Contactless Electroreflectance Spectroscopy Research Articles

Electro-modulation and surface photovoltage spectroscopy with semi-transparent graphene electrodes

Investigating the fundamental properties of new semiconductor materials and structures requires the use of a wide range of optical spectroscopy techniques. Among them, there are those that exploit the use of electrodes that are semi-transparent to light, a requirement arising from the need to either apply external perturbation to the studied material or sense the effects generated by light excitation. In this work, semi-transparent graphene electrodes were obtained by chemical vapour deposition of the graphene on a copper substrate and transferring these layers to a sapphire substrate. The fabricated electrodes were tested in contactless electroreflectance, soft-contact electroreflectance, and surface photovoltage spectroscopy on III-V epilayers (InP and GaAs) and van der Waals crystals (MoS2 and WSe2), reference industry-standard and emerging semiconductors. The obtained results clearly show that graphene electrodes are suitable for these measurements and have many advantages over the commonly used approaches to ensure transparent conducting layers in optoelectronic devices. On the other hand, compared to indium‑tin oxide, their transparency covers a wide spectral range, including ultraviolet (UV), revealing another important advantage. We also demonstrate that further structuring of the graphene layer enables maximizing the transparency up to the limit imposed by the hosting substrate.

Toward h-BN/GaN Schottky Diodes: Spectroscopic Study on the Electronic Phenomena at the Interface.

Hexagonal boron nitride (h-BN), together with other members of the van der Waals crystal family, has been studied for over a decade, both in terms of fundamental and applied research. Up to now, the spectrum of h-BN-based devices has broadened significantly, and systems containing the h-BN/III-V junctions have gained substantial interest as building blocks in, inter alia, light emitters, photodetectors, or transistor structures. Therefore, the understanding of electronic phenomena at the h-BN/III-V interfaces becomes a question of high importance regarding device engineering. In this study, we present the investigation of electronic phenomena at the h-BN/GaN interface by means of contactless electroreflectance (CER) spectroscopy. This nondestructive method enables precise determination of the Fermi level position at the h-BN/GaN interface and the investigation of carrier transport across the interface. CER results showed that h-BN induces an enlargement of the surface barrier height at the GaN surface. Such an effect translates to Fermi level pinning deeper inside the GaN band gap. As an explanation, we propose a mechanism based on electron transfer from GaN surface states to the native acceptor states in h-BN. We reinforced our findings by thorough structural characterization and demonstration of the h-BN/GaN Schottky diode. The surface barriers obtained from CER (0.60 ± 0.09 eV for GaN and 0.91 ± 0.12 eV for h-BN/GaN) and electrical measurements are consistent within the experimental accuracy, proving that CER is an excellent tool for interfacial studies of 2D/III–V hybrids.

Probing Defects in MoS2 Van der Waals Crystal through Deep‐Level Transient Spectroscopy

The electrical performance of transition metal dichalcogenides (TMDCs) is strongly affected by the quality of electrical metal contacts and the formation of electrically active defects. Herein, deep‐level transient spectroscopy (DLTS) is used for direct probing of deep‐level defects in the bandgap of single MoS2 van der Waals crystal. Standard DLTS temperature spectra reveal a deep‐level trap located at about 0.36 eV below the conduction band edge. This trap is tentatively attributed to sulfur vacancies and localized on the electronic band structure of MoS2, obtained within the density functional theory (DFT), and matched with experimentally studied electronic band structure, by absorption and contactless electroreflectance (CER) spectroscopy.

Contactless electroreflectance spectroscopy with a semitransparent capacitor made of a silver mesh of ultrathin lines

Semitransparent electrodes operating in the broad spectral region are needed for many applications and one of them is contactless electroreflectance (CER) spectroscopy. In this work, the technology of printed electronics has been applied to fabricate mesh-like semitransparent electrodes composed of ultrathin (10 µm) silver lines of different densities. These electrodes have been printed on borosilicate glass and used in CER measurements. It has been shown that the CER signal is linearly proportional to the applied voltage and inversely proportional to the distance. In addition, a shadow effect has been observed for some distances because of non-transparent silver paths. Finally, a phenomenological formula has been proposed to describe the intensity of the CER signal in the capacitor with the semitransparent electrode. Moreover, the mesh-like printed electrodes have been applied to measure CER spectra for van der Waals crystals and perspectives for further development of semitransparent electrodes for CER measurements have been discussed.

