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Abstract
The room-temperature and temperature-dependent photoluminescence spectra from as-grown and annealed porous polysilicon in the pure oxygen atmosphere have been measured and analyzed. The energy of blue emissions (B band) is independent on the measurement temperature, however; the intensity of the B band is decreased with the increasing measurement temperature. With the increasing measurement temperature, the band gap emissions of as-grown and annealed porous polysilicon at 300 °C show redshift. From the evolution of intensity with the measurement temperature, there are two different non-radiative recombination processes. At the low temperature range between 11 K to 80 K, the thermal quenching behaviors are due to the influence of the surface states. At the higher temperature range (from 80 K to 300 K), however, nonradiative recombination processes can be attributed to the thermal escape. We believe that the understanding of the defects is very beneficial for the application of the porous polysilicon in the optoelectronic device field.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
To address deteriorating environmental issues, such as burgeoning carbon emission and consumption of petrochemical energy, light-emitting diodes (LEDs) and solar cells are considered to applied. [1,2] Due to the high stability, non-toxicity, high carrier mobility and well-established fabrication technique, silicon, characterized with the indirect band gap, is currently one of the most potential materials in the LEDs and solar cells field. [3–6] For the indirect materials, at least one phonon must be introduced to conserve the momentum in the absorption or emission [7]. However, silicon nanocrystals (nc-Si) have a direct band gap and a unique property of controlling the optical band gap through using the quantum confinement effect. [8,9] Porous polysilicon has been studied and confirmed to be consisted of the SiO2/SiOx matrix with randomly distributed nc-Si, which is an important semiconductor with the lower reflectivity and is widely used in optoelectronic field. Many heterojunctions devices based on nc-Si have been applied and investigated [10–15]. In the fabrication processes of porous polysilicon, the defects, such as surface defects and dangling bonds, have been introduced. The introduction of defects can tune the energy band structure of heterojunctions. [16] For example, after the defects are introduced into heterojunctions the potential barrier is improved to impede the electron transportation from n-type materials to porous polysilicon and the hole transportation from porous polysilicon to n-type materials. The aggregation of carriers in the heterojunction interface will generate an additional electric field to affect the performance of heterojunctions. In addition, the introduction of defects can shorten the diffusion length of carriers and increases the captured probability of carriers [17]. So, it is necessary to investigate the defects in the porous polysilicon. The investigation of the room-temperature and temperature-dependent photoluminescence (PL) is an effective approach to understand the defects, the radiative and nonradiative recombination processes of excited carriers occurred in the semiconductor [18–21].
In the present works, we have fabricated porous polysilicon by hydrothermal etching the polysilicon. The room-temperature and temperature-dependent PL have been measured and recorded. Through analyzing the PL data, the origins of emission peaks, and the recombination mechanisms have been studied in detail.
2. Experiment
The polysilicon wafers in the present work are boron-doped with a resistivity of 1.0 Ohm cm, which are cleaned by the standard RCA method. The porous polysilicon have been fabricated through hydrothermally etching the polysilicon wafers in the mixed solution of hydrofluoric acid (HF, 13.0 mol l−1) and ferric nitrate (Fe(NO3)3, 0.03 mol l−1). After the polysilicon wafer is placed into an autoclave with the mixed solution, the autoclave is sealed and placed in a furnace. Keeping at the temperature of 140 °C for 40 minutes, the autoclave is naturally cooled down to room temperature. The sample is then taken out and washed with de-ionized water and absolute ethyl alcohol in turn. A layer of dark black materials can be found on the surface of polysilicon wafers, which is called as porous polysilicon. In order to investigate the defect properties, the porous polysilicon have been annealed in the pure oxygen at the temperature of 300 °C, 500 °C and 900 °C for 1 hour, respectively.
The morphology and microstructure of porous polysilicon are characterized by a field emission scanning electron microscope (FE-SEM, JSM 6700F) and a high-resolution transmission electron microscope (HR-TEM, JEM-2100), respectively. The room-temperature and temperature-dependent PL spectra are measured and recorded using a double grating spectrofluorometer (HORIBA, FL3-22), which is equipped with a closed-cycle helium cryostat (Janis CCS-100) and a digital temperature controller (LakeShore-325) to provide continuous temperature variation from ∼11 K to ∼300 K.
