Luminescence properties of defects in GaN

Michael A. Reshchikov; Hadis Morkoç

doi:10.1063/1.1868059

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Luminescence properties of defects in GaN

Michael Reshchikov

2005, Journal of Applied Physics

https://doi.org/10.1063/1.1868059

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Abstract
AI

Gallium nitride (GaN) and its alloys are critical semiconductor materials used in various applications, particularly in visible and ultraviolet light emission and high-power electronics. Understanding the defects within GaN, notably point defects and their impact on luminescence properties, is essential for further enhancing the performance of devices. This paper discusses the relevance of luminescence techniques in identifying these defects, highlights existing controversies in the literature regarding their characterization, and emphasizes the need for a thorough review of defect-related luminescence to facilitate device improvement.

Key takeaways
AI

Understanding and reducing defects in GaN is critical for enhancing device performance.
Point defects in GaN lead to excess dark current, noise, and reduced efficiency in devices.
Dislocation densities in GaN on sapphire substrates range from 10^9 to 10^10 cm^-2, complicating fabrication.
Luminescence is a powerful tool for identifying point defects, especially in wide-band-gap semiconductors.
The review critically analyzes defect-associated luminescence, aiming to clarify controversial findings in GaN literature.

Figures (123)

FIG. 1. Radiative transitions associated with major doping impurities (see Sec. V) and unintentionally introduced defects (Sec. IV) in GaN. For the VcgaOn complex, two charge states are shown (Sec. IV B). Transitions re- sulting in the GL2 and RL2 bands are assumed to be internal and the related defect levels are unknown (Sec. IV F).

FIG. 2. Formation energies as a function of Fermi level for native point defects in GaN. Ga-rich conditions are assumed. The zero of Fermi level corresponds to the top of the valence band. Only segments corresponding to the lowest-energy charge states are shown. Adapted with permission from Limpijumnong and Van de Walle, Phys. Rev. B 69, 035207 (2004). Copy- right (2004) by the American Physical Society.

FIG. 3. Transition levels for native defects in GaN, determined from forma- tion energies displayed in Fig. 2. Adapted with permission from Limpijum- nong and Van de Walle, Phys. Rev. B 69, 035207 (2004). Copyright (2004) by the American Physical Society. Similar to the case of GaAs,”” vacancies in GaN are multiply charged defects, and several defect levels may ap- pear in the energy gap (Fig. 3). A gallium vacancy (Vc¢,) may be the dominant native defect in n-type GaN, whereas the nitrogen vacancy (Vy) may be abundantly formed in p-type

FIG. 4. Calculated formation energies and ionization levels for the defects in GaN in the Ga-rich case. The dashed lines correspond to isolated point defects and the solid lines to defect complexes, respectively. Reprinted with permission from Mattila and Nieminen, Phys. Rev. B 55, 9571 (1997). Copyright (1997) by the American Physical Society.

TABLE II. Calculated acceptor ionization energies (in meV) for wurtzite (wz) and zinc-blende (zb) GaN.

FIG. 6. Calculated formation energy of interstitial hydrogen in wurtzite GaN as a function of Fermi level. Es=0 corresponds to the valence-band maximum, and formation energies are referenced to the energy of a H, molecule. Reprinted with permission from Van de Walle, Phys. Status Solidi B 235, 89 (2003).

FIG. 7. Calculated formation energies of hydrogenated Ga vacancies in GaN as a function of Fermi energy. The formation energies of the isolated vacancy (V2), of interstitial Ht and H™, and of the Si donor are also in- cluded. Reprinted with permission from Van de Walle, Phys. Rev. B 56, 10020 (1997). Copyright (1997) by the American Physical Society.

FIG. 8. Schematic of the main transitions in n-type GaN in conditions of PL. Electron-hole pairs (6én=p) are created with the rate G by optical exci- tation. The photogenerated holes are captured by radiative acceptors (one acceptor level A; is shown), nonradiative defects (S), and form excitons (Ex). The level for excitons is conventional, meaning only that some energy is required to dissociate the excitons. The solid and dotted lines show the transitions of electrons and holes, respectively. Optical transitions are shown by the straight solid lines; recombination of holes at nonradiative centers with free electrons is shown with a dashed line. Rates for all the transitions are noted.

The probability of thermal activation of holes from an acceptor, Q,. can be obtained from a detailed balance as where g is the degeneracy factor of the acceptor level, Ny is the density of states in the valence band, m, is the effective mass of the holes in the valence band, v, is the hole thermal velocity, and o,; is, by definition, the hole-capture cross sec- tion of the ith acceptor. An analysis of Eqs. (6) and (8) shows that the temperature dependence of PL intensity for the ith acceptor involves a region of thermal quenching with an ac- tivation energy Ea, at temperatures T satisfying the condition (1-7, i) TR, Q,>1. Variation of the parameter 7, in the region of thermal quenching of PL related to ith acceptor can be ignored if the quenching regions for different defects do not overlap significantly. Parameters Ey, and C,; (or oi) can be obtained by fitting Eq. (6) to the experimental dependence of the PL intensity in the region of thermal quenching of the PL band related to the ith acceptor. Note that the value of Ey. calculated from the fit to the experimental data using Eq. (6) is somewhat different from the ionization energy obtained from the slope of the Arrhenius plot of I?4(T-!) due to the temperature dependence of Ny and C,;. The temperature de- pendence of the acceptor energy level may also lead to some discrepancy between the value of E,. at temperatures of PL quenching and the value of E, (T=0) obtained from the low- temperature spectroscopy. An analysis of Eqs. (6)-(8) also indicates that the inte- grated PL intensity for the given recombination channel at a It is evident from Eqs. (6) and (7) that the temperature dependence of the PL intensity in n-type GaN is largely de- termined by exciton dissociation and thermal escape of holes from the defects to the valence band. Although the exact expressions for the exciton dissociation can also be derived,”® it is much easier to account for the contribution of the exciton dissociation into the defect-related PL (the term TrexQ ex) by taking the temperature dependence of the inte- grated exciton emission intensity from the experiment.°”

