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[Rahmat Hadi Saputro](https://orcid.org/0000-0001-7035-1680), Tatsuro Maeda, Kaoru Toko, [Ryo Matsumura](https://orcid.org/0000-0003-2303-4978), [Naoki Fukata](https://orcid.org/0000-0002-0986-8485)

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This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Electronic Materials, copyright © 2024 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsaelm.4c00399[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[High Doping Activation (≥1020 cm−3) in Tensile-Strained n‑Ge Alloys Achieved by High-Speed Continuous-Wave Laser Annealing](https://mdr.nims.go.jp/datasets/43d0a744-8aad-42cd-a4cf-b4f83aa8ab2c)

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High doping activation (≥1020 cm–3) in tensile-strained n-Ge alloys achieved by high-speed continuous wave laser annealingRahmat Hadi Saputro1, 2, Tatsuro Maeda3, Kaoru Toko2, Ryo Matsumura1,*, Naoki Fukata1, 2,*.1 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2 Graduate School of Science and Technology, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan3 National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8560, Japan* Corresponding authorsEmails: MATSUMURA.Ryo@nims.go.jp, FUKATA.Naoki@nims.go.jpABSTRACTGermanium-based materials are essential for the integration of Group IV optoelectronics on silicon devices. In addition to tensile strain, high n-type doping is critical, as it provides abundant carriers for recombination, potentially enabling higher photoemissions from Ge-based materials. We report here record-high 68% doping activation on n-Ge with ≥1020 cm–3 carrier density. This study centers on Sb-doped n-type Ge-on-insulator thin films with Si or Sn alloying, grown using high-speed continuous wave laser annealing (CWLA). Crystal mapping revealed the growth of polycrystalline n-GeSn and n-GeSi thin films with grain sizes up to 4 μm in diameter. Micro-PL measurements showed the PL intensity of n-Ge to be enhanced by the alloying of Sn and Si, with peak intensity 1.5 and 3 times higher for n-GeSn and n-GeSi, respectively. Raman peak redshift and broadening are observed in the samples, indicating high tensile strain and n-type doping. The measured carrier density of CWLA-grown films aligns well with the PL intensity trend, suggesting the process to have promise for achieving electrically improved Ge-based thin films. Keywords: germanium, thin film, polycrystalline growth, laser annealing, impurity doping 1. INTRODUCTIONThe exploration of Group IV-based photonics aims to create integrated data communication and processing on a single chip, offering energy-efficient solutions for applications such as data centers and telecommunications. The unique properties of Ge make it well suited for these optoelectronic components that employ near-infrared wavelengths.1–5 One of the main challenges in Ge-based optoelectronic materials is that of enhancing light emission efficiency. The indirect bandgap nature of Ge historically limits its efficacy as a light emitter. Incorporating tensile strain and n-type doping in Ge holds promise for quasi-direct emission by modulating the bandgap and increasing carriers for radiative recombination.6–9 However, the problem is that n-type Ge formation is somewhat more challenging than p-type Ge formation due to its tendency to become p-type with point-defect assisted hole-conduction and limited solubility of Group V elements in Ge.10,11 There have been several attempts to grow highly n-doped Ge thin films using ion implantation12, solid-phase crystallization (SPC)13, flash lamp annealing (FLA)14, and pulse laser annealing (PLA)15. Several groups have studied the growth of Ge-based thin film on insulators for this purpose, with liquid-phase crystallization (LPC) being a promising strategy for increasing process throughput. LPC by rapid thermal annealing (RTA) in several seconds has been reported, but shows limited incorporation.16 On the other hand, the nanosecond-order process by PLA limits the crystal grain size to several nanometers.15,17 Therefore, microsecond annealing time appears to be effective for improving doping during LPC and achieving a large-grained Ge-on-insulator. We have recently developed a high-speed continuous-wave laser annealing (CWLA) method for improving the crystallization of GeSn18, SiGe19, and