Aluminum nitride (AlN) crystal is one of the key frontier materials in semiconductors due to its superior properties such as ultra-wide band gap, deep ultraviolet transparency, high thermal conductivity, high sound velocity, and high temperature stability. AlN is an ideal substrate for gallium nitride (GaN)-based power semiconductor devices and deep ultraviolet photoelectronic devices, and is also a key material in deep ultraviolet photoelectronic and surface acoustic wave devices. Different AlN crystal growth related technologies have been developed since 1960s. Among them, a PVT AlN crystal technology is relatively mature, and several manufacturers are able to provide 2 inch AlN wafers. However, a high growth temperature (i.e., 2 300 ℃) used in this PVT AIN technology makes the growth of larger PVT AlN wafer extremely hard, and induces some impurities doping and relevant absorption in the ultraviolet region. Moreover, PVT AlN is hard to fulfill controlled doping. These factors and extremely high cost restrict the application.Hydride vapor phase epitaxy (HVPE) is considered an alternative to PVT AlN due to its potential advantages in terms of large-area wafer preparation, controlled doping and cost. Recent studies dealt with the preparation HVPE AlN. This review briefly introduced the crystal structure and fundamental properties, application scenarios, and requirements of AlN crystals. The growth technologies of AlN crystals were concisely categorized, and the HVPE method was elaborated in terms of principle, structure of the growth zone, temperature, atmosphere, substrates, process, application technology, advantages, and disadvantages. In addition, the main challenges for HVPE AlN technology were also proposed.The structural factors of HVPE growth zone (i.e., the horizontal or vertical configuration, the location and direction of gas outlet, the position and direction of substrate, and the size of the growth chamber) are coupled with process factors (i.e., flow rate, pressure and temperature), resulting in complicated flow and temperature fields. A reasonable flow field can create a uniform laminar flow pattern on the substrate surface, which is crucial for epitaxy, crystal quality and uniformity. For instance, a horizontal HVPE system, in which the gas flow direction is horizontal and perpendicular to the substrate surface, can produce a mirror-like AlN epilayer on a c-plane sapphire substrate at 1 100 ℃ with a growth rate of 1.1 μm·h-1 and obtain FWHM of planes (0002) and as low as 40 arcsec and 45 arcsec, respectively. A higher growth temperature can accelerate chemical reaction on the substrate surface and epitaxy. Meanwhile, a high temperature can significantly increase the migration rate of active Al and N atoms in the growth plane, thus promoting two-dimensional growth and crystal quality. HVPE AlN epilayers grown at 1 550 ℃ with a growth rate of about 20 μmh-1 can obtain a FWHM of planes (0002) and of 102 arcsec and 219 arcsec, respectively.The V/III ratio affects the concentration of growth atoms, diffusion distance, reaction rate and growth mode, significantly affecting the crystal quality and surface morphology of HVPE AlN. In general, the V/III ratio of HVPE AlN growth process ranges from 10 to 300. AlN film grown under V/III ratio of 150 can obtain FWHM values of planes (0002) and of 64 arcsec and 648 arcsec, respectively. When the V/III ratio is 150 and 300, the threading dislocation (TD) density can reach 8.9×106 cm-2 and 5.9×107 cm-2, respectively.The materials, crystal orientation and polarity of substrate are a basis for the crystal quality, morphology and performance of HVPE AlN epilayer. Except for commonly used sapphire, Si and SiC substrate, GaN or AlN templates are used to further reduce TD density. Especially, an association of patterned substrates with a lateral epitaxial overgrowth technology is proven as an effective way in reducing TD density. The optimization of growth process, such as surface nitridation of the substrate, buffer layer growth, adjustment of growth procedures, etc., is also investigated to improve the crystal quality of HVPE AlN epilayers.As an alternative to PVT AlN wafers, HVPE AlN wafers can enter into application only if an effective lift-off technology is established. There are only three processes, namely, chemical etching, mechanical lift-off, and self-seperation methods.This review also introduces HTCVD AlN technique, which is developed and can be regarded as a prototype or a variant of HVPE AlN. It also has the same advantages as HVPE AlN. Moreover, HTCVD AlN can grow at 1 700 ℃ and avoid harmful Si and O doping of AlN crystal, which happens in HVPE AlN process, leading to a significant promotion of AlN crystal quality. In addition, the HTCVD also adopts a safe Ar carrier and a solid AlCl3 source instead of explosive H2 and corrosive HCl and Cl2. Summary and prospects There are still some problems in HVPE AlN that need to be solved in the future.The most promising application of HVPE AlN is a large-area free-standing substrate, but the prerequisite for its realization is the development of effective lift-off technology. It is a long-standing challenge and a technical barrier for HVPE AlN application. A fundamental technological tactics to solve the lift-off problem is an idea regarding substrate technology, such as using patterned substrates with a large depth ratio, using template or buffer layers made of two-dimensional materials, or using the both. Little work on HVPE AlN epilayers with a diameter of 2 inches, a thickness of 100 μm and a TD density of 106 cm-2 has been reported yet due to the limitations of heteroepitaxy and growth temperatures below 1 400 ℃. The coupled temperature and flow fields as well as the position of the gas-solid interface can affect the uniformity and crystal quality of the AlN epilayers. A problem is an intense stress field caused by heteroepitaxy, which varies with the size of the epilayer. HVPE AlN epilayers with a thickness of several microns can crack, and those with a thickness of several tens of microns can cause a substrate fracture. A solution to the stress problem also can be innovative substrate technologies such as patterned substrates and two-dimensional material buffer layers.In addition, achieving n-type and p-type doping in HVPE AlN with a high carrier concentration and a high mobility remains a challenge. Specific issues such as nonpolar surface epitaxy and parasitic reactions also need further studies.