Neutralization of shallow donors

HYDROGEN NEUTRALIZATION OF SHALLOW-DONOR IMPURITIES IN ARSENIC-DOPED EPILAYERS ON SILICON... 

Evidence for hydrogen plasma neutralization of shallow donors in high purity (about 5 x 1014 cm-3) n-type GaAs epitaxial layers has been obtained by Pan et al. (1987a) from the observation of the discrete magneto-PTIS spectrum of the donors near 5 meV. This technique is very sensitive and it allowed to observe a decrease (between 10 and 50 percent) of the net... 

Shallow levels play an important part in electronic conductivity. Shallow donor levels lie close to the conduction band in energy and liberate electrons to it to produce n-type semiconductors. Interstitial metal atoms added to an insulating ionic oxide often act in this way because metal atoms tend to ionize by losing electrons. When a donor level looses one or more electrons to the conduction band, it is said to be ionized. The energy level representing an ionized donor will be lower than that of the un-ionized (neutral) donor by the same amount as required to move the electron into the conduction band. The presence of shallow donor levels causes the material to become an w-type semiconductor. 

The same sort of considerations will apply to vacancies. For instance, an anion vacancy may give rise to a set of shallow donor levels just below the lower edge of the conduction band. If the vacancy is created by removing a neutral nonmetal atom from the crystal, the electrons that were on the anion are transferred to the conduction band to produce an n-type semiconductor. The energies of neutral and ionized vacancies are slightly different. 

The exposure of n-type LPE GaAs layers to a hydrogen plasma for three hours at 300°C induces a neutralization of five deep electron traps at c - 0.13 eV, c - 0.36 eV, c - 0.38 eV, c - 0.54 eVand c - 0.73 eV (Pearton and Tavendale, 1982). The thermal stability of these neutralized centers is lower than for EL2 neutralization and can be compared with the shallow donors one. 

In MBE grown GaAs three dominant electron traps are usually observed Ml at c - 0.17 eV, M3 at c - 0.28 eV and M4 at c - 0.45 eV. Exposure of MBE grown material to a hydrogen plasma for 30 minutes at 250°C completely passivates these three deep levels as shown in Fig. 10 (Dautremont-Smith et al., 1986). After five minute anneals at 400°C or 500°C, the passivation remains complete while the shallow donors are fully reactivated. A five minute annealing at 600°C partially restores the electrical activity of M3. Therefore the thermal stability of the neutralization of deep levels in MBE material is much higher than in other materials and is compatible with most technological treatments. 

A comparison of Fig. 5.12 and the defect density in Fig. 5.9 shows that the total density of the band tail electrons - neutral donors plus occupied intrinsic band tail states-is about ten times less than the density of deep defects induced by the doping. This is a remarkable result because it implies that almost all the donors are compensated by deep defects. However, before considering the consequences of this observation, it is helpful to discuss an alternate experimental technique for measuring the density of shallow electrons or holes, because of the possibility that ESR is missing some of the carriers due to electron pairing or broadening of the resonance. 

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