Optical signature of Mg-doped GaN: Transfer processes

Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26–3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes's rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 $\pm$ 5 and 195 $\pm$ 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.

Figure 1

Low-temperature (2 K) PL spectra of not intentionally doped (NID) and fully thermally activated GaN:Mg samples with varying Mg content. The spectral range displays the full spectrum from the free excitons (FXA, FXB), over the donor and acceptor bound excitons (DBX, ABX), towards the spectral region of the donor-acceptor pair transitions (DAP).

Figure 2

Low-temperature (2 K) PLE (a) and PL (b) spectra of a lowly doped ($8\times {10}^{17}{\mathrm{cm}}^{-3}$) and fully thermally activated GaN:Mg sample. PLE spectra with detection energies at the spectral positions of DAP1 and DAP2 reveal several excitation channels of the DAP that can be correlated to their corresponding PL transitions of the free excitons (FX) as well as donor (DBX) and acceptor (ABX) bound excitons.

Figure 3

Low-temperature (2 K) PLE spectra of the $8\times {10}^{17}{\mathrm{cm}}^{-3}$ Mg-doped GaN sample in close energetic vicinity to the DAP1 and DAP2 spectral positions show a structured absorption edge which allows to determine the donor binding energies of the three most prominent donors to $48\pm 5$, $61\pm 5$, and $118\pm 5$ meV by a linear approximation (grey lines). The spectra have been vertically shifted for clarity.

Figure 4

Temperature-resolved PL spectra in the spectral range of the DAP1+2 transition measured for the $8\times {10}^{17}{\mathrm{cm}}^{-3}$ doped GaN:Mg sample. At $\sim$40 K, the appearance of the (e, A) transition can be observed that coexists with the DAP luminescence at higher temperatures, while thermalization of the LO phonon replica of the bound excitons (BX) occurs. Spectra have been vertically shifted for clarity.

Figure 5

The binding energy as a function of the localization energy yields a linear dependence for the donor and acceptor bound excitons.[53] The proportional constants yield 0.11 for the donor and 0.77 for the acceptor bound excitons.

Figure 6

Temperature-resolved PL spectra of the bound (ABX1-3, DBX2-3) and free (FXA) excitons of the $8\times {10}^{17}{\mathrm{cm}}^{-3}$ doped GaN:Mg sample. Spectra have been vertically shifted for clarity.

Figure 7

Intensity of the donor bound excitons DBX2-3 (a) and the acceptor bound excitons ABX1-3 (b) over a temperature range from 3–50 K. The resulting activation energy $E_{\mathrm{act}}$ is a measure of the thermalization process. Numbers in parentheses represent the errors.

Figure 8

PL and PLE spectra of the $8\times {10}^{17}{\mathrm{cm}}^{-3}$ Mg-doped GaN sample in the spectral range of the free (FX) and bound (DBX, ABX) excitons. The drop lines indicate the detection energy. Spectra have been vertically shifted for clarity.

Figure 9

(a) Transients of the bound excitons ABX1 and DBX3. Solid lines show the response function deconvoluted data. (b) Lifetimes ${\tau }_{D1,A1}$ and ${\tau }_{D2,A2}$ of DBX2, DBX3, ABX1-3 as a function of the localization energy $E_{\mathrm{loc}}^{3/2}$. Solid lines represent fits based on a model by Rashba and Gurgenishvili[76] and numbers in parentheses represent the errors.