En esta tesis se investigó el crecimiento de AlN sobre sus-tratos de zafiro por el método MOVPE para su posterior utilización como pseudo-substrato en la fabricación de tran-sistores de alta movilidad de electrones (HEMT). Los experi-mentos de crecimiento se llevaron a cabo a 50 − 100 hPa y una temperatura 950 − 1200 ◦C. Como precursores del grupo III y grupo V y gas portador se utilizaron TMAl, NH3, N2 y/o H2, respectivamente. La evaluación de la calidad de las películas se realizó ópticamente por reflectometría in-situ, estructuralmente mediante mediciones de difracción de ra-yos-X y morfológicamente por mediciones AFM y SEM. Se estudiaron principalmente dos tipos de estrategia de creci-miento, en una etapa y en dos etapas. Para ambas estrate-gias de crecimiento se estudió la influencia de la configuración de entrada (convencional - invertida) de los gases (precur-sores) al reactor. El impacto de la presión parcial de los pre-cursores del grupo III y grupo V a V/III constante se estudió para el caso de la estrategia de crecimiento en una etapa. En el caso del crecimiento de AlN en dos etapas se investigó el efecto del tiempo de nucleación a temperatura constante de 950 ◦C y la temperatura de crecimiento en el rango de 1140 − 1200 ◦C en el crecimiento de las películas de AlN. De los resultados obtenidos de las distintas experiencias se puede concluir en forma general y sólo para el rango de los valores utilizados para los distintos parámetros de crecimiento de AlN, que no es posible optimizar la calidad cristalina y la suavidad de la superficie de las películas al mismo tiempo con sólo variar un parámetro de crecimiento. / In this thesis the growth of AlN on sapphire substrates by the method MOVPE for later use as pseudo-substrate in the manufacture of transistors high electron mobility tran-sistors (HEMT) was investigated. Growth experiments were carried out at 50 − 100 hPa and a temperature 950 − 1200 ◦C. As precursors of group III and group V and carrier gas, TMAl, NH3, and N2/H2, were used, respectively. The evaluation of the quality of the films was carried out optically by in-situ reflectometry, structurally by diffraction of X-ray (XRD) and morphologically by AFM and SEM measurements. Principally, two types of growth strate-gies were studied, one stage and two stages. For both growth strategies the influence of the gas(precursors) input configuration (conventional - inverted) to reactor was investigated. The impact of the partial pressure of group III and group V precursors at V/III constant was studied for the case of the one stage growth strategy. In the case of AlN growth, by means of the two stages growth strategy, the effect of nucleation time at a constant temperature of 950 ◦C and growth temperature in the range of 1140 − 1200 ◦C in the growth of AlN films was investi-gated. From the results obtained from the diverse experien-ces, it can be concluded, in general and only for the range of values used for the various parameters of growth of AlN, that is not possible to optimize the crystalline quality and surface smoothness of the films at the same time with the only variation of one growth parameter.

Understanding the magnitude of the temperature in AlGaN/GaN heterostructure fi eld e ffect transistors(HFETs) is a critical aspect of understanding their reliability and providing proper thermal management. At present, most models used to determine the temperature rise in these devices are based on continuum based heat conduction. However, in such devices, the heat generation region can be on the order of or smaller than the phonon mean free path of the heat carriers, and thus, such models may under predict the temperature. The aim of this work is towards building a multiscale thermal model that will allow for the prediction of heat transport from ballistic-diffusive phonon transport near the heat generation region and diffusive transport outside of this zone. First, a study was performed to determine the appropriate numerical solution to the phonon Boltzmann transport equation followed by its integration into a multiscale thermal scheme. The model, which utilizes a Discrete Ordinates Solver, was developed for both gray and non-gray phonon transport. The scheme was applied to the solution of speci fic test problems and then finally to the electrothermal modeling of AlGaN/GaN HFETs under various electrical bias conditions.

