Research On The Temperature Dependence Of Nitrogen-vacancy Color Centers In Diamond And The Fabrication Of Diamond Deep-UV Photodetectors

As a member of ultra-wide band gap semiconductor materials,diamond（E_g=5.47 eV）has huge potential values in the fields of new-generation deep ultraviolet optoelectronic devices,high voltage and high-power electronic devices,due to its superior properties like high thermal conductivity,high carrier mobility,high breakdown field and high chemical stability.Moreover,due to its excellent photon emission stability and long spin coherence time,the nitrogen impurities induced nitrogen-vacancy（NV）color centers in diamond has attracted extensive investigation in the field of nanoscale temperature sensing with high temperature precision and high spatial resolution.At the same time,as a deep-levels defect within the diamond bandgap,NV centers can become carrier recombination center easily,which seriously limit the performance of diamond-based detectors.Therefore,reducing the nitrogen impurity content in diamond is of great significance to improve the quality of single crystal diamond and then to promote the application of diamond in the field of detectors.In this thesis,single crystal diamond was epitaxially grown on HPHT diamond through microwave plasma chemical vapor deposition（MPCVD）method.The temperature-dependent characteristics of diamond NV centers with different nitrogen concentrations were studied,then,based on the single crystal diamond with low NV defect centers,diamond-based all-carbon deep-UV photodetectors were fabricated by using graphene as electrodes,in which graphene was produced by nickel metal catalyzed in-situ conversion.In this thesis,single crystal diamond containing uniformed distributed NV center is obtained by introducing N₂ into the CVD growth process.Photoluminescence（PL）spectroscopy analysis shows that the intensity of the zero-phonon line（ZPL）of NV center increases first and then decreases with the increase of nitrogen concentrations,indicating that the continuous increase of nitrogen content will inhibit the formation of NV center in diamond.In addition,according to the Raman spectra and the PL spectra of NV centers,when the temperature increased from 80 K to 300 K,the ZPL of NV centers（NV⁰ and NV^-）has the corresponding changes of position shift,decrease of amplitude and the broadening of FWHM,which can be attributed to the enhanced interaction of electron-phonon scattering.The ZPL of NV center shifts with the change of temperature,indicating the energy level of NV centers depend on temperature.For traditional semiconductors,the temperature dependence of energy levels can be described by Varshni model,which,in contrast,is inapplicable to diamond because diamond has a very high Debye temperature and a small thermal expansion coefficient.Therefore,a modified Varshni formula is proposed to fit the characteristics of energy level of NV centers to temperature in diamond.According to the result,the modified Varshni model has higher fitting accuracy than the traditional Varshni model,which indicates that the temperature-dependent property of diamond NV center is an inherent property of NV center instead of affecting by nitrogen concentration.Finally,based on the temperature-dependent energy level shift of NV centers,a diamond temperature sensor is proposed.The detection accuracy of this diamond sensor can reach up to98%,indicating the possibility of diamond temperature sensing based on NV center.In this thesis,all-carbon“solar blind”photodetectors were fabricated on CVD diamond with low NV defect centers using graphene as electrodes,in which graphene was in-situ converted through high temperature annealing with Ni as catalyst.Three different interdigital electrode structure were formed on three as-grown single crystal diamond surface,all of the electrode fingers were designed with 100μm in width and2 mm in length,and 100μm,150μm,200μm in space,respectively.According to the analysis of Raman spectra,the typical strong G-band at 1580 cm^-1 and 2D-band at2700 cm^-1 of graphene can be clearly observed,which reveals that the top surface of diamond has been converted to graphene.The current-voltage（I-V）characteristic and the spectral response of the fabricated diamond detectors were tested and analyzed.The results showed that good Ohmic contact was formed between the graphene electrode and diamond.Meanwhile,at the same bias,the dark current of the detector decreases with the increase of electrode spacing.When the electrode spacing is 150μm,the detector shows large photocurrent and the light/dark ratio reaches over 100.In term of the spectral response,the response peak of the detector appears at around 220 nm with the value high up to248 A/W.The response inhibition ratio of the diamond detector at 220 nm to the wavelength of 280 nm is R₂₂₀/R₂₈₀=32.In addition,the diamond detector with electrode spacing of 150μm achieved the maximum optical conductivity gain of 1.4×10³,and the detection rate（D*）was 3.1×10¹⁴ cm·hz^1/2·w^-1/2 at an applied bias of 15V,which was 1～2 orders higher than the other two devices.So,the high performance of the photodetector can be attributed to the preparation of high-quality single crystal diamond with low NV defect centers and the properly designed about electrode structure.