Modulation Effects of Hydrogen on Structure and Photoluminescence

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Modulation Effects of Hydrogen on Structure and Photoluminescence of GaN Nanowires Prepared by PECVD Yu-Hang Ji, Ruzhi Wang, Xiao-Yu Feng, Yuefei Zhang, and Hui Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05532 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Modulation Effects of Hydrogen on Structure and Photoluminescence of GaN Nanowires Prepared by PECVD Yu-Hang Jia, Ru-Zhi Wang*a, Xiao-Yu Fenga, Yue-Fei Zhangb, Hui Yana

a

College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China. b

Inst. Microstruct. & Property Adv. Mat., Beijing University of Technology, Beijing 100124, China.

Abstract：： Gallium nitride (GaN) nanowires (NWs) were grown on Si(100) substrates at different hydrogen gas flow rates, using plasma enhanced chemical vapor deposition. The size and morphology of the GaN NWs could be modulated by controlling the hydrogen atmosphere. The diameters of the GaN NWs ranged from 53 to 221 nm, and their morphology transformed from hexagonal prism to triangular pyramid upon changing the hydrogen atmosphere conditions. The modulation effects originated from the competitive equilibrium between surface and interfacial diffusion induced by hydrogen in the vapor-liquid-solid growth mechanism. The photoluminescence spectra of the different GaN NWs may have originated from two metabolic peaks, which were closely related to their nanostructures. These findings provide a simple and convenient method to modulate the structure of GaN NWs, and may further the application of GaN in nano-optoelectronic devices.

*To

whom correspondence should be addressed: [email protected];

ACS Paragon Plus Environment


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Introduction Gallium

nitride

(GaN)

nanowires

(NWs)

are

one-dimension

semiconductors with a wide band gap (3.4 eV) and excellent thermal stability. They have potential application in blue light emitting diodes (LEDs)1, laser diodes (LDs)2, and ultraviolet (UV) photodetectors3. In these applications, the controlled growth of GaN NWs and an understanding of the growth mechanism are fundamental to achieve the desired structure and performance. There are various reported ways to prepare GaN NWs, most of which use ammonia (NH3) as the N precursor4-5 which is caustic and hazardous to human health and the environment. GaN NWs are typically synthesized using metal particle-assisted growth via the vapor-liquid-solid (VLS) growth mechanism6-7. One of the most important parameters for controlling the growth and therefore the structure is the gas flow rate8. Lim et al. studied the role of hydrogen as a carrier gas in GaN synthesis by chemical vapor deposition (CVD).9 Jung et al. studied the high temperature etching effect of hydrogen in GaN synthesis by metal-organic chemical vapor deposition (MOCVD)10. However, the effects of hydrogen on modulating the GaN structure are still not clear from these studies. Most III–V NWs grown experimentally exhibit a wurtzite structure with a hexagonal prism morphology11-13, although a triangular pyramid morphology is occasionally observed.7 We previously reported a low cost and green method for the large-scale preparation of high quality GaN NWs, by plasma-enhanced chemical vapor deposition (PECVD) without using ammonia.14-15 However, it


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is also difficult to regulate the size and morphology of the GaN NWs using only these simple process parameters. In the current study, we prepared GaN NWs by PECVD. The structure of the GaN NWs could be significantly modulated by simply changing the hydrogen flow rate. The modulation mechanism of the hydrogen is presented. The structural effect of hydrogen on the photoluminescence (PL) properties has been observed. These findings provide a simple and effective method for the controllable preparation of GaN NWs. Experiments GaN NWs were fabricated by PECVD in a flow of mixed nitrogen and hydrogen gases. A mixture of Ga2O3 (purity 99.999%) and carbon (purity 99.99%) powders with a Ga2O3: C mass ratio of 1: 12 was placed in the middle of an Al2O3 boat, which in turn was placed in the heater zone of a horizontal tube furnace. Silicon(100) wafer was used as the substrate, and a 15 nm thin film of Au was positioned above the Al2O3 boat. A N2 flow rate of 40 standard cubic centimeters per minute (sccm) acted as a protection gas while the furnace temperature was increasing. The furnace was heated to the desired temperature for 50 min, and then maintained at 880 °C for 60 min under a stable flow of different ratios of hydrogen and nitrogen. We named the GaN NWs synthesized at hydrogen flow rates of 0 sccm, 10 sccm, and 20 sccm as sample a, sample b and sample c, respectively. The furnace was allowed to cool to room



