BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to epitaxial films in general, and in particular to a method for growing germanium epitaxial films on silicon substrates.

2, Description of Related Art

There are two conventional methods for growing single-crystal germanium films on silicon substrates using ultra-high vacuum chemical vapor deposition (UHV-CVD). The first method allows a germanium film to be grown directly on top of a silicon layer, and the second method uses a silicon and silicon germanium buffer layer at the interface.

Since the first method is very selective, a germanium film on!y grows on a silicon layer and not on any exposed dielectric material. The problem with the first method is that the resultant germanium film is very rough and has a substantially high defect density. Compared with the first method, the second method is not selective at all Thus, although the second method overcomes the roughness problem, the resultant germanium film occurs on an underlying silicon layer as well as an underlying dielectric layer. The growth on the underlying silicon layer is desired but the growth on the underlying dielectric layer is not.

The present disclosure provides an improved method for growing selective germanium epitaxial films. SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a silicon substrate is initially preconditioned with hydrogen gas. The temperature of the preconditioned silicon substrate is then decreased, and germane gas is flowed over the preconditioned silicon substrate to form an intrinsic germanium seed layer. Next, a mixture of germane and phosphine gases can be flowed over the intrinsic germanium seed layer to produce an n-doped germanium seed layer. Otherwise, a mixture of diborane and germane gases can be flowed over the intrinsic germanium seed layer to produce a p-doped germanium seed layer. At this point, a bulk germanium layer can be grown on top of the doped germanium seed iayer.

AU features and advantages of the present invention will become apparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

Figure 1 is a high-level process flow diagram of a method for growing germanium epitaxial films, In accordance with a preferred embodiment of the present invention; and

Figure 2 depicts a silicon substrate having a germanium epitaxial film, in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF A PREFEREED EMBODIMENT

The desired smooth and highly selective germanium layer is formed using a UHV-CVD system. The formation cart be completed by using the following gases: hydrogen, 100% sllane (SiHi), 100% germane (GeH ₄ ), 1% diborane (B ₂ H ₆ ) and 1% phosphine (PH ₃ ).

Referring now to the drawings and in particular to Figure 1, there is illustrated a high-level process flow diagram of a method for growing germanium epitaxial films, in accordance with a preferred embodiment of the present invention. Starting at block 10, a silicon substrate is preconditioned with hydrogen gas at 750 EC for 60 minutes, as shown in block 11. The pressure of hydrogen is controlled at 3E ^-4 mBar.

Next, the temperature is ramped down from 750 EC to 350 EC in 200 minutes in vacuum at 2 EC per minute, as depicted in block 12.

Germane gas at 1.5E ^' ' ¹ mBar is flowed over the preconditioned silicon substrate for 120 minutes, as shown in block 13. This stop initiates the layer-by-layer growth for approximately the first ten layers of defect-free single-crystal germanium Can intrinsic germanium seed layer.

The temperature is then ramped back up from 350 EC to 600 EC in 125 minutes in vacuum at 2 EC per minute, as depicted in block 14.

A 1:3 mixture of phosphine and germane gases at 6E ^-4 mBar is flowed over the intrinsic germanium seed layer for 30 minutes, as shown in block 15. This in-situ doped germanium growth step produces approximately 150 A of an n-doped germanium seed layer with a phosphorus concentration of IE ²¹ atoms/cm ³ . Some of the phosphorus diffuses into the underlying intrinsic germanium seed layer (from block 13) and reduces the stress in the underlying intrinsic germanium seed layer. The stress in the germanium is initially created by the lattice mismatch between germanium and silicon from the intrinsic germanium seed layer and silicon substrate, respectively. At this point, an uniform bulk single-crystal germanium film having an extremely low level of defects can be grown on top of the n-dopcd germanium seed iayer, as depicted in block 16. For example, germane gas at 1 ,5E ^{- 3} mBar can be flowed over the the n-doped germinamum seed layer for 480 minutes to produce appioximately 1 urn of an intrinsic germanium layer. If desired, the bulk germanium layer may be in-situ n-doped or p-doped by injecting some phoshiπe or diborane, respectively, along with the germane gas.

If a p-doped germanium seed layer is desired in block 15, the phosphϊne gas can be replaced by diborane gas with slightly different conditions. For example, a 1:1 mixture of diborane and germane gases at 6E ^-4 mBar is flowed over the intrinsic germanium seed layer for 30 minutes in order to produce a p-doped germanium seed layer. This rn-siϊu doped germanium growth step produces approximately 150 Δ of a p-doped layer with a boron concentration of 5E ²¹ atoms/cm'.

The steps shown in blocks 13 and 15 provide an intrinsic (first) germanium seed layer and a doped (second) germanium seed layer, respectively, which are key to the growth of a smooth bulk germanium film layer. The mechanism behind the steps shown in blocks 13 and 15 is that some of the phosphorus diffuses into the underlying germanium and reduces the stress in the underlying germanium. The stress is generated by the lattice mismatch between germanium and silicon. The reduction of stresses results in a smooth bulk germanium growth.

The smooth defect-free germanium film can be used to produce germanium photodetectors with lower dark currents than can be produced with a typical process that does not use the doped step in block 15. The disclosed method also decreases the sensitivity of germanium growth to mask si/e. With a typical germanium growth, the final germanium thickness varies with the size of the mask opening to the silicon substrate. The overall germanium smoothness, decreased sensitivity to pattern size, and lower dark current make the disclosed method preferred for overall process integration. With reference to Figure 2, there is depicted a silicon substrate having a germanium epitaxial film, in accordance with apreferred embodiment of the present invention, As shown, an intrinsic germanium seed layer 22 is grown on top of a silicon substrate 21. A bulk germanium film 24, which can be intrinsic or doped, is grown on top of a doped germanium seed iayer that rests on top of intrinsic germanium seed layer 22.

One conventional method for improving overall germanium smoothness and for decreasing germanium growth sensitivity to mask size is to use a silicon-germanium buffer layer. The usage of a silicon buffer layer, however, is not selective and results in germanium growth over exposed nitride and oxide regions as well as over exposed silicon regions. Frequently, nitride or oxide layers are used to mask regions where no germanium growth is desired, and a loss of growth selectivity requires additional processing to remove germanium from the tops of exposed nitride or oxide regions. The method of the present invention does not use any silicon containing buffer layer and therefore provides a very selective germanium growth. The simultaneous smooth germanium growth, low sensitivity to pattern size, and high growth selectivity are key benefits from the steps shown in blocks 12 and 13.

As has been described, the present invention provides an improved method for growing germanium epitaxial films. The method of the present invention can be used to grow single-crystal germanium films that are very selective and yet very smooth and defect-free. The improved germanium growth process allows for simpler processing and yields lower dark currents in germanium P-i-N photodiodes.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.