Electrochemical Capacitance-voltage Technique Research Articles

Fabrication and characterization of heavily doped n-type GaAs for mid-infrared plasmonics

N-type doping in GaAs is a self-limited process, rarely exceeding a carrier concentration level of 1019 cm−3. Here, we investigated the effect of intense pulsed light melting on defect distribution and activation efficiency in chalcogenide-implanted GaAs by means of positron annihilation spectroscopy and electrochemical capacitance–voltage techniques. In chalcogenide-doped GaAs, donor–vacancy clusters are mainly responsible for donor deactivation. Using positrons as a probe of atomic scale open volumes and DFT calculations, we have shown that after nanosecond pulsed light melting the main defects in heavily doped GaAs are gallium vacancies decorated with chalcogenide atoms substituting As, like VGa–nTeAs or VGa–nSAs. The distribution of defects and carriers in annealed GaAs follows the depth distribution of implanted elements before annealing and depends on the change in the solidification velocity during recrystallization.

Investigation of p+ emitter formation for n-type silicon solar cells application

This work reports on the study of boron diffusion in n-type silicon from preform source for p+ emitter formation. Using quartz tube furnace, three main parameters are investigated that are: drive-in temperature, drive-in time and temperature ramp-up time. It is found that the sheet resistance as measured by the four point probe method (4-pp) decreases from 93 Ω/⎕ to 24 Ω/⎕ as the drive-in temperature increases from 850 °C to 950 °C, likewise, it decreases from 57 Ω/⎕ to 38 Ω/⎕ as the drive-in time increases from 20 min to 40 min. The electrically active boron dopant profiles as measured by the electrochemical capacitance voltage technique (ECV) exhibit surface concentrations below the solid-solubility limit of boron in silicon for all the studied emitters, the maximum surface and peak boron concentrations of 1.30 × 1020 and 1.58 × 1020 atoms cm−3 were measured respectively on the emitter which is formed at a drive-in temperature of 950 °C. Furthermore, it is revealed from the secondary ion mass spectroscopy measurements (SIMS) the presence of the boron rich layer (BRL) on its surface. The Hall Effect measurement method is used for measuring sheet resistance and sheet charge carrier concentration.

Active and buffer layers of GaN HEMT: ECV profiling and 2DEG calculation

ABSTRACT Gallium nitride transistors with high electron mobility (HEMT) have been studied by means of electrochemical capacitance–voltage (ECV) technique. The capacitance–voltage characteristics at various frequencies were measured; the free charge carrier depth distributions as well as the intensity of quantum well filling by charge carriers were obtained. The features of 2D electron gas (2DEG) appearance in GaN HEMT, related to spontaneous and piezoelectric polarisation, as well as factors affecting the density of free charge carriers in channel and the performance of GaN HEMTs were described in detail. The efficiency of ECV technique for quality control of both active and buffer layers of GaN HEMT heterostructures is shown. The calculation of 2DEG density for various technological parameters of the interface, such as mutual orientation of AlGaN/GaN layers and composition of Al x Ga1-x N, is carried out taking into account piezoelectric effects.

Contactless, nondestructive determination of dopant profiles of localized boron-diffused regions in silicon wafers at room temperature

We develop a photoluminescence-based technique to determine dopant profiles of localized boron-diffused regions in silicon wafers and solar cell precursors employing two excitation wavelengths. The technique utilizes a strong dependence of room-temperature photoluminescence spectra on dopant profiles of diffused layers, courtesy of bandgap narrowing effects in heavily-doped silicon, and different penetration depths of the two excitation wavelengths in silicon. It is fast, contactless, and nondestructive. The measurements are performed at room temperature with micron-scale spatial resolution. We apply the technique to reconstruct dopant profiles of a large-area (1 cm × 1 cm) boron-diffused sample and heavily-doped regions (30 μm in diameter) of passivated-emitter rear localized-diffused solar cell precursors. The reconstructed profiles are confirmed with the well-established electrochemical capacitance voltage technique. The developed technique could be useful for determining boron dopant profiles in small doped features employed in both photovoltaic and microelectronic applications.

