RU2054748C1 - Method of determination of profile of concentration of current carriers in semiconductor structures with usage of semiconductor-electrolyte contact - Google Patents

Abstract

FIELD: electronics. SUBSTANCE: method of determination of profile of concentration of current carriers in semiconductor structures with usage of semiconductor-electrolyte contact includes local layer by layer etching of structure, measurement of current of electrochemical reaction and calculated determination of concentration of current carriers. Measurement of current of electrochemical reaction is performed while semiconductor structure is illuminated by light with energy of quantum equal to energy of transition (λ₃-λ₁) of Brillouin zone of semiconductor material of structure. Illumination intensity amounts to 10¹³-10¹⁴ cm^-2s^-1. External voltage under which photoflux is equal to zero is applied simultaneously with illumination to semiconductor structure. EFFECT: improved authenticity of determination of profile of concentration of current carriers. 5 dwg, 1 tbl

Description

The invention relates to semiconductor technology and can be used to determine the concentration profile of current carriers in multilayer semiconductor structures based on GaAs, Si, solid solutions of compounds A ₃ B ₅ , including superlattices.

A known method for determining the concentration profile of current carriers in semiconductor structures by the capacitance-voltage method with an electrochemical etching probe [1]
The disadvantages of the method are the limitations in determining the concentration profile of current carriers in the surface layer with a thickness less than the space charge region (SCR) at zero bias, as well as the impossibility of determining the concentration profile of current carriers in ultrathin structures with thicknesses smaller than the width of the SCR at zero bias.

A known method for determining the concentration profile of current carriers in semiconductor structures by measuring the photocurrent of electrochemical etching [2] in which the concentration profile of current carriers in an n ⁺ n i-type semiconductor structure is determined from measurements of the photocurrent in the semiconductor-electrolyte contact when an anode potential is applied to the semiconductor, which reduces the concentration of electrons on the surface and increasing the concentration of photogenerated minority carriers of hole currents. The etching depth of the semiconductor was determined by the anode Faraday current.

 In this way, it is fundamentally impossible to determine the concentration of the main current carriers (in this case, electrons) in the semiconductor structure, since the measured value is the current of the photogenerated minority charge carriers, which depends on the light intensity and the recombination losses of these carriers in the SCR and at the heterointerfaces. The method cannot also be used to determine the concentration profile in ultrathin regions of the structure, the thickness of which is less than the SCR width, due to the presence of energy bending of zones in the SCR, which is necessary for the separation of photogenerated charge carriers during the flow of a photoelectrochemical current.

The prototype of the proposed method is a method for determining the concentration profile of current carriers in semiconductor structures [3] according to which the concentration profile of current carriers, for example electrons, can be determined from the current of the electrochemical reaction of the contact electrolyte semiconductor by the calculation formula
i _n An _o l ^-y z ^-αlΔE g ^{/ KT} , (1) where i _n current of the electrochemical reaction;
n _o determined electron concentration;
y _z bending of energy zones on the surface of the semiconductor;
α transfer coefficient of the electrochemical reaction;
Δ E _{g the} change in potential jump in the Helmholtz layer during the course of the reaction with respect to its equilibrium value.

 The calculation formula (1) is used as a prerequisite for developing a method for measuring the concentration profile of current carriers in semiconductor structures.

Analysis of the calculation formula (1) shows that the main disadvantage of the prototype method is its low accuracy due to the presence of zones of energy bending (y _s ) at the interface, the practical and calculated determination of which is extremely time-consuming and has a large error. The method is not applicable for determining the concentration profile of current carriers in multilayer semiconductor structures with ultrathin layers due to the presence of energy bending of zones extending over distances, in many cases larger than the doping regions of ultrathin layers in the measured structure.

 The technical result of the invention is to increase the accuracy of determining the concentration profile of current carriers and providing the ability to determine the concentration profile of current carriers in multilayer ultrathin structures.

The technical result is achieved by the fact that according to the method for determining the concentration profile of current carriers in a semiconductor structure using a semiconductor-electrolyte contact, including local layer-by-layer etching of the structure, measuring the current of the electrochemical reaction and calculating the concentration of current carriers, measuring the current of the electrochemical reaction is carried out when the semiconductor structure is illuminated with light with a quantum energy equal to the transition energy (λ ₃ -λ ₁ ) of the Brillouin zone of a semiconductor material structure, while the illumination intensity is 10 ¹³ 10 ¹⁴ cm ^-2 s ^-1 , and simultaneously with the illumination, an external voltage is applied to the structure at which the photocurrent is zero.

