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Properties and applications of MBE grown AlGaN
Materials Science and Engineering B50 (1997) 212 – 218
Properties and applications of MBE grown AlGaN M. Stutzmann *, O. Ambacher, A. Cros, M.S. Brandt, H. Angerer, R. Dimitrov, N. Reinacher, T. Metzger, R. Ho¨pler, D. Brunner, F. Freudenberg, R. Handschuh, Ch. Deger Walter Schottky Institut, Technische Uni6ersita¨t Mu¨nchen, Am Coulombwall, D-85748 Garching, Germany
Abstract AlGaN epitaxial films have been grown on sapphire by plasma-induced molecular beam epitaxy (MBE) over the entire composition range from GaN to AlN. Structural and optical properties of the alloys have been investigated by X-ray diffraction (XRD), transmission electron and atomic force microscopy, Raman scattering, ellipsometry, optical transmission, and subgap absorption spectroscopy. Electron spin resonance has been used to study the dependence of intrinsic paramagnetic defects on Al mole fraction. N- and p-type doping with Si and Mg, respectively is found to become increasingly difficult with increasing Al content because of a continuous shift of the donor and acceptor levels deeper into the bandgap. Apart from the use of AlGaN as cladding layers in light emitting diodes, applications in MODFET transistors, solar blind photodetectors, surface acoustic wave devices and Bragg reflectors appear interesting and will be discussed briefly. © 1997 Elsevier Science S.A. Keywords: Molecular beam epitaxy; AlGaN; Properties
1. Introduction
2. Sample preparation
Compared to the considerable amount of research done on GaN, the alloy systems InGaN and AlGaN have received much less attention so far. Among these alloys, InGaN with small In concentrations is necessary for blue light emitters and has been investigated much more than InGaN with concentrations of In exceeding 30% [1,2]. The same is true for the AlGaN system, where alloys with Al contents up to 20% have been reasonably well studied because of their use in light emitting diodes and MODFET transistors, but again much less work has been done for higher Al contents [3,4]. In addition, most of the alloy studies published so far have dealt with epitaxial films deposited by MOCVD. Here, we report on a systematic study of AlGaN alloys spanning the entire compositional range from GaN to AlN and deposited by molecular beam epitaxy (MBE), with the aim to obtain a consistent set of data for the structural, optical and electronic properties of these alloys.
Samples have been deposited in a TECTRA three chamber MBE system equipped with standard effusion cells for Ga and Al and with an Oxford CARS 25 plasma source for N. C-plane sapphire was used as the substrate. Deposition occured directly on the cleaned and nitridized substrate without a buffer layer at a growth rate of 600 nm h − 1. The flux of metal atoms to
* Corresponding author. Tel.: + 49 89 28912760; fax: +49 89 28912737; e-mail: [email protected]
Fig. 1. High resolution X-ray diffraction spectra of MBE-grown AlGaN alloy films with different composition.
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M. Stutzmann et al. / Materials Science and Engineering B50 (1997) 212–218
Fig. 2. Raman spectra of AlGaN showing the dependence of the (a) A1(TO) and the E2 mode and (b) of the A1(LO) mode on Al content.
the substrate was kept constant at 8 × 1014 cm − 2 s − 1, the nitrogen pressure in the growth chamber was 4× 10 − 5 mbar (base pressure 2× 10 − 10 mbar). N2 with a purity of 6.0 and additional purification via adsorption filters was used at a flow rate of 2 sccm. The substrate temperature was varied between 840°C for GaN and 1000°C for AlN. The typical thickness of AlGaN epitaxial films discussed below was 1 – 1.5 mm.
