EL2 Trap Research Articles

Variations of electron traps in bulk n-GaAs by rapid thermal processing

Rapid thermal processing (RTP) using halogen lamps for liquid encapsulated Czochralski grown GaAs doped with Si has been studied by deep-level transient spectroscopy, capacitance-voltage, and photoluminescence measurements. RTP is performed at 700, 800, and 900 °C for 6 s with and without SiO2 encapsulation. Three electron traps ED1(Ec −0.26 eV), ED2(Ec −0.49 eV), and ED3(Ec −0.55 eV) are produced depending on RTP conditions. The trap ED1 is observed in all RTP samples, and its depth profiles vary with RTP conditions. The decrease of the shallow donor concentration occurs for RTP samples above 800 °C, especially for those without encapsulation. It is thought that these results, which are peculiar to the RTP method, are related to the production of As interstitials and As vacancies by the large thermal stress induced by the rapid heating for RTP. The decrease of the concentration of trap EL2(Ec −0.81 eV) is not observed with RTP as reported in the furnace processing. This is due to the effect of the short-time processing of RTP.

Electron traps in AlGaAs grown by molecular-beam epitaxy

Using deep-level transient capacitance spectroscopy we have investigated deep electron traps in n-AlGaAs grown by molecular-beam epitaxy (MBE). The thermal activation energies of seven traps, labeled ME1–ME7, observed in this study increase with increasing Al content(x) up to the direct-indirect crossover point (x∼0.42), but show only a small change with further increases in Al content. Traps ME4–ME7 are dominant in samples with x≤0.2. Traps ME4–ME6 strongly depend on the growth ambient. The concentration of ME7 is almost independent of the ambient in the growth chamber but decreases rapidly with increasing growth temperature. ME7 is a native defect and can almost certainly be identified with the trap EL2 observed in bulk and vapor-phase epitaxially grown GaAs. Traps ME4–ME6 are probably formed by impurities involving oxygen such as CO, H2O, and AsO in the growth ambient. All of the traps, ME5–ME7 are clearly responsible for a decrease in the photoluminescence intensity of MBE grown AlGaAs.

Optical Assessment of the Interaction Between EL2 and EL6 Levels in Boron Implanted GaAs

The majority optical cross section and the concentration of levels with optical response induced by boron implantation at 1010 ions/cm2 in GaAs, have been obtained using the technique labelled Optical Isothermal Transient Spectroscopy, OITS, in an energy range between 0.8 eV and 1.4 eV. The experimental optical data measured at different annealing temperatures, show only the presence of the EL2 and EL6 electron trap levels. The optical behaviour found is in complete agreement with the existence of an interaction between these two deep levels, EL2 and EL6. Besides, this interaction has been found to be the origin of the U-band in the DLTS spectra and of the anomalous dependence of the level concentration on the annealing temperature deduced from the OITS and DLTS spectra.

Observation of bias-dependent capture-emission processes in MBE-grown GaAs layers

The capture-emission process in a semiconductor sample was greatly affected by a varying electric field in DLTS analysis. The sample was an n-type GaAs layer grown on a 〈100〉-oriented seimi-insulating substrate by molecular beam epitaxy. An E c − 0.5 eV electron trap level was found under lower bias voltage (−6 V) while both E c − 0.40 eV and E c − 0.50 eV traps were observed under higher bias voltage (−7.0 to −8.0 V). Emission rates have been measured that increased with increasing bias voltage from 0 V to −5 V and then decreased when biases were higher than −5 V. In the capacitance transient, when it revealed a delay, two levels, namely at −0.4 and −0.5 eV, appeared in the DLTS signals. However, the EL2 trap ( E c − 0.78 eV), found on another sample, revealed monotonically increasing emission with increasing bias. An explanation of these phenomena was proposed in light of the interaction of these two levels due to the electric field effect.

