Growth of AlGaN Film on Si ( 111 ) Substrate

At present, the applications of AlxGa1 xN are extensive, such as for visible-blind ultraviolet detectors, laser diodes, light emitting diodes (LEDs) and HEMTs. In this paper, Al0.25Ga0.75N and Al0.32Ga0.68N films have been grown on 2 in Si (111) substrates by MOCVD. The low-temperature (6 K) photoluminescence (PL) spectrum and XRD rocking curve measurements have been employed to study the crystal quality of samples, and the phonon replica peak can be observed, which indicate that the samples have better quality in a small-localized region. The surface morphology of samples was investigated by AFM and the result of wavy surface agrees with the deduction from XRD rocking curve measurements. The sheet resistance mappings have been shown, and it indicates the nonuniformity of AlGaN film on Si (111) will increase sharply as the Al content increases.


Introduction
GaN and related Group-III nitrides grown by metal-organic chemical-vapor deposition(MOCVD)can be applied to high power, high frequency and high-temperature electronic devices [1][2][3] as well as light emitting diodes (LEDs) and laser diodes.Ternary AlGaN in particular is an important one with wide direct band gap, which has a broad range of applications in ultraviolet (UV) optoelectronic devices [4][5][6].Due to the limited availability of bulk GaN or AlN substrates for homoepitaxial growth, Generally GaN is grown on sapphire, SiC and Si substrates.The improvement of the power density of the device is limited by Sapphire substrate's bad thermal conduction.Meanwhile, the high cost of SiC substrates hinders the application of GaN material grown on SiC.Compared with sapphire and SiC, Si is the best alternative for its low cost, good thermal conductivity and ability to be integrated with the mature Si-based processing techniques [7,8].However, for the large lattice mismatch and the large coefficient of thermal-expansion (CTE) mismatch between GaN (AlN) and Si, the growth of high quality, thick crack-free AlGaN on Si substrate is a difficult task, and the cracks and high defect density seriously degrade the performance of GaN based devices on Si substrates.There are few reports on AlGaN growth on silicon substrates [9][10][11].In this study, the epitaxy of AlGaN on 2 in Si (1 11) substrates was performed in a horizontal MOCVD reactor.The low-temperature (6K) photoluminescence (PL) spectrum and XRD rocking curve measurements have been employed to study the crystal quality of samples.We also investigate the surface morphology of AlGaN epilayers on Si (111) substrate by AFM.The sheet resistance mappings of samples indicate the nonuniformity of AlGaN film on Si (111) will increase sharply as the Al content increases.

Results and Discussion
The HR-XRD 2ș-Ȧ scan for the sample has been measured.Using Vegard's law, the Al content in the AlxGa1-xN film can be estimated to be 25% and 32% respectively.The low-temperature (6 K) continuous-wave (cw) PL spectrum of samples is presented in Fig. 1.Besides the pits, the wavy surface is also observed as shown in the set of Fig. 3.The stepping curve is the white line trace in AFM measurements.The wavy surface has a low density of pits and the flat surface has a high density of pits.Taking AlGaN/Si, the big mismatch system into consideration, the probable interpretation is that the formation of the wavy surface is a way to accommodate strain energy and the formation of pits is a way to release the tensile pressure, which is all caused by CTE or lattice mismatch between AlGaN and Si.When the Al content reaches the threshold value, may be 30%, the function of accommodation will switch to stress release and led finally to the formation of pits.In addition, the sheet resistance mappings of the samples are shown in Fig. 4,

Conclusion
AlGaN film has been grown on Si (111) substrates.
AlxGa1-xN epilayers were grown by MOCVD on 2-in Si (111) substrates.The Si substrates were degreased by hot H2SO4 solutions for 5 min, NH3•H2O:H2O2:H2O (1:1:5) solutions for 5 min, HCl: H2O2:H2O (1:1:5) solutions for 5 min in due order, and then etched with HF (2%) for 0.5 min to remove the surface oxide layer.This procedure results in an oxide-free, hydrogen-terminated Si surface.Trimethylaluminum (TMA), Trimethylgallium DOI: 10.1051/ 06016 (2016) , matecconf/2016 MATEC Web of Conferences 61 610 2016 APOP (TMG), and ammonia were used as Al, Ga, and N sources, respectively.H2 was used as a carrier gas.After preparation, Si (111) substrates were heated under H2 ambient at 1060 for 3 min to clean its surface prior to growth.Pre-deposition of Al was used to prevent the formation of SiNx.In the initial stage of growth, a 60nm thick HT-AlN layer was grown, which is used as nucleation layer, and then 20 periods of AlN/GaN (2nm/3nm) superlattices was grown.The introduction of such superlattices aims at creating an additional compressive strain during the growth or the cooling down of the sample, in order to counterbalance the tensile strain in nitrides deposited on silicon[12].Finally, another 30-40nm HT-AlN buffer layer was grown.Following the composite buffer layer, 500 nm Al0.25Ga0.75Nepilayers and 500 nm Al0.32Ga0.68Nepilayers were grown for sample A and B respectively.The samples were investigated by AFM and XRD rocking curve measurements.UV PL was performed in a home-made vacuum spectroscope.The excitation source was a frequency quadrupled Ti: sapphire laser.The sheet resistance mappings of samples were measured by LEI 1500 series contactless resistance mapping system.

Fig. 1 .Fig. 2 .
Fig.1.the low-temperature (6 K) continuous-wave(cw) PL spectrum of samples It exhibits the main free excition emission line at 4.058 ev and 4.235 ev respectively.And the value of content deduced by band gap is in accordance with XRD's.Two weaker peaks are observed at each curve.The energy separation between the successive emission

Fig. 4 .
Fig.4. the sheet resistance mappings of samples Obviously, the nonuniformity increases sharply as the Al content increases.Although the 5% nonuniformity of sample A is just passable, the value can still be reduced for preparation of devices.
The low-temperature PL spectrum shows two clear phonon replica peaks which indicate the samples have better quality in a small-localized region.The film preferred orientation (001) represents a deviation by analysis of XRD rocking curve.AFM measurements show two type of surface morphology, further analysis indicate that there may be a critical Al content value for the formation of surface pits.In addition, the wafer nonuniformity increases sharply as the Al content increases.