CdGeAs2 Crystals Research Articles

The Electronic Structure and Optical Properties of CdGeAs2 Crystal: A DFT and HSE06 Study

The electronic structural and optical properties of CdGeAs2 crystals are calculated by using the Perdew–Burke–Ernzerhof (PBE) functional within generalized gradient approximation (GGA) and the Heyd–Scuseria–Ernzerhof (HSE06) functional. The results show that CdGeAs2 is an optical crystal with a direct bandgap of 0.71 eV by using the HSE06 functional method, which is closer to the experimental value. The Mulliken population and differential charge density analysis indicate that the Ge–As and Cd–As bonds have covalent properties, and that the covalent bond of Cd–As is visibly stronger than that of the Ge–As bond. The optical properties show that the CdGeAs2 crystal has strong absorption and reflection in the ultraviolet region and strong transmittance in the infrared region. The average static refractive index of CdGeAs2 is 2.96, and the static birefractive index is 0.08. The results show that CdGeAs2 is an excellent optical material of potential applications in the middle and far infrared.

Structural, electronic and optical properties of CdGeAs2 with hybrid density functional (HSE06)

We calculate the structural, electronic and optical properties of the CdGeAs2 with the HSE06 functional. The results show that the structural parameters and band gap calculated by the HSE06 functional mixed with 25% Hatree-Fock exchange are closer to the experimental data. Comparing the band structures of CdGeAs2 and GaAs, it is concluded that CdGeAs2 is a direct band gap semiconductor. Based on the Bader charge and the electronic localization function, the properties of bonding between different atoms are analyzed. In addition, we explain the main spectral characteristics with the electronic structure, it provides a theoretical basis for in-depth understanding of the optical properties of CdGeAs2, and guides significance for the further theoretical research and practical application of CdGeAs2 crystals.

Synthesis, growth and mechanical properties of mid-infrared nonlinear optical crystal CdGeAs2

CdGeAs2 crystal is a promising nonlinear optical material for second harmonic generation (SHG) and optical parameter oscillation (OPO) applications in mid-infrared range. In this work, CdGeAs2 polycrystalline was synthesized using temperature oscillation method and CdGeAs2 single crystals with diameters of 30 mm were grown by modified vertical Bridgeman method. The synthesized polycrystalline was characterized by X-ray diffraction (XRD). The composition and transmission of wafers cut from a single crystal were characterized by energy dispersive spectrometer (EDS) and IR Prestige-21 spectrophotometer. The mechanical properties such as Young's modulus and hardness of (001), (100), (102) and (112) faces of CdGeAs2 were measured by nanoindentation technique. The measured Young's moduli established a relationship with the bonding direction, i.e., the closer the indentation direction was to the bonding direction, the stronger the stiffness will be.

Об особенностях примесного энергетического спектра арсенидов

The impurity energy spectrum of non-doped bulk crystals of n-type GaAs, InAs, CdSnAs2, CdGeAs2 and CdTe, ZnO is investigated basing on the findings from the quantitative analysis of baric and temperature dependences of kinetic coefficients. It`s found that in the above-listed semiconductors the deep level donor center corresponds to the intrinsic defect of vacancy in the anion sublattice. The energy level positions relative to conduction band edge and the pressure coefficients of energy gaps between them and corresponded conduction band bottom are determined. The shift of deep donor center energy levels towards the depth of the conduction band is observed with a decrease in the forbidden band width in above-listed semiconductors.

Growth and characterization of Cr-doped CdGeAs2 crystal

Cr-doped CdGeAs2 polycrystal was synthesized by a single-temperature region combined with mechanical and temperature oscillation method, and Cr-doped CdGeAs2 single crystal with Φ17mm×30mm was grown by the improved Bridgman method. The crystal structure, electrical properties and doping content of Cr-doped CdGeAs2 were characterized by high-resolution XRD, Hall-effect and ICP, respectively. In the XPS measurement, the high-resolution scanning spectra of the Cr-doped CdGeAs2 crystals were obtained for the first time, and the binding energies of Cr 2p3/2 are 575.44eV and 576.50eV, and the binding energies of Cr2p1/2 are 583.06eV and 585.14eV by peak separation and fitting. Finally, FTIR was used to characterize the infrared properties of Cd-doped CdGeAs2 crystals. The results show that the transmittance near 5.5μm increases with a small amount of doping compared to undoped crystal, and reaches a maximum 42% when the Cr content is 0.057%. When the Cr content increases to 0.158%, the transmittance decreases slightly, and while the content reaches 0.324%, the transmittance of the whole wavelengths decreases sharply. Appropriate amount of Cr can reduce the absorption of CdGeAs2 near 5.5μm, improve its infrared transmittance and broaden its applications.

Surface treatments of CdGeAs2 single crystals

The performances of second harmonic generation (SHG) and optical parametric oscillator (OPO) in CdGeAs2 crystal are strongly influenced by surface quality. In this paper, the surfaces of samples were treated by mechanical polishing (MP), chemical polishing (CP), chemical–mechanical polishing (CMP) and CP following CMP closely (CMP + CP). Then, the surface state was characterized by optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM measurements show that an ultra-smooth surface is achieved after CMP + CP treatment and the roughness value is 0.98 nm. Meanwhile, the roughness of the surfaces treated by MP, CP and CMP are 4.53, 2.83 and 1.38 nm, respectively. By XRD rocking curves, the diffraction peak which belongs to the wafer treated by CMP + CP is the highest in intensity and best symmetrical in shape. XPS analysis indicates that Ge4+ proportions of GeO2 in total Ge content of CdGeAs2 wafers’ surface after MP, CP, CMP and CMP + CP treatment are 27.6%, 42.8%, 6.1% and 30.3%, respectively.

