Method and apparatus for growing a gallium nitride boule

This patent application claims the benefit of prior filed copending provisional patent application serial No. 60/159,163 filed on Oct. 13, 1999 and entitled Growth of Gallium Nitride Boule.

1. Field of the Invention

This invention relates to the field of semiconductor manufacture, and more specifically to the making of gallium nitride (GaN) from the reaction of gallium (Ga) vapor with gaseous anhydrous ammonia (NH 3 ).

2. Description of the Related Art

Gallium nitride is a well-known refractory nitride optoelectronic material from the group of III nitrides having an energy gap that covers the spectral range from red to deep ultraviolet. Among its known uses are color displays, color copying, optical storage, power and microwave electronics, optoelectronics, and radiation detection from infrared to X-rays.

One of the first instances of GaN synthesis was by Juza and Hahn when ammonia was passed over hot gallium to produce small needles and platelets of GaN. Later, Maruska and Tietjen used chemical vapor deposition to make a large area of crystalline GaN on sapphire.

For commercial GaN device applications, metalorganic chemical vapor deposition (MOCVD) has been used to produce super bright blue light-emitting diodes (LEDs).

Without limitation thereto, prior methods of making GaN include the above-defined MOCVD process, a reactive sputtering process wherein atoms or ions of a solid material target are ejected into a gas phase by a momentum exchange with energetic particles, the growth of GaN by the vapor phase epitaxy (VPE) method, and the growth of GaN by the molecular beam epitaxy (MBE) method.

VPE is a chemical vapor deposition method that is carried out in a hot wall reactor at up to atmospheric pressure wherein gallium monochloride (GaCl) is synthesized upstream in the reactor by reacting HCl gas with liquid Ga metal at from 800 to 900-degrees C. GaCl is then transported to a substrate where the GaCl reacts with NH 3 at 900 to 1100-degrees C. to form GaN via the following reaction.

GaCl+NH 3 ⇄GaN+HCl+H 2

The primary precursor gases that are employed during growth of GaN by the MOCVD process are Ga(CH 3 ) 3 or Ga(C 2 H 5 ) 3 and NH 3 , while GaCl 3 and NH 3 are also used for the growth of GaN by the halide VPE method.

MBE is a thin-film deposition process in which beams of atoms or molecules react on a clean surface of a single-crystalline substrate that is held at a high temperature under an ultrahigh vacuum of better than 10 −10 torr. GaN has been grown by MBE using NH 3 as the source of molecular nitrogen (N 2 ) wherein the NH 3 is decomposed on the surface of a substrate by pyrolysis at from 700 to 900-degrees C., and wherein N reacts with Ga to form GaN. The growth mechanism of GaN by MBE is believed to consist of the thermal activation of NH 3 and surface reaction mitigated dissociation followed by reaction of N with Ga to form GaN.

Reference can be made to the two publications entitled “Gallium Nitride (GaN) 1” and “Gallium Nitride (GaN) II”, by Jacques I. Pankove and Theodore D. Moustakas, volumes 50 and 57 of “Semiconductors and Semimetals”, Academic Press, copyright 1998 and 1999, for a discussion of GaN.

It is against the above generally stated background that the present invention was made.

A high quality GaN boule 60 (i.e., a carrot-shaped and generally synthetically formed mass of GaN having the structure of a single crystal) is epitaxially grown by reacting a vapor of the metal Ga with the gas NH 3 at a high temperature of about 1200-degrees C., which high temperature causes the NH 3 to dissociate into the two elements N and H.

While NH 3 is know, as a result of measurements made by the present inventor, to require a temperature of about 1400 degrees C. in order to produce a dissociation of the NH 3 , in accordance with the present invention the presence of Ga vapor acts as a catalyst that lowers the dissociation temperature of NH 3 to about 750-degrees C.

In accordance with this invention, a seed 51 of GaN is placed within a growth furnace 30 that is heated to about 1200-degrees C. An input stream of Ga vapor and an input stream of NH 3 gas are then incident on the GaN seed. Again as a result of measurements made by the present inventor, it was determined that GaN does not decomposed at 1200-degrees C. in the presence of NH 3 .

