Tunable Luminescent A-SiN_xO_y Films with High Internal Quantum Efficiency and Fast Radiative Recombination Rates

Abstract

In this work, we systematically investigated the N_x bonding defects that induced high photoluminescence internal quantum efficiencies (PL IQEs) and very fast radiative recombination processes in amorphous silicon oxynitride (a-SiN_xO_y) systems. The luminescent N‒Si‒O bonding-related defect states were checked for the XPS, EPR, and temperature-dependent steady-state PL (TD-SSPL) properties. The PL IQEs were calculated from PL quantum yields through the principle of planar geometry optics, and then confirmed by the TD-SSPL properties. The radiative recombination rates [k_r(R)] were determined by combining the PL IQE values and ns-PL lifetimes obtained from time-resolved PL measurements. Both the PL IQE, exceeding 72%, and the fast k_r(R) (~10⁸ s⁻¹) are proportional to the concentration of N_x defects, which can be explained by N‒Si‒O bonding states related to the quasi-three-level model, suggesting the possible realization of stimulated light emission in a-SiN_xO_y systems.

1. Introduction

As one of the traditional semiconductors, silicon (Si) is widely used in today’s microelectronic, photovoltaic, and optoelectronic technologies [1,2,3]. In Si-based monolithic optoelectronic integrated circuits, the most difficult work is to realize an efficient Si-based lighting source. However, silicon is not a suitable luminescent material, and its indirect band gap limits light emission efficiency. Therefore, Si-based luminescent materials (including Si alloys and nanostructured Si) have been actively investigated over the last two decades, with an interest in improving the PL external quantum efficiency (PL EQE, or called PL quantum yield, PL QY) and internal quantum efficiency (PL IQE), and understanding the radiative recombination mechanisms of the light emission [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22].

Abundant previous works have reported on the improved light emission efficiency from colloidal Si QDs, and the results obtained from colloidal Si QDs were rather high (PL QY, 43–90%) [6,7,8]. However, only a few works have focused on the light emission efficiency in nanocrystal-Si embedded Si alloys. The PL EQE from thermal annealed nc-Si embedded Si nitride films, and PL IQE from nc-Si embedded Si dioxide films have been achieved at ~7% [10] and 59% ± 9% [11], respectively. On the other hand, although much attention has been paid to Si alloys [13,14,15,16,17], promising results are still lacking. Our group has found that O atoms’ impurity induced a significant enhancement of PL intensities in a-SiN_x films, and confirmed the new luminescent N_x defect centers [18,19,20]. Visible light emission devices with phosphorus-doped n-a-SiN_xO_y/p-Si heterojunction structure have also been realized [21]. Recently, PL EQE values of 8.38% have been achieved from tunable luminescent a-SiN_xO_y films [22].

In this work, we systematically analyzed the PL internal quantum efficiencies and fast radiative recombination processes of a-SiN_xO_y films. We checked the N‒Si‒O bonding defects using XPS, EPR, and temperature-dependent steady-state PL (TD-SSPL) measurements. The PL IQEs were calculated from the measured PL QY values using the principle of planar geometry optics, and then confirmed by the TD-SSPL properties. At the PL peak energy (E_PL) of 2.55 eV, we achieved a high PL IQE of 72% in a-SiN_xO_y systems. Combing with the obtained PL IQEs and ns-PL lifetimes, the fast radiative recombination rates (~10⁸ s⁻¹) from a-SiN_xO_y films have also been determined. Both the PL IQE values and the fast radiative recombination rates from a-SiN_xO_y films for various stoichiometries are proportional to the related concentration of N_x defect states, which can be explained by a quasi-three-level model, suggesting the possible realization of stimulated light emission in a-SiN_xO_y films.

2. Materials and Methods

2.1. Material Fabrication

A-SiN_xO_y films with various thicknesses (10 nm–4 μm) and the controlled a-SiN_x were deposited on roughened quartz and p-Si substrates by using PECVD, by processes described in detail elsewhere [22]. N/Si ratios were effectively controlled through controlling the gas flow rate R (R = NH₃/SiH₄) by changing the ammonia flow. As the PL intensities increased rapidly when the film thickness increased, we chose ensemble samples with the same thickness of ~500 nm as the research objects in this article.

