Ion beam modification studies of InP based multi quantum wells

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 1810–1815 www.elsevier.com/locate/nimb

Ion beam modification studies of InP based multi quantum wells S. Dhamodaran a,*, G. Devaraju a, A.P. Pathak a, A. Turos b,c, D.K. Avasthi d, R. Kesavamoorthy e, B.M. Arora f a

School of Physics, University of Hyderabad, Central University (P.O.), Hyderabad 500 046, India b Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland c Soltan Institute of Nuclear Studies, 05-400 Swierk/Otwock, Poland d Inter University Accelerator Centre, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110 067, India e Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 602 103, India f Tata Institute of Fundamental Research, Homibhabha Road, Colaba, Mumbai 400 005, India Received 15 October 2007; received in revised form 6 December 2007 Available online 12 January 2008

Abstract Effects of heavy ion irradiation on InP based structures have been discussed. In specific, recent results on heavy ion irradiation of InP and InGaAs/InP multi quantum wells have been studied as a function of incident ion fluence and electronic energy loss. Studies involve photoluminescence (PL), high resolution X-ray diffraction (HRXRD), raman spectroscopy (RS) and atomic force microscopy (AFM). The heavy ion induced modifications in InP have been analyzed and damage creation has been correlated with a possible mechanisms. Multi quantum wells were investigated before and after irradiation and band gap engineering has been demonstrated using swift heavy ions for the first time. Band gap shift as a function of incident ion fluence has been observed and a maximum shift of 82 nm for a fluence of 1  1013 ions/cm2 has been reported. Possible applications in optoelectronic devices are highlighted. Ó 2008 Elsevier B.V. All rights reserved. PACS: 68.65.–k; 78.55.–m; 61.72.Dd; 68.37.Ps; 61.80.Jh Keywords: Quantum wells; Photoluminescence; HRXRD; AFM; Irradiation

1. Introduction InP and InxGa1 xAs/InP are indispensable materials used in longer wavelength optical communications [1,2]. Ion beam processing of semiconductors is essential for device applications. In specific quantum well intermixing (QWI) is of interest from the band structure modification point of view. Ion implantation is considered to be advantageous for QWI among the other available techniques [3,4]. A range of techniques have been demonstrated to generate intermixing at the interfaces, which include, impurity induced interdiffusion [5], disordering using various dielectric cap layers [6], ion implantation [7], focused ion

*

Corresponding author. E-mail address: [email protected] (S. Dhamodaran).

0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.106

beam induced intermixing [8],. . . etc. High energy ion beam (energy > 1 MeV/nucleon) induced band gap engineering of semiconductor quantum wells has not been reported to our knowledge. Due to the negligible lateral straggling and long range of the high energy ions it will be useful to modify multi quantum wells [9]. We report modifications induced by high energy ions and compare with low energy work available in literature. For the present case the electronic energy loss to the nuclear energy loss ratio used is 40 and 250 and the choice of the ions Ag and Au. The choice of Ag and Au ions is based on previous report that heavy ions are useful in creating electronic energy loss induced modifications of bulk InP [10]. The choice of energies is from the calculations which indicate that the electronic energy loss is constant in the entire thickness of the multi quantum wells and hence the modifications would be uniform. The characterization techniques involve

S. Dhamodaran et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1810–1815

photoluminescence (PL), high resolution XRD (HRXRD), Raman spectroscopy and atomic force microscopy (AFM) which are chiefly used to characterize semiconductors. The structural modification of bulk, InP and band structure, interface and surface modifications of InGaAs/InP multi quantum wells have been investigated. 2. Experimental details The InP samples were commercially purchased and the InGaAs MQW samples were grown by metal-organic chemical vapor deposition (MOCVD) on (0 0 1) oriented semi-insulating InP substrates. The samples used in the present work are listed in 1 with the names or identifications (IDs). The irradiation was performed at room temperature as given in Table 1. The ion beam was magnetically scanned over a 1  1 cm2 area on the sample surface for uniform irradiation. Photoluminescence studies were carried out at room temperature (295 K) as well as at low temperature (18 K). PL was excited either with Ar ion laser (488 nm) or with YAG laser (532 nm) and detected with a LN2-cooled InAs detector after dispersing with 2/3 m McPherson monochromator. HRXRD experiments have been performed using the Philips X’Pert system with a Cu Ka radiation. Profiles of symmetric and asymmetric reflections were recorded in x/2h scans after optimizing the tilt and azimuthal angles. The samples were subjected to rapid thermal annealing (RTA) in order to minimize the irradiation induced damages. As-grown and high energy irradiated InGaAs/InP multi quantum wells were annealed at 700° C for 60 s in N2 (with a flow of 1000 SCCM) atmosphere using RXV6 RTP system (AET Thermal Inc.). For the M-I samples, polished InP wafers were used for proximity capping and the MQWs were placed upside down during annealing. For the M3-I samples plasma enhanced chemical vapor deposition (PECVD) silicon nitride (Si3N4) cap were used, which was etched using buffered-HF after annealing.

