With the development of first-generation silicon semiconductor materials and the second-generation GaAs semiconductor materials, the application of its devices has also reached the limit. As a representative of the third-generation semiconductor materials, aluminum nitride (AlN) has the advantages of wide band gap, high critical breakdown electric field, high thermal conductivity, good ultraviolet transmittance and strong radiation resistance^[1⇓-3]. Therefore, AlN crystals have broad application prospects in the fields of high-power high- frequency electronic devices and deep ultraviolet light- emitting devices^[4⇓-6]. The research of AlN crystals has become a frontier and hot spot in the field of semiconductors. The melting point of the AlN single crystal is very high. The theoretically calculated melting point is 2800 ℃, and the dissociation pressure corresponding to the melting point temperature is as high as 20 MPa. Therefore, it is difficult to grow AlN crystals by melt method under normal pressure. A variety of methods have been developed to grow AlN crystals, including aluminum metal nitridation, solution, hydride vapor phase epitaxy, and physical vapor transport^[7-8]. The PVT method has the advantages of simple growth process, fast growth rate, good crystal integrity and high safety, which is the most successful method for growing bulk AlN crytal^[9-10].

However, in the process of growing AlN crystals by PVT method, it is difficult to maintain ideal thermodynamic equilibrium conditions, thus crystal defects are inevitably generated, and trace impurities exist in the crucible and raw materials, forming Al vacancies, gap oxygen, and carbon interstitial atoms, etc. These defects or vacancies can form midgap energy states in the band gap, which affects the photoexcitation and emission characteristics, hinders band edge transitions, thereby reducing the performance of AlN crystals based deep ultraviolet (DUV) optoelectronic devices^[11⇓-13]. Moreover, these impurities can significantly reduce the transport characteristics such as the carrier mobility of the AlN crystal, thereby limiting its application in microelectronics such as AlGaN based high electron mobility transistor (HEMT). Recently, many methods have been used to reduce defects and impurities to improve crystal quality of AlN by PVT method. Guguschev, et al^[11] optimized the geometry of the growth device, the raw material purification process and growth parameters to reduce the impurity content of the AlN crystal. Hartmann, et al^[14] investigated the effects of different supersaturation on the quality and impurity content of AlN crystals. Our research group studied the effects of different temperature gradients on the growth of AlN crystals, and found that the radial temperature gradient was the driving force for expanding the diameter of AlN crystals. However, these reports mainly focus on the effects of growth devices, growth process parameters, etc. Further improvements in wafer quality after growth processing have not been investigated. In recent years, it has been reported that high-temperature annealing technique can eliminate the tilt component between the buffer layer grains and realize the reconstruction of AlN, and thus applying to manufacture high-quality, low stress AlN film or efficient DUV-LED device by MOCVD method^[12⇓-14]. High temperature annealing technique has received much attention due to its effectiveness in improving the integrity of AlN crystals. However, studies on high temperature annealing of AlN wafers obtained by cutting and polishing AlN boules grown by PVT have not been reported.

To reduce the defects of AlN crystal grown by PVT and the residual stress during subsequent cutting, grinding and polishing, high temperature annealing technique was used to process AlN sample. In this study, the effects of high temperature annealing on the crystal quality, optical properties, stress, and impurity of AlN sample were investigated in detail. It was found that the impurity content of AlN sample decreased under appropriate annealing process, and the crystal quality was significantly improved.

AlN crystals were grown in a home-designed RF- heated furnace with a high purity nitrogen atmosphere by PVT method, and the growth crucible was made of high purity tungsten. The specific growth parameters were described in the literature^[15]. The AlN crystals grown by our research group are shown in Fig. 1(a). The grown AlN bulk single crystal was cut into a 500 μm thick sample and then polished. The polished AlN sample is shown in Fig. 1(b). To ensure experimental comparability, the same sample was cut into three small sample and annealed in a high purity nitrogen atmosphere at 1400, 1600 and 1800 ℃, and a schematic view of high temperature annealing process is shown in Fig. 2. These annealed samples were named AlN-1400, AlN-1600 and AlN-1800.

