STRAIN AND DAMAGE MEASUREMENTS IN ION IMPLANTED AlxGa 1. _xas/gaas SUPERLATTICES

Transcription

1 STRAIN AND DAMAGE MEASUREMENTS IN ION IMPLANTED AlxGa 1 _xas/gaas SUPERLATTICES A. H. HAMDI*, J. L. TANDON**, T. VREELAND, JR!, AND M.-A. NICOLET* *California Institute of Tehnology, Pasadena, CA **Applied Solar Energy Corporation, City of Industry, CA ABSTRACT Strain measurements in Al Ga As/GaAs superlatties have been arried x 1 -x 15-2 out before and after Si ion implantation. For doses up to 5x1 em, no atomi intermixing of the sub layers i s observed by baksattering spetrometry. However, with x-ray roking urve measurements, signifiant hanges in the strain profiles are deteted for implantations with doses as low as 7xlo 12 m- 2 Interpretation of the roking urves suggests that low-dose implantations release strain in the AlxGa _xas sublayers. The strain profile 1 reovery of the implanted samples, upon annealing at 42 C, implies that the damage aused by implantation i s largely reversible. INTRODUCTION Superlatties omposed of thin alternating layers of different ompound semiondutors offer unique possibilities in the oneption and fabriation of novel optial and eletroni devies [1,2]. To exploit these strutures, loalized and ontrolled doping by ion implantation onstitutes a desirable proessing method. Reently, a few studies have explored ion implantation in AlAs/GaAs, InGaAs/GaAs and GaAsP/GaP superlatties [3-6]. The primary objetive of these studies is to demonstrate that superlatties an be doped by implantation without materially degrading the superlattie. Underlying these studies are the fundamental questions of the nature of the damage reated by an ion irradiation in a superlattie, and its evolution upon subsequent thermal annealing. Almost al l superlatties possess strain in one or both sublayers beause of the lattie mismath of the onstituents. By measuring this strain as a funtion of depth, and monitoring its evolution after implantation and annealing, aurate information on the umulative effets of defets on the superlattie an be obtained. In this paper, the tehnique of x-ray roking urves is used to make suh measurements. The hanges in the roking urves of AlxGa 1 _xas/gaas superlatties are measured as a funtion of the dose after Si ion implantations, and also upon thermal annealing. The urves are interpreted in terms of hanges in the strain after using the kinemati model of x-ray diffration [7]. The results help in understanding the onset of damage reated by ion implantation. Mat. Res. So. Symp. Pro. Vol Materials Researh Soiety

2 32 EXPERIMENTAL AlxGa 1 _xas/gaas (x =.88) strained-layer-superlattie (SLS) strutures were prepared by metal organi hemial vapor deposition. Alternating layers (1 eah) of AlxGa 1 _xas and GaAs were grown with thiknesses of 14 and 27 A respetively, on (1) GaAs substrates. Si ions were implanted at room temperature with an energy of 2 kev. To minimize hanneling during implantation, the inoming beam was oriented 7 wit respet to the sample's surfae normal The range of the ions is about 2 A with a range straggling of 9 A whih plaes the Si and the damage profiles fully within the superlattie. The implantation doses ranged from7x1 12 to 5x1o 15 Si/m 2. Thermal annealing was arried out, on a few samples, in a forming gas atmosphere (85% N % H 2 ) at 42 C for one hour. X-ray roking urve measurements were made in the (4) refletion, using a double-rystal diffratometer with FeKa. The x-ray beam was ollimated 1 and rendered nearly monohromati by a (4) refletion from a high quality (1) GaAs rystal (the first rystal). The beam spot was adjusted to.5 mm x 1 mm by slits plaed between the first rystal and the sample. The measured roking urves were fitted with alulated urves using Speriosu's model of kinematial theory of x-ray diffration in thin epitaxial layers [7]. In this model, the diffration by the substrate is treated dynamially. For fitting, the struture fators of Al _ 88 Ga_ 12 As (= 117.4) and GaAs (= 157.) were alulated using the tabulated atomi sattering fators [8]. Baksattering spetrometry measurements were also arried out using a 2 MeV He beam. All data refer to room temperature unless stated otherwise. RESULTS AND DISCUSSION Effet of Si Ion Implantation Figure 1 shows the measured (dashed line) and the alulated (solid line) roking urves for the as-grown AlxGa 1 _xas/gaas SLS struture. The angle is plotted relative to the Bragg angle (eb) of the substrate peak, Psub, at e =. The refleting power, plotted on the vertial axis, is normalized with respet to the intensity of the inoming x-ray beam. The periodiity in the struture fators and in the strain of the layers in the SLS generates the subsidiary peaks observed in the roking urves. The displaement of the peak P from Psub measures the average strain in the SLS. For symmetri refletion, as in this ase, the equa l separation between the subsidiary peaks measures the average thikness of one period in the SLS. The alulated urve in Fig. la was obtained using the strain distribution in Fig. lb. The details of the alulations are given elsewhere U,9]. From Fig. lb, the number

