Vacancies in SiC nanopowders

doi:10.1016/S0921-5107(00)00466-9

Materials Science and Engineering: B

Volume 77, Issue 2, 31 August 2000, Pages 147-158

https://doi.org/10.1016/S0921-5107(00)00466-9 Get rights and content

Abstract

Origin of vacancies in the large-sized SiC nanocrystals (higher than 10 nm) has been investigated using theoretical band structure calculations and experimental electronic paramagnetic resonance (EPR) measurements. Influence of geometry sizes on appearance of concrete vacancy has been studied. The theoretical approach includes self-consistent norm-conserving pseudopotential band energy calculations and geometry structure optimisation. The performed calculations show that the presence of the vacancies is a necessary attribute of the SiC nanocrystallites. Moreover, the type and concentration of the vacancies are dependent on the nanoparticle geometry. We have revealed that spin-polarised states of intracrystallite vacancies differ essentially from vacancies in the bulk crystals. A comparison between the performed theoretical simulations and obtained EPR experimental data shows the possibility of using the proposed methods for prediction of vacancy appearance in the binary nanocrystallites and possibility for their operation.

Introduction

Investigation of semiconductor nanocrystals has recently caused considerable experimental and theoretical interest [1], [2], [3], [4], [5], [6]. There are many investigations devoted to changes of the band structure due to confinement-induced quantization of the continuous band states [7], [8]. Traditionally, a main method for study of the nanocrystallites consists of the use of different optical methods: fluorescence [9], Raman scattering [10], photon echo [11], and electroluminescence [12]. All these works demonstrate an influence of the crystalline size decrease on the changes of the electronic structure and the corresponding optical spectra [13] due to the size-scaling of electronic transitions and electron–phonon interactions, including anharmonic electron–phonon interactions. One of the clearer influences of the mentioned effects on the band energy gap was demonstrated in Ref. [14].

All the known theoretical methods can be classified into two main groups. First, consider the crystalline from the bulk long-range ordering approach [15]. Most of the calculations within a framework of the bulk-like approach were carried out using the semi-empirical band energy calculations. In the second category, the nanocrystals are built as a large molecule and a molecular cluster approach is applied [16]. Unfortunately, both of the approaches usually neglect the presence of vacancies. However, even from simple thermodynamical considerations, one can expect the appearance of the vacancies in such partially ordered materials.

The vacancies may play an essential role in the observed electronic effects, first of all in the electron carrier dynamics fast-response optics, conductivity, etc. This is also experimentally demonstrated by electronic paramagnetic resonance (EPR) spectra in the different semiconducting nanocrystallites [17], [18], [19], [20]. The possibility of operation by properties of vacancies could open a new era in opto-electronic and computer devices. Nanocrystalline SiC powders are promising because their vacancy concentration can be varied. By changing the size of corresponding nanocrystallites, one can make use of certain properties such as conductivity, spin-orientation phenomena or non-linear optical second-order effects. These properties are largely dependent on vacancies in the SiC nanopowders.

The origin of these vacancies is not yet fully understood. These vacancies warrant investigation not only because they are a general feature of solid state physics, but also due to conceivable practical benefits, since intrinsic vacancies can significantly change the optical and electronic properties of SiC nanocrystallites. One of the main problems that should be resolved in order to use the vacancies as materials for optoelectronics is a problem of relation between nanocrystallite sizes and vacancy types. Contribution of interface fragments should be clarified.

In order to clarify the role of interfaces and of the associated vacancies, we propose a complex experimental and theoretical approach. For investigations, SiC nanocrystals of relatively large size (10–30 nm) were chosen. The main reasons for such selection are the following:

•
technology of the SiC nanocrystallite preparation with the desired sizes is better developed comparing with other semiconducting materials [21];
•
relatively large sizes (10–30 nm) allow one to use one-electron solid-state calculation methods in the k-space;
•
high mechanical and chemical stability of the SiC; and
•
the possibility of effective variation of energy gaps due to the possibility of changing degrees of hexagonal-like and cubic phases.

For the theoretical simulation, we modify the approach developed for the partially disordered materials, such as glass [22], organic molecules [23], and disordered crystals [24].

The main steps of the mentioned approach are as follows:

•
band energy calculations within a framework of the local spin density approximation (LSDA) with different ratios of cubic and hexagonal-like phases;
•
taking into account the surrounding amorphous-like background;
•
geometry optimisation of near-surface states; and
•
study of the correlation between nanocrystallite sizes and structural peculiarities of the investigated composites;

We will show that the presence of the vacancies favours achievement of a minimum of total energy, U_tot, and stabilises structural configuration. A more sensitive approach to such kinds of vacancies involves EPR spectra with the doubling bonds.

Thus, the main goals of the present work are the following:

•
experimental investigations of the vacancies using the EPR method;
•
optimisation of the interface;
•
evaluation of an influence of the nanocrystallite sizes on the vacancy-induced spin-polarised wavefunctions; and
•
comparison of the proposed theoretical approach with the experimentally obtained EPR data.

