Damage buildup and annealing characteristics in Be-implanted InAs0.93Sb0.07 film

Nuclear Instruments and Methods in Physics Research B 285 (2012) 11–17

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Damage buildup and annealing characteristics in Be-implanted InAs0.93Sb0.07 film Q.W. Wang, C.H. Sun, S.H. Hu, L.M. Wei, J. Wu, Y. Sun, G.J. Hu, G. Yu, X. Chen, H.Y. Deng ⇑, N. Dai ⇑ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 21 February 2012 Received in revised form 30 April 2012 Available online 12 May 2012 Keywords: III–V Semiconductors Ion-implantation HRXRD Microstructure

a b s t r a c t Damage buildup in 80 keV Be-implanted InAs0.93Sb0.07 epitaxial layer grown by liquid epitaxy growth (LPE) with the implantation fluences ranging from 1  1013 to 4  1015 cm2 have been detailedly investigated by high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The implantation-induced nonlinear maximum perpendicular strain em as a function of the Be fluence was deduced. Microstructural variation created by damage buildup was analyzed. The characteristics of annealing on the lattice damage were also studied. The created damages can be recovered by rapid thermal annealing at 500 °C for samples with the fluence below 1.0  1015 cm2, but nano-sized residual damages still existed when the fluence reaches 4.0  1015 cm2 due to the poor recrystallization of the small disorder region. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The ternary alloy InAs1-xSbx is a promising material for infrared optoelectronic device applications such as infrared laser, LED and photodetector [1–6], due to its better stability than popularly-used HgCdTe. Meanwhile, with Sb composition changing the bandgap of InAs1-xSbx covers both the mid-infrared (3–5 lm) and the longwavelength infrared (8–12 lm) atmospheric windows where optical absorption goes to minima [7]. In addition, the material is also potentially useful for the high-speed device such as high mobility transistors due to its high electron mobility [8,9]. Ion implantation has been widely used for intentional impurity doping to modify the semiconductor physical characteristics for device purposes. Zn, Mg, and Be elements with two valence electrons are often used as the p-type dopants for III–V compound semiconductors. Owing to light atom mass and low damage introducing rate, Be ion is a good choice for p-type doping in InAs1-xSbx through ion implantation. Unfortunately, ion implantation is a non-equilibrium technique, which often causes lattice damages. Especially, with damage accumulation, phase transformation of crystalline-to-amorphous might occur when the fluence is above a threshold value. Although most of the damages can be removed by rapid thermal annealing, some heavy lattice damages cannot be eliminated, especially when an amorphous phase occurs, which will degrade device’s performance seriously. Therefore the mechanism of the damages formation and recovery in implanted semiconductor is of great scientific and technological interest. Lots of works have been done on the ⇑ Corresponding authors. Tel.: +86 021 25051418; fax: +86 021 65830734 (H.Y. Deng), tel.: +86 021 65169633; fax: +86 021 65830734 (N. Dai). E-mail addresses: [email protected] (H.Y. Deng), [email protected] (N. Dai). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.05.003

damage buildup and annealing characteristics for bulk semiconductors such as Si, Ge, GaAs, GaSb, InAs and InSb [10–14]. Very recently, Gonzalez-Arrabal had reported characteristics of lattice disorder and strain in Mn-implanted InAs film [15]. However, to our best knowledge, there have been fewer reports on characteristics about microstructure variation in the Be-implanted InAs1-xSbx epitaxial layer before and after annealing. In this paper, InAs0.93Sb0.07 epitaxial layer grown by liquid epitaxy growth (LPE) was implanted with Be ions. The implantation energy was 80 keV and the fluence varied from 1.0  1013 to 4.0  1015 cm2. The characteristics of damage buildup and its removal by rapid thermal annealing at temperature 500 °C were investigated by high resolution X-ray diffraction and TEM. The implantation-induced nonlinear maximum perpendicular strain em as a function of the Be fluence was obtained. The nonlinear strain was analyzed in terms of point defects. In addition, we found that the caused damages can be recovered by rapid thermal annealing at 500 °C for samples with the fluence below 1.0  1015 cm2. Nano-sized residual damages after annealing at 500 °C, however, were still observed when the fluence reaches 4.0  1015 cm2. Those residual damages might originate from poor recrystallization of the small disorder region.