Ga is a solid at room temperature, Ga becomes a liquid at about 30-degrees C., and Ga becomes a vapor at the temperature that is within the growth-furnace.

An upward-facing, shower head-shaped, manifold 10 of unique construction is provided to uniformly distribute the Ga vapor and the NH 3 gas to the interior of the growth furnace at a location that is generally below and spaced from the bottom of the GaN seed, for example, spaced by a distance of from about 0.5 to about 10 mm. GaN vapor is thus formed within this space, generally adjacent to the surface of the boule.

Ga vapor is flushed or carried into the growth furnace by way a controlled stream of nitrogen (N 2 ). At the location of the exterior surface of the GaN seed, the Ga vapor reacts with the NH 3 gas to epitaxially form solid GaN on the exterior surface of the GaN seed, and to also form H 2 .

While it is possible that some GaN vapor forms within the growth furnace, and then deposits epitaxially on the GaN seed, it is believed that most of the solid GaN epitaxially forms on the GaN seed from a combination of the Ga vapor with the N that results from the decomposition of the NH 3 . In other words, most of the Ga vapor moves away from the manifold and toward the surface of the GaN seed, whereupon the Ga vapor combines with N and then deposits on, and thereby epitaxially grows, the boule.

The manifold flat top surface 11 includes a plurality of physically-spaced nozzles 13 , each nozzle having a generally circular-shaped orifice. A heated chamber 21 is formed directly under this top surface, and this heated chamber includes the nozzles. Liquid Ga is compressed, using a motorized pipette 39 , and the compressed Ga vapor is then piped, under pressure, into the nozzles, along with nitrogen. Within the hot nozzles the Ga vaporizes, as the Ga vapor is mixed with a controlled amount of nitrogen gas.

Gaseous NH 3 is also supplied under pressure to portions of the hot chamber that surround the Ga nozzles, so as to flow through areas 131 of the manifold top flat surface that surround each Ga nozzle. As a result of the relatively high temperature of about 1200-degrees C. that exists at and above the manifold's top surface, GaN vapor moves upward toward the exterior surface of the GaN seed.

FIG. 1 is a partial side section view of a shower-head manifold 10 in accordance with the invention, this figure showing five nozzles 13 through which Ga vapor flows, this figure showing a first supply pipe 14 that supplies Ga liquid and N 2 gas to the five flow interconnected Ga nozzles, and this figure showing a second supply pipe 20 that supplies gaseous NH 3 through NH 3 nozzle portions 131 of the manifold that surround the Ga nozzles.

FIG. 2 is a top view of the shower-head manifold of FIG. 1, this figure showing eight generally parallel nozzle members 22 , each nozzle member having a linear plurality of individual Ga vapor nozzles that alternate with a plurality of individual NH 3 nozzles.

FIG. 3 is a side section view of a growth-furnace 30 in accordance with the invention, this growth furnace having the manifold of FIGS. 1 and 2 located within the bottom half of the furnace.

FIG. 1 is a partial side section view of a hot or heated shower head manifold 10 in accordance with the invention. Without limitation to a specific geometric orientation,.in an embodiment of the invention hot manifold 10 included a generally flat and horizontal planar upper face that is indicated by numeral 11 . That is, nozzle face 11 lies in an X-Y plane of coordinate system 12 , wherein the Z axis of coordinate system 12 indicates the upwardly-directed vertical axis.

Five nozzles 13 are shown in FIG. 1 through which Ga vapor and N 2 gas flows in an upward or Z-direction. A first supply pipe 14 supplies Ga vapor and N 2 gas to nozzles 13 , which nozzles 13 are flow interconnected by way of conduits 15 . , As will be appreciated, Ga liquid turns to vapor by the time it reaches manifold 10 . Without limitation thereto, in an embodiment of the invention each nozzle 13 included a generally centrally located and circular-shaped orifice 16 having a diameter of about 0.5 mm. Also without limitation, an example center-to-center distance 19 between adjacent nozzles 13 was about 5 mm.

