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Stress analyses of GaN film manufactured by ECR plasma-enhanced chemical vapor deposition
Vacuum 86 (2012) 1517e1521
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Stress analyses of GaN film manufactured by ECR plasma-enhanced chemical vapor depositionq Fu Silie a, *, Chen Junfang a, Gao Peng a, Wang Chun-ann b a b
School of Physics and Communication Engineering, South China Normal University, Guangzhou 510006, China School of Electronic and Information, GuangDong Polytechnic Normal University, 510665, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2011 Received in revised form 21 December 2011 Accepted 21 December 2011
The residual stress in GaN film grown on (0001) a-Al2O3 substrate at 450e500 C by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECRePECVD) is investigated. Macro deformation analysis reveals a low level of compressive stress, from 0.46 GPa to 1.03 GPa in GaN/Sapphire. Low growth temperature and high N2:TMG flow ratio contributes to decreasing of residual stress. A blue shift for the edge peak in photoluminescence analysis (PL) is related to compressive stress. Roughness statistics and AFM morphology of GaN film show a fine smoothness and uniform surface. All results demonstrate that ECR-PECVD process is favorable for depositing GaN films at low temperature. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: GaN film ECRePECVD Residual stress X-ray diffraction Photoluminescence
1. Introduction GaN and its related III-nitride compounds represent a class of wide bandgap semiconductors, which have wide applications in field effect transistors, blue and ultraviolet light emitting devices. As one of practical plasma sources, electron cyclotron resonance plasma is generated as a consequence of microwave energy absorption by electrons through cyclotron resonance. High density of reactive species inside contributes to depositing film at a relatively low temperature [1]. The knowledge of stress state in epitaxial GaN is important for better understanding its property and improving nitride-based devices. Macro deformation analysis [2], X-ray diffraction [3] and Raman shift analysis [4] etc have been developed to evaluate the stress in films. It is well known that GaN films are usually grown on heterostructural substrates. Undesired compressive or tensile stress will be caused by mismatch of lattice constant and coefficient of thermal expansion (CTE), as well as by coalescence of grains or islands [5]. Furthermore, V/III flow ratio also has effect on stress in film [6]. Different stress type was found in GaN film grown on ZnO under Ga-rich and N-rich conditions [7]. Stress get rise to crystal quality problems, such as high density of structural imperfections q Project supported by Chinese NSF [10575039]. * Corresponding author. E-mail address: [email protected] (F. Silie). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.021
(dislocations or grain boundaries [8]), presence of an uncontrolled deep center which is responsible for strong yellow luminescence band [9], a shift of band-edge peak, and so on. The study of stress found that large tensile stress exists in GaN epilayers grown on Si and 6HeSiC while a small compressive stress appears in the film grown on sapphire [10]. Tensile stress results in a decrease of energy band gap while compressive strain causes an increase of bandgap [11]. Increasing the thickness of GaN film can relax the stress. High level of stress, about 1 GPa in 1 mm GaN was reported in Ref. [12]. A nearly stress-free GaN film less than 80 MPa was achieved as thickness is 30 mm [13]. Furthermore, introduction of AlN buffer layer can reduce, or even eliminate stress when its thickness reached a certain high level [11]. For ECRePECVD process, GaN film is deposited at a relatively low temperature. Therefore, stress in GaN layer is expectedly lower than other growth methods. Low stress is favorable for improving GaN film quality. In this paper, GaN film was manufactured on (0001) a-Al2O3 substrate by ECRePECVD at low temperature. Trimethylgallium (TMG, Ga(CH3)3) and nitrogen are introduced as Ga and N precursors, respectively. XP1 stylus profiler is employed to investigate the residual stress through scanning macro curvature of GaN surface. The result show there is a low level of compressive stress in films. Finally, x-ray diffraction (XRD), room temperature photoluminescence (PL), and atomic force microscope (AFM) are used to analyze the influence of stress on properties of GaN film.
