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Diffusion and impurity segregation in hydrogen-implanted silicon carbide A. Barcz, M. Kozubal, R. Jakieła, J. Ratajczak, J. Dyczewski, K. Gołaszewska, T. Wojciechowski, and G. K. Celler Citation: Journal of Applied Physics 115, 223710 (2014); doi: 10.1063/1.4882996 View online: http://dx.doi.org/10.1063/1.4882996 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Splitting kinetics of Si 0.8 Ge 0.2 layers implanted with H or sequentially with He and H J. Appl. Phys. 104, 113526 (2008); 10.1063/1.3033555 Role of strain in the blistering of hydrogen-implanted silicon Appl. Phys. Lett. 89, 101901 (2006); 10.1063/1.2345245 Investigation of the cut location in hydrogen implantation induced silicon surface layer exfoliation J. Appl. Phys. 89, 5980 (2001); 10.1063/1.1353561 Copper gettering at half the projected ion range induced by low-energy channeling He implantation into silicon Appl. Phys. Lett. 77, 972 (2000); 10.1063/1.1289062 Defect formation and annealing behavior of InP implanted by low-energy 15 N ions J. Appl. Phys. 83, 738 (1998); 10.1063/1.366746 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 209.183.185.254 On: Wed, 26 Nov 2014 21:51:34

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Page 1: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

Diffusion and impurity segregation in hydrogen-implanted silicon carbideA. Barcz, M. Kozubal, R. Jakieła, J. Ratajczak, J. Dyczewski, K. Gołaszewska, T. Wojciechowski, and G. K.Celler Citation: Journal of Applied Physics 115, 223710 (2014); doi: 10.1063/1.4882996 View online: http://dx.doi.org/10.1063/1.4882996 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Splitting kinetics of Si 0.8 Ge 0.2 layers implanted with H or sequentially with He and H J. Appl. Phys. 104, 113526 (2008); 10.1063/1.3033555 Role of strain in the blistering of hydrogen-implanted silicon Appl. Phys. Lett. 89, 101901 (2006); 10.1063/1.2345245 Investigation of the cut location in hydrogen implantation induced silicon surface layer exfoliation J. Appl. Phys. 89, 5980 (2001); 10.1063/1.1353561 Copper gettering at half the projected ion range induced by low-energy channeling He implantation into silicon Appl. Phys. Lett. 77, 972 (2000); 10.1063/1.1289062 Defect formation and annealing behavior of InP implanted by low-energy 15 N ions J. Appl. Phys. 83, 738 (1998); 10.1063/1.366746

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Page 2: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

Diffusion and impurity segregation in hydrogen-implanted silicon carbide

A. Barcz,1,2,a) M. Kozubal,1 R. Jakieła,2 J. Ratajczak,1 J. Dyczewski,2 K. Gołaszewska,1

T. Wojciechowski,2 and G. K. Celler3

1Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland2Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland3Institute for Advanced Materials, Devices, and Nanotechnology (IAMDN)/Department of Materials Scienceand Engineering, Rutgers University, New Brunswick, New Jersey 08901, USA

(Received 28 March 2014; accepted 30 May 2014; published online 11 June 2014)

Diffusion and segregation behavior of hydrogen and oxygen in silicon carbide subjected to H

implantation and subsequent annealing were studied with a number of analytical techniques

including Secondary Ion Mass Spectrometry (SIMS), Rutherford backscattering spectrometry in

channeling geometry, field emission scanning electron microscopy, optical microscopy,

cross-sectional transmission electron microscopy, and atomic force microscopy. Hþ implantation

was carried out with energies of 200 keV, 500 keV, or 1 MeV to doses of 1� 1016, 1� 1017, or

2� 1017 ion/cm2, and thermal treatment was conducted in flowing argon for 1 to 2 h at temperatures

of 740, 780, 1000, or 1100 �C. The process of migration and eventual loss of hydrogen in a point

defect regime is postulated to proceed to a large extent through ionized vacancies. This conclusion

was derived from the observed substantial difference in H mobilities in n- vs. p-type SiC as the

population of ionized vacancies is governed by the Fermi-Dirac statistics, i.e., the position of the

