Concentration Of Deep Defects Research Articles

Sodium in Cu(In, Ga)Se2 Solar Cells: To Be or Not to Be Beneficial

In this work, the dual effect of Na in the copper indium gallium diselenide (CIGS) absorber is studied. Na doping exhibits a drastic improvement of the cell efficiency mainly by an increase in the open‐circuit current and fill factor. However, when the Na content is too high, it can deteriorate the cell properties and performance. Nonetheless, it is still beneficial when compared with a cell without Na inside. Here, we therefore use electron backscatter diffraction, atom probe tomography, and admittance spectroscopy to understand why Na starts to deteriorate the properties when inserted excessively. More exactly, it is shown that a moderate addition of Na strongly decreases the open‐circuit voltage (VOC) deficit, whereas the opposite happens when the Na content is too high. Interestingly, this cell with deteriorated VOC value and characterized by a very high Na content exhibits a high density of not only grain boundaries and dislocations but also of Na‐rich clusters. These clusters may host a high concentration of deep defects and trap the electrons within the CIGS bulk. At this stage, Na still has its beneficial effects, but the cell properties start to deteriorate compared to the cell with a moderate content of Na, especially when found as clusters in the bulk. In conclusion, this work clearly demonstrates the dual effect of Na in the CIGS absorber.

Photoluminescence of bulk α -In2Se3 crystals irradiated by high-energy electrons

The photoluminescence (PL) spectra of bulk α-In2Se3 crystals before and after 10 MeV electrons irradiation with the 1015 and 1017 cm−2 fluences were studied in the temperature range from 7 to 340 K. Three main types of radiative recombinations corresponding to band-to-tail (BT), deep defects, and band-to-band (BB) recombination were manifested in the non-irradiated α-In2Se3 crystals. Also recombinations that can be associated with exciton recombinations at temperatures below 45 K are observed. After electron irradiation, noticeable changes in the PL spectra are observed. We detected a slight increase in activation energy of the BT recombination. An increase in the concentration of deep defects is also noted. The significant decrease in PL intensity of the BB recombinations indicates the formation of non-radiative recombination centers after electron irradiation with the 1017 cm−2 fluence. Our study may be useful for understanding the effects of high-energy electrons irradiation on the performance of electronic and photovoltaic devices based on α-In2Se3.

Capacitance characterization of GaP/Si superlattice grown by time-modulated PECVD

The work is devoted to study of GaP/Si superlattice grown on n-type GaP wafer by time-modulated plasma-enhanced chemical vapor deposition. Deep-level transient spectroscopy showed appearance of defects in μc-GaP layer. Addition of silicon layers 0.5 nm thick leads to decreasing of deep defect concentration. However, further increasing of silicon content in superlattice leads to appearance of new deep defects in structure.

Electrophoretic deposition of ZnO nanostructures: Au nanoclusters on Si substrates induce self-assembled nanowire growth

Fil: Sandoval, Claudia. Universidad Nacional de Tucuman. Facultad de Ciencias Exactas y Tecnologia. Departamento de Fisica. Departamento de Nanomateriales y Propiedades Dielectricas; Argentina

Influence of Ga-notch position on recombination processes in Cu(In,Ga)Se2-based solar cells investigated by means of photoluminescence

One of the consequences of the deposition of Cu(In,Ga)Se2 (CIGSe) absorber by a three stage process is a non-uniform Ga distribution. It takes the form of the so-called Ga-notch and is considered to be crucial for achieving highly efficient CIGSe solar cells. However, the influence of this sequential element co-evaporation on defect related properties of the absorber is not fully understood. In this paper, we use voltage dependent photoluminescence (PLV) to investigate the impact of a different Ga-notch position on recombination processes in CIGSe-based solar cells. The most striking difference between investigated samples is the increased sensitivity of the photoluminescence signal to blue light, as the position of the Ga-notch moves away from the CdS/CIGSe interface. Such metastable behavior of PLV characteristics and its close correlation with changes observed in capacitance–voltage curves suggest an increased concentration of deep defects in the top CIGSe layer. We propose that the observed changes of PLV characteristics can be explained by electrical field redistribution within the junction due to charging of deep metastable defects.

