Journal of Crystal Growth 496–497 (2018) 31–35
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Improved quality of In0.30Ga0.70As layers grown on GaAs substrates using undulating step-graded GaInP buffers Kuilong Li, Wenjia Wang
T
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College of Science, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
A R T I C LE I N FO
A B S T R A C T
Communicated by K. Jacobs
High quality In0.30Ga0.70As layers on GaAs substrates were obtained using compositional undulating step-graded Ga1-xInxP (x = 0.48–0.78) buffers grown by metal-organic chemical vapor deposition. The density of threading dislocation is about 4.0 × 106 cm−2 and the root-mean-square roughness is only 5.20 nm, which is much better than those grown on the conventional step-graded metamorphic buffer. On one hand, the reversed GaInP layers reduce the distribution imbalance of α dislocations between the (1 1 1) and (−1 −1 1) slip planes, which promote dislocation glide. On the other hand, the inserted tensile strained GaInP layers change the direction of dislocation glide and facilitate dislocation annihilations, which effectively confine the threading dislocations in the buffer. Overall, this work provides a promising way to obtain virtual substrates for the achievement of desired metamorphic devices.
Keywords: A1. X-ray diffraction A1. Stresses A3. Metalorganic chemical vapor deposition B2. Semiconductor III-V materials
1. Introduction In group III-V material systems, heteroepitaxial semiconductor layers play an important role to fulfill the ideal bandgap engineering for the realization of the desired devices. However, the growth technique has been greatly limited by the acceptable substrates, which hinders the growth of defect free high-quality heterostructures in the case of large lattice misfit. In these lattice-mismatch structures, the device layers might be grown on the substrates directly or employing an intermediate buffer layer, which is normally defined as metamorphic buffer. During the growth of the metamorphic buffer, the accumulated strain can be relieved by the formation of dislocations once the layer thickness is above the critical thickness. Unfortunately, the threading dislocations (TDs) may travel vertically through the buffer into the active device layers acting as nonradiative recombination or scattering centers to reduce the carrier lifetime [1,2], which is detrimental to the optoelectronic and electric devices, such as solar cells, lasers, and light emitting diodes (LEDs). Therefore, it is essential and crucial to confine the TDs in the metamorphic buffer by optimizing the growth conditions and buffer structures. An ideal metamorphic buffer should not only be compatible with the target device in the growing process, but also relax the strain sufficiently, prevent the TDs from propagating into the device layers efficiently, and have an acceptable surface roughness. Various metamorphic buffer structures have been implemented in order to realize the targets mentioned above, including thick composition-uniform
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Corresponding author. E-mail address:
[email protected] (W. Wang).
https://doi.org/10.1016/j.jcrysgro.2018.05.014 Received 15 March 2018; Received in revised form 11 May 2018; Accepted 13 May 2018 Available online 14 May 2018 0022-0248/ © 2018 Elsevier B.V. All rights reserved.
buffers, compositionally linear-graded [3,4], step-graded [5,6], or reverse-graded buffers [7,8], and digitally graded buffers [9], especially the compositionally step-graded structures have been proved to reduce the TD density efficiently due to the interfaces formed between the adjacent steps. Metamorphic Ga1-xInxP buffers with lattice constant overlapping from GaP to InP can be grown on GaP, GaAs, Ge, or InP substrates to achieve the goal lattice transition. It has been utilized in many applications as “virtual” substrates, such as in Ga1-xInxP/GaP LEDs [10], Ga1-xInxP/GaAs heterojunction bipolar transistors [11], and Ga1-xInxP/GaAs high electron mobility transistors [12]. Besides those, in the triple-junction Ga0.52In0.48P/GaAs/In0.30Ga0.70As solar cells [13], the 2.20% lattice mismatch between In0.30Ga0.70As and GaAs layers is overcome using the compositionally step-graded metamorphic Ga1xInxP (x = 0.48–0.78) buffer. Although the TD density in the In0.30Ga0.70As layer has been reduced significantly by suppressing phase separation in the GaInP buffer, the buffer structure needs to be further optimized to satisfy the requirements of In0.30Ga0.70As solar cells. In this work, a compositionally undulating step-graded Ga1-xInxP (x = 0.48–0.78) buffer was employed by inserting some reverse step GaInP layers with tension strain in the common compressive stepgraded Ga1-xInxP buffer. The experimental results demonstrated that it improved the quality of In0.30Ga0.70As layers profoundly, not only smoothening the surface but also reducing the TD density largely. The strain relaxation mechanism was also investigated from the tilt generation and transmission electron microscopy (TEM) results. The
Journal of Crystal Growth 496–497 (2018) 31–35
K. Li, W. Wang
Fig. 1. The schematic structures of GaInP buffers for the SG sample (left) and USG sample (right).
