xSnx Layers Research Articles

Ge1−xSnx layers with x∼0.25 on InP(001) substrate grown by low-temperature molecular beam epitaxy reaching 70 °C and in-situ Sb doping

Ge1−xSnx with x∼25%, group-IV alloy semiconductor, is highly attracted to mid-infrared (MIR) photodetector applications because of its narrow bandgap with ∼0.25 eV, the compatibility to the Si integrated circuit platform, and lattice matching of Ge0.75Sn0.25 to InP substrate. Although the heteroepitaxial growth of Ge0.75Sn0.25 on InP has been reported, the basic physical properties of Ge0.75Sn0.25 have not yet been clarified. In this study, we discuss three topics: (1) understanding the crystalline growth features of Ge0.75Sn0.25 on InP, (2) revealing the thermal stability of Ge0.75Sn0.25, and (3) developing a carrier-control technique. First, we found that a low-temperature molecular beam epitaxy could achieve a Ge0.75Sn0.25 layer with superior crystallinity without dislocations. However, low Sn-content Ge1−xSnx regions are locally formed during the Ge0.75Sn0.25 heteroepitaxy; the origins of the low Sn-content Ge1−xSnx regions were discussed by transmission electron microscopy analysis. In addition, we found that lowering the growth temperature from 100 °C to 70 °C also effectively reduces the area of the low Sn-content Ge1−xSnx regions. Next, we found that Ge0.75Sn0.25 layers grown at 70 °C sustain thermal stability up to 200 °C without causing crystalline degradations. Finally, we demonstrated the in-situ Sb doping to Ge0.75Sn0.25. We found that the undoped and in-situ Sb-doped Ge0.75Sn0.25 layers exhibited p-type and n-type conduction, respectively, with carrier concentrations of order 1019 cm−3 by Hall effect measurement. This study suggests the MIR photodetector application is practically possible using low-temperature processes with process temperatures lower than 200 °C.

Effect of Growth Temperature on Crystallization of Ge1−xSnx Films by Magnetron Sputtering

Ge1−xSnx film with Sn content (at%) as high as 13% was grown on Si (100) substrate with Ge buffer layer by magnetron sputtering epitaxy. According to the analysis of HRXRD and Raman spectrum, the quality of the Ge1−xSnx crystal was strongly dependent on the growth temperature. Among them, the GeSn (400) diffraction peak of the Ge1−xSnx film grown at 240 °C was the lowest, which is consistent with the Raman result. According to the transmission electron microscope image, some dislocations appeared at the interface between the Ge buffer layer and the Si substrate due to the large lattice mismatch, but a highly ordered atomic arrangement was observed at the interface between the Ge buffer layer and the Ge1−xSnx layer. The Ge1−xSnx film prepared by magnetron sputtering is expected to be a cost-effective fabrication method for Si-based infrared devices.

Влияние инжекционных эффектов на электрические свойства гетеропереходов pSi--nSi1--xSnx

We studied the current-voltage characteristic of pSi--nSi1--xSnx structures in the temperature range of 293--393 K so as to find out the role of injection phenomena during electrical property formation in pSi--nSi1--xSnх heterojunctions derived from the Si1--xSnx (0 ≤ x ≤ 0.04) solid solution. We established that the current-voltage characteristic of such heterojunctions consists of two typical segments. We determined that an exponential function describes the first current-voltage characteristic segment well. In all current-voltage characteristics the exponential curve is followed by sublinear segments that do not depend on temperature. We show that the theory of injection depletion effect describes these segments well. We used the sublinear segment of the current-voltage characteristic to determine the value of the parameter a, which can be employed to calculate deep level impurity concentration leading to the appearance of the sublinear segment. We prove that the structure under investigation may be considered to be a pSi--nSi1--xSnx--n+Si1--xSnx (0 ≤ x ≤ 0.04) junction with a high-resistance nSi1--xSnx layer. The analysis results make it possible to conclude that charge carrier dissipation on both complex aggregates and nanoinclusions plays a significant role in forming electrophysical properties in the Si1--xSnx (0 ≤ x ≤ 0.04) solid solution and that epitaxial films of Si1--xSnx (0 ≤ x ≤ 0.04) solid solutions derived on silicon substrates are efficient promising materials for developing diodes operating under double injection

