Study of III-nitrides heterostructures grown by plasma-assisted molecular beam epitaxy (PAMBE)

Dalam tesis ini,fokus adalah kajian mengenai pertumbuhan struktur hetero III-nitrid untuk membangunkan sistem MBE yang baru. In this thesis, the focus is on the studies of the growth of III-nitride heterostructures for the purpose of developing the new PAMBE system

PA-MBE GaN-Based Optoelectronics on Silicon Substrates

Dalam penyelidikan ini, epitaksi alur molekul berbantukan plasma nitrogen frekuensi radio (RF) digunakan untuk menumbuhkan bahan galium nitrid (GaN) di atas substrat Si(111) dengan penggunaan aluminium nitrid (AlN) yang ditumbuhkan pada suhu tinggi sebagai lapisan penimbal. In this project, radio-frequency (RF) nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) technique was used to grow GaN-based layers on Si(111) substrate using high temperature grown AlN as buffer layer

Investigation of Wear Mechanism of Gallium Nitride

The optoelectronic properties of gallium nitride (GaN) has been extensively studied for decades, which has facilitated its application in many different areas, cementing it as one of the most important semiconductor materials in the world. However, in comparison to the study of its optoelectronic properties, there are few studies of its mechanical properties - especially the tribological performance. Knowing the tribological properties of GaN, such as friction and wear, is crucial for understanding its machinability, the implementation of GaN in MEMS, solar cells, and other devices, as well as the wear performance of these GaN-based devices when working under harsh environments. In our study, we reveal that GaN has an ultralow wear nature, and that its wear rate can approach that of diamond. We also discover that the wear rate of GaN is affected by its crystallographic orientation, humidity, and composition.For the crystallographic orientation dependence, we look into the physics by both experimental and computational methods. We demonstrate that both the friction coefficient and wear rate of GaN exhibits a 60° periodicity. We conclude that these periodic variations of wear rate and friction coefficient are the results of a periodic variation of the energy barrier.The moisture dependent wear mechanism of GaN has been investigated under dry, low humidity, and high humidity environments. The results show that the wear rate of GaN perfectly follows an increasing of the humidity which spans over two orders of magnitude when the testing environment switches from dry nitrogen to humid lab air. On the contrary, the friction coefficient gave a contrary response, i.e., the lowest friction coefficient was found under low humidity environment, dry nitrogen had the highest friction coefficient, and the humid environment had its friction in the middle. Various characterization techniques, including SEM/EDS, AFM and TEM were employed to interrogate the worn surfaces under each condition. Based on the results, we hypothesize that the wear under dry nitrogen environment is adhesive in nature whereas grooving abrasive wear dominates the wear behavior of GaN under a humid environment.The compositional study of GaN wear revealed that by alloying different elements into the GaN system, one can not only tune the bandgap, but also modify the wear rate. This finding can be useful for applications and design that require suitable electronic properties while keep the wear rate within an acceptable range.Furthermore, during the investigation of the GaN wear mechanism, we discovered that the tribological sliding can also alter the surface band bending of this material. Our results demonstrate that the environment, number of sliding cycles, and normal loads can effectively tune the surface band bending of GaN. This finding shows the capability of mechanical dynamic contact for surface electronic property modification, which can be used in various applications, such as gas sensing, photocatalysis, and photochemistry.Understanding of the wear mechanism of GaN as well as the shear-induced band bending on GaN can remarkably promote the applications of GaN in various fields other than the optoelectronic area. This also reinforces the important message that tribology is not only a discipline that focuses on investigation of protective coating and lubrication but also can be used in device design and fabrication

Correlating X-ray microanalysis and cathodoluminescence data from III-nitride semiconductors

