Dislocation reduction of InAs nanofins prepared on Si substrate using metal-organic vapor-phase epitaxy

InAs nanofins were prepared on a nanopatterned Si (001) substrate by metal-organic vapor-phase epitaxy. The threading dislocations, stacked on the lowest-energy-facet plane {111}, move along the SiO2 walls, resulting in a dislocation reduction, as confirmed by transmission electron microscopy. The dislocations were trapped within a thin InAs epilayer. The obtained 90-nm-wide InAs nanofins with an almost etching-pit-free surface do not require complex intermediate-layer epitaxial growth processes and large thickness typically required for conventional epitaxial growth.


Background
The semiconductor industry has followed Moore's law for over 30 years with gate dimensions of transistors approaching the nanometer scale (<100 nm) [1]. The nanofin channels that are partly wrapped around these gates offer more effective control than that in traditional planar devices [2]. InAs is a direct-bandgap transition semiconductor material with high electron mobility [1][2][3][4]. InAs on a Si substrate is expected to allow side-by-side integration of InAs optoelectronics with conventional Si-based complementary metal oxide semiconductor products using monolithic epitaxial technology [1][2][3][4][5][6][7][8]. Unfortunately, the hetero-epitaxy of InAs nanofins deposited on Si presents challenges, including the dislocations on the nanofin surface and positioning of the nanofin. These dislocations are generated due to an 11.6% lattice constant mismatch (a Si = 5.43 Å, a InAs = 6.06 Å), thermal expansion coefficient mismatch, and polarity mismatch [6][7][8]. For a zinc-blende structure deposited on a diamond structure, the dislocations generated near the InAs/Si interface, such as mistfit dislocations, threading dislocations, and antiphase domain boundaries, are annihilated or multiplied during the epitaxial process [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. A previous report indicated that a threading dislocation density above 10 7 cm −2 for a 900-nm-thick epilayer decreased to 10 6 cm −2 due to the self-annihilation of dislocations when the film thickness was increased to more than 4 μm for a III-V epilayer deposited on a Si substrate [8,18]. A III-V epilayer on a miscut Si (001) substrate has fewer threading dislocations and antiphase domain boundaries than those of an epilayer on an untilted Si (001) substrate [6,17]. Previous studies have attempted to reduce the dislocation density for a III-V epilayer deposited on a Si (001) substrate using methods such as thermal cycle annealing and the utilization of short-period strained intermediate-layer superlattices [6][7][8][9][10]. During thermal cycle annealing treatment, an epilayer is subjected to large temperature oscillations and thus periodically switches between the compressed and tensile states, reversing the motion of the dislocations. However, thermal cycle annealing processes are time-consuming. Strained intermediate-layer superlattices effectively decrease the threading dislocation density by generating additional stress, but their insertion leads to poor reproduction. In previous reports, the density of dislocations in an epilayer deposited on a planar Si substrate could be decreased by about two orders of magnitude by mixed thermal cycle annealing steps and strained intermediate-layer superlattice epitaxial growth processes [6][7][8][9][10]. An alternative technology for effectively reducing dislocation density is a kind of selective epitaxial growth on patterned Si substrate, which has the capability to produce a low-threading-dislocation surface using a simple epitaxial growth process. Yamaguchi et al. reported a model for the etching-pit density that could be reduced as the pattern trench width decreases [19].
In this work, InAs nanofins were deposited on nanopatterned Si (001) with SiO 2 as sidewalls. As confirmed by transmission electron microscopy (TEM), the highaspect-ratio (aspect ratio = trench height / trench width) SiO 2 sidewalls produce an almost etching-pit-free InAs surface. The 90-nm-wide InAs nanofins with almost etching-pit-free surfaces represent a breakthrough for a III-V material for use in the advanced Si-based semiconductor industry.

