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Comparison of different pathways in metamorphic graded buffers on GaAs substrate: Indium incorporation with surface roughness
Applied Surface Science 324 (2015) 304–309
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Comparison of different pathways in metamorphic graded buffers on GaAs substrate: Indium incorporation with surface roughness Rahul Kumar a,∗ , P. Mukhopadhyay b , A. Bag a , S. Kr. Jana a , A. Chakraborty c , S. Das a , M. Kr. Mahata a , D. Biswas c a
Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur 721302, India Rajendra Mishra School of Engineering Entrepreneurship, Indian Institute of Technology, Kharagpur 721302, India c Department of Electronics & Electrical Communication Engineering, Indian Institute of Technology, Kharagpur 721 302, India b
a r t i c l e
i n f o
Article history: Received 25 August 2014 Received in revised form 8 October 2014 Accepted 27 October 2014 Available online 6 November 2014 Keywords: MBE Metamorphic buffer Surface roughness Cross hatch pattern RSM
a b s t r a c t In this work, compositionally graded In(Al,Ga)As metamorphic buffers (MBs) on GaAs substrate have been grown by MBE through three different paths. A comparative study has been done to comprehend the effect of underlying MB on the constant composition InAlAs healing layer by analyzing the relaxation behaviour, composition and surface morphology of the grown structures. The compositional variation between the constant composition healing layers on top of graded MB has been observed in all three samples although the growth conditions have been kept same. Indium incorporation rate has been found to be dependent on underlying MB. By combining the result of atomic force microscopy, photo-luminescence and X-ray reciprocal space mapping, varying surface roughness has been proposed as the probable driving force behind different Indium incorporation rate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction III–V high electron mobility transistor (HEMT) or in particular InGaAs/InAlAs based HEMT has shown great promise in the field of high speed and low power digital applications [1]. InGaAs/InAlAs based HEMT structure grown on lattice matched InP substrate has many advantages like high low-field mobility and high electron saturation velocity [2]. However, InP substrates are more expensive, brittle and available in only small areas compared to GaAs substrates [3,4]. Very high lattice mismatch (around 3.7%) between GaAs and InGaAs/InAlAs heterostructure (lattice matched to InP) obstruct the direct growth of later on the GaAs substrate. Even 1% lattice mismatch can result in a very high dislocation density (>109 cm−2 ), disastrous for electronic and optoelectronic devices [5]. Compositionally graded metamorphic buffers have been widely utilized to accommodate the lattice mismatch and to produce a template for further growth of active layers having low defect densities [6]. Different buffer routes have been studied previously employing compositionally graded ternary alloys such as InGaAs [7], InAlAs [8] and quaternary alloys such as InAlGaAs [9], InGaAsP [10] and AlGaAsSb [11]. Among them, In(Ga,Al)As buffer have been
∗ Corresponding author. Tel.: +91 9800155190. E-mail addresses: [email protected], [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.apsusc.2014.10.155 0169-4332/© 2014 Elsevier B.V. All rights reserved.
rigorously studied as substitutes for InP substrates [12,13] because of their equivalent performance in devices and the freedom of choice for the indium content in the field of the band gap engineering [4,14]. Irrespective of the buffer approach used, the main aims of the MB growth are to produce highly relaxed buffer with a smooth surface and to minimise the device degrading threading dislocation density (TDD) [15]. Highly relaxed MB is essential as any strain in the active layers can severely degrade the transport properties of the device. Dislocations can act as non-radiative recombination centres which degrade carrier life time. Also, dislocations create Coulombic potential which degrade the mobility. Strain non-uniformity introduced by alloy separation leads to roughness at heterointerface. This interface roughness broadens the optical emission spectra for optoelectronics devices and obstructs the smooth carrier transport along the channel in electronic devices. Al containing MBs, InAl(Ga)As, are preferable for their better electrical isolation [16]. At the same time, they have inferior crystalline quality than InGaAs due to clustering effect. Clustering effect is the result of the large difference in the bond energy between In As and Al As [17]. Suitable pathways for MB growth have been adopted considering above mentioned two facts. Fig. 1 depicts the three different pathways used in this work by the band-gap vs lattice constant diagram. Fig. 2 shows the nominal structure of the grown samples.
