Geometry Controlled White Light Emission and Extraction in CdS

Article Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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Geometry Controlled White Light Emission and Extraction in CdS/ Black-Si Conical Heterojunctions Arijit Sarkar,† Ajit K Katiyar,‡ Subhrajit Mukherjee,‡ Sudarshan Singh,‡ Sumit K Singh,‡ Amal Kumar Das,‡ and Samit K Ray‡,§,* †

Advanced Technology Development Center and ‡Department of Physics, Indian Institute of Technology, Kharagpur 721302, India S.N. Bose National Center for Basic Sciences, Sector-III, Salt Lake, Kolkata 700098, India

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§

ABSTRACT: The light emission characteristics of Si nanocrystallites on a chemically etched black-Si surface, coupled with its excellent light extraction feature of nanocone geometry is reported for potential use in future Si-based CMOS-compatible light-emitting applications. As a proof of concept, n-CdS/p+-black-Si conical-heterojunction arrays, fabricated by depositing n-CdS film on vertically standing nanoporous Si nanocones, exhibiting intense white light emission, have been studied. Si nanocone arrays of 1 to 3 μm height were fabricated using a metal-assisted chemical etching technique followed by CdS deposition by pulsed laser deposition. A broad EL emission covering the entire visible spectrum and extending up to the NIR wavelength region (450 to 860 nm) has been achieved at room temperature and low forward bias condition. Improved light extraction characteristics in the nanocone textured geometry of black Si over a planar heterojunction have been corroborated by optical simulation analysis. The enhanced light emission characteristics of n-CdS/p+-black-Si conical-heterojunction arrays at a low bias voltage may open up a new opportunity for CMOS-compatible, low-power-consuming, and highly efficient phosphor-free white LEDs in the near future. KEYWORDS: black Si, CdS/Si heterojunction, conical heterojunction, white LED, electroluminescence, COMSOL simulation, LED simulation

1. INTRODUCTION Silicon-based optoelectronic devices and their monolithic integration for future photonic integrated circuits (PICs) require CMOS compatibility in terms of their fabrication and operation. A large number of photonic devices based on silicon-like modulators and photodetectors have already been reported in the literature, but achieving a Si-based optical source is still a major challenge.1−3 The main bottleneck on this context is the inherent indirect band gap nature of bulk Si, exhibiting poor near-infrared (NIR) emission.4,5 Over the last several years, new pathways have been explored to develop Sibased light emitters using various Si nanostructures such as nanowires,6−8 quantum dots,1,9−11 and Group IV alloys.12,13 One-dimensional (1D) Si nanostructure arrays such as nanowires are preferred for these kinds of applications because of their ability to form versatile templates, which assist in fabrication of radial heterojunctions.4,14−16 In particular, vertically oriented cone-like nanostructures termed as black silicon, attractive due to its excellent antireflection feature, are promising for Si-based photovoltaic technology.2,5,17−19 The conical black-Si (bSi) nanostructures containing Si nanocrystallites on the surface not only exhibit an excellent light trapping behavior but also have the capability of providing intense light emission and enhanced light extraction.6,20−22 It has already been demonstrated that texturing or patterning the © XXXX American Chemical Society

surface of GaN- and InGaN-based LEDs enhances the light extraction efficiency by reducing the total internal and Fresnel’s reflections suffered by the emitted light rays.23−27 Therefore, a conical p-n heterojunction between cone-like bSi nanostructures with an appropriate direct and wide band gap semiconductor appears potentially attractive for efficient Sibased light-emitting devices. Cadmium sulfide (CdS), an n-type, group II−VI semiconductor with a direct band gap of 2.42 eV, is attractive for optoelectronic applications because of its excellent optical performance in the visible wavelength range and high stability.28−31 In recent years, 1D n-CdS/p-Si conical heterojunctions have already been studied for fabrication of photovoltaic/photosensing devices using Si nanocones/nanowires.15,32 Hayden et al. reported light emission in a single CdS/Si radial heterojunction fabricated on a crystalline Si nanowire core.33 The observed narrow emission mainly originates due to the recombination of injected carriers in the CdS shell having no contribution from Si nanowires, limiting its spectral emission centered at 528 nm with full width at half-maximum of only 20 nm.33 Received: October 11, 2018 Accepted: December 26, 2018 Published: December 26, 2018 A

