Deep-level Spectroscopy Research Articles

Acoustic spectroscopy of deep levels in Cr-doped GaAs using light beam generated SAW

A light-beam-generated space-charge inhomogeneity was used for the first time as a probing tool to study the behavior of deep levels in high-resistivity, Cr- doped GaAs using acoustic transient spectroscopy. The space-charge inhomogenity produced by the illumination of photoresistive piezoelectric semiconductors in a high-frequency electric field can generate, in proper conditions, both the surface and transversal or longitudinal acoustic wave. The temperature dependence of the time development of such a generated surface acoustic wave (SAW) after illumination by pulsed monochromatic light was used to investigate deep levels in high-resistivity GaAs using the technique of acoustic deep-level transient spectroscopy (A-DLTS) [P. Bury and I. Jamnický, Proc. World Congress on Ultrasonics, Berlin, 1995, p. 535]. Deep-level parameters such as activation energy and capture cross-section were determined. [Work supported by a Grant of the Slovak Ministry of Education, No 1/1310/96.]

Photo-Induced Current Transient Spectroscopy of Deep Levels in ZnTe and Mg0.21Zn0.79Te Crystals

Photo-Induced Current Transient Spectroscopy (PICTS) was used to characterize deep impurity levels in P-doped MgxZn1—xTe alloys (x = 0 and 0.21). The current transient data recorded at different temperatures (100 ≤ T ≤ 300 K) were analyzed to generate PICTS spectra. The analysis uses a three-gate method that provides access to values of the lifetime comparable to the decay period, similar to the case of DLTS. The detected traps were found to have depths ranging from 0.11 to 0.96 eV. Some of these traps are tentatively assigned to P (0.30 eV), Cu (0.11 to 0.15 eV) and Si or C (0.39 eV).

Electrical characteristics and deep-level admittance spectroscopies of low-temperature grown GaAs p-i-n structures

The properties of low-temperature grown GaAs are studied via the electrical characterization of p-i-n structures with part of the intrinsic layer grown at 300 °C. Comparisons are made between the low-temperature and normally grown samples. The current of the low-temperature sample is about two orders of magnitude higher than that of the normally grown sample in both forward and reverse bias. From temperature-dependent analysis, the leakage current of the low-temperature sample is contributed by the recombination current through defect levels around the midgap, from which a recombination lifetime of 9.4×10−12 s was obtained. By using admittance spectroscopy we observed a dominant electron level at 0.60 eV with a corresponding capture cross section of 1.0×10−13 cm2 that was not observed in the normally grown sample; thus it is believed to be introduced by the As-rich low-temperature layer.

Current noise spectroscopy of deep energy levels in photoconductors

According to a barrier photoconductance theory recently developed, two current noise sources can be envisaged, related respectively to the fluctuation of the barrier height (photoinduced noise component) and to the trapping–detrapping processes in shallow states within the photoconducting material (g–r and 1/f noise component). It is shown that the first noise component, which can be separated by the second one, gives information on the energy gap and on the photoionization cross section of the deep energy levels of the photoconducting material. In particular for thin films it will be shown that the photoionization cross section of the deep energy levels varies inversely to the total number of photons impinging on the photoconductor and proportionally to the noise power spectral density as the photon energy changes. Such relationship suggests that the wavelength dependence of the noise power spectrum provides the correction to be taken into account if the constant photocurrent method is used and the variations of the minority carrier lifetime with the photon energy cannot be disregarded. Results concerning the energy gap and its temperature dependence, in the interval ranging from the room temperature to 200 K, are reported and compared with the results found in the literature for CdS based samples. The photoionization cross section of the deep centers as a function of the photon energy, at room temperature, is reported for the same sample. Such quantities have been obtained by measuring the spectral density of the photoinduced noise vs wavelength at constant photoconductance value. As for the other kinds of noise spectroscopy, the main advantage of the present method is to work out in the operative condition of the semiconductor device.

Evidence For Non-Correlation Between The 0.15 eV And 0.44 eV Cu-Related Acceptor Levels In GaAs

AbstractWe present investigations on the two dominating acceptor levels observed in Cu-diffused GaAs which have frequently been attributed to the two ionization levels of a double CuGa acceptor. We employed plasma hydrogenation and lithium diffusion followed by reverse-bias and zero-bias annealing to passivate and subsequently reactivate the Cu-related acceptor levels. Deep-level current-transient spectroscopy measurements reveal that the two levels are independently reactivated, strongly indicating that they arise from different defects.