Rapid-Thermal-Processing Induced Deep Level Traps and their Spatial Distribution in MBE GaAs

ABSTRACTRapid thermal processing (RTP) using halogen lamps for a Si-doped molecular beam epitaxial (MBE) n-GaAs layers was investigated by deep level transient spectroscopy. RTP was performed at 700°C, 800°C and 900°C for 6 s. Two electron traps NI ( Ec-0.5-0.7eV) and EL2 (Ec - 0.82 eV) are produced by RTP at 800 and 900°C.The peculiar spatial variations of the Nl and EL2 concentration across the MBE GaAs films are observed. The larger concentrations of the trap N1 and EL2 are observed near the edge of the samples, and the minima of N1 and EL2 concentration lie between the center and the edge of the sample. It seems that these spatial variations of N1 and EL2 concentration are consistent with that of the thermal stress induced by RTP. Furthermore, the EL2 concentration near the edge of the sample is suppressed by the contact with the GaAs pieces on the edge around the sample during RTP.

Analysis of the near-intrinsic and extrinsic photocapacitance due to the EL2 level in boron-implanted GaAs

A detailed analysis of the photocapacitance signal at the near-band and extrinsic energetic ranges in Schottky barriers obtained on horizontal Bridgman GaAs wafers, which were implanted with boron at different doses and annealed at several temperatures, has been carried out by using the optical isothermal transient spectroscopy, OITS. The optical cross sections have been determined as well as the quenching efficiency of the EL2 level which has been found to be independent of the annealing temperature. Moreover, the quenching relaxation presents two significant features: (i) a strong increase of the quenching efficiency from 1.35 eV on and (ii) a diminution of the quenching transient amplitude in relation with that shown by the fundamental EL2 level. In order to explain this behavior, different cases are discussed assuming the presence of several energy levels, the existence of an optical recuperation, or the association of the EL2 trap with two levels located, respectively, at Ev+0.45 eV and Ec−0.75 eV. The theoretical simulation, taking into account these two last cases, is in agreement with the experimental photocapacitance data at low temperature, as well as at room temperature where the EL2 filling phototransient shows an anomalous behavior. Moreover, unlike the previous data reported for the EL2 electron optical cross section, the values found using our experimental technique are in agreement with the behavior deduced from the theoretical calculation. The utilization of the OITS method has also allowed the determination of another level, whose faster optical contribution is often added to that of the EL2 level when the DLOS or standard photocapacitance is used.

Deep states in GaAs LEC crystals

Deep level defects in n-GaAs LEC crystals have been investigated by computerised deep level spectroscopy. We observed two electron traps with energies of 0.83 eV (EL2) and 0.38 eV (EB6). The effects of high temperature (850°C) heat treatment on the grown-in defects were also studied. The results showed that three electron traps with emission activation energies of 0.14, 0.35 and 0.44 eV (EI1) were generated, and that the 0.38 eV (EB6) level was removed as a consequence of the anneal. It is believed that the first two levels are reported for the first time. It was also found that the main electron trap in GaAs, 0.83 eV (EL2), was reduced in concentration as a result of annealing. The depth distribution of this electron trap was measured taking into account the nonionized front region of the depletion layer. Radial distributions of the EL2 trap and shallow donors have also been characterised. Both profiles were W-shaped.

Study of electron traps in n-GaAs resulting from infrared rapid thermal annealing

Electron traps in n-GaAs resulting from infrared rapid thermal annealing (IRTA) have been studied by deep-level transient spectroscopy (DLTS). An electron trap with an activation energy of 0.20 eV from the conduction band, termed EN1, is introduced by IRTA above 800 °C. This trap formation is closely related to a rapid heating stage in an IRTA process. The EN1 concentration changes similarly to the concentration of the dominant midgap electron trap EL2 versus the variation of annealing temperature, annealing time, or encapsulating films. It is proposed that the IRTA-related EN1 trap is ascribed to defect complexes, including AsGa antisite defects, such as VAsAsGa.