DOI: 10.1021/acsaelm.8b00001 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Electronic Materials

Figure 1. (a) Cross-sectional FESEM image of Si nanocone (bSi) arrays, (b) high-resolution TEM image of a single Si nanocone, (c) SAED pattern from Si nanocone, (d) cross-sectional FESEM image of CdS/bSi conical-heterojunction arrays, (e) high-resolution TEM image of a single CdS/bSi nanocone conical heterojunction, and an (f) SAED pattern from the CdS/bSi nanocone heterostructure. deposition (PLD) using a commercially purchased CdS target. The device fabrication was finally completed via depositing an electrically conducting layer of aluminum-doped ZnO (AZO) acting as a top transparent electrode using the same PLD system followed by thermal evaporation of a thick aluminum (Al) layer for the bottom electrode. The morphologies of the fabricated bSi templates and CdS/bSi conical-heterojunction arrays were studied with a ZEISS SUPRA 40 field emission scanning electron microscope (FESEM) and a JEOL JEM-2100F transmission electron microscope (TEM). The crystalline properties of deposited CdS films and fabricated conical heterojunctions were investigated using a Phillips Xpert MRD X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å). The photoluminescence (PL) experiments were performed using a He− Cd laser (325 nm) as the excitation source at ambient atmosphere. To record and detect the PL signals, a TRIAX 320 monochromator and Hamamatsu R928 photomultiplier detector were used. The electrical measurements of the conical heterojunction were studied using a Keithley 4200-SCS semiconductor parameter analyzer. The biasdependent room-temperature electroluminescence (EL) under carrier injection into the devices was also performed using the above stated optical configuration.

Here we report enhanced light emission characteristics of nCdS/p+-bSi conical-heterojunction arrays using nanocone textured Si templates. The fabricated conical-heterojunction diode exhibits superior rectifying behavior and efficient yellowish white electroluminescence at room temperature on application of a low forward bias, without demanding any complex color mixing processes or phosphor coatings. The simulation supports significantly improved light extraction behavior in conical black Si as compared to the planar heterojunction. The study indicates that an n-CdS/p+-bSi conical heterojunction is a potential candidate for the development of a CMOS-compatible, low-power-consuming, highly efficient white LED that can be integrated into future photonic integrated chips (PIC).

2. EXPERIMENTAL DETAILS Pieces of 2 × 2 cm2 cut from commercially available one-side-polished p+-Si (100) wafers (boron-doped, resistivity ≈ 0.001−0.005 Ω-cm) were cleaned through a standard RCA cleaning process and then dipped in dilute HF for few minutes. The detailed fabrication process of bSi nanostructures has been reported elsewhere.14 In brief, cleaned Si substrates were instantaneously immersed into a solution composed of an optimized amount of HF and HAuCl4 for about 60 s to form a homogeneous film of gold nanoparticles (Au NPs). Thereafter, the Si pieces with Au NPs on its surface were submerged in an etchant solution consisting of an optimized concentration of HF and H2O2 to form cone-like Si nanostructures on the polished side of the substrates. The shiny surface of the Si wafers turned dark black and were highly absorbing under optimized etching for 30 min. The etched Si pieces were then rinsed repeatedly with deionized (DI) water to wash away the excess etchant solution, and the Au NPs are dissolved by aqua regia treatment. Finally, fabricated black-Si pieces were dipped in 2% HF for several minutes to get rid of any native oxide layer and were dried under N2 flow. The n-CdS/p+-bSi conical heterojunction has been fabricated by depositing CdS film on p+-bSi nanocone arrays by pulsed laser

3. RESULTS AND DISCUSSIONS 3.1. Structural Characteristics. The morphology of the bSi nanocones fabricated on a Si wafer is presented in the typical cross-sectional FESEM image of Figure 1(a). The micrograph exhibits dense and vertically standing cone-shaped Si nanostructures with varying height and base dimensions formed uniformly on Si wafers. Nanocones are found to be 1 to 3 μm in height and 500 to 700 nm in diameter at the base, whereas the tips are a few nanometers in diameter. Figure 1(b) presents the cross-sectional TEM image of an individual Si nanocone, which further confirms the dimensions of fabricated nanocones. The inset of Figure 1(b) also reveals the formation of porous Si at the outer surface of nanocones as a result of chemical etching. Though the outer shell is found to be porous B


