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GaN
The research and application of GaN materials is currently the frontier and hot spot of global semiconductor research. It is a new type of 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 third-generation semiconductor materials, which is the successor to the first generation of Ge and Si and the second generation of GaAs, InP compound semiconductor materials. It has a wide direct bandgap, strong atomic bond, high thermal conductivity, good chemical stability (hardly corroded by any acid), and strong anti-radiation ability. It is used in optoelectronics, high temperature, and high power devices, and high-frequency microwave devices.
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, which is the highest (0.5 or 0.43) among III-V compounds. Under atmospheric pressure, GaN crystals generally have a hexagonal wurtzite structure. It has 4 atoms in a cell, and the atomic volume is about half of GaAs. Because of its high hardness, it is also a good coating protection material.
At room temperature, GaN is insoluble in water, acid, and alkali, but dissolves at a very slow rate in a hot alkali solution. NaOH, H2SO4, and H3PO4 can corrode poor-quality GaN relatively quickly 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.
There are two main crystal structures of GaN, namely the wurtzite structure and the sphalerite structure.
GaN wurtzite structure diagram
The electrical characteristics of GaN are the main factors affecting the device. Unintentionally doped GaN is n-type in all cases, and the electron concentration of the best sample is about 4×1016/cm3. Generally, the prepared P-type samples are highly compensated.
Many research groups have been engaged in this research work. Among them, Nakamura reported that the highest GaN mobility data is μn=600cm2/v·s and μn=1500cm2/v·s at room temperature and liquid nitrogen temperature, respectively, and the corresponding current carrying of the sub-concentrations are n=4×1016/cm3 and n=8×1015/cm3. The electron concentration values of MOCVD deposited GaN layers reported in recent years are 4×1016/cm3, <1016/cm3; the results of plasma-activated MBE are 8×103/cm3, <1017/cm3.
The concentration of undoped carriers can be controlled in the range of 1014-1020/cm3. In addition, through the P-type doping process and the low-energy electron beam irradiation or thermal annealing treatment of Mg, the doping concentration can be controlled in the range of 1011-1020/cm3.
The characteristics of GaN that people pay attention to are aimed at its application in blue and violet light-emitting devices. Maruska and Tietjen first accurately measured the direct gap energy of GaN as 3.39 eV. Several groups have studied the dependence of the GaN band gap on temperature. Pankove et al. estimated an empirical formula for the temperature coefficient of the band gap: dE/dT = -6.0×10-4eV/k. Monemar measured the basic band gap as 3.503eV±0.0005eV, Eg=3.503+(5.08×10－4T2)/(T－996) eV at 1.6kT. In addition, many people study the optical properties of GaN.
The GaN material series has a low heat generation rate and high breakdown electric field and is an important material for the development of 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 breakthroughs in key thin film growth technologies, a variety of GaN heterostructures have been successfully grown. New devices such as Metal Field Effect Transistor (MESFET), Heterojunction Field Effect Transistor (HFET), and Modulation Doped Field Effect Transistor (MODFET) have been fabricated from GaN materials. Modulation-doped AlGaN/GaN structure has high electron mobility (2000cm2/v·s), high saturation velocity (1×107cm/s), and low dielectric constant. It is the preferred material for making microwave devices. GaN has a wider bandgap (3.4eV) and sapphire with other materials used as the substrate, which has good heat dissipation performance and is conducive to the operation of the device under high power conditions.
The GaN material series is an ideal material for short-wavelength light-emitting devices. The band gap of GaN and its alloys covers the spectral range from red to ultraviolet. Since Japan developed a homojunction GaN blue LED in 1991, InGaN/AlGaN double heterojunction ultra-bright blue LEDs and InGaN single quantum well GaN LEDs have come out one after another. 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 many years of blue LEDs. Blue light-emitting devices have a huge application market in high-density optical disk information access, all-optical display, laser printers, and other fields. With the continuous in-depth research and development of III-nitride materials and devices, the GaInN ultra-high blue and green LED technology has been commercialized. Now major companies and research institutions in the world have invested heavily in the development of blue LEDs.
In 1993, Nichia first developed a high-brightness GaInN/AlGaN heterojunction blue LED with a luminous brightness exceeding LCD. Using Zn-doped GaInN as the active layer, the external quantum efficiency reached 2.7%. The peak wavelength was 450nm, and the product was commercialized. In 1995, the company introduced a commercial GaN green LED product with a light output power of 2.0mW and a brightness of 6cd, with a peak wavelength of 525nm and a half-width of 40nm. Recently, the company used its blue LED and phosphorescent technology to launch a white light solid light-emitting device product 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 applications of high-brightness LEDs mainly include automotive lighting, traffic signals, and outdoor road signs, flat-panel gold displays, high-density DVD storage, blue and green light submarine communications, etc.
After the successful development of III-nitride blue LEDs, the focus of research began to shift to the development of III-nitride blue LED devices. Blue LED has broad application prospects in the fields of optical control measurement and high-density optical storage of information. Nichia is currently the world leader in the field of GaN blue LEDs, and its GaN blue LEDs have a continuous working life of 10,000 hours at 2mW at room temperature. Using sapphire as the substrate, HP has successfully developed an optical ridge waveguide refractive index guided GaInN/AlGaN multi-quantum well blue LED. CreeResearch is the first company to report on the CWRT blue laser made on SiC.
Following companies such as Nichia, Cree Research, and Sony, Fujitsu announced the development of an InGaN blue laser, which can be used in CW at room temperature. Its structure is grown on a SiC substrate and uses a vertical conduction structure (P-type and n-type). Type contacts are made on the top and back of the wafer respectively), this is the first report of a CW blue laser with a vertical device structure.
In terms of detectors, GaN ultraviolet detectors have been developed with a wavelength of 369nm, and its response speed is 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 early warning.
For GaN materials, the heteroepitaxial defect density is quite high due to the unresolved substrate single crystal for a long time, but the device level is already practical. In 1994, Nichia Chemical made 1200mcd LEDs. In 1995, they made Zcd blue (450nm LED) and green 12cd (520nm LED). In 1998, Japan formulated a 7-year plan to develop LEDs using wide-bandgap nitride materials. The goal is to develop a high-energy ultraviolet LED that is sealed in a fluorescent tube and emits white light by 2005. The power consumption of this white LED is only 1/8 of that of incandescent lamps and 1/2 of that of fluorescent lamps, and its lifespan is traditional 50 times to 100 times that of fluorescent lamps. This proves that the development of GaN materials has been quite successful and has entered the stage of practical application. The formation of InGaN alloys, InGaN/AlGaN dual junction LEDs, InGaN single quantum well LEDs, and InGaN multiple quantum well LEDs have been successfully developed. In the future, it can be combined with AlGaP and AlGaAs red LEDs to form a bright-brightness full-color display. In this way, the white light source mixed with the three primary colors also opens up new application areas, and the era characterized by high reliability and long life LED 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 material growth and device technology, and become a new generation of high temperature and high power devices.
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