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Patent US8123859 - Method and apparatus for producing large, single-crystals of aluminum nitride - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA method and apparatus for producing bulk single crystals of AlN having low dislocation densities of about 10,000 cm−2 or less includes a crystal growth enclosure with Al and N2 source material therein, capable of forming bulk crystals. The apparatus maintains the N2 partial pressure at greater than...http://www.google.com/patents/US8123859?utm_source=gb-gplus-sharePatent US8123859 - Method and apparatus for producing large, single-crystals of aluminum nitrideAdvanced Patent SearchPublication numberUS8123859 B2Publication typeGrantApplication numberUS 12/841,350Publication dateFeb 28, 2012Filing dateJul 22, 2010Priority dateDec 24, 2001Also published asUS7776153, US20060005763, US20080006200, US20110011332Publication number12841350, 841350, US 8123859 B2, US 8123859B2, US-B2-8123859, US8123859 B2, US8123859B2InventorsLeo J. Schowalter, Glen A. Slack, J. Carlos RojoOriginal AssigneeCrystal Is, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (100), Non-Patent Citations (169), Referenced by (3), Classifications (19), Legal Events (1) External Links: USPTO, USPTO Assignment, EspacenetMethod and apparatus for producing large, single-crystals of aluminum nitride
US 8123859 B2Abstract
A method and apparatus for producing bulk single crystals of AlN having low dislocation densities of about 10,000 cm−2 or less includes a crystal growth enclosure with Al and N2 source material therein, capable of forming bulk crystals. The apparatus maintains the N2 partial pressure at greater than stoichiometric pressure relative to the Al within the crystal growth enclosure, while maintaining the total vapor pressure in the crystal growth enclosure at super-atmospheric pressure. At least one nucleation site is provided in the crystal growth enclosure, and provision is made for cooling the nucleation site relative to other locations in the crystal growth enclosure. The Al and N2 vapor is then deposited to grow single crystalline low dislocation density AlN at the nucleation site. High efficiency ultraviolet light emitting diodes and ultraviolet laser diodes are fabricated on low defect density AlN substrates, which are cut from the low dislocation density AlN crystals. Bulk crystals of ZnO may also be produced using the method.
1. A method of producing single-crystal AlN in a predominantly N2-filled environment, the method comprising:
providing Al and N2 vapor in a crystal growth enclosure, the crystal growth enclosure comprising a selective barrier configured to substantially prevent passage of Al vapor and permit passage of N2 vapor therethrough;
maintaining in the crystal growth enclosure, (i) an N2 partial pressure greater than stoichiometric pressure relative to the Al, and (ii) a total vapor pressure at super-atmospheric pressure;
providing at least one nucleation site in the crystal growth enclosure;
cooling the nucleation site relative to other locations in the crystal growth enclosure;
generating a flow of N2 vapor past the nucleation site; and
depositing the Al and N2 vapor under conditions capable of growing single crystalline AlN originating at the nucleation site.
2. The method of claim 1, wherein cooling the nucleation site comprises disposing the crystal growth enclosure within a temperature gradient.
3. The method of claim 2, further comprising moving the crystal growth enclosure through the temperature gradient while depositing the Al and N2 vapor.
4. The method of claim 3, wherein the crystal growth enclosure is moved through the temperature gradient at a rate of approximately 0.5 mm/h to approximately 3 mm/h.
5. The method of claim 1, wherein substantially no Al vapor diffuses from the crystal growth enclosure while depositing the Al and N2 vapor.
6. The method of claim 1, wherein the nucleation site is seeded.
7. The method of claim 1, wherein providing the Al and N2 vapor comprises subliming solid source material.
8. The method of claim 7, wherein the solid source material comprises polycrystalline AlN.
9. The method of claim 1, wherein providing the Al and N2 vapor comprises vaporizing Al and N2 from solid AlN.
10. The method of claim 1, wherein providing the Al and N2 vapor comprises injecting a source gas.
11. The method of claim 1, wherein the selective barrier comprises a bore extending through a wall of the crystal growth enclosure.
12. The method of claim 1, wherein the selective barrier comprises a seal disposed in the crystal growth enclosure, the seal configured to substantially prevent passage of vapor therethrough when vapor pressures interior and exterior to the crystal growth enclosure are at mechanical equilibrium, and to permit passage of vapor therethrough when the vapor pressures are not at mechanical equilibrium.
