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Patent US8088220 - Deep-eutectic melt growth of nitride crystals - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsIn accordance with various embodiments, crystalline structures are formed by providing, at a growth temperature, a liquid comprising AlN and having a quality factor greater than approximately 0.14 and forming solid AlN from the liquid, the growth temperature being lower than the melting point of AlN...http://www.google.com/patents/US8088220?utm_source=gb-gplus-sharePatent US8088220 - Deep-eutectic melt growth of nitride crystalsAdvanced Patent SearchPublication numberUS8088220 B2Publication typeGrantApplication numberUS 12/126,334Publication dateJan 3, 2012Filing dateMay 23, 2008Priority dateMay 24, 2007Also published asUS20090050050Publication number12126334, 126334, US 8088220 B2, US 8088220B2, US-B2-8088220, US8088220 B2, US8088220B2InventorsGlen A. Slack, Sandra B. SchujmanOriginal AssigneeCrystal Is, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (105), Non-Patent Citations (133), Referenced by (2), Classifications (18), Legal Events (1) External Links: USPTO, USPTO Assignment, EspacenetDeep-eutectic melt growth of nitride crystalsUS 8088220 B2Abstract In accordance with various embodiments, crystalline structures are formed by providing, at a growth temperature, a liquid comprising AlN and having a quality factor greater than approximately 0.14 and forming solid AlN from the liquid, the growth temperature being lower than the melting point of AlN.
RELATED APPLICATIONS This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/931,683, filed May 24, 2007, the entire disclosure of which is hereby incorporated by reference.
GOVERNMENT SUPPORT This invention was made with United States Government support under 70NANB4H3051 awarded by the National Institute of Standards and Technology. The United States Government has certain rights in the invention.
FIELD OF THE INVENTION The technology disclosed herein relates generally to fabrication of semiconductor single crystals, in particular the growth of nitride semiconductor single crystals from a molten solution.
BACKGROUND The interest in nitrides of the group-III metals such as gallium (Ga), indium (In) and aluminum (Al) has risen during the past decade because of their potential as high-frequency/high-power electronic materials and blue, violet and ultraviolet optoelectronic materials for devices such as light-emitting diodes (LEDs), laser diodes (LDs), and photo detectors. Such devices include active areas consisting of layers composed of solid solutions of, e.g., gallium nitride (GaN), indium nitride (InN), and/or aluminum nitride (AlN) in various proportions. In various devices, some or all of the layers are doped with one or more impurity species, e.g., n-type or p-type dopants. The layers are typically epitaxially grown on a substrate that provides the template for the ordering of the atoms in the layers. The layers may be lattice-matched to the substrate, i.e., have an atomic spacing (or lattice parameter) closely or identically matched to that of the substrate, in order to avoid the formation of crystalline defects, such as dislocations, that may negatively affect the performance of the devices.
Several types of materials may be used to fabricate substrates for nitride-based semiconductor heterostructures. For devices employing GaN or Ga1-xInxN (herein interchangeably referred to as InGaN or GaInN), a highly desirable substrate would be a large-area GaN single-crystal wafer. While several methods to grow GaN crystals have been proposed, none of them appears to be commercially feasible to fabricate large-area bulk crystals of GaN. Sapphire is a popular conventional substrate material because relatively high-quality, inexpensive sapphire substrates are commercially available. However, for a number of reasons, sapphire is not particularly suited for GaN epitaxy. Single-crystal substrates of silicon carbide (SiC) present an attractive alternative to sapphire due to their close lattice match to AlN/GaN in the plane perpendicular to the c-axis (i.e., the �c-plane�) and their high thermal conductivity. In addition, SiC substrates can be made electrically conducting, which is attractive for some applications, including fabrication of LEDs and LDs. However, wurtzite SiC (matching the wurtzite crystal structure of GaN) is not available and the lattice mismatch along the c-axis between GaN and both 4H and 6H SiC is substantial. In addition, the chemical bonding between the group IV elements of the SiC and the group III or group V elements of the nitrides may deleteriously create nucleation problems leading to electronic states at the interface.
