Abstract:
A highly heat conductive layer is combined with or placed in the vicinity of the optical waveguide region of active semiconductor components. The thermally conductive layer enhances the conduction of heat away from the active region, which is where the heat is generated in active semiconductor components. This layer is placed so close to the optical region that it must also function as a waveguide and causes the active region to be nearly the same temperature as the ambient or heat sink. However, the semiconductor material itself should be as temperature insensitive as possible and therefore the invention combines a highly thermally conductive dielectric layer with improved semiconductor materials to achieve an overall package that offers improved thermal performance. The highly thermally conductive layer serves two basic functions. First, it provides a lower index material than the semiconductor device so that certain kinds of optical waveguides may be formed, e.g., a ridge waveguide. The second and most important function, as it relates to this invention, is that it provides a significantly higher thermal conductivity than the semiconductor material, which is the principal material in the fabrication of various optoelectronic devices.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to improvements in optoelectronic devices, such as laser diodes, light emitting diodes, semiconductor optical amplifiers, vertical cavity laser diodes, etc., and more specifically, the invention uses a highly thermally conductive (HTC) dielectric in optoelectronic devices. 
     2. Description of Related Art 
     Heat generation in any electronic semiconductor device is almost always deleterious to the device performance. The reason for this is basically the heat tends to “smear” or broaden the electron energy distribution and increase the decay rate of free carriers that contribute to the gain. The net effect in optoelectronic devices is that the optical gain decreases significantly with temperature. In devices such as laser diodes, light emitting diodes and optical amplifiers, these thermal effects can be catastrophic. The most common way to solve the problem is to conduct the heat away to a heat sink that is approximately at the ambient temperature. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a highly thermally conductive layer for use in an optoelectronic device. 
     The present invention uses a highly heat conductive layer in combination with or in the vicinity of optical waveguides in active semiconductor components. The thermally conductive layer enhances the conduction of heat away from the active region, which is where the heat is generated in active semiconductor components. In the invention, this layer is placed so close to the optical region that it may also function as a waveguide. This layer then can cause the active region to be nearly the same temperature as the ambient or heat sink. However, the semiconductor material itself should be as temperature insensitive as possible and therefore the invention combines a highly thermally conductive dielectric layer with improved semiconductor materials to achieve an overall package that offers improved thermal performance. The semiconductor materials include InGaAs strained quantum wells on GaAs substrates and AlGaInAs/InGaAs quantum wells on InP substrates. Advances in the types of semiconductor materials and the dielectric materials may be applied to this invention. Also, these materials can be used for improving thermal issues on vertical cavity surface emitting lasers (VCSELs) and other optoelectronic devices where thermal generation and low heat conduction is significant. 
     The highly thermally conductive dielectric in this invention serves two basic functions. First, it provides a lower index material than the semiconductor device so that certain kinds of optical waveguides may be formed, e.g., a ridge waveguide. The second and most important function, as it relates to this invention, is that it provides a significantly higher thermal conductivity than the semiconductor material, which is the principal material in the fabrication of various optoelectronic devices. One goal of the invention is to operate these devices without thermoelectric coolers. Normally, the optoelectronic devices need some kind of temperature control in order to operate properly in an optical communications system. The temperature control apparatus is usually cumbersome and adds significant complexity to the system. 
     An embodiment of the present invention comprises the use of a highly thermally conductive dielectric as the material used in one or both of the mirrors in the design of a vertical cavity semiconductor laser (1.3-1.6 μm wavelength of operation). In the case of long-wavelength vertical cavity laser diodes, heat generation is a significant detriment to the successful operation of the laser. By using a highly thermally conductive dielectric Bragg mirror, heat from the active region can be quickly conducted to the heat sink, and thus for a given operating current, the operating temperature of the device can be significantly reduced. 
     The uses of the invention include: 1) fiber optic communication devices: lasers, high intensity light emitting diodes, and optical amplifiers for telecommunications, networking, or data transport within a system area network; 2) high sensitivity detection of optical signals for field deployment, etc.; 3) DOD fly by fiber and avionics; and 4) any optoelectronics where thermal performance is a significant issue and thermoelectric (TE) coolers are not appropriate. The above devices have numerous military and commercial applications in communications, sensors, surveillance and storage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the structure of an embodiment of the highly thermally conductive (HTC) layer of the present invention. 
     FIG. 2 shows a ridge waveguide utilizing an HTC layer. 
     FIG. 3A shows the structure of the highly thermally conductive dielectric for a Bragg mirror in a vertical cavity laser. 
