Neutral Shallow Donors Research Articles

Contributions to the optical linewidth of shallow donor-bound excitonic transition in ZnO

Neutral shallow donors in zinc oxide (ZnO) are spin qubits with optical access via the donor-bound exciton. This spin–photon interface enables applications in quantum networking, memories, and transduction. Essential optical parameters which impact the spin–photon interface include radiative lifetime, optical inhomogeneous and homogeneous linewidth, and optical depth. We study the donor-bound exciton optical linewidth properties of Al, Ga, and In donors in single-crystal ZnO. The ensemble photoluminescence linewidth ranges from 4 to 11 GHz, less than two orders of magnitude larger than the expected lifetime-limited linewidth. The ensemble linewidth remains narrow in absorption through samples with an estimated optical depth up to several hundred. The primary thermal relaxation mechanism is identified and found to have a negligible contribution to the total linewidth at 2 K. We find that inhomogeneous broadening due to the disordered isotopic environment in natural ZnO is significant, contributing 2 GHz. Two-laser spectral hole burning measurements indicate that the dominant mechanism, however, is homogeneous. Despite this broadening, the high homogeneity, large optical depth, and potential for isotope purification indicate that the optical properties of the ZnO donor-bound exciton are promising for a wide range of quantum technologies, and motivate a need to improve the isotope and chemical purity of ZnO for quantum technologies.

Properties of Donor Qubits in ZnO Formed by Indium-Ion Implantation

Shallow neutral donors (D$^{0}$) in ZnO have emerged as a promising candidate for solid-state spin qubits. Here, we report on the formation of D$^{0}$ in ZnO via implantation of In and subsequent annealing. The implanted In donors exhibit optical and spin properties on par with $\textit{in situ}$ doped donors. The inhomogeneous linewidth of the donor-bound exciton transition is less than 10 GHz, comparable to the optical linewidth of $\textit{in situ}$ In. Longitudinal spin relaxation times ($T_1$) exceed reported values for $\textit{in situ}$ Ga donors, indicating that residual In implantation damage does not degrade $T_1$. Two laser Raman spectroscopy on the donor spin reveals the hyperfine interaction of the donor electron with the spin-9/2 In nuclei. This work is an important step toward the deterministic formation of In donor qubits in ZnO with optical access to a long-lived nuclear spin memory.

Higher-order Zeeman effect of Mg-related donor complexes in silicon

Magnesium (Mg) atoms in interstitial positions of a silicon host lattice form double donor centers. The binding energy of the neutral $\mathrm{M}{\mathrm{g}}^{0}$ state corresponds to midinfrared wavelengths. Due to its interstitial character, the outer orbitals of Mg atoms can form specific bonds with different trace elements in the silicon crystal. These occur as single and double donors that become detectable by infrared spectroscopy when the concentration of either Mg or the dopant coupled to it is sufficiently large. Infrared absorption spectroscopy at high magnetic fields allows us to observe the Zeeman effect of several shallow Mg-related donor complexes and neutral Mg donors, very similar to those observed for hydrogenlike centers. The quadratic Zeeman effect indicates a hydrogenlike center structure of the excited states and spatial localization of complexes with a single excess electron in its outer shell, which are formed by Mg with an impurity atom. In contrast, Mg complexes formed by bonds with two electrons are closer to substitutional double donors in silicon.

Zinc vacancy related emission in homoepitaxial N-doped ZnO microrods

Vertically aligned N-doped ZnO microrods have been homoepitaxially grown on a high-quality ZnO template via chemical vapor transport method without employing any catalysts. In spite of the incorporation of nitrogen, the as-grown microrods exhibited excellent crystalline and optical quality, and the behavior of intrinsic point defects and impurity-related defects have been investigated in detail by the temperature-dependent photoluminescence spectra. The emissions at 3.377, 3.368, and 3.363eV are ascribed to free excitons, surface bound excitons and excitons bound to neutral shallow donors: zinc interstitials, respectively. The emissions at 3.359, 3.311, and 3.240eV correspond to the radiative recombination of excitons bound to neutral acceptors, free electrons to acceptors and donor acceptor pairs, all of which are originated from the same shallow acceptor of zinc vacancy clusters with its energy level of about 126meV above the valence band maximum. The isolated zinc vacancy is a deep acceptor, which results in the emission at 3.318eV and the green band emissions. This study suggests that the intrinsic point defects play an extremely important role in optical properties of ZnO besides the external dopants and the understanding of intrinsic defects is crucial to get stable p-type conductivity and finally realize a ZnO homojunction for optoelectronic applications.

