Patent ID: 12227873

DETAILED DESCRIPTION

Embodiments of the present invention enable the fabrication of high-quality single-crystal AlN bulk crystals (i.e., boules and/or substrates) that undergo significant diameter expansion during crystal growth.FIGS.1A-1Care schematic illustrations of various crystals, and associated parameters thereof, relevant to embodiments of the present invention.FIG.1Adepicts an exemplary seed crystal100having a diameter102, a front surface104, and a back surface106. While the seed crystal100is depicted as cylindrical with circular surfaces, the seed crystal100is not limited to such shapes. As such, diameter102generally refers to the largest lateral dimension of the seed crystal100, and may therefore correspond to a “width” or “maximum width,” e.g., for seed crystals100having non-circular shapes. In various embodiments of the invention, the seed crystal100has a thickness ranging from approximately 0.1 mm to approximately 3 mm. Typically, the front surface104is exposed to the incoming vapor utilized for crystal growth, and the resulting crystal extends from the front surface104. The seed crystal100may be mounted within the growth apparatus via the back surface106(see, e.g.,FIG.2). Depending upon the seed-mounting procedure, the seed crystal100may have an exposed growth surface that is equal to or less than the area of the front surface104(i.e., a portion of the front surface104may be covered or otherwise prevented from receiving the incoming vapor). Herein, references to “seed diameter” or “seeded diameter” refer to the diameter of actual area of the seed crystal100exposed for growth thereon (i.e., the “seed area” or “seeded area”), even if that area is less than the total area of front surface104. In addition, the seed diameter or seeded diameter may have a shape different from that of the actual surface104of the seed crystal100itself, resulting from, e.g., the masking or otherwise occlusion of a portion of surface104. For example, the seeded diameter may be circular while the actual surface104is non-circular or vice versa.

FIG.1Bis a schematic depiction of a crystal (or “crystalline boule” or “boule”)108resulting from crystal growth on seed crystal100(e.g., via a vapor-phase transport technique such as sublimation-recondensation). Note that the crystal108does not terminate at a pointed tip, but rather has a relatively planar surface due to the initiation of growth on the seed crystal100. The crystal108has an initial seeded diameter110(i.e., the diameter of the seeded area of the crystal, which may correspond to the diameter of the initial seed crystal or a portion thereof) and, due to diameter expansion during growth, may be described as a geometrical combination of a frustum112and a dome114, the frustum112resulting from diameter expansion during growth and the dome114resulting from, at least in part, the shape of the thermal field within the growth chamber. The frustum112may (but need not) be, for example, right, circular, and conical. The dome114may (but need not) be, e.g., a spherical cap or a spherical segment. In various embodiments, the dome114may have the form of a cone (e.g., with a rounded tip) or a truncated cone (e.g., a frustum tapering in the opposite direction from that of the frustum112). As shown, the diameter (or other lateral dimension) of the crystal may increase, due to diameter expansion to a maximum crystal diameter116. The curvature of the dome114may increase as the radial thermal gradient utilized during crystal growth increases. As such, crystals108having small (or even substantially non-existent) domes114may result from the use of small radial thermal gradients during crystal growth. That is, in accordance with embodiments of the invention, the radial thermal gradient may be adjusted (e.g., during growth) to decrease the size of dome114or to virtually eliminate dome114entirely. Note that, since crystals108are grown from seed crystals100, they are larger, and contain more usable, high-quality volume (e.g., for the production of single-crystal AlN wafers) than similar crystals grown by unseeded growth. (Unseeded growth is typically reliant upon spontaneous nucleation, which can introduce excessive numbers of defects and/or non-uniformity in crystalline orientation.) As disclosed herein, crystals108, being produced by seeded growth, may also incorporate at least a portion of the seed crystal100itself therein.

FIG.1Cis a cross-sectional view of an exemplary crystal108. As shown, the crystal108has a total length118that encompasses both the frustum and dome sections of the crystal. The total length118includes both an expansion length120(i.e., the length of the diameter-expanded volume of the crystal in the growth direction, e.g., perpendicular from the surface of the seed crystal100) and a dome length122. In various embodiments of the invention, the crystal108may include a portion124in which the diameter is not expanded (due to, for example, deliberate modification of the radial thermal gradient and/or diameter expansion sufficient to reach the interior wall of the growth apparatus), and portion124may have a length126that contributes to total length118. Portion124may be, e.g., cylindrical, or may have one or more flat surfaces (e.g., may have the shape of a hexagonal prism (for example, having sides parallel to the m-planes {1-100})). Portion124may be present but is not necessarily present in embodiments of the present invention. When present, portion124may have a diameter that is substantially equal to the maximum, or expanded, diameter116, as shown inFIG.1C. The frustum112also has an expansion height, or slant height,128, which is measured along the surface of the diameter-expanded volume of the crystal. It is readily apparent fromFIG.1Cthat, in the absence of diameter expansion, the expansion height128and the expansion length are equivalent.

FIG.1Dis a schematic diagram of another exemplary crystal108produced in accordance with embodiments of the present invention. As shown, the crystal108ofFIG.1Dis similar to the crystal108ofFIG.1C, except that the “straight” portion124having a substantially constant diameter is longer than the expansion length120, due to rapid initial expansion of the crystal (resulting from, for example, use of one or more of the techniques in accordance with embodiments of the invention detailed herein). Crystals108as shown inFIG.1Dmay beneficially provide a large crystalline volume from which many wafers having substantially the same diameter may be produced.

In various exemplary embodiments, the expansion length120may range from approximately 1%, 2%, 3%, 5%, or 10% to approximately 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of total length118, while the length126may range from approximately 0%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 5%, or 10% to approximately 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of total length118, and the dome length122may range from approximately 0%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, or 5% to approximately 20%, 25%, 30%, 35%, 40%, or 45% of total length118(while, as shown inFIGS.1C and1D, the sum of the expansion length120, length126, and dome length122is equal to 100% of the total length118).

FIGS.1E and1Fare additional schematic illustrations of various crystals, and associated parameters thereof, relevant to embodiments of the present invention.FIG.1Eschematically depicts how wafers may be sliced from the crystal108in accordance with various embodiments of the invention. As shown, a wafer130-1and a wafer130-2having a larger diameter may be sliced from the crystal108substantially parallel to the seed crystal100. In other embodiments, wafers may be sliced from the crystal108along other directions, even substantially perpendicular to the plane of the seed crystal100. Wafers sliced from crystal108having a diameter greater than that of the seed crystal100(e.g., wafer130-2) may be subsequently utilized as seed crystals for additional growth of larger-diameter crystals, for example as disclosed in U.S. patent application Ser. No. 16/008,407, filed on Jun. 14, 2018 (the '407 application), the entire disclosure of which is incorporated by reference herein. Wafers sliced from the portion124of a crystal108as shown inFIGS.1C and1Dmay have substantially the same diameter as each other.

FIG.1Fis a schematic sectional view of the crystal108depicting (in shading) expanded area132, i.e., the cross-sectional area of the crystal108exceeding the seeded area110and resulting from diameter expansion.FIG.1Falso depicts an exemplary expansion angle134, which corresponds to the angle between the normal of the seed crystal and the plane of the expansion height128. In various embodiments, the expansion angle134may be, but is not necessarily, substantially constant during the entire growth of the crystal108. That is, the plane of the expansion height128need not be linear. When wafers are cut from the crystal108, the expanded area132therefore corresponds to an annular region extending inward from the outer edge of the wafer, and the seeded area corresponds to a central area of the wafer having approximately the size and shape of that of the seed crystal100utilized to grow the crystal108. Thus, at least within the frustum112of a diameter-expanded crystal108, a wafer separated from a portion of the boule farther away from the seed crystal100will have a larger expanded area132than one cut from the boule closer to the seed crystal100, while the seeded areas of both wafers may be approximately the same in size and shape. On the other hand, multiple wafers separated from a portion124of a crystal108will have approximately the same expanded areas132and approximately the same seeded areas.