Research in group III- nitride semiconductors has seen major developments during the last couple of decades. One of the materials that satisfy requirements for optoelectronic devices in the ultra-violet (UV) spectral range and high power, high frequency electronic devices is the AlGaN. Performance and reliability of these devices will strongly depend on the electronic properties of epitaxial layers which are critically affected by structural defects and unintentional and intentional doped impurities. This thesis presents research on III- nitride semiconductors, in particular AlGaN and GaN materials. It is focused on characterization of AlGaN materials and the effects of n- and p-type doping, AlN content, occurrence of defects and crystal orientation on its quality. Different electron microscopy techniques are used to investigate luminescence, composition and doping properties of semiconductor structures and their correlation with surface features. The main techniques used for the characterization consisted of cathodoluminescence spectroscopy (CL) for the probing of luminescence properties, secondary electron (SE) and backscattered electron (BSE) imaging for investigation of the sample morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The type of growth method and choice of substrate have a great influence on the surface morphology and luminescence homogeneity of the AlGaN layer, with compositional inhomogeneity of the MBE samples confirmed only on sub μm level but having lower emission intensity compared to MOCVD samples. The thesis presents detailed steps of a procedure to quantify trace elements and investigates the associated challenges. The whole process of measurement optimization for Mg and Si dopants is described and final recipe on how to measure the concentration of major (alloy) and minor Si/Mg (dopant) elements is presented. A systematic study of polar and semipolar n-type doped AlGaN/AlN layers grown on sapphire by (MOCVD) with varied Si/group-III ratios in the gas phase was accomplished. The AlN incorporation was higher in the polar samples and the highest values of Si incorporations were observed for the polar samples with the highest Si/III ratios, while saturation of Si incorporation was seen for the semipolar samples at higher Si/III ratios. CL point spectra showed how changes in the relative intensity of the NBE peaks and impurity transitions depend strongly on the growth conditions and surface orientations. The semipolar samples showed better compositional homogeneity. A study was also performed on AlGaN:Mg samples to study the impurity transitions and luminescence properties of non LED epilayer samples grown on MOCVD AlN/sapphire templates and more complicated LED structures with different numbers of MBE-grown layers. MBE samples showed superior quality to other combinations of MBE and MOCVD structures, mainly due to problems associated with the transfer of sample between different reactors and the introduction of impurities that will form different defects within the material. Finally, some proposals for future work are presented.Research in group III- nitride semiconductors has seen major developments during the last couple of decades. One of the materials that satisfy requirements for optoelectronic devices in the ultra-violet (UV) spectral range and high power, high frequency electronic devices is the AlGaN. Performance and reliability of these devices will strongly depend on the electronic properties of epitaxial layers which are critically affected by structural defects and unintentional and intentional doped impurities. This thesis presents research on III- nitride semiconductors, in particular AlGaN and GaN materials. It is focused on characterization of AlGaN materials and the effects of n- and p-type doping, AlN content, occurrence of defects and crystal orientation on its quality. Different electron microscopy techniques are used to investigate luminescence, composition and doping properties of semiconductor structures and their correlation with surface features. The main techniques used for the characterization consisted of cathodoluminescence spectroscopy (CL) for the probing of luminescence properties, secondary electron (SE) and backscattered electron (BSE) imaging for investigation of the sample morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The type of growth method and choice of substrate have a great influence on the surface morphology and luminescence homogeneity of the AlGaN layer, with compositional inhomogeneity of the MBE samples confirmed only on sub μm level but having lower emission intensity compared to MOCVD samples. The thesis presents detailed steps of a procedure to quantify trace elements and investigates the associated challenges. The whole process of measurement optimization for Mg and Si dopants is described and final recipe on how to measure the concentration of major (alloy) and minor Si/Mg (dopant) elements is presented. A systematic study of polar and semipolar n-type doped AlGaN/AlN layers grown on sapphire by (MOCVD) with varied Si/group-III ratios in the gas phase was accomplished. The AlN incorporation was higher in the polar samples and the highest values of Si incorporations were observed for the polar samples with the highest Si/III ratios, while saturation of Si incorporation was seen for the semipolar samples at higher Si/III ratios. CL point spectra showed how changes in the relative intensity of the NBE peaks and impurity transitions depend strongly on the growth conditions and surface orientations. The semipolar samples showed better compositional homogeneity. A study was also performed on AlGaN:Mg samples to study the impurity transitions and luminescence properties of non LED epilayer samples grown on MOCVD AlN/sapphire templates and more complicated LED structures with different numbers of MBE-grown layers. MBE samples showed superior quality to other combinations of MBE and MOCVD structures, mainly due to problems associated with the transfer of sample between different reactors and the introduction of impurities that will form different defects within the material. Finally, some proposals for future work are presented

