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The mechanism of zero-bias photocurrent (ZBPC) in semi-insulating GaAs:Cr bulk crystals is studied by kinetics measurements. A simple quantitative model describing the time behaviour of the photogenerated electron concentration is developed. This model confirms our previous qualitative suggestion [J. Electrochem. Soc. 141 (1994) 2533] that the ZBPC is due to the random defectdensity fluctuations (RDDF) in the material investigated. The ZBPC polarity inversion observed in some ranges of ZBPC low temperature spectra [J. Electrochem. Soc. 141 (1994) 2533] is also explained in frame of the model proposed. It is shown that the negative ZBPC is carried out by electrons driven by electric fields connected with previously generated holes trapped in the potential ‘hills’ of the valence band. # 2003 Elsevier Science B.V. All rights reserved.

The concentration of the intrinsic mid-gap donor EL2 in semi-insulating GaAs is known to be dependent on melt composition during crystal growth and on post-growth thermal treatment of crystals. A detailed analysis of data emerged from monitoring the production process of state-of-the-art GaAs substrates revealed discrepancies to the compensation model. By means of a combination of methods for quantitative point defect analysis we can show that EL2 concentration depends also on compensation ratio or position of Fermi level, respectively.

Semi-insulating silicon carbide single crystals have been grown using high temperature chemical vapor deposition without vanadium doping. The resistivity of standard and exploratory 2″ diameter substrates has been analysed topographically with 1 mm lateral resolution with an improved contactless resistivity mapping technique. Absolute resistivity values are ranging from below 1 × 105 to above 1 × 1012 Ω cm. The lateral homogeneity of state-of-the-art material is very satisfactory, whereas strong localized inhomogeneities in exploratory materials underscore the decisive advantage of a nondestructive and topographic resistivity evaluation in supporting the optimization of the crystal growth procedure.

A neural network (NN) method has been proposed as a new algorithm for extraction of defect centers parameters in semi-insulating materials from experimental data obtained by photoinduced transient spectroscopy (PITS). The new algorithm is applied to investigate irradiation-induced defect centers in high resistive silicon. The folds on the PITS spectral surface formed due to the presence of defect levels are best fitted with a two-dimensional approximation function with implementation of the NN learning process. As a result, the Arrhenius plots for defect centers are obtained and the parameters of these centers are determined.

Photocurrent was measured as a function of wavelength for Cl-doped semi-insulating CdTe specimens with evaporated Au electrodes. It is found that the photocurrent decreased with decreasing temperature. This behavior of the photocurrent is different from that usually observed in other semiconductor specimens. Employing the theory, which was proposed for the amorphous semiconductor, we attempted to explain this phenomenon. From the photocurrent spectral peak at various temperatures, the band gap of CdTe was deduced and an empirical equation for the temperature dependence of the band gap EG(T) was obtained as EG(T)=EG(0)-αT2/(T+β), α=5.21×10-4 eV/K, β=37.5 K.

Our calculations to investigate the role of stoichiometry of GaAs are based on the assumption of thermodynamic equilibrium of point defects at an elevated temperature and then the redistribution of the charge states of the quenched-in defects at room temperature. The effect of stoichiometry of semi-insulating (SI)-GaAs crystal on carrier concentration is well known, especially the change from p-type to SI behavior as the crystal changes from Ga rich to As rich. Our detailed calculations confirm this, but in addition show that the sudden decrease in carrier concentration is followed by an equally sharp increase before becoming relatively constant at a value typical of As rich SI substrates. We are not aware of measurements that have detected this increase in carrier concentration, presumably due to difficulty in accurately controlling the crystal stoichiometry. However, this effect is likely to have an enormous impact in device processing, and hence, needs to be further looked into experimentally. Also, since the primary purpose for developing this model is to make it available for GaAs Technology Computer Aided Design (TCAD) tools, we have only made those assumptions that ensure effortless integration with them. We also discuss the calibration methodology for use of the model in TCAD based process and device design.

