Resonant photoemission study of Ti interaction with GaN surface

Surface Science 600 (2006) 873–879 www.elsevier.com/locate/susc

Resonant photoemission study of Ti interaction with GaN surface I.A. Kowalik a,*, B.J. Kowalski a, P. Kaczor a, B.A. Orlowski a, E. Lusakowska a, R.L. Johnson b, L. Houssiau c, J. Brison c, I. Grzegory d, S. Porowski d a Institute of Physics, Polish Academy of Sciences, Aleja Lotniko´w 32/46, PL-02-668 Warsaw, Poland Universita¨t Hamburg, Institut fu¨r Experimentalphysik, Luruper Chausse 149, D-22761 Hamburg, Germany c University of Namur, FUNDP-LISE, Rue de Bruxelles 61, B-5000 Namur, Belgium Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29, PL-01-141 Warsaw, Poland

b

d

Received 4 October 2005; accepted for publication 5 December 2005 Available online 6 January 2006

Abstract Ti/GaN interface formation on GaN(0 0 0 1)-(1 · 1) surface has been investigated by means of resonant photoelectron spectroscopy (for photon energies near to Ti 3p ! 3d excitation). The sets of photoelectron energy distribution curves were recorded for in situ prepared clean GaN surface and as a function of Ti coverage followed by post-deposition annealing. Manifestations of chemical reactions at the Ti/GaN interface were revealed in the valence band spectra as well as in the Ga 3d core level peak—the discerned contribution of Ti 3d states to the valence band turned out to be similar to that reported in the literature for titanium nitride. The interaction between Ti and N was further enhanced by post-deposition annealing. The study was complemented with SIMS and AFM measurements.
2005 Elsevier B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Interface states; Titanium; Gallium nitride; Metal–semiconductor interface

1. Introduction The problem of metal/semiconductor interface formation is one of the issues in surface science which have been continuously attracting considerable interest for several decades. Atomic and electronic structures of various interfaces were thoroughly investigated by means of experimental and theoretical methods. As a consequence, several models describing such systems were developed [1]. Simultaneously, knowledge about metal/semiconductor interface formation mechanisms turned out to be very useful for applications. It was helpful in designing rectifying or ohmic junctions, in predicting properties of interfaces occurring in electronic devices. Progress in electronic technology has been inspiring investigations of new metal/semiconductor interfaces. Interest in titanium–gallium nitride system

*

Corresponding author. Tel.: +48 22 843 66 01; fax: +48 22 843 09 26. E-mail address: [email protected] (I.A. Kowalik).

0039-6028/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.12.016

seems to be an example of such intermixing of motivations related to basic research and applications. Semiconducting nitrides of the group III elements, such as gallium nitride, are considered as very promising materials for optoelectronic devices working in the green– blue–ultraviolet range of the light spectrum (light-emitting diodes and lasers) [2–4]. Moreover, possible applications in high power and high temperature electronic devices also inspire intensive studies of GaN and its ternaries. For all these applications, metal/semiconductor contacts are indispensable elements of the devices. The ohmic contacts should be thermally stable and should have low resistivity. This usually needs formation of a complex structure formed of several layers with carefully optimized properties and playing different roles during the contact operation. They usually contain a barrier layer which blocks metal atom diffusion into the semiconductor. For contacts on n-GaN, titanium is considered as one of the best elements for preparation of such a layer [5]. It is highly reactive and forms very stable titanium nitride. Therefore processes

