N-type Ge Research Articles

Development of the Ge-MDST instrument structure with an induced p-type channel

The conditions for the growth of n-type Ge conduction layers by the HW CVD method with the parameters required to create a Ge-TIR transistor with an induced p-type channel are determined. The conditions of deposition by electron beam deposition and subsequent annealing of layers of a high-k dielectric ZrO2:Y2O3 are optimized, allowing to achieve a leakage current of 5 × 10–6 A/cm2. For the developed instrument structure, some parameters of the Ge-TIR transistor were calculated, such as the channel length, the maximum voltage between the drain and the source, and the breakdown voltage.

Fabrication of penetrating pores in epitaxial Ge-on-Si through preferential etching along threading dislocations

Epitaxial Ge-on-Si possesses a high density of threading dislocations (TDs) due to the lattice mismatch and difference in thermal expansion coefficient. By employing the lattice distortion at the TDs, we demonstrate that penetrating pores along TDs can be formed in both p- and n-type heteroepitaxial Ge layers through a preferential etching. It has been found that the preferential etching at TD sites takes place in the porosification process of Ge-on-Si samples and is independent of the doping type and concentration. The penetrating pores follow the path of the TD lines and can penetrate the entire Ge layer of 1.3 μm to further porosificate the Si substrate. The effects of anodic current density and total etching duration have been thoroughly investigated on forming penetrating pores at TD sites. The dissolution mechanism in the porosification process has been revealed by dissolution valence calculation and recorded potential curves. Our findings shed light on Ge perforation in both p- and n-type Ge-on-Si and show great potential in Si-based integrated photonics and microelectronics.

Strain-dependent grain boundary properties of n-type germanium layers

Polycrystalline Ge thin films have attracted considerable attention as potential materials for use in various electronic and optical devices. We recently developed a low-temperature solid-phase crystallization technology for a doped Ge layer and achieved the highest electron mobility in a polycrystalline Ge thin film. In this study, we investigated the effects of strain on the crystalline and electrical properties of n-type polycrystalline Ge layers. By inserting a GeOx interlayer directly under Ge and selecting substrates with different coefficients of thermal expansion, we modulated the strain in the polycrystalline Ge layer, ranging from approximately 0.6% (tensile) to − 0.8% (compressive). Compressive strain enlarged the grain size to 12 µm, but decreased the electron mobility. The temperature dependence of the electron mobility clarified that changes in the potential barrier height of the grain boundary caused this behavior. Furthermore, we revealed that the behavior of the grain boundary barrier height with respect to strain is opposite for the n- and p-types. This result strongly suggests that this phenomenon is due to the piezoelectric effect. These discoveries will provide guidelines for improving the performance of Ge devices and useful physical knowledge of various polycrystalline semiconductor thin films.

Charge transport in n-type As- and Sb-hyperdoped Ge

This paper presents a systematic study of the charge transport behavior of heavily doped n-type Ge layers with As and Sb. A nonequilibrium method ion implantation followed by milliseconds flash lamp annealing is applied to synthesize the n++ Ge layers (Ge:As and Ge:Sb). The resulting materials contain free electrons with a density above 3 × 1019 cm−3 and mobility more than 220 cm2/(V s). Quantum corrections to the conductance in a magnetic field are observed at low temperatures. Weak localization persists up to 30 K in Ge:Sb, while only up to 10 K in Ge:As. Using the Hikami–Larkin–Nagaoka model to fit the magnetoconductance data, we obtain the phase coherence length lϕ of the hyperdoped Ge samples in the range of 70–163 nm. This study may pave a way to explore possible applications for quantum technologies utilizing As- and Sb-hyperdoped Ge.

Multiple conduction bands with strong valley anisotropy yielding ultrahigh thermoelectric power factors in n-type elemental Ge

Materials with high power factors are of great importance for thermoelectric power generation. However, currently most state-of-the-art thermoelectric compounds with excellent ZT values mainly benefit from thermal conductivity reduction. Herein, we report an ultrahigh power factor of 64.8 μWcm−1K−2 at 673 K in phosphorus-doped n-type elemental Ge. This is achieved because of the presence of multiple conduction bands with strong valley anisotropy that ensures both large band degeneracy and high electron mobility. By contrast, for the gallium-doped p-type Ge whose electrical properties are governed by one valence band, the optimized power factor is only 26.2 μWcm−1K−2, far less than that of Ge1-xPx. Consequently, a decent ZT of ∼0.5 at 873 K is achieved for n-type elemental Ge. Our work highlights the importance of multi-valley electronic structure and band anisotropy in optimizing thermoelectric performance.

