Masatoshi Kitagawa

Effects of discharge parameters on deposition rate of hydrogenated amorphous silicon for solar cells from pure SiH4 plasma

A systematic deposition of hydrogenated amorphous silicon films from pure SiH4 plasma was made in a capacitively coupled rf glow‐discharge system by changing anode–cathode spacing d and chamber pressure p simultaneously. The data of the deposition rate in the p‐vs‐d space had two boundaries. One was pd=const. The other seems to be pd2=const. The rf plasma can stably sustain between the boundaries. The boundaries are discussed with rf power per SiH4 molecule and with overlapping Paschen’s lines of various fragments, especially H2 due to the SiH4 dissociation. We found the optimum conditions in which the deposition rate was more than 10 A/s without large photo‐induced degradation. 10% efficient p‐i‐n solar cells were achieved with the intrinsic layer deposition rate of 3.9 A/s and more than 6% efficiency with 10 A/s.

Interaction of hydrogenated amorphous silicon films with transparent conductive films

The effects of the deposition temperature on the interaction of the hydrogenated amorphous silicon films with indium–tin–oxide and tin–oxide films have been investigated in the temperature range 150–300 °C, using Auger electron spectroscopy, secondary ion mass spectrometry, and scanning electron microscopy. It was found that the constituent atoms such as indium and tin are detected in the thin amorphous silicon films deposited. Around the interface between the transparent conductive films and amorphous silicon films the formation of oxidized silicon was also observed. The depth distributions of indium in the amorphous silicon films are strongly dependent upon the deposition temperature while those of tin are almost independent. From results on the observation of the surface morphology of the amorphous silicon films, it has been shown that those depth distributions may be much affected by the initial covering over substrates with the amorphous silicon films.

Low Temperature Growth of Amorphous and Polycrystalline Silicon Films from a Modified Inductively Coupled Plasma

A conventional inductive rf discharge is modified by inserting a discharge antenna in a plasma vessel with magnetic multipole confinement, which gives a high-density (~1011 cm-3) silane plasma at very low pressures (~1 mTorr). This new type of inductively coupled plasma (ICP) enables high-rate deposition (~1 nm/s) of a-Si:H films at low substrate temperatures of ~100°C, which have the photoconductivity of 10-5–10-4 S/cm and the dark conductivity of 10-10–10-9 S/cm. Moreover, microcrystalline or polycrystalline silicon films are formed on glass substrates at moderate temperatures of 200–300°C where the dark conductivity becomes comparable to the photoconductivity and the X-ray diffraction pattern shows sharp peaks corresponding to the silicon crystalline surfaces. Mass spectrometric measurements of the highly dissociated silane plasma show unique radical compositions; ~90% of ions are hydrogen species (H3+, H2+, H+) while the density of neutral radicals (SiH3, SiH2, SiH) is lower than that of ionic radicals (SiH3+, SiH2+, SiH+, Si+). Thus, the main precursor of film growth from high-density plasmas may be ionic radicals rather than neutral radicals.

Low-Temperature Preparation of Pb(Zr, Ti)O3 Thin Films on (Pb, La)TiO3 Buffer Layer by Multi-Ion-Beam Sputtering

Ferroelectric lead-zirconate-titanate (PZT) thin films were successfully fabricated by the multi-ion-beam sputtering technique in an oxygen ambient at a low substrate temperature of 415 o C. By inserting lead-lanthanum-titanate (PLT) buffer layers between substrates and PZT films, the perovskite-PZT thin films could be epitaxially grown on (100)MgO, (100)Pt/MgO and (111)Pt/Ti/SiO 2 /Si substrates. These films, even at thickness values of as low as 630 A, showed excellent ferroelectric properties with a remanent polarization of 20 μC/cm 2 , coercive field of 200 kV/cm, and a relative dielectric constant of 700

Antiferroelectric PbZrO3 thin films prepared by multi‐ion‐beam sputtering

Antiferroelectric PbZrO3 thin films have been fabricated by a multi‐ion‐beam sputtering technique at a substrate temperature as low as 415 °C. Single crystal perovskite PbZrO3 films oriented along the a axis could be epitaxially grown on (100)MgO, (100)Pt/MgO substrates using a PbTiO3 buffer layer. The PbZrO3 films achieved high dielectric constants of about 400, which are almost 2.4 times larger than that of bulk PbZrO3. The measurements of D–E hysteresis loops and Curie temperature demonstrated the antiferroelectric to ferroelectric phase transition of PbZrO3 films with a thickness of 1770 A, while for PbZrO3 films of 875 A the phase transition could not be clearly observed.