Fermi level. For higher doses, a well defined buried planar zone forms in SiC at the maximum of

deposited energy, comprising numerous microvoids and platelets that are trapping sites for

hydrogen atoms. At a certain temperature, a more or less complete exfoliation of the implanted

layer is observed. For a 1 MeV implant heated to 1100 �C in nominally pure argon, SIMS profiling

reveals a considerable oxygen peak of 1016 O atoms/cm2 situated at a depth close to that of the peak

of the implanted Hþ. Similarly, 1100 �C annealing of a 200 keV implant induces the formation of a

thin oxide (4 nm), located at the interface between the implanted layer and the substrate as

evidenced by both SIMS and HRTEM. The measurements were taken on the part of the sample that

remained un-exfoliated. In view of a lack of convincing evidence that a hexagonal SiC might

contain substantial amounts of oxygen, further investigation is under way to elucidate its presence in

the irradiation-damaged films. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4882996]

I. INTRODUCTION

Silicon carbide is an excellent material for high tempera-

ture, high power, and high breakdown voltage electronic devi-

ces. Due to the wide bandgap energy ranging from 3 eV for

6H polytype to 3.2 eV for 4H-SiC, typical leakage currents

are orders of magnitude lower than in silicon, with the temper-

ature at which hexagonal SiC becomes intrinsic exceeding

�800 �C. Such properties are critical for power switching

devices as the specific on-resistance scales inversely with the

cube of the breakdown electric field.1 In addition, SiC is the

only compound semiconductor which can be thermally oxi-

dized to form a uniform and planar SiO2 oxide.2 This enables

fabrication of metal-oxide-semiconductor field effect transis-

tors (MOSFET), insulated gate bipolar transistors (IGBT),

and MOS-controlled thyristors (MCTs). Excellent thermal sta-

bility promises long-term reliable operation at high tempera-

tures, but it also presents problems in certain fabrication steps,

such as selective doping, where impurities must be introduced

by ion implantation due to exceedingly low diffusion coeffi-

cients of common dopants at reasonable temperatures.

Application of high temperature annealing (>1600 �C) often

leads to severe degradation of surface morphology as well as

incorporation of unwanted contaminants.3,4

Hydrogen, an otherwise ubiquitous gaseous element,

plays a special role in processing of SiC–based devices. In

the chemical vapor deposition techniques, hydrogen occurs

in nearly all reactants, from CH4/H2 and C3H8/H2 precursors,

NH3 or B2H6/H2 dopant sources, to HCl/H2—an in situ sub-

strate etchant.5 Moreover, addition of H2 to the plasma

greatly improves the uniformity of reactive ion etching.6

Finally, hydrogen passivation of Al and B acceptors in SiC

as well as its susceptibility to trapping on defects in an analo-

gous manner as it is observed in (hydrogenated) silicon are

well documented in the literature.7–11

Most of the published research has concentrated on the

trap-driven incorporation of hydrogen in both 6H and 4H

SiC polytypes, and there is only one paper, published in

1978, which addresses classical diffusivity and solubility of

hydrogen in silicon carbide.12 The authors have employed ei-

ther the 3H isotope–tritium produced in a nuclear reactor or2H deuterium to trace their in- and out-diffusion kinetics in

different powdered SiC materials. They found hydrogen sol-

ubility relatively high, �10�2 per Si atom at 1000 �C, and,

more interestingly, its value actually decreasing when the

temperature goes up.a)[email protected]

0021-8979/2014/115(22)/223710/9/$30.00 VC 2014 AIP Publishing LLC115, 223710-1

JOURNAL OF APPLIED PHYSICS 115, 223710 (2014)

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Page 3: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

Low-mass projectiles traveling through matter exhibit

narrow concentration depth distribution peaked at the end of

their path; the same applies for the corresponding damage

profile. This feature has been successfully employed to pro-

duce a highly damaged zone at a well defined depth, in

which the crystalline lattice is very fragile. When additional

energy is provided by heating, this zone can fracture, gener-

ating surface blisters and flakes of material detached from

the bulk crystal, or under well engineered conditions an

exfoliation and transfer of a crystalline layer may occur.