Ge(Sn) growth on Si(001) by magnetron sputtering

The semi-conductor Ge1―xSnx exhibits interesting properties for optoelectronic applications. In particular, Ge1―xSnx alloys with x ≥ 0.1 exhibit a direct band-gap, and integrated in complementary-metal-oxide-semiconductor (CMOS) technology, should allow the development of Si photonics. CMOS-compatible magnetron sputtering deposition was shown to produce monocrystalline Ge1―xSnx films with good electrical properties at low cost. However, these layers were grown at low temperature (< 430 K) and contained less than 6 % of Sn. In this work, Ge1―xSnx thin films were elaborated at higher temperature (> 600 K) on Si(001) by magnetron sputtering in order to produce low-cost and CMOS-compatible relaxed pseudo-coherent layers with x ≥ 0.1 exhibiting a better crystallinity. Ge1―xSnx crystallization and Ge1―xSnx crystal growth were investigated. Crystallization of an amorphous Ge1―xSnx layer deposited on Si(001) or Ge(001) grown on Si(001) leads to the growth of polycrystalline films. Furthermore, the competition between Ge/Sn phase separation and Ge1―xSnx growth prevents the formation of large-grain Sn-rich Ge1―xSnx layers without the formation of β-Sn islands on the layer surface, due to significant atomic redistribution kinetics at the crystallization temperature (T = 733 K for x = 0.17). However, the growth at T = 633 K of a highly-relaxed pseudo-coherent Ge0.9Sn0.1 film with low impurity concentrations (< 2 × 1019 at cm―3) and an electrical resistivity four orders of magnitude smaller than undoped Ge is demonstrated. Consequently, magnetron sputtering appears as an interesting technique for the integration of optoelectronic and photonic devices based on Ge1―xSnx layers in the CMOS technology.

Formation and optoelectronic property of strain-relaxed Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSny double heterostructures on a boron-ion-implanted Ge(001) substrate

We have investigated the formation and optoelectronic properties of strain relaxed Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSny double heterostructures on ion-implanted Ge substrates. The strain relaxation of Ge1−x−ySixSny and Ge1−xSnx epitaxial layers was achieved using an ion-implanted Ge substrate. The maximal degree of strain relaxation (DSR) of the Ge1−xSnx layers was evaluated to be 46%. In addition, we obtained a sharp and strong peak in the photoluminescence (PL) spectra from the sample with a DSR of 46%, while no strong peak was detected from a sample with a smaller DSR (22%). From the theoretical calculation of the energy band structure and the measurement temperature dependence of the PL intensity, the sharp and strong peak can be explained by the transition from an indirect to direct bandgap semiconductor due to the increase of the DSR and a concomitant increase of the Γ-valley electron population. Moreover, the PL intensity increases by the improvement of the crystallinity by a post deposition annealing process.

Operation of thin-film thermoelectric generator of Ge-rich poly-Ge1-xSnx on SiO2 fabricated by a low thermal budget process

A thin-film thermoelectric generator composed of p- and n-type poly-Ge1−xSnx (x ∼ 0.02) on a Si(001) covered with SiO2 has been successfully fabricated by low thermal budget processes (under 300 °C) and demonstrated for the first time. Both the crystallization and dopant activation were simultaneously performed using pulsed UV laser irradiation in flowing water. A recorded activation ratio of Sb in the poly-Ge1−xSnx enabled a relatively high power factor (9.2 μ Wcm−1 K−2 at RT), which is comparable to the counterparts of n-type Ge1−xSnx layers epitaxially grown on InP(001).

