Laser Beam Induced Current Measurements Research Articles

Inductively Coupled Plasma-Induced Electrical Damage on HgCdTe Etched Surface at Cryogenic Temperatures

Plasma etching is a powerful technique for transferring high-resolution lithographic patterns into HgCdTe material with low etch-induced damage, and it is important for fabricating small-pixel-size HgCdTe infrared focal plane array (IRFPA) detectors. P- to n-type conversion is known to occur during plasma etching of vacancy-doped HgCdTe; however, it is usually unwanted and its removal requires extra steps. Etching at cryogenic temperatures can reduce the etch-induced type conversion depth in HgCdTe via the electrical damage mechanism. Laser beam-induced current (LBIC) is a nondestructive photoelectric characterization technique which can provide information regarding the vertical and lateral electrical field distribution, such as defects and p–n junctions. In this work, inductively coupled plasma (ICP) etching of HgCdTe was implemented at cryogenic temperatures. For an Ar/CH4 (30:1 in SCCM) plasma with ICP input power of 1000 W and RF-coupled DC bias of ∼ 25 V, a HgCdTe sample was dry-etched at 123 K for 5 min using ICP. The sample was then processed to remove a thin layer of the plasma-etched region while maintaining a ladder-like damaged layer by continuously controlling the wet chemical etching time. Combining the ladder etching method and LBIC measurement, the ICP etching-induced electrical damage depth was measured and estimated to be about 20 nm. The results indicate that ICP etching at cryogenic temperatures can significantly suppress plasma etching-induced electrical damage, which is beneficial for defining HgCdTe mesa arrays.

Compressive Current Response Mapping of Photovoltaic Devices Using MEMS Mirror Arrays

Understanding the performance and aging mechanisms in photovoltaic devices requires a spatial assessment of the device properties. The current dominant technique, electroluminescence, has the disadvantage that it assesses radiative recombination only. A complementary method, laser beam-induced current (LBIC), is too slow for high-throughput measurements. This paper presents the description, design, and proof of concept of a new measurement method to significantly accelerate LBIC measurements. The method allows mapping of the current response map of solar cells and modules at drastically reduced acquisition times. This acceleration is achieved by projecting a number of mathematically derived patterns on the sample by using a digital micromirror device (DMD). The spatially resolved signal is then recovered using compressed sensing techniques. The system has fewer moving parts and is demonstrated to require fewer overall measurements. Compared with conventional LBIC imaging using galvanic mirror arrangements or xy scanners, the use of a DMD allows a significantly faster and more repeatable illumination of the device under test. In this proof-of-concept instrument, sampling patterns are drawn from Walsh–Hadamard matrices, which are one of the many operators that can be used to realize this technique. This has the advantage of the signal-to-noise ratio of the measurement being significantly increased and thus allows elimination of the standard lock-in techniques for signal detection, reducing measurement costs, and increasing measurement speed further. This new method has the potential to substantially decrease the time taken for measurement, which demonstrates a dramatic improvement in the utility of LBIC instrumentation.

Photocatalytic activity of Cr-doped TiO2 nanoparticles deposited on porous multicrystalline silicon films

This work deals with the deposition of Cr-doped TiO2 thin films on porous silicon (PS) prepared from electrochemical anodization of multicrystalline (mc-Si) Si wafers. The effect of Cr doping on the properties of the TiO2-Cr/PS/Si samples has been investigated by means of X-ray diffraction (XRD), atomic force microcopy (AFM), photoluminescence, lifetime, and laser beam-induced current (LBIC) measurements. The photocatalytic activity is carried out on TiO2-Cr/PS/Si samples. It was found that the TiO2-Cr/PS/mc-Si type structure degrades an organic pollutant (amido black) under ultraviolet (UV) light. A noticeable degradation of the pollutant is obtained for a Cr doping of 2 at. %. This result is discussed in light of LBIC and photoluminescence measurements.

