A-plane sapphire

The structure is bonded to a substrate 24 which is chosen to have a coefficient of thermal expansion that is selected for providing the resultant read-out chip assembly with an effective coefficient of thermal expansion that is approximately the same as an HgCdTe detector array 36. The substrate material may be GaAs (4.5-5.9 x 10"6 m/mK), CdTe, Ge (5.5-6.4 x 10"6 m/mK), and a-plane sapphire (3.5-7 5 x 10" m/mK) where the coefficients of thermal expansion are given in parentheses. The coefficients of thermal expansion for silicon, HgCdTe and epoxy are 1.2 x 10"6 m/mK, 3.8-4.5 x 1 O 6m/mK and 30-50 x 10"6 m/mK, respectively. Next, the substrate 16 is removed and aluminium pads 34a are formed. Indium bumps 34b are cold welded to corresponding indium bumps 36b. 

Fig. 7.10. Maximum, root mean square, and average roughness Amax, Rq, and fta, respectively (derived from 10 X 10 pm2 AFM scans) of 300 nm thick ZnMgO films (PLD from a ZnO 4wt. %MgO target) on a-plane sapphire in dependence on oxygen partial pressure during PLD growth. Lowest film surface roughness is obtained around 10 3 mbar. The lines are drawn to guide the eye. Measured by G. Zimmermann...

Fig. 7.11. AFM scan images (10x10pm2) of roughness optimized ZnO (top, 1.4 tm thick), ZnO 4wt. %MgO (center, 250nm thick on ZnO buffer), and MgO (bottom, 400 nm thick) films on a-plane sapphire with average roughness f a of 1.45, 0.41, and 1.23 nm, respectively. Measured by G. Zimmermann...

Fig. 7.12. AFM scan images (10 x 10 pm2) of mobility optimized, 1.4 pm thick ZnO thin films on a-plane sapphire. The Hall mobility at 300 K correlates with the grain size. The average roughness and the carrier concentration are 12.5mm/1.6 x 1016 cm 3 (top) and 8.38nm/3.1 x 1016 cm 3 (bottom), respectively. Measured by G. Zimmermann...

Fig. 7.18. Temperature dependence of the Hall mobility (top) and of the carrier concentration (bottom) of undoped PLD ZnO thin films on a-plane sapphire grown at different oxygen partial pressures (see legends). Note the different temperature scales. The film grown at highest pressure shows an unusual metal-like temperature dependence of the carrier concentration for T < 90K. By H. von Wenckstern [59]...

Fig. 7.19. Photoluminescence spectra (2K) of PLD ZnO thin films on a-plane, c-plane, and r-plane sapphire substrates [63]. All films were grown at about 650°C and at 1.6 x 10 2 mbar oxygen pressure. The FWHM of the most intense donor bound exciton peaks D°X of the ZnO films are 1.4 meV on a-plane sapphire, 1.7 meV on c-plane sapphire, and 2.6 meV on r-plane sapphire. The spectral resolution of the PL setup was 1 meV at 3.35 eV...

Figure 7.19 shows PL spectra recorded at 2K for 2.2, 0.7, and 1.5 pm thick PLD ZnO films on a-plane, c-plane, and r-plane sapphire, respectively [63], The full widths at half maximum (FWHM) of the most intense bound exciton peaks are 1.4, 1.7, and 2.6 meV for a-, c-, and r-sapphire, respectively. The film on a-plane sapphire shows the narrowest FWHM among the films under investigation and the free A-exciton (Xa) is most clearly resolved, thus indicating best structural properties of ZnO on a-plane sapphire. The ZnO films on a- and c-plane sapphire grow c-axis textured, whereas films on r-plane sapphire grow a-axis oriented with the ZnO c-axis being in-plane, as demonstrated already in Fig. 7.4. The PL spectrum of the film on r-plane sapphire shows no phonon replica, probably due to the changed ZnO orientation.

Fig. 7.20. Comparison of PL spectra at 2 K of a 2.2 /um thick ZnO film on a-plane sapphire and of a ZnO bulk single crystal grown by seeded chemical vapor deposition (Eagle Picher), both (0001) oriented. The PLD film was deposited by a 4-step PLD process [51] and shows a PL spectrum very similar to that of the single crystal. The energies of the assigned luminescence peaks are given in [eV]. The spectral resolution of the PL setup is 1 meV at 3.35 eV [63]...

Fig. 7.21. CL line scan at 9 K of the intensity of the bound exciton transitions 16 to Ig measured at the cross-section of a 2.2 pm thick ZnO film on a-plane sapphire from the sapphire substrate to the ZnO film surface. The inset shows the corresponding SEM image of the cross section. Measured by J. Lenzner...

Table 7.6. Comparison of band gap energies Es, exciton binding energies Exb, and 1LO and 2LO phonon energies hwilo and Ri lo, calculated from the PL peak energies of a ZnO single crystal (Eagle Picher) and a PLD ZnO thin film on a-plane sapphire at 2 K, as given in Fig. 7.20 [63]...

Fig. 7.25. CL intensity, Hall mobility, and carrier concentration at 300K of PLD ZnO thin films on a-plane sapphire show maxima around 1 mbar background gas pressure of O2, N2O, and N2 during growth at 100 mm target substrate distance, indicating the growth condition for ZnO thin film scintillators [89]. The average surface roughness is considerably increased at the high growth pressure of 1 mbar...