Experimental Calibration of Sn‐Related Varshni Parameters for High Sn Content GeSn Layers

AbstractFrom temperature‐dependent photoluminescence of a single Ge0.84Sn0.16 sample, Sn‐related Varshni parameters are extracted and used as input parameters in an 8‐band k·p model, and predict direct bandgap energies of high Sn content GeSn alloys with concentration varying from 8% to 16%. Then, exhaustively compared are the calculated direct Γ‐valley bandgap energies to photoluminescence results of 13% and 16% Sn content alloys and to direct bandgap energies found in literature. The agreement between k·p modeling and experimental datasets is close to 3% for different strain conditions of GeSn epilayers. The outcome of this study is an 8‐band k·p model with a fixed set of parameters, the model being self‐sufficient to describe the direct bandgap of Ge1−xSnx layers with x varying from 8% to 16% for large ranges of strain and temperature.

Influence of Sn precursors on Ge1−xSnx growth using metal-organic chemical vapor deposition

We have investigated the metal-organic chemical vapor deposition (MOCVD) of Ge1−xSnx epitaxial layers using tetrakis-dimetyl-amino-tin (TDMASn), which is expected to be more easily decomposable than other Sn precursors such as tri-butyl-vinyl-tin (TBVSn). We found that the deposition rate of Sn atoms using TDMASn is higher than that using TBVSn at a growth temperature as low as 300 °C. Also, we examined the MOCVD growth of a high-Sn-content Ge1−xSnx layer with TDMASn at a low temperature below 300 °C, and we found that the Sn content saturated at about 5%. The volume of precipitated Sn grains on the surface increases with the partial pressure of Sn precursor, the reason why the substitutional Sn content in Ge1−xSnx is limited despite increasing the partial pressure of a Sn precursor would not be because of the deposition-rate limitation of Sn by surface chemical reaction but the precipitation of Sn in this MOCVD condition with TDMASn.

Synthesis of heavily Ga-doped Si1−xSnx/Si heterostructures and their valence-band-offset determination

Heavily doped Si1−xSnx alloys are strongly desired for next-generation group-IV electronics. A polycrystalline growth study has been conducted on heavily Ga-doped amorphous Si0.9Sn0.1 layers deposited on a tensile-strained Si-on-insulator substrate. P-type conduction and a high Hall hole concentration over 1020 cm−3 are observed for grown Si1−xSnx layers with the initial Ga concentration of 1021 cm−3. In addition, we find that the valence-band offset of the Si1−xSnx relative to Si (ΔEv) is determined by the substitutional Sn content regardless of the hole concentration in the grown layers. Even for the substitutional Sn content less than 5%, a relatively high value of ΔEv (∼0.5 eV) could be obtained. These results suggest that with heavily Ga-doped Si1−xSnx alloys could become a p+-source material, which would provide benefits for Si-based tunnel field-effect transistors.

Optoelectronic properties of high-Si-content-Ge1−x–ySixSny/Ge1−xSnx/Ge1−x–ySixSny double heterostructure

The optoelectronic properties of Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSny double heterostructures pseudomorphically grown on a Ge substrate were investigated. The photoluminescence (PL) intensity of the sample with Ge0.66Si0.23Sn0.11 cladding layers is three times larger compared to PL from structure with a Ge cladding layer, which can be attributed to higher energy band offsets at both conduction and valence band edges at the Ge0.91Sn0.09/Ge0.66Si0.23Sn0.11 interface. The PL spectrum of the sample with the Ge0.66Si0.23Sn0.11 cladding layer at room temperature can be deconvoluted into four components, and the origins of these components can be assigned to direct and indirect transitions by measuring the temperature dependence of each component’s intensity. In addition, we examined the formation and characterization of strain-relaxed Ge1−x−ySixSny/Ge1−xSnx/Ge1−x−ySixSny double heterostructures to relieve the compressive strain in the Ge1−xSnx layer. Stacking faults were observed in the Ge1−xSnx and Ge1−x−ySixSny layers. The PL peak intensity of the strain-relaxed Ge1−xSnx layer decreases by a factor of 1/20 compared to the PL peak intensity of the double heterostructure pseudomorphically grown on a Ge(001) substrate. In addition, PL intensity can be increased by post-deposition annealing owing to decreasing defects.

