Deep Donor Defect Research Articles

Study of Band Structure at the Zn(S,O,OH)/Cu(In,Ga)Se2 Interface via Rapid Thermal Annealing and Their Effect on the Photovoltaic Properties

This study focused on understanding the mechanisms of the photovoltaic property changes in Zn(S,O,OH)/Cu(In,Ga)Se2 solar cells, which were fabricated via annealing, using reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), low temperature photoluminescence (LTPL), and secondary ion mass spectroscopy (SIMS). A pinhole-free Zn(S,O,OH) buffer layer was grown on a CIGS absorber layer using the chemical bath deposition (CBD). When the Zn(S,O,OH) film was annealed until 200 °C, the Zn-OH bonds in the film decreased. The band gap value of the annealed film decreased and the valence band offset (VBO) value at the Zn(S,O,OH)/CIGS interface with the annealed film increased. Both results contribute to the conduction band offset (CBO) value at the Zn(S,O,OH)/CIGS interface and, in turn, yield a reduction in the energy barrier at the interface. As a result of the annealing, the short circuit current (JSC) and quantum efficiency (QE) values (400-600 nm) of the cell increased due to the improvement in the electron injection efficiency. However, when the Zn(S,O,OH) film was annealed at 300 °C, the cell efficiency declined sharply due to the QE loss in the long wavelength region (800-1100 nm). The SIMS analysis demonstrated that the Cu content in the CIGS bulk decreased and the Cu element also diffused into CIGS/Mo interface. Through LTPL analysis, it was seen that the considerable drop of the Cu content in the CIGS bulk induced a 1.15 eV PL peak, which was associated with the transition from a deep donor defect to degrade the quality of the CIGS bulk. Accordingly, the series resistance (RS) and efficiency of the cell increased.

An Extended Unified Schottky-Poole-Frenkel Theory to Explain the Current-Voltage Characteristics of Thin Film Metal-Insulator-Metal Capacitors with Examples for Various High-k Dielectric Materials

Historically, there is a controversy regarding the current-voltage (I-V) characteristics of thin film MIM (metal-insulator-metal) capacitors, which is quite frequently modeled by either the Schottky model or the Poole-Frenkel model. In this paper, the author points out that the two models actually can be unified. The physics underlying this model involves a non-uniform distribution of deep donor defect states such that a very large quantity of defect states exist at the two interface of the MIM capacitor while the density of defect states in the insulator bulk is relatively low, resulting in an M/n-i-n/M structure. This unified Schottky-Poole-Frenkel model can be further extended to include other effects like space charge limited current and tunneling. The effect of trap limited space charge limited current is also discussed. Examples of the application of this theory will be provided for MIM capacitors based on various high-k dielectric materials like tantalum oxide, titanium oxide, zirconium oxide and aluminum oxide.

Investigation of Near-IR Emission from Hydrogenated Nanocrystalline Silicon – The Oxygen Defect Band

AbstractHydrogenated nanocrystalline silicon (nc-Si:H), a mixture of nanometer sized crystallites and amorphous silicon tissue, demonstrates a photoluminescence band centered at ∼ 0.7 eV, which emerges in response to annealing at an onset temperature of ∼ 200–300 °C. This temperature range correlates well with hydrogen effusion spectroscopy studies, and evidence suggests thermal liberation of hydrogen from grain boundary regions allows oxidation of crystallite surfaces during annealing. We tentatively attribute the 0.7 eV PL in nc-Si:H to deep donor defect states related to oxygen precipitates, and argue for the possible involvement of dislocations inside of crystallites to accompany these precipitates.

Study on the Defect Energy Levels of High Resisitivity In-doped CdZnTe Crystals

Samples with different In dopant concentrations were grown by Low Pressure Vertical Bridg- man Method.Low temperature photoluminescence(PL)spectra,Deep Level Transient Spectroscope (DLTS)and high resistivity Hall test were used to study major defects in high resistivity In-doped CdZnTe crystal and its possible compensating mechanism.The PL spectra showed that in the In-doped CdZnTe samples of high resistivity,In dopants occupied Cd vacancies,which would exist in undoped CdZnTe crystal,forming shallow donor defect[In~+_(Cd)],located at E_c-18meV.and the[In~+_(Cd)]interacted with[V~(2-)_(Cd)]to form a complex defect[(In~+_(Cd)-V~(2-)_(Cd))-]at E_v+163meV.The DLTS results showed that a deep level donor defect was found at 0.74eV below the conduction band,representing probably the energy level of antisite defect[Te_(Cd)].The results indicated that the electrical properties of In-doped CdZnTe crystals were dominated by a comprehensive compensating consequence among In donor de- fects,deep level donor defect Te antisites,intrinsic acceptor defect Cd vacancies and other impurities acceptor defects.

