Session L19: Focus Session: Dopants and Defects in Semiconductors II

Highly Enriched $^{28}$Si -- a New Testbed for Impurity and Defect Structure

We have recently found that many optical transitions in Si, including those of shallow impurity bound excitons, and the electronic ground state to excited state transitions of shallow donors and acceptors, are remarkably sharper in highly enriched $^{28}$Si than in natural Si, due to the removal of inhomogeneous isotope broadening. This work is now being extended to deeper defects, many of which have been studied for decades in natural Si and were until now thought to be well understood. In $^{28}$Si, due to the narrowness of the individual transitions, changing the isotopic species of the defect constituents results in well-resolved components, rather than the `isotope shift' of the broad, unresolved inhomogeneously broadened lines observed in natural Si. This results in an `isotopic fingerprint' of the defect, revealing not only the participation of a given element in the defect, but also the number of atoms of that element which are involved. We have recently shown [1] that a well known Cu-containing defect with a no-phonon luminescence line at $\sim $1014 meV, which was thought to be a Cu-pair, and for which ab-initio calculations [2] based on a pair-model appeared to agree convincingly with experiment, in fact contains four Cu atoms. A related center at $\sim $944 meV, modelled in the past as a different configuration of a Cu-pair [3], was shown to contain three Cu atoms [4]. We have now found that the 944 meV center also contains one Ag atom, and that another defect exists which contains two Cu and two Ag atoms. We will show that high resolution spectroscopy in highly enriched $^{28}$Si produces many more surprising results regarding the actual constituents of well known deep centers in Si. \newline [1] M.L.W. Thewalt et al., Physica B \underline {401-402}, 587 (2007). \newline [2] S.K. Estreicher et al., Phys. Rev. Lett. \underline {90}, 035504 (2003). \newline [3] S.K. Estreicher, D. West and M. Sanati, Phys. Rev. B \underline {72}, 121201 (2005). \newline [4] A. Yang et al., Physica B \underline {401-402}, 593 (2007).   

Nitrogen-hydrogen complexes in ZnO: A possible route toward p-type conductivity.

Zinc oxide (ZnO) is a wide band gap II-VI semiconductor with optical, electronic, and mechanical applications. The lack of reliable $p$-type doping, however, has prevented it from competing with other semiconductors such as GaN. In this talk, I describe the successful incorporation of nitrogen-hydrogen (N-H) complexes in ZnO during chemical vapor transport (CVT) growth, using ammonia as an ambient. The N-H bond-stretching mode gives rise to an infrared (IR) absorption peak at 3150.6 cm$^{-1}$. Isotopic substitutions for hydrogen and nitrogen result in the expected frequency shifts, thereby providing an unambiguous identification of these complexes. The N-H complexes are stable up to $\sim $700\r{ }C. The introduction of neutral N-H complexes could prove useful in achieving reliable $p$-type conductivity in ZnO.   

Vibrational lifetimes of O-H stretch modes in MgO and ZnO

Hydrogen is an important and omnipresent impurity in a wide class of oxides. A more complete understanding of the role of hydrogen in wide-bandgap oxides such as MgO and ZnO is crucial for further development of oxide-based optoelectronics. We have measured for the first time the vibrational lifetime of the O-H stretch mode associated with the Mg$^{2+}$ vacancy in MgO for the charge state, V$_{OH}$-, and the neutral state, V$_{OH}$, using picosecond transient bleaching spectroscopy. For the V$_{OH}$ center we find the lifetime ($\sim $11 ps) is longer than for the charged defect state ($\sim $5 ps). These lifetimes are almost an order of magnitude shorter than in covalent semiconductors Si and Ge [1]. Similar measurements will be presented for interstitial hydrogen in ZnO. Our results provide new insight into the coupling of the ionic surroundings to the O-H vibration within the crystal lattice. [1] M. Budde et al., PRL 87, 145501 (2001).   

