Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

2.5 Behaviour of amorphous Ge contacts to monocrystalline silicon

The performance of evaporated amorphous Ge films (thickness $\text{～-500}\text{A}\text{?}$) contacts to etched n-and p-type silicon crystals of different resistivities are discussed. The Ge was 3 Ωcm n-type was also used but gave no difference), and as external contact to the Ge film an Au layer was evaporated. The behaviour of the aGeSi junction seems to be largely governed by interface effects (and thus depends on surface preparation), as is often the case with metal-Si junctions, but Ge gives more reproducible and less time-varying results.In the process of clarifying the function of the contact the following structures were investigated (1) aGe_pⁿSiIn (Hg), where the latter is an ohmic contact, (2) amSi_pⁿSi-metal where amSi is a surface region of the crystal which has been rendered amorphous by ion bombardment, (3) aGe_pⁿSi-metal. I-V and C-V measurements were performed. From the results we conclude that aGeSi junctions act as low-resistance contacts when fed by electron or hole currents from the crystal. The currents (holes and electrons) that are injected into the crystal from the film are limited by barriers to small current densities, usually in the range 10^?6 A cm^?2. It is suggested that the small hole currents are explained by an increase in the hole barrier, effected by positive charges at the interface or in the Ge film, which are built up when positive carriers (holes) are injected by the contact.     

Amorphous silicon carbide heterojunction solar cells on p-type substrates

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO₂ passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow (PH₃) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current-voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH₃-flow as pure a-Si:H(n).     

Sol-gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells

Sol-gel grown polycrystalline Al doped zinc oxide (AZO) thin films have been deposited on Si wafers, microscopy slide glass and fluorine doped tin oxide coated glass substrates using the spin coating technique. The atomic ratio of Al:Zn in the films is 0.2. From the X-ray diffraction investigations it is found that the preferential growth of (100) reflection peak has taken place in the 450, 550 and 600 °C annealed films. Scanning electron microscopic study has shown that the films contain well-defined grains arranged in a closely packed array. The resistivity of the 500 °C annealed film is measured to be 5 × 10^− 1 Ω cm. The films have exhibited excellent optical transmittance (~ 90%) in the 400-1100 nm wavelength range. Refractive indices (n = 1.9-1.95) of the films on Si wafer are independent of the annealing temperature. Thickness of the films produced at 4000 rpm is in the range of 58-62 nm. The refractive index and thickness of these films are nearly appropriate to cause destructive interference after reflection from front emitters of solar cells. These films have demonstrated a reflectivity value of about 3% at a wavelength of 700 nm. The AZO coated silicon solar cells possess V_oc and I_sc values of 573 mV and 237 mA, respectively.     

Rapid thermal-plasma annealing of ZnO:Al films for silicon thin-film solar cells

Rapid thermal annealing of sputter-deposited ZnO and Al-doped ZnO (AZO) films with and without an amorphous silicon (a-Si) capping layer was investigated using a radio-frequency (rf) argon thermal plasma jet at atmospheric pressure. The resistivity of bare ZnO films on glass decreased drastically from 10⁶ to 10³ Ω·cm at maximum surface temperatures T_max above 650 °C, whereas the resistivity increased from 10^− 4 to 10^− 3-10^− 2 Ω·cm for bare AZO films. On the other hand, the resistivity of AZO films with a 30-nm-thick a-Si capping layer remained below 10^− 4 Ω·cm, even after TPJ annealing at a T_max of 825 °C. X-ray diffraction and X-ray photoemission electron studies revealed that the film crystallization of both AZO and a-Si layers was promoted without the formation of an intermixing layer. Additionally, the crystallization of phosphorous- and boron-doped a-Si layers at the sample surface was promoted, compared to that of intrinsic a-Si under identical plasma annealing conditions. The role of the a-Si capping layer on sputter-deposited AZO and ZnO films during TPJ annealing is demonstrated. The effects of the mixing of phosphorous and boron impurities in a-Si:H during TPJ annealing of flat and textured AZOs are also discussed.     

Realization of high mobility p-type co-doped ZnO: AlN film with a high density of nitrogen-radicals

High mobility p-type ZnO:AlN thin films have been efficiently realized by utilizing pre-activated nitrogen (N) plasma sources with an inductively coupled dual target co-sputtering system. High density of N-plasma-radicals was generated with an additional RF power applied through a ring-shaped quartz-tube located inside the chamber during co-sputtering process. The AlN codoped ZnO film shows excellent p-type behavior with a high mobility and a hole concentration of 154 cm^2°V^− 1s^− 1 and about 3 × 10^18°cm^− 3 at 600 °C, respectively. Electrical properties of p-n homo-junction devices based on p-type ZnO film are also discussed.     