The process of exfoliation and layer transfer that is

assisted by high dose Hþ implantation has become a dominant

method of forming silicon on insulator (SOI). Known as the

Smart CutTM technology, the method depends on implanting

Hþ into a “donor” wafer that is coated with a film of thermal

SiO2, fusion bonding of such a wafer to another oxidized

“handle” wafer, and finally a heat treatment that leads to split-

ting of the wafer pair along the plane of maximum implanta-

tion damage.13 The final product, after some surface

smoothing and thermal annealing is a thin layer of single crys-

talline Si separated by a layer of amorphous SiO2 from a bulk

support wafer. SOI wafers are in widespread use for high per-

formance microprocessors (Si films 5–100 nm thick), and for

power and high voltage devices (Si films 0.3–2 lm thick).14

The process of Hþ induced blistering and/or exfoliation

requires a sufficiently high dose of implanted hydrogen—

typically of the order of 5� 1016 to 1017 cm�2, and it is

observed not only in Si but also in many other crystalline

materials, including Ge, SiC, GaN, GaAs, InP, and Al2O3

(sapphire).

The first aim of this paper is to investigate the behavior

of hydrogen implanted into 4H-SiC. The data show that the

background doping of SiC with n or p type impurities has a

significant impact on redistribution of implanted H during

thermal anneals. The second aim is to describe the presence

of oxygen in the SiC crystalline lattice and its gettering by

Hþ implantation induced damage.

II. EXPERIMENTAL

The material under investigation was a research grade n-

type 4H-SiC from Cree, cut 4� off-axis, with resistivity in the

range of 0.015–0.028 X�cm. Additionally, n (N doping) or p

(Al doping) type SiC layers were grown on two of the sam-

ples. This process was performed using metal-organic chemi-

cal vapor deposition (MOCVD) technique at the Institute of

Electronic Materials Technology (ITME) with ammonia

(NH3) or trimethylaluminum (TMA) precursors. Optimum

growth conditions for high quality, high electrical activity of

Al-doped film were substrate temperature 1620 �C, overall

pressure 75 millibars, and flow rates equal to 20, 12, and

80 ml/min for silane, propane, and H2, respectively.

Hþ implantations were conducted at room temperature

(RT) using a NEC 3SDH-2 Pelletron tandem accelerator.

Samples were tilted by �7� relative to the ion beam axis to

prevent ion channeling. Irradiation was carried out with ener-

gies of 200 keV, 500 keV, or 1 MeV and fluences of 1� 1016,

1� 1017, or 2� 1017 ion/cm2. Post-implantation thermal

annealing was performed in a quartz furnace in flowing argon

ambient for 1 to 2 h at temperatures of 740, 780, 1000, or

1100 �C.

Since most of the data in this work rely on the credibility

of Secondary Ion Mass Spectrometry (SIMS) measurements,

special attention was paid to the accuracy and reproducibility

of the analyses. H and O concentration profiles were meas-

ured with a SIMS Cameca IMS 6F microanalyser using a

15 keV Csþ mass-filtered primary ions of intensity �300 nA.

The beam was focused into a spot �30 lm in diameter and

rastered over a nominal area of 100 lm� 100 lm. The

selected probing field was 8 lm in diameter in the center of

the crater. Since the detectability of “ambient” species is lim-

ited mostly by the rate of their adsorption on the sample sur-

face, care was taken to minimize residual pressure. Prior to

analysis, the samples were baked in situ for 2 h at

100–150 �C, titanium sublimation pump was activated for 2

min and the cryo-shield surrounding the sample holder was

filled with liquid nitrogen. Under these conditions, the pres-

sure in the sample chamber amounted to 1� 1.5� 10�10 Torr

with the beam on. The background signal was found to

decrease monotonically during bombardment, probably due

to the removal by backscattered Cs ions of O and

H-containing species adsorbed on the first immersion lens. As

a reference, in silicon the minimum background value of

5� 8� 1015 O/cm3 is routinely reached in our system—a

value unattainable in other SIMS laboratories. This is mainly

because the analysis chamber of our instrument is equipped

with an ion pump instead of a turbomolecular one used in a

majority of spectrometers of this type.

