Enhancement in electron transport and light emission efficiency of a Si nanocrystal light-emitting diode by a SiCN/SiC superlattice structure
© Huh et al.; licensee Springer. 2013
Received: 15 October 2012
Accepted: 17 December 2012
Published: 5 January 2013
We report an enhancement in light emission efficiency of Si nanocrystal (NC) light-emitting diodes (LEDs) by employing 5.5 periods of SiCN/SiC superlattices (SLs). SiCN and SiC layers in SiCN/SiC SLs were designed by considering the optical bandgap to induce the uniform electron sheet parallel to the SL planes. The electrical property of Si NC LED with SiCN/SiC SLs was improved. In addition, light output power and wall-plug efficiency of the Si NC LED with SiCN/SiC SLs were also enhanced by 50% and 40%, respectively. This was attributed to both the formation of two-dimensional electron gas, i.e., uniform electron sheet parallel to the SiCN/SiC SL planes due to the conduction band offset between the SiCN layer and SiC layer, and an enhanced electron transport into the Si NCs due to a lower tunneling barrier height. We show here that the use of the SiCN/SiC SL structure can be very useful in realizing a highly efficient Si NC LED.
Until now, lots of research have been devoted towards the development of Si-based light sources that could enable the integration of photonics with Si microelectronics[1–3]. Si-based light sources could reduce the fabrication cost because their compatibility with a conventional complementary metal-oxide semiconductor (CMOS) technology is better than any other light source such as conventional GaAs- and GaN-based light emitters. Despite a lot of efforts for the realization of Si-based light sources with high efficiency, luminescence efficiency from Si-based light sources is still very low due to an indirect bandgap nature of the bulk Si[4, 5]. Recently, because of this obstacle for realizing efficient Si-based light sources, Si nanocrystals (NCs) have, therefore, attracted the most attention as promising light sources for the next generation of Si-based nanophotonics[6–8]. Si NCs showed a quantum confinement effect that increased in the overlapping of electron–hole wave functions, leading to an enhancement in luminescence efficiency. Another advantage for light sources using Si NCs is that the optical bandgap can be easily tuned by changing the size of NCs. This implies that Si NCs are of particular interest as a light source, covering the whole visible wavelength range.
Si NCs have been generally synthesized into insulating matrices. Because of this, disadvantages appear in realizing an efficient Si NC light-emitting diode (LED). To realize efficient Si NC LEDs, therefore, following required factors such as the formation of Si NCs with high density, surrounding matrix, and design of an efficient carrier injection film should be addressed. We and others have recently demonstrated an in situ growth of well-organized Si NCs in a Si nitride (SiN x ) matrix by conventional plasma-enhanced chemical vapor deposition (PECVD) and have achieved a reliable and stable tuning of the wavelength ranging from near infrared to ultraviolet by changing the size of Si NCs[8, 10, 11]. SiN x as a surrounding matrix for Si NCs can provide advantages over generally used Si oxide films because of the in situ formation of Si NCs at low temperature, small bandgap, and clear quantum confinement dependence on the size of Si NCs. These merits can meet the requirements for the current CMOS technology such as compatibility with integration and cost-effectiveness. To inject the carriers into the Si NCs, polysilicon, indium tin oxide (ITO), and semitransparent metal films have been generally used as contact materials[12–14]. However, the photons generated from the Si NCs could be absorbed because the photons passed through these contact materials to escape out from the Si NC LEDs. A suitable carrier injection layer is, therefore, very crucial for enhancing the light emission efficiency of Si NC LEDs. In previous results[15, 16], we grew the amorphous SiC(N) film with an electron density up to 1019 cm−3 using a PECVD at 300°C and demonstrated that the amorphous SiC(N) film could be a suitable electron injection layer to improve the light emission efficiency of Si NC LEDs.
Recently, alternative methods such as surface plasmons (SPs) by nanoporous Au film or Ag particles that could enhance the luminescence efficiency from the Si NCs and external quantum efficiency of a Si quantum dot LED were reported. These approaches, however, need complicated wet etching and annealing processes to apply SP coupling. They also have disadvantages in realizing an efficient Si NC LED, such as having an impractical structure for LED fabrication and absorption of light escaping out from the LED at the metal layer. A reliable, simple, and practical device design without additional processes is, hence, very crucial in the fabrication and an enhancement of the light emission efficiency of Si NC LED. In this work, we present the concept that can uniformly transport the electrons into the Si NCs by employing 5.5 periods of SiCN/SiC superlattices (SLs) specially designed for an efficient electron transport layer, leading to an enhancement in the light emission efficiency of Si NC LED. A SiCN film in 5.5 periods of SiCN/SiC SLs was designed to have a higher optical bandgap than that of SiC to induce a two-dimensional electron gas (2-DEG), i.e., uniform electron sheet, at the interface between the SiCN and SiC layers due to the conduction band offset between these two layers. The electrical characteristic of Si NC LED with the SLs was improved. Moreover, light emission efficiency and wall-plug efficiency (WPE) of the Si NC LED with the SLs were also enhanced by 50% and 40%, respectively.
The Si NCs used here were embedded into a SiN x matrix with a thickness of 50 nm and were in situ grown by PECVD, in which Ar-diluted 10% SiH4 and NH3 was used as the source of reactants. The plasma power, chamber pressure, and substrate temperature for the growth of Si NCs were fixed at 5 W, 500 mTorr, and 250°C, respectively. The size of Si NCs embedded into a SiN x was around 4 nm, which was confirmed by high-resolution transmission electron microscopy (HRTEM). No post annealing process was performed to create the Si NCs into the SiN x matrix after the growth. SiCN (3 nm)/SiC (3 nm) SLs at 5.5 periods doped with phosphorous (P) was deposited on the Si NCs which were embedded into the SiN x matrix at 300°C by a PECVD. The SiCN/SiC SLs were grown by changing the flow rates of CH4 and NH3 sources while the flow rate of SiH4 was fixed. An amorphous SiC film (approximately 40 nm) doped with P that is used as an electron injection layer was deposited on the 5.5 periods of SiCN/SiC SLs. An ITO layer (100 nm) used as a transparent current spreading layer was deposited at 150°C on an amorphous SiC film and then annealed at 300°C for 30 min in a pulsed laser deposition chamber to improve the electrical property and optical transparency. Right after the deposition of ITO, the Si NC LED samples were etched using an inductively coupled SF6/O2 plasma and standard photolithographic technique until the Si layer was exposed. Finally, a Ni/Au (30/120 nm) layer was deposited for the top and backside contacts using thermal evaporation. A mesa-type Si NC LED with 5.5 periods of SiCN/SiC SLs with an area of 300 × 300 μm2 was fabricated, and Si NC LED without SiCN/SiC SLs was also fabricated for comparison.
Results and discussion
We demonstrate the fabrication of Si NC LED with 5.5 periods of SiCN/SiC SLs. SiCN/SiC SLs at 5.5 periods was designed by considering the optical bandgap to form the uniform electron sheet parallel to the SL planes. The electrical property of Si NC LED with 5.5 periods of SiCN/SiC SLs was improved. Moreover, light output power and WPE of the LED with 5.5 periods of SiCN/SiC SLs were also enhanced by 50% and 40%, respectively, which were ascribed to the formation of uniform electron sheet and enhancement in electron transport in Si NCs. We show here that the SiCN/SiC SL structure can be used to realize a highly efficient Si NC LED.
This work was supported by the Converging Research Center Program through the Converging Research Headquarter for Human, Cognition and Environment funded by the Ministry of Education, Science and Technology (grant code 2011 K000655).
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