WO2014021623A1

WO2014021623A1 - Method and device for measuring internal quantum efficiency of an optical element

Info

Publication number: WO2014021623A1
Application number: PCT/KR2013/006869
Authority: WO
Inventors: 심종인; 한동표; 정현돈
Original assignee: 주식회사 에타맥스
Priority date: 2012-07-31
Filing date: 2013-07-31
Publication date: 2014-02-06

Abstract

A method for measuring the efficiency of an optical element is disclosed. The intensity of the light emitted from the optical element is measured by applying an injection current to the optical element, a relative radiative efficiency is calculated from a ratio of the intensity of the emitted light to the injection current, the maximum relative radiative efficiency and the maximum injection current corresponding to the maximum relative radiative efficiency are obtained, a reference injection current for minimizing an amount of change of a recombination coefficient in an active layer of the optical element in correspondence with a carrier density change in the active layer of the optical element is extracted from data of injection currents that are equal to or less than the maximum injection current and data of relative radiative efficiencies that are equal to or less than the maximum relative radiative efficiency, a reference internal quantum efficiency of the optical element is calculated from the reference injection current, and internal quantum efficiencies of the optical element in various injection currents are calculated from the reference internal quantum efficiency.

Description

Method and device for measuring internal quantum efficiency of optical device

The present invention relates to an optical device, and more particularly, to a method and apparatus for measuring the internal quantum efficiency of a light emitting diode.

In general, a light emitting diode (LED) is widely used as a light source because of its small size, low power consumption, and high reliability. The light emitting diode uses compound semiconductors such as InGaSaP, AlGaAs, GaAlP, GaP, InGaAlP, GaN. The light emitting diode includes an N-type semiconductor layer composed of a compound semiconductor, an active layer on the N-type semiconductor layer, and a P-type semiconductor layer on the active layer. A light-emitting diode (LED) is a kind of p-n junction diode, and is a semiconductor device using electroluminescence, a phenomenon in which light is emitted when a voltage is applied in a forward direction. The central wavelength of light emitted from the light emitting diode is determined by the bandgap energy (Eg) of the semiconductor used.

As a method of measuring internal quantum efficiency at a specific temperature (eg, room temperature) of a light emitting diode, a temperature dependent electroluminescence (TDEL) method is most commonly used. This method uses the excitation current at cryogenic temperatures (approx.IIntensity of emitted light according toPRelative luminous efficacy defined as the ratio ofη)(In other words,η = P / I) Is the maximum, that is, the maximum relative luminous efficiency (η _max) (In other words,η _max = P _max / I _maxMaximum excitation current withI _max), The internal quantum efficiency (η _IQE) Is assumed to be 100%. Also, any excitation current (for example, at room temperature)IInternal quantum efficiency atη _IQE) Is the relative luminous efficiency (η = P / I) And maximum relative luminous efficacy at cryogenic temperatures (η _max = P _max / I _max) Ratio, i.e. (P / I) / ( P _max / I _max) Is obtained. However, the cryogenic temperature can be assumed to be 100% internal quantum efficiency is limited to the case where the maximum value of the relative luminous efficiency gradually increases to a specific maximum value as the temperature is lowered. In addition, changing the temperature from cryogenic to room temperature takes a very long time (about 5-6 hours) and requires expensive equipment for temperature testing. Due to the size limitation of the chamber of the device for temperature testing, only a small portion of the wafer has to be cut and measured, so the internal quantum efficiency of the entire wafer cannot be measured. External quantum efficiency defined by (number of photons exiting into free space per unit time) / (number of electrons injected into an optical element per unit time)η _EQE) Can be measured experimentally. External quantum efficiencyη _EQE) Is the internal quantum efficiency (η _IQE) And light extraction efficiencyη _extractionSince the internal quantum efficiency can be measured, it is possible to separately measure the internal quantum efficiency and the light extraction efficiency.

The present invention is to provide a method and / or apparatus capable of measuring the internal quantum efficiency of the light emitting diode.