The orientation of a wafer or seed crystal may be selected from a boule or other crystal during slicing via, for example, x-ray diffraction measurements and/or other materials characterization enabling identification of the orientation of the crystal; such techniques are known to those of skill in the art and may be performed without undue experimentation. In accordance with embodiments of the invention, a newly sliced wafer or seed may be polished to reduce surface roughness and remove cutting artifacts and/or damage. The polarity of a wafer or seed crystal may also be identified and selected chemically. For example, the polarity may be identified and selected via exposure of the wafer or seed to a basic or acidic solution, which will roughen an N-polarity face while leaving an Al-polarity face smooth, as detailed in the '407 application.

FIG.2depicts a crystal-growth apparatus200suitable for the growth of single-crystal AlN in accordance with various embodiments of the present invention. As shown, apparatus200includes a crucible205positioned on top of a crucible stand210within a susceptor215. Both the crucible205and the susceptor215may have any suitable geometric shape, e.g., cylindrical. During a typical growth process, an AlN boule220(e.g., a crystal108) is formed by condensation of a vapor225that includes, consists essentially of, or consists of the elemental precursors of the AlN boule220, i.e., Al and N atoms and/or N2molecules. In typical embodiments, the vapor225arises from the sublimation of a source material230, which may include, consist essentially of, or consist of the polycrystalline AlN source material described above. The AlN boule220may form on and extend from a seed crystal235. (Alternatively, the AlN boule220may nucleate upon and extend from a portion of the crucible205itself.) The seed crystal235may be a single crystal (e.g., a polished wafer) including, consisting essentially of, or consisting of AlN. In various embodiments, the seed crystal235has a diameter (or width or other lateral dimension) of at least approximately 10 mm, at least approximately 25 mm, at least approximately 35 mm, at least approximately 40 mm, or even at least approximately 50 mm. In various embodiments, the seed crystal235has a diameter (or width or other lateral dimension) of approximately 50 mm or less, approximately 100 mm or less, or approximately 150 mm or less, and/or single-crystal AlN grown therefrom has a diameter (or width or other lateral dimension) of approximately 150 mm or less. In various embodiments, the crystalline orientation (i.e., the normal to the exposed plane (e.g., c-plane)) of the seed crystal235is substantially parallel to the c-axis. In other embodiments, the crystalline orientation of the seed crystal235is at least approximately 5°, or even at least approximately 100 away from the c-axis; the orientation of the seed crystal235may be toward a non-polar direction. In various embodiments, the crystalline orientation of the seed crystal235may be no more than approximately 30°, or no more than approximately 20°, away from the c-axis.

The crucible205may include, consist essentially of, or consist of one or more refractory materials, such as tungsten, rhenium, tantalum carbide, and/or tantalum nitride. As described in the '135 patent and the '153 patent, the crucible205may have one or more surfaces (e.g., walls) configured to selectively permit the diffusion of nitrogen therethrough and selectively prevent the diffusion of aluminum therethrough.

As shown inFIG.2, during formation of the AlN boule220, a polycrystalline material240may (but does not necessarily) form at one or more locations within the crucible205not covered by the seed crystal235. However, the diameter (or other radial dimension) of the AlN boule220may expand, i.e., increase, during formation of the AlN boule220, thereby occluding the regions of polycrystalline material240(if present) from impinging vapor225and substantially limiting or even eliminating their growth. As shown inFIG.2, the diameter of the AlN boule220may expand to (or even start out at, in embodiments utilizing larger seed crystals235) be substantially equal to the inner diameter of the crucible205(in which case no further lateral expansion of the AlN boule220may occur).

The growth of the AlN boule220along a growth direction245typically proceeds due to a relatively large axial thermal gradient (e.g., ranging from approximately 5° C./cm to approximately 100° C./cm) formed within the crucible205. A heating apparatus (not shown inFIG.2for clarity), e.g., an RF heater, one or more heating coils, and/or other heating elements or furnaces, heats the susceptor215(and hence the crucible205) to an elevated temperature typically ranging between approximately 1800° C. and approximately 2300° C. Prior to the onset of growth, the crucible205and its contents (e.g., seed crystal235and source material230) may be held at a temperature approximately equal to the desired growth temperature for a predetermined soak time (e.g., between approximately 1 hour and approximately 10 hours). In various embodiments, this soak at temperature stabilizes the thermal field within the crucible205, promotes effective nucleation on the seed crystal235, and promotes high-quality transition from nucleation to bulk growth of the single-crystalline AlN.

The apparatus200may feature one or more sets of top thermal shields250, and/or one or more sets of bottom axial thermal shields255, arranged to create the large axial thermal gradient (by, e.g., better insulating the bottom end of crucible205and the source material230from heat loss than the top end of crucible205and the growing AlN boule220). During the growth process, the susceptor215(and hence the crucible205) may be translated within the heating zone created by the heating apparatus via a drive mechanism260in order to maintain the axial thermal gradient near the surface of the growing AlN boule220. One or more pyrometers265(or other characterization devices and/or sensors) may be utilized to monitor the temperature at one or more locations within susceptor215. The top thermal shields250and/or the bottom thermal shields255may include, consist essentially of, or consist of one or more refractory materials (e.g., tungsten), and may be quite thin (e.g., between approximately 0.125 mm and 0.5 mm thick). As detailed in the '612 patent, the top thermal shields250and/or the bottom thermal shields255may be arranged in various configurations and/or have various characteristics (i.e., different numbers of shields, different spacings between shields, different thicknesses, different sized apertures defined therethrough, different sizes, etc.) in order to form a variety of different axial and radial thermal gradients within the crucible205and thus, the growth of the AlN boule220(e.g., the growth rate, the degree of radial expansion during growth, if any, etc.).

In various embodiments, the crucible205has a lid270with sufficient radiation transparency to enable at least partial control of the thermal profile within the crucible205via the arrangement of the top thermal shields250. Furthermore, in embodiments featuring a seed crystal235, the seed crystal235is typically mounted on the lid270prior to the growth of AlN boule220. The lid270is typically mechanically stable at the growth temperature (e.g., up to approximately 2300° C.) and may substantially prevent diffusion of Al-containing vapor therethrough. Lid270generally includes, consists essentially of, or consists of one or more refractory materials (e.g., tungsten, rhenium, and/or tantalum nitride), and may be fairly thin (e.g., less than approximately 0.5 mm thick).

As shown inFIG.2, each of the top thermal shields may have an opening275therethrough. The opening275normally echoes the geometry and/or symmetry of the crucible205(e.g., the opening275may be substantially circular for a cylindrical crucible205). The size of each opening275may be varied; typically, the size(s) range from a minimum of 10 mm to a maximum of approximately 5 mm (or even 2 mm) less than the diameter of the crucible205.

For example, in an embodiment, five thermal shields250, each having a diameter of 68.5 mm and an opening size (diameter) of 45 mm, are used. The thickness of each of the thermal shields250is 0.125 mm, and the thermal shields250are spaced approximately 7 mm from each other. At a typical growth temperature of 2065° C., this shield arrangement results in a radial thermal gradient (measured from the center of the semiconductor crystal to the inner edge of the crucible) of 27° C./cm. Of course, this value is merely exemplary, and those of skill in the art may arrange thermal shields to achieve a range of different radial thermal gradients without undue experimentation.

Embodiments of the present invention enable even higher rates of diameter expansion of the AlN crystal via augmentation of the radial thermal gradient resulting from the arrangement of thermal shields250. (For avoidance of doubt, the techniques detailed herein in accordance with embodiments of the invention enable higher rates of diameter expansion of growing AlN single crystals, while preserving crystal quality (and therefore, the production of AlN single crystals having larger crystal augmentation parameters, as detailed herein), than do techniques detailed in the '612 patent.) In general, techniques in accordance with embodiments of the invention increase the radial thermal gradient via tailored heating of the edges of the growing crystal and/or altering the condensing vapor to enhance lateral growth of the crystal. In conventional techniques, often the conventional wisdom is the suppression of the radial thermal gradient in order to, e.g., minimize the curvature of the leading edge of the growing crystal. The conventional wisdom in the art also tends to emphasize the maintenance of a substantially uniform temperature in the radial direction during crystal growth. Embodiments of the invention contradict such conventional wisdom in order to further enhance diameter expansion (for example, beyond that achievable merely by the arrangement of external thermal shields, even in combination with external differential heating and insulation techniques) while maintaining high crystalline quality of the resulting bulk crystal.