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2. Experimental details 2.1. The growth process of GaN film with ECRePECVD Schematic diagram of ECRePECVD system is shown in Fig. 1. It includes a F14 20 cm resonance room and a F35 50 cm reaction chamber. In our experiments, the working gas N2 is regulated by a mass flow controller (MFC) and fed into the resonance room through a distributor ring held on the front wall of resonance room. The reactive gas is regulated by another MFC and fed into the reaction chamber by pure N2 carrier through a distributor ring held on the front wall of reaction chamber. Two sets of concentric magnet coils supply a divergence-type magnetic field. The energy of 2450-MHz TE10 microwave is strongly absorbed by electrons in the ECR zone around 875 Gauss through cyclotron resonance. High density plasma in the reaction chamber, including N2*, Nþ 2 *, N*, Ga*, CH* etc (as shown in Fig. 3), is thus diffused to the sample table, which is located at the downstream z ¼ 25 cm of reaction chamber to avoid the affection of magnetic field. The spatial distribution of plasma density in reaction chamber is diagnosed by a Langmuir probe and shown in Fig. 2. Langmuir probe was made of F0.5 mm tungsten, which is inserted into a stainless steel tube and insulated by ceramics. Measurement of spatial distribution shows that plasma density around the sample table is about 2.5 1010 cm3. Analyses of radial uniformity show that the uniformity of plasma density within a radius of 6 cm at z ¼ 25 cm is about 96%. More details about spatial distribution in this ECRePECVD system can be found in Ref. [14]. The optical emission spectroscopy (OES) of N2/TMG ECR plasma near the sample table is shown in Fig. 3.Typical spectrum of nitrogen ECR plasma, including first negative series P Pþ 2 of Nþ ð1n : B2 þ first positive series of N2 2 u /X g Þ, P þ ð1p : B3 Pg /A3 u Þ, and second positive series of N2 ð2p : C 3 Pu /B3 Pg Þ [15] are appeared in the Fig. 3. The lifetime of Q molecular nitrogen in such C3 u state is too short to contribute to GaN growth. First negative series, such as 391.4 and 427.8 nm in correP sponding with B2 þ u state also do not contribute to GaN growth [16]. First positive series corresponding to molecular nitrogen is in a wide range of 545e700 nm. The lifetime of first positive is considered to be about several seconds. Such excited nitrogen in first positive series is favorable for depositing GaN. Strong Ga* 403.2 nm and 417.2 nm spectral lines indicate that TMG is significantly dissociated in ECR plasma even under the ambient temperature condition. Generally, these kinds of species obtained under the self-heating plasma
Fig. 1. Schematic diagram of ECRePECVD system.
Fig. 2. Spatial distribution of ECR plasma in reaction chamber.
condition are radical in the process of growing GaN film. It contributes to chemical reaction occurring at a relatively low temperature. In our process, 10-min nitridation procedure in high N2-ambient at about 350 C is adopted to cover a thin nitridation layer on surface in advance, and then two step growth is employed to grow GaN film. First the growth temperature is increased to grow GaN buffer layer at about 400 C for 15 min. Then the temperature is further increased to a certain degree such as 500 C and a GaN epilayer is slowly grown for one and half an hour to ensure fine crystal quality. Finally an hour N2-ambient annealing at about 550 C is applied after the growth is completed. Residual stress induced by mismatch in thermal expansion coefficient and lattice constant is unavoidable. However, stress related to lattice mismatch can be reduced by nitridation procedure and buffer layer. In addition, thermal stress caused by heating can be relaxed by annealing treatment. 2.2. The residual stress analyses of GaN film by XP1 stylus profiler AMBIOS XP1 stylus profiler was employed to investigate the stress in GaN film. The scanning principle of XP1 is similar to optical cantilever of atomic force microscope. To measure the film thickness, a step must be etched on surface in advance. A flat clip used to fix the sample on table in ECRePECVD can left a small blank area to create a step on sapphire for measuring film thickness. The film Stress s is measured by the level of bend or curl through the following well-known Stoney equation:
Fig. 3. Optical emission spectroscopy of N2/TMG ECR plasma.
F. Silie et al. / Vacuum 86 (2012) 1517e1521
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Table 1 Data about GaN and sapphire (extracted from Refs. [12,17]). Crystal
Lattice constant (Å)
GaN
Wurtzite a ¼ 3.1878, e c ¼ 5.185 Hexagonal a ¼ 4.7589, (0001)/(0001) c ¼ 12.991 16.02%
a-Al2O3
s¼
Lattice mismatch E v (room temp.) (GPa) 295 425
1 E ts2 ; 6R 1 y tf
CTE ( 106/ C)
0.25 aa ¼ 4.997 ac ¼ 4.481 0.30 aa ¼ 8.31 ac ¼ 8.5
(1)
where ts and tf are thickness of substrate and film, respectively. Film thickness can be acquired through the step-curve. R is the radius of curvature of film, and E/(1 y) denote elastic constant of substrate, in which E and y is Young’s Modulus and Poisson’s Ratio of sapphire, respectively. This result may be verified by using the equation below for radius of curvature R:
R ¼
L2 8B
ðif L[BÞ;
(2)
where B is the bow (maximum between the trace and its chord) and L is chord length (scan length). To investigate the stress of GaN film, a pre-stress scan of substrate was done before film growth. Calculation of R is carried out by comparing original profile of substrate and post-stress profile of GaN epilayer at the same place. In our experiment, the scan length is fixed at 5 mm. According to the analysis of spatial distribution of plasma density, the plasma density is considerable uniformity in such small size, so the effect of inhomogeneous thickness on film profile can be ignored in our experiment. 3. Results and discussions (0001) sapphire with one side polished is employed to grow GaN film. The size of substrate is 10 10 0.5 mm. Young’s Modulus of sapphire is 425 GPa and Poisson’s Ratio is 0.3. More detail data about GaN and its substrate are listed in Table 1. The step-curves of GaN films are shown in Fig. 4. Thickness of GaN films is calculated through measuring the height of step. Film profiles of GaN films are shown in Fig. 5. Tensile or compressive
Fig. 5. Scanning curves of GaN films.
