WO1992001827A1

WO1992001827A1 - Oriented diamond crystals

Info

Publication number: WO1992001827A1
Application number: PCT/US1991/005099
Authority: WO
Inventors: Michael W. Geis; Nikolay Efremow; Henry I. Smith
Original assignee: Massachusetts Institute Of Technology
Priority date: 1988-06-03
Filing date: 1991-07-19
Publication date: 1992-02-06

Abstract

A method for producing oriented diamond films on a substrate. Diamond seed crystals are suspended in a slurry and applied to the susbstrate surface. Removing the slurry fluid produces seed crystals whose (111) crystal planes are substantially similarly oriented to the substrate. An oriented polycrystalline diamond film may be grown about the seed crystals by CVD.

Description

ORIENTED DIAMOND CRYSTALS

Related Application This application is a continuation-in-part of U.S. application Serial No. 202,877, filed June 3, 1988, by Michael . Geis et al. and entitled "Oriented Diamond Crystal", the entire contents of that application being incorporated herein by reference.

Field of the Invention This invention relates to the fabrication of semiconductor material for use in electronic devices, and more particularly, to the fabrication of diamond semiconductor films.

Background of the Invention Diamond is a material with semiconductor properties that are superior to Silicon (Si) , Germanium (Ge) or Gallium Arsenide (GaAs) , which are now commonly used. In particular, diamond provides a higher band gap, a higher breakdown voltage and a greater saturation velocity which produces a substantial increase in its projected cutoff frequency and maximum operating voltage compared to devices fabricated from Si, Ge, or GaAs. Furthermore, diamond has the highest thermal conductivity of any solid at room temperature and excellent conductivity over a temperature range up to and beyond 500°k. Diamond therefore holds the potential for efficient semiconductor operation at high frequency and power. Finally, diamond, by virtue of its small molecular size compared to other materials, provides a smaller neutron cross section which reduced its degradation rate in radioactive environments. Unfortunately, however, the advantages of diamond as a semiconductor have not been exploited for various reasons.

Although natural diamonds may be of device quality, they are of limited supply and size. Furthermore, most natural diamonds are insulators, and thus introduction, or doping, of electrically active impurities such as boron by ion implantation is required to render them useful as semiconductors. Doping via ion implantation has proven to be problematic in diamond. A review of this method may be found in Vavilov et al. "Electronic and Optical Processes in Diamond" copyright 1975, Nauka, Moscow.

A more practical approach is to synthesize device quality diamonds on a- desirable substrate by chemical vapor deposition (CVD) . In this technique, a gaseous mixture including a carbon supply, usually provided by methane and hydrogen, is pyrolized, or injected into a high frequency plasma, above a substrate surface. Radicals containing carbon react to produce diamond crystals on the substrate, while the hydrogen present is converted to atomic hydrogen which preferentially etches away graphite and thereby leaves a film which is predominately diamond. This method also allows for the possibility of doping by introducing electrically active impurities into the environment above the substrate which are then trapped in the diamond lattices so synthesized. The quality and grpwth rate of diamond films produced by CVD on substrates has been found in the past to be dependent on the nature of t e substrate. In fact, it had been thought that only whe:_ the substrate is diamond itself may device quality films be grown. In this technique, known as homoepitaxy, the orientation of newly deposited diamond is coincident with that of the substrate. This method, however, imposes the disadvantages of cost, size and availability of natural diamonds. Furthermore, in the cases where films have been successfully formed not using diamond as the substrate, they have proven to be largely inhomogeneous, i.e, polycrystalline and exhibiting excessive crystal defects. The growth of diamond films about diamond seed crystals has been practiced in our laboratory. The advantage to this approach is that the seeds may be small, and may take advantage of more readily available natural or synthetic diamond grits. Furthermore, the rate of diamond growth by CVD is enhanced by employing seeds. Another method involves polishing silicon substrates with diamond powder and growing diamond films by CVD about the crystal residue which remains. In both techniques, the seed crystals are completely unoriented and the resulting films are polycrystalline and inhomogeneous.

Summary of the Invention In a first aspect, the invention features a method for forming an electrical device having a diamond-like film with a substantially uniform crystalline orientation. The method includes orienting seed crystals on a substrate surface. A slurry fluid is applied to the substrate surface and incorporated in the slurry fluid are seed crystals to substantially separate the seed crystals. Each of the seed crystals has at least one common crystalline plane. The slur*. / fluid is gently removed to enable preferential orien^! ation of seed crystals on the substrate such that tι< ^~- common crystalline planes of the crystals are oriented similarly with respect to the substrate. A film about the seed crystals. The film is formed of crystalline regions having the common crystalline planes oriented similarly with respect to the substrate. In various embodiments: the common crystalline planes are the (111) planes. The common crystalline planes of the crystals are substantially parallel to the surface of the substrate on which each of the crystals is supported. The common crystalline planes of the crystals are substantially parallel to the complementary crystalline planes of the substrate. The crystals have a cubic crystalline structure. The crystals are selected from the group consisting of diamond and boron nitride. The crystals have the (111) planes exposed, and/or the crystals have the (100) planes exposed.