3. Structural properties Upon inspection with an atomic force microscope, all AlGaN samples exhibit a rather smooth surface morphology with an rms roughness of about 5 nm. Fig. 1 shows high-resolution X-ray diffraction (XRD) spectra of Alx Ga1 − x N samples with x between zero and one. The width of the (002) Bragg reflex increases from about 50 arcsec for GaN to about 200 arcsec for AlN, with a significantly larger width for alloys with an Al content of about 80%, indicating a reduced structural quality in this alloy range. Based on the accurate determination of the alloy composition by elastic recoil detection, we were able to show that the positions of the reflexes follow Vegard’s law, i.e. a linear interpolation between the lattice constants of pure GaN (a = 0.31892 nm, c= 0.51850 nm) and AlN (a = 0.3112 nm, c = 0.4982 nm) within experimental accuracy. Note, however, that a biaxial compressive strain of about 0.4 GPa is present in all of our samples and has to be taken into account for a correct interpretation of the XRD data. Details are described in [5]. Raman spectra of the AlGaN alloy films are shown in Fig. 2 both for the spectral region around 550–650 cm − 1 of the A1(TO) and E2 modes and around 750– 850 cm − 1 of the A1(LO) mode. These spectra were obtained in a triple micro-Raman spectrometer for backscattering geometry under illumination with the 476.5 nm line of an Ar + -ion laser. The A1(LO) mode exhibits a one-mode behavior with a significant upward
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bowing for intermediate Al contents. The dependence of the E2 mode is more complex, suggesting a twomode behavior with a splitting of about 50 cm − 1 between the vibrations derived from the GaN and AlN pure modes. The variation of the weaker A1(TO) Raman mode with alloy composition is masked by the stronger E2 peaks and cannot be followed clearly with the present set of samples. However, our results in Fig. 2 (a) are compatible with a single mode behaviour with downward bowing for intermediate Al concentrations. It is obvious that the reduced structural quality of AlGaN samples with Al contents between 60 and 80% also affects the Raman spectra: the line width of the dominant Raman peaks increases from several cm − 1 in pure GaN and AlN to several tens cm − 1 in these AlGaN alloys. A more extensive discussion of Raman spectra can be found in [6].
4. Optical properties The optical properties of the AlGaN-alloys have been investigated by conventional UV-VIS transmission measurements, photothermal deflection spectroscopy (PDS) and UV ellipsometry in the energy range between 1 and 17 eV. The room temperature transmission data are summarized in Fig. 3. With increasing Al-content, the absorption edge shifts from 3.4 to 6.2 eV. Note, however, that the edge is less steep for the alloy compositions x=0.5 and x =0.86, in accordance with
Fig. 3. Optical transmission spectra of AlGaN at room temperature.
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gap, we obtain the following dependence on alloy composition x at room temperature: Egap(x)= x6.13 eV+ (1− x)3.42 eV− bx(1−x) with a bowing parameter of b= 1.3 eV deduced from a least squares fit to the data points in the inset of Fig. 5. This value is in reasonable agreement with the bowing parameter of about 1 eV published by Koide et al. [3]. Taking advantage of a UV ellipsometer at the Berlin synchrotron facility BESSY I, the same set of samples has also been used for the determination of the pseudodielectric functions of AlGaN over a wide energy range (3–17 eV). As an example, Fig. 6. summarizes the results obtained for the imaginary part of the dielectric function, showing the dependence of the interband critical points on UV photon energy. As expected the E1 and E2 transitions shift to higher energies with increasing Al content, however less pronounced than the fundamental edge transition E0. The spectra of all samples also show a yet unidentified transition at a photon energy of about 13 eV. A more detailed discussion of these data will be published elsewhere [10].
Fig. 4. Energy dependence of the refractive index at 300 K for alloy samples with different aluminium concentrations.
