The photoluminescence (PL) properties of silicon-on-insulator (SOI) samples, modified by the Si+ self-ion-implantation (SII) into Si thin film followed by annealing, have been well investigated. The well-known W-line can also be observed in SII SOI samples, its emitting behavior and structural evolution have been discussed in this article. The parallel PL pattern trend and the similar changes of temperature-dependent intensity suggest that luminescence center of I1 and I2 peaks located in the near-infrared band originates from the same interstitial-clusters (InCs). The PL peak at 1.762 eV can be ascribed to the quantum confinement (QC) from small-sized Si nanocrystals. Based on the electron spin resonance (ESR) experiments and the variation of normalized PL intensities at different annealing temperature (TA), the neutral oxygen vacancy (NOV) [O3≡Si-Si≡O3] is proposed to be responsible for the blue emission of P2 and P3 peaks, whose intensity can be restrained by the existence of the paramagnetic E1' defects [O3≡Si+]. The density of E1' defect is found to reduce with the increase of annealing temperature (TA). Our results provide a useful method to identify the origin of luminescence centers and pave a way for the application of new type optical defects on silicon based light emitting devices (LEDs).
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Due to the nature of indirect bandgaps, the achievement of optical transition in silicon requires the assistance of phonons, which results in relatively low quantum yields of free exciton emissions and long radiative time in this crystal, even at liquid He temperature [1, 2]. Consequently, considerable efforts have been expended to overcome the optoelectronic drawbacks of silicon itself for achieving the great goal of the microelectronic and optoelectronic integration on one silicon chip [3–5]. Many approaches, such as porous silicon, bandgap modification, and defect engineering, have been applied recently to enhance the light emitting efficiency of silicon. With the fulfillment of these methods, strong luminescence phenomena in a wide range of light-emitting regions from blue light to mid-infrared (IR) have been demonstrated in this kind of indirect bandgap semiconductor.
As a traditionally microelectronic process, the ion implantation is also an effective mean to construct the radiation recombination centers (so-called optical defects), which can distribute uniformly in certain thick range of silicon crystal and create some sub-bands in the middle of forbidden band. The existence of these sub-bands facilitates the optical transition probability and the radiative recombination within the frame of momentum conservation due to its broadening momentum space in the forbidden band. Therefore, several kinds of ion, such as P, H, He, C, B, Cu, Fe, and Ni, etc. [6, 7] have been employed, one and after the other, to modulate the optical properties of silicon crystal. Among the multiple implanted-ion elements, the Si+ self-ion-implantation (SII) into silicon has aroused the more extensive interesting due to the introduction of “pure” defect, that is, only interstitials and little vacancies dominate the original defects by this modulation method . This conduces to manipulate the formation and quantity of certain optical center. On the other hand, the SII implantation can also produce most of all the optical defect types which have reported in the literatures, including the non-phonon luminescence lines D1-D4, W, G, S, and R, etc., which exhibit great potential in optoelectronic applications. The origin of W line is always on debate, though recent reports claimed that the small interstitial like I2C or single interstitial could be responsible for this sharp PL peak [9, 10]. The D1-D4 line and G line may seem more promising, but their complex preparation process and relatively weak intensity also impede their development [3, 11]. Nonetheless, the high light emission efficiency LED based on a W peak (located at 1218 nm) has been prepared successfully by J.M. Bao et al . Although the luminescence is emitted only through defects created during implantation, the external efficiency exceeded the order of ~10−3 and brought promise to the future of SII. Emission at relatively high recording temperature (TR) (~280 K) also promises D1 band an alternative choice for fabricating the Si-based room-temperature light emitting diodes (LEDs) [11, 13]. However, emitted only below room temperature of SII bulk Si sample has always be a problem for its practical application. In Si+ ion-implantation SiO2 samples, PL properties in the visible region can be observed at room temperature by different kinds of point defect luminescence centers through implantation process [14–17]. This process can also help to form the Si nanocrystal (Si-NCs) for its luminescence in the red light to near-infrared region [18–21]. Whereas these visible emission deviate from the modern communication band. The previous research on the light-emitting of ion-implanted silicon-based materials mainly focused on the infrared emission characteristics in bulk silicon or visible emission in SiO2, but there are few reports on the luminescence properties of SOI.
