Abstract:
An LED optimized for use in low-cost gas or other non-solid substance detection systems, emitting two wavelengths (“colors”) of electromagnetic radiation from the same aperture is disclosed. The LED device emits a light with a wavelength centered on an absorption line of the target detection non-solid substance, and also emits a reference line with a wavelength that is not absorbed by a target non-solid substance, while both wavelengths are transmitted through the atmosphere with low loss. Since the absorption and reference wavelengths are emitted from the same exact aperture, both wavelengths can share the same optical path, reducing the size and cost of the detector while also reducing potential sources of error due to optical path variation.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present invention is related to semiconductor electro-optical device design. More particularly, present invention teaches a structure of a light emitting semiconductor device that can be used as a source of electromagnetic radiation of two different wavelengths, suitable for implementation of highly efficient, low-cost gas detection system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas sensors have applications spanning much of the human experience. Research, industrial, safety, consumer, medical, and military applications abound. Many of gases of interest have absorption spectral features in the mid infrared that can be used to detect and differentiate between these gases. The combination of atmospheric transparency, strong spectral signatures, and availability of efficient light sources and detectors makes the mid-infrared (mid-IR) an attractive wavelength region for gas spectroscopy. For the purpose of the present disclosure, the discussion below is referring to gases, while it would be easily discovered by a skilled artisan that all principles disclosed below can be equally applied to the detection of liquids and gels; the materials that can be all together characterized as non-solid substances. 
         [0003]    A great many gas absorption spectroscopy systems have been developed. [“Differential absorption spectroscopy”, U. Platt, J. Stutz, Springer Verlag Berlin Heidelberg, 2008] Systems that measure the complete mid-IR absorption spectrum provide the most information, but they can be expensive, power hungry, and physically large. Although these systems can detect multiple gases, they are poorly suited for many gas detection applications, such as leaks detection, environment monitoring etc. where cost, power consumption, and size are paramount factors. 
         [0000]    Alternatively, systems can be designed to use only two wavelengths: an absorption wavelength that is absorbed by the gas under investigation, and a reference wavelength that is not. A comparison of the intensity of the absorption and reference wavelengths reveals the presence and concentration of the target gas. For many applications, an additional requirement is that neither wavelength should be absorbed by standard atmospheric gases. This permits the detection of target gas(es) diluted in the atmosphere. 
         [0004]    There are many ways to generate the absorption and reference wavelengths. One is to use a single broadband light source, such as a glow bar, and wavelength selective filters. The selective filters of each of the above wavelengths are altered using mechanical actuator system (for example, a rotating disc with holes where the filters are physically mounted). This approach is attractive in that the probe and reference wavelengths originate from the same physical space, and any variations in the input optical power can be expected to affect both absorption and reference wavelengths in a similar way. It is unattractive in that most of the broadband optical power is wasted and that only low frequency variations in the source intensity can be compensated for due to the slow mechanically actuated filter system. 
         [0005]    Another option improves power efficiency by utilizing two independent and relatively narrow spectrum light sources. If the narrow light sources are Light Emitting Diodes (LEDs), then they can be alternated rapidly using a pseudorandom sequence. A correlation analysis of the detector signal with the pseudorandom sequence greatly reduces noise in this approach. Power efficiency is also improved due to the efficiency of LEDs compared to broadband sources and the fact that only useful wavelengths are generated. 
         [0006]    The Gallium Antimonide (GaSb) material system can produce LEDs covering the spectral range of the infrared (IR) light with the wavelength from 2 to 12 urn, permitting the detection of many gases, including methane, a commercially and environmentally important gas. 
         [0007]    The approach of using two narrow band light sources, such as LEDs, has some downsides as well. In particular, the optical complexity of the system increases, the light of the two wavelengths does not share the same optical path, and the sources operate at independent, possibly different temperatures. The increased optical complexity adversely impacts the cost and the size of the detector, and as a result of the entire system. Any variations in the optical alignment due to mechanical vibrations or thermal imbalance may produce measurement and detection errors. While these errors can be theoretically “averaged away” using correlation analysis, the result could be slower measurements and increased power consumption. 
         [0008]    The independence and possible differing of the temperatures of the two light sources is a more fundamental problem. The efficiency and central emission wavelength of all existing LEDs is dependent on temperature; these effects are particularly evident for mid-IR LEDs since they use narrow band gap semiconductor materials whose properties are very sensitive to temperature. Further, the temperature of the LED is affected by self-heating from its own bias current. A pseudorandom variation in LED current results in a correlated pseudorandom variation in the wavelength of the emitted light and its intensity. A correlation analysis is then not effective in countering this subtle problem. 