Growth and characterizations of CdGeAs2 single crystal by descending crucible with rotation method

By the method of descending crucible with rotation, crack-free CdGeAs2 single crystals of Φ15 mm × 50 mm were grown in a furnace with three independent heating zones after optimizing the temperature field, and the descending and rotational speed to meet the need of CdGeAs2 crystal growth. The properties of as-grown crystal were characterized by a variety of techniques. The results of X-ray diffraction (XRD) show that there are two cleavage faces, which are (110) and (101). The peaks are in high intensity and good symmetry, which demonstrates that the crystal is integral in structure and well crystallized. The energy-dispersive spectrometry results indicate that the wafer of the CdGeAs2 crystal is closer to the stoichiometry. The IR transmittance of the wafer is ~48.6 % at 5.5 μm, and the maximum value is up to 51.6 % in the range of 2.3–18.0 μm. Etch pits of (001) face are observed and the density of the etch pits is evaluated to be 1 × 105 cm−2.

Tunable Middle Infrared Radiation with CdGeAs2 Crystal

Efficient continuous wave tunable mid-infrared radiation by the process of second harmonic generation of tunable CO2 laser radiation using a CdGeAs2 (CGA) crystal has been reported. Its phase matched angular tuning characteristic at the CO2 laser wavelength, phase matched angular acceptance and spectral acceptance have been measured. Considering effective figure of merit, CGA crystal has been found to be 90 times more efficient than ZnGeP2 crystal at 10.6 µm.

Generation of tunable mid-IR radiation by second harmonic in a CdGeAs2 crystal

Tunable mid-IR radiation is obtained during second harmonic generation of tunable CO2-laser radiation using a CdGeAs2 crystal. Its angular tuning characteristics at the CO2- laser wavelength, angular acceptance angle and spectral acceptance are measured. For second harmonic generation at 10.6 μm, the conversion efficiency in the CdGeAs2 crystal is 90 times higher than that in the ZnGeP2 crystal.

Effect of thermal annealing on the optical properties of CdGeAs2 wafers

A crack-free CdGeAs2 single crystal 15mm in diameter and 50mm in length was grown in a three-zone tubular furnace by the modified vertical Bridgman method. During the annealing processes, the effects of treatments with different atmosphere, different temperatures and time were investigated. The as-grown and annealed wafers were characterized using X-ray diffractometer (XRD), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and IR microscope. Conclusively, the results confirm annealing could improve the optical qualities of as-grown CdGeAs2 crystal. The best result was obtained under cover-up with CdGeAs2 polycrystalline powder at 450°C for 150h and the IR transmittance of the wafer measured by FTIR was up to 48.65% nearby 5.5μm and exceeded 50% in the range of 8–12μm. Additionally, the monolithic homogeneity of the crystal has also been greatly improved after annealing under cover-up with polycrystalline powder.

Synthesis, structures, and electrophysical properties of single crystals of solid solutions CdGeAs2:Mn(x) and Cd0.964Zn0.036GeAs2:Mn(x)

Single crystals of the solid solutions CdGeAs2:Mn(x) and Cd0.964Zn0.036GeAs2:Mn(x) have been grown by the vertical Bridgman method. An X-ray diffraction study has demonstrated that Cd0.964Zn0.036GeAs2 (I), Cd0.964Zn0.036GeAs2:Mn (1.5 wt%) (II), and Cd0.964Zn0.036GeAs2:Mn (2.18 wt %) (III) retain the CdGeAs2 structure (tetragonal system, space group I\(\bar 4\)2d). The unit cell parameters of the solid solutions are as follows: a = b = 5.934(1) A, c = 11.219(2) A for I; a = b = 5.919(1) A, c = 11.204(2) A for II; and a = b = 5.918(1) A, c = 11.208(2) A for III. Many of Mn atoms in II and III occupy interstitial sites in the crystal lattice. Selected electrical and magnetic properties of single crystals of CdGeAs2:Mn(x) are discussed.

Electron paramagnetic resonance and electron‐nuclear double resonance study of Mn2+ ions in CdGeAs2 crystals

AbstractElectron paramagnetic resonance (EPR) and electron‐nuclear double resonance (ENDOR) have been used to characterize Mn2+ (3d5) ions in CdGeAs2 crystals grown by the horizontal Bridgman method. These samples were p‐type with room‐temperature hole concentrations ranging from 2 × 1015 cm–3 to 5 × 1016 cm–3. The Mn2+ ions substitute for the Cd2+ ions in this tetragonal chalcopyrite lattice, and form “deep” isoelectronic centers. Their g values and crystal‐field parameters are g ‖ = 2.0025, g ⊥ = 2.0011, D = 118.8 MHz, a + 2/3F = 18.4 MHz, and a = –0.3 MHz. The hyperfine and nuclear electric quadrupole parameters for the 55Mn nucleus are A ‖ = –154.94 MHz, A ⊥ = –155.69 MHz, and P = 0.622 MHz. Hyperfine parameters for one set of nearby Cd nuclei and two sets of nearby As nuclei were also determined using ENDOR. No evidence was found in these doped bulk crystals to suggest that Mn4+ ions substitute for Ge4+ ions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Urbach rule used to explain the variation of the absorption edge inCdGeAs2 crystals

The absorption edge has been studied over the temperature range from 5 to 300 K in both p-type and n-typebulk CdGeAs2 crystals. An exponential distribution of states, or Urbach tail,below the direct band edge produces a linear dependence ofd [log(α)]/d (hν) with incidentphoton energy hν. The slope of this edge is nearly independent of temperature in p-type samples, butvaries with temperature in n-type samples. These differences in behaviour areexplained by a model that includes both thermal (phonon) and structural disordercontributions to the exponential tail of states. The structural disorder contributionwas found to be dominant