Characterizations of GaN film growth by ECR plasma chemical vapor deposition

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 3325–3331

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Characterizations of GaN film growth by ECR plasma chemical vapor deposition Silie Fu, Junfang Chen , Hongbin Zhang, Chaofen Guo, Wei Li, Wenfen Zhao School of Physics and Communication Engineering, South China Normal University, Higher Education Mega center, Guangzhou 510006, China

a r t i c l e in fo

abstract

Article history: Received 10 September 2008 Received in revised form 6 February 2009 Accepted 14 March 2009 Communicated by R. Bhat Available online 6 April 2009

The electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR–MOPECVD) is adopted to grow GaN films on (0 0 0 1) a-Al2O3 substrate. The gas sources are pure N2 and trimethylgallium (TMG). Optical emission spectroscopy (OES) and thermodynamic analysis of GaN growth are applied to understand the GaN growth process. The OES of ECR plasma shows that TMG is significantly dissociated in ECR plasma. Reactants N and Ga in the plasma, obtained easily under the self-heating condition, are essential for the GaN growth. They contribute to the realization of GaN film growth at a relatively low temperature. The thermodynamic study shows that the driving force for the GaN growth is high when N2:TMG41. Furthermore, higher N2:TMG flow ratio makes the GaN growth easier. Finally, X-ray diffraction, photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN films. The results demonstrate that the ECR–MOPECVD technology is favorable for depositing GaN films at low temperatures. & 2009 Elsevier B.V. All rights reserved.

PACS: 61.72.Uj 52.50.Sw 52.25.Kn Keywords: A1. OES A1. Thermodynamic analysis A3. ECR–MOPECVD B1. GaN

1. Introduction The wide direct band-gap gallium nitride is a promising material for the construction of short-wavelength optoelectronic devices [1]. Furthermore, its outstanding stability in thermal and chemical enables it to operate at high temperatures and in hostile environments, such as petroleum probing and aerospace. At present, the prevailing growth techniques of GaN are metalorganic chemical vapor deposition (MOCVD), molecule beam epitaxy (MBE) as well as halide vapor phase epitaxy (HVPE). As one of the novel growth techniques, the electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR–MOPECVD) has been used to make high quality and compactly structured film at low temperatures [2]. ECR plasmas are generated as a consequence of microwave energy absorption by the electrons through cyclotron resonance. High degree of ionization, high concentration of active species, and large volume uniform plasma are excited under low gas pressure. The emission spectra of the ECR plasma carry useful intrinsic information. Identification of the excited species in the ECR plasma is important to understand the growth mechanism of

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E-mail address: [email protected] (J. Chen). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.03.040

ECR–MOPECVD. Optical emission spectroscopy (OES) is a useful and non-contact tool to analyze the information. The application of the OES for plasma-assisted-depositing GaN films has been discussed in literatures [3,4]. In Ref. [5], the OES analysis for ECR–MBE growth of GaN was used to explain the decrease of Ga emission intensity under a deeper positive bias condition by suppressing the Ga desorption. The dependence of the GaN growth rate on the electron temperature in the ECR plasma was studied in Ref. [6]. This study suggested that electron temperature can be regarded as a candidate of control parameters in plasmaassisted CVD. These studies on the nitrogen species in plasmaassisted MBE growth of GaN illustrated that the predominant species are the nitrogen atoms in radio frequency (RF) plasma sources instead of the nitrogen molecules in ECR sources [7,8]. Thermodynamics analysis is a useful way to understand and explain the process of film growth, particularly the gas–solid interface growth. Thermodynamic analysis on growth of group nitrides was discussed in Refs. [9,10]. It is well known that the thermodynamics of GaN film growth is related to the gases source, substrate type, pressure, and growth temperature [11]. For lowpressure ECR plasma, diffusion is the main mechanism of various particles propagation, and the quasi-thermodynamic equilibrium can be reached at the gas–solid interface. In this paper, the ECR-MOPECVD is adopted to grow the GaN on (0 0 0 1) sapphire at a low-substrate temperature.

