Abstract:
The present invention relates to a method of mass fabricating a hyperboloid-drum element which is uniform in size and with the diameter of an active layer (active region or gain medium) ranging from tens of nm to less than a few μm, and to an element fabricated thereby. According to the present invention, the fabrication method of the hyperboloid-drum element comprises forming an epitaxial layer which includes an n-type semiconductor joined with a p-type semiconductor on a substrate and an active region near a border region and a boundary between the n-type semiconductor and the p-type semiconductor; and etching the epitaxial layer into a shape of the hyperboloid-drum having the minimum diameter at the active region by an ion-beam etching method. The hyperboloid-drum element fabricated in accordance with the present invention has advantages of uniformity in size and good reproducibility.

Description:
FIELD OF THE PRESENT INVENTION  
       [0001]     The present invention relates to a method of fabricating a structure with a size ranging from nanometers to micrometers, and in particular, to a mass fabrication method for a hyperboloid-drum element of uniform size with an active layer (region or gain medium) diameter ranging from tens of nm to less than a few μm and the element fabricated thereby.  
       BACKGROUND OF THE INVENTION  
       [0002]     Recently, examination of elements of nanoscale size has been regarded as an important topic in the scientific field due to their unusual electrical and optical characteristics. At present, worldwide studies on nanoscale phenomena have been extensively carried out and published, especially regarding the fabrication methods and characteristics of an electronic element or an optical device in nanometer scale consisting of the quantum dot.  
         [0003]     For the case of a semiconductor laser in particular, the fabrication of a nano-laser using the quantum dot needs a reflecting mirror placed on top and bottom of the quantum dot made by a self-assembled growth (SAG) method in the active region of a conventional laser structure where resonation occurs. And the nano-laser has been made by electron beam lithography and etching having a high resolution among the conventional fabrication method for the semiconductor laser.  
         [0004]     However, it is difficult to achieve ideal characteristics because of the difficulties in acquiring process reproducibility and uniformity in the size, the shape, and the position of the quantum dot during the self-assembled growth process. In particular, for the fabrication of a single photon source (SPS), though it is ideal to locate a single quantum dot in a microcavity, i.e., the active region of the element, there is no certain method to achieve the ideal condition in the mass scale. Furthermore, it is difficult to achieve the metallic deposition process on the cap for electrical pumping due to the small size of MESA ranging from tens of no to hundreds of nm formed by the self-assembled growth method.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with the present invention a reproducible method is provided to mass produce a hyperboloid-drum structure using ion-beam etching.  
         [0006]     Further in accordance with the present invention a hyperboloid-drum structure with an active region of adjustable size is provided to be used for producing an optical element and an electronic element in nanoscale.  
         [0007]     In an exemplary embodiment of the present invention, a hyperboloid-drum element includes a p-type semiconductor and an n-type semiconductor joined at their boundary, and an active region which is formed near a border region including the boundary. In order to have a minimum diameter at the active region, the hyperboloid-drum element is in a shape such that the diameter decreases gradually from the outer end of the n-type semiconductor and the p-type semiconductor as it nears the border region.  
         [0008]     An intrinsic semiconductor can be placed between the n-type semiconductor and the p-type semiconductor and joined with them. The active region in this case is formed near the border region which includes the intrinsic semiconductor and each of the boundaries of the intrinsic semiconductor between the n-type semiconductor and the p-type semiconductor.  
         [0009]     The diameter of the active region is featured with the range of tens of nm to several μm. A base material can be selected from the group consisting of GaAs, GaN, ZnSe, SiC, and InP.  
         [0010]     In another exemplary embodiment of the present invention, a hyperboloid-drum element comprises an active region having a quantum well structure; an n-type barrier layer and a p-type barrier layer formed on both surfaces of the active region, respectively; an n-type distributed Bragg reflector (DBR) placed outside of the n-type barrier layer; and a p-type distributed Bragg reflector (DBR) placed outside of the p-type barrier layer and in a shape such that a minimum diameter occurs at the active region and the diameter decreases gradually with distance from each of the distributed Bragg reflectors toward the active region so that the quantum dot is positioned at the active region.  