FIG.3illustrates one technique for enhancing the radial thermal gradient in accordance with embodiments of the invention. In the embodiment ofFIG.3, one or more internal shields (or baffles)300are disposed within the crucible205proximate the edge of the growing AlN boule220. In accordance with various embodiments, the internal shields300transfer heat from the walls of the crucible205toward the edge of AlN boule220, raising the temperature thereof, and the internal shields300also retain the heat in proximity to the edge of the AlN boule220. In this manner, the internal shields300augment, or increase, the radial thermal gradient within the crucible205, resulting in enhanced lateral crystal growth and increased diameter expansion of AlN boule220. In various embodiments of the invention, the number of internal shields300disposed within the crucible205ranges from 1 to 10, or even 1 to 15. The present inventors have found that the use of internal shields300within the crucible205enables more rapid diameter expansion of the AlN boule220and, relatedly, the formation of larger boules, with more usable volume for the fabrication of substrates therefrom, than is possible with conventional growth techniques, despite the conventional wisdom that additional objects disposed within the growth crucible tend to deleteriously disrupt crystal growth and/or act as nucleation centers for extraneous, parasitic growth of polycrystalline or otherwise unusable material.

In various embodiments of the invention, the internal shields300include, consist essentially of, or consist of one or more refractory materials (e.g., tungsten and/or TaC), and may be quite thin (e.g., between approximately 0.125 mm and 0.5 mm thick). In other embodiments, one or more of the thermal shields may have a greater thickness, e.g., ranging from approximately 1 mm to approximately 3 mm. In various embodiments, the density (and concomitant impact on the thermal field proximate the shield) of one or more of the internal shields300may vary. For example, one or more of the internal shields300may have a density ranging from approximately 10% full density to approximately 100% full density (as an example, the 100% full density of tungsten is approximately 19.3 g/cm3). Thin foils of refractory materials having different densities and/or thicknesses are commercially available and may be provided without undue experimentation. In various embodiments, an internal shield having a larger thickness and/or a larger density may transfer more heat, and therefore increase the radial thermal gradient, more than such shields having smaller thicknesses and/or smaller densities.

As shown inFIG.3, the outer boundary of the internal shields300may conform substantially to the shape and size of the interior wall of the crucible205, and the size of the central openings in the internal shields300may vary to accommodate diameter expansion (e.g., expected diameter expansion) of the AlN boule220. For example, the central openings of the internal shields300may increase as the distance of the individual shields away from the seed crystal235increases, at least when the internal shields300are positioned where it is desired or expected for the AlN boule220to undergo diameter expansion. In addition, the density and/or thickness of the individual internal shields300may vary (e.g., increase) as a function of distance away from the seed crystal235, at least when the internal shields300are positioned where it is desired or expected for the AlN boule220to undergo diameter expansion. Such increases may compensate for the loss of volume of the shields300having larger central openings. In various embodiments, at positions where it is desired or expected for the AlN boule220to not undergo diameter expansion, the central opening size, density, and/or thickness of the internal shields300may be substantially constant.

Similarly, in regions in which more rapid diameter expansion is desired, the spacing between the internal shields300may be decreased, compared to regions in which diameter expansion is not desired or expected (e.g., to as large a degree). Example spacings between the internal shields300may range from approximately 1 mm to approximately 50 mm, or from approximately 5 mm to approximately 10 mm.

In accordance with embodiments of the invention, the internal shields300may be mounted within the crucible205via a variety of different approaches. For example, the internal shields300may be held by or affixed to the interior surface of the crucible205at their outer edges. The internal shields300may each be rested on a platform or pedestal within the crucible205(e.g., extending from the inner wall thereof), or the internal shields300may rest at their central openings on an internal support extending from the top surface of the crucible proximate the seed crystal235. (The internal support is not depicted inFIG.3for clarity, but may echo the outer shape of the crystal and either be in contact therewith or spaced away therefrom; the inner edges of the internal shields300may contact the internal support and therefore be supported thereby.) The internal support, which may include, consist essentially of, or consist of one or more of the same materials as internal shields300, may have the shape of a truncated cone having a first inner diameter at its upper end (i.e., the end proximate the seed crystal235) approximately equal to, or even less than the diameter of seed crystal235, and a second, larger, inner diameter at its lower end. In various embodiments, the inner diameter of the internal support may increase at a rate substantially equal to or greater than the (e.g., desired or expected) expansion rate and/or angle of the AlN boule220. That is, in embodiments of the invention, the AlN boule220may not contact (at least, not fully contact except at one or more discrete points) the interior surface of the internal support (i.e., the internal support may not contact and fit snugly around the AlN boule220). In other embodiments, the inner diameter of the internal support may increase at a rate smaller than the expected expansion angle of the AlN boule220(i.e., the expansion angle that would otherwise occur given the growth parameters in the absence of the internal support), and thus the internal support may restrict the expansion angle and rate of expansion of the AlN boule220to desired values defined by the geometry of the support.

In various embodiments, all or a portion of the internal support may be conical (i.e., have a diameter that increases in a direction away from the seed crystal235), e.g., at positions where it is desired or expected for the AlN boule220to undergo diameter expansion. For example, all or a portion of the internal support may have the shape of a frustum having a smaller-diameter top opening to accommodate the seed crystal235, and which flares out to accommodate the diameter-expanded AlN boule220. In various embodiments, all or a portion of the internal support may be cylindrical (i.e., have a diameter than is substantially constant as a function of distance away from the seed crystal235), e.g., at positions where it is desired or expected for the AlN boule220to not undergo diameter expansion. In one example, the internal support may be partially conical and partially cylindrical, echoing the diameter change of portions112and124of the crystal108shown inFIG.1C.

InFIG.3, the interior shields300are depicted as extending around the AlN boule220approximately parallel to the plane of the seed crystal235(i.e., approximately perpendicular to the lateral growth direction) and/or to the top or bottom surface of the crucible205, but in various embodiments the internal shields300are oriented at other angles. For example, one or more of the interior shields300may extend approximately perpendicular to the plane of the seed crystal235or at an inclined angle thereto (e.g., at an angle ranging from approximately 5° to approximately 850 with respect to the plane of the seed crystal and/or to the top and/or bottom surface of the crucible). In an example embodiment, one or more of the interior shields300may be oriented at an angle approximately perpendicular to the plane defined by the expansion height of the AlN boule220(i.e., approximately perpendicular to the edge of the frustum of the crystal)—for example, one or more of the internal shields300may extend approximately perpendicular from the inclined edge of the internal support. In various embodiments, one or more of the interior shields300may be oriented at angles different from those at while one or more of the other interior shields300are oriented. In various embodiments, the interior shields300merely influence the thermal field within the crucible205(e.g., increase the radial thermal gradient at one or more points and/or regions) and do not contact the AlN boule220itself during growth thereof.