ECV profiling of GaN HEMT heterostructures

AlGaN/GaN high electron mobility transistor (HEMT) heterostructures were investigated by means of the electrochemical capacitance-voltage technique. The concentration profiles of free charge carriers across the samples were experimentally obtained. The features of 2D electron gas (2DEG) appearing in GaN HEMT in relation to spontaneous and piezoelectric polarization were described in detail, along with the factors affecting the density of free charge carriers in the channel and the performance of GaN HEMTs. The 2DEG density for various technological parameters of the interface, such as the mutual orientation of the AlGaN/GaN layers and AlxGai-xN composition, was calculated accounting for piezoelectric effects.

ECV Doping Profile Measurements in Silicon Using Conventional Potentiostat

Formation of p–n junctions by phosphorus diffusion from liquid dopant in Si was investigated using a custom-built procedure for performing electrochemical capacitance–voltage (ECV) measurements. The feasibility of using a potentiostat equipped with an impedance module was investigated using an algorithm and experimental setup developed for the ECV technique. Using impedance spectroscopy and amperometry methods in a controlled manner, the dopant concentration was measured in the depth range of 600 nm to 900 nm. Sequential etching in NH4F (0.1 M) electrolyte using current–time (I–t) testing under ultraviolet (UV) illumination was applied to etch the surface of n-type silicon and estimate the diffusion depth. Initial current–voltage (I–V) tests were used to determine the bias voltage. The results of the proposed method were compared with those obtained using a commercially available ECV profiler. A conventional parallel equivalent circuit model of the Schottky junction was used to describe the electrolyte–silicon barrier, leading to good agreement between the proposed method and commercial ECV analysis results.

ECV profiling of GaAs and GaN HEMT heterostructures

AlGaAs/InGaAs/GaAs and AlGaN/GaN HEMT heterostructures were investigated by means of electrochemical capacitance-voltage technique. A set of test structures were fabricated using various doping techniques: standard doping, δ-doping GaAs pHEMT and nondoping GaN HEMT. The concentration profiles of free charge carriers across the samples were experimentally obtained. The QW filling was analyzed and compared for different mechanisms of emitter doping and 2DEG origins.

Electrochemical capacitance voltage measurements in highly doped silicon and silicon-germanium alloys

The electrochemical capacitance voltage technique has been used on highly boron doped SiGe and Si layers. Although the boron concentration is constant over the space charge depth, the 1/C2 versus voltage curves are not linear. They indeed present a negative curvature. This can be explained by the existence of deep acceptors which ionise under a high electric field (large inverse voltage) and not at a low inverse voltage. The measured doping concentration in the electrochemical capacitance voltage increases strongly as the inverse voltage increases. Thanks to a comparison with the boron concentration measured by secondary ions mass spectrometry, we show that the relevant doping concentrations in device layers are obtained for small inverse voltage in agreement with the existence of deep acceptors. At the large inverse voltage, the measured doping can be more than twice larger than the boron concentration measured with a secondary ion mass spectroscopy.

Analysis of p‐type SiOx layers as a boron diffusion source for n‐type c‐Si substrates

We evaluate the use of p‐type silicon oxide (p‐SiOx) dielectric layers as a boron diffusion source for n‐type crystalline silicon (c‐Si) substrates. The p‐SiOx layers grown on n‐type c‐Si substrates by plasma enhanced chemical vapor deposition using a gas mixture of He/hexamethyldisiloxane/CO2/B2H6 are thermally stable and do not peel off during annealing up to 1050 °C, making them effective diffusion sources. The layers were examined before and after annealing with characterization techniques including spectroscopic ellipsometry and secondary ion mass spectrometry. We observe that there is a reduction in the thickness of the p‐SiOx layer after annealing by about 50%, and that boron diffuses into the n‐type c‐Si substrate, forming a p+ layer, limited by the formation of a carbon‐rich layer above the c‐Si surface. The concentration of holes in the diffused region was measured by the electrochemical capacitance–voltage technique, and it was found that essentially all the boron that diffused into the n‐type c‐Si was active, unaffected by the presence of carbon and oxygen atoms. The concentration of carriers can be controlled by the initial thickness of the p‐SiOx layers and the depth of the p+/n junction can be controlled by the time of annealing. A surface carrier concentration of 3 × 1019 at cm−3 and a sheet resistance of the order of 120 Ω sq−1 was obtained upon annealing at 1050 °C for 30 min.

