Processing-Induced Electrically Active Defects in Black Silicon

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Processing-Induced Electrically Active Defects in Black Silicon Nanowire Devices Stefania Carapezzi, Antonio Castaldini, Fulvio Mancarella, Antonella Poggi, and Anna Cavallini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00600 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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ACS Applied Materials & Interfaces

Processing-Induced Electrically Active Defects in Black Silicon Nanowire Devices Stefania Carapezzi,* † Antonio Castaldini, † Fulvio Mancarella,  Antonella Poggi  and Anna Cavallini. * † † Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, Bologna, 40127, Italy.  Institute for Microelectronics and Microsystems, CNR-IMM, Via P. Gobetti 101, 40129, Bologna, Italy. Corresponding Authors: * Address correspondence to [email protected], and to [email protected]. ABSTRACT: Silicon nanowires (Si NWs) are widely investigated nowadays for implementation in advanced energy conversion and storage devices, as well as many other possible applications. Black Silicon (BSi)-NWs are dry etched NWs that merge the advantages related to lowdimensionality with the special industrial appeal connected to deep reactive ion etching (RIE). In fact, RIE is a well established technique in microelectronics manufacturing. However, RIE processing could affect the electrical properties of BSi-NWs by introducing deep states into their forbidden gap. This work applies deep level transient spectroscopy (DLTS) to identify electrically active deep levels and the associated defects in dry etched Si NW arrays. Besides, the

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successful fitting of DLTS spectra of BSi-NWs based Schottky barrier diodes (SBDs) is an experimental confirmation that the same theoretical framework of dynamic electronic behavior of deep levels applies in bulk as well as in low dimensional structures like NWs, when quantum confinement conditions do not occur. This has been validated for deep levels associated to simple point-like defects as well as for deep levels associated with defects with richer structures, whose dynamic electronic behavior implies a more complex picture. KEYWORDS: Black Silicon, silicon nanowire arrays, top-down nanofabrication, dry etching, nanodevices, deep levels, DLTS 1. Introduction BSi holds a special place among renewable energy materials. This is accounted for by its distinctive features compared to usual silicon surfaces, like low reflectivity, enhancement of the area of the chemically active surface, and non-wettability. 1 Presently, its applications span from micro-electro-mechanical systems (MEMS),

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gas, chemical and biological sensing,

3-5

to drug

delivery, 6 solar cells, 7 anodes for high energy lithium ion batteries, 8,9 H2-production by photoelectrochemical splitting of water, surface.

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as well as a bactericidal medium and a “self-cleaning”

The tuning of the nanofabrication strategies allows to design a variety of different

nanostructures over BSi surfaces, even BSi-NW arrays. In this latter case, the usual BSi properties are merged with the ones associated to Si NWs, enlarging the realm of possible nanotechnological applications. Si NWs are key elements of the developing nanoscaled technology.

12-24

Efficient integration

and economic fabrication are of fundamental importance to boost the transition from their implementation in proof-of-concept devices to industrial applications. A technological-appealing way towards low-cost mass production is represented by wafer-scale manufacturing of BSi-NW


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arrays by top-down fabrication approaches. integration concepts in device architectures

25

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Additionally, NW ensembles allow for novel

and enhancement of the signal-to-noise ratio.

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RIE 28-30 plays a crucial role as a top-down nanofabrication method for BSi-NWs. 25 In fact, RIE is a successful tool for creating features with complex designs and very high aspect ratios down to nano-scale sizes.

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Besides, it possesses the capability of integrating itself in the

existing silicon-related industry. However, in view of the realization of devices based on BSiNWs it is of paramount importance to study how RIE processing affects the electrical properties of BSi processed layers by introducing deep states into the forbidden gap. In general, the efficiency of nanoscaled devices is limited well below its theoretical value by the electrically active defects generated during nanofabrication procedures. In fact, these defects are responsible of macroscopic current inhomogeneities, low breakdown voltages, shunts, as well they act as enhanced recombination centers. 33-36 The purpose of the present work has been to detect and identify the specific kind of electrically active defects that an established top-down approach to Si NWs fabrication like RIE may introduce. To this scope we have exploited the most widely used technique for characterizing deep states in bulk semiconductors, that is DLTS, which possesses many advantages like high sensitivity and quick indication of all the majority carrier traps in a single temperature scan.