         [0009]    In the light of the above, the best possibility for an LED-based gas detection system would be to use a single narrow-band light source along with a single wideband detector. This design has many advantages, a simplified optical arrangement being the most evident. However, in order to achieve such system, that single narrow-band light source must emit two different wavelengths of light from the same physical space. LEDs can do this by means of extreme temperature shifts, but achieving these temperature shifts is slow and introduces errors due to the varying temperature of the LED. Pseudorandom correlation becomes impractical at the low speeds achievable with temperature based wavelength tuning. 
         [0010]    There is a need, therefore, for a light source that would produce a dual color, narrow band light emission from the same physical space, with independent or correlated control of the light intensity of each color. It is further desired that the said dual color light source is simple, low cost and easy in both mounting and control. Each wavelength, or a “color”, of a dual light color source should respectively correspond to an absorption and transmission spectra of a gas targeted for detection in a certain gas detection system. 
         [0011]    The present invention teaches the design of such dual color LED that emits the light of one of the two pre-selected wavelengths in the IR region of the spectra, depending on the polarity of the applied external electric bias. These and other advantages of the present invention will be more readily understood from the following discussion taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  represents the prior art dual color, three-electrode LED schematic layer structure and terminal layout. 
           [0013]      FIG. 2  represents the schematic layer structure and terminal layout of the dual color LED of present invention. 
           [0014]      FIGS. 3A-3C  demonstrate three possible designs of the active regions of the dual color LED of the present invention. 
           [0015]      FIG. 4  depicts schematic band diagram of the n-p-n embodiment of the dual color LED of present invention in the absence of the external electric bias, and an equivalent circuit representation of the same. 
           [0016]      FIG. 5  depicts schematic band diagram of the p-n-p embodiment of the dual color LED of present invention in the absence of the external electric bias, and an equivalent circuit representation of the same. 
           [0017]    A schematic band diagrams of the n-p-n embodiment of the dual color LED of present invention under different polarities of the external electric bias are presented in  FIG. 6 . 
           [0018]      FIG. 7  schematically represents the epitaxial layer structure of the example dual color LED device as per the teachings of the present invention. 
           [0019]      FIG. 8  provides the current-voltage characteristic of the example dual color LED device as per the teachings of the present invention. 
           [0020]      FIG. 9  provides the emission spectra characteristics of the example dual color LED device as per the teachings of the present invention at different bias polarities. 
       
    
    
     PRIOR ART BY THE SAME INVENTORS 
       [0021]    Dual color LEDs have been demonstrated by the same group of inventors in the past for the infrared scene projection applications [S. Jung, S. Suchalkin, G. Kipshidze, D. Westerfeld, E. Golden, D. Snyder and G. Belenky, Applied Physics Letters 96 (19) (2010).] The schematic layout of the prior art device is presented in  FIG. 1 . In order to establish independent control of the intensities of the two wavelengths, three terminals were required: a common anode  10  and two independent cathodes  20  and  30 . While these three terminals permit independent control of the two wavelengths, the third terminal greatly complicates fabrication, increases cost, and reduces yield. 
         [0022]    A three-electrode LED, or, more precisely, Light Emitting Triode (LET) had an active region comprising two parts  40  and  50 , separated by an injection layer  60 . Each part  40  and  50  contained an active area with efficient direct radiative electron-hole recombination, emitting the light of different wavelength (1.9 um for active region part  40  and 2.9 um for active region part  50 ) within a single device. The intensity ratio between two colors was controlled by two voltages: between the cap layer  70  and injection layer  60  (color  1 ) and between injection layer  60  and the substrate  80  (color  2 ). The surface isolation between a common anode  10  and the one of the independent cathodes  20  contacting the cap layer  70  was achieved using passivation dielectric layer  90 . 
         [0023]    In order to perform its function, an LET comprised three electrical terminals  10 ,  20  and  30  providing the contacts, respectively, to the injection layer  60 , cap layer  70  and substrate  80 . The first two contacts  10  and  20  were positioned on the epi-layer side of the structure, in order to provide efficient light emission from the substrate  80  side of the structure. Accordingly, these two contacts  10  and  20  were positioned at laterally different areas. Since the electric current spreading in the LED epilayers was not effective, especially at high currents, the emitting areas of the light of colors  1  and  2  also had different lateral shape. 
         [0024]    In the optical gas sensor system, the emission area of the LED is imaged onto the detector. If an LET from this example was used in such a system, different emitting areas for the absorption and reference wavelengths would lead to additional complications, as they would be imaged to different regions on the detector surface. 