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2. Experimental procedure
Fig. 1. Lattice constant vs band gap diagram for arsenide material system with Si and Ge. Different arrows show the different pathways used in this work.
It has been reported for InGaN alloys that the roughness at the growth front modulates the indium incorporation rate [18]. However, to the best of author’s knowledge, there has been no report in the literature regarding the relation between indium incorporation and roughness at the growth front for the III-As ternary/quaternary alloys; although the change in indium incorporation with the residual strain in the III-As layer has been reported earlier [19].The study of variation in indium incorporation rate due to different growth front roughness introduced by different MBs is the prime objective of this work. Surface morphology for different MB approaches has been observed. Strain relaxation in the MB and the effect of the MB on the structural properties of the overlying layer has been studied. The observation of relaxation behaviour as well as compositional analysis of MBs has been performed using reciprocal space map (RSM) analysis. Generally, Strain relaxation, tilt information, lattice parameters and alloy compositions are described more efficiently by RSM than rocking curves. Surface morphological study of the grown samples has been done by atomic force microscopy (AFM) images. Compositional variation in top InAlAs layer has been studied with Room temperature photoluminescence (RTPL) analysis by observing the emission wavelength shift.
All structures studied in this work have been grown on epiready semi-insulating GaAs (1 0 0) substrates by the solid source arsenide molecular beam epitaxy (MBE) equipped with an arsenic valve cracker. The arsenide chamber is a part of four-chamber cluster-tool MBE. MBs have been grown by three different routes in this work, including InGaAs, InAlGaAs and InAlAs graded MBs. All structures have an InAlAs healing layer followed by InGaAs cap layer on the top. MB in the first sample (A11) comprises of graded InGaAs layers followed by graded InAlAs layers. In the second sample (A12) graded InAlGaAs layers have been used instead of graded InGaAs. Lastly, in the third sample (A13), graded InAlAs was directly grown on GaAs substrate. The MB growth started with nominal indium mole fraction of 0.1. The growth conditions for final InAlAs graded buffer and healing layers have been kept same in all the samples. To avoid the oxidation of the top surface, a thin InGaAs cap layer has been grown on the top in all the structures. Substrate cleaning has been done to remove the native oxide layer at around 590 ◦ C under As environment. Reflection highenergy electron diffraction (RHEED) pattern changes from dim to bright streaky pattern which indicates the removal of oxides from the substrate surface. Further 20 ◦ C growth temperature increment for 3 min confirms the complete removal of surface oxide. Suitable V/III ratios and growth temperatures have been adopted for binary, ternary and quaternary alloys so that the indium incorporation rate with the surface roughness can be studied. Growth was monitored by in-situ RHEED system (15 keV and 1.52 amp). During the entire growth process, a streaky 2 × 4 RHEED pattern has been observed for A11 and A12 indicating the As-rich surface reconstruction and 2-dimensional growth. A rougher surface has been indicated by RHEED pattern for A13. Surface morphology of the samples on 10 m × 10 m areas has been investigated by AFM (Agilent Tech, model no. 5500) in intermittent contact mode. High resolution X-ray diffraction (HRXRD) measurements have been performed using a BEDE D1 (Jordon Val˚ line and a ley) diffractometer equipped with Cu K␣1 (1.54056 A) Ge (0 0 4) channel-cut crystal. We have described the measurement setup details for double and triple axis (TA) omega-2theta as well as RSM measurements in our earlier work [20]. All the symmetric (0 0 4) and asymmetric (2 2 4) measurements of A11 and A12 have been done in TA configuration. Due to lower intensity of diffracted beam for A13, asymmetric (2 2 4) measurements in TA configuration have not given any meaningful data. Asymmetric (2 2 4) measurements of A13 have been done in double axis (DA) configuration. RTPL measurements have been performed on RPM 2000 nanometrics PL setup using 405 nm diode laser and 420 nm long pass filter.
3. Results and discussion
Fig. 2. Nominal structures of the grown samples.