Carrier Dynamics and Photoexcited Emission Efficiency of ZnO:Zn Phosphor Powders

Nonstoichiometric ZnO with an excess of Zn atoms (ZnO:Zn) has a long history of use as a green/monochrome phosphor in electron-excited vacuum fluorescent and field emission displays. We previously studied the external quantum efficiency of such ZnO:Zn powders under continuous-wave photoexcitation and found that the efficiency depended sensitively on excitation density [\textit{Appl. Phys. Lett. }\textbf{91}, 011902 (2007)]. Here we study experimentally the time-integrated quantum efficiency and the time-resolved photoluminescence decays of both band edge and defect emission of ZnO:Zn powders under femtosecond pulsed excitation of varying intensity. The results are discussed in terms of a rate equation model which describes energy transfer between band edge and radiative defect levels, as well as nonradiative centers.   

Study of the carrier concentration dependent photoluminescence of Ga-doped ZnO thin films grown by molecular-beam epitaxy

The undoped and Ga-doped ZnO thin films were grown on $r$-sapphire using molecular-beam epitaxy (MBE) system. The samples of electron concentrations ranging from 1.4x10$^{18}$ to 3.4x10$^{19}$ cm$^{-3}$ were grown and studied. The RT PL peaks show a monotonic red shift from 3.280 to 3.229 eV with the increase of electron concentration, which is attributed to the band-gap narrowing effect. The red-shifted peak values have been fitted. The evolution of the LT PL spectra were studied and discussed. The free exciton emission at 3.371 eV, the first Ga-level-related peak at 3.313-3.321 eV, and the second Ga-level-related peak at 3.359 eV (assigned as the Ga D$^{0}$X peak) are competing with each other in the LT PL spectra. These three kinds of peaks are dominating in the lightly (or undoped), mediate, and heavily doped ZnO:Ga samples, respectively. From the experiments, we conclude that there are two Ga levels in ZnO. In the lightly doped sample, the Ga atoms contribute to the first Ga level around 3.32eV. When the Ga incorporation reaches some critical amount, Ga atoms form the second Ga level in ZnO at 3.359 eV.   

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

Extensive theoretical studies based on first-principles have been carried out for the mechanism of magnetism in ZnO thin films and nanowires. It has been identified that the observed magnetism is introduced by Zn vacancy and is affected by its concentration. The main source of the magnetic moment comes from the unpaired 2p- electrons in oxygen sites around the Zn vacancy, instead of Zn 3d electrons. Moreover, Zn vacancy is more energetically favorable to reside on the surface, and its formation energy is found to be less than that of oxygen vacancy that does not introduce any magnetism. These findings suggest that the main vacancy species is Zn vacancy as expected by experiments. The present theoretical study not only provides some deep understandings for the experimentally observed magnetism in un-doped ZnO samples, but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO structure for bio-magnetic applications.   

Magnetism of Undoped and Co-Doped TiO2 Clusters

TiO$_{2}$ is a widely used optically active material, and transition-metal doped TiO$_{2}$ has attracted much attention in spin electronics. Recently, it has been argued that ferromagnetism is a universal feature of nanoparticles of nonmagnetic oxides, and our focus is on doped and undoped TiO$_{2}$ nanoclusters. The clusters are examined with TEM, AFM, MFM, and hysteresis loops and zero field cooled magnetization curves were measured by SQUID magnetometry. Both doped and undoped films display hysteresis and magnetic order in the investigated temperature range of 5K to 400~K. The ordering temperature is above 400 K, and~both magnetization and coercivity are enhanced in~the out-of-plane direction. Undoped TiO$_{2}$ particles exhibit a nominal moment of about 0.2 $\mu _{B}$ per surface atom. Small Co concentrations have little effect on the magnetism of the particles. Higher Co doping percentages, about 8{\%}, yield proteretic (clockwise) loops, indicating~the formation of CoO. It has~been suggested that the magnetic moment of 'nonmagnetic' oxide thin films is a surface effect, and the comparison of different particle sizes yields a similar picture for our particles. Our renormalization-group modeling assumes indirect exchange interactions between scarce magnetic moments and yields a logarithmic dependence of the ordering temperature on the particle size - This research is supported by NSF MRSEC and NCMN.   

