Blue Electroluminescent Al₂O₃/Tm₂O₃ Nanolaminate Films Fabricated by Atomic Layer Deposition on Silicon

Abstract

Realization of a silicon-based light source is of significant importance for the future development of optoelectronics and telecommunications. Here, nanolaminate Al₂O₃/Tm₂O₃ films are fabricated on silicon utilizing atomic layer deposition, and intense blue electroluminescence (EL) from Tm³⁺ ions is achieved in the metal-oxide-semiconductor structured luminescent devices based on them. Precise control of the nanolaminates enables the study on the influence of the Tm dopant layers and the distance between every Tm₂O₃ layer on the EL performance. The 456 nm blue EL from Tm³⁺ ions shows a maximum power density of 0.15 mW/cm². The EL intensities and decay lifetime decrease with excessive Tm dopant cycles due to the reduction of optically active Tm³⁺ ions. Cross-relaxation among adjacent Tm₂O₃ dopant layers reduces the blue EL intensity and the decay lifetime, which strongly depends on the Al₂O₃ sublayer thickness, with a critical value of ~3 nm. The EL is attributed to the impact excitation of the Tm³⁺ ions by hot electrons in Al₂O₃ matrix via Poole–Frenkel mechanism.

1. Introduction

Traditional electronic integrated circuits have been facing with a bottleneck in terms of power consumption, speed, and signal crosstalk as the communication frequency and bandwidth rise to a higher level. One possible solution is the optoelectronic integration which realizes photonic technologies on silicon chips [1,2,3,4]. However, applicable Si-based light sources have been unsolved for a long time. Rare earth (RE) ions are generally efficient luminescence centers in various matrixes. Nowadays diverse RE-doped insulating materials have been developed for the applications in solid state lasers and phosphors [5,6,7,8,9]. However, it has been widely known that the mismatch in the coordination structure and atomic size of silicon (tetrahedron) and RE ions (octahedron) limit the desired spectroscopic performance due to the clustering of RE ions in the Si host [10,11]. Aiming for the realization of compact Si-based optoelectronics, electroluminescence (EL) from RE³⁺ ions has been extensively reported in many compounds, such as SiN_x, TiO₂, and ZnO [12,13,14,15]. However, the efficiencies of the devices based on the aforementioned materials are far from practical utilization. One of the limitations is the large leakage current. RE-implanted SiO₂ MOS-structured light-emitting devices (MOSLEDs) have attracted much attention due to their notable EL efficiency and silicon compatibility [16,17]. In comparison, similar devices based on Al₂O₃ nanofilms present much lower working voltage, and comparable efficiency in our previous study, while their EL performance needs more exploration [18,19]. Blue emission, which has the highest photon energy (2.6–2.7 eV) of the three primary colors, is of great importance in display and lighting. Tm³⁺ ions have present efficient blue emissions in various matrixes including ZnS, ZnO, fluorophosphate, and many other oxides and fluorides [20,21,22,23]. The reported achievements are mostly focused on photoluminescence (PL), by virtue of upconversion to convert infrared photons to blue emission [20,24]. For practical application, electrically excited devices are urgently needed. Whether high-energy blue photons can be generated in this prototype device is still unknown. Using Tm-doped Al₂O₃ might exploit the merits of both oxides to realize efficient blue EL from Tm³⁺ ions.

Atomic layer deposition (ALD) is a monatomic vapor deposition technique achieved by alternating saturated gas–surface reactions, based on which the film can be deposited in a self-limited growth mode and exhibits superior homogeneity and excellent uniformity [25,26,27,28,29]. This technique supplies a convenient way to devise nanolaminates with optimal performance. In this work, we fabricate nanolaminate Al₂O₃/Tm₂O₃ films which function as blue EL layers in the Si-based MOSLEDs. The EL intensity and decay lifetime are compared by changing the Al₂O₃ or Tm₂O₃ sublayer cycles. The influence of the Tm clustering and interaction concerning the Al₂O₃ or Tm₂O₃ cycles are explored respectively. The 456 nm blue EL from Tm³⁺ ions shows a maximum power density of 0.15 mW/cm². The device characteristics are in good consistence with the previous reports on the excitation mechanism and the critical interlayer thickness for the cross-relaxation among adjacent dopant layers.

