Infrared electroluminescence from erbium-doped spark-processed silicon

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Journal of Luminescence 127 (2007) 339–348 www.elsevier.com/locate/jlumin

Infrared electroluminescence from erbium-doped spark-processed silicon$ Kwanghoon Kim1, Rolf E. Hummel Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA Received in revised form 11 December 2006; accepted 14 December 2006 Available online 25 January 2007

Abstract The infrared (IR) electroluminescence (EL) of erbium-doped spark-processed silicon (sp-Si) was investigated. For this, a device was constructed which consisted of a silicon wafer on which an erbium layer was vapor deposited, followed by spark-processing and rapid thermal annealing for 15 min at 900 1C in air. The metallization consisted of a 200 nm Ag layer (above the spark-processed area) and a 50 nm thick Al film (on the ‘‘back side’’), containing a window through which the light could escape. Maximal light emission occurred near 1.55 mm, that is, at a wavelength where commonly used fiber optical materials have their minimum in energy loss. The processing parameters for most efficient light emission were an Er thickness of 200–300 nm, a spark-processing time of about 30 s, an n-type Si wafer having a low (3–5 O cm) resistivity, an operating temperature near room temperature, and an operating voltage between 25 and 40 V under reverse bias. The results are interpreted by postulating an energy transfer from sp-Si to the Er3+ ions involving the first excited state 4I13/2 to ground state 4I15/2. Further, impact excitation and hot electrons that are accelerated into the erbium doped sp-Si by the applied field (100 kV/cm) are considered. r 2007 Elsevier B.V. All rights reserved. PACS: 78.60.Fi Keywords: Electroluminescence; Spark-processing; Silicon; Infrared; Erbium

1. Introduction Spark-processing is a technique that allows the modification of some vital properties of materials, in our specific case, those of silicon (Si) [1]. As an example, sparkprocessing creates a substance that allows Si to exhibit strong room-temperature photoluminescence (PL), electroluminescence (EL) and cathodoluminescence (CL) in the visible spectral range. This observation is important since Si is known to be an indirect band gap material whose luminescence capabilities are generally negligible. Sparkprocessing is a dry technique (unlike many other methods that are utilized to create porous Si with the aim to induce $ Based on a dissertation submitted to the University of Florida by K. Kim as partial fulfillment for the Doctor of Philosophy (Part III). Corresponding author. Tel.: +1 352 392 6667; fax: +1 352 392 0326. E-mail address: [email protected]fl.edu (R.E. Hummel). 1 New address: Microchip Technology Inc., 21015 S.E. Stark Street, Gresham, OR 97030, USA.

0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.12.009

PL). Shortly, high-frequency (several kilohertzs), highvoltage (several thousand volts), and low average current (several milliamperes) unipolar electric pulses involving about 16 kHz are applied for a certain length of time between the substrate (e.g., a Si wafer as cathode), and a counter-electrode, for example, a tungsten wire tip. This leads to electric pulses about 0.02 ms in length that are repeated every 60 ms. The resulting substance is a grayishlooking layer on (and in) the Si substrate which luminesces strongly in the green and ultraviolet/blue area of the electromagnetic spectrum [1]. The present paper extends our investigations to electroluminescence of erbium-doped spark-processed silicon (sp-Si). An EL device was constructed and its characteristic features and optimal parameters for efficient light emission were determined. Since silicon is transparent in the infrared (IR) region, the light is made to escape the device through a window on the untreated (not spark-processed) side of the device which makes its application for integrated optoelectronics, in

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K. Kim, R.E. Hummel / Journal of Luminescence 127 (2007) 339–348

