There are many open questions remaining to be answered concerning silicon nanocrystals.
In this chapter, we identify some experiments that could clarify our understanding. We also highlight a few ideas and devices suggested by our work.
1. Concentration effects in silicon nanocrystal ensembles.
As described in chapter 2, the photoluminescence decay dynamics of dense ensembles of silicon nanocrystals are suspected to be dominated by efficient internanocrystal energy transfer processes. The details of these processes have been essentially swept under the rug by the widespread use of the stretched exponential function to describe time-resolved photoluminescence measurements. The development of a physical model that predicts the dynamics of photoluminescence is an important challenge for the silicon nanocrystal research community. However, the existing data set is not sufficient to distinguish between the different models that have been proposed. There is a good opportunity for the right experiment to have a significant impact on the discussion.
Our method of measuring the decay rate while changing the local density of optical states would provide a powerful experimental protocol for reexamining photolumines- cence decay in silicon nanocrystals. In particular, we suggest a study in which the concentration of nanocrystals embedded in a staircase-etched oxide is controlled by varying the implantation dose over several samples. An extensive data set could be collected by monitoring the decay lifetime as a function of wavelength, local density of states, excitation rate, and concentration. These data could then be used to construct an improved model for energy transfer processes in silicon nanocrystal ensembles.
2. Developing an internal quantum efficiency metrology protocol for materials evaluation.
The internal quantum efficiency for radiation is an important measure of sample qual- ity for all optical materials. It would be useful to develop an optimized standard
3. Time-resolved bandedge electroluminescence in a silicon nanocrystal feled.
The bandedge electroluminescence signal observed in our silicon nanocrystal feled deserves further study. We have suggested that light emission occurs only for gate bias transitions that correspond to the collapse of the inversion layer in the channel.
Time-resolved measurements will be able to establish whether this is actually the case.
These measurements may also clarify the contribution of stored charges to the band edge signal. The decay of the anticipated band edge electroluminescence pulse may correspond to the discharging time scale for the nanocrystal ensemble if the stored charge contributes directly to recombination. If the light is generated only by the inversion layer electrons, the decay dynamics may be more rapid. It might also be interesting to look for bandedge electroluminescence in the control devices that do not have nanocrystals in the gate oxide.
4. Temperature dependent electroluminescence in a silicon nanocrystalfeled.
The field-effect electroluminescence mechanism has so far provided an adequate con- ceptual framework for our experiments, but the underlying carrier transport mecha- nism is unclear. Simulations suggest that tunneling processes would be too slow to explain the time constants that we observe. If transport instead occurs by charge hopping between defect states in the gate oxide, we should be able to measure a char- acteristic exponential temperature dependence in the electroluminescence signal. For example, in Poole-Frenkel conduction [172, 175], the current density is:
JP−F = qen0μENexp
−qe
kT
φB−
qeEN
πN
,
where the current density φB is the barrier height and EN is the electric field in the oxide.
In this experiment, the silicon nanocrystal feled would be mounted in a cryostat and the optoelectronic characterization performed for this thesis would be repeated
at a range of low temperatures. It will be important in this work to carefully account for the known temperature dependent emission characteristics of silicon nanocrystals caused by the singlet-triplet exchange interaction in the excitons.
5. Field-effect electroluminescence in nonsilicon quantum dots.
We have suggested that field-effect electroluminescence could be used to electrically pump nonsilicon semiconductor quantum dots. However, the mechanism may rely on some particular material property of silicon or SiO2. It would be interesting to design and fabricatefeleds using III–IV or IV–VI quantum dots to demonstrate that we have indeed developed a new general class of light emitting devices. In view of the performance limits calculated in section 4.11, afeled that contained direct gap nanocrystals seems especially promising.
6. Single photon electroluminescence in a silicon nanocrystalfeled.
Our devices were intentionally fabricated with large gate contact pads in order to allow convenient access to the silicon nanocrystal ensemble with free space optics.
It would be interesting to scale a silicon nanocrystal feled down to the very small device area regime. We suggest that this could be accomplished by focused ion beam milling. Our existing devices could be nearly arbitrarily reduced in size by defining reduced gate contact pad regions within the existing polysilicon gate layer. It should be straightforward to make a feledwith a gate area less than 100 nm2. At this size scale, it is statistically possible to address a single silicon nanocrystal.
If field-effect electroluminescence were accomplished at the single nanocrystal level, we could observe electrically pumped “photon on demand” single photon light emission.
By further controlling the injection of additional carriers, it may even be possible to drive the emission into a regime where the statistics are determined by the gate bias modulation rather than by the spontaneous emission lifetime. One method would involve intentionally quenching the exciton shortly after creating it in the nanocrystal by injecting an additional carrier to induce Auger recombination.
7. An extensive parametric study of silicon nanocrystalfeledelectroluminescence.
In this thesis, we have studied fewer than 1% of thefeleddevices that were fabricated during our collaboration with Intel Corporation. We have now developed a suite of
Imaging the electroluminescence of larger devices at different operating frequencies might reveal if electron drift velocity is important in the charge injection process.
Larger devices might exhibit reduced electroluminescence at the center of the gate contact pad.
Finally, we suggest attempting a time-resolved measurement of the first few cycles of electroluminescence to look for nonequilibrium light emission processes. For exam- ple, the gate bias could be held at −6 V for several seconds before applying series of 100 μs alternating bias pulses. In these measurements, the transient photolumi- nescence measurements made during our optical memory experiments should provide a useful guideline for choosing sufficiently long gate bias dwell times. Based on the signal levels we have observed in steady state for modulation at 1 kHz, this type of measurement should be practical with integration times of order 10 min.
8. A feledpumped silicon nanocrystal sensitized erbium laser.
An electrically pumped silicon compatible laser is a “holy grail” in silicon photonics.
We propose that silicon nanocrystal sensitized erbium could be used as an electrically pumped gain medium in a waveguide integrated feled structure (figure 6.1). The low index of refraction of silicon nanocrystal doped SiO2 in comparison to silicon and other potential electrical contact materials suggests the use of a “slot waveguide”
design [219]. In this structure, the guided mode can be highly confined in a low index region.
The slot waveguide structure is ideally suited for the field-effect electroluminescence mechanism. The gate oxide of thefeledwill form the slot while the gate contact and a thin film substrate will form the cladding layers. The long decay lifetime of erbium should allow the gain medium to be inverted by intermittent electrical pumping. It will be very important to control losses in the contact cladding layers in order to achieve net gain.
n+
ptype channel ntype gate
~8nm Control Oxide
~5nm Si NC:Er
~2nm Tunnel Oxide
140nmp-Si DRAIN
Si Substrate Buried Oxide
140nmn-Si
GROUND GATE
SOURCE
Si nanocluster Er3+
Figure 6.1. A schematic diagram of an electrically pumped silicon nanocrystal sensitized erbium horizontal slot waveguide laser. The erbium ions are pumped by energy transfer from nanocrystals that are excited by field effect electroluminescence.
In this thesis, we have demonstrated that silicon nanocrystals embedded in SiO2 con- stitute a fully cmos compatible optical material system. We have shown through photo- luminescence experiments that dense silicon nanocrystal ensembles can be formed in well defined layers using ion implantation, and that these layers can emit light with very high internal quantum efficiency. We have fabricated optoelectronic devices that have allowed us to experimentally contribute to the understanding of charge dependent processes in silicon nanocrystal ensembles. And finally, we have discovered and developed a new electrical ex- citation mechanism that significantly adds to the promise of silicon nanocrystals for silicon photonics.