FIG. 1 also shows a second supply pipe 20 that supplies pressurized anhydrous NH 3 gas to the internal portion 21 of hot manifold 10 that surround Ga nozzles 13 . This NH 3 gas flows through the nozzle or nozzle space 131 that surrounds each Ga nozzle 13 .

FIG. 2 is a top view of generally circular shower head manifold 10 , this figure showing eight generally parallel and linear nozzle members 22 , each nozzle member 22 having a plurality of individual Ga nozzles 13 , each Ga nozzle having an adjacent NH 3 nozzle or nozzle space 131 .

Without limitation thereto, members 22 are preferably evenly spaced by a distance 23 of about 5 mm. Also without limitation thereto, the diameter 34 of circular manifold 10 is in the range of from about 1 to about 2.5 cm.

Manifold 10 is preferably constructed of a metal that will withstand the processing temperatures of the present invention, and of a metal that is compatible with the materials being processed by the present invention, examples being stainless steel and molybdenum.

By operation of manifold 10 , and as a result of a temperature of about 1200-degrees C., a generally uniform gaseous distribution that includes Ga vapor, NH 3 gas, N 2 gas, and H 2 gas is produced in the area 18 that is directly above the top surface 11 of manifold 10 . Generally at this location 18 , GaN vapor is produced, the GaN vapor then moving or migrating upward in the Z-direction.

It is believed that at 1200 -degrees C. any decomposition of GaN into Ga and N is in equilibrium with the formation of GaN. Also, Ga vapor reaching the GaN seed reacts rapidly with NH 3 to form an epitaxial layer of GaN on the surface of the seed.

FIG. 3 is a side section view of a growth-furnace 30 in accordance with the invention wherein manifold 10 of FIGS. 1 and 2 is located within the bottom half of furnace 30 . Without limitation, furnace 30 is in the form of a hollow cylinder having a closed top and bottom, and whose central axis 31 extends vertically in the Z-direction, the Z-direction height 32 of furnace 30 being at least 10 cm, and the diameter 33 of furnace 30 being about 10 cm for growing a boule having a 1 cm diameter. Of course, for the growing of larger size boules, the size of furnace 30 is increased.

Furnace 30 is heated in a well-known manner by a water-cooled radio frequency (RF) induction heater 34 that encircles furnace 30 . A water cooling jacket 35 is provided, cooling jacket 35 having a water inlet 36 and a water outlet 37 . In a manner that is not critical to the invention, RF heater 34 is controlled by means not shown, so as to controllable reach and then maintain a temperature of about 1200-degrees C. within the interior volume 38 of furnace 30 . Furnace 30 includes two inlets 14 and 20 and one outlet 141 .

As shown, and in an embodiment of the invention, above-described hot manifold 10 is mounted at a fixed position in the lower half of furnace 30 , generally centered on central axis 31 . Liquid Ga is compressed, using motorized pipette 39 , and the Ga liquid is supplied, under pressure, to conduit 14 , along with a pressurized flow of N 2 . Gaseous NH 3 is also supplied, under pressure, to the interior volume 21 of hot manifold 10 by way of conduit 20 .

A vertically-extending hollow cylinder silicon carbide coated (SiC-coated) graphite susceptor 40 is concentrically located generally in the vertical center of furnace 30 . In a well known manner, susceptor 40 is induction heated by operation of RF heater 34 . Without limitation thereto, susceptor 40 is preferably surrounded by a relatively long vertical cylinder of porous graphite hollow cylinder thermal insulator 50 that is also centered upon axis 31 . Insulator 50 is provided to minimized heat loss from the cylindrical susceptor heating element 40 that is concentrically located inside of thermal insulator 50 .

