Suppression of Rotational Twins in Epitaxial B12P2 on 4H-SiC

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Suppression of Rotational Twins in Epitaxial B P on 4H-SiC C. D. Frye, C. K. Saw, Balabalaji Padavala, Neelam Khan, R. J. Nikolic, and J. H. Edgar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00867 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Crystal Growth & Design

Suppression of Rotational Twins in Epitaxial B12P2 on 4H-SiC C.D. Frye,1,2 C.K. Saw,1 Balabalaji Padavala,2 Neelam Khan,3 R.J. Nikolic,1 and J.H. Edgar2* 1. Center for Micro and Nano- Technologies, Lawrence Livermore National Laboratory, Livermore, CA 94550 2. Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506 3. School of Science and Technology, Georgia Gwinnet College, Lawrenceville, GA 30043

Abstract B12P2 was grown epitaxially on 4H-SiC using two different substrate miscuts: a standard 4° miscut toward the [11-20] and a custom miscut 4° toward the [1-100]. Epitaxy on substrates miscut to the [11-20] resulted in highly twinned B12P2 films with a rotational twin density of approximately 70% twin orientation I and 30% twin orientation II. In contrast, epitaxy on substrates tilted toward the [1-100] produced films of >99% twin orientation I. A H2 etch model is used to explain the 4H-SiC surface morphology for each miscut prior to epitaxy and demonstrate how the surface steps influence the nucleation of B12P2 twin orientations. Surface steps on substrates miscut to the [11-20] tend to be zig-zagged with steps rotated 60° from one another producing B12P2 crystals that nucleate in orientations rotated by 60°, hence forming rotationally twinned films. Steps on substrates tilted to the [1-100] tend to be parallel resulting in crystallographically aligned B12P2 nucleation.

Laue photographs taken of B12P2 films grown at 1300 °C on 4H-SiC miscut (a) 4° toward the [1-100] and (b) 4° toward the [11-20]. The X-ray beam is incident normal to the (0001) face of the film and substrate. Red circles enclose 4H-SiC spots while blue squares surround the B12P2 spots. Subscripts “I” and “II” denote the two twin orientations.

*James H. Edgar 1005 Durland Hall Manhattan, KS 66506 Phone: (785)-532-4320 Email: [email protected]

ACS Paragon Plus Environment


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Suppression of Rotational Twins in Epitaxial B12P2 on 4H-SiC C.D. Frye,†,‡ C.K. Saw,† Balabalaji Padavala,‡ Neelam Khan,¶ R.J. Nikolic,† and J.H. Edgar∗,‡ Center for Micro and Nano- Technologies, Lawrence Livermore National Laboratory, Livermore, CA 94550, Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, and School of Science and Technology, Georgia Gwinnett College, Lawrenceville, GA 30043 E-mail: [email protected]

∗ To

whom correspondence should be addressed Livermore National Laboratory ‡ Kansas State University ¶ Georgia Gwinnett College † Lawrence

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Abstract B12 P2 was grown epitaxially on (0001) 4H-SiC using two different substrate miscuts: a ˉ and a custom miscut 4◦ toward the [1100]. ˉ Epitaxy standard 4◦ miscut toward the [1120] ˉ resulted in highly twinned B12 P2 films with a rotational on substrates miscut to the [1120] twin density of approximately 70% twin orientation I and 30% twin orientation II. In contrast, ˉ epitaxy on substrates tilted toward the [1100] produced films of >99% twin orientation I. A H2 etch model is used to explain the 4H-SiC surface morphology for each miscut prior to epitaxy and demonstrate how the surface steps influence the nucleation of B12 P2 twin orientations. ˉ tend to be zig-zagged with steps rotated 60◦ Surface steps on substrates miscut to the [1120] from one another producing B12 P2 crystals that nucleate in orientations rotated by 60◦ , hence ˉ forming rotationally twinned films. Steps on substrates tilted to the [1100] tend to be parallel resulting in crystallographically aligned B12 P2 nucleation.