In various embodiments: the substrate is non-diamond. The substrate is a metal, silicon or quartz.

In various further embodiments: the method includes the step of cleaning the seed crystals before the step of incorporating the seed crystals in a slurry. The cleaning includes etching the diamond crystals. The cleaning includes cleaning the crystals in NaN0₃. The NaNo₃ is at about 300°C. The seed crystals are greater than about 1.0 and less than about 100 μm.

In various embodiments: the slurry fluid is a mixture of water and soap. The slurry fluid is a mixture of hydrocarbon oil and trichloroethylene. The slurry fluid is sulfuric acid. The slurry fluid includes an etchant for diamond. lln further /arious embodiments: the substrate is a smooth*single plarαe. The surface of the substrate is patterned to all w preferential orientation of the seed crystals with respect to rotation about an axis perpendicular to the common planes. The substrate includes pits distributed over the surface. The substrate includes gratings. The seed crystals are substantially symmetric faceted crystals. The seed crystals are about the size of the textured features. The film approaches single crystal diamond.

In various embodiments: after the removing step, crystals not similarly oriented are removed by applying to the substrate and seed crystals, glue-type material, drying the glue-type material and washing the substrate. The steps of orienting the seed crystals and removing crystals not similarly oriented may be repeated to provide a desired density of crystals on the substrate.

In another aspect, the invention features a method for forming a diamond-like film with a substantially uniform crystalline orientation. The method includes selecting cubic seed crystals having a preformed shape and size and preparing a substrate by patterning the substrate to form surfaces complementary to the shape and size of the cubic seed crystals. The seed crystals are oriented on the substrate surface by applying a slurry fluid to the substrate surface and incorporating in the slurry fluid seed crystals to substantially separate the seed crystals. Each of the seed crystals has at least one common crystalline plane. The slurry fluid is gently removed to enable preferential orientation of seed crystals on the substrate such that the common crystalline planes of the crystals are oriented similarly with respect to the substrate and the rotational orientation of the seed crystals is defined by the patterned surfaces. A film is grown about the seed crystals. The film is formed of crystalline regions having the common crystalline planes oriented similarly with respect to the substrate. Preferably the common crystalline planes of the crystals are substantially parallel to the surface of the substrate on which each of the crystals is supported. The common crystalline planes of the crystals are substantially parallel to the complementary crystalline planes of the substrate. The substrate is silicon. The common crystalline planes are the (ill) planes. The patterned features are pits having exposed the (111) planes of the substrate. The seed crystals are faceted diamond seed crystals, having substantially symmetric shape. The film approaches single crystal diamond. Brief Description of the Drawings A preferred embodiment of the present invention is described in the accompanying drawings, in which: Fig. 1 is a flow diagram showing four preferred steps of the oriented diamond crystal process of the present invention;

Fig. 2 is a perspective view of a diamond crystal seeded onto a flat substrate in practice of process steps 1-3 of Fig. 1, showing a preferred crystal orientation on the substrate;

Fig. 3 is a graphic representation of an X-ray diffraction intensity pattern obtained from diamond crystals oriented on a fused silica substrate according to the present invention, where X-ray intensity is plotted on the Y-axis against the ^"Bragg angle 20; Fig. 3a is the x-ray diffraction signal from randomly oriented seed crystals, suspended in a paste.

Fig. 4 is a graphic representation of the distribution of the orientations of the crystals' (111) p .anes relative to the substrate planes, before (dotted ine) and after (solid line) step 3 of Fig. 1, where diffraction intensity is plotted on the Y-axis in arbitrary units and degree of tilt of crystals* (111) planes relative to the substrate planes is plotted along the X-axis; Fig. 4a is an illustration of a diamond seed crystal with its (111) axis displaced by θ from the substrate normal; Fig. 5a is a schematic view of seed crystals on a substrate surface before step 4 of the present invention; Fig. 5b is a schematic view showing growth of the seed crystals during the early stage of step 4 of the present invention; Fig. 5c is a schematic view showing formation of a crystal film after step 4 of the present invention;

Figs. 6-6a are schematic views, respectively, of a random crystalline film and an oriented film.

Fig. 7 is a perspective view of a flat surface, seeded with diamond crystals in practice of the present invention wherein the orientation of the (111) planes is depicted;

Fig. 8 is a perspective view of a grated surface, seeded with diamond crystals in practice of the present invention wherein certain planes and directions are depicted;

Fig. 9 is an x-ray diffraction pattern from a substrate as in Fig. 8 seeded with faceted crystals. Fig. 10 is a perspective view of a pitted substrate, included faceted diamond seed crystals, while Fig. 10a is an enlarged view of the boxed region "a" of Fig. 10 and

Fig. 11 is a side cross-sectional view of a vertical semiconductor device having an oriented polycrystalline film grown in practice of the present invention on a conductive substrate.

Description of the Preferred Embodiment Referring to Figure 1, there is provided a flow diagram of the preferred method for oriented diamond crystal growth, including suspending seed crystals in a slurry applied to the surface of a prepared substrate, gently removing the slurry fluid, for example, by heating the substrate to evaporate the fluid, and growing a film of diamond crystals on the treated substrate, by CVD.

