MULTILAYERED AMORPHOUS SILICON STRUCTURES

HAL Id: jpa-00226712

https://hal.archives-ouvertes.fr/jpa-00226712

Submitted on 1 Jan 1987

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MULTILAYERED AMORPHOUS SILICON STRUCTURES

M. Hirose, S. Miyazaki

To cite this version:

M. Hirose, S. Miyazaki. MULTILAYERED AMORPHOUS SILICON STRUCTURES. Journal de

Physique Colloques, 1987, 48 (C5), pp.C5-589-C5-596. �10.1051/jphyscol:19875128�. �jpa-00226712�

MULTILAYERED AMORPHOUS SILICON STRUCTURES

M. HIROSE and S. MIYAZAKI

Department of Electrical Engineering, Hiroshima University, Higashihiroshima, 724, Japan

Ultra-thin multiple layered structures which consist of hydrogenated amorphous silicon (a-si:H) and silicon nitride (a-Sil-XNX:H) have been systematically studied as a new class of semiconductor superlattices. The layer thickness can be controlled on an atomic scale, and hence the optical and electrical properties have been interpreted by assuming the quantized states in the potential well layers, as in the case of crystalline semiconductor superlattices. The existence of the quantized levels in the a-Si:H well layer has been directly demonstrated by observing the resonant tunneling current through a-Si:H/a-Si3N4:H double barrier structures. Novel thin film transistors (TFTs) in which the ~ - s ~ : H / ~ - S ~ ~ - ~ N ~ : H multilayer is employed as an active region have been designed and fabricated.

1. INTRODUCTION

Each of amorphous semiconductors composed of random network has its own short range order in terms that the nearest neighbour interatomic distance or bond length is basically identical to that of the corresponding crystalline phase. Artificially designed periodic potential structures produced by amorphous semiconductor multilayers are, therefore, regarded as an amorphous superlattice. In particular, ultra-thin multilayered structures consisting of hydrogenated amorphous silicon (a- Si:H) and silicon-based compounds such as a-Sil -xCx:H, a-Sil -xNx:H, a-Sil -xOx:H and a-Sil _xGeX:H have been extensively studied[l -51. In this paper, we describe recent studies on the structural, optical and electrical properties of a-Si:H/a-Sil-XNX:H multilayers and demonstrate the novelty of this material.

2. PREPARATION OF MULTILAYERED STRUCTURES

Ultra-thin a-Si:H and a-Sil-xNx:H layers were alternatively deposited by the glow discharge decomposition of an SiH4 gas and an SiH4+NH3 gas mixture, respectively. The nitrogen content X in a-Sil_xNx:H was controlled by changing the gas molar fraction of [NH31/[SiH4]. The preparation method of the multiple layered structures is described in detail in a previous work[&]. For the purpose of obtaining the abrupt interface and of preparating the uniform ultra-thin layers with desired chemical compositions, the deposition time for the individual layer growth was kept much longer than the residence time of the reactive gases in the reactor.

In addition, the glow discharge was turned off at each step of the individual layer deposition, and the reaction chamber was pumped down to 10-3 Torr and purged with hydrogen gas.

3. STRUCTURE OF AMORPHOUS SUPERLATTICES

Evaluation of amorphous multilayers was performed by Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), x-ray diffraction and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875128

JOURNAL DE PHYSIQUE

I I I

a-S~:H (z~A)I~-s~~N,:H(IooA)

-

2 ND 30 PERIODS

-

- -

1 , . . 1 1 1 1 1 1 1 1 1 1 1

1.5 2.0

25

2 6 (DEGREES)

Fig. 1 X-ray diffraction intensity vs scattering angle (lower scale) and lattice spacing (upper scale) for an a-Si:H(25 A)/a-Si3N4:H(100 A) multilayer.

transmission electron microscopy[l-31. X-ray diffraction of the multilayers clearly shows the periodic nature associated with the well defined superstructure. Figure 1 represents a typical diffraction intensity curve plotted against scattering angle.

The specimen was designed so as to contain 3 0 periods of 25 A thick a-Si:H well layer and 100 A a-SigN4:H barrier. The x-ray diffraction measurements for the multilayers deposited on c-Si substrates were carried out by using CuKa ( A =1.5418 A) radiation. In the figure, the first Bragg peak is not observed since it must appear at a very low angle of about 0.7 degrees. The layer spacing is determined to be 124 A from the positions of the Bragg peaks observed in Fig. 1.