Introduction Icosahedral boron phosphide (B12 P2 )—one of two semiconducting boron phosphide compounds (the other being zincblende BP)—has several properties that make it an attractive electronic material: a wide bandgap of 3.35 eV, 1 strong resistance to chemical attack, and a reported ability to “self-heal” from intense, high-energy electron bombardment which prevents it from agglomerating defects or becoming amorphous. 2–4 These properties are especially valuable for radiation applications such as radioisotope batteries, radiation detectors, or electronics in high radiation environments. However, poor material quality has prevented fabrication of B12 P2 devices, and B12 P2 crystal growth development is still in its early stages. Since native B12 P2 substrates are unavailable, B12 P2 has most commonly been grown heteroepitaxially on a variety of substrates by chemical vapor deposition (CVD). 5–12 Although Si has been the most prevalent substrate owing to its ubiquity and affordability, 4H-SiC has distinct advantages as a substrate for B12 P2 heteroepitaxial growth. These include 1) a relatively low lattice constant mismatch of about 2.5% when the a-lattice constant of B12 P2 (5.99 Å) is compared to 2



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the length 2×a of 4H-SiC (2×3.073 Å = 6.146 Å) and 2) a higher thermal stability of 4H-SiC over that of Si which decreases auto-doping and increases the temperature window in which B12 P2 can be deposited. However, one drawback of using 4H-SiC as a substrate is that it has hexagonal ˉ only has rhombohedral symmesymmetry (space group P63 mc) while B12 P2 (space group R3m) try, which—as will be described below—can result in twin defects. These defects must be avoided because when domains of different twin orientations meet, they form twin boundaries which are 2-dimensional defects and act as charge carrier scattering centers or worse introduce defect energy levels into the band gap.

Figure 1: The B12 P2 unit cells depicted in the rhombohedral system and hexagonal system using black lines. The H and R subscripts denote hexagonal and rhombohedral indices. Boron icosahedra are centered on the vertices of the rhombohedral unit cell. Boron atoms are the smaller green spheres; phosphorus atoms are the larger red spheres. ˉ space group, the B12 P2 crystal structure can be represented by Belonging to the R3m both rhombohedral and by hexagonal coordinates.

The rhombohedral and hexagonal unit

cells for B12 P2 are depicted using black lines in Figure 1. The two unit cells are crystallographically aligned to one another to show the relationship between the two representations. (Note: the rhombohedral structure in Figure 1 shows B atoms outside of the unit cell to illustrate how B12 icosahedra are located at the vertices of the rhombohedral unit cell.) While rhombohedral coordinates reflect the true symmetry of the crystal, hexagonal indices are often easier to visualize and are convenient to use when relating a B12 P2 crystal to a crystal in the hexagonal system such as during heteroepitaxy of B12 P2 on (0001) 4H-SiC. For example, the crystallographic relationship between B12 P2 and 4H-SiC is more intuitively un3


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ˉ B12 P2 ||(0001)4H-SiC [1100] ˉ 4H-SiC than when derstood using hexagonal indices: (0001)B12 P2 [1100] using a combination of rhombohedral indices for B12 P2 and hexagonal indices for 4H-SiC: ˉ B P ||(0001)4H-SiC [1100] ˉ 4H-SiC . In turn, the (hkil) Bravais-Miller hexagonal coor(111)B12 P2 [121] 12 2 dinate system is used throughout this report. While using the hexagonal (hkil) system is convenient for B12 P2 , it is imperative to keep in mind that the hexagonal indices for the 4H-SiC and B12 P2 are not one-to-one interchangeable. B12 P2 still only has 3-fold rotational symmetry about the (0001) ˉ are not equivalent directions in B12 P2 like they are in 4H-SiC. axis. Therefore, [1100] and [1100] ˉ Likewise, (1100) and (1100) are also not equivalent crystal planes in B12 P2 . Rotational twinning is common during heteroepitaxy of (111) zincblende crystals such as GaP, 13 GaAs, 14 and GaSb 15 on (111) Si, where the bilayer stacking sequence of the zincblende structure can continue that of the Si substrate to form an untwinned domain or rotate 180◦ about the (111) axis via a stacking fault at the epitaxial interface to form a twinned domain. In these cases, twinning is not caused by a symmetry mismatch between the film and the substrate (both have 3-fold rotational symmetry) but because of the stacking fault at the epitaxial interface. Twin formation of B12 As2 , another icosahedral boride, grown on on-axis (0001) hexagonal SiC substrates is also well-documented in the literature. 16–20 Similarly to that of the zincblende/Si system, twin formation on an ideal, perfectly on-axis (0001) hexagonal SiC crystal may not be caused by the symmetry mismatch between the B12 As2 and the SiC because the nucleating epitaxial crystal only "sees" a single SiC bilayer on the crystal surface, which by itself only has 3-fold symmetry. To illustrate, Figure 2 a) and b) depicts two B12 P2 ˉ B P ||(0001)4H-SiC [1100] ˉ 4H-SiC and twin orientations on a single SiC bilayer: (0001)B12 P2 [1100] 12 2 ˉ ˉ 4H-SiC . These two B12 P2 orientations will be referred (0001)B12 P2 [1100] B12 P2 ||(0001)4H-SiC [1100] to as twin orientation I and twin orientation II, respectively. 21 Here, the bonding configuration of the B12 P2 to the SiC is different for each twin orientation. In practice, however, even for a (0001) hexagonal SiC crystal that is nominally cut on-axis, multiple SiC bilayers will still be exposed on the substrate surface. 22 If B12 P2 has a preferred/more energetically favorable rotational orientation on a single SiC