As a result of practicing the steps of the above invention, a diamond crystal film can be grown on a substrate. In particular, steps 1-3 of this procedure produce oriented seed crystals, i.e., a desired crystalline orientation of seed crystals on the substrate is effected such that a common planes of the crystals is oriented similarly with respect to the substrate. For example, the seed crystals may be oriented such that particular crystalline planes of the seed crystals, the (111) planes, are substantially parallel to the plane or planes of the substrate surface on which the crystals are supported. These seed crystals may be, for example, diamond or boron nitride or any other crystal whose crystal structure and behavior is similar to diamond. In Step 4, a single crystal film is grown around these oriented seeds preferably by CVD. The crystalline orientation of the film with respect to the substrate is the same as the seed crystals from which the film is grown since the crystal planes exposed to the reaction chamber are the same for all seeds. Thus, a plurality of oriented seed crystals, having common planes oriented similarly with respect to the substrate results in a film formed of grown crystalline regions all having the same crystalline orientation. The resultant film is typically smoother than films grown from seeds having random orientations since the rate of crystal growth is dependent on the crystal planes. Furthermore, for doped films, for use in semiconductor diamond devices, films formed from oriented seed crystals produce a more uniform doping concentration throughout the film, since dopant incorporation is also dependent on the particular crystalline planes which is grown.

In preparation for practice of the first step of the invention, synthetic or natural diamond grit is prepared for use, preferably by immersion in a bath of boiling sulfuric acid and ammonium persulfate at approximately 300°C for 10 minutes. Thereafter, after the wash solution is decanted from the grit, the grit is next rinsed with deionized water, concentrated hydrofluoric acid and again with deionized water. The grit is thus made ready for use in the first step of the present invention. Grit sizes from less than 2μm up to lOOμ have proven to yield satisfactory results in laboratory tests of the present invention. In some embodiments, as will be discussed further below, it is preferable that the crystals be faceted, i.e., have substantially symmetric shapes such as cubic, square, pyramidal or triangular shapes. Preferably the seeds are of fairly uniform size and spacial distribution on the substrate.

It has been found that the prepared diamond seed crystals tend to adhere to one another, forming clumps which may inhibit their orientation on the prepared substrate. The first step of the present method is directed to separating or declumping, the seed crystals. In particular, the washed seeds are suspended in a slurry, preferably by mixture of 0.1 grams of grit per 10 ml of a 40,000:1 solution of water in soap, such as common microelectronics cleaning soap. Alternate slurry solutions include silicone base diffusion pump oil in trichloroethylene. Still other slurry solutions include hydrocarbon oil or sulfuric acid. The slurry mixture is then subjected to an ultrasonic vibratory mixer to suspend the grit. It has also been found advantageous to clean the crystals with an etchant such as NaN0₃ that, we believe, finishes the diamond surface by removing small defects (e.g., around 500 angstroms) and dissolving very small crystals (generally, not detectable by examination with optical or scanning electron microscope) included as an impurity in a batch of crystals of selected size. The temperature and concentration and the duration of treatment are selected so that the diamond surfaces are finished but without excessive pitting of the surfaces. The substrate, which may be, for example, a standard Si wafer, is preferably prepared by cleaning in an oxygen plasma asher. Other substrates having metal or quartz surfaces also may be used. In some preferred embodiments, as will be further discussed below, the substrate surface may be etched to form a complementary shape to that of the seed crystals to effect rotational orientation. The substrate is wetted across^" its surface by application of the slurry (step 2) in any conventional manner, such as by use of a dropper or the like.

Application of the slurry is carried out gently to avoid substantially modifying the substrate by, for example, scratching the substrate with the diamond grit acting as an abrasive. The slurry fluid may also be applied to the substrate and the seed crystals lightly sprinkled onto the fluid.

Next, slurry solvent is gently removed. For example, the wafer is gently heated (step 3) until the solvent is removed and the surface is visually dry. Preferably the heating is accomplished slowly to effect gradual evaporation but avoid boiling or rapid evaporation that may disturb the seed crystals and disturb the preferential orientation. Such heating has been achieved by use of an ordinary laboratory hot plate, where the substrate is placed on the hot plate surface and is raised to approximately 200°C for approximately two minutes. It will also be understood that the slurry solvent may be air dried, or evaporated gradually under vacuum.

The diamond growth CVD process (step 4) involves heating the substrate to 900°C and passing a gaseous mixture of, for example, 99% hydrogen and 1% methane, through a 2.75 GHz discharge above the substrate surface to create a plasma. Since pure diamond is an insulator, it may be necessary to introduce or dope electrically active impurities to render the diamond useful as a semiconductor. To create boron-doped crystal films for use in P-type semiconductor devices, B₂H₆ may be added to the gas during CVD at chosen concentrations in the parts per million range. To create phosphorus-doped crystal films for use in N-type semiconductor devices, PH₃ may be added to the gas during CVD at chosen concentrations in the parts per million range. As a result, diamond films may be produced with a chosen level of doping.

