Zn-rich Conditions Research Articles

Achieving highly-enhanced UV photoluminescence and its origin in ZnO nanocrystalline films

ZnO is an efficient luminescent material in the UV-range ∼3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PL, are assigned to Zni related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The O2 ambient was found to introduce nominal Oi centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of ∼40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zni-related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects.

Facile, Noncyanide Based Etching of Spray Deposited Cu2ZnSnS4 Thin Films for Secondary Phase Removal

The coexistence of secondary phases in the quaternary compound kesterite Cu2ZnSnS4 (CZTS), a promising photovoltaic absorber, is a major problem while synthesizing under Zn or Cu rich conditions. A large variety of secondary phases, such as CuS, Cu1.8S, Cu2S, Cu2SnS3, and ZnS exist on the surface unless they are not removed by dedicated surface treatment before the annealing step. Under a carefully chosen concentrations of the starting precursors (usually, Zn-poor and Sn-rich) for spray pyrolyzed CZTS, the fraction of ZnS is minimized. However, under such growth conditions, binary Cu-sulfides become dominant. In this work, a selective noncyanide based chemical etching procedure is demonstrated prior to the deposition of the buffer layer. The absorber surface was treated with hydrogen peroxide that is known to remove Cu1.8S, Cu2S, and allied secondary phases to a large extent as compared to conventional KCN based techniques. By this treatment, the optical band gap is changed from 1.8 eV to most suitable 1....

Growth and characterization of indium-doped Zn3P2 bulk crystals

In this paper, we report the crystal growth of indium-doped zinc phosphide bulk crystals to obtain n-type conduction. The crystal growth experiments were carried out by unidirectional solidification from In–Zn–P ternary solution, and n-type Zn3P2 bulk crystals were successfully obtained. It was also revealed that the electrical properties of indium-doped Zn3P2 crystals could be controlled by heat treatment under controlled partial pressure of phosphorus or zinc. The relationship between the electron concentration and the partial pressure of zinc or phosphorus even in indium-doped Zn3P2 can be understood on the basis of defect equilibria, as in the case of undoped Zn3P2. However, the controllable range of the electron concentration was 1010–1013 cm−3, which was too low in terms of the concentration of doped indium. This is due to the low formation energy of the intrinsic acceptor, interstitial phosphorus, and carrier compensation should be discussed. According to recent reports on ab initio calculation, a weakly n-type Zn3P2 is expected to be formed under extremely Zn-rich growth conditions. The results obtained in this study coincide with the prediction based on calculations, and provide useful knowledge for the formation of the p–n junction of Zn3P2.

Breakthrough of the p-type doping bottleneck in ZnO by inserting an ultrathin ZnX (X = S, Se and Te) layer doped with NX or AgZn

The worldwide problem of p-type doping in ZnO is investigated based on first-principles calculations by combining the standard density functional theory and hybrid functional methods. We find that p-type doping can be realized by inserting an ultrathin ZnX (X = S, Se and Te) layer, doped with NX or AgZn, into ZnO to form short-period (ZnO)m/(ZnX)n (m > n) superlattices. The formation energy is the lowest for NX or AgZn in the ZnX layer. The Zn-rich (Zn-poor) condition is favourable for the formation of the NX (AgZn) defect. Compensation by the native defects can be avoided for the Ag-doped (ZnO)m/(ZnX)n under the Zn-poor condition. The N (Ag) acceptor activation energy can be reduced from 0.45 (0.43) eV in ZnO to 0.33 (0.32) eV in (ZnO)5/(ZnS)1, 0.20 (0.24) eV in (ZnO)5/(ZnSe)1 and 0.12 (0.13) eV in (ZnO)5/(ZnTe)1, which is caused by the ZnX-monolayer modulation to the local structure around the NX or AgZn defect and the high-lying p-derived valence bands. Moreover, the band gaps can be tuned from 3.40 eV of ZnO to 3.21 eV of (ZnO)5/(ZnS)1, 2.41 eV of (ZnO)5/(ZnSe)1 and 2.26 eV of (ZnO)5/(ZnTe)1, which is promising for the integration of ZnO-based white light-emitting diodes.

