CN105606588A - Raman scattering method for measuring GaN thermal expansion coefficient - Google Patents

Abstract

A Raman scattering method for measuring the thermal expansion coefficient of GaN, comprising the following steps: 1) scribing and sampling the sample, and cleaning; 2) performing a variable temperature Raman test on the sample; 3) performing linear fitting on the test results; extracting the linear fitting combined slope and intercept. According to the extracted results, combined with the Gruneisen parameters The physical meaning of the physical meaning, to achieve the test and characterization of the thermal expansion behavior of solid materials; the present invention uses variable temperature Raman scattering technology, using Raman scattering to obtain the relationship between the Raman phonon frequency shift and temperature, can accurately realize the measurement of GaN, AlN and InN and other Group III nitride epitaxial layer thin film binary and multi-element alloy systems for non-destructive testing and characterization, avoiding the damage to the sample and the more complex formula derivation and mathematical calculation in the general characterization method; due to the shape and size of the sample There is no strict requirement, and the thermal expansion behavior of various semiconductor materials can be tested conveniently. The method is simple, easy to implement, and has small errors.

Description

A Raman Scattering Method for GaN Thermal Expansion Coefficient Measurement

technical field

The invention belongs to the technical field of testing methods for thermal expansion behavior of solid materials, and in particular relates to a Raman scattering method for measuring the thermal expansion coefficient of GaN, which is used for the measurement and characterization of the thermal expansion coefficient of GaN thin film materials. The relationship between frequency shift and temperature is analyzed, combined with the expansion law of solid materials, the relationship between the thermal expansion behavior of materials and the variable temperature Raman scattering frequency shift is processed, so as to obtain the relevant information about the thermal expansion behavior of GaN thin film materials.

Background technique

Group III nitride binary and multi-element alloy optoelectronic materials are ideal candidates for realizing solid-state lighting (Solid-State Lighting, SSL), and also for realizing semiconductor light-emitting diodes (Light-emitting Diode, LED), laser diodes (Laser Diode, LD), high It is an ideal material for brightness white light lighting engineering and energy saving and emission reduction. InN and its related Group III nitride ternary alloys (InAlN, InGaN) have direct and continuously adjustable bandgap characteristics between 0.7eV-6.2eV, and have a very wide spectral range, covering green light , blue light and ultraviolet spectrum, become the key material for making solid luminescent active region, and has great application prospects. From the material point of view, it is very difficult to produce blue light and ultraviolet light, and InGaN is the only luminescent material that can realize these two wave bands, and it matches very well with the spectrum of sunlight (0.4eV-4.0eV), making In _x Ga _1-x N has broad application prospects in the photovoltaic industry, especially in the fields of array solar cells, blue-green LEDs and white LEDs.

Under the condition of constant external pressure, the phenomenon of expansion of an object due to temperature change is called "thermal expansion". Most substances increase in volume when the temperature rises, and shrink in volume when the temperature decreases (that is, the phenomenon of thermal expansion and contraction). Under the same conditions, gases expand the most, liquids expand second, and solids expand the least. There are also a few substances within a certain temperature range. When the temperature rises, their volume decreases instead (that is, the phenomenon of thermal contraction and cold expansion). From the perspective of molecules, when the temperature of an object rises, the average kinetic energy of molecular motion increases, the distance between molecules also increases, and the volume of the object expands accordingly; when the temperature decreases, the average kinetic energy of molecules decreases when the object cools , so that the distance between molecules is shortened, so the volume of the object will be reduced. And because the average kinetic energy of solid, liquid and gas molecular motion is different, there are also significant differences from the macroscopic phenomenon of thermal expansion. We know that when the temperature changes, the volume of the material will expand and shrink, and its ability to change is the thermal expansion coefficient of the material as the volume change caused by the unit temperature change under isobaric conditions (the pressure p is constant). The coefficient of expansion characterizes the physical quantity introduced by the degree of change in length, area, and volume of an object when it is heated. It is the general term for linear expansion coefficient, surface expansion coefficient and volume expansion coefficient.