The present invention provides a method for measuring the efficiency of an optical device. The method includes applying an excitation current ( I ) to an optical device to measure the intensity P of emitted light from the optical device; Calculating a relative luminous efficiency η from the ratio P / I of the intensity P of the emitted light to the excitation current I ; Obtaining a maximum relative luminous efficiency and a maximum excitation current corresponding to the maximum relative luminous efficiency; From the data of the excitation current below the maximum excitation current and the data of the relative luminous efficiency below the maximum relative luminous efficiency, the change amount of the recombination coefficient in the active layer of the optical element is small with respect to the carrier concentration change in the active layer of the optical element. Extracting a reference excitation current I _ref to be; Calculating a reference internal quantum efficiency η _{IQE, ref} of the optical device at the reference excitation current; And calculating, from the reference internal quantum efficiency η _{IQE, ref} , the internal quantum efficiency η _IQE of the optical device at various excitation currents.

The extracting of the reference excitation current may include a curve of the second parameter y with respect to the first parameter x.

Extracting the reference excitation current I _ref from the second parameter y at the point where the derivative of b with respect to x becomes the minimum, wherein the first parameter x is

, The second parameter (y) is

Where I is the excitation current, I _normal is the standard excitation current below the maximum excitation current, P is the intensity of the emitted light, and P _normal is the intensity of the standard emitted light at I _normal .

The internal quantum efficiency η _{IQE, ref} of the optical device at the reference excitation current I _ref is

, Wherein, a _ref is the reference excitation current (I _ref) the value of a, b _ref is the b value of the in the reference excitation current (I _ref), and P _ref are (I _ref) the reference exciting current in The intensity of the emitted light at

The internal quantum efficiency η _IQE of the optical device at the various excitation currents I

Where η is the relative luminous efficiency calculated by P / I at the excitation current I , η _ref is the relative luminous efficiency calculated by P _ref / I _ref at the reference excitation current I _ref .

The present invention provides an apparatus for measuring efficiency of an optical device. The apparatus includes an optical measuring unit for applying an excitation current to the optical device to measure the intensity of the emitted light from the optical device; And extracting a reference excitation current at which the second derivative value of the second parameter y curve is minimized relative to the first parameter x, calculating an internal quantum efficiency of the optical device at the reference excitation current, and calculating the reference excitation current. And an operation unit for calculating the internal quantum efficiency of the optical device at various excitation currents from the internal quantum efficiency of the optical device in (1), wherein the first parameter (x)

, The second parameter (y) is

The reference excitation current I _ref is a curve of the second parameter y with respect to the first parameter x.

Is the excitation current at the point where the derivative of b with respect to x becomes the minimum.

The internal quantum efficiency η _{IQE, ref} of the optical element at the reference excitation current I is

Using the concept of the present invention, the internal quantum efficiency can be measured nondestructively within a short time (about 5 minutes) immediately after the production of the light emitting diode, and the internal quantum efficiency can be separated from the external quantum efficiency in the chip or package state Can be. Since the efficiencies of the light emitting diodes can be separated, it is possible to easily diagnose the cause of defects in the production of the light emitting diodes.

1 is a view for explaining an efficiency measuring device of an optical device according to an embodiment of the present invention.

2 is a flow chart for calculating the internal quantum efficiency according to an embodiment of the present invention.

3 is a graph of the intensity of emitted light measured by injecting an excitation current into a light emitting diode.

FIG. 4A is a graph of relative light emission efficiency η with respect to the excitation current obtained from FIG. 3.

4B is a conceptual diagram corresponding to FIG. 4A.

5 is an x-y graph according to a model of the present invention.

6 shows two linear equations from the data at two points A1 and A2 measured immediately adjacent.

Figure 7 shows the results of a and b for x and the derivatives therefor.

8 is a graph of the calculated excitation current-internal quantum efficiency.

9 is a view for explaining the details of the operation unit according to an embodiment of the present invention.