stress type is determined by the concave or convex of scanning curves. Moreover, roughness of films can be measured by the scanning curves. The residual stress calculated from the comparison to scanning curve of substrate and GaN epilayer are listed in Table 2. Note that high sharp peaks appeared in Fig. 5 are due to dusts on the surface during scanning. It is found that there is compressive stress existed in all of GaN films deposited under different growth process. As we know, for the case of GaN/sapphire, the lattice of epitaxial GaN rotates by 30 with respect to that of sapphire. Atomic spacing of the substrate parallel to a of GaN is only 2.75 Å [18], which is shorter than 3.1878 Å of GaN. Additionally, the thermal expansion coefficient of GaN is smaller than that of sapphire both in c plane direction and [0001] direction. Both lattice and thermal mismatch lead to compressive stress inevitably. In our experiments, the compressive stress is in the range from 0.46 GPa to 1.03 GPa under the condition of T ¼ 450e500 C and N2:TMG flow ratio varied from 10:1 to 14:1. The results indicate that low temperature and high N2:TMG flow ratio will cause the trend of low residual stress. Roughness statistics in Table 2 shows that the average roughness Ra is varied from 36 Å to 60 Å. It demonstrates the roughness of GaN films grown by ECRePECVD maintain fine smoothness even under different growth process. Such a smoothness and compact of surface is comparable to those of MOCVD technology which based on heating decomposition and grown at an extremely high temperature (up to 1050 C). To investigate crystal quality of GaN film, XRD analysis is carried out and the result is shown in Fig. 6, XRD analysis shows that the peak of Sapphire (0006) is at 2q ¼ 41.60 and the peak of GaN (0002) is at 2q ¼ 34.48 . The GaN peak turns sharp and strong with the increase of N2:TMG flow ratio. It means that the crystal quality of GaN films gets better with a high ratio of N2:TMG. A 140 full width of half maximum (FWHM) of the GaN (0002) peak is obtained at N2:TMG ¼ 14:1. Table 2 Growth condition, stress and PL data of GaN films. Sample Growth parameter Growth N2:TMG flow Temp. ( C) ratio (sccm)
Fig. 4. Step-curves of GaN films.
1# 2# 3# 4#
500 500 480 450
10:1 11.5:1 12.5:1 14:1
Thickness Roughness Average Edge peak (nm) tf (mm) Ra (Å) stress (GPa) 0.65 1.4 1.3 0.9
66 39 36 43
1.03 0.78 0.65 0.46
362.5 e 363 364
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F. Silie et al. / Vacuum 86 (2012) 1517e1521
Fig. 6. XRD of GaN films with different N2:TMG flow ratio. Fig. 8. The spectral intensity of Nþ 2 391.3 nm and 427.8 nm dependence on N2:TMG flow ratio.
To investigate the effect of residual stress on optical property of GaN film, room temperature photoluminescence analysis of GaN films was carried out. As shown in Fig. 7, the edge peaks of 1#, 3#, 4# GaN are located at 362.5 nm, 363 nm and 364 nm, which have a blue shift about 3.0 nm, 2.5 nm and 1.5 nm with respect to theoretical value (365.5 nm), respectively. Compressive strain is responsible for such blue shift. This type of stress is related to N2:TMG flow ratio. Lower stress strength is corresponding to higher N2:TMG flow ratio. OES measurements in section 2.1 have indicated that there are certain level concentration of Nþ 2 ions in ECR plasma, such as 391.4 nm and 427.8 nm in corresponding with P Pþ þ 2 B2 þ u /X g . Study of the spectral emission intensity of N2 391.3 nm and 427.8 nm dependence on N2:TMG flow ratio shows that both of the spectral intensity increases with increasing of N2:TMG flow ratio (as shown in Fig. 8). Higher N2:TMG flow ratio þ means more Nþ 2 ions in the ECR plasma. These N2 ions are accelerated by plasma sheath self-built near the float sample table and results a bombardment on sample. Bombardment of higher concentration of Nþ 2 ion give rise to increasing of vacancies in GaN film [19]. High atomic vacancies concentration in GaN will reduce the Young’s modulus of GaN film [20]. As a result, the residual stress is weakened as N2:TMG flow ratio increase. Study of Residual stress in GaN film grown on Si (111) by MOCVD also showed the same
Fig. 7. The edge peak of GaN films, and inset is the PL spectra.
phenomenon that the residual stress decreases with increasing V/ IIII ratio [21]. Using the following relation between luminescence shift and stress we obtain the value of stress:
DEPL ¼
dEPL s; ds
(3)
where dE/ds is the linear pressure coefficient (25 meV/GPa or 2.69 nm/GPa for GaN/sapphire system [10]). For the 4 # sample, the residual stress calculated from the relationship is 1.5/ 2.69 ¼ 0.56 GPa, which is consistent with 0.46 GPa measured by Xp1. Furthermore, Compressive stress will cause a slight convex of epifilm and lengthen the axial lattice constant. The c lattice constant can be obtained by Hooke’s law:
s¼
c c0 E c c0 295 c c0 ¼ ¼ 1180 ðGPÞa ; c0 v c0 0:25 c0
(4)
where c0 is the bulk value of the lattice constant (5.185 Å). For the 4# sample, the c lattice constant is 5.187 Å.