Group III nitride epitaxial layer thin film materials, due to the lack of intrinsic substrate materials, are often grown on substrate materials such as sapphire, SiC and Si by heteroepitaxial methods, metal organic chemical vapor deposition (Metal -organicChemicalVaporDeposition, MOCVD) is a growth method commonly used in heteroepitaxial technology. Due to the large lattice mismatch and thermal expansion coefficient mismatch with the substrate material, there is a high dislocation density (including line dislocation density and plane dislocation density) in the epitaxial layer film material, the order of magnitude Generally about 10 ⁹ -10 ¹⁰ /cm ² . In the growth process, in order to reduce the high dislocation density and polarization effect in the epitaxial film caused by lattice mismatch, various methods and means are usually adopted, such as two-step method (that is, nitridation of the substrate and GaN buffer layer technology), low temperature AlN nucleation layer plus high temperature AlN nucleation layer technology, AlGaN/GaN superlattice structure, etc. Even so, the dislocation density in the GaN epitaxial film grown by MOCVD is still as high as 10 ⁸ /cm ² . Figure 1 is a typical process of epitaxially growing GaN thin films on sapphire substrates.

It can be seen from Figure 1 that temperature plays a very important factor in the growth of GaN epitaxial film. From about 500°C at the beginning of growing the GaN buffer layer to 1025°C at the growth of the GaN epitaxial layer, the temperature range during this period is large, and the thermal shrinkage of various materials under temperature changes is different. Therefore, it is very important to accurately measure the thermal expansion coefficient of materials. important.

The coefficient of thermal expansion is defined as α=ΔV/(V*ΔT), where ΔV is the change in the volume of the object under a given temperature change ΔT, and V is the volume of the object. Strictly speaking, this expression is only a differential approximation of the differential definition when the temperature range is not large; the accurate definition requires that ΔV and ΔT are infinitely small, which also means that the thermal expansion coefficient is usually not constant in a large temperature range. When the temperature range is not very large, α is a constant. Using it, the volume expansion of isotropic solids and liquids can be expressed as follows:

V(T)＝V ₀ (1+3αΔT)

For an object that can be approximately regarded as one-dimensional, the length is the decisive factor to measure its volume. At this time, the coefficient of thermal expansion can be simplified and defined as: the ratio of the increase in length under a unit temperature change to the original length, which is the coefficient of linear expansion.

For three-dimensional anisotropic substances, there are linear expansion coefficients and volume expansion coefficients. For example, the graphite structure has significant anisotropy, so the linear expansion coefficient of graphite fiber also exhibits anisotropy, which shows that the thermal expansion coefficient parallel to the layer direction is much smaller than that perpendicular to the layer direction.

The measurement of the coefficient of thermal expansion of solids is generally carried out with a thermal dilatometer. Its working principle is to place the sample under the control of a certain temperature program (rising/falling/constant temperature and its combination), and measure the change process of the sample length with temperature or time. In the actual measurement process, different types of replaceable sample holders (quartz, alumina, etc.) are provided for different samples, with different test temperature ranges, and are used in the fields of ceramic materials and metal materials. It mainly tests the thermal expansion/contraction phenomenon of solid materials.

GaN is an extremely stable, hard compound with a high melting point. The melting point is about 1700°C. It has high hardness and is brittle. GaN crystals generally have a hexagonal wurtzite structure, and the epitaxial growth temperature is about 850-1150°C. At room temperature, GaN is insoluble in water, acids and bases, and dissolves very slowly in hot alkaline solutions. NaOH, H ₂ SO ₄ and H ₃ PO ₄ can quickly corrode GaN with poor quality. GaN is unstable at high temperature under HCl or H ₂ gas, but it is most stable under N ₂ gas. It is a good coating protection material.

There are many disadvantages in using a thermal dilatometer to test the thermal expansion coefficient of GaN epitaxial film: 1) GaN material has high hardness and high brittleness, making it difficult to make strips/standard samples; 2) GaN has a small thermal expansion coefficient, so during the heating process, The size increase effect caused by volume expansion is small, and it is difficult to measure accurately; 3) Most importantly, GaN materials are mostly grown by heteroepitaxial growth, and the thickness of the thin film material is generally small (a sapphire substrate with a thickness of 430 microns, In addition to the buffer layer/nucleation layer, etc., the thickness of epitaxially grown GaN is generally about 600 microns), and this layered structure will have an adverse effect on the measurement results.

