**4. AFM study of Al(Ga)N**

#### **4.1 AlN growth condition optimization**

Due to its potential for many applications, the growth and characterization of AlN has been a subject of much interest. Great progresses on AlN material growth and device fabrication have been attained. The first electroluminescent emission at wavelength of 200 nm from a p-i-n AlN

InN is usually grown by MBE due to its low growth temperature. Chad et al. grew InN films in two different growth regimes and characterized the surface morphology by AFM, as shown in Fig. 10(a) (the N-rich regime) and Fig. 10(b) (the In-droplet regime). Clear growth steps were observed for InN grown in N-rich regime and Indium droplets were observed

Fig. 10. AFM micrographs of (a) a 1.5- μm -thick InN layer grown in the In-droplet regime

Due to its potential for many applications, the growth and characterization of AlN has been a subject of much interest. Great progresses on AlN material growth and device fabrication have been attained. The first electroluminescent emission at wavelength of 200 nm from a p-i-n AlN

Fig. 9. AFM micrographs of typical InN layer.

when it was grown in the In-droplet regime (Gallinat et al., 2006).

and (b) a 1- μm -thick InN layer grown in the N-rich regime.

**4. AFM study of Al(Ga)N** 

**4.1 AlN growth condition optimization** 

homojunction LED operating at 210 nm was realized (Taniyasu et al., 2006). The first AlN metal-semiconductor-metal photodetector with a peak responsivity at 200 nm was. obtained (Li et al., 2003). Significant improvement of the AlN crystal quality using Lateral Epitaxial Overgrowth (LEO) of AlN on sapphire substrates was also demonstrated (Chen et al., 2006). This resulted in 214 nm stimulated emission, the shortest wavelength stimulated emission reported in semiconductor materials (Shatalov et al., 2006).

AlN is widely used as the buffer layer for Ultraviolet (UV) Light Emitting Diodes (LED) and High Electron Mobility Transistors (HEMT) grown on SiC substrates currently due to its small lattice constant, wide bandgap and high thermal conductivity (Chen et al. 2009a; Chen et al. 2009b). It is therefore necessary to develop an uncomplicated method for growing high quality, thick and crack-free AlN on SiC substrates. AFM images of the AlN grown in 3D and 2D modes are shown in Figs. 11(a) and 11(b), respectively. Islands are present in Fig. 11(a), showing that this material has a 3D growth mode. Steps with a height difference of one (0001) AlN monolayer can be seen in Fig. 11(b), showing that the growth mode of this material is 2D. The RMS surface roughness of the 3D AlN over 5 × 5 μm� area is 0.517 nm, while that of the 2D AlN is 0.151 nm.

Fig. 11. AFM images of AlN grown at different growth modes: a) three-dimensional, b) twodimensional.

High quality AlN films were grown by switching between the established 2D and 3D AlN growth modes, a method we call modulation growth (MG) (Chen et al. 2008;). The structure of a MG AlN sample is shown in Fig.12. First, a 300 nm 3D AlN layer was grown on the SiC substrate. Then a 2D 200 nm AlN layer was grown. Subsequently, this 3D-2D period was repeated twice. The total thickness of the resulting film is 1.5 µm. The surface of the AlN grown by 3D-2D modulation growth is very smooth. Fig. 13 shows a 5 × 5 μm� AFM scan of the surface with an RMS surface roughness of 0.132 nm. Well defined steps and terraces indicate a step-flow growth mode, and can be observed in all parts of the wafer. The height difference between terraces corresponds to one monolayer of (0001) AlN. No step terminations were observed over the scanned area indicating a low density of Threading

AFM Application in III-Nitride Materials and Devices 199

Epitaxial lateral overgrowth (ELOG) and similar techniques have proved to be an effective method to reduce the TD in GaN (Detchprohm et al., 2001; Weimann et al., 1998) and improve the devices performance. However, ELOG is difficult to undertake with AlN, and even with AlGaN, due to the high sticking coefficient of Al adatom, thus causing a low lateral growth. For the first time, Chen et al. reported the pulsed lateral epitaxial overgrowth (PLOG) of AlN films over shallow grooved sapphire substrates (Chen et al., 2006). The PLOG approach at temperatures around 1150 °C enhances the adatom migration thereby significantly increasing the lateral growth rates. This enables a full coalescence in wing