Since its unique structural characteristics, SOI materials show great application advantages in electrical properties, and the sandwich structure of top-layer silicon and buried oxide silicon also provides research potential for the infrared-to-visible emission characteristics of silicon-based materials. Moreover, the SOI preparation process and ion-implantation technology are perfectly integrated with current silicon-based electronic devices and CMOS production technology. Hence, the exploration of the luminescence properties of ion-implantation SOI provides a novel idea for the development of modern electronic devices. In this work, the photoluminescence (PL) properties of the Si+ SII SOI samples have been systematically investigated in the range from the visible to near-infrared bands under different modified conditions, new type optical defects, which show great application potential, have also been demonstrated in the modified SOI wafer.
SOI structures prepared by SIMOX method were used as substrates. The top P-type silicon in  crystal orientation with 220 nm thickness and 10 Ω∙cm resistivity, lay above the insulating buried oxide layer of ~340 nm thickness. Si+ was implanted into top silicon with implanting energy of 130 keV by a MEVVA source ion implanter, the implanting dose of 1.0 × 1015, 1 × 1016, 5 × 1016, and 1 × 1017 cm−2 and all the implantation experiments were conducted with an ion beam at an incident angle of 7° with respect to the normal of the Si film to avoid the channeling effect. Samples were annealed at 400, 500, 600, 700, 800, and 900 °C in a N2 ambience for 30 min. In the IR region, PL spectra were measured from 7 to 273 K and luminescence excited using an Ar laser tuned to 457 nm and a liquid-nitrogen cooled InGaAs detector used to record signals. In the visible region, PL signature was recorded by a SS-RF540 spectrometer at room temperature using a 270 nm ultraviolet (UV) as the irradiation source, UV light was dispersed by using a double-grating monochromator (Gemini-180, Horiba Scientific, Edison, NJ, USA) from a 150 W xenon lamp. PL signals were detected by a photomultiplier tube detector. Electron spin resonance (ESR) signals were acquired at 2 K using a Bruker EMX plus 10/12 (Bruker Corp., Billerica, MA, USA), equipped with an Oxford ESR910 Liquid Helium cryostat spectrometer (Oxford Instruments plc, Abingdon, UK), operating at a low microwave power of 2.0 mW to avoid saturation.
3. Results and discussion
In Fig. 1, the PL spectrum of SII SOI sample annealed at 800 °C is dominated mainly by a peak of D1 located at 0.813 eV (1525 nm). This peak is commonly attributed to the luminescence of dislocation, in which exciton recombination occurs via geometric dislocations, or accumulations of point defects generate in stress fields near dislocations in silicon samples [22, 23]. Although there is an inverse dependence between TR and D1 band intensity, this luminescence peak could be even detected near room temperature . The inset of Fig. 1 shows TR dependence of the intensity of D1 band range from 10 to 200 K. Since the higher TR result in more intense phonon vibrations, recombination efficiency decrease and the intensity of D1 band decreases monotonously with the increase of TR.
The PL intensity of the 400 °C annealed samples was multiplied 10 times and some peaks clearly domain these PL spectra, as shown in Fig. 1. A weak peak labeled as EHLTO and located at 1.075eV can be attributed to the electron-hole liquid (EHL), the mechanism of this this peak is attributed to the recombination of electrons and holes. Usually, electrons and holes are bound in free excitons (FE) by Coulombic attractions. With increasing the density of free excitons, the excitons tend to dissociate into plasma, then this kind of free exciton plasma phase can be described as electron-hole plasma (EHP). The excited carriers are confined within oxide layers of sandwich-like SOI structure by the presence of diffusion barrier, which restrains the recombination rate at the interfaces, and results in the accumulation of extremely high density of electrons and holes . Once temperature decreases below a critical value (~30 K), massive FE or EHP transform into EHL.
