crystalline silicon heterostructure

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physics& Technology37 (1996) 723-726

IR selective sensitivity of amorphous/crystalline silicon heterostructure Boris G. Budaguan, Alexei A. Sherchenkov *, Arcadi A. Aivazov Department of Microtechnology, Institute of Electronic Technology, Moscow, Russian Federation

Received 11 December1995

Abstract

An IR detector based on a-Sin-type)/c-Si(p-type) heterostructure is reported to have selective sensitivity in the wavelength range of 1000-1100 nm with a maximum at 1080 nm. The energy band diagram of the heterostructure is considered taking into account the existence of the inteffacial layer. The analysis of the photogeneration and carrier transport processes in the a-Si(n-type)/c-Si(p-type) heterostructure has been carried out. It is shown that the interracial layer and defects on the a-Si/c-Si boundary control the spectral photosensitivity characteristic.

Recent investigations have suggested that amorphous semiconductor/monocrystalline silicon heterostructures are effective for the production of a number of photoelectronic devices [I-4]. However, there is small progress in using amorphous silicon and well developed crystalline Si technology for IR detector fabrication. Though there are studies of the a-Si:H/c-Si heterostructure from the viewpoint of its application to many devices [1,2,5-8], the physics of the a-Si/c-Si heterojunction is far from being understood. A detailed analysis of the energy band diagram of the a-Si/c-Si heterostructure is necessary to understand the fundamental device physics. In this paper, we study the energy band diagram of the a-Si(n-type)/c-Si(p-type) heterostructure and analyze the photogeneration and carrier transport processes in the structure.

* Corresponding author.

The a-Si films with the thickness of 1.6-3.7 I~m were deposited by the RF magnetron sputtering of monocrystalline n-type Si with a resistivity of 0.01 fl cm. The deposition was conducted through the mask on monocrystalline p-type Si substrates with a resistivity of 20 l l cm. The substrates were heated before the deposition to the temperatures T~ = 20, 200, 300 and 500°C. After the deposition samples were annealed at temperature Ta = 200°C for 30 min in vacuum. Next, the aluminium contacts were deposited through the mask on the top of a-Si film and c-Si substrate surfaces. The spectral dependences of the photosensitivity of the resulting a-Si(n-type)/c-Si(p-type) structures were measured in the wavelength range of 500-1200 nm at room temperature [9,10]. The results of the photosensitivity measurements for different T~ are shown in Fig. 1, It is seen from Fig. 1 that the increase of Ts from room temperature to 500°C shifts the position of the photosensitivity

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B.G. Budaguan et al. / lnfrared Physics & Technology 37 (1996) 723-726

50

60

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Vb=0v '" " Vb = 0.5 V -- -40

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1000

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maximum from X = 900 to k = 1080 nm. It is also seen that the value of the relative photosensitivity at X = 1080 nm is sufficient for the production of IR detectors. To analyze the photosensitivity spectral dependences presented above, it is necessary to consider the energy band diagram of the a-Si(n-type)/c-Si(ptype) structure. The reverse current-voltage characteristic of our structure is shown in Fig. 2. As can be seen from the figure there is an initial region with rapid growth of the reverse current with a following slow increase at Vb > 5 V. It was shown [7] that the

-7 0000000000000000

-9 -10

0

i

i

i

600

800

lO00

1200

Wavelength, nm

Fig. 1. Photosensitivityspectra of a-Si(n-type)/c-Si(p-type) structure for d = 1.6 p,m, Vb = 19 V, Ta = 200°C and different Ts.

-11

400

i

i

i

i

5

l0

15

20

25

Vb, V Fig. 2. Reverse current-voltage characteristic of a-Si(n-type)/cSi(p-type) structure for d = 1.6 p.m, Ts = 500°C and Ta = 200°C.

Fig. 3. Photosensitivityspectra of a-Si(n-type)/c-Si(p-type) structure for d = 1.6 p,m at different biases Vb.

absence of the reverse current saturation is connected with the presence of the additional layer on the a-Si/c-Si interface. So, the existence of the interfacial layer must be taken into consideration when analysing the energy band diagram. This layer may be a very thin native oxide existing on the silicon substrate surface. On the other hand, interface defect states may be caused by the positive ion bombardment of the c-Si surface from the plasma during a-Si deposition. Fig. 3 shows the relative photosensitivity spectra at different reverse biases V b for the a-Si/c-Si structure prepared at T~ = 500°C. As one can see from Fig. 3 the increase of the reverse bias shifts the relative sensitivity maximum from h = 840 to X-1080 nm. This shift of the maximum correlates with the change of the dependence I(V b) for Vb > 5 V. In Fig. 4 we present the energy band diagram of the a-Si(n-type)/c-Si(p-type) heterostructure. The Fermi level in c-Si was estimated to be 0.25 eV above the top of the valence band. The Fermi level in amorphous silicon, Sa, was determined from the dark conductivity activation energy measurements and was 0.55 eV below the bottom of the conduction band. It shows that the deposited a-Si films are slightly n-type. The values of electron affinities for c-Si (×e) and a-Si (×a) were taken as 4.05 and 3.93 eV [7], respectively. The bandgaps for c-Si and a-Si were taken as 1.12 and 1.4 eV [11], respectively. The

B.G. Budaguan et aL / Infrared Physics & Technology 37 (1996) 723-726

I qVd c-Si

l

iege

Xc

a-Si

~t>OI xa i-

Qss

',I l >

Fig. 4. Energy band diagram of a-Si(n-type)/c-Si(p-type) structure with interfacial layer at equilibrium.