Among the many physical properties of semiconductor materials, optical properties are one of the most important physical properties. Exploring the interaction process between the electromagnetic radiation field and semiconductors can provide information on the crystal structure, energy band structure, and motion laws of electrons and phonons of semiconductor materials. Optical methods have always been one of the most important and effective means to characterize semiconductor materials.

What Raman scattering provides is the information of the lattice vibration inside the material. It characterizes the material composition, crystal quality, residual stress and the number of free carrier concentrations of the material in the form of inelastic scattering. Crystal defects, grain size reduction, amorphous structure, and residual stress in crystals can all shift the peak of the Raman frequency shift, broaden the peak, and change the symmetrical shape of the peak. The behavior of Raman phonon peaks at different temperatures contains information about optical lattice waves generated by atomic vibrations inside materials. From the perspective of atomic vibration, thermal expansion belongs to the anharmonic effect of atomic vibration, and the relationship between the expansion of optical lattice wave volume and optical lattice wave conforms to the Gruneisen (Greenneisen) parameter, which is a small change with temperature constant. Therefore, the measurement of the GaN thermal expansion coefficient can be realized by testing the temperature-varying Raman scattering of the GaN epitaxial film.

Contents of the invention

In order to overcome above-mentioned deficiencies in the prior art, the object of the present invention is to provide a kind of Raman scattering method of GaN thermal expansion coefficient measurement, adopt the characterization means of new temperature-variable Raman scattering, by within a certain temperature range (with the range of cooling medium and appropriate change), GaN epitaxial layer thin film Raman scattering phonon peak frequency shift behavior with temperature; use software fitting method to obtain the slope of the dω/dT line; extract the thermal expansion coefficient of the GaN epitaxial layer thin film from the slope of the straight line. Using the method of variable temperature Raman scattering, it can accurately realize the non-destructive detection and characterization of the thermal expansion behavior of GaN, AlN, InN and other Group III nitride epitaxial layer thin films binary and multi-component alloy systems, and solve the technical problems in measuring the thermal expansion coefficient of GaN , helps to reduce experimental errors and test costs, and has no potential safety hazards; it has the characteristics of simple method, convenient use, low cost and easy popularization.

In order to achieve the above object, the technical solution adopted in the present invention is: a Raman scattering method for GaN thermal expansion coefficient measurement, comprising the following steps:

First, sample the GaN epitaxial layer film sample and clean it;

Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface;

Second, conduct variable temperature Raman tests on GaN epitaxial layer thin film samples;

The sample is placed on the test table of the Raman scattering instrument, the test surface is smooth, and the rough surface is the back; before the test, it is tested with an Ar ⁺ laser with a wavelength of 514.5nm at room temperature. In the polarization mode, measure the frequency shift value of the phonon vibration mode E ₂ (high) in the GaN epitaxial film;

Before the formal test, the instrument parameters must be calibrated first. After the matching grating is selected, the zero position of the grating is calibrated by the position of the main peak of the standard silicon single crystal (520cm ^-1 ), and the spectrum can be collected after the calibration is completed. After the measurement, the data processing is carried out through the instrument's built-in software. First, the baseline is cut off, and then the data is fitted through the mixed function of Lorentz and Gaussian to obtain the information of the peak position and the full width at half maximum. The Raman scatterometer The model of the variable temperature table is Linkam-Examina-THMS600, the variable temperature range is from 83K to 503K, the step size is 52.5K, and the accuracy can be controlled within 0.1K. When doing low temperature experiments, it is connected to a liquid nitrogen tank, and the cooling medium uses liquid nitrogen to cool down. .

