Surface modification of indium phosphide by 100 MeV ...

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Surface modification of indium phosphide by 100 MeV iron ions R. L. Dubey

a b

b

b

, S. K. Dubey , A. D. Yadav & D. Kanjilal

c

a

St. Francis Institute of Technology, Mt. Poinsur, Borivali, Mumbai, 400103, India b

Department of Physics, University of Mumbai, Mumbai, 400098, India c

Inter University Accelerator Centre, New Delhi, 110067, India Version of record first published: 23 Apr 2013.

To cite this article: R. L. Dubey , S. K. Dubey , A. D. Yadav & D. Kanjilal (2013): Surface modification of indium phosphide by 100 MeV iron ions, Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology, DOI:10.1080/10420150.2013.789027 To link to this article: http://dx.doi.org/10.1080/10420150.2013.789027

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Radiation Effects & Defects in Solids, 2013 http://dx.doi.org/10.1080/10420150.2013.789027

Surface modification of indium phosphide by 100 MeV iron ions R.L. Dubeya,b *, S.K. Dubeyb , A.D. Yadavb and D. Kanjilalc

Downloaded by [Radhekrishna L Dubey] at 22:02 23 April 2013

a St.

Francis Institute of Technology, Mt. Poinsur, Borivali, Mumbai 400103, India; b Department of Physics, University of Mumbai, Mumbai 400098, India; c Inter University Accelerator Centre, New Delhi 110067, India (Received 16 October 2012; final version received 17 March 2013)

Surface modification of indium phosphide (InP) irradiated with 100 MeV 56 Fe7+ ions for various ion fluences ranging from 5 × 1012 to 2 × 1014 cm−2 was studied using Raman scattering, Fourier transform infrared (FTIR) and atomic force microscopy. The shift in the Raman peak revealed the presence of stress in the irradiated samples. The average crystallite size obtained by analyzing Raman spectra with the phononconfinement model decreased with ion fluence. The observed interference fringes in the Fourier transform reflectance spectra of irradiated InP indicate the presence of disorder and defects in the surface region of irradiated InP. The power spectral density obtained from Atomic Force Microscopy (AFM) measurements showed the surface evolution due to diffusion process up to ion fluence 5 × 1013 cm−2 ; however, surface diffusion process was less dominant at higher ion fluences. Keywords: swift iron ions; indium phosphide; Raman scattering; FTIR; AFM

1.

Introduction

Indium phosphide (InP) is an direct band gap compound semiconductor widely used in optoelectronic and microwave devices (1). The properties of InP can be further tailored by swift heavy ion irradiation. Irradiation of InP with Au+ , Pb+ , Kr+ and Xe+ ions produced disorder region as well as large number of defects (2–6). Both disorder region and defects changed the properties of the irradiated samples. Under certain conditions, latent tracks in InP due to high electronic excitation have been observed. However, limited studies on the transition metal (i.e. Fe and Co) ion irradiation of InP have been reported (7–10). Atomic force microscopic studies of 100 MeV ironirradiated InP showed the presence of nano-sized hillocks on the surface after irradiation. The size and density of nano structures were found to depend on the ion fluence (7). High Resolution X-ray Diffraction (HRXRD) studies revealed the presence of the damaged layer peak in addition to the substrate peak at higher fluence 2 × 1014 cm−2 . The screw dislocations induced in the irradiated samples was found to vary with ion fluence (7). The XRD studies of 100 MeV iron ion-irradiated InP showed the decrease in crystallite size from 201.88 to 139.80 nm for ion fluences 5 × 1012 to 2 × 1014 cm−2 (8). In the present work, the effects of 100 MeV 56 Fe7+ ion irradiation on InP for various ion fluences varying from 5 × 1012 to 2 × 1014 cm−2 have been investigated by the Raman scattering, Fourier transform infrared (FTIR) and atomic force microscopy techniques. The concentration of iron ions for the highest fluence 2 × 1014 cm−2 estimated from the Stopping *Corresponding author. Email: [email protected] © 2013 Taylor & Francis

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R.L. Dubey et al.

and Range of Ions in Matter (SRIM) calculation (11) was found to be 0.006 at%, which is very small to give rise to the spin effect in InP. The ratio of electronic energy loss, Se, and nuclear energy loss, Sn, for 100 MeV Fe ions in InP is ∼526 at the surface (11). Therefore, the surface modification of InP is expected due to electronic energy deposition.

