I. Introduction
The research and application of GaN materials is the forefront and hotspot of global semiconductor research. It is a new semiconductor material for the development of microelectronic devices and optoelectronic devices. Together with semiconductor materials such as SIC and diamond, it is known as the first generation of Ge and Si. Third-generation semiconductor materials after semiconductor materials, second-generation GaAs, and InP compound semiconductor materials. It has a wide direct band gap, strong atomic bonds, high thermal conductivity, good chemical stability (nearly corroded by any acid) and strong anti-irradiation ability in optoelectronics, high temperature and high power devices and high There are broad prospects for the application of frequency microwave devices.
Table 1 Characteristics of brazed ore GaN and sphalerite GaN
Second, the characteristics of GaN materials
GaN is a very stable compound and a hard, high melting point material with a melting point of about 1700 ° C. GaN has a high degree of ionization and is the highest (0.5 or 0.43) among III-V compounds. At atmospheric pressure, GaN crystals are generally hexagonal wurtzite structures. It has 4 atoms in a cell, and its atomic volume is about half that of GaAs. Because of its high hardness, it is a good coating protection material.
2.1 Chemical properties of GaN
At room temperature, GaN is insoluble in water, acids and bases and dissolves at very slow rates in hot alkaline solutions. NaOH, H2SO4, and H3PO4 can corrode poor quality GaN faster, and can be used for defect detection of these low-quality GaN crystals. GaN exhibits unstable characteristics at high temperatures under HCL or H2 gas, and is most stable under N2 gas.
2.2 Structural properties of GaN
Table 1 lists the characteristics comparison of wurtzite GaN and sphalerite GaN.
2.3 Electrical properties of GaN
The electrical properties of GaN are the main factors affecting the device. Unintentionally doped GaN is n-type in each case, and the best sample has an electron concentration of about 4 x 1016/cm3. The P-type samples prepared under normal conditions are all highly compensated.
Many research groups have been engaged in research work in this area. Among them, Nakamura reported that the highest mobility data of GaN is μn=600cm2/v·s and μn=1500cm2/v·s at room temperature and liquid nitrogen temperature, respectively. The subconcentration is n = 4 × 10 16 /cm 3 and n = 8 × 10 15 /cm 3 . The electron concentration values ​​of the MOCVD deposited GaN layer reported in recent years are 4 × 10 16 /cm 3 , < 1016 / cm 3 ; the results of plasma activated MBE are 8 × 103 / cm 3 , < 1017 / cm 3 .
The undoped carrier concentration can be controlled in the range of 1014 to 1020/cm3. In addition, the doping concentration can be controlled in the range of 1011 to 1020/cm3 by the P-type doping process and the low energy electron beam irradiation or thermal annealing treatment of Mg.
2.4 Optical properties of GaN
The characteristics of GaN that people are concerned with are aimed at its application in blue and violet light emitting devices. Maruska and Tietjen first accurately measured the GaN direct gap energy of 3.39 eV. Several groups have studied the dependence of GaN bandgap on temperature. Pankove et al. estimated an empirical formula for the bandgap temperature coefficient: dE/dT=-6.0×10-4 eV/k. Monemar determined a basic band gap of 3.503 eV ± 0.0005 eV and an Eg = 3.503 + (5.08 x 10-4 T2) / (T-996) eV at 1.6 kT.
In addition, many people have studied the optical properties of GaN.
