growth strategy for controlling dislocation densities and crystal morphologies of single crystal...
TRANSCRIPT
Diamond & Related Materials 33 (2013) 71–77
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Diamond & Related Materials
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Growth strategy for controlling dislocation densities and crystal morphologies ofsingle crystal diamond by using pyramidal-shape substrates☆
Alexandre Tallaire ⁎, Jocelyn Achard, Ovidiu Brinza, VianneyMille, Mehdi Naamoun, François Silva, Alix GicquelUniversité Paris 13, Sorbonne Paris Cité, Laboratoire des Sciences des Procédés et des Matériaux (CNRS UPR 3407), 93430 Villetaneuse, France
☆ Originally presented at the International ConfereMaterials.⁎ Corresponding author.
E-mail addresses: [email protected], ale(A. Tallaire).
0925-9635/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.diamond.2013.01.006
a b s t r a c t
a r t i c l e i n f oAvailable online 23 January 2013
Keywords:Plasma assisted CVDSingle crystal diamondDislocationsDefectsGrowth
The growth of millimetre-thick diamond single crystals by plasma assisted CVD is complicated by the formationof unepitaxial defects, particularly at the edges of the crystal. These defects tend to encroach on the top surfacehence limiting themaximum thickness to typically a fewhundreds ofmicrometres. Dislocations are another typeof defects that are also particularly formed at the edges of the crystal. They thread through the diamond film,strongly affecting its characteristics. The growth on pyramidal-shape substrates having different angles andorientations was carried out in an attempt to solve those issues. It was found that the pyramidal-shape tendsto disappear after a certain thickness is grown. The inclined faces of the pyramid not only helped in preservingthe crystal morphology over a large thickness but also deviated dislocations towards the edges of the crystal,hence limiting their occurrence at the surface. Using this strategy, millimetre-thick diamond single crystalspresenting a reduced dislocation density were successfully grown.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Diamond is a promising material for making power electronic de-vices [1] especially thanks to a high breakdown field and an excep-tionally high thermal conductivity. However the development ofsuch devices has been hampered by the availability of large-sizethick defect-free single crystal diamonds. The diamond-based powerdevices reported so far can reach high blocking voltages or high currentdensities but once the device is scaled-up with electrode dimensions ofa fewmm2 in order to draw currents of several tens of Amps, the devicesuffers from current leakage or premature breakdown related to crystaldefects [2,3]. Hence the larger the contact area, the higher is the proba-bility to find defects such as dislocation bundles [4,5].
Dislocations can generate levels in the bandgap of diamond dueto the presence of carbon dangling bonds in the core [6]. The stressfield that surrounds dislocations is high enough to affect light prop-agation and to cause birefringence, thus plaguing the use of diamondfor optical windows or Raman lasers [7]. Dislocations in ChemicallyVapour Deposited (CVD) diamond grown onto (001)-oriented sub-strates usually propagate along the growth direction [8,9] to formthe so-called “threading dislocations” (TD) with a typical density ofthe order of 104–106 cm−2, several orders of magnitude higherthan for the best High Pressure High Temperature (HPHT) diamonds
nce of Diamond and Carbon
rights reserved.
[10]. They have two main origins: (i) dislocations already present inthe substrates that extend into the CVD layer, (ii) new dislocationsformed at the substrate/layer interface due to the presence of stress,stacking faults or polishing grooves. These latter are also particularlygenerated from the sharp edges of cubic-shape substrates. A typicalsquare pattern corresponding to the imprint of the substrate is indeedusually observed under cross-polarisers or after surface etching [11]. Byusing an adapted surface treatment such as plasma etching in order toremove surface defects [12,13], their density can be decreased but onlyto a limited extent. Their propagation direction can also be changed byalternating growth parallel and perpendicular to the initial substrate'ssurface [14] or by using off-axis substrates with an angle higher than10° [15,16] since, in the latter case, it becomes more energeticallyfavourable for dislocations to follow the b110> direction. Howevertheir density still remains too high for the development of CVD diamondelectronic devices.