Ge20 thin films on insulators. The high-speed CWLA is the process of laser irradiation, melting, then re-crystallization in a microsecond time. The process involves film melting that caused by the rise of temperature induced by laser irradiation. This approach has also successfully enhanced the photoluminescence of polycrystalline Ge on insulators by combining both tensile strain (about 0.5%) and n-type doping.21 The results of these studies suggest improved doping activation on tensile-strained Ge to be the key to achieving enhanced photoemission of Ge-based materials. It has been reported that co-doping techniques, such as the use of phosphorus (P) with antimony (Sb)22 or P with nitrogen (N)23, increase the n-type doping density in Ge. Higher doping is expected to increase the probability of carrier recombination, consequently enhancing optical activity. Our idea to improve radiative emission is to utilize high-speed CWLA on Sb-doped Ge on an insulating substrate, which has been proven to induce crystallization with tensile strain and n-type doping.21 In this study, we describe the effects of alloying with two other Group IV elements (Sn or Si). The addition of Sn or Si is expected to improve the doping activation through the alloying of Ge with different atomic size element. We focus on investigating the effect of alloying on the crystallization process and the improvement of doping activation to enhance the optical activity of Ge-based thin films on insulators.2. EXPERIMENTAL METHODSSample preparationThe quartz substrates (thickness 500 μm) were coated with a 50-nm silicon nitride (SiNx) underlayer by sputtering a Si3N4 target at room temperature. Sb-doped Ge thin films with thickness of 100 nm were then grown by molecular beam deposition at 450 ˚C. The Sb Knudsen cell was fixed at 350 ˚C for all samples. This deposition method was chosen for its advantageous in precisely controlling growth parameters such as temperature and the composition of elements involved in the growth.27 For the samples with added Si or Sn, the deposition rate was modulated at the ratio of X:Ge = 1:9, where X is Si or Sn, by adjusting the Knudsen cell temperature. After the Ge-based thin film depositions, 50 nm of SiNx top interface layers and a thick SiO2 capping layer (about 1 μm) were deposited by sputtering at room temperature to suppress the ablation and aggregation of liquid Ge during annealing.21 After deposition, the samples were mounted in the CW laser annealing chamber with N2 ambient (O2 < 100 ppm). A Nd:YVO4 solid-state laser with a wavelength of 532 nm was used with this system. The annealing was performed by moving the stage such that the laser scanned the sample at a high vscan speed of 13 - 15 m/s (corresponding to about 1.3 - 1.5 μs annealing time) while also shifting the laser horizontally in 5-μm steps to scan the whole surface, as shown in Figure 1(a) below. The laser beam has a Gaussian profile with diameter of approximately 20 μm. The laser scan overlapped to expect a uniform heat distribution during annealing. CWLA was performed at various laser powers (Elaser) to induce crystallization. After the CWLA process, the capping layers were carefully removed in a dilute hydrofluoric acid solution. Material characterization The chemical composition of the thin films was characterized by SIMS (for the Sb dopant) and AES (for Ge, Si, and Sn). A scanning electron microscope equipped with an EBSD attachment was used for crystal characterization (acceleration voltage: 15 kV). The PL spectra were acquired with a micro-PL system equipped with an InGaAs detector. Photoexcitation was induced by a 647-nm laser, focused on the sample surface at a diameter of approximately 1 μm. The PL spectra were carefully verified by varying the excitation power. All the spectra were obtained under the same conditions. Excitation at 20 mW was chosen to prevent local heating. The carrier concentrations were obtained by Hall effect measurements using the 4-probe Van der Pauw method and measured at a magnetic field of 0.5 T. The samples were isolated into a clover-leaf pattern prior to measurement, to suppress current leakage. The data for each sample was averaged over five measurements. Micro-Raman spectroscopy was also performed to investigate the in-plane strain effect using an excitation wavelength of 532 nm. The light was focused via a 100× objective