Nanocrystallized Ge-Rich SiGe-HfO2 Highly Photosensitive in Short-Wave Infrared

Group IV nanocrystals (NCs), in particular from the Si–Ge system, are of high interest for Si photonics applications. Ge-rich SiGe NCs embedded in nanocrystallized HfO2 were obtained by magnetron sputtering deposition followed by rapid thermal annealing at 600 °C for nanostructuring. The complex characterization of morphology and crystalline structure by X-ray diffraction, μ-Raman spectroscopy, and cross-section transmission electron microscopy evidenced the formation of Ge-rich SiGe NCs (3–7 nm diameter) in a matrix of nanocrystallized HfO2. For avoiding the fast diffusion of Ge, the layer containing SiGe NCs was cladded by very thin top and bottom pure HfO2 layers. Nanocrystallized HfO2 with tetragonal/orthorhombic structure was revealed beside the monoclinic phase in both buffer HfO2 and SiGe NCs–HfO2 layers. In the top part, the film is mainly crystallized in the monoclinic phase. High efficiency of the photocurrent was obtained in a broad spectral range of curves of 600–2000 nm at low temperatures. The high-quality SiGe NC/HfO2 matrix interface together with the strain induced in SiGe NCs by nanocrystallization of both HfO2 matrix and SiGe nanoparticles explain the unexpectedly extended photoelectric sensitivity in short-wave infrared up to about 2000 nm that is more than the sensitivity limit for Ge, in spite of the increase of bandgap by well-known quantum confinement effect in SiGe NCs.


Introduction
There is considerable interest in nanostructured materials from the group IV Si-Ge system for photonics applications [1][2][3][4][5][6][7][8][9][10]. However, the most important inconvenience of this system is the low light absorption-emission efficiency of bulk Si-Ge with an indirect band gap that counts against the long-held goal of integrated group IV photonics. This inconvenience can be solved by nanostructuring (quantum confinement) combined with strain [11][12][13] or by exploiting other crystalline structures different from Fd-3m diamond [14,15], such as metastable hexagonal phase [16]. Other solutions are to develop plasmonic structures [17], to fabricate structures with GeSi quantum dots (QDs) embedded in microresonators [18], or by employing hydrogenation technique for passivating detrimental defects [19], all these enabling the enhancement of GeSi and Ge QDs photoluminescence. Another proposed route targeted the host matrix considering its role in the Ge QD growth kinetics and morphology (QDs density and separation distances) and in the light absorption of Ge QDs embedded in matrix, i.e., better Si 3 N 4 instead of SiO 2 [20]. For photodetectors based on nanocrystals (NCs)/QDs from the Si-Ge system, it was shown that the photoresponse is enhanced by NCs formation (and strain) and is size-dependent, the spectral sensitivity limit being controlled by NC size [21][22][23][24]. Additionally, photocurrent enhancement can be further induced by field effects [23,25] or achieved by coating Ge QDs with metal shells [26]. Photoconductive properties can also be boosted, besides quantum confinement effect in Ge NCs/QDs, by exploiting the Ge related defects/traps, most of them being localized states from NC/oxide matrix interface, by trapping photogenerated holes on these defects/traps and thus increasing the lifetime of electrons [27][28][29]. Recently, a phototransistor with Ge QDs decorated on a single Si-nanowire channel integrated on an SOI platform was reported, its spectral response at room temperature ranging from 1200 to 1700 nm [30].
On the other hand, short wave infrared (SWIR) extended spectral photocurrent is ensured by stabilization of Ge-rich alloy SiGe NCs [25] against fast Ge diffusion [31] as we showed for Ge-rich SiGe NCs embedded in TiO 2 . This is achieved by preparing NCs surrounded by SiO 2 thin layers that have a protective role against Ge fast diffusion, resulting in the formation of Ge-rich SiGe NCs. Thus, SWIR spectral sensitivity (cut-off wavelength) was pushed up to 1700 nm for cooled structures (100 K), and up to 1400 nm at room temperature [25], in comparison with Ge NCs embedded in TiO 2 for which a cut-off of 1250 nm was obtained [23]. Density functional theory (DFT) computations of the energy gap of alloy GeSi NCs with high Ge content for finding bandgap diameter dependence for different Ge contents and bigger diameters in agreement to the experimental data can be used for the design and characterization of GeSi NCs-based optical sensors [32]. Based on all these results, recently, we developed an optical sensor system with a photoactive layer of GeSi NCs in the SiO 2 matrix that discriminates between different slippery road conditions, to be mounted on a platform for warning drivers in due time and at sufficient distance [33]. The sensor has 360-1350 nm sensitivity and 10 2 -10 3 signal/noise ratio at room temperature. As shown in the present paper, the SWIR photoresponse can be further improved by using nanocrystalline HfO 2 as the embedding matrix instead of SiO 2 or TiO 2 . HfO 2 is a high-k oxide, highly CMOS compatible, and highly scalable. The nanocrystalline HfO 2 as matrix creates a good separation between SiGe NCs and thus the SiGe NCs/HfO 2 NCs matrix interface is of high quality [34]. Using HfO 2 as a matrix is beneficial as it allows the obtaining of SiGe NCs-HfO 2 films with good NCs surface passivation by HfO 2 oxide matrix.
In this paper, films of Ge-rich SiGe NCs embedded in nanocrystallized HfO 2 were prepared by magnetron sputtering deposition of 3-layers stacks of cap HfO 2 /SiGeHfO 2 active layer/buffer HfO 2 /on Si followed by rapid thermal annealing (RTA). The structures present a cut-off wavelength extended in SWIR of about 2000 nm in the broad 600-2000 nm spectral photocurrent of the structures at low temperatures, due to photocarrier generation in the heterojunction of the embedded Ge-rich SiGe NCs with 0.6% evaluated strain and Si substrate. The cap and buffer are very thin HfO 2 layers hampering the fast diffusion of Ge from the active layer, without blocking the electrical contact to the substrate and top contacts.

