Photoluminescence spectra of nitrogen implanted GaSe crystals

ARTICLE IN PRESS

Journal of Luminescence 128 (2008) 1551–1555 www.elsevier.com/locate/jlumin

Photoluminescence spectra of nitrogen implanted GaSe crystals M. Karabuluta,, G. Bilira, G.M. Mamedova, A. Seyhanb, R. Turanb a

b

Department of Physics, Kafkas University, 36100 Kars, Turkey Physics Department, Middle East Technical University, Ankara, Turkey

Received 11 October 2007; received in revised form 13 February 2008; accepted 21 February 2008 Available online 29 February 2008

Abstract GaSe single crystals were N-implanted along c-axis with ion beams of 1014 and 1016 ions/cm2 doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states. r 2008 Elsevier B.V. All rights reserved. PACS: 71.35.y; 78.55.m Keywords: GaSe; N implantation; Bridgman; Exciton photoluminescence

1. Introduction GaSe is a layered semiconductor in which each layer consists of four covalently bounded sheets in the Se–Ga– Ga–Se sequence, while the layers are bounded with van der Waals forces. These crystals have been investigated as promising materials in photoelectronic devices [1]. The perfect mirror surfaces of these crystals with no dangling bonds do not require extra mechanical or chemical procedures for the measurements. Because of this, it is easy to obtain junctions on these crystals, which may increase the potential application area of these crystals [2,3]. For example, GaN layer was obtained on GaSe base by thermal and plasma nitridization [1]. The photoluminescence (PL) investigations of GaN–GaSe hetherojunctions obtained with both methods showed that the emission spectrum was in the 2.10–3.46 eV energy interval, which is characteristic to GaSe and GaN. Also, with the surface oxidization studies of layered GaSe and InSe, the possibility of forming metal–oxide–semiconductor (MOS) structures has been investigated [2]. By oxidizing GaSe Corresponding author. Tel.: +90 474 212 3073; fax: +90 474 212 2706.

E-mail address: [email protected] (M. Karabulut). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.02.014

crystals, GaO3–Ga2Se3–GaSe stable structures with new potential application areas were shown to form [2]. The cathodoluminescence spectrum of this system covers a wide energy interval of 1.4–4.0 eV. Through similar studies, AIIIBVI based materials with modified surface structures have been studied for different applications such as optical gates, radiation detectors and gas sensors [1–3]. Ion implantation provides another mean of obtaining different structures on the surfaces of layered crystals with different advantages [3]. Due to the weak van der Waals interactions between the layers, heavy doping can be achieved by ion implantation method. In these crystals, a greater concentration of dopant atoms may be obtained between the layers and activation of these dopant atoms may take place as in intercalation effect [4–6]. Also, the penetration depth of the atoms may be higher due to the channeling effect [3]. Hence, heavily doped crystals with thicker surface layers can be obtained by ion implantation method. When GaSe and GaS crystals were implanted with Ar+, + B , Kr+, P+ ions of different doses (3.75  1013– 6.25  1015 ions/cm2), it was observed that the electrical conductivity and the intensity of exciton PL decreased [7]. In N, Si, and Ge implanted GaSe crystals, it was seen that

ARTICLE IN PRESS M. Karabulut et al. / Journal of Luminescence 128 (2008) 1551–1555

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750 500 250 0 750 500 250 0 450 300 150 0 150 100 50 0

temperatures. The PL spectrum of undoped GaSe was also measured for comparison. 2. Experimental GaSe crystals were grown using conventional Bridgman method from a stoichiometric mixture of high purity Ga (99.9999%) and Se (99.999%) in evacuated quartz ampoules (104 Torr) whose inner walls were coated with graphite [12,13]. For the ion implantations, a layer of 2  6  0.05 cm was cleaved from the ingot along c-axis with a razor blade. Three samples were obtained from this layer. A Varian DF4 type ion implantation system was used for nitrogen implantation [8,9]. One of the samples

B A

Undoped 40 K C 50 K

PL Intensity (a.u.)

PL Intensity (a.u.)

the radiation defects partially disappeared after annealing and the atom complexes became electrically active. The defects caused by ion implantation were observed to decrease after annealing. Also, the conductivity of crystals implanted with different atoms was dependent on the type of atom [8,9]. The effect of N implantation on GaSe crystals can be seen more clearly in exciton PL spectra as was observed for thermal doping [10] and annealing [11]. The formation of different hetherojunctions can be clearly seen in the PL spectrum. In this work, with these purposes, we have investigated the PL spectra of N-implanted GaSe crystals (implanted by ion beams of 1014 and 1016 ions/cm2 doses having energies of 60 and 100 keV, respectively) at different

70 K

90 K

0 6 4 2 0

120 K

30 15 0

1.4

1.6 1.8 hν (eV)

PL Intensity (a.u.)