lens, and the absolute Raman shift was calibrated using the optical phonon (300 cm–1) of a single-crystal Ge (100) substrate.Figure 1. (a) Schematic diagram of sample preparation by molecular beam deposition and continuous-wave laser annealing. (b) Depth profiling of the Sb concentration on as-deposited samples obtained by SIMS measurement. (c) Top-view optical microscope image of the annealed film with low-energy annealing (1.0 W) and (d) ablation effect with high-energy annealing (3.2 W).3. RESULTS AND DISCUSSIONFigure 1(a) illustrates the experimental procedure for CWLA processing of an as-deposited Ge-based thin film. Three samples were prepared for laser annealing: n-Ge with and without Si or Sn alloying. A sandwiched SiNx interface was employed due to its advantages over the SiO2 interface in enabling large-grained polycrystalline films.21,24 For n-type Ge growth, initial dopant atomic concentration is a crucial parameter. It is also known that Sb tends to segregate toward the film surface during molecular beam deposition.25,26 We therefore investigated the dopant distribution of all the samples in the depth direction. Figure 1(b) shows the Sb profile in Ge films deposited at 450 ˚C with constant Sb flux from the K-cell maintained at 350 ˚C. Here, the Sb atoms are uniformly distributed throughout the as-deposited Ge-based films, and the dopant concentration was observed to be at a similar level in all samples. The absence of Sb segregation is possibly attributable to the suppression of dopant re-evaporation due to the optimized deposition and K-cell temperatures, similar to epitaxially-grown films in p-Ge(100) substrate.27 On the other hand, the intended ratio of Si (or Sn) over Ge at 1:9 fluctuates throughout the films (Supporting Figure S1). The average chemical compositions, estimated from AES measurements, are summarized in Table 1. The value was extracted up to a depth of 80 nm to avoid inaccuracy at the interface.Table 1. Chemical composition of the as-deposited films Sample x [Sb] (x1020 cm–3) n-Ge1-xSix 0.13 3.2 n-Ge1-xSnx 0.09 3.9 n-Ge - 3.8Subsequently, these samples were CW laser-annealed at various powers to induce crystallization. The effect of laser annealing with Elaser of 1 W changed the contrast of the film’s surface, as shown in Figure 1(c), giving evidence of structural changes in the film after annealing. The laser was scanned on these samples from the top to the bottom of the indicated region and shifted horizontally. Increasing laser power may induce better crystallization, but the melted Ge commonly shows strong aggregation during liquid-phase crystallization.18–21 Figure 1(d) shows exposed substrate on more than half of the annealed area due to the melted film that breaks up into islands under high laser power (Elaser: 3.2 W). All the prepared samples underwent a similar phenomenon, which limits the annealing window for crystallization at high laser power. It is therefore important to investigate the crystallinity of samples annealed within acceptable laser power.Crystal mapping analysis of the Ge-based thin film was performed to evaluate its crystallinity. The inverse pole figures (IPF) obtained by EBSD analysis, shown in Figures 2(a-d) for n-SiGe samples and Figures 2(e-h) for n-GeSn samples, show the growth of polycrystalline Ge-based thin film alloys using the CWLA process. Here, the as-deposited samples are considered as polycrystalline despite the tiny grain size (< 0.2 µm) because the depositions were performed under high temperature (450°C), which may induce crystallization. The randomly occurring crystal orientation resulted from spontaneous nucleation induced by laser power. This growth behavior is commonly observed in crystal growth on insulating substrates.13,21,28–30 In these figures, the crystal grain boundary is shown as black lines. It also found that the grain distribution follows the laser path, as indicated by the presence of amorphous lines between the laser scans at lower powers (Figure 2(b) & 2(f)). The Gaussian distribution of the laser spot caused this non-uniform grain distribution at lower power; however, the overlapping laser spot at higher power successfully suppressed these amorphous lines (Figures 2(d) & 2(h)). More importantly, the crystal grain size of all the samples significantly increased as the laser power was raised, as shown in Figure 2(i). Grain size