M EASUREMENT AND C HARACTERIZATION

The crystalline quality of grown samples, a-plane GaN on patterned m-plane sapphire and TELOG a-plane GaN on γ-plane sapphire were characterized by X-ray diffraction (XRD). Additionally, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to observe the surface morphology, growth mode, and defect distribution of the epitaxial film.

The scanning electron microscopy also provided the ability of observing the cross-sectional image of these epitaxial samples. Specially, the etching effects of TELOG a-plane GaN in KOH-ethylene glycol solution under different conditions are clear and unequivocal in scanning electron microscopy. This offered the important information of clarifying the etching characteristics including etching direction, etching rate, and most of all, etching mechanism.

The capable of imaging at a significantly higher resolution of Transmission electron microscopy can help in determining the defects distribution and arrangement.

In accompany with the electron diffraction patterns, the types of defects in a-plane GaN on m-plane sapphire and a-plane GaN on γ-plane sapphire are clarified.

Optical properties of these a-plane GaN samples were investigated by uncontact and nondestructive photoluminescence spectroscopy and cathodoluminescence spectroscopy. The luminescence results can be categorized to show the lighting behavior and the crystalline quality respectively. Photoluminescence spectroscopy, in addition, provided the information of light extraction ability of KOH-ethylene glycol solution etched TELOG a-plane GaN MQWs. Cathodoluminescence spectroscopy mapping of the cross-sectional image of a-plane GaN on m-plane sapphire and top-plane of a-plane GaN on γ-plane sapphire revealed the relative threading dislocation density and crystalline quality of each specific region, including the original templates or lateral overgrown layers.

3.6 References

[3.01]Lei, T., Ludwig, K.F., Jr and Moustakas, T. (1993) Journal of Applied Physiology, 74, 4430.

[3.02]Powell, R.C., Lee, N.E., Kim, Y.-W. and Green, J. (1993) Journal of Applied Physiology, 73, 189.

[3.03]Tansley, T.L., Goldys, E.M., Godlewski, M., Zhou, B. and Zuo, H.Y. (1997) The contribution of defects to the electrical and optical properties of GaN, in GaN and Related Materials (ed. S. Pearton), Gordon and Breach, Amsterdam, pp. 233–295.

[3.04]Leszczynski, M. (1999) Common Crystal Structure of the Group III-Nitrides (ed.

J.H. Edgar), IEE EMIS Data Review Series, No. 23, INSPEC, London, p. 6.

[3.05]Liu, L. and Edgar, J.H. (2002) Substratesfor gallium nitride epitaxy. Materials

Science & Engineering R: Reports, 37, 61–127.

[3.06]M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, S. P. DenBaars, (2002) Applied Physics Letter, 81, 469.

[3.07]D. N. Zakharov, Z. L. Weber, (2005) Physical Review B, 71, 235334.

[3.08]M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, S. P. DenBaars, (2002) Applied Physics Letter, 81, 1201.

Table 3.01. Crystallographic relationship between GaN films and sapphire substrates.

Figure 3.01. Projection of bulk basal plane sapphire and GaN cation positions for the observed epitaxial growth orientation. The circles mark Al atom positions and the dashed lines show the sapphire basal plane unit cells. The open circles mark the N-atom positions and solid lines show the GaN basal plane unit cell.

Figure 3.02. Projection of bulk a-plane sapphire and basal plane GaN cation positions for the observed epitaxial growth orientation. The solid circles mark the Al atom positions and the dashed lines show the sapphire a-plan unit cells. The open circles mark the N-atom positions and the solid lines show the GaN basal plane unit cell.

Figure 3.03. Schematics of crystallographic relationship between a-plane GaN and γ-plane sapphire.

Figure 3.04. Projection of bulk γ-plane sapphire and a-plane GaN anion and cation positions for the observed epitaxial growth orientation.

Figure 3.05. (a) Sapphire γ-plane stacking sequence showing O atoms in larger clear circles and Al atoms in smaller, filled circles. (b) The atomic arrangement on three layers (the uppermost one is O, immediately below is Al and third layer down is another O layer) on the γ-plane of sapphire.

Figure 3.06. Depiction of c-plane, a-plane, m-plane, γ-plane in GaN.

Figure 3.07. Trench-patterned m-plane sapphire substrate for a-plane GaN overgrowth.

Figure 3.08. The MOCVD growth procedure for a-plane GaN on m-plane sapphire.

Figure 3.09. Schematics of TELOG a-plane GaN technique.

Figure 3.10. The MOCVD growth procedure for a-plane GaN by TELOG technique.

Figure 3.11. Conditions of KOH etching of TELOG a-plane GaN and InGaN/GaN MQWs.

Chapter 4

Result I – Nonpolar a-Plane GaN on Patterned m-Plane Sapphire

4.1 The Epitaxial Mode and Morphology under Different Level of V/III Ratio

Te artificially-etched exposed symmetric c-plane vertical sidewalls and m-plane terraces of sapphire (as in figure 3.07) during MOCVD growth resulted in two kinds of GaN crystallites; each of the crystallite has unique growth orientation and terminal planes and outlines. First, on-axis XRD scans figure 4.01 indicated that pure a-plane GaN was carried out under the V/III ratio of 9000, 1800, and 350. Figure 4.02 ~4.04 showed that the selective area epitaxy of c-plane GaN was grown laterally only from the artificially-etched c-plane sapphire sidewalls. On the contrary, deposition of GaN was completely absent on the m-plane terraces. It’s confirmed^[4.01] experimentally that the crystallographic orientation relationships of GaN epitaxial stripes grown from c plane sidewalls of trench-patterned m-plane sapphire are (11-20)GaN∥(10-10)sapphire, [0001]GaN∥[0001]sapphire, and [10-10]GaN∥[11-20]sapphire.

The SEM images in figure 4.05~4.07 show clearly the representative cross-sectional morphology of GaN stripes selectively grown on trench-patterned m-plane sapphire. The shapes of GaN stripes were a strong function of the V/III ratio.

Figure 4.05 shows the sample carried out with a V/III ratio of 9000 for 30 min. GaN grew from the c-plane trench sidewalls only with faster growth rate along +c-direction

than that in a-direction, and therefore the upward planes are inclined by 58.5^° with respect to the trench sidewalls. Additionally, the upwardly inclined plane was tentatively attributed to (11-22) planes, since the growth front was constructed by the competition between +c-planes and a-planes of GaN.

Figure 4.06 revealed that when the V/III ratio was decreased to 1800, the growth mode is similar to that with a V/III ratio 9000 for 30 min. The reduction of V/III ratio led to a more conspicuous increase of growth rate in both +c and a direction of GaN which leads to the coalescent tip and level-raising respectively. As a matter of fact, the upwardly inclined (11-22) planes are maintained.

Next, a different growth mode was presented under a still lower V/III ratio, i.e.

350, with the same growth time. As in figure 4.07, the growth along the +c direction advanced overwhelmingly faster than that in a direction and was stymied by itself while these planes coalescence in +c direction. This formed the upwardly flat a-plane.

Something must be mentioned that the growth along the -c direction was almost stationary, no matter how much the V/III ratio vary, from 9000 to 350. Furthermore, the growth on the (10-10) sapphire terraces has been completely forbidden.

The schematic depictions of the shape of GaN stripes under different growth V/III ratio 9000, 1800 and 350 were in figure 4.08 ~ 4.10. As in these figures, the blue-color region are grown GaN. It depicts out that the lower III-V ratio leads to higher growth rate of +c-direction and suppresses the revelation of a-planes. In figure 4.10, the slow growth rate of GaN –c-direction is also depicted.

Figure 4.11 showed the dramatically-altered growth mode and crystal orientations by decreasing the value of V/III ratio to 72. The GaN grown laterally

(from c-plane sidewalls) and vertically (from m-plane terraces) were obtained simultaneously which revealed in figure 4.12. The GaN stripes grown from c-plane sapphire sidewalls performed similar behavior with that at V/III ratio of 350, but it played another role as the mask for the epitaxial lateral overgrowth (ELO) of GaN on the m-plane sapphire terraces. These well-defined in-situ-forming GaN-masks in [11-20]sapphire direction suppressing the growth of (10-11) GaN on m-plane sapphire terraces and leading to pure growth of (11-22) GaN, and the in-plane epitaxial relationships of (11-22) GaN on m-plane terraces are (11-22)GaN∥(10-10)sapphire, [10-10]GaN∥[1-210]sapphire, and [1-21-1]GaN∥[0001]sapphire[4.01]. It’s worth noting that the gradual growth of lateral-GaN limited the epitaxial lateral overgrowth of (11-22) GaN. As a result, the (11-22) GaN shows no noticeable wing-region over the lateral-GaN but it contains 32∘inclined +c-planes with respect to the upwardly flat (11-22) plane.

4.2 Defects Distribution and Clarification

Figure 4.13 shows the STEM image of GaN under a V/III ratio of 72 on a lower terrace/trench width ratio (2μm/4μm) substrate. The growth mode is equivalent to that of higher terrace/trench width ratio (i.e. 4μm/2μm); meanwhile, the stacking faults were localized in the GaN grown from the m-plane terraces. The orientation of stacking faults in the GaN on the m-plane terraces, completely parallel to the inclined (0001) GaN planes, are in close correlation with its growth rate along the +c-direction, which advances overwhelmingly faster than that in a-direction.

In figure 4.14, the bright field cross-sectional TEM image shows the distribution of stacking faults and threading dislocations in (11-22) GaN on m-plane

terraces and epitaxial lateral overgrowth (11-22) GaN, respectively. By correlating the growth mechanism in section 4.1 for the V/III ratio of 72 with the transmission electron microscopy observations, the reasons of defects formation are attributed to atomic-rearrangement-related deposition and strain.

The stacking faults in (11-22) GaN on m-plane sapphire terraces are caused by the growth front which advances too fast to completely fill each atomic layer parallel to (0001) plane of GaN, and thus leads to a series of stacking faults parallel to each other in (0001) plane. Figure 4.15 and figure 4.16 show the dark field cross-sectional TEM images with different g vectors. The visibility of specific type of stacking faults is different under these two kinds of g vector. However, it needs more careful experiments and observations to determine the type of stacking faults exactly.

The epitaxial lateral overgrowth (11-22) GaN region in figure 4.14 showed some threading dislocations started from the local region near the corner-edge of m-plane terraces and penetrated directly through the (11-22) GaN bulk layer to the surface. The distribution of these threading dislocations did not spread far into the (11-22) GaN bulk layer in [0001] direction. In this observation, we concluded here that the strain field introduced by the interaction the epitaxial lateral overgrowth (ELO) of GaN stripes from c-plane sapphire sidewalls and the epitaxial growth of GaN on the m-plane sapphire terraces. Fortunately, the slow growth rate of epitaxial lateral overgrowth of GaN along –c-direction prevented the distribution of threading dislocations extending into the lateral overgrowth region. Here the slow growth rate can be regarded as smaller interaction forces between ELO-GaN and GaN stripes from c-plane sapphire and led to the confined strain field.

Figure 4.17 depicted out the cross-sectional cathodoluminescence image of the

epitaxial lateral overgrowth (ELO) of GaN stripes from c-plane sapphire sidewalls.

By correlating the mechanism of cathodoluminescence spectroscopy and the crystalline quality, the bright, clear light region represents the high crystalline quality of epitaxial lateral overgrowth region of GaN. Obviously, the infinitesimal region which contained threading dislocations and stacking faults do almost no effect on the crystalline quality of lateral overgrowth region of GaN.

Apparently, the narrower terraces did effectively restrain the volume of semipolar (11-22) GaN growing from m-plane sapphire terraces; however, the growth of (11-22) GaN cannot be eliminated by decreasing width of m-plane terraces. To summarize, the growth mode of GaN on trench-patterned m-plane sapphire is strongly depends on the V/III ratio during MOCVD growth rather than the terrace/trench width ratio of the patterned substrate.