Experimental details

Single-crystalline (001) n-type InP substrates were irradiated with 100 MeV56 Fe7+ ions at fluences ranging from 5 × 1012 to 2 × 1014 cm−2 at room temperature, using the 15 UD Pelletron facilities at Inter University Accelerator Centre (IUAC), New Delhi. Ion beam scanning was used to irradiate the whole sample surface in a uniform way. In order to prevent the heating during irradiation, the beam current was held at 3–4 pnA (particle nano-ampere) and the sample was mounted on the copper target ladder with silver paste giving a good thermal conduction between them. The Raman scattering measurements were performed at room temperature on the LabRAM HR Raman spectrometer using the 514.5 nm line of an argon ion laser in the back scattering geometry. Fourier transform infrared absorption and reflection spectra for non-irradiated and irradiated samples were recorded in the spectral region 6000–400 cm−1 using the Fourier transforms infrared spectrometer (Jasco FTIR-610). The far infrared Fourier transform reflection spectra were recorded on NICOLET MEGANA – 550 in the spectral range 400–50 cm−1 . The surface morphology of non-irradiated and irradiated samples was examined using atomic force microscopy from Digital Instruments (Nanoscope-III) with a silicon nitride tip under ambient conditions in the tapping mode.

3.

Results and discussion

Figure 1 shows the experimental and phonon-confinement model (PCM) fitted firstorder Raman spectra of non-irradiated InP and samples irradiated for ion fluences 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 . The spectrum of non-irradiated sample

-1

344.28 cm

Intensity (arb. units)


2.

(a)

Non-irradiated, L = •

(b)

1 x 10 cm , L= 45 nm

13

-2

13

-2

14

-2

14

-2

(c)

5 x 10 cm , L= 41 nm

(d)

1 x 10 cm , L= 36 nm

(e)

2 x 10 cm , L= 33 nm

320

340

360

380

400

-1

Wavenumbers (cm ) Figure 1. First-order LO Raman mode of 56 Fe7+ ion-irradiated InP at various ion fluences fitted with PCM as described by Equation (2): •, experimental data; –, PCM fit to data. L (nm) is the phonon coherence length as determined by the fit to the data.


Radiation Effects & Defects in Solids

3

shows a strong peak at 344.28 cm−1 , which corresponds to characteristic Longitudinal Optical (LO) Raman modes of crystalline InP. The LO Raman peak is observed at 344.17, 343.69, 342.48 and 342.35 cm−1 for ion fluences 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 , respectively. It is clear from Figure 1 that the intensity of Raman peak decreased and the peak position was shifted toward the lower wave number as compared with the Raman spectra of the non-irradiated InP sample. The shifts in the peak positions of Raman spectra can be affected by the residual stress as well as by phonon confinement. The contributions due to two effects can, however, be deconvoluted (12). The spatial correlation model related to k-vector relaxation-induced damage shows that, when disorder is introduced into the crystal lattice by irradiation, the correlation function of the phonon vibrational modes become finite due to the induced defects and the momentum k = 0 selection rule is relaxed (13). Consequently, the phonon modes shifts toward lower frequencies and broaden asymmetrically as the ion fluence is increased (14). The stress (σ ) induced in the irradiated samples was determined from the shift of the Raman LO peak using the following relation (15): σ (MPa) = −250ω (cm−1 ),

(1)

where ω = ωs − ω0 . In this expression, ω0 and ωs denote the wave numbers of the LO peak in the non-irradiated and irradiated samples, respectively. The irradiation-induced stress was found to be 27.2, 145.5, 450.0 and 482.5 MPa for the ion fluences of 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 , respectively. The increase in irradiation-induced stress with the ion fluence is due to the increase in disorder and defects in the surface region of irradiated InP samples. The PCM developed by Richter et al. (16) was used to evaluate the phonon coherence length or the average size of crystallites.Assuming a constant coherence length (L) in the scattering volume, the intensity of the first-order Raman mode in the scattering volume is given by the following relation:

2π/a0

I(ω) = 0

exp(−q2 L 2 /16π 2 )4π q2 dq , [ω − ω (q)]2 + (0 2)2

(2)

where ω (q), a0 and 0 are the phonon dispersion (17), lattice constant of InP and Raman intrinsic line width of crystalline InP, respectively. By fitting the experimental Raman LO mode with PCM, we have obtained the phonon coherence length (Table 1), L, of the irradiated samples. The Raman LO mode of the non-irradiated InP sample is symmetric with an infinite phonon coherence length due to the long-range periodicity of the InP lattice. After irradiation, asymmetry in the LO Raman mode increased due to the presence of disorder in the irradiated sample. The phonon coherence length of 45 nm is obtained for ion fluence 1 × 1013 cm−2 , indicating the presence of crystallite with an average size of 45 nm. The average crystallite size is reduced to 33 nm for the highest ion fluence 2 × 1014 cm−2 (Table 1). In a similar study of 65 MeV oxygen ion-irradiated Si carried by Bogle et al. (18), the phonon coherence length was 85 and 10 nm for the ion fluences of 1 × 1013 and 1.5 × 1014 cm−2 , respectively. For the GaP nanowires of diameter ∼20 nm, the LO phonon mode was downshifted by ∼4 cm−1 with respect to its position in bulk (19). In the present study, the ion fluence of 2 × 1014 cm−2 leads to an average crystallite size of 33 nm and downshift of the LO phonon mode by ∼2.3 cm−1 . Thus, PCM fitting suggests the presence of nanometer-sized undamaged crystalline regions in the as-irradiated InP. The ratios of the area under the Raman Table 1.