Fig. 9. Surface morphology of 4# GaN film (5000 nm 5000 nm).
F. Silie et al. / Vacuum 86 (2012) 1517e1521
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4. Conclusions
Fig. 10. Data statistic of area and height distribution rate.
In addition, there is a wide yellow band present in the inset. It is centered at 550 nm, 542 nm, 525 nm (about 2.2 eV) with a range of 480e700 nm (1.8e2.5 eV) for 1#, 3#, 4# GaN samples respectively. Such yellow band is associated with transitions from conduction band or shallow donor level to deep acceptor level. Ga vacancies (VGa) resulted from high Nþ 2 ion bombardment are responsible for deep acceptor level. As we know, N vacancies are associated with shallow donor level in GaN film [22]. Lower N2:TMG flow ratio means higher concentration of N vacancies in GaN films. High concentration of N vacancies will enhance radioactive compound between VN shallow donor level and VGa deep acceptor level. As a result, strong yellow band appeared in PL spectrum as 1# GaN shown in Fig. 7. The morphology of 4# GaN is shown in Fig. 9. Its average size of granules is about 70 nm. As we know, the growth of GaN film on aAl2O3 substrate is a kind of heterogeneous expitaxy. Study of kinetics and dynamics in film growth pointed out that growth model of heterogeneous expitaxy is usually three-dimensional islands growth [23]. First Three-dimensional islands are formed (VolmereWeber model), and then two-dimensional layers are grown layer by layer (layer-by-layer model) to form a smooth surface. Statistic of area distribution rate (ADR) and height distribution rate (HDR) shows that ADR is 57.9% when HDR is at the maximum percentage 22.7% and height is 35.6 nm. The ADR of height higher than 23.7 nm counts for 86.7% of the total area. Additionally, the HDR of height in range of 27.7 nme43.5 nm counts for 62% of the total height (as shown in Fig. 10). The result indicates that sample 4# is inclined to grow with layer-by-layer model and the surface is compact and uniform. This result is consistent with the PL analysis. High N2:TMG flow ratio will weaken the stress between epilayer and substrate. Accordingly, the structure defects are reduced.
GaN films are grown on (0001) a-Al2O3 substrate in an ECRePECVD system at T ¼ 450e500 C. The N and Ga precursors are pure N2 and trimethylgallium, respectively. The ex situ XP1 stylus profiler is employed to investigate residual stress through scanning macro curvature of GaN surface. The results show that lattice and thermal mismatch lead to compressive stress for GaN grown on sapphire. The compressive stress range from 0.46 GPa to 1.03 GPa under the condition of T ¼ 450e500 C and N2:TMG flow ratio varied from 10:1 to 14:1. Low temperature and high N2:TMG flow ratio will cause the trend of low residual stress. Compressive stress is responsible for blue shift of the edge peak with respect to theoretical value in PL analysis. Lower stress strength is corresponding to higher N2:TMG flow ratio. Higher N2:TMG flow ratio further weakens the yellow band present in PL. Finally, Roughness statistics and AFM morphology of GaN epifilm show a fine smoothness and uniform surface, which demonstrate from other hand that ECRePECVD is favorable for depositing GaN at low temperature.