         [0011]     The diameter of the active region is featured with the range of tens of nm to several μm, and GaAs is the base material of the active region.  
         [0012]     Also, an n-type AlGaAs layer is provided as the n-type barrier layer, and a p-type AlGaAs layer is provided as the p-type barrier layer. The distributed Bragg reflector can be deposited with alternating layers of Al 0.3 Ga 0.7 As with a high refractive index, and Al 0.9 Ga 0.1 As with a low refractive index, and each of the layers is λ/4 in thickness.  
         [0013]     In an exemplary embodiment of the present invention, a fabrication method of the hyperboloid-drum element comprises forming an epitaxial layer which is made by joining an n-type semiconductor and a p-type semiconductor on the substrate and includes an active region near a border region and a boundary between the n-type semiconductor and the p-type semiconductor; and etching the epitaxial layer into the shape of the hyperboloid-drum having the minimum diameter at the active region by the ion-beam etching method.  
         [0014]     The substrate can be made of a base material selected from the group consisting of GaAs, GaN, ZnSe, SiC, and InP.  
         [0015]     The etching the epitaxial layer includes manufacturing a photoresist mask using photolithography, and etching the epitaxial layer into a shape of the hyperboloid-drum through the manufactured mask. It is preferable in the etching process to have an acute angle between the incident ion beam and the normal direction of the substrate where the epitaxial layer is formed.  
         [0016]     Corrosive gases such as BCl 3  and Cl 2  can be used in the etching process, and an inert gas ion beam is preferable as the ion beam.  
         [0017]     After the ion-beam etching, wet etching can be carried out to prevent damage to the sample surface by the ion-beam etching process. To prevent natural oxides from forming on the surface, a surface treatment can be done by ammonium sulfide treatment. And in order to prevent natural oxides from forming on the surface after the ion-beam etching, a plasma treatment can be possible by using one or more gases selected from the group consisting of N 2 , H 2 , and NH 3 .  
         [0018]     In another exemplary embodiment of the present invention, a fabrication method of the hyperboloid-drum element comprises forming an epitaxial layer having the active region on a substrate, and etching the epitaxial layer into the shape of the hyperboloid-drum having the minimum diameter at the active region by the ion-beam etching method. The forming the epitaxial layer includes forming an n-type distributed Bragg reflector on an n +  doped substrate; forming an n-type barrier layer on top of the n-type distributed Bragg reflector; forming an active region with a quantum well on top of the n-type barrier layer; forming a p-type barrier layer on top of the active region; and forming a p-type distributed Bragg reflector on top of the p-type barrier layer.  
         [0019]     The etching the epitaxial layer includes manufacturing a photoresist mask by using photolithography; and etching the epitaxial layer into the shape of the hyperboloid-drum through the manufactured mask. It is preferable in the etching process to have an acute angle between the incident ion beam and the normal direction of the substrate where the epitaxial layer is formed.  
         [0020]     The fabrication method of the hyperboloid-drum element includes further coating the outer surface of the p-type distributed Bragg reflector with polyimide and to planarize it; and forming an electrode by etching the polyimide and depositing Cr/Au.  
         [0021]     In the fabrication method of the hyperboloid-drum element of the present invention described above, it is possible to adjust the size easily and to acquire reproducibility for mass production due to etching the active region with the quantum well into the size of tens to hundreds of run without using the quantum dot in the SAG method. Furthermore, it is easy to deposit a metal electrode on the top surface of the hyperboloid-drum because the top surface can be made in microscale, though the active region of the hyperboloid-drum structure in this case is in nanoscale. Therefore, it is possible to do electric pumping as well as optical pumping. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0023]      FIG. 1  is a schematic diagram of a hyperboloid-drum element according to an exemplary embodiment of the present invention.  