In various embodiments of the invention, atmospheric plasma is utilized to enrich the source vapor phase within the crucible205with nitrogen atoms and concentrate such atoms preferentially at the lateral edge of the growing crystal. The excess nitrogen produced by the plasma process promotes increased lateral growth of the AlN crystal at rates exceeding those enabled by the mere introduction of nitrogen gas (or a nitrogen-containing gas) itself, even at super-atmospheric growth pressures. As shown inFIG.4, nitrogen gas (and/or a nitrogen-containing gas) may be introduced into the crucible205via one or more nozzles400in order to provide excess nitrogen proximate the edge of the growing AlN boule220. In addition, one or more plasma electrodes410are disposed proximate to (e.g., at a distance ranging from approximately 0.3 cm to approximately 1 cm away from) the edge of the AlN boule220(e.g., where the edge is expected to reach or be during growth) and/or to the internal support (if present). As shown, the electrodes410may be arranged at an angle to accommodate diameter expansion of the AlN boule220, but in other embodiments the electrodes410may be arranged in other configurations (e.g., parallel to the crucible walls or at an angle and then parallel to the crucible walls). AC or DC current may be applied to the electrodes410via a high-frequency current source (which may be incorporated with the RF source utilized for crystal growth), and a pulsed electric arc may be generated via high-voltage discharge at the electrodes410. The nitrogen gas from nozzles400may flow proximate or through the electrodes410and be converted into a plasma that envelops all or a portion of the edge of the AlN boule220. This nitrogen plasma significantly and preferentially increases the amount of nitrogen within the expanded portion of the AlN boule220, increasing its lateral growth rate (and therefore enhancing the diameter expansion of AlN boule220).

In various embodiments, the electrodes410may be operated, and the resulting plasma formed, uniformly during most of significantly all of the growth of the AlN boule220. In other embodiments, the electrodes410may be operated only during one, two, or more intervals during the growth, and the plasma may not be present between such intervals. In yet other embodiments, the current applied to the electrodes410may be varied one or more times during the growth to increase or decrease the amount of plasma produced during particular points of the growth process. In this manner, the rate of diameter expansion of AlN boule220, and the resulting shape thereof, may be influenced by the presence or absence of the plasma, and/or of the level of power supplied to the electrodes410.

Embodiments of the present invention enable the growth of AlN single crystals having masses, volumes, and/or rates of diameter expansion greater than those enabled by conventional techniques. For example, embodiments of the invention enable the formation of AlN single crystals having large crystal augmentation parameters (CAPs), where the CAP, in mm is defined as:

CAP=AE-ASLE=π4×LE⁢(dE2-dS2)
where AEis the expanded area (i.e., the cross-sectional area of the portion of the crystal having the maximum diameter116inFIGS.1B and1C) in mm2, dEis the expansion diameter (i.e., the maximum diameter116inFIGS.1B and1C) in mm, ASis the seeded area (i.e., the cross-sectional area of the portion of the crystal having the seeded diameter110inFIGS.1B and1C) in mm2, dSis the seed diameter (i.e., the seed diameter110inFIGS.1B and1C, which may correspond to the minimum diameter of the crystal) in mm, and LEis the expansion length (i.e., length120inFIGS.1C and1F) of the crystal in mm. In accordance with embodiments of the invention, the CAP value provides a superior measure of diameter expansion, normalized to crystal length, than the expansion angle134(seeFIG.1F), as the expansion angle may vary during growth of the crystal and/or may be difficult to measure.

Embodiments of the present invention enable the growth of AlN single crystals having CAPs unattainable utilizing conventional techniques, due at least in part to faster diameter expansion during crystal growth. Embodiments of the invention also maintain high crystal quality, notwithstanding the faster diameter expansion during crystal growth.FIG.5is a photograph of an exemplary AlN single crystal500grown in accordance with embodiments of the invention. AlN single crystals in accordance with embodiments of the invention may have CAP values greater than 20, greater than 40, greater than 60, greater than 80, greater than 90, greater than 100, greater than 150, greater than 500, greater than 1000, or even greater than 1500 (herein, all CAP values are in units of mm unless otherwise indicated), while AlN crystals produced by conventional techniques and AlN crystals reported in the literature have computed CAP values less than 20 (e.g., between 10 and 15, or even less). Conventional growth techniques incapable of fast diameter expansion require much longer growths (and concomitantly larger expansion lengths and smaller CAPs) to achieve large expanded areas of the resulting AlN single crystals. Thus, embodiments of the invention facilitate the faster, more economical production of large, high-quality AlN crystals (e.g., single-crystal AlN wafers) from small seed crystals. For example, the crystal500ofFIG.5has a CAP of 45, thereby illustrating the superiority of embodiments of the present invention over conventional techniques. In accordance with various embodiments, the CAP of AlN single crystals may be no greater than approximately 1600, or no greater than approximately 1700, or no greater than approximately 2000.

Table 1 below reports various CAP values for a variety of different crystals produced by the present inventors, as well as the ratios (in %) of various dimensional parameters for the crystals as shown inFIGS.1B-1D. In Table 1, crystals #1-#4 and #10-#15 had the shape of crystal108depicted inFIG.1B(i.e., with no “straight” portion124), while crystal #5 had the shape of crystal108depicted inFIG.1C(i.e., with a longer frustum112and corresponding expansion length) and crystals #6-#9 had the shape of crystal108depicted inFIG.1D(i.e., with a short expansion length and longer straight portion124).

TABLE 1Ratios, %BouleExpansionStraightExpansionLength/Length/Height/Length/ExpandedTotalTotalExpandedCAPDiameterLengthLengthDiameterCrystal #(mm)(118/116)(120/118)(126/118)(120/116)1803344014.552923564022.583994454023.4441325434018.465754219378.0861570381180.3871059522610.938777482490.969314544731.92101223550017.6511914750023.53121102750013.64131384250020.8314286672047.87151104442018.51

Embodiments of the invention also enable the fabrication of AlN single crystals having unusually large masses and/or volumes compared to conventional AlN crystals. For example, AlN single-crystal boules grown in accordance with embodiments of the present invention may have amass greater than approximately 78 g, greater than approximately 100 g, greater than approximately 120 g, or greater than approximately 140 g, greater than approximately 220 g, or even greater than approximately 240 g. In accordance with various embodiments, the mass may be less than approximately 350 g, or less than approximately 300 g. When larger seeds are utilized, AlN single-crystal boules grown in accordance with embodiments of the present invention may have even larger masses, e.g., greater than approximately 300 g, greater than approximately 500 g, greater than approximately 800 g, greater than approximately 1000 g, or even greater than approximately 1200 g. In accordance with various embodiments, the mass may be less than approximately 1500 g, or less than approximately 1400 g. Thus, exemplary ranges of boule mass in accordance with embodiments of the present invention include, but are not limited to, approximately 78 g—approximately 1300 g, approximately 78 g-approximately 300 g, and approximately 380 g—approximately 1300 g.

Correspondingly (and assuming a constant boule density of 3.255 g/cm3for AlN), AlN single-crystal boules grown in accordance with embodiments of the present invention may have a volume greater than approximately 24 cm3, greater than approximately 30 cm3, greater than approximately 50 cm3, greater than approximately 70 cm3, greater than approximately 75 cm3, or greater than approximately 80 cm3. In accordance with various embodiments, the volume may be less than approximately 100 cm3, or less than approximately 90 cm3. When larger seeds are utilized, AlN single-crystal boules grown in accordance with embodiments of the present invention may have even larger volumes, e.g., greater than approximately 100 cm3, greater than approximately 200 cm3, greater than approximately 300 cm3, or even greater than approximately 350 cm3. In accordance with various embodiments, the volume may be less than approximately 500 cm3, or less than approximately 400 cm3. Thus, exemplary ranges of boule volume in accordance with embodiments of the present invention include, but are not limited to, approximately 24 cm3-approximately 400 cm3, approximately 24 cm3-approximately 80 cm3, and approximately 120 cm3-approximately 400 cm3.

FIG.6Ais a plot showing the distribution of mass (and standard deviation thereof) of over 1200 different AlN single-crystal boules grown in accordance with embodiments of the present invention with seed crystals having diameters of approximately 52 mm or less. Using such seed crystals, as shown, the mass of the boules ranges from approximately 70 g to over approximately 250 g.FIG.6Bis a plot showing the distribution (and standard deviation) of the calculated boule volumes of the over 1200 different AlN single-crystal boules. Such volumes, in these example embodiments, range from approximately 20 cm3to approximately 80 cm3. As demonstrated, AlN single-crystal boules grown in accordance with embodiments of the present invention have larger mass and/or volume than those produced using conventional techniques. The values reported inFIGS.6A and6Bscale up accordingly when larger seed crystals are utilized for growth in accordance with embodiments of the present invention, and thus the values reported inFIGS.6A and6Bshould not be interpreted as limiting embodiments of the present invention. The present inventors have achieved boules having larger masses and volumes, as detailed above, using larger seed crystals.

