Article pubs.acs.org/JPCC
Crystallization Effects of Nanocrystalline GaN Films on Field Emission Wei Zhao,† Ru-Zhi Wang,*,† Zhi-Wei Song,† Hao Wang,† Hui Yan,† and Paul K. Chu‡ †
Laboratory of Thin Film Materials, College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China ABSTRACT: Nanocrystalline GaN films are fabricated on Si substrates by pulsed laser deposition to achieve enhanced field emission (FE) based on microstructural engineering. In the process, the microstructure including crystallization and orientation can be conveniently and directly modulated by the temperature. XRD reveals that the fabrication temperature affects the crystallinity and grain size of the GaN films. A strong correlation between the microstructure and FE properties is also observed. The turn-on electric field decreases from 30.5 to 2.3 V/μm, and the FE current density increases by two to three orders of magnitude based on the microstructure of the GaN films. A qualitative grain boundary conduction model is proposed to explain the strong correlation between the microstructure and FE properties. The results demonstrate the importance of microstructural effects on FE, and they must be considered in the design and production of FE GaN devices. GaN can be dramatically improved,23,24 but up to now, there has not been a systematic study on the influence of the microstructure on the FE properties of GaN films produced on Si. In this work, a series of nanocrystalline GaN films is prepared on Si by pulsed laser deposition (PLD). The microstructure, including crystallization and orientation, can be controlled by the deposition temperature. The turn-on field of the nanocrystalline GaN film with the optimal microstructure is lower than that of conventional film GaN materials and even comparable to those of 1D GaN materials. Our results demonstrate that the FE properties of GaN films depend strongly on the microstructure and an enhancement mechanism is postulated and discussed.
1. INTRODUCTION High-performance cold field electron emitters have applications in a wide range of field emission (FE) based devices such as ultrathin flat-panel displays,1 microwave power amplifiers,2 electron microscopes3 as well as gas and mass sensors.4,5 Wide band gap semiconductors such as diamond, boron nitride (BN), and aluminum nitride (AlN) have potentials in thin-film field emitters because of their low or even negative electron affinity6−8 and large band bending.9 GaN is a particularly promising wide band gap field emitter because of its low electron affinity (2.7 to 3.3 eV) as well as its excellent physical and chemical stability.10,11 Sapphire and SiC are commonly used as the substrates to fabricate GaN12,13 and Si is usually not recommended as a substrate for III−V nitrides because of the large lattice mismatch that affects the microstructure (crystallization and orientation) of the deposited layer. However, the use of Si as substrate for FE devices can pare fabrication costs and render integration of field emitters into Si-based integrated circuits possible. Therefore, it is crucial to study the microstructure effects of FE GaN devices produced on a Si substrate. The electrical properties of semiconductor materials are closely related to the microstructure of the emitters.14 In the case of diamond-like carbon (DLC), the grain size reduction in the transition from microcrystalline toward nanocrystalline improves the FE properties.15,16 The content of sp3 carbon atoms is one of the most important factors determining the FE properties of DLC films.17−19 Therefore, electron emission can be enhanced by controlling crystallization during fabrication. Furthermore, GaN has favorable piezoelectric and spontaneous polarization properties due to the wurtzite crystal structure, which dramatically affects the electrical properties.11,20−22 By fine-tuning crystallization and orientation, the FE properties of © 2012 American Chemical Society
2. EXPERIMENTAL DETAILS The GaN films were deposited on (100) n-type Si wafers by PLD. A GaN target with a diameter of 20 mm was loaded into the deposition chamber and held by a rotating target holder for uniform ablation. The chamber was filled with N2 gas to a working pressure of 1 Pa. Laser ablation was conducted using a KrF excimer laser (248 nm, 10 ns) at an energy density of 2.0 J/cm2 for 10 min at a pulse frequency of 10 Hz. The ablated species were deposited onto the Si substrate placed at a distance of 65 mm from the target. The GaN films were deposited at different substrate temperature of 1000 to 900, 800, 700, 600, and 500 °C (samples designated as a−f, respectively). The crystal structure of the films was determined by X-ray diffraction (XRD, Bruker AXS D8 Advance) with Cu Kα Received: November 11, 2012 Revised: December 24, 2012 Published: December 24, 2012 1518
dx.doi.org/10.1021/jp311155y | J. Phys. Chem. C 2013, 117, 1518−1523
The Journal of Physical Chemistry C
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radiation and X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was applied to examine the surface composition. Atomic force microscopy (AFM, NT-MDT Solver P47) and scanning electron microscopy (SEM, Hitachi S4800) were employed to investigate the surface morphology, and the FE properties were determined at room temperature using a parallel-plate diode configuration at a pressure of 5 × 10−7 Pa or lower. A Si wafer (0.001 Ω·cm) was used as the anode, and the cathode and anode (10 × 10 mm2) were separated by a glass fiber. The I−V curves were acquired on a Keithley 2410. To remove adsorbates from the cathode surface, the measurements were conducted for several cycles until the J−E characteristics became stable and reproducible. Figure 2. XRD pattern obtained from samples b to f. The insert shows the XRD patterns acquired from sample a.
