Structural and Optical Properties of ZnO Nanotips Grown on GaN Using Metalorganic Chemical Vapor Deposition

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO||(0002)GaN and (101̄0)ZnO||(101̄0)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni’s equation to study the variation of energy bandgap versus temperature.     

Electrical and optical properties of lodine doped CdZnTe layers grown by metalorganic vapor phase epitaxy

Electrical and photoluminescence properties of iodine doped CdZnTe (CZT) layers grown by metalorganic vapor phase epitaxy have been studied. Doped layers showed an n-type conductivity from the Zn composition x=0 (CdTe) to 0.07. Above x=0.07, resistivities of doped layers increased steeply up to 10⁶ Ω-cm. Resistivities of doped CZT layers were higher than those of undoped layers above x=0.6. Photoluminescence intensity of doped layers increased compared to undoped layers. Doped CdTe and ZnTe layers showed neutral donor bound exciton emission lines at the exciton related region. Also, these layers showed an increase in emission intensity at the donor acceptor pair recombination bands. Sharp emission lines were observed in doped CZT layers at around 1.49 eV. These emission lines were considered to be originated from GaAs substrates which were optically excited by the intense emission from doped CZT layers.     

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd_1−xZn_xTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)_h of the CZT detector measured by DC photoconductivity is 7.0 × 10⁻⁴ cm²/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D⁰, X) or neutral acceptors (A⁰, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D⁰, X) depends linearly on the incident laser intensity, while (A⁰, X) has a nonlinear dependence.     

Low-Temperature Photoluminescence Study of CdTe:In Crystals Annealed in Molten Bismuth

We used a low-temperature photoluminescence (PL) technique to investigate CdTe:In crystals after annealing in molten bismuth (Bi). The two annealed samples showed different resistivities after the treatment. For both samples, we observed very strong emissions in the excitonic spectral region and revealed fine structures of exciton emissions in the PL spectrum. In the sample with high resistivity, we found one ionized donor-bound exciton peak, (D⁺,X), that we ascribed to incorporated Bi atoms occupying Cd sites in the CdTe. The temperature dependence of the (D⁺,X) peak emission had an associated activation energy of 3.59 meV for the exciton bound to this ionized donor. Meanwhile, a donor–acceptor pair peak at 1.5315 eV, which was absent from the PL of the low-resistivity sample, suggested the likelihood of some Bi atoms occupying Te sites in the high-resistivity sample. Our findings highlight the need for detailed investigation of annealing conditions to ensure precise control of the electrical properties of the material during annealing in molten Bi.     

On the origin of the luminescence band with hνm=1.5133 eV in gallium arsenide

The excitation-level dependence of intensities of the luminescence bands with hν_m=1.5133, 1.5141, and 1.5153 eV in semi-insulating GaAs crystals at 4.2 K was examined. The dependences obtained for all three bands are identical. The analysis of these results indicates that, in this material, the luminescence band with hν_m=1.5133 eV is related to the annihilation of the exciton-impurity complexes D ⁺ X (excitons X being bound to ionized shallow donors D ⁺).     

Optical spectroscopy of free excitons in a CuInS<Subscript>2</Subscript> chalcopyrite semiconductor compound

The spectra of reflectance and luminescence of high-quality CuInS₂ single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy ～1.5356 eV and doubly degenerate peak BC at the energy ～1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A _UPB at ～1.5361 eV and A _LPB at ～1.5347 eV, with a half-width ～1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be ～18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.     

Annealing effects of a high-quality ZnTe substrate

The sharp photoluminescence (PL) and optical-reflection spectra in the bandedge region of the high-quality nondoped ZnTe substrate (100) were observed at 4.2 K. Free exciton, associated with lower and upper polaritons (EX^L and EX^U) at 2.382 eV and 2.381 eV, respectively, were clearly observed. This meant that this substrate was high quality. The intensity of a bound exciton peak (2.375 eV), which is caused by a Zn vacancy, of a neutral acceptor decreased with an increase of the Zn vapor pressures.     

