Lasing with Pumping Levels of Si Nanocrystals on Silicon Wafer
© The Author(s). 2016
Received: 14 July 2016
Accepted: 26 October 2016
Published: 15 November 2016
It is reported that the silicon nanocrystals (NCs) are fabricated by using self-assembly growth method with the annealing and the electron beam irradiation processes in the pulsed laser depositing, on which the visible lasing with higher gain (over 130 cm−1) and the enhanced emission in optical telecommunication window are measured in photoluminescence (PL). It is interesting that the enhanced visible electroluminescence (EL) on silicon nanocrystals (Si-NCs) is obviously observed by the naked eyes, and the light-emitting diode (LED) of the Si-NCs with external quantum efficiency of 20% is made on silicon chip in our laboratory. A four-level system is built for emission model in nanosilicon, in which the PL and EL measurement and transmission electron microscope (TEM) analysis demonstrate that the pumping levels with shorter lifetime from the rising energy of the Si quantum dots due to the quantum confinement effect occur, and the electronic localized states with longer lifetime owing to impurities bonding on Si-NCs surface are formed in the crystallized process to produce the inversion of population for lasing, where the optical gain is generated.
The prominent position of silicon follows from a unique combination of advantageous properties such as the availability of larger single crystals, higher purity, effective conductivity engineering, a matching insulator, and importantly natural abundance. Even though the optical properties of crystalline silicon are relatively poor due to its indirect bandgap precluding the efficient emission, silicon light source is of key importance for integrated Si optoelectronics material and devices [1–6], in which as a potential material for Si light source, silicon nanocrystals (Si-NCs) have received a lot of interest concerning enhancements of their emission [7–16].
It has been established that the spectrum of the photoluminescence (PL) emission from Si-NCs shifts to the blue due to the quantum confinement (QC) effect related to the rising electronic states which have shorter lifetime (in nanosecond (ns) scale) [17, 18] to be suitable for pumping levels, due to the Heisenberg principle related to ⊿t ~ h/⊿E  in the QC effect. The results of experiment and calculation indicate the quantization-related open of the bandgap and the enhancement of the radiative recombination rate, as momentum conservation is gradually relaxed with decrease of nanocluster size.
On the other hand, the impurities of oxygen and nitrogen or the interface and defects lead to the forming of the electronic localization states into the bandgap due to broken symmetry of system, whose energy position refers to Si–O and Si–N bonds on curved surface (CS) related to smaller nanoclusters called CS effect [19, 20]. Here, the electronic localization states have a longer lifetime (in microsecond (μs) scale) [21, 22] to be suitable for producing inversion of population in the levels. Therefore, in the article, the energy band becomes discrete for the “band” conception to cease instead of building a four-level system involving the QC electronic states such as the pumping level from the Si-NCs, the higher emission level of the electronic localized states due to n-doped on the Si-NC surface, the lower emission level of p-doped states, and the valence band level.
We fabricate the silicon nanocrystals by using the pulsed laser deposition (PLD) method with the annealing and the electron beam irradiation processes  in which the annealing time and the plasmonic interaction play important roles . The photoluminescence (PL) and the electroluminescence (EL) measurement and the transmission electron microscope (TEM) analysis in the Si-NC structures demonstrate that the pumping states with a shorter lifetime from the Si quantum dots (QDs) occur, which are verified in their direct emission in shorter wavelength with the QC effect, and on the other hand, the electronic localized states with longer lifetime owing to impurities bonding on the Si-NCs surface are formed in the crystallized process to produce the inversion of population for lasing, in which the optical gain in PL emission is measured and the visible emission in EL on Si-NCs is obviously observed by the naked eyes. The light-emitting diode (LED) of Si-NCs with external quantum efficiency of 20% is made on silicon chip in our laboratory.
The methods for fabricating the Si-NCs are self-assembly from silicon-rich silicon oxide matrices [25–28], plasma synthesis [29, 30], and colloidal chemistry [31, 32] in tradition. The successful self-assembly growth method with the annealing and the electron beam irradiation processes on the amorphous films prepared by PLD process is better to form the silicon nanocrystals in narrower size distribution and is easy to control the impurities on the Si-NC surface, which are in the pure-physics processes with less factors disturbing the fabrication.
Experiments and Results
Another interesting method for fabricating the silicon nanocrystals is in the photons interaction. In Fig. 1c, at first, the amorphous silicon film is formed in the PLD process. Subsequently, a pulsed Nd:YAG laser is used to irradiate the amorphous silicon film, in which the crystallizing process for producing QDs forms by controlling interaction of the photons and the plasmonic vibration called laser annealing. In the physical process, the standing wave and the lattice pattern with characteristic of the Wigner crystal [33, 34] in the plasmonic vibration are observed. In the furnace annealing, high-temperature annealing of the substoichimetric film (typically 900~1100 °C) produces a phase separation between Si and SiO x (with x < 2) for forming Si QDs (Fig. 1d). The dimensions, crystallinity, and size distribution of the Si-NCs depend on the Si excess, the temperature, and the annealing time.
