Changes in Surface Morphology and Photoluminescence Spectrum during Photoelectrochemical Etching of ( 0001 ) nGaN on Sapphire

Surface morphological and 10K photoluminescence (PL) spectral changes during photoelectrochemical (PEC) etching for (0001) face of n-GaN on sapphire are reported. The etching is initiated from the step edges and the etch pit density increases to 5×10 cm−2 which is larger than the total dislocation density of GaN sample. Subsequently, the neighboring etch pits are connected to form triangular hillocks which are surrounded by {112̄0} facets. Then the whisker structure is formed by way of mesh morphology. The whisker density is nearly equal to the dislocation density including edge dislocation (2×10 cm−2). The donor-bound exciton peak of as-grown n-GaN redshifts 24 meV by the whisker formation, indicating the relaxation of compressive strain and the contribution of deeper impurity bound excitons. The broad peaks of lower energies than 3.35 eV are enhanced by the whisker formation. [DOI: 10.1380/ejssnt.2010.392]


I. INTRODUCTION
GaN and related materials are attractive for high temperature and robust electron and optoelectronic devices because GaN has the properties of wide (3.4 eV at room temperature) and direct bandgap, high electron mobility, and high thermal and chemical stability.The photoelectrochemical (PEC) etching is a wet method based on the dissolution of the semiconductor layer during the photoexcitation in basic solution [1].It has been reported that the PEC etching of (0001) face of GaN on sapphire gives a high density (10 9 cm −2 ) whisker with a diameter of several tens of nm and each whisker has a dislocation [2].Recently we applied the PEC etching to increase the surface area of GaN by roughening in fabricating GaN based dye-sensitized solar cell [3].Compared to the flat as-grown surface, the surface area increased by about 10 2 times by PEC etching.
In this study, the surface morphological change from the as-grown surface to whisker during PEC etching is observed by atomic force microscopy (AFM).In addition, 10K photoluminescence (PL) spectral change is also reported.

II. EXPERIMENTAL
The 2 µm thick Si doped n-GaN were grown on the 2 µm thick undoped GaN by metalorganic vapor phase epitaxy (MOVPE) on (0001) sapphire substrate.The carrier concentration was 3×10 18 cm −3 .After depositing Al/Au ohmic contact on n-GaN, the PEC etching was performed in 0.02 M KOH aqueous solution under the irradiation of 365 nm light from Hg-Xe lamp filtered by 10% CuSO 4 aqueous solution.The intensity of excitation light was changed from 1.5 mWcm −2 to 20 mWcm −2 .
The diameter of spot size was 4 mm.The etching periods were 1 min at 1.5 mWcm −2 first, followed by 1 min at 11 mWcm −2 , 20 min at 20 mWcm −2 , 10 min at 20 mWcm −2 and 90 min at 20 mWcm −2 .The surface morphology for GaN surface of each period was observed by the tapping mode of AFM (Nano Scope IIIa).The PL measurement was done at 10K by the excitation of He-Cd laser (325 nm and 0.6 Wcm −2 ).The spectrum was measured by the optical fiber and photodiode array (Ocean Photonics HR2000 with a resolution of 0.8 nm).

III. RESULTS AND DISCUSSION
Figure 1 shows the typical photocurrent profile with the time of PEC etching at the excitation of 20 mWcm −2 .Integrated electron charge calculated from photocurrent was also shown in the right-hand axis.Totally 4 µm thick GaN layer including the 2 µm thick n-GaN and the 2 µm thick undoped GaN was almost etched to leave broken whiskers and high-resistive thin GaN layer on sapphire around 80 min etching.
Figure 2 shows the AFM images of the surface for as-  corresponds to the total thickness of GaN including 2 µm thick n-GaN and 2 µm thick undoped GaN.Almost GaN whiskers are nearly free standing on sapphire by relaxing the compressive strain of GaN layer which is caused by the difference of thermal expansion coefficients between GaN and sapphire.
Figure 4 plotted the densities of growth and etch pit, triangular hillock, mesh, and whisker, which were observed during PEC etching of GaN, as a function of spent charge amount.At the initial stage, etch pit density (5×10 10 cm −2 ) was larger than the dislocation density including edge dislocations (2×10 9 cm −2 ).This result indicates the formation of etch pit occurs independently of dislocation.Then the three etch pits are connected to form the triangular hillock, resulting in a decrease by about one third in hillock density.Finally the density of whisker approaches the as-grown growth pit density which is the density of the screw and mixed dislocations.Therefore, as so far reported [2,4], the holes produced in the whisker by the excitation during PEC etching are trapped around dislocations and do not contribute to further etching.As a result, a dislocation remains in a whisker.
Figure 5 shows the comparison of PL spectrum at 10K for as-grown n-GaN and free standing whiskers.In the PL spectrum of as-grown n-GaN, the I 2 peak at 3.47 eV by the shallow donor-bound excitons is dominant and weak Y 2 peak at 3.4 eV and Y 6 peak at 3.3 eV can be seen.The origins of Y 2 and Y 6 peaks are considered as the exciton bound to the structural defect [5].In the PL spectrum of free standing GaN whiskers, the band edge peak redshifted about 24 meV and Y 6 peak around 3.29 eV was clearly observed.Figure 6 shows the plot of redshift energy for the band edge peak and the intensity ratio of band edge peak to Y 6 peak during PEC etching, as a function of spent charge amount.With the increase in spent charge amount, the redshift energy increased.It is reported for GaN nanocolumns that the donor-bound exciton peaks are located at 3.4-3.45eV.These GaN nanocolumns are grown on Si substrate and are not separated from Si substrate.Therefore, their peak energies change with the degree of tensile strain depending on the growth condition [6].In the present study, the redshift is reasonable for the band-edge peak of GaN whisker compared with that of as-grown n-GaN because the GaN whiskers are separated from the sapphire substrate and are free from the biaxial compressive strain caused by the difference of thermal expansion coefficient between GaN and the (0001) sapphire substrate [7,8].However, the observed redshift of about 24 meV is larger than the 15 meV estimated from the relaxation of biaxial compressive strain.Furthermore, the broader band-edge peak observed in the GaN whisker possibly suggests deeper donor levels related to surface defects such as nitrogen vacancy in the bound exciton peak, because the surface area in the whisker becomes larger than that in the as-grown GaN.In addition, the intensity ratio of Y 6 peak increases by the whisker formation.This is due to the trapping of excitons to the structural defects formed in the neighborhood of dislocation and/or dislocation itself in a smaller volume of whisker compared to the GaN layer.

IV. CONCLUSIONS
We studied the surface morphological change and 10K PL spectral change from the as-grown surface to whisker during PEC etching.For the as-grown surface, growth pits located at the step-crossing and step-terminating positions were observed with the density of 5×10 8 cm −2 .These pits are related to the screw and mixed dislocaitons.For the initial stage etching surface, the etching was initiated from the step edges and the etch pit density increased to 5×10 10 cm −2 , which is larger than the total dislocation density of GaN sample.The etching occurs independently of dislocation.For the etched surface, the three etch pits are connected to form the triangular hillock which are surrounded by {11 20} facets.The mesh morphology is formed by deeping holes surrounded by triangular hillocks.The whisker density is nearly equal to the dislocation density including edge dislocation and finally becomes nearly the screw and mixed dislocation density.For the PL spectrum at 10K, the band edge exciton peak redshifted 24 meV and Y 6 peak was clearly observed by the whisker formation.The 24 meV peak shift is larger than the 15 meV estimated from the relaxation of biaxial compressive strain caused by the difference of thermal expansion coefficient between GaN and sapphire substrate.This indicates the band edge peak of free standing whisker originates not from shallow donor bound excitons but from deeper impurity bound excitons.With the increase in spent charge amount, the intensity ratio of Y 6 peak to band edge peak increased.This shows the trapping of excitons to the structural defects formed in the neighborhood of dislocation and/or dislocation itself increases in a small volume of whisker compared to the GaN layer.
FIG.4: Densities of growth and etch pit, triangular hillock, mesh, and whisker, observed during PEC etching of GaN, as a function of spent charge amount.

FIG. 6 :
FIG.6: Plots of redshift energy for band edge peak and the intensity ratio of band edge peak to Y6 peak during PEC etching, as a function of spent charge amount.