GROUP III NITRIDE SUBSTRATE, METHOD OF MAKING, AND METHOD OF USE
Embodiments of the present disclosure include techniques related to techniques for processing materials for manufacture of group-III metal nitride and gallium based substrates. More specifically, embodiments of the disclosure include techniques for growing large area substrates using a combination of processing techniques. Merely by way of example, the disclosure can be applied to growing crystals of GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic and electronic devices, lasers, light emitting diodes, solar cells, photo electrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, and others.
Latest SLT Technologies, Inc. Patents:
- Oxygen-doped group III metal nitride and method of manufacture
- GROUP III NITRIDE SUBSTRATE AND METHOD OF MAKING
- Large area group III nitride crystals and substrates, methods of making, and methods of use
- Group III nitride substrate, method of making, and method of use
- Reusable nitride wafer, method of making, and use thereof
This application is a divisional of co-pending U.S. patent application Ser. No. 16/882,219, filed May 22, 2022, which claims the benefit to U.S. provisional application No. 62/975,078, filed Feb. 11, 2020. Each of the aforementioned related patent applications is herein incorporated by reference.
BACKGROUND FieldThis disclosure relates generally to techniques for processing materials for manufacture of gallium-containing nitride substrates and utilization of these substrates in optoelectronic and electronic devices. More specifically, embodiments of the disclosure include techniques for growing large area substrates using a combination of processing techniques.
Description of the Related ArtGallium nitride (GaN) based optoelectronic and electronic devices are of tremendous commercial importance. The quality and reliability of these devices, however, is compromised by high defect levels, particularly threading dislocations, grain boundaries, and strain in semiconductor layers of the devices. Threading dislocations can arise from lattice mismatch of GaN based semiconductor layers to a non-GaN substrate such as sapphire or silicon carbide. Grain boundaries can arise from the coalescence fronts of epitaxially-overgrown layers. Additional defects can arise from thermal expansion mismatch, impurities, and tilt boundaries, depending on the details of the growth of the layers.
The presence of defects has deleterious effects on epitaxially-grown layers. Such effects include compromising electronic device performance. To overcome these defects, techniques have been proposed that require complex, tedious fabrication processes to reduce the concentration and/or impact of the defects. While a substantial number of conventional growth methods for gallium nitride crystals have been proposed, limitations still exist. That is, conventional methods still merit improvement to be cost effective and efficient.
Progress has been made in the growth of large-area gallium nitride crystals with considerably lower defect levels than heteroepitaxial GaN layers. However, most techniques for growth of large-area GaN substrates involve GaN deposition on a non-GaN substrate, such as sapphire or GaAs. This approach generally gives rise to threading dislocations at average concentrations of 105-107 cm−2 over the surface of thick boules, as well as significant bow, stress, and strain. Reduced concentrations of threading dislocations are desirable for a number of applications. Bow, stress, and strain can cause low yields when slicing the boules into wafers, make the wafers susceptible to cracking during down-stream processing, and may also negatively impact device reliability and lifetime. Another consequence of the bow, stress, and strain is that, during growth in m-plane and semipolar directions, even by near-equilibrium techniques such as ammonothermal growth, significant concentrations of stacking faults may be generated. In addition, the quality of c-plane growth may be unsatisfactory, due to formation of cracks, multiple crystallographic domains, and the like. Capability to manufacture substrates larger than 2 inches is currently very limited, as is capability to produce large-area GaN substrates with a nonpolar or semipolar crystallographic orientation. Most large area substrates are manufactured by vapor-phase methods, such as hydride vapor phase epitaxy (HVPE), which are relatively expensive. A less-expensive method is desired, while also achieving large area and low threading dislocation densities as quickly as possible.
Ammonothermal crystal growth has a number of advantages over HVPE as a means for manufacturing GaN boules. However, the performance of ammonothermal GaN crystal growth processing may be significantly dependent on the size and quality of seed crystals. Seed crystals fabricated by HVPE may suffer from many of the limitations described above, and large area ammonothermally-grown crystals are not widely available.
Lateral epitaxial overgrowth (LEO) is a method that has been widely applied to improvement in the crystallographic quality of films grown by vapor-phase methods. Several authors have disclosed methods for performing lateral growth from sidewalls of thin-film GaN layers on non-GaN substrates. However, to the best of our knowledge, analogous methods have not yet been disclosed for bulk growth of GaN, including ammonothermal growth, and the methods that have been disclosed for sidewall LEO of thin-film GaN are impractical for bulk GaN.
Due to at least the issues described above, there is a need for substrates that have a lower defect density and are formed by techniques that improve the crystal growth process. Also, from the above, it is seen that techniques for improving crystal growth are highly desirable.
SUMMARYEmbodiments of the present disclosure include a free-standing group III metal nitride crystal. The free-standing crystal comprises a wurtzite crystal structure, a first surface having a maximum dimension greater than 40 millimeters in a first direction, an average concentration of stacking faults below 103 cm−1; an average concentration of threading dislocations between 101 cm−2 and 106 cm−2, wherein the average concentration of threading dislocations on the first surface varies periodically by at least a factor of two in the first direction, the period of the variation in the first direction being between 5 micrometers and 20 millimeters, and a miscut angle that varies by 0.1 degree or less in the central 80% of the first surface of the crystal along the first direction and by 0.1 degree or less in the central 80% of the first surface of the crystal along a second direction orthogonal to the first direction. The first surface comprises a plurality of first regions, each of the plurality of first regions having a locally-approximately-linear array of threading dislocations with a concentration between 5 cm−1 and 105 cm−1, the first surface further comprises a plurality of second regions, each of the plurality of second regions being disposed between an adjacent pair of the plurality of first regions and having a concentration of threading dislocations below 105 cm−2 and a concentration of stacking faults below 103 cm−1, and the first surface further comprises a plurality of third regions, each of the plurality of third regions being disposed within one of the plurality of second regions or between an adjacent pair of second and having a minimum dimension between 10 micrometers and 500 micrometers and threading dislocations with a concentration between 103 cm−2 and 108 cm−2.
So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.
DETAILED DESCRIPTIONAccording to the present disclosure, techniques related to techniques for processing materials for manufacture of group-III metal nitride and gallium based substrates are provided. More specifically, embodiments of the disclosure include techniques for growing large area substrates using a combination of processing techniques. Merely by way of example, the disclosure can be applied to growing crystals of GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photo electrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, and others.