     FIG. 3B shows the reflection spectrum of the Bragg mirror. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there are essentially five layers in the HTC layer. The layer  1  is the adhesion layer. The thickness dimension of the adhesion layer is on the order of tens of angstroms. This material can be any number of reactive materials with oxygen. Its main purpose is to bind any III-V oxides with the subsequent layers. This material should be a semiconductor that is reactive with oxygen since a metal would tend to be very lossy for an optical signal. An example of material used in the adhesion layer is amorphous silicon. Other possible materials might be germanium. 
     For guided wave devices, layer  2  is a dielectric waveguide layer that provides optical confinement for the guided optical mode. This dielectric layer would represent the cladding of an optical waveguide. All of the core of the guide and possibly a portion of the upper cladding of the waveguide would reside in the semiconductor epitaxial layer below. One would try to optimize the thickness of layer  2  so as to reduce its thermal resistance while at the same time allowing for the optical mode intensity to decrease so that subsequent layers do not introduce excessive optical loss. This layer would be comprised of a low optical loss material such as SiO2, Al2O3, Ta2O5, etc. 
     Layer  3  is the thickest layer and is made of a highly heat conductive layer relative to the epitaxial material of a optoelectronic device to which the HTC layer is attached. This film can be comprised of any number of materials such as amorphous silicon, diamond, BeO, AlN or any other dielectic material that is electrically non-conductive but possesses high heat conductivity. This material is the most critical to the thermal performance since it is quite dose to the active region and meant to be substantial in thickness relative to the other layers so that it conducts heat away from the active area. It is not necessary for this layer to be low optical loss, however it is preferred. The thickness of layer  2  will determine the amount of optical loss that can be tolerated in layer  3 . In the case that layer  3  is made up of a Bragg reflector that is made of highly thermally conductive and low optical loss materials, then layer  2  may or may not be needed. A Bragg reflector is a series of high and low index layers that are each one-quarter wavelength thick (where the wavelength is the free space wavelength divided by the index of refraction). Bragg reflectors are typically made up 5 to 50 pairs of high and low index material. 
     Layer  4  is a stress compensation layer that is needed to compensate for the film built up in layers  1 ,  2 , and  3 . Layer  4  reduces any tendency of the film to delaminate due to differences in thermal expansion coefficients, i.e., it compensates for stress in the overall film stack. The net effect is that the process yields are significantly improved with this layer. In order to reduce stress and make a symmetric structure with regard to built in stress, this layer is made to be the same thickness as layer  2 . However, other materials and thickness could be used. Again factors affecting the choice of thickness and material should be weighed against any increase in thermal resistance. Ideally, this layer would be made of a highly thermally conductive material that adhered well to layer  3  and any subsequent layers and the thickness would be chosen to reduce stress from the deposition of layers  2  and  3 . 
     Layer  5  is provided in the case when resist or metal adhesion to the fourth layer is poor and so the fifth layer is usually made of the same material as the first layer. The fifth layer promotes adhesion and is provided for the ease of any subsequent processing steps. A material that can be used for layer  5  is amorphous silicon. The thickness of this layer is on the order of tens of angstroms. 
     One embodiment of the invention relates to an improvement in a ridge waveguide semiconductor optical amplifier (SOA). Other devices that can share this same basic ridge waveguide structure, except for the end facets, are laser diodes and an edge-emitting light emitting diode (LED). Semiconductor optical amplifiers are fiber optic devices that provide gain in optical networks. They can compensate for loss in the serial transmission of data such as a long haul fiber optic network or they can compensate for distribution loss such as the parallel transmission of cable TV signals. The SOA has been used extensively in switching systems and routers by overcoming loss from splitters as well as performing as the basic switch. The SOA has a very short carrier lifetime and thus can be switched in a nanosecond. Also, the SOA provides an extremely high extinction (on/off) ratio, up to 50 dB with reverse bias, that is a monotonic function of the applied voltage and thus ideally suited for switching applications. 
     A semiconductor optical amplifier utilizing the HTC layer and state of the art epitaxial materials is shown in FIG. 2. A ridge waveguide  20  is affixed to a thermally robust epitaxial material  22  (e.g., strained-layer InGaAs/AlInGaAs/InP quantum wells (QWs)). HTC dielectric layer  24  is affixed to the ridge waveguide  20 . An electrically conductive layer  26  is applied to the top of the ridge waveguide  20  and the top portion of the HTC  24 . The thermally robust layer  22  is attached to a semiconductor layer  28 . Layer  28  is much thicker than layers  20 ,  22 ,  24 ,  26  and  30  since the major portion of it would be the semiconductor substrate, approximately 500 μm thick. Another electrically conductive layer  30  is applied to the bottom of epitaxial material  28 . The other layers ( 20 ,  22 ,  24 ,  26  and  30 ) are no more than a few microns thick in total. 