conjunction with a waveguide structure quite feasible. The characteristic wavelength of the Er doped sp-Si EL device lies conveniently near 1.55 mm, that is, at a wavelength at which the energy loss of conventional fiber optic cables is minimal. 2. Experimental procedure The devices consisted of a 1  0.5 in2 (2.54  1.27 cm2), 400 mm thick, n-type (1 0 0), phosphorous-doped Czochralski grown (3–5 O cm) silicon wafer. The samples were first ultrasonically cleaned for 15 min each by consecutive exposure to trichloroethylene, electronic grade acetone, and electronic grade methanol followed by a 15 min deionized (DI) water rinse. The samples were then dried under N2 atmosphere at room temperature. Second, an erbium layer (purity 99.9999%, 0.1 mm thick) was laid down on top of the surface-oxidized silicon wafer utilizing physical vapor deposition in a vacuum of about 106 Torr (1.33  104 Pa). The deposited metal thickness was checked using a thickness monitor that was mounted in the same plane as the samples. Third, a high-frequency, high-voltage, and low-current spark was applied by a spark generator delivering 10–15 kV, 14–16 kHz, and 5–10 mA of spark voltage, frequency, and current, respectively. A tungsten tip was used as an anode. The spark gap was 2 mm. The abovementioned silicon wafer was used as the ground for the system. Spark-processing was conducted at atmospheric pressure in air. Effective incorporation of oxygen, erbium activation, and diffusion of erbium into the sp-Si necessitates rapid thermal annealing (RTA) of the samples in air (or in O2). Various combinations of temperatures and annealing times were carried out as outlined below. In most cases, erbiumdoped sp-Si was heated in air, at 900 1C for 15 min, unless otherwise stated. In order to exploit EL from Er-doped sp-Si two metallization layers need to be deposited to provide Ohmic contacts (see Fig. 1). At first, a silver layer (about 200 nm in thickness) was vapor deposited on the spark-processed area. This layer provides not only a metallic contact but

Fig. 1. Schematic representation of a typical EL device utilizing erbiumdoped spark-processed silicon. The light escapes through the window at the ‘‘bottom’’ of the device. The Ag contact reflects the EL light back into the device and towards the window.

also reflects the light which is generated near and in the spark-processed area towards the window. Second, the ‘‘back side’’ contact (opposite from the spark-processed area) was prepared. The Si surface was roughened using a small grit silicon carbide sand paper followed by etching off the native oxide, applying a commercial buffered HF solution. The window area was masked utilizing Al foil. Subsequently, Al (purity 99.999%), about 50 nm in thickness was vapor deposited on this area in a vacuum of about 106 Torr (1.33  104 Pa). The completed device is depicted in Fig. 1. It may be argued that the light from this device could be made to escape through the silver (top) layer if it were only thin enough. However, the spark-processed area is generally quite porous. As a result, the thin silver film would not cover the entire spark-processed area which in turn reduces the intensity of the light emission. On the other hand, a thicker silver layer does not allow enough light to transmit due to absorption. Thus, for wavelength below the energy gap (1.1 eV), the ‘‘bottom window’’ design is a better alternative (see in this context also Ref. [2]). The EL was stimulated by passing a pulsed direct current, having a square wave function (frequency 200 Hz, 30% duty cycle) through the device whereby the positive terminal is connected to the bottom Al electrode (see Fig. 1). (No light emission is observed for opposite biasing conditions.) Pulsing was necessitated to prevent excessive heating and thus, destruction of the device. Typically about 25 V was applied. The EL spectra of the device were measured by utilizing an infrared monochromator (blazed at 830 nm) in conjunction with a Peltier cooled (30 1C) InGaAs photocell. 3. Results and discussion Fig. 2 depicts a typical EL spectrum for Er-doped sp-Si for the device described in the previous section. A maximal output intensity near 1550 nm (0.81 eV) with a FWHM of 41 nm is discernible. The spectrum has a characteristic light emission of the first excited state 4I13/2 to ground state 4I15/2 transition of the Er3+ ion [3]. It is further observed that the EL spectrum is a convolution of several peaks, probably as a result of Stark splitting of the excited states and ground states. Fig. 2 also contains a PL spectrum of the same device by stimulating the Er-doped sp-Si with the 488 nm line of an Ar ion laser. Compared to the EL spectrum, the PL spectrum is somewhat broader (FWHM ¼ 44 nm) and slightly shifted to shorter wavelengths having a peak wavelength near 1540 nm. This may be an indication of different microenvironments for the Er3+ ions that are excited optically compared to electrically. Moreover, the EL peak intensity is about 35% smaller than that for PL if one takes the same input power (200 mW/cm2) into consideration (as is done in Fig. 2). Specifically, the laser power density was measured with a power meter whereas the electrical power density input was calculated by

