Conductivity Of GaN Research Articles

Investigation of defect states in AlGaN/GaN high electron mobility transistors by small-signal admittance analysis

We have performed a small-signal admittance analysis to extract trap parameters in an AlGaN/GaN high electron mobility transistor-on-Si. Whereas the admittance in the accumulation- and depletion-bias regimes is primarily due to the interface traps, the admittance near the threshold voltage and below is due to mono-energetic traps inside GaN. The density extracted for threading dislocation-related 1D traps at ≈0.22 eV below the GaN conduction band edge is similar to that previously reported by reverse-biased gate leakage analysis of the analyzed device. Our analysis suggests additional 1D traps of comparatively lower density ≈4×1014cm−3 but considerably large capture cross section ≈8×10−14cm2 in the GaN layer at ≈0.31 eV below the conduction band. The AlGaN trap density is considerably larger near the AlGaN/GaN interface than in the bulk AlGaN. The AlGaN traps mainly contribute to the voltage stretch; their admittance contribution is negligible. Gate leakage dominates the conductance at lower frequencies.

First-principles study on thermal transport properties of GaN under different cross-plane strain

With the development of power devices towards high integration and power, the electrical and thermal stresses per unit area become more concentrated and intensified. The impact of cross-plane strain resulting from inverse piezoelectric effect and thermal expansion on power devices cannot be ignored. Cross-plane strain has a substantial influence on the thermal properties of GaN. However, the research on the influence of cross-plane strain on the thermal conductivity of GaN has not been reported in the literature. Based on the first-principles calculation method and the phonon Boltzmann transport equation, the influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is systematically studied in this study. The thermal conductivity of GaN has anisotropy and increases significantly with the decrease in temperature. The thermal conductivity of GaN at room temperature under the free state is calculated to be 257 and 275 W/(m·K) for in-plane (k⊥) and cross-plane (k∥) directions, respectively. Compared with previous theoretical reports, our calculation results are more consistent with the existing experiment values. Under the state of cross-plane strain, the lattice thermal conductivity changes remarkably. In detail, the average thermal conductivity at room temperature decreased by 35 % under a 5 % cross-plane tensile strain state, while it increased by 11.5 % under a 5 % cross-plane compressive strain state. According to the calculation results, the influence of cross-plane on lattice thermal conductivity is mainly due to the change in phonon lifetime. The analysis of two mechanisms of phonon lifetime suppression indicates that the cross-plane strain will significantly change the frequency of the high-frequency optical acoustic branch. This result leads to a change in the phonon scattering process and thus affects the phonon lifetime. Besides, the anisotropy of thermal conductivity changes under different strain values, which may be due to the weakened piezoelectric polarization effect induced by strain.

Atomistic Origins of Various Luminescent Centers and n-Type Conductivity in GaN: Exploring the Point Defects Induced by Cr, Mn, and O through an Ab Initio Thermodynamic Approach.

GaN is a technologically indispensable material for various optoelectronic properties, mainly due to the dopant-induced or native atomic-scale point defects that can create single photon emitters, a range of luminescence bands, and n- or p-type conductivities. Among the various dopants, chromium and manganese-induced defects have been of particular interest over the past few years, because some of them contribute to our present-day light-emitting diode (LED) and spintronic technologies. However, the nature of such atomistic centers in Cr and Mn-doped GaN is yet to be understood. A comprehensive defect thermodynamic analysis of Cr- and Mn-induced defects is essential for their engineering in GaN crystals because by mapping out the defect stabilities as a function of crystal growth parameters, we can maximize the concentration of the target point defects. We therefore investigate chromium and manganese-induced defects in GaN with ab initio methods using the highly accurate exchange-correlation hybrid functionals, and the phase transformations upon excess incorporation of these dopants using the CALPHAD method. We also investigate the impact of oxygen codoping that can be unintentionally incorporated during crystal growth. Our analysis sheds light on the atomistic cause of the unintentional n-type conductivity in GaN, being ON-related. In the case of Cr doping, the formation of CrGa defects is the most dominant, with an E +/0 charge transition at E VBM + 2.19 eV. Increasing nitrogen partial pressure tends to enhance the concentration of CrGa. However, in the case of doping with Mn, several different Mn-related centers can form depending on the growth conditions, with MnGa being the most dominant. MnGa possesses the E 2+/+, E +/0, and E 0/- charge transitions at 0.56, 1.04, and 2.10 eV above the VBM. The incorporation of oxygen tends to cause the formation of the MnGa-VGa center, which explains a series of prior experimental observations in Mn-doped GaN. We provide a powerful tool for point defect engineering in wide band gap binary semiconductors that can be readily used to design optimal crystal growth protocols.

