Tamara B. Fehlberg

Characterisation of Multiple Carrier Transport in Indium Nitride Grown by Molecular Beam Epitaxy

Transport properties of two distinct electron species in indium nitride grown by molecular beam epitaxy (MBE) have been measured. Variable field Hall and resisitivity voltages were used in a quantitative mobility spectrum analysis (QMSA) to extract the concentrations and mobilities of the two electron species, attributed to the bulk electrons and a surface accumulation layer. Single magnetic field data corresponds to neither electron species. The bulk electron distribution has an extracted average mobility of 3570 cm2/(V s) at 300 K, which rises to over 5100 cm2/(V s) at 150 K. Bulk electron concentration in the sample is 1.5 ×1017 cm-3 . The surface electrons have a higher sheet charge density and an order of magnitude lower average mobility than those in the bulk.

Transport Studies of AlGaN/GaN Heterostructures of Different Al Mole Fractions With Variable

The influence of passivation by silicon nitride (SiNx) of different stress states (compressive, neutral, and tensile) is presented as a function of varying Al mole fraction (x). All types of SiNx passivant induced, as expected, an increase in 2-D electron gas (2DEG) concentration. In addition, however, the 2DEG mobility increased after passivation for the low-x (0.15) sample, and the more tensile the film stress is, the greater the relative increase. This led to a very highly measured 2DEG mobility of 2360 cm2V-1s-1 at 300 K. In higher x samples, however, mobility was decreased after passivation, increasingly so as x increased, and to a varying extent with different stresses. It is apparent from the results that there is a complex relationship between the stress in the SiNx layer, the mole fraction, and the transport properties of the 2DEG. Thus, tailoring of the passivant deposition conditions, and not simply passivant dielectric choice, to optimize transport properties, is critical for a given passivant, deposition tool, and Al mole fraction, to maximize device performance.

\hbox{SiN}_{x}

Magnetic-field dependent Hall-effect measurements and mobility spectrum analysis were employed to study anisotropic transport in N-polar GaN/Al0.3Ga0.7N heterostructures grown on vicinal sapphire substrates. The significant anisotropy in the mobility in the parallel and perpendicular directions to the miscut direction was accompanied by a slight anisotropy in charge density. A single electron species was found in the direction parallel to the steps resulting from growth on the vicinal substrates; while in the perpendicular direction two distinct electrons peaks were evident at T≤150 K. The lower average mobility in the perpendicular direction is attributed to interface roughness scattering.

Passivation Stress

We have investigated the ion sensitivity of ungated AlGaN/GaN heterostructure-based devices and found that these devices are sensitive and selective to the negative ion concentration in the solution. Such selectivity towards negative ions can be employed in cell-based biosensor applications via detection of negative ion transport through the cell membrane/ion channels. Compatibility of living cells and AlGaN/GaN heterostructures for this application has therefore been investigated qualitatively and quantitatively by flow cytometry. Although the mortality rate increases marginally with Al composition, this effect is not strong. This provides much-needed flexibility in designing biosensors by enabling Al mole fraction to be selected on the basis of optimum heterostructure properties. The results of these investigations are very promising for cell-based biosensor development.

Two-dimensional electron gas transport anisotropy in n-polar GaN/AlGaN heterostructures

In this work, we present results of a magneto-transport study of two-dimensional electron gas transport in N-polar GaN/AlGaN heterostuctures grown on misoriented sapphire substrates. It is shown that the multi-atomic “steps” resulting from epitaxial growth on misoriented substrates leads to significantly lower average two-dimensional electron gas (2DEG) mobility in the direction perpendicular to the interfacial steps which is accompanied by significant broadening of the 2DEG mobility distribution.