         [0024]      FIGS. 2A  to  2 E are process diagrams showing a sequence to fabricate a hyperboloid-drum element according to an exemplary embodiment of the present invention.  
         [0025]      FIGS. 3A and 3B  are schematic diagrams which define a forming angle of a hyperboloid depending on an angle between an incident ion beam and a substrate in a fabrication process according to an exemplary embodiment of the present invention.  
         [0026]      FIG. 4  is a schematic diagram of the chemically assisted ion-beam etching system used in an experiment for the present invention.  
         [0027]      FIG. 5  is a photograph showing the hyperboloid-drum device according to an exemplary embodiment of the present invention.  
         [0028]      FIG. 6  is a graph which shows variation of sidewall angle (α) depending on the variation of angle (θ) between an incident ion beam and a substrate in a chemically assisted ion-beam etching process.  
         [0029]      FIG. 7  is a graph showing that shapes of the waist (an active region) of a hyperboloid-drum element and a top surface region (where metal is to be deposited) vary with the variation in size of the CAIBE photoresist (PR) mask, depending on angle (θ) between an incident ion beam and a substrate.  
         [0030]      FIG. 8  shows scanning electronic microscope images of a hyperboloid-drum fabricated according to an exemplary embodiment of the present invention.  FIG. 8 ( b ) is a magnified image of the active region shown in  FIG. 8 ( a ), and  FIG. 8 ( d ) is a magnified image of the active region shown in  FIG. 8 ( c ).  
         [0031]      FIG. 9  is a graph which shows variation of an optical power versus bias current of a hyperboloid-drum element with an active region diameter of 600 nm fabricated according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]     Reference will now be made in detail to embodiments of the present invention, examples of which may be illustrated in the accompanying drawings.  
         [0033]      FIG. 1  is a schematic diagram of the hyperboloid-drum element according to an exemplary embodiment of the present invention.  
         [0034]     Referring to  FIG. 1 , a hyperboloid-drum element has an active region  33  with a quantum well structure, and an n-type barrier layer  31  and a p-type barrier layer  35  are placed on both surfaces of the active region  33 , respectively. And, ann-type distributed Bragg reflector (DBR)  20  is placed on the outside of the n-type barrier layer  31 , and a p-type distributed Bragg reflector (DBR)  40  is placed on the outside of the p-type barrier layer  35 .  
         [0035]     The active region  33  has a quantum well structure such that both surfaces of a non-doped GaAs layer are surrounded with the barriers  31 ,  35  including the non-doped AlGaAs layer with a higher energy level and which restricts the holes and the electrons coming through the distributed Bragg reflectors  20  and  40  doped to be n-type and p-type, respectively.  
         [0036]     The n-type distributed Bragg reflector  20  has a structure with alternating layers of the deposited Al 0.3 Ga 0.7 As layer  21  and  23  with a high refractive index, and the Al 0.9 Ga 0.1 As layer  22  and  24  with a low refractive index. Similarly, the p-type distributed Bragg reflector  40  has a structure with alternating layers of the deposited Al 0.3 Ga 0.7 As layer  41  and  43  with a high refractive index, and the Al 0.9 Ga 0.1 As layer  42  and  44  with a low refractive index. Each of these layers is λ/4 in thickness, and is preferably formed with a linearly varying aluminum mole fraction in order to reduce the series resistance between them.  
         [0037]     On the other hand, the structure can be deposited by a metal organic chemical vapor deposition (MOCVD) method on the n+ doped GaAs substrate  10 . And, the structure is in the shape of the hyperboloid-drum which has a minimum diameter at the active region and of which diameter decreases gradually away from the distributed Bragg reflectors  20  and  40  or the substrate  10  toward the active region  33  so that the quantum dot is positioned at the active region. Such a structure of the hyperboloid-drum can be fabricated by ion-beam etching.  
