Compared with the first and second generation semiconductor materials, the third generation semiconductor materials exhibit higher breakdown field strength, higher saturated electron drift velocity, outstanding thermal conductivity, and wider band gap, suitable for manufacturing of electronic devices with high frequency, high power, radiation resistance, corrosion resistant properties, 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 preparing blue-green laser, radio frequency (RF) microwave and power electronic devices. It has broad application prospects in laser display, 5G communication, phased array radar, aerospace, etc. Hydride vapor phase epitaxy (HVPE) method is the most promising method for growth of GaN crystals due to its simple growth equipment, mild growth conditions and fast growth rate. Due to the widely used quartz reactors, unintentionally doped GaN obtained by HVPE method inevitably has donor impurities (Si and O). Therefore, the grown GaN shows n-type electrical properties, high carrier concentration and low conductivity, which limits its application in high-frequency and high-power devices. Currently, doping is the most common method to improve the electrical performance of semiconductor materials, through which different types of GaN single crystal substrates can be obtained with different dopants to improve their electrochemical characteristics and meet the different needs of market applications. In this article, the basic structure and properties of GaN semiconductor crystal material are introduced, and the recent progress of the high quality GaN crystals grown by HVPE method is reviewed; and the doping characteristics, dopant types, growth process and the influence of doped atoms on the electrical properties of GaN are introduced. Finally, the challenges and opportunities faced by the HVPE method to grow doped GaN crystals are briefly described, and the future developments in several directions are prospected.
QI Zhanguo, LIU Lei, WANG Shouzhi, WANG Guogong, YU Jiaoxian, WANG Zhongxin, DUAN Xiulan, XU Xiangang, ZHANG Lei. Progress in GaN Single Crystals: HVPE Growth and Doping. Journal of Inorganic Materials, 2023, 38(3): 243-255 DOI:10.15541/jim20220607
(a) Hexagonal unit call (left) and the bond structure of GaN (right), with green balls indicating Ga atoms and blue balls indicating N atoms; (b) Polar face (left), non-polar face (middle) and one kind of semi-polar faces (right) of GaN crystal
Fig. 3
Photos and characterization of GaN crystals grown by HVPE
(a) 2-inch 2.5 mm thick GaN crystal; (b) (0002) surface high-resolution XRD pattern; (c) (10¯12) high-resolution XRD pattern; (d) Image of GaN wafers; (e) CL image (dislocation density ~5×106 cm-2); (f) AFM image (RMS<0.2 nm in the range of 10 μm×10 μm)
High carrier concentration; anti-surfactant effect
High power and high current optoelectronic devices (LED, LD)
Ge
GeCl4/Ge3N4
Little effect on lattice structure and stress, causing no morphological deterioration, higher carriers concentration than that of Si-doped; creating cavities inside the sample
(a) Structure of Si-doped HVPE-GaN reactor; (b) Image of 800 μm- thick Si-doped HVPE-GaN; (c) Distribution of free carrier concentration along the diameter of Si-doped HVPE-GaN
(a) Structure of Ge-doped HVPE-GaN reactor; (b) Morphologies of Ge-doped HVPE-GaN: crystallized in H2 carrier gas (left), crystallized in N2 carrier gas (middle), distribution of free carrier concentration along the diameter of Ge-doped HVPE-GaN
(a) Schematic of the HVPE system for growth of Mg doped GaN using MgO[53]; (b) Hole concentration measured at room temperature as a function of Mg concentration[55]; (c) Compensating donor concentration (Nd) and acceptor concentration (Na) as a function of Mg concentration[55]
(a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72]
(a) Formation energy versus Fermi level for CGa and CN in GaN: Ga-rich conditions (left), and N-rich conditions (right)[79]; (b) CN impurity model in GaN[79]; (c) Optical transitions of CN in GaN[79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82]
Doping is essential in the growth of bulk GaN substrates, which could help control the electrical properties to meet the requirements of various types of GaN-based devices. The progresses in the growth of undoped, Si-doped, Ge-doped, Fe-doped, and highly pure GaN by hydride vapor phase epitaxy (HVPE) are reviewed in this article. The growth technology and precursors of each type of doping are introduced. Besides, the influence of doping on the optical and electrical properties of GaN are presented in detail. Furthermore, the problems caused by doping, as well as the methods to solve them are also discussed. At last, highly pure GaN is briefly introduced, which points out a new way to realize high-purity semi-insulating (HPSI) GaN.
The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition (MOCVD)-GaN template by halide vapor phase epitaxy (HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence (PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.
LIUN, JIANGY, XIAOJ, et al.
