Ambipolar Diffusion Length Research Articles

Photomagnetoelectric effect of high resistivity CdTe

Photomagnetoelectric (PME) effect of high resistivity CdTe and CdZnTe for nuclear detector applications is measured. Ambipolar diffusion lengths of 9 μm and 25 μm for CdTe and CdZnTe respectively are obtained at room temperature. From the temperature dependence of PME short circuit current measurements, a peak at T = 240 K is found for THM CdTe when the excitation light is chopped at the frequency of 135 Hz. The behaviour is explained by the reemission of trapped carrier at the recombination centre by optical excitation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study of the photovoltaic effect in thin variable-gap semiconductor layers

The report presents the results of a theoretical investigation of the photovoltaic effect in thin variable-gap semiconductor layers whose thickness is comparable with the ambipolar diffusion length. The cases of layers of extrinsic and intrinsic types of conductivity with a linear coordinate dependence of band-gap are analyzed in detail. The layers are assumed to be subjected to illumination by highly absorbed light from the wide-gap surface. It is shown that, due to the presence in the photovoltage of both the field and Dember components, sign inversion points may occur on the spectral dependence of the photovoltaic effect. The possibilities of practical use of the specific features of the photovoltaic effect in thin variable-gap layers on the basis of CdHgTe solid solution are discussed.

Deposition of HWCVD poly-Si films at a high growth rate

The process parameters for high growth rate poly-silicon films by hot-wire chemical vapour deposition have been explored. A four-wire hot wire assembly has been employed for this purpose. High silane to hydrogen flow ratios and high wire temperatures are the key process parameters to achieve high growth rate and growth rates higher than 5 nm/s can be achieved. The process conditions to incorporate high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures poly-Si films deposited at 1.3 nm/s showed high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as i-layer, n–i–p solar cell on stainless steel substrate without back reflector showed an efficiency of 4.4%.

Defects and transport properties of electron-irradiated microcrystalline silicon with successive annealing

We present a systematic study of isochronal annealing of electron-irradiated microcrystalline silicon thin films using a set of complementary characterisation techniques: electrically detected magnetic resonance, steady-state and modulated photocurrent, constant photocurrent method and steady-state photocarrier grating. These techniques allowed monitoring of the evolution of defect densities, optical absorption, as well as transport and electronic properties such as dark conductivity, mobility–lifetime product of electrons and ambipolar diffusion length. We show how and to what extent defects can be removed upon subsequent isochronal annealing steps. While the hole transport properties are restored after annealing, we find the values for the electron transport properties remain deteriorated.

Process Parameters for Poly-Silicon Deposition at a High Growth Rate (1-7 nm/s) by Hot-Wire Chemical Vapour Deposition

AbstractHigh silane to hydrogen flow ratios and optimum wire temperatures are the key process parameters to achieve high growth rate poly-silicon films by hot wire chemical vapour deposition (HWCVD) using a four-wire hot-wire assembly. Four tungsten wires, 4 cm apart from each other, were used as catalytic filaments. Growth rates higher than 7 nm/s have been achieved at a substrate temperature of ∼510°C. The increase in deposition rate was accompanied by deterioration of two physical properties i.e., decrease in photoresponse and increase in oxygen incorporation in the film, which is attributed to high porosity in the material that is commonly observed in these high growth rate materials. The process conditions to incorporate a high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures, poly-Si films deposited at 1.3 nm/s showed a high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as the i-layer in an n-i-p solar cell on a stainless steel substrate, without back reflector, showed an efficiency of 4.4 % and a high open circuit voltage of 0.58 V, which is attributed to effective passivation of defects and dopants by incorporated hydrogen.

Polycrystalline silicon deposited on glass by subatmospheric-pressure chemical vapour deposition at a high rate

Abstract Amorphous silicon films have been deposited on glass by subatmosphericpressure chemical vapour deposition and then crystallized by solid-phase crystallization. The structural and electrical properties of these polycrystalline silicon films are presented in this work. Good crystalline quality at a deposition pressure of about 400 mbar has been achieved as well as values of the mobilitylifetime product above 10−5cm2V−1 and an ambipolar diffusion length near 200 nm. Depending upon the deposition temperature and pressure, growth rates of up to 20 μm h−1 can be obtained. In-situ doping with arsenic and boron has been studied using Hall effect measurements. High mobilities around 45cm2V−1s−1 have been attained for highly n-type doped samples and mobilities in the range from 20 to 30 cm2 V−1 s−1 for boron-doped samples.

Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when |n| is below a certain threshold value |nth|. In the simplified case when d/L≫1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found |nth|≈0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of |n|. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells.