Fig. 2. The (0 0 4) rocking curves of InGaAs layers of the SG and USG samples with the incident X-ray beam along the [1 1 0] and [1 −1 0] directions.
compositionally undulating metamorphic buffer provides a promising way to overcome the lattice mismatch and fulfill the desired device bandgap engineering.
3. Results and discussions In zinc blend semiconductors, the compressive misfit strain is mainly relieved by 60° dislocations [15], which can be divided into two types- α dislocations and β dislocations with group III and V atoms at their cores and line vector along the [1 −1 0] and [1 1 0] directions [16], respectively. The strain along the [1 1 0] direction is mainly relieved by α dislocations while in the perpendicular direction β dislocations make great contributions for the strain relaxation. Fig. 2 shows the (0 0 4) rocking curves of both samples along the 〈1 1 0〉 directions. Obviously, the full width at half maximum (FWHM) of the rocking curves of the USG sample is much smaller than those of the SG sample in both [1 1 0] and [1 −1 0] directions, specifically 491 arc sec and 832 arc sec for the SG sample while 366 arc sec and 583 arc sec for the USG sample along the [1 1 0] and [1 −1 0] directions, respectively. The broadening of the rocking curves is primarily attributed to the dislocations with line vector normal to the incident X-ray beam, that is to say, the FWHM measured at the [1 1 0] azimuth reflects the α dislocation density while that at [1 −1 0] corresponds to β dislocations [17]. Since the dislocation density is proportional to the square of FWHM [18], the insertion of reversed GaInP layers decreased the density of α and β dislocations by about 45% and 51%, respectively. Given in Fig. 3, the PL spectra of the InGaAs layers show that the energy bandgap is nearly the desired 1.04 eV in both samples, but the PL peak intensity of the SG sample is much smaller than that of the USG sample, indicating much more TDs existed in the InGaAs layer of the SG sample acting as nonradiative recombination centers, which is consistent with the XRD results displayed in Fig. 2. In addition, the symmetric (0 0 4) RSM gives the lattice parameter a⊥ perpendicular to the (0 0 1) surface of the sample, while asymmetric (2 2 4) RSM with incident X-ray beam along the [1 1 0] and [1 −1 0] directions give the in-plane lattice parameters a[110] and a[1 − 1 0], respectively. Then the corresponding lattice constant of the fully relaxed layer would be determined by the 1−υ υ elastic law as a o = 1 + υ a⊥ + 1 + υ (a[1 1 0] + a[1 − 1 0] ) , where υ is the Poisson ratio. Consequently, the degree of strain relaxation in each 〈1 1 0〉 direction can be expressed by R= (a < 1 1 0 > −a substrate)/(a o−a substrate) . The results demonstrated that the strain relaxation rate is almost identical for both samples, 97.5% in the [1 1 0] direction and 92.3% in the [1 −1 0] direction for the SG sample while 98.1% and 93.0% for the USG sample along the [1 1 0] and [1 −1 0] directions, respectively, implying that the reduction of the TD density in the USG sample is not resulted from the difference of the relieved strain. Therefore, the lower TD density in the USG sample is closely associated with the buffer structure.
