Igor Beinik

Electrical properties of ZnO nanorods studied by conductive atomic force microscopy

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, we present a complementary electrical characterization of individual upright standing and lying ZnO nanorods using conductive atomic force microscopy (C-AFM). Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps. The current maps were recorded simultaneously with the topography acquired by contact mode AFM. Further, C-AFM was utilized to determine the local current-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Current-voltage characterization revealed a characteristic similar to that of a Schottky diode. Detailed discussion of the electrical properties is based on local I-V curves, as well as on the 2D current maps recorded from specific areas.

Conductive Atomic-Force Microscopy Investigation of Nanostructures in Microelectronics

Conductive atomic-force microscopy (C-AFM), where a conductive, biased probe is scanned in contact mode across the surface under investigation is one of the most prominent scanning probe microscopy based techniques to study electrical properties of dielectric and semiconducting thin films on the nanometer scale. The technique, originally developed to evaluate the homogeneity in gate dielectrics is also successfully applied to study electrical and electronic properties of semiconductor nanostructures. The chapter starts with the discussion of the technical implementation of the technique (both under ambient conditions and in ultra-high vacuum) and the experimental peculiarities due to contact mode. The concepts of two-dimensional current maps acquired at constant tip-to-sample bias and local current voltage maps will be introduced for the example of thin silicon gate oxide and high-k dielectric thin films. Applicability of C-AFM to semiconductor nanostructures is demonstrated for supported semiconductor nanowires and free standing nanorods. Characterization of antiphase defects in ternary alloys and ZnO based multilayer varistor films show the technique’s potential for device evaluation. An outlook is devoted to the so-called photoconductive AFM where photocurrents are detected under simultaneous illumination with monochromatic light.

Photoresponse from single upright-standing ZnO nanorods explored by photoconductive AFM

Summary Background: ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, photoconductive atomic force microscopy (PC-AFM) has been applied to investigate transient photoconductivity and photocurrent spectra of upright-standing ZnO nanorods (NRs). With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have also performed time-resolved photoluminescence measurements macroscopically. Results: Persistent photoconductivity from single ZnO NRs was observed for about 1800 s and was studied with the help of photocurrent spectroscopy, which was recorded locally. The photocurrent spectra recorded from single ZnO NRs revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV. This value is at least 100 meV lower than the band-gap energy determined from the photoluminescence experiments. Conclusion: The obtained results suggest that the photoresponse in ZnO NRs under ambient conditions originates preferentially from photoexcitation of charge carriers localized at defect states and dominates over the oxygen photodesorption mechanism. Our findings are in agreement with previous theoretical predictions based on density functional theory calculations as well as with earlier experiments carried out at variable oxygen pressure.

Conductive atomic force microscopy study of InAs growth kinetics on vicinal GaAs (110)

Conductive atomic force microscopy has been used to investigate the effect of atomic hydrogen and step orientation on the growth behavior of InAs on GaAs (110) misoriented substrates. Samples grown by conventional molecular beam epitaxy exhibit higher conductivity on [11¯0]-multiatomic step edges, where preferential nucleation of InAs nanowires takes place by step decoration. On H-terminated substrates with triangular terraces bounded by [11¯5]-type steps, three-dimensional InAs clusters grow selectively at the terrace apices as a result of a kinetically driven enhancement in upward mass transport via AsHx intermediate species and a reduction in the surface free energy.

Scanning probe microscopy-based characterization of ZnO nanorods

We apply scanning probe microscopy (SPM) to study the morphology and electrical properties of vertical zinc oxide nanorods grown by hydrothermal methods on silicon substrates. It is demonstrated that - against the intuition - SPM techniques can indeed be used to study such fragile high-aspect ratio semiconductor nanorods. Atomic-force microscopy (AFM) operating in tapping mode yields - via the analysis of the height histograms calculated from AFM images - easy access to the height fluctuations in the nanorod ensemble. High-resolution AFM images reveal the three-dimensional shape of the nanorods including transition facets between the (0001) top terrace and the {10-10} side facets. Further, we were able to acquire current-voltage curves of individual nanorods by conductive atomic force microscopy (CAFM) operating in contact mode.