Intrinsic Lattice Defects Research Articles

Growth Defects in Light Emitting InGaN/GaN Structures

Influence of the intrinsic lattice defects and background impurities on the properties of nitride GaN, InN, and InGaN compounds and parameters of the LED InGaN/GaN/Al 2 O 3 heterostructures with an active region based on multiple InGaN quantum wells is studied. It is shown that preferential accumulation of donor-type defects in GaN, InN, and InGaN compounds is connected with the peculiarities of energy position of the charge neutrality level CNL in the band spectra of these compounds. Influence of the sapphire substrate on the defect formation in nitride layers, the use of profiled sapphire substrate to reduce the dislocation density in an active region of the InGaN/GaN/Al 2 O 3 heterostructure, and the use of polar and semi-polar planes to reduce the Stark effect are considered.

Charge Neutrality in Semiconductors: Defects, Interfaces, Surface

The results of the experimental and theoretical studies of charge neutrality in semiconductor materials are summarized. It is shown that the charge neutrality level determines the electronic properties of semiconductors saturated by intrinsic lattice defects, the electronic state of the semiconductor surface, and, to a large extent, the height of the metal/semiconductor barrier as well as the energy band discontinuities in semiconductor heteropairs.

ChemInform Abstract: Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth‐Abundant Solar Cell Absorbers

AbstractReview: emphasis on recent theoretical studies; 137 refs.

Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth‐Abundant Solar Cell Absorbers

The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2 . Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn +SnZn ], [VCu +ZnCu ] and [ZnSn +2ZnCu ] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn +SnZn ] clusters, especially in Cu2ZnSnS4. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.

Structural and magnetic properties of iron in graphite

Pure and impure graphite powders, and two samples of highly oriented pyrolytic graphite (HOPG) have been studied by X-ray diffraction, electron microscopy, magnetometry and 57Fe Mössbauer spectrometry in order to identify the distribution of iron impurities and establish their role in the weak ferromagnetism of some of these materials. Pure graphite powder (99.9995%) shows straightforward diamagnetic behavior, with quantum oscillations at 4K. The impure (>99%) powder gives a ferromagnetic signal exhibiting little temperature dependence in the temperature range 1.8–300K, due to the presence of roughly isometric inclusions of metallic iron and cementite (Fe3C) about 100nm in size. These inclusions are not superparamagnetic. The HOPG-ZYA and -ZYB samples show similar, but highly anisotropic ferromagnetic signals with a saturation magnetization of 1–5×10−3Am2kg−1, due to inclusions of thin Fe3C platelets in the graphite sheet with a diameter of ≈1μm and diameter to thickness ratio >10. These platelets form Frisbee-shaped rings around primary lens-shaped inclusions of cubic (Ti,V)C. It is important to account quantitatively for the presence of these metallic, iron-rich inclusions before claiming that the origin of any weak ferromagnetic behavior of graphite is related to intrinsic lattice defects.

Erratum to: Half-metallic spin polarized electron states in the chimney-ladder higher manganese silicides MnSi1 − x (x = 1.75–1.73) with silicon vacancies

The electronic and optical properties of the higher manganese silicides, known as the Nowotny “chimney-ladder” phases, are investigated by means of first-principles density-functional calculations and compared with the experimental results. According to our calculations, Mn4Si7 is an intrinsic semiconductor and a nonmagnetic material, while Mn11Si19 and Mn15Si26 are metals and half-metallic weak ferromagnets, in agreement with recent experimental findings. The role of intrinsic lattice defects, namely vacancies, common for such open crystal structures, is clarified. The vacancies in Mn4Si7 lead to a metallization of the electronic spectrum and to a magnetic state of half-metallic nature. In Mn11Si19 and Mn15Si26 an effect of vacancies is that the average magnetic moment per Mn atom is almost four times larger than that in stoichiometric higher manganese silicides. The origin of this magnetic moment enhancement is explained by the variation of the Mn coordination numbers. The calculated optical spectra are in good agreement with available experimental data. We find a relatively high (1.2°) polar magneto-optical Kerr effect in Mn11Si19 and Mn15Si26 in the infrared region 0.1−0.3 eV. It is shown that such magneto-optical peaks are formed by the low energy plasmons.

Surface Origin of High Conductivities in UndopedIn2O3Thin Films

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In{2}O{3}, and SnO{2} is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In{2}O{3} samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In{2}O{3} can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O{2} partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In{2}O{3} thin films.

