Silicon Interface Passivation Research Articles

Interface Engineering by Intermediate Hydrogen Plasma Treatment Using Dc-PECVD for Silicon Heterojunction Solar Cells

The high open-circuit voltage (VOC) in silicon heterojunction solar cells is attributed to the superior amorphous–crystalline silicon interface passivation. In this work, we investigated a method called intermediate hydrogen plasma treatment (I-HPT) in which the intrinsic amorphous silicon is exposed to hydrogen plasma in between the deposition using direct current (d.c.) plasma-enhanced chemical vapor deposition. This method can not only enhance the surface passivation but also find industrial application due to its ease of integration in the production line. Fourier transform infrared spectroscopy reveals that I-HPT makes the bulk of the amorphous matrix more disordered. We anticipate that the improvement in passivation comes from the diffusion of hydrogen from the bulk to the interface and shifting of the film closer to the “amorphous-to-microcrystalline” transition regime. Our optimized I-HPT method can obtain an implied VOC of 746 mV without annealing, which corresponds to a low surface recombination velocity of 3.2 cm/s. Therefore, we propose the I-HPT method as an alternative to high-temperature annealing which can reduce a fabrication step and processing time. The I-HPT films characterized by Raman spectroscopy do not show any hydrogen-induced crystallization. We have also demonstrated the proof of concept by applying I-HPT to a silicon heterojunction solar cell which shows ∼15 mV increase in VOC in the device and an absolute increase in efficiency by 0.3% as compared to a cell with no HPT.

(Invited) Challenges in III-V Surface Passivation and Some Potential Solutions

In many applications, III-V surfaces lie at the buried or embedded interface of an insulator/III-V or metal/III-V junction. The chemical and physical properties of these III-V interfaces are typically different from the uniform III-V bulk crystal structures. Thus, it is difficult to predict the interface behavior (e.g. electronic structure and doping efficiency) on the basis of the established bulk-crystal physics and chemistry. III-V device interfaces often have an amorphous structure and extend over several atomic layers, instead of having a monocrystalline and atomically sharp structure. Reasons for that are discussed in this presentation.Therefore, it is not surprising that the III-V interfaces are a weak part of many practical devices (e.g. laser diode, infrared photodiode, micro-LED, HEMT) causing degradation in the current as well as future III-V applications. III-V interfaces include a high density of point defects, as compared to the uniform bulk crystals areas, which cause the electronic defect levels around the band gap at the interfaces, which further cause the electrical and optical losses. Use of III-V crystals is expected to increase in the areas like LED lighting, infrared optoelectronics, and high-speed transistor circuits, where it is not possible to utilize the Si crystals.Thus it is relevant to investigate and develop the III-V interface properties. The surface or interface passivation means a decrease in the defect-level density or/and in the effects of defect levels. It can be expected that the III-V passivation development requires a multistep procedure combining benefits of the different methods.Some commonly used passivation methods (e.g. wet chemistry, hydrogen forming gas, ALD self cleaning) are summarized in this talk. Our group has studied the issue whether it is useful to include ultrahigh-vacuum (UHV) treatments in a large arsenal of different passivation methods. The traditional surface science has been based on UHV technology. Our group has approached the III-V passivation challenge from a surface-science viewpoint. We have investigated in particular the clean surface properties and the surface oxidation in UHV conditions. Our group has addressed the question whether it is beneficial to pre-oxidize III-V surfaces intentionally because it is very difficult to avoid oxidation of III-V device surfaces. Our results support the previous findings that proper oxidation of III-V interfaces is a potential method to improve the devices performance.We have taken a step toward combining the surface science with the electrical characterization of III-V interfaces to find interconnections between electrical properties and surface-science ones. The high-resolution synchrotron x-ray photoelectron spectroscopy (XPS) has been used. Because XPS is a widely used method, we have addressed specific challenges in XPS analysis and that how the XPS analysis can be developed by including theoretical ab initio calculations. In this talk, it is also attempted to make a comparison to the silicon interface passivation.

Correlative analysis of embedded silicon interface passivation by Kelvin probe force microscopy and corona oxide characterization of semiconductor.