Achieving porous germanium from both p- and n-type epitaxial Ge-on-Si via bipolar potentiostatic etching

Due to the large surface specific area, tunable porosity and refractive index, porous germanium (Ge) has wide applications in optoelectronics, photovoltaics, anode materials in batteries. Electrochemical etching is the most used approach to fabricate porous Ge. However, previous work focused on bulk Ge wafer etching, which is not suitable for device integration on a Si substrate, and illumination had to be used for n-type porous Ge formation. In this paper, we show that by using bipolar potentiostatic etching, porous Ge with the thickness of more than 100 nm and the pore diameter in the range of 10 to 100 nm, can be obtained from both p- and n-type Ge-on-Si layers without illumination. The pore formation mechanism is interpreted by hydrogen evolution effect and Space Charge Region (SCR) model where pore nucleation, growth and dissolution are determined by the total etching time. Our work paves the way for porous Ge applications in Si based integrated photonics and microelectronics.

Reducing specific contact resistivity for n-type germanium using laser activation process and nano-island formation

This study presents a laser activation process (LAP) for germanium (Ge) to improve the electrical performance of n-type Ge devices. The LAP highly activated the dopant and created a shallow junction in Ge. We also investigated a triple contact of titanium (Ti)/nickel (Ni) nano-island/Ge to reduce contact resistivity and enhance the tunneling current. The results showed that the LAP with a fluence of 140 mJ/cm2 effectively activated the dopant, resulting in a high forward current density and a low ideality factor of the n+-p junction diode. The triple contact of Ti/Ni nano-island/Ge showed the lowest specific contact resistivity, indicating an increase in the tunneling current. The Ni nano-island contact showed the best overall electrical performance, attributed to the boosted electric field and the lower density of states at the interface. The results show that combining multiple approaches, including the optimized laser activation process and triple contact formation, can significantly reduce the contact resistance on n-type Ge, providing a promising approach for improving performance.

Consistent optical and electrical determination of carrier concentrations for the accurate modeling of the transport properties of n-type Ge

A consistent methodology is presented to extract carrier concentrations in n-type Ge from measurements of the infrared dielectric function and the Hall effect. In the case of the optical measurements—usually carried out using spectroscopic ellipsometry—the carrier concentration is affected by the doping dependence of the conductivity effective mass, which is computed using a model of the electronic density of states that accounts for non-parabolicity and is fit to electronic structure calculations. Carrier concentrations obtained from Hall measurements require a knowledge of the Hall factor, which is arbitrarily set equal to unit in most practical applications. We have calculated the Hall factor for n-Ge using a model that accounts for scattering with phonons and with ionized impurities.We show that determinations of the carrier concentration n using our computed effective mass and Hall factor virtually eliminates any systematic discrepancy between the two types of measurement. We then use these results to compute majority carrier mobilities from measured resistivity values, to compare with measurements of minority carrier mobilities, and to fit empirical expressions to the doping dependence of the mobilities that can be used to model Ge devices.

High-electron mobility P-doped polycrystalline GeSn layers formed on insulators at low temperatures

Despite its long history, synthesizing n-type polycrystalline Ge layers with high-electron mobility on insulating substrates has been difficult. Based on our recently developed solid-phase crystallization technology, here, we have demonstrated the highest recorded electron mobility (450 cm2 V−1 s−1) for Ge-based polycrystalline thin films on insulating substrates. The underlayer type and small amount of Sn addition were the key parameters controlling both the density and barrier height of the grain boundaries in the P-doped polycrystalline Ge layers. The low growth temperature (≤400 °C) allowed us to develop a GeSn layer on a heat-resistant polyimide film, which exhibited the highest electron mobility (200 cm2 V−1 s−1), as a semiconductor thin film synthesized directly on a flexible substrate. These achievements herald the development of high-performance polycrystalline Ge-based devices on inexpensive glass and flexible plastic substrates.

