In this paper, the dielectric properties of indium phosphide (InP) are investigated under a strong microwave field. By introducing a re-entrant coaxial cavity, the strong microwave field is constructed, and the dielectric properties of the material are monitored using the cavity perturbation method. It is shown that the dielectric properties of InP changes obviously under the given strong microwave field. From the experimental results and theoretical analysis, we conclude that the nonlinear behaviour is caused by a kind of non-thermal microwave effect. The experimental setup and method could also be applied to predict the consequences of non-thermal microwave effects of other high power microwave materials under strong microwave field.

The electromagnetic properties of material have crucial guiding significance to their application. There are various types of external factors that affect the electromagnetic properties of materials, including ionizing radiation and electromagnetic radiation. Since the 1950s, a large number of experiments and theories have been proposed to investigate the effects of ionizing radiation on materials.1–10 It is shown that the electrical properties of pentacene, a kind of organic semiconducting material, has changed when exposed to ionizing radiation.1 An increment in bulk dielectric trap densities has been observed when hafnium and gadolinium oxide gate dielectrics were exposed to the ionizing radiation.2 The shape, size, and height of the hillocks of InP were found to vary with respect to ion fluence; and the amount of disorder in the irradiated samples increased as the ion fluence increased.5 The Raman peak shift, the disorders, and defects in the surface region increased after the InP sample was irradiated.8 

Due to the rapid development of high power microwave technology and the increasing miniaturization of the modern circuit, the microwave field intensity between the materials increases significantly. Therefore, the influence of the microwave field on materials has been extensively studied by scholars.12–28 It is demonstrated that microwave field actives ClTE centers of CdTe:Cl single crystals, which results in an increase in the intensity of photoluminescence line of excitons bound at the corresponding CLTE donor centers and the experimental results indicate that efficient modification of defect structure of the material under investigation because of microwave radiation.21–23 Short-term microwave treatment is shown to cause long-term nonmonotonic changes in spectral characteristics, which results from the modification of the materials structure.24 The relaxation of internal mechanical strains is proved to be an important factor affecting the property of semiconductor, which changes after being stimulated by microwave.25 In Ref. 26, two mechanisms are discussed for the capacitance variations of a thin ferroelectric film-based capacitor under elevated microwave power. Experimental results confirmed that electric field effect is the dominating mechanism in the average capacitance variation.

It can be seen from the above researches that the interaction mechanism between microwave field and material is mainly focused on the experimental exploration, and most of the investigations are based on the optical and photoluminescence methods. The microwave field effect is studied by detecting the photoluminescence of the materials before and after microwave irradiation.17–24 The non-thermal microwave effect is proved to exist as one of the possible interaction mechanisms.14,17–19,22,23,26 However, the deep interaction mechanisms are not clear enough.

Indium phosphate (InP) is a kind of foundation material for high power microwave and optoelectronic devices, which has received a widespread attention.6–11 It shows that heavy iron radiation can change the structure of the material,8 which may affect the physical properties of the material.9,10 Moreover, these surely result in catastrophic failures of devices and finished system. When the microwave field strength in the environment reaches a certain threshold value, whether it will affect the microwave characteristics of InP is worth thinking about and studying.

Here, we present a work-ability experimental setup and method to explore the possible interaction mechanism between microwave electric field and InP material. Our experimental technique is based on a multimode re-entrant coaxial cavity. The TEM004 mode is used to stimulate the strong electric field environment while the TEM005 mode is for testing the performance of the material before and after the interaction with the microwave electric field. The two modes are excited by two different swept frequency microwave sources, respectively. The structure schematic of the re-entrant coaxial cavity is given in Fig. 1(a). By adjusting the size of the cavity, the TEM004 and the TEM005 modes work at the frequencies around 2.45 GHz and 3.13 GHz, respectively. There is no doubt that many interference modes exist in the designed cavity, which may influence our experimental results. Therefore, four slits in the longitudinal direction of the cavity are made to suppress the TE modes. Additionally, four grooves on the sides of the slits are made to avoid the forming cut-off waveguide in the depth direction of the slit, as is shown in Fig. 1(b). The cavity parameters are as follows: 2a = 13 mm, 2b = 45 mm, 2c = 4.5 mm, L = 199 mm, d = 2.5 mm, while the width of grooves and slits are 10 mm and 0.5 mm, respectively.