A vapor phase method for the growth of high-resistivity CdTe:Cl single crystals is described. The photoluminescence spectra of these crystals are studied as a function of the dopant content. The energy level position which determines the conductivity of these single crystals was determined from electric and photoelectric investigations.

The semi-insulating (SI) behavior of InP wafers prepared with the phosphorus-vapor-pressure controlled annealing method has been investigated. Usually, the carrier concentrations in undoped InP wafers were decreased from the order of 10 15 to 10 1 cm -3 after annealing, but a prominent SI property was not obtained due to insufficient native deep levels for pinning the Fermi level. Transport properties of extremely low Fe doped InP wafers with Fe concentration of 1.5 × 10 15 cm -3 were converted from conductive to semi-insulating after anneals of either at 950 °C for longer than 20 h or at higher than 940 °C for 40 h, under the phosphorus vapor pressure of 0.1 MPa. The compensation mechanism by Shockley will explain not only the electrical activation of Fe after annealing, but also the decrease of shallow donors which are related to native defects such as phosphorus vacancies. In this paper, effective annealing conditions for realizing the reproducible production of extremely low Fe doped SI InP have been investigated systematically. Furthermore, a compensation mechanism will also be proposed in connection with it.

Semi-insulating (SI) InP wafers of 2 and 3 in. diameters have been prepared by annealing undoped LEC InP at 930 degreesC for 80 h under pure phosphorus ambient (PP) and iron phosphide ambient (IP). The electrical uniformity of annealed undoped SI wafers, along with a Fe-doped as-grown SI LEC InP wafer, has been characterized by whole wafer PL mapping and radial Hall measurements. Defects in these wafers have been detected by photo-induced current transient spectroscopy (PICTS). The results indicated that the uniformity of IP wafer is much better than that of PP wafer and as-grown Fe-doped Si InP wafer. There are fewer traps in undoped SI InP IP wafer than in as grown Fe-doped and undoped SI InP PP wafer, as evidenced by PICTS. The good uniformity of the IP wafer is related to the nonexistence of high concentration of thermally induced defects. The mechanism for this phenomenon is discussed based on the results. (C) 2002 Elsevier Science B.V. All rights reserved.

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to study the electronic properties of grown-in point defects in semi-insulating (SI) GaAs bulk crystals grown by liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) methods. The novel method, which represents the significant improvement of the earlier PITS technique, relies on the extraction of trap parameters from the digitally recorded photocurrent transients by means of the correlation procedure. The influence of the crystallisation conditions on the defect structure of high resistivity GaAs is compared. In the VGF material the concentrations of traps P6 (0.05 eV), P7 (0.08 eV), P12 (0.18 eV), EL14 (0.22 eV) and EL3 (0.58 eV), tentatively identified as V As −/0 , Ga As 0/− , B As −/−− , V Ga –V As and O i –V As , respectively, are found to be much lower than in the LEC material.

The temperature dependence of the subpicosecond carrier transport in SI-InP has been studied in detail using the THz radiation between 20 and 300 K. It is found that the phase reversal of the waveform occurs at ∼120 K. The reversal indicates the change of the carrier transport direction and is discussed on the basis of the drift-diffusion model.

Dependences of the spectral peak position of edge photoluminescence, its half-width, resistivity, charge carrier mobility in crystals of semi-insulating undoped GaAs on the carbon concentration NC at 77 K (3.0×1015 cm−3≤NC≤4.3×1016 cm−3) were studied. The dependences observed are explained by the interaction of charge carriers with ionized impurity atoms and with structural defects.

In the frame of a research project aimed at obtaining high-resistivity CdTe crystals, the authors report on the preparation of (nominally) undoped semi-insulating (SI) CdTe crystals. The method is based on the use of polycrystalline CdTe charges synthesized from 7 N pure Cd and Te elements, in such a way as to ensure an atomic stoichiometric ratio (Cd/Te) equal to 1 within the weighing error. The charges are further stoichiometrically adjusted by subliming off residual excess components through effusion in semi-open ampoules vented to vacuum. By making use of these charges, undoped SI CdTe ingots, made up of large single-crystal grains, have been grown from both vapour, by closed-ampoule physical vapour transport and melt, by liquid encapsulation Bridgman technique. The yields in SI material turned out to be 100% for vapour-grown samples and about 50% for melt-grown ones. Preliminary results of this research work are presented here and briefly discussed.

