| At room temperature, the energy band gap of AlGaN material varies from 3.4 to 6.2 eV. Therefore, AlGaN material is useful for making light emitters and photodetectors (PDs) in the ultraviolet (UV) wavelength region. UV-PDs are important devices that can be used in a variety of applications, such as covert communications, early missile threat detection, chemical and biological threat detection, flame detection, power line monitoring, UV environmental monitoring, UV spectroscopy, and UV astronomy. Various kinds of structures of UV-PDs are proposed, such as p-n junction diodes, p-i-n diodes, Schottky barrier diodes, and metal-semiconductor-metal (MSM) PDs. Since the fabrication of MSM UV-PDs is simple and compatible with field effect transistor technology, MSM UV-PDs have been paid much attention. For the non-polar a-plane AlGaN material, the absence of polarization field in the growth direction and the intrinsic in-plane polarization anisotropy can be exploited to enhance the radiative efficiency and the intrinsic polarization sensitivity. Thus, the non-polar a-plane AlGaN material becomes a promising candidate for realizing UV polarization-sensitive photodetectors (UV-PSPDs). Potential applications for the UV-PSPDs include solid state light sources, optical storage, biophotonics, polarized-light detection, and narrow band light detection. In this paper, the non-polar a-plane GaN and AlGaN epi-layers were successfully grown on the semi-polar r-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD). In order to improve the crystal quality for the non-polar a-plane GaN and AlGaN epi-layers, the process parameters for these epi-layers were systematically optimized. Moreover, the effects of Si-doping and Mg-doping on the structural, electrical and optical properties of the non-polar a-plane GaN epi-layers, the polar c-plane and the non-polar a-plane AlGaN epi-layers were studied in detail, respectively. Furthermore, the surface properties for the polar c-plane AlN and AlGaN epi-layers were quantitatively analyzed, respectively. In addition, the property for MSM GaN-based PSPD was preliminarily investigated. The major research contents and results achieved in this paper were listed as follows:1. The non-polar (1120) a-plane GaN epi-layers were grown on the semi-polar (1102) r-plane sapphire substrates by MOCVD with the low temperature (LT) GaN and AlN nucleation layer, respectively. The optimization of V/III ratio, TMGa flow rate, and reactor pressure should be helpful to improve the surface morphology and crystal quality for the non-polar (1120) a-plane GaN epi-layer. Moreover, the surface morphology, crystal quality, and structural anisotropy for the non-polar (1120) a-plane GaN epi-layer were improved by using the AlGaN interlayer, especially the Al-composition-graded AlGaN interlayer. With increasing SiH4 flow rate, the ionization efficiency for Si dopant increased, leading to the increase of electron concentration for the non-polar (1120) a-plane GaN epi-layer at room temperature. The formation of VGa in the non-polar (1120) a-plane GaN epi-layer was boosted by the relatively high Si-doping level. Therefore, the yellow luminescence for the non-polar (1120) a-plane GaN epi-layer was enhanced. Since the defect density was increased by Mg-doping, the surface morphology for the non-polar (1120) a-plane GaN epi-layer was degraded. On the other hand, the crystal quality for the non-polar (1120) a-plane GaN epi-layer along c-direction was hardly affected by Mg-doping. However, both the crystal quality along m-direction and structural anisotropy for the non-polar (1120) a-plane GaN epi-layer were deteriorated due to Mg-doping.2. The non-polar (1120) a-plane AlGaN epi-layers were grown on the semi-polar (1102) r-plane sapphire substrates with the LT AlN nucleation layer and high temperature (HT) AlN buffer layer, respectively. For the non-polar (1120) a-plane AlGaN epi-layer, increasing the V/â…¢ ratio for the HT AlN buffer layer and decreasing the thickness for the AlGaN buffer layer seemed to be helpful to improve the surface morphology for the AlGaN epi-layer. The dislocations formed in the growth of HT AlN buffer layer were effectively suppressed by the introduction of the AlGaN interlayer. Therefore, the crystal quality and structural anisotropy were significantly improved with the AlGaN interlayer, especially the Al-composition-graded AlGaN interlayer inserted. However, the crystal quality for non-polar (1120) a-plane AlGaN epi-layer was not positively related to its surface morphology.3. The effects of Si-doping on the structural, electrical, and optical properties for the polar (0001) c-plane and non-polar (1120) a-plane AlGaN epi-layers were studied in detail. The results showed that Si-induced compressive strain relaxation in the non-polar (1120) a-plane AlGaN samples can be promoted by the structural anisotropy as compared with the polar counterparts. The crystal quality for both the polar (0001) c-plane and non-polar (1120) a-plane AlGaN epi-layers with the increase of Si-doping level was attributed to the Si-induced enhancement in the opportunity for the dislocations to interact and annihilate. With increasing SiH4 flow rate from 0 to 40 sccm, the acceptor-like dopants in both the polar (0001) c-plane and non-polar (1120) a-plane AlGaN epi-layers increased, which promoted the compensation of the native defects. Thus, the blue luminescence of the polar (0001) c-plane and non-polar (1120) a-plane AlGaN epi-layers was enhanced. In addition, the surface morphology, crystal quatliy, and electrical property for Mg-doped polar (0001) c-plane AlGaN epi-layer were remarkably improved by the AIN buffer layer and the Al-composition-graded AlGaN interlayer.4. The surface properties for the polar (0001) c-plane AIN and AlGaN epi-layers were quantitatively studied by angle-resolution X-ray photoelectron spectroscopy. The results suggested that the air-exposed surfaces of the polar (0001) c-plane AIN and AlGaN epi-layers were inevitably oxidized to Al-oxide- and Ga-oxide containing layers. Moreover, more Al-O bonds existed on the air-exposed surfaces of the polar (0001) c-plane AlGaN epi-layers with relatively high Al composition due to the large chemical affinity of aluminum to oxygen. In particular, the as-deposited polar (0001) c-plane AIN epi-layer was indeed a nitrogen-deficient film near the surface since N atoms were partially replaced by O atoms. With increasing the Al composition, the Ga Auger effect for the as-deposited polar (0001) c-plane AlGaN epi-layer was greatly suppressed because of the less Ga-N bonds. Furthermore, the Al composition in the as-deposited polar (0001) c-plane AlGaN epi-layers was demonstrated to be non-uniform since Al atom is more easily oxidized than Ga atom, especially for the polar (0001) c-plane AlGaN epi-layer with relatively high Al composition.5. The polar (0001) c-plane Al0.28Gao.72N/Al0.45Ga0.55N multiple quantum wells (MQWs) was grown on the polar (0001) c-plane sapphire substrate by MOCVD. Moreover, the structural and optical properties for the Alo.28Gao.72N/Al0.45Ga0.55N MQWs were characterized and studied, respectively. The characterization result showed that the internal quantum efficiency for the Alo.28Gao.72N/Al0.45Ga0.55N MQWs was 18% at room temperature (RT). By separating the AlInGaN MQWs absorption region from the multiplication region, the quantum efficiency and responsivity for the GaN-based PD were greatly increased, and the cut-off wavelength can be optionally tuned. Furthermore, the threshold breakdown voltage for the GaN-based PD proposed in this paper was effectively decreased. Based on the non-polar (1120) a-plane GaN epi-layer grown on the semi-polar (1102) r-plane sapphire substrate by MOCVD, the MSM GaN-based UV-PSPD was fabricated. The performance for the MSM GaN-based UV-PSPD was also characterized and analyzed. The result suggested that the responsivity for the MSM GaN-based UV-PSPD was 0.31 mA/W at 10 V. In addition, a maximum polarization sensitivity ratio of 1.5 was achieved.
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