2. Experimental InAs0.93Sb0.07 epitaxial layer was grown on unintentionally doped (1 0 0) InAs substrate by a conventional horizontally-sliding graphite boat at 500 °C. The detailed growth equipment and growth process were described elsewhere [16,17]. The thickness of InAs0.93Sb0.07 film was 4.4 lm. All the samples for implantation were cut from the same epitaxial wafer. Implantation of 80 keV Be

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ions was performed at nominal room temperature with the fluence ranging from 1  1013 to 4  1015 cm2. The current density was 250 nA/cm2 in order to avoid the beam-induced heating effect. To minimize the channeling effect, the Be-ion incident beam is at 7° off the (1 0 0) direction. The computer simulation results of the distribution of Be ions and implantation-induced vacancies were calculated by SRIM2008 [18]. The rapid thermal annealing process was performed at 500 °C in the protection of Ar ambient at one atmosphere pressure for 10 s. During the rapid annealing process, the samples were covered by another InAs0.93Sb0.07 epitaxial wafer in order to minimize the evaporation of the group-V elements. Using the Bruker D8 Discover diffractometer, the HRXRD spectra were measured by the Cu Ka1 line with the wavelength of 0.15406 nm. The cross-sectional TEM images were measured on the specimens prepared by conventional polishing and Ar ion milling techniques. During the Ar ion milling, the specimens were cooled with liquid nitrogen to keep implantation-induced damages stable. The TEM measurement was operated with the electron at 200 kV.

3. Result and discussion Fig. 1a shows the HRXRD patterns of the as-grown InAs0.93Sb0.07/InAs samples by LPE in 2h-x scan mode. From 20° to 70°, only (2 0 0) and (4 0 0) peaks can be observed, which indicates that the epitaxial layer is good single crystal with (1 0 0)

direction paralleling to the (1 0 0)-oriented InAs substrate. To clarify the fine structure, the detailed (4 0 0) diffraction peaks were given in the Fig. 1b in logarithmic scale. The asymmetry of InAs1-xSbx (4 0 0) peak originates from the misfit dislocation layer near the interface between film and substrate due to the strain relaxation. The (4 0 0) peak position of the InAs1-xSbx film is at 60.81°, the corresponding Sb composition x is about 0.07 calculated by Vegard’s law. The full-width at half-maximum (FWHM) of (4 0 0) peak for the epi-layer is 0.04°, which is comparable to the corresponding value of the substrate (0.02°). To evaluate the film quality, the (4 0 0) rocking curve (RC) was also presented in the Fig. 1c. The RC exhibits nearly perfect Gauss function’s shape and was fitted very well by Gauss function as shown by the red solid line. Its FWHM is about 0.08°, indicating the high quality of the epi-layer. The inset in Fig. 1a is the cross-sectional SEM micrograph of sample delineated by A–B solution. The interface is clear and abrupt at micrometer resolution, showing that the thickness of the film is 4.4 lm. The thickness of film is much larger than that of the 80 keV Be-implanted layer of about 0.6 lm calculated by SRIM 2008. Therefore, the misfit dislocations near the interface between the InAs0.93Sb0.07 film and the InAs substrate will not affect the damage buildup during the implantation. Computer simulation results of the distributions of implanted atoms and implantation-induced vacancies for 80 keV Be ions in the InAs0.93Sb0.07 epitaxial layer were obtained by SRIM 2008. During the simulation, 1  104 Be ions were taken into account, and the density of InAs0.93Sb0.07 was chosen to be 5.675 g/cm3 as the linear interpola-

Fig. 1. (a) HRXRD patterns of the as-grown InAs0.93Sb0.07/InAs structure in 2h-x scan mode. The inset shows the cross-sectional SEM micrograph of the InAs0.93Sb0.07/InAs structure etched by A–B solution. The detailed (4 0 0) diffraction peaks (b) and the rocking curve (c) of epitaxial layer.