2.2. Characterization of A-SiN_xO_y Thin Films

The chemical compositions and atomic scale defect states were confirmed by the XPS (Thermo ESCALAB 250, ThermoFisher Scientific, Waltham, MA, USA) and the EPR (Bruker EMXplus, X-band, Bruker, Billerica, MA, USA) measurements. The TD-SSPL and PL excited (PLE) properties were measured by a Fluorolo-3 system (HORIBA Jobin Yvon, Paris, France) in a computer-controlled Delta 9023 oven (State College, PA, USA) under various temperatures, using a 75W Xe lamp (λ_exc = 250‒800 nm) and a He‒Cd laser (λ_exc = 325 nm) as light sources. The optical band gaps (E_opt) were obtained from transmittance measurements (Shimadzu UV-3600, Shimadzu Corp., Hadano, Kanagawa, Japan). The refractive indexes (n) were measured using a spectroscopic ellipsometer (Jobin Yvon UVISEL, HORIBA Jobin Yvon, Paris, France). Both the E_opt and n of a-SiN_xO_y samples are listed in

Table 1. The optical parameters, E_PL (E_exc > E_opt), PL QEs, and ns-PL lifetimes of a-SiN_xO_y films.

R	Eopt (eV)	EU Edge (eV)	EPL (eV)	ΔEstokes (eV)	n	N* (%)	η (%)	ε (%)	τmeas(ns)
0.3	2.93	2.86	2.12	0.74	2.265	7.64	1.57	20.5	5.18
0.5	3.15	3.02	2.23	0.79	1.966	10.77	4.33	40.2	6.07
0.8	3.47	3.14	2.36	0.78	1.904	11.63	7.76	65.5	7.12
1.5	3.98	3.38	2.55	0.83	1.889	11.85	8.38	72.1	7.79
2.5	4.50	3.66	2.81	0.85	1.837	12.67	5.61	44.3	6.31
5	4.62	3.75	2.91	0.84	1.803	13.25	4.84	36.5	4.91

. A FLS980 (Edinburgh Instrument Ltd., Edinburgh, UK) equipped with an EPL375 pulse diode laser (λ_exc = 375 nm, pulse width ~53 ps, repetition rate ~20 MHz), and a TCSPC (resolution time ~100 ps), were used to record the time-resolved PL.

3. Results and Discussion

3.1. Identification of the N‒Si‒O Defect States

The existence of N‒Si‒O bonding configurations in a-SiN_xO_y samples was confirmed by the XPS measurements [19,20,22]. The concentrations of N and O are 35–49% and 2.2–5.5%, respectively, for a-SiN_xO_y films with various R. To further identify the related atom-scale defect states, we performed EPR measurements. Figure 1a displays the measured EPR absorption spectrum of the a-SiN_xO_y films (R = 1.5), which has a value of g = 2.0030. We considered all possible typical trivalent Si dangling bonds (Si DBs) [23], and the nitroxide-like N_x center [24], which is the herald of a true a-SiN_xO_y phase [25]. Then we decomposed the measured EPR signals and obtained the related g values and Gauss line width (∆H_pp), as shown in the Figure 1a inset. The densities of the coexisting Si DBs and N_x defects of a-SiN_xO_y films with various R can be calculated by doubly integrating all the measured and fitted first derivative EPR signals, which are plotted in Figure 1b.

3.2. PL Properties of the A-SiN_xO_y Thin Films

For SSPL properties, with a rise in R, the PL peak positions exhibit a blue shift, and are tunable in the visible range (2.12–2.91 eV) under the excitation wavelength λ_exc = 325 nm (He‒Cd laser), as shown in Figure 2a. Generally, the relationship between PL IQE (ε) and PL EQE (η) can be described as $\epsilon =\eta /N^{\ast }$. Here $N_{}^{\ast }$ denotes the light extraction factor. We directly measured the absorption photon numbers and the emitted photon numbers in a calibrated integrating sphere, thus obtaining PL EQEs (PL QYs) exceeding 1.5% for tunable luminescent a-SiN_xO_y films [22]. From the principles of planar geometry optics, we know that the emitted photons generated inside the samples are partially influenced by total internal reflection, and most of the generated photons are trapped inside the samples, since the refractive index of a-SiN_xO_y samples (n) is much higher than the air index ($n_{air}$). Thus, the first part of light extraction factor $N_1^{\ast }$ can be calculated as [26,27]:

$$N_1^{\ast }\approx \left[1-{\left(\frac{n-n_{air}}{n+n_{air}}\right)}^2\right]\times \frac{1}{2}\left(1-\sqrt{1-{\left(\frac{n_{air}}{n}\right)}^2}\right).$$

(1)

The remaining photons inside the sample should directly stroke or be internally reflected onto the rough substrate surface, which can also be scattered in all directions, and then emit out from the top surface [26,27], thus contributing the second and third parts of the light extraction factor $N_2^{\ast }$ and $N_3^{\ast }$, respectively. The rest of the contributions were too weak to separate out, and should be ignored. Therefore, the light extraction factor was defined as $N^{\ast }\approx {\displaystyle \sum _{x=1}^3{N}_x^{\ast }}$, and the calculated PL IQEs exceeding 20% should be obtained in the visible range, as shown in Figure 2b and Table 1. For various R, the variation tendency of ε was consistent with the variation tendency of N_x defect densities, indicating that luminescent N_x defects are responsible for the high PL internal quantum efficiencies in our a-SiN_xO_y systems.