Threading dislocations in GaN are known to act as strong non-radiative recombination centers which can severely limit the efficiency of GaN-based LEDs and laser diodes. Non-radiative recombination generates local heating which may lead to faster device degradation (Cao et al., Microelectronics Reliability, 2003, 43(12), 1987-1991). In high-power applications, GaN-based devices suffer from decreased efficiency with increasing current density, known as droop. There is evidence suggesting a correlation between dislocation density and the magnitude of droop in LEDs (Schubert et al., Applied Physics Letters, 2007, 91(23), 231114). For GaN-based laser diodes there is a well-documented negative correlation between dislocation density and mean time to failure (MTTF) (Tom iya et al., IEEE Journal of Selected Topics in Quantum Electronics, 2004, 10(6), 1277-1286), which appears to be due to impurity diffusion along the dislocations (Orita et al., IEEE International Reliability Physics Symposium Proceedings, 2009, 736-740). For electronic devices, dislocations have been shown to markedly increase the leakage current (Kaun et al., Applied Physics Express, 2011, 4(2), 024101) and reduce the device lifetime (Tapajna et al., Applied Physics Letters, 2011, 99(22), 223501-223503) in HEMT structures. One of the primary advantages of using bulk GaN as a substrate material for epitaxial thin film growth is a large reduction in the concentration of threading dislocations in the film. Therefore, the dislocation density in the bulk GaN substrate will have a significant impact on the device efficiency and the reliability.
As noted above, lateral epitaxial overgrowth (LEO) is a method that has been widely applied to improvement in the crystallographic quality of films grown by vapor-phase methods. For example, methods whereby GaN layers were nucleated on a sapphire substrate, a SiO2 mask with a periodic array of openings was deposited on the GaN layer, and then GaN was grown by metalorganic chemical vapor deposition (MOCVD) through the openings in the SiO2 mask layer, grew laterally over the mask, and coalesced. The dislocation density in the areas above the openings in the mask were very high, similar to the layer below the mask, but the dislocation density in the laterally-overgrown regions was orders of magnitude less. This method is attractive because it can be applied to large area substrates, significantly reducing their dislocation density. Similar methods, with variations, have been applied by a number of groups to vapor-phase growth of GaN layers. These methods are variously referred to as LEO, epitaxial lateral overgrowth (ELO or ELOG), selective area growth (SAG), and dislocation elimination by epitaxial growth with inverse pyramidal pits (DEEP), or the like. In the case of essentially all variations of this method, it is believed that a thin heteroepitaxial GaN layer is grown on a non-GaN substrate, a patterned mask is deposited on the GaN layer, and growth is re-initiated in a one-dimensional or two-dimensional array of openings in the mask. The period or pitch of the growth locations defined by the openings in the mask is typically between 2 and 100 micrometers, typically between about 5 and 20 micrometers. The individual GaN crystallites or regions grow and then coalesce. Epitaxial growth may then be continued on top of the coalesced GaN material to produce a thick film or “ingot.” A relatively thick GaN layer may be deposited on the coalesced GaN material by HVPE. The LEO process is capable of large reductions in the concentration of dislocations, particularly in the regions above the mask, typically to levels of about 105-107 cm−2. However, very often the laterally-grown wings of the formed LEO layer are crystallographically tilted from the underlying substrate (“wing tilt”), by as much as several degrees, which may be acceptable for a thin-film process but may not be acceptable for a bulk crystal growth process, as it may give rise to stresses and cracking as well as unacceptable variation in surface crystallographic orientation.
Several factors limit the capability of the LEO method, as conventionally applied, to reduce the average dislocation density below about 105 to 107 cm−2, or to reduce the miscut variation across a 50 or 100 mm wafer to below about 0.1 degree. First, the pitch of the pattern of openings formed in the mask layer tends to be modest, but larger pitches may be desirable for certain applications. Second, c-plane LEO growth is generally performed in the (0001), or Ga-face direction, which creates at least two limitations. One limitation is that M-direction growth rates tend to be lower than those of (0001)-direction growth rates and semipolar (10-11) facets often form, with the consequence that the overall crystal diameter decreases with increasing thickness and making coalescence of large-pitch patterns difficult. In addition, another limitation is that growth in the (0001) direction tends to exclude oxygen, in contrast to growth in other crystallographic directions. As a consequence, there may be a significant lattice mismatch between a (0001)-grown HVPE crystal used as a seed and the crystal grown upon it by another technique. In addition, if semipolar facets form during the LEO process there may be a significant variation in oxygen (or other dopant) level, giving rise to lateral variations in the lattice constant and stresses that can cause cracking in the LEO crystal itself or in a crystal grown on the latter, used as a seed.
Variations of the LEO method have been disclosed for other group III metal nitride growth techniques besides HVPE. In a first example, Jiang, et al. (U.S. No. 2014/0147650, now U.S. Pat. No. 9,589,792) disclosed a process for ammonothermal LEO growth of group-III metal nitrides, replacing the mask layer in typical vapor-phase LEO-type processes (SiO2 or SiNx) by a combination of an adhesion layer, a diffusion-barrier layer, and an inert layer. In a second example, Mori, et al. (U.S. No. 2014/0328742, now U.S. Pat. No. 9,834,859) disclosed a process for LEO growth of group-III metal nitrides in a sodium-gallium flux. However, in this method the coalescing crystallites typically have prominent semipolar facets, leading to significant lateral variation in the impurity content of coalesced crystals, and the thermal expansion mismatch between the coalesced nitride layer and a hetero-substrate, which includes a different material than the coalesced nitride layer, may cause uncontrolled cracking.
Several authors, for example, Linthicum et al. (Applied Physics Letters, 75, 196, (1999)), Chen et al. (Applied Physics Letters 75, 2062 (1999)), and Wang, et al. (U.S. Pat. No. 6,500,257) have noted that threading dislocations in growing GaN normally propagate predominantly in the growth direction and showed that the dislocation density can be reduced even more than in the conventional LEO method by growing from the sidewalls of trenches in thin, highly-defective c-plane GaN layers rather than vertically through windows in a patterned mask. These methods have been extended to nonpolar- and semipolar-oriented thin GaN films by other authors, for example, Chen et al. (Japanese Journal of Applied Physics 42, L818 (2003)) and Imer et al. (U.S. Pat. No. 7,361,576). However, to the best of the inventors' knowledge, sidewall LEO methods have not yet been extended to growth of bulk GaN, nor to the growth of N-sector GaN. In particular, we have found that different methods that those used in the thin film studies work best to form trenches several hundred microns deep with pitches on the millimeter scale and produce some unexpected benefits.
