Dopant atoms, activating

The activation energy Ea - defined as Ec - Ey for the conduction band (and analogously for the valence band), can be used to assess the presence of impurities. Due to their presence, either intentional (B or P dopant atoms) or unintentional (O or N), the Fermi level shifts several tenths of an electron volt towards the conduction or the valence band. The activation energy is determined from plots of logafT) versus 1/7, with 50 < 7 < 160°C. For undoped material Ea is about 0.8 eV. The Fermi level is at midgap position, as typically Eg is around 1.6 eV. 

Finally, we tried to activate dopant atoms using pulsed laser irradiation, which is effective in lowering the process temperature. The light source was a 308-nm XeCl excimer laser, which is a standard source for crystallizing a-Si films used in the LTPS process.19 A test sample of 76 nm thickness, prepared from the copolymerized solution (l-wt% phosphorus, 30-min UV irradiation, 500 °C 2hr annealing), was irradiated using a XeCl laser at various intensities to activate the dopant atoms. Figure 5.19 shows the relationship between the... 

PH3 [93] Molecules adsorb at dangling bonds and adsorption is self-limiting. Subsequent annealing can incorporate and electrically activate P dopant atoms into the surface. Applications in doped atomic scale devices [18] and quantum computation [20]. 

The Hall effect provides a measure of the net carrier concentration of the dopants. Depending on the depth of the dopants, the activation of the impurity can be very much reduced. For example, Mg in GaN forms a level at 250 meV above the valence band, and the percentage of activation of the magnesium atoms at room temperature is about 1%. DLTS provides a measure of the deep states within the bandgap of the semiconductor. However, it only provides the activation energy and the impurity concentration, and it does not give the exact nature of the impurity concerned. Implantation experiments are required to correlate known impurities with the energy levels measured by DLTS. 

This Datareview discusses the redistribution of typical dopant atoms in GaN during the implant activation anneal. Secondary ion mass spectroscopy (SIMS) spectra of the impurity profiles (impurity concentration versus depth into the sample) before (as-implanted) and after annealing are presented. SIMS analysis is the primary method of characterising impurity distributions in semiconductors [2], This information can be used to roughly estimate a diffusivity, D , of the dopant at the temperatures studied by invoking the relationship... 

More recently, Faughnan and Hanak (1983) have used spectral response data to determine that the concentration of acceptors is —1019 cm-3 for p-type a-Si H layers containing — 1021 boron atoms cm -3 (as determined by SIMS) for a doping efficiency of — 1 %. Dresner (1983) has estimated that the doping efficiency of boron in a-Si H is — 0.1 % for films containing between 1019 and 1021 boron atoms cm-3. Thus, more recent estimates of the doping efficiency are in the range 0.1-1.0%. Apparently, many of the dopant atoms do not go into electronically active substitutional sites. 

The second key observation is that the intersections between a twin boundary and a crystal surface represent chemically activated sites (and mechanically soft areas) (Novak and Salje 1998a, 1998b). It appears safe to assume that similarly activated sites exist also at the intersection of APBs and dislocations with the surface (e.g. Lee et al. 1998, Hochella and Banfield 1995). Besides the obvious consequences for the leaching behaviour of minerals, these key observations lead to the hypothesis of confined chemical reactions inside mesoscopic patterns. The idea is as follows as the surface energy is changed near mesoscopic interfaces, dopant atoms and molecules can be anchored near such interfaces. Some particles will diffuse into the mineral and react with... 

The doping concentration is often confused with the carrier concentration. In uniformly and moderately doped substrates the two are virtually identical at room temperature. This is no longer true when the substrate is heavily doped or when a diffused or ion-implanted layer is measured. Even if all the dopant atoms are electrically active, the carrier concentration in heavily-doped material is lower than the doping concentration, as described by Fermi-Dirac statistics if). This is further aggravated if, for example, the ion-implanted layer is not wholly activated or if there is a steep doping gradient. It is important that one is aware of these difficulties. 

This technique gives the total dopant concentration, not just the electrically active portion. We mention it here, even though it is not an electrical characterization technique in the sense that the others are, because it is routinely used to characterize the dopant concentration and depth of ion-implanted and diffused layers. When the dopant atoms are electrically active, then SIMS is found to give results very close to those obtained from spreading resistance measurements ( 9). When electrical activation is not complete, then there will be significant deviations between SIMS and SRP or C-V data. 

SRP. Spreading resistance profiling measures the response of the dopant atoms that reside at electrically active sites in the lattice. Combining NDP and SRP allows one to distinguish dopants, such as boron, that ire activated into electrically active sites from those located in nonactivated sites, such as in precipitates or interstitials. Therefore, the techniques can be used to select treatment methods that best activate the boron dopant and to provide information on the regions where non-electrically active dopant resides. 

In the case of doping by ion implantation, rii can be determined from the ion dose if the range distribution of the implanted ions is known. The concentration D of impurity atoms in a doping configuration has been directly measured by EXAFS studies (Knights et al, 1977) indirectly it can be estimated by monitoring the electrical activity of these atoms. However, only ionized dopant atoms ( d) are electrically active, and we may define... 

Ion implantation is a low temperature process which will not usually disturb earlier impurity distributions which may have been placed in the material. An anneal cycle must, however, be considered in order to electrically activate the dopant atoms. 

Imaging lattice defects (dislocations, stacking faults) and dopant striations that actively influence the diffusion length, for example, by a Cottrell atmosphere of dopant atoms... 

EDMR has allowed several different unusual experiments. For example, neutral arsenic dopants interacting with a 2D electron gas have been studied with continuous-wave EDMR at 9.7 GHz and 94 GHz. The Anderson-Mott transition between conduction by sequential tunneling through isolated dopant atoms, and conduction through thermally activated impurity Hubbard bands has been studied in arrays of a few arsenic dopant atoms in a silicon transistor. Single erbium spins with resolved hyperfine structure have been electrically detected after resonant optical excitation. The use of the valley degree of freedom has been eonsidered with dopants in silicon both experimentally and theoretically. The quantum confinement due to silicon nanowires may inerease the temperatures where silicon donor quantum devices ean operate. ... 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

---