Patent Publication Number: US-2007096171-A1

Title: Semiconductor laser device that has the effect of phonon-assisted light amplification and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a semiconductor laser device and, more particularly, to a semiconductor laser device that has the effect of phonon-assisted light amplification and a method for manufacturing the same.  
      2. Description of Related Art  
      Indirect bandgap materials are not used in any existent electro-luminescence semiconductor laser as media for light amplification. The reason is that structures capable of producing sufficient light amplification have not yet been developed from indirect bandgap materials like silicon. Because silicon ICs develop fast, in order to expand the applications of silicon in the field of electro-optics, the development of silicon lasers is much demanded.  
      Recently, much research has been devoted to the silicon material itself or the light emission or even light amplification phenomenon of silicon material, e.g., porous silicon, extrinsic doping, silicon dioxide ion implantation, erbium-doped silicon dioxide, SiGe or SiSn alloy, nanometer grain silicon, quantum confinement structure, coated light emission organic layer, grown GaN, and so on. The above or other techniques, however, cannot be easily integrated with the ULSI technology, causing much difficulty in commercialization. On the other hand, because the technique of using structures such as defect loops or MOS junctions to accomplish high-efficiency light emission of silicon is consistent with the ULSI technology, electronic devices can be easily integrated with the light source on the same silicon chip. Although the above structures can achieve an external light emission efficiency of 10 −2 , no current-excited lasing phenomenon has ever been observed.  
      Photo-excited gain phenomenon of silicon material has been realized already, and photo gain phenomenon has been observed in silicon nanometer grains. DC-operated photo-excited silicon Raman lasers have currently been developed by the Intel. It is difficult for silicon materials to realize current-excited lasing phenomena. The reason is that Si is an indirect bandgap material having low electro-luminescence light emission efficiency. The light emission rate of direct bandgap materials is about 10 9 /s, the non-radiation rate thereof is about 10 5 /s. The non-radiation rate of silicon material is approximately equal to that of direct bandgap materials, but the radiation rate thereof is about 10 4 /s, only one tenth of the non-radiation rate. Therefore, how to increase the radiation rate and decrease the non-radiation rate of indirect bandgap materials like Si to achieve high-efficiency light emission and to further produce Si laser devices is a very important topic in this industry.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a semiconductor laser device that has the effect of phonon-assisted light amplification and a method for manufacturing the same, which produce laser light of silicon and enhance the light emission efficiency by that has the effect of phonon-assisted light amplification.  
      Another object of the present invention is to provide a semiconductor laser device that has the effect of phonon-assisted light amplification and a method for manufacturing the same, which conform to existent IC fabrication processes and thus facilitate commercialization.  
      Another object of the present invention is to provide a semiconductor laser device that has the effect of phonon-assisted light amplification and a method for manufacturing the same, whereby the manufactured semiconductor laser device has a simple structure, a small size, and an easy fabrication process. Therefore, the production cost can be reduced, and the market competitiveness can be enhanced.  
      To achieve the above objects, the present invention provides a method for manufacturing a semiconductor laser device that has the effect of phonon-assisted light amplification. The method comprises the steps of: providing a clean semiconductor silicon substrate; etching the semiconductor silicon substrate to remove a native oxide on the surface of the semiconductor silicon substrate; forming a silicon dioxide nanometer particle layer and a thin oxide on the semiconductor silicon substrate; and forming a conductive layer on the nanometer particle layer. The nanometer particle layer has a plurality of holes. The silicon dioxide nanometer particle layer is formed on the thin oxide, or the silicon dioxide nanometer particle layer is formed on the semiconductor silicon substrate and exposes in the atmosphere to form another thin oxide right on the semiconductor silicon substrate. Part of the surface of the semiconductor silicon substrate will be exposed out of the holes. The exposed surface of the semiconductor silicon substrate will contact the atmosphere to form the above thin oxide. An electrode layer can further be formed on the back face of the semiconductor silicon substrate to accomplish electro-luminescence of the semiconductor silicon substrate by means of current flow. Here, a MOS junction is utilized to enhance accumulation of carriers. Migration of metal atoms in the nanometer particle layer is utilized to form the MOS junction of nanometer structure so as to have the tunneling current phenomenon in nanometer range for current conduction. The light amplification efficiency can therefore be enhanced.  
