RU73126U1 - TWO-COLOR LED WITH INTEGRATED THERMAL REFRIGERATOR FOR THE MIDDLE INFRARED SPECTRUM - Google Patents

Abstract

The inventive utility model relates to devices emitting electromagnetic waves in the middle infrared range (1.6-5.0 μm), and can be used in portable devices for optical spectroscopy of gases and liquids in the maximum intensity range of characteristic absorption lines of chemical substances. The main areas of application are environmental monitoring, medical diagnostics and continuous monitoring of technological processes. The objective of the claimed utility model is the implementation of a compact two-color high-performance LED emitter for the mid-IR spectral region with a design that allows thermal stabilization of radiation.

This problem is solved by developing a two-color LED with an integrated thermo-refrigerator and a thermistor. The product includes two LED chips with maximum radiation at different wavelengths. Chips are made on the basis of heterostructures. grown on GaSb substrates with a quaternary GaInAsSb solid solution as an active region and a wide-gap AlGaAsSb boundary layer for the range 1600-2400 nm, or heterostructures grown on InAs substrates with a binary InAs, a triple InAsSb or quadruple InAsSbP layer as an active region and a wide-gap bounding layer InAsSbP for the range of 2700-5000 nm.

The implementation of a two-channel thermostabilized emitter for the mid-IR region of the spectrum allows developers of portable analyzers to significantly improve the compactness, reliability, and service life of their products.

Description

The utility model relates to devices emitting incoherent electromagnetic waves in the mid-infrared range (1.6-5.0 microns). and can be used for optical spectroscopy of gases and liquids in the range of maximum intensity of characteristic absorption lines of chemicals. The main areas of application are environmental monitoring, medical diagnostics and continuous monitoring of technological processes.

The main characteristic absorption lines of natural and industrial gases and liquids, such as methane CH ₄ , carbon dioxide CO ₂ , water H ₂ O, and others lie in the mid-infrared spectral range 1.6–5.0 μm. These absorption bands are unique to each chemical like fingerprints and are well known in modern molecular spectroscopy.

Currently, there are portable optical analyzers in which dispersion (thermal) sources are used as a source of infrared radiation (http://www.bhkinc.com ). In them, a wire or other conductive element is heated by a flowing electric

current and radiates in a wide spectral range. Special optical filters cut out the desired spectral range. Such IR emitters can be considered as a functional analog of LEDs for the mid-IR region of the spectrum.

Thermal sources of infrared radiation as a key element of portable optical sensors have several disadvantages:

- Low efficiency. A large electric power is consumed, and a very small part is used from a wide spectrum of thermal radiation.

- Poor performance. Such a source really cannot be modulated electrically, therefore, mechanical modulators are used to provide selective amplification of the detector signal.

- Big sizes. The compactness of such a sensor is limited by the need to use additional filters, modulators, as well as a large dissipated thermal power.

There are sources of average IR radiation based on re-radiation (photoluminescence), which can also be considered as an analogue. In them, a polycrystalline PbSe layer is deposited on a transparent window. Short-wavelength radiation in the region of 0.9 μm from the GaAs LED excites photoluminescence in a lead selenide layer with a wide spectrum and a maximum of about 4.3 μm. The desired spectral range of radiation is cut out using an optical filter, www.optico.ru In general, the above disadvantages (low speed, the need to use filters and the use of a small part of the total radiation spectrum) remain valid for this type of IR emitters. The disadvantages should also be added significantly higher

the cost of emitters based on re-radiation compared to heat sources.

The results of measuring gas concentration using an optical analyzer are influenced by factors such as changes in temperature and optical transparency of the medium. Therefore, the optimal measuring circuit should contain two channels - measuring (at the wavelength of maximum absorption of the measured gas) and reference (at the wavelength, where the absorption of this gas is minimal). Measurement of the differential difference of the signals from the measuring and reference source ensures the stability of the results when changing the main external factors. Optimum reliability and stability of measurements is ensured in the case of using a two-beam circuit with temperature stabilization.

The objective of the proposed technical solution was to create a new two-color LED with a built-in thermal refrigerator for the mid-IR region of the spectrum. A key element of the two-color LED module is LED heterostructures based on narrow-gap A ³ B ⁵ semiconductor materials. The designs of LED heterostructures are shown in Fig. 1. The wavelength of the LED radiation is determined by the band gap, that is, the composition of the solid solution in the active region. For LEDs emitting in the range 1700-2400 nm (Fig. 1 a), in the active region, a GaInAsSb quaternary solid solution with an indium content of 0-25% is used. A wide-gap AlGaAsSb solid solution with an aluminum content of 64% is used as an electronic limitation. The structures are grown on GaSb substrates by liquid phase epitaxy (LPE) or gas phase epitaxy from organometallic compounds (MOCVD). For LEDs emitting in the range of 2700-5000 nm

(Fig. 16), InAsSb (P) quaternary or ternary solid solution with a phosphorus content of 0-15% (2700-3400 nm) or antimony 0-20% (3400-5000 nm) is used in the active region. A wide-gap InAsSbP solid solution with a phosphorus content of 50% is used as an electronic limitation. The structures are grown on InAs substrates by gas phase epitaxy from organometallic compounds (MOCVD). Using photolithography, heterostructures are used to form LED chips ranging in size from 0.3 × 0.3 mm to 1.0 × 1.0 mm. Then the chip is mounted on a ceramic or silicon substrate ranging in size from 0.8 × 0.8 mm to 2.0 × 2.0 mm (Fig. 2).

