Negative magnetoresistance in silicon doped with manganese
Objects and methods of research
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2 Objects and methods of research
Most atoms in solids do not have a magnetic moment, but there are a number of transition elements in which the internal d inhabited only partially filled, and therefore, these atoms have a non-zero magnetic moment. If clusters are formed on the basis of such atoms, then the magnetic moment of each atom interacts with the magnetic moments of other atoms in such a way that it can align all atoms in one direction along a certain symmetry axis of the cluster. Such a cluster has a sufficiently large total magnetic moment [8 – 10]. Therefore, it is of great interest to study the formation of clusters based on elements of the transition group in the crystal lattice of a semiconductor. Choosing of manganese as impurity atom, was dictated by the fact that, firstly, manganese is a unique paramagnetic atom that has the electronic str ucture … 3d5 4s2 with the spin, and secondly, the manganese atom has the Mn++(3d5) state with a small ionic radius (0.96 Å), which is smaller than the Si bond ling’s (1.77 Å). Therefore, mang anese atoms in silicon are located mainly in interstitial-site state and have the largest diffusion coefficient. These mentioned parameters of manganese atoms in the silicon lattice seem to stimulate the formation of clusters [11]. Indeed, as was shown in [12-14] using the electron spin resonance (ESR) method, nanoclusters consisting of 4 neutral manganese atoms were found in manganese-doped silicon during slow cooling after diffusion. However, controlling the cooling rate did not make it possible to obtain samples with reproducible parameters. The composition, structure, size, magnetic moment, and magnetic properties of silicon with such nanostructures have also not been practically studied. Although the modern diffusion technology used in electronics does not require impurity diffusion according to a given mechanism, the development of nanotechnology and especially the development of technology for the formation of nanoclusters of impurity atoms with a controlled structure, composition, and their distribution in the crystal volume suggests the need for diffusion of impurity atoms mainly along the interstitial mechanism. Since only diffusion through interstices allows one to control the state of impurity atoms in the lattice and their interaction with each other and defects, and most importantly, under certain thermodynamic conditions, it allows the formation of nanoclusters with different structures and compositions [15]. One of the real ways to perform inter-nodal diffusion is to perform low-temperature and step diffusion. By low-temperature diffusion one must understand such diffusion process, in which thermally equilibrium concentration of vacancies (Nv) in lattice must be essentially less than concentration of introduced impurity atoms (N) in lattice. In the process of high- temperature diffusion on silicon surface up to 20 μm depth [16] intensive silicide formation occurs in the temperature range T=1030÷1120 ºC as well as essential erosion of material surface. During silicide formation (Mn4Si7) practically all interstitial atoms are captured and also diffusion rate of impurity manganese atoms in silicon decreases. Therefore, these facts significantly limit the possibility of using high- temperature doping to form clusters of impurity atoms in the lattice, which requires a more detailed study of this process. At the same time it is necessary to obtain an answer about the possibility of using the obtained diffusion parameters of impurity atoms during high- temperature diffusion under conditions of low-temperature and step diffusion process. In order to obtain compensated silicon with specified and reproducible electrophysical parameters the low-temperature stepwise doping method was used. The essence of our developed low-temperature stepwise diffusion is as follows. The samples under study and the diffusant - pure metallic manganese of a certain mass (determined by the ampoule volume) are in evacuated quartz ampoules (ampoule pressure ~10-6 mmHg.), which are introduced into the diffusion furnace at T=300 K. It has been determined in advance that the temperature of the furnace at the location of the ampoule is E3S Web of Conferences 401 , 05094 (2023) CONMECHYDRO - 2023 https://doi.org/10.1051/e3sconf/202340105094 2 gradually increased at a rate of 5 deg/min [17]. The samples are heated to a temperature of T=550÷700 ºC and held at this temperature for t=10÷20 min, then t he temperature is raised at a rate of 15÷16 ºC/min to a value of T=960÷1100 ºC (Figure 1). The samples are held at this temperature for 5÷10 min, then the ampoules are removed from the oven and cooled at a rate of 200 ºC/sec. Fig. 1. Technology of low-temperature step-by-step method of silicon doping with manganese atoms It is known from the literature [18-20] that the ESR method is often used to investigate the state of manganese atoms in the silicon crystal lattice. Manganese atoms are paramagnetic centres with spin S=5/2 (3d5 4s0) and depending on the doping condition can be found in the crystal lattice of silicon in the states Mn0 (3d5 4s2), Mn+ (3d5 4s1), Mn++ (3d5 4s0) or [MnB]+. If we consider that manganese atoms in silicon create two donor energy levels E1=EC- 0.27 eV and E2=EC-0.5 eV [19] then in compensated samples p-Si =(5÷10)·103 Ohm·cm, in which the Fermium energy equals EF=EV+(0.38÷0.45) eV, all injected manganese atoms are mainly in double positively ionized state Mn++. As the Fermi level shifts towards conduction zone the concentration of atoms in [Mn]++ state decreases and correspondingly the concentration of atoms in Mn+ and Mn0 states increases, and in overcompensated samples manganese atoms are mainly in Mn0 and Mn+ states. The following samples were made to investigate the state of manganese atoms by ESR. I-party. Compensated silicon samples p-Si samples n-Si II-party. Compensated and overcompensated silicon samples with similar resistivities doped with manganese atoms by conventional high-temperature diffusion technology. To clarify the nature of the state of manganese atoms in the silicon lattice they were also investigated by means of ESR spectra on Broker at T=77 K. To record the ESR spectra a spectrometer operating in the 3 cm wavelength range was used. Integral sensitivity of the device was ~5·1010 spin/Gs and the accuracy of detection w as up to 0,001%. The precise determination of the g-factor of the observed spectrum was carried out using a marker line with g=2.0024. E3S Web of Conferences 401 , 05094 (2023) CONMECHYDRO - 2023 https://doi.org/10.1051/e3sconf/202340105094 3 |