The PECVD parallel plate reactor used in this experiment was a prototype reactor used in prior investigations. The reactor has been previous described in detail [9] . The gases and reactants used in the reactor during the experiment were ultrapure argon (99.999% Scott Specialty Gases), pure H₂S (99.5% Matheson Tri-Gas), GeCl₄ and SnCl₄ (99.99% and 98% respectively, Strem Chemicals).

The gas phase resulting from the vapor pressures of the GeCl₄ and SnCl₄ liquids provided the Ge and Sn components for the Ge_xSn_yS_z thin films. The vapor pressure of the GeCl₄ at 22˚C was sufficient enough to drive the mass flow controller allowing the set flow of GeCl₄ to reach the reactor without heating. The vapor pressure of the SnCl₄ was less under the same temperature conditions, and its flow required the application of heat tape to the reagent canister and gas line. The canister was kept at 40˚C and the line 45˚C by means of two separate power sources. Also for some of the depositions, ultrapure argon was bubbled through the SnCl₄, to provide sufficient backing pressure for control by the mass flow controller.

For all experiments, the pressure inside the chamber was maintained between 194.7 and 197.3 Pa by means of an automatic control valve supplying argon to the pump to control the pumping speed. The gases were fed through the top platen and showerhead in a continuous flow. The power of the plasma was kept at a constant 22 W for all depositions. The temperature of the substrate ranged from 140˚C - 150˚C.

Doubly polished pure crystalline silicon wafers in the 111 orientation, and glass microscope slides were used as the target substrates. The silicon wafer was broken up into equally sized pieces and then they, along with the glass microscope slides were placed on an unbroken silicon wafer and consistently oriented in the chamber on the grounded electrode. Thicknesses of the resultant films were determined by an Ambios Technologies XP-100 profilometer.

The flow rates of the GeCl₄ and H₂S reactants were kept constant for each deposition. H₂S was kept at 12.0 sccm and the GeCl₄ at 1.0 sccm. The argon flow rates varied, depending on the deposition, in order to keep the total gas pressure in the reactor constant between depositions. The SnCl₄ flow rate was the main variable throughout the course of the entire experiment. As indicated previously, some of the depositions used argon backing on the SnCl₄ line and the flow rates were corrected for this mixture. The flows of the depositions under these parameters ranged from 3 sccm to 20 sccm. There were also a few depositions which used only SnCl₄ without any argon backing and the flow rates were calculated based only on the SnCl₄ physical properties. The flows of the depositions under these parameters ranged from 5 sccm to 15 sccm.

Film composition and properties were determined using a variety of methods. The chemical make-ups of the films, in atomic percentages, were determined using an Oxford Model 7353 EDS coupled with a LEO Model 1430 SEM. Raman spectroscopy was used to determine the molecular composition of the films. Raman spectra were collected with a Jobin-Yvon Horiba HR-800 Raman microscope using the 647.1 nm line of a Lexel Model 95L-K2 Kr-ion laser for excitation. These two techniques were combined in order to confirm the identities of the species present in the films. A Bruker D8 Discover XRD, Cu-Kα radiation, was used to determine crystallinity of the films.

RRAM devices were constructed by depositing three layers of material, in succession, on a standard microscope slide cut in half lengthwise. The three layers were: a lower aluminum electrode layer, a thin film semiconductor layer, and a silver electrode layer on the top. The construction was performed by first depositing two aluminum lines, roughly 6 mm wide and separated by ~5 mm along the length of a microscope slide using a Denton Deskpro magnetron sputter system. The slides were masked with aluminum foil to create the two-line pattern on the slide. The thickness of the deposited lines was roughly 0.5 micrometers. Next, the slides with the aluminum lines were masked with aluminum foil so that ~6 mm on opposing ends of each line was covered by the foil. This was done to provide a conductive contact point for future testing of the devices. Next the masked slides were placed into the PECVD chamber and the Ge_xSn_yS_z semiconductor layer was deposited on the exposed parts of the glass slides including the parts of the aluminum lines that were exposed. The thickness of the chalcogenide semiconductor layer was typically 100 - 200 nm. The final layer of the devices consisted of two 5 mm wide, 0.4 µm thick, silver lines deposited on top of the Ge_xSn_yS_z semiconductor layer, perpendicular to the aluminum lines. These provided the upper contacts for the RRAM devices, and the silver for the formation of the conductive bridging in the RRAM. When all three layers were present, in the structure described, a total of four devices are created at the intersections of the silver and aluminum lines with the Ge_xSn_yS_z layer sandwiched between the aluminum and silver line intersection.