Large lattice relaxation deep levels in neutron-irradiated GaN

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) measurements have been carried out in neutron-irradiated n-type hydride-vapor-phase-epitaxy-grown GaN. A defect center characterized by a DLTS line, labeled as N1, is observed at EC−ET=0.17eV. Another line, labeled as N2, at EC−ET=0.23eV, seems to be induced at the same rate as N1 under irradiation and may be identified with E1. Other defects native to wurtzite GaN such as the C and E2 lines appear to enhance under neutron irradiation. The DLOS results show that the defects N1 and N2 have large Frank-Condon shifts of 0.64 and 0.67eV, respectively, and hence large lattice relaxations. The as-grown and neutron-irradiated samples all exhibit the persistent photoconductivity effect commonly seen in GaN that may be attributed to DX centers. The concentration of the DX centers increases significantly with neutron dosage and is helpful in sustaining sample conductivity at low temperatures, thus making possible DLTS measurements on N1 an N2 in the radiation-induced deep-donor defect compensated material which otherwise are prevented by carrier freeze-out.

Electrical and multifrequency EPR study of nonstoichiometric defects in 4H-SiC

Two paramagnetic intrinsic defects P and ND1 have been studied in both C-rich n-type 4H-SiC and undoped semi-insulating (s.i.) 4H-SiC in the dark and under illumination of the s.i. sample with light at 140 and 37GHz in the temperature interval from 4.2 to 77K. Photo EPR and Hall effect measurements indicate that P is a deep donor defect localized at EC−1.15eV, while ND1 is shallow donor defect localized at EC−0.07eV.Based on the observed 13C hyperfine structure, and the C-rich growth conditions, we identify the P center with the silicon vacancy, while ND1, which exhibited a strong central hyperfine interaction with one 13C atom, is attributed to the carbon antisite CSi. Considering that the defects have spin S=12, C3V symmetry of the EPR spectrum and appear to be donor-like defects, P is attributed to the silicon vacancy in the −3 charge state (VSi3−), whereas ND1 is suggested to be the carbon antisite in the single negative charge state (CSi−).

Proton implantation effects on electrical and optical properties of undoped AlGaN with high Al mole fraction

Electrical and optical properties of undoped n-AlGaN films with Al composition close to 40% were studied before and after implantation of various doses of 100 keV protons. In the virgin samples, the electrical properties were determined by deep donor defects with an energy level near 0.25 eV from the conduction band edge and a concentration of ∼1018 cm−3. Other deep centers present had energy levels of 0.12, 0.3, and 0.45 eV. The luminescence spectra were dominated by two defect bands near 2.3 and 3.6 eV. Proton implantation significantly decreased the concentration of major donors even at the lowest doses of 1012 cm−2. For higher doses the Fermi level became progressively deeper and the data indicated complexing of defects present in the sample with either primary radiation defects or/and hydrogen introduced by implantation. The effect of the proton implantation on the intensity of luminescence bands was complicated but overall the intensity of defect bands was increased with implantation.

Carbon-doped MOCVD InP is semi-insulating up to 700°C

InP : C layers grown by metal organic vapour phase epitaxy at 500°C and doped with CCl 4 remain semi-insulating following anneals up to 700°C. The compensation of grown-in C P acceptors ([C P]∼5×10 18 cm −3) is attributed to the presence of C–C deep donor defects, revealed by Raman scattering. [C P] and the concentration of grown-in C P–H pairs both decrease with increasing temperatures. C In, V InH 4 or P In shallow donors are not detected. Reductions in the internal random electric fields upon annealing lead to narrowing of the C P localised vibrational mode. Samples annealed at 800°C become lightly n-type (∼10 16 cm −3).

