ELECTRICAL INSTABILITY PHENOMENA IN IRREGULAR SILICON STRUCTURES

​ЯВЛЕНИЯ ЭЛЕКТРИЧЕСКОЙ НЕУСТОЙЧИВОСТИ В НЕРЕГУЛЯРНЫХ КРЕМНИЕВЫХ СТРУКТУРАХ

Мурадов Намиг Мамедгусейн

доктор философии по техническим наукам, Национальное аэрокосмическое агентство Азербайджанское специальное конструкторское бюро космического приборостроения,

Азербайджан, г. Баку

ABSTRACT

The article describes the results of studies of the phenomena of electrical instability and inductance, and their physical mechanisms are given. Conductivity models are  discussed and it is shown that the most important parameter determining the nature of conductivity is the grain size, which depends on the type of dopant, film thickness, temperature of the growth process, crystallographic orientation, at which the equilibrium number of carriers localized at the boundaries is equal to their total number.

Based on the theory of relaxation of barrier layers at grain boundaries and frequency measurements of resistance, relaxation times and energies of deep traps were calculated, which correspond to the experimental values ​​known from the literature.

A study of the structure of a metal-tunnel oxide –n+ semiconductor revealed the phenomenon of surface-barrier current instability, caused by the filling and emptying of surface states by carriers injected from the n+ region.

АННОТАЦИЯ

В статье описываются результаты исследований явлений электрической неустойчивости и индуктивности, приводятся их физические механизмы. Обсуждены модели проводимости и показано, что важнейшим параметром, определяющим характер проводимости, является размер зерна, зависящий от типа легирующей примеси, толщины пленки, температуры процесса выращивания, кристаллографической ориентации, при котором равновесное число локализованных на границах носителей равно их полному числу.

На основе теории релаксации барьерных слоев на границах зерен и проведенных частотных измерений сопротивлений вычислены времена релаксации и энергии глубоких ловушек, которые соответствуют известным из литературы экспериментальным значениям.

Исследованием структуры металл-туннельный окисел –п⁺ - полупроводник обнаружено явление поверхностно-барьерной неустойчивости тока, обусловленное заполнением и опустошением поверхностных состояний инжектируемыми из п⁺ -области носителями.

Keywords: Amorphous material, electrical phenomena, silicon, polycrystalline, irregular structure.

Ключевые слова: Аморфный материал, электрические явления, кремний, поликристалл, неупорядоченная структура.

Introduction. From a constructive point of view, the most widespread rectification sandwich structures are metal-semiconductor-metal-type [1], metal-dielectric-semiconductor and metal-dielectric-metal-type [2,3] forms. At this time, transition voltages are reached.

The phenomenon of electrical conversion with memory is of great importance for modern electronics. They can be used to create electrically rewritable memory devices that store information when a power source is turned on. Currently, elements based on PST conversion and memory effects are more promising.

Research object and methodology. In 1963, the first information on the observation of NR (negative resistance) in glassy semiconductors gave a great impetus to the creation of various converter devices based on chalcogenide materials [4]. However, the instability of the electrical properties, caused by the complexity of the exact reproduction of the necessary composition of the material, limited their wide application. It was determined that the reliability of the devices is greatly affected by maintaining the exact composition of the amorphous material. A 2 at.% change in the composition of one of the components of the glass significantly reduces the conversion effect of memory devices [5]. In addition, large-scale inhomogeneity can occur in glassy semiconductors (as a result of the inherent instability of amorphous materials and their tendency to crystallization) [6]. Another reason for unreliability is the active interaction of most chalcogenous materials with the materials that determine their detection. Polycrystalline metallic electrodes initiate the recrystallization of the active materials of the vitreous converter. In order to eliminate the effect of the electrode material on the semiconductivity, a gap of hard-melting metal, such as vanadium or titanium, is placed between them, but this leads to technological or structural complexity of the device [7].

For example, the issue of resistance variation of planar PS-resistors with a certain value of current pulses was considered in [8, 9]. However, the measuring range of the resistance of such resistors is small, so this phenomenon is conventionally called the effect. Polycrystalline refers to the deficiency of silicon layers (both undoped and device doped) in the lack of bistable conversion effect of the memory. The study of PSL (polycrystalline silicon layers) with small dopant atom concentration seems to be very relevant and promising. In these, the presence of space charge region (SCR) at the grain boundary and the control of the grain size while growing the layers allow to obtain a wide set of physical properties. These include PSL grown at the same time as monocrystalline layers with moderate doping levels in a single epitaxial growth process [10]. PSl can be used for the creation of various functional elements, silicon can serve to prepare logic and analog IS active elements for signal amplification and processing.