Potential fluctuations, diffusion length and lateral photovoltage in hydrogenated amorphous silicon and silicon–germanium thin films

Abstract The lateral photovoltage (LPV) has been measured between coplanar electrodes by illuminating hydrogenated amorphous silicon samples at various positions with a red laser spot. We find that the LPV decreases at higher temperatures and increases upon light soaking. Similar results are obtained for hydrogenated amorphous silicon–germanium alloys. The diffusion length of carriers in our samples is measured by the steady-state photocarrier grating technique and the LPV is calculated. The calculated LPV is much smaller than that experimentally measured. We propose that the potential fluctuations present in the samples might be responsible for the large LPV. We have measured the ambipolar diffusion lengths in a-Si : H and a-SiGe : H samples by the SSPG technique. The ambipolar diffusion length decreases with increasing germanium incorporation in the films while keeping the other deposition parameters the same. This decrease in the ambipolar diffusion length with increasing germanium incorporation is attributed to the increase in the DOS as evident from our CPM measurements. Moreover, for all the samples studied, we found that the diffusion length decreases with LS and this is also explained on the basis of the rise in the DOS. These findings are as expected and are in agreement with the published results (Ritter et al. 1987, Weiser and Ritter 1989, Sakata et al. 1997). A LPV is observed in all the samples studied. The LPV decreases upon increasing the temperature of measurement but increases upon LS. In all cases the magnitude of the LPV is found to be much larger than expected from the measured L. The observations are explained on the basis of the potential fluctuations present in the material. These arise from the heterogeneities, that is non-uniform distribution of hydrogen and variation in silicon and germanium concentrations from point to point in the a-SiGe: H film. In our model, the effect of these potential fluctuations is twofold. The electrons and holes become separated spatially in the presence of the potential fluctuations. This tends to reduce their recombination probability and might increase L. Secondly, there is an accumulation of localized charges in the states that coexist above the percolation edge with the extended states. Although these charges do not participate in conduction, they will give a contribution to the LPV. Other factors, for example band bending at the surface might also contribute by separating the carriers. Hence larger potential fluctuations are likely to give a large LPV, since both N and L in equation (14) are expected to be larger. Since the presence of a bias light is expected to reduce the potential fluctuations, the LPV should also be reduced. This explanation of the LPV in terms of potential fluctuations agrees with the observation that the value of L measured by SSPG decreases as the light intensity increases (Weiser and Ritter 1989). Further, we note that SSPG is always carried out in the presence of light and, therefore, is expected to give a smaller L than its value in the dark. In order to test this, we measured the LPV in the presence of bias light and found that the LPV decreases when the bias light is on (see figure 12). The increase in LPV after LS in all the samples can also be explained by this model. Since LS increases the potential fluctuations (Hauschildt et al. 1982, Agarwal 1996, Agarwal et al. 1996), the increase in the LPV is understandable. Thus we can explain all our data by the potential fluctuation model. A more quantitative analysis is not possible at this stage. It is, however, clear that a more heterogeneous sample should show a larger LPV since the potential fluctuations are more. This is an interesting observation which could be useful in comparing the quality of samples prepared under different conditions. This requires more experimentation.

Heterogeneous growth of microcrystalline silicon germanium

Abstract Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10 −5 /Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm 2 has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 A/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).

Light-soaking and annealing kinetics of majority and minority carrier mobility–lifetime products in a-Si:H

A study on undoped a-Si:H of the light-soaking/annealing process of majority as well as minority carrier properties and their relations to defect density is presented. Defect density was measured by the constant photocurrent method (CPM), electron mobility–lifetime product was deduced from photoconductivity and hole mobility–lifetime product was obtained from measurements of ambipolar diffusion length by steady-state photocarrier grating technique (SSPG). We confirm the already known hysteresis behaviour in the dependence of electron mobility–lifetime product on defect density, and report a new and opposite hysteresis-like dependence of the hole mobility–lifetime product during the light-soaking/annealing process. The changes in the mobility–lifetime products of both carriers and concomitant changes in the defect density and in the Fermi level are discussed on the basis of recombination kinetics and structural order effects.

Importance of the transport isotropy in μc-Si:H thin films for solar cells deposited at low substrate temperatures

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) ( L ∥) and by surface photovoltage (SPV) ( L ⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.

Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy

We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.

Electronic and topographic properties of amorphous and microcrystalline silicon thin films

Electronic properties of microcrystalline silicon (μc-Si) thin films prepared by different techniques are presented and compared to that of device-grade, undoped, hydrogenated amorphous silicon (a-Si:H). It is found that whatever the preparation technique, the conductivity of μc-Si is significantly larger and the mobility-lifetime products and ambipolar diffusion lengths of optimised layers can be higher than in a-Si:H. In addition, no light-induced degradation of electronic properties is observed. Local topographic and electrical probing results on μc-Si films are also shown. The surface roughness of μc-Si samples depends on the preparation technique, but a common aging phenomenon in the local electrical probing is found and described.