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Trimethylgallium (TMG) (Ga(CH3)3) and nitrogen are introduced as reactive gas and working gas, respectively. Optical emission spectroscopes are employed to investigate the emission spectra of the species in N2 plasma, and in N2 and TMG hybrid plasma. The thermodynamics analysis on the growth process of the GaN film is studied, based on the results of the OES. Finally, X-ray diffraction, room temperature photoluminescence (PL), and atomic force microscope (AFM) are used to analyze crystal quality of GaN film.

2. Experimental details 2.1. The process of GaN film grown with ECR–MOPECVD In our experiments, the 2450 MHz TE10 microwaves, propagating in the BJ22 waveguide, is transmitted into resonance room through a quartz widows. Two sets of concentric magnet coils supply a divergence-type magnetic field. The intensity of magnetic field and the location of the resonance zone are adjusted by the magnet coil current (Im). The energy of microwave (PW) is strongly absorbed by the electrons in the ECR zone around 875 G. 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 chamber. The reactive gas TMG is fed into the reaction chamber by another MFC and a distributor ring. In the resonance room, the gas particles are ionized, dissociated or excited through collisions. Electrons and heavier ions are accelerated and then extracted out off the resonance room together because of the ambipolar diffusion. The highdensity plasma in the reactive chamber, including active species such as N*2, N+*2, N*, Ga*, CH*, etc., is thus diffused to the sample table, which is located at the downstream z=25 cm to avoid the affection of the magnetic field. In such a condition, a quasithermodynamic equilibrium can be satisfied. The analyses of the spatial ECR plasma distribution in the reaction chamber show that the plasma density around the sample table is about 2.5  1010 cm3. More details about this ECR–PECVD system and the spatial plasma density distribution in the system can be found in Refs. [12,13].

The key effects on the GaN film growing are microwave power, gas pressure, growth temperature as well as N2: TMG flow ratio. These parameters are related to the growth rate, surface morphology, film quality etc. Two-step growth process, like the method adopted by normal MOCVD technique, was employed to grow GaN film by ECR– MOPECVD. Prior to loading into the vacuum chamber for growth, the sapphire substrates were chemically cleaned by ultrasonic in sulfuric acid solution, rinsing with demi-water, and then drying in oven. A 10 min nitridation procedure is applied for the substrates to cover a thin nitridation layer of a few monomolecular on surface prior to the growth. And then two-step growths are employed to grow the GaN film. First the substrate temperature is increased to grow GaN buffer layer. The main function of the GaN buffer layer is to improve the crystal match between the substrate and epilayer, which otherwise should be 14% without buffer. The substrate temperature on the buffer layer is further increased to a certain degree such as 500 1C and a GaN epilayer is slowly grown for one and half an hour to ensure fine crystal quality. Finally, an hour N2-ambient annealing is applied after the growth is completed. Note that hydrogen is not used for growth as a cleaning/etching gas, because the plasma itself has the function of cleaning objects placed inside. 2.2. Optical emission spectroscopy of the ECR plasma The emission spectra of the ECR plasma are measured by a monochrometer with a 1200 grooves/mm grating. A typical spectrum of nitrogen ECR plasma includes first negative series P P Q of N+2 (1n: B 2 +u-X 2 +g), first positive series of N2 (1p: B 3 g3P+ 3Q 3Q A u), and second positive series of N2 (2p: C u-B g) [14], as shown in Fig. 1. The ionization energy of nitrogen is 15.63 eV and dissociation energy is 24.32 eV. As shown in Fig. 1, some intense spectral lines of second positive series, such as 337.1, 357.7, and 380.5 nm can be observed in the scope of 340–545 nm. The lifetime of molecular nitrogen in Q such C 3 u state is too short to contribute to the GaN growth. The spectral lines are to be quenched before reaching the substrate surface through a float plasma sheath. Several strong spectral

Fig. 1. Typical optical emission spectroscopy of the nitrogen ECR plasma.