Scanning Laser-Beam-Induced Current Measurements of Lateral Transport Near-Junction Defects in Silicon Heterojunction Solar Cells

We report the results of scanning laser-beam-induced current (LBIC) measurements on silicon heterojunction solar cells that indicate the length scale over which photogenerated carriers are sensitive to local defects at the amorphous silicon/crystalline silicon heterojunction interface. The defects were intentionally created with focused ion beam irradiation, enabling us to study how defects at a predefined and known location affect carrier collection and transport in neighboring regions where the device remains pristine. The characteristic length scale over which carriers in the pristine areas of the device are vulnerable to loss via recombination in the adjacent defective region increases to over 50 μm as the device is forward biased. For photocarriers generated near the amorphous-crystalline interface, LBIC measurements suggest that lateral transport in the near-junction inversion layer in the c-Si is an important transport mechanism.

Laser beam induced current measurements of Cd1−xZnxS/CdTe solar cells

The role of the window layer in Cd1−xZnxS/CdTe photovoltaic (PV) device structures was investigated using a triple wavelength laser beam induced current (LBIC) technique. Full pn structures were deposited using metal organic chemical vapour deposition (MOCVD). The triple wavelength LBIC technique allowed for the diagnostic analysis of carrier collection uniformity at 3 wavelength dependent photon penetration depths, δp. An assessment of the Cd1−xZnxS window layer film thickness and surface coverage showed a correlation between lateral device performance and window layer thickness distribution. Islands of thick Cd1−xZnxS were shown to increase localised blue absorption, correlating to isolated regions of increased red and IR photoresponse. Overall device open-circuit voltage, Voc is shown to demonstrate a linear dependence on thick (∼300nm) Cd1−xZnxS surface coverage. Voc and shunt resistance Rsh losses attributed to the window layer were found to jointly contribute to the overall cell efficiency losses. A non-uniform pin-hole distribution, leading to Rsh loss, was found to be independent of the overall Cd1−xZnxS thickness distribution; thus the triple wavelength LBIC measurement was able to identify causes for the observed losses in both Voc and Rsh.

Scribing of Thin-Film Solar Cells with Picosecond and Femtosecond Lasers

Continuous growth of the thin-film solar technology market stimulates the development of versatile technologies for large-scale patterning of thin-film materials on rigid and flexible substrates, and laser technologies are a promising method to accomplish the scribing processes. In this study we compare picosecond and femtosecond pulse laser scribing for thin-film solar cells. For this we selected a Nd:YVO4 mode locked picosecond laser with the pulse duration of 10 ps and a Ti: sapphire laser with the pulse duration of 300 fs. We concentrated on so-called P3 step of laser scribing to expose the molybdenum back-contact. The visual quality of the scribes was controlled with optical and scanning electron microscopes. The conversion efficiency tests, LIT (lock-in thermography) and LBIC (laser-beam induced current) measurements were performed on the laser-scribed complete working solar cells of prefabrication stage. The damage-free exposure of molybdenum layer was possible in complex thin-film structure with both pulse durations. LIT and LBIC measurements did not show evidence of internal shunt formation near the scribed zone using lasers with both pulse durations. The efficiency tests confirmed an insignificantly higher solar cell performance after femtosecond laser scribing, although at both pulse durations marginal reduction in solar cell efficiency was observed.

Direct Estimation of the Electron Diffusion Length in Dye-Sensitized Solar Cells

The diffusion length is a key parameter that controls the electron collection efficiency in dye-sensitized solar cells (DSCs). In this work, we carry out a direct estimation of this parameter by means of the laser beam-induced current (LBIC) technique. The DSC devices are prepared on transparent conducting glass substrates, which were divided in two electrically isolated parts by means of a groove. The LBIC measurement is conducted by moving a highly focused laser spot over the DSC across the groove and monitoring the open-circuit voltage yielded by the solar cell. The resulting voltage profile can be fitted to a simple diffusion-recombination model such that the electron diffusion length can be extracted. Measurements carried out on DSC with various oxides (TiO2/ZnO) and electrolytes (organic, ionic-liquid) yield diffusion lengths in the 10–35 μm range, with longer values found for higher illumination and for cells of better efficiency.