1.2

2.0

B

240 K

1.2

2.2

A

600 1014 ions/cm2 400 57 K 200 C 0 300 80 K 200 100 0 120 90 K 80 40 0 30 150 K 15

1.6

1.4

1.8 hν (eV)

2.0

2.2

A

450 1016 ions/cm2 300 46 K 150 C 0 450 50 K 300 150 0 150 60 K 100 50 0 21 120 K 14 7 0 4 210 K 2

B

0 1.2

1.4

1.6

1.8 hν (eV)

2.0

2.2

Fig. 1. The PL spectra of (a) undoped, (b) 1014 ions/cm2 N-implanted and (c) 1016 ions/cm2 N-implanted GaSe crystals measured at different temperatures.

ARTICLE IN PRESS M. Karabulut et al. / Journal of Luminescence 128 (2008) 1551–1555

800 600

A

200

600 400

B

C

0

A

1014 ions/cm2

200

B

C

0 350 175

A

1016 ions/cm2

1.2

1.5

C

B B1

1.8

0 1.5

2.5

2.0 hν (eV)

3. Results and discussions 800 600

hνA = 2.107 eV

Undoped

400

0 600

1014 ions/cm2

400

hνA = 2.107 eV

A

hνB = 2.080 eV B

200 0 400

A

hνB = 2.080 eV B

200 PL intensity (a.u.)

Fig. 1 shows the PL spectra of undoped and Nimplanted (1014 and 1016 ions/cm2) GaSe crystals measured at different temperatures. The PL spectra measured at T ¼ 50 K are given in more detail in Fig. 2. As seen, the PL spectra of undoped and N-implanted GaSe crystals consist of characteristic intrinsic and extrinsic regions [10,11,14–17]. The intrinsic region of the PL spectra contain a main peak denoted as A, and two peaks at low temperatures denoted as B and B1 while the extrinsic region contain a broad band labeled as C, which is also observed at low temperatures. The undoped, the 1014 ions/cm2 N-implanted and 16 10 ions/cm2 N-implanted samples used for PL measurements were obtained from the same GaSe crystal and they have the same band structure. However, as seen from Fig. 2, there are some changes in the PL spectrum of 1016 ions/ cm2 N-inplanted GaSe. The intrinsic PL band in undoped and 1014 ions/cm2 N-implanted sample consists of a high intensity A band located at hn ¼ 2.107 eV and a B band observed in the long wavelength tail of this band at hn ¼ 2.080 eV. In the 1016 ions/cm2 N-implanted GaSe sample, the PL spectrum shifts to higher energy (3 meV); the A band is observed at hn ¼ 2.110 eV, the B band is observed at hn ¼ 2.104 eV and a new peak labeled as B1 is observed at an energy position of hn ¼ 2.074 eV. In the PL spectra of all three samples an extrinsic broad band labeled as C in figures is observed. The energy position of this band is hn ¼ 1.5 eV. The temperature dependences of the PL intensity of the A band are given in Fig. 3. It is seen that the intensity of this band decreases linearly with temperature at low temperatures for all three samples. At TX100 K the intensity decreases slowly with increasing temperature. As seen from Figs. 1 and 2, the intensity of this peak decreases as the implantation dose increases. In undoped and Ge doped GaSe crystals, the direct free excitons (n=1) were shown to be dominant in the photoconductivity spectra

T = 50 K

Undoped

400 PL intensity (arb.units)

was not intentionally implanted while the other two samples were implanted by bombarding the crystal surfaces perpendicular to c-axis with ion beams of 60 and 100 keV of doses 1014 and 1016 ions/cm2, respectively. The distribution of N ions in N-implanted GaSe crystals were shown to have a Gaussian shape through projected range by TRIM calculation [9]. The color of the surfaces of the Nimplanted samples was yellowish compared to the red color of the undoped GaSe crystal surface. The PL spectra were recorded using a Hamamatsu C7041 multichannel detector. The excitation was provided by a 532 nm Nd:YAG laser. The intensity of the laser beam was 0.3 W/cm2. The luminescence was focused with two convex lenses on the entrance slit of an Oriel MS257 model monochromator with four gratings. A He cryostat was used for the measurements to provide a temperature interval of 10–300 K.

1553

1016 ions/cm2

hνA = 2.110 eV

A

hνB = 2.104 eV hνB1 = 2.074 eV B B1

200 0 2.06

2.08 hν (eV)

2.10

2.12

Fig. 2. (a) The PL spectra of undoped and N-implanted GaSe crystals at 50 K. The inset given is the C band. (b) The A and B bands of PL spectra at 50 K.