Nature of the native deep localized defect recombination centers in the chalcopyrite and orthorhombic AgInS2

We studied the deep photoluminescence (PL) emission in polycrystalline chalcopyrite and orthorhombic AgInS2. In both phases several PL bands were detected at 8 K. On the energy scale these deep PL bands are positioned according to a regular pattern. This is explained as being due to electron-hole recombination within very close deep donor-deep acceptor pairs, with different distances between donor and acceptor defects. The deep donor defect is an interstitial silver Agi and the native deep acceptor defect appears to be situated at the Ag or In place. The two different crystal modifications also cause slightly different distances between donor and acceptor defects in the AgInS2 lattice and, as a result of this, different spectral positions of the deep PL bands. It is shown that these deep localized donor–acceptor pairs can be reasonably efficient radiative recombination centers up to distances of 5.3 Å between the deep donor and the deep acceptor and, thus, up to six distinct deep PL bands are visible in AgInS2. The deep donor-deep acceptor pair model is confirmed also by the temperature quenching experiments and by the excitation power dependences.

Investigation of the formation of the 2.8 eV luminescence band in p-type GaN:Mg

The stability of defects present in GaN:Mg has been investigated using photoluminescence (PL) spectroscopy. The two dominant defect-related PL emission bands in p-type GaN were investigated, the blue band at 2.8 eV and the ultraviolet (UV) emission band at 3.27 eV. The intensity of the 3.27 eV PL band increases with increasing resistivity, whereas the 2.8 eV PL band intensity increases with a decrease in resistivity. The luminescence data is explained by a model whereby the concentration of luminescent centers depends on the Fermi level position. The shallow donor responsible for the UV band is attributed to hydrogen, whereas the deep donor defect responsible for the 2.8 eV band is attributed to a nitrogen vacancy complex.

The Metastability of Deep Donor Defects in Semiconductor Compounds

There is a whole class of deep donor defects which exhibit a metastable character, i.e. transform into a metastable state after a specific photo-excitation. Typical examples are the EL2 defect in GaAs and the DX center in several ternary III-V compounds. We shall present other defects which behave similarly, demonstrating that this metastable character is not specific of a defect but is a common behaviour of a class of defects. We shall also demonstrate that for all these defects the thermal excitation of electrons takes place in the L conduction band and that the metastable character is a direct consequence of this. This leads to a new concept of a metastable state, a result of an electronic metastability due to a band structure effect and not to a strong electron-phonon interaction.

The EL2-like metastable defect and the n- to p-type transition in silicon planar-doped GaAs

Through photo-Hall measurements at temperatures below about 120 K, we have observed the presence of a deep donor defect, with characteristics similar to those of the EL2 center, in planar-doped GaAs samples grown by molecular beam epitaxy at 300 °C. We have shown that this EL2-like center can account for the remarkable photoreleasing of electrons and holes into the conduction and valence bands, respectively. The two different kinds of carriers accumulate in two spatially separated channels, which can secondarily account for the n- to p-type transition we have observed.

Electrical evaluation of InP surface damage by reactive ion etching using mixture of ethane (C 2H 6) and hydrogen (H 2)

The electrical properties of the (100) InP surface were studied based on the fabrication process of a buried heterostructure (BH) laser diode (LD). The current-voltage ( I- V) and capacitance-voltage ( C- V) characteristics were measured for Schottky barriers on the surface at each step of the process. The characteristic variation during the process showed the C 2H 6/H 2-reactive ion etching (RIE) induced damage on the near-surface region: a layer resulting from phosphorus desorption, a low p-concentration layer due to hydrogen passivation, and deep donor defects with electrical drift phenomena. It was found that the damage layers were removed by annealing after the chemical etching which produced almost the same surface as that immediately before the BH regrowth.

Zone melt growth of GaAs for gamma ray detector applications

Vertical Zone Melt Growth (VZM) is being investigated as a technique for preparation of bulk GaAs crystals with applications as ambient temperature gamma ray detectors/spectrometers. Zone refining (repeated passes of a molten zone across a crystal) and zone levelling (one pass of a molten zone which is gallium rich) are being investigated for improving the purity of bulk GaAs and lowering the deep level donor defect (EL2) concentration. GaAs which has been zone refined and GaAs which has been zone levelled with a gallium rich melt have been grown, characterized and processed as detectors. In this paper, we discuss details of the VZM growth of GaAs and the crystalline and electronic properties achieved by zone levelling and zone refining.

