## 2 Dynamic Nuclear Polarization in Semiconductor Nano-and Microstructures
### Optically Induced DNP in Semiconductors
#### 2.1.4 Optical Spin Orientation in Various Semiconductors  
Since the first successful experiment [14], discussed in the previous Subsection, there has been a steady interest in theoretical and experimental investigation of optical spin orientation in various semiconductors. Among them, c-Si is an advantageous material for spintronics because of its long spin relaxation time and dominating technology. The dynamic polarization of \({}^{29}\)Si nuclei in phosphorus-doped (\(N_{\rm d}=1.5\times 10^{13}\) cm\({}^{-3}\)) \(n\)-type c-Si induced by optical pumping was investigated in Reference [27]. During illumination of c-Si, the magnetization of \({}^{29}\)Si nuclei becomes gradually established, following an exponential law with a time constant equal to the spin-lattice relaxation time \(T_{1}\) of \({}^{29}\)Si nuclei. Limiting relative polarization of the \({}^{29}\)Si nuclei obeyed the following equation [27]:  
\[\frac{\langle I_{2}\rangle}{I_{0}}\ =\ 1-s\frac{\left|\gamma_{e}\right|}{ \gamma_{n}}\frac{n_{ph}}{n_{0}+n_{ph}}. \tag{13}\]  
With the concentrations \(n_{\rm ph}\) of photoexcited and \(n_{0}\) of thermal electrons, correspondingly. Without light, the nuclear magnetization equaled its equilibrium value. When the concentrations of photoexcited electrons was much higher than that of thermal electrons, the saturation value was described by Equation (13) in agreement with experiment (see Figure 7). The limiting degree of nuclear polarization was determined by the spin polarization of both the photoexcited and thermal electrons in the conduction band. The presence of rare earth impurities (Gd) in combination with shallow impurities (P) resulted in increased optical pumping efficiency, because the rare earth impurities increased the ionization energy of the shallow impurities [28]. More recently, the dynamic-nuclear-polarization capabilities of optical pumping at 4 K and a magnetic field of 7 T were investigated with a \({}^{29}\)Si-enriched sample. Using NMR spectroscopy, the \({}^{29}\)Si polarization as high as up to 0.25% was demonstrated [29]. The investigations of above-bandgap excitations look promising for further enhancement of \({}^{29}\)Si polarization in the sample area close to the exposed surface. However, c-Si is an indirect bandgap semiconductor and it was soon clear that higher nuclear polarizations could be generated in direct bandgap materials. The main focus of solid-state optical pumping therefore shifted to _direct bandgap_ semiconductors soon after the pioneering experiment of [PERSON] [14], which was analyzed in Section 2.1.3. The stationary spin orientation of photoelectrons excited by circularly polarized light in Ga\({}_{\text{x}}\)Al\({}_{\text{1-x}}\)As, as detected by the degree of the circular polarization of photoluminescence, was determined by the value of the nuclear magnetization [30]. The optical method manifested a very high sensitivity (signal-to-noise ratio of the order of 100:1 when the total number of photoelectrons constituted 10\({}^{9}\)) and could be applied to samples at microscale. Optically-pumped, directly-detected NMR experiments on the bulk crystalline InP (c-InP) were performed [31] by using irradiation of a sample of undoped c-InP wafer from a diode laser to enhance the spin polarization of \({}^{31}\)P nuclei near the sample surface. The spin polarization was found to depend on the light polarization. The spin polarization magnitude and polarization transfer efficiency of the from the semiconductor substrate to the overlayer (Figure 8) were high enough to permit solid state NMR measurements on sub-nanomole quantities of molecules in the overlayer [31]. The optically induced dynamic nuclear polarization in CdS was observed by radio-frequency detection of the \({}^{113}\)Cd nuclear magnetic resonance signal for \(T<2\) K. At this temperature, the optically induced spin polarization was negative when compared to the thermal polarization, irrespective of the helicity of the pumping light [32].