Modulation of Schottky barrier at metal/Ge contacts by phosphoric acid coating and excimer laser annealing

Germanium is an attractive material for advanced devices such as high speed transistors and high efficiency optical devices. To realize the high performance operation, low resistive contacts to Ge are required. However, the Fermi level in Ge at metal/Ge contacts is often pinned near the valance band edge. This Fermi level pinning generates high Schottky barrier heights at metal/Ge contacts regardless the work function of the metals, which results in low resistive contacts to p-type Ge but high resistive contacts to n-type Ge. Thus, a technique for modulating the Schottky barrier heights at metal/Ge contacts is desired. For this purpose, effects of surface treatments of Ge using phosphoric acid coating and excimer laser annealing (LA) were investigated. After LA without phosphoric acid coating, the reverse current of the Al/n-type Ge contacts is increased by an order of magnitude. On the other hand, the reverse current is significantly increased by three orders of magnitude after LA with phosphoric acid coating. Here, the Schottky barrier heights are decreased from 0.56 to 0.50 eV and from 0.59 to 0.49 eV after LA for samples without and with phosphoric acid coating, respectively. These phenomena are attributed to formation of very thin GeOx surface layers and very shallow doped layers having very high P concentrations, respectively. This technique is useful for modulation of the Fermi level pinning for metal/Ge contacts with the low thermal budget.

N-Type Polycrystalline Germanium Layers Formed by Impurity-Doped Solid-Phase Growth

The carrier mobility of polycrystalline Ge thin-film transistors has significantly improved in recent years, raising hopes for the realization of next-generation electronic devices. Here, we adapted advanced solid-phase crystallization, which achieved the highest hole mobility of the polycrystalline semiconductor layer, to Ge layers doped with n-type impurities (P, As, and Sb). The type and amount of dopants had marked effects on the growth morphology and electrical properties of the Ge layers because they altered the activation energies in crystal growth, dopant activation rates, and grain boundary properties. In particular, P doping was effective in increasing the grain size (25 μm) and lowering the grain boundary barrier height (20 meV), which improved the electron concentration (8.0 × 1018 cm–3) and electron mobility (380 cm2 V–1 s–1) in n-type polycrystalline Ge layers. The electron mobility is greater than that of most semiconductor layers synthesized at low temperatures (≤500 °C) on insulators, and this will pave the way for advanced electronic devices, such as multifunctional displays and three-dimensional large-scale integrated circuits.

A Harmonic Radar Tag With High Detection Range Utilizing Ge FinFETs CMOS Technology

This study fabricated and verified a germanium (Ge) fin field-effect transistor (FinFET) on developed GeSOI platform. The Ge FinFETs were demonstrated for radio-frequency (RF) applications with a harmonic radar tag. The relation between the detection range (Rd), received power (Pr), and threshold voltage (Vth) of a diode was qualitatively discussed. To meet the requirement of a low Vth, two kinds of Ge FinFETs with different metal gates were fabricated and compared. After the evaluation of electrical characteristics of n-type and p-type Ge FinFETs with different fin numbers, a tag for harmonic radar was designed by integrating diode-connected TiN gate 1-fin Ge FinFETs (Vth = 0.1 V) with a high impedance antenna. The RF performance was evaluated at 9.4 GHz and 18.8 GHz. The results indicated a 50 % improvement in the Rd as compared to the tag using a commercial Schottky diode. Therefore, the proposed low-Vth Ge FinFET on developed GeSOI platform for complementary metal–oxide–semiconductor (CMOS) is promising for high-sensitivity harmonic radar applications.

(Digital Presentation) Ge-on-insulator Platform for Mid-infrared Photonic Integrated Circuits