Structure and Properties of Silicon Titanium Oxide Films Prepared by Plasma-Enhanced Chemical Vapor Deposition Method

We have developed a novel material of silicon titanium oxide with high dielectric constant and low leakage current. The silicon titanium oxide films were prepared by plasma-enhanced chemical vapor deposition (plasma CVD) with the gas mixture of TiCl4-SiH4-N2O. It was found that these films have intermediate properties of silicon dioxide and titanium dioxide by varying the ratio of SiH4/(SiH4+TiCl4). The properties of silicon titanium oxide nearly corresponded to those of Ta2O5 when the gas mixture ratio was 0.075, with a dielectric constant of 18 and a leakage current density of 2.5×10-8 A/cm2 at 1 MV/cm.

Reflection micro-Fresnel lenses and their use in an integrated focus sensor.

Blazed reflection micro-Fresnel lenses and their use in an integrated focus sensor are proposed. Theoretical analysis indicates that the optical characteristics of reflection Fresnel lenses can be improved compared with a conventional transmission micro-Fresnel lens. These reflection Fresnel lenses were fabricated using electron-beam lithography and exhibited the diffraction-limited focusing characteristics with 71% high efficiency. The focus sensor has a folded optical path and includes a beam splitter integrated with thin film components, such as a reflection elliptical Fresnel lens and a quadrant photodetector. The reflection elliptical Fresnel lens in the focus sensor exhibiting excellent astigmatic characteristics agreed with the theoretical results, and the focus error signal was detected. This sensor can be developed as the optical head of an optical disk system.

Low‐temperature crystallization of hydrogenated amorphous silicon films in contact with evaporated aluminum electrodes

Measurements are reported on x‐ray diffraction and Auger electron spectroscopy studies of Al films on phosphorous‐doped hydrogenated amorphous silicon (a‐Si:H) prepared by rf glow discharge deposition. Infrared absorption and current‐voltage characteristics of and Al electrode a‐Si:H p‐i‐n diodes are also presented. Si and Al Auger depth profile data show that Si was detected at the free surface of the 5000‐A Al film on the 1500‐A a‐Si:H film after a 185 °C anneal, while a constant Si signal was detected in the Al film from its free surface to the interface with the a‐Si:H after a 200 °C anneal. The 200 °C anneal reduced the p‐i‐n diodes to resistors. The a‐Si‐H in contact with the Al film crystallized at 300 °C. A 360 °C anneal prior to the Al deposition raised the recrystallization temperature above 300 °C. The evolution of hydrogen is proposed as a mechanism for lowering the recrystallization temperature in a–Si:H.

Thomson scattering measurements of electron temperature and density in an electron cyclotron resonance plasma

Electron temperature Te and density ne in the source region of an electron cyclotron resonance discharge have been measured by incoherent Thomson scattering of the beam from a 0.5 J yttrium aluminum garnet laser. This is the first experiment in which this technique, routinely used on fusion plasmas, has been applied to a processing plasma. Measurements were made in an argon discharge at pressures from 0.3 to 2 mTorr and microwave powers from 250 to 1000 W. Velocity distributions were measured both parallel and perpendicular to the magnetic field and a slight anisotropy of electron temperature was observed for low‐pressure discharges. Temperatures in the range of 1–5 eV and densities in the range of 2–10×1017 m−3 were measured. Te and ne were found to strongly depend on pressure but only weakly on the input power and discharge magnetic field. No deviations from a Maxwellian velocity distribution were observed.

Interaction between n‐type amorphous hydrogenated silicon films and metal electrodes

The interactions of Al and NiCr electrodes with hydrogenated n‐type amorphous silicon films have been investigated in a temperature range of 100–350 °C. It was found that pits were produced in the a‐Si films at temperatures above 170 °C in the case of Al electrodes, while they were not observed in the case of NiCr electrodes even if heat‐treated at a temperature of 350 °C. From the Auger electron spectroscopy measurements, it was shown that a marked interdiffusion of Al and Si occurs. The sheet resistance of the a‐Si films began to increase at temperatures above 170 °C. The contact resistivity between the a‐Si films and Al electrodes could not be determined in the case of a‐Si films with usual low conductivity. For the specimens with high conductivity [σ∼1(Ω cm)−1], the value of about 1×10−3Ω cm2 was obtained.