     The latest advances in quantum confined technology (strained-layer InGaAs/AlInGaAs/InP QWs) and the HTC dielectric technology of the present invention are integrated, as shown in FIG. 2, to achieve a high gain thermally robust semiconductor optical amplifier (SOA) at 1.3 μm and 1.5 μm. Each technology significantly improves the thermal performance and thus, by combining the two, an unprecedented thermal performance for SOAs is achieved. This enables the operation of a high gain SOA without a thermoelectric cooler, which is presently the limitation in laser module lifetime. 
     The temperature dependence of quantum well devices arises in large part due to thermal excitation of electrons out of the quantum wells (QWs) and over the barrier regions. Since only electrons within QWs contribute to optical gain, such thermal excitations of the carriers out of the QWs degrades performance at high temperature. The InGaAs/AlInGaAs material system allows one to build QWs with a large activation energy barrier to thermal emission (i.e., a large “conduction band offset”), which significantly reduces device temperature dependence. 
     An additional advantage of these materials is their compatibility with strained layer epitaxy. By growing quantum wells which are strained (&lt;2% lattice-mismatched), one can improve the optical gain characteristics. Strained layer epitaxy allows for the suppression of non-radiative recombination mechanisms such as Auger and intravalence band recombination processes which are significant at these wavelengths of interest. These non-radiative recombination mechanisms decrease the gain and are quite temperature sensitive in less thermally robust materials such as InGaAsP/InP which is the current material system for laser diodes used in the 1.3 to 1.6 μm wavelength range. 
     Under high current situations, the highly thermally conductive layer allows heat to be quickly dissipated near the active region of the SOA device. This technology has enabled high gain in a SOA using double heterostructure InGaAsP/InP material. The InGaAsP/InP double heterostructure is a thermally inferior material with respect to InGaAs/AlInGaAs/InP quantum wells in terms of the band offsets (380 meV versus 680 meV) and Auger recombination rates (worse by almost two orders of magnitude). 
     High performance lasers at 1.3 μm have been successfully demonstrated using the strained layer AlInGaAs QW. The quantity To is a parameter that describes the temperature sensitivity of a laser diode device. 
     In general the threshold gain varies in proportion to an exponential function of the temperature exp(T/To), where T is the temperature in degrees on the Kelvin scale. The exponential function represents a potentially very strong dependence on temperature when To is small: a doubling of To reduces any increases (as a ratio) in threshold due to temperature by the square root. By using AlInGaAs QWs, these lasers have demonstrated a doubling of To to 80 K versus an InGaAsP/InP multiple quantum well (MQW) with To˜40 K. To is defined as Ith(ΔT)=Itho(exp(ΔT/To)). Ith is the threshold current needed to turn on the laser diode. The temperature difference, ΔT, represents the temperature difference between the test temperature and, typically, the ambient temperature. Thus, a doubling of To reduces any increases (as a ratio) in threshold due to temperature by the square root. These lasers can operate without thermoelectric coolers and have excellent performance characteristics. Owing to the increased carrier level and the dependence of nonradiative recombination mechanisms (e.g., Auger and intravalence band absorption) on carrier and temperature, a high gain SOA is more difficult to make temperature insensitive than a laser diode. It is in this situation, that the real advantage of combining the InGaAs/AlInGaAs quantum well (QW) technology with the highly thermally conductive dielectric is apparent. 
     Multiple strained-layer quantum well technology can be used to fabricate a polarization independent SOA structure. By growing quantum wells which are strained (lattice-mismatched), one can preferentially select the optical polarization to be amplified. Changing the QW strain changes the amplified polarization state. By judiciously combining both tensile and compressively strained QWs, one can create amplifiers with polarization-insensitive gain characteristics. This attribute is essential for using components in conventional optical fiber systems, in which the optical polarization state varies in a random, time-dependent fashion. Lattice mismatch can give rise to compressive or tensile strain. Often in a semiconductor crystal lattice, the properties of the crystal are very dependent on the orientation of an applied field. In the case of tensile or compressive strain within the plane of the active layers of InGaAsP, AlInGaAs, or InGaAs the optical gain can be enhanced for the TE mode (applied compressive strain) or the TM mode (applied tensile strain). For example see M. A. Newkirk et al “1.5 μm Multiquantum-Well Semiconductor Optical Amplifier with Tensile and Compressively Strained Wells for Polarization-Independent Gain,” IEEE Photonics Technology Letters, vol. 4, no. Apr. 4, 1993, pp. 406-408. 
