On the Control of the Fixed Charge Densities in Al2O3-Based Silicon

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Article

On the Control of the Fixed Charge Densities in AlO Based Silicon Surface Passivation Schemes 2

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Daniel Kai Simon, Paul Matthias Jordan, Thomas Mikolajick, and Ingo Dirnstorfer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06606 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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ACS Applied Materials & Interfaces

On the Control of the Fixed Charge Densities in Al2O3 Based Silicon Surface Passivation Schemes Daniel K. Simon*,1, Paul M. Jordan1, Thomas Mikolajick1,2, Ingo Dirnstorfer1 1) NaMLab gGmbH, Nöthnitzer Str. 64, D-01187 Dresden, Germany 2) Chair of Nanoelectronic Materials, TU Dresden, D-01062 Dresden, Germany KEYWORDS fixed charges, surface passivation, atomic layer deposition, interface modification, aluminum oxide, hafnium oxide, silicon oxide

ABSTRACT

A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes. Al2O3 nanolayers grown by atomic layer deposition contain negative fixed charges. Electrical measurements on slant-etched layers reveal that these charges are located within 1 nm distance to the interface with the Si substrate. When inserting additional interface layers, the fixed charge density can be continuously adjusted from 3.5·1012 cm-2 (negative polarity) to zero and up to 4.0·1012 cm-2 (positive polarity). An HfO2 interface layer of one monolayer or more reduces the negative fixed charges in Al2O3 to zero. The role of HfO2 is, thus, described as an ‘inert spacer’ controlling the distance between Al2O3

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and the Si substrate. It is suggested that this spacer alters the non-stoichiometric initial Al2O3 growth regime, which is responsible for the charge formation. Based on the good passivation properties of this charge-free HfO2/Al2O3 stack, negative or positive fixed charges can be formed by introducing additional thin Al2O3 or SiO2 layers between the Si substrate and this HfO2/Al2O3 capping layer. All stacks provide very good passivation of the silicon surface. The measured effective carrier lifetimes are between 1 and 30 ms. This charge control in Al2O3 nanolayers allows the construction of zero fixed charge passivation layers as well as layers with tailored fixed charge densities for future solar cell concepts and other field-effect based devices.

INTRODUCTION Nowadays high efficiency solar cells require excellent surface passivation. In the last decade, Al2O3 nanolayers emerged into the photovoltaic industry as passivation material for p-type silicon.1 The excellent passivation performance of Al2O3 deposited by means of plasmaenhanced chemical vapor deposition2 or atomic layer deposition (ALD)3 is based on two effects. First, a chemical passivation is provided by hydrogen diffusion towards the interface during a post deposition annealing.4 At the interface, hydrogen saturates open bonds5 and reduces the density of interface traps (Dit), which act as recombination centers.6 Second, Al2O3 provides a high density of fixed charges (Qf) with negative polarity in the range of 1012 – 1013 cm-2.7–13 These charges repel electrons from the silicon surface and thus suppress surface recombination by field-effect passivation. By the introduction of Al2O3 passivation layers the open circuit voltage and the efficiency of silicon solar cells are significantly improved.10,14–16 In general, higher fixed charge densities reduce the surface recombination. In low injection conditions, the surface recombination velocity is inversely proportional to Qf2.17 On p-type Si the


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negative fixed charges in Al2O3 lead to an excellent passivation over a broad injection level range.10,12 However, in n-type Si, an additional near surface recombination channel appears due to a hole inversion layer formed by the negative charges in Al2O3. At some point within the inversion layer the ratio of electron and hole densities becomes equal to the inverse ratio of their respective capture cross sections. The recombination rate at defects is at its maximum at this point. This near surface recombination causes a reduced lifetime in the low injection range,17–19 which is the operation range of a typical solar cell. To avoid the near surface recombination channel, charged dielectric passivation layers are usually combined with Si substrates doped in opposite polarity, e.g. Al2O3 is applied on p-type but not on n-type Si. Consequently, solar cells require two different passivation materials. To simplify the solar cell process route a symmetrical passivation scheme simultaneously for both sides would be very attractive. This is possible with zero fixed charge dielectric passivation layer, which were demonstrated recently with Al2O3 in combination with SiO2,9,20 HfO29 or Al:SiO221 interface layers suppressing the field-effect passivation. Though these stacks do not provide field-effect passivation, high carrier lifetimes in the millisecond range are achieved due to an excellent chemical surface passivation. Zero fixed charge passivation layers are deposited by means of ALD since this method provides best control of interface properties in the nanometer range. In pure Al2O3 passivation the fixed charges are located within the first nanometers of the layer. Secondary-harmonic spectroscopy7 and corona charge measurements8 show that the fixed charge density remains constant, while decreasing the film thickness down to the nanometer range. Furthermore, a linear correlation of the flat band voltage (Vfb) and the oxide thickness was found in capacitance-voltage [C(V)] measurements.22,23 This linear correlation is also consistent


