Measurements of the avalanche breakdown potential on n-type
gallium arsenide electrodes in contact with water or aqueous
solutions of NaN03 and NaCl are reported. Calculations are presented
of the dielectric constant and of field strength in the
Helmholtz and the space charge layer of the semiconductor.
The interfacial electrode capacitance of the GaAs electrode is · a
complicated function but can successfully be approximated with the
equation describing the capacitance of the p-n junction in the
solid state

A. Wolkenberg

English

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CR 0 AT IC A CHEMIC A ACT A 44 (1972) 53 CCA-682 541.135.4 Conference Paper Avalanche Breakdown on a Semiconductor n-type Gallium Arsenide Electrode* A. Wolkenberg Institute of ELectron Te_chnoLogy, Warsaw, Poiand Received Nov embe r 10, 1971 Measurements of the avalanche breakdown potential on n-type gallium arsenide electrodes in contact with water or aqueous solutions of NaN03 and NaCl are reported. Calculations are pre-sented of the dielectric constant and of field strength in the Helmholtz and the space charge layer of the semiconductor. The interfacial electrode capacitance of the GaAs electrode is · a complicated function but can successfully be approximated with the equation describing the capacitance of the p-n junction in the solid state. INTRODUCTION The electrochemical properties of a semiconductor electrode may be considered with assumption that the rate of the electrode reaction is deter-mined by the stage of the electrochemical reaction on the semiconductor surface1 relative to gallium arsenide. A second way of considering the electrode process is the assumption that the reaction rate is determined by the stage of carrier transfer through the space charge layer formed as a result of polarization in the semiconductor, and that the stage of the electrochemical reaction is limited by the inflow of carriers on the semiconductor - electrolyte interface2• In this case the courses of the resultant reactions would be distinctly and strongly dependent on the electrical ipraperties of the semicondiuctor-electrolyte interface. Acceleration of the electrochemical electrode reaction should follow as result of decreasing of the dielectric constant of the electro-lyte. To larger values of the dielectric constant there would correspond higher values of the electrode potential 8 Ub, at which a marked start of the electrochemical reaction would follow. On the basis of this assumption it may be stated that the breakdown potential 8 Ub, should assume the highest values in the pure solvent (largest dielectric constant), and with incre&sing activity of the electrolyte it should diminish. In two electrolytes with equal activities the breakdown potential should be proportional to their dielectric constants: EXPERIMENTAL The experiments were carried out in an electrolytic vessel in a system as described previously2•3• Deionized . water and p.a. reagents were used for the in-vestigations. n-Type gallium arsenide .oriented along the (111) axis was used with * Based on a lecture prec:ented at the 22nd Meeting of the InterriationaL Society of Electrochemistry, Dubrovnik, Yugoslavia, September 1971. 54 A. WOLKENBERG mobility µ = 2000 cm2/V. sec and with a concentration - 1 X 1018/cm3 which cor-responds to the resistance Q = 0.003 ohm. cm and - 2 X 1017/cm3 what equals to the resistivity Q = 0.02 ohm. cm. The ohmic junction to the surface of gallium arsenide was carried out by vacuum evaporation of layer of indium, the heating of this layer under hydrogen, and subsequently soldering the wire by an indium solder. The measurements were carried out at a constant temperature of 250 C under argon, which was simultaneously used for intensive mixing of the near-electrode space. The purity of the argon was 99.999°/o. In Fig. 1 the method of fixing the electrode in the glass cork is represented schematically. The real surface of the investigated electrode was 0.385 cm2• Ac-count was taken of the coefficient of roughness of 1.3. Fig. 1. Stopper for GaAs electrode. 