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occurring during formation of Ti/GaN interface have been intensively studied [5–14] and important pieces of information were acquired. Introduction of Ti layer significantly decreases the contact resistance [11]. This effect has been attributed to the formation of a degenerate n-type surface layer. It occurs due to formation of nitrogen vacancies in GaN surface layer because of interaction between titanium and nitrogen atoms [12]. The vacancies are electrically active as donors. Such a heavily doped semiconductor surface in contact with a highly conductive metal (TiN conductivity is 1.3 times higher than conductivity of Ti) constitutes the necessary component of a tunneling junction. The contact resistance can also decrease due to Ti-induced reduction of a native oxide on GaN surface [13]. In spite of the collected knowledge about interaction between Ti and GaN, further investigations are still necessary in order to describe Ti/GaN interface formation under various conditions. The results would be valuable for the abovementioned applications. On the other hand side, GaN doped with transition metal atoms may change into a ferromagnetic semiconductor [15–18]. Intense search for such materials and thorough studies of their properties are related to prospects of spin-based electronics development. The interactions determining magnetic properties of the crystals depend on the position of the magnetic ions in the lattice as well as their charge state [19]. Thus, knowledge of the phenomena leading to incorporation of transition metal atoms (including Ti) into the GaN lattice is important also for this field. In this paper, we present the results of the resonant photoemission study of interaction between titanium and the surface of a gallium nitride bulk crystal. This interaction was monitored during the Ti/GaN interface formation. The system was formed by stepwise deposition of Ti on the clean, ordered surface of GaN crystal. The photoemission measurements were performed after each stage of deposition. Such an experimental procedure enabled us to collect information about Ti–GaN interaction for very small amounts of titanium, at the very beginning of the interface formation. This stage of the system fabrication is particularly important for its final properties. Afterwards, changes in the interface structure were observed as a function of the Ti layer thickness. As a consequence, the results presented in this paper show a substantially different aspect of Ti/GaN interface formation and structure than those previously described in the abovementioned literature. However, this work is also a complementary contribution to the investigations of Ti/GaN system. Further information about Ti distribution in the subsurface layer of GaN was obtained by secondary ion mass spectrometry (SIMS) measurements. The morphology of the system obtained by Ti deposition and annealing of the sample under UHV conditions was studied ex situ by atomic force microscopy (AFM). Taking under consideration all the acquired results we proposed a scenario of Ti/GaN interface formation.

2. Experimental details The bulk crystals of GaN investigated in the reported experiments were grown from solution of atomic nitrogen in high temperature (
1500 C) liquid gallium under high N2 pressure (
10–15 kbar). They were grown as platelets with (0 0 0 1) orientation (the c-axis perpendicular to the platelet) [20], the thickness of 0.1–0.2 mm and the lateral size of a few mm. The samples were almost free of dislocations (dislocation density of 10–100 cm2). This determined conditions of Ti interaction with GaN surface markedly different from those occurring on heteroepitaxial GaN layers [21]. The Ti atoms cannot diffuse into the crystal by dislocation decoration. Thus, we believe that our results describe properly Ti interaction with a clean GaN surface. The concentration of free electrons in the high pressure grown GaN is about 1019 cm3 and their mobility is about 100 cm2/V s [4]. The surfaces of such GaN samples are polar and two opposite crystal surfaces are not equivalent. In this paper we present the results of investigations of N-polar surfaces. The surface polarity was determined by room temperature etching in aqueous solutions (10–1 N) of KOH and NaOH [22,23]. Resonant photoelectron spectroscopy (RPES), the technique applied in the reported experiments, allows us to reveal the contribution of the electrons of transition metal atoms or rare earth atoms to the valence band, due to use of the Fano effect [24]. In such an experiment the radiation energy is tuned to the energy of an intra-ion transition involving the partly filled shell (e.g. 3p ! 3d for transition metals). The quantum interference of two processes: one leading to a discreet final state and the second–to a state of continuum, results in modulation of the transition probability described by the Fano formula [25]. For a system containing titanium atoms, we consider: • the intra-ion excitation (to the discreet state): Ti 3p6 3d2 þ hm ! Ti 3p5 3d3 ; • the direct photoemission of an electron from the 3d orbital (the transition to a continuum state): Ti 3p6 3d2 þ hm ! Ti 3p6 3d1 þ e . Thus, resonant enhancement of the photoelectron emission from the Ti 3d shell occurs for photon energy close to the energy of Ti 2p ! 3p excitation. In this study, analysis of the photoemission spectra measured on and off resonance enabled us to determine Ti 3d contribution to the valence band of the system. Its shape was compared with those reported in the literature for Ti nitrides. The resonant photoemission experiments were performed in the HASYLAB synchrotron radiation laboratory in Hamburg (Germany) at the E1 beamline which consists of the FLIPPER II plane grating monochromator covering the photon energy range of 15–200 eV (synchrotron radiation is coming from DORIS III storage ring).

