Doping of nitride semiconductors

n-type doping

Nitride semiconductors are easy to dope n-type; in fact they often exhibit unintentional n-type conductivity.  Nitrogen vacancies were long thought to be the source of this unintented doping; we know know that nitrogen vacancies are unlikely to form in n-type nitrides (see Native defects).  Unintentional impurities, such as oxygen or silicon, are a more consistent explanation for the observed n-type conductivity. 

Oxygen exhibits a very interesting behavior in nitrides.  In GaN, it behaves as a shallow acceptor, but when the band gap is increased (either under hydrostatic pressure, or by alloying with AlN), the oxygen undergoes a transition to a deep center (a so-called DX center).  In this new configuration, which is illustrated at right, oxygen actually behaves as a deep acceptor, i.e., it becomes a compensating center.  Note that the oxygen atom (the red ball) moves away from its normal, substitutional site by almost 1 angstrom. 

Oxygen becomes a deep level in AlGaN when the Al concentration exceeds about 30%.  Our calculations have shown that silicon does not undergo the DX transition.  Silicon should therefore act as a shallow donor in AlGaN up to very high Al concentrations.  Note, however, that oxygen tends to be unintentionally incorporated in compound semiconductors with high Al content;  and any oxygen that is present in AlGaN with more than 30% Al will act as a compensating center!

"DX center formation in wurtzite and zinc-blende AlGaN", Chris G. Van de Walle, Phys. Rev. B 57, 2033 (1998). 

"Doping of AlxGa1-xN", C. Stampfl and Chris G. Van de Walle, Appl. Phys. Lett. 72, 459 (1998). 

"Metastability of oxygen donors in AlGaN", M. D. McCluskey, N. M. Johnson, Chris G. Van de Walle, D. P. Bour, M. Kneissl, and W. Walukiewicz, Phys. Rev. Lett. 80, 4008 (1998). 

p-type doping

Magnesium is the most common p-type dopant. We have found that the limits in the hole concentration are mainly due to Mg solubility, not to incorporation of Mg on other sites. We have performed extensive investigations of interactions between acceptors and hydrogen (see below). Studies of other acceptor dopants indicate that none has characteristics which are uniformly better than those of Mg -- except for beryllium, which may have a somewhat higher solubility as well as a lower ionization energy.  Being a small atom, however, beryllium easily incorporates in interstitial positions, where it acts as a donor.  We are currently investigating how to circumvent this problem.

For more information: 

"Defects and doping in GaN", J. Neugebauer and C. G. Van de Walle, in Proceedings of the 22th International Conference on the Physics of Semiconductors, Vancouver, 1994, edited by D. J. Lockwood (World Scientific Publishing Co Pte Ltd., Singapore), p. 2327. 

"Chemical trends for acceptor impurities in GaN", J. Neugebauer and Chris G. Van de Walle, J. Appl. Phys. 85, 3003 (1999). 

Hydrogen in GaN

The figure shows total energy surfaces for hydrogen in the positive and negative charge states in GaN, as well as for hydrogen interacting with magnesium. Hydrogen behaves as a donor in p-type GaN, and as an acceptor in n-GaN. This behavior is similar to other semiconductors, including the negative-U character of the impurity (meaning that the neutral charge state is never stable); the value of U, however, is the largest ever reported for any defect or impurity: -2.4 eV. Hydrogen molecules are energetically unfavorable. Hydrogen forms electrically neutral complexes with both acceptor and with donor impurities. However, the formation energies indicate that hydrogen incorporation in n-type material is very low. In p-type material, hydrogen is readily incorporated, and  in fact enhances the incorporation of Mg dopants.

"Hydrogen in GaN: novel aspects of a common impurity", J. Neugebauer and C. G. Van de Walle, Phys. Rev. Lett. 75, 4452 (1995). 

"Role of hydrogen in doping of GaN", J. Neugebauer and C. G. Van de Walle, Appl. Phys. Lett. 68, 1829 (1996). 

"Hydrogen and acceptor compensation in GaN"; C. G. Van de Walle et al. in Properties, Processing and Applications of Gallium Nitride and Related Semiconductors, edited by J. Edgar, S. Strite, I. Akasaki, H. Amano, and C. Wetzel, EMIS Datareview Series No. 23 (INSPEC, IEE, 1999), pp. 317-321.