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ˉ Figure 2: (a) (0001) face view and (b) (1ˉ 120) face view of two B12 P2 twin orientations bonded to a single SiC bilayer demonstrating two different bonding arrangements of B12 P2 to SiC. (c) (0001) ˉ face view and (d) (1ˉ 120) face view of two B12 P2 twin orientations bonded to different bilayers of a 4H-SiC crystal. The larger blue atoms are Si while the smaller brown atoms are C. In (c) and (d), the bonding arrangement between each B12 P2 twin and 4H-SiC bilayer is identical, but the twins are rotated 180◦ (or equivalently 60◦ or 300◦ ) relative to each other due to the symmetry mismatch between B12 P2 and 4H-SiC. bilayer, the rotational symmetry mismatch then plays a direct role in twinning if multiple bilayers are exposed. The hexagonal structure of the 4H-SiC is composed of stacks of bilayers that are rotated 180◦ (or equivalently 60◦ or 300◦ ) relative to one another, which is highlighted using red lines in Figure 2 d). So, even for a fixed B12 P2 /SiC bilayer orientation, B12 P2 twins can still form if both rotations of bilayers are exposed to the substrate surface as demonstrated in Figure 2 c) and d). In Figure 2 c) and d), both twin orientation I and twin orientation II have identical bond configurations to the underlying 4H-SiC; the only difference is the B12 P2 rotation and SiC bilayer rotation. Furthermore, Zhang et al. 18,23 and Padavala et al. 24 found that twins could be suppressed 5


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during epitaxy of B12 As2 and BP on (0001) 4H-SiC when the substsrate is miscut 4-7◦ toward ˉ the [1100]. This suggests that nucleation and thus twinning on miscut 4H-SiC is mediated by the substrate step structure which is dependent on the symmetry of the crystal. ˉ and on In this work, B12 P2 was grown on (0001) 4H-SiC substrates miscut 4◦ to the [1100] ˉ under the optimized conditions described in an earlier publicasubstrates miscut 4◦ to the [1120] tion. 12 An idealized etch model as postulated by Zhang et al. 18,25 is presented in detail to explain how the step morphology of the substrates evolves during an in-situ H2 etch and to describe how ˉ B12 P2 twins nucleate on the substrate surfaces. By selecting the miscut to be toward the [1100], the surface morphology can be engineered to suppress rotational twin formation during epitaxy. ˉ has a twin density of The data shows that B12 P2 grown on 4H-SiC substrates miscut to the [1120] approximately 70% orientation I and 30% orientation II. In contrast, B12 P2 grown on substrates ˉ miscut to the [1100] have a twin density of less than 1% orientation II.

Experimental Methods ˉ or B12 P2 films were deposited by CVD on 4H-SiC substrates miscut 4◦ toward either the [1120] ˉ the [1100] at 1300 ◦ C for 30 min using the process described in our previous publication. 12 The films had a nominal thickness of 2 μ m. These two miscuts were chosen because substrates miscut ˉ are the industry standard substrates for 4H-SiC homoepitaxy while substrates miscut to the [1120] ˉ suppress twinning in B12 As2 epitaxy. On-axis (0001) substrates were neglected 7◦ to the [1100] from this study because twinning of epitaxial borides on these substrates is well documented. The surface morphology of the films were imaged and characterized by scanning electron microscopy (SEM) while the root mean squared roughness (RRMS ) was measured by white light interferometry. XRD 2θ -ω scans were taken on both films to confirm that the out-of-plane epitaxial relationship was (0001)B12 P2 ||(0001)4H-SiC . The film quality was assessed by XRD rocking curves. To determine the in-plane epitaxial relationship and reveal the orientation of the films as well as the amount of twinning, transmission Laue photographs were taken, and the diffracted spots were indexed. For

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the Laue photographs, a white spectrum X-ray beam was generated from a Mo source, and the Xray beam was collimated to a diameter of 0.7 mm. The photographs were recorded in a 16-bit format so that spot intensities could be measured. In addition to the Laue photographs, XRD pole figures were taken to further characterize twinning in the samples. To evaluate the surface morphology just before B12 P2 epitaxy, substrates of each miscut type were prepared by the same procedure used during epitaxy. That is, they were loaded into the growth chamber, etched at 1650 ◦ C for 20 min, and cooled to 1300 ◦ C. Then 6 sccm of PH3 was flowed over the substrates for 2 min as is done immediately before epitaxy, but instead of introducing B2 H6 to initiate growth, the PH3 flow was terminated and the substrates were cooled to room temperature and removed from the chamber. The surfaces of each miscut type after the surface preparation were imaged by atomic force microscopy (AFM) to characterize the surface step morphology.

