Abstract:
The present invention is disclosed that a device capable of normal incident detection of infrared light to efficiently convert infrared light into electric signals. The device comprises a substrate, a first contact layer formed on the substrate, an active layer formed on the first contact layer, a barrier layer formed on the active layer and a second contact layer formed on the barrier layer, wherein the active layer comprises multiple quantum dot layers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a photodetector apparatus, particularly to a quantum dot infrared photodetector apparatus. 
         [0003]    2. Description of the Prior Art 
         [0004]    In recent years, there are many researches associated with the development of quantum dot infrared photodetector, because it has the characteristics of low cost and high-temperature operation etc. The basic function of quantum dot infrared photodetector has been improved, and its performance has also been increased greatly. 
         [0005]    Conventionally, quantum dot means that the nano-grade semiconductor raw materials having the narrower energy gap are imbedded into the semiconductor materials having the wide energy gap to form the quasi-zero-dimensional nano-material. The transitions of electrons among different states can be restrained by the quasi-zero-dimensional degree of freedom for electron state. Therefore,the quantum dot infrared photodetector formed by the quantum dot has the characteristics of low dark current, high responsivity and high-temperature operation etc. 
         [0006]    In addition, the quantum dot infrared photodetector can absorb normal incident infrared due to three-dimensional bound states. The quantum dot infrared photodetector has the advantage of simplified manufacturing process compared to the quantum well infrared photodetector (QWIP) which cannot absorb normal incident light. And it is advantageous for the manufacturing of medium-wavelength (3 μm to 5 μm) photodetector because of larger energy gap difference of quantum dot structure. Thus, the development of quantum dot infrared photodetector is attracted a lot of interest by people. As for the quantum dot infrared photodetector technology, it was disclosed in “Applied Physics Letter 73, 963 (1998)” and “Applied Physics Letter 73, 3153 (1998)” These two prior papers disclosed the basic structure and embodiments of quantum dot infrared photodetector technology. Even the basic concept of quantum dot infrared photodetector technology had been revealed earlier in 1996, such as in the “Semiconductor Science Technology 11, 759 (1996)”. It was also much earlier compared to the first patent disclosing quantum dot infrared photodetector technology as U.S. Pat. No. 6,239,449 “Quantum dot infrared photodetectors (QDIP)”. 
         [0007]    The early quantum dot infrared photodetector technology was limited by self-assembled quantum dot process. Thus, in order to solve the adjustment problem of the detecting wavelength, the dots-in-a-well structure was disclosed in “Applied Physics Letter 79, 3341 (2001)”. In the dots-in-a-well structure, the thickness of quantum well could be employed to change the detecting wavelength, which was advantageous for the application of quantum dot infrared photodetector. However, the addition of quantum well structure would influence the quantum efficiency of the component. 
         [0008]    Because of the above-mentioned reasons, the development of quantum dot infrared photodetector has been paid more attention day by day. In order to respond the future demand of photodetector technology, it is necessary to develop relevant technology of infrared photodetector, to reduce the manufacturing cost, manpower and time, and achieve the purpose of energy saving and carbon reduction effectively. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is disclosed that a device capable of normal incident detection of infrared light to efficiently convert infrared light into electric signals. The device comprises a substrate, a first contact layer formed on the substrate, a quantum dot active layer formed on the first contact layer, a barrier layer formed on the quantum dot active layer and finally a second contact layer formed on the barrier layer, wherein the active layer comprises a plurality of quantum dot layers. Wherein, the active layer comprises a first quantum well layer formed on the barrier layer, a quantum dot layer formed on the first quantum well layer, a confinement enhancing layer formed on the quantum dot layer, a second quantum well layer formed on the enhancement layer, and a barrier layer formed on the second quantum well layer. 
         [0010]    Generally speaking, the quantum efficiency of quantum dot photodetector is less than 1%, and the invention can improve it by an order of magnitude to several percentages. 
         [0011]    Compared to the existed prior art, the invention can raise the quantum efficiency to about 20 times. 
         [0012]    The invention can increase the signal/noise ratio of detecting signal. It can raise the sensitivity of infrared detection under the same working temperature, or raise the working temperature under the same sensitivity. 
         [0013]    The major application of the invention is used to detect the black body radiation of object itself, which means the object under room temperature can be detected without the existence of any light source. 
         [0014]    The invention can improve the quantum dot structure in quantum well. The invention can add a high energy gap in quantum dot, so that the electrons in quantum dot will get a stronger quantum confinement effect. It can increase the infrared absorption efficiency of electrons, in order to improve the performance of device. 
         [0015]    The advantage and spirit of the invention can be understood further by the following detailed description of invention and attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0017]      FIG. 1  shows a preferred embodiment of the invention. 
           [0018]      FIG. 2  shows a preferred embodiment of the quantum dot active layer of the invention. 
           [0019]      FIG. 3  shows a preferred embodiment of the invention for the quantum dot active layer having a plurality of quantum dot layers. 
           [0020]      FIG. 4  shows the comparison curves of the invention and the prior art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The invention relates to a quantum dot infrared photodetector apparatus, which mainly comprises the following components: 
         [0022]    As shown in  FIG. 1 , a semiconductor substrate  101  is provided, which is an undoped GaAs semiconductor substrate  101 . 
