Understanding spectroscopic phonon-assisted defect
features in CVD grown 3C-SiC/Si(1 0 0) by modeling
and simulation
Devki N. Talwar
a,*, Z.C. Feng
baDepartment of Physics, Indiana University of Pennsylvania, 975 Oakland Avenue, Indiana, PA 15705-1087, USA bGraduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei 106, Taiwan, ROC
Received 12 December 2003; accepted 9 February 2004
Abstract
New phonon-assisted defect features are observed using photoluminescence (PL) and Raman scattering spectros-copy on 3C-SiC/Si(1 0 0) films grown by chemical vapor deposition (CVD) technique. The ultraviolet excitation room-temperature (RT) PL-Raman spectra show a luminescence band near 2.3 eV due to RT recombination over the 3C-SiC indirect band gap. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC we also observed weaker impurity modes near 620, 743 and 833 cm1. These frequencies are compared with the results of Green’s function simulations of impurity modes with plausible defect structures to best support the observed Raman features as well as modes of some prototypical defect center.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: 3C-SiC, Raman scattering; Green function; Simulation
1. Introduction
Silicon carbide (SiC), owing to its excellent electrical, mechanical and chemical properties, is experiencing a renewed wave of interest in elec-tronic applications with feasibility of use in ex-treme environments [1–4]. Continuing progress in the growth of bulk and epitaxial SiC films with defined doping levels and device processing has resulted in successful fabrication of several
elec-tronic devices with the same quality standards as of the Si technology.
Chemicalvapor deposition (CVD) has been a successful and reliable method for the epitaxial growth of SiC films on Si and SiC substrates. Despite significant differences in the lattice con-stants (20%) and thermalexpansion coefficients (8%) between 3C-SiC and Si, large area hetero-epitaxial 3C-SiC/Si(1 0 0) layers are successfully grown by CVD technique [5–8]. The large lattice mismatch and high deposition temperature re-quired for the growth of SiC films are primarily responsible for the fundamental material related problems including the residual strain, thermal
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Corresponding author. Fax: +1-724-3573804. E-mail address:talwar@iup.edu(D.N. Talwar).
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expansion coefficient, low energy defect formation etc. The epitaxially grown films are usually n-type with high donor concentrations [9–11]. Recent studies by scanning electron microscopy (SEM) have revealed high density of lattice defects such as stacking faults and anti-phase boundaries [12,13]. As the reliability of semiconductor devices is strictly correlated to the presence of defects, the key to a successfulSiC device technology demands
(i) the controlof doping, (ii) the knowledge of
defect states within the materialinfluencing the lifetime of devices and (iii) the site-selectivity of both intrinsic and doped defects.
The purpose of the present paper is to report the results of an extensive spectroscopic (photolumi-nescence (PL) and Raman scattering) investiga-tions on 3C-SiC/Si(1 0 0) films grown by CVD technique (cf. Section 2). We have performed both ultraviolet (UV) excitation (325 nm) PL-Raman measurements in the backscattering geometry [14] and Raman scattering with an excitation of 406.7 nm line from a Kr ion laser [15]. The room-tem-perature (RT) PL-Raman spectra exhibited a luminescence line near 2.3 eV due to RT recombi-nation over the 3C-SiC indirect band gap. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC we have also observed
weaker impurity modes near 620, 743 and 833 cm1.
These frequencies are compared with the results of Green’s function simulations (cf. Section 3) of impurity modes with plausible defect structures to best support the observed Raman features as well as the modes of some prototypicaldefect center.
2. Experimental results
The 3C-SiC samples used in the present spec-troscopic studies are epitaxially grown on Si(1 0 0) substrate by CVD method. The CVD 3C-SiC/Si films are generally reported to be under tensile strain. Measurements on the sample curvature have revealed that the sign of internal stress be-comes abruptly negative when the initial stage of carbonization of Si substrate prior to growth is
carried out at temperatures lower than 1050°C.
Our CVD method includes the preparation of an initialSiC layer from carbonization and
sub-sequent growth of SiC. The details of the growth process are described elsewhere [6]. It has been shown that the initialgrowth of 1–3 lm layer contained high density of dislocations, stacking fault, micro-cracking etc. After a certain thickness of growth, a very good and high quality 3C-SiC/ Si(1 0 0) layers are obtained.
2.1. Combined PL-Raman spectroscopy under UV excitation
In an earlier study Shim et al. [16] reported that an indirect band gap 3C-SiC/Si materialcan not exhibit luminescence at room temperature. If excitation comes from the interface side of the free 3C-SiC film (etching away Si substrate) a broad PL band near 2.4 eV with a weak line at 3.0 eV is observed. The authors of Ref. [16] speculated the origin of these bands as either due to recombina-tion center associated with the OH group or localized states near the interface.