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edge (NBE) emission from the CdS thin film.28,30 Comparatively, a lower-intensity broad peak observed at ∼714 nm is commonly observed in polycrystalline CdS films deposited by physical techniques.34,35 The red emission peak at ∼714 nm can be attributed to the trapped electron transition from surface states such as Cd vacancy to the valence band of CdS.34−36 The PL emission from the CdS/bSi nanocone conical heterojunction shows contribution from both CdS films as well as porous Si nanocones. For a better insight on the individual contributions, the experimentally acquired PL spectrum of the fabricated conical heterojunction was deconvoluted into three Gaussian peaks centered at 512, 644, and 714 nm and is presented in Figure 3(b). The position and full width at half-maximum (fwhm) of the deconvoluted peaks in the PL spectrum of the conical heterojunction are in agreement with the individual PL peaks obtained for Si nanocones and CdS thin films exhibited in Figure 3(a). Digital photographs of the PL emissions from bSi, CdS film, and the CdS/bSi conical heterojunction are presented in Figure 3(c), (d), and (e), respectively. A noticeable reddish emission from bSi, a greenish emission from CdS film, and a yellowish white emission from the conical heterojunction are in agreement with the PL spectra presented in Figures 3(a) and (b). 3.3. Electrical Characteristics. The fabricated n-CdS/p+bSi conical-heterojunction-based electroluminescent device is presented schematically in Figure 4(a). The room-temperature current−voltage (I−V) characteristics of the device in the dark condition are shown in Figure 4(b). The asymmetric nature of the I−V characteristics reveals the formation of a conical p-n heterojunction between the p+-bSi core and n-CdS shell. The I−V curve shows that the conical p-n heterojunction diode between n-CdS and p+-bSi has a rectifying behavior with a rectification ratio of about 125 at ±3 V bias. In order to compare the electrical behavior, a control device with an nCdS/p+-Si planar heterojunction configuration has also been fabricated, and the corresponding I−V characteristic is exhibited in Figure 4(b). The control device shows a rectification ratio of 48 at ±3 V bias. Thus, n-CdS/p+-bSi conical heterojunctions exhibit a superior rectification behavior with a lower reverse saturation current in comparison to the control device. The I−V characteristic of a real diode is governed by the Richardson-Schottky diode equation given as37,38

with a pore size of ∼2 to 5 nm, the core remains crystalline, which is ascertained through the selective area electron diffraction (SAED) pattern shown in Figure 1(c). Figure 1(d) exhibits the cross-sectional FESEM micrograph of a CdS film deposited on bSi nanocones forming a conical p-n heterojunction. The image shows a conformal and uniform coverage of Si NCs by 50 to 80 nm thick CdS film. A typical cross-sectional TEM image in Figure 1(e) further confirms the conformal and uniform coverage of CdS film on Si nanocones. A closer look at the heterojunction between bSi and CdS film reveals the formation of a rough interface, which is a result of the presence of a thin porous layer containing Si nanocrystallites at the bSi nanocone surface. The SAED pattern of the CdS/bSi heterojunction is presented in Figure 1(f) showing the diffraction rings because of the polycrystalline nature of the CdS film along with the diffraction spots of the crystalline Si at the background. The (002), (110), and (112) planes of wurtzite CdS are noticed in the SAED pattern. This observation has been corroborated using a grazing angle (2°) incidence X-ray diffraction pattern of the heterostructure as shown in Figure 2. Numerous distinct diffraction peaks at 2θ =

Figure 2. XRD spectra of the fabricated CdS/bSi conical heterojunction at 2° grazing angle incidence.