Sample Preparation
Photosensitive films of Ge-rich SiGe NCs embedded in nanocrystallized HfO 2 are prepared by magnetron sputtering deposition (Surrey Nanosystems Gamma 1000 equipment, Surrey NanoSystems Ltd., Newhaven, United Kingdom) of 3-layers stack of cap HfO 2 /SiGeHfO 2 active layer/buffer HfO 2 /Si wafer followed by RTA (Annealsys AS-Micro rapid thermal prcessing system, Annealsys, Montpellier, France) for nanostructuring. The (Si 1-x Ge x ) 1-y (HfO 2 ) y layer is obtained by co-deposition from 3 independent plasma controllers of targets of Si (15 W DC power), Ge (10 W DC), and HfO 2 (45 W RF), the estimated volume composition (based on deposition rates) being x = 70% and y = 50%. Buffer and cap HfO 2 layers that are cladding the active SiGe-based layer are deposited by sputtering from the HfO 2 target. Substrates of cleaned p-Si with 7-14 Ωcm resistivity were used, and sputtering was made in Ar atmosphere at 4 mTorr. Buffer HfO 2 and cap HfO 2 layers serve to reduce the Ge loss by fast diffusion from the photoactive SiGe region. After nanocrystallization by RTA at 600 • C for 8 min in Ar (6N purity) atmosphere, SiGe NCs and HfO 2 NCs are formed. The resulting structure has layer thicknesses of 160 nm for the SiGe NCs-HfO 2 photosensitive layer, and 17 and 9 nm for buffer and cap HfO 2 layers, respectively. For photoelectrical measurements, Al coplanar contacts with 5 × 2 mm 2 areas and 2 mm gap between them were deposited by vacuum evaporation on top of annealed structures.
Spectral photoresponsivity was measured by using the lock-in amplifier technique, under illumination with monochromatic light modulated at 120 Hz, in a dedicated set-up (more details in [25]). Second-order blocking was ensured by longwave pass filters (550, 1000, 1150, and 1250 nm cut-on wavelengths). Measurements were performed in both photovoltaic regimes (0 V voltage) and by applying 1 V bias voltage.