This dissertation is focused on the research efforts to develop the growth, processing, and modeling technologies for GaN-based Heterojunction Field Effect Transistors (HFETs). The interest in investigating GaN HFETs is motivated by the advantageous material properties of nitride semiconductor such as large band gap, large breakdown voltage, and high saturation velocity, which make it very promising for the high power and microwave applications. Although enormous progress has been made on GaN transistors in the past decades, the technologies for nitride transistors are still not mature, especially concerning the reliability and stability of the device. In order to improve the device performance, we first optimized the growth and fabrication procedures for the conventional AlGaN barrier HFET, on which high carrier mobility and sheet density were achieved. Second, the AlInN barrier HFET was successfully processed, with which we obtained improved I-V characteristics compared with conventional structure. The lattice-matched AlInN barrier is beneficial in the removal of strain, which leads to better carrier transport characteristics. Furthermore, new device structures have been examined, including recess-gate HFET with n+ GaN cap layer and gate-on-insulator HFET, among which the insertion of gate dielectrics helps to leverage both DC and microwave performances. In order to depict the microwave behavior of the HFET, small signal modeling approaches were used to extract the extrinsic and intrinsic parameters of the device. An 18-element equivalent circuit model for GaN HFET has been proposed, from which various extraction methods have been tested. Combining the advantages from the cold-FET measurements and hot-FET optimizations, a hybrid extraction method has been developed, in which the parasitic capacitances were attained from the cold pinch-off measurements while the rest of the parameters from the optimization routine. Small simulation error can be achieved by this method over various bias conditions, demonstrating its capability for the circuit level design applications for GaN HFET. Device physics modeling, on the other hand, can help us to reveal the underlying physics for the device to operate. With the development of quantum drift-diffusion modeling, the self-consistent solution to the Schrödinger-Poisson equations and carrier transport equations were fulfilled. Lots of useful information such as band diagram, potential profile, and carrier distribution can be retrieved. The calculated results were validated with experiments, especially on the AlInN layer structures after considering the influence from the parasitic Ga-rich layer on top of the spacer. Two dimensional cross-section simulation shows that the peak of electrical field locates at the gate edge towards the drain, and of different kinds of structures the device with gate field-plate was found to efficiently reduce the possibility of breakdown failure.

AlGaN/GaN high electron mobility transistors (HEMTs) have appeared as attractive candidates for high power, high frequency, and high temperature operation at microwave frequencies. In particular, these devices are being considered for use in the area of high RF power for microwave and millimeter wave communications transmitter applications at frequencies greater than 100 GHz and at temperatures greater than about 150 °C. However, there are concerns regarding the reliability of AlGaN/GaN HEMTs. First of all, thermal reliability is the chief concern since high channel temperatures significantly affect the lifetime of the devices. Therefore, it is necessary to find the solutions to decrease the temperature of AlGaN/GaN HEMTs. In this study, we explored the methods to reduce the channel temperature via high thermal conductivity diamond as substrates of GaN. Experimental verification of AlGaN/GaN HEMTs on diamond substrates was performed using micro-Raman spectroscopy, and investigation of the design space for devices was conducted using finite element analysis as well. In addition to the thermal impact on reliability, environmental effects can also play a role in device degradation. Using high density and pinhole free films deposited using atomic layer deposition, we also explore the use of ultra-thin barrier films for the protection of AlGaN/GaN HEMTs in high humidity and high temperature environments. The results show that it is possible to protect the devices from the effects of moisture under high negative gate bias stress testing, whereas devices, which were unprotected, failed under the same bias stress conditions. Thus, the use of the atomic layer deposition (ALD) coatings may provide added benefits in the protection and packaging of AlGaN/GaN HEMTs.

The unique properties, such as large direct bandgap, excellent thermal stability, high μH × ns, of III-nitrides make them ideal candidates for both optoelectronic and high-speed electronic devices. In the past decades, great success has been achieved in commercialization of GaN based light emitting diodes (LEDs) and laser diodes (LDs). However, due to the lack of native substrates, thin films grown on sapphire or SiC substrates have high defect densities that degrade the device performance and reliability. Conventional epitaxy lateral overgrowth (ELO) can reduce dislocation densities down to ∼10-6 cm-2 in the lateral growth area, but requires ex situ photolithography steps. Hence, an in situ method using a SiNx interlayer (nano-scale ELOG) has emerged as a promising technique. The GaN templates prepared by this method exhibit a very low dislocation density (low-10-7 cm-2) and excellent optical and electrical properties. As a cost, such high quality GaN templates containing SiN, nanonetworks are not suitable for heterojunction field effect transistor (HFET) applications due to degenerate GaN:Si layer which serves as parallel conduction channel. This dissertation discusses the growth of low dislocation density GaN templates, by using the in situ SiNx nanonetwork for conductive templates, and the AIN buffer for semi-insulating templates. On SiN x nanonetwork templates, double-barrier RTD and superlattice (SL) exhibited negative differential resistances. Moreover, the injection current of Blue LEDs (450 nm) was improved ∼30%. On semi-insulating GaN templates, nearly lattice matched AlInN/AIN/GaN HFETs were successfully demonstrated and exhibited ∼ 1600 cm2/Vs and 17 600 cm2/Vs Hall mobilities at 300 K and 10 K, respectively. Those mobility values are much higher than literature reports and indicate that high quality HFETs can be realized in lattice matched AlInN/AIN/GaN, thereby solving the strain related issue. The attempt to use InGaN as the 2DEG channel has also been successfully implemented. A Hall mobility (1230 cm2/Vs) was achieved in a 12 nm InGaN channel HFET with AlInGaN barrier, which demonstrates the viability of InGaN channel HFETs.