3. RESULTS AND DISCUSSION XPS is used to determine the composition of the samples. Figure 1 shows a general scan in the binding energy range from
content improves at a higher temperature. The full width at half-maximum (fwhm) of the (002) peak shows an obvious drop providing direct evidence that the nanocrystalline GaN grain size increases at a high substrate temperature. The modification of crystal structure is attributed to the better lattice arrangement resulting from increased lattice vibration at a high temperature. To observe the evolution of the GaN films surface morphology, we show the AFM images of the GaN surfaces in Figure 3. A large quantity of small nanoscale protrusions can be observed from the surface, and they greatly enhance the local electric field.24 As the temperature is increased, the diameter of the surface protrusions decreases from 500 to 100 nm, and the corresponding root-mean-square surface roughness values of samples f−a are 50.3, 51.8, 33.6, 16.8, 11.0, and 3.9 nm, respectively. XRD and AFM reveal that the deposition temperature dramatically affects the microstructure and surface morphology of the GaN films. The thickness of the GaN films ranges from 220 to 260 nm, and it may affect the FE characteristics. Zhao et al.29 studied the FE properties of ta-C films with different thicknesses, and a thickness-independent phenomenon was observed. However, Lu et al.30 observed that the FE properties of BaSrTiO3 film were thickness-dependent. The present study reveals that the film-thickness dependence stems from the microstructural features associated with the different thicknesses. Sugino et al.31 studied the thickness effects of BN nanofilms, and in BN films thicker than 20 nm, the turn-on field of the electron emission decreased because of increasing surface roughness rather than thickness modulation. Hence, it appears that the film thickness change alone cannot affect the FE properties of traditional cathodes thicker than 20 nm. In our experiments, because the films are all thicker than 200 nm, the thickness effects can be ignored. The J−E characteristics are shown in Figure 4a, and the corresponding Fowler−Nordheim (F−N) plots are given in Figure 4b. All samples exhibit a clear FE behavior. The FN plots of all samples in Figure 4b exhibit a linear relationship in the high-field region, suggesting that the emission current should originate from a quantum mechanical tunneling process. In this work, the turn-on field Eon and threshold field Eth are defined at an emission current density of 1 μA/cm2 and 0.1 mA/cm2, respectively. The values of Eon and data from other GaN nanostructured materials reported previously are summarized in Table 1. The Eon of the microstructure modulated nanocrystalline GaN film (sample b) is smaller
Figure 1. XPS spectra of nanocrystalline GaN film. The insert shows the O 1s and Ga 3d core-level XPS spectra.
