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The SiH4 and GeH4 reactant gases used for depositing microcrystalline SiGe films could be simultaneously decomposed when acted cooperatively on the plasma and the assistant CO2 laser in the laser-assisted plasma enhanced chemical vapor deposition system. The carrier mobility of the 80 W laser-assisted SiGe films was significantly increased to 66.8 cm2/V-s compared with 2.22 cm2/V-s of the non-laser-assisted SiGe films. The performances of the resulting p-Si/i-SiGe/n-Si near-infrared photodetectors were improved due to the high quality and high carrier mobility of the laser-assisted SiGe films. The maximum photoresponsivity and the maximum quantum efficiency of the photodetectors with 80 W laser-assisted SiGe films were respectively improved to 0.47 A/W and 68.5% in comparison with 0.31 A/W and 46.5% of the photodetectors with non-laser-assisted SiGe films.
©2013 Optical Society of America
1. Introduction
Recently, the near-infrared photodetectors have been widely developed and applied in optical communication systems and biomedical sensors. Although the III-V-based semiconductor photodetectors exhibit a higher and faster responsivity [1–4], they suffer from a higher fabrication cost and difficult integration with Si-based integrated circuits (ICs). The SixGe1-x materials are suitable for fabricating devices used in communication systems operating in the near infrared region from a wavelength of 0.8 μm to 1.6 μm [5,6]. Furthermore, the resulting devices reveal advantages of low cost and easy integration with existing Si-based technology [7–10]. Consequently, the amorphous SiGe films are commonly deposited using a conventional plasma enhanced chemical vapor deposition (PECVD) system. However, the amorphous SiGe films suffer from a lower mobility and a smaller absorption coefficient in the near-infrared region. To improve the performances of the SiGe-based devices, the microcrystalline SiGe films are proposed and deposited using a high frequency plasma enhanced chemical vapor deposition system [11], electron cyclotron resonance plasma enhanced chemical vapor deposition system [12], hot wire chemical vapor deposition system [13], and capacitively coupled plasma enhanced chemical vapor deposition system [14]. To obtain microcrystalline structure, the deposited SiGe films were then post-annealed at high temperature [15]. In this work, the microcrystalline SiGe films were deposited using the designed laser-assisted plasma enhanced chemical vapor deposition (LAPECVD) system and without post-annealing process. The performances of the deposited SiGe films and the resulting p-Si/i-SiGe/n-Si near-infrared photodetectors were measured and analyzed.
2. Experiments
In the LAPECVD system, the external CO2 laser with a wavelength of 10.6 μm was guided into the chamber of the conventional PECVD system through a ZnSe window [16]. Since the silane (SiH4) and the germane (GeH4) had a higher absorption coefficient at the wavelength of 10.6 μm [17,18], the CO2 laser was used in the designed LAPECVD system to assist the decomposition of the reactant gases. Therefore, both the plasma and the CO2 laser could be simultaneously utilized to more completely decompose the SiH4 and GeH4 reactant gases in the LAPECVD system.
The reactant gas of 95% hydrogen-diluted silane (SiH4) was used as the material source of both the p-Si films and the n-Si films. The diborane (B2H6) and the 99% hydrogen-diluted phosphine (PH3) were used as the acceptor and donor dopant sources of the p-Si films and the n-Si films, respectively. During the deposition procedure, the flow rate of SiH4 reactant gas, PH3 donor source, and B2H6 acceptor source was fixed at 40 sccm, 5 sccm and 20 sccm, respectively. For depositing the n-Si films and the p-Si films with 80 W CO2 laser power, the associated working pressure was 0.4 torr and 0.9 torr, and the associated RF power of the system was 40 W and 20 W, respectively. During the deposition of the SiGe films in the LAPECVD system, the flow rate of the 95% hydrogen-diluted SiH4 and the pure GeH4 was fixed at 40 sccm and 4 sccm, respectively. The base pressure and the working pressure were 10−6 torr and 0.5 torr, respectively. The RF power of the system was kept at 20 W, while the CO2 laser power was varied to study the function of the CO2 laser assistance.
