Groups-II and -I Acceptors in Group-IV Crystals

80 K) and all these pairs exhibit a separation close to 2meV, the value of the separation of the lTg+ and ir7+ acceptor levels. 

A good linear correlation was found between the integrated absorption of the strongest RT line centred at 347 meV and the neutral acceptor concentration obtained from Hall effect measurements of five natural lib diamonds [43]. This was later converted into a RT calibration factor of this band of 1 x 1014 cm-1, assumed to be valid for B concentrations up to a few 1018 cm-3. For larger B concentrations going up to 1 x 102°cm 3, a calibration factor of about one order of magnitude larger was obtained by correlation with SIMS measurements on CVD diamonds [64], These calibration factors are discussed in the review by Thonke [177], 

A pi/2 spectrum, similar to the one observed in silicon, should be observed at energies a few meV above the ionization energy. However, such a spectrum has not been clearly identified. 

An ESR spectrum denoted C-NL1 corresponding to J = 3/2 has been detected in lib diamond at 1.4 K under a uniaxial stress 0.5GPa and it has been attributed to neutral B [3]. 

In 677-SiC, B replaces a Si atom and its ionization energies in the three non-equivalent sites measured by admittance spectroscopy are 0.27, 0.31, and 0.38 eV [56], In undoped and boron-doped p-type 6H-SiC samples, a photoionization spectrum with a temperature-dependent threshold between 0.5 and 0.7eV, and a maximum at 1.75 eV has been reported [83]. The difference between the threshold energy and the electrically-measured ionization energy of B (0.3-0.4eV) is attributed to lattice relaxation. This photoionization spectrum is correlated with the observation near LHeT of three narrow absorption lines at 2.824, 2.863, and 2.890 eV tentatively attributed to excitons bound to neutral B at the three possible sites in 6H-SiC. 

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In predominantly covalent compounds the behavior of impurities is similar to that in covalent elemental crystals. For example, in gallium arsenide, group I or II elements on Ga sites or group III or IV elements on As sites will act as acceptors, while group IV or V elements on Ga sites and group VI or VII elements on As sites will act as donors. Thus, a group IV element such as Si may act as either a donor or acceptor depending upon which sublattice it enters ... 

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