Development and characterization of zone melt growth GaAs for gamma-ray detectors

GaAs is a potentially attractive material for room temperature X-ray and /spl gamma/-ray spectrometers. To date, the only high resolution GaAs devices were produced by epitaxial growth. The usefulness of detectors made from bulk grown semi-insulating (SI) GaAs has been limited by low charge collection efficiency caused, it is believed, by the high density of EL2 deep donor defects. Vertical zone melt (VZM) growth of GaAs has recently been developed at the Naval Research Laboratory. Zone refining and zone leveling techniques were used with VZM to reduce the level of impurities and the EL2 defects in bulk SI-GaAs. Schottky barrier and PIN diodes have been fabricated from the newly grown material. These devices were characterized using /spl alpha/ particles and /spl gamma/-rays. In this paper, the measurements and analysis of the first VZM GaAs devices are presented and compared with commercially available GaAs. The intent is to test the hypothesis that high purity, low defect GaAs material growth could lead to improved radiation detectors.

Semi-insulating GaAs layers grown by molecular-beam epitaxy

Temperature dependent conductivity and Hall-effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi-insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi-insulating GaAs, has been evaluated. A similar activation energy is observed in as-grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.

Persistent Photo-Hall Phenomenon and Near-Bandedge Absorption in Lightly N-Type Lec GaAs

ABSTRACTThe novel infrared imaging technique for assessment of undoped semi-insulating (SI) GaAs substrates known as Reverse Contrast is known to result from absorption from point defects whose concentrations approximately anti-correlate with those of EL2, deep donor defects. The absorption that occurs within - 65 meV of Eg in cooled samples is sufficiently strong that commercial wafers can be mapped with simple infrared CCTV imaging equipment. RC defects are thought to be very deep acceptors with an ionization energy close to the Conduction Band. Concentrations of RC defects are not measurable in SI GaAs as they are un-ionized in the dark. Like EL2 defects they can be photo-quenched by irradiation with mid-gap light.In this paper, we present novel Hall Effect measurements on very lightly Te-doped n-type GaAs where at least a fraction of RC defects are ionized. A permanent increase in the carrier concentration is observed after photo-quenching corresponding to the bleaching of RC defects. The carrier concentration returns to its initial value at the same temperature at which the absorption of RC defects is recovered. This result allows a calibration for the absorption coefficient to be found.

Ultraviolet photoluminescence from undoped and zn doped AlxGa1−xN with x between 0 and 0.75

The origin of the abrupt decrease withx both in electron concentration and in mobility of AlxGa1−xN in the range ofx between 0.4 and 0.6 was investigated by photoluminescence from epitaxial layers covering the entire range of alloy composition. Band edge luminescence from undoped layers was observed out to anx value of 0.75. Whenx was larger than 0.2, a peak 0.2–0.5 eV below the band edge peak was also observed both from undoped and from Zn doped samples. This is tentatively ascribed to an unidentified acceptor (or acceptors) related to the presence of Al. No luminescence which could be attributed to a deep native donor defect was observed in semi-insulating AlxGa1−xN layers. When Zn was added in the lowx range of the alloys, a broad band 0.5–0.8 eV below the band edge peak was observed as well as a relatively narrow peak.

Donors in semiconductors and metastability.

We observed that excited or ground effective-mass states of deep or shallow donor defects linked to the different conduction bands of the semiconductor can exhibit long lifetimes characteristics of their depth from the corresponding band. This way the metastable behaviors of the so-called DX center and EL2 defects in GaAs and related alloys can be understood without the need to introduce distorted atomic configurations of these defects.

Donor‐related edge luminescence in ion‐implanted ZnTe

AbstractSingle crystal samples of ZnTe are implanted at 77 K with chlorine, bromine, copper, or silver ions of 100 keV energy. The low temperature cathodoluminescence spectra of the sample are recorded as a function of post‐implantation isochronal (30 min) annealing temperature in vacuum in the range 50 to 500 °C and compared with the spectra of similarly‐annealed unimplanted and argon‐implanted reference samples. Chlorine, bromine, and copper each induce similar changes in the emission spectrum and, in addition to enhancing strongly donor‐acceptor pair bands at 547.3 (CII) and 553.6 nm (BII), produced new edge emission bands at 530.2, 535.2, and 530.6 nm, respectively. These three new bands are each assigned to a transition from a deep donor level to the valence band. The deep donor defects are attributed to the complexes (VZn2ClTe), (VZn2BrTe), and (VZn2Cui) or (VZn 2CuTe), with electron binding energies of 65, 88, and 69 meV, respectively. In contrast, annealing of silver‐implanted ZnTe causes emission spectrum changes comparable to those induced by argon ion bombardment and produces no effects which can be attributed to the silver impurity.