Nuclear Instruments and Methods in Physics Research B 212 (2003) 473–476 www.elsevier.com/locate/nimb
Ion beam induced modification of lattice strains in In0:1Ga0:9As/GaAs system S.V.S. Nageswara Rao a, A.K. Rajam a, Azher M. Siddiqui b, D.K. Avasthi b, T. Srinivasan c, Umesh Tiwari c, S.K. Mehta c, R. Muralidharan c, R.K. Jain c, Anand P. Pathak a,* a
School of Physics, University of Hyderabad, Central University (P.O.), Hyderabad 500 046, Andhra Pradesh, India b Nuclear Science Centre, Post Box No. 10502, Aruna Asaf Ali marg, New Delhi 110067, India c Solid State Physics Laboratory, Timarpur, Delhi 110 054, India
Abstract The effects of 150 MeV Ag ion irradiation on the molecular beam epitaxially grown In0:1 Ga0:9 As/GaAs samples have been studied using high resolution X-ray diffraction (HRXRD). Our earlier experiments suggest that the compressive strain will decrease due to ion beam mixing effects in an initially strained system. Similarly a tensile strain will be induced in an initially lattice matched system. These studies are being performed to explore the possibility of spatial bandgap tuning for the integration of optoelectronics circuits. Here we present a systematic study to understand the effects of ion fluence and the initial strain/layer thickness on the modification of strain. Strain measurements are performed using HRXRD by determining the angular shift in the layer peak position with respect to that of the substrate peak. Substrate peak broadens with ion fluence due to the implantation effects at the end of the ion range (13 lm). However, the thin InGaAs layer and the substrate region near to this layer will not be affected. It is shown that the Swift Heavy Ion induced mixing can alter the lattice strain at room temperature without loss of the quality of the structure. 2003 Elsevier B.V. All rights reserved. Keywords: SLS; Strain; RBS; Channeling; Ion beam mixing; Bandgap tuning
1. Introduction In recent years, the intermixing of III–V semiconductor quantum wells and superlattices has been investigated extensively. In this paper, we discuss the study of effects of ion beams on strained layer superlattice (SLS). Basically a su-
*
Corresponding author. Tel.: +91-40-23010181; fax: +91-4023010181/23010227/23010120. E-mail address:
[email protected] (A.P. Pathak).
perlattice [1,2] is a periodic array of lattices one upon the other with different composition and hence different lattice parameter. If we grow an epilayer with different lattice constant on the substrate, because of the fact that the atoms of the epilayer will have a tendency to sit on the places where the substrate atoms are sitting, it will lead to a tensile or compressive strain depending on whether the lattice constant of the epitaxial material is larger or smaller than that of the substrate material. If this lattice mismatch is so high that it cannot withstand the strain then the structure will
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01458-7
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get relaxed into a dislocated structure. Another factor which also may lead to the dislocations in the material is the thickness of the layers. It is observed that the critical thickness [1] for an In 0:1 Ga0:9 As/GaAs layer is around 300 A. The effect of strain on the bandgap is most interesting aspect of SLS in both basic as well as applied research view points. The basic need of studying these materials is to make materials with tunable bandgap for the integration in optoelectronic devices. We have grown thin In0:1 Ga0:9 As layer of different thicknesses on GaAs substrate using molecular beam epitaxy (MBE) technique. Because of the lattice mismatch between InGaAs and GaAs, the strain along various symmetry directions is developed in this structure. As a result, the symmetry points and directions of the crystal are modified and hence will lead to a change in the bandgap of the material along those directions. Extensive amount of experimental and theoretical work [3–5] has been done for monitoring and controlling these effects. The ion-beam mixing technique has been extensively used to modify the strain in the superlattices. Low energy ion beams have been used to introduce strain in the lattice matched systems. But the Swift Heavy Ion (SHI) beam mixing is more suitable because we can confine the interface mixing to a narrow region. Here, the energy of the SHI is transferred to the lattice in an inelastic fashion. The ‘‘Thermal Spike Model’’ gives a reasonable explanation to this energy transfer mechanism. This model assumes that the inelastic loss of energy produces very high temperature, which exists for a very short time which in turn is responsible for the diffusion in the local molten state of the material. The material can not
come back to its equilibrium state because of rapid quenching. Another advantage of this high energy irradiation is, there will not be any damage in the epitaxial layer as the energetic ion will directly go and sit in the substrate. Damage in the substrate region can be annealed out by rapid thermal annealing. Here we have different In0:1 Ga0:9 As/GaAs strained layer structures with different thicknesses which have been irradiated and characterized by high resolution X-ray diffraction (HRXRD).