13. The method of claim 1, wherein maintaining the N2 partial pressure greater than stoichiometric pressure relative to the Al comprises maintaining the N2 partial pressure at greater than two times the stoichiometric pressure relative to the Al.
14. The method of claim 1, further comprising providing hydrogen in the crystal growth enclosure, the hydrogen constituting up to approximately 5% of a vapor mixture comprising the hydrogen and the Al and N2 vapor.
15. The method of claim 1, wherein a diffusion length of Al transport is less than 2 cm.
16. The method of claim 2, wherein the temperature gradient is formed at least in part by the arrangement of metallic baffles outside of the crystal growth enclosure.
17. The method of claim 16, wherein (i) a first set of metallic baffles is disposed at a proximal end of the crystal growth enclosure, (ii) a second set of metallic baffles is disposed at a distal end of the crystal growth enclosure, and (iii) a number of metallic baffles in the first set is different from a number of metallic baffles in the second set.
18. The method of claim 16, wherein at least one of the metallic baffles defines a hole therethrough.
19. The method of claim 2, wherein the thermal gradient is greater than 100�/cm.
20. The method of claim 2, wherein the thermal gradient is an axial thermal gradient.
21. The method of claim 1, wherein depositing the Al and N2 vapor forms a bulk AlN single crystal, and further comprising slicing or cutting the bulk AlN single crystal to form a single-crystal AlN substrate.
22. The method of claim 21, further comprising removing surface damage from a surface of the single-crystal AlN substrate.
23. The method of claim 21, further comprising depositing an epitaxial layer on a surface of the single-crystal AlN substrate.
24. The method of claim 23, wherein the epitaxial layer comprises AlxGayIn1-x-yN, wherein 0≦x≦1 and 0≦y≦1-x. Description
This application is a continuation of U.S. patent application Ser. No. 11/265,909, filed Nov. 3, 2005, which is a continuation of U.S. patent application Ser. No. 10/910,162, filed Aug. 3, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/324,998, filed Dec. 20, 2002, issued as U.S. Pat. No. 6,770,135, which claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/344,672, filed Dec. 24, 2001. The entire disclosures of each of these applications are incorporated herein by reference.
Several types of materials are routinely used to form semiconductor substrates. Sapphire is popular, because relatively high-quality, inexpensive sapphire substrates are commercially available. However, sapphire is far from being an ideal substrate for GaN epitaxy. Its lattice mismatch to GaN is large (about 16%), it has little distinction between the + and − [0001] directions which can give rise to +/−c-axis domains in epitaxial films of GaN, and its differential thermal expansion can lead to cracking during the cooling process after the device fabrication process. In spite of those problems, recently, Nichia Ltd (Japan) has announced the production of the first violet laser with commercial possibilities (more than 10,000 hours of operating life) using sapphire substrates. Currently, LDs (Laser Diodes) are selling for around $2,000 apiece. Using sapphire substrates leads to a costly fabrication process since it requires growing buffer layers and using Lateral Epitaxial Overgrowth techniques (LEO). Even though this announcement is very promising, Nichia's lasers still have problems. Some sources claim that heat builds up in these lasers as they shine. Sapphire, with a very low thermal conductivity, traps that heat, a fault that may trigger burnout down the road. To build an even more durable blue laser, Nichia and others are investigating other alternatives such as free-standing substrates. In this technique, the substrate is removed after a thick GaN layer is grown atop the sapphire. This method leaves the GaN as the base for building the laser. This base should be better at dissipating heat, in addition to matching the alloy layers above. However, this alternative may increase fabrication cost.
It may therefore be desirable to provide alternative substrates such as AlN, for fabricating nitride-based (e.g., GaN) commercial devices. A sublimation-recondensation technique was developed for AlN crystal growth by Slack and McNelly (G. A. Slack and T. McNelly, J. Cryst. Growth 34, 263 (1976) and 42, 560 (1977), hereinafter the “Slack reference”). In this technique, polycrystalline source material is placed in the hot end of a crucible while the other end is kept cooler. The crystal nucleates in the tip and grows as the crucible is moved through the temperature gradient. This approach demonstrated relatively slow crystal growth of 0.3 mm/hr while the crystal growth Chamber was maintained at 1 atm (100 kPa) of N2. To make such substrates commercially feasible, it would be desirable to increase the growth rate. A number of researchers skilled in art have examined the possibility.