TD-SSPL measurements are always used to identify the PL mechanisms, and we check the related PL IQE values, as the radiative recombination makes a dominant contribution to the recombination processes at low temperatures, which means PL IQE is nearly equal to 100% [5,6,12]. Figure 3 shows the TD-SSPL properties for the R = 1.5 samples from 8 K to 300 K. One can see that the E_PL keeps nearly stable and is independent on the measurement temperatures. The PL profiles were observed no appreciable change under various measurement temperatures. Such phenomenon indicates that the carrier recombination through defect states, which is different from those through band tail levels. It will be discussed in detail later. As shown in Figure 3 insert, the integrated PL intensity [I_PL(T)] keeps nearly stable at low measurement temperature range [T< 80 K, here we called I_PL(T₀)], and then decreased rapidly as the measurement temperature rises up from 80 K to 300 K, indicating the increasing domination of nonradiative recombination in this temperature range. We estimated the TD-PL IQE by using the thermal ionization model [10,13]:

$$I_{PL}(T)=\frac{I_{PL}(T_0)}{1+B\exp (-E_a/KT)},$$

(2)

where B is inversely proportional to the radiative recombination rates and E_a denotes the activation barrier energy. From the thermal ionization model, I_PL(T) was well fitted with B = 10 and E_a = 57 meV, which was similar to the reported results [10]. The PL IQE for a-SiN_xO_y films with R = 1.5 estimated from TD-SSPL measurements is 74.8%, which is consistent with the calculated values (ε ~72.1%) from directly measured PL QYs, and is much higher than those from nc-Si-embedded a-SiO_x samples [11].

3.3. Recombination Rates of A-SiN_xO_y Thin Films

To further analyze the obtained high light emission efficiencies, we intensively investigated the ns-PL decay properties and the recombination rates [k(R)]. Figure 4a shows the ns-TRPL decay spectra of a-SiN_xO_y samples at room temperature. We fitted the PL decay curves and obtained the ns-PL lifetimes (τ_meas) by [28].

$$I(t)={{\displaystyle \sum _{i=1}^n{A}_n\exp (-t/{\tau }_n)}}_{}with{\tau }_{meas}=\frac{A_1{\tau }_1^2+A_2{\tau }_2^2+A_3{\tau }_3^2}{A_1{\tau }_1+A_2{\tau }_2+A_3{\tau }_3}.$$

(3)

The a-SiN_xO_y samples for different R have an average value of about 6.23 ns at RT. Generally, k(R) can be expressed as $k\left(R\right)=k_r\left(R\right)+k_{nr}\left(R\right)$, where k_r(R) and k_nr(R) denote the radiative recombination rates and nonradiative recombination rates of a-SiN_xO_y samples with various R, respectively. The radiative rates and nonradiative rates can be described as

$$k_r\left(R\right)=\frac{1}{{\tau }_r\left(R\right)}=\frac{\epsilon \left(R\right)}{{\tau }_{meas}\left(R\right)}with{k}_{nr}\left(R\right)=\frac{1}{{\tau }_{nr}\left(R\right)}=\frac{1-\epsilon \left(R\right)}{{\tau }_{meas}\left(R\right)},$$

(4)

where ε(R) denotes the PL IQEs, and τ_r(R) and τ_nr(R) denote the radiative and nonradiative lifetimes, respectively. Combining the obtained ε(R) with the measured ns-PL lifetimes, k_r(R) and k_nr(R) were calculated from Equation (4), as shown in Figure 4b. The k_r(R) have an average value of about 0.8 × 10⁸ s⁻¹ under RT, which can be compared with that of direct band gap materials (such as CdSe NCs, k_r ~10⁸ s⁻¹ [29]). We found that the variation tendency of the radiative recombination rates was also consistent with the variation tendency of N_x defect densities, thus contributing to our understanding of the domination of N‒Si‒O bonding defect states in fast radiative recombination processes.

3.4. A-SiN_xO_y Quasi-Three-Level Systems

We further intensively studied the N_x defect features of luminescent a-SiN_xO_y films by analyzing the variation tendency of PL peak positions (E_PL) for change of excitation photon energies (E_exc), comparing with those of the controlled a-SiN_x films.