The RRAM memory devices, constructed with the chalcogenide semiconductor layer, with different percentages of Sn, were tested by constructing current vs voltage curves for each. An ideal RRAM memory device would, at a certain voltage, change nearly instantaneously from a low conductance to much higher conductance condition. This occurs when the Ag upper layer is sufficiently positively charged relative to the Al bottom layer, that Ag ions move through the semiconductor layer and form conductive bridges between the Al and Ag layers. If the conductance changes by two to three orders of magnitude, this is sufficient for both “off” and “on” memory states. To determine if this behavior is present for the devices constructed with different Sn percentages, current vs voltage curves were gathered. A Keithley Model 2602 SourceMeter was connected to the Ag and Al electrodes using small flat alligator clips. A graphical program was written in LabVIEW to control the voltage sweeps and simultaneously collect the current measurements. The voltage limits were determined based on the amount of current change seen in the initial sweeps. In some cases higher voltage level sweeps ±3 V were required to observe significant changes in the current. In other cases significant changes in the current were observable with a sweep of only ±1 V.

The compositions of the thin films of Ge_xSn_yS_z deposited with PECVD were found to be close to the desired values of 2%, 8%, 15%, 26%, and 34% Sn. As mentioned previously, these thin films are the semiconductor film that separates the Al and Sn electrodes in the RRAM devices. Based on EDS measurements the atomic ratio of (Ge + Sn)/S varied between 0.6 and 0.9 (average 0.69) and in some cases residual Cl was also measured.

The XRD spectra of these films are shown in Figure 1. The thickness of each of these films was roughly on the order of 1 - 2 microns. For the case with 2% tin, the film is found to be amorphous as there are no sharp peaks presented in the spectrum, but as the atomic concentration of Sn in the thin films increases, peaks assignable to either GeS₂ or SnS₂ begin to appear in the spectrum [8] [10] [11] . As the percentage of Sn increases the peaks generally become sharper and narrower, and reflect the published values for crystalline SnS₂. The JCPDS stick spectrum for SnS₂ is given in Figure 1 as well for comparison. Crystalline GeS₂ also has significant peak at a 2θ of 15 degrees and so some of the intensity in that XRD peak for high Sn content could be due to GeS₂.

The Raman spectra of these films are presented in Figure 2. Note that the prominent peak in the Raman spectrum of nearly pure GeS₂ appears at about 342 cm⁻¹, and is associated with the stretch for the tetrahedral GeS₄ structure [12] [13] . When Sn is added to the deposition, a new peak appears at about 312

cm⁻¹. As the amount of Sn increases to 8%, a broad peak at around 312 cm⁻¹ becomes more prominent in the spectrum, and the peak at 342 cm⁻¹ broadens and becomes less intense. As the Sn composition increases to 15%, the maximum is now slightly less than 310 cm⁻¹, but the spectrum is still fairly broad. At both 26% and 34% Sn, the spectrum has narrowed more significantly with just one prominent peak likely assignable to primarily Sn₂S₃ with some SnS₂ at about 307 cm⁻¹ [8] [14] . The broadening and subsequent narrowing of the peaks indicate that the film becomes less ordered at low concentrations of Sn such as at atomic percentages of 2% and 8%, but at higher concentration of 26% and 34%, the films have increasing long range order. The 15% Sn thin film represents that case where some ordering has occurred due to the presence of the tin sulfides, but there is still significant disorder present due to the germanium.

As described previously in the methods section, simple Resistive RAM devices were constructed with an aluminum lower electrode, a silver upper electrode, and the Ge_xSn_yS_z semiconductor thin film layer sandwiched between the lower and upper conductive layers. The conductivity of each of the devices was measured as a function of the voltage difference between the upper Ag electrode and the lower Al electrode. The voltage was first increased so that the potential of the Ag was positive relative to the Al electrode, and then the relative voltage on the Ag electrode was lowered so it was eventually negative relative to the Al electrode. For the first few cycles, the voltage limits were often set between +3.0 V and −3.0 volts. Presumably, this is necessary for pathways and silver ions to form and move through the semiconductor layer. After a few cycles, typically 4 or 5, the voltage limits are reduced to a narrower range. These current-voltage curves (I-V curves) were collected over several cycles to determine the repeatability and durability of the RRAM device. The I-V curves generated from devices containing 5% Sn, 15% Sn, and 25% Sn are presented in Figure 3. In all three cases, the I-V curves are from voltage sweeps 53, 54, 55, and 56 in each device.

The I-V curves made with thin films with low concentrations of Sn have I-V curves where the current increase is somewhat complicated as the sweep voltage increases. Although the conductivity only increases slightly until 0.3 V, at that point it starts to rise more quickly with a moderate slope. At the much higher concentration of Sn of 25%, there is almost no conductivity hold-off and the current increases linearly with a higher slope than for the low Sn percentage case. At intermediate concentrations of Sn, around 15% Sn, the I-V curves are now more nonlinear, and the conductivity does not change significantly until the voltage sweep rises beyond 1 V. Once it reaches higher than a volt, then the conductivity increases linearly with a high slope and increases by more than 1 order of magnitude in 0.3 - 0.4 volts. For the 15% case as the voltage is increased above some threshold value the conductivity increases sharply, and then decreases as the voltage is lowered to negative values. At a low enough negative value the current again snaps back to the low values encountered at the beginning of the positive sweep. The I-V pattern is somewhat repeatable but changes slightly from one I-V sweep to the next.