Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

The transition to energy efficient smart grid and wireless communication with improved capacity require the development and optimization of next generation semiconductor technologies and electronic device components. Indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with bandgap energies ranging from 0.7 to 6.0 eV, facilitating their utilization in optoelectronics and photonics. The GaN-based blue light-emitting diodes (LEDs) have enabled efficient and energy saving lighting, for which the Nobel Prize in Physics 2014 was awarded. GaN and AlN have high critical electric fields, high saturation carrier velocities and high thermal conductivities, which make them promising candidates for replacing silicon (Si) in next-generation power devices. The polarization-induced two-dimensional electron gas (2DEG), formed at the interface of AlGaN and GaN has enabled GaN-based high electron mobility transistors (HEMTs). These devices are suitable for high-power (HP) switching, power amplification and high-frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for next-generation 5G and 6G communication systems, radars, satellites, and a plethora of other related applications.

Despite the immense efforts in the field, several material related issues still hinder the full exploitation of the unique properties of GaN-based semiconductors in HF and HP electronic applications. These limitations and challenges are related among others to: i) poor efficiency of p-type doping in GaN, ii) lack of linearity in AlGaN/GaN HEMTs used in low-noise RF amplifiers and, iii) MOCVD growth related difficulties in achieving ultra-thin and high Alcontent AlGaN barrier layers with compositionally sharp Al profiles in AlGaN/GaN HEMTs for HF applications.

In this PhD thesis, we address the abovementioned issues by exploiting the hot-wall MOCVD combined with extensive material characterization. Main results can be grouped as follows:

i) state-of-art p-GaN with room-temperature free-hole concentrations in the low 10¹⁸ cm^-3 range and mobilities of ~10 cm²/Vs has been developed via in-situ doping. A comprehensive understanding of the growth process and its limiting factors, as related to magnesium (Mg), hydrogen (H) and carbon (C) incorporation in GaN is established. Further improvement of p-type doping in as-grown GaN:Mg is achieved by using GaN/AlN/4H-SiC templates and/or by modifying the gas environment in the growth reactor through the introduction of high amounts of hydrogen (H₂) in the process. Using advanced scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) we have established an improved comprehensive model of the pyramidal inversion domain defects (PIDs) in relation to the ambient matrix. First experimental evidence that Mg is present at all interfaces between PID and matrix allows for more accurate evaluation of Mg segregated at the PID, necessary for understanding the main limiting factor for p-type conductivity in GaN against alternative compensating donor or passivation sources.

ii) Compositionally graded AlGaN channel layers in AlGaN/(Al)GaN HEMTs with various types of compositional grading have been developed, and graded channel devices were compared with conventional AlGaN/GaN HEMT indicating improved linearity. The first large signal measurements in Europe of a graded channel AlGaN/GaN HEMT has been carried out demonstrating improved linearity figure of merit IM₃ by 10 dB compared to conventional Fe-doped GaN buffer devices. These results are showing state-of-the-art performance and pave the way for novel highly linear GaN receivers.

iii) Ultrathin (sub-10nm) and high Al-content (>50%) AlGaN barrier GaN HEMT structures have been developed with 2DEG carrier densities ~1.1×10¹³ cm^-2 and mobilities ~1700 cm²/Vs. Advanced characterization with atomic precision involving STEM and energy dispersive X-ray spectroscopy (EDS), has allowed experimental determination of the Al profiles and has revealed deviations from the nominally intended structures. Such deviations are found also in different source materials including commercial HEMT epistructures grown by MOCVD. The implications of the Al-profile deviations are critically analyzed in terms of 2DEG properties and device fabrication and performance. The capabilities and the limitations of MOCVD processes, related to growth of compositionally sharp and ultrathin high-Al-content AlGaN layers on GaN have been evaluated and their prospects in HF have been assessed.

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.

Herein, metal-organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] approximate to 2 x 10(19) cm(-3) is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as-grown Mg-doped GaN layers is certified by a room-temperature hole concentration of p approximate to 2 x 10(17) cm(-3) in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated Mg-Ga acceptor.

