Semi-insulating Semiconductors Research Articles

Non-destructive mapping of electrical properties of semi-insulating compound semiconductor wafers using terahertz time-domain spectroscopy

With an increasing demand for THz electronics applications, it is essential to obtain the electrical properties of semiconductor materials used in their fabrication. This study presents an approach to estimate the electrical properties of semi-insulating compound semiconductors using terahertz time-domain spectroscopy (THz-TDS). By employing THz-TDS in transmission mode, the resistivity and carrier concentration of semi-insulating Silicon Carbide (SiC) and Indium Phosphide (InP) wafers are non-destructively measured. The study utilizes the simplified Drude model and the Nelder-Mead algorithm to estimate the electrical properties from the THz-TDS transmission (S21) measurement data. The resistivity values of the SiC and InP samples, (1.42 ± 0.15) × 105 Ω cm and (3.0 ± 0.1) × 107 Ω cm, respectively, are in accordance with the manufacturer specifications. Moreover, the feasibility of non-destructive mapping of the electrical properties of the wafers is demonstrated, offering a novel tomographic inspection approach for online monitoring. This technique holds significant promise for enhancing production yields in the semiconductor industry.

Alpha radiation induced space charge stability effects in semi-insulating silicon carbide semiconductors compared to diamond

Although the use of semi-insulating silicon carbide material for radiation detection purposes has been previously demonstrated, its use in practical applications has been inhibited by space charge stability issues caused by defect concentrations within the material, the so called polarisation effect, by which the count rate and resultant spectrum changes with irradiation time.This is a result of the charge carriers generated during irradiation filling deep level defects within the material, causing space charge buildup and de-activating that trap level until the trapped charge is re-emitted. Consequently, the time dependence of the polarisation effect has been determined by a combination of parameters that can be influenced during operation, namely the incident radiation intensity, ambient light, temperature and bias. The material properties have also been considered through the use of materials with different defect capture cross sections, concentrations and energy level.A thorough characterisation of the alpha irradiation induced polarisation phenomenon in semi-insulating silicon carbide has been conducted to demonstrate that stable operation detectors are in fact possible with this material. The effects were compared to single crystal diamond and polycrystalline diamond, which are known to exhibit similar polarisation issues.The polarisation rate as an effect of incident flux, bias and temperature was determined, with the depolarisation rate as a function of ambient light and bias also demonstrated. Consequently it has been shown that stable operation can be maintained for detectors made from semi-insulating SiC material of active thickness 350μm at incident alpha radiation fluxes of <0.7 alphas per second per mm2 with high operating biases (>±400V). Furthermore, polarisation can be suitably managed or reduced through the use of light illumination and elevated temperatures (373K).

Analysis of trapping and de-trapping in CdZnTe detectors by Pockels effect

In this contribution we introduce a method of deep level spectroscopy in semi-insulating semiconductors demonstrated on detector-grade bulk CdZnTe. The method is based on the measurements of temporal and temperature evolution of the electric field profile in studied samples, which is very sensitive to a change of occupancy of deep levels. The measurement of the electric field is based on the linear electro-optic (Pockels) effect using the InGaAs avalanche photodiode with fast response. The internal electric field profile in studied samples significantly changes under various external conditions represented by the application of the bias and pulsed illumination with below-bandgap light. From the knowledge of the electric field behavior and using a standard analysis based on thermally induced transitions of electrons and holes from the deep levels to the conduction and valence bands, respectively, it is possible to get activation energies of the energy levels, their types (donor or acceptor) and corresponding capture cross-sections. By this method we have found deep levels responsible for the polarization of CdZnTe detector under high photon-fluxes. Identified deep levels eV, eV and eV can capture the photo-generated holes and thus form a positive space charge, which is responsible for polarization of the detector.

Specific features of the photoconductivity of semi-insulating cadmium telluride

The effect of local illumination providing a high level of free-carrier injection on the conductivity of a sample of semi-insulating cadmium telluride and on the properties of ohmic contacts to the sample is studied. It is found that, irrespective of the illumination region, the contact resistance of ohmic contacts decreases and the concentration of majority carriers in the sample grows in proportion to the illumination intensity. It is shown that inherent heterogeneities in crystals of semi-insulating semiconductors can be studied by scanning with a light probe.

Modeling of Semiconductor Detectors Made of Defect-Engineered Silicon: The Effective Space Charge Density

The effective space charge density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> is the average density of carriers over the depletion layer in a semiconductor diode and is measured from the capacitance-voltage curve extrapolated to full depletion <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> . Full semiconductor modeling has been performed for PIN diodes made of materials with a large density of generation-recombination (g-r) centers, such as irradiated or semi-insulating semiconductors. The results show that this extrapolation method can give incorrect values for the introduction rate of charged traps and g-r centers with large irradiation fluence. This is because the introduction of midgap g-r centers moves the Fermi level toward midgap which allows existing traps to change their ionization state. We propose an alternative approach to evaluate the effective density from the C-V characteristics without the need to evaluate the depletion voltage.

