www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2008) 2031–2036

Nanostructures on SiC surface created by laser microablation

L. Fedorenko a,*, A. Medvid’ b, M. Yusupov a, V. Yukhimchuck a, S. Krylyuk a, A. Evtukh a

a V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41 Prospect Nauky, Kiev 03028, Ukraineb Riga Technical University Latvia, 14 Azenes Str., LV-1048 Riga, Latvia

Received 19 February 2007; received in revised form 9 August 2007; accepted 10 August 2007

Available online 19 August 2007

Abstract

Silicon carbide (SiC), as it is well-known, is inaccessible to usual methods of technological processing. Consequently, it is important to search

for alternative technologies of processing SiC, including laser processing, and to study the accompanying physical processes. The work deals with

the investigation of pulsed laser radiation influence on the surface of 6H–SiC crystal. The calculated temperature profile of SiC under laser

irradiation is shown. Structural changes in surface and near-surface layers of SiC were studied by atomic force microscopy images,

photoluminescence, Raman spectra and field emission current–voltage characteristics of initial and irradiated surfaces. It is shown that the

cone-shaped nanostructures with typical dimension of 100–200 nm height and 5–10 nm width at the edge are formed on SiC surface under nitrogen

laser exposure (l = 0.337 mm, tp = 7 ns, Ep = 1.5 mJ). The average values of threshold energy density hWthni at which formation of nanostructures

starts on the 0 0 0 1 and 0 0 0 1̄surfaces of n-type 6H–SiC(N), nitrogen concentration nN ffi 2 � 1018 cm�3, are determined to be 3.5 J/cm2 and

3.0 J/cm2, respectively. The field emission appeared only after laser irradiation of the surface at threshold voltage of 1000 Vat currents from 0.7 mA

to 0.7 mA. The main role of the thermogradient effect in the processes of mass transfer in prior to ablation stages of nanostructure formation under

UV laser irradiation (LI) was determined. We ascertained that the residual tensile stresses appear on SiC surface as a result of laser microablation.

The nanostructures obtained could be applied in the field of sensor and emitting extreme electronic devices.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.16.Mk, 81.07.Bc, 79.20.Ds

Keywords: Silicon carbide; Laser ablation; Nanostructures; Field emission; Photoluminescence; Raman spectra

1. Introduction

As it is well-known, silicon carbide due to its unique

electrical, radiative, chemical, and mechanical stability is

widely used as a promising material for extreme electronics.

However, at the same time SiC is actually inaccessible to usual

methods of technological processing used for the majority of

traditional semiconductors (Si, Ge, GaAs, etc.): thermal

diffusion of impurities, annealing, and chemical etching.

Consequently, it is important to search for alternative

technologies of processing SiC, including laser processing,

and to study the accompanying physical processes.

Present opportunities to control the intensity and durations

of exposure of localized LI and to select the degree of

absorption in a material, allow to consider LI as a prospective

* Corresponding author. Tel.: +380 44 5256477; fax: +380 44 5258342.

E-mail address: lfedor@isp.kiev.ua (L. Fedorenko).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.08.048

tool for modification of SiC. A number of studies on exposure

of silicon carbide to LI are available: laser annealing of

implanted layers in SiC [1], laser stimulated recrystallization of

amorphous a-SiC in crystalline c-SiC [2], formation of ohmic

contacts [3,4], and laser ablation [5]. The last has been used to

deposit thin SiC films [6] and to stimulate etching [7].

In a solid at the ablation process the focused laser beam

usually forms a cone-shaped crater the typical depth of which

is several tens of microns [8]. In our previous studies the

thermal gradient effect (TGE) has been found to cause

morphological and structural transformations in semicon-

ductors such as InSb [9], Si [10], CdTe [11] at LI intensities

in the range of fundamental absorption (hn > Eg for the given

material) insufficient to start the ablation process. The effect

at LI is manifested as transfer of dopants and transformation

of intrinsic defects without a phase transition in the material.

Theoretical considerations of the TGE [12] show that

direction of displacements of atoms is defined by the

covalent radius of the dopant and its local environment. The

L. Fedorenko et al. / Applied Surface Science 254 (2008) 2031–20362032

processes mentioned above proceed in the solid phase and are

predominant in ablation. When laser energy density is

increased a dynamic phase transition starts leading to

microablation of the material and morphological changes

of the surface.

The present study was aimed at examining structural and

morphological changes in the near-surface layer and on the

surface of 6H–SiC before and at initial stages of microablation

in the range of laser intensities insufficient to cause visually

observable changes on the surface.