Atom probe tomography study of internal interfaces in Cu2ZnSnSe4 thin-films

We report on atom probe tomography studies of the composition at internal interfaces in Cu2ZnSnSe4 thin-films. For Cu2ZnSnSe4 precursors, which are deposited at 320 °C under Zn-rich conditions, grain boundaries are found to be enriched with Cu irrespective of whether Cu-poor or Cu-rich growth conditions are chosen. Cu2ZnSnSe4 grains are found to be Cu-poor and excess Cu atoms are found to be accumulated at grain boundaries. In addition, nanometer-sized ZnSe grains are detected at or near grain boundaries. The compositions at grain boundaries show different trends after annealing at 500 °C. Grain boundaries in the annealed absorber films, which are free of impurities, are Cu-, Sn-, and Se-depleted and Zn-enriched. This is attributed to dissolution of ZnSe at the Cu-enriched grain boundaries during annealing. Furthermore, some of the grain boundaries of the absorbers are enriched with Na and K atoms, stemming from the soda-lime glass substrate. Such grain boundaries show no or only small changes in composition of the matrix elements. Na and K impurities are also partly segregated at some of the Cu2ZnSnSe4/ZnSe interfaces in the absorber, whereas for the precursors, only Na was detected at such phase boundaries possibly due to a higher diffusivity of Na compared to K. Possible effects of the detected compositional fluctuations on cell performance are discussed.

C–H complex defects and their influence in ZnO single crystal**Project supported by the National Natural Science Foundation of China (Grant No. 61474104).

Infrared absorption local vibration mode (LVM) spectroscopy is used to study hydrogen related defects in n-type ZnO single crystal grown by a closed chemical vapor transport (CVT) method under Zn-rich growth conditions, in which carbon is used as a transport agent. Two C–H complex related absorption peaks at 2850 cm−1 and 2919 cm−1 are detected in the sample. The formation of the C–H complex implies an effect of carbon donor passivation and formation suppression of H donor in ZnO. The influence of the complex defects on the electrical property of the CVT-ZnO is discussed based on Hall measurement results and residual impurity analysis.

Characterization of Cu2ZnSnSe4 (CZTSe) nanoparticles synthesized via solvothermal method for solar cell applications

Stannite Cu2ZnSnSe4 (CZTSe) nanoparticles were synthesized using a simple solvothermal process at low temperatures using ethylenediamine as a solvent. The structures and morphologies of CZTSe nanoparticles were characterized as functions of various zinc salts in the precursors. Nanosized CZTSe particles, formed with zinc chloride as the source of zinc and obtained at 240 °C after 30 min of the aforementioned process, exhibited a single CZTSe phase. Increasing the processing duration to 6 h markedly increased the crystallinity of heat treated powders. All particles that were obtained after 6 h 240 °C from various zinc salts exhibited a single tetragonal phase with particle sizes in the range of 30–60 nm. CZTSe nanoparticles that were obtained from zinc chloride and zinc acetylacetonate had better crystallinity compared to particles obtained from other zinc salts. The structural variation in CZTSe that was synthesized from zinc chloride was observed as a function of Cu/(Zn + Sn) ratio. A Cu poor and Zn rich conditions resulted into a highly crystalline CZTSe phase with relatively smooth and closely packed morphology. The CZTSe nanoparticles, obtained from ZnCl2, were evaluated for potential use in solar cells via preparing precursor ink. The contents of the precursor ink were fixed to Cu/(Zn + Sn) = 0.8 and Zn/Sn = 1.2, and the ink was coated onto Mo substrates via drop casting. The GIXRD patterns of the film showed single CZTSe phase without any binary phases throughout the film. These results demonstrated that the as formed CZTSe nanoparticles are suitable for use as absorber layers in low-cost solar cells.

Calculation of Defect Concentrations and Phase Stability in Cu$_2$ ZnSnS$_4$ and Cu$_2$ ZnSnSe$_4$ From Stoichiometry

The application of the quarternary compounds Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ZnSnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CZTS) and Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ZnSnSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CZTSe) as efficient solar cell absorber materials is dependent on the complex behavior of the large variety of intrinsic lattice defects. In this paper, a canonical approach is presented and applied to calculate the defect concentrations and the position of the Fermi level for CZTS and CZTSe at a given temperature as a function of stoichiometry (or a combination of stoichiometry and elemental chemical potentials). With the defect concentrations, the chemical potentials (which are generally not experimentally accessible) can be calculated, allowing the relation of sample composition to the phase stability of CZTS and CZTSe with respect to secondary compounds. Based on the model, it is shown that the stable CZTS with off-stoichiometric composition requires both Cu-poor and Zn-rich conditions, while the compositional space corresponding to stable CZTSe is wider than for CZTS. Additionally, the determination of the Fermi level directly relates the desired p-type conductivity to phase stability in both materials. The method used in this study is applicable to a wide range of complex materials.

