Hengfang Zhang

Group-III nitrides semiconductor materials have been widely developed for application in optoelectronic and electronic devices. GaN-based high-electron mobility transistors (HEMTs) continue to attract strong interest for application in radio frequency electronics and power switches. Nitrogen(N)-polar HEMTs have a number of advantages over their gallium(Ga)-polar counterparts, such as the feasibility to fabricate low resistive ohmic contacts, an enhanced carrier confinement with a natural back barrier, as well as high device scalability. Recently, epitaxial N-polar GaN layers have been demonstrated on different substrates such as GaN, sapphire and SiC with miscut angles in order to suppress the presence of hexagonal hillocks onto the surface of N-polar GaN epilayers. However, N-polar GaN epitaxy on on-axis substrate still remains a challenge. The formation of large hexagonal hillocks and the poor crystalline quality of the epitaxial films prevents further progress. In this study, we report epitaxial GaN layers grown onto on-axis and 4o off-cut SiC (000) substrates by hot-wall MOCVD. GaN epilayers are grown simultaneously on both substrates employing N-polar AlN nucleation layers (AlN-NLs). The surface morphology, crystal quality and polarity of the GaN epilayers on the two types of substrates is investigated ­in relation to the AlN NL growth mode. For GaN films onto on-axis substrates mixed polarity was observed regardless of the AlN-NLs growth mode. GaN on AlN NLs with island growth exhibits smooth surface with many pits while with 2D growth AlN NLs shows rougher and step-like surface. However, for off-axis samples, the polarity and morphology of GaN films were not significantly affected by AlN-NLs growth mode. The surface roughness of GaN reduces significantly with decreasing the growth temperature of GaN, while the polarity is changed from N-polar to mixed.

Rosalia Delgado Carrascon

GaN has continued to gain scientific attention as one of the most promising wide-band gap semiconductor materials for high-frequency and power devices. The quality of GaN layers is strongly influenced by the lattice and thermal mismatches with the substrate, resulting in strain and defects which limit the device performance. To solve these issues, homoepitaxy seems to be the best approach to reduce dislocation density in the GaN layer providing superior crystalline material quality for the production of vertical electronic power devices. An alternative to the use of GaN bulk substrates is to obtain high-quality coalescence overgrowth templates from patterned-grown GaN nanowires (NWs). In this work, we study the dependence of the NW reformation and coalescence experimental growth conditions on the quality and morphology of a subsequent 2-micron thick GaN layer grown by Metal Organic Chemical Deposition (MOCVD). The structural quality of the homoepitaxial GaN films has been evaluated by high-resolution X-ray diffraction (HRHRD), revealing a dislocation density in the order of 2x10⁷ cm^-2. Morphological studies performed with Atomic Force Microscopy (AFM) over an area of 10 x 10 μm² have shown smooth surfaces with the root mean square (RMS) values between 0.12-0.15 nm. Atomically step-liked surface area where no spiral steps have been observed, indicating step-flow growth mode. The sample with the smoothest surface had the lowest screw dislocation density, the lowest value of strain along the c-axis and the highest transmittance. The thermal conductivity measurements performed by transient thermoreflectance (TTR) revealed that the smoothest sample exhibits an out-of-plane thermal conductivity of k = 206 W/mK at room temperature, approaching the GaN bulk value. The improvement in the quality of the GaN layers is believed to arise from the optimization of the reformation process where different annealing and nucleation conditions have been employed. We conclude that annealing at an optimized temperature in presence of ammonia followed by a nucleation under high V/III ratio leads to the best properties among all studied samples.

Dat Q. Tran

In this work we study the thermal conductivity of Al_xGa_1-xN layers over the entire composition range using transient thermoreflectance (TTR) technique . Weshow that the thermal conductivity of Al_xGa_1-xN sharply decreases with increasing Al content and already at x ≈0.15 it is more than one order of magnitude smaller than that of GaN layer of the same thickness. At higher Al content nearly constant values of the thermal conductivity is measured up to x = 0.7. The obtained data are in a good agreement with the theoretical calculations based on Debye-Callaway formalism. In these calculations, all resistive phonon-scattering processes are taken into account, namely the Umklapp phonon-phonon scattering, the phonon-point-defect scattering, the phonon-dislocation scattering and the phonon-boundary scattering, and the virtual-crystal model is applied. In the virtual-crystal model, the phonon scattering by alloy disorder is treated as the phonon-point-defect scattering. By separating the contributions of different scattering processes we find that the Al composition dependence of the thermal conductivity of AlGaN follows the shape of the strength of the phonon-alloy-disorder scattering which appears to be the dominant scattering mechanism in this material. As a consequence of alloy doping, the AlGaN surface becomes severely degraded that gives rise to increasing thermal boundary resistance (TBR) at Au/AlGaN interface. This experimental result cannot be simply explained by the phonon mismatch model which predicts an almost linear Al composition dependence.