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3. Results and discussion 3.1. InP wafers The HRXRD (0 0 2) scans of In-U, In-I1, In-I2 and InI3 samples are given in Fig. 1(a) and (b). The In-I1 and InI2 (Ag ion irradiated) samples indicate no damage peak whereas, the In-I3 (Au ion irradiated) sample shows damage peak(s) at the lower angle side of the substrate peak. Irradiation of InP wafers reported in literature have shown damage peak at the higher angle side of the substrate peak [11,12]. The damage peaks were deconvoluted and the strain values with respect to the substrate peak calculated to be 0.34% and 0.721%. The possible mechanism of damage by Au ions would be ‘P’ loss from the InP lattice. Preferential loss of ‘P’ induces local compressive strain due to ‘In’ rich conditions giving rise to peaks at the lower angle side. Recent report on 180 MeV Au ion irradiated InP sample characterized using extended X-ray absorption finestructure (EXAFS) indicates the presence of chemical disorder in the form of In–In bonding apart from In–P bonding [13]. This supports our assumption of ‘P’ loss and ‘In’ rich lattice of the damaged region. In spite of the high electronic energy loss no damage peak has been observed for Ag ion irradiated samples. It is clear that heavy ions like Au create high lattice damage as seen by HRXRD but intermediate mass like Ag (of energy as high as 200 MeV) does not affect the X-ray diffractogram up to a fluence of 1  1013 ions/cm2. The above conjecture also highlights that each single heavy ion create damages where as in the case of intermediate or low mass ions the damage is a collective effect of the fluence used. 4. Multi quantum wells 4.1. Photoluminescence The irradiated MQW samples did not show any luminescence until after annealing. This would be due to the

Table 1 Sample name/IDs and description Sample name/IDs

Description

In-U In-I1 In-I2 In-I3 M-U M-I1 M-I2 M-I3 M-U-A7 M-I1-A7 M3-U M3-I1 M3-I2 M3-I1A M3-I2A

High purity undoped InP wafer of 500 lm thick InP-U sample irradiated using 100 MeV Ag8+ ions with a fluence of 1  1013 ions/cm2 InP-U sample irradiated using 200 MeV Ag13+ ions with a fluence of 1  1013 ions/cm2 InP-U sample irradiated using 200 MeV Au12+ ions with a fluence of 7  1012 ions/cm2 As-grown InP(20 nm)/In0.55Ga0.45As (20 nm)/InP(0 0 1) MQW of 15 periods M-U sample irradiated using 150 MeV Ag12+ ions with a fluence of 1  1013 ions/cm2 M-U sample irradiated using 200 MeV Au13+ ions with a fluence of 7  1012 ions/cm2 M-U sample irradiated using 100 MeV Ag8+ ions with a fluence of 1  1013 ions/cm2 M-U sample subjected to RTA at 700° C M-I1 sample subjected to RTA at 700° C As-grown InP(20 nm)/In0.23Ga0.77As (5 nm)/InP(0 0 1) MQW of 15 periods M3-U irradiated using 100 MeV Au ions with a fluence of 5  1012 ions/cm2 M3-U irradiated using 100 MeV Au ions with a fluence of 1  1013 ions/cm2 M3-I1 sample subjected to RTA at 700° C M3-I2 sample subjected to RTA at 700° C

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a

M-U

@ 18K

Intensity (arb.units)

Intensity (arb.units)