Third, GaN material growth
The growth of GaN material is achieved by the chemical reaction of Ga and NH3 decomposed by TMGa at high temperature. The reversible reaction equation is:
Ga+NH3=GaN+3/2H2
Growing GaN requires a certain growth temperature and requires a certain NH3 partial pressure. Commonly used methods include conventional MOCVD (including APMOCVD, LPMOCVD), plasma enhanced MOCVD (PE-MOCVD), and electron cyclotron resonance assisted MBE. The required temperature and NH3 partial pressure are successively reduced. The equipment used in this work is AP-MOCVD, the reactor is horizontal and has been specially modified. Using domestic high-purity TMGa and NH3 as source material, DeZn is used as P-type dopant source, (0001) sapphire and (111) silicon are used as substrate for high-frequency induction heating, and low-resistance silicon is used as heating element. High purity H2 is used as a carrier gas for the MO source. High purity N2 was used as the regulation of the growth zone. HALL measurement, twin diffraction, and room temperature PL spectroscopy were used as mass characterizations of GaN. There are two key problems in order to grow perfect GaN. One is how to avoid the strong parasitic reaction of NH3 and TMGa, so that the two reactants are deposited more completely on sapphire and Si substrates, and the second is how to grow perfectly. Single crystal. In order to achieve the first purpose, a variety of airflow models and various forms of reactors were designed. Finally, a unique reactor structure was finally explored. GaN was grown on the substrate by adjusting the distance between the TMGa pipe and the substrate. . At the same time, in order to ensure the quality and repeatability of GaN, a silicon susceptor is used as a heating body to prevent violent reaction of NH3 and graphite at high temperatures at high temperatures. For the second problem, a conventional two-step growth method, a high temperature treated sapphire material, first grows a GaN buffer layer of about 250 A0 at 550 ° C, and then grows a perfect GaN single crystal material at 1050 ° C. For the growth of a GaN single crystal on a Si substrate, an AlN buffer layer was first grown at 1150 ° C, and then GaN crystals were grown. Typical conditions for growing this material are as follows:
NH3: 3L/min
TMGa: 20μmol/minV/III=6500
N2: 3 ~ 4L / min
H2: 2<1L/min
It is common to use Mg as a dopant to grow P-type GaN. However, after the material is grown, high-temperature annealing is performed at about 800 ° C and under an N 2 atmosphere to achieve P-type doping. In this experiment, Zn was used as a dopant, DeZ2n/TMGa=0.15, and the growth temperature was 950 °C. The GaN single crystal grown at high temperature was cooled with the furnace, and Zn had the ability of P-type doping. Therefore, when the intrinsic concentration is low, P-type doping is expected.
However, the Ga source used in MOCVD is TMGa, which also produces by-products, which is harmful to the growth of GaN films, and growth at high temperatures, although advantageous for film growth, is liable to cause diffusion and phase separation of the multi-phase film. Nakamura and others improved the MOCVD device. They first used TWO-FLOWMOCVD (double beam MOCVD) technology, and applied this method for a lot of research work and achieved success. A schematic diagram of double beam MOCVD growth is shown in Figure 1. In the reactor, a main gas stream consisting of H2+NH3+TMGa, which passes through the quartz jet at a high speed parallel to the substrate, and another route H2+N2 forms a secondary gas stream vertically sprayed toward the surface of the substrate, in order to change the direction of the main gas stream, so that the reactants The surface of the substrate is in good contact. The GaN film grown directly on the α-Al2O3 substrate (C surface) by this method has an electron carrier concentration of 1×10 18 /cm 3 and a mobility of 200 cm 2 /v·s, which is the best value for directly growing the GaN film. .
Fourth, the application of GaN materials
4.1 GaN-based new electronic device
Figure 1 Dual-flow MOCVD grown GaN device
Figure 2 Comparison of performance between GaN-based devices and CaAs and SiC devices
The GaN material series has low heat generation rate and high breakdown electric field, and is an important material for developing high-temperature high-power electronic devices and high-frequency microwave devices. At present, with the progress of MBE technology in the application of GaN materials and the breakthrough of key thin film growth technologies, many heterostructures of GaN have been successfully grown. New devices such as metal field effect transistors (MESFETs), heterojunction field effect transistors (HFETs), and modulation doped field effect transistors (MODFETs) were fabricated from GaN materials. Modulated doped AlGaN/GaN structure has high electron mobility (2000cm2/v·s), high saturation speed (1×107cm/s), low dielectric constant, and is a preferred material for fabricating microwave devices; GaN The wide band gap (3.4eV) and sapphire and other materials are used as substrates, and the heat dissipation performance is good, which is beneficial to the device under high power conditions. Figure 2 shows the performance of GaN electronic devices compared with GaAs and SiCMESFETs. It can be seen from the figure that GaN-based electronic devices have good application prospects.