Different PL spectra of a-SiN_xO_y films with R = 0.8 under different E_exc are given in Figure 5a. We found that the E_PL of a-SiN_xO_y films hardly varied (~0.04 eV) for change of excitation photon energies, whether E_exc > E_opt or E_exc < E_opt. We also see that band shape profiles are not changed for different E_exc; this is a typical feature of luminescence related to defect states. For the PL spectra of the controlled a-SiN_x films in Figure 5b, in the case of E_exc < E_opt, the E_PL of a-SiN_x films has a blue shift (~0.31 eV); when E_exc rises, in the case of E_exc > E_opt, E_PL stays nearly constant, exerting the typical PL characteristics of the radiative recombination from band tail states carrier transition.

Then we calculated the Stokes shifts from conduction band tails (E_{U Edge}) to PL peak positions ($\Delta E_{stokes}=E_U{{}_{}}_{Edge}-E_{PL}$). It is generally known that PLE spectra represent the state density distribution of luminescence excitation states (conduction band E_C); thus we can analyze the excitation processes and PL excitation mechanisms from PLE spectra. Excitation states of amorphous semiconductors involve band tail states of E_C extended in the band gap; this represented the E_{U Edge} of the E_C tail, which is defined as the boundary threshold value energy of PLE. E_C tail width E_U can also be estimated by subtracting the E_{U Edge} from the E_C (E_opt) of samples, i.e., $E_U=E_{opt}-E_{U_{}Edge}$. The E_PL continues to have a blue shift with a sustained rise in R for tunable luminescent a-SiN_xO_y films in the visible range. Firstly, we noticed that a blue shift occurred to the E_PL and E_opt was broadened with the increase of R, while E_{U Edge} would shrink in the direction of E_C (E_opt) energy level with the increase in N content, and the position of N‒Si‒O defect states energy level in a-SiN_xO_y films was fixed under E_{U Edge}. For a-SiN_xO_y films with different R, Stokes shift has no variation with changes in E_opt, and converges to a stable value ($\Delta E_{stokes}$~0.8 eV), as shown in Table 1. On the other hand, Robertson et al. reported that the Si‒N bond would form a bonding state (σ) and an anti-bonding state (σ^*), and E_opt was calculated from the differences between these two energy levels in the a-SiN_x band gap [30]. For a-SiN_x with x < 1.2, the position of E_C bottom would basically not change with the content of N, while the movement of the valence band (E_V) in the top Si‒N bond caused a change of E_opt. With the increase in N content (x > 1.25), the E_C bottom of a-SiN_x films was gradually replaced by Si‒N anti-bonding states with a higher energy position, and the E_V top was gradually replaced by a lone-pairs state of N 2p; at this point, the increase in E_opt was mainly decided by the movement of the E_C bottom to a higher energy level. For our a-SiN_xO_y films, from the XPS measurements, the x of N have a range of about 0.54–0.96 with various R. Therefore, combining the factors of the aforementioned aspects, we assumed that the E_PL of N‒Si‒O-related defect states would shift through control of N content (varying with changes in E_opt), which is caused by the movement of the E_V top in the band gap in a-SiN_xO_y films.

Therefore, based on the research into luminescent N_x defects and related ns-TRPL decay above, we put forward a quasi-three-level systems model to explain the obtained high PL IQEs (ε ~72%) and fast radiative recombination rates (k_r ~10⁸ s⁻¹) in a-SiN_xO_y systems, which is distinctly different from those of band tail related a-SiN_x systems [13]. The typical PL mechanisms of band tail state carrier transitions are shown in Figure 6a. When E_exc is less than E_opt (E_exc < E_opt), excitation state carriers (E₁) relax to a deeper energy level, and achieve recombination luminescence among bands after thermalization. With the rise in E_exc, excitation state carriers will occupy a higher energy level of conduction band tail (from E₁ to E₂), so that E_PL should move to the location of the high energy level by degrees, i.e., E_PL blue shift. In case E_exc > E_opt, E_PL tends to stay stable and is independent of E_exc. However, for a-SiN_xO_y systems, as shown in Figure 6b, the radiative recombination processes should be divided into two steps: firstly, excitation state electrons relax to the band tail of conduction band in the process of nonradiative recombination, and are caught by N‒Si‒O relevant defect center after thermalization; then electrons transit from N‒Si‒O defect states to the valence band to conduct radiative recombination transition, resulting in highly efficient light emission and fast radiative recombination rates.

4. Conclusions

In conclusion, the PL internal quantum efficiencies and radiative recombination mechanisms have been investigated in tunable luminescent a-SiN_xO_y systems. PL IQEs of 72% have been achieved, which is much higher than those of nanocrystal Si-embedded Si oxide films. Fast radiative recombination rates (~10⁸ s⁻¹) have also been achieved. We discussed and put forward the PL mechanisms of luminescent N‒Si‒O defect-related quasi-three-level systems, which suggested the possibility of stimulated light emission in a-SiN_xO_y films.