Acceptors In CdTe Research Articles

Application of the difference spectroscopy for studying of complex acceptors in CdTe

AbstractWavelength modulation of the excitation source combined with the analysis of the differential PL signal have been applied to studying of electronic spectra of acceptors in compensated CdTe:Cl, CdTe:Ag,Cl single crystals. For the Cl‐related complex acceptor with an activation energy of ∼121 meV (A‐center), the energies of eight excited states are measured. In the case of tetrahedral AgCdacceptor the splitting of 2P3/2 (Γ8) and 2S3/2 (Γ8) states has been revealed for AgCd centres located at a short distances (5‐7 nm) from a hydrogen‐like donor (ClTe). A method allowing a simple symmetry test for acceptors in a diverse zinc‐blende compound semiconductor is proposed. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (TD) AgCd acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P3/2 (Г8) and 2S3/2 (Г8) states is clearly observed for AgCd centers located at a short distance (5–7 nm) from a hydrogen-like donor (ClTe). This splitting results from the reduction of the TD symmetry taking place when the acceptor is a member of a donor–acceptor pair. For the Cl-related complex acceptor with an activation energy of ∼121 meV (A-center), the energies of eight excited states are measured. It is shown that this defect produces low-symmetry central-cell correction responsible for the strong splitting of S-like TD shells. The energy spectrum of the Bi-related shallow acceptor with an activation energy of ∼36 meV is measured as well. The spectrum obtained differs drastically from the hydrogen-like set of levels, which indicates the existence of repulsive low-symmetry perturbation of the hydrogen-like Coulomb potential. It is also shown that the spectra of selectively excited PL recorded for a macroscopic ensemble of distant donor–acceptor pairs allow one to detect the low symmetry of acceptors of a given type caused by their complex nature or by the Jahn–Teller distortion. This method does not require any additional (external) field and is applicable to acceptors in diverse zinc-blende compound semiconductors.

Numerical Simulation of Copper Migration in Single Crystal CdTe

With better CdTe material properties and more effective contacts, the compensated Cu doping is crucial for enhancing the performance of CdTe solar cells. Moreover, the metastable behavior of CdTe devices is associated with Cu migration. To understand the formation of Cu acceptors in CdTe and Cu-related metastabilities, we developed a one-dimensional diffusion-reaction simulator to study the kinetics of Cu-related defects in CdTe material. A detailed kinetic model that explicitly describes the evolutions of point defects was developed and applied for the first time. Migration of intrinsic and Cu-related extrinsic defects in single crystal CdTe (sx-CdTe) bulk has been investigated in time-space domain self-consistently with free carrier transport. Cu profiles achieved from simulations match with the experimental profiles quantitatively. The simulation results provide explanations of limited Cu incorporation (also known as the Cu solubility limits) and the development of Cu compensation in course of annealing and cooling processes. Although the one-dimensional simulation has intrinsic limitations when applied to poly-crystalline CdTe solar cells, the results suggest that the approach still has strong potential in better understanding the performance and the metastabilities of poly-crystalline CdTe photovoltaic devices.

Complex acceptors in CdTe:Cl investigated by differential spectroscopy

The acceptor states in CdTe:Cl single crystals are investigated using a technique based on the analysis of differential spectra of selectively excited donor-acceptor pair photoluminescence. The splitting of 2P 3/2(Γ8) and 2S 3/2(Γ8) states of the AgCd tetrahedral acceptor taking place upon a decrease in the donor-acceptor separation is found. The energies of seven excited states of the complex acceptor with an activation energy of ∼121 meV are determined.

First-principles-based analysis of the influence of Cu on CdTe electronic properties

The maximum voltage of CdTe solar cells is limited by low majority carrier concentration and doping difficulty. Copper that enters from the back contact can form both donors and acceptors in CdTe. It is empirically known that the free carrier concentration is several orders lower than the total Cu concentration. Simplified thermodynamic models of defect compensation after Cu introduction can be found in literature. We present a first-principles-based analysis of kinetics of defect formation upon Cu introduction, and show that Cui is mobile at room temperature. Calculations of properties of Cui–VCd and Cui–CuCd complexes show that the neutral Cui–CuCd complex is mobile at elevated temperatures, while formation of the VCd–Cui complex is unlikely because it transforms into the CuCd defect.

