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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.

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E- and upper E+ valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emission is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.

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.

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, B x Ga 1 − x N 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. [ABSTRACT FROM AUTHOR]

The electronic band structure of Ga(PAsN) with a few percent of nitrogen is calculated in the whole composition range of Ga(PAs) host using density functional methods including the modified Becke-Johnson functional to correctly reproduce the bandgap and unfolding of the supercell band structure to reveal the character of the bands. Relatively small amounts of nitrogen introduced to Ga(PAs) lead to the formation of an intermediate band below the conduction band, which is consistent with the band anticrossing model, widely used to describe the electronic band structure of dilute nitrides. However, in this study, calculations are performed in the whole Brillouin zone and they reveal the significance of the correct description of the band structure near the edges of the Brillouin zone, especially for the indirect bandgap P-rich host alloy, which may not be properly captured with simpler models. The influence of nitrogen on the band structure is discussed in terms of the application of Ga(PAsN) in optoelectronic devices such as intermediate band solar cells, light emitters, as well as two color emitters. Additionally, the effect of nitrogen incorporation on the carrier localization is studied and discussed. The theoretical results are compared with experimental studies, confirming their reliability. [ABSTRACT FROM AUTHOR]

Selection of a suitable chemical etching method for threading dislocations density estimation is crucial for the structural characterization of epilayers grown by MOVPE and thus further for device fabrication. In this work, threading dislocations density in single layer and Al x Ga 1 − x N/GaN periodical heterostructures has been investigated by means of chemical etching and XRD analysis. For chemical etching, two types of agents have been used: phosphoric acid (220 ° C / 7.5 min) and a molten mixture of KOH-NaOH (440 ° C / 2.5 min). Estimation of dislocation density has been carried out on the basis of SEM images. It was shown that defect-selective etching in molten alkaline solution is more effective than etching in phosphoric acid and provides more accurate data that are in agreement with data obtained from XRD characterization. [ABSTRACT FROM AUTHOR]

Interband optical transitions in highly mismatched ZnTe1−xOx and Zn1−yCdyTe1−xOx alloys with Cd content y = 0.1 and 0.32 and oxygen content x < 0.016 grown on ZnTe substrates were studied by photoreflectance (PR) and photoluminescence (PL) in a broad temperature range. The incorporation of oxygen into a Zn(Cd)Te matrix results in a splitting of the conduction band (CB) into two E− and E+ subbands forming a semiconductor with an intermediate band. In ZnTeO, only the E− band could be probed by PR and there was no PL signal. An addition of Cd atoms to form a ZnCdTeO quaternary alloy significantly improves the optical quality as evidenced by an emergence of an E+ related transition in the PR spectra and the appearance of a PL emission related to the E− band visible up to 260 K. Moreover, for Cd content above 25%, a change in the E− band character is observed from localized O-like to CB-like. The analysis of a PR signal shows a strong reduction of the temperature dependence of the energy gap of Zn(Cd)TeO alloys compared to ZnTe. The temperature related reduction of the bandgap shift with increasing O content is well explained by the band anticrossing interaction between the temperature dependent conduction band of the host Zn(Cd)Te matrix and the temperature independent energy of highly localized O states. [ABSTRACT FROM AUTHOR]

Photoreflectance (PR) and microwave-reflectance photoconductivity-decay (μ-PCD) have been applied to study the surface band bending in Mg-doped InN layers with various Mg concentrations (∼1 × 1017 to ∼7 × 1019 cm−3) and thicknesses (∼400 to ∼3600 nm). A PR resonance related to the band-to-band transition at the Γ point below the surface electron accumulation is observed for moderately doped samples ([Mg]: 1.3 × 1017 cm−3 to 8.7 × 1017 cm−3). Samples with the Mg doping concentration in the range of ∼5 × 1018 ≤ [Mg] ≤ 7 × 1019 cm−3 are p-type and show very weak or no PR resonance. It is shown that the results correlate with the surface layer depletion thickness. The strongest PR signal is observed for thick depletion layers. The results are confirmed by μ-PCD measurements that show the longest carrier lifetimes for moderately doped samples with the thickest surface depletion layer. [ABSTRACT FROM AUTHOR]