Optical Properties of Wurtzite InN and Related Alloys

In dieser Arbeit werden die optischen Eigenschaften von Wurtzitstruktur InN und verwandten ternären InGaN und AlInN, sowie quaternären AlInGaN Legierungen untersucht. Der Schwerpunkt wird auf die Charakterisierung mittels spektroskopischer Ellipsometrie gelegt. Die auf Si(111) Substraten gewachsenen InN-Proben und die Kohlstoff dotierten InN-Proben sind im Spektralbereich vom mittleren Infrarot bis hin zum Vakuum-Ultraviolett untersucht worden. Die Elektronenkonzentration für die InN-Proben wird durch selbstkonsistentes Lösen (der Ellipsometriedaten Analyse im Infrarotbereich und der Anpassung des Absorption Ansatz) bestimmt. Die intrinsische spannungsfreie Bandlücke für InN Proben wird unter Berücksichtigung von Vielteilcheneffekten wie der Bandlückenrenormierung und der Burstein-Moss-Verschiebung, sowie dem Einfluss der Verzerrung für die Bandlücke bestimmt. Die k*p-Methode wird verwendet, um die Verschiebung der Bandlücke (beeinflusst durch Verzerrung) zu berechnen. Es wird demonstriert, dass eine Erhöhung des Kohlenstofftetrabromid (CBr4) Drucks während des Wachstumsprozess, die Elektronenkonzentration in den InN-Proben erhöht. Die Indium-verwandten Legierungen wurden im Spektralbereich des nahen Infrarot bis zum Vakuum-Ultraviolett untersucht. Das analytische Modell, der dielektrichen Funktion im Spektralbereich 1-10 eV, für die Indium-verwandte Legierungen wird präsentiert. Durch die Anpassung des analytischen Modells an die experimentellen dielektrischen Funktionen, werden die Bandlücke und die Übergangsenergien im Hochenergie-Bereich evaluiert. Die Bowing-Parameter der spannungsfreien Bandlücke für die ternären Systeme InGaN und AlInN werden bestimmt. Es wird demonstriert, dass der Bowing-Parameter für AlInN von der Komposition der Legierung abhängig ist. Die Kenntnis von Bowing-Parametern für die ternären Legierungen ermöglicht die Entwicklung einer empirischen Gleichung, zur Berechnung der Bandlücke in quaternären Legierungen. Alle experimentell durch Ellipsometrie bestimten Bandlücken der untersuchten Legierungen werden durch ab-initio Daten unterstützt.In this work, optical properties of wurtzite structure InN and related ternary InGaN and AlInN, as well as quaternary AlInGaN alloys were investigated. The spectroscopic ellipsometer was used as the main characterization tool for the analysis of the optical properties. The InN samples grown on Si(111) substrates, as well as carbon doped InN samples were investigated from mid-infrared up to vacuum-ultraviolet spectral range. The electron concentration for InN samples were evaluated by solving a self-consistent problem that includes the IR-SE ellipsometry data analysis and the imaginary dielectric function around the band gap calculation. The intrinsic strain-free band-gap was estimated after taking into consideration a band-gap renormalization and Burstein-Moss shift, as well as a strain influence on the band gap. The k*p method was used to calculate the strain induced band-gap shift. From the analysis, it was shown that for the carbon doped InN samples the electron concentration increases linearly by increasing the CBr4 dopant pressure during the MBE growth process. The In-related alloys were investigated from near-infrared up to vacuum-ultraviolet spectral range. The analytical model of the dielectric function in the spectral range 1-10 eV was presented. From the fit of the analytical model to the experimental dielectric functions, the band gaps and high-energy inter-band transitions were estimated. The strain-free band-gap bowing parameters for ternary InGaN and AlInN alloys were obtained. It was demonstrated, that the bowing parameter for AlInN is composition dependent. With the knowledge of the bowing parameters of ternary alloys, it was possible to develop an empirical equation that allows to estimate the band gap for a quaternary AlInGaN alloy. All experimentally obtained band gaps are in good agreement with the ab-initio calculated values