Ingemar Persson

Polarity control for high electron mobility transistors (HEMTs) based on GaN/AlN interfaces opens up for tunable spontaneous polarization fields. This is highly relevant for realizing improved charge carrier confinement, low ohmic contacts, and device scalability. However, polarity determination is challenging owing to inhomogeneous surfaces leading to different polarity domains. A number of techniques have been utilized to determine the polarity, including wet chemistry etching, convergent beam electron diffraction (CBED), high angle annular scanning transmission electron microscopy (HAADF STEM), and kelvin probe force microscopy (KPFM). The most powerful technique for a thorough investigation of the mechanism of polarity control is HAADF STEM, as it can be employed to directly visualize the atomic structure from the substrate to the surface. In this report, we explain the mechanism behind polarity inversion in AlN/GaN interfaces and demonstrate how different polarity domains coexist.

Alexis Papamicail

Ultra-wide bandgap semiconductors have attracted a lot of attention the past few years due to their enhanced electrical and optical properties compared to the existing wide bandgap semiconductors. Especially β-Ga₂O₃ has been a topic of high interest because of its attractive properties (mainly the high breakdown electrical field, ~9 MV cm^-1 and good control of n-type doping and conductivity at room temperature) which make it a potential candidate for use in UV detectors and high-power electronic devices in the future. Employing β-Ga₂O₃ in high power devices requires a thorough understanding of its elastic, electronic and optical properties both at room temperature and at elevated temperatures. In this study, we investigated the temperature dependence of lattice parameters and strain evolution along different directions in the monoclinic lattice of bulk β-Ga₂O₃. The thermal expansion coefficients are determined and found to be in good agreement with recently published results. Furthermore, we determine the lattice parameters of β-Ga₂O₃ thin films grown on c-sapphire by x-ray diffraction radial scans and reciprocal space mapping and estimate the strain values along the main crystallographic directions. Finally, the phonon frequency shifts in β-Ga₂O₃ are determined from MIR ellipsometry data using the eigendielectric polarization model. Employing a newly developed theory on strain-stress relationships in crystals with monoclinic symmetry we correlate strain and phonon frequency shifts and determine the phonon deformation potentials for the first time.

Nerijus Armakavicius

Spectroscopic ellipsometry is a technique highly sensitive to the complex dielectric function of materials which are governed by their atomic and electronic structures. Thus, various physical properties can be accessed from the analysis of the ellipsometry data. Performing ellipsometry in an ultra-wide spectral range spanning from terahertz (THz) up to ultraviolet (UV) can provide rich information about the thin layers within the sample, since the physical properties, governing the complex dielectric function, changes with the photon energy. In the visible-UV (Vis-UV) spectral range, the main contributions to the dielectric function of semiconductors comes from electronic excitations, such as band-to-band and excitonic transitions. In the infrared (IR) spectral range, photon energies are too low for electronic excitations, while the crystal lattice vibrations come into play. Therefore, IR ellipsometry allows studying optical phonons in the semiconductors. In addition, free charge carriers may be also accessed through phonon-plasmon coupling effects. Going into the THz range, the contribution of the free charge carriers to the dielectric function of conductive layers becomes a dominant term. With the help of the optical Hall effect, induced by the external magnetic field, THz ellipsometry can provide properties of the free charge carriers. In this work, we show the capabilities of the ultra-wide range spectroscopic ellipsometry (THz-IR-Vis-UV), achieved by employing several instruments, for studying semiconductor layers with the examples of the group III nitrides and their heterostructures.

