Electrical characteristics of amorphous GaAs-n-crystalline Si heterojunctions

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering B34 (1995) 27 31

B

Electrical characteristics of amorphous GaAs-n-crystalline Si heterojunctions A. Fennouh,

K. Aguir, H. Carchano,

J.L. Seguin

Electronique et Physico-chimie des Couches Minces, Universit~ d'Aix-Marseille 111, Facultk des Sciences et Techniques de St-J~r6me, Case A62, 13397 Marseille Cedex 20. France Received 18 July 1994

Abstract

Capacitance-voltage (C V) characteristics and temperature dependence of current density-voltage ( J - V ) characteristics of as-grown amorphous gallium arsenide (a-GaAs) thin film heterojunctions formed on n-type crystalline silicon (c-Si(n)) have been measured. It has been found that the depletion layer of a-GaAs-c-Si(n) junction spreads in the c-Si side. The forward current, for bias voltages less than 0.4 V, shows voltage and temperature dependence expressed as exp ( - Eaf/k T)exp (A V), where Eaf and A are constants independent of voltage and temperature. This current may be ascribed to a multi-tunnelling capture-emission phenomena. The reverse current is proportional to e x p ( - Ea~/kT)(V) 1/2, where Ear is a constant. This current is probably limited by a generation process.

Keywords: Gallium arsenide; Silicon; Heterojunctions

1. Introduction Amorphous silicon (a-Si) technology continues to have a major impact in a wide range of applications [1,2]. By enabling the manufacture of devices such as thin-film transistors (TFTs) and photodiodes through plasma deposition over large areas at relatively low temperatures, this technology has reformed the fields of photovoltaics and photoreceptors and, more recently, flat-panel displays and facsimile machines. Moreover, recent investigations have shown that a-Si is effective in forming good heterojunctions with crystalline materials such as Si [3,4], GaAs [5] and CdTe [6]. Various models have been proposed to explain the electrical characteristics of these structures. Amorphous I I I - V semiconductors, and GaAs in particular, seem to be useful as optoelectronic devices working in the visible wavelength region. However, not so much interest is being shown towards amorphous I I I - V semiconductors. During the last decade, the main interest of our laboratory has been concentrated on the structure and electrical properties of amorphous gallium arsenide (a0921-5107/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved

S S D I 0921-5107(95)01221-4

GaAs) deposited by r.f. sputtering [7-10]. Attempts to improve the quality of the films and to apply them to optoelectronic devices have been undertaken. These activities have allowed the best deposition conditions to be obtained leading to stoichiometric a-GaAs films with high intrinsic properties. Thus high resistivity (108 f2 cm) amorphous GaAs thin films was obtained on glass substrates at deposition temperatures Ts varying from 303 to 563 K in pure argon at a pressure of 6.67 Pa with an electrode separation of 3 cm. The sputtering power was adjusted between 35 and 40 W in order to fix a constant d.c. self bias voltage of 800 V [11]. It has been found that at Ts -- 563 K the composition of the films is stoichiometric and the conductivity activation energy Ea reaches its maximum value (0.6 eV). At lower temperatures, the material contains an excess of As and is n-type-like, whilst at temperatures higher than 563 K, an excess of Ga results in a p-type-like material [11,12]. Using the space charge limited current method, a density of states at mid gap of 3 x 1016 e V - I C m - 3 was calculated. These states extended over a range of

28

A. Fennouh et al. ,' Materials Seienee and Engineering B34 (1995) 27 31

0.14 eV above the Fermi level. Optical measurements have shown a widening of the optical gap from 1.1 to 1.55 eV where T~ is increased from 303 to 563 K [13]. Moreover, the realization of electronic devices using a-GaAs requires film deposition on conductive substrates which either are polycrystalline (metals) or monocrystalline (semiconductors). The study of the heterojunctions obtained can be useful in investigating amorphous-crystalline junction properties in order to improve heterojunctions devices. This paper reports on the electrical properties of a-GaAs thin films deposited on n-type crystalline Si wafers by means of sputtering in Ar. Capacitance voltage (C V) and current density voltage ( J - V ) characteristics are discussed in the case of a simple structure of an undoped a-GaAs-c-Si(n) heterojunction.

