Optically cooled platform for thermal management applications

Abstract

A semiconductor device comprising a waveguide having a core, said core having inserted therein one or more layers of nanoemitters.

Claims

1. A semiconductor device comprising a waveguide having a core, made of silicon nitride, sandwiched in between two opposingly located cladding layers, made of aluminum nitride; said core having inserted therein one or more layers of nanoemitters, wherein said nanoemitters, when excited with emission at certain wavelength, produce photoluminescence such that the short-wavelength anti-Stokes photoluminescence prevails over the long-wavelength Stokes photoluminescence.

2. The semiconductor device of claim 1, wherein said nanoemitters are quantum dots.

3. The semiconductor device of claim 2, wherein said quantum dots are made of CdSeS/ZnS.

4. The semiconductor device of claim 2, wherein said quantum dots are made from InP, CdSe, CdTe, PbS, and PbSe.

5. The semiconductor device of claim 1, wherein said nanoemitters are quantum wells.

6. The semiconductor device of claim 1, wherein said nanoemitters are quantum wires.

7. The semiconductor device of claim 1 further including a plurality of waveguides in contact with one or more layers of active electronic components; said plurality of waveguides are configured into one or more three-dimensional stacks of optically cooling planes with layers of active electronic components being cooled in between.

8. The semiconductor device of claim 1 further including a plurality of optical waveguides in contact with one or more layers of active electronic components; said optical waveguide of claim 1 implemented in a two-dimensional photonic-crystal waveguide platform as a slab photonic-crystal waveguide; said plurality of optical waveguides implemented in a plurality of two-dimensional photonic-crystal waveguide platforms; and said plurality of two-dimensional photonic-crystal waveguides arranged as three-dimensional stacks of photonic-crystal optically cooling planes with layers of active electronic components being cooled in between.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.

(2) FIG. 1 illustrates an energy-level scheme for a radiation-balanced laser.

(3) FIG. 2 illustrates a SiO.sub.2/AlN/Si.sub.3N.sub.4/AlN-based waveguide structure with embedded QDs for an embodiment of the present invention.

(4) FIG. 3A depicts a room temperature absorption spectrum of CdSeS/ZnS QDs in hexane.

(5) FIG. 3B shows temperature-dependent photoluminescence (PL) at 350 nm excitation of CdSeS/ZnS QDs in hexane.

(6) FIG. 3C shows temperature-dependent PL at 658 nm excitation of CdSeS/ZnS QDs in hexane.

(7) FIG. 4A is a cross-sectional SEM image of a waveguide with a QD layer in the middle of the Si.sub.3N.sub.4 core for an embodiment of the present invention.

(8) FIGS. 4B and 4C are optical microscope images (left) and the fluorescence from the corresponding area at 450 nm excitation (right). Top and bottom panels are top-down and cross-sectional views respectively.

(9) FIG. 5 is an SEM image of CdSeS/ZnS QDs spin-coated on Si.sub.3N.sub.4 at 200 rpm for 15 seconds for an embodiment of the present invention.

(10) FIG. 6 illustrates CdSeS/ZnS QDs clusters fluorescence on the glass at an excitation wavelength of 457 nm for an embodiment of the present invention

(11) FIG. 7 illustrates CdSeS/ZnS QDs fluorescence on a layer of SiO.sub.2 nanoparticles at an excitation wavelength of 457 nm for an embodiment of the present invention.

(12) FIG. 8A is a schematic illustration of the measurement setup for luminescent thermometry.

(13) FIG. 8B is a plot of a signal for various components vs. time.

(14) FIG. 9 illustrates photoluminescence thermometry signal phase change with ambient temperature.

DETAILED DESCRIPTION OF THE INVENTION

(15) Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.

(16) Cooling a semiconductor device using anti-Stokes photoluminescence presents many challenges. These include the requirements for the properties/quality of the material (such as high thermal conductivity, high efficiency of anti-Stokes luminescence, high external quantum efficiency, strong coupling between excitons and LOPs, and negligible background absorption) that are not easy to meet in one specific material.

