Donald L. Dorsey

Analysis of the residual stress distribution in AlGaN/GaN high electron mobility transistors

A comparative analysis of the residual stress distributions across the conductive channel of Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is presented. Stress was measured by means of micro-Raman spectroscopy and micro-photoluminescence (PL). Raman measurements probed the volume average of the stress through the GaN layer whereas the stress near the GaN surface (AlGaN/GaN heterointerface) was acquired via PL. By combining Raman, PL, and x-ray diffraction, a self-consistent method was developed to accurately determine the variation in magnitude of stress throughout the thickness of the GaN layer. Based on this framework, it is observed in AlGaN/GaN HEMTs that a depth variation in the GaN residual stress occurs near the gate and ohmic electrodes. At these regions, the stress near the AlGaN/GaN interface (or GaN surface) exhibits a tensile shift compared to the stress averaged through the entire thickness of GaN. Across the conductive channel (away from the metal pads), the bulk average stres...

Thermometry of AlGaN/GaN HEMTs Using Multispectral Raman Features

In this paper, we utilize micro-Raman spectroscopy to measure temperature and stress in state-of-the-art AlGaN/GaN HEMTs. A rigorous discussion on the physical accuracy, precision, and precautions for diverse Raman thermometry methods is developed. Thermometry techniques utilizing shifts in a single Raman Stokes peak position underpredict the channel temperature due to induction of operational thermoelastic stress in operating devices. Utilizing the change in phonon linewidth by employing a proper reference condition gives true temperature results. Making use of frequency shifts in both the E2(high) and A1(LO) phonon modes offers accurate and time-efficient means to determine the state of temperature and thermal stress in operating AlGaN/GaN HEMTs presuming that linear relations between phonon frequencies and temperature/stress are well determined. Useful applications of this method such as monitoring stress in GaN wafers between fabrication steps and Raman thermography on AlGaN/GaN HEMTs are demonstrated.

The Impact of Bias Conditions on Self-Heating in AlGaN/GaN HEMTs

The thermal response of AlGaN/GaN high electron mobility transistors directly correlates with the overall performance and reliability of these devices. In general, a hot spot develops near the drain end of the gate electrode during power dissipation. The device channel temperature was examined via micro-Raman spectroscopy under various bias conditions where power dissipation levels were identical. Under these bias conditions, difference in internal states (sheet carrier density and electric held distribution) within the device alters the heat generation profile across the channel. High Vds conditions lead to significantly higher channel temperature compared to that for low Vds conditions although the power dissipation is kept constant. Experimental results show ~13°C deviation between Vds = 45 V and Vds = 7 V cases when the power dissipation is 4.5 W/mm. This suggests that bias conditions may have a relatively signihcant impact on device reliability and that this effect must be considered when building thermal models of devices under operation or undergoing accelerated life testing.

Electrical and structural dependence of operating temperature of AlGaN/GaN HEMTs

Abstract Understanding the distribution of the considerable heat generated in the active region of high power AlGaN/GaN high electron mobility transistors (HEMTs) at the sub-micron length scales relevant to the failures being observed in these devices is crucial for understanding device performance and reliability. In addition, electrical bias conditions and structural characteristics such as field plates alter the electric field distribution and thermal path within the device leading to changes in the heat generation profile across the channel. This in turn influences the value and location of the device peak temperature and the channel to ambient (or case or base-plate) thermal resistance. The channel temperature distribution of AlGaN/GaN HEMT structures with and without source connected field plates were examined via micro-Raman spectroscopy and coupled electro-thermal simulation. For both type of structures, high V ds conditions lead to significantly higher channel temperature compared to that for low V ds conditions for the same power dissipation level . This is important because the industry standard Arrhenius relation assumes the total power is sufficient to describe the device channel temperature and that the bias condition is irrelevant [1] . We explore the level of agreement between modeling and experiment, and also the extent to which variability in input parameters for the modeling affects model results. We show that operating bias condition has a significant role in device reliability by altering value and location of the peak temperature, which then alters the type and rate of thermally induced degradation taking place at critical locations such as the drain side corner of the gate. Specifically, care must be taken when extrapolating results of an accelerated life test to usage conditions at dissimilar bias conditions to consider if the results will be applicable.

The impact of mechanical stress on the degradation of AlGaN/GaN high electron mobility transistors

Coupled electro-thermo-mechanical simulation and Raman thermometry were utilized to analyze the evolution of mechanical stress in AlGaN/GaN high electron mobility transistors (HEMTs). This combined analysis was correlated with electrical step stress tests to determine the influence of mechanical stress on the degradation of actual devices under diverse bias conditions. It was found that the total stress as opposed to one dominant stress component correlated the best with the degradation of the HEMT devices. These results suggest that minimizing the total stress as opposed to the inverse piezoelectric stress in the device is necessary in order to avoid device degradation which can be accomplished through various growth methods.