3 - ' Psub --- Me as (a) -Cal. 1. Po 1..2 Qj a 3: u; a.. '-.1 ' :::> u u i '+- a.. :: '- o._.1 (b) (Deg) Depth (ktlo /i) Figure 1 (a). X-ray roking urves of Fe Ka (4) refletion for an 1 as-grown AlxGa 1 _xas / GaAs SLS struture with 1 periods. The alulated urve (solid line) was obtained using the depth-strain distribution shown in (b). of periods in the SLS are verified to be equal to ten. The aeraqe thiknesses of the GaAs and the AlxGa 1 _xas sublayers are 27 and 14 A respetively. The averaoe perpendiular strains relative to the substrate are.% and.25% respetively. The remaining disrepany between the measured and the alulated urves as observed in Fig. 1a has been attributed to thikness variations in the periods of the SLS and to the nonabrupt interfaes of the sub layers [9]. Thus, from the analyses of the roking urve measurements, the strain profiles in the SLS are aurately determined. Roking urves on the AlxGa 1 _xas/gaas SLS samples were obtained after Si implantations with doses ranging from 7 x 1 12 to 5 x 1 15 m- 2 (see Fig. 2). The intensity of subsidiary peaks, exept P, diminishes. Also P shifts to an inreased angular position p'. I Sine the separation between Psub and P measures the average strain in the damaged SLS, the average strain learly inreases after this implantation. Further insight into the hange in strain upon implantation is provided by fitting the measured roking urve with a alulated (solid line) one (see Fig. 2a). To this end, the strains of the sublayers in the model of Fig. 1a were hanged and alulations were made iteratively to obtain a best

4 I.3 T,--,.---, -- - Meas. Psub (a) (b) -Cal. 1. p' 1. e.2 Q; u; 3: Q_.1 '- _g :::l CJl.S2 "" u.1 Q_ '- a:; :: GaAs - AlxGa 1 _xas I '----'---'----'---A (Deg) Depth (kilo 14) Figure 2. Fe Ka 1 (4) roking urves of an AlxGa 1 _xas/gaas SLS struture implanted with 2 kev Si ions to a dose of 7 x 1 12 m- 2. The alulated urve was obtained using the sublayers strain distribution shown in (b). fit. The strain profile used in this fit of Fig. 2a is shown in Fig. 2b. It is important to note that the damage indued by the low dose implantation onsidered here redues the strain in the AlxGa 1 _xas sublayers. On the other hand, the strain inreases in the GaAs sublayers, so that the net effet is an overall inrease in the average strain of the SLS. A similar inrease in the strain of the bulk GaAs after implantation of a orresponding dose has been previously reported [1]; a redution of strain in a single AlxGa 1 _xas layer ha s also been measured after implantation with similar doses [11]. The urrent data are thus onsistent with those measurements. It shou ld be pointed out that an equally good fit in Fig. 2a ould be obtained by interhanging the strains in the AlxGa 1 _xas and GaAs sublayers in Fig. 2b. This is beause of a small differene between the strains in the two sublayers for this partiular ase. From measurements performed on single AlxGa 1 _xas layers, the strain profile in Fig. 2b appears more probable. The average strain in the SLS strutures inreases as the implantation dose rises above 7 x 1 12 Si/m 2. The measured average strain as a funtion of dose is shown in Fig. 3. The average strain initially inreases with dose and tends to saturate, to a level of.44% beyond a dose of 5 x 1 14

5 kev Si + A1. 88 Ga.IZ As /GaAs SLS.4.3 Vi ' E in A 88 Ga.l 2 As as- grown sub layer Average strain in as -grown SL S Dose (Si/m 2 ) Figure 3. The average strain in the damaged SLS strutures as a funtion of Si ion dose. The average strain was measured by the angular separation of P 1 from P b using the relation su ' lie =- <E:"- >tane 8 Si/m 2. A nonlinear behavior of strain has also been observed in bulk GaAs. Strain measurements arried out on single AlxGa 1 As layers on GaAs show that -x 13 the strain in these layers dereases with doses up to 1x1 Si/m 2, and then inreases [11]. It would be onstrutive to pursue suh investigations in bulk material to understand the nonlinear behavior of strain with dose in SLS strutures. Thermal Annealing The reovery of the strain profiles of the implanted SLS strutures upon annealing is demonstrated in Fig. 4. X-ray roking urves were obtained before and after annealing at 42 C for one hour of a sample implanted with 1 x 1 14 Si/m 2. In the urve of the as-implanted sample (Fig. 4a), the angular separation between the peaks Psub and P is about twie that in Fig. 2a beause of the higher dose (see Fig. 3). Another striking feature of Fig. 4a is that the peak P 1 is more pronouned than in Fig. 2a. The intensity of this peak is related to the differene in the strain between the sublayers of the SLS [12]. The low intensity of P 1 in Fig. 2a thus is due to the small differene in the strains of the sublayers, as shown in Fig. 2b.