0 to 600 eV. The binding energies of the spectra are referenced to that of the C 1s peak (284.8 eV). The nanocrystalline GaN film consists of Ga, N, C, and O, and the corresponding photoelectron peaks are Ga 3d (19.8 eV), Ga 3p (105.4 eV), Ga 3s (159.8 eV), and N 1s (397.6 eV). Because of prior exposure to air, the O 1s and C 1s core level peaks are also present. The insert in Figure 1 shows O 1s and Ga 3d XPS spectra. The O 1s peak at 531.3 eV is attributed to chemisorbed oxygen25 and the Ga 3d core level yields peak positions at 20.6 to 20.8 eV for Ga−O bonding and 19.6 to 19.8 eV for Ga−N bonding.26,27 Therefore, the Ga 3d peak at 19.8 eV can be assigned to nitride. The O 1s and Ga 3d peaks confirm that the samples are gallium nitride and not oxide. XRD is carried out to determine the crystal structure of the GaN films. The GaN film deposited at below 500 °C exhibits a typical amorphous pattern. As shown in Figure 2, when the temperature reaches 600 °C, the characteristic diffraction peaks of GaN begin to appear as a broad peak around 35°. As the temperature is increased to 800 °C, the peaks at (100), (002), and (101) can be indexed to hexagonal wurtzite GaN,28 indicating the development of a crystalline structure. When the temperature is further increased to 1000 °C, only the (002) peak can be observed implying that the GaN film is preferentially oriented in the c-axis direction. The diffraction intensity increases as the deposition temperature is increased from 500 to 1000 °C, indicating that the nanocrystalline GaN 1519
dx.doi.org/10.1021/jp311155y | J. Phys. Chem. C 2013, 117, 1518−1523
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Figure 3. AFM images of (a) sample a, (b) sample b, (c) sample c, (d) sample d, (e) sample e, and (f) sample f.
than that of the conventional film GaN materials32−34 and even comparable to those of 1D GaN materials,28,35−42 indicating efficient FE from the GaN films with the optimal microstructure. The detailed information about the F−N plot of GaN films is necessary to understand the microstructure effects on field electron emission. According to the F−N field electron emission theory, the emission current is related to the applied electric field by the following relationship:43 ⎛ αA β 2 ⎞ ⎛ I ⎞ Bϕ3/2d ln⎜ 2 ⎟ = − + ln⎜ ⎟ ⎝V ⎠ βV ⎝ ϕd 2 ⎠
enhancement factor and effective emission area can be calculated from the intercept k and slope b of the F−N plot. Because the GaN films are thick enough so that the quantum structural effects can be neglected24,44−46 and without chemisorbed materials, the surface effective barrier ϕ can be considered as a constant. If βA and αA are normalized to 1 for sample a, then the field-enhancement factor ratio βr can be calculated from the following relationship: βr =
(1)
βx βA
=
kA k Bϕ2/3d · 2/3 = A kx kx Bϕ d
(2)
and the effective emission area ratio αr can be calculated from the following
where ϕ is the surface effective barrier, d is the distance between the anode and cathode, β is the field enhancement factor, α is the effective emission area, and A and B are constants corresponding to 1.54 × 10−6 A(eV)V−2 and 6.83 × 107 (eV)−3/2 V cm−1, respectively. By linear fitting, the field-
AβA 2 αx exp(bx)ϕd 2 exp(bx) = · = αr = 2 2 αA Aβx exp(bA)ϕd exp(bA)βr 2 1520
(3)
dx.doi.org/10.1021/jp311155y | J. Phys. Chem. C 2013, 117, 1518−1523
The Journal of Physical Chemistry C
Article
Figure 5. (a) Field-enhancement factor ratio βr and (b) effective emission areas ratio αr calculated from the linear fit of the FN plots.
Figure 4. FE characteristics of samples a−f: (a) FE current density as a function of the applied electric field (J−E). The insert shows the variation in Eon and Eth observed from the GaN films. (b) Corresponding FN plots of ln(I/V2) versus 1/V.
Figure 5 shows the fitted βr (Figure 5a) and αr (Figure 5b) values of GaN films. With the exception of sample f, the variation in βr is consistent with the surface roughness. However, the result does not obey the common FE rule; that is, the lower the βr, the better the FE performance. The variation in αr is consistent with the FE properties. Therefore, the combined information imparted by βr and αr confirms that the FE properties arise from the microstructure of the GaN films in lieu of the surface morphology. Hence, detailed information concerning the microstructure of the GaN films is crucial to the understanding of the FE mechanism. On the samples prepared at a relatively low temperature (