In the ElOG process for AlN, first a 0.3-μm-thick AlN layer was grown on the sapphire substrates using a migration-enhanced low pressure MOCVD process. Standard photolithography was then used to form 2 μm wide-masked stripes, with 4-10 μm wide openings oriented along the AlN < 11�00 > directions. Reactive ion etching was used to remove the AlN and sapphire in the openings area to form 4-10μm-wide and 0.7-μm-deep trenches. This grooved sapphire/AlN sample served as the template for the subsequent pulsed lateral epitaxial overgrowth. TMA and NH3 were used as the precursors, and growth was carried out at 1150 °C. For the PLOG process, the NH3 supplied to the reactor was pulsed (pulse durations 6 to 12 seconds) while the TMA flow was kept constant during the growth. Fig. 14 shows a cross-sectional SEM image of a fully coalesced PLOG-AlN film with a 4- μm -wide trench (4- μm -wide support mesa). The SEM image clearly shows lateral growth in the < 112�0 >direction. The sidewall and the top surface are very smooth. A complete triangular void forms upon the trench after the coalescence of the stripes when thickness of

the film reaches 6 μm . This indicates the lateral to vertical growth rate ratio is 1:3.

Fig. 14. Cross sectional SEM image of AlN film grown on grooved template by PLOG.

After coalescence, the AlN surface became flat over the whole wafer. A 10 × 10 μm� atomic force microscopy (AFM) scan shows an RMS surface roughness of 0.5 nm. Fig. 15 shows a 6 × 6 μm� AFM scan image covering one mask period of the PLOG-AlN surface including both the region directly grown on top of the support mesa and the region laterally grown over the trench. As seen from the well-defined steps and terraces, a two dimensional stepflow growth is dominant over the entire wafer under these PLOG growth conditions.

regions as wide as 4 to 10 µm.

Fig. 12. Schematic of AlN grown with 3D-2D modulation growth method.

Fig. 13. AFM image of AlN grown with the 3D-2D modulation method.

Dislocation (TD) with screw character, which was confirmed by on-axis and off-axis X-ray rocking curves. The Full Widths at Half Maximum (FWHM) of the (002) and (102) peaks were 86 and 363 arc sec, respectively. The off-axis (105) and (201) peaks were also measured and have FWHM of 225 and 406 arc sec, respectively. The narrow symmetric and asymmetric FWHM of the X-ray data for AlN suggest a low threading dislocation (TD) density, with a small proportion of TDs with screw components.

An atomically flat surface can be obtained when AlN growth is performed at temperatures higher than 1300 °C (Imura et al., 2007). However, the dislocation density is still as high as 1 × 10 cm���. Therefore, the crystalline quality of AlN requires further improvement, particularly when it is grown on sapphire substrates.

Fig. 12. Schematic of AlN grown with 3D-2D modulation growth method.

Fig. 13. AFM image of AlN grown with the 3D-2D modulation method.

density, with a small proportion of TDs with screw components.

particularly when it is grown on sapphire substrates.

Dislocation (TD) with screw character, which was confirmed by on-axis and off-axis X-ray rocking curves. The Full Widths at Half Maximum (FWHM) of the (002) and (102) peaks were 86 and 363 arc sec, respectively. The off-axis (105) and (201) peaks were also measured and have FWHM of 225 and 406 arc sec, respectively. The narrow symmetric and asymmetric FWHM of the X-ray data for AlN suggest a low threading dislocation (TD)

An atomically flat surface can be obtained when AlN growth is performed at temperatures higher than 1300 °C (Imura et al., 2007). However, the dislocation density is still as high as 1 × 10 cm���. Therefore, the crystalline quality of AlN requires further improvement, Epitaxial lateral overgrowth (ELOG) and similar techniques have proved to be an effective method to reduce the TD in GaN (Detchprohm et al., 2001; Weimann et al., 1998) and improve the devices performance. However, ELOG is difficult to undertake with AlN, and even with AlGaN, due to the high sticking coefficient of Al adatom, thus causing a low lateral growth. For the first time, Chen et al. reported the pulsed lateral epitaxial overgrowth (PLOG) of AlN films over shallow grooved sapphire substrates (Chen et al., 2006). The PLOG approach at temperatures around 1150 °C enhances the adatom migration thereby significantly increasing the lateral growth rates. This enables a full coalescence in wing regions as wide as 4 to 10 µm.