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3.1.2. Variation of spark-processing time Fig. 4 depicts the IR EL intensity at 1550 nm as a function of spark-processing time. The samples were prepared as described above (see also caption of Fig. 4), and measured under identical spectrometer conditions. It is observed that with increasing spark-processing time, the IR

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3.1.1. Concentration of erbium in sp-Si Fig. 3 presents the EL luminescence of erbium-doped spSi as a function of the initial thickness of the erbium layer. It is observed that the IR EL intensity steeply increases up to a layer thickness of about 100 or 200 nm, after which it moderately increases further. The slope change in the IR EL intensity above 100 nm, suggests that there is a limit to the concentration of Er3+ ions that can be optically activated in the sp-Si matrix. It is also suggested that precipitation of erbium may take place, as seen in the infrared PL of erbium-doped sp-Si [4].

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Fig. 3. Infrared electroluminescence peak intensity (at 1550 nm) of erbium-doped spark-processed silicon as a function of erbium concentration, 900 1C RTA for 15 min in air.

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knowing the applied voltage (25 V), the current density (28 mA/cm2), and the duty cycle (30%). The inset of Fig. 2 depicts the IR EL spectrum of a control sample, that was identically prepared as described above but without spark processing. No major light emission, that is, only ‘‘noise’’ is observed from this control sample, under either biasing conditions. In other words, spark processing is the key process to generate IR EL for erbium-doped silicon.

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Wavelength (nm) Fig. 2. Infrared electroluminescence spectrum at room temperature for erbium-doped spark-processed silicon prepared using 0.1 mm erbium metal deposited on a silicon wafer (n-type, 5 O cm), a spark time of 30 s in air, a spark gap of 2 mm and 900 1C rapid thermal annealing for 15 min in air. Aluminum (E50 nm) on the back side of the wafer and silver (E200 nm) on the top of devices was deposited by physical vapor deposition as electrical contacts. The dashed line indicates a photoluminescence spectrum of erbium-doped spark-processed silicon for comparison. The inset depicts the infrared ‘‘emission’’ of a control device (noise) prepared as in Fig. 2 but without spark-processing.

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Spark Processing Time (s) Fig. 4. Infrared electroluminescence intensity of erbium-doped sparkprocessed silicon monitored at 1550 nm as a function of spark-processing time. Initial erbium thickness: 200 nm; 900 1C rapid thermal annealing for 15 min in air.

EL intensity increases sharply in its early stages and peaks around 30–40 s. Spark-processing for more than 50 s results in a sharp decrease in IR EL intensity. Above 80 s of sparkprocessing the EL signal is barely detectable. In order to further investigate the influence of sparkprocessing time, energy dispersive X-ray spectroscopy (EDS) was utilized applying an accelerating voltage of 15 kV and spark-processing times of 10, 40, 70, and 100 s (see Fig. 5). Each spectrum was normalized to its maximal Si concentration. It is observed that essentially no erbium can be detected after 70 and 100 s of spark-processing

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3.1.3. Type of substrate In order to investigate a possible influence of the wafer type, IR EL devices were fabricated using n-type and p-type silicon having resistivities in the same range. Specifically, Czochralski grown (1 0 0) 3–5 O cm n-type silicon and alternatively Czochralski grown (1 0 0) 1–3 O cm

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whereas a moderate erbium concentration is seen after 40 s spark-processing. The largest erbium concentration is detected after only 5 s spark-processing. In this case the erbium is probably still deposited on the free surface, but not in the SiOx matrix. It is suggested that the increase in IR EL intensity in the 1–40 s range is due to number of factors. First, the increase in area of the halo region (the area around the actual spark-processed region) results in an increase in the light emitting area, and therefore an increase in EL intensity. Second, as the spark-processing time is moderately increased, the number of emitting centers, the surface morphology, and the thickness of the surface oxide layer are optimized, which all result in an enhanced EL intensity. It is additionally suggested that for higher spark-processing times, the thicker oxide layer starts to negatively affect the electrical conduction and thus spark-processing efficiency, and/or that the erbium concentration is reduced by continued ablation during spark-processing.

