ELSEVIER Applied Surface Science 92 (1996) 124-127

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aoot surface science

Characteristics of PECVD GaN thin films

C.H. Lee *, C.T. Chela Graduate School of Materials Science and Engineering, Cheng Kung University, Tainan, Taiwan, ROC

Received 12 December 1994; accepted for publication 4 March 1995

Abstract

The characteristics of GaN thin films deposited by PECVD on Si are investigated. The compositional, chemical, electrical and optical properties are characterized as functions of deposition temperature and RF power. The resistivity, chemical etching rate and refractive index are analyzed and discussed in terms of the film composition.

1. I n t r o d u c t i o n

Among III-V compounds, GaN crystallizes into a wurtzite structure and has a direct band gap energy of about 3.36 eV. For optoelectronic applications, it is supposed to be an efficient light emitting material for blue emission. However, because of the presence of the native donors of high concentration nitrogen vacancies ( > 10 ~s cm-3), it is very difficult to compensate for the p-type impurities to fabricate a p -n junction. Consequently, under the limited condi- tions for application to LED, most of the GaN light emitting diodes are made of MIS structures. Since MIS diodes are operated by means of injection mechanisms different from those for p -n junction diodes, the basic requirements of the semiconductor crystal quality are not as critical as those for p -n junctions. In addition, for the fabrication of detectors and photo-voltaic cells, both polycrystalline and amorphous semiconductor materials can be used. Therefore, GaN thin films made by means of a non-equilibrium process, such as plasma enhanced

* Corresponding author.

chemical vapor deposition can be of interest for the various applications to optoelectronic devices in a similar manner as a-Si:H for solar cells and inte- grated circuits. For various reasons, in the recent years, GaN thin films have been investigated and prepared by different methods including photo-chem- ical vapor deposition [1], RF sputtering [2], atomic layer epitaxy [3], MBE [4], plasma enhanced CVD [5] and MOCVD [6,7]. Though limited success has been achieved in obtaining a p-type layer by doping with Mg or Be as an acceptor element from MOVPE growth of GaN on saphire, most of the low tempera- ture processes produce high resistivity non-stoichio- metric GaN. Perhaps because of application limita- tion, the highly resistive GaN thin films from most of the low temperature processes were not well characterized. Therefore, this work was initiated to study the nature of the origin of the high resistivity of GaN prepared by plasma enhanced CVD on Si substrates.

2. Experimental

The experimental work of this study is carried out in a quartz reaction chamber with the substrate placed

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C.H. Lee, C.T. Chen / Applied Surface Science 92 (1996) 124-127 125

on the resistance heated pedestal for GaN deposition in the temperature range of 300°C to 500°C under a low pressure between 1 to 5 Torr using triethylgal- lium and nitrogen as reactants [8]. The deposition reaction is assisted by the plasma generated by a 500 W, 13.6 MHz radio frequency wave generator with two semicircular plate electrodes placed outside the reaction tube and above the top plane of the pedestal. Properties including the solid state composition, and electrical, optical and chemical aspects are character- ized as functions of process parameters. The resistiv- ity, etch rate and refractive index are briefly dis- cussed in terms of the film composition.

3. Results and discussion

The composition of the films is determined by means of AES spectroscopic analysis. The results in terms of G a / N ratio as functions of RF power and substrate temperature are shown respectively in Figs. 1 and 2. When the RF power is low, the ionization rate of TEG is apparently not high enough to match with that of the ionized nitrogen, N +, available under the experimental conditions, which results in a low ratio of Ga/N. However, for high RF power, the ionization rates of both reactants are high enough to yield a more stoichiometric GaN compound. In the case of the substrate temperature effect, it is apparent

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Fig. 2. Solid phase Ga /N ratio and resistivity versus deposition temperature.

that in the low temperature range with medium RF power there is enough ionized gallium to react with N + to result in a near stoichiometric composition. At high substrate temperatures on the other hand, there appear to be more active N + species on the substrate surface for a reaction resulting in nitrogen rich Gab/.

The resistivity of the films grown under the same set of experimental conditions is determined by I - V measurements of the metal /GaN/Si(MIS) struc- tures. The data as functions of the RF power and substrate temperature are also plotted with the G a / N composition data in Figs. 1 and 2 respectively. As shown in Fig. l, the resistivity decreases from 1012 13 • cm for N-rich GaN films grown with 20 W of RF power down to about 101° l~. cm for growth with more than 80 W, while the composition G a / N ratio changes from about 0.65 to near stoichiometry. It is apparent that the resistivity is controlled by the composition of the films. Namely, the near stoichio- metric GaN has low resistivity similarly to other III-V compound semiconductors while the nonstoi- chiometric GaN becomes semi-insulating due to the defect structures of Ga deficiency. As to the effect of the substrate temperature on the resistivity by the data in Fig. 2, the resistivity increases with growth temperature from 107 f t - cm at 200°C to more than 10 l° f t . cm at 350°C, whereas the solid phase com- position G a / N ratio correspondingly decreases from 1.1 to about 0.55. This means that the stoichiometric

126 C.H. Lee, C.T. Chen /Applied Surface Science 92 (1996) 124"127

condition controls the resistivity in a similar manner as the effect of the RF power. Namely, the Ga-rich GaN films have a lower resistivity of about 10 7

~ - cm for growth at 200°C, while the resistivity of nitrogen rich GaN is higher because of the defect structure due to Ga deficiency for growth at higher temperatures.

Since the GaN thin films are desirable for semi- conductor processing application, the etching charac- teristics are important for photolithographic work. In this work, the etching experiments are carried out at 22 5: 2°C by using both 85% H3PO 4 and 0.5M KOH solutions as etchants. The etch rates as functions of growth temperature and RF power are plotted in Figs. 3 and 4 respectively. In general, the etch rate of 85% HaPO 4 is faster than that of the solution of KOH (0.5M). The etch rates from both solutions decrease with the increase of growth temperature (Fig. 3) and they both increase with the RF power (Fig. 4). Chemical etching is a dissolution process which should be related to either the density or the nature against chemical reaction. This means that

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Fig. 3. Etch rate and refractive index of GaN thin films versus deposition temperature.

either the density or the stoichiometric condition dominates the etching characteristics. Namely, the films of either higher density or lower Ga /N ratio would have a smaller etch rate because of a slower reaction against chemical etching.

The refractive index is determined from ellipsom- etry measurements, and also plotted in terms of growth temperature and RF power as shown in Figs. 3 and 4. The indices values show an opposite trend from the etching rates, but are weakly dependent on both the process parameters. The refractive index can be related to either crystallinity or material density. In this case, the nonstoichiometric GaN has larger refractive index because it has a greater density. In addition, the defect structures due to the unsaturated bonds are more effective to deflect the traveling lightwaves resulting in a greater refractive index.

4. C o n c l u s i o n

For GaN by PECVD on Si with a fixed reactant ratio and reaction pressure, the solid phase composi-

C.H. Lee, C.T. Chen /Applied Surface Science 92 (1996) 124-127 127

tion G a / N ratio varies from 1.0 to 0.5 strongly depending on RF power and temperature. The resis- tivity decreases from 10 .2 l l . cm for the GaN films (Ga/N---0 .65) to 10 .° l-l. cm for those near stoi- chiometry. The refractive index and resistance to chemical etching in both H3PO 4 and ](OH solutions also change with the process parameters in a similar manner.

Acknowledgements

The authors wish to thank T.F. Siao and S.L. Lung for their help in preparation of the manuscript. This work is sponsored by National Science Council for ROC (#NSC80-0404-E-006-37).

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