Besides a reduced dislocation density the synthesis of device-gradediamond also requires that very thick crystals are grown so that theycan be used as substrates in a vertical geometry [17]. However thegrowth of millimetre-thick crystals is further complicated by the forma-tion of unepitaxial crystals at the corners and edges of the crystal where(111) and (110) faces are formed. After a thickness of typically 500 μm,these defects overgrowand tend to encroach on the top surface imposingthe growth run to be aborted. The polycrystalline rim must be laser-cutaway and the surface re-polished before CVD growth is resumed whichis time-consuming and causes the formation of newdefects. The additionof nitrogen to the gas phase is a possible way to reduce twinning and de-fect formation for obtaining thicker films [18] but it is unsuitable withdevice-grade diamond since this impurity acts as a deep donor.
Fig. 1. Schematics of the initial diamond substrate before polishing (a) and of substrates polished into a pyramidal-shape (b) type A, 20° {100}-misoriented, (c) type B,20° {110}-misoriented.
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Hence there is a strong need for developing a process that helpsgrowing millimetre-thick CVD diamond crystals with a reduced dislo-cation density. In this paper it is proposed that the substrate's shapecan be engineered in such a way that dislocations are deviated awayfrom the top (001) surface and so that the influence of sharp edgesof the substrate on both dislocation generation and unepitaxial defectformation is inhibited. Pyramidal-shape substrates with varying an-gles and orientations were prepared and the results are discussed interms of achievable thickness and dislocation distribution.
2. Experimental details
Ib HPHT diamond substrates (3×3×1.5 mm3) having both theirlateral and top faces oriented along the {100} directions were used(Fig. 1a). They were then polished into a pyramidal-shape for whichthe lateral sides of the pyramid were inclined by an off-angle of 20°,30° or 40° along either the {100} directions (type A substrates;Fig. 1b) or the {110} directions (type B substrates; Fig. 1c); i.e. eitherthe lateral sides or the corners of the substrate. The square top of the
Fig. 2. Optical images and 3D representation of the sample grown onto 20° {100}-misorientthe CVD layer is (a) 90 μm, (b) 270 μm, (c) 500 μm.
pyramid had an area of approximately 200×200 μm2. The angles anddimensions of the pyramids were measured using confocal laser mi-croscopy (CLM) (Keyence VK9700).
Diamond growth was carried out using plasma-assisted CVD ontothe pyramidal-shape substrates under our optimized growth condi-tions [19]. These typically include a high plasma power density(100 W/cm3), a temperature of around 850 °C and a methane con-centration of 5%. Under these conditions the growth rates of the(100), (111) and (113) crystallographic planes can be described bythe parameters α=1.8, β=1.1, and γ=4 [20]. The growth wassometimes interrupted and resumed in order to observe the evolu-tion of crystal shape.
The crystals were characterised by photoluminescence (PL) imag-ing with a DiamondView™ equipment that uses near band-edge UVlight as an excitation source (around 225 nm). The crystal dimensionswere measured and Scanning Electron Microscopy (SEM) was alsoused to observe the crystal shape with a large depth of field using aZEISS field effect gun system. To reveal dislocations, the sampleswere etched inside the CVD reactor using H2/O2 plasma (98/2) at a
ed pyramidal-shape substrate after several growth interruptions. The total thickness of
Fig. 3. Laser microscopy images of the crystal grown on a 20° {100}-misoriented pyramidal-shape substrate (CVD thickness of 300 μm) showing a) edge, lateral and top faces;(b) a lateral rough face.
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pressure of 200 mbar, a power of 3 kW and a temperature of about800 °C for a few minutes. Dislocations reaching the surface areevidenced by square etch-pits [21]. Birefringence images were ac-quired by observing the crystal under cross polarisers in a home-made set-up. Freestanding CVD diamond plates were obtained bylaser cutting to remove the HPHT substrate followed by polishing.