References [1] Fu SL, Chen JF, Zhang HB, Guo CF, Li W, Zhao WF. J Cryst Growth 2009; 311(12):3325e31. [2] Krost A, Dadgar A, Strassburger G, Clos R. Phys Stat Sol (a) 2003;200(1):26e35. [3] Cui JP, Wang XF, Duan Y, He JX, Zeng YP. Chin Phys Lett 2008;25(6):2277e80. [4] Ma CH, Huang JH, Chen H. Thin Solid Film 2002;418(12):73e8. [5] Dadgar A, Poschenrieder M, Bläsing J, Fehse K, Diez A, Krost A. Appl Phys Lett 2002;80:3670e2. [6] Kusaka K, Hanabusa T, Tominaga K. Vacuum 2004;74(3e4):613e8. [7] Minegishi T, Suzuki T, Harada C, Goto H, Cho MW, Yao T. Curr Appl Phys 2004; 4(6):685e7. [8] Molnar RJ, Singh R, Moustakas TD. Appl Phys Lett 1995;66:268e70. [9] Nakamura S, Mukai T, Senoh M. J Appl Phys 1994;76:8189. [10] Zhao DG, Xu SJ, Xie MH, Tong SY. Appl Phys Lett 2003;83(4):677e9. [11] Rieger W, Metzger T, Angerer H, Dimitrov R, Ambacher O, Stutzmann M. Appl Phys Lett 1996;68(7):970e2. [12] Barghout K, Chaudhur J. J Mater Sci 2004;39:5817e23. [13] Hermann M, Gogova D, Siche D, Schmidbauer M, Monemar B, Stutzmann M, et al. J Cryst Growth 2006;293:462e8. [14] (a) Chen JF, Fu SL, Lai QX, Xiang PF, Li Y, Wu XQ. Vacuum 2006;81:49e53; (b) Fu SL, Chen JF, Hu SJ, Wu XQ, Lee Y, Fan SL. Plasma Sources Sci Technol 2006;15:187e92. [15] Herzberg G. Molecular spectra and molecular structure. In: Spectra of diatomic molecules, vol. I. Beijing: Science Press; 1983 [in Chinese] p. 355. [16] Botchkarve A, Salvador A, Sverdlov B, Myoung J, Morkoc H. J Appl Phys 1995; 77:4455e8. [17] Liu L, Edgar LH. Mater Sci Eng 2002;R37:61e127. [18] Powell RC, Lee NE, Kim YW, Greene JE. J Appl Phys 1993;73:189e204. [19] Molnar RJ, Moustakas TD. J Appl Phys 1994;76:4587e95. [20] Xie SF, Chen SD, Soh AK. Chin Phys Lett 2011;28(6):066201. [21] Fu Yankun, Daniel AG, Ryan H. J Vac Sci Technol A 2000;18:965e7. [22] Glaser ER, Kennedy TA, Doverspike K, Rowland LB, Gaskill DK. Phys Rev B 1995;51(19):13326e36. [23] (a) En-ge Wang. Prog Phys (in Chinese) 2003;23:1e61; (b) Wang En-ge. Prog Phys (in Chinese) 2003;23:145e91.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Stress analyses of GaN film manufactured by ECR plasma-enhanced chemical vapor depositionq Fu Silie a, *, Chen Junfang a, Gao Peng a, Wang Chun-ann b a b
School of Physics and Communication Engineering, South China Normal University, Guangzhou 510006, China School of Electronic and Information, GuangDong Polytechnic Normal University, 510665, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2011 Received in revised form 21 December 2011 Accepted 21 December 2011
The residual stress in GaN film grown on (0001) a-Al2O3 substrate at 450e500 C by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECRePECVD) is investigated. Macro deformation analysis reveals a low level of compressive stress, from 0.46 GPa to 1.03 GPa in GaN/Sapphire. Low growth temperature and high N2:TMG flow ratio contributes to decreasing of residual stress. A blue shift for the edge peak in photoluminescence analysis (PL) is related to compressive stress. Roughness statistics and AFM morphology of GaN film show a fine smoothness and uniform surface. All results demonstrate that ECR-PECVD process is favorable for depositing GaN films at low temperature. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: GaN film ECRePECVD Residual stress X-ray diffraction Photoluminescence
1. Introduction GaN and its related III-nitride compounds represent a class of wide bandgap semiconductors, which have wide applications in field effect transistors, blue and ultraviolet light emitting devices. As one of practical plasma sources, electron cyclotron resonance plasma is generated as a consequence of microwave energy absorption by electrons through cyclotron resonance. High density of reactive species inside contributes to depositing film at a relatively low temperature [1]. The knowledge of stress state in epitaxial GaN is important for better understanding its property and improving nitride-based devices. Macro deformation analysis [2], X-ray diffraction [3] and Raman shift analysis [4] etc have been developed to evaluate the stress in films. It is well known that GaN films are usually grown on heterostructural substrates. Undesired compressive or tensile stress will be caused by mismatch of lattice constant and coefficient of thermal expansion (CTE), as well as by coalescence of grains or islands [5]. Furthermore, V/III flow ratio also has effect on stress in film [6]. Different stress type was found in GaN film grown on ZnO under Ga-rich and N-rich conditions [7]. Stress get rise to crystal quality problems, such as high density of structural imperfections q Project supported by Chinese NSF [10575039]. * Corresponding author. E-mail address: [email protected] (F. Silie). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.021
(dislocations or grain boundaries [8]), presence of an uncontrolled deep center which is responsible for strong yellow luminescence band [9], a shift of band-edge peak, and so on. The study of stress found that large tensile stress exists in GaN epilayers grown on Si and 6HeSiC while a small compressive stress appears in the film grown on sapphire [10]. Tensile stress results in a decrease of energy band gap while compressive strain causes an increase of bandgap [11]. Increasing the thickness of GaN film can relax the stress. High level of stress, about 1 GPa in 1 mm GaN was reported in Ref. [12]. A nearly stress-free GaN film less than 80 MPa was achieved as thickness is 30 mm [13]. Furthermore, introduction of AlN buffer layer can reduce, or even eliminate stress when its thickness reached a certain high level [11]. For ECRePECVD process, GaN film is deposited at a relatively low temperature. Therefore, stress in GaN layer is expectedly lower than other growth methods. Low stress is favorable for improving GaN film quality. In this paper, GaN film was manufactured on (0001) a-Al2O3 substrate by ECRePECVD at low temperature. Trimethylgallium (TMG, Ga(CH3)3) and nitrogen are introduced as Ga and N precursors, respectively. XP1 stylus profiler is employed to investigate the residual stress through scanning macro curvature of GaN surface. The result show there is a low level of compressive stress in films. Finally, x-ray diffraction (XRD), room temperature photoluminescence (PL), and atomic force microscope (AFM) are used to analyze the influence of stress on properties of GaN film.