2. Experiments All the samples investigated in this paper were grown on (0 0 1) GaAs substrates with 15° miscut towards (1 1 1)A by the Aixtron 200/4 horizontal low pressure (100 mbar) metal-organic chemical vapor deposition (MOCVD) system. Large substrate miscut has been proved to be useful to soften the surface morphology and reduce dislocation density of the metamorphic buffer [14]. The group-III precursors include trimethylgallium (TMGa) and trimethylindium (TMIn), while PH3 and AsH3 were used as the group V sources, and palladium diffused H2 was as the carrier gas. The structures of the conventional step-graded (SG) and undulating step-graded (USG) metamorphic Ga1-xInxP (x = 0.48–0.78) buffers are shown in Fig. 1. The overall growth schedule was as follows: firstly, the GaAs substrates were annealed at 600 °C for 5–10 min under AsH3 to remove native oxides; secondly, a 100 nm thick GaAs layer was deposited at 610 °C to smooth the surface; thirdly, the metamorphic GaInP buffer was grown at 610 °C; lastly, a 1.0 μm thick InGaAs cap layer lattice-matched to the top buffer layer was grown at 675 °C. No interruption was employed in the process of material growth. The average growth rate of the GaInP buffer was about 0.56 nm/s and the V/III ratio was kept at a constant about 108 by simultaneously adjusting the PH3- and In- fluxes. The strain relaxation and crystalline quality of the GaInP buffers and top In0.30Ga0.70As layers were characterized using a X-ray diffractometer (a Bruker D8 system with a 2 kW sealed X-ray tube and 0.154 nm wavelength). Tilt between the epilayers and the substrate along the [1 1 0] direction was characterized by the symmetric (0 0 4) reciprocal space mapping (RSM). The surface morphology and rootmean-square (RMS) roughness were observed using a Veeco Dimension 3100 atomic force microscopy (AFM) system with the Nanoscope IIId controller in tapping mode. The crystal microstructures and dislocation distributions were evaluated by transmission electron microscopy (TEM-a Tecnai G2 F20 S-Twin microscope operated at 200 kV) in cross section (XTEM) and plan view (PVTEM). XTEM samples were cleaved along 〈1 1 0〉 directions. The photoluminescence (PL) measurements were performed by an RPM 2000 mapping system utilizing a 980 nm excitation laser.
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Table 1 Four slip systems of α dislocations for the heteroepitaxy of (0 0 1) zinc-blend semiconductors. System
Line vector
Glide plane
Burgers vector
S1 S2 S3 S4
1/2a[1 −1 0] 1/2a[1 −1 0] 1/2a[1 −1 0] 1/2a[1 −1 0]
(1 1 1) (1 1 1) (−1 −1 1) (−1 −1 1)
1/2a[1 0 −1] 1/2a[0 1 −1] 1/2a[1 0 1] 1/2a[0 1 1]
Fig. 5. Illustration of the asymmetry in misfit components at the growth surface between the (1 1 1) and (−1 −1 1) planes on a (0 0 1) substrate with a miscut toward (1 1 1)A. The b(1 1 1) and b( −1 −1 1) represent the Burgers vectors of α dislocations in the (1 1 1) and (−1 −1 1) planes, respectively.
Fig. 3. Photoluminescence spectra of the InGaAs cap layers of the SG and USG samples measured at room temperature.
Fig. 4(a) and (b) exhibit the RSMs of (0 0 4) planes of both samples with the incident X-ray beam along the [1 1 0] direction. Obviously, tilt was generated between the epilayers and the substrate, generally about 0.37° for the USG sample and 0.46° for the SG sample. α dislocations, as one type of 60° dislocations, have four 1/2a 〈1 1 0〉 {1 1 1} slip systems for (0 0 1) heteroepitaxy as listed in Table 1. The Burgers vector of α dislocation can be resolved into three components shown in Fig. 5, i.e., a screw component bscrew parallel to the dislocation line direction at the interfaces, a misfit component bmisfit perpendicular to the dislocation line in the growth plane, and a tilt component btilt normal to the surface, which may introduce tilt in the epilayers. In the (1 1 1) glide planes, the tilt component of α dislocation is along the [0 0 1] direction whereas along the [0 0− 1] direction in the (−1 −1 1) glide planes. If α dislocations distribute equally in both planes, the tilt component would be canceled out, otherwise, a tilt would be generated. Therefore, herein the tilt in both samples imply an uneven distribution of α dislocations between the (1 1 1) and (−1 −1 1) planes. The substrate misorientation leads to an asymmetry in the magnitude of the misfit component at the interfaces as displayed in Fig. 5, increasing the resolved shear stress (RSS) which is proportional to the Schmid factor on the (1 1 1) slip systems whereas decreasing that on the (−1 −1 1) slip planes [16]. Meanwhile, the activation