These phenomena may be related to the electronic localized states at 2 eV from Si–O–Si bridge bonds formed in dilute oxygen and the electronic localized states at 1.78 eV from Si=O bonds formed in concentrated oxygen . After increasing annealing time to 20 min, the sharper peak near 600 nm in the PL spectrum is observed on the Si-NC structures prepared in dilute oxygen in which Si–O–Si bonds are easy to generate on the Si QDs, but they gradually disappear after annealing for 30 min, as shown in Fig. 5e. It is interesting that the very sharper peak near 700 nm with the higher gain (over 130 cm−1) and the full width at half maximum of 0.5 nm is measured on the Si-NC structures in the Purcell cavity prepared in concentrated oxygen for forming Si=O bond after annealing for 20 min, as shown in Fig. 5f, and the inset shows the evolution of the peak intensity as a function of the optically pumped sample length measured by using the various stripe length method for getting optical gain. Here, the Purcell cavity prepared by the PLE process is important for resonating and selecting mode of emission in Fig. 5f.
Results and Discussion
We fabricate the silicon nanocrystals by using a self-assembly growth method with the annealing and the electron beam irradiation processes on the amorphous silicon film prepared by using PLD. The silicon nanocrystals have been one of the most important developments in the field, as they offer strong carrier confinement and modification of the energy levels through quantization, as well as the ability to use their surface as a further design parameter, from which we provide a emission model with the four-level system. In the PL, EL measurement and the TEM analysis on the Si-NC structures, it is demonstrated that the pumping levels with higher energy due to the QC effect of Si QDs and the electronic localized states with longer lifetime owing to impurities bonding on Si-NC surface are produced in the crystallized process for the inversion of population in lasing. The optical gain in PL emission is measured in the Si-NC structures, and the visible EL on the Si-NCs is obviously observed by the naked eye. The light-emitting device on the Si-NCs with external quantum efficiency of 20% is made on silicon chip in our laboratory. If one could have better controlling for the nature of the photoactive impurities on the Si-NCs or the photoactive defects in Si crystallization, and increase their density for building the inverse population levels, thereby increasing the spectral density of emission, a true all-silicon laser will be in reach on Si chip.
Fabrication of Silicon Quantum Dots Under Irradiation of Electron Beam
The amorphous silicon film was exposed under electron beam with 0.5 nA/nm2 for 5~30 min in Tecnai G2 F20 system, in which coherent electron beam from field-emission electron gun, accelerated by 200 kV, has higher energy and better coherence. After irradiation under electron beam for 15 min, the silicon quantum dot (Si QD) structures are built and embedded in SiO x (with x < 2) or Si y N x (with x < 4 and y > 3) amorphous film prepared in oxygen or nitrogen gas, respectively, in the PLD device. Physical process of fabricating Si QDs under irradiation of electron beam is shown in Fig. 1, in which it is interesting that the Si QDs with the spherical shape gradually grow up after irradiation of electron beam for suitable time, whose size range is from 2 to 5 nm.
Laser Annealing Process
A pulsed Nd:YAG laser at 1064 nm is used to make annealing on the amorphous film of silicon, in which the temperature sensor indicates that the temperature is about 900 °C on the area exposed under laser beam. In the laser annealing process, besides the thermal action, it is important that coupling between photons and plasmons produced by nanosecond pulsed laser on Si surface forms resonance to transfer the energy to the atoms for crystallizing. It is very interesting that the lattice pattern of plasmonic standing wave is observed in coupling between photons and plasmons as shown in the inset of Fig. 1c, which is similar with the Wigner crystal structure.
Electroluminescence Measurement on the LED with Si QDs
The construction of silicon QD LED is shown in Fig. 6a, in which the PIN structure involves the bottom buffer silicon layer, the top Si layer doped with sulfur, and the medium layer with Si QDs doped with oxygen. In the LED device, the positive pole is connected on the gold film under the P-type Si layer and the negative pole is connected with the ITO film on the N-type Si top layer. The electroluminescence (EL) spectra is measured on the LED, in which the bright light is observed by the naked eyes, whose external quantum efficiency in PL emission is over 20% and the threshold current is about 50 mA/mm2.
This work was supported by the National Natural Science Foundation of China (Grant No.11264007, 61465003).
HW is a main writer and researcher who wrote the main manuscript text and prepared Figs. 1, 2, 3, 4, 5, 7, and 8, provides new ideas, designs investigation plan in research, is a main researcher in experimental work, takes part in the preparing process of the samples, and makes measurement of the PL and the EL spectra on the samples; LS is a main researcher in experimental work, who prepared Figs. 2, 3, 4, and 6, and takes part in the SEM and TEM measurement on the samples; HZ is a main researcher in experimental work, who prepared Figs. 4, 5, and 6, and takes part in measurement of the PL and the EL spectra on the samples; WK is a main researcher in experimental work, who prepared Fig. 6, and takes part in the PL and the EL spectra measurement on the samples; QC is a main researcher in experimental work, who prepared Figs. 5 and 6, and takes part in the PL and the EL spectra measurement on the samples; ZQ is a main researcher and writer who revised the main manuscript and prepared Fig. 8. All authors reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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