summary
Compared with the first and second generation semiconductor materials, the third generation semiconductor materials have higher breakdown field strength, electron saturation rate, thermal conductivity and wider band gap, which is more suitable for the preparation of high frequency, high power, radiation-resistant and corrosion-resistant electronic devices, optoelectronic devices and light-emitting devices. As one of the representatives of the third generation of semiconductor materials, gallium nitride (GaN) is an ideal substrate material for blue-green lasers, RF microwave devices and power electronic devices, and has broad application prospects in laser display, 5G communication, phased array radar, aerospace and other aspects. The Hydride vapor phase epitaxy (HVPE) method has become the mainstream method for preparing GaN crystals due to its simple growth equipment, mild growth conditions and fast growth rate. Due to the widespread use of quartz reactors, the unintentional doping of GaN obtained by HVPE growth inevitably has donor-type impurities Si and O, which make them exhibit n-type electrical properties, high carrier concentration and low conductivity, which limits its application in high-frequency and high-power devices. Doping is the most common way to improve the electrical properties of semiconductor materials, and different types of GaN single crystal substrates can be obtained by doping and utilizing different dopants to improve their electrochemical characteristics and meet the different needs of market applications. This paper introduces the basic structure and properties of GaN semiconductor crystal materials, and reviews the main research progress of HVPE to grow high-quality GaN crystals in recent years. The doping characteristics, dopant types, growth process, and influence of doped atoms on electrical properties of GaN were described in detail. Finally, the challenges and opportunities of HVPE growth doped GaN single crystal are briefly described, and the future development prospect of GaN single crystal is prospected.
As a representative of the third generation of semiconductor materials, group III nitride has great application prospects in the fields of optoelectronics and microelectronics, and the related material growth and device development have received extensive attention from researchers, and have made great progress. Compared with the first and second generation semiconductor materials, the third generation semiconductor materials based on silicon carbide (SiC), zinc oxide (ZnO), gallium nitride (GaN) and aluminum nitride (AlN) have higher breakdown electric field, electron saturation rate, thermal conductivity and wider band gap, which is more suitable for the development and manufacturing of high-frequency, high-power, radiation-resistant, corrosion-resistant electronic devices, optoelectronic devices and light-emitting devices.
Compared with indirect bandgap semiconductor SiC and ZnO with p-type doping problems, GaN, as a representative of the third generation of semiconductor materials, belongs to direct bandgap semiconductors, which have many excellent properties such as wide band gap, high breakdown voltage, high thermal conductivity, and small dielectric constant. Due to its excellent optical, electrical, mechanical properties and thermal stability, it has been widely used in optoelectronic devices such as blue-green lasers, radio frequency microwave devices and power electronic devices, and has important applications in laser display, 5G communication, phased array radar and smart grid, and has gradually become the core supporting material of the third-generation semiconductor industry.
According to the different substrate materials, GaN is divided into homoepitaxy growth and heteroepitaxy growth. When GaN materials are grown epitaxically on heterogeneous substrates, due to the mismatch between lattice constant and thermal expansion coefficient between heterogeneous substrates and newly grown GaN, heteroepitaxy will cause strong stress in the epitaxial layer, resulting in cracks. In addition, the electrical properties and structural properties of the heterogeneous substrate will affect the crystalline quality (surface morphology, defect density, internal stress) of the epitaxial material, and the crystal quality obtained is poor (large wafer curvature and high dislocation density) compared with the homogeneous epitaxy. Homogeneous epitaxy can make up for the lack of heteroepitaxy and grow high-quality crystals. Since GaN epitaxial growth has a strong dependence on substrate quality, it is impossible to significantly improve the quality of new growth crystals, and high-quality substrates are required to make up for it, and how to obtain large-size, high-quality GaN single crystals is still the focus of current research.
Compared with ammonia thermal method, co-solvent method and other methods, the hydride vapor phase epitaxy (HVPE) method has simple equipment, low cost, fast growth rate, and the GaN single crystal obtained by growth has large size, good uniformity, easy to control photoelectric properties, and has become a research hotspot at home and abroad, and is also the most widely used and promising GaN single crystal commercial growth method. Due to the use of HVPE quartz reactor, the donor-type impurities Si and O are inevitably incorporated into the GaN epitaxial growth process, and some intrinsic defects inside GaN are also donor-type, so that unintentional doping of GaN presents n-type electrical properties. The unintentional doping of GaN has a high background carrier concentration, low conductivity and large fluctuation range, which limits its further R&D application.
In order to compensate for the lack of unintentional doping of GaN electrical properties and better utilize the excellent properties of GaN, it is necessary to grow or dope it with high purity. GaN materials with different electrical characteristics can be obtained by doping them, improving their electrochemical properties and broadening their application fields. This article systematically reviews the principle and latest research progress of HVPE growth and doping of GaN crystals, introduces the effects of various types of doping protons on the growth of GaN single crystals, and prospects the development trend of HVPE growth and doping of GaN crystals.
