InP(100) diodes

Thin Solid Films 256 ( 1995) 80-84

ELSEVIER

Electrical behaviour of epitaxial SrF,/InP( 100) diodes B. Mombelli, LEMME,

A. Elfajiri,

UniversitP de Bordeaux

G. Couturier,

A. S. Barr&e

I, 3.51 Cows de Irr LibPmtion.

33405 Trrlence Cedes,

Frunce

Received I5 October 1993; accepted 20 July 1994

Abstract In this paper, it is shown that the conductivity of SrF, layers epitaxially grown onto n-type InP( 100) substrates is about IO-l6 S cm-’ at room temperature with an activation energy close to 0.7 eV. An ionic contribution only appears for temperatures higher than 380 K. The electrical properties of MIS structures (metal/SrF,/n-type InP( 100)) were investigated by means of capacitance-voltage characteristics (C( V)) and admittance spectroscopy measurements. The results strongly depend on the cleaning process applied to the semiconductor surface. The classical cleaning process (H,SO, H,O, H,O + HF deoxidization) did not allow the accumulation regime to be reached in the semiconductor. In this case the trap density is greater than IO’* eV_’ cm-l. On the other hand, it was demonstrated that treatment with ammonium sulphide improves the qualities of the device. In particular, the density of states can be reduced to IO” eV-’ cmm2 and the accumulation regime is achieved. Keywords:

Dielectric

properties;

Electrical

properties

and measurements;

1. Introduction The passivation of III-V semiconductors is of great technological interest for the production of microelectronic (junction field effect transistors [JFET], heterojunction field effect transistors [HFET] etc.) and optoelectronic (lasers, photodiodes etc.) devices. Currently, to obtain metal-insulator-semiconductor field effect transistors (MISFET), the carrier mobility near the semiconductor surface has to be high and the surface state density N,, has to be as low as possible. Contrary to the case of Si, native oxides on InP surfaces do not allow reduction of the surface state density N,, below 1012- lOI eV_’ cm-*. Recently, deposition of Al203 has been found to give some positive results [ 11, but many problems remain unresolved: leakage current becomes higher when exposed to the atmosphere and charges could be trapped in the insulator. Barriere et al. [2] have proposed changes in both the nature of the insulator and the method of growth. They have investigated physico-chemical [ 31 and electrical [4] properties of oxidized InP single crystals under fluorine gas. This chemical treatment leads to the formation of a thin film of InF, on the semiconductor surface. This 0040-6090/95/$9.50($21995 ~ Elsevier Science S.A. All rights reserved SSDI 0040-6090(94)06301-X

Fluorine;

Strontium

process pushes the insulator-semiconductor interface back into the bulk of the semiconductor. Consequently this technique allows elimination of the native oxides from the InP surface. By means of this passivation technique a modulation of the surface potential is observed for a thin film of thickness less than 100 nm. However N,, remains very high ( > lOI* eV_’ cm-‘) and the accumulation regime is never reached. Beyond a thickness greater than 100 nm, the films flake off. Consequently this passivation technique seems to be unsuitable for MISFET devices. At the same time, Group IIA cubic fluorides, such as BaF, [5], CaFz [6], SrF, [7, 81 and a solid solution Ba, _.\-Sr,F,+ , (x = 0.825), which exactly match the InP lattice parameter [9], have recieved increased attention as insulating materials. Thin films of these compounds can be epitaxially grown onto the InP substrates by molecular beam epitaxy (MBE). Unfortunately, the solid solution does not have a congruent sublimation under vacuum [IO]. As a consequence, it has been preferred simply to study SrF,/InP( 100) structures. The relative lattice parameter mismatch between these two compounds is I. 19%) at room temperature (a InP = 5.869, as+ = 5.799 A) and it is only 0.82% at the growth temperature (T = 270 “C).

B. Momhelli

el al.

I Th

SrF,/n-type InP( 100) structures, prepared by sublimation of SrF, powders under a classical vacuum (lo-’ Torr) exhibit a modulation of the surface potential in the semiconductor. However, for low sweep voltages, the C(V) characteristics show a marked hysteresrs which has been interpreted in terms of the ionic transport in the insulating thin films. The ionic transport was confirmed by admittance spectroscopy measurements performed on metal/SrF,/metal structures [ 111. Moreover. it has been shown that the composition of the layers was modified after ageing in air. These observations seem to be related to the polycrystalline state of the films. To improve both the crystallinity of the films and the surface state of the substrates before the growth of the films new structures were prepared under ultra-high vacuum (UHV). TEis paper deals with the electrical properties of these devices. It will be shown that the surface potential modulation is strongly dependent on the substrate cleaning process.

