Discussion

These experimental observations clearly indicate that high-temperature growth of the BexZn1−xO films on a highly-mismatched substrate causes: (i) Be composi-

tional fluctuation; (ii) phase separation; and (iii) increased film mosaicity. These are all associated with the atomic movement of Be and strain relaxation at different

Tg. It is worth noting the different types of strain induced by the different lattice mismatch of ZnO/BeO and between the alloy film and underlying substrate. As a result, during the growth, it is expected that the interface region has more elastic strain energy than the upper layers of the film. With such an elastic strain distribu- tion near the interface, Be atoms tend to move up into the more relaxed regions by a compositional pulling effect [198]. Subsequently, less Be composition can be present in the interface region, giving rise to the formation of an undoped ZnO-like interface. As Tg increases, this relaxation process is promoted by providing more energy for Be movement. As a result, the Be composition in the film is divided roughly into Be- depleted and Be-rich regions in the BexZn1−xO alloy films grown at Tg ≥ 600 ◦C. Further relaxation with higherTg can lead to more mosaic spread in the alloy films. In addition, thermally-driven Be atoms moving to more relaxed areas are randomly dissociated from the alloy lattices. This causes increasing structural deterioration in the alloy film. Based on further investigation presented in Chapters 6 and 7, it was found that the dissociation/diffusion of Be in BexZn1−xO alloy films is induced by

thermal annealing of overT = 600◦_{C [101]. Klingshirn et al. [58, 59] predicted the}

diffusion of Be and the resulting segregation in local areas of BexZn1−xO/ZnO-based

QW structures. Hence, we suggest that the compositional inhomogeneity and phase separation of the BexZn1−xO alloys originate by the coupling of strain relaxation

5.4 Conclusion

In summary, the influence of the growth temperature on the formation of crystalline BexZn1−xO films grown onc-Al2O3substrates has been investigated using synchrotron XRD, TEM, and PL measurements. The XRD and HRTEM results show that a single-phase BexZn1−xO alloy film with the highest Be composition,

x = 0.25, was obtained atTg = 400◦C. ForTg ≥600◦C, phase separation into Zn- rich and a Be-rich phases were observed together with compositional inhomogeneity of Be in the alloy films. This corresponds to the XRD BeZnO(0002) peak splitting and broad PL emission bands. These results are attributed to a compositional pulling or gradient effect and wider distribution of mosaic structure in the high- temperature-grown films. The structural fluctuation is caused by strain relaxation in conjunction with thermal displacement of Be during the high-temperature growth of the BexZn1−xO alloy films on the highly-mismatched substrate.

Formation of a Degenerate Interface in Highly-

Mismatched BZO

6.1 Introduction

Wide band gap ZnO-based materials have many versatile properties such as large exciton binding energy (60 meV), unintentional n-type conductivity, high transparency in the near ultra-violet and visible ranges, and the high natural abun- dance) [4–8, 33, 58, 199]. These materials also provide opportunities for the fab- rication of many optoelectronic devices such as laser diodes (LDs), light emitting diodes (LEDs), high electron mobility transistors (HEMTs), UV sensors, and their nanoscale structures in device applications [3, 5, 30, 153, 200]. However, there are persistent issues in using ZnO-based materials which include: conversion to p-type conductivity due to the position of the charge neutrality level (CNL), i.e., the en- ergy at which defect states change from donor-type to acceptor-type with reference to the Fermi level, being above conduction band minimum; thermal instability at high temperature (above 600◦_{C); and the distribution of thermally-created defects}

such as interstitial Zn (Zni), Zn vacancy (VZn), interstitial oxygen (Oi), and oxygen vacancy (VO) [119, 201, 202]. The formation of such intrinsic defects, which creates states in the band gap of ZnO, strongly depends on the experimental conditions, namely, thermodynamic equilibrium [8].

BexZn1−xO ternary alloys have also been considered as promising candidate

for the effective band gap engineering of ZnO [183]. This is because both ZnO and BeO possess wurtzite (WZ) crystal structures with a large difference in band gap energy. However, highly different physiochemical nature (e.g.atomic size, elec- tronegativity, and formation enthalpy) of constituent atoms in the alloy system and mechanical stress of their heterostructures readily induce phase fluctuation un-

der thermodynamic equilibrium. Therefore, thermodynamic phase stabilization of highly mismatched alloys causes phase separation and the formation of multi-phase crystallinity [27, 169, 190, 203, 204]. Understanding the thermodynamics of Be and following phase transitions in the alloy system is essential to control such solid-state reactions and the resulting physical and chemical properties of the material.

In this chapter, the influence of thermal treatment on an inhomogeneous BexZn1−xO (BZO) film grown on Al2O3(0001) and annealed at temperatures (TA) up to 950 ◦_{C is explored. Thermal annealing induces significant recrystallization}

via strain relaxation and atomic redistribution to form a multi-phase crystalline film. This results in the considerable micro-segregation and mass transport of BeO and ZnO phases within the film combined with the formation of a crystalline ZnO interface layer. X-ray photoemission spectroscopy (XPS) along with infrared (IR) reflectance measurements and simulations clearly show the out-diffusion of Be atoms and precipitation of lattice point defects in the BZO films with increasingTA. Highly conductive layers emerge at the BZO/Al2O3 interface for TA ≥ 800 ◦C. As a con- sequence, carrier concentration, conductivity, and density-of-state averaged effec- tive masses of the degenerate interfaces are separately determined by applying a two-layer model for the high-temperature annealed BZO films. Accumulation of Be (Zn) at the surface (interface) is addressed by the migration of cations during high-temperature annealing based on a counter-diffusion mechanism to elucidate the formation of the highly degenerate interface layer in the alloy film.