1 GaN crystal
1.1 GaN crystal structure and defects
GaN single crystal is solid at room temperature and pressure, with three crystal structures, namely hexagonal zinc structure, cubic sphalerite structure and rock salt ore structure. The structure of fibrine zincite at room temperature and pressure is a thermodynamically stable structure, which belongs to the P63mc space group and is the most common crystal structure of GaN single crystals. In a zincite GaN structure, six Ga atoms and six N atoms are present in each unit cell. In the unit cell, each Ga atom is surrounded by the nearest four N atoms, forming a coordination tetrahedron; Similarly, each N atom is surrounded by the nearest four Ga atoms to form a tetrahedral coordination, so the fiber-zinite structure GaN can also be understood as two sets of hexagonal lattice mantles formed with a stable thermodynamic structure (see Figure 1(a)).
Due to the special symmetry of the hexagonal structure, the hexagonal GaN crystal system can be expressed by the three-axis Miller index (hkl), or by the four-axis Miller-Bravi index (hkil), where i=- (h+k), although the two representations have the same effect, but compared with the Miller index, the Miller-Bravey index is more common and intuitive, and is widely used.
In GaN crystals, due to the non-central symmetry of the fibrinite structure, GaN crystals observed in different directions show different faces: in the c-axis direction, that is, the face pointed to in the [0001] direction is the Ga surface, and the face in the [000-1] direction is the N surface (there is a significant difference in performance, the Ga surface is more stable than the N surface). Due to the influence of crystal structure, the C-axis ([0001] direction of the crystal has polarity. According to the difference in the angle between the crystal plane and the [0001] direction, the crystal plane of GaN is divided into three categories: the first is a polar plane perpendicular to the [0001] base vector, also known as the c plane, the base plane or (000m) surface; The second is a non-polar surface parallel to the [0001] base vector, in fact there are only two non-polar surfaces, namely m-plane {10-10} and a-plane {11-20}; The third is a semipolar plane with an angle between 0° and 90° to the [0001] base vector (see Figure 1(b)). According to the powder X-ray diffraction results of the GaN sample, only a limited number of crystal planes actually contain atoms, and the potential semipolar faces are: {10-10}, {10-12}, {10-13}, {10-14}, {10-15}, {11-22}, {11-24}, {20-21}, {20-23}, {30-32}, {31-30}, {21-32} and {21-33}; At present, semipolar surfaces represented by {10-10}, {10-13}, {10-14}, {11-22}, {20-21} and {31-30} have been discovered and studied.
The atoms in a perfect crystal are arranged in strict periodic rules, but the occurrence of defects during the growth process not only destroys the integrity of the crystal structure, but also affects the properties of the crystal. Therefore, studying the generation, interaction and performance of defects in crystals is of great significance to improve the photoelectric performance of GaN crystals and improve the efficiency and stability of GaN-based devices. According to the scale and morphology, defects are divided into four types [4]: zero-dimensional defects, that is, point defects, related to the position of a single atom, such as vacancy (VGa, VN), interstitial atoms (Ni, Gai, interstitial impurity atoms), substitute atoms (NGa, GaN, alternative heterogeneous atoms), doped GaN is to form point defects through impurity atoms in the crystal to affect the photoelectric properties of the crystal. Different point defects play as donor, recipient or other electronic impurities, and common donors in GaN include Si, Ge on the Ga grid position and O, S, Se, etc. at the N grid position; Common recipients in GaN are Mg, Ca, Zn on the Ga grid position and Fe, C, Si, Ge, etc. at the N grid position. One-dimensional defects, also known as line defects, are related to a certain direction, such as dislocations; Two-dimensional defects, also known as surface defects, are related to a certain crystal plane, such as grain boundaries, crystal planes, stacking layer faults; Three-dimensional defects, also known as bulk defects, are volume-related, such as cavities, cracks, and pits.
The band gap of GaN is as high as 3.4 eV, which determines that GaN materials have unique advantages in near-ultraviolet and blue-green photoelectric devices. High electron mobility and high saturation electron rate mean that GaN can be used to make high-speed electronic devices, especially the high carrier mobility in two-dimensional electron gas, which makes GaN-based HEMT devices widely used. Moreover, compared with the first and second generation semi-conductor materials such as Si and GaAs, the high thermal conductivity and breakdown field strength of GaN make GaN-based devices work under high power in harsh environments, and the application prospects are broader.