Defect Luminescence of Yag Nanopowders and Crystals

Defect Luminescence of Yag Nanopowders and Crystals Undoped and rare-earth-ion-doped Y3Al5O12 (YAG) nanopowders are prepared by the sol-gel low-temperature combustion method. The luminescence characteristics of the YAG, YAG: Ce, YAG:Pr, and YAG: Ce/Pr nanopowders are compared with those of the single crystals. The luminescence band peaking around 3.1 eV is complex and excited at about 3.6 eV, 3.9 eV and 4.3 eV. The 3.1 eV emission was peculiar to all the samples studied. The Stokes shift of this band is ~0.5 eV. The decay time of the ~3.1 eV emission at 80 K is ~14 ns and the slower decay (afterglow) components are practically absent. The 3.1 eV luminescence was suggested to arise from different intrinsic lattice defects.

Intrinsic Ferromagnetism in Nanocrystalline Mn-Doped ZnO Depending on Mn Concentration

The physical properties of Zn(1-x)Mn(x)O nanoparticles synthesized by thermal decomposition are extensively investigated by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman light scattering and Hysteresis measurements. XRD and XPS spectra reveal the absence of secondary phase in nanocrystalline ZnO doped with 5% or less Mn; and, later confirms that the valance state of Mn to be 2+ for all the samples. Raman spectra exhibit a peak at 660 cm(-1) which we attribute to the intrinsic lattice defects of ZnO with increasing Mn concentration. Overall, our results demonstrate that ferromagnetic properties can be realized while Mn-doped ZnO obtained in the nanocrystalline form.

Identification of the prime optical center inGaN:Eu3+

We identify a dominant light-emitting center in ion-implanted $\text{GaN}:{\text{Eu}}^{3+}$ for which the lattice damage has been completely healed, according to x-ray diffraction and Rutherford backscattering spectrometry measurements, by high-temperature, high-pressure annealing. This center is likely to be the isolated substitutional ${\text{Eu}}_{\text{Ga}}$ defect. It lacks a ``subgap'' excitation band and therefore has no state in the GaN band gap, shows threefold splitting of its ${^{7}\text{F}}_{2}$ level, with two sublevels nearly degenerate, and exhibits a long, single-exponential luminescence decay. Competing luminescent centers of GaN:Eu involve this prime center with intrinsic lattice defects, one of which may also be responsible for the GaN yellow band.

Influence of silver clusters on the light absorption in CdBr2-Ag crystals

Experimental studies of the absorption spectra of CdBr2-Ag photochromic crystals revealed several absorption bands positioned close to the intrinsic absorption edge. It is suggested that impurity silver ions, neutral silver atoms and intrinsic lattice defects form nanoclusters which are responsible for the absorption peaks within the transparency range of the crystal. The effect of UV radiation on the absorption profile is also discussed in the framework of the proposed model.

Defect engineering in implantation technology of silicon light-emitting structures with dislocation-related luminescence

Results obtained in development of physical foundations of ion implantation technology for fabrication of silicon light-emitting structures (LESs) based on dislocation-related luminescence and intended for operation at wavelengths close to ∼1.6 μm are summarized. The development of the concept of defect engineering in the technology of semiconductor devices makes it possible to determine the fundamental aspects of the process of defect formation; reveal specific features of the emission spectra related to changes in the implantation conditions of Er, Dy, Ho, O, and Si ions and the subsequent annealing; and design light-emitting structures with a desirable spectrum of luminescent centers and extended structural defects. The technological conditions in which only a single type of extended structural defect (Frank loops, perfect prismatic loops, or pure edge dislocations) is introduced into the light-emitting layer are found, which enables analysis of the correlation between the concentration of extended defects of a certain type and the intensity of lines of the dislocation-related luminescence. The key role of intrinsic point lattice defects in the origination and transformation of extended structural defects and luminescent centers responsible for the dislocation-related luminescence is revealed. It is found that the efficiency of luminescence excitation from the so-called D1 centers, which are of particular interest for practical applications, varies by more than two orders of magnitude between structures fabricated using different technological procedures. High-efficiency silicon light-emitting diodes with room-temperature dislocation-related luminescence have been fabricated.

