Deep Acceptor Level Research Articles

Impact of alloying duration of an electrodeposited Cu/Sn/Zn metallic stack on properties of Cu2ZnSnS4 absorbers for thin‐film solar cells

AbstractThe impacts of preheating of an electrodeposited Cu/Sn/Zn (CTZ) stack precursor on structural changes of the CTZ precursor and the impact on structural and electric properties of the finally obtained Cu2ZnSnS4 (CZTS) films are discussed in detail. We found that preheating for relatively long durations improved the qualities of CZTS films: these films were composed of large grains and had compact and flat surface morphologies. The best solar cell with efficiency of 8.1% was obtained on the basis of a CZTS film derived from the CTZ precursor preheated for 200 min. The external quantum efficiency response of the cell indicated efficient utilization of photons with relatively long wavelength regions because of its good structural and electronic properties. On the other hand, a short circuit current density–temperature property of one of the best cells in this study suggested that the CZTS film had deep acceptor levels and/or an appreciable energy barrier to the Mo back contact. Moreover, an open circuit voltage–temperature property of the corresponding device showed activation energy of 1.18 eV, indicating preferential occurrence of CdS–CZTS interface recombination. Copyright © 2015 John Wiley & Sons, Ltd.

Time-dependent electric field in Al/CdTe/Pt detectors

Al/CdTe/Pt detectors are very attractive devices for high-resolution X-ray spectroscopy, even though they suffer from bias-induced time instability (polarization). Polarization phenomena cause a progressive time-degradation of the spectroscopic performance of the detectors, due to hole trapping and detrapping from deep acceptor levels that directly control the electric field distribution. In this work we present experimental investigations on the electric field profile of planar Al/CdTe/Pt detectors by means of Pockels effect measurements. The time/temperature dependence of the electric field was investigated in a long time window (up to 10h) and the correlation with the reverse current transients was also studied. Two energy levels (0.62eV and 1.16eV) of the deep hole traps were measured, in agreement with our previous results obtained through electrical and spectroscopic approaches.

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl4GaIn3Se2S6 layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35eV and in the temperature range of 16–45K. A broad PL band centered at 2.20eV was observed at T=16K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9mWcm−2 range. Radiative transitions from shallow donor level located at 10meV below the bottom of conduction band to moderately deep acceptor level located at 180meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030nm at room temperature revealed the presence of indirect transitions with 2.22eV band gap energy.

Electrical characterization of Si doped AlN films synthesized by pulsed laser deposition

The electrical properties of thin AlN films doped with Si (AlN:Si) have been investigated. The films were synthesized on Si substrates at 800 °C by pulsed laser deposition in low-pressure nitrogen ambient. The AlN:Si films exhibit non-ohmic I-V characteristics and the current through these films is controlled by space charge limited current. The C-V dependence of metal-insulator-silicon (MIS) structures with AlN:Si films exhibits an excess capacitance around zero bias voltage. This excess capacitance indicates the presence of deep acceptor levels situated at the boundaries of adjacent grains in the AlN:Si films. The Si donor density in the AlN:Si films, estimated from the 1 MHz C-V characteristics, is of the order of 10 18 cm −3 . The impedance measurements of these AlN:Si structures at different test voltage frequencies reveal that the charge transport mechanism is dominated by either thermally-activated hopping or electron tunneling from occupied to nearest unoccupied deep levels.

Oxygen defects in phosphorene.

Surface reactions with oxygen are a fundamental cause of the degradation of phosphorene. Using first-principles calculations, we show that for each oxygen atom adsorbed onto phosphorene there is an energy release of about 2eV. Although the most stable oxygen adsorbed forms are electrically inactive and lead only to minor distortions of the lattice, there are low energy metastable forms which introduce deep donor and/or acceptor levels in the gap. We also propose a mechanism for phosphorene oxidation involving reactive dangling oxygen atoms and we suggest that dangling oxygen atoms increase the hydrophilicity of phosphorene.