Substrate 101 may have a surface threading dislocation density less than about 107 cm−2, less than about 106 cm−2, less than about 105 cm−2, less than about 104 cm−2, less than about 103 cm−2, or less than about 102 cm−2. Substrate 101 may have a stacking-fault concentration below about 104 cm−1, below about 103 cm−1, below about 102 cm−1, below about 10 cm−1 or below about 1 cm−1. Substrate 101 may have a symmetric x-ray rocking curve full width at half maximum (FWHM) less than about 500 arcsec, less than about 300 arcsec, less than about 200 arcsec, less than about 100 arcsec, less than about 50 arcsec, less than about 35 arcsec, less than about 25 arcsec, or less than about 15 arcsec. Substrate 101 may have a crystallographic radius of curvature greater than 0.1 meter, greater than 1 meter, greater than 10 meters, greater than 100 meters, or greater than 1000 meters, in at least one, at least two, or in three independent or orthogonal directions.
Substrate 101 may comprise regions having a relatively high concentration of threading dislocations separated by regions having a relatively low concentration of threading dislocations. The concentration of threading dislocations in the relatively high concentration regions may be greater than about 105 cm−2, greater than about 106 cm−2, greater than about 107 cm−2, or greater than about 108 cm−2. The concentration of threading dislocations in the relatively low concentration regions may be less than about 106 cm−2, less than about 105 cm−2, or less than about 104 cm−2. Substrate 101 may comprise regions having a relatively high electrical conductivity separated by regions having a relatively low electrical conductivity. Substrate 101 may have a thickness between about 10 microns and about 100 millimeters, or between about 0.1 millimeter and about 10 millimeters. Substrate 101 may have a dimension, including a diameter, of at least about 5 millimeters, at least about 10 millimeters, at least about 25 millimeters, at least about 50 millimeters, at least about 75 millimeters, at least about 100 millimeters, at least about 150 millimeters, at least about 200 millimeters, at least about 300 millimeters, at least about 400 millimeters, or at least about 600 millimeters.
Large-area surface 102 (
Referring again to
Referring to
Other methods besides the lift-off procedure described above may be used to form the patterned mask layer 111, including shadow masking, positive resist reactive ion etching, wet chemical etching, ion milling, and nanoimprint lithography, plus variations of the negative resist lift-off procedure described above.
In certain embodiments, patterned mask layer(s) 111 are deposited on both the front and back surfaces of substrate 101.
In an alternative embodiment, as shown in
Trenches 115 are then formed in exposed regions 120 of the substrate 101 through the openings 112 (or “windows”) formed in patterned mask layer 111, as shown in
The surfaces and sidewalls of the nascent trenches 114 may contain damage left over from the laser ablation process. In certain embodiments, substrate 101, containing nascent trenches 114, is further processed by wet etching, dry etching, or photoelectrochemical etching in order to remove residual damage in nascent trenches 114 as shown in
The substrate 101 with the masked, patterned trenches 115 is then used as a substrate for bulk crystal growth, for example, comprising ammonothermal growth, HVPE growth, or flux growth. In the discussion below the grown GaN layer will be referred to as an ammonothermal layer, even though other bulk growth methods, such as HVPE or flux growth, may be used instead. In certain embodiments, comprising ammonothermal bulk growth, patterned substrate 101 may then be suspended on a seed rack and placed in a sealable container, such as a capsule, an autoclave, or a liner within an autoclave. In certain embodiments, one or more pairs of patterned substrates are suspended back to back, with the patterned large area surfaces facing outward. A group III metal source, such as polycrystalline group III metal nitride, at least one mineralizer composition, and ammonia (or other nitrogen containing solvent) are then added to the sealable container and the sealable container is sealed. The mineralizer composition may comprise an alkali metal such as Li, Na, K, Rb, or Cs, an alkaline earth metal, such as Mg, Ca, Sr, or Ba, or an alkali or alkaline earth hydride, amide, imide, amido-imide, nitride, or azide. The mineralizer may comprise an ammonium halide, such as NH4F, NH4Cl, NH4Br, or NH4l, a gallium halide, such as GaF3, GaCl3, GaBr3, Gal3, or any compound that may be formed by reaction of one or more of F, Cl, Br, I, HF, HCl, HBr, Hl, Ga, GaN, and NH3. The mineralizer may comprise other alkali, alkaline earth, or ammonium salts, other halides, urea, sulfur or a sulfide salt, or phosphorus or a phosphorus-containing salt. The sealable container (e.g., capsule) may then be placed in a high pressure apparatus, such as an internally heated high pressure apparatus or an autoclave, and the high pressure apparatus sealed. The sealable container, containing patterned substrate 101, is then heated to a temperature above about 400 degrees Celsius and pressurized above about 50 megapascal to perform ammonothermal crystal growth.
As a point of reference,
Ammonothermal group III metal nitride layer 213 may have a thickness between about 10 micrometers and about 100 millimeters, or between about 100 micrometers and about 20 millimeters.
In certain embodiments, ammonothermal group III metal nitride layer 213 is subjected to one or more processes, such as at least one of sawing, lapping, grinding, polishing, chemical-mechanical polishing, or etching.
In certain embodiments, the concentration of extended defects, such as threading dislocations and stacking faults, in the ammonothermal group III metal nitride layer 213 may be quantified by defect selective etching. Defect-selective etching may be performed, for example, using a solution comprising one or more of H3PO4, H3PO4 that has been conditioned by prolonged heat treatment to form polyphosphoric acid, and H2SO4, or a molten flux comprising one or more of NaOH and KOH. Defect-selective etching may be performed at a temperature between about 100 degrees Celsius and about 500 degrees Celsius for a time between about 5 minutes and about 5 hours, wherein the processing temperature and time are selected so as to cause formation of etch pits with diameters between about 1 micrometer and about 25 micrometers, then removing the ammonothermal group III metal nitride layer, crystal, or wafer from the etchant solution.
