Sub-picosecond electron dynamics in polycrystalline diamond films

Highlights

• Sub-picosecond electron dynamics is sensitive to morphology and content of non-diamond carbon phase

• Dynamics consists of two competitive processes with opposite signs

• Instantaneous increase of absorption on free carriers is observed in diamond grains

• Filling the π states of non-diamond carbon phase leads to a slower gradual increase in transmission

Abstract

Polycrystalline diamond, as a composite material, primarily consists of sp³ and sp² hybridized carbon phases in different ratio. The amount of grain boundaries, sp² non-diamond carbon phase and other defects and impurities play a crucial role in relaxation and recombination of photo-excited charge carriers in diamond films. The transient transmission measurements are performed using non-degenerate pump and probe technique using few-cycle laser pulses with temporal resolution of ~15 fs. We show that the initial processes occurring after interaction of light with diamond membranes are sensitive to the sp³/sp² ratio phases. We propose a simple microscopic model of electron relaxation via sub-gap energy levels caused by surface states, sp² carbon phase and defects of the crystal lattice of diamond grains.

Introduction

Polycrystalline diamond (PCD) films have been investigated for their unique properties which are determined by the combination of the diamond crystal (sp³ hybridization) bulk and surface features where non-diamond carbon phases (sp¹ and sp² hybridizations) are preferentially localized. Here, a precise controlling of electrical and optical properties via growth conditions is required for a variety of applications. These include biosensing and bioelectronics due to the diamond biocompatibility and non-cytotoxicity [[1], [2], [3], [4], [5], [6], [7], [8]]. Furthermore, quantum defects in diamond crystal lattice are used for quantum optics [9], for sensitive detection of magnetic fields [10,11] and nonlinear optics [12].

Self-supporting diamond membranes of various thickness, grain size, surface roughness and content of non-diamond carbon phase (i.e. different ratio of sp³ and sp² hybridized phases) can be prepared by chemical vapor deposition (CVD). The crystal size in such films then strongly influences the initial dynamics of excited charge carriers [13] – a key parameter for optical and optoelectronic applications. Regarding to electronic properties, the composition of the films leads to a fairly complex manifold of electronic states which can differ both in energy and spatial positions. They affect the microscopic electronic behavior and in turn the optical and electronic response of the films.

Previously we studied recombination processes of photoexcited electrons in micro- and nanocrystalline diamond (MCD and NCD) films in dependence on the sub-gap energy states as well as on the film's morphology. The properties of sub-gap energy levels were studied using ultrafast time-resolved photoluminescence spectroscopy including temperature and air pressure dependences of the luminescence decay times [[14], [15], [16], [17]]. In combination with other experimental methods (Raman spectroscopy and scanning electron microscopy), the influence of morphology and amount of the non-diamond sp² phase on recombination and nonlinear optical response was studied [[18], [19], [20], [21], [22], [23], [24], [25], [26]]. It was also shown, that diamond grain boundaries largely affect nonlinear optical phenomena as well as coherent phonon dynamics. Measurements of transient transmission were performed with degenerate pump and probe technique where the ps − ns dynamics of the defect centers was studied [27]. Ultrafast sub-picosecond relaxation of photo-excited charge carriers attributed to the separation of electrons and holes on grain boundaries was observed. However, the temporal resolution of the experiments did not allow detailed investigation of ultrafast dynamics of these processes and their dependence on diamond grains size and the amount of sp² carbon phase between the grains.

Time-resolved laser spectroscopy has been used as an efficient tool for investigation of relaxation and recombination processes of charge carriers in diamond materials. Recent progress in ultrafast laser technology opened the possibility to shift the time resolution of measurements down to few femtoseconds. In this way a new insight into the initial carrier relaxation processes can be obtained. In this paper, we use the few-cycle laser pulses to achieve this very high time resolution in the study of polycrystalline diamond films. We extend our previous study on time-resolved photoluminescence spectroscopy [18] by employing non-degenerate femtosecond pump and probe spectroscopy with time resolution of ~15 femtoseconds to monitor the very initial relaxation of photoexcited charge carriers. Initial dynamics after the excitation appear to change dramatically depending on growth conditions and composition of the fabricated diamond membranes. Near-infrared probing in a non-degenerate pump and probe scheme allows us to monitor the filling of sub-gap states (π states) which exhibit luminescence in this spectral region [18]. We observe an interplay between an instantaneous increase of absorption due to populating diamond volume states and slower absorption bleaching due to relaxation within π states of the sp² carbon phase. This type of dynamics was not observed in previous degenerate transient transmission experiments investigating similar samples [27].