Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-11-15
2003-09-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S045013, C372S050121
Reexamination Certificate
active
06618418
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a dual III-V nitride laser structure with a sapphire substrate and, more particularly, to a dual III-V nitride laser structure with a trench into a thick current spreading layer to separate the dual lasers to reduce thermal cross-talk.
Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their ends by cleaved surfaces 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. 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. Layers with excess electrons are said to be n-type, i.e. negative, while 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 “inject” them into the active layers, where electrons recombine with holes to produce light. Optical feedback provided by the cleaved mirrors allows resonance of some of the emitted light to produce coherent “lasing” through the one mirrored edge of the semiconductor laser structure.
The III-V nitrides make possible diode lasers that operate at room temperature and emit shorter-wavelength visible light in the blue-violet range under continuous operation. The III-V nitrides comprise compounds formed from group III and V elements of the periodic table. The III-V nitrides can be binary compounds such as gallium nitride (GaN), aluminum nitride (AlN), or indium nitride (InN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN), and quarternary alloys such as aluminum gallium indium nitride (AlGaInN).
These materials are highly promising for use in short-wavelength light emitting devices for several important reasons. Specifically, the AlGaInN 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 high electric current and 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 group III-V nitride semiconductor layers grown on a sapphire substrate will emit light in the near ultra-violet to visible spectrum within a range including 360 nm to 650 nm.
The shorter wavelength of blue/violet 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 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.
This type of laser is employed in communication systems, laser xerography, and other applications where the device's small size, low operating current, and other characteristics are beneficial. The performance of many devices, such as laser printers and optical memories, can be improved by the incorporation of multiple laser beams. For example, laser printers which use multiple beams can have higher printing speeds and/or better spot acuity than printers which use only a single beam.
Two lasers or dual lasers can be fabricated on the same substrate to provide closely spaced, independently addressable laser beams for such applications.
In virtually all of the applications of these lasers, it is necessary to modulate the output of the laser. Where a number of solid state lasers are integrated onto a single substrate, it is almost always necessary to modulate the output of each laser independently. In some applications this modulation is at a very high frequency, in others it is at a low frequency, and in still others this frequency will vary. Since lasing depends on a current flowing into the active layer, one obvious way to modulate the light output of a laser is to modulate the driving current. In fact, varying the driving current is presently the most common and conventional way in which a laser's output is modulated. However, this method of modulation has a number of distinct drawbacks and disadvantages, among which is transient heating of the chip on which the laser or lasers are formed.
Heat is generated through voltages drops across the metal electrode/semiconductor interfaces, which have a finite resistance, and through voltage drops across the resistive semiconductor layers. Energy is also introduced into the active region of the laser by injecting electrons into the conduction band and/or holes into the valence band. Electrons relax into the lowest energy state of the conduction band and holes relax into the lowest energy state of the valence band through non-light emitting processes and release their energy in the form of heat.
When a laser diode is switched from the OFF to the ON state, e.g. by forward biasing the laser diode at a constant current above threshold, laser operation is obtained very quickly (typically ~ns), while the device temperature continues to increase until it reaches equilibrium (typically several hundred &mgr;s). This transient heating, or heating that changes over time, can cause the light output of that III-V nitride blue laser diode and any adjacent III-V nitride blue laser diodes to drop as the threshold current of the laser device increases with temperature.
The desired separation between adjacent laser diodes in a dual laser structure may be 20 microns. Under these circumstances, the heat dissipated during the operation of one laser increases the temperature in the active region of the other laser. This is known in the art as thermal cross-talk. Thermal cross-talk renders the power output of the neighboring laser unpredictable and erratic.
High speed and high resolution printing requires laser devices with little or no fluctuations of the output power. For example, the variation in the laser light output required for red and IR laser diodes for printing applications is smaller than 4% and those requirements would be similar for III-V nitride blue laser diodes.
Another related consequence of transient heating of a laser is wavelength variation. Essentially, the operating wavelength of a laser diode is dependent on the temperature of the laser diode. If the temperature varies, the wavelength of operation will vary. Thermal cross-talk from one laser diode will change the wavelength of the light emitted from an adjacent laser diode.
Group III-V nitride blue lasers are particularly susceptible to thermal cross-talk due to the poor thermal conductivity of the sapphire substrate and the relatively high electric power consumption of III-nitride base laser device. For example, AlGaInN laser devices have threshold currents in the order of 50 mA and operating voltages of 5 V (compared to about 15 mA and 2.5 V for red lasers).
One method of reducing thermal cross-talk is to separate the dual lasers in the semiconductor structure with a groove extending between the two lasers into the substrate, as taught in U.S. Pat. No. 5,805,630 to Valster et al. The two red/IR lasers are formed from gallium arsenide semiconductor layers and the substrate that the groove extends into is also gallium arsenide.
Unfortunately, removing a significant portion of the substrate as taught in that patent weakens the structural integrity of the entire semiconductor structure and makes it more susceptible to breakage.
Furthermore, a III-V nitride blue laser structure typical
Kneissl Michael A.
Northrup John E.
Al-Nazer Leith A
Ip Paul
Propp William
Xerox Corporation
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