Fabrication And Characterisation Of Terahertz Semiconductor Detectors,Emitters And Power Amplifiers

Terahertz?THz?wave refers to electromagnetic radiation with frequency ranged from 0.1 to 10 THz?1 THz = 1000 GHz?.Its frequency is located between microwave and infrared.Due to great difficulties in generation and detection,terahertz radiation is still wrapped in mystery.Investigation and unstanding of terahertz radiation are very deficient that people often call it "terahertz gap".In recent years,terahertz wave attract much attention because it is found has many potential applications in fields like,national defense,ultra high speed wireless communication,phase analysis,universal spectroscopy and security check.However,the application of terahertz wave is facing the problem of lacking suitable terahertz sources and detectors.The available terahertz equipments or devices are mostly bulky,expensive or require very low temperature,which made them not suitable for large-scale applications.Thanks to the rapid development of semiconductor material growth technology and micro/nano fabrication technology in recent years,it has been possible for semiconductor devices to act as THz emitters or detectors.Because of the advantages of low cost,small volume,low power consumption,high efficiency and integratable,THz semiconductor devices has been acknowledged to be an important direction for the development and application of THz in the future.This thesis focused on the development status of domestic and foreign terahertz technologies,fabricated THz semiconductor devices and systematically studied their working principle,DC and high frequency characteristics and performance improvement methods.These THz devices include planar detectors,emitters,and power amplifiers.The main research contents are as follows.1)In this paper,the improvement of the responsivity of planar terahertz semiconductor detector-self switching diode?SSD?is studied.SSD has a small parasitic capacitance which make its operating frequency as high as 1.5 THz.However,the response of SSD is not high enough.In this paper,the improvement of SSD responsivity is investigated through filling its trenches with different dielctric materials and compare DC and RF performances.Experimental results show that covering SSD with PMMA can significantly improve its on/off ratio in I-V curves,improve device power linearity and greatly enhance its responsivity at high frequency.The measured highest responsivity is 1650 mV/mW,which is ten times higher than the origin SSD without covering PMMA.2)Fabrication of SSD by dry etching was investigated and the performances of fabricated devices are studied.For the first time,we purposed SiO as low damage dry etching mask.The rectification characteristic of SSD is depends on channel and trench with nanoscale widths.Previous studies mainly use wet etching to realize trenches,which have the drawbacks including poor repeatability and rough edges.Thought dry etching was using to prepare SSD,the reported performances of devices are not good.We found that dry etching mask has obvious effect on the proformance of fabricated SSD.Thermally evaporated SiO dry etching mask is purposed to solve those problems in organic photoresist mask and in organic mask deposited in plasma environment.The thermally evaporated SiO has small crystal size,good flatness,and high dry etching resistance,which make it very suitable for dry etching.The most important advantage of SiO is that the thermal evaporation deposition method does not cause damage to the surface of semiconductor.So,it can improve the performance of devices whose carriers are located near the surface?such as two-dimensional electron gas?.In this thesis,by comparing with by spin coated PMMA mask and sputtered SiO₂ mask,the SSD fabricated with SiO hard mask show good morphology and better high-frequency performance.The responsivity of the devices fabricated with SiO mask at 220 GHz is 1 to 2 orders of magnitude higher than that of the other two masks,and the equivalent noise power is 1 orders of magnitude less.The Hall measurement results show that the concentration and mobility of carriers in the channel prepared with SiO mask are higher than those prepared with SiO₂ mask.These results fully demonstrate the role of SiO in reducing the influence of etching mask on the substrate.3)The effects of surface oxide layer treatment on the width of surface depletion region and the Fermi level pinning effect were investigated.There are a large number of surface states in InGaAs due to the surface oxidation which have a direct impact on the width of the SSD channel.In this paper,the relationship between the conductivity of SSD and the channel width is measured,and a surface depletion region width of 46 nm is obtained.Furthermore,it is found that the width of surface depletion region was increased to 74 nm after removed the surface oxide layer with acid,and the width decreased to about 35 nm after oxygen plasma treatment.These results show that the width of depletion layer in InGaAs will change with surface state,which should be related to the changes of the surface state density.4)The fabrication