The role of an amorphous SiC:H `buffer' in the high-performanceμc-SiC:H/a-SiC:H/poly-Si heterojunction solar cells

High-efficiency solar cells have been developed using relatively simple processing at low temperatures up to 300°C. The cells studied were p⁺ μc-SiC:H/p a-SiC:H (buffer)/n poly-Si and n⁺ μc-SiC:H/n a-SiC:H (buffer)/p poly-Si heterojunctions fabricated by the electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD) method. The thin amorphous buffer layer played an important role in improving the photovoltaic performance. The optimization of the buffer layer thickness resulted in a conversion efficiency of η=15.4% under AM1 solar simulated radiation of 100 mW/cm²     

Optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films

The present work reports on the optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films. Optical transmission experiments have shown that the optical properties of the films are affected by thermal annealing when T_a>600 °C, due to emission of hydrogen bonded to silicon. Although the electrical properties of Al/a-SiC:H Schottky diodes are invariant for T_a?400 °C, for higher T_a these properties are improved with the optimum result achieved at . At this annealing temperature the linear log I-V characteristics span about eight orders of magnitude and the ideality factor is 1.09±0.04, making these diodes very interesting for many potential applications. For higher T_a (>600 °C) the electrical properties of Al/a-SiC:H Schottky diodes deteriorate with complete degradation at . For temperatures up to 600 °C this behavior is attributed to relaxation of the strain in the amorphous network which is possibly combined with weak hydrogen emission for temperatures up to 600 °C, leading to an optimum material quality. For further increase of T_a (>600 °C) the observed deterioration of the electrical properties of Al/a-SiC:H Schottky diodes is due to the intensive emission of hydrogen atoms bonded to silicon that cause voids in the amorphous network. These results are also supported by the experimental values of the room temperature apparent barrier height of the Al/a-SiC:H junction ?_bRT and its temperature coefficient γ.     

Mechanical stress in PECVD a-SiC:H: aging and plasma treatments effects

Hydrogenated amorphous silicon carbide (a-SiC:H) deposited by PECVD is one of the most promising dielectric diffusion barrier available in Cu—Ultra low k interconnections due to its low dielectric constant and good barrier ability. In this work, the mechanical stress evolution with time of a-SiC:H film exposed to room atmosphere is studied and compared with the behavior observed on other PECVD dielectrics (SiN, SiO₂, SiCN). For as-deposited a-SiC:H samples, a strong stress evolution with time toward compression is observed and the results are interpreted mainly in terms of surface reactivity and silanol buildup. Infrared spectroscopy analysis allows to confirm that the mechanical stress evolution and the OH content are linked. An oxidation of the hydrogenated amorphous silicon carbide film with time is also observed. Different plasma treatments (He, O₂ or H₂) are tested on a-SiC:H films to limit the stress drift with time. Each plasma treatments are able to limit the stress evolution of a-SiC:H films but the mechanisms are different in each case: densification of the film with He plasma treatment, formation of a dense oxide at the surface with O₂ plasma treatment and passivation of dangling bonds with H₂ plasma treatment.     

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40 MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiO_x:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiO_x:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density J_sc. The increased J_sc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680 nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.     

本征微晶硅薄膜的结构演变及其对太阳电池光伏性能的影响

采用高压射频等离子体增强化学气相沉积(RF-PECVD)方法制备本征硅薄膜和n-i-P结构太阳电池,研究了氢稀释率对本征硅薄膜的电学特性和结构特性的影响.采用光发射谱(OES)和喇曼(Raman)散射光谱研究了处于过渡区的本征硅薄膜的纵向结构演变过程.结果表明:光发射谱和喇曼散射光谱可以作为研究硅薄膜的纵向结构演变有效手段.随着氢稀释率的增加,硅薄膜从非晶相向微晶相过渡时,其纵向结构的改变会严重影响硅薄膜太阳电池的光伏性能.     