Group III-Sb Metamorphic Buffer on Si for p-Channel all-III-V CMOS: Electrical Properties, Growth and Surface Defects

ABSTRACTGroup III-Sb compound semiconductors are promising materials for future CMOS circuits. Especially, In1-xGaxSb is considered as a complimentary p-type channel material to n-type In1-xGaxAs MOSFET due to the superior hole transport properties and similar chemical properties in III-Sb’s to those of InGaAs. The heteroepitaxial growth of In1-xGaxSb on Si substrate has significant advantage for volume fabrication of III-V ICs. However large lattice mismatch between InGaSb and Si results in many growth-related defects (micro twins, threading dislocations and antiphase domain boundaries); these defects also act as deep acceptor levels. Accordingly, unintentional doping in InGaSb films causes additional scattering, increase junction leakages and affects the interface properties. In this paper, we studied the correlations between of defects and hole carrier densities in GaSb and strained In1-xGaxSb quantum well layers by using various designs of metamorphic superlattice buffers.

Compensation mechanism of DX-like center in neutron transmutation doped GaN

Neutron transmutation doped (NTD)-GaN, which is irradiated with fast and thermal neutrons at fluxes of 6.7×1018cm−2 and 1.4×1019cm−2, respectively, keeps the high resistivity of 108Ωcm at room temperature. The energy level estimated from the temperature dependence of carrier concentration by high temperature alternating current Hall effect measurements is 960meV. This energy is attributed to the nitrogen interstitial (Ni) as a deep acceptor level located below the bottom of the conduction band, whose level is close to that of the nitrogen split interstitial. In Rutherford backscattering spectroscopy/channeling measurements using 1.5-MeV H+ ions, the lattice displacement of N atoms is observed, suggesting the existence of Ni. Neutron-transmuted DX-like center of Ge as a donor is compensated by both Ni and 14C acceptors generated from the (n,p) reaction of 14N. The origins of high resistivity after the neutron irradiation are attributed to these acceptors.

Density and energy level of a deep-level Mg acceptor in 4H-SiC

Reliably determining the densities and energy levels of deep-level dominant acceptors in heavily doped wide-band-gap semiconductors has been a topic of recent discussion. In these discussions, the focus is on both Hall scattering factors for holes and distribution functions for acceptors. Mg acceptor levels in 4H-SiC seem to be deep, and so here the electrical properties of Mg-implanted 4H-SiC layers are studied by measuring Hall effects. The obtained Hall scattering factors are not reliable because they drop to less than 0.5 at high measurement temperatures. Moreover, the Fermi–Dirac distribution function is unsuitable for examining Mg acceptors because the obtained acceptor density is much higher than the concentration of implanted Mg atoms. However, by using a distribution function that includes the influence of the excited states of a deep-level acceptor, the density and energy level of Mg acceptors can be reliably determined.

Effects of impurity-band conduction on thermoelectric properties of lightly doped p-type CoSb3

Experimental data on the thermoelectric properties of lightly doped p-type CoSb3 reported by Mandrus et al. [Phys. Rev. B 52, 4926 (1995)], Arushanov et al. [Phys. Rev. B 56, 1911 (1997)], and Dyck et al. [Phys. Rev. B 65, 115204 (2002)] have been analyzed, incorporating both the impurity-band conduction and the nonparabolicity of the valence band. The concentrations of a resonant acceptor, a deep acceptor, and a compensating donor level have been considerably corrected from those obtained in the original studies by Arushanov et al. and Dyck et al. The impurity-band conduction in the deep acceptor level has been proved to be the dominant conduction mechanism at low temperatures in the sample of Dyck et al. while that in the resonant acceptor level has been proved to be the dominant one in the other samples. Especially, the impurity-band conduction has been proved to be predominant up to as high as 400 K in the sample of Mandrus et al. As a result of incorporating the effects of both the impurity-band conduction and the nonparabolicity, the agreement of the simulated Seebeck coefficient with the experimental one has been excellently improved.