In various embodiments (and as demonstrated by, e.g., Table 1 above), AlN single-crystal boules grown in accordance with embodiments of the invention have ratios of boule length (i.e., total length118inFIGS.1C and1F) to maximum diameter (i.e., maximum crystal diameter116inFIGS.1B and1C) ranging from approximately 0.3 to approximately 0.7, or ranging from approximately 0.35 to approximately 0.66. In various embodiments AlN single-crystal boules grown in accordance with embodiments of the invention have ratios of expansion length (i.e., expansion length120inFIGS.1C and1F) to maximum diameter (i.e., maximum crystal diameter116inFIGS.1B and1C) falling into one of two different ranges, depending upon the rapidity of the diameter expansion. For example, AlN single-crystal boules grown in accordance with embodiments of the invention having small expansion lengths (e.g., as shown inFIG.1D; for example, boules having ratios of expansion length to total length ranging from approximately 0.5% to approximately 5%, or approximately 1% to approximately 4%) may have ratios of expansion length to maximum diameter ranging from approximately 0.002 to approximately 0.02, or from approximately 0.003 to approximately 0.02, or from approximately 0.003 to approximately 0.01. In another example, AlN single-crystal boules grown in accordance with embodiments of the invention having larger expansion lengths (e.g., as shown inFIGS.1B and1C; for example, boules having ratios of expansion length to total length ranging from approximately 15% to approximately 80%, or from approximately 30% to approximately 70%) may have ratios of expansion length to maximum diameter ranging from approximately 0.08 to approximately 0.5, or from approximately 0.1 to approximately 0.3, or from approximately 0.15 to approximately 0.25.

The values of both ratios are lower than those previously achieved in the art and demonstrate the superiority of AlN single-crystal boules grown in accordance with embodiments of the present invention compared to those produced using conventional techniques. For example, boules in accordance with embodiments of the present invention enable the fabrication of greater numbers of large-diameter AlN single-crystal wafers per total boule length, i.e., the single-crystal AlN is more beneficially distributed within the boule, at least from the standpoint of large wafer production. The crystals produced in accordance with embodiments of the invention are therefore more economical, and enable production of larger wafers therefrom, when compared to conventional crystals and production techniques therefor.

In accordance with embodiments of the invention, the seed diameter may range from approximately 5 mm to approximately 100 mm, approximately 5 mm to approximately 52 mm, or approximately 52 mm to approximately 100 mm. The total boule length may range from approximately 18 mm to approximately 50 mm, approximately 18 mm to approximately 35 mm, or approximately 30 mm to approximately 50 mm. The maximum crystal diameter may range from approximately 17 mm to approximately 120 mm, approximately 17 mm to approximately 65 mm, or approximately 65 mm to approximately 120 mm. These values are exemplary and should not be interpreted as limiting embodiments of the present invention.

Moreover, single-crystal AlN boules fabricated in accordance with embodiments of the invention exhibit high crystal quality, notwithstanding the high rates of diameter expansion utilized during their formation. For example, boules fabricated in accordance with embodiments of the invention exhibit threading dislocation densities less than 105cm−2, or even less than 3×104cm−2, as confirmed by x-ray topography measurements. Moreover, such low defect densities are approximately the same in peripheral, expanded regions of the boules and the central portions of the boules.

One or more substrates (or “wafers”) may be separated from AlN boule220by the use of, e.g., a diamond annular saw or a wire saw, after crystal growth. In an embodiment, a crystalline orientation of a substrate thus formed may be within approximately 2° (or even within approximately 1, or within approximately 0.5°) of the (0001) face (i.e., the c-face). Such c-face wafers may have an Al-polarity surface or an N-polarity surface, and may subsequently be prepared as described in U.S. Pat. No. 7,037,838, the entire disclosure of which is hereby incorporated by reference. In other embodiments, the substrate may be oriented within approximately 2° of an m-face or a-face orientation (thus having a non-polar orientation) or may have a semi-polar orientation if AlN boule220is cut along a different direction. The surfaces of these wafers may also be prepared as described in U.S. Pat. No. 7,037,838. The substrate may have a roughly circular cross-sectional area with a diameter of greater than approximately 50 mm. The substrate may have a thickness that is greater than approximately 100 μm, greater than approximately 200 μm, or even greater than approximately 2 mm. The substrate typically has the properties of AlN boule220, as described herein. After the substrate has been cut from the AlN boule220, one or more epitaxial semiconductor layers and/or one or more light-emitting devices, e.g., UV-emitting light-emitting diodes or lasers, may be fabricated over the substrate, for example as described in U.S. Pat. Nos. 8,080,833 and 9,437,430, the entire disclosure of each of which is hereby incorporated by reference.

AlN bulk crystals (e.g., boules and/or wafers) produced in accordance with embodiments of the present invention may have etch pit density measurements (i.e., etching measurements that reveal defects such as threading dislocations intersecting the surface of the crystal) ranging from approximately 5×103cm−2to approximately 1×104cm−2. AlN crystals in accordance with embodiments of the present invention may have a density of threading edge dislocations ranging from approximately 1×103cm−2to approximately 1×104cm−2and a density of threading screw dislocations ranging from approximately 1 cm−2to approximately 10 cm−2, e.g., a total threading dislocation density less than approximately 104cm−2. When measured via x-ray diffraction, x-ray rocking curves (e.g., along (0002) and/or (10-12)) of AlN crystals in accordance with embodiments of the invention may have full width at half maximum (FWHM) values less than 50 arcsec (e.g., ranging from approximately 30 arcsec to approximately 50 arcsec, or from approximately 40 arcsec to approximately 50 arcsec), or even less than 40 arcsec (e.g., ranging from approximately 20 arcsec to approximately 40 arcsec, approximately 30 arcsec to approximately 40 arcsec, or approximately 20 arcsec to approximately 35 arcsec). As measured by secondary ion mass spectroscopy (SIMS), AlN single crystals in accordance with embodiments of the invention may have carbon concentrations of approximately 1.8×1016cm−3-5×1017cm−3, as well as oxygen concentrations of approximately 1×1017cm−3-7.9×1017cm−3. In various embodiments, the carbon concentration may range from approximately 1.8×1016cm−3to approximately 5×1016cm−3. The thermal conductivity of AlN single crystals in accordance with embodiments of the invention may be greater than approximately 290 Watts per meter-Kelvin (W/m·K), as measured by the American Society for Testing and Materials (ASTM) Standard E1461-13 (Standard Test Method for Thermal Diffusivity by the Flash Method), the entire disclosure of which is incorporated by reference herein, and provided by a commercial vendor such as NETZSCH Inc. of Exton, Pennsylvania.