Third, extract the coefficient of thermal expansion from the processing of variable temperature Raman data;

The sample is taken out of the Raman test system, and the relationship between the measured phonon peak frequency shift and temperature is linearly fitted to obtain the slope of the dω/dT line, and the following theory is used for analysis and processing:

1) According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\&Center\; Dot;\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$

In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, the meaning of the Gruneisen parameter, represents the change in the frequency shift of the Raman scattering phonon peak caused by the change of the volume, and the change of the volume contains the behavior and information related to thermal expansion;

2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that

In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so:

ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT)

It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the thermal expansion coefficient α of the material can be extracted from the value of the slope of the line -3αγω ₀ .

The beneficial effects of the present invention are:

The key of the present invention is to adopt the method of Raman scattering, utilize under variable temperature situation, the characteristic that Raman scattering phonon peak frequency shift changes with temperature, combine the correlation between material thermal expansion behavior and phonon peak frequency shift, by analyzing dω/ The information in the slope of the dT line is used to analyze the thermal expansion behavior in GaN epitaxial thin films.

The invention adopts the temperature-variable Raman scattering technology, uses Raman scattering to obtain the relationship between the Raman phonon frequency shift and temperature, and performs non-destructive characterization of the thermal expansion behavior of GaN. It can accurately realize the non-destructive detection and characterization of the thermal expansion behavior of GaN, AlN, InN and other III-nitride epitaxial layer thin films binary and multi-component alloy systems, through the consistency between the thermal expansion coefficient and the origin mechanism of optical wave phonons inside the material , combined with the physical meaning of the Gruneisen parameter, the relationship between the two is cleverly combined, avoiding the damage to the sample and the more complicated formula derivation and mathematical calculation in the general characterization method; Therefore, it is convenient to test the thermal expansion behavior of various semiconductor materials. The formula is simple, the physical meaning is clear, it is easy to implement, and the error is small.

Description of drawings

FIG. 1 is a flow chart of growing a GaN epitaxial film by MOCVD technology in the prior art.

Fig. 2 is a schematic diagram of Raman phonon peaks that may appear in wurtzite-structured GaN under different backscattering modes of the present invention.

Fig. 3 is a linear fitting coordinate diagram of a pair of test data according to the embodiment of the present invention.

Fig. 4 is a coordinate diagram of linear fitting of test data according to Embodiment 2 of the present invention.

Fig. 5 is a coordinate diagram of linear fitting of test data according to Example 3 of the present invention.

detailed description

The invention adopts the temperature-variable Raman scattering method to perform non-destructive measurement and test on the thermal expansion behavior of the GaN epitaxial film. Combined with the test principle and test process, three examples are given.

Embodiment one

A Raman scattering method for GaN thermal expansion coefficient measurement comprises the following steps:

Step 1, sampling and cleaning the GaN epitaxial film sample.

Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface;

Step 2, conduct variable temperature Raman test on the GaN sample;

1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm;

2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K (the boiling point of liquid nitrogen), and start the test when the temperature is stable;

Step 3, process the measured variable temperature Raman data, and extract the coefficient of thermal expansion therein;

1) Take the sample out of the Raman test system, and use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature. The obtained results are shown in Figure 3, and the dω/dT straight line is obtained Slope, analyzed and processed using the following theory:

According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\&Center\; Dot;\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$

In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion;

2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that

In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so:

ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT)

It can be seen from the above formula that by measuring the relationship between the Raman phonon frequency shift and the temperature within a certain temperature range, a linear fitting is performed, and the value of the thermal expansion coefficient α of the material can be extracted from the value of the slope of the line _-3αγω0 ;

The calculation process is as follows: As can be seen from the figure above, dω/dT is the slope of the straight line in Figure 3 -0.0110, and the intercept is ω ₀ =573.46; for the Raman phonon mode E ₂ (high), its Gruneisen parameter γ value is about is 1.47, establish an equivalent relationship:

-3ω ₀ αγ＝-0.0110

It can be seen from the calculation that the thermal expansion coefficient α=4.35×10 ^-6 /K, which is slightly different from the average value of the GaN thermal expansion coefficient in the a direction and the c direction in the table below, which is 4.38×10 ^-6 /K.