Methods
InAs nanofin growth was performed in an Aixtron (Herzogenrath, Germany) metal-organic vapor-phase epitaxy reactor using H 2 as the carrier gas, with a total flow of 6,000 sccm. A nanoscale shallow trench isolation pattern, with the narrowest width below 100 nm, and a miscut of 6°towards the 1 1 À 0 Â Ã direction on p-type Si (001) wafers were adopted in this work. For patterned substrate fabrication, a 235-nm-thick SiO 2 film was formed by thermal oxidation on a Si (001) substrate. The SiO 2 mask patterns were produced using 193-nm immersion lithography and reactive ion beam etching on a (001)oriented Si substrate. The nanopatterned Si substrates were then cleaned in a 1:1:7 solution of reagent-grade HCl/ H 2 O 2 /H 2 O for 5 min prior to 1 vol.% HF for 1 min. After a thermal desorption procedure, low-temperature InAs nucleation layers were selectively grown on a patterned Si (001) substrate at 623 K and a pressure of 37.5 Torr. Hightemperature InAs buffer layers were then selectively grown at 823 K and a pressure of 75.0 Torr. The surface morphology was studied using field-emission scanning electron microscopy (SEM). High-resolution cross-sectional TEM images were obtained using an FEI Tecnai F20 microscope (FEI, Hillsboro, OR, USA) operated at 200 kV. Figure 1a,b shows a cross-sectional TEM image and the diffraction pattern of InAs deposited on a one-SiO 2 -sidewall-patterned Si (001) substrate, respectively. The lattice constant of the InAs epilayer is about 5.96 Å. The arrows indicate the threading dislocations. A large number of dislocations were generated at the InAs/Si interface; the dislocation density is above 10 9 cm −2 . In Figure 1a, the dislocations settle at an incline angle to the Si (001) surface, and some dislocations are blocked by the one-SiO 2 -sidewall pattern. However, this one-SiO 2 -sidewall pattern cannot effectively prevent the dislocations from prorogating upward. Figure 1c shows a TEM image of InAs grown on a 200-nm-wide patterned Si (001) substrate with an aspect ratio of 1.2. The dislocations are blocked within the initial epilayer with the two-SiO 2 -sidewall-patterned Si substrate. The diffraction pattern of the InAs epilayer on a 200-nm-wide trench-patterned Si (001) substrate is shown in Figure 1d. The lattice constant is about 6.03 Å, which is similar to that of natural InAs (6.06 Å). The d 002 value (d 002 = 3.029 Å) of InAs on nanopatterned Si substrate is similar to that of normal InAs (d 002 = 3.030 Å). A low-residual-strain InAs epilayer was obtained using the nanopatterned Si substrate. In addition, the diffraction pattern shows no twin spots, confirming a lack of twin defects. According to the diffraction pattern, the epitaxial direction of InAs is along the [001] direction. The InAs has a coherent direction with the (001)-oriented Si substrate. Figure 2a,b shows a TEM image and the diffraction pattern of InAs grown on a 90-nm-wide trench-patterned Si (001) substrate with an aspect ratio of 2.5, respectively. The 11 À 0 Â Ã direction is the short axis of the InAs nanofins. The lattice constant of the InAs epilayer is about 6.04 Å. Trenches with an aspect ratio of 2.5 are effective in stopping the extension of dislocations. The dislocations generated in the zinc-blende (InAs) and diamond structure (Si) interface include misfit and threading dislocations. Misfit dislocations are generated from the InAs/Si interface and are parallel to the growth plane [15]. Theoretical models have proposed the possibility of strain relief by decreasing the initiating epitaxial areas to the nanoscale regime [19][20][21][22][23][24]. The effect of strain transfer and dilution for finite dimensional nanopatterned substrate is considered as the linear dimension of individual nanoscale areas is reduced to the point where the stress is completely relieved before sufficient energy is accumulated to create a dislocation. The almost dislocation-free epilayer is possibly demonstrated when the epilayer was deposited on a nanoscale epitaxial area. However, we observed the dislocations, as shown in Figure 2a. The stain energy in the trench middle region is higher than the trench edge region [20][21][22][23]. The dislocations are generated in the trench middle region due to strain relaxation. Then, the dislocations extended to the trench edge region. Figure 2c shows a cross-sectional TEM image along the 11 Ã direction (the short axis of the nanofins) for the epilayer deposited on submicron-patterned Si (001) substrate [25]. Misfit segments can lie along the 11 À 0 Â Ã direction in the (001) growth plane, while threading segments rise up on the {111} plane in the same direction [6,25]. Threading dislocations on the {111} plane are a result of the planes' lower facet surface energy [13,14]. Threading dislocations make a 45°angle with the underlying Si (001) substrate. The threading dislocations can be stopped by the SiO 2 sidewalls on the Si (001) substrate. It is anticipated that almost all the dislocations will be blocked when the aspect ratio is greater than tan45°(aspect ratio > 1 [width × tan45°]). For InAs deposited on a 90-nm-wide trench-patterned Si (001) substrate, the dislocations can be trapped at a thickness below approximately 90 nm. As shown in Figure 2c, a displacement-type moiré pattern of the InAs initial deposition layer, which reflects the usual characteristic of a regular misfit direction, is observed at the InAs/Si interface. The InAs deposited on the nanopatterned Si (001) is almost dislocation-free on the top epilayer region. The pattern aspect ratio of 2.5 is good enough to block the dislocations from propagating upward.

