ARTICLE pubs.acs.org/JPCC
Enhanced Field Emission from GaN and AlN Mixed-Phase Nanostructured Film Zhi-Wei Song, Ru-Zhi Wang,* Wei Zhao, Bo Wang, and Hui Yan Laboratory of Thin Film Materials, College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China ABSTRACT: A zinc-blende (ZB) AlN and wurtzite (WZ) GaN mixed-phase (AGMP) nanostructured film was prepared by pulsed laser deposition on n-type Si (100) substrates. Compared with single-phase AlN or GaN film for field emission properties, the turn-on field of AGMP film is lowered from 17.8 to 1.2 V/μm at 1 μA/cm2, and the FE current density of AGMP is raised up about four orders of magnitude. The striking FE enhancement effect of the mixed-phase structure may be partially originated from an efficient electron step-transport in mixed phase, which provides a most favorable emission energy level and greatly advances the supply of effective FE electrons. In addition, the AGMP nanofilm may help with electron transport, which is very different from the mixtures of ZB and WZ nanowires blocking electron transport in the recent report [Nano Lett. 2011, 11, 2424].
1. INTRODUCTION Field emission (FE), as one of the applications of nanostructural materials, has great commercial interest in display and other vacuum electron devices. Compared with bulk materials, 1D nanostructures material, such as nanowires1 and nanotubs,2,3 are relatively unstable and can be easily deformed during operation specially under large current and high power density conditions due to ineffective thermal dissipation.4,5 Therefore, the studies of stable FE thin-film emitters have attracted great attention. To improve FE properties of these film emitters, some ideas have been put forward, such as FE enhancement by an ultrathin wide band gap semiconductor layer,6 modulating quantum structure of nanofilm79 and polarization field of oriented nanostructured films.10 In addition, the electrical properties of semiconductor materials are closely related to the crystal structure of the samples.11 For instance, Teii et al.1214 observed that the phase assignments in the BN film remarkably affect surface morphologies and electrical properties as well as FE characteristics. Therefore, it is possible to improve the FE characteristics by engineering the crystal structure of semiconductor film emitters. Very recently, Thelander et al.15 reported that for InAs nanowires the mixtures of zinc-blende (ZB) and wurtzite (WZ) phases can exhibit up to two orders of magnitude higher resistivity than its single-phase structure, which is considered to result from the polarization charges at the WZ/ZB interface.16 It indicates that the mixture phase may block electron transport. However, in contrast with the results of nanowires, in this work, we found that a ZB AlN and WZ GaN mixed-phase (AGMP) nanostructured film exhibited up to four orders of magnitude larger FE current than that of single-phase AlN or GaN film; it means the WZ and ZB mixture phase film may help with electron transport. Furthermore, the Hall test also proved that the AGMP can be r 2011 American Chemical Society
decreased down to two orders of magnitude lower resistivity than ZB AlN or WZ GaN. An FE enhancement mechanism by the step-transport in mixed-phase nanostructured film was also presented. In general, sapphire and SiC can be used for substrates of GaN epitaxial layer due to the little lattice mismatch and the thermal expansion coefficient. However it would be highly advantageous to grow GaN on silicon substrates due to potential integration between GaN high power electronics and silicon technologies17,18 for FE applications, and silicon is a promising substrate because of its high quality, low cost, and good conductivity. So, in this Article, we deposit GaN epitaxial layer on silicon substrates.
2. EXPERIMENTS AND MEASUREMENTS The AGMP nanostructured film was deposited on (100)oriented n-type Si substrates by pulsed laser deposition (PLD). The atom ratio is Al:Ga 1:1. Single-phase WZ GaN and ZB AlN films were also deposited for comparison. The PLD target was prepared with GaN and AlN powders using pressing and sintering. The target was ablated using a KrF excimer laser with a wavelength of 248 nm, pulse duration of 10 ns, energy density of 5.0 J/cm2, and laser frequency of 10 Hz. The ablated species were deposited onto the Si substrate placed at a distance of 85 mm from the target and heated to 880 °C. During deposition, N2 gas was introduced into the growth chamber to a working pressure of 1 Pa. The films were produced using deposition time from 5 min to obtain thicknesses of 120 nm. Received: October 21, 2011 Revised: December 22, 2011 Published: December 22, 2011 1780
dx.doi.org/10.1021/jp210103k | J. Phys. Chem. C 2012, 116, 1780–1783
The Journal of Physical Chemistry C
ARTICLE
Figure 1. XRD pattern obtained from GaN, AlN, and AGMP.