The GaN boule that is grown in accordance with the present invention is indicated in FIG. 3 by numeral 60 . Boule 60 is physically grown on a GaN seed 51 that is attached to or fixed to the bottom end 52 of a metal shaft 53 , for example a stainless steel shaft 53 , that extends generally coincident with axis 31 .

The upper end 54 of shaft 53 is attached to or fixed to an electrically operated rotator/puller motor means 55 , As boule 60 grows, motor means 55 is operable to bidirectionally move shaft 53 in a vertical direction, as shown by arrow 62 , and/or to bidirectionally rotate shaft 53 , as shown by arrow 61 . Rotary movement 61 is provided to homogenize the growth of GaN over the whole external area of seed 51 . Examples of rotary movement 61 and linear movement 62 are from about 1 to about 100 revolutions per minute, and from about 0.3 to about 1.0 mm per hour, respectively.

Under equilibrium conditions, the vertical spacing or vertical separation of the top surface 11 of manifold 10 from the bottom surface of the growing boule remains substantially constant at from about 0.3 to about 1 mm. Motor means 55 provides simultaneous circular movement 61 and vertical movement 62 , motor means 55 being controlled by means not shown so as to maintain this vertical spacing substantially constant. For example, optical sensing means or capacitive sensing means, not shown, are provided to monitor this vertical spacing and to control motor means 55 in a manner to maintain the desired constant spacing.

As a feature of the invention, the growth of boule 60 can be momentarily interrupted by inserting a puff of N 2 into Ga vapor supply line 14 , thus leaving boule 60 and manifold 10 in an NH 3 environment that prevents the net decomposition of GaN.

As a feature of the invention, single crystal boules 60 of Ga 1-x Al x N can be grown by adding aluminum (Al) vapor to the Ga vapor. Similarly, by adding indium (In) vapor to the Ga vapor a single crystal boule 60 of In 1-x Ga x N can be grown. The growth of In-rich InGaN preferably requires a graded concentration of In within the boule in order to reduce the strain that results from a lattice mismatch of InN and GaN. Of interest is the use of the invention to grow a boule 60 of Al 1-x Ga xy In 1-y N that matches the lattice constant of GaN.

It is important that a good seed 51 be provided. Such a seed can be produced by lateral epitaxial overgrowth over SiO 2 strips or SiN 4 strips, as taught in O. H. Nam et al, Applied Physics Letters, 71, 2638, 1997.

Another technique of producing a seed 51 is to use a ZnO single crystal that has a mismatch of about 2-percent in lattice constant with respect to GaN. In this case, it may be desirable to coat the ZnO with GaN at a low temperature in order to prevent decomposition of the ZnO, followed by the high temperature growth of GaN.

Another seed technique is to make a van-der-Waals seed/substrate having a good lattice match to GaN. Two such seed/substrate candidates are MoS 2 and WS 2 which have a lattice mismatch of about 0.6-percent to GaN. These seeds are mica-like crystalline layers of SMS that are weakly bonded between adjacent S layers, where M stands for M o or W.

A preferred seed method starts with a single crystal platelet fabricated by Unipress in accordance with S. Porowski, “Near Defect Free GaN Substrates”, MRS Soc. Symp. Proc., Vol. 537, G1.3, 1999. This Unipress platelet is grown from liquid Ga under high pressure of N 2 . The resulting platelet is about 1 square centimeter in area and contains few defects; i. e., about 10 2 defects per square centimeter as compared to about 10 8 defects per square centimeter when MOCVD techniques are used.

Once a boule 60 is grown to a cross-sectional area of about 1 square centimeter, the boule can be sliced, to thereby form low defect wafers. These wafers can then be assembled, using x-ray diffraction guidance, into a well oriented larger size wafer that can then be used as larger seed for the growth of a much larger boule in accordance with this invention.

The invention has been described above while making reference to various embodiments of the invention. However, it is known that other skilled in the art, upon learning of this invention, will readily visualize yet other embodiments that are within the spirit and scope of this invention. Thus, the above detailed description is not to be taken as a limitation on the spirit and scope of this invention.