However, most artisans in this field have based their work on rate equations derived by Dryburgh (Estimation of maximum growth rate for aluminum nitride crystals by direct sublimation, J. Crystal Growth 125, 65 (1992)), which appear to overestimate the growth rate of AlN and, in particular, suggest that the maximal growth conditions are near stoichiometric vapor conditions, i.e., the Al and N2 partial pressures should be adjusted so that the Al partial pressure is twice that of the N2. This for example, is a significant teaching of U.S. Pat. Nos. 5,858,085; 5,972,109; 6,045,612; 6,048,813; 6,063,185; 6,086,672; and 6,296,956; all to Hunter. In addition, the art teaches that the N2 partial pressure should be maintained at less than atmospheric pressure.
Additional AlN work was completed by Segal et al. (A. S. Segal, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, A. D. Roenkov, M. G. Ramm, Yu. A. Vodakov, “On mechanisms of sublimation growth of AlN bulk crystals,” J. Crystal Growth 211, 68 (2000)). While this work was published subsequently to the conception of the present invention, it appears to be the first peer reviewed publication to suggest that Dryburgh's growth equations are incorrect. Segal et al., however, teach growth conditions and experiments that are open, which allows the Al vapor to escape. Disadvantageously, it would be difficult to grow large boules of AlN this way since: (i) control of growth would be difficult (since it would be non-uniform across the surface), (ii) a large amount of Al would be wasted, (iii) the excess Al in the rest of the furnace would create problems because of its high reactivity, and (iv) it would be difficult to maintain high differences in temperature (ΔT) between the source and growing crystal surface.
The present invention includes a method and apparatus for producing aluminum nitride (AlN) substrates that advantageously have a relatively small lattice mismatch (around 2.2%) with GaN, and have an almost identical thermal expansion from room temperature to 1000� C. Furthermore, AlN crystals grown by the present method have a low dislocation density of 10,000 cm−2 or less and may be grown to have a radial dimension exceeding 10 mm and a length exceeding 5 mm. These AlN crystals also advantageously have the same wurtzite (2H) crystal structure as GaN and nominally the same type of piezoelectric polarity. Also, the chemical compatibility with GaN is much better than that of the SiC. In addition, AlN substrates cut from the bulk crystals tend to be attractive for AlxGa1-xN devices requiring higher Al concentration (e.g., for high temperature, high power, radiation hardened, and ultra-violet (UV) wavelength applications).
An aspect of the present invention is the recognition that, contrary to the teachings of the prior art, pressures above atmospheric pressure may be utilized to advantageously produce single AlN crystals at relatively high growth rates and crystal quality. The invention includes controlling the temperature difference between an AlN source material and growing crystal surface, the distance between the source material and the growing crystal surface, and the ratio of N2 to Al partial vapor pressures. The invention includes increasing the nitrogen pressure beyond the stoichiometric pressure to force the crystal to grow at a relatively high rate due to the increased reaction rate at the interface between the growing crystal and the vapor. This increase in the growth rate has been shown to continue with increasing N2 partial pressure until diffusion of Al from the source to the growing crystal (i.e., the negative effects of requiring the Al species to diffuse through the N2 gas) becomes the rate-limiting step. The present invention also includes a technique for establishing suitable nitrogen pressures for crystal growth, as will be discussed in greater detail hereinbelow. Moreover, use of such higher-pressure nitrogen may have the added benefit that it reduces the partial pressure of aluminum inside the growth crucible. This may lead to longer crucible lifetimes and reduced wear on the rest of the furnace owing to a reduction in corrosion due to the Al vapor which may escape the crucible inadvertently.
The present invention also includes an apparatus (including a crystal growth enclosure or crucible) capable of providing a relatively sharp thermal profile, i.e., a relatively large thermal gradient along a relatively short axial distance, for sublimation and recondensation/nucleation. The crucible is configured to operate at an internal super-atmospheric pressure. During operation, the crystal nucleates at a nucleation site (e.g., at the tip of the crucible), and grows at a relatively high rate as the crucible moves relative to the temperature gradient. As used herein, the term ‘axial’ refers to a direction relative to the apparatus of the present invention, which is substantially parallel to push rod 17 as shown in FIG. 2. Moreover, the term ‘nucleation site’ refers to a location of either seeded or unseeded crystal growth.
