METHOD OF PRODUCING LARGE GaAs AND GaP INFRARED WINDOWS

Abstract

IR window slabs of GaP greater than 4 inches diameter, and of GaAs greater than 8 inches diameter, are grown on a substrate using Hydride Vapor Phase Epitaxy (HVPE), preferably low pressure HVPE (LP-HVPE). Growth rates can be hundreds of microns per hour, comparable to vertical melt growth. GaAs IR windows produced by the disclosed method exhibit lower absorption than crystals grown from vertical melt near 1 micron, due to reduced impurities and reduced growth temperatures that limit the solubility of excess arsenic, and thereby reduce the EL2 defects that cause high absorption near one micron in conventional GaAs boules. Silicon wafers can be used as HVPE substrates. For GaAs, layers of GaAsP that vary from 0% to 100% As can be applied to the substrate. EMI shielding can be applied by adding a dopant during the final stage of growth to provide a conductive GaAs or GaP layer.

Claims

1. An infrared window comprising a GaAs slab having a slab largest dimension that is greater than eight inches or a or GaP slab having a slab largest dimension that is greater than four inches, said slab being formed from a substrate wafer of single crystal silicon to which at least one layer of GaAs or GaP has been applied by HVPE, the substrate wafer being subsequently removed from the slab.

2. The infrared window of claim 1, wherein the slab largest dimension is greater than 12 inches.

3. The infrared window of claim 1, wherein the slab is substantially round, having a slab diameter of greater than eight inches.

4. The infrared window of claim 3, wherein the slab diameter is greater than 12 inches.

5. The infrared window of claim 1, wherein the slab is at least 2 mm thick.

6. The infrared window of claim 1, further comprising an electrically conductive layer of doped GaAs or GaP applied to the slab.

7. The infrared windows of claim 1, wherein the substrate wafer is a wafer of single crystal silicon.

8. The infrared window of claim 1, further comprising an anti-reflective coating applied to the slab.

9. A method of producing a slab of GaAs suitable for forming an infrared (IR) transparent window having a window largest dimension that is greater than eight inches or a slab of GaP suitable for forming an infrared (IR) transparent window having a window largest dimension that is greater than eight inches, the method comprising: preparing a substrate having a substrate largest dimension; placing the substrate within a reaction chamber of an HVPE reactor; applying gases to a surface of the substrate within the chamber, whereupon at least two of the gases react with each other to form a GaAs or GaP layer via HVPE, thereby growing the slab of GaAs or GaP on the substrate, the slab being a GaAs slab having a largest dimension greater than eight inches or a GaP slab having a largest dimension greater than four inches; causing the slab to grow to a desired thickness by continuing to apply the gases to a surface of the slab; and removing the substrate from the slab of GaAs or GaP.

10. The method of claim 9, wherein the slab largest dimension of the slab formed on the substrate is greater than 12 inches.

11. The method of claim 9, wherein the substrate is substantially round, having a diameter of greater than eight inches.

12. The method of claim 11, wherein the substrate diameter is greater than 12 inches.

13. The method of claim 9, wherein the desired thickness is at least 2 mm.

14. The method of claim 13, wherein a time required to grow the slab to the desired thickness is no greater than 1 week.

15. The method of claim 9, further comprising, during a phase of forming the slab, including a dopant gas as one of the gases that are applied to the surface of the slab, thereby forming an electrically conductive layer of doped GaAs or GaP on the slab.

16. The method of claim 15, wherein the dopant gas contains silicon.

17. The method of claim 15, wherein the dopant gas is applied to the surface of the slab during a final phase of forming the slab.

18. The method of claim 15, wherein the dopant gas is applied to the surface of the slab during an initial phase of forming the slab.

19. The method of claim 9, further comprising applying a layer of doped GaAs or doped GaP to a surface of the slab after the slab has been removed from the HVPE reactor.

20. The method of claim 9, wherein the substrate is a wafer of single crystal silicon.

21. The method of claim 20, wherein the slab is a GaAs slab, and wherein the method further comprises: forming a layer of GaP on the silicon substrate; applying layers of GaAsP onto the GaP wafer while gradually reducing a proportion of phosphorous in the gases, so that the applied layers transition from GaP to GaAs; and after the slab has been formed, grinding the silicon substrate and the layers of GaAsP off of the slab.

22. The method of claim 9, wherein a sacrificial layer is included between the slab and the substrate, and wherein removing the substrate from the slab includes removing the sacrificial layer, thereby freeing the slab of GaAs or GaP from the substrate.

23. The method of claim 22, wherein the sacrificial layer is chemically removed.

24. The method of claim 22, wherein the sacrificial layer is an ion-implanted damage layer forming a cleavage plane, and wherein the sacrificial layer is removed due to separating of the cleavage plane upon application thereto of thermal stress.

25. The method of claim 9, further comprising, after the slab has grown to the desired thickness, applying an anti-reflective coating to the slab.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 is a perspective view showing a substrate wafer placed within an LP-HVPE reactor;

[0044] FIG. 2 is a graph that illustrates the decreased absorption near 1 micron of GaAs grown via LP-HVPE as compared to GaAs from a boule grown from melt;

[0045] FIG. 3 is a cross-sectional side view illustrating a method of growing GaAs slabs by HVPE on a silicon substrate, whereby layers of GaAsP are applied to the silicon substrate having a ratio of As to P that varies gradually from 0% to 100% As;

[0046] FIG. 4A is a cross-sectional side view illustrating an embodiment where a sacrificial layer is included between the GaAs or GaP slab and the silicon substrate; and

[0047] FIG. 4B is a cross-sectional side view illustrating the embodiment of FIG. 4A after the sacrificial layer has been removed, freeing the GaAs or GaP slab from the substrate.

DETAILED DESCRIPTION

[0048] The present disclosure is a method of making GaP slabs having largest dimensions greater than 4 inches and GaAs slabs having largest dimensions greater than 8 inches, and preferably up to 12 inches or more, that are sufficiently thick to be structurally competent when used as IR windows, and wherein the GaAs slabs that are produced using the disclosed method have good transparency at wavelengths near one micron.

[0049] It will be understood that the present disclosure is not limited to round GaAs and GaP slabs, and that the term diameter should be interpreted to mean largest dimension unless otherwise required by context.

[0050] More specifically, the present disclosure teaches using Hydride Vapor Phase Epitaxy (HVPE) as a method of making GaAs and GaP IR window slabs having diameters of 4 inches, 8 inches, or even 12 inches or more. In embodiments, low pressure HVPE (LP-HVPE) is used to make the slabs.

[0051] With reference to FIG. 1, in embodiments a substrate wafer 100 is placed within a reaction chamber 102 and subjected to two gases 104, 106 that react with each other at the surface of the substrate 100 to form GaAs or GaP, thereby growing a crystal on the substrate. In the illustrated example, HCl gas 108 flows over a reservoir of liquid gallium 110 to form GaCl gas 104 that is applied vertically to the substrate 100 by vertical nozzles 112. At the same time, AsH.sub.3 gas 114 is applied horizontally to the surface of the substrate 100 by a horizontal nozzle 116. In the illustrated example, the substrate 100 rests on a substrate holder 118 that is rotated by H.sub.2 gas flow 120 which flows through a support tube 122 into a susceptor 124 and across groves on the underside of the substrate holder 118, causing it to rotate during the epitaxial growth process. In similar embodiments, PH.sub.3 gas 114 is applied in lieu of AsH.sub.3 so as to grow a crystal of GaP on the substrate 100.

[0052] As with other epitaxial methods, HVPE has been used for growing thin layers on small wafers of GaAs and GaP for use in integrated circuits and other electronic devices, as well as for electro-optic applications such as solar cells. Unlike most epitaxial growth methods, HVPE, and especially LP-HVPE, can provide GaAs or GaP growth rates of hundreds of microns per hour, which is comparable to vertical melt growth rates, and thereby sufficient in a practical sense for producing GaAs and GaP slabs that are thick enough to be structurally competent as IR windows.

[0053] Furthermore, the thermodynamics and kinetics of the HVPE growth process (and the resulting crystal quality) are independent of diameter and layer thickness. Consequently, large-diameter substrates 100, and a reactor having a reaction chamber 102 and substrate holder 118 that are large enough to accommodate them, are fundamentally all that is required for growing GaAs and GaP window slabs of 12 diameter and larger. For some applications, full-thickness plates of GaAs or GaP can be produced using LP-HVPE in less than a week. Also, this approach eliminates the material waste and fabrication cost of slicing and shaping thick plates from bulk crystal boules grown from the melt.

[0054] With reference to FIG. 2, it is notable that HVPE-grown GaAs windows, and especially LP-HVPE-grown GaAs windows 202, typically exhibit significantly lower absorption than melt-grown commercial off-the-shelf (COTS) GaAs crystals 200, especially at wavelengths near 1 micron. In part, this is because GaAs grown using HVPE exhibits a greatly reduced concentration of impurities due to the use of ultra-high purity gas phase precursors. More importantly, native defects are reduced because the reduced growth temperatures of HVPE limit the solubility of excess arsenic that manifests itself as arsenic-on-gallium anti-sites. It is these so-called EL2 defects that tend to cause high absorption losses in conventional, melt-grown GaAs windows at wavelengths near one micron. In the illustrated example, a GaAs wafer approximately 1 mm thick grown vertically from melt and sliced from a boule transmits less than 35% at one micron, whereas a GaAs layer of similar doping and thickness grown by LP-HVPE transmits more than 50% at the same wavelength.

[0055] Silicon wafers can be used as substrates 100 for HVPE growth of large GaAs and GaP IR window slabs. Si wafers are inexpensive, are of extremely high crystalline quality (dislocation-free), and are commercially available in very large diameters (12 standard, 18 custom, and up to 24 has been demonstrated). Si has the same diamond-like crystal structure and an almost identical lattice constant to GaP, allowing GaP slabs to be grown easily on a silicon substrate.

[0056] The lattice constant for GaAs differs somewhat from silicon. However, the small difference in lattice constant between Si and GaAs can be managed through the use of thick and gradual buffer layers (made possible by the very high growth rate of HVPE). With reference to FIG. 3, for example, the ratio of As to P of a GaAsP seed layer can be varied from 0% to 100% arsenic, thereby beginning with a layer of GaP 300 applied to a silicon substrate 100, and then applying a series of layers of GaAsP 302, 304, 30\6, 308, 310 that gradually transition from GaP 302 to GaAs 312.

[0057] Once the slab 312 has been grown, the Si substrate, and any intervening buffer layers 300-310 are removed from the slab 312. In some embodiments, these layers are simply ground and polished off of the slab 312. In other embodiments, with reference to FIG. 4A, a sacrificial layer 400 is included between the slab 312 and the substrate 100 with any buffer layers 300-310. In some of these embodiments, after the HVPE process is complete, the sacrificial layer 400 is chemically removed, thereby releasing the slab 312 from the substrate 100 (and any buffer layers 300-310), as is illustrated in FIG. 4B. In other of these embodiments, an ion-implanted damage layer 400 creates a defined cleavage plane that separates under applied thermal stresses to release the slab 312 from the silicon substrate 100.

[0058] In some embodiments, EMI shielded GaAs and GaP windows are produced by adding a dopant such as Si to the epitaxial gasses during the final stage of HVPE growth. In other embodiments, EMI shielded GaAs and GaP windows are produced by applying a layer of doped GaAs or GaP to the window in a subsequent deposition step.

[0059] Embodiments further include applying an anti-reflective coating to the window slab or to the doped conducting layer (if present).

[0060] The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application. This specification is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.

[0061] Although the present application is shown in a limited number of forms, the scope of the invention is not limited to just these forms, but is amenable to various changes and modifications. The disclosure presented herein does not explicitly disclose all possible combinations of features that fall within the scope of the invention. The features disclosed herein for the various embodiments can generally be interchanged and combined into any combinations that are not self-contradictory without departing from the scope of the invention. In particular, the limitations presented in dependent claims below can be combined with their corresponding independent claims in any number and in any order without departing from the scope of this disclosure, unless the dependent claims are logically incompatible with each other.