         [0023]    As shown in  FIG. 1 , a first contact layer  102  is provided, which is an n + -type GaAs layer  102  doped with Group V element formed on the semiconductor substrate  101 , and the thickness is about 500 nm. 
         [0024]    As shown in  FIG. 1 , a barrier layer  103  is formed on the first contact layer  102 . The barrier layer  103  is an undoped GaAs layer  103  or a AlGaAs layer  103 , wherein the content of aluminum (Al) is 0% to 35%. The thickness of barrier layer  103  is about 10˜150 nm. The thickness of barrier layer  103  is about 53 nm in the embodiment. 
         [0025]    Then, as shown in  FIG. 1 , an active layer  104  is formed on the barrier layer  103 . 
         [0026]    Finally, as shown in  FIG. 1 , a second contact layer  105  is formed on the active layer  104 , and the second contact layer is an n + -type GaAs layer  105  doped with Group V element. 
         [0027]    The previous active layer  104  is piled by a quantum dot layer  104 , thus the active layer  104  includes the followings: 
         [0028]    As shown in  FIG. 2 , a first quantum well layer  201  is formed on the barrier layer  103 , and then the first quantum well layer  201  is an undoped In 0.15 Ga 0.85 As layer, wherein the content of indium (In) is about 0% to 20%. Normally, the thickness of the first quantum well layer  201  is about 0 nm to 15 nm, here the thickness of the first quantum well layer  201  is about 2 nm in the embodiment. As shown in  FIG. 2 , a quantum dot (In(Ga)As QD) layer  202  is formed on the first quantum well layer  201 , and the density of quantum dot (In(Ga)As QD) layer  202  shall be larger than 1×10 9 cm −2 . 
         [0029]    As shown in  FIG. 2 , a confinement enhancing layer  203  is formed on the quantum dot layer  202 . The confinement enhancing layer  203  is an undoped Al 0.3 Ga 0.7 As layer  203 , wherein the content of aluminum (Al) can be 0% to 35%. The thickness of the confinement enhancing layer  203  is about 2 nm to 5 nm. The thickness of the confinement enhancing layer  203  is about 2.5 nm in the embodiment. 
         [0030]    As shown in  FIG. 2 , a second quantum well layer  204  is formed on the confinement enhancing layer  203 . The second quantum well layer  204  is an undoped In 0.15 Ga 0.85 As layer  204 , wherein the content of indium (In) can be 0% to 20%. The thickness of the second quantum well layer  204  is about 0 nm to 15 nm. The thickness of the second quantum well layer  204  is about 4.5 nm in the embodiment. 
         [0031]    As shown in  FIG. 2 , a barrier layer  205  is formed on the second quantum well layer  204 . The barrier layer  205  is an undoped GaAs layer  205  or AlGaAs layer  205 . The thickness of the barrier layer  205  is about 150 nm. The thickness of the barrier layer  205  is about  53  nm in the embodiment. 
         [0032]    As shown in  FIG. 3 , a quantum dot active layer  314  is added on the existing active layer  104 . The quantum dot active layer  314  comprises the first quantum well layer  301 , a quantum dot layer  302  formed on the first quantum well layer  301 , a confinement enhancing layer  303  formed on the quantum dot layer  302 , a second quantum well layer  304  formed on the confinement enhancing layer  303 , a barrier layer  305  formed on the second quantum well layer  304 , and a second contact layer  105  formed on the barrier layer  305 . 
         [0033]    Thus, a plurality of quantum dot layers can be added in the quantum dot infrared photodetector of the invention, such as the active layer  104  or the quantum dot active layer  314  is added between the barrier layer and the contact layer. In the invention, about 3 layers to 100 layers of quantum dot layers can be added totally. 
         [0034]    As shown in  FIG. 4 , the curves of bias voltage versus peak responsivity are drawn. Compared to the existed technology for the quantum dot infrared photodetector, the invention can raise the quantum efficiency to about 20 times. The quantum efficiency of common quantum dot photodetector is less than 1%, and the invention can improve it by an order of magnitude to several percentages. The invention can increase the signal/noise ratio of detecting signal. It can raise the sensitivity of infrared detection under the same working temperature, or raise the working temperature under the same sensitivity. The working temperature of the invention can reach 200 K compared to the typical absolute temperature of 77 K (boiling temperature of liquefied nitrogen). 
         [0035]    The major application of the invention is used to detect the black body radiation of object itself, which means the object under room temperature can be detected without the existence of any light source. The invention is used in military applications mainly at early stage, which can be used for the object detection in the battle space, hot tracing missile and early warning system in battlefield. With the progress of technology recently, the similar portable system which uses the basic principle of the invention can be applied to general security surveillance, fire monitoring and fire detection, specific gas discharge monitoring, and nondestructive inspection of semiconductor process and cancer cell detection. 
         [0036]    The invention is mainly to improve the quantum dot structure of in the quantum well. The invention can add a high energy gap in quantum dot, so that the electrons in quantum dot will get a stronger quantum confinement effect. It can increase the infrared absorption efficiency of electrons, in order to improve the performance of component. 
         [0037]    It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.