identification by the TSL OIM analyzer attached to the EBSD equipment involves calculating the area of each segmented grain in the polycrystalline structure and determining an equivalent circular diameter that would have the same area. The average grain size is then determined from these equivalent diameters. It was found that the average grain size saturated at 1.6 - 2 μm before encountering the aggregation problem at higher powers. The addition of Sn or Si into n-Ge gives different effects to its crystal quality. According to the phase diagram, while Si exhibits complete solid solubility in germanium Ge31, Sn has a low thermal equilibrium solid solubility of approximately 1% in Ge32. For the n-GeSn samples, the significant decreases in average grain size at Elaser> 2200 mW is likely caused by Sn precipitation in higher annealing energy. In this study, large crystal grains with diameters up to 4 μm were obtained on n-GeSn using the CWLA method, demonstrating its advantages over the pulse laser annealing (PLA) method (up to 0.8 μm)15,33. The larger grain size can be attributable to the longer annealing time, in the order of microseconds when using CWLA, in contrast to the much shorter nanosecond times using PLA. Unlike PLA that involves rapid annealing through short bursts of high-energy pulses, CWLA enables longer annealing times due to the continuous delivery of laser energy, which is advantageous for obtaining crystal growth time.  Regarding the applications of  poly-Ge, it has been reported that the decrease in carrier mobility due to grain boundaries is relatively small, attributed to the lower grain boundary barrier of Ge compared to Si.28 And there are indeed examples of highly mobile device operation reported for poly-Ge MOSFETs.14 Also, the device size will be scaled down to sub-micrometer size to be utilized in on-chip optical devices.Figure 2. (a)-(d) Sets of 70°-tilted SEM images and inverse pole figures (IPF) of the n-GeSi films before and after CWLA process at Elaser: 2400-2800 mW. Direction of the laser scan and shift are indicated by arrows. (e)-(h) SEM and IPF of n-GeSn films with CWLA at Elaser: 1800 - 2200 mW. (i) Average crystal grain size of the CWLA-grown films as a function of Elaser. Inset image indicates the crystal orientation of (a)-(h). Dotted line indicates measurement resolution.The optical activity of the CWLA-grown Ge-based alloys was investigated by means of micro-PL measurements. Figure 3(a) shows room-temperature PL spectra obtained from the large-grained (1.6 - 2 μm) polycrystalline samples. As expected, broad PL peaks were obtained in all the samples due to their polycrystalline structure. We consider the obtained PL spectrum not as a single peak, but as an overlay of multiple peaks attributed to emissions arising from the coupling of electrons in the Γ-valley and L-valley, and heavy-holes (HH) and light-holes (LH) in the valence band. The sudden drop in PL intensity at about 0.57 eV appears to be the InGaAs detector’s cut-off point. The CWLA-grown n-Ge was found to exhibit higher PL intensity than the as-deposited n-Ge due to improved crystal quality. Moreover, the PL intensity of CWLA-grown n-Ge was enhanced by the alloying of Sn and Si, at about 1.5 times higher for CWLA-grown n-GeSn, followed by n-GeSi at about three times higher, making it the highest among the samples. Without neglecting the possibility of non-radiative recombination by high doping, the results show the dominance of radiative recombination that might indicate the emission through quasi-direct bandgap mechanism. CWLA dependence on PL intensity was also observed in the Ge-based alloys shown in Figure 3(b) for n-GeSn and in Figure 3(c) for n-GeSi. The rising trend of PL intensity with increased Elaser is possibly attributed to the increase of carrier concentration that caused by the improved crystal quality, as previously shown in Figure 2. In Figure 3(c), the notable increase at 2400 mW is most likely related to the threshold where quasi-Fermi energy level surpasses the Γ-valley energy level, allowing carrier injection for direct band transition. The PL spectra were measured at the same 20 mW of excitation power. Low excitation power was chosen to prevent structural damage to the thin films, since measurements at an excitation power of 30 mW or higher leave a dark mark on the sample’s surface, caused by sample burn