Analysis of results. PS – resistors [10] – are made according to the technology described in [11]. 5 μm thick layers were grown on silicon substrates with r-type conductivity (KDB-10). On them, LDS parts with different configurations, the topology of which is shown in Figure 1, were formed due to oxidation, photolithography and STI. During the deposition process, the layers were doped with phosphorus to a concentration of 10¹⁶ sm^-3. The specific resistances measured in monoparts were in the range of 0.2 - 0.5 Om-sm.

The true concentration of charge carriers was calculated from the grain volume based on the C – V characterization method and fast surface state charge separation, measurement of the temperature dependence of the Hall effect, interpretation of low-temperature (77 k) VAX with the Sottky-type interdevice barrier model, and small grain (0.3 mkm) for layers, this value was 1.5x10¹⁴sm ^-3, when the average size of grains is increased to 5 μm, it increases to 1.3x1015sm ^-3.

Below is the result of mono- and bistable transitions detected in weakly doped layers in strong electric fields.

The VAX of the PS resistors was measured on the board with a tungsten needle probe assembly and observed on a PNXT (a device for observing the characteristics of transistors). A typical VAX of PS-resistors, which has a NR part, is given in Fig.1. When reaching a certain value of the threshold voltage, the resistance of the PS-resistors decreases sharply and becomes low-resistance with a jump, which is maintained up to a certain minimum holding current. The subsequent decrease in current returns the PS-resistors to high ohms. As can be seen, VAX is symmetrical with respect to the polarity of the applied voltage.

After electrical shaping, that is, after releasing a sufficiently high current (~ 300 mA) from the layer, it turns from monostable to bistable, in other words, a memory effect occurs. PS - the resistance of the resistors decreases by an average of one order from the initial value (in the order of 100 kOm), and the conversion voltage is 5 - 15 V.

а

б

Figure 1. Monostable (a) and bistable (b) conversion effect in layers with an irregular structure based on LDS

The transformation occurs in all types of layers with different initial resistance. In the case that the converting devices are characterized by the ratios of the resistances in the YO and AO cases, then it would be reasonable to select small-grained layers with maximum initial resistance before heat treatment for research. The studied PS-resistors range in length from 10 μm to 850 μm, unlike converters based on amorphous materials, the threshold voltage and fields depend non-linearly on the layer length (Figure 2).

In order to study the statistical and dynamic parameters of the transformation effect in a wide temperature range, the samples are soldered to a plane metallic body. Aluminum electrodes are coated on both the PST and the p+- layer with a high concentration of phosphorus, and also on the bottom layer of difficult-to-melt metals, which serves to prevent the diffusion of aluminum atoms into the layer. When switching PS-resistors with rectangular pulses, the transition characteristic has two parts, the first part corresponds to the delay time τ₃, when the device remains in the YO state after the pulse, and the second part corresponds to the connection time τ v, when the layer changes to the AO state.

It decreases depending on whether the impulse given during deceleration of the coupling exceeds the threshold voltage. For example, when the pulse exceeds the threshold voltage by 1.5 times, it decreases to 20 μs during deceleration. Typical values of the switching time (when the pulse amplitude is twice the threshold voltage) do not exceed 200 n.sec. When the transducer in the YO state is affected by single rectangular pulses with an amplitude close to the threshold voltage, the generation of electrical oscillations is observed in a number of samples. The frequency of relaxation oscillations does not exceed 50-100 kHz. With the increase of the pulse amplitude, the number of oscillations increases, they fill the entire pulse and PSTs become AO. The period of the dances is found from the following relationship:

                                                                               (1)

Here R_H – limiting resistance = 500 Ohm; S – mounting capacity (S=20 pF); U₂ = 45 V – supply voltage; U_ost = 10 V; U_hud = 15 V. According to (3.1) oscillation frequency f = 300 MHs. In real devices, the frequency is three orders of magnitude lower. This shows that the generation of oscillations is not related to the external circuit of the MM device, but to the electronic processes from the area of space charges.

The obtained results show that PST based on LDS are reprogrammable memory elements by electrical erasure of information. Under the influence of light, the MM part of PST in VAX disappears in the photoresistor mode, and to restore it, it is necessary to increase the applied voltage. The increase in breakdown voltage is about 1V compared to the dark value of the voltage.

In [12], inductive phenomena observed in locally formed PSL in the process of epitaxial growth of monocrystalline layers are described. The VAX of PSTs is of S-shaped type with MM, in the frequency range of 0.465-10 MHz, the transition of the conductivity from the capacitive to the inductive character of the reactive collector was observed. Under the influence of light, the inductance was held again, and at this time the NR part disappeared in VAX.