The effect of growth conditions, point defects and hydrogen on the electronic structure and properties of p-type (Al,N) codoped ZnO: A first principles study

The effects of point defects, hydrogen, and growth conditions on the electronic structure and properties of the (Al,N) codoped p-type ZnO have been investigated using the first principles method. The obtained results showed that the AlZn–NO–VZn complex is a shallow acceptor that can play an important role in achieving the p-type conductivity in the (Al,N) codoped ZnO films. Our results showed also that the electrical conductivity type in the (Al,N) codoped ZnO films strongly depends on the donor/acceptor concentrations ratio. The codoped ZnO films prepared under both Zn-rich and O-rich growth conditions with a donors/acceptors ratio of 1:2 have a p-type conductivity, while those prepared with a ratio of 1:1 cannot be p-type unless if they are prepared under O-rich conditions. The achieved p-type quality depends also on the used nitrogen doping source. To prepare p-type ZnO film of high quality using the (Al,N) codoping method, the use of NO or NO2 is recommended. The presence of donor defects such as oxygen vacancies and hydrogen will significantly affect the electronic properties of the (Al,N) codoped ZnO films, and if the concentration of these defects in the sample is high enough, the material can be easily converted to n-type.

Intrinsic defects and Na doping in Cu2ZnSnS4: A density-functional theory study

The quaternary Cu2ZnSnS4 (CZTS) is a promising material for the thin-film solar cell applications. However, because CZTS is only stabilized in a very small chemical-potential region and the defect formation is much more complicated compared to the ternary systems, it requires careful control of the thin-film solar cell preparation. We have performed a density-functional theory investigation on the intrinsic defects, defect complexes and Na doping in this complicated system. Our calculations show that the disorder and nonstoichiometry in the Cu–Zn plane induces a bandgap shrinkage of the material. From our calculated results for various defects and the doping rules, we rationalize why the experiments obtain high conversion efficiencies for the thin-film solar cells under the Cu-poor and Zn-rich conditions. The most possible defects which fulfill the experimental Cu-poor and Zn-rich conditions are VCu, rather than ZnCu. It is necessary to keep the Sn chemical potential moderate during the CZTS thin-film solar cell growth because the Sn-involved defects and defect complexes are generally harmful. We also find that Na doping is expected to increase the conductivity and improves the conversion efficiency of CZTS solar cells. The present study provides valuable insights into the defect physics of the complicated CZTS system and a theoretical basis for exploring its practical applications as thin-film solar cell absorbers.

Mobility of indium on the ZnO(0001) surface

The mobility of indium on the Zn-polar (0001) surface of single crystal ZnO wafers was investigated using real-time x-ray photoelectron spectroscopy. A sudden transition in the wettability of the ZnO(0001) surface was observed at ∼520 °C, with indium migrating from the (0001¯) underside of the wafer, around the non-polar (11¯00) and (112¯0) sidewalls, to form a uniform self-organized (∼20 Å) adlayer. The In adlayer was oxidized, in agreement with the first principles calculations of Northrup and Neugebauer that In2O3 precipitation can only be avoided under a combination of In-rich and Zn-rich conditions. These findings suggest that unintentional In adlayers may form during the epitaxial growth of ZnO on indium-bonded substrates.

First-principles study of p-type conductivity of N-Al/Ga/In co-doped ZnO

N-mono doped and N-Al/Ga/In co-doped ZnO were investigated by first-principles calculations. We have studied the structural and electrical properties, the p-type conductivities and the defects formation energies of different doped structures. The results showed that the Fermi level moved downward into the valance band and showed p-type conductivity after doping. From the hole carriers analysis, it was found that the hole carriers concentration in N-doped ZnO was larger than the others and had a greater hole effective mass, while the co-doping ZnO had a better carriers transfer character, and the (N,Ga)-ZnO structure showed optimal mobility. The defects formation energy of N-ZnO was relatively high; however, the value of the (N-Al)-ZnO structure on both the O-rich and Zn-rich conditions was low, implying a feasible dual-doped approach.

