate of Ge with respect to Si is due to both the high stopping power of germanium atoms and the low mobility of point defects within the collision cascades. The amorphization of Ge has been modeled by employing the critical damage energy density model in a large range of implantation energies and fluences both at RT and LNT. The experimental results for implantation at LNT were fitted using a critical damage energy density of ∼1 eV/atom. A fictitious value of ∼5 eV/atom was obtained for the samples implanted at RT, essentially because at RT the damage annihilation plays a non-negligible role against the crystalline-amorphous transition phase. The critical damage energy density model was found to stand also for other ions implanted in crystalline Ge (Ar and Ga). [ABSTRACT FROM AUTHOR]

Abstract: In this work we propose an alternative methodology to study B diffusion in crystalline Ge. We enhance B diffusion by means of passing implants in such a way to increase the point-defects distribution through the sample, well above the equilibrium value. A comparison between B diffusion occurring under implantation with different ions or after post-implantation annealing allowed to discern any possible role of ionization effects on B diffusion. Indeed, B diffusion is demonstrated to occur through a point-defect-mediated mechanism. The diffusion mechanism is hence discussed. These results are a key point for a full comprehension of the B diffusion in Ge. [Copyright &y& Elsevier]

We addressed the issues related to quantitative carrier profiling by scanning capacitance microscopy (SCM) on doped layers with different dimensions, starting from thick (∼5 μm) uniformly B-doped Si layers, down to Si/Si1-xGex/Si quantum wells with nanometric width. We preliminarly discussed the influence of the SCM hardware on the quantification, by comparing the analyses performed on Si calibration standards with two different atomic force microscopes, i.e., DI3100 by Veeco and XE-100 by PSIA, equipped with different SCM sensors. Furthermore, both concentration and spatial resolution are demonstrated by measurements on specially designed samples containing B-doped quantum wells of Si0.75Ge0.25 layers strained between Si films. Measurements were taken both on cross-sectioned samples and on beveled ones. A SCM spatial resolution of 1 nm with a concentration sensitivity from 5% to 10% was demonstrated by quantitative majority carrier profiling on the B-doped Si/Si0.75Ge0.25/Si heterostructures. [ABSTRACT FROM AUTHOR]

The authors of the above paper call into question recent evidence on the properties of selfinterstitials, I, in Ge [Cowern et al., Phys. Rev. Lett. 110, 155501 (2013)]. We show that this judgment stems from invalid model assumptions during analysis of data on B marker-layer diffusion during proton irradiation, and that a corrected analysis fully supports the reported evidence. As previously stated, I-mediated self-diffusion in Ge exhibits two distinct regimes of temperature, T: high-T, dominated by amorphous-like mono-interstitial clusters--i-morphs--with self-diffusion entropy ≈30 k, and low-T, where transport is dominated by simple self-interstitials. In a transitional range centered on 475 °C both mechanisms contribute. The experimental I migration energy of 1.8±0.26 eV reported by the Müunster group based on measurements of self-diffusion during irradiation at 550 °C

High doping regimes of B implanted Ge have been accurately characterized combining Hall effect technique and nuclear reaction analysis. Preamorphized Ge was implanted with B at 35 keV (spanning the 0.25–25×1020 B/cm3 concentration range) and recrystallized by solid phase epitaxy at 360 °C. The Hall scattering factor and the maximum concentration of active B resulted rH=1.21 and ∼5.7×1020 B/cm3, respectively. The room-temperature carrier mobility was accurately measured, decreasing from ∼300 to 50 cm2/V s in the investigated dopant density, and a fitting empirical law is given. These results allow reliable evaluation for Ge application in future microelectronic devices. [ABSTRACT FROM AUTHOR]