Fabrication of 2-inch free- standing GaN substrate on sapphire with a combined buffer layer by HVPE
Free-standing GaN substrates are urgently needed to fabricate high-power GaN-based devices. In this study, 2-inch free-standing GaN substrates with a thickness of ~250 μm were successfully fabricated on double-polished sapphire substrates, by taking advantage of a combined buffer layer using hydride vapor phase epitaxy (HVPE) and the laser lift-off technique. Such combined buffer layer intentionally introduced a thin AlN layer, using a mix of physical and chemical vapor deposition at a relatively low temperature, a 3-dimensional GaN interlayer grown under excess ambient H2, and a coalescent GaN layer. It was found that the cracks in the epitaxial GaN layer could be effectively suppressed due to the large size and orderly orientation of the AlN nucleus caused by pre-annealing treatment. With the addition of a 3D GaN interlayer, the crystal quality of the GaN epitaxial films was further improved. The 250-μm thick GaN film showed an improved crystalline quality. The full width at half-maximums for GaN (002) and GaN (102), respectively dropped from 245 and 412 to 123 and 151 arcsec, relative to those without the 3D GaN interlayer. The underlying mechanisms for the improvement of crystal quality were assessed. This method may provide a practical route for fabricating free-standing GaN substrates at low cost with HVPE.
KIMT, JUNGY H, SONGJ, et al.
High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates
The influences of laser scanning speed on the structural and optical properties of thin GaN films separated from sapphire substrates by excimer laser lift-off
Journal of Physics D: Applied Physics, 2009, 42(4):045414.
\n Laser lift-off of GaN from sapphire substrates has become a viable technique to increase the brightness of GaN-based light-emitting diodes (LEDs). The LEDs free from sapphire exhibit high luminous efficiency by placing highly reflective electrode on the back side. The devices serve low series resistance together with low thermal resistance taking advantages of the vertical structure. Thinner epitaxial structure is desired to serve better device performance, however, cracks in the film after the lift-off limits the minimum thickness. In this paper, successful laser lift-off of very thin GaN with the thickness down to 4 µm is described. The established laser lift-off system utilizes homogenized beam-profile of the employed neodymium-doped yttrium aluminium garnet (Nd:YAG) third harmonic laser in which optimization of the laser fluence minimizes the thickness of the decomposed GaN. It is also revealed by calculation that the compressive stress in the thin GaN is increased by reducing the thickness. It is demonstrated that the lattice of the GaN is relaxed after the laser lift-off, which is confirmed by photoluminescence (PL) and X-ray diffraction (XRD) measurements. In addition, reduction of the wafer bowing of GaN on sapphire is experimentally confirmed after the laser irradiation with the formation of metal Ga in between the interface. \n
FUJIKURAH, KONNOT, SUZUKIT, et al.
Macrodefect-free, large, and thick GaN bulk crystals for high-quality 2-6 in. GaN substrates by hydride vapor phase epitaxy with hardness control
Japanese Journal of Applied Physics, 2018, 57(6):065502.
To produce high-quality GaN (0001) substrates with a low threading dislocation density (TDD) and a small off-angle variation, we have developed a technique named the “maskless-3D method.” This method, which is applied during GaN boule growth by hydride vapor phase epitaxy (HVPE), induces three-dimensional (3D) growth on a normal GaN (0001) seed substrate. We showed that by an appropriate choice of HVPE conditions, and without using a mask, the 3D growth shape was controlled to eliminate the c-plane and thereby suppress the propagation of dislocations from the seed. Subsequently, two-dimensional (2D) growth was carried out on the 3D structure. This 2D growth area was machined to produce a 2 inch GaN substrate with a TDD of about 4 × 105 cm−2 and an off-angle variation of 0.05°. We also confirmed that it was possible to insert the 3D growth area twice, thereby further reducing the TDD to 104 cm−2.
MIKAWAY, ISHINABET, KAGAMITANIY, et al.
Recent progress of large size and low dislocation bulk GaN growth
Gallium Nitride Materials and Devices XV, 2020, 11280:1128002.
This paper describes the fabrication of a 2 inch free standing porous GaN crystal film and the application in the growth of relaxed crack-free thick GaN.
YUR, WANGG, SHAOY, et al.
From bulk to porous GaN crystal: precise structural control and its application in ultraviolet photodetectors
Journal of Materials Chemistry C, 2019, 7(45):14116.
This paper describes the nucleation mechanism of GaN crystal growth on porous GaN/sapphire substrates. The growth behavior of epitaxially grown GaN on porous substrates is studied in detail for the first time at the nucleation stage.
CAIL L, FENGC J.
First-principles study on the electronic structure and optical properties of GaN with Mg doped
Journal of North China Institute of Science and Technology, 2019, 16(4):120.
