DX Centers Research Articles

Ionization of the sulfur-related DX Center in In 1 − x Ga x P in an electric field

Using deep-level transient spectroscopy and capacitance-voltage measurements, the effect of electric field on ionization of the sulfur-related DX center is investigated for n-In1 − xGaxP layers (x = 0.65) grown by vapor-phase epitaxy. It is shown that, upon increasing the electric field strength from 1.3 × 104 to 1.9 × 105 V/cm, the activation energy of the center decreases from ∼0.38 to ∼0.26 eV, and the ionization process is governed by multiphonon tunneling. By the estimated tunneling time of the defect, it is concluded that the DX center corresponds to a model of large lattice relaxation.

Experimental observation of “configuration” modes of bistable centers in CdF2:In crystals

New “configuration” modes, which were predicted by us for CdF2:In crystals, have been revealed at the frequencies ν1 ≈ 32.4 cm−1 and ν2 ≈ 96.3 cm−1 for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m1m2/(m1 + 2m2) of the In ion (m1) and two F ions (2m2) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF2 (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N1) and shallow (N2) states. The obtained ratio N2/N1 ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (e′) and imaginary (e″) parts of the dielectric constant has been found (at a frequency of 25 cm−1, Δe′ ≈ 0.2 and Δe″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF2:In, related to the impurity lattice modes, has also been revealed.

IntrinsicDXCenters in Ternary Chalcopyrite Semiconductors

In III-V and II-VI semiconductors, certain nominally electron-donating impurities do not release electrons but instead form deep electron-traps known as "DX centers." While in these compounds, such traps occur only after the introduction of foreign impurity atoms, we find from first-principles calculations that in ternary I-III-VI2 chalcopyrites like CuInSe2 and CuGaSe2, DX-like centers can develop without the presence of any extrinsic impurities. These intrinsic DX centers are suggested as a cause of the difficulties to maintain high efficiencies in CuInSe2-based thin-film solar-cells when the band gap is increased by addition of Ga.

Degradation and their Recovery Behavior of Irradiated GaAlAs LEDs

The degradation and recovery behavior of device performance on GaAlAs LEDs (Light emitting diodes) irradiated by 2-MeV electrons and 70-MeV protons are investigated. The reverse current increases after irradiation, while the capacitance decreases. The device performance degradation is proportional with fluence. For electron irradiation, fluence rate is also effective for degradation. Low fluence rate shows more large degradation compared to high fluence rate resulting from heat impact in bulk. DLTS measurement reveals the DX center in epitaxial substrate, and this spectrum increases with fluence. The radiation damage of proton is larger than that of electron irradiation, which is caused by the difference of mass and possibility of nuclear collision for the formation of lattice defects. After irradiation, the device performance recovers by thermal annealing.

Compensating defects in Si-doped AlN bulk crystals

The rather low n-type conductivity observed in Si-doped sublimation-grown AlN bulk crystals is explained by the formation of high concentrations of compensating defects. The model is based on the experimental verification of a shallow impurity band formed by Si donors and the presence of acceptor-like electron traps within 1 eV below the conduction band edge. Further it is suggested that the majority of the Si donors is compensated by deep acceptors in the lower half of the band gap. This compensation model is an alternative to the controversially discussed assumption of Si DX center formation.

Radiation damages of GaAlAs LEDs by 70-MeV proton and 2-MeV electron irradiation

The electric characteristics of proton and electron irradiated Light emitting diodes are investigated. The reverse current increased proportional with the proton fluence, while the capacitance decreased for increasing irradiation. DLTS measurements revealed the DX center and the spectrum grows for higher irradiation fluence. For electron irradiations, the shape of the I/V and C/V characteristic did not changed, although for higher irradiation fluence rates, the reverse current increased and the capacitance decreased.

Optical properties of compositionally graded InxAl1–xAs/GaAs heterostructures

We report the photoluminescence (PL) properties of compositionally graded InxAl1–xAs (0.13<x<xc) lattice mismatched heterostructures grown on GaAs substrates by Molecular Beam Epitaxy (MBE). Two series of epilayers (xc=0.34 and xc=0.37) were examined in this work by photoluminescence, including step graded and various doping concentrations. At room temperature, the PL spectra of the two samples show the presence of an intense band B attributed to band–band transition in InAlAs active layer. At low temperature (T<200 K), a broad band labelled C appears instead of band B in the PL spectra of the tow samples. This band which is 0.3 eV below the conduction band of the active layer corresponds to the radiative recombination from both native defects during growth and deep levels in InAlAs as DX centers. The PL line width of the band C increases with temperature in both samples. The PL peak of GaAs substrate was buried by other bands when the indium composition and the doping concentration increase.