10 shows the ratio of the relative luminous efficiencies at multiple temperatures to the maximum relative luminous efficiency near the absolute temperature.

11 shows internal quantum efficiencies at multiple temperatures obtained according to the model of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosure may be made thorough and complete, and to fully convey the spirit of the invention to those skilled in the art. In addition, since it is in accordance with the preferred embodiment, reference numerals presented in the order of description are not necessarily limited to the order.

Embodiments according to the spirit of the present invention discloses a method for nondestructively measuring the internal quantum efficiency of the optical device (for example, a light emitting diode) at room temperature or constant temperature. The carrier of the light emitting diode is excited by the excitation current and loses energy in the form of emitted light through recombination.

1 is a view illustrating an internal quantum efficiency measuring apparatus 100 of an optical device according to an embodiment of the inventive concept. Referring to FIG. 1, the apparatus 100 for measuring internal quantum efficiency of an optical device includes a central control unit 110, a current supply unit 120 for applying an excitation current to the optical device 140, and an optical measuring unit 130. Include. The optical measuring unit 130 may include an optical sensor 131 and an optical fiber 132. The optical device 140 may be a light emitting diode chip or a packaged light emitting diode.

The central controller 110 controls the operation of the optical measuring unit 130 and collects the intensity of the emitted light from the optical device 140 according to the application of the excitation current through the current supply unit 120 to improve the internal quantum efficiency of the optical device. Can be calculated The central controller 110 may include a calculator 111 for calculating the internal quantum efficiency of the optical device. The optical measuring unit 130 may exchange necessary data with the central control unit 110. The central controller 110 may transmit a control signal to the current supply unit 120 so that a current having a required intensity can be injected into the optical device 140. In addition, the light measuring unit 130 may detect the emitted light from the optical device 140, generate a predetermined electrical signal corresponding to the intensity of the emitted light, and transmit it to the central controller 110.

3 to 8, a method of calculating the internal quantum efficiency of the optical device according to the change of the intensity of the excitation current in the calculation unit of FIG. 1 will be described.

2 and 3, the intensity P of the emission light of the light emitting diode may be measured by injecting an excitation current I into the light emitting diode. (S10)

As an indicator of the performance of the light emitting diode, an external quantum efficiency η _EQE is mainly used. The external quantum efficiency may be expressed as a product of light extraction efficiency ( η _extaction ), injection efficiency ( η _injection ) and radiative efficiency ( η _radiative ), as shown in Equation 1 below.

Equation 1

Here, the product of the luminous efficiency ( η _radiative ) and the injection efficiency ( η _injection ) is defined as an internal quantum efficiency ( η _IQE ), and the luminous efficiency ( η _radiative ) is generally a carrier-rate _square as in Equation (2). expressed as a carrier rate equation. Therefore, the external quantum efficiency η _EQE may be expressed as a product of the internal quantum efficiency η _IQE and the light extraction efficiency η _extaction , as in Equation 2.

Equation 2

Here, the external quantum efficiency η _EQE is (the number of photons escaped into free space per unit time) / (the number of electrons injected into the optical element per unit time), and the internal quantum efficiency ( η _IQE ) is (active layer of the optical element per unit time). The number of photons generated by / in the number of electrons injected into the optical device per unit time, and the injection efficiency ( η _injection ) are (the number of electrons injected into the active layer of the optical device per unit time) / (injected into the optical device per unit time) The number of electrons), the luminous efficiency ( η _radiative ) is the number of photons generated in the active layer of the optical device per unit time / the number of electrons injected into the active layer of the optical device per unit time, and the light extraction efficiency ( η _extaction ) The number of photons that escaped into the free space per hour) / (the number of photons generated in the active layer of the optical device per unit time). In the carrier rate equation representing the luminous efficiency of Equation 2, A is the non-luminescence recombination coefficient, B is the luminescence recombination coefficient, N is the carrier concentration of the active layer. A and B are expressed as functions of N. Hereinafter, the concept of relative light emission efficiency using an increase rate of the relative light amount instead of the external quantum efficiency by the absolute light amount described above is used.