Study of the crystal features of mc‐Si PV cells by laser beam induced current (LBIC)

AbstractIn this work we present LBIC measurements of a set of commercial multicrystalline silicon samples manufac‐tured by different suppliers. The LBIC measurements were obtained with a home‐made system, using several excitation wavelengths, equipped with an autofocus system, and controlling the reflected light. This system keeps constant the distance between the microscope objective and the sample, despite the bowing and the surface roughness characteristic of this type of samples. For the calculation of the Leff maps one needs to consider the distribution of the reflected light, which the maps usually show contrasts corresponding to the different grain orientations. The LBIC maps present networks of dark lines, corresponding to regions with high carrier capture rates. The dark line network does not necessarily match the grain boundaries (GB) revealed in the optical images, but many of them are intragarin defects. The trapping activity of the GB is known to depend on the GB angle. Usually, small angle GBs are decorated with defects that make them electrically active, while the large angles GBs are less decorated, and present a weak electrical activity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatially distributed model for the analysis of laser beam induced current (LBIC) measurements of thin film silicon solar modules

A 3D distributed model is developed and implemented based on circuit analysis software for the investigation of spatial variation in performance due to the distributed nature and non-uniformity of solar cell properties. This is applied to LBIC measurements where it is used for sensitivity analysis of the measurements with respect to certain parameters in series connected thin film PV modules. The model is used to explain the differences in dark and illuminated measurements, which clearly shows the illuminated LBIC signal is largely dependent on the homogeneity of the background illumination. The dark LBIC is largely affected by the shunt resistance of the neighbouring cells rather than by the signal strength of the cell under test. It is required to bring the cell into limiting conditions, which then gives a signal one order of magnitude stronger than that in the non-limiting case. The simulations are validated against measurements taken in these regimes.

Laser-beam-induced current mapping of spatial nonuniformities in molecular beam epitaxy As-grown HgCdTe

The formation of dislocations and corresponding built-in electric fields in molecular beam epitaxy (MBE)-grown HgCdTe can have a major impact on the performance and yield of photodetectors fabricated from this material. This paper investigates the presence of such built-in electric fields arising from dislocation segregation in MBE as-grown HgCdTe, and their subsequent removal via a low-temperature Hg-saturated anneal. The electrical properties and surface morphology of an HgCdTe layer grown on a thin CdTe buffer layer are compared with those of an HgCdTe layer grown directly on the CdZnTe substrate. Laser-beam-induced current (LBIC) imaging is a nondestructive technique capable of mapping built-in electric fields present in a semiconductor material, which, in the present case, has been used to reveal dislocation distributions present in as-grown, unintentionally doped, MBE-grown Hg0.71Cd0.29Te. Two-dimensional scanning LBIC measurements at 160 K allow spatial mapping of electric fields across the HgCdTe wafer. Subsequent isothermal annealing of the wafer in an Hg atmosphere has been found to decrease the magnitude of the built-in electric fields to below the LBIC detection limit. However, of particular note, is that before and after annealing, crosshatch patterns can be seen using Nomarski microscopy, with the crosshatching being predominantly in the [01\(\bar 1\)] direction and, to a lesser extent, in the [\(\bar 2\)31] and [\(\bar 2\)13] directions. Defect-decoration etching of the annealed wafer reveals dislocation banding parallel to the [01\(\bar 1\)] direction, which closely resembles the contrast observed in the LBIC image of the wafer before annealing. These Nomarski and LBIC images are compared with those of a second wafer, which incorporates a 40-nm CdTe buffer layer. The second wafer does not show significant Nomarski or LBIC contrast, indicating a flat, electrically uniform as-grown layer. Variable magnetic-field Hall measurements at 77 K and quantitative mobility-spectrum analysis (QMSA) indicate predominately p-type conduction with a doping density of 2×1015 cm−3 in the as-grown layer. After Hg annealing at 240°C, no LBIC signals are observed at 160 K, and Hall measurements at 77 K indicate the presence of two n-type carriers, with a combined doping density of 2×1015 cm−3. Double-crystal x-ray diffraction measurements show no evidence of twinned crystal volumes in the layers before or after annealing, or any change in the full-width at half-maximum (FWHM) (41 arcsec) of the (422) reflection. The similarity between the dislocation density distribution, as revealed by defect decoration, and the LBIC image suggests that Hg out-diffusion during growth is expedited in regions of high dislocation concentration, thus creating a nonuniform Hg vacancy-acceptor concentration. The as-grown acceptor concentration, in turn, modulates the hole concentration, creating p+/p− junctions and built-in electric fields in the material. Low-temperature annealing in a saturated-Hg atmosphere does not remove the crosshatch patterns or dislocation banding, but it fills the Hg vacancies, revealing the uniformly distributed n-type background, thus reducing the magnitude of any built-in electric fields. The LBIC mapping of MBE as-grown HgCdTe samples is, thus, capable of revealing defect distributions that would otherwise require a destructive technique, such as defect-decoration etching, to determine.