Optical behavior of the U band in relation to EL2 and EL6 levels in boron-implanted GaAs

The majority-carrier optical emission cross section and the concentrations of the levels induced by boron implantation at 1010 ions/cm2 in GaAs have been obtained usng a new optical isothermal capacitance transient spectroscopy in the photon energy range 0.8–1.4 eV. The experimental optical data obtained at different annealing temperatures show only the presence of the EL2 and EL6 electron trap levels. Moreover, they are in complete agreement with the existence of an interaction between these two deep trap levels. Besides, this interaction has been found to be at the origin of U band in the deep level transient spectroscopy (DLTS) spectra and of the anomalous dependence on the annealing temperature of the level concentrations deduced from the optical technique and DLTS spectra. off

Evidence for two energy levels associated with EL2 trap in GaAs

By photocapacitance technique, applied to n-type LEC-grown GaAs, two energy levels:E v +0.45 eV andE c −0.75 eV are identified, for the first time, as being associated with the EL2 trap. As follows from the analysis of photo-EPR results on highly resistive GaAs crystals, the same energy levels can be attributed to the arsenic antisite defect, AsGA. In view of these findings, it is argued that the occupied EL2 level corresponds to the neutral charge state of AsGa defect.

Inhomogeneity of the deep center el2 in GaAs observed by direct infra-red imaging

The distribution of the dominant deep trap EL2 in 7.5cm diameter crystals of semi-insulating GaAs is studied by whole slice infrared imaging. Very significant fluctuations in the neutral EL2 concentration ([EL2]ℴ) are observed, corresponding at most to variations in [EL2]ℴ of up to 80%. The different sorts of fine structure, namely cell structure and bands of high infrared absorption ("sheets" and “streamers”) lying in (110) planes running down the growth directions, are described.

AsGa antisite defects in GaAs

Recent experimental results concerning the anion antisite defect AsGa in GaAs are discussed. AsGa appears to be a double donor with ionization energies corresponding to the Fermi level pinning energies at GaAs surfaces. It gives rise to the dominant midgap trap EL2, showing its characteristic photo-quenching effects. Evidence has been found indicating AsGa to be a killer center for band-gap light emission. During dislocation motion antisite defects are produced so that they may be involved in laser degradation processes.

EFFECTS OF GROWTH CONDITIONS ON DEEP-LEVEL DEFECTS IN VPE AND LPE GaAs

Studies of the deep-level defects in the VPE and LPE GaAs layers under various growth conditions have been made in this work, using DLTS and C-V techniques. In the VPE grown GaAs, characterization of grown-in defects vs. Ga/As ratio (i.e., 2/1, 3/1, 4/1, and 6/1) were made on GaAs layers grown on , , and substrates. The two common electron traps observed in these samples were due to the EB-4 (i.e., Ec -0.71 eV) level and the EL-2 (i.e., E -0.83eV) level. The density of these two electron traps was found to depend on the stoichiometry as well as substrate orientation. For examples, in the orientation samples, the density of EL2 was found to decrease with increasing Ga/As ratio (e.g., NT = 1.2x1014cm-3 for Ga/As = 2/1, and reducing to 5.4x1011 cm-3 for Ga/As = 6/1). A similar result was also observed in the EB-4 electron trap. Low temperature (230°C) thermal annealing and recombination enhanced annealing studies on these samples showed significant reduction in the density of EB-4 trap and slight reduction in EL-2 trap density. For the LPE grown GaAs layers, two growth temperatures (700 and 800°C) and two temperature dropping rates (l°C/min. and 0.4°C/min.) were used. The DLTS and C-V measurements were made on these samples, and the results showed that only EB-4 electron trap (i.e., Ec-0.71 eV) was observed in the LPE GaAs samples with 1°C/min. dropping rate. Details of our DLTS analysis of the grown-in defects vs. growth conditions in both LPE and VPE grown GaAs layers will be discussed in this paper.

Relation between Cr-Level and Main Electron Trap (EL2) in Boat-Grown Bulk GaAs

In order to characterize the Cr (Chromium)-level in bulk GaAs, a normal DLTS method was applied to boat-grown bulk GaAs which was Cr- and O (Oxygen)-doped but did not become semi-insulating. In the lightly-compensated region, no Cr signal was observed, probably because it is covered by the signal from the main electron trap EL2. In the moderately-compensated region, both signals of Cr and El2 were observed, but the trap concentration of EL2 was greatly reduced. In the heavily-compensated region, a large Cr-trap signal was observed, but no signal of EL2 could be observed even when the minority carrier injection was suppressed. Since Cr occupies a Ga site and forms deep acceptors, these results can be understood by considering that the Cr doping reduces the number of Ga vacancies and as a result the number of main electron traps EL2 which are produced by a Ga deficit is also reduced.