GaN heterostructures prepared on vicinal-cut sapphire (0 0 0 1) substrates

ARTICLE IN PRESS

Journal of Crystal Growth 308 (2007) 252–257 www.elsevier.com/locate/jcrysgro

Crystal growth and characterization of AlGaN/GaN heterostructures prepared on vicinal-cut sapphire (0 0 0 1) substrates R.W. Chuanga,b,c,, C.L. Yua,b,c, S.J. Changa,b,c, P.C. Changd, J.C. Lina,b,c, T.M. Kuana,b,c a

Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC b Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, ROC c Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC d Department of Electronic Engineering, Nan Jeon Institute of Technology, Yan-Hsui Township, Tainan County 737, Taiwan, ROC Received 2 November 2006; received in revised form 11 July 2007; accepted 8 August 2007 Communicated by T.F. Kuech Available online 24 August 2007

Abstract We report the growth of AlGaN/GaN heterostructures on conventional c-axis-oriented and vicinal-cut (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition (MOCVD). By maintaining the same growth conditions using these two different sapphire substrates, we were able to achieve comparably lower dislocation density and improved crystal quality for the AlGaN/GaN heterostructure grown on 11-tilt sapphire substrate. Furthermore, the growth on 11-tilt sapphire substrate followed a step-flow mode without the formation of two-dimensional islands, and holes (defects) therefore could not be formed through incomplete island coalescence. The corresponding dark leakage current of epilayers grown on 11-tilt sapphire substrate was two orders of magnitude smaller compared to the one on c-axis-oriented substrate. r 2007 Published by Elsevier B.V. PACS: 81.10; 73.40; 68.65 Keywords: A3. MOCVD; B1. AlGaN/GaN; B1. Vicinal-cut sapphire; B3. Schottky diode

1. Introduction In recent years, an unimaginable progress has been achieved in GaN-based optical devices such as light emitting diodes (LEDs) [1,2] and laser diodes (LDs) [3,4]. In addition, the excellent physical and electrical properties of GaN and its associated compounds such as wide bandgap (AlInGaN varies from 0.78 to 6.2 eV), high break down fields (106 V/cm), and high saturated electron drift velocity (107 cm/s), have made them well suited for hightemperature, high-power and high-frequency electronic devices [5,6]. However, relatively poor material quality of III-nitride films compared to other traditional III–V semiconductors inevitably degrades the device performance Corresponding author. Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan, ROC. Tel.: +886 6 2757575; fax: +886 6 2345482. E-mail address: [email protected] (R.W. Chuang).

0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.08.015

somewhat to a certain degree. Conventional GaN-based devices are usually grown on top of c-face (0 0 0 1) sapphire substrate. Even though incorporating an additional lowtemperature GaN or AlN nucleation layer could improve the crystal quality of GaN epitaxial layer [7,8], nevertheless, the lowest achievable threading dislocation density of these samples still lingered in the range of 109–1012 cm2 due to large differences in lattice constant and thermal expansion coefficient between the epitaxial layer and the underlying sapphire substrate. Therefore, being able to further enhance the performance of GaN-based devices certainly rests upon our ability to devise novel growth techniques to trim the threading dislocation density. Epitaxial lateral overgrowth (ELOG) was proposed earlier to meet this challenge [9]; however, additional process and difficulty in modifying the existing epitaxy were inevitably added to this task. Here, we propose to use the vicinal-cut sapphire substrate in place of the conventional c-axis-oriented sapphire substrate for purpose of

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reducing the dislocation density. Previously, Grudowski et al. [10] found that the higher photoluminescence intensity could be obtained when GaN layer was grown on mis-oriented substrate. Furthermore, Fatemi et al. [11] also observed a significant enhancement in the electrical and crystalline properties of GaN layers grown on vicinal-cut sapphire substrate. Since then, several experimental results concerning the growth of GaN layers on vicinal-cut substrates have been reported in reputable scientific journals [12,13]. Similar to AlGaAs/GaAs system, the formation of twodimensional electron gas (2DEG) in AlGaN/GaN system also serves as the basis for high carrier mobility devices, such as heterostructure field effect transistors (HFETs). The existence of AlGaN/GaN heterojunction with high conduction band offset and high piezoelectricity also brings about high sheet carrier density and high room temperature mobility. However, the AlGaN/GaN HFETs which had been demonstrated in the past produced a very large Schottky-gate leakage current compared to the ideal Schottky-gate reverse current [14]. Therefore, improving the crystal quality of AlGaN/GaN heterostructure becomes an essential task for achieving a dramatic reduction in the leakage current. As mentioned earlier, one of plausible ways to reduce threading dislocation and also to improve the crystal quality of III-nitride epitaxial layers is to use vicinal-cut sapphire substrate. However, so far to our knowledge no report on AlGaN/GaN heterostructure preparing on vicinal-cut sapphire substrate has ever been found in the literature. Therefore, in this study we hereby report the growth of AlGaN/GaN heterostructure on vicinal-cut sapphire substrate and then use it to fabricate AlGaN/GaN Schottky diodes. Structural and electrical properties of the prepared samples will also be discussed.