Another CVD method of growing diamond is to flow a mixture of hydrogen and a hydrocarbon gas, such as methane, past a hot tungsten filament located in close proximity to the substrate surface. This method of diamond CVD growth was described originally by S. Matsumoto et al. in the Japan Journal of Applied Physics, vol. 21, page L183, (1982), with recent elaborations by Hirose and Terasawa, in Japan. J. Appl. Phys. vol, 25, p. L519 (1986) . Referring now to the perspective view of Fig. 2, a diamond crystal seeded onto a flat substrate in practice of process steps 1-3, and prior to step 4, is conceptually presented, 'lere, the desired orientation of the (111) planes of the seed crystal is shown with respect to the substrate surface. In particular, it has been found that seeded crystals after the heating of step 3 and prior to the CVD of step 4, are oriented such that the (111) planes of the crystals (using the Miller indices referencing method) will lie parallel to the substrate planes on which the crystals are supported.

Turning to Figure 3, a graphic representation of an X-ray diffraction pattern obtained from diamond crystals applied to a silicon substrate by the present method, and substantially as oriented in Fig. 2 , is shown, where X-ray intensity is plotted in arbitrary units on the Y-axis and diffraction angle 2Θ is plotted on the X-axis, where 2Θ represents the Bragg angle. It will be understood by those skilled in the art that diffraction from the (111) planes of the crystals is almost exclusively experienced, as represented by the extreme spike at 43.9°. This graph therefore clearly shows that the method of steps 1-3 indeed brings about the desired orientation shown in Fig. 2. For comparison, in Fig. 3a, the x-ray diffraction from randomly oriented diamond seeds suspended in a paste is shown. As can be seen, diffraction signals from various planes such as the 220, 311 and 400 planes are detected, indicating that the crystals are randomly oriented. Turning next to a graphic representation of Fig.

4, the distribution of crystals with (111) crystal planes oriented relative to the substrate planes is presented before and after the heating of step 3. In this graph, X-ray diffraction intensity is plotted along the Y-axis in arbitrary units, while the degree of tilt of the crystals' (111) planes relative to the substrata planes is plotted along the X-axis. Thus, the graph Represents the variation of the (111) planes (from-* 4- tor8-μm-diam^> diamond seeds as a function of the angle θ measured from the substrate normal. The schematic Fig. 4a is of a diamond seed with its (111) axis displaced (tilted) by θ from the substrate normal. The schematic mildly curvilinear dotted line in Fig. 4 represents diffraction intensity after the step 2 application of the seed crystals to a substrate but before the heating of step 3, and shows some amount of (111) planes orientation. The sharply parabolic curve (in solid line) represents vastly improved diffraction intensity after the fluid removal annealing of step 3, suggestive of vastly improved crystal orientation. It will thus be appreciated that the heating process of step 3 substantially improves the conformity of the seed crystal orientation on the substrate surface. The film growth process of the invention (step 4) is shown in detail in Figs. 5a-c. Fig. 5a is a schematic view of seed crystals on a substrate surface in practice of the invention, after step 3 but before step 4. These crystals, as seeded, obtain the preferred orientation shown in Fig. 2. In Fig. 5a, the preferred orientation of the seed crystals is indicated by the horizontal hash marks drawn across the cross-section of the crystals.

Fig. 5b is a schematic representation of the early stage of the CVD process of step 4, where it will be seen that the seeded crystals become enlarged as new diamond material is deposited on the seed surface. The new diamond material is also favorably oriented in coincidence with the orientation of the seed crystals, as indicated by the horizontal hash marks of Fig. 5b, coincident wit the hash marks of 5a.

As showi, in Figure 5c, the resulting grown crystals merge ar.d a crystalline film is formed upon completion of the CVD process. The film thickness is preferably greater than the average separation between adjacent seeds. This oriented film is characterized by a multiplicity of crystallites A, B, C grown from individual seeds a, b, c, all of whose (111) planes are similarly oriented. At the point where adjacent crystals A, B, C merge, there is the possibility that crystal defects may occur due to the differing rotational orientation of the seeds about their (111) axis. These defects are shown schematically in Figure 5c as the substantially vertical lines intermediate the grown seed crystals.

Referring to Figs. 6 and 6a, a randomly oriented crystal film, formed, for example, from seed crystals having random orientations is compared to an oriented film formed from seed crystals having uniform crystal planes orientation according to the invention are shown, respectively. The oriented films according to the invention have advantages over randomly oriented polycrystalline films. In diamond, dopant incorporation during crystal growth is a strong function of orientation. For example, incorporation of boron, a p- type dopant, is 100 times higher on (111) planes than on (100) planes during low pressure diamond growth (Buin et al. J. Cryst. Growth 31, 44 (1975)). Similar results have been reported for phosphorus n-type dopant. As illustrated in Fig. 6, the resultant film grown from randomly oriented crystals includes regions 20 having higher dopant (illustrated by dots) concentrations and regions 22 having lower dopant concentrations. By contrast, as indicated in Fig..6a, oriented films formed according to the invention ha-sre a substa: tially more uniform dopant concentration?throughout _he film.