4.3 Summary

In this chapter the growth mechanism and epitaxial mode of a-plane GaN on trench-patterned m-plane sapphire by MOCVD under different V/III ratio was demonstrated. For V/III ratio from 350 up to 9000, the epitaxial lateral growth of GaN from the +c-plane sidewalls and forming high crystalline quality a-plane GaN is dominated. Under low level V/III ratio of 72, the growth of lateral-GaN accompanied the aroused (11-22) GaN on m-plane sapphire terraces. This unexpected appearance of semipoalr (11-22) GaN can be suppressed by optimizing the V/III ratio. On the other hand, lower terrace/trench width ratio doesn’t interdict it. By optimizing the level of V/III ratio during MOCVD growth and appropriate terrace/trench width ratio of patterned m-plane sapphire substrate, we expect that a high crystalline quality

nonpolar a-plane GaN epilayer on m-plane sapphire is achievable.

4.4 References

[4.01] X. Ni, et al. Appl. Phys. Lett.90, (2007) 182109.

Figure 4.01. XRD θ-2θ spectrum of a-plane epitaxy on trench-patterned m-plane sapphire.

Figure 4.02. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 9000.

Figure 4.03. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 1800.

Figure 4.04. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 350.

Figure 4.05. The SEM images a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 9000. The corresponding crystal planes and orientations are depicted.

Figure 4.06. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 1800. The corresponding crystal planes and orientations are depicted.

Figure 4.07. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 350. The corresponding crystal planes and orientations are depicted.

Figure 4.08. Schematics of the epitaxial mode of a-plane GaN on m-plane sapphire under the V/III ratio of 9000 by MOCVD.

Figure 4.09. Schematics of the epitaxial mode of a-plane GaN on m-plane sapphire under the V/III ratio of 1800 by MOCVD.

Figure 4.10. Schematics of the epitaxial mode of a-plane GaN on m-plane sapphire under the V/III ratio of 350 by MOCVD.

Figure 4.11. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 72.

Figure 4.12. The SEM images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 72. The corresponding crystal planes and orientations are depicted.

Figure 4.13. The cross-sectional TEM image of GaN grown by MOCVD under the V/III ratio of 72 with terrace/trench width 2μm/4μm. The corresponding crystal planes and orientations are depicted.

Figure 4.14. The cross-sectional TEM image of GaN grown by MOCVD under the V/III ratio of 72 with terrace/trench width 2μm/4μm. The corresponding crystal planes, orientations and defects distribution are depicted.

Figure 4.15. The cross-sectional TEM image of GaN grown by MOCVD under the V/III ratio of 72 with terrace/trench width 2μm/4μm. The corresponding g vector is along [0001] direction.

Figure 4.16. The cross-sectional TEM image of GaN grown by MOCVD under the V/III ratio of 72 with terrace/trench width 2μm/4μm. The corresponding g vector is along [-2110] direction.

Figure 4.17. The CL images of a-plane GaN grown by MOCVD on trench-patterned m-plane sapphire under the V/III ratio of 1800.

Chapter 5

Result II –Trenched epitaxial lateral overgrowth (TELOG) a-Plane GaN LED on γ-Plane Sapphire

5.1 The Epitaxial Mode and Morphology

The preparation process flow of the patterned a-plane GaN template on γ-plane sapphire for high crystalline quality TELOG a-plane GaN is shown in Figure 3.09.

While the stripe-patterned a-plane template GaN was ready for further regrowth, growth with MOCVD under different level of V/III ratio was introduced in order to achieve the smooth coalescence surface.

Under the suggestion from the experiments and results in Chapter 4, we got the possible optimal growth conditions of a-plane GaN grown on γ-plane sapphire, i.e. the lower V/III ratio which leads the faster advancing rate of the growth front in c-direction. To observe the growth mechanism in detail, the MOCVD regrowth was stopped before the complete coalescence of the a-plane GaN films. The SEM images of cross-sectional TELOG a-plane GaN by MOCVD under different level of V/III ratio and at different growth time were shown in figure 5.01~ 5.03. In Figure 5.01 and 5.02, the MOCVD regrowth conditions were controlled at V/III ratio of 500, 980^°C, 30 minutes and 120 minutes respectively. Under such conditions, the advancing rate of the growth front in a-direction is comparable to that of +c-direction while the advancing rate of the growth front in –c-direction is scarcely comparable to them. The

characterization of advancing rate of each direction can be done by dividing the length extending out of the seed-region by growth time, i.e. 30 minutes in figure 5.01. The divergence of these growth rates seems quite indifferent to the final morphology, however, as to the longer growth time, the unexpected outcome shows.