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2. Experimental details 2.1. The growth process of GaN film with ECRePECVD Schematic diagram of ECRePECVD system is shown in Fig. 1. It includes a F14 20 cm resonance room and a F35 50 cm reaction chamber. In our experiments, the working gas N2 is regulated by a mass flow controller (MFC) and fed into the resonance room through a distributor ring held on the front wall of resonance room. The reactive gas is regulated by another MFC and fed into the reaction chamber by pure N2 carrier through a distributor ring held on the front wall of reaction chamber. Two sets of concentric magnet coils supply a divergence-type magnetic field. The energy of 2450-MHz TE10 microwave is strongly absorbed by electrons in the ECR zone around 875 Gauss through cyclotron resonance. High density plasma in the reaction chamber, including N2*, Nþ 2 *, N*, Ga*, CH* etc (as shown in Fig. 3), is thus diffused to the sample table, which is located at the downstream z ¼ 25 cm of reaction chamber to avoid the affection of magnetic field. The spatial distribution of plasma density in reaction chamber is diagnosed by a Langmuir probe and shown in Fig. 2. Langmuir probe was made of F0.5 mm tungsten, which is inserted into a stainless steel tube and insulated by ceramics. Measurement of spatial distribution shows that plasma density around the sample table is about 2.5 1010 cm3. Analyses of radial uniformity show that the uniformity of plasma density within a radius of 6 cm at z ¼ 25 cm is about 96%. More details about spatial distribution in this ECRePECVD system can be found in Ref. [14]. The optical emission spectroscopy (OES) of N2/TMG ECR plasma near the sample table is shown in Fig. 3.Typical spectrum of nitrogen ECR plasma, including first negative series P Pþ 2 of Nþ ð1n : B2 þ first positive series of N2 2 u /X g Þ, P þ ð1p : B3 Pg /A3 u Þ, and second positive series of N2 ð2p : C 3 Pu /B3 Pg Þ [15] are appeared in the Fig. 3. The lifetime of Q molecular nitrogen in such C3 u state is too short to contribute to GaN growth. First negative series, such as 391.4 and 427.8 nm in correP sponding with B2 þ u state also do not contribute to GaN growth [16]. First positive series corresponding to molecular nitrogen is in a wide range of 545e700 nm. The lifetime of first positive is considered to be about several seconds. Such excited nitrogen in first positive series is favorable for depositing GaN. Strong Ga* 403.2 nm and 417.2 nm spectral lines indicate that TMG is significantly dissociated in ECR plasma even under the ambient temperature condition. Generally, these kinds of species obtained under the self-heating plasma
Fig. 1. Schematic diagram of ECRePECVD system.
Fig. 2. Spatial distribution of ECR plasma in reaction chamber.
condition are radical in the process of growing GaN film. It contributes to chemical reaction occurring at a relatively low temperature. In our process, 10-min nitridation procedure in high N2-ambient at about 350 C is adopted to cover a thin nitridation layer on surface in advance, and then two step growth is employed to grow GaN film. First the growth temperature is increased to grow GaN buffer layer at about 400 C for 15 min. Then the temperature is further increased to a certain degree such as 500 C and a GaN epilayer is slowly grown for one and half an hour to ensure fine crystal quality. Finally an hour N2-ambient annealing at about 550 C is applied after the growth is completed. Residual stress induced by mismatch in thermal expansion coefficient and lattice constant is unavoidable. However, stress related to lattice mismatch can be reduced by nitridation procedure and buffer layer. In addition, thermal stress caused by heating can be relaxed by annealing treatment. 2.2. The residual stress analyses of GaN film by XP1 stylus profiler AMBIOS XP1 stylus profiler was employed to investigate the stress in GaN film. The scanning principle of XP1 is similar to optical cantilever of atomic force microscope. To measure the film thickness, a step must be etched on surface in advance. A flat clip used to fix the sample on table in ECRePECVD can left a small blank area to create a step on sapphire for measuring film thickness. The film Stress s is measured by the level of bend or curl through the following well-known Stoney equation:
Fig. 3. Optical emission spectroscopy of N2/TMG ECR plasma.
