Plasma-cracked supply of group V and group VI elements for low temperature epitaxy

Journal of Crystal Growth 136 (1994) 157—161 North-Holland

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Plasma-cracked supply of group V and group VI elements for low temperature epitaxy T. Hariu, S. Yamauchi

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S.F. Fang

2

T. Ohshima 1 and T. Hamada

1

Department of Electronic Engineering, Tohoku University, Sendai 980, Japan

Plasma-cracking of group V (N, As, Sb) and group VI (Se) molecules into excited atoms has been confirmed by optical emission spectroscopy. The larger increase of density of group V elements than that of group III elements with increasing RF power can explain the shift of optimum V/Ill supply ratio for the epitaxial growth of 111—V compounds. Plasma-cracking of nitrogen molecules in nitrogen-hydrogen mixed plasma through Penning effect is useful to grow better quality p-type ZnSe layers.

1. Introduction Source materials with high reactivity are required to grow better quality epitaxial crystal layers at a lower temperature. The high reactivity, however, often brings problems such as toxicity, inflammability, etc. Hydrides of group V and VI elements would be good source materials without toxicity, although some of them require higher temperature for thermal decomposition than the growth temperature of the substrates. Their elemental sources, when supplied through thermal vaporization as used in MBE, consist of such multi-atomic molecules as As4, As2, Se6, Se4, Se2, etc., which are much less reactive compared with their atOmic states. The purpose of this paper is to describe the plasma-cracked supply of group V and group vi elements, which is confirmed by optical emission spectroscopy (OES) of plasma. Although the present results were obtained in the growth in plasma, which we call plasma-assisted epitaxy, the plasma-cracking of source materials into excited atoms or radicals can be employed for source

1

Now with Oki Electric Co., Hachioji, Japan.

2

Now with Texas Instruments, Dallas, Texas, USA.

cells in CBE or MBE to achieve lower temperature epitaxy and effective doping.

2. Plasma-cracking of group V and VI elements

The growth chambers for Ill—V compounds [1] and ZnSe [21 have been described elsewhere in detail. OES can detect active species supplied through plasma without disturbing the plasma system and in fact excited atomic states of group V and group VI elements were observed as shown in fig. 1. In molecular beam epitaxy (MBE), thermal crackers of group V and group VI elements have been successfully employed to improve the electronic properties of GaAs [3,4], InAs [5,6] and ZnSe [7,81.However, in these cases they are used to increase the dimer components (As2, Se2) instead of As4 and Sea. The atomic species produced in plasma, which are more reactive than dimers, have been useful in growing better quality layers at a low temperature with less V/Ill or VI/Il supply ratio [1,9,10]. OES can also detect the excited states of group III elements, but the increase of density of excited group V atoms with increasing RF power

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V and group VI elements for LTE

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was stronger than that of excited atomic group III elements, as shown in fig. 2. The latter increase is almost parallel to the increase in excitation of the host gas element, and simply corresponds primarily to the increase of electron density in plasma, while the former increase is more enhanced by plasma-cracking.

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It is to be noted that these active species sometimes react mainly with host plasma gas to produce new species, although their exact effect on growth behavior has not been clarified. For example, in hydrogen plasma OES detected SeH, although hydride of As or Sb has not been observed, and in nitrogen plasma OES detected SeN. The increase of emission intensities of atomic Se and SeN in nitrogen plasma, SeH in hydrogen-nitrogen mixed plasma, and intensities of other species are shown in fig. 3 as a function of Se-source temperature. The emission lines of Se and SeN in hydrogen—nitrogen mixed plasma are merged into hydrogen emission lines and then it was difficult to derive their correct dependence. However, the remarkable decrease of N~ line is associated with the formation of SeN by the reaction between SeH and N~. Nitrogen gas is composed of very stable dimer molecules and its cracking into atoms has not been confirmed by OES in pure nitrogen plasma when the plasma was excited with 13.56 MHz RF power at a pressure around 4 mTorr (although OES detected excited N2 and Nt). However when the nitrogen gas is mixed with about 20% hydrogen, emission lines of atomic nitrogen were clearly

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3. The effect of plasma-cracking on optimum

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observed, as shown in fig. 4, and this mixed plasma was found to be suitable in getting p-type ZnSe with higher conductivity and better photoluminescence property [2]. The plasma-cracking of nitrogen molecules in mixed plasma is most likely caused through Penning effect; energy transfer from excited hydrogen to nitrogen molecules. Although such plasma-cracking through Penning effect was also detected in nitrogen—helium mixed plasma, the growth in this mixed plasma has not resulted in better quality ZnSe [2].

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WAVELENGTH (nm) Fig. 4. Optical emission spectra of (H2 +N2) mixed and N2 plasma with different ~N2 /(PH2 + ~N2) partial pressure ratios;

(a) 60%, (b) 80% and (c) 100%.

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T Hariu et al.

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methods such as MBE and good quality layers can be obtained with a wider range of V/Ill supply ratio [9,12]. However, when we want to reduce the growth temperature, the V/Ill supply ratio should be more precisely controlled at a lower temperature, because the electronic properties of grown layers depend much more critically upon the supply ratio. Fig. 5 shows such a

of group Vand group VI elements for LTE

critical dependence at low temperature and a weak dependence at higher temperature for InSb and InAs. The optimum V/Ill supply ratio for growth of high quality epitaxial layers shifts to lower values with increasing RF power supply to excite plasma, as shown in fig. 6 for InSb and InAs. This shift of optimum V/Ill supply ratio is caused by the

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steeper increase of density of excited group V atoms relative to group III atoms with increasing plasma power, as described in section 2.

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4. Conclusions

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been by optical emission spectroscopy. Such confirmed cracking and chemical activation of source materials can avoid the use of toxic or in-

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flammable further materials the(CBE), epitaxial as sometimes used temperature. in chemibeamreduce epitaxy and growth can be employed to This chemical activation is also useful to enhance the sticking probability of doping impurity, which is otherwise very low, as found in the growth of p-type ZnSe : N.

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References

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[1] H. Takei, T. Hamada and T. Hariu, J. Crystal Growth 115 (1991) 309. [21 T. Hamada, T. Hariu and S. Ono, Japan. J. AppI. Phys. 32 (1993) 674.

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Fig. 6. Effect of applied RF power and V/Ill supply ratio on the electrical properties of (a) InSb and (b) InAs grown at 270°C.

[3] J.N. Neave, P. Blood and BA. Joyce, Appl. Phys. Letters 311. [4] 36 H. (1980) Kuenzel and K. Ploog, AppI. Phys. Letters 37 (1980)

416. 4239. [5] BR. Hancock and H. Kroemer, J. Appi. Phys. 55 (1984) [6] S. Kalem, J. AppI. Phys. 66 (1989) 3097. [7] D.A. Cammack, K. Shahzad and T. Marshall, AppI. Phys. 56 (1990) 845. [8] H. Cheng, J.M. DePuydt, M. Hasse and J.E. Potts, AppI. Phys. Letters 56 (1990) 848. [9] K. Matsushita, T. Sato, Y. Sato, Y. Sugiyama, T. Hariu and Y. Shibata, IEEE Trans. Electron Devices ED-31 (1984) 1092. [10] T. Ohshima, S. Yamauchi and T. Hariu, Japan. J. Appi. Phys. 28 (1989) L13. [11] T. Hamada, Master Thesis, Tohoku University (1993). [12] S.F. Fang, K. Matsushita and T. Hariu, Appi. Phys. Letters 54 (1989) 1338.