Investigation of delta-doped pHEMT InGaAs/GaAs/AlGaAs structures by the electrochemical capacitance-voltage technique

In pHEMT devices, a two dimensional (2D) conducting channel representing a quantum well (QW) and adjacent layers of wide-gap semiconductors are fabricated on an undoped material which provides a high mobility of free carriers in the channel. During the development of pHEMT structures it is always very important to estimate and control the free charge carriers concentration, that is not a trivial task because of its redistribution which causes changing of the electric field distribution and, respectively, the energy spectrum of states in the 2DEG channel. By using the modern ECV technique, the pHEMT InGaAs/GaAs/AlGaAs structures were investigated. Free charge carriers concentration profiles were obtained, and the location of 5-layers and the carrier concentration in the QW were estimated.

Impact of acceptor concentration on the resistivity of Ni/Au p‐contacts on semipolar (20–21) GaN:Mg

The p‐type doping of GaN with Mg, in particular doping of p++ cap layers and its influence on the resistivity of Ni/Au contacts on semipolar (20–21) GaN, has been investigated. For this purpose, we have compared GaN:Mg grown on several semipolar and polar orientations with respect to the acceptor concentration NA measured by electrochemical capacitance voltage techniques. For the same Mg precursor flow and Mg/III ratio, we observe very similar acceptor densities NA of up to 1 × 1019 cm−3 for (0001), (20–21), (20–2–1), and (11–22) GaN:Mg. Furthermore, the impact of the II/III ratio for the p++ cap layer (NA > 1 × 1019 cm−3) on I–V characteristics of Ni/Au (20 nm/30 nm) contacts on (20–21) oriented GaN:Mg has been investigated. Ohmic I–V characteristics were observed for Mg/III ratios >1 × 10−2. Specific contact resistivities as low as 2.4 × 10−3 Ω cm2 could be achieved. Inclusion of this p++ cap layer resulted in reduced turn on voltages in light emitting diodes with 450 nm emission wavelength.

Raman scattering study of vibrational modes and hole concentration in GaxMn1−xSb

The Raman scattering study of vibrational modes and hole concentration in a ferromagnetic semiconductor Ga1−xMnxSb grown by Mn ion implantation, deposition and post-annealing has been presented. The experiments are performed both in implanted and unimplanted regions before and after etching the samples. The Raman spectra measured from the unimplanted region show only GaSb-like phonon modes. On the other hand, the spectra measured from the implanted region show additional phonon modes approximately at 115, 152, 269, 437 and 659 cm−1. The experimental results demonstrate that the extra modes are associated with surface defects, crystal disorder and blackish layer that is formed due to Mn ion implantation, deposition and annealing processes. Furthermore, we have determined the hole concentration as a function of laser probing position by modeling the Raman spectra using coupled mode theory. The contributions of GaSb-like phonon modes and coupled LO-phonon plasmon mode are taken into consideration in the model. The hole-concentration-dependent CLOPM is resolved in the spectra measured from the implanted and nearby implanted regions. The hole concentrations determined by Raman scattering are found to be in good agreement with those measured by the electrochemical capacitance–voltage technique.

Passivation of Interfacial States for GaAs- and InGaAs/InP-Based Regrown Nanostructures

The interfacial charge density of regrown structures was studied for several different material systems: GaAs, InGaAs/InP, and InAlAs-InGaAs superlattice structures on InP. The particular application of interest is in the fabrication of nanoscale devices. Such structures require a very low density of interfacial charge at their exposed surfaces in order to avoid Fermi-level pinning and subsequent lateral carrier depletion across the structure. (110)-Oriented samples, mimicking the exposed sidewalls of nano-etched structures, were plasma-etched using a variety of gas-phase chemistries. The interfacial charge density at regrown interfaces was studied using capacitance-voltage (CV) and electrochemical CV techniques after in situ and ex situ pretreatments and epitaxial regrowth. The minimum interfacial charge densities obtained for these material systems were < 10 11 cm -2 . Preferential regrowth around etched nanopillars was demonstrated for InP-based structures.

On application of electrochemical capacitance–voltage profiling technique for n-type SiC

The behaviour of n-6H SiC in HF- and KOH-based electrolytes is studied. It is shown that at the room temperature the dissolution of SiC in these electrolytes is accompanied by a number of side effects, such as passivation of the etched surface with an anodic film or surface roughening. On the basis of the results obtained, an etching method employing repetitive use of both electrolytes is proposed for accurate characterization of n-SiC with the electrochemical capacitance–voltage technique.