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Through DLTS measurements it is possible to determine the defect concentration Nt, activation energy Et and capture cross-section t associated to a deep level, where Et and  t are called the “defect signature” or “thermodynamic fingerprints” of the defect. Once Nt, Et and  t are known, the effect of the associated defect on transport properties can be deduced through the Shockley– Read–Hall statistics. 37


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A series of technical difficulties have to be dealt with to apply DLTS to probe NWs. In case of DLTS measurements over the present BSi-NWs arrays, the challenging step has been the realization of BSi-NWs SBDs, due to the low density of NWs and large gap-distances between adjacent NWs. It is worth noting that DLTS was never applied before, to our knowledge, to characterize low density vertical aligned NW arrays (VANAs), and DLTS measurements have been reported only for very high density chemical-vapor-deposition grown Si NWs.

38

The

experimental procedure we have adopted is suitable to perform DLTS measurements over generic VANA systems and not only over BSi-NWs. Thus, it paves the way to investigate electrically active defects in NWs grown by a wide variety of methods. 2. Results and Discussion 2.1 BSi-SBDs Fabrication The BSi-NWs were fabricated by time-multiplexed alternating RIE etching, also called Bosch processing,

39-41

of substrates of Czochralski (CZ) crystalline silicon in an inductively coupled

plasma (ICP) etcher (Figure S2 in Supporting Information, SI). The p-Si substrates had resistivity of 1-10 Ω·cm. A purposeful tuning of the Bosch process‟ parameters induced selfmasking conditions („nanomasking‟), resulting in spontaneous production of random clusters over the substrates („nanomask‟). The nanomasking was attained by unbalancing the etching time compared to the passivation time. This led to the incomplete removal of the passivating layer and to the fabrication of the nanomask during the first etching step (Figure 1c1). Subsequent alternate cycles of etching and passivation steps generated randomly distributed BSiNWs (Figure 1). Sulfur hexafluoride (SF6) was the etchant gas while octafluorocyclobutane (C4F8) was the passivation gas. Scanning electron microscopy (SEM) characterization allowed to determine that the density of BSi-NWs was about 3.6 x 108 NWs·cm-2, their average length


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about 4.4 μm and their average diameter of 230 nm. Further details about

the BSi

nanostructuring are reported in SI.

Figure 1. (a) – (h) Schematic model of the BSi-NW array fabrication. Starting from the asgrown substrate (a), (b) shows the passivation step, that is the deposition of the passivation layer, while (c1) - (c2) illustrate the etching step, subdivided into removal of the passivation layer (c1) and the etching of the Si substrate (c2). In particular, (c1) shows the nanomask deposited on Si


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surface as a consequence of the incomplete removal of the passivation layer. Further repetitions of passivation ((d) and (f)) and etching steps ((e1) – (e2) and (g1) – (g2)) allow to elongate the nanostructures. A final etching (h) removes the protective polymer layer from the BSi-NWs. (i) SEM micrograph of the cross section of BSi-nanostructured surface. Prior to the realization of the BSi-NWs SBDs, the BSi-NWs were encapsulated in a filler insulating substance to prevent the formation of shunting paths between the top contact and the back contact on the substrate. The filling material played also the role of an inactive supporting medium.