Results Surface Morphology of B12 P2 Films SEM micrographs of the two films reveal that both films are composed of sub-micron, faceted ˉ grains (Figure 3). Although the RRMS roughness (see Table 1) of the film grown on the [1100] misˉ miscut substrate (16 nm), this is still rough cut (7 nm) is much smaller than the film on the [1120] compared to other heteroepitaxial crystal systems which can be an order of magnitude smoother. As observed previously, 12 small secondary nuclei (not shown in SEM) are scattered around the films, possibly from homogeneous nucleation in the gas phase.

Out-of-plane X-ray Diffraction XRD 2θ -ω scans of B12 P2 films grown on each substrate miscut as well as a 4H-SiC substrate without a B12 P2 film are shown in Figure 4. From the XRD spectra, B12 P2 on both substrates

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ˉ Figure 3: SEM micrographs of B12 P2 films grown on 4H-SiC miscut 4◦ to the [1100] (left photo) ◦ ˉ and 4H-SiC miscut 4 to the [1120] (right photo). The scale bar is 3 μ m and is the same for both photos.

Table 1: RRMS roughness (nm) as measured by optical interferometry, XRD rocking curve FWHM (arcsec), and % twin orientation I as measured by Laue spot intensity for B12 P2 films grown on ˉ ˉ and 4H-SiC miscut 4◦ to the [1120]. 4H-SiC miscut 4◦ to the [1100]

Substrate Miscut ˉ 4◦ to [1100]

ˉ 4◦ to [1120]

(Custom)

(Standard)

RRMS Roughness (nm)

7

16

FWHM (arcsec)

1080

1050

% Twin Orientation I

>99%

70%

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have an out-of-plane epitaxial relationship of (0001)B12 P2 ||(0001)4H-SiC . Rocking curves (Figure S1) were also taken about the (0003) B12 P2 peak on both films with FWHM values of 1080 arscec ˉ ˉ miscut substrates, respectively. and 1050 arcsec for films grown on the [1100] miscut and [1120] Rocking curve measurements are a direct measure of the mosaicity of the films, i.e. in this case, small misalignments of the (0003) B12 P2 plane from different crystal domains throughout the film. Similar values for the FWHM should be expected for films grown under the same conditions regardless of the amount of twinning present, as the (0003) planes of either twin rotation are parallel with respect to the (0001) plane of the 4H-SiC and should have little influence on the mosaicity. (0 0 0 3 ) B

1 2

P

( 0 0 0 4 ) 4 H - S iC 2

(0 0 0 6 ) B

1 2

P 2

L o g In te n s ity ( a .u .)

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[1 1 0 0 ] M is c u t

[1 1 2 0 ] M is c u t

S u b s tra te

2 0

2 5

3 0

3 5

4 0

4 5

5 0

5 5

6 0

6 5

7 0

7 5

8 0

2 θ( ) o

Figure 4: XRD 2θ -ω scans of a bare 4H-SiC substrate and B12 P2 films grown on 4H-SiC misˉ ˉ cut toward the [1100] and [1120]. The out-of-plane epitaxial relationship of B12 P2 to 4H-SiC is (0001)B12 P2 ||(0001)4H-SiC as is indicated by the {000l} family of planes for both B12 P2 and 4H-SiC.