The concentration of threading dislocations in the surface of the window regions 215 may be less than that in the underlying substrate 101 by a factor between about 10 and about 104. The concentration of threading dislocations in the surface of the window regions 215 may be less than about 108 cm−2, less than about 107 cm−2, less than about 106 cm−2, less than about 105 cm−2, or less than about 104 cm−2. The concentration of threading dislocations in the surface of wing regions 217 may be lower, by about one to about three orders of magnitude, than the concentration of threading dislocations in the surface of the window regions 215, and may be below about 105 cm−2, below about 104 cm−2, below about 103 cm−2, below about 102 cm−2, or below about 10 cm−2. Some stacking faults, for example, at a concentration between about 1 cm−1 and about 104 cm−1, may be present at the surface of the window regions 215. The concentration of stacking faults in the surface of wing regions 217 may be lower, by about one to about three orders of magnitude, than the concentration of stacking faults in the surface of the window regions 215, and may be below about 102 cm−1, below about 10 cm−1, below about 1 cm−1, or below about 0.1 cm−1, or may be undetectable. Threading dislocations, for example, edge dislocations, may be present at coalescence fronts 219, for example, with a line density that is less than about 1×105 cm−1, less than about 3×104 cm−1, less than about 1×104 cm−1, less than about 3×103 cm−1, less than about 1×103 cm−1, less than about 3×102 cm−1, or less than 1×102 cm−1. The density of dislocations along the coalescence fronts may be greater than 5 cm−1, greater than 10 cm−1, greater than 20 cm−1, greater than 50 cm−1, greater than 100 cm−1, greater than 200 cm−1, or greater than 500 cm−1.
In certain embodiments, the process of masking and bulk group III nitride crystal growth is repeated one, two, three, or more times. In some embodiments, these operations are performed while the first bulk group III metal nitride layer remains coupled to substrate 101. In other embodiments, substrate 101 is removed prior to a subsequent masking and bulk crystal growth operation, for example, by sawing, lapping, grinding, and/or etching.
In certain embodiments, the edge of free-standing ammonothermal group III metal nitride boule 413 is ground to form a cylindrically-shaped ammonothermal group III metal nitride boule. In certain embodiments, one or more flats is ground into the side of free-standing ammonothermal group III metal nitride boule 413. In certain embodiments, free-standing ammonothermal group III metal nitride boule 413 is sliced into one or more free-standing ammonothermal group III metal nitride wafers 431, as shown in
More complex patterns are also possible and may be advantageous, for example, in being more resistant to cracking or cleaving. The pattern 502 may be elongated in one direction compared to another orthogonal direction, for example, due to the free-standing laterally-grown group III metal nitride boule 413 being sliced at an inclined angle relative to the large-area surface of a free-standing ammonothermal group III metal nitride boule 413. The pattern 502 of locally-approximately-linear arrays of threading dislocations may be characterized by a linear array of threading dislocations (
Referring again to
The arrays may be elongated in one direction compared to another orthogonal direction, for example, due to the boule being sliced at an inclined angle relative to the large-area surface of a free-standing ammonothermal group III metal nitride boule. The pattern of locally-approximately-linear arrays 419 of threading dislocations may be characterized by a pitch dimension L, or by pitch dimensions L1 and L2 in two orthogonal directions, between about 5 micrometers and about 20 millimeters or between about 200 micrometers and about 5 millimeters. In certain embodiments, the pattern of locally-approximately-linear arrays 419 of threading dislocations is approximately aligned with the underlying crystal structure of the group III metal nitride, for example, with the locally-approximately-linear arrays lying within about 5 degrees, within about 2 degrees, or within about 1 degree of one or more of <10-10>, <11-20>, or [0 0 0±1] or their projections in the plane of the surface of the free-standing ammonothermal group III nitride boule or wafer. The linear concentration of threading dislocations in the pattern may be less than about 1×105 cm−1, less than about 3×104 cm−1, less than about 1×104 cm−1, less than about 3×103 cm−1, less than about 1×103 cm−1, less than about 3×102 cm−1, or less than about 1×102 cm−1. The linear concentration of threading dislocations in the pattern may be greater than 5 cm−1, greater than 10 cm−1, greater than 20 cm−1, greater than 50 cm−1, greater than 100 cm−1, greater than 200 cm−1, or greater than 500 cm−1.
The concentration of threading dislocations in the wing regions 417 between the locally-approximately-linear arrays of threading dislocations may be below about 105 cm−2, below about 104 cm−2, below about 103 cm−2, below about 102 cm−1, or below about 10 cm−2. The concentration of threading dislocations in the surface of the window regions 415 may be less than about 108 cm−2, less than about 107 cm−2, less than about 106 cm−2, less than about 105 cm−2, or less than about 104 cm−2. The concentration of threading dislocations in the surface of the window regions may be higher than the concentration of threading dislocations in the surface of the wing regions by at least a factor of two, by at least a factor of three, by at least a factor of ten, by at least a factor of 30, or by at least a factor of 100. The concentration of threading dislocations in the surface of the window regions may be higher than concentration of threading dislocations in the surface of the wing regions by less than a factor of 104, by less than a factor of 3000, by less than a factor of 1000, by less than a factor of 300, by less than a factor of 100, or by less than a factor of 30. In some embodiments the boundary between the window regions 415 and the wing regions 417 may be decorated with dislocations, for example, with a line density between about 5 cm−1 and 105 cm−1. The concentration of threading dislocations, averaged over a large area surface of the free-standing ammonothermal group III nitride boule or wafer, may be below about 107 cm−2, below about 106 cm−2, below about 105 cm−2, below about 104 cm−2, below about 103 cm−2, or below about 102 cm−2. The concentration of stacking faults, averaged over a large area surface of the free-standing ammonothermal group III nitride boule or wafer, may be below about 103 cm−1, below about 102 cm−1, below about 10 cm−1, below about 1 cm−1, or below about 0.1 cm−1, or may be undetectable. In some embodiments, for example, after repeated re-growth on a seed crystal with a patterned array of dislocations and/or growth to a thickness greater than 2 millimeters, greater than 3 millimeters, greater than 5 millimeters, or greater than 10 millimeters, the positions of the threading dislocations may be displaced laterally to some extent with respect to the pattern on the seed crystal. In such a case the regions with a higher concentration of threading dislocations may be somewhat more diffuse than the relatively sharp lines illustrated schematically in
The free-standing ammonothermal group III metal nitride boule or wafer may have a large-area crystallographic orientation within 5 degrees, within 2 degrees, within 1 degree, within 0.5 degree, within 0.2 degree, within 0.1 degree, within 0.05 degree, within 0.02 degree, or within 0.01 degree of (0001)+c-plane, (000-1) —c-plane, {10-10} m-plane, {11-20} a-plane, {11-2±2}, {60-6±1}, {50-5±1}, {40-4±1}, {30-3±1}, {50-5±2}, {70-7±3}, {20-2±1}, {30-3±2}, {40-4±3}, {50-5±4}, {10-1±1}, {10-1±2}, {10-1±3}, {21-3±1}, or {30-3±4}. The free-standing ammonothermal group III metal nitride boule or wafer may have an (h k i I) semipolar large-area surface orientation, where i=−(h+k) and/and at least one of h and k are nonzero.
