Device-quality Films Research Articles

Anneal treatment to reduce the creation rate of light-induced metastable defects in device-quality hydrogenated amorphous silicon

We observe a dramatic reduction in the Staebler–Wronski metastable defect creation efficiency in device-quality films of hydrogenated amorphous silicon after they undergo a 20 min anneal treatment at 350 to 400 °C. After several hours of rapid degradation with a high intensity pulsed laser, there are about half as many total dangling bond defects in the annealed samples as in unannealed control samples. This reduction is observed in both 1.02-μm- and 0.22-μm-thick films, indicating it is not a surface-related artifact. The improved stability is likely caused by H motion, which restructures the Si–Si network and H-related nanovoids.

Pulsed laser deposition film of a donor–acceptor–donor polymer as possible active layer in devices

A molecule having a ketone group between two thiophene groups was synthesized. Presence of alternating electron donating and accepting moieties gives this material a donor-acceptor-donor (DAD) architecture. PolyDAD was synthesized from DAD monomer by oxidative polymerization. Device quality films of polyDAD were fabricated using pulsed laser deposition technique. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectra (FTIR) data of both as synthesized and film indicate the material does not degrade during ablation. Optical band gap was determined to be about 1.45 eV. Four orders of magnitude increase in conductivity was observed from as synthesized to pulsed laser deposition (PLD) fabricated film of polyDAD. Annealing of polyDAD films increase conductivity, indicating better ordering of the molecules upon heating. Rectifying devices were fabricated from polyDAD, and preliminary results are discussed.

A low-temperature crystallization path for device-quality ferroelectric films

We show a path for low-temperature crystallization of device-quality solution-processed lead zirconate titanate films. The essential aspect of the path is to circumvent pyrochlore formation at around 300 °C during temperature increase up to 400 °C. By maintaining enough carbon via pyrolysis at 210 °C, well below the temperature for pyrochlore formation, Pb2+ can be reduced to Pb0. This leads to the lack of Pb2+ in the film to suppress the development of pyrochlore, which accounts for the usual high-temperature conversion to perovskite. Films on metal, metal/oxide hybrid, and oxide bottom electrodes were successfully crystallized at 400–450 °C.

Fabrication of device quality films of high loaded PPy/MWCNT nanocomposites using pulsed laser deposition

Polypyrrole (PPy) – multiwalled carbonnanotubes (MWCNT) nanocomposites with various MWCNT loading were prepared by in situ inversion emulsion polymerization technique. High loading of the nano filler were evaluated because of available inherent high interface area for charge separation in the nanocomposites. Solution processing of these conducting polymer nanocomposites is difficult because, most of them are insoluble in organic solvents. Device quality films of these composites were prepared by using pulsed laser deposition techniques (PLD). Comparative study of X-ray photoelectron spectroscopy (XPS) of bulk and film show that there is no chemical modification of polymer on ablation with laser. TEM images indicate PPy layer on MWCNT surface. SEM micrographs indicate that the MWCNT’s are distributed throughout the film. It was observed that MWCNT in the composite held together by polymer matrix. Further more MWCNT diameter does not change from bulk to film indicating that the polymer layer remains intact during ablation. Even for very high loadings (80 wt.% of MWCNT’s) of nanocomposites device quality films were fabricated, indicating laser ablation is a suitable technique for fabrication of device quality films. Conductivity of both bulk and films were measured using collinear four point probe setup. It was found that overall conductivity increases with increase in MWCNT loading. Comparative study of thickness with conductivity indicates that maximum conductivity was observed around 0.2 μm.