M-I1-A7

Unirradiated

100 MeV Ag

200 MeV Ag

x 10 -800

-600

-400

-200

0

200

400

600

1400

1450

Omega (arcsec)

1500

1550

1600

1650

Wavelength (nm) Fig. 2. LT-PL spectra of M-U and M-I1-A7 samples.

b

200 MeV Au

Intensity (arb.units)

M3-U M3-I1A (x 15) M3-I2A (x100) Gauss fit of M3-I2A

Intensity (Arb.Units)

Damage peaks

-800

-600

-400

-200

0

200

400

600

Omega (arcsec) 1100

Fig. 1. HRXRD (0 0 2) scans of (a) In-U, In-I1 and In-I2 and (b) In-I3.

1150

1200

1250

1300

1350

Wavelength (nm) Fig. 3. LT-PL spectra of M3-U and M3-I1A and M3-I2A samples.

presence of large density of non-radiative recombination centers associated with the damage produced during irradiation. The low temperature photoluminescence (LT-PL) spectra of M-U and M-I1-A7 (as grown and irradiated and annealed lattice matched) samples are given in Fig. 2. The peak position of M-I1-A7 shifted to lower wavelength of about 23 nm with respect to M-U sample. This is a clear indication of band gap change after irradiation. The broad spectrum is an undesirable one, but a slight colour change at the surface for M-I1-A7 sample was observed. This is probably due to InP proximity capping which is also responsible for broad PL spectra. Optimizing the annealing conditions would help in getting intense and narrow PL spectra. Fig. 3 shows the LT-PL spectra of M3U and M3-I1A and M3-I2A (as grown and irradiated and subsequently annealed tensile strained) samples. It is clear

from the spectra that the PL peak shifts to lower wavelength side as a function of incident ion fluence. The band gap increase has been attributed to the interfacial mixing induced higher tensile strain in the layers. It is also to be noted that the luminescence intensity and peak width of M3-I1A is good compared with the M3-U. The peak intensity of M3-I2A has two Gaussian profiles indicating residual defects and annealing conditions are to be optimized. Yet the shift clearly indicates the possibility of band gap engineering as a function of incident ion fluence. Band gap engineering in similar samples using low energy ions and/or annealing have been demonstrated by few groups [14–17]. Band gap modification from as low as 20 nm [14] to as high as 100 nm [15], depending on experimental con-

S. Dhamodaran et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1810–1815

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ditions, has been demonstrated using low energy ion beams. In comparison, present study demonstrates band gap engineering using swift heavy ions as high as 82 nm for the first time. Band gap engineering using low energy ions is achieved by introducing point defects into the active region, which allows atomic diffusion to take place between the layers. In the present case the band gap modification has been reported using swift heavy ions where the electronic energy loss is 15 keV/nm. Hence, the band gap engineering may be attributed solely to the electronic energy loss induced modifications [9]. Such studies are useful for spatial band gap engineering of optoelectronic devices in a controlled manner.

Such results are comparable and supportive with the present studies. Fig. 5 shows the (0 0 4) scans of M-U, M-I1 and M-I2 samples. The broad satellite peaks for M-I2 samples indicates huge degradation of the interface. The strain value was less for M-I2 sample as compared with M-I1. The effect of electronic energy loss and fluence are to be explored in detail. The strain values calculated from HRXRD for M3-U, M3-I1A and M3-I2A samples indicate strain modification upon irradiation and subsequent annealing. The diffusion of atomic species across the interface leads to such strain modifications. This underlines that the band gap modifications depend strongly on the interfacial modifications.

4.2. High resolution XRD

4.3. Raman spectroscopy

Fig. 4 shows the (0 0 4) scans of M-U, M-I1 and M-I3 (as grown and Ag ion irradiated lattice matched) samples where the peak shift is clearly visible while the substrate peak for all the samples match. This peak shift is an indication of compressive strain induced in the samples upon irradiation. Intense and ordered peaks were observed for M-I1 and M-I3 samples also but with slight broadening of the satellite peaks. The samples were subjected to RTA to reduce the point defects created due to irradiation. Upon annealing the interface mixing induced disorder is observed from the vanishing of satellite peaks. The M-I1A7 sample shows a huge reduction in the intensity and broadening of the satellite peaks. The average compressive strain of 0.2041% in M-I1 sample has been reduced to 0.1509% upon annealing at 700° C as compared with MU. This implies that annealing results in remixing by preferential diffusion of elements to maintain the lattice parameter of the layer matched to the substrate. Preferential diffusion and mixing by maintaining the lattice parameter close to that of the substrate has been reported [18,19].

