Shallow Donors in GaN and Related Compounds

The electron effective mass in GaN is now reasonably well established by cyclotron resonance measurements [14-16] asm, = (0.22 0.0 l)m, and the low frequency dielectric constant (appropriately averaged spatially) e(0) = 9.5 0.2, from infrared refractive index and optic phonon energy measurements [17]. We can therefore derive a reliable value for the hydrogenic donor ionisation energy of EDH = (33.0 1.5) meV which compares well with IR absorption measurements, giving Ed = (35 1) meV [18] (see below). The discrepancy is readily explained in terms of a small chemical shift. 

Skromme et al [24] claim to have identified a weak transition in the low temperature spectra of undoped MOVPE and MOMBE-grown GaN films as a two-electron replica of the D°X transition (in which the donor is left in its 2s excited state) which yields a value of 22 meV for the ls-2s separation and, therefore, a donor ionisation energy ED = 29 meV. 

The literature abounds with reports of thermal activation energies for shallow donors in GaN, obtained from Hall effect measurements over a range of temperatures, above and below room temperature, though their interpretation is rendered problematic by a number of complicating factors. At low temperatures there is clear evidence for impurity band conduction (see, for example, [31]) which severely limits the temperature range over which data may usefully be fitted to the standard equation for free carrier density n in terms of the donor density ND and compensating acceptor density NA  

The results of these pressure measurements can be transferred to the AlxGai.xN system, suggesting that 0, though a shallow donor for low values of x, will form a deep trapping level for x 0.4. This is qualitatively in accord with the experimental observations reported below of carrier freeze-out for x 0.2. Si is predicted to act as a shallow donor for Al fractions up to at least 0.56, though experimental data suggest that Si-doped alloys also show significant carrier freeze-out. 

From the above account, it will be apparent that our knowledge of shallow donors in GaN is still far from complete. In particular, the controversy over the origin of n-type conduction in undoped material 

---

A8.4 Shallow donors in GaN and related compounds A8.5 Acceptors in GaN and related compounds A8.6 Theory of codoping of acceptors and reactive donors in GaN... 

---