2. Materials and Methods

The nanolaminate Al₂O₃/Tm₂O₃ films were grown on <100>-oriented phosphorous–doped silicon (n-Si) substrates with the resistivity of 2–5 Ω·cm and a thickness of 500 μm (CETC-46 Ltd., Tianjin, China), which were cleaned through the standard RCA process before growth. The ALD equipment was a 4-inch chamber system (Nano Tech Savannah 100, Cambridge, MA, USA). Trimethylaluminum [TMA, Al(CH₃)₃, 99.999+%] and Tm(THD)₃ (THD = 2,2,6,6-teramethyl-3,5 heptanedionate, 99.9%, Strem Chemicals, Inc., Newburyport, MA, USA) were used as the metal precursors for Al₂O₃ and Tm₂O₃, while ozone was used as the oxidant. N₂ was used as the carrier and purge gas with a flow rate of 20 sccm. During the growth, the pulse time of TMA, Tm(THD)₃, and ozone was 0.015 s, 2 s, and 1.8 s, respectively. The TMA was maintained at room temperature while the Tm precursor was heated at 170 °C. The pipelines and the substrates were maintained at 190 °C and 325 °C. The growth rates for the Tm₂O₃ and Al₂O₃ films were 0.216 Å/cycle and 0.79 Å/cycle, respectively.

In order to investigate the luminescent characteristics of nanolaminate Al₂O₃/Tm₂O₃ films, a series of devices concerning the Tm₂O₃ dopant cycles and the Al₂O₃ interlayer cycles were fabricated as shown in

Table 1. The corresponding experimental parameters for clarity.

Sample Label	Tm3+ (at%)	Al2O3:Tm2O3 Cycle Number
AOT-1	0.69	13:1
AOT-2	1.37	13:2
AOT-4	2.64	13:4
AOT-6	3.83	13:6
AOT-8	4.95	13:8
AOT-d05	2.46	7:2
AOT-d1	1.37	13:2
AOT-d2	0.69	26:2
AOT-d3	0.45	40:2
AOT-d4	0.35	52:2
AOT-d6	0.23	78:2

. The total cycle numbers were adjusted correspondingly to obtain the luminescent films with a thickness of ~50 nm. The thickness of the film was measured by an homemade ellipsometer with a 632.8 nm He-Ne laser at an incident angle of 69.8°. As the thickness variation from the designed value for the nanolaminates are quite small (less than 3%), the nominal Tm concentrations are used to quantify the doping levels. All Al₂O₃/Tm₂O₃ films were subsequently annealed at 800 °C in N₂ atmosphere for 1 h to reduce defects and activate Tm³⁺ luminescence. Then, 120 nm TiO₂/Al₂O₃ nanolaminate films consisting of 2 nm Al₂O₃ and 8 nm TiO₂ sublayers were grown by ALD on Al₂O₃/Tm₂O₃ films as the protective layers. Afterwards, ~100 nm ZnO:Al₂O₃ films were grown by ALD as the transparent conductive electrodes, which were lithographically patterned into 0.5 mm circular dots. Finally, 100 nm Al electrodes were deposited on the back side of the Si substrates by thermal evaporation, and annealed afterwards in vacuum at 250 °C for 0.5 h to realize ohmic contact.

The PL spectra from the luminescent nanolaminates were excited by a 355 nm laser. For EL and Current–Voltage (I–V) measurements, the devices were activated by means of a Keithley 2410 SourceMeter unit (Keithley Instruments Inc., Cleveland, OH, USA), with the negative voltage connecting to n-Si substrates. The PL and EL signals were detected by a monochromator (Zolix λ500, Zolix Instruments Co., Ltd, Beijing, China) and a Si photomultiplier connected to a Keithley 2010 multimeter (Keithley Instruments Inc., Cleveland, OH, USA). Photographic images were collected by a digital camera through a 20-fold objective microscope. Time-resolved photoluminescence (TRPL) was measured by a SR430 multi-channel scaler (Stanford Research Systems Inc., Sunnyvale, CA, USA) with a 355 nm laser working in the pulse mode. The decay lifetime of the EL emission was measured by the SR430 multichannel scaler, excited by a high-voltage amplifier equipped with a digital function signal generator (DG5072, RIGOL Technology Co., Ltd, Beijing, China). All the above measurements were performed at room temperature.

3. Results and Discussion

The Tm₂O₃ films deposited by ALD can be crystalized into Tm₂O₃ phase even without annealing, while the Al₂O₃ films are amorphous after annealing at 800 °C. However, the nanolaminate Al₂O₃/Tm₂O₃ film with the highest Tm content (AOT-8) is amorphous after annealing at 800 °C, therefore the nanolaminate structure restricts the grain growth of the dopant Tm₂O₃ layers. Figure 1a shows the PL spectra from the nanolaminate Al₂O₃/Tm₂O₃ films. The PL peaks at 456 nm are attributed to the transition of ¹D₂→³F₄ in Tm³⁺ ions [20,21,22]. The inset of Figure 1a presents the comparison of the PL intensities of all samples, which decrease with the Tm₂O₃ dopant layers. Due to the common cluttering characteristics of RE ions, with the increase of Tm content, the number of activated Tm³⁺ ions decreases and the cross relaxation between Tm³⁺ ions further reduce the radiative transitions [30,31]. For TRPL results shown in Figure 1b, the decay lifetime of these PL emissions from Tm³⁺ ions also decreases with the Tm content, which coincides with the PL intensities. The inset gives the fitting values of the PL decay lifetime, which are in the range of 0.13–1.25 μs. The PL decay lifetime decreases rapidly as the Tm dopant layers rise to 4. The cross relaxation and concentration quenching contribute to the nonradiative recombination and decrease the luminescence lifetime.