The orientations of these films were carried out by X-ray diffraction (XRD, Bruker AXS D8 Advance), using Cu Kα radiation to investigate the crystalinity and crystal orientation of the films. Atomic force microscopy (AFM, NT-MDT SolverP47) was employed to investigate the surface morphology, and the FE characteristics were measured in the chamber evacuated to 5 107 Pa at room temperature. A Si wafer (0.001 Ωcm) was used as an anode electrode, and the cathode and anode (5 5 mm2) were separated by two glass fibers at a distance of 14 μm. The IV curves were acquired using a Keithley 2410. The carrier concentration, the resistivity, and the type of impurity of AlN, GaN, and AGMP films were measured with a four-point probe by the Hall test system (Nanometrics HL5500).
3. RESULTS AND DISCUSSION The XRD patterns of GaN, AlN, and AGMP nanostructured films are depicted in Figure 1. The diffraction peak from the (0002) plane of the hexagonal WZ GaN is observed from singlephase GaN and AGMP film. The cubic ZB AlN is obtained from single-phase AlN and AGMP. Therefore, in the case of AGMP, both WZ GaN and ZB AlN have been grown. The formation of the AGMP structure has also been theoretically approved by Purton et al.19 with Monte Carlo simulations. The JE characteristics are shown in Figure 2a, and the corresponding FowlerNordheim (FN) plots are given in Figure 2b. The turnon field Eon is defined at an emission current density of 1 μA/ cm2. The results show that compared with single-phase AlN or GaN film, the FE properties of the AGMP film are dramatically improved. The Eon is lowered from 17.8 to 1.2 V/μm, and the FE current is raised about four orders of magnitude from 1.5 108 to 0.5 104 A/cm2 at 3 V/μm. The FN plots in Figure 2b show a linear relationship in the high-field region for all samples, suggesting that the emission current should be originated from a quantum mechanical tunneling process. For investigation of the FE enhancement mechanism of the AGMP film, it is necessary to consider the effects of surface potential barrier Φsur and the field-enhancement factor β, which are the key factors affecting FE properties in semiconductor films.10,12 The β of the emitter (surface morphologies) and the Φsur can be estimated10,20 from the experimental data. From the surface morphology (because the field-enhancement factor β is closely dependent on the surface roughness measured by AFM in Figure 3, we simply assume that β = β0R, where β0 is constant
Figure 2. FE characteristics of GaN, AlN, and AGMP: (a) FE current density as a function of the applied electric field and (b) corresponding FN plots.
Figure 3. AFM images of (a) GaN film, (b) AlN film, and (c) AGMP film.
and is related to the film’s surface structure), the calculated ratio of β for AlN, GaN, and AGMP is 1.0:4.0:6.2. Moreover, the relation of three structures for surface potential barrier (Φsur) can be drawn by the FN equation21 ! ! β2 E2 Bϕ3=2 J ¼A exp ð1Þ ϕ βE 1781
dx.doi.org/10.1021/jp210103k |J. Phys. Chem. C 2012, 116, 1780–1783
The Journal of Physical Chemistry C
ARTICLE
Table 1. Carrier Concentration, Resistivity, and Type of Impurity for AlN, GaN, and AGMP carrier concentration
Figure 4. Schematic of: (a) field emission enhancement mechanism from AGMP films and (b) energy band structure of electron steptransport process.