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Wavelength (nm) Fig. 6. Infrared electroluminescence spectra of n-type (dashed curve) and p-type (solid curve) wafers. (1 0 0) n-type wafer (3–5 O cm resistivity). Applied voltage: 25 V. Current: 101 mA. The (1 0 0) p-type wafer has a 1–3 O cm resistivity. Applied voltage: 25 V. Current: 163 mA.

p-type silicon were utilized which underwent identical processing and measuring procedures. EL was observed in both cases when the top silver contact was negatively biased (Fig. 6) but no EL was observed in either case with a positively biased top. No

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significant EL peak position change is discernible between the obtained spectra. However, the IR EL device on n-type silicon has a 30% higher irradiance at 1550 nm than the ptype device. The output power/input power (external) efficiencies are 0.0073% (n-type) and 0.0044% (p-type), respectively. (The output power was obtained by measuring the absolute irradiance with a calibrated optical detector.) Fig. 7 depicts the reverse current–voltage characteristics for n-type (circles) and p-type (squares) for erbium-doped sp-Si EL devices. These curves reveal increasing currents when a negative voltages are applied, which are characteristics for quasi-rectifying devices. It is observed that the IR EL devices fabricated on p-type silicon pass slightly higher currents than the n-type for the same voltages. In short, n-type silicon is slightly more efficient for erbium-doped sp-Si IR EL devices.

3.1.4. Measuring temperature The IR EL intensity at 1550 nm under 25 V bias was monitored, as a function of operating temperature for an erbium-doped, n-type sp-Si EL device (see Fig. 8). For comparison, the intensity of the 1420 nm wavelength (background) is also presented. It is observed that the IR EL intensity decreases sharply with increasing device temperature from room temperature to about 50 1C after which the decrease is smaller. In contrast, the EL intensity at 1420 nm remains essentially unchanged. This suggests that only the EL peaks related to Er3+ ions are affected by the device temperature. It is further suggested that a higher device temperature results in an increase in the nonradiative recombination rate of generated phonons. During heating and cooling of the device, no hysteretic effect and no spectral shift of the IR EL was observed (not shown for brevity).

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Temperature (°C) Fig. 8. Infrared electroluminescence intensity of erbium-doped sparkprocessed silicon as a function of operating temperature at 1550 nm (peak wavelength) and 1420 nm (background wavelength) for 25 V bias. The inset depicts some pertinent I–V curves which relate to the main graph. (The arrows on the curves mark the approximate breakdown voltages for the different temperatures.)

The temperature dependence of the reverse current– voltage characteristics is depicted in the inset of Fig. 8. A positive temperature coefficient is observed, that is, the (negative) breakdown voltage increases with increasing temperature. (This is demonstrated in the inset of Fig. 8 by arrows.) Concomitantly, the device current increases at a given voltage with decreasing temperature (see inset). Now, it is known that impact ionization is characterized by a positive temperature coefficient. This suggests that impact ionization plays a decisive role in EL for erbiumdoped sp-Si.

3.1.5. Applied voltage and power Fig. 9 presents IR EL spectra for erbium-doped sp-Si measured at different excitation voltages. It is observed, as expected, that the IR EL intensity increases with increasing driving voltage. No spectral shift of the maximal peak wavelength is observed. The IR EL intensity at 1550 nm as a function of applied voltage is depicted in the inset of Fig. 9. It is observed that beyond a threshold voltage of about 3 V, the IR EL intensity increases rapidly with applied voltage and eventually levels off beginning at 25 V. The saturation of the IR EL intensity beyond 25 V is believed to be due to the finite number of available states into which electrons can be excited. A further increase in applied voltage (near 52 V) results in a breakdown of the device, as seen in Fig. 11. Fig. 10 shows the relationship between the IR EL intensity of erbium-doped sp-Si and the applied power. It is observed that this intensity sharply increases initially and then levels off. For explanation of this behavior, the same arguments as above can be put forward.

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Fig. 11. Current–voltage characteristics of erbium-doped spark-processed silicon. An n-type wafer having 5 O cm has been used. Pulsed DC with 200 Hz frequency and 30% duty cycle was utilized.

