Citation: Journal of Vacuum Science & Technology B 32, 050601 (2014); doi: 10.1116/1.4892519
View online: http://dx.doi.org/10.1116/1.4892519
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/32/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Direct growth of a 40 nm InAs thin film on a GaAs/Ge heterostructure
by metalorganic chemical vapor deposition
Hung-Wei Yu, Tsun-Ming Wang, Hong-Quan Nguyen, Yuen-Yee Wong, and Yung-Yi Tu
Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan
Edward Yi Changa)
Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan and Department of Electronic Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan
(Received 21 May 2014; accepted 24 July 2014; published 5 August 2014)
In this paper, the authors directly grew an InAs thin film (40 nm) by metalorganic chemical vapor deposition on GaAs/Ge substrates by using flow-rate modulation epitaxy with an appropriate V/III ratio. The growth of a high-quality InAs thin film with periodic 90misfit dislocations was related to a uniform monolayer In atom distribution at the InAs/GaAs interface. The In monolayer effec-tively minimized the difference between surface energy and strain energy, producing a stable inter-face during material growth. The authors also found that a tightly controlled V/III ratio can improve the quality of the InAs islands on the GaAs/Ge heterostructures, though it is not the key factor in InAs thin-film growth.VC 2014 American Vacuum Society.
[http://dx.doi.org/10.1116/1.4892519]
I. INTRODUCTION
In recent years, much progress has been made in growing III–V materials on GaAs or Si substrates for high-speed device applications.1,2 Some III–V compound semiconduc-tors, such as In(Ga)As-based materials, have high electron mobility because of their direct, narrow energy bandgaps. However, the lattice constants of InAs and Ge differ by 7.2%, which generates many InAs islands (Stranski–Krastanov mode, S–K mode) and misfit dislocations at the interface, degrading electron mobility. Some traditional growth techni-ques executed by molecular beam epitaxy, including linearly graded, step-graded and direct growth, were used to suppress stacking fault formation, mixing with 60 and 90 misfit dis-locations at the interface.3,4In general, the above-mentioned methods induce 90complete dislocation formation, and pre-vent gliding of 60 misfit dislocations at the interface. Although these approaches can effectively release compres-sive strain with 90 complete dislocations (90misfit disloca-tions), they require thicker buffer layers, from several hundreds of nanometers to several micrometers; it is very dif-ficult to directly form thin InAs epitaxial layers on highly mis-matched substrates.
However, thin InAs epitaxy on substitutional substrates has been explored by using the epitaxial transfer method.5In particular, Koet al.5also pointed out that the electron mobil-ity in InAs field-effect-transistors increases with the increase in InAs thickness, and reaches a maximum with an InAs
thickness of 40–50 nm. In our previous work, low-antiphase domain and smooth GaAs epitaxy on a Ge/Si substrate with the graded-temperature arsenic prelayer was demonstrated by metalorganic vapor phase epitaxy.6Therefore, the contin-uous development of an epitaxial technique that can grow a thin InAs layer (40 nm) directly on GaAs/Ge or other sub-stitutional substrates without a buffer layer while maintain-ing high relaxation and a smooth surface is an important criterion for the development of high-speed III–V electronic devices on Ge/Si substrate.
II. EXPERIMENTAL METHODS
All samples in this study were grown by a low-pressure metal organic chemical vapor deposition (MOCVD, EMCORE D180) using trimethylgallium, trimethylindium, and arsine (AsH3) as the source materials. Ge (100)
sub-strates with six degree off toward the [111] direction were used, and GaAs epitaxy was grown on them using the graded-temperature arsenic prelayer. The detailed descrip-tion of the growth of GaAs epitaxy on Ge substrate can be found in Ref.6. Prior to growth, the substrate was annealed in hydrogen at 650C for 60 s and then cooled down to the growth temperature of 350C in hydrogen. The graded-temperature arsenic prelayer was formed while the substrate temperature increased from 350 to 420C. Then, the GaAs epitaxy was grown on the Ge substrate by low-temperature epitaxy (450C). The growth temperature for the InAs thin films was kept at approximately 450C, similar to the growth temperature of the GaAs epitaxy. Besides, some researchers showed that the control of V/III ratio is an
a)Author to whom correspondence should be addressed; electronic mail:
important parameter for InAs nanostructure growth, spe-cially quantum dots and nanowires.7–9The V/III ratio during material growth strongly affects the activation energy of the surface and the coverage of the precursors. Therefore, in order to improve the quality of the InAs grown on the GaAs/ Ge heterostructure, the V/III ratio was adjusted in this study from 16 to 225 (samples A–F). To deposit the InAs interfa-cial layer, we grew some samples with flow-rate modulation epitaxy (FME), whose the gas-flow sequence consists of four steps in one cycle, as shown in Fig.1. Our results show that using FME growth with an appropriate V/III ratio (sample G) effectively produced InAs thin films with high relaxation and smooth surface morphology on the GaAs/Ge hetero-structure. The surface morphology of the InAs thin film on the GaAs/Ge heterostructure was examined using atomic
mission electron microscopy, JEOL ARM-200). The crystal-line quality and strain relaxation of the grown samples were estimated using high resolution x-ray diffraction.