High-order phonon anharmonicity and thermal conductivity in GaN

High-order phonon anharmonicity and thermal conductivity in GaN

Weak effects of electron-phonon interactions on the lattice thermal conductivity of wurtzite GaN with high electron concentrations

Weak effects of electron-phonon interactions on the lattice thermal conductivity of wurtzite GaN with high electron concentrations

Impact of post-deposition annealing on the electronic properties of Al2O3/GaN interface by first-principles study

The electronic properties of Al2O3/GaN interface are investigated by first-principles calculations to explore the effects of post-deposition annealing (PDA) in O2 or N2 ambient. It is found that for the initial Al2O3/GaN interface without post-annealing, there are traps with a wide energy range in the GaN band gap, mainly caused by dangling bonds (DBs) of Ga, Al, and N atoms. However, with proper PDA in O2 ambient, the trap states induced by Al DBs completely merge into GaN conduction band, while the trap states induced by Ga DBs and N DBs completely merge into GaN valence band. Therefore, the interface shows a cleaner band structure. Nevertheless, excessive interface O content can lead to trap states around the valence band maximum induced by N DBs and O DBs. For PDA in N2 ambient, the trap states induced by Al DBs and Ga DBs can be completely suppressed under appropriate interface N content. Unfortunately, deep and shallow traps caused by N DBs can be introduced in the GaN band gap. This study systematically reveals the physical mechanism by which PDA in O2 ambient compared to N2 can significantly reduce the interface state density and improve threshold voltage stability of the device.

Opposite atom dependence of isotope engineering of thermal conductivity in bulk and 2D GaN

Isotope engineering has been shown to be an effective means of regulating thermal conductivity. In this work, we studied the isotope engineering of thermal conductivity in bulk and 2D GaN, and diametrically opposite atom isotope dependence is found. That is, Ga isotope has a large effect (77%) on bulk GaN, while the effect of N isotope on the thermal conductivity is negligible. In 2D GaN, however, N isotope effect (20%) is more significant than that of Ga. Understanding of the different isotope dependence is achieved by deeper insight. Due to the relative magnitude of scattering rate, isotopic scattering influences the thermal conductivity of bulk and 2D GaN in different frequency regions, leading to the opposite atom dependence.

On the thermal conductivity anisotropy in wurtzite GaN

GaN-based power devices operating at high currents and high voltages are critically affected by the dissipation of Joule heat generated in the active regions. Consequently, knowledge of GaN thermal conductivity is crucial for effective thermal management, needed to ensure optimal device performance and reliability. Here, we present a study on the thermal conductivity of bulk GaN in crystallographic directions parallel and perpendicular to the c-axis. Thermal conductivity measurements are performed using the transient thermoreflectance technique. The experimental results are compared with a theoretical calculation based on a solution of the Boltzmann transport equation within the relaxation time approximation and taking into account the exact phonon dispersion. All factors that determine the thermal conductivity anisotropy are analyzed, and the experimentally observed small anisotropy factor is explained.

Improvement of Mg-Doped GaN with Shutter-Controlled Process in Plasma-Assisted Molecular Beam Epitaxy

Mg-doped GaN was grown by plasma-assisted molecular beam epitaxy (PAMBE) on a Fe-doped GaN template substrate by employing a shutter-controlled process. The transition from n-type to p-type conductivity of Mg-doped GaN in relation to the N/Ga flux ratio was studied. The highest p-type carrier concentration in this series was 3.12 × 1018 cm−3 under the most N-rich condition. By modulating the shutters of different effusion cells for the shutter-controlled process, a wide growth window for p-type GaN was obtained. It was found that the presence of Mg flux effectively prevents the formation of structural defects in GaN epi-layers, resulting in the improvement of crystal quality and carrier mobility.