Since mid-infrared (MIR) wavelengths have a great potential for optical communication, sensing, and quantum information, Si-based MIR photonic integrated circuits (PICs) have been developed by leveraging Si photonics technology for near-infrared wavelengths. However, the transparency wavelength window of Si is from 1.2 µm to 8 µm, limiting the available wavelengths in the MIR spectrum. Ge is emerging as a waveguide material to overcome this difficulty because Ge is transparent in the entire MIR spectrum. A Ge-on-Si waveguide is one of the promising platform for a MIR PICs. We have also proposed a Ge-on-insulator (GeOI) platform for MIR integrated photonics [1]. The strong optical confinement in a GeOI waveguide enables an ultracompact MIR PIC. Using wafer bonding and smart-cut, a GeOI wafer was successfully fabricated [2]. As a result, we have demonstrated various Ge passive devices, thermo-optic phase shifters, and modulators on a GeOI platform [3][4].The propagation loss of a GeOI waveguide is one of the issues. The crystal defects induced by hydrogen implantation for smart-cut generates holes in a Ge film. As a result, evening using an n-type Ge donor wafer for smart-cut, a final Ge layer tends to be p-type. Since free-hole absorption in Ge is significant, the propagation loss of a p-type GeOI becomes large. To suppress hole generation, the optimization of the hydrogen implantation conditions was conducted. We found that the higher the implantation energy was, the deeper the center of the defect position was from a Ge surface [5]. When the implantation energy increases from 80 keV to 160 keV, the defects can be removed from a 300-nm-thick Ge device layer, enabling an n-type GeOI wafer. As a result, a low-loss GeOI waveguide with a propagation loss of 2.3 dB/cm was demonstrated.The monolithic integration of Ge passive waveguides and photodetectors (PDs) is also essential for a MIR PIC. We examined the defect-assist photodetection mechanism in a GeOI waveguide with a lateral PIN junction. When a reverse voltage was applied to the PIN junction, the substantial photocurrent was observed at a wavelength of 2 µm for which Ge is transparent. The defect-assist photodetection is expected to be enhanced due to the large intrinsic carrier density in Ge. As a result, the responsivity of 0.25 A/W was obtained at -5 V.In conclusion, we have developed a GeOI photonics platform for MIR wavelengths. We have achieved a low-loss Ge waveguide, PDs, and optical modulators, the indispensable building blocks for large-scale MIR PICs. The functionality of a GeOI platform can be extended with emerging materials including graphene and phase change materials.This work was partly based on the results from the project JPNP13004, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and supported by JST-Mirai Program Grant Number JPMJMI20A1, JSPS KAKENHI Grant Number JP20H02198, and the Canon Foundation.Reference[1] Z. Zhao, C.-M. Lim, C. Ho, K. Sumita, Y. Miyatake, K. Toprasertpong, S. Takagi, and M. Takenaka, “Low-loss Ge waveguide at the 2-µm band on an n-type Ge-on-insulator wafer,” Opt. Mater. Express, vol. 11, no. 12, pp. 4097–4106, 2021.[2] J. Kang, X. Yu, M. Takenaka and S. Takagi, “Impact of thermal annealing on Ge-on-Insulator substrate fabricated by wafer bonding,” Materials Science in Semiconductor Processing, vol. 42, Part 2, pp. 259-263, 2016.[3] M. Takenaka et al., “Heterogeneous CMOS Photonics Based on SiGe/Ge and III–V Semiconductors Integrated on Si Platform,” IEEE J. Sel. Top. Quantum Electron., vol. 23, no. 3, pp. 64–76, May 2017.[4] T. Fujigaki, S. Takagi, and M. Takenaka, “High-efficiency Ge thermo-optic phase shifter on Ge-on-insulator platform,” Opt. Express, vol. 27, no. 5, p. 6451, Mar. 2019.[5] Z. Zhao et al., “Low-loss Ge waveguide at the 2-µm band on an n-type Ge-on-insulator wafer,” Opt. Mater. Express, OME, vol. 11, no. 12, pp. 4097–4106, Dec. 2021.[6] Z. Zhao, C.-P. Ho, K. Toprasertpong, S. Takagi, and M. Takenaka, “Monolithic Germanium PIN Waveguide Photodetector Operating at 2 μm Wavelengths,” Optical Fiber Communication Conference (OFC2020), W4G.3, San Diego, 8–12 March 2020.