It should be noted that although the GaN thermal expansion coefficient is often divided into the a direction and the c direction in the academic circle, there is no distinction between the a direction and the c direction for the Gruneisen parameters γ and ω ₀ , so these two constants are used to Calculating the thermal expansion coefficient of GaN is related to the comprehensive characterization of the overall thermal expansion behavior of the material itself;

Main Physical Properties of Group III Nitride Semiconductor Materials (300K)

Embodiment two

A Raman scattering method for GaN thermal expansion coefficient measurement comprises the following steps:

Step 1, sampling and cleaning the GaN epitaxial film sample;

Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface;

Step 2, conduct variable temperature Raman test on the GaN sample;

1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm;

2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K (the boiling point of liquid nitrogen), and start the test when the temperature is stable;

Step 3, process the measured variable temperature Raman data, and extract the coefficient of thermal expansion therein;

1) Take the sample out of the Raman test system, and use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature. The obtained results are shown in Figure 4, and the dω/dT straight line is obtained Slope, analyzed and processed using the following theory:

According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\·\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$

In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion;

2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that

In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so:

ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT)

It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the material thermal expansion coefficient α can be extracted from the value of the slope of the line -3αγω ₀ :

The calculation process is as follows: as shown in Figure 4, dω/dT is the slope of the straight line in Figure 4 -0.0111, and the intercept is ω ₀ =574.40; for the Raman phonon mode E ₂ (high), its Gruneisen parameter γ value It is about 1.47, establishing an equivalence relationship:

-3ω ₀ αγ＝-0.0111

It can be seen from the calculation that the thermal expansion coefficient α=4.34×10 ^-6 /K, which is relatively close to the average value of the GaN thermal expansion coefficient in the a direction and the c direction in the table below: 4.38×10 ^-6 /K;

It should be noted that although the GaN thermal expansion coefficient is often divided into the a direction and the c direction in the academic circle, there is no distinction between the a direction and the c direction for the Gruneisen parameters γ and ω ₀ , so these two constants are used to The calculation of the thermal expansion coefficient of GaN is related to the comprehensive characterization of the overall thermal expansion behavior of the material itself.

Main Physical Properties of Group III Nitride Semiconductor Materials (300K)

Embodiment three

A Raman scattering method for GaN thermal expansion coefficient measurement comprises the following steps:

Step 1, sampling and cleaning the GaN epitaxial film sample;

Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface;

Step 2, performing temperature-varying Raman tests on the GaN epitaxial layer thin film sample;

1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm;

2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K (the boiling point of liquid nitrogen), and start the test when the temperature is stable;

Step 3, process the measurement data and extract the coefficient of thermal expansion;

1) Take the sample out of the Raman test system, and use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature. The obtained results are shown in Figure 5, and the dω/dT straight line is obtained The slope is then analyzed and processed using the following theory:

According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\·\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$

In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion;

2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that

In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so:

ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT)

It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the material thermal expansion coefficient α can be extracted from the value of the slope of the line -3αγω ₀ :

The calculation process is as follows: As can be seen in Figure 5, dω/dT is the slope of the straight line in Figure 5 -0.0111, and the intercept is ω ₀ =570.20; for the Raman phonon mode E ₂ (high), its Gruneisen parameter γ value It is about 1.47, establishing an equivalence relationship:

-3ω ₀ αγ＝-0.0111

It can be seen from the calculation that the thermal expansion coefficient α=4.41×10 ^-6 /K, which is relatively close to the average value of the GaN thermal expansion coefficient in the a direction and the c direction in the table below: 4.38×10 ^-6 /K;

It should be noted that although the GaN thermal expansion coefficient is often divided into the a direction and the c direction in the academic circle, there is no distinction between the a direction and the c direction for the Gruneisen parameters γ and ω ₀ , so these two constants are used to The calculation of the thermal expansion coefficient of GaN is related to the comprehensive characterization of the overall thermal expansion behavior of the material itself.