2. Experimental

procedure

2.1. ,\irmple prepcrration

und physic0 -chemical

churLlctrri-_crtions

Two cleaning processes of the substrates were tested. The first one has been extensively described [ 12, 131: the degreasing, etching (H*SO,, H,02, H,O (2:l:l)) and deoxrdizing (HF-ethanol 10%) of the substrates. This cleaning process leads to a (4 x 2)In stabilized reconstructed surface after annealing under UHV at relatively low temperature (350 “C, 30 min) [ 121. The second cleaning process uses a phase of sulfurization ((NH,),S,) rather than of deoxidization. This treatment leads to only one monolayer of sulfur on the semiconductor surface. The RHEED (reflective high energy electron diffraction) images show (1 x 1) patterns of InP substrates. It has been previously shown that -he ((NH,)?S,) treatment leads to a greater metal dependence of the Schottky barrier height [ 141 and to a reduction of the interface state density in MIS diodes [ 151. It also leads to a reduction of the surface recombination velocity [ 161 and thus to an increase of the photoluminescence intensity of the semiconductor [ 171. The films were obtained by sublimation of a high purit:ir powder (Suprapure Merck) under UHV at 1200 ‘C; a platinum crucible was used to heat the powder. The growth rate was 0.1 nm s-’ and the substrate temperature was 270 ‘C. Whatever the cleaning process, the growth of SrF, thin films onto InP( 100) substrates was tridimensional, minimizing the surface free energy of the layers [ 181. Because of the high value of the stiffness coefficient of

Solid Films 256 (1995) 80 -84

XI

the fluoride (the Young’s modulus of SrF, is 101.3 GPa) the layers were not constrained, even after deposition of a second monolayer [ 191. Thus, an incommensurate type interface is obtained. The physico-chemical properties of the layers were mainly deduced from Rutherford backscattering spectroscopy (RBS) of 2 MeV ‘He- particles [8]. The channeling effect was used to determine the epitaxial quality of the structures. The results indicate that the films are indeed stoichiometric and crystallized and that they have the same orientation as the substrates. For a thickness less than 400 nm, the more the thickness of the layer increases the better the crystallinity. Beyond 400 nm, the layers flake off. This is due to the thermal dilatation coefficients differing between InP and SrF,. 2.2. Electrical

measurements

The electrical properties of SrF, thin films were deduced from current-voltage (I- V) and admittance measurements of Al/SrF,/(n= 4 x 10” cmm3)InP MIM diodes. The real ( Y,(w)) and imaginary ( Y,(tn)) parts of the admittance were recorded on a large frequency range ( lo-‘- 10h Hz) with a fully computerized Solartron 1174 frequency response analyzer. The insulator-semiconductor interfaces were mainly characterized by capacitance-voltage (C( I’)) measurements of Al/SrF2/(n = 10lh cm m3)InP( 100) MIS diodes. The 1 MHz C(V) characteristics were performed by means of a Booton capacitance meter, in the 77-300 K temperature range. The electrical properties of SrF,/ InP interfaces were also investigated by the conductance method using a lock-in amplifier (PAR model 124). In the following, the structures obtained with the first cleaning process will be called SrF,/InP and those which were sulfurized will be called SrF,/( S)InP.

3. Results and discussion 3.1. Electrical

properties

of’ SrF,

thin ,film.s

The real and imaginary parts of the admittance of a Al/SrF,/n’InP diode are shown in Fig. 1. In the high frequency range, Y,(tu) is controlled by the bulk capacitance, which is temperature independent. A dielectric constant of 6.5 is found for SrF, thin films. This value is in good agreement with the value found by M. V. Subrahmanya et al. [20] who previously studied the real (E’) and the imaginary parts (f:“) of the dielectric constant of SrFz single crystals. They have shown that E’ varies from 6.55 to 6.5 and E” from 2.7 x 10-I to 1.1 x lo-’ when the frequency increases from 100 to 300 kHz.