 The essence of the proposed method, which allows to achieve the goal, is to reduce the energy bending of the zones to a constant and, in the optimal case, minimum value that does not depend on the properties of the electrolyte and the material of the semiconductor. In the proposed method, the change in the bending of the energy zones in the surface layer of the semiconductor is achieved by applying an external electric voltage, and due to a change in the magnitude of the external voltage, the bending of the energy zones is reduced to zero. The achievement of the zero value of the bending of the energy bands of the semiconductor (rectification of the energy bands at the heteroboundary) is controlled by a photocurrent, for which, simultaneously with the action of an external voltage, the semiconductor is irradiated with strongly absorbed light that generates charge carriers near the surface.

In the presence of a bend of energy zones at the surface, these charge carriers are separated by the SCR field, as a result of which a photocurrent occurs with a closed external circuit. In the absence of bending of zones on the surface, this process stops, as a result of which the photocurrent drops to zero. Thus, the method of photocurrent controls the zero bending of zones on the surface of the semiconductor. Since light is absorbed in a thin surface region in the region of strong light absorption by a semiconductor, the zero bending of the zones is controlled in the same region. Thus, the thickness of the surface layer where light is absorbed determines the accuracy of the method and, accordingly, the applicability of the method for measuring the concentration profile of current carriers in ultrathin semiconductor structures. The maximum absorption coefficient, when absorption occurs at a distance of ≅ 10 nm, is achieved at the transitions (λ ₃ -λ ₁ ) of the Brillouin zone of the semiconductor material. Therefore, for strong light absorption in a semiconductor (at a distance of ≅ 10 nm from the surface), the energy of the light quantum should be equal to the energy gap (λ ₃ -λ ₁ ) of the Brillouin zone of the semiconductor. This quantum energy of light is both the minimum and nominal value in the proposed range of quantum energy values. Limitations on the upper limit (not more than 0.5 kT), where k is the Boltzmann constant; T absolute temperature) due to considerations of mechanical strength, chemical stability, the absence of crystallographic changes that may result from exposure to light, etc. those. factors that characterize the method as non-destructive. The energy transitions (λ ₃ -λ ₁ ) of the Brillouin zone can be calculated according to the developed schemes and are known for many semiconductors. The depth of light absorption is also known for many materials, for example, for GaAs at a wavelength of λ 280 nm, d 8.7 nm, for GaP at λ 270 nm d 9 nm, for InP at λ 270 nm d 8.1 nm. The illumination intensity during measurements should be weak (10 ¹³ .10 ¹⁴ cm ^-2 s ^-1 ) in order to eliminate the effect of lighting on the semiconductor potential in the electrolyte and errors caused by the Dembar photo-emf.

 Under conditions when the bending of energy zones on the surface is close to zero, it is necessary to choose electrolytes that ensure the flow of reaction currents near the potentials of the planar zones of the semiconductor. Such electrolytes can be acid solutions in which the hydrogen evolution reaction proceeds. In these electrolytes, ultraviolet light is practically not absorbed. Radox systems that do not absorb ultraviolet light can also be selected.

 The calculation formula for determining the concentration profile of current carriers, which is used as a prototype method, does not disclose approaches for its practical implementation. Until now, the practical implementation of the method has not been carried out, therefore, it is an independent task, the solution of which is not trivial due to the need to find the optimal approach in building a sequence of techniques and in finding the techniques themselves.

 From the patent and scientific and technical literature, a set of techniques is not known, including ranges of values of individual process parameters, as well as a sequence of techniques that would unambiguously lead to the achievement of the goal. The independence and originality of the approach to solving the goal, from which the stated sequence of technical methods follows, allows us to assume that the proposed method meets the criterion of "inventive step".

 In addition, from the technical and patent literature it is known that at present there are no methods for measuring the concentration profile of current carriers in ultrathin multilayer semiconductor structures that provide the same or higher measurement accuracy than the described method.

 The design of the method is simpler and cheaper compared to installations of a similar purpose (SIMS secondary ion mass spectroscopy). The proposed method does not have theoretical and practical limitations when used to measure the concentration profile in semiconductor structures of various ratings.

 These circumstances suggest that the proposed method has novelty, inventive step and industrial applicability.

In FIG. 1 shows a diagram of a measuring installation; in FIG. 2 the dependence of the hydrogen evolution current for gallium arsenide in a 10% aqueous KOH solution. For different values of the concentration of current carriers no in gallium arsenide
curve 1 for n _o 8.3 · 10 ¹⁴ cm ^-3 ,
curve 2 for n _o 1.4 · 10 ¹⁶ cm ^-3 ,
curve 3 for n _o 3.7 · 10 ¹⁶ cm ^-3 .

In FIG. 3 shows the distribution profile of the concentration of current carriers and the distribution profile of dopants in the structure of gallium arsenide of the type n ⁺ i δ _p i n + obtained by molecular beam epitaxy (MBE), where
curve 1 profile of the distribution of the concentration of current carriers, measured by the proposed method,
curve 2 the distribution profile of the dopant silicon obtained by the method of secondary ion mass spectroscopy (SIMS),
curve 3 distribution profile of the dopant of beryllium obtained by SIMS.