FIG.7Ais a schematic view of a UV LED700fabricated on an AlN substrate in accordance with embodiments of the present invention. As shown, the UV LED700features a set of layers epitaxially grown over an AlN substrate705and two top-side metal contacts710,715. Specifically, immediately above the substrate is a 500 nm layer720of undoped (i.e., unintentionally doped) AlN, topped with a bottom contact layer725of n-doped (with Si at a concentration of 2×1018cm−3) Al0.83Ga0.17N that is 500 nm thick. Above the bottom contact layer725is a multiple-quantum-well (MQW) layer730featuring five sets of a 2 nm thick Al0.78Ga0.22N quantum well and a 6 nm thick Al0.85Ga0.15N barrier, all of which are undoped. Above the MQW layer730is a 10 nm thick electron-blocking layer formed of undoped Al0.95Ga0.05N. Above the electron-blocking layer is an undoped graded layer735graded from Al0.95Ga0.05N to GaN over a thickness of 30 nm. Finally, over the graded layer735is a 10 nm thick p-doped (with Mg at a concentration of 1×1019cm−3) GaN cap layer740. The p-metal layer710is formed over the cap layer740, while the n-metal layer715is formed over the bottom contact layer725(revealed by etching away the overlying structure, for example).FIG.7Bis a plan-view photograph of the UV LED700ofFIG.7Awhen emitting light at approximately 230 nm. Devices such as that depicted inFIGS.7A and7Bexhibited output powers between 20 μW and 500 μW at currents of 20 mA and at room temperature, continuous wave (CW) operation. Such output powers are indicative of external quantum efficiencies ranging from 0.02% to 0.5% in the wavelength range of 228 nm to 238 nm.

After formation of the electrodes (e.g., contacts710,715), the resulting light-emitting device may be electrically connected to a package, for example as detailed in U.S. Pat. No. 9,293,670, filed on Apr. 6, 2015 (the '670 patent), the entire disclosure of which is incorporated by reference herein. A lens may also be positioned on the device to transmit (and, in various embodiments, shape) the light emitted by the device. For example, a rigid lens may be disposed over the device as described in the '670 patent or in U.S. Pat. No. 8,962,359, filed on Jul. 19, 2012, or in U.S. Pat. No. 9,935,247, filed on Jul. 23, 2015, the entire disclosure of each of which is incorporated by reference herein. After packaging, any remaining portion of the substrate may be removed.

In accordance with embodiments of the invention, various techniques for partial or complete substrate removal may be utilized if desired. For example, etching techniques, such as electrochemical etching techniques described in U.S. patent application Ser. No. 16/161,320, filed on Oct. 16, 2018, the entire disclosure of which is incorporated by reference herein, may be utilized. In other embodiments, techniques like those utilized in U.S. patent application Ser. No. 15/977,031, filed on May 11, 2018, may be utilized.

AlN crystals, and wafers produced therefrom, in accordance with embodiments of the present invention may also advantageously exhibit high levels of UV transparency, even at deep-UV wavelengths, for example as described in U.S. patent application Ser. No. 16/444,147, filed on Jun. 18, 2019 (the '147 application), the entire disclosure of which is incorporated by reference herein. For example, embodiments of the invention include techniques for the control and reduction of carbon content in the source material utilized to grow the AlN single crystal and UV-transparency enhancement via thermal treatments, as detailed below.

In various embodiments, the polycrystalline AlN ceramic may be fabricated in accordance with the techniques described in U.S. Pat. No. 9,447,519 (the '519 patent), the entire disclosure of which is incorporated by reference herein, i.e., a “pellet-drop” technique using high-purity Al pellets melted in the presence of nitrogen to form AlN polycrystalline ceramic material. In various embodiments, the ceramic is broken up into fragments to facilitate removal of much of the carbon therefrom. The ceramic may be fragmented by, e.g., application of mechanical force. The present inventors have found that, surprisingly, much of the carbon present in the polycrystalline AlN ceramic remains on smaller fragments and/or dust (e.g., particles having large aggregate surface area and/or having diameters less than about 2 mm) resulting from the fragmentation process, while larger fragments (e.g., ones having widths, diameters, or other lateral dimensions ranging from 0.5 cm to 2 cm) exhibit smaller carbon concentrations. In various embodiments, the fragments of the AlN ceramic may be separated on the basis of size using one or more sieves, and/or compressed air or another fluid (e.g., nitrogen or an inert gas such as argon) may be applied to the fragments to minimize or reduce the amount of dust or other particles thereon. For example, as reported in the '147 application, the entire disclosure of which is incorporated by reference herein, after fragmentation and separation, the larger fragments have carbon concentrations that range from approximately 5 ppm to approximately 60 ppm, with an average carbon concentration of approximately 26 ppm. In stark contrast, the resulting powder and smaller fragments have carbon concentrations that range from approximately 108 ppm to approximately 1800 ppm, with an average carbon concentration of approximately 823 ppm.

Thus, in accordance with various embodiments of the invention, one or more of the larger fragments of the AlN polycrystalline ceramic, once separated from the smaller fragments and powder, may be utilized directly as the source material for formation of single-crystal AlN (as detailed above). In other embodiments, one or more (typically more) of the fragments are collected and placed into a crucible (e.g., a tungsten (W) vessel) for subsequent heat treatment. (While in preferred embodiments only the larger fragments of the polycrystalline AlN ceramic are heat treated, embodiments of the invention do encompass heat treatment of the entire, unfragmented ceramic.)

In various embodiments, the optional subsequent preparation stage involves an annealing and densification treatment of at least a portion of the polycrystalline AlN ceramic (e.g., one or more larger fragments thereof) to form high-quality polycrystalline AlN source material. In accordance with various embodiments of the invention, the AlN ceramic (or portion thereof) may be heated to a first temperature T1ranging from 1100° C. to 2000° C. and held at temperature T1for a time period t1of, for example, 2 hours to 25 hours. Thereafter, the ceramic (or portion thereof) may be heated to a higher second temperature T2(e.g., a temperature ranging from 2000° C. to 2250° C.) and held at temperature T2for a time period t2of, for example, 3 hours to 15 hours. During the heat treatment, the ceramic (or portion thereof) is annealed and densified to form a polycrystalline AlN source material that may be utilized in the subsequent formation of single-crystal AlN bulk crystals. Because the polycrystalline AlN source material is generally approximately stoichiometric AlN with low concentrations of impurities, it may be used to form an AlN bulk crystal without further preparation (e.g., without intermediate sublimation-recondensation steps).

In an alternative heat treatment in accordance with embodiments of the invention, a longer ramp to temperature T2is utilized in place of the first annealing step at temperature T1. In accordance with various embodiments of the invention, the AlN ceramic (or portion thereof) may be ramped to temperature T2(e.g., a temperature ranging from 2000° C. to 2250° C.) over a time period t1ranging from, for example, 5 hours to 25 hours. Thereafter, the ceramic (or portion thereof) may be held at temperature T2for a time period t2of, for example, 3 hours to 25 hours. During the heat treatment, the ceramic (or portion thereof) is annealed and densified to form a polycrystalline AlN source material that may be utilized in the subsequent formation of high-quality single-crystal AlN bulk crystals. Because the polycrystalline AlN source material is generally approximately stoichiometric AlN with low concentrations of impurities, it may be used to form an AlN bulk crystal without further preparation (e.g., without intermediate sublimation-recondensation steps).

In various embodiments, the carbon concentration of the polycrystalline AlN source material, as measured by instrumental gas analysis (IGA), ranges from approximately 3.0×1018cm−3to approximately 1.8×1019cm−3, approximately 3.8×1018cm−3to approximately 1.2×1019cm−3, or even from approximately 3.0×1018cm−3to approximately 9.0×1018cm−3. After the optional densification heat treatment, the density of the polycrystalline AlN source material, as measured by pycnometry at room temperature, may be approximately equal to that of single-crystal AlN, i.e., approximately 3.25 g/cm3to 3.26 g/cm3. In various embodiments, the measured density of the AlN ceramic without the densification heat treatment may be lower, e.g., approximately 2.95 g/cm3to approximately 3.20 g/cm3. In various embodiments, after the optional densification heat treatment, the polycrystalline AlN source material typically has an amber color and is composed of fairly large grains (e.g., average grain diameter ranging from approximately 0.1 mm to approximately 5 mm).

Referring back toFIG.2, in accordance with embodiments of the invention, one or more internal parts of the crystal-growth apparatus200(e.g., the crucible205, the susceptor215, and/or the crucible stand210) may be annealed before crystal growth and formation of AlN boule220, and such annealing may advantageously decrease the carbon concentration in the AlN boule220. In various embodiments, the one or more internal parts of the crystal-growth apparatus200may be annealed at, for example, a temperature ranging from approximately 1000° C. to approximately 1800° C. for a time period of approximately 5 hours to approximately 50 hours.