In certain embodiments, a large-area surface of a free-standing ammonothermal group III metal nitride crystal or wafer has a crystallographic orientation that is miscut from {10-10} m-plane by between about −60 degrees and about +60 degrees toward [0001]+c-direction and by up to about 10 degrees toward an orthogonal <1-210> a-direction. In certain embodiments, a large-area surface of the free-standing ammonothermal group III metal nitride crystal or wafer has a crystallographic orientation that is miscut from {10-10} m-plane by between about −30 degrees and about +30 degrees toward [0001]+c-direction and by up to about 5 degrees toward an orthogonal <1-210> a-direction. In certain embodiments, a large-area surface of the free-standing ammonothermal group III metal nitride crystal or wafer has a crystallographic orientation that is miscut from {10-10} m-plane by between about −5 degrees and about +5 degrees toward [0001]+c-direction and by up to about 1 degree toward an orthogonal <1-210> a-direction. The free-standing ammonothermal group III metal nitride crystal or wafer may have a stacking fault concentration below 102 cm−1, below 10 cm−1, or below 1 cm−1, and a very low dislocation density, below about 105 cm−2, below about 104 cm−2, below about 103 cm−2, below about 102 cm−2, or below about 10 cm−2 on one or both of the two large area surfaces.
The free-standing ammonothermal group III metal nitride boule or wafer may have a symmetric x-ray rocking curve full width at half maximum (FWHM) less than about 200 arcsec, less than about 100 arcsec, less than about 50 arcsec, less than about 35 arcsec, less than about 25 arcsec, or less than about 15 arcsec. The free-standing ammonothermal group III metal nitride boule or wafer may have a crystallographic radius of curvature greater than 0.1 meter, greater than 1 meter, greater than 10 meters, greater than 100 meters, or greater than 1000 meters, in at least one, at least two, or in three independent or orthogonal directions.
In certain embodiments, at least one surface of the free-standing ammonothermal group III metal nitride boule or wafer has atomic impurity concentrations of at least one of oxygen (O), and hydrogen (H) above about 1×1016 cm−3, above about 1×1017 cm−3, or above about 1×1018 cm−3. In certain embodiments, a ratio of the atomic impurity concentration of H to the atomic impurity concentration of 0 is between about 0.3 and about 1000, between about 0.4 and about 10, or between about 10 and about 100. In certain embodiments, at least one surface of the free-standing ammonothermal group III metal nitride boule or wafer has impurity concentrations of at least one of lithium (Li), sodium (Na), potassium (K), fluorine (F), chlorine (CI), bromine (Br), or iodine (I) above about 1×1015 cm−3, above about 1×1016 cm−3, or above about 1×1017 cm−3, above about 1×1018 cm−3. In certain embodiments, the top and bottom surfaces of the free-standing ammonothermal group III metal nitride boule or wafer may have impurity concentrations of O, H, carbon (C), Na, and K between about 1×1016 cm−3 and 1×1019 cm−3, between about 1×1016 cm−3 and 2×1019 cm−3, below 1×1017 cm−3, below 1×1016 cm−3, and below 1×1016 cm−3, respectively, as quantified by calibrated secondary ion mass spectrometry (SIMS). In another embodiment, the top and bottom surfaces of the free-standing ammonothermal group III metal nitride boule or wafer may have impurity concentrations of O, H, C, and at least one of Na and K between about 1×1016 cm−3 and 1×1019 cm−3, between about 1×1016 cm−3 and 2×1019 cm−3, below 1×1017 cm−3, and between about 3×1015 cm−3 and 1×1018 cm−3, respectively, as quantified by calibrated secondary ion mass spectrometry (SIMS). In still another embodiment, the top and bottom surfaces of the free-standing ammonothermal group III metal nitride boule or wafer may have impurity concentrations of O, H, C, and at least one of F and CI between about 1×1016 cm−3 and 1×1019 cm−3, between about 1×1016 cm−3 and 2×1019 cm−3, below 1×1017 cm−3, and between about 1×1015 cm−3 and 1×1019 cm−3, respectively, as quantified by calibrated secondary ion mass spectrometry (SIMS). In some embodiments, the top and bottom surfaces of the free-standing ammonothermal group III metal nitride boule or wafer may have impurity concentrations of H between about 5×1017 cm−3 and 1×1019 cm−3, as quantified by calibrated secondary ion mass spectrometry (SIMS). In certain embodiments, at least one surface of the free-standing ammonothermal group III metal nitride boule or wafer has an impurity concentration of copper (Cu), manganese (Mn), and iron (Fe) between about 1×1016 cm−3 and 1×1019 cm−3. In a specific embodiment, the free-standing ammonothermal group III metal nitride boule or wafer has an infrared absorption peak at about 3175 cm−1, with an absorbance per unit thickness of greater than about 0.01 cm−1.
The free-standing ammonothermal group III metal nitride crystal or wafer may be characterized by a wurtzite structure substantially free from any cubic entities or other crystal structures, the other structures being less than about 0.1% in volume in reference to the substantially wurtzite structure.