The schematic for the multilayered devices is shown in Figure 2a. The EL spectrum from the MOSLED based on the Al₂O₃/Tm₂O₃ nanolaminate with 2 cycles of Tm dopant (AOT-2) is presented in Figure 2b. The EL emissions mainly exhibit several peaks at the wavelengths of 368, 456, 474, and 802 nm, which originate from the radiative transitions from the ¹D₂, ³F₄, ¹G₄, and ³H₄ excited states to the ³H₆ ground state in Tm³⁺ ions, respectively, as sketched in the inset of Figure 2b [21,22,23]. It is noteworthy that the EL emissions at 456 nm and 474 nm are dominating and the blue light is easily seen by naked eyes, as shown in Figure 2c. These images were taken by a digital camera from this AOT-2 MOSLED at different injection currents. The blue EL emission gradually brightens with the increase of the injection current from 10 µA to 80 µA.

Figure 3a shows EL spectra from the MOSLEDs based on the Al₂O₃/Tm₂O₃ films with different Tm dopant cycles at an injection current of 5 µA. The concentrations of Tm dopant are from 0.69% to 4.95%, respectively. The spectra exhibit four peaks at 368, 458, 474, and 802 nm as mentioned above. The inset shows that the 456 nm blue EL intensity increases with the Tm dopant cycles up to 2 and then decreases due to concentration quenching. The EL presents higher tolerance for Tm clustering than the PL performance. The dependence of the 456 nm EL power density on the injection current density are shown in Figure 3b. Generally, the EL intensity presents a linear relationship with the injection current density. A power density up to 0.15 mW/cm² was obtained from the optimal MOSLED at a current density of 2.87 A/cm². Initially, the EL output power density increases as the Tm dopant cycles increases to 2, due to the increase of the excitable Tm³⁺ ions. The further decline of the power density with the Tm dopant cycle is attributed to the clustering and cross relaxation which reduce the number of excited Tm³⁺ ions [30,31]. The efficiency and output power are lower than the previously reported devices based on the Tb and Yb doped Al₂O₃ nanolaminates [18,19]. As the energy of the blue photon is higher than that of the green EL from Tb³⁺ ions and the near-infrared one from Yb³⁺ ions, the excitation possibility of the radiative transitions within Tm³⁺ ions should be lower which leads to the limited efficiency and output power. In addition, the visible EL from the RE-doped SiO₂ is stronger than the devices in this work [32]. The higher working voltage needed for luminescence in SiO₂ evidences the necessity of high electrical field for excitation of the photon with higher energy, which is adverse to practical application. However, this EL output power density is superior to the EL devices based on the RE-doped ZnO as the leakage current is greatly restricted comparatively [13].

Figure 4a,b shows the dependence of blue (456 nm) EL intensities, together with the injection current, on the applied voltages for the nanolaminate MOSLEDs based on different Al₂O₃/Tm₂O₃ films. All devices exhibit a typical I–V characteristic of the MOS structure, i.e., the current starts with a low background one under the low electric field, then exponentially increases with the voltage [16,17,18,19]. The difference on the leakage currents mainly depends on the process of device procedures, coming from the electrons hopping through the defects within the matrix. At this stage, no hot electrons are generated in the Al₂O₃/Tm₂O₃ conduction band with no EL emissions. Afterwards, the injection current increases exponentially with the applied voltage and the conduction mechanism is dominated by the Poole–Franked (P–F) mode until the device breakdown [18,19]. In the P–F conduction mode the plot of the ln(J/E) versus E^1/2 features a linear relationship (J is the current density and E is the electric field). As shown in Figure 4c, for all Al₂O₃/Tm₂O₃ MOSLEDs the P–F plots work in the EL-enabling voltages, with the threshold voltage of around 40 V (~3 MV/cm). The slopes of the linear plots of the P–F injections are similar while the little difference is caused by the slight variation of the injection current as mentioned above. Therefore, for the EL excitation, electrons are firstly injected into the conduction band of Al₂O₃ by trap-assisted tunneling and accelerated to gain energy under high electric field. These hot electrons excite the doped Tm³⁺ ions from the ground state to higher levels by inelastic collision. After the nonradiative relaxation, the radiative transitions in the Tm³⁺ ions from the excited state to ground state generate the characteristic EL emissions [20,21,22].

