Abstract
We report the efficient enhancement of light emission from silicon crystal by covering the silicon surface with an ultrathin (several nm) SiO2 layer. The photoluminescence of Si band edge emission (1.14 μm band) at room temperature is enhanced by two orders of magnitude. Compared with a p-Si/n-Si diode, light emission from a p-Si/SiO2/n-Si diode by current injection via direct tunneling is enhanced by more than 3 orders of magnitude. The light-emission enhancement is attributed to the diminishment of nonradiation recombination at the surface/interface and to the space confinement of the carrier recombination. The simple structure and low operating bias (approximately 1 volt) of our light emitting diodes supply a new choice for realizing efficient current injection light source in silicon compatible with conventional ULSI technology.
The physical curiosity and the increasing need for fully silicon-compatible optoelectronics device triggered extensive study on developing Si-based light-emitting materials and devices such as porous silicon [1–3], nanocrystalline silicon (Si-nc) buried in SiO2 [4–7], and dislocation engineering [8, 9]. Porous silicon can be a relatively efficient emitter but its process lacks compatibility with conventional ultralarge scale integration (ULSI) technology. Si-nc buried in SiO2 usually requires a high-working bias (5 Volt to several decades Volt), and light-emission is due to impact ionization, leading to a fast degradation of the oxide matrix due to the hot electrons transport, and making it difficult for fabricating efficient current injection devices such as light emitting diodes (LEDs) and laser diode (LDs). The simplest way is to use an essentially bulk material, because a bulk material is not only compatible to ULSI technology, but also possible for efficient current injection, especially for those when relatively high power (accordingly high injection current) is preferred. However, the key issue is, as bulk silicon is an indirect band semiconductor and therefore a poor light emitter, how to increase the radiation recombination by diminishing the nonradiation recombination. Ng et al. [8] reported the enhanced light-emission by implantation of boron into the surface of an n-type Si substrate and attributed it to the carriers confinement at the loop-dislocation induced from implantation of boron and the following thermal treatment. Kittler et al. [9] demonstrated that the efficient light-emissions (with the external and internal quantum efficacy up to 0.026% and 2%, resp.) can be realized in a p-type Si substrate as well as in an n-type Si substrate by implantation of phosphorus or boron at the respectively surface, but attributed the enhanced light-emission to a high Shockley-Read-Hall lifetime. It is well-known that a carrier confinement and/or a diminishment of the nonradiation process will improve the radiation recombination. An insulate layer in a semiconductor will no doubt prevent the transport of carriers at lower energy, meanwhile, if the insulate layer is thin enough, carriers can go through it by direct tunneling; hence, a balance between the carrier confinement and efficient current injection can be setup. In this work, we report that the efficient light-emission at room temperature can be realized in silicon LEDs with a p-Si/SiO2/n-Si structure. Our photoluminescence (PL) results indicate that the enhancement of light-emission from silicon is close-related with the existence of an ultrathin SiO2 layer.
The device structure of the p-Si/SiO2/n-Si LED is schematic shown in Figure 1(a). The substrate is an n-type (phosphorous-doped) Si (100) with the resistivity of 0.1 Ωcm and a thickness of 0.3 mm. The surface of the substrate was thermal-oxidized to form a SiO2 layer with thickness of several nm, then an 1 μm thick p-type Si layer (boron-doped) with resistivity of 0.01 Ωcm was grown on the SiO2 layer in a vacuum of Torr. by use of a RF magnetron sputtering system coupled with a load-lock. To form an ohmic-electrode, phosphorous implantation was performed on the n-Si rear surface at an energy of 30 keV. Before depositing Al electrode on the p-side and Ti/Au one on the n-side, 820°C thermal annealing for 1 hour in an Ar atmosphere was performed for making both the p- and n-type impurities active. These ways p-Si/SiO2/n-Si diodes were fabricated with chip sizes of mm. For comparison, a p-n diode without the SiO2 layer was prepared by erasing the native SiO2 on the substrate surface with 10% HF etching liquid just before loading the substrate into the vacuum chamber to form the p-type layer. Figure 1(b) shows the plot of a p-Si/SiO2/n-Si diode with SiO2 nominal thickness of 1.5 and a p-n diode, respectively. The forward current of the former diode is clearly lower at same bias compared with the latter due to the insulate SiO2 layer. In the p-Si/SiO2/n-Si diode, carriers may transport through the ultrathin SiO2 layer by direct tunneling [10, 11], which will be further described in next sections. Figure 2 shows the room temperature (RT) electroluminescence (EL) spectra taken from the p-Si front side of the two diodes with a forward current of 100 mA, respectively. Both spectra have peaks centered at around 1138 nm (1.09 eV) originating from the Si band to band recombination. Compared with the p-n diode, EL intensity of the p-Si/SiO2/n-Si diode increased to more than three orders of high magnitude. Figure 3 gives the emission power (also taken from the p-Si front side) and power density dependence of the injection current density. The typical power densities at 4.4 A/cm2, 22 A/cm2, and 57 A/cm2 are 1249 μW/cm2, 5942 μW/cm2, and 13276 μW/cm2, respectively. The external quantum efficiency and power efficiency at 4.4 A/cm2 (0.97 V) are calculated to be 0.026% and 0.027%, respectively, and they slightly decrease to 0.021% and 0.017% at 57 A/cm2 (1.23 V), indicating a stable performance in a wide range of injection current. The external quantum efficiency is comparable to those in [8] (0.02%) and [9] (0.02–0.026%). In the reported result from Si-nc/SiO2 multilayer light-emitting device [7], the highest power efficiency of 0.17% for devices of such type was obtained at 4.9 × 10−4 mA/cm2 with a power density of μW/cm2, but it dramatically declined to 0.003% at 20 mA/cm2 with a maximum power density of 2.6 μW/cm2.