Fig. 6 shows the Raman spectra of M-U, M-I1 and M-I2 samples. There exist two regions of interest, (i) 200– 300 cm 1 and (ii) 300–400 cm 1. First region corresponds to the InAs and GaAs while the second corresponds to InP and GaP regions. The modes observed around 230 cm 1 belong to InAs type and around 255 cm 1 to GaAs type modes. The modes observed around 350 cm-1 belong to InP and around 380 cm 1 to the GaP type modes. As seen in Fig. 6 the M-U sample shows broad bands corresponding to InAs, GaAs and InP regions and no modes around 380 cm 1 for GaP. This is a clear indication that no mixing has occurred in the as-grown sample. Raman spectra for M-U sample for the InAs and GaAs (200–300 cm 1) region is broad. The same was observed on several spots of the sample and the reason for the broadness is not clear at present. But a deconvoluted spectrum indicates the possibility of LO and TO mode overlapping. Peaks with FWHM higher than 3 cm 1 were only

M-U M-I3 M-I1

Intensity (Arb.Units)

Intensity (arb.units)

M-U

M-I1

M-I2

-500

-250

0

250

500

Omega (Arcsec) Fig. 4. (0 0 4) HRXRD scans of M-U, M-I1 and M-I3 samples.

-2000

-1500

-1000

-500

0

500

1000

1500

2000

Omega (arcsec) Fig. 5. (0 0 4) HRXRD scans of M-U, M-I1 and M-I2 samples.

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was fixed at 3 cm 1. The M-I1 sample shows a broad InAs mode with very low intensity. A sharp GaAs mode is observed for M-I1 which has been shifted to higher frequency in comparison with the M-U sample indicating a compressive strain in the layer. The InP type LO mode shifts to lower frequency as arsenic incorporates in to the InP layer. It is also evidenced from the InAsP mode developed around 305 cm 1 [20] along with the InP type TO mode around 320 cm 1. GaP mode observed around 380 cm 1 is a clear indication of mixing. In any case interfacial mixing as indicated by HRXRD results is confirmed by Raman. The GaP mode observed in irradiated samples and not in the as-grown one gives an additional proof apart from the peaks shifts of the Raman modes.

Intensity (arb.units)

M-I2

M-I1

M-U 200

220

240

260

280

300

320

340

360

380

400

-1

Raman Shift (cm ) Fig. 6. Raman Spectra of M-U, M-I1 and M-I2 samples.

considered since the slit width of the monochromator corresponding to the spectral line width in terms of FWHM

4.4. Atomic force microscopy In case of bulk semiconductors only a slight increase of surface roughness observed by AFM whereas, the change in roughness was quite high in the case of multilayers.

Fig. 7. Surface morphology of samples (a) M-U (b) M3-U (c) M-I1 and (d) M-I2.

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Fig. 7(a)–(d) shows the AFM images of M-U, M3-U, M-I1 and M-I2 samples, respectively and the roughness values are given in the Figure. Few spike/dot like features are seen in the irradiated and/or annealed samples which varies from 30 to 150 nm in size and 2–10 nm in height. Size and height of such dots increase with the increase electronic energy loss. Such features were not matching with the fluence indicating that the features are formed by defect clusters and not due to single ion impact. In the case of bulk samples such features were even very few in numbers. Certain cases it is also seen that these dots tend to align along specific directions (Fig. 7(c)). The origin of these may be due to the smaller thickness of the top InP layer where high electronic energy loss is deposited. Further studies are in progress for a detailed understanding. 5. Conclusion We have studied the structural modification of bulk semiconductors using HRXRD and band structure, interface and surface modifications of MQWs upon heavy ion irradiation and subsequent annealing. For InP Au ion indicates structural damage which indicates the role of ion mass to be crucial in such damage formation. The ‘P’ loss upon Au ion irradiation in InP is a possible mechanism of local strain in the damaged region. The irradiation induced band gap engineering has been demonstrated for the first time using swift heavy ions. HRXRD and Raman spectroscopy have been utilized to understand the interface modifications. AFM was useful in understanding the surface modifications and further analysis are being carried out for a better understanding. The mechanism for the observed modifications is possibly the enhanced diffusion of atomic species across the interface upon ion irradiation and annealing. Possible applications of such studies have been highlighted and further TEM measurements on these samples have been planned. Acknowledgements S.D. thanks CSIR, New Delhi for the award of Senior Research Fellowship. G.D. thanks IUAC for the fellowship through a UFUP project sanctioned to APP. This

