Laser Beam Induced Current Research Articles

Spatially resolved photocurrent measurements of microstructured a-Si:H solar cells

The variation of electronic properties in an amorphous (a)-Si:H solar cell film deposited on a textured substrate is measured with sub-micrometer resolution (<500 nm). The experiments are performed using a high resolution laser beam induced current (LBIC) set-up where in addition to the photocurrent the light reflection is monitored. Fluctuations in photocurrent are correlated with surface structures of the cell. These variations are mainly caused by variations in the light intensity due to the lateral structure and lateral variation of the local electric field in the pin-junction. The results are discussed including resolution limits of the apparatus and atomic force image of the solar cell.

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.

Junction depth measurement in HgCdTe using laser beam induced current (LBIC)

A new, nondestructive junction depth measurement technique for HgCdTe photovoltaic devices is investigated. The technique uses a scanning laser microscope to obtain laser beam induced current (LBIC) data from which information regarding the junction depth is extracted, and is applicable to both homojunction and heterojunction diodes. For implanted heterojunction photodiodes, the position of the n-p junction relative to the heterojunction is an important factor determining completed device performance, with blind photodiodes resulting if the n-p junction is incorrectly placed. At present, the only methods available for junction depth determination (e.g., secondary ion mass spectroscopy and differential Hall) are destructive and not applicable as routine process monitoring techniques. It is envisaged that the development of a nondestructive routine process monitoring procedure will help improve yield and reduce the cost of HgCdTe photovoltaic devices. In this paper, experimental and theoretical results are presented in order to assess the sensitivity of the new technique to the effects of junction doping density, illumination wavelength, frontside/backside illumination, and test structure geometry.

Laser Beam Induced Current Characterization of High Efficiency Chalcopyrite Solar Cells

Laser Beam Induced Current (LBIC) techniques has been employed to investigate hidden microdefects and longitudinal non-uniformities over the illuminated surface of CIS and CIGS thin-film solar cells. By the laser scan the obtained two-dimensional and thre

Characterization of novel mono- and bifacially active semi-transparent crystalline silicon solar cells

This paper presents the latest cell results for semi-transparent mono- as well as bifacially active POWER (Polycrystalline Wafer Engineering Result) solar cells of different cell sizes on Cz and multicrystalline silicon substrates. Top efficiencies of 10.4% for monofacial and 12.9% for bifacial cells are reported. Attention has been paid to apply a fully industrially compatible production process. It uses dicing saw based mechanical texturization of the front and rear side of the silicon wafer and screen printing metallization. In the POWER solar cell concept, perpendicular grooves on the front and rear side create holes with a variable diameter at their crossing points. This results in a partial optical transparency of the solar cell. In this study, holes of 200 /spl mu/m diameter lead to a transparency of 16-18% on average for the total cell area. The cell characteristics for the different cell types are compared by means of illuminated and dark current-voltage (I-V), spectral response, and Laser Beam Induced Current (LBIC) measurements. While bifacial POWER cells need a more elaborate production process, they reveal better I-V characteristics and a higher efficiency as compared to monofacial cells. This is mainly explained by a better surface passivation due to an active emitter and a passivating silicon nitride ARC both on the front and rear surface.

Laser beam induced current imaging of reactive ion etching induced n-type doping in HgCdTe

The nondestructive optical characterization technique of laser beam induced current (LBIC) has been used to illustrate the effects of reactive ion etching (RIE) of mid-wavelength infrared n-type HgCdTe. RIE may be used as a method of n-p junction formation, as a means of forming n+ ohmic contacts to wider bandgap HgCdTe, or for mesa isolation etching of epilayers for HgCdTe detectors and emitters. Along with experimental measurements of the LBIC phenomena, this paper introduces the simulation of LBIC signals using a commercial semiconductor device modeling package. A number of LBIC maps are presented for different wafer processing conditions, with the results being explained using the simulation software. The experimental and calculated results bring to light a number of previously unreported characteristics associated with the LBIC phenomena, including the effect of junction depth, temperature, and grading of the junction region. In addition to the LBIC technique confirming the presence of an n+ region after RIE processing, it also provides information regarding the depth of the n+ region and lateral extent of the doping.

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).

Characterisation of reactive-ion-etching-induced type-conversion in p-type HgCdTe using scanning laser microscopy

In this work we characterise the n-type converted region occurring in both vacancy and extrinsically doped p-type Hg 1− x Cd x Te ( x ≈ 0.3) after standard reactive-ion-etch (RIE) process. The laser beam induced current (LBIC) technique is used to characterise parameters such as lateral and vertical conversion depth. Furthermore, by fitting a theoretically determined LBIC signature to the measured LBIC over the temperature range 80–300 K, it is possible to estimate the donor level density of the n-type converted region.

