AVS2011 Session EM+SS-FrM: Surfaces and Materials for Next Generation Electronics

To avoid defects such as polytype inclusions or dislocations, the epitaxial growth of 3C-SiC requires a lattice-matched, perfect substrate. One solution to this problem is to use step-free mesas of 4H-SiC as 3C-SiC lattice templates [1]. In this work, we describe the formation of large area step-free mesas and the subsequent nucleation and growth of 3C-SiC layers. A powerful array of tools were utilized to characterize the properties of these layers including Nomarski microscopy, secondary electron microscopy (SEM), atomic force microscopy and X-Ray diffractometry. Micro-photoluminescence (µ-PL) was employed to investigate the presence of electronic defects and identification of polytype, enabling us to obtain information about the structural and electronic properties on a micron-sized length scale.

On-axis 4H-SiC substrates were initially patterned and reactive ion etched to produce hexagonal and square shaped mesas with varying widths ranging from 40 to 400 µm (400% greater area than previous reports), and heights from 2 to 5 µm. Homoepitaxial layers were grown on the mesas in an Aixtron VP508 horizontal hot-wall chemical vapor deposition reactor using the standard chemistry of silane and propane in order to grow out the steps on the mesas. The homoepitaxial layers were terminated at the mesa step edge and further growth is prohibited. The films were grown at 2µm/hr and the pressure and temperature were 100 mbar and 1580°C, respectively. The yields of 200 µm width 4H-SiC step-free mesas was ~95%. Heteroepitaxial 3C-SiC was grown 2 µm thick on the homoepitaxy (4H-SiC) by means of reducing the growth temperature to 1450°C, while maintaining 100mbar.

Under Nomarski evaluation, ~ 18% of the 200 µm wide 3C-SiC mesas appeared to be step-free. Micro-PL maps were used to confirm the presence of 3C-SiC, where uniform 3C-SiC was detected across the entire mesas. X-ray rocking curves also indicated 3C-SiC, with the FWHM of the SiC (111) being ~21", indicating good quality material.

Yield maps for the 200 and 400 µm mesas will be presented. In addition, real color PL imaging will be used to determine the types of defects within the mesas which displayed lower PL intensity regions of 3C-SiC. Lastly, initial results of Schottky rectifiers performance made on the layers will given.

References

[1] J. A. Powell, et al., Appl. Phys. Lett. 77, 1449 (2009).

The III-Nitride material system has demonstrated its potential for a broad range of optoelectronic and electronic applications. However there are no reports of efficient III-Nitride tunnel junctions due to the large band gaps in this material system. In this work, we show that with unique properties such as the polarization, tunneling can be enhanced using band bending over smaller distances in nitride heterostructures, leading to record reverse and forward tunneling current density for the III-nitride material system.

We have designed and demonstrated GaN/InGaN/GaN tunnel junction with a record high current density of 118 A/cm² at a reverse bias of 1 V by utilizing a 6.4 nm thin In_0.33Ga_0.67N barrier material. N-polar p-GaN/In_0.33Ga_0.67N/n-GaN heterostructure designed for tunneling close to zero bias was grown by plasma assisted molecular beam epitaxy by choosing the critical thickness of InGaN barrier appropriately. The tunnel junction sample shows five orders of magnitude higher current at a reverse bias of 1 V as compared to a standard p+/n+ GaN sample indicating efficient tunneling across the InGaN barrier. The tunneling turn-on close to zero bias, and maximum current density of 9.1 kA/cm² achieved in this work demonstrates the potential of polarization-engineered tunnel junctions.

Two distinct regimes of transport are identified based on the temperature dependent I-V measurements. At lower reverse bias, defect assisted tunneling with strong temperature dependence is found to dominate. In this regime, a plot of ln (J/E) vs E^1/2 shows a linear behavior suggesting a Frenkel- Poole emission mechanism due to the high field in the InGaN quantum well. A direct band to band tunneling regime resulting in weak temperature dependence that arises from band gap variation with temperature is observed from a reverse bias of 1 V. A decrease in current density is observed with increase in temperature in the range of 77- 150 K and this can be attributed to the presence of band tail states which has been observed previously in In face InGaN.

We discuss the design of these quantum well tunnel junctions. Athough higher indium compositions yield higher band to band tunneling probability, calculations using a simplified Kane model reveal that the wider depletion region in n GaN due to higher band offset considerably reduces the net tunneling probability. Calculations also reveal the need for very high doping in the n GaN layer so as to minimize the depletion region thickness in order to achieve very high current densities in such polarization charge assisted tunnel junctions. These calculations can guide future tunnel junctions with better performance characteristics.

The development of organic electronics calls for new tools to study organic thin films. By measuring the frequency noise experienced by a cantilever near a surface, we are able to microscopically probe organic materials. In previous work, we used an Atomic Force Microscope to measure frequency noise due to dielectric fluctuations as a function of cantilever height and voltage over a thin film of polyvinyl acetate. In parallel, we have developed a zero-free parameter linear-response theory of thermally induced dielectric fluctuations that successfully describes our observations.¹

Having understood dielectric fluctuations, we are now investigating fluctuations induced by carrier motion in polymeric semiconductors. Charge transport in these devices is not well understood. Previous work indicates that the ratio of diffusion constant to mobility in these materials violates what is predicted by the Einstein relation. In addition, there is an ongoing controversy on the charge density and electric field dependence of mobility. While the correlated-disorder model correctly predicts the electric field dependence of mobility, models that predict a density dependence of mobility rely on uncorrelated site-to-site energies.

A resolution of these controversies calls for new tools to study carrier motion in organic semiconductors. By measuring the frequency fluctuations experienced by a cantilever near a surface, we are able to microscopically probe carrier motion in organic materials. Comparing our observations over a poly(3-hexylthiophene) transistor to a calculation based on free diffusion, we find that while theory overestimates the observed fluctuations, it predicts the correct spectral shape and distance dependence of the fluctuations. Even at high gate bias, the observed cantilever frequency fluctuations differ from what we expect based on free diffusion and on the measured carrier mobility. This discrepancy indicates a breakdown of the Einstein relation. Further we present a number of different charge hopping models. We find that the predicted cantilever frequency noise is very sensitive to the details of the model, indicating that frequency noise spectra are a vital tool for selecting appropriate charge transport models.

[1] Nikolas Hoepker, Swapna Lekkala, Roger F. Loring, John A. Marohn (manuscript in preparation). Quantifying Dielectric Fluctuations over Polymer Films Using an Atomic Force Microscope.

Engineering molecular superstructures on metals opens great possibilities for the control and exploration of complex nanosystems for technological applications. Of particular interest is the use of chiral molecules, such as alanine, to build self-assembled nanoscale structures for the trapping of the two-dimensional free electron gas of a metal. In the present work, molecules of D- or L-alanine were deposited on Cu(111). Scanning tunneling microscopy and spectroscopy revealed the formation of a uniform network of hexagonal pores of average diameter ~1.2 nm. Each pore acts as a quantum corral by confining the two-dimensional electron gas of the Cu(111) surface state. Furthermore, excess alanine molecules were trapped at the inner perimeter of the hexagonal pore, and were observed as rotating or immobile spatial states. This study demonstrates the engineering of one of the smallest quantum confined structure, and the dynamics of molecular motion within these potential wells.

Acknowledgements: This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357