Rutherford backscattering spectrometry (RBS) in chan-

neling geometry was performed using aforementioned 3SDH-

2 Pelletron tandem accelerator with 2.5 MeV Heþþ and a 170�

scattering angle. The surface morphology of the structures was

analysed by field emission scanning electron microscopy

(FE-SEM) using Auriga—Zeiss instrument, an optical micro-

scope with the Nomarski contrast and an Innova Veeco atomic

force microscopy (AFM) system with amplitude and phase

modulation. Cross-sectional transmission electron microscopy

(XTEM) specimens were prepared using focused ion beam

(FIB) and were investigated in the JEM-2100 transmission

electron microscope operating at 200 kV.

III. RESULTS

For the sake of clarity, this section has been divided

according to the applied dose and/or the energy of the

implanted hydrogen ions.

A. Low dose–low energy

SIMS depth profiles of hydrogen implanted at an energy

of 200 keV with a fluence of 1016/cm2 are shown in Fig. 1

for p-type and n-type SiC samples subsequently annealed in

argon ambient at 1000 and 1100 �C. The Al-doped p-type

layer �3 lm in thickness was epitaxially grown on a Cree

n-type, Si-face 4H-SiC substrate using TMA as a precursor;

the resultant Al concentration amounted to 7 � 1018/cm3. As

for the n-type material, it is the substrate itself that was

exploited because the intended nitrogen doping level in the

MOCVD film turned to be an order of magnitude lower than

223710-2 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 4: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

that in the substrate wafer. Thus, within the adopted notation,

SiC is synonymous with n-SiC throughout this paper.

For the lower annealing temperature of 1000 �C, the

measured H profile in p-SiC extends to the surface while that

in the n-type semiconductor exhibits a well defined cut-off

front at a depth of �0.5 lm. After the 1100 �C anneal, the

effect appears much more pronounced; here the peak concen-

trations as well as the corresponding total hydrogen losses dif-

fer by a factor of 40. In order to verify such a strong

dependence of hydrogen mobility on the type of doping, simi-

lar measurements were conducted on MOCVD specimens

fabricated by a different Lab, with basically the same out-

come. Also, on one occasion, a semi-insulating material (of

unknown origin) was characterized, showing a similar behav-

ior to the n-doped semiconductor.

The lowest measurable H concentration of about

1018/cm3 may be regarded as a background level (bkg) for

these particular analytical conditions. This could have been

significantly improved by applying smaller crater area/higher

primary beam flux thereby increasing the removal rate of the

material over the rate of adsorption of the H-containing mole-

cules on the investigated surface. Such procedure would, how-

ever, inevitably degrade the depth resolution, i.e., the ability

to distinguish regions with rapidly changing concentrations.

B. High dose–low energy

SIMS profiles are influenced by the fact that the fluence

of 1017 cm�2 at 200 keV exceeds the threshold for exfolia-

tion. The structural changes in the implanted samples are

well illustrated in the Nomarski optical interference contrast

micrographs shown in Fig. 2. The sample surface heated to

740 �C remains featureless, but after 780 �C thermal anneal

small blisters appear on the surface. When even higher tem-

perature of 1100 �C is used, the blisters burst and thin flakes,

whose thickness corresponds approximately to the projected

range Rp of the implanted Hþ are detached from the surface.

If exfoliation is incomplete, some flakes are still partially

attached to the bulk crystal. Alternatively some flakes are

completely removed, but still sit on the surface. SEM micro-

graphs of Fig. 3(a) show changes in contrast in the sample

annealed at 780 �C, which are likely caused by the subsur-

face blisters; some of them cause rounded cracks visible on

the surface. Fig. 3(b) shows the edge of a flake that remains

attached to the substrate after heating to 1100 �C and a

freshly exposed “new” surface.

Rutherford backscattering and ion channeling data in

Fig. 4 confirm the microstructural changes induced by ther-

mal annealing of H-implanted material. A peak in the chan-

neled spectrum of as-implanted crystal at about 550 keV

represents the lattice damage near Rp of 200 keV H ions.

FIG. 1. Hydrogen SIMS profiles in p type and n type SiC H-implanted at

200 keV to a dose of 1 � 1016/cm2 and annealed at 1000 �C and 1100 �C for

1 h in argon.

