Optical Transient Current Spectroscopy Research Articles

Deep levels in semi-insulating InP obtained by annealing under iron phosphide ambiance

Deep levels in semi-insulating (SI) InP obtained by annealing in iron phosphide (IP) ambiance have been characterized by optical transient current spectroscopy (OTCS). Compared with the OTCS result of the SI InP prepared by annealing in pure phosphorus (PP) ambiance, the IP SI InP presents only two traps with activation energies of 0.20 and 0.63 eV, respectively. The results suggest that the diffusion of Fe-atoms suppresses the formation of a few defects in the IP SI InP. The nature of deep levels in the IP and PP SI InP has been discussed on the basis of these results. The relation between material property and defects in those SI InP has also been revealed.

Photoluminescence assessment of undoped semi-insulating InP wafers obtained by annealing in iron phosphide vapour

We have investigated the photoluminescence mapping characteristics of semi-insulating (SI) InP wafers obtained by annealing in iron phosphide ambience (FeP2-annealed). Compared with as-grown Fe-doped and undoped SI InP wafers prepared by annealing in pure phosphorus vapour (P-annealed), the FeP2-annealed SI InP wafer has been found to exhibit a better photoluminescence uniformity. Radial Hall measurements also show that there is a better resistivity uniformity on the FeP2-annealed SI InP wafer. When comparing the distribution of deep levels between the annealed wafers measured by optical transient current spectroscopy, we find that the incorporation of iron atoms into the SI InP suppresses the formation of a few defects. The correlation observed in this study implies that annealing in iron phosphorus ambience makes Fe atoms diffuse uniformly and occupy the indium site in the SI InP lattice. As it stands, we believe that annealing undoped conductive InP in iron phosphide vapour is an effective means to obtain semi-insulating InP wafers with superior uniformity.

Properties of deep levels and photorefractive effect in GaAs/AlGaAs multiple quantum wells grown in low temperature

We report on the properties of GaAs/AlGaAs multiple quantum wells grown at medium temperature range 300–400°C. The samples exhibit both of sharp exciton absorption and ultrafast carrier lifetime, which are suitable as photorefractive devices. Photoluminescence, absorption spectroscopy, optical transient current spectroscopy and pump-probe technique were used to characterize the materials grown on GaAs (0 0 1) on-axis or miscut substrates. Using a YAG : Nd laser as a pump source, the quenchable characteristics of EL2-like defects were observed in electroabsorption for the first time. By optimizing growth parameters and device structure, the transient diffraction efficiency approaches 1.2% and 4.5% in non-degeneration four-wave mixing under Frantz–Keldysh and Stark geometry, respectively.

Annealing-induced evolution of defects in low-temperature-grown GaAs-related materials

Infrared absorption spectroscopy, optical transient current spectroscopy (OTCS), and photoluminescence (PL) spectroscopy are used to investigate the annealing induced evolution of defects in low-temperature (LT)-grown GaAs-related materials. Two LT samples of bulk GaAs (sample A) and ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple-quantum-well (MQW) structure (sample B) were grown at 220 and 320 \ifmmode^\circ\else\textdegree\fi{}C on (001) GaAs substrates, respectively. A strong defect-related absorption band has been observed in both as-grown samples A and B. It becomes weaker in samples annealed at temperatures above 600 \ifmmode^\circ\else\textdegree\fi{}C. In sample A, annealed in the range of 600--800 \ifmmode^\circ\else\textdegree\fi{}C, a large negative decay signal of the optical transient current (OTC) is observed in a certain range of temperature, which distorts deep-level spectra measured by OTCS, making it difficult to identify any deep levels. At annealing temperatures of 600 and 700 \ifmmode^\circ\else\textdegree\fi{}C, both ${\mathrm{As}}_{\mathrm{Ga}}$ antisite and small As cluster-related deep levels are identified in sample B. It is found that compared to the As cluster, the ${\mathrm{As}}_{\mathrm{Ga}}$ antisite has a larger activation energy and carrier capture rate. At an annealing temperature of 800 \ifmmode^\circ\else\textdegree\fi{}C, the large negative decay signal of the OTC is also observed in sample B. It is argued that this negative decay signal of the OTC is related to large arsenic clusters. For sample B, transient PL spectra have also been measured to study the influence of the defect evolution on optical properties of LT ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ MQW structures. Our results clearly identify a defect evolution from ${\mathrm{As}}_{\mathrm{Ga}}$ antisites to arsenic clusters after annealing.