The influence of Yb 3+ doping on the upconversion luminescence of Pr 3+ in aluminum oxide based powders prepared by combustion synthesis

The effect of ytterbium (Yb 3+) doping on the upconversion (UC) emission of praseodymium (Pr 3+) doped in aluminum oxide based powders prepared by combustion synthesis is reported for near-infrared excitation ( λ = 980 nm). Our experimental results show that the crystalline structure and the UC emission changes with the Yb 3+ concentration. The sample containing only Pr 3+ (1.0 wt.%) did not show any UC signal and the UC emission profiles of the samples containing Pr 3+ (1.0 wt.%) and Yb 3+ (0.5, 2.0 wt.%) are quite different. The sample containing 0.5 wt.% of Yb 3+ has five emission lines in the visible range associated to Pr 3+ 4f–4f transitions, 3P 0 → 3H 4 (∼497 nm), 3P 0 → 3H 5 (∼525 and ∼550 nm), 3P 0 → 3H 6 (∼620 nm) and 3P 0 → 3F 2 (∼650 nm). We believe that the UC process has its origin in energy transfer from Yb 3+ ions to Pr 3+ ions in Pr 0.83Al 11.83O 19 phase. The sample containing 2.0 wt.% of Yb 3+ has only one emission line in the visible range peaked at ∼507 nm which we believe has its origin in cooperative UC emission due to excited Yb 3+ pairs in YbAlO 3 phase. The samples containing Yb 3+ also present UC emission lines in the near-infrared which are assigned to intrinsic lattice defects.

The charge neutrality level and the fermi level pinning in A3N (BN, AlN, GaN, InN) nitrides

The electronic band structure and position of the charge neutrality level (CNL) in BN, AlN, GaN, and InN compounds with cubic and hexagonal lattices are calculated within the density functional theory (DFT-GGA). It is shown that the charge neutrality level is shifted from the middle of the BN and AlN forbidden band to the upper half of the GaN forbidden band and to the allowed energy region in the InN conduction band as the cation atomic weight increases. This determines semiinsulating properties of BN and AlN, n-type conductivity of GaN, and n +-type conductivity of InN upon saturation of these materials by intrinsic lattice defects due to hard radiation.

Growth and characterization of Cu-catalyzed ZnO nanowires

Cu-catalyzed ZnO nanowires have been synthesized by thermal evaporation of Cu- Zn powder alloys in a water vapor atmosphere. The diameters of the nanowires can be controlled by the amount of water vapor introduced into in the reaction tube. Raman spectroscopic characterization reveals two additional modes at 621 and 671cm−1 which are indicative of the existence of host-lattice defects caused by Cu doping or other intrinsic lattice defects that formed during the nanowire growth. Field emission and photoluminescence (PL) properties of Cu-catalyzed ZnO nanowires have also been investigated using nanowires with a diameter of 20 nm and 200 nm. The field emission results show that the thinner nanowires have a lower turn-on electrical field than the thicker ones, whilst both nanowires exhibit a similar field emission enhancement factor in the high electric field region. In the PL spectra, we have observed a blue-shift of the UV emission peak and a red-shift of the green emission peak when the nanowire diameters decreased from 200 nm to 20 nm.

Photoluminesence and FTIR study of ZnO nanoparticles: the impurity and defect perspective

AbstractWe studied the possible correlations between defects and photoluminescence spectra in ZnO nanoparticles of sizes ranging from 43 nm to 73 nm in diameter. The defects and impurity contents were characterized by Fourier‐transform infrared (FTIR) spectroscopy. The results show fewer carboxylate and hydroxyl impurities for particles of larger sizes. No significant variation in oxygen vacancy content was found among samples. Annealing in vacuum at 300 °C significantly reduces the carboxylate and hydroxyl impurities in the samples. The total luminescence intensity (UV + visible) increases as the particle size grows for both the unannealed and annealed samples. This suggests that both types of luminescence are subject to non‐radiative quenching by near surface defect centers, possibly carboxylate and hydroxyl impurities. There may be quenching due to intrinsic lattice defects too. It is found that annealing in vacuum enhances the visible luminescence both absolutely and relative to the UV exciton luminescence. In addition to the 2.5 eV green luminescence peak, a peak centered at 2.8 eV can also be resolved, espeically for the 43 nm sample. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and luminescence characteristics of quartz from pegmatites