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3.2. Current–voltage characteristics Fig. 11 depicts the current–voltage characteristics of an erbium-doped sp-Si device. It is observed to be similar to the rectifying characteristics of a diode with the exception that the reverse current is larger than for an ideal rectifying device (near-zero current). It is further observed that the current increases substantially with applied negative voltage (quasi-rectifying I–V characteristics). At large values of reverse bias, the current increases dramatically and eventually displays breakdown. The breakdown voltage is seen in Fig. 11 to be about 52 V. At this voltage a current of 180 mA is flowing.

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Fig. 12. Current–voltage characteristics of erbium-doped spark-processed n-type silicon as a function of wafer resistivity. Selected wafers having 0.02, 1.5, 5.2, 13, 25, and 40 O cm have been used.

The current–voltage dependence on wafer resistivity is depicted in Fig. 12. Selected n-type Si wafers having resistivities of 0.02, 1.5, 5.2, 13, 25, and 40 O cm, respectively, have been used. Each sample was prepared, applying the same processing parameters and measuring conditions. It is observed that as the wafer resistivity increases, the reverse device current decreases rapidly. In fact, the device prepared on a 40 O cm silicon wafer depicts clearly a rectifying I–V characteristics. On the other hand, the device fabricated utilizing a 0.02 O cm silicon wafer is observed to have Ohmic-like I–V characteristics. In short, with increasing resistivity, the current–voltage characteristic gradually changes from

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Ohmic type to rectifying type. The IR EL intensity dependence on wafer resistivity is presented in Fig. 13 for identically spark-processed samples under 15 V bias. It is observed that the intensity initially decreases strongly with increasing wafer resistivity and eventually decreases to a lesser extent for wafers having intensity above 10 O cm. No EL was observed from the sample fabricated using a 0.02 O cm wafer. 3.2.1. Indium-tin-oxide (ITO) contact Finally, the Ag contact (Fig. 1) was replaced by an ITO layer which is known to be transparent as well as reasonably conductive, in order to elucidate if light emittance through the top surface would improve the efficiency. For this, an ITO film about 200 nm thick was laid down on the spark-processed, Er-doped, and annealed Si surface by pulsed laser deposition applying a bias of 20 V between target and sample. It is observed that the spectrum depicted in Fig. 14 bears little resemblance to the abovepresented spectra. Specifically, the peak position is shifted from 1550 to 1535 nm under same electrical excitation condition as for the Ag contact. Further, the peak is slimmer (8 nm FWHM compared to 44 nm FWHM for Ag contact) and the device is less stable, drains a higher current and displays an abrupt change in device current at 25 V. In short, an ITO contacted device yields less desirable features than that depicted in Fig. 1. It is proposed that the mechanisms for light emission is different for the two contact types. 4. Further discussion and conclusions The previous section has demonstrated that from Erdoped, sp-Si an EL device can be built which emits in the

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infrared (1550 nm) under reverse biased conditions, which entails that the Si substrate is biased positively. The process parameters that resulted in the most efficient EL of erbium-doped sp-Si were found to be 100 nm of initial erbium layer thickness, 30 s of spark-processing time, n-type wafer, and 3 O cm wafer resistivity. These parameters are a good indication that an enhancement of the number of optically active erbium ions are paramount for efficient EL [5]. For EL devices, the wafer resistivity plays an important role for current pathways. Specifically, a lower resistivity wafer enables a higher EL intensity (Fig. 13) indicating that the electrical characteristics of the bulk silicon are relevant to both, the conduction and the EL performance. In contrast, in the case of photoluminescence, the silicon wafer resistivity does not play a major role to affect the emission intensity [4]. The emission spectra in Fig. 9 shows that for erbiumdoped sp-Si neither a shift of the peak at 1.55 mm nor other erbium related peaks around 1.55 mm are observed when the bias voltage on the device is varied. Erbium-doped silicon-rich silicon oxide (SRSO), as well as silicon nanoclusters and silica also display EL in the breakdown regime and also only under reverse bias [6,7]. In contrast to this, erbium-doped crystalline silicon, porous silicon, and silicon nanocrystals reveal EL under both forward and reverse bias [5,7–11]. Some recent results involving nc-Si on erbium-doped SRSO or silica showed EL emission under forward bias [12,13]. Even though EL under forward bias is possible, the intensities of the latter are 10–20 times less than that under reverse bias [11]. Moreover, EL under reverse bias is more stable [5]. Luminescence mechanisms involving carrier-mediated processes and resonant Auger recombination [11,14] as well as exciton recombination [11,15] have been proposed as