Fig. 4. Lateral view of the pyramidal-shape substrate during growth illustrating thedisappearance of the inclined lateral face as the film thickens.
3. Evolution of crystal shape — type A substrates
Optical images of the crystal grown onto a type A substrate withoff-angles of 20° are presented in Fig. 2 after three successive growthruns corresponding to a total deposited thickness of 90, 270 and500 μm respectively. A graphical 3D representation is also given toget a better view of the evolution of the crystal shape. The lateralfaces of the pyramid (pink colour) are rapidly disappearing to the fa-vour of the white top (001) face that expands. Moreover new facesare formed at the edge (blue colour) that are roughly but not exactlyaligned with (113) faces found at the corners. These also tend to dis-appear as the growth proceeds and the final shape of the CVD crystalis no different to what would have been obtained if a non-polishedcubic-shape substrate had been used. {113}-type faces at each of the4 corners of the crystal are visible which is consistent with our previ-ously reported results [22] under similar conditions where {113}faces exhibit the slowest growth rate. The corners are twinned forlong deposition times (Fig. 2c).
The lateral faces which were initially smooth are becoming roughas the growth proceeds, showing square kinks and a “fish-scale”mor-phology as illustrated in Fig. 3b. Indeed it is known that during CVDdiamond growth under these conditions, steps are directed towardsb110> directions. For example the well-known pyramidal-shape de-fects that sometimes overgrow on (100) surfaces always have theirfour sides oriented in the b110> directions [23].
The newly formed edge faces (blue colour) show a few striationsthat are running parallel to each other but remain relatively smooth(Fig. 3a). It was observed though that these faces are slightly curvedespecially near the corner. This could be explained by the relativelydifferent temperatures experienced by the top of the crystal that pen-etrates into the plasma compared to the bottom of the crystal whichis placed onto the cooled substrate holder. Moreover growth of thelateral face near the corners might be curbed by the presence of twin-ning and defects at the corners. Nevertheless it can be observed thateven for thick films these twinned sectors are not encroaching onthe top surface that remained smooth and preserved by the presenceof the inclined faces of the pyramid. Hence the use of pyramidal-shape substrates constitutes a real advantage to prevent such de-fects from propagating to the top surface. For a similar thickness,
better morphologies are obtained compared with common cubic-shape substrates.
The growth rates of the lateral and edge crystal faces can be eval-uated from the measurement of the evolution of the size of thecentral square on the microscopy picture [24,25]. According to thesketch of Fig. 4, the displacement of the lateral face is:
Dlat ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiD2norm þ d2
q� sin θ1 þ θ2ð Þ ð1Þ
where d is the enlargement of the central square, Dnorm is the normalgrowth of the top (001) sector which can be directly measured usinga calliper; θ1 is the off-angle of the inclined face (20°) and
θ2 ¼ tan−1 Dnorm.
d
n oð2Þ
The growth rates of the lateral and edge faces were calculatedto be 32.5 and 31.5 μm/h respectively while the growth rate ofthe top sector was 13.5 μm/h. The minimum thickness requiredfor the inclined faces to disappear, i.e. when the central squarereaches the edges of the crystal, was found to be about 490 μm.This reasonably agrees with the observed crystal morphology ofFig. 2c, for which after a deposited thickness of 500 μm, the in-clined faces are very small.
Fig 5. (a) Sketch of the pyramidal-shape substrate and CVD film. The 2 laser-cut planes perpendicular to the surface are shown. (b) Complete PL image of the sliced sample and(c) magnified region.
