1. Field of the Invention
This invention relates to InGaN/GaN light emitting diodes (LEDs) and laser diodes (LDs), and more particularly to nonpolar III-nitride LEDs in which the wavelength can be controlled by selecting the barrier thickness of multiple quantum wells (MQWs).
2. Description of the Related 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 fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These compounds are referred to herein as Group III nitrides, or III-nitrides, or just nitrides, or by the nomenclature (Al, B, Ga, In)N. Devices made from these compounds are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride 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 cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in Group-III nitride optoelectronic devices is to grow the devices on nonpolar planes of the crystal. For example, in GaN crystals, such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN are the {11-20} family, known collectively as a-planes, and the {10-10} family, known collectively as m-planes.
The other cause of polarization is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al, B, Ga, In)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer lattice matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
The advantage of using nonpolar planes over c-plane nitrides is that the total polarization will be reduced. There may even be zero polarization for specific alloy compositions on specific planes. Such scenarios will be discussed in detail in future scientific papers. The important point is that the polarization will be reduced compared to that of c-plane nitride structures.
Although high performance optoelectronic devices on nonpolar on-axis m-plane GaN have been demonstrated, it is difficult to obtain long wavelength emission from InGaN/GaN MQWs grown on m-plane GaN. This is probably due to the low In incorporation of the InGaN/GaN MQWs. The emission wavelength of devices grown on m-plane is typically 400 nm, while the emission wavelength of devices grown on c-plane is 450 nm at the same growth condition. Reducing the growth temperature increases the In incorporation; however, crystal quality would be degraded. This would be a significant problem for applications such as blue, green, yellow, and white LEDs.
The present invention describes a technique for the growth of nonpolar III-nitride light emitting devices in which the emission wavelength from the devices can be controlled by the barrier thickness of the MQWs in the devices. For example, the present invention has obtained blue and green emission without the effect of polarization.