Surprisingly, given the lattice mismatch between HVPE GaN and ammonothermal GaN, results of use of the herein-disclosed techniques show that ammonothermal lateral epitaxial overgrowth is capable of producing thick, large-area GaN layers that are free of cracks. In certain embodiments, the free-standing ammonothermal group III metal nitride crystal or wafer has a diameter larger than about 25 millimeters, larger than about 50 millimeters, larger than about 75 millimeters, larger than about 100 millimeters, larger than about 150 millimeters, larger than about 200 millimeters, larger than about 300 millimeters, or larger than about 600 millimeters, and a thickness greater than about 0.1 millimeter, greater than about 0.2 millimeter, greater than about 0.3 millimeter, greater than about 0.5 millimeter, greater than about 1 millimeter, greater than about 2 millimeters, greater than about 3 millimeters, greater than about 5 millimeters, greater than about 10 millimeters, or greater than about 20 millimeters, and is substantially free of cracks. By contrast, we find that ammonothermal growth on large-area, un-patterned HVPE GaN seed crystals leads to cracking if the layers are thicker than a few hundred microns, even if a patterning process had been used to form the HVPE GaN seed crystal.
A free-standing ammonothermal group III metal nitride wafer may be characterized by a total thickness variation (TTV) of less than about 25 micrometers, less than about 10 micrometers, less than about 5 micrometers, less than about 2 micrometers, or less than about 1 micrometer, and by a macroscopic bow that is less than about 200 micrometers, less than about 100 micrometers, less than about 50 micrometers, less than about 25 micrometers, or less than about 10 micrometers. A large-area surface of the free-standing ammonothermal group III metal nitride wafer may have a concentration of macro defects, with a diameter or characteristic dimension greater than about 100 micrometers, of less than about 2 cm−2, less than about 1 cm−2, less than about 0.5 cm−2, less than about 0.25 cm−2, or less than about 0.1 cm−2. The variation in miscut angle across a large-area surface of the free-standing ammonothermal group III metal nitride crystal or wafer may be less than about 5 degrees, less than about 2 degrees, less than about 1 degree, less than about 0.5 degree, less than about 0.2 degree, less than about 0.1 degree, less than about 0.05 degree, or less than about 0.025 degree in each of two orthogonal crystallographic directions. The root-mean-square surface roughness of a large-area surface of the free-standing ammonothermal group III metal nitride wafer, as measured over an area of at least 10 □m×10 □m, may be less than about 0.5 nanometer, less than about 0.2 nanometer, less than about 0.15 nanometer, less than about 0.1 nanometer, or less than about 0.10 nanometer. The free-standing ammonothermal group III metal nitride wafer may be characterized by n-type electrical conductivity, with a carrier concentration between about 1×1017 cm−3 and about 3×1019 cm−3 and a carrier mobility greater than about 100 cm2/V-s. In alternative embodiments, the free-standing ammonothermal group III metal nitride wafer is characterized by p-type electrical conductivity, with a carrier concentration between about 1×1015 cm−3 and about 1×1019 cm−3. In still other embodiments, the free-standing ammonothermal group III metal nitride wafer is characterized by semi-insulating electrical behavior, with a room-temperature resistivity greater than about 107 ohm-centimeter, greater than about 108 ohm-centimeter, greater than about 109 ohm-centimeter, greater than about 1010 ohm-centimeter, or greater than about 1011 ohm-centimeter. In certain embodiments, the free-standing ammonothermal group III metal nitride wafer is highly transparent, with an optical absorption coefficient at a wavelength of 400 nanometers that is less than about 10 cm−1, less than about 5 cm−1, less than about 2 cm−1, less than about 1 cm−1, less than about 0.5 cm−1, less than about 0.2 cm−1, or less than about 0.1 cm−1.
In some embodiments, the free-standing ammonothermal group III metal nitride crystal or wafer is used as a seed crystal for further bulk growth. In one specific embodiment, the further bulk growth comprises ammonothermal bulk crystal growth. In another specific embodiment, the further bulk growth comprises high temperature solution crystal growth, also known as flux crystal growth. In yet another specific embodiment, the further bulk growth comprises HVPE. The further-grown crystal may be sliced, lapped, polished, etched, and/or chemically-mechanically polished into wafers by methods that are known in the art. The surface of the wafers may be characterized by a root-mean-square surface roughness measured over a 10-micrometer by 10-micrometer area that is less than about 1 nanometer or less than about 0.2 nanometers.
A wafer may be incorporated into a semiconductor structure. The semiconductor structure may comprise at least one AlxInyGa(1-x-y)N epitaxial layer, where 0≤x, y, x+y≤1. The epitaxial layer may be deposited on the wafer, for example, by metalorganic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE), according to methods that are known in the art. At least a portion of the semiconductor structure may form a portion of a gallium-nitride-based electronic device or optoelectronic device, such as a light emitting diode, a laser diode, a photodetector, an avalanche photodiode, a photovoltaic, a solar cell, a cell for photoelectrochemical splitting of water, a transistor, a rectifier, and a thyristor; one of a transistor, a rectifier, a Schottky rectifier, a thyristor, a p-i-n diode, a metal-semiconductor-metal diode, high-electron mobility transistor, a metal semiconductor field effect transistor, a metal oxide field effect transistor, a power metal oxide semiconductor field effect transistor, a power metal insulator semiconductor field effect transistor, a bipolar junction transistor, a metal insulator field effect transistor, a heterojunction bipolar transistor, a power insulated gate bipolar transistor, a power vertical junction field effect transistor, a cascode switch, an inner sub-band emitter, a quantum well infrared photodetector, a quantum dot infrared photodetector, and combinations thereof. The gallium-nitride-based electronic device or optoelectronic device may be incorporated into a lamp or a fixture, such as a luminaire. The gallium-nitride-based electronic device or optoelectronic device, after singulation, may have lateral dimensions of at least 0.1 millimeter by 0.1 millimeter. The gallium-nitride-based electronic or optoelectronic device may have a maximum dimension of at least 8 millimeters and, for example, may comprise a laser diode. The gallium-nitride-based electronic or optoelectronic device may be entirely free of dislocations throughout its volume. For example, at a dislocation density of 104 cm−2, a substantial fraction of 0.1×0.1 mm2 devices could be expected to be free of dislocations. At a dislocation density of 102 cm−2, a substantial fraction of 1×1 mm2 devices could be expected to be free of dislocations. The gallium-nitride-based electronic or optoelectronic device may be entirely free of stacking faults throughout its volume. For example, at a stacking fault density of 1 cm−1, a substantial fraction of 10×1 mm2 stripe-shaped devices, such as laser diodes with nonpolar or semipolar large area surfaces and c-plane facets, could be expected to be free of stacking faults.
