Lattice Dynamical Investigations on DX Center Localised Vibrational Modes in AlxGa1−xAs and AlxGa1−xAs:Si

We find the DX centers in Si-doped AlAs for the first time. The activation energy is measured as 0.56 eV from deep level transient spectroscopy (DLTS). The DX centers in n-AlAs exhibit a large capture energy 0.5 eV and a persistent photoconductivity. These properties are similar to those of the DX centers in AlxGa1−xAs with x∼0.3. However, the carrier concentration in the DX centers revealed by DLTS is not linearly proportional to Si donor concentration. This result is interpreted by the band structure that the DX center level lies at 30 meV above the X-conduction band (CB) minima and at 150 meV below the L-CB minima. The DX center is found not to be associated with the X-CB minima, but the L-CB minima.

A deep-level transient spectroscopy (DLTS) technique is reported for determining the capture cross-section activation energy directly. Conventionally, the capture activation energy is obtained from the temperature dependence of the capture cross section. Capture cross-section measurement is often very doubtful due to many intrinsic errors and is more critical for nonexponential capture kinetics. The essence of this technique is to use an emission pulse to allow the defects to emit electrons and the transient signal from capture process due to a large capture barrier was analyzed, in contrast with the emission signal in conventional DLTS. This technique has been applied for determining the capture barrier for silicon-related DX centers in AlxGa1−xAs for different AlAs mole fractions.

Deep-level admittance spectroscopy (DLAS) of DX centers in AlxGa1−xAs:Sn (0.2<x<0.6) reveals the presence of three levels SN1, SN2, and SN3 related to the Sn donor. While SN1 and SN3 are observed in all the samples, SN2 is prominently seen only in the indirect band-gap samples. The conventional capacitance deep-level transient spectroscopy (DLTS) is found to be unsuitable for the study of the DX center in AlxGa1−xAs:Sn with x>0.35 because of the strong freeze-out of free carriers in these samples. Even in the case of low AlAs mole fraction samples (x<0.35), the DLTS technique fails to reveal all the levels observed by DLAS and provides information only on the SN3 level.

The donor-related deep electron traps in Te-doped Alx Ga1−x Sb on GaSb substrate were investigated by deep level transient spectroscopy, capacitance-voltage, photocapacitance, and Hall-effect measurements. Deep electron traps were not detected in the Al composition range 0≤x<0.2, but were detected in the higher range of x. The concentration of the deep electron traps increases steeply with x and then saturates. The concentration also increases linearly with donor concentration for the same Al composition. In the temperature-dependent Hall-effect measurement, both shallow donor and deep donor levels were observed. The deep donor is dominant for x≥0.4, and the thermal activation energy E0 increases dramatically from 6 to 110 meV in the range of 0.2<x≤0.5. Persistent photoconductivity was observed for x≥0.3 at temperatures below 100 K. All the experimental results indicate that the deep electron traps in Te-doped Alx Ga1−x Sb are quite similar to the DX center in Alx Ga1−x As.

The field effect has been investigated on the deep levels and DX centers in molecular-beam epitaxilly-grown AlxGa1−xAs on n+ GaAs for several compositions. We have measured the isothermal and isofield capacitance transients for the major energy levels with the applied potential in the range from −0.1 to −3 V. In samples with x=0.5, two electron trap levels (E1 and E2) were detected. While E1 was strongly field dependent (changed from 0.499 to 0.287 eV) E2 was practically unchanged (0.305 eV) with respect to energy, cross section, and peak broadening. Contrary to this in pure GaAs (samples x=0), we observed three energy levels, one hole trap (E1=0.239 eV), one electron trap (E2=0.512 eV), and the usual EL2 trap (E3=0.72 eV). At this composition, however, levels E1 and E2 were absent at low reverse fields (at −1 V) and EL2 remained unchanged at 0.72 eV. We have considered the recent theories of Pool–Frenkel effect and tunneling for electron–phonon interaction to interpret the results. In particular, for AlxGa1−xAs with x=0.5, the electron–phonon coupling factor (S) was found to be about 4 and the deep centers were identified as neutral to repulsive.

In this work we have applied the admittance spectroscopy technique to characterize the DX centers in AlxGa1−xAs alloys doped with silicon. Our experimental results reveal the existence of two DX centers related to silicon in AlxGa1−xAs alloys, named DX-I and DX-II centers, with thermal activation energies of 0.370 and 0.415 eV, respectively. These values are lower than those obtained by other authors using capacitance techniques. To explain this disagreement it should be noticed that capacitance techniques can be affected by the nonexponential behavior of the thermal emission transients of the DX centers in AlxGa1−xAs alloys.