FIG. 9. Calculated temperature dependencies of the PL QE for three radia- tive recombination channels in GaN: excitonic (ex) and via two acceptors (Al and A2). The dependences were calculated using Eqs. (6)-(10) with the following parameters: Max(0)=0.2; 7a1(0) =0.2; ma()= 0.08; TRexQ ex =250 exp(—10 meV/kT), 7g,=10°s, 7R2=5X10s, C,;=10% cms, Cp=4X 1077 cm? st, Ex, =0.34 eV, Eq, =0.8 eV. Reprinted with permis- sion from Reshchikov and Korotkov, Phys. Rev. B 64, 115205 (2001). Copyright (2001) by the American Physical Society.

FIG. 11. An example of the CC diagram and resulting PL spectrum. The measure of the electron-phonon coupling is the Huang-Rhys factor, S.°4 The larger the S, the stronger the electron-phonon coupling is and the broader the PL band is. The so-called configuration coordinate (CC) model®-*" j often used to illustrate and even quantitatively describe re- combination involving a localized carrier. The simplest one- dimensional CC diagram is illustrated in Fig. 11. In this dia- gram, adiabatic potentials represent the total potential energy, including electron and nuclear, of the defect. The equilibrium positions of the ground and excited states are displaced according to the strength of the electron-phonon coupling. In the simplest case, the electronic state interacts with a single localized vibrational mode of frequency wp. In general, the phonon frequencies in the ground and excited states may be different and are represented by the terms w4 and w5, respectively. At low temperatures (kT <fw9) optical transitions take place from the zero vibrational level of the excited state, and the maximum of the emission (Ex) COT: responds to the solid vertical line AB indicated in Fig. 11. After emitting a photon, the system relaxes to the zero vibra- tional level of the ground state by emitting several phonons with energies /w§ in the crystal (transition B -C). Probabil- ity of optical transition from the zero level of the excited state to the n vibrational level (n=0,1,2,...) of the ground state is determined by the Poisson distribution

FIG. 10. Schematic representation of a recombination of the localized hole with a free electron, resulting in emission of a photon and several phonons.

FIG. 12. An example of the CC diagram and resulting PLE spectrum. D. Photoluminescence excitation spectra

FIG. 16. Temperature-induced variation of the intensity of the YL band in the region of its quenching. The solid curves demonstrate fits by using Eqs. (5)-(7). Reprinted with permission from Reshchikov and Korotkov, Phys. Rev. B 64, 115205 (2001). Copyright (2001) by the American Physical Society.

FIG. 15. Temperature dependencies of QE of the YL, BL, and exciton emission at excitation density of 210 W/cm2. Reprinted with permis- sion from Reshchikov and Korotkov, Phys. Rev. B 64, 115205 (2001). Copyright (2001) by the American Physical Society.

FIG. 17. Intensity of the YL, BL, and UVL bands at 15 K in undoped GaN grown by MOCVD. The curves are calculated using Eq. (17). Reprinted with permission from Korotkov et al., Physica B 273, 80 (1999). Copyright (1999) by Elsevier.

FIG. 18. Steady-state PL spectrum (solid curve) and time-resolved PL spec- tra (points) at time delays of 10~, 10-, and 10~ s for undoped GaN sample grown by MOCVD. Reprinted with permission from Korotkov et al., Physica B 273, 80 (1999). Copyright (1999) by Elsevier. The time-resolved PL spectra for different delay times are shown in Fig. 18. No difference is seen between the SSPL and TRPL spectra for small delay times.'®® With in- creasing delay time, no shift or change in the band shape is observed within the experimental accuracy of <30 meV. Similar results have been reported in Refs. 76 and 191. The PL bands originating from the DAP transitions involving deep donors are expected to redshift significantly with delay time.’?°° This is a result of the faster recombination in close pairs, which contribute to the high-energy side of the band

FIG. 19. Temperature dependence of the effective lifetime of the YL (2.2 eV), BL (2.9 eV), and UVL (3.27 eV) bands in undoped GaN grown by MOCVD. Reprinted with permission from Korotkov et al., Physica B 325, 1 (2003). Copyright (2003) by Elsevier.

FIG. 20. PLE spectra for the Y L in the 2.5-m-thick MBE-grown GaN layer at 15 and 295 K. The solid curve is a Gaussian fit with maximum at 3.32 eV and FWHM of 0.52 eV. The broken line is a Gaussian fit with maximum at 3.19 eV and FWHM of 0.40 eV. Reprinted with permission from Resh- chikov et al., Mater. Res. Soc. Symp. Proc. 693, 16.19 (2002).

FIG. 21. One-dimensional CC diagram for the YL in GaN.

FIG. 22. PL spectra from a freestanding GaN template at 15 and 295 K. Exciton part at 15 K is cut in order to present better the defect-related bands. See Ref. 222 for details of the exciton part.

FIG. 24. Dependence of the integrated intensity of the YL and GL bands on excitation intensity. The points are experimental data obtained from decon- volution of the spectra shown in Fig. 23 onto two bands (YL and GL). The solid lines represent a fit with the following parameters: Cy; =10°8 cm3s™, Cy9=2.5x 10"? cm3s, C,;=3.3x 1077 cms, Cy9=1.5 x 1077 cm3 sl B=5x107 cm?s*t, Na=t. 5x 1045 cm, no=13 x 10!6 cm-3. Also shown are the FWHM and position of maximum (after deduction of 2 eV for appearance) of the experimentally observed broad band (points) and the same values simulated by using the modeled shapes of the YL and GL bands and their relative contributions. Reprinted with per mission from Reshchikov et al., Appl. Phys. Lett. 81, 4970 (2002). Copy- right (2002) by the American Institute of Physics.