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with charges located at the interface between Al2O3 and substrate.24 However, some studies also report additional negative25 or positive26,27 volume charges in the order of 1019 cm-3. When volume charges reach this high density, they also contribute to the field-effect passivation. The origin of the interface charges in Al2O3 is still under discussion. Naumann et al. used insitu x-ray photoelectron spectroscopy (XPS) to investigate the initial growth of ALD-grown Al2O3 prior and after annealing. The authors found that initial growth was non-stoichiometric. Within the first nanometers from the silicon surface the O/Al ratio changes from 10 to 3/2 with the latter corresponding to stoichiometric Al2O3.28 The oxygen excess at the interface is suggested to contribute as oxygen interstitial (Oi) defects. First principle calculations of defect formation energies revealed that negatively charged Oi26 and three times negatively charged aluminum vacancies (VAl)29 have the lowest formation energy and are most stable in crystalline Al2O3.30 Kimoto et al. investigated the Al coordination within Al2O3 by electron energy loss spectroscopy.31 Tetrahedral and octahedral coordinated Al is present in Al2O3.32 However, the tetrahedral coordination is dominant near the interface and this dominance is attributed to the coordination of the aluminum silicate interface layer, where Si atoms are incorporated in SiO4 tetrahedra.31 According to Lucovsky, the local molecular structure of amorphous Al2O3 contains tetrahedrally coordinated Al in AlO4/2− units and octahedrally coordinated Al3+ in a ratio of 3:1 to assure charge neutrality.32 Due to the negative charges of the tetrahedral coordination, it is suggested that the gradient in coordination is responsible for the negative fixed charges in Al2O3.11,27 As the tetrahedral AlO4/2- units are O-rich, this finding is consistent with the increased O/Al ratio found in XPS measurements. Furthermore, the lack in octahedral Al3+ could be interpreted as defects, similar to three times negatively charged VAl in crystalline Al2O3.


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Other groups identified dipoles at the SiO2/Al2O3 interface.33 34 Interface dipoles also shift Vfb and provide field-effect passivation for the silicon surface.35 According to Kita et al., dipoles are formed at the interface of SiOx and high-k dielectrics because of an oxygen displacement due to different oxygen areal densities.34 This article is focused on charge control in Al2O3-based passivation layers with very low surface recombination velocities. The charges are modified by introducing thin HfO2 and SiO2 interface layers and adjusting the stack sequence. The role of these interface layers for the formation of fixed charges is discussed.


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EXPERIMENTAL SECTION High quality, shiny etched (both sides) floatzone (100) wafers were used. The 150 mm boron doped p-type and phosphorus doped n-type Si wafers had a resistivity of 1-3 Ωcm and a thickness of 250 µm. As sample pretreatment, only megasonic DI-water cleaning was applied, which did not remove the native oxide from the substrate surfaces. The wafers were cleaved into four equal sized pieces. The passivation layer stacks were symmetrically and simultaneously deposited on both wafer sides by means of ALD in an Oxford Instruments OpAL system without vacuum breaks at 150 °C. Three different ALD materials were used: (I) Al2O3 was grown using a trimethylaluminum/water

process,

(II)

