Switching behavior in II-IV-V2 amorphous semiconductor systems

Journal of Non-Crystalline Solids 116 (1990) 191-200 North-Holland

191

S W I T C H I N G B E H A V I O R IN II-IV-Vz A M O R P H O U S S E M I C O N D U C T O R S Y S T E M S

Kug Sun H O N G and Robert F. SPEYER Division of Ceramic Engineering and Science, New York State College of Ceramics at Alfred University, Alfred, N Y 14802, USA

Received 19 June 1989 Revised manuscript received 2 October 1989

The electrical characteristics and switching behavior of amorphous ternary semiconducting C d G e A s 2 were studied under DC and AC conditions. The samples tested were fabricated as as-quenched bulk compounds, roller splat-quenched ribbons, and Ar-sputtered thin films. Vapor deposited Ag, Au and A1 behaved similarly as electrical contacts while the commercial Ag-paste electrodes revealed variable contact resistance. The threshold electric field of ribbon sample was - 5 × 103 V / c m , smaller than - 1.5 x 104 V / c m for the sputtered thin films. The electrical band gap of amorphous C d G e A s 2 was determined to be 1.2 eV (n-type). The "forming" process was believed to occur during the first switching cycle whereby highly conductive amorphous channels formed through the width of the samples.The effects of current on switching were investigated and shown not to be significant. The response time for switching decreased approximately exponentially with the applied voltage, The thermal assisted electronic model was postulated for the O F F - O N transition in these materials.

I. Introduction

Switching and breakdown behavior of amorphous semiconductors have been studied during the past two decades. Many attempts to explain the switching mechanism have been made with chalcogenide glasses [1-4]. Further, a large number of thin film and bulk substances such as metal sulfides, metal oxides, and semiconductors, elemental or in compound form, were investigated in both amorphous and crystalline states [5-8]. Proposed switching models have been classified into thermal, electronic, and electronic modified thermal. The assigned mechanism depends on whether the ON-state arises from Joule heating causing an exponential decrease in conductivity (thermal model), from processes such as carrier tunneling, double injection and impact ionization (electronic model), or electronic initiated by thermal (electronic modified thermal) [9]. The switching mechanism is still controversial. The electronic nature of the threshold switching process was discussed in detail elsewhere [10-13]. In the present paper are presented the electrical 0022-3093/90/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland)

characteristics and switching behavior of the amorphous ternary semiconductor CdGeAs 2 compounds belonging to the I I - I V - V 2 system, which show promise for application in the areas of photovoltaic energy conversion and non-linear optical devices [14]. Crystalline CdGeAs 2 adopts the chalcopyrite structure which is a superlattice of the zincblende structure with the c/a axis-ratio approximately equal to 2. The tetrahedral coordination implies that the bonding is primarily covalent with sp 3 hybrid bonds prevalent, although there is some ionic character present because the atoms are different. In amorphous CdGeAs 2, two cations, Cd and Ge, are bound tetrahedrally to a common anion, As, and there are no lone-pair electrons [15] in contrast to chalcogenide glasses [16]. The electrical band gap has been measured as Eg~ = 1.1 eV from electrical conductivity measurements [17], and the optical band gap is E~"p= - 0.75 eV from the absorption edge [18]. By contrast, typical chalcogenide glasses such as Te40 As35Ge6Sit8 show Eg I = - 0.5 eV and Eg p = 1.0 eV [19-21].

