Abstract:
An evaluation apparatus is taught to nondestructively characterize the electroluminescence behavior of the semiconductor-based or organic small-molecule or polymer-based light-emitting material as the finished light-emitting device functions through electroluminescence. An electrode probe is used to temporarily form a light-emitting device through forming an intimate electrical contact to the surface of the light emitting material. A testing system is provided for applying an electrical stimulus to the electrode probe and temporarily formed device and for measuring the electrical and optical/electroluminescence response to the electrical stimulus. The electrical and optical properties of the light-emitting material can be nondestructively determined from the measured response. Optionally a light stimulus is used to perform the photoluminescence characterization together with the electroluminescence characterization, and both characterizations can be performed at the same sample location or/and at the wafer level.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to light-emitting materials characterization or evaluation, and more specifically to the optical and electrical characterization of the electroluminescence materials which can be used for fabricating light-emitting diode (LED) or organic light-emitting diodes (OLED). Though the invention will be described with particular reference to the quantum well-based LED materials system, the invention is not so limited, and it will also find applications in optical, electrical and optoelectronic evaluation of all type light-emitting materials systems which are semiconductor-based or organic small-molecule or polymer-based, as well as p/n junction, semiconductor laser structures, and the like.  
       DESCRIPTION OF RELATED ART  
       [0002]     Breakthroughs in artificial lighting from semiconductor-based or organic small-molecule- or polymer- based light-emitting materials system have led to tremendous opportunities in modern society. Solid-state lighting (SSL) based on semiconductor light-emitting diode (LED) is an emerging new-generation lighting technology which meets the worldwide trend for more energy efficient and environment-friendly usage of the finite energy. Organic small molecule and polymer-based LEDs (OLEDs) have the advantages of cost-efficiency, mechanical flexibility and large area, which offer the potential to revolutionize the flat-panel display industry, and therefore change how and where people can access the information through displays in TVs, computers and portable electronic devices. However, these lighting technologies are still too expensive for wide-spread applications. In order to reap the benefits of these exciting technologies, innovative and high-yield manufacturing equipment and tools are needed to overcome the hurdles to widespread market penetration.  
         [0003]     The light-emitting or luminescence from both semiconductor-based and organic small-molecule or polymer-based materials systems are produced by electrical current injection through radiative recombination of excess electrons and holes in the emissive/active layers. Correspondingly this type luminescence is usually called electroluminescence. In order to optimize the material growth recipe or control quality for further device fabrication, it is of great interest to nondestructively characterize the electroluminescence behavior of the material at the wafer level as the finished device (LED or OLED) functions through electroluminescence.  
         [0004]     The prior art for non-destructively characterizing the light-emitting materials is the photoluminescence (PL) mapping method. This method combines conventional PL, which utilizes a light beam to excite the carriers inside the investigated structure and measure the spectrum of the emitted light, with a scanning stage. Both the intensity and the peak wavelength uniformity across the whole wafer can thus be acquired and used for evaluation. The excess electron-hole pairs in photoluminescence and electroluminescence are photo-excited and electrically injected respectively. While the photoluminescence is mainly determined by the optical properties of the material, the electroluminescence is determined by a number of factors, such as the optical properties and physical structures of the optically active layers, the electrical properties of two conductive regions which are used for cathode and anode contacts, and the properties of the electrical contacts through which the electrical current injected. It is well known in the art that photoluminescence is not equivalent to electroluminescence. High photoluminescence efficiency is necessary but not sufficient for a good light-emitting materials or wafers. A wafer with high photoluminescence efficiency may or may not exhibit high electroluminescence efficiency and hence produce good LEDs/OLEDs. Thus, there remains an unfulfilled need for non-destructively characterizing the light-emitting materials which can be used for improved quality control at the wafer level.  
         [0005]     The prior art also does not teach effective means for separating out the various factors attributed to a poor electroluminescence which could result from the failure at any layers of the material structure, or from the problem caused by device (LED/OLED) fabrication. Though device fabrication and device level test could teach a mean to correlate the different factors with the device performance or the electroluminescence, such approach is expensive in terms of personnel time and material cost, and sometimes it still can be difficult or even impossible to trace failure factors.  
         [0006]     In view of these disadvantages, it would be useful to have a nondestructive characterization method that preferably is performed at the wafer level and closely resembles the electroluminescence and LED/OLED device operation, and that has the ability to independently evaluate the relative contributions or effects on electroluminescence of various sample regions such as active emissive layer, anode and cathode conductive layers.  