HfO2

based

on

a

tetrakis(ethylmethylamino)-

hafnium/water process and (III) SiO2 was deposited with a silanediamine/oxygen-plasma process using a remote plasma source. The steady-state growth per cycle values (GPC) were 1.1, 1.2 and 1.3 Å/cycle, respectively. Slant-etched samples were slowly dipped into 1 % hydrofluoric acid and rapidly pulled out again to achieve a gradient from zero to the as-deposited thickness (~ 35 nm). After slant-etching an additional HfO2 capping layer was deposited to suppress the leakage current. All samples were activated by post-deposition annealing in a SHS 2800 furnace (AST Electronic GmbH) in forming gas (N2 : H2 = 9 : 1). A plateau temperature of 350 °C was held for 10 min. The effective minority carrier lifetime (τeff) was determined at an injection level (∆n) in the range of 1·1015 cm-3 by means of microwave detected photoconductivity (MDP) using a MDPmap

setup

from

Freiberg

Instruments

GmbH.36

Subsequently

metal-insulator-

semiconductor (MIS) structures were formed by depositing Al electrodes through a hard mask (diameter: ~ 200 µm) on top of the passivation layers. The electrodes were evaporated in a thermal evaporator system (custom built, BESTEC GmbH). Thermal evaporation was used to


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avoid radiation damage known from electron beam evaporation.37 These MIS structures were used for C(V) measurements. The flat band voltage and the oxide capacity (Cox) were measured at high frequency (800 kHz) with a Keithley 4200 semiconductor characterization system. All measurements were done at room temperature. Scanning transmission electron microscopy (TEM) images were prepared in a Zeiss Libra 200 MC CsSTEM system using an acceleration voltage of 200 kV. For the TEM analysis, the layer was deposited on polished Czochralski Si. The sample preparation comprised Au-Pd and Pt depositions. Further details on the experimental setup can be found in Ref. 9, 36, 38.

Figure 1. Flat band voltages are shown as functions of EOT determined on slant-etched oxides with HfO2 capping layers. The linear fit (solid lines) of the experimental data revealed fixed charges with a density of 3·1012 (neg.), 1·1012 (pos.) and 0.2·1012 cm-2 (neg.) for Al2O3, SiO2 and HfO2 (according to Equation 2), respectively. All fit curves had the same intercept on the vertical axis (Vfb = -0.6 V). The insets illustrate the layer stacks. The grey line and the dashed lines serve as guides to the eye.


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RESULTS AND DISCUSSION Location of fixed charges In the first step, the concentration and spatial distribution of fixed charges were investigated in HfO2, Al2O3 and SiO2 layers. For this experiment, the layers were slant-etched to realize a thickness gradient from zero to about 30 nm on the sample. Since C(V)-measurements require low ohmic contributions, the slant-etched Al2O3 and SiO2 layers were capped with a thick HfO2 layer. This capping layer reduced the leakage current and enabled stable measurements even for very low Al2O3 and SiO2 thicknesses smaller than 1 nm. This approach was different from many other investigations, which used slant-etched SiO2 layers to characterize an overlying high-k capping oxide.27,39 In this work the investigations focused on the slant-etched layer itself. Figure 1 shows the measured flat band voltages as functions of the equivalent oxide thickness (EOT). Selected C(V)-curves can be found in Supporting Information S2. The EOT was calculated from the measured oxide capacity as follows:24 ‫= ܱܶܧ‬

ఌబ ఌೄ೔ೀమ

(1)

஼೚ೣ

where εSiO2 is the relative permittivity of SiO2 and ε0 is the permittivity of vacuum. The EOT values are related to the k-values of the materials, which were determined to be 6.3, 14.2 and 6.6 for SiO2, HfO2 and Al2O3, respectively. These k-values are in good accordance with other studies on ALD-grown layers (see Supporting Information S1).40–42 The introduction of EOT was required since materials with different permittivities were combined. The correlation of flat band voltage and EOT is described by:22,27,39,43 ܸ௙௕ = ߶୫ୱ + ‫ ܱܶܧ‬൤−‫ݍ‬

ொ೔೙೟೐ೝ೑ೌ೎೐ ఌబ ఌೄ೔ೀೣ

ఘ

൨ + ‫ ܱܶܧ‬ଶ ൤−‫ ݍ‬ଶఌ೓೔೒೓షೖ ൨− ఌ బ ೄ೔ೀೣ

௤ொ೏೔೛೚೗೐ ௗమ ఌబ

ௗ

ቂ ఌ + ఌభቃ మ

భ

(2)

where q stands for the elementary charge. In previous study, the work function differences (ϕms) of Al and Si were determined to be -0.6 V and 0.2 V for p- and n-type silicon, respectively.9