F. Silie et al. / Vacuum 86 (2012) 1517e1521
1519
Table 1 Data about GaN and sapphire (extracted from Refs. [12,17]). Crystal
Lattice constant (Å)
GaN
Wurtzite a ¼ 3.1878, e c ¼ 5.185 Hexagonal a ¼ 4.7589, (0001)/(0001) c ¼ 12.991 16.02%
a-Al2O3
s¼
Lattice mismatch E v (room temp.) (GPa) 295 425
1 E ts2 ; 6R 1 y tf
CTE ( 106/ C)
0.25 aa ¼ 4.997 ac ¼ 4.481 0.30 aa ¼ 8.31 ac ¼ 8.5
(1)
where ts and tf are thickness of substrate and film, respectively. Film thickness can be acquired through the step-curve. R is the radius of curvature of film, and E/(1 y) denote elastic constant of substrate, in which E and y is Young’s Modulus and Poisson’s Ratio of sapphire, respectively. This result may be verified by using the equation below for radius of curvature R:
R ¼
L2 8B
ðif L[BÞ;
(2)
where B is the bow (maximum between the trace and its chord) and L is chord length (scan length). To investigate the stress of GaN film, a pre-stress scan of substrate was done before film growth. Calculation of R is carried out by comparing original profile of substrate and post-stress profile of GaN epilayer at the same place. In our experiment, the scan length is fixed at 5 mm. According to the analysis of spatial distribution of plasma density, the plasma density is considerable uniformity in such small size, so the effect of inhomogeneous thickness on film profile can be ignored in our experiment. 3. Results and discussions (0001) sapphire with one side polished is employed to grow GaN film. The size of substrate is 10 10 0.5 mm. Young’s Modulus of sapphire is 425 GPa and Poisson’s Ratio is 0.3. More detail data about GaN and its substrate are listed in Table 1. The step-curves of GaN films are shown in Fig. 4. Thickness of GaN films is calculated through measuring the height of step. Film profiles of GaN films are shown in Fig. 5. Tensile or compressive
Fig. 5. Scanning curves of GaN films.
stress type is determined by the concave or convex of scanning curves. Moreover, roughness of films can be measured by the scanning curves. The residual stress calculated from the comparison to scanning curve of substrate and GaN epilayer are listed in Table 2. Note that high sharp peaks appeared in Fig. 5 are due to dusts on the surface during scanning. It is found that there is compressive stress existed in all of GaN films deposited under different growth process. As we know, for the case of GaN/sapphire, the lattice of epitaxial GaN rotates by 30 with respect to that of sapphire. Atomic spacing of the substrate parallel to a of GaN is only 2.75 Å [18], which is shorter than 3.1878 Å of GaN. Additionally, the thermal expansion coefficient of GaN is smaller than that of sapphire both in c plane direction and [0001] direction. Both lattice and thermal mismatch lead to compressive stress inevitably. In our experiments, the compressive stress is in the range from 0.46 GPa to 1.03 GPa under the condition of T ¼ 450e500 C and N2:TMG flow ratio varied from 10:1 to 14:1. The results indicate that low temperature and high N2:TMG flow ratio will cause the trend of low residual stress. Roughness statistics in Table 2 shows that the average roughness Ra is varied from 36 Å to 60 Å. It demonstrates the roughness of GaN films grown by ECRePECVD maintain fine smoothness even under different growth process. Such a smoothness and compact of surface is comparable to those of MOCVD technology which based on heating decomposition and grown at an extremely high temperature (up to 1050 C). To investigate crystal quality of GaN film, XRD analysis is carried out and the result is shown in Fig. 6, XRD analysis shows that the peak of Sapphire (0006) is at 2q ¼ 41.60 and the peak of GaN (0002) is at 2q ¼ 34.48 . The GaN peak turns sharp and strong with the increase of N2:TMG flow ratio. It means that the crystal quality of GaN films gets better with a high ratio of N2:TMG. A 140 full width of half maximum (FWHM) of the GaN (0002) peak is obtained at N2:TMG ¼ 14:1. Table 2 Growth condition, stress and PL data of GaN films. Sample Growth parameter Growth N2:TMG flow Temp. ( C) ratio (sccm)
Fig. 4. Step-curves of GaN films.
1# 2# 3# 4#
500 500 480 450
10:1 11.5:1 12.5:1 14:1
Thickness Roughness Average Edge peak (nm) tf (mm) Ra (Å) stress (GPa) 0.65 1.4 1.3 0.9
66 39 36 43
1.03 0.78 0.65 0.46
362.5 e 363 364
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Fig. 6. XRD of GaN films with different N2:TMG flow ratio. Fig. 8. The spectral intensity of Nþ 2 391.3 nm and 427.8 nm dependence on N2:TMG flow ratio.