Furthermore, crystallites of a .andomly oriented film grow at different rates resulting in an uneven surface, again as illustrated in Figs. 6-6a. When the film is oriented, formed from oriented seed crystals, the seed crystals grow at substantially the same rate, forming a smoother film surface. Oriented films of high quality will have electrical properties approaching those of single crystal diamond films, i.e., the grain boundaries are reduced or absent and the defects between adjacent crystallites (crystal portions grown from adjacent seeds) are not electrically active, i.e., they don't create an insulating barrier.

The following example is illustrative.

EXAMPLE 1 The following technique has been used to grow (111) oriented diamond films on smooth substrates using oriented diamond seeds. The degree of orientation of the film is defined by the initial orientation of the seeds. The initial orientation was effected by the seed size and the cleaning procedure. Over 90% of the seeds exhibit a (111) orientation with a tip angle of less than 0.25°. Further growth of diamond on these seeds does not affect the orientation, and diamond films obtained by such growth allow smoother surfaces and more controlled doping. The seed crystals are diamond grit obtained by pulverizing either natural (Micronized natural diamond with sizes ranging from 0.25 to 100 μm in diameter was obtained from Dubbledee Corp, 100 Stierli Court, Mount Arlington, NJ 07856) or synthetic (Micronized synthetic diamond was obtained from Specialty Materials Dept. General Electric, Worthington, OH 43085) single-crystal diamonds. Seeds were cleaned in either molten NaN0₃

(700°C) (S. Tolansky R.F. Millet, and J. Punglia, Philos. Mag. 26, 1275 (1972)) or a solution of H₂S0₄ and (NH₄)₂S₂0₈ at 200°C, rinsed in cor-.entrated HF and deionized water, and vacuum dried. They were then suspended in a slurry using any of several liquid carriers, including isopropyl alcohol, a 1:1000 solution of soap (Electronic grade soap. Micro, from International Products Corp. P.O. Box 118, Trenton, NJ 08601.) and water, or a 1:100 solution of Dow Corning 705 diffusion pump oil and trichloroethane. The slurry was spread across a silicon substrate and the fluid evaporated. The loosely adherent seeds were then removed by light ultrasonic cleaning in water or isopropyl alcohol. Diamond films were then grown on the attached seeds by either the plasma (D.D. Rathman et al. in Diamond and Diamond-Like Materials Synthesis, Ed. H. Johnson, A.R. Badzian, and M.W. Geis (Materials Research, Pittsburgh, PA, 1988, p. 115.) or the hot filament method (S Matsumoto, Y. Sato, N. Ka o, and N. Setaka, Jpn.J.Appl.Phys.21, L183 (1982).)

The orientation of films grown from attached seeds were characterized as a function of deposition and growth parameters. A lone (111) peak, as shown in Fig. 3 indicated that the diamond seeds have a (111) orientation. As shown in Fig. 4, the angular distribution of the (111) crystal axes of the seeds about the substrate normal can be used as a measure of the quality of the orientation. This distribution can be characterized by measuring the x-ray diffraction intensity from the (111) planes as a function of the angle measured from the substrate normal. Figure 4a schematically shows a single seed with its (111) axis θ degrees from the substrate normal. The half-width angle at half the maximum x-ray peak intensity, HWHM, is defined as the tip angl.. and is used to characterize the textuire. Ih' these experiments, the tip angle was found to increase with a d v rease in seed size. For seeds cleaned in H₂S0₄ and (NH₄)₂S₂0_g the tip angle varied from ±3.5° for 0- to 0.25-μm-diam seeds to less than ±0.25° for seeds larger than 20 μm. Seeds cleaned in NaN0₃ have tip angles about half of those cleaned with H₂S0₄ and (NH₄)₂S₂O_g. Optical and scanning electron microscope examination determined that for the films grown from seeds larger than 10 μm, over 90% of the seeded crystals on the substrate have a (111) orientation. In these experiments, in films grown from seeds less than lμm in diameter a smaller fraction, <50%, of the crystallites have (111) orientation and these have a tip angle (~3.5° HWHM) . Both the attached seeds and diamond films grown from them are strongly adherent to the substrate. In these experiments, the orientation was independent of the substrate material, being the same on Si, fused quartz, and chrome-coated substrates. Diamond obtained by growth on unseeded substrates or substrates scratched with diamond seeds had no measurable crystalline orientation. Roughening the seed surfaces by etching in air at 700°C caused the tip angle to increase and the fraction of seeds having a (111) orientation to decrease. Smoothing the diamond facets by cleaning in NaN0₃ decreases the tip angle and increases the fraction of seeds having a (111) orientation. Other crystalline materials that are known to form facets also exhibit orientation. Graphite, hexagonal boron nitride, and NaCl have tip angles from 3° to 5° for crystals lOμ and larger in diameter. These tip angles are an order of magnitude larger than those obtained with the same size diamond seeds.