2. Experiment

The experimental set-up for modification of SiC consisted of

a single-mode nitrogen N2-laser (l = 0.34 mm, tp = 7 ns,

E = 30 mJ), a multi-mode nitrogen N2-laser with transversal

discharge (l = 0.34 mm, tp = 5 ns, E = 1.5 mJ), a YAG-

laser (l = 1.064 mm, tp = 10 ns, E = 50 mJ, l = 0.532 mm,

tp = 10 ns, E = 5 mJ), a system of automatic scanning of the

focused beam and a system of visualization of the active region

on the basis of a CCD camera. Exposure of surface to pulsed LI

depending on composition of the surrounding medium and

orientation of the crystal was studied. 6H–SiC(N), nitrogen

concentration nN ffi 2 � 1018 cm�3 samples in the form of

plates with orientation of surfaces 0 0 0 1, 0 0 0 1̄ and the size

0.5 cm � 0.5 cm � 0.04 cm grown by modified Lely techni-

ques were examined.

Structural changes in the near-surface layers and morphology

on surface SiC were examined by a Nanoscope IIIa series

Dimension 3000, digital instruments atomic force microscope

(AFM). Photoluminescence (PL) spectra were measured at

excitation by wavelength of 337.4 nm of a nitrogen laser in the

range of temperatures 5–300 K. Raman spectra at reflection were

excited by at wavelength of 487.9 nm of an Ar+-laser at a room

temperature, pressure 1 atm and 60% humidity and registered by

a diffraction spectrometer equipped with cooled photoelectron

multiplier in the photon counting mode. Frequencies of known

emission bands of Ar+-laser plasma were used to determine more

exactly the Raman band frequencies.

Field emission properties of the modified surface were

studied from current–voltage characteristics (CVC) in vacuum

chamber under stable pressure of 10�6 Torr.

3. Numerical simulation of 6H–SiC ablation

The process of 6H–SiC ablation by nanosecond pulsed

radiation of N2-laser [13] was calculated numerically within the

limits of the thermal model [14,15] assuming a sharp interface

between the condensed and vapor phases under conditions of

surface evaporation. The laser–SiC interaction is considered

with account for the liquid–solid phase transition. Absorption

of laser radiation by vapor target reducing energy density of

light reaching the SiC surface is neglected. We also confine our

consideration to a one-dimensional model. In the framework of

the thermal model the laser–target interaction including

melting–solidification, vaporization, and energy transport can

be described by the temperature field T(x,t) obeying the heat

diffusion equation:

½cþ LmdðT � TmÞ��

@T

@t� n

@T

@x

�

¼ @

@x

�K

@T

@x

�þ aAIðtÞexp

� Z x

0

aðTÞ dx0�: (1)

Here Lm is the latent heat of melting (per unit volume), Tm the

melting temperature, I(t) the laser pulse power density, a and A

the optical absorption coefficient and the absorptivity of the

target material at the laser wavelength used, respectively; c the

heat capacity (per unit volume), and K is the thermal con-

ductivity. The term with d-function, in Eq. (1) insures continuity

of the heat flow with regard to release or absorption of the latent

heat at the liquid–solid boundary. Dependence of optical and

thermal parameters of SiC and their changes at the melting

point temperature are taken into account.

Eq. (1) is given in a coordinate system fixed to the target

surface moving with the velocity v of surface recession (the

surface is always at x = 0, x-axis is directed inward the target).

According to the Clausius–Clapeyron equation recession

velocity v depends on the surface temperature T and can be

written in a simplified way as

n ¼ n0exp

�� Ln

RT s

�(2)

where Ln is the molar latent heat of vaporization, Ts the surface

temperature, R the universal gas constant and v0 is the pre-

exponential factor depending on the target material. Initial and

boundary conditions are taken as

Tðx!1; tÞ ¼ Tðx; t ¼ 0Þ ¼ T0; K@T

@x

����x¼0

¼ rLvv

M(3)

where T0 is the initial temperature of the sample, r the density

and M is the molar mass.

Eqs. (1)–(3) were solved numerically by the finite-difference

method. The spatial and time steps were 20 nm and 100 ps,

respectively. The thermal and optical properties of SiC have not

been adequately explored yet over a wide range of

temperatures. The following values:

Tm ¼ 3100 K; Lm ¼ 5000 J=cm3; Ln ¼ 4� 105 J=mol;

n0 ¼ 108 cm=s; C ¼ 3 J=cm3 K; r ¼ 3:2 g=cm3;

M ¼ 40:0 g=mol

of SiC parameters are used here [16,17].

Thermal conductivity of solid SiC is approximated as

K(T) = 1160/T (W/cm K). Optical parameter values of the solid

phase (T � Tm) are chosen as a = 4 � 103 cm�1 and A = 0.7

[18].