On hydrogen transport VPE-grown CdTe epilayers for fabrication of 1–100 keV X-ray detectors

The hydrogen transport vapour-phase epitaxy growth of CdTe on ZnTe/GaAs hybrid substrates for the realisation of detector-grade epilayers is reported. Growth rates close to 10 μm/h were achieved by growing CdTe at around 770°C and with the source temperature at 827°C. The crystalline structure of the as-grown epilayers was investigated by double crystal X-ray diffraction measurements. Despite the large epilayer/substrate lattice misfit, the crystalline perfection of CdTe layers rapidly improves with the thickness; indeed, FWHM values of the (4 0 0)CdTe peak down to 59 arcsec are obtained for ∼30 μm thick samples; this value compares well with results published in the literature for best epitaxial CdTe. Correspondingly, mirror-like and nearly featureless surfaces were observed. CdTe samples grown below 650°C are p-type and low resistive [ ρ≤20 Ω cm at room temperature (RT)], whilst at temperatures >650°C the layers are n-type. In this case, resistivity ∼10 6 Ω cm are obtained for growth between 675°C and 700°C, but lower values result above 700°C. However, resistivity in the 10 4–10 5 Ω cm range are achieved in the latter case by increasing the epilayer thickness up to 30 μm. This behaviour is ascribed to the occurrence of Ga-related donors in CdTe, the latter originating by the diffusion of Ga atoms from the GaAs substrate. These donors are almost compensated by residual acceptors in CdTe, giving rise to the medium/high resistivity values observed for some of the samples. The detection capability of the material has been demonstrated by time-of-flight (TOF) measurements performed on a device made by an n-CdTe epilayer. The analysis of the TOF collected charge as a function of the applied reverse voltage gives μτ≈4.9×10 −6 cm 2/V for this material. Such low value arises from a large electron trapping in CdTe epilayers, consistently with the ∼10 18 cm −3 density of compensated (positively charged) donors.

Calculated Electric Field Gradients and Electronic Properties of Acceptors in CdTe

Linearised augmented plane wave (LAPW) calculations are performed in order to determine the electric field gradients (EFG) induced by group V and group Ib acceptors in CdTe at the nearest neighbour (NN) atomic site. Experimentally, the EFG are measured by the perturbed γγ-angular correlation spectroscopy using the radioactive probe isotopes 111In and 77Br. Besides the identification of the experimentally observed defects, the LAPW calculations provide information about fundamental properties of the EFG. For the group Ib acceptors, the dependence of the EFG on the NN distance is opposite to the expectation from the simple point charge model.

Defect complexes formed with Ag atoms in CdTe, ZnTe, and ZnSe

Using the radioactive acceptor 111Ag for perturbed γγ-angular correlation (PAC) spectroscopy for the first time, defect complexes formed with Ag are investigated in the II–VI semiconductors CdTe, ZnTe, and ZnSe. The donors In, Br and the Te-vacancy were found to passivate Ag acceptors in CdTe via pair formation, which was also observed in In-doped ZnTe. In undoped or Sb-doped CdTe and in undoped ZnSe, the PAC experiments indicate the compensation of Ag acceptors by the formation of double broken bond centres, which are characterised by an electric field gradient with an asymmetry parameter close to η=1. Additionally, a very large electric field gradient was observed in CdTe, which is possibly connected with residual impurities.

The Incorporation and Complex Formation of Ag Acceptors in CdTe

AbstractUsing the radioactive isotope 111Ag, the incorporation of Ag into CdTe is investigated by photoluminescence spectroscopy (PL) and the perturbed yy-angular correlation technique (PAC). PL is used to demonstrate the incorporation as a substitutional acceptor (AgCd) by monitoring the intensity of the donor-acceptor transition, corresponding to an acceptor level of EV + 108 meV, which decreases just with the half life of the radioactive decay of the 111Ag isotope. Since the isotope 111Ag also serves as a probe atom for PAC experiments, it is possible to confirm the incorporation of Ag atoms on lattice sites with cubic symmetry. Additionally, in CdTe a defect complex is detected, which is assigned to a AgCd-VTe pair. A second defect complex is observed in CdTe doped with In, which is assigned to an InCd-AgCd pair. By PAC experiments with the probe 111In that are performed in Ag doped CdTe, the formation of the InCd-AgCd pair is confirmed besides the well known formation of the InCd-VCd pair. The migration energy of the cation vacancy is estimated on the basis of an isothermal annealing sequence.

