The enhanced recombination and external quantum efficiency (EQE) of the multi-color metal-oxide-semiconductor light-emitting diodes (MOSLEDs) made on the SiOx film with buried Si quantum dots (Si-QDs) grown by plasma-enhanced chemical vapor deposition are demonstrated. By shrinking Si-QD size from 4.2 to 1.8 nm with increasing RF plasma power from 20 to 50 W, these MOSLEDs enhance the maximal electroluminescent (EL) power from 0.1 to 0.7 μW. This is mainly attributed to the enhanced recombination rate by enlarging the overlap between electron and hole wave-functions. As evidence, the photoluminescent lifetime is significantly shortened from 5 µs to 0.31µs due to the enhanced direct recombination in smaller Si-QDs. The corresponding power-current slope and EQE are observed to increase from 0.09 to 5.7 mW/A and from 1.9 × 10−5 to 2.4%, respectively. The EL enhancement originates from shorter wavelength and stronger carrier confinement within Si-QDs with smaller size, as confirmed by the increased barrier height at the ITO/SiOx:Si-QD interface from 1.05 to 3.62 eV. The smaller and denser Si-QDs result in a current endurance to operate the MOSLED at breakdown edge with highest power conversion efficiency, thus providing a maximal blue-light EL power at 0.7 μW with the highest EQE of 2.4%.
©2013 Optical Society of America
The Si-rich SiOx film with buried silicon quantum dots (Si-QDs) has attracted much attention in the past decade due to its potential application in metal-semiconductor-oxide light emitting diodes (MOSLEDs) with the CMOS compatible processes for all-Si based optical interconnect applications. The controversy on mechanisms of the light emission from Si-QDs has been solved due to the observation of the quantum confinement effect (QCE) and its coincidence with the modeling by the non-phonon assistant carrier recombination rate. Koch et al. and Prokes reported that the light emission of Si-QDs is based on the radiative recombination occurred at surface defect states surrounding the Si-QD core [1, 2]. Another possible mechanism has attributed the luminescence to the band-to-band transition with Si-QDs with QCE by Delley et al. . The latter one has been supported by the Si-QD size-dependent luminescence [4–6]. Park’s group employed a theoretical formula of E(eV) = 1.56 + 2.40/d2 to obtain the relationship between the photoluminescence (PL) peak wavelength and the Si-QD size for Si-QDs embedded in the Si-rich SiNx film . In addition, a similarly theoretical formula of E(eV) = 16.89 + 23.9/d2 for the PL of the Si-QDs embedded in the SiNx matrix with varying the Si-QDs size also be reported by Nguyen et al . In addition, de Boer and associates change the largest average size of Si-QD from 5.5 nm to 2.5 nm to make the PL detuned from 990 nm to 750 nm . Later on, the nearly coincident photoluminescence (PL) and electroluminescence (EL) of n+-Si/SiOx:Si-QDs/p+-Si LEDs were reported . The Si-QD related EL is more complicated due to the interactions among the confinement geometry, the valley degeneracy, the crystallographic orientations, and the quantum Stark effect on intraband transitions. Instead of the Si-QDs, the carriers trapped in defects may also contribute to the PL and EL at different wavelengths [8–10]. The weak-oxygen bond and neutral oxygen vacancy related defects cause the PL at 415-455 nm . In addition, the Si = O bonds at the interfaces between Si-QDs and SiOx matrix also contribute to the PL at 700-710 nm . The radiative oxide defects for Eu3+ doped in the SiOx films offer the PL ranging between 400 and 800 nm . Nevertheless, the quantum efficiency of the light emission from Si-QDs or radiative defects is strongly correlated with the carrier transport through SiOx layer [11–13]. The Si-QDs embedded in SiO2 layer could dominate the charge retention performance , which could be strongly affected by annealing [15, 16].