         [0007]     The present invention contemplates such characterization or evaluation method and apparatus.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with one aspect of the present invention, a nondestructive LED/OLED probe is disclosed. With this probe, a temporary LED (or OLED) device will be formed in the light-emitting material through a well-defined electrical contact, with the contact defining the device area. The formed LED/OLED device is nondestructive, instant, and can be used for wafer-level and micrometric-scale investigations on light-emitting materials. By applying an electrical stimulus to this LED/OLED probe, the electroluminescence behavior of the material can be characterized at the wafer level, as the finished device (LED or OLED) functions through electroluminescence. The disclosed LED/OLED probe also has the ability to characterize the anode and cathode conductive layers in the light-emitting material. With a means for applying an electrical stimulus in the probe and a means for measuring the electroluminescence and electrical response, the electroluminescence and electrical properties of various regions in the light-emitting materials, such as active emissive layer, anode and cathode conductive layers, can be independently evaluated.  
         [0009]     Preferably, spring loaded contact probes, or other similar means, will be used as electrodes in the disclosed LED/OLED probes with which the force of electrodes on the material can be properly controlled to avoid possible mechanical damage to the tested material. Though metal probes can be used in the present invention, the tip of electrodes is preferably made from an elastically-deformable electrically-conductive material and has flat surface with well-defined contact area. The elastically-deformable electrically-conductive material is used to ensure that an intimate contact between the electrodes and the light-emitting material is formed and the temporary LED (or OLED) device area is well-defined. Optionally, a concentric dot and ring electrodes, which are also made from an elastically-deformable electrically-conductive material, are used in the disclosed LED/OLED probe.  
         [0010]     The light-emitting materials can be any type of semiconductor-based or organic small-molecule- or polymer-based materials systems in which the luminescence is produced by electrical current injection. Due to possible structure difference in different light-emitting material systems, corresponding variation in LED/OLED probe structures is disclosed.  
         [0011]     In accordance with another aspect of the present invention, methods for determining the electroluminescence and electrical properties of various regions in the light-emitting materials are disclosed. The electroluminescence and electrical properties to be characterized include light emission intensity, peak wavelength, wavelength variation, spectrum half width and electrical/optical energy conversion efficiencies, as well as electrical properties of different layers.  
         [0012]     In accordance with yet another aspect of the present invention, an apparatus for evaluating the light-emitting material is disclosed. The apparatus is developed based on the disclosed LED/OLED probe. With a means for applying an electrical stimulus in the probe and a means for measuring the electroluminescence and electrical response, the electroluminescence and electrical properties of various regions in the light-emitting materials can be nondestructively and independently evaluated. The advantage of the present invention is that the corresponding evaluation can be done at the wafer level yet as the finished device (LED or OLED) functions through electroluminescence.  
         [0013]     Preferably, the apparatus includes a translation means for relatively translating the LED/OLED probe and the test material. The translation stage can be controlled at wafer-level or micrometric scale. With a scanning stage, the above evaluations can be used to determine the wafer-level uniformity about light emission intensity, peak wavelength, wavelength variation, spectrum half width and electrical/optical energy conversion efficiencies, as well as electrical properties of different layers. The micrometric-scale scanning capability could be used for localized investigations such as defect influence on device performance.  
         [0014]     Optionally, the apparatus includes a light injection means which can be used to characterize the photoluminescence. The photoluminescence will be measured by the same means for measuring the electroluminescence in the apparatus. The advantage of adding the option of photoluminescence measurement is that both photoluminescence and electroluminescence can be measured from the same sample location which can provide an extra dimension of information to evaluate the material. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention may take forms in various components and arrangements of components, and various steps and arrangements of steps. The drawings are only for the purposes of illustrating a preferred embodiment and are not to be constructed as limiting the invention.  
         [0016]      FIG. 1  is a schematic illustration of one embodiment of a nondestructive LED/OLED probe.  
         [0017]      FIG. 2  is a schematic drawing of one embodiment of electrode arrangement (hexagonal) used in  FIG. 1 .  
         [0018]      FIG. 3  is a schematic drawing of one embodiment of electrode arrangement (concentric) used in  FIG. 1 .  
         [0019]      FIG. 4  is a schematic illustration of one embodiment of a nondestructive LED/OLED probe with enhanced electrical contacts.  
         [0020]      FIG. 5  is a schematic illustration of one embodiment of the light-emitting material evaluation apparatus using the nondestructive LED/OLED probe. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention will be described with the reference to the accompanying figures wherein like reference numbers correspond to like elements.  