B. Mombeili et al. I Thin Solid Films 256 (1995) SO-84

-6

-12

-14 -2

(a)

-1

0

1

1% IWW -3

Fig. 2. I(V) characteristics of a metal/( 150 nm thick)SrF,/n+InP (100) structure (S = 0.5 mm2) for various temperatures.

s=osmm~ d=lSOnm

I

-13 1 -4 @I

-3

-2

-I

o

I WF

2

3

4

5

6

The results of the I- V measurements are shown in Fig. 2. The currents are temperature activated and do not follow an ohmic law. The Poole-Frenkel effect is probably responsible for the charge transfer mechanism. By assuming the simplest case of only one type of traps in the band gap of the insulator, the current is then expressed by:

(WI

Fig. 1. (a) Real Y,(W) and (b) imaginary Y,(o) parts of the admittance of a (S = 0.5 mm’)Al/( 150 nm thick)SrF,/n+InP( 100) structure vs. frequency for various temperatures. Insert shows log(g) vs. T-’ characteristic.

In the high frequency range, the increase of Y,.(m) is due to the resistance of the electrodes. In the medium frequency range, Y,(o) is controlled by the dielectric losses which obey to the classical law Y,(o) = (jAo)“. At low frequencies and low temperatures, Y,(w) tends towards a constant value proportional to the electronic conductivity CJ of the bulk of the SrF, thin films. For T = 337 K, 0 was found to be as low as lo-l5 S cm-‘. This value is close to the conductivity of the better insulating materials, like SiO,. As it is shown on the Arrhenius diagram (see the insert in Fig. 1) the conductivity is thermally activated and the activation energy AE is about 0.7 eV. The resistivity at room temperature, close to lOI R cm, was deduced by extrapolation from the Arrhenius plot. For temperatures over 400 K, an ionic contribution is observed. The decrease of Y,(w) at low frequencies is due to interface phenomena, which are also responsible for the increase of Y(o)/0 (see Fig. l(b)). However, the total conductivity remains very low, even at 472 K. It must be pointed out that the conductivity of the films prepared at 170 “C under UHV, is about three orders of magnitude smaller than the conductivity of films prepared under classical vacuum [21]. Moreover, in this latter case an ionic conductivity was observed even at room temperature.

(6 - BE”*) 2kT where E and 4 are respectively the applied electric field and the ionization energy of the traps, B = q3’2 (7cEO&,)“2, E, being the high frequency dielectric constant (a, = n’). In Fig. 3 are plotted log(Z/I/) vs. VI’*. The obtained straight lines confirm the hypothesis of a Poole-Frenkel mechanism. The high frequency dielectric constant E,, deduced from the slopes of the straight lines, is very close to the expected value (E, = 2.07). Finally, the ionization energy of the traps, deduced from the slope of log(Z/V) vs. T-’ (see the insert in Fig. 3) is 4 = 1.5 eV. This value agrees well with the AE activation energy deduced from admittance measurements, AE = 4/2. In conclusion, we report that the dielectric breakdown of the films is higher than lo6 V cm-‘.

-26

”

. .

384 K .

.

.

.

.

.

Fig. 3. Log(l/V) vs. V ‘I* transformations Insert shows log(l) vs. T-‘.

of the curves

of Fig. 2.

B.

3.2. Fluoride-semiconductor

Mombelli et al. 1 Thin Solid Films 256 (1995) X0&84

interfaces

3.2. I. The case of SrF211nP(100) 3.2.1 1. Capacitance -voltage characteristics. Typical C(V) characteristics at 300 K and liquid nitrogen temperature of a Al/SrF,/nInP( 100) diode are reported in Fig. 4. curves (a) and (b) respectively. A relatively large modulation of the surface potential in the semiconductor is clearly observed. The hysterisis is small compared to that of devices prepared under classical vacuum. This is due to the decrease of the ionic conductivity However, C(V) characteristics do not have the expected behaviour. In particular at 300 K and for positive bias, the measured capacitance is smaller than the insulator capacitance (C,, = 850 pF). This shows that the accumulation regime is never reached. At 77 K, the capacitance is strongly reduced. For negative bias the capacitance is smaller than the theoretical one (C’“,,, = 215 pF). This shows that the semiconductor is in deep depletion. For positive bias, the drop of capacitance shows that surface states are contributing to the capacitance at 300 K. At 77 K the relaxation times of these states are probably so high that they cannot follow the 1 MHz a.c. signal. Thus. the C(V) characteristic at 300 K is not a “high frequency” characteristic and the density of states N,, cannot be deduced from Terman’s method [22]. However, it is still possible to have an order of magnitude of N,, by comparing the capacitance values at 301) and 77 K which can be expressed as: C&J,