Ion beam synthesis of low resistivity contacts in amorphous silicon-based materials

Low resistivity layers have been formed at low process temperatures, by high dose Co⁺ ion implantation in to hydrogenated amorphous silicon (a-Si:H) and amorphous silicon carbide (a-SiC:H). The lowest resistivities, of the order of 10 ohms/Sq, have been observed for the carbon-free films (a-Si:H) and can be obtained at annealing temperatures of <250°C. Schottky barrier contacts to the a-Si:H films exhibit near ideal behavior with low leakage currents, of the order 10⁻⁹ A·cm⁻². The electrical properties of the amorphous films are assessed as a function of ion dose, dose rate and annealing conditions, with a view to optimizing these parameters.     

Genuine wide-bandgap microcrystalline emitter Si-HBT with enhancedcurrent gain by suppressing homocrystallization

The energy bandgap of microcrystalline silicon (μc-Si) emitter prepared by the plasma CVD method for Si-HBTs was investigated. The μc-Si films directly deposited on c-Si substrates were confirmed to have almost the same energy bandgap as c-Si because of μc-Si crystallization, resulting in formation of a homojunction. In order to suppress such a homojunction formation, a c-Si surface modification method using an a-SiC thin layer was proposed. The a-SiC layer was confirmed to have the effect of producing an abrupt and uniform heterojunction. A current gain as high as 523 was obtained by using the a-SiC thin layer, which was 24 times larger than that without the a-SiC layer     

Influence of hydrogen dilution and substrate temperature on growth of nanocrystalline hydrogenated silicon carbide films deposited by RF sputtering

Hydrogenated nanocrystalline silicon carbide (nc-SiC: H) thin films were prepared by radiofrequency magnetron sputtering. Deposition was effectuated in plasma of Argon and Hydrogen mixture with several proportions (30-80% H₂) and at different substrate temperatures (ambient, 500 °C). The films microstructure was studied by means of FTIR and Raman spectroscopy. These two techniques helped us to have an idea on the composition of our samples and the existing species. A comparative study of the obtained results has allowed us to make conclusions about the role of both hydrogen dilution and substrate temperature on deposition of layers with good parameters in terms of crystallinity and optical properties. These observations were correlated with those obtained by diffraction and high-resolution TEM.     

p型微晶硅薄膜的沉积及其在微晶硅薄膜太阳电池中的应用

采用等离子体化学气相沉积(PECVD)方法制备了硼掺杂微晶硅薄膜和微晶硅薄膜太阳电池.研究了乙硼烷含量、p型膜厚度及沉积温度对硼掺杂薄膜生长特性和高沉积速率的电池性能的影响.通过对p型微晶硅薄膜沉积参数的优化,在本征层沉积速率为0.78nm/s的高沉积速率下,制备了效率为5.5%的单结微晶硅薄膜太阳电池.另外,对P型微晶硅薄膜的载流子疏输运机理进行了讨论.     

Effects of deposition pressure on the microstructural and optoelectrical properties of B-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by hot-wire chemical vapor deposition

We report on the effects of deposition pressure P_d on the growth and properties of the B-doped nanocrystalline silicon (nc-Si:H) thin films grown by hot-wire chemical vapor deposition (HWCVD) at very high hydrogen dilution of 98.8%. We found that the crystallinity of nc-Si:H or μc-Si:H films is not only determined by hydrogen dilution but also the concentration ratio of atomic H to SiH₃ ([H]/[SiH₃]) on the growing surface which is varied with deposition pressure P_d. Furthermore, there is a threshold of [H]/[SiH₃] ratio which we name as overfull hydrogen (OH). When the [H]/[SiH₃] ratio is lower than the threshold OH ([H]/[SiH₃]<OH), the crystallinity of the nc-Si:H or μc-Si:H films increases with increasing [H]/[SiH₃] ratio. But when the [H]/[SiH₃] ratio is higher than the threshold OH ([H]/[SiH₃]>OH), the crystallinity decreases with increasing [H]/[SiH₃] ratio. Finally, the high conductivity of 4.22 S cm⁻¹ of the B-doped nc-Si:H thin film deposited at 15 Pa is obtained.     