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temperature after the reaction had finished. The resulting sample was obtained on the silicon substrate. The structure and phase purity of the product were determined by X-ray diffraction (XRD, BRUKER D8 ADVANCE). The morphologies of the products were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S4800) equipped with an energy dispersive X-ray spectrometer (EDS). The morphologies of single NWs were characterized by FESEM (Hitachi SU8020). The geometry and crystallinity of the samples were characterized by high-resolution transmission electron microscope (HRTEM, Tecnal G2 F30 S-TWIN) equipped with a selected area electron diffraction (SAED) apparatus. PL spectra of the samples were measured at room temperature using the 325 nm line of a He–Cd laser as the light source. Results and discussions Fig. 1 shows FESEM images of GaN NWs synthesized at different hydrogen flow rates. Fig. 1(a), (b) and (c) show GaN NWs grown at hydrogen flow rates of 0 sccm, 10 sccm, and 20 sccm, respectively. Insets show the morphologies of single NWs. NWs with different morphologies and sizes were obtained by varying the flow rate of the hydrogen atmosphere. Increasing the hydrogen flow rate caused the diameter of the NWs to increase and then decrease, and the structure of the NWs changed from hexagonal prism (Fig. 1(a)) to triangular pyramid (Fig. 1(b) and (c)). Fig. 1(d), (e) and (f) show EDS


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spectra of sample a, sample b and sample c, respectively. The atomic content of Ga was less than that of N, and small amounts of oxygen impurities were present in all samples. The XRD patterns of samples prepared at different hydrogen flow rates are shown in Fig. 1(g). The XRD patterns of all samples were in agreement with the standard peaks of bulk GaN with a hexagonal structure (JCPDS 50-0792), and weak peaks of Au were also detected.14 No other peaks of crystalline impurities, such as C or Ga2O3, were detected. Fig. 1(g) also shows that the relative intensity of the (100) peak in sample a was lower than that in sample b and sample c. This suggests that the content of the exposed (100) crystal surface in sample a may have been less than that in the sample b and sample c.



Si Au

Si Au

(d) Ga 10.03 N 14.69 O 5.65 Si 69.40 Au 0.22

N

Ga O

0

Ga

2

3

4

5 6 keV

7

8

9

Si Au

10 11

Ga 14.24 N 28.99 O 3.88 Si 52.85 Au 0.04

N Ga

0

Ga

Au Ga Au

Au

1

2

3

4

5 6 keV

7

8

9

O

Au

(f)

O

Ga 30.66 N 40.14 O 3.83 Si 24.85 Au 0.51

Ga Au

1

(e)

N

10 11

0

Ga

Au

1

2

3

4

5 6 keV

7

8

9

Au

10 11

002GaN 101GaN

(g) Intensity(a.u.)

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100GaN

111Au

Sample a

Sample b

Sample c

30

32

34

36

38

40

2θ(°)

Fig. 1. FESEM images of (a) sample a, (b) sample b and (c) sample c. Insets show morphologies of single NWs, with scale bars of 50 nm. EDS spectra of (d) sample a, (e) sample b and (f) sample c. (g) XRD patterns of sample a, b and c.

Further structural information about the GaN NWs was obtained from HRTEM images and corresponding SAED patterns of individual GaN NWs. TEM images of individual GaN NWs of two samples are shown in Fig. 2(a) and (d). The diameters of the two individual NWs were about 70 nm and 120 nm, respectively. Sample a exhibited a hexagonal structure and sample c exhibited a triangular structure, which is consistent with results from Fig. 1(a) and (c). HRTEM images are shown in Fig. 2(b) and (e). The d-spacings


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between adjacent lattice fringes were about 0.284 nm and 0.274 nm, respectively, which corresponded to the d-spacing of (100) planes in standard wurtzite-type GaN.16-17 This indicates that both NWs had grown along the [100] direction. The corresponding SAED patterns in Fig. 2(c) indicate that the individual GaN NWs were single-crystalline structures.

Fig. 2(a) Typical bight-field TEM image, (b) HRTEM image, and (c) SAED pattern of sample a. (d) Typical bight-field TEM image, (e) HRTEM image, and (f) SAED pattern of sample c.

Fig. 3 shows diameter distributions of GaN NWs prepared at different hydrogen flow rates. By measuring over 100 NWs from randomly recorded FESEM images, average diameters of the samples were found to vary from 53 nm to 211 nm. The histograms were subjected to Gaussian fitting. When the



hydrogen flow rate was increased from 0 sccm to 10 sccm with a constant nitrogen flow rate of 40 sccm, the average diameter of the GaN NWs increased (Fig. 3(a) and (b)). However, when the hydrogen flow rate was increased from 10 to 20 sccm with the same nitrogen flow rate, the average diameter of the GaN NWs decreased (Fig. 3(b) and (c)).