Recently, the research has been emphasized on the SiOx based MOSLEDs with broadband tunable EL wavelengths in visible region. Furthermore, the MOSLEDs with different matrices, such as SiNx, SiOxNy, etc, are demonstrated to enhance the carrier injection because these matrices have the lower barrier height [17–20]. The external quantum efficiency (EQE) of MOSLEDs suffers from a limited tunneling probability of carriers passing through the SiOx matrix. The EQE of Si-QD embedded in MOSLEDs of up to 0.2% was reported by several groups [21–24]. Marconi’s group employed the nanocrystalline-Si/SiO2 multilayers to fabricate the LEDs. The EQE of these devices is 0.2% with an operating voltage of 36 V . Lin and associates used Si nanopillars to enhance the Fowler-Nordeim tunneling and reduce the effective barrier height, providing that the EQE of MOSLEDs is up to 0.1% . In addition, they also detuned the size of Ni nanodots as the metal mask to form the different-sized Si nanopillars which control the PL wavelength ranging from 826 nm, to 874 nm . Under the pulse operation, Nishimura and associates reported a maximal EQE of up to 0.8% . Gelloz et al. further observed that the EQE of LEDs with the buried Si-QDs under the continuous-wave operation can be enhanced up to 1.1% by the post-anodized electrochemical oxidation of porous silicon . The EQE of up to 0.2% for the graded Si-QD LEDs with the Si-QD/SiO2 multilayer is achieved by Anopchenko et al. . Per’alvarez’s group observed that the EQE of MOSLEDs with the Si-rich SiOx film grown by ion implantation and PECVD are 10−3% and 0.1%, respectively . In addition, the recent work further reported that pointed that the higher barrier height at the ITO/SiOx:Si-QD interface and the lower capacity of carrier storage within Si-QDs for MOSLEDs could be prompt their power conversion ratio to 2.35 × 10−2% . Lin and associates further employed the nano-structures, such as nano-pillars, nano-pyramids, etc., to enhance the EL intensity of MOSLEDs [30, 31]. The EQE of MOSLEDs with an n-ZnO/SiO2-Si QDs-SiO2/p-Si heterostructure is improved up to 4.3 × 10−2% by Sun’s group . More recently, the EQE of Si-QD related EL device has been improved to 8.6% by using the organic layer as a host matrix [33, 34].
By properly tuning RF plasma powers under plasma-enhanced chemical vapor deposition (PECVD) and lengthening the annealing duration, the multicolor EL of Si-QD embedded in Si-rich SiOx MOSLEDs with variable Si-QD sizes are demonstrated, and a maximal EQE for the MOSLED containing smallest Si-QDs is reported. In this work, the significantly shortened PL lifetime and the mechanism related to the enhanced EL power are elucidated. Besides, the carrier tunneling and their correlation between the Si-QD sizes and the barrier height at ITO/SiOx interface are studied.
2. Experiment setup
The 450-nm thick SiOx films were deposited on (100)-oriented p-type Si substrates with a resistivity of 1 × 10−3 Ω-cm by a PECVD system under constant SiH4 and N2O fluences of 40 sccm and 125 sccm, respectively, at a chamber pressure of 100 Pa and a deposition temperature of 200°C. The RF plasma power is detuned from 20 to 50 W at the 10 W increment for changing the Si-QD sizes and the PL/EL wavelengths . Afterwards, the SiOx samples were encapsulated annealed in a quartz furnace with flowing N2 atmosphere at 1100°C for 2.5 or 90 min to induce Si-QDs precipitation. To fabricate a MOSLED, a 100-nm-thick indium tin oxide (ITO) film with a contact diameter of 0.8 mm was sputtered on the top of the SiOx surface, and a 200-nm-thick Al film was evaporated at the bottom of the p-Si substrate. In our previous work, the resistivity of the ITO films is improved to 1.2 × 10−4 Ω-cm after annealing at 450°C for 10 min . Therefore, all samples were post-annealed in a quartz furnace at 450°C under N2 ambient for 10 min to improve the ITO resistivity [36, 37]. Under the illumination with a 1-ns gain-switched GaN laser diode at wavelength of 405 nm, the full-band time-resolved PL (TRPL) analysis with detection wavelengths ranging from 410 nm to 900 nm was resolved by a photomultiplier (Hamamatsu R928) with switching response of 2-3 ns. The current-voltage (I-V) analysis of MOSLEDs was performed by using a programmable electrometer (Keithley, model 237). The room-temperature EL was measured by driving the MOSLEDs above the Fowler-Nordheim (F-N) tunneling threshold voltage. The EL power detection ranging from 300 nm to 900 nm in silicon integral sphere head (ILX, OMH-6703B) connected with power multimeter (ILX, OMM-6810B) was performed. The schematic diagram of SiOx MOSLEDs with buried Si-QDs is shown in Fig. 1 . By taking the SiOx sample with smallest Si-QDs as an example, the buried Si-QD size has a relatively uniform distribution within Si-rich SiOx films. The average size and the standard deviation of the Si-QD embedded in SiOx film decrease from 4.2 ± 0.4 to 1.8 ± 0.2 nm with the RF plasma power increasing from 20 to 50 W, respectively.
