Transition from Amorphous Semiconductor to Amorphous Insulator in Hydrogenated Carbon−Germanium Films Investigated by Raman Spectroscopy

Abstract

Amorphous hydrogenated carbon-germanium films (a-Ge X C Y :H) were fabricated by plasma chemical vapor deposition in an audio frequency (af) three-electrode reactor using tetramethylgermane (TMGe) as a source compound. Two types of the material, namely semiconducting (a-S) and insulating (a-I) films characterized by quite different electronic properties, were produced. For example, electrical conductivity at room temperature, T room , amounts to approximately 10 -18 S/m (with activation energy E A ≈ 0.9 eV) and 10 -4 S/m (E A ≈ 0.3 eV) for a-I and a-S, respectively. This very drastic change in the electronic structure of the a-Ge X C Y :H films can be caused by a very small variation of the plasma deposition conditions and therefore it is termed a-I-a-S transition. The previous attempts to explain the nature of the a-I-a-S transition from the molecular and supermolecular point of view (quantitative chemical microanalysis, XPS and IR spectrscopies, TEM, SEM, electron diffraction, and AFM) have failed. In this paper Raman spectroscopy has been used to this end. It has been found that the carbon structure does not determine the electronic properties of the films and sp 2 sites are not responsible for the a-I-a-S transition. This conclusion is supported by investigations on a-Ge X C Y :H films, in which sp 2 sites are purposely created. On the other hand, a drastic difference in the Ge atoms dispersion in the a-I and a-S has been found. In the a-I films a great amount of Ge atoms is molecularly dispersed, whereas in the a-S films the vast majority of Ge atoms is in the form of a-Ge several-nanometersized clusters. It is suggested that these clusters play a crucial role in the formation of the amorphous semiconductor network and they are responsible for the a-I-a-S transition.

Transition from amorphous semiconductor to amorphous insulator in hydrogenated carbon–germanium films investigated by IR spectroscopy

“…Most probably ion bombardment of depositing film, which is responsible for deposition of a-S material, promotes phase separation. As a result, the density of amorphous germanium clusters is sufficient to influence the material electronic structure [7]. This hypothesis has been strongly supported by impedance spectroscopy measurements carried out over a broad range of temperatures [3].…”

“…An important insight has been provided by the results of Raman spectroscopy investigations [7]. It has been shown, against expectations, that carbon structures are not responsible for the observed change in electronic structure of the material, and the creation of sp 2 bonds cannot be correlated with the a-I/a-S transition.…”

Conventional pyrolysis of [CCSiMe2CCGeMe2]n polymer precursor for Si/Ge/C materials

“…Moreover, solubility of carbon in silicon is very low (10 17 cm À3 at the melting point) and in germanium extremely low (10 8 cm À3 ). For example, Vilcarromero et al [23] found that during rf-co-evaporation of Ge and C targets for production of Ge 1Àx C x films germanium atoms were dispersed in sp 3 hybridized carbon and over x > 0.2 sp 2 carbon domains were formed; López et al [24] observed in thin SiGeC films produced by ArF laser CVD of disilane, germane and ethene an independent incorporation of Ge and C atoms; Hu et al [22] produced Ge 1Àx C x films by rf sputtering in which germanium and carbon clusters were built in a composite amorphous material; Kazimierski et al [25] fabricated by plasma CVD in an audio frequency three electrode reactor insulating and semiconducting a-Ge x C y :H thin films using (CH 3 ) 4 Ge. The films possessed a composite structure of germanium cluster, chains and nanocrystals embedded in sp 3 /sp 2 carbon surroundings.…”

MOCVD Synthesis of Germanium Filled “Diamondlike” Carbon Nanotubes and Microtubes from Organogermanium Precursors and Their Field‐Emission Properties

“…Also note that this line has an asymmetric contour and its half-width increases to 14 cm -1 which could be caused by partial disorder in the subsurface layer as well as by an internal stress. [49] [54] were found. It is also important that no…”