FIG. 23. Room-temperature PL spectrum of a freestanding GaN template (Ga face) at different excitation densities. Points are experimental data (only every tenth point is shown for clarity); solid curves - fit using modeled Y L and GL bands with their relative contribution given in Fig. 24. Reprinted with permission from Reshchikov et al., Appl. Phys. Lett. 81, 4970 (2002). Copyright (2002) by the American Institute of Physics.

FIG. 25. PL spectrum in the defect range taken for below-band excitation at different photon energies. The excitation density is about 0.2 W/cm2. Re- printed with permission from Reshchikov et al., Appl. Phys. Lett. 78, 3041 (2001). Copyright (2001) by the American Institute of Physics.

FIG. 26. Position of the broad band maximum, FWHM, and sample trans- parency for the freestanding GaN template as a function of the incident light energy at room temperature. The excitation density is about 0.2 W/cm?. The full squares and triangles are for the Ga face. The open squares and triangle: are for the N face. The solid lines are guides to the eye. Reprinted with permission from Reshchikov et al., Appl. Phys. Lett. 78, 3041 (2001). Copyright (2001) by the American Institute of Physics.

FIG. 27. PLE spectra for the YL in the freestanding GaN template at dif- ferent temperatures. The solid curve is a Gaussian fit with maximum at 3.32 eV and FWHM of 0.52 eV. Reprinted with permission from Reshchikov et al., Mater Res. Soc. Symp. Proc. 693, 16.19 (2002).

FIG, 28. PL spectrum from the freestanding GaN template excited with below-band-gap energy (3.44 eV) at 50 K. Points— experiment. Dashed lines— Gaussians peaking at 1.86 and 2.27 eV and having FWHM of 390 meV. The solid curve is the sum of two Gaussians. While the Y L band intensity under resonant excitation in the MBE-grown sample was completely independent of tem- perature (Fig. 20), it sharply changes in the temperature range of 30-50 K in the freestanding template, being tem- perature independent at higher and lower temperatures (Fig. 27).!°° An activation energy of 15 meV, determined from a

FIG. 29. Steady-state PL spectra (solid curves) obtained from a freestanding GaN under excitation with pulsed nitrogen (SS-N) and cw He-Cd (SS- HeCd) lasers, and time-resolved PL spectra at different time delays (points). T=15 K. Reprinted with permission from Reshchikov et al., Appl. Phys. Lett. 83, 266 (2003). Copyright (2003) by the American Institute of Physics.

FIG. 31. Population of an acceptor with holes after excitation pulse in the DAP model (Ref. 79) with the same parameters as in Fig. 30. Note that W,,a, covers the range estimated for different acceptors in GaN. Reprinted with permission from Reshchikov et al., Physica B 340-342, 444 (2003). Copy- right (2003) by Elsevier.

FIG. 30. PL intensity decay of the UVL (3.25 eV), BL (2.9 eV), and GL-YL (at 2.2 eV) in freestanding GaN grown by HVPE. The curves are calculated using Eq. (17) with the following parameters: Np =1.8x10'® cm, ap =2.4 nm, and Wyax=10°, 3 107, and 10° s, as shown in the figure. Re- printed with permission from Reshchikov et al., Physica B 340-342, 444 (2003). Copyright (2003) by Elsevier. Several mechanisms may lead to persistent PL. The most common ones are DAP transitions, ° potential fluctuations,” and trapping and detrapping by shallow traps.”” We may completely neglect potential fluctuations in the studied

FIG. 33. PL lifetime (points) as a function of inverse temperature for the main PL bands in freestanding GaN: UVL band (at 3.28 eV), BL band (2.9 eV), GL band (2.43 eV), and YL band (2.21 eV). Variation of the inverse free-electron concentration (solid line) is also shown for comparison. Re- printed with permission from Reshchikov et al., Physica B 340-342, 448 (2003). Copyright (2003) by Elsevier.

FIG. 32. Steady-state (SS) PL spectrum (solid line) and PL spectrum at time delays of 10-*, 107°, 10, and 10? s (open points) in freestanding GaN at 100 K. Effective lifetime is also shown (closed points). Reprinted with per- mission from Reshchikov et al., Physica B 340-342, 448 (2003). Copyright (2003) by Elsevier.

FIG. 35. UVL band in undoped GaN at 15 K. Points are from experiment The dashed line is a modeled shape of the zero-phonon line (ZPL). The solic line is a fit accounting for LO phonon replicas having intensities of 0.4 0.097, 0.022, and 0.004 with the shapes identical to the modeled shape o: the ZPL. The ZPL and donor-bound-exciton peaks are located respectivels at 3.286 and 3.475 eV in this undoped GaN layer grown by MBE or sapphire.

FIG. 37. PL spectrum of the UVL band at different temperatures in free- standing GaN (empty squares) and in the GaN overgrown on the freestand- ing GaN template by MBE (filled circles). Excitation density is 100 W/cm?. Reprinted with permission from Reshchikov et al., Appl. Phys. Lett. 79, 3779 (2001). Copyright (2001) by the American Institute of Physics.

FIG. 36. Transformation of the UVL band (e-A related) in GaN with temperature.

FIG. 38. Positions of the ZPL of the UVL band in undoped GaN as a function of excitation intensity in the steady-state PL (filled points, bottom scale) and time delay after excitation pulse (open points, top scale). Squares-30-um-thick GaN layer on sapphire substrate, triangles— 200-uym- thick freestanding GaN template, both grown by HVPE. The e-A line is detected at ~3.282 eV while the DAP line position varies from 3.253 to 3.269 eV depending on experimental conditions. Reprinted with permission from Reshchikov et al., Physica B 340-342, 444 (2003). Copyright (2003) by Elsevier.