Fig. 7.27. SEM images of ZnO thin films on a-plane sapphire grown in 1 mbar oxygen. Both films show very high UV CL intensity, but very different film morphology due to different growth conditions [90]...

Fig. 7.33. Typical SEM images of PLD grown ZnO nanowires (lOOmbar Ar, 840° C) on a-plane sapphire with Au colloides as nucleation sites [142]...

The shift of the A line in the epilayers has been connected with the variation of the lattice parameters of GaN [1,11,12], The shift of this line was also measured in samples subjected to hydrostatic pressure (see Datareview A3.1). Combination of all these data permits one to obtain the whole series of excitonic deformation potentials [6,16], Two sets of data are available which are consistent with each other and are given in TABLE 1. The discrepancies between them are linked to the differences in the values of the stiflhess coefficients of GaN used by the authors. Gil and Alemu [6] in their work subsequent to the work of Shan et al [16] used data not available when Shan et al calculated their values. The notations are the same and are linked to the relationship with the quasi cubic model of Pikus and Bir [17], Deformation potentials as and a6 have been obtained by Alemu et al [8] who studied the anisotropy of the optical response in the growth plane of GaN epilayers orthorhombically distorted by growth on A-plane sapphire. For a detailed presentation of the theoretical values of deformation potentials of GaN we refer the reader to Suzuki and Uenoyama [20] who took the old values of the stiflhess coefficients of GaN [21]. 

Growth on (1120) A-plane sapphire yields an epitaxial relationship of (0001)g,n//(1 120)ai2O3 with inplane orientations of [1120]g.n//[0001]Ai2O3 and [1120]g,n//[1120]ai2O3 [2,4], This results in approximately a 30% lattice mismatch for both in-plane orientations by comparing the oxygen sublattice distance with the GaN lattice. 

Gomez-De Arco L, Lei B, Cronin BS, Zhou C (2008) Resonant micro-Raman spectroscopy of aligned single-walled carbon nanotubes on a-plane sapphire. Appl Phys Lett 93 123112 Jorio A, Pimenta MA, Souza Filho AG, Saito R, Dresselhaus G, Dresselhaus MS (2003) Characterizing carbon nanotube samples with resonance Raman scattering. New J Phys 5 139.1-139.17... 

Plot of grain boundary displacement versus anneal time for growth of a-plane sapphire seeds into 500 ppm Ti-doped and undoped aluminas at 1600°C. In comparison to results obtained previously with a lower purity powder, mobilities in undoped material are increased. However, the most striking feature is the increased migration rate for the Ti-doped material. 

In this chapter, we review recent studies of optical phonons in relation to the anisotropic strain in GaN heteroepitaxial layers and quantum dots (QDs) with nonpolar orientations. For reasons of comparison, results on anisotropically strained c-plane GaN films grown on a-plane sapphire is also presented where appropriate. The application of GIRSE and Raman scattering to the studies of optical phonon frequencies of anisotropic wurtzite GaN is described to reveal the phonon mode behavior. The assessment of anisotropic strain components in a-plane GaN films and their evolution with thickness... 

Further, the presence of anisotropic distortion of the basal plane of a-plane wurtzite layers, [s 7 Sy ), will lift-off the degeneracy of the x and e Sy. Therefore, the IR dielectric functions Sx and Sy provide access to the frequencies and broadening parameters of the TO and LO phonons with Ei symmetry polarized along the x = [1120] and y = [1100] directions. In other words, the splitting of the TO and LO with Ei symmetry that is predicted theoretically by Equation 9 [14,16] can be obtained from the IR eUipsometry data analysis. Note, that polar c-plane GaN heteroepitaxial layers that experience anisotropic distortion of the basal plane, for instance when grown on a-plane sapphire [29] will also allow assessment of the Ei phonon splitting [17, 18]. In this case, the optical measurement will depend on the orientation of the plane of incidence and incident polarization with respect to the two in-plane directions X = [1120] and y = [1100]. The standard eUipsometry measurement for non-c-plane-oriented and anisotropically strained wurtzite crystals is inapplicable and the generalized eUipsometry approach is needed. 

Figure 9.15 Experimental (dots) and calculated (solid lines) GIRSE spectra of a polar c-plane GaN film on a-plane sapphire for different angles between the plane of incidence and the GaN [1120] direction ...

Splitting of the E2 phonon have been also observed for anisotropically strained c-plane films on a-plane sapphire [17, 18]. A subtle difference of 0.5 cm in the E2 frequency was determined when measuring in z(xx)z and z(yx)z configurations. This is in agreement with the theoretical prediction... 

Kato et al. [163] and Iwata et al. [164] have performed similar studies on ZnO heteroepitaxial layers grown using plasma-assisted MBE and radical-source MBE techniques. Kato et al. [163] used (112 0)a-plane sapphire substrates and high-temperature growth with low-temperature buffer layers for high-quality undoped ZnO epitaxial films. They obtained electron mobilities as high as 120 cm s and... 

Figure 2.22 Schematic diagram of atom positions for basal ZnO grown on a-plane sapphire. The dots mark the O-atom positions and the dashed lines show the sapphire a-plane unit cells. The open circles markZn-atom positions and the solid lines show the ZnO basal-plane unit cell.

Figure2.23 Stereographic projections ofX-ray poie figure results fortheZnO (10 11) reflection for a ZnO film grown on (a) (inset) c-plane sapphire substrate and (b) a-plane sapphire substrate.

---