process of GaN nano devices was studied,and a terahertz detector based on GaN SSD was successfully fabricated.As a representative of the new generation of wide band gap semiconductor,GaN has the advantages of high temperature resistance,high breakdown resistance and high radiation resistance.The characteristics of GaN SSD are studied in this thesis.The contact resistance is reduced to 1.1× 10-4 ? cm-2 after optimizing the ohmic contact process.SSDs with trench width as small as 30 nm and depth as large as 135 nm was achieved by using dry etching with SiO mask.The high frequency measurements show that the operating frequency of GaN SSD is as high as 220 GHz with a good linear response.This proves that a GaN THz detector has been successfully fabricated.In addition,the research direction of GaN SSD in the future are also presents.5)A high frequency and high power terahertz source based on planar Gunn diode?PGD?is fabricated,and a two-dimensional resonant cavity based on coplanar waveguide?CPW?is proposed.PGD on semiconductor heterojunction has been reported with fundamental frequency higher than 300 GHz,which is much higher than the traditional vertical Gunn diode.However,the RF power of PGD is still very low.In this thesis,we studied the optimization of semiconductor heterojunction substrate structure,device structure and fabrication process,which greatly enhanced the negative differential resistance?NDR?performance.The current peak to valley ratio is 1.25.With the aid of 3D electromagnetic simulation,a two-dimensional resonant cavity based on CPW is proposed,and applied to PGDs.The device show output frequency of more than 100 GHz and output power of 0.8 mW,which is much higher than other PGDs as reported in literatures.This is for the first time that the output power of PGD reach the level of miliwatt.The phase noise of fabricated THz source is-107 dBc/Hz,which is 30 dB lower than that in literature reporte.The stability of output frequency of the device is also tested,and the bias drift is only 0.21 GHz/V.6)The fabrication of GaN PGD is tried.The main problems in GaN Gunn diodes and the direction of investigation in future are discussed.Due to the wide band gap,high temperature resistance,high breakdown resistance and high carrier saturation drift velocity,GaN is very suitable for the fabrication of high frequency and high power Gunn devices.In this thesis,the preparation process of GaN PGD was studied.The prepared device show obvious decrease in current increase rate at the electric field of 7 V/?m.The realization of GaN terahertz source is a world-class problem.In this paper,the problems in GaN PGD are studied and some suggestions for future research are proposed.7)A planar power amplifier with operating frequency higher than 110 GHz has been successfully fabricated by using a planar Gunn device.Power amplifier is one of the key elements in terahertz systems.Although high mobility field effect transistor?HEMT?based power amplifier have a cut-off frequency as high as 0.6 THz,but its channel length must be shortened to 25 nm.Such requirements make the device fabrication become very difficult,device prices become very high,and not suitble to large-scale applications.Other power amplifiers,like heterojunction bipolar transistor?HBT?and resonant tunneling diode?RTD?have the problems of signal distortion at high input power.In this thesis,for the first time,we proposed the proformance of planar Gunn diode as power amplifier by applying its NDR effect.By improving the NDR effect of device,a planar Gunn amplifier with a power gain of 17 dB and operating frequency of more than 110 GHz was fabricated on InGaAs substrate.The 0 dB compression point of device is 0 dBm,which is much higher than that of HBT and RTD.We also found that the optimum working frequency of plane Gunn amplifier is inversely proportional to the channel length.The channel length of device with optimum working frequency up to 0.6 THz is 300 nm,which is about 10 times of that in HEMT with the same working frequency.This can significantly reduce the difficulty in device fabrication,thereby greatly reduce the cost of device.The working stability of the planar Gunn amplifier was also tested.Although no cooling techniques like thinning substrate and mounting chip to heat sinks was used,the device show good stability with a power gain of 16 dB in a 2 hour measurement with fluctuation less than 0.8 dB.8)Two-dimensional side gate transistor?SGT?is fabricated and its working principle,equivalent circuit,DC transfer and output characteristics are studied.The structure and working mechanism of SGT are similar to SS.Based on the measurements of SGTs,we can be obtain much more informations such as,threshold voltage,switching ratio,carrier mobility,defect density,trench capacitor than from SSDs.Studying the relationship of SGT characteristics will help us to the improve the performance of SSD.In this thesis,the SGT with a channel width of 90?150 nm was successfully fabricated.The device with a channel width 135 nm show a threshold voltage of-0.8 V.The optimal operating voltage of these device is 0.5 V and the maximum transconductance was 24.1 ?S.