We report a correlative analysis between corona oxide characterization of semiconductor (COCOS) and Kelvin probe force microscopy (KPFM) in a study of embedded silicon surfaces in the field of chemical and field-effect passivation. The COCOS approach gives access to the defect density, the total charge contained in the passivation stack, and the potential barrier. Based on the COCOS parameters, we could probe by KPFM to analyze the influence of the passivation stack upon the surface photovoltage. Thus, KPFM emerges as a valuable method to access chemical and field-effect passivation directly. We confirm that it is possible to differentiate by KPFM the influence of local band bending (i.e., field-effect passivation) from the effects due to the local recombination rates (i.e., chemical passivation). The measurements were carried on five different passivation layers of different thicknesses, precisely, 10.5nm SiO2, 50nm SiN, 7nm Al2O3, 7nm HfO2, and a double layer of 7nm Al2O3 below 53nm Ta2O5. Based on our correlative analysis, we could identify by KPFM that HfO2 displays the best chemical passivation properties. Additionally, we confirm that using an anti-reflective coating such as a Ta2O5 layer on top of Al2O3 causes the chemical passivation to deteriorate. Finally, for p-type silicon, SiN appears to be the worst case in terms of field-effect passivation.

Silicon interface passivation studied by modulated surface photovoltage spectroscopy

We demonstrate that the modulated surface photovoltage spectroscopy (modulated SPS) technique can be applied to investigate interface states in the bandgap, i.e. interface passivation, of crystalline silicon coated with a downshift layer such as hydrogenated aluminum nitride with embedded terbium ions by suppressing straylight with a cut-off filter. Different hydrogen contents influence the surface photovoltage spectra at photon energies below the bandgap of crystalline silicon. Modulated SPS reveals that at higher hydrogen content there is a lower signal and, thus, a lower density of surface defect states. Our experiments show that modulated SPS can become a powerful tool for characterizing defect states at interfaces which cannot be easily studied by other methods.

Improved amorphous/crystalline silicon interface passivation for silicon heterojunction solar cells by hot-wire atomic hydrogen during doped a-Si:H deposition

Intrinsic/doped stacked hydrogenated amorphous silicon (a-Si:H) are widely used passivation layers for amorphous/crystalline silicon (a-Si/c-Si) heterojunction solar cells. This work reports that hot wire chemical vapor deposition of doped a-Si:H can significantly modify the property of the underlying intrinsic a-Si:H (a-Si:H(i)) as well as a-Si/c-Si interface passivation, which stems from the in-diffusion of highly reactive atomic hydrogen. Fourier transform infrared spectroscopy, spectroscopic ellipsometry and Raman analyses indicate that the underlying a-Si:H(i) films become more compact and less defected as a result of network reconstruction during doped a-Si:H capping. After this reconstruction, underdense a-Si:H(i) films obtained superior passivation quality than widely used dense layers, despite the inferior quality in the initial state. Effective minority carrier lifetime of c-Si passivated by underdense a-Si:H(i) was 19.9 ms, much higher than 15.2 ms in the case of using dense a-Si:H(i). The porous structure of underdense a-Si:H(i) facilitates hydrogen diffusion towards a-Si/c-Si interface and hence a rapid reduction of interface defect densities occurs, accounting for the better passivation quality. SHJ solar cells (160 μm, 156 × 156 mm2) with industry-compatible process were fabricated, yielding the efficiency up to 23.0% with high Voc values of 741 mV.

Improved Process Stability on an Extremely Thin Amorphous/Crystalline Silicon Interface Passivation Layer by Using Predeposition on the Chamber Wall