Microscopic Aspects Of Charge Transport In Hydrogenated Microcrystalline Silicon

ABSTRACTCharge transport in hydrogenated microcrystalline silicon (µc-Si:H) is determined by structure on several size scales: i) local atomic arrangement (<1 nm), ii) crystalline grains and their boundaries (1-10 nm), iii) grain aggregates or columns (0.1-1 µm) and finally iv) features comparable to layer thickness (0.1-10 µm). We first summarize our experimental results concerning these effects: differences of conductivities of grains and amorphous tissue measured locally by conductive AFM, transport anisotropy observed by comparing dark conductivity and ambipolar diffusion length parallel and perpendicular to the substrate, and finally thickness dependence of transport parameters (e.g. dark conductivity activation energy and prefactor). Most of these phenomena can be described by using a novel model of the µc-Si:H growth leading to a structure known as Voronoi adjacency network. The model is based on the nearest neighbor constrained growth. To our knowledge, the Voronoi structure is the first structural model able to predict structure and transport properties of the µc-Si:H and it may become a basis for the future predictive model of µc-Si:H based solar cells.

Variable frequency photoconductive grating method

The theoretical treatment of the steady-state photocarrier grating method is extended to include the frequency dependence of the chopping frequency, ω. It is shown that measurements of the characteristic parameter, β, as a function of ω yield both the ambipolar diffusion length, L, as well as the carrier lifetime, τ, for a fixed angular setting. An alternative experimental setup using a Pockel cell and polarizing cube as beam splitter is suggested to facilitate measurements at higher frequencies.

Fresnel corrections to measurements of ambipolar diffusion length

The steady state photocarrier grating method is re-examined to take account of Fresnel reflection losses at the surface. With the present conventional experimental method, the key experimental quantity β is the ratio of two lock-in amplifier readings of the conductance of the specimen with and without photocarrier grating fringes, respectively. It is shown that because of the change in optical polarization between the two readings a multiplicative factor on the measured β is necessary. Both experimental and computed results show that this can have an effect on the derived ambipolar diffusion length and surface recombination velocity.

Charge transfer mechanism in high-purity silicon-basedn +-π-p + structures

Results of experimental investigations into current-voltage characteristics of n+-π-p+ structures based on high-purity silicon doped with boron are presented. It is shown that the I–V characteristic at high injection levels is described by Stafeev's theory, when the thickness d of the π-region is smaller than or close to three ambipolar diffusion lengths La. For a current density J above (3–50) A/cm2 at varying temperatures, the I–V characteristic obeys a relation usually occurring in the case where charge-carrier recombination in the n+ and p+-regions dominates that found in the π-region. The effect of mutual electron-hole scattering on the behavior of the I–V characteristic is evident at J>630 A/cm3. For d/La=9, the I–V characteristic at high injection levels is treated by the theory of double carrier injection into a semiconductor with consideration for diffusion corrections. In the temperature range below 200–276 K, the I–V curve for all samples studied exhibits a linear dependence on J followed by a portion corresponding to a maximum occupancy of recombination levels by injected electrons. Here the Fe or Au donor levels presumably act as recombination levels. The electron and hole capture areas (cross sections) by recombination centers are roughly estimated.

In situ investigation of polymorphous silicon deposition

Polymorphous silicon (pm-Si:H) consists of an amorphous silicon matrix in which silicon crystallites with nanometer sizes are embedded. This material combines the optical absorption of amorphous silicon (a-Si:H) with electrical transport properties close to those of microcrystalline silicon (μc-Si:H). pm-Si:H thin films were prepared by radio frequency plasma enhanced chemical vapor deposition. To get a better insight in the growth process, we measured the plasma electrically and by means of optical emission spectroscopy. The optical properties of the films were measured by spectroscopic ellipsometry (SE). The results are consistent with the proposal that the growth precursors of pm-Si:H differ from those of μc-Si:H and a-Si:H and that the silicon particles are formed in the gas phase. High resolution transmission electron microscopy (HRTEM) revealed the presence of crystallites with diamond and hexagonal structure embedded in the amorphous matrix. The ambipolar diffusion length in pm-Si:H is a factor of three longer than in optimized a-Si:H as concluded from diffusion induced time resolved microwave conductivity (DTRMC) measurements.

Electronic transport in hydrogenated microcrystalline silicon: similarities with amorphous silicon

Undoped hydrogenated microcrystalline silicon (μc-Si:H) layers were grown by the very high frequency glow discharge (VHF-GD) technique under various deposition conditions. The electronic transport properties under illumination were investigated by means of steady-state photoconductivity and steady-state photocarrier grating methods. Similarly to hydrogenated amorphous silicon (a-Si:H), power law dependencies as a function of the generation rate are observed for the photoconductivity, for the ambipolar diffusion length, and for the parameter b (indicating the Fermi level). For μc-Si:H, as for a-Si:H, nearly constant product of (mobility × recombination time) of majority and minority carriers is observed as a function of the parameter b. Based on these similarities, we assume that the electronic transport model developed for a-Si:H remains valid for μc-Si:H.