It should be noted that even the tenfold intensity magnify the noise signals in PL spectra, the peak located at 1.018 eV (1218 nm) could be also detected with a relatively high annealing temperature (TA) of 400 °C. The origin of W-line has been discussed for decades and it is typically attributed to a stable configuration formed by I3C defect with the lowest energy in Si lattice [25, 26]. Generally, the W-line reaches its maximum intensity after annealing at 275 °C, with increasing TA tending to a W-line intensity decline and its LC, the I3C defects introduced by ion-implantation, evolve to another LC, a tetra-interstitial cluster (I4C) defect, which is highly symmetrical and produces triplet excited states within the Si lattice . Here, I4C defects normally emit at 1193 nm (1.039 eV) and the peak labeled X-line. However, in this case, no signals are detected from X peak in 400 °C annealed samples, which appeared inconsistent with previous studies . Yang et al. have found that the maximum W-line intensity exists at a depth of ~800 nm, using reactive ion etching (RIE), which indicates that the LC of W-center is centralized at this depth. Furthermore, the sample they prepared was implanted with an energy of 300 keV and the project range (Rp) of maximum defect density distribution calculated by TRIM roughly 400 nm . This means that the annealing process drives the W-center to migrate to a deeper position beneath the Rp. These Si+ introduced by implantation are unstable, during the annealing process, small size defects tend to migrate to the interface between top Si and BOX to repair lattice. This explains why the W-center is located at about twice depth below the Rp and also the absence of an X-line in the sample annealed at 400 °C. The Rp of the present sample is ~130 nm in top Si film, suggesting that the annealing process prompt interstitial Si+ to migrate to Si/SiO2 interface. These interstitial Si+ ions assemble at or across the interface of Si-buried silicon oxide (BOX), which fails to form X-centers, which would also be the reason why the W-line possesses relatively weak intensity.
Original fitting experiments performed here indicated that the line-shape of the peak located around 0.971 eV (1277 nm) is convoluted, and this signature could not be constructed from only one type of LC. Further fitting operations emphasize that the 0.965 eV line (1285 nm) and 0.977e V line (1269 nm), labeled as I1 and I2 respectively, are actually the basic features for forming the intense 0.971 eV peak, as shown in Fig. 1. Commonly, the luminescence centers (LCs) of I-bands, which consist of multiple emission peaks, are attributed to I-type point-defect complexes or large interstitial-clusters (InCs) possessing multiple configurations. The formation of these defects originates from excess Si+ interstitials which were introduced by implantation process . By measuring the transient supersaturation during Ostwald ripening, Cowern et al. found the minima of cluster formation energy (Efc) at n = 4 and 8, where n is the cluster size, in other words, the clusters possess stable structure in I4-Cs or I8-Cs . Notably, the two luminescence peaks I1 and I2 overlap each other and they appear to be lying over a broad envelope (peak). These spectral features of broadening in PL spectra have been usually observed in samples having extended defects, and are associated with the quantum confinement (QC) of carriers in the regions with high strain surrounding defects . This suggests that the material contains I-rich regions where I-type point-defect complexes and larger InCs form.
The Arrhenius plot of the intensity of implantation samples annealed at 400 °C versus TR from 10 to 200 K is shown in Fig. 2. PL intensities of each peak are normalized. The W-line can be detected as low as 80 K, whereas the I1 and I2 peaks can be detected at 120 K, higher than W-line, indicating better thermal stability of their LCs. The maximum intensities of I1 and I2 both appears at ~40 K. Beyond this critical temperature (40 K for both I1 and I2), PL intensities decrease because of thermal scattering and exciton dissociation. However, when TR is lower than 40 K, the probability of exciton thermal ionization decreases with decreasing TR, result in fewer excitons recombination from their original traps, therefore, they are difficult to be captured by other, deeper, exciton traps, including LCs of I1 and I2, which result in the lower PL intensities. This phenomenon of competition for capturing excitons between optical centers and other traps explain the abnormal intensity growth below 40 K. The I1 and I2 intensity possess similar temperature-dependent intensity pattern changes, meaning the LCs of these two peaks may originate from same the InCs defect configuration. In other words, these two sharp-peaks are due to excitons jumping to the same ground state from different excited states in the same LC. In addition, during PL testing, I1 and I2 disappear simultaneously when TR increases to 120 K. As a result, the origins of I1 and I2 peaks are concluded to relate to radiative combination from different excited states to the same ground state of InCs, possibly the I8Cs.