We have utilized the isothermal capacitance transient spectroscopy and modulated function waveform analysis to investigate the deep levels and DX centers in molecular beam epitaxially grown AlxGa1−xAs/GaAs Schottky diodes deposited on n+ GaAs, with Al composition ranging from x=0 to 0.5, and implanted with Si. In the concentration range x=0.19 to 0.50, two major electron trap levels (E1 and E2) were detected, which gradually changed with composition. For example, E1 changed from 0.393 to 0.339 eV and E2 changed from 0.136 to 0.287 eV. However, in pure GaAs, three major trap levels with energy E1 (hole)=0.226 eV, E2 (electron)=0.496 eV, and E3 (EL2)=0.74 eV were observed. Apparently, these levels are governed by the deep levels known as DX centers.

Evidence for two Si-related DX like centers in Al xGa 1− xAs and GaAs

Deep levels in Si-doped Al xGa 1 − xAs layers

Alloy splitting of Te-DX in AlxGa1−xAs analysis using the deep level transient spectroscopy technique

Field effect studies have been performed on the deep levels in molecular beam epitaxially grown AlxGa1−xAs (with x=0.385) and InxGa1−xAs (with x=0.1) on n+-GaAs. We have measured the isothermal and isofield capacitance transients for the major energy levels with the applied electric field [here, field means normalized field, F(norm)=F(appl)×10−5] in the range −0.5 to −5 V/cm. The results of our investigation suggest that the applied field has a distinct effect on thermal emission rate, capture cross section and activation energy. In particular, the Arrhenius plots showed a nonlinear behavior that could be associated with electron–phonon interactions. Furthermore, the activation energy versus field plots indicate a linear behavior, illustrating a complex nature of the deep levels some of which, E1 in AlxGa1−xAs and E3 in InxGa1−xAs, can be characterized as DX centers.

DX-like centers in n-type Al-doped ZnS1−xTex grown by molecular-beam epitaxy

DX centers in selenium doped AlxGa1−xAs with two values of the aluminum content, x=0.34 and 0.48, are carefully analyzed by three different techniques: deep level transient spectroscopy (DLTS), admittance spectroscopy, and the capacitance voltage transient technique (CVTT). We use conceptual differences between these techniques to extract important information about the nature of the DX centers. Good agreement is found between the capacitance transients recorded during the DLTS measurements and those obtained by CVTT at every point in the space charge region. From that, we conclude that is the very nature of the DX centers the solely responsible for the anomalies found in DLTS results. The main cause for these anomalies is the thermal dependence of the electron capture rate of these centers. CVTT curves also reveal that no electric field enhanced emission processes take place for these centers. For our analysis of the shape of the DLTS and admittance spectroscopy curves we conclude that several DX levels exist, according to the alloy broadening theory. Finally, some simulations of the DLTS spectra were made. These calculations reveal the important effect of experimental parameters such as the filling pulse duration, the velocity of the temperature scan, and the initial conditions of the occupation factor of the deep levels on the DLTS curves.

The characterization of deep levels generated by donor dopants (DX centers) requires reliable deep level transient spectroscopy (DLTS) data. Because in AlGaAs and GaAsP the electron thermal emission from DX centers produces strong nonexponential capacitance transients, blind DLTS analysis may lead to erroneous trap parameter determinations. In this work the DLTS sampling conditions to be used for proper DX center characterization are examined. It is concluded that, only under constant tb/ta windowing ratios, self-consistent trap parameters are obtained.

DX centers in In-mixed AlGaAs alloys are analyzed by deep level transient spectroscopy and capacitance vs temperature measurements. The addition of In to Si-doped AlGaAs, with x=0.21 and 0.30, shifts the Si-DX center to a shallower position. Under hydrostatic pressure, DX centers deepen again into the band gap. The DX center shift, and consequently, the reduction of the DX center electron occupancy, when In is added, is due to an increase of the Γ to L energy difference. In terms of band-gap energy and DX center depth, adding 1% In is equivalent to a 1% Al reduction. Then, In mixing does not offer any new benefit to minimize DX center effects in AlGaAs-based heterojunction devices.