diffusion voltage, Vd, estimated from this was equal to 0.46 V which correlates with presented in Ref. [5]. The charge space distribution in the case intermediate layer is determined from the neutrality condition

Qa+Qsc+Qs~=O

diagram the data with the charge-

(1) where Qa, Q~c, Qs~ are space charges of a-Si, c-Si and interface defect states, respectively. As c-Si is p-type in our case, so Q~ is supposed to be positive and in c-Si exists a region with a negative space charge. According to Eq. (1), the space charge Qa in a-Si is negative when Qss > Q~c and positive when Q~ < Q~c- The positive charge is originated from the positively ionized donors impurity while the negative charge can be determined by the high density of states near the midgap of a-Si [12]. These states behave like negatively charged acceptors when the Fermi level shifts towards the conduction band. When the low reverse bias is applied to this structure the interfacial layer and defect states on the a-Si/c-Si interface obstruct spreading of the depletion layer in c-Si [7]. So the rapid increase of reverse current in Fig. 2 at Vb < 5 V the is connected with the dependence of the potential barrier on the applied bias. At Vb > 5 V the interface charge, Qs~, becomes sufficiently small for the depletion layer to begin to spread in the c-Si side and the rise of the reverse current slows down. The energy band diagram model considered above will be used now to explain photosensitivity spectra of the a-Si(n-type)/c-Si(p-type) structure. For this purpose it is necessary to consider the distribution of photogenerated carriers in the a-Si film and c-Si

725

substrate at different illumination wavelengths. Using the spectral dependence of the absorption coefficient of a-Si [11] we estimated that the film with the thickness of 1.6 g,m effectively absorbs the light with h < 850 nm. Therefore, the holes photogenerated in a-Si at these wavelengths will participate in the photocurrent and will determine the photosensitivity spectrum of the structure. The light with h > 850 nm penetrates into the c-Si substrate where it is effectively absorbed and generates excess charge carriers. Thus, the minority of electrons photogenerated in p-Si substrate will result in the appearance of photosensitivity in the longer wavelength range. As can be seen from Fig. 3, the photosensitivity spectrum for Vb = 0 has a maximum at X = 840 nm. In the absence of the applied voltage the depletion layer in a-Si is small and is determined by the build-in field due to the space charge distribution. In this case as the hole diffusion length in a-Si is too small (about 500 ,~) [12] only holes generated near the a-Si/c-Si interface take part in the photocurrent. So the holes generated by the light with X < 800 nm at a larger distance than the diffusion length from the interface do not take part in the photocurrent. On the other hand, the photogenerated electrons in c-Si at h > 850 nm also cannot take part in the photocurrent due to their recombination on positively charged states of the interfacial layer. Thus, a maximum of the photosensitivity at h = 840 nm is observed in Fig. 3. The applied reverse bias Vb = 0.5 V leads to the decrease of the positive charge Qss on the interracial layer and thus to the participation of photogenerated electrons in the c-Si substrate in the photocurrent. As a result we see the shift of the photosensitivity maximum to longer wavelengths with the increase of Vb. On the other hand, there is no cut-off change in the wavelength range of 500-700 nm at Vb > 0.5 V. In this case the electric field strength in the a-Si film with d - 1 . 6 Ixm is high enough ( E > 3 × 103 V / c m ) to sweep off all photogenerated holes from the film. Taking into account that in a-Si ixe >> ixh (where ixe, ix h are the mobilities of electrons and holes, respectively) the photocurrent from c-Si becomes predominant which determines the photosensitivity maximum at h = 1080 nm. A small value of Q~ is also supported by the slowing down of the reverse current rising above 5 V in Fig. 2. In the

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B.G. Budaguan et aL / lnfrared Physics & Technology 37 (1996) 723-726

framework of this model the appearance of the photosensitivity peak in the long wavelength region with increasing Ts can be explained by the decrease of the density of states on the a-Si/c-Si interface. Thus, the existence of an interfacial layer and high defect density on the a-Si/c-Si interface controis the spectral photosensitivity characteristics. The lowering of the interface defect density leads to the IR sensitivity of the presented structure. So controlling the interface during the fabrication of the aSi/c-Si structure provides the monitoring of the detector characteristics in the wavelength range of 600-1100 nm.

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[2] H. Matsuura and Y. Hatanaka, J. Appl. Phys. 71 (1992) 2315. [3] N. Benhard, B. Frank, B. Movaghar and G.H. Bauer, Phil. Mag. B 70 (1994) 1139. [4] K. Aguir, A. Fennouh, H, Carchano, J.L. Seguin, B. Elhadadi and F. Lalande, Thin Solid Films 257 (1995) 98. [5] H. Matsuura, T. Okuno and K. Tanaka, J. Appl. Phys. 55 (1984) 1012. [6] H. Mimura and Y. Hatanaka, Appl. Phys. Lett. 45 (1984) 452. [7] H. Mimura and Y. Hatanaka, Jpn. J. Appl. Phys. 26 (1987) 60. [8] H. Matsuura, J. Appl. Phys. 64 (1988) 1964. [9] B.G. Budaguan, A.A. Aivazov and O.N. Stanovov, Phil. Mag. B 66 (1992) 355. [10] B.G. Budaguan, A.A. Aivazov and O.N. Stanovov, J. Phys.: Condens Matter. 5 (1993) 6965. [11] A.K. Batabyal, P. Chaudhuri, S. Ray and K. Barua, Thin Solid Films. 112 (1984) 51. [12] A. Madan and M.P. Shaw, The Physics and Applications of Amorphous Semiconductors (Academic Press, New York, 1988).