the broader Bragg reflexes of these films. A quantitative analysis of the interference pattern in the transmission data following Freeman and Paul [7] allows us to deduce the energy dependence of the refractive index as given in Fig. 4. The static refractive index n(0) at room temperature is found to decrease linearly with Al content x following the relation n(0) = 2.29− 0.33 x. At higher energies, n(hn) increases as expected for a direct bandgap semiconductor close to the band edge [8]. Further details, e.g. the temperature dependence of the refractive index, will be published elsewhere [9]. A precise knowledge of n(hn) is for example necessary for the design of Bragg reflectors based on AlGaN multilayers. The dependence of the optical absorption close to the band edge and in the subgap region of the alloys can be seen in the PDS spectra of Fig. 5. GaN shows a sharp absorption edge followed by an exponential defect absorption tail at a level of about 100 cm − 1. With increasing Al concentration the band edge becomes increasingly flatter, particularly for the ‘critical’ alloy composition of x= 0.86, whereas the subgap defect absorption remains unchanged within a factor of three. We have used the energy E4.8 at which the absorption coefficient reaches a value of 104.8 cm − 1 as a measure for the effective room temperature band gap of the alloys. (In GaN, E4.8 lies about 20 meV above the energy of the free A-exciton for the entire temperature range 4–300 K. [9]) Using this definition for the band
5. Dopants and defects We next discuss doping of AlGaN with Si and Mg. Not intentionally doped samples have a residual donor concentration of about 5× 1017 cm − 3 probably due to
Fig. 5. Subgap absorption spectra of AlGaN obtained by photothermal deflection spectroscopy. The inset shows the dependence of the fundamental band gap at 300 K on Al content, b is the bowing parameter discussed in the text.
M. Stutzmann et al. / Materials Science and Engineering B50 (1997) 212–218
Fig. 6. Imaginary part of the pseudo-dielectric function measured by UV ellipsometry.
oxygen contamination. The carrier density can be determined by standard Hall measurements up to Al contents of about 40%. For higher Al contents, the carrier density and their mobility become too low to perform a Hall analysis. Optimized undoped GaN grown in our MBE system has a mobility of 350 cm2 V − 1 s − 1 at 300 K. This value decreases rapidly to about 100 cm2 V − 1 s − 1 for an Al concentration of 10% and down to 10 cm2 V − 1 s − 1 for alloys with 30% Al. The effective mass state of the residual dopants gives rise to an electron spin resonance signal which is shown in Fig. 7 for various Al contents between 0 and 40%. The effective mass donor state in GaN has a g-value of g = 1.9455 (magnetic field perpendicular to the c-axis) which increases to about g =1.9656 for x= 0.38. The resonances exhibit a slight anisotropy of Dg =0.003 with respect to the c-axis of the films. Our observations for MBE-grown AlGaN are in good agreement with data reported by Carlos et al. for MOCVD material [11]. As in GaN, n-type doping of AlGaN with Si is quite efficient and can be achieved over the entire composition range. In our experiments, we used a Si flux of about 1012 cm − 2 s − 1 to obtain an Si concentration of about 5 x 1020 cm − 3 in the films. Fig. 8 summarizes the main results obtained so far. For small Al concentrations below 15%, an Si concentration of 1019 cm − 3 is sufficient to cause a quasi-metallic conductivity at room temperature. Our results basically agree with those of Korakakis et al. also included in Fig. 8 [12]. The
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conductivity activation energy in MBE-grown GaN with lower Si content is close to 20 meV. For higher Al concentrations, we observe a rapid decrease of the conductivity, due to a degradation of the electron mobility and, more importantly, due to a shift of the Si donor level deeper into the bandgap. The data in Fig. 8 suggest a more or less linear dependence of the donor binding energy on Al content, with a value larger than 300 meV for pure AlN. p-Type doping with Mg is much more difficult. In general a Mg flux comparable to the Ga or Al flux is needed to incorporate about 1020 cm − 3 Mg atoms into the epitaxial films. In pure MBE GaN samples this is enough to obtain more than 1018 cm − 3 free holes at room temperature without the necessity of an additional acceptor activation step as required in MOCVD material. The hole mobility is of the order of 10 cm2 V − 1 s − 1 at 300 K. As shown in Fig. 9, the conductivity activation energy of p-type GaN is 170 meV, similar to MOCVD samples. As in n-type AlGaN, the electrical characteristics of Mg-doped AlGaN degrade rather rapidly with Al content: Fig. 9 suggests a shift of the acceptor level to 360 meV above the valence band edge for as little as 27% Al. Thus, Hall measurements in p-type AlGaN quickly become impossible as the Al content is increased. However, using thermopower, the p-type conductance of Mg-doped AlGaN still can be demonstrated clearly (see inset in Fig. 9).
Fig. 7. Electron spin resonance signal of the residual effective mass donor in nominally undoped AlGaN for Al concentrations up to 40%.
