摘要:

λ-DNA Hind III was denatured by HCl, renatured by NaOH, and completely digested by DNase I sequentially. The produced mixture was separated by gel filtration, and one component was obtained which showed resistance to the digestion of DNase 1. The results of atomic force microscopy (AFM) studies on this component indicated that an unusual form of DNA was presented, with the height 13 times that of double-stranded DNA. The renatured and partially digested λ-DNA Hind III was also studied by AFM, and the images revealed both the duplex DNA and the unusual DNA. By comparison with known results, it is suggested that the unusual DNA is a multistranded DNA rather than a simple supercoil.

ISSN:1071-1023    

DOI:10.1116/1.589563

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

The Si(111)7×7surface is exposed at room temperature to atomic hydrogen and studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) in ultrahigh vacuum. For increasing exposures, the LEED pattern of the surface changes in well defined steps to a1×1pattern. The STM images of the1×1surface appear rough and disordered. The1×1pattern and rough surface are consistent with an etching of several surface layers by the atomic hydrogen. Heating the1×1surface to 560 °C and cooling to room temperature produces a surface with a∛×∛R30°LEED pattern. The STM images of this surface are consistent with a distribution of adatoms on an ideal Si(111) surface. Further heating results in a7×7surface.

ISSN:1071-1023    

DOI:10.1116/1.589564

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

This article attempts to describe a novel high vacuum scanning force microscope (HV-SFM) using a self-oscillating piezoelectric cantilever in frequency modulation (FM) mode. Since no external deflection sensor or external vibrator is needed, the new HV-SFM is very simple and easy to handle in vacuum conditions in comparison with conventional systems using optical sensors. FM detection is used to detect the force gradient acting on the end of the piezoelectric cantilever because it gives higher response speed in vacuum conditions compared to the commonly used slope detection. The unimorph cantilever consists of a 1.0 μm thickPb(Ti,Zr)O3(PZT) layer on aSiO2elastic base, which becomes self-oscillating when an ac voltage is applied to the piezoelectric layer. The 160 μm long piezoelectric cantilever has successfully been oscillated at its natural resonance frequency of 117 kHz by connecting its piezoelectric layer directly into the FM loop, which applies positive feedback to the piezoelectric charge current caused by cantilever vibration. The shift in oscillation frequency of the FM loop, or the shift of cantilever resonance frequency due to force gradient acting on the cantilever end, is measured as a SFM feedback signal. At a frequency shift of 80 Hz, the force gradient and the force acting on the cantilever tip were calculated as 0.012 N/m and 0.02 nN, respectively, from an obtained force curve. At that set point, high-resolution images of a gold film have been obtained. The constructed FM detection noncontact SFM with a self-oscillating PZT cantilever has proven to be stable and easy to handle in vacuum conditions.

ISSN:1071-1023    

DOI:10.1116/1.589565

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

Atomic force microscopy measurements have been performed in order to study mercury cadmium telluride thin films grown on hybrid substrates by means of a new method based on an isothermal vapor phase epitaxy process (iso-VPE). The morphology of the samples, grown both on sapphire and on silicon 2 in. substrates, was observed at the different steps of the iso-VPE process. In the case of the silicon substrate the absence of interdiffusion with the deposited film was detected by means of a cross sectional atomic force microscopy analysis of the sample. Moreover, the surface quality of the iso-VPE grown thin films has been found to be comparable with that of samples grown on hybrid substrates by means of conventional techniques like liquid phase epitaxy. Fourier-transform infrared measurements were also performed in order to prove the high optical goodness of the samples.

ISSN:1071-1023    

DOI:10.1116/1.589566

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

Quantum wires were fabricated byshallow implantationof Ga ions from a focused-ion-beam source into a modulation-dopedAlxGa1−xAs/GaAsheterostructure. This type of implantation reduces crystal damage and keeps the implanted ions away from the two-dimensional electron gas. An electron mobility as high as5.04×105 cm2/V swas obtained for 10-μm-long wires with an effective width of 0.152 μm, which is much higher than any previously reported values. The electron ballistic lengths in the wires were determined from measurements of the bend and transfer resistance and agreed well with the calculated elastic mean free paths. The transfer resistance versus magnetic field profiles exhibited electron focusing peaks associated with good specularity at the boundary.