Heuristic rules for group IV dopant site selection in III–V compounds

Abstract The use of column IV dopants in III–V compounds is of great interest because they are easy to handle and in the case of Si, readily available in most molecular beam epitaxy (MBE) chambers. A column IV dopant can act as a donor or an acceptor or both depending on its size relative to the cation and anion sites, the substrate orientation and the growth conditions. In this article, we present heuristic rules for predicting group IV dopant site selection in III–V compounds based on the relative covalent radii of the dopant to the host lattice sites, types and relative numbers of surface dangling bonds and the growth conditions. Preliminary predictions show excellent agreement with available experimental data.

Compositional analysis of mixed–cation-anion III–V semiconductor interfaces using phase retrieval high-resolution transmission electron microscopy

Employing exit‐plane wave function (EPWF) reconstruction in high‐resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III‐V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub‐lattices. Specifically, we use the focal‐series reconstruction technique, which retrieves the complex‐valued EPWF from a thru‐focus series of HRTEM images. A study of interfaces in Al0.4Ga0.6As–GaAs and In0.25Ga0.75Sb–InAs heterostructures using focal‐series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub‐lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In–Ga and As–Sb contents across ultra‐thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In0.25Ga0.75Sb–InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs–GaSb–InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic‐level compositional analysis of heterostructures with two species per sub‐lattice, hitherto unfeasible with existing HRTEM methods.

Transient stress characterization of AlGaN/GaN HEMTs due to electrical and thermal effects

Abstract In this paper, we present finite element simulation results of the transient stress response of an AlGaN/GaN high electron mobility transistor (HEMT). The modeling technique involves a small-scale electro-thermal model coupled to a large-scale mechanics model to determine the resulting stress distribution within a device operated under radio frequency (RF) conditions. The electrical characteristics of the modeled device were compared to experimental measurements and existing simulation data from literature for validation. The results show critical regions around the gate Schottky contact undergo drastically different transient stresses during pulsed operation. Specifically, stress profiles within the AlGaN layer around the gate foot print (GFP) undergo highly tensile electro-thermal stresses while stresses within the AlGaN outside the gate connected field plate (GCFP) towards the drain contact undergo highly tensile electrical stress and compressive thermoelastic stress. It is shown AlGaN/GaN HEMTs undergo large amounts of cyclic loading during typical transient operation. Based on these findings, transient failure mechanisms may differ from those previously studied under DC operation due to large amount of cyclic loading of a device around the gate structure.

Quantifying stoichiometry of mixed-cation-anion III-V semiconductor interfaces at atomic resolution

Employing the focal-series reconstruction technique in high-resolution transmission electron microscopy, we obtained true atomic resolution images of interfacial disorder within cation and anion sublattices across interfaces in an InGaSb∕InAs heterostructure. This enabled independent quantitative mapping of changes in the In–Ga and As–Sb contents across interfacial regions ∼0.6nm in width. A comparison of the cation and anion sublattice images revealed that intermixing at the InGaSb‐on‐InAs interface is confined to the In–Ga sublattice. Also, atomic scale roughness within the As–Sb sublattice of the InAs‐on‐InGaSb interface was discerned. This approach is general, permitting atomic-scale compositional analysis of heterointerfaces with two species per sublattice.

Demonstration of high mobility and quantum transport in modulation-doped β-(AlxGa1-x)2O3/Ga2O3 heterostructures

In this work, we demonstrate a high mobility two-dimensional electron gas (2DEG) formed at the β-(AlxGa1-x)2O3/Ga2O3 interface through modulation doping. Shubnikov-de Haas (SdH) oscillations were observed in the modulation-doped β-(AlxGa1-x)2O3/Ga2O3 structure, indicating a high-quality electron channel formed at the heterojunction interface. The formation of the 2DEG channel was further confirmed by the weak temperature dependence of the carrier density, and the peak low temperature mobility was found to be 2790 cm2/Vs, which is significantly higher than that achieved in bulk-doped Beta-phase Gallium Oxide (β-Ga2O3). The observed SdH oscillations allowed for the extraction of the electron effective mass in the (010) plane to be 0.313 ± 0.015 m0 and the quantum scattering time to be 0.33 ps at 3.5 K. The demonstrated modulation-doped β-(AlxGa1-x)2O3/Ga2O3 structure lays the foundation for future exploration of quantum physical phenomena and semiconductor device technologies based on the β-Ga2O3 material s...