Zeeman levels of the shallow lithium acceptor and band parameters in cadmium telluride

We have measured the energies and the Zeeman splitting of excitations of the non-ionized Li acceptor in CdTe by means of far-infrared absorption spectroscopy in order to determine the valence band parameters of the host material (0<or=B<or=7T, B//(100) and B//(110), 1.5K<or=T<or=4.2K). We have studied in detail the splitting of the G line (1S3/2( Gamma 8) to 2P32/( Gamma 8)) using polarized light; g factors for initial and final states are given. Shifts of the transition energies caused by interactions with phonons are identified and separated by a critical comparison of the excitation energies measured for Li with results published for other acceptors in CdTe. Using the recently developed theory of Schmitt and co-workers, a set of valence band parameters of the host material is obtained, which reproduces very well both the excited acceptor states at B=OT and the Zeeman patterns. With a value of the static dielectric constant epsilon =10.31 taken from the literature the Baldereschi-Lipari set amounts to mu =0.68, delta =0.13, R0=30.9 meV. The Luttinger parameters are determined as gamma 1=4.14, gamma 2=1.09, gamma 3=1.62 and kappa =0.46. We compare these numbers with results obtained by different methods and published previously in the literature.

Oxygen doping in CdTe, CdS and ZnS

Properties of isoelectronic oxygen in II–VI semiconductors were studied by photoluminescence (PL) measurements. A donor-to-acceptor peak, which was confirmed by measurements of the peak shifts with excitation density, was observed in the PL spectra from oxygen-doped CdTe, CdS, and ZnS. It was found that oxygen can act as an acceptor in CdTe, CdS, and ZnS as well as in ZnSe. It was reconfirmed that oxygen in ZnTe acts as an isoelectronic trap. Charge transfer from host lattice to oxygen atom may play an important role in oxygen acting as an acceptor. From the charge-transfer model, it can be qualitatively interpreted that there are two roles for oxygen in II–VI compounds, as an acceptor or as a trap, and these roles are classified by ionicity of the compounds.

Evidence of a deep donor in CdTe

We report low temperature photoluminescence measurements on nominally undoped and chlorine-doped bulk CdTe and undoped as well as indium-doped MBE CdTe layers. From the magnetic field dependence of the free-to-bound transitions, we obtained the effective mass of the conduction band electron and the binding energies of acceptors in CdTe. From the analysis of the energetic shifts and splittings of the 1.4 eV luminescence band, we distinguish two recombinations of completely different nature. The 1.452 eV line is a (D 0, A 0) and (e, A 0) transition, where the acceptor is propable Cu and has a binding energy of 147.2±0.3 meV. The luminescence near 1.475 eV is assigned with a (D 0, h) transition. The donor is deep ( E D = 131 meV) and most likely a Cl I-Cl Te complex.

Excited states of acceptors in CdTe and ZnTe

Higher excited states of acceptors in CdTe and ZnTe are obtained by using the Baldereschi and Lipari spherical model including the cubic correction and the central-cell effect. This is done by solving the coupled radial equations by the finite-element method with Arnoldi's algorithm, which gives several (≅20) low-lying states simultaneously. Our procedure allows one to determine very accurately the host band-structure parameters. In the case of CdTe we obtain the Luttinger parameters γ 1 = 5.30, γ 2 = 1.62, γ 3 = 2.10 and the dielectric constant ϵ 0 = 9.3. For ZnTe we obtain γ 1 = 3.80, γ 2 = 0.86, γ 3 = 1.32 and ϵ 0 = 9.4.

Excited Acceptor States in II–VI and III–V Semiconductors

AbstractHigher excited states of acceptors in CdTe, ZnTe, and GaAs are obtained by using the Baldereschi and Lipari spherical model including the cubic correction and the central‐cell effect. This is done by solving the coupled radial equations by the finite‐element method with Arnoldi's algorithm, which gives several (≈ 20) low‐lying states simultaneously. The procedure permits to determine very accurately the host band‐structure parameters. In the case of II–VI compounds, the Luttinger parameters γ1 = 5.30, γ2 = 1.62, γ3 = 2.10 and the dielectric constant ϵ0 = 9.3 for CdTe and, γ1 = 3.80, γ2 = 0.86, γ3 = 1.32 and ϵ0 = 9.4 for ZnTe are obtained. For GaAs, γ1 = 7.20, γ2 = 2.15, γ3 = 3.05, and ϵ0 = 12.49 are obtained.

Hydrogen-acceptor interaction in CdTe and ZnTe studied by photoluminescence

Photoluminescence studies on CdTe and Znte implanted with hydrogen and deuterium at 150keV in the fluence range 10 14–10 15 cm -2 are reported. In CdTe the introduction of hydrogen (deutrium) makes the recombination lines of excitons bounds to residual acceptors to disappear. The donor bound excitons are not affected. In ZnTe the intensity of the principal acceptor-bound exciton (Cu) is much reduced. In both crystals a new sharp line (H) appears with an adjacent structure on the low energy side. Short time annealings in the temperature range 150–250°C lead to the disappearance of the sharp lines (H) and to the recovery of the initial spectrum. These results shows that hydrogen is able to interact with acceptors in CdTe and ZnTe leading to the formation of a new exciton trapping centre. Possible microscopic models for this isoelectronic-like complex are proposedl

Electronic properties of a shallow complex acceptor in CdTe.

