H.-J. Buhlmann

Selective GaN Epitaxy on Si(111) Substrates Using Porous Aluminum Oxide Buffer Layers

Epitaxial GaN layers were grown by metallorganic vapor phase epitaxy on Si(111) substrates by using aluminum oxide (AlOx) buffer layers. The AlOx was obtained by anodic oxidation of an evaporated Al film. High-resolution X-ray diffraction and room temperature photoluminescence measurements were used to investigate the grown GaN layers. Epi-growth of GaN on the AlOx stripes was shown to be possible, while no growth took place on the neighboring Si surface, hereby demonstrating perfect selectivity.

Epitaxy of Gallium Nitride by HVPE Using Low Temperature Intermediate Buffer Layers Deposited by MOVPE

We investigated the use of different GaN and AlN buffer layers pre-deposited at low temperature (LT) for the growth of thick GaN layers by Hydride Vapor Phase Epitaxy (HVPE). The layers obtained on LT buffer layers grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) show an excellent homogeneity, good crystalline quality (X-Ray Diffraction (XRD) Full Width at Half Maximum (FWHM) values around 650 arcsec) and the surface morphology (characterized by optical microscopy and atomic force microscopy measurements) is significantly improved compared to layers grown directly on sapphire. The partial replacement of the carrier gas nitrogen by hydrogen leads to a further improvement of the surface morphology and crystalline quality, reaching best XRD FWHM values of 350 arcsec. In contrast, experiments using an amorphous LT-HVPE buffer layer grown in situ show rather poor results.

Influence of the device-width on the accuracy of quantization in the integer quantum Hall effect

Using a cryogenic current comparator bridge, several Hall bar samples of different widths were compared to check whether size effects may deteriorate the accuracy of quantization in the integer quantum Hall effect. No width dependence of the quantized Hall resistance was observed in the range from 1 mm down to 10 /spl mu/m. The difference between the quantized Hall resistances of the i=2 and i=4 plateaus of the various samples was found to be smaller than the measurement uncertainty which is typically less than 1 part in 10/sup 9/. >

Properties of alloyed AuGeNi-contacts on GaAs/Ga/AlAs-heterostructures

The technique of contacting GaAs/AlGaAs heterostructures using an evaporated AuGeNi alloy has been refined with emphasis on optimum contact properties in the quantum Hall regime. The resistance of the electrical contacts to two-dimensional electron gas has been measured in various heterostructures with two different methods. The results of the first method using a transmission line and the second one using the quantum Hall effect as well as the good time and thermal cycling stabilities obtained demonstrate the high quality of the contacts. The contact resistance in the quantum Hall regime is found to be lower than 1 Omega . >

In-situ monitoring of GaN growth by hydride vapor phase epitaxy

In-situ reflectivity measurements of the growth surface during deposition in a Hydride Vapor Phase Epitaxy system are presented. The GaN growth rate increases linearly with the HCl flow and increases monotonically with the ammonia flow. Following the replacement of the carrier gas nitrogen by hydrogen, the growth rate initially increases, passes through a maximum at a H-2 concentration of 0.15 and then decreases.

Comparison of the quantized hall resistance in different GaAs/Al/sub x/Ga/sub 1-x/As heterostructures

GaAs/AlGaAs heterostructures with mobilities varying between 15 and 50 T/sup -1/ have been successfully fabricated and contacted. The quantized Hall resistances of seven different samples have been compared with an uncertainty of less than 1 part in 10/sup 8/ using a potentiometric measurement system. Preliminary results for the relative differences of the QHR measured on three different GaAs samples are shown. No sample dependence has been found at a precision level of 1 part in 10/sup 8/. >

The quantum Hall effect as a standard to define the laboratory unit of resistance

A measurement system has been developed to determine the resistance of integer quantum Hall plateaux relative to a room-temperature reference resistor network of nominally the same values. Silicon MOSFET and GaAs-AlGaAs samples have been successfully fabricated and measured. The results confirm that the integer quantum Hall effect (QHE) may be used to monitor a group of standard resistors comprising the laboratory unit of resistance. They demonstrate the feasibility of defining the laboratory unit of resistance by adopting a value for the quantum Hall resistance h/e2.

A.C. measurements of edgeless currents in a Corbino ring in the quantum Hall regime

Abstract Measurements have been made of radial currents in a two dimensional electron gas in the quantum Hall regime using a Corbino ring geometry. A small solenoid is used to modulate the flux linking a high mobility GaAsAlxGa1−xAs Corbino ring, thereby inducing an alternating radial current flow. Plateaus of current value are observed giving a clear indication that quantization in this geometry occurs for bulk, opposed to edge, charge transport. We find that the Corbino ring behaves as a quantised a.c. current source over a wide range of loading resistance values and several decades of frequency. The internal impedance of that current source is much larger than the Hall resistance and is dominated by the internal capacitance of the ring.

Investigation of DX centers in modulation-doped field-effect transistor-type Al 0.3 Ga 0.7 As/GaAs heterostructures using a Fourier-transform deep level transient spectroscopy system

Al0.3Ga0.7As:Si/GaAs modulation-doped field-effect transistor-type heterostructures were grown using two different growth temperatures (500 and 620°C) and three doping modes (δ-doping, pulse-doping, and uniform-doping). Deep level transient spectroscopy (DLTS) measurements were performed on these structures using a new Fourier-analysis method. Up to four DLTS peaks, related to the different possible configurations of the nearest Al and Ga neighbors around each DX site, were observed. Both the growth temperature and the doping-mode are found to affect the DLTS spectra, in particular the number of observed peaks and their width. These results are interpreted in terms of the different mobilities of the Si doping atoms on the surface during growth.

Influence of infrared illumination on precision measurements of the integer quantum Hall effect

The preliminary result reported here unambiguously shows that infrared illumination of a nonfully quantized QHE sample can restore its ideal behaviour. Differences as large as 0.1 ppm in the Hall resistance have disappeared when the sample is illuminated by 150 ms pulses of infrared light. In parallel, the contact resistances of nonideal terminals have been found to decrease with the illumination time, although some high resistance contacts did not show any dependence on illumination. In addition, the illumination leads to an increase of the critical current, although the way this increase is taking place seems to be nonuniversal.