FIG. 39. PL decay for the DAP transitions (filled points) and e-A transitions (empty points) of the UVL band in freestanding GaN template. Reprinted with permission from Reshchikov et al., Physica B 340-342, 444 (2003). Copyright (2003) by Elsevier.

FIG. 40. ODMR spectra at 24 GHz detected on the UVL band (at 3.27 eV) for several orientations of the applied magnetic field in the (11-20) plane (0° refers to the c axis). The thick black curves (displaced vertically for clarity) are simulations of the low-field line shapes for resonance SA. Reprinted with permission from Glaser et al., Phys. Rev. B 68, 195201 (2003). Copy- right (2003) by the American Physical Society.

FIG. 41. g values of ODMR signal SA in homoepitaxial Si-doped GaN layer (circles) and those found previously for Mg shallow acceptors in Mg-doped GaN heteroepitaxial layers (squares) as a function of the angle (6) between magnetic-field vector and the c axis. The dashed curves are fits to the data [see Eq. (30)]. Reprinted with permission from Glaser et al., Phys. Rev. B 68, 195201 (2003). Copyright (2003) by the American Physical Society.

FIG. 46. Room-temperature PL spectrum of GaN layer grown by HVPE on sapphire (H966 for us) excited at i.w,,,=3.41 eV. The points are experimen- tal data taken at P.,,=1 (1) and 0.001 W/cm? (2). The curves are self- consistent deconvolution of the experimental curves into two bands (a RL with a maximum at 1.92 eV and a GL with the maximum at 2.39 eV). Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 680, E5.6 (2001).

FIG. 47. Room-temperature PL spectra of GaN layers grown by MBE on sapphire. Oscillations with a period of 100-150 meV are caused by inter- ference from the 1.5-2.5-um-thick films. Sample svtl703 is a representative sample with a strong Y L band in the PL spectrum.

FIG. 48. PL spectra from Ga-rich GaN layers grown by MBE on sapphire. T=15K.

FIG. 49. Effect of excitation density, P,,,, on low-temperature PL spectrum of the Ga-rich GaN layer. Position and shape of the bands are independent of excitation density with accuracy of + (5-10) meV. The curves are shifted arbitrarily in the vertical direction (log scale).

FIG. 51. PL spectra of the Ga-rich GaN layer (sample svt591) at different temperatures.

FIG. 50. Variation of quantum efficiency of the RL2, GL2, and exciton emission bands in the Ga-rich GaN layer with an excitation density at 15 K.

FIG, 52. PL intensity (in log scale) vs inverse temperature for the RL2 and GL2 bands. The solid lines are calculated curves using Eq. (32) with the activation energy (Ey) of 115 meV (1) and 100 meV (2) for the RL2 and GL2 bands, respectively.

FIG. 53. Temperature dependence of the peak position of the RL2 and GL2 bands in Ga-rich GaN.

FIG. 54. Temperature dependence of the FWHM of the RL2 and GL2 bands in Ga-rich GaN. The curves are fit by Eq. (26) with the following param- eters: W(0)=250 meV and fw) =23 meV (GL2); W(0)=300 meV and hw) =33 meV (RL2).

FIG. 55. PLE spectra at 15 K for the RL2 and GL2 bands in Ga-rich GaN. The points are experimental data. Curve 1 represents a part of the PLE spectrum for the RL2 band in the sample svt591 measured with excitation by Xe lamp. Curve 2 is the PLE spectrum of the sample svt369 at 295 K and represents the noise signal. Curves 3 and 4 are calculated Gaussians with maxima at 3.10 eV (3) and 3.38 eV (4) and FWHM of 372 meV (3), and 204 meV (4). Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 680, E5.6 (2001).

FIG. 57. PL spectra of undoped GaN samples grown by MOCVD on sap- phire. T=200 K.

FIG. 56. An example of the CC diagram for the GL2 and RL2 bands in Ga-rich GaN. Transition 1 corresponds to excitation of the e-h pair; transi- tion 2 corresponds to capture of carrier by defect in its excited state and relaxation of the lattice around the defect; PL involves emission of a photon (vertical transition 3) and relaxation of the lattice followed by emission of phonons (transition 4); thermal excitation of the defect up to crossover point (transition 5) results in nonradiative recombination with emission of many phonons (transitions 6 and 4). E, is the barrier height for the nonradiative recombination. Zero-phonon energy is Ey=E,—E.

The highest QE at low temperature. ‘Critical temperature above which the quenching takes ple At 300 K or at T,, whichever is smaller. n.e.=nonexponential TABLE III. Parameters of the PL bands in undoped GaN analyzed in our laboratory.

FIG. 58. Normalized AL band in GaN films grown by MBE on 10-um HVPE layer (sample svt857) and on 200-um HVPE freestanding template (sample svt844) at 15 and 295 K.

TABLE IV. Low-temperature parameters of the PL bands and related defects in doped GaN.

FIG. 59. Low-temperature PL spectrum of undoped and Zn-doped GaN layers grown by HVPE on sapphire. The intensity is normalized at maxi- mum of the BL band.

FIG. 62. Low-temperature PL spectra at different excitation intensities from high-resistivity GaN:Zn layer grown by HVPE on sapphire (front side).

FIG. 61. PL spectra of high-resistivity Zn-doped GaN grown on sapphire (front side) at different temperatures. Excitation density is 0.1 W/cm?. Re- printed with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 693, 12.10 (2002).

FIG. 63. PL intensity decay of the BL (at 2.9 eV) in undoped (1011), Si-doped (1721), and two Zn-doped (560 and 1394) GaN layers grown by HVPE on sapphire. Note the logarithmic scales. Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 693, 12.10 (2002).

FIG. 66. Variation of the energy of the DAP transitions vs the reciproca DAP separation for the Mg,, acceptor in GaN. Reprinted with permissior from R. Stepniewski et al., Phys. Status Solidi B 210, 373 (1998).

