Research On Polarity Modulation And Strain Engineering In Flexible Free-standing AlN Films

In recent years,AlN,an ultra-wide bandgap semiconductor material,is receiving more and more attention and flourishing.Due to its ultra-wide bandgap(6.2 e V),good piezoelectric properties,ultra-high ultrasonic transmission speed,very high Baliga figure-of-merit and critical electric field,making AlN an ideal material for high-frequency high-power electronic,piezoelectric/acoustic wave,and deep-ultraviolet optoelectronic devices.However,AlN still faces both challenges of material growth and device preparation,and there are many difficulties to overcome.In particular,the large bandgap leads to high activation energy of the dopant,high contact and on-resistance,and poor device performance making AlN-based devices far below the theoretically predicted limit of performance.In addition,based on theoretical predictions,AlN with specific polarity,i.e.,N-polar or Al-polar,is very important to optimize transport characteristics of devices,by impacting built-in electric fields,interfacial or face charges,etc.However,limited by different fabrication processes,there are still problems such as the difficulty to control polarity of AlN and the lack of relevant mechanistic studies,as well as the lack of direct experimental evidence that proves the direct effect of intrinsic polarity on AlN devices.Therefore,the developing of AlN with high crystalline quality and tunable bandgap and polarity as well as enhancing the performance of AlN-based devices,such as ohmic contact and on-resistance by various means,are the focus of studies.Recently,the introduction of strain engineering in ultra-wide bandgap materials to modulate their properties has become a hot topic of research.For example,it has been reported that the bandgap of diamond can be"metallized"by applying elastic strain through strain engineering to allow ultra-wide bandgap semiconductors to have the bandgap comparable to silicon or even reduced to 0 e V.Elastic strain can also modulate the performance of electronic devices,providing new degrees of freedom for device performance enhancement and novel device design.However,there are few reports on the realization of strain engineering in flexible AlN single crystals and subsequent practical cases of strain engineering in AlN-based devices.To overcome the above challenges,this thesis firstly combines an epitaxial growth technique with insertion of oxide buffer layers and a transfer process of free-standing thin films to prepare a flexible high-quality free-standing AlN film that can be heterogeneously integrated with arbitrary substrates.The crystalline quality,surface,and cross-sectional morphology of the AlN films before and after the transfer process were systematically investigated.Moreover,the flexibility of the flexible AlN films and the contact resistance improved by ion implantation were investigated by in situ techniques.Then the transfer process was used to realize the control of AlN polarity and to achieve the tuning of AlN-based Schottky diode device performance.In order to reduce the bandgap of AlN and improve the device performance,we also propose a method that can continuously modulate the bandgap of flexible AlN material by strain engineering,which can tune the band gap of AlN from 6.2 e V to 4.8 e V,and achieve the improvement of the photoelectric response performance of flexible AlN-based deep UV photodetector,improving the responsiveness by 161%,reducing response time by 31%,and decreasing the dark current.Our works in this thesis provide effective theoretical and experimental support to solve the issues of poor crystallization quality of heterogeneous integrated thin films,large bandgap,limited device performance,and the lack of related research on polarity modulation and strain engineering in AlN.The specific research contents of this thesis are arranged as follows:(1)We first present a method to successfully prepare a flexible free-standing AlN film,which can realize heterogeneous integration with different substrates such as Si and flexible substrates,by transfer process of free-standing thin film with wet etching and liquid-phase exfoliation.This free-standing AlN film has good crystalline quality,smooth surface,and can be heterogeneously integrated with any supported substrate.Based on it,the optimized etching transfer process combined with photolithography and dry etching process succeeds in the transfer and heterogeneous integration of complete AlN devices and large-size films.Using focused ion beam and in situ techniques,an in situ bending experiment of a free-standing AlN film was designed and showed that the free-standing AlN film possessed unexpected flexibility and could achieve spontaneous recovery after very large bending angles(up to 180°),and no significant fracture was observed after several bending runs.Meanwhile,in situ ion implantation was undertaken on free-standing AlN films using the focused ion beams,and Ga ions were found to be successfully implanted on the surface of the AlN films by in situ characterization.Compared with current reports of AlN ohmic contacts formed by rapid thermal process,a smaller specific contact resistance of only 0.013Ω·cm~2 was achieved in the implanted region.The results provide a solid basis for the subsequent preparation of AlN devices and strain engineering applications.(2)A simple flipping process was proposed for the polarity control of free-standing AlN films.Then the polarization behavior and surface states of AlN with different polarities were verified by chemical etching,piezoelectric force microscopy and Kelvin force microscopy,respectively.Based on this,AlN-based Schottky diode devices were further prepared,and it was found that the ohmic contact and Schottky diode devices prepared on the two polarities of AlN films have different contact resistance and different electrical transmission characteristics.More importantly,by extraction and calculation,we find that the surface potential difference measured by the scanning probe measurement techniques and the potential height difference extracted from the SBD devices are consistent with each other,which can provide direct experimental evidence for the modulation of device performance by AlN polarity.Various experimental results show that the opposite surface polarization charge present at the surface in N-polar Schottky diode devices pulls down the barrier height between the metal-semiconductor contacts compared to Al-polar devices.The reduction in barrier height(approximately 0.2 e V)results in a 87%decrease in on-state resistance,an order of magnitude increases in current density,and a 55%decrease in ohmic contact resistance,which is expected to be used to improve AlN-based device performance and design new devices.(3)The strain engineering based on flexible free-standing AlN film was developed and found the optical bandgap of the AlN is tuned by introducing bending stress,which can continuously reduce the bandgap of AlN from 6.2 e V to 4.8 e V.The solar-blind deep-ultraviolet photodetectors based on strain engineering in AlN represent an enhanced photoelectric response.Firstly,to improve the photo response of the AlN-based photodetector,a vertical structure photodetector with smaller electrode spacing was prepared,achieving higher responsivity and faster time response than conventional planar structure detectors.The flexible AlN-based deep-ultraviolet photodetector was also modulated by bending stress,resulting in a 161%improvement in responsivity and 31%decrease in response time,as well as a decrease in dark current at bending state.This enhanced performance could originate from the modulation of the bandgap and the piezo-phototronic effect,which influences the migration of carriers during the optoelectronic processes.Our work provides an effective method for tuning the performance of AlN materials and devices by strain engineering,which is promising to solve the current challenges of AlN such as high activation energy of n/p doping and limited device performance.