1 - GaAs electrode; 2 - washers; 3 - ground joint; 4 - supporting pipe; 5 - rubbers; 6 - wire. The measurement of the electrode potential was carried out with respect to a calomel electrode. A platinum electrode with 6 cm2 of surface was used for pola-rization. Measurements of the dependence of the electrode potential on the applied current intensity (direct current method) was performed after stabilization of the stationary potential of the electrode in complete darkness. RESULTS AND DISCUSSION As a result -of the performed investigations the potential - polarization current density curves were obtained, shown in Figs. 2-4. The anodic polarization determines, the barrier direction for the created p-n junction, cathodic polarization denotes the conduction direction. M ~ ."" . II f c.,., . 32 u ' 2 · j . '/ 'j r1 ~6 ~ ~ l/•it B ,J ,r. 24 -2.0 -!,6 -1.2 . -0.S -0,4 ~ t'l""' VL f/ fJ. ~~ o.~ 0,8 12 1.6 2,0 2A 2,8 (VJ 3, /. ~, -s -16 J1 -24 I I/ -32· I, II Id -4 0 Fig. 2. Polarization curves of n-type GaAs ND= 1X101•/cm•. e H20; O 1 M NaNOs; X 0.01 M NaNOs; 0 0.1MNaN03;/;:,.1 M NaCl. SEMICONDUCTOR GaAs ELECTRODE 32 mA ·1 CiiiI 24 I f 16 I - B 2.0 -i.6 -i.2 -o,s -0.4 / _v (~ 0,8 1,6 2,4 3,2 4,0(V]4, s Oj or -8 -l0 0" -. /. " "-Fig. 3. Dependence of polarization curves on carriers concentration in 0.1 M NaN03• 0 GaAs., 1 X 101•/cm•; e GaAsn 2 X 1017/cmS. ""' I °/ c;;;T 24 11 Vo 16 ' ' 8 ~ (Y 2,4 -2,0 -i.6 -1.2 -o.& -Q4 .)j .,.-- o.s 1,6 2,4 3,2 "4,0 o4.8 5,6(¥]6, J I -8 I I I I I I I I ~ I . -ffi -24 ~ -32 4 Fig. 4. Polarization curves on GaAs., 2 X 10"/cm• in 0 0.1 M NaNOa; e 0.01 M NaN03. 55 Determining the breakdown potential 5 Ub as the potential at which a positive .increase of the field strength ·occurs ~n the space charge layer in the semiconductor until causing an avalanche generation of carriers2,3, we can observe that the highest breakdown ·potential occurs in the case of a pure solvent, that is, deionized water. With increasing concentration of the solution (NaNOJ the breakdown potential decreases. At equal concentrations (1 M NaN03 and 1 M NaCl) the breakdown potential is greater in 1 M NaN03 than in 1 M NaCl. A- similar phenomenon ·Occurs in the case of cathodic polarization in the conduction direction. In order to establish the proper potential affecting the field strength in the space charge layer a measurement of the potential of planar bands was carried out and the absolute value of the breakdown potential was assumed to be the sum of the potential of planar bands and of the break-down potential for each solution under investigation. In Table I the values of the breakdown ,potential 5 ub are given. The assumption that the field intensity causing avalanche breakdown in the space charge 'iay~r in gallium arsenide is constant and equal according to Sze4 Fb = 4 X 105 V/cm, the occurrence of a pure p:nocess of avalanche breakdown in the ca:_yf so strongly doped semiconductor is problematic 56 A. WOLKENBERG TABLE I Breakdown Potential of GaAs Electrode Potential of Potential Breakdown Electrolyte cpb - qlB = Q cut-off potential [V] [V] [V] deionized H20 -- 0.9 + 2.7 3.6 0.01 M NaN03 -0.85 + 2.44 3.29 0.1 M NaN03 - 0.8 + 2.25 3.05 1 M NaN03 - 0.75 + 1.70 2.45 1 M NaCl - 0.70 + 1.65 2.35 and the occurrence of Zener or tunnel effect becomes probable. The men-tio·ned value of the field intensity in the space charge layer corresponds to the breakdown potential, according to Sze5 - 4.4 V, it is larger than the values obtained in the case of the investigated electrolytes. The sequence of electrolytes with decreasing values of breakdown potential coincides with decreasing values of the dielectric constant (Table II)6. TABLE II Dieiectric Constant at Hetmhoitz Doubie Layer Dielectric F . Solution Dielectric FH constant ~ constant [V/cm] of Helmholtz Fm double layer deionized water 71.8 2.78 x 105 7.6 1.2 0.01 M NaN03 71.5 2.2 x 105 6 1.3 0.1 M NaNOa 69.7 1.75 x 105 4.8 1.4 1 M NaNOs 60.3 0.62 x 105 1.7 1.75 1 M NaCl 59.0 0.43 x 105 1.2 1.8 On the basis of Shockley's7 views that one of the reasons for breakdown in p-n junctions are local increases of the electric field intensities on the semiconductor surface on the contact boundary with a dielectric having a