FIG. 65. Low-temperature (4.2 K) PL spectrum of a Mg-doped GaN sample. Reprinted with permission from R. Stepniewski et al., Phys. Status Solidi B 210, 373 (1998).

FIG. 64. Room-temperature concentration of free holes in Mg-doped GaN as determined from the Hall effect vs concentration of Mg obtained from SIMS measurements. The solid line is a guide to the eye. Reprinted with permission from Obloh et al., J. Cryst. Growth 195, 270 (1998). Copyright (1998) by Elsevier.

FIG. 67. Expected typical amplitude of potential fluctuations in p-type GaN:Mg with the combined concentration of donors and acceptors N; =10!8, 10'9, and 107° cm=.

FIG. 68. Schematic representation of bands and defect levels in the presence of long-range potential fluctuations under conditions of PL. The arrows indicate diagonal transitions resulting in redshifted PL bands. Photogener- ated electrons and holes are shown as filled and empty circles, respectively. in the case of DAP transitions,*"*°!°3!”°!® where the param- eter y was estimated from Eq. (34). Equations (36) and (37) are obtained assuming that after excitation the photogener- ated electrons and holes relax quickly to the potential minima and maxima of the conduction band and the valence band, respectively (Fig. 68). Then they recombine with the probability determined by the degree of overlap of the wave functions of the recombining carriers.®°>"? In the case of the DAP recombination, the photogenerated carriers are first captured by donors and acceptors, resulting in an additional terms in Eq. (37). In both cases, the spatial separation of the recombining carriers results in diagonal transitions with re- duced energy and slower dynamics. Increasing the excitation intensity flattens the potential fluctuations according to Eq. (34), and the PL bands shift to higher photon energies, ap- proaching the shape and position ascribed to uncompensated semiconductor.°”**3!6319 Tt follows from the above argu- ments that Eqs. (36) and (37) can be used only when the wave function of at least one carrier is large enough, namely, comparable to the spatial size of potential fluctuations.

FIG. 69. Estimated average slope of potentials in compensated p-type GaN:Mg with the combined concentration of donors and acceptors N; =10!8, 1019, and 10° cm>.

FIG. 72. Low-temperature PL spectra of semi-insulating GaN:Mg sample at different excitation intensities. Reprinted with permission from Reshchikov et al., Phys. Rev. B 59, 13176 (1999). Copyright (1999) by the American Physical Society. While the UVL band in GaN: Mg shifted notably with excitation intensity only in selected samples,” the significant shift of the BL band, typically of the order of 0.1 eV, has been observed as its characteristic Feature, 5°:82:294,296-300,304,307-310,322,323,329.330 chifts as high as approximately 0.2 eV have been reported in investigations where the excitation intensity chan anged several orders of mag- nitude, as depicted in Fig, 72.52°%410522 While nearly all

FIG. 70. Low-temperature PL spectrum of GaN:Mg grown by MBE at different excitation intensities. Note the gradual shift of the BL from 2.85 to ~2.95 eV with increasing excitation density up to 0.3 W/cm? above which the UVL band and the A°X line at 3.45 eV emerge with roughly quadratic dependence of their intensities on the excitation density.

FIG. 71. Low-temperature PL spectra of semi-insulating GaN:Mg sample at different excitation intensities. Reprinted with permission from Reshchikov et al., Phys. Rev. B 59, 13176 (1999). Copyright (1999) by the American Physical Society.

FIG. 73. Intensity dependence of the PL spectra in p-type GaN:Mg (a) measured with a cw-HeCd laser at up to 2 kW/cm?, (b) measured with the third harmonic of a pulsed Nd:YAG (yttrium aluminum garnet) laser with pulse intensities up to 5 MW/cm2. Reprinted with permission from Eckey et al., J. Appl. Phys. 84, 5828 (1998). Copyright (1998) by the American Institute of Physics.

FIG. 76. Low-temperature PL spectrum of the As-implanted GaN, showing the no-phonon (NP) line and its phonon replicas. Reprinted with permission from Chen and Skromme, Mater. Res. Soc. Symp. Proc. 743, L11.35 (2003).

FIG. 78. CL spectra of a cubic GaN crystal at 5 K. (a) and (b) correspond t different points on the same crystal. The solid lines in the insets are fits t the data points, using Gaussians. The FWHM of the exciton line amounts t 8 meV (a) and 11 meV (b). Reprinted with permission from Menniger et al Phys. Rev. B 53, 1881 (1996). Copyright (1996) by the American Physice Society.

FIG. 82. PL spectra of the 80-ym-thick GaN layer for o polarization (solid line) and 7 polarization (dotted line). The intensities of the donor-bound excitons (double peak at about 3.477 eV) and acceptor-bound excitons (at about 3.473 eV) are substantially lower in the z-polarized spectrum com- pared to the o polarization because the bound A excitons are dipole- forbidden in the E|lc geometry. Note that due to biaxial compressive stress in GaN grown on sapphire all peaks in this sample are shifted to higher energy by 7 meV as compared to homoepitaxial GaN layers. Reprinted with permission from Paskov et al., Phys. Status Solidi B 228, 467 (2001).

FIG. 80. A schematic of the exciton energy levels in wurtzite GaN in (a) an uncoupled hydrogenlike isotropic model, (b) including the effect of aniso- tropy, (c) including the effects both of anisotropy and intersubband coupling, and (d) including anisotropy, intersubband coupling, and polaron correc- tions. Reprinted with permission from Rodina et al., Phys. Rev. B 64, 115204 (2001). Copyright (2001) by the American Physical Society.