K.S. Hong, R.F. Speyer / Switching behavior in I I - I V- V, amorphous semiconductor systems

192

2. E x p e r i m e n t a l

Bulk, splat-ribbon, and thin film amorphous CdGeAs 2 samples were prepared by the quenching of liquids sealed in fused silica ampoules, roller splat-quenching, and Ar-sputtering. Bulk amorphous CdGeAs: compounds were fabricated by water quenching stoichiometric liquids, vacuum-sealed in carbon-coated (pyrolyzed) fused silica ampoules (inner diameter of 7 mm), after motorized mixing for 24 h. Splat-ribbon samples were made by injection of molten bulk compounds onto the surface of a rotating metal wheel under a nitrogen atmosphere. Thin films were obtained by Ar-sputtering of bulk compounds onto silicate glass substrates. Splat roller quenched samples were - 3 x 5 mm 2 with thickness of 14, 25 and 31 p~m, varied by adjusting liquid temperature and injection speed (i.e. N 2 pressure and injection hole diameter). In the case of bulk samples, the samples were ground to 15-40 pm thickness using a micropolisher. The amorphous nature of as-quenched samples was confirmed using X-ray diffraction, as shown in fig. 1. Electrode contacts were made by evaporation deposition on the sample surface covered by a masking aluminum sheet. The Ag-paste electrodes were brushed on. Samples were tested in the plane parallel form, which consisted of the compound sandwiched between two metal electrodes. The circuit for electrical measurements was made with the sample and an external resistor in series. The voltage-current characteristics were measured using Hewlett Packard * 4140B pA m e t e r / D C voltage source, a Keithley ** 617 programmable electrometer, and a Tektronix ~ 2235 oscilloscope. Data acquisition was made via a Digital §§ professional 350 computer. 3. R e s u l t s and d i s c u s s i o n

The switching behaviour of these compounds was investigated under DC and AC conditions * ** § §§

Hewlett-Packard Co., Fairport, NY 14450, USA. Keithley Instruments, Inc., Cleveland, OH, USA. Tektronix, Inc., Beaverton, O R 97077, USA. Digital Equipment Co., Maynard, MA 01754, USA.

most were performed with DC since switching voltage has been shown to increase with decreasing pulse width for a square pulse [22]. The switching voltage measured with DC is conventionally defined as the "threshold voltage". The "overvoltage" refers to the amount of switching voltage which exceeds the threshold voltage under AC or pulse conditions. The voltage-current ( V - I ) characteristics were measured, varying the electrode areas and the sample thicknesses to determine which mechanism was dominant between electronic and thermal as well as the "forming" (electronic modified thermal) effects [2,23-25]. The switching response times were measured with constant increasing voltage (voltage ramp) and instantaneous voltage (voltage pulse and hold). In addition to the threshold voltage for switching as mentioned above, the effects of current on switching or breakdown behavior were studied. Voltage-current characteristics were measured with different external resistors (ballast resistance) in series with the sample. More reliable results could be obtained with the bulk and splat-ribbon samples of amorphous CdGeAs 2 than with thin films, presumably due to chemical composition deviation and mechanical defects (weak spots). In some samples of bulk amorphous CdGeAs 2, microcracks were believed to exist due to non-uniform thermal stress distributions which developed during quenching inside the fused silica-tube. Most experiments were performed with splat-ribbon samples. The V - I Characteristics of bulk samples behaved the same as those of splat-ribbon samples using vapor deposited contacts. The temperature dependence of the electrical conductivity of differently heat-treated (fig. 1) amorphous CdGeAs 2 is shown in fig. 2. The electrical band gap of amorphous CdGeAs 2 was calculated as 1.2 eV using the logarithmic form of the equation: o = o 0 e x p ( - E g ~ / 2 kT). The more pronounced dependence of conductivity on temperature for the amorphous form as compared with the crystalline forms can be clearly observed from the figure. The band gaps of partially as well as fully devitrified CdGeAs 2 were calculated as Eg I = 0.4 eV. The conductivity of partially crystallized CdGeAs 2 ( - 5 0 % crystallinity) was lower