Doping for Speed: Colloidal Nanoparticles for Thin-Film Optoelectronics

Solution-processable semiconductor materials ranging from conjugated polymers and small organic molecules to colloidal inorganic nanoparticles are being studied for applications in both low-cost solar cells and photodetectors. High-quality thin films of many inorganic semiconductors can be prepared by techniques such as chemical vapor deposition, molecular beam epitaxy, and atomic layer deposition. In contrast, preparing device-quality films of inorganic materials from colloidal solutions can be more difficult due to the challenge of achieving well-defined doping, controlled trap densities, and reproducible surface chemistry. Nevertheless, solution deposition using colloidal precursors is an attractive goal because of the potential for low-cost, large-area processing. In recent years, a great deal of effort has focused on the colloidal synthesis of wide-band-gap metal oxides such as TiO(2), visible-absorbing II-VI compounds such as CdSe, and small-band-gap materials such as PbSe. Much of the work on visible- and IR-absorbing photodetectors has been done on materials containing metals such as Cd and Pb. A new paper in this issue demonstrates photoconductive detectors made from Cu-containing In(2)S(3) nanoplates. The incorporation of Cu into the In(2)S(3) nanoplates leads to a significant decrease in the lifetime of the photoexcited carriers, resulting in significantly faster response times for the photodetectors processed from colloidal solution.

Hydrazine-based deposition route for device-quality CIGS films

A simple solution-based approach for depositing CIGS (Cu–In–Ga–Se/S) absorber layers is discussed, with an emphasis on film characterization, interfacial properties and integration into photovoltaic devices. The process involves incorporating all metal and chalcogenide components into a single hydrazine-based solution, spin coating a precursor film, and heat treating in an inert atmosphere, to form the desired CIGS film with up to micron-scaled film thickness and grain size. PV devices (glass/Mo/CIGS/CdS/i-ZnO/ITO) employing the spin-coated CIGS and using processing temperatures below 500 °C have yielded power conversion efficiencies of up to 10% (AM 1.5 illumination), without the need for a post-CIGS-deposition treatment in a gaseous Se source or a cyanide-based bath etch. Short-duration low-temperature ( T < 200 °C) oxygen treatment of completed devices is shown to have a positive impact on the performance of initially underperforming cells, thereby enabling better performance in devices prepared at temperatures below 500 °C.

Optimization of growth conditions of ZnO nano thin films by chemical double dip technique

Zinc oxide (ZnO) nano thin films have been deposited by the chemical double-dip technique using aqueous ZnSO4 and NaOH solutions. The ZnO films were characterized in terms of surface morphology by x-ray diffraction, energy-dispersive x-ray analysis (EDX), the use of a scanning electron microscope (SEM) and atomic force microscope (AFM) for surface morphology. The films exhibited a smooth morphology. The chemical states of oxygen and zinc in the ZnO nano thin films were also investigated by x-ray photoelectron spectroscopy (XPS). In the present investigations, highly textured ZnO thin films with a preferential (002)-orientation were prepared on glass substrates. The deposition conditions were optimized to obtain device-quality films for practical applications.

Cluster-eliminating filter for depositing cluster-free a-Si:H films by plasma chemical vapor deposition

A cluster-eliminating filter is developed to reduce a volume fraction VF of amorphous silicon nanoparticles above approximately 1 nm in size (referred to as a cluster) incorporated into a-Si:H films. The filter reduces the VF value by using the difference between a sticking probability of clusters and a surface reaction probability of SiH3 radicals, which are the predominant deposition radicals. By employing the filter together with a cluster-suppressed plasma chemical vapor deposition reactor, the VF value is reduced below 1∕180 compared to that for the conventional device quality films. Such cluster-free a-Si:H films have an extremely small hydrogen concentration associated with Si-H2 bonds below 5.46×10−3at.%.

Maskless lateral epitaxial over growth of GaN films on in situ etched sapphire substrates by metalorganic chemical vapor deposition

It has been reported recently that the sapphire substrate can be etched inside a metalorganic chemical vapor deposition reactor through GaN decomposition-induced reaction, and that device-quality GaN films can be grown on thus etched substrates with residual gallium droplets acting as nucleation sites [Hao et al., Appl. Phys. Lett 84 (2004) 4041]. In the present study, a maskless lateral epitaxial over growth (LEO) method is proposed to produce GaN films with reduced dislocation density through the use of in situ etched sapphire substrates. The microstructure of such GaN films was investigated by transmission electron microscopy and scanning electron microscopy, and the effect of lateral over growth has been confirmed. Dislocations in such GaN films were further analyzed through etch-pits density experiment and it has been found that this maskless LEO method is very effective to reduce pure edge dislocation density in the GaN films.