XTEM Analysis
The general cross-section view of samples is given in Figure 1a which reveals the 160 nm thick SiGe NCs-HfO 2 photosensitive layer cladded by buffer and cap HfO 2 layers. The HfO 2 /SiGe NCs-HfO 2 /HfO 2 /SiO 2 /on Si structure has a thickness of 9 nm/160 nm/17 nm/2 nm with an error of ±1 nm. HRTEM images in the bottom part of the annealed structure near the interface with the Si substrate (Figure 1b,c) show the nanocrystallization of HfO 2 with tetragonal/orthorhombic structure (0.293-0.295 nm lattice fringes) in both buffer HfO 2 and SiGe NCs-HfO 2 layers. Figure 1c evidences a SiGe NC by the (111) 0.325 nm lattice fringes corresponding to a high Ge atomic composition of 87%. HRTEM image in Figure 1d shows that the SiGe NCs-HfO 2 layer is formed of both SiGe and HfO 2 NCs with sizes from 3 to 7 nm, the average size being 4 nm. The top HfO 2 layer has a monoclinic structure ( Figure 1e). SAED pattern from Figure 1f reveals the cubic structure of SiGe NCs by 0.321 ± 0.002 nm (111) reflections. The presence of tetragonal HfO 2 is also evidenced by the 0.294 (111) reflections [34], while monoclinic HfO 2 structure is shown by 0.284 (111) reflections [35]. If we consider the SAED value of 0.321 ± 0.002 nm, then the Ge concentration of SiGe NCs is 57% ± 13%, close to the atomic Ge concentration of about 50% estimated based on deposition rate calibration.

XRD Investigations
The diffractogram measured on the annealed sample is given in Figure 2. Most intense maxima correspond to different crystallographic planes of the tetragonal T HfO 2 phase. Monoclinic HfO 2 was not detected (according to PDF 01-078-0049, most intense lines are at 28.35 and 31.65 deg). This is in good agreement with HRTEM results, i.e., nanocrystallization of HfO 2 with T structure (0.293-0.295 nm lattice fringes) evidenced along with a great thickness of structure (~177 nm) in both buffer HfO 2 (17 nm thickness) and SiGe NCs-HfO 2 layer (160 nm), while the thin top HfO 2 cap layer (9 nm) has monoclinic structure. This means that the tetragonal T component is dominant in the structure, in good agreement with the global XRD curve.

XRD Investigations
The diffractogram measured on the annealed sample is given in Figure 2. Mo tense maxima correspond to different crystallographic planes of the tetragonal T phase. Monoclinic HfO2 was not detected (according to PDF 01-078-0049, most in lines are at 28.35 and 31.65 deg). This is in good agreement with HRTEM results, i.e., and 88% for the (220) peak. The Ge atomic concentration in SiGe NCs found by XRD and HRTEM analyses, higher than the mean value in the active SiGeHfO2 layer, can be explained by a fast diffusion of Ge than Si to form SiGe NCs. However, the strain induced by HfO2 nanocrystallization can influence the estimation of composition in SiGe NCs. The average size of SiGe NCs calculated by using the full width at half-maximum of the (111) and (220) peaks is 6.1 and 4.9 nm, respectively in good agreement with the HRTEM results (3-7 nm).

Raman Scattering Analysis
Raman spectra measured on the RTA sample with laser excitation wavelengths of 633 and 325 nm are presented in Figure 3a and are compared with the Raman spectrum measured on bulk Si (633 nm excitation). The sample spectra are normalized to the intensity of the Ge-Ge peak, while the Si spectrum is normalized to the intensity of peak corresponding to the 2TA mode of the Si wafer (~301 cm −1 ). The 633 nm spectrum presents the 2TA mode of Si substrate and a shoulder positioned around 291 cm −1 together with a broad band (centre at ~269 cm −1 ) as evidenced by the deconvolution from Figure 3b. The maximum positioned at ~291 cm −1 corresponds to Ge-Ge vibration mode in Ge-rich SiGe NCs, being shifted to lower energy due to the quantum confinement and stress effect. We evaluated the strain in SiGe NCs as being 0.6% [2] by considering a Ge content of 70% in the SiGe NCs. However, two maxima with low intensity corresponding to SiGe 111 and 220 lines of the cubic structure are measured. These SiGe maxima corresponding to SiGe NCs are better evidenced by long-time scans around the SiGe 111 and 220 lines (Figure 2-insets) [36]. Ge concentration x of Si 1−x Ge x NCs evaluated from the maxima positions by linear interpolation between positions corresponding to pure Si and Ge is 95% Ge for the (111) peak and 88% for the (220) peak. The Ge atomic concentration in SiGe NCs found by XRD and HRTEM analyses, higher than the mean value in the active SiGeHfO 2 layer, can be explained by a fast diffusion of Ge than Si to form SiGe NCs. However, the strain induced by HfO 2 nanocrystallization can influence the estimation of composition in SiGe NCs. The average size of SiGe NCs calculated by using the full width at half-maximum of the (111) and (220) peaks is 6.1 and 4.9 nm, respectively in good agreement with the HRTEM results (3-7 nm).