Low temperature saturation of p–n junction laser beam induced current signals

To date, most studies of the use of laser beam induced current (LBIC) for non-destructive characterisation of photodiodes have been qualitative and/or empirical, due in part to the difficulty of isolating the influence of the large number of material and device parameters on which the LBIC signal is dependent. The development of methodologies whereby these parameters can be determined quantitatively from the LBIC measurements is important for the technique to gain wider acceptance. This work describes, for the first time, the specific experimental conditions under which some of the variable parameters can be eliminated, substantially reducing the complexity of the analysis. In particular, temperature dependence of the peak-to-peak LBIC measurements on p–n junctions is examined for the first time, revealing that a saturation state is reached at low temperatures. When measurements are performed under these saturation conditions, the peak-to-peak LBIC signal becomes independent of doping density and bulk recombination parameters, allowing other parameters including device geometry to be examined with less ambiguity. This concept represents a crucial step towards a quantitative procedure for extraction of p–n junction material and device parameters using LBIC.

Two-dimensional resolution of minority carrier diffusion constants in different silicon materials

Hall measurements are a common method to determine the majority charge carrier diffusion constant. But the diffusion constant of the minority carriers D n , the more interesting parameter in photovoltaics, is rather hard to detect. In this paper we introduce a method to determine D n locally resolved and mapped in two dimensions. For that purpose the local diffusion length L diff, which can be calculated from LBIC (laser beam induced current) measurements, has been combined with the local bulk lifetime τ b received by μ-PCD (microwave-detected photo conductance decay) measurements. We evaluated the diffusion constants of the minority charge carriers D n for different p-type silicon materials with a resolution of 100 μm. The measurements were carried out on solar cells before and after remote plasma hydrogen passivation in order to get an impression of the diffusion constant dependency on hydrogen incorporation.

Laser beam induced current as a tool for HgCdTe photodiode characterisation

A non-destructive optical characterisation technique is used for the investigation of Mercury Cadmium Telluride (HgCdTe) photovoltaic devices. The technique uses a scanning laser microscope to obtain Laser Beam Induced Current (LBIC) data from which it may be possible to extract information such as junction depth, array uniformity, and other material and device parameters. LBIC has been previously used only as a qualitative technique, but in this work the procedure is being developed into a quantitative tool. At present the only junction depth profiling techniques are destructive, while array uniformity can only be examined after bonding to readout circuits. In this paper we present both theoretical and experimental results which show that LBIC can be employed as a quantitative tool for device characterisation. The primary measure of performance of IR detectors is the zero bias dynamic resistance junction area product, R 0 A. LBIC measurements indicate that the peak LBIC signal varies by a factor of ≃2 for long wavelength infrared photodiodes for which the R 0 A varies between 70 Ω cm 2 (acceptable for operation and the lower bound of typical values) and 8 Ω cm 2 (unacceptable and typical for poor quality diodes).