This value is in good agreement with the layer thickness (125 A) estimated from the deposition rates of the individual layers. Also, the measured full width at half maximum (FWHM) in the 2 8 scan is about 0.1 degrees, indicating that the deposited layers are smooth and parallel on an atomic scale and the a-Si:H/a-Si3N4:H interface is abrupt enough to reveal quantum size effect. Additional information on the layered structure is derived from x-ray rocking curves obtained by holding the specimen around the Bragg angle. Figure 2(a) shows a rocking curve for the 2nd Bragg peak observed in Fig. 1. A sharp peak of the FWHM'L0.04 degrees strongly supports the existence of the well defined layered structure. However, there exists a weak satellite peak at about +0.06 degrees away from the main peak. This subpeak is clearly observed in the rocking curve optically aligned for the subpeak (Fig. 2(b)).

This implies the presence of monolayer steps and/or local mosaic structures in the multilayers[21. The monolayer steps possibly existing in the multilayers could be ascribed to the fact that Si-N-Si bond formation at the interfaces does not necessarily proceed through the same growth process for the case of a-Si:H deposition on a-Si3N4:H and for a-Si3N4:H on a-Si:H. Also, a surface smoothing mechanism arising from enhanced surface diffusion of adsorbed species might be involved in the plasma deposition process, resulting in surprisingly flat interfaces.

4. OPTICAL PROPERTIES 4.1 OPTICAL BANDGAP

For the well-defined, ultra-thin layered structures in which the potential well width is in the range of a carrier de Broglie wavelength and the inelastic carrier

diffusion length exceeds the well width, a blue shift of the optical absorption edge can be observed since the effective optical bandgap of the well layer increases due to the quantization effect of the extended states. Figure 3 represents the optical bandgap Eopt for a-Si:H/a-Si3N4:H multilayers measured as a function of well layer thickness. In determining the optical absorption coefficient of the a-Si:H well layer, the effective refractive index of the layered structure was taken into account instead of using the refractive index of bulk a-Si:H. The optical bandgap of amorphous silicon multilayers exhibits a systematic blue shift when the well layer thickness decreases from 50 to 8 A[1,3,41, in consistence with the increase in the conductivity activation energyr51. The measured optical gap is wellexplainedby assuming that the optical absorption occurs through the transition between the first quantized states in the conduction and valence band of the a-Si:H well layer. The calculated curve in the figure was%,obtained by assuming the electron effective mass of m:=0.6m0 and the hole mass mc=mo, where mo is the free electron mass. In addition, the band discontinuity of 1.7 eV is employed for both the conduction and valence band. The band alignment was directly determined by the valence band spectra of the heterojunctions[61. It is unlikely that the undesirable incorporation of nitrogen into the a-Si:H well layer causes an increase in Eopt,because the slope of the Tuac plot

a

increases from 830 to 1 1 0 0 ( e ~ c m ) - ~ / ~ with decreasing the a-Si:H well layer thickness. If the a-Si:H layer is contaminated with nitrogen, the slope must be reduced in disagreement with the observed result.

4.2 PHOTOLUMINESCENCE

Photoluminescence (PL) spectra for a-Si:H ( 8 % 400 A)/a-Si3N4:H (100 A ) multilayers consist of an emission band arising only from a-Si:H well layers when the specimen is excited with 514.5 nm light from an ~r' ion laser. The spectrum for the multilayer having a well layer thickness of more than 100 A was basically similar to that for bulk a-Si:H, while the spectrum for the thinner well width than 50 A was different from the bulk one. The blue shift of peak energy and broadening of FWHM were observed as the well layer thickness decreases below 50 A. Note that the luminescence occurs through radiative recombination of electrons and holes trapped by the band tail states[7]. The quantization in the extended states modifies

JOURNAL DE PHYSIQUE

0

Experimental

_- I

-

Theoretical

WELL LAYER THICKNESS ( A )

Fig. 3 The optical bandgap Eopt and the slope of the Tauc plot

fi,

as a function of well layer thickness for a-Si:H/a-SigN4:H(100 A) superlattices. Solids line indicates the calculated Eopt by assuming the electron effective mass m;=0.6m0 and the hole mass m~=l.OmO.

the band tail states of the two dimensional system: The quantized states might be accompanied with their own tail states arising from random potential in the a-Si:H well layer. This introduces a new PL band at a higher energy side in addition to the emission band for bulk a-Si:H, and hence the FWHM of the emission spectrum apparently becomes broad. The integrated PL intensity increases with decreasing the well layer thickness below 50 A as shown in Fig. 4, in which the differences in the total a-Si:H layer thickness and in the effective refractive index of each sample are carefully taken into account to determine the integrated PL intensity for the same amount of absorption of the excited laser light. The result indicates that the decrease in the well layer thickness reduces the nonradiative recombination through gap states because the tunneling probability of carriers.to deep lying nonradiative states is decreased when a very thin a-Si:H is sandwithed with barriers[71.

The luminescence quenching by electric field applied perpendicularlyto the multilayers was studied, in order to reveal the existence of the well-defined a-Si:H quantum well to confine electrons in the a-Si:H/a-Si3N4:H multilayers. The absence of built-in electric field in the superlattices and the spatial extent of carrier wave functions in the well layer are also clarified by PL quenching experiments.

Figure 5 shows the luminescence intensity in the a-Si:H/a-SigN4:H multilayers as a function of applied electric field perpendicular to the layered structures. In the range of the well layer thickness from 400 to 100 A, the luminescence quenching due to the applied field becomes more pronounced with decreasing the well layer thickness, indicating that photogenerated holes and electrons are spatially separated. For smaller well layer thicknesses less than 50 A, the quenching effect becomes less pronounced. In particular for the well widths less than 25 A, the quenching effect is hardly observed because the photocarriers in the quantized levels can be no longer spatially separated even at an electric field strength of

lo6

V/cm. Therefore, it is likely that the spatial extent of carrier wave function is larger than 25 A. Note that the clear observation of PL quenching by external electric field is a direct consequence of the absence of built-in field induced by interface defects.

- - - -

-

a - -

O

W

- + ^L- -

[r:

-

C3 -

'5' 'lib

^{I I I}

^l

^{l I l l l 1 1 1}

Z ^{50 100 500}

WELL LAYER THICKNESS ( A )

Fig. 4 Integrated PL intensity a.s a function of well layer thickness.

Fig. 5 Integrated PL intensity as a function of applied field for a-Si:H/

Si3N4:H(100 A) multilayers.

JOURNAL DE PHYSIQUE

5. RESONANT TUNNELING

In order to directly prove the existence of the quantization effect in the amorphous multilayers, the current transport across the a-Si:H/a-Si3N4:H double barriers was examined, because the resonant tunneling of electrons through the quantized states in the a-Si:H well will yield current bumps on the current-voltage characteristics. Figure 6 represents measured current-voltage characteristics of a double barrier system with a well width of 4 0 A, together with the calculated electron transmission coefficient inthe the double barrier. The calculation of electron transmission coefficient in the double barriers was carried out, based on the WKB approximation[8] for the band profile with the conduction band discontinuity of 1.7 eV determined by XPS[6]. We assumed that electrons in the symmetric double barrier system at zero bias have a thermal energy of kT/2 and that externally applied bias is divided among the a-Si3Nq:H barrier layers and the phosphorus doped a-Si:H well layer by considering the respective film thickness and permittivity. No significant structure or current bump is observed at 288 K because of the thermal smearing effect, while at 77 K the current bumps are clearly observed at applied biases of about 0.2, 0.4 and 0.9 volts. The current density is less sensitive to temperature as expected for the tunneling transport. Note that the voltages corresponding to these current bumps are in excellent agreement with theoretically predicted biases at which the resonant tunneling takes place when the electron effective mass m: is chosen as 0.6m0. It must be emphasized that the electrqn effective mass obtained from the resonant tunneling experiments is consistent with that determined from the optical bandgap data for a-Si:H/a-Si3N4:H multilayers as shown in Fig. 3. Unfortunately, sharp resonant tunneling can not be detected in the

lo-*

WELL LAYER THICKNESS 1

Fig.

n

m

0.5 1.0 1.5

APPLIED BIAS ( VOLTS )

6 Measured current-voltage characteristics of a double barrier with the well layer thickness Lw=L+O A. Metal contact is positively biased. Also, the calculated electron transmission coefficient through the double barrier T*T at 77 K is plotted against applied bias.

the conduction electrons in a-Si:H can be quantized even in the potential well with a layer thickness of 40 A, indicating that the free electron wave function in a-Si:H must extend over at least 40 A in consistence with the result of electric field quenching of luminescence.

6. APPLICATIONS

Based on better understanding of the quantum size effect in the multilayers, novel thin film devices such as superlattice TFTs, EL diodes and solar cells have been designed and fabricated [9-111. The superlattice TFT is expected to have unique features as follows: (1) The electrons confined in an a-Si:H well layer exhibit quasi-two-dimensional natures and the mobility could be enhanced. (2) The surface band bending in the electron accumulation layer is moderated by the presence of a- S~I-~N,:H barrier layers and hence the drop of field effect mobility due to the interface states could be reduced.

The superlattice TFTs with active layers consisting of a-Si:H/a-Sil-,Nx:H superlattices were fabricated in a reverse staggered electrode structure. Figure 7 shows the field effect mobility measured as a function of well layer thickness for the fixed a-Sil-,NX:H barrier width of TO A. The mobility remains almost unchanged (0.13 cm2/vsec) when the well layer thickness is reduced down to 50 A, below which it starts to increase and saturates around 0.70 cm2/vsec at well widths of less than 25 A. The well thickness at which the mobility starts to increase corresponds to the onset of the quantization in the a-Si:H conduction band as shown in Fig. 3. This appears to indicate that the quantized electrons less interact with the band tail states which deteriorate the mobility[9].

WELL LAYER THICKNESS ( A )

Fig. 7 Electron field effect mobility as a function of well layer thickness for superlattice TFTs.

JOURNAL DE PHYSIQUE

7. CONCLUSIONS

It is demonstrated that a new class of semiconductor superlattices and quantum well structures are fabricated from the alternative deposition of a-Si:H and a-Sil-x Nx:H. It is found that the a - s i : H / a - ~ i ~ - ~ ~ ~ : H interface is abrupt on an atomic scale and that there are no significant interface states which induce the built-in electric field in the system and deteriorate the luminescence efficiency from the a- Si:H well layer. For the well layer thickness below 50 A, the existence of quantum size effects has been demonstrated from the blue shift of the optical absorption edge as well as of the luminescence peak energy and from the increase in the luminescence intensity. The photoluminescence quenching by electric field perpendicularly applied to a-Si:H/a-Sil-X~X:H multilayers has indicated that the spatial extent of electron wave function in the a-Si:H well is larger than 25 A. The direct evidence for the existence of the quantized levels in a-Si:H well layer has been obtained by observing the resonant tunneling current through a-Si:H/a-Si3N4:H double barriers. The superlattice TFTs have shown the enhancement of field effect mobility.

References

[I] B. Abeles and T. Tiedje, Phys. Rev. Lett. 51 (1983) 2003.

[2] B. Abeles, T. Tiedje, K.S. Liang, H.W. Deckman,.H.C. Stasiewsky, J.C. Scanlon and P.M. Eisenberger, J. Non-Cryst. Solids 66 (1984) 351; M. Hirose and S.

Miyazaki, Oyo Buturi 56 (1987) 186.

[31 J. Kakalios, H. Fritzche, N. Ibaraki and S.R. Ovshinsky, J. Non-Cryst. Solid 66 (1984) 339.

[&I

S. Miyazaki, N. Murayana, M. Hirose and M. Yamanishi, Technical Digest of the

1st Intern. Photovoltaic Science and Engineering Conf. (Kobe, 1984) p. 425.

151 C.R. Wronski, P.D. Persans and B. Abeles, Appl. Phys. Lett. 49 (1986) 596.

[61 T. Hayashi, S. Miyazaki and M. Kirose, Extended Abstracts of the 19th Conf.

Solid State Devices and Materials (Tokyo, 1987) to be published.

[71 R. A. Street, J.C. Knights and D.K. Biegelsen, Phys. Rev. B18 (1978) 1880.

[81 S. Miyazaki, Y. Ihara and M. Hirose, Extended Abstracts of the 6th Intern.

Conf. on Solid State Devices and Materials (Tokyo, 1986) p. 675.

[91 M. Tsukude, S. Akamatsu, S. Miyazaki and M. Hirose, Jpn. J. Appl. Phys. 26 (1986) L111.

[I01 D. Kruangam, T. Endo, M. Deguchi, W. Guang-Pu

,

H. Okamoto and Y. Hamakawa, Optoelectronics 1 (1 986) 67.

[Ill M. Hirose, N. Murayama, S. Miyazaki and Y. Ihara, Proc. of MRS Spring Meeting (Palo Alto, 1986) p. 405; H. Tarui, T. Matuyama, S. Tsuda, Y. Hishikawa, T.

Takahama, S. Nakayama, N. Nakamura, T. Fukatsu, S. Nakano, M. Ohnishi and Y.

Kuwano, Extended Abstracts of the 6th Intern. Conf. on Solid State Devices and Materials (Tokyo, 1986) p. 687.