Formation of ultra-low resistance contact with nickel stanogermanide/heavily doped n+-Ge1−xSnx structure

We examined the formation of Ni(Ge1−xSnx)/n+-Ge1−xSnx contacts with Sb- and P-doped Ge1−xSnx epitaxial layers with various electron concentrations and investigated the crystalline structure and contact resistivity. The fully maintained strain and no serious degradation of the crystallinity were confirmed in the Sb-doped Ge1−xSnx layer after Ni stanogermanidation, while the Sn content slightly decreased. The contact resistivity of Ni(Ge1−xSnx)/Sb-doped Ge1−xSnx interface achieved a value lower than the P-doped samples, in spite of high Sn contents with a Ni germanidation temperature of 350 °C. The ultra-low contact resistivity as low as 10−9 Ω cm2 was obtained with the Ni(Ge1−xSnx)/Sb-doped Ge0.935Sn0.065 sample with an electron concentration of 1.8 × 1020 cm−3.

Reduced thermal conductivity of a Ge1−xSnx layer formed on a self-assembled Sn nanodot template

Self-assembled Sn nanodots were formed on SiO2 and the crystallinity and Sn composition of a Ge1−xSnx polycrystal formed on the Sn nanodots were evaluated. The average diameter of the Sn nanodots was 17.1 nm, and their density was 1.5 × 109 cm−2. Raman spectroscopy revealed that the crystallinity of Ge1−xSnx increased from 52.29% to 86.82% as the Ge deposition temperature increased from 100 °C to 225 °C. However, agglomeration degraded the surface roughness when the Ge deposition temperature was high. By contrast, the Sn composition reduced from 12.84 to 4.78%. For both high Sn composition and a flat surface, lowering the Ge deposition temperature was found to be crucial. The impact of the crystallinity and Sn composition on thermal conductivity was investigated using the time-domain thermoreflectance method. Thermal conductivity increased with the increase in crystallinity, and it reached 2.305 W m−1 K−1 for Ge1−xSnx with a crystallinity of 86.82%. Ge1−xSnx alloy had lower thermal conductivity than that of Ge at the same crystallinity.

Epitaxial GeSn: impact of process conditions on material quality

The electrical and optical material properties of epitaxial Ge1−xSnx and SiyGe1−x−ySnx are of high interest for novel device applications. However, the limited Sn solubility in Ge makes the epitaxial growth of Ge1−xSnx and SiyGe1−x−ySnx challenging. Most of the literature describing the epitaxial growth is for Ge2H6 and SnCl4 as Ge and Sn precursors, respectively. A more recent publication deals with the epitaxial growth of high-quality Ge1−xSnx with the more conventional GeH4. In this manuscript, we compare the structural and optical material quality of Ge1−xSnx, epitaxially grown on Ge virtual substrates as a function of growth pressure, growth temperature, the choice of the carrier gas (H2 or N2) and the choice of the Ge precursor (GeH4 versus Ge2H6). The best material quality in terms of surface morphology and photoluminescence characteristics is obtained if GeH4 is used as a Ge precursor. For Ge1−xSnx grown with Ge2H6 and at atmospheric pressure, pyramidical defects can be seen and there is a risk for uncontrolled local Sn agglomeration. The pyramidical defects are not observed on Ge1−xSnx layers grown at reduced pressure, but the highest achievable substitutional Sn concentration is lower. No pyramidical defects are found for Ge1−xSnx layers grown with GeH4 and the issue of uncontrolled local Sn agglomeration does not appear.