AFM topographical and phase contrast images of all the samples are shown in Fig. 3. The RMS roughness value of A11 is the least whereas A13 has the maximum RMS roughness value. The difference in surface roughness of all three samples can be explained by cation (In, Al and Ga) surface mobility during growth. At growth temperature less than 600 ◦ C, In, Al and Ga have different surface mobilities at the growth front. This difference even increases at low V/III ratio. The difference of surface mobilities between In and Al is higher than In and Ga. This gives the rougher surface topography of InAlAs relative to InGaAs and InAlGaAs. For InAlGaAs, surface roughness is intermediate to both InGaAs and InAlAs. Moreover, enhanced clustering effect in A13 can result in rougher surface morphology [8].
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Fig. 3. AFM topographical images of (a) A11 (drawn directions are showing the orientation of the stripes), (b) A12, (c) A13 (TD denotes the threading dislocations) and phase contrast images of (d) A11 (e) A12 (f) A13.
The cross hatch pattern, a signature of low mismatched system, has been reported to be the combined effect of buried misfit dislocation and surface diffusion of adatoms [21,22]. Composition fluctuations in the layer of ternary compound have also been proposed as an alternative explanation [23]. One to one correspondence between cross-hatch and misfit dislocation has been shown in [24]. However, in contrary, Hsu et al. [21,25] has reported 1–2 order larger period of cross-hatch than misfit dislocation spacing. Overall, the origin of cross hatch and their mapping with the misfit dislocation have been a matter of debate in research community. However, almost all has conceded the misfit dislocation as the root cause of cross hatch pattern. A strong anisotropy of cross hatch surface morphology can be seen in A11 (stripes are elongated in a particular direction). Stripes in two perpendicular directions
(1 1 0 and 1 1¯ 0) are related to the asymmetry in the formation of two perpendicular group of dislocations, namely ␣ and  dislocations [26]. Any anisotropy in stripes along these two perpendicular directions is related to the different rate of formation of ␣ and  dislocations. A11 and A12 show the cross hatch pattern whereas A13 does not show this pattern. Presence of Al leads to more randomised nucleation and consequently a rough and irregular growth front. Cross hatch pattern is more regular in A11 than A12, as A11 avoids the Al containing buffer at the initial graded layers. Phase lag in the phase contrast image is the result of energy dissipation by the probe tip and sample interaction. Higher phase lag for Al containing buffer (A12 and A13) in the phase contrast image can be the result of either inhomogeneity due to phase separation (driven by randomised nucleation) or enhanced surface roughness. Threading
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dislocations (TDs) can be seen in AFM topographical image of A13. Deep circular dishes and the rim of increased thickness around the circumference can be seen. TD creates a strain field around the dislocation line. To minimise the strain energy due to this strain field, surface changes its morphology. Fig. 4 shows the HRXRD RSMs of all three samples in triple axis (TA) 0 0 4 configuration. The solid line passing through the centre of RSM contour was obtained by a Gaussian fit to the RSM data at Kz values. This line is parallel to the Kz axis or [0 0 4] direction indicating that grown epi-layers has no detectable net tilt with respect to the substrate orientation. Further, symmetric omega-2theta measurements have been taken at four different sample azimuths (ϕ), namely 0◦ , 90◦ , 180◦ and 270◦ , to confirm the tilt. Peak separation between epi-layer and substrate in all four measurements has been same confirming that there is no tilt. Fig. 5 shows grazing incidence (GI) asymmetric 224 RSMs of A11 and A12 in TA configuration; and for A13 in double axis configuration. The solid line passing through the centre of RSM contour was obtained by a Gaussian fit to the RSM data at few representative Ks value for 224 RSM points. In the 224 RSMs, vertical thin line (in the [0 0 4] direction) shows the locus of the distribution of RSM peaks, if the samples were fully strained; whereas inclined thin line (in the [2 2 4] direction) shows the distribution of peaks, if the samples were fully relaxed. Any peak between these two lines indicates the partially relaxed layer. From asymmetric RSM, Strain relaxation with the composition and thickness can be understood. For fully relaxed epitaxial layers, RSM peaks are expected to fall on fully relaxed line (along [2 2 4 direction]) shown in Fig. 5. If a layer is more relaxed; its peak will be more close to this inclined line. The RSM peaks of the MB layers are more close to the inclined line for A11 than A12. This shows that InGaAs MB relaxes more completely than the Al containing MB. The difference in peierls force, related to the atomic bonding, is the possible reason behind this phenomenon [8]. From the symmetric and asymmetric RSM, relaxation and composition of the top InAlAs has been calculated. Out-of-plane lattice constant and mismatch of the top InAlAs layer has been calculated from symmetric RSM. Inplane lattice constant and mismatch of the top InAlAs layer has been calculated from symmetric and asymmetric RSM. These calculated values are listed in Table 1.