The two-color LED module (Fig. 3) consists of:

- building - 9

- thermo-refrigerator - 10

- thermistor - 11

- measuring LED chip - 12

- reference LED chip - 13

- reflector or cover with window - 14

The module pin numbers in Fig. 3 are indicated by the numbers 1-8.

The module is mounted in a 9 mm standard TO-5 package with 6 or 8 pins (9). A 3 × 3 mm (10) thermo-refrigerator (Peltier element) embedded in the housing provides a temperature difference between the hot and cold ends without a load of at least 60 degrees. A temperature sensor - a thermistor (11) is glued to the cold end of the thermo-refrigerator. Two LED chips emitting at different wavelengths (12 and 13) are already glued onto the free surface of the cold end of the thermo-refrigerator. The anode and cathode of the measurement LED are connected

gold wires with the external terminals of module 1 and 2. The anode and cathode of the reference LED are connected to terminals 3 and 4. The terminals of the thermo refrigerator are connected to external terminals 5 and 6. The terminals of the thermistor are connected to external terminals 7 and 8. When using a 9 mm case with 6 leads the cathodes of the two chips are soldered directly to the case. A lid with a window with a diameter of 9 mm or a parabolic reflector with a window with a diameter of 15 mm is welded to the body to narrow the radiation pattern of the LED.

The device operates as follows.

Conclusions 1 and 2 of the LED module are connected to a switching power supply unit of the measuring channel. Pins 3 and 4 are connected to the pulse power supply of the reference channel. Pins 5 and 6 are connected to the temperature controller, and more precisely to the power circuit of the thermo-refrigerator. Pins 7 and 8 are connected to the feedback circuit of the temperature controller. An electronic circuit including switching power supplies of two LED channels and a temperature controller is not part of the claimed design. Different versions of such an electronic circuit can be implemented by users of a two-color LED module for the mid-IR region of the spectrum. In this case, the circuit should provide the operating modes indicated in the technical passport of a particular LED module.

First you need to select and set the desired temperature. To do this, the current of the thermo-refrigerator is smoothly increasing (terminals 5 and 6) while monitoring the temperature through measuring the resistance of the thermistor (terminals 7 and 8). A specific calibration curve of the thermistor is attached to the individual technical data sheet of the module. When the selected temperature is reached, it is necessary to enable stabilization mode of this

temperature using a negative feedback circuit. When the temperature stabilization process is completed, it is necessary to turn on the pulse power of the measuring and reference LED chips. The duration of the pulses and the amplitude of the current are selected in accordance with the intervals of permissible values stated in the technical data sheet. It is advisable to choose a ratio of the currents of the measuring and reference channel, which provides a zero differential signal at the receiver at zero concentration of the measured substance. Then an increase in the concentration of the substance in the medium will lead to an excess of the signal from the reference channel over the signal from the measuring channel. The differential difference will be proportional to the concentration of the measured substance.

A prototype of the inventive device was manufactured and tested. This two-color LED module with a built-in thermal refrigerator for the mid-IR region is characterized by compactness and unity of design. In 9 mm size there are two different (measuring and reference) LED emitters, a thermo-refrigerator and a thermosensor. In this regard, the inventive device has no direct analogues in the mid-IR region of the spectrum, where for the implementation of a two-channel circuit based on thermal or re-emitting sources, significantly larger optical systems are used. The small distance between the chips provides the same operating conditions for two channels, which gives an important advantage of this design over conventional two-channel circuits. The maximum wavelength of the radiation and the half-width of the radiation spectrum is determined by the parameters of the heterostructure itself, and not by an external optical filter. The lifetime of LEDs 80,000-100,000 hours is significantly longer than the life of others

types of sources of infrared radiation. The well-known process of slow degradation of the power of semiconductor LEDs occurs identically for two chips of the same type of structure, which ensures the stability of the differential signal for 8-10 years. The design allows you to choose the total temperature for two emitters in the range of -10 ÷ 20 ° C and maintain it constantly with minimal electrical power. The small size and high efficiency of the thermo-refrigerator allows you to maintain a temperature close to room temperature with a constant current of about 10 mA. A two-color LED module with a built-in thermal refrigerator for the mid-IR spectral range fulfills all the requirements of portable optical gas analysis.

The positive effect of the claimed utility model in comparison with heat sources and sources based on reradiation for the middle IR region consists in an extended service life of up to 10 years of continuous operation (80,000-100,000 hours), increased speed (rise and fall times of less than 50 ns), no need use of additional optical filters. The flexibility of heteroepitaxy technology and the miniature size of the LED chips made it possible to realize a unique design, where a thermo-refrigerator, a thermosensor and two emitters are combined in one standard 9 mm case. For considered as analogues of heat sources and sources based on re-radiation for the middle IR region, the implementation of such a miniature design is in principle impossible. The inventive utility model allows developers of portable gas analyzers to significantly improve the compactness, reliability and durability of their products.

Claims (2)

1. A two-color LED with a built-in thermal refrigerator for the mid-IR spectral region (1600-5000 nm), which includes two LED chips with emission maxima at different wavelengths based on heterostructures grown on GaSb substrates with four GaInAsSb solid solution as the active region and a wide-gap AlGaAsSb boundary layer for the range 1600-2400 nm, or heterostructures grown on InAs substrates with a binary InAs, a triple InAsSb or a quadruple InAsSbP layer as an active region, and a wide-gap InAsSbP boundary layer for I range 2700-5000 nm, placed in a single LED housing with built-in thermo-refrigerator and thermistor. 2. The device according to claim 1, characterized in that the said two-color LED in a single housing includes a two-channel thermostabilized circuit.