smaller dielectric constant than semiconductor, it may be stated that at mean field strength, space charge layer and Helmhol.tz layer lower than the intensity of breakdown field ~b the field strength in ·the space charge layer achieves the value sufficient to start the breakdowin Fb = 4 X 105 V/cm. In Table II the values of the ~leotr:ic constant are given calculated in the Helmholtz layer, resulting fro: / the following equations : (FH + Fsp. ch)/2 =Fm . . (1) where /'so/es= FH/Fs (2) FH - field strength in the Helmholtz layer of thickness wrr = 3 X X 10-s cm [V/cm], Fsp.ch. - field strength in the space charge layer in GaAs, SEMICONDUCTOR Ga As ELECTRODE 57 Fm - mean field strength in the space charge layer and in the Helm-holtz layer for w 111 = wll + W 8p. ch . The order of calculated value of dielectric constant in the Helmholtz layer in the case of pure water is the most similar as that on the basis of data compiled in Table II. The order of magnitude of the values obtained corresponds to the data on the structure of a double layer elaborated by Bockris8 and Conway9. This conformity speaks rather in favour of recognizing the correctness of the reasoning p;:-esented that the process determining the reaction rate on the semiconducting electrode is carrier transport through the space charge layer . The mean values of the electric field s trength in the total space charge and Helmholtz layers were fr.om 1.2 times to 1.8 times smaller than the field strength in the semiconductor depending on the chemical composition of the solution. The cathodic (conduction) direction indicates a similar depen-dence of the electrode potential on the propert ies of the solution which is another analogy with the .p-n devices. The experimental examination .of the potential distribution was made by recording polarization curves on an electrode of gallium arsenide of high purity and high resistivity. (Figs. 3 and 4). The potential breakldown in 0.1 M NaN03 increases of about 1.5 V (F·ig. 3) that is 1.5 times as much. For p-n junction4 in a solid state the potential breakdown is 2.4 times as much in the case of the discussed differences of concentrat ions. Taking into account the presented argumentation we may conclude that the dielectric constant in double layer as a result of th.e action of higher electrical field is smaller ~nd equals to 3 but not 4.8 (Table II). The increase of the polarization potential of anode may also cause the passivation of the surface of semiconductor electrode (Fig. 4) . When the surfaces of the electrode were very accurately polished, also the passive films were subject to rather hard breakdown; in range of anode polarization, in the solution of higher concentration (in 0.1 M NaNO,i), higher potential of breakdown was observed, than in the more diluted solution (0.01 M NaNO,). It confirms the previous observation that the area of investigation of clean properties of semiconductor origin is limited to some lower anode potentiaJ" in the solutions not enabling the passivation of the surfaces. The measurements of the electrode capacity in the case of n-p junction present should point out the dependence of the capacity from the electrode potential analogous to the dependence of the capacity of the junction in solid state devices. In this case it should be listed experimentally the obtained results with the theoretical curve drawn on the basis of the equation of the unitary capacity of the p-n junction4 : C - dQ = 1/ qo • Eo •Es • ND sp. ch - d<l> V 2 I <l>bi ± <l>E J where q0 - elementary load = 1.6 X 10-10, i;0 - dielectric constant of vacuum = 8.854 X 10-12 F/m, E8 - dielectric constant of semiconductor, ND - donors concentration (or acceptors) in m'"'s, (3) 58 A. WOLKENBERG <I>bi - diffusion potential (built in) of real, p-n junction at the given concentration of donors (acceptors) , <DE - electrode potential (positive in the anode area, negative in the cathode area upon the diffusion potential <I>1i;). In Fig. 5 introduced are the results of the measurem ents of the capacity of electrode of GaAs of the n-type Nn = 1 X 1018/ cm3 as regards the curve HO' Ho0 ~ ~ ..__ i - -~~ ~LD ' 20 0 0 0 ' l / ..