In-plane X-ray Diffraction Laue photographs were taken to both reveal the in-plane epitaxial relationship and characterize the presence and amount of rotational twinning in the films. Figure 5 shows the diffraction patterns generated from a broad spectrum Mo X-ray beam incident normal to the (0001) of the substrate and ˉ ˉ miscut substrates are depicted film. B12 P2 films grown on the [1100] miscut substrate and [1120] 9


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in Figure 5 (a) and (b), respectively. All of the diffraction spots in Laue photos have been indexed, ˉ but for clarity, only select spots have been labeled here. In each photograph, the 4H-SiC {101ˉ 1} ˉ spots. To distinguish between spots are circled, and squares are placed around the B12 P2 {202ˉ 1} the two rotational variants of B12 P2 , a subscript of “I” or “II” was included on the B12 P2 spot labels ˉ spots to represent spots from each respective twin orientation. The alignment of the B12 P2 {202ˉ 1} ˉ spots confirms that the inalong a straight line from the center beam spot to the 4H-SiC {101ˉ 1} ˉ ˉ plane epitaxial relationship is (0001){1100} B12 P2 k(0001){1100} 4H-SiC . There were no extraneous spots in the photos, which would indicate that another B12 P2 orientation is present. If another ˉ peak over the (0003) in orientation were present, for example from a possible overlapping (0112) Figure 4, it was too minor to be detected above the background noise level in these measurements.

Figure 5: Laue photographs taken of B12 P2 films grown at 1300 ◦ C on 4H-SiC miscut (a) 4◦ ˉ ˉ toward the [1100] and (b) 4◦ toward the [1120]. The X-ray beam is incident normal to the (0001) face of the film and substrate. Circles enclose 4H-SiC spots while squares surround the B12 P2 spots. Subscripts “I” and “II” denote the two twin orientations. The most notable difference in the diffraction patterns between the two vicinal substrates is the ˉ ˉ I spots in the B12 P2 film on the [1100] miscut substrate and the appearance presence of only {202ˉ 1} ˉ I and {202ˉ 1} ˉ II spots in the film on the [1120] ˉ miscut substrate. Thus, suppression of both {202ˉ 1} ˉ of rotational twinning on [1100] miscut substrates is clearly demonstrated. Since the X-ray beam for the Laue photographs was 700 μ m in diameter, the beam was assumed to be far larger than 10



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any twin domain and that a statistically significant amount of each twin was present in the beam ˉ miscut sample, one twin orientation line. Although two twin orientations are present in the [1120] is more prevalent than the other, as indicated by the difference in intensity between the two sets of ˉ miscut sample. To quantify the amount of each variant in the films, the intensity spots on the [1120] ˉ II spots were integrated. The ratio of (202ˉ 1) ˉ I :(202ˉ 1) ˉ II was ≈7:3 or about ˉ I and (202ˉ 1) of the (202ˉ 1) ˉ I . Upon closer inspection to the [1100] ˉ ˉ spots above the noise 70% (202ˉ 1) tilted sample, no {202ˉ 1} from air scattering could be observed on the Laue photograph, even at an exposure time of four hours. An XRD pole figure confirmed the dominance of a single twin orientation; however, it also contained a very faint signal from a second twin orientation [Figure S2 (a)]. Therefore, a small amount of the second variant could be present throughout the film. Nonetheless, by integrating ˉ satellite peaks corresponding to each rotational variant in the pole the intensities of the {011ˉ 2} ˉ figures, the film grown on the [1100] miscut SiC is >99% variant I. As in the Laue photograph for ˉ miscut sample, both twin orientations are clearly present in the pole figure [Figure S2 the [1120] (b)].

Discussion Mechanism of Rotational Twin Suppression A “reverse” step-flow model is described here to explain how different types of step edges form on each type of 4H-SiC miscut during the in-situ H2 etch. The shape and orientation of these steps influence the nucleation sites of the two rotational variants of B12 P2 . In this model, carbon atoms are presumed to etch to produce methane (CH4 ), and silicon atoms are etched into silane (SiH4 ) molecules. In the etch processes described below, the term “dangling bonds” is used, but the dangling bonds are assumed to actually be terminated by H atoms as the atoms are etched into the reaction products. In short, “dangling bonds” simply refers to C and Si bonds that are not bound to the substrate.

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ˉ Etching of 4H-SiC miscut 4 ◦ to the [1100]

Figure 6: (a) a cross-sectional view of an ideal vicinal (0001) 4H-SiC substrate miscut 4◦ to the ˉ [1100]; the surface steps are equally spaced apart. A perspective view of the terminating C atoms of the two step types are shown above the upper cross-sectional view where dangling bonds are represented by bonds to white atoms. After an ideal in-situ H2 etch, the quickly etching orangelabeled steps bunch together under a slowly etching green-labeled step as depicted in (b). Figure 6 depicts two cross-sectional views of a vicinal (0001) 4H-SiC substrate miscut 4◦ to the ˉ ˉ direction. In Figure 6 (a), the steps are equally spaced apart in an [1100] as viewed along the [1120] idealized unetched surface. In each unit cell length along the [0001], four step risers exist. Within each unit cell height, there are two types of step edges, each with terminating atoms in different bonding configurations. In one type (labeled with green arrows in Figure 6), the terminating C atoms have a single dangling bond and are bound to the rest of the substrates by three bonds. In the second type (labeled with orange arrows), the terminating C bonds have two dangling bonds and are only bound to the surface by two bonds. A close-up perspective view of the terminating C atoms and their respective bonding configurations are shown above the cross-sectional view in Figure 6 (a) with dangling bonds shown as bonds to white atoms (the white atoms exist in the figure to clearly demonstrate the location of the dangling bonds). Since the C atoms in the second (orange) type step are bound less strongly to the substrate because they only have two bonds to the substrate, one would expect that these C atoms would etch more readily during the in-situ H2 etch 12