Enhancement-mode thin film transistor with nitrogen-doped ZnO channel layer deposited by laser molecular beam epitaxy

The thin film transistors (TFTs) based on nitrogen doped zinc oxide (ZnO) were investigated by laser molecular beam epitaxy. The increase of ZnO films' resistivity by nitrogen doping was found and applied in enhancement mode ZnO-TFTs. The ZnO-TFTs with a conventional bottom-gate structure were fabricated on thermally oxidized p-type silicon substrate. Electrical measurement has revealed that the devices operate as an n-channel enhancement mode and exhibit an on/off ratio of 10⁴. The threshold voltage is 5.15 V. The channel mobility on the order of 2.66 cm² V^− 1 s^− 1 has been determined.     

Chemical fabrication of p-type Cu2O transparent thin film using molecular precursor method

A transparent p-type Cu₂O thin film of 50 nm thickness was successfully fabricated by means of a solution-based process involving the thermal reaction of molecular precursor films spin-coated on a Na-free glass substrate. The precursor solution was prepared by the reaction of an isolated Cu²⁺ complex of ethylenediamine-N, N, N′, N′-tetraacetic acid with dibutylamine in ethanol. The Cu₂O thin films resulting from heat treatment of the precursor film at 450 °C for 10 min in Ar gas at a flow rate of 1.0 L min⁻¹ were characterized by X-ray diffraction which indicated a precise cubic lattice cell parameter of a = 0.4265(2) nm, with a crystallite size of 8(2) nm. X-ray photoelectron spectroscopy peaks, attributable to the O 1s and Cu 2p_3/2 level of the Cu₂O film were found at 532.6 eV and 932.4 eV, respectively. An average grain size of the deposited Cu₂O particles of ca. 200 nm was observed via field-emission scanning electron microscopy. The optical band edge evaluated from the absorption spectrum of the Cu₂O transparent thin film was 2.3 eV, assuming a direct-transition semiconductor. Hall Effect measurements of the thin film indicated that the single-phase Cu₂O thin film is a typical p-type semiconductor, with a hole concentration of 1.7 × 10¹⁶ cm⁻³ and hole mobility of 4.8 cm² V⁻¹ s⁻¹ at ambient temperature. The activation energy from the valence band to the acceptor level determined from an Arrhenius plot was 0.34 eV. The adhesion strength of the thin film on the Na-free glass substrate was also determined as a critical load (Lc1) of 2.0 N by means of a scratch test. The method described is the first example of fabrication and characterization of a p-type Cu₂O transparent thin film by a wet process.     

Reactive magnetron sputtering from a composite target for large area BaPbO3 thin film electrode

Radio frequency reactive magnetron sputtering from a composite target made of PbO pellets uniformly positioned on a metallic Ba disc has been utilized for BaPbO₃ electrode deposition on 150 mm Si wafers. The reactive sputtering process has been analyzed in relation to sputtering parameters for composite targets with different percentage of PbO coverage. The process optimization method for in situ crystallized BaPbO₃ thin film fabrication has been emphasized. The growth of BaPbO₃ films has been discussed from the viewpoint of the BaO-PbO phase diagram and thermodynamics of Ba_n + 1Pb_nO_3n + 1 (n = 1, 2, ∞) phase formation. The microstructure analysis of the deposited films has been performed with atomic force microscopy and wide-angle X-ray diffraction (XRD) techniques. The grazing angle XRD measurements reveal the formation of a Ba₂PbO₄ phase in the film fabricated at 450 °C. The Ba₂PbO₄ phase content decreases with decreasing substrate temperature. The BaPbO₃ film deposited at a substrate temperature of 430 °C on naturally oxidized (001) Si wafers shows an electrical resistivity of 1.13 mΩ·cm. The BaPbO₃ films deposited on SiO₂ (native oxide)/Si wafer do not exhibit a preferred orientation whereas use of (111) Pt/SiO₂/Si substrate results in highly (111)-oriented films.     

Direct bonding of silicon wafers with a diffusion layer

The possibility of solid-phase direct bonding of silicon wafers having p ⁺-or n ⁺-type diffusion layers with a high surface dopant concentration has been demonstrated for the first time. Pis’ma Zh. Tekh. Fiz. 24, 1–5 (March 26, 1998)     

Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Fabrication of n-Zn1−xGaxO/p-(ZnO)1−x(GaP)x thin films and homojunction