3. Results and discussion
Under forward bias, the electrons and holes inject into Si-QDs by F-N tunneling through the Si-rich SiOx host matrix from ITO and p-Si substrate, respectively. With the RF power increasing from 20 to 50 W, the turn-on electric field increases from 3.2 to 9.2 MV/cm and from 2.6 to 8.8 MV/cm after annealing for 2.5 and 90 min, respectively. The increasing Si-QD size could lead to an enhancement of the tunneling current. That is, the lower turn-on electric field is observed in MOSLEDs with enlarged Si-QDs. The compliance voltage dropped by lengthening the annealing time of SiOx film before fabricating MOSLEDs, which originates from the thickness shrinkage of SiOx film after long-term annealing. By defining a transmission coefficient of T(Ex) as a function of the energy Ex incident on the barrier at metal/oxide interface, and then summing up over all possible energies using the Wentzel-Kramers-Brillouin (WKB) approximation in the absence of Schottky effect with xti = 0Å (with ignored temperature variation and barrier shrinkage), the F-N tunneling current density, JF-N, can be expressed as :Fig. 2 can be extracted from the ln(JF-N/E2) vs. E plot, in which the intersection point of two fitted lines represents the turn-on electric field and the barrier height can be calculated from the descending slope.
With a forward (positive) bias applied from Al contact (p-type Si) to ITO contact (SiOx gate), electrons are injected from ITO gate contact and holes are injected from Al contact of the MOSLED device operated at an accumulation condition. With a relatively large bias added at p-type Si side, the band diagram of MOSLEDs is analogous to that of the p-type MOS diode operated at the accumulation state, as shown in Fig. 3 . With increasing RF plasma powers, the enhancing turn-on electric field accompanied with the enlarging interfacial barrier height from 1 eV to 3.6 eV is observed no matter annealing the SiOx at short or long duration, as shown in Fig. 2. The existence of buried Si-QDs leads to a decreased turn-on voltage of the F-N tunneling and creates a tunneling path for carriers from Si substrate to ITO contact. A larger RF plasma power suppresses the Si-rich condition to shrink the Si-QDs in more stoichiomatric SiOx ilm, which provides a larger interfacial barrier height for carrier tunneling from ITO to SiOx.