B migration in Si and Ge matrices raised a vast attention because of its influence on the production of confined, highly p-doped regions, as required by the miniaturization trend. In this scenario, the diffusion of B atoms can take place under severe conditions, often concomitant, such as very large concentration gradients, non-equilibrium point defect density, amorphous-crystalline transition, extrinsic doping level, co-doping, B clusters formation and dissolution, ultra-short high-temperature annealing. In this paper, we review a large amount of experimental work and present our current understanding of the B diffusion mechanism, disentangling concomitant effects and describing the underlying physics. Whatever the matrix, B migration in amorphous (α-) or crystalline (c-) Si, or c-Ge is revealed to be an indirect process, activated by point defects of the hosting medium. In α-Si in the 450-650 °C range, B diffusivity is 5 orders of magnitude higher than in c-Si, with a transient longer than the typical amorphous relaxation time. A quick B precipitation is also evidenced for concentrations larger than 2 × 1020 B/cm3. B migration in α-Si occurs with the creation of a metastable mobile B, jumping between adjacent sites, stimulated by dangling bonds of α-Si whose density is enhanced by B itself (larger B density causes higher B diffusivity). Similar activation energies for migration of B atoms (3.0 eV) and of dangling bonds (2.6 eV) have been extracted. In c-Si, B diffusion is largely affected by the Fermi level position, occurring through the interaction between the negatively charged substitutional B and a self-interstitial (I) in the neutral or doubly positively charged state, if under intrinsic or extrinsic (p-type doping) conditions, respectively. After charge exchanges, the migrating, uncharged BI pair is formed. Under high n-type doping conditions, B diffusion occurs also through the negatively charged BI pair, even if the migration is depressed by Coulomb pairing with n-type dopants. The interplay between B clustering and migration is also modeled, since B diffusion is greatly affected by precipitation. Small (below 1 nm) and relatively large (5-10 nm in size) BI clusters have been identified with different energy barriers for thermal dissolution (3.6 or 4.8 eV, respectively). In c-Ge, B motion is by far less evident than in c-Si, even if the migration mechanism is revealed to be similarly assisted by Is. If Is density is increased well above the equilibrium (as during ion irradiation), B diffusion occurs up to quite large extents and also at relatively low temperatures, disclosing the underlying mechanism. The lower B diffusivity and the larger activation barrier (4.65 eV, rather than 3.45 eV in c-Si) can be explained by the intrinsic shortage of Is in Ge and by their large formation energy. B diffusion can be strongly enhanced with a proper point defect engineering, as achieved with embedded GeO2 nanoclusters, causing at 650 °C a large Is supersaturation. These aspects of B diffusion are presented and discussed, modeling the key role of point defects in the two different matrices. [ABSTRACT FROM AUTHOR]

Ge nanoclusters (NCs), synthesized by ion implantation and annealing up to 900 °C, result small (∼2 nm) and amorphous in Si3N4, crystalline and much larger in SiO2. The NCs ripening and crystallization kinetics in Si3N4 is retarded by larger interfacial energy and lower diffusivity of Ge in comparison to SiO2. Ge NCs absorb light more efficiently when embedded in Si3N4 than in SiO2. A significant effect of the barrier height on absorption was evidenced, in agreement with effective mass theory predictions. The smaller bandgap of Ge NCs embedded in Si3N4 and their closeness is promising features for light harvesting applications. [ABSTRACT FROM AUTHOR]

Abstract: In this work we compare the B electrical activity in crystalline (c-Ge) and preamorphized Ge (PAI-Ge), in order to elucidate the activation mechanisms involved in the two cases and evidence the possible advantages of an approach over to the other. With this aim, we independently measured the hole fluence and the sheet resistance, thus extracting the carrier mobility, as a function of the implanted B fluence. In particular, we evidenced that it is possible to reproduce the metastability of the PAI process implanting B in c-Ge at very high fluences. However, by properly choosing the implantation conditions in c-Ge, in such a way to disable dynamic annealing during implantation, the activation of B can be raised up to the level attainable in PAI-Ge also for lower B fluences. Finally, the thermal evolution of the formed junction was tested, evidencing a high stability under annealing up to 550°C in both c- and PAI-Ge. [Copyright &y& Elsevier]