On the contrary by using a sensitive Renishaw UV Raman-PL microscope system we have real-ized a direct detection of room-temperature PL spectra in a 16 lm 3C-SiC/Si(1 0 0) CVD sample. The results displayed in Fig. 1a for the energy range 2.0–3.75 eV reveala band near 2.3 eV originating from 3C-SiC––a representative of the PL emissions across the indirect band gap. The
Raman scattering features (a sharp band3.7 eV
and a weak line near 3.62 eV) are also visible.
Raman shift plotted in Fig. 1b has clearly re-vealed the 3C-SiC longitudinal optical (LO)
phonon at C point (980 cm1 with a FWHM of
14 cm1) along with second-order Raman features
between the energy range of 1500–1800 cm1. The
perusalof Fig. 1a reveals that our 2.3 eV PL line of 3C-SiC/Si(1 0 0) has a full width at half maxi-mum (FWHM) of about 0.11 eV––much nar-rower (6 times) than reported in Ref. [16]. This indicates the high quality of 3C-SiC film from our CVD samples. The Raman scattering signals are generally much weaker than the PL emission. However the combined PL-Raman spectrum in Fig. 1a exhibiting Raman mode intensities at the same scale with the PL lines is a further testimony of the high crystalline quality of the 3C-SiC/Si materials.
2.2. Raman spectra
In addition to the Raman active 3C-SiC modes, we have observed new phonon-features in many 3C-SiC/Si(1 0 0) samples. In Fig. 2 the results of Raman scattering are displayed from a CVD 3C-SiC/Si(1 0 0) sample excited by the 406.7 nm line
from a Krþ laser at 300 K. Clearly, the strong
Raman lines characteristic of Si substrate [TO(C) mode] and 3C-SiC [LO(C) mode] are observed. By magnifying the intensity 50 times the weaker
modes at 620, 743 and 833 cm1become visible. In
Fig. 3 we have plotted Raman spectra from three 3C-SiC/Si(1 0 0) samples, with different
thick-nesses. Two modes at 620 and 833 cm1 are
ob-served from the thinnest film of 4 lm (cf. Fig. 3a). In the 7 lm 3C-SiC/Si sample, besides the 620
cm1 mode, additionalline at 680 cm1 is seen.
However, for the thickest 16 lm film, the features
at 620, 680 and 833 cm1, are not so obvious.
Comprehensive Green’s function simulations (cf. Section 3) of impurity modes with plausible defect structures are performed to best correlate the phonon-assisted Raman features to defects and to prototypicalcenter.
Fig. 2. Room-temperature Raman scattering from a CVD 3C-SiC/Si(1 0 0) excited under 406.7 nm. If magnified 50 times the weaker modes at 620, 743 and 833 cm1are clearly observed.
Fig. 3. Room-temperature Raman spectra from three CVD 3C-SiC/Si(1 0 0) samples of different thickness.
2.0 2.5 3.0 3.5 PL Raman 325 nm, 300 K (16 µm) CVD 3C-SiC/Si(100) INTENSITY (a.u.) ENERGY (eV) 500 1000 1500 2000 325 nm, 300 K (16µm) CVD 3C-SiC/Si(100) 2nd order LO(Γ) Intensity (a.u.) Raman Shift (cm-1) (a) (b)
Fig. 1. Combined PL and Raman spectra, under UV (325 nm) excitation from a CVD 3C-SiC/Si(1 0 0) sample with film thickness of 16 lm: (a) PL-Raman is displayed in the energy range of 2.0–3.75 eV and (b) Raman shift is plotted in wave number.
3. Theoretical
3.1. Dynamical properties of defects in 3C-SiC The study of localized vibrational modes (LVMs) in compound semiconductors has pro-vided us with valuable information about the site-selectivity and chemical nature of defects. Except for H-related modes the LVM data for intrinsic or extrinsic defects in 3C-SiC is rather sparse. Green’s function theory is used to study the impurity modes of both isolated and complex defects in 3C-SiC. The results are evaluated for plausible defect structures to best support the observed Raman features as well as a prototypical center.
3.1.1. Isolated single substitutional defects
In zinc-blende type materials the light impurities occupying either the cation or anion site vibrates in
the triply degenerate F2 mode causing high
fre-quency infrared and Raman active LVMs. The heavier impurity, with appropriate ‘impurity–host
interactions’, may give rise to Raman active A1, E
and F2 type ‘quasi-localized’ modes. Earlier, we
have analyzed LVMs due to several isolated defects in III–V compounds and established valuable trends in the force-constant variations relating them to the redistribution of electron charge den-sity in the impurity–host bonding. These trends in the bonding mechanism are important to establish the role of intrinsic and extrinsic defects in 3C-SiC.