Theoretical study on the thermal transport and its tunability of a-plane trilayer GaN

Two-dimensional (2D) a-plane gallium nitride, a non-layered 2D material, has promising applications in photoelectric nanodevices due to its direct band bandgap. Herein, employing molecular dynamics simulations, we studied the thermal transport properties of a-plane trilayer GaN, and the temperature, together with strain modulation on the thermal conductivity of the system. The a-plane trilayer GaN shows anisotropic thermal conductivity with 70.22 Wm−1K−1 and 41.81 Wm−1K−1 along zigzag- and armchair- directions respectively at room temperature when extrapolated to infinite size. In addition, the thermal conductivity of trilayer GaN exhibits decreasing trend in response to the increase of temperature. The thermal conductivity decreases monotonically with the increased compressive uniaxial and biaxial strain, while it shows an up-then-down trend under tensile strain. The tunability of thermal conductivity under biaxial strain is much larger than that of uniaxial strain. The phonon density of states is further investigated to understand the behavior of thermal conductivity. The tunability of the system thermal conductivity will expand its applications in thermal management and nanodevices.

Interface state density distribution near conduction band edge at Al2O3/Mg-ion-implanted GaN interface formed after activation annealing using AlN cap layer

An interface state density (Dit) distribution near the conduction band edge (EC) at the Al2O3/Mg-ion-implanted GaN interface was measured after ion implantation, annealing with an AlN protective cap, and cap layer removal. Mg ions were implanted into n-GaN with a Si concentration of 6 × 1017 cm−3 at a maximum Mg concentration of 2 × 1017 cm−3, resulting in the maintenance of the n-type conduction in GaN even after the activation of Mg dopants. Activation annealing was carried out at 1250 °C for 1 min using an AlN cap layer. The complete removal of the AlN cap layer was accomplished by wet etching, which was confirmed by x-ray photoelectron spectroscopy. The photoluminescence spectrum showed donor–acceptor-pair emission after annealing, indicating the activation of Mg acceptors. By applying the capacitance–voltage method to a completed metal–oxide–semiconductor diode, we derived a continuous distribution of relatively low Dit below 5 × 1012 cm−2 eV−1, which increased monotonically toward EC in the range from EC − 0.15 to EC − 0.45 eV. Compared with the Dit distribution of the as-implanted sample, the density of the discrete level at EC − 0.25 eV generated by divacancies markedly decreased upon 1250 °C annealing.

An Explicit Thermal Resistance Model Regarding Self-Heating Effect of AlGaN/GaN High Electron Mobility Transistor

This paper introduced an accurate empirical model for the thermal resistance of a single-finger AlGaN-GaN high electron mobility transistor (HEMT) on three different substrates including Sapphire, SiC and Si. The model reckons the constant thermal conductivity of GaN and substrate, thickness of host substrate layers, gate length (Lg) and width (Wg). The model plausibility is verified by comparing it with DC channel temperature measurement method and charge controlled based device modeling which agrees very favorable observation of the model data. Having nimble expression for the channel temperature is of inordinate importance in the field of designers of power device and monolithic microwave integrated circuits. Proposed model gives a variety of inquiries that would be impossible or impractical to do using time-consuming numerical simulations.