III. RESULTS AND DISCUSSION
Figures2(a)and2(b)illustrate cross-sectional high-reso-lution transmission electron microscopy (HRTEM) images of an InAs thin film on the GaAs/Ge heterostructure grown by MOCVD (sample G). The epitaxial thicknesses of the GaAs and InAs layers were about 116 and 40 nm, respec-tively. It can be seen that periodic 90 misfit dislocations (arrowed) were formed at the InAs/GaAs interface, as shown in Fig.2(b). A model for the formation of 90misfit dislocation was established,10indicating that 90misfit dis-locations could be formed by combining Frank and Shockley partial dislocations. The effect of the atomic arrangement at the interface versus 90 misfit dislocations will be discussed later. Due to the rapid formation of the InAs thin film with periodic 90 misfit dislocations on the GaAs surface, the difference between the surface energy and strain energy could become small when the InAs epi-taxial thickness exceeded the critical thickness.11 This behavior implies that atoms from the precursors were likely deposited uniformly on the GaAs surface, not on the distort-ing atomic structure, and eventually form a continuous InAs thin film on the GaAs/Ge heterostructure. On the other hand, from the selective-area diffraction pattern taken from the InAs/GaAs interface, the satellite spots surrounding the primary beam can be clearly seen, as shown in Fig. 2(c). These diffraction spots, close to the primary beam, repre-sent the InAs epitaxial structure, indicating its good crystal-line quality.
Figure3illustrates the reciprocal space map (RSM) of an InAs thin film on the GaAs/Ge heterostructure (sample G)
FIG. 1. (Color online) Schematic illustration of the growth process for the InAs epitaxy (flow-rate modulation epitaxy growth together with an appro-priate V/III ratio of 32) on a GaAs/Ge heterostructure.
FIG. 2. (a) Cross-sectional TEM image of the InAs thin film on the GaAs/Ge heterostructure, (b) HRTEM image of the InAs/GaAs interface, and (c) selected
area diffraction pattern from the InAs/GaAs interface.
obtained by using a Ge (224) crystal analyzer scan. According to the RSM result, it is very difficult to distin-guish between the GaAs and Ge epitaxial structures because of their similar lattice constants and the larger scan range we used in this study. Therefore, the RSM result shows that there are only two regions appearing in the Q(z)direction for
the InAs/GaAs/Ge heterostructure. The theoretical values for Q(z)and Q(x)in this RSM can be calculated (Q(z)¼ 0.6603;
Q(x)¼ 0.4669), and relaxation line was also generated
(shown by the red line in Fig.3). The core of the InAs peak below the relaxation line indicates compressive strain along the growth direction. The degree of relaxation for an InAs thin film on a GaAs/Ge heterostructure can be defined by the following equation:
The degree of relaxation¼ ak asub afull asub
100%; (1) where asub is the lattice constant of the Ge substrate
(5.658 A˚ ) and afull is the lattice constant of the InAs
(6.058 A˚ ). The Bragg angle was determined from the peak position of the InAs epitaxy. The difference of lattice pa-rameters between the in-plane and off-plane was calculated to obtain the exact value ofa//¼ 6.018 A˚ . It can be
demon-strated that a high relaxation of 90% for a 40 nm InAs thin
film on a GaAs/Ge heterostructure was achieved. As judged from the HRTEM and RSM results, a thin InAs layer (40 nm) with high relaxation was grown by MOCVD on the GaAs/Ge heterostructure without a buffer layer.
Figure 4 illustrates a high-angle annular dark field (HAADF) image of the InAs/GaAs interface region taken along the [010] zone axis (sample G). The experimental result shows that the InAs/GaAs interface is clearly identi-fied as containing two atomic phases, each made by a differ-ent type of atom. The atomic arrangemdiffer-ent in the InAs thin film, grown on a 2 MLs thick wetting layer, was slightly distorted along the growth direction. Because of the distorted atomic arrangement, we assumed that some specific In atoms above the wetting layer filled positions with lower energy,12 where each In atom sits in the middle region of the regular In and As column as shown in Fig. 4. This behavior can effectively reduce the difference in lattice constants between GaAs and InAs epitaxial layers and enhance the InAs epitax-ial quality. It implies that the formation of 90misfit disloca-tion is closely related to the In atom distribudisloca-tion at the interface when the InAs epitaxial thickness exceeds the criti-cal thickness. As mentioned above, the success of the growth of a thin InAs film on the GaAs/Ge heterostructure depends
FIG. 3. (Color online) RSM of the InAs thin film on the GaAs/Ge
hetero-structure measured by using a Ge (224) crystal analyzer scan.
FIG. 4. (Color online) HAADF image of the InAs/GaAs interface taken along the [010] zone axis.
