Patent Application
20080197358
The present invention is directed, in general, to a semiconductor device having one or more III-V crystals and methods of manufacturing thereof.
III-V crystals are of interest in a number of optical and electronic applications because these crystals can have a wide band gap. E.g., in certain opto-electronic devices, III-V crystals can be used to emit light at wavelengths (e.g. blue light) that would otherwise be difficult to attain. It can be difficult, however, to grow epitaxial layers of III-V crystals, or to grow such crystals to a suitable size for practical application. It can also be difficult to grow epitaxial layers of III-V crystals on particular types of crystalline substrates, e.g., silicon, because the lattice constant of the III-V crystals does not match the lattice constants of the substrates. In particular, growing a group III-V crystal on a substrate with different lattice constants typically caused defects to be formed in the group III-V crystal. If a grown III-V crystal has an excessive number of defects, then the opto-electronic properties of the crystal will suffer (e.g., decreased light emitting efficiency, or substantially increased or decreased electrical resistivity).
One embodiment is a device. The device comprises a substrate having crystal-support-structures thereon and a III-V crystal. The III-V crystal is on a single contact region of one of the crystal-support-structures. An area of the contact region is no more than about 50 percent of a surface area of the III-V crystal.
Another device embodiment comprises the III-V crystal. The III-V crystal has a uniform crystal orientation except for a defect region that is located centrally from said III-V crystal. The defect region occupies less than about 10 percent of a total volume of the III-V crystal.
Another embodiment is a method of manufacturing a device. The method comprises forming crystal-support-structures on a substrate, wherein each of the crystal-support-structures has a pre-defined contact region. The method further comprises growing a III-V crystal on the contact region of one of said crystal-support-structures, an area of the contact region being no more than about 50 percent of the surface area of the grown III-V crystal.
The invention is best understood from the following detailed description, when read with the accompanying FIGUREs. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The present inventions benefit from the recognition that the number of defects in III-V crystals can be reduced by initiating the growth of the crystals on micro- or nano-sized crystal-support-structures. As crystal's growth extends beyond a contact region with the support-structure, the problem of there being a lattice mismatch between the crystal and support-structure is reduced because the bulk of the crystal is in free space, air, or another gas/fluid. Because the bulk of the crystal contacts at most, a non-solid, the number of defects in the final crystal is reduced. Moreover, the defects can be localized to the vicinity around the contact region of the crystal-support-structure.
The term III-V crystal as used herein refers to a crystal that is substantially an alloy containing having at least one type of atom from Group 13 (i.e., group 3A using the American convention) and one type of atom from Group 15 (i.e., group 5A using the American convention) of the Periodic Table of Elements (IUPAC convention for Group labels). The term III-nitride crystal as used herein refers to a crystal that is substantially an alloy containing at least one type of atom from Group 13 and nitrogen atoms. Exemplary III-V crystals include crystals formed substanitally of the binary alloys GaN, AlN, InN, and other alloys such as AlxGa1-xN, InxGa1-xN, In A1-xN, and AlxInyGa1-x-yN. III-V crystals include crystals of such alloys that are intrinsically or extrinsically doped, e.g., with conductivity altering impurity atoms.
The term crystal-support-structure as used herein refers to a predefined raised feature on a surface, wherein the structure has at least one lateral dimension that is about 1 mm or less and preferably smaller than about 0.1 mm. The crystal-support-structures can be nanostructures (at least one lateral dimension about 1 micron or less) or microstructures (at least one lateral dimension about 1 millimeter or less). The term contact region refers to the area of the nanostructure that directly contacts the III-V crystal or that the growth or the III-V crystal is initiated from.
One embodiment of the present invention is a method of manufacturing an apparatus.
In some cases, the crystal-support-structures 110 are formed from the substrate 115. E.g., a substrate 115 composed of a substrate (e.g., silicon, sapphire) can be patterned (e.g. photolithography) and etched (e.g., reactive ion etching), or micro-machined, using conventional processes. The crystal-support-structures 110 and the substrate 115 can both be made of crystalline silicon. Silicon is desirable for its low cost and the ability to pattern or machine silicon via numerous commercially available processes. In some cases, it is also desirable for the crystal-support-structures 110, the substrate 115, or both, to be made of electrically conductive material (e.g., silicon or extrinsically doped silicon).