(a)
(b)
Photoluminescence (PL), excited with the second-harmonic generation (SHG) 532 nm line of an Nd:YVO4 laser, was performed for understanding the mechanism of the strong enhancement of light-emission. It was found that PL (excited and collected from the p-side) intensity from the p-Si/SiO2/n-Si structure as described above is two orders of magnitude higher than that from the p-Si/n-Si one indicating that the thin SiO2 layer plays a very important role. We further investigated the SiO2/Si samples with PL (excited and collected from the SiO2 side) measurements. Figure 4 shows the PL spectra of three n-type Si substrates at room temperature marked as A–C. Substrates A and B have a SiO2 layer on the surfaces with nominal thickness of 0 and 1.5 nm, respectively, and substrate C is that of B after thermally treated at 820°C for 1 hour in an Ar atmosphere. All of the spectra have a band-to-band emission peak centered at around 1140 nm (1.09 eV). Compared with sample A, the peak intensities of B and C increased 28 and 98 times, respectively. Besides, we found that the PL intensity increased with the SiO2 thickness and gradually saturated; for instance, the PL intensity was doubled for a substrate with a 10 nm-thick SiO2 layer compared with the substrate C. However, a thick SiO2 layer will make it difficult for efficient current injection.
In an indirect band semiconductor, the nonradiative recombination processes are usually dominant over radiative ones. The nonradiation processes include the recombination via an energy (deep) level within the band gap (Shockley-Read-Hall recombination) and the Auger recombination. Energy (deep) levels within the band gap also appear at the semiconductor surface/interface and act as nonradiation recombination centers (surface effects). The difference between substrate B and A is the existing of a covering SiO2 layer or not. As ultrathin SiO2 has a band gap of 8.95 eV [12], A SiO2 layer can prevent carriers from diffusing to the surface (except those by tunneling effect) and meanwhile make the carrier density near the SiO2 layer effectively higher. This is a kind of carrier confinement hence the related nonradiation recombination may be diminished and the radiation one increased. However, the SiO2/Si interface may produce new nonradiation centers. It is likely that the thermal treatment effectively improves the interface states and further enhances the radiation combination as shown by sample C.
In our p-Si/SiO2/n-Si diode with forward bias, it is considered that carriers may go through the ultrathin SiO2 layer by direct tunneling. It was shown in [10] that the direct tunneling is dominant in SiO2/Si with ultrathin nm. At the SiO2/Si interface, there exist a valence band offset of 4.25 eV and a conduction band offset of 3.13 eV [11] making the tunneling possibility for holes much smaller than electrons. The electron-tunneling current is usually dominant over the hole current. However, in a system where hole concentration is much higher than the electron concentration, things will be different. Cai and Sah [11] demonstrated the dominant hole tunneling current at low gate voltage in a p+gate pMOST. Our devices are such designed with the holes concentration in the p-Si layer being two orders of magnitude higher than the electron concentration in the n-Si substrate so that the amount of direct tunneling holes is comparable (if not dominant) to that of direct tunneling electrons. In this case, the direct tunneling minority carriers may respectively recombine radiatively, in a more effective way, with the majority carriers “blocked” near the both sides of the SiO2 barrier. A full understanding of the mechanism requires further investigations. We would like to point out that in the conventional silicon technology, the ultrathin native SiO2 layer of a silicon substrate is etched out before homoepitaxial growth of another silicon layer to form a p-n junctions, and this will make the silicon diode a poor light emitter.
We also would like to point out that the above diode structure is only a fundamentally simple one, as only a uniform ultrathin SiO2 layer is used on the Si substrate. A substrate covered with periodic patterned (stripes or dots) thick (thick enough for negligible vertical current injection through the thick part at low bias) and ultrathin SiO2 layer will not increase the resistance dramatically (essentially only doubled) for the vertical current injection through the ultrathin SiO2 layer as described above, meanwhile, the lateral carrier confinement and photonic-crystal effects can be expected for the light-emitting device with a properly designed period of thick/thin SiO2 layers.
In conclusion, we demonstrate the efficient enhancement of light-emission from a p-Si/SiO2/n-Si diode with an ultrathin SiO2 layer. The enhancement of light-emission from Si is attributed to the diminishment of nonradiation recombination at the surface/interface and the space confinement of carrier recombination due to the existence of the ultrathin SiO2 layer. The simple structure and low-operating bias (~1 V) of our light-emitting diodes supply a new choice for realizing efficient current injection light source in silicon compatible with conventional ULSI technology.
Acknowledgments
The authors thank K. Nakajima, M. Miyamoto, N. Ohishi, and K. Tanaka for their technical assistance in the device fabrication, PL and EL measurements.