The semiconductor material and electrode structure of the photoconductive antenna affect the terahertz wave characteristics. To improve the performance of the photoconductive antenna, the semiconductor material must exhibit short carrier life, large mobility and high breakdown electric field^[9]. The main difference between GaAs and LT-GaAs is that the epitaxial growth temperature of LT-GaAs is 250 ℃ while the GaAs growth temperature is about 600 ℃^[10]. The growth environment of LT-GaAs enables sub-picosecond carrier lifetime and higher breakdown field strength^[11]. Therefore, it has high electron mobility, fast acquisition time of electrical signals, high breakdown voltage and high resistance, and is an ideal material for fabricating photoconductive antennas^[12].

If the LT-GaAs is directly grown on a GaAs substrate by molecular beam epitaxy (MBE)^[13], together acting as the substrate of a strip conductor, this will generate a large loss when transmitting terahertz waves. In addition, LT-GaAs crystal cannot directly grow on the dielectric layer due to lattice matching, which is required for crystal growth. It has been reported in literature that LT-GaAs film can be obtained using this structure of epitaxial wafers, which composed of Ga_0.7Al_0.3As, LT-GaAs, Ga_0.7Al_0.3As, AlAs sacrificial layers, GaAs buffer layers and GaAs substrate^[14]. Here, the AlAs sacrificial layer is corroded using HF solution^[15]. However, this method is not without difficulties. HF solution is both highly corrosive and highly toxic^[16]. In addition, the photoconductive antenna substrate containing a 50nm-thick Ga_0.7Al_0.3As layer results in poor coupling efficiency of the THz wave into the microstrip line.

In this work, the epitaxial wafer structure consists of LT-GaAs, AlAs sacrificial layer, GaAs buffer layer and SI-GaAs substrate, as shown in Fig.1(a). The AlAs sacrificial layer is corroded to obtain a single LT-GaAs film as shown in Fig.1(b). The growth of the epitaxial wafer is carried out by MBE epitaxial equipment and the substrate is a 2-inch SI-GaAs^[17]. The deposition rate of the materials is less than 1 000 nm· h^-1, ensuring that the deposition quality and deposition rate can be precisely controlled. To improve the crystal growth quality, a GaAs buffer layer of 80 nm thickness is first grown at a temperature of 580 ℃, and then a 200-nm-thick AlAs sacrificial layer is lattice-matched epitaxially grown at a temperature of 550 ℃, followed by the growth of a 2-μ m-thick GaAs layer at low temperature. The growth temperature 250 ℃ is used for LT-GaAs followed by an annealing temperature of 550 ℃ for 15 minutes. The lattice constant of GaAs is close to that of AlAs and does not cause serious lattice mismatch.

Fig.1

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	Fig.1 LT-GaAs epitaxial wafer (a) Epitaxial wafer structure (b) Selective corrosion diagram

For safety, the selective corrosion of LT-GaAs epitaxial wafers is carried out by using HCl solution instead of HF solution. HCl solution is a mix of 38% concentrated HCl with deionized water, and the mass fraction is obtained after calculation. Experimental result shows that the higher the mass fraction of HCl is, the faster the sacrificial layer will erode, but the reaction rates of HCl and LT-GaAs film also increase, so it is not suitable to acquire single-layer LT-GaAs film. Therefore, for finding appropriate HCl solution, the following research has been done.

A 2-inch epitaxial film was separated into 2× 2 mm blocks, cleaned with alcohol and placed in sealed test tubes filled with prepared HCl solution of different mass fraction ranging from 0.89% to 26.45%. The corroded depth of the AlAs sacrificial layer was measured using an optical microscope and the corrosion rate was calculated according to corrosion time. For the experimental statistics, the corrosion rate is the average of six results. Fig.2 (a) shows the relationship between corrosion rate of AlAs and HCl mass fraction at room temperature. The corrosion rate of the AlAs sacrificial layer increases little in the mass fraction range 3.54% to 13.57%, and it has significantly improved above 15.94%.