In FIG. 4 shows the distribution profile of the concentration of current carriers and the distribution profile of dopants in the structure of gallium arsenide of the n ⁺ i δ _p in ^{+ type} obtained by the MBE method,
where curve 1 is the distribution profile of the concentration of current carriers, measured by the proposed method;
curve 2 distribution profile of the doping impurity of silicon obtained by SIMS,
curve 3 distribution profile of the dopant of beryllium obtained by SIMS.

In FIG. 5 shows the distribution profile of the concentration of current carriers n _o for single-crystal indium phosphide obtained by the proposed method.

The table shows the results of determining the concentration profile of current carriers by the proposed method for the type of structure of gallium arsenide for type n ⁺ i δ _p in ⁺ obtained in the epitaxial process using MOS technology. The contact area is 0.123 cm ² .

 The installation for implementing the method consists of a power battery 1, a high-pressure xenon lamp 2, type DKSh-1000, a modulator 3, quartz lenses 4 and 5, a monochromator 6, type MDR-2, a potentiostat 7, type P5848, a cell with an optically transparent window 8 containing an electrolyte a semiconductor structure 9, a counter electrode 10, a comparison electrode 11, an AC amplifier 12 of type U2-8, a synchrodetector 13 of type UPI-1, a recorder 14 of type KSP-4, and an auxiliary generator 15 of type G3-56. The feedback circuit between the output signal from the synchrodetector 13 UPI-1 and the potentiostat 7 P5848 is introduced into the measurement circuit. Feedback contains switch 16.

The determination of the concentration of current carriers n _o by the proposed method is as follows. The semiconductor structure 9 with an ohmic clamping contact is placed in a cell with an optically transparent quartz window 8 containing an electrolyte. For compounds of type A ³ B ⁵ , a 10% KOH solution can serve as an electrolyte.

 As the counter electrode 10, a platinum or carbon electrode is used. The reference electrode 11 is a silver chloride electrode. The light source is a high-pressure xenon lamp 2 of the DKSSh-1000 type, the light of which is modulated by modulator 3 at a low frequency of 20 Hz. The light from the lamp is focused by a quartz lens 4 on the entrance slit of an MDR-2 type monochromator 6, and by a lens 5 on a semiconductor structure 9 in the cell. The photocurrent is measured in the potentiostatic mode, the cell potential is set by potentiostat 7 of type P5848.

 The signal proportional to the semiconductor photocurrent is amplified by a U2-8 type AC amplifier 12 at a light frequency modulation frequency of 20 Hz, then it is detected by a UPI-1 type synchrodetector 13 and recorded on a KSP-4 type 14 recorder. The output installation captures the photocurrent and the dark reaction current on the semiconductor (hydrogen evolution reaction). Feedback has been introduced into the measurement circuit between the output signal from the synchrodetector 13, which is proportional to the photocurrent, and the potentiostat 7. Feedback provides automatic setting of the semiconductor potential, at which the photocurrent is zero. Thus, when feedback is turned on, the magnitude of the monitored photocurrent is zero, and the setup automatically determines the magnitude of the dark reaction current on the semiconductor at a potential that provides zero photocurrent.

 The auxiliary generator 15 connected to the light modulator 3 and the synchrodetector 13 serves to match the phase of the modulated light and the phase of the synchrodetector 13.

The currents of the electrochemical reaction for semiconductors with different electron concentrations n _{o are} significantly different (Fig. 2), which makes it possible to calibrate the current i from the value n _o for the same material under certain etching conditions and the amount of bending of the zones on the surface y _z 0. V As a result of such a calibration, the dependence i (n _o ) for each of the studied semiconductor materials is constructed according to formula (1) and the value of n _o for multilayer structures with a resolution in thickness equal to the depth of light absorption in a given semiconductor is determined water tank. The magnitude of the zone bending (y _z 0) is measured by the photoelectrochemical current method with an accuracy of CT / e (≈25 mV), which determines the accuracy of measuring the concentration of charge carriers by at least half an order of magnitude. The concentration profile of charge carriers is determined by layer-by-layer photoelectrochemical or chemical etching of the semiconductor structure.

 PRI me R 1. Structures of gallium arsenide obtained by the MOS technology.

The submicron structures obtained by the MOS technology grown on an n-type gallium arsenide substrate contained the following regions: n ⁺ i δ _p in ⁺ , where the δ _p region according to the technical growth conditions had a thickness of the order of 10 nm and a doping level ≥ 10 ¹⁸ cm ^-3 . The boundaries of the regions of growth and concentration of charge carriers in each of the regions were determined. The concentration profile for these samples was determined by photoelectrochemical etching in a 10% KOH solution and measuring the electrochemical current at zero bending of the zones on the surface of the structure using the method described above.

