Room temperature magneto-optic effect in silicon light-emitting diodes
Résumé
In weakly spin-orbit coupled materials, the spin-selective nature of recombination can give rise to large magnetic-field effects, for example on the electro-luminescence of molecular semiconductors. While silicon has weak spin-orbit coupling, observing spin-dependent recombination through magneto-electroluminescence is challenging: silicon's indirect band-gap causes an inefficient emission , and it is difficult to separate spin-dependent phenomena from classical magneto-resistance effects. Here we overcome these challenges and measure magneto-electroluminescence in silicon light-emitting diodes fabricated via gas immersion laser doping. These devices allow us to achieve efficient emission while retaining a well-defined geometry thus suppressing classical magnetoresis-tance effects to a few percent. We find that electroluminescence can be enhanced by up to 300% near room temperature in a seven Tesla magnetic field, showing that the control of the spin degree of freedom can have a strong impact on the efficiency of silicon LEDs. Introduction Spintronic effects in systems with weak spin-orbit coupling have attracted considerable attention due to their rich fundamental physics and potential for device applications. 1-4 A class of these effects can be measured optically, 5-7 providing direct insight into phenomena such as spin-dependent recombination, where only the singlet state of an electron-hole pair can recombine radiatively back to the ground state. Since external magnetic fields can change the spin statistics and energy levels in the sample, magneto-electroluminescence (MEL) effects have been seen as the hallmark of spin-dependent recombination phenomena, and have given important insight into the role of spin in organic materials used for light-emitting diodes (LEDs). 8-10 These spintronic effects can then be harnesseed, to provide very senstive magnetic field sensors, sensible to external magentic fields of only a few mTesla comparable with the fluctuating hyperfine fields inside organic materials 11-13 or to engineer new light-emitting device architectures through reverse intersystem crossing. 14 Like molecular semiconductors, silicon has weak spin-orbit coupling, but emission is much less efficient due to sil-icon's indirect band-gap, making analogous magneto-optic studies challenging, and requiring careful engineering to prepare efficient light-emitting diodes. 15-17 In addition, observing spin-dependent magneto-electroluminescence in silicon requires that the magnetic field and device currents are parallel to effectively suppress classical magnetoresistance (MR) contributions which can enhance MR in silicon up to spectacular values even at room temperature. 18-22 Here we address both of these challenges by developing a new fabrication method for efficient silicon light-emitting diodes using an original doping technique, gas immersion laser doping (GILD), and investigate spin-dependent recom-bination in silicon LEDs (SiLEDs). The GILD process 23-26 allows us to reach doping levels well beyond the solubility threshold which, as we describe below, gives rise to efficient emission, while retaining the well-defined planar geometry necessary to align electric and magnetic fields. Using our SiLEDs, we find that when classical MR effects are suppressed, electroluminescence can be substantially enhanced under a magnetic field near room temperature. We explain this phenomenon using a model of spin-dependent recombination 28-30 of electron-hole pairs and use our analysis to estimate the exchange energy of weakly bound excitons in silicon. Our experiments provide an optoelectronic approach to probe the spin statistics of carriers in silicon-a material which is an excellent candidate for scalable spin quantum computing. 31-33 They also highlight the importance of controlling the spin degree of freedom for the efficiency of silicon light emitting devices. Results Description of the system: We start by describing the fabrication procedure of the GILD SiLEDs [Fig. 1-a] and the physical mechanism behind their enhanced efficiency, before discussing the MEL response of these devices. The Si light-emitting diodes were prepared by doping two 2 × 2 mm 2 spots with opposite polarities p+/n+ on a n-Si [100] substrate of resistivity 45 Ωcm and thickness 700 µm using the GILD technique [Fig. 1-a]. A precursor gas PCl 3 (BCl 3) for n+ (p+) doping is injected into an ultra-high vacuum chamber, where it saturates the chemisorbtion sites on the Si surface. The substrate is then melted by a pulsed excimer XeCl 308nm laser with a 25 ns pulse duration.
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