FIG. 2. Nomarski optical images of

the SiC surface after 200 keV H-

implantation to 1 � 1017/cm2, annealed

in argon at (a) 740 �C for 1 h; (b) 780 �Cfor 2 h; and (c) 1100 �C for 1 h.

223710-3 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 5: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

Thermal annealing at 740 �C is insufficient to form blisters,

but many point defects closer to the surface are eliminated

by such a thermal treatment; therefore, the channeling yield

between the surface and the main damage peak is reduced.

After 780 �C anneal, blisters form, as already seen in the op-

tical micrograph of Fig. 2. These blisters strain and distort

the crystal lattice, and this is reflected in the channeling spec-

trum labeled “bubbles” which is shifted up for all energies.

Finally, the spectrum after 1100 �C anneal, marked “flakes”

is an average of exfoliated and unexfoliated regions, thus the

amount of damage within the probing He beam area is

reduced.

Defects characteristic of high dose Hþ implantation

form even before the heat treatment. These are hydrogen

induced platelets, often known as HIPs (disk-shaped voids of

nanometer dimensions filled with H atoms bonded to Si

walls of the voids). During the subsequent heat treatment

these HIPs grow and coalesce with neighboring microvoids

by Ostwald ripening into larger blisters that eventually burst

producing free-standing flakes of crystalline material.14 The

diffusion data obtained after 740 and 780 �C are likely influ-

enced by the internal microstructure of HIPs and blisters.

These data should be qualitatively different from diffusion

profiles obtained for lower fluences, at which voids do not

form at any temperature. Fickian out-diffusion is reduced or

entirely prevented by trapping of H in the microvoids or

HIPs. However, a fraction of hydrogen atoms enters the

underlying semiconductor due to “classical” equilibrium dif-

fusion, where the Y-axis value of the inflexion points visible

on the right side of the main H peak represents the solid solu-

bility of hydrogen in an unperturbed 4H-SiC crystal (Fig. 5).

Fitting these “tails” to adequate erfc functions makes it, in

principle, possible to extract the associated diffusion coeffi-

cients D.

XTEM photograph of sample irradiated at lower dose of

1016 cm�2 and heated to 1100 �C appears virtually feature-

less and suggests that no extended defects form (Fig. 6(a)).

This is in accordance with the corresponding RBS channeled

spectra showing little or no damage in as-implanted and

1100 �C annealed sample, respectively (not shown). At 10

� higher dose, the initial damaged zone visible as a pair of

dark blurred lines (Fig. 6(b)) transforms, upon heating to

780 �C, into a planar layer with well-defined boundaries

(Fig. 6(c)). Fig. 6(d) shows a thin, ribbon-like oxide film

beneath the granular zone. Inspection of other parts of the

FIG. 3. FE-SEM images of SiC

implanted with 200 keV Hþ ions with

fluence of 1� 1017 ion/cm2 and subse-

quently annealed at (a) 780 �C for 2 h

or at (b) 1100 �C for 1 h in argon

(taken at an angle of 45�).

FIG. 4. Rutherford backscattering spectrometry in channeling geometry

(RBS/C) spectra of 200 keV H-implanted SiC subsequently annealed in ar-

gon in various temperatures for 1 h, except for the sample annealed at

780 �C for 2 h.

FIG. 5. SIMS depth profiles of H and O in 200 keV 1� 1017 ion/cm2

H-implanted SiC and annealed at 740 �C or 780 �C for 1 h or 2 h,

respectively.

223710-4 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 6: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

specimen proves that this oxide is not continuous; in fact

there is no trace of oxygen in the SIMS profile (Fig. 5). The

lamella from the sample annealed at 1100 �C was cut away

at an area that remained un-exfoliated. The perturbed, porous

layer resembles that from the previous picture while the ox-

ide appears thicker and continuous (Fig. 6(e)). HRTEM

imaging in Fig. 6(f) shows distortions of atomic planes in the

vicinity of dark spots related, presumably, to voids or other

extended defects.

