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Abstract
Femtosecond laser hyperdoped silicon, also known as the black silicon (BS), has a large number of defects and damages, which results in unstable and undesirable optical and electronic properties in photonics platform and optoelectronic integrated circuits (OEICs). We propose a novel method that elevates the substrate temperature during the femtosecond laser irradiation and fabricates tellurium (Te) hyperdoped BS photodiodes with high responsivity and low dark current. At 700 K, uniform microstructures with single crystalline were formed in the hyperdoped layer. The velocity of cooling and resolidification is considered as an important role in the formation of a high-quality crystal after irradiation by the femtosecond laser. Because of the high crystallinity and the Te hyperdoping, a photodiode made from BS processed at 700 K has a maximum responsivity of 120.6 A/W at 1120 nm, which is far beyond the previously reported Te-doped silicon photodetectors. In particular, the responsivity of the BS photodiode at 1300 nm and 1550 nm is 43.9 mA/W and 56.8 mA/W with low noise, respectively, which is valuable for optical communication and interconnection. Our result proves that hyperdoping at a high substrate temperature has great potential for femtosecond-laser-induced semiconductor modification, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The silicon photodetector is one of the most critical elements in a silicon photonics platform as an integrated optical receiver due to its low cost, low noise, and great compatibility with the current complementary metal-oxide-semiconductor (CMOS) fabrication processes [1–3]. It is well known that long wavelengths are used for the information transmission in silicon photonic systems in optical communication and optical interconnection, particularly at 1300 nm and 1550 nm. However, due to the 1.12-eV bandgap of silicon, it is unable to respond to a characteristic near-infrared (IR) communication band. To achieve silicon-based IR detection, narrow-gap semiconductors, such as Ge, InGaAs, and PbS, are usually embedded into the Si platform either through epitaxial growth or via bonding methods. This will, however, increase the processing complexity and compromise the CMOS compatibility [4]. Intriguingly, hyperdoping is an effective method to extend the absorption range of silicon. Many recent studies have reported that femtosecond laser hyperdoping can broaden the dopant energy level of the hyperdoped silicon (BS) into an intermediate band [5,6]. Then, two sub-bandgap IR photons can be used to excite an electron from the valence band to the conduction band [7,8]. Given its broadband absorption capability and low cost, hyperdoped silicon is of prime interest for silicon-based detectors [4,9,10] for security and telecommunication, and for highly-efficient photovoltaic cells that harvest additional energy from the sub-bandgap spectrum [11–13].
Despite the excellent IR absorption, the application of hyperdoped silicon in silicon-based detections still faces some problems. Inevitably, the crystal lattice is extremely damaged by a high concentration of undesirable defects and damages incorporated into the silicon [14–16]. These defects induce a high density of recombination centers and the resultant internal leakage, which greatly reduce the responsivity and increase the dark current of the device. To date, several studies were devoted to improving the crystallinity of hyperdoped silicon. The approaches included femtosecond laser irradiation followed by thermal annealing [17], ion implantation followed by proper annealing [18,19], and the co-doping of supersaturated nitrogen and sulfur [20]. Nevertheless, defects and dislocations still formed in the sample, thereby hindering the industrialization.
In this work, we report an optimized method that elevates the silicon substrate temperature during the femtosecond laser irradiation and manufactures highly responsive Te-hyperdoped BS photodiodes. When the temperature increases, a uniform surface microstructure with an excellent single-crystal hyperdoped layer is formed in the sample surface, which helps to enhance the photoresponse and the stability. The n-n$\def\upmu{\unicode[Times]{x00B5}}^+$ BS photodiode prepared at 700 K through annealing has a responsivity of 120.6 A/W at 1120 nm, which is the highest reported value for a Te-doped silicon photodiode, to the best of our knowledge. Particularly, the sub-bandgap responsivity of the BS photodiode is 43.9 mA/W and 56.8 mA/W at 1300 nm and 1550 nm with low noise, respectively, which is valuable for optical communication and interconnection. Our research provides new ideas for the development of silicon-based detectors.