Fig. 4. (a) Green’s function calculations of the gap modes as a function of force-constant change parameter t for impurities occupying the C site in 3C-SiC. (b) Green’s function calculations of the impurity (localized and gap) modes as a function of force-constant change parameter u for defects occupying the Si site in 3C-SiC.
In the Green’s function frame work [17] we have simulated local and gap modes as a function of force-constant parameter tðuÞ for severaldefects occupying Si or C sites in 3C-SiC (see: Fig. 4a–b). The light impurities on Si-sites not only provide high frequency LVMs but also gap modes that emerge from the top of the acousticalbranch (see: Fig. 4a). In addition to the ‘quasi-localized’ modes for heavy impurities on C-sites, our simulation
predict F2-type gap modes emerging from the
minimum of the opticalband (see: Fig. 4b). If the appropriate force-constant variations are included our simulations predicted local, gap
modes for BSi(990 cm1,620 cm1), CS(1000
cm1, 620 cm1), N
Si(985 cm1,620 cm1) and
gap modes for NC(638 cm1F2), and SiC (648
cm1 F
2;622 cm1 A1). Although the calculated
impurity are infrared and Raman active most of them fall close to the phonon continuum of 3C-SiC. This might be the reason why no such vibrations are detected by opticalspectroscopy. Again, despite the relatively high frequency of
the C-antisite CSi (1000 cm1) mode it is not
observed experimentally because the center is both electrically and optically inactive.
3.1.2. Nearest-neighbor pair defects
A prototypicalDIdefect center in SiC
responsi-ble for the sharp emission lines in the low temper-ature photoluminescence (LTPL) is well known for
decades. The DI is observed in as-grown material
after quenching from growth temperature as well as in epitaxial layers grown by CVD or molecular
beam epitaxy (MBE) methods. The sharp Li
emis-sion lines are followed by characteristic
phonon-assisted structures around 83 meV (664 cm1) in
the phonon band gap. These facts suggests that DI
center has an intrinsic nature and is formed by vacancies, antisites and/or interstitials introduced by the damage. Severalmodels have been proposed
for the DI center including a divacancy; carbon
divacancy; VSi–NC complex, etc. Using Greens
function theory we have calculated impurity modes
for severalnearest-neighbor pair defects (VSi– NC;
SiC–Csi) of C3vsymmetry in 3C-SiC.
As reported in Section 3.1.1 the LVM of an
iso-lated CSi defect (1000 cm1) fal l s cl ose to the
maximum phonon frequency xm of 3C-SiC, while
the isolated SiCdefect provides two impurity modes
in the phonon gap––a triply degenerate F2 mode
(648 cm1) and a non-degenerate A
1mode (622
cm1) (see: Fig. 4a). The pairing of antisite defects
SiC–CSilifts the degeneracies of the F2modes. With
an appropriate stiffening between the impurity bond
the F2 gap mode splits up into two modes 651
cm1 (A
1) and 647 cm1 (E). These results
com-pare favorably well with our Raman data and are consistent with the observed characteristic
phonon-assisted structure in the phonon gap of the DIcenter.
4. Summary and conclusions
In summary, we have carried out a compre-hensive theoreticalstudy to understand the spec-troscopic phonon-assisted defect features observed in CVD grown 3C-SiC/Si(1 0 0) samples. Our UV (325 nm) excitation PL-Raman spectra exhibited 2.3 eV luminescence band due to RT recombina-tion over the 3C-SiC indirect band gap. Raman scattering studies are made with an excitation of 406.7 nm from a Kr ion laser, in the back scat-tering geometry using a triple spectrometer. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC new impurity-activated features are observed in many CVD grown 3C-SiC/Si samples. Green’s function calculations of impurity modes are used to modelthe defect structures that best supported the observed Ra-man features as well as modes of a prototypical center.
Acknowledgements
The authors wish to express their sincere thanks to Dr. J.A. Powell for the growth of the samples used in this study, to Drs. D. Pitt, K. Williams and K.T. Yue for the use of Raman instruments, and to Professors W.J. Choyke, I. Ferguson and C.C. Yang for their support and help throughout this project. The work of one of the authors (DNT) was supported by the Cottrell College Science Award # CC4600 (Research Corporation), and by the NationalScience Foundation Grant No. ECS-9906077.
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