due to local heating. Excitation power dependence of the PL intensity (Supporting Figure S2) was confirmed in all samples at up to 20 mW of excitation power. According to our previous study,21 the PL peak analysis of CWLA-grown n-Ge indicating the dominant indirect band transition on the PL intensity and the peak shift toward lower energy is attributable to high tensile strain (about 0.5%). In our experiments, the peak position of the CWLA-grown n-GeSi was obtained at 0.68 eV. Here, we consider that the apparent peak shifts are not attributed to actual shift of peaks, but rather due to changes in the intensity ratios of peak components. Specifically considering the emissions arising from the coupling of electrons in the Γ-valley and L-valley, and heavy- (HH) and light-holes (LH) in the valence band. Meanwhile, the PL peak position on the n-GeSn is highly influenced by Sn incorporation.5,34 The peak position, obtained at about 0.66 eV, corresponds to 6 - 7% Sn incorporation, which is reasonable compared to the initial 9% Sn incorporation, shown above in Table 1. Figure 3. (a) Photoluminescence (PL) spectra of the crystallized n-GeSi, n-GeSn, and n-Ge. PL spectra of as-deposited n-Ge are also shown for comparison. (b) PL intensity of n-GeSn and (c) n-GeSi as a function of Elaser. The dotted line indicates the PL intensity of as-deposited film.Figure 4. Normalized Raman spectra of the crystallized n-GeSi, n-GeSn, and n-Ge films. The numbers indicate the peak position of the corresponding spectrum. The inset shows the wide-range Raman spectra of the n-GeSi samples.The normalized Raman spectra of CWLA-grown samples are summarized in Figure 4. Here, a single peak around the Ge-Ge phonon mode is observed in the n-Ge and n-GeSn samples, while the n-GeSi also exhibited peaks attributable to the Si-Ge phonon mode, as shown in the inset. The Ge-Ge peak is shifted toward a lower wavenumber in all the samples, indicating Sn or Si incorporation in the grown films. It is known that Raman shifts can significantly change not only due to strain, but also due to doping concentration and alloying. In this study, the tensile strain in the thin films is imposed by the difference in thermal expansion’s coefficient with the substrate, suggesting that alloying by approximately 9-13% would not greatly alter the amount of strain. The Raman shift of n-Ge in this study was observed at a similar position (295.5 cm–1) to our previously reported data21. The previous study compares the Raman shift of undoped i-Ge and Sb-doped n-Ge. The i-Ge sample exhibited peak position at 297 cm-1, which corresponds to about 0.56% of high tensile strain, and the excess shift on the n-Ge sample down to 295.5 cm-1 indicating the effect of doping incorporation. Moreover, asymmetric broadening at lower wavenumbers is also prominent in the n-GeSi sample. This asymmetric broadening is a signature of the Fano effect.35 This occurs due to interference between a discrete state (optical phonon) and continuum states (inter-band excitation), indicating the presence of free electrons from the electrically activated Sb atoms. Generally, it is possible to estimate the electrically active doping concentration using the Fano equation with nanostructured Ge36,37 or polycrystalline Si38. However, with the incorporation of Si and tensile strain on the films, there are many unknown parameters that will need to be uncovered and taken into account for calculating the doping concentration in these samples. Nevertheless, peak broadening is consistent with the PL result, which may indicate that the enhanced photoluminescence is related to higher carrier concentrations. Figure 5. (a) Fabrication process of the cloverleaf-shaped thin films for the Hall effect measurement. (b) Carrier density of the crystallized n-GeSi, n-GeSn, and n-Ge before and after the CWLA process. (c) Carrier density of the n-GeSi as a function of Elaser. The dotted line indicates the initial Sb dopant level. Inset image is a top-view microscopic image of the cloverleaf-shaped film. (d) Benchmarking of the obtained doping activation rate.Finally, the electrical properties of the Ge-based alloy films were measured by Hall effect measurements using the Van der Pauw method, in which the current, voltage, and magnetic field were measured to extract Hall coefficient and carrier concentration. The samples are patterned into