Referring to FIGS. 2 and 4A, the relative luminous efficiency η can be calculated by (intensity of emitted light) / (excitation current), that is, P / I. (S20) From FIG. 4B, the maximum relative luminous efficiency ( η _max ) and the maximum excitation current I _max corresponding to the maximum relative luminous efficiency are obtained (S30).

4b shows the maximum relative luminous efficiency (η _maxIn the region of excitation current below), the change in relative light emission efficiency is almostη _radiativeIs determined by Light extraction efficiencyη _extaction) Is determined by the shape of the optical device, so it can be treated as a constant value that does not change with the injection current. In addition, in the step of starting to flow a current to the optical device, the electrons injected from the electrode first reach the active layer region having the lowest potential energy and start to recombine, so the injection efficiency is close to 100%. The development of the following equations is considered in the section that can be treated as a constant because the change in the injection efficiency and the light extraction efficiency is sufficiently smaller than the change in the luminous efficiency according to the current change. At this time, the considered excitation current (I) Is the maximum relative luminous efficiency (η _max) Corresponding to the maximum excitation current (I _max) That is, the range of data considered first in the present invention is 0 <I<I _maxIs the range (hereinafter considered).

When the light emission intensity P represents the light emission recombination process as a dichotomous recombination process of the free carrier as shown in Equation 3, the emission light intensity P may be expressed as a product of the square of the emission recombination coefficient B and the carrier concentration N of the active layer. Can be.

Equation 3

here,V _aIs the volume of the light emitting diode active layer,qIs the charge of the electron,η _c Denotes the optical coupling efficiency between the light emitting element and the light receiving element. Standard excitation currentI _normal) Is the maximum excitation current (I _max) Excitation current as a standard for normalization required for analysis below. Standard excitation currentI _normalIntensity of emitted light measured inP) Is the standard emission intensity (P _normalIt is defined as Standard excitation currentI _normalRelative luminous efficacy at)η _normal) (In other words,η _normal = P _normal / I _normalIs defined as). In the following embodiments, the maximum relative luminous efficiency (η _max) Corresponding to the maximum excitation current (I _max) The standard excitation current (I _normal)I _normal=I _max) Data was obtained. Intensity of standard emission light (P _normal) Can be expressed by the following equation (4).

Equation 4

Here, N _normal is the carrier concentration at the standard excitation current I _normal . By dividing Equation 3 by Equation 4, the carrier concentration N may be expressed by only the intensity P of emission light and the emission recombination coefficient B , which is expressed as Equation 5 below.

Equation 5

On the other hand, from the equations (2) and (3), the excitation current I can be expressed as shown in the following equation (6).

Equation 6

In the standard excitation current ( I _normal ), it is represented by the following equation (7).

Equation 7

By dividing Equation 6 by Equation 7, the excitation current I can be expressed by an expression relating to the emission recombination coefficient B , the non-emission recombination coefficient A , and the carrier concentration N. This is expressed as in Equation 8.

Equation 8

Substituting the carrier concentration N of equation (5) into equation (8), it is expressed by the following equation (9).

Equation 9

If Equation 9 is simply expressed, it is expressed as Equation 10.

Equation 10

here,

,

Where a is a function of carrier concentration N , which varies with A (N), B (N). b is a constant that does not change with carrier concentration ( N ). In other words, when the mathematical model represented by Equation 10 is applicable, it means that the coefficient of b should be a constant. In Equation 10, I / I _normal and

Are externally measurable values. At this time,

,

to be. Therefore, referring to FIG. 5, from the intensity characteristic of the excitation current-emission light of the light emitting diode, y = I / I _normal ,

You can get the xy graph defined by.

a AndbThe rate of change of is very small compared to the rate of change of x and y. Thus, referring to FIG. 6, two linear equations (from the data x1, y1, x2, y2) at two points A1 and A2 measured immediately adjacent (y _One = ax _One + bx _One ² Andy ₂ = ax ₂ + bx ₂ ² ) Can be obtained. Solving the binary first-order equation, the interval between the two points A1 and A2aAndbCan be obtained. SuchaAndbIs another point (i.e. 0 <I<I _maxCan also be obtained. 7 is for x in the consideration intervalaAndbShows the result.