It is known that the semiconductor analogue of inductance is negative capacitance, which are related to each other by the following formula [13]:

                                                                         (2)

                                                                                      (3)

Figure 2. Dependence of the boundary area of the transformation on the length of the LDS layer

Figure 3. Oscillations of the voltage during the application of single impulses with an amplitude greater than the threshold voltage to the LDS layers. On the vertical axis - 10 V division, on the horizontal axis - 2 mks / division

а)

b)

Figure 4. Dark VAX of P layers under the influence of light (b) with and without negative resistance

Here, the quality of the inductance, which is equal to QL, is the lifetime of the charge carriers, and T is the period of the external alternating voltage.

As can be seen from (2) when

                                                                                              (4)

and when           

                                                                                              (5)

Based on the experimental results of the frequency dependence of the conductivity, the dependence of ln(r₁ – r) on the inverse value of the frequency for PST in the YO case is in the form of a test line with two straight lines. The test shows that the change of the reaction time 1 to the new reaction time is related to the deep reaction centers. Estimating the reaction time according to two sloping lines gave the values t₁ = 4x 10^-7 s and t₂ = 2.4x 10^-8 seconds. These times correspond to two deep trap levels (0.12 and 0.44 eV) obtained experimentally by different authors [14–16] in PSL.

Based on the analysis of the literature and conducted studies, the mechanism of inductive effect generation in irregular layers containing SCR and deep traps from the grain boundary has been proposed [17].

It is known that the capacitance character of the reactive collector of conductivity changes to inductance as a result of the capture and generation of charge carriers in the r-p transitions that hold deep traps [18]. The presence of barrier layers and deep traps at the boundary of the grains, as well as the analysis of light dependences of temperature, frequency and C – V characteristics allow us to say that the inversion of capacitance into inductance is related to the filling and discharging processes of deep traps.

Indeed, in the absence of charge carrier injection, the unfilled traps do not participate in the conductivity and do not affect the capacity of the structures due to their electroneutrality. The breakdown of the barrier layers leads to the appearance of carriers with high mobility in SCR, which begin to be occupied by traps.  

                                                                                                 (6)

It is determined by the expression. Here vдр – drift speed of charge carriers j – current density; - coefficient of occupation of electrons in deep traps.

At small slip voltages, the current density is low and the condition is satisfied. - is the circular frequency, f is the frequency of the measured signal.

PSL – what is essentially the case of YO, when shear stress is applied, the free charge carriers injected into the carrier-depleted layers begin to be occupied by deep traps at the grain boundary. At small levels of injection, deep traps () are large during relaxation and satisfy the condition. Therefore, the traps are unable to follow the variation of the changing signal and have no effect on the reactive nature of the conductance. Capacity remains a ratio. As the shear stress increases, the injection level increases. At this time, the probability of the charge carriers with large mobility to be caught by deep traps increases, which leads to a decrease in their reaction time. At a certain level of injection, the reactive sum of the conductance becomes zero and the condition is satisfied. The frequency of the measurement signal becomes smaller than the capture frequency during the further increase of the shear stress. At this time, the deep traps do not have time to carry out the process of capture and generation of charge carriers during the period of change of the alternating signal, which causes the current to lag behind the phase voltage and, thus, inductance.

The decrease of the current under the influence of illumination, which leads to the disappearance of the S-shaped part, indicates that the polysilin layers have a negative photoconductivity. Such events are also related to the presence of deep traps. It is known [19] that skin-level centers in semiconductors can have several charge states, which correspond to different degrees of localization of the wave function.

The charge state of the center when n > 1 can be taken into account in the one-electron band scheme by introducing a correction level of an electron that can reside in the conduction band. In this case, the possibility of negative photoconductivity can occur when the conduction electron absorbs a photon and passes from the band state to the local state, which leads to a decrease in the current and the disappearance of the NR part [20].

Results

Based on the conducted research, it can be concluded that locally formed polysilicon layers form a functional element with nonlinear voltage-capacitance characteristics with two stable conductivity states whose parameters can be controlled by voltage and light.

Literature review and experimental studies have allowed us to clarify the physical properties of inductance phenomena in polycrystalline silicon layers, which are related to deep levels of charge and discharge at grain boundaries. The reactive nature of the conductance of PST depends on whether the processes of capture and generation of charge carriers by deep traps can keep up with the frequency of the external alternating signal or not. At low levels of injection, the capture frequency is low, so the reactive conductance is capacitive in nature, while at higher levels of injection, the capture frequency increases and exceeds the frequency of the applied drift. In this case, the traps can follow its change, which causes the current to lag behind the voltage in phase, which indicates the formation of inductance. Due to the intergranular amorphous layer in irregularly structured layers, there is also a barrier mechanism, as well as a jump character of conductivity, which is more evident at low doping levels and occurs even at room temperature in small-grained layers.

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