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GaN/AlGaN heterostructures with small Al contents are of interest for solar blind photodetectors and high frequency/high power modulation doped field effect transistors (MODFETs). So far, most of the corresponding device structures have been deposited by MOCVD [14], but MBE grown devices have also demonstrated promising properties [15]. The layout of a MODFET with 2 mm source-drain distance and a 1 mm gate is shown in Fig. 13. At present, the performance of our devices is still limited by the small electron mobility (200 cm2 V − 1 s − 1) at the GaN/AlGaN interface. Since similar transistors prepared from MOCVD material work quite well, we are optimistic that similar results will also be achieved with MBE AlGaN in the near future.
Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft and the Bayerische Forschungsstiftung are gratefully acknowledged. Fig. 8. Electrical conductivity and conductivity activation energy of Si-doped AlGaN as a function of Al concentration. Results of Korakakis et al. [12] are shown for comparison.
6. Applications of MBE-grown AlGaN Finally, we discuss briefly potential applications of AlGaN in electronic devices or components. Above, we have already mentioned the possibilty of using AlGaN multilayers in Bragg reflectors, e.g. for use in vertically emitting lasers. Based on the optical properties of the alloys shown in Figs. 3 and 4, it is straightforward to calculate the reflectivity of AlGaN multilayers consisting of two sets of layers with different Al content and thickness [13]. Fig. 10 shows the result of such a simulation for Al0.9Ga0.1N/Al0.5Ga0.5N stacks consisting of 20 or 30 periods, respectively. Reflectivities of more than 95% should be achievable by suitable variation of layer thicknesses over a wide spectral range between 250 and 450 nm wavelength. A very promising application of AlGaN films with high Al content is the use in surface acoustic wave (SAW) devices. As an example, Fig. 11 shows the basic components of a SAW delay line or filter, using MBE grown AlGaN or AlN on sapphire as the active layer. First transfer characteristics of such devices are depicted in Fig. 12. Although the insertion losses are still very high (mainly because of non-optimized contacts, but also because of surface roughness and limited layer thickness), a big advantage is the high speed of sound and the possibility to operate the devices at high temperatures without the loss of piezoelectricity.
Fig. 9. Temperature dependence of the dark conductivity of AlGaN doped with 5 ×1019 cm − 3 Mg. The inset shows a thermopower plot for the sample with 27% Al.
M. Stutzmann et al. / Materials Science and Engineering B50 (1997) 212–218
Fig. 10. Simulated reflectivity spectra of AlGaN Bragg reflectors. See text for details.
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Fig. 12. Transfer characteristics of SAW devices with 75 and 100% Al content in the active layer. The corresponding velocities of the Rayleigh wave are 5380 and 5780 m s − 1.
Fig. 11. Schematic drawing of a surface acoustic wave delay line.
References [1] A. Osamura, K. Nakajima, Y. Murakami, P.H. Shingu, A. Ohtsuki, Solid State Commun. 11 (1972) 617. [2] S. Nakamura, T. Mukai, Jpn. J. Appl. Phys. 31 (1992) L1457. [3] Y. Koide, H. Itoh, M.R.H. Khan, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Appl. Phys. 61 (1987) 4540. [4] S. Yoshida, S. Misawa, S. Gonda, J. Appl. Phys. 53 (1982) 6844. [5] H. Angerer, O. Ambacher, M. Stutzmann, T. Metzger, R. Ho¨pler, E. Born, A. Bergmaier, G. Dollinger, Mat. Res. Soc. Conf. Proc., MRS, San Francisco, 1997 (in press). [6] A. Cros, H. Angerer, O. Ambacher, M. Stutzmann, R. Ho¨pler, T. Metzger, Solid State Commun. 104 (1997) 35. [7] E.C. Freeman, W. Paul, Phys. Rev. B20 (1979) 716. [8] P.Y. Yu, M. Cardona, Fundamentals of Semiconductors, Springer, Berlin, 1996, p. 258
Fig. 13. Layout of a GaN/AlGaN MODFET with a 1 mm gate.