Electrodeposition of ternary CuAgSe thin films: Use of the electrochemical quartz crystal microbalance approach for real-time phase quantification

A detailed electrochemical study of the ternary Cu + Ag + Se system on Pt substrate using the techniques of cyclic voltammetry (CV), hydrodynamic voltammetry at Pt-rotated disc electrode (RDE), anodic stripping has identified optimum conditions for the electrodeposition of ternary CuAgSe thin semiconductor films used as active membranes in developing Cu(II)-ion-selective sensors. A combined coulometry-quartz crystal microbalance (EQCM) protocol has been developed which allows real-time phase composition analysis of the target ternary CuAgSe and admixture with either Cu 2Se or Ag 2Se binary selenides. The EQCM data have provided valuable information concerning the mechanistic details of the CuAgSe-electrodeposition. The molar ratio of Cu:Se in the electrolytic bath is shown to be of crucial importance for depositing device quality films. Different mechanisms dominate when this ratio is increased from 1:5 to 1:1, from one-step diffusion-controlled electrochemical process (Cu:Se = 1:5) to a mixed electrochemical and chemical pathway for Ag + assimilation (Cu:Se = 1:1).

GaN films and GaN‐based light emitting diodes grown on the sapphire substrates with high‐density nano‐craters formed in situ metalorganic vapor phase epitaxial reactor

A novel process has been developed to grow GaN films and GaN-based light emitting diodes on the sapphire substrates with high-density nano-craters formed on the surface of the substartes in situ metalorganic vapor phase epitaxial (MOCVD) reactor. The whole process is a single and integrated MOCVD run. Firstly a thin GaN layer is grown on the sapphire substrate, and then it is etched away by only flowing the H2 into the reactor at the higher temperature. It is found, amazingly, that the thermal decomposition of GaN can induce the chemical etching of sapphire, and result in the formation of high-density nano-craters on the surface of the substrates. Finally the device-quality GaN films could be grown on the etched sapphire substrate with the residual gallium droplets as nucleation sites. A preliminary result shows that the output power of GaN light-emitting diodes built on the in situ etched sapphire substrates could be increased by 30%. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation chemistry of high-density nanocraters on the surface of sapphire substrates with an in situ etching and growth mechanism of device-quality GaN films on the etched substrates

An efficient method has been investigated to grow GaN films with sapphire substrates being treated in situ metalorganic chemical vapor deposition reactor for a special effect rather than simple thermal cleaning. First, a thin GaN layer is grown on the sapphire substrate. And then it is almost etched away by thermal decomposition. It is found that the decomposition of GaN induces the decomposition of sapphire resulting in the formation of high-density nanocraters on its surface. Finally the device-quality GaN film is regrown on the etched substrate with residual gallium droplets as nucleation sites. The chemistry of the etching process and the mechanism of the final GaN growth process have been discussed. The distinct feature of this method is the in situ formation of high-density nanocraters on the surface of the substrate. A rough interface between the substrate and GaN can improve the efficiency of the light-emitting diode built on it greatly.

Advances in plasma-enhanced chemical vapor deposition of silicon films at low temperatures

The properties of silicon films prepared by plasma-enhanced chemical vapor deposition (PECVD) at low temperatures (<400 °C) are controlled by the physical and chemical processes that occur in the gas phase, at the top-most film surface, within the first several monolayers of the surface, and even well into the film bulk. Recent advances in understanding these processes have led to several new developments in silicon PECVD. The advances include: (i) novel concepts for depositing high-rate, device-quality silicon films, and (ii) deposition phase diagrams for optimizing silicon films for high-stability, high-performance devices. In situ and real time probes of the gas phase, the film surface, and its sub-surface have played key roles in these advances.