The EL decay lifetime of the 456 nm EL from different nanolaminate Al₂O₃/Tm₂O₃ MOSLEDs is measured under pulse excitation mode. The decay curves are shown in Figure 5a, which are close to the single exponential decay function. The decay lifetime decreases from 4.02 µs to 0.53 µs with the increase of Tm dopant cycles, as shown in Figure 5b. These values of EL decay lifetime are several times larger than that of PL decay lifetime shown in Figure 1b, and keep decreasing with the Tm doping concentration, which comes from the cross relaxation and concentration quenching caused by the excess Tm³⁺ ions. These phenomena again mean that the tolerance on the concentration quenching in EL performance is higher than that in PL.

In the RE-doped Al₂O₃ MOSLEDs, the Al₂O₃ sublayer thickness affects the cross relaxation between excited RE ions, and the acceleration distance for injected electrons. In order to investigate the effect of the distance between Tm₂O₃ dopant layers, a series of MOSLEDs were fabricated in which the Al₂O₃ sublayer thickness varied from 0.5 nm to 6 nm while the Tm dopant cycles was fixed at 2. Figure 6a shows the dependence of the blue EL intensity on the injection current. Here, the EL intensities are divided by the cycle numbers to present the emissions from every Tm dopant cycle. With the increase of the thickness of Al₂O₃ sublayer, the contribution of a single Tm dopant cycle to the EL intensity firstly increases and then saturates as the Al₂O₃ interlayer thickness reaches 3 nm. Figure 6b presents the tendency. This phenomenon has been observed in our previous reports with a similar value, concerning the nonradiative interaction among excited RE³⁺ ions and the acceleration distance for the injected electrons [18,19]. Therefore, it is a common characteristic for the luminescent RE³⁺ ions in an Al₂O₃ matrix that the distance for the presence of nonradiative interaction and adequate electron acceleration is around 3 nm.

Furthermore, the decay lifetimes for these MOSLEDs are shown in Figure 6c, whose correlation with the Al₂O₃ interlayer thickness is summarized in Figure 6d. Similar to the EL intensity, the decay lifetime increases from 1.18 to 7.41 μs with the Al₂O₃ interlayer thickness increasing from 0.5 nm to 3 nm, and saturates at higher distances. The reduction of the decay lifetime at higher Tm doping concentrations is still ascribed to the increase of nonradiative cross relaxations between the two closely located Tm³⁺ dopant layers as mentioned above, with the similar critical Al₂O₃ interlayer thickness of 3 nm [19,33]. Considering the total EL intensities, the optimal Al₂O₃ interlayer thickness in these MOSLEDs is 2 nm. It should be noted that there is little difference between the total EL emission from nanolaminate Al₂O₃/Tm₂O₃ MOSLEDs with 1 nm and 2 nm Al₂O₃ interlayers. The effect of more dopant ions is offset by the relative lowered excitation efficiency. This optimal doping concentration is also consistent with previous reports (around 1 at%) on the RE doped luminescent materials [18,33].

The blue EL intensities (output powers) from our prototype devices are quite low and incapable of practical application. This work confirms the potential to realize blue EL from Al₂O₃/Tm₂O₃ nanolaminates by ALD. Moreover, the devices are fabricated entirely by ALD, which is characterized by the precise control of the film deposition over large substrates, and the compatibility with Si-based CMOS technology. Therefore, MOSLEDs based on Al₂O₃/Tm₂O₃ nanolaminates can be easily expanded for mass-production. The challenging deficiencies are the low EL efficiency and output power, the high working voltage, and the limited injection current. Further optimization can be achieved by adopting a thicker Al₂O₃/Tm₂O₃ luminescent layer with more optimal dopant structure and a less resistant protective layer with higher dielectric constant, to obtain a higher emission intensity.

4. Conclusions

Blue EL is demonstrated from nanolaminate Al₂O₃/Tm₂O₃ MOSLEDs fabricated by ALD. The emission at 456 nm from Tm³⁺ ions exhibits a power density of 0.15 mW/cm². The decrease of the EL intensity and decay lifetime due to the clustering and cross-relaxation of the Tm³⁺ ions is observed by adjusting the Tm₂O₃ dopant cycles. The decay lifetime for the Tm³⁺ ions under optical excitation is in the range of 0.13–1.25 μs while under electrical excitation, the decay lifetime increases to 1.13–4.02 μs. The EL is attributed to the impact excitation of the Tm³⁺ ions by hot electrons in the Al₂O₃ matrix via the P–F mechanism. Consistent with the previous results, a critical Al₂O₃ interlayer thickness of ~3 nm for the nonradiative interaction among excited Tm³⁺ ions and the acceleration distance of the injected electrons works. This work could contribute to the development of Si-compatible RE-doped light sources by modifying the dopant structure in the nanolaminates to achieve efficient emissions.