HVPE growth method for 1.2 GaN
Crystallization of GaN is a rather challenging process, melting at extremely high temperatures (> 2500 °C) and the N2 pressure required for uniform melting is expected to be higher than 6 GPa, so GaN growth is currently not possible directly from the melt. At present, the growth methods of GaN include HVPE method, co-solvent method, ammonia heat method, high-pressure solution growth method (HNPS) and chemical vapor deposition (CVD). Compared with traditional methods such as ammonia heat method and solvent method, HVPE method has the advantages of mild growth conditions, low growth equipment requirements, fast growth rate (up to hundreds of microns per hour), high process repeatability, easy doping, etc., and has become the most widely used method for the commercial preparation of GaN, and is also considered to be the most potential method for growing GaN crystals. The growth rate of the HVPE method mainly depends on the geometry of the reactor, the flow rate of the source gas and the growth temperature. The use of HVPE method can quickly grow a thick film with low dislocation density, its disadvantage is that it is difficult to accurately control the film thickness, and the reaction gas HCl is corrosive to the equipment, which affects the further improvement of the purity of GaN materials.
HVPE is a gas-phase-based growth method. The main mechanism is that GaCl formed by the reaction of metal Ga with HCl in the low temperature region (~850 °C) is transported to the substrate surface in the high temperature region (~1040 °C) at a pressure lower than 1 atmosphere, and the reaction formula is as follows (the reactor structure is shown in Figure 2).
HVPE growth GaN has two growth modes: low temperature (LT) mode and high temperature (HT) mode. Films grown in these modes vary significantly depending on surface roughness, pit density and shape, and growth stress values. The surface is smooth in HT mode, but the growth stress is high, and cracks are easy to occur. The surface is rough in LT mode with a high density of V-shaped pits, but this film has no cracks.
At present, the most commonly used substrates for the preparation of GaN devices are SiC, sapphire (Al2O3), AlN and other heterogeneous substrate materials, but due to the existence of lattice mismatch and thermal expansion coefficient mismatch between the heterogeneous substrate and GaN, the quality and performance of the grown crystals will inevitably affect and reduce the service life and reliability of the device. Homogeneous substrates reduce stress and cracking and improve their performance.
The growth process has a great influence on crystal quality. The crystal quality of GaN can be effectively improved by adjusting temperature, flow, and V/III during growth. Due to the lack of homogeneous substrates, heteroepitaxy is still the mainstream choice for GaN crystal growth, and it is particularly important to solve the stress caused by mismatch in the process of heterogeneous epitaxy. Among them, the most serious impact is the cracking between GaN and the heterogeneous substrate due to lattice mismatch and thermal mismatch, which restricts the complete acquisition of large-size single crystals. Etching pretreatment of the substrate and the addition of buffer layer can also reduce the density of defects (dislocations) in the grown crystals and improve the crystal quality of GaN. Porous substrates are a simple method to achieve low dislocation density in semiconductor growth technology, which provides a reliable application for the heteroepitaxy growth of lattice mismatch materials, significantly reduces the stress generated during the heteroepitaxy process, and improves the optical quality of the epitaxial layer. In 2021, Liu et al. successfully obtained a 2-inch crack-free self-supporting GaN with high crystal quality on a sapphire substrate by combining a low-temperature AlN buffer layer and a 3D GaN intermediate layer, using laser lift-off technique (LLO), further improving the crystal quality of GaN epitaxial films. In LLO operation, the excitation radiation passes through the sapphire, is absorbed by the GaN at the interface and rapidly divided into metal Ga and N2, and then the resulting N2 expansion separates the two sides of the interface, completing the separation of GaN. Operating parameters such as laser scanning speed, excitation intensity, and ambient pressure conditions all affect the quality of the separated GaN material and need to be precisely adjusted. After laser emission, the compressive stress in the GaN film mainly comes from the thermal mismatch between the GaN thin film and the sapphire substrate. The evaporative pressure and stress relief of N2 formed by the decomposition of the interface GaN cause cracking, and the laser peeling of GaN can be achieved more easily by increasing the thickness of GaN to reduce the compressive stress.
1.3 HVPE method growth
Progress of GaN crystals The crystal growth of GaN has been steadily advancing, and foreign institutions have led the research and development of companies such as the Institute of Polish Physics, Japan's Mitsubishi, Sumitomo, SCIOCS, and Kyma in the United States; Mainland China started late in the field of GaN semiconductor materials, but it has a lot of relevant basic research technology reserves, among which Suzhou Navi and Zhong gallium 2-inch GaN crystals have been mass-produced, and Shandong University, the 46th Research Institute of China Electronics Technology Group Co., Ltd. and other units have also made great progress.