Optical properties of undoped and iodine-doped CdTe

A comprehensive study of the properties of undoped and iodine-doped CdTe structures by photoluminescence (PL) and photoreflectance (PR) is reported. Undoped bulk CdTe and iodine-doped CdTe layers grown by metalorganic molecular beam epitaxy on (lOO)-oriented CdTe and (211)B-oriented GaAs substrates with electron concentrations ranging from 10¹⁴ to mid-10¹⁸ cm^-3 were included in this study. Lineshape modeling of 80KPL and PR spectra indicated the presence of both free exciton and donor-hole transitions at the higher doping levels. Strong PL and PR signals were also observed at room temperature. If only a single transition is considered for the analysis of the 300K spectra, the PL emission peak and the PR transition energy both exhibit a strong dependence on electron concentration for doped layers. However, lineshape modeling of the room-temperature spectra indicated the presence of multiple transitions consisting of free exciton and direct band-to-band transitions. The use of two transitions resulted in a constant value of bandgap over the entire range of conductivities studied. A strong correlation remained between the broadening of the PR and PL spectra and excess carrier concentration N_D-N_A. In addition, the E₁ transition energy measured by PR was found to vary dramatically with growth conditions.     

Effects of heat treatment on the 0.8 eV photoluminescence emission in GaAs grown by molecular beam epitaxy at low temperatures

We report 0.8 eV photoluminescence (PL) emission of GaAs grown at low temperatures between 325 and 400°C by molecular beam epitaxy. Effects of heat treatments of the 0.8 eV emission are compared with those of the 1.467 eV sharp bound exciton lines. This allows us to attribute the 0.8 eV emisson to the As-V_Ga center. We discuss the assigning of the As_i-V_Ga center to the well-known EL6. The PL intensity variation of 0.68 eV EL2 and 0.8 eV As_i-V_Ga seen in substrate materials is explained in terms of dislocation−mediated As_i-V_Ga transformation to EL2 whereas the PL intensity variation of 0.8 eV As_i-V_Ga for molecular beam epitaxy layers can be attributed to the growth condition.     

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band‐edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)₂PbI₄ are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 10³ cm s^?1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low‐lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.     

The role of lead in growing Ga1−X InXAsYSb1−Y solid solutions by liquid-phase epitaxy

The electrical properties of Ga_1?X In_XAs_YSb_1?Y (X=0.14–0.27) solid solutions grown from a Pb-containing solution-melt were investigated for the first time. Three acceptor levels were found to exist, specifically, a shallow level with the activation energy E _A1≈0.008–0.015 eV, and two deep levels E _A2≈0.024–0.033 eV and E _A3≈0.07 eV. It is demonstrated that the use of Pb makes it possible to obtain undoped solid solutions with a low concentration of defects and impurities and with high carrier mobility.     

Low temperature photoluminescence of lightly Si-doped and undoped MBE GaAs

A semiquantitative low temperature (~4 K) photoluminescence technique has been used to estimate the residual carbon concentration in MBE GaAs to be ~2 × 10¹⁴ cm^?3 . This value agrees well with the total compensating acceptor concentration calculated from electrical measurements and thus confirms that carbon is the dominant residual acceptor in MBE GaAs. In high purity MBE GaAs films grown at substrate temperatures between 545°C and 625°C a band of at least nine luminescence peaks is observed in the 1.471 eV to 1.491 eV spectral region. These peaks can be correlated with the most prominent "defect-induced" bound exciton peaks in the 1.504 eV to 1.511 eV spectral region and their transition energies follow the empirical relation $$hv_{C_{As}^o ,X} - hv_{d,X} = 0.38[hv_{e,C_{As}^o } - hv_{e,d} ].$$ This suggests that the luminescence in these two spectral regions have a common origin at a set of “defect-complexes” which involve carbon impurities.     