Turning now to Fig. 7, a flat substrate surface, seeded with diamond crystals, is shown in perspective view. These crystals are indicated to have dissimilar orientations with respect to rotation about an axis perpendicular to the (111) planes. It will be appreciated that these crystals may have irregular shapes. However, regular tetrahedral crystal shapes are drawn in Fig. 7 for ease of indicating the orientations of the (111) planes. In some embodiments, the seed crystals are faceted as shown, i.e., they have regular, symmetric shapes. Growth of diamond films by CVD from such crystals may lead to crystal defects at the point at which film growth from adjacent crystals meet. This leads to an oriented crystal film of many crystals (i.e., polycrystalline film) grown from adjacent seeds whose (111) planes are all similarly oriented. Nevertheless, and quite surprisingly, these defects are not so substantial as to be disabling. An alternative embodiment of the present invention is to apply the method of steps 1-4 to a surface which has been previously patterned, for example, in the form of a grating, as disclosed in the perspective view of Figure 8 to allow preferential orientation of the seed crystals with respect to rotation about an axis perpendicular to the common planes of the crystals similarly oriented with respect to the substrate. Employing a grated substrate surface orients the seed crystals with respect to rotation relative to each other and thus reduces or eliminates the crystal defects arising from orientaition mismatch between adjacent seed crystals. In this manner a polycrystalline film with fewer defect boundaries may be formed. In particular embodiments, the shape or morphology of the substrate grating is formed complementary to the shape of the seed crystals by exposing the crystalline planes of the substrate that is complementary to the exposed planes of faceted seed crystals. For example, a silicon substrate may be formed with a grating having exposed on the walls of the grating, the (111) crystal planes and the seed crystals are faceted preferably to form symmetric seeds having exposed the (111) crystal planes. Thus, the shape of the grating is inherently complementary to the shape of the crystals. For crystalline substrates, such as silicon, the perimeter of the gratings formed run along the crystalline planes of the wafer to provide a smooth grating, the walls of which are formed from the (111) planes of the silicon. For example, the perimeter of the gratings are oriented with respect to the 110 axis of the silicon wafer. It will also be understood that the gratings may be formed by lithographic techniques (as discussed below with respect to etched pits) . Preferably the crystals are of a size that approximates that of the grating. Seed crystals, applied in a slurry, and after evaporation, are oriented such that the (111) crystal face of the crystal and the substrate are in face-wise arrangement. The (111) planes of the seed crystal is thus being parallel to the planes of the (111) substrate surface, i.e., the wall of the grating that supports the seed crystals. The crystalline orientation of the crystals, i.e., the (111) planes being parallel to the substrate surface supporting the crystals is achieved by the phenomenon discussed above and obtained by the process of Fig. 1, while the rotational orientation of the seeds is achieved by the complementary shape of the seeds and the textured substrate.

Referring now to Fig. 9, an x-ray diffraction pattern is shown as a function of substrate rotation, from the (311) diamond planes of diamond seed crystals (about 100 microns) oriented on a silicon surface that was etched with a grating such that the (111) planes is exposed. The grating was approximately lOOμm in width at the surface of the substrate with adjacent gratings separated by 100-200 microns. As shown in Fig. 9, the diffraction maxima from the (311) planes is detected predominantly only at regular orientations of the substrate as the substrate is rotated, indicating that the majority of the crystals are similarly oriented in the gratings.

Referring now to Figs. 10-lOa, in another embodiment employing a substrate with a patterned film, the substrate may be patterned with a series of preformed pits and faceted seed crystals applied to the substrate such that they are positioned within the pits. Preferably the pits have a shape complementary to the shape of preformed crystals. For example the seed crystals may be faceted diamond crystals faceted along the (111) and/or (100) planes and have a substantially symmetric crystalline shape, such as square, triangular or cubic-trapezoidal .shapes. The pits are then formed with complementary shapes.

Referring now to Fig. 10a, preferably the substrate is a silicon substrate having cubic-form pits, the walls of which are exposed (111) planes. The seed crystals may be oriented within the pits such that the (111) planes of the crystals mates in face-wise manner with the (111) planes exposed in the substrate pit. Fig. 10a illustrates the use of symmetric crystals having a cubic-trapezoidal shape with the (111) and (100) planes exposed. In this manner, the seed crystal is oriented such that the (111) planes is parallel to the contacting planes of the substrate and the rotational orientation of the various crystals seeded on the substrate are defined. This embodiment has the advantage of enabling selecting the density of oriented seed crystals on the substrate and the proximity of adjacent seed crystals to enable fine tuning of the final grown film. Preferably the seed crystals are either tetrahedral or triangular diamond seed crystals of about 10 to 100 micron. The pits are dimensioned to have a size that approximates that of the seed crystals. The opening of the substrate pits are typically about 100 microns for crystals about 75 microns. The pits are separated, for example, between 100 to 200 microns. Good results have been obtained with the seed crystals and pits sized such that the top of the crystal, when seated in the pit is approximately even with the planes of the unpitted substrate surface. Using smaller crystals has the advantage that less crystal growth is needed to form a film with a uniform surface.