Above the melting temperature SiC is a solution of carbon

in liquid silicon. Therefore, in the temperature range T � Tm

parameter values approaching those of liquid silicon are

used: a = 106 cm�1, A = 0.3, K = 0.6 W/cm K. The temporal

profile of the laser pulse was assumed to have the form

I(t) = (W/tp)sin2(pt/2tp) for 0 � t � 2tp and I(t) = 0, where W

Fig. 1. The spatial-temporal evolution of temperature in SiC, calculated for the

case of 10 ns N2-laser pulse of energy density 10 J/cm2.

Fig. 2. AFM images of 6H–SiC(N): irradiated by N2-laser in single-mode

operation (a), in multi-mode operation (b), and friction mode of AFM image (c)

(arrows indicate the area irradiated in multi-mode operation).

L. Fedorenko et al. / Applied Surface Science 254 (2008) 2031–2036 2033

is the pulse energy density (J/cm2), tp is the laser pulse

duration.

The calculated temperature profile of SiC under laser

irradiation of the intensity of 1.0 GW/cm2 is shown in Fig. 1.

The solid line is the isotherm corresponding to T = Tm and

defines the melting front. According to the calculations, the local

temperature of the sample reaches its maximum value of 5000 K

towards the end of the pulse when the melting front reaches its

maximum depth of 1.3 mm. Velocity of the melting front essen-

tially decreases as the surface temperature reaches its maximum

value. It can be explained as a result of heat being taken away due

to the intensive surface evaporation. For the same reason the

maximum temperature is reached under the surface, which

makes possible to form the liquid phase in the bulk of SiC by

focusing the laser beam far beneath the surface. After completion

of the laser pulse the melted layer is rapidly cooled down to the

temperature Tm and, as a result, the crystallization front shaped

by the isotherm of the liquid phase plateau (Fig. 1) is formed.

After that proceeds solidification and the liquid–solid front

moves toward the surface at a rate of about 3.5 m/s. Our numeri-

cal simulations show, that, due to a high rate of evaporation and

subsequent removal of the latent heat of vaporization at the

melting temperature, the surface of the SiC sample is cooled off

below the melting point. For that reason the liquid phase at the

surface becomes overcooled and solidification begins the front of

crystallization moving from the surface inward at the rate of

1.2 m/s. This value is in agreement with the relation

L0mvc ¼ Lvvoexp

�� Lv

RTm

�

(vc is the velocity of the front of crystallization and L0m is the

molar latent heat of crystallization) resulting from the balance

between latent heats of crystallization and evaporation at the

liquid–solid and the solid–vapor phase boundaries. Thus, the

front of crystallization moves to the melted layer from two

directions—the surface and the bulk towards the maximum

temperature.

4. Results and discussion

Exposure of the surface of 6H–SiC to N2-laser pulses

(l = 0.337 mm, tp = 7 ns) of below-threshold of energy density

W < hWthdi, where hWthdi is an average value of a damage

threshold of a material, promotes formation of nanocrystal

structures. Two thresholds of energy density on the SiC surface

were established in our experiment: (1) hWthdi is average value

of energy density of a threshold of appearance of the observable

damages with the naked eye; (2) hWthni is threshold of

nanostructure formation, that is detected only by AFM analysis.

AFM analysis of the spatial distribution and the size of

Fig. 3. The PL spectra of 6H–SiC(N) crystal before (line), and after LI in

single-mode operation at different temperature: at energy density W below

(hWthni �W � hWthdi) (hollow circles) and above (W � hWthdi) the ablation

threshold (daggers).

L. Fedorenko et al. / Applied Surface Science 254 (2008) 2031–20362034

nanocrystals after exposure to pulsed N2-laser radiation [19]

had shown certain regularity of the location with respect to the

centre of the laser spot in case of single-mode generation

(Fig. 2a) at energy density WhWthni � W � hWthdi, and a

random nature in the case of multi-mode generation (Fig. 2b) at

laser energy density W close to the damages threshold

W � hWthdi, when consequences of the ablation could not be

seen by eyes yet. In the last case random size and location of

nanocrystals result from the interference of laser modes instable

at transversal pumping discharge of the laser.