Measured compositions and laser emission wavelengths of GaXIn1−XAsyP1−y lpe layers lattice-matched TO InP Substrates

The alloy compositions of Ga_XIn_1−XAs_yP_1−y LPE layers lattice-matched to InP substrates have been determined by electron microprobe analysis. The composition data are well repre-sented by x = 0.40y + 0.067y². The emission wavelengths of lattice-matched Ga_XIn_1−XAs_yP_1−y/InP double-heterostructure diode lasers have been measured at 300 and 80 K. The photon energies for laser emission at 300 K are given by hΝ(eV) = 1.307 − 0. 60y + 0.03y². The emission energies at 80 K are 57 meV higher. This work was sponsored by the Department of the Air Force.     

High performance aluminum arsenic intraband resonant microwave devices

We report on GaAs/AlAs triple-barrier quantum well intraband (TBQWI) heterostructures grown by molecular beam epitaxy (MBE) on n⁺ GaAs substrate. Heterostructure quality was evaluated by X-ray diffraction and photoluminescence spectrum measurements. The position of the broad peak near 65.84° corresponds well to the diffraction from the (4 0 0) face of AlAs layers assuming intensity of total AlAs spacers and barriers. The 10K photoluminescence (PL) data has a strong peak at 8140 Å. The PL spectrum is dominated by a sharp peak centered at the emission energy of 1.52 eV attributed to the energy of e1-hh bond exciton of GaAs layer. TBQWI heterostructures were grown and processed into resonant tunneling diode (RTD). Room temperature electrical measurement of the TBQWI RTD yielded maximum peak to valley current ratio (PVCR) of 120 with peak current density (J_p) of 2.1 kA/cm². The high PVCR of this GaAs/AlAs TBQWI RTD is, to the better of our knowledge, one of the higher PVCRs obtained in any intraband tunnel device.     

Enhanced Trion Emission in Monolayer MoSe2 by Constructing a Type-I Van Der Waals Heterostructure

Trions, quasi-particles consisting of two electrons combined with one hole or of two holes with one electron, have recently been observed in transition metal dichalcogenides (TMDCs) and drawn increasing attention due to potential applications of these materials in light-emitting diodes, valleytronic devices as well as for being a testbed for understanding many-body phenomena. Therefore, it is important to enhance the trion emission and its stability. In this study, a MoSe₂/FePS₃ van der Waals heterostructure (vdWH) with type-I band alignment is constructed, which allows for carriers injection from FePS₃ to MoSe₂. At low temperatures, the neutral exciton (X⁰) emission in this vdWH is almost completely suppressed. The I_Trion/I_x0 intensity ratio increases from 0.44 in a single MoSe₂ monolayer to 20 in this heterostructure with the trion charging state changing from negative in the monolayer to positive in the heterostructure. The optical pumping with circularly polarized light shows a 14% polarization for the trion emission in MoSe₂/FePS₃. Moreover, forming such type-I vdWH also gives rise to a 20-fold enhancement of the room temperature photoluminescence from monolayer MoSe₂. These results demonstrate a novel approach to convert excitons to trions in monolayer 2D TMDCs via interlayer doping effect using type-I band alignment in vdWH.     

Magnetooptical properties of a single CdMnSe/CdMgSe quantum well

The spectra of photoluminescence and reflectance in magnetic fields up to 7 T are studied for a 3.8 nm semimagnetic CdMnSe quantum well confined by two CdMgSe barriers. A noticeable magnetic shift in the σ⁺-polarized emission line of the heavy exciton to low energies and a decrease in the halfwidth of the line by more than one-half are detected with increasing magnetic field. It is established that a localized magnetic polaron is formed, with the polaron energy of 19.8 meV determined from the change in the degree of circular polarization in magnetic field. A σ^?-polarized emission line is observed in magnetic fields ranging from 0.4 to 2 T. This line can be interpreted as being produced by the complex of two electrons, with oppositely directed spins, and a heavy hole, i.e., by the trion X ^? or the exciton localized at a donor, D ⁰ X. The binding energy of such complex is 10 meV.     

