VORRICHTUNG ZUR KONTINUIERLICHEN MESSUNG DER DICKE

In a device for the continuous measurement of thickness of a material deposit produced on a bearer material by means of a coating machine, the deposit thickness on the measuring electrode (9) of a reference probe (50) is determined electrically by detecting the alternating current signal on the measuring electrode (9). The measuring electrode (9) is located between two metal housings (1) provided with inlet openings (4, 5) and is applied with opposite alternating currents shifted in phase by 180 degrees, the amplitudes of said alternating currents being adjusted at the start of measurement such that no signal occurs at the measuring electrode (9). The material deposit produced on the measuring electrode (9) located in the vicinity during the coating of a bearer material causes an offset of the electric field in the reference probe (50) and thus a measuring current in the measuring electrode (9) connected to an evaluation circuit

GaAs bipolar transistors with a Ga0.5In0.5P hole barrier layer and carbon-doped base grown by MOVPE.

GaAs bipolar transistors with a 50-A-thick lattice-matched Gasub0.5Insub0.5P layer between emitter and base acting as a hole repelling potential barrier in the valence band have been fabricated from films grown by metalorganic vapor phase epitaxy (MOVPE). The 1000-A-thick base of these transistor structures was doped with carbon to 2 x 10high19 cmhighminus3 resulting in a base sheet resistance of 250 Omega. Carbon has been chosen because of its low diffusivity. Using the barrier layer as an etch stop we fabricated mesa-type broad-area devices. The output characteristics of the devices is ideal with very small offset voltages and infinite Early voltages. Common emitter current gains of up to 70 at 10high4 A/square centimetre collector current density have been obtained. The current gain is clearly higher than the one calculated for a bipolar junction transistor with the same doping profile because the base-emitter hole current is suppressed by the Gasub0.5Insub0.5P potential barrier in the valence band. It was found that the electron injection across the emitter-base junction is not affected because of the small band offset in the conduction band

High performance GaInP/GaAs hole barrier bipolar transistors -HBBTs-

The HBBT may be understood as an advanced variant of the well-known heterojunction bipolar transistor. A thin GaInP film (10-20 nm) embedded between emitter and base layer acts as an efficient hole barrier. Combined with several inherent advantages of GaInP the HBBT leads to an attractive alternative to the conventional AlGaAs/GaAs HBT. We report on HBBT devices with different layer sequences and geometries. HBBT devices exhibit almost constant current gain over a wide range of collector current. Maximum oscillation frequencies above 100 GHz are achieved. Applications in the microwave frequency range will be discussed. GaInP/GaAs HBBTs are well described by a T-like small signal equivalent circuit including bias and geometry scaling. A direct parameter extraction method and first load-pull simulations are reported

A novel GaAs bipolar transistor structure with GaInP-hole injection blocking barrier.

GaAs bipolar transistors of different emitter types have been fabricated from MOCVD grown lattice matched Gasub0.5Insub0.5P/GaAs layer structures using carbon for heavy base doping (p equals 2x10high19 cmhighminus3). Besides conventional heterojunction bipolar transistors we also investigated tunneling emitter bipolar transistors having 2 and 5 nm thin GaInP layers between emitter and base, which act as a hole repelling potential barrier in the valence band. Current gains up to 115 have been obtained at collector current densities of 10high4 A/qcm even for this heavy base doping. All devices show an almost ideal output characteristics with large Early voltage and small offset voltage. From the temperature dependence of the collector current a small effective conduction band barrier at the heterointerface is determined which hardly affects electron injection into the base

AlGaInP/GaInAs/GaAs MODFET devices - candidates for optoelectronic integrated circuits

Modulation doped field effect transistors (MODFETs) with a phigh+-GaAs gate structure were fabricated from AlGaInP/GaInAs/GaAs heterostructures for the first time. Self-aligned devices with a gate length of 0.7 mym exhibit drain currents of 330 mA mmhigh-1, transconductances of 200 mS mmhigh-1 and very small gate currents below 50 nA even at gate-source voltages of - 5 V. Furthermore, large gate-drain diode breakdown voltages in excess of 30 V are observed. Short gatelength devices (0.3 mym) show a good r.f. performance with a current gain cut-off frequency of 60 GHz and a maximum frequency of oscillation of 140 GHz

OMVPE-grown AlxGa1-x0.5In0.5P/InGaAs MODFET structures - growth procedure and hall properties.

Modulation doped (AlsubxGasub1minussubx)sub0.5 Insub0.5P/InsubyGasub1minussubyAS/GaAs single quantum well structures have been grown by low pressure metal organic vapour phase epitaxy and characterized by Hall measurements and cathodoluminescence. The high conduction band offset in this structure is favourable for the fabrication of MODFET structures with high two-dimensional electron gas densities in the InsubyGasub1minusyAs channel. Electron sheet densities up to 2.75x10high12 cmminus2 have been obtained for x is equal 0.4 and y is equal 0.3 with appropriate doping. The structures have excellent Hall properties at room temperature and at 77 K. The critical interface between obtained for x equal 0.4 and y equal 0.3 with appropriate doping. The structures have excellent Hall properties at room temperature and at 77 K. The critical interface between the InGaAs channel layer and and the AlGaInP barrier layer has been improved by placing a thing AlGaAs interlayer (1.8 nm thick) between th ese two layers. This arrangement provides an abrupt interface between quantum well and interlayer and suppresses the interaction of the two-dimensional electron gas in the quantum well with the non-ideal interface between the interlayer and the AlGaInP barrier layer. Cathodoluminescence studies revealed that the mechanism of strain relaxation in these structures is distinctly different from the mechanism observed in AlGaAs/InGaAs/GaAs heterostuctures grown by molecular beam epitaxy

Growth of thick films CdTe from the vapor phase

CdTe films with a thickness of 100 µm were grown by molecular beam epitaxy (MBE) on semi-insulating LEC GaAs (0 0 1) substrates. The films were characterized by X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), I/V characteristics and detector measurements using a 241Am source. The grown layers were highly oriented as revealed from X-ray pole figure measurements and high-resolution XRD experiments. Low temperature PL measurements showed the formation of A-center complexes including Ga and In acting as donors. Furthermore, the distribution of these impurities in the films was determined by spatially resolved PL measurements. XPS demonstrates the formation of a TeO2 overlayer due to Cd depletion at the surface of the films. The electrical measurements including I/V characteristics and detector measurements resulted in resistivity values of around 3×10(exp 8) ohm cm and a µt product of electrons of 10(exp -5) cm2/V

Photoluminescence microscopy investigation of lattice defects in epitaxial heterostructures

Spectrally selective photoluminescence microscopy has been used to investigate non-radiative defects and crystal imperfections in lattice-matched AlGaAs/GaAs quantum well structures grown by metalorganic vapour phase epitaxy and molecular beam epitaxy. Defects are selectively imaged in different epilayers of the heterostructures, using the compositional dependence of the luminescence wavelengths LambdasubPL. The correlation of LambdasubPL with individual layers allows one to analyse generation, propagation and blocking of defects during epitaxial growth, thereby discriminating growth-induced defects against imperfections caused by substrate imperfections. The propagation and gradual modification of defects from the bottom to the top of a sequence of layers are clearly observed. The influence of GaAs buffer layers, short period superlattices and AlGaAs barrier layers on the quality of subsequently deposited layers will be demonstrated