Parameter related to surface modification of InP obtained from the Raman spectra.

Ion fluence (cm−2 ) AI /A0 Coherence length (nm)

Non-irradiated

1 × 1013

5 × 1013

1 × 1014

2 × 1014

1 ∞

0.72 45

0.70 41

0.44 36

0.43 33

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R.L. Dubey et al.

Reflectance (a.u.)

(f) (e) (d) (c) (b) (a)


1600 1400 1200 1000 800 600 -1 Wavenumber (cm )

400

Figure 2. FTIR reflectance spectra of InP samples: (a) non-irradiated and irradiated with 100 MeV 56 Fe7+ ions for ion fluences (b) 5 × 1012 , (c) 1 × 1013 , (d) 5 × 1013 , (e) 1 × 1014 and (f) 2 × 1014 cm−2 .

peaks of non-irradiated sample (A0 ) and irradiated samples (AI ) are given in Table 1. It can be seen from Table 1 that the ratio of area (AI /A0 ) decreases with increase in ion fluence. This indicates the decrease in the degree of crystallinity of the InP surfaces after irradiation. Figure 2 shows the Fourier transform reflectance spectra of non-irradiated InP and the samples irradiated with various ion fluences. The spectrum recorded for ion fluence 5 × 1012 cm−2 (Figure 2(b)) exhibits very weak interference fringes in the spectral region 1600–400 cm−1 , indicating that the refractive index of the irradiated layer is almost that of the underlying InP substrate. For ion fluence 1 × 1013 cm−2 more pronounced interference fringes appeared (Figure 2(c)). Furthermore, with increase in ion fluence the amplitude increases and the position of the maxima and minima shifts (Figure 2(c– f)). The weak interference fringes seen in the reflectance spectra of 5 × 1012 cm−2 suggest that the sample irradiated for this ion fluence mainly contains point defects and defect complexes. However, these defect complexes overlap and form an amorphous layer at higher ion fluences. This follows that the refractive index of the irradiated layer increases with increase in ion fluence as the thickness of the irradiated layer is nearly constant with ion fluence (20). The refractive index of the irradiated layer was estimated from the fringe shift using the following equation: n=

1 , 2d(υ1 − υ2 )

(3)

where n is refractive index, d is the thickness of the irradiated layer and υ1 , υ2 are the wave number of successive maxima or minima of the interference fringes. The refractive index of InP irradiated with ion fluences of 5 × 1012 , 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 was found to be 3.32, 3.37, 3.43, 3.51 and 3.64, respectively. Whereas refractive index of the non-irradiated InP calculated at 1000 cm−1 was 2.79. Figure 3 show the representative far infrared Fourier transform reflectivity spectra of the non-irradiated and sample irradiated with 100 MeV 56 Fe7+ ions. It shows the occurrence of plasma resonance at 318 cm−1 (Figure 3(a)) and plasma edges nearer to the Transverse Optical (TO) and LO modes at 307 and 345 cm−1 , respectively. The plasma edges are found to be shifted toward the lower frequency region with increase in ion fluence (Figure 3(b)–(d)). The shift of plasma edges indicates the reduction of carrier concentration in the sample as ion fluence progresses. Figure 4 shows the representative Fourier transform infrared absorbance spectra of non-irradiated and samples irradiated with ion fluences of 1 × 1014 and 2 × 1014 cm−2 . The spectrum of non-irradiated sample shows two absorption peaks at 2336 and 2360 cm−1 (Figure 4(a)). The samples irradiated up to ion fluence 5 × 1013 cm−2 showed similar results as that of non-irradiated InP with increased absorption. The spectra of 1 × 1014 cm−2 showed two very sharp peaks at 2339 and 2358 cm−1 (Figure 4(b)) corresponding to energy