First-principles determination of the electronic structure of native point defects and impurities in rutile TiO$_{2}$

Density-functional theory calculations are used to determine the electronic structures of native point defects (Ti interstitials and O vacancies) and dopants (Al and Nb) in rutile TiO$_{2}$. The calculated densities of states (DOS) show that in pristine and defective structures that contain charged Ti interstitials or O vacancies, the lower conduction bands of the defective structures are shifted up in energy relative to the perfect structure. This shift leads to a broader lower conduction band that more readily promotes the formation of shallow donor levels. This effect is more pronounced in the case of Ti interstitials. We also find that the charge state of the Ti interstitial influences the extent of orbital overlap. The case of Al dopants is much more complex since Al can either be a donor or an acceptor. In the case of Al and Nb substitutional defects, the calculated DOS is similar to the DOS of the pristine structure.   

Group III-A Acceptor-Hydrogen interactions in SnO$_{2}$

Using first-principles calculations we investigate the role of hydrogen in the passivation of p-type dopants in SnO$_{2}$. We focus on group III-A elements, including Al, Ga, and In and investigate the stability of these impurities when substituting Sn under hydrogen-free and hydrogen-rich conditions. Hydrogen effectively passivates the acceptors and removes their electrical activity. Based on calculated binding and migration energies we discuss conditions under which hydrogen can be removed and acceptor activation can take place. We also calculate the stretch-mode vibrational frequencies associated with the hydrogen-impurity complexes, providing a signature for experimental identification in vibrational spectroscopy. We conclude that the group III-A elements studied are suitably shallow acceptors for p-type doping and that the presence of interstitial H will not impede, and potentially enhance, p-type doping of SnO$_{2}$.   

Investigation of Electroluminescent Degradation in doped ZnS phosphors

We present optical and EXAFS data on a series of ZnS samples doped with Cu, Mn and Cl. These materials (30 micron particles) have a strong electroluminescence (EL) when subjected to a 100V square-wave voltage. At 100 kHz, the luminescence decays significantly in a 20 hr period. We show that this degradation can partially be reversed by annealing the sample and that this can be repeated several times. In addition the EL emission centers reoccur at the same points in the 30 micron particles after the anneal. The optimum annealing temperature is about 180C, but varies slightly for different wavelengths. Surprisingly an anneal at somewhat higher temperatures (240C) dramatically reduces the EL intensity. The EXAFS studies show that the local structure about Cu continues to look like CuS for ``as made", EL degraded, rejuvenated samples (annealing at 180C), and thermally degraded samples (annealed at 240C). This means that most of the Cu is in the relatively inert CuS precipitates, and does not change significantly with EL degradation or annealing. Thus the EL active sites must be dilute. We discuss some possible models.   

Theoretical Study of native defects in CdGeAs$_2$

First-principles results are presented for various native defects \textit{viz} : V$_{\rm{Cd}}$, V$_{\rm{Ge}}$, V$_{\rm{As}}$, Cd$_{\rm{Ge}}$, Ge$_{\rm{Cd}}$, Ge$_{\rm{As}}$ and As$_{\rm{Ge}}$ in CdGeAs$_2$ under different growth conditions. The defects were calculated by constructing a 64 atom supercell in the full potential linearized muffin-tin orbital implementation of the density functional theory under the local density approximation (LDA). Calculations of the energy of formation show that antisites should be the most abundant type of defect. The LDA band gap is adjusted to experimental band gap by introducing a non-local orbital dependent constant potential shift to the $s$-orbitals of Cd and Ge and $d$-orbitals of Cd within the LSDA+U approach. The defect transition levels for different charge states are calculated. The calculations support the earlier suggestion that Ge$_{\rm{As}}$ is a shallow acceptor. The calculated transition levels are found to be significantly different form corresponding defects levels of ZnGeP$_2$. The defect levels are interpreted in a simple molecular-orbital theory and compared with the available experimental data.