Boron influence on bandgap and photoluminescence in BGaN grown on AlN

Since the BGaN alloy is considered a promising material in the wide range of optoelectronic applications, a detailed study of its band structure and optical properties is highly demanded. Here, BxGa1−xN layers with 0.5%, 1.1%, and 1.2% B were grown by metalorganic vapor-phase epitaxy on AlN/sapphire templates and investigated by structural and optical methods. The bandgaps of the investigated alloys were examined by contactless electroreflectance (CER) spectroscopy. Because no GaN layer is present in the investigated samples, the detected CER resonances do not overlap with the GaN-related signal, which is typical for BGaN layers grown on GaN templates. Thus, the energy of the bandgap-related transition in BGaN samples can be unambiguously determined from the resonances observed in the CER spectra. The boron-induced redshift of the bandgap was determined to be about 60 meV/% B for the studied samples. By means of photoluminescence measurements, the deteriorating optical quality of samples with increasing boron content is shown as the decreasing bandgap- to defect-related emission intensity ratio. What is more, the defect-related emission is shifted from typical for GaN yellow range to the red and is located at 1.9 eV for all BGaN samples.

Nitrogen-related intermediate band in P-rich GaNxPyAs1\u2212x\u2212y alloys

The electronic band structure of phosphorus-rich GaNxPyAs1−x−y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. It is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate band (E− band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E+ band using contactless electroreflectance spectroscopy.

Engineering of electric field distribution in GaN(cap)/AlGaN/GaN heterostructures: theoretical and experimental studies

Polarization engineering of GaN-based heterostructures opens a way to develop advanced transistor heterostructures, although measurement of the electric field in such heterostructures is not a simple task. In this work, contactless electroreflectance (CER) spectroscopy has been applied to measure the electric field in GaN-based heterostructures. For a set of GaN(d = 0, 5, 15, and 30 nm)/AlGaN(20 nm)/GaN(buffer) heterostructures a decrease of electric field in the GaN(cap) layer from 0.66 MV cm−1 to 0.27 MV cm−1 and an increase of the electric field in the AlGaN layer from 0.57 MV cm−1 to 0.99 MV cm−1 have been observed with the increase in the GaN(cap) thickness from 5–30 nm. For a set of GaN(20 nm)/AlGaN(d = 10, 20, 30, and 40 nm)/GaN(buffer) heterostructures a decrease of the electric field in the AlGaN layer from 1.77 MV cm−1 to 0.64 MV cm−1 and an increase of the electric field in the GaN layer from 0.57 MV cm−1 to 0.99 MV cm−1 were observed with the increase in the AlGaN thickness from 10–40 nm. To determine the distribution of the electric field in these heterostructures the Schrödinger and Poisson equations are solved in a self-consistent manner and matched with experimental data. It is shown that the built-in electric field in the GaN(cap) and AlGaN layers obtained from measurements does not reach values of electric field resulting only from polarization effects. The measured electric fields are smaller due to a screening of polarization effects by free carriers, which are inhomogeneously distributed across the heterostructure and accumulate at interfaces. The results clearly demonstrate that CER measurements supported by theoretical calculations are able to determine the electric field distribution in GaN-based heterostructures quantitatively, which is very important for polarization engineering in this material system.