ν=5/2fractional quantum Hall state in low-mobility electron systems: Different roles of disorder

We report the observation of a fully developed fractional quantum Hall state at $\nu=5/2$ in GaAs/Al$_{x}$Ga$_{1-x}$As quantum wells with mobility well below $10^{7}$\thinspace cm$^{2}$/Vs. This is achieved either by strong illumination or reducing the barrier Al composition without illumination. We explain both results in terms of screening of the ionized remote impurity (RI) potential by nearby neutral shallow donors. Despite the dramatic improvement in the transport features, the energy gap $\Delta_{5/2}$ is limited to a rather small value ($\sim100$ mK), which indicates that once the RI potential is well screened and the $5/2$ state emerges, the size of $\Delta_{5/2}$ is limited by the mobility, i.e., by background impurities.

Influence of irradiation with γ-ray photons on the photoluminescence of Cd0.9Zn0.1Te crystals preliminarily subjected to the intense radiation of a neodymium laser

The effect of the preliminary treatment of Cd0.9Zn0.1Te crystals with high-power pulses of neodymium laser radiation (the power density is ≤1.8 MW/cm2, at a wavelength of 532 nm) on the low-temperature (5 K) photoluminescence induced by γ-ray radiation (the dose was Φγ = 5 kGy) is studied. The luminescence bands are related to radiation-stimulated donor-acceptor pairs, which include shallow neutral donors and neutral cadmium vacancies stimulated by γ-ray irradiation, the transition of free electrons to neutral cadmium vacancies formed by radiation, and the annihilation of excitons bound to the above vacancies. It is shown that, in the crystals preliminarily treated with laser radiation, the intensity of the γ-ray-stimulated luminescence bands is significantly lower than in crystals not subjected to laser radiation. This fact is accounted for by a decrease in the concentration of cadmium vacancies generated by the γ-ray radiation as a result of their annihilation during the course of their interaction with laser-stimulated defects, in particular, as a consequence of their recombination at laser-stimulated interstitial cadmium atoms.

Dynamics of donor bound excitons in ZnO

Comprehensive time-resolved photoluminescence measurements are performed on shallow neutral donor bound excitons (D0Xs) in bulk ZnO. It is found that transients of the no-phonon D0X transitions (I6-I9 lines) are largely affected by excitation conditions and change from a bi-exponential decay with characteristic fast (τf) and slow (τs) time constants under above-bandgap excitation to a single exponential one, determined by τs, under two-photon excitation. The slow decay also dominates transients of longitudinal optical phonon-assisted and two-electron-satellite D0X transitions, and is attributed to “bulk” D0X lifetime. The fast component is tentatively suggested to represent effects of surface recombination.

Photoluminescence evaluation of the quality of Cd0.9Zn0.1Te detectors doped with different indium concentrations

AbstractWith the aim to determine an optimal indium (In) concentration in Cd0.9Zn0.1Te:In nuclear radiation detectors for a good material quality, low‐temperature photoluminescence (LTPL) measurements have been performed at 5 K in the energy range of 1.2–1.7 eV. The LTPL spectrum of initial sample contained a neutral acceptor bound exciton (A0X) peak at 1.635 eV, donor–acceptor pair (DAP) peak at 1.606 eV with its 1LO phonon replica, and deep‐level defects (D) peak at 1.455 eV. In doping leads to the appearance of a new relatively intensive peak at 1.652 eV that is caused by a shallow neutral donor bound exciton (D0X) due to In substituting cadmium vacancies (VCd). At In concentrations of about 8 × 1016–1017 cm−3 the value of $I_{{\rm D}^{{\rm 0}} {\rm X}} /I_{{\rm D}} $ ratio is maximal, which indicates a low density of detrimental carrier traps and, therefore, high‐material quality. We believe that at such concentrations the majority of as‐grown Cd vacancies will be substituted by In atoms and, therefore, noted concentrations are optimal, which was confirmed by the correlation with mobility–lifetime product.