(17) In one aspect, the present invention provides an alternative concept of a cooling hybrid device, where nanosized emitters (nanoemitters) or quantum well (QW) layers, quantum dot (QD) layers, quantum wires grown in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence are inserted into an optical waveguide structure made of a wider-bandgap thermally conducting material. The emitters may be optically pumped by the external laser emission and provide cooling, similar to the laser cooling scheme of rare-earth-doped glasses optically pumped with external laser light. This embodiment of the present invention overcomes the known problem of very low absorption of the pump light in the latter scheme by putting the emitters inside a resonant cavity and, thus, employing the effect of cavity-enhanced absorption of the pump light.

(18) The present invention, in an alternate embodiment, provides an optical cooling device in the form of an optical dielectric waveguide 200 that consists of an aluminum nitride (AlN) layer 210, Si.sub.3N.sub.4 layer 220 and an AlN layer 230 on a substrate 240 such as SiO.sub.2. A plurality of layers 250, 251 and 252 may be inserted into the core Si.sub.3N.sub.4 waveguide layer 220 as shown in FIG. 2. As further shown, layers 250, 251 and 252 are comprised of a plurality of nanoemitters, quantum dots, quantum wells, or quantum wires 260-265.

(19) The materials of the optical waveguide are characterized by very high thermal conductivity, and the waveguide structure will serve as an effective heat sink for active electronic components and active optical cooling plane applications.

(20) The wide-bandgap materials of the optical waveguide of the present invention have very low background absorption over a very wide wavelength range. The optical and thermal properties (for polycrystalline forms) of the materials are shown in Table 1.

(21) TABLE-US-00001 TABLE 1 Material properties of a prototype cooling device Material κ, W/(m .Math. K) n at 532 nm E.sub.g, eV λ at E.sub.g, nm Si.sub.3N.sub.4 30**   2.026* ~5 eV 248 AlN 140-180** 1.929* ~6 eV 206 SiO.sub.2 1.38 1.5 ~8 eV 155

(22) AlN thin film deposition can be done by numerous techniques which include physical vapor deposition (PVD), chemical vapor deposition (CVD), and DC/RF reactive sputtering. CVD has been demonstrated for both AlN and Si.sub.3N.sub.4 thin films. In an alternate embodiment, the entire structure of the optical waveguide shown in FIG. 2 may be fabricated in one run using CVD on a SiO.sub.2 substrate.

(23) For enhanced directional extraction of spontaneous emission out of the cooling device, PhCs may be used to control spontaneous emission due to the 2D PBG effect, not attainable in conventional dielectric-waveguide-based cavities. When implemented in a 2D PhC-waveguide platform, the concept of cooling 2D PhC semiconductor devices may be realized as slab PhC waveguides fabricated e.g in cubic photonic lattices and terminated with mirror facets. PhC waveguides can be formed by removing one or more rows of PhC air holes.

(24) In all embodiments, the cooling performance may be achieved by extracting a substantial part of the anti-Stokes spontaneous emission of the embedded nanoemitters from the cavity. Anti-Stokes spontaneous luminescence caused by single-photon phonon-assisted carrier excitation may be achieved using InP, CdSe, CdTe, PbS, and PbSe quantum dots (QDs).

(25) In active optical cooling applications, the external pumping light can be supplied to the cooling device through a properly aligned and rigidly attached output optical fiber of an external pigtailed semiconductor laser. Similarly, the light output can be collected into a fiber attached to the other end of the cooling device cavity.

(26) The geometry of a single device allows for the fabrication of an array of such devices on a common substrate, thus making a cooling semiconductor plane structure that can be co-fabricated with a layer of active electronic components being cooled, for example, using a Si-on-AlN wafer-bonding process. Furthermore, the cooling plane structures such as optical waveguide 200 may be organized into a 3D stack of optically cooling planes with layers of active electronic components being cooled in between. In the case of a 3D stack of optically cooling planes, the waveguide structure of the cooling planes may be placed between multiple layers of active electronics components being cooled, which makes the substrate in FIG. 2 unnecessary.

(27) The fabrication process for the cooling planes is compatible with CMOS technology. Moreover, the embodiments of the present invention are applicable to any waveguide structure, including cylindrical waveguide or conventional optical fiber.