To investigate the effect of residual stress on optical property of GaN film, room temperature photoluminescence analysis of GaN films was carried out. As shown in Fig. 7, the edge peaks of 1#, 3#, 4# GaN are located at 362.5 nm, 363 nm and 364 nm, which have a blue shift about 3.0 nm, 2.5 nm and 1.5 nm with respect to theoretical value (365.5 nm), respectively. Compressive strain is responsible for such blue shift. This type of stress is related to N2:TMG flow ratio. Lower stress strength is corresponding to higher N2:TMG flow ratio. OES measurements in section 2.1 have indicated that there are certain level concentration of Nþ 2 ions in ECR plasma, such as 391.4 nm and 427.8 nm in corresponding with P Pþ þ 2 B2 þ u /X g . Study of the spectral emission intensity of N2 391.3 nm and 427.8 nm dependence on N2:TMG flow ratio shows that both of the spectral intensity increases with increasing of N2:TMG flow ratio (as shown in Fig. 8). Higher N2:TMG flow ratio þ means more Nþ 2 ions in the ECR plasma. These N2 ions are accelerated by plasma sheath self-built near the float sample table and results a bombardment on sample. Bombardment of higher concentration of Nþ 2 ion give rise to increasing of vacancies in GaN film [19]. High atomic vacancies concentration in GaN will reduce the Young’s modulus of GaN film [20]. As a result, the residual stress is weakened as N2:TMG flow ratio increase. Study of Residual stress in GaN film grown on Si (111) by MOCVD also showed the same
Fig. 7. The edge peak of GaN films, and inset is the PL spectra.
phenomenon that the residual stress decreases with increasing V/ IIII ratio [21]. Using the following relation between luminescence shift and stress we obtain the value of stress:
DEPL ¼
dEPL s; ds
(3)
where dE/ds is the linear pressure coefficient (25 meV/GPa or 2.69 nm/GPa for GaN/sapphire system [10]). For the 4 # sample, the residual stress calculated from the relationship is 1.5/ 2.69 ¼ 0.56 GPa, which is consistent with 0.46 GPa measured by Xp1. Furthermore, Compressive stress will cause a slight convex of epifilm and lengthen the axial lattice constant. The c lattice constant can be obtained by Hooke’s law:
s¼
c c0 E c c0 295 c c0 ¼ ¼ 1180 ðGPÞa ; c0 v c0 0:25 c0
(4)
where c0 is the bulk value of the lattice constant (5.185 Å). For the 4# sample, the c lattice constant is 5.187 Å.
Fig. 9. Surface morphology of 4# GaN film (5000 nm 5000 nm).
F. Silie et al. / Vacuum 86 (2012) 1517e1521
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4. Conclusions
Fig. 10. Data statistic of area and height distribution rate.
In addition, there is a wide yellow band present in the inset. It is centered at 550 nm, 542 nm, 525 nm (about 2.2 eV) with a range of 480e700 nm (1.8e2.5 eV) for 1#, 3#, 4# GaN samples respectively. Such yellow band is associated with transitions from conduction band or shallow donor level to deep acceptor level. Ga vacancies (VGa) resulted from high Nþ 2 ion bombardment are responsible for deep acceptor level. As we know, N vacancies are associated with shallow donor level in GaN film [22]. Lower N2:TMG flow ratio means higher concentration of N vacancies in GaN films. High concentration of N vacancies will enhance radioactive compound between VN shallow donor level and VGa deep acceptor level. As a result, strong yellow band appeared in PL spectrum as 1# GaN shown in Fig. 7. The morphology of 4# GaN is shown in Fig. 9. Its average size of granules is about 70 nm. As we know, the growth of GaN film on aAl2O3 substrate is a kind of heterogeneous expitaxy. Study of kinetics and dynamics in film growth pointed out that growth model of heterogeneous expitaxy is usually three-dimensional islands growth [23]. First Three-dimensional islands are formed (VolmereWeber model), and then two-dimensional layers are grown layer by layer (layer-by-layer model) to form a smooth surface. Statistic of area distribution rate (ADR) and height distribution rate (HDR) shows that ADR is 57.9% when HDR is at the maximum percentage 22.7% and height is 35.6 nm. The ADR of height higher than 23.7 nm counts for 86.7% of the total area. Additionally, the HDR of height in range of 27.7 nme43.5 nm counts for 62% of the total height (as shown in Fig. 10). The result indicates that sample 4# is inclined to grow with layer-by-layer model and the surface is compact and uniform. This result is consistent with the PL analysis. High N2:TMG flow ratio will weaken the stress between epilayer and substrate. Accordingly, the structure defects are reduced.
GaN films are grown on (0001) a-Al2O3 substrate in an ECRePECVD system at T ¼ 450e500 C. The N and Ga precursors are pure N2 and trimethylgallium, respectively. The ex situ XP1 stylus profiler is employed to investigate residual stress through scanning macro curvature of GaN surface. The results show that lattice and thermal mismatch lead to compressive stress for GaN grown on sapphire. The compressive stress range from 0.46 GPa to 1.03 GPa under the condition of T ¼ 450e500 C and N2:TMG flow ratio varied from 10:1 to 14:1. Low temperature and high N2:TMG flow ratio will cause the trend of low residual stress. Compressive stress is responsible for blue shift of the edge peak with respect to theoretical value in PL analysis. Lower stress strength is corresponding to higher N2:TMG flow ratio. Higher N2:TMG flow ratio further weakens the yellow band present in PL. Finally, Roughness statistics and AFM morphology of GaN epifilm show a fine smoothness and uniform surface, which demonstrate from other hand that ECRePECVD is favorable for depositing GaN at low temperature.
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