      Besides, the present invention also provides a semiconductor laser device that has the effect of phonon-assisted light amplification, which comprises a semiconductor silicon substrate, a silicon dioxide nanometer particle layer, a thin oxide, a conductive layer, and an electrode layer. The silicon dioxide nanometer particle layer and the thin oxide are formed on the semiconductor silicon substrate. The silicon dioxide nanometer particle layer has a plurality of holes. The diameter of these holes is 0.5 nm to 1 μm. The silicon dioxide nanometer particle layer is on the thin oxide, or the thin oxide is exposed out of the holes of the silicon dioxide nanometer particle layer. The conductive layer is formed on top of the nanometer particle layer, and the electrode layer is formed on the back face of the semiconductor silicon substrate so that a voltage can be applied across the nanometer particle layer to drive atoms in the conductive layer to migrate to the oxide exposed out of the surface of the semiconductor silicon substrate. Carrier confinement in nanometer range can thus be achieved to enhance the electro-luminescence efficiency of the semiconductor laser device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which:  
       FIG. 1  is a flowchart of a method for manufacturing a semiconductor laser device using phonon-assisted light amplification technique of the present invention;  
       FIGS. 2A  to  2 E are cross-sectional views showing the steps of the method for manufacturing a semiconductor laser device of the present invention;  
       FIG. 3  is a voltage versus current diagram before and after migration of silver atoms of the semiconductor laser device of the present invention;  
       FIG. 4  is an optical power versus current diagram of the semiconductor laser device of the present invention;  
       FIG. 5  is a light emission spectrum diagram under different currents of the semiconductor laser diode of the present invention;  
       FIG. 6  is a more detailed light emission spectrum near the center of the spectrum shown in  FIG. 5 ; and  
       FIG. 7  is a light emission spectrum diagram measured at two different times under the same current of the semiconductor laser diode of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention provides a semiconductor laser device that has the effect of phonon-assisted light amplification and a method for manufacturing the same, which makes use of the DC-operated phonon-assisted light amplification technique corresponding to silicon energy gap at room temperature. The phonon-assisted stimulated emission mechanism of silicon material is similar to the stimulated Raman scattering mechanism, both emitting photons and phonons. The stimulated Raman scattering, however, absorbs a high-energy photon to emit a lower-energy photon and phonon. The phonon-assisted stimulated emission emits photons and phonons through the recombination of electron-hole pair. Moreover, the phonons generated by the phonon-assisted light emission are at the band edge because of the momentum difference between electron and hole. The present invention utilizes nanometer particles to provide carrier confinement so that injected electrons and holes can easily form excitons at the junction of silicon and silicon dioxide, hence enhancing phonon-assisted light emission to produce light amplification. The idea of using nanometer structure to achieve carrier confinement can derive several kinds of designs of nanometer structure, and applies to all indirect bandgap materials including element and compound semiconductors such as Si, Ge, SiGe, SiC, GaP, and AlAs, and can further cover several kinds of wavelengths including communication bands, visible light, and UV light.  
       FIG. 1  is a flowchart of a method for manufacturing a semiconductor laser device using phonon-assisted light amplification technique of the present invention. First, a clean semiconductor silicon substrate is provided (Step S 100 ). The semiconductor silicon substrate is then etched to remove the native oxide on the surface of the semiconductor silicon substrate (Step S 200 ). Next, a silicon dioxide nanometer particle layer is formed on the semiconductor silicon substrate (Step S 300 ). Subsequently, the sample is exposed in the atmosphere and a thin oxide is formed on the surface of the semiconductor silicon substrate because oxygen and moisture can reach the surface through the holes among the silicon dioxide nanometer particle (Step S 400 ). Finally, a conductive layer is formed on the nanometer particle layer (Step S 500 ). A semiconductor laser device is thus formed.  
       FIGS. 2A  to  2 E are cross-sectional views showing the steps of the method for manufacturing a semiconductor laser device of the present invention. First, as shown in  FIG. 2A , a 2-inch n-type semiconductor silicon substrate  10  manufactured by the float zone method is used. The carrier lifetime is 500 μs to 1 ms. Acetone, methyl alcohol, and deionized water are separately used to clean the surface of the semiconductor silicon substrate  10 . An oxide etchant is then used to remove the native oxide on the surface of the semiconductor silicon substrate  10 .  