         [0038]     The hyperboloid-drum made by the etching process reduces the active region  33 , and can therefore lower the threshold current necessary to start the emission in the laser device with such a structure. In particular, when the active region  33  is fabricated as small as nanoscale to be a quantum dot, the quantum restriction effect occurs, and such a device can be used in the application for a single photon source and a single electron transistor.  
         [0039]     An AuGe/Ni/Au layer deposited on the bottom surface of the n-type GaAs substrate  10  forms an n-type electrode  12 , and the Cr/Au layer deposited on the top surface of the p-type distributed Bragg reflector  40  forms a p-type electrode  53 . After the p-type distributed Bragg reflector  40  is coated with polyimide and planarized, the p-type electrode  53  is made by deposition of the Cr/Au layer on the etched top surface of the p-type distributed Bragg reflector  40 .  
         [0040]     In the element with the etched shape hyperboloid-drum, the size of the uppermost layer is microscale where the metal electrode is deposited for electrical pumping, while the active region  33  is in nanoscale. Therefore, it is possible to use a conventional photolithography process for patterning to deposit the metal electrode. In the case of a cylinder shape instead of the hyperboloid-drum shape, it is extremely difficult to use the patterning on the surface in nanoscale by the conventional photolithography process due to the same diameter of the uppermost layer with that of the active region  33  in nanoscale. Therefore, the etched shape of the hyperboloid-drum makes it possible to use optical pumping immediately after the etching process, and the electrical pumping after depositing the electrode by the post process.  
         [0041]      FIGS. 2A  to  2 E are process diagrams showing a sequence to fabricate the hyperboloid-drum element according to an examplary embodiment of the present invention.  
         [0042]     In this embodiment, an epitaxial layer with the active region  33  is formed on the substrate. The hyperboloid-drum element is made by the ion-beam etching process to shape the epitaxial layer into the hyperboloid-drum of which minimum diameter is occurred at the active region  33 . The epitaxial layer is deposited on the substrate by the metal organic chemical vapor deposition (MOCVD) method.  
         [0043]     Referring to  FIGS. 2A  to  2 E, the formation of the epitaxial layer undergoes the following steps.  
         [0044]     First, the n-type distributed Bragg reflector  20  is formed on the n+ doped substrate  10 . To form the n-type distributed Bragg Reflector  20 , each λ/4 thick layer is deposited with alternating Al 0.3 Ga 0.7 As layers  21  and  23  with a high refractive index, and Al 0.9 Ga 0.1 As layers  22  and  24  with a low refractive index. Each layer can be formed with a linearly varying aluminum mole fraction in order to reduce the series resistance between them.  
         [0045]     Then, the active region  33  is formed on the n-type distributed Bragg reflector  20 . The active region  33  has the quantum well structure such that the AlGaAs layers as the barriers are placed on both sides of the non-doped GaAs layer.  
         [0046]     Then, the p-type distributed Bragg reflector  40  is formed on the active region  33 . To form the p-type distributed Bragg reflector  40 , each λ/4 thick layer is deposited with the alternating Al 0.3 Ga 0.7 As layer  41  and  43  with a high refractive index and Al 0.9 Ga 0.1 As layer  42  and  44  with a low refractive index. Each layer can be formed with a linearly varying aluminum mole fraction in order to reduce the series resistance between them.  
         [0047]     An n-type electrode  12  is deposited on the outer surface of the substrate with the epitaxial layer made by the process. To form the n-type electrode  12 , an AuGe/Ni/Au layer is deposited on the outer surface of the substrate  10 . Heat treatment is carried out to form the Ohmic contact in the temperature range of 400° C. to 500° C., for example at 425° C. where the AuGe, Ni, and Au are alloyed.  
         [0048]     Then, in order to etch the layered structure into the shape of the hyperboloid-drum, a photoresist mask is made by first using photolithography, and then the epitaxial layer is etched as illustrated in  FIG. 2E  with the photoresist mask by the ion-beam etching process.  