(28) FIG. 4A shows a cooling device that consists of a symmetric SiO.sub.2/Si.sub.3N.sub.4/SiO.sub.2 optical dielectric waveguide grown 400 on Si substrate 420 with layers of commercially available CdSeS/ZnS QDs 440 inserted into the middle of Si.sub.3N.sub.4 core 450 which is made of opposing layers 410 and 412. With the SiO.sub.2 materials of the optical waveguide claddings 430 and 432 characterized by very high thermal resistivity, the optical cooling effect is created in the waveguide core with embedded QDs and detected by noncontact luminescent thermometry.

(29) CdSeS/ZnS QDs have been acquired with photoluminescence (PL) peak at 630 nm (FIG. 3B) and a double-peaked absorption at 575 nm and 615 nm near the bandgap edge (FIG. 3A) at room temperature corresponding to two closely spaced excitation levels in QDs, which is a necessary condition for anti-Stokes refrigeration. The QDs demonstrated relatively strong anti-Stokes PL as shown in FIG. 3C.

(30) The layer structure for an 8-μm-wide waveguide was 1-μm SiO.sub.2/6-μm Si.sub.3N.sub.4/1-μm SiO.sub.2 on a Si substrate 420 as shown in FIG. 4. The total thickness was slightly greater than 8 μm. QD layer 440 was embedded in the middle of Si.sub.3N.sub.4 core 450. It consisted of CdSeS/ZnS QDs sandwiched by 20 nm-thick SiO.sub.2 layers (not visible in the figure) which are expected to contribute to thermal insulation of the QDs from the Si.sub.3N.sub.4 core 450. After growing the bottom half structure of the waveguide, the sample was unloaded and was covered by the QD colloid and left in the atmospheric environment until the complete vaporization of hexane. Then, the sample was reloaded into the reactor and the top half of the waveguide was grown on it.

(31) FIG. 5 is an SEM image of CdSeS/ZnS QDs that were spin-coated on the bottom half of the waveguide structure. After spin coating, the solvent of the colloidal solution (hexanes) was removed by its evaporation at room temperature under the atmospheric condition. With no patterning of the Si.sub.3N.sub.4 layer, the particles and clusters are randomly distributed on the surface. Further experiments with quantum dots showed that they tend to cluster on flat surfaces. The clusters' fluorescence is shown on a glass surface in FIG. 6. A rough surface is required to prevent the agglomeration of the quantum dots during the drying process. Fluorescence of individual CdSeS/ZnS quantum dots was achieved on a layer of 50-nm-diameter SiO.sub.2 nanoparticles, as shown in FIG. 7. The embodiments of the present invention may be enhanced by optimization of the surface as well as colloidal concentration and spin rpm in coating for better uniformity.

(32) Other techniques such as dip coating may be used to improve the spatial uniformity of the QDs if the agglomerated QDs interact. If they don't interact, their agglomeration presents no obstacle.

(33) The optical cooling effect of the present invention has been demonstrated using a novel original version of the luminescent thermometry method of relative temperature measurements where laser cooling or heating effect correlated with a phase φ change of the fluorescence signal relative to the probe signal registered by a lock-in amplifier. It has been found that φ changing sign from positive to negative corresponded to local cooling effect induced by the pump laser.

(34) FIG. 8A is a schematic illustration of the measurement setup for the luminescent thermometry of the present invention. Components include pump laser 810 at 658 nm for pumping and probe laser 820 at 405 nm for probing. Also included are a photodetector 830, chopper 840, and lock-in amplifier 850. Probing and pumping laser beams are combined by dichroic beam-splitter 860 and coupled into waveguide 880 by objective 870. Probing and pumping lasers are modulated by chopper 840 in such a way that they exhibit 180-degree phase shift (one is on when the other is off). Filters block pumping and probing light and allow predominantly fluorescence and anti-Stokes emission collection.

(35) FIG. 8B represents signal plots vs time of the pump laser, QD temperature, probe laser, reference of the lock-in amplifier, and measured fluorescence/anti-Stokes emission vs. time.

(36) When the cooling by anti-Stokes effects begins with the QDs at the middle of the core, induced by the pumping starting at the time t.sub.1 (the first row in FIG. 8B), the temperature of the sample drops down (the second row in FIG. 8B). At time t.sub.2, when the pumping laser is off, the cooling process is terminated and the sample starts to warm due to heat transfer from the environment. At that moment, the probing laser is on (the third row in FIG. 8B) and the fluorescence can be measured. The fluorescence intensity decreases with increasing temperature as shown in FIG. 3B. The probing and pumping lasers cannot be turned on together because of the fluorescence and anti-Stokes emission interference.