Investigation of defects in low-temperature-grown GaAs using optical transient spectroscopy

Optical transient current spectroscopy (OTCS) has been used to investigate defects in the low-temperature-grown GaAs after postgrowth rapid thermal annealing (RTA). Two samples A and B were grown at 220°C and 360°C on (0 0 1) GaAs substrates, respectively. After growth, samples were subjected to 30 s RTA in the range of 500–800°C. Before annealing, X-ray diffraction measurements show that the concentrations of the excess arsenic for samples A and B are 2.5×10 19 and 1×10 19 cm −3, respectively. It is found that there are strong negative decay signals in the optical transient current (OTC) for the annealed sample A. Due to the influence of OTC strong negative decay signals, it is impossible to identify deep levels clearly from OTCS. For a comparison, three deep levels can be identified for sample B before annealing. They are two shallower deep levels and the so-called As Ga antisite defect. At the annealing temperature of 600°C, there are still three deep levels. However, their structures are different from those in the as-grown sample. OTC strong negative decay signals are also observed for the annealed sample B. It is argued that OTC negative decay signals are related to arsenic clusters.

Evolution from point defects to arsenic clusters in low-temperature grown GaAs/AlGaAs multiple quantum wells

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorption spectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiple quantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 350°C by molecular beam epitaxy on miscut (3.4° off (0 0 1) towards (1 1 1) A) (0 0 1) GaAs substrate. After growth, the sample was subjected to 30 s rapid thermal annealing in the range of 500–800°C. It is found that the integrated PL intensity first decreases with the annealing temperature, then gets a minimum at 600°C and finally recovers at higher temperatures. OTCS measurement shows that besides As Ga antisites and arsenic clusters, there are several relatively shallower deep levels with excitation energies less than 0.3 eV in the as-grown and 500°C-annealed samples. Above 600°C, OTCS signals from As Ga antisites and shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenic atoms group together to form arsenic clusters during annealing.

Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers

At a medium substrate temperature of 400 °C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz–Keldysh geometry. A peak electroabsorption of 2100 cm−1 is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 °C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 °C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 °C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 °C has better performance of the photorefractive effect.

Studies of the origin of the yellow luminescence band, the nature of nonradiative recombination and the origin of persistent photoconductivity in n-GaN films

It is shown that in n-GaN samples with a strong yellow luminescence band the deep levels spectra measured by optical transient current spectroscopy (OTCS) is always dominated by a hole trap at E v+0.85 eV which is absent in the spectra of the samples with weak yellow luminescence. Based on the results of recent theoretical calculations it is suggested that these traps could be gallium vacancies V Ga and that the centers responsible for yellow luminescence are formed by pairing such vacancies with shallow donors. The results indicate that the yellow luminescence band could be the dominant recombination channel in the samples with strong yellow luminescence band whereas in the samples with weak yellow luminescence recombination this role is taken over by nonradiative recombination centers. Deep levels transient spectroscopy (DLTS) measurements revealed the existence of a barrier for capture of electrons for electron traps at E c−0.55 eV (0.15 eV) and E c−0.8 eV (about 0.35 eV) and a possible role of such traps in persistent photoconductivity at room temperature in n-GaN is discussed.