2. Sample preparation Conventional r.f. sputtering equipment was used to deposit thin films of a-GaAs on n-type crystalline Si wafers (c-Si(n)) with an impurity concentration of about 5 x 1015 cm 3. An n + Si layer was diffused on the back side of the wafers to provide an ohmic contact. The wafers were cleaned by trichloroethylene and acetone, then boiled in pure H 2 S O 4 and HNO3 and the oxide etched off in dilute HF. a-GaAs films of about 500 nm in thickness were deposited on c-Si wafers by sputtering a water-cooled monocrystalline disc target of undoped G a A s by means of Ar plasma. The sputtering conditions were as follows: (a) the base pressure was about 10 4 Pa; (b) the pressure of Ar was 6.67 Pa; (c) The r.f. input power was 25 W corresponding to a d.c. self bias voltage Vdc of 600 V; (d) the substrate temperature was maintained at 150 °C. The deposited films thus obtained presented a slight excess of As (XAs = 0.51) For higher temperatures, the films are microcrystalline. In order to fabricate a simple heterojunction diode, an ohmic contact was formed on the back side of the c-Si wafer by evaporating Ag. Au was evaporated on the deposited a-GaAs film with an area of 5 x 10 3 cm 2. Au contact is described as an ohmic contact with a-GaAs [14] and this property has been verified in our laboratory, This structure is schematically shown in Fig. 1. T h e - C V characteristics of the devices were measured using an H P 4275A multi-frequency L C R meter and the J - V characteristics were performed using an H P 4140B picoammeter d.c. voltage source. For measurements of J V characteristics, the structures are set in a liquid-nitrogen-cooled cryostat which allows measurements to be carried out in the range 123-423 K.

V

'

lAu

Lea

la.GaAs

I n c-Si

I ....

I

--~

~'

Cc

-T-

-'2"-

Fig. 1. Structure and equivalent circuit of the heterojunctions studied.

3. Experiments 3.1. C V charaeteristics of a-GaAs c-Si(n) structure Fig. 2 shows the measured high frequency (1 MHz) C V characteristics of the a-GaAs c-Si(n) heterojunction. Forward direction refers to a positive bias applied to a-GaAs. This characteristic, similar to the C V characteristic of a-Si-c-Si(n) [15], is a typical high frequency differential capacitance of a heterojunction with a crystalline n-type substrate. Fig. 3 shows the C 2(V) dependence, which was plotted from the data of Fig. 2; the filled symbols represent the measured capacitance C and the open symbols represent the depletion layer capacitance Cc in c-Si deduced by the following assumption. The total capacitance (C) of the junction is given by

1

1

1

= Q + C-~

(1)

where Ca is the capacitance of the a-GaAs layer. Frequencies used are high enough to neglect the dielectric relaxation process in undoped a-GaAs which presented a minimum resistivity of about 107 f~ cm [11,16]. Hence we can consider the depletion layer extending in the c-Si(n) side regardless of that of the a-GaAs side. Indeed, a heterojunction formed by depositing a-GaAs on a heavily doped crystalline G a A s (1.7 x 1019 c m - 3 ) wafer showed a constant value capacity of a-GaAs alone in the frequency range of 10 K H z to 1 M H z from 5 V to 5 V bias voltages. This suggests that the 25

I

I

I

20

E 15

10

I

I

-4

-2

0

VOLTAG E(V)

Fig. 2. C V characteristics of the a-GaAs c-Si(n) heterojunction.