Optimization of Al/a-SiC:H optical sensor device by means of thermal annealing

The optimization of optoelectronic properties of Al/a-SiC:H Schottky diodes grown as Al/a-SiC:H/c-Si(n) structures is studied by means of thermal annealing of a-SiC:H thin films. According to the spectral response of the Schottky diodes the measured quantum efficiency, η_measured, increases with increasing annealing temperature (400–600 °C), whereas η_measured decreases for T_a>600 °C. For T_a=600 °C, optimum material quality of a-SiC:H films is achieved and the spectral response of the Al/a-SiC:H/c-S(n) structures present very high and almost constant values (η_measured80%) for the whole range of wavelengths from 500 up to 850 nm. These results show that our Al/a-SiC:H/c-S(n) structures can be very attractive as optical sensors. Diffusion length calculations as well as the mobility by lifetime product (μτ)_p of the minority carriers (holes) of a-SiC:H films present a dependence on T_a similar to that of the measured quantum efficiency. Finally, the quantum efficiency of films processed with T_a=675 °C is found to increase when the Al/a-SiC:H/c-S(n) structures are exposed to hydrogen, a result that could be promising for the construction of a hydrogen detection sensor.     

New precursors for CVD copper metallization

A novel CVD copper process is described using two new copper CVD precursors, KI3 and KI5, for the fabrication of IC or TSV (Through Silicon Via) copper interconnects. The highly conformal CVD copper can provide seed layers for subsequent copper electroplating or can be used to directly fabricate the interconnect in one step. These new precursors are thermally stable yet chemically reactive under CVD conditions, growing copper films of exceptionally high purity at high growth rates. Their thermal stability can allow for elevated evaporation temperatures to generate the high precursor vapor pressures needed for deep penetration into high aspect ratio TSV vias. Using formic acid vapor as a reducing gas with KI5, copper films of >99.99 atomic % purity were grown at 250 °C on titanium nitride at a growth rate of > 1500 Å/min. Using tantalum nitride coated TSV type wafers, ∼ 1700 Å of highly conformal copper was grown at 225 °C into 32 μm × 5 μm trenches with good adhesion. With ruthenium barriers we were able to grow copper at 125 °C at a rate of 20 Å/min to give a continuous ∼ 300 Å copper film. In this respect, rapid low temperature CVD copper growth offers an alternative to the long cycle times associated with copper ALD which can contribute to copper agglomeration occurring.     

Deposition of highly efficient microcrystalline silicon solar cells under conditions of low H2 dilution: the role of the transient depletion induced incubation layer

This paper addresses the plasma deposition of highly efficient microcrystalline silicon (μc‐Si:H) p‐i‐n solar cells under conditions of high SiH₄ utilization and low H₂ dilution. It was established that the transient depletion of the initially present SiH₄ source gas induces the formation of an amorphous incubation layer that prevents successful crystallite nucleation in the i‐layer and leads to poor solar cell performance. The effect of this transient depletion induced incubation layer on solar cells was made visible through dedicated solar cell deposition series and selected area electron diffraction measurements. Applying a gas flow procedure at plasma ignition it was succeeded to prepare state‐of‐the‐art μc‐Si:H material and solar cells under low hydrogen dilution conditions, highlighted by μc‐Si:H solar cells of up to 9·5% efficiency prepared using an undiluted source gas flow consisting solely of SiH₄. Copyright © 2007 John Wiley & Sons, Ltd.     

掺杂室沉积本征微晶硅材料及其在太阳能电池中的应用

在掺杂P室采用甚高频等离子体增强化学气相沉积（VHF—PECVD）技术,制备了不同硅烷浓度条件下的本征微晶硅薄膜．对薄膜电学特性和结构特性的测试结果分析表明：随硅烷浓度的增加,材料的光敏性先略微降低后提高,而晶化率的变化趋势与之相反;X射线衍射（xRD）测试表明材料具有（220）择优晶向．在P腔室中用VHF—PECVD方法制备单结微晶硅太阳能电池的i层和p层,其光电转换效率为4．7％,非晶硅／微晶硅叠层电池（底电池的p层和i层在P室沉积）的效率达8．5％．     

Influence of substrate on the growth of microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition

Microcrystalline silicon (μc-Si) thin films are widely used for silicon thin film solar cells, especially in the high performance tandem solar cells which comprise an amorphous silicon junction at the top and a μc-Si junction at the bottom. One of the major factors affecting the photovoltaic properties of μc-Si thin film solar cells of thin films is the quality of the μc-Si thin films. In this work, we investigated the effect of substrates on the crystallization characteristics and growth behaviors of μc-Si thin films grown by the plasma enhanced chemical vapor deposition method (PECVD), and found that substrates have a strong effect on the crystallization characteristics of μc-Si thin films. In addition, the growth rate of μc-Si thin films was also highly influenced by the substrates. Three types of substrates, quartz glass, single crystalline silicon and thermally oxidized single crystalline silicon, were used for growing μc-Si thin films from SiH₄/H₂ with a flow rate ratio 2:98 at different temperatures. Crystallization characteristics of these μc-Si thin films were studied by Raman scattering and X-ray diffraction techniques.     

低温高速率沉积非晶硅薄膜及太阳电池

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,保持沉积温度在125℃制备非晶硅薄膜材料及太阳电池。在85 Pa的低压下以及400～667 Pa的高压下,改变Si H4浓度和辉光功率等沉积参数,对本征a-Si材料的性能进行优化。结果表明,在高压下,合适的Si H4浓度和压力功率比可以使a-Si材料的光电特性得到优化,并且薄膜的沉积速率得到一定程度的提高。采用低压低速和高压高速的沉积条件,在125℃的低温条件下制备出效率为6.7%的单结a-Si电池,高压下本征层a-Si材料的沉积速率由0.06～0.08 nm/s提高到0.17～0.19 nm/s。     

Microstructure evolution of hydrogenated silicon thin films

This paper describes the microstructure evolution of hydrogenated silicon films containing various amounts of hydrogen. Microcrystalline silicon films were produced when the hydrogen content of the films was adjusted by using the diluted hydrogen and hydrogen atom treatment methods. Polycrystalline silicon films having grain sizes in the micrometre range were deposited at low temperatures (250°C) by electron cyclotron resonance chemical vapour deposition with the hydrogen dilution method. The micro crystalline and polycrystalline films were characterized by NMR, FTIR, Raman, X-ray and optical spectroscopy and electrical measurements. The results suggest the possibility of even larger grain silicon films suitable for high-performance solar cells which avoid the fundamental difficulties of amorphous Si:H solar cells.     

Free electron laser annealing of silocon carbibe

We have applied a free electron laser (FEL) to crystallize amorphous silicon carbide (a-SiC) and to remove the damage and activate the dopant of a damaged layer of nitrogen implanted cubic silicon carbide (3C-SiC) films at room temperature. The FEL has two main characteristics, wavelength tunability and ultrashort-pulse operation (~10 ps) with intense peak power (~MW). The wave-length was selected at the energy of the Si-C stretch mode in order to excite the lattice vibration directly. We observed the crystallization of a-SiC occurs at room temperature when irradiation with a 12.6 μm FEL. The present results indicate that FEL annealing (12.6 μm: transverse optical mode, 10.3 μm: longitudinal optical mode) is effective for recrystallization and activation of an ion-implanted SiC films.     

Industrial high‐rate (∼5 nm/s) deposited silicon nitride yielding high‐quality bulk and surface passivation under optimum anti‐reflection coating conditions

High‐quality surface and bulk passivation of crystalline silicon solar cells has been obtained under optimum anti‐reflection coating properties by silicon nitride (a‐SiN_x:H) deposited at very high deposition rates of ∼5 nm/s. These a‐SiN_x:H films were deposited using the expanding thermal plasma (ETP) technology under regular processing conditions in an inline industrial‐type reactor with a nominal throughput of 960 solar cells/hour. The low surface recombination velocities (50–70 cm/s) were obtained on p‐type silicon substrates (8·4 Ω cm resistivity) for as‐deposited and annealed films within the broad refractive index range of 1·9–2·4, which covers the optimum bulk passivation and anti‐reflection coating performance reached at a refractive index of ∼2·1. Copyright © 2005 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: Thin‐film free‐standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin‐film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS‐HJ). High crystallographic quality mono‐Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free‐standing mono‐Si (FMS) films were processed into solar cells by creating a‐a‐Si/c‐Si heterojunction. In our preliminary experiments a thin‐film FMS‐HJ solar cell with 9.6% efficiency was realized in a 20‐μμm‐thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.