C. High dose–high energy

Implantations of Hþ at 500 keV and 1 MeV provide us

with an interesting comparison. 1 MeV sample was implanted

with 1017 cm�2 and this is below the threshold for void crea-

tion and eventual exfoliation. However, at 1000 �C a charac-

teristic shape of the initially Gaussian distribution suggests

agglomeration of hydrogen at the perturbed region rather than

out-diffusion (Fig. 7). At 500 keV the implanted dose of

2� 1017 cm�2 was above threshold for exfoliation during high

temperature thermal annealing. At 800 �C the H in-depth

profile remains virtually unchanged relative to the

as-implanted one, consistent with H being trapped in the

microvoids yet the very top of the peak takes a

quasi-rectangular form. The latter feature can be appreciated

in the inset, in which adequate scaling was chosen (Fig. 8).

Thus, in both cases, the hydrogen atoms exhibit a tendency to

segregate at the maximum of damage density with no detecta-

ble loss to the ambient. With further increase of temperature

the whole implanted film breaks away. Fig. 9 shows a peak at

the new post-exfoliation surface, representing hydrogen that

was left after crystal split along the depth of maximum dam-

age (approximately the same as the depth of maximum H con-

centration, although there is a small difference between the

two, with the H peak somewhat deeper) and did not escape

through the surface. The other curve plotted on the graph

refers to similar situation after exfoliation by 200 keV ions

1017/cm2 and 1100 �C annealing. Here also the zero depth on

the X axis relates to the original depth at which the layer sepa-

ration have occurred. In a detailed study of implanted silicon,

the fracture location was identified as coinciding with the

maximum damage density produced by energetic H ions.15

FIG. 6. XTEM images of SiC sub-

jected to H-implantation (a)–200 keV

with a dose of 1� 1016 ion/cm2

annealed at 1100 �C for 1 h in argon,

(b)–200 keV with a fluence of 1� 1017

ion/cm2 as-implanted, (c) and (d)–as in

Fig. 6(b), annealed for 2 h at 780 �C,

(e) and (f)–200 keV 1� 1017 ion/cm2,

annealed for 1 h at 1100 �C.

223710-5 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 7: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

D. Oxygen gettering

Fig. 10(a) shows a very surprising profile of oxygen in

4H-SiC implanted with 1 MeV, 1017 cm�2 Hþ. It appears

that implantation induced defects acted as gettering centers

for oxygen present in the SiC crystal lattice. Peak values of

over 1020 cm�3 O atoms are indicated by SIMS in a narrow

band around an 11 lm depth, coincident with the projected

range Rp of implanted H. It is impractical to obtain XTEM

data at this large depth. The only possibility at hand to ac-

quire some information on the morphology of the

oxygen-containing layer was to cleave the sample and obtain

AFM scans. Fig. 11 taken over an area of 2 � 2 lm2 in a

phase mode AFM operation confirms that a distinct planar

layer about 0.1 lm thick forms at the interface between the

implanted volume and the underlying substrate. Note that the

relief of the cleaved planes above the buried layer (closer to

the initial surface) and below (bulk SiC) differ considerably.

Similar effect is observed in a 200 keV implant heated

to 1100 �C. As this sample undergoes partial exfoliation,

both SIMS (Fig. 10(b)) and HRTEM (Figs. 6(e) and 6(f))

analyses were performed at an area where no sign of disinte-

gration was observed. A very thin continuous oxide (4 nm),

comprising �5 � 1015 oxygen atoms/cm2 is located at the

interface between the implanted layer and the substrate.

IV. DISCUSSION

In an ideal configuration when the in-depth profile of

implanted ions can be expressed by a Gaussian distribution

with variance DRp2 and diffusion process obeys the Fick’s

continuity equation with a diffusion coefficient D, then the

heat treatment for time t results in another Gaussian function,

with a variance

X ¼ DRp2 þ 4Dt: (1)

Note that the rule of additivity of variances or second

moments holds true for any two convoluted distributions, not

FIG. 7. SIMS depth profiles of 1 MeV 1� 1017 ion/cm2 H-implanted SiC,

annealed at 1000 �C for 1.5 h in argon.

FIG. 8. Hydrogen and oxygen SIMS depth profiles of the 500 keV 2� 1017

ion/cm2 H-implanted SiC before and after 800 �C annealing for 1.5 h in ar-

gon. The inset shows the same curve scaled so that the (�rectangular) fea-

ture at the H peak is emphasized.