The devolopment of group-III nitride semiconductor technology continues to expand rapidly over the last two decades. The indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with a wide bandgap range, spanning from infrared(IR) to deep-ultraviolet (UV), enabling their utilization in optoelectronic industry. The GaN-based light-emitting diode (LED) is already the commercial solution for efficient and energy saving lighting. Additionally, the physical properties of these materials such as the high critical electric field, the high saturation carrier velocity and the high thermal conductivity, make them promising candidates for replacing silicon (Si), and other wide-bandgap semiconductors such as silicon carbide (SiC) in power devices. More importantly, the polarization-induced two-dimensional electron gas (2DEG), forming at the interfaces of these semiconductors, led to the fabrication of the GaN-based high electron mobility transistor (HEMT). This device is suitable for high power (HP) switching, power amplifiers and high frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for 5G communication systems, radars, satellites and a plethora of other related applications.

Achieving the efficient GaN blue LED (Nobel Prize in Physics 2014), came as a result of (partially) solving several material issues of which, p-type GaN was of crucial importance. Since 1992, a lot of effort is being dedicated on the understanding and overcoming of the limitations hindering efficient p-type conductivity and low hole mobility in metal-organic chemical vapor deposition (MOCVD) grown p-GaN. The limitations arise from the fact that magnesium (Mg) is the only efficient p-type dopant for GaN so far and only a very small percentage ∼2% of the incorporated Mg is active at room temperature. More limitations come from its solubility in GaN and the crystal quality deterioration and formation of inversion domains (IDs) at high doping levels. Free-hole concentrations in the low 10¹⁸ cm^-3 range with mobilities at ∼10 cm²V^-1s^-1 demonstrate the state-of-art in MOCVD grown p-GaN, still leaving a wide window for improvement. Another intensively investigated topic is related to the aluminum gallium nitride (AlGaN)/GaN HEMTs. High electron density and mobility of the 2DEG in the range of 10¹³ cm^-2 and ∼2400 cm²V^-1s^-1 respectively, are reported. Interface engineering, addition of interlayers and backbarriers are only some of the modifications introduced at the basic AlGaN/GaN HEMT structure in order to achieve the aforementioned values. Nevertheless, fundamental phenomena can still be revealed by special characterization techniques and provide a deeper understanding on the causal factors of theHEMT’s macroscopic properties.

The main research results presented in this licentiate thesis are organized in three papers:

In paper I we perform an in-depth investigation of the Mg-doped GaN growth by hot-wall MOCVD. We strive for exploiting any possible advantages of the hot-wall MOCVD on the growth of high-quality p-GaN relevant for use in HP devices. Additionally, we aim to gain a comprehensive understanding of the growth process and its limiting factors. The effects of growth conditions on the Mg, hydrogen (H) and carbon (C) incorporation in GaN are approached from the gallium (Ga)-supersaturation point of view. Control of the bis(cyclopentadienyl) magnesium (Cp2Mg)/trimethylgallium(TMGa) ratio, the V/III ratio and the growth temperature, resulted in high quality p-GaN growth on AlN/4H-SiC templates, showing state-of-the-art electrical properties. In paper II, we manage to increase the free-hole concentrations in as-grown GaN:Mg in two different ways, either by growing the GaN:Mg layer on a GaN/AlN/4H-SiC template, or by modifying the gas environment of the growth. It is shown that using a GaN/AlN/4H-SiC template results in higher carrier concentration and large improvement of the as-grown p-GaN resistivity. More importantly, the high amount of hydrogen (H2) flow during GaN:Mg growth, results in higher amount of non-passivated Mg in the as-grown layers allowing for high free-hole concentration and significantly lower resistivity in the as-grown p-GaN. Paper III focuses on the effect of aluminum (Al)-content variation in the barrier layer of AlGaN/GaN HEMTs. The THz-optical Hall effect (OHE) measurements revealed a peak of the 2DEG mobility followed by a decrease above certain value of Al%. We correlate this effect with the electron effective mass (m_eff) variation and draw conclusions about the mobility limiting mechanisms. In the low-Al regime, the mobility decreases because of the increase in m_eff while, in the high-Al regime, the mobility is limited by the lower carrier scattering time.