         [0022]     With reference to  FIG. 1 , a light-emitting material testing method is described in accordance with one embodiment of the nondestructive LED/OLED probe. The nondestructive LED/OLED probe  10  operates upon an associated light-emitting sample  20 . The associated sample  20  typically has a plurality of layers  22 ,  24 ,  26  which are usually grown on a substrate  28 . In  FIG. 1 , a gallium nitride (GaN) based LED structure is exemplarily shown, which includes a sapphire or silicon carbide (SiC) substrate  28 , a p-type GaN region  22  and a n-type GaN region  26 . The sandwiched layer between the p-type GaN region  22  and the n-type GaN  26  are AllnGaN (Al: aluminum, In: indium) multi-quantum-well barriers  24  which comprise the active/emissive region of LED device. Electrically the p-type GaN region  22  and the n-type GaN region  26  can be briefed as two conductive layers since they are mainly developed for anode and cathode contacts to inject the electrical current. Certainly, the invention is not limited in application to the exemplary GaN-based LED structure shown in  FIG. 1 , the associated sample  20  can be any type of semiconductor-based or organic small-molecule- or polymer-based light-emitting materials systems which can be used for fabricating the light-emitting diodes (LEDs) or organic light-emitting diodes (OLEDs). The invention will also find applications in characterizing p/n junction, semiconductor laser structures, and the like.  
         [0023]     With continuing reference to  FIG. 1 , the nondestructive LED/OLED probe  10  consists of electrodes  12  and  16 . Electrodes  12  and  16  are used to temporarily form electrical contacts  21  and  27  to the p-type GaN. Well-defined LED devices  25  and  23  are simultaneously formed in the active layer at the places right below the corresponding contacts  21  and  27 . Preferably, the force of electrodes on the material is properly controlled to avoid possible mechanical damage to the tested material. One exemplary approach to control the electrode force is to use spring  14  loaded electrodes  12  and  16 . Though metal probes, such as commercially available pogo probes, can be used in the present invention, the tip of electrodes  12 ,  16  is preferably made from an elastically-deformable electrically-conductive material and has flat surface with well-defined contact area. Together with means for controlling the electrode force, the elastically-deformable electrically-conductive material is used to ensure that intimate contacts  21  and  27  are formed between the electrodes and the light-emitting material and the areas of the temporary LED (or OLED) devices  23  and  25  are well-defined. The elastically-deformable electrically-conductive material can be a conductive elastomer or a conductive polymer.  
         [0024]     With continuing reference to  FIG. 1 , and with further reference to  FIGS. 2-3 , the electrode  12  preferably consists of multiple contact points  21  which uniformly surround the electrical contact  27 . Though  FIG. 2  shows one embodiment of electrode  12  arrangement with six contact points  21 , the invention is not limited in application to this embodiment. The number of contact points  21  can be of any number starting from two. The contact shapes are not necessarily the circular as shown in  FIGS. 2 and 3 , any other shapes, such as triangular, rectangular, square, pentagon, and the like, can be used. Though the size of contact  27  is preferably smaller than the size of contact  21 , they are not necessarily the same. Optionally, a concentric dot  27  and ring  21  structure are used for electrodes  16  and  12  respectively. The concentric dot  27  and ring  21  electrodes are also preferably made from an elastically-deformable electrically-conductive material which can be a conductive elastomer or a conductive polymer. The dimensions for contacts  21 ,  27  of these electrodes can be of any size, from a few microns to a few centimeters, or even bigger, which should be determined by the purpose of each specific application. For the purpose of high scanning resolution, a smaller contact size of electrodes is preferred.  
         [0025]     With ongoing reference to  FIG. 1 , a proper electrical stimulus will be applied between electrodes  16  and  12  by the electrical stimulus means  30  to intentionally forward bias the temporary LED  23  and reverse bias the temporary LED  25 . In the exemplary GaN-based sample  20  drawn in  FIG. 1 , a positive bias is needed to realize the above bias condition. The electrical stimulus can be DC current or voltage, AC voltage combined with a DC bias voltage, AC voltage, or the like. If it is small, the electrical stimulus goes mainly through the conductive layer  22 . As the electrical stimulus increases to certain value, the electrical stimulus begins to mainly go through the temporary LED  23 , conductive layer  26  and the temporary LED  25 . At this condition, the temporary LED  23  begins to emit light as a finished device (LED or OLED) functions through electroluminescence. The electrical stimulus is usually limited to a certain value to avoid possible breakdown or electrical heating which could cause damage to the material. The electrical measurement means  32  and the optical measurement means  40  measure the electrical and optical response to the applied electrical stimulus and determine from the response one or more properties of the test sample  20 . The light emission intensity, electroluminescence characteristics, peak wavelength, wavelength variation, spectrum half width and electrical/optical energy conversion efficiencies, electrical properties of different layers, or the like, can be determined from these measurements.  