As illustrated the crystal-support-structures 110 can be configured as pillars. The pillars can be cylindrically shaped, although other shapes (e.g., cones, cubes, prisms, pyramids etc . . . ) can be used. In some cases, the crystal-support-structures 110 are configured as closed-cells (e.g., hexagonally shaped cells). Crystal-support-structures 110 may be configured as closed cells that can be interconnected to other closed cells or be free-standing. Hexagonal shaped closed-cells are sometimes preferred because this structure is easier to manufacture than e.g., circular cells, and because this structure is inherently mechanically stable and able to distribute mechanical stresses across the entire structure. Example configurations and methods of fabricating of pillar-shaped and close-cell-shaped crystal-support-structures 110 configured as various types of microstructures or nanostructures that could serve as a crystal-support-structure 110 are further discussed in U.S. patent application Ser. Nos. 10/835,639 to Kroupenkine filed Apr. 30, 2004 and 11/460,901 to Kroupenkin filed Jul. 28, 2006 which are both incorporated by reference herein in their entirety.
A plurality of pillar- or closed-cell-shaped crystal-support-structures 110 can form a micro- or nanostructured surface 125 on the substrate 115. The crystal-support-structures 110 on the surface 125 can be evenly spaced apart from each other, or variably spaced apart. In some cases, a lateral separation 127 between adjacent pillar-shaped crystal-support-structures 110 is greater than a lateral thickness of the grown III-V crystal (e.g., thickness 310 in
It is sometimes desirable to have one or more groups 130 of crystal-support-structures 110 on the substrate 115. E.g. about 10 to 100 crystal-support-structures 110 configured as pillars or interconnected closed-cells are more closely spaced to adjacent members of their group 130 than to other crystal-support-structures 110 in different groups 132. Such configurations can facilitate the formation of larger crystals, e.g., a thin film III-V crystal that contacts a plurality crystal-support-structures 110 in the group 130.
As further illustrated in
In some cases, the contact region 140 is located on an upper surface 145 of the crystal-support-structures 110, and may comprise the entire upper surface 145 of the structure 110. Having the contact region 140 on the upper surface 145 facilitates the formation of large crystals on individual one, or groups 130, of the crystal-support-structures 110. Having the contact region 140 on the upper surface 145 can also be advantageous when the objective is to form an array of crystals all in the same plane, e.g., the same plane as the substrate 115 that the structures 110 are formed on. In others cases, however, the contact region 140 can be located on a sidewall 150 of the crystal-support-structures 110. Having the contact region 140 on the sidewall 150 can facilitate the formation of crystals that bridge adjacent crystal-support-structures 110.
It is advantageous for the pre-defined contact region 140 to be small compared to the desired final size of the grown crystal, because this helps to minimize the extent of defects in the crystal. A small contact region 140 is facilitated by having the contact region 140 on the upper surface 145 of crystal-support-structures 110 with a small lateral dimension 160. E.g., when the crystal-support-structure 110 is configured as a pillar, at least one dimension that is about 1 mm or less can be a lateral dimension 160 corresponding to the diameter of the pillar.
The diameter 160 is carefully designed to balance several factors. It is desirable to make the diameter 160 small so as to minimize crystal defects by minimizing the size of the contact region 140. However, the diameter must be large enough to provide mechanical support and stability to the crystal. In some cases the diameter 160 must be large enough to permit electrical conduction through the crystal-support-structures 110. In some preferred embodiments, the pillar-shaped crystal-support-structure 110 has a diameter 160 ranging from about 100 to 300 nanometers.
In some cases, it is desirable for the height 165 of the structure 110 to be at least about 2 times greater than a vertical thickness of grown III-V crystal (e.g., the thickness 312 depicted in
It is desirable to configure the contact region 140 to facilitate the seeding and growth of an epitaxial III-V crystal. In some cases, the contact region 140 has a substantially similar lattice geometry of atoms as the lattice geometry of atoms in the III-V crystal to be grown thereon. Selecting a lattice geometry for the contact region 140 to correspond to that of the crystal helps promote the epitaxial growth of the crystal with a minimum of defects. In some preferred embodiment, a crystal lattice of the contact region 140 and said III-V crystal 210 are both substantially hexagonal geometries. E.g., when the crystal has a hexagonal crystal structure (e.g., a III-nitride crystal like GaN or InN), then crystal-support-structures 110 can be formed from a silicon wafer, or a silicon layer on the substrate 115, that has a (111) orientation. In such instances, the contact region 140 located on the upper surface 145 of the structure 110 will have a hexagonal lattice arrangement of atoms. Similarly, when the crystal has a cubic crystal structure (e.g., GaAs or InP), then crystal-support-structures 110 can be formed by vertically etching a silicon wafer-substrate having a (100) orientation. In such instances, the contact region 140 on the upper surface 145 of the structure 110 will have a cubic lattice arrangement of atoms.
