Michael J. Manfra

Inhomogeneous spatial distribution of reverse bias leakage in GaN Schottky diodes

The reverse bias leakage current in macroscopic GaN Schottky diodes is found to be insensitive to barrier height. Using a scanning current–voltage microscope, we show that the reverse bias current occurs at small isolated regions, while most of the sample is insulating. By comparing the current maps to topographic images and transmission electron microscopy results, we conclude that reverse bias leakage occurs primarily at dislocations with a screw component. Furthermore, for a fixed dislocation density, the V/III ratio during the molecular beam epitaxial growth strongly affects reverse leakage, indicating complex dislocation electrical behavior that is sensitive to the local structural and/or chemical changes.

Direct imaging of reverse-bias leakage through pure screw dislocations in GaN films grown by molecular beam epitaxy on GaN templates

Excess reverse-bias leakage in GaN films grown by molecular beam epitaxy on GaN templates is correlated with the presence of pure screw dislocations. A scanning current–voltage microscope was used to map the spatial locations of leakage current on high quality GaN films under reverse bias. Two samples with similar total dislocation density (∼109 cm−2) but with pure screw dislocation density differing by an order of magnitude were compared. We found that the density of reverse-bias leakage spots correlates well with pure screw dislocation density, not with mixed dislocation density. Thus, pure screw dislocations have a far more detrimental impact on gate leakage than edge or mixed dislocations.

High-mobility AlGaN'GaN heterostructures grown by molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy

We report on the growth and transport properties of high-mobility two-dimensional electron gases (2DEGs) confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy. We have grown samples over a broad range of electron densities ranging from ns=6.9×1011 to 1.1×1013 cm−2, and at T=4.2 K, observe a peak mobility of 53 300 cm2/V s at a density of 2.8×1012 cm−2. Magnetotransport studies on these samples display exceptionally clean signatures of the quantum Hall effect. Our investigation of the dependence of 2DEG mobility on carrier concentration suggests that the low-temperature mobility in our AlGaN/GaN heterostructures is currently limited by the interplay between charged dislocation scattering and interface roughness.

Effect of growth stoichiometry on the electrical activity of screw dislocations in GaN films grown by molecular-beam epitaxy

The impact of the Ga/N ratio on the structure and electrical activity of threading dislocations in GaN films grown by molecular-beam epitaxy is reported. Electrical measurements performed on samples grown under Ga-rich conditions show three orders of magnitude higher reverse bias leakage compared with those grown under Ga-lean conditions. Transmission electron microscopy (TEM) studies reveal excess Ga at the surface termination of pure screw dislocations accompanied by a change in the screw dislocation core structure in Ga-rich films. The correlation of transport and TEM results indicates that dislocation electrical activity depends sensitively on dislocation type and growth stoichiometry.

High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy

We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (∼20 μm) GaN templates are characterized by low threading dislocation densities (∼5×108 cm−2) and by room temperature resistivities of ∼108 Ω cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x⩽0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns⩽2×1012 cm−2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ∼2×1...

Electron mobility exceeding 160000cm2∕Vs in AlGaN∕GaN heterostructures grown by molecular-beam epitaxy

We report on the transport properties of a two-dimensional electron gas (2DEG) confined in an AlGaN∕GaN heterostructure grown by plasma-assisted molecular-beam epitaxy on a semi-insulating GaN template prepared by hydride vapor phase epitaxy with a threading dislocation density of ∼5×107cm−2. Using a gated Hall bar structure, the electron density (ne) is varied from 4.1to9.1×1011cm−2. At T=300mK, the 2DEG displays a maximum mobility of 167000cm2∕Vs at a sheet density of 9.1×1011cm−2, corresponding to a mean-free-path of ∼3μm. Shubnikov–de Haas oscillations, typically not observed at magnetic fields below 2T in GaN, commence at B=0.6T.

Impact of Si doping on radio frequency dispersion in unpassivated GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy

We report on the effect of Si doping on the transient behavior of unpassivated high-power GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy on 6H–SiC. The incorporation of Si into the heterostructure barrier is found to reduce the level of radio frequency dispersion as compared to undoped structures. In some devices which incorporate Si doping of the barrier, the pulsed and steady-state current–voltage characteristics coincide, and gate lag is found to be insignificant. More typically, ∼90% of the dc value of drain current is restored at 1 μs after pulsing the gate from pinch off to VGS=0 V. Significant gate lag is observed in devices that are not doped with Si. In the undoped structure, the drain current reaches only ∼70% of the dc value within 1 μs. The transient behavior in the two designs is attributed to the same defect state with activation energy of 0.22 eV. Dispersion reduction is correlated with an increase of gate leakage current in Si-doped devices.

Dynamics of trapped charge in GaN/AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy

We report on the dynamics of trapped charge in unpassivated GaN/AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy. Trap states are probed using a transient channel-current technique. By tailoring the gate pulse depth and width, this method allows selective probing of different trapping centers. We have identified at least two different trap centers that influence the current dynamics in our structures. In addition, the charge emission from the faster trap is found to have a clear square-root dependence on the applied electric field. This unambiguous field dependence allows us to isolate the mechanisms responsible for emission. We also identify the trapping mechanism and estimate the characteristic time required to fill available trap states and a lower bound for the trap density.

Scaling of Majorana Zero-Bias Conductance Peaks

We report an experimental study of the scaling of zero-bias conductance peaks compatible with Majorana zero modes as a function of magnetic field, tunnel coupling, and temperature in one-dimensional structures fabricated from an epitaxial semiconductor-superconductor heterostructure. Results are consistent with theory, including a peak conductance that is proportional to tunnel coupling, saturates at 2e^{2}/h, decreases as expected with field-dependent gap, and collapses onto a simple scaling function in the dimensionless ratio of temperature and tunnel coupling.

Noise Suppression Using Symmetric Exchange Gates in Spin Qubits

We demonstrate a substantial improvement in the spin-exchange gate using symmetric control instead of conventional detuning in GaAs spin qubits, up to a factor of six increase in the quality factor of the gate. For symmetric operation, nanosecond voltage pulses are applied to the barrier that controls the interdot potential between quantum dots, modulating the exchange interaction while maintaining symmetry between the dots. Excellent agreement is found with a model that separately includes electrical and nuclear noise sources for both detuning and symmetric gating schemes. Unlike exchange control via detuning, the decoherence of symmetric exchange rotations is dominated by rotation-axis fluctuations due to nuclear field noise rather than direct exchange noise.