Dopant behavior in heavily doped polycrystalline Ge1−xSnx layers prepared with pulsed laser annealing in water

A low-temperature process for the formation of heavily doped polycrystalline Ge (poly-Ge) layers on insulators is required to realize next-generation electronic devices. In this study, we have systematically investigated pulsed laser annealing (PLA) in flowing water for heavily doped amorphous Ge1−xSnx layers (x ≈ 0.02) with various dopants such as B, Al, Ga, In, P, As, and Sb on SiO2. It is found that the dopant density after PLA with a high laser energy is reduced when the oxidized dopant has a lower oxygen chemical potential than H2O. As a result, for the p-type doping of B, Al, Ga, and In, we obtained a high Hall hole density of 5 × 1019 cm−3 for PLA with a low energy. Consequently, the Hall hole mobility is limited to as low as 10 cm2 V−1 s−1. In contrast, for As and Sb doping, because the density of substitutional dopants does not decrease even after PLA with a high energy, we achieved a high Hall electron density of 6 × 1019 cm−3 and a high Hall electron mobility simultaneously. These results indicate that preventing the oxidation of dopant atoms by water is an important factor for achieving heavy doping using PLA in water.

High n-type Sb dopant activation in Ge-rich poly-Ge1−xSnx layers on SiO2 using pulsed laser annealing in flowing water

Heavy n-type doping in polycrystalline Ge (poly-Ge) is still under development owing to the low solid solubility and the low activation ratio of group-V dopants in Ge. To solve this problem, we have investigated ultra-short (55 ns) laser pulse annealing in flowing water for Sb-doped amorphous Ge1−xSnx layers (x ≈ 0.02) on SiO2. It is found that fully melting a Ge1−xSnx layer down to the Ge1−xSnx/SiO2 interface leads to a large grained (∼0.8 μmϕ) growth, resulting in not only a high electrical activation ratio (∼60%) of Sb atoms in the polycrystals but also a high electron density around 1020 cm−3. As a result, the electron mobility in the Ge-rich poly-Ge1−xSnx layers exceeds that in single-crystalline Si even in the region of a high electron density around 1020 cm−3. The low thermal budget process opens up the possibility for developing Ge1−xSnx based devices fabricated on 3D integrated circuits as well as flexible substrates.

Free-running Sn precipitates: an efficient phase separation mechanism for metastable Ge1\u2212xSnx epilayers

The revival of interest in Ge1−xSnx alloys with x ≥ 10% is mainly owed to the recent demonstration of optical gain in this group-IV heterosystem. Yet, Ge and Sn are immiscible over about 98% of the composition range, which renders epilayers based on this material system inherently metastable. Here, we address the temperature stability of pseudomorphic Ge1−xSnx films grown by molecular beam epitaxy. Both the growth temperature dependence and the influence of post-growth annealing steps were investigated. In either case we observe that the decomposition of epilayers with Sn concentrations of around 10% sets in above ≈230 °C, the eutectic temperature of the Ge/Sn system. Time-resolved in-situ annealing experiments in a scanning electron microscope reveal the crucial role of liquid Sn precipitates in this phase separation process. Driven by a gradient of the chemical potential, the Sn droplets move on the surface along preferential crystallographic directions, thereby taking up Sn and Ge from the strained Ge1−xSnx layer. While Sn-uptake increases the volume of the melt, single-crystalline Ge becomes re-deposited by a liquid-phase epitaxial process at the trailing edge of the droplet. This process makes phase separation of metastable GeSn layers particularly efficient at rather low temperatures.

