Albrecht Fischer

Raman spectroscopy—A versatile tool for characterization of thin films and heterostructures of GaAs and AlxGa1−xAs

It is shown that Raman spectroscopy can provide useful information on characteristic properties of thin crystalline films of compound semiconductors. Crystal orientation, carrier concentration, scattering times of charge carriers, composition of mixed crystals and depth profiles can be studied in thin layers and heterostructures of GaAs and AlxGa1−xAs. The advantages and disadvantages of Raman scattering compared to conventional characterization methods are discussed.

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED, SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and AlxGa1−xAs films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and AlxGa1−xAs by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.

GaAs Substrate Preparation for Oval-Defect Elimination during MBE Growth

We have developed a new method for GaAs substrate preparation which significantly reduces the formation of oval defects during MBE growth of selectively doped n-AlxGa1-xAs/GaAs heterostructures. The method simply requires treatment in H2SO4 after mechano-chemical polishing in NaOCl solution and generation of a protective surface oxide during In soldering. Routinely a density of oval defects of less than 200 cm-2 is achieved for 2 µm thick heterostructures. The efficiency of the new preparation procedure is demonstrated by 2DEG mobilities in excess of 106 cm2/(Vs) at 6 K obtained with a spacer width as narrow as 18 nm.

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦Ts≦600[°C]) practicable forn-type layer growth. Using a constant As4 to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingTs. Growth at 500° ≦Ts≦600[°C] produces high-qualityn-GaAs: Ge films (ND/NA≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [GeAs] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [CAs] decreases thus leading to the well-establishednonlinear behaviour of the (NA/ND vs. 1/Ts plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([GeAs]/[GeGa]) vs. 1/Ts plot which has exactly the same slope as the (PAs2/PGa) vs. 1/Ts plot derived from vapour pressure data of As2 and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.

Improvedp/n junctions in Ge-doped GaAs grown by molecular beam epitaxy

Abstractp/n junctions in Ge-doped GaAs consisting ofp-type layers with extremely smooth surfaces and low compensationn-type layers were fabricated by molecular beam epitaxy (MBE). During growth the site occupancy of Ge was controlled by varying the substrate temperature fromTs<500°C (n-type layers) toTs>600°C (p-type layers) at aconstant As4 to Ga flux ratio of 2. This yields stable growth conditions for the generation of Ga and As vacancies, respectively.

High-mobility two-dimensional electron Gas from delta-doped asymmetric AlxGa1−xAs/GaAs/AlyGa1−yAs quantum wells

A new delta (δ)-doped asymmetric AlxGa1-xAs/GaAs/AlyGa1-yAs quantum well structure is designed and grown by molecular beam epitaxy which provides a high-mobility two-dimensional electron gas with µ = 3.2×105 cm2/Vs at 5 K. The key feature of the new structure is that no bound states for electrons are found in the asymmetric well if the width is smaller than the critical value. The undesired persistent photoconductivity effect due to deep donors in n-type AlxGa1-xAs: Si does not exist in the structure. Important for high transconductances in field-effect transistor is the narrow spacing between the two-dimensional electron gas and the crystal surface.

High‐throughput high‐yield fabrication of selectively doped AlxGa1−xAs/GaAs heterostructures by molecular beam epitaxy

A new etching procedure for GaAs substrates and a modified configuration of selectively doped n‐AlxGa1−xAs/GaAs heterostructures have been developed for molecular beam epitaxy to reduce the density of oval defects and the epitaxial growth time. The new simplified substrate preparation method reduces the oval defect density to less than 100 cm−2 and allows storage of prepared substrates in air under dust‐free conditions for several weeks without any degradation. Modification of the layer structure reduces the throughput time to less than 30 min per wafer including epitaxial growth, wafer exchange, heat and cool times. High mobilities of the two‐dimensional electron gas and excellent reproducibility are achieved. The new process uses the favorable growth rate of <1 μm/h for GaAs and thus allows growth of nearly 500 high‐quality heterostructure wafers for device fabrication without exposure of the growth chamber to atmosphere for effusion cell recharging.

Electric-Field-Enhanced Extrinsic Photoluminescence in AlGaAs-GaAs Single-Quantum Wells

The low-temperature photoluminescence of a 5 nm AlxGa1-xAs-GaAs single-quantum well in an electric field normal to the quantum-well plane has been investigated. When only the GaAs quantum well is photoexcited, the luminescence exhibits a low-energy shift under both reverse and forward bias. This low-energy shift is accompanied by a line broadening. The experimental results are explained by two competing radiative recombination processes, i.e. intrinsic free-exciton recombination and recombination between n=1 electrons and neutral acceptor-like centers near the heterojunction interfeces. The application of an electric field displaces the hole distribution in the quentum well more strongly than the electron distribution and thus enhances the recombination process, including the acceptor-like centers near the hetero-junction interfaces.

Electrical Properties of GaAs Overgrown by Molecular Beam Epitaxy on Gallium Ion Implanted Substrates

We describe the electrical properties of GaAs overgrown by molecular beam epitaxy on 100 kV Ga+ ion implanted substrates. Room-temperature carrier mobilities of the overgrown layers maintain good values for Ga doses up to 1014/cm2. With increasing Ga dose, the conductances of the overgrown layers shows a significant reduction, which is attributed to carrier depletion caused by ionized deep acceptors in the Ga-implanted layer. This effect can be almost entirely screened out when GaAs layers are overgrown directly on Ga-implanted semi-insulating substrates.