Raman Scattering Analysis
Raman spectra measured on the RTA sample with laser excitation wavelengths of 633 and 325 nm are presented in Figure 3a and are compared with the Raman spectrum measured on bulk Si (633 nm excitation). The sample spectra are normalized to the intensity of the Ge-Ge peak, while the Si spectrum is normalized to the intensity of peak corresponding to the 2TA mode of the Si wafer (~301 cm −1 ). The 633 nm spectrum presents the 2TA mode of Si substrate and a shoulder positioned around 291 cm −1 together with a broad band (centre at~269 cm −1 ) as evidenced by the deconvolution from Figure 3b. The maximum positioned at~291 cm −1 corresponds to Ge-Ge vibration mode in Ge-rich SiGe NCs, being shifted to lower energy due to the quantum confinement and stress effect. We evaluated the strain in SiGe NCs as being 0.6% [2] by considering a Ge content of 70% in the SiGe NCs. The broad band centred at ~269 cm −1 (Figure 3b) is the signature of the SiGe disordered component [25]. The 325 nm spectrum reveals only the contribution of Ge-rich SiGe NCs in nanocrystallized HfO2 matrix due to stronger absorption in the SiGe-containing layer (Electronic Supplementary Information of Ref. [37]).

Spectral Photosensitivity
The spectral photocurrent efficiency measured in the photovoltaic (0 V) regime at 100 and 300 K is illustrated in Figure 4. One can see that at 100 K the samples are sensitive in visible-near-infrared-short wave infrared (VIS-NIR-SWIR) from 600 nm up to about 2000 nm. The photoresponsivity at 100 K extends in SWIR at longer wavelengths than expected for bulk Ge-rich SiGe. This can be explained by strain induced in SiGe NCs that drastically reduces the bandgap, but also by crystalline disorder in SiGe NCs that results in extension of the electronic states distribution into the bandgap. We have to mention that the spectral photocurrent efficiency is slightly increased by only 17% by applying a voltage bias of 1 V in comparison to 0 V bias. As can be seen in Figure 4, the photocurrent spectrum is also shifted to longer wavelengths by increasing the measurement temperature at 300 K, but the efficiency is strongly reduced due to the increase of the photocarrier recombination probability. We showed in previous reports on group IV-based NCs that the spectral photocurrent efficiency is much increased at low temperatures (100-200 K) than at room temperature, and it has a weak dependence on temperature in contrast to the 200-300 K interval for which it strongly decreases with the increase of temperature (e.g., in [38]). The lowering of the efficiency at high temperature reduces the photocurrent below the sensitivity of the used measurement setup for wavelengths longer than 1800 nm. The SWIR photosensitivity is due to the light absorption in the heterojunction region of Ge-rich SiGe NCs separated by a very thin HfO2 buffer layer from the Si substrate [25,39]. The high sensitivity detected in our samples is due to the high-quality interface of SiGe NCs with the nanocrystallized HfO2 matrix. The beneficial role of using nanocrystallized HfO2 matrix we also proved in gate HfO2/floating gate of Ge QDs in HfO2/tunnel HfO2 memory capacitors with high memory performance by obtaining floating gates consisting of a single layer of well-separated Ge QDs in nanocrystallized HfO2, Ge QDs being located at the crossing of the HfO2 NCs boundaries [34]. The broad band centred at~269 cm −1 (Figure 3b) is the signature of the SiGe disordered component [25]. The 325 nm spectrum reveals only the contribution of Ge-rich SiGe NCs in nanocrystallized HfO 2 matrix due to stronger absorption in the SiGe-containing layer (Electronic Supplementary Information of Ref. [37]).