253

17 nm undoped-Al0.22Ga0.78N cap layer 250 nm undoped-GaN channel layer 1.1 µm Mg : GaN semi-insulating layer 2.2 µm undoped-GaN layer 25 nm LT-GaN nucleation layer 0° or 1° vicinal-cut sapphire

Fig. 1. Schematic structure of the samples used in this study.

Table 1 Measured FWHM for (0 0 2) and (1 0 2) XRD of samples A and B

Sample A Sample B

XRD (0 0 2) FWHM (arcsec)

XRD (1 0 2) FWHM (arcsec)

216 295

378 464

point probes were used to make contact with the semiconductor sample surface based on the Van der Pauw technique. Schottky diodes were then fabricated thereafter on these two samples. We deposited Ti(13 nm)/Al(100 nm) as the ohmic contact, and then thermally annealed at 600 1C for 16 min. Ni(40 nm)/Au(100 nm) was served as the Schottky contact. The diameter of fabricated circular Schottky diodes was kept at 400 mm. Current–voltage (I–V) and capacitance–voltage (C–V) measurements were conducted using a Keithley-4200 semiconductor parameter analyzer and an HP4284 LCR meter, respectively. 3. Results and discussion

2. Experiments Epitaxial sample grown on an otherwise c-plane (0 0 0 1) sapphire substrate with a vicinal-cut angle of 11 toward the a-planeð1 1 2¯ 0Þ using metalorganic chemical vapor deposition (MOVCD) is hereafter referred to as sample A. Prior to the growth, these sapphire substrates were first heated to 1100 1C in a stream of hydrogen to clean the substrate surface. Each epi-sample consists of a 25-nm-thick GaN nucleation layer, a 2.2-mm-thick un-doped GaN buffer layer, a 1.1-mm-thick Mg-doped semi-insulating carrier confinement layer, a 0.25-mm-thick un-doped GaN channel layer, and a 17-nm-thick un-doped Al0.22Ga0.78N cap layer. Fig. 1 depicts the schematic structure of the samples used in this study. For comparison, identical AlGaN/GaN heterostructure was also prepared on c-axis-oriented (i.e. 01-tilt) (0 0 0 1) sapphire substrate, is hereafter referred to as sample B. Notice that exactly the same growth conditions were used when preparing the epitaxial layers on these two different substrates. The as-grown AlGaN/GaN epitaxial layers were then characterized by high-resolution X-ray diffraction (HRXRD), Scanning Probe Microscopy (SPM) and Hall-effect measurement. For Hall measurement, four

We first performed HRXRD rocking curve measurements of symmetric (0 0 2) and asymmetric (1 0 2) diffractions to characterize the structural properties of samples A and B. Table 1 lists the measured values of full-width-at-halfmaximum (FWHM) of symmetrical (0 0 2) and asymmetrical (1 0 2) XRD spectra of samples A and B. The FWHM values of the (0 0 2) and (1 0 2) XRD peaks thus determined for sample A were 216 and 378 arcsec, respectively. In contrast, the FWHM values of (0 0 2) and (1 0 2) XRD peaks for sample B were 295 and 464 arcsec, respectively. In other words, narrower (0 0 2) and (1 0 2) XRD peaks were achieved for the AlGaN/GaN epitaxial layers grown on 11-tilt sapphire substrate (i.e., sample A). It was known that the symmetrical XRD FWHM was most sensitive to the screw and/or mixed dislocations, while the asymmetrical XRD FWHM was related to the other dislocation such as edge dislocation [15,16]. Therefore, a narrower peak width of symmetrical and asymmetrical HRXRD rocking curves suggests that the crystalline quality of AlGaN/GaN films grown on vicinal-cut sapphire substrate is better than that of the same epi-structure grown on c-axis-oriented sapphire substrate. It is understood that changing the twist