Fabrication and characterization of III-nitride nanophotonic devices

Doctor of PhilosophyDepartment of PhysicsHongxing JiangIII-nitride photonic devices such as photodetectors (PDs), light emitting diode (LEDs), solar cells and optical waveguide amplifiers were designed, fabricated and characterized. High quality AlN epilayers were grown on sapphire and n-SiC substrates by metal organic chemical vapor deposition and utilized as active DUV photonic materials for the demonstration of metal-semiconductor-metal (MSM) detectors, Schottky barrier detectors, and avalanche photodetectors (APDs). AlN DUV PDs exhibited peak responsivity at 200 nm with a very sharp cutoff wavelength at 207 nm and extremely low dark current (<10 fA), very high breakdown voltages, high responsivity, and more than four orders of DUV to UV/visible rejection ratio. AlN Schottky PDs grown on n-SiC substrates exhibited high zero bias responsivity and a thermal energy limited detectivity of about 1.0 x 1015 cm Hz1/2 W-1. The linear mode operation of AlN APDs with the shortest cutoff wavelength (210 nm) and a photocurrent multiplication of 1200 was demonstrated. A linear relationship between device size and breakdown field was observed for AlN APDs. Photovoltaic operation of InGaN solar cells in wavelengths longer than that of previous attainments was demonstrated by utilizing InxGa1−xN/GaN MQWs as the active layer. InxGa1-xN/GaN MQWs solar cells with x =0.3 exhibited open circuit voltage of about 2 V, a fill factor of about 60% and external quantum efficiency of 40% at 420 nm and 10% at 450 nm. The performance of InxGa1-xN/GaN MQWs solar cell was found to be highly correlated with the crystalline quality of the InxGa1-xN active layer. The possible causes of poorer PV characteristics for higher In content in InGaN active layer were explained. Photoluminescence excitation studies of GaN:Er and In0.06Ga0.94N:Er epilayers showed that Er emission intensity at 1.54 µm increases significantly as the excitation energy is tuned from below to above the energy bandgap of these epilayers. Current-injected 1.54 µm LEDs based on heterogeneous integration of Er-doped III-nitride epilayers with III-nitride UV LEDs were demonstrated. Optical waveguide amplifiers based on AlGaN/GaN:Er/AlGaN heterostructures was designed, fabricated, and characterized. The measured optical loss of the devices was ~3.5 cm−1 at 1.54 µm. A relative signal enhancement of about 8 dB/cm under the excitation of a broadband 365 nm nitride LED was achieved. The advantages and possible applications of 1.54 µm emitters and optical amplifiers based on Er doped III-nitrides in optical communications have been discussed

Luminescence study of III-nitride semiconductor nanostructures and LEDs

In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively.In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively

Growth and Characterization of Indium Nitride Layers Grown by High-Pressure Chemical Vapor Deposition