---ft .~ 4 - - -Ho-• -i,4 -1,2 -~.O -0,S -0,6 -OA -0,2 O D,2[V)O,.<I Fig. 5. Capacity vs. potential on GaAsn 1 X 10••/ cm•. 1 - in 1 M NaN03 ; 2 - theoretical curve experimental points for 0.1 M NaNOa ; 3 - in 0.01 M NaN03 ; 4 - in H 20. drawn on the basis of the equation (3). For the NaNO,. solutions of the con-centration of 0.01 M to 0.1 M the theoretical curve is between the expe-r imental curves. With the increase of the concentration the measured capacity increases (1 M NaN03 ), with the dilution the measured capacity decreases (H"O). Analogous reaction of the capacity - potential relationship was observed on the electrode of GaAs0 - N0 = 2 X 1017/cm3 (Fig. 6) and on the electrode of Sip - NA = 4 X 1Q19/cm3 (Fig. 7). Considering the capacity of the electrode as regards an electroly te Cm as a sum of serial capacities of a double layer and a layer of a space charge, we have where Cm - measured capacity, C"p.ch. - capacity . of the space charge layer of semiconductor, Ce - capacity of a double layer of an electrolyte. (4) SEMICONDUCTOR GaAs ELECTRODE 59 ' ' ,_, -,. ~ \. r---....~ -"""'- . • ' - " v ,., 0 0 0 0 0 0 0 ,._ 0 0 , -1,4 -1,0 -0,0 -0,2 0 0,2 0,6 f,O f,4 1,8 2,2(V]2,6 Fig. 6, Capacity vs, potential on GaAs0 2 X 1017/cm•, 1 - in 0.1 M NaNo,; 2 - theoretical curve; 3 - in 0,01 M NaN03, HO " ................ -""" • ... J - 2 v v 0 D .... ' J -1,4 -1,0 -0,6 -0,2 o 0,2 0,6 to f,4 1,s [v] 2,t Fig, 7. Capacity vs. potential on Siv 4 X 10" /cm'. 1 - theoretical curve; 2 - in 0.1 M NaNO,, Here we can see that the measured capacity Cm should yield the equation Cm ~ esp.ch. and since the value Cc is negligible as being very large in comparison with e sp.ch. On the basis of measured values we may assume that the capacity of the layer of spatial charge under the influence of the increase of concentration ,of the elect:mlyte increases a·bove the value com-puted with the equation (3). This increase of capacity being the result of local growth of carrier concentration near the semiconductor surface. The · enlargement of the electrolyte concentratiion leads to the increase of den-sity of charges on the interface semiconductor-electrolyte. To conclude we may state that the present mechanism of electrolytic reaction is a reasonable interpretation of the real course of the process, although the occurrence of the tunnel mechanism cannot be excluded. The dependence of the GaAs electrode capacitance on the potential is rather complicated but in first approach equation of capacity of p-n junction gives quite good conformity of theory with experimental results for solution concentrations of 0.01 M to 0.1 M. 60 A. WOLKENBERG REFERENCES 1. H. Geris ch er, Surface Science 18 (1969) 97 . 2. A. W o 1 k e n b e r g, ELectron TechnoLogy 2 (1969) 3. 3. A. W o 1 k e n b erg, ELectron TechnoLogy 3 (1970) 32. 4. S. M. Sze, Physics AppL. Letters 8 (1966) 111. 5. S. M. Sze, Physics of Semiconductors, Wiley-Interscience, 1969. 6. J. Barthe I, Z. Phys. Chem. NF. 71 (1970) 115. 7. W. Sh o ck I e y, Solid State ELectronics 2 (1961) 35. 8. J. 0' M. Bock r is, Proc. Roy. Soc. London A 274 (1963) 55. 9. B. E. Conway, Trans. Faraday Soc. 47 (1951) 756. IZVOD ElektriCki proboj na elektrodi n-tipa GaAs A. WoLkenberg Opisana su mjerenja elektrickog proboja (avalanche breakdown) na elektrodama n-tipa GaAs u kontaktu s deioniziranom vodom te vodenim otopinama NaN03 i NaCl. Pokazano je da potencijal proboja pada s porastom koncentracije NaN03. Izraeunati su i dielektricka konstanta i elektricno polje u Helmholtzovom dijelu dvosloja te u sloju prostornog naboja. S porastom koncentracije elektrolita opada vrijednost konstante i polja u Helmholtzovom sloju, a raste elektrieno polje u sloju prostornog naboja. Iako je zavisnost totalnog kapaciteta GaAs elektrode u kontaktu s elektrolitnom otopinom kompleksna funkcija, ipak se moze dobro aproksimirati jednadzbom za kapacitet p - n spoja u cvrstom poluyodieu. INSTITUT ZA ELEKTRONSKU TEHNOLOGIJU V ARSA VA, POLJSKA Primljeno 10. studenoga 1971. 

Avalanche Breakdown on a Semiconductor n-type Gallium Arsenide Electrode

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