Fabrication, Properties And Growth Mechanism Of GaN Materials

GaN, the third-generation semiconductor materials, has shown great prospect in applications of short wavelength blue and ultraviolet (UV) light-emitting devices (LEDs), microwave devices and high-power semiconductor devices, due to its unique properties such as broad direct bandgap, high thermal conductivity, high electron saturated mobility, high thermal stability, and so on. Many countries have put a lot of manpower, material and financial resources to study it. The premise of the research and development of GaN-based devices is the growth of high quality GaN materials, so the study of GaN materials receives extensive attention of physical, chemical, materials scientists. To date, blue-green LEDs, lasers (LDs), UV detectors and electronic devices, such as Metal Semiconductor Field Effect Transistor (MESFET), High Electron Mobility Transistor (HEMT), HeteroJunction Bipolar Transistor (HBT), Metal Oxide Semiconductor Field Effect Transistor (MOSFET)and so on, have been fabricated by GaN-based materials.With the rapid development of microelectronic and optoelectronic devices, the integrated degree is higher and higher and the size of the devices is smaller and smaller. Therefore, it is of great significance to fabricate nanodevices using nano-size materials with excellent and unique properties. The theories and experiments have proved that one-dimension GaN nanostructures can significantly improve the properties of the blue / green / UV optical and electrical devices. So one-dimension GaN materials are thought to be a kind of promising materials. The morphology and microstructure of the GaN nanostructures determine their physical and chemical properties. Single crystal GaN nanostructures especially nanowires with high purity and high quality must be first synthesized. Then various properties and microstructures of them should be studied, which provides a solid foundation for the next applications. In order to promote the development of GaN optoelectronic nano-devices with better optical and electrical properties, the appropriate doping is necessary. Various methods are tried to achieve GaN nanostructures with different dopants and study the effect of dopants on the optical, electrical and magnetic properties of the nanostructures. Although a lot of pioneering works have been done on the synthesis, doping, microstructure, growth mechanism and physical properties of the GaN nanostructures, it is still in the initial research stage. It is very difficult to find a simple controllable method to synthesize high-quality one-dimensional GaN nano-materials with unique features and good properties. Therefore there is necessary to study one-dimensional GaN materials in detail.One-dimension GaN materials were synthesized on Si (111) substrates by two-step growth technique through magnetron sputtering and ammoniating. The effects of different growth conditions on the morphology of GaN nanostructures were discussed mainly. The structure, morphology, compositions and optical properties of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), x-ray photoelectron spectroscopy (XPS) scanning electron microscope (SEM) transmission electron microscope (TEM) and photoluminescence (PL). The main contents were as follows:1. Synthesis of one-dimensional Dy-doped GaN nanostructuresRare earth doped Ga2O3 films were deposited on the Si (111) substrates through co-sputtering, then annealed under flowing ammonia atmosphere and one-dimension Dy doped GaN nanostructures with high quality were synthesized. The results of XPS suggest that the compositions of the products are Ga, N and a small amount of Dy. The results of XRD and FTIR show that the products have a hexagonal wurtzite crystal structure, and crystal lattice is slightly expand due to the doping of Dy. Through the PL results, the characteristic transition of 4f inner electrons of Dy3+ from 4F9/2 to 6H13/2 is discovered at 576nm. All the results prove the synthesis of one-dimension Dy-doped GaN nanostructures. The effects of ammoniating temperature, ammoniating time, doping concentration, buffer layer and substrates on the one-dimension Dy-doped GaN nanostructures are also discussed. The results are as follows:Ammoniating temperature has a great influence on the quality and structure of one-dimension Dy-doped GaN nanostructures. With the increase of temperature from 950℃to 1050℃, the crystal quality of the samples improves firstly, and then drops. The morphology of the samples varies from a small number of nanowires to a small number of nanorods nanowires, then to a large number of single crystal nanowires with high aspect ratio and finally to a few micron cylinder aggregates. These changes in morphology are due to the atomic mobility at different temperature. At a low temperature, the atomic mobility is low. There is not enough energy for atoms to move to the best position, which results in the growth of a small number of nanowires. At a high temperature, the atomic mobility increases. Atoms obtain enough energy to move to growth position. The amount of nanowires increases. However, when the temperature increases to a certain value, the diameter of nanowires becomes big due to the fact that the increase rate of lateral mobility of atoms is faster than vertical mobility. Also the length of the nanowires becomes short and the amount is small, which is due to the decompose or desorption of GaN at high temperature.Ammoniating time has a significant effect on the one-dimensional Dy-doped GaN nanostructures. With the increase of time, the morphology of the products varies from nanorods to nanowires, then to nanorods. The diameters of nanostructures increase gradually. The crystal quality of nanostructures improve first and then drops. The optical properties have similar changes. When the ammoniating time is short, the atoms do not have enough time to migrate to the best power position. when the ammoniating time to reach a certain value, all the atoms have sufficient time to move to the growth position and become a member of nanostructures. When time continues to increase, no new atom migrates and the decomposition of GaN is continuing. So the decomposition rate of GaN is bigger than formation rate, resulting in growth of shorter nanostructures. At the same time, the newborn GaN still