Silicon Doping Dependence of n-Type Al0.5Ga0.5 N Layers Grown by Metalorganic Chemical Vapor Deposition.

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Silicon Doping Dependence of n-Type Alo.5Ga,,5NLayers Grown by Metalorganic Chemical Vapor Deposition LiLiang(+ &)I, ZhangRong(% $ ? ) I , XieZili(&8A)'*, ZhangYu(% k ) ' v 2 , Xiu Xiangqian Liu Bin (3d & ) I , Zhou Jianjun ( A & % ) I , Chen Lin (i% *)I, Gong Haimei ( &2$&)3, Zheng Youdou (gF4k.t)' Yu Huiqiang ( & S $ S ) ' , Han Ping (6 ? ) I , ( 1 . Key Laboratory of Advanced Photonic and Electronic Materials , Department of Physics , Nanjing University, Nanjing 210093, China; 2 . Department of Physics, School of Applied Science, University of Science and Technology Beijing , Beijing 100083, China ; 3. Shanghai Institute of Technical Physics , Chinese Academy of Sciences , Shanghai 200083, China )

($%$~a)',

Abstract: The electrical, structure and optical properties of Si-doped Alo,5G%,SNepilayers with a thickness of about 0.5 p n grown on sapphire substrates using an A1N buffer by metalorganic chemical vapor deposition were reported. Hall-effect measurements showed that n-type Alo,,G%.,N was achieved achieved with an electron concentration of 1 . 2 x 1019cm-3 and mobility of 12 cm2.Vs-' at room temperature. The electron concentration increases with an increase of Si doping level. Lattice constant of c and Raman shift of Si-doped Alo.,G%.,N epilayers with various SiH4 flow rates was studied by Xray diffraction (XRD) and Raman scattering spectrum. With increasing SiH4 flow rate, the decrease of the lattice constant of c and the frequency of E z phonon implies gradual relaxation of the stress in the epilayers .

Key words: AlGaN; Si doping; MOCVD CLC number: 0649.1 ; TN304.2 Document code : A

Wide-band-gap Al-rich AlGaN alloy systems are attractive materials for fabrication of electronic and optoelectronic devices, such as high-power field effect transistors, solid-state lighting for general illumination and solar blind UV photodetectors. In such applications, highly conductive n-type Al,Gal - .N alloys with A1 content as high as 0 . 5 are needed. Although Sidoping has been proved to be an efficient way to achieve n-type A1,Gal N alloys"*21, it is still difficult to achieve highly conductive n-type Al,Gal -,N : Si films with x > 0. 5. The carrier concentration is found to markedly decrease with increasing aluminum mole fraction z in the 1aye1-s'~'~' . Various explanations for the n-type doping problems in Al,Gal-.N with increasing A1 content have been proposed, such as an increase in compensation by acceptor traps associated with unintentional i m p ~ r i t i e s ',~ ]dislocation density and native defectsr6', donor activation energy[71, and the occurrence of oxygen-DX center transition"] . However, it has also been reported that use of Si as --I

Article ID: 1002 - 0721 (2007) - 0349 - 04

surfactant during metalorganic chemical vapor deposition (MOCVD) growth is shown to improve structural quality by reducing the defect density"] . In this work, the Si-doping effect of n-type Ab5 Ga,,,N layers on the structure, electrical and optical properties was investigated. The high-resolution X-ray diffraction (HRXRD) was used to evaluate the changes in their strain states and structural characteristics, Raman scattering spectra was used to study the stress relaxation by Si incorporation performed with one undoped and two Si-doped samples with carrier concentration in the range from 3.0 x 10" up to 1.2 x lOI9

1 Experimental Si-doped Ab.,Ga.,N epilayers with a thickness of about 0 . 5 pm were grown on c-plane sapphire substrates using A1N buffer by low-pressure ( 1.33 x Pa) MOCVD. The growth was initiated by an A1N nucleation layer of about 20 nm thickness grown at 570 "c. After that, Si-doped AL.,G*.,N layers were de-