Philipp Kuehne

The optical Hall effect (OHE) is a physical phenomenon which manifests itself as optical birefringence caused by a static external magnetic field acting on free charge carriers, and can be quantify by generalized spectroscopic ellipsometry. Depending on the free charge carrier parameter i.e. carrier concentration, mobility and effective mass, the optimal spectral range to detect the OHE may lay between the mid-infrared to the terahertz (THz). For free charge carriers in 2‑dimensional electron gases in group-III nitrides, exposed to magnetic fields of several tesla, the OHE is best studied at THz frequencies. Here we employed an 8T magnet together with the THz frequency-domain ellipsometer of the Terahertz Materials Analysis Center at Linköping University [1] to study the temperature dependent free charge carrier properties in GaN based high electron mobility structures (HEMTs) with different barrier materials (AlGaN and InAlN). Analysis reveals negligible variations in carrier concentration and a strong temperature dependence of the carrier mobilities. However, the effective mass parameter significantly changes with temperature. We discuss possible causes, such as magnetic field orientation, conduction band non-parabolicity, penetration of the electron wave function into the barrier, additional conduction channels and polaronic effects. Finally, we analyze the temperature-dependence of the carrier scattering, employing models that include the temperature dependence of the effective mass.

Ragnar Ferrand­-Drake Del Castillo

In the pursuit of higher operation frequencies of GaN HEMTs with aim on achieving D-band lateral and vertical downscaling is required. The T- (or mushroom-) gate geometry in relation to field plate gates, enable a lower parasitic capacitance to the source and drain contacts. Here the methods employed for achieving a successful T-gate design with gate lengths down to 20-30 nm are discussed. The desired dimensions are achieved using Electron beam lithography (EBL) combined with a hard mask during etching process.

Yin Zeng

AlGaN/GaN HEMTs has been great developed since its wide band gap energy and high sheet charge density, which result in high breakdown voltage and large channel current. However, the catastrophic failure of AlGaN/GaN device caused by complex physic mechanisms like impact ionization, punch-through effect is still a significant limit of reliability in high power and frequency applications. Thus, sustainable breakdown condition under DC measurement and the Pulsed IV measurement were applied to study the static and dynamic breakdown characteristics of GaN based devices. In DC measurement, breakdown mechanism showed a different voltage pattern vs. temperature. In dynamic measurement, premature gate leakage breakdown caused by trap effect were found. The measurements will be used to extract a compact model for use in CAD.

Kai-Hsin Wen

It is challenging to achieve low-resistive ohmic contacts to III-nitride semiconductors due to their wide bandgap. Two approach have been evaluated in this work and all the experiments were performed on AlGaN/GaN heterostructures. The fabricated ohmic contacts were recess etched, metallized with a Ta/Al/Ta stack, and annealed at 550-575^◦C. Re-metallization was performed by wet etching the annealed contacts, followed by re-deposition of a metal stack and annealing. The purpose was to increase the amount of N vacancies in the AlGaN, which are responsible for the contact formation. A minimum contact resistance of 0.41 Ωmm was achieved with this method. In the second approach, different sputtered bottom Ta thickness (10/15/20 nm) was investigated in order to improve the sidewall coverage. The minimum contact resistance was found to be 0.6 Ωmm. The reason was assumed that the thickness of sputtered Ta should be thinner than the evaporated Ta due to its higher density. The Advanced Spice Model (ASM) for GaN HEMTs is a surface potential based physical compact model. Physical effects such as, self-heating, temperature dependence, trapping effects, DIBL, noise model and non-linear access region resistance are all incorporated with the 2DEG core model. Among all the existing models, ASM is preferred owing to its physical based formulation and the scalability, which enables to simulate precisely in RF and power region.

David Adolph

MOCVD has become the standard method of growing epi-layers of III-Nitride semiconductor. However, MBE possesses some interesting characteristics that may complement MOCVD in this context, e.g. (1) In-situ monitoring techniques resulting in excellent control of the growth process; (2) Good control of interface abruptness; (3) Ultra-high vacuum and high purity materials leads to lower levels of contaminants, (4) p-GaN does not need a high-temperature activation anneal to become electrically conductive, due to the low hydrogen levels, (5) doping profiles do not suffer from memory effects, (6) lower growth temperatures facilitate high indium content materials. We have now refurbished an EPI-930 equipped to a III-nitride MBE. It is equipped with (In,Ga,Al)(Si,Mg) solid effusion cells and two Nitrogen plasma sources. We will grow mainly on 4H-SiC substrates and on GaN/AlN/4H-SiC(0001)-templates. The long-term goal for our work is to grow complex heterostructures suitable for electrical and optical devices, such as HEMTs operating well beyond 100 GHz as well as VCSELs operating at blue wavelengths. We are aiming at HEMTs with regrown contacts and In-containing channels since this is expected to give a high electron velocity. Pure InN-layers is predicted to have a very high electron velocity (107cm/s). The EPI-930 is located inside MC2 Nanofabrication Laboratory which is a fully equipped cleanroom with state-of-the-art processing equipment which will be used to fabricate the HEMT-structures. Material characterization will be performed with XRD, SEM, AFM, SIMS and TEM. It is also possible to perform advanced Ellipsometry end Photoluminescence in cooperation with Linköping University. We will also use advanced electrical measurements both at room temperature and cryogenic temperatures with the equipment operational at the Microwave Electronics Laboratory.