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than the C atoms in the other step type which are bound by an extra bond. During an ideal surface etch, both types of steps should etch back (toward the left as depicted in Figure 6). Because C atoms with two dangling bonds can be etched more readily, these steps etch faster than the other type. As the orange steps catch up to the nearest green step, the steps bunch together and the step etch rate becomes limited by the green step edge etch rate. The difference in etch rate of the two step types is exaggerated in Figure 6, showing that the green steps do not etch while the orange steps do. The etch mechanism may even be slightly more complicated, following a morphology similar to that described by Flidr et al. 26 However, regardless of the actual etch rates, as long as the orange steps etch more quickly, the final result will be the step structure depicted in Figure 6 (b) [with a corresponding surface step structure depicted in Figure S3 and Figure 9 (d)]. Further details on how the surface structure forms can be found in the supplementary information.

Figure 7: (a) a cross-sectional view of an ideal vicinal (0001) 4H-SiC substrate miscut 4◦ to the ˉ where the surface steps are equally spaced apart. The two step types are marked with yellow [1120] and blue arrows. (b) a top view of a SiC bilayer [outlined in a red dashed box in (a)] from each of the two step types. A solid yellow line represents a step edge of type I while a solid blue line represents a step edge of type II. Dotted yellow and blue lines exist to lead the eye between the cross-sectional and top views. A detailed etched process for each bilayer is depicted in Figures S4 and S5 for steps type I and II, respectively.

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ˉ Etching of 4H-SiC miscut 4 ◦ to the [1120] ˉ The same method of step etching that was applied to substrates miscut to the [1100] can be applied ˉ to the substrates that are miscut to the [1120]. As was done in the previous section, the starting point is a cross-sectional view of a 4H-SiC substrate ideally miscut 4◦ so that all step risers are ˉ equally spaced apart. A cross-sectional view of the steps as viewed along the [1100] is shown in Figure 7 (a). Unlike in Figure 6 (a) where the atomic bonding configuration is evident when viewed ˉ the steps here are essentially indistinguishable from this viewpoint. Nonetheless, along the [1120], as with the other miscut, there are still two types of step edges with different bonding configurations, denoted here as step type I (marked with yellow arrows) and step type II (marked with blue arrows). To better visualize the terminating bonds on the two types of step edges, bilayers from each step type (enclosed by red dashed rectangles) are depicted below the cross-sectional view and ˉ in a top view in Figure 7 (b). It is important to keep in mind in the top are viewed along the [0001] view of these bilayers that all of the Si atoms have dangling bonds pointing out of the page while the C atoms are bonded to Si atoms beneath them with bonds pointed directly into the page. At a glance, the two step types in the top view appear to be similar, but they are rotated 180◦ about the [0001] from one another, or equivalently, they are rotated by 60◦ . Following the same method for ˉ ideally etching the step edges as was done for the [1100] miscut, i.e. atoms with only one dangling do not etch while atoms with two dangling bonds do etch, the step edges etch into the morphology depicted in Figure 7 (b). A detailed step-by-step explanation of the step morphologies produced from this method of etching is layed out in the supplementary information and a schematic of the surface structure is depicted in Figure S6 (b) and Figure 9 (a).

