Abstract:
A quantum dot optoelectronic device has an overgrown layer containing antimony (Sb). The optical characteristics and thermal stability of the optoelectronic device are thus greatly enhanced due to the improved crystal quality and carrier confinement of the quantum dot structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optoelectronic device; more particularly, relates to n quantum dot optoelectronic device having an overgrown layer containing antimony (Sb) to improve optical characteristics and thermal stability. 
     DESCRIPTION OF THE RELATED ART(S) 
     A general quantum dot optoelectronic device on a GaAs substrate is fabricated by covering a layer of InGaAs or InAlAs on quantum dots to improve size uniformity of quantum dots and to extend light emission wavelength after growing InAs quantum dots on a GaAs substrate. Yet, by doing so, the confinement potential of the carriers in quantum dot structure is lowered by the InGaAs; or, the light emission efficiency is reduced due to the poor crystal quality of InAlAs grown at low temperature. Besides, growing a heterostructure containing indium (In), like InGaAs or InAlAs, on a GaAs substrate may induce crystal defects if the lattice mismatch is too large, which adversely affect the performance of the optoelectronic device. 
     Shown in  FIG. 1  is an energy band diagram of a heterostructure (prior art) consisting of InAs quantum dot  41 , InGaAs  42  and GaAs  43 . For the optoelectronic device with an InGaAs overgrown layer on quantum dots, carriers confined in the quantum dots are apt to escape from the quantum dots by thermal excitation owing to the lower barrier of the InGaAs overgrown layer. Thus, the performance of the optoelectronic device at high temperature is seriously degraded. 
     SUMMARY OF THE INVENTION 
     The present invention proposes a quantum dot optoelectronic device having an Sb-containing overgrown layer to improve the material quality and carrier confinement so as to enhance its optical characteristics and thermal stability. 
     The present invention is a quantum dot optoelectronic device having an Sb-containing compound, including an epitaxy substrate, an epitaxy buffer layer deposited on the epitaxy substrate, at least one quantum dot structure deposited on the epitaxy buffer layer, an overgrown layer deposited on the at least one quantum dot structure, and a surface layer deposited on the overgrown layer, where the overgrown layer is made of a material containing Sb and is a layer of Al x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  or InAl x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  or multiple layers of Al x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  and InAl x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z , where 0≦x, y, m, z≦1. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The present invention will be better understood from the following detailed description of the preferred embodiment according to the present invention, taken in con junction with the accompanying drawings, in which 
         FIG. 1  is the energy band diagram of the prior art. 
         FIG. 2  is the schematic structure of the present invention; 
         FIG. 3A  is the schematic structure of the first embodiment; 
         FIG. 3B  is the energy band diagram of the first embodiment; 
         FIG. 3C  is the emission spectra of the prior art and first embodiment; 
         FIG. 3D  is the plot showing the temperature-dependent integrated emission intensity of the prior art and first embodiment; 
         FIG. 4A  is the schematic structure of the second embodiment; and 
         FIG. 4B  is the emission spectra of the second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is provided to understand the features and the structures of the present invention. 
       FIG. 2  shows a schematic structure of the present invention. As shown in the figure, the present invention is a quantum dot optoelectronic device  1  having an Sb-containing compound, comprising an epitaxy substrate  11 , an epitaxy buffer layer  12 , at least one quantum dot structure  13 , an overgrown layer  14  and a surface layer  15 . 
     The epitaxy substrate  11  is made of GaAs, InP, GaN, sapphire, SiC, Ge or Si; and is an N-type substrate, a P-type substrate or an undoped substrate. 
     The epitaxy buffer layer  12  is deposited on the epitaxy substrate; and is an N-type layer, a P-type layer or an undoped layer. It can also be a composite layer consisting of multiple layers of different materials depending on the type of devices. 
     The quantum dot structure  13  is deposited on the epitaxy buffer layer  12 . The quantum dot structure  13  is made of InAs, InN, InSb, AlSb, InGaAs, InAlAs, InGaN, InAlN, InGaSb, InAsSb, InAlSb, InGaAsSb or InGaAsNSb. 