29

A. Fennouh et al. / Materials Science and Engineering B34 (1995) 27 31

30

I

J(AI m n~)

I

I

25 o

I

o.oo, p

I

I

I

I

I9

I

20

x t~

~"

15

E ,,.~u 1 0 (.9 5

10.9 a ~ 0

I -,

I

0 VOLTAGE (V)

-4

-2

2

Fig. 3. Plot of inverse of square of capacitance vs. applied voltage for a-GaAs c-Si(n) heterojunction. Solid symbols represent the capacitance measured; open symbols represent the capacitance deduced from using Eq. (1).

redistribution of majority carriers in the a-GaAs films cannot respond to an a.c. voltage as high as 1 MHz. This value may be considered as the geometric capacitance C~ of an MIS structure as if the a-GaAs layer were dielectric. Therefore, the depletion layer capacitance Cc can be obtained by using Eq. (1). The value of Ca, 23 nF, which has been used to calculate Cc from Eq. (1) is the saturated capacitance with the forward bias. As shown in Fig. 3 (open symbols), the dependence shows a linear relationship for voltages V less than - 1 V. The deviation from linearity in the low voltage region may be considered to be due to the interface states between c-Si and a-GaAs [17]. The impurity concentration (4.5 x 10 ~s c m - 3 ) deduced from the slope of cc2(V) is close to that of the c-Si used in this work. Consequently, this indicates that the depletion layer spreads only into the c-Si side. 3.2. Energy band diagram of a - G a A s - c - S i ( n ) heterojunction Assuming that the present undoped a-GaAs-c-Si(n) heterojunction can be compared with an abrupt heterojunction model, the energy band diagram of Fig. 4 can

,,,=o.o,.vy

";:,;"'%I E¢I

ir/j

7

" -1

0 1 VOLTAGE (V)

2

Fig. 5. Plots of log of current density J vs. voltage V applied to heterojunction at different temperatures: (1) 213 K; (2) 243 K; (3) 273 K; (4) 298 K; (5) 332 K; (6) 353 K; (7) 372 K; (8) 392 K; (9) 402 K.

be established. The conduction-band discontinuity is expressed by [18] AEc = XI - )(2

(2)

where X1 and X2 are the electron affinities of c-Si(n) and a-GaAs respectively. Using X] = 3.98 eV, a value deduced from the studies of a-GaAs-c-Si(p) heterojunctions [19], and X2 = 4.05 eV, AEc was found to be 0.07 eV, indicating that the main-band discontinuity occurs in the valence band. In fact AEc + Ev = Eg2 - Egl, where Eg 2 = 1.4 eV is the optical gap of a-GaAs (deduced by the measurements of Carchano et al. [13]) and Eg 1 = 1.12 eV is the band gap of c-Si(n). The energy difference 6, between the Fermi level and the bottom of the conduction band of c-Si(n) was found to be 0.23 eV from the equation c5~ = kTln(Nc/Nd), where k is Boltzmann's constant, T the temperature, Nc the effective density of states (2.8 X 1019 c m - 3 ) in the conduction band and Nd the impurity concentration of the donors (4 x 10 is cm 3), determined by the C - V measurements on an A u - c Si(n) Schottky diode. ~2 = 0.6 e g [11,16] is considered as the activation energy of dark conductivity of aGaAs. 3.3. J - V characteristics

,-"1 AEv:O.21eV

nc-Si(I-2ohm cm-I)

10.13 -2

~,

Eo2

a-GaAs(lO7ohm cm-J)

Fig. 4. Energy band diagram of a-GaAs-c-Si(n) heterojunction at equilibrium.

J - V characteristics of the a-GaAs-c-Si(n) heterojunctions have been measured as a function of temperature in the range 213-402 K and of voltage in the - 1.5 V < V < + 1.5 V range. Fig. 5 shows the temperature dependence of the current from which it can be

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A. Fennouh et al. / Materials Science and Engineering B34 (1995) 27 31

seen that the a-GaAs c-Si(n) junctions exhibit a marked rectification characteristic for the whole range of the measured temperatures. 3.3.1. Forward J - V characteristics

Rectification characteristics are generally described by either the diffusion model, the emission model, or the recombination model [20] from which a relation between J and V is given by

where T is the measuring temperature, and tl the temperature independent diode factor. The diode factor can take any value between 1 ~< ~l ~< 2 depending on the transport properties of the junction. However, in our case as shown in Fig. 5, for voltages less than 0.4 V, it was found that the gradient of the forward current on a semi-log scale was almost constant for various temperatures. Then, in the forward current, the voltage dependence of the junction current cannot be approximated by the typical function in Eq. (3) and is rather expressed as J = Jo[exp(A V)]