[9] D. Brunner, H. Angerer, E. Bustarret, F. Freudenberg, R. Ho¨pler, R. Dimitrov, O. Ambacher, M. Stutzmann, J. Appl. Phys. (in press). [10] T. Wethkamp, K. Wilmers, N. Esser, W. Richter, O. Ambacher, H. Angerer, G. Jungk, R.L. Johnson, M. Cardona, Thin Solid Films (in press). [11] W.E. Carlos, J.A. Freitas Jr., M. Asif Khan, D. Olson, J.N. Kuznia, Phys. Rev. B48 (1993) 17878. [12] D. Korakakis, H.M. Ng, K.F. Ludwig, T.D. Moustakas, Mat. Res. Soc. Symp. Proc. 449 (1996) 233.
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[13] O. Ambacher, M. Arzberger, D. Brunner, H. Angerer, F. Freudenberg, N. Esser, T. Wethkamp, K. Wilmers, W. Richter, M. Stutzmann, MRS Internet J. Nitride Semicond. Res. 2 (1997) 22.
.
[14] M. Asif Khan, Q. Chen, M.S. Shur, B.T. Dermott, J.A. Higgins, J. Burm, W. Schaff, L.F. Eastman, Electron. Lett. 32 (1996) 357. [15] Z. Fan, S.N. Mohammad, O. Atkas, A.E. Botchkarev, A. Salvador, H. Morkoc, Appl. Phys. Lett. 69 (1996) 1229.
Properties and applications of MBE grown AlGaN M. Stutzmann *, O. Ambacher, A. Cros, M.S. Brandt, H. Angerer, R. Dimitrov, N. Reinacher, T. Metzger, R. Ho¨pler, D. Brunner, F. Freudenberg, R. Handschuh, Ch. Deger Walter Schottky Institut, Technische Uni6ersita¨t Mu¨nchen, Am Coulombwall, D-85748 Garching, Germany
Abstract AlGaN epitaxial films have been grown on sapphire by plasma-induced molecular beam epitaxy (MBE) over the entire composition range from GaN to AlN. Structural and optical properties of the alloys have been investigated by X-ray diffraction (XRD), transmission electron and atomic force microscopy, Raman scattering, ellipsometry, optical transmission, and subgap absorption spectroscopy. Electron spin resonance has been used to study the dependence of intrinsic paramagnetic defects on Al mole fraction. N- and p-type doping with Si and Mg, respectively is found to become increasingly difficult with increasing Al content because of a continuous shift of the donor and acceptor levels deeper into the bandgap. Apart from the use of AlGaN as cladding layers in light emitting diodes, applications in MODFET transistors, solar blind photodetectors, surface acoustic wave devices and Bragg reflectors appear interesting and will be discussed briefly. © 1997 Elsevier Science S.A. Keywords: Molecular beam epitaxy; AlGaN; Properties
1. Introduction
2. Sample preparation
Compared to the considerable amount of research done on GaN, the alloy systems InGaN and AlGaN have received much less attention so far. Among these alloys, InGaN with small In concentrations is necessary for blue light emitters and has been investigated much more than InGaN with concentrations of In exceeding 30% [1,2]. The same is true for the AlGaN system, where alloys with Al contents up to 20% have been reasonably well studied because of their use in light emitting diodes and MODFET transistors, but again much less work has been done for higher Al contents [3,4]. In addition, most of the alloy studies published so far have dealt with epitaxial films deposited by MOCVD. Here, we report on a systematic study of AlGaN alloys spanning the entire compositional range from GaN to AlN and deposited by molecular beam epitaxy (MBE), with the aim to obtain a consistent set of data for the structural, optical and electronic properties of these alloys.
Samples have been deposited in a TECTRA three chamber MBE system equipped with standard effusion cells for Ga and Al and with an Oxford CARS 25 plasma source for N. C-plane sapphire was used as the substrate. Deposition occured directly on the cleaned and nitridized substrate without a buffer layer at a growth rate of 600 nm h − 1. The flux of metal atoms to
* Corresponding author. Tel.: + 49 89 28912760; fax: +49 89 28912737; e-mail: [email protected]
Fig. 1. High resolution X-ray diffraction spectra of MBE-grown AlGaN alloy films with different composition.