Embodiments of the present invention have demonstrated that the problem with diffusion described above can be circumvented, at least in part, by providing a net flow of gas from the source (11, FIG. 2) towards the growing crystal (7, FIG. 2) greater than that caused simply by the evaporation and recondensation process. This may be obtained when a thin wall tungsten crucible (9, FIG. 2) is used, and it may also be possible to obtain this effect with other crucible materials that are pervious to nitrogen gas, or with other types of selective barriers such as openings 20, 21, described hereinbelow with respect to FIGS. 3-5. Nitrogen is able to diffuse through thin-walled tungsten (W) crucibles at fairly high rates at the crystal growth temperatures (˜2300� C.) used. The diffusion rate of Al through the W walls is much lower. Thus, under equilibrium conditions, the partial pressure of nitrogen inside the crucible is identical to that outside (e.g., in chamber 2, FIG. 2) the crucible, while the total pressure of gasses inside the crucible may be higher due to the partial pressure of the Al vapor.
However, once crystal growth is initiated and the AlN source is maintained at a higher temperature than the growing crystal, the nitrogen partial pressure at the cool end (e.g., location 19 of the growing crystal 7) of the crucible tends to become greater than at the hot end, while the opposite is true for the aluminum pressure. Meanwhile, the total gas pressure inside the crucible remains substantially uniform throughout to maintain mechanical equilibrium. (As used herein, the term ‘mechanical equilibrium’ refers to instances in which the total vapor pressure interior and exterior to the crystal growth chamber are substantially equal.) Thus, the nitrogen partial pressure at the cold end 19 of the crucible tends to exceed the nitrogen pressure outside the crucible (within chamber 2) while the opposite is true at the hot end. For this reason, nitrogen tends to diffuse out of the crucible at the cold end and diffuse into the crucible at the hot end resulting in a net flow of the gas mixture in the crucible from the AlN source toward the growing crystal.
As mentioned hereinabove, an aspect of the present invention was the understanding of the applicable surface kinetics. The rate at which Al and N incorporate into the growing AlN has generally been modeled in two approaches. In the first, it is assumed that the nitrogen molecules N2 have a relatively low condensation coefficient compared to the Al atoms due to a configurational bather. In the second approach, it is assumed that the N2 molecules physisorb onto the AlN surface. These N2 molecules are then assumed to be kinetically hindered in the production of N atoms that can then incorporate into the AlN crystal. We have modeled both approaches and both models lead to identical results.
ⅆ [ Al ] ⅆ t = β Al P Al - C Al [ Al ] - B ( [ Al ] - K s [ N ] ) . ( 1 ) Likewise, the rate of change of the surface concentration [N] of nitrogen atoms is
In these equations, the first term represents the addition of molecules from the vapor. It is assumed that all of the Al atoms stick but only a fraction γ of the N2 molecules condense on the surface. The term βi represents the modified Hertz-Knudsen factor which is proportional to the square root of mass of the ith species (where i represents either Al or N2) divided by the temperature. The condensation coefficient γ in this model is not subscripted since we assume that it only applies to the N2 molecules.
[Al]�[N]=K s, (3)
Significantly, the only parameters appearing in this cubic equation are the Langmuir evaporation rate EL and the stoichiometric nitrogen pressure Ps. It is rather remarkable that, in the limit of very large B, of the five free parameters that appear in Eqs. (1) and (2), only two parameters are needed in Eq. (4) to determine the net flux at any nitrogen and aluminum partial pressure. Both of these parameters have been determined experimentally although there is substantial uncertainty in the Langmuir evaporation rate as pointed out earlier.
The value of FAlN is linear in ΔT for ΔT≦5100� C. We find that the linear dependence of FAlN on PN2 is valid (within 1%) between 0 and 200 bars of nitrogen gas for the experimentally determined values of EL and Ps in the temperature range 2000� C.<TH<2500� C. We have determined
A o(ΔT=50� C.,T H=2300� C.)=0.156 mm−hr−1−bar−1, (7)
using the data of Bolgar et al. (A. S. Bolgar, S. P. Gordienko, B. A. Ryklis, and V. V. Fesenko, in “Khim. Fiz. Nitridov”, p. 151-6 (1968) [Chem. Abstr. 71, 34003j (1969)]). for the Langmuir evaporation rate (259 mm/hr) and the equilibrium, stoichiometric nitrogen pressure determined from the JANAF tables (M. W. Chase et al. “JANAF Thermochemical Tables”, Third Edition (1985), see 0.1. Phys. Chem. Ref. Data 14, Supplement No. 1 (1985)), Ps(2300� C.)=0.13 bar. This equilibrium pressure would lead to an effective growth rate of 18.9 m/hr if all the nitrogen and aluminum stuck to the surface without re-evaporation and PAl=2PN2=Ps, such as shown in FIG. 1. These calculations assume that the area of the evaporating surface and that of the growing crystal are equal and that diffusion effects may be neglected. This last assumption is rather important and, as FIG. 1 shows, ceases to be true at high enough N2 pressure.
