DX Centers In GaAs Research Articles

First Principle Calculation of the DX-CenterGround-States in GaAs, AlxGa1-xAs Alloysand AlAs/GaAs Superlattices

The atomistic nature of the deep donor levels referred to as DX centers in GaAs, Al x Ga 1- x As alloys and AlAs/GaAs superlattices is investigated by applying the ab initio self-consistent pseudopotential method to 64-atom super cells. The total energy and force calculations reveal that the most stable state is the T d -distorted atomic configuration around a substitutional donor with the neighbouring bond relaxation given by the effective ionic radius (0.7% for Si G a ). On the contrary, a negatively charged donor with broken-bond configuration is shown to give a metastable state and not the ground state even under high hydrostatic pressures. It is found that a deep A 1 level attributable to the DX center is induced by a simple substitutional donor with the obtained optimal lattice configuration. The theory successfully explains experimental results on the thermal and optical ionization energies for the DX centers.

Comment on "Energetics of DX-center formation in GaAs and AlxGa1-xAs alloys"

The prediction of Chadi and Chang [Phys. Rev. B 39, 10 063 (1989)] that the DX center in GaAs is a negatively charged defect is investigated. It is found from ab initio self-consistent pseudopotential calculations that the proposed negatively charged center is a metastable state and not a ground state at hydrostatic pressures up to 60 kbar.

Structure and dynamics of the DX center in GaAs:Si.

We have carried out local-density functional cluster calculations on the ${\mathrm{Si}}_{\mathrm{Ga}}^{\mathrm{\ensuremath{-}}}$ defect in GaAs. We find that the distortion proposed by Chadi and Chang involving a large Si movement along 〈111〉 breaking an Si-As bond has the same energy as a simple breathing distortion. The calculated local vibratory modes of one Chadi-Chang structure do not agree with those recently assigned to DX from an infrared-absorption experiment. On the other hand, the calculated triplet mode due to the other structure above is in reasonable agreement with these observations. The effective charge of the second type of defect is about three times that of the first. It is proposed that both defects coexist with the Chadi-Chang one being almost infrared inactive.

Si DX centers in GaAs at large hydrostatic pressures

A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of SiGa and SiDX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.

Spatial Correlation of Impurity Charges Induced by Coulomb Interactions - Application to DX Centers in GaAs

The electron mobility enhancement observed in heavily doped GaAs under hydrostatic pressure is interpreted in terms of spatial correlation between the donor charges within partially occupied system of impurities induced by strong inter-donor Coulomb interaction. A simple analytic theory is given for both DX 0 and DXmodels of the impurity state. The mobility is shown to increase together with pressure in both models. Estimates of the energy of the DX level are strongly perturbed by the inter-donor Coulomb interactions.

Pressure-induced shallow-to-deep donor-state transition in 119Sn-doped GaAs observed by Mössbauer spectroscopy.

The Sn DX center in GaAs, a deep donor state of Sn, has been observed by M\"ossbauer measurements at high pressure. The size of the pressure-induced Sn DX M\"ossbauer resonance compared to the net conduction-electron concentration at zero pressure provides evidence that the Sn DX center localizes two or three electrons in the ground state.

Pressure dependence of electron scattering by DX centres in GaAs and AlGaAs

Abstract The electron mobility of heavily n-dopped GaAs and AlGaAs increases rapidly with applied hydrostatic pressure as carriers are trapped at DX centres. This has been taken as evidence against the Chadi and Chang (CC) negative charge state model of the DX centre. We argue that DX− centres are formed close to d+ centres in highly doped samples, and that the mobility data is in fact fully consistent with the CC model, when such dipole-like correlations are included.

Electrical properties of DX centers in GaAs and AlGaAs

Abstract The DX center, the lowest energy state of the donor in AIGaAs with x < 0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AIGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 × 1019 cm−3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed.

DX centers in III–V compound and alloy semiconductors as studied by hydrostatic pressure experiments

The metastable nature of DX centers in GaAs and related alloys is shown with particular emphasis on the role of the conduction band structure and the large DX-center lattice relaxation, based on DLTS and persistent photoconductivity measurements under hydrostatic pressure. The observation of similar deep centers in GaInP and AlGaSb under hydrostatic pressure is also described. The chemical trends of DX centers in GaAs under pressure suggest the importance of further study of the microscopic relaxation around the center.

DX center in Ga1-xAlxAs alloys.

From the electron-emission and -capture variations with the alloy composition, we deduce that the DX center in ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As alloys is linked to the L conduction band. This is verified by the study of the DX center in GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As superlattices, where it is observed that the DX center does exist only in GaAs-AlAs superlattices such that the L miniband which replaces the L band of the AlAs barrier is at the same energy as the bottom of this L band, although the X conduction band has been replaced by a \ensuremath{\Gamma} miniband. This property can only find an explanation in the effective-mass approximation and cannot be justified in large--lattice-relaxation models. Within this model the DX level is the effective-mass state of the doping impurity associated with the L band which undergoes a shallow-deep instability due to intervalley mixing. This description implies a small, if any, electron-phonon interaction. From our electron-paramagnetic-resonance studies we conclude that the large threshold for optical ionization involves a transition to a higher state than the bottom of the conduction band and must probably be attributed to an internal transition. The barrier for electron capture is explained as originating from a change of the local force constants of the defect with its charge state. The variation of the capture barrier height with alloy composition is quantitatively accounted for by electron emission from the bottom of the conduction band into the L band, followed by a multiphonon-emission process over a constant barrier.

Ion implantation in semiconductors studied by Mössbauer spectroscopy

The application of Mossbauer spectroscopy as an extremely sensitive characterization technique for ion-implanted semiconductors, is illustrated. Factors influencing the final landing site of implanted ions are first reviewed, as well as ion beam induced material modifications. Recent applications of Mossbauer spectroscopy in this field are then discussed including the study of supersaturated solutions of Sb and Sn in Si, the formation of epitaxial and buried silicides and the search for the DX-center in GaAs.

Study of DX Centers in GaAs1−xPx :Te

ABSTRACTA capacitance study of GaAs1−xPx : Te. x &gt; 0.2, obtained by vapor phase epitaxy, has revealed two DX centers, characterized by thermal activation energies ΔEA = 0.18 eV and ΔEB = 0.39 eV. and photoionization energies 0.6 eV and 1.1 eV, respectively. Both traps show strong non-exponential behavior in thermal emission and capture processes. It is shown that correct parameter values for DX centers can be determined from measurements of the transition region width. This method, which allows us to circumvent alloy effects, has been used to study DX center parameters as a function of the phosphorus content. It is demonstrated that the level B is linked to the X minimum, and that the barrier height for electron capture at this trap, with respect to the indirect minimum, is independent of alloy composition.