Non-uniformity of Hg diffusion in p-type HgCdTe

Hg atoms have been found to diffuse non-uniformly into p-type HgCdTe epilayers, in which the acceptor is an Hg vacancy. Laser beam induced current (LBIC) images show that fast diffusion paths exist in the samples annealed under Hg at critical vapor pressure. Conversely, slow diffusion paths exist in the samples which were ion-implanted and then annealed in a nitrogen atmosphere. The density of both paths is 1−5 × 10 5 cm −2. Dislocation densities increase in the Hg diffused layers, but their positions seldom coincide with the LBIC images, indicating that dislocations are not the origin of abnormal diffusion paths. It was found that localized stresses significantly influence Hg diffusion. Ion implantation changes the stress in the surface layers from compressive to tensile stress. This change might lead to opposite types of paths.

Estimation of doping density in HgCdTe p-n junctions using scanning laser microscopy

Quantitative assessment of p- to n- type conversion due to reactive ion etching (RIE) of p-type Hg0.71Cd0.29Te is presented using laser-beam-induced-current (LBIC) measurements. For the RIE processing conditions used (390 mT, CH4/H2, 0.4 W/cm2), n-type conversion was observed in extrinsic arsenic-doped p-type Hg0.71Cd0.29Te which had previously undergone a Hg anneal to eliminate Hg vacancies. Effective doping density of the n-type converted region is determined by fitting a theoretically determined LBIC signature to the measured LBIC signal over a temperature range 80–300 K. Effective n-type doping density is the only fitting parameter used in the simulation, which was carried out using a commercial semiconductor device modeling package (SEMICAD™ DEVICE). This noncontact experimental technique promises to be a useful tool in the characterization of p-n junction diodes in HgCdTe, and for studying the precise nature of p to n conversion in p-type HgCdTe.

Electrical fluctuations in HgCdTe introduced during quenching after annealing

HgCdTe epilayers used in infrared detectors have to be quenched after annealing in order to control the p-type carrier concentrations. It was found that μm-scale electrical fluctuations are introduced in the epilayer and around threading dislocation clusters during the quenching. Differential Hall measurements revealed an n-type region in the first 1 μm of the epilayer, followed by the p +-type (10 17 cm −3) region which gradually decreases with depth to 10 16 cm −3 in the next 5 μm. The fluctuating layer thickness decreased when the cooling rates increased. LBIC (laser-beam-induced current) images showed that p +-type electrical fluctuations and their dependence on the cooling rates are also observed around the threading dislocation clusters. The fluctuations are more than five times deeper than the former surface fluctuations. Simple estimations for the cooling process of the epilayers can qualitatively explain the experimental results.

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).

A discrete element model of laser beam induced current (LBIC) due to the lateral photovoltaic effect in open-circuit HgCdTe photodiodes

The non-destructive optical characterization technique of Laser-Beam-Induced-Current (LBIC) imaging has proven useful in qualitatively assessing electrically active defects and localized non-uniformities in HgCdTe materials and devices used for infrared photovoltaic arrays. To further the development of a quantitative working model for LBIC, this paper focuses on the application of the technique to photovoltaic structures that are represented by a discrete element equivalent circuit. For this particular case the LBIC signal arises due to the lateral photovoltaic effect in non-uniformly illuminated open-circuit photodiodes. The outcomes of the model predict all of the experimentally observed geometrical features of the LBIC image and signal. Furthermore, the model indicates that the LBIC signal has an extremely weak dependence on the p-n junction reverse saturation current, and shows a linear dependence with laser power. This latter feature map be useful for non-contact measurement of the quantum efficiency of individual photodiodes within a large two-dimensional focal plane array. The decay of the LBIC signal outside the physical boundary of the p-n junction is of the same form as the roll-off in the short circuit photoresponse and, therefore, can be used to extract the diffusion length of minority carriers. Experimental data is obtained from an arsenic implanted p-on-n junction fabricated on MBE grown Hg/sub 1-x/Cd/sub x/Te material with an x-value of 0.3. The p-on-n diode is shown to be uniform and of high quality with an R/sub 0/A product of 1/spl times/10/sup 8/ /spl Omega//spl middot/cm/sup 2/ at 77 K. The validity of the simple model developed in this paper, is confirmed by the excellent agreement with experimental results. Consequently, the LBIC technique is shown to be an appropriate diagnostic tool for non-contact quantitative analysis of semiconductor materials and devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reconstruction of Semiconductor Doping Profile from Laser-Beam-Induced Current Image

In this paper, the authors study the reconstruction of a semiconductor doping profile or, equivalently, the equilibrium potential, from its LBIC (laser-beam-induced current) image. For the one-dimensional case, the authors first characterize the attainable class of current measurements, and from this they show the nonuniqueness of the inverse problem. Then the reconstruction of the equilibrium potential is reduced to finding two constants subject to some constraints. A reconstruction algorithm is established based on a least squares formulation of the problem. The case of noise-collapsed data is also discussed. For a special case of two-dimensional domain, the authors apply the one-dimensional algorithm supplemented with a correction from the other spatial direction to establish an alternate direction iteration algorithm for reconstruction of the two-dimensional equilibrium potential. The authors also present some numerical examples to illustrate the reconstruction results by these algorithms.