Conditions were optimized for growing semi-insulatingn-type Cd1 – xZnxTe (x = 0.0002 and 0.1) crystals.

For a semiconductor material, the characterization of its electronic band structure is very important for analyzing its physical properties and applications in semiconductor-based devices. Photoreflectance spectroscopy is a contactless and highly sensitive method of characterizing electronic band structures of semiconductor materials. In the photoreflectance spectroscopy, the modulation of pumping laser can cause a change in material dielectric function particularly around the singularity points of joint density of states. Thus the information about the critical points in electronic band structure can be obtained by measuring these subtle changes. However, in the conventional single-modulated photoreflectance spectroscopy, Rayleigh scattering and inevitable photoluminescence signals originating from the pumping laser strongly disturb the line shape fitting of photoreflectance signal and influence the determination of critical point numbers. Thus, experimental technique of photoreflectance spectroscopy needs further optimizing. In this work, we make some improvements on the basis of traditional measurement technique of photoreflectance spectroscopy. We set an additional optical chopper for the pumping laser which can modulate the amplitude of the photoreflectance signal. We use a dual-channel lock-in amplifier to demodulate both the unmodulated reflectance signals and the subtle changes in modulated reflectance signals at the same time, which avoids the systematic errors derived from multiple measurements compared with the single-modulated photoreflectance measurement. The combination of dual-modulated technique and dual-channel lock-in amplifier can successfully eliminate the disturbances from Rayleigh scattering and photoluminescence, thus improving the signal-to-noise ratio of the system. Under a visible laser (2.33 eV) pumping, we measure the room-temperature dual-modulated photoreflectance spectrum of semi-insulating GaAs in a region from near-infrared to ultraviolet (1.1 ～6.0 eV) and obtain several optical features which correspond to certain critical points in its electronic band structure. Besides the unambiguously resolved energy level transition of E0 and E0+0 around the bandgap, we also obtain several high-energy optical features above the energy of pumping laser which are related to high-energy level transitions of E1, E1+1, E0' and E2 in the electronic band structure of GaAs. This is consistent with the results from ellipsometric spectroscopy and electroreflectance spectroscopy. The results demonstrate that for those high-energy optical features, the mechanism for photoreflectance is that the photon-generated carriers modulate the build-in electric field which affects the overall electronic band structures, rather than the band filling effect around those critical points. This indicates that dual-modulated photoreflectance performs better in the characterization of semiconductors electronic band structure at critical point around and above its bandgap.

The effect of surface treatment in various etching solutions with subsequent depositions of different metals (Au, Pt, In) is studied on semi-insulating CdTe. Properties of the contacts are investigated by the current–voltage characteristics measurements using three-terminal technique. We show that thermal deposition of In on the surface treated in HI-solution results in the best electric contacts for semi-insulating n-type CdTe. Chemically deposited gold in combination with treatment in HBr-solution proves to be the best on p-type material.

Native defect levels in ternary compound ${\mathrm{Tl}}_{6}$${\mathrm{I}}_{4}\text{S}$ single crystals were studied by low-temperature photoluminescence (PL) and photoconductivity (PC) measurements. From the PL measurements, a broad emission band centered at 1.64 eV was observed at low temperatures. The peak can be decomposed using two standard Gaussian functions to reveal two emission bands at 1.55 and 1.66 eV. The PL peak at 1.55 eV is attributed to donor-acceptor pair recombination between a sulphur vacancy $({\mathrm{V}}_{\mathrm{S}})$ deep donor $({E}_{d}=0.57\text{eV})$ and an antisite defect $({\mathrm{S}}_{\mathrm{I}})$ shallow acceptor $({E}_{a}=20\text{meV})$. The 1.66-eV emission band is attributed to self-activated luminescence involving a defect complex and is described using a configuration coordinate model. Within this framework, the 1.66-eV emission band is associated with a S vacancy donor bound to a Tl vacancy acceptor that forms a ${\mathrm{V}}_{\mathrm{S}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{Tl}}$ Schottky pair. The photoconductivity spectra show the presence of a deep donor level located at 0.46 eV below the conduction-band edge, in good agreement with that measured by PL spectroscopy.