FIG. 81. Schematic diagram of the internal structure of X, and Xp, excitons in a wurtzite GaN. The allowed polarizations are given in parentheses. The thin lines correspond to dipole-forbidden states. Reprinted with permission from Paskov et al., Phys. Rev. B 64, 115201 (2001). Copyright (2001) by the American Physical Society. Figure 82(a) presents the PL spectrum from the 80-y1m- thick GaN layer in the commonly used configuration of PL experiment in which the wave vector of the emitted light k is parallel to the crystalline c axis. Figure 82(b) shows the PL spectra taken in another configuration, namely, for the case where the emission k vector is perpendicular to the c axis, and two polarizations of PL, determined by the orientation of the electric field E, are distinguishable: o polarization (E 1c) and z polarization (E llc). Free-exciton peaks at Elic appear at lower energies than at E Lc. The small shifts in the amount of 0.53+0.03 and 0.24+0.08 meV for the A and B

FIG. 83. A high-resolution PL spectrum of a homoepitaxial MOCV D-grown GaN. Reprinted with permission from Kornitzer et al., Phys. Rev. B 60, 1471 (1999). Copyright (1999) by the American Physical Society.

FIG. 84. Low-temperature PL spectrum of freestanding GaN template. Re- printed with permission from Wysmolek et al., Phys. Status Solidi B 235, 36 (2003).

FIG. 85. PL spectra taken at 5, 15, and 25 K in the two-electron satellit region. The dotted line overlapping the 5-K spectrum represents the best fit Emission-peak assignments apply to the indicated spectrum only except fo mission peaks connected by a dotted line. The spectrum intensities mea sured at 15 and 25 K were divided by 4 and 6, respectively, to minimiz overlapping. Reprinted with permission from Freitas, Jr. et al., Phys. Rev. E 66, 233311 (2002). Copyright (2002) by the American Physical Society. A series of lines observed between 3.44 and 3.46 eV (Figs. 84 and 85) is due to two-electron satellite (TES) transitions.’224” 484 The TES transitions occur when an ex- citon bound to a neutral donor (D°X) recombines and leaves the donor in an excited state. The TES energy for a particular

FIG. 86. Low-temperature PL spectra measured with the excitation power densities, from top to bottom, 61.1, 8.4, 0.80, and 0.08 W/cm*. The inset represents the linear dependence of the excitons’ binding energy with the neutral-donor binding energy. Reprinted with permission from Freitas, Jr. et al., Phys. Rev. B 66, 233311 (2002). Copyright (2002) by the American Physical Society.

FIG. 87. PL spectra of the exciton bound to a neutral acceptor (A°X,) in GaN recorded at different values of magnetic field oriented at 35° to the c axis. The inset shows PL spectrum of GaN in the range of excitonic transi- tions. Reprinted with permission from Stepniewski et al., Phys. Rev. Lett. 91, 226404 (2003). Copyright (2003) by the American Physical Society.

FIG. 88. Time-resolved PL spectrum of GaN:Zn at 9 K. Two lines of exci- tons bound to Mg acceptor (AygX) and Zn acceptor (AzpX) are at 3.467 and 3.456 eV, respectively. Inset: comparison of time-integrated spectra of GaN:Zn at 8 and 31 K. Reprinted with permission from Korona et al., Phys. Status Solidi B 235, 40 (2003).

TABLE V. Classification and typical characteristics of the Y; lines in GaN. Reprinted with permission from Reshchikov et al., Physica B 340-342, 440 (2003 Copyright (2003) by Elsevier.

FIG. 91. Positions of the DBE and Y; peaks in different samples. The samples, including etched ones, are numbered 1,2,3..., on this figure. For example, numbers 23, 24, and 25 correspond to the same sample 605: as- grown, etched in phosphoric acid, and PEC etched, respectively. Note that Y.=Y5, Y4=Y4, and Y7=Y; on this figure when the doublets are resolved.

FIG. 92. 1x 1-~m AFM images of the GaN sample 426. (a)— as-grown; (b)— after etching in boiled aqua regia for 5 min; (c), (d), (e), (f)— after etching in H3PO, at 160°C for 5 s, 15 s, 1 min, and 30 min, respectively. The vertical scale is 30 nm for all images. Density of the etch pits is about 10!° cm~ in (c) and (d). Note that the depth of the pits did not increase with increasing etching time, remaining under 30 nm, and we could not detect any reduction of the layer thickness even after 1 h or longer etching time.

FIG. 90. Low-temperature PL spectra from different GaN layers grown on sapphire by MBE. The spectra are normalized at maximum and shifted along the energy axis (from 1 to 7 meV) so that the DBE peak position is the same for all samples (3.470 eV). Reprinted with permission from Resh- chikov et al., J. Appl. Phys. 94, 5623 (2003). Copyright (2003) by the American Institute of Physics.

FIG, 93. Effect of etching of Ga-polar GaN in boiled aqua regia for 6 min and in H3PO, at 160°C for 5 s and 30 min. P.,,~=100 W/cm’. See the surface morphologies for this sample in Fig. 92.

FIG. 96. Evolution of PL intensity at 3.36 eV with time of HeCd laser exposure. T=15 K. P.,,=200 W/cm?. e. Effect of temperature. With increasing temperature, all the Y; peaks quenched with activation energies in the range between 10 and 30 meV at temperatures below 100 K. Typi- cal transformation of the PL spectrum in Ga-polar film is shown in Fig. 100. The Y,, Y2, Y4, and Y, lines revealed two components.°°”°"**7 The low-energy components (Y7 at 3.21 eV, Y, at 3.350 eV, Y> at 3.400 eV, and Y; at about 3.45 eV) quenched first, giving way to the high-energy compo- nents (Y4 at 3.23 eV, Yj at 3.362 eV, Y5 at 3.407 eV, and Y7 at 3.46 eV) which quenched at higher temperatures (Fig. 100). The quenching of these lines could be fitted using Eq. (32) with E, =18 and 42 meV for the Y, and Y; components and E,=15+5 and 120420 meV for the Y; and Y7 compo-

FIG, 94, Low-temperature PL spectrum of the N-polar GaN layer grown by MBE on sapphire substrate, P ...=300 W/cm. A piece of the sample was etched in H3PO, at 100°C for 10 s and another piece of the sample was etched by PEC method. d. Effect of excitation intensity. The PL intensity of most of the Y; lines increased approximately linearly with increas-

FIG. 95. Transformation of PL spectrum with time of exposure by HeCd laser at 15 K. P¢=200 W/cm. The sample was etched in H3PO, at 160 °C for 1 min and subsequently exposed to air for 20 days. Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 743, L11.3 (2003).

