János Makai

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.

Realisation of quantitative Makyoh topography using a digital micromirror device

Makyoh topography (MT) is an optical characterisation tool for flatness testing of mirror-like surfaces. In MT, the surface is illuminated by a collimated light beam, and the reflected image is detected on a screen placed some distance away from the sample. Because of the focussing/defocussing action of the surface undulaations, the image shows intensity variations related to the sample morphology. In its original form, MT is qualitative only. By inserting a structured mask (e.g., a grid) into the path of the illuminating beam, the surface topography can be calculated by the integration of the gradients obtained by the determination of the displacements of the grid node positions, compared to a reference flat, similarly to a wavefront sensor. A DMD provides an easy and verstile way of realisation of such a structured mask. In this paper, we report on a quantitative MT set-up using a programmed DMD. Possibilities of the realisation of different mask patterns are analysed. The results are compared to interferometry.

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.

Enhancement of the low level detection limit of radiometric quality photovoltaic and photoconductive detectors

When a current-to-voltage (I/V) converter is used with a photodetector, the ratio of the photodetector impedance (shunt impedance) to the input impedance of the detection circuit defines the ratio of the photogenerated current that will produce the output voltage. Ideally, the input impedance should be negligible in comparison with the shunt impedance as only in this case will the whole photogenerated current produce the output signal. With high quality, high band gap detectors in most of the cases this requirement is automatically met, but if lower band gap detectors are used this requirement may not be satisfied.The input impedance increases linearly with increasing feedback impedance of the I/V converter; i.e. at a sufficiently high feedback impedance value the output signal will deviate from the ideal case—it will be lower. Since the measurement of low intensity radiation requires a high feedback impedance, the lower the irradiance value on the detector the higher this deviation will be. If all the parameters of the detection system are kept constant during a measurement series, only calibration is required to achieve low uncertainty, but if any of the impedances or the gain of the operational amplifier changes during a measurement series this will result in increased measurement error. It should be noted that the values of these impedances can change rapidly with temperature. By applying a bootstrap circuit to the detector the impedance of the bootstrapped detector—depending on the frequency of the operation—will be 3–6 decades higher, compared with the detector alone; i.e. bootstrapping virtually increases the impedance of the detector. Consequently, this error will appear only at a much higher sensitivity range, greatly reducing this problem. Here we analyse the bootstrap solution.

Electrochemical defect profiling for semiconductor heterostructures

A special selective electrochemical etching-based apparatus is presented which is appropriate for use in the in situ observation of the defects in heterostructures. The working of the set-up is demonstrated on InGaAs/ GaAs(001) heteroepitaxial systems where the epitaxial layer thickness is above the critical layer thickness. With incremental layer removal, the depth profile of the dislocation density is mapped and the measured density of defects which is inversely proportional to the layer thickness is in agreement with the theoretical model.

CURRENT-TO-VOLTAGE CONVERTER FOR LINEARITY CORRECTION OF LOW SHUNT RESISTANCE PHOTOVOLTAIC DETECTORS

The most accurate absolute calibration of radiometric transfer standard detectors is carried out by cryogenic radiometers at a usually high irradiance level. A radiometric scale can be maintained by these calibrated detectors in a given irradiance range if they are linear or their nonlinearity is predictable in this range. The linearity of photovoltaic quantum detectors can be affected among others by their shunt resistance and the applied circuitry. Many of the radiometric quality large‐area photodiodes exhibit very low shunt resistance in comparison to Si detectors. When used with a traditional current‐to‐voltage converter the requirement of the ideal short‐circuit operation is not always met. Consequently, part of the photoinduced current flows through the shunt resistance of the detector as a shunt current, and only the rest produces the output voltage. This shunt current is highly temperature dependent and is generally proportional to the output voltage causing a nonlinear deviation of the output sig...

The Application of an Optical Biasing Method to Determine Temperature-dependent Nonlinearity of Photovoltaic Ge Detectors

The differential spectral responsivity method has been used to determine the linearity of large-area Ge detectors near room temperature. The nonlinearity caused by using a traditional transimpedance amplifier is discussed and the applicability of a bootstrapping technique to compensate for the low shunt resistance of the photodiode is shown. The detectors were found to be linear up to a saturation limit which is nearly independent of wavelength. The temperature dependence of the saturation level is also described.

Low radiation level detection with room temperature InAs detector

Recently, room temperature or near room temperature InAs detectors are widely used in laser warning receivers, process control monitors, temperature sensors, etc. requiring linear operation over many decades of the sensitivity range. The linearity of zero biased Si, InGaAs and Ge detectors is thoroughly discussed in the literature, contrary to InAs detectors. In an earlier work of the authors it has been demonstrated that applying a bootstrap circuit to a Ge detector – depending on the frequency of the operation – will virtually increase the shunt resistance of the detector by 3–6 decades compared to the detector alone. In the present work, a similar circuitry was applied to a room temperature InAs detector, the differences between the bootstrapped Ge and bootstrapped InAs detector are underlined. It is shown, how the bootstrap circuit channels the photogenerated current to the feedback impedance decreasing with many decades the detectable low level limit of the detector – I/V converter unit. The linearity improvement results are discussed as a function of the chopping frequency, calculated and measured values are compared, the noise sources are analyzed and noise measurement results are presented.

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.

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.