Contactless electroreflectance studies of surface potential barrier in AlGaN/n‐AlGaN structures with various Al concentrations

Contactless electroreflectance spectroscopy has been applied to study the surface potential barrier in AlxGa1−xN/n‐AlxGa1−xN structures with 0 ≤ x ≤ 0.25 grown by metaloorganic vapor phase epitaxy. A strong band‐to‐band transition followed by Franz–Keldysh oscillation (FKO) was clearly observed for all samples. The value of built‐in electric field was determined from the period of FKO, and the surface potential barrier was calculated with knowledge of the thickness of undoped AlxGa1−xN layer. For a set of GaN/n‐GaN structures with different thickness of undoped GaN layer (30–120 nm) an analysis of built‐in electric field gives the surface potential barrier of 0.59 ± 0.03 eV. For AlxGa1−xN(60 nm)/n‐AlxGa1−xN structures with 0.09 ≤ x ≤ 0.25 the surface potential barrier does not vary with Al concentration within the experimental accuracy. In this case the position of the Fermi level at the surface has been estimated to be ∼1.4 eV. The increase of surface potential barrier from 0.59 eV for GaN to 1.4 eV for AlGaN with 0.09 ≤ x ≤ 0.25 is attributed to low concentration of n‐type non‐intentional residual dopants in the undoped AlxGa1−xN layer as well as the presence of Al atoms at the surface and their higher sensitivity to oxidation and/or creation of defects states which are not present at GaN surfaces.

Contactless electroreflectance spectroscopy of ZnO/ZnMgO quantum wells: Optical transitions and Fabry–Perot features

Contactless electroreflectance (CER) spectroscopy was applied to study optical transitions in ZnO/ZnMgO quantum wells (QWs) of various widths and barrier contents (Mg = 10, 20, and 30%) grown on sapphire substrates with a Zn0.9Mg0.1O or ZnO buffer layer. Spectral features related to absorption in ZnO QW region and ZnMgO barrier were clearly observed in CER spectra. The spectral position of these features correlates very well with photoluminescence peaks related to emission from ZnO QW and ZnMgO barrier. Besides CER resonances, which are associated with optical transitions, some spectral features related to the Fabry–Perot (F–P) interference effect were clearly observed for these samples. They appear for these structures due to a strong contrast of refractive index between the sapphire and Zn(Mg)O layers and they are observed in the region of transparency for Zn(Mg)O epilayers. For some samples the F–P related signal is quite strong and interferes with CER resonances related to QW transitions. However the two contributions (i.e., CER signal related to optical transitions and the Fabry–Perot effect) can be recognized when the proper analysis of CER spectra is performed. This analysis is presented and discussed in this paper.

Contactless Electroreflectance Spectroscopy of ZnO with Different Polarization-Direction Probe Beams

Contactless electroreflectance (CER) spectra of ZnO bulk has been measured at 300K. It is observed the difference between the CER spectra using by the polarization E of probe light perpendicular ( Ec) and parallel ( Ec) to the c-axis of the m-plane ZnO. In addition, a mercury lamp was focused on the sample to reduce its strength of electric field. It was observed that the CER spectrum was blue shifted with Hg lamp on. Hence, the observed features were attributed to excitonic transitions. The experimental spectra were fitted by Lorentzian lineshapes. The energies of the A ( B), B(A), and C transitions can be determined.

Growth-Temperature Dependence of Wetting Layer Formation in High Density InGaAs/GaAs Quantum Dot Structures Grown by Droplet Epitaxy

We present a study of the growth-temperature-dependent wetting layer formation in self-assembled InGaAs/GaAs quantum dot structures formed by droplet epitaxy at elevated growth temperatures up to 500 °C. The energies of electron heavy-hole and light-hole transitions of the InGaAs wetting layer is investigated by contactless electroreflectance spectroscopy. A clear indication of a strong growth-temperature influence on the wetting layer morphology is observed. At the lowest growth temperature no wetting layer could be detected at all. The influence of the substrate temperature on structural and optical properties of the dots is also discussed.