In various embodiments of the invention, the concentration of carbon within the AlN boule220may be decreased via the introduction of one or more gettering materials within the crucible205prior to and during growth of the AlN boule220. The gettering materials may be introduced as a portion or all of one or more of the components of the crystal-growth apparatus200(e.g., the crucible205, a liner situated within the crucible205and proximate an interior surface or wall thereof, the susceptor215, and/or the crucible stand210), and/or the gettering materials may be introduced as discrete masses of material within the crystal-growth apparatus200. The gettering materials may be disposed between the source material230and the growing AlN boule220in order to, e.g., getter or absorb contaminants such as carbon from the vapor flowing toward the AlN boule220(i.e., toward the seed crystal235). In various embodiments, the gettering materials are stable at and have melting points greater than the growth temperature (e.g., greater than approximately 2000° C.) and have low vapor pressures to prevent contamination of the growing AlN boule220with the gettering materials themselves. In various embodiments, a gettering material has a eutectic melting point with AlN that is greater than the growth temperature (e.g., greater than approximately 2000° C.). Examples of gettering materials in accordance with embodiments of the present invention include boron (melting point of approximately 2300° C.), iridium (melting point of approximately 2410° C.), niobium (melting point of approximately 2468° C.), molybdenum (melting point of approximately 2617° C.), tantalum (melting point of approximately 2996° C.), rhenium (melting point of approximately 3180° C.), and/or tungsten (melting point of approximately 3410° C.). In various embodiments, the gettering material (or the component of the apparatus200or portion thereof) may include, consist essentially of, or consist of one or more non-tungsten materials having melting temperatures of at least approximately 2300° C.

After growth of the AlN boule220, the AlN boule220may be cooled down to approximately room temperature for subsequent removal from the crystal-growth apparatus200. For example, the AlN boule220may be cooled in a two-stage process as described in the '519 patent. However, in various embodiments of the invention, the AlN boule220may simply be cooled down from the growth temperature in a single stage, at an arbitrary rate, as the heat treatment detailed below obviates the need for the two-stage process of the '519 patent. In fact, in various embodiments of the present invention, the AlN boule220is cooled down from the growth temperature to approximately room temperature at a high rate (e.g., greater than 70° C./hour, greater than 80° C./hour, greater than 100° C./hour, greater than 150° C./hour, greater than 200° C./hour, greater than 250° C./hour, greater than 300° C./hour, greater than 400° C./hour, or even greater than 500° C./hour; in various embodiments, the rate may be no more than 2000° C./hour, 1500° C./hour, or 1000° C.) without any “controlled cooling” achieved via application of power to the heating elements of crystal-growth apparatus200. In various embodiments of the invention, gas (e.g., nitrogen and/or an inert gas) is flowed within the crystal-growth apparatus200at a high rate (e.g., a rate approximately equal to or higher than any gas-flow rate utilized during crystal growth) in order to cool the AlN boule220. For example, the gas-flow rate utilized during crystal growth may be approximately 4 slm or less, approximately 3 slm or less, approximately 2 slm or less, or approximately 1 slm or less. The gas-flow rate utilized during crystal growth may be approximately 0.1 slm or more, approximately 0.5 slm or more, approximately 1 slm or more, or approximately 2 slm or more. In various embodiments, the gas-flow rate utilized during cooling may be approximately 5 slm or more, approximately 10 slm or more, approximately 15 slm or more, approximately 20 slm or more, or approximately 25 slm or more. The gas-flow rate utilized during cooling may be approximately 30 slm or less, approximately 25 slm or less, approximately 20 slm or less, approximately 15 slm or less, or approximately 10 slm or less. In addition, in embodiments of the invention, the crucible205(and thus the AlN boule220therewithin) may be moved to an edge of the hot zone, or above the hot zone, formed by the heating elements of the crystal-growth apparatus200in order to more rapidly cool the AlN boule220.

Advantageously, the high-rate cooling of AlN boule220minimizes or eliminates the formation of cracks within the AlN boule220, particularly when the AlN boule220has a diameter of approximately 50 mm or greater. However, the high cooling rate may also result in deleteriously high UV absorption within the AlN boule220at one or more wavelengths (e.g., wavelengths around approximately 310 nm), as described in the '147 application.FIG.3Adepicts the UV absorption spectrum for an exemplary AlN boule220cooled quickly from the growth temperature as detailed herein. For example, the UV absorption spectrum of an exemplary AlN boule220cooled quickly from the growth temperature may exhibit an elevated peak at approximately 310 nm that impairs the UV transparency of the crystal over a wide range of wavelengths, and the UV absorption coefficient may be greater than 20 cm−1over the entire wavelength range of 210 nm to 400 nm. The UV absorption coefficient may also be greater than 30 cm−1over the wavelength range of 210 nm to 380 nm. Thus, in accordance with various embodiments of the present invention, control of various impurity concentrations such as carbon during the growth of and within the resulting AlN crystal may be insufficient to achieve low UV absorption coefficients, particularly at deep-UV wavelengths (e.g., between 210 nm and 280 nm, between 230 nm and 280 nm, or between 210 nm and 250 nm).

After cooling to room temperature, the AlN boule220, or a portion thereof, may be heat treated to further improve its UV transparency, particularly at deep-UV wavelengths. For example, one or more wafers may be separated from AlN boule220, as detailed herein, and one or more of the wafers may be heat treated for improvement of UV transparency. The ensuing description refers to the heat treatment of the AlN boule220, but it should be understood that only one or more portions of the boule (e.g., one or more wafers) may be heat treated, rather than the entire boule. In addition, the heat treatments detailed herein may be performed on various different AlN crystals (e.g., AlN single crystals), even if not initially grown and cooled as detailed herein, in order to improve UV absorption.

In various embodiments of the invention, the AlN boule220is annealed in a heating apparatus (e.g., a furnace such as a resistive furnace or a radio-frequency (RF) furnace) configured for substantially isothermal or quasi-isothermal heating. The interior of the furnace (at least in the heated, or “hot” zone), as well as any hardware (e.g., a platform or other support) within the furnace, may include, consist essentially of, or consist of one or more refractory materials (e.g., W or another refractory metal) having a melting point exceeding about 2800° C., or even exceeding about 3000° C. In various embodiments, the interior of the furnace (at least in the heated, or “hot” zone), and the hardware (e.g., a platform or other support) within the furnace, may be free of carbon, carbon-based or carbon-containing materials, graphite, quartz, alumina, and/or molybdenum. Before the AlN boule220is placed within the furnace, the furnace may undergo a bake-out run at high temperature to reduce or minimize the presence of any contaminants therewithin. For example, the furnace may be heated to about 2600° C. under vacuum for a time period of, e.g., approximately 0.5 hours to approximately 2 hours. After the furnace has cooled, the AlN boule220may be placed within the furnace, which may then be filled with nitrogen gas at a pressure of, e.g., approximately 1 bar to approximately 2 bars. The AlN boule220may be placed “loosely” (i.e., not attached, adhered, or fastened to) on a platform within the furnace that may include, consist essentially of, or consist of W or another refractory metal. In various embodiments, the loose placement of the AlN boule220reduces or substantially eliminates stresses due to any differential thermal expansion between AlN boule220and the platform.