Silicon in AlN: shallow donor and DX behaviors

AbstractIn unintentionally Si‐doped AlN bulk samples, an electron paramagnetic resonance (EPR) spectrum with characteristics of a shallow donor, previously assigned to the shallow Si donor, was observed at room temperature in darkness. Temperature dependent studies of the EPR signal showed that Si is a DX center in AlN. However, with the negatively charged DX– state determined to be only ∼78 meV below the neutral shallow donor state, Si should behave as a shallow dopant in AlN at normal device operating temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Shallow donor and DX states of Si in AlN

In unintentionally Si-doped AlN, the electron paramagnetic resonance (EPR) spectrum of the Si shallow donor (g=1.9905) was observed in darkness at room temperature. The temperature dependence of the EPR signal suggests that Si in AlN is a DX center with the DX− state lying at ∼78 meV below the neutral shallow donor state. With such relatively small thermal activation energy, Si is expected to behave as a shallow dopant in AlN at normal device operating temperatures.

Positive and Negative Donor Impurity in an InAs/AlAs Quantum Wire

Binding energies of positive and negative charged donor impurities in an InAs/AlAs cylindrical quantum wire are investigated. Numerical calculations are performed using the variational procedure within the single band effective mass approximation. We assume that the impurity is located at the axis of the wire. The interband optical transition with and without the exciton is computed as a function of wire radius. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. Neutral shallow donors comprise a positively charged donor and a single bound electron. It is observed that (i) negative trions have a higher binding energy than positive trions, (ii) the binding energy of the heavy-hole exciton is much larger than that of the light-hole exciton due to different hole mass values (iii) the exciton binding energy and the interband emission energy are both increased when the radius of the cylindrical quantum wire is decreased and (iv) the effect of exciton influences the interband emission energy. Our results are in good agreement with the recent published results. Keywords: Quantum wire; Impurity level; Binding energy; Excitons. © 2010 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. DOI: 10.3329/jsr.v2i3.4715 J. Sci. Res. 2 (3), 433-441 (2010)

Neutral shallow donors near a metallic interface

The effect of a metallic gate on the bound states of a shallow donor located near the gate is studied. We calculate the energy spectrum as a function of the distance between the metallic gate and the donor and find an anti-crossing behavior in the energy levels for certain distances. We show how a transverse electric field can tune the average position of the electron with respect to the metallic gate and the impurity.

Binding Energy of D− and D0 Centers Confined by Spherical Quantum Dots

We study a negative donor center, a neutral donor in a spherical Gaussian potential quantum dot by using the matrix diagonalization of Hamiltonian within the effective-mass approximation. We calculate the energy E(D−) as functions of Gaussian potential size and depth, the same calculations as performed with the parabolic approximation. The dependence of the ground state of the neutral shallow donor and the negatively charged donor on the dot size and the potential depth is investigated.

Donor Centers in a Gaussian Potential

We study a neutral donor center (D0) and a negatively charged donorcenter (D−) trapped by a quantum dot, which is subjected to a Gaussianpotential confinement. Calculations are made by using the method ofnumerical diagonalization of Hamiltonian within the effective-massapproximation. The dependence of the ground state of the neutral shallowdonor and the negatively charged donor on the dot size and the potentialdepth is investigated. The same calculations performed with the parabolicapproximation of the Gaussian potential lead to the results that arequalitatively and quantitatively different from each other.