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Table 1 lattice constant, percentage relaxation, lattice mismatch and composition of the healing layer, calculated from RSMs.
Relaxation (in %) Composition Parallel mismatch (%) Perpendicular mismatch (%) Cl (in Ang) al (in Ang)
A11
A12
A13
76.5 34% 1.9349 3.1261 5.83011 5.76277
76 39.5% 2.2572 3.6518 5.85983 5.78099
– – – 3.8895 5.87327 –
Table 2 Percentage relaxation, strain and composition of the healing layer, calculated from the combination of symmetric RSM and RTPL results.
Relaxation (in %) Composition Parallel strain Perpendicular strain
A11
A12
A13
87 37.5 −3.390 × 10−3 3.418 × 10−3
81.5 41.5 −5.523 × 10−3 5.596 × 10−3
84 45.7 −5.181 × 10−3 5.275 × 10−3
Measured RTPL from all the samples are shown in Fig. 6. PL Peaks of the healing layers are shown by arrows. The MB in A13 comprises of InAlAs grading from almost 0.1 mol fraction of In. Moreover, indium incorporation rate is highest in A13, as the HRXRD and PL results (Tables 1 and 2) suggest. The combined effect of these two can be responsible for the distinct spectrum of A13. The spectrum of A13 might contain the peaks of initial graded InAlAs MB which will be otherwise absent in other two samples due to the low band gap of InGaAs and InAlGaAs. Also, the two consecutive steps in MB seem to be compositionally well separated to give two separate peaks in the PL emission spectra. Composition, relaxation lattice constant and strain in the healing layer have been calculated from combination of symmetric HRXRD scan and PL result. Calculated values are listed in Table 2. Small deviation in Tables 1 and 2 results could be related to the dopant dependent band gap narrowing in the PL result [27]. A large shift in the emission wavelength and hence composition between the top InAlAs layers of all the samples has been observed. This result suggests that indium incorporation rate in all the samples are not the same.
Fig. 4. Symmetric (0 0 4) RSMs in TA configuration of all the samples.
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Fig. 5. Asymmetric (2 2 4) RSMs of A11 and A12 in TA, A13 in DA configuration.
Composition calculated from RSM (listed in Table 1) and PL (listed in Table 2) suggests the same. Indium content of healing layer drastically changes between these three samples, even though the growth parameters have been kept same during the growth of these layers in all structures. As the calculated strain in the A12 and A13 are almost same, compositional variation among samples is not due to residual strain in the epi-layers. These results
suggest that the final layer Indium composition has strong dependence on the underlying MB at the growth conditions used in our study. The possible reason behind the different Indium composition in all three samples is the different surface roughness. Higher roughness at the growth front provides more stable nucleation sites than the flat surfaces. At the rougher growth front, Indium is fast incorporated compared to Indium adatom diffusing on smooth growth front [18]. Moreover, the rougher growth front may result in more Indium incorporated alloy as increased surface roughness results in enhanced trapping of incident Indium. 4. Conclusions In(Al,Ga)As MBs on GaAs substrate with different pathways have been grown. A suitable growth strategy has been adopted to study the indium incorporation rate with growth front roughness. A comprehensive analysis of the grown samples using AFM, HRXRD and RT-PL has been presented. Strain relaxation, surface morphology and compositional variation in these MBs have been studied. Al containing MBs have been found to produce rougher surface and inferior relaxation properties. Change in indium incorporation rate, in the samples grown with different buffer strategies, has been observed. Probable reason for this change has been reported to be the varying surface roughness at the growth front. Acknowledgements The authors express their gratitude to the financial support of ‘ENS’ project, Department of Electronics and Information Technology (DeitY), Government of India. They are also thankful to central research facility (CRF) of IIT, Kharagpur. References
Fig. 6. RTPL spectra of grown samples.