Crystallization Of Tensile Strained n-Type Ge By Continuous Wave Laser Annealing

In recent years, germanium (Ge) has attracted a lot of attention for the development of next generation devices due to its higher carrier mobilities compared with silicon (Si) and its compatibility for complementary metal-oxide-semiconductor (CMOS) applications. It is widely known that Ge is an indirect-band semiconductor like Si. However, by introducing tensile strain, the 136 meV difference between direct and indirect gaps can be reduced. Furthermore, in the case of 0.2-0.3% tensile strained Ge, n-type doping in the order of 1019 cm-3 were expected to be resulted in quasi direct-band light emission around 1550 nm wavelength. [1] The realization of this tensile strained n-Ge is promising for the integration of light sources on next generation Ge-based devices. Here, we focused on n-type Ge deposition using molecular beam epitaxy (MBE) method and have succeeded in realizing highly Sb-doped epitaxial n-Ge films by modulating the deposition temperatures. [2] Moreover, we have recently reported the crystallization of amorphous Ge by high-speed continuous wave laser annealing (CWLA), which more importantly also resulted in the introduction of 0.55-0.62% tensile strain. [3] In this study, we applied the similar annealing technique for crystallization of Sb-doped Ge toward the realization of tensile strained n-type Ge films.Sb-doped poly-crystalline Ge films with high Sb concentration (approximately 1019 cm-3) and thickness of about 100 nm were deposited on quartz substrate by molecular beam deposition at 450ºC substrate temperature. Then, 300 nm of SiO2 capping layer was deposited by sputtering in room temperature, such that the sample structure become that shown in Fig.1(a). The samples then annealed in the CWLA system equipped with Nd:YVO4 solid state laser with wavelength of 532 nm as the light source, which scan laser light at the speed (vscan) of 800 m/min. Also, the laser is focused to 20 μm diameter and shifted at 5 μm to scan the samples. Here, the laser power (Elaser) was changed from 300 to 1000 mW. After laser annealing, the capping layer were removed then the samples were characterized by micro-Raman spectroscopy.The annealed sample surface shows brighter contrast compared to the as-deposited samples, as shown in Fig.1(b) and (c), which indicate the structural change of Ge layer. Raman spectra measured at surface of annealed Ge layers are summarized in Fig.2. Here, Ge-Ge peak around 300.2 cm-1 confirmed on the samples, indicating the crystallization of the Ge layers. This peak is not shifted in low Elaser but largely shifted in high Elaser at up to -2.4 cm-1 for undoped and -3.6 cm-1 for the Sb-doped Ge films. This difference could be explained by the liquid phase recrystallization after Ge layer melted in high Elaser (≥600 mW). During recrystallization process, tensile strain is accumulated upon cooling as the tensile strain amount (0.43-0.63%) is agree with the thermal expansion value of Ge between solidification temperature and room temperature (-0.60%). This mechanism is similar to the crystallization of amorphous Ge films on quartz substrates in our previous work. [3] These results show the successful crystallization of tensile strained n-Ge layer by the CWLA method. This finding will be useful for the growth of Ge-based films and a promising step toward the development of CMOS-integrated optoelectronics. Further investigation of the annealing conditions and its relations with the properties of Sb-doped Ge films will be discussed in the main presentation. References Liu, J., Kimerling, L. C. & Michel, J. Semicond. Sci. Technol. 27, (2012).Saputro, R.H., Matsumura, R. & Fukata, N. Cryst. Growth & Des. 21, 6523–6528 (2021).Matsumura, R. & Fukata, N. ECS J. Solid State Sci. Technol. 9, 063002 (2020). Figure 1

Temperature and doping-dependent interplay between the direct and indirect optical response in buffer-mediated epitaxial germanium

The structural and optical properties of buffer mediated epitaxial germanium (Ge) layer were investigated and compared with bulk n-type and p-type Ge substrates. An interconnected dual-chamber molecular beam epitaxy (MBE) system was used to grow a 280 nm thin Ge epilayer on (100)GaAs substrate with an intermediate AlAs buffer layer. The lattice-matched, abrupt Ge/AlAs heterointerface was analyzed using cross-sectional transmission electron microscopic analysis, and no elemental interdiffusion was detected via secondary ion mass spectrometry. A strong direct gap transition, compared to the indirect gap transition, and a series of phonon-assisted transitions was observed by photoluminescence (PL) spectroscopy. In addition, the intensity of the direct gap recombination decreases with decreasing PL measurement temperatures, which was ascribed to the reduced density of Γ-valley electrons available for recombination at lower temperature. Furthermore, the intensity ratio between the direct and indirect optical transition drastically decreases with decreasing temperature in both n-type epitaxial and p-type bulk Ge. An empirical relation in both direct and indirect peak position with temperature was established. The observed strong luminescence in 280 nm thick epitaxial Ge at room temperature is vital for Ge-based photonic devices. In addition, the quality of the epitaxial Ge layer grown via MBE is on par with bulk Ge substrates.