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga $({\mathrm{Mg}}_{\mathrm{Ga}})$ in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, ${\mathrm{Li}}_{\mathrm{Zn}}$, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using ${\mathrm{Na}}_{\mathrm{Zn}}$ in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.

Defects in heteroepitaxial CdHgTe/Si layers and their behavior under conditions of implanted p +-n photodiode structure formation

The impurity-defect structure of heteroepitaxial CdxHg1 − xTe/Si (0.35 < x < 0.39) layers grown by molecular beam epitaxy for the creation of arsenic-ion-implanted p+-n junctions has been studied by the photoluminescence method. It is established that full realization of the possibilities of p+-on-n photodiode structures based on the CdHgTe/Si system is hindered by uncontrolled doping of the material that leads to the formation of both shallow (impurity level energy ∼10 meV) and deep (∼50 meV) acceptor levels.

Structural and electronic properties of cubic boron nitride doped with zinc

Structural and electronic properties of Zn-doped cubic boron nitride (cBN) were investigated via first principle calculation based on density functional theory. Our simulation suggests that Zn can substitute for both B (ZnB) and N (ZnN) atom; ZnB is energetically favorable, and ZnN can only be prepared under B-rich conditions. ZnB induced a shallow acceptor level; however, the large difference in electronegativity between Zn and N makes the acceptor level strongly localized, which reduces effective carrier density. In the case of ZnN, both deep acceptor levels within band gap and shallow acceptor levels at the top of valence band were induced, which produced more free carriers than ZnB. The calculated results account for experimental results of enhanced electric conductivity of Zn-doped cBN films prepared under B-rich conditions.

Characterization of deep acceptor level in as-grown ZnO thin film by molecular beam epitaxy

We report deep level transient spectroscopy results from ZnO layers grown on silicon by molecular beam epitaxy (MBE). The hot probe measurements reveal mixed conductivity in the as-grown ZnO layers, and the current—voltage (I—V) measurements demonstrate a good quality p-type Schottky device. A new deep acceptor level is observed in the ZnO layer having activation energy of 0.49 ±0.03 eV and capture cross-section of 8.57 × 10−18 cm2. Based on the results from Raman spectroscopy, photoluminescence, and secondary ion mass spectroscopy (SIMS) of the ZnO layer, the observed acceptor trap level is tentatively attributed to a nitrogen-zinc vacancy complex in ZnO.

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = INBE/IDLE can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.

High-Performance a-IGZO Thin Film Diode as Selector for Cross-Point Memory Application

We present amorphous indium-gallium-zinc oxide Schottky diodes with unprecedented current densities of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at forward biases of 1.5 and 5 V, respectively. The diode presents a high rectification ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> at ±2 V, which is essential for suppressing the sneak current of not-selected cells in the memory array. In addition, we show that the diode complies with the demanding performance of memory applications. The device degradation, given by a 30% reduction of its forward current after 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s of continuous bias stress or 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> pulses cycles, was studied via I-V and C-V measurements and can be attributed to trapping of electrons at deep acceptor levels, which increases the diode built-in potential. Finally, we show that the device is stable upon thermal stress at 300 °C for 1 h, which opens the possibility of further processing and integration with the memory cell.

Defect levels in Cu2ZnSn(SxSe1−x)4 solar cells probed by current-mode deep level transient spectroscopy

Defect levels in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have been investigated by current-mode deep level transient spectroscopy. Experiments were carried out on two CZTSSe cells with photoconversion efficiencies of 4.1% and 7.1% measured under AM 1.5 illumination. The absorber layer of the 4.1% efficiency cell was prepared by annealing evaporated ZnS/Cu/Sn stacked precursor under S/Se vapor, while the absorber of the 7.1% efficiency cell was prepared by co-evaporation of the constituent elements. The 4.1% efficiency CZTSSe cell with a S/(S + Se) ratio of 0.58 exhibited two dominant deep acceptor levels at Ev + 0.12 eV, and Ev + 0.32 eV identified as CuZn(-/0) and CuSn(2-/-) antisite defects, respectively. The 7.1% efficiency cell with purely Se composition S/(S + Se) = 0 showed only one shallow level at Ev + 0.03 eV corresponding to Cu-vacancy (VCu). Our results revealed that VCu is the primary defect center in the high-efficiency kesterite solar cell in contrast to the detrimental CuZn and CuSn antisites found in the low efficiency CZTSSe cells limiting the device performance.