Received date: 2006- 11 - 16; revised date: 2007-04- 16 Foundation item: Special Funds for Major State Basic Research Project 973 (2006CB604905, 2006CB604907 ) ; National Natural Science Foundation of China (6039072, 60476030, 60421003, 60676057); Great fund of Ministry of Education of China (10416) ; the Research Fund for the Doctoral Program of Higher Education of China (20050284004) and Nature Science Foundation of Jim& Province (BK2005210, BK2006126) Blogrnphy: Li Liang (1980 - 1, Male, Doctor; Research interest: 111-nitride growth by MOCVD * Correaponding author (E-mail : xzl @ nju .edu .cn)

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posited at 1100 "c. The metalorganic sources used were trimethylgallium ( TMGa) for Ga and trimethylaluminum ( TMAl ) for A1 . Blue ammonia and silane ( SiH,) were used as nitrogen and silicon sources, respectively. The Si-dopant concentration was intentionally varied by changing the SiH4 flow rate from 2 . 4 to 8.1 nmol-min-I, while keeping other conditions the same. The A1 content of AlGaN layers was obtained by double-crystal X-ray diffraction ( DCXRD ) measurement using Vegard' s law. The c-lattice constant was analyzed in 8/28 scan mode of DCXRD to measure the residual strain. HRXRD using w-mode scan was employed to evaluate the structural properties. For the measurement of the micro-Raman scattering spectra, polarized light from the 488.0 nm line of an Ar-ion laser was focused down to 1 pm diameter of the samples at room temperature. The polarization of the scattered light wan not identified. The electrical properties of all samples were evaluated by Hall measurements in Van der Pauw geometry at room temperature. Atomic force microscopy ( AFM ) was performed to investigate the morphological features of samples, using a Digital Instruments Nanoscope IIIa instrument, operated in contact mode. The root mean square (rms) surface roughness was calculated using the AFM software.

2 Results and Discussion Fig. 1 shows XRD pattern measured in 8/28 scan mode. As the Si-doping concentration increases, the 28 position shifts to higher angle, indicating that the c-lattice constant becomes smaller. From the 28 value, the residual strain, A c/ co, was analyzed, where co is the lattice constant along c axis for fully relaxed Ab,,G%,,N alloys, calculated by the bulk lattice constants of GaN and AlN, using Vegard's law. Taking account of the AlGaN films under the strain, the inplane stress ( a//) can be roughly estimated by the simple expression for the elasticity"01 , (c-co)B B afl=

CO

*'".,Ga".P

(0002) v

". d .

v

.2 . 3 3

.

(3) Sample B (2) Sample A ( 1 ) Sample C

(3)

A -

(2) (1)

In order to further investigate the Si-doping effect on the residual stress of the Alo.sG%., N alloys, the Raman scattering spectra of all samples was measured. AlGaN alloys have a hexagonal wurtzite structure belonging to the C& space group with two formula units per primitive cell. A group theory analysis of lattice vibrations at the Brillouin zone center predicts the following four Raman active phonon modes : A ( z ) + E I ( x ,y ) + 2 E 2 , where the x , y and z in parentheses represent the polarization direction. The polar modes, A I ( 2 ) and E l ( x ,y ) are polarized along the z optical axis and in the basal ( x , y ) plane, respectively. Since A l and E l symmetries are both Raman and IR active, they split into longitudinal ( L O ) and transverse (TO) components. The frequency of the E 2 Raman mode is linearly dependent on stress and not sen.sitive to doping level of the samples due to its transverse optical character, and is therefore a measure of the residual strain state of the sample. Higher phonon frequencies imply compressive stress , whereas lower phonon frequencies imply tensile stress (as compared to the relaxed phonon frequency). Fig. 2 shows the Raman spectra in the E 2 spectral region in t (x , - ); scattering configuration for all samples. Sample A has

Y

where B is the bulk modulus (200 GPa) and Y the Poisson ration (0.38)"01. As seen in Table 1, the residual strain in the undoped film is compressive strain, while that in the film doped with n 1 . 2 x 10'' cm-3 changing into tensile strain. The rocking curve of the (0002) plane of all samples was measured by the HRXRD. It is known that the symmetric (ooo2)rocking curve is broadened by screw and mixed dislocations. As listed in Table 1, the sample A has the minimum value of the full width at half maximum (FWHM), indicating that it has a reduced dislocation density.