For F-N tunneling, the calculated barrier height is effective barrier height at the interface between ITO gate and the whole SiOx:Si-QD film. Therefore, the effective barrier height is determined by the material characterization of SiOx film. Even though, the barrier height of SiOx/ITO junction is smaller than that of a pure SiO2/ITO junction (3.7 eV), indicating that the O/Si composition ratio is still below 2.0. The band diagrams of the MOSLEDs with Si-QDs embedded in the SiOx grown with different RF plasma powers are illustrated in Fig. 3, where the barrier height between ITO and SiOx grown with different RF plasma powers are depicted in the inset of Fig. 3. For example, the barrier heights of MOSLEDs with buried Si-QDs with sizes of 4.2 nm and 1.8 nm are 1.05 and 3.62 eV, respectively. It is straightforward that the band diagram bends more in smaller size of Si-QDs, indicating that a larger external electric field is needed to turn on the F-N tunneling mechanism in the MOSLEDs with smaller Si-QDs embedded in the SiOx. In our case, the O/Si composition ratio of the SiOx film enlarges with increasing RF plasma power. In addition, the excess Si concentration has a significant effect on the turn-on voltage of MOSLEDs . The less excess Si in the SiOx film provides fewer tunneling paths to make the current enhance. The SiOx film also enlarges its resistance with increasing O/Si composition ratio because the SiOx film gradually becomes non-stoichiometric to approach a pure SiO2 matrix. Therefore, the carriers need a higher turn-on voltage to be injected into Si-QDs. Moreover, the effective barrier height is varied with changing Si-QD size. That is because the smaller Si-QDs decrease the effective dielectric constant and enhance the barrier height of F-N tunneling to degrade overall tunneling probability .
Pavesi and Turan have simulated the individual states of the electron and hole energy levels with increasing Si-QD size. Their results have shown that the variation of conduction band is more than that of valence band for Si-QD size of smaller than 6 nm, and the quantum confinement effect of Si-QDs larger than 6 nm gradually diminishes . These results clearly shown that the Fermi level in Si-QD moves upward with decreasing Si-QD size. The conduction band moves upward with shrinking Si-QD size, whereas the valence band moves downward with a smaller shift. Their simulation supports an upward movement of the Fermi level for smaller Si-QDs. Without bias, the larger bending degree of energy band at the Si-QD/SiO2 interface is observed for samples with smaller Si-QDs grown at higher RF plasma powers.
Alternatively, there is another simulation work attributing the increasing interfacial barrier height of Si-QDs with decreasing size to the down-shifted Fermi level, from which the difference between the Fermi level of Si-QDs and that of ITO decreases making the bending degree of the energy band less significant without external bias. The Fermi level would down-shift by shrinking Si-QD size because the asymmetrical shift of the conduction band and the valence band of Si-QDs. Due to the quantum confinement effect, the up-shift of the conduction band toward the vacuum band is less significant (actually, this shift can be ignored) than the down-shift of the valence band when decreasing the Si-QD size . This results in an enlarged energy band (Eg) with down-shifted Fermi level for smaller Si-QDs, providing an enlarged interfacial barrier height by decreasing the Si-QD size. With the decrease of the size of Si-QDs by enlarging the RF power or lengthening the post annealing time, the Fermi level of Si-QDs down-shifts and the difference between the Fermi level of Si-QDs and that of ITO decreases making a less significant bending degree of energy band. Correspondingly, the effective barrier height ФB increases to assist carriers F-N tunneling from ITO to Si-QDs through SiO2 matrix. This inevitably induces the increased turn-on electrical fields (Eturn-on), as shown in Fig. 2.
Note that both models can successfully elucidate the decrease of the effective barrier height by enlarging Si-QD size and density results in a better carrier injection (it also depends on the resistance of the films). The overlap between electron and hole wave functions in both real and momentum space will increase in smaller Si-QDs with a higher probability of non-phonon assisted radiative recombination. Furthermore, the density of Si-QDs with smaller sizes is larger than that with larger sizes. Hence, the number of the active Si-QDs contributing to the luminescence is also increased as the size of Si-QDs reduces. The aforementioned mechanisms explain why a lower carrier injection as well as smaller current could lead to a larger external quantum efficiency and higher EL (see Fig. 5) observed from smaller Si-QDs even with a larger turn-on electrical field Eturn-on, as shown in Fig. 2.