At W � hWthni (i.e., before, or close to formation of

nanocrystals) the AFM images in the ‘‘friction mode’’ show

changes of the phase composition at the peripheral parts of

irradiated region in surface, see arrows close to nanocrystals

(Fig. 2c). The energy density threshold hWthni of nanocrystal

formation was determined by hWthni = 3.5 J/cm2 and 3.0 J/cm2

for 0 0 0 1 and 0 0 0 1̄ surface, correspondingly, the values of

hWthdiwere 5.6 J/cm2 and 5.0 J/cm2, correspondingly at single-

mode N2-laser operation. Obviously, the nanocrystallites

formation at the 6H–SiC(N) surface becomes possible in the

range of hWthni � W < hWthdi due to shift of the maximum

temperature from the surface to the bulk according to our

theoretical simulation. Thus, the conditions of a local pressure

increasing could be realized that is sufficient for solid–liquid

phase transition, impossible for SiC in open air. It was detected

no difference between the threshold levels hWthni and hWthdiand either the environment (vacuum, air, inertial gas) of the

irradiated surface with the N2-lasers.

The AFM images also show the absence of nanocrystal

formations on the surface of 6H–SiC(N) crystals after exposure

of the first (l = 1.06 mm) and the second (l = 0.53 mm)

harmonics of YAG laser radiation regardless to energy density.

The threshold of energy density of YAG-LI hWthdi with regard

to visually observable changes of the 6H–SiC(N) surface have

been determined as hWthdi = 60 J/cm2 (l = 1.06 mm) and

hWthdi = 25 J/cm2 (l = 0.53 mm). We detected no difference

between the threshold levels hWthdi and either the environment

(vacuum, air, inertial gas) or orientation of the irradiated

surface with the YAG-laser (l = 1.06 mm, l = 0.53 mm).

Given below PL and Raman spectra data were obtained in

case of single-mode operation.

On PL spectra (Fig. 3) one may see some broadening of the

hnmax = 2.64 eV PL band at 80 K and a significant increase of

the hnmax = 2.75 eV and hnmax = 2.92 eV PL band intensities in

case of hWthni < W � hWthdi at T = 5 K, 80 K, 300 K. At levels

W > hWthdi the narrowing of the hnmax = 2.64 eV PL band and

decreasing of the hnmax = 2.75 eV PL band intensity is evident

while the intensity of the hnmax = 2.92 eV PL band, in

distinction from the case of W � hWthdi, does not change. In

the first case increasing of the hnmax = 2.75 eV band intensity

can be caused by growth of N concentration in the near-surface

layers due to the drift under TGE conditions [2]. Direction of

the drift of nitrogen atoms N, according to theoretical

considerations [12,20], is turned against of the temperature T

gradient, from hot to cold layers of the lattice, because in our

case the size (the covalent radius) of the dopant atom N is

smaller than the effective size of the atoms of the host material

SiC, i.e., from maximum temperature Tmax to the surface, and to

the bulk. Redistribution is also suggested by the sharp

boundaries of the irradiated areas of the surface (marked by

arrows on the AFM ‘‘friction mode’’ image in Fig. 2c). The

distance of mass transfer calculated within the model shows

that the depth of redistribution of the dopants is �1 mm. The

Fig. 5. Current–voltage characteristic (left) and the corresponding Fowler–

Nordheim plot (right) for the case of LI in multi-mode operation W � hWthdi.

L. Fedorenko et al. / Applied Surface Science 254 (2008) 2031–2036 2035

absorption depth of the laser radiation (l = 0.34 mm) exciting

photoluminescence is of the order of 2.5 mm. So it is possible to

conclude, that a principal cause of laser stimulated changes of

the n = 2.75 eV PL band could be the growth of N concentration

under TGE conditions and substitution of carbon atoms C in a

thin near-surface layer of 6H–SiC. The broadening of the

hnmax = 2.64 eV band is possibly related to deformation of the

lattice bands in a thin near-surface layer of 6H–SiC(N) due to

exposure of laser radiation, broadening of the phonon spectrum

and the spectrum of luminescent transitions, accordingly.

Appearance of the hnmax = 2.92 eV PL band could possibly be

connected with weakening of restrictions for indirect PL band–

band transitions; however, finding out the reason it retains at

W > hWthdi requires further examination.

In the case of higher energy density (W > hWthdi) the

decrease of the hnmax = 2.75 eV band intensity and narrowing

of the hnmax = 2.64 eV band are caused by the increasing of

concentration of lattice defects due to laser ablation with

corresponding rise of a competitive nonradiative recombination

channel. PL spectra obtained for 0 0 0 1 and 0 0 0 1̄ surface

were similar.

Raman spectra (Fig. 4) also suggests appreciable structural

changes in the near-surface layer 6H–SiC(N) at exposure to LI.

