Strain measurements in Al_xGa_(1−x) As/GaAs superlattices have been carried out before and after Si ion implantation. For doses up to 5 × 10^(15) cm^(−2), no atomic intermixing of the sublayers is observed by backscattering spectrometry. However, with x-ray rocking curve measurements, significant changes in the strain profiles are detected for implantations with doses as low as 7 × 10^(12) cm^(−2). Interpretation of the rocking curves suggests that low-dose implantations release strain in the Al_xGa_(1−x) As sublayers. The strain profile recovery of the implanted samples, upon annealing at ∼ 420°C, implies that the damage caused by implantation is largely reversible

Hamdi, A. H.

Nicolet, M-A.

Tandon, J. L.

Vreeland, T., Jr.

Caltech Authors - Main

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 Technology, Pasadena, CA 91125 
**Applied Solar Energy Corporation, City of Industry, CA 91744 
ABSTRACT 
Strain measurements in Al Ga 1 As/GaAs superlattices have been carried x -x 15 -2 
out before and after Si ion implantation. For doses up to 5x10 em , no 
atomic intermixing of the sub layers i s observed by backscattering spectrom-
etry. However, with x-ray rocking curve measurements, significant changes 
in the stra in profiles are detected for implantations with doses as low as 
7xlo 12 cm- 2 Interpretation of the rocking curves suggests that low-dose 
implantations release strain in the AlxGa 1_xAs sublayers. The strain profile 
recovery of the implanted samples, upon annealing at ~ 420°C, implies that 
the damage caused by implantation i s largely reversible. 
INTRODUCTION 
Superlattices composed of thin alternating layers of different compound 
semiconductors offer unique possibilities in the conception and fabrication 
of novel optical and electronic devices [1,2]. To exploit these structures, 
localized and controlled doping by ion implantation constitutes a desirable 
processing method. Recently, a few studies have explored ion implantation 
in AlAs/GaAs, InGaAs/GaAs and GaAsP/GaP superlattices [3-6]. The primary 
objective of these stud ies is to demonstrate that superlattices can be doped 
by implantation without materially degrading the superlattice. Underlying 
these studies are the fundamental questions of the nature of the damage created 
by an ion irradiation in a super lattice, and its evolution upon subsequent 
thermal annealing. 
Almost al l superlattices possess strain in one or both sublayers because 
of the lattice mismatch of the constituents. By measuring this strain as a 
function of depth, and monitoring its evolution after implantation and anneal-
ing, accurate information on the cumulative effects of defects on the super-
lattice can be obtained. In this paper, the technique of x-ray rocking curves 
is used to make such measurements. The changes in the rocking curves of 
AlxGa 1_xAs/GaAs superlattices are measured as a function of the dose after 
Si ion implantations, and also upon thermal annealing. The curves are inter-
preted in terms of changes in the stra in after using the kinematic model 
of x-ray diffraction [7]. The results help in understanding the onset of 
damage created by ion implantation. 
Mat. Res. Soc. Symp. Proc . Vol. 37. • 1985 Materials Research Society 
320 
EXPERIMENTAL 
AlxGa 1_xAs/GaAs (x = .88) strained-layer-superlattice (SLS) structures 
were prepared by metal organic chemical vapor deposition. Alternating layers 
(10 each) of AlxGa 1_xAs and GaAs were grown with thicknesses of 140 and 270 A 
respectively, on (100) GaAs substrates. Si ions were implanted at room tem-
perature with an energy of 200 keV. To minimize channeling during implanta-
tion, the incoming beam was oriented ~ 7° wit~ respect to the sample's surface 
normal~ The range of the ions is about 2000 A with a range straggling of 
~ 900 A which places the Si and the damage profiles fully within the super-
lattice. The implantation doses ranged from7x10 12 to 5x1o15 Si/cm2. Ther-
mal annealing was carried out, on a few samples, in a forming gas atmosphere 
(85% N2 + 15% H2) at ~ 420°C for one hour. 
X-ray rocking curve measurements were made in the (400) reflection, using 
a double-crystal diffractometer with FeKa1. The x-ray beam was collimated 
and rendered nearly monochromatic by a (400) reflection from a high quality 
(100) GaAs crystal (the first crystal). The beam spot was adjusted to 
~ 0.5 mm x 1 mm by slits placed between the first crystal and the sample . The 
measured rocking curves were fitted with calculated curves using Speriosu's 
model of kinematical theory of x-ray diffraction in thin epitaxial layers [7]. 
In this model, the diffraction by the substrate is treated dynamically. For 
fitting, the structure factors of Al _88Ga_ 12As (= 117.4) and GaAs (= 157.0) 