The role of point defects related with carbon impurity on the kink of log J–V in GaN-on-Si epitaxial layers

Carbon impurity as point defects makes key impact on the leakage in GaN-on-Si structures. GaN-based epitaxial layers with different point defects by changing carbon-doped concentration were used to investigate the point defects behavior. It was found that leakage mechanisms correspond with space-charge-limited current models at low voltages, and after 1st kink, electron injection from silicon to GaN and PF conduction play a key role in the leakage of both point defects case with low carbon and high carbon doped. In addition, high carbon in GaN-on-Si epitaxial layers obtained lower leakage and larger breakdown voltage. The slope of log J–V has two kinks and effective energy barrier E a has two peaks, 0.4247 eV at about 300 V and 0.3485 eV at about 900 V, respectively, which is related to accepted states and donor states related with carbon impurity. While the slope of log J–V has one kink and effective energy barrier E a has one peak, 0.4794 eV at about 400 V of low carbon in GaN-on-Si epitaxial layers, indicating only field-induced accepted ionized makes impact on leakage. The comparative results of more donor trap density in high carbon indicate point defects related with carbon impurity play a key role in the kinks of log J–V slope.

MOCVD Growth and Characterization of Be-Doped GaN.

Beryllium has been considered a potential alternative to magnesium as a p-type dopant in GaN, but attempts to produce conductive p-GaN:Be have not been successful. Photoluminescence studies have repeatedly shown Be to have an acceptor level shallower than that of Mg, but deep Be defects and other compensating defects render most GaN:Be materials n-type or semi-insulating at best. Previous reports use molecular beam epitaxy or ion implantation to dope GaN with Be, almost exclusively. Due to the high toxicity of Be organometallics, reports of GaN:Be by metal-organic chemical vapor deposition (MOCVD) have been largely absent. Here, we report a systematic study of growth of GaN:Be by MOCVD. All doped samples show the established UV band and yellow luminescence signature of GaN:Be, and growth conditions resulting in high-quality GaN with stable Be incorporation were established. Our results show that the MOCVD growth technique allows for Be incorporation pathways that have not been possible with previous growth methodologies and is highly promising in achieving p-type conductivity in GaN:Be.

Accurate estimation of thermal properties for AlGaN/GaN HEMTs on diamond

For accurate estimation of the thermal characteristics of AlGaN/GaN high electron mobility transistors on diamond, the anisotropic thermal conductivities of polycrystalline diamond (PCD) and GaN with their thickness dependences are included in the finite element simulation model, of which the PCD thermal conductivity kPCD is calculated through detailed analyses of the grain sizes in the directions parallel and vertical to the interface and relative phonon-grain boundary scatterings. In addition, the GaN thermal conductivity kGaN is the result of an approximate solution of the phonon Boltzmann transport equation. To investigate the effects of anisotropic GaN and PCD thermal conductivity, four cases of thermal conductivity models are compared: anisotropic kGaN and kPCD, anisotropic kGaN with constant kPCD, constant kGaN with anisotropic kPCD, and constant kGaN and kPCD. Then the maximum channel temperature simulated from the four above-mentioned models and the errors relative to the case of anisotropic kGaN and kPCD are investigated with respect to thermal boundary resistance, GaN layer thickness, gate pitch, heat source width, and power dissipation. The results show that the combination of anisotropic kPCD and constant kGaN overestimates the maximum channel temperature rise, and using constant kPCD and anisotropic kGaN underestimates the maximum channel temperature rise for almost all the simulations in the text. However, the channel temperature stemming from constant kPCD and kGaN is generally closer to the case with anisotropic PCD and GaN thermal conductivity.

Mechanism of high conduction on the N polar face of GaN

High conductivity on the N-face of GaN has long been observed. For many years, sapphire was the basic substrate used for growing GaN layers. Hence, such conductivity was associated with a high concentration of defects at the GaN/sapphire interface. Due to recent advances in GaN growth techniques, bulk GaN in polar and non-polar orientations has become available. In this study, we compare GaN grown on sapphire with bulk GaN. It is found by numerical fitting that the sheet resistance of GaN layers on sapphire substrates is determined by both bulk GaN conduction and polar-induced screening-charge conduction. The same high conductivity on the N-faces of GaN layers is observed even without the interface with sapphire substrates, i.e., when the latter are peeled off and the GaN layers are chemically mechanically polished. A further comparison of three samples with different orientations (c-, a-, and m-planes) suggests that the observed high conductivity is caused by a mobile screening charge attracted to the N-face by the polar charge present on it. This conclusion follows from the fact that no high conductivity is observed in the samples with non-polar orientations.