To determine the band alignment at the GaSb1-xBix/GaSb interface, a set of GaSb1-xBix/GaSb quantum wells (QWs) of various widths (7, 11, and 15 nm) and contents (Bi ≤ 12%) were grown by molecular beam epitaxy and investigated by photoreflectance (PR) spectroscopy. In PR spectra, the optical transitions related to both the ground and the excited states in the QW were clearly observed. It is a direct experimental evidence that the GaSb1-xBix/GaSb QW is a type-I QW with a deep quantum confinement in both the conduction and valence bands. From the comparison of PR data with calculations of energies of QW transitions performed for the varying valence band offset (VBO), the best agreement between experimental data and theoretical calculations has been found for the VBO ∼50 ± 5%. A very similar VBO was obtained from ab initio calculations. These calculations show that the incorporation of Bi atoms into a GaSb host modifies both the conduction and valence band: the conduction-band position changes linearly at a rate of ∼15–16 meV per % Bi and the valence band position changes at a rate of ∼15–16 meV per % Bi. The calculated shifts of valence and conduction bands give the variation of VBO between GaSb1-xBix and GaSb in the range of ∼48%–52%, which is in good agreement with conclusions derived from PR measurements. In addition, it has been found that the electron effective mass reduces linearly with the increase in Bi concentration (x): m e f f G a S b B i = m e f f G a S b − 0.2 x , where m e f f G a S b is the electron effective mass of GaSb. Moreover, a strong photoluminescence (PL) was observed and a negligible Stokes shift (less than a few meV) between the PL peak and the fundamental transition in the PR spectrum was detected for all QWs at low temperatures. It means that the investigated QWs are very homogeneous, and the carrier localization for this alloy is very weak in contrast to other dilute bismides. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed investigation of the low temperature emission properties of ZnSeO alloys by means of photoluminescence, micro-photoluminescence, and time-resolved photoluminescence. We show that the low temperature broad photoluminescence spectrum of ZnSeO attributed to the recombination of localized excitons is composed of sharp lines related to individual trapping states. Based on studies of photoluminescence thermal quenching from individual trapping states and photoluminescence dynamics, the mechanism of nonradiative recombination in ZnSeO alloys is discussed. Moreover, an unexpected decrease of the low temperature Stokes shift with increasing oxygen content is observed in contrast to what has been reported for GaAs based highly mismatched alloys. The possible origin of this effect is proposed. [ABSTRACT FROM AUTHOR]

The last missing piece of the puzzle for the full functionalization of group IV optoelectronic devices is a direct bandgap semiconductor made by CMOS compatible technology. Here, we report on the fabrication of GeSn alloys with Sn concentrations up to 4.5% using ion implantation followed by millisecond-range explosive solid phase epitaxy. The n-type single crystalline GeSn alloys are realized by co-implantation of Sn and P into Ge. Both the activation of P and the formation of GeSn are performed during a single-step flash lamp annealing for 3 ms. The bandgap engineering in GeSn as a function of the doping level and Sn concentration is theoretically predicted by density functional theory and experimentally verified using ellipsometric spectroscopy. We demonstrate that both the diffusion and the segregation of Sn and P atoms in Ge are fully suppressed by millisecond-range nonequilibrium thermal processing. [ABSTRACT FROM AUTHOR]

Optical transitions in atomically thin MoS2 samples made by sulfidation of a metallic molybdenum layer have been studied by photoreflectance (PR) and photoacoustic (PA) spectroscopy. The obtained spectra are compared with PR and PA spectra of bulk MoS2. It is shown that the absorption edge observed in the PA spectrum shifts to blue when moving from the bulk MoS2 to the atomically thin MoS2 layers, whereas the direct optical transitions at the K point of the Brillouin zone (A and B transitions), which are observed in the PR spectrum, do not shift spectrally in a significant manner. On the other hand, the AH transition, which is related to the direct optical transition at the H point of the Brillouin zone and is typical of bulk MoS2, is not observed for atomically thin MoS2 layers. Moreover, a strong and broad PR resonance related to the band nesting (C transition) is identified in the PR spectra of studied samples. In this case, C and CH transitions are observed for bulk MoS2, while only a C transition is observed for atomically thin MoS2. [ABSTRACT FROM AUTHOR]