         [0049]     Ion beam etching systems such as RIE (reactive ion etching), CAIBE (chemically assisted ion beam etching), and ICP (inductive coupled plasma) are used. The basic components of the system are a vacuum chamber and an ion generator by an imposed DC or RF bias. The essence of these etching processes is the dry etching process that etches a sample by using the kinetic energy of the ion decomposed from the gas through the ion generator. Due to the linear motion of the ions in general, the etched shape can be changed by the change of the angle between the ion beam and the sample.  
         [0050]     In order to etch the layered structure into the shape of the hyperboloid-drum in this embodiment, the target object to be etched is tilted with a given angle (θ) from the incident direction of the ion beam. The tilt angle θ is defined as the angle between the incident direction of the ion beam and the normal direction of the substrate, and can be properly selected in the range 0° to 90° to etch the layered structure into the shape of the hyperboloid-drum.  
         [0051]     Also, the photoresist mask is patterned circularly by using the photoresist in this embodiment, and it plays a role of an etching mask, i.e., the etching is carried out on the region not covered with the mask and the region covered with the mask is not etched.  
         [0052]     In the etching process, corrosive gases such as BCl 3  and Cl 2  are chemically used for the etching by an argon ion (Ar + ) beam. The corrosive gases play a role of helping the ion etching by reacting chemically with the surface of the sample such as GaAs or AlGaAs to form a chemical compound which is easily broken and separated by the ion beam. The role of these corrosive gases can have an influence on the etching rate and the roughness of the etched surface.  
         [0053]     The shape of the hyperboloid-drum can be etched by adjusting the angle between the ion beam and the substrate, the temperature of the sample, the distance between the ion source and the sample, and the flow rate of the corrosive gas.  
         [0054]     After the chemically assisted ion-beam etching process, wet etching is carried out slightly to compensate for the damage to the sample surface resulting from the chemically assisted ion-beam etching process. To prevent natural oxides from forming on the surface, a surface treatment can be done by plasma treatment using gases such as N 2 , H 2 , or NH 3 , or by an ammonium sulfide treatment using various solutions. Combination of the plasma treatment and the ammonium sulfide treatment can also be possible.  
         [0055]     After this process, the polyimide  51  is coated on the whole sample, and planarized to deposit the metal electrode on the hyperboloid-drum element. Then, the p-type electrode  53  is made by the deposition of the Cr/Au layer on the etched top surface after etching the polyimide  51  to expose the top surface of the element.  
         [0056]      FIGS. 3A and 3B  are schematic diagrams defining the forming angle of the hyperboloid that depends on the angle between the incident ion beam and the substrate.  
         [0057]     In  FIG. 3 , α is the angle of the sidewall inclination of the element measured from the substrate normal. The etched sidewall has an outward-tapered (α&lt;0°) shape when the tilt angle θ is small, as shown in  FIG. 3A . Increase in the angle θ reduces the magnitude of the sidewall angle α and it is possible to have a vertical sidewall (α=0°) at a certain tilt angle θc. The inward-tapered sidewall profile (α&gt;0°) shown in  FIG. 3B  is obtained when the tilt angle θ is bigger than θc.  
         [0058]     The bottom part in  FIG. 3B  is the shadow region  60 , which is an etched surface under the shadow of the mask for the ion beam. This shadow effect is used to obtain a nano-structure of the hyperboloid-drum in accordance with the present embodiment: a combination of the normally etched inward-tapered region and the outward-tapered shadow region results in the nano-structure of the hyperboloid-drum. H is the total height of the etched MESA, i.e., the height of the hyperboloid-drum, while h is the height from the active region to the top surface excluding the shadow region.  
         [0059]     Although the embodiment describes the fabrication of a hyperboloid-drum structure by etching GaAs as a base material, the present invention is not limited to the base material and includes every structure that can be made to the hyperboloid-drum shape by chemically assisted ion-beam etching. Therefore, it is possible to fabricate the hyperboloid-drum structure from the base material such as GaN, ZnSe, SiC, and InP, which falls within the spirit and scope of the present invention.  