In this work, the performances of the deposited laser-assisted SiGe films were investigated and compared with the SiGe films deposited by the conventional PECVD system under the same deposition conditions except the laser assistance. The grazing incident X-ray diffraction (GI-XRD) was used to measure the crystalline structure of the deposited SiGe films and to estimate the average grain size. The nanoclusters embedded in the laser-assisted SiGe films were verified to be of a single crystalline structure as measured by a high-resolution transmission electron microscopy (HRTEM). The energy-dispersive spectrometer (EDS) was used to measure the Si and Ge contents in the deposited SiGe films. The electrical performances of the SiGe films, as well as the p-Si and n-Si films, were measured using a Hall measurement. The Fourier transform infrared (FTIR) spectrometry was used to measure the absorption of the Si-H and Ge-H bonds in the deposited SiGe films, from which the hydrogen concentrations (NH) of the deposited SiGe films were estimated. To examine the Steabler-Wronski effect (SWE) [19] of the deposited SiGe films and the resulting p-Si/i-SiGe/n-Si near-infrared photodetectors, the samples were reexamined after light-soaking under the condition of 3-suns AM1.5G 300 mW/cm2 for 8 hours. The current-voltage (I-V) characteristics of the resulting near-infrared photodetectors were measured by an Agilent 4156C semiconductor parameter analyzer. Using a monochromator and an Xe lamp source, the spectral photoresponsivity and the quantum efficiency of the near-infrared photodetectors were measured.
The schematic configuration of the p-Si/i-SiGe/n-Si near-infrared photodetectors was shown in Fig. 1 . A 150-nm-thick aluminum (A1) metal layer was firstly deposited on the glass substrates as the n-type electrode. A 25-nm-thick n-type Si film, a 300-nm-thick i-SiGe film, and a 20-nm-thick p-type Si film were then sequentially deposited on the A1 layer using the LAPECVD system. After the deposition of the p-Si/i-SiGe/n-Si structure, a 100-nm-thick indium-tin-oxide (ITO) layer was deposited on the p-Si film using a RF sputter system, serving as the transparent electrode.
3. Experimental results and discussion
The electron mobility and electron concentration of the laser-assisted n-Si films, determined by Hall measurement, were 10.9 cm2/V-s and 1.11 × 1018 cm−3, respectively. The laser-assisted p-Si films had hole mobility of 2.22 cm2/V-s and hole concentration of 3.68 × 1017 cm−3. Moreover, the electrical properties of the resistivity, carrier mobility and carrier concentration of the SiGe films deposited without and with CO2 laser assistance are listed in Table 1 . The carrier mobility of the laser-assisted SiGe films increased with an increase of the CO2 laser power. The carrier mobility of 66.8 cm2/V-s for the laser-assisted SiGe films deposited by 80 W laser power was much higher than the value of 2.22 cm2/V-s for the non-laser-assisted SiGe films. The carrier concentration of the laser-assisted SiGe films slightly increased with an increase of the laser power. Compared with the carrier concentration of 2.56 × 1015 cm−3 for the non-laser-assisted SiGe films, the carrier concentration of 8.43 × 1014 cm−3 was obtained for the SiGe films deposited under 80 W laser power. The associated resistivity of the resulting SiGe films decreased with an increase of the CO2 laser power due to the increase of the carrier mobility. To understand the above-mentioned phenomena, the crystalline structure of the deposited SiGe films was measured using a glancing incident angle x-ray diffraction (GI-XRD). Figure 2 shows the obtained results. It was found that the non-laser-assisted SiGe films deposited by a conventional PECVD system revealed an amorphous structure. In contrast, for the laser-assisted SiGe films deposited with various CO2 laser power, the XRD spectra exhibited three peaks situated at the diffraction angle of 27.8þ, 46.2þ and 54.8þ, which corresponded to the (111), (200), and (311) orientation of the crystalline SiGe of a diamond-cubic structure, respectively. It could also be seen that the full width at half maximum (FWHM) of the (111) peak was narrowed down, indicating an improvement in the crystallinity, for SiGe films deposited with an increased CO2 laser power. In other words, crystalline SiGe films could be deposited by the assistance of CO2 laser in the LAPECVD systems. The average grain size (B) of the deposited SiGe films can be estimated from the Scherrer’s formula [20]:
Table 1. Electrical Properties of SiGe Films Deposited by Various CO2 Laser Powers
Fig. 2 The X-ray diffraction patterns of the SiGe films deposited by various laser assistance powers.
