DOI: 10.1021/cg1007473
Epitaxial Growth of High Quality Nonpolar InN Films on LiGaO2 Substrates
2011, Vol. 11 664–667
Guoqiang Li*,†,‡ and Hui Yang† †
College of Materials Science and Engineering, Southern China University of Technology, Guangdong 510641, China, and ‡Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom Received June 4, 2010; Revised Manuscript Received August 2, 2010
ABSTRACT: A new system for obtaining high quality nonpolar InN films has been demonstrated. It is found that a-plane InN epitaxially grows on LiGaO2 (001) with high phase purity and smooth surface. The in-plane epitaxial relationships are InN [0001]// LiGaO2 [100], and InN [1100]// LiGaO2 [010]. The interface between the substrate and a-plane InN is atomically abrupt. The threading dislocation density drops dramatically from 2 1010 cm-2 of its initial growth stage to 3 109 cm-2 when the InN film thickness reaches 30 nm, which we believe is driven by dislocation merging or annihilation near the nucleation layer. The band edge emission from its photoluminescence spectra of as-grown a-plane InN is located at 0.72 eV. This novel high quality nonpolar a-plane InN on a LiGaO2 (001) system opens up a new possibility for high-efficiency nonpolar InN devices.
Introduction Among the Group-III nitride family, InN has recently been attracting enormous attention due to its excellent optoelectronic properties, which allow it to be used for fabrication of infrared detectors, high speed electronic devices, and so on.1-6 At present, InN devices are mainly grown along the [0001] c-axis of hexagonal InN, where strong spontaneous and strained-induced piezoelectric polarization exists.7,8 The internal electric field caused by this polarization leads to carrier separation in the quantum wells (so-called quantumconfined Stark effect QCSE) and hence a reduced quantum efficiency, in other words, a reduced radiative combination probability of the carriers.9 One way to eliminate QCSE from occurring is to grow nonpolar hexagonal InN planes including (1120) a-InN and (1100) m-InN planes. Actually, the application of nonpolar orientated Group-III nitrides films for device fabrication is one of the main interests in this field and is now being highly pursued globally. In contrast to the exciting progress that has been achieved up to now for the growth of nonpolar GaN films,10-12 the development of nonpolar InN has been very slow, mainly because of the lack of suitable lattice-matched substrates. Although r-plane sapphire has been used to grow nonpolar InN films,13-18 the critical requirement for the duration of nitridation pretreatment of the substrate makes this route not very practical, and the large lattice mismatch between r-plane sapphire and nonpolar InN leads to low quality of the epitaxy. A free-standing GaN template has also been proposed as a substrate for m-plane InN epitaxy recently.9,10,20 Its high price will hugely raise the cost of InN device fabrication. High quality nonpolar InN grown on lattice-matched and costeffective substrate is urgently desired. LiGaO2 belongs to the space group of Pna21 that has an orthorhombic unit cell with dimensions a = 5.402 A˚, b = 6.372 A˚, and c = 5.007 A˚.21 The commercial production of bulk LiGaO2 single crystals has been mature for quite a long time. *To whom corresponding should be addressed. Tel.: þ86-20-87112957. Fax: þ86-20-87112957. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 03/02/2011
LiGaO2 (001) was reported for the growth of polar c-plane GaN (0001) films due to their closely matched lattice constants.22-24 However, a detailed study of LiGaO2 structure allows us to notice that LiGaO2 (001) would also provide superb lattice matches to the nonpolar (1120) a-InN plane with its lattice constants 2aInN = 6.140 A˚ (3.6% lattice mismatch against bLiGaO2) and c=5.703 A˚ (5.5% lattice mismatch against aLiGaO2), as illustrated by Figure 1. This indicates that LiGaO2 (001) would very possibly be an extraordinary candidate for epitaxial growth of nonpolar a-plane InN! Our previous work has proven that pulsed laser deposition (PLD) is a very promising technique for growth of high quality Group-III nitride films even at low temperature thanks to the high kinetic energy of Group-III precursors24-27 and is used for epitaxial growth of InN films for our study. In this work, we report for the first time the synthesis of high quality a-plane InN films on LiGaO2 (001) substrates. This nonpolar InN shows high crystallinity with low defect density and strong band edge emission at 0.72 eV. This a-plane InN/LiGaO2 (001) system opens up a new possibility for high-efficiency nonpolar InN devices. Experimental Section As-received LiGaO2 (001) substrates were annealed at 1000 °C in air for 4 h to obtain an atomically flat surface before growth. After being cleaned in organic solvents to remove surface contamination, the substrates were introduced into the ultrahigh vacuum (UHV) PLD chamber with a background pressure of 3.0 10-10 Torr. A Q-switch pulsed Nd:YAG laser with a working wavelength of 355 nm and pulse width 5 ns was used to ablate an In metal target (99.9999% purity) with an average pulse energy of 120 mJ and a pulse repetition of 30 Hz. The laser beam was introduced into the chamber at an angle of 45° and focused on the rotating In target. The ablated In species were supplied onto the substrates that were kept at 300 °C and mounted 5 cm away from the target. The substrate temperature was measured by a thermocouple embedded in the substrate holder beneath the substrate mounting area. During the growth, high purity N2 with a pressure of 5 10-6 Torr was supplied through an inert gas purifier and a radio frequency plasma radical generator operated at 320 W. In situ reflection high-energy electron diffraction (RHEED) was used to monitor the growth condition during the whole course. r 2011 American Chemical Society
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Figure 1. Atomic models for (a) LiGaO2 (001) and (b) InN (1120) plane, where lattice constants and each type of atom are denoted.
Figure 2. (a) The RHEED pattern with the electron beam incidence along InN [1100] and (b) the AFM image of a 100 nm thick a-plane InN film grown on LiGaO2 (001) substrate. The rms value of this surface is 3.06 nm. As-grown InN films were evaluated by X-ray diffraction (XRD with Cu KR1 X-ray source) rocking curves for crystallinity and by atomic force microscopy (AFM) for surface morphology. Hall effect measurements were also performed at room temperature to obtain carrier characteristics. Transmission electron microscopy (TEM) was employed to characterize the interface between the substrates and InN films and microstructure of InN films. Electron energy loss spectroscopy (EELS) collections were carried out under scanning TEM mode, where a spot formed by a 0.6 nm diameter electron beam was used to scan across the interface with an interval of 1 nm between two spots.
Results and Discussion Figure 2a shows a typical RHEED pattern for InN films grown on LiGaO2 (001) substrates with the electron beam incidence along InN [1100], which is a good indication of a-plane InN film growth. The streaky RHEED pattern also suggests a smooth surface of the as-grown InN films. AFM image of a 100 nm thick a-plane InN film is given by Figure 2b. The root-mean-square (rms) value of this surface is as low as 3.06 nm, which is consistent with our RHEED observation. Figure 3a plots the XRD θ-2θ profile of the 100 nm thick InN on LiGaO2 (001) substrate. Two sharp peaks related to InN (1120) and LiGaO2 (001), as indicated in this figure, are clearly seen. On the contrary, evidence of a InN (0002) peak which should be sitting at about 31.35° is not detected at all, suggesting high phase purity of a-plane InN films. This high phase purity should be attributed to the enhanced migration ability of Group-III precursors ablated by the pulsed laser that suppresses the formation of nucleation of polar-phase (0002) InN. The crystallinity of the as-grown a-plane InN films with various film thicknesses is studied by (1120) X-ray rocking curves. The full width at half-maximum (fwhm) of these curves which is used to evaluate the crystallinity was summarized in
Figure 3. (a) XRD θ-2θ profile of a 100 nm thick InN on LiGaO2 (001) substrate, and (b) fwhm values of (1120) XRD rocking curves for as-grown a-plane InN films with various film thicknesses.
Figure 4. (a) A bright field XTEM image at low magnification, from which we can notice that the interface between LiGaO2 (001) substrate and a-plane InN film (indicated by the white arrow) is abrupt, and the thickness of the film is about 180 nm. (b) A dark field XTEM image from the same sample at higher magnification under the two-beam diffraction condition of g = [1100], where the dashed line indicates the place where dislocation density suddenly drops.
Figure 4b. We notice that the crystallinity of the films shows an improved tendency with the increase in film thickness, though at a different improving rate. The 30-nm-thick InN film has a relatively low crystallinity with the fwhm equal to 1.20°. The crystallinity improves quite slowly when the thickness is below 30 nm, then improves dramatically at 30 nm thickness, and finally keeps almost constant crystallinity for thicker films with the fwhm around 0.30°, which is smaller than the recently reported ones for nonpolar a-plane InN.19,20,28 In order to further understand the structure for a-plane InN films grown on LiGaO2 (001) substrates, cross-sectional transmission electron microscopy (XTEM) samples of a-plane InN films grown on LiGaO2 (001) substrates were made by mechanical polishing followed by low-energy and low-angle ion milling (Fischione 1010 Low Angle Ion Milling & Polishing System), ending up with the sample edge thickness of about 15 nm. The XTEM samples were then put into a JEOL 3000F
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Figure 6. A PL spectrum collected at 15 K from a 100 nm a-plane InN film on LiGaO2 (001). Figure 5. A high resolution XTEM image around the interface of a-plane InN films grown on LiGaO2 (001), where the atomically sharp interface between InN epitaxy and LiGaO2 substrate can be clearly seen, and the measured spacings indicate the in-plane epitaxial relationship of InN [0001]//LiGaO2 [100].
field emission gun TEM working at a voltage of 300 kV, which gives a point-to-point resolution of 0.17 nm. Figure 4a is a bright field XTEM image at low magnification, from which we can notice that the interface between LiGaO2 (001) substrate and a-plane InN film is abrupt, indicating the suppression of interfacial reaction between the film and substrate which usually takes place for MBE or MOCVD growth where higher growth temperature is required. Figure 4a also shows a flat top surface of the as-grown a-plane InN film that is consistent with our RHEED and AFM observations. We can also obtain the thickness of the film from this XTEM image. It is about 180 nm after 20 min growth, corresponding to a growth rate of 540 nm/h. Weak-beam dark field imaging is a powerful technique to study dislocations inside materials. Figure 4b is a dark field XTEM image under the two-beam diffraction conditions of g=[1100], where threading dislocations can be clearly observed from their sharp contrast against the matrix. It can be easily found that in the first 30 nm away from the interface, the InN film is highly distorted by a high density of threading dislocations of 2 1010 cm-2, which was obtained via the following calculation n ð1Þ d ¼ lt where l is the actual width along the interface, n is the number of threading dislocations in this l width region, and t is the thickness of the XTEM sample in this region which was measured by energy filtered TEM to be 15 nm. Above this highly distorted region, the TEM image becomes much clearer, as seen from Figure 4a, with a dramatic decrease in threading dislocation density to 3 109 cm-2. This sharp decrease in dislocation density accounts for the improvement of crystallinity at 30 nm film thickness revealed by X-ray rocking curves. We believe that the sharp reduction of dislocation density is primarily driven by dislocation merging or annihilation near the nucleation layer.29 Figure 5 shows a high resolution XTEM image around the interface of a-plane InN films grown on LiGaO2 (001). The measured spacings confirm the epitaxial relationships are InN(1120)//LiGaO2 (001), and InN [0001]//LiGaO2 [100]. EELS attached to the microscope has been used to evaluate the concentration distribution across the interface as indicated by the dashed line in Figure 3b. We detect neither O diffusing from the substrate to the epitaxy nor N precursors reacting
with the substrate. The chemical ratio between In and N is strictly 1:1 in as-grown InN film. Clearly, there is no interfacial layer between the substrate and the film, and the interface is atomically abrupt. Obviously, the interfacial reaction is completely suppressed by the application of low temperature growth for PLD, which is essential for reduction of dislocation density in the as-grown InN films. Figure 6 shows the photoluminescence (PL) spectrum collected at 15 K from a 100 nm a-plane InN film on LiGaO2 (001). The emission is dominated by the band edge emission at 0.72 eV. This is 0.02 eV higher than the 0.70 eV for c-plane InN.30,31 One possibility of this difference might be due to the absence of strong internal electrostatic fields of as-grown a-plane InN.7 Hall effect measurements were carried out on the as-grown nonpolar InN at room temperature. We found that its electron concentration is about 6.2 1018 cm-3, and the electron mobility is 293 cm2 V-1 s-1. This relatively high electron cencentration reminds us that the blueshift of the PL spectrum may also be very possibily related to band tailing effects which are strongly associated with carrier concentration and structural disorders.32,33 Actually, previous investigations have repeatedly reported band tailing effects on polar and nonpolar InN.16,19,33 Meanwhile, our finding of 0.72 eV is slightly higher than the reported 0.70 eV of a-plane InN films grown on r-plane sapphire using MBE,15,16 and much higher than the reported 0.62-0.65 eV of a-plane InN films grown on r-plane sapphire by MOCVD.17,18 Apart from the possible carrier concentration difference among these films, or equally the influence from band tailing effects as mentioned above, the different strain states caused by the different substrates ((001) LiGaO2 in this work vs r-plane sapphire as reported) might also play a role. The use of (001) LiGaO2 as a substrate in our case is beneficial to obtain higher quality a-plane InN as a consequence of better matched lattice constants between them. We noticed that the growth technique also affects the band edge emission of as-grown a-plane InN. The growth temperature decreases in the order of MOCVD, MBE, and PLD. It probably indicates that a lower growth temperature helps to relieve strain generated in InN caused by both lattice and thermal mismatches between substrate and epilayer, and consequently improves a-plane InN quality. The fwhm value of as-grown a-plane InN is as small as 33 meV, and the emission related to defects which is located at lower energy (about 0.67 eV) is very weak. All these results verify that this aplane InN/LiGaO2 (001) system would be a very good approach for realizing high-efficiency InN devices. Summary In summary, we have found a-plane InN epitaxially grown on LiGaO2 (001) with high phase purity and smooth surface.
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The as-grown a-plane InN shows high crystallinity. The fwhm for (1100) XRC from a 100 nm a-plane InN is as small as 0.30°. The in-plane epitaxial relationship are InN [0001]// LiGaO2 [100], and InN [1100]// LiGaO2 [010]. The interface between the substrate and a-plane InN is atomically abrupt. XTEM images indicate that threading dislocation density drops dramatically from 2 1010 cm-2 of its initial growth stage to 3 109 cm-2 when InN film thickness reaches 30 nm, which we believe is driven by dislocation merging or annihilation near the nucleation layer. The as-grown a-plane InN exhibits excellent band edge emission at 0.72 eV. The successful growth of high quality a-plane InN on LiGaO2 (001) indicates that LiGaO2 (001) is a superior substrate to r-plane sapphire for growth of a-plane InN due to better matched lattice constants. The successful growth of high quality a-plane InN also suggests that PLD is an ideal technique for growth of nonpolar nitrides at least for two reasons. One is its enhanced migration ability of precursors by the pulsed laser that suppresses the formation of nucleation of polar-phase InN. The other is its suppressed interfacial reaction by the application of low temperature growth for PLD which is critical to reduce dislocation density and relieve strain in InN films caused by both lattice and thermal mismatches between substrate and epilayer. The successful growth of aplane InN on LiGaO2 (001) provides a novel approach for synthesis of high quality nonpolar InN films, which possibly opens a new solution for realizing high-efficiency InN devices. Acknowledgment. Special thanks to Dr. H. L. Zhang from Oxford University and Prof. S. C. Mu from Wuhan University of Technology for their fruitful suggestions and assistance. This work is jointly supported by Royal Society of the UK, National Science Foundation of China, and Central University Fund for Fundamental Research of SCUT.
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