42,43

The encapsulation procedure consisted in spin-coating of epoxy-based negative

tone SU8 2002 photoresist over the BSi-NWs. The SU8 2002 resulted an appropriate choice because it has high filling coverage,

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high thermal stability 42 and low soft-baking temperature

(further information in SI). The high thermal stability of the filling material is of major importance when dealing with DLTS measurements, where the temperature T runs from 80 K to more than 300 K. The curing temperature too, is a crucial point. In fact temperature treatments could affect the samples by diffusing impurities because the solubility of contaminants increases with T. Further details on the encapsulation procedure are reported in SI. The spin-coating and curing of SU8 2002 over Si NW arrays resulted in a faultless filling as well in polymer remnants over the NW tips (Figure 2a1 and a2). In order to expose the NW tips before the metallization process, the samples were RIE processed for 2 minutes with an O2 flow rate (Figure 2b). Finally, the samples were dipped in 5% HF for 10 s to obtain oxide-free NW tips. The metal contacts were evaporated over the top surface of the samples, while galliumpainted ohmic contacts were realized on the back-substrates.


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Figure 2. Integration procedure of BSi-NWs with photoresist SU8 2002. (a1)-(a2) SEM micrographs of BSi-NWs array after spin-coating and curing of the photoresist. (a1) Plan view at low magnification showed that the filling procedure was successful, with complete coverage of substrate. (a2) At higher magnification it was possible to observe some residuals of SU8 2002 at NWs‟ tips. (b) After RIE processing for 2 minutes with an O2 flow these residuals were satisfactorily removed.


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2.2 DLTS analysis Three kind of samples were investigated by DLTS: i) a SBD where the metal barrier was deposited over an as-grown silicon substrate (sample A), ii) a SBD realized on a standard Boschprocessed substrate where no BSi-NWs were fabricated (sample B), and iii) BSi-NWs SBDs (samples C). The different processing conditions for samples B and C have been listed in Table S1 in SI. Figure S3 in SI shows the Current-Voltage measurements performed on these samples. The comparison of the DLTS results on samples B and C has given the unique possibility to gain an insight on the specific damage, if any, associated to the BSi-NWs fabrication procedure compared to the one generated by standard Bosch processing of the silicon substrate. 2.2.1 Point-like defect analysis Literature reports

44-51

that the damage induced by RIE varies with the depth from the

processed surface (Figure S1 in SI). Thus, to compare the generation of deep levels during the BSi-NWs fabrication with the case when their growth was inhibited, the same region had to be probed by DLTS in samples B and C. To this aim preliminary Capacitance-Voltage measurements were performed and the depletion depth versus applied voltage was plotted (Figure S4 in SI). Accordingly, the BSi-NWs SBDs were biased at a polarization voltage Vr = - 4 V with a filling pulse Vfill = 0 V, while the standard Bosch-processed substrate was biased at Vr = - 0.5 V with a filling pulse Vfill = 0 V. The capacitance values as well as the DLTS signals of the BSi-NWs SBDs had to be corrected by taking into account their series resistances,

52

while

this correction was negligible for samples A and B (Figure S3 and further information in SI).


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The DLTS measurements were performed by a Lock-in-type SULA double boxcar spectrometer. This means that the filtering of the capacitance transients was realized through a two-point subtraction method. While the temperature slowly ran, two gates sampled the capacitance signal at times t1 and t2 and output the capacitance difference C = C(t2) - C(t1). In this case, the simple relationship 𝑒𝑝 = ln 𝑡2 /𝑡1 / 𝑡2 − 𝑡1 = 𝜏𝑟𝑒𝑓 −1 links the thermal emission rate ep, or probability for emission of holes into valence band from intragap levels, and the reference time constant of the rate window ref.

37,52

Considering our DLTS system sensitivity

and the doping density (1.4 x 1015 - 1.5 x 1015 cm-3) of the samples, the defect detectability limit was estimated to be about (3 – 6) x 1011 defects·cm-3.

