Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix
© Yamada et al.; licensee Springer. 2014
Received: 19 December 2013
Accepted: 4 February 2014
Published: 12 February 2014
We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 1019 cm-3 to 3.7 × 1017 cm-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.
Solar cells that use nanomaterials have attracted interest for their potential as ultra-high efficiency solar cells . The conversion efficiency limit of a single-junction solar cell strongly depends on the band gap of the absorber layer, which is known as the Shockley-Queisser limit . To overcome the efficiency limit, various types of quantum dot solar cells, such as quantum size effect type, intermediate band type, and multiexciton generation type, have been proposed [3–5]. The quantum size effect type utilizes the phenomenon that the band gap of a material can be tuned by controlling the diameter of quantum dots, including the periodically arranged narrow-gap quantum dots in a wide-gap dielectric matrix. The fabrication of an amorphous silicon dioxide (a-SiO2) matrix including size-controlled silicon quantum dots (Si-QDs) was reported by Zacharias et al. . The size-controlled Si-QDs can be formed by annealing a superlattice with silicon-rich silicon oxide layers and stoichiometric silicon oxide layers, which is called a silicon quantum dot superlattice structure (Si-QDSL). Since this report was published, silicon quantum dots embedded in various wide-gap materials, such as amorphous silicon carbide (a-SiC), amorphous silicon nitride (a-Si3N4), and hybrid matrices, have been reported [4, 7–11]. Further, the quantum size effect can be observed from the measurement of photoluminescence spectra or absorption coefficients [12–14]. The Bloch carrier mobility in a Si-QDSL with an a-SiC matrix is higher than that in a Si-QDSL with an a-SiO2 or an a-Si3N4 matrix . The barrier height between a-SiC and Si quantum dots is lower than those of the other two materials, resulting in the easy formation of minibands . Moreover, the crystallization temperature of a-SiC is lower than those of the other materials. Therefore, in this study, we focus on a Si-QDSL with an a-SiC matrix.
High-temperature annealing above 900°C is needed to fabricate a Si-QDSL with an a-SiC matrix. Several problems, such as dehydrogenation, formation of leakage paths by the crystallization of the a-SiC phase, and dopant diffusion in the intrinsic layer of the solar cell structure, occur during high-temperature annealing, resulting in the degradation of solar cell performance. The dehydrogenation problem has been addressed by hydrogen plasma treatment (HPT) . The crystallization of the a-SiC phase can be prevented by incorporating a small amount of oxygen in the a-SiC matrix . Niobium-doped titanium dioxide (TiO2:Nb) can be used as a phosphorus (dopant) diffusion barrier layer for the Si-QDSL solar cell . Using these techniques, an efficiency of 0.39% has been achieved in Si-QDSL solar cells fabricated on insulator substrates . Some researchers have reported the electrical properties of silicon quantum dot solar cells [20, 21]. However, clear evidence of the contribution from Si-QDs has not yet been reported because of poor device quality. To improve device quality, the collection efficiency of the photogenerated carrier should be improved. For this purpose, further reduction of the defect density in the Si-QDSL layers and improvement of the p/i interface is significantly important.
In this study, the dependence of hydrogen concentration and defect density in Si-QDSL films on the process temperature of HPT was investigated. Diffusion coefficients of hydrogen in Si-QDSLs for several treatment temperatures were estimated by secondary ion mass spectrometry (SIMS). Hydrogen incorporation was also investigated by Raman scattering spectroscopy. In addition, spin densities were measured by electron spin resonance (ESR) spectroscopy, and the optimal temperature was explored. The influence of HPT on the surface of Si-QDSLs was also investigated. The surface morphologies of Si-QDSLs after HPT were measured by atomic force microscopy (AFM), and the thicknesses of the surface damaged layers were estimated by spectroscopic ellipsometry and cross-sectional transmission electron microscopy (TEM). The etching of the surface damaged layer was performed by reactive ion etching (RIE) using a tetrafluoromethane and oxygen (CF4 + O2) gas mixture.