A simple photocathode based on graphene nanoplatelets glued on semi-insulating (s.i.) GaAs was fabricated and used for field electron emission in a diode configuration. The graphene nanoplatelets act as field emitter array with low turn-on field. The photomodulation was achieved with a GaAs photoswitch in series to the bottom of the graphene nanoplatelet emitters. The position and power of the laser illumination (800 nm) was not critical for photomodulation.

We demonstrate a novel optically tunable photosensitive capacitor (PSC) made from high-purity semi-insulating 4H-SiC. Photosensitive capacitors can provide continuously variable reactive tuning in RF circuitry or enable capacitive-optical sensing applications. Unlike varactors, PSCs often do not require a DC bias voltage to operate. To demonstrate the effect, we fabricated several 1cm x 1cm square photocapacitor devices from bulk material using metal-evaporated Ti/Au contacts using a simple planar parallel-gap geometry. IV curves were taken of the devices using an HP-4145B semiconductor parameter analyzer to verify Schottky behavior as a function of DC bias. The samples were then illuminated with pulsed below-bandgap 470 nm and 590 nm high intensity LED light sources. The resulting data demonstrated an increase in capacitance, Cs, and a drop in resistance, Rs, with increasing optical intensity incident on the device. The observed shifts in both Cs and Rs were repeatable. At a measurement frequency of 33 kHz. Cs increased from its nominal value of 186.7 pF to 575.6 pF while Rs dropped from 150.0 kΩ to 22.4 kΩ. This demonstrates the existence of the photocapacitance effect in high-purity semi-insulating 4H-SiC and thus warrants further investigation. The underlying phenomenon of the effect is suspected to be light interaction with the dominant deep level traps through the Shockley–Read–Hall (SRH) recombination mechanism.

Semi-insulating (SI) silicon carbide (SiC) was evaluated as a candidate material for dielectric substrate for patch antennas suitable for monolithic antenna integration on a SiC semiconductor chip. Computer simulations of the return loss were conducted to design microstrip patch antennas operating at 10 GHz. The antennas were fabricated using SI 4H-SiC substrates, with Ti-Pt-Au stacks for ground planes and patches. A good agreement between the experimental results and simulation was obtained. The radiation performance of the designed SiC based patch antennas was as good as that normally achieved from antennas fabricated using conventional RF materials such as FR4 and Rogers. The antennas had the gain around 2 dBi at 10 GHz, which is consistent with the conventional antennas of a similar size.

Cu diffusion into semi-insulating GaAs was performed at 1150°C. The PICTS spectra of the diffused GaAs show several Cu-related peaks. The height of the characteristic low-temperature peak (CuA, T≈150 K) is proportional to the total Cu concentration determined by AES. The appearance and the peak heights of further Cu-related peaks in the spectra are obviously a function of the Cu concentration.

The n-p conversion of the conduction type and a decrease in resistivity to 102 Ω cm at 300 K were revealed upon proton irradiation (5 MeV, 300 K, D≈2×1017 cm−2) of semi-insulating GaAs:Cr (ρ≈(3–4)×108 Ω cm). Temperature dependences of ρ for heavily irradiated samples indicate a hopping conduction in the temperature range of 400–120 K, with the transition to the conduction with variable-range hopping at T≤120 K. The effects of electronic switching were found in low-resistivity proton-irradiated GaAs:Cr at about 20 K. The isochronous annealing of radiation defects in the temperature range of 20–750°C was investigated.

Temperature dependence of the THz radiation from n-, p-, and semi-insulating- (SI-) InP surfaces excited by ultrashort laser pulses has been studied in detail. It is found that the polarity reversal of the waveform occurs at 120 K for SI-InP whereas it does not for n- and p-InP. This result is interpreted in terms of the crossover of the two radiation mechanisms, i.e. the current surge effect due to a built-in surface field and the photo-Dember effect: the former is effective at high temperatures and the latter is effective at low temperatures.