     The laser diode and edge-emitting light emitting diode applications share the same ridge waveguide configuration as in FIG.  2 . The only differences with a SOA are the end facets of the ridge waveguide. The laser diode would have a cleaved facet perpendicular to the waveguide; thus a cleaved facet mirror would be formed as in any conventional laser diode. The edge emitting light emitting diode would have one facet ( the one non-output facet) with significant optical loss to suppress lasing. 
     Another embodiment of the invention is the use of a specific high thermally conductive dielectric layer in a Bragg mirror for the purpose of reducing the operating temperature of a vertical cavity semiconductor laser diode. Heat generation leading to higher operating temperatures is an important problem to be solved in long wavelength (1.3-1.6 μm). In developing an appropriate mirror for a long wavelength vertical cavity, several important criteria need to be imposed. First, the mirror ideally would consist of highly thermally conductive dielectric materials while maintaining low optical loss. Second, the various mirror materials should posses large index differences so as to reduce the overall thickness of the mirror, which would reduce the overall thermal resistance. Third, the should have a very high reflectivity. Ideally, this reflectivity should be greater than 99.5%. The present invention is the first to combine the highly thermal conductivity material dielectric mirrors, that utilize AlN and hydrogenated amorphous Si, with the thermally robust AlGaInAs gain medium. 
     FIG. 3A illustrates the structure of the Bragg mirror,  100 , with the novel use of AlN (aluminum nitride) and hydrogenated amorphous Si as the pair of materials that comprise the Highly Thermally Conductive Bragg mirror layers. Bragg mirror  100  represents alternating pairs of materials, namely the AlN and the hydrogenated amorphous silicon, where the thickness of each layer is given by λ/(4 n) where λ is the wavelength of operation of the laser and n is the optical index of refraction. The function of  100  is to reflect the optical laser light back towards the active region  102 . Region  101  represents the p or n type semiconductor material such as InP or AlInGaAs. The function of  101  is to provide current carrier injection into region  102 . Region  102  represents the active region that is made up of quantum wells of AlInGaAs or InGaAsP. The function of  102  is to provide optical gain to the laser light that propagates back and forth between Bragg mirrors  100  and  104 . Region  103  represents the n or p type semiconductor material (opposite the type in  101 ). Region  103  provides current injection of the opposite carrier type to that of region  101 ; thus, by injecting carriers of opposite polarities into the active region  102 , a population inversion is created in  102  and thus optical gain is provided. 
     Region  104  is the second Bragg mirror comprised of semiconductor material either InGaAsP/InP pairs or GaAs/AlGaAs pairs of materials. Region  105  is the semiconductor substrate from which the mirror  104  and regions  103 , 102 , and  101  are grown. The substrate  105  can be either InP or GaAs. The ohmic contacts to provide electrical current to the device are provided by  106  and  107 . Region  108  is a heat sink, comprising a large metal mount, preferably copper or a diamond submount, that conducts away the heat generated by the active region  102  and regions  101 ,  103 ,  104 ,  105 ,  106 , and  107 . The highly thermally conductive Bragg mirror  100  conducts the heat away from the active region where the laser light is present more efficiently than another type of Bragg mirror such as another semiconductor Bragg mirror. 
     FIG. 3B shows the reflection spectrum of the Bragg mirror. The AlN was chosen for its high thermally conductivity and low optical loss. The hydrogenated amorphous silicon was chosen for its high optical index of refraction (˜3.5) and low optical loss. The peak reflectivity at 1.3 μm is approximately greater than 99%. The number of dielectric pairs used in the mirror was 4 pairs of AlN and amorphous Si (the substrate was GaAs). The mirror used in the calibration of the reflectivity measurement was a Newport ER.2 mirror. The specified mirror reflectivity at 1.3 μm is 98% according to the Newport catalog. The measured reflectivity appears to be high enough to begin fabrication of a high quality thermally robust long wavelength vertical cavity semiconductor laser. Of course, as the device is further developed, the reflectivity will be improved and optimized (i.e. more mirror pairs will be added as needed) to achieve the necessary performance. The reflection measurement was performed by using a white light source coupled into a 50 μm core/125 μm cladding multi-mode fiber and then coupled into a 3 dB multi-mode directional coupler. One of the output ports was connected to a GRIN lens collimator. The light was retroreflected from the mirror under test and maximized for coupling. The other port on the 3 dB coupler was connected to the input of a Hewlett-Packard Optical Spectrum Analyzer. The calibration mirror was the Newport ER.2 mirror. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.