       SUMMARY OF THE INVENTION 
       [0025]    Provided is an LED optimized for use in low-cost gas detection systems, the LED emitting two wavelengths (“colors”) of infrared light from the same aperture. The dual color LED is useful in gas detection since one device emits a wavelength centered on an absorption line and also emits a reference wavelength that is not absorbed by a target gas, while both wavelengths are transmitted through the atmosphere with low loss. The presence of target detection gas is determined by comparing the intensities of the absorption and reference wavelengths after the light has traversed a sample. 
         [0026]    The LED device simplifies and reduces the cost of absorption based gas detectors. Since the absorption and reference wavelengths are emitted from the same exact aperture, both wavelengths can share the same optical path, reducing the size and cost of the detector while also reducing potential sources of error due to optical path variation. 
         [0027]    The high modulation speed of the LED of the present invention permits switching between the colors rapidly in a pseudorandom sequence so that correlation analysis can significantly reduce error without increasing cost. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The schematic layout of the dual-color LED optimized for a gas detection system is depicted in  FIG. 2 . The layout comprises a substrate  180  of a first carrier type (p-type or n-type), an epitaxially grown transition layer  160  of a second carrier type (n-type or p-type, respectively), and an epitaxially grown cap layer  170  of the first carrier type. A first epitaxially grown active region  140  emitting a light of a first wavelength is sandwiched between the substrate  180  and the transition layer  160 . A second epitaxially grown active region  150  emitting a light of a second wavelength is sandwiched between the cap layer  170  and the transition layer  160 . It is noted that the first wavelength is shorter than the second wavelength, in order to avoid absorption of the light of the second wavelength by the first active region  140 . A metal contact  120  is deposited on top of the cap layer  170 , and another metal contact  130  shown in the picture as two disconnected portions, but optionally connected outside of the picture plane is deposited on the back side of the substrate  180 . The back side of the substrate is defined as a side opposite to the side on which epitaxial growth of the semiconductor layers is performed. An external bi-directional power source  110  can be further connected to contacts  120  and  130  to achieve the light emission from the device. 
         [0029]    The exact detailed structures of the active regions  140  and  150  are not essential for the present invention and could be discovered by an artisan skilled in the art. The three most conventional band diagrams corresponding to the possible active regions structures are shown in  FIG. 3 .  FIG. 3   a  demonstrates the schematic band diagram (without external electric bias) of a bulk active region comprising a single semiconductor material.  FIG. 3   b  demonstrates the schematic band diagram (without external electric bias) of a quantum well active region comprising a semiconductor material of a first band gap with inclusion(s) of a second semiconductor material of a second band gap, where the second band gap is narrower than the first band gap, the inclusion(s) of the second material having thickness(es) small enough to create (a) quantum well(s) for carrier localization.  FIG. 3   c  demonstrates the schematic band diagram (without external electric bias) of a superlattice active region, which is similar to the quantum well active regions where the inclusions of the narrow band semiconductor materials are separated by short distances, which allows for a sub-band creation that is common for multiple quantum wells. The black arrows in  FIG. 3  illustrate the optical transition (radiative electron-hole recombination process). 
         [0030]    A schematic band diagram (without external bias) of the dual-color LED of one of the embodiments of the present invention is shown in  FIG. 4 . The embodiment comprises the substrate  180  of n-type, the transition layer  160  of p-type, and the cap layer  170  of n-type. For simplicity, the first and the second active regions  140  and  150  with bulk structure of  FIG. 3   a  are shown. Note that due to typical difference in the effective masses of electrons and holes, the Fermi energy  200  of the structure lays slightly higher than the valence band edge in p-material of the transitional layer  160 , and also slightly above the conduction band edge in n-materials of the substrate  180  and the cap layer  170 . A circuit schematic representation  300  of the structure is also shown. 
         [0031]    A key aspect of the dual-color LED of this n-p-n embodiment is the design of the p-type transitional layer  160 . The top of the valence band of the active regions  140  and  150  and the transitional layer  160  are aligned in order to facilitate the hole transport. As a result, the core operating principle of the device is realized: that each active region emits light under forward bias, and conducts current easily when reverse biased without emitting light. 
         [0032]    A schematic band diagram (without external bias) of the dual-color LED of one of the embodiments of the present invention is shown in  FIG. 5 . The embodiment comprises the substrate  180  of p-type, the transition layer  160  of n-type, and the cap layer  170  of p-type. For simplicity, the first and the second active regions  140  and  160  with bulk structure of  FIG. 3   a  are shown. Note that due to typical difference in the effective masses of electrons and holes, the Fermi energy  200  of the structure lays slightly higher than the conduction band edge in n-material of the transitional layer  160 , and also slightly above the valence band edge in p-materials of the substrate  180  and the cap layer  170 . A circuit schematic representation  400  of the structure is also shown. 
