This section provides background information and introduces information related to various aspects of the disclosure that are described and/or claimed below. These background statements are not admissions of prior art.
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established, for example, in the fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), CVD, metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HYPE).
As shown in FIG. 1, GaN and its alloys are most stable in the hexagonal wurtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. FIG. 1 illustrates an example c-plane 2, m-plane 4, and a-plane 6. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the wurtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the wurtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, related art c-plane quantum well structures in III-nitride (III-N) based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction can significantly degrade the usefulness of these III-N materials.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes, the m-planes and a-planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent non-polar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. However, growth of GaN semiconductor wafers with a non-polar surface remains difficult. Accordingly, there exists a need to increase the efficiency and improve operating characteristics for III-nitride based optoelectronic and electronic devices, for example LEDs.