where A and B are constants, J, ϕ, E, and β are the field-emission current density, effective surface potential barrier (work function), the electrical field intensity, the voltage applied to the samples, and the field enhancement factor, respectively. Here (Bϕ3/2)/β is corresponding to the slope of FN curves in Figure 2b. Therefore, the Φsur can be estimated from eq 1. The calculated ratio of Φeff for AlN, GaN, and AGMP is 1:4.46: 3.45. On the basis of the above results and the reported experimental data (the value of ΦGaN and βGaN are 3.4 and 1.0 102),10,11 if the FE enhancement is only resulted from the enhancement of the geometric structure and the decrease in the surface potential barrier, then the calculated ratio of J for AlN, GaN, and AGMP is 11.2:1:124.7 at 3 V/μm. The theoretical prediction is similar to the experimental ratio of AlN and GaN, but it is far deviated from the experimental ratio of AlN or GaN and AGMP. It is well known that the FE current is dependent on interior electron supply and the probability of surface tunneling.7 From the above analysis, it is clear that the improved FE properties of the AGMP not only resulted from the enhancement effect of the surface tunneling. The additional FE enhancement should be due to a high efficient electron supply in mixed-phase nanostructure. Here an electron step-transport model is supposed to explain the high efficient electron supply. A two-stage FE mechanism is assumed for the AGMP film, as shown in Figure 4: (1) electron step-transport from the substrate to the interface of surface/ vacuum (the quantum well structure at the surface) and (2) electron FN tunneling from surface potential barrier. For the FE enhancement of second process, it has been discussed as the above. Here we focus only on the FE enhancement effect of steptransport of the interior electron supply. In general, field electron emission locates at the CB of semiconductors.22 It indicates that if there are enough electrons
resistivity (Ω/sq)
(/cm3)
ZB-AlN WZ-GaN
66.73 6023
9.784 10 5.976 1015
n n
AGMP
0.2672
2.97 1018
n
type of impurity 14
accumulated in the CB of AlN with low or negative electron affinity23 in the AGMP, the surface FN tunneling will be easier. How do lots of electrons transfer from Si substrate to the quantum well structure at the surface? Because of the thickness of 120 nm for the AGMP, the quantum resonant tunneling may be impossible. Therefore, we suppose that there is an efficient electron step-transport from Si substrate to the surface of the AGMP (as shown in Figure 4), which is possible for the different energy band structure of ZB AlN and WZ GaN. The transition energy of step-transport can be obtained from the thermal activation energy.11,15 In the step-transport, the electron first transported into the CB of GaN and next accumulated at the CB of AlN. Compared with electron transport in a single-phase GaN or AlN film, the AGMP film has two benefits to enhance fieldemission properties. First, it always makes the electron locate at the most favorable energy level (in this case for CB of AlN) for surface FN tunneling; Second, because of the reduction of electron-transition energy by the step-transport, more electrons can be accumulated in the quantum well near at the surface, which greatly increases interior electron supply for surface FN tunneling. To demonstrate further the possibility of the step transport, the Hall measurement was also adopted to examine the carrier concentration, the resistivity, and the type of impurity of AlN, GaN, and AGMP (seen from Table 1). It is clear that for the mixed-phase structure the surface resistivity has been lowered remarkably from 6023Ω/sq to 0.2672Ω/sq. It indicates that the AGMP nanofilm may help to electron transport, which is very different from the mixtures of WZ and ZB nanowires blocking electron transport in the recent report.15 One possible mechanism is spontaneous polarization charges at the nanowires WZ/ZB interfaces, which suppress carrier accumulation at the nanomises surface, as discussed by Dayeh et al.,16 that would create a sawtooth potential in the conduction and valence bands15 and block electron transport with high densities of twin planes and stacking faults along the length of a nanowire. However, more electron channels perpendicular to films surface may be supplied by the step-transport in the AGMP nanofilms, which is favorable for electron transport and field electron emission. From Table 1, the carrier concentration of the AGMP film was far more than that of AlN or GaN film, and the impurities for AlN, GaN, and AGMP film all are n type. On the basis of the experimental parameters of Table 1, we have calculated the energy difference of the CBM and the Fermi level at 300 K (room temperature) in Table 2. It shows that there is smaller energy difference for the AGMP than that for ZB AlN or WZ GaN. Because the doping or defects energy levels should be above the Fermi energy level for semiconductors at room temperature, the difference of the doping or defect energy levels and CBM will be