-

Raman shifVcm-'

Fig.2 Raman spectra with z ( x , - ); configuration in the E 2 spectral region for samples A , B and C

Li L et a l . Silicon Doping Dependence of n-type Al,,,Ga,,,N Layers Grown

351

Table 1 Representative characteristics obtained from XRD, Raman and Hall measurements for a series of undoped (Sample C) and Si-doped A10.5Ga,,.SN(Sample A , B ) used in this study Samples

A B C

Fig.3

Flow rates of S i b / ( nmol * min

2.4 8.1 Undoped

)

Residual strain Ac/co

Biaxial FWHM in (0002) stress/GPa w scan (arcmin)

2 . 0 ~ 0.105 - 3 . 9 ~ 1 0 - ~ -0.205 1 . 8 ~ 1 0 ' ~ 0.947

31.2 41.4 46.2

Phonon frequency Ezlcm-'

Mobility/(cm2*Vs-') and concentration/cm -

588.1 584.5 589.1

Mob.: 12.3 Con. : 3 . 0 10'' ~ Mob.: 12.0Con.: 1 . 2 ~ 1 0 ' ~ Semi-insulating

.

AFM images of samples C ( a ) and A ( b ) The rms surface roughness values measured from the 10 pm x 10 pm images were ( a ) 38.2 nm and (b) 31.8 nm

an E 2 mode frequency of 588.1 cm-' ; Sample B , 584.5 cm-'; and Sample C , 589.1 em - '. The E 2 mode frequencies monotonically decrease with increasing Si-doping concentration. This frequency shift verifies the presence of the gradual relaxation of the biaxial compressive stress in the films. The residual stress in the film is believed to change to tensile stress at the SiH4 flow rate of 8.1 nmol*min-'. The electrical properties of all samples were evaluated by Hall measurements in Van der Pauw geometry at room temperature. The ohmic contacts were formed by the Ti/Al/Ni/Au and subsequent annealing at 850 "c in N2 ambient. The electron concentration and electron mobility of all samples are listed in Table 1. The undoped film is semi-insulating and the elecGa, N increases with the tron concentration of SiH, flow rate. At the highest doping level, we achie-

ved the highly conductive A&,,G%,,N film with the electron concentration 1.2 x lOI9 cm-3 and the mobility of 12. 0 cm2 * Vs-' . Achieving highly conductive Al, Gal I N alloys with high A1 contents is known to be very challenging since oxygen impurity incorporation as well as the presence of dislocations can enhance the formation of cation vacancies during the nitride crystal growth to form energetically stable VAl or VA1 - ON complexes, which act as acceptors and compensate the a-type cond~ctivity'~].It has also been reported that

the A1 atoms or Al-containing molecules may have a small diffusion length on the surface of the low temperature nucleation layer, so the size of the AlGaN islands that appeared in the initial stage of the growth is small'"]. Therefore, it causes the AlGaN layers with high-density dislocations and a rough surface. Si can act as surfactants, which modify the surface energy in films on mismatched underlying layers. The absorption of Si atoms appeared to enhance the surface mobility of the group 111 atoms"21. Figs. 3( a ) and ( b ) show AFM images of sample C and A . The measured rms surface roughness over the 10 pm x 10 pm scans decreases from 38.2 nm for sample C , to 31 . 8 nm for sample A , indicating an improvement of morphological features of samples, consistent with previous rocking curve results. The results of the X-ray photoelectron spectra (XPS) showed that the oxygen concentration also decreased in Si-doped samples. It is thus assumed that doping with Si acting as surfactants may decrease the oxygen impurity and dislocation densities G q . 5N films, which is essential for attaining in' highly conductive Al-rich epilayers . ,

3

Conclusion

The highly conductive Si-doped Alo Ga,,, N film with the electron concentration of 1.2 x lOI9 cm-3 and the mobility of 12.0 cm2* Vs- was achieved. By employing XRD and Raman spectroscopy, a systematic

352

study was performed to assess the Si-doping effect of the relaxation of residual stress in Ab.,Ga,,sN films grown by MOCVD. As the SiH4 flow rate increases, the lattice constant c is found to decrease continuously and the compressive stress becomes relaxed. Hall-effect measurements showed that the electron concentration increased with Si-doping level.

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