were calculated using the tabulated atomic scattering factors [8]. 
Backscattering spectrometry measurements were also carried out using 
a 2 MeV He beam. All data refer to room temperature unless stated otherwise. 
RESULTS AND DISCUSSION 
Effect of Si Ion Implantation 
Figure 1 shows the measured (dashed line) and the calculated (solid line) 
rocking curves for the as-grown AlxGa 1_xAs/GaAs SLS structure. The angle ~0 
is plotted relative to the Bragg angle (eB) of the substrate peak, Psub, at 
~e = 0. The reflecting power, plotted on the vertical axis, is normalized 
with respect to the intensity of the incoming x-ray beam . The periodicity in 
the structure factors and in the strain of the layers in the SLS generates 
the subsidiary peaks observed in the rocking curves . The displacement of the 
peak P
0 
from Psub measures the average strain in the SLS. For symmetric re-
flection, as in this case, the equa l separation between the subsidiary peaks 
measures the average thickness of one period in the SLS . The calculated 
curve in Fig . la was obtained using the strain distribution in Fig. lb. The 
details of the calculations are given elsewhere U,9]. From Fig. lb, the number 
-' 
321 
0.3 100.0 
--- Me as 
Psub 
(b) (a) 
10.0 
-Calc. 
Po 
~ 
~ 
0 
0 
c 
0.2 
Qj 1.0 ·a 
3: u; 
0 
a.. '-0.1 0 
0' 
:::> ~ u u i5 
~ 0.01 c 0 .1 . '+- Q) 
Q) a.. 
0:: '-
Q) 
o._ 
0.001 
0 .0001 0 .0 
-0.5 -0.25 0 .0 0 .25 0 .5 0 .0 1.0 2 .0 3.0 4.0 5.0 
68 (Deg) Depth (ktlo /i) 
Figure 1 (a). X-ray rocking curves of Fe Ka 1 (400) reflection for an 
as-grown AlxGa 1_xAs / GaAs SLS structure with 10 periods. The calculated 
curve (solid line) was obtained us ing the depth-strain distribution 
shown in (b) . 
of periods in the SLS are verified to be equal to ten. The a~eraqe thicknes-
ses of the GaAs and the AlxGa 1_xAs sublayers are 270 and 140 A respectively. 
The averaoe perpendicular strains relative to the substrate are 0.0% 
and 0.25% respectively. The remaining discrepancy between the measured and 
the calculated curves as observed in Fig. 1a has been attributed to thickness 
variations in the periods of the SLS and to the nonabrupt interfaces of the 
sub layers [9]. Thus, from the analyses of the rocking curve measurements, 
the strain profiles in the SLS are accurately determined. 
Rocking curves on the AlxGa 1_xAs/GaAs SLS samples were obtained after Si 
implantations with doses ranging from 7 x 1012 to 5 x 1015 cm-2 (see Fig. 2). 
The intensity of subsidiary peaks, except P
0
, diminishes. Also P
0 
shifts to 
an increased angular position p' . 
0 
I 
Since the separation between Psub and P 
0 
measures the average strain 
in the damaged SLS, the average strain clearly increases after this implanta-
tion. Further insight into the change in strain upon implantation is provided 
by fitting the measured rocking curve with a calcu lated (solid line) one 
(see Fig. 2a). To this end, the strains of the sublayers in the model of 
Fig. 1a were changed and calculations were made iteratively to obtain a best 
322 
100.0 I 0 .3 T ,--,.---, 
-- - Meas . Psub 
-Calc. 
(a) (b) 
10.0 
p' 
~ 0 0 
~ 1.0 c 0 ·e 0.2 
Q; u; 
3: '-0 0.1 _g Q_ 
:::l 
CJl .S2 
c ""0 
c 
u 0 .01 Q) 
~ Q_ 
'-a:; Q) 
0:: 0... 
0.001 
0.1 
--- GaAs 
- AlxGa 1_xAs 
0.0001 1 I 
0.25 0 .5 
0 .0 '----'---'----'---A-~ 
0 .0 1.0 2.0 3 .0 4.0 5.0 -0.5 -0.25 0.0 
68 (Deg) Depth (kilo 14) 
Figure 2. Fe Ka1 (400) rocking curves of an AlxGa 1_xAs/GaAs 
SLS structure implanted with 200 keV Si ions to a dose of 
7 x 1012 cm- 2. The calculated curve 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 induced by the low dose implantation 
considered here reduces the strain in the AlxGa 1_xAs sublayers. On the other 
hand, the strain increases in the GaAs sublayers, so that the net effect is an 
overall increase in the average strain of the SLS. A similar increase in the 
strain of the bulk GaAs after implantation of a corresponding dose has been 
previously reported [10]; a reduction of strain in a single AlxGa 1_xAs layer 
ha s also been measured after implantation with similar doses [11]. The cur-
rent data are thus consistent with those measurements. It shou ld be pointed 
out that an equally good fit in Fig. 2a could be obtained by interchanging 
the strains in the AlxGa 1_xAs and GaAs sublayers in Fig. 2b. This is because 
of a small difference between the strains in the two sublayers for this par-
ticular case. From measurements performed on single AlxGa 1_xAs layers, the 
st rain profile in Fig. 2b appears more probable. 
The average strain in the SLS structures increases as the implantation 
dose rises above 7 x 1012 Si/cm 2. The measured average strain as a function 