Figure 3. Typical DLTS spectra of sample B (full circles symbols; reverse bias = - 0.5 V) and of a typical sample C (open square symbols; reverse bias = - 4.0V). Emission rate ep = 11.6 s-1. Figure 3 shows typical DLTS spectra of samples B and C. DLTS spectra of the reference sample A did not show any detectable peaks, meaning that the concentration of deep levels, if any, in the as-grown substrates is lower than (3 – 6) x1011 defects·cm-3. Two DLTS peaks were


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found in the standard Bosch-processed substrate (Figure 3). The thermal emission rates ep (T2corrected) of the corresponding deep levels are plotted versus the inverse temperature in Figure 4. Their characteristic parameters are listed in Table 1. These two levels (labeled HB1 and HB2) have activation enthalpies equal to EV + 0.38 eV and EV + 0.67 eV, respectively, EV being the top of the valence band. Two DLTS peaks were as well found in the BSi-NWs SBDs (Figure 3). Their Arrhenius plot is shown in Figure 4, while their thermodynamic fingerprints are listed in Table 2. These two levels (labeled HC1 and HC2) possess activation enthalpies equal to EV + 0.54 eV and EV + 0.60 eV, respectively. Indeed, a partial peak was also present at low temperatures (T ≲ 150 K) in case of BSi-NWs. It was not possible to plot its correspondent Arrhenius plot and to extract its defect signatures. However, we were able to exclude that this peak was related to a single shallow level. Further discussion is reported in SI. The thermodynamics fingerprints of levels HB2 and HC2 almost coincide (Figure 4, Table 1 and 2), suggesting that they originate from the same defect. We availed ourselves of „RIE Damage‟ literature and in general of DLTS results on bulk Si to identify it, because the energy levels of NWs recover the values of bulk materials when quantum confinement conditions do not occur, 38,53 as it is the present case. Awadelkarim and co-workers 54 observed the hole trap H3 at EV + 0.65 eV in CZ Si wafers reactive-ion-etched with CHF3-Ar gas. Hamamoto and co-workers 49

reported on levels located at EV + 0.6 and 0.66 eV in CZ Si reactive-ion-etched with Cl2-SiCl4

gas. A review about majority carrier trap levels induced by RIE in p-silicon is summarized in Table S2 in SI. Hole trap levels situated about mid-gap have been found not only in RIEprocessed silicon. In high-dose, low-energy H-ion-implanted, B-doped CZ silicon Bruni and coworkers

55

found the hole trap H(0.67) at EV + 0.67 eV. Miksic and co-workers

56

related this

level to the level H(0.68) at EV + 0.68 eV which they detected in silicon processed under similar


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conditions. The level H(0.67) was tentatively identified

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as the complex VH2, composed of a

vacancy and two hydrogen atoms. The comparison of the Arrhenius plots of levels HB2, HC2, H(0.67)

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and H(0.68)

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(Figure

4) evidences that they are very close. A minor difference is represented by a small scattering in the values of the apparent capture cross sections. This suggests that these deep centers belong to the same family. The identification of the defect responsible for levels HB2 and HC2 as the complex VH2 is strengthened from two facts. First, hydrogen is known to permeate up to microns below dry etched surfaces even when it is not an ingredient of the etching recipe.

45

In

fact, hydrogen is commonly available sub specie of H2O at the base pressures currently in use in dry etching chambers.

45

The pressure conditions during Bosch processing of samples B and C

have been reported in Table S1 in SI. Second, in both samples B and C the zones probed by DLTS range in depth from ~ 800 nm to 1 m from the surface (Figure S4 in SI). Consequently, they belong to the so called ‟defect reaction region‟, the region where vacancies generated in the ‟displacement damage region‟ diffuse. 47


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Figure 4. Thermal emission rates (-T2 corrected) of the levels HB1 and HB2 (violet empty circle symbols; standard Bosch-processed substrate) and HC1 and HC2 (pink full circle symbols; BSiNWs SBDs) related to the present work, are plotted versus the inverse temperature together with the literature results of Henry and co-workers Awadelkarim and co-workers workers

61

54

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(level H(0.54), black full square symbols),

(levels H1 and H3, red dash-dotted lines), Watanabe and co-

(level RH1, dark yellow short-dotted line), Bruni and co-workers

olive empty star symbols), Miksic and co-workers

56

55

(level H(0.67),

(levels H(0.68) and H(0.52), purple

asterisks), Simoen and co-workers 62 (levels 0.619, 0.504, 0.423 and 0.401, gray dotted lines).