Strain engineering in epitaxial Ge1−xSnx: a path towards low-defect and high Sn-content layers

The plastic strain relaxation of CVD-grown Ge1−xSnx layers was investigated in x = 0.09 samples with thicknesses of 152, 180, 257, 570, and 865 nm. X-ray diffraction-reciprocal space mapping was used to determine the strain, composition, and the nature of defects in each layer. Secondary ion mass spectrometry was used to examine the evolution of the compositional profile. These results indicate that growth beyond the critical thickness results in the spontaneous formation of a relaxed and highly defective 9% Sn layer followed by a low defect 12% Sn secondary layer. We find that this growth method can be used to engineer thick, strain-relaxed, and low defect density layers. Furthermore we utilize this strain-dependent Sn incorporation behavior to achieve Sn compositions of 17.5%. Photoluminesence of these layers produces light emission at 3.1 μm.

Self-organized lattice-matched epitaxy of Si1−xSnx alloys on (001)-oriented Si, Ge, and InP substrates

The crystal growth of single-crystalline Si1−xSnx layers with various Sn contents and analytical comparisons of their fundamental physical properties are strongly desired for next-generation group-IV electronics. In the present study, Si1−xSnx layers with varying Sn contents (1%−40%) were grown on various substrates [(001)-oriented Si, Ge, or InP] by solid-phase epitaxy. Crystallographic and composition analyses indicated that the grown Si1−xSnx layers were nearly lattice-matched to the substrates. When grown on Si, Ge, and InP substrates, the substitutional Sn contents were ∼1%, ∼20%, and ∼40%, respectively. Hard X-ray photoelectron spectroscopy revealed a valence-band offset resulting from the Sn substitution. The offset exhibited an upward-bowing tendency when plotted against the Sn content. The Si0.78Sn0.22/n-type Ge junction displayed rectifying diode characteristics with the ideality factor of 1.2.

In situ phosphorus-doped Ge1−xSnx layers grown using low-temperature metal-organic chemical vapor deposition

We report the metal-organic chemical vapor deposition method for epitaxy of n+-Ge and Ge1−xSnx layer with an electron concentration as high as 2 × 1019 cm−3 by in situ phosphorus (P) doping. In this study, we examined MO precursors of tertiary-butyl-germane (t-BGe), tri-butyl-vinyl-tin (TBVSn), and two kinds of P precursors of tri-ethyl-phosphine (TEP) and tertiary-butyl-phosphine (t-BP). We have investigated crystalline and electrical characteristics of P-doped Ge and Ge1−xSnx layers. In the case of using TEP, the P-doped Ge0.98Sn0.02 epitaxial layer grown at 320 °C on virtual Ge substrate was demonstrated, in which the chemical P-incorporation as high as 1 × 1019 cm−3 and the full electrical activation of P. By using t-BP instead of TEP, the chemical P-incorporation as high as 8.1 × 1019 cm−3 was achieved for Ge epitaxial layer grown at 300 °C. It is found that the activation energy of the growth rate of Ge and P with t-BGe, TEP, and t-BP was estimated to be 1.0−1.2, 2.1, and 1.1 eV, respectively. The precursor combination of t-BGe and t-BP has a wider temperature window for a lower growth temperature. In situ P-doping with t-BP enables us to grow the Ge0.975Sn0.025 epitaxial layer with an electron concentration as high as 1.7 × 1019 cm−3 at the temperature as low as 300 °C. For both cases of TEP and t-BP, we observed that increasing Sn content significantly reduces the P-incorporation, which is not common phenomenon in a conventional CVD growth.

Control of Ge1−x−ySixSny layer lattice constant for energy band alignment in Ge1−xSnx/Ge1−x−ySixSny heterostructures

The energy band alignment of Ge1−xSnx/Ge1−x−ySixSny heterostructures was investigated, and control of the valence band offset at the Ge1−xSnx/Ge1−x−ySixSny heterointerface was achieved by controlling the Si and Sn contents in the Ge1−x−ySixSny layer. The valence band offset in the Ge0.902Sn0.098/Ge0.41Si0.50Sn0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge1−x−ySixSny layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge0.64Si0.21Sn0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge1−xSnx layer on a strain-relaxed Ge1−x−ySixSny layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.