In a specific embodiment, the method also deposits an n-type contact 639, and a p-type contact 637 as shown in
Referring now to
Referring now to
Individual die, for example, light emitting diodes or laser diodes, may be formed by sawing, cleaving, slicing, singulating, or the like, between adjacent sets of electrical contacts. Referring again to
The methods described herein provide means for fabricating large-area group III metal nitride substrates, albeit having some potentially defective regions. The methods described herein provide means for fabricating high-performance light emitting diodes and/or laser diodes that avoid potential issues associated with defective regions in the large-area group III metal nitride substrates.
The above sequence of steps provides a method according to an embodiment of the present disclosure. In a specific embodiment, the present disclosure provides a method and resulting crystalline material provided by a high pressure apparatus having structured support members. Other alternatives can also be provided where steps are added, one or more steps are removed, or one or more steps are provided in a different sequence without departing from the scope of the claims herein.
EXAMPLESEmbodiments provided by the present disclosure are further illustrated by reference to the following examples. It will be apparent to those skilled in the art that many modifications, both to materials, and methods, may be practiced without departing from the scope of the disclosure.
Example 1A c-plane oriented bulk GaN crystal grown by HVPE, approximately 0.3 millimeters thick, was provided for use as a substrate 101 for patterning and ammonothermal crystal growth. A 100-nanometer-thick layer of TiW was sputter-deposited as an adhesion layer on the (000-1)N-face of the substrate, followed by a 780-nanometer-thick inert layer comprising Au. A 6-micrometer-thick Au layer was then electroplated on the sputtered layer, increasing the thickness of the inert layer (e.g., blanket mask 116). Using AZ-4300 as a photoresist (e.g., photoresist layer 103), a pattern comprising linear arrays of 3-micrometer-wide by 1-centimeter-long slits (e.g., openings 112), with a pitch diameter of 1200 micrometers was defined. A wet-etch process was performed, using a commercial TFA gold etching solution at room temperature, as shown schematically in
A patterned, trenched c-plane-oriented bulk GaN substrate 101 was prepared by a similar procedure as that described in Example 1. The patterned substrate was placed in a silver capsule along with a 15%-open-area baffle, polycrystalline GaN nutrient, NH4F mineralizer, and ammonia, and the capsule was sealed. The ratios of GaN nutrient and NH4F mineralizer to ammonia were approximately 1.69 and 0.099 respectively, by weight. The capsule was placed in an internally-heated high pressure apparatus and heated to temperatures of approximately 666 degrees Celsius for the upper, nutrient zone and approximately 681 degrees Celsius for the lower, crystal growth zone, maintained at these temperatures for approximately 215 hours, and then cooled and removed. Ammonothermal GaN filled in most of the volume in the trenches, grew through the linear openings in the patterned mask on the HVPE GaN substrate, grew laterally, and coalesced fully, forming an ammonothermal GaN layer approximately 1200 micrometers thick with a smooth top surface. Two parallel cuts were made in the ammonothermal GaN layer, perpendicular to both the surface and the patterns, resulting in a bar-shaped test specimen with m-plane surfaces. One m-plane surface of the test specimen was polished and examined by optical microscopy, as shown in
A patterned, trenched c-plane-oriented bulk GaN substrate was prepared by a similar procedure as that described in Examples 1 and 2, and the final group III metal nitride layer 213 is shown in
A patterned, trenched c-plane-oriented bulk GaN substrate was prepared by a similar procedure as that described in Examples 1 and 2 but with a pitch of 800 micrometers. The patterned, trenched substrate was placed in a silver capsule along with a 15%-open-area baffle, polycrystalline GaN nutrient, NH4F mineralizer, and ammonia, and the capsule was sealed. The ratios of GaN nutrient and NH4F mineralizer to ammonia were approximately 1.71 and 0.099 respectively, by weight. The capsule was placed in an internally-heated high pressure apparatus and heated to temperatures of approximately 668 degrees Celsius for the upper, nutrient zone and approximately 678 degrees Celsius for the lower, crystal growth zone, maintained at these temperatures for approximately 485 hours, and then cooled and removed. Ammonothermal GaN filled in most of the volume in the trenches of the trenched substrate, grew through the linear openings in the patterned mask on the HVPE GaN substrate, grew laterally, and coalesced fully, forming an ammonothermal GaN layer approximately 980 micrometers thick with a smooth top surface. The HVPE GaN substrate was removed by grinding, and the resulting free-standing ammonothermal GaN substrate was polished and chemical-mechanical polished. The free-standing ammonothermal GaN substrate was then characterized by x-ray diffraction, using a PANalytical X′Pert PRO diffractometer using an electron energy of 45 kV with a 40 mA line focus, a 0.0002 degree step, a 1 sec dwell time, an Ge(220) mirror, a slit height of 1.0 mm and a slit width of 1.0 mm, at nine different locations across the substrate. The results of an analysis of the formed GaN substrate are summarized in
A c-plane oriented bulk GaN crystal grown by HVPE, approximately 0.3 millimeters thick, was provided for use as a substrate for patterning and ammonothermal crystal growth. A 100-nanometer-thick layer of TiW was sputter-deposited as an adhesion layer on the (000-1)N-face of the substrate, followed by a 780-nanometer-thick inert layer comprising Au. A 6-micrometer-thick Au layer was then electroplated on the sputtered layer, increasing the thickness of the inert layer. A pattern was formed on the N-face of the substrate using a frequency-doubled YAG laser with nano-second pulses. The pattern comprised domains of m-trenches, with linear openings oriented approximately 50-60 micrometers wide and parallel to <10-10>, with a pitch of 1200 micrometers. The patterned substrate was then placed in a stirred beaker with concentrated H3PO4. The beaker was heated to approximately 280 degrees Celsius over approximately 30 minutes, held at this temperature for approximately 60 minutes, and cooled. A cross section of a trench formed by this procedure, having a depth of approximately 200 micrometers and a width at the top of approximately 80 micrometers, is shown in