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Table 1. Results of DLTS investigations on standard Bosch-processed substrate (sample B).

Activation energy Level

ΔHp a)

Activation energy ΔHp

b)

Capture cross section σp a)

[eV]

[eV]

[cm2]

HB1

0.38

0.379 c)

1 x 10-15

HB

-

0.3

-

HB2

0.67

0.65

1 x 10-15

Capture cross section σp

Level density NT a)

b)

Level density NT

b)

[cm-3]

[cm-3]

3.7 x 1012

7.4 x1012 c)

2.3 x 10-19

-

4 x 1012

5.5 x 10-14

1.8 x 1013

1.8 x 1013

[cm2] 1 x 10-15

c)

a)

As extracted by fitting the Arrhenius plot of T2-corrected ep versus the inverse temperature.

b)

As extracted by fitting DLTS spectra according to point defect model.

c)

As extracted by fitting after the model of Omling and co-workers, 57 broadening parameter S = 32 meV. Table 2. Results of DLTS investigations on Si NWs of the BSi-nanostructured surface (sample C).

Activation energy Lev el

ΔHp a)

Activation energy ΔHp

b)

Capture cross section σp a)

[eV]

[eV]

[cm2]

HC1

0.54

0.535 c)

9 x 10-13

HC

-

0.33

-

HC2

0.60

0.60

2 x 10-14

Capture cross section σp

Level density NT a)

b)

Level density NT

b)

[cm-3]

[cm-3]

4.5 x 1012

9.4 x1012 c)

1.6 x 10-16

-

1.8 x 1012

2.3 x 10-14

9 x 1012

8.9 x 1012

[cm2] 6 x 10-13

c)

a)

As extracted by fitting the Arrhenius plot of T2-corrected ep versus the inverse temperature.

b)

As extracted by fitting DLTS spectra according to point defect model.

c)

As extracted by fitting after the model of Omling and co-workers, 57 broadening parameter S = 33 meV.


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2.2.2 “Zero barrier analysis”

Figure 5. Fits (red solid lines) to the experimental DLTS spectra (green open circle symbols) of (a) standard Bosch-processed substrate (reverse bias of - 0.5 V) and (b) BSi-NWs SBDs (reverse bias of – 4 V) for emission rate of ep = 23.2 s-1. The deconvoluted point-like peaks (labeled HB2 and HB for sample B, HC2 and HC for samples C; blue short dashed lines) and Gaussian broadened peaks (labeled HB1 for sample B, HC1 for sample C; purple short dotted lines) have been separately simulated. Peaks HB1‟, HC1‟ (black dash-dotted lines) are point-like spectra simulated assuming the same defect signatures of the experimental lines HB1, HC1 respectively. The DLTS spectrum for a level within the bandgap and corresponding to a point-like defect is described by:


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∆𝐶 𝐶0

𝑁

= 2𝑁𝑡 ∙ 𝑒𝑥𝑝 −𝑒𝑝 𝐸𝑡 , 𝜎𝑡 , 𝑇 𝑡1 − 𝑒𝑥𝑝 −𝑒𝑝 𝐸𝑡 , 𝜎𝑡 , 𝑇 𝑡2

(1)

𝑎

𝐸𝑡

𝑒𝑝 𝐸𝑡 , 𝜎𝑡 , 𝑇 = 𝛾𝑆𝑖 𝜎𝑡 𝑇 2 𝑒𝑥𝑝 − 𝑘

(2)

𝐵𝑇

where Si is a constant dependent on material properties (for p-Si Si = 2.6 x 1021 cm-2K-2s-1), kB is the Boltzmann constant and Na is the doping density.

