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Author: Yong Liu Publisher: ISBN: Category : Languages : en Pages : 170
Book Description
Hydrogenated amorphous silicon germanium (a-SiGe:H) films and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with small amount of H2, was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, and quantum efficiency were characterized during the process of pursuing good materials. High-quality material was successfully fabricated with the ECR-PECVD technique at high growth rates. The device we made with 1.47 eV band gap has a fill factor of 64.5%. With the graded band gap and graded doping techniques, 70% fill factor was achieved when the band gap was graded from 1.75 to 1.47 eV. We also got 68% fill factor with the band gap graded form 1.75 to 1.38 eV.
Author: Yong Liu Publisher: ISBN: Category : Languages : en Pages : 170
Book Description
Hydrogenated amorphous silicon germanium (a-SiGe:H) films and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with small amount of H2, was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, and quantum efficiency were characterized during the process of pursuing good materials. High-quality material was successfully fabricated with the ECR-PECVD technique at high growth rates. The device we made with 1.47 eV band gap has a fill factor of 64.5%. With the graded band gap and graded doping techniques, 70% fill factor was achieved when the band gap was graded from 1.75 to 1.47 eV. We also got 68% fill factor with the band gap graded form 1.75 to 1.38 eV.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Hydrogenated amorphous silicon germanium films (a-SiGe:H) and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large-scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert-gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. Preferential attachment of hydrogen to silicon, clustering of Ge and Si, and columnar structure and buried dihydride radicals make the film intolerably bad. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with a small amount of H[sub 2], was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, I-V curve, and quantum efficiency were characterized during the process of pursuing good materials. The microstructure of the a-(Si, Ge):H material was probed by Fourier-Transform Infrared spectroscopy. They found that the advantages of using helium as the main plasma species are: (1) high growth rate--the energetic helium ions break the reactive gas more efficiently than hydrogen ions; (2) homogeneous growth--heavy helium ions impinging on the surface promote the surface mobility of the reactive radicals, so that heteroepitaxy growth as clustering of Ge and Si, columnar structure are reduced; (3) surface hydrogen removal--heavier and more energetic helium ions break the Si-H much easier than hydrogen ions. The preferential attachment of Si-H to Ge-H is reduced. They also found that with the small amount of hydrogen put into the plasma, the superior properties of a-(Si, Ge):H made from pure hydrogen dilution plasma were still maintained. These hydrogen ions help to remove the subsurface weakly bonded hydrogen and buried hydrogen. They also help to passivate the Ge-dangling bond.
Author: Matthew Alan Ring Publisher: ISBN: Category : Languages : en Pages : 180
Book Description
Ion bombardment is inherent in the growth of amorphous materials by conventional PECVD methods, such as electron cyclotron resonance (ECR) of radio-frequency (rf) discharge. In these methods plasma ions are necessary to decompose the source gases; however, these ions also impinge upon the growing film surface, imparting their energy to the material. In conventional deposition techniques it is difficult to isolate the effects of the ions so a novel approach is taken in this research where an ECR ion source is attached to a "hot-wire" deposition reactor. This unique reaction system allows researchers to vary the ion bombardment density and energy to study the effects of the ion bombardment on the resulting material. Throughout this research, HW-ECR materials outperformed the HWCVD materials deposited at identical substrate temperatures for photovoltaic applications. The addition of ion bombardment saw a substantial decrease in Urbach energy, hydrogen microstructure, and a corresponding increase in photoconductivity and photosensitivity, regardless of band-gap. During the experiments, the ECR microwave power applied to the reactor was adjusted and showed that as band-gap decreased, less ion energy was required to show improvements in material quality. In addition, a lower filament temperature was required as band-gap decreased to maintain a high photoconductivity. The first ever solar cell devices having intrinsic layers deposited by HW-ECR were deposited in this work, and show that this deposition method can produce solar cells with performance on par with current PECVD and HWCVD materials. In addition, these devices are deposited at much higher growth rates, reducing fabrication time dramatically. The standard growth model fails to address the role of ion bombardment; however, J. Robertson of Cambridge University has proposed a model for the role of hydrogen ions in the removal of excess hydrogen. The results presented here support that model, and in addition, neutral ions, such as helium, are shown to improve the material. Helium ions are postulated to aide mostly in the surface diffusion of reactive radicals during deposition, as helium is a non-reactive chemical species.