The full-band TRPL from 410 nm to 900 nm for all SiOx:Si-QDs have been analyzed. In principle, the full-band TRPL trace of the SiOx:Si-QDs with a continuously distributed radiative lifetime is contributed by all Si-QDs with different sizes. Because of the inhomogeneous broadening luminescence of Si-QD, the full-band TRPL decay traces of SiOx:Si-QDs should be analyzed by using a multi-exponential decay function shown in Eq. (1). However, the discrete multi-exponential decay function cannot perfectly fit the TRPL trace due to the continuous Si-QD size distribution. Therefore, the stretched exponential function is employed to evaluate the lifetime dispersion effect of the SiOx:Si-QDs sample, as given byFig. 4(a) . After lengthening the annealing time up to 90 min, the TRPL lifetime is lengthened by 2-3 times due to the enlarged Si-QD size after precipitation. Although the oxygen-related defects also induce another nanosecond blue-band emission mechanism, it can be excluded since its TRPL lifetime is as short as 10 ns [8, 43]. The shortening of TRPL lifetime by two orders of magnitude is strongly correlated with the shrinking Si-QD size from 4.2 to 1.8 nm. In smaller Si-QDs, the TRPL lifetime is significantly shorter because of a better overlap between wave functions of electron and hole states due to quantum confinement effect. A non-phonon assisted electron-hole recombination procedure within narrower momentum overlapped region occurs to provide higher recombination rate with enhanced probabilities of the direct transition and the increased EQE . This explains the observation of the higher EQE in blue-light MOSLED. This evidence further supports the direct radiative recombination in Si-QD core with a strong quantum confinement effect. For the SiOx:Si-QDs samples grown by increasing RF plasma powers, the lifetime dispersion factor (β) increases from 0.63 to 0.75 and from 0.61 to 0.7 after 2.5-min and 90-min annealing, respectively. The enhanced lifetime dispersion phenomenon occurs with enlarging Si-QD size distribution, thus providing a reducing value of the lifetime dispersion factor b. For example, the dispersion factor of 0.63 for SiOx samples grown with the RF plasma power of 20 W has a radiative lifetime ranged between 7 and 24.5 μs. In contrast, the lifetime dispersion shrinks to range between and ns for the SiOx samples grown with the RF plasma power of 50 W.
When increasing RF plasma powers from 20 to 50 W, the maximal EL power of the Si-QDs embedded in SiOx MOSLEDs made by the 2.5 min annealed SiOx samples slightly enlarges from 7.5 to 75 nW. In contrast, the EL power significantly enlarges by one order of magnitude when lengthening the annealing time of SiOx films from 2.5 to 90 min. As a result, the maximal EL power can be enlarged up to 0.7 μW if the RF plasma power increases to 50 W and the annealing time lengthens to 90 min. The corresponding power-current slope also increases from 0.05 to 73.87 and 0.09 to 5.7 mW/A for the MOSLEDs with SiOx films annealing at 2.5 and 90 min, respectively. Such an enhancement can be attributed to either the improved carrier transport under an enhanced tunneling environment, or the increasing Si-QD density in SiOx films grown at larger RF plasma and annealed at lengthened duration. To further discriminate the main contribution from two possibilities, the time resolved PL has been performed for obtaining the lifetime and recombination rate of carriers in the SiOx samples prepared at different recipes . The increasing trend of optical power-current slope is strongly correlated with the increment of Si-QD density, which is proportional to the normalized PL intensity per thickness, indicating the increasing number of Si-QDs. However, the turn-on currents of MOSLEDs with the 2.5 min and 90 min annealed SiOx samples decrease from 53.9 to 0.2 μA and 357.9 to 8.6 μA, respectively. The electric field across the SiOx film beyond turn-on is dominated by F-N tunneling mechanism. The smaller Si-QD provides a deeper quantum well to enlarge the barrier height between the conduction band of Si-QDs and oxide which leads to a better confinement for carriers, yet the recombination rate is still limited by the less overlapped wave-functions between electron and hole. The P-I curves also show the degradation of current endurance in Si-QDs based MOSLEDs grown with the decreasing RF power, which is related to the high turn-on nearly the breakdown edge of SiOx. By defining the power conversion ratio (PCR) as ηPCR = Popt/Ibias × Vbias with Vbias, Ibias, and Popt denoting the biased voltage, the biased current and the constant optical output power, respectively. The PCR of these devices increases from −67 dB to −38dB after annealing.