(37) The slope of the fluorescence signal is related to the temperature change in the waveguide: negative slope corresponds to cooling by the pump and then warming due to the environment (the fifth row in FIG. 8B). In the case of the laser warming effect, the graph would be the opposite: a positive slope would indicate warming by the pump and then cooling due to the environment. Note, that the QD temperature signal and the corresponding fluorescence signal are shifted by a half period (T/2) relative to the probe signal, which means that the pump and fluorescent signals should be 90-degree shifted in phase.

(38) Lock-in amplifier 850 was used to determine such a phase change of the fluorescence signal relative to the original probe signal. The signal collected by the detector, S.sub.1=A.sub.1 sin (ωt+φ), can be written as
S.sub.1=P.sub.s,pump+P.sub.s,probe+S.sub.w,  (1)

(39) where P.sub.s,pump and P.sub.s,probe are the measured scattered powers from the pump and probe lasers in the setup, respectively, and S.sub.w is the fluorescence in the waveguide. The P.sub.s,pump and P.sub.s,probe are greatly attenuated by a band-pass filter to increase portion of S.sub.w collected by the photodetector. The reference signal S.sub.2 that the chopper generates for the lock-in amplifier with the angular frequency wand the phase shift set equal to zero can be written as S.sub.2=A.sub.2 sin (ωt). Ultimately, lock-in amplifier 850 measures the autocorrelation, A, expressed as
A=|A|.Math.exp(iφ)∝∫.sub.t.sup.t+TS.sub.1.Math.S.sub.2dt′˜∫.sub.t.sup.t+TS.sub.w.Math.S.sub.2dt′  (2)

(40) In general A is a complex number which is proportional to the amplitude of signal S.sub.w and indicating its phase shift φ. The amplitude of S.sub.w reflects the decay rate of the fluorescence that includes the information on cooling and warming rates of the given nanoemitters, directly related to anti-Stokes effects for laser cooling. In the Fourier transform of the fluorescence intensity decreasing with time (corresponds to laser cooling when the pump laser is on and to sample warming when the pump laser is off) as predicted in FIG. 5B, φ in Eq. (2) measured by the lock-in amplifier must be negative. In contrast, the fluorescence intensity increasing with time (corresponds to laser warming when the pump laser is on and to sample cooling when the pump laser is off) would induce positive φ.

(41) The evidence of optical cooling at elevated temperatures of the waveguide, is consistent with the strong temperature dependence of anti-Stokes photoluminescence as shown in FIG. 3C. The phase φ change with increasing temperature of the waveguide environment from 270 K to 350 K was registered. This dependence is shown in FIG. 9.

(42) Phase φ changing sign at the temperature about 320 K was observed which is direct evidence of local optical cooling. The physical mechanism can be explained in the following way. As can be seen in FIG. 3C, the PL peak occurs at 630 nm. The pump laser available for the experiments emitted at 656 nm. This corresponds to 0.078 eV anti-Stokes shift. Since the thermal energy kT at room temperature of 25.7 meV was much smaller than the anti-Stokes shift of 78 meV, the efficiency of phonon-exciton coupling was very low and insufficient for laser cooling. That problem was solved by heating the waveguide environment to the temperature above 320 K, matching kT with the anti-Stokes shift. This allowed phonon-exciton coupling to be enhanced and laser cooling to be observed.

(43) The strong temperature dependence of anti-Stokes PL shown in FIG. 3C is consistent with the predicted enhancement of anti-Stokes PL with increasing temperature. Near-unity quantum efficiency of QDs is the necessary condition for laser cooling. The room-temperature quantum efficiency of CdSeS/ZnS QDs over 50% for UV excitation was reported. Moreover, a ˜1.5 to ˜2 times increase in quantum efficiency has recently been reported in CdSe/CdS QDs with the excitation wavelength increasing from UV to Vis. These observations combined with the strong temperature enhancement of anti-Stokes PL (FIG. 3C), make near-unity quantum efficiency of CdSeS/ZnS QDs excited at 658 nm at the waveguide temperatures higher than 320 K a reasonable assumption.

(44) While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above-described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.