A patterned, trenched c-plane-oriented bulk GaN substrate was prepared by a similar procedure as that described in Example 5, except that a higher power was used for the laser so that slots were formed that fully penetrated the substrate. After etching with concentrated H3PO4 at approximately 280 degrees Celsius for approximately 30 minutes, the width of the slots was approximately 115 micrometers. The patterned substrates were placed in a silver capsule along with a 15%-open-area baffle, polycrystalline GaN nutrient, NH4F mineralizer, and ammonia, and the capsule was sealed. The ratios of GaN nutrient and NH4F mineralizer to ammonia were approximately 1.74 and 0.099 respectively, by weight. The capsule was placed in an internally-heated high pressure apparatus and heated to temperatures of approximately 667 degrees Celsius for the upper, nutrient zone and approximately 681 degrees Celsius for the lower, crystal growth zone, maintained at these temperatures for approximately 500 hours, and then cooled and removed. Ammonothermal GaN filled in most of the volume in the trenches of the trenched substrate, grew through the linear openings in the patterned mask on the HVPE GaN substrate, grew laterally, and coalesced fully, forming an ammonothermal GaN layer approximately 2010 micrometers thick with a smooth top surface. The surface of the ammonothermal GaN layer was lightly etched and was examined by optical microscopy. An optical micrograph of the layer is shown in
While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Claims
1. A method for forming a group III metal containing nitride free-standing crystal, comprising:
- depositing a patterned mask layer on a first surface of a substrate, wherein the substrate is selected from one of single-crystalline group-III metal nitride, gallium-containing nitride, and gallium nitride, and has a concentration of threading dislocations less than 108 cm−2 and a concentration of stacking faults less than 104 cm−1, and the patterned mask layer comprises an array of openings that have a pitch in a first direction between 5 micrometers and 20 millimeters;
- removing portions of the substrate exposed within the array of openings to form trenches in the substrate, the trenches having a depth below the first surface of greater than 50 micrometers; and
- performing a bulk crystal growth process using the substrate as a seed crystal.
2. The method of claim 1, wherein the trenches have a width between 10 micrometers and 200 micrometers and a length between 100 micrometers and 50 millimeters.
3. The method of claim 1, wherein the patterned mask layer comprises an inert layer overlying an adhesion layer, wherein the adhesion layer comprises one or more of Ti, TiN, TiNy, TiSi2, Ta, TaNy, Al, Ge, AlxGey, Cu, Si, Cr, V, Ni, W, TiWx, TiWxNy and has a thickness between 1 nanometer and 1 micrometer and the inert layer comprises one or more of Au, Ag, Pt, Pd, Rh, Ru, Ir, Ni, Cr, V, Ti, or Ta and has a thickness between 10 nanometers and 100 micrometers.
4. The method of claim 3, wherein the pattered mask layer further comprises a diffusion barrier layer between the adhesion layer and the inert layer, the diffusion barrier layer comprising one or more of TiN, TiNy, TiSi2, W, TiWx, TiNy, WNy, TaNy, TiWxNy, TiWxSizNy, TiC, TiCN, Pd, Rh, or Cr, and having a thickness between 1 nanometer and 10 micrometers.
5. The method of claim 1, wherein the bulk crystal growth process comprises:
- placing the substrate in a sealable container along with a group III metal source, a mineralizer and ammonia;
- heating the sealable container to a temperature above 400 degrees Celsius, thereby causing an ammonothermal group III metal nitride material to grow within the plurality of trenches, within the plurality of openings, outward through the plurality of openings, and subsequently laterally over the patterned mask; and
- pressurizing it to a pressure above 50 megapascal for a duration of at least 100 hours.
6. The method of claim 1, wherein the bulk crystal growth process causes a group III metal nitride material to grow laterally over the patterned mask layer and coalesce to form one or more coalescence fronts, wherein the one or more coalescence fronts comprise a pattern of locally-approximately-linear arrays of threading dislocations that have a concentration between 5 cm−1 and 105 cm−1.
7. The method of claim 1, wherein the openings in the patterned mask layer are formed by a lithography process and the trenches underlying the openings are formed by a wet etching process.
8. The method of claim 1, wherein the openings in the patterned mask layer and the trenches are formed by a laser process.
9. The method of claim 8, wherein the laser process comprises scanning over individual locations repetitively to form openings in the mask layer and trenches having predetermined dimensions.
10. The method of claim 8, wherein the laser process comprises scanning over an entire pattern on the first surface repetitively to form openings in the mask layer and trenches having predetermined dimensions.
11. The method of claim 8, wherein residual damage in the trenches is removed by a wet etching or by a photoelectrochemical etching process.
12. The method of claim 1, wherein the substrate comprises single-crystal gallium nitride, the first surface has a crystallographic orientation within about 5 degrees of (000-1) -c-plane, and the trenches are formed by an etching process comprising H3PO4.
13. The method of claim 1, wherein the openings of the patterned mask layer has a shape selected from round, square, rectangular, triangular, and hexagonal, and has a size between 1 micrometer and 5 millimeters.
14. The method of claim 1, wherein the openings of the patterned mask layer comprise slits, the slits having a width between 1 micrometer and 5 millimeters and a length between 5 micrometers and 20 millimeters.
15. The method of claim 14, wherein adjacent rows of slits are displaced along a direction of the slits with respect to one another.
16. The method of claim 14, wherein the arrangement of the slits reflect a hexagonal symmetry of the substrate.
17. The method of claim 1, wherein the trenches penetrate the entire thickness of the substrate.
18. The method of claim 6, further comprising forming a free-standing group III metal nitride boule from the laterally-grown group III metal nitride material.
19. The method of claim 18, further comprising preparing at least one free-standing group III metal nitride substrate from the free-standing group III metal nitride boule.
20. The method of claim 1, wherein the bulk crystal growth is initiated from sidewalls of the trenches.
Type: Application
Filed: Jun 8, 2023
Publication Date: Oct 5, 2023
Applicant: SLT Technologies, Inc. (Los Angeles, CA)
Inventors: Wenkan JIANG (Walnut, CA), Mark P. D'EVELYN (Vancouver, WA), Derrick S. KAMBER (Camas, WA), Dirk EHRENTRAUT (Camas, WA), Jonathan D. COOK (Santa Barbara, CA), James WENGER (Vancouver, WA)
Application Number: 18/331,719
