Preparation of room temperature terahertz detector with lithium tantalate crystal and thin film Open Access

The terahertz (THz) region of electromagnetic spectrum is often regarded as the final unexplored area of spectrum and presents a challenge for both electronic and photonic technologies.^1,2 Great promotions of THz research have been obtained in the last ten years including high-power THz wave generation, THz time domain spectroscopy, THz imaging and so on.^3–5 THz detectors are playing a key role in wide range of applications such as material identification, imaging, tomography, secured communication, medical detection.^6–10 It's necessary to develop terahertz detectors with high sensitivity, low coat, room temperature operating, and low complex characteristics.

The THz detectors can be used for passive or active imaging.¹¹ Most THz detectors for passive imaging application are highly complex and bulky, and require extreme levels of cooling. There is also no easy way to establish the radiation spectral content. For THz imaging application, room temperature direct THz radiation detection detector for active imaging applications is preferred. Pyroelectric thermal detectors are excellent candidates for broadband THz detector operating at room temperature. The detectors utilize permanently poled ferroelectric crystals, which generate a charge as the crystal heats up by absorbing incident radiation. The pyroelectric material, especially lithium tantalite (LiTaO₃), has been successfully used in infrared region detection,^12,13 and shows great potentials in the direct detection detectors for active imaging applications. But using LiTaO₃ material to fabricate THz thermal detector is seldom reported and noise equivalent power (NEP) of the detectors are near 10⁻⁹ ∼ 10⁻⁸ W.^14,15

The output current of pyroelectric detector is directly proportional to the changing rate of temperature of material.¹⁶ So using pyroelectric thin film instead of crystral would effectively reduce the thermal capacity and increase the device sensitivity, but the pyroelectric coefficient of thin film is lower than that of crystal, which is not beneficial to obtain high voltage signal output. Therefore, terahertz detectors with both lithium tantalite crystal and thin film are designed, simulated, and fabricated in this paper to obtain high sensitivity device with low cost, real-time, small bulk, room temperature operating performances.

Voltage responsivity (R_v), an important character of THz detector, can be simply expressed as the following equation:¹⁷ 

where η is absorptivity of THz absorption layer, p is pyroelectric coefficient of LiTaO₃ material, A the area of element, R the effective resistance in the output circuit of detector and G the effective thermal conductance coupling the detector to a constant temperature heat sink, ω is the operation frequency, C is the electrical and H the thermal capacity of the detector element. At low operation frequency, for the bulk pyroelectric detector, the R_v of detector can be simply modified to:¹⁸ 

in which, C_th is the thermal capacitance and C_th = c_thAd (where c_th is the volume specific heat, A and d is the area and thickness of the detector element). From the equation, the smaller the thickness d of detector, the larger is the voltage responsivity R_v. The thickness of normal LiTaO₃ crystal slice is over 100 microns, so it's necessary to reduce the thickness of the LiTaO₃ crystal to improve the thermal response of detector. By chemical mechanical polishing techniques, the thickness of lithium tantalate crystal slice reduces to about 40 microns. But the rough crystal surface and polish trace would affect the pyroelectric parameters, the surface morphology and crystal thickness should be modified subtly.

Strong oxidizing solution (H₂O₂) mixed with KOH is used to modify the polished LiTaO₃ crystal. The etching process could be described as the following chemical reaction equation,

in which, the value of x is related to solution temperature, oxidant concentration and pH value. The solution temperature, oxidant concentration and pH value would remarkably affect the etching speed of crystal, and the relative removing speed of LiTaO₃ crystal at different conditions are shown in Fig. 1. In Fig. 1(a), the removing speed increases as the solution temperature increasing and reaches the maximum value when the temperature is 150 °C. Because the boiling point of hydrogen peroxide is 151.4 °C, hydrogen peroxide would vaporize or resolve and reduce the oxidizing function when the solution temperature is higher than the boiling point. Fig. 1(b) indicates the etching speed of solution (pH:10, temperature 150 °C, etching time:1 hour) with different oxidant concentration. The etching of LiTaO₃ crystal became faster when the adding more oxidizing solution, the higher contention solution (>30%) mixed with KOH is unstable for the etching process. As shown in Fig. 1(c), there is an optimized etching speed of LiTaO₃ crystal when the pH value is controlled between 10 and 12, lower or higher KOH concentration will reduce the corroding speed.

Another important function of the corroding solution is reducing surface roughness of LiTaO₃ crystal. Scanning electronic microscopy (SEM) images of surface morphology of the crystal slice are shown in Fig. 2. The image of Fig. 2(b) (after oxidizing solution etching) indicates more flat and clean surface than the crystal without oxidizing solution etching shown in Fig. 2(a), which is suitable to enhance the adhering with electrode and improving pyroelectric parameter.

The LiTaO₃ thin film was fabricated by an improved sol-gel process in our lab. High concentration LiTaO₃ precursor solution could be obtained with mixing lithium acetate and tantalum ethoxide in a 1, 2-Propylene glycol solution, then the precursor was spun on substrate and annealed at 700 °C to be crystalline. The detail fabricated process of the film could be referenced in another report.¹⁹ 

X-Ray Diffraction (XRD) analysis was carried out to study the crystalline phase formation and preferred orientation of crystal growth of LiTaO₃ thin film. The XRD patterns, shown in Fig. 3, indicate that the films crystallized well at 700 °C annealing temperature and the preferred orientations of the film are (012), (116) and (110). The size of crystalline particles could be examined by SEM test. From the former image shown in Fig. 2(b), the size of LiTaO₃ particles is very uniform and reaches ∼12 nm. Then the annealed thin film (at 700 °C, 5 min) was polarized at 20 V for 30 min to obtain pyroelectric characteristic. The hysteresis loop diagram of LiTaO₃ thin films is plotted in Fig. 4. The remanent polarization of the film is 10.71 μC·cm⁻² and the coercive electric field is 19.1 kV·cm⁻¹.