24.9, 26.5, 28.2, 36.6, 43.7, 47.8, 51.8, and 54.6° analogous to (100), (002), (101), (102), (110), (103), (112), and (004) crystallographic planes of wurtzite CdS (JCPDS card, file no. 41-1049) are observed. In addition to CdS diffraction features, strong (004) and (002) peaks due to the bSi core are also observed at 2θ = 69 and 32.9°, respectively. 3.2. Photoluminescence Characteristics. To examine the optical emission characteristics of the n-CdS/p+-bSi heterojunction, PL experiments were performed at room temperature. The normalized PL spectra of fabricated bSi, control CdS thin film, and the CdS/bSi heterojunction are shown in Figure 3(a). Under 325 nm excitation, the PL spectrum recorded from bSi has a single broad peak centered at ∼644 nm, originating from the quantum confinement of carriers in Si nanocrystals formed on the superficial porous portions of bSi nanocones. Emissions with analogous energy locations have been reported for porous Si nanostructures by different research groups.6−8,32 The PL emission from PLD deposited control CdS film exhibits an intense sharp peak in the green region (∼512 nm) and a broad peak centered at ∼714 nm. The peak at ∼512 nm near the elementary absorption edge of bulk CdS is attributed to the near band

I = Iseq(V − IR s)/ ηkT

(1)

Where I is the current, Is is the reverse saturation current, V is the voltage, Rs is the series resistance, k is the Boltzmann constant, T is the temperature, and η is the diode ideality factor. The value of diode ideality factor can be obtained using the equation8 η=

q ∂V kT ∂ln I

(2)

The calculated low-field diode ideality factor (η) from eq 2 is found to be 3.2 for the conical heterojunction and 4.4 for the control device, signifying the nonideal behavior of both of the fabricated heterojunction diodes. A somewhat higher value of η may be a result of the rough interface between porous Si and CdS film or owing to the presence of a thin SiO2 layer at the interface, both formed during the fabrication of bSi by chemical etching.8,14,32 The inset in Figure 4(b) exhibits the best fitted I−V curve of the conical heterojunction using the C


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Figure 3. (a) Normalized room-temperature PL spectra of bSi and CdS film on c-Si substrate and CdS/bSi conical-heterojunction arrays. (b) Deconvoluted room-temperature PL spectrum of CdS/bSi heterojunction. Digital photographs of PL emission from (c) as prepared bSi, (d) CdS film on planar Si substrate, and (e) CdS/bSi conical heterojunction.

Figure 4. (a) Schematic diagram illustrating coaxial AZO/n-CdS/p+-bSi conical-heterojunction-based LED. (b) Room-temperature I−V characteristics of then-CdS/p+-bSi conical and CdS/Si planar heterojunctions under the dark condition. The inset exhibits the fitted I−V curve for the conical heterojunction following a power law IαVm agreeing to the space-charge-limited-current (SCLC) model.

Table 1. Comparison of Electrical Performances and Operating Voltages of Different CdS/Si-Heterojunction-Based LEDs Reported in the Literature device structure CdS/porous Si heterojunction CdS/Si nanoporous pillar array CdS/Si nanopillar array CdS:In/Si nanopillar array CdS/black-Si conical heterojunction

forward current (mA cm−2)

reverse saturation current (mA cm−2)

rectification ratio

diode ideality factor (η)

operating voltage (V)

references

15.5@ 5 V 15 @ 3 V 33 @ 3 V 23.64 @ 3 V

5.7 @ 5 V 4.2 @ 3 V 0.5 @ 3 V 0.29 @ 3 V

3.6 66 125

3.19

4.0−12.0 6.0 5.0−9.0

49 31 50

5.0−30.0

this work

equation IαVm, with the value of m as 2 and the proportionality constant as 1.44. Thus, the charge transport of the conical heterojunction can be explained using the space-chargelimited-current (SCLC) model.39,40 Upon an increase in the bias, space charges related to surface and trap states are developed, which influences the current conduction within the diode and results in nonideal I−V characteristics.41 This kind of behavior is commonly observed in one-dimensional semiconductor systems such as nanowires.42 The electrical

performance of our fabricated conical-heterojunction device is comparable with those found in other recently published works on similar kind of device configuration, which are represented in Table 1. The tabulated data clearly indicates that the fabricated conical-heterojunction diode has superior electrical performance, when compared to other recently published results. 3.4. Electroluminescence Characteristics. Achieving visible electroluminescence (EL) emission on applying a D