Influence of iron on defect concentrations and device performance for Cu(In,Ga)Se2 solar cells on stainless steel substrates

ABSTRACTThin film Cu(In,Ga)Se2 (CIGSe) solar cells deposited on flexible steel foil substrates by coevaporation are investigated. Iron diffusion from the steel foil substrate material into the CIGSe absorber layers is studied by secondary ion mass spectroscopy (SIMS) for different maximum absorber growth temperatures and solar cell back‐contact configurations. The optimization of the absorber growth temperature, the introduction of a diffusion barrier layer at the back contact, and the adjustment of the sodium doping lead to device efficiencies above 14%. Defect spectroscopy by means of admittance measurements shows a clear correlation of the deep defect concentration with the Fe content in the CIGSe absorber layer and a correlation between the sodium incorporation and the net carrier concentration. It is found that too much Na supports the diffusion of Fe, inducing an increased deep defect concentration. The Fe‐induced deep defects, which form a broad acceptor‐type defect band at 0.44 eV above the valence band maximum, are identified as the cause of bulk recombination, limiting the device performance in these solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

Characterization of defects in undoped non c-plane and high resistance GaN layers dominated by stacking faults

Deep defects in undoped c- and a-planes GaN layers grown by MOVPE on sapphire substrates were investigated by photoluminescence (PL), photocurrent (PC) and thermally stimulated current spectroscopy (TSC). In a-plane GaN the PL spectra are dominated by basal stacking faults and donor–acceptor pair recombinations correlated signals, whereas the near band-edge emission is the dominating in c-plane GaN layers. In TSC, no evidence is found for a strong trap emission of a-plane GaN correlated with the basal stacking fault-related PL signal. But, the high concentration of stacking faults in a-plane GaN layers induces a high concentration of deep defects with activation energies of 220 and 320 meV causing the DAP transition in Pl, the broad peak B in PC and TSC trap emissions in the temperature range T2. The trap at 220 meV is well known from c-plane GaN and related to dislocations, which support the assumption that this deep trap is introduced by the stacking faults in a-plane GaN.

Effects of deep defect concentration on junction space charge capacitance measurements

Expressions for the small signal capacitance and pulse bias-induced large signal capacitance transient of an abrupt n+-p junction in the presence of one or more deep traps of arbitrary concentrations are presented. The expressions show that a simple inverse relationship between the measured small signal capacitance and the junction’s space charge depletion width is not expected. Unexpected effects on defect capture cross section and defect concentration determination, when applied to deep level transient spectroscopy simulations, are also reported. Experimental results of both temperature dependent capacitance and transient capacitance measurements performed on electron irradiated Si n+-p junctions are presented and analyzed in the context of these expressions. Modeling of the measured signals using these expressions is shown to be in good agreement with the experiment.

Influence of deep level defects on electrical compensation in semi-insulating InP materials

In this paper, we analyze and compare electrical compensation and deep level defects in semi-insulating (SI) materials prepared by Fe-doping and high temperature annealing of undoped InP. Influence of deep level defects in the SI-InP materials on the electrical compensation has been studied thermally stimulated current spectroscopy (TSC). Electrical property of the Fe-doped SI-InP is deteriorated due to involvement of a high concentration of deep level defects in the compensation. In contrast, the concentration of deep defects is very low in high temperature annealed undoped SI-InP in which Fe acceptors formed by diffusion act as the only compensation centre to pin the Fermi level, resulting in excellent electrical performance. A more comprehensive electrical compensation model of SI-InP has been given based on the research results.

Influence of the Ga-content on the bulk defect densities of Cu(In,Ga)Se 2

We fabricate a series of CIGS absorber layers with Ga-contents x=Ga/(In+Ga) ranging from x=0 (CuInSe 2) to 1 (CuGaSe 2) by single layer coevaporation. The open circuit voltages V oc of the completed solar cells increase with increasing Ga-content but not proportional to the change of the band gap energy E g of the CIGS-layers. In contrast to the behaviour at Ga-contents exceeding x≈0.3 the difference E g−q V oc decreases from x=0 to x≈0.3, thus, having a minimum at x≈0.3. We determine defect concentrations in the absorber of these cells by admittance spectroscopy. These bulk defects also have a minimum concentration at x≈0.3. This low concentration of deep defects reduces recombination losses and thus, the difference of E g−q V oc. In addition, we fabricated absorber layers with a three stage process and an average Ga content x of approximately 0.3. The resulting solar cells have a lower defect concentration and a higher open circuit voltage than solar cells from single stage processes with x≈0.3. We find a correlation between volume defect concentrations and the difference E g−q V oc suggesting that volume defects determine the open circuit voltage of CIGS solar cells in the whole composition range from CuInSe 2 to CuGaSe 2 and when using different absorber processes.