In this research the growth of InN epilayers by high-pressure chemical vapor deposition (HPCVD) and structural, optical properties of HPCVD grown InN layers has been studied. We demonstrated that the HPCVD approach suppresses the thermal decomposition of InN, and therefore extends the processing parameters towards the higher growth temperatures (up to 1100K for reactor pressures of 15 bar, molar ammonia and TMI ratios around 800, and a carrier gas flow of 12 slm). Structural and surface morphology studies of InN thin layers have been performed by X-ray diffraction, low energy electron diffraction (LEED), auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS) and atomic force microscopy (AFM). Raman spectroscopy, infrared reflection, transmission, photoluminescence spectroscopy studies have been carried out to investigate the structural and optical properties of InN films grown on sapphire and GaN/sapphire templates. InN layers grown on a GaN (0002) epilayer exhibit single-phase InN (0002) X-ray diffraction peaks with a full width at half maximum (FWHM) around 200 arcsec. Auger electron spectroscopy confirmed the cleanliness of the surface, and low energy electron diffraction yielded a 1×1 hexagonal pattern indicating a well-ordered surface. The plasmon excitations are shifted to lower energies in HREEL spectra due to the higher carrier concentration at the surface than in the bulk, suggesting a surface electron accumulation. The surface roughness of samples grown on GaN templates is found to be smoother (roughness of 9 nm) compared to the samples grown on sapphire. We found that the deposition sometimes led to the growth of 3 dimensional hexagonal InN pyramids. Results obtained from Raman and IR reflectance measurements are used to estimate the free carrier concentrations, which were found in the range from mid 10^18 cm-3 to low 10^20 cm-3. The optical absorption edge energy calculated from the transmission spectra is 1.2 eV for samples of lower electron concentration. The Raman analysis revealed a high-quality crystalline layer with a FWHM for the E2(high) peak around 6.9 cm^-1. The results presented in our study suggest that the optimum molar ratio might be below 800, which is due to the efficient cracking of the ammonia precursor at the high reactor pressure and high growth temperature

太陽光発電材料としての有機金属化学堆積法によるInxGa1-xN 薄膜の電子状態および欠陥評価に関する研究

筑波大学 (University of Tsukuba)201

Growth and Characterization of Indium Nitride Layers Grown by High-Pressure Chemical Vapor Deposition

In this research the growth of InN epilayers by high-pressure chemical vapor deposition (HPCVD) and structural, optical properties of HPCVD grown InN layers has been studied. We demonstrated that the HPCVD approach suppresses the thermal decomposition of InN, and therefore extends the processing parameters towards the higher growth temperatures (up to 1100K for reactor pressures of 15 bar, molar ammonia and TMI ratios around 800, and a carrier gas flow of 12 slm). Structural and surface morphology studies of InN thin layers have been performed by X-ray diffraction, low energy electron diffraction (LEED), auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS) and atomic force microscopy (AFM). Raman spectroscopy, infrared reflection, transmission, photoluminescence spectroscopy studies have been carried out to investigate the structural and optical properties of InN films grown on sapphire and GaN/sapphire templates. InN layers grown on a GaN (0002) epilayer exhibit single-phase InN (0002) X-ray diffraction peaks with a full width at half maximum (FWHM) around 200 arcsec. Auger electron spectroscopy confirmed the cleanliness of the surface, and low energy electron diffraction yielded a 1×1 hexagonal pattern indicating a well-ordered surface. The plasmon excitations are shifted to lower energies in HREEL spectra due to the higher carrier concentration at the surface than in the bulk, suggesting a surface electron accumulation. The surface roughness of samples grown on GaN templates is found to be smoother (roughness of 9 nm) compared to the samples grown on sapphire. We found that the deposition sometimes led to the growth of 3 dimensional hexagonal InN pyramids. Results obtained from Raman and IR reflectance measurements are used to estimate the free carrier concentrations, which were found in the range from mid 10^18 cm-3 to low 10^20 cm-3. The optical absorption edge energy calculated from the transmission spectra is 1.2 eV for samples of lower electron concentration. The Raman analysis revealed a high-quality crystalline layer with a FWHM for the E2(high) peak around 6.9 cm^-1. The results presented in our study suggest that the optimum molar ratio might be below 800, which is due to the efficient cracking of the ammonia precursor at the high reactor pressure and high growth temperature