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Three different charges were considered, the areal charge density at the interface to the Si/SiOx substrate (Qinterface), the volume charge density in the high-k layer (ρhigh-k) and the areal density of the dipole charges (Qdipole) at the interface to the Si/SiOx substrate. The dipole is defined by two separated charges with a distance of d1 and d2 w.r.t. the corresponding interface and the permittivities of the two materials.43 The areal charge density at the interface to Si is usually described as fixed charges. Charges within the ultra-thin SiOx interface layer were neglected in accordance to literature.23,25,39 According to Equation 2, Qdipole and ϕms result in an offset in Vfb, which is independent of EOT. For HfO2, the measured Vfb was almost independent of EOT (Figure 1). A fit of the experimental data with Equation 2 revealed a very low density of interface charges of about 0.2·1012 cm-2 (negative polarity). Interface charge densities below 0.5·1012 cm-2 will be referred to as zero fixed charge in the following. Volume charges would have resulted in parabolic dependence of Vfb, which was not visible. Assuming a measurement error of ± 100 mV, the density of volume charges could be estimated to be about 2·1017 cm-3. With this value, volume charges are negligible for the field-effect passivation. It should be noted that in HfO2 the charge density strongly depends on the applied process conditions, e.g. the annealing temperature44 or the precursor chemistry.45,46 For Al2O3 with HfO2 capping layer, a strong linear correlation of Vfb and EOT was found with a positive slope. The fit of the data with Equation 2 revealed negative interface charges with a density of 3.0·1012 cm-2. Al2O3 was free of volume charges within the detection limit (~1017 cm-3). This approach based on the assumption that the HfO2 capping layer was charge free and without influence on Vfb. Hence, the Vfb shift was fully attributed to charges in Al2O3. However, the HfO2 capping layer created an offset of EOT. As a consequence, the lowest


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measured EOT value was about 7 nm, which corresponded to the thickness of HfO2. The Al2O3 thickness was zero at this point. The benefit of the slant-etch oxide with HfO2 leakage barrier was the possibility to measure the Vfb vs. EOT curves down to very low Al2O3 thicknesses. Equation 2 implied that the interface charges were located at the interface of the Si/SiOx substrate and the dielectric layer. Consequently, Vfb was expected to sharply increase when Al2O3 appeared below HfO2, i.e. when Qinterface changed from 0.2·1012 cm-2 (HfO2) to 3.5·1012 cm-2 (Al2O3) in Equation 2. A Vfb change of about 1 V is visible at about 7 nm EOT in Figure 1. However, the data rather shows a gradual transition from the linear Al2O3 function (black) towards the linear HfO2 function (blue). This transition appeared within an EOT range of about 0.7 nm (7.6 - 8.3 nm EOT) and is illustrated by a grey line serving as guide to the eye. Within this transition area, the Al2O3 layer fully built up its charges. Thus, the fixed charges in Al2O3 were located within a physical thickness of about 1.0 nm. This observation matched with reported fixed charge measurements on very thin Al2O3 layers.7,8 For SiO2 with HfO2 capping layer, a linear function with negative slope was found. The SiO2 layer was free of volume charges within the measurement accuracy (~1017 cm-3). The fixed charge density was fitted to be about 1.0·1012 cm-2 with positive polarity. This value is close to values reported in the literature for plasma ALD-grown SiO2, though the reported charge densities are slightly larger (~2.0·1012 cm-2).9,47 The origin of these charges is still under discussion. In thermally grown SiO2 the fixed charges are related to oxygen vacancies.48 As plasma ALD-grown SiO2 was found to be rather stoichiometric,49 it was suggested the positive fixed charges were formed by remaining hydroxyl groups or charges at the Si substrate interface.50