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4. Dislocation propagation direction and distribution:
In order to observe dislocation propagation, the CVD crystalgrown in 3 successive runs shown in Fig. 2 was laser cut along theb100> direction into a 1 mm-thick slice as shown in Fig. 5a. On thePL image of Fig. 5b the 2 growth interruptions are clearly visible leav-ing blue lines parallel to the surface. These are related to preferentialuptake of impurities such as boron, nitrogen and silicon at thegrowth resumption as previously reported [26]. In the magnified PLimage of Fig. 5c, inclined dark blue streaks that correspond to the lu-minescence originating from dislocations are visible. It is clear thatthe lateral faces of the engineered substrate has successfully changedthe propagation direction of threading dislocations from b100> to adirection close to b110> which is consistent with the resultsreported by Davies et al. [16].
In order to reveal dislocations and evaluate their distribution an-other CVD film, 750 μm-thick, was grown onto a 20° type A substratein a single run in order to avoid the formation of defects due togrowth interruptions and subsequently H2/O2 plasma etched. InFig. 6, the central part of the crystal exhibits a high etch-pit densityindicating that many dislocations have thread straight through theCVD layer from the top square of the pyramid. The other part of thecrystal around the central square shows a reduced etch-pit density.It is believed that the growth onto pyramidal-shape substrateshelped decreasing dislocation density because of two main reasons:first, dislocations were deviated towards the edges of the crystal andcould no longer emerge at the surface; second the absence of sharpedges which for cubic-shape substrates are an efficient source of
Fig. 6. (a) Image of a 750 μm-thick CVD film grown onto 20° {100}-misoriented pyramidalcentre illustrating that there is a central square with a much higher etch-pit density.
dislocations. In fact the imprint of the substrate edges usually re-vealed as a square contour by etching could not be seen in thiscase. Nevertheless a frame of etch-pits can be distinguished aroundthe central square indicating that the boundary between 2 growthsectors acts as an efficient source of dislocations (see Fig. 6b). Despitethis promising result, the dislocation content remains high and fur-ther work is required to evaluate more precisely the improvement,using heavier techniques such as combined X-Ray topography andTransmission Electron Microscopy (TEM).
The CVD filmwas then prepared into a freestanding plate by remov-ing the substrate. The film exhibited high transparency (Fig. 7a). By PL,no evidence of nitrogen-related centres was found (normally leading tored emission) but a dim blue luminescence around the central squarecorresponding to the top of the pyramid was observed (Fig. 7b).Cross-polarisers analysis showed that stress has built-up around thissquare leading to the appearance of a bright cross (Fig. 7c). Hence byusing this growth technique, thicker CVD single crystals with lower dis-location density can be obtained although the initial presence of thepyramid led to a heterogeneous distribution of stress and defects inthe final crystal. The growth history of the crystal can be evidencedusing luminescence techniques such as the DiamondView™.
5. Variation of the pyramid's off-angle and orientation:
CVD diamond growth was also carried out on substrates presentingoff-angles of 20, 30 and 40° both along the b100> (type A) and b110>directions (type B). The films were grown at the same normal rate
-shape substrate after plasma etching to reveal dislocations. (b) Magnified area in the
Fig. 7. (a) Image of a freestanding CVD plate grown onto a 20° {100}-misoriented pyramidal-shape substrate (the circular feature in the centre is the substrate holder); (b) PL imageshowing the presence of a central square; (c) birefringence image acquired in the centre of the crystal.
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(13 μm/h) leading to a deposited thickness of 300 μm in order to com-pare the obtained morphologies presented in Fig. 8.
For the growth on type B substrates (Fig. 8d, e, and f), no extrafaces were formed; the existing lateral faces grew keeping a smoothsurface. The previously reported “fish scale” morphology which canbe detrimental since impurities tend to easily incorporate at stepedges was not observed.
It can be noted that the size of the central square on top of thepyramid decreases with increasing the off-angle for both types ofsubstrates which implies that the lateral and edge growth rates areinfluenced by the pyramidal shape angle. In order to more preciselyquantify this, the different growth rates of lateral and edge facesfor type A substrates and lateral faces for type B substrates were cal-culated using Eqs. (1) and (2) and the results are reported in Fig. 9.