2. Experimental section
A Te film with a thickness of 50 nm was thermally evaporated onto a 400-${\upmu }$m-thick high-resistivity (3-5 k$\Omega \cdot$cm) n-type silicon (100) wafer after cleaning by the RCA standard method. The Te-coated silicon wafers were mounted on a three-axis translation stage controlled by a computer in a vacuum chamber. A heating device was installed in the vacuum chamber to control the temperature of the substrate silicon wafer at 300 K, 500 K, and 700 K. 700 K was the highest temperature tested because of the melting point of Te (T$_{m}$=725 K). Heated wafers were processed in a nitrogen (N$_{2}$) atmosphere by a Ti:sapphire femtosecond laser (central wavelength: 800 nm; pulse duration: 120 fs; repetition rate: 1 kHz) doping system at a fluence of 1.3 kJ/m$^{2}$. The laser pulses were focused by a lens with a focal length of 50 cm at a normal incident with the sample. The femtosecond laser spot moved on the sample surface at a scan speed of 1 mm/s and a line spacing of 50 ${\upmu }$m. The laser processing area of each sample was a square of 8 mm $\times$ 8 mm. The spatial profile of the laser pulse was nearly Gaussian with a laser spot diameter around 164 ${\upmu }$m on the sample surface. A half-wave plate (HWP) and a Glan-Taylor polarizer (GTP) were used to continuously vary the incident energy. The polarization of the laser pulse was controlled by the GTP. Each spot on the sample surface was exposed to 200 laser pulses. Te-hyperdoped silicon photodiodes were fabricated with these BS samples. Rapid thermal annealing at 773 K for 10 minutes in a nitrogen flow could be used to activate the hyperdoped carriers. Aluminum films were thermally evaporated on both surfaces of the sample to make electrodes for the ohmic contact.
The morphology of the irradiated surface microstructures was obtained with a field emission scanning electron microscopy (SEM) and an atomic force microscopy (AFM). The crystalline form was analyzed by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) in Shanghai Doesun Energy Technology Co., Ltd.. A secondary ion mass spectrometry (SIMS) were performed to characterize the Te concentration profiles in Eurofins Scientific (Shanghai) Co., Ltd.. The responsivity of the device was measured using a 250 W tungsten halogen lamp, and the light passed through a grating monochromator with a spectral resolution of 0.2 nm, a high-pass filter, and a chopper connected to a lock-in amplifier (Stanford SR830). The light was finally focused on the surface of the device with a size smaller than the active area of the device. The applied reverse bias voltage was 5 V before annealing process, and 2 V after annealing process. We used a substitution method to determine the responsivity. Two calibrated commercial photodetectors with different response wavelength ranges from Thorlabs Inc. were used for comparison with the samples. One was a silicon photodetector (DET36A/M) (350 nm - 1100 nm) and the other was a germanium photodetector (DET50B/M) (700 nm - 1800 nm). The actual responsivity was calibrated by Thorlabs, Inc.. The current-voltage (I-V) characteristics of the BS photodiodes were acquired with a Keithley 2410 source meter.
3. Results and discussion
Homogeneous microstructures are formed after irradiation by the femtosecond laser, as presented in Figs. 1(a)–1(c). Macroscopically, a uniform surface microstructure generally indicates a uniform surface hyperdoping, which creates a large n-n$^{+}$ junction area with a high quality. This leads to a uniform spectral absorption and responsivity of the BS photodiode. At 300 K and 500 K, stripes and droplets are observed in the microstructure at the edge and at the center of the laser spot, respectively. At 700 K, the stripes disappear. When the sample is irradiated by the femtosecond laser, ablation occurs when the surface temperature exceeds a critical value [21]. At low substrate temperatures like 300 K and 500 K, a higher laser fluence is required to reach the critical temperature for ablation to occur. At the center of the laser spot, droplets form because the fluence is above the ablation threshold. At the edge of the laser spot, stripes form where the fluence is near the ablation threshold. However, for a high substrate temperature (700 K), the decrease of ablation threshold [22,23] brings the temperature over the critical threshold in the entire irradiated area, which makes the stripes disappear. Additionally, the depth of the different structures in Figs. 1(a)–1(c) can be analyzed statistically with AFM measurements, as shown in Fig. 1(d). When the substrate temperature increases, the distribution of the depth of the microstructures formed is narrower and the surface microstructures are more homogeneous. Previously, we established that modifications of the surface morphology induced by the femtosecond laser irradiation are the result of the competition between periodic surface structuring stimulated by the interference of the incident light with the surface plasmon polaritons (SPPs) and surface smoothing stimulated by the melting of the surface [24]. At elevated temperatures, surface smoothing dominates in the competition mechanism so that homogeneous surface microstructures are formed. Related researches have been studied for various materials [25–28].
Fig. 1. (a)–(c) SEM images of silicon surfaces irradiated with the laser fluence of 1.3 kJ/m$^{2}$ at various substrate temperatures: (a)300 K, (b)500 K and (c)700 K. The black arrow indicates the direction of the laser polarization. The yellow arrow represents the direction of the movement of the laser spot on the sample surface. (d) Box chart of the depth statistics of the structures in (a)–(c) obtained from the AFM measurements.