         [0026]     With continuing reference to  FIG. 1 , and with further reference to  FIG. 4 , the nondestructive LED/OLED probe shown in  FIG. 1  can be further enhanced with a third electrode which is configured differently according to the different material structures. The substrate  28  in different light-emitting material systems can be identified as conductive or nonconductive/insulated. For example, in GaN-based LED structures, sapphire substrates are usually nonconductive while highly doped SiC substrates are usually conductive. For the material systems with conductive substrate, the conductive layers  26  and substrate  28  are usually electrically connected. A third electrode  56 , which is connected electrically to the substrate, is preferably used. Under this test setup, the electrical stimulus will pass through the loop formed by electrical wire  19 , electrode  16 , contact  27 , temporary LED  23 , conductive layer  26 , conductive substrate  28 , electrode  56 , and electrical wire  54 . Together with electrode  16 , the electrode  12  can be used to evaluate the electrical properties of the conductive layer  22 . During test, the electrode  12  can also be used as guard ring configuration to reduce the noise and possible current leakage. The use of guard ring configuration of the electrode  12  is straightforward and known in the art. When the substrate of the material systems is insulated, a third electrode  58  can be used to electrically connect the conductive layer  26  at the edge of wafer. Correspondingly, under this test setup, the electrical stimulus will pass through the loop formed by electrical wire  19 , electrode  16 , contact  27 , temporary LED  23 , conductive layer  26 , electrode  58 , and electrical wire  52 . Again, together with electrode  16 , the electrode  12  can be used to evaluate the electrical properties of the conductive layer  22 , and it can also be used as guard ring configuration to reduce the noise and possible current leakage. Though only one contact for electrodes  56  or  58  is drawn in  FIG. 4 , the electrode  56  or  58  can be multiple contact points. To ensure a good electrical contact, the electrode  58  and  56  are preferably made from an elastically-deformable electrically-conductive material which can be a conductive elastomer, or a conductive polymer, or a metal sheet with conductive adhesive. The advantage of using the third electrode  56  or  58  in the present invention is that, besides it uses a smaller electrical stimulus, the current-voltage characteristics of the light-emitting material and the electrical properties of conductive layer  26  can be further determined at the wafer level as the finished device (LED or OLED) functions.  
         [0027]     With continuing reference to  FIGS. 1, 4  and  5 , the optical measurement means  40  is used to detect the spectrum and light intensity of the electroluminescence generated by the temporary LED  23  under the stimulus of the electrical stimulus means  30 . It includes an optical detector  44  which can be a photomultiplier tube, a photodiode, a diode array, charge coupled device (CCD), intensified CCD, or the like, and also preferably includes a light-collecting lens  42 , optical fiber coupling (no shown), a dispersive component such as a monochromator, spectrograph, or the like. Though the optical measurement means  40  is drawn at the same side of the sample where the nondestructive LED/OLED probe is located. The present invention is not limited in the application to this configuration. Depending on material structure of the sample  20 , the optical measurement means  40  can be located at any side of the sample.  
         [0028]     With continuing reference to  FIGS. 1-4 , and with further reference to  FIG. 5 , an apparatus embodiment is shown in  FIG. 5 . The apparatus consists of the above disclosed nondestructive LED/OLED probe  10 , electrical stimulus means  30 , electrical measurement means  32 , optical measure means  40 , probe control means  66 , sample stage  60  and preferably the stage translation means  56 . The probe control means  66  is used to load and unload the nondestructive LED/OLED probe  10  to form the well-defined LED device in the sample  20 ; it will also control the contact force of electrodes to avoid possible mechanical damage to the sample  20 . Sample stage  60  provides the place to hold sample and will also have the means to produce good electrical contacts for electrodes  58  and  56 .  
         [0029]     Optionally, the apparatus in  FIG. 5  includes a light injection means  70  which can be used to characterize the photoluminescence. The photoluminescence will be measured by the same optical measurement means  40  for measuring the electroluminescence in the apparatus. The advantage of adding the option of photo-luminescence measurement is that both photoluminescence and electroluminescence can be measured from the same sample location which can provide an extra dimension of information to evaluate the material. The photoluminescence measurement is straightforward and known in the art, but the combination of photoluminescence and electroluminescence measurement at the same sample place at the wafer level is the new art in the present patent.  
         [0030]     Preferably, the associated sample  20  is mounted on the stage  60  which is driven by a stage translation means  62 . The sample  20  can be moved laterally with respect to the LED/OLED probe  10 . In this way, the lateral inhomogeneities about electrical, electroluminescent and photoluminescent characteristics of the sample  20  can be probed.  
         [0031]     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed descriptions. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of appended claims or the equivalents thereof.