ISSN:1071-1023    

DOI:10.1116/1.589350

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

The property of electroluminescent porous silicon (PS) diodes as surface-emitting cold cathodes were investigated. The experimental PS diodes consist of thin Au films, PS,n+-type Si substrates, and ohmic back contacts. When a positive bias voltageVPSis applied to the Au electrode with respect to the substrate, electrons are uniformly emitted through the Au contact as well as photons. The cold electron emission characteristics are presented here in terms of the PS layer thickness dependence, effects of rapid thermal oxidation (RTO), and electroluminescence (EL) characteristics. It was demonstrated that both the decrease in the PS layer thickness(dPS)and the introduction of RTO treatment are useful for a significant improvement in the emission characteristics, and that the emission current and efficiency for a RTO-treated diode withdPS=3μm reach 450 μA/cm2and 0.2%, respectively, atVPS=27 V. It is also shown that in every case, the Fowler–Nordheim scheme holds in the bias voltage dependence of the emission current. The emission mechanism based on the high-field effect near the outer surface of PS layer, is discussed in relation to the visible EL emission.

ISSN:1071-1023    

DOI:10.1116/1.589351

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

A study of the field emission from Spindt-type metallic integrated single microtips and arrays of microtips has been carried out. Measurements of field emission patterns,I/Vcurves, current stability, energy distributions against both applied voltage and angle of emission, and high frequency noise were made throughout the seasoning process. These measurements show that the emitters are not conventional metal emitters but undergo an evolution that depends on their past history such as operating time and exposure to ambient gases. In particular, the energy distributions showed three distinct behaviors; (1) a large and even dominating part of the total emission current could come from low-energy secondary electrons generated by collisions of the emitted electrons with the grid, particularly in the beginning of the seasoning process. After seasoning, the electrons were predominantly emitted from either, (2) multiple discrete bands nearer to the Fermi level that displace with applied voltage, or, (3) from the standard conduction band at the Fermi level. This invalidates the strict use of simple Fowler–Nordheim theory for analyzing the field emission from these emitters. A model based essentially on the field emission from nanoprotrusions all along the surface of the conical shank of the deposited tip is proposed which is consistent with all the experimental results. The spatial distribution of the noncontrolled nanoprotrusions evolves from the shanks to the microtip apex regions during the seasoning process.

ISSN:1071-1023    

DOI:10.1116/1.589352

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

The emission properties of the pyramidal-shaped diamond after hydrogen and oxygen surface treatments were discussed. There was a shift inC1sbinding energy peak in the x-ray photoelectron spectrum due to the surface treatments. The lowest turn-on voltage in the emission current versus anode voltage characteristics is observed after the hydrogen treatment. The emission barrier height ratios against the as-prepared surface are found to be 0.68 for hydrogen treated and 2.1 for oxygen treated surfaces, respectively, from the slopes of the Fowler–Nordheim characteristics. The change in the barrier height is speculated to be explained by the dipole formed at the diamond surface. The emission stability is also confirmed, and this remarkable feature was obtained for a hydrogen-treated surface.

ISSN:1071-1023    

DOI:10.1116/1.589353

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

A technique for fabricating self-aligned gate structures onto pre-existing silicon field emitter arrays is introduced in detail in this article. The gate-controlled field emission characteristics of the resulting devices are also described.

ISSN:1071-1023    

DOI:10.1116/1.589354

出版商:American Vacuum Society    

年代:1997

数据来源: AIP

摘要:

The mechanism for the lateral growth of ultrathin GaAs whiskers is discussed in connection with the vapor–liquid–solid growth model. The observed growth rate shows that the migration of the source material plays an essential role in such whisker growth.

ISSN:1071-1023    

DOI:10.1116/1.589355

出版商:American Vacuum Society    

年代:1997

数据来源: AIP