In 2018, Fujikura et al. in Japan successfully achieved the preparation of 2~6-inch GaN body crystals without large defects through HVPE on the basis of a new crystal hardness control. Dislocation is a major feature of crystal quality, Fujimoto et al. used SiO2 hexagonal mask for two-step smooth surface growth, which effectively improved the lattice curvature and crystal quality of the obtained GaN crystals, and the dislocation density was reduced to 6.8×105 cm-2. Yoshida's team successfully obtained a 2-inch GaN substrate with a dislocation density of 4×105 cm−2 by eliminating the C plane to inhibit the propagation of seed crystal dislocations by using a three-dimensional growth zone to eliminate the C-plane, and further reduced the dislocation density to 104 cm−2 by growing the three-dimensional growth zone twice. In 2020, Mitsubishi Corporation of Japan prepared a GaN single crystal substrate with low dislocation density (1.4×103 cm-3) on ammonia hot GaN seed crystals by HVPE method. Jae-Shim et al. used a two-step growth method to release thermal stress between the sapphire substrate and the epitaxial GaN layer, and obtained a 2-inch bowless self-supporting GaN wafer for high brightness light-emitting diodes (HB-LEDs) by LLO, three-step polishing, and inductively coupled plasma reactive ion etching (ICP-RIE).
GaN has developed a 2-inch GaN self-supporting substrate product with dislocation densities as low as 4×105 cm-2 to 7×105 cm-2, and has begun mass production and sales. It provides Si-doped 2-inch high conductivity GaN self-supporting substrates for blue-green lasers and vertical GaN power devices; C-doped 2-inch semi-insulated GaN self-supporting substrates are available for the fabrication of high-performance microwave RF devices. The State Key Laboratory of Crystal Materials of Shandong University has also conducted research on the growth and processing of GaN single crystals. GaN above 900 °C is prone to decomposition and is prone to form porous structures. A 2-inch porous GaN substrate and a 2-inch self-supporting porous GaN single crystal film were successfully prepared by high-temperature annealing, and the influence of annealing time and annealing temperature on the surface morphology, optical and electrical properties of porous GaN was studied in detail. The porous structure makes the growth interface form voids, effectively block the dislocation, reduce stress and achieve separation from the substrate, and for the first time grow on a high-temperature annealed porous substrate and obtain high-quality self-peeling GaN single crystals. The nucleation stage growth behavior of epitaxy GaN on the prepared porous substrate was studied in detail. Recently, the team used HVPE to grow 2-inch GaN single crystals with a thickness of up to 2.5 mm and a smooth surface without pits. The key technology of 2-inch single crystal homogeneous epitaxial growth was overcome, and the surface half-peak width of GaN single crystal (0002) was 48 arcseconds, the width of (10-12) plane half-peak was 67 arcseconds, and the dislocation density (DD) was as low as 5×106 cm-2. The processed samples are microscopicly flat and have good crystal quality (see Figure 3), specific research papers, detailed reports later.
Compared with other methods, HVPE has received widespread attention in the commercial field for growing GaN with fast speed, low cost, and simple equipment process. In recent years, driven by national policies and market conditions, the research on HVPE preparation of GaN has been steadily advanced, with outstanding results and good development prospects.
2 Doping and progress of HVPE-GaN
Electrical performance is the core parameter of GaN single crystal substrates, and it is also the key to determine whether GaN single crystal substrates can achieve a wide range of applications. The resistance of conventional GaN crystals is generally low, limiting their use in high-frequency, high-power devices. Doping is a common means of regulating the electrical properties of GaN wafers, and impurities and defects can create energy levels within the bandgap that affect the physical and chemical properties of the host material. Different doping sources will have different effects on GaN crystals, produce different electrical characteristics (n-type, p-type, semi-insulating type), and are used in different fields.
2.1 N-type GaN growth
The early preparation of GaN was mainly unintentional doping, due to the presence of intrinsic defects (such as N vacancies) within GaN and the inevitable release of donor-type impurities (Si and O) by the use of quartz reaction chambers, which showed the electrical properties of n-type. Si and O of unintentional doping of GaN belong to shallow donor impurities, substrate carrier concentration in the range of 1016~1017 cm-3, in low carrier concentration samples, Si concentration is higher than O concentration, and in higher carrier concentration materials, O concentration is higher than Si concentration, electron concentration decreases with the increase of GaN thickness, resistivity fluctuation range is relatively large, performance is unstable, not suitable for high power (optoelectronic and electronic) vertical devices, further doping is required to meet the needs of device manufacturing. The carriers on the n-type GaN substrate obtained by doping can be efficiently transmitted throughout the device, significantly improving the power and efficiency of the device, and are used in the fabrication of high-power vertical devices.