3. Results and discussion
Many fine cracks and larger flakes can be observed on the surface of polysilicon from Fig. 1(a). In the previous works, the average reflectance of polysilicon is ∼38% at the wavelength range between 200 nm and 800 nm. In order to reduce the reflectance of polysilicon, the porous polysilicon have been fabricated through hydrothermal etching method. [22] The SEM image of as-grown porous polysilicon is shown in Fig. 1(b). From Fig. 1(b) the porous structure can be observed and the average size of pores can be calculated to be ∼40 nm. To investigate the fine structure of obtained porous polysilicon, a piece of dark materials on the surface is carefully cleaved from the sample and studied by the TEM and HR-TEM, respectively, and shown in Fig. 1(c) and Fig. 1(d). In the Fig. 1(d), a zone with the distinct lattice fringe can be observed. The distance of the fringe is ∼0.239 nm. By comparing to the PDF card (JCPDS: 01-089-9055) in the XRD system, the lattice fringe is corresponding to the (020) plane of silicon. The silicon nanocrystal has been encapsulated by SiO2 or SiOx. [20] So, it can be concluded that a porous layer, which is consisted of a SiO2 or SiOx (1<x<2) matrix and silicon nanocrystal, has been fabricated after the polysilicon has been hydrothermally etched.
Fig. 1. (a) SEM image of polysilicon. (b) SEM image, (c) TEM image and (d) HRTEM image of as-grown porous polysilicon.
The defects in the porous polysilicon can seriously affect the performance of devices based on the porous polysilicon. Through analyzing the PL spectra of semiconductor, the defects can be determined. Figure 2(a) shows the PL spectra from the as-grown and annealed porous polysilicon. In the present works, all PL spectra are recorded under an excitation wavelength of 270 nm from a Xenon lamp. We can find that two wide emission bands from PL spectrum of as-grown porous polysilicon, which are located between ∼300 nm and ∼530 nm, and between ∼530 nm and ∼800 nm, respectively. For convenience, two emission bands are named as blue emission band (B band) and green emission band (G band), respectively. In the short wavelength direction of B band, there are two small sharp peaks located at ∼377 nm and ∼395 nm, respectively, which might be ascribed to the existence of Fe3+. The B band becomes dominant with increasing annealing temperature. In addition, for the porous polysilicon with the annealing temperature of 300 °C and 500 °C, a platform can be observed in the B band. For the annealed sample at 900 °C, the platform in the B band is disappeared. And the left shoulder of B band is higher than the right shoulder of B band. So, it is believed that B band should be originated from two different emission mechanisms, respectively, which are attributed to the emissions from the luminescence centers in the silicon suboxide layers and the silicon oxide layers.
Fig. 2. (a) PL at 300 K (b) PL at 11 K from porous polysilicon annealed at the different temperature.
From the G band for as-grown sample, a stronger peak (∼618 nm) with a weak peak (∼665 nm) can be observed. With increasing the annealing temperature, the G band gradually becomes weak. From the PL spectra of porous polysilicon annealed at 500 °C and 900 °C, however, the peaks of ∼618 nm vanish and the peaks of ∼665 nm become dominant. Two emission mechanisms can be attributed to the G band, which might be the band gap emission of silicon nanocrystal and surface states, respectively. In the previous work [20], we had concluded that the peaks of ∼665 nm, which are from porous silicon annealed at argon atmosphere, were attributed to defects in the interface of silicon nanocrystal and SiOx layers. In the present work, however, the porous polysilicon is annealed in the oxygen atmosphere. From the PL data, we think that silicon nanocrystal and SiOx layer are disappeared after the porous polysilicon are annealed at 500 °C and 900 °C in the oxygen atmosphere. So, the peak of ∼665 nm might not originate from the defects in the interface of silicon nanocrystal and SiOx layers.
Figure 2(b) gives the PL spectra of porous polysilicon at the measurement temperature of 11 K. It can be seen from the low temperature PL spectra, there are still two emission bands, namely B band and G band. With increasing the annealing temperature, B bands become dominant, which positions do not vary. In the B band, two sharp peaks, which are attributed to Fe3+, can still be observed. The variation of G band with the measurement temperature is similar to Fig. 2(b). However, the peaks, which are ascribed to band gap of silicon nanocrystal, show blueshift.