The device structure of typical pyroelectric detectors is three layers, with LiTaO₃ material sandwiched between two metal electrodes. In order to improve the device absorption in terahertz region, a thin layer of carbon nanotube is covered on the top electrode surface by spin coating method. Because the THz detector basing on pyroelectric material is a kind of thermal sensitive detector, the heat conducting information of device structure should be obtained. With finite element method, three dimension detector structures with LiTaO₃ crystal and thin film were set up and simulated. The structure of THz crystal detector, from top to bottom, is THz absorber (1 μm)/ Al electrode (100 nm)/ LiTaO₃ crystal (2 mm × 6 mm, thickness 40 μm)/ Pt electrode (150 nm). This structure is adhering on Al₂O₃ substrate with two pieces of ethoxyline resin (1 mm × 2 mm), and the device structure is shown in Fig. 5(a). The device structure of thin film detector from top to bottom is THz absorber (1 μm)/ Al electrode (100 nm)/ LiTaO₃ thin film (100 μm × 100 μm, thickness 2 μm)/ Pt electrode (150 nm), and the device is supported by two Si₃N₄ layer (300 nm) arm. SiO₂ layer under the Si₃N₄ layer acts as sacrificial layer and is etched with buffered oxide etch to form microbridge structure. When adding a heat flux of 6.7 × 10⁻¹³W/μm² on those detectors, the temperature changing maps of two devices are shown in Fig. 5(b) and 5(d). It's obvious the temperature increasing value of microbridge is higher than that of crystal device and the temperature uniformity is better for the microbridge device. Those are effective to improve the performance of THz detector based on LiTaO₃ thin film with low pyroelectric coefficient.

Both kinds of THz detectors were fabricated with former pyroelectric materials and simulated structure, and bonded with a pre-amplifier by two spun gold before test. Because it's a direct detection detector, far infrared laser (the FIR molecule CH₃OH, the pump line is 9P36, radiation frequency is 2.52 THz) acted as terahertz source and oscilloscope was used to display the voltage signal coming from the pre-amplifier. The detecting frequency could be controlled precisely by chopper, and for example, the waveform of response voltage (V_s) signal at 100 Hz frequency was shown in Fig. 6. The noise voltage (V_n) of pyroelectric detector was measured with dynamic signal analyzer (SR785) at room temperature after removing ambient radiation. The noise voltage of was several hundreds nV and would reduce with the increasing of test frequency.

The energy of terahertz wave radiation on the detector could be identified with a terahertz power meter and the THz energy radiation on detectors (P) could be calculated with the following equation:

in which, P₀ is the test value of terahertz dynamometer, A_d is the area of THz detector, A_s is the area of terahertz dynamometer incident aperture, and α is the chopper coefficient. It's uneasy to identify the uniformity of laser beam energy in A_d and A_s, the energy radiation calculated from equation (4) may be a little lower than the real value. The power of the far infrared laser is 129 mW according to the stand terahertz dynamometer. So the 2.52 THz wave radiation power on the LiTaO₃ crystal slice detector and LiTaO₃ thin film detector are 6.8 mW and 5.69 μW, respectively.

With the value of response voltage, noise voltage and incident energy, the NEP of THz detectors can be calculated with the expression:

The value of response voltage and NEP parameters at different frequency of two terahertz detectors are shown in Fig. 7. Obviously, the NEP of detector basing on pyroelectric crystal is much lower than that of device with LiTaO₃ thin film material. The lowest NEP value reaches 6.8 × 10⁻⁹ W and 8.9 × 10⁻⁸ W for crystal device and thin film device, respectively. The experiments result don't match with the former heat conduction simulation models, which indicate that the conversion ratio from temperature changing to charge of device with LiTaO₃ thin film is much lower than the device with pyroelectric crystal. From Fig. 7(a) and 7(b), the NEP parameter of detectors has an optimizing value near 30 Hz chopper frequency and would increase at higher frequency. The noise voltage of THz detectors drops quickly at lower chopper frequency and then becomes stable value at high frequency. The value of V_s slides down promptly at high frequency range, so that there is an optimizing operating frequency range for those detectors.

Room temperature terahertz detectors basing on pyroelectric material was reported in this paper. The LiTaO₃ crystal slice was polished to 40 micron thickness by chemical mechanical polishing techniques. Then by oxidizing solution etching, surface morphology of crystal slice became more flat and crystal thickness reduced. The LiTaO₃ thin film was fabricated by an improved sol-gel process, in which high concentration LiTaO₃ precursor solution was obtained with mixing lithium acetate and tantalum ethoxide in a 1, 2-Propylene glycol. Some characters of both materials were tested and those materials were used to fabricate THz detectors. Three dimension models of detectors were built to simulate the temperature increasing map of two devices at a fixed heat flux. Terahertz devices were fabricated and tested with 2.52 THz far infrared laser. The lowest NEP value for terahertz detector using pyroelectric crystal material reaches 6.8 × 10⁻⁹ W, which is lower than the detector with LiTaO₃ thin film. The performance of THz detector with microbridge structure and LiTaO₃ thin film can be enhanced by improving the pyroelectric parameter of thin film.

This work was supported by the Program for New Century Excellent Talents in University (NCET-10-0299), National Science Foundation of China (No. 61235006) and Fundamental Research Funds for the Central Universities (ZYGX2011X012).