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By extracting the thermodynamic

fingerprints of a defect level from its Arrhenius plot and substituting them in Equations (1) and (2), it is then possible to simulate the corresponding DLTS line. We applied the above delineated procedure to the defect levels HB1 and HB2 for sample B, HC1 and HC2 for sample C. Noticeably, the experimental line HB1 did not overlap to the corresponding simulated line labeled HB1‟ (Figure 5a). The same applied to lines HC1 and HC1‟ (Figure 5b). Actually, lines HB1 and HC1 exhibited peak-shapes broader than those of point-like defects. The DLTS line broadening has been reported in literature for deep centers in semiconductor alloys,

57

for point

defect clusters generated in plastically deformed Si as well as in ion-implanted Si, and for dislocations.

58

In bulk silicon is then a distinctive hallmark of defects whose structure is more

complex than point-like defects. By comparing the experimental data to the simulated lines, it is apparent that the broadening effect is symmetrical for both peaks HB1 (Figure 5a) and HC1 (Figure 5b). Omling and coworkers

57

modeled the symmetrical broadening of a DLTS peak by replacing the capacitance

transient for a point defect level 𝐶 𝑡 = 𝐶0 𝑒𝑥𝑝 −𝑒𝑝 𝐸𝑡 𝑡 with the integral 37 𝐶 𝑡 =

∞ 0

𝐶0 𝑒𝑥𝑝 −𝑒𝑝 𝐸 𝑡 𝑔 𝐸 𝑑𝐸

(3)


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assuming negligible the temperature dependence and the broadening of the capture cross section. The spectral distribution of single levels was described by a Gaussian distribution centered at the mean energy value E0t

𝑔 𝐸 =

1

∙ exp − 2𝜋𝑆

𝐸−𝐸0𝑡 2

(4)

2𝑆 2

where 2𝑆 2ln2 was the full width at half maximum (FWHM) of the spectral distribution. S was termed as the „broadening parameter‟. It is worth noting that the above approach represents a ‟zero barrier analysis‟

58

in case of

DLTS lines related to extended defects, since it does not consider the capture barrier E due to the Coulomb interaction of electrons occupying the extended defect states. In most cases, E is proportional to the total occupation of the extended defect through the constant of proportionality

. Anyway Schröter and co-workers 58 have demonstrated that “the analysis according to Omling and co-workers

57

indeed yields“, for certain ranges of the parameters S and , the same

thermodynamic fingerprints than the more complete model accounting for both the spectral distribution of single levels and the capture barrier. This has been numerically validated for S = 30 meV and  = 1 eV. 58 To gain further insight we fitted the DLTS spectra of BSi-NWs SBDs and standard Boschprocessed substrate. The fit analysis was optimized by fitting the data for three different emission rates (Table S3 in SI). Figure 5 shows the fitting curves (red solid lines) and the experimental spectra (green open circle symbols) for the emission rate of ep = 23.2 s-1. Only through the deconvolution procedure was possible to individuate the presence of a third (point-like) line for both the standard Bosch-processed substrate and the BSi-NWs SBDs (peak HB and HC,


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respectively, in Figure 5), and to extract its relevant thermodynamic parameters (Table 1 and 2, and Table S3 in SI). The DLTS lines resulted to be the convolution of two point-like peaks (HB2 and HB, HC2 and HC) and one Gaussian-broadened peak (HB1, HC1) for both samples B and C. It is noteworthy to observe that level HB shows almost overlapping fingerprints with level BT2 in Vuillaume and co-workers, 59 and the same applies for level HC and the level at EV + 0.33 eV in Castán and co-workers 60 (Table S2 in SI). We would like to highlight that the defect signatures extracted as fitting parameters from the DLTS spectra showed an excellent agreement with the ones determined from the Arrhenius plots, and this for each peak HB1 and HC1 (Tables 1 and 2), which confirmed the validity of the Gaussian-broadening description

57

for them. Besides, the fit analysis allowed the evaluation of

the associated broadening parameter values. It resulted that S(HB1) = 32 meV for level HB1 and S(HC1) = 33 meV for level HC1. Both these values are then in the same range of magnitude in which the “zero-barrier analysis” and the model after Schröter and co-workers to yield the same description.

