BACKGROUND

Aluminum nitride (AlN) holds great promise as a semiconductor basis material for numerous applications, e.g., opto-electronic devices such as short-wavelength LEDs and lasers, dielectric layers in optical storage media, electronic substrates, and chip carriers where high thermal conductivity is essential, among many others. In principle, the properties of AlN will allow light emission in the 200 nm wavelength region to be achieved. But many practical difficulties should be addressed for such devices to become commercially practicable.

For example, bulk AlN crystals often exhibit a substantial amount of cracking, which results in crystal separation before, during or after the crystal is formed into a wafer. If the crystal cracks or fully separates, it is very difficult or sometimes impossible to use it as a reliable substrate for device fabrication. Most of the commercially available machines for epitaxy, photolithography and other device processing require perfectly shaped, round wafers with uniform thickness. Any crack, even ones that do not result in wafer separation, will impair commercial usefulness. Therefore, the cracking problem in AlN crystal growth has crucial importance for the further development of nitride-based electronics.

In addition, many opto-electronic applications will require transparent wafers. While AlN is intrinsically transparent at optical wavelengths between 210 and 4500 nm, macroscopic defects such as cracks and inclusions significantly scatter light and reduce the apparent transparency in this important optical region. Elimination of cracks and inclusions is of critical importance to the development of ultraviolet light emitting diodes (LEDs), for example.

DESCRIPTION OF THE INVENTION

Brief Summary of the Invention

Microvoids (MVs) in AlN single crystals are a type of crystallographic defect that can be classified as an inclusion. The lateral (i.e. parallel to the c-plane) size of the MV defects typically varies from 0.1 to 3 micrometers (μm). Under low resolution (<100×) optical microscopy, the MVs appear to be spherically shaped inclusions. However, under higher 200×) resolution optical microscopy, as well as when imaged using Atomic Force Microscopy (AFM), the MVs appear to have a hexagonal shape well aligned with (i.e., approximately parallel to) the c-plane. During chemical treatment (chemical etching or chemical-mechanical polishing), MVs intersecting an AlN surface leave shallow pits with a round or hexagonal shape. Also, AlN material with a significant density of MVs tends to etch faster (during chemical etching) than the material with somewhat lower MV density. Therefore, the MVs can behave as heterogeneous inclusions in the AlN solid phase. The MVs may be associated with a gas-phase segregation, e.g. of oxygen, nitrogen, or hydrogen. MVs containing segregated gaseous species can lower the stacking symmetry of the AlN parallel to the c-axis thus representing stacking faults. In crystals grown by sublimation-recondensation (such as described in U.S. Pat. No. 6,770,135, the entire disclosure of which is hereby incorporated by reference), the volume density can be about109cm−3and the surface density of MVs (i.e., MVs which either intersect the surface after cutting or are within 1 μm of the surface) can be about105cm−2.

We have found that reducing the MV density in AlN ameliorates numerous problems related to cracking during crystal growth, etch pit generation during the polishing, reduction of the optical transparency in an AlN substrate, and, possibly, growth pit formation during epitaxial growth of AlN and/or AlGaN. The ability to address these problems by reducing MV formation facilitates practical crystal production strategies and the formation of large, bulk AlN crystals with low defect densities—e.g., a dislocation density below104cm−2, an inclusion density below104cm−3(MVs are a type of inclusion) and/or a MV density below104cm−3. The invention also facilitates fabrication of single-crystal wafers of AlN ready for epitaxial growth and oriented within 2° of the (0001) face (c-face) with the Al polarity, and having a shallow pit density of less than100cm−2. Embodiments of the invention further facilitate formation of single-crystal wafers of AlN that are at least 2 cm in diameter having an optical absorption coefficient of less than 1 cm−1over any part of the wavelength range where AlN is intrinsically transparent spanning from 210 nm to 4,500 nm. Embodiments of the invention also permit fabrication of substantially crack-free boules of single-crystal AlN having diameters larger than 2 cm and lengths greater than 1 cm. The term “wafer,” as utilized herein, refers to a self-supporting substrate and/or portion of a boule of material such as AlN. The term “substantially crack-free,” as utilized herein, refers to a crack density of less than 5 cracks per boule or volume of single crystal material which is approximately 2 cm diameter by 1 cm in length, or less than 10 cracks per two-inch (2″) diameter wafer, where the cracks are typically less than approximately 2 cm in length. Alternatively, it can refer to approximately zero cracks in a crystal boule or portion thereof, or can refer to a finite crack density insufficient to cause separation of a portion of the crystal boule during growth, cool-down, slicing, or other handling procedures.

Accordingly, in a first aspect the invention features a method of growing single-crystal AlN including providing in a crystal growth enclosure a vapor comprising Al and N2, and depositing the vapor as single-crystalline AlN having a microvoid density less than approximately104cm−3. In an embodiment, the partial pressure of N2in the crystal growth enclosure may be maintained at a level greater than a stoichiometric pressure relative to the Al. The partial pressure of N2may be within the range of 1-50 bar.

One or more of the following features may be included. A growth rate of the single-crystalline AlN in any crystallographic direction may be within the range of approximately 0.1 to approximately 2 mm/hr. A push rate less than or equal to the intrinsic growth rate of the single-crystalline AlN may be maintained. The push rate may be at least 0.1 mm/hr. Deposition of the single-crystalline AlN may originate at a seed crystal disposed within the crystal growth enclosure and oriented such that a direction of maximum growth rate of the single-crystalline AlN is rotated at least approximately 10° away from a c-axis of the single-crystalline AlN and toward a non-polar direction. The oxygen concentration in the vapor may be less than 300 parts per million (ppm) atomic percent, and the hydrogen concentration in the vapor may be less than 0.5%. The crystal growth enclosure may include tungsten, and the growth temperature may be less than approximately 2350° C. The crystal growth enclosure may include at least one of tantalum and carbon, and the growth temperature may be less than approximately 2750° C. The temperature gradient along a length of the crystal growth enclosure may be greater than approximately 5° C/cm and less than approximately 100° C/cm.

The method may further include, following deposition, the step of slicing an AlN wafer from the single-crystalline AlN. The cross-sectional area of the AlN wafer may be approximately equal to that of the single-crystalline AlN, and may be approximately circular with a diameter greater than approximately2cm.

One or more of the following features may be included. The method may include the step of annealing the AlN wafer at a first temperature and a first pressure. The first temperature may be greater than approximately 2000° C. and the first pressure may be greater than approximately 35 bar. The first temperature may be less than approximately 2350° C. After annealing, the microvoid density in a center region of the AlN wafer may be less than the density in an edge region of the AlN wafer, and may be less than approximately 104cm−3, or even approximately zero.

The method may include the step of polishing the AlN wafer, wherein after polishing the AlN wafer has an etch pit density less than approximately 100 cm−2. In some embodiments, the method includes the step of depositing an epitaxial layer on the AlN wafer, wherein the epitaxial layer has a surface pit density less than approximately 100 cm−2. In another embodiment, the microvoid density of the single-crystalline AlN is greater than zero and the single-crystalline AlN is substantially crack-free.

In another aspect, the invention features a bulk AlN crystal having a dislocation density below 104cm−2and an inclusion density below 104cm−3. In an embodiment, the microvoid density of the crystal is less than approximately 104cm−3.

In yet another aspect, the invention features a single-crystal wafer of AlN oriented within 2° of the (0001) face with the Al polarity, the wafer having a shallow pit density less than 100 cm−2. In an embodiment, the wafer is substantially crack-free. The microvoid density in a center region of the wafer may be less than the density in an edge region of the wafer. For example, the microvoid density in a center region of the wafer may be approximately zero and the microvoid density in an edge region of the wafer may be less than approximately 104cm−3.

In another aspect, the invention features a single-crystal wafer of AlN having an optical absorption coefficient of less than 5 cm−1at all wavelengths in the range spanning 500 nm to 3,000 nm. In an embodiment, the wafer is substantially crack-free. A microvoid density in a center region of the wafer may be less than a microvoid density in an edge region of the wafer. The microvoid density in a center region of the wafer may be approximately zero and the microvoid density in an edge region of the wafer may be less than approximately 104cm−3. In an embodiment, the optical absorption coefficient may be less than 1 cm−1at all wavelengths in the range spanning 500 nm to 3,000 nm. The wafer may have a diameter greater than approximately 2 cm.

In another aspect, the invention features a single-crystal wafer of AlN having an optical absorption coefficient less than 1 cm−1at any wavelength in the range spanning 210 nm to 4,500 nm and a diameter greater than approximately 2 cm.

In yet another aspect, the invention features a boule of crack-free, single-crystal AlN having a diameter larger than 2 cm and a length greater than 1 cm. In an embodiment, the boule may have a microvoid density in the range of greater than zero to less than approximately 104cm−3.

