Orientation-patterned Semiconductors

An older method is the “stack-of-plate” method: several GaAs wafers with alternating crystal orientations are mechanically mounted or bonded to each other [1]. This technique allows for large apertures as required for high-energy applications, but not for small orientation periods (thus not for all desirable phase-matching applications) and not for easy mass production. Therefore, this technique could not find a wide range of applications in nonlinear optics.

Another approach is epitaxial growth of patterned films on a suitable template [2, 6], serving as a pre-oriented substrate. The template itself may be fabricated with wafer stacking (see above), but that again limits the achievable poling period.

Far smaller periods can be realized with lithographic techniques. One of them [3] works as follows:

• It starts with the bonding of two carefully aligned wafers with inverted phase domains; the quality of the bonding can be improved with additional bonding layers grown with OMCVD. One of the wafers contains an edge stop layer, which allows one to remove all but a very think layer after bonding. This results in a GaAs wafer having a thin (few μm) domain-inverted layer on top.
• With optical lithography, one can then remove a periodic pattern of stripes of that think layer, resulting in an orientation-patterned template.
• This can then be used for subsequent growth of thick layers e.g. with HVPE.

A later developed all-epitaxial approach is based on GaAs/Ge/GaAs heteroepitaxy (polar-on-nonpolar epitaxy):

• One first grows with MBE a thin layer of GaAs with uniform domain orientation, which is slightly misoriented from the (100) orientation of the surface.
• Then one applies a regular pattern of thin germanium stripes (≈3 nm thickness) with optical lithography.
• Further growth of GaAs up to a thickness of e.g. 5 μm leads to antiphase domains on the Ge-covered regions. (Without the above mentioned misorientation of the substrate, antiphase domains would be obtained only with 50% probability.)

For growing samples to a substantial thickness, relatively fast growth techniques such as organometallic vapor phase epitaxy (OMVPE) or hydride vapor phase epitaxy (HVPE) or liquid phase epitaxy (LPE) are required. Molecular beam epitaxy (MBE), for example, would be far too slow; it suitable only for generating the thin patterned template.

The quality of the thick epitaxially grown material can be very high, as long as sufficiently precise vertical domain propagation can be maintained over the required thickness. (Growth conditions need to be optimized accordingly, e.g. via various growth parameters such as temperature.) The film thickness and hence the beam aperture is often limited to a few hundred microns, although some growth techniques have been optimized for obtaining even devices with a thickness of the order of 1 mm [12] as required for high-power devices. Such device thicknesses are possible even for domain periods of the order of 30 μm.

Epitaxial techniques can also be applied to waveguide structures, resulting in an intense interaction which is not limited by beam divergence. One can thus obtain relatively efficiency frequency conversion at low power levels. For waveguides, a large device thickness is usually not required.

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