Main Physical Properties of Group III Nitride Semiconductor Materials (300K)

Claims (4)

1. A Raman scattering method for GaN thermal expansion coefficient measurement, is characterized in that, comprises the steps: First, sample the GaN epitaxial layer film sample and clean it; Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface; Second, conduct variable temperature Raman tests on GaN epitaxial layer thin film samples; The sample is placed on the test table of the Raman scattering instrument, the test surface is smooth, and the rough surface is the back; before the test, it is tested with an Ar ⁺ laser with a wavelength of 514.5nm at room temperature, at x(yy) In the polarization mode, measure the frequency shift value of the phonon vibration mode E ₂ (high) in the GaN epitaxial film; First, calibrate the instrument parameters, select the supporting grating, and calibrate the zero point position of the grating through the position of the main peak of the standard silicon single crystal. After the calibration is completed, the spectrum is measured, and after the measurement is completed, the data is processed through the instrument's own software. , first cut off the baseline, and then fit the data through the mixed function of Lorentz and Gaussian to obtain the information of peak position and full width at half maximum. From 83K to 503K, the step size is 52.5K, and the accuracy can be controlled within 0.1K. When doing low temperature experiments, a liquid nitrogen tank is connected, and the cooling medium uses liquid nitrogen to cool down; Third, process the measured variable temperature Raman data and extract the coefficient of thermal expansion; 1) The sample is taken out from the Raman test system, and the relationship between the measured phonon peak frequency shift and temperature is linearly fitted to obtain the slope of the dω/dT line, and the following theory is used for analysis and processing: According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\·\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$ In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion; 2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so: ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT) It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the thermal expansion coefficient α of the material can be extracted from the value of the slope of the line -3αγω ₀ . 2. the Raman scattering method of a kind of GaN thermal expansion coefficient measurement according to claim 1, is characterized in that, comprises the following steps: Step 1, sampling and cleaning the GaN epitaxial film sample; Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface; Step 2, conduct variable temperature Raman test on the GaN sample; 1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm; 2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K, and start the test when the temperature is stable; Step 3, process the measured variable temperature Raman data, and extract the coefficient of thermal expansion therein; 1) Take the sample out of the Raman test system, use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature, obtain the slope of the dω/dT line, and then use the following theory to analyze and deal with: According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\&Center\; Dot;\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$ In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion; 2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}={\mathrm{\ω}}_0{(1+3\mathrm{\α}T)}^{-\mathrm{\γ}}$ In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so: ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT) It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the material thermal expansion coefficient α can be extracted from the value of the slope of the line -3αγω ₀ : The calculation process is as follows: dω/dT is the slope of the fitted line -0.0110, and the intercept is ω ₀ =573.46; for the Raman phonon mode E2(high), its Gruneisen parameter γ value is about 1.47, establishing an equivalent relation: -3ω ₀ αγ＝-0.0110 It can be seen from the calculation that the thermal expansion coefficient α=4.35×10-6/K, which is slightly different from the average value of the GaN thermal expansion coefficient in the a-direction and c-direction 4.38× ^10-6 /K in the table below. 3. the Raman scattering method of a kind of GaN thermal expansion coefficient measurement according to claim 1, is characterized in that, comprises the following steps: Step 1, sampling and cleaning the GaN epitaxial film sample; Use a diamond glass knife to slice the grown GaN film sample with a diameter of 2 inches to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface, and the a-plane GaN epitaxial layer The film is placed in a CVD chamber with a vacuum degree of 5.0×10 ^-3 mbar, and nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the film surface; Step 2, conduct variable temperature Raman test on the GaN sample; 1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm; 2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K, and start the test when the temperature is stable; Step 3, process the measured variable temperature Raman data, and extract the coefficient of thermal expansion therein; 1) Take the sample out of the