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diminished contact area of crystalline particles, depleting the effective cross sectional area of the highly conductive path. The diminished contact area also increased the conduction path length as mobile carriers were forced to follow haphazard crystallite bridges between front and rear contacts. Hence the calculated conductivity (o = LIAR) of partially crystallized CdGeAs 2 was lower than the fully crystallized form. The sign of the thermal E M F (hot point probe) measurement on amorphous CdGeAs 2 indicated n-type conductivity. The switching behavior of amorphous CdGeAs 2 varied substantially as a function of electrode materials used, as shown in fig. 3. The point VTH represents the threshold voltage, the point of transition between O F F and ON states. The point VMH is the " m i n i m u m holding voltage" representing the O N - O F F transition. Samples of - 3 x 5 m m 2 area were used with 1 m m diameter circular electrodes to avoid gas discharge along the surface. Repeated switching cycles were observed with the samples using vapor deposited Au, Ag, and A1, but the samples with one side or both

sides using Ag-paste as electrode exhibited a breakdown behavior (memory effect) after switching, terminating the switching behavior upon subsequent cycles. This breakdown is considered due to at least partial devitrification of the sample (Txtal - 410 o C). Its V-I behavior after breakdown was similar to polycrystalline samples. The Ag-paste electrodes played the role of an external resistor, causing the voltage across the sample to drop. When the sample switched from a low conductance (OFF) to a high conductance (ON) state under abnormally high applied voltage, the contact resistance at the interface disappeared due to a n o n - m e t a l - m e t a l transition [27], resulting in an excessive overvoltage applied to the sample. High current passed through the sample, which caused breakdown by Joule heating. The OFF-state current was dependent on electrode area and switching history as shown in fig. 4. Initially, current increased linearly with voltage in the ohmic region, and then increased exponentially before switching (near vertical portion). In this non-ohmic region, the exponential increase in

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showed the OFF-state current acting more independently of electrode area, regardless of extent of switching iteration. When amorphous CdGeAs 2 underwent its first switching, an intermediate, "forming" process was assumed to occur: a highly conductive region ("channel") in the amorphous matrix was formed due to structural alterations. These channel(s) became the dominant conduction mechanism and made contact area irrelevant. This structural alteration is not interpreted as full ordering to the crystalline state. Rather, under a high electric field, the ions or tetrahedral units changed their positions and bonding status (e.g. bonding angle), which was frozen prior to the first O N - O F F transition, presumably not unlike that which would occur at temperatures above the glass transition temperature. The threshold voltage vs. sample thickness was investigated with two different voltage ramps. As shown in fig. 5, for a slow voltage ramp (1 V/s), the threshold voltage increased non-linearly with increase in sample thickness. Under the rapid voltage ramp ( - 1 0 3 V / s ) the threshold voltage also increased non-linearly but with a greater thickness sensitivity (larger slope). For both cases it is assumed that the temperature of the samples rose by Joule heating caused by leakage current prior to switching. More substantive heating is expected to occur at lower applied voltages under a slow voltage ramp as compared with the more rapid one, hence threshold voltages were observed at lower values. The trend of threshold voltages had a smaller value of slope in the slow as compared with the more rapid ramping case, since the accumulated heating effects over considerably longer times had a more substantial effect on the slow ramping case with increasing thickness. The temperature of the sample surface was lower than that inside, since heat flowed out through the metal electrodes. The thick samples had a higher inside temperature. Thus, it is assumed that this higher temperature resulted in the observed non-linearity in the curves, in which thicker samples had more severe heating effects, causing the slumping (non-linear) increase in threshold voltage with increasing thickness. Thus, it follows that the greatest sample voltage