Abstract: Transmission Electron Microscopy was combined with Time Resolved Reflectivity to study the amorphous–crystalline (a–c) interface evolution during Solid Phase Epitaxy Regrowth (SPER) of Si0.83Ge0.17 films deposited on Si by Molecular Beam Epitaxy and amorphized with Ge+ ion implantation. Starting from the Si/SiGe interface, a 20nm thick layer regrows free of defects with the same SPER rate of pure Si. The remaining SiGe regrows with planar defects and dislocations, accompanied by a decrease of the SPER velocity. The sample was also studied after implantation with B or P. In these cases, the SPER rate raises following the doping concentration profile, but no difference in the defect-free layer thickness was observed compared to the un-implanted sample. On the other hand, B or P introduction reduces the a–c interface roughness, while B–P co-implantation produces roughness comparable to the un-implanted sample. [Copyright &y& Elsevier]

B diffusion measurements are used to probe the basic nature of self-interstitial point defects in Ge. We find two distinct self-interstitial forms—a simple one with low entropy and a complex one with entropy ~30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket—we name it a morph. Computational modeling suggests that morphs exist in both self-interstitial and vacancylike forms, and are crucial for diffusion and defect dynamics in Ge, Si, and probably many other crystalline solids. [ABSTRACT FROM AUTHOR]

Abstract: B diffusion in crystalline Ge is investigated under equilibrium and non-equilibrium conditions in a large temperature range (200–800°C), in order to discriminate the role of self-interstitials (Is) and the energy barriers involved in the microscopic mechanism of B migration. To this aim, we copiously furnished Is by 200 or 300keV H+ irradiation, and performed a direct comparison with B diffusion in thermal conditions at the same temperature (T). The diffused profiles of B were simulated assuming the kick-out model, and the extracted parameters (migration length, λ, and formation rate of mobile B, g) indicated that the B diffusion is always mediated by Is showing different features at low and high T regimes. For T lower than 600°C the thermal generation of Is is negligible and the only barrier to g (measured to be ∼0.1eV) is due to the Is migration and B mobile formation. At T higher than 600°C, the thermal generation of Is starts to overcome the Is supply from the irradiation, and the activation energy of g increases to 3.0–3.5eV. The migration length in the low-T regime has the largest value (∼20nm), while it decreases down to 1–2nm by increasing T, showing a negative activation energy of ∼−0.64eV, compatible with a dissociation process which stops the diffusion event. In this regard, we observed that the mobile B migration length depends only on T, regardless of the point defects concentration. These results and the energy barriers measurements contribute to a further comprehension of the B diffusion and point defects in crystalline Ge. [Copyright &y& Elsevier]

The effect of O implantation in crystalline Ge on the density of native point defects has been investigated through transmission electron microscopy and B diffusion experiments. Annealing at 650 °C following O implants produces a band of defects (∼5-10 nm), compatible with GeO2 nanoclusters (NCs). A clear shape transformation from elongated to spherical forms occurs within 2 h, concomitant with a transient enhanced diffusion of B. A large injection of self-interstitials from GeO2 NCs, giving a vacancy undersaturation, and a long-range migration of self-interstitials are evidenced and discussed. [ABSTRACT FROM AUTHOR]

Hall effect measurements in the 4–300 K temperature range have been used to investigate the electrical properties of B doped Si1−xGex layers (with 0≤x≤0.2) grown on Si(100) by MBE. The Hall concentration and mobility of strained and relaxed Si1−xGex layers have been converted into carrier concentration and drift mobility using the appropriate Hall scattering factor rH that has been determined by the comparison between the chemical B concentration profile (measured by secondary ion mass spectrometry) and the Hall carrier concentration. The mobility of both relaxed and strained layers was equal to that of Si and independent of the Ge concentration for x<0.2 and B concentration ≥1018 cm−3. The Hall scattering factor as a function of temperature has been determined. [Copyright &y& Elsevier]