      Next, as shown in  FIG. 2B , silicon dioxide nanometer particles are suspended in an isopropanol solvent to make a silicon dioxide nanometer particle suspension solution, which is then spin-coated on the surface of the cleaned semiconductor silicon substrate  10 . Here, the diameter of the silicon dioxide nanometer particles is 8 to 12 nm. The silicon dioxide nanometer particles occupy about 25.5 wt % of the original solution, and the solvent is isopropanol or methyl alcohol. If water is used as the solvent, the silicon dioxide nanometer particles cannot be coated on the semiconductor silicon substrate. After spin-coating, a soft bake at 90° C. is preformed for 2 min to remove the solvent (isopropanol or methyl alcohol) so as to form a silicon dioxide nanometer particle layer  20  on the surface of the semiconductor silicon substrate  10 .  
      As shown in  FIG. 2C , a 200 nm conductive metal such as Al is evaporated at the back face of the semiconductor silicon substrate  10  to be used as an electrode layer  30 , forming a conductive end of the semiconductor silicon substrate  10 . The thickness of the electrode layer  30  is 100 nm to 500 nm.  
      Subsequently, as shown in  FIG. 2D , because the exposure step has a significant influence to the performance of the manufactured device, the above device is placed in the atmosphere for 5 days to form several holes in the silicon dioxide nanometer particle layer  20 . Oxygen and moisture can therefore touch the surface of the semiconductor silicon substrate  10  to form a thin oxide  11  of about 0.5 nm to 5 nm thick after a period of time.  
      Finally, as shown in  FIG. 2E , silver paste is coated on the silicon dioxide nanometer particle layer  20  to be used as a conductive layer  40 . The coated area is about 1 mm 2 . The size of the sliced device is 4 mm 2 . A gold wire is bonded to the conductive layer  40  and connected to a current source.  
      The manufactured semiconductor laser device that has the effect of phonon-assisted light amplification according to this embodiment of the present invention is a MOS junction diode having nanometer structures, as shown in  FIG. 2E . The semiconductor laser device comprises a semiconductor silicon substrate  10 , a silicon dioxide nanometer particle layer  20 , an electrode layer  30 , and a conductive layer  40 . The thickness of the silicon dioxide nanometer particle layer is about 100 nm, and can be 0.5 to 1000 nm. The silicon dioxide nanometer particle layer  20  is disposed between the conductive layer  40  and the semiconductor silicon substrate  10  to have an insulation effect. The silicon dioxide nanometer particle layer  20  has several holes  21 , which can be circular or bar-shaped, and of uniform or nonuniform sizes. The diameter of the holes can be 0.5 nm to 1 μm. Part of the surface of the semiconductor silicon substrate  10  is exposed out of the holes. The exposed surface of the semiconductor silicon substrate  10  has a thin oxide  11 . The current can flow through the conductive layer  40 . The material of the conductive layer  40  can be metal, doped semiconductor, or doped dielectric. Electro-luminescence of silicon is accomplished by means of current flow. There are two kinds of electro-luminescence: DC and non-DC. The back face of the semiconductor silicon substrate  10  is coated with Al to be used as the electrode layer  30 . A voltage can therefore applied across the nanometer particle layer  20  to form a MOS junction for carrier accumulation. Through migration of metal atoms of the nanometer particle layer  20 , metal nanometer wires are formed to form a MOS junction with nanometer structures. The phenomenon of tunneling current happens in nanometer range to conduct current, thereby enhancing the light emission efficiency of the semiconductor laser device. Besides, the thin oxide can first be formed on the semiconductor silicon substrate  10 , and the silicon dioxide nanometer particle layer is then formed on the thin oxide to achieve the same effect.  
      After a voltage is applied across the semiconductor laser device, silver atoms of the conductive layer will be attracted by the electric field to move among nanometer particles so as to form silver nanometer wires and reach the thin oxide generated on the semiconductor silicon substrate that is placed in the atmosphere. The contacts will form a MOS tunneling current junction.  
       FIG. 3  is a voltage versus current diagram before and after migration of silver atoms of the semiconductor laser device of the present invention. Before the formation of silver nanometer wires, there is only a small current. When the device is biased above 7.5 V, a large current flows through, meaning the silver nanometer wires have formed a closed circuit. Hereafter, the current keeps stable, and the voltage-current relationship is the same as that of a common MOS tunnel diode.  