The temperature within the furnace may then be ramped to the desired annealing temperature at a ramp rate of, e.g., approximately 1° C./min to approximately 50° C./min. In various embodiments, the annealing temperature is between approximately 2100° C. and approximately 2500° C., e.g., approximately 2400° C. In various embodiments, the annealing temperature is between approximately 2150° C. and approximately 2400° C. The present inventors have found that lower annealing temperatures (e.g., about 2000° C.) are generally insufficient to improve the UV transparency of AlN boule220at deep-UV wavelengths to the desired level. Once the desired annealing temperature has been achieved, the AlN boule220is annealed at that temperature for a time period of, for example, approximately 0.5 hour to approximately 100 hours, approximately 0.5 hour to approximately 5 hours, or approximately 1 hour. After annealing, the temperature of the furnace is slowly ramped down to an intermediate temperature (for example, between approximately 800° C. and approximately 1200° C., e.g., approximately 1000° C.) at a rate ranging between approximately 60° C./hour and approximately 120° C./hour. For example, the furnace may be cooled from an exemplary annealing temperature of 2200° C. to 1000° C. over a time period of 15 hours. Such slow cooling may be achieved via controlled application of heat with the furnace (e.g., at low power levels). Thereafter, the furnace may be turned off, and the furnace and the AlN boule220may be allowed to cool to room temperature. Thus, in various embodiments of the invention, the entire annealing cycle, including the cool-down therefrom, of the AlN boule220is performed in substantially isothermal or quasi-isothermal conditions.

FIG.8is a graphical comparison of a UV absorption spectrum800, corresponding to a conventional UV absorption spectrum reported in the '519 patent and a UV absorption spectrum810of an AlN single crystal fabricated and annealed in accordance with embodiments of the present invention. As shown, over the entire range of wavelengths, the crystal in accordance with embodiments of the invention exhibits a lower absorption coefficient, and the spectrum is substantially constant (or “flat”) for wavelengths between 210 nm and 280 nm. At about 230 nm, the crystal in accordance with embodiments of the invention has an absorption coefficient of less than 10 cm−1(in the depicted example, approximately 7 cm−1-8 cm−1), which is dramatically lower than the results achieved in the '519 patent. In addition, the slope of the absorption coefficient as a function of wavelength near the band edge is much steeper, as described in more detail below.

As mentioned above, embodiments of the present invention include and enable the production of single-crystal AlN having a steep drop-off in the absorption coefficient near the band edge, i.e., AlN having a low Urbach energy. The “Urbach tail” is the exponential part of the absorption coefficient curve near the optical band edge, and is related to crystalline disorder and localized electronic states extending into the band gap.

The spectral dependence of the absorption coefficient (a) and photon energy (hv) is known as Urbach empirical rule, which is given by the following equation:

α=α0⁢exp⁡(hvEU)
(see Franz Urbach, “The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids,” Phys. Rev. 92 (1953) 1324, the entire disclosure of which is incorporated by reference herein). α0is a constant, and EUis the Urbach energy, i.e., the energy of the band tail. The above equation may be rewritten as:

ln⁢⁢α=ln⁢⁢α0+(hvEU)
and the Urbach energy may be determined from the slope of the line when ln (α) is plotted as a function of the incident photon energy hv; on such a plot, ln (α0) is the y-intercept of the line and thus corresponds to ln (α) at a theoretical zero photon energy. Specifically, the Urbach energy is the inverse of the slope.

FIG.9is a plot used to determine the Urbach energy of the sample from the '519 patent having the absorption spectrum800presented inFIG.8as well as the Urbach energy of the sample in accordance with embodiments of the invention having the absorption spectrum810presented inFIG.8. As shown, the slope of the resulting curve900for the inventive sample is much steeper (the slope is approximately 4.7/eV) and results in an Urbach energy of approximately 0.21 eV in the range of photon energies of 5.85 eV to 6.00 eV. In stark contrast, the curve910corresponding to the sample having the absorption spectrum800exhibits a slope of approximately 0.5/eV, which results in an Urbach energy of approximately 2.0 eV. In accordance with embodiments of the invention, the present inventors have fabricated samples having Urbach energies ranging from approximately 0.2 eV to approximately 1.8 eV, e.g., from approximately 0.21 eV to approximately 1.0 eV, which are significantly lower than those of conventional samples and samples reported in the literature.

In general, UV absorption spectra (and Urbach energies derived therefrom) may be determined by measuring reflections of incident light on a sample using a spectrometer. For example, the UV absorption spectra of samples in accordance with embodiments of the invention were measured using a V-670 (Class I) spectrometer and X-Y stage from Jasco Corporation. 52 points per sample were measuring utilizing a two-axis stage controller from Chuo Precision Industrial Co., Ltd. Wavelengths from 200 nm to 800 nm were measured, but measurements up to wavelengths of 2000 nm may be acquired utilizing this set-up. The absorption spectrum of a sample having a thickness L is estimated based on the light incident on the sample and the light transmitted by the sample, taking into account the light reflected back toward the light emission from both surfaces of the sample. The thickness L may be measured using, for example, a gauge (e.g., ACANTO, CERTO, METRO, or SPECTO length gauges, and associated GAGE-CHEK evaluation electronics, available from Heidenhain Corp. of Schaumburg, IL) or an optical system such as the ULTRA-MAP 100B or ULTRA-MAP C200, available from MicroSense, LLC of Lowell, MAFIG.10summarizes this calculation, and absorption coefficient α at a particular wavelength of incident light λ may be calculated from:

ITI0=(1-R)2⁢e-α⁢⁢L
where ITis the intensity of the transmitted light and I0is the intensity of the incident light. The reflectance R may be determined from:

R=(n-1n+1)2
where the refractive index n may be determined from the dispersion formula:

n2-1=2.1399+1.3786⁢λ2λ2-0.17152+3.861⁢λ2λ2-15.032
and where dispersion formula is provided from J. Pastrňã{acute over (k)} and L. Roskovcovã, “Refraction index measurements on AlN single crystals,” Phys. Stat. Sol. 14, K5-K8 (1966), the entire disclosure of which is incorporated by reference herein.

The improved UV absorption spectra of embodiments of the present invention enable enhanced performance of light-emitting devices (e.g., lasers and light-emitting diodes (LEDs)) fabricated on AlN substrates having the improved spectra, particularly at short wavelengths.FIG.11is a graph of LED device emission intensity as a function of wavelength for simulated LEDs emitting at about 217 nm. The top curve1100is the emission intensity as a function of wavelength for an LED fabricated on a substrate having the improved absorption spectrum enabled by embodiments of the present invention—in this example, the UV absorption spectrum810depicted inFIG.8. The bottom curve1110corresponds to the same LED structure fabricated on a substrate having the absorption spectrum800ofFIG.8). As shown inFIG.11, the emission intensity enabled by embodiments of the present invention is increased by nearly a factor of two at the peak emission wavelength of about 217 nm and is higher over the entire wavelength range.FIG.12is a graph of the same spectra over a smaller wavelength range, in which the relative intensities of the LEDs have been independently normalized to the same value in order to demonstrate the narrower intensity peak of the device in accordance with embodiments of the present invention. This narrower peak enables superior LED performance. The simulations for the devices depicted inFIGS.11and12indicate that the emission power for the device in accordance with embodiments of the invention will be increased by at least 1.6× for the substrate thickness of 0.55 mm utilized in the simulations. Due to the improved UV absorption, this advantage will be larger for larger substrate thicknesses. In addition, when reflectors are utilized to reflect light emitted by the device into a preferred direction, the power of the device will increase for each pass through the substrate traveled by the reflected light. For example, the improvement in device emission power enabled by embodiments of the present invention may be approximated as 2×(1.6)3, or approximately 8×, when reflected light traverses the substrate having the improved absorption spectrum three times.

The growth of bulk single crystals has been described herein primarily as being implemented by what is commonly referred to as a “sublimation” or “sublimation-recondensation” technique wherein the source vapor is produced at least in part when, for production of AlN, crystalline solids of AlN or other solids or liquids containing AlN, Al or N sublime preferentially. However, the source vapor may be achieved in whole or in part by the injection of source gases or the like techniques that some would refer to as “high-temperature CVD.” Also, other terms are sometimes used to describe these and techniques that are used to grow bulk single AlN crystals in accordance with embodiments of the invention. Therefore, the terms “depositing,” “growing,” “depositing vapor species,” and like terms are used herein to generally cover those techniques by which the crystal may be grown pursuant to embodiments of this invention.

The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.