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Fig. 15. Schematic representation of the possible nano-silicon particles (NSP) related Er3+ excitation mechanism in the EL process of an Au/ SRSO:Er/n+-Si system under reverse bias (after Ref. [6]).

possible mechanisms. Fig. 15 depicts a schematic representation of the possible nano-silicon particles (NSP) related Er3+ excitation mechanism in the EL process of an Au/SRSO:Er/n+-Si system under reverse bias. A similar mechanism could be considered for erbium-doped sp-Si. However, this mechanism occurs under carrier injection mode at forward bias. Thus, recombination of excitons is not the suggested mechanism for EL in erbium-doped spSi. Instead impact excitation may be the preferred pumping mode under reverse bias condition for erbium-doped sp-Si. The voltage, VB, for impact excitation, that is, avalanche breakdown in silicon generally follows the equation [16]: VB ¼

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where e is the dielectric permittivity, F is the breakdown field, q is the electron charge, and ND is the dopant concentration. The EL intensity dependence on wafer resistivity exhibits this behavior as seen in Fig. 13, thus suggesting avalanche breakdown. Further, the temperature dependence of the reverse characteristics exhibits a positive temperature coefficient as is typically observed for impact ionization.

Fig. 11 shows an I–V characteristic of an erbium-doped sp-Si device. The overall shape is semi-symmetrical with signs of rectification in reverse bias. In reverse bias, tunneling of electrons into the sp-Si may occur, especially at higher bias voltages, thus contributing to the current. The ‘‘softness’’ of the I–V characteristics for erbium-doped sp-Si EL devices is possibly due to field enhancement, caused by the surface features that constitute the halo region around the sparked spot. Indeed, the low threshold voltage for EL in erbium-doped sp-Si supports this suggestion. The higher conductivity of the erbium-doped sp-Si compared to the native oxide (SiOx/SiO2), could also be a contributing factor for current flow. The EL peak position of erbium-doped sp-Si was found to be different from that for PL. The difference in peak wavelength between infrared PL and EL may be the result of optically and electrically excited erbium centers in different microenvironments within the active layer. The different roles of the sp-Si in EL and PL processes may be responsible for this fact. For PL almost all photo-generated carriers stem from the sp-Si. Thus, the PL intensity is strongly affected by spark-processing. In contrast, for EL of erbium-doped sp-Si, the sp-Si related Er3+ excitation is just one of the various Er3+ excitation paths. Other possible Er3+ excitation processes for EL are: (1) injected electrons and holes. They can be captured by defects in Si and sp-Si, and their non-radiative recombination can excite Er3+; (2) the hot carriers generated in the breakdown process of the sp-Si impact Er3+ and excite them directly. One could argue that a generation of electron–hole pairs would occur due to the high electric field in the erbiumdoped layer. The energy released through recombination of these electron–hole pairs could in turn excite erbium atoms. However, no emission bands related to the recombination of electron–hole pairs in the wavelength range from 0.8 to 1.7 mm was observed [4]. Thus, the above suggestion is excluded. Moreover, the fact that no EL is observed in devices with ohmic characteristics makes it unlikely that a recombination of electron–hole pairs in nanoparticles is primarily responsible for the observed infrared light emission. The 1.55 mm EL is instead interpreted to be caused by erbium ions that are excited through collisions with energetic electrons and accelerated by the electric field. In other words, the excitation mechanism in these devices is very likely to be due to impact by energetic carriers. Moreover, the observation that a rectifying contact is necessary for electron injection into the spark-processed layer is indicative that a hot electron process is active. Whether erbium is excited directly by hot carriers impact or through energy transfer from sp-Si excited by hot carriers, is not distinguishable. In any case, sp-Si has a role in allowing a high current density to exist and hence an erbium excitation. The spark-processed layers can be used to generate hot electrons that are accelerated into the erbium-doped sp-Si by the large applied field. These electrons enable efficient