FIG. 5. (Color online) AFM images of the InAs epitaxy with different V/III ratios on a GaAs/Ge heterostructure; (a) V/III: 16, (b) FME together with an
on uniform atomic deposition at the interface during initial growth. Based on the HAADF image, we demonstrated that the high surface mobility of In atoms, leading to more uni-form In coverage at the interface, can effectively decrease the surface energy.13Since the surface energy is lower than the distorting energy during the InAs/GaAs/Ge heterostruc-ture growth, a stable interface condition without stacking faults between the InAs and GaAs epitaxial layers can be generated.
The effect of V/III ratio on the quality of InAs epitaxy on the GaAs/Ge heterostructure was also investigated. Figure5
and Table I illustrate the surface morphology and surface roughness with different V/III ratios of InAs epitaxial layers deposited on GaAs/Ge heterostructures as observed by AFM. The optimum V/III ratio of 32 (sample C) can effec-tively improve the surface roughness (4.5 nm) of the InAs/ GaAs/Ge heterostructure. Higher V/III ratios (32), imply-ing that more arsenic atoms exist durimply-ing the InAs growth, lead to poor surface morphology due to 3D growth forma-tion11,14 where a mixture of 60 and 90 dislocations were generated. At lower V/III ratios (<32) and lower growth temperatures, AsH3pyrolysis is incomplete under this
con-dition, providing inadequate arsenic atoms during growth. This growth condition will result in liquid–metal droplets on the surface and then increase surface roughness. Actually, we also observed that a distinct link between indi-vidual InAs islands was formed under lower V/III ratios of 32, as shown in Figs. 6(a) and 6(b). It was obvious that InAs island growth and partial islands coalescence occurred. At higher V/III ratios (225), many stacking faults and threading dislocations formed and reached the island sur-face. These results indicate that producing high-quality InAs islands requires very tight control of the V/III ratio during growth. However, a good V/III ratio alone does not guarantee the formation of an InAs thin film on a GaAs/Ge heterostructure, as shown in Fig.6.
The smooth surface morphology is necessary for high-speed device applications due to reduced interface carrier scat-tering. The results of sample G (in TableI) and Fig.2(b)have shown that the FME growth technique with appropriate V/III ratio can rapidly generate periodic 90 misfit dislocations at
FIG. 6. (Color online) TEM images of the InAs epitaxy with different V/III ratios on a GaAs/Ge heterostructure; (a) V/III: 16, (b) V/III: 32, and (c) V/III: 225. growth conditions, 40 nm InAs film with periodic 90 misfit dislocations
grown on a GaAs/Ge heterostructure.)
Samples Growth temperature (C) Chamber pressure (Torr) V/III ratios Surface roughness (nm) A 450 40 16 7.9 B 450 40 24 6.2 C 450 40 32 4.5 D 450 40 45 5.1 E 450 40 90 8.7 F 450 40 225 7.1 G 450 40 32 1.5
FIG. 7. (Color online) XRD analysis results of the InAs epitaxy without
FME (sample C) and with FME (sample G) on a GaAs/Ge heterostructure.
the interface, reducing the surface roughness from 4.5 nm (sample C) to 1.5 nm. Figure7illustrates XRD analysis results of InAs thin films with and without FME on the GaAs/Ge het-erostructure. The strong XRD peak at about 66 (2h) is the GaAs/Ge heterostructure, and the InAs peak position is about 61. The InAs peak of sample G is sharper than that of sample C. This seems to indicate that the crystal quality of InAs epi-taxy with periodic 90misfit dislocations (sample G, FWHM: 500 arc sec) was much better than sample C without FME.
IV. CONCLUSIONS
We have demonstrated that, as compared with tradi-tional growth technique,3,4 very thin (40 nm) and smooth (1.5 nm) InAs films can be grown directly on GaAs/Ge heterostructures by MOCVD. Under optimum growth pa-rameters, periodic 90 misfit dislocations can be generated at the InAs/GaAs interface to relax the misfit strain (strain relaxation 90%) during the InAs/GaAs/Ge heterostruc-ture growth. We also found that the 90 misfit dislocation formation was closely related to the In atom distribution at the interface when the InAs epitaxial thickness exceeded a critical thickness. Uniform In coverage above the wetting layer promoted the formation of 90 misfit dislocations and produced a stable interface between the GaAs and InAs epitaxial layers. These results demonstrate that high-relaxation InAs epitaxy with periodic 90 misfit dislocations effectively reduces the difference between the surface energy and strain energy during growth, promot-ing the formation of an InAs thin film on a GaAs/Ge het-erostructure. In addition, we also demonstrated that a
tightly controlled V/III ratio improves the quality of InAs islands grown on the GaAs/Ge heterostructure; however, it is not the only key factor controlling InAs thin-film growth.
ACKNOWLEDGMENTS
This work was sponsored by the NCTU-UCB I-RiCE program, Ministry of Science and Technology, Taiwan, and TSMC, under Grant No. MOST 103-2911-I-009-302.
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![Figure 4 illustrates a high-angle annular dark field (HAADF) image of the InAs/GaAs interface region taken along the [010] zone axis (sample G)](https://thumb-ap.123doks.com/thumbv2/9libinfo/7582106.126530/4.918.479.832.85.369/figure-illustrates-annular-haadf-interface-region-taken-sample.webp)