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Fig. 2. (a) HRXRD spectrum near (4 0 0) peaks of Be-implanted InAs0.93Sb0.07/InAs structure (2  1014 cm2). The (b) shows the normalized HRXRD spectra near (4 0 0) peak of samples with different fluences.

tion of InAs 5.667 and InSb 5.7747 g/cm3. The ratio of In, As, and Sb component in InAs0.93Sb0.07 was taken to be 1:0.93:0.07. The angle of the incident beam was 7° off the surface normal. The mean projected range Rp is 0.28 lm and the corresponding straggling range DRp is 0.12 lm. Both the Be-distribution and the vacancy-distribution show a Gaussian-like shape with the center located at 0.29 and 0.20 lm, respectively. In order to investigate the lattice change of microstructure due to implantation, the HRXRD was performed along (1 0 0) direction on Be-implanted InAs0.93Sb0.07 film in 2h-x scan mode to monitor (4 0 0) diffraction peak. As shown in Fig. 2a, a new peak appeared at the left side of (4 0 0) diffraction peak after Be implantation at fluence 2  1014 cm2. To investigate the evolution of the new peak with the variation of fluences, the HRXRD spectra near (4 0 0) diffraction peak were performed for all the samples with the fluence ranging from 1  1013 to 4  1015 cm2. The detailed results are presented in Fig. 2b, where, to compare the new peaks clearly, all the curves are normalized with respect to the (4 0 0) peak of the InAs0.93Sb0.07 film. When the fluence is below 4  1013 cm2, the (4 0 0) peak is almost the same as that of the as-grown film. When the fluence reaches 6  1013 cm2, however, a new peak shows up at the lower-angle side near (4 0 0) peak. The new peak grows up and, at the same time, shifts further away from zero D2h with the increasing of the fluence. Its intensity reaches to the maximum at the fluence of 4  1014 cm2. When the fluence is above 1  1015 cm2, its intensity starts to reduce and its peak position to shift toward zero D2h a little. The new peaks were attributed to implantation-induced strain in the implanted layer [19–21]. It is commonly believed that the implantation-induced lattice point defects are mainly responsible for the strain [20–24]. For the 80 keV Be-implanted InAs0.93Sb0.07 case, we found that the strain also originates from point defects. To investigate the damage buildup, the maximum perpendicular strain is calculated using the new peaks in Fig. 2b by the expression em = Dhcot(hB) [21], where hB is the Bragg reflection angle of the InAs0.93Sb0.07 epitaxial layer and Dh the angular deviation after implantation. For samples with fluences above 1  1014 cm2, the positions of the new peak were directly specified from the HRXRD spectra in Fig. 2b. The positions of the new peaks caused by Be-implantation at the fluences of 6  1013 and 1  1014cm2 were extracted by least-square Gaussian fit to the two-peak experimental data where the dominant peak at zero D2h comes from the (4 0 0) diffraction of the as-grown InAs0.93Sb0.07 film. The fitting results are shown in Fig. 3b and c, respectively. The calculated result is shown in Fig. 3a, which

shows non-linear characteristic. When the fluence is below 2  1013 cm2, almost no strain is detectable. In the fluence range from 4  1013 to 4  1014 cm2, the strain increases up quickly. The largest em is about 0.095% at the fluence 4  1014 cm2. When the fluence is above 1  1015 cm2, however, the strain shows quasisaturation and even become a little lower. The non-linear behavior can be explained by the Hecking model in terms of point defects accumulation. According to Hecking’s model, the radiation damage at fixed depth can be described by two differential equations [25,26]:

dnpd npd dna npd ¼ ðPpd  Rpd npd Þ þ Cnpd ð1  Þ dNI npd ð1  na Þ dNI 1  na

ð1Þ

and

dna ¼ ðPa þ Ga na Þð1  na Þ dNI

ð2Þ

where npd is relative concentration of point defects (both isolated and clustered defects are taken into account), na is relative amount of amorphous damage and NI is ion fluence. The total relative radiation damage is n = npd + na. For our case, the point defects are predominant according to the TEM results. Hence only Eq. (1) is taken into account and point defects accumulated with fluences model can be written as [26]:

dnpd npd ¼ ðPpd  Rpd npd Þ þ Cnpd ð1  Þ dNI npd npd Þ ¼ P pd eRpd NI þ Cnpd ð1  npd

ð3Þ

In the first term PpdRpdnpd, Ppd is the cross section for point defect production and Rpd the parameter for point defect recombination describes the defect production and quasi-saturation behavior. The second term characterizes the non-recombination clusters of point defects with parameter C up to a saturation concentration npd . According to the TEM results, the influence of clusters is small and can also be neglected. Therefore, take C = 0 and the analytic solution for simplified Eq. (3) can be obtained as:

npd ¼ A

Ppd ð1  eRpd NI Þ Rpd

ð4Þ

where A is constant. We supposed that the maximum perpendicular strain is proportional to the relative concentration of point defects in the heaviest damaged region, and the maximum strain em with

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Fig. 3. (a) The curve of maximum perpendicular strain as a function of Be ion fluence. (b) and (c) show the fit to the new peaks caused by Be-implantation at the fluence of 6  1013 and 1  1014 cm2, respectively.

the relative concentration of point defects npd is expressed by the equation:

em ¼ knpd ¼ kA

Ppd ð1  eRpd NI Þ Rpd

ð5Þ

where k is the proportionality coefficient and npd is replaced with Eq. (4). Using Eq. (5) the curve of maximum strain em was fitted, and the fitting result is shown in Fig. 3a. The fitting curve agrees well with the experimental data over the whole range and parameter Rpd obtained by fitting is 9.6  1015 cm2. The involved physical mechanism can be explained as follows: When the fluence is below 2  1013 cm2, the crystal damage is very slight and only contains simple point defects such as Frankel pairs with low concentration. As a result, almost no strain can be detected in this Be-implantation range. However, in the fluence range from 4  1013 to 4  1014 cm2, the strain increases quickly mainly due to the increasing of point defects concentration. When the fluence is above 4  1014 cm2, the concentration of point defects reaches quasi-saturation due to the defect recombination caused by dynamic annealing.

To confirm our explanation and investigate the evolution of the defects, the HRTEM in the heaviest damaged region for the samples with fluences 1  1013, 2  1014, 4  1014 and 4  1015 cm2 were carried out with electron beam paralleling to (0–11) direction. The HRTEM of the sample at fluence 1  1013 cm2 is shown in the Fig. 4a, which lattice fringes are clear and the dots contrast are a little uniformed. That indicates the implanted layer is only slightly damaged. The corresponding Fast Fourier Transform (FFT) pattern is shown in the inset of Fig. 4a, which coincides with the (0–11) diffraction pattern of zinc-blende structure as marked in the inset. The HRTEM at fluence 2  1014 cm2 is shown in Fig. 4b, and we can see the dot contrast is not uniform compared with Fig. 4a. This indicates the increasing of point defects in this region, and the corresponding HRTEM can be simulated by multislice method [27]. Besides these point defects no extended defects are observed at this fluence, which confirmed that the strain mainly originates from point defects. The corresponding FFT pattern is also shown in the inset of Fig. 4b. The FFT pattern only contains fundamental lattice refection patterns and no other points and halo patterns are observed, which also confirmed that no ex-

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Fig. 4. (a), (b), (c) and (d) HRTEM images in the heavily damaged region near Rp for samples with fluences 6  1013, 2  1014, 4  1014 and 4  1015 cm2. The insets show the corresponding FFTs. (e), (f), (g) and (h) show the stacking fault and typical disordered regions in squares marked as ‘‘1’’, ‘‘2’’, ‘‘3’’ and ‘‘4’’ in (c) and (d), respectively. Insets in (c) and (d) shows the corresponding FFTs.