The work is organised as follows: In Section 2, experimental details concerning specimen preparation and the EPR measurements are presented. Section 3 presents methodology of the band energy structure calculations. Then, Section 4 presents the main physical mechanisms of the nanocrystallite topology optimisation as well as results devoted to the size-dependent band energy structure calculations. We demonstrate that the vacancies (carbon or silicon) are a necessary attribute of the interfaces. The comparison of the experimentally evaluated and theoretically calculated spin-polarised wavefunctions is given in Section 5.

Section snippets

Specimen features

The ultrafine SiC powders were synthesised by laser pyrolysis of a gaseous mixture (SiH₄, C₂H₂). Consequent to annealing up to 1800°C, the properties of the particular defects are dependent on the crystalline structural changes. For every nanoparticle, we have used X-ray diffractometry to determine the degree of hexagonality, H, defined as a ratio of the hexagonal phase to the total (cubic+hexagonal) presence. The transmission electron microscopy data have been used in order to determine the

Crystalline structure

The SiC nanopowders consist of the crystalline of the α-phase (SiC-6H) and the β-phase (SiC-3C). Previous nuclear magnetic resonance measurements [25] gave, for the investigated specimens, the following ratio: about 60% α-SiC and 27% β-SiC. Amorphous-like SiC is presented in the proportion of 12%. Moreover, the performed microscopic measurements show that the sizes of the nanocrystallites were varied in the range 10–35 nm and sizes of the grains within 35–90 nm.

Silicon carbide occurs in several

Methods of structure optimisation

For performing the total energy minimisation to receive the optimised geometry, we have added to the electronic part of the total energy also the ionic contribution, U_totⁱ, calculated by means of an Ewald summation: $U_{tot}^{i} = ∑ i,l ∑ m,n Z_{m} Z_{n} / r_{im} − r_{ln}$ where $r_{im} = r_{i} + r_{m}$

The total energy minimisation was performed using the total energy derivative procedure relative to the interatomic distances, bond and torsion angles in order to obtain the more stable geometry states.

The geometry optimisation is started from

Vacancy appearance

Our calculations have shown that the carbon defects seem to be more preferable. Silicon defects are less probable than those of carbon. From a comparison of Fig. 9, Fig. 10, one can see that a region with the commensurable nanoparticle (d) and grain (L) sizes is more preferable for the defect formation. The latter correlates well with the existence of maximal interface thickness (see Fig. 6). For all the vacancy types in this region, we observe a drastic increase of the appropriate vacancy

Conclusions

Using a complex approach that includes experimental EPR measurements and self-consistent band energy calculations within the norm-conserving nonlocal pseudopotential method with the LSDA approach, we have carried out calculations of the vacancy spin-polarised state appearance versus the nanoparticle and grain sizes. We have found essential dependence of the carbon vacancy concentration in the SiC nanopowders versus the nanoparticle and grain sizes.

The procedure of interface geometry

References (48)

I.V. Kityk et al.
Phys. Lett.
(1996)
O.I. Shpotyuk et al.
J. Non-Crystal. Solids
(1997)
I.V. Kityk et al.
Phys. B: Cond. Matter
(1999)
I.V. Kityk et al.
Solid State Commun.
(1996)
D. Yean et al.
J. Phys. Chem. Solids
(1971)
W. Wang et al.
J. Phys. Chem.
(1994)
N.A. Hill et al.
Phys. Rev. Lett.
(1995)
A. Mews et al.
Phys. Rev.
(1996)
C.R. Kagam et al.
Phys. Rev. Lett.
(1995)
D.J. Norris et al.
Phys. Rev.
(1996)

A.P. Alivisatos

Science

(1996)

C.B. Murray et al.

J. Am. Chem. Soc.

(1993)

H. Weller

Angew. Chem. Int. Ed. Engl.

(1993)

A. Haselbarth et al.

Chem. Phys. Lett.

(1993)

J.R. Heath et al.

J. Chem. Phys.

(1994)

R.W. Schoenlen et al.

Phys. Rev. Lett.

(1993)

V.L. Colvin et al.

Nature

(1994)

W.L. Wilson et al.

Science

(1993)

S.H. Tolbert et al.

Phys. Rev. Lett.

(1994)

A.I. Ekimov et al.

J. Opt. Soc. Am.

(1993)

P.E. Lippens et al.

Semicond. Sci. Technol.

(1991)

W.L. Warren et al.

Phys. Rev. Lett.

(1990)

M. Suzuki et al.

J. Am. Ceram. Soc.

(1995)

X. Li et al.

Mat. Sci. Eng.

(1996)

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Vacancies in SiC nanopowders

Abstract

Introduction

Section snippets

Specimen features

Crystalline structure

Methods of structure optimisation

Vacancy appearance

Conclusions

Phys. Lett.

J. Non-Crystal. Solids

Phys. B: Cond. Matter

Solid State Commun.

J. Phys. Chem. Solids

J. Phys. Chem.

Phys. Rev. Lett.

Phys. Rev.

Phys. Rev. Lett.

Phys. Rev.

Science

J. Am. Chem. Soc.

Angew. Chem. Int. Ed. Engl.

Chem. Phys. Lett.

J. Chem. Phys.

Phys. Rev. Lett.

Nature

Science

Phys. Rev. Lett.

J. Opt. Soc. Am.

Semicond. Sci. Technol.

Phys. Rev. Lett.

J. Am. Ceram. Soc.

Mat. Sci. Eng.