Fig.2

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	Fig.2 Corrosion rate image (a) Relationship between the corrosion rate of AlAs and HCl mass fraction at room temperature; (b) Relationship between corrosion rate and temperature and concentration of HCl solution

Next, the corrosion rate as a function of both temperature and concentration of HCl solution was examined. A DF-101S heated thermostatic magnetic stirrer (Beijing Kewei Yongxing Instrument Co. Ltd.) ensures temperature homogeneity. According to the relationship between the corrosion rate and concentration at normal temperature, the maximum, minimum, and intermediate concentrations of HCl solution were selected for investigation. The HCl solution with an intermediate concentration of 13.57% was chosen because the corrosion rate changes most slowly during the selective corroding of the epitaxial wafers with HCl solutions at concentrations of 3.54% to 13.57%, and the 13.57% HCl solution corrosion rate was maximum in this concentration range. To restrict volatilization of HCl during the heating process, the HCl solution occupies 90% of the space of the entire closed tube and the heating temperature was only from 25 ℃ to 73 ℃.

Fig.2(b) shows the relationship between corrosion rate and temperature and concentration of HCl solution. In general, an increase in temperature results in an increase in corrosion rate. A marked difference is observed in the rate of increase of corrosion rate for the HCl solution with mass fraction of 13.57% and 26.45%. Here, the former exhibits a linear increase in corrosion rate as a function of temperature, while the latter exhibits an exponential increase. The corrosion rate of the 0.89% HCl solution evolves somewhat differently. When the temperature reaches 73 ℃, the corrosion rate begins to decrease. The reason for this phenomenon is that the mass fraction of HCl solution decreases during heating due to the evaporation of HCl. At high concentrations of HCl in solution this evaporation does not significantly affect the HCl mass fraction.

Fig.3 shows scanning electron microscope (SEM) images of LT-GaAs films selectively corroded by 0.89%, 13.57% and 26.45% mass fractions of HCl solution. Both the surfaces of the LT-GaAs thin film obtained by selective corroding with HCl solution with mass fraction of 0.89% and 26.45% are poor quality. Whereas it was found that a 13.57% HCl solution produced repeatable, good quality surfaces.

Fig.3

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	Fig.3 Microscopic morphologies of corroded LT-GaAs films (a) 0.89% (b) 13.57% (c) 26.45% HCl solution

Based on the above experimental results, a 13.57% HCl solution occupying 90% of the volume of the test tube and placed in a 73 ℃ temperature water bath was used for selectively corroding the epitaxial wafer to efficiently obtain high quality LT-GaAs thin films. Then, after cleaning using deionized water and then alcohol, the prepared LT-GaAs thin film is placed on a silicon wafer. The alcohol is driven off by a slow flow of air (N₂). Finally, the LT-GaAs film is bonded to the silicon wafer by the van der Waals force^{[18, 19]}. Fig.4(a) and (b) are a two-dimensional and a three-dimensional image of the transferred and bonded LT-GaAs film, respectively, which are taken by an atomic force microscope (AFM), and the scanning area is 15 μ m× 15 μ m^[20].

Fig.4

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	Fig.4 Scanning diagram of LT-GaAs thin film after transfer bonding (a): Two-dimensional image; (b): Three-dimensional image

To demonstrate whether the flatness of the film after transfer bonding is compatible with the requirements of the THz on-chip system, the following experiment is performed. At first, the silicon wafer bonded with the LT-GaAs film was made into a photoconductive antenna structure by means of photolithography, vacuum evaporation and the like. Then, the two electrodes of the photoconductive antenna were connected to the wires. Finally, the prepared photoconductive antenna was examined using a terahertz time-domain spectroscopy system based on asynchronous sampling. Fig.5 is terahertz time-domain and frequency domain spectra generated by the antenna, respectively. The generation of the terahertz signal proves that the single-layer LT-GaAs film preparation, transfer and bonding processes are successful, and can meet the production requirements of the THz on-chip system.

Fig.5

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	Fig.5 Terahertz signal generated by antenna (a): Time domain spectrum; (b): Frequency domain spectrum