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The Vfb function at very low SiO2 thicknesses (~ 7 nm EOT) was difficult to evaluate since the interface charge densities in SiO2 and HfO2 were similar and thus the Vfb difference became small. However, the data indicated that the charge formation also occurred on a length scale below 1 nm physical SiO2 thickness. Some measurement points occurred above -0.7 mV, which are considered to be measurement flyers. For all three oxides, Vfb was identical at 0 nm EOT (-0.6 V). According to Equation 2, this value is the sum of the work function difference and the dipole induced voltage shift. The work function different was constant in this study, since neither the electrode material nor the substrate doping was changed. Considering the different dipole formation mechanisms,34 it appeared very unlikely to find identical dipole contribution for all investigated oxides. Though dipoles could not be excluded their contribution to the flat band shift was at least very unlikely. Assuming the existence of neither dipoles nor volume charges, only interface charges exist, which will be termed fixed charges for simplicity in this work. With this assumption, Equation 2 was simplified and the fixed charge densities were determined with:24 ܳ௙ =

஼೚ೣ ௤

൫߶௠௦ − ܸ௙௕ ൯.

(3)

It should be noted, that an error in the work function difference only caused a systematic error in Qf for all samples without falsifying the relative dependencies.


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Figure 2. Fixed charge densities are shown for SiO2 and Al2O3 as functions of the HfO2 interface ALD cycle number x. The insets illustrate the layer stacks. The blue dashed line marks the value of pure HfO2. Dashed lines serve as guides to the eye.

Control of fixed charges The fixed charge density in Al2O3 can be controlled by a thin HfO2 interface layer.9 This is shown in Figure 2 (further details in Supporting Information S3). Pure Al2O3 had negative fixed charges with a density of 3 – 4·1012 cm-2. After adding three ALD cycles of HfO2 interface layer, the charges were reduced to about 1.0·1012 cm-2. Three cycles of HfO2 corresponded to less than one monolayer, assuming a growth rate of 1.2 Å/cycle. When further increasing the interface layer thickness, the fixed charge density reduced below 0.5·1012 cm-2 and converged to the value found for pure HfO2 (0.2·1012 cm-2, horizontal dashed line in Figure 2). The HfO2 interface layer can also be applied to SiO2 with positive charges. SiO2 had a fixed charge density of about 1.7·1012 cm-2, which is in good agreement with the literature.9,20 By introducing just one cycle of HfO2 the charge density dropped by about 30 %. When further increasing the HfO2 thickness up to 10 cycles the charges further reduced to values in the range


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of 0.5 – 1.0·1012 cm-2. After 10 cycles of HfO2, the stack was still slightly positively charged and had not fully converged to the properties of pure HfO2. The HfO2 interface layer transformed negatively and positively charged dielectrics to layers with close to zero fixed charge density. This charge control cannot be explained by compensation of charges by additional charges with opposite polarity. In HfO2, the charge density was too low for charge compensation. The data rather suggested that HfO2 acted as a barrier for charge formation. In Al2O3, an HfO2 barrier thickness of roughly one monolayer (~ 0.5 nm) was required for charge suppression. The reduction of charges in SiO2 occurred on a longer length scale. The measured charge densities were found to be independent of the substrate doping. Similar densities of fixed charges were measured on p- and n-type Si.

Figure 3. TEM image of a zero fixed charge HfO2/Al2O3 stack.

Figure 3 shows a TEM image of a zero fixed charge stack with a 10 cycles HfO2 interface layer and a 20 nm Al2O3 capping layer. The Si substrate had a smooth surface covered by interfacial SiOx with a thickness of about 1 nm. A dark line above the SiOx showed the presence of a fully closed HfO2 layer with a thickness of 1.5 – 1.8 nm. This value was close to the thickness expected from 10 ALD cycles with a steady state growth of 1.2 Å/cycle. All oxide


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layers were fully amorphous. The grainy structure of Al2O3 was an artefact caused by electron beam irradiation during the TEM measurement. Subsequently, the HfO2/Al2O3 stack was used to investigate the charge formation. The HfO2/Al2O3 stack with 10 cycles of HfO2 was free of fixed charges. To form the charges, either an Al2O3 or a SiO2 interface layer was introduced underneath this stack. Figure 4 illustrates the achieved fixed charge densities as a function of ALD cycles (the corresponding Vfb vs. EOT curves are shown in Supporting Information S3 Figure S3). Already the first ALD cycle of Al2O3 or SiO2 considerably formed fixed charges with negative or positive polarity, respectively. However, the charge forming process was different for both materials.