Zn-poor Cu2 ZnSnSe4 thin films and solar cell devices

This work reports the synthesis of photovoltaic-grade Cu2ZnSnSe4 thin films with Zn-poor compositions (Zn/Sn < 1) prepared by selenization of sputter deposited metallic precursors. This contrasts with most other reports where up to now fabrication of photovoltaic-grade Cu2ZnSnSe4 requires Cu-poor and Zn-rich compositions, which often favor the formation the ZnSe secondary phase. Synthesis of Zn-poor or stoichiometric films opens up prospects for preparing device-grade films while minimizing formation of common secondary phases. While the maximum device efficiency is modest (2.5%), it is notable because it falls outside the optimum compositional range. Annealing conditions including temperature, chalcogen source, and system pressure were varied to determine their influence on film and device properties. Phase characterization confirms that Cu2ZnSnSe4 is the main phase present. Reasons for the formation of device-grade films outside the ideal compositional range are discussed, including the fact that initial film growth is under Zn-rich conditions, and that films are Cu-poor in relation to Zn (Cu/Zn < 2), both of which may potentially give rise to the necessary electronic defects for a photovoltaic-grade absorber layer. Finally, the importance reporting other compositional ratios, especially Cu/Zn and Cu/Sn, instead of the more common ratio Cu/(Zn + Sn), is expounded for this class of materials.

Ab initio study of native point defects in ZnO under pressure

We investigate the formation enthalpies and transition energy levels for several native point defects in B1 phase of ZnO under applied hydrostatic pressure using density functional theory. The formation volume decreases gradually with increasing pressure, and increases linearly with the number of electrons adding to the system. In negatively charged state, the calculated formation enthalpy decreases with pressure, suggesting an increase in the equilibrium defect concentration. The behavior of the positively charged state is on the contrary, consistent with the results of the formation volume. In particular, the formation enthalpy of oxygen vacancy increases with pressure, which makes the defect formation harder under pressure. Under Zn-rich conditions, the “negative-U” phenomenon of oxygen vacancy, which appears under ambient conditions, vanishes with further increase in pressure when the Fermi enthalpy is close to the conduction band minimum.

GGA + U study of native point defects in ZnRh2O4

ZnRh2O4 spinel is the one of the most promising transparent conducting oxides for applications in optoelectronic technology. Energy levels and formation energies of native point defects, i.e. vacancies (V), interstitials (I), and cation antisites in ZnRh2O4 were analysed. Generalized gradient approximation (GGA) was supplemented by the +U on-site corrections imposed on d(Rh) and p(O) states. The inclusion of the pronounced distortions of the anion sublattice was necessary to obtain the correct band gap. U was treated as a free parameter, which allowed for the systematic study of the U-induced changes of the defect states. A diagram of the thermodynamic phase stability of ZnRh2O4 was obtained. ZnRh is the dominant acceptor that can be responsible for the observed p-type conductivity in ZnRh2O4. The low formation energy of ZnRh can make the intentional n-doping difficult. In the O-rich conditions the second important acceptor is VZn. The two dominant donors that can compensate ZnRh in Zn-rich and O-rich conditions are VO and RhZn, respectively. Growth conditions leading to the lowest concentrations of native defects were identified. Due to the pronounced occupancy dependence of the +U term, VO and RhZn are ‘negative-Ueff’ centres.

Co-evaporation process study of Cu2ZnSnSe4thin films byin situlight scattering andin situX-ray diffraction

Multi-stage co-evaporation processes for the growth of Cu2ZnSnSe4 (CZTSe) thin films are investigated with time-resolved in situ angle-dispersive X-ray diffraction (in situ XRD). Different preparation protocols were applied and controlled by in situ laser light scattering (in situ LLS). The composition of the deposited layers was adjusted by making use of a stoichiometric transitions in the LLS signal at the point where the Cu content equals the [Zn + Sn]. The ability of in situ XRD to distinguish between CZTSe and ZnSe is used to develop new processes that minimize the formation of ZnSe as a secondary phase. At high temperatures, an initially grown ZnSe layer forms at the Mo interface, which may not be incorporated into the CZTSe due to Zn-rich preparation conditions. By using lower temperatures at the beginning, CZTSe growth starts directly and a heating step restores the high temperature of the substrate. Thus, the formation of secondary phases is diminished without losing the benefits of higher preparation temperature. The ZnSe growth is reduced and the formation of a continuous layer is not observed.