energy of dislocation nucleation is inversely proportional to the RSS [19,20]. The Schmid factor is about 0.43 and 0.28 for the (1 1 1) and (−1 −1 1) planes, respectively. As a result, α dislocations in the (1 1 1) planes suffering from high RSS are expected to dominate the dislocation nucleation process and then result in a net tilt generation. However, the experiment results is much smaller than the value predicted by the LeGoues model which assumed that the strain relaxation process of the epilayer with composition graded buffer is dislocation nucleation limited [21]. Besides those, by taking the effects of geometry and strain relaxation mechanism into account, Ayers has proposed a model based on dislocation glide to weigh the amount of tilt, and the tilt in the [1 1 0] direction is expressed as [22]
a −c ⎧ Δω = tan−1 tan[ 2 (θ1 + θ2)] s + ⎨ as ⎩
4
δ btilt ⎫ misfit ⎬ ⎭
∑ bi i=1
(1)
where Δω is the tilt, θ1 and θ2 are the angles that the (0 0 1) substrate inclines to [1 0 0] and [0 1 0], respectively, i corresponds to the ordinal number of slip systems, δi represents the strain relieved by the ith slip
Fig. 4. Symmetric (0 0 4) RSM of the USG sample (a) and SG sample (b) with projection of the incident beam on (0 0 1) surface along the [1 1 0] direction. 33
Journal of Crystal Growth 496–497 (2018) 31–35
K. Li, W. Wang
that impacts of pre-existing parallel dense α dislocations to β dislocations. Consequently, in comparison with α dislocations, more β dislocations are introduced in each sample just as demonstrated by the FWHMs results of the rocking curves shown in Fig. 2. In addition, the density of α dislocations in the USG sample is smaller than that of the SG sample, which can weaken the negative effects of α dislocations on the glide of β dislocations in the USG sample. Eventually, the reduction of α dislocations contributes to decreasing the density of β dislocations. On the other hand, as displayed in Fig. 6, the RMS roughness of the USG sample is reduced from about 9.0 nm to 5.2 nm compared to the SG sample. The insertion of the reversed GaInP layers smoothens the surface significantly. The tensile GaInP layers abate the surface strain fluctuations which makes the strain field distributed more uniformly. Then during the growing process, the surface atoms were promoted to diffuse to locate at the favorite sites and lower the surface roughness. In turn, the improved surface morphology results in a lower possibility to impede the gliding of dislocations. Besides those discussed above, the reversed tensile GaInP layers actually increase the probability of dislocation interactions in the form of annihilation. XTEM micrograph shown in Fig. 7(a) exhibits that most of the dislocations in the USG sample are confined in the metamorphic buffer and distinctively located at the interfaces. In the USG metamorphic buffer, during the growing process of the compressive 200 nm layer, the compressive strain was not relieved sufficiently at the initial stage, which make the tensile strain at the interfaces between the 200 nm/100 nm layers was not as large as we designed. However, the small tensile strain still play an important role in improving the surface morphology by abating the surface strain fluctuations and then facilitated dislocation glide and residual strain relaxation. In turn, rarely new dislocations were generated at the interface, and the component difference between the compressive and tensile strain layers was small. As a result, no obvious interface was observed in the XTEM micrograph Fig. 7a. Meanwhile, the density of TDs in the top InGaAs layer shown in Fig. 7(b) is about 4.0 × 106 cm−2 estimated over several (1–3) × (1–3) μm2 TEM images using a statistical method, which can satisfy the demand of solar cells. In comparison, the TD density for the SG sample obtained from the statistical data over several (1.5–3) × (1.5 3) μm2 PV-TEM images is about 1.0 × 107 cm−2 as displayed in Fig. 7(c). During the strain relaxation process of metamorphic buffers, the line direction of dislocations may rotate 90° from compressive stress field to tensile stress field, and the glide direction of dislocations does alter simultaneously. Consequently, the change of dislocation glide at the interfaces between the compressive and tensile GaInP layers in the USG sample enhances dislocation annihilations, performed much like a superlattice structure, and high quality In0.30Ga0.70As layers are obtained using the USG
system, btilt is the tilt component of the Burgers vector, and bmisfit is the misfit component of the Burgers vector. Since the substrate miscut is known in our case, the formula above can be simplified as