Where K = 0.9 is a proportional constant, λ = 1.54 Å is the X-ray wavelength, D is the FWHM value, and θ is the diffraction angle. The FWHM value of 0.47þ for the (111) peak of the SiGe films deposited with 80 W laser power was deduced from the XRD measurement and correspondingly the average grain size of 19.2 nm was calculated using Eq. (1). Since the crystallinity was improved by an increase of CO2 laser power, the increase of the associated carrier mobility was naturally attributed to the crystalline structure improvement of the SiGe films deposited with a higher CO2 laser power.
To further reveal the microscopic crystalline structure, the laser-assisted SiGe films were examined using a high-resolution transmission electron microscope (HRTEM). Figure 3(a) shows the HRTEM image of the laser-assisted SiGe films deposited with 80W CO2 laser power. It clearly revealed several nano-crystals showing lattice fringes with a spacing of about 3.19 Å. Figure 3(b) shows the selective area electron diffraction pattern of the laser-assisted SiGe films deposited with 80 W CO2 laser power. The complete and incomplete diffraction rings formed from distinctive spots indicated the crystallographic planes of (111), (220), and (311). These experimental results were consistent with the experimental GI-XRD results shown in Fig. 2.
Fig. 3 (a) High-resolution transmission electron microscopy image and (b) selective area electron diffraction pattern of laser-assisted SiGe films.
To analyze the Si and Ge contents in the SiGe films, the SiGe films were measured using energy-dispersive spectrometer (EDS) and the results for films deposited with various laser powers were listed in Table 2 . It could be seen from the Table 2 that the Si content in the deposited SiGe films was decreased with an increase of the CO2 laser power. Particularly, the films deposited with 80 W CO2 laser power had a Si fraction of 0.535 (i.e. the film Si0.535Ge0.465). In contrast, the non-laser-assisted films contained Si of a fraction 0.569 (i.e. Si0.569Ge0.431). The above-mentioned variation trends of the film composition implied different decomposition rate of SiH4 and GeH4 upon the assistant influence of the CO2 laser. More exactly, the decrease of the Si content in the SiGe films deposited with a higher laser power was attributed to the induced higher decomposition rate of the GeH4 at the same CO2 laser power compared with that of the SiH4.
Table 2. Contents and Optical Energy Bandgap of the SiGe Films Deposited by Various CO2 Laser Powers
Using an ultraviolet/visible (UV/VIS) Hitachi 4100 spectrophotometer, the transmittance T and the reflectivity R of the deposited SiGe films were obtained. The absorption coefficient α was determined from the equation , where d was the thickness of the SiGe films. For indirect bandgap semiconductors, the optical energy bandgap (Eopt) could be obtained from the absorption coefficient α using the following equation [21]:
where hν is the photon energy, and C is a constant. The optical energy bandgap of the various SiGe films was determined by plotting the (αhν)1/2 versus hν, as shown in Fig. 4 , and then extrapolating the linear part to the photon energy axis. The associated optical energy bandgap of the various SiGe films was listed in Table 2. It was found that the optical energy gap of the non-laser-assisted SiGe films was larger than that of the laser-assisted SiGe films, which was partly due to the larger energy bandgap of the amorphous SiGe films compared to that of the crystalline SiGe films with the same composition. Moreover, the Si content in the laser-assisted SiGe films decreased with an increase of the CO2 laser power. In general, the energy bandgap of the SiGe films with a lower Si content was smaller than that with a higher Si content. Consequently, the decrease of the optical energy bandgap of the SiGe films with an increase of the CO2 laser power could be caused by the better crystalline structure and the decrease of the Si content in the resulting SiGe films.To clarify the difference in microscopic structure of the SiGe films deposited under various CO2 laser power, the Steabler-Wronski