Methods for determining deep defect concentration from dependence ofexcess carrier density and lifetime on illumination intensity

Two methods are proposed for determining the deep defect concentration from dependences of excess majority carrierconcentration and carrier lifetime on, respectively, illumination intensity and injection level.The methods are based on saturation of the excess majority carrier density with increasingillumination intensity and on an abrupt decrease in the lifetimeof majority carriers with their increasing excess concentration,which takes place as a result of filling of the defect level by minority carriers.In contrast to the well known injection-levelspectroscopy, both the methods make it possible to determine thedensity of a defect without knowing any of its parameters, such asenergy level or recombination coefficient of electrons and holes.These methods are applied to boron-doped single-crystal siliconwith radiation-induced deep defects of the phosphorus-vacancy,oxygen-vacancy and carbon-oxygen complex types.It is shown that with these methods it is possible to determine the density ofonly those deep defects which control free carrier density andlifetime and give rise to a significant difference between theexcess concentrations and lifetimes of electrons and holes.The analysis is based on the assumption that(i) the density of deep defects is independent of the illumination intensity,(ii) processes of generation-recombination via deep defects aredescribed within the Shockley-Read-Hall recombination theory and(iii) recombination via other defects, band-to-band recombination, Augerrecombination, etc, are negligible.

Mechanisms for the anomalous dependence of carrier lifetime on injection level and photoconductance on light intensity

Recent experiments indicated an anomalous dependence of carrier lifetime on injection level and of photoconductance on carrier photogeneration rate. In this work mechanisms for these phenomena are proposed which are based on the effects of anomalous increase of carrier lifetime and filling of the defect level by minority carriers, respectively. Distinct from previously known mechanisms, which considered two types of deep defects, traps and recombination centers, our mechanism may take place in semiconductors containing only one type of deep defect, which is a recombination center. It was shown that the anomalous injection-level dependence of lifetime occurs only when the semiconductor is exactly compensated by recombination centers. This conclusion differs from that of the previous model, which concluded that carrier trapping is responsible for the earlier phenomenon. It is shown that the injection-level dependence of carrier lifetime can be used as an experimental tool to determine the deep defect concentration.

Reduction of deep defect concentration in chlorine-doped ZnSe by after-growth thermal treatment

Deep-level transient spectroscopy was applied for investigating the effect of after-growth annealing on the concentration of deep defects in Cl-doped ZnSe epilayers. The samples were grown by molecular beam epitaxy on (100)-GaAs substrates, employing ZnCl2 as the dopant source. The ZnSe:Cl epilayers were annealed in Zn-rich atmosphere at temperatures 400–650 °C for 18–60 h, respectively. As a consequence of the thermal treatment, the total concentration of deep defects in the material was significantly reduced. In particular, annealing almost completely eliminated the dominant defects in the as-grown material (located 0.51 eV below the edge of the conduction band), which strongly supports the identification of the origin of these defects as zinc-vacancy complexes. We also note that the annealing process introduces a small concentration of new defects. The activation energies for electron emission and capture of these traps are 0.24 and 0.17 eV, respectively, indicating that this level lies at 0.07 eV below the conduction band.

Investigations on deep traps in GaAs and (AlxGa1−x)As bulk layers grown by metalorganic vapour-phase epitaxy using the new alternative arsenic precursor diethyl-tert-butylarsin

Metalorganic vapour-phase epitaxy (MOVPE) growth experiments for GaAs and (AlGa)As have been performed as a function of growth temperature and VIII ratio using the new alternative precursor diethyl-tert-butylarsin (DEtBAs) in combination with trimethylgallium (TMGa) and trimethylaluminium (TMAl). The incorporation of deep traps has been investigated by means of photoluminescence (PL) and deep level transient Fourier spectroscopy (DLTFS). In (AlGa)As layers grown with DEtBAs a drastic reduction of the deep broad band luminescence between 1.5 and 2 eV is observed in dependence of the growth conditions. A probably deep complex defect involving an As vacancy is observed in (AlGa)As layers grown with DEtBAs. In GaAs layers grown with DEtBAs or AsH3 the dominant deep trap is the EL2 defect. Deep level photoluminescence studies show a reduced incorporation of the EL2 defect as compared to AsH3 grown layers, due to the smaller VIII ratio. The obtained low deep defect concentration in particular for GaAs layers underlines the great potential of the model precursor DEtBAs as substitute for the highly toxic AsH3 in MOVPE.

Interfacial defect creation by interstitial lithium doping of amorphous hydrogenated silicon

We study interstitial doping in amorphous hydrogenated silicon (a-Si:H) by indiffusion of lithium at 230 °C using secondary ion mass spectrometry, photothermal deflection spectroscopy (PDS), and electron spin resonance. Lithium is distributed nonuniformly in the films with peak concentrations within 300 nm of either interface. Lithium doping introduces up to 1018 paramagnetic defects per cc in a-Si:H (g=2.0061, ΔHp.p. =5.2 G), equal to the concentration of deep defects created as measured by PDS. The nature of these defects is discussed.