In more detail, the EQE of the MOSLEDs grown with different RF plasma powers are plotted as a function of current density as shown in Fig. 5 , which is defined as the ratio of the output photon number to the input electron number,Fig. 5. That is because the MOSLEDs under an operation of higher current injection easily contribute to a rise on device temperature. In addition, the impact ionization of hot carriers for devices easily occurs under an operation of the higher electric field . The additional kinetic energy makes the electron-hole pairs separate from but not recombine with each other. Therefore, this phenomenon also contributes to the EQE degradation of MOSLEDs under the higher biasing current and electric field. In addition, the EQE of MOSLEDs also has a reduced phenomenon when the recombination mechanism is dominated by the defect or Auger recombination. The dominated recombination could be determined by the Z-parameter analysis [46, 47]. The Eq. (3) is based on the hypothesis of the Boltzmann statistics of carriers and an absence of leakage currents . The Z-parameter could be obtained by the P-I curve of MOSLEDs, which is described as
The Z-parameter varies between 1 and 3 for different recombination mechanisms. Typically, the Z parameters are 1, 2, and 3 for the defect, radiative, and Auger related recombinations, respectively. The non-integer Z-parameter results from the mixed contribution of two different recombination mechanisms. For example, the Z-parameter between 2 and 3 represents the mixed contribution of the radiative and Auger related recombinations. When Z-parameter gradually approaches the specific integers, it represents that the specific recombination mechanism of MOSLEDs is gradually dominated. Figure 6(a) shows the Z-parameter as a function of biased currents for the MOSLEDs with 2.5-min annealed SiOx samples grown at different RF plasma powers, respectively. At lower bias currents, the dominated recombination for MOSLEDs with 2.5-min annealed SiOy grown at different RF plasma powers is radiative recombination with a corresponding Z-parameter of approximately 2. However, the Z-parameter gradually approaches 3 at extremely high bias, indicating that the Auger recombination becomes to dominate MOSLEDs. The Z-parameter for MOSLEDs with 90-min annealed SiOx samples has a similar trend, as shown in Fig. 6(b). In particular, the Auger recombination becomes more significant for the MOSLEDs with 90-min annealed SiOx grown at different RF plasma powers. The Auger recombination even starts at lower biased currents. Therefore, the EQE of MOSLEDs with 90-min annealed SiOx samples is apparently lower than that of devices with 2.5-min annealed SiOx samples.
The annealing time dependent EL spectra blue-shifts with enlarging RF plasma power, as shown in Fig. 7 . The biased current of MOSLEDs with SiOx films grown at larger RF plasma powers is decreased, because the O/Si composition ratio of the SiOx layer is increased dramatically to make the host matrix seriously isolated. The required turn-on current significantly reduces from 53.9 to 0.2 μA as the RF plasma power enlarges from 20 W to 50 W. In opposite, the EL intensity is enlarged by 40 times for the blue-light MOSLED even biases at smaller current. This is strongly correlated with the optical power-current slope discussed previously. For the MOSLED made by low-plasma grown SiOx with buried Si-QDs, two peak wavelengths of 480 and 681 nm are observed. After 90 min annealing, the latter peak has a red-shifted phenomenon due to the QCE. These two peaks compete and evolute each other as the biased current increases. The growth of SiOx films at higher RF plasma powers enables the precipitation of smaller Si-QDs in SiOx, which provides the EL color shifted from red to blue color even with short-term annealing. In particular, the long-term annealing makes Si atoms obtain more energies to diffuse a longer length to form larger Si-QDs, hence the corresponding EL spectrum slightly broadens toward long wavelengths and attenuates its EL intensity at shorter wavelengths accordingly.