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M. Stutzmann et al. / Materials Science and Engineering B50 (1997) 212–218
Fig. 2. Raman spectra of AlGaN showing the dependence of the (a) A1(TO) and the E2 mode and (b) of the A1(LO) mode on Al content.
the substrate was kept constant at 8 × 1014 cm − 2 s − 1, the nitrogen pressure in the growth chamber was 4× 10 − 5 mbar (base pressure 2× 10 − 10 mbar). N2 with a purity of 6.0 and additional purification via adsorption filters was used at a flow rate of 2 sccm. The substrate temperature was varied between 840°C for GaN and 1000°C for AlN. The typical thickness of AlGaN epitaxial films discussed below was 1 – 1.5 mm.
3. Structural properties Upon inspection with an atomic force microscope, all AlGaN samples exhibit a rather smooth surface morphology with an rms roughness of about 5 nm. Fig. 1 shows high-resolution X-ray diffraction (XRD) spectra of Alx Ga1 − x N samples with x between zero and one. The width of the (002) Bragg reflex increases from about 50 arcsec for GaN to about 200 arcsec for AlN, with a significantly larger width for alloys with an Al content of about 80%, indicating a reduced structural quality in this alloy range. Based on the accurate determination of the alloy composition by elastic recoil detection, we were able to show that the positions of the reflexes follow Vegard’s law, i.e. a linear interpolation between the lattice constants of pure GaN (a = 0.31892 nm, c= 0.51850 nm) and AlN (a = 0.3112 nm, c = 0.4982 nm) within experimental accuracy. Note, however, that a biaxial compressive strain of about 0.4 GPa is present in all of our samples and has to be taken into account for a correct interpretation of the XRD data. Details are described in [5]. Raman spectra of the AlGaN alloy films are shown in Fig. 2 both for the spectral region around 550–650 cm − 1 of the A1(TO) and E2 modes and around 750– 850 cm − 1 of the A1(LO) mode. These spectra were obtained in a triple micro-Raman spectrometer for backscattering geometry under illumination with the 476.5 nm line of an Ar + -ion laser. The A1(LO) mode exhibits a one-mode behavior with a significant upward
213
bowing for intermediate Al contents. The dependence of the E2 mode is more complex, suggesting a twomode behavior with a splitting of about 50 cm − 1 between the vibrations derived from the GaN and AlN pure modes. The variation of the weaker A1(TO) Raman mode with alloy composition is masked by the stronger E2 peaks and cannot be followed clearly with the present set of samples. However, our results in Fig. 2 (a) are compatible with a single mode behaviour with downward bowing for intermediate Al concentrations. It is obvious that the reduced structural quality of AlGaN samples with Al contents between 60 and 80% also affects the Raman spectra: the line width of the dominant Raman peaks increases from several cm − 1 in pure GaN and AlN to several tens cm − 1 in these AlGaN alloys. A more extensive discussion of Raman spectra can be found in [6].
4. Optical properties The optical properties of the AlGaN-alloys have been investigated by conventional UV-VIS transmission measurements, photothermal deflection spectroscopy (PDS) and UV ellipsometry in the energy range between 1 and 17 eV. The room temperature transmission data are summarized in Fig. 3. With increasing Al-content, the absorption edge shifts from 3.4 to 6.2 eV. Note, however, that the edge is less steep for the alloy compositions x=0.5 and x =0.86, in accordance with
Fig. 3. Optical transmission spectra of AlGaN at room temperature.
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gap, we obtain the following dependence on alloy composition x at room temperature: Egap(x)= x6.13 eV+ (1− x)3.42 eV− bx(1−x) with a bowing parameter of b= 1.3 eV deduced from a least squares fit to the data points in the inset of Fig. 5. This value is in reasonable agreement with the bowing parameter of about 1 eV published by Koide et al. [3]. Taking advantage of a UV ellipsometer at the Berlin synchrotron facility BESSY I, the same set of samples has also been used for the determination of the pseudodielectric functions of AlGaN over a wide energy range (3–17 eV). As an example, Fig. 6. summarizes the results obtained for the imaginary part of the dielectric function, showing the dependence of the interband critical points on UV photon energy. As expected the E1 and E2 transitions shift to higher energies with increasing Al content, however less pronounced than the fundamental edge transition E0. The spectra of all samples also show a yet unidentified transition at a photon energy of about 13 eV. A more detailed discussion of these data will be published elsewhere [10].