Current simulation of symmetric contacts on CdTe

This article presents the calculated current–voltage characteristics of symmetric Metal–Semiconductor–Metal configurations for Schottky, Ohmic, and injecting-Ohmic contacts on high resistivity CdTe. The results clearly demonstrate that in the wide band-gap, semi-insulating semiconductors, such as high resistivity CdTe, the linearity of the I–V curves cannot be considered a proof of the ohmicity of the contacts. It is shown that the linear I–V curves are expected for a wide range of contact barriers. Furthermore, the slope of these linear curves is governed by the barrier height, rather than the bulk doping concentration. Therefore the deduction of bulk's resistivity from the I–V curves may be false.

Simulation of Schottky and Ohmic contacts on CdTe

This study focuses on finite element method simulations of various contacts on high resistivity, “semi-insulating,” semiconductors in general, and detector grade CdTe, in particular. The results of this study confirm that the simplified approach assuming depletion approximation is not suitable for analyses of semi-insulating semiconductors. Furthermore, the space charge in such devices is usually dominated by the free charge carriers. The forward current in Schottky diodes may be much higher than expected from the bulk majority carrier concentration. It is shown that the minority current is often much higher than the majority current. These results indicate that a full computer simulation is vital in order to describe the microscopic conditions inside the device, as well as the macroscopic behavior of the device.

Effect of Intrinsic Defects in High-Purity Semi-Insulating 4H-SiC on Reverse Current–Voltage Characteristics of Schottky Barrier Diodes

Schottky barrier diodes are fabricated using high-purity semi-insulating 4H-SiC. Under certain measurement conditions, the reverse current–voltage (IR–V) characteristics of the diodes exhibit a peak, with the diodes appearing to behave as negative resistance diodes. To investigate the effect of intrinsic defects in the 4H-SiC on the IR–V characteristics, the transient reverse currents of the diodes are measured. Without any assumptions regarding defects, a graphical peak analysis method, discharge current transient spectroscopy (DCTS), which uses the isothermal transient reverse current, can determine the densities and emission rates of defects. From simulation of the IR–V characteristics using the densities and emission rates of intrinsic defects determined by DCTS, the effect of the intrinsic defects in high-purity semi-insulating 4H-SiC on the IR–V characteristics of its Schottky barrier diodes is elucidated. It is found that DCTS is suitable for determining the densities and emission rates of electrically active defects in semi-insulating semiconductors.

Characterization of Traps in Semi-Insulating 4H-SiC by Discharge Current Transient Spectroscopy

In order to characterize traps in semi-insulating 4H-SiC that is regarded as an attractive semiconductor for X-ray detectors, we apply discharge current transient spectroscopy (DCTS) that is a graphical peak analysis method based on the transient reverse current of a diode. We have found at least three types of traps whose emission rates at 373 K are 4.9×10-3, 8.3×10-3 and 8.0×10-2 s-1. Since it is difficult to characterize traps in semi-insulating semiconductors by transient capacitance methods, it is demonstrated that DCTS is a powerful method for determining the densities and emission rates of traps in semi-insulating semiconductors.

Modification of charge compensation in semi-insulating semiconductors by high energy light ion irradiation

High energy light ions have been used to engineer the electrical properties of semi-insulating InP and GaAs by creation and annihilation of native defects. Due to light mass, high energy ions lose most of their energy during flight inside the material by electronic excitation, which also modifies the charge states of the defect. Imbalance in charge compensation caused by irradiation induced defect engineering results in the modification of resistivity of the semi-insulating samples. The activation energies of the defect levels responsible for the modified resistivity have been estimated from the measurement of thermally activated currents. It is shown that the modified charge compensation is due to annihilation and/or creation of native defects depending on the material (GaAs or InP) and the fluence of irradiated ions. It has also been observed that annealing does not have any effect on modified compensation in InP, but in the case of GaAs, compensation recovers after thermal annealing.