Fig. 5. (a) HRXRD (4 0 0) peaks for as-implanted (1  1015 cm2), annealed for 10 s and as-grown sample. (b) HRXRD (4 0 0) peaks for as-implanted (4  1015 cm2), annealed for 10, 20 s and as-grown samples.

tended defect exists at this fluence. However, when the fluence reaches 4  1014 cm2, the HRTEM shows the uniformity of the

dot contrast is larger than that of 2  1014 cm2. Meanwhile, the stacking faults are observed. Besides these stacking faults no amor-

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Fig. 6. Cross-sectional TEM micrographs of as-implanted (4  1015 cm2) epitaxial layer (a) and annealed for 20 s (b).

InAs0.93Sb0.07

phous components are found, which is also confirmed by the FFT in the inset. Fig. 4e shows the magnified image of the stacking fault structure in the square marked as ‘‘1’’ in Fig. 4c in detail. These stacking faults may originate from the dynamic annealing, which also observed in Se-implanted GaAs [28]. This indicates that dynamic annealing play an important role for Be-implanted InAs0.93Sb0.07 at the nominal room temperature. The Fig. 4d shows besides point defects and stacking faults observed at fluence 4  1014 cm2, disorder regions are formed at the fluence 4  1015 cm2. The corresponding FFT in the inset shows fundamental lattice refection patterns and the ambiguous halo pattern as marked by the arrow. This indicates most of the region is defected-crystalline and amorphous component arises at this fluence. Fig. 4f shows the magnified image of the stacking fault structure corresponding to the region marked as ‘‘2’’ in detail, which is similar to that in Fig. 4e. Fig. 4g and h present HRTEM images of the

regions marked as ‘‘3’’ and ‘‘4’’ in Fig. 4d, respectively, where typical small disorder regions are shown in detail. Apparently, the small disorder regions are almost in amorphous phase, which are also confirmed by FFTs where only the ambiguous center point and the halo can be observed as shown in the insets in Fig. 4g and h. Since these disordered regions are primary amorphous nuclei caused by implantation, the strain of experimental data is not strictly saturated as the fitting results but becomes a little lower when the fluence is above 1  1015 cm2 as shown in Fig. 3a. During the fitting only point defects accumulation is taken into account and the amorphous damage is neglected. Since the small disordered regions in nearly amorphous phase make no contribution to the intensity of X-ray diffraction, the intensity of the new peaks decreases in Fig. 2b. Meanwhile, the disorder regions partially relax the strain, leading to shift of the peak toward zero D2h. The implanted samples were annealed at 500 °C for 10 s in Ar ambient to remove the damages and to activate doped acceptors. During the rapid annealing process, the sample surface was covered by that of another InAs0.93Sb0.07 epitaxial wafer to prevent the loss of the group-V elements As and Sb (they have high saturation vapor pressure). After annealing, HRXRD was performed again in 2h-x scan mode to monitor the change of the new peak near (4 0 0) diffraction peak. As we expected, all the new peaks disappeared in the fluence range from 1  1013 to 1  1015 cm2. Take the sample Be-implanted at fluence 1  1015 cm2 as an example. To compare the change of the peaks clearly, (4 0 0) peaks of as-implanted, annealed, and as-grown samples are presented in Fig. 5a. It shows clearly that, after annealing, the peak of Be-implanted sample is the same as that of the as-grown one. This indicates the implantation induced defects can be recovered at temperature 500 °C for 10 s. For the sample Be-implanted at fluence 4  1015 cm2, the defects can’t be recovered thoroughly after annealing under the same condition. The (4 0 0) diffraction peaks of as-implanted, annealed for 10 and 20 s, and as-grown samples are shown in Fig. 5b. As marked by the red arrow in the figure, there is still a weak peak at the lower angle side, indicating that the rapid annealing can’t eliminate the defects. The residual peak still exists with an annealing time increasing to 20s. Thus, we expect that, at high fluence above 4  1015 cm2, more stable defects or dislocations are formed, which cannot be removed by a simple annealing treatment. The recovery of the residual damages could be realized by increasing annealing temperature and time. However, higher temperature and longer time are likely to result in fast evaporation of group-III elements and redistribution of Be ions, which is harmful for the material used for optoelectronic devices. Therefore room temperature Be-ion implantation for InAs1-xSbx

