Epitaxial Growth Of Composition Well-controlled AlGaN On Al Substrate By Pulsed Laser Depostion

AlGaN semiconductor ternary alloys varied in Al compositions from 0 to 1, can achieve the bandgap ranging from 3.4 eV to 6.2 eV, spanning the continuously adjustable wavelength from 365 nm to 200 nm, which are promising materials for the fabrication of ultraviolet(UV) light-emitting diodes(LEDs), UV detectors, high electron mobility transistors(HEMTs), etc. To fabricate highly-efficient devices, the growth of high-quality AlGaN epitaxial films is of paramount importance. To date, the substrates for AlGaN epitaxy are sapphire, Si, etc. However, there are relatively larger lattice mismatches existing between these substrates and AlGaN, which hamper the growth of high-quality AlGaN epitaxial films. The utilization of Al substrates can well circumvent these meeting problems. On the one hand, the lattice mismatch between Al(111) and AlGaN(0002) is in the range of 8.9%~11.4%, which is beneficial to the nucleation of AlGaN on Al during the initial growth. On the one hand, by optimizing the epitaxial growth process, AlGaN epitaxial films with well-controlled Al composition can be obtained on Al substrates.Recently, AlGaN epitaxial films are usually grown by reactive magnetron sputtering(RMS), metal organic chemical vapor deposition(MOCVD), molecular beam epitaxy(MBE), atomic layer deposition(ALD), pulsed laser deposition(PLD), etc. For RMS, it requires relatively high growth temperature and shows low vacuum. For MOCVD, it also requires relatively high growth temperature and there is an irreversible pre-reaction between TMAl and NH3. For MBE, it needs high growth temperature and there is an interfacial reaction existing between film and substrate. For ALD, it shows a relatively low growth rate and low vacuum. In this regard, PLD can overcome the meeting problems mentioned above. The plume plasma produced by pulsed laser ablation of the target has high kinetic energy, which can make thin films growth with sharp and abrupt hetero-interfaces at very low temperature and suitable vacuum viable. Therefore, the interfacial reaction between the epitaxial film and the substrate can be effectively suppressed. Meanwhile, there is no irreversible pre-reaction existing in this case and the PLD system is flexible for controlling. Therefore, PLD is suitable for the epitaxial growth of AlGaN film on Al substrate. However, due to the highly directional distribution of the precursor on substrate produced by traditional PLD, the poor thickness homogeneity of film is usually obtained. In this case, laser rastering integrated in PLD can overcome this problem and is employed to epitaxial growth of AlGaN films with well-controlled Al composition. The defect formation and annihilation mechanisms of AlGaN epitaxial film grown on Al substrate by PLD and the corresponding growth mechanism are investigated in depth. After investigation, the main contents and conclusions for the dissertation are shown as follows:Firstly, epitaxial growth of high-homogeneity crack-free AlN film on Al substrate by PLD. By studying the effect of pulsed laser energy, growth pressure, growth temperature, substrate nitridation and cooling rate on the crystalline quality, surface morphology, stress state and interfacial property, the high-quality AlN epitaxial film has been grown on Al substrate. Although Al is thermally active at high temperature, the plume plasma produced by pulsed laser ablation of the target has high kinetic energy, which can make thin film growth at low temperature viable, and thereby effectively suppress the interfacial reaction between AlN and Al. This eventually leads to the growth of high-quality AlN epitaxial film with sharp and abrupt hetero-interfaces. Additionally, effect of laser rastering technique integrated in PLD on the thickness homogeneity and surface root-mean-square(RMS) homogeneity is carefully studied. It is found that by adjusting the laser ratering parameters, the scanning location and rate of pulsed laser are changed, and thereby the spatial distribution of plume plasma is changed accordingly, which can realize the homogeneous deposition of AlN plasmas on Al substrates ultimately. Based on the characterizations, the defect formation and annihilation mechanisms of AlN epitaxial film grown on Al substrate by PLD, and the corresponding growth mechanism are well investigated in depth. The nitridation of Al substrates not only can release the stress, but also can provide an AlN template for subsequent growth of AlN epitaxial film; the high-energy effect of PLD that can assist the specie overcoming the energy barrier to migration, diffusion, nucleation, and growth; while the pulse effect of PLD that can ensure enough time for specie to migration to its equilibrium position. All of these three aspects enhance the two-dimensional(2D) growth of AlN epitaxial film.Secondly, epitaxial growth of high-homogeneity crack-free AlGaN film with well-controlled Al composition on Al substrate by PLD. Based on the optimized growth parameters of AlN epitaxial on Al substrate by PLD, AlGaN epitaxial films with various Al compositions can be grown on Al substrate by PLD through optimizing the target and growth temperature. This work proposes that GaN plasmas initially deposit on Al substrates in the form of GaN lattices, and then Al atoms diffuse into GaN lattices and replace some of Ga atoms, leading to the formation of AlGaN. Meanwhile, due to the different diffusions of Al in various temperatures, the replacement of Al with Ga is varied, and thereby the different AlGaN in Al compositions. Furthermore, by studying the effect of growth process on the stress state, interfacial property and Al composition of AlGaN epitaxial growth, it is noted that the AlGaN epitaxial films grown by the combination of low-temperature(LT) and high-temperature(HT) growth show high-quality. LT growth can benefit to the nucleation of films during the initial growth and also can release the stress formed due to the lattice mismatch between film and substrates by the formation of high-density of 2D islands. HT growth not only can enhance the diffusion of Al to form high Al composition AlGaN, where the Al composition is same as that grown only by HT, but also can promote the migration of species and the coalescent of 2D islands. This results in the annihilation of dislocations by changing the direction of dislocation motion and dislocation reactions, etc., leading to the formation of 2 μm-thick high-quality AlGaN epitaxial films.Finally, epitaxial growth of high-homogeneity crack-free GaN film on Al substrate by PLD. Based on the optimized PLD growth parameters, effect of crystal buffer layer and amorphous inserted layer on the crystalline quality, surface morphology, stress state, and interfacial property is well studied. It is found that the combination of AlN crystal buffer layer and AlN amorphous inserted layer can achieve high-quality GaN epitaxial film. On the one hand, AlN not only can barrier the Al atoms from diffusing into GaN lattice, but also can reduce the dislocation density due to the small lattice mismatch between Al N and Al than that between GaN and Al. On the other hand, the Al N amorphous inserted layer can release the stress and trap the dislocations from extending to subsequent layer. Both of these two aspects lead to the growth of high-quality crack free Ga N epitaxial film. Apparently, the nucleation mechanism of Ga N grown on AlN/Al hetero-structure is hence proposed.To conclude, the as-grown AlGaN epitaxial film with well-controlled Al composition is potential for the fabrication of highly-efficient UV LEDs, UV detectors, HEMTs, etc. Furthermore, the defect formation and annihilation mechanisms of AlGaN epitaxial film grown on Al substrate by PLD obtained in this work are guidelines for the epitaxial growth of AlGaN.