58

58

has been tested

In standard Bosch-processed substrate defect signatures similar

to level HB1 were reported for level H1 in Awadelkarim and co-workers,

54

level RH1 in

Watanabe and co-workers, 61 level at EV + 0.401 in Simoen and co-workers 62 (Figure 4, Table 1, and Table S2 in SI). The fingerprints of level HC1 in BSi-NWs SBDs well overlap instead to those of level H(0.54) reported by Henry and co-workers

63

in plasma etched (PE) B-doped

silicon substrates (Figure 4, Table 2, and Table S2 in SI). But this is the first time that a line broadening in dry etched silicon as well in BSi-NWs is successfully modeled after the ‟zerobarrier analysis‟. 57 The broadening parameter S is linked to the intrinsic features of the correspondent defect as well as to the lattice disorder. 64-66 After Das and co-workers, 64 the quantities a = S / E0t and b =


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S / kBTpeak constitute the natural scales to evaluate the lattice disorder, where the weak disorder region corresponds to a < 0.1, b < 1, the strong disorder region to a > 0.1, b > 1. The same analysis applied to standard Bosch-processed substrate yielded a(HB1) = 0.08 and b(HB1) = 1.9, to BSi-NWs SBDs a(HC1) = 0.06 and b(HC1) = 1.8. This means that both lines HB1 and HC1 show 1) similar corresponding values for a, b, and that 2) these values fall in the region from weak to strong disorder. It is noteworthy to notice that the extracted values of a, b for HB1 and HC1 are compatible with those found in plastically deformed silicon. 65,66 The “zero-barrier analysis” associated to dislocations,

57

65,66

has been applied to interpret broadening features of DLTS lines

to point defect clusters generated by plastic deformation of Si,

67

and to interstitial clusters in Si-implanted Si. 68 Indeed, we can rule out that levels HB1 and HC1 are associated with extended defects, as these were documented only in the ‟displacement damage region„,

45,47,69

which extends itself at maximum 100 nm under the etched surface

47

(Figure S1). In both standard Bosch-processed substrate and BSi-NWs SBDs the regions probed by DLTS lay well beyond this zone (Figure S3). A single pair of DLTS defect signatures (so called B lines) were detected in Si-implanted p-type silicon, at EV + 0.33 eV (B1 line) and at EV + 0.52 eV (B2 line). They were associated to Sii-clusters formation.

68

Both these lines showed

quite broad peaks that were modeled after the ‟zero barrier analysis‟ with a Gaussian spectral energy distribution. The extracted broadening parameter S(B1) of B1 line ranged from about 17 to 25 meV, while the broadening parameter S(B2) for B2 line varied from about 12 to 27 meV. 68 Also, their activation enthalpies showed ±0.05 eV variations associated to different processing conditions and to differences in defect environment, cluster size distribution and/or electric field. 68


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We wish to point out that the B lines share some of their main features with the levels HB1 and HC1. Both B lines and levels HB1 and HC1 exhibit symmetrical DLTS line-broadening that may be modeled after the ‟zero barrier analysis‟. The activation enthalpy of level HB1, EV + 0.38, is within + 0.05 eV from the level B1, as well the activation enthalpy of level HC1, EV + 0.54, is within + 0.02 eV from the level B2. Besides, the DLTS-probed region in both samples B and C belongs to the ‟defect reaction region‟, where interstitials defects as well as vacancies diffuse from the ‟displacement damage region„.

47

This suggests a common origin related to point-

defect-clusters for level HB1 in standard Bosch-processed silicon substrate and level HC1 in BSi-NWs SBDs, Sii-clusters being possible candidates according to recent findings from literature. 68 Conclusions In this work we investigated the electronic level scheme of BSi-NWs obtained by top-down approach through Bosch processing. Specifically, by DLTS we analyzed the impact over the electrically active defects in dry etched silicon surfaces when BSi-NWs growth is purposefully induced or avoided. We evidenced that the deepest energy level is the point defect level HB2/HC2, which was detected in BSi-NWs as well in standard Bosch-processed silicon substrate when their growth was inhibited. We associated it to the defect VH2 detected in lowenergy H-ion-implanted Si.