To form a pitted substrate having pits of a complementary shape to the seed crystals, a silicon wafer may be provided having on its exposed outer surface the (100) planes. Onto the wafer surface is deposited a 1,000 angstrom thick silicon dioxide film and thereafter evaporated a metal layer such as a 500 angstrom chromium or aluminum layer, for example by electron beam evaporation techniques. A photoresist is applied to the metal layer and the photoresist exposed through a mask having a desired pattern to form a desired pattern of pits (as in Figs. 9-9a or a grating as in Fig. 8). After exposure, the photoresist is developed and the underlying metal layer etched. The silicon dioxide layer is removed by employing an etchant aqueous solution of HF or reactive ion etching in a fluorine containing gas. The silicon is etched with an etchant that selectively removes silicon along the (100) planes leaving exposed the (111) planes. Such etchants include sodium hydroxide, tetramethyl ammonium hydroxide, ethyl diamine/pyrocatechol or hydrazine. It will be understood that pits of complementary shape and size may be formed by other methods and on other substrates, including non- crystalline substrates.

Prior to applying the seed crystals, it is preferable to clean the crystals in a NaN0₃ at less than 600°C (to avoid pitting the surface of the crystals) , preferably about 300°C to finish the surface of the faceted diamond crystals and remove irregularities and dissolve rogue crystals.

To apply the crystals, the substrate surface is gently wetted with concentrated sulfuric acid such that it forms a thin film over the substrate. The seed crystals are then applied by gently sprinkling onto the sulfuric acid film. Finally, the sulfuric acid is evaporated, gently and without boiling, by heating to about 300°C. It has been observed by microscopy that as the sulfuric acid evaporates, the crystals are drawn into the pits. Crystallographic analyses have indicated that the crystals are oriented such that the (111) planes is oriented in face-wise manner with respect to the (111) planes exposed in the silicon substrate pits.

In a variation of the method, a small amount of tetramethyl ammonium hydroxide may be added to the sulfuric acid to slightly etch the silicon substrate as the seed crystals are oriented, and thus accommodate burrs or irregularities on the surface of the faceted crystals. An etchant for diamond such as NaN0₃ may also ' be included in the slurry fluid.

Seed crystals applied to pitted substrates show an improvement in orientation by about a factor of two (measured by the half-width at half maximum of the x-ray diffraction peaks as in Fig. 3) over crystals applied to grated surfaces (as in Fig. 8) .

In a further embodiment, the seeding process may be carried out iteratively. After a first seeding of the substrate such as a pitted substrate with seed crystals, a glue in the form of a photoresist (Selusol AZ thinner and Novolak photoresist polymer) preferably in dilute solution (0.1% by weight) is gently applied, for example, with an eye dropper to a substrate surface and allowed to spread thereover and dry. After the glue has dried, a light cleaning is carried out by gentle agitation with an ultrasonic cleaner and rinsing with water. It has been found that this process particularly removes crystals that are poorly oriented in pits or gratings etched into the substrate surface while not substantially removing those crystals which are properly oriented. We propose that the glue, which forms a thin film (believed to be less than 2 to 4,000 angstroms) on the substrate, fixes the oriented crystals more firmly since a greater surface area between the crystals and the substrate exist when the faces of the (111) planes are parallel and face-wise, compared to randomly oriented crystals. After the rinsing step, the substrate may be reseeded and the gluing procedure discussed above, repeated to increase the density of oriented crystals on the substrate to a desired level.

The present invention may be favorably employed in the growth of vertical semiconductor devices. These devices are characterized by vertical current flow through the device. Turning now to Fig. 11, there is shown a side cross-sectional view of a vertical semiconductor device having an oriented polycrystalline film grown on an ungrated conductive substrate (such as of nickel or carbon) , created by the present method of crystal film growth. In Fig. 11, the now familiar grain boundaries of the textured film will be seen, where it will be appreciated that a grating pattern has been etched into the surface of the prepared polycrystalline film, and where the device is provided with an emitter, base, and collector.

As will be further appreciated, the vertical axes of the crystals of the textured film are within a few degrees of the substrate normal, where rotational orientations about the normal axis have not been controlled. For purposes of this device, the film has been doped with boron sufficient to render it a suitable semiconductor. Ohmic contacts may be created by conventional means. Finally, metal is evaporated on all horizontal grating surfaces without metallizing the vertical walls of the grating, thus creating a Schottk base and contacts on the tops of the grating.

This vertical device is only one of several devices which may be created in practice of the present invention. The preferred process is as follows:

1. growing an oriented diamond-like polycrystalline film on a conducting substrate of nickel, carbon, or the like; 2. creating ohmic contact surfaces;

3. creating a grating in the grown film with ion-beam assisted etching; and

4. evaporating metal (such as aluminum) on the horizontal surfaces of the grating to form an emitter and base.

The technique of ion beam etching of step 3. above has been particularly set forth in the Journal of Vacuum Science and Technology, vol. 313, p. 416 (1985). Finally, as discussed it should be appreciated that in randomly oriented crystal films, as provided by prior art methods, substantial doping variations ocseur between crystals in the film. This occurs because the doping concentration is dependent upon the orientation of the growing crystal. By providing a film in which the crystal planes are consistently orienred, however, the doping concentration in the present invention is both uniform and predictable.