The PL spectra range from 1.6 to 2.4 eV of SOI SII samples shows that, when SOI samples are implanted with doses of 1 × 1016, 5 × 1016, and 1 × 1017 cm−2 and then annealed at 800 °C in N2 ambience for 1 h, the peak located at 2.123 eV is overlapped by two different peaks, as shown in Fig. 3(a). The sub-bandgap energy of interstitial defect LCs in Si lattices induced by ion-implantation is generally lower than 1.1 eV (bandgap of bulk Si). Yet, during the annealing procedure, amorphous SiO2 in Si+ ion-implanted BOX layer generate nanoscale sized crystal particles. The bandgap of these Si nanocrystals (Si-NCs) increases because of QC effect, this leads to short wavelength PL near or within the visible region . The P1 peak undergoes a redshift from 1.792 to 1.727 eV with increasing implantation dose; the normalized PL intensities from different doses are exhibited in the inset of Fig. 3(a). This phenomenon is interpreted by QC, apart from temperature and time of annealing process, excess Si+ introduced into the SiO2 can also have an impact on the Si-NC size; the relatively larger Si-NCs can be obtained by high implantation doses . Due to QC, bandgap increases with reducing Si-NC size, meaning the redshift occurs when increases implantation dose.
The parallel bimodal behavior of P2 (2.123eV) and P3 (2.173eV) peaks in different implantation does implies similar emission origins for these luminescence peaks. No shifts occur with increasing implantation dose, although PL intensity increases with increasing the implantation dose. Figure 4 shows ESR spectra of the samples annealed at different TA. ESR curves a-d exhibit two features at different g factors (Landé g-factor) of 2.0057 and 2.0004. The first feature is the superposition of spectra due to amorphous silicon centers (α-Si), which may be associated with weak oxygen and also Pbo [∙Si≡Si3] centers. The second feature is probably associated with paramagnetic E1' [O3≡Si∙] centers, a positively charged non-radiative center similar to E1' center, predominantly in buried SiO2 layer, these kind of point defect is not very stable in high temperature annealing, such that defect density decreases with thermal annealing and almost disappear below 700 °C (curve d) . From the normalized PL intensity of peaks P2 and P3 annealed from 400 to 900 °C that shown in Fig. 3(b), it is clear that the intensity of each peak increases gradually with increasing TA and reaches maximum intensity at 600 °C. Nevertheless, a successive intensity decrease appears when TA is higher than 600 °C, which relate to the disappearance of the second feature in ESR signals from E1' centers. This phenomenon is affected by diamagnetic NOV and E1' defects in SiO2 films , both of these defects are induced by implantation process. The excess Si induced by ion-implantation process with oxygen vacancies, which should be responsible for the blue emission of P2 and P3 peaks, and the density of E1' centers decrease with increasing TA and they disappear when TA is higher than 600 °C. Therefore, in the TA range below 700 °C, with the decrease of the non-radiative recombination E1' center, the PL intensity increases constantly at higher temperatures. However, when TA is higher than 700 °C, oxygen vacancies start to resolve, this leads to the rapid decrease of their PL intensity. Consequently, the 600 °C is an optimal TA value for the P2 and P3 peaks to emit the blue light, since most of the E1' centers have been annihilated while the structure and quantity of NOV center remain stable at this temperature.
Photoluminescence properties of the SII modified SOI samples have been well studied. With the increase of annealing temperature, the migration of interstitial Si+ ion into the deeper lattice and the unique structure of SOI induce the decrease of W-line intensity and the absence of X line. Recording temperature dependence on the normalized PL intensity of I1 and I2 peaks, indicates that both peaks share the same origination and InCs configuration constructed by the Si interstitials. The modified SOI samples with relatively higher Si+ ion implantation dose exhibit better light-emitting behavior in the visible band. With the increase of implantation dose, the P1 peak undergoes a remarkable red-shift from 1.792 eV to 1.727 eV due to the decrease of quantum confinement resulting from the increase of Si-NC sizes. According to the ESR experiments and the normalized PL intensity variation of P2 and P3 peaks with the annealing temperature, the diamagnetic neutral oxygen vacancy can be identified to be responsible for these two PL peaks emitting with blue light. The paramagnetic E1' center is found to restrain the luminescence of the neutral oxygen vacancy, while this depression effect decreases with the increase of the annealing temperature. Our results indicate that the Si+ self-implantation can create new optical centers for the SOI samples in both visible and infrared band, one can manipulate the optical properties of these Si thin film by modulating the self-implantation and subsequent annealing processes, this provides us an alternative way to achieve the light emitting device on SOI wafer with high efficiency.
National Natural Science Foundation of China (11564043, 11504322, 11704330); Application Basic Research Project (2016FB002); Reserve Talents project of Academic Lead of Yunnan Province (2017HB001).
The authors thank Liang Chang-Neng for the help of PL measurement at School of Physics and Astronomy, Yunnan University. A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS.
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