[12,13]. Analysis of the temperature dependences of the peak intensity, energy position and FWHM of the A band observed in the PL spectra of undoped and N-implanted crystals, and consideration of the band structure of GaSe show that this band is related to the direct free excitons (DFE) [14–18]. Similar behavior of DFE peak intensity in the PL spectrum was also observed in phosphorous doped GaSe and in annealed GaSe crystals [10,11]. The temperature dependence of the PL intensity of the B band in undoped and N-implanted GaSe crystals is given in Fig. 4. The intensity of this band for all three samples decreases exponentially with temperature according to IPLexp(DE/kBT). The activation energies were calculated from the fits, which are given as solid lines in Fig. 4. The activation energies of the B band are calculated as

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800

Undoped

Undoped 600

102

400 101 200 100

0 0

100

200

14

300 PL Intensity (a.u.)

PL Intensity (a.u.)

800 1014 ion s/c m2 600 400 200 0

102

100

200

18

20

22

24

26

1014 ions/cm2

101 100

10 0

16

12

14

16

18

300 1016 ions/cm2

800 1016

ion s/c

102

m2

600 400

101

200

100

0

16 0

100

200

300

T (K) Fig. 3. Temperature dependence of the PL intensities of the A emission bands for undoped and N-implanted GaSe crystals.

DE ¼ 0.024 eV for 1014 ions/cm2 N-implanted GaSe crystal and DE ¼ 0.027 eV for 1016 ions/cm2 N-implanted GaSe crystal. The B band is associated with the recombination of direct excitons bound (DBE) to the centers formed by impurities and dopant atoms [10,11,14–17]. The activation energy for 1014 ions/cm2 N-implanted GaSe crystal is very close to the energy difference between the A and B bands (huA–huBE27 meV, Fig. 2b). However, for 1016 ions/cm2 N-implanted GaSe crystal the energy difference between the A nad B bands is 6 meV. The activation energy DE is related to the binding energy as DhuEEA–EB ¼ Eb. The ionization energy of the centers to whom excitons are bound can be calculated from Ei ¼ DE/0.115 eV [14–17]. Hence, the ionization energies of these centers are calculated to be Ei=234 and 52 meV for the 1014 ions/ cm2 N-implanted and 1016 ions/cm2 N-implanted GaSe crystals, respectively. This indicates that excitons are bound to different centers in 1014 ions/cm2 N-implanted and 1016 ions/cm2 N-implanted GaSe crystals. Similar shallow states were also observed previously [8]. The energy position of B1 peak observed in PL spectra of N-implanted samples is 36 meV below the A band. According to the band structure of GaSe, the energy

18

20 103/T (K)

22

Fig. 4. Temperature dependence of the PL intensities of the B emission bands for undoped and N-implanted GaSe crystals. Fits are given as solid lines.

position of this band corresponds to indirect exciton states. The energy difference between the direct and indirect exciton states is given as 35 meV in Ref. [16]. As a result of the interaction with the dopants and disorders, the wave vector ~ ? c) is satisfied and selection rules for optical transition (E thus, the indirect free exciton (IDFE) band, the B1 band, is observed [14–18]. We believe that N atoms lay between the layers as a result of intercalation, and formation of new complexes with Se atoms and vacancies takes place. Similarly to the effect of intercalation of H atoms in GaSe and InSe [6], the DFE peak broadens and shifts to higher energy (3 meV) as a result of N intercalation. The C band observed in the PL spectra of undoped and N-implanted GaSe crystals is shown to have Gaussian line shape, which is also given as an inset in Fig. 2a. This band is due to the irradiative transitions to the deep centers and has been explained by the Configuration Coordinate (CC) model [10,11]. 4. Conclusion The PL spectra of undoped and N-implanted GaSe crystals were measured as a function of temperature. The

ARTICLE IN PRESS M. Karabulut et al. / Journal of Luminescence 128 (2008) 1551–1555

PL spectra consist of two regions. The intrinsic region of the PL spectra contains a main peak denoted as A, and two peaks at low temperatures denoted as B and B1 while the extrinsic region contains a wide band labeled as C which is also observed at low temperatures. It was observed that as the N-implantation dose increased the exciton PL intensity clearly decreased and the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states. The extrinsic C band is associated with the irradiative recombination of deep acceptor centers. References [1] O.A. Balitskii, V.P. Savchyn, P.J. Stakhira, N.N. Berchenko, Vacuum 67 (2002) 69. [2] V.P. Savchyn, V.B. Kytsai, Thin Solid Films 361 (2000) 123. [3] J.W. Mayer, L.E. Eriksson, J.A. Daries, Ion Implantation in Semiconductors, Academic Press, New York, London, 1970.

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