4.2 GaN-based optoelectronic devices
The GaN material series is an ideal short-wavelength light-emitting device material, and the band gap of GaN and its alloys covers the spectral range from red to ultraviolet. Since the development of homojunction GaN blue LEDs in Japan in 1991, InGaN/AlGaN double heterojunction ultra-bright blue LEDs and InGaN single quantum well GaN LEDs have been introduced. At present, Zcd and 6cd single quantum well GaN blue and green LEDs have entered the mass production stage, thus filling the gap in the market for blue LEDs for many years. The development history of LEDs marked by luminous efficiency is shown in Figure 3. Blue light-emitting devices have a huge application market in the fields of information access, all-optical display, and laser printers for high-density optical disks. With the deepening of research and development work on III-nitride materials and devices, GaInN ultra-high-light blue and green LED technology has been commercialized, and now the world's major companies and research institutions have invested heavily in the development of blue LEDs. The ranks of competition.
In 1993, Nichia first developed a high-brightness GaInN/AlGaN heterojunction blue LED with a luminance greater than lcd. Using Zn-doped GaInN as the active layer, the external quantum efficiency was 2.7%, the peak wavelength was 450 nm, and the product was realized. Turn. In 1995, the company introduced a commercial GaN green LED product with a light output of 2.0 mW and a brightness of 6 cd, with a peak wavelength of 525 nm and a half-width of 40 nm. Recently, the company used its blue LED and phosphorescent technology to introduce a white solid-state light-emitting device with a color temperature of 6500K and an efficiency of 7.5 lumens/W. In addition to Nichia, HP, Cree and other companies have launched their own high-brightness blue LED products. The market for high-brightness LEDs is expected to jump from $386 million in 1998 to $1 billion in 2003. Applications for high-brightness LEDs include automotive lighting, traffic signals and outdoor signposts, flat-panel gold displays, high-density DVD storage, and blue-green-to-submarine communications.
After the successful development of Group III nitride blue LEDs, the focus of research began to shift to the development of Group III nitride blue LED devices. Blue LED has broad application prospects in the fields of high-density optical storage of light control and information. At present, Nichia is the world leader in GaN blue LED field, and its GaN blue LED has a lifetime of 2mW at room temperature exceeding 10,000 hours. Based on sapphire, HP has successfully developed a refractive index guided GaInN/AlGaN multi-quantum well blue LED. Cree and Fujitsu use SiC as a substrate material to develop Group III nitride blue LEDs. CreeResearch is the first to report a CWRT blue laser fabricated on SiC.
To the device structure. Following the companies of Nichia, CreeResearch and Sony, Fujitsu announced the development of an InGaN blue laser that can be used at room temperature in CW. Its structure is grown on SiC substrates and uses vertical conduction structures (P-type and n- The type contacts are fabricated on the top and back sides of the wafer, respectively, which is the first reported vertical device structure CW blue laser.
In terms of detectors, GaN UV detectors have been developed with a wavelength of 369 nm and a response speed comparable to that of Si detectors. But research in this area is still in its infancy. GaN detectors will have important applications in flame detection and missile warning.
Fifth, the application prospects of GaN
For GaN materials, since the substrate single crystal has not been solved for a long time, the heteroepitaxial defect density is quite high, but the device level has been put into practical use. In 1994 , Nichia Chemical Institute made 1200mcd LED. In 1995, it made Zcd blue light (450nm LED) and green light 12cd (520nm LED). In 1998, Japan formulated a 7-year plan to develop LEDs with wide band gap nitride materials. The goal is to develop a high-energy UV LED that is sealed in a fluorescent tube and emits white light by 2005. This white LED consumes only 1/8 of the energy of an incandescent lamp and is 1/2 of a fluorescent lamp. Its life is traditional. 50 to 100 times the fluorescent lamp. This proves that the development of GaN materials has been quite successful and has entered the practical stage. The formation of InGaN-based alloys, InGaN/AlGaN double-junction LEDs, InGaN single quantum well LEDs, and InGaN multiple quantum well LEDs have been successfully developed. InGaNSQWLED6cd high-brightness pure green tea color and 2cd high-brightness blue LED have been produced. In the future, it can be realized by combining bright light full-color display with AlGaP and AlGaAs red LEDs. Such a white light source mixed with three primary colors also opens up new fields of application, and an era characterized by highly reliable, long-life LEDs will come. Both fluorescent lamps and light bulbs will be replaced by LEDs. LED will become the leading product, and GaN transistors will also develop rapidly with the development of materials and device technology, becoming a new generation of high temperature frequency and high power devices.
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