GSMBE Growth And Characterization Of Fundamental And QCL Materials

This dissertation mainly focused on the gas source molecular beam epitaxy (GSMBE) technology and the optimization growth of quantum cascade laser (QCL) materials. The growth conditions and properties of III-V group fundamental materials were also investigated as the high material quality required in QCL fabrication. The main results achieved in this work could be summarized as follows:1. High quality III-V group materials lattice-matched with InP and GaAs were grown by GSMBE, including InP, InGaAs, InAlAs on InP, and AlGaAs on GaAs. The best FWHM of XRD of the three kinds of ternaries is only wider than that of the substrates, indicating coherent growth and high single crystal qualities. Since the performance of QCLs is influenced remarkably by the quality of the InGaAs layers that act as quantum wells, the crystal qualities, electrical and optical properties of InGaAs were investigated with different growth temperatures. Likewise, the surface defect density and electrical properties dependent on growth temperature of InP layers were investigated. The results show that the best growth temperature for InP is 40℃lower than InGaAs, implying that the growth temperature has to be regulated during the GSMBE growth of QCL structures containing InP layers.2. The InP based In_xGa_1-xAs (x > 0.53) and In_xAl_1-xAs (x < 0.52) strained system were investigated by using XRD. The critical thickness versus material composition was evaluated. The relaxation ratio of strained layers was accurately calibrated by the integration of symmetric and asymmetric XRD. It was found that the composition intervals from the full strained layers to the full relaxed ones were quite narrow for both materials, and the span of x was less than 0.1.3. The Si incorporation behaviors in InP-based InGaAs, InAlAs and GaAs-based AlGaAs were investigated respectively for full composition ranges. The doping concentration of InGaAs layers were not affected by its composition, whereas, InAlAs and AlGaAs layers both exhibited a doping valley, locating at the direct-indirect bandgap crossover. At the both sides of InAlAs doping valley, there were other two doping gaps which showed high electrical resistance. The doping valley could be interpreted by both energy band theory and Hall effect measurement theory. The rise of donor ionization energy of Si-doped InAlAs was revealed near the doping valley by employing Hall effect measurement dependent on temperature, which gave us direct proof for the abnormal behavior of the donor energy at the crossover points.4. Composition uniformity of InGaAs and InAlAs lattice-matched layers grown on 2" InP substrates was studied. The composition fluctuation less than 0.1% was achieved for both materials, which guaranteed the uniformity for the growth of device structures. A monolayer control method was developed by using superlattice samples and XRD. This method was applied to calibrate the growth rate for both lattice-matched and strain-compensated QCL active cores. The thickness error between the real active core and the design is less than 2%.5. The origin of the surface defects of layers was studied. The defect density was reduced from 10³/cm² to 10/cm² by using the cells with special temperature zone for Ga and In, which laid an excellent material foundation for the development of high quality lasers.6. Several kinds of F-P and DFB QCL structures were grown by GSMBE, during which the device performance was investigated and compared for different injector doping concentration, as well as binary or ternary waveguide cladding. Several multi-mode F-P QCLs in the range of 5-10μm were realized for the first time in Asia. The devices could operate in CW mode at LT and pulsed mode at RT. It was shown that low threshold current density could be achieved by reducing the injector doping concentration, and the waveguide cladding of InP had better thermal properties than InAlAs. Single-mode DFB-QCLs with the wavelength of 7.4, 7.6, 7.7, 8.4μm were also fabricated for the first time in Asia. The threshold current densities at LT and RT were as low as 574A/cm² (70K) and 970A/cm² respectively. CW operation in both F-P and DFB QCLs was achieved at 135K. N₂O gas sensing has been demonstrated by using our own DFB-QCLs as light sources.