cloverleaf-shaped structures to isolate the measurement area, as shown in Figure 5(a). This isolation was implemented to ensure the reliability of the measurement results by suppressing current leakage. Figure 5(b) shows the measured highest carrier density of CWLA-grown samples compared to the as-deposited result for each sample. The contrast in carrier density of as-deposited n-GeSn is likely caused by the crystal quality, in which the poor quality of as-deposited state might be resulted from phenomena such as Sn precipitation during deposition. The carrier density of CWLA samples with Sn or Si alloying is consistent with the pattern of PL intensity (Figure 3(a)). The higher electron density increases the likelihood of electron-hole pairs recombining, significantly enhancing photoluminescent emissions. Furthermore, the Elaser dependence of carrier density on n-GeSi samples is obtained, as shown in Figure 5(c). This trend might also explain the Elaser dependence on PL intensity as previously shown in Figure 3(c). In a general case, in which the film is relaxed and non-highly doped, a GeSi might have lower PL intensity than a GeSn or a Ge films. Here, we reported that in a tensile strained and highly n-doped Ge-based alloys the pronounced PL peak for n-GeSi compared to n-GeSn was also attributable to its higher carrier density.Figure 5(d) summarizes the activated carrier concentration and the initial impurity concentration of n-type Ge-based thin films grown using various methods. Our work with the high-speed CWLA method exhibited the highest carrier concentration among the polycrystalline n-Ge films (at an impurity level of 1020 cm–3). Although the single crystal epitaxial n-Ge films have higher carrier concentration than our polycrystalline n-Ge, a higher carrier concentration was achieved after incorporating Si or Sn. The highest doping activation at up to 68% was achieved by incorporating Si into n-Ge film. The incorporation of Sn into n-Ge also improved the carrier concentration, with comparable results to PLA-grown n-GeSn15, which used a similar Sb-doping and liquid-phase growth mechanism. The high carrier concentration combined with high tensile strain achieved by high-speed CWLA is of potential use for optoelectronic applications of Ge-based thin films.4. CONCLUSIONSWe grew Sb-doped n-type Ge thin films with Si and Sn alloying using high-speed CW laser annealing. Crystal mapping analysis showed growth of polycrystalline Ge-based thin films with an average grain size of 1.6 - 2 μm. Large grain sizes of up to 4 μm in diameter were obtained using the high-power CWLA process. The optical activity of the CWLA-grown films was investigated using micro-PL measurements. The PL intensity of n-Ge was enhanced by the alloying of Sn and Si, with a peak intensity about 1.5 times higher for n-GeSn, followed by n-GeSi at about three times higher. The Raman peak redshift and asymmetric broadening at the lower wavenumbers of the Ge-Ge phonon mode suggest higher doping activation. The measured carrier densities of CWLA samples with Sn or Si alloying are consistent with the patterns of PL intensities, indicating that high carrier concentration increases the likelihood of electron-hole pair recombination and enhances photoluminescence. The incorporation of Si in n-Ge exhibited the highest doping activation (about 68%) among the n-type polycrystalline Ge films. These results suggest that the alloying process shows promise for achieving electrically improved n-type Ge-based thin films for optoelectronic device applications.SUPPORTING INFORMATIONSee supporting information for the chemical composition measured by AES (Figure S1) and excitation power dependence of the PL intensity (Figure S2). (PDF)ACKNOWLEDGMENTSThis work was supported by JSPS Kakenhi (Grants no. JP20K14796 and JP23K13370), the Murata Science Foundation, and the World Premier International Research Center Initiative (WPI-Initiative). A part of this work was also supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM),” MEXT, Japan, proposal number 23NM5001, 23UT1050, and 23UT0030.AUTHOR DECLARATIONSThe authors declare no conflicts of interest associated with this manuscript.REFERENCES(1)  Wirths, S.; Geiger, R.; Von Den Driesch, N.; Mussler, G.; Stoica, T.; Mantl, S.; Ikonic, Z.; Luysberg, M.; Chiussi, S.; Hartmann, J. 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