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Figure 5. (a) Room-temperature electroluminescence spectra of an n-CdS/p+-bSi conical heterojunction when varying forward biases are applied. The photograph of the EL emission at 15 V forward bias is shown in the inset. (b) The deconvoluted EL spectrum of n-CdS/p+-bSi conicalheterojunction arrays under 15 V forward bias at room temperature. (c) Integrated EL intensity plotted with respect to different applied forward bias. (d) The black dot with chromaticity coordinates (0.35, 0.41) plotted in the CIE 1931 (x, y) chromaticity diagram exhibiting the emission from the n-CdS/p+-bSi conical-heterojunction arrays is in the white light region.

forward bias to n-CdS/p+-bSi heterojunctions is essential to its application in white light-emitting diodes (LEDs). A yellowish white colored EL emission could be detected easily even by the naked eye on applying a forward bias to the device. The EL spectra of an n-CdS/p+-bSi conical heterojunction with varying applied forward biases from 5 to 30 V are recorded and represented in Figure 5(a). On application of forward bias, a broad band emission, covering the entire visible wavelength range starting from 500 to 860 nm in the near-infrared (NIR) region, is observed. Detectable EL emission is noticed on applying a bias of 5 V, and with an increase in bias, the increased electron injection across the junction results in a gradual increase in EL intensity. The inset in Figure 5(a) presents the digital photograph of the EL emission from the device at 15 V forward bias. In order to have a proper insight at the exact reason behind the broad band EL emission, the recorded EL spectrum at 10 V bias was deconvoluted using a Gaussian function and exhibited in Figure 5(b). The observed broad emission spectrum comprises three individual sub-bands centered at 530, 652, and 735 nm. The deconvoluted EL energy peaks agree quite well qualitatively with PL spectra shown in Figure 3(b). Thus, the peaks centered at 530 and 735 nm in the EL spectra originate from their combination of injected carriers within the CdS shell. Specifically, the 530 nm peak originates because of the radiative recombination of electrons and holes at the band edges of CdS. On the other hand, the peak at 735 nm is related to the radiative recombination of electrons trapped in the surface states (Cd vacancy) with the holes in the valence band of CdS. Moreover

the peak centered at 652 nm is due to the recombination of quantum confined carriers within Si nanocrystallites formed on the porous Si surface of the Si nanocone core beneath the CdS shell. Some notable differences in energy between the deconvoluted EL and PL spectra are clearly evident. There is a minor red shift in the location of each peak in the EL spectra. Additionally, the intensity and full width at half-maximum (fwhm) of the peaks are different when compared with those in the PL spectra. The deviation may arise because of different excitation mechanisms involving the two processes. For PL, the external excitation is an optical source, and the emission depends entirely on the optical properties of the materials involved. However, EL emission is caused by an external current injection and collectively depends on the heterojunction band alignment, field-induced band bending, optical properties of the active light-emitting layers, work function of the electrodes, and their interfacial properties.37,43 Moreover, on completion of fabrication of the entire device structure, multiple interfaces are formed among the electrodes, bSi surface, CdS film, and AZO top contact. These interfaces introduce unavoidable interfacial traps and defects within the device. On application of an external bias, the recombinations of carriers at these interfacial defects and trap states may also contribute to the broadening of the emission peak. The integrated EL intensity of the recorded broad EL emission from fabricated n-CdS/p+-bSi heterojunctions was plotted as a function of applied forward bias, which is presented in Figure 5(c). With a gradual increase in the applied bias, the increase in integrated intensity is roughly linear from E


