2068 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 19, OCTOBER 1, 2006 Enhancement of Performance of Si Nanocrystal Light-Emitting Diodes by Using Ag Nanodots Chul Huh, Jae-Heon Shin, Kyung-Hyun Kim, Chel-Jong Choi, Kwan Sik Cho, Jongcheol Hong, and Gun Yong Sung Abstract—Effects of Ag nanodots on silicon nanocrystal (nc-Si) light-emitting diodes (LEDs) are investigated. The electrical prop- erty of the nc-Si LED with Ag nanodots was enhanced compared to that of the nc-Si LED without ones. This was attributed to the increase in the electric field due to the formation of Ag nanodots at the contact interface, indicating that the current could flow more efficiently from the indium tin oxide layer to n-SiC film. The for- mation of Ag nanodots with a size of nm was confirmed by using a high-resolution transmission electron microscope analysis. Moreover, light output power of the nc-Si LED with Ag nanodots was enhanced. Index Terms—Ag nanodot, light-emitting diode (LED), silicon nanocrystal (nc-Si), silicon nitride. R ECENTLY, much researche has been devoted towards the Si-based microphotonics due to the application for light emitters such as light-emitting diodes (LEDs) and laser diodes [1]. If we could get a full spectrum in the visible wavelength by employing silicon nanocrystals (nc-Si), we could realize a full- color display and could reduce the fabrication cost because the compatibility with a conventional Si technology is better than any other materials such as conventional GaAs- and GaN-based materials. It is well known that bulk silicon has poor lumines- cence efficiency due to the indirect nature of its bandgap and is, thus, inefficient for light sources. However, if the size of nc-Si is smaller than the free exciton Bohr radius of bulk Si ( 4.6 nm), the light emission efficiency could be much enhanced due to an increase in overlapping of electron–hole wave functions, that is, a quantum confinement effect [2]. In order to synthesize the nc-Si, the Si-rich oxide (SRO) ma- trix has been widely used as the surrounding matrix [1], [3]–[5]. There are, however, disadvantages in using the SRO film as the surrounding matrix. The nc-Si in SRO film needs relatively high processing temperature for the formation and crystallization. Moreover, the electrons would be trapped in localized levels in the bandgap of nanocrystals produced the oxygen atoms, re- sulting in an uncontrollable tuning of the emission wavelength of nc-Si [6]. Another obstacle for the SRO matrix is that the high operating voltage needs to inject the current into nc-Si due to a huge barrier mismatch between Si and SRO. In our previous re- sult [7], well-organized nc-Si in a silicon nitride film instead of the silicon oxide film was grown by a plasma-enhanced chem- ical vapor deposition (PECVD). The results demonstrated that Manuscript received May 18, 2006; revised July 27, 2006. This work was supported by the Ministry of Information and Communication in Korea. The authors are with the IT Convergence Technology Research Division, Electronics and Telecommunications Research Institute (ETRI), Daejeon 305- 700, Republic of Korea (e-mail: chuh@etri.re.kr; gysung@etri.re.kr). Digital Object Identifier 10.1109/LPT.2006.883249 the nc-Si in silicon nitride films showed a clear quantum con- finement effect depending on the size of nc-Si, indicating that the bandgap of nc-Si could be controlled from the near infrared to the ultraviolet range. Moreover, the protype nc-Si LEDs were fabricated by employing an amorphous n-SiC film and indium tin oxide (ITO) current spreading layer to enhance the current injection into the nc-Si in silicon nitride matrix [8]–[10]. It was found that the amorphous n-SiC film could be used as an elec- tron injection layer into the nc-Si. The light output power is, however, still low to use in real application areas. Hence, the systematic investigation on the metal contact to an n-SiC film should be done to further increase the performance of the nc-Si LEDs. In this letter, we investigated the effects of the formation of Ag nanodots at the contact interface between the ITO layer and an n-SiC film on performance of the nc-Si LEDs. With the formation of nano-sized Ag dots at the interface between n-SiC film and ITO layer, the electrical property and light output power of nc-Si LEDs were enhanced due to an increase in the elec- tric field at the interface, meaning that the current flow from the ITO layer to SiC film could be enhanced compared to that of the nc-Si LEDs without Ag nanodots. The nc-Si in silicon nitride matrix with a thickness of 40 nm was grown by a PECVD. The silane and ammonia gases were used for growing the nc-Si in silicon nitride film, respectively. P-type silicon wafers doped with boron (around cm ) were used as substrates. The plasma power, chamber pressure, and substrate temperature for the growth were fixed at 5 W, 500 mTorr, and 250 C, respectively. The flow rate of siliane and ammonia gases was 50 and 10 sccm, respectively. In order to make a p-n junction structure, an amorphous n-type SiC film with a thickness of 250-nm doped with phosphorus was grown on the silicon nitride film at 300 C by a PECVD. The silane and CH gases were employed for growing amorphous n-SiC films, respectively. Tri-methyl-phosphite metal–organic source was used as a doping source. The samples were annealed at 950 C for 2 min to activate the dopant sources. The nc-Si LEDs with an area of 300 300 m were fabri- cated as follows. The n-type SiC and silicon nitride films were etched using inductively coupled SF –O plasma until the Si layer was exposed. The very thin Ag layer with a thickness of 2.5 nm was deposited on an n-SiC film via a thermal evaporation. A 100-nm-thick ITO layer was then deposited on an Ag layer by using a pulsed laser deposition. After deposition of the Ag and ITO layers, the samples were annealed at 500 C for 30 min under vacuum to form the Ag nanodots at the interface between ITO and n-SiC films. Finally, a Ni–Au (30/100 nm) film was deposited for the top and backside contacts. The nc-Si LEDs without an Ag layer were also fabricated for the comparison. 1041-1135/$20.00 © 2006 IEEE