Results and discussion
A schematic diagram of the dislocation-trapping mechanism is shown in Figure 3. Compared with the rectangularplanar-shaped trench bottom, the concave-shaped trench bottom helps the formation of double steps, resulting in the suppression of antiphase domain boundaries [12]. The concave-shaped bottom, related to the double-step formation energetics on the substrate surface, facilitates surface step creation. Compared with InAs on a Si (111) substrate for a large number of vertical upward dislocations, the InAs on a Si (001) substrate can avoid the vertical upward dislocations. Compared to a one-SiO 2 -sidewall-patterned Si substrate, a high-aspect-ratio nanopatterned Si substrate with two SiO 2 sidewalls can decrease the dislocation density from about 10 9 cm −2 to almost 0.
In previous reports, the etching-pit density of the epilayer deposited on a planar Si substrate was reduced by more than one order of magnitude to about 1.2 × 10 6 cm −2 using thermal cycle annealing and strained intermediatelayer superlattices [9,10]. Another study reported an etching-pit density of 2 × 10 6 cm −2 for epilayer deposited on a planar Si substrate using GeSi intermediate layers [26]. Another study found an etching-pit density for an epilayer deposited on submicron domain-patterned and planar Si substrates of 6 × 10 5 and 4 × 10 7 cm −2 , respectively [27]. The molten KOH etching was used to examine the dislocation behavior on the epilayer surface [15,28,29]. In this study, the samples were immersed in molten KOH solution at 633 K for 2 min, and then, the standard cleaning steps were used to clean the sample surface. Figure 4a shows a SEM image of an InAs nanofin surface. The InAs nanofin was selectively deposited in individual open Si surfaces bounded by the SiO 2 mask. The length of the nanofins is above 4.5 μm. A high-resolution TEM image of the InAs nanofin surface is shown in Figure 4b. The two images indicate an almost etching-pit-free InAs nanofin surface. Corresponding to Figure 2a,c, the dislocations generated from the InAs/Si interface were trapped, and thus, an almost etching-pit-free surface was created, as shown in Figure 4a,b. Gao et al. reported vertical InP nanowires epitaxially grown on Si (111) by metal-organic vapor-phase epitaxy (MOVPE) [30]. Liquid indium droplets were formed in situ and used to catalyze deposition. Recently, Miao et al. demonstrated a catalyst-free InP nanowire growth on Si (001) by MOVPE [31]. The density and size of the nanowires can be controlled, but the positions of the  nanowires are difficult to define. In this study, we used the immersion lithography technique to define the nanoscale epitaxial position. InAs nanofins (a nanostructure similar to nanowires) on a nanopatterned Si (001) substrate are suitable for the advanced commercial integrated circuit industry. InAs deposited on a nanopatterned Si substrate is a viable option for a good template for III-V epilayer integrated circuits for use with Si-based systems.

Conclusions
The 90-nm-wide InAs nanofins were selectively deposited on a nanopatterned Si (001) substrate with SiO 2 as sidewalls. As confirmed by TEM, the InAs deposited on the nanopatterned Si (001) shows an almost dislocationfree top epilayer region. The dislocations, originating from the InAs/Si interface, extended along the {111} facet plane and were terminated by two SiO 2 sidewalls. When the aspect ratio is greater than 1, the dislocations can be trapped within an initial thickness (thickness < 100 nm). The 90-nm-wide InAs nanofin surface is almost etching-pit-free. InAs nanofins, with a width of about 90 nm, have the potential to be incorporated into the advanced Si-based complementary metal oxide semiconductor technology platform, allowing InAs material to be integrated with Si (001) substrates.