Janos Balazs

Tunability of the optical band edge in thin PbS films chemically deposited on GaAs(100)

The optical properties of chemical-solution-deposited thin films of lead sulfide (PbS) were investigated using infrared transmission and photoluminescence spectroscopies. The synthesized films are characterized by a wide range of microstructures, from 15 nm nanocrystals up to monocrystals. Energy bandgap values for bulk and nanostructured layers varied from 0.41 eV up to 0.48 eV, respectively. Blueshifts in both absorbance and emission peaks of the nanostructured layers were obtained due to quantum size effects. The optical properties of the films are shown to be size-dependent, with the band edge increasing with temperature.

Spectral characteristics of InGaAsP/InP infrared emitting diodes grown by LPE

InGaAsP/InP double heterostructure infrared emitting diodes were grown by liquid phase epitaxy. Eleven different diodes were fabricated with optimal spacing of their peak emission wavelengths in order to have sufficient overlapping of their spectra. The spectral characteristics of these different wavelength InGaAsP/InP infrared emitting diodes were studied systematically. The spectral bandwidth of the LEDs was usually broader than theoretically predicted. The broadening of the emission spectra of the LEDs on the long wavelength side can be associated with nonuniform active layer composition. Investigation of the cross-section of the LED structures showed that their active layer had a thin layer at the InP interface with shifted composition. The band gap difference between the bulk and the transition layer was found to be in correlation with the long wavelength side broadening of the emission spectra of the corresponding LEDs.

Influence of LPE growth conditions on the electroluminescence properties of InP/InGaAs(P) infrared emitting diodes

Abstract Liquid phase epitaxial growth of InP/InGaAs(P) infrared emitting diodes was studied systematically. Small area surface emitting LED chips were prepared to cover completely the 1100–1700 nm wavelength ranges. Nine different diodes were fabricated with optimal spacing of the peak emission wavelengths in order to have sufficient overlapping of their spectra. Efficient LEDs with narrow spectra have been realised by careful selection of the layer structure and growth conditions. Thick active layers with constant composition and abrupt interfaces have been grown for each emission wavelength. The long wavelength diodes (1275–1675 nm) were grown with additional quaternary layer(s) to prevent the melt-back of the active layer by InP melt. Low growth temperature (590°C) was used to prepare the LED structures. In the spectral range of 1250–1520 nm higher growth temperatures (625–645°C) were necessary to improve the device parameters. Such phenomena confirm the existence of the miscibility gap in the InGaAsP quaternary crystal system. Quaternary layers grown in the middle of the immiscibile region showed non-uniform composition and ragged interfaces at the upper heterojunction.

Liquid phase epitaxy growth and characterization of Ga1-xInxAsySb1-y quaternary alloys

Band gap, solid phase composition and lattice mismatch data are presented characterising Ga 1-x In x As y Sb 1-y layers grown by liquid phase epitaxy (LPE) on oriented GaSb substrate. Nearly lattice-matched (|Δa/a < 0.15%) Ga 1-x In x As y Sb 1-y layers were grown over the range 0 < x < 0.20. The lattice-mismatch was estimated from X-ray diffraction measurements. The X-ray diffraction pattern of the lowest band gap (0.55 eV) Ga 0.80 In 0.20 As 0.17 Sb 0.83 layer showed strong compositional grading, indicating the presence of a miscibility gap near this composition in this material system. The composition of the grown epitaxial layers was determined by electron probe microanalysis in the wavelength dispersive mode. Band gap energies down to 0.55 eV were obtained on different composition samples from infrared transmission measurements. The measured compositional dependence of the band gap exhibits smaller bowing than calculated using the correlated function expansion technique.

Noise characteristics of bootstrapped photovoltaic and photoconductive detectors

If linearity of a semiconductor photodetector is a critical issue in an application like in radiometry, spectrophotometry, etc. the detectors are used in the so called current measurement mode. In this mode the detector is directly connected to a current-to-voltage converter, consequently in an ideal situation all the photogenerated charge carriers appear on the feedback resistor of the converter and produce a voltage drop. Photovoltaic detectors are short circuited by the converter, the built in electric field of the detector transfers the generated carriers to the converter. On the other hand photoconductive detectors should be biased externally by a constant voltage to produce an internal field for the transport of the charge carriers. Some applications need great area detectors, that especially in the IR wavelength region show low shunt resistance values. Ge, InAs, InSb, PbS, PbSe, HgCdTe, etc., have many decades lower shunt resistance values than that of a Si or InGaAs detector operating in the visible or near infrared region. If high sensitivity is needed than a high value of the feedback resistor is required that decreases the closed loop gain, consequently the settling accuracy. Bootstrapping of the detectors virtually increases the shunt resistance and so restores the decreased closed loop gain and settling accuracy. The bootstrapping technique will be shown and the noise characteristics will be analyzed and compared to the traditional solutions.

Surface-State Density Evaluation Problems in MNOS Structures

The capacitance-voltage investigations are of peculiar importance in the study of interface properties in MIS structures. In our research work on MNOS memory structures we investigated the quasi-static and high-frequency C-V characteristics of MNOS capacitors and found that the usual evaluation methods give contradictory results.

Photoluminescence investigation of GaAs quantum nanostructures

We have studied the optical properties of droplet epitaxial quantum dots and quantum rings in the GaAs/GaAlAs system. Photoluminescence spectra measurements were performed in the temperature range from 4 to 300 K. The differences between the observed spectra and the electronic structure of quantum dots and quantum rings are briefly discussed.

Electrical and photoelectrical behaviour of CdTe structures

Abstract The electrical and photoelectrical behaviour of Au/n-CdTe junctions prepared on CdTe monocrystalline substrates and CdTe epitaxial layers grown on n + GaAs substrates were studied. The electrical and photoresponse properties depended very strongly on the parameters of the compensated high-resistive layer at the CdTe surface formed by annealing during preparation.

Electrical and photoelectrical behavior of Au/n-CdTe junctions

The electrical and photo electrical behavior of Au/n-CdTe junctions prepared no CdTe mono crystalline substrates were studied. Both properties depended strongly on the parameters of the compensated high-resistive layer at the CdTe surface formed by annealing in air during preparation.

Characterization of the van Cittert monochromator-based IR spectrophotometer

The van Cittert system consists of two identical monochromators coupled together as a subtractive system. The usual narrow slit between the two monochromators is replaced by a wide one and a part of this wide slit is covered by a lamella mounted on a fine resolution translation stage. The wide slit transmits a wide spectral range but the lamella filters out a narrow range like a notch filter. The second monochromator collects and recombines the transmitted radiation at its exit slit. Behind the exit slit and the beam forming elements can the sample be positioned. By the translation of the lamella the center wavelength of the notch can be shifted over the whole spectral range covered by the wide slit. Carrying out spectrophotometric measurements the total transmitted power is measured at every stop of the lamella. From this array of measurement results the transmittance or the reflectance of the sample can be calculated as a function of the wavelength.