Several modifications and variations of the present invention are possible when considered in the light of the above teachings. It is therefore understood that the scope of the present invention is not to be limited to the details disclosed herein, may be practiced otherwise than is as specifically described, and is intended only to be limited by the claims appended hereto:

What is claimed is:

Claims

Claims 1. A method for forming an electrical device having a diamond-like film with a substantially uniform crystalline orientation, comprising: orienting seed crystals on a substrate surface by, applying a slurry fluid to said substrate surface and incorporating in said slurry fluid seed crystals to substantially separate said seed crystals, each of said seed crystals having at least one common crystalline plane; and gently removing the slurry fluid to enable preferential orientation of seed crystals on said substrate such that said common crystalline planes of the crystals are oriented similarly with respect to said substrate; and growing a film about said seed crystals, said film being formed of crystalline regions having said common crystalline planes oriented similarly with respect to said substrate.

2. The method of claim 1 wherein said common crystalline planes are the (111) planes.

3. The method of claim 2 wherein said common crystalline planes of said crystals are substantially parallel to the surface of said substrate on which each of said crystals is supported.

4. The method of claim 2 wherein said common crystalline planes of said crystals are substantially parallel to the complementary crystalline plai _»s of said substrate. ^t/f

5. The method of claim 2 wherein said seed crystals have a cubic crystalline structure.

6. The method of claim 5 wherein said crystals are selected from the group consisting of diamond and boron nitride.

7. The method of claim 6 wherein said crystals have the (111) planes exposed.

8. The method of claim 6 or 7 wherein said crystals have the (100) planes exposed.

9. The method of claim 1 wherein said substrate is non-diamond.

10. The method of claim 9 wherein said substrate is a metal.

11. The method of claim 9 wherein said substrate is silicon.

12. The method of claim 9 wherein said substrate is quartz.

13. The method of claim 1 further comprising the step of cleaning said seed crystals before the step of incorporating said seed crystals in a slurry.

14. The method of claim 13 wherein said cleaning comprises etching said diam nd crystals.

15. The method of claim 14 wherein said cleaning comprises cleaning said crystals in NaN0₃. 1 16. The method of claim 15 wherein the NaNos is

2 at about 300°C.

1 17. The method of claim 1 wherein said seed

2 crystals are greater than about 10 and less than about

3 100 μm.

1 18. The method of claim 1 wherein said slurry

2 fluid is a mixture of water and soap.

1 19. The method of claim 1 wherein said slurry

2 fluid is a mixture of hydrocarbon oil and

3 trichloroethylene.

1 20. The method of claim 1 wherein said slurry

2 fluid is sulfuric acid.

1 21. The method of claim 1 wherein said slurry

2 fluid includes an etchant for diamond.

1 22. The method of claim 3 wherein said substrate

2 is a smooth single plane.

1 23. The method of claim 3 wherein the surface of

2 said substrate is patterned to allow preferential

3. orientation of said seed crystals with respect to

4 rotation about an axis perpendicular to said common

5 planes.

1 24. The method of c.^" ιim 21 wherein said substrate

2 includes pits distributed o- ^■ ar said surface.

25. The method of claim 21 wherein said substrate includes gratings.

26. The method of claims 23-24 wherein said seed crystals are substantially symmetric faceted crystals.

27. The method of claim 26 wherein said seed crystals are about the size of said patterned features.

28. The method of claim 1 or 23 wherein said film approaches single crystal diamond.

29. The method of claim 1 or 23 further comprising, after said removing step, removing crystals not similarly oriented by applying to said substrate and seed crystals, glue-type material, drying said glue-type material and washing said substrate.

30. The method of claim 29 further comprising repeating said steps of orienting said seed crystals and removing crystals not similarly oriented to provide a desired density of crystals on said substrate.

31. A method for forming a diamond-like film with a substantially uniform crystalline orientation, comprising: selecting cubic seed crystals having a preformed shape and size, preparing a substrate by patterning said substrate to form surfaces complementary to said shape and size of said cubic seed crystals; and orienting seed crystals on said subst.cate surface by. - so ¬ applying a slurry fluid to said substrate surface and incorporating in said slurry fluid seed crystals to substantially separate said seed crystals, each of said seed crystals having at least one common crystalline plane; gently removing the slurry fluid to enable preferential orientation of seed crystals on said substrate such that said common crystalline planes of the crystals are oriented similarly with respect to said substrate, and the rotational orientation of said seed crystals is defined by said patterned surfaces, and growing a film about said seed crystals, said film being formed of crystalline regions having said common crystalline oriented similarly with respect to said substrate.

32. The method of claim 31 wherein said common crystalline planes of said crystals are substantially parallel to the surface of said substrate on which each of said crystals is supported.

33. The method of claim 32 wherein said common crystalline planes of said crystals are substantially parallel to the complementary crystalline planes of said substrate.

34. The method of claim 33 wherein said substrate is silicon.

35. The method of claim 34 wherein said common crystalline planes are the (111) planes.

36. The method of claim 35 wherein said patterned features are pits having exposed the (111) planes of said substrate.

37. The method of claim 36 wherein said seed crystals are faceted diamond seed crystals, having substantially symmetric shape.

38. The method of claim 31 wherein said film approaches single crystal diamond.