(4)

where A is a constant independent of temperature. These experimental results suggest that the forward currents in the voltage range 0.1 0.4 V are mainly dominated by tunnelling mechanisms. The data of Fig. 5 were analyzed using Eq. (4), resulting in the temperature dependence of the pre-exponential factor J0 shown in Fig. 6, Jo was obtained by extrapolating the forward current curves to zero voltage. Since localized states are quasi-continuously distributed in the mobility gap of

J0(A/mm^2) 1 0 .7

1 0 "8

a-GaAs [13], a multi-tunnelling process should predominate in the present system. Hence, J0 should change exponentially with T according to this multi-tunnelling process [21]. However, Jo obtained in our study is found to vary exponentially with - I / T as shown in Fig. 6. Namely, the relation Jo ' z exp

kT ]

holds between J{) and T, where AE~,r is an activation energy. This inconsistency was solved by considering the multi-tunnelling capture emission process proposed by Matsuura et al. [22] for amorphous on crystalline heterojunctions. In the multi-tunnelling capture emission model, Jo is expressed as

(6) where B is a constant independent of applied voltage and temperature, V~h the thermal velocity, (7,, (O-p) the capture cross-section of electrons (holes), Nc (Nv) the effective density of states in the conduction band (valence band) of a-GaAs, and Ev, ET, Ec, and Ev, the energies of the Fermi level, trapping level, the conduction band, and the valence band of a-GaAs respectively. The first term of Eq. (6) shows an electron emission rate and the second term shows a hole capture rate. The activation energy Ear= 0.34 eV obtained from Fig. 6 is different from the activation energy d = (Ev - Ev) = 0.8 eV [11,16] of the dark conductivity of a-GaAs. This suggests that an electron emission process dominates the carrier transport mechanism. The first term in the right-hand side of Eq. (6) determines the magnitude of Jo. Since no current flows in the junction when V = 0, as shown in Fig. 5, the net forward current density is described by J = J0[exp(A V)] -- 1 3.3.2. Reverse J

1 0 .9

1

0 "10

1 0 "11

i

2

i

3

i

i

4

1000IT

Fig. 6. Temperature dependence of the extrapolated values or Jo.

(5)

(7)

V characteristics

From the equation of J (Eq. (7)), a saturated value of the reverse current is expected to be Jo. However, Fig. 5 shows that the reverse current exceeds the value of Jo, indicating that the reverse current should be limited by another transport mechanism. Furthermore, the activation energy E~r=0.55 eV of the reverse current at - 0 . 1 V is different from E~,- for the forward bias characteristics. Fig. 7 shows the reverse current to increase as a function of (V) j'2, which was replotted from the data of Fig. 5 at room temperature. When the generation cur-

A. Fennouh et al. / Materials Science and Engineer#~g B34 (1995) 27 31

(3) The reverse current, described as e x p ( - E ~ r / kT)(V) ~'2, may be reasonably ascribed to a generation current.

JR(A / mm 2) I

I

I

I

I

1

References

0.8

0.6

0.4

31

I

.

1.4

Fig. 7. Reverse current density voltage characteristics at room temperature.

rent is taken into account in the depletion region, this current should be proportional to the width o f the depletion region that varies with (V) 1/2. The data show a good linearity in the - 1.2 V < V < - 0 . 5 V range. This linearity is observed for all experimental temperatures in this work. This indicates that this reverse current is probably limited by the generation current in the depletion region [20,23].

4. Conclusion Measured C V and J - V characteristics of an aGaAs c-Si(n) heterojunction were investigated. The results obtained are summarized as follows. (1) From the C V measurements, the depletion layer of an a-GaAs-c-Si(n) junction was considered to spread in the c-Si side. (2) The forward current was expressed as J = Joexp(AV) for voltages less than 0.4 V, where Jo is proportional to exp( - E~t-/kT). In this region, the forward current is attributed to an emission of electrons tunnelled from c-Si into gap states in the a-GaAs.

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