Reactive ion etching for mesa structuring in HgCdTe

Both wet chemical and dry plasma etching techniques have been investigated for mesa structuring in n- and p-type HgCdTe. Scanning electron microscopy (SEM) confirms the isotropic nature of a bromine-based wet chemical etching solution, and the anisotropic profile that results from reactive ion etching. Laser-beam-induced-current (LBIC) measurements reveal no significant modifications to the electrical properties for chemically etched HgCdTe material, but clearly demonstrate a p- to n-type conversion in p-type samples and n+ doping in n-type samples for reactive ion etching (RIE) (processing conditions: 400 mTorr, CH4/5H2, 0.4 W/cm2). LBIC measurements following low-temperature (200 °C) mercury annealing of RIE-processed samples indicate the full restoration of electrical properties to that of the initial as-grown wafers, thus preserving the beneficial aspects of RIE for anisotropic mesa structuring in HgCdTe.

Mercury annealing of reactive ion etching induced p- to n-type conversion in extrinsically doped p-type HgCdTe

Mercury annealing of reactive ion etching (RIE) induced p- to n-type conversion in extrinsically doped p-type epitaxial layers of HgCdTe (x=0.31) has been used to reconvert n-type regions created during RIE processing. For the RIE processing conditions used (400 mT, CH4/H2, 90 W), p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200 °C for 17 h, LBIC measurements clearly indicated that no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a uniform p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA−ND=2×1016 cm−3, μ=350 cm2 V−1 s−1).

Reactive Ion Etching (RIE) Induced p- to n-Type Conversion in Extrinsically Doped p-Type HgCdTe

AbstractMercury annealing of reactive ion etching (RIE) induced p- to n-type conversion in extrinsically doped p-type epitaxial layers of HgCdTe (x=0.3 1) has been used to reconvert n-type conversion sustained during RIE processing. For the RIE processing conditions used (400mT, CH4/H2, 90 W) p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200°C for 17 hours, LBIC measurements clearly indicated no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA-ND=2×1016 cm−3,.=350 cm2.V−1. S−1).

Reactive Ion Etching (RIE) Induced p- to n-Type Conversion in Extrinsically Doped P-Type hgcdte

AbstractMercury annealing of reactive ion etching (RIE) induced p- to n-type conversion in extrinsically doped p-type epitaxial layers of HgCdTe (x=0.31) has been used to reconvert n-type conversion sustained during RIE processing. For the RIE processing conditions used (400mT, CH4/H2, 90 W) p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200°C for 17 hours, LBIC measurements clearly indicated no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA-ND=2× 1016 cm−3, μ=350 cm2.V−1.−1).

Spectroscopic Laser Scanning Analysis of Photo-Induced Current on a-Si Solar Cells

Spectroscopic analysis of the laser beam induced current (LBIC) has been carried out as a powerful tool for characterizations of spacial homogeneity and surface and junction interfaces in amorphous silicon (a-Si) solar cells. We have found the photocurrent inhomogeneity of a-Si solar cells and analyzed the origins of the fluctuation using the spectroscopic LBIC measurement. There are three kinds of fluctuation patterns, i.e., spikes based on the dust and scratch of the film, aligned sinusoidal fluctuation with short period upon the glass substrate thickness inhomogeneity, and relatively long period oscillation based on the a-Si junction interface situation. It has also been revealed that a spectroscopic technique varing the wavelength of probe light makes it possible to know the in-depth profile of opto-electronic properties as well as optical properties of semiconductor devices.