B12 P2 Nucleation on 4H-SiC Steps ˉ Zhang et al. 23,27 found that for B12 As2 growth on 4H-SiC miscut 7◦ to the [1100], the B12 As2 crystal likely nucleates on the slowly etching unbunched, isolated green-labled step riser and grows laterally outward. Here, B12 P2 is presumed to ideally nucleate at the same location and in the same way due to its structural similarity to B12 As2 . A plausible heterostructure of B12 P2 nucleation on 14



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ˉ such a solitary green step edge of a 4H-SiC substrate miscut 4◦ to the [1100] is shown in Figure 8. Only one twin orientation can nucleate on a particular step edge and the epitaxial relationship is ˉ 4H-SiC . ˉ B P k(0001)[1100] locally fixed where (0001)[1100] 12 2

Figure 8: A cross-sectional view of an ideal B12 P2 nucleation site is depicted with B12 P2 nucleating at a solitary slowly etching green step edge. A plausible epitaxial bonding arrangement is shown on the step edge. ˉ two competing nucleation sites (isolated green step edges) For substrates miscut to the [1120], form on the stepped surfaces, which are a result of how the two step types etched. A wide field view of the surface schematic from Figure S6 is depicted in Figure 9 (a), illustrating the zig-zag step structure. These nucleation sites are rotated by 60◦ which is the same degree of rotation between the two B12 P2 rotational variants. Since B12 P2 can nucleate on either of the two step edges, a B12 P2 nucleus on one step edge will be rotated by 60◦ relative to a B12 P2 nucleus on the ˉ will competing step edge. Therefore, B12 P2 films grown on 4H-SiC substrates miscut to the [1120] be twinned as was determined by the Laue photographs. A schematic of the nucleation of the two twin orientations is illustrated in Figure 9 (b) (triangles were added to the B12 P2 structures to help clarify the rotation of the crystal structure). ˉ A wide field view of the surface steps on substrates miscut to the [1100] is illustrated in Figure 9 ˉ miscut substrates, all of the steps and thus all of the nucleation sites are (d). Unlike in [1120] parallel to one another, and therefore, the crystallographic directions of all B12 P2 nuclei must all also be parallel [see Figure 9 (e)]. Regardless of which step edge B12 P2 nucleates on, only one twin orientation forms and the film will be untwinned.

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Evidence for Hypothesized Step Morphologies

Figure 9: Schematic depictions of the ideally etched surface morphology of 4H-SiC substrates ˉ and to (d) the [1100]. ˉ Cartoon B12 P2 nuclei are placed on nucleation miscut 4◦ to (a) the [1120] ˉ and (e) [1100] ˉ sites (solitary green steps) for (b) [1120] miscut substrates. Triangles are added to the B12 P2 structures as a visual aid to identify the rotational orientation of the nuclei. AFM images ˉ and (f) [1100] ˉ of the etched surface for the (c) [1120] miscut substrates (RRMS was 5 nm for both surfaces). The ideal etch mechanism outlined above makes several assumptions: the starting surface is perfectly stepped, the surface only etches at step edges, and perfect control over the step etch rate is achieved. To evaluate whether the ideally etched surface is representative of the actual etched surfaces used during growth of the B12 P2 films in this study, substrates of each miscut type were prepared in the same way as those used to grow the B12 P2 films. Then, the substrates were imaged by AFM. The substrates were etched at 1650 ◦ C for 20 min in H2 and then cooled to 1300 ◦ C where 6 sccm of PH3 was flowed for 2 min. Typically, B2 H6 would be introduced into the system to initiate B12 P2 nucleation, but in this study, the PH3 flow was stopped after the 2 min, and the 16



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substrates were cooled back down to room temperature under H2 flow. AFM images were taken of the surface morphology of the etched substrates and are shown in Figure 9 (c) and (f). While the surfaces are not the perfectly etched steps depicted in the schematics, there is definitely a qualitative ˉ resemblance between the actual and ideally etched surfaces. The striations on the [1100] miscut ˉ miscut substrate are all largely parallel as predicted by the etch model, and the surface of the [1120] substrate has zig-zag-like structures with step edges that are rotated by about 30◦ in each direction ˉ as the etch model predicts. from the (1120) For both of the etched substrates, the surface is more hillocked than it is a smoothly descending series of atomic steps (RRMS was 5 nm for both surfaces). The deviation from the ideal etch model results from less-than-optimal etch and surface preparation conditions. For hillocks to form, etching must take place at locations other than the step edges. The surface etching at 1650 ◦ C is likely too high of a temperature, and the etch is too aggressive. At such extreme temperatures, surface atoms away from the step edges may etch or even thermally dissociate away from the substrate. A lower temperature should promote etching more similar to the idealized etch model that was outlined in this report. Although the surfaces are hillocked, the substrate is still macroscopically ˉ stepped in the direction of the miscut. In the case of the substrate miscut to the [1100], even if ˉ ˉ opposing twin orientations nucleate on the (1100) side of the hillock and the opposing (1100) side ˉ of the hillock, the twin that has nucleated on the (1100) side should still dominate. The twin orienˉ tation that has nucleated on the (1100) side of the hillock can grow laterally out and away from the ˉ substrate due to substrate crystal’s tilt. When the twin orientation that has nucleated on the (1100) side of the hillock grows laterally out, it will terminate as it grows toward and effectively into the substrate. Twins nucleating on each side of a hillock may be the source of the slight detection ˉ of a second twin orientation in the pole figure of the [1100] miscut film. With optimized etching ˉ of [1100] miscut substrates and subsequent nucleation on more ideal surface steps, the secondary twin orientation could be further suppressed, approaching nucleation of 100% twin orientation I. ˉ miscut substrate, there should be an equal For B12 P2 growth on the ideally etched [1120] amount of each twin orientation; however, the film is about 70% twin orientation I. The cause