2. Experimental We have grown different In0:1 Ga0:9 As/GaAs strained single layers using MBE at SSPL, Delhi. Table 1 Details of samples and their ID Sample ID
Specification
Growth technique
Source
4201
)/GaAs In0:1 Ga0:9 As(100 A
MBE
2601 4101
In0:1 Ga0:9 As(250 In0:1 Ga0:9 As(400
SSPL, Delhi ’’ ’’
210 – – – 450 –
6.2 7.5 7.4 7.2 7.1 7.5 6.7
’’ ’’
Table 2 Details of irradiations Sample ID
Irradiation details Energy (MeV)
Fluence (ions/cm2 )
4201 2601
150 150
4101
150
1 · 1013 5 · 1012 , 1 · 1013 , 2 · 1013 1 · 1013
Table 3 Details of layer thickness, In content and strain percentage ) Sample Layer thickness (A Indium content (x)% 4201 (100 A) 100 7.2 4201I (1 · 1013 ) ) 2601 (250 A 2601512 2601113 2601213 ) 4101 (400 A 4101I (1 · 1013 )
)/GaAs A )/GaAs A
New name
4201113 2601512, 2601113, 2601213 4101113
e%
De
De%
0.9769 0.8418 1.0174 1.0039 0.9904 0.9634 1.0174 0.9094
0.1351
13.82
0.0135
1.3269
0.027 0.0538 0.108
2.6538 5.2879 10.615
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Table 1 lists details of the samples along with their identifications (ID). The samples have been irradiated by 150 MeV Ag ion beam delivered from
475
15 MV Pelletron Accelerator at NSC, Delhi. The irradiation details are given in Table 2. The strain analysis of these samples has been performed with HRXRD at SSPL, Delhi. The XRD data has been fitted using MADMAX simulation software.
3. Results and discussion The measured values of thicknesses and compositions were observed different from the nominal values. A detailed information is given in Table 3. Fig. 1 shows the HRXRD spectra of 4101U (unirradiated) and 4101I (irradiated). It shows a clear shift in layer peak which directly indicates the decrease in the strain (compressive), while the broadening in the substrate peak indicates the damage in the substrate material which can be annealed out. This is in fact the implantation damage at the end of the energetic particle range
0
0
-1
-1
REFLECTIVITY
REFLECTIVITY
Fig. 1. HRXRD spectra of samples 4101U and 4101I.
-2 -3 -4 -5
-0.5
(a)
-4
0 ANGLE (deg)
0.5
1
-6 -0.8
0
0
-1
-1
-2 -3
-5
-5
-0.4
-0.2 0 0.2 ANGLE (deg)
0.4
0.6
-0.2 0 0.2 ANGLE (deg)
0.4
0.6
0.8
-0.6
-0.4
-0.2 0 0.2 ANGLE (deg)
0.4
0.6
0.8
-3 -4
-0.6
-0.4
-2
-4
-6 -0.8
-0.6
(b)
REFLECTIVITY
REFLECTIVITY
-3
-5
-6 -1
(c)
-2
-6 -0.8
0.8
(d)
Fig. 2. (a)–(d) MADMAX–HRXD fittings (2601U, 2601512, 2601113 and 2601213, respectively).
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ence. The dependence of thickness on the ionbeam fluence is clearly shown in the Table 3. 4. Conclusions The ion mixing behavior of semiconductor strained structures has been studied. It has been shown that SHI will alter the strain in the strained lattices. We have found that the epitaxial layer will not be damaged during the swift ion irradiation. The gradual diffusion of In towards the substrate results the decrease in the In composition with the increase in ion fluence. It is shown that the strain is modified during the ion irradiation and is decreasing with the increase in ion fluence. Hence it implies that high energy ion irradiation will tune the strain without destroying the quality of the material. Acknowledgements Fig. 3. (a) A plot of In composition as a function of ion beam fluence for 2601 sample, (b) a plot of compressive strain as a function of ion beam fluence for 2601 sample.
(13 lm). For the sample 2601 which is irradiated by different fluences, it is observed that the strain is decreasing with the increase in ion fluence. Refer Figs. 2(a)–(d) which show the MADMAX– HRXRD fittings of 2601. Fig. 3(a) shows the plots of the In composition as a function of ion fluence. It shows that the In composition decreases with the ion fluence. It is due to the fact that the increase in ion beam irradiation will lead to excess diffusion of In into the substrate region. Fig. 3(b) shows the plot of the strain (compressive) versus the ion fluence, which tells us the compressive strain will decrease with the increase in beam flu-
A.P.P. and A.K.R. thank the DRDO New Delhi and IUC DAEF Kolkata for providing a research project. S.V.S.N.R. thanks CSIR for providing SRF. References [1] S.T. Picraux, B.L. Doyle, J.Y. Tsao, in: T.P. Pearsall (Ed.), Semiconductors and Semimetals, Strained-layer Superlattices: Materials Science and Technology, Vol. 33, Academic Press, New York, 1991. [2] G.C. Osbourn, J. Appl. Phys. 53 (1982) 1586. [3] S. Charabonneu, P.J. Poole, P.G. Pive, M. Buchanan, R.D. Goldberg, I.V. Mitchell, Nucl. Instr. and Meth. B 106 (1995) 457. [4] J.W. Wan, D.A. Thompson, J.G. Simmons, Nucl. Instr. and Meth. B 106 (1995) 461. [5] W. Xia, S.N. Hsu, C.C. Han, S.A. Papert, B. Zhu, C. Cozzolino, P.K.L. Yu, S.S. Lau, Nucl. Instr. and Meth. B 59–60 (1991) 491.