Referring to FIG. 7, both a and b are variables depending on the carrier concentration N. From equations (3) and (10), the x-axis is monotonically proportional to the carrier concentration ( N ). As it described above, according to the model of the present invention, a is one appropriate to be represented as a function of the carrier concentration (N), b is to be represented by a constant which does not depend on the carrier concentration (N). Here, it is assumed that the reference point is a point where the rate of change of the carrier concentration N of b is the smallest (the point that can be expressed as a constant), that is, the point where the derivative of b with respect to x becomes the minimum (that is, the point close to zero). In other words, the second derivative of the second parameter y curve relative to the first parameter x is minimal. At this reference point, the model presented in Equation 2 may best fit. The excitation current I at the reference point is defined as the reference excitation current I _ref . (S40) Therefore, at the reference excitation current I _ref , the reference internal quantum efficiency η _{IQE, ref} is represented by the model shown in Equation 2. ) Can be expressed by Equation 11 (S50).

Equation 11

Here, a _ref , b _ref , and P _ref are the intensity a of the parameter a , the parameter b and the emission light P at the reference excitation current I _ref , respectively.

By using the reference internal quantum efficiency η _{IQE, ref} , the internal quantum efficiency η _IQE at the remaining various excitation currents I can be obtained. Referring to Equation 12, it is possible to obtain the measured overall excitation current ( I ) _-internal quantum efficiency ( η _IQE ) characteristics. (S60)

Equation 12

Here, η and η _ref is the relative luminous efficiency are measured at each excitation current (I) and a reference excitation current (I _ref). 8 is a graph of the excitation current-internal quantum efficiency calculated by Equation (12). Very consistent with the graph of FIG. 2 actually measured.

Referring to FIG. 9, calculation of the efficiency of the optical element of the calculating unit 111 constituting the central control unit of FIG. 1 will be described. The calculation unit 111 according to the embodiment of the inventive concept may perform the above-described steps of FIG. 2. The calculation unit 111 may include a data input unit 112, an external quantum efficiency calculator 113, a maximum excitation current calculator 114, a reference excitation current strength extractor 115, a reference internal quantum efficiency calculator 116, and The internal quantum efficiency calculation unit 117 may be included.

The data input unit 112 collects the intensity P of the emitted light output from the light measuring unit 130. The relative light emission efficiency calculator 113 calculates the relative light emission efficiency from the ratio P / I of the intensity P of the emitted light to the excitation current I as in S20. The maximum excitation current calculator 114 obtains the maximum relative light emission efficiency and the corresponding maximum excitation current I _max from the relative light emission efficiency data in the relative light emission efficiency calculator 113 as in S30. The reference excitation current extractor 115 extracts the reference excitation current I _ref as in S40. The reference internal quantum efficiency calculation unit 116 calculates the internal quantum efficiency η _IQE.ref of the optical element at the intensity of the reference excitation current as in S50. The internal quantum efficiency calculator 117 calculates the internal quantum efficiency η _IQE at various excitation currents as in S60.

10 shows the ratio of relative luminous efficiencies at multiple temperatures to relative luminous efficiencies near an absolute temperature. Relative luminous efficiency saturates at or near absolute temperature. Accordingly, the energy loss due to the non-luminescent recombination ( A ) is very small compared to the light emitting recombination of the carrier near the absolute temperature means that the internal quantum efficiency can be assumed to be 100%. 10 is a measurement result of the internal quantum efficiency obtained for a plurality of temperatures through the temperature dependent electroluminescent efficiency (TDEL) method based on the above point.