The electrical properties of gallium nitride (GaN) substrate are crucial to the performance of vertical power devices. Bulk GaN substrates with carrier concentrations in the range from 6.7 × 1017 to 1.7 × 1019 cm−3 are grown by hydride vapor phase epitaxy. All samples show no obvious tensile stress regardless of the carrier concentration. Moreover, the mobility of Si-doped high-quality bulk GaN is superior to the GaN template with higher dislocation density at the same carrier concentration. The influence of carrier concentration on the performance of ohmic contact on N-face of Si-doped GaN is also carefully studied by circular transfer length measurement and rapid thermal annealing methods. The specific contact resistivity decreases monotonically with increase of carrier concentration, while it increases with the annealing temperature. The N-face contact becomes non-ohmic when the annealing temperature exceeds the limit value, which increases with the carrier concentration. The sample with carrier concentration of 1.7 × 1019 cm−3 still showed ohmic behavior after annealing at 450 °C. These results are not only useful to improve the electrical properties of N-type bulk GaN substrate, but also provide a potential solution for improving the efficiency of vertical devices in the future.
OSHIMAY, YOSHIDAT, WATANABEK, et al.
Properties of Ge-doped, high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy
\n Low-resistivity p-type GaN films, which were obtained by N2-ambient thermal annealing or low-energy electron-beam irradiation (LEEBI) treatment, showed a resistivity as high as 1×106 Ω·cm after NH3-ambient thermal annealing at temperatures above 600°C. In the case of N2-ambient thermal annealing at temperatures between room temperature and 1000°C, the low-resistivity p-type GaN films showed no change in resistivity, which was almost constant between 2 Ω·cm and 8 Ω·cm. These results indicate that atomic hydrogen produced by NH3 dissociation at temperatures above 400°C is related to the hole compensation mechanism. A hydrogenation process whereby acceptor-H neutral complexes are formed in p-type GaN films was proposed. The formation of acceptor-H neutral complexes causes hole compensation, and deep-level and weak blue emissions in photoluminescence. \n
AMANOH, KITOM, HIRAMATSUK, et al.
P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI)
Japanese Journal of Applied Physics, 1989, 28(12A):L2112.
\n Distinct p-type conduction is realized with Mg-doped GaN by the low-energy electron-beam irradiation (LEEBI) treatment, and the properties of the GaN p-n junction LED are reported for the first time. It was found that the LEEBI treatment drastically lowers the resistivity and remarkably enhances the PL efficiency of MOVPE-grown Mg-doped GaN. The Hall effect measurement of this Mg-doped GaN treated with LEEBI at room temperature showed that the hole concentration is ∼2·1016cm-3, the hole mobility is ∼8 cm2/V·s and the resistivity is ∼35 Ω·cm. The p-n junction LED using Mg-doped GaN treated with LEEBI as the p-type material showed strong near-band-edge emission due to the hole injection from the p-layer to the n-layer at room temperature. \n
TONGY Z, LIF, YANGZ J, et al.
Electrical property and annealing characteristics of heavy Mg-doped GaN films
GaN crystals were grown by hydride vapor phase epitaxy (HVPE) and doped with C. The seeds were high-structural-quality ammonothermally crystallized GaN. The grown crystals were highly resistive at 296 K and of high structural quality. High-temperature Hall effect measurements revealed p-type conductivity and a deep acceptor level in the material with an activation energy of 1 eV. This is in good agreement with density functional theory calculations based on hybrid functionals as presented by the Van de Walle group. They obtained an ionization energy of 0.9 eV when C was substituted for N in GaN and acted as a deep acceptor.
FREITASJR J A, TISCHLERJ G, KIMJ H, et al.
Properties of Fe-doped semi-insulating GaN substrates for high-frequency device fabrication
Carbon doping is a viable approach for compensating the unintentional donors in GaN and achieving semi-insulating substrates necessary for high-frequency, high-power devices. In this work, bulk material properties and point defects are studied in mm-thick free-standing carbon-doped GaN to understand the efficacy of the carbon dopant. Temperature-dependent Hall measurements reveal high resistivity and low carrier concentrations at temperatures as high as 560 degrees C in a 6x10(17)cm(-3) C-doped sample, and electron paramagnetic resonance (EPR) indicates that carbon acts as the compensating defect. Photoluminescence, in agreement with photo-EPR, suggests that the compensating center is C-N; however, additional defects, which possibly limit compensation, are formed at carbon concentrations greater than 5x10(17)cm(-3).
ZHOUD, NIY, HEZ, et al.
Investigation of breakdown properties in the carbon doped GaN by photoluminescence analysis
Elimination of macrostep-induced current flow nonuniformity in vertical GaN PN diode using carbon-free drift layer grown by hydride vapor phase epitaxy
Schematic diagram of GaN<sup>[<xref ref-type="bibr" rid="b4">4</xref>]</sup>
(a) Hexagonal unit call (left) and the bond structure of GaN (right), with green balls indicating Ga atoms and blue balls indicating N atoms; (b) Polar face (left), non-polar face (middle) and one kind of semi-polar faces (right) of GaN crystal ...