Origin of the defect states at ZnS/Si interfaces

Electrical characterisation of silicon surfaces contaminated by a zinc-sulphide overlayer has been carried out by forming Schottky diodes on the silicon after the ZnS has been etched off. The techniques include current-voltage, capacitance-voltage, and deep-level transieni spectroscopy. The Schottky diodes show clear “memory” of the presence of the ZnS overlayer and the electrical characteristics are far from ideal. Five deep levels in the sub-surface region of the silicon are detected, corresponding to the Zn+, Zn++, S−, S−− states and probably to a Zn−B complex (p-type). Diffusion of the zinc and sulphur into the silicon is therefore confirmed and this diffusion is thought to create a compensated layer at the interface. These impurity states control the electrical characteristics of the surface in these diodes.

Temperature-time duality and deep level spectroscopies

Relaxation of deep levels in semiconductors is studied through capacitance transients. We explore the temperature-time duality relationship which is inherent in such thermal relaxation processes. Using duality considerations we show the existence of four distinct spectroscopies. We demonstrate that the techniques for spectroscopic evaluation of capacitance transients are based on differential operators and provide a novel interpretation to spectroscopy. We extend this approach to higher order spectroscopy. Two families of higher order spectroscopy are analyzed using the formalism of temperature-time duality and differential operators. From duality considerations we have suggested a novel deep level spectroscopy as well as various improvements in line shapes and spectroscopic quality of existing techniques.

Transient current spectroscopy of deep levels in semi-insulating polycrystalline silicon

Poole–Frenkel emission by transient current measurements in semi-insulating polycrystalline silicon containing 35 at. % O has been investigated. In the presence of intense electric fields the conductivity of the material is strongly enhanced by the emission of free carriers. By applying square wave voltage signals, the field strength is modulated, thus producing current transients. By the analysis of these transients we have measured the emissivity of the traps responsible for the Poole–Frenkel effect as a function of temperature. The emissivity shows an activated behavior and the corresponding binding energy of the trap results equal to ≊0.64 eV.

Computer interpretation of phenomenological models of deep-level spectroscopy

There is a need for an easily visualized representation of the models employed in deep-level spectroscopy. A comparative analysis is given of the methods described in the literature for indicating the results of deep-level spectroscopy in order to identify deep centers. A computer approach is suggested for interpreting a model reflecting the activation energy, the capture cross section, and the type of deep levels, and also the multilevel nature of the majority of deep centers.

Low-frequency oscillations in semi-insulating GaAs and InP: deep level domain spectroscopy

Low-frequency-current oscillations in semi-insulating GaAs and InP, believed to be caused by the transit of high-field domains produced by the field-enhanced trapping of electrons, were investigated with the aim of finding the best method of extracting information on deep levels from the temperature dependence of the power spectra of the current, a procedure known as "deep-level domain spectroscopy." With low voltages applied to our specimens of undoped GaAs the conduction was "ohmic", the oscillations were fairly coherent over a range in temperature of about 300–350 K, and since the peaks in the spectra all had the same activation energy within experimental scatter, they were concluded to be harmonics determined by the shape of the high-field domains, with the activation energy corresponding to the trap causing the high-field domains. For higher voltages (above a "knee" in the l–V curve), the oscillations were much larger in amplitude and complex. As in our previous work with InP, the spectra after baseline removal contained peaks with differing activation energies. These peaks are believed to be due to deep levels other than those involved in the field-enhanced trapping process (EL2 in GaAs and an Fe-related level in InP:Fe). Rather than conclude that several traps independently cause high-field domains, it is suggested that the occupancy of such traps will be modulated by the passage of a high-field domain and this will produce the observed peaks. A simplified model is treated analytically. In practice, peaks were observed with differing activation energies only when the oscillations were incoherent and therefore had a broad spectral content. The oscillations occur at sufficiently low average fields to be a potentially serious problem for integrated circuits made on semi-insulating substrates. The maximum entropy (or "all poles") method was compared with windowed fast Fourier transform methods for use in deep-level domain spectroscopy. Results are presented for specimens of undoped and Cr-doped GaAs, and for Fe-doped InP.

An efficient and low cost optical excitation system: application to deep-level spectroscopy

A simple, efficient and low cost optical excitation system (OES) with a semiconductor optical emitter (LED or laser) is described. Construction modifications of such a system suitable for a bath-type cryostat are presented. Some representative results of application of the OES to deep-level spectroscopy (DLS) in semi-insulating GaAs are demonstrated.

Hall effect and deep level transient spectroscopy measurements in indium selenide doped with chlorine

Abstract The temperature dependence of the Hall effect has been studied on InSe single crystals doped with Cl. The electron mobility can be explained by combining the homopolar optical-phonon and the ionized impurity scatterings. Moreover, two electron traps of depths 0.66 and 0.30 eV have been investigated by deep-level transient-capacitance spectroscopy measurements (DLTS). The first level is present in both pure and doped samples, while the second level only appears in doped samples.

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.