drop occurred near the electrodes, where the electrical conductivity was lower than in the interior since electrical conductivity decreased exponentially with decreasing temperature (fig. 2). The threshold electric field in these near-electrode contact regions is expected to be much larger than the calculated value of - 5 × 103 V / c m from the overall sample thickness. The average threshold voltage of the thin films ( - 1 ~m thick) was - 1.5 x 10 4 V / c m . However, some of the thin films showed breakdown after switching and the others showed varying minimum holding voltages (MHV). The switching response times were measured as a function of a moderate voltage ramp and a near-instantaneous voltage ramp, both to a setting level-voltage. For the moderate voltage ramp, voltage was increased at a constant rate (1 V / s ) to a setting level-voltage and the elapsed times (starting at the onset of the setting level-voltage) before switching are indicated in fig. 6(a). For both cases, the switching response times decreased exponentially with increase in setting level-voltages. The temperature of samples undoubtedly increased during the elapsed time and the increase of internal sample temperature resuited in a decrease in internal resistivity. As before, the remaining voltage drop appeared mainly across the colder interface regions whose thicknesses decreased with time due to spreading of the higher temperature interior. As the thickness of these regions decreased, the electric field within them increased, ultimately causing switching to occur after shorter times. The switching process itself is interpreted as an electronic process (e.g. impact ionization, tunneling), which was brought about by thermal effects. The extended time periods (1-25 s) observed prior to switching clearly indicate the presence of a thermal mechanism in the switching process. Figure 6(b) shows the response times decreasing with level-voltages for near-instantaneous level-voltages as in the slow leveling ramp case. However, higher values of applied voltage at the switching point could be measured than in the slow voltage ramp case, since switching would occur during the slower voltage ramp (to higher voltages). In the slow voltage ramp experiments,

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leakage current generated heating occurred both during the ramp as well as during the hold, whereas for instantaneous level-voltages, heating was effectively present only during the hold. To investigate the effect of current through the sample, various external resistors were connected with the samples in series. The applied voltages to be circuit and currents were measured using a constant voltage ramp (1 V / s ) as shown in fig. 7. The calculated threshold voltages across the samples ( I = Vapplied/(Rext . . . . 1 q- Rsample), Vsample = Vapplie d ),( R sample//( R ext . . . . 1 -b R sample)) f r o m

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of I and V~pplied from fig. 7 are - 1 3 . 1 V (13.1 V, 13.1 V, 13.2 V, 13.1 V). Therefore, increasing current did not effect threshold voltages across the sample during the O F F - O N transition. F r o m the figure, it can be seen that the current at the threshold voltage increased with increase in external resistance. However, all currents exceeding 2 mA caused breakdown. In the ON-state, the

current increased linearly with the applied voltage because the sample resistance settled down to a constant low state after switching (Ohm's law). The portion of the traces returning to the OFF-state initially dropped linearly, region I, following Ohm's law. As current continued to drop toward the OFF-state, two distinct elbows were observed, initially to a steeper slope at region II, and then to the almost vertical region III, representing the O N - O F F transition. The elbow between region II and III is believed to indicate the minimum holding voltage. The minimum holding voltage changed with the current and voltage ramp. When the external resistance was low (1.0 kf~), the channel diameter was interpreted as large enough for high current flow due to large Joule heating.The resistance at the colder contact area was the dominant barrier for current flow (region I). The transition from regions I to II is interpreted to represent a shrinkage of the channel diame-

Fig. 8. Oscillograms of the switching behavior of amorphous CdGeAs2 under AC (60 Hz) conditions. (a) normal switching (hor. 5 V/div., vert. 10 V/'div.); (b) after breakdown (hor. 5 V/div., vert. 10 V/div); (c) low applied (hor. 15 V/div., vert. 5 ms/div.); (d) high applied voltage (hor. 15 V/div., vert. 5 ms/div.).

K.S. Hong, R.F. Speyer / Switching behavior in 11-1V- ~ amorphous semiconductor systems

ter to the point where its resistance became significant. When the applied voltage was decreased to a critical value, the current dropped abruptly (region III). In the case of high external resistance (i.e. 5.0 k~2), current was restricted by the external resistor, so that in the ON-state, the current was not high enough to enlarge the channel diameter; therefore the equivalent of region II is not observed. Figure 8 shows switching behavior under AC conditions. In figs. 8(a) and 8(b), the horizontal divisions represent the externally applied voltage and the vertical divisions represent the voltage measured across the sample. Region I in fig. 8(a) shows the voltage collapse across the sample during switching to the ON-state. This behavior was symmetrical on the negative applied voltage side. The high conductance state after breakdown is shown in fig. 8(b). After breakdown, the voltagecurrent characteristics were observed to behave like the comparatively conductive polycrystalline form, yet microstructural confirmation of such a phase transformation by TEM has to date proven difficult [23]. In figs. 8(c) and (d), the horizontal divisions represent time and the vertical divisions represent voltage. In fig. 8(c), the curve with greater amplitude is the externally applied voltage and the shorter curve is the voltage measured across the sample. The peak applied voltage was increased in fig. 8(d) (not shown in the figure), not only to apply a high maximum voltage to the sample in the ON-state, but also to generate a steep voltage ramp. The overshoot effect was observed at the O F F - O N transition [28] (point A in fig. 8(d)), similar to the behavior of a ZnO-varistor [29], but not as significantly in fig. 8(c). The maxima of voltage across the sample traces in figs. 8(c) and (d) represent the onset of the ON-state portion of the switching cycle. The ensuing drop in voltage across the sample differed depending on the amplitude of applied voltage. In fig. 8(c), the drop in sample voltage is comparatively gradual throughout the ON state, whereas a sharp drop followed by a flat portion (point B) is observed in fig. 8(d). It is believed that the increasing applied voltage acted to increase the channel diameter [25] (and to

199

decrease the thickness of the colder contact regions) by Joule heating, causing the gradual voltage drop shown in fig. 8(c). The sharp applied voltage increase caused a rapid increase in channel diameter, with a corresponding rapid drop in sample voltage, to the point where the channel diameter was adequately broad so as not to be a significant current inhibitor. The increase in sample voltage at point C in fig. 8(d) is considered an effect of the two competing resistance mechanisms [30], that due to the cold contact regions and that due to channel diameter. In fig. 8(d), the flat portion (point B) was taken as a region in which channel diameter resistance was negligible. As applied voltage decreased, decreasing channel diameter once again became significant and caused a momentary increase in measured sample voltage. This phenomena is not as clearly observed in fig. 8(c) since a comparatively gradual increase in channel diameter did not ever make its resistance negligible. Although the time scales for the AC measurements were one order of magnitude less than those of DC measurements, thermal effects are still considered to play a significant role as described above. It is believed that after a finite number of voltage cycles, the sample interior temperature reached some slightly oscillating elevated temperature, well above room temperature. Since amorphous samples have demonstrated and exponential temperature dependence of conductivity, these slight temperature oscillations at elevated temperatures are expected to have a substantive effect on conductivity. The minimum holding voltage is not denoted by the AC characteristic voltage traces since the sample voltage dropped at that point with applied voltage.

4. Conclusions

In the case of bulk samples, some cracking developed by non-uniform internal thermal stress distributions. Ar-sputtered thin films did not exhibit reliable electrical property data due to compositional deviation and mechanical defects. Reli-

200

K.S. Hong~ R.F. Speyer /Switching behavior in I I - 1 V - V 2 amorphous semiconductor systems

able data was obtained using splat-ribbons of amorphous CdGeAs 2. The electrical conductivity of CdGeAs 2 increased exponentially with the temperature in both amorphous and crystalline states, but more sharply in the former. The electrical band gap of amorphous CdGeAs 2 was determined to be 1.2 eV (n-type). The OFF-state current was proportional to electrode area in the virgin samples but not in previously switched samples. Thus, it was concluded that an intermediate "forming" process occurred during (first) switching, where conductive amorphous channels were created. The threshold voltage was not proportional to sample thickness, which implies Joule heating of the sample. A threshold electric field for switching existed because the applied voltage manifested itself in a high, localized electric field in the colder region near the electrodes which maintained the remaining off-state voltage drop after Joule heating eliminated all other resistive regions. The final switching action is interpreted as electronic whereby carrier tunneling and/or impact ionization caused high current to break through these near-contact resistive barriers. The existence of a thermal dependence on switching was confirmed by long periods of time (1-25 s) at applied holding voltages prior to switching. Interpretation of AC measurements indicate thermal effects altering the sample voltage behavior under 60 Hz applied voltages.

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