In order to evaluate the crystalline quality and structural perfection of AlN before and after thermal annealing, high-resolution X-ray diffraction (HR-XRD) and Raman spectrum were carried out. In addition, the optical properties of the crystals in terms of impurity- related below-bandgap transitions were investigated by room temperature photoluminescence (PL) and absorption spectrum measurements (Varian UV-VIS-NIR cary 50). The Electron backscattered diffraction (EBSD) pattern obtained from the scanning electron microscope (SEM) image was magnified 8000 times. The EBSD maps were acquired by a step size of 500 nm, and the grid was 120×120 points. Secondary ion mass spectroscopy (SIMS) depth profiling was performed using a CAMECA IMS-6f with a 50 nA Cs+beam over a 180 μm × 180 μm area.

EBSD is an effective method to characterize crystal orientation, the plastic strain existing in the lattice and related information in crystal materials. The EBSD measurement is performed on the AlN without annealing to determine its crystal orientation and crystal quality. Fig. 3(a) is a Kikuchi diffraction pattern of the AlN sample. As seen from the figure, AlN sample has a highly oriented crystal structure along the (11¯20) direction, and the pole figure (Fig. 3(c)) also proves that the crystal is in the (11¯20) direction. Fig. 3(b) shows the band slope (BS) of an AlN crystal from EBSD mapping data. The mapped colors concentrate in a mixture of green and yellow, indicating that the BS value changes by less than 75. These results indicate that the AlN sample without annealing have a high crystallographic integrity.

The quality of AlN crystals without and with annealing are characterized by high-resolution X-ray diffraction (HRXRD) rocking curves (Fig. 4). According to the relationship between the Burger's vector and the XRD Bragg peak width, the threading dislocations in the AlN crystal are dominated by pure edge type dislocations. Therefore, the FWHM of the (10¯12) plane can directly reflect the AlN crystal quality. Table 1 is the FWHM of the (10¯12) plane of symmetry ω-scan of the AlN crystal without and with annealing, and the FWHM value (80, 79 and 97 arcsec at 1400, 1600, and 1800 ℃, respectively) of the (10¯12) plane of the annealing sample is significantly lower than that of the sample without annealing (104 arcsec). The edge dislocations density(ρ_e) can be calculated by the following formula:

Where β is the FWHM of planes, |b_a| is the Burgers vector lengths equated to a-axial lattice constants. Table 1 is the calculated dislocation density. It can be seen from the table that the dislocation density of the sample after high temperature annealing is reduced to some extent. The AlN sample annealed at 1400 ℃ has the lowest dislocation density, a 41% reduction, as compared to the sample without annealing. During the annealing process, the higher N₂ background gas pressure makes the transport of other substances to the surface of AlN less, and the atoms on the surface of AlN are mainly migrated and recombined. When the annealing temperature is moderate (1400-1600 ℃), Al atoms and N atoms have enough activity, and some of Al atoms and N atoms will migrate to the original vacancies and defects, thus reducing point defects and line defects. When the annealing temperature is too high (1800 ℃), Al atoms and N atoms have very high activity, and the atoms on the AlN surface will migrate disorderly, instead of reducing the dislocation. FWHM of the sample annealed at 1600 ℃ is almost the same as that at 1400 ℃ (80 arcsec), thus 1400-1600 ℃ is the optimum annealing temperature range.

Raman spectroscopy is an effective method for evaluating crystal structure, quality, and residual stress. Fig. 5 shows the Raman spectra of AlN samples without and with annealing at different temperatures. It can be observed that there are four peaks E₂ (low), A₁ (TO), E₂ (high) and E₁ (TO) associated with the (11¯20) plane of wurtzite structure AlN. The in-plane residual stress (σ_a) can be derived by using Eq. (2):

Where Δω [E₂(high)] is the strain-induced Raman frequency shift for E₂(high) mode and the biaxial stress coefficient k roughly equals to 2.4 cm^-1·GPa^-1. The E₂(high) mode peaks of the AlN samples without and with annealing all locate at 656.23 cm^-1, which coincides well with the reported value (657.4 cm^-1) of freestanding AlN crystal. These results indicate that the stress value of the small-sized AlN sample obtained by homo-epitaxial reaches the minimum, and the effect of high-temperature annealing on small-sized sample stress is small. However, the impact on the stress of large-sized AlN samples remains to be further studied.

Fig. 6 shows the optical absorption spectra of the AlN crystals with and without annealing. There is an obvious absorption peak at 634 nm (1.96 eV) with inset in Fig. 6(a). The strong absorption peak of the AlN crystal at 634 nm (1.96 eV) may be due to the defect energy levels formed by carbon and oxygen.