Alternative substrates for commercial fabrication of nitride-based devices are necessary to mitigate the above-described limitations of sapphire and SiC substrates. Specifically, the physical and electronic properties of AlN�its wide energy band gap, high-breakdown electric field, extremely high thermal conductivity, and low optical absorption in the visible and ultraviolet spectra�make this material desirable for a wide variety of semiconductor applications as a substrate material. In addition, both SiC and sapphire have a large thermal-expansion mismatch with GaN between room temperature and typical temperatures used for epitaxy of the nitrides (approximately 1100� C. or higher), while AlN and GaN are very well thermally matched over this same temperature range.
Recently, fabrication of AlN crystals having very low dislocation densities has been demonstrated by the sublimation-recondensation method described in, e.g., commonly-owned co-pending U.S. patent application Ser. No. 11/503,660 (the �'660 application�), the entire disclosure of which is hereby incorporated by reference. While the sublimation-recondensation method is extremely useful in the growth of pure, high-quality, large-diameter crystals of AlN, the material is usually insulating. For some applications, doped semiconducting material is advantageous, and growth of doped crystals from a melt may be easier because applying a nitrogen overpressure in the melt vessel may maintain the doping elements mixed in the molten AlN solution. In the sublimation-recondensation method, elements with very different vapor pressures compared to that of Al tend to be non-uniformly incorporated (or even not incorporated) as the crystal grows. The sublimation-recondensation technique uses a solid-vapor-solid transition at a temperature of approximately 2300� C. In comparison, a liquid-solid or a solid-liquid-solid transition from a molten phase including AlN may provide higher growth rates. One of the growth-rate limiting factors in the sublimation-recondensation method is the fact that the dissociation energy of nitrogen gas (N2) is 9.75 electron volts (eV), and most of the nitrogen present in the vapor is in N2 form. The growth rate from the vapor phase is limited by the incorporation rate of atomic nitrogen (N), which is present in smaller amounts, into the solid AlN. In the liquid phase, most or all of the nitrogen is present as atomic N (and at concentrations approximately 104 times higher than in the vapor phase used in sublimation-recondensation), facilitating higher growth rates of AlN. However, the melting temperature of AlN is in the vicinity of 2750� C., where the dissociation pressure of nitrogen is 8 bars. Thus, a typical pure AlN melt-to-AlN solid transition requires higher temperature and higher gas pressure than the sublimation-recondensation method.
SUMMARY A technique is provided for forming single-crystal materials from a melt via the utilization of pseudo-binary deep-eutectic systems. Such systems include a liquid phase whose �eutectic� melting temperature is lower than the melting temperature of either of the two pure materials in the pseudo-binary solution. As utilized herein, �pseudo-binary� refers to systems that mimic typical binary solutions of the form A-B, but wherein the A and B species are each compounds of at least two elements or ions. Single crystals of, e.g., AlN, are fabricated at the high growth rates enabled by the use of a liquid starting material (and its concomitant high free N concentration) but at lower temperatures and pressures than those required by the solidification of a pure AlN molten phase. Preferred pseudo-binary systems include those in which the solid solubility of each compound in the other (the �mutual solid solubility�) is very low, i.e., less than 5% or even approximately zero. These systems enable single crystals of high purity to be formed from the pseudo-binary melt, as the solidification of one member of the pseudo-binary pair will not incorporate appreciable amounts of the other. Moreover, dopant species may be incorporated into the growing crystal with a high degree of uniformity and at high concentrations.