FIG. 97. Low-temperature PL spectrum of Ga-polar GaN layer grown by MBE on top of the HVPE-grown 10-ym-thick GaN on sapphire substrate. Excitation density was attenuated from 100 to 1 W/cm? by neutral density filters. The spectra are shifted arbitrary along the vertical axis for better viewing. Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 693, 16.28 (2002).

FIG. 99. Dependence of the PL peak shift on excitation density at 15 K.

FIG. 100. Temperature dependence of PL spectrum at P .,,=100 W/cm? for as-grown Ga-polar GaN layer by MBE on top of the HVPE-grown 10-um- thick GaN on sapphire substrate. Temperatures are 15, 35, 60, 100, 140, 195 K (solid curves), 25, 50, 80, 120, 160 K (dashed curves), and 240 K (dotted curve). Reprinted with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 693, 16.28 (2002). supports the assignment of the. Yy line to excitons bound to inversion domain boundaries.~° 7 Namow Ga-polar domains in N-polar media might create high and narrow potential humps near the surface due to essentially different band bending near the surface in Ga- and N-polar films.°°? How- ever, this remains to be confirmed before it can be treated with sufficient level of confidence. The exact potential pro- file may be sample dependent, explaining the slightly differ-

FIG. 98. Low-temperature PL spectrum of N-polar GaN layer grown by MBE on sapphire substrate. The spectra are normalized at maximum. Re- printed with permission from Reshchikov et al., Mater. Res. Soc. Symp. Proc. 743, L11.3 (2003). a. The 3.45-eV line (Y,;). The 3.45-eV peak, labeled as Y,, appeared only in the N-polar films containing a high density of inversion domains (~10'' cm~) and relatively low den- sity of dislocations (~10° cm~).°°°*! Schuck et al.°2° re- lated this peak to the inversion domain boundaries. A very small Huang-Rhys factor’” for the Y,; peak (about 0.02)

FIG. 101. Temperature dependence of positions of the FE (DBE) and Y; peaks in GaN samples. For the sample 605 the DBE and FE peaks were unresolved and the position of their common maximum is plotted. The solid curves show the shift of the GaN band gap (Ref. 473) corrected to the following positions of lines at T =0: 3.479 eV (FE); 3.459 eV (Y{); 3.410 eV (Y5); 3.351 eV (Yq); 3.363 eV (Y]); 3.305 eV (Yg); 3.195 eV, 3.210, and 3.225 eV (Y7 and Y3). Reprinted with permission from Reshchikov et al., J. Appl. Phys. 94, 5623 (2003). Copyright (2003) by the American Institute of Physics.

FIG. 102. Low-temperature PL spectrum from GaN layers grown by MBE on sapphire substrate.

FIG. 103. Low-temperature PL spectrum of Ga-polar GaN layers grown by MBE on sapphire substrate subjected to different treatments [rapid thermal annealing (RTA), etching in hot H3PO, and PEC etching].

TABLE VI. Comparison of dislocation densities (from TEM data) and relative intensities of the Y; and DBE lines (from PL spectrum) in GaN layers grown by MBE on sapphire substrate

FIG. 104. Low-temperature (15 K) PL spectra of five GaN layers grown by MBE on sapphire. All the spectra are measured under identical conditions. Excitation density is 30 W/cm?. The spectra have been shifted by up to 10 meV to compensate different strain-related shifts and align the DAP peaks. See characteristics of these samples in Table VI. Reprinted with permission from Reshchikov et al., Mater Res. Soc. Symp. Proc. 798, Y 5.66 (2004). In numerous investigations involving PL in GaN, the artificial oil-related lines with their characteristic properties, presented below, were inadvertently attributed to emission from GaN, 12%283:323,522-534 The energy position of these lines falls into the range of 3.358-3.370 and 3.298-3.313 eV. These lines have often been detected in undoped GaN where no emission typical for GaN was observed2?"°” or in p-type GaN where the near-band-edge emission was absent or very weak, !90.783:323,529 4 few clearly observed phonon replicas of the 3.31-eV line (separated by about 70 meV which is smaller than the LO phonon energy in GaN by about 20 meV) have been recorded.°22">”°*”" A t low temperatures, the 3.36-eV line is stronger and sharper than the 3.31-eV line. With increasing temperature, the 3.36-eV_ line quenches faster than the 3.31-eV line,229922-524926.529.530 and the acti- vation energy of quenching does not exceed 30 meV.°352653F Increasing the measurement temperature re-

FIG. 105. PL spectra of oil from mechanical pump. Phonon replicas of the J4 peaks are separated by 70 meV.

FIG. 107. Variation of the blue band intensity with time (recorded at 2.8 eV). The intensity at “zero time” is normalized to 100% for all excitation densities. The bleaching with time has fast and slow components. The latter is nearly independent of the excitation density. Reprinted with permission from Reshchikov et al., Appl. Phys. Lett. 78, 177 (2001). Copyright (2001) by the American Institute of Physics.

FIG. 110. Variation of PL spectrum with temperature for the GaN layel exhibiting unstable BL and Y L bands. Excitation density is 10-? W/cm? (a such low-excitation density the bleaching caused by UV exposure was neg- ligible). The spectra were taken without high-intensity UV exposure.