Spectral Photosensitivity
The spectral photocurrent efficiency measured in the photovoltaic (0 V) regime at 100 and 300 K is illustrated in Figure 4. One can see that at 100 K the samples are sensitive in visible-near-infrared-short wave infrared (VIS-NIR-SWIR) from 600 nm up to about 2000 nm. The photoresponsivity at 100 K extends in SWIR at longer wavelengths than expected for bulk Ge-rich SiGe. This can be explained by strain induced in SiGe NCs that drastically reduces the bandgap, but also by crystalline disorder in SiGe NCs that results in extension of the electronic states distribution into the bandgap. We have to mention that the spectral photocurrent efficiency is slightly increased by only 17% by applying a voltage bias of 1 V in comparison to 0 V bias. As can be seen in Figure 4, the photocurrent spectrum is also shifted to longer wavelengths by increasing the measurement temperature at 300 K, but the efficiency is strongly reduced due to the increase of the photocarrier recombination probability. We showed in previous reports on group IV-based NCs that the spectral photocurrent efficiency is much increased at low temperatures (100-200 K) than at room temperature, and it has a weak dependence on temperature in contrast to the 200-300 K interval for which it strongly decreases with the increase of temperature (e.g., in [38]). The lowering of the efficiency at high temperature reduces the photocurrent below the sensitivity of the used measurement setup for wavelengths longer than 1800 nm. The SWIR photosensitivity is due to the light absorption in the heterojunction region of Ge-rich SiGe NCs separated by a very thin HfO 2 buffer layer from the Si substrate [25,39]. The high sensitivity detected in our samples is due to the high-quality interface of SiGe NCs with the nanocrystallized HfO 2 matrix. The beneficial role of using nanocrystallized HfO 2 matrix we also proved in gate HfO 2 /floating gate of Ge QDs in HfO 2 /tunnel HfO 2 memory capacitors with high memory performance by obtaining floating gates consisting of a single layer of well-separated Ge QDs in nanocrystallized HfO 2 , Ge QDs being located at the crossing of the HfO 2 NCs boundaries [34]. The broad spectral photosensitivity of the investigated photosensor based on Ge-rich SiGe NCs can have multiple applications such as monitoring of slippery road conditions, internet of things, and biomedical applications.

Conclusions
We have shown that by embedding Ge-rich SiGe NCs in nanocrystallized HfO2, the 3-layers stack of cap HfO2/SiGe NCs in HfO2 active layer/buffer HfO2/Si wafer has enhanced sensitivity in SWIR. Specific to this 3-layers stack is the nanocrystallized HfO2 with tetragonal/orthorhombic structure that was revealed beside the monoclinic phase in both buffer HfO2 and SiGe NCs-HfO2 layers. The high-efficiency photocurrent with broad 600-2000 nm spectral response in VIS-NIR-SWIR measured at 100 K shows extended cut-off wavelength to 2000 nm. This is explained by the high quality of the SiGe NC/HfO2 interface together with the strain induced in Ge-rich SiGe NCs (3-7 nm diameter and 0.6% evaluated strain) that drastically reduces the bandgap.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest. The broad spectral photosensitivity of the investigated photosensor based on Ge-rich SiGe NCs can have multiple applications such as monitoring of slippery road conditions, internet of things, and biomedical applications.

Conclusions
We have shown that by embedding Ge-rich SiGe NCs in nanocrystallized HfO 2 , the 3-layers stack of cap HfO 2 /SiGe NCs in HfO 2 active layer/buffer HfO 2 /Si wafer has enhanced sensitivity in SWIR. Specific to this 3-layers stack is the nanocrystallized HfO 2 with tetragonal/orthorhombic structure that was revealed beside the monoclinic phase in both buffer HfO 2 and SiGe NCs-HfO 2 layers. The high-efficiency photocurrent with broad 600-2000 nm spectral response in VIS-NIR-SWIR measured at 100 K shows extended cut-off wavelength to 2000 nm. This is explained by the high quality of the SiGe NC/HfO 2 interface together with the strain induced in Ge-rich SiGe NCs (3-7 nm diameter and 0.6% evaluated strain) that drastically reduces the bandgap.