In this study, in situ plasma diagnostic systems of optical emission spectrometry (OES) and quadrupole mass spectrometry (QMS) were used to monitor an extremely thin (5–10 nm) intrinsic amorphous/crystalline Si interface passivation film as deposited using plasma-enhanced chemical vapor deposition (PECVD). We observed a dramatic improvement in the quality of the passivation layer with a chamber background environment, even with a background pressure of 1E-6 Torr. When the chamber walls were coated (at a predeposition time of approximately 150 min) to a specified thickness of a few hundred micrometers, the minority carrier lifetime increased by more than 26 times, as compared with an insufficiently coated counterpart with a predeposition of approximately 30 min (from approximately 30 μs to 800 μs). In this predeposition process, the tendency of species to concentrate could be systematically obtained using the in situ OES system, with the chamber environment monitored using residual gas analysis and threshold ionization mass spectrometry of QMS. We found that the optimal predeposition time was 150 min, which enabled the OES intensity of Si*/SiH*, Hβ/Hα, and Hα/SiH* to become stable. OES and QMS were incorporated in this study and were validated using the Fourier-transform infrared spectroscopy absorbance spectra on the films. The plasma condition was stabilized and the minority carrier lifetime was improved to 800 μs. The proposed predeposition process stabilized the chamber environment and gas discharge. Therefore, the optimized value of the minority carrier lifetime could be consistently obtained. A transmission electron microscopy photograph showed a compact a-Si:H layer (of approximately 10 nm) on the c-Si substrate interface passivation layer with a void-free and crystallites-free interface after a predeposition time of 150 min.

Physical mechanism and optimal design of silicon heterojunction solar cells

Silicon heterojunction (SHJ) solar cells are crystalline silicon wafer-based photovoltaic devices fabricated with thin-film deposition technology. The SHJ solar cells hold great potential for large-scale deployment for high conversion efficiencies with low-cost manufacturing. Recently Kaneka Corporation has fabricated an interdigitated-back-contact (IBC) SHJ solar cell with a certified 26.33% conversion efficiency in a large area (180.4 cm2), which is a world record for any 1-sun crystalline silicon wafer-based solar cell. The key feature of SHJ solar cells is the impressive highopen-circuit voltages (Voc) achieved by the excellent amorphous/crystalline silicon interface passivation. Generally, in SHJ solar cells, the boron doped hydrogenated amorphous silicon [(p)a-Si:H] serves as hole collector and the phosphorus doped hydrogenated amorphous silicon [(n) a-Si:H] functions as electron collector. In order to improve the lateral carrier transport of these layers, transparent conductive oxides (TCOs) are usually deposited on both sides of the solar cell. Therefore the parameters such as the heterointerface passivation quality, doping concentration and thickness of the a-Si:H doped layer, and work function of the transparent conductive oxide layer are the key factors that affect the performances of SHJ solar cells. Enormous research efforts have been devoted to studying the effects of the aforementioned influencing parameters on the photovoltaic characteristics of SHJ solar cells. Some research groups have addressed the physical mechanism behind the limitation of the solar cell efficiency. Owing to the insight into the physical mechanism some guidelines for optimally designing the high-performance solar cells in future are obtained. It seems therefore important to summarize the research efforts devoted to the physical mechanism and optimal design of SHJ solar cells.In the present review, we mainly discuss three important issues: 1) the amorphous/crystalline silicon interface passivation; 2) the Schottky barrier resulting from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer; 3) the screening length that is required to efficiently shield the parasitic opposing band from bending originating from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer. The numerical simulation and optimal design of SHJ solar cells are analyzed, and three strategies that may improve the solar cell performances are presented: 1) a hybrid SHJ solar cell structure with a rear heterojunction emitter and a phosphorus-diffused homojunction front surface field; 2) replacing the (p)a-Si:H doped layer by higher doping efficiency microcrystalline silicon alloys such as c-Si:H, c-SiOx:H or c-SiCx:H; 3) replacing the (p)a-Si:H doped layer by higher work function transition metal oxides such as MoOx, WOx or VOx. Finally, the research progress and future development of SHJ solar cells are also described.

Annealing induced amorphous/crystalline silicon interface passivation by hydrogen atom diffusion

Post-annealing is an efficient method to improve passivation quality of the amorphous/crystalline silicon configuration and it’s been widely used in fabrication of amorphous/crystalline silicon heterojunction solar cells. In this study, hydrogenated amorphous silicon thin films are deposited on n type monocrystalline silicon, using a single chamber radio-frequency plasma-enhanced chemical vapor deposition system. After passivation the best result with effective minority carrier lifetime (τeff) exceeding 1600 μs is achieved. Fourier transform infrared spectrum is utilized to investigate the structure evolution after annealing. The result shows that, the improved passivation quality is attributed to interface dangling bonds saturated by H atoms diffusion during annealing, the reason of passivation quality affected by annealing and film thickness is also proposed.

