Contactless Electroreflectance Research Articles

Fermi level and bands offsets determination in insulating (Ga,Mn)N/GaN structures

The Fermi level position in (Ga,Mn)N has been determined from the period-analysis of GaN-related Franz-Keldysh oscillation obtained by contactless electroreflectance in a series of carefully prepared by molecular beam epitaxy GaN/Ga1−xMnxN/GaN(template) bilayers of various Mn concentration x. It is shown that the Fermi level in (Ga,Mn)N is strongly pinned in the middle of the band gap and the thickness of the depletion layer is negligibly small. For x > 0.1% the Fermi level is located about 1.25–1.55 eV above the valence band, that is very close to, but visibly below the Mn-related Mn2+/Mn3+ impurity band. The accumulated data allows us to estimate the Mn-related band offsets at the (Ga,Mn)N/GaN interface. It is found that most of the band gap change in (Ga,Mn)N takes place in the valence band on the absolute scale and amounts to −0.028 ± 0.008 eV/% Mn. The strong Fermi level pinning in the middle of the band gap, no carrier conductivity within the Mn-related impurity band, and a good homogeneity enable a novel functionality of (Ga,Mn)N as a semi-insulating buffer layers for applications in GaN-based heterostuctures.

Contactless electroreflectance studies of the Fermi level position at the air/GaN interface: Bistable nature of the Ga-polar surface

In this paper we show that the surface Fermi level of Ga-polar GaN exhibits a bistable behavior allowing it to be located at two distinct energetic positions at the air/GaN interface which is unusual for other III–V semiconductors such as GaAs or GaSb. To determine the Fermi level position at the air/GaN interface we perform contactless electroreflectance measurements on specially designed UD+ structures [GaN(undoped)/GaN(highly doped)/substrate] doped by Si and Mg. Analyzing the period of Franz-Keldysh oscillation we determine the built-in electric field in the undoped (U) layer. These studies coupled with numerical solutions of the Poisson equation allowed us to determine the position of the Fermi level at the air/GaN interface. We observe a change in the band bending correlated to different Fermi level positions in the doped (D+) layer. We show that depending on the doping type in the D+ layer the Fermi level at the air/GaN interface is located in the upper or lower singularity of surface density of states (SDOS) for Si or Mg doping of D+ layer, respectively. We support our findings with the density functional theory calculations of the SDOS and the dependence of the Fermi level position on the doping concentration in the bulk of a GaN slab.

Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

Ge and its alloys are attractive candidates for a laser compatible with silicon integrated circuits. Dilute germanium carbide (Ge1−xCx) offers a particularly interesting prospect. By using a precursor gas with a Ge4C core, C can be preferentially incorporated in substitutional sites, suppressing interstitial and C cluster defects. We present a method of reproducible and upscalable gas synthesis of tetrakis(germyl)methane, or (H3Ge)4C, followed by the design of a hybrid gas/solid-source molecular beam epitaxy system and subsequent growth of defect-free Ge1−xCx by molecular beam epitaxy (MBE). Secondary ion mass spectroscopy, transmission electron microscopy and contactless electroreflectance confirm the presence of carbon with very high crystal quality resulting in a decrease in the direct bandgap energy. This technique has broad applicability to growth of highly mismatched alloys by MBE.

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.

Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS2, MoSe2, WS2, and WSe2. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS2, MoSe2, WS2, and WSe2 crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an AH transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and AH transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (BH transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the BH transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

Electromodulation spectroscopy of direct optical transitions in Ge1−xSnx layers under hydrostatic pressure and built-in strain

Unstrained Ge1−xSnx layers of various Sn concentration (1.5%, 3%, 6% Sn) and Ge0.97Sn0.03 layers with built-in compressive (ε = −0.5%) and tensile (ε = 0.3%) strain are grown by molecular beam epitaxy and studied by electromodulation spectroscopy (i.e., contactless electroreflectance and photoreflectance (PR)). In order to obtain unstrained GeSn layers and layers with different built-in in-plane strains, virtual InGaAs substrates of different compositions are grown prior to the deposition of GeSn layers. For unstrained Ge1−xSnx layers, the pressure coefficient for the direct band gap transition is determined from PR measurements at various hydrostatic pressures to be 12.2 ± 0.2 meV/kbar, which is very close to the pressure coefficient for the direct band gap transition in Ge (12.9 meV/kbar). This suggests that the hydrostatic deformation potentials typical of Ge can be applied to describe the pressure-induced changes in the electronic band structure of Ge1−xSnx alloys with low Sn concentrations. The same conclusion is derived for the uniaxial deformation potential, which describes the splitting between heavy-hole (HH) and light-hole (LH) bands as well as the strain-related shift of the spin-orbit (SO) split-off band. It is observed that the HH, LH, and SO related transitions shift due to compressive and tensile strain according to the Bir-Pikus theory. The dispersions of HH, LH, and SO bands are calculated for compressive and tensile strained Ge0.97Sn0.03 with the 8-band kp Hamiltonian including strain effects, and the mixing of HH and LH bands is discussed. In addition, the dispersion of the electronic band structure is calculated for unstrained Ge1−xSnx layers (3% and 6% Sn) at high hydrostatic pressure with the 8-band kp Hamiltonian, and the pressure-induced changes in the electronic band structure are discussed.