The first-order Raman spectra of the 6H polytype used for the

studies in the geometry of scattering back only the modes with

symmetries A1 and E2 [21] are active. At the centre of the

Brilluoin zone the TO mode has symmetry E1 and is forbidden

in Raman scattering at the given geometry. However, due to a

deviation from the exact 1808 geometry at measuring the

spectrum a weak band at the frequency of 799.0 cm�1 is present

(Fig. 4). Apart from it the spectrum contains two bands at

frequencies 791.0 cm�1 and 769.8 cm�1 corresponding to

modes with symmetry E2. The nanocrystals formed on the

surface of SiC after laser processing may have a different

structure and other polytype of SiC. The contribution to Raman

scattering from the near-surface layers is by some orders greater

than contribution from the nanocrystals due to the rather

considerable depth of absorption of laser radiation 1/

Fig. 4. Raman spectra of 6H–SiC(N): before (line) and after LI in single-mode

operation: (hWthni � W � hWthdi) (hollow circles), W � hWthdi (daggers).

a = 4 � 10�3 cm compared to average thickness j of the

nanocrystallite layer, j � 200 nm. Therefore, it seems impos-

sible to extract the signals from the latter. However, the

frequencies of the corresponding Raman bands from samples

with nanocrystals are by 1.5 cm�1 lower. This effect may

suggest that the near-surface layer of SiC is slightly stretched

compared to the rest of the sample. It has to be noticed that

Raman bands are shifted only from samples irradiated with

intensities above the threshold of nanocrystal formation that

allow to propose that a tension rises due to the presence of

nanocrystals on the surface. Raman spectra were measured only

for 0 0 0 1 surface.

The quantum-dimensional nature of nanocrystal formations

were revealed on CVC and corresponding Fowler–Nordheim

(F–N) plots in studies of field emission from surface areas of

6H–SiC(N) irradiated at W < hWthdi. The field emission data

were obtained in the case of multi-mode N2-laser operation.

No field emission was found from original samples. The field

emission appeared from irradiated samples at threshold voltage

of 1000 V at currents from 0.7 mA to 0.7 mA. The current–

voltage characteristic and the corresponding F–N plot of electron

field emission from SiC nano-tips are presented in Fig. 5.

The curve follows F–N tunneling through the triangle barrier

of a wide band gap semiconductor-vacuum interface [22]. A

field enhancement factor b 10 and effective area of field

emission a 10�11 cm2 are found from the F–N plot.

However, the average electronic field (�105 V cm�1) is not

enough for effective electron field emission from 6H–SiC, i.e.,

the work function f 4.5 eV [23]. Therefore, an additional

factor corresponding to the electric field enhancement is

the nano-tip curvature radius. One can see (Fig. 1) that the

curvature radius of a single tip is about 3 nm, therefore, the

average additional field enhancement factor b* 6. As

determined from the F–N plot, the effective work function is

fef 4.65 eV.

5. Conclusion

The nanostructured formations on the surface of 6H–SiC(N)

at irradiation by N2-laser (l = 0.337 mm) are obtained.

L. Fedorenko et al. / Applied Surface Science 254 (2008) 2031–20362036

The accumulative character of the nanostructure formation

is ascertained.

The values of threshold energy density hWthni at which

formation of nanostructures starts on the 0 0 0 1 and 0 0 0 1̄

surfaces of n-type 6H–SiC(N) are determined to be 3.5 J/cm2

and 3.0 J/cm2, respectively. The values of threshold energy

density hWthdi of visually observed damages were 5.6 J/cm2

and 5.0 J/cm2, accordingly.

Analysis of AFM data, PL spectra, and Raman spectra

shows:

T

he process of the nanocrystal formation on 6H–SiC(N)

surfaces exposed to N2-laser radiation (l = 0.337 mm) is

accompanied by formation of the liquid phase and the solid–

liquid phase transition.

T

he features of nanostructure formation and PL spectra of

the 0 0 0 1 and 0 0 0 1̄ surfaces of n-type 6H–SiC(N) are

similar.

T

he absence of appreciable influence of oxygen on the

processes of nanostructure formation on silicon carbide SiC

surface under nitrogen laser exposure.

The results obtained suggest of a defining role of the thermal

gradient mechanism of mass transfer in the initial stage of the

nano-hill formation process on SiC surfaces being important for

development of laser nanotechnologies for SiC.

Examination of field emission CVC from SiC surfaces after

exposure laser radiation has shown the tunneling nature of the

emission current and the increase of emission efficiency from

nanostructured SiC surfaces compared with a non-irradiated

surface.

Nevertheless, the degree of the nanostructure change, the

chemical composition of nanocrystals and surrounding layers,

the character of dopant redistribution, and reconstruction of

defects still remain unclear and require further examination.

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