Effects of electrode geometry and polarity on the occurrence of negative peaks in optical transient current spectroscopy applied to semi-insulating gallium arsenide

Optical transient current spectroscopy (OTCS) involves observing the transient decay of “photoconductivity” and is of interest as a means of characterizing semi-insulating GaAs wafers. We report experimental results which show that the OTCS spectrum and, in particular, the occurrence of negative peaks is strongly dependent on the shape, size and electrode configuration of the specimen, as well as on the magnitude of the applied voltage and (with asymmetric electrodes) its polarity. A negative peak sometimes changes to a current oscillation with increasing voltage, probably indicating field enhanced trapping. Negative peaks may be observed with sandwich structures with a transparent electrode covering the illuminated surface. This shows that a recent proposal that surface states are involved is at least not always correct. It is suggested that several mechanisms can give negative peaks, and probably more than one has done so under the various experimental conditions which have been used. The present work shows that any practical application of OTCS for ingot qualification will be dependent on standardization of specimen geometry, electrodes and operating parameters.

Comparison of isothermal and boxcar methods of analysis of photocurrent transients

Analysis of photocurrent transients to obtain information on deep levels in semi-insulating gallium arsenide is known as optical transient current spectroscopy (OTCS). Mostly a boxcar amplifier technique has been employed. It is suggested that, because the geometries which have been used may not produce the physics assumed in the method as originally proposed, the assumptions on which the boxcar technique is based may not be satisfied. To test this, digitally recorded transients over a range of temperature were analysed using an improved isothermal analysis method and a software-simulated boxcar. The latter was found to give anomalous trap signatures, particularly when 'negative transients' were concerned, and to resolve fewer levels than the isothermal method. Analysis of the parameters of the decays observed by the isothermal method showed that the boxcar assumptions were indeed not satisfied.

Low-frequency oscillations in semi-insulating InP

Goronkin and Maracas (IEDM Conf. Proc., Dec. 1984, p. 182) reported a method of obtaining information on deep levels in GaAs by analyzing the temperature dependence of low-frequency current oscillations observed in semi-insulating material. In the present work, a modified version of their method was applied to Fe compensated InP. Power density spectra showed one main peak and several less well defined. An Arrhenius plot of log(T2/2f) versus 1/T gave an activation energy of 0.47 eV for the main peak, close to the value found by optical transient current spectroscopy (OTCS). The method is complementary to OTCS in that the same equipment is used and information is obtained more easily on levels which exhibit field-enhanced trapping.

Optical Transient Current Spectroscopy of Deep Levels in Semi‐Insulating Indium Phosphide

Deep levels in iron‐compensated semi‐insulating were investigated by a form of digital optical transient current spectroscopy (OTCS). The decay of the transient photocurrent following periods of illumination was analyzed by a numerical method not previously applied to the problem. This could resolve a transient into a sum of exponential decays with time constants spanning a range of 104. Each time constant corresponds to the release of electrons or holes from a particular deep level. The temperature dependence gave information on the activation energy of the corresponding deep levels. By digitizing the entire transient, rather than recording two points, or their difference, as in the original OTCS method, only one sweep of temperature is required. Since the present algorithm determines the amplitudes and time constants of the exponential decays simultaneously, base‐line induced artifacts and related problems encountered in previous work are avoided. The temperature range was limited by excessive dark current above about 340 K, and by “negative transients” and low‐frequency oscillations below about 260 K. In the accessible range, nine deep levels were resolved. One level, previously unobserved in any type of , may be due to phosphorus vacancies. Of the remaining eight levels, one had previously been identified by OTCS in semi‐insulating . The remaining seven had not previously been detected in semi‐insulating but had activation energies close to levels previously found using capacitance deep level spectroscopy in doped (i.e., conducting) material. “Negative transients” are defined as transients in which the current initially undershoots the steady‐state value. Low frequency oscillations were observed in the same range of temperature, depending upon the applied voltage. It is tentatively proposed that a dependence on voltage of both positive and negative transients and of the low‐frequency oscillations arises from field‐enhanced trapping. The oscillations are attributed to traveling high‐field domains caused by hot‐electron trapping following the original Ridley model.

