We have identified piezoelectric fields in strained GaInN/GaN quantum well p-in structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga 0.84 In 0.16 N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be ͑0001͒, corresponding to the Ga face.

O and Si donors in GaN are studied by Raman spectroscopy under hydrostatic pressure p. The ground state of O is found to transfer from a shallow level to a deep gap state at p . 20 GPa reminiscent of DX centers in GaAs. Transferred to Al x Ga 12x N we predict that O induces a deep gap state for x . 0.40. In GaN:Si no such state is induced up to the highest pressure obtained ͑p 25 GPa͒ equivalent to x 0.56 in Al x Ga 12x N and possibly higher. We attribute this distinction to the lattice sites of the dopants. O substituting for N is found to be the origin of high free electron concentration in bulk GaN crystals. [S0031-9007(97)03179-7] PACS numbers: 71.55. Eq, 62.50.+ p, 72.20.Jv, 78.30.Fs Donors in III-V compound semiconductors are of special interest because they can assume both extended or localized states, i.e., they can be metastable [1][2][3][4]. In the most thoroughly studied system, GaAs, either a high free carrier concentration n, alloying with AlAs, or application of hydrostatic pressure p can induce a transition from a shallow hydrogenic state of the dopant to a strongly localized one of the same impurity. Many different donor species, e.g., Si Ga (group-IV element on group-III site) and S As (group-VI on group-V site), transform into the nonhydrogenic configuration at very similar characteristic transition pressures [3,4]. In addition, metastability effects, such as a limited free electron concentration and persistent photoconductivity, have been found and interpreted with activation barriers between the different configurations of the donor. All of these effects have been associated with a so-called DX center [2].In GaN we find that the O donor dopant shows characteristic features of a DX defect when hydrostatic pressure is applied. Si, in contrast, behaves like a hydrogenic donor. This distinction is attributed to the actual lattice site of the impurity, and this effect is extremely pronounced in this compound semiconductor system. The pressure experiments can directly be transferred from GaN to the Al x Ga 12x N system predicting a strongly localized gap state of O for higher Al concentrations.Dopant impurities typically can induce both resonant and hydrogenic defect levels in the electronic band structure. In many cases only the hydrogenic level is relevant. Under certain conditions, however, a charge transfer from a quasihydrogenic state to a strongly localized neutral charge state ͑D 0 ͒ can occur [5]. In addition, as proposed by Chadi and Chang [1], a structural relaxation of the donor impurity in the vicinity of the transition conditions can lead to an activation barrier between the two states. This widely accepted model of a structural relaxation explains the metastability and the activation barrier between the different states of DX centers. Promoted by the transfer of electrons, this new strongly localized state (DX) can be the ground state [6]. Ab initio calculations reproduce the experimental observations in GaAs, including the very similar transition conditions found for all substituti...

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Photoluminescence investigations on undoped n-type GaN layers grown on 6H-SiC and sapphire reveal the presence of residual acceptors with a binding energy of 230 meV. Their presence in high temperature vapor phase epitaxy grown layers is strongly correlated with the graphite susceptor containing the Ga. Mg as a contamination can be ruled out. In metal organic vapor phase epitaxially grown layers, the metal organic are probably the source of the carbon contamination. It is concluded that carbon on nitrogen sites introduces the most shallow acceptor in GaN. The experimental observations are supported by an estimate of the acceptor binding energy using effective-mass-theory.