The observed growth rate in the Slack reference for TH=2300� C. and TC˜2200� C. run in 0.95 bar of N2 plus 0.05 bar of H2 was 0.3 mm/hr. This should be compared with the theoretically determined growth rate of 0.32 mm/hr. This is a remarkable agreement given the uncertainties in the experimental data for the Langmuir evaporation rate and in the measured growth rate by Slack and McNelly. Note that there are no adjustable parameters in the way that this theory was developed, the theory only depends on the experimentally determined equilibrium pressures and the measured Langmuir evaporation rates. Note also that the experiment was conducted in a crucible where the growing crystal surface was smaller than the evaporating AlN source material. This tends to lead to an amplification of the observed growth rate. It should also be noted that these equations predict a theoretical growth rate of 0.020 mm/hr at the stoichiometric nitrogen pressure (0.13 bar) at a source temperature of 2300� C. and a ΔT of 50� C.
Turning now to FIG. 2, the apparatus (e.g., furnace) of the present invention is described. As shown, the furnace includes a heating source 6, such as a radio-frequency coil, that induces an electro-magnetic (EM) field within growth chamber 2. This EM field couples with a metallic susceptor (push tube) 3 located concentrically inside the coil and provokes heat generation by the Joule effect on it. (Although in a useful embodiment, susceptor/tube 3 is cylindrical, i.e., has a circular axial cross-section, as used herein, the terms ‘tube’ or ‘tubular’ also include tubes of non-circular axial cross-section.) The relative position and dimension of the push tube with respect to the shielding elements and coil creates a thermal gradient along the walls of the susceptor 3, i.e., in the axial direction. A crucible 9 is disposed concentrically within tube 3, and includes the highly purified source material 11 at a proximal end thereof (polycrystalline AlN) and eventually, the growing AlN crystal 7 at the distal end (e.g., at tip 19) thereof.
An aspect of this invention is the arrangement of the shielding elements in the system that allows setting an adequate thermal gradient along the walls of push tube 3. As shown, there are two distinct sets of shielding elements around the push tube. The first set includes two concentric open joint tungsten cylinders 8, each having a thickness of less than about 0.005″ in a desired embodiment. The skilled artisan will recognize that other refractory metals such as molybdenum or rhenium may be substituted for W. The term ‘open-joint’ refers to the cylinders 8 having an open longitudinal seam (i.e., the cylinders to not extend a full 360 degrees) so that there is no continuous electrical path around the cylinder. This helps to prevent the cylinders from coupling to the rf fields and becoming too hot and/or absorbing power intended for heating the crucible. Both the open-joint and the thickness are chosen to minimize the coupling between these metallic parts and the 450 MHz rf electromagnetic field. In addition, cylinders 8 are preferably axially shorter than rf coil 6 and located nominally in the center of the coil 6 (e.g., both concentrically with coil 6 and equidistantly from both (axial) ends thereof) to avoid inducing local non-axial-symmetric hot spots on shields 8. The second set of shields 10 desirably include pyrrolitic boron nitride (pBN) cylinders (e.g., approximately five to seven cylinders, in particular embodiments), which in desirable embodiments, are at least 0.050 inches (1.3 mm) thick and several centimeters longer than rf coil 6. While a purpose of the pBN shields 10 is to thermally insulate the tube 3 to obtain the desire temperature in the work zone, the mission of the metallic shields 8 is two-fold. Shields 8 serve as heat reflectors causing the temperature in the center of the hot zone to be much higher than at the coil ends. In addition, they serve to protect the push tube 3 from picking up boron generated by the continuous sublimation of the pBN shields 10. (Boron has a eutectic point with tungsten at the growth temperatures used in this invention if the boron concentration exceeds some value. Once the boron picked up by the push tube is higher than that value, a liquid phase tends to form on the skin of the push tube leading to its failure.) The shielding arrangement described hereinabove advantageously produces a sharp thermal gradient on the crucible 9 (e.g., over 100� C./cm) as its tip 19 moves axially beyond the metallic shields 8. As discussed hereinabove, this relatively large gradient has been shown to facilitate large growth rates. As also shown, the shields 8, 10 may be enclosed in a dielectric tube (5) fabricated from quartz or other suitable material, to insulate the coil 6 from the metallic elements of the system and prevent arcing, etc. with them.