of dose is shown in Fig. 3. The average strain initially increases with dose 
and tends to saturate, to a level of ~ 0.44% beyond a dose of ~ 5 x 1014 
0.5 200 keV Si + 
A1. 88Ga .IZ As /GaAs SLS 
~ 0.4 
~ 0 
c 
0 0.3 
Vi 
Q) 
0' 
2 0.2 
Q) 
~ 
0.0 12 
10 
Figure 3. 
a function 
the angular 
• 
--------- E in A~ 88 Ga.l 2 As 
as- grown sub layer 
Average strain in as -grown SL S 
1014 
Dose (Si/cm2 ) 
The average strain in the damaged SLS structures as 
of Si ion dose. The average strain was measured by 
separation of P 1 from P b using the relation 
0 su ' 
·lie =- <E:"- >tan e8 
323 
Si/cm 2. A nonlinear behavior of strain has also been observed in bulk GaAs. 
Strain measurements carried out on single AlxGa 1 As layers on GaAs show that -x 13 
the strain in these layers decreases with doses up to 1x10 Si/cm 2, and 
then increases [11]. It would be constructive to pursue such investigations 
in bulk material to understand the nonlinear behavior of strain with dose in 
SLS structures. 
Thermal Annealing 
The recovery of the strain profiles of the implanted SLS structures upon 
annealing is demonstrated in Fig. 4. X-ray rocking curves were obtained be-
fore and after annealing at 420°C for one hour of a sample implanted with 
1 x 10 14 Si/cm2. In the curve of the as-implanted sample (Fig. 4a), the 
angular separation between the peaks Psub and P
0 
is about twice that in 
Fig. 2a because of the higher dose (see Fig. 3). Another striking feature of 
Fig. 4a is that the peak P~ 1 is more pronounced than in Fig. 2a. The inten-
sity of this peak is related to the difference in the strain between the sub-
layers of the SLS [12]. The low intensity of P~ 1 in Fig. 2a thus is due to 
the small difference in the strains of the sublayers, as shown in Fig. 2b. 
324 
100.0 1 
(a) 1 
psub 
' (b) ~ 
10.0 p :: 
~ 0 
p' 
0 :: SLS 
/: lxl0
14 
Si/cm
2 
, : 
0 '• . ,, 
II ~ I 
11 •• 
I ; ; ~ 
;• )' 
Anneal 
420°C lhr . 
~ 1. 0 
Q) ' o I 3 
0 
Q. 
0"1 0 .1 ' ;. \ .. :; .S: 
u 
_.<!! 
'+-
Q) 
n::: 
p~l i \ 
0.01 ~ ' I I ( 
0. 0 01 
'I ' ' 
. f , .. ':~~· 
• If 
O.OOOIL_ __ _L ____ ~--~-----
: 'f'.fl· •' ,. 
- ' 
- 0 .5 -0. 2 5 0 .0 0.25 0 .5 -0.5 -0.2 5 0.0 0.2 5 
6.8 (Deg) 6. 8 (D eg ) 
Figure 4. Fe Ka 1 (400) x-ray rocking curves of an AlxGa 1_ As / GaAs 14 2 X 
sample implanted with 1 x 10 Si/cm , (a) before annealing 
(b) after annealing, at 420°C for one hour. 
0 .5 
After annealing, the rocking curve (Fig. 4b) reverts very nearly to that of 
the as-grown sample (Fig. 1a), which implies an almost complete recovery of 
the strain profile of the implanted SLS structure. 
This investigation establishes that within the resolution of the rocking 
curves, thermal annealing restores the original state of the SLS. This con-
clusion was tested by conducting backscattering spectrometry measurementsf12] . 
Insignificant atomic intermi xing in depth was observed up to 5 x 1015 Si/cm2 
after irradiation and subsequent heat treatment. We conclude that the rever-
sible strain alteration produced by the implantation is due to damage gener-
ated within the sublayers. 
CONCLUSIONS 
Ion implanted AlxGa 1_xAs/GaAs superlattices have been investigated before 
and after annealing. Strain profile measurements in these strained-layered-
structures have been carried out by x-ray rocking curves. The technique is 
sensitive enough to detect and measure changes in the strain below 0.1% 
induced by implantations with doses as low as 7 x 1012 Si / cm2. 
The analyses of the rocking curves show that in these SLS structures 
325 
the initial sta93 of implantation reduces the strain in the AlxGa 1_xAs sub-
layers. In addition, backscattering analyses establish that up to doses of 
5 x 1015 Si/cm2, long-range displacements of atoms are not dtectable. This 
observation is further substantiated by the recovery of the SLS structure up-
on annealing at a relatively low temperature of 420°C. 
The study of implanted SLS structures, as carried out in this paper, 
opens up new avenues in investigating the fundamental aspects of ion-solid 
interactions. In particular, details of damage creation and ion mixing 
could be clarified by conducting studies similar to this one on highly 
strained SLS structures with atomically sharp interfaces. 
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 
financial support by the Office of Naval Research under contract 
N00014-84-C-0736 through Rockwell International is acknowledged as well. 
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English

Strain and Damage Measurements in Ion Implanted Al_xGa_(1−x)As/GaAs Superlattices

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Strain and Damage Measurements in Ion Implanted Al_xGa_(1−x)As/GaAs Superlattices

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