Yasuhiro Shiraki

Electrical conductivity of disordered layers in GaAs crystal produced by ion implantation

The electrical conductivity of disordered layers in GaAs produced by ion implantation has been studied as a function of ion species, dose level, energy of implanted ions, and anneal temperature. The resistivity of the implanted layer, ρ, at room temperature can be expressed by a single relation, ρ=const×NDD−3, even for different masses and energies of implanted ions, where NDD is the total number of displacements of host atoms per unit volume due to nuclear collisions with implanted ions. At a temperature above 180 K the resistivity is proportional to exp(B/T) and below 180 K it changes as exp(B/T1/4). The conduction mechanism of the implanted layers is discussed based upon the hopping conduction in disordered or amorphous semiconductors. The annealing behavior of the implanted layers was also studied. The activation energy for recovery of the resistivity ρ was about 0.35 eV, which is quite small compared with the migration energies of stray atoms in ordered crystals. These results indicate that the impla...

Growth and characterization of atomic layer doping structures in Si

A new method to fabricate atomic layer doping structures in silicon using a combination of molecular‐beam epitaxy and solid‐phase epitaxy is developed. The antimony dopants are restricted to a thickness less than the resolution limit of secondary ion mass spectrometry, and exhibit extremely sharp C‐V carrier profiles of less than 2 nm for a single Sb monolayer (ML). Hall and resistivity data show full activation for 0.1 ML and saturation at 8×1013 electrons cm−2. Measurements down to 4 K show metallic conduction for highly doped samples (above 0.05 ML) two‐carrier conduction for intermediate levels, and strong freezeout for low amounts of Sb (below 0.01 ML). The two‐dimensional electron gas is shown to exist by angular dependence of magnetoresistance.

Atomic layer doped field‐effect transistor fabricated using Si molecular beam epitaxy

Atomic layer doped field‐effect transistors (ALD FETs) have been fabricated, in which n‐type, δ‐doped, and p‐type doped regions are used as a conductive channel and a punchthrough stopper, respectively. It is shown that the ALD FET shows a high transconductance while the punchthrough current is sufficiently suppressed, indicating that the device is promising as a short‐channel FET for the next generation of ultralarge scale integration.

Optical studies of deep‐center luminescence in CdS

Red luminescence in CdS has been studied in as‐grown and ion‐implanted crystals. An IR band consisting of two emission bands, IR1 and IR2, is produced by ion implantation. The higher‐energy band, IR1, has the same origin as the R band observed in some as‐grown crystals. A comparison of the R and IR1 bands in CdS with self‐activated (SA) luminescence in ZnS and GaAs shows that they are SA luminescence at 1.7 eV at low temperatures in CdS. The temperature variations in peak energy, half‐width, and intensity of luminescence are discussed, and the behavior is explained in terms of a configurational coordinate model. A vibrational energy of 0.025 eV, which is about 70% of the LO phonon energy in CdS, is calculated for the excited state of the center. The study of the excitation dependence of the center shows that the luminescence is due to an electronic transition within a localized center and not due to a donor‐acceptor pair recombination. The center has an excitation band about 0.22 eV below the band gap. Th...

Molecular beam and solid-phase epitaxies of silicon under ultra-high vacuum

Abstract Low temperature growth of silicon epitaxial layers has been performed by two methods: by molecular beam epitaxy and by solid-phase crystallization of amorphous films deposited onto crystalline substrates under ultra-high vacuum.

Ion Implantation of Nitrogen into Cadmium Sulfide

Ion implantation of nitrogen into n‐type cadmium sulfide has been investigated with energy up to 200 keV at dose levels 1015–1017 cm−2. The measurements of I‐V, C‐V characteristics, and photoelectric properties of diodes made on implanted CdS have given evidence for p‐type layer formation. EPR studies have clarified the nitrogen center and lattice‐defect centers produced by the ion implantation. Current of the implanted diodes can be expressed by I=I0[exp(eV/nkT) −1] and the parameter n was remarkably reduced from 10 to 2.7 by a 10‐min anneal at 400°C. Overannealing degraded the diode characteristics. The peculiar C‐V characteristics and the negative photovoltage were observed for the implanted diode, which can be understood only by presuming a series structure of the p‐n junction formed by nitrogen implantation and a Schottky barrier on a p‐type surface. The relative spectral response of photovoltage of the diode had a peak at 800 mμ, i.e., at 0.85 eV below the band edge after annealing below 400°C. The ...

Polycrystalline‐silicon thin‐film transistors on glass

Thin‐film transistors are fabricated on polycrystalline silicon on transparent glass using molecular beam deposition at low temperatures. It is found that the transistor characteristics and stability of this device, together with the low‐temperature processes utilized, afford potential application to flat display panels.

Low‐temperature formation of polycrystalline silicon films by molecular beam deposition

Textured polycrystalline‐silicon films are obtained by molecular beam deposition under ultrahigh vacuum at temperatures between 400 and 700u2009°C. Thinner films have a 〈110〉 preferred orientation, whereas a 〈100〉 preferred orientation is dominant in films with a thickness of above 1 μm. The deviation of the textured axes decreases from 20° to 10° with raising substrate temperature from 400 to 700u2009°C. A depth‐dependent preferred orientation inside the film is also observed.

Amorphous GaP produced by ion implantation

Two types of non-crystalline states (“disordered” and “amorphous”) of GaP were produced by using ion implantation and post annealing. A structural-phase-transition-like annealing behaviour from the “disordered” state to the “amorphous” state was observed. n nThe ion dose dependence and the annealing behaviour of the atomic structure of GaP implanted with 200 keV − N+ ions were studied by using electron diffraction, backscattering and volume change measurements. The electronic structure was also investigated by measuring optical absorption and electrical conductivity. n nThe implanted layer gradually loses the crystalline order with the increase of the nitrogen dose. n nThe optical absorption coefficient α and electric conductivity σ of GaP crystals implanted with 200 keV−N+ ions of 1 × 1016 cm−2 were expressed as αhν = C(hν − E0)n and log σ = A − BT-14, respectively. Moreover, the volume of the implanted layer increased about three percent and the electron diffraction pattern was diffused halo whose intensity monotonically decreases along the radial direction. These results indicate that the as-implanted layer has neither a long range order nor a short range order (“disordered state”). n nIn the sample implanted at 1 × 1016 cm−2, a structural phase-transition-like annealing stage was observed at around 400°C. That is, the optical absorption coefficient α abruptly fell off from 6 × 104 to 7 × 103 cm−1 and the volume of the implanted layer decreased about 2% within an increase of less than 10 degrees in the anneal temperature. Moreover, the short range order of the lattice structure appeared in the electron diffraction pattern. According to the backscattering experiment, the heavily implanted GaP was still in the non-crystalline state even after annealing. n nThese facts lead us to believe that heavily implanted GaP, followed by annealing at around 400°C, is in the “amorphous” state, although as-implanted Gap is not in the “amorphous” state but in the “disordered” state.

Low temperature MBE growth of high quality AlGaAs

Abstract High quality AlGaAs MBE layers have been successfully grown at very low temperatures (