Pressurize chamber to 1 bar with a gas comprising about
Place crucible in tube, with tip 19 in high temperature
region at proximal end of shield 8
move crucible tip towards distal end of chamber 2
Crystal growth initially involves evacuating 30 the chamber 2 (FIG. 2). e.g., to pressures on the order of 0.01 mbar (1 Pa) using a vacuum pump. The chamber 2 is then refilled 32 with nitrogen gas. This step is preferably repeated 34 several times to minimize oxygen and moisture contamination. The chamber is then pressurized 36 to about 1 bar (100 kPa) with nitrogen gas which is preferably mixed with a small amount of hydrogen. For example, a gas including about 95-100% N2 and 0-5% H2 is useful. Exemplary embodiments use about 3% H2 and 97% N2 due to commercial availability of gas premixed at these percentages. Polycrystalline AlN source material 11 is placed 37 at a proximal end of crucible 9. The crucible 9 may then be evacuated and sealed, or may be provided with openings 20, 21 as described hereinabove. The crucible is then disposed 38 concentrically within tube 3, with tip 19 in the high temperature region of the furnace (i.e., nominally within the proximal end of shield 8). The temperature is then ramped 40 to bring the tip of the crucible to a temperature of approximately 1800� C., in particular embodiments, within about 15 minutes. At the end of this temperature ramp, the gas pressure is set and maintained 42 at a predetermined super-atmospheric pressure, and the temperature is ramped 44 to a final crystal growth temperature, e.g., in about 5 hours. During this ramping 44, the pressure is continuously adjusted 46, e.g., using a vent valve (not shown) to maintain it at that fixed value. The goal of this ramping 44 is to enhance the purity of the source material 11 by permitting part of the oxygen still contained within it to diffuse out of the crucible (e.g., through the crucible walls). This diffusion occurs because the vapor pressure of the aluminum suboxides (such as Al2O, AlO, etc.), generated due to the presence of oxygen in the source material, is known to be higher than that of Al over AlN for the same temperature.
In particular embodiments, a value of 18 psi has been used to demonstrate growth rates of 0.9 mm/hr with a source 11 to crystal surface 7 separation of approximately 2 cm and the use of either CVD W (Chemical Vapor Deposited Tungsten) or powder metallurgy W crucibles (such as described in commonly assigned U.S. patent application Ser. No. 10/251,106, entitled, Power Metallurgy Tungsten Crucible for AlN Crystal Growth, which is fully incorporated by reference herein). The source to growing crystal surface distance may vary during the growth run if the area of the growing crystal surface is different than the surface area of the source material and the growth rate (i.e., axial rate of movement of the crucible through the temperature gradient) may need to be adjusted to account for this change. However, typically the surface area of the source and growing crystal surface will be kept nominally constant and approximately the same size so that the separation between the source and growing crystal surface will remain about constant during most of the growth.
The basic procedures for fabricating light emitting diodes (LEDs) and laser diodes (LDs) in compound semiconductors are well known. Typically, a p-n junction is formed by growing p-type and then n-type (or vice-versa) epitaxial layers on an appropriate substrate. Metal contacts (more desirably, ohmic contacts) are attached to the p-type and n-type semiconductor layers. The LED or LD functions by passing current through the device in the “forward direction.” That is, a power source pushes electrons out of the n-type semiconductor toward the p-type semiconductor and holes out of the p-type semiconductor toward the n-type semiconductor. The electrons and holes recombine with each other to produce photons at a wavelength that is determined by the bandgap energy of the semiconductor region where the recombination is occurring and may be shifted to somewhat higher energies by quantum confinement effects as well as by strain and impurities in the recombination region. Many factors can affect the efficiency of the LEDs. However, a very significant factor is the efficiency with which electrons and holes, once either generated or pushed out of their equilibrium positions, recombine to produce the desired radiation. In addition, LDs require the creation of an optical cavity to allow amplification by stimulated emission of the appropriate photons. This efficiency is improved by defining the region of recombination, often by creating one or more quantum wells. However, defects such as dislocations provide non-radiative recombination sites and will also reduce the recombination efficiency. Most importantly, once the density of non-radiative recombination centers gets higher than the typical diffusion length, of carriers (the holes and electrons) before they recombine, the loss in efficiency will become very significant and, as the density of defects is increased even further, the device will either perform badly (very inefficient) or not at all.