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ACS Applied Electronic Materials 5 up to 15 V, but thereafter, the rate of increase in the intensity shows a saturation tendency. The saturation tendency of the integrated EL intensity may be a result of the large Joule heating or exhaustion of recombination centers at a higher current injection condition, deteriorating the EL performance of the LED.8,37 The CIE (Commission Internationale de l’É clairage) 1931 chromaticity coordinate of the observed EL spectra obtained at 15 V from the fabricated n-CdS/p+-bSi conical heterojunction is represented with a black dot in the chromaticity diagram in Figure 5(d). The corresponding chromaticity coordinates are x = 0.35 and y = 0.41, suggesting a yellowish white light emission that is near the pure white light chromaticity coordinates (x = 0.33, y = 0.33). Further, the value of the CIE color rendering index (CRI) has been found to be 92 and the correlated color temperature (CCT) to be 4243K for our fabricated devices. The performance of the fabricated nanocone heterojunction LEDs is compared in Table 1 in terms of operating voltage with performances described in other recently published reports regarding similar device structures. The result reveals a relatively lower-powerconsuming n-CdS/p+-bSi heterojunction device operating over a wide range of applied biases. The origin of broad electroluminescence in a fabricated conical heterojunction has been elucidated by the help of an energy band alignment diagram at zero and moderate forward biases, which is presented in Figure 6(a) and (b), respectively. For the n-CdS/p+-bSi heterojunction case, the electron affinities for χ(Si), χ(Siporous), and χ(CdS) have been taken as 4.05, 3.69, and 4.15 eV, respectively, from the literature.8,44 Whereas, the band gap values for Eg(Si), Eg(Siporous), and Eg(CdS) have been taken as 1.12, 1.93, and 2.42 eV, respectively.8,44 The above electron affinities and band gap values result in a conduction band offset (ΔEc) for electrons of 0.46 eV, whereas the offset for holes (ΔEv) is estimated to be 0.95 eV at the Si/CdS interface. On applying a suitable high forward bias, holes from p+-Si are injected into CdS, and electrons from n-CdS are injected into Si as shown in Figure 6(b). The recombination of holes and electrons in the porous Si gives rise to an emission at 652 nm. Similarly, electrons and holes along the band edge of CdS recombine radiatively to emit photons of 530 nm. On the other hand, a fraction of the injected electrons on applying the bias are trapped because of the surface states (Cdvac) situated at ∼0.7 eV below the conduction band of CdS.44 These trapped electrons undergo radiative recombination with holes in the valence band of CdS, giving rise to an emission at 735 nm.44 The combined effect of these transitions results in a broad EL emission from 500 to 860 nm. 3.5. Optical Simulation. The effect of conical configuration on the light extraction from the fabricated n-CdS/p+ -bSi conical-heterojunction LED has been analyzed by a threedimensional finite element method (FEM) based on Fresnel’s equation using COMSOL MULTIPHYSICS (version 5.2, R.F. module) simulation software. For simulation, the dimensions of the Si cone in the array include a base diameter of 500 nm, an apex diameter of 50 nm, and a height of 1.0 μm. Over this Si nanocone array, a CdS film that is 50 nm thick has been added to form a conical heterostructure. The effective refractive index (neff) of the Si nanocone has been obtained from the effective medium approximation (EMA) theory15 q 1/ q neff (h) = [f (h)nSiq + {1 − f (h)}nCdS ]

Figure 6. Energy band diagram of CdS/p+-bSi conical heterojunction at (a) zero bias and (b) forward bias, demonstrating different transition and recombination processes.

where the value of q is 2/3, and nSi and nCds are, respectively, the refractive indices of bulk Si and CdS. The f(h) is the fill factor, defined as the areal ratio of the nanostructure to the total substrate surface at a height (h), and has been obtained from the FESEM image of the Si nanocone arrays. For the calculation of neff, the values of nSi = 3.85 and nCds = 2.46 at λ = 650 nm have been used.45,46 The relative permittivity is expressed as εr = (n − ik)2, while relative permeability (μr) and conductivity (σ) are 1 and 0, respectively. The value of the effective refractive index (neff) of Si nanocones changes gradually along the height of the cones from tip to base. The structure-dependent smooth gradient of neff introduces complex light management characteristics, which strongly influence the diffraction and total internal reflection properties of light, providing an effective light propagation through the conical textured CdS/bSi heterostructure. The simulation provides the electric field distribution within and surrounding the CdS/bSi conical heterostructure and its tip area, where the plane wave is incident at the bottom edge of the structure and propagates along the +z-direction, remaining invariant in the other two directions. In the case of a fabricated LED, the light emission originates at the Si/CdS interface under the forward bias condition. However, as a result of the limitation in

(3) F


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⎯→ ⎯

Figure 7. Electric field intensity distribution (| E |) due to plane wave propagation through (a) CdS/bSi conical heterostructure and a (b) planar CdS/Si heterostructure. (c) The wave propagation spectrum considering the optical band gap of CdS/bSi conical heterostructure.

