Study On MOCVD Growth Of High Al Fraction AlGaN And P-type Doping Of AlGaN Films

Ternary aluminum gallium nitride(AlGaN)as a third generation of new wide band-gap semiconductor material is one of the important representatives with continuously adjustable band-gap between 3.4eV(GaN)and 6.2eV(AlN).The corresponding wavelength can cover the band range of 200nm-365nm according to the band gap of AlGaN.So AlGaN is an ideal material for the fabrication of ultraviolet light emitting diode(UV-LED),laser diode(LD)and other optoelectronic devices in the UV spectral region.As the increase of the AIN mole fraction x of the AlxGa1-xN alloys,it can be utilized for fabrication of deep-UV light emitting diodes(DUV-LEDs)and laser diodes(LDs),whose source area needs AlGaN epilayer with high AIN mole fraction.Therefore,the growth and characterization of AlGaN with high AIN mole fraction has became one of the research focuses in recent years.Furthermore,compared to the doping level in GaN,p-type doping efficiency in AlGaN alloys is relatively low.The main physical reason is that with the increase of AIN mole fraction,the forbidden bandwidth of AlGaN alloys increases,acceptor level becomes deep,and activation energy increases monotonously,which result in the decrease of activation efficiency and carriers concentration.So the fabrication of ohmic contact is very difficult,which can not meet the requirements for the fabrication of UV-LED device.Therefore,it is urgent to study p-type AlGaN with low resistance and high activation efficiency.In this paper,a series of epitaxial AlGaN films with high AIN mole fraction are grown on GaN/sapphire substrates with high temperature AIN(HT-AlN)as the interlayer by metal organic chemical vapor deposition(MOCVD)technique.And the physical properties of the films are systematically studied.Then,the p-type AlGaN films are grown,the physical properties are also researched.Finally,the stability and electronic properties of the inversion domains in AlGaN:Mg films are theoretically studied.All above results could provide technical scheme for the growth of high quality AlGaN films,and it is of great significance for the growth and application of high performance UV LED device structure.The main research contents and results are listed as follow:(A)Epitaxial growth and physical properties of AlGaN films with high AIN mole fraction1.High AIN mole fraction AlxGa1-xN films with x varying from 0.33 to 0.79 were grown on GaN/sapphire templates with the high temperature AIN interlayer(HT-AlN)by metal organic chemical vapor deposition(MOCVD).A way of decreasing TMGa/TMAl molar ratio to improve the Al incorporation efficiency is discovered.The best crystalline quality,among these AlxGa1-xN alloys,can be obtained for an AIN mole fraction x = 0.55,where the full-width at half-maximum of(0002)diffraction peak in Al0.55Ga0.45N is measured to be 259 arc sec by high resolution X-ray diffraction(HRXRD).2.The screw threading dislocation(TDs)density of Al0.55Ga0.45N film is 2×108 cm-2 evaluated by transmission electron microscope(TEM),which agrees with the calculation results from Williamson-Hall plots.Moreover,cross-sectional TEM indicates that the HT-AlN interlayer could sufficiently reduce the threading dislocations(TDs)through generation of V trenches in the HT-AlN interlayer due to the lateral overgrowth of initial AlGaN sub-grains.Thus,the TDs propagate along the V trenches,then bend into basal planes and annihilate with other dislocations.The crystal quality of AlGaN epitaxial layer is greatly improved.3.The study of optical properties indicates that the obvious S-shape of temperature dependence on emission peak position is observed for Al0.55Ga0.45N,which is attributed to exciton localization with energy(Elox)?14.95 meV at 10 K,resulting from potential fluctuation and band tail states.The time-resolved photoluminescence(TRPL)spectrum show a bi-exponential decay at low temperature.The fast decay time implies the presence of the radiative recombination of localized excitons,while relative long one is assigned as being due to certain trapping and detrapping processes originating from cation vacancy complexes,such as V?-ON,V?-SiN,V?-Xi.(B)Epitaxial growth and physical properties of Mg doped AlGaN films1.Mg-doped AlxGa1-xN(0.23?x?0.57)films have been MOCVD grown on GaN/sapphire templates with high temperature AIN interlayer(HT-AlN)by uniform Mg doping method on the basis of unintentionally doped AlGaN films.A combination of secondary ion mass spectrometry(SIMS)and transmission electron microscopy(TEM)indicates the H is beneficial to increase the incorporation of Mg atoms.While the flow rate of Cp2Mg is too large,due to the limitation of solid solubility of Mg,Mg clusters are formed,which leads to the formation of inversion domains,and then the film was transformed into N-polar film growth.On one hand,the subsequent Mg incorporation is reduced;On the other hand,since Mg-H complex can not be formed,the H concentration is reduced.Moreover,with the increase of Cp2Mg flow rate,the initial formation position of inversion domains moves towards the AlGaN/HT-AlN interface.2.Hall experiment measurements show that Hall activation conduction can be realized in AlxGa1-xN(x=0.23,0.35 and 0.57)samples with Mg doping concentration of?1020 cm-3;At room temperature,the hole carrier concentrations decreases from 1018 cm-3(x=0.23)to 1016cm-3(x=0.57).According to the dependence of hole concentration with temperature,the Mg acceptor activation energies of Al0.23Ga0.77N:Mg ? Al0.35Ga0.65N:Mg films are 172meV and 242meV,respectively,which is caused by the coulomb potential overlapping of the acceptors and the coulomb potential shielding caused by the high concentration of free carriers,leading to the decrease of the acceptor activation energy.While the Cp2Mg flow rate is further increased,AlGaN alloys turned into N-polarity growth mode.Due to the poor film quality,the amount of inversion domains and intrinsic defects(VN3+)increase,So the self-compensation degree increase,and the resistivity rapidly increases by orders of magnitude.3.The catoluminescence spectra(CL)indicates that the near band-edge(NBE)emission is quenching for Mg doped AlxGa1-xN(x=0.23,0.35 and 0.57),there are two kinds of donor-acceptor pairs(DAP),which are ascribed to the bound exciton recombination between the shallow donor(O)and deep acceptor(V? complex)1-,and the donor(VN3+)and Mg neutral acceptor,respectively,leading to the increase of compensation degree,which is the main reason for the reduction of hole concentration.(C)First principle study on the inversion domain structures1.Based on the first principle calculation,the stability of the inversion domain structures and its corresponding un-inverted structure in GaN and AIN films have been studied.The results indicate that Mg clusters easily cause inversion domain structures in GaN;the Mg layer on(0001)base plane of the pyramidal inversion domains(PIDs)easily cause growth polarity inversion in AIN,and IDB*structures are easily formed in both GaN and AIN.On this basis,the effect of A1 atom distribution on the stability of the inversion domains has been discussed.2.Based on the first principle calculation,we have obtained the energy band diagram of the inversion domain structures.And the formation mechanisms of energy levels at the bottom of conduction band and the top of valence band are revealed by calculation of electron wave function and projected density of state at bottom of conduction band and top of valence band in the inversion domain structures.