где η глубина травления, см;
n квантовый выход фототока с учетом отраженного света, n 1;
М молекулярный вес полупроводника, г · экв^-1;
Q заряд, пропущенный через межфазную границу раздела полупроводник электролит, К/см²;
Z число электронов в фотоэлектрохимической реакции разложения полупроводника, Z 6;
ρ плотность полупроводника, г/см³, для арсенида галлия ρ 5,317 г/см³;
F число Фарадея, к · г · экВ^-1.The etching depth was determined by the formula
η

where η is the etching depth, cm;
n quantum output of the photocurrent taking into account the reflected light, n 1;
M molecular weight of the semiconductor, g · equiv ^-1 ;
Q charge passed through the semiconductor electrolyte interface, K / cm ² ;
Z is the number of electrons in the photoelectrochemical decomposition reaction of the semiconductor, Z 6;
ρ semiconductor density, g / cm ³ , for gallium arsenide ρ 5.317 g / cm ³ ;
F is the Faraday number, k · g · ecV ^-1 .

The structure was illuminated with light with a wavelength of 280 nm. The results of determining the concentration profile for the structure are presented in the table. It follows from the table that the cathodic current remained almost constant along the depth of the n ^{+ region} , which indicates a uniform doping of this region. The depth of the i-region was 122.3 nm. This region also had uniform alloying. For the δ _{p region} , the hole concentration was high and amounted to ≈ 10 ¹⁸ cm ^–3 , while its thickness was d9.8 nm. The subsequent i-region had a thickness of 205.1 nm and a concentration of n _o 5 · 10 ¹⁵ cm ^-3 . The i-region was followed by the n ⁺ region with a carrier concentration of n _o ≈ 5 · 10 ¹⁸ cm ^-3 .

 Comparison of the obtained profile of the concentration of charge carriers with SIMS data showed the reliability of the proposed method and good quantitative agreement of the results.

 PRI me R 2. Structures of gallium arsenide obtained by MBE technology.

 Layer-by-layer etching of these structures was carried out by the chemical method.

Chemical etchants had the composition, ml: H ₃ PO ₄ 60 H ₂ O ₂ 30 HClO ₄ 30 H ₂ O 240 or H ₂ O ₂ 30 H ₂ O 100 H ₃ PO ₄ 10
The etching rate was determined by the loss of weight of the semiconductor and amounted to 0.5-2 μm / min. For each of the experiments, the etching rate was determined by the satellite sample. After etching of each layer, the structure was thoroughly washed twice with distilled water. Zero bending of the zones in the semiconductor electrolyte contact and the magnitude of the electrochemical current were controlled in the same way as in the previous case; a 10% KOH solution was used. The structure was illuminated with light with a wavelength of 280 nm.

 The concentration profile measurements are shown in FIG. 3 and 4 for two different structures. Good quantitative agreement with SIMS data was also obtained for these structures.

 PRI me R 3. InP single crystals.

The concentration profile of uniformly doped InP single crystals with an electron concentration of n _o 2 · 10 ¹⁶ cm – ^{3 was} determined. Layer-by-layer etching of InP was carried out by a chemical method in a solution of HCl HO ₃ (1: 1) at room temperature. A 10% KOH solution was used as an electrolyte. The sample was illuminated with light with a wavelength of 275 nm. The results of measuring the electron concentration profile n _o are shown in FIG. 5. It is seen that for the studied InP thicknesses, the electron concentration is constant, which corresponds to the data of either the InP sample.

 Technical and economic advantages of the proposed method compared to the prototype consist in higher measurement accuracy, the method provides a determination of the concentration profile of current carriers in ultrathin semiconductor structures, the design of the method does not require expensive components. The method is simple and reliable in operation, therefore, it will be widely used at industry enterprises.

Claims (1)

THE METHOD FOR DETERMINING THE PROFILE OF CONCENTRATION OF CURRENT CARRIERS IN SEMICONDUCTOR STRUCTURES USING THE CONTACT OF SEMICONDUCTOR-ELECTROLYTES, which includes local layer-by-layer etching of structures, measurement of the current of the electrochemical reaction and the calculation of the concentration of charge carriers near the conductors, which differ in and the measurement of the current of the electrochemical reaction is carried out when the semiconductor structure is illuminated with light with an energy of cable lying in the range from the transition energy λ ₃ -λ ₁ Brillouin zone of the semiconductor material of the structure to an energy of 0.5 kT, with the illumination intensity of 10 ^{1 3} - 10 ^{1 4} cm ^{- 2} s ^{- 1} and simultaneously with the illumination to the semiconductor structure apply an external voltage at which the photocurrent is zero.

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