Semi-insulating (SI) GaAs is now being reconsidered as a promising material for radiation detectors, mostly due to greatly improved quality of the material. In this paper we shall describe the properties of the state-of-the-art SI GaAs crystals grown by LEC method as relevant for such applications. Specifically, we shall concentrate on the assessment of the spectra and density of residual impurities, on the measurements of deep levels spectra and on studying the impact of these centers on material parameters. The spectra of deep centers as studied by PICTS and DLTS indicate that, in LEC crystals grown from stoichiometric melts, besides the most prominent EL2 center, other electron and hole traps are usually observed. By comparing the DLTS and PICTS results we show that PICTS often tends to overestimate the relative contributions of traps shallower than EL2. Two approaches to decreasing EL2 concentration in LEC grown crystals have been proposed in literature: to grow crystals from Ga-rich melts and to anneal the material at high temperatures. We have studied the changes in deep level spectra and transport parameters for both cases. In annealing experiments the possibility of thick samples modification by arsenic diffusion was also investigated.

A buried p-layer in a GaAs MESFET channel is successfully formed by (Si, Mg) co-implantation and rapid thermal annealing process. The effects of co-implantation dose on the measured electrical activity are investigated; and the results on MESFET implantation profiles are also discussed, when Si and Mg are co-implanted in the semi-insulating (SI) GaAs substrates. These modified profiles are calculated by considering electrical activation and diffusion of both Si and Mg in the SI GaAs. The results are compared with an n-layer measured by the C – V and I – V techniques.

A gamma-ray detection performance of the semi-insulating (SI) GaAs detectors was studied in the temperature range 210–300 K. The reduction of detector noise was observed as expected, as well as a decrease of the operation temperature-induced changes in the charge collection process that are strongly material dependent. The most pronounced improvement in the collection process was observed for chromium-doped SI GaAs.

A particle detector made of semi-insulating GaAs is used for preliminary study of X-ray imaging. A standard technology developed in our laboratory is applied to prepare this detector. The detector works at room temperature in the photon counting mode. Results show that the quality of images taken at chosen exposure range is limited only by the photonic noise from used X-ray source 241Am.

We report here the preliminary results on liquid encapsulated Czochralski (LEC) semi-insulating (SI) GaAs detectors after γ irradiation. The detectors have been irradiated with 1.17 and 1.33 MeV photons from 60 Co with doses up to 19.2 kGy. Total doses correspond to the photon flux by the order of 10 13 γ/mm 2 incident on the device. The detectors before and after exposure to various radiation doses have been characterised. The I – V characteristics for both polarities and pulse-height spectra versus voltage applied at the temperature of 295 K using 122 keV 57 Co γ-source were measured. A significant increase of charge-collection efficiency after irradiation of detectors was observed.

A combination of complex surface capacitance mapping and sheet resistance mapping is applied to establish the origin of resistance variations on semi-insulating (SI) 6H–SiC substrates. The direct correlation between the capacitance quadrature and the sheet resistance is found in vanadium-doped SI samples. Regions with low capacitance quadrature show high sheet resistance. This indicates, associated with the nonhomogeneity of sheet resistance on the substrate, that the quality of crystallization is not good enough, which also leads to resistivity nonhomogeneity when comparing with different types of deep defects. According to the capacitance mapping, the region with bad crystallization quality has a high radio absorption coefficient. Another correlation is established between the capacitance in-phase and sheet resistance for the vanadium-doped sample. In this sample, the capacitance in-phase map shows not only the surface topography, but also the same distribution trend as the sheet resistance, namely, regions of high capacitance in-phase reveal high sheet resistance.

Variations in the spectra of edge-emission photoluminescence of semi-insulating undoped GaAs crystals as a result of heat treatment for 20–90 min at 900°C were studied. It is shown that heat treatment substantially affects (transforms) the excitonic component of the spectrum. The observed changes in the spectra are related to variations in the impurity composition of the crystals studied. These variations are shown to be caused by heat treatment.