In the ElOG process for AlN, first a 0.3-μm-thick AlN layer was grown on the sapphire substrates using a migration-enhanced low pressure MOCVD process. Standard photolithography was then used to form 2 μm wide-masked stripes, with 4-10 μm wide openings oriented along the AlN < 11�00 > directions. Reactive ion etching was used to remove the AlN and sapphire in the openings area to form 4-10μm-wide and 0.7-μm-deep trenches. This grooved sapphire/AlN sample served as the template for the subsequent pulsed lateral epitaxial overgrowth. TMA and NH3 were used as the precursors, and growth was carried out at 1150 °C. For the PLOG process, the NH3 supplied to the reactor was pulsed (pulse durations 6 to 12 seconds) while the TMA flow was kept constant during the growth.

Fig. 14 shows a cross-sectional SEM image of a fully coalesced PLOG-AlN film with a 4- μm -wide trench (4- μm -wide support mesa). The SEM image clearly shows lateral growth in the < 112�0 >direction. The sidewall and the top surface are very smooth. A complete triangular void forms upon the trench after the coalescence of the stripes when thickness of the film reaches 6 μm . This indicates the lateral to vertical growth rate ratio is 1:3.

Fig. 14. Cross sectional SEM image of AlN film grown on grooved template by PLOG.

After coalescence, the AlN surface became flat over the whole wafer. A 10 × 10 μm� atomic force microscopy (AFM) scan shows an RMS surface roughness of 0.5 nm. Fig. 15 shows a 6 × 6 μm� AFM scan image covering one mask period of the PLOG-AlN surface including both the region directly grown on top of the support mesa and the region laterally grown over the trench. As seen from the well-defined steps and terraces, a two dimensional stepflow growth is dominant over the entire wafer under these PLOG growth conditions.

AFM Application in III-Nitride Materials and Devices 201

Fig. 16. AFM scans of 500 nm thick Al0.5Ga0.5N grown on a 15 nm AlN nucleation layer grown at (a) 625 °C, (b) 650 °C, and (c) 675 °C and on a low temperature 525 °C AlN NL with

Subsequently, it was observed by AFM that the direct growth of an 1μm layer of AlGaN on AlN homoepitaxial layers always results in a roughened morphology. To facilitate heteroepitaxial strain relaxation while preserving structural quality. Z. Ren et al. adopted a design of step-graded layers consisting of three superlattices (SLs 1, 2, and 3) with average Al compositions of 0.90, 0.73, and 0.57. Each SL was composed of ten periods of Al�Ga���N ���� Å)/ Al�Ga���N (150 Å) (x/y=1.0/ 0.8, 0.8/ 0.65, and 0.65/ 0.50) (Ren et al., 2007). Surface morphology after the growth of SL 1 (Figs. 17(a) and (d)) and SL 2 (Figs. 17(b) and (e)) indicates that pseudomorphic growth persists with an atomically smooth surface under a step-flow growth mode. During the growth of SL 3, surface morphology underwent a fundamental change with the appearance of large 1–2 μm plateaus or platelets separated by deep trenches or clifflike edges with a height of ~100 nm (Fig. 17(c)) even though step

thicknesses of (d) 10 nm, (e) 15 nm, and (f) 30 .

flow was still maintained locally (Fig. 17(f)) (Ren et al. 2007).

Fig. 15. AFM image acquired from the PLOG-AlN trench and support mesa regions. Arrows point to intersections of dislocations at the surface.

Height differences of 0.27 nm between terraces correspond to one monolayer of (0001) AlN (c/2=0.25nm). The overgrown region has long parallel atomic steps without step terminations, indicating a reduced TD density. At the coalescence point, two threading dislocations marked by arrows are seen in Fig. 15 as step terminations in the AFM image. Another TD is observed in the support area in the same image, implying higher TD density in the support mesa region than that of the trench region. The step termination density, corresponding with either pure screw or mixed screw-edge character, measured by AFM, is 8.3 × 10�cm�� for the scan area shown in Fig. 15.

#### **4.2 AlGaN material growth**

The growth of AlxGa1-xN can be carried out on sapphire substrates with a thin low temperature (LT) AlN nucleation layer (Wickenden et al., 1998; Wang et al., 2007 ; Koide et al., 1988) or thick high temperature (HT) AlN template (Sun et al., 2004 ; Mayes et al., 2004 ; Fischer et al., 2004). Grandusky et al. studied the effect of LT-AlN nucleation layer growth conditions for the growth of high quality AlxGa1-xN layers on sapphire (Grandusky et al., 2007). The conditions of the LT AlN had a dramatic effect on the morphology and crystalline quality of the overgrown AlxGa1-xN layers. The effect of growth temperature and thickness of the LT AlN nucleation layers on the overgrown Al0.5Ga0.5N is shown in Fig. 16. Fig. 16(a) shows a smooth surface with a RMS roughness of 1 nm over 10 × 10 µm, whereas Figs. 16(b) and (c) show rough surfaces with 3D crystallites clearly evident. As demonstrated in Fig. 16, extreme changes in the surface morphology are seen with further increase in temperature of the nucleation layer (NL). For a lower growth temperature of 525 °C for the AlN NL, little change was seen in the surface morphology. The best surface morphology was the sample with a thickness of 15 nm as can be seen from Figs. 16(d)–(f). Large AlGaN surface crystallites were observed when the nucleation layer was too thin, as shown in Fig. 16(d). When the layer was thicker than 15 nm, slight roughening of the surface could be seen in Fig. 16(f), as well as a broadening of the (0002) rocking curve.

Fig. 15. AFM image acquired from the PLOG-AlN trench and support mesa regions. Arrows

Height differences of 0.27 nm between terraces correspond to one monolayer of (0001) AlN (c/2=0.25nm). The overgrown region has long parallel atomic steps without step terminations, indicating a reduced TD density. At the coalescence point, two threading dislocations marked by arrows are seen in Fig. 15 as step terminations in the AFM image. Another TD is observed in the support area in the same image, implying higher TD density in the support mesa region than that of the trench region. The step termination density, corresponding with either pure screw or mixed screw-edge character, measured by AFM, is

The growth of AlxGa1-xN can be carried out on sapphire substrates with a thin low temperature (LT) AlN nucleation layer (Wickenden et al., 1998; Wang et al., 2007 ; Koide et al., 1988) or thick high temperature (HT) AlN template (Sun et al., 2004 ; Mayes et al., 2004 ; Fischer et al., 2004). Grandusky et al. studied the effect of LT-AlN nucleation layer growth conditions for the growth of high quality AlxGa1-xN layers on sapphire (Grandusky et al., 2007). The conditions of the LT AlN had a dramatic effect on the morphology and crystalline quality of the overgrown AlxGa1-xN layers. The effect of growth temperature and thickness of the LT AlN nucleation layers on the overgrown Al0.5Ga0.5N is shown in Fig. 16. Fig. 16(a) shows a smooth surface with a RMS roughness of 1 nm over 10 × 10 µm, whereas Figs. 16(b) and (c) show rough surfaces with 3D crystallites clearly evident. As demonstrated in Fig. 16, extreme changes in the surface morphology are seen with further increase in temperature of the nucleation layer (NL). For a lower growth temperature of 525 °C for the AlN NL, little change was seen in the surface morphology. The best surface morphology was the sample with a thickness of 15 nm as can be seen from Figs. 16(d)–(f). Large AlGaN surface crystallites were observed when the nucleation layer was too thin, as shown in Fig. 16(d). When the layer was thicker than 15 nm, slight roughening of the surface could be seen

point to intersections of dislocations at the surface.

8.3 × 10�cm�� for the scan area shown in Fig. 15.

in Fig. 16(f), as well as a broadening of the (0002) rocking curve.

**4.2 AlGaN material growth** 

Fig. 16. AFM scans of 500 nm thick Al0.5Ga0.5N grown on a 15 nm AlN nucleation layer grown at (a) 625 °C, (b) 650 °C, and (c) 675 °C and on a low temperature 525 °C AlN NL with thicknesses of (d) 10 nm, (e) 15 nm, and (f) 30 .

Subsequently, it was observed by AFM that the direct growth of an 1μm layer of AlGaN on AlN homoepitaxial layers always results in a roughened morphology. To facilitate heteroepitaxial strain relaxation while preserving structural quality. Z. Ren et al. adopted a design of step-graded layers consisting of three superlattices (SLs 1, 2, and 3) with average Al compositions of 0.90, 0.73, and 0.57. Each SL was composed of ten periods of Al�Ga���N ���� Å)/ Al�Ga���N (150 Å) (x/y=1.0/ 0.8, 0.8/ 0.65, and 0.65/ 0.50) (Ren et al., 2007). Surface morphology after the growth of SL 1 (Figs. 17(a) and (d)) and SL 2 (Figs. 17(b) and (e)) indicates that pseudomorphic growth persists with an atomically smooth surface under a step-flow growth mode. During the growth of SL 3, surface morphology underwent a fundamental change with the appearance of large 1–2 μm plateaus or platelets separated by deep trenches or clifflike edges with a height of ~100 nm (Fig. 17(c)) even though step flow was still maintained locally (Fig. 17(f)) (Ren et al. 2007).

AFM Application in III-Nitride Materials and Devices 203

LED grown on PSS showed a higher internal quantum efficiency due to a dislocation density reduction by epitaxial lateral overgrowth technology (Tadatomo et al., 2001; Yamada et al., 2002 ; Hsu et al., 2004). Besides the elimination of threading dislocations due to the lateral growth of GaN on top of PSS, researchers believe that PSS improve the light extraction. It is well known that the large difference of refractive index between semiconductor and air leads to trap of large percentage of light emitting from LED (Lee et al., 2005). Thus, it is important to design and characterize the geometry of the PSS. AFM is

AFM images of PSS with different specs are shown in Figs. 19(a) and (b). The bump shapes, depths and widths could be characterized accurately. The average bump depth, pitch and gap of the patterns seen in Fig. 19(a) are 1.1 μm, 3 μm and 0.2 μm, respectively. While the average bump depth, pitch and gap of the patterns seen in Fig. 19(b) are 0.3 μm, 6 μm and 3 μm,

(a) (b) Fig. 19. (a) One patterned sapphire substrate with a cross sectional line to show the height, width and shape of the bumps. (b) 3-dimensional AFM image of another PSS with triangular

Fig. 20 (a) is the surface morphology of an LED grown on a PSS. There is a p++ layer on the top of the LED surface. The bumps on the surface are caused by heavily doped Mg on the surface, usually rooted from a screw type dislocation. In the manufacturing of LEDs it is important to verify that the surface is free of pits, which implies the p-layer has coalesced well and sealed all the V-pits from the underneath layer of InGaN. Fig. 20(b) is an example of a LED with pits on the surface. LEDs with un-coalesced surfaces with pits usually have

Screw type related pits on the GaN surface have been confirmed to be the source of reverse leakage in GaN films (Law et al., 2010). Fig. 21(a) shows an AFM topograph of a GaN sample grown by molecular beam epitaxy (MBE), in which the screw type TD related pits could be observed. Fig. 21(b)–(d) show conductive atomic force microscopy (CAFM) images obtained at dc biases of -14, -18, and -22 V of the area in Fig. 21(a). Fig. 21(b) shows several small, dark features that correspond to localized reverse-bias leakage paths observable

the only available tool to characterize the bump shape accurately so far.

respectively.

pyramid cone shaped bumps.

high leakage currents.

**5.2 As grown LED surface and leakage characteristics** 

Fig. 17. AFM images of AlGaN superlattices on bulk AlN. The scan area of (a), (b), and (c) is 10 × 10 μm�, and (d), (e), and (f) is 1 × 1 μm�. (a) and (d), (b) and (e), and (e) and (f) are taken after the growth of superlattices 1, 2, and 3, respectively. Root-mean-square (rms) roughness from each scan is labeled.