Energy barrier between states of a bistable center in photochromic crystals CdF2:Ga and CdF2:In

The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF2:In and CdF2:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF2:In and CdF2:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ∼10 and ∼500 meV, respectively.

DX-like centers in InAs∕GaAs QDIPs observed by polarization-dependent Fourier transform infrared spectroscopy

In order to decrease dark current density and improve spectral response tunability of quantum dot infrared photodetectors (QDIPs), it is critical to understand how dopants are incorporated into quantum dot active regions. In this article, polarization-dependent Fourier transform infrared absorbance spectroscopy is used to measure intraband absorption in InAs∕GaAs QDIP heterostructures featuring different molecular beam epitaxy remote doping schemes. In addition to a QD absorbance peak near 90meV, a peak at 405meV is observed. This peak at 405meV demonstrates signature characteristics of DX centers in III-V semiconductors, such as spherical symmetry, thermal activation of trapped carriers with increasing temperature, and dependence of absorbance peak magnitude (and thereby DX center concentration) on doping concentration and doping scheme. A conduction band energy diagram for the observed DX-like center and the corresponding optical ionization process is proposed. In addition, the effect of these DX-like c...

CdF 2 crystals with bistable impurity centers as media of the real-time holography

Bistable (DX) impurity centers in semi-conductors provide opportunity for writing reversible phase hologram due to photo-induced change of the center state. The non-traditional ionic CdF 2 semi-conductor doped with some column-III elements (Ga, In) that form DX centers is promising materials for the real-time holography over a wide time range from seconds to ∼100 ns or less with the temperature used as the controlling parameter. Processes of photo- and thermal-induced conversion of bistable center states as mechanisms of writing and decay of holographic gratings are discussed. Holographic parameters of CdF 2:Ga and CdF 2:In based media are considered as well as their possible use in optical devices.

Electron statistics in CdF2 semiconductor crystals with DX centers

The degree of compensation and ionization energy of two-electron DX centers in CdF2: In and CdF2: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF2: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF2 crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF2: In crystals, the degree of compensation is smaller than that in CdF2: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF2: Ga semiconductor. For a total charged-impurity concentration of ∼1020 cm−3, the width of the impurity band in CdF2: Ga is not likely to exceed ∼0.02 eV.

Capture barrier for DX centers in gallium doped Cd1−xMnxTe

We report on the capture barrier for the gallium related DX center in Cd0.99Mn0.01Te. In order to determine the barrier height, two methods were applied: an analysis of the persistent photoconductivity decay and the optical deep level transient spectroscopy technique. Over a range of temperatures varying from 77to105K, the capture barrier height, deduced from the decay time constants of photoconductivity, has been found to be equal to 0.22eV. An apparent hole trap, observed with the optical deep level transient spectroscopy, was attributed to the thermally activated capture cross section of a DX center with a 0.23eV capture barrier. The obtained data are close to 0.21eV, the value of the capture barrier which we determined earlier with the help of the deep level transient spectroscopy method.

Metastabilities in the electrical characteristics of Au‐CdMnTe Schottky contacts

AbstractSchottky Au‐Cd0.99Mn0.01Te:Ga contacts were studied by the use of the capacitance – voltage measurements and thermally stimulated capacitance technique. Capacitance was recorded within 77 K–400 K temperature range. Persistent photocapacitance was observed at temperatures as high as 260 K. Obtained results can be explained assuming the presence of DX centers in the semiconductor and the interface states at the metal–semiconductor interfacial layer. The low temperature photocapacitance has been attributed to the presence of DX centers whereas that observed at higher temperatures has been linked with the interface states. The presence of the interface states has been confirmed by the thermally stimulated capacitance studies performed at various reverse biases. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Probing the magnetic ground state of theDX−center inCd1−xMnxTe:Inusing muon spin relaxation

Muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements have been made on ${\mathrm{Cd}}_{0.85}{\mathrm{Mn}}_{0.15}\mathrm{Te}:\mathrm{In}$ and ${\mathrm{Cd}}_{0.86}{\mathrm{Mn}}_{0.14}\mathrm{Te}:\mathrm{Al}$. The spin glass transition region has been studied using $\ensuremath{\mu}\mathrm{SR}$ and low-temperature magnetic susceptibility measurements. Persistent photoconductivity (PPC) has been shown to exist only in the In-doped sample, the effect of PPC on the spin glass transition temperature using bulk magnetic susceptibility has also been studied, indicating the presence of bound magnetic polarons. $\ensuremath{\mu}\mathrm{SR}$ on the Al-doped sample clearly shows the spin glass transition, however, the presence of the DX center, which causes PPC when doping with In donors, perturbs the muon response. Particular attention is paid to the possibility of the DX center trapping muonium, preventing the detection of the spin glass transition. PPC does not induce a change in the muon response, however, we find continuous illumination of the sample allows the observation of the spin glass transition. We suggest that these results can be explained by assuming the presence of multiple DX centers. The muon acting as a local magnetic probe of the DX center also suggests the ground state is diamagnetic.