       EXPERIMENTAL EXAMPLE  
       [0060]     The CAIBE (Chemically assisted ion-beam etching) system for the experiment to make the element of the hyperboloid-drum is schematically shown in  FIG. 4  and will be described in detail hereinafter. The system has a dual-grid Kaufman-type ion source  72  with a diameter of 3 cm.  
         [0061]     For this experiment, the substrate fixed to the substrate holder  76  is rotated at 25 rpm, and its temperature is kept constant to assure etch uniformity and reproducibility. The tilt angle θ, which is the angle between the incident ion beam and the normal direction of the substrate, is adjusted to achieve the desired etched sidewall profile.  
         [0062]     Also, the system has four nozzles  74  for gas injection, which are located near the substrate. The tips of the nozzles  74  are tilted with the substrate, so that the geometry of gas feeding does not change with the tilt angle. The gas flow rates for Ar, Cl 2 , and BCl 3  are 5, 2, and 3 sccm, respectively.  
         [0063]     While changing the tilt angle θ for a given beam energy and current, the distance between the substrate and ion source is kept constant at 13 cm to maintain the same beam profile.  
         [0064]     The system has a load-lock chamber  75  and a turbomolecular pump CTMP)  78 , which is rotated at 26,700 rpm. The background pressure is ˜1×10 −6  Torr, and the pressure for CAIBE is ˜5.2×10 −4  Torr.  
         [0065]     The hyperboloid-drum element is fabricated by the CAIBE method using the experiment system and is shown in  FIG. 5 .  
         [0066]     The structure of the hyperboloid-drum device was fabricated on an n-type GaAs substrate grown by the metal organic vapor-phase epitaxy method. The structure consists of two distributed Bragg reflector (DBR) mirrors surrounding a one-λ cavity, which has three 80 Å GaAs quantum wells, Al 0.3 Ga 0.7 As barriers, and spacers. The thickness of one-λ cavity is 269.4 nm. There are 38 periods in the n-type bottom mirror and 21.5 periods in the p-type top mirror. The mirrors consist of alternating 419.8 Å Al 0.15 Ga 0.85 As layers and 488.2 Å Al 0.95 Ga 0.05 As layers. Between the layers, a 200 Å-thick, linearly graded AlGaAs layer was grown. The p-type and n-type distributed Bragg reflector mirrors were doped to a dose bigger than 10 18  cm −3  with C and Si, respectively. The height of the nano-structure of the Hyperboloid-drum is 8 μm.  
         [0067]     A masking layer ˜1.7 μ/m thick for CAIBE was fabricated with AZ5214 PR using a Karl Suss MJB3 contact aligner and a contact mask. The damage induced by the CAIBE process were removed by a H 2 SO 4  polishing process, in which the samples were dipped into a H 2 SO 4 :H 2 O 2 :H 2 O=1:8:1000 solution for 5 seconds. Subsequently, sulfur passivation followed to improve the intensity and lifetime of the laser. For the sulfur treatment, a 6% excess sulfur-containing (NH 4 ) 2 S x  solution was used at 60° C., and the samples were dipped into the solution for 8 minutes. These sulfur-treated samples were loaded immediately into the plasma-enhanced chemical vapor deposition chamber of a downstream type, and they were prebaked first at 300° C. for 30 minutes in an NH 3  environment before deposition of a Si 3 N 4  layer. This temperature treatment helps the excess sulfur not bonded to the GaAs surface to sublimate. After the sulfur passivation, a polyimide coating, etching for planarization, and evaporation of Cr/Au and AuGe/Ni/Au for p and n contacts followed. The polyimide layer reinforces the fragile nano-structure of the hyperboloid-drum.  
         [0068]      FIG. 6  shows the variation of the sidewall angle α with varying the tilt angle θ between the incident ion beam and the substrate.  