A shallow complex acceptor in CdTe with a binding energy of 56.4\ifmmode\pm\else\textpm\fi{}0.5 meV is reported. A bound-exciton (BE) line at 1.5901 eV is found to be associated with this acceptor, the most shallow acceptor-related bound exciton discovered so far in CdTe. Two-hole photoluminescence satellites recorded with resonant excitation in the 1.5901-eV bound-exciton line reveal several s-like excited states of the acceptor hole, of which the 2S replica interacts strongly with phonons in the range of the two-${\mathrm{LO}}^{\mathrm{\ensuremath{\Gamma}}}$-phonon mode. A complicated phonon spectrum in the optical-phonon range is found for this bound exciton. Further, the two-hole spectrum contains additional electronic lines, not seen for substitutional acceptors in tetrahedral symmetry. Two such lines, corresponding to hole energies 7.3 and 33.3 meV above the acceptor ground state, are interpreted as excited states associated with the component of the ${\ensuremath{\Gamma}}_{8}$ hole state split by the local strain field, and the 2${P}_{3/2}$ excited hole state, respectively. Dye-laser-excited photoluminescence excitation spectra similarly reveal excited states to the bound exciton, particularly a doublet at \ensuremath{\approxeq}2.6 meV above the 1.5901-eV lowest BE state. These excited BE states are interpreted as states split off from the lowest at 1.5901 eV by the local axial strain field at the defect.

Properties of nitrogen acceptor in CdTe: Energy spectrum and interaction with hydrogen

The energy levels of the nitrogen acceptor in CdTe were determined on nitrogen implanted single crystals. With low implantation (10 13 atoms cm -2), a low temperature annealing (250°C, 1 h) leads to a complete recovery of the luminescence spectra. Excitation spectra of the donor-acceptor pair band associated with nitrogen allows one to identify the internal excitation transitions. By fitting to effective mass theoretical models the ground state energy is set at 55.3±0.3 meV. Annealing under a H 2 atmosphere leads to a disappearance of the nitrogen DAP band, which is restored after a second normal annealing. The effect depends on H 2 pressure and annealing time. This reversible phenomenon is thought to be due to a direct interaction between the oppositely charged species H + and N - leading to neutralization of the acceptors.

Identification of the isolated deep Ni acceptor in CdTe and ZnTe: comparison with isomorphous systems

Single crystals of CdTe and ZnTe doped with nickel have been investigated by electron spin resonance. Among other signals they exhibit an isotropic line near mod g mod =1.4 which is identified as being due to the isolated, ionised Ni acceptor state, NiCd+(3d9) and NiZn+(3d9). Central hyperfine interaction with 61Ni and ligand hyperfine interaction with 125Te could be resolved for CdTe:Ni. Intra d-shell optical transitions of Ni+ in CdTe were also observed. The general properties of the Ni(d8/d9) acceptor in cubic IIB-VI and III-V compounds are pointed out and are contrasted with those of the copper acceptor in IIB-VI compounds. A tentative prediction for the Ni acceptor level in CdTe is made.

Excited states of Ag and Cu acceptors in CdTe

We report on resonantly excited luminescence (REL) at 1.8 K and infrared absorption (IRA) between 6–12 K of CdTe doped with Ag and Cu impurities. The luminescence was excited with a tunable dye laser. Two hole transitions up to 6 S 3 2 state have been observed, the bound exciton lines have been identified. 2 P 5 2 states have been observed in IRA measurements. The binding energies as deduced from REL experiments are 107.5 meV for Ag and 146 meV for Cu. The fitted values of the valence band parameters are Ro = 27 ± 3 meV μ = 0.73 ± 0.03 and δ = 0.12 ± 0.01.

Identification of Cu and Ag acceptors in CdTe

We report here on the identification of the two dominant acceptor levels in high purity p type CdTe, with Cu and Ag on Cd site. This identification is based on back doping experiments coupled with electrical measurements and photoluminescence studies. Cu and Ag can form easily complex centers when a supersaturation is achieved. The way of obtained good doping without complexation, is explained. The principal bound exciton lines are at 1,5896 eV (Cu) and at 1,5885 eV (Ag). The precise hole binding energies obtained from optical data are E A ( Cu) = 146 meV and E A ( Ag) = 108 meV.