Photoluminescence from Cu2ZnSnS4 thin films with different compositions fabricated by a sputtering-sulfurization method

Photoluminescence (PL) from Cu2ZnSnS4 (CZTS) thin films fabricated by a sputtering-sulfurization method with considerably different Cu/(Zn+Sn) ratios of 0.60–4.21 and Zn/Sn ratios of 0.29–1.68 was investigated with respect to the defect physics. The ranges include the Cu-poor and Zn-rich (Cu/(Zn+Sn)= 0.78 and Zn/Sn=1.55) CZTS composition, which is determined as a suitable composition for application as a solar cell absorber. The origins of the luminescence are attributed to band-to-impurity (BI), band-to-tail (BT), and band-to-band (BB) recombinations at approximately 1.23, 1.35, and 1.48eV, respectively, which are typically observed from heavily doped and highly compensated semiconductors as a result of temperature dependence, excitation power dependence, and spectral shapes of the PL spectra. Although the BT and BB band emissions had no strong dependence on the film compositions, an intense BI band emission was dependent on the film composition and observed for Cu/Sn≤2.0. The compositional dependence indicates that a deep acceptor level is formed in CZTS with Cu/Sn≤2.0, which is thus considered to be a suitable composition range for the absorber of a highly efficient solar cell.

Electrical Behavior of Mg in Mg-Implanted 4H-SiC Layer

Because Al and B (elements of group III) in SiC are acceptors with deep energy levels and these acceptors cannot reduce the resistivity of p-type SiC very much, Mg (an element of group II) that may emit two holes into the valence band is investigated. Annealing at 1800 ◦C makes a Mg-implanted layer p-type. It is found that an Mg acceptor level in 4H-SiC is too deep to reliably determine the density and energy level of the Mg acceptor using the frequently used occupation probability, i.e., the Fermi Dirac distribution function. Using the distribution function that accounts for the in uence of the excited states of a deep-level acceptor, the density and energy level of Mg acceptors can be determined to be approximately 1× 10 cm−3 and 0.6 eV, respectively. These values are considered to be reliable because they agree well with the Mg implantation condition.

ComprehensiveAb InitioStudy of Doping in Bulk ZnO with Group-V Elements

Despite the lack of reproducible experimental confirmation, group-V elements have been considered as possible sources of p-type doping in ZnO in the form of simple and complex defects. Using ab initio calculations, based on state-of-the-art hybrid exchange-correlation functional, we study a wide range of defects and defect complexes related with N, P, As, and Sb impurities. We show that none of the candidates for p-type doping can be considered a good source of holes in the valence band due to deep acceptor levels and low formation energies of compensating donor defects. In addition, we discuss the stability of complexes in different regimes.

Electrical Properties of Mg-Implanted 4H-SiC

Because Al and B (elements of III group) in SiC are deep-level acceptors and these acceptors cannot reduce the resistivity of p-type SiC very much, Mg (element of II group) that may emit two holes into the valence band is investigated. A p-type 4H-SiC layer is obtained by 1800 °C annealing of the Mg-implanted layer, not by 1600 and 1700 °C annealing. It is found that a Mg acceptor level in 4H-SiC is too deep to determine the reliable density and energy level of the Mg acceptor using the frequently-used occupation probability, i.e., the Fermi-Dirac distribution function. Using the distribution function including the influence of the excited states of the Mg acceptor, therefore, its density and energy level can be determined to be approximately 1×1019 cm-3 and 0.6 eV, respectively. Judging from the Mg implantation condition, the obtained values are considered to be reliable.