Light-emitting diodes based on phosphorescent guest/polymeric host systems

Fabrication of polymer light-emitting diodes based on emission from the phosphorescent molecule fac tris(2-phenylpyridine) iridium doped into a poly(vinyl carbazole) host are reported. Several spin-coating solvents were evaluated for deposition of the polymer layer; toluene and chlorobenzene were found to consistently produce device-quality films with sufficient incorporation of the dopant. For single-layered devices with Mg0.9Ag0.1 cathodes, the luminance efficiency at 20 mA/cm2 was measured to be 8.7 Cd/A for devices processed from chlorobenzene. This efficiency could be increased by over a factor of two with a trilayered device geometry consisting of the doped polymer layer, a hole-blocking layer, and electron transport layer. Further increases in efficiency, up to 30 Cd/A and 8.5% external quantum efficiency, were observed when a second dopant of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was added to the polymer emitter layer.

Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method

The applicability of photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chemical vapor deposited and physical vapor deposited CuGaSe2 (CGSe) thin films is investigated. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a-Si theory of PDS and CPM analysis, a two-layer system consisting of a defect-rich near-surface layer and a less defective bulk layer is found in device-quality films. Optical improvements due to the Ga+Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near-surface defect density remains unchanged.

Photoinduced defects in wide-gap materials: Hydrogenated amorphous silicon-carbon and silicon-nitrogen films

Abstract Wide-bandgap hydrogenated amorphous silicon-carbon and silicon-nitrogen films having optical gaps in the range 1.9–4.0eV have been deposited by the 13.56 MHz plasma-enhanced chemical vapour deposition technique from SiH4 + C2H2 (+H2), SiH4 + NH3 (+H2), gas mixtures. The deposition conditions have been chosen so as to obtain device-quality films already successfully applied in optoelectronic technology. The films have been light soaked in the average weather conditions solar spectrum (air mass 2.0) for times up to 90 h, monitoring the absorbed energy. The optical properties have been measured after each occurrence of damage, and the defect density evolution due to light soaking was determined by photothermal deflection spectroscopy. An increase in the density of defects with light exposure was observed in all samples strongly dependent on carbon or nitrogen gas sources, plasma conditions and initial properties.

Amorphous silicon films and solar cells deposited by HWCVD at ultra-high deposition rates

The deposition conditions for hydrogenated amorphous silicon, deposited by hot wire chemical vapor deposition, are linked to the film structure as we increase deposition rates ( R d) to >100 Å/s. At low R d (<20 Å/s), films with optimal properties are deposited under low silane depletion conditions, and all measures of structure (X-ray diffraction, Raman spectroscopy, H evolution, small-angle X-ray scattering (SAXS)) indicate a compact material. At high R d (>100 Å/s), optimum films are deposited under silane depletion conditions as high as 75–80%, and all structural properties except for the SAXS results once again indicate a compact material. We relate changes in the film electronic structure (Urbach edge) with increasing R d to the increase in the SAXS signals, and note the invariance of the saturated defect density versus R d, discussing reasons why these microvoids do not play a role in the Staebler–Wronski effect for these films. Finally, we present device results over the whole range of R d that we have studied and suggest why, at high R d, device quality films can be deposited at such high silane depletions.