Determination of the electron trap level in Fe-doped GaN by phonon-assisted conduction phenomenon

We acoustically measured the energy level for thermally activated conduction (TAC) in high-resistivity Fe-doped GaN using the non-contacting antenna-transmission acoustic-resonance method. The acoustic attenuation takes a maximum at a specific temperature, where the TAC is accelerated with the help of phonon energy. The Debye-type relaxation is thus observed for acoustic attenuation, and its activation energy (0.54 0.04 eV) was determined with attenuation measurements at various frequencies and temperatures. This value agrees with the E3 level in GaN, indicating that thermally associated conduction originates from the E3 trap level. We also measured the five independent elastic constants at high temperatures.

P-type conductivity and suppression of green luminescence in Mg/N co-implanted GaN by gyrotron microwave annealing

Co-implantation of Mg with N has been shown to improve p-type conductivity in Mg-implanted GaN. Achievement of p-type material still requires temperatures beyond the thermodynamic stability of GaN, however. In this study, we present results of implantation and anneal activation of GaN, co-implanted with Mg and N or Mg only by repeated, short thermal cycles of 1350 °C using a high-power gyrotron microwave source with a quasi-gaussian intensity profile. Spatial variations in optical and electrical properties of the resulting films are characterized by photoluminescence and diode I–V and C–V measurements. Resistive Mg/N co-implanted and annealed material shows dominant luminescence of the VN-related green luminescence (GL2) band at 2.37 eV and relatively lower intensity acceptor-related ultraviolet luminescence (UVL) at 3.27 eV. However, a material showing p–n diode behavior shows higher-intensity UVL luminescence and suppression of the GL2 band, permitting observation of the yellow luminescence (YL) present in the as-grown GaN. The YL is attributed to unintentionally introduced CN–ON complexes and is commonly observed in GaN grown by metalorganic chemical vapor deposition but is typically absent in implanted/annealed GaN. Co-implanted material is compared to material implanted only with Mg and annealed under the same conditions, which shows p-type activation, but contains persistent GL2 luminescence post-anneal and lowers maximum hole concentration.

Monotonic variation in carbon-related defects with Fermi level in different conductive types of GaN

This study examined the photoluminescence (PL) of samples of GaN and found that the intensity of the peak of luminescence related to carbon impurities in p-GaN changed with the doping concentration of Mg. However, the results of a secondary ion mass spectrometry test showed that the concentration of carbon impurities did not change correspondingly. Moreover, we observed changes in the relative strength of the peak related to carbon impurities in the PL spectra of a series of samples of n-type conductive GaN. This suggests a connection between the behavior of carbon-related defects and the conductivity of GaN. The results show that the variation in carbon-related defects was monotonic. As the Fermi level approached the conductive band, carbon-related defects that generated higher-energy photonics became more dominant in the PL spectra in the series from p-type to n-type GaN.

Understanding of impact of carbon doping on background carrier conduction in GaN* *Project partially supported by the National Key Research and Development Program of China (Grant No. 2017YFB0402800), the Key Research and Development Program of Guangdong Province, China (Grant No. 2020B010174003), the National Natural Science Foundation of China (Grant No. U1601210), and the Natural Science Foundation of Guangdong Province, China

The impact of carbon doping on the background carrier conduction in GaN has been investigated. It is found that the incorporation of carbon can effectively suppress the n-type background carrier concentration as expected. Moreover, from the fitting of the temperature-dependent carrier concentration and mobility, it is observed that high nitrogen–vacancy (VN) dominates the background carrier at room temperature which consequently results in n-type conduction. The doping agent (carbon atom) occupies the nitrogen site of GaN and forms CN deep acceptor as revealed from photoluminescence. Besides, a relatively low hole concentration is ionized at room temperature which was insufficient for the compensation of n-type background carriers. Therefore, we concluded that this background carrier concentration can be suppressed by carbon doping, which substitutes the N site of GaN and finally decreases the VN.