Electrical and optical properties of oxygen doped GaN grown by MOCVD using N2O

Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N₂O as oxygen dopant source. The layers were deposited on 2″ sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N₂) as carrier gas. Prior to the growth of the films, an AIN nucleation layer with a thickness of about 300? was grown using trimethylaluminum. The films were deposited at 1085°C at a growth rate of 1.0 μm/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N₂O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4×10¹⁶ cm⁻³ to 4×10¹⁸ cm⁻³ as measured by Hall effect. The observed carrier concentration increased with increasing N₂O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emission an exciton bound to a shallow donor. With increasing N₂O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments.     

Solution‐Processed Thermally Activated Delayed Fluorescent OLED with High EQE as 31% Using High Triplet Energy Crosslinkable Hole Transport Materials

Solution‐processed organic light‐emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) material as emitter have attracted much attention because of their low cost and high performance. However, exciton quench at the interface between the hole injection layer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and emitting layer (EML) in devices can lead to low device performance. Here, a novel high triplet energy (2.89 eV) and crosslinkable hole‐transporting material grafted with oxetane groups, N,N‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐3,5‐di(9H‐carbazol‐9‐yl)benzenamine (Oxe‐DCDPA)), as crosslinked hole transport layer (HTL) into the interface of PEDOT:PSS layer and EML is proposed for prevention of exciton quenching, and among the reported devices with single HTL in solution‐processed TADF‐OLED, the highest external quantum efficiency (EQE)/luminous efficiency (η_L) of 26.1%/94.8 cd A^?1 and 24.0%/74.0 cd A^?1 are achieved for green emission (DACT‐II as emitter) and bluish‐green emission (DMAC‐TRZ as emitter), respectively. Further improvement, using double HTLs, composed of N,N′‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy))‐hexylphenyl)‐N,N′‐diphenyl‐4,4′‐diamine with high hole mobility and Oxe‐DCDPA with high triplet energy, leads to the highest EQE/η_L of 30.8%/111.9 cd A^?1 and 27.2%/83.8 cd A^?1 for green emission and bluish‐green emission, respectively. These two devices show the high maximum brightness of 81 100 and 70 000 cd m^?2, respectively.     

Electrical and photoluminescence properties of Ge-doped n-type GaAs Grown by molecular beam epitaxy

Ge-doped n-type MBE GaAs has been studied for the doping range 6.7 × 10¹⁵ to 1.5 × 10²⁰cm^-3 and the compensation ratio inferred from the mobility variation with free-carrier concentration. The doping achieved for a given Ge source temperature is an order of magnitude greater than generally reported and this is attributed to use of a source of large surface area. Photoluminescence studies at 4°K for lightly doped specimens show the usual bound exciton, band-to-C_AS and band-to-Ge_AS peaks and their LO phonon replicas. However, with Ge doping exceeding 10¹⁸cm^-3 broad deep-level peaks develop centered at 1.3257 eV moving towards 1.255 eV, with half widths of about 115 meV. Whether these peaks are related to the broad-band photoluminescence centered at 1.20 eV (20°K) that has been reported earlier for Ge doped Bridgmangrown and epitaxially vapor grown GaAs, is not known. Since the energy displacement is considerable, it is possible that the centers responsible differ in the MBE grown material.     

Hydrogen-containing donors in silicon: Centers with negative effective correlation energy

The reconstruction of shallow-level hydrogen-containing donors in Si is studied. The donors are formed by implantation of low-energy (300 keV) hydrogen ions into the experimental samples and subsequent heat treatment at 450°C. The experiments are carried out for Ag-Mo-Si Schottky diodes and diodes with a shallow (～1 μm) p⁺-n junction. The concentration and distribution of the donors are determined by applying the method of C–V characteristics at a frequency of 1.2 MHz. An analysis of the temperature dependence of the equilibrium electron concentration shows that the reconstruction of the hydrogen-containing donors can be described under the assumption of recharging of a center with negative effective correlation energy (U < 0). The transformation between two equilibrium configurations of a double hydrogen donor (D _B ⁺⁺ ? D _A ⁰ ) proceeds with the Fermi level position E_F = E _c ? 0.30 eV. The reconstruction of the donors from a neutral to a doubly charged state (D _A ⁰ → D _B ⁺⁺ ) which is stimulated by the capture of minority carriers, is observed at room temperature.