5

levels 0.290 and 0.293 eV, respectively. However, in the spectra of 2 × 1014 cm−2 four absorption peaks at 2328, 2345, 2357 and 2366 cm−1 (Figure 4(c)) corresponding to energy levels 0.289, 0.291, 0.293 and 0.294 eV appeared. Iron acts as a deep accepter in InP and thus compensates the background donor concentration at lower ion fluences and at higher fluences the InP:Fe layer may be formed. The two-dimensional atomic force microscopy images (Figure 1 of Ref. 7) have been used for the power spectral density (PSD) analysis. The PSD curves of 100 MeV Fe7+ ionirradiated InP surfaces for the fluence ranging from 5 × 1012 to 2 × 1014 ions cm−2 are shown in Figure 5. The PSD spectra shown in Figure 5 can be divided into two distinct regions (i) increasing intensity of PSD function with plateau in the lower spatial frequency regions (q < q0 ) and (ii) decreasing intensity of PSD in the higher spatial frequency regions (q > q0 ). The plateau height in the lower spatial frequency regions indicates the roughening of ion-irradiated surfaces. The value of plateau height extracted from Figure 5 was found to be 1.0 × 105 , 1.7 × 106 , 1.6 × 106 , 4.0 × 105 and 6.4 × 106 nm4 for ion fluences 5 × 1012 , 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 , respectively. However, the plateau height of non-irradiated sample was 2.0 × 102 nm4 . The initial increase in plateau height is due to swift heavy-ion-induced mass transport phenomena through atomic displacements leading to the formation of nanostructure on the surface of irradiated InP. A continuous swift heavy ion bombardment at higher fluence (1 × 1014 cm−2 ) would lead to overlapping of the nanostructures giving rise to the surface smoothening effect. However, further ion bombardment would again result in the formation of new structures on the smoothen

0.8 0.7

Reflectivity

0.6

(a)

0.5 0.4

(b)

0.3

(c)

0.2

(d)

0.1 0.0 200

250

300 350 400 45 0 Wavenumbre (cm–1)

500

Figure 3. Far infrared reflectance spectra of InP sample: (a) non-irradiated and irradiated with 100 MeV 56 Fe7+ ions for ion fluences (b) 5 × 1012 , (c) 1 × 1013 and (d) 2 × 1014 cm−2 .

Absorbance


Radiation Effects & Defects in Solids

1.40 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 0.90

1 2 3

4

1 2

(c) (b)

(a) 2220 2280 2340 2400 2460 2520 2580 Wavenumber (cm–1)

Figure 4. FTIR absorbance spectra of InP samples: (a) non-irradiated and irradiated with 100 MeV 56 Fe7+ ions at the fluences (b) 1 × 1014 and (c) 2 × 1014 cm−2 .

R.L. Dubey et al.


Power spectral density g (nm4)

6 8

10

Virgin 12 5x10 13 1x10 13 5x10 14 1x10 14 2x10

7

10

6

10

5

10

4

10

3

10

2

10

1

10

0

10 -1 10

0

1

2

10 10 10 Spatial frequency q (mm–1)

Figure 5. Plot of PSD function vs. spatial frequency of 100 MeV 56 Fe7+ ion-irradiated InP surfaces at room temperature showing the dependence on the ion fluences.

surface (8). For higher spatial frequency region, PSD curve displays power-law dependence (21). γ = Aq−p .

(4)

The slope (p) of PSD function for higher spatial frequency region (q > q0 ) has been estimated by the fitting equation (4). The value of p was found to be 3.25, 3.43, 3.52, 2.77 and 2.99 for ion fluences 5 × 1012 , 1 × 1013 , 5 × 1013 , 1 × 1014 and 2 × 1014 cm−2 , respectively. The q−4 behavior, characteristic of surface diffusion, was observed up to ion fluences 5 × 1013 cm−2 , whereas q−2.77 and q−2.99 behavior for higher ion fluences 1 × 1014 and 2 × 1014 cm−2 may be due to stochastic roughening processes (22).

4.

Conclusions

The irradiation of n-type InP samples with 100 MeV 56 Fe7+ ions was carried out for various ion fluences ranging from 5 × 1012 to 2 × 1014 cm−2 . The shift in the Raman peak with the ion fluence showed the presence of stress after irradiation. The average crystallite size obtained by analyzing Raman spectra with PCM was found to decrease with increase in ion fluence. FTIR reflectivity revealed the increase in the irradiation-induced defects and disorders in the surface region of InP after irradiation. PSD analysis of Atomic Force Microscopy images showed surface diffusion process dominant up to ion fluence 5 × 1013 cm−2 ; however, at higher ion fluences 1 × 1014 and 2 × 1014 cm−2 surface smoothening may be due to stochastic roughening processes. Acknowledgements The authors are thankful for the financial support received from IUAC, New Delhi, under the UFUP-35320 scheme and to the members of the Pelletron group for providing iron ion beam.

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Radiation Effects & Defects in Solids (3) (4) (5) (6) (7) (8) (9)


(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)

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