FIG. 1.

(a) Schematic diagram of the cavity. (b) Machining stereogram of the cavity.

FIG. 1.

(a) Schematic diagram of the cavity. (b) Machining stereogram of the cavity.

Close modal

The re-entrant coaxial cavity is simulated in high-frequency structure simulator (HFSS), as shown in Fig. 2. The strongest electric field of the TEM004 mode locates at L/7, 3*L/7, 5*L/7 and 7*L/7 (location of the gap) of the cavity, while the strongest electrical field of TEM005 mode locates at L/9, 3*L/9, 5*L/9, 7*L/9 and 9*L/9 (location of the gap) of the cavity. It can be seen that the electric field strength at the cavity gap, where the sample is located, is the strongest for both modes. From Fig. 2, if the stimulus signal is injected in through a probe at around the 5*L/7 of the cavity, the TEM004 and the TEM005 modes will be both activated. By adjusting the power level of the stimulus signal, the electric field strength around the gap will change accordingly. However, the stimulus port should be adjusted to a good matching state so that the power signal can be injected into the cavity as much as possible.

FIG. 2.

Electrical field distribution of the two modes in the cavity. TEM004 mode (left) and TEM005 mode (right).

FIG. 2.

Electrical field distribution of the two modes in the cavity. TEM004 mode (left) and TEM005 mode (right).

Close modal

Based on the designed multimode re-entrant coaxial cavity, a test system for studying the nonlinear behaviour of InP material is built, as shown in Fig. 3. The test system is mainly composed of two parts: the electric field stimulation part and the material property test part. The electric field stimulation part mainly consists of the VNA 1, two isolators, a power amplifier with a maximum gain of 53 dB, a directional coupler and a probe. The stimulus signal can be injected into the cavity through port 3 by the probe, which is supplied and amplified by the VNA 1 and amplifier respectively. The directional coupler is used to extract the reflection of the stimulus signal from port 3, which can be used to monitor the power level of the stimulus signal. The material property test part mainly consists of the VNA 2, two band-stop filters, and two loops. Through the two loops, the testing signal is injected into and coupled out of the cavity for testing the dielectric performance of the materials under test. In addition, two band-stop filters with about 60 dB attenuation in the stop band, are introduced to isolate the signals coming from both modes (TEM004 and TEM005).

FIG. 3.

Physical model for studying the nonlinear behavior of InP.

FIG. 3.

Physical model for studying the nonlinear behavior of InP.

Close modal

For the convenience of testing, a quartz tube is utilized to load the InP sample. Therefore, the influence of the tube should be excluded first. According to the physical model (Fig. 3), a measurement system is implemented and the measurement system is utilized to explore the quartz tube and InP finally. The detailed measurement procedures are as follows.

First of all, the empty quartz tube is placed at the gap of the cavity, where the electric field is strong. A wide band signal with a small power level (-40 dBm) is produced by VNA 1. Measure and search the minimum value of the transmission curve from VNA 1. Then the center frequency of the VNA 1 is set to the frequency value corresponding to the minimum value of the previous search, and the working bandwidth is set to 100 MHz, as shown in Fig. 4. The transmission curve S21 shown in Fig. 4 actually represents the reflection of the stimulus signal. As can be seen that only the narrow band frequency signal around 2.43 GHz is input into the cavity within one sweep time. The sweep time (t) of the VNA 1 is set to 1.1 s. In this way, the stimulus signals are similar to an impulse signals mode, and the time required to obtain a strong electric field in the gap of the cavity is much less than the sweep time t, eliminating the effect of microwave thermal effects.

FIG. 4.

Reflection of port 3 with the quartz tube at the gap of the cavity.

FIG. 4.

Reflection of port 3 with the quartz tube at the gap of the cavity.

Close modal

Then, the dielectric property of the quartz tube is detected by the material property test part. A wide band signal with a center frequency is around the resonant frequency of the mode TEM005 is supplied by VNA 2, and the transmission curve S21 is obtained from VNA 2. Search the maximum value of the curve and reset the sweep frequency range from the maximum value to the maximum value, namely point frequency sweeping. The sweep time is set to 2 t. In this way, we can monitor the change of the resonant peak quickly.