Si doping and Ge doping are the most common ways to implement n-type GaN. In HVPE, there are many options for Si doping sources. First of all, like MOVPE, gas sources such as silane can be considered, but due to the poor thermal stability of silane, it will be quickly resolved before reaching the substrate, which is not the best choice for Si doping; Solid Si can be used as a doping source to react with HCl to generate SiHCl3, at high temperature, converted to SiCl2, and then transported to the growth area, because the Si sheet changes after the reaction to affect the control of doping amount, Lipski F through HVPE with Si-Ga solution as both Si source and Ga source successful preparation to obtain Si doped GaN; SiH2Cl2 has higher thermal stability, and SiH2Cl2 is currently used as the most common doping source, and the GaN grown by HVPE has good crystal quality (see Figure 4(a) for device structure). Si atoms are shallow benefactors in GaN and can increase the Fermi level of GaN, so higher Si doping concentrations can improve the performance of ohmic contact. Moreover, proper Si doping does not affect the high structural quality of the obtained HVPE-GaN crystals. However, Si impurities have anti-surfactant effect, and with the increase of doping concentration, the monoatomic layer SiGaN3 will be formed on the surface of GaN, introducing repulsive electric dipole moments, hindering the continued growth of GaN on the surface, resulting in deterioration of surface morphology, which in turn limits the increase of Si concentration. Due to the interaction between Si atoms and linear dislocations, Si doping also causes dislocations in GaN materials to tilt during dislocation climbing, thereby introducing tensile stress and causing problems such as warpage and cracking of GaN, reducing the thickness of the critical layer. Tensile strain caused by Si doping is widely present in GaN, AlGaN, and AlN, independent of the growth technique used. The lower the dislocation density, the weaker the influence of Si doping and carrier concentration on tensile stress. The use of high-quality seed crystals as substrates can effectively reduce the dislocation density of GaN materials and reduce the existence of tilt dislocations, thereby alleviating the tensile stress inside Si-doped GaN. Xia et al. found that the mobility of Si-doped high-quality mass GaN was better than that of GaN substrates with higher dislocation densities at the same carrier concentration. Doping with Si can obtain a highly conductive n-type HVPE-GaN crystal with a free carrier concentration very uniform in the c plane (see Figure 4(b, c) with only a small deviation of carrier concentration at the wafer edge).
In addition to Si, Ge is a very promising n-type GaN-doped atom, compared to Si doping, Ge is a surfactant during GaN growth, and its doping does not increase the dislocation density to prevent deterioration of surface morphology during growth. The atomic half-diameter of Ge is close to that of Ga atom, and the addition of Ge impurities has less influence on GaN lattice structure and stress than Si impurities. GeCl4 is an excellent choice for Ge doping sources in GaN growth (the equipment structure is shown in Figure 5(a)), Iwinska et al. found that during the growth process in the H2 environment, due to the formation of Ge droplets on the surface of the growing crystal (Ge melting point is lower than 950 °C), the growth of the crystal is hindered, resulting in the formation of pits in the crystal, the pit density increases with the increase of Ge concentration, when the supply of Ge precursors is stopped, the pits may overgrow laterally, affecting the corresponding performance, to N2 The gas is the carrier gas, and it can be undisturbed during the crystallization process to obtain high-quality Ge-doped GaN (free carrier distribution without fluctuations) (see Figure 5(b)). Dislocation tilt does not depend on the type of dopant, and like Si, Ge has the same effect on the stress evolution of n-type GaN, and dislocation tilt that causes tensile stress occurs during epitaxial growth, mainly due to the rise of Ga vacancies. Using GeCl4 as the doping source, Oshima's team used HVPE growth to obtain GaN crystals, showing that GaN still has excellent performance even at high Ge-doping concentrations, and is a very promising preparation method for n-type GaN. The carrier concentration of GaN can be increased to more than 1018 cm-3 by Si doping and Ge doping, meeting the needs of high-power (optoelectronic and electronic) vertical devices; Through the growth and research of n-type GaN, it is helpful to further develop and improve the properties of GaN, among which reducing the dislocation density and alleviating the stress during the growth process is essential for the preparation of high-reliability and high-performance electronic optoelectronic devices of n-type GaN, which plays an important role in promoting the application of GaN crystals.
2.2 p-type GaN growth
P-type GaN can be used in the preparation of high-efficiency optoelectronic devices such as blue-green light-emitting diodes and laser diodes and excellent thermoelectric devices, but its preparation is difficult and its late start restricts the development and application of p-type GaN base devices. High doping concentration p-type GaN requires improved carrier injection efficiency of (i) luminescent p-n junctions; (ii) current diffusion in light-emitting structures; (iii) ohmic contact parameters to reduce the operating voltage and tolerate the higher forward currents required for the high output power operation of the optical source. After Mg doped into GaN and the remaining H atoms in the GaN crystal to form a Mg-H neutral complex, causing hole compensation, resulting in the passivation effect of Mg, losing its main role, resulting in high resistance, until 1989, Amano et al. used low-energy electron beam irradiation (LEEBI) epitaxial treatment of GaN doped with Mg to obtain low-resistance p-type GaN samples that really opened the research of P-type GaN. At present, Mg doping is the only method to obtain the p-type conductivity in GaN so far, and the lattice constant and unit cell volume of the system after Mg doping GaN increase, while the band density increases, and the valence band top of the system moves in the direction of high energy, and enters the Fermi energy level above, resulting in GaN showing p-type conductivity, and its electrical properties are closely related to the Mg doping dose and annealing process.