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(edge dislocation density) and the tilt (screw dislocation density) angles in the atomic arrangements of the material may induce a change in the dislocation density. In fact, a small tilt between the GaN plane and substrate plane may result in smaller lattice mismatch [17]. In other words, based on the symmetric (0 0 2) and asymmetric (1 0 2) diffraction results, the tilt and twist of the epilayers could be significantly reduced if the vicinal-cut sapphire substrate was used (i.e., sample A), and this in turn helped to improve the overall structural quality of AlGaN/GaN epilayers. SPM was also used to characterize these two samples. Figs. 2(a) and (b) show SPM images (5  5 mm2) of samples A and B, respectively. Several dark spots were clearly visible in both figures, which could well be due to the threading dislocation in the epitaxial layers. Based on these

Fig. 2. SPM images (5  5 mm2) of AlGaN/GaN epitaxial layers grown on (a) 11- and (b) 01-tilt sapphire substrates.

two figures, it was found that threading dislocation densities were tabulated to be around 1.98  109 and 4.28  108 cm2 for AlGaN/GaN epitaxial layers grown on 01-tilt and 11-tilt sapphire substrates, respectively. A reduction in dislocation density was somewhat related to the change in the tilt and twist of the epilayers. Furthermore, sample A clearly exhibited a step-flow growth pattern (ordered terrace array) on its surface morphology. The root mean square (RMS) roughness of samples A and B were approximately 1.087 and 0.227 nm, respectively. The surface of AlGaN/GaN films grown on c-axis-oriented substrate (sample B) was comparably smoother than those grown on the vicinal substrate (sample A), showing the growth mode had been changed from spiral-type to step-flow [18]. Therefore, a step-flow growth mode governing the epitaxial growth on a vicinalcut sapphire helps to suppress the formation of twodimensional islands, this in turn prevents holes from occurring due to incomplete island coalescence, and dislocations are thereby minimized [19,20]. Next, Figs. 3(a) and (b), respectively, depict the measured temperature-dependent mobility data for samples A and B. Notice that the mobility decreases in response to an increase in temperature for both samples. The sheet resistivity for both samples is around 500 O/&. These results appear reasonable since the AlGaN cap layer employed in our study is un-doped. It was also found that the maximum mobilities obtained were 5890 and 5490 cm2/V-s at 120 K while the room temperature mobilities were 1100 and 1210 cm2/V-s for samples A and B, respectively. The observed phenomenon was mainly due to the dominant effect of scattering mechanisms, such as piezoelectric, acoustic-mode deformation potential, and polar optical phonon at high temperature. To further analyze these data, the experimental curves relating mobility (m) to temperature (T) were fitted using the calculated mobility with relevant factors which are limited by different scattering mechanisms according to Matthiesen’s rule. Figs. 3(a) and (b) also show the scatteringlimited mobility curves obtained by considering different scattering events and an eventual statistical fit to the m vs. T characteristics for both samples. A good fit is realized in the low-temperature range, but it fails to achieve with a reasonable accuracy in the high-temperature regime. Therefore, other than the aforementioned scattering mechanisms, an additional scattering term such as dislocation scattering must also be taken into account in order to improve the overall fit accuracy, especially for a mobility trend in the high-temperature regime. It is known that dislocations can induce strain fields and polarization charge densities, and so it is vitally important to study their effects on the carrier transport of a 2DEG. Notice that it is possible to express the mobility limit as the function of crystal defect in the form of C/T1.5 at high temperature [21], where C is a fitting constant. Our work showed that the resultant fitting curve agreed well with the experimental data when these four mechanisms were taken

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255

100 Sheet Carrier Concentration (cm-2)

105 polar phonon

acoustic

Mobility (cm2/Vs)

piezoelectric 104

crystal defect 2.05x107/T1.5

103

sample A sample B

10

sample A 102 100

100 200

300

400

500

200

300

400

500

Temperature (K)

Temperature (K) Fig. 4. Temperature-dependent sheet carrier concentration of samples A and B.

105

polar phonon

acoustic

Mobility (cm2/Vs)

piezoelectric 104 crystal defect 1.45x107/T1.5 103

sample B 102 100

200

300

400

500

Temperature (K) Fig. 3. Temperature-dependent mobility of (a) sample A and (b) sample B.

into account. As what the temperature-dependent mobility data has shown in Figs. 3(a) and (b), the fitting constant C calculated equals to 2.05  107 and 1.45  107 for samples A and B, respectively. A larger value of C for sample A again suggests better crystalline quality of AlGaN/GaN epilayers [22]. Fig. 4 presents Hall sheet carrier concentration as function of temperature for both samples. At room temperature, the sheet carrier density is 10.3  1012 and 9.83  1012 cm2 for samples A and B, respectively. An increase in sheet carrier density is accompanied by a decrease in electron mobility, when Fig. 4 along with Figs. 3(a) and (b) are compared and assessed. Carrier distribution profile is an important factor for heterostructural devices. One can use C–V measurement to determine carrier distribution profile [23] ns ¼