         [0033]    A key aspect of the dual-color LED of this p-n-p embodiment of  FIG. 5  is the design of the n-type transitional layer  160 . The bottom of the conduction band of the active regions  140  and  150  and the transitional layer  160  are aligned in order to facilitate the electron transport. As a result, the core operating principle of the device is realized: that each active region emits light under forward bias, and conducts current easily when reverse biased. 
         [0034]      FIG. 6  illustrates this core operating principle in case of the p-n-p embodiment of  FIG. 4 . This embodiment is selected for illustration without limiting the scope of the present invention; a skilled artisan can apply the illustrated principle to the n-p-n embodiment without any difficulty. In  FIG. 6 , two schematic band diagrams of  FIG. 4  are shown under different bias polarities. The polarities are depicted by symbols “+” and “−” in circles. Here again, the simplest case of bulk active regions are shown without limiting the scope of the invention. When positive bias is applied to the cap layer  170  with respect to the substrate  180 , the first active region  140  is forward biased and produces the light, while the second active region  150  is reverse biased and conducts current due to the hole tunneling from the valence band of the transitional layer  160  into the conduction band of the cap layer  170  under high electric field. Respectively, when negative bias is applied to the cap layer  170  with respect to the substrate  180 , the second active region  150  is forward biased and produces the light, while the first active region  140  is reverse biased and conducts current due to the hole tunneling from the valence band of the transitional layer  160  into the conduction band of the substrate  180  under high electric field. 
       Example Structure 
       [0035]    In order to validate the teachings of the present invention, a dual-color LED optimized for the gas detection system was fabricated using GaSb material system. The layout structure of this example device is presented in  FIG. 7 . The bulk substrate  180  of n-type GaSb material was obtained from a commercial vendor. A first active region  140  emitting the light of the wavelength of ˜8 um (photon energy ˜0.15 eV) was grown using Molecular Beam Epitaxy (MBE). Next, the transitional layer  160  comprising a p-type GaSb material was grown by MBE on top of the first active region  140 . After that, the second active region  150  emitting the light of the wavelength of ˜4 um (photon energy ˜0.3 eV) was grown by MBE on top of the transitional layer  160 , followed by the cap layer  170  comprising n-type In 0.1 GaAs 0.088 Sb composition growth by MBE. 
         [0036]    Fabrication of this exemplary dual-color LED included a deep etched 400 um wide round mesa with top annular contact deposited onto the substrate. The device was mounted epi-layer side down; the emission was out-coupled through the substrate surface. 
         [0037]    As illustrated above, when conducting the electric current, one active region of the device is under forward bias and emits light, while the other is under reverse bias and conducts without emitting light. The question in this approach is whether a reverse-biased p-n junction can transmit high current without permanent damage. Reverse biased diodes have been successfully used as a connection region in type I cascaded lasers and LEDs [S. Jung, S. Suchalkin, G. Kipshidze, D. Westerfeld, G. Belenky, Photonic Technology Letters, v. 25 (23), pp. 2278-2280 (2013), R. Q. Yang, Y. Qiu, Journal of Applied Physics, v. 94(11), pp. 7370-7372(2003)]. More than 10 milliwatts of the optical power at room temperature was obtained from a two-cascade LED where the current was injected into second cascade through the reverse biased GaSb p-n junction. The bias voltage for this two-cascade device was increased by a factor of ˜2 which indicates low voltage loss across the reverse biased unction between the cascades. No damage was observed. As long as the bandgap of the active regions materials of the dual-color LED of the present invention is less than the bandgap of GaSb, low voltage losses are expected on the reverse biased heterostructure. 
         [0038]    The I-V characteristic of the fabricated exemplary dual-color LED is presented in  FIG. 8 . Here, positive voltage corresponds to the higher potential of the substrate. As expected above, there is no voltage blocking by a reverse-biased p-n junction in any direction of the current. 
         [0039]    The emission spectra of the fabricated exemplary dual-color LED is presented in  FIG. 9 . When positive bias is applied to the substrate, the device emits light that peaks at the photon energy around 0.15 eV. When negative bias is applied to the substrate, the device emits light that peaks at the photon energy around 0.28 eV. 
         [0040]    Thus, the example validates that the present invention enables the two-terminal, dual-color LED emitting the light of two different wavelengths in the IR region of the spectra, depending on the polarity of the allied electric signal, from the same physical aperture. Such a device is optimized for usage in the low-cost, simple and yet reliable gas detection systems that can find wide range of applications, including household natural gas/methane/carbon monoxide detection systems and pipeline monitoring systems.