51,37

We found also two broadened lines, HB1 in dry etched Si and

HC1 in BSi-NWs, differing in activation enthalpy, capture cross section and density. The ‟zerobarrier analysis‟

57

was successfully applied to fit both the lines HB1 and HC1, allowing to

assess their broadening quantitatively. Indeed, only a report on DLTS line broadening in ‟RIE damage‟ literature exists,

63

with just a rough estimate of this broadening. Furthermore, we

showed how the lines HB1 and HC1 share features in good agreement with those reported for the


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Page 20 of 30

B-lines associated to Sii-clusters in Si-implanted silicon. 68 Our findings demonstrate that, among the processing factors which could negatively impact on electrical properties of devices based on BSi-NWs, hydrogen permeation seriously affects these devices by introducing the VH2 defect. By comparing DLTS results from BSi-NWs SBDs and from standard Bosch-processed substrate we also evaluated that the fabrication of NWs affects mainly the signatures of the broadened lines HB1 and HC1, even if they share common origin related to point-defect-clusters. Finally, we wish to highlight that DLTS is a smart method to extract the temperature dependence of the emission rates of deep levels. Its theoretical basis is a semiclassical description of the dynamic electronic behavior of deep levels in bulk semiconductors. By assuming that the Boltzmann statistics describes the free carrier concentrations in a nondegenerate semiconductor, and that the Fermi-Dirac statistics rules the occupancy of a majority carrier trap, then the principle of detailed balance under equilibrium condition yields the temperature behavior of the trap emission rate. Thus, the successful fitting of DLTS spectra of BSi-NWs SBDs has given the first experimental demonstration that the same description of the dynamic electronic behavior of deep levels applies also in case of low dimensional structures like NWs, at least when no quantum confinement condition occurs. This has been validated for deep levels associated to point-like defects as well as for defects with richer structure leading to DLTS line broadening, for which the more complex model represented by the ‟zero-barrier analysis‟ 57 applies. Supporting Information. Fabrication details of BSi-NWs (samples C). Description of standard Bosch-processing applied on Si substrate that inhibited BSi-NWs growth (sample B). BSi-NWs encapsulation procedure. Current-Voltage measurements on standard Bosch-processed substrate and BSi-NWs SBDs. Series resistance effect in BSi-NWs SBDs. Depletion depth in standard


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Bosch-processed substrate and in BSi-NWs SBDs. Discussion of DLTS spectra of BSi-NWs SBDs for T < 150 K. Review Table of hole trap levels detected in RIE processed silicon. Fitting parameters for the DLTS spectra of the standard Bosch-processed substrate and of the BSi-NWs SBDs. This material is available free of charge via the Internet at http://pubs.acs.org. Present Addresses:  Advanced Research Center for Electronic Systems ‘‘E. De Castro” (ARCES) and Departement of Electronics (DEI), University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES 1. Liu, X.; Coxon, P. R.; Peters, M.; Hoex, B.; Cole, J. M.; Fray, D. J. Black Silicon: Fabrication Methods, Properties and Solar Energy Applications Energy Environ. Sci. 2014, 7, 3223–3263. 2. Barillaro, G.; Nannini, A.; Piotto, M. Electrochemical Etching in HF Solution for Silicon Micromachining Sens. Actuators, A 2002, 102, 195–201. 3. Stewart, M. P.; Buriak, J. Chemical and Biological Applications of Porous Silicon Technology Adv. Mater. (Weinheim, Ger.) 2000, 12, 859–869. 4. Angelescu, A.; Kleps, I.; Mihaela, M.; Simion, M.; Neghina, T.; Petrescu, S.; Moldovan, N.; Paduraru, C.; Raducanu, A. Porous Silicon Matrix for Applications in Biology Rev. Adv. Mater. Sci. 2003, 5, 440–449.


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Processing-Induced Electrically Active Defects in Black Silicon

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