2 , qK s o A2 ½dð1=C 2 Þ=dV 

(1)

K s o A , C

(2)

W¼

where Ks is the relative dielectric constant of semiconductor (9.5 for GaN, 9.0 for AlN), eo is the dielectric constant of free space, A is the area of the Schottky contact, C is the measured capacitance, and W is a depth measured from surface. The insets of Figs. 5(a) and (b) show measured capacitance–voltage (C–V) characteristics of samples A and B, respectively. When the applied reverse bias is high (being more negative), the measured capacitance corresponding to the depletion of 2DEG along the heterointerface approaches a minimum value. At his juncture the AlGaN layer is mostly depleted due to the presence of surface states. As we decrease the reverse bias (being less negative) by shifting toward the positive bias regime, a plateau of capacitance is observed which corresponds to a situation when the 2DEG approaches its equilibrium concentration. As the 2DEG capacitance vanishes for large bias voltage (being more positive), the capacitance measured is uniquely that of the Schottky diode. For large positive bias voltages, the capacitance is actually seen to diverge when the Schottky diode turns on. The carrier distribution profiles obtained from these C–V curves can thus be determined. Notice that the presence of 2DEG density at the AlGaN/GaN interface with undoped AlGaN layer is solely due to the piezoelectric properties of III-nitrides. As shown in Fig. 5(a) and (b), the concentration spike corresponding to the position of AlGaN/GaN hetero-interface occurs at around 18 nm for both samples. The result indicates the majority of carriers are indeed well confined. It was also found that 2DEG carrier concentration was higher for sample A, which agreed well with the result of Hall measurement. In fact, higher sheet carrier concentration of sample A evidently could be benefited from an improved crystalline quality of epilayers grown on 11-tilt vicinal-cut sapphire substrate. Lastly, according to in Figs. 5(a) and (b) notice that as the device becomes highly reverse biased (being more negative), the resultant capacitance fully decaying down to zero is not observed. To explain this observation, we believe the answer is somewhat

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100

5

2

1

Gate Current (A)

3

measured at 1MHz

2

1

10-4

10-6

10-8

0 -2

-1 0 Bias Voltage (V)

10-10 -10

0 10

Sample A Sample B

10-2

4 Capacitance (nF)

Electron Concentration (x1019 cm-3)

Sample A 3

20

30 Depth (nm)

40

50

-8

-6

-4

-2

0

2

4

Bias Voltage (V) Fig. 6. Measured I–V characteristics of the two samples.

Sample B 3

4 Capacitance (nF)

Electron Concentration (x1019 cm-3)

5

3

2

1

2

Measured at 1MHz

1

0 -2

-1 Bias Voltage (V)

0

0 10

20

30

40

50

Depth (nm) Fig. 5. Carrier distribution profiles of (a) sample A and (b) sample B determined from C–V measurements. The inset shows the respective C–V characteristics.

related to the trapping of electrons by the surface states present either in the bulk or at the surface of AlGaN. The presence of these charged surface states prevents the AlGaN layer from fully depleted, which therefore contributes to a remnant capacitance measured. The measured I–V characteristics of Schottky diodes fabricated on both samples A and B are shown in Fig. 6. The reverse leakage current of sample A was much smaller than that of sample B. With 10 V reverse bias, the reverse leakage currents of samples A and B measured were 1.52  106 and 2.05  104 A, respectively, showing a reduction of reverse leakage current by around two orders of magnitude. Using thermionic theory [24], it was found that Schottky barrier height and ideality factor of sample A were 0.78 and 1.97 eV, respectively, while the Schottky barrier height and ideality factor for sample B were, respectively, determined as 0.69 and 2.16 eV. A large ideality factor implies the current transport mechanism involved may not be purely a thermionic emission alone, instead, the tunneling becomes a dominant current trans-