Modified Exponential Fit of Optically Induced Transient Currents in Semi‐Insulating Semiconductors

In the original model for optical transient current spectroscopy for deep levels in semi‐insulating it was assumed that the currents were due to release of electrons from traps in the depletion region of a reverse biased Schottky diode contact and their transit through the depletion layer. For the geometries normally used, a more realistic model involves processes in a neutral semiconductor. In this case retrapping effects are now shown to lead to a modified form of an exponentially decaying transient. Using an isothermal method of decomposition of the digitized transient into the decays due to separate deep levels it was found that a better fit is obtained using the new form, and anomalous activation energies are avoided.

Optical Transient Current Spectroscopy for Trapping Levels in Semi‐Insulating LEC Gallium Arsenide

Optical transient current spectroscopy (OTCS) is a member of the class of deep level transient spectroscopy (DLTS) techniques. Its advantage is that, unlike other DLTS techniques, it can be applied to semi‐insulating (which is the starting material for integrated circuit (IC) fabrication). The aim is to obtain information on deep trapping levels, which may be of fundamental interest and is certainly of practical interest since it can help in diagnosing the suitability of the material for IC fabrication. We have investigated the method using different device geometries, electrode materials, surface preparations, and crystals. Various “peaks” in the OTCS spectrum were identified, including EL2, the center responsible for the semi‐insulating property in undoped LEC . One is of particular interest, since it is a negative peak, i.e., the current increases (instead of decreasing) with time (after the sudden drop when the light ceases) until it settles at its steady‐state dark value. The amplitude of this peak was found to be much bigger with thinned sandwich structures than with planar electrode structures. Evidence was obtained that this peak is associated with crystal damage produced by abrasion or polishing. The current theory of OTCS due to Hurtes et al. and Martin et al. is based on the Sah et al. treatment of depletion regions. It is pointed out that it may not necessarily apply with typical electrode geometries. In particular, the model that this theory provides for negative peaks is not the only possible one. Two further models are proposed. Negative peaks could be due to an electrically neutral semiconductor model in which more majority carriers are trapped due to illumination. They could also be given by an insulator model in which the current is due to thermal release of carriers from traps that then travel a certain way before recapture.

Deep Levels in Semi‐Insulating LEC GaAs Before and After Silicon Implantation

The deep trapping levels present before ion implantation of silicon into the semi‐insulating LEC starting material were investigated using optical transient current spectroscopy (OTCS). MESFET channel current deep level transient spectroscopy (DLTS) was used for the implanted material. With a silicon nitride layer used to encapsulate the for postimplantation annealing and with implantation directly into the , it was found that of seven or more deep levels seen in the semi‐insulating substrate prior to silicon implantation only the level believed to be EL12 remained. On implanting through a thin encapsulating layer and annealing under , only EL2 was found. With a silicon dioxide layer as an encapsulant, two traps remained and two apparently unreported levels appeared.

Characterization of deep levels in semi-insulating gallium arsenide

Two traps with activation energies ofE c – 0·47 eV andE v + 0·79 eV have been detected in semi-insulating GaAs:Cr through optical transient current spectroscopy (otcs) in the temperature range 300–450 K. The latter trap gives rise to rising current transients which result in a negative peak in theotcs spectrum. The theoretical expressions for current transients have been derived.

OTCS study of defect state in Fe-doped semi-insulating InP induced by ion irradiation and thermal annealing

Deep level centers in LEC-grown semi-insulating InP before and after 60 keV proton or N+2 ion irradiation and thermal annealing have been characterized by means of an optical transient current spectroscopy (OTCS) and their effects on resistivity are comparatively investigated. Two centers with activation energies of 0.31 and 0.65 eV were resolved in the substrate. It was found that ion irradiation induced a defect center with an activation energy of 0.19 eV and thermal annealing at 500 C and above induced two centers with activation energies of 0.45 and 0.81 eV. The decrease of resistance observed after these treatments is shown to be explained from the behavior of induced center.