Author: Pio Capezzuto Publisher: Elsevier ISBN: 0080539106 Category : Science Languages : en Pages : 339
Book Description
Semiconductors made from amorphous silicon have recently become important for their commercial applications in optical and electronic devices including FAX machines, solar cells, and liquid crystal displays. Plasma Deposition of Amorphous Silicon-Based Materials is a timely, comprehensive reference book written by leading authorities in the field. This volume links the fundamental growth kinetics involving complex plasma chemistry with the resulting semiconductor film properties and the subsequent effect on the performance of the electronic devices produced. Focuses on the plasma chemistry of amorphous silicon-based materials Links fundamental growth kinetics with the resulting semiconductor film properties and performance of electronic devices produced Features an international group of contributors Provides the first comprehensive coverage of the subject, from deposition technology to materials characterization to applications and implementation in state-of-the-art devices
Author: Jianhua Zhu Publisher: ISBN: Category : Languages : en Pages : 124
Book Description
High quality [mu]c-SiGe:H is a very promising alternative for high efficiency photovoltaic device. The preparation of this material requires very high hydrogen dilution ratio in PECVD system. The deposition rate was greatly limited by this high hydrogen ratio. In this work, ECR PECVD technique is used to deposit [mu]c-SiGe:H material. The growth rate can be greatly enhanced by taking advantage of the high plasma density and low ion energy features of ECR and extremely high hydrogen dilution ratios are no longer necessary for [mu]c-SiGe:H growth. Films with good crystallinity were prepared at hydrogen dilution ratio as low as 1:15. An intensive study has been completed for the [mu]c-SiGe:H with 0 to over 75% Ge incorporated. The optical bandgap shrinks with the incorporation of Ge into the material. Raman spectra and the increase of activation energy and photosensitivity indicates the deterioration of crystallinity by adding Ge to the Si structure. Solar cell devices using [mu]c-SiGe:H as the active layer were deposited on stainless steel substrates. Fill factors over 55% were achieved for [mu]c-SiGe devices with less than 35% Ge. An [mu]c-Si buffer layer between n+ and[mu]c-SiGe:H n layer was used in the device design and this buffer layer revealed to be very beneficial to the device performance and the growth rate of [mu]c-SiGe:H active layer. C-V measurements showed that the accidental oxygen leakage can raise the doping level to the order of 1E17cm−3. ppm level TMB can be mixed in the source gas to very effectively reduce the N-type doping brought by oxygen. Short circuit current was increased by the TMB counter doping. The minority carrier diffusion length was estimated from reversed bias QE and C-V measurements. In the [mu]c-SiGe:H devices fabricated by ECR PECVD, the hole diffusion length is several tenth micrometers. The accidental doping in the [mu]c-SiGe:H deteriorates the device performance by decreasing the minority carrier diffusion length. Compensating doping of TMB can increase minority carrier diffusion length L[subscript p] and improve short circuit current, and hence improve the conversion efficiency of solar cell device.
Author: Kevin A. McComber Publisher: ISBN: Category : Languages : en Pages : 136
Book Description
The integration of photonics with electronics has emerged as a leading platform for microprocessor technology and the continuation of Moore's Law. As electronic device dimensions shrink, electronic signals encounter crippling delays and heating issues such that signal transduction across large on-chip distances becomes increasingly more difficult. However, these issues may be mitigated by the use of photonic interconnects combined with electronic devices in electronic-photonic integrated circuits (EPICs). The electronics in proposed EPIC designs perform the logic operations and short-distance signal transmission, while photonic devices serve to transmit signals over longer lengths. However, the photonic devices are large compared to electronic devices, and thus the two types of devices would ideally exist on separate levels of the microprocessor stack in order to maximize the amount of silicon substrate available for electronic device fabrication. A CMOS-compatible back-end process for the fabrication of photonic devices is necessary to realize such a three-dimensional EPIC. Back-end processing is limited in thermal budget and does not present a single-crystal substrate for epitaxial growth, however, so high-quality crystal fabrication methods currently used for photonic device fabrication are not possible in back-end processing. This thesis presents a method for the fabrication of high-quality germanium single crystals using CMOS-compatible back-end processing. Initial work on the ultra-high vacuum chemical vapor deposition of polycrystalline germanium on amorphous silicon is presented. The deposition can be successfully performed by using a pre-growth hydrofluoric acid dip and by limiting the thickness of the amorphous silicon layer to less than 120 nm. Films deposited at temperatures of 350° C, 450° C, and 550° C show (110) texture, though the texture is most prevalent in growths at 450° C. Poly-Ge grown at 4500 C is successfully doped n-type in situ, and the grain size of as-grown material is enhanced by lateral growth over a barrier. Structures are fabricated for the growth of Ge confined in one dimension. The growths show faceting across large areas, in contrast to as-deposited poly-Ge, corresponding to enhanced grain sizes. Growth confinement is shown to reduce the defect density as the poly-Ge grows. When coalesced into a continuous film, the material grown from 1 D confinement exhibits a lower carrier density and lower trap density than as-deposited poly-Ge, indicating improved material quality. We measure an increased grain size from as-deposited poly-Ge to Ge grown from ID confinement. Single-crystal germanium is grown at 450° C from confinement in two dimensions. Such growths exhibit faceting across the entire crystal as well as the presence of E3 boundaries ({111} twins), with many growths showing no other boundaries. These twins mediate the growth of the crystal, as they serve as the points for heterogeneous surface nucleation of adatom clusters. The twins can form after the crystal nucleates and are strongly preferred in order to obtain appreciable crystal growth rates. We model the growths from the confining channels in order to find the optimum channel geometry for large, uniform, single-crystal growths that consistently emerge from the channel. The growths from 2D confinement show lower trap density than those from 1 D confinement, indicating a further enhancement of the crystal quality due to the increased confinement. This method of single-crystal growth from an amorphous substrate is extensible to any materials system in which selective non-epitaxial deposition is possible.
Author: Yong Liu
Author: Yong Liu
Author: 
Author: Matthew Alan Ring
Author: Pio Capezzuto
Author: Tien H. Nguyen
Author: James Robert Doyle
Author: Jianhua Zhu
Author: 
Author: Todd William Schroeder
Author: Kevin A. McComber