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Comparison of different pathways in metamorphic graded buffers on GaAs substrate: Indium incorporation with surface roughness Rahul Kumar a,∗ , P. Mukhopadhyay b , A. Bag a , S. Kr. Jana a , A. Chakraborty c , S. Das a , M. Kr. Mahata a , D. Biswas c a
Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur 721302, India Rajendra Mishra School of Engineering Entrepreneurship, Indian Institute of Technology, Kharagpur 721302, India c Department of Electronics & Electrical Communication Engineering, Indian Institute of Technology, Kharagpur 721 302, India b
a r t i c l e
i n f o
Article history: Received 25 August 2014 Received in revised form 8 October 2014 Accepted 27 October 2014 Available online 6 November 2014 Keywords: MBE Metamorphic buffer Surface roughness Cross hatch pattern RSM
a b s t r a c t In this work, compositionally graded In(Al,Ga)As metamorphic buffers (MBs) on GaAs substrate have been grown by MBE through three different paths. A comparative study has been done to comprehend the effect of underlying MB on the constant composition InAlAs healing layer by analyzing the relaxation behaviour, composition and surface morphology of the grown structures. The compositional variation between the constant composition healing layers on top of graded MB has been observed in all three samples although the growth conditions have been kept same. Indium incorporation rate has been found to be dependent on underlying MB. By combining the result of atomic force microscopy, photo-luminescence and X-ray reciprocal space mapping, varying surface roughness has been proposed as the probable driving force behind different Indium incorporation rate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction III–V high electron mobility transistor (HEMT) or in particular InGaAs/InAlAs based HEMT has shown great promise in the field of high speed and low power digital applications [1]. InGaAs/InAlAs based HEMT structure grown on lattice matched InP substrate has many advantages like high low-field mobility and high electron saturation velocity [2]. However, InP substrates are more expensive, brittle and available in only small areas compared to GaAs substrates [3,4]. Very high lattice mismatch (around 3.7%) between GaAs and InGaAs/InAlAs heterostructure (lattice matched to InP) obstruct the direct growth of later on the GaAs substrate. Even 1% lattice mismatch can result in a very high dislocation density (>109 cm−2 ), disastrous for electronic and optoelectronic devices [5]. Compositionally graded metamorphic buffers have been widely utilized to accommodate the lattice mismatch and to produce a template for further growth of active layers having low defect densities [6]. Different buffer routes have been studied previously employing compositionally graded ternary alloys such as InGaAs [7], InAlAs [8] and quaternary alloys such as InAlGaAs [9], InGaAsP [10] and AlGaAsSb [11]. Among them, In(Ga,Al)As buffer have been
∗ Corresponding author. Tel.: +91 9800155190. E-mail addresses: [email protected], [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.apsusc.2014.10.155 0169-4332/© 2014 Elsevier B.V. All rights reserved.
rigorously studied as substitutes for InP substrates [12,13] because of their equivalent performance in devices and the freedom of choice for the indium content in the field of the band gap engineering [4,14]. Irrespective of the buffer approach used, the main aims of the MB growth are to produce highly relaxed buffer with a smooth surface and to minimise the device degrading threading dislocation density (TDD) [15]. Highly relaxed MB is essential as any strain in the active layers can severely degrade the transport properties of the device. Dislocations can act as non-radiative recombination centres which degrade carrier life time. Also, dislocations create Coulombic potential which degrade the mobility. Strain non-uniformity introduced by alloy separation leads to roughness at heterointerface. This interface roughness broadens the optical emission spectra for optoelectronics devices and obstructs the smooth carrier transport along the channel in electronic devices. Al containing MBs, InAl(Ga)As, are preferable for their better electrical isolation [16]. At the same time, they have inferior crystalline quality than InGaAs due to clustering effect. Clustering effect is the result of the large difference in the bond energy between In As and Al As [17]. Suitable pathways for MB growth have been adopted considering above mentioned two facts. Fig. 1 depicts the three different pathways used in this work by the band-gap vs lattice constant diagram. Fig. 2 shows the nominal structure of the grown samples.