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lines of first negative series, such as 391.4 and 427.8 nm are also easily observed in the spectrum. Those nitrogen ions in correP sponding B 2 +u state do not contribute to the GaN growth [15]. First positive series due to molecular nitrogen is in a wide range of 545–700 nm. The lifetime of first positive is considered to be about several seconds. This means that such excited nitrogen in first positive series will have considerable long time to react with the Ga reactants. Additionally, the emission intensities of first negative series and second series are much stronger than that of first positive series. This demonstrates that high excited level state molecules and molecular nitrogen ions are dominant in the ECR plasma [16]. In addition, a weak N* 410.9 nm line found in the spectrum demonstrates that there exist weak dissociation collisions (N*2+e-N+N*+e) in the ECR plasma. The OES of nitrogen and TMG hybrid plasma are shown in Fig. 2. Strong Ga* 403.2 and 417.2 nm spectral lines indicate that TMG is significantly dissociated in ECR plasma at the ambient temperature condition. From the above discussions, the main ionization and dissociation collisions processes in the N2/TMG hybrid ECR plasma include:

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2.3. The thermodynamic analysis of GaN growth For the GaN grown in our ECR–MOPECVD system, TMG is used as the Ga source; The N source is N2. The a-Al2O3 (0 0 0 1) substrate is located at the downstream in the reaction chamber, where there is a fine uniformity in radial direction and far away from magnetic fields. The assumption that the movement of the particles is not affected by the magnetic field but mainly by a diffusion process is satisfied. The ECR plasma containing Ga and N reactants diffuses from the ECR zone to the reaction chamber and then travel to the sample table. Reactant species are absorbed onto the substrate surface. The following chemical reaction occurs on the substrate surface: GaðgÞ þ NðgÞ ¼ GaNðsÞ,

(1)

where g and s denote gas phase and solid phase, respectively. In such a process, a quasi-thermodynamic equilibrium is realized on the gas–solid interface. The equilibrium constant expression of the reaction is 1 . pGa pN

(2)

N2 þ e ! N2  þ 2e;

Kp ¼

N2 þ e ! N2 þ þ 2e;

The condition of restriction obtained from the reaction is as follows:

N2 þ e ! N þ N þ e;

p0Ga  pGa ¼ p0N  pN ,



(3)

where p0Ga and p0N denote the respective input partial pressures, and pGa and pN denote the equilibrium partial pressures. The driving force of the reaction Dp is given as follows:



ðCH3 Þ3 Ga ! Ga þ 3CH þ 3H: Generally speaking, these kinds of reactive species obtained under the self-heating plasma condition are radical in the process of growing GaN film. It contributes to chemical reaction occurring at a relatively low temperature; otherwise it should take place at an extremely high temperature. For example, the growth temperature of MOCVD process adopted heating decomposition method can be up to 1050 1C.

Dp ¼ p0Ga  pGa p0Ga ð1  xÞ þ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p0Ga ð1  xÞ2 þ 4=K p

¼ p0Ga  2  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p0Ga 2 1 þ x  ð1  xÞ þ 4=ðK p p0Ga Þ2 , ¼ 2

Fig. 2. The OES of the N2/TMG ECR plasma.

(4)

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where x=p0N/p0Ga denotes the input V/III ratio. The equilibrium constant of the reaction is given by

DG0 ¼ RT lnðK p Þ.

(5)

As in Ref. [17]

DG0 ¼ A þ B=T þ CT ln T þ DT þ ET 2 ,

(6)

where A ¼ 2:090  102 ; D ¼ 9:189  10

2

;

B ¼ 8:484  102 ; E ¼ 2:570  10

11

C ¼ 2:547  103 , .

The driving force Dp for GaN growth is shown in Fig. 3 as a function of the original input N2:TMG flow ratio for several input pressure of p0Ga at 723 K. The driving force Dp depends on input N2:TMG flow ratio and input partial pressure of Ga(g). It decreases as the original input pressrue of TMG decreases. For a certain p0Ga, Dp is very low if N2:TMGo1 and it turns stronger as N2:TMG increases until N2:TMG=1. The driving force Dp will become constant when N2:TMG41 and the value is very high. In such a high N2:TMG condition, the GaN growth is much earlier.

p0Ga = 0.1Pa

Driving force Δp/Pa

0.10 0.08 0.06 0.04 0.02

p0Ga = 0.01Pa P0Ga = 0.001Pa

0.00 10-3

10-2

10-1

100

101

102

103

N2/TMG flow ratio Fig. 3. The driving force for the GaN growth, as a function of original N2/TMG flow ratio at 723 K.