Ga doped ZnO (GZO) and GaP codoped ZnO (GPZO) thin films of different concentrations (1–4 mol%) have been grown on sapphire substrates by RF sputtering for the fabrication of ZnO homojunction. The grown films have been characterized by X-ray diffraction (XRD), photoluminescence (PL), Hall measurement, energy dispersive spectroscopy (EDS), time-of-flight secondary ion mass spectrometer (ToF-SIMS), UV–Vis–NIR spectroscopy and atomic force microscopy (AFM). Unlike in conventional codoping, here we directly doped (codoped) GaP into ZnO to realize p-ZnO. The Hall measurements indicate that 2 and 4% GPZO films exhibit p-conductivity due to the sufficient amount of phosphorous incorporation while all the monodoped GZO films showed n-conductivity as expected. Among the p-ZnO films, 2% GPZO film shows low resistivity (2.17 Ωcm) and high hole concentration (1.8 × 10¹⁸ cm^?3) by optimum incorporation of phosphorous due to best codoping. Similarly, among the n-type films, 2% GZO shows low resistivity (1.32 Ωcm) and high electron concentration (2.02 × 10¹⁹ cm^?3) by optimum amount of Ga incorporation. The blue shift and red shift in NBE emission observed from PL acknowledged the formation of n- and p-conduction in monodoped and codoped films, respectively. The neutral acceptor bound exciton recombination (A⁰X) observed by low temperature PL for 2% GPZO confirms the p-conductivity. Further, the high concentration of P atoms than Ga observed from ToF-SIMS (2% GPZO) also supports the p-conductivity of the films. The fabricated p–n junction with best codoped p-(ZnO)_0.98(GaP)_0.02 and best monodoped n-Zn_0.98Ga_0.02O films showed typical rectification behavior of a diode. The diode parameters have also been estimated for the fabricated homojunction.     

As-grown textured zinc oxide films by ion beam treatment and magnetron sputtering

This work presents as-grown textured ZnO:Al films by rf magnetron sputtering initiated by pre-treatment of glass substrate with mixed argon and oxygen ions. A 650 nm thick of this film exhibits surface texture features with lateral size around 500 nm; the resistivity is below 5 × 10⁻⁴ Ω · cm and the transparency in the near-infrared spectral range is high (> 80% at 1000 nm). Microcrystalline silicon thin film solar cells grown on the textured glass exhibit excellent light trapping effect with a short circuit current density of 18.2 mA/cm².     

The effects of impurity and temperature for transparent conducting oxide properties of Al:ZnO deposited by dc magnetron sputtering

Aluminum-doped zinc oxide films (ZnO:Al) were deposited on Si wafers and glass substrates by dc magnetron sputtering from a ZnO target mixed with 2 wt% Al₂O₃ for photovoltaic films. The effect of base pressure, additional oxygen, and substrate temperature were studied in detail. By dc magnetron sputtering at room temperature, the resistivity and the average transmittance in visible range was 2.3 × 10⁻³ Ω cm and 77.3%, respectively. And these were improved up to 3.3 × 10⁻⁴ Ω cm and 86% at the substrate temperature of 400 °C by high deposition rate and low impurity ambient. The mobility and the carrier concentration were improved by the increased preferred orientation of (002) plane and grain size of film with increasing deposition temperature. This advanced AZO film with good resistivity and transmittance can be expected as the front TCO of thin film solar cells.     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

High-Quality YBa2Cu3O7−x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7?x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7?x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

Epitaxial growth and properties of cubic Zn0.7Mg0.3O films on TiN-buffered Si(100) substrates by in situ pulsed laser deposition

A novel cubic Zn_0.7Mg_0.3O film on silicon substrate is conducted by KrF excimer pulsed-laser ablation system. By introducing a thin TiN buffer, layer-by-layer growth of cubic Zn_0.7Mg_0.3O film epilayer has been realized. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the Zn_0.7Mg_0.3O films were strongly affected by the TiN buffer layer. The Zn_0.7Mg_0.3O film obtained at an optimal buffer layer exhibited high quality and good surface. For the metal-insulator-metal (MIM) structure of Pt/Zn_0.7Mg_0.3O (200 nm)/TiN (20 nm)/Si (400 μm) prepared at the optimal growth conditions achieved a very low leak current density of ∼10⁻⁶ A cm⁻² at an electric field of 9 × 10⁵ V cm⁻¹ and the permittivity (?_r) of about 8.1, agreed well with that of acquired MgO film and MgO single crystal.     

Electrodeposition of SmS optical thin films on ITO glass substrate

SmS optical thin films were deposited on the surface of ITO glass with an electrodeposition method using aqueous solution containing SmCl₃·6H₂O and Na₂S₂O₃·5H₂O. The phase composition was analyzed by X-ray diffraction (XRD) and microstructure of the film was characterized by atomic force microscope (AFM). It is showed that SmS thin film could be obtained in the solution with n(Sm)/n(S) = 1:4, pH = 4.0 and annealing in Ar atmosphere at 200 °C for 0.5 h. The as-prepared thin films on the ITO glass exhibit a dense microstructure. The band gap of the thin film has been found to be 3.6 eV.