0.25 0.20 0.15 0.10 0.05

Raletive Frequency

(a)

0.30

0.35

(b)

0.30 0.25 0.20 0.15 0.10 0.05 0.00

0.00 0

10

20

30

40

50

60

70

80

Diameter (nm)

90 100 110

Raletive Frequency

0.35

0.35

Raletive Frequency

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(c)

0.30 0.25 0.20 0.15 0.10 0.05 0.00

0

50 100 150 200 250 300 350 400 450 500

Diameter (nm)

0

20 40 60 80 100 120 140 160 180 200 220

Diameter (nm)

Fig. 3. Histograms showing diameter distributions of (a) sample a, (b) sample b, and (c) sample c.

The above results show that the morphology and size of the GaN NWs were affected by the hydrogen atmosphere. Two morphologies and distinct sizes of GaN NWs were obtained simply by controlling the hydrogen flow rate. To better understand the hydrogen-modulated growth mechanism of the GaN NWs, a growth model is presented showing the effects of hydrogen on the GaN NW structure. SEM images in Fig. 4(a) and (b) show that the GaN NWs possessed a ball-like catalyst at the top of each NW, which was consistent with the VLS growth mechanism.14, 18-19 The hydrogen-modulated growth of the GaN NWs can be considered in terms of the thermodynamic equilibrium of a liquid droplet on the top of a growing NW, which is significantly affected by


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the NW growth angle.19 The three-phase boundary is the preferred nucleation site for GaN NWs.9 By considering an alloy droplet on the top of a growing NW, the three-phase line of contact can adjoin the flat growth front. The interfacial tensions βL (liquid–vapor), βS (solid–vapor), and βSL (solid–liquid) at point A in the three-phase line are configured as shown in Fig. 4(a) and (b). For the triangular pyramid morphology, the interfacial tensions at point A are not in balance because βSL < βL < βS, in which βSL is normal to βS. During the growth of NWs, a stable three-phase line (separating the vapor, liquid, and solid phases) is supported by a sufficiently high surface energy of the liquid alloy in the droplet at the top. However, introducing a high-energy hydrogen plasma activates the lateral surface of the NW. The effect of hydrogen ions results in the downward force of surface energy βS being much higher than the sum of the upwards forces βL + βSL. Therefore, the three-phase line will move downward slightly. In other words, the NWs grow by wetting the sidewall, and more area is wetted during this process. The absorption of precursors and thus nucleation will be accelerated. In the case of the reductive NW, the droplet contact angle δ increases with decreasing NW diameter (Fig. 4(a)). It then causes an inward force, forming a narrowing inclined facet (Fig. 4(a)). The higher surface energy results in the growth rates of the different lateral facets are not in equilibrium, so the three narrower planes tend to disappear. This gradually leads to the formation of a triangular cross section (Fig. 4(d)).20 Overall, these effects cause the NW diameter to become smaller and smaller,



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eventually forming a triangular pyramid NW (Fig. 4(c)). In contrast, when the hydrogen plasma is not involved in the growth process, the downward interfacial tension of surface energy βS is balanced with the sum of the upwards tensions βL + βSL. Under this thermodynamic equilibrium, the three-phase line is relatively stable, the droplet contact angle δ is 0°, and the growth rates of the lateral facets are similar. This leads to the growth of a hexagonal NW with a constant diameter (Fig. 4(b)). In addition to the NW morphology, the NW size was also affected by hydrogen (Fig. 4(e)). Hydrogen contributed to the reduction of Ga2O3 as well as acting as a carrier gas. The reduction reaction can be given by14: Ga2O3 + H+ + N+ + C → GaN + H2O+ COX

(1)

In reaction (1), Ga2O3 is reduced by hydrogen plasma during synthesis of the GaN NWs. Increasing the hydrogen flow rate causes more Ga2O3 to be reduced. This suggests that when the nitrogen flow rate is stable, the N(V)/Ga(III) ratio will increase with increasing hydrogen flow rate, and the higher V/III ratio results in a higher surface growth rate.21 Thus, the NW diameter increased upon increasing the hydrogen flow rate from 0 sccm to 10 sccm. Excess hydrogen reduces supersaturation of the nucleation sources and therefore suppresses NW growth.9 The increase in hydrogen flow rate also leads to a high density of hydrogen ions, whose etching effect suppresses the GaN NW diameter.22 This


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may have been the cause of the lower diameter NWs when the hydrogen flow rate was increased from 10 sccm to 20 sccm. Hydrogen in PECVD significantly affected the morphology and size of the GaN NWs. Compared with conventional CVD,9 hydrogen had a more pronounced effect on the growth of GaN NWs by PECVD.