Pressure behaviour of the UV and green emission bands in ZnO micro‐rods

AbstractThe pressure behavior of the ultraviolet (UV) and green emission bands in ZnO tetrapod‐like micro‐rods has been investigated at 300 and 70 K, respectively. The pressure coefficient of the UV band at 300 K is 24.5 meV/GPa, consistent with that of the band gap of bulk ZnO. However, the pressure coefficient of the green band is 25 meV/GPa, far larger than previous literature reports. The green band in this work originates from Cu‐related emission, as confirmed by the fine structure observed in the spectra at 10 K. The pressure coefficients of four phonon replicas of the free exciton emission (FX) at 70 K are 21.0, 20.2, 19.8, and 19.3 meV/GPa, respectively. The energy shift rate of the FX emission and the LO phonon energies is then determined to be 21.4 and 0.55 meV/GPa. The pressure coefficient of the neutral donor bound exciton (D0X) transition is 20.5 meV/GPa, only 4% smaller than that of FX. This confirms that the D0X emission corresponds to excitons bound to neutral shallow donors. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd1−xZnxTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)h of the CZT detector measured by DC photoconductivity is 7.0 × 10−4 cm2/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D0, X) or neutral acceptors (A0, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D0, X) depends linearly on the incident laser intensity, while (A0, X) has a nonlinear dependence.

Time-resolved spectroscopy of excitons bound at shallow neutral donors in HVPE GaN

Time-resolved photoluminescence (TRPL) data for temperatures 2–150 K are presented for two thick HVPE samples grown in two different laboratories. The samples both have residual O and Si shallow donor concentrations in the 10 16 cm −3 range. The radiative decay time for neutral donor-bound excitons (DBEs) related to these donors is found to be about 300 ps. The decay of the DBEs at longer decay times is found to be related to feeding from the free exciton–polariton states. At elevated temperatures the decay of the DBE is very similar to the free exciton decay.

Muonium states in II–VI zinc chalcogenide semiconductors

A systematic experimental study of muonium states in the binary compounds ZnSe, ZnS and ZnTe is presented. Deep muonium states are observed in ZnSe and ZnS but not in ZnTe. In ZnTe, only the diamagnetic signal is seen. The temperature dependence of the muonium and muon fractions is measured in the three materials by muon spin rotation and, for ZnTe, also by RF-μSR. In none of the three compounds any indication of the neutral donor state, known from ZnO, CdS and others, is found. We conclude that in the Zn chalcogenides the neutral (interstitial) muonium configuration is more stable than the neutral (anion bonded) shallow donor configuration. The significance of the results is discussed with respect to the behaviour of hydrogen.

Time-resolved Spectroscopy of Excitons Bound at Shallow Neutral Donors in HVPE GaN

AbstractTime-resolved photoluminescence (TRPL) spectroscopy has been performed in the temperature range 2 K to 300 K on thick (300 μm – 1 mm) nominally undoped bulk HVPE grown GaN layers. The PL spectra are dominated by the free excitons (FEs) and the two neutral donor bound excitons (DBEs) at about 3.4712 eV and 3.4721 eV at 2 K, assigned to the residual O and Si donors, respectively. The zero-phonon decay curves for both FEs and DBEs are nonexponential, with a fast initial decay and a slower tail at longer times. The initial decay time of the FE is about 130 ps at 2 K in such samples, the corresponding initial decay time for the DBEs for O and Si is close to 300 ps. The different lines corresponding to the so called two-electron transitions for the DBE show a different behaviour, with a longer decay time than the corresponding principal DBE states. The LO phonon replicas of the DBEs also have a decay time in excess of 1 ns in lightly doped samples.

Coherent optical control of spin-spin interaction in doped semiconductors

We provide a theory of laser-induced interaction between spins localized by impurity centers in a semiconductor host. By solving exactly the problem of two localized spins interacting with one itinerant exciton, an analytical expression for the induced spin-spin interaction is given as a function of the spin separation, laser energy, and intensity. We apply the theory to shallow neutral donors (Si) and deep rare-earth magnetic impurities (Yb) in III-V semiconductors. When the photon energy approaches a resonance related to excitons bound to the impurities, the coupling between the localized spins increases, and may change from ferromagnetic to antiferromagnetic. This light-controlled spin interaction provides a mechanism for the quantum control of spins in semiconductors for quantum information processing; it suggests the realization of spin systems whose magnetic properties can be controlled by changing the strength and the sign of the spin-spin interaction.