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for the increased amount of that twin orientation is unknown, but there are several possible causes. One explanation for the discrepancy could be the deviation from a perfect 4H-SiC polytype (the manufacturer claimed the substrate to be 90% 4H-SiC). Another explanation could be that the ˉ substrate is not miscut perfectly toward the [1120]. An alternative is that the etch model is incomplete or that there are other nucleation mechanisms that result from the non-ideal etch. Future experiments should be performed to investigate this further.

Conclusions ˉ B12 P2 films were grown on (0001) 4H-SiC substrates miscut 4◦ to the [1100] and 4H-SiC substrates ˉ miscut 4◦ to the [1120]. The films were characterized by XRD 2θ -ω scans to confirm the out-ofplane epitaxial relationship, XRD rocking curves to evaluate the crystal quality, Laue photography to determine the in-plane epitaxial relationship and characterize the amount of rotational twinning, and XRD pole figures to further assess rotational twinning. The epitaxial relationship for B12 P2 ˉ B P k(0001)h1100i ˉ 4H-SiC and more grown on both types of substrate miscut was (0001)h1100i 12 2 ˉ B P k(0001)[1100] ˉ 4H-SiC for films grown on 4H-SiC substrates miscut specifically (0001)[1100] 12 2 ˉ ˉ to the [1100] (neglecting the small amount of the twin orientation II). Films grown on [1100] ˉ miscut substrates had miscut substrate were >99% twin orientation I while films grown on [1120] a twin density of 70% twin orientation I and 30% twin orientation II. An idealize etch model ˉ was described to explain the suppression of twin formation on substrates miscut to the [1100] by explaining how step edges form on 4H-SiC after performing an in-situ H2 etch and then how B12 P2 nucleates on those step edges. The etch model was qualitatively supported by AFM images of the etched 4H-SiC surfaces. Since rotational twinning was suppressed on 4H-SiC miscut toward the ˉ [1100], this miscut proves to be a superior substrate for heteroepitaxial growth of B12 P2 . In future work, a more detailed study focused on the etch conditions of the 4H-SiC and the nucleation of B12 P2 should be done whereby the etch mechanism and B12 P2 nucleation can be characterized by scanning tunnelling microscopy. Such a study would enable both a better under-

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standing of the etch mechanism and a better etch recipe that prevents hillock formation and creates a stepped 4H-SiC surface that more closely resembles the ideally etched surface from the model described in this work. Such a surface is desirable for controlled nucleation but also because an atomically smooth and abrupt interface between B12 P2 and SiC is desired for p-B12 P2 /n-SiC heterojunction diodes. A study on the evolution and propagation of the twin orientations by crosssectional transmission electron microscopy would also be beneficial to understanding how twins form and grow within the film. Further, electrical measurements of the carrier mobility on twinned and untwinned samples could demonstrate the influence of twin planes on the transport of charge carriers in the epitaxial films. The modification and engineering of the step structure of other types of hexagonal substrates such as AlN or GaN could also potentially be used to control the nucleation and suppression of rotational twins in 3-fold symmetry crystals such as B12 P2 or even in other materials such as cubic (111) compounds.

Acknowledgement We would like to thank Dr. Tina Salquero (Associate Professor of Chemistry) University of Georgia for the use of Innova AFM for surface studies. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DEAC52-07NA27344, LLNL-JRNL-731996-DRAFT. Material growth was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-SC0005156.

Supporting Information Available Rocking curves; Pole figures; Additional figures to help depict H2 etching mechanism of 4H-SiC This material is available free of charge via the Internet at http://pubs.acs.org.

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Graphical TOC Entry

B12 P2 epitaxial layers grown by CVD on standard (0001) 4H-SiC substrates miscut 4◦ toward the [112ˉ 0] resulted in rotationally twinned films. In contrast, twinning in B12 P2 layers was suppressed to

Suppression of Rotational Twins in Epitaxial B12P2 on 4H-SiC

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