... [4]
(a) Hexagonal unit call (left) and the bond structure of GaN (right), with green balls indicating Ga atoms and blue balls indicating N atoms; (b) Polar face (left), non-polar face (middle) and one kind of semi-polar faces (right) of GaN crystal ...
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
The influences of laser scanning speed on the structural and optical properties of thin GaN films separated from sapphire substrates by excimer laser lift-off
(a) Structure of Si-doped HVPE-GaN reactor; (b) Image of 800 μm- thick Si-doped HVPE-GaN; (c) Distribution of free carrier concentration along the diameter of Si-doped HVPE-GaN ...
... [38]
(a) Structure of Si-doped HVPE-GaN reactor; (b) Image of 800 μm- thick Si-doped HVPE-GaN; (c) Distribution of free carrier concentration along the diameter of Si-doped HVPE-GaN ...
Blocking growth by an electrically active subsurface layer: the effect of Si as an antisurfactant in the growth of GaN
(a) Structure of Ge-doped HVPE-GaN reactor; (b) Morphologies of Ge-doped HVPE-GaN: crystallized in H2 carrier gas (left), crystallized in N2 carrier gas (middle), distribution of free carrier concentration along the diameter of Ge-doped HVPE-GaN ...
... [47]
(a) Structure of Ge-doped HVPE-GaN reactor; (b) Morphologies of Ge-doped HVPE-GaN: crystallized in H2 carrier gas (left), crystallized in N2 carrier gas (middle), distribution of free carrier concentration along the diameter of Ge-doped HVPE-GaN ...
Investigation of pits in Ge-doped GaN grown by HVPE
... (a) Schematic of the HVPE system for growth of Mg doped GaN using MgO[53]; (b) Hole concentration measured at room temperature as a function of Mg concentration[55]; (c) Compensating donor concentration (Nd) and acceptor concentration (Na) as a function of Mg concentration[55] ...
Thermodynamic analysis of the gas phase reaction of Mg-doped GaN growth by HVPE using MgO
... (a) Schematic of the HVPE system for growth of Mg doped GaN using MgO[53]; (b) Hole concentration measured at room temperature as a function of Mg concentration[55]; (c) Compensating donor concentration (Nd) and acceptor concentration (Na) as a function of Mg concentration[55] ...
... [55] ...
Influence of microstructure on the carrier concentration of Mg-doped GaN films
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
Characteristics of semi-insulating, Fe-doped GaN substrates
Iron as a source of efficient Shockley-Read-Hall recombination in GaN
2
2016
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
Optical nonlinearities and carrier dynamics in Fe doped GaN single crystal
2
2014
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
Effect of Fe-doping on nonlinear optical responses and carrier trapping dynamics in GaN single crystals
2
2015
... (a) Resistivity as a function of reciprocal temperature for samples doped with Mn, C, and Fe[8]; (b) Formation energy versus Fermi level for FeGa, FeN and Fei in GaN in different charge states, under Ga-rich conditions[68]; (c) Carrier concentration and Hall mobility versus Fe concentration in GaN films co-doped with Si and Fe[70]; (d) Resistivity versus inverse temperature for samples doped with Fe at various Fe concentrations[63]; (e) Schematic diagram of the energy levels and carrier decay processes of Fe-doped GaN[71]; (f) Carrier trapping time for Fe-doped GaN bulk crystals[72] ...
... (a) Formation energy versus Fermi level for CGa and CN in GaN: Ga-rich conditions (left), and N-rich conditions (right)[79]; (b) CN impurity model in GaN[79]; (c) Optical transitions of CN in GaN[79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82] ...
... [79]; (c) Optical transitions of CN in GaN[79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82] ...
... [79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82] ...
Two charge states of the CN acceptor in GaN: evidence from photoluminescence
... (a) Formation energy versus Fermi level for CGa and CN in GaN: Ga-rich conditions (left), and N-rich conditions (right)[79]; (b) CN impurity model in GaN[79]; (c) Optical transitions of CN in GaN[79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82] ...
Growth and properties of intentionally carbon-doped GaN layers
... (a) Formation energy versus Fermi level for CGa and CN in GaN: Ga-rich conditions (left), and N-rich conditions (right)[79]; (b) CN impurity model in GaN[79]; (c) Optical transitions of CN in GaN[79]; (d) Defect density as a function of C concentration[81]; (e) Temperature-dependent resistivity for C doped GaN[82] ; (f) Concentrations of carbon, oxygen, and silicon in C-doped GaN layers versus the input mole fraction of pentane[82] ...
... [82] ...
Current collapse and the role of carbon in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy
Elimination of macrostep-induced current flow nonuniformity in vertical GaN PN diode using carbon-free drift layer grown by hydride vapor phase epitaxy