Figure 5.02 depicts out the unexpected deep-extending coalescence boundary and accompanies the stripe-like voids at the end of the coalescence boundary on the terminal surface. These defects do actually affect the further epitaxial growth of n-GaN, superlattices, MQWs, and p-GaN and lead to unrecoverable results. Thus the growth condition was tuned to a lower level of V/III to achieve a completely coalescence GaN with smooth terminal surfaces.

The regrowth result is shown in figure 5.03, where the epitaxial lateral growth GaN was completely coalescence and performed a perfect thickness without deep-extended boundaries and stripe-like voids. The formation of the stick-like shaped voids is due to the fast advancing rate of +c-direction of GaN and the quickly shrinking of the space between that growth front and the original seed-wall. The small crevice prevents the sources of MOCVD from diffusing into the void the thus leaves it at the bottom of the thick GaN epitaxial layer. The dimension of the residual voids are approximately 3μm in height and 300~400nm in width. These stick-like shaped voids showed no tremendous to the optical properties enhancement for its small dimension in width and far away from the position of the MWQs. However, the size of these voids is large enough to allow the chemical solution to penetrate deeply into it and perform some specific etching effects and enhancement to the optical properties of the MQWs. The related results are provided in chapter 6 here of this thesis.

Additionally, the morphology of the terminal surface of the TELOG a-plane GaN

revealed some information about the growth mode, crystalline quality and most of all, the defects distribution. As can be seen in figure 5.04, terminal surface of the seed region are much rougher than that of the wing region. The differences of roughness lead the contrast of the surface image which does a great help in distinguishing the pattern direction after the completely coalescence regrowth.

By correlating the transmission electron microscopy observations with the terminal surface morphology, the distribution of dislocations explains the relationship.

In figure 5.05, the dislocations are initiated from the seed region, the original a-plane GaN template on γ-plane sapphire, and extended upward toward the terminal surface.

This would be the main cause of the formation of surface morphology in figure 5.04.

The wing regions, in contrast, are almost free of dislocations and perform a perfect crystalline quality. The nature of epitaxial lateral growth of these regions leads to the smooth terminal surface and extremely low dislocation density.

The specific optical properties of the TELOG a-plane GaN and InGaN/GaN MQWs are seriously affected by the in-situ formed surface morphology characteristics. The corresponsive data and discussions are available in this chapter at section 5.3.

As for the coalescence behavior that cause the void in TELOG a-plane GaN, the defect here be named as “coalescence boundary” in figure 5.06 telling the complete story. The epitaxial lateral growth rates of GaN along +c and –c direction are fast and slow respectively, and lead to the wider wing region and a narrower one. The planes of each growth front met together and heal over the terminal surface. The slight difference of GaN rotation angle from c-plane sapphire sidewall cause the mismatch of lattice structure and forming the so called “coalescence boundary”. The

coalescence boundary extends from the pinnacle of the voids and penetrates the GaN layer with the ends at the terminal surface. The existence of the coalescence boundary acts as the indication of the +c/–c polarity of the growth front and becomes an important index while the TELOG a-plane GaN was etched in KOH-ethylene glycol chemical solution. Moreover, the coalescence boundary, a relatively weak region, would play a special role in the whole KOH-ethylene glycol chemical solution etching

coalescence boundary extends from the pinnacle of the voids and penetrates the GaN layer with the ends at the terminal surface. The existence of the coalescence boundary acts as the indication of the +c/–c polarity of the growth front and becomes an important index while the TELOG a-plane GaN was etched in KOH-ethylene glycol chemical solution. Moreover, the coalescence boundary, a relatively weak region, would play a special role in the whole KOH-ethylene glycol chemical solution etching