migrated to the position of crystal nucleus and start a new round growth, resulting the formation of the thicker nanostructures.The doping of Dy element reduces the atomic mobility, also blocks the vertical growth of nanostructures. At the optimum growth conditions, the length of the GaN nanostructures becomes shorter with the increase of the doping concentration. The morphology of GaN nanostructures varies from nanowires to nanorods and finally to nanoparticles.The growth temperature of GaN nanostructures drops with the use of Au buffer layer. At 950℃, many nanowires with twoies were synthesized. One morphology with high Au content is curve and has a big diameter. The other with low Au content is straight and thin. At high temperature (1000℃), the nanowires have the same morphology of short nanowires and congregate together, indicating that the catalysis of Au become weak relatively. In addition, the substrate is also important to the growth of GaN nanostructures. At 950℃, quartz substrate is more suitable for the growth of one-dimension Dy doped GaN nanostructures growth. The specific growth conditions need further research.In summary, the optimum growing conditions of one-dimensional Dy-doped GaN nanostructures are of ammoniating temperature at 1000℃and ammoniating time at 15min, the best doping concentration needs further studies.2. Synthesis of one-dimension Tb-doped GaN nanostructuresUsing co-sputtering and two-step growth technique, one-dimension Tb-doped GaN nanostructures were synthesized on Si (111) substrates. XRD, FTIR, SEM, EDS, HRTEM and PL are employed to characterize the structure, morphology, composition and optical properties. When the Tb layer thickness is 5nm, a large quantity of GaN nanowires with hexagonal wurtzite crystal structure were synthesized at 950℃for 15min. These nanowires with a curve morphology have diameters of about 20~100nm and lengths of ten microns. They disorderly covered the entire surface of the substrates. EDS spectrum showed that the main compositions of nanowires are Ga, N and Tb of about 2at. %. The internal structure information of one nanowire given by HRTEM shows fewer dislocations and defects. The distance between two adjacent crystal faces is slightly larger than the corresponding distance of undoped GaN. The FTIR results showed that the Ga-N bond absorption peak located at 558.94cm-1, which is consistent with the reported position. Except for the common UV emission peak, there is also a green emission peak at the 544nm corresponding to 5D4-7F3 characteristic transitions of 4f inner electrons of Tb3+. Another peak located at 413nm maybe relates with Tb. These results indicate the synthesis of one-dimension Tb-doped GaN nanostructures.The morphology of nanowires changed with the ammoniating temperature, ammoniating time and doping concentration. At 900℃, a small amount of nanowire-clusters distributed on the substrate surface. At 950℃, a large quantity of nanowires covered the whole substrate surface. At 1000℃, some aggregations of nanowires and nanocentrums with a small amount are found on the substrates, which is due to the decomposition of GaN. When the time increases from 10min to 20min, the amount of nanowires increases gradually. And the nanowires ammoniated for 15min have the most clean surface. Likely Dy doping, the doping of Tb also reduces the atomic mobility and hinders the the vertical growth of nanostructures. With the increase of doping concentration, the morphology of the nanostructures changed from nanowires to nanorods, finally to nanoparticle films.3. Synthesis of one-dimension GaN nanowires with Au nano-dot templateOne-dimension GaN nanowires were synthesized on Si (111) substrates using Au nanodot template. First, Au films with a certain thickness were deposited on Si (111) substrates by DC magnetron sputtering technique, then annealed in Ar gas to form Au nanodot template. Finally, Ga2O3 films were deposited on Au template and ammoniated to synthesize one-dimension GaN nanostructures. XRD, FTIR, SEM, HRTEM and PL are used to characterize the structure, morphology and optical properties of the products. The results show that the GaN nanowires have hexagonal wurtzite crystal structure and possess good optical properties. Ammoniating temperature, time and buffer layers affect the morphology of nanowires greatly. The samples synthesized at 950℃for 15min have the best morphology, while the thickness of Au films is 30nm. 4. Magnetic field controlled magnetron sputtering to deposit GaN filmsBased on the theory of a charged particle movement in electromagnetic field and the magnetic mirror field, a magnetic field was added under the substrates, which changed the distribution of magnetic field in sputtering space and then changed the sputtering parameters. GaN films were form by two-step growth technique. That is, Ga2O3 films were first deposited on Si substrates and then ammoniated in a tube quartz furnace. The results indicate that the applied magnetic field enhances the sputtering rate and the crystalline degree of Ga2O3 films. Single-crystal wurtzite structure GaN films with high density and good optical properties were obtained through ammoniating Ga2O3 films at 1050℃. As a result of the added magnetic field, the ammoniating temperature to obtain high quality single crystal GaN film with high quality increases.5. The primary research of one-dimension GaN nanostructure growth mechanismFor the first time, defect-energy aggregation confined growth theory model was proposed to explain the one-dimension GaN nanostructure growth mechanism Through observation and analysis of the experimental phenomenon, nanostructures grown from some special sites on the substrate surface are found. This phenomenon is due to the use of buffer layer, which results in the energy re-distribution on Si substrates and forms some defect aggregates. Many unsaturated bonds in these aggregates make the surface energy increase. In order to minimize the total energy of the system, atoms are preferential to aggregate in the sites of defects and form crystal nucleus. Under the role of the surface free energy, these crystal nucleus finally grew up to one-dimension GaN nanostructures. In the growth of the GaN nanostructrures, new defect-energy aggregations will change its growth direction and lead to the curve and bifurcate nanostructures.