Figure 4. Fixed charge densities are shown as functions of the ALD cycle number of a SiO2 or an Al2O3 interface layer below a zero fixed charge HfO2/Al2O3 stack. In case of SiO2, a maximum charge density of 4.0·1012 cm-2 (pos. polarity) was achieved at 15 ALD cycles. In case of Al2O3, the fixed charge density converged to 3.5·1012 cm-2 (neg. polarity). The horizontal dashed lines mark the value of the pure oxides. The inset illustrates the layer stacks. Dashed lines serve as guides to the eye.


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In case of Al2O3, the fixed charge density converged to a value of 3.5·1012 cm-2 after about 10 cycles of Al2O3 interface layer. This density was similar to the value determined for pure Al2O3 (horizontal dashed line in Figure 4). The charge formation within Al2O3 saturated after 10 cycles, which corresponded to about 1 nm. This thickness scale was in good agreement with the Vfb measurements on slant-etched structures (Figure 1), where the linear trend built up within 1 nm thickness. The observed fixed charge modification in Al2O3 (Figure 2 and 3) was consistent with models based on stoichiometry deviation and changes in Al coordination as discussed in the introduction chapter. The predominant formation of tetrahedrally coordinated Al in AlO4/2- units at the interface required a physical connection to the tetragonal oriented Si/SiOx interface.31,32 When this connection was interrupted by HfO2 with 8-fold coordinated Hf-atoms, the growth in tetrahedral coordination is not enhanced at the interface. Hence, the charge formation in the HfO2/Al2O3 stack is suppressed because of the Al2O3 growth occurs on an amorphous HfO2 interface layer instead of crystalline Si. On the other side, when Al2O3 is deposited on top of the Si/SiOx substrate and below the zero fixed charge HfO2/Al2O3 stack, the formation of tetrahedral coordinated Al is reinforced. Within this model, the HfO2 layer acts as an ‘inert spacer’, which defines the charge formation by the distance between the Al2O3 layer and the crystalline Si substrate without interacting with the material. The charge formation within SiO2 was overlapped by a second effect. Positive charges were quickly formed after the first cycles of SiO2 interface layer. However, already after five cycles, the charge density exceeded the values known from pure SiO2 (horizontal dashed line in Figure 4). The density peaked at 4.0·1012 cm-2 after 15 cycles of SiO2. When further increasing the SiO2 interface layer thickness, Qf reduced to 3.0·1012 cm-2 at 50 cycles of SiO2. An


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extrapolation of the curve suggested that the charge density of pure SiO2 could be reached after more than 100 cycles (~10 nm) of SiO2. The peak in charge density is related to the presence of HfO2 in the stack. In previous work a SiO2/Al2O3 stack without HfO2 was investigated and it was found that the charge density monotonously increased with the SiO2 thickness (discussion in Supporting Information S4 Figure S7).9 The simple ‘inert spacer’ model, which was proposed for Al2O3, failed to explain the observations in the SiO2/HfO2/Al2O3, stacks. The data rather suggested that HfO2 was involved in the charge formation. Bersuker et al. reported that Hf-based high-k materials can cause an oxygen deficiency in a SiO2 interface layer.51 The authors found a correlation between the initial SiO2 interface layer thicknesses and the density of positive fixed charges after the HfO2 deposition and annealing.48 Oxygen vacancies in SiO2 are positively charged and shift the total charge into positive direction. This effect could explain the remaining positive charges in SiO2 with 10 cycles of HfO2 interface layer (Figure 2). The additional positive charges could also explain the maximum of positive charges in Figure 4, when assuming an increasing charge formation up to 15 cycles of SiO2 and a reduced influence of these charges when the SiO2/HfO2 interface shifts away from the Si surface. On the other side, the slant-etched sample with SiO2 and HfO2 capping layer could be consistently described without assuming an additional charge creation. The discrepancy in charge formation could possibly be explained by different process sequences (additional data in Supporting Information S4). In Figure 1, HfO2 was deposited on homogeneous SiO2, which was slant-etched. In Figure 4 HfO2 was deposited on thin ALD-grown SiO2, which might had an increased effective surface for chemical interactions due to initial island growth.