 Björn Hult

Wide bandgap III-Nitride HEMTs have become a feasible alternative to Si and SiC power MOSFETs in power electronic converters by virtue of the high breakdown field of GaN and high electron mobility of the III-Nitride heterostructures. Today, lateral GaN-on-Si HEMTs are normally used as power electronic switching devices in the 650V and 900V classes. However, we’re investigating whether lateral GaN-on-SiC HEMTs with blocking voltages over 1000V can compete with current GaN-on-Si, Si, and SiC power devices. GaN heterostructures grown on SiC have shown to be an attractive alternative to the conventional GaN-on-Si for fabricating power electronic HEMTs due to the high thermal conductivity of SiC, as well as to the improved lattice matching of GaN grown on SiC. Furthermore, a new method of growing GaN on SiC which have been developed by SweGaN have the potential to enhance the breakdown strength of GaN. In this work, we aim to fabricate metal-insulator-semiconductor (MIS) HEMTs on SweGaN-grown epitaxial heterostructures (QuanFine) with a double field plate structure to reduce leakage currents, which in turn should result in higher blocking voltages. We also aim to create models using TCAD simulations to predict how the different field plates influence the electric fields and leakage currents

Ding Yuan Chen

The impact of different carbon-doped concentration in AlGaN/GaN/AlGaN back-barrier high electron mobility transistors (HEMTs) is investigated. Three epi-wafers with different carbon-doped concentration of 3×10¹⁷, 1.5×10¹⁷, and 0.7×10¹⁷ cm^-3 in back-barrier are grown by MOCVD (denoted as High-C, Mid-C, and Low-C respectively). HEMTs with the gate length of 100 nm and 200 nm are processed by passivation-first method and low annealing temperature recessed Ta-ohmic contact. Three wafers present similar electron mobilities of 2120 cm²/Vs, sheet resistance of 290 Ω/sq, and carrier density of 1×10¹³ cm^-2 after HEMTs processing. The High-C and Mid-C show better two-dimensional electron gas (2DEG) confinement with a lower drain induced barrier lowering (DIBL) value of ~4.5 mV/V than Low-C. Pulsed-IV characterization indicates that the High-C suffers larger current collapse (~40%) due to sever trapping effect in highly doped AlGaN back-barrier. Three wafers show comparable extrinsic f_T of 50 GHz and f_max of 110 GHz. Active load-pull measurement at 10 GHz demonstrates that the Low-C offer highest output power (~1.86 W/mm). The Mid-C, which has good 2DEG confinement, small trapping effect, and decent output power, can be considered as an acceptable carbon doping concentration for AlGaN back-barrier epi-design.

 Ding Yuan Chen

The impact of thin Al_0.5Ga_0.5N top barrier (6 nm) on un-intentional doped (UID) QuanFINE v1, Fe-doped QuanFINE v1, and Fe-doped QuanFINE v2 high electron mobility transistors (HEMTs) is studied. Three epitaxial wafers with 260 nm-thick GaN buffer design are grown by MOCVD and are denoted as TB-QF1, TB-FeQF1, and TB-FeQF2, respectively. Epi-layers including a GaN cap, an Al_0.5Ga_0.5N barrier, an AlN exclusion layer, and an AlN nucleation layer are identical. SiNx passivation-first process is used to fabricate the HEMTs with the gate length of 100, 200, and 500 nm. Low annealing temperature Ta-based recessed ohmic contact is formed with the contact resistance of 0.24 ohm-mm. Consistence two-dimensional electron gas (2DEG) performance is sustained after the HEMTs processing with the mobility of 1700 cm²/Vs, sheet resistance of 295 Ω/sq, and carrier density of 1.2×10¹³ cm^-2. DC, pulsed-IV, small-signal, and large-signal characterization are performed.