Characterization of deep centers in semi-insulating SiC and HgI2: Application of discharge current transient spectroscopy

In order to characterize traps in semi-insulating 4H-SiC and HgI2 regarded as an attractive semiconductor for X-ray detectors, we apply discharge current transient spectroscopy (DCTS), which is a graphical peak analysis method based on the transient reverse current of a diode. In high-purity semi-insulating 4H-SiC whose diode may detect X-rays or γ-rays, DCTS can detect two types of trap species, and can determine those densities and emission rates. The emission rates of detected traps are close to each other, suggesting that thermally stimulated current (TSC) may not distinguish between these two types of traps. In semi-insulating HgI2, the densities and emission rates of two types of trap species are also determined by DCTS. Therefore, it is demonstrated that DCTS is a powerful method for determining the densities and emission rates of traps in semi-insulating semiconductors

Effect of temperature- and composition-dependent deep level energies on electrical compensation: Experiment and model of theCd1−xZnxTesystem

The effect of temperature and composition dependences of deep level ionization energies in semi-insulating compound semiconductors is investigated and illustrated in the example of the ternary ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{Te}$ system. Those dependences are determined by the behavior of the band extrema in an absolute energy scale and the actual nature of a particular defect, which controls its relation to the host crystal states. Examples for the interpretation of experimental temperature and composition dependences of critical charge transport parameters are given.

The Space Charge Wave Induced Hall Effect

A mechanism is proposed according to which the Hall effect arises due to the space charge wave rectification in semi-insulating semiconductors.

Second harmonic generation and rectification of space-charge waves in photorefractive crystals

A theory explaining the effects of second harmonic generation and rectification of space-charge waves (SCWs) in both space and time in photorefractive crystals and semi-insulating semiconductors is formulated for the first time. The theory predicts two mechanisms of SCW second harmonic excitation. An experimental technique is proposed for second harmonic detection, and the formulas required for interpreting the experimental results are derived. It is found that the effect of complete rectification of SCWs may lead to a change in the current passing through the sample by tens of percent. The results of calculations are in qualitative agreement with preliminary experimental results.

Photocurrent Transients in Presence of a Double Impurity in Semi-Insulating Semiconductors

A phenomenological model is developed to study the transient behaviour of the photocurrent due to detrapping of carriers from a double impurity in semi-insulating semiconductors. The model predicts a peak in the transient curve at constant temperature. An Arrhenius plot of tp (the time at the peak) yields the second ionization energy of the double impurity. The activation energy determined from the photoinduced current transient spectroscopy (PICTS) analysis corresponds to the first ionization energy of the impurity. The model explains the appearance of a negative peak in the PICTS spectrum which was reported by previous authors on a number of semiconductors. Experimental investigation of the current decay curves on MgxZn1—xTe (x = 0.10, 0.14, 0.30) showed that for x = 0.10 the behaviour of the photocurrent decay at different temperatures and the PICTS spectrum can be interpreted in terms of the double impurity model. The ionization energies of the impurity were determined to be 0.082 ± 0.007 eV and 0.12 to 0.13 eV.

Photocurrent Transients in Presence of a Double Impurity in Semi-Insulating Semiconductors

Photocurrent Transients in Presence of a Double Impurity in Semi-Insulating Semiconductors

Displacement measurement and surface profiling using semi-insulating photoconductive semiconductors and linearly frequency-ramped lasers

Target displacement measurement and surface profiling are demonstrated experimentally using coherence frequency domain reflectometry and the photo-emf optical difference frequency sensor that was recently shown to generate steady state dc photocurrents linearly proportional to the optical frequency differences between two laser beams interfering inside semi-insulating photoconductive semiconductors. The simplicity and spatial adaptivity of photo-emf optical frequency sensors eliminate the stringent requirements of optical alignment and calculation-intensive signal processing found in conventional coherence frequency domain reflectometry systems.

Gain spectra of beam coupling in photorefractive semiconductors

The photorefractive recording dynamics of two-beam coupling in semi-insulating semiconductors by beams with slightly different frequencies are studied theoretically and experimentally. The influences of bulk absorption, Gaussian beam profiles, and experimental geometry on the temporal response are analyzed. These effects act to narrow the bandwidth. Measurement of the material photorefractive time constant is discussed.

Crystal-temperature dependence of picosecond two-beam coupling gains in semi-insulating semiconductors

Temperature dependences of time-resolved picosecond beam coupling gains and losses in semi-insulating undoped GaAs and CdTe:Ga are investigated experimentally at a wavelength of 1.064 μm. In the temperature range of 283–363 K a weak temperature dependence of the free-carrier energy transfer gain is observed at high fluence excitation, while photorefractive beam coupling gains and nonlinear absorption losses are found to exhibit no clear temperature dependence. A numerical simulation of the dynamics of photoexcited carriers and space-charge fields is also performed in order to examine the experimental results.

Nonlocal Photorefractive Screening from Hot Electron Velocity Saturation in Semiconductors.

Intervalley scattering of hot electrons during high-field transport in transverse-field photorefractive quantum wells induces a nonlocal optical response in which photoinduced changes in the refractive index are spatially shifted relative to the optical stimulus, providing an avenue for optical gain. We demonstrate that the onset of the photorefractive phase shift coincides with the onset of velocity saturation. This nonlocal response is the high-resistivity consequence in semi-insulating semiconductors of the Gunn effect mechanism.