effect of these films upon persistent light exposure was also examined. The concentration of the hydrogen bonded with Si and Ge within the SiGe films was estimated from the measured Fourier transform infrared (FTIR) spectra. Figure 5(a) and 5(b) shows the FTIR absorption spectra of the Si-H bonds and the Ge-H bonds, respectively. In the Figs. the full lines are the Gaussian fitting curves. The absorption spectra for the Si-H rocking mode [Si-H(r)] (640 cm−1) extend from the wavenumber of 500 cm−1 to 800 cm−1 and the spectra for the Ge-H stretching mode[Ge-H(s)] (1800 cm−1) locate from 1800 cm−1 to 2000 cm−1 [22,23]. The hydrogen concentration NH could be estimated by integrating the absorbance of the associated mode band and described as [24]:
where A is a proportional constant (oscillator strength), α is an absorption coefficient, and ω is an angular frequency. According to the previous reports [25,26], the A value of the Si-H(r) and Ge-H(s) was 1.6 × 1019 cm−2 and 5.0 × 1019 cm−2, respectively. The NH value of the non-laser-assisted SiGe films and the laser-assisted SiGe films deposited with various CO2 laser powers was listed in Table 3 . It was found that the NH value of the laser-assisted SiGe films was smaller than that of the non-laser-assisted SiGe films. This decrease of the hydrogen concentration was attributed to the more complete decomposition of SiH4 and GeH4 by simultaneous application of plasma and CO2 laser in the LAPECVD system compared with the case of only plasma applied in the PECVD system. Furthermore, the NH value of the laser-assisted SiGe films decreased with an increase of the CO2 laser power, because more complete decomposition of SiH4 and GeH4 could be obtained using a higher CO2 laser power. It was also found that the hydrogen concentration in Si-H bonds was one order of magnitude higher than that in the Ge-H bonds. It could be deduced that the hydrogen in the environment of the LAPECVD system more preferred to bond with Si than with Ge. Furthermore, the light-soaking was performed under 3-suns AM1.5G 300 m W/cm2 for 8 hours on the non-laser-assisted SiGe films and the laser-assisted SiGe films deposited with 40 W and 80 W laser powers. The FTIR absorption spectra of all these light-soaked samples were measured and shown in Fig. 5 for comparison with the unsoaked samples. The corresponding hydrogen concentration of the light-soaked samples were deduced and listed in Table 3. According to the experimental results, the NH reduction ratio of 5% induced by the light soaking for the Si-H and Ge-H bonds of the laser-assisted SiGe films deposited with 80 W CO2 laser power was much lower than that of 15% for the non-laser-assisted SiGe films. This phenomenon verified that the laser-assisted SiGe films were more stable upon light soaking.Fig. 5 (a) Si-H bonds and (b) Ge-H bonds bonding configuration of various SiGe films before and after light- soaking.
Table 3. Hydrogen Concentration of Various SiGe Films Without and With Light Soak
Using the above-mentioned deposition conditions, the laser-assisted n-Si film, laser-assisted p-Si film, and the non-laser-assisted and laser-assisted SiGe films were deposited for fabricating the near-infrared photodetectors of the structure shown in Fig. 1. The dark current as a function of reverse bias voltage of the various fabricated photodetectors is shown in Fig. 6 . It was found that the dark current of the photodetectors with laser-assisted SiGe films was smaller than that of the photodetectors with non-laser-assisted SiGe films. The dark current of the photodetectors with non-laser-assisted SiGe film operated at −2V was about 3.5μA. The dark current of the photodetectors with laser-assisted SiGe film deposited under 80 W CO2 laser power was 0.3 μA, which was one order of magnitude smaller than that of the photodetectors with non-laser-assisted SiGe films. Furthermore, the dark current was decreased with an increase of the CO2 laser power. According to the experimental GI-XRD results, it could be deduced that the improvement mechanism of the dark current was attributed to the crystalline improvement caused by the function of CO2 laser.