Raman test system, use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature, obtain the slope of the dω/dT line, and use the following theory for analysis and processing : According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\·\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$ In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon The peak Gruneisen parameter, which represents the change of the volume change causes the change of the Raman scattering phonon peak frequency shift, and the change of the volume contains the behavior and information related to thermal expansion; 2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}={\mathrm{\ω}}_0{(1+3\mathrm{\α}T)}^{-\mathrm{\γ}}$ In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, and the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so: ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT) It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the material thermal expansion coefficient α can be extracted from the value of the slope of the line -3αγω ₀ : The calculation process is as follows: the slope of the dω/dT line is -0.0111, and the intercept is ω ₀ =574.40; for the Raman phonon mode E2(high), its Gruneisen parameter γ value is about 1.47, and an equivalent relationship is established: -3ω ₀ αγ＝-0.0111 It can be seen from the calculation that the coefficient of thermal expansion α=4.34×10 ^-6 /K, which is relatively close to the average value of GaN thermal expansion coefficient in the a direction and c direction in the table below 4.38×10 ^-6 /K. 4. the Raman scattering method of a kind of GaN thermal expansion coefficient measurement according to claim 1, is characterized in that, comprises the following steps: Step 1, sampling and cleaning the GaN epitaxial film sample; Use a diamond glass knife to slice the grown 2-inch GaN film sample to make a sample with a size of about 1cm×1cm. The GaN epitaxial layer film is cleaned on the surface by placing the a-plane GaN epitaxial layer film In a CVD furnace chamber with a vacuum degree of 5.0×10 ^-3 mbar, nitrogen gas with a flow rate of 60-100 liters/min is introduced at room temperature to remove scratches and surface attachments on the surface of the film; Step 2, performing temperature-varying Raman tests on the GaN epitaxial layer thin film sample; 1) Place the cleaned GaN epitaxial film sample on the Raman scattering test bench, adjust the Raman scattering instrument, connect the variable temperature platform and the liquid nitrogen tank, and set the test temperature range and step length; the excitation light source is Ar ⁺ , and the laser wavelength is 514nm; 2) Set the polarization mode to Slowly turn on the variable temperature cooling device, cool down the entire test system to 77K, and start the test when the temperature is stable; Step 3, process the measured variable temperature Raman data, and extract the coefficient of thermal expansion therein; 1) Take the sample out of the Raman test system, use Origin8.0 software to linearly fit the relationship between the measured phonon peak frequency shift and temperature, obtain the slope of the dω/dT line, and then use the following theory to analyze and deal with: According to the definition of Gruneisen parameters: $\frac{dl\mathrm{no}\mathrm{\ω}}{d\ln V}=\frac{V}{\mathrm{\ω}}\&Center\; Dot;\frac{d\mathrm{\ω}}{dV}=-\mathrm{\γ},$ It can be seen that: $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}$ In the above formula, V is the volume, V ₀ is the volume at absolute 0K, ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; according to the definition, γ is the phonon Peak Gruneisen parameter, which represents the change of Raman scattering phonon peak frequency shift caused by the change of volume, and the change of volume contains the behavior and information related to thermal expansion; 2) According to the previous formula V(T)=V ₀ (1+3αΔT), it can be seen that $\mathrm{\ω}={\mathrm{\ω}}_0{\left(\frac{V}{V_0}\right)}^{-\mathrm{\γ}}={\mathrm{\ω}}_0{(1+3\mathrm{\α}T)}^{-\mathrm{\γ}}$ In the above formula, T is the temperature in K; ΔT is the temperature change; ω ₀ is the phonon intrinsic frequency shift, that is, the frequency shift at absolute 0K; ω is the phonon frequency shift; γ is the phonon peak Gruneisen parameter; It is the coefficient of thermal expansion, its value is small, the order of magnitude of the typical value is about 10 ^-5 ~ 10 ^-6 /K, so: ω＝ω ₀ (1+3αT) ^-γ ≈ω ₀ (1-3αγT) It can be seen from the above formula that by measuring the relationship between Raman phonon frequency shift and temperature in a certain temperature range, linear fitting is performed, and the value of the thermal expansion coefficient α of the material can be extracted from the value of the slope of the line -3αγω ₀ : The calculation process is as follows: the slope of the dω/dT line is -0.0111, and the intercept is ω ₀ =570.20; for the Raman phonon mode E2(high), the Gruneisen parameter γ value is about 1.47, and an equivalent relationship is established: -3ω ₀ αγ＝-0.0111 It can be seen from the calculation that the thermal expansion coefficient α=4.41×10 ^-6 /K, which is relatively close to the average value of GaN thermal expansion coefficient in the a direction and c direction in the table below 4.38×10 ^-6 /K.