Since the Mg doping appears near the Fermi level, the top of the valence band enters above the Fermi level, so that GaN presents P-type conductivity, and after doping Mg, the valence band and conduction band bandwidth are narrowed, and the locality is enhanced, the valence band and conduction band of GaN move in the direction of high energy, and the upward movement of the conduction band is larger than that of the valence band, resulting in an increase in the band gap. Through the analysis of the Mg-doped GaN dielectric function, it is found that a new set of dielectric peaks are introduced in the high-energy and low-energy distinctions, which are related to the transition of the Mg protogen. At the same time, the introduction of Mg also shifts some of the original dielectric peaks to high energy.
MgO has a melting point of about 2800 °C, and the vapor pressure is basically the same as that of quartz, which is an attractive material for Mg-doped sources in HVPE systems, and MgCl is transported to the substrate by reacting with HCl for doping (see Figure 6(a) for reactor structure). Ohnishi et al. used MgO as a doping source to adjust the Mg doping concentration by controlling the HCl flow to achieve the growth of HVPE of Mg-doped GaN, and studied the electrical properties and structural defects of the p-type GaN layer with a Mg concentration of 8.0×1018~8.3× 1019 cm−3. Mg doping concentrations above 5×1019 cm-3 result in self-compensation and lead to a decrease in free hole concentration, which is not conducive to obtaining high hole concentrations and low resistivity p-type GaN (see Figure 6(b)), and the Hall effect measurement surface at different temperatures forms a cone inverse domain (PID) in samples heavily doped with Mg, the Mg atoms in PID are inactive, do not act as acceptors, inhibit the increase in acceptor concentration, and compensate for the increase in donor concentration, which in turn leads to a decrease in hole concentration (see Figure 6( c))。
Due to the late start, complex process, and difficult doping of p-type GaN, the research progress is slow, and the ionization energy of Mg is large (about ~180 meV), which limits the concentration of hole carriers in Mg-doped GaN and affects its electrical properties. However, due to its unique optoelectronic properties for the production of light-emitting devices, p-type GaN has attracted increasing attention from researchers, and the process of preparing p-type GaN by HVPE method is relatively lacking, and in-depth research and improvement of its growth method and mechanism will further expand the application of GaN light-emitting devices.
2.3 Semi-insulated GaN growth
Devices such as High electron mobility transistors (HEMTs) must be prepared on a semi-insulated GaN substrate to overcome signal loss due to parasitic capacitance. There are two ways to grow semi-insulated GaN in HVPE. Most HVPE devices use quartz components, inadvertently adding Si and O, resulting in n-type conductivity, so resistivity can be improved by designing new HVPE devices to remove quartz from the reactor to obtain high-purity GaN crystals; In another method, a deep level dopant can be intentionally added to compensate for unintentional free electrons, generally by using deep level impurities (Fe, Mn, C) to compensate the background shallow donor (Si impurities and O impurities) (the corresponding semi-insulating GaN wafer morphology is shown in Figure 7), high concentrations of shallow donors require higher concentrations of compensation impurities, which may reduce the inherent properties of the material, so it is also critical to reduce the concentration of intrinsic donor impurities in the crystal.
Bockowski et al. measured the activation energies of the Mn, C, and Fe dopants in GaN at a high physical energy level of 1.8, 1, and 0.6 eV, respectively, with the highest resistivity of Mn doped and the lowest resistivity of Fe doped (see Figure 8(a)).