Fig. 4. Energy dependence of the refractive index at 300 K for alloy samples with different aluminium concentrations.
the broader Bragg reflexes of these films. A quantitative analysis of the interference pattern in the transmission data following Freeman and Paul [7] allows us to deduce the energy dependence of the refractive index as given in Fig. 4. The static refractive index n(0) at room temperature is found to decrease linearly with Al content x following the relation n(0) = 2.29− 0.33 x. At higher energies, n(hn) increases as expected for a direct bandgap semiconductor close to the band edge [8]. Further details, e.g. the temperature dependence of the refractive index, will be published elsewhere [9]. A precise knowledge of n(hn) is for example necessary for the design of Bragg reflectors based on AlGaN multilayers. The dependence of the optical absorption close to the band edge and in the subgap region of the alloys can be seen in the PDS spectra of Fig. 5. GaN shows a sharp absorption edge followed by an exponential defect absorption tail at a level of about 100 cm − 1. With increasing Al concentration the band edge becomes increasingly flatter, particularly for the ‘critical’ alloy composition of x= 0.86, whereas the subgap defect absorption remains unchanged within a factor of three. We have used the energy E4.8 at which the absorption coefficient reaches a value of 104.8 cm − 1 as a measure for the effective room temperature band gap of the alloys. (In GaN, E4.8 lies about 20 meV above the energy of the free A-exciton for the entire temperature range 4–300 K. [9]) Using this definition for the band
5. Dopants and defects We next discuss doping of AlGaN with Si and Mg. Not intentionally doped samples have a residual donor concentration of about 5× 1017 cm − 3 probably due to
Fig. 5. Subgap absorption spectra of AlGaN obtained by photothermal deflection spectroscopy. The inset shows the dependence of the fundamental band gap at 300 K on Al content, b is the bowing parameter discussed in the text.
M. Stutzmann et al. / Materials Science and Engineering B50 (1997) 212–218
Fig. 6. Imaginary part of the pseudo-dielectric function measured by UV ellipsometry.
oxygen contamination. The carrier density can be determined by standard Hall measurements up to Al contents of about 40%. For higher Al contents, the carrier density and their mobility become too low to perform a Hall analysis. Optimized undoped GaN grown in our MBE system has a mobility of 350 cm2 V − 1 s − 1 at 300 K. This value decreases rapidly to about 100 cm2 V − 1 s − 1 for an Al concentration of 10% and down to 10 cm2 V − 1 s − 1 for alloys with 30% Al. The effective mass state of the residual dopants gives rise to an electron spin resonance signal which is shown in Fig. 7 for various Al contents between 0 and 40%. The effective mass donor state in GaN has a g-value of g = 1.9455 (magnetic field perpendicular to the c-axis) which increases to about g =1.9656 for x= 0.38. The resonances exhibit a slight anisotropy of Dg =0.003 with respect to the c-axis of the films. Our observations for MBE-grown AlGaN are in good agreement with data reported by Carlos et al. for MOCVD material [11]. As in GaN, n-type doping of AlGaN with Si is quite efficient and can be achieved over the entire composition range. In our experiments, we used a Si flux of about 1012 cm − 2 s − 1 to obtain an Si concentration of about 5 x 1020 cm − 3 in the films. Fig. 8 summarizes the main results obtained so far. For small Al concentrations below 15%, an Si concentration of 1019 cm − 3 is sufficient to cause a quasi-metallic conductivity at room temperature. Our results basically agree with those of Korakakis et al. also included in Fig. 8 [12]. The
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conductivity activation energy in MBE-grown GaN with lower Si content is close to 20 meV. For higher Al concentrations, we observe a rapid decrease of the conductivity, due to a degradation of the electron mobility and, more importantly, due to a shift of the Si donor level deeper into the bandgap. The data in Fig. 8 suggest a more or less linear dependence of the donor binding energy on Al content, with a value larger than 300 meV for pure AlN. p-Type doping with Mg is much more difficult. In general a Mg flux comparable to the Ga or Al flux is needed to incorporate about 1020 cm − 3 Mg atoms into the epitaxial films. In pure MBE GaN samples this is enough to obtain more than 1018 cm − 3 free holes at room temperature without the necessity of an additional acceptor activation step as required in MOCVD material. The hole mobility is of the order of 10 cm2 V − 1 s − 1 at 300 K. As shown in Fig. 9, the conductivity activation energy of p-type GaN is 170 meV, similar to MOCVD samples. As in n-type AlGaN, the electrical characteristics of Mg-doped AlGaN degrade rather rapidly with Al content: Fig. 9 suggests a shift of the acceptor level to 360 meV above the valence band edge for as little as 27% Al. Thus, Hall measurements in p-type AlGaN quickly become impossible as the Al content is increased. However, using thermopower, the p-type conductance of Mg-doped AlGaN still can be demonstrated clearly (see inset in Fig. 9).