FIG. 1 is a phase diagram of the pseudo-binary system AlN�Ca3N2, an example of a shallow eutectic system with a low quality factor;
DETAILED DESCRIPTION The phase diagram of the AlN�Ca3N2 pseudo-binary system is depicted in FIG. 1. The minimum liquidus temperature (i.e., the eutectic temperature) is approximately 1393� C. A quality factor Q may be defined as:
where Tlm is the lower of the two melting points of the pure compounds in the pseudo-binary and ΔT, the �temperature depression,� is the difference between Tlm and the eutectic temperature, where all temperatures are in degrees Kelvin. For the AlN�Ca3N2 system shown in FIG. 1, the quality factor Q is approximately 0.033. Herein, we shall term the pseudo-binary systems with Q≧0.14 as �deep-eutectic systems,� 0.10≦Q≦0.14 as �intermediate systems,� and 0.01≦Q<0.10 as �shallow-eutectic systems.�
The AlN�Ca3N2 system may be characterized as a shallow-eutectic system. Its low Q value implies that very little AlN is dissolved in the liquid phase at the eutectic temperature. Such shallow-eutectic systems are not useful for the production of commercial-quality, large single crystals. The size of any crystals formed from this system is limited by the low solubility of the AlN in Ca3N2, as well as the low increase in solubility as the temperature is raised above the eutectic temperature of 1393� C. Moreover, attempts to fabricate single crystals from such systems may even result in explosions caused by the high dissociation pressure of Ca3N2 as the temperature is increased over the eutectic point. Many nitride compounds, e.g., Ca3N2 and Mg3N2 form shallow or intermediate pseudo-binary systems with AlN, and hence are not suitable for the fabrication of commercial-quality single-crystal AlN. Such compounds have relatively low melting points, high vapor pressures, and form low Q systems with AlN. Other AlN-containing systems, e.g., AlN�Li3N, do not exhibit single eutectic points and thus form at least one intermediate ternary compound, e.g., one incorporating Al, Li, and N.
In a deep-eutectic pseudo-binary system, there is no fundamental distinction between the solvent and the solute. In growth systems with low Q values (e.g., shallow eutectic systems) the solvent has a much lower melting point than the solute, which facilitates the growth of crystals at a temperature near the melting point of the solvent. However, the solubility of the solute in the solvent is generally quite small, as in the case of the AlN�Ca3N2 system described above. Thus, deep-eutectic systems may advantageously be utilized for the formation of large single crystals of high purity.
A second advantage of high-Q, deep-eutectic systems is that the vapor pressure of the two member compounds at the eutectic temperature is much lower than their vapor pressures at their own, individual melting points. By contrast, in the AlN�Ca3N2 system, the vapor pressure of Al is very low at the growth temperature, but that of Ca is almost the same as that over pure Ca3N2. High vapor pressure may lead to individual components evaporating instead of remaining in the melt, changing the composition of the liquid. This composition change in the melt may lead to non-uniform incorporation of the components in the solid crystal. Table 1 shows several compounds that form deep eutectic systems with AlN, their melting points and the calculated eutectic temperatures and Q values.
Scandium nitride has a cubic, rock salt structure. In ScN the Sc-to-N distance is (ao/2)=2.256 Å, where ao is the cubic lattice parameter. The Sc atoms are octahedrally coordinated to six N atoms. Scandium forms other compounds with N in which the Sc atoms are tetrahedrally coordinated to N, which is the same as the coordination of Al with N in AlN. In AlN, the Al coordination is tetrahedral with an Al-to-N distance of 1.893 Å. The distance ratio (2.121 Å/1.893 Å)=1.12 is sufficiently close to unity that some ScN may be soluble in AlN. The ScN�AlN phase diagram is very similar to the one in FIG. 2. It has a eutectic temperature of 2048� C. at a composition of 27 mole percent AlN and 73 mole percent ScN. The mutual solid solubilities are no more than a few mole percent and may even be approximately zero, as shown in FIG. 2. No Sc�Al�N ternary compounds are known. Cooling a ScN�AlN melt containing between 100% AlN and 28% AlN will precipitate nearly pure AlN crystals before it solidifies at 2048� C.