In GaN, as a transition metal (TM), the Fe2+/3+ charge conversion energy levels are close to the middle of the band gap, and this effect is used by heavy Fe doping to achieve semi-insulating performance, which is applied to electronic and optoelectronic devices, and is also the most commonly used doping source for researchers to prepare semi-insulating GaN. The incorporation of Fe will make the GaN crystal form a deeply central center, and the excited hole compensation part of the electrons generated due to intrinsic defects reduces the concentration of free carriers (electrons) in GaN, so that the resistivity at room temperature is increased to 3.6×108 Ω·cm, thereby giving the material high resistance characteristics (semi-insulation); With the incorporation of Fe, the resistivity in the GaN crystal gradually increases (see Figure 8(d)), and the relaxation effect of the internal residual stress of the GaN epitaxial layer on the sapphire substrate becomes more and more significant with the increase of Fe doping concentration. Iron-doped GaN has good thermal stability and the resistivity remains largely unchanged even at annealing temperatures of 1050 °C. However, when the Fe concentration is too high (≥1×1018 cm-3), the introduction of impurities may lead to an increase in defect density and deterioration of structural quality. Fe on FeN and gap configuration Fei has very high formation energies compared to FeGa (see Figure 8(b)), and Fe atoms incorporated into GaN typically occupy the Ga position in the GaN lattice. Fe2+ and Fe3+ are present at high concentrations together, while only Fe3+ is present in lower concentrations of doped. Since the ion radius of Fe3+ is smaller than the ion radius of Ga3+, and the Fe-N bond is shorter than the Ga-N bond, the length of the Ga-N bond near Fe increases, resulting in a slight increase in the a and b values and a slight decrease in the c value of the unit cell after doping.
Since the resistivity of Fe-doped GaN is controlled by Fe impurities' compensation of unintentionally doped shallow donor impurities, both carrier concentration and mobility in GaN decrease with increasing Fe concentration (see Figure 8(c)). By reducing the concentration of background impurities, the Fe concentration required to achieve semi-insulating electrical characteristics can be significantly reduced. When GaN is excited by two-photons, the free electrons generated by light are captured by Fe3+[4E(G)], and Fe3+ is ionized to Fe2+, and due to the Coulomb action between Fe2+ and the hole, Fe2+[5T2(D)] captures holes into an excited state of Fe3+[4E(G)]. These carrier trapping processes provide additional paths for carrier recombination, reducing the lifetime of photogenerated carriers (see Figure 8(e)). Fe3+ + 2hω→ Fe3++ eCB + hVB → Fe2++ hVB → (Fe3+) * Due to carrier trapping effects, the lifetime of carriers is significantly shortened and decreases linearly with increasing Fe concentration. At high Fe-doped concentrations (1×1019 cm-3), the equivalent carrier lifetime can be reduced to 10 ps, nearly three orders of magnitude faster than Si-doped and non-doped GaN crystals (see Figure 8(f)). However, due to the parasitic precipitation of Fe and the upper limit of doping of Fe concentration, the lifetime of the carrier is not reduced indefinitely.
Fe doping in HVPE is commonly used as ferrocene (Cp2Fe), which is used with an aerator to add the source material to the HVPE gas mixture, but Cp2Fe can cause unintentional doping of carbon into the material. Fe can also be used as a dopant in HVPE in the form of a pure metal (flowing HCl gas through the pure metal). The essence of both is to react with HCl to form FeCl2 and transport to the substrate as a doping substance on the HVPE growth surface.
Iwinska et al. used ammonia heat GaN as seed crystals, using solid Fe as the doping source, and grew GaN crystals by HVPE method, and obtained GaN crystals co-doped with Fe and Mn. Freitas et al. [75] adopted a new iron precursor Fe2O3 as a dopant through HVPE (to avoid absorption of C from organic metal sources) to compensate for the ubiquitous Si and O shallow donor impurities, and grew thick independent iron-doped semi-insulating GaN layers on a GaN/sapphire substrate. The Fe concentration in GaN decreases with the increase of growth rate, and when epitaxial growth is carried out on Fe-doped GaN as a substrate, it can be added to the epitaxial growth of undoped GaN through solid-phase diffusion, surface segregation or vapor phase diffusion, which affects the performance of the device. Fe will produce parasitic deposits in GaN doping, limiting the further increase of sample doping concentration, how to overcome this problem, improve the doping concentration of Fe is still the focus of research.
C is another good semi-insulating GaN dopant, and C-containing gases such as CH4, C2H4, and C5H12 are often used as doping sources in HVPE. It is well known that C impurities in GaN, not only as donors, but also as acceptors (the relationship between CGa and CN formation energy and Fermi energy levels in different environments is shown in Figure 9(a)), when the C concentration is lower than 1×1019 cm-3, C atoms occupy N atomic positions (CN) in the GaN crystal structure (see Figure 9(b)), manifesting as deep acceptors, however, with an excess of C doping concentration, a large number of Ga position C (CGa) is formed in GaN as a donor, Compensates for CN, thereby reducing the concentration of deep receptors. CN produces a yellow luminescent band around 2.2 eV and a blue band near 2.9 eV (see Figure 9(c) for the CN transition emission process). Although C doping produces dopant concentration-related deficits (see Figure 9(d)), it does not affect the stress and dislocation enhancement of GaN crystals, GaN materials can maintain good crystal quality even if C impurity concentrations exceed 1×1019 cm-3, and moderate carbon doping may even improve crystal quality by reducing the marginal dislocation density more strongly. Room temperature resistivity up to 1010 Ω·cm can be obtained by controlling the input partial voltage of the C precursor to adjust the C doping concentration (see Figure 9(e, f)). In addition, detailed photoionization spectroscopy studies have shown that C impurities are related to the trap center in the HEMT device, which will cause the current collapse of the device, and CN acts as a deep acceptor to compensate for n-type background impurities, thereby suppressing leakage current under high electric fields and increasing breakdown voltage; When the doping concentration is excessive, the compensation of the n-type background impurity by the deep energy level receptor is inhibited by the CGa-CN self-compensation effect, resulting in a decrease in breakdown voltage.