8

12

N2:TMG Flow ratio 16 20 24

1.4

temperature flow ratio

1.2 Growth rate (um/h)

28

According to thickness measured by XP1 profiler as well as growth time recorded, the growth rate is obtained and shown in Fig. 4 as a function of N2:TMG flow ratio and growth temperature. It indicates that there is no direct relationship between temperature and growth rate. High temperature dose not mean high growth rate because dissociation and ionization of nitrogen can easily take place even under the self-heating plasma condition. But lower N2:TMG flow ratio (namely higher p0Ga) will cause the trend of high growth rate. This phenomenon is consistent with the thermodynamic analysis of GaN growth. Comparison to the results of X-ray diffraction (XRD) and PL discussion followed, such low N2:TMG condition is not ideal for depositing high-quality GaN film. Consideration of the quality of GaN film, decreasing N2:TMG flow ratio to speed up growth rate is not proposed. Even in a high ratio of flow condition such as 28:1, the growth rate can still reach 0.6 mm/h. Furthermore, the growth rate can be maintained at high ratio level about 0.4 mm/h in the 360–500 1C temperature range.

3. Results and discussion Using the OES of N2 and TMG hybrid plasma and the thermodynamic analysis of GaN growth, several optimized growth conditions are chosen to grow the GaN film. Under these conditions, highly dense and uniform plasma rich in Ga and N reactants are obtained as desired. Finally, X-ray diffraction, room temperature photoluminescence (PL) and atomic force microscope are applied to investigate crystal quality, morphology and roughness of the GaN film. The results are shown in Figs. 5–7, respectively. XRD analysis shows that the peak of GaN (0 0 0 2) is at 2y=34.481, and the peak of Sapphire (0 0 0 6) is at 2y=41.601. As shown in Fig. 5, the peak of GaN (0 0 0 2) turns sharp and strong with the increase in N2:TMG ratio. It means that the crystal quality of GaN gets better with a high ratio of N2:TMG. These results agree well with those in Fig. 3, in which a 140 full width of half maximum (FWHM) of the peak is obtained at N2:TMG=28:2. It indicates that there is an excellent advantage for ECR– MOPECVD technology to deposit GaN film even at a low-growth temperature.

32 Sapphire (0006)

600

1.4

500

1.2 Growth rate (um/h)

4

2.4. The growth rate of GaN films

Intensity (a.u)

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GaN(0002)

400

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

100

0.2

0.2

0

300 200

4#28:2 3#26:4 2#24:6 1#20:10

30

360

380

400 420 440 460 Temperature (°C)

480

500

520

Fig. 4. Growth rate as a function of temperature and N2:TMG flow ratio.

40

50 60 2θ/degree

70

Fig. 5. The XRD of GaN with various N2:TMG flow ratios: p=0.15 Pa, Pw=550W, Im=160 A, and T=723 K.

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70

6# GaN

60

Intensity (a.u)

50 40 30

5# GaN

20 10

4# GaN

0 300

400

500

600

700

800

Wavelength (nm) Fig. 6. PL spectrum of GaN films.

Fig. 7. AFM surface morphology of 4# GaN film. (a) 1000  1000 nm2 and (b) 500  500 nm2.