A high-quality crystalline structure is expected since the density of the recombination centers is greatly reduced, which will then improve the responsivity of the photodetectors. The high crystal quality of the samples obtained with a high substrate temperature was confirmed through TEM, HRTEM, and SAED, as summarized in Fig. 2. Cross-sections of a single droplet on the Te-hyperdoped silicon surface at 300 K, 500 K, and 700 K were analyzed. More specifically, bubbles with a diameter around 200 nm are formed at 300 K inside the core of the droplet and many potholes are formed on the surface of the droplets (Fig. 2(a)) as a result of the phase explosion [29] and resolidification after irradiation by the femtosecond laser. In addition, the highly disordered lattice and the blurred boundaries of the surface potholes are shown in Fig. 2(b). Meanwhile, the SAED pattern in Fig. 2(c) also indicates a poor crystallization. When the substrate temperature increases, the size of the bubbles and the potholes decreases and the crystal quality improves, as shown in Figs. 2(d)–2(i). Particularly, the surface profile of the droplet of the substrate kept at 700 K is smooth and the inside bubbles become smaller. Few potholes are formed on the surface of the droplet with a clear surface boundary. Especially, in Figs. 2(h) and 2(i), perfect single crystals remain even after an extreme processing by the femtosecond laser.
Fig. 2. Overview cross-sectional TEM images of a single droplet on the hyperdoped silicon surface with bubbles (blue dashed round), high-resolution TEM images of a pothole on the droplet boundary (red square), and the SAED pattern of the surface of the Te-hyperdoped silicon at different substrate temperatures: (a)–(c)300 K, (d)–(f)500 K, and (g)–(i)700 K.
Our results indicate that hyperdoping with the femtosecond laser at an elevated temperature has a positive impact on the crystallinity due to the slower resolidification. After the femtosecond laser irradiation, the material is rapidly thermalized through electron-phonon scattering on the timescale of picoseconds. When the temperature of the irradiated area exceeds a critical value, the ablated material is removed, resulting in a decrease of the average energy of the target and a stabilization of the surface temperature around the critical temperature [21]. Subsequently, the processed area cools down from the critical value to the substrate temperature on the timescale of nanoseconds through thermal diffusion and ablation [30]. By heating the sample, the temperature difference between the melting surface and the substrate is decreased, thereby lowering the cooling rate of the irradiated area and the resolidification speed. Compared with traditional processes where the rapidly resolidificated lattice is dramatically disordered [31,32], the longer lasting molten phase leads to a high-quality crystalline structure [33].
Generally, it is difficult to produce a single crystal and hyperdoping at the same time with femtosecond laser processing. We obtained a highly-ordered single crystal as well as a supersaturated concentration of the dopant in the surface layer of the sample using the high-substrate-temperature treatment. Figure 3 shows the Te concentration profiles measured by SIMS in the silicon samples irradiated at 300 K and 700 K. The doping concentration decreases when the depth increases. The reported equilibrium solid-solubility limit of Te in crystalline silicon is 3.5$\times 10^{16}$cm$^{-3}$ at room temperature which is indicated by a dash-dotted line [34]. However, the maximum Te concentration measured was 8.2$\times 10^{19}$cm$^{-3}$ in the top 20 nm after femtosecond laser irradiation at 700 K, in contrast with a value of 1.2$\times 10^{20}$cm$^{-3}$ at 300 K in the top 20 nm. Since the evaporative removal is more intense at an elevated temperature, the overall doping concentration of the sample at 700 K is slightly lower than at 300 K. However, the Te concentration is still more than 3 orders of magnitude above the solid-solubility limit. Meanwhile, a shallow n-n$^{+}$ heterojunction [10,35] is formed due to the steep decrease of the Te concentration with the depth. This favors the rectification of the photodiode, and reduces the dark current due to a larger barrier in a built-in electric field which is stronger at a short distance. This has great potential for the preparation of practical n-n$^{+}$ photodetectors.
Fig. 3. SIMS profiles for the Te-hyperdoped silicon samples irradiated with the laser fluence of 1.3 kJ/m$^{2}$ at 300 K and 700 K.
Given the uniform surface microstructures, the high single crystal quality, and the n-n$^{+}$ heterojunction, the BS processed at 700 K has a high potential for photoelectric detection. For each substrate temperature of 300 K, 500 K, and 700 K, the responsivity curves of several photodiode samples were exhibited in Fig. 4. Aluminum films were thermally evaporated on both surfaces of the photodiodes to provide electrodes for the ohmic contacts. Figure 4 shows that the responsivity of higher-temperature samples is generally better than that of lower-temperature samples. Although the observable noise appears in the short (400-600 nm) and IR (1200-1600 nm) wavelength ranges, the temperature-dependent responsivity can still be distinguished. The dark-current-voltage curves are shown in the inset, which indicates that all samples have good rectification characteristics with an n-n$^{+}$ heterojunction. Moreover, the elevated substrate temperature can increase the doping concentration gradient, improve the lattice quality, and reduce the defects, which will suppress dark current of the photodiode. The dark current density of the 700-K sample was reduced by nearly an order of magnitude with values of 5.0 ${\upmu }$A/cm$^{2}$, 9.1 ${\upmu }$A/cm$^{2}$, and 22.2 ${\upmu }$A/cm$^{2}$ at −2 V, −3 V, and −5 V, respectively. Therefore, compared to conventional (300-K processed) BS photodiodes, high-temperature (700-K processed) BS photodiodes have significantly reduced noise in the IR band after 1200 nm, as shown in Fig. 4.