The pseudo-binary system YN�AlN has a Q-factor of 0.210. Several studies have shown that there are no intermediate compounds in the YN�AlN pseudo binary phase diagram. See, e.g., U.S. Pat. No. 4,547,471 to Huseby et al., and W. Y. Sun, Z. K. Huang, T. Y. Tien, and T. S. Yen, Mater. Letters, Vol. 3-4, pp. 67-99 (1991), the entire disclosures of which are hereby incorporated by reference. Huseby et al. found no intermediate compounds at 1900� C. to 2000� C. while Sun et al. found none at 1850� C. The estimated tetrahedral Y-to-N distance is 2.34 Å for Y dissolved in AlN. This is 24% larger than that of AlN, so the solid solubility of YN in AlN, and vice-versa, will be quite small, i.e., less than a few mole percent, or even approximately zero. Thus, a useful temperature range for growing AlN crystals from a AlN-YN melt is 2300� C. to 2055� C. with an AlN content of the melt of 47 to 28 mole percent.
Embodiments of the present invention may be utilized to advantageously getter oxygen impurities from the desired single-crystal material. For the case of a eutectic material 360 including AlN and YN, the YN will getter oxygen during growth of single-crystal AlN by the reaction YN+�Al2O3→�Y2O3+AlN. At the eutectic temperature (in this example 2055� C.), the Gibbs free energy difference (ΔG) for this reaction to form one mole of AlN is −148 kJ/mol, so Y2O3 is more stable than Al2O3. During the deep-eutectic growth of AlN, the Y2O3 remains in solution in the liquid YN�AlN eutectic material 360 and the newly grown AlN single crystal has a lower oxygen concentration than that of source material 370. The lower limit in the final oxygen concentration in the single-crystal material may be determined by the purity of the other compound in eutectic material 360. Other nitrides possess favorable ΔG values and may therefore beneficially getter oxygen during AlN growth from the melt, for example, ScN (ΔG=−126 kJ/mol), LaN (ΔG=−104 kJ/mol), and GdN (ΔG=−94 kJ/mol). In contrast, nitrides such as TiN (ΔG=95 kJ/mol) and ZrN (ΔG=13 kJ/mol) may not getter oxygen due to unfavorable ΔG values.
Horizontal-boat-zone-melting (HBZM): in this method, two rods of solid material, e.g., AlN, are connected through a hot, molten zone of a liquid pseudo-binary phase held between them. See W. D. Lawson and S. Nielsen, �Preparation of Single Crystals�, Academic Press, New York, 1958, pp. 18-20, the entire disclosure of which is hereby incorporated by reference. One rod is a single crystal; the other is a polycrystalline source material. As the molten zone is moved the crystal grows incorporating material from the polycrystalline rod. The deep-eutectic material systems described herein may also be utilized in HBZM to fabricate single crystals of, e.g., AlN.
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Growth, (Jan. 25, 2006), vol. 287, No. 2, pp. 372-375.* Cited by examinerReferenced byCiting PatentFiling datePublication dateApplicantTitleUS8222650Nov 12, 2009Jul 17, 2012Crystal Is, Inc.Nitride semiconductor heterostructures and related methodsUS8323406Jan 17, 2008Dec 4, 2012Crystal Is, Inc.Defect reduction in seeded aluminum nitride crystal growthClassifications U.S. Classification117/67, 117/54, 252/518.1, 117/81, 117/77, 117/73, 117/11, 117/64, 117/78, 117/76International ClassificationC30B19/00, C03B19/06Cooperative ClassificationC30B13/02, C30B29/403, C30B13/10European ClassificationC30B13/02, C30B13/10, C30B29/40BLegal EventsDateCodeEventDescriptionOct 22, 2008ASAssignmentOwner name: CRYSTAL IS, INC., NEW YORKFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SLACK, GLEN A.;SCHUJMAN, SANDRA B.;REEL/FRAME:021722/0120;SIGNING DATES FROM 20081020 TO 20081021Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SLACK, GLEN A.;SCHUJMAN, SANDRA B.;SIGNING DATES FROM 20081020 TO 20081021;REEL/FRAME:021722/0120RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google