         [0069]     The abscissa and the ordinate in  FIG. 6  represent the tilt angle θ between the ion beam and the sample and the sidewall angle α, respectively. The direction of the ion beam is set to be vertical from top to bottom so that the sample is only tilted to adjust the angle between the ion beam and the sample. Also, the voltage and the current in the index box of  FIG. 6  indicate the intensity of the ion beam, which can be represented by the product of the voltage and the current.  
         [0070]     A negative α indicates an outward-tapered sidewall, and a positive α indicates an inward-tapered sidewall. All sidewalls are outward-tapered at θ=0°, except that fabricated with the 750 eV, 30 mA beam. For a given beam, the sidewall angle α increases when the tilt angle θ increases, and it is in the range of 15 to ˜25° at θ=50°. For a fixed θ, the sidewall angle α increases with increasing the beam energy and current.  
         [0071]     According to this condition, it is ascertained that the transition in the shape from the trapezoid (at negative α) to the hyperboloid-drum (at positive α) occurs with the variation of the angle between the ion beam and the substrate.  
         [0072]      FIG. 7  displays that the shape of the waist (the active region) of the hyperboloid-drum element and the top surface region (where metal is to be deposited) varies with the variation in the size of the CAIBE photoresist (PR) mask.  
         [0073]     The process gas mixture for this experiment was Ar:Cl 2 :BCl 3 =5:2:3 with a total flow rate of 10 sccm. The beam energy, beam current, tilt angle θ, and etch time for CAIBE were 500 eV, 20 mA, 50°, and 27.5 min, respectively. The substrate temperature was kept constant during the CAIBE process at 20° C. (triangle), 40° C. (lozenge), and 60° C. (circle). At these temperatures, the effect of the substrate temperature on the erosion rate of mask is negligible, and the fabricated nano-structure of the hyperboloid-drum have top surfaces of nearly the same size, as shown in  FIG. 7 .  
         [0074]     The observed loss on the mask diameter after CAIBE is approximately 1.6 μm. The diameter of the active region for a given substrate temperature increases in proportion to the mask size. When the substrate temperature is 20° C., the diameter of the active region for a mask size of 5.4 μm is ˜900 nm. The diameter of the active region decreases with increasing the substrate temperature because the desorption rates of reaction by-products increase with temperature. The diameter of the active region is reduced to ˜200 mm when the substrate temperature is 60° C.  
         [0075]     The SEM images of the fabricated hyperboloid-drum element are shown in  FIG. 8 . The structures shown in FIGS.  8 ( a ) and  8 ( c ) are fabricated at a substrate temperature 60° C. with mask sizes of 5.7 μm and 5.2 μm, respectively. Magnified images thereof are shown in FIGS.  8 ( b ) and  8 ( d ), respectively. Both structures have the same etched height of ˜8 μm. The active region diameters are 600 nm and 95 nm in FIGS.  8 ( b ) and  8 ( d ), respectively, demonstrating that a fabrication of the hyperboloid-drum element with a nanoscale active region is possible by adjusting the mask size.  
         [0076]     Referring to  FIG. 8 , it is possible to accurately control the size of the mesa top by the size of the photoresist mask where the metal electrode is deposited depending on the etching condition and the size of the active region where the optical emission occurs. The size of the active region can be easily controlled by adjusting the size of the etching mask because the top region of the element is roughly linearly proportional to the size of its active region, depending on the size of the etching mask. Not only can this hyperboloid-drum element control the size of the active region in nanoscale where the emission occurs, but it also has an advantage for the electrical laser emission due to the large top area of the mesa where the electrode is deposited. The temperature in the index box indicates the temperature of the sample. The temperature of the sample can be controlled by heating/cooling the holder where the sample is put.  
         [0077]     For the hyperboloid-drum element fabricated by an exemplary embodiment of the present invention, the light-current-voltage curves for the element with the active region diameter of 600 nm are shown in  FIG. 9 .  
         [0078]     The emission of the optical element can be confirmed though the early characteristics of the light power vs. current of the test element are not significant.  
         [0079]     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concept herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention, as defined in the appended claims.