FIG. 9. SIMS depth profiles of hydrogen remaining in the substrate after

exfoliation which occurred after annealing for 1 h in samples: (500 keV,

2� 1017/cm2,1000 �C) and (200 keV, 1� 1017/cm2, 1100 �C).

223710-6 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 8: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

necessarily Gaussian. Here, as one can see in Fig. 1, the

effect of annealing manifests itself in apparent reduction of

the hydrogen concentration rather than in broadening of the

initial profiles. This is especially visible for the curve corre-

sponding to an 1100 �C anneal of the p-type silicon carbide.

Evidently, the process of migration and eventual loss of

hydrogen is much more complex than that described by

Fick’s Law with a unique value of D. In fact, the magnitude

of the “D” coefficient, referred to sometimes by its historical

designation as a diffusion constant depends on several fac-

tors: local concentration of solute species, temperature, inter-

nal electric field, strain, or stoichiometry (im)balance of the

host material.

One important circumstance of implanting H atoms as

well as other light elements is that their in-depth profile

closely coincides with that of accompanying defects. It is

suggested that what we really observe in the SIMS graphs

are hydrogen atoms bound to these defects as the H content

considerably exceeds the expected solid solubility in an

untreated material. XTEM micrograph in Fig. 6(a) indicates

that extended defects do not form under this dose/energy

condition.

Further we postulate that upon heating, a fraction of H

atoms are being released from the defect traps and move by

undergoing “normal” diffusional hops while the crystal lat-

tice is being progressively restored. As these processes pro-

ceed simultaneously, mathematical modeling would be

difficult if not impossible to perform due to a lack of

adequate kinetic parameters.

The observed substantial difference in hydrogen mobili-

ties in n- vs. p-type semiconductor may only be attributed to

a significantly different concentration of vacancies, through

which the H atoms are supposed to relocate. In the present

case, it must concern the ionized vacancies as their popula-

tion is subject to Fermi-Dirac statistics, i.e., to the Fermi

level position.

The above finding provides a clear observation of the

Fermi level effect on diffusion in silicon carbide. This topic

received special attention with respect to gallium arsenide

and related compounds16 and, understandably, to silicon.17

An excellent introduction to the problem the reader may find

in Ref. 18. In GaAs, the concentration of ionized gallium

vacancies was found to increase with the position of Fermi

level; the highest concentration, due to its lowest formation

energy was proposed for triple ionized gallium vacancy

VGa3þ. In Ref. 10, the authors did observe dependence of

deuterium mobility in SiC powders on the concentration of

certain contaminants, including aluminum, but their study

was focused on the application of this material to the high

temperature shielding of nuclear reactors. The time of these

experiments coincided with the invention of the physical

vapor transport process enabling epitaxial growth19 of rela-

tively large single crystal SiC so, presumably, the authors

have ignored semiconducting properties of “carborundum.”

We should mention also the work by Svensson et al.20 where

behavior of several impurities was investigated in 6H-SiC

FIG. 10. Hydrogen along with oxygen

SIMS depth profiles of the (a) 1 MeV

and (b) 200 keV H-implanted SiC

before and after 1100 �C annealing for

1 h in argon. Measurement on sample

(b) was performed on an un-exfoliated

surface.

FIG. 11. Phase mode AFM image over 2 lm � 2 lm on a cleaved plane of

1 MeV, 1017 ion/cm2 H-implanted SiC.

223710-7 Barcz et al. J. Appl. Phys. 115, 223710 (2014)

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Page 9: Diffusion and impurity segregation in hydrogen-implanted silicon carbide

epitaxial films. Enhanced diffusion of hydrogen injected into

near-surface of p-type material by plasma discharge or

low-energy implantation has been recognized in the past, for

example see a detailed review by Deak, Gali, and Aradi.10

The authors concentrate on a microscopic aspect of interac-

tion of hydrogen atoms with defects and p-type dopants.

However, the observed trapping of Hþ and passivation of

acceptors would retard rather than enhance the mobility of

hydrogen and, hence, cannot explain the increased H diffu-

sivity relative to n-type semiconductor. Reactivation of pas-

sivated acceptors occurs at relatively low temperatures of

530–600 K indicating the dissociation of acceptor-H com-

plexes.11 Therefore, at higher temperatures, as in this present

work, when the lattice is supposed to restore completely, the

only factor promoting fast relocation of H atoms is a persist-

ing surplus of vacancies that have been generated during the

growth of the p-type layer.

