000002175324.pdf - KMOU Repository

수소화물 증기상에서 에피택시를 사용하여 자립형 GaN 박막 성장에 대한 조사. 본 논문에서는 ZnO를 템플릿 층으로 사용하여 사파이어 기판 위에 III-V족 화합물 반도체인 GaN 물질을 성장시키고, 구조적 및 광학적 조사를 수행하였다. 고품질 GaN 기판의 성장은 이러한 재료에 의해 제한됩니다.

2장에서는 본 연구에서 제작된 독립형 GaN 기판을 성장시키고 평가한다. V/III) 변형에 의한 GaN 성장에 적합한 구조적 및 광학적 성장 조건. 그리고 5장에서는 ZnO의 극성을 이용한 화학적 식각을 통해 독립형 GaN 기판을 제작하였고, 3장과 4장에서 GaN의 성장 조건을 확인하였고, 마지막으로 6장에서 확인하였다.

Chapter1. Introduction

Sapphire is the most commonly used substrate for epitaxial growth of the III nitrides as described in the previous section. Aksaki grew the first smooth surfaces of GaN films in 1986, demonstrating that two-dimensional nucleation in nitride epitaxy could be achieved, thereby significantly improving the electrical and luminescent properties of the films. The following figure 1.3 shows the effect of load and the growth surface on the direction of the spontaneous and piezoelectric polarization.

In particular, self-doping from the back side of the GaAs substrate has been a serious problem for GaN growth [37]. According to SAG, the side faces are covered with SiO2 except the top part as shown in FIG. 1.6 (b). A thick layer of GaN grown by HVPE can be spontaneously separated from the substrate by applying compressive stress concentration on the seeds due to purposely formed voids.

While the other approaches require complex processes such as deposition of mask materials and growth of GaN template before growth of thick-GaN with HVPE. And based on the stability of ZnO with different polarity should also be checked out.

FIGURE 1.1 Bandgap and chemical bond lengths of compound semiconductors that emit in the visible range of the electromagnetic spectrum

Chapter2. Experimental

The HCl and carrier gases flowing through the tubes, the materials inside the tube, and the temperature at the reaction point determine the reaction products. The final reaction leaves behind the desired material, which is deposited on a substrate that is usually rotated at the end of the tube. The temperature at the Ga source zone and the GaN growth zone for the reaction was determined to be 850°C and 1040°C, respectively, as shown in Fig.

If the transferred energy exceeds the material's work function, the emitted electron can leave the solid. When the energy of the emitted electron is less than approx. 50 eV, it is referred to by convention as a secondary electron (SE) or simply a secondary. Most of the emitted secondaries are produced much deeper in the material, and suffer additional inelastic collisions, which lower their energy and trap them in the interior of the solid.

In most currently available SEMs, the energy of the primary electron beam can range from a few hundred eV up to 30 keV. The values of δ and η will change during this burst, but it produces photomicrographs that can vary in appearance and information content as the energy of the primary beam changes.[1]. For the X-ray scans, a beam of parallel and monochromatic X-rays of wavelength λ is incident on a crystal at an angle θB, Bragg angle, which is measured between the direction of the incident beam and the crystal plane in question.

For asymmetric (hkl) Bragg reflections, ω-2θ scan direction also runs radially from the origin (000) of the reciprocal space along Ghkl (FIG. 2.3a) (ii) In the ω scan, the detector is fixed in position with wide open entrance slits and the sample is rotated, i.e. Photoluminescence is one of the characterization methods to evaluate the optical properties of compound semiconductors. The list shows a schematic description of the related transitions and energy positions within the gap of the defect.

The energy position of the luminescence lines and bands can depend on voltage in GaN thin layers, temperature and excitation intensity. Therefore, in TABLE 2.2 the energy positions corresponding to the stress-free GaN at low temperature are given. The same crystal structure and especially the small lattice mismatch of only about 1.8% (c-axis) and 0.4% (a-axis), the fact that ZnO can be easily etched makes it very attractive.

FIGURE 2.2 Schematic illustration of HRXRD geometry

Chapter4. Optimization of growth condition for high quality GaN grown on ZnO template

I used N2 as carrier gas to protect ZnO from etching of the growth conditions, optimized growth temperature of 750°C from 10,500°C and changing the flow rate of V(NH3)/III(HCl) ratio from 10 to 80, and then identified with growing conditions. 4.3) Effect of GaN growth temperature on ZnO growth. 4.3.1) ZnO etching rate after GaN growth. Because ZnO can be easily etched by the growth temperature and HCl and NH3, which are commonly used in the HVPE growth of GaN. First, I optimized the growth temperature for GaN grown on a Zn-polar ZnO template. Nevertheless, in this study, GaN film was grown below 1000 °C for the direct growth of GaN on ZnO.