FIG. 108. Low-temperature PL spectrum of undoped GaN before and after UV illumination during 150 min with P,,,=0.3 W/cm?. Excitation density during the PL scan is 0.003 W/cm?.

FIG. 109. Evolution of PL intensity at 15 K with UV exposure time for unstable YL and BL bands (YL-u and BL-u, respectively) and stable PL bands (YL and BL) in undoped GaN layers. P..,=0.3 W/cm?.

FIG, 111. Schematic diagram showing the band bending and the vacuur level (Eyqc) near the surface of GaN in vicinity of the metal tip.

FIG. 113. Evolution of the YL intensity at room temperature in changing ambient for the MBE-grown GaN layer.

FIG. 112. Schematic diagram showing the band bending near the surface o! GaN in dark (a) and after UV light pulse (b). Electrons are shown with solic circles and holes— with empty circles. The Fermi level Ep is shown as the same for electrons and holes in nonequilibrium case (b) in assumption tha recombination in bulk is much faster than recombination over the barrier and concentration of traps in the depletion region is much less than density of the surface states.

FIG. 114. Evolution of the near-band-edge emission intensity at room tem- perature under different ambient conditions for the MBE-grown GaN. Re- printed with permission from Reshchikov et al., Mater Res. Soc. Symp. Proc. 743, L11.2 (2003).

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Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steadystate and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN. GaN is widely used in light-emitting devices such as blue and white LEDs and lasers 1. It is also a very promising material for future high-power electronic devices 2-4. In order to achieve high breakdown voltage, the concentration of point defects unintentionally introduced during the material growth must be significantly reduced. Unlike extended defects, which are easy to detect and identify, point defects in GaN are not well understood, and their detrimental role in electronic devices needs to be determined. Hydride vapor phase epitaxy (HVPE) is a superior technique to obtain high-quality GaN with a low density of structural defects and a low concentration of point defects 5. According to secondary ion mass-spectrometry (SIMS) analyses, undoped GaN grown by HVPE contains low concentrations of impurities. In particular, the concentration of carbon, which creates deep-level defects, can be reduced down to 10 15 cm −3 (ref. 6), and the concentrations of oxygen and silicon (both acting as shallow donors in GaN) can be reduced down to 2 × 10 16 and 2 × 10 15 cm −3 , respectively 7-9. All these values are at the detection limit of SIMS measurements. To detect and quantify impurities and native defects with lower concentrations, photoluminescence (PL) can be used 10. For this, the defect-related PL bands must be reliably identified and their characteristic parameters be found. Another challenge for the evaluation of the electrical properties of GaN grown by HVPE on sapphire substrates is the presence of a parasitic conductive layer near the GaN/sapphire interface 11. This degenerate GaN layer shunts the conductivity of the bulk GaN and may introduce significant errors in the concentration of free electrons when it is determined with the Hall effect method at room temperature. The actual concentration of free electrons and their mobility in bulk GaN can be calculated by applying a two-layer model when the Hall effect is studied for a wide range of temperatures 12. The concentration of uncompensated shallow donors in the vicinity of the GaN surface can also be estimated from capacitance-voltage measurements 13. PL has an advantage over the electrical measurements as being a nondestructive method without the need for contact fabrication.

Photoluminescence of polycrystalline hexagonal boron nitride (hBN) was measured by means of time-and energy-resolved spectroscopy methods. The observed bands are related to DAP transitions, impurities and structural defects. The excitation of samples by highenergy photons above 5.4 eV enables a phenomenon of photostimulated luminescence (PSL), which is due to distantly trapped CB electrons and VB holes. These trapped charges are metastable and their reexcitation with low-energy photons results in anti-Stockes photoluminescence. The comparison of photoluminescence excitation spectra and PSL excitation spectra allows band analysis that supports the hypothesis of Frenkel-like exciton in hBN with a large binding energy. Optical and fluorescence properties of hexagonal boron nitride deserved particular interest during last decade since the first observation of an intense far-UV exciton emission 1, 2 making this material a candidate for use in new light-emitting devices. A successful synthesis of high-purity single crystal samples initially at high-P,T 1, 2 and later at atmospheric conditions 2 enabled this achievement. Accordingly, understanding of the electronic band and exciton structure of hBN becomes even more important issue as it serves a basic system for single-wall 3, 4 and multi-wall boron nitride nanotubes . Despites of many efforts in the past, the electronic properties of hBN remain largely unknown. They have been earlier studied by luminescence 1, 10-17 , optical reflectance and absorption 18-21 , photoconductivity 22 , x-ray emission , inelastic x-ray scattering 26-28 , x-ray absorption , or electron energy loss spectroscopy. After all, a large spread of band gap energies reported in literature, ranging from 3.1 to 7.1 eV 14 , is currently explained by sample quality and related to experimental methods used. Recently, arguing on generally high luminosity of the free exciton luminescence, Watanabe et al 1 assumed that hBN is a direct band gap material. They have measured the band gap energy of 5.971 eV and inferred an exciton binding energy of 0.149 eV that corresponds to the Wannier exciton model. However this result is in a large disagreement with the most recent theoretical calculations using the so-called all electrons GW approximation . They predict hBN to be an indirect band gap material with a band gap energy of 5.95 eV and a lowest direct interband transitions at 6.47 eV 34 . Moreover, Arnaud et al. has deduced a huge exciton binding energy of 0.72 eV and predicted that the low-lying exciton in hBN belongs to the Frenkel type. The intense free exciton luminescence observed in single crystal hBN is explained by a large oscillator strength of merged excitonic transitions 34 . In view of many disagreements further confrontation between experiment and theory will continue. In these conditions, more experimental data concerning electronic and related G. Zimmerer, Nuclear Instruments and Methods in Physics Research Section A:

Luminescence properties of defects in GaN

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