port mechanism instead, such as direct tunneling or trapassisted tunneling. A relatively smaller ideality factor obtained for sample A seems reasonable owing to a reduced dislocation density which is related to trap-assisted tunneling [25]. Based on the aforementioned results, we conclude a larger Schottky barrier height, smaller ideality factor, and smaller leakage current realized for sample A are attributed to an overall better crystalline quality and comparably fewer dislocations. In other words, a noticeable improvement in the crystalline quality of AlGaN/GaN heterostructure and a better rectifying behavior have all been successfully achieved when the epitaxial MOCVD growth is performed using the vicinal-cut sapphire substrate. 4. Conclusions Al0.22Ga0.78N/GaN heterostructures were prepared on vicinal-cut sapphire substrate. When compared with AlGaN/GaN grown on the conventional c-axis sapphire substrate, it was found that AlGaN/GaN prepared on vicinal-cut sapphire substrate exhibited narrower XRD FWHM, lower dislocation density, rougher surface because of the step-flow growth mode, and larger lowtemperature mobility. The achievable lower leakage current, larger barrier height and smaller ideality factor were evidently benefited by fabricating the AlGaN/GaN Schottky diodes on vicinal-cut sapphire substrates. References [1] S. Nakamura, IEEE J. Sel. Top. Quantum Electron. 3 (1997) 435. [2] D.S. Wuu, W.K. Wang, W.C. Shih, R.H. Horng, C.E. Lee, W.Y. Lin, J.S. Fang, IEEE Photon. Technol. Lett. 17 (2005) 288. [3] S. Nakamura, Semicond. Sci. Technol. 14 (1999) R27. [4] S. Nakamura, G. Fasol, The Blue Laser Diode, first ed, Springer, Berlin, 1997 Germany. [5] R. Li, S.J. Cai, L. Wong, Y. Chen, K.L. Wang, R.P. Smith, S.C. Martin, K.S. Boutros, J.M. Redwing, IEEE Electron. Dev. Lett. 20 (1999) 323.

ARTICLE IN PRESS R.W. Chuang et al. / Journal of Crystal Growth 308 (2007) 252–257 [6] M. Akita, S. Kishimoto, T. Mizutani, IEEE Electron. Dev. Lett. 22 (2001) 376. [7] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [8] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [9] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [10] P.A. Grudowski, A.L. Holmes, C.J. Eiting, R.D. Dupuis, Appl. Phys. Lett. 69 (1996) 3626. [11] M. Fatemi, A.E. Wickenden, D.D. Koleske, M.E. Twigg Jr., J.A. Freitas, R.L. Henry, R.J. Gorman, Appl. Phys. Lett. 73 (1998) 608. [12] S.W. Kim, H. Aida, T. Suzuki, Phys. Stat. Sol. (c) 1 (2004) 2483. [13] X.Q. Shen, H. Matsuhata, H. Okumura, Appl. Phys. Lett. 86 (2005) 021912. [14] W. Saito, M. Kuraguch, Y. Takada, K. Tsuda, I. Omura, T. Ogura, IEEE Trans. Electron. Dev. 51 (2004) 1913. [15] M.G. Cheong, K.S. Kim, C.S. Oh, N.W. Namgung, G.M. Yang, C.H. Hong, K.Y. Lim, D.H. Lim, A. Yoshikawa, Appl. Phys. Lett. 77 (2000) 2557.

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[16] Y. Fu, Y.T. Moon, F. Yun, U. Ozgur, J.Q. Xie, S. Dogan, H. Morkoc, C.K. Inoki, T.S. Kuan, L. Zhou, D.J. Smith, Appl. Phys. Lett. 86 (2005) 043108. [17] T. Lei, K.F. Ludwig Jr., T. Moustakas, J. Appl. Phys. 74 (1993) 4430. [18] J.C. Lin, Y.K. Su, W.H. Lan, T.M. Kuan, W.R. Chen, Y.C. Cheng, W.J. Lin, Y.C. Tzeng, H.Y. Shin, Mater. Sci. Eng. B—Solid State Mater. Adv. Technol. 128 (2006) 107. [19] R. Grousson, V. Voliotis, N. Grandjean, J. Massies, M. Leroux, C. Deparis, Phys. Rev. B 55 (1997) 5253. [20] M. Shinohara, M. Tanimoto, H. Yokoyama, N. Inoue, Appl. Phys. Lett. 65 (1994) 1418. [21] H. Tang, W. Kim, A. Botchkarev, G. Popovici, F. Hamdani, H. Morkoc, Solid-State Electron. 42 (1998) 839. [22] J.C. Lin, Y.K. Su, S.J. Chang, W.R. Chen, R.Y. Chen, Y.C. Cheng, W.J. Lin, J. Crystal Growth 285 (2005) 481. [23] S.M. Sze, Physics of Semiconductor Devices, second ed, Wiley, New York, 1981. [24] K.M. Tracy, P.J. Hartlieb, S. Einfeldt, R.F. Davis, E.H. Hurt, R.J. Nemanich, J. Appl. Phys. 94 (2003) 3939. [25] W. Saito, M. Kuraguchi, Y. Takada, K. Tsuda, I. Omura, T. Ogura, IEEE Trans. Electron. Dev. 52 (2005) 159.