Fig. 4. Responsivity at room temperature of photodiode samples, made from Te-hyperdoped silicon obtained at 300 K, 500 K and 700 K, respectively, for a reverse bias voltage of 5 V. Four 700-K samples with identical processing conditions were displayed, and two samples of other temperatures were displayed as references. The inset shows one of the dark-current-voltage (I-V) curves of the Si:Te photodiodes for each substrate temperature.
Based on the optimal substrate temperature (700 K), we further optimized the post-processing of the BS photodiodes. Rapid thermal annealing at 773 K for 10 minutes in a nitrogen flow was used to activate the hyperdoped carriers and to produce the high responsivity, as shown in Fig. 5. In detail, the responsivity after annealing is 41.7 A/W, 120.6 A/W, 43.9 mA/W and 56.8 mA/W at 600 nm, 1120 nm, 1300 nm, and 1550 nm, respectively. Such excellent properties are far beyond the previously reported Si:Te photodetectors. Furthermore, it can be seen that the responsivity curve after annealing is smoother and more stable in the IR range (1200-1600 nm) than before annealing, which indicates lower noise. The applied optimized reverse bias is 5 V before annealing, and 2 V after annealing. Even though the bias voltage is smaller after annealing, the maximum responsivity is 7.3 times higher than the maximum responsivity of 16.5 A/W before annealing, and the IR responsivity could achieve the same order of magnitude as the IR responsivity before annealing. Because of the smaller dark current brought by the smaller optimized bias voltage after annealing, the noise can be greatly suppressed in the IR band. The detectivity (D*) value is about 2.54$\times 10^{9}$ Jones at 1550 nm.
Fig. 5. Responsivity at room temperature of the 700-K Te-hyperdoped silicon photodiodes before and after rapid thermal annealing (773 K, 10 mins). The applied optimized reverse bias is 5 V before annealing, and 2 V after annealing. A commercial silicon photodetector with a reverse bias voltage of 12 V and a commercial germanium photodetector with a reverse bias voltage of 5 V are also shown, for reference.
In addition, we displayed a commercial Si photodetector with a reverse bias voltage of 12 V for comparison as a reference. For all photodiodes, the responsivity in 700-1600 nm was calibrated by a commercial Ge photodetector which is indicated by a dash-dotted line in Fig. 5. The BS:Te photodiode after annealing is two orders of magnitude more responsive than the traditional commercial Si photodetector in the range of 600-1100 nm, and has a more significant improvement at a longer wavelength.
The high responsivity, the low dark current, and the low noise of the single crystal BS photodiode are attributed to high degree of lattice order and the hyperdoping of Te. The highly-crystalline structure greatly reduces the density of carrier recombination centers, which reduces the scattering and increases the carrier mobility. The intermediate band formed by hyperdoping ensures an efficient IR photon absorption and photoelectric conversion. Moreover, the steep and shallow doping depth forms an n-n$^{+}$ junction, and the collection of carriers is enhanced by the near-surface depletion region.
4. Conclusion
In summary, we developed a Te-hyperdoped BS photodiode with the highest responsivity yet reported for Si:Te photodiodes and with a low dark current, a low noise and a high stability. These excellent properties are achieved by elevating the substrate temperature during the femtosecond laser irradiation that yields uniform surface microstructures, a highly crystalline surface layer, the hyperdoping of Te, and a steep shallow n-n$^{+}$ heterojunction. After annealing, at a reverse bias voltage of 2 V, the photodiode has a responsivity of 41.7 A/W, 120.6 A/W, 43.9 mA/W and 56.8 mA/W at 600 nm, 1120 nm, 1300 nm, and 1550 nm, respectively. The noise is highly suppressed especially in the IR band. In addition, we analyzed how the elevated substrate temperature affects the performance of the BS photodiode by slowing down the cooling velocity after irradiation by the femtosecond laser and reducing the ablation threshold. Our results prove that hyperdoping at a high substrate temperature has great potential for the laser-induced modification of semiconductors, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits. Through further optimization of the post-processing parameters, better comprehensive properties of photodetectors can be expected.
Funding
National Natural Science Foundation of China (11574158, 11874227, 11974192); Higher Education Discipline Innovation Project (B07013); Changjiang Scholar Program of Chinese Ministry of Education (IRT_13R29).
Disclosures
The authors declare no conflict of interest.
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