Fig. 8. SEM images of the 300 μm-thick CVD single crystals grown on type A substrates with(e) 30° and (f) 40°.
For growth on type A substrates, the rough lateral faces (red starsymbols) have the highest growth rate and quickly disappear where-as edge faces (blue triangular symbols) remain for longer times. Thecriterion for disappearance of these faces can be defined as:
Vlat > Vnorm � cosθ1 ð3Þ
where Vlat and Vnorm represent the growth rate of the inclined face andtop face respectively and θ1 is the angle of the pyramid. It is representedby the dotted line in Fig. 9 and it is fulfilled whatever the off-angle issince growth rates of lateral and edge faces are always greater than it.Hence, after a given deposited thickness the final shape of the crystaltends to be that expected for growth on a classic cubic-shape substrate.In other words the benefit of growing onto pyramidal-shape substrates
an angle of (a) 20°, (b) 30° and (c) 40°; and type B substrates with an angle of (d) 20°,
Fig. 9. Evolution of the growth rate of different faces as a function of pyramid's off-angle. (a) Type A substrates, (b) type B substrates. The minimum thickness required for disap-pearance of the inclined faces is also given (open symbols) as well as the critical growth rate required (dotted line).
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in terms of dislocation density and inhibition of crystal defects willend. This minimum thickness (green open round symbols) varieswith the angle and orientation of the pyramid. The highest thicknessof 2.2 mm required for having total disappearance of lateral faces isobtained with an off-angle of 40° and type B substrates. Neverthe-less, as illustrated in Fig. 8f, the crystal's morphology is plagued bydefects. In this case the very steep angles of the pyramid reach thebottom of the substrate due to its limited thickness and it is believedthat the high thermal gradients between the top and base of the pyr-amid are responsible for the poor morphology. The best compromiseis thus the use of a pyramidal shape substrate oriented along theb110> direction with angles of 20° or 30°. In such conditions,it was possible to grow a CVD single crystal with a thickness of1.7 mm while limiting defect formation at the edges. An image ofthe freestanding sample is given in Fig. 10. Further measurementsare on their way to assess its dislocation density.
6. Conclusion
In this paper, the influence of engineered substrate shape onto thegrowth of CVD diamond was studied in order to decrease dislocationdensities and improve the morphology of thick films.
The use of pyramidal-shape substrates with angles above 20°allowed bending dislocation towards the edges of the crystal, limitingtheir emergence at the surface. Further work must be carried out toquantify the degree of improvement, in particular using combinedX-ray topography and TEM. The second benefit concerns the crystalmorphology since twinning at the edges of the crystal that usually en-croach on the top surface was inhibited.
Fig. 10. 1.7 mm thick freestanding CVD sample grown using a pyramidal-shape substratewith a misorientation of 20° along the b110> direction.
Theeffect of different orientations andangles of the pyramidal-shapesubstrate on the growth of a CVD layer was studied. Whatever the sub-strate shape, the lateral faces of the pyramid tend to disappear duringgrowth; hence the final crystal shape is not different from that obtainedif a cubic substrate was used. Theminimum thickness required for com-plete disappearance of these faces increaseswith the angle and is higherfor off-angles along the b110> direction, reaching up to 2.2 mm. Usingthis growth strategy it was possible to growmillimetre-thick high puritydiamond crystalswith an improvedmorphology. In the future, this tech-niquewill be applied to the growth of boron-dopedmaterial that is suit-able for fabricating electronic devices.
Acknowledgements
This work was financially supported by the French Labex SEAM(Sciences and Engineering for Advanced Materials and devices),CGI (Commissariat Général à l'Investissement), ANR (Agence Nationalede la Recherche) through the project CROISADD no. ANR-11-ASTR-020and DGE (Direction Générale des Entreprises) through the projectDIAMONIX no. 08 2 90 6066.
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