Stefan Nemeth

Growth and characterization of low-temperature grown GaN with high Fe doping

We succeeded in growing highly Fe-doped GaN films by solid-source molecular beam epitaxy using an electron-cyclotron-resonance microwave nitrogen plasma. The substrate temperature was in the range of 380–520 °C. The samples were analyzed by x-ray diffraction and transmission electron microscopy, and showed hexagonal (wurtzite) or cubic (zincblende) structure or a mixture of both phases. The Fe concentration was on the order of 1019 cm−3 and extended x-ray absorption fine structure data show that the Fe is substituting the Ga in GaN. The magnetization measurements as a function of temperature reveal ferromagnetic properties below 100 K for the sample grown at the lowest temperature.

Extended wavelength InGaAs on GaAs using InAlAs buffer for back-side-illuminated short-wave infrared detectors

We conducted an experimental study of back-side-illuminated InGaAs photodiodes grown on GaAs and sensitive in the short-wave infrared up to 2.4 μm. Standard metamorphic InGaAs or IR-transparent InAlAs buffers were grown by molecular-beam epitaxy. We studied dark current and photocurrent as a function of buffer thickness, buffer material, and temperature. A saturation of the dark current with buffer thickness was not observed. The maximum resistance area product was ∼10 Ω cm2 at 295 K. The dark current above 200 K was dominated by generation–recombination current. A pronounced dependence of the photocurrent on the buffer thickness was observed. The peak external quantum efficiency was 46% (at 1.6 μm) without antireflective coating.

Molecular beam epitaxy and characterization of heterostructures for magnetic sensing applications

quantum well structures of high quality were grown by molecular beam epitaxy. The structural and electrical quality was characterized by x-ray diffractometry, Raman spectroscopy and Hall transport measurements. When an optimized buffer layer was used on a GaAs substrate, electron mobilities of at 300 K and at 20 K were routinely achieved for undoped structures. These excellent transport properties were utilized in a sensitive magnetoresistive sensor.

Compositional dependence of AlAsySb1−y ternaries on the ratio of Sb/As fluxes and on the substrate temperature

We present a systematic study of the compositional dependence of AlAsySb1−y layers, grown by molecular beam epitaxy, on the ratio of As to Sb fluxes and on the substrate temperature. The initial results clearly demonstrate that variations in the composition can be observed by changing the above‐mentioned parameters. The emphasis of this communication is on understanding the qualitative trends of these dependencies.

Fluorescence x-ray absorption fine structure study on local structures around Fe atoms heavily doped in GaN by low-temperature molecular-beam epitaxy

A local structural transition in heavily Fe-doped GaN films related to the magnetic properties has been revealed by fluorescence x-ray absorption fine structure (XAFS) analysis. The structural transition is explained (or considered to be induced) by the change in the degree of hybridization between Fe 3d and N 2p states, which can be evaluated by x-ray absorption near edge structure spectra. The XAFS analysis indicates that the present diluted magnetic semiconductor based on GaN can be fabricated by electron cyclotron resonance microwave plasma-assisted low-temperature molecular-beam epitaxy.

Molecular Beam Epitaxial Growth of Bulk AlAs0.16Sb0.84 and AlAs0.16Sb0.84/InAs Superlattices on Lattice-Matched InAs Substrates

AlAs0.16Sb0.84 ternaries and AlAs0.16Sb0.84/InAs superlattices have been grown by molecular beam epitaxy lattice-matched on InAs substrates. The compositional dependence of AlAsy Sb1-y on the ratio of Sb4 to As2 fluxes as well as on the substrate temperature has been investigated and can be explained qualitatively. The growth of AlAs0.16Sb0.84/InAs superlattices is described and illustrated by the results of reflection high-energy electron diffraction measurements. The crystalline quality of the superlattices is demonstrated by means of transmission electron microscopy and X-ray diffraction measurements.

Growth and tunneling spectroscopy study of Fe/(GaAs, AlAs)/Ga1-xMnxAs ferromagnet/semiconductor heterostructures

Abstract Fully expitaxial tunneling structure Fe/Semiconductor/(Ga,Mn)As was grown and investigated. With semi-insulating (Ga,Mn)As as electrode, strong temperature dependent structure was observed in I–V characteristic. The electronic band structure in the contact region is discussed and the result is explained as inelastic tunneling via impurity states.

InAs/(GaIn)Sb superlattices for IR optoelectronics: Strain optimization by controlled interface formation

Abstract The structural properties of InAs/(GaIn)Sb and (InGa)As/GaSb superlattices (SLs), grown by solid-source molecular-beam epitaxy on (0 0 1) GaAs substrates using a strain relaxed GaSb or InAs buffer layer or directly on (0 0 1) InAs substrates, were analyzed by high-resolution X-ray diffraction and Raman spectroscopy. The residual strain within the SL was found to depend critically on the type of interface bonds, which can be either InSb- or GaAs-like. Thus, to achieve lattice matching to the buffer layer or substrate by strain compensation within the SL stack, the controlled formation of the interface bonds is vital. On the other hand, minimization of the residual strain is shown to be a prerequisite for achieving a high photoluminescence yield and high responsivities for InAs/(GaIn)Sb SL based IR detectors.

Extended-wavelength InGaAs on GaAs hybrid image sensors

Short wavelength infrared (SWIR) photovoltaic diode structures made of InGaAs material were grown on GaAs by means of molecular beam epitaxy. Growth quality and composition of the layers are determined by HRXRD. The electrical characterization is performed by Current-Bias characterization (proposal) and spectral resolved measurements to determine the resistance area product (R0A) and the spectral responsivity (R) of diodes. The processing is performed with standard photolithography and micro-structuring techniques aiming at the production of 1D and 2D infrared camera arrays. The diced IR sensor is flip chip assembled on a Silicon read out integrated circuit (ROIC). Linear arrays of 256 pixels with 25 μm pitch were fabricated as well as focal plane arrays (FPA) of 256 × 320 pixel with 30 μm pitch. Measures of electrical interconnection yield will be shown. Functionality is proven for different applications up to 2.5 μm wavelength.

InGaAs on GaAs extended wavelength linear detector arrays

We report on In(80%)GaAs line scan sensors with 128 pixels on 50 micrometers pitch for use as thermo-electrically-cooled spectroscopic sensor in the short-wave infrared (1 - 2.5 micrometers ).