Fig. 7. Electron spin resonance signal of the residual effective mass donor in nominally undoped AlGaN for Al concentrations up to 40%.
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GaN/AlGaN heterostructures with small Al contents are of interest for solar blind photodetectors and high frequency/high power modulation doped field effect transistors (MODFETs). So far, most of the corresponding device structures have been deposited by MOCVD [14], but MBE grown devices have also demonstrated promising properties [15]. The layout of a MODFET with 2 mm source-drain distance and a 1 mm gate is shown in Fig. 13. At present, the performance of our devices is still limited by the small electron mobility (200 cm2 V − 1 s − 1) at the GaN/AlGaN interface. Since similar transistors prepared from MOCVD material work quite well, we are optimistic that similar results will also be achieved with MBE AlGaN in the near future.
Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft and the Bayerische Forschungsstiftung are gratefully acknowledged. Fig. 8. Electrical conductivity and conductivity activation energy of Si-doped AlGaN as a function of Al concentration. Results of Korakakis et al. [12] are shown for comparison.
6. Applications of MBE-grown AlGaN Finally, we discuss briefly potential applications of AlGaN in electronic devices or components. Above, we have already mentioned the possibilty of using AlGaN multilayers in Bragg reflectors, e.g. for use in vertically emitting lasers. Based on the optical properties of the alloys shown in Figs. 3 and 4, it is straightforward to calculate the reflectivity of AlGaN multilayers consisting of two sets of layers with different Al content and thickness [13]. Fig. 10 shows the result of such a simulation for Al0.9Ga0.1N/Al0.5Ga0.5N stacks consisting of 20 or 30 periods, respectively. Reflectivities of more than 95% should be achievable by suitable variation of layer thicknesses over a wide spectral range between 250 and 450 nm wavelength. A very promising application of AlGaN films with high Al content is the use in surface acoustic wave (SAW) devices. As an example, Fig. 11 shows the basic components of a SAW delay line or filter, using MBE grown AlGaN or AlN on sapphire as the active layer. First transfer characteristics of such devices are depicted in Fig. 12. Although the insertion losses are still very high (mainly because of non-optimized contacts, but also because of surface roughness and limited layer thickness), a big advantage is the high speed of sound and the possibility to operate the devices at high temperatures without the loss of piezoelectricity.
Fig. 9. Temperature dependence of the dark conductivity of AlGaN doped with 5 ×1019 cm − 3 Mg. The inset shows a thermopower plot for the sample with 27% Al.
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Fig. 10. Simulated reflectivity spectra of AlGaN Bragg reflectors. See text for details.
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Fig. 12. Transfer characteristics of SAW devices with 75 and 100% Al content in the active layer. The corresponding velocities of the Rayleigh wave are 5380 and 5780 m s − 1.
Fig. 11. Schematic drawing of a surface acoustic wave delay line.
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Fig. 13. Layout of a GaN/AlGaN MODFET with a 1 mm gate.
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