Finally, the test results of the empty quartz tube are obtained as shown in Fig. 5. Fig. 5(a) presents the test results when the VNA 1 is working with an output power of -40 dBm. And Fig. 5(b) shows the test results when VNA 1 is working with an output power of -5 dBm. These results show that when we change the output power of VNA 1 from -40dBm to -5dBm, the microwave field strength around the quartz tube changes, but the resonance peak does not change.

FIG. 5.

Test results of the quartz tube. (a) Test results obtained when the VNA 1 works with an output power of -40 dBm and (b) test results when the VNA 1 works with an output power of -5dBm.

FIG. 5.

Test results of the quartz tube. (a) Test results obtained when the VNA 1 works with an output power of -40 dBm and (b) test results when the VNA 1 works with an output power of -5dBm.

Close modal

The cavity perturbation method (CPM) has been one of the most common methods to determine the dielectric properties of materials.26,27 In this work, we detect the dielectric property of the materials under test by the CPM. According to the CPM, as expressed in equations (1) and (2), the real part of the dielectric constant is determined from the shift of the resonant frequency, while the imaginary part is the reason of the change of the quality factor of the cavity.26 

ωω0ω0=ε0(ε1)ΔEE0*dV4W
(1)
1Q1Q0=ε0εΔEE0*dV4W
(2)

In equation (1) and (2), ω0 and ω are the angular resonant frequencies before and after the perturbation, respectively. Q0 and Q are the quality factors before and after the perturbation. E0 and E are the electric field strength before and after the perturbation in the cavity. W is the total store energy in the cavity. ε′, ε″ are the real and imaginary parts of the dielectric constant, respectively.

Thus, considering the resonant peak of the cavity loaded with the quartz tube under different power levels are still the same, we conclude that the real part of the dielectric constant of the quartz tube has no nonlinearity in the given strong electromagnetic field environment. Therefore, the effect of quartz tubes on the dielectric properties of InP sample can be excluded.

Place the InP sample in the quartz tube and research on InP materials. Fig. 6 depicts the reflection of port 3 (the stimulus signals) when the InP sample (loading in the quartz tube) is located at the gap of the cavity. It shows that only the narrow band frequency signal around the frequency of 2.43 GHz could be injected into the cavity, which can create the strong microwave field around the InP sample. Fig. 7 presents the dielectric property testing results of the InP sample. Fig. 7(a) shows the test results when VNA 1 is working with an output power of -40 dBm while Fig. 7(b) shows the test results when the VNA 1 is working with an output power of -5 dBm. There are two special points (A and B) in Fig. 7(b), which means the resonant peak changes when the given power signals (-5 dBm) are injected into the cavity. That is to say, the real part of the dielectric constant of the InP sample changes under different microwave field. According to the sweep time of the both VNAs (the sweep time of VNA 2 is two times of VNA 1), it is found that only when the power signal is injected into the cavity, the nonlinear behaviour of the phenomenon appears. Moreover, when the microwave field strength applied to the InP material is weak, the two special points A and B back to normal, as shown in Fig. 7. Therefore, we can conclude that the nonlinear behaviour is not caused by microwave thermal effect, but by some inherent characteristics of the InP material.

FIG. 6.

Reflection of port 3 with the InP sample (loading in the quartz tube) at the gap of the cavity.

FIG. 6.

Reflection of port 3 with the InP sample (loading in the quartz tube) at the gap of the cavity.

Close modal
FIG. 7.

Test results of the InP sample. (a) Test results obtained when the VNA 1 works with an output power of -40 dBm and (b) test results when the VNA 1 works with an output power of -5 dBm.

FIG. 7.

Test results of the InP sample. (a) Test results obtained when the VNA 1 works with an output power of -40 dBm and (b) test results when the VNA 1 works with an output power of -5 dBm.

Close modal

In conclusion, our work presents an approach to study the effect of microwave field on InP material. Experimental results show that the microwave field actually affects the dielectric property of InP. The sweep frequency mode of two VNAs helps to prove that the nonlinear behavior of the InP is caused by microwave non-thermal effect. The method can also be employed to predict the consequences of microwave non-thermal effect of other high power microwave materials under strong microwave field.

This work was supported by the National Natural Science Foundation of China (Grant No. 61671123 and 61001027).

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