     The overgrown layer  14  contains antimony (Sb); is deposited on the quantum dot  13 ; is an N-type layer, a P-type layer or an undoped layer; and is made of a layer of Al x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  or InAl x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  or multi-layers of Al x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z  and InAl x Ga 1-x (N y P 1-y ) m As z Sb 1-m-z , where s, y, z and m are a number between 0 and 1. 
     The optoelectronic device has one period of quantum dot active region consisting of  13  and  14 , or multiple periods of  13  and  14  separated by a spacer layer. 
     The surface layer  15  is formed on the overgrown layer; is an N-type layer, a P-type layer or an undoped layer. It can also be a composite layer consisting of multiple layers of different materials depending on the type of devices. 
     Thus, a novel quantum dot optoelectronic device having an Sb-containing overgrown layer is obtained, which is applied to a quantum dot light emitting diode, a quantum dot laser, a quantum dot optical detector, a quantum dot optical amplifier, a single electron transistor, a quantum dot memory, or an active layer of an optical switch. 
     Shown in  FIG. 3A  is a schematic structure showing the first embodiment of this invention. As shown in the figure, a quantum dot optoelectronic device  1   a  comprises an epitaxy substrate  11   a , an epitaxy buffer layer  12   a , a quantum dot layer  13   a , an overgrown layer  14   a  and a surface layer  15   a , where the epitaxy substrate  11   a , the epitaxy buffer layer  12   a , and the surface layer  15   a  are made of GaAs; the quantum dot structure  13   a  is made of InAs; and the overgrown layer  14   a  is made of InGaAsSb. 
       FIG. 3B  is an energy band diagram of the first embodiment of this invention, showing the band gaps of the quantum dot  21 , the overgrown layer  22  and GaAs  23 . The confinement potential for the carriers is increased by the InGaAsSb overgrown layer compared to the prior arts because both the band gap of the InGaAsSb overgrown layer moves upward with respective to the band gap of InAs quantum dot. Therefore, thermal e escape of electrons and holes out of the quantum dot is reduced. 
       FIG. 3C  shows the room temperature photo luminescence spectra of the prior art  24 , whose overgrown layer is InGaAs, and first embodiment of this invention  25 . From the first spectrum  24  and the second spectrum  25 , it is clearly observed that, by using the overgrown layer of InGaAsSb, higher luminescence intensity is obtained. 
       FIG. 3D  is a plot showing the temperature-dependence of the integrated photoluminescence intensity of the prior art  26  and the first embodiment of this invention  27 . It is clearly observed that the first embodiment of this invention, which has an overgrown layer of InGaAsSb on InAs quantum dots, exhibits less intensity degradation as the temperature increases. The activation energy deduced from this figure for the first embodiment of this invention is 406 meV compared to 243 meV for the prior art, indicating the superiority of this invention. 
       FIG. 4A  is a schematic structure showing the second embodiment of this invention. As shown in the figure, a quantum dot optoelectronic device  1   b  comprises an epitaxy substrate  11   b , an epitaxy buffer layer  12   b , a quantum dot layer  13   b , an overgrown layer  14   b  and a surface layer  14   b , and a surface layer  15   b , where the epitaxy substrate  11   b , the epitaxy buffer layer  12   b , and the surface layer  15   b  are made of GaAs; the quantum dot structure  13   b  is made of InAs; and the overgrown layer  14   b  is made of InAlAsSb. 
       FIG. 3B  shows the room temperature photoluminescence spectra of the prior art  31 , whose overgrown layer is InAlAs, and the second embodiment of this invention  32 . The luminescence intensity of the quantum dot structure is obviously improved by the use of the InAlAsSb overgrown layer  14   b  because Sb serves as a surfactant that reduces the crystal defects in aluminum containing materials. Moreover, band gap of InAlAsSb is high enough so that the energy difference between the ground and first excited state emission of quantum dots is not reduced significantly by the incorporation of Sb into the overgrown layer. 
     The preferred embodiments disclosed herein are not intended to limit the scope of the invention. Therefore, simple modifications or duplicating the quantum dot layer based on the same principles of this invention are all within the scope of the present invention.