Sensitivity of Fermi level position at Ga-polar, N-polar, and nonpolar m-plane GaN surfaces to vacuum and air ambient

Ga-polar, N-polar, and nonpolar m-plane GaN UN+ structures have been examined in air and vacuum ambient by contactless electroreflectance (CER). This technique is very sensitive to the surface electric field that varies with the Fermi level position at the surface. For UN+ GaN structures [i.e., GaN (undoped)/GaN (n-type)/substrate], a homogeneous built-in electric field is expected in the undoped GaN layer that is manifested by Franz–Keldysh oscillation (FKO) in CER spectra. A clear change in FKO has been observed in CER spectra for N-polar and nonpolar m-plane structures when changing from air to vacuum ambient. This means that those surfaces are very sensitive to ambient atmosphere. In contrast to that, only a small change in FKO can be seen in the Ga-polar structure. This clearly shows that the ambient sensitivity of the Fermi level position at the GaN surface varies with the crystallographic orientation and is very high for N-polar and nonpolar m-plane surfaces. This feature of the N-polar and nonpolar m-plane surfaces can be very important for GaN-based devices grown on these crystallographic orientations and can be utilized in some of the devices, e.g., sensors.

Bi-induced acceptor level responsible for partial compensation of native free electron density in InP1−xBix dilute bismide alloys

Deep level transient spectroscopy (DLTS) has been applied to study electron and hole traps in InPBi alloys with 2.2 and 2.4% Bi grown by molecular beam epitaxy. One donor-like trap with the activation energy of 0.45–0.47 eV and one acceptor-like trap with activation energy of 0.08 eV have been identified in DLTS measurements. For the reference sample (InP grown at the same temperature), the deep donor trap has also been observed, while the acceptor trap was not detected. According to the literature, the deep donor level found in InP(Bi) at 0.45–0.47 eV below the conduction band has been attributed to the isolated PIn defect, while the second trap, which is observed only for Bi containing samples at 0.08 eV above the valence band can be attributed to Bi clusters in InPBi. This acceptor level was proposed to be responsible for the observed partial compensation of native free electron density in InPBi layers. It is also shown that the deep donor traps are active in photoluminescence (PL). A strong radiative recombination between donor traps and the valence band are observed in PL spectra at energy 0.6–0.8 eV, i.e. ~0.47 eV below the energy gap of InPBi, which is determined by contactless electroreflectance.

Surface potential barrier in m-plane GaN studied by contactless electroreflectance

Contactless electroreflectance (CER) is used to study the surface potential barrier in m-plane GaN UN+ [GaN ( nm)/GaN:Si] structures grown by using molecular beam epitaxy. Clear bandgap-related transitions followed by Franz–Keldysh oscillations (FKO) have been observed in the CER spectra of all samples at room temperature. The built-in electric fields in the undoped cap layers have been determined from the FKO period. From the built-in electric field and the undoped GaN layer thickness, the Fermi level location at the air-exposed m-plane GaN surface has been estimated as 0.42 ± 0.05 eV below the conduction band.

Determination of composition of non-homogeneous GaInNAs layers

Dilute nitride GaInNAs alloys grown on GaAs have become perspective materials for so called low-cost GaAs-based devices working within the optical wavelength range up to 1.6μm. The multilayer structures of GaInNAs/GaAs multi-quantum well (MQW) samples usually are analyzed by using high resolution X-ray diffraction (HRXRD) measurements. However, demands for precise structural characterization of the GaInNAs containing heterostructures requires taking into consideration all inhomogeneities of such structures. This paper describes some of the material challenges and progress in structural characterization of GaInNAs layers. A new algorithm for structural characterization of dilute nitrides which bounds contactless electro-reflectance (CER) or photo-reflectance (PR) measurements and HRXRD analysis results together with GaInNAs quantum well band diagram calculation is presented. The triple quantum well (3QW) GaInNAs/GaAs structures grown by atmospheric-pressure metalorganic vapor-phase epitaxy (AP-MOVPE) were investigated according to the proposed algorithm. Thanks to presented algorithm, more precise structural data including the nonuniformity in the growth direction of GaInNAs/GaAs QWs were achieved. Therefore, the proposed algorithm is mentioned as a nondestructive method for characterization of multicomponent inhomogeneous semiconductor structures with quantum wells.