6 (a) 1. p :: p' :: SLS.,, ' /: lxl 14 Si/m 2 II I 11 I ; ; 1., : ; )' 3 Q. ' o I 1 psub "1.1 ' ;..S: u _.<!! '+- n::: pl i \.1 I I ('. 1 'I ' '. f,..': If,. : 'f'.fl ' ' (b) Anneal 42 C lhr. \.. :; O.OOOIL L (Deg) 6. 8 (Deg ) - '.5 Figure 4. Fe Ka 1 (4) x-ray roking urves of an AlxGa 1 _ As / GaAs 14 2 X sample implanted with 1 x 1 Si/m, (a) before annealing (b) after annealing, at 42 C for one hour. After annealing, the roking urve (Fig. 4b) reverts very nearly to that of the as-grown sample (Fig. 1a), whih implies an almost omplete reovery of the strain profile of the implanted SLS struture. This investigation establishes that within the resolution of the roking urves, thermal annealing restores the original state of the SLS. This onlusion was tested by onduting baksattering spetrometry measurementsf12]. Insignifiant atomi intermi xing in depth was observed up to 5 x 1 15 Si/m 2 after irradiation and subsequent heat treatment. We onlude that the reversible strain alteration produed by the implantation is due to damage generated within the sublayers. CONCLUSIONS Ion implanted AlxGa 1 _xas/gaas superlatties have been investigated before and after annealing. Strain profile measurements in these strained-layeredstrutures have been arried out by x-ray roking urves. The tehnique is sensitive enough to detet and measure hanges in the strain below.1% indued by implantations with doses as low as 7 x 1 12 Si / m 2. The analyses of the roking urves show that in these SLS strutures

7 325 the initial sta93 of implantation redues the strain in the AlxGa 1 _xas sublayers. In addition, baksattering analyses establish that up to doses of 5 x 1 15 Si/m 2, long-range displaements of atoms are not dtetable. This observation is further substantiated by the reovery of the SLS struture upon annealing at a relatively low temperature of 42 C. The study of implanted SLS strutures, as arried out in this paper, opens up new avenues in investigating the fundamental aspets of ion-solid interations. In partiular, details of damage reation and ion mixing ould be larified by onduting studies similar to this one on highly strained SLS strutures with atomially sharp interfaes. ACKNOWLEDGMENTS The authors thank Y. C. M. Yeh, D. A. Smith, and A. Mehta at Applied Solar Energy Corporation for supplying the as-grown samples. Partial finanial support by the Offie of Naval Researh under ontrat N14-84-C-736 through Rokwell International is aknowledged as well. REFERENCES 1. Y.-H. Wu, M. Werner, and S. Wang, Appl. Phys. Lett., 45, 66 (1984). 2. D. Arnold, J. Kl em, T. Henderson, M. Morko, and L. PErikson, App 1. Phys. Lett., 45, 764 ( 1984). 3. J. J. Coleman, P. DDapkus, C. G. Kirkpatrik, M. D. Camras, and N. Holonyak, Jr., Appl. Phys. Lett., 4, 94 (1982). 4. D. R. Myers, R. M. Biefeld, I. J. Fritz, S. T. Piraux, and T. E. Zipperian, Appl. Phys. Lett., 44, 152 (1984). 5. G. W. Arnold, S. T. Piraux, PS. Peery, D. R. Myers, and L. R. Dawson, Appl. Phys. Lett., 45, 382 (1984). 6. S. T. Piraux, G. W. Arnold:-D. R. Myers, L. R. Dawson, R. M. Biefeld, I. J. Fritz, and T. E. Zipperian, IBMM'84, Ithaa, New York (July 16-2, 1984}; proeedings in Nul. Instr. Meth. B (in press). 7. V. S. Spe.riosu and T. Vreeland, Jr., J. Appl. Phys. 56, 1591 (1984}. 8. J. A. Ibers and W. C. Hamilton, eds., International Tables X-Ray Crystallography, Vol. IV, (Kymoh, Birmingham, 1974). 9. A. H. Hamdi, V. S. Speriosu, J. L. Tandon, and M-A. Niolet, Phys. Rev. B (in press). 1. V. S. Speriosu, B. M. Paine, M-A. Niolet, and H. L. Glass, Appl. Phys. Lett. 4, 64 (1982). 11. A. H. Hamdi, J. L. Tandon, T. Vreeland, Jr., and M-A. Niolet, (to be published). 12. A. H. Hamdi, J. L. Tandon, and M-A. Niolet, 8th Intl. Conf. on the Appliation of Aelerators in Researh and Industry, Denton, Texas, (November 12-14, 1984); proeedings in Nul. Instr. Meth. B.