It is also well known that the addition of In to III-N LED and LD devices helps to localize the free carriers away from non-radiative defects in recombination region probably through the formation of composition fluctuations where regions with higher In content have a somewhat smaller bandgap (and hence, are a lower energy region for the free carriers to localize). This allows visible LEDs (emitting in the wavelength range from 400 to 550 nm) to function even though the dislocation density is very high (exceeding 108 cm−2) (see, for example, J. Y. Tsao, Circuits and Devices Magazine, IEEE, 20, 28 (2004)). These high dislocation densities are the result of growing nitride epitaxial layers on foreign substrates such as sapphire (crystalline Al2O3) or SIC. The wavelength of emission from pure GaN is approximately 380 nm and decreases to 200 nm for pure AlN. Wavelengths in between these two end points are achieved by alloying AlN with GaN. InN can also be introduced to make a AlxGayIn1-x-yN alloy. While success has been achieved in making LEDs at wavelengths down to 250 nm, the efficiencies of these devices remains quite poor (<1%) (A. Khan, “AlGaN based Deep Ultraviolet Light Emitting Diodes with Emission from 250-280 nm,” presented at the International Workshop on Nitride Semiconductors in Pittsburgh, Pa. (Jul. 19, 2004)).
Furthermore, low substrate dislocation densities appear to be critical to the fabrication of high efficiency ultra-violet (UV) light emitting diodes and UV laser diodes particularly at wavelengths shorter than 340 nm (S. Tomiya, et al., “Dislocations in GaN-Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers,” Phys. Stat. Sol. (a) 188 (1), 69-72 (2001); A. Chitnis et al., “Milliwatt power AlGaN quantum well deep ultraviolet light emitting diodes,” Phys. Stat. Sol. (a) 200 (1), 88.101 (2003); M. Takeya, et al., “Degradation in AlGaInN lasers,” Phys. Stat. Sol. (c) 0 (7), 2292-95 (2003). Nitride semiconductor layers grown on sapphire substrates typically have dislocation densities of about 108-1010 cm−2, and recently, S. Tomiya et al. have produced ELO GaN substrates with dislocation densities of about 106 cm−2, as discussed in the aforementioned journal article. Furthermore, high efficiency ultraviolet (UV) light emitting diodes and UV laser diodes (LDs) require layers of AlxGayIn1-x-yN with relatively high aluminum concentration.
Higher efficiency, light-emitting diodes (LEDs) have been demonstrated on the AlN substrates prepared by the method of the present invention (R. Gaska, presentation at the Fall Materials Research Society Meeting, Boston, Mass. (2003); and L. J. Schowalter et al., “Native, single-crystal AlN substrates,” submitted to Phys. Stat. Sol. (c) Jul. 18, 2004). These results demonstrate over an order of magnitude improvement in photoluminescence efficiency on the low defect layers grown on the substrates cut from bulk crystals of the present invention than those results obtained on high defect density epitaxial layers grown on sapphire (which has a defect density greater than 1,000,000 dislocations cm−2). The improved results are believed to be due to the low density of dislocations (10,000 cm−2 or less) that are observed in substrates cut and prepared from bulk AlN crystals grown by the present invention. This low dislocation density was measured by Synchrotron White Beam X-ray Topography (SWBXT) (B. Raghothamachar et al., J. Crystal Growth 250, 244-250 (2003); and L. J. Schowalter et al., Phys. Stat. Sol. (c) 1-4 (2003)/DOI 10.1002/pssc.200303462).