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2. Experimental procedure
Fig. 1. Lattice constant vs band gap diagram for arsenide material system with Si and Ge. Different arrows show the different pathways used in this work.
It has been reported for InGaN alloys that the roughness at the growth front modulates the indium incorporation rate [18]. However, to the best of author’s knowledge, there has been no report in the literature regarding the relation between indium incorporation and roughness at the growth front for the III-As ternary/quaternary alloys; although the change in indium incorporation with the residual strain in the III-As layer has been reported earlier [19].The study of variation in indium incorporation rate due to different growth front roughness introduced by different MBs is the prime objective of this work. Surface morphology for different MB approaches has been observed. Strain relaxation in the MB and the effect of the MB on the structural properties of the overlying layer has been studied. The observation of relaxation behaviour as well as compositional analysis of MBs has been performed using reciprocal space map (RSM) analysis. Generally, Strain relaxation, tilt information, lattice parameters and alloy compositions are described more efficiently by RSM than rocking curves. Surface morphological study of the grown samples has been done by atomic force microscopy (AFM) images. Compositional variation in top InAlAs layer has been studied with Room temperature photoluminescence (RTPL) analysis by observing the emission wavelength shift.
All structures studied in this work have been grown on epiready semi-insulating GaAs (1 0 0) substrates by the solid source arsenide molecular beam epitaxy (MBE) equipped with an arsenic valve cracker. The arsenide chamber is a part of four-chamber cluster-tool MBE. MBs have been grown by three different routes in this work, including InGaAs, InAlGaAs and InAlAs graded MBs. All structures have an InAlAs healing layer followed by InGaAs cap layer on the top. MB in the first sample (A11) comprises of graded InGaAs layers followed by graded InAlAs layers. In the second sample (A12) graded InAlGaAs layers have been used instead of graded InGaAs. Lastly, in the third sample (A13), graded InAlAs was directly grown on GaAs substrate. The MB growth started with nominal indium mole fraction of 0.1. The growth conditions for final InAlAs graded buffer and healing layers have been kept same in all the samples. To avoid the oxidation of the top surface, a thin InGaAs cap layer has been grown on the top in all the structures. Substrate cleaning has been done to remove the native oxide layer at around 590 ◦ C under As environment. Reflection highenergy electron diffraction (RHEED) pattern changes from dim to bright streaky pattern which indicates the removal of oxides from the substrate surface. Further 20 ◦ C growth temperature increment for 3 min confirms the complete removal of surface oxide. Suitable V/III ratios and growth temperatures have been adopted for binary, ternary and quaternary alloys so that the indium incorporation rate with the surface roughness can be studied. Growth was monitored by in-situ RHEED system (15 keV and 1.52 amp). During the entire growth process, a streaky 2 × 4 RHEED pattern has been observed for A11 and A12 indicating the As-rich surface reconstruction and 2-dimensional growth. A rougher surface has been indicated by RHEED pattern for A13. Surface morphology of the samples on 10 m × 10 m areas has been investigated by AFM (Agilent Tech, model no. 5500) in intermittent contact mode. High resolution X-ray diffraction (HRXRD) measurements have been performed using a BEDE D1 (Jordon Val˚ line and a ley) diffractometer equipped with Cu K␣1 (1.54056 A) Ge (0 0 4) channel-cut crystal. We have described the measurement setup details for double and triple axis (TA) omega-2theta as well as RSM measurements in our earlier work [20]. All the symmetric (0 0 4) and asymmetric (2 2 4) measurements of A11 and A12 have been done in TA configuration. Due to lower intensity of diffracted beam for A13, asymmetric (2 2 4) measurements in TA configuration have not given any meaningful data. Asymmetric (2 2 4) measurements of A13 have been done in double axis (DA) configuration. RTPL measurements have been performed on RPM 2000 nanometrics PL setup using 405 nm diode laser and 420 nm long pass filter.
3. Results and discussion
Fig. 2. Nominal structures of the grown samples.