Electronic band structure of compressively strained Ge1−xSnx with x < 0.11 studied by contactless electroreflectance

Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge1−xSnx layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8 ± 0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV).

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 and theoretical studies of band gap and spin-orbit splitting in InP1−xBix dilute bismide with x ≤ 0.034

Contactless electroreflectance is applied to study the band gap (E0) and spin-orbit splitting (ΔSO) in InP1−xBix alloys with 0 < x ≤ 0.034. The E0 transition shifts to longer wavelengths very significantly (−83 meV/% Bi), while the E0 + ΔSO transition shifts very weakly (−13 meV/% Bi) with the rise of Bi concentration. These changes in energies of optical transitions are discussed in the context of the valence band anticrossing model and ab initio calculations. Shifts of E0 and E0 + ΔSO transitions, obtained within ab-initio calculations, are −106 and −20 meV per % Bi, respectively, which is in a good agreement with experimental results.

Surface photovoltage and modulation spectroscopy of E− and E+ transitions in GaNAs layers

Surface photovoltage (SPV) spectra were measured for GaN0.014As0.986 layers at room temperature and compared with room temperature photoreflectance (PR) and contactless electroreflectance (CER) measurements. Spectral features related to E− and E+ transitions were clearly observed in SPV spectra at energies corresponding to PR and CER resonances. In this way it has been shown that SPV spectroscopy is an alternative absorption-like technique to study both the E− and E+ transitions in dilute nitrides. The observation of E+ transition in SPV spectra means that it is a direct optical transition at the Γ point of GaNAs band structure which can be explained by the band anticrossing interaction between the localized states of N and the extended conduction band states of the GaAs host.

Identification of nitrogen- and host-related deep-level traps in n-type GaNAs and their evolution upon annealing

Deep level traps in as-grown and annealed n-GaNAs layers (doped with Si) of various nitrogen concentrations (N = 0.2%, 0.4%, 0.8%, and 1.2%) were investigated by deep level transient spectroscopy. In addition, optical properties of GaNAs layers were studied by photoluminescence and contactless electroreflectance. The identification of N- and host-related traps has been performed on the basis of band gap diagram [Kudrawiec, Appl. Phys. Lett. 101, 082109 (2012)], which assumes that the activation energy of electron traps of the same microscopic nature decreases with the rise of nitrogen concentration in accordance with the N-related shift of the conduction band towards trap levels. The application of this diagram has allowed to investigate the evolution of donor traps in GaNAs upon annealing. In general, it was observed that the concentration of N- and host-related traps decreases after annealing and PL improves very significantly. However, it was also observed that some traps are generated due to annealing. It explains why the annealing conditions have to be carefully optimized for this material system.

Position of fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN heterostructures without and with AlN layer

Position of Fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN transistor heterostructures without and with AlN layer were studied experimentally using contactless electroreflectance and theoretically solving Schrodinger-Poisson equation with various surface boundary conditions. It has been observed that the thin AlN layer changes very strongly the distribution of electric field in this heterostructure but the Fermi level position on Al0.2Ga0.8N surface does not change significantly. Its position is the same within experimental uncertainly (i.e., ∼0.5 eV below conduction band) for both bulk Al0.2Ga0.8N and Al0.2Ga0.8N/GaN heterostructures.

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.

Contactless electroreflectance studies of free exciton binding energy in Zn1-xMgxO epilayers

Contactless electroreflectance (CER) has been applied to study optical transitions in Zn1-xMgxO layers with magnesium concentration ≤44%. CER resonances related to free exciton and band-to-band transitions were clearly observed at room temperature. For ZnO the two transitions are separated by the energy of ∼65 meV, which is attributed to the free exciton binding energy in ZnO. Due to magnesium incorporation, the CER resonances broaden and shift to blue. The energy separation between excitonic and band-to-band transitions increases up to ∼100 meV when the magnesium concentration reaches 22%. For larger magnesium concentrations, CER resonances are significantly broadened and the excitonic transition is no longer resolved in the CER spectrum.

Influence of AlN layer on electric field distribution in GaN/AlGaN/GaN transistor heterostructures

Distribution of built-in electric field in GaN/AlGaN/GaN transistor heterostructures without and with AlN layer was studied theoretically (solving the Schrodinger and Poisson equation) and experimentally (measuring contactless electroreflectance (CER) spectra and analyzing the AlGaN-related Franz-Keldysh oscillation). It is shown that the AlN layer changes very strongly the distribution of electric field in such heterostructures. This change can be very well predicted if the surface boundary conditions for self-consistent Schrodinger-Poisson calculations in GaN/AlGaN/GaN heterostructures are known. These conditions can be determined/verified by CER measurements of AlGaN-related Franz-Keldysh oscillation, which depends on the built-in electric field in AlGaN layer.