The carrier transport and recombination through small Si-QDs are less efficient than those through large Si-QDs, because the less Si-rich SiOx host environment inevitably degrades the tunneling of carriers and the probability of tunneling through small Si-QDs is significantly reduced. With enlarging biased current, the EL contributed by larger Si-QDs gradually saturates and the overflowed carriers lead to the enhanced recombination in smaller Si-QD, as shown in Fig. 8 . The inset of Fig. 8 depicts the linear relationship between the EL intensity and biasing currents. The output power as a function of biased currents can be written as PEL = ηext × Ibias × (1.24/λEL). For MOSLEDs with buried Si-QDs of different sizes, ηext and λEL are varied with the Si-QD sizes. The same phenomenon between EL intensity and biased voltages has been reported by Irrera et al . The large-size Si-QD dependent EL grows and saturates earlier at lower biased condition, yet the EL power linearly increases before its saturation at higher biases.
The color of EL pattern shown in Fig. 9 varies from red to blue by increasing RF plasma powers during PECVD growth. Growing the SiOx at low RF power, the EL pattern is dark because of a low EL power for larger Si-QDs due to the greatly reduced wave-function overlap and recombination rate of electrons and holes. In contrast, the EL patterns become brighter with steeper P-I slope for MOSLEDs made on the SiOx prepared with lengthening annealing time. The long-term annealing provides brighter patterns due to the endurance of these MOSLEDs under higher current injection. The EL pattern of a 40-W grown SiOx based MOSLED reveals slightly green at lower bias but becomes a white-light pattern at higher current due to the EL spectral broadening and blue-shift effects. The brightest EL is observed from the MOSLEDs with smallest Si-QDs embedded in SiOx. The MOSLED with the 2.5-min annealed SiOx film shows a purely blue color but that with the 90-min annealed SiOx film varies to a light blue pattern due to the contribution of the slightly larger Si-QDs.
The carrier recombination rate and external quantum efficiency enhanced multicolor emission from SiOx based MOSLEDs with buried Si-QDs are demonstrated. With the RF plasma power increasing from 20 to 50 W during PECVD growth, the Si-rich condition is controlled to shrink the Si-QD size in non-stoichiomatric SiOx films and to provide a large interfacial barrier height for carrier confinement within small Si-QDs. The MOSLED made on a 2.5-min annealed SiOx film with buried Si-QDs slightly enlarges its maximal EL power from 7.5 to 75 nW. In contrast, the EL power significantly enlarges by one order of magnitude to 0.7 μW when the annealing time lengthens up to 90 min. The TRPL analysis reveals that the PL lifetime shortens from 5 µs to 0.31µs with shrinking the Si-QD size from 4.2 to 1.8 nm. This is attributed to a larger overlap between electron and hole wave functions, which essentially leads to a faster non-phonon-assisted direct carrier recombination in smaller Si-QDs. The turn-on currents of MOSLEDs after annealing for 2.5 and 90 min decrease from 53.9 to 0.2 μA and from 357.9 to 8.6 μA, respectively. The smaller Si-QD provides a deeper quantum well to enlarge the barrier height at ITO/SiOx interface, which leads to a better carrier confinement for carriers at cost of a less electron-hole wave-function overlap. The EQE of the Si-QD embedded in SiOx MOSLED grown with enlarging RF plasma powers significantly increases from 1.9 × 10−5 to 2.4% after annealing for 2.5 min. After annealing up to 90 min, the same devices slightly degrade the EQE by one order of magnitude but enlarge the EL power due to the enhanced carrier injection. The growth at higher plasma powers enables the precipitation of smaller Si-QDs in SiOx, which provides the EL color shifted from red to blue even with short-term annealing. In particular, the long-term annealing makes Si atoms obtain more energies to diffuse the longer length to form larger Si-QDs so that the EL spectrum slightly broadens toward long wavelengths and attenuates its EL intensity at shorter wavelengths accordingly. Such an all Si-based multicolor MOSLED with enhanced on IQE and EQE is fully compatible with current Si fabrication process, which could be used as an on-chip transmitter in the next-generation optical interconnect network to improve the chip-to-chip transmission performance.
This work was partially supported by the National Science Council, Taiwan, R.O.C. and the Excellent Research Projects of National Taiwan University, Taiwan, R.O.C., under grants NSC 100-2221-E-002-156-MY3, NSC 101-2622-E-002-009-CC2, NSC 101-ET-E-002-004-ET and 99R80301.
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