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Room temperature photoluminescence analysis of GaN films deposited under different conditions is shown in Fig. 6. Growth conditions and some measured parameters are listed in Table 1. The result indicates that a sharp edge peak of 4# GaN is located at 364 nm. This is consistent with the theoretical value (365.5 nm). A drift about 1.5 nm is caused by stress. Stress generated by the mismatch between epilayer and substrate is responsible for the drift. This type of stress is mainly related to V/III ratio. Lower stress strength is corresponding to higher V/III ratio. The drift in the figure indicates that high V/III ratio is suitable for GaN growth. In addition, there is a wide yellow band present in Fig. 6. It is centered at 550, 542, and 525 nm (about 2.2 eV) with a range of 480–700 nm (1.8–2.5 eV) corresponding to 4#, 5#, and 6# GaN samples, respectively. This yellow band is associated with transitions from conduction band or shallow donor level to deep acceptor level [18]. From the above OES discussions, strong P P spectral lines of first negative series (B 2 +u-X 2 +g) found in the spectrum means that there is lots of high-energy N+2 in the ECR plasma. In the process of GaN growth in ECR plasma, these N+2 ions (o20 eV) diffused into downstream will be accelerated by plasma sheath self-built near the float sample table and results a bombardment on sample surface. High-energy ion bombardment gives rise to Ga vacancies (VGa) [19], which are responsible for deep acceptor level. GaN film deposited under Ga-rich ambience (such as 6# GaN, N2:TMG=20:2) has high concentration of N vacancies (namely VN). N vacancies are associated with shallow donor level in the GaN film [20,21]. 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 6# GaN shown in Fig. 6. In order to compare different V/III ratio on GaN surface, AFM surface morphologies of 4# and 5# GaN are shown in Figs. 7 and 8, respectively. Statistic of area and height distribution rate (HDR) is also shown in Fig. 9. The growth of GaN on a-Al2O3 substrate is a kind of heterogeneous expitaxy. The study of kinetics and dynamics in film growth pointed out that growth model of heterogeneous expitaxy is usually three-dimensional islands growth [22,23]. First three-dimensional islands are formed (Volmer-Weber model), and then two-dimensional layers are grown layer by layer (layer-by-layer model) to form a smooth surface. Comparison between Figs. 7 and 8 shows that the surface of 4# GaN is compact and uniform than that of 5# GaN, which is grown with lower N2:TMG rate. This result is consistent with the PL analysis. High V/III ratio will weaken the stress between epilayer and substrate. Accordingly, the structure defects are reduced. Roughness statistics shows that the average size of granules is about 70 nm for sample 4, larger than that of sample 5, whose is about 50 nm. In addition, the root of square mean of surface roughness (Rq) is 0.14 nm. Such a smoothness of surface is comparable to those of MOCVD technology which must be grown at an extremely high temperature. Statistic of area distribution rate (ADR) and height distribution rate indicates that ADR is 52.8% when HDR is at the maximum percentage 6.3% and height is 40 nm for 4# GaN, and 52.7% at the maximum HDR 8.3% when height is 41 nm for 5# GaN. For the 4# GaN, ADR of height higher than 28.7 nm counts for 95% of the total area, and 32.2 nm for 5# GaN at the same percent. Additionally, the FWHM of HDR is 16 nm for sample 4, and 12 nm for sample 5, what means that sample 4 is better than sample 5 regarding to preferential orientation and uniform granule. Sample 5 is inclined to grow with layer-by-layer model.

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Table 1 GaN film growth conditions. Sample

Temperature (1C)

N2:TMG (sccm)

Microwave power (W)

Gas pressure (Pa)

Edge peak (nm)

Yellow band (nm)

4 5 6

450 480 500

28:2 25:2 20:2

550 550 550

0.15 0.15 0.15

364 364 363

550 542 525

Fig. 9. Data statistic of area and height distribution rate (1000  1000 nm2). (a) 4# GaN and (b) 5# GaN.

Fig. 8. AFM surface morphology of 5# GaN film. (a) 1000  1000 nm2 and (b) 500  500 nm2.

4. Conclusions GaN films are grown on (0 0 0 1) a-Al2O3 substrate in an ECR–MOPECVD system at T=723 K. The gas sources are pure N2 and TMG. Optical emission spectroscopy and thermodynamic analysis of the GaN growth are applied to understand the GaN growth process. The optical emission spectroscopes show that high excited level state molecules and nitrogen molecular ions are dominant in the ECR plasma and there exists a weak quantity of

N*. TMG is significantly dissociated in the ECR plasma. The reactants N and Ga in the plasma, obtained under the selfheating plasma condition, are essential for the GaN growth. They contribute to the realization of the GaN film growth at a relatively low temperature. Thermodynamic analysis of the GaN growth shows that the driving force Dp for GaN growth is small when N2/TMGo1, but becomes a large constant force when N2/TMG41. Furthermore, Higher N2/TMG ratio makes the GaN growth easier. Finally, X-ray diffraction, room temperature photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN film. The XRD shows that the peak of GaN (0 0 0 2) is at 2y=34.481, being sharper and more intense as the N2/TMG flow ratio increases. A 140 FWHM of the peak is obtained at N2:TMG=28:2. The PL analysis shows that a sharp edge peak located at 364 nm and a wide yellow band centered at 550 nm with a range of 480–700 nm. The surface morphology shows that the surface of GaN grown with high V/III ratio is compact and smooth. These results demonstrate that the

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quality of GaN films grown by ECR–MOPECVD is comparable to those by the MOCVD technology under extremely high temperatures.

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