For the p-Si/i-SiGe/n-Si near-infrared photodetectors shown in Fig. 1, the 300-nm-thick i-SiGe absorption films were deposited with non-laser assistance or with laser assistance under various CO2 laser powers, respectively. As shown in Fig. 2, the microcrystalline SiGe films deposited with CO2 laser assistance were obtained. Furthermore, the crystalline structure of the laser-assisted SiGe films was improved with an increase of CO2 laser power. Figure 7(a) shows the photoresponsivity as a function of wavelength for various p-Si/i-SiGe/n-Si near-infrared photodetectors. The photoresponsivity R was defined as:
where Iph (A) and Po(W) are the measured photocurrent and the illuminating incident optical power, respectively. In the measurement, a Perkin Elmer Optoelectronics 300 W Xenon light source was used as the illumination source, from which the light of various wavelength was selected using a monochrometer. The photocurrent was measured using an Agilent 4156C semiconductor parameter analyzer. It was found from Fig. 7(a) that the maximum photoresponsivity of the photodetectors with the SiGe absorption layer deposited by a higher CO2 laser power shifted toward a longer wavelength. It was attributed to the decrease of the silicon content and the associated optical energy bandgap of the SiGe films deposited with an increase of the CO2 laser power as listed in Table 2. The wavelength of maximum photoresponsivity of the photodetectors with non-laser-assisted SiGe absorption film was 835 nm and shifted to 850 nm for the photodetectors with 80 W CO2 laser-assisted SiGe absorption film. Moreover, the photoresponsivity of the photodetectors increased with an increase of the CO2 laser power used in the deposition of the SiGe absorption layer, which was attributed to the crystalline improvement of the SiGe absorption films deposited with an increase of the CO2 laser power. As shown in Fig. 7(a), compared with the maximum photoresponsity of 0.31 A/W for the photodetectors with non-laser-assisted SiGe absorption film, the maximum photoresponsivity of 0.47 A/W was obtained for the photodetectors deposited using 80 W CO2 laser power.Fig. 7 (a) The photoresponsivity and (b) the spectral quantum efficiency of the p-Si/i-SiGe/n-Si near-infrared photodetectors with various SiGe films.
The corresponding spectral quantum efficiency η of the p-Si/i-SiGe/n-Si near-infrared photodetectors could be drawn out using the following definition formula:
where e and hν are the electron charge and the photon energy, respectively. Figure 7(b) shows the resultant spectral quantum efficiency of the various photodetectors, which varies similarly to the spectral photoresponsivity shown in Fig. 7(a). The maximum quantum efficiency of 68.5% of the photodetectors with SiGe absorption film deposited using 80 W CO2 laser power was larger than that of 46.5% for the non-laser assisted SiGe absorption film.4. Conclusions
Comparing with the conventional PECVD method, the SiH4 and GeH4 reactant gases were decomposed by the plasma in combination with the assistance of the CO2 laser during the deposition process of the SiGe films in the LAPECVD system. With the assistance of the CO2 laser, the reactant gases were more completely decomposed. For a fixed RF power, the higher the CO2 laser power, the more complete the decomposition. Therefore, the hydrogen concentration, existed in the form of the Si-H and Ge-H bonds, in the SiGe films was reduced when deposited using a higher CO2 laser power. Consequently, the hydrogen concentration reduction ratio induced by light soaking was decreased in the SiGe films deposited with an increase of the CO2 laser power. From the light-soaking, GI-XRD and HRTEM experimental results, it was concluded that SiGe films with microcrystalline structure and high stability were deposited using the laser assisted PECVD system. Using the non-laser-assisted SiGe films and the various laser-assisted SiGe films as the absorption film, the p-Si/i-SiGe/n-Si near-infrared photodetectors were fabricated. It was demonstrated that the performances, such as the dark current, the spectral photoresponsivity, and quantum efficiency were improved for the photodetectors with SiGe absorption films deposited by a higher CO2 laser power. These experimental results verified that high quality SiGe films and high performance near-infrared photodetectors could be obtained using the CO2 laser assisted PECVD (i.e. LAPECVD) system.
Acknowledgments
The authors gratefully acknowledge the support from the National Science Council of Taiwan, Republic of China Under Contract No.NSC-99-2221-E006-106-MY3, NSC-99-2221-E006-208-MY3, and NSC-100- 3113-E005-003-CC2, and the Advanced Optoelectronic Technology Center of the National Cheng Kung University.
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