AFM topographical and phase contrast images of all the samples are shown in Fig. 3. The RMS roughness value of A11 is the least whereas A13 has the maximum RMS roughness value. The difference in surface roughness of all three samples can be explained by cation (In, Al and Ga) surface mobility during growth. At growth temperature less than 600 ◦ C, In, Al and Ga have different surface mobilities at the growth front. This difference even increases at low V/III ratio. The difference of surface mobilities between In and Al is higher than In and Ga. This gives the rougher surface topography of InAlAs relative to InGaAs and InAlGaAs. For InAlGaAs, surface roughness is intermediate to both InGaAs and InAlAs. Moreover, enhanced clustering effect in A13 can result in rougher surface morphology [8].
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Fig. 3. AFM topographical images of (a) A11 (drawn directions are showing the orientation of the stripes), (b) A12, (c) A13 (TD denotes the threading dislocations) and phase contrast images of (d) A11 (e) A12 (f) A13.
The cross hatch pattern, a signature of low mismatched system, has been reported to be the combined effect of buried misfit dislocation and surface diffusion of adatoms [21,22]. Composition fluctuations in the layer of ternary compound have also been proposed as an alternative explanation [23]. One to one correspondence between cross-hatch and misfit dislocation has been shown in [24]. However, in contrary, Hsu et al. [21,25] has reported 1–2 order larger period of cross-hatch than misfit dislocation spacing. Overall, the origin of cross hatch and their mapping with the misfit dislocation have been a matter of debate in research community. However, almost all has conceded the misfit dislocation as the root cause of cross hatch pattern. A strong anisotropy of cross hatch surface morphology can be seen in A11 (stripes are elongated in a particular direction). Stripes in two perpendicular directions
(1 1 0 and 1 1¯ 0) are related to the asymmetry in the formation of two perpendicular group of dislocations, namely ␣ and  dislocations [26]. Any anisotropy in stripes along these two perpendicular directions is related to the different rate of formation of ␣ and  dislocations. A11 and A12 show the cross hatch pattern whereas A13 does not show this pattern. Presence of Al leads to more randomised nucleation and consequently a rough and irregular growth front. Cross hatch pattern is more regular in A11 than A12, as A11 avoids the Al containing buffer at the initial graded layers. Phase lag in the phase contrast image is the result of energy dissipation by the probe tip and sample interaction. Higher phase lag for Al containing buffer (A12 and A13) in the phase contrast image can be the result of either inhomogeneity due to phase separation (driven by randomised nucleation) or enhanced surface roughness. Threading
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dislocations (TDs) can be seen in AFM topographical image of A13. Deep circular dishes and the rim of increased thickness around the circumference can be seen. TD creates a strain field around the dislocation line. To minimise the strain energy due to this strain field, surface changes its morphology. Fig. 4 shows the HRXRD RSMs of all three samples in triple axis (TA) 0 0 4 configuration. The solid line passing through the centre of RSM contour was obtained by a Gaussian fit to the RSM data at Kz values. This line is parallel to the Kz axis or [0 0 4] direction indicating that grown epi-layers has no detectable net tilt with respect to the substrate orientation. Further, symmetric omega-2theta measurements have been taken at four different sample azimuths (ϕ), namely 0◦ , 90◦ , 180◦ and 270◦ , to confirm the tilt. Peak separation between epi-layer and substrate in all four measurements has been same confirming that there is no tilt. Fig. 5 shows grazing incidence (GI) asymmetric 224 RSMs of A11 and A12 in TA configuration; and for A13 in double axis configuration. The solid line passing through the centre of RSM contour was obtained by a Gaussian fit to the RSM data at few representative Ks value for 224 RSM points. In the 224 RSMs, vertical thin line (in the [0 0 4] direction) shows the locus of the distribution of RSM peaks, if the samples were fully strained; whereas inclined thin line (in the [2 2 4] direction) shows the distribution of peaks, if the samples were fully relaxed. Any peak between these two lines indicates the partially relaxed layer. From asymmetric RSM, Strain relaxation with the composition and thickness can be understood. For fully relaxed epitaxial layers, RSM peaks are expected to fall on fully relaxed line (along [2 2 4 direction]) shown in Fig. 5. If a layer is more relaxed; its peak will be more close to this inclined line. The RSM peaks of the MB layers are more close to the inclined line for A11 than A12. This shows that InGaAs MB relaxes more completely than the Al containing MB. The difference in peierls force, related to the atomic bonding, is the possible reason behind this phenomenon [8]. From the symmetric and asymmetric RSM, relaxation and composition of the top InAlAs has been calculated. Out-of-plane lattice constant and mismatch of the top InAlAs layer has been calculated from symmetric RSM. Inplane lattice constant and mismatch of the top InAlAs layer has been calculated from symmetric and asymmetric RSM. These calculated values are listed in Table 1.