Samples of pegmatite quartz from several localities in Norway and Namibia were investigated by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) measurements, and trace-element analysis (ICP-MS) to obtain information about their structure and luminescence characteristics. The defect structure and trace-element composition of the pegmatite quartz samples that were studied differ from those of quartz of other origin (hydrothermal, igneous and metamorphic quartz). EPR measurements reveal an almost complete lack of intrinsic lattice defects associated with O or Si vacancies (e.g., E′ center, O 3− 2 center), whereas some trace elements (Al, Ti, Ge, Li) are apparently enriched and form paramagnetic centers. The results indicate that there possibly is a redistribution of alkali ions during electron irradiation. The diamagnetic [AlO 4 /M + ] 0 center transforms into the paramagnetic [AlO 4 ] 0 center, while the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO 4 ] 0 and [TiO 4 ] 0 to form paramagnetic centers ([TiO 4 /Li + ] 0 , [GeO 4 /Li + ] 0 ). These defects result in a specific luminescence behavior, which is very similar for all samples. In general, quartz from pegmatites shows homogeneous CL without growth zoning or internal structures suggesting constant physicochemical conditions during crystal growth. The CL emission is dominated by a transient bluish-green CL, which disappears after 60–100 s of electron irradiation. The two main emission bands centered at 505 nm (2.45 eV) and 390 nm (3.18 eV) are probably related to alkali-compensated, trace-element centers in the quartz structure. Other CL emission bands, which are characteristic features of igneous, metamorphic, or hydrothermal quartz (e.g., at 450 nm = 2.75 eV, 580 nm = 2.14 eV, 650 nm = 1.91 eV) are almost completely lacking. This fact indicates that the defects responsible for these CL emissions are absent in pegmatite quartz.

Disorder effects near a magnetic instability in CePtSi1−xGex (x=0, 0.1)

The magnetic susceptibility and nuclear magnetic resonance $(\mathrm{NMR})$ linewidth have been measured in the heavy-fermion alloys ${\mathrm{CePtSi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$, $x=0$ and $0.1$, to study the role of disorder in the non-Fermi-liquid $(\mathrm{NFL})$ behavior of this system. The theoretical $\mathrm{NMR}$ line shape is calculated from disorder-driven $\mathrm{NFL}$ models and shows the same essential features as the observed spectra. Analysis of $^{29}\mathrm{Si}$ and $^{195}\mathrm{Pt}$ $\mathrm{NMR}$ linewidths strongly suggests the existence of locally inhomogeneous susceptibility in both materials, and agrees with the widths of the local susceptibility distributions estimated from the susceptibility fits to the disorder-driven $\mathrm{NFL}$ models. Disorder-driven mechanisms can also explain the $\mathrm{NFL}$ behavior in ${\mathrm{CePtSi}}_{0.9}{\mathrm{Ge}}_{0.1}$; the $\mathrm{NMR}$ spectra do not, however, distinguish between the Kondo-disorder and Griffiths phase models. We find that stoichiometric $\mathrm{CePtSi}$ and $\mathrm{Ge}$-doped ${\mathrm{CePtSi}}_{0.9}{\mathrm{Ge}}_{0.1}$ show similar degrees of magnetic disorder, although a narrower distribution of local susceptibilities in $\mathrm{CePtSi}$ allows Fermi-liquid behavior to appear below $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The residual resistivity reported in $\mathrm{CePtSi}$ is relatively large, which indicates a significant level of intrinsic lattice defects and seems to be consistent with the disorder observed in the $\mathrm{NMR}$ spectra.

Femtosecond pulsed laser micromachining of single crystalline 3C–SiC structures based on a laser-induced defect-activation process

A femtosecond pulsed Ti:sapphire laser with a pulse width of 120 fs, a wavelength of 800 nm and a repetition rate of 1 kHz was employed for direct write patterning of single crystalline 3C–SiC thin films deposited on Si substrates. The ablation mechanism of SiC was investigated as a function of pulse energy. At high pulse energies (>1 µJ), ablation occurred via thermally dominated processes such as melting, boiling and vaporizing of single crystalline SiC. At low pulse energies, the ablation mechanism involved a defect-activation process that included the accumulation of defects, formation of nano-particles and vaporization of crystal boundaries, which contributed to well-defined and debris-free patterns in 3C–SiC thin films. The interactions between femtosecond laser pulses and the intrinsic lattice defects in epitaxially grown 3C–SiC films led to the generation of nano-particles. Micromechanical structures such as micromotor rotors and lateral resonators were patterned into 3C–SiC films using the defect-activation ablation mechanism.

Chemical origin of the yellow luminescence in GaN

The influence of ion-beam-produced lattice defects as well as H, B, C, N, O, and Si, introduced by ion implantation, on the luminescence properties of wurtzite GaN is studied by cathodoluminescence spectroscopy. Results indicate that intrinsic lattice defects produced by ion bombardment mainly act as nonradiative recombination centers and do not give rise to the yellow luminescence (YL) of GaN. Experimental data unequivocally shows that C is involved in the defect-impurity complex responsible for YL. In addition, C-related complexes appear to act as efficient nonradiative recombination centers. Implantation of H produces a broad luminescent peak which is slightly blueshifted with respect to the C-related YL band in the case of high excitation densities. The position of this H-related YL peak exhibits a blueshift with increasing excitation density. Based on this experimental data and results reported previously, the chemical origin of the YL band is discussed.