FIG. 6 is a schematic cross-sectional view of a portion of a 260 nm laser diode fabricated using the method of the present invention. Initially, low defect density AlN substrate 40, which is prepared using the method discussed above, is polished by chemical mechanical polishing (CMP). Polished substrate 40 is then introduced into a conventional organo-metallic vapor phase epitaxy system (OMVPE). The surface of the low defect density AlN substrate is cleaned to remove any oxide or other impurities on the crystal surface. Cleaning is effected by heating the substrate at a temperature of 1150� C. for 20 min under ammonia plus nitrogen or hydrogen plus nitrogen flow prior to growth. An epitaxial layer 42 of AlN having a thickness of about 100 nm is then grown on substrate 40 to improve the surface quality of AlN substrate 40 before starting to grow the device layers. Next an undoped AlxGa1-xN buffer layer 44 having a thickness of approximately 1 μm is grown atop the epitaxial AlN layer 42 to relieve lattice mismatch through the formation of misfit dislocations. Formation of threading dislocations, which will continue to propagate through the device layers, is minimized by grading x from 0 to the final value of 0.5 (50% Al concentration). Onto buffer layer 44, a 1 μm thick layer 46 of Si-doped AlxGa1-xN (x=0.5) is grown. This layer 46 makes the n-type contact to the LD. A 50 nm thick layer 48 of Si-doped AlxGa1-xN (x=0.6) is then grown onto the Si-doped AlxGa1-xN (x=0.5) layer 46, followed by the growth of a 10 nm thick layer 50 of undoped AlxGa1-xN (x=0.5). Onto layer 50 is grown a 50 nm thick layer 52 of Mg-doped AlxGa1-xN (x=0.6) followed by the growth of a 1 μm thick layer 54 of Mg-doped AlxGa1-xN (x=0.5). After the growth steps, substrate 40 (now with epitaxial layers 42, 44, 46, 48, 50, 52 and 54 on it) is slowly ramped down from the growth temperature of about 1100� C. and removed from the OMVPE system. The top surface of epitaxial layer 54 is then coated with a metal contact layer 56 for the p-type semiconductor, and metal layer 56 is coated with a photoresist (not shown), which is then developed. The photoresist is removed where the n-type metal contact 58 will be formed. The substrate plus epitaxial layers plus metal layer are then etched such that the semiconductor is removed down to the n-type layer 46, which will be used for the n-type metal contact 58. A second coating of photoresist (for lift off) (not shown) is deposited, which is then patterned and removed where the n-type contacts are desired. The n-type metallization is complete, and the metal coating adjacent the second photoresist layer is removed by lift off to produce the desired wafer. Laser facets are achieved by cleaving the wafer. These facets may optionally be coated to protect them from damage. Wire bonding contacts (not shown) are made to the p-type and n-type metal layers and the laser diode is packaged appropriately.
The growth of the bulk single crystal of AlN has been described primarily herein as being accomplished by what is commonly referred to as a “sublimation” technique wherein the source vapor is produced at least in part when crystalline solids of AlN or other solids or liquids containing AlN, Al or N sublime preferentially. However, as also disclosed herein, the source vapor may be achieved in whole or in part by the injection of source gases or like techniques that some would refer to as “high temperature chemical vapor deposition”. Also, other terms are sometimes used to describe these and other techniques that are used to grow bulk single AlN crystals according to this invention. Therefore, the terms “depositing”, “depositing vapor species” and like terms will sometimes be used herein to generally cover those techniques by which the crystal may be grown pursuant to this invention.
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Growth, (Jan. 25, 2006), vol. 287, No. 2, pp. 372-375.Referenced byCiting PatentFiling datePublication dateApplicantTitleUS8344409 *Jul 7, 2011Jan 1, 2013Epistar CorporationOptoelectronic device and method for manufacturing the sameUS8946736Dec 31, 2012Feb 3, 2015Epistar CorporationOptoelectronic device and method for manufacturing the sameWO2015038398A1 *Sep 4, 2014Mar 19, 2015Nitride Solutions, Inc.Bulk diffusion crystal growth process* Cited by examinerClassifications U.S. Classification117/89, 117/105, 117/108, 117/942, 117/109International ClassificationH01L33/02, C30B29/04, H01L33/00, C30B11/00Cooperative ClassificationC30B29/403, H01L33/0075, C30B11/003, H01L33/025, C30B23/00, H01L2924/0002European ClassificationC30B29/40B, H01L33/00G3C, C30B23/00, C30B11/00FLegal EventsDateCodeEventDescriptionAug 31, 2010ASAssignmentOwner name: CRYSTAL IS, INC., NEW YORKFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SCHOWALTER, LEO J.;SLACK, GLEN A.;ROJO, J. 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