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The final experiment focused on the role of SiO2 at the silicon surface and below the ALD grown oxides. Interfacial SiOx is linked to the oxide growth process and reported by many groups. It was found to be present even after removing the native oxide prior the Al2O3 deposition.52–55 The oxide was also visible in the TEM cross-section of the HfO2/Al2O3 stack (Figure 3). To better understand the role of this layer a zero fixed charge stack with 10 cycles of HfO2 interface layer and a 20 nm Al2O3 capping layer was investigated. An additional SiO2 interlayer was introduced between HfO2 and Al2O3. Figure 5 shows the determined fixed charge density for increasing thickness of the SiO2 interlayer. Note, spectroscopic ellipsometry measurements revealed similar GPC values of 1.3 Å/cycle for ALD-grown SiO2 in this stack configuration as for the SiO2/HfO2/Al2O3 stack in Figure 4 (see Supporting Information S4 Figure S6).

Figure 5. Fixed charge densities are shown as a function of the ALD cycles number of the SiO2 interlayer. In this stack no charges were formed up to a SiO2 thickness of 50 ALD cycles. The inset illustrates the layer stack.


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Without SiO2 interlayer the fixed charge density was about 0.5·1012 cm-2 (neg. polarity). The additional SiO2 interlayer should reinforce the fixed charge formation, if the charges are formed by a reaction between Al2O3 and SiO2. However, no significant charge formation was found with increasing interlayer thickness. Up to 50 cycles of SiO2 (~6.5 nm) the measured charge densities scattered within 0.0 – 0.7·1012 cm-2 (neg. polarity). As for the other experiments, the same results were found on p- and n-type Si. This experiment confirmed that the charge formation was linked to a reaction between Al2O3 and Si rather than Al2O3 and SiO2. Charges are not formed independently of the silicon substrate. This finding was in contradiction to other studies, which reported Si–O–Al dipoles formed between Al2O3 and interfacial SiOx on the silicon substrate.33–35 However, the absence of dipoles was in accordance with the conclusion drawn from the slant-etch series, where the thickness dependency of Vfb was attributed to charges at the interface and not to dipoles between the two oxides. As the fixed charge formation is linked to the silicon substrate, it is not possible to create highly charged layers with multiple HfO2–SiO2–Al2O3 sequences.

Passivation Performance For all samples the effective minority carrier lifetime was measured. The measurement was done at high injection level (∆n ~ 1015 cm-3) in order to qualify the surface passivation without the influence of the near surface recombination, which depended on the fixed charge density and polarity of substrate doping.17,18 Figure 6 presents the carrier lifetime data as a function of the interface layer thickness for the sample series shown in Figure 4 (see Supporting Information S3 Figure S4). For the Al2O3/HfO2/Al2O3 stacks the carrier lifetimes were independent of the Al2O3


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interface thickness with values around 8 ms and around 20 ms on p- and n-type Si, respectively. The large difference between values measured on p- and n-type Si substrates could be explained by the higher intrinsic bulk lifetime of n-type Si. The lifetime data of the SiO2/HfO2/Al2O3 stacks showed a clear influence of the SiO2 interface layer. When increasing its thickness to 20 cycles, τeff decreased to about 1 ms and 4 ms for p- and n-type Si, respectively. This point corresponded to the maximum charge density within this sample series. Therefore, the carrier lifetime reduced though the field-effect passivation was very high. Thicker SiO2 interfaces did not further deteriorate the lifetime. These lifetimes still proved a very good surface passivation; however, the recombination rate was almost a factor of 10 higher than in those stacks having an Al2O3 interface layer.

Figure 6. MDP measurement of carrier lifetimes (∆n ~ 1015 cm-3) on the stacks shown in Figure 4. Increasing of SiO2 layer thickness reduced the carrier lifetime while the increase of the Al2O3 thickness had no influence. The inset illustrates the layer stack. Dashed lines serve as guides to the eye.


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The negative influence of SiO2 on the effective minority carrier lifetime was also observed in the sample series with HfO2/SiO2/Al2O3 stacks (Figure 5). The general trend was a carrier lifetime reduction with increasing SiO2 thickness. Poor lifetime values of ALD-grown SiO2 in the range of few 100 µs are also reported in the literature47 and linked to high Dit values.9,21,47,50 In contrast to ALD-grown SiO2, the presence of HfO2 hardly influenced the carrier lifetime values. In the HfO2/Al2O3 stack (Figure 2), the introduction of up to 10 cycles of HfO2 resulted in carrier lifetimes of more than 5 ms and 20 ms for p- and n-type Si, respectively. The high carrier lifetime in Al2O3 layers and HfO2/Al2O3 stacks were related with very low Dit values in the range of a few 1010 eV-1cm-2.9 This high level of chemical passivation guarantees an excellent surface passivation even in absents of fixed charges and field-effect passivation.