Improved amorphous/crystalline silicon interface passivation for heterojunction solar cells by low-temperature chemical vapor deposition and post-annealing treatment

In this study, hydrogenated amorphous silicon (a-Si:H) thin films are deposited using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system. The Si-H configuration of the a-Si:H/c-Si interface is regulated by optimizing the deposition temperature and post-annealing duration to improve the minority carrier lifetime (τeff) of a commercial Czochralski (Cz) silicon wafer. The mechanism of this improvement involves saturation of the microstructural defects with hydrogen evolved within the a-Si:H films due to the transformation from SiH2 into SiH during the annealing process. The post-annealing temperature is controlled to ∼180 °C so that silicon heterojunction solar cells (SHJ) could be prepared without an additional annealing step. To achieve better performance of the SHJ solar cells, we also optimize the thickness of the a-Si:H passivation layer. Finally, complete SHJ solar cells are fabricated using different temperatures for the a-Si:H film deposition to study the influence of the deposition temperature on the solar cell parameters. For the optimized a-Si:H deposition conditions, an efficiency of 18.41% is achieved on a textured Cz silicon wafer.

Improved amorphous/crystalline silicon interface passivation with two-step intrinsic layers

The implied Voc value of the sample obtained using the two-step i-layers process was much higher than that obtained using the traditional one step i-layer process.

Amorphous/crystalline silicon interface defects induced by hydrogen plasma treatments

Excellent amorphous/crystalline silicon interface passivation is of extreme importance for high-efficiency silicon heterojunction solar cells. This can be obtained by inserting hydrogen-plasma treatments during deposition of the amorphous silicon passivation layers. Prolonged hydrogen-plasmas lead to film etching. We report on the defect creation induced by such treatments: A severe drop in interface-passivation quality is observed when films are etched to a thickness of less than 8 nm. Detailed characterization shows that this decay is due to persistent defects created at the crystalline silicon surface. Pristine interfaces are preserved when the post-etching film thickness exceeds 8 nm, yielding high quality interface passivation.

All in-situ GaSb MOS structures on GaAs (001): Growth, passivation and high-k oxides

The GaSb-on-GaAs growth was optimized for the fabrication of metal-oxide-semiconductor (MOS) capacitors (Caps) with low interface state trap density (Dit) using in-situ deposited amorphous silicon (a-Si) interface passivation layer (IPL) and high-k oxides. The best top surface with the average roughness Ra=0.37nm and with spiral type “step-flow” growth mode was observed in the GaSb structure with the initial 0.5μm grown at 410°C and the top 0.5μm grown at 485°C. N- and p-type GaSb MOSCaps with reasonable capacitance–voltage(C–V) characteristics at room temperature (RT) were demonstrated using all in-situ 0.5nm a-Si IPL and 10nm Al2O3+HfO2 or Al2O3. A-Si IPL was found essential for n-MOSCaps but not in the case of p-MOSCaps where comparable C–V characteristics with a similarly low Dit=1–2×1012cm−2eV−1were demonstrated without IPL.

Crystalline silicon interface passivation improvement with a-Si1−xCx:H and its application in hetero-junction solar cells with intrinsic layer

Excellent passivation of an n-type Czochralski crystalline silicon surface is made possible by the deposition of hydrogenated silicon carbide (Si1−xCx:H) layers in the electron cyclotron resonance chemical vapor deposition. We investigate the structural effect with various CH4/SiH4 dilution ratios, and the lowest effective surface recombination velocity (21.03 cm/s) that can be obtained. We also demonstrate that the Voc can be improved more than 200 mV by inserting Si1−xCx:H layers to form hetero-junction with intrinsic thin layer (HIT) solar cells. The conversion efficiency of the planar HIT solar cell with μc-Si emitter can reach 13%.