In 2021, Lai Yun of University of Shanghai for Science and Technology and Luo et al. of Gallium Semiconductor Technology Co., Ltd. [85] successfully prepared four-inch self-supporting semi-insulating GaN wafers using HVPE with a concentration of 5% methane gas as a doping source, and the resulting wafers had high quality (dislocation density less than 106 cm-2, resistivity > 109 Ω·cm). Lyons used optical experiments and mixed density functional theory calculations to study the properties of C doped GaN in HVPE growth, and confirmed that the photoluminescence measurement results showed that the yellow luminescence band changed with C concentration, indicating that the properties of C in GaN changed with the increase of C content.
Semi-insulating GaN has high dark state resistance, good photoelectric characteristics, piezoelectric characteristics and strong radiation resistance, and has a wide range of applications and good development momentum. The use of HVPE doped Fe, C and other impurities to realize the growth of semi-insulated GaN, with simple method and high crystal quality, is widely favored by researchers and has high research value and commercial value.
3 Conclusion and outlook
As the research of Si materials gradually reaches its physical limits, GaN has attracted widespread attention because its excellent properties are considered to be one of the preferred materials for the semiconductor industry in the future. As the third generation of wide bandgap semiconductor material, GaN has the advantages of corrosion resistance, high breakdown voltage, high electron mobility and high chemical stability, and is an ideal substrate material for the preparation of lasers (LD), photodiodes (LEDs), high electron mobility transistors (HEMT), radio devices (RF) and power electronic devices, and is widely used in photovoltaic power generation, laser display, rail transit, phased array radar and 5G communication and other production and life as well as national defense and security fields. Compared with other GaN preparation methods, HVPE method has a wide range of application prospects because of its fast growth rate, mild growth conditions and low growth cost, and is one of the current research priorities. Due to the widespread use of quartz cavity in HVPE, there are inherent donor impurities (Si, O) in non-doped GaN, which makes it exhibit n-type conductive properties, often causing parasitic voltage, current leakage and other problems, and due to low resistivity and large fluctuation range, it is not suitable for direct application in the manufacture of actual devices. Through the use of different dopants as dopant sources in the preparation process, different types of doped GaN can be obtained, improving their electrical properties and expanding the application range (see Table 1). N-type GaN can be obtained by Si doping and Ge doping, which increases the carrier concentration of GaN to more than 1018 cm-3 to meet the needs of high-power (optoelectronic and electron) vertical devices; The p-type GaN obtained by Mg doping can be used in the fabrication of light-emitting devices due to its unique photoelectric properties. The use of Fe, C and other high-resistance semi-insulating GaN obtained by the preparation of main impurities has the potential ability to manufacture transverse conductive devices, such as HEMT, its preparation process is simple, the performance is excellent, and the reliability of the long-term operation of the device is improved, and it has a very wide application prospect, which has become the research focus of scientific researchers.
At present, the growth of GaN crystal HVPE has a common problem of crystal growth, that is, the study of growth process precedes the study of growth mechanism. With the continuous development of GaN growth technology, the lack of growth mechanism will also limit the further improvement of crystal growth technology. Therefore, the research of growth process and mechanism must be two-pronged, combining theory with practice to promote the improvement and progress of HVPE-GaN crystals. For the doping of GaN, it is necessary to further reduce the defects of the crystal material itself, improve the doping level, and optimize the crystal performance. With the improvement of HVPE-GaN crystal growth and doping process, and the realization of large-size, high-quality, high-performance GaN crystals, GaN substrate materials will be more widely used in high-power, high-frequency communication and other fields.
Source: Journal of Inorganic Materials
Authors: QI Zhanguo1, LIU Lei1, WANG Shouzhi1, WANG Guodong1, YU Jiaoxian2, WANG Zhongxin1, DUAN Xiulan1, XU Xiangang1, ZHANG Lei1(1. State Key Laboratory of Crystal Materials, Institute of New Generation Semiconductor Materials, Shandong University; 2. Qilu University of Technology (School of Materials Science and Engineering, Shandong Academy of Sciences)