SUMMARY AND CONCLUSION In this article the fixed charge control in Al2O3-based passivation stacks was investigated. Negative charges in Al2O3 could be suppressed by an HfO2 interface layer with a thickness of roughly one monolayer. The HfO2/Al2O3 stacks provided excellent surface passivation even without field-effect passivation. The measured carrier lifetimes reached more than 5 ms and 20 ms on p- and n-type silicon, respectively. The role of the HfO2 interface layer was described as an ‘inert spacer’, which separated the Al2O3 layer from the Si surface. By introducing the spacer, the charge formation in Al2O3 was blocked. When introducing an Al2O3 layer below the spacer, the negative fixed charges emerged again. Measurements on slant-etched Al2O3 layers showed that the fixed charges were formed within the first 1.0 nm from the interface to the Si substrate. It was suggested that the presence of


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HfO2 controlled the charge formation by a change of the non-stoichiometric initial Al2O3 growth regime (first ALD cycles). According to literature, this non-stoichiometric initial growth regime results in a deviation of Al coordination and O/Al ratio close to the Si substrate. Since this growth regime is substantially based on the physical contact to the substrate, the ‘inert spacer’ controlled the fixed charge formation by controlling the influence of the silicon substrate on the ALD growth process. The HfO2 interface layer was also applied to SiO2 stacks. Similar to the Al2O3-based stacks, the interface layer allowed reducing or enhancing the density of fixed charges, depending on whether SiO2 was located above or below the HfO2. However, experimental data could only be explained by a chemical interaction of the involved materials, i.e. the simple ‘inert spacer’ model was not valid for stacks involving SiO2 and HfO2. It was suggested that additional charges were formed by the creation of oxygen vacancies in the SiO2 interface layer. Finally, the role of the interfacial SiOx in the charge formation process was investigated. The data showed that no charges were formed at a SiO2/Al2O3 interface, which was separated from the Si surface by an HfO2 interface layer of 10 ALD cycles (~1 nm). This result confirmed that the charge formation required the contact to silicon. Models assuming dipoles between SiO2 and the high-k materials are not compatible with the observations. Based on these results, stacking of fixed charges was impossible. The doping of silicon had no influence; all the measured fixed charges were similar on p- and n-type silicon. By introducing Al2O3 or SiO2 interface layers underneath an HfO2/Al2O3 stack, the fixed charge density could be continuously controlled within a broad range from 3.5·1012 cm-2 (negative polarity) to zero charges and further to 4·1012 cm-2 (positive polarity). This broad adjustable range of fixed charge density was reached by only changing the first nanometer of the


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passivation stack. At the same time, all stacks provided excellent surface passivation resulting in carrier lifetimes in the millisecond range. With these properties the stacks are very attractive candidates for zero fixed charge passivation layers as well as layers with tailored fixed charge densities for future solar cell concepts. Strategies discussed in this article can also be applied to a broad range of field-effect based MIS devices using electrostatic doping,56 e.g. in carbon based transistors57 or GaN MIS heterojunction devices,58 where the gate dielectrics consist of HfO2, SiO2 or Al2O3.59

ASSOCIATED CONTENT Supporting Information Raw measurement data of the dielectric materials, the slant-etched layers and the layer stacks. Detailed discussion on plasma ALD-grown SiO2 including TEM, spectroscopic ellipsometry and fixed charge densities measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources This work was funded by the German Federal Ministry of Education and Research BMBF and the Federal Environment Ministry BMU (MWT-plus, no. 03SF0420A).

ACKNOWLEDGEMENT Jan Gärtner and Talha Chohan (both NaMLab gGmbH, Dresden) are gratefully acknowledged for experimental support. Uwe Mühle (Fraunhofer IKTS-MD, Dresden) is gratefully acknowledged for the TEM measurements.

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On the Control of the Fixed Charge Densities in Al2O3-Based Silicon

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