Influence of deposition temperature on the structure of 3,4,9,10-perylene tetracarboxylic dianhydride thin films on H-passivated silicon probed by Raman spectroscopy

Abstract

Raman spectroscopy was used to characterize the structural order in thin organic films of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). Films of the same average thickness were grown by organic molecular beam deposition on hydrogen-passivated p-type silicon (1 0 0) substrates at different growth temperatures between 230 and 470 K. The Raman spectra of all samples exhibit four external vibrational modes, that occur as a consequence of the arrangement of the PTCDA molecules in a crystalline environment. The full width at half maximum of these phonon lines decreases with increasing temperature of the substrate during deposition. A similar tendency is also observed for the Raman-active internal molecular modes. In addition, with increasing deposition temperature the diffusely scattered light background in the Raman spectra increases, as well as the photoluminescence background in the high frequency range. We relate the observed spectral changes to an increase in the size of the crystalline domains within the films with increasing deposition temperature, an effect that is macroscopically reflected by an enhanced degree of surface roughness. The different quality of the crystalline PTCDA domains was also complementary revealed by X-ray diffraction measurements.

Introduction

Thin films of organic semiconductors have already proved a great potential for optoelectronic devices [1]. Progress has greatly benefited from controlled film deposition under clean ultra-high vacuum (UHV) conditions: organic molecular beam deposition (OMBD). Additional potential for future applications lies in combining organic and inorganic semiconductors for electronic devices. Device performance will markedly be influenced by the degree of structural order in the organic films which to a large extent controls the electronic properties (such as carrier mobilities) as well as the optical ones. The structural order itself is expected to depend critically on the growth parameters, in particular on the substrate temperature.

The film formation properties of the archetype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a variety of inorganic substrates have already been studied by a number of experimental and theoretical methods [1], [2], [3], [4]. The most commonly used substrate materials have been weakly interacting ones, such as metals (e.g. Ag and Au), highly oriented pyrolytic graphite, and insulators (NaCl, SiO₂, Al₂O₃) [1], while investigations using the more reactive surfaces of more common inorganic semiconductors such as silicon and gallium arsenide have been scarce.

On clean GaAs(1 0 0) [5] or Si(1 0 0) [6] surfaces a strong chemical interaction occurs between the anhydride groups of the molecules at the interface and the substrate, predominantly with the dangling bonds and defects. This usually results in the formation of small crystalline domains, in nearly random azimuthal orientation. Improved structural order is observed when reactive semiconductor substrate surfaces are passivated, e.g. by reaction with chalcogen or hydrogen atoms in the case of GaAs and Si substrates, respectively. The passivation induces a reduction of chemically active sites mainly by saturating dangling bonds [7]. Hence the work to be presented here concentrates on the deposition of PTCDA on hydrogen terminated Si(1 0 0), for the latter see [8]. Substrate temperature is a further important parameter that can control structural order. It was indeed already observed by X-ray diffraction (XRD) that the crystallite size drastically increased as temperature was raised from 320 to 370 K [9] when PTCDA was deposited on Si(1 0 0) wafers with natural oxide.

We have previously demonstrated that Raman spectroscopy is a versatile tool to characterize PTCDA films [10], [11]. Raman spectra provide information on the internal vibrational modes of PTCDA molecules as well as on external vibrational modes (phonons). The latter were detected for all films deposited on passivated semiconductor substrates at frequencies between 40 and 125 cm⁻¹. These Raman active modes are of rotational–vibrational (libronic) origin in the PTCDA molecular crystal which has monoclinic symmetry (space group C_2h⁵) with two molecules per unit cell (for a detailed discussion see [12]).

The first Raman study of the influence of substrate temperature on the growth process of PTCDA films on hydrogen-passivated (H-passivated) (1 0 0) oriented silicon surfaces to be reported here, reveals marked changes in the PTCDA phonon spectra. These changes can be correlated with the degree of structural order using additional XRD data.