Fig. 7. (a) Cross-sectional HRTEM micrograph of Be-implanted InAs0.93Sb0.07 (4  1015 cm2) after annealing for 20 s in the regions without nano-sized damages. (b) The detailed HRTEM micrograph of nano-sized damage.

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p-doping should be carried out at a fluence below 4  1015 cm2 to avoid unrecoverable damages. To confirm HRXRD results and investigate the annealing characteristics microscopically, cross-sectional TEM images of the samples implanted at fluence 4  1015 cm2 and annealed for 20 s are shown in Fig. 6a and b, respectively. As shown in Fig. 6a, the as-implanted sample shows a heavily damaged region with thickness of 0.4 lm about 0.15 lm beneath the surface. The center of the heavily damaged region is around the Rp ± DRp. After annealing most of damages are recovered except those nano-sized residual damages observed as the dark spots, as shown in Fig. 6b. The nano-sized damages are probably responsible for the residual peaks in Fig. 5b. To analyze the detailed microstructure after annealing, cross-sectional HRTEM measurements were also performed. As shown in Fig. 7a, the HRTEM image shows that the lattices in most parts of the sample are defects-free, indicating that to a large degree the implantation-induced damages has already been recovered by the annealing process at 500 °C. Fig. 7b shows the details of the residual nano-sized damages in the heavily damaged region. The moiré fringe in nano-sized damage spot was observed and marked with the white lines in the figure, which is caused by the overlapping layers with different lattice spacing and a small included angle [29]. It indicates that this damage region is still crystalline but it contains a distorted layer rotated from the normal orientation. At the same time lattice spacing of a distorted layer is a little different from that of normal crystalline. The formation of these nano-sized residual damages is correlated with the poor recrystallization of the small disorder regions. 4. Conclusion In conclusion, we have observed the damage buildup in Be-implanted InAs1-xSbx epitaxial layer with the fluences from 1  1013 to 4  1015 cm2. The nonlinear curve of the implantation-induced strain was obtained. The largest strain is 0.095% at the fluence 4  1014 cm2. The strain was attributed to implantation-induced lattice point defects. When the fluence is above 1  1015 cm2 the small disordered regions are observed. The implantation-induced defects can be removed by rapid annealing process at 500 °C for 10 s when the fluence is below 1  1015 cm2. But some nano-sized residual damages still existed after annealing at 500 °C even for 20 s when the fluence reaches 4.0  1015 cm2. It might originate from to the poor recrystallization for the disordered regions.

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Acknowledgements The authors wish to thank J.L. Wang and Prof. P.P. Chen for the help of HRXRD measurement and J.M. Li from Institute of Semiconductor, CAS. for ion implantation. This work was supported in part by National 973 Project (Nos. 2012CB934300, and 2012CB619200), National Science Foundation in China (Nos. 10804117, 11174307 and 11074265), Shanghai City of Committee for Science and Technology (Nos. 08ZR1421900, and 11DZ1140500), and the CAS/ SAFEA International Partnership Program for Creative Research Teams. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

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