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impact excitation of Er3+ ions incorporated into the sp-Si. Assuming that the voltage is applied across the high resistive layer underneath the electrodes (about 200 nm), the electric field in this region is estimated to be about 100 kV/cm at the threshold voltage of 5 V, which is large enough for electron impact excitation of Er atoms to occur. Taking all presented results into account, it is proposed in Fig. 16 that in areas with sufficient field enhancement, facilitated by the existing surface features, electrons are injected with sufficient energy into the sp-Si, where they generate excited electrons by impact ionization. These excited electrons then return to the ground state via radiative and non-radiative pathways. These energies are transferred to the Er3+ luminescence center and excite electrons from the ground state (4I15/2) to the excited state (4I11/2 or 4I13/2). Finally, the energies dissipate radiatively to the ground state having a 1.54 mm emission. Above some critical field, the fixed positive charge created by impact ionization causes significant enhancement of the cathode field. This results in an increased electron injection and impact ionization, which eventually leads to a runaway current and destructive breakdown. Erbium-doped sp-Si EL devices have been found in this research to have an external power efficiency better than 0.007%. This value is about 2 orders of magnitude smaller than what has been reported for erbium-doped crystalline

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silicon devices [17]. The most successful results claim a roomtemperature emission with an external quantum efficiency of 0.1% in a megahertz-modulated Er3+-based LED [18]. In order to obtain efficient EL, not only the current should easily flow through the device and excite the optically active erbium centers, but also the medium itself should not interfere with the emission. To improve the emission intensity, various attempts for finding better suited materials as emission windows were made. sp-Si has a rough and porous surface. Thus, it is impossible to make a window, which is not covered with electric contact materials at the spark-processed side of the sample. A conducting material is needed, which can cover the entire area of light emission. ITO was considered as a contact electrode material, because of its transparency and reasonable conductivity. But ITO’s transparency occurs mostly in the visible range. For l ¼ 1550 nm, the index of refraction for ITO is n ¼ 1.28 and the damping constant is k ¼ 0.12. Using Eq. (2), Ref. [19], W¼

l , 4pk

(2)

it can be calculated that ITO has a characteristic penetration depth, W, of 1.02 mm at 1550 nm. For comparison, silicon with n ¼ 3.42 and kE0 at 1550 nm, the penetration depth is extremely large, that is, silicon is

Fig. 16. Suggested model for the 1.54 mm infrared EL mechanism. (1) Electrons are injected into the spark-processed silicon by tunneling. (2) Electrons are accelerated to high velocities leading to impact ionization. (3) Impact ionization results in the generation of excited electrons. (4) Excited electrons excite Er3+ from the ground state to the 4I11/2 or 4I13/2 state. (5) Electrons relax radiatively to the ground state from the 4I13/2 state.

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transparent in the infrared. Thus, silicon is utilized as an emission window in this research. In conclusion, erbium-doped sp-Si was found to exhibit EL at room temperature near 1550 nm having a weak temperature dependence of the EL signal. The devices were found to be stable with respect to operating time and environmental conditions. The parameters under which most efficient EL devices can be achieved have been established. Acknowledgments We are indebted to Wacker Siltronic (Dr. H. Ru¨fer) for supplying the Si wafers. We also thank Thierry Dubroca for building the infrared measurement system. References [1] R.E. Hummel, in: H.S. Nalwa (Ed.), Silicon-Based Materials and Devices, Vol. I: Materials and Processing, Academic Press, New York, 2001. [2] N. Shepherd, R.E. Hummel, Phys. Status Solidi A—Appl. Res. 197 (2003) 222. [3] H. Ennen, J. Schneider, G. Pomrenke, A. Axmann, Appl. Phys. Lett. 43 (1983) 943. [4] K. Kim, R.E. Hummel, J. Appl. Phys. 100 (2006) 043114.

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