Epitaxial Growth Of GaN Films On The Si Substrates And The Preparation Of MSM UV Detectors

The 3rd generation semiconductor Gallium nitride(GaN)is a binary III / V direct bandgap semiconductor with strong radiation and high temperature resistance.Its wide band gap of 3.4 eV affords it special properties including small size,high sensitivity and adaptability to extreme environment for applications in missile warning,ultraviolet communication and flame detection.Owing to the lack of homogeneous epitaxial substrates for GaN material,the technology of heteroepitaxy is generally used.Silicon(Si)become one of the most promising substrate materials and research hotspot taking advantage of low cost and good electrical/thermal conductivity.Nevertheless,the high lattice mismatch(17%)and thermal mismatch(-115%)between GaN and Si render GaN film high density dislocation,which stand in the way of the development of GaN ultraviolet(UV)photodetector on Si substrate.In this study,the method of molecular beam epitaxy(MBE)technology for preparing Al buffer layer combined with pulsed laser deposition(PLD)technology for epitaxial growth of GaN film was presented.As a result,the Al buffer layer with 0.9% lattice mismatch with Si substrate was epitaxial grown by MBE,followed by GaN epitaxial growth carried out by PLD.Furthermore,metal-semiconductor-metal(MSM)UV photodetectors was designed and fabricated.Firstly,the Al buffer layer with a low dislocation density is grown on the Si substrate by MBE.The nucleation mechanism of Al buffer layer was studied by the characterization of crystal quality and surface morphology of Al buffer layer with different thickness.On the basis of this,the two-dimensional growth of the Al buffer layer is promoted by adjusting on the growth temperature.During the process,the dislocation is annihilated and the crystal quality is improved.The Al buffer layer with a low dislocation density is realized on the Si(111),and the X-ray rocking curve(X-ray Rocking Curve,XRC)FWHM of Al(111)is 0.25 °.Then,high quality GaN film was continually grown by PLD.The effect of Al buffer layer on the epitaxial growth of GaN films was studied in detail,and the stress and dislocation control mechanism of GaN films were demonstrated.For one reason,the high energy GaN particles generated by PLD react with Al atoms introduced by Al buffer layer to form AlGaN,which effectively releases the stress.For another,in control of the thickness of the Al buffer layer,Al droplet on the surface as the nucleation center of GaN continuously absorb GaN particles to promote epitaxial growth.The GaN film with(0002)and(10-12)XRC FWHM of 0.46 ° and 0.67 ° was prepared by the synergy effect of the two parts.Finally,the interdigital electrodes was designed and the GaN-based UV photodetector was finished after study on device features.Considering the influence of width and spacing of the electrodes on the device performance,an interdigital electrode utilizing high work function metal Ni/Au,with a length of 260 ?m and a width of 5 ?m was designed.The preparation of MSM-type UV photodetector was completed by photolithography,evaporation electrode and lift-off.Under the 5V applied bias,the dark current is 3.57 nA and the response time is 2 ms@20 Hz,the responsivity reach the peak of 0.24 A/W at the wavelength of 358 nm,then begin to decline rapidly,indicating that the detector has a good "solar blind" feature.Hence,the finished device exhibits low dark current,quick response and high ultraviolet/visible inhibition ratio.In addition,the gain mechanism existing in the detector was revealed.The gain mechanism of enhanced photo responsivity can be attributed to the additional current generated by the decrease of the barrier height resulted from the hole trapping effect on the metal/semiconductor interface.Overall,the MSM UV photodetector was fabricated using high quality GaN film epitaxial grown on Si substrate.The method and conclusion provide a new way to improve the performance of UV photodetectors.