First-principles study of p-type nitrogen/aluminum co-doped zinc oxide

First-principles calculations have been performed to investigate the electronic structures and the defects formation energies of the pure and nitrogen/aluminum (N-A1) co-doped zinc oxide. The calculated results of the pure ZnO using LDA+U method show band gap and lattice parameters are in reasonable agreement with previous data. Incorporation of N into ZnO host induces distortions of the lattice. According to the results, the Fermi-level of Al doped ZnO shift upward into conduction band and show n-type conductivity, while codoped structures show p-type conductivity. The defects formation energies of Al doped structure on Zn-rich and O-rich conditions are low. When codoped with N, the complexes exhibit high defects formation energy on O-rich condition, indicating it is difficult to realize p-type ZnO in this case. On Zn-rich condition, the codoped structures display lower defects formation energies and show p-type conductivity. From hole carriers analysis, it is shown that the carriers in N-Al doped ZnO possess a better transfer character. All these results indicate that codoped ZnO structures are beneficial for the realization of p-type ZnO.

Control of metal salt ratio and MoS2 layer thickness in a Cu2ZnSnS4 thin film solar cell

Cu2ZnSnS4 (CZTS) absorber layers were prepared from copper acetate, zinc acetate, tin chloride, and thiourea in a solution of methanol, ethylenediamine, and ethanolamine using a sol–gel spin-coating method. Sol–gel precursor solutions were prepared with different metal salt ratios, and the effects on film growth, optical properties, and crystal properties of CZTS films were investigated. In addition, the role of sulfurization temperature on grain size of CZTS and formation of the MoS2 layer at the CZTS and Mo interface were investigated. By changing the metal salt ratio in the precursor solution, a Zn-rich and Cu-poor condition in the CZTS film was obtained. By preparing Zn-rich and Cu-poor CZTS film with a thinner MoS2 layer at a lower sulfurization temperature, the CZTS cell efficiency was improved, and a conversion efficiency of 1.22 % was obtained.

Light induced phase change in Cu2−xZn1.3SnS4 thin films

Cu2ZnSnS4 and its alloy based thin film solar cells have shown better photovoltaic performance under Cu-poor and Zn-rich conditions. However, the effect of Cu-stoichiometry on the coexistence of kesterite (KS), stannite and/or partially disordered kesterite (PD-KS) phases and their influence on photovoltaic performance is not clearly understood. Raman studies were carried out on Cu2−xZn1.3SnS4 (x = 0, 0.3, and 0.5) thin films by changing the intensity of the incident laser beam. It was observed that both Cu-stoichiometry and incident laser beam intensity induce a disorder in the system. Disorder induced transformation of KS (I4¯) to PD-KS (I4¯2m) is explained by Raman studies.

In situXRD investigation of Cu2ZnSnSe4thin film growth by thermal co-evaporation

The deposition of a Cu2ZnSnSe4 (CZTSe) thin film with a multi-stage co-evaporation process is investigated with time-resolved in situ X-ray diffraction (in situ XRD). For the experiment a novel setup intended for in situ analysis of thin film deposition processes was used. The in situ data confirm the former observation that CZTSe growth is delayed with deposition of only Cu2−xSe and ZnSe in the initial process stage and provide new insight into the evolution of the appearing phases. In Zn-rich deposition conditions, ZnSe deposited at the beginning may not be consumed by the growing CZTSe but remain as an unreacted layer at the interface to the Mo back contact. Cu2−xSe growth starts with the formation of a Cu rich phase, which is reduced to a Cu poor phase in the process. Furthermore, our results show that in situ XRD at elevated temperatures is able to distinguish between ZnSe and CZTSe and that it can be used for the detection of ZnSe as secondary phase.