      Not all regions covered by silver paste have currents flowing through them. Currents only flow through the regions with silver nanometer wires. Other regions keep insulating. Silicon below the part of the thin oxide of the semiconductor silicon substrate touched by the silver nanometer wires will have a larger potential. Therefore, more energy band bending of the conduction band and the valence band in the semiconductor silicon substrate will be formed in this region. Majority carriers will thus accumulate in this region to achieve 3D confinement of majority carriers. Moreover, the tunneling current will bring minority carriers for this region, and other insulating regions will have no minority carriers, hence achieving confinement of minority carriers. The above two kinds of confinement cause overlap of the wave functions of electron and hole in space so that phonons can more easily interact with electron-hole pairs to accomplish the light emission effect and meet the requirement of conservation of momentum of the light emission mechanism of indirect bandgap materials. Therefore, this confinement region can provide light amplification through current excitation.  
       FIG. 4  is an optical power versus current diagram of the semiconductor laser device of the present invention. As can be seen from the figure, there is an obvious change of efficiency at 56 mA (corresponding to a current density of 7.13 A/cm 2 ).  FIG. 5  is a light emission spectrum diagram under different currents of the semiconductor laser diode of the present invention. The spectrum at the threshold current is a spontaneous emission spectrum, showing a smooth bell curve identical to photoluminescence and electro-luminescence spectra. When the current exceeds the threshold current, many peaks appear in the spectrum, meaning there is light amplification caused by stimulated emission in many frequency bands. All the above operations and measurements are under continous-wave (cw) situation and conducted at room temperature. The measurement had been repeated for several weeks. Moreover, the time period of each operation keeps for 5 to 10 hours.  
      These peaks of light amplification primarily exist near the center of the spontaneous emission spectrum because of the larger optical gain at the center than at two ends of spectrum.  FIG. 6  is a more detailed light emission spectrum near the center of the spectrum shown in  FIG. 5 . As can be more clearly seen in this figure, many peaks appear after the current exceeds the threshold current.  
       FIG. 7  is a light emission spectrum diagram measured at two different times under the same current (80 mA) of the semiconductor laser diode of the present invention. The reason is that there is no special design of resonance cavity in the semiconductor laser device. This phenomenon is similar to a random laser. Light travels a certain distance in the silicon material and goes back to the light emission region to gain light amplification. The reflection faces in the optical path can be silver paste conductive layer, space between particles in the nanometer particle layer, or silver nanometer wires. Light is reflected back to the regions for carrier confinement and light emission to obtain optical gain so as to achieve light amplification. Therefore, the amplification of light is accomplished by means of phonon-assisted stimulated emission. If the laser cavity is formed, then the emission spectrum will not be random. Instead, the lasing wavelength/spectrum can be controlled by the laser cavity.  
      The present invention has successfully utilized the current conduction structure of a nanometer structure MOS junction diode to produce laser lights at the infrared band corresponding to the energy gap of the silicon material.  
      In this nanometer structure MOS junction diode, the minority-carrier lifetime change with the injection current. It is estimated that the radiative-recombination rate is about ten times that of silicon substrate. The internal light emission efficiency can be higher than 60%. The reason why the light emission efficiency increases is that the nanometer particles provide confinement of carriers in space to more easily form excitons. Therefore, the phonon-assisted stimulated emission mechanism can be enhanced to accomplish the electro-luminescence phenomenon.  
      Besides, the present invention uses the coating of silicon dioxide nanometer particles to form an insulating layer that has no any damage to the silicon material. These nanometer particles won&#39;t deteriorate under the conventional semiconductor manufacturing processes, and conform to existent IC manufacturing processes, hence facilitating monolithic integration of electronic chip and light emission device and expanding the application range of silicon chip and silicon material. Moreover, the present invention has a simple structure, an easy manufacturing process, a low production cost, and a small volume, and can be integrated with the IC industry, hence being of large practical value. Furthermore, the SiO2 nanoparticles can be applied to other types of substrates for light emission at the bandgap of other semiconductors different from Si. Because the SiO2 nanoparticles cause no any change to the substrate underneath, this technique can be easily used for other types of semiconductors.  
      Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.