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Table 1 lattice constant, percentage relaxation, lattice mismatch and composition of the healing layer, calculated from RSMs.
Relaxation (in %) Composition Parallel mismatch (%) Perpendicular mismatch (%) Cl (in Ang) al (in Ang)
A11
A12
A13
76.5 34% 1.9349 3.1261 5.83011 5.76277
76 39.5% 2.2572 3.6518 5.85983 5.78099
– – – 3.8895 5.87327 –
Table 2 Percentage relaxation, strain and composition of the healing layer, calculated from the combination of symmetric RSM and RTPL results.
Relaxation (in %) Composition Parallel strain Perpendicular strain
A11
A12
A13
87 37.5 −3.390 × 10−3 3.418 × 10−3
81.5 41.5 −5.523 × 10−3 5.596 × 10−3
84 45.7 −5.181 × 10−3 5.275 × 10−3
Measured RTPL from all the samples are shown in Fig. 6. PL Peaks of the healing layers are shown by arrows. The MB in A13 comprises of InAlAs grading from almost 0.1 mol fraction of In. Moreover, indium incorporation rate is highest in A13, as the HRXRD and PL results (Tables 1 and 2) suggest. The combined effect of these two can be responsible for the distinct spectrum of A13. The spectrum of A13 might contain the peaks of initial graded InAlAs MB which will be otherwise absent in other two samples due to the low band gap of InGaAs and InAlGaAs. Also, the two consecutive steps in MB seem to be compositionally well separated to give two separate peaks in the PL emission spectra. Composition, relaxation lattice constant and strain in the healing layer have been calculated from combination of symmetric HRXRD scan and PL result. Calculated values are listed in Table 2. Small deviation in Tables 1 and 2 results could be related to the dopant dependent band gap narrowing in the PL result [27]. A large shift in the emission wavelength and hence composition between the top InAlAs layers of all the samples has been observed. This result suggests that indium incorporation rate in all the samples are not the same.
Fig. 4. Symmetric (0 0 4) RSMs in TA configuration of all the samples.
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Fig. 5. Asymmetric (2 2 4) RSMs of A11 and A12 in TA, A13 in DA configuration.
Composition calculated from RSM (listed in Table 1) and PL (listed in Table 2) suggests the same. Indium content of healing layer drastically changes between these three samples, even though the growth parameters have been kept same during the growth of these layers in all structures. As the calculated strain in the A12 and A13 are almost same, compositional variation among samples is not due to residual strain in the epi-layers. These results
suggest that the final layer Indium composition has strong dependence on the underlying MB at the growth conditions used in our study. The possible reason behind the different Indium composition in all three samples is the different surface roughness. Higher roughness at the growth front provides more stable nucleation sites than the flat surfaces. At the rougher growth front, Indium is fast incorporated compared to Indium adatom diffusing on smooth growth front [18]. Moreover, the rougher growth front may result in more Indium incorporated alloy as increased surface roughness results in enhanced trapping of incident Indium. 4. Conclusions In(Al,Ga)As MBs on GaAs substrate with different pathways have been grown. A suitable growth strategy has been adopted to study the indium incorporation rate with growth front roughness. A comprehensive analysis of the grown samples using AFM, HRXRD and RT-PL has been presented. Strain relaxation, surface morphology and compositional variation in these MBs have been studied. Al containing MBs have been found to produce rougher surface and inferior relaxation properties. Change in indium incorporation rate, in the samples grown with different buffer strategies, has been observed. Probable reason for this change has been reported to be the varying surface roughness at the growth front. Acknowledgements The authors express their gratitude to the financial support of ‘ENS’ project, Department of Electronics and Information Technology (DeitY), Government of India. They are also thankful to central research facility (CRF) of IIT, Kharagpur. References
Fig. 6. RTPL spectra of grown samples.
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