Electric field effect on the electron emission from Te-DX in Al xGa 1− xAs

The present work is aimed to investigate the electron emission from DX centers in tellurium-doped Al x Ga 1− x As with aluminium composition x = 0.40 using deep-level transient spectroscopy. Evidence that Te-DX exhibits an alloy splitting is shown from the temperature-dependent emission at a given reverse bias. The Poole–Frenkel effect was evoked to explain the electric field-enhanced emission. A discrepancy to this model is observed from experimental results. An attempt to explain this discrepancy as due mainly to the short-range character of the Te-DX-related potential.

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

The paper reports on a deep level transient spectroscopy analysis of Te-related DX centers in AlxGa1−xAs with aluminum composition x=0.40. As was shown from experimental results, the state energy of this trap shows a microscopic structure due to an alloy effect. A theoretical analysis based on a multi-step scheme has been made to explain the electron emission from a multiconfigurate center. Using this model, we derived the binding energies of the splitted Te-DX states in the alloy material studied.

Deep levels and low-frequency noise in AlGaAs∕GaAs heterostructures

The low-frequency noise is studied in different AlGaAs∕GaAs heterostructures under electric biases as a function of the temperature. The heterostructures differ in the two-dimensional electron-gas density and the mobility. In addition to the thermal noise, different contributions are found and characterized (1∕f and generation-recombination noises). The presence of the generation-recombination noise is linked to defects in the interface between the channel and electrical contacts. Thermal activation energy values are deduced from Arrhenius plots and related to AsGa antisite defects and DX centers. The 1∕f noise is also characterized and its origin is identified as being the carrier mobility fluctuation dominated by lattice phonons.

Large lattice relaxation deep levels in neutron-irradiated GaN

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) measurements have been carried out in neutron-irradiated n-type hydride-vapor-phase-epitaxy-grown GaN. A defect center characterized by a DLTS line, labeled as N1, is observed at EC−ET=0.17eV. Another line, labeled as N2, at EC−ET=0.23eV, seems to be induced at the same rate as N1 under irradiation and may be identified with E1. Other defects native to wurtzite GaN such as the C and E2 lines appear to enhance under neutron irradiation. The DLOS results show that the defects N1 and N2 have large Frank-Condon shifts of 0.64 and 0.67eV, respectively, and hence large lattice relaxations. The as-grown and neutron-irradiated samples all exhibit the persistent photoconductivity effect commonly seen in GaN that may be attributed to DX centers. The concentration of the DX centers increases significantly with neutron dosage and is helpful in sustaining sample conductivity at low temperatures, thus making possible DLTS measurements on N1 an N2 in the radiation-induced deep-donor defect compensated material which otherwise are prevented by carrier freeze-out.

Generation-recombination noise in pseudomorphic Modulation-doped Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs micro-Hall devices

The noise spectrum in micro-Hall devices based on pseudomorphic Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs modulation-doped heterostructures was measured between 4 Hz and 65 kHz, allowing components due to thermal, 1/f, and generation-recombination to be characterized. Applying deep level noise spectroscopy (DLNS) in the temperature range of 77-300 K to analyze the generation-recombination part of the spectrum, two electron traps contributing to noise density were identified. An emission activation energy of 474 meV was measured for the dominant trap, corresponding to the well-known DX center originating from the AlGaAs barrier. The other deep level, with an emission activation energy of 242 meV, is probably related to defects in the InGaAs layer. The structures under investigation resulted in high-performance micro-Hall devices: a supply-current-related sensitivity up to 725 V/spl middot/A/sup -1//spl middot/T/sup -1/ at 77 K independent of bias current, a signal-to-noise sensitivity of 155 dB/spl middot/T/sup -1/ and a detection limit of 340 pT/spl middot/mm/spl middot/Hz/sup -1/2/ at 77 K were measured.

Photoinduced submillimeter-wave amplitude diffraction grating in a semiconducting CdF2:Ga crystal

It is shown that exposure of an additively colored CdF2:Ga crystal with bistable DX centers that is slowly cooled to 150 K to blue-green light through a slotted mask produces a submillimeter-wave diffraction grating, which persists for a long time at temperatures of 160–240 K. It is also shown that the diffraction grating induced in a sample is an amplitude grating. The absorption of submillimeter waves in illuminated regions of the sample is associated with the conductivity due to the transition of impurity centers to a metastable donor state. In the n-i-n-i-type conducting structure obtained, the conductivity of n-type regions at 225 K amounts to σ ′ ≈ 0.24 Ω−1 cm−1.