It is not necessary for each of the crystal-support-structures 110 to have identical shape or dimensions. It is also not necessary that separations between the crystal-support-structures 110 are identical to each other or that said structures are all interconnected or all free-standing. However, such unitary configurations are sometimes preferred for ease of manufacture.
The initial portion of the crystal 210 that forms directly on the contact region 140 is termed the defect region 215. The term defect regions 215 as used herein is defined as that portion of the crystal 210 having a substantially elevated concentration of lattice defects, e.g., dislocations or disinclinations. Typically, the concentration of the various types of lattice defect, e.g., threading defects, would be expected to be at least about 10 or more times higher, or even about 100 or more times higher in the defect region 215 than in other regions of the same crystal 210. Because the crystal 210 can grow outward in all directions from the defect region 215 (except the contact region), the defect region 215 is typically centrally located in the crystal 210. That is, the defect region 215 is located in a central portion 216 of the crystal that is away from one or more a lateral edge 217 of the crystal 210.
The defect region 215 is typically adjacent to the contact region 140. The defect region 215 can be identified and its total volume substantially quantified by inspecting plan or cross-sectional views of transmission electron microscope (TEM) images of the grown III-V crystal 210. For instance, in a III-nitride crystal 210 (e.g., InN or GaN), the defect region 215 can include one or more thread defects 220 in the crystal's hexagonal shape that project vertically above the contact region 140, but not laterally. Examples of such threading defects in a III-V crystal are presented in F. A. Ponce and D. P. Bour, Nature 386:351-59 (1997), which is incorporated by reference herein in its entirety. In some embodiments of the device 100, the defect region 215 occupies less than about 10 percent of a total volume of the III-V crystal 210. In some cases, thread defects 220, and hence defect region 215, do not extend all the way to the other side 222 of the crystal (e.g., the side opposite to the side that is adjacent to the contact region 140). In other cases, the thread defect 220 density of crystal 210 adjacent to the side 222 opposite to the contact region 140 is lower than the thread defect 220 density of the crystal 210 adjacent to the contact region 140.
In some preferred embodiments of the device 100, the III-V crystal 210 has a low defect density, except for the defect region 215 located adjacent to the contact region 140. The term low defect density as used herein is defined as a threading defect 220 density of less than about 1×108 cm2. The defect region 215 generally has a threading defect 220 density of more than about 1×108 cm−2. In some embodiments the threading defect 220 density in the defect region 215 is about 10, and in some cases 100 times, greater than the threading defect 220 density elsewhere in the crystal 210. E.g., in some embodiments, a threading defect 220 density in the defect region 215 is about 1×108 cm−2 or greater and a thread defect density of the III-V crystal 210 outside of the defect region 215 is less than about 1×107 μm2, and more preferably, less about 1×106 μm−2.
The relative rates of vertical and lateral growth of the crystal 210 can be further controlled by adjusting the relative amounts of group-III atoms 225 and group-V atoms 230 that the contact region 140 is exposed to. Growing the III-V crystal 210 can include exposing the contact region 140 to group-III atoms 225 and to group-V atoms 230 in a ratio that promotes a faster lateral growth than a vertical growth of the III-V crystal 210. E.g., when growing III-nitride crystals like GaN or InN, exposing the contact region 140 to group-III atoms 225 and group-V atoms 230 in a ratio of greater than about 1:1 (e.g., group-III atoms:group-V atoms) promotes faster lateral growth than vertical growth. The ratio can similarly be adjusted to promote a faster vertical growth of crystal over lateral growth by e.g., changing the ratio to less than about 1:1.
In some preferred embodiments, the epitaxial growth of the III-V crystal 210 is facilitated through the use of molecular beam epitaxy (MBE) process. E.g., consider the case when a III-nitride crystal 210 is grown on silicon crystal-support-structure 110. The MBE process can include a radiofrequency plasma using a feed gas of N2 (about 0.1 to 1 sccm) and plasma power of about 250 to 400 Watts. Different feed gases could be used to generate group-V atoms 230 other than nitrogen. The MBE process can further include a metal source (e.g., an effusion cell) whose temperature (e.g., about 900 to 1050° C. for Ga, about 700 to 850° C. for In, about 1050 to 1250° C. for Al) is adjusted to a range that permits different group-III atom 225 fluxes. The relative amounts of group-III atoms 225 and to group-V atoms 230 presented to the contact region 140 can be adjusted by changing the flow rate of the feed gases, the temperature of the metal source, or both.
The MEE process can further include adjusting the temperature of the substrate 115, or entire device 100, to facilitate the formation of epitaxial crystal 210. It is important for the substrate's 115 temperature to not be too high as to evaporate the group-III atoms 225 from the contact region 140. It is also important for the substrate's temperature to not be too low as to promote the deposition of metal droplets of group-III atoms 225. The appropriate temperature of the substrate 115 depends upon which group III atoms 225 are being used. E.g., for In, Ga, and Al, the substrate's 115 temperature preferably ranges from about 350 to 500° C., about 650 to 800° C., and about 700 to 900° C., respectively.
One skilled in the art would appreciate that the particular conditions of the MBE process would be adjusted to facilitate the presentation of the group III-atoms and group V-atoms. One skilled in the art would also understand that techniques other than MBE, e.g., vapor phase epitaxial methods, could be used.
The III-V crystal 210 can comprise more than one layer. For instance after forming a crystal of the desired lateral thickness, the MBE process can be adjusted to promote the vertical growth of the crystal 210. Moreover, the feed gas or metal source can be changed to form III-V crystal layers having different compositions. E.g., as illustrated in
In some cases, the growth of the crystal 210 is terminated such that there are individual III-V crystals 210 separately formed on each of the crystal-support-structures 110. In such cases, the lateral separation 127 between the adjacent individual crystal-support-structures 110 is greater than a lateral thickness 310 of the grown III-V crystal 210. In cases where the crystal's shape approximates that of a circular plate (e.g., a hexagonal plate such as formed in certain III-nitride crystals), then the crystal's lateral thickness 310 corresponds to a diameter of the crystal 210. E.g., in some cases, the diameter 160 of pillar-shaped crystal-support-structures 110 is about 10 percent or less than the average lateral thickness 310 (e.g., diameter) of the grown III-V crystal 210. In some cases each one of the pillar-shaped crystal-support-structures 110 each have diameters 160 that are about 10 percent or less of a lateral thickness 310 than individual ones of the III-V crystal 210 located on the individual crystal-support-structure 110. In some embodiments, the separated crystals 210 (e.g., GaN or InN) have a lateral thickness 310 of from about 1 to 50 microns, and a vertical thickness 312 of from about 1 to 20 microns.
As noted above, the extent of defects in the crystal 210 can be minimized and localized by configuring the crystal 210 to touch only one solid surface, the contact region 140, while the bulk of the crystal contacts air (or whatever other gaseous or liquid medium that surrounds the crystal). It is preferable for the area of the contact region 140 to be substantially less than a surface area 320 of the grown crystal 210. In some preferred embodiments, an area 325 of the contact region 140 is no more than about 50 percent of a surface area 320 of the grown III-V crystal 210 (e.g., the final crystal produced by the growing process). The term surface area 320 as used herein refers to the total surface area of a side 330 of the grown III-V crystal 210 that touches the contact region 140. In some preferred embodiments of the device 100 the area 325 of the contact region 140 is no more than about 50 percent, more preferably no more than about 10 percent, and even more preferably, no more than about 1 percent, of a surface area 320 of the grown III-V crystal 210
In some embodiments of the device 100, the growth of the crystals 210 (
As shown in
In some cases, the merged layer 510 can be directly used as a light emitting or detecting component of the device 100. Defect-free regions 520 of the layer 510 laying between the crystal-support-structures 110 can be further processed to form, e.g., a light emitting, light detecting or transistor devices. In other cases, such as shown in
In some embodiments of the device 100, the crystal-support-structures 110 remain as a component part of the device 100. E.g., electrically conductive crystal-support-structures 110 can be configured to provide an electrical connection to the III-V crystal 210. The crystal-support-structures 110 can also serve to hold a plurality of the III-V crystals 210 in a common plane 610 during further fabrication steps of the device, such as illustrated in
As further illustrated in
In other embodiments of the device 100, the crystal-support-structures 110 are removed and therefore these structures 110, and the substrate 115, are not components of the device 100. In such embodiments, further steps are taken to separate the III-V crystal 210 from the crystal-support-structures 110.
The III-V crystals 210 adhered to the adhesive layer 910 can then be coupled to conductive lines similar to that discussed above in the context of
Although the embodiments have been described in detail, those of ordinary skill in the art should understand that they could make various changes, substitutions and alterations herein without departing from the scope of the invention.