Damage at hydrogenated amorphous/crystalline silicon interfaces by indium tin oxide overlayer sputtering

Damage of the hydrogenated amorphous/crystalline silicon interface passivation during transparent conductive oxide sputtering is reported. This occurs in the fabrication process of silicon heterojunction solar cells. We observe that this damage is at least partially caused by luminescence of the sputter plasma. Following low-temperature annealing, the electronic interface properties are recovered. However, the silicon-hydrogen configuration of the amorphous silicon film is permanently changed, as observed from infra-red absorbance spectra. In silicon heterojunction solar cells, although the as-deposited film's microstructure cannot be restored after sputtering, no significant losses are observed in their open-circuit voltage.

In Situ Deposited HfO2 with Amorphous-Si Passivation as a Potential Gate Stack for High Mobility (In)GaSb- Based P-MOSFETs

Electrical and structural properties of GaSb metal-oxide-semiconductor capacitors with in-situ deposited HfO2 gate dielectric and in-situ deposited amorphous silicon interface passivation layer are presented. Capacitance-voltage characteristics with low C-V stretch out, reduced hysteresis, lower EOT is obtained in a temperature range of 3500C-5500C post deposition annealing in forming gas. The gate leakage current of the gate stack was ≤ 1μA/cm2. p-MOSFETs fabricated on epitaxially grown strained (In)GaSb quantum channel (ε=1.6%) channels using this all in-situ high-k gate stack showed a maximum ION=23mA/mm for a 3μm gate length device

Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition. We show a dramatic improvement in passivation when H2 plasma treatments are used during film deposition. Although the bulk of the a-Si:H layers is slightly more disordered after H2 treatment, the hydrogenation of the wafer/film interface is nevertheless improved with as-deposited layers. Employing H2 treatments, 4 cm2 heterojunction solar cells were produced with industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm2 cells.

Metal-oxide-semiconductor field-effect-transistors on indium phosphide using HfO2 and silicon passivation layer with equivalent oxide thickness of 18 Å

In this letter, we demonstrate the electrical properties of metal-oxide-semiconductor capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs) on InP using atomic layer deposited HfO2 gate dielectric and a thin silicon interface passivation layer (Si IPL). Compared with single HfO2, the use of Si IPL results in better interface quality with InP substrate, as illustrated by smaller frequency dispersion and reduced hysteresis. MOSFETs with Si IPL show much higher drive current and transconductance, improved subthreshold swing, interface-trap density and gate leakage current with equivalent oxide thickness scaling down to 18 Å.

A high performance In0.53Ga0.47As metal-oxide-semiconductor field effect transistor with silicon interface passivation layer

In this letter, we demonstrate a high performance In0.53Ga0.47As channel n-type metal-oxide-semiconductor field effect transistor with silicon interface passivation layer (IPL) and HfO2 gate oxide. Owing to the effectiveness of Si IPL on improving the interface quality, good device characteristics have been obtained, including the peak transconductance of 7.7 mS/mm (Lg=5 μm and Vd=50 mV), drive current of 158 mA/mm (Lg=5 μm, Vgs=Vth+2 V, and Vd=2.5 V), and the peak effective channel mobility of 1034 cm2/V s. As an important factor on device design, the impact of silicon IPL thickness on the transistor characteristics has been investigated.

Effects of Anneal and Silicon Interface Passivation Layer Thickness on Device Characteristics of In[sub 0.53]Ga[sub 0.47]As Metal-Oxide-Semiconductor Field-Effect Transistors

In this letter, we demonstrate an In 0.53 Ga 0.47 As channel n-type metal-oxide-semiconductor field-effect transistor (MOSFET) with silicon interface passivation layer (IPL) and HfO 2 gate oxide. The effects of the source/drain activation temperature, postdeposition annealing temperature, and thickness of silicon IPL on the transistor characteristics have been investigated. The results suggest that the annealing temperatures are critical for determining the transistor performances. Even though In 0.53 Ga 0.47 As is easier to obtain the unpinned surface Fermi level compared to GaAs, applying silicon IPL still improves the In 0.53 Ga 0.47 As nMOSFET characteristics significantly through engineering the HfO 2 /In 0.53 Ga 0.47 As interface quality.