Crystallization Of Tensile Strained n-Type Ge By Continuous Wave Laser Annealing

Germanium has attracted a lot of attention for the development of next generation electronic devices. Moreover, the small difference of band gap at Γ-point and L-point gives interesting properties for optical applications as well. Introducing tensile strain to a Ge film was known to reduce those band gap difference, although it is very challenging to transform Ge into direct-band semiconductor. In this research, we investigated the growth of tensile strained n-type Ge films by continuous wave laser annealing, which is promising approach to realize quasi direct-band recombination.

Annealing induced carrier activation and PL enhancement in Ge-on-Si grown by low temperature MBE

A series of phosphorus-doped Ge-on-Si were grown by molecular beam epitaxy and then annealed at different temperatures to investigate dopant activation and diffusion. The X-ray diffraction and reciprocal space mapping results show that an in-plane tensile strain up to 0.26% was introduced by annealing. Hall effect measurement results revealed that annealing can effectively activate the dopants, and 700 °C is an optimized annealing temperature. High mobility of 300–1000 cm2/V·s for n-type Ge is achieved at room temperature. Secondary ion mass spectrum profile shows that the diffusion coefficient for these P-doped Ge samples is on the order of 10-14 cm2/s after annealing at 700 °C. Significant photoluminescence enhancement in the samples annealed at 700 °C was observed and is attributed to the crystalline quality improvement, residual tensile strain and n+ doping.

Single-Walled Carbon Nanotube-Germanium Heterojunction for High-Performance Near-Infrared Photodetector.

In this research, we report on a high-performance near-infrared (near-IR) photodetector based on single-walled carbon nanotube-germanium (SWCNT-Ge) heterojunction by assembling SWCNT films onto n-type Ge substrate with ozone treatment. The ozone doping enhances the conductivity of carbon nanotube films and the formed interfacial oxide layer (GeOx) suppresses the leakage current and carriers’ recombination. The responsivity and detectivity in the near-IR region are estimated to be 362 mA W−1 and 7.22 × 1011 cm Hz1/2 W−1, respectively, which are three times the value of the untreated device. Moreover, a rapid response time of ~11 μs is obtained simultaneously. These results suggest that the simple SWCNT-Ge structure and ozone treatment method might be utilized to fabricate high-performance and low-cost near-IR photodetectors.

Spatial-dependence cross-examination method of the Seebeck effect applied to Ge surface

In this article, a spatial-dependent evaluation method is presented in order to visualize the changing behavior of several parameters such as temperature and electrical potential in semiconductor materials. If well-known experimental standard methods enable solid measurements and results, space-dependent simulations enable a good step-by-step follow-up of the mechanisms. The original combination of three complementary methods (analytical, numerical and experimental) is presented here as a whole package to extract the Seebeck coefficient of Germanium for small temperature variations above room temperature. An experimental negative temperature effect on the Seebeck coefficient for n-type Ge was successfully interpreted by the influence of minority carriers at low doping level, showing by the way some limitations of the analytical and numerical model.

Distribution of charge carrier concentrations in epitaxial Ge and GeSn layers grown on n-=SUP=-+-=/SUP=--Si(001) substrates

Heteroepitaxial Ge or Ge1-xSnx layers were grown by hot-wire chemical vapor deposition on Si(001) substrates doped heavily with a donor (As or Sb) impurity. The same layers were also grown on high-resistance Si(001) substrates for comparison. The depth profiles of carrier concentration were measured in both types of layers using the capacitance-voltage method, and carrier mobilities were measured additionally by the Hall effect method in layers on high-resistance silicon. It was found that the layers grown on high-resistance substrates were p-type, while the layers grown in the same regimes on heavily doped substrates were n-type with electron concentration n=(4-9)·1016 cm-3 in Ge layers and n=(2-4)·1017 cm-3 in GeSn layers. It was established experimentally and theoretically that the effect of autodoping of Ge and GeSn layers is lacking in the hot-wire chemical vapor deposition method. In our view, the growth of n-type Ge and GeSn layers on n+-Si(001) substrates doped heavily with a donor (As or Sb) impurity is associated with the segregation of this impurity in the process of growth of a buffer Si layer and its subsequent incorporation into growing Ge or GeSn layers. Keywords: epitaxy, doping, Ge, Si, Sn, concentration.