To further investigate the origins of each emission peaks in the PL spectra, the temperature-dependent PL spectra from four porous polysilicon are recorded and shown in Fig. 3. There are still two sharp peaks which are attributed to Fe3+. For four porous polysilicon, the position of peaks located at ∼446 nm is independent on the measurement temperature, however, the intensity of which is dependent on the measurement temperature and decreases with increasing the measurement temperature. For as-grown porous polysilicon and annealed porous polysilicon at 300 °C, G bands are dependent on the measurement temperature. With increasing the measurement temperature, the intensity of G bands is decreasing and the position shows redshift.
Fig. 3. Temperature-dependent PL from porous polysilicon annealed at the different temperature. (a) As-grown, (b) 300 °C, (c) 500 °C, and (d) 900 °C.
For the as-grown porous polysilicon and annealed porous polysilicon at 300 °C, the evolution of position and intensity of G bands are shown in Fig. 4. Figures 4(a) and 4(b) illuminate the position energy of G bands as a function of measurement temperature. The evolution can be expressed and fitted by Varshni Eq. (1). [23,24]
where $E_{0}$ is the band gap value at 0 K, $\alpha$ is the temperature coefficient, and the value of $\beta$ is close to the Debye temperature of materials. From the evolution of G bands in Fig. 4(a), it can be seen that the fitting curve matches the experimental data very well. The parameters can be obtained, such as $E_{0} = 2.126{\pm}0.002$ eV, $\alpha = 0.45{\pm}0.02$ meV/K, and $\beta = 82.9{\pm}17$ K. For the evolution of porous polysilicon annealed at 300 °C in Fig. 4(b), the parameters can be obtained by using Eq. (1):$E_{0} = 2.121{\pm}0.002$ eV, $\alpha = 0.64{\pm}0.06$ meV/K, and $\beta = 142.9{\pm}39$ K. The evolution can be ascribed to the combination of thermal expansion and electron-phonon interaction from the band gap with the variation of the measurement temperature [25,26].Fig. 4. (a) Temperature-dependent band gap values from as-grown porous polysilicon and (b) temperature-dependent band gap values from porous polysilicon annealed at 300 °C. (c) Temperature-dependent intensity from as-grown porous polysilicon and (d) temperature-dependent intensity from porous polysilicon annealed at 300 °C.
Figure 4(c) shows the temperature-dependent intensity of G bands from as-grown porous polysilicon. As can be seen from Fig. 4(c), the evolution about the intensity vs 1000/T is gradually decreased from 11 K to 80 K, and followed by a stronger exponential decreased from 80 K to 300 K. The evolution in Fig. 4(d) is similar to Fig. 4(c). It is clear that there are two different nonradiative processes to be responsible for those appearances from the PL quenching due to the variation of measurement temperature. Equation (2) can be taken into account a two-step quenching mechanism, the PL intensity as a function of measurement temperature can be expressed as [27,28]:
4. Conclusion
In summary, we have fabricated the porous polysilicon by hydrothermal etching the polysilicon in the solution of HF and Fe(NO3)3. The room-temperature and temperature-dependent PL spectra from as-grown and annealed porous polysilicon in the pure oxygen atmosphere have been measured and recorded. Through analyzing the PL data, the left and right shoulders B band are ascribed to the emissions from the luminescence centers in the silicon suboxide layers and the silicon oxide layers, respectively. The position of B bands is independent on the measurement temperature, however, the intensity of B band is decreased with the increasing measurement temperature. And the G band is originated from the band gap emission of silicon nanocrystal and surface states. With increasing the measurement temperature, the band gap emissions of as-grown and annealed porous polysilicon at 300 °C show redshift. From the evolution of intensity with the measurement temperature, there are two different non-radiative recombination processes at the different measurement temperature. It is believed that understanding of the defects is very important to apply the porous polysilicon in the optoelectronic device field.
Funding
Key Scientific Research Project of Colleges and Universities in Henan Province (21A140021); Science and Technology Department of Henan Province (152300410173).
Acknowledgement
The authors are grateful to Professor Xin Jian Li from Zhengzhou University for his contributions to this work. This work was supported by the Research Project for Basic and Forefront Technology of Henan Province (152300410173) and the Key Research Project for Science and Technology of the Education Department of Henan Province (21A140021).
Disclosures
The authors declare no conflicts of interest.
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