Fig. 4(a) SEM image of sample a, with inset showing the growth model of a hexagonal prism NW. (b) SEM image of sample c, with inset showing the growth model of a triangular pyramid NW. (c) Schematic diagram of the transformation from a hexagonal to triangular cross section. (d) Schematic diagram of the growth of NWs at different hydrogen flow rates.

PL tests were used to study the optical properties of the GaN NWs. Pure GaN



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exhibits ultraviolet luminescence (UVL) at room temperature.23 Fig. 5 shows a UVL peak at about 367 nm to 384 nm for all three samples. Sample a exhibited a lower emission intensity, indicating NWs of lower quality. Introducing hydrogen caused the UVL peak intensity to increase, indicating NWs of higher quality. The peak positions of all three samples were red-shifted compared with that of pure GaN (365 nm). This may have been caused by defects such as Ga vacancies (VGa),24 which are one of the main causes of a reduced GaN bandgap. The EDS spectra in Fig. 1 showed less Ga atoms than N atoms, suggesting the existence of VGa. The fitting line of the PL spectrum suggests that the strong PL peaks may have originated from two separate peaks, namely the PL peaks of intrinsic GaN and O-doped GaN. The PL properties of O-doped GaN were reported by Monemar et al.25 Peak A (labeled P(A) in Fig. 5) is consistent with the intrinsic peak (about 361 nm). Peak B (labeled P(B) in Fig. 5) at 377‒382 nm is consistent with the main residual acceptor introduced by O doping. The relative intensity of P(B) increased with increasing Ga NW diameter. This may be explained from the XRD patterns in Fig. 1, in which the relative intensity of the (100) peak in sample a was much lower than that in samples b and c. The intensity of the exposed (100) diffraction also increased with increasing GaN NW diameter, and this naturally modulates its PL properties. The PL peak intensities of different epitaxial planes have been reported for m-plane and c-plane ZnO epitaxial films.26


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P(A)

362

UVL

P(B)

377 D=87nm Sample a

Intensity(a.u.)

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381 361

D=53nm Sample c

382 361

D=221nm Sample b

300 325 350 375 400 425 450 475 500

Waveleng(nm)

Fig.5 PL spectra for various NWs diameters.

Conclusions GaN NWs were fabricated on Si(100) substrates, by PECVD at hydrogen flow rates ranging from 0 sccm to 20 sccm. FESEM images showed that the morphology and size of the NWs were sensitive to the hydrogen atmosphere. Thus, the presence of hydrogen significantly affected the GaN NW structure. Hexagonal prism or triangular pyramid NWs were obtained in the presence and absence of a hydrogen atmosphere, respectively. A thermodynamic equilibrium model was presented to explain the effects of hydrogen on the morphology and size of the GaN NWs. The modulation effect originated from the competitive



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equilibrium between surface and interface diffusion induced by hydrogen in the VLS growth mechanism. Different sized GaN NWs exhibited different PL spectra. Those with larger diameters exhibited prominent UVL intensity, indicating higher crystal quality with fewer defects. A red shift in the PL spectra of the GaN NWs were observed compared with pure GaN, which originated from the presence of Ga vacancies in the NWs. The fitted PL spectra indicated that the PL of O defects in GaN NWs could be modulated by the epitaxial planes of the GaN NWs. These findings provide a simple and convenient method for tailoring the morphology and size of GaN NWs, which may advance the application of GaN NWs in nano-optoelectronic devices.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51472010 and 11774017), the science and technology key projects preferred funding for Beijing Overseas talents and Beijing municipal high level innovative team building program (No. IDHT20170502).The authors thank Aidan G. Young, PhD for improving the English text in revised manuscript..


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52x57mm (300 x 300 DPI)



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52x57mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


115x81mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

289x203mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


115x81mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

82x58mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37x27mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23x27mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27x31mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

37x27mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23x27mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

27x31mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55x36mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

297x209mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


118x83mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

118x83mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


85x119mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

85x116mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


82x164mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

322x87mm (150 x 150 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


152x198mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

44x29mm (300 x 300 DPI)


Page 40 of 40

Modulation Effects of Hydrogen on Structure and Photoluminescence

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