Growth-Temperature Dependence of Wetting Layer Formation in High Density InGaAs/GaAs Quantum Dot Structures Grown by Droplet Epitaxy

We present a study of the growth-temperature-dependent wetting layer formation in self-assembled InGaAs/GaAs quantum dot structures formed by droplet epitaxy at elevated growth temperatures up to 500 °C. The energies of electron heavy-hole and light-hole transitions of the InGaAs wetting layer is investigated by contactless electroreflectance spectroscopy. A clear indication of a strong growth-temperature influence on the wetting layer morphology is observed. At the lowest growth temperature no wetting layer could be detected at all. The influence of the substrate temperature on structural and optical properties of the dots is also discussed.

Electromodulation spectroscopy of optical transitions and electric field distribution in GaN/AlGaN/GaN transistor heterostructures with various AlGaN layer thicknesses

AbstractGaN/AlGaN/GaN field effect transistor (FET) heterostructures with various thicknesses of AlGaN layer (10, 20, and 30nm) were investigated by electromodulation spectroscopy [i.e. contactless electroreflectance (CER) and photoreflectance (PR)]. In PR spectra optical transitions in GaN cap, AlGaN and GaN buffer layers were clearly observed whereas in CER spectra the optical transition in GaN buffer layer was not observed due to the screening of band bending modulation in CER spectroscopy by the two dimensional electron gas at the AlGaN/GaN interface. The transition related to GaN‐cap was observed ∼0.1 eV above the energy gap of GaN due to the quantum confinement in the surface GaN quantum well. The built‐in electric field in AlGaN layer was determined from the analysis of AlGaN‐related Franz‐Keldysh oscillation. For the sample with 10, 20, and 30nm thick AlGaN layer the electric field has been determined from CER(PR) measurements to be 0.30(0.27), 0.46(0.39) and 0.78(0.63) MV/cm, respectively. The smaller electric field, which is obtained from PR measurements, results from the photovoltaic effect which is eliminated in CER spectroscopy. The electric field distribution in the full heterostructure (GaN(cap) layer, AlGaN layer, and near the AlGaN/GaN interface in GaN(buffer) layer) has been found comparing theoretical calculations for the full heterostructure with the electric field for AlGaN layer obtained from CER measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

AbstractThe authors present the application of contactless electroreflectance (CER) spectroscopy to study optical transitions in low dimensional semiconductor structures including quantum wells (QWs), step-like QWs, quantum dots (QDs), quantum dashes (QDashes), QDs and QDashes embedded in a QW, and QDashes coupled with a QW. For QWs optical transitions between the ground and excited states as well as optical transitions in QW barriers and step-like barriers have been clearly observed in CER spectra. Energies of these transitions have been compared with theoretical calculations and in this way the band structure has been determined for the investigated QWs. For QD and QDash structures optical transitions in QDs and QDashes as well as optical transitions in the wetting layer have been identified. For QDs and QDashes surrounded by a QW, in addition to energies of QD and QDash transitions, energies of optical transitions in the surrounded QW have been measured and the band structure has been determined for the surrounded QW. Finally some differences, which can be observed in CER and photo-reflectance spectra, have been presented and discussed for selected QW and QD structures.

Contactless electroreflectance study of E and E + ΔSO transitions in In0.53Ga0.47BixAs1−x alloys

Energies of E0 and E0 + ΔSO transitions in In0.53Ga0.47BixAs1−x alloys with 0 < x ≤ 0.036 have been studied by contactless electroreflectance spectroscopy at room temperature. It has been clearly observed that the E0 transition shifts to longer wavelengths (∼50 meV/% of Bi), while the E0 + ΔSO transition is approximately unchanged with changes in Bi concentration. These changes in the energies of optical transitions are discussed in the context of the valence band anticrossing model as well as the common anion rule applied to III-V semiconductors.

Application of contactless electroreflectance to study the epi readiness of m-plane GaN substrates obtained by ammonothermal method

The authors have applied contactless electroreflectance (CER) spectroscopy to study the epi readiness of m-plane GaN substrates obtained by the ammonothermal method. It has been clearly observed that the CER resonance, which is related to the energy gap transition, appears for samples with a well-polished surface. The sharpness of this resonance is directly related to the surface quality. The broadening of energy gap transition can be used as a parameter to quantify the surface quality. For samples polished with optimal conditions, this broadening (γpol) is close to the broadening observed for the cleaved GaN surface (γclev) with m-plane orientation (150–190 vs 135 meV). The quality of the polishing process can be evaluated by analyzing the γclev/γpol ratio, where γclev/γpol=1 corresponds to an excellent polishing process. In the authors’ case, this ratio has been determined to be close to 1 for well-polished samples.