Electrochemical capacitance-voltage characterization of plasma-doped ultra-shallow junctions

Ultra-shallow Si p+n junctions formed by plasma doping are characterized by electrochemical capacitance-voltage (ECV). By comparing ECV results with those of secondary ion mass spectroscopy (SIMS), it is found that the dopant concentration profiles in heavily-doped p+ layer as well as junction depths measured by ECV are in good agreement with those measured by SIMS. However, the ECV measurement of dopant concentration in the underlying lightly doped n-type substrate is significantly influenced by the upper heavily-doped layer. The ECV technique is also easy to control and reproduce. The ECV results of ultra-shallow junctions (USJ) formed by plasma doping followed by different annealing processes show that ECV is capable of reliably characterizing a Si USJ with junction depth as low as 10 nm, and dopant concentration up to 1021 cm−3. Also, its depth resolution can be as fine as 1 nm. Therefore, it shows great potential in application for characterizing USJ in the sub-65 nm technology node CMOS devices.

Near-infrared intersubband absorption in nonpolar cubic GaN∕AlN superlattices

Optical absorption spectra related to intersubband transitions in molecular beam epitaxially grown nonpolar cubic-GaN∕AlN superlattices were observed in the spectral range of 1.5–2.00μm. The background doping was measured using an electrochemical capacitance-voltage technique and found to be on the order of 1018cm−3. This doping level yields a Fermi energy level slightly above the ground state energy level enabling intersubband transitions to occur. The existence of the intersubband transition is verified in several samples with different well widths. The observed peak position energy of the intersubband transition is compared to those calculated using a transfer matrix method.

1.37 - 2.90 Micron Intersubband Transitions in GaN/AlN Superlattices

ABSTRACTIntersubband transitions in the spectral range of 1.37-2.90 °Cm is observed in molecular beam epitaxy grown Si-doped GaN/AlN multiple quantum wells using a Fourier-transform spectroscopy technique. A blue shift in the peak position of the intersubband transition is observed as the well width is decreased. A sample with a well width in the order of 2.4 nm exhibited the presence of three bound states in the GaN well. The bound state energy levels are calculated using a transfer matrix method. An electrochemical capacitance voltage technique is used to obtain the three dimensional carrier concentrations in these samples which further enable the calculation of the Fermi energy level position. Devices fabricated from these GaN/AlN quantum wells are found to operate in the photovoltaic mode.

Determination of doping profiles on bevelled GaAs structures by Raman spectroscopy

A method for determination of doping concentration profiles of GaAs multilayer structures on a bevelled surface by Raman spectroscopy is presented. By scanning the laser beam along the bevel we obtained micro-Raman spectra in different depth positions in the structure. Calculated I TO/ I LO intensities determine the doping concentration in these points for values above 3×10 16 cm −3. The results are compared with electrochemical capacitance–voltage technique and secondary ion mass spectrometry. Some specific problems are discussed.

Improvement of the Electrochemical Profiling Technique of Carrier Concentration in Cu(InGa)Se2 Thin Film Solar Cells

In our previous works, we measured the carrier concentration profile of Cu(InGa)Se2 (CIGS) films using the electrochemical capacitance–voltage (ECV) method with ethylenediaminetetraacetic acid (EDTA). However, we occasionally observed deposition of Se onto the CIGS films after the etching, which limited the reproducibility of the measurement. In this study, we tried to improve the ECV technique and successfully measured the carrier concentration profile for the film with a wide Cu/III ratio range by the addition of sulfurous acid into the EDTA electrolyte. Moreover, the results of these measurements revealed that the carrier (hole) concentrations of the CIGS films ranged between 1016 and 1018 cm-3 and that the carrier concentration changed with the growth conditions. Based on the above results, it is believed that this new measurement technique is a useful and powerful tool for characterization of CIGS thin films.

Carrier Profiling of a Heterojunction Bipolar Transistor and p–i–n Photodiode Structures by Electrochemical C–V Technique

Carrier profiling of GaAs/GaAlAs multilayer structures (heterojunction bipolar transistor and heterojunction p-i-n photodiode) by electrochemical capacitance-voltage technique is described. Optimum measurement parameters (electrolyte type, etching and measurement voltages, etc.) are established. The results are compared with those of spreading resistance profiling and transmission electron microscopy. Some specific problems are discussed.