This result indicates that the growth of GaN on ZnO is possible in a low temperature range from 750 0C to 850 0C. The effects of growth temperature on the crystallinity of a 4μm thick GaN film were investigated by high-resolution X-ray diffraction (HRXRD). The full width at half maximum values (FWHM) of (0002) X-ray ω-rocking curves were measured to evaluate the structural quality of GaN layers.

But the growth rate of GaN is affected by the reactor pressure. (change in atmospheric pressure due to weather change; 30 mTorr). 4.4.2) Surface morphology. It is clear that the surface morphology and etching rate of ZnO are independent of the V/III ratio. The effects of the V/III ratio on the crystallinity of GaN films were investigated by high-resolution X-ray diffraction (HRXRD).

When the V/III ratio is greater than 10, the FWHM values of GaN are narrower than the ZnO template layer, when the V/III ratio is 50 and 80, the XRC FWHM was significantly narrower than that of ZnO. The broad luminescence band at about 2.9eV is assigned to an acceptor-bound exciton (Zn doped GaN) [6] Based on the morphology, XRC and PL results, I determined the optimal V/III ratio as 50 for HVPE GaN on Zn-polar ZnO template. The effects of growth temperature and V/III flow ratio on the surface morphology and chemical stability and crystallinity of GaN films subsequently grown by HVPE are investigated.

As a result, the growth temperature of 8500C and the V/III ratio of 50 are determined as the optimal growth condition for the direct growth of GaN on the ZnO template.

TABLE 4.1 Growth temperature dependence on ZnO etching rate and GaN growth rate

Chapter5. Fabrication of Freestanding GaN substrate

In this chapter, I will discuss the self-separation and fabrication of thick GaN grown directly on ZnO template. The ZnO layer was removed in the HVPE reactor, NH4 gas atmosphere (850°C to 1 minute) was carried out by etching. And the GaN layer was grown again under the GaN growth conditions (TG: 10500C, V/III ratio ~50). And the re-grown free-standing GaN substrate was compared. 5.3) Fabrication sequence of the self-separated FS-GaN substrate FIG.

Although the thickness is different, the FWHM values of the two samples are similar. Low-temperature (12K) PL spectra taken from the top surface of the final free-standing samples as shown in Fig. Sample-(b) shows a lower intensity at 2.9 eV and the UV peak at 3.364 eV dominates the spectrum, indicating a higher optical quality of sample-(b).

A new technique for fabricating free-standing GaN substrate by direct growth on ZnO template has been developed. The free-standing GaN film grown on Zn-polar ZnO template is demonstrated by HVPE without the aid of GaN protective cap layers.

FIGURE 5.1 Fabrication sequence of the Self-separated FS-GaN substrate by chemical etching

Chapter6. Summary and conclusion

As a result, self-separation technique through ZnO template layer was developed for fabricating FS-GaN substrates. The thick GaN film grown on the ZnO template layer was separated from the c-sapphire substrate by gas phase chemical etching. This thesis proved that direct growth of GaN on ZnO template is useful to fabricate next generation FS-GaN substrate and devices.

Appendix A

지금까지 도와주신 많은 분들께 감사드립니다. 특히 저를 지도해주시고 많은 관심과 지도를 해주신 장지호 교수님 감사드립니다. 아직은 새벽이 어색하지만 지금 이 시간에도 감사하다는 말을 전하고 싶습니다.

양민 교수님, 김홍승 교수님에게도 감사드립니다. 나누나, 승준 등 다 나열하기는 어렵지만 모두 감사드립니다. 마지막으로 항상 제 옆에 계시고, 인내심을 갖고 보살펴주신 부모님과 형에게 진심으로 감사하다는 말씀 전하고 싶습니다.

Molecular Beam Epitaxy of GaSb Layers on GaAs(001) Substrates Using Three-Step ZnTe Buffer Layers, W. Molecular Beam Epitaxy of GaSb Layers on ZnTe/GaAs: Influence of the Chemical Composition of the ZnTe Surface. Improving the crystallinity of MBE grown GaSb epilayer by using three-step ZnSe1-YTe Y buffer layer, S.Y.

Molecular Beam Epitaxy of ZnSe/ZnTe Superlattice Buffer Layers for Antimonide Thin Film Growth, S.Y. Molecular Beam Epitaxy of GaSb on ZnTe/GaAs – Influence of the chemical composition of ZnTe surface, W. Molecular Beam Epitaxy of ZnSe/ZnTe Superlattice Buffer Layers for the Growth of GaSb thin film on GaAs substrate, S.Y.

Hall mobility enhancement of Te-doped GaSb films grown on ZnTe buffer layer by molecular beam epitaxy, S.Y.