Application of contactless electroreflectance to III-nitrides

Contactless electroreflectance (CER) spectroscopy was applied to study (i) the type of surface band bending, (ii) the built-in electric field, and (iii) the Stokes shift in selected III-nitrides samples. The surface band bending was studied for truly bulk GaN crystals with various types of conductivity obtained by the ammonothermal method and GaN epilayers doped by Si and Mg. It has been shown that the shape of CER resonance, which is related to the band-to-band absorption, is sensitive to the type of surface band bending. This feature of semiconductor surface can be determined in contactless manner from CER measurements. The built-in electric field was studied for AlGaN/GaN heterojunction structures. From the period of AlGaN-related Franz-Keldysh oscillation the value of electric field has been determined to be 0.27 MV/cm. This value is much smaller that the polarization-related electric field in this system and results from the Fermi level pining at AlGaN surface and the Fermi level position at AlGaN/GaN interface. The Stokes shift was studied for InGaN/GaN multi quantum wells. This shift has been found to be ∼ 50 meV at room temperature.

Contactless and soft contact electroreflectance of AlGaN/GaN and GaN/AlGaN/GaN field effect transistor heterostructures

AbstractAlGaN/GaN field effect transistor (FET) structures with a two dimensional electron gas (2DEG) at the interface were investigated by electromodulation spectroscopy [contactless electroreflectance (CER), and soft contact electroreflectance (SCER)]. It has been clearly shown that in contactless mode of electroreflectance (i.e., CER spectroscopy) the 2DEG screens the band bending modulation in GaN layer from the external electromodulation which is generated by the capacitor‐like system. In a result only the optical transition in AlGaN layer and no signal, which could be associated with the optical transition in GaN buffer layer, is observed in CER spectra. This screening is not present in soft contact mode of electroreflectance (i.e., SCER spectroscopy). It has been clearly shown that the soft contact mode of electrorefletance works very well for AlGaN/GaN FET structures with a 2 nm thick GaN cap layer. In this mode of electroreflectance measurements the signal from GaN buffer layer was clearly observed whereas in CER spectra this signal was not detected. In CER spectra only the AlGaN‐related transition and the GaN cap‐related transition were observed. The last transition was observed at the energy of ∼3.65 eV because of quantum confinement in GaN cap layer, i.e., a surface quantum well in fact. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy

The band-edge property and built-in electric fields of two different Al(x)Ga(1-x)N/GaN(AlGaN/GaN) heterostructures (HSs) with and without an additional AlGaN inserted layer were studied by thermoreflectance (TR), photoluminescence (PL), and contactless electroreflectance (CER) techniques. The PL spectra characterize the band-edge luminescence property of GaN. Free exciton transitions of AlGaN and GaN were probed experimentally by TR. Prominent Franz-Keldysh oscillations (FKOs) of GaN and the opposite FKO phase of AlGaN were simultaneously detected by the additional AlGaN inserted sample with CER owing to the enhancement effect of built-in electric fields of GaN and AlGaN with the same polarity direction. Optoelectronics properties of the two HSs were characterized by the experimental analyses.

Evidence for Fermi level shift in GaInAs/GaAs quantum wells upon nitrogen incorporation

GaInAs/GaAs and GaInNAs/GaAs quantum well (QW) structures grown by metalorganic vapor phase epitaxy have been studied by contactless electroreflectance spectroscopy. In addition to the N-related redshift of QW transitions, an increase in the electric field in the GaAs cap layer has been observed after the incorporation of nitrogen atoms into the GaInAs QW. This observation is associated with the tendency of the Fermi level shift to a given energy in the GaInNAs QW region due to N-related defects.