In the past it has been demonstrated that the carbon concentration of large semi-insulating (SI) GaAs single crystals grown by the conventional liquid encapsulation Czochralski (LEC) technique can be controlled by several methods including variations of growth parameters. It was the aim of the present paper to clarify which of the relationships of LEC growth could be used for a carbon control in the VCz-method characterized by the application of an inner chamber made from graphite to avoid selective As evaporation. In detail this comprised a study of the influence of several growth parameters like the water content of the boric oxide, the composition of the working atmosphere, the gas flow, a titanium gettering and additions of gallium oxide. As a result, for the first time carbon concentrations down to ≈ 1014 cm-3 were obtained in 3'' (75 mm) diameter VCz crystals.

Venezia Tecnologie SpA (ENI Group), Via delle Industrie 39, I-30175 P. Marghera (VE), Italy~Received 2 December 1999!Elastic energy absorption measurements versus temperature on semiconducting, semi-insulating~SI!, andFe-doped InP are reported. A thermally activated relaxation process is found only in the SI state, which isidentified with the hopping of H atoms trapped at In vacancies. It is proposed that the presence of In vacanciesin InP prepared by the liquid encapsulated Czochralski method is due to the lowering of their energy by thesaturation of the P dangling bonds with H atoms dissolved from the capping liquid containing H

Variable-temperature current–voltage has been used to study the conduction properties of Fe-doped semi-insulating (SI) InP in the as-grown and annealed states. It is found that the trap-filling (TF) process disappears gradually with lengthening of annealing time. This phenomenon is explained by the decrease of the concentration of the empty Fe deep level (Fe3+) that is caused by the thermally induced donor defect formation. The TF process cannot be observed in annealed undoped and long-time annealed Fe-doped SI InP material. The breakdown field of annealed undoped and Fe-doped SI InP is much lower than that of as-grown Fe-doped InP material. The breakdown field decreases with decreasing of temperature indicating an impact ionization process. This breakdown behavior is also in agreement with the fact that the concentration of the empty deep level in annealed InP is lowered.

Using the two-dimensional reciprocal space mapping, we have clearly established the relative positions of the arsenic atom (EL2 defect) corresponding to its stable and metastable configurations. It is shown that lowering only the temperature of the sample, before transition, induces transformation of the lattice. Characteristic temperature (120–140 K) below which such transformation takes place is the same as for thermal recovery of the defect. It is suggested that this temperature related lattice relaxation might promote a further transition to the metastable state. Full reciprocal space maps of the x-ray diffracted intensity are shown for the whole substrate-layer system.

Semi-insulating InP single crystals double doped with Zn and Fe and single doped with Fe were grown by the Czochralski technique. Resistivity and Hall coefficient were measured at temperatures between 230 K and 420 K. The Hall coefficient shows an extremum at about 250 K and a change of sign to positive below about 235 K in InP:(Zn+Fe). The effect is discussed in terms of the strength of electron and hole mixed conductance at various temperatures on account of a shift of the Fe deep level with temperature.

The lateral homogeneity of the electrical resistivity p of semi-insulating GaAs substrates is measured with high resolution using contactless capacitive mapping. The improved technique is capable of imaging mesoscopic p fluctuations correlated with the cellular structure of the dislocation density. The results compare favorably with data obtained by point contact topography. A rneasurement and statistical analysis procedure is described that allows an individual evaluation of the macro- and mesocopic contributions to the total on-wafer p variation.

We present a study of the ZnO/GaAs SI junction and its possible application in radiation detectors. Ion beam sputtered ZnO films obtained in a H 2 environment show low resistivity and very good optical transmission in the visible range. Current–voltage measurements reveal an ohmic behaviour of the ZnO contact on semi-insulating GaAs. To carry out an evaluation of the injection properties of the ZnO contact a ZnO/GaAs/Schottky structure has been realised. Under high reverse electric field (≈4×10 4 V/cm) low minority injection from the ZnO contact has been observed. The same structure used as light detector has shown an efficiency of 34% for a bias of 600 V at a wavelength of 670 nm.

Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs.