Introduction
Cubic GaN (c-GaN) epilayers grown by molecular beam epitaxy (MBE) show outstanding electrical and optical features [Reference As, Schikora, Greiner, Lübbers, Mimkes and Lischka1,Reference Schikora, Hankeln, As, Lischka, Litz, Waag, Buhrow and Henneberger2], demonstrating the high potential of c-GaN for the realization of blue emitting laser diodes. Gain measurements in undoped cubic material reveal gain values which are comparable to that of the best hexagonal GaN (h-GaN) [Reference Holst, Eckey, Hoffmann, Broser, Schöttker, As, Schikora and Lischka3]. Particularly, epitaxially grown layers of c-GaN lend themselves to the production of cleaved laser cavities and optically excited stimulated emission from such cleaved facetts has already been observed [Reference Holst, Hoffmann, Broser, Schöttker, As, Schikora and Lischka4]. In order to fabricate almost any device it is necessary to carry out controlled doping of GaN in order to realize both n-type and p-type GaN material. In hexagonal GaN the principal p-type dopant is Mg and n-type dopant is Si.
In this paper we summarize recent doping experiments of cubic GaN epilayers by Mg and Si. Secondary ion mass spectroscopy (SIMS), low temperature photoluminescence (PL) and temperature dependent Hall-effect measurements are used to study the incorporation, the optical and electrical properties of Mg- and Si-doped samples.
Experimental
Cubic GaN films are grown by rf-plasma assisted MBE on semi-insulating GaAs (001) substrates at a substrate temperature of 720°C. The growth rate is about 0.07 μm/h and the thickness of the layers is about 1 μm, respectively. Elemental Mg and Si are evaporated from commercial effusion cells at source temperatures between 260°C and 450°C, and 750°C and 1150°C, respectively. The concentration and depth distribution of Mg is measured by secondary ion mass spectroscopy (SIMS) using Mg implanted calibrated standards for quantification, and an O2+ primary beam of 6 keV. Photoluminescence (PL) measurements are performed in a He bath cryostat at 2 K. Luminescence is excited by the 325 nm line of a cw HeCd UV laser with a power of 3 mW and measured in a standard PL-system. Hall-effect measurements are performed using square shaped samples (Van der Pauw geometry) in a cryostat between 240 K to 380 K, at a magnetic field of 0.3 T and with the sample in the dark.
Results and Discussion
Mg doping of cubic GaN
The 2 K photoluminescence spectra of cubic GaN epilayers grown at different Mg source temperatures TMg is shown in Fig.1. The lowest spectrum is that of an undoped reference sample grown before introducing Mg into the chamber. The near band edge luminescence of the reference sample is dominated by an excitonic transition X at 3.26 eV and an omnipresent donor-acceptor pair transition (D0,A0) at 3.15 eV [Reference As, Schmilgus, Wang, Schöttker, Schikora and Lischka5]. The nature of the involved shallow donor and acceptor is not identified up to now. The corresponding binding energies are 25 meV and 130 meV, respectively. Mg-doping at source temperatures TMg below 300°C results in the appearence of a donor acceptor transition (D0,A0 Mg) at3.04 eV. At about 60 K this transition thermalizes to the corresponding band acceptor transition (e, A0 Mg) at about 3.07 eV. The donor participating in this line has the same ionization energy as the one observed in the undoped sample. From the energetic position an acceptor activation energy of EMg = 0.230 eV is estimated. This energy level is in excellent agreement with recent theoretical calculations of MgGa for c-GaN [Reference Neugebauer, Walle and Symp6] and is somewhat lower than that for the corresponding value of 0.265 eV for h-GaN [Reference Kaufmann, Kunzer, Maier, Obloh, Ramakrishnan, Santic and Schlotter7]. Detailed characteristics of the temperature and intensity dependence of the PL spectra will be presented elsewhere. At Mg concentrations higher than 1018 cm−3 (TMg higher than 350°C) the low energy side of the spectrum will be dominated by a broad blue emission band centered at about 2.8 eV, which is modulated due to interference fringes of the PL light (upper curves in Fig.1). The nature of this deep transition is unknown up to now, however it seems that also in c-GaN Mg is incorporated at different lattice sites or forms complexes at high Mg flux. A recent publication on h-GaN attributes this blue band to Mg-VN complexes [Reference Kaufmann, Kunzer, Maier, Obloh, Ramakrishnan, Santic and Schlotter8]. Rapid thermal annealing experiments in N2 atmosphere (30 s) show that the responsible defect for the blue band is thermally stable up to an annealing temperature of 1100°C.
Fig.1: Low temperature 2 K photoluminescence spectra of Mg doped cubic GaN epilayers grown by different Mg source temperature TMg (X excitonic transition, (D°,A°) omnipresent donor-acceptor pair transition). The dashed lines indicate the shallow Mg-related transitions (D°,A°Mg) and (e,A°Mg). The dashed arrow below 2.95 eV indicates the deep Mg-related blue band, which is modulated due to interference fringes of the PL light.
The full squares in Fig.2 show the amount of incorporated Mg measured by SIMS versus the beam equivalent pressure of Mg (BEPMg). The Mg concentration remains below about 5*1018 cm−3 and is nearly independent on the arrival rate of supplied Mg, which varied by four orders of magnitude. This behaviour is similar to that observed for MBE growth of hexagonal GaN [Reference Guha, Bojarczuk and Cardone9] and GaAs [Reference Wood, Destimone, Singer and Wicks10] and is expected to be due to the high vapor pressure of Mg. Depth profile measurements show a homogenious distribution of Mg within the top epilayer and an accumulation of Mg by one order of magnitude at the GaN/GaAs interface [Reference As, Simonsmeier, Schöttker, Frey, Schikora, Kriegseis, Burkhardt and Meyer11]. This may be due to the increased number of structural defects near the interface incorporated as a result of the large lattice mismatch between substrate and epilayer.
Fig. 2: Integral PL intensity of various observed transitions (open symbols), Mg concentration measured by SIMS (full squares), and free hole concentration pHall at room temperature versus beam equivalent pressure (BEPMg) of the offert Mg flux.
Hall-effect measurements of the Mg-doped cubic GaN epilayers show p-type conductivity without postgrowth treatments. For the sample grown at a Mg source temperature of TMg = 300°C the measured free hole concentration pHall and hole mobility at room temperature is about 3*1016 cm−3 and 215 cm2/Vs, respectively. This low hole concentration is expected due to the high ionization energy of Mg ( [Mg]SIMS = 1.7*1018cm−3 ). Under the assumption of compensation the temperature dependence of the concentration of free holes yields for the shallow Mg acceptor an activiation energy of EMg= 0.110 ± 0.020 eV [Reference As12]. The corresponding value for hexagonal GaN is 160 ± 5 meV [Reference Kim, Salvador, Botchkarev, Aktas, Mohammad and Morcoc13]. In both cases the thermal activation energies are much lower than the optical activation energies. The reason for this may be either due to a strong electron lattice coupling [Reference Ridley14] since the Bohr radius of the ”shallow” effective mass like acceptor state is only 6.3 Å or due to potential fluctuations [Reference Dewsnip, Orton, Lacklison, Flannery, Andrianov, Harrison, Hooper, Cheng, Foxon, Novikov, Ber and Kudriavtsev15]. The room temperature free hole concentration pHall versus the beam equivalent pressure of Mg (BEPMg) is depicted in Fig.2 by full circles. In contradiction to the Mg concentration measured by SIMS no increase of pHall vs. BEPMg is seen, indicating that an additional compensating donor may be incorporated during Mg-doping.
This assumption is constrained by our PL-measurements. Using Gaussian functions for the different transitions to fit the measured spectra, the integral intensities of the different PL-lines are estimated and also depicted in Fig. 2 versus BEPMg (open symbols). Both the shallow transitions, (D0, A0 Mg) and (e, A0 Mg), as well as the deep blue band show a clear increase with increasing Mg-flux. Therefore all three transitions are related to the incorporation of Mg into c-GaN. However, whereas the shallow transitions seems to saturate at higher Mg-flux, this is not the case for the blue band. Since the measured hole concentration is nearly independent of the supplied Mg amount we conclude that the blue band may act as a compensating deep donor center. This conclusion is in agreement with similar observations made in h-GaN [Reference Eckey, von Gfug, Holst, Hoffmann, Schineller, Heime, Heuken, Schön and Beccard16]. For p-type GaN theoretical calculations also show that Mgi, MgN or Mg-VN may act as compensating deep donors. Since the formation energies of all these Mg-related defects are low enough they may be possible candidats for the blue band in GaN.
Si-doping of cubic GaN
The optical properties of Si doped cubic GaN are shown in Fig. 3. At 2 K the spectrum of the sample grown with the lowest Si-flux (source temperature TSi= 750°C) is dominated by the excitonic transition X at 3.26 eV and the omnipresent donor-acceptor pair transition (D°,A°) at 3.15 eV [Reference As, Schmilgus, Wang, Schöttker, Schikora and Lischka5]. With increasing Si flux a clear shift to higher energies of the (D°,A°) and an increase of the full width at half maximum (FWHM) is observed. For samples grown at TSi > 1025°C both lines merge to one broad band. This behavior is similar to the one observed in the well known case of GaAs heavily doped with Si or Te and can well be described by electron-impurity interactions (band tailing), shrinkage of the band gap due to exchange interaction among free carriers and conduction band filling effects [Reference Abramov, Abramova, Verbin, Gerlovin, Grigorév, Ignatév, Karimov, Novikov and Novikov17, Reference De-Sheng, Makita, Ploog and Queisser18]. A detailed analysis of the PL-spectra will be published elsewhere.
Fig. 3: Low temperature PL spectra of Si doped cubic GaN epilayers grown with different Si source temperatures TSi .With increasing Si source temperature the (D°,A°) transition broadens and shifts to higher photon energies.
For Si doped cubic GaN epilayers the free electron concentration at room temperature (measured by Hall-effect) is depicted in Fig. 4 as a function of the Si-effusion cell temperature (full triangles). One clearly sees that the free electron concentration nearly exactly follows the Si-vapor pressure curve (full line in Fig.4) [Reference Souchiere and Binh19]. This indicates a constant sticking coefficient of Si at the growth temperature chosen. Nearly all Si atoms are incorporated at Ga sites and act as shallow donors. Temperature dependent Halleffect measurements further show that c-GaN grown at 720°C and a Si source temperature TSi ≥ 1025°C are totally degenerated with a maximum free electron concentration of 5*1019 cm−3 and an electron mobility of 75 cm2/Vs, respectively. This clearly demonstrates the ability of controlled n-type doping of cubic GaN by Si up to concentrations which are necessary for the fabrications of laser diodes.
Fig. 4: Room temperature free electron concentration measured by Halleffect (full triangles) versus Si source temperature. The full line represents the vapor presure curve of Si after [Reference Souchiere and Binh19].
Conclusions
Cubic GaN films grown by rf-plasma assisted MBE on semi-insulating GaAs (001) substrates are doped by Mg and Si, yielding p- and n-type conductivity, respectively. A Mg related shallow donor-acceptor transition at 3.04 eV with an optical acceptor activation energy of EA= 0.230 eV is observed by low temperature PL. At Mg concentrations above 1018 cm−3 a broad blue band dominates the PL spectra and indicates that also in cubic GaN Mg is incorporated at different lattice sites or forms complexes. Hall-effect measurements show that this complex may act as a compensating donor. Si-doping follows exactly its vapor pressure curve, indicating a constant sticking coefficient in the investigated temperature range. With increasing Si-concentration a continuous increase and broadening of the near-band luminescence is measured. Sidoped c-GaN epilayers are n-type with electron concentrations up to 5*1019 cm−3.
Acknowledgments
The authors acknowledge the support of their work by ”Deutsche Forschungsgemeinschaft”, project number As (107/1-1).