11 shows internal quantum efficiencies at multiple temperatures obtained according to the model of the present invention. The results obtained in FIG. 8 are very consistent with the actual measurement data of FIG. 10.

With the concept of the present invention, a short period of time immediately after the production of the light emitting diode in which (about 5 minutes) to be measured the internal quantum efficiency nondestructively, the internal quantum efficiency from the external light emission efficiency (η) of the chip or package state ( η _IQE.ref ) can be measured separately.

By separating the efficiencies of the light emitting diodes, it is possible to easily diagnose the cause, such as where the defects are caused when the light emitting diodes are produced.

Claims

Measuring an intensity P of the emitted light from the optical device by applying an excitation current I to the optical device;

Calculating a relative luminous efficiency η from the ratio P / I of the intensity P of the emitted light to the excitation current I ;

Obtaining a maximum relative luminous efficiency and a maximum excitation current corresponding to the maximum relative luminous efficiency;

From the data of the excitation current below the maximum excitation current and the data of the relative luminous efficiency below the maximum relative luminous efficiency, the change amount of the recombination coefficient in the active layer of the optical element is small with respect to the carrier concentration change in the active layer of the optical element. Extracting a reference excitation current I ref to be;

Calculating a reference internal quantum efficiency η IQE, ref of the optical device at the reference excitation current; And

Calculating the internal quantum efficiency ( η IQE ) of the optical device at various excitation currents from the reference internal quantum efficiency ( η IQE, ref ).
The method according to claim 1,

The extracting of the reference excitation current may include a curve of the second parameter y with respect to the first parameter x.
Extracting the reference excitation current I ref from the second parameter y at the point where the derivative of b with respect to x becomes the minimum at

The first parameter (x) is
, The second parameter (y) is
Where I is the excitation current, I normal is the standard excitation current below the maximum excitation current, P is the intensity of the emitted light, and P normal is the intensity of the standard emission light at I normal .
The method according to claim 2,

The internal quantum efficiency η IQE, ref of the optical device at the reference excitation current I ref is
, Wherein, a ref is the reference excitation current (I ref) the value of a, b ref is the b value of the in the reference excitation current (I ref), and P ref are (I ref) the reference exciting current in Efficiency measurement method of optical element which is intensity of emitted light in
The method according to claim 2,

The internal quantum efficiency η IQE of the optical device at the various excitation currents I
Where η is the relative luminous efficiency calculated by P / I at the excitation current I , η ref is the relative luminous efficiency calculated by P ref / I ref at the reference excitation current I ref Efficiency measurement method.
An optical measuring unit measuring an intensity of emitted light from the optical device by applying an excitation current to the optical device; And

Extract the reference excitation current at which the second derivative of the second parameter (y) curve is minimized compared to the first parameter (x), calculate the internal quantum efficiency of the optical device at the reference excitation current, and at the reference excitation current A calculation unit for calculating the internal quantum efficiency of the optical device at various excitation currents from the internal quantum efficiency of the optical device of

The first parameter (x) is
, The second parameter (y) is
Where I is the excitation current, I normal is the standard excitation current below the maximum excitation current, P is the intensity of the emitted light, and P normal is the intensity of the standard emission light at I normal .
The method according to claim 5,

The reference excitation current I ref is a curve of the second parameter y with respect to the first parameter x.
The efficiency measuring device of an optical element which is an excitation current at a point where the derivative of b with respect to x becomes minimum.
The method according to claim 6,

The internal quantum efficiency η IQE, ref of the optical element at the reference excitation current I is
, Wherein, a ref is the reference excitation current (I ref) the value of a, b ref is the b value of the in the reference excitation current (I ref), and P ref are (I ref) the reference exciting current in Efficiency measuring device of optical element which is intensity of emitted light in
The method according to claim 7,

The internal quantum efficiency η IQE of the optical device at the various excitation currents I
Where η is the relative luminous efficiency calculated by P / I at the excitation current I , η ref is the relative luminous efficiency calculated by P ref / I ref at the reference excitation current I ref Efficiency measuring device.