Significant enhancement of absorption is observed with annealing temperature increasing, indicating that the annealing process is beneficial to improve the quality of AlN crystal. The relationship of the molar absorption coefficient(α) and photon energy(E) is listed below：

Where B is the proportional constant, E_g is the optical band gap of semiconductor material, and the value of m is related to semiconductor material and transition type. When m=1/2, it corresponds to the dipole transition allowed by direct bandgap semiconductors. By Lamberbier’s law:

Where b is sample thickness, c is concentration, where bc is a constant, if B₁=(B/bc)^1/2, Then formula (3) can be written:

According to the correlation curves of (αE)² and E, as shown in Fig. 6(b), the band gap of AlN with annealing is higher than that without annealing. The band gaps are 3.90, 3.93, 3.91, and 4.05 eV for the AlN without and with annealing at 1400, 1600, and 1800 ℃. The variation trend of the band gap is shown in the inset in Fig. 6(b). The band gap values of the samples are all smaller than the theoretical band gap value of AlN, which may be due to crystal defects or impurities induced intermediate levels associated with the conduction band resulting in a decrease in band gap^[16]. The deep ultraviolet absorption coefficient of AlN crystals mainly depends on the impurities C and O^[9,17]. According to density functional theory calculations and experiments, it is shown that C impurities exist in the form of CN in AlN crystals, resulting in a very strong ultraviolet absorption peak at about 265 nm (4.7 eV)^[18]. The C mainly comes from the graphite insulation. The possible gas phase substances are CN, CO or C_xH_y. The recombination of V_Al and O impurity defects leads to absorption at 280-380 nm (3.3-4.3 eV)^[19]. O is mainly vapor-phase Al₂O sublimed from raw materials at the growth interface, and is affected by factors such as the raw material particle size, sintering process, and purity of the chamber. Therefore, AlN crystals with high UV transmittance can be obtained by suppressing and eliminating the content of C and O impurities. The band gap increases from 3.90 eV for AlN crystal without annealing to about 4.05 eV for AlN crystal with annealing at 1800 ℃. The increase in band gap is attributed to the reduction of defects and impurities^[20], which confirms that high temperature annealing improves the crystal quality of the AlN sample.

The impurity incorporation in the AlN crystals is characterized by SIMS. As shown in Fig. 7, three main impurities of silicon, carbon and oxygen are detected in the AlN crystals. The carbon mainly comes from the graphite insulating, and the silicon and oxygen elements are mainly derived from the AlN raw material. The silicon concentration of the AlN samples without and with annealing at 1600 ℃ is 1×10¹⁷ cm^-3 without substantial change. After annealing, the carbon concentration decreases from 2.5×10¹⁹ to 2.0×10¹⁹ cm^-3, and the oxygen concentration increases from 3.1×10¹⁹ to 3.5×10¹⁹ cm^-3. In this experiment, intermediate frequency heating and graphite insulation system are used. C mainly comes from the graphite insulation in the growth equipment, thus it is inevitable that the C concentration is relatively high. O is mainly the gas phase Al₂O₃ sublimated from AlN raw material at the growth interface. At present, the inhibition and elimination of C and O impurities are in progress. The decrease of carbon concentration may result in an increase in the band gap of the AlN crystal, which is consistent with the optical absorption results. The increase of oxygen concentration after annealing may lead to the enhanced absorption of AlN crystals at 634 nm (1.96 eV). The increase of oxygen concentration after annealing may be attributed to the residual of the insulating material, and a higher purity insulating material is used in subsequent experiments.

In conclusion, AlN crystals grown by the PVT method were annealed in the temperature range of 1400 ℃ to 1800 ℃ under nitrogen atmosphere. The crystal quality of these AlN crystals was improved significantly after thermal annealing. The minimum FWHM of (10¯12) is 79.92 arcsec after annealed at 1400 ℃. SIMS measurements show that the thermal annealing process reduces C impurities. The band gap increases from 3.90 eV (unannealed AlN crystal) to about 4.05 eV (1800 ℃-annealed AlN crystal). The increase in the band gap is attributed to the reduction of defects and impurities, further demonstrating the improved crystal quality. From these results, it is concluded that the high temperature annealing technique can effectively improve the quality of AlN crystals.