a −c ⎧ + Δω = tan−1 tan θ s ⎨ as ⎩
4
δ btilt ⎫ misfit ⎬ ⎭
∑ bi i=1
(2)
where θ is the substrate miscut angle, 15° in our samples. For this model, mainly two cases are taken into consideration based on a different ratio of slip systems participating in the strain relaxation process to determine the tilt. One is that all the four slip systems take part in the strain relaxation equally, and the other is only the dislocations in the most stressed slip systems play roles to relieve strain. The calculation results demonstrate that the tilt is about 0.36° in the first case and 1.38° for the second case. The experimental tilt result of the USG sample is 0.37°, almost identical to the calculation result in the first case, indicating that all the four slip systems of α dislocation in the USG sample were activated to relieve strain and the strain relaxation process is dislocation glide dominated rather than dislocation nucleation. In comparison, the tilt in the SG sample is 0.46°, which is between the extreme cases. Therefore, the insertion of the reversed tension-strained GaInP layers can ease the imbalance of the misfit strain stress distributed between different slip systems, and make dislocations distributed more uniformly which is beneficial to facilitate dislocation glide and reduce dislocation density. Eventually, the density of α dislocations in the USG sample is smaller than that of the SG sample. In lattice mismatched epilayers, the formation of misfit dislocations (MDs) is primarily induced by the gliding of TDs, which facilitates the strain relaxation process. Unfortunately, the MDs existed in the perpendicular direction of TDs may hinder the propagation of TDs [23,24]. The strain field associated with a MD decays with a distance r from the MD by 1/r. In particular, at the interfaces, the strain field of the MDs reduces the effective driving force imposed on the TDs once a moving TD approaches the orthogonal MDs. Then the gliding of TDs is blocked to hinder the formation of MDs and in turn, more dislocations need to be generated to relieve the residual strain. During the growing process of metamorphic GaInP buffers, in comparison with β dislocations, α dislocations are normally firstly nucleated due to their lower nucleation energy [25]. Meanwhile, substrate miscut towards (1 1 1)A makes α dislocations distributed parallel along the [1 −1 0] directions while β dislocations in both (1 −1 1) and ( −1 1 1) planes crossed over with each other [26]. This is confirmed from the cross-hatched surface morphology which is closely associated with the dislocations shown in Fig. 6(a) and (b). Then, the parallel-distributed α dislocations build a strong wall of strain field blocking β dislocation glide. That is to say, the impedance of β dislocations against α dislocations is not as serious as
Fig. 6. AFM images of the SG sample (a) and USG sample (b) with scan size 40 μm × 40 μm. 34
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Fig. 7. (a) XTEM micrograph of the USG sample with the diffraction vector g = 〈2 2 0〉; (b) and (c) the PVTEM micrographs of the USG and SG sample, respectively.
buffer.
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4. Conclusions In this paper, high quality ∼1.04 eV In0.30Ga0.70As layers with about 2.20% lattice mismatch to GaAs were obtained utilizing compositional undulating step-graded GaInP buffers grown by MOCVD. The insertion of reversed GaInP layers can reduce the distribution imbalance of α dislocations between the (1 1 1) and (−1 −1 1) slip planes, which facilitate dislocation glide. In addition, the improved surface morphology and enhanced dislocation annihilations induced by tensile GaInP layers significantly decrease the TD density. Hence, the compositional undulating graded buffer is valid to confine the dislocations and provides a promising way to fulfill the lattice transition as virtual substrates for desired semiconductor devices. Acknowledgements This work is supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2017LF022), the National Natural Science Foundation of China (Grant No. 31600597). References [1] C.L. Andre, J.J. Boeckl, D.M. Wilt, A.J. Pitera, M.L. Lee, E.A. Fitzgerald, B.M. Keyes, S.A. Ringel, Appl. Phys. Lett. 84 (2004) 3447. [2] C.L. Andre, D.M. Wilt, A.J. Pitera, M.L. Lee, E.A. Fitzgerald, S.A. Ringel, J. Appl. Phys. 98 (2005) 014502. [3] M. Natali, F. Romanato, E. Napolitani, D. De Salvador, A.V. Drigo, Phys. Rev. B 62 (2000) 11054.
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