Forty-period hydrogenated amorphous silicon oxycarbide with a silicon-rich composition (a-Si0.56C0.32O0.12:H)/hydrogenated amorphous silicon oxycarbide (a-Si0.40C0.35O0.25:H) superlattice was deposited on quartz substrates using very-high frequency plasma-enhanced chemical vapor deposition. The source gases were silane (SiH4), monomethylsilane (MMS), hydrogen (H2), and carbon dioxide (CO2). The flow rates of MMS, H2, and CO2 were fixed as 1.7, 47.5, and 0.4 sccm, respectively. SiH4 was intermittently flowed during the deposition of silicon-rich layers. Plasma power density, plasma frequency, deposition temperature, deposition pressure, and electrode distance were 13 mW/cm2, 60 MHz, 193°C, 20 Pa, and 3 cm, respectively. The thicknesses of silicon-rich layers and stoichiometric layers were 5 and 2 nm, respectively. The films were thermally annealed at 900°C for 30 min under a forming gas (3% H2 + 97% N2) atmosphere to form Si-QDs. Film thicknesses of post-annealed samples were 250 ± 10 nm.
After annealing, the samples were exposed to hydrogen plasma to terminate dangling bond defects accompanying hydrogen atoms in the Si-QDSL. The flow rate of H2, plasma power density, plasma frequency, process pressure, and electrode distance were 200 sccm, 2.60 W/cm2, 60 MHz, 600 Pa, and 3 cm, respectively. The treatment temperature was varied from 200°C to 600°C. To evaluate the hydrogen diffusion coefficient in the Si-QDSL, the samples were treated at 300°C for 20 min, 400°C for 10 min, 500°C for 3 min, and 600°C for 1 min. The depth profiles of the hydrogen concentration were measured by SIMS. In the measurements, Ce+ ions were used to measure the hydrogen depth profiles. Also, the depth was calibrated by the etching rate of the Si-QDSL. Crystalline silicon was used as the standard sample to evaluate the hydrogen concentration. The accuracy of the hydrogen concentration by the SIMS measurement was ± 40%. In addition, for measurements of Raman scattering spectra and ESR, treatment temperature was varied from 200°C to 600°C and the treatment time was fixed at 60 min.
The thicknesses of surface damaged layers formed by 60-min HPT were estimated by spectroscopic ellipsometry and cross-sectional TEM. The surface morphologies of Si-QDSLs after a 60-min HPT were measured by AFM. The etching of the surface damaged layer was performed by RIE using CF4 + O2 gas (4% O2 + 96% CF4). The gas flow rate, process pressure, and plasma power density were 10 sccm, 4 Pa, and 0.221 W/cm2, respectively. The surface morphologies after etching were evaluated by AFM and spectroscopic ellipsometry.
Results and discussion
From the depth profiles of Si-QDSLs for a treatment temperature of 600°C, hydrogen concentration was found to drastically decrease. Saturation hydrogen concentration after sufficient treatment was estimated at approximately 1.0 × 1021 cm-3, indicating that the hydrogen concentration at the surface drastically decreases because the loss of adsorbed hydrogen atoms is dominant at high temperatures.
where NTotal- DB, NSi-DB, and NC-DB are the densities of total dangling bonds (Total-DBs), Si-DBs, and C-DBs, respectively. y is the ratio of NC-DB to NSi-DB and x is the composition ratio of C to Si. is the average g value of Si-DB surrounded by (12 - n) Si atoms and n C atoms, is the average g value of a C-DB surrounded by (12 - n) Si atoms and n C atoms, and g(x, y) is the measured g value. Although this expression is derived for an a-Si1-xC x alloy system, it is believed to be valid for Si-QDSL with an a-SiC matrix, which can be considered as an approximately homogeneous material, since the dangling bond defect density in Si-QDs is much lower than that of the a-SiC matrix, and the dangling bonds on Si-QD surfaces are passivated by the a-SiC matrix. An average composition ratio of 0.40 was used.
Thicknesses estimated by fitting of the spectroscopic ellipsometry measurements of Si-QDSLs
Thicknesses estimated by fitting of the spectroscopic ellipsometry measurements of surface-etched Si-QDSLs
Hydrogen plasma treatment temperature dependences of defect densities and hydrogen concentrations in Si-QDSLs as well as the surface morphologies of Si-QDSLs were investigated. Hydrogen could be quickly incorporated as the treatment temperature increases. On the other hand, dehydrogenation of hydrogen atoms terminating the dangling bonds is dominant during high-temperature treatments. The optimal treatment temperature was found to be approximately 400°C, and a defect density of 3.7 × 1017 cm-3 was achieved, which is comparable to the defect density of a typical a-SiC:H film. In addition, damaged layer was found to form on the surface by HPT; this damaged layer can be easily removed by RIE without additional damage to the sample. Thus, HPT and damaged layer removal process are very important for the fabrication of Si-QDSL solar cells.
This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy Trade and Industry of Japan.
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