Microcrystalline Silicon for Solar Cells at High Deposition Rates by Hot Wire Cvd

AbstractWe have explored which deposition parameters in Hot Wire CVD have the largest impact on the quality of microcrystalline silicon (μc-Si) made at deposition rates (Rd) &lt; 10 Å/s for use in thin film solar cells. Among all parameters, the filament temperature (Tfil) appears to be crucial for making device quality films. Using two filaments and a filament-substrate spacing of 3.2 cm, μc-Si films, using seed layers, can be deposited at high Tfil (∼2000°C) with a crystalline volume fraction &lt; 70-80 % at Rd's &lt; 30 Å/s. Although the photoresponse of these layers is high (&lt; 100), they appear not to be suitable for incorporation into solar cells, due to their porous nature. n-i-p cells fabricated on stainless steel with these i-layers suffer from large resistive effects or barriers, most likely due to the oxidation of interconnected pores in the silicon layer. The porosity is evident from FTIR measurements showing a large oxygen concentration at ∼1050 cm-1, and is correlated with the 2100 cm-1 signature of most of the Si-H stretching bonds. Using a Tfil of 1750°C, however, the films are more compact, as seen from the absence of the 2100 cm-1 SiH mode and the disappearance of the FTIR Si-O signal, while the high crystalline volume fraction (&lt; 70-80 %) is maintained. Using this Tfil and a substrate temperature of 400°C, we obtain an efficiency of 4.9 % for cells with a Ag/ZnO back reflector, with an i-layer thickness of only ∼0.7 μm. High values for the quantum efficiency extend to very long wavelengths, with values of 33 % at 800 nm and 15 % at 900 nm, which are unequalled by a-SiGe:H alloys. Further, by varying the substrate temperature to enable deposition near the microcrystalline to amorphous transition (‘edge’) and incorporating variations in H2 dilution during deposition of the bulk, efficiencies of 6.0 % have been obtained. The Rd's of these i-layers are 8-10 Å/s, and are the highest to date obtained with HWCVD for microcrystalline layers used in cells with efficiencies of ∼6 %.

Optical and Structural Studies of InGaN Layers and GaN/InGaN MQWs Using TPIS-MOCVD

AbstractInGaN device quality films and their related heterostructures play a critical role in the development of nitride devices. InGaN growth needs to be performed at much lower temperatures than GaN growth, due to the lower dissociation temperature of InN. Furthermore, decomposition of ammonia becomes less efficient with decreasing temperature due to high kinetic barrier for breaking N-H bonds and InGaN growth needs high NH3/TMIn ratio. We investigated the optical and structural properties of InGaN bulk layers and GaN/InGaN MQWs using thermally precracked ion supplied metalorganic chemical vapor deposition (TPISMOCVD) system. The temperature range for this study was 670 -770°C. In a low NH3 flow condition, In metal droplet appeared on the surface on InGaN layer in conventional MOCVD system, but it disappeared in TPIS-MOCVD system. An increasement of In mole fraction in InGaN could be achieved even in low NH3 flow. As the NH3 flow rate and the InGaN growth temperature decreased, In metal droplet was more effectively reduced by ammonia precracking. The quality of InGaN/GaN MQWs was evaluated with high resolution XRD and TEM.

大気圧プラズマＣＶＤ法による太陽電池用アモルファスＳｉの超高速成膜

Further investigations to improve the quality of the amorphous silicon (a-Si) films fabricated at extremely high deposition rate by atmospheric pressure plasma CVD were performed. The electrical and optical properties of the films were investigated with the measurements of photo and dark conductivity and optical absorption, and the relationship between each deposition parameter and the film properties was studied. It became clear that the optimum input power and the deposition gap existed for both of increasing the deposition rate and improving the film properties. Furthermore, the optical gap (E opt) could be controlled by the hydrogen concentration in the plasma and the substrate temperature. As a result of optimizing the deposition parameters, it became possible to improve the film quality under the ultrahigh-rate deposition condition. The maximum deposition rate of the device-quality a-Si film was 0.28μm/s. At this deposition condition, it might be possible by scanning substrate to form a-Si film of the thickness of 0.3μm on the substrate of 1m×1m area in 200 seconds, which was more than 10 times faster than the conventional plasma CVD process. E opt of the device-quality film was in the range of 1.58≤E opt≤1/68V, , which was similar to that of a-Si film fabricated by the conventional plasma CVD at low deposition rate.