Spatially resolved characterization of HgCdTe materials and devices by scanning laser microscopy

Spatially resolved characterization of HgCdTe materials and p-n junction diodes using scanning laser microscopy is reviewed. Several techniques that yield spatial maps of electrical inhomogeneities in HgCdTe material and non-uniformities in various performance parameters of p-n junctions fabricated using these materials have been developed. Many of the techniques are non-destructive, or can be made such with minor changes in sample preparation, and are scalable to large full wafers. A high-resolution and non-destructive technique called 'laser beam induced current (LBIC)' has been developed for spatial mapping of electrically active regions in HgCdTe. When applied to unprocessed HgCdTe material, LBIC images represent spatial distributions of electrically active defects including inclusions, strain, damage, precipitates, stacking faults, twin boundaries, dislocation clusters, bandgap and doping variations. Device structures such as p-n junctions are special cases of electrically active regions, therefore the LBIC technique lends itself to a non-destructive study of p-n junction arrays without requiring any direct electrical contact to the individual elements of the array. The remote contacting, especially using pressure contacts, makes the application of the LBIC technique non-destructive, allowing testing at various stages of device processing to identify particular processing procedures that need optimization. LBIC has also been used to spatially map electrical non-uniformities at the HgCdTe surface near its interface with an insulating passivation layer. Besides LBIC imaging, scanning laser microscopy has been used for several other applications. For HgCdTe p-n junctions, applications include photoresponse mapping (uniformity, active area and diffusion length determination, contact bonding effects), spatially resolved light-induced degradation, and avalanche photodiode properties (ionization coefficients, localized breakdown). A key technique currently in development is the non-contact diode R0A determination based on trapping mode photoconductivity. Examples of scanning laser microscopy on HgCdTe materials are infrared transmission mapping (thickness and bandgap variations), photoluminescence mapping, and minority carrier lifetime mapping (distribution of recombination centres).

Investigation of the crystalline and electrical qualities of EPR photovoltaic silicon flat plates

The EPR polysilicon ribbons are obtained from 3.5–5N purity grade silicon powder melting. Being assigned to the photocell manufacture we study the texture by X-ray diffraction, ECP (Electron Channeling Pattern) and TEM (Transmission Electron Microscopy) to reveal the crystal orientations of the grains and prove the possible existence of specific orientations adapted to the best photovoltaic conversion efficiency such as on (331), (110), (111). Moreover, we listed the possibility to reduce the (111) orientation with a monocrystalline seed. It appears that the crystal growth is essentially anisotropic and that only the orientation of the grain and then of (331) planes parallel to the ribbon surface may be considered as dominant at last. The crystalline seed is acting only at the crystalline start. The electrical characterizations were executed essentially by EBIC (Electron Beam Induced Current) and LBIC (Laser Beam Induced Current) methods. Two interface types were observed: one parallel and constitued by ∑3 and ∑9 twins, show low electrical activity, the other curved with no defined index and higher electrical activity.

Identifiability of Semiconductor Defects from LBIC Images

The identifiability of defects in a semiconductor from its laser-beam-induced current (LBIC) image is studied. It is shown that the LBIC technique is reliable for detecting any spatial defects in the semiconductor material. Continuous dependence of the current measurements on the spatial defects is proved, and sensitivity is also discussed in certain cases.

Remote contact LBIC imaging of defects in semiconductors

A high resolution and nondestructive optical technique, called “laser-beam-induced current (LBIC)”, has been developed for spatial imaging of electrically active defects in liquid phase epitaxial (LPE) HgCdTe alloy semiconductor. The technique consists of mapping the induced current between two remote contacts on the sample as a function of the incident-focused laser beam position. The induced current is a result of the charge separating effect of built-in fields in the vicinity of defects in semiconductors. The low laser power does not damage the sample, and the resolution of the technique is limited by the diffusion length of the carriers in the material. Also, since device structures such as p-n junctions are special cases of electrically active regions, LBIC imaging has been utilized to study the opto-electronic properties of these structures in a nondestructive manner, without requiring any electrical contact to the active elements. In addition, LBIC has been utilized to obtain electrical nonuniformities at the HgCdTe semiconductor surface near its interface with a ZnS passivation layer.

Simple modeling techniques for analysis of laser beam induced current images

Spatially resolved laser beam induced current measurements yield large volumes of data. We present here a data reduction technique based on simple emulation of the laser beam induced current (LBIC) response expected from a distribution of charge in a medium. A model for the current expected when scanning over a buried charged sheet allows some LBIC data to be reduced to a geometric model for a cracked sample. The results obtained are sufficiently encouraging to indicate that such modeling methods can reduce the often complex LBIC images obtained from HgCdTe material to a digest of geometric and electrical properties.