The origin of the oxygen that is gettered by the defects is

not well understood. There is very little experimental data on

oxygen in single crystalline SiC. SIMS data of Han et al.21 are

for 3C-SiC grown at a relatively low temperature of 1000 �Con a Si substrate. Data show high concentration of oxygen, up

to 1020 cm�3, but the crystalline quality of the material is in

doubt. It is clear that highly defective material may contain a

large concentration of oxygen atoms trapped on the defects.

There are no published experimental data to our knowledge

on the oxygen content of 4H-SiC. The situation is different on

the computational side. Several groups have investigated theo-

retical models of oxygen in cubic (3C-SiC) and hexagonal

(4H-SiC and 6H-SiC). Di Ventra and Pantelides22 have done

first-principles calculations of oxygen stability, diffusion, and

precipitation in cubic SiC, and they found very low solubility

of O in SiC. Gali et al.23 have done ab initio calculations to

determine possible configurations of the isolated oxygen im-

purity atoms in both 3C-SiC and in 4H-SiC. They identified O

on the carbon site, Oc, as the most stable configuration in the

4H-SiC lattice, and correlated this with the oxygen-related

peaks measured by deep level transient spectroscopy (DLTS).

They also predicted oxygen concentration as high as

1018 cm�3 in heavily n-type doped 4H-SiC. Muto et al.24 con-

firmed experimentally the presence of oxygen atoms on C

sites using electron energy loss spectroscopy. However, there

are no data on the oxygen concentration in 4H-SiC bulk single

crystals. Our gettering data suggest that there is a significant

amount of oxygen that may be released from the trap sites by

the radiation damage and that can diffuse and become

re-trapped near the plane of maximum damage either at the

point defects or at H-induced micro-voids.

In silicon, or more specifically in silicon on insulator

(SOI), it is possible to in-diffuse some oxygen from the am-

bient and through a thin layer of single crystalline silicon,

<200 nm, to the buried oxide, where it reacts with Si at the

buried Si/SiO2 interface. This process, known as ITOX

(Internal Oxidation), was used in the early days of SOI tech-

nology to improve the quality of the buried oxide formed by

oxygen implantation.25 To obtain a measurable increase in

the thickness of the buried oxide, very aggressive oxidation

was required at temperatures >1350 �C, close to the Si melt-

ing point at 1412 �C. Whether a similar effect can be

achieved in 4H-SiC is rather doubtful. This would require

significant solubility of O in the SiC lattice and a relatively

high diffusion coefficient at 1100 �C anneal temperature.

Diffusion of typical dopants in SiC is extremely low even at

1600 �C, so it would be surprising to see such mobility of ox-

ygen atoms in 4H-SiC. But the SIMS data of Fig. 10 seem to

indicate that oxygen is present and can be gettered to the

heavily damaged zone during the 1100 �C anneal.

Similar phenomenon of agglomeration of oxygen at

hydrogen implantation-induced defects was observed and

intensively investigated in both CZ and FZ silicon.26–30 The

result of these studies remains far from satisfactory even if

the solubilities and diffusivities of oxygen in this material

are very well known.

V. CONCLUSIONS

– We have found substantial differences in diffusivity of

hydrogen implanted into silicon carbide depending on the

type of doping of the semiconductor. The observed

enhanced mobility of hydrogen in the p-type material is

postulated to originate from a surplus of ionized vacancies

as their population is governed by the Fermi-Dirac statis-

tics, i.e., the position of the Fermi level.

– Application of higher irradiation doses leads to irreversible

formation of a well defined planar zone of microcavities,

bubbles, and other extended defects comprising large

amounts of agglomerated hydrogen. At sufficiently high

temperatures the heavily implanted layer tends to exfoliate

from the substrate.

– Within the perturbed film, SIMS analysis revealed the

presence of oxygen, in quantities largely exceeding its

expected content in 4H-SiC.

ACKNOWLEDGMENTS

The research was partially supported by the European

Union within European Regional Development Fund, through

a grant “Innovative Economy”: (No. POIG.01.03.01-00-

159/08, "InTechFun").

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