Deep Donor and Acceptor Levels Induced by High Temperature and long time Annealing in LEC Gallium Arsenide

ABSTRACTThe effects of high temperature isothermal annealing on the electrical properties of donor and acceptor defects in n-type LEC GaAs are investigated. The annealing experiments are performed under As-rich atmosphere at 1000°C for 1–4 and 16 hours followed by a very quick quenching into cold water of the quartz ampoules containing the samples. The donor and acceptor levels are detected respectively by standard (DLTS) and optical (ODLTS) deep level spectroscopy. DLTS results show the presence of one single donor level present in unannealed and annealed samples at Ec - 0.79eV which is identified as the well known electron trap EL2 Only the sample annealed for 16 hs exhibits the presence of a new electron trap named TAI at Ec - 0.32eV. The appearance of TAI is correlated in one hand with the evolution of EL2 concentration and in the other hand to the effect of long duration (16 hs) of the treatment. For acceptor levels, two hole traps HT1 and HT2 are detected respectively at EV + 0.18 eV and EV+ 0.28 eV. HT1 is detected only in samples annealed for 4 and 16 hs and HT2 is detected in all studied samples. Photoluminescence (PL) measurements show the presence of the 1.44 eV band corresponding to gallium antisite GaAs defect. This band observed in unannealed and annealed samples shows that GaAs remains stable even after thermal annealing at lOOO°C for 16 hs and it is correlated with the presence of HT2.

Transient spectroscopy of deep levels in thin semiconductor films

Characterization of deep levels in thin-film semiconductors by capacitive pulse transient spectroscopy has been analyzed and verified by measurement. Numerical calculations show that highly resistive thin films create a degradation of signal and possible sign inversion of the signal. Experiments on silicon on insulator and molecular beam epitaxy GaAs Schottky barrier capacitors verify the calculated signal deviation.

Spontaneous current oscillations in optically pumped semi-insulating InP

Current oscillations arising from voltage-controlled negative differential resistivity are observed in optically pumped semi-insulating InP at low temperature. The presence of high-field domains sweeping through the sample are detected using photorefractive four-wave mixing. Direct evidence of the role of defects in these current oscillations is presented using deep-level photodiffractive spectroscopy. A deep hole trap with an energy Ev +85 meV is observed. The current oscillations occur only when this defect level is partially occupied.

Deep-level spectroscopy by analysis of isothermal capacitance transients

Deep-level spectrometry of semiconductor has been carried out by measuring isothermal capacitance transients. From the recorded transients, DLTS signals have been reconstructed by software to give Arrhenius plots. Transients have been acquired in two time ranges to increase the emissivity interval on which to observe the trap decay. The method has been tested on Mo/GaAs Schottky barriers and a comparison has been carried out with results obtained on the same diodes using a commercial DLTS system.

Thermoelectric effect spectroscopy of deep levels—application to semi-insulating GaAs

A new method, thermoelectric effect spectroscopy, is proposed for the analysis of deep levels in semi-insulating materials. Besides the information on energy position and relative concentrations of traps, the main advantage of the method is its ability to determine the trap sign, i.e., the method is capable to resolve electron traps from hole traps. The proposed method is very simple for application. It is also shown how in combination with other techniques it can give a complete picture about trap-filling dynamics during low-temperature transient phenomena caused by illumination. The applicability and validity of the method are demonstrated on GaAs in which both electrons and hole traps are found.

Nonlinear spectroscopy of deep levels in wide-gap II–VI semiconductors

Experimental data of nonlinear spectroscopy of intrinsic two-photon (TPA) and impurity two-step (TSA) light absorption in wide-gap semiconductors, illustrating new materials science possibilities in the investigation of band and defect states of the crystals, are discussed. A new field in TPA spectroscopy of band edge deformation by lattice defects or crystal perfection TPA metrology is revealed. Nonlinear deep level active spectroscopy (DLAS) determined by saturated laser modulation of quasistationary TSA via deep levels (DLs) is presented as a method of the investigation and parameter metrology of DLs. The influence of the intrinsic stoichiometric defects on a number of technologically different II–VI crystals is presented experimentally.

Photorefractive characterization of deep level compensation in semi-insulating GaAs

We show that photorefractive beam coupling can be used for deep level spectroscopy in semi-insulating GaAs. In four samples cut from different locations in a single boule of GaAs, we find the same degree of compensation as determined from absorption spectra, Hall measurements, and the photorefractive effect. In the most compensated of our samples, the sign of the photorefractive beam coupling changes from electron to hole dominated as the optical wavelength is changed from 1.06 to 1.15 μm. Furthermore, we show that absorption spectra for semi-insulating GaAs can be used to choose samples for optimal photorefractive behavior at wavelengths of 0.9–1.5 μm.