The present invention relates to an indium gallium aluminum nitride-based vertical cavity surface emitting laser (xe2x80x9cVCSELxe2x80x9d) structure or an indium gallium aluminum nitride-based light emitting diode (xe2x80x9cLEDxe2x80x9d) structure and, more particularly, to a p-n tunnel junction for current injection for the indium gallium aluminum nitride-based semiconductor VCSEL or LED structure.
Monolithic solid state semiconductor lasers are very desirable light sources for high speed laser printing, optical fiber communications and other applications. Recently, there has been an increased interest in vertical cavity surface emitting lasers although edge emitting lasers are currently used in the vast majority of applications. A common laser structure is a so-called xe2x80x9cedge emitting laserxe2x80x9d where light is emitted from the edge of the monolithic structure of semiconductor layers. A laser structure is a xe2x80x9cVCSELxe2x80x9d where the light is emitted from the surface of the monolithic structure of semiconductor layers.
Vertical cavity surface emitting lasers are very desirable light sources for high speed laser printing, optical fiber communications, optical sampling and other applications. VCSELs have several advantages over edge emitting lasers including an emitted beam with a small angular divergence, a circular, anastigmatic beam and ease of fabrication into one or two dimensional arrays.
Vertical cavity surface emitting lasers generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at opposite semiconductor layers that act as mirrors. The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.
An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to xe2x80x9cinjectxe2x80x9d them into the active layers, where electrons recombine with holes to produce light. Optical feedback provided by the opposite semiconductor layers allows resonance of some of the emitted light to produce coherent xe2x80x9clasingxe2x80x9d through either the top surface or the bottom surface of the semiconductor laser structure.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V semiconductors, comprise elements selected from group III, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as indium gallium aluminum nitride (InGaAlN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.
These materials are particularly suited for use in short-wavelength light emitting devices for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
Semiconductor laser structures comprising nitride semiconductor layers grown on a sapphire substrate will emit light in the ultra-violet to visible spectrum within a range including 280 nm to 650 nm.
The shorter wavelength violet of nitride based semiconductor laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers. The emission wavelength of GaN-based lasers and LEDs with an AlGaN or AlInGaN active region can be tuned into the UV range of the spectrum. Emission wavelength around 340 nm and 280 nm are particularly interesting for the optical excitation of biomolecules in bacteria, spores and viruses, which can be applied e.g. in bioagent detection systems.
P-type doping of InGaAlN layers is a key problem in the realization of GaN-based devices. It is difficult to achieve a high hole concentration in AlGaN alloys since the ionization energy of Mg acceptors is relatively high (xcx9c200 meV for Mg in GaN) and increases even further with higher Al content (xcx9c3 meV per % Al). Therefore, p-doped waveguide and cladding layers contribute significantly to the series resistance of the nitride-based laser structure, which results in higher operating voltages. Even in today""s currently best violet nitride lasers, the operating voltages are on the order of 5 to 6 V, which is 2 to 3 V above the laser emission energy. For UV laser and LEDs, which require even higher Al compositions, the series resistance is going to be even larger. For a UV laser structure emitting around 340 nm, the required Al composition for the cladding layers would be around 30%. The increase in Mg acceptor activation energy in the AGaN layer would result in an almost an order of magnitude drop in hole concentration compared to a Mg-doped GaN film.
In addition, the optimum growth temperatures for Mg-doped AlGaN layers is typically lower than the growth temperatures for Si-doped or un-doped AlGaN films, because of the improved Mg incorporation efficiency at lower temperatures. However, the structural quality of nitride-based semiconductor layers is reduced, when grown at a lower temperature, which deteriorates the structural and electronic properties of the upper cladding layers and upper waveguide layers in a III-V nitride laser structure.
Furthermore, GaN:Mg, InGaN:Mg, short period AlGaN/GaN superlattice layers or bulk AlGaN layers doped with Mg are used as waveguiding layers in GaN-based laser diodes structures. These Mg-doped layers have a significant absorption loss particularly in the blue to ultraviolet spectrum that a nitride based laser will emit light. For laser diodes operating close to the band gap of GaN ( less than 400 nm), this leads to increased distributed loss and consequently to increased threshold current densities.
It is an object of this invention to provide a nitride based VCSEL or LED structure with a reduced number of p-type semiconductor layers.
According to the present invention, a p-n tunnel junction between a p-type semiconductor layer and a n-type semiconductor layer provides current injection for an nitride based vertical cavity surface emitting laser or light emitting diode structure. The p-n tunnel junction reduces the number of p-type semiconductor layers in the nitride based semiconductor VCSEL or LED structure which reduces the distributed loss, reduces the threshold current densities, reduces the overall series resistance and improves the structural quality of the laser by allowing higher growth temperatures.
The relative thinness of the tunnel junction semiconductor layers allows the VCSEL or LED to emit light through the layers. The use of n-type current spreading and contact layers allows the VCSEL or LED to use a metallic annular electrode, rather than a semi-transparent electrode.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.