electric field intensity distribution (| E |2 ) has been presented at a particular wavelength of 650 nm for both the planar as well as CdS/bSi heterostructure. This wavelength has been chosen, because it is the most intense wavelength constituting subbands in the EL spectra (Figure 5(a), (b)) of the fabricated LED device. Figures 7(a) and (b) compare the electric field

characteristic wave features such as diffraction and interference effects have been taken into consideration.25,26,47 The conical surface of CdS/bSi induces multiple types of scattering at the cone surface while reducing the total internal reflection, thereby enhancing the overall wave propagation.23−27 It is to be noted that the maximum field distribution and propagation are at the apex of the cones, which reveals that the cone-like structure leads to an enhanced field distribution and propagation. But this phenomenon is absent in case of a planar CdS/Si heterojunction, where the back reflection from the interface reduces the overall wave propagation.23−27,47,48 The unique light scattering behavior of the conical structured LED results from the combined effect of a diffraction of waves at the apex and edges of the cones along with the reduced total internal reflection and complex interference phenomena within the cones. 23−27,47,48 The field distribution and wave propagation results demonstrate that the conical textured CdS/bSi heterostructure is superior to the planar heterostructure, when compared in terms of light propagation and extraction. So it is expected that the light extraction is dominant in a conical structured heterojunction, as compared to the planar heterojunction following its generation through the recombination of an electron−hole pair upon an applied bias. The wave propagation spectrum of the CdS/bSi heterostructure as a function of wavelength has been simulated and is presented in Figure 7(c). The simulated spectrum reflects the band edge transitions of the fabricated CdS/bSi heterostructure, which are responsible for the light emission, and the results agree well with the experimentally obtained PL spectrum (Figure 3(b)).

distribution (| E |2 ), originating from the plane wave propagation in CdS/bSi and planar CdS/Si heterostructures with a color index. The results show that the conical texture effectively enhances the propagation of the electromagnetic wave over the planar wave. To analyze this observation,

4. CONCLUSION Conical n-CdS/p+-bSi conical-heterojunction arrays have been fabricated on Si nanocones to study the light emission characteristics of Si CMOS-compatible devices. A strong red

simulation tools, we considered normal light incidence from the bottom surface and specifically estimated the electric field distribution and light propagation through interior and exterior of the nanocone geometry. It should be noted that the overall electric field extracted outside the semiconductor boundary qualitatively depends on the surface geometry only, irrespective of the position of light source. The top edge of the structure has a port boundary condition but without any excitation to ensure complete absorption of the approaching waves. To find the numerical solution, the electric field is considered to vary along the direction of propagation in the following way ⃗ E ⃗( r ⃗) = E0⃗ e−i(K · r )⃗

(4)

⎯⎯⎯⎯→

⎯⎯→

where E0 is the amplitude of the incident plane wave, and K is the wave propagation vector in vacuum. For comparison, a planar CdS/Si heterostructure with 50 nm CdS film on Si has also been simulated to visualize the impact of nanostructure geometry on the wave propagation. To analyze the impact of conical geometry on light propagation and the effective extraction capability of the fabricated CdS/bSi heterojunction LED, the cross-sectional ⎯→ ⎯

⎯→ ⎯

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luminescence is observed from Si nanocrystallites on the surface of black Si prepared by metal-assisted chemical etching. The electrical characteristics of the fabricated conical heterojunction exhibit superior performance, when compared with other recent reports with similar device structure. A broad band emission ranging from 450 to 860 nm covering the entire visible spectrum and extending up to the NIR region is observed from the fabricated n-CdS/p+-bSi conical heterojunctions on applying a forward bias as low as 5 V. The EL emission from the device appears to be of yellowish white in color visible to the naked eye. Optical simulation reveals that the conical texturing in the CdS/bSi conical heterostructure results in an enhanced light extraction capability over the planar device. Fabricated n-CdS/p+-bSi conical heterojunctions with superior diode characteristics and low-powerconsuming EL behavior are promising for applications in CMOS-compatible, phosphor-free white LEDs for photonic integrated chips.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Samit K Ray: 0000-0002-8099-6690 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial financial support from MHRD IMPRINT “USR” project grant no. 3-18/2015-T.S.-I (Vol. IV) and DST-Meity supported NNetRA “NRK” project grant no. 5(1)/2017NANO. The collaboration and discussion with Prof. A. K. Raychaudhuri, S. N. Bose National Center for Basic Sciences, Kolkata, are gratefully acknowledged.

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Geometry Controlled White Light Emission and Extraction in CdS

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