Changes in surface composition of GaN by impurity doping

Thin Solid Films 287 (1996) 184-187

ELSEVIER

Changes in surface composition of GaN by impurity doping T. M o r i ~, T. O h w a k i ~, Y. T a g a a , . , N. S h i b a t a b, M . K o i k e b, K. M a n a b e b a Toyota Central Research and Development Laboratories Inc., Nagakute-cho, Aichi 480-! !, Japan b Toyoda Gosei Co., Ltd., Nishikasugai-gun, Aichi 452, Japan

Received 7 August 1995; accepted 31 January 1996

Abstract Changes in the surface composition of GaN films by p- and n-type doping were studied using both Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It was found that the surface composition of the GaN films was affected by impurity doping, i.e. the surface of the p-type GaN film was enriched with gallium and that of the n-type GaN film with nitrogen, compared with that of the undopcd GaN film. The surface composition of the GaN films by AES agrees with that by XPS after taking the contamination layer thickness into account. The experimental results thus obtained were discussed by taking account of the formation energies of native defects in p- and n-type GaN. Keyword,,: Augerelectron spectroscopy; Semiconductors; Surface composition; X-ray photoelectron spectroscopy

1. Introduction Much effort has been devoted to the study of a high-efficiency blue GaN light omitting diode (LED) and the potentiality of a blue GuN laser diode (LD) [ 1,21. Recently, the bright blue LEDs of GuN wore reported [ 3 ], but many technical problems still remain unsolved in GaN devices II,2]. Among them, low ohmic contact resistance to p-type GaN is key technology for high emission efficiency and low driving voltage in blue GaN LED. Furthermore, contact resistance will be more important in GuN LD, because LD will require a high current density to operate. However, the final ohmic contact structure to p-type GuN is not yet determined because of the large work function of p-type GaN. On the other hand, it is more essential to know actual surface features of GaN such as chemical composition and morphology prior to ohmic contact formation. Surprisingly, there exists no systematic study of surface characterization of GuN using an analytical method. On the other hand, Neugebauer and Van de Waile [4] recently calculated formation energies of native defects in GuN with the first-principle total energy method, and coneluded the difference of vacancy formation between p- and n.type GuN, In the present work, we analyzed the changes in surl~ce compositions of undoped, p-type and n-type GaN films using AF~Sand XPS. Then, we discussed the prcsen! experimental "Com~ponding author 0040,609019615151~ ¢~ 19o,6El~vler Science S A All rights re,~l~,ed PII S0040.6091) ( 96 ) 08 7 ~ I. I

data taking accounts of the calculation results obtained by Neugebauer and Van de Walle [4].

2, Experimental Three kinds of specimens of undoped, p-type and n-type GaN films, were used in the present experiment. The specimens are schematically shown in two dimensions in Fig. I. First, AIN buffer film [5,6] was prepared on sapphire substrate at 400 °C, followed by the formation of undoped GaN film with a thickness of 2 000 nm at 1 150 °C. Finally, undopod, p-type, and n-type GaN films with a thickness of 500 nm were grown at 1 150 °C. Magnesium and Si were doped in GaN fihns as p- and n-type dopants and the concentrations of Mg and Si were I x 102o and 1 × I0 Is cm -3, respectively. These GaN and AIN films were grown by the metalorganic vapor phase epitaxy (MOVPE) method, where trimethylgallium, trimethylaluminum, ammonia, monosilane and hisundoped, p-,n- GuN; 500nm undoped GaN; 2000nm AIN Sapphire

Fig I Schenlatic slruclut~c of the specimens.

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T. Mori et aL /Thin Solid Films 287 (1996) 184-187

(c)n-~pe

Fig. 2. The scanning electron microscope image of p-type GaN film. The magnification is 50 000.

cyclopentadienyl magnesium were used as Ga, AI, N, Si and Mg sources, respectively. After the growth, the surface treatment of low-energy electron beam irradiation [7] or heat treatment [ 8 ] was performed on the p-type GaN film to obtain highly p-type GaN film. All specimens were rinsed in organic solvents before AES and XPS analyses. AES spectra were measured with JEOL JAMP- 10SX with a base pressure of 5 × 10 -7 Pa. The accelerating voltage was 5 kV, the typical electron beam current was 1.5 × 10 -'~ A and the electron probe diameter was 100 p,m. XPS spectra were measured with ULVAC-PHI PHI5500 using the Mg Ka line. The pressure during the measurements was under 9 × 10- 8 Pa. The surface morphologies of GaN films were first examined by scanning electron microscopy (SEM). It can be seen from the SEM image of the p-type GaN film shown in Fig. 2 that the surface is ahnost smooth except a lot of small holes. We also examined the surfaces of undoped and n-type GaN films, and lbund that they are smoother than that of the ptype GaN film. We concluded that the surface was sufficiently flat to discuss the surface composition change by AES and XPS.

3. Results and discussion

Figs. 3 and 4 show typical Auger electron spectra and Xray photoelectron spectra of (a) undoped, (b) p-type, and (e) n-type GaN films. These spectra show the presence of gallium and nitrogen, but Mg and Si were not detected because their concentrations were under AES sensitivities. On the contrary, there exists foreign atoms such as carbon and oxygen, which may arise from the surface contamination layer. Table I shows the peak intensities, which are the peakto-peak amplitudes of AES spectra and the peak areas of XPS spectra, normalized by the nitrogen peak intensities. Table I reveals that the surface of p-type GaN film is contaminated more heavily by carbon and oxygen than the others. This contamination was found to consist chiefly of organic corn-

MVV N I KLL LMM o ' ' 12'oo Kinetic Energy [oV] Fig. 3. Typical Auger electron spectra of (a) undoped GaN, (b) p-type GaN and (c) n-type GaN films. These spectra are normalized by the peak-topeak amplitudes of N KLL. ] Ga 2p~2 [

Ga LMM I N ls Ga 3d

-

o,)

LJ lo o s;o Binding Energy [eV]

o

Fig. 4. X-ray photoelectron spectra of (a) undoped GaN, (b) p-type GaN and (c) n-type GaN films, These spectra are normalized by the peak area of Nls.

pound from the analysis of the O Is peak of XPS spectra. This contamination is due to the surface treatment of a lowenergy electron beam irradiation or heat treatment for obtaining highly p-type GaN film. The other noticeable aspect of Table I is that the gallium intensity differs with each specimen. 'Fable 2 shows the gallium intensities normalized by that of the undoped GaN film, where the gallium intensities of AES and XPS are respectively denoted by R,, and Rx. It can be seen from Table 2 that the p-type GaN film has larger R,, and R~ than the undoped GaN film, and the n-type GaN film has smaller ones. Since the AES and XPS peak intensities are affected by the thickness of contamination layers on specimens, the effects of the contamination layer on Ra and R,, have to be estimated. The effects of the contamination layer can be easily estimated by a general method [91. We will first describe the modification of Ra and next describe that of R,,. The peak intensity was assumed to be proportional to the factor e x p ( - d / A ) , where d is the contamination layer thickness and A is the escape depth of Auger electrons. The escape depth was V'3/2 times as large as the inelastic mean free path in AES because of a reflection angle of 30° in the present

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T. Mori et al. /Thin Solid Films 287 (1996) 184-187

Table I The peak-to-peak amplitudes of AES spectra and the peak area of XPS spectra of undoped, p-type and n-type GaN films, which are normalized by the nitrogen P ~

....

,....

XPS

AES

Ga LMM N KLL C KLL O KLL

Undoped

P-type

N-type

0.66 + 0.01 1.00 0.27 5:0.01 0.12 :t: 0.01

0.95 + 0.02 1.00 0.70 + 0.06 0.70 + 0.03

0.40 5:0.01 1.00 0.38 5:0.06 0.11 5:0.01

Ga 3d N Is . O Is

.

Undoped

P-type

N-type

1.00 !.00 . 0.36

1.43 1.00

0.96 1.00

1.65

0.37

Undoped

P-type

N-type

!.00 !.00

i.43 1.24

0.95 0.94

.

Table 2 The gallium intensities normalized by that of undoped GaN film, which were obtained from both AES and XPS spectra

XPS

AES

Ri R/

Undoped

P-t~pe

N-type

1.~.,05:0.01 1.00 + 0.01

1.44 + 0.03 1.32 + 0.03

0.61 + 0.01 0.76 5:0.01

R, R,'

R,, ~ak.to-peak amplitude of Ga KI.,L obtained from AES; R~, peak area of Ga Is obtained from XPS.R/and Rx' are modified intensities of Ra and R,, respectively, for the contamination layer thickness

experimental geometry. The inelastic mean free paths were obtained from the formula proposed by Tanuma et al. [ 10], we used the following inelastic mean free path values, i.e. 2.00 nm for Ga LMM, 0.96 nm for N KLL and 0.47 nm for Ga MVV. Because of two peaks in the gallium spectra (Ga LMM and Ga MVV) with different escape depths, it was possible to deduce the difference in thickness of the contamination layer. The p-type GaN film had a contamination layer 0.14 am thicker than that of the undoped GaN film; n-type GaN film had a contamination layer 0.36 nm thinner than that of the undoped GaN film. We calculated the ratio R,,' of Raexp( - d/Ao,)/exp( - d/AN) of each specimen to that of undoped GaN film, where ~o~ and ANare the escape depths of Ga LMM and N KLL, respectively, Next we calculated the modified peak area ratios in XPS spectra, R~', in a similar manner as described above, The escape depth is 1/¢2 times as large as the inelastic mean free path because of a reflection angle of 45* in the present experimental geometry. We used the following inelastic mean free path values, i.e. 2.23 nm for Ga 3d, 1.69 nm for N Is and 0,55 nm for Ga 2p31z [ 10], As shown in the Table 2, the Rt~' and R~' of the p-type GaN film are bigger than unity and those of the n-type are smaller. Both AES and XPS results indicated that the p-tyre GaN film had a gallium-rich surface, and the n-type GaN film had a nitrogen-rich surface. This can be restated that tile p-type impurity doping increases gallium on the surface and that the n-type impurity doping increasesi~itrogen. Quantitatively, the p-type AES results agreed with XPS results; the n-type AES results showed larger changes in surface composition ratio than XPS results. Electron beam irradiation may stimulate the gallium reduction of n-type GaN film in AES measurement. A key to the understanding of these surface composition changes lies in the native defects in GaN. In fact, Neugebauer

and Van de Walle have studied the electronic structure, atomic geometry, and formation energies of native defects in GaN using the first-principle total energy calculation [4]. According to them, dominant native defects in GaN are vacancies, furthermore, nitrogen vacancy dominates in p-type GaN and gallium vacancy dominates in n-type GaN. This indicates that the gallium-rich surface on the p-type GaN film is due to the nitrogen vacancy, and that the nitrogen-rich surface on the n-type GaN film is due to the gallium vacancy. The present results shown in Table 2 can be qualitatively explained by the theoretical calculation results obtained by Neugebauer and Van de Walle. From a practical point of view, the surface composition changes obtained here are useful in improving the ohmic contact with GaN. This is because the surface composition affects a chemical reaction of GaN with metal, and because the chemical reaction is necessary to lower the ohmic contact resistance with p-type GaN.

4. Conclusion The impurity doping into GaN films ires been found to influence the surface composition. The p-type GaN film had a gallium-rich surface and the n-type GaN film had a nitrogenrich surface. These experimental results have been explained by the formation energy of native defects in p- and n-type GaN.

Acknowledgements We express appreciation to Drs. M. Hirose and N. Isomura for XPS measurement and to Dr. M. Tada for SEM measurement.

T. Mori et al. / Thin Solid Fibns 287 (1996) 184-187

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