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work has been partially supported by Indo Polish joint scientific and technological cooperation programme. We are thankful to UGC–SAP for providing funds to purchase the InP samples. References [1] J.C. Campbell, in: Chinlon Lin (Ed.), Optoelectronic Technology and Lightwave Communication Systems, Van Nostrand Reinhold, New York, 1989, p. 363. [2] Y.G. Wey, D.L. Crawford, K. Giboney, J.E. Bowers, M.J. Rodwell, P. Silvestre, M.J. Hafich, G.Y. Robinson, Appl. Phys. Lett 58 (1991) 2156. [3] P.G. Piva, P.J. Poole, M. Buchanan, G. Champion, I. Templeton, G.C. Aers, R. Williams, Z.R. Wasilewski, E.S. Koteles, S. Charbonneau, Appl. Phys. Lett. 65 (1994) 621. [4] L.V. Dao, M.B. Johnston, M. Gal, L. Fu, H.H. Tan, C. Jagadish, Appl. Phys. Lett. 73 (1998) 3408. [5] D.G. Deppe, N. Holonyak, J. Appl. Phys. 64 (1988) R93. [6] H.H. Tan, J.S. Williams, C. Jagadish, P.T. Burke, M. Gal, Mater. Res. Soc. Symp. Proc. 396 (1996) 823. [7] J.D. Ralston, A.L. Moretti, R.K. Jain, F.A. Chambers, Appl. Phys. Lett. 50 (1987) 1817. [8] J.P. Reithmaier, A. Forchel, IEEE J. Quant. Electron. 4 (1998) 595. [9] S. Dhamodaran, A.P. Pathak, A. Turos, G. Sai Saravanan, S.A. Khan, D. K. Avasthi, B.M. Arora, Nucl. Instr. and Meth. B, in press. [10] W. Wesch, A. Kamarou, E. Wendler, Nucl. Instr. and Meth. B 225 (2004) 111. [11] A. Turos, J. Gaca, M. Wojcik, L. Nowicki, R. Ratajczak, R. Groetzschel, F. Eichhorn, N. Schell, Nucl. Instr. and Meth. B 219– 220 (2004) 618. [12] R.L. Dubey, S.K. Dubey, A.D. Yadav, S.J. Gupta, S.D. Pandey, T.K. Gundu Rao, T. Mohanty, D. Kanjilal, Nucl. Instr. and Meth. B 257 (2007) 287. [13] C.S. Schnohr, P. Kluth, A.P. Byrne, G.J. Foran, M.C. Ridgway, Nucl. Instr. and Meth. B 257 (2007) 293. [14] D. Barba, B. Salem, D. Morris, V. Aimez, J. Beauvais, M. Chicoine, F. Schiettekatte, J. Appl. Phys. 98 (2005) 54904. [15] C. Carmody, H.H. Tan, C. Jagadish, J. Appl. Phys. 93 (2003) 4468. [16] H. Peyre, F. Alsina, J. Camassel, J. Pascual, R.W. Glew, J. Appl. Phys. 73 (1993) 3760. [17] O. Hulko, D.A. Thompson, J.A. Czaban, J.G. Simmons, Semicond. Sci. Technol. 21 (2006) 870. [18] S.J. Yu, H. Asahi, S. Emura, S. Gonda, K. Nakashima, J. Appl. Phys. 70 (1991) 204. [19] S.J. Yu, A.J. Takizawa, K. Asami, S. Emura, S. Gonda, H. Kubo, C. Hamaguchi, Y. Hirayama, J. Vac. Sci. Technol. B 9 (1991) 2683. [20] N.P. Kekelidze, G.P. Kekelidze, Z.D. Makharadze, J. Phys. Chem. Solids 34 (1973) 2117.