= C,,,’ + ]C,,(&

+

x3

For a 2.5 V bias, C,,, k = 720 pF and C,, k = 390 pF, thus C,, = 720pF and C,, = 5 nF, which leads to a density of states N,, = 3 x IO” eV-’ cm- ‘. 3.2.1.2. Conductance measurements. The electrical behaviour of MIS diodes for negative bias was investigated by measuring the conductance (Y,.(T, (I))) and the capacitance (C,( T, w) = Y, ( T, CO)/w). Measurements were done from 77 to 300 K and for various frequencies varying from 10’ to lo5 Hz, the bias was maintained at 2 V. Results are shown in Fig. 5. For T < 215 K, and as already mentioned above, the capacitance is smaller than Cm,“; thus the semiconductor is in deep depletion. The first step which appears at 215 K is assigned to the formation of the inversion layer. This process is dominated by the thermal generation of electron-hole pairs. At high temperatures, Y,( T, w) and Ci( T, w) present respectively a maximum and a second step at about the same temperature. The maxima and steps shift towards high temperatures as the frequency measurement increases. This behaviour looks like a Debye relaxation and is attributed to carrier transitions between the interface traps and the semiconductor bands. The traps may be located either in the insulator or at the interface. They could be due to a continuum of states [23] or to a single trap level situated in the insulating material. In the latter case, it has been demonstrated that such a trap gives rise to an apparent uniform density of states [24].

Gl-’

CT,‘,= C;,’ + CT,;‘(&) where C,,(4,) and C,, are respectively the capacitance of the semiconductor for a surface potential Cp,and the capacitance of the surface states.

C W) 800 ,

(b)

Fig. 4 1 MHz capacitanceevoltage (C(V)) characteristics at (a) 300 K and (b) 77 K of a (S = 1 mm’)A1/(67 nm thick)SrF,/n-InP (100) structure. The sweep rate is 5 x IO-’ V s-l.

Fig. 5. (a) Conductance Y,( T, w) and (b) capacitance C,( T. co) of the MIS structure presented in Fig. 4 vs. the temperature for various frequencies varying from IO* to IO5 Hz. A bias voltage of -2 V is applied to the structure.

B. Mombelli et ul. I Thin Solid Films 256 (1995) 80-84

84

of states is greater than lOI eV_’ cmp2. This does not allow the accumulation regime to be reached. On the other hand, if we replace the deoxidation phase by a sulfurization phase, then the density of states can be lowered to 10” eV-’ cmb2 and the accumulation regime is satisfactorily achieved. This suggests that InP-MISFET devices can be obtained. Work is now in progress to improve further the quality of the fluoride thin films.

C (PF)

Acknowledgements -4

-3

-2

-I

0

I

2

3

4

v (Volts)

Fig. 6. 1 MHz C(V) characteristics at (a) 300 K and (b) 77 K of a (S = 0.78 mm*)Al/( 150 nm thick)SrF,/n-(S)InP( 100) structure. The sweep rate is 10-s V s-‘.

Using the classical representation of a MIS diode, (see the insert in Fig. 5) it is possible to deduce the density of states N,, from the real part G,,, of the surface state conductance. G,,, is calculated from Y,(o) and Yi(~) by removing the effect of the capacity Ci, [25]. Assuming a single trap level at the interface, classical relations then give the N,, value; N,, z 8kTG,,,/q2w where G,,, is the maximum of the G,,,, it is found N,, z lOI eV ’ cme2. 3.2.2. Case of SrFJ(S)InP(lOO) The density of states of SrF,/InP( 100) interface is too high to lead to competitive MIS devices. To improve the quality of the insulator-semiconductor interface a sulfurization of the substrates has been performed. The results are presented below. The C(V) characteristics at 300 and 77 K of a Al/SrF,/ n-(S) InP( 100) diode are reported in Fig. 6. A wide modulation of the surface potential in the semiconductor is obtained. Contrary to the case of SrF,/InP interfaces, the measured capacitance for positive bias is very close to the Ci, value, either at 300 or 77 K. Now the accumulation regime seems to be reached. A determination of the density of states by Terman’s method leads to N,, z 10” eV’ cme2. For negative bias and low temperature (77 K), the capacitance remains smaller than the theoretical value of C,i,, showing that the semiconductor is again in a deep depletion regime but this is not surprising at this temperature. 4. Conclusion It has been shown that SrF, thin films, with a resistivity p z lOI R cm at room temperature, can be epitaxially grown onto InP( 100) substrates. The ionic conductivity of these films is very small and it is observed only for temperatures higher than 380 K. Although a classical cleaning of InP( 100) substrates leads to a good expitaxy of the fluoride layers, the density

The authors are grateful to Prof. J. Salardenne and Dr. H. Ricard for helpful discussions. This work has been supported by the Centre National d’Etudes des Telecommunications (CNET).

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