I.A. Kowalik et al. / Surface Science 600 (2006) 873–879

3. Results and discussion In order to achieve the aim of the experiment—the description of the Ti/GaN interface formation—sets of photoelectron energy distribution curves (EDC) were recorded for clean GaN surface, then for subsequent stages of the system formation. The spectra were normalized to the monochromator output and corrected for photon flux variations. The first Ti deposition corresponded to a for˚ thick layer (2.7 A ˚  1 ML). Next, the layer mation of 2.7 A ˚ . Then, the samthickness was stepwise increased to 6.75 A ple was annealed in a few stages at 100–150 C. The photoemission spectra were measured for several photon energies in the range from 34 to 60 eV. Thus, changes in the electronic structure of the system corresponding to the increasing Ti 3d contribution could be revealed due to spectral feature intensity changes related to Ti 3p–3d resonance. Fig. 1 presents the set of the energy distribution curves, covering the valence band and Ga 3d core level, measured after the last Ti evaporation (with the Ti layer corresponding to 2.6 ML). The spectra were normalized with respect to the photon beam intensity. The energy scale origin was set at the Fermi level (as measured for a thick Ti layer). The background of secondary electrons was subtracted using the Shirley method. The collected spectra show that the valence band of the system consists of two main maxima: at the edge of the band with the onset at the Fermi level (marked as A in the picture) and at about 6.2 eV (C). A weaker shoulder can be discerned between them, at about 3.8 eV (B). The sharp onset of the spectra at the Fermi energy suggests metallic properties of the layer probed in the photoemission experiment. The Ga 3d peak occurs at the binding energy of 20 eV, although its intensity is strongly reduced due to deposited Ti layer (see Fig. 2). The shape of the valence band definitely does not correspond to that of metallic titanium [26–28] or of a simple superposition of pure Ti with clean GaN. It is rather similar to the shape of the valence band revealed for TiN [29,30]. As a consequence, we expect that the first maximum, at the edge of the valence band, is dominated by

18

hν [eV]

16

C

A

Int. [arb. u.]

Ti/GaN - EDC spectra taken after evaporation (2.6 ML) 15

B

Ei = 0.5 eV Ei = 6.2 eV

10 CIS curves 5 0

14

Intensity [ arb. u.]

The photoelectron spectrometer is equipped with a cylindrical mirror electron energy analyzer (CMA). The energy resolution in our experiment was typically about 0.2 eV. The radiation spot size at the sample was about 0.3 · 1 mm2. The GaN (0 0 0 1) surface was prepared in situ by repeated cycles of Ar+ ion sputtering (E = 600 eV) in the direction perpendicular to the sample surface and annealing at 500 C under UHV conditions. The surface crystallinity was assessed by low energy electron diffraction (LEED) technique, which indicated hexagonal (1 · 1) surface symmetry. The surface conditions were also controlled by spectroscopy of the Ga 3d core level. Ti was deposited stepwise from a calibrated source in a UHV chamber directly attached to the spectrometer.

875

44 48 36 40 Photon Energy [eV] Ga 3d

12 60 49

10 48 47.5 8 47 46.5 46 6 45.5 45 43

4 40 39.5 39 2 38 37 34

0

-5

0

5 10 15 20 Binding Energy [eV]

25

Fig. 1. The set of the EDCs measured for 34–60 eV energy range for GaN(0 0 0 1) surface after deposition of 2.6 ML Ti.

Ti 3d states. The second one consists of hybridized Ti 3d and N 2p states. This supposition can be verified by analysis of the spectra measured for photon energies close to the Ti 3p–3d resonance (shown in Fig. 1). Indeed, the intensity of the first maximum (A) strongly increases when the photon energy achieves 46 eV (see the inset of Fig. 1). The photon energy dependence of the maximum C is markedly weaker. The inset in Fig. 1 shows the photoemission intensity as a function of photon energy for the maxima A and C. The curves were derived from the spectra of Fig. 1. Their shapes clearly correspond to Fano profiles associated with the Ti 3p–3d resonance. Summarizing, the analysis of the data shown in Fig. 1 clearly proves that Ti related electronic states of the Ti/GaN system can be discerned by means of resonant photoelectron spectroscopy at Ti 3p–3d transition. It also enables us to determine the photon energies corresponding to the extremes of the Fano profile in the photoemission vs. photon energy curve, i.e. to minimum and maximum intensity of emission from Ti 3d related states. Fig. 2 shows photoemission spectra taken after each stage of the Ti/GaN system preparation procedure at the photon energies corresponding to the resonance (Fig. 2a) and the antiresonance (Fig. 2b). Changes in the subsequent spectra enable us to follow the modifications of electronic states distribution caused by deposition of Ti. Comparing the corresponding spectra from the left and right panel of Fig. 2, we can reveal contribution of Ti 3d to the electronic structure of the system. The spectra obtained for the clean surface of GaN show the valence band shape similar to that observed previously

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EDCRES

Ti/GaN 22 20 18

hν = 46 eV

Ti/GaN

Annealing time [h] (Temperature [°C]) C Ti3d+N2p

22

A B

18

4(150)

E

2(150)

16 2.5(100)

2.5(100)

Intensity [ arb. u.]

Intensity [ arb. u.]

D 4(150)

14 1(100)

12 Ti layer thickness [ML]

8

hν = 38 eV

Annealing time [h] (Temperature [°C])

20

16 2(150)

10

EDCANTIRES

2.6 2.1

6 1.6

1(100)

14 Ti layer thickness [ML] 12

2.6

10 2.1 8 1.6

6 1

1

4

4

Ga 3d valence band

2 clean

0 a

-5

Ga 3d

2

valence band clean

0

5

10

15

20

0

25

Binding Energy [eV]

b

-5

0 5 10 15 20 Binding Energy [eV]

25

Fig. 2. Selected photoemission energy distribution curves obtained from GaN surface subsequently covered with Ti. The curves were taken for (a) hm = 46 eV (the photon energy corresponding to maximum of the resonant Fano profile) and (b) hm = 38 eV (the antiresonant photon energy corresponding to the minimum of the Fano profile).

for GaN(0 0 0 1) surface [31]. It extends from 2 to 11.5 eV and consists of two main parts: from the edge to 8.5 eV and below this binding energy. The main feature of the valence band at about 5.5 eV derives from N p states contribution to the density of states. The next maximum, near 9.5 eV, primarily consists of the hybridized Ga s and N p states [32]. The Ga 3d peak has the maximum at 20.4 eV. The spin orbit splitting (0.46 eV [33]) cannot be resolved. A weak shoulder occurring just above the Ga 3d corresponds to hybridized N 2s states. The difference between the curves obtained for 46 and 38 eV is much smaller than the changes appearing after Ti deposition. Thus, the contribution of the photon energy dependence of the emission from the valence band of GaN to resonance–antiresonance difference of Ti/GaN spectra is negligible. Deposition of 1 ML of Ti leads to changes on the Fermi edge—a clearly visible peak A appears. Such an increase of the photoelectron intensity is indicative for metallic properties of the system. The two remaining features of the valence band gradually merge into one maximum C at the energy of about 6.2 eV. After next three deposition stages the valence band intensity increases and simultaneously the feature A in the vicinity of the Fermi energy becomes more visible with a sharper onset. After the third deposition stage (2.1 ML) a small feature (B) at the binding energy of about 3.8 eV appears. The overall decrease of photoemission intensity, noticeable after the last Ti deposition (2.6 ML), is probably related to surface roughness increasing during Ti deposition process. An interaction of Ti atoms with the GaN surface was also assessed by spectroscopy of Ga 3d core level. After the first stages of Ti deposition, the Ga 3d signal decreased

and the additional shoulder, marked as D in Fig. 2, appeared about 1 eV below the Ga 3d core level energy. With the increase of Ti layer thickness, the Ga 3d signal consistently decreased and it disappeared when Ti layer thickness reached 2.6 ML. The annealing of the sample covered with the Ti layer caused a substantial change in the density of states distribution of the valence band. This suggests a significant influence of annealing on interactions between atoms at the interface. During the annealing both principal maxima of the valence band (C at about 6.2 eV and B at about 3.8 eV) increased. We also observed a minor change in the photoemission just below the Fermi edge—the maximum A became narrower. The additional feature D, close to the Ga 3d peak, decreased after long annealing (about 6 h), but the Ga 3d peak reappeared in the spectrum. On the other hand, a new peak (E) emerged at about 22.5 eV as a result of annealing. Similar changes can also be noticed in Fig. 3. It shows the differences between spectra taken at the resonance and antiresonance energies. The corresponding curves were normalized only to the monochromator output and photon flux variations. Such a procedure resulted in the difference spectra with satisfactory cancellation of the Ga 3d peak. This justifies the assumption that changes in the photoionisation cross-section of the states other than Ti 3d are negligible in the relatively narrow photon energy range (8 eV) and they do not contribute to the shape of the difference spectra in the considered binding energy range. Thus, the curves shown in Fig. 3 reproduce the shape of Ti 3d contribution to the electronic states distribution of Ti/GaN system. The changes caused by contribution of Ti 3d states

I.A. Kowalik et al. / Surface Science 600 (2006) 873–879

Annealing time [h](Temperature [°C])

12 4(150)

10

2(150)

Intensity [arb.u.]

2.5(100)

8

1(100) Ti layer thickness [ML]

6

4

2.6 2.1 1.6

2

0 -5

1 clean

0 5 Binding Energy [eV]

10

Fig. 3. The set of the resonance–antiresonance difference curves obtained for GaN surface covered with about 2.6 ML of Ti and annealed at 100– 150 C.

are clearly discernible at the vicinity of the Fermi edge and, markedly weaker, deeper in the valence band region. The maximum at the Fermi level increased after subsequent Ti depositions. The height of the broad feature for the binding energy range of 3–8 eV is much less dependent on the amount of Ti. Annealing resulted in some decrease of the maximum A, while the broad features at 3.8 and 6.2 eV changed the relative intensity. Making use of the acquired data we can undertake an attempt to describe the Ti/GaN interface formation process. For increasing Ti coverage, the valence band of the system becomes similar to that of TiN [29,30]. Moreover, the first Ti deposition leads to appearance of an additional component in the Ga 3d feature (peak D in Figs. 1 and 2). Its energy position corresponds to that of metal-like Ga occurring on clean GaN surface after Ar+ ion sputtering. This confirms that Ti deposition induces GaN surface disruption with nitrogen atoms reacting with titanium atoms. The relative contribution of unbound Ga to the Ga 3d feature markedly increases during Ti deposition, indicating efficient replacement of Ga by Ti in chemical bonds with nitrogen. However, the total intensity of the Ga 3d feature monotonically decreases with increasing thickness of the Ti layer. Thus, unbound Ga atoms remain close to the interface. The interpretation based on formation of TiN at the Ti/GaN interface can also be supported by comparison of the substantially different Gibbs formation energies of GaN (118 kJ/mol) and TiN (347 kJ/mol) [34]. Neckel et al. [35] showed that the first maximum of the valence band of TiN consists mainly of Ti 3d states. The second maximum corresponds to a mixture of Ti 3d and

N 2p states. Figs. 1–3 clearly show that intensity of the first maximum discernible in our spectra changes for the photon energies close to 46 eV in accordance with Ti 3p–3d resonance. The resonant enhancement of the deeper part of the valence band formed after Ti deposition is much weaker. This supports our assumption that TiN is formed due to deposition of Ti on GaN. Nevertheless, we have to admit that some metallic Ti still occurs on the sample and contributes to the maximum A. The EDC curves measured after annealing allow us to observe changes in the interaction of Ti atoms with GaN surface at higher temperatures. Fig. 2a shows that the annealing of Ti/GaN caused increase of the peak C with a shoulder B (characteristic of TiN [29]), while the intensity of A remained almost unchanged. Thus, the relative intensity of the peaks A and C became similar to that observed for titanium nitride [29,30]. This observation suggests that Ti atoms (contributing only to A) diffused into the crystal and formed TiN with strong peak C. The annealing enhanced this process. After long annealing, the component D of Ga 3d feature (in Fig. 2b) decreased indicating strong reduction of the amount of Ga in the layer probed by photoemission experiment. Appearance of some new weak features (like E) at photon energy of 38 eV suggests that remained Ga atoms changed their chemical bonding. Photoemission spectra in the constant-initial-state (CIS) mode were also measured in order to verify presence of the Ti 3d contribution in the peak A (at the binding energy of 0.5 eV) (Fig. 4). The photon energy and the electron kinetic

18 16 14

Ti/GaN

CIS

E= 0.5 eV

experimental data AFTER EVAPORATION experimental data AFTER ANNEALING Fano shape fit

12 Intensity [arb.u.]

Ti/GaN ERES - EANTIRES

877

10 8 6 4 2 0 32 34 36 38 40 42 44 46 48 50 Photon Energy [eV]

Fig. 4. Constant-initial-state (CIS) spectra taken for the initial state binding energy (Ei) of 0.5 eV. The curves were measured after last evaporation and after annealing at 150 C. Both curves are shown with a Fano lineshape fit (the linear background was subtracted).

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100000

Ti on GaN Ga

+

Intensity (Cnts)

10000

1000

100 +

N

+

GaN 10 +

Ti + TiN

1 0

100

200 300 400 Acquisition Time (Sec)

500

Fig. 5. The SIMS measurement results obtained ex situ after Ti deposition of 2.6 ML and after annealing.

ToF (Fig. 5). Apart from the expected sample constituents like Ga+ and N+ and GaN+, the Ti+ and TiN+ signals clearly appear for the sample surface and the subsurface region. Marked difference between profiles of Ti and TiN ions suggests that the latter do not form only by interaction of secondary Ti+ and N+ ions above the sample surface. We believe that part of them, at least, originate from TiN formed at the interface. Morphology of the surface obtained by Ti deposition on GaN and annealing under UHV conditions was investigated ex situ by atomic force microscopy (AFM). A fragment of Ti/GaN surface is shown in Fig. 6. The surface turned out to be covered by a continuous, atomically smooth layer with some, unidentified, terrace-like features of 5–20 nm height and lateral size up to 250 nm. 4. Summary The bulk GaN(0 0 0 1)-(1 · 1) sample surface gradually covered with Ti atoms was investigated in situ by means of resonant photoemission spectroscopy. It has been confirmed that such system forms a reactive interface. The interface formation process, as deduced from the acquired data, proceeds in the following stages: (a) first Ti deposition (about 1 ML) causes surface disruption—unbound Ga appears on it; (b) further Ti deposition leads to simultaneous formation of a Ti–N compound and to increase of amount of metallic Ti covering the GaN surface; (c) annealing process enhances reaction of Ti with N and the valence band of the system becomes similar to that observed for titanium nitride.

Acknowledgement Fig. 6. Surface morphology of 2.6 ML of Ti deposited on GaN (0 0 0 1)(1 · 1) and annealed under ultra high vacuum conditions (obtained by AFM).

energy were scanned simultaneously so that the photoemission from electronic states at the fixed binding energy level were observed. The analysis of the data confirmed that the intensity of the peak A behaves in accordance with a Fanotype profile characteristic of Ti. In Fig. 4, we compare the CIS curves measured after the last Ti evaporation and after annealing at 150 C with corresponding Fano profiles (the linear background was subtracted). After evaporation (the black squares) the Fano profile has maximum at the photon energy of about 46 eV, which corresponds well to Ti 3p ! 3d transition. Annealing led to a change of the Fano profile, including the energy shift by 1.5 eV. This confirms change in the chemical binding of Ti due to annealing. A depth profile of the chemical composition of the obtained Ti/GaN system was ex situ investigated by SIMS—

This work was supported by MSRIT (Poland) Research Projects no. 1 P03B 053 26, 1 P03B 116 28, 72/E-67/SPB/ DESY/P-03/DWM68/2004-2006 and by the IHP-Contract HPRI-CT-1999-00040/2001-00140 of the European Commission. References [1] H. Lu¨th, Surfaces and Interfaces of Solid Materials, Springer-Verlag, Berlin Heidelberg, 1998, p. 423. [2] J.W. Orton, C.T. Foxton, Rep. Prog. Phys. 61 (1998) 1. [3] A. Pelzmann, C. Kirchner, M. Mayer, V. Schwegler, M. Schauler, M. Kamp, K.J. Ebeling, I. Grzegory, M. Leszczynski, G. Nowak, S. Porowski, J. Cryst. Growth 189–190 (1998) 167. [4] S. Porowski, J. Cryst. Growth 166 (1996) 583. [5] S. Noor Mohammad, J. Appl. Phys. 95 (2004) 4856. [6] J.K. Kim, H.W. Jang, J.-L. Lee, J. Appl. Phys. 91 (2002) 9214. [7] A.G. Baca, F. Ren, J.C. Zolper, R.D. Briggs, S.J. Pearton, Thin Solid Films 308–309 (1997) 599. [8] B.P. Luther, S.E. Mohney, T.N. Jackson, Semicond. Sci. Technol. 13 (1998) 1322.

I.A. Kowalik et al. / Surface Science 600 (2006) 873–879 [9] D.-F. Wang, F. Shiwei, C. Lu, A. Motayed, M. Jah, S.N. Mohammad, K.A. Jones, L. Salamanca-Riba, J. Appl. Phys. 89 (2001) 6214. [10] Z.M. Zaho, R.L. Jiang, P. Chen, D.J. Xi, H.Q. Yu, B. Shen, R. Zhang, Y. Shi, S.L. Gu, Y.D. Zheng, Appl. Phys. Lett. 79 (2001) 218. [11] M.E. Lin, Z. Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, H. Morkoc, Appl. Phys. Lett. 64 (1994) 1003. [12] S. Ruvimov, Z. Liliental-Weber, J. Washburn, K.J. Duxstad, E.E. Haller, Z.-F. Fan, S.N. Mohammad, W. Kim, A.E. Botchkarev, H. Morkoc¸, Appl. Phys. Lett. 69 (1996) 1556. [13] B.P. Luther, S.E. Mohney, T.N. Jackson, M. Asif Khan, Q. Chen, J.W. Yang, Appl. Phys. Lett. 70 (1997) 57. [14] J. Dumont, E. Monroy, E. Mun˜oz, R. Caudano, R. Sporken, J. Cryst. Growth 230 (2001) 558. [15] M.L. Reed, M.K. Ritums, H.N. Stadelmaier, M.J. Reed, C.A. Parker, S.M. Bedair, N.A. El-Masry, Mater. Lett. 51 (2001) 500. [16] M.L. Reed, N.A. El-Masry, H.N. Stadelmaier, M.K. Ritums, M.J. Reed, C.A. Parker, J.C. Roberts, S.M. Bedair, Appl. Phys. Lett. 79 (2001) 3473. [17] M. Zajac, R. Doradzinski, J. Gosk, J. Szczytko, M. LefeldSosnowska, M. Kaminska, A. Twardowski, M. Palczewska, E. Grzanka, W. Gebicki, Appl. Phys. Lett. 78 (2001) 2470. [18] S. Sonoda, H. Hori, Y. Yamamoto, T. Sasaki, M. Sato, S. Shimizu, K. Suga, K. Kindo, IEEE Trans. Magn. 38 (2002) 2859. [19] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [20] I. Grzegory, M. Bockowski, B. Lucznik, S. Krukowski, Z. Romanowski, M. Wroblewski, S. Porowski, J. Cryst. Growth 246 (2002) 177.

879

[21] S. Porowski, I. Grzegory, D. Kolesnikov, W. Lojkowski, V. Jager, W. Jager, V. Bogdanov, T. Suski, S. Krukowski, J. Phys.: Condens. Matter 14 (2002) 11097. [22] J.L. Rouviere, J.L. Weyher, M. Seelmann-Eggebert, S. Porowski, Appl. Phys. Lett. 73 (1998) 668. [23] S. Krukowski, S.Z. Romanowski, I. Grzegory, S. Porowski, J. Cryst. Growth 189–190 (1998) 159. [24] Ph. Durand, I. Paidarova´, F.X. Gade´a, J. Phys. B 34 (2001) 1953. [25] U. Fano, Phys. Rev. 124 (1961) 1866. [26] Y. Fukuda, F. Honda, J.W. Rabalais, Surf. Sci. 91 (1980) 165. [27] E. Bertel, R. Stockbauer, T.E. Madey, Phys. Rev. B 27 (1983) 1939. [28] T. Kaurila, J. Va¨yrynen, J. Electron Spectrosc. Relat. Phenom. 82 (1996) 165. [29] R.D. Bringans, H. Ho¨chst, Phys. Rev. B 30 (1984) 5416. [30] G.G. Fuentes, P. Prieto, C. Morant, C. Quiro´s, R. Nu´n˜ez, L. Soriano, E. Elizalde, J.M. Sanz, Phys. Rev. B 63 (2001) 075403. [31] B.J. Kowalski, L. Plucinski, K. Kopalko, R.J. Iwanowski, B.A. Orlowski, R.L. Johnson, I. Grzegory, S. Porowski, Surf. Sci. 482–485 (2001) 740. [32] R.W. Hunt, L. Vanzetti, T. Castro, K.M. Chen, L. Sorba, P.I. Cohen, W. Gladfelter, J.M. Van Hove, J.N. Kuznia, M. Asif Khan, A. Franciosi, Physica B 185 (1993) 415. [33] R.A. Beatch, E.C. Piquette, T.C. McGill, MRS Int. J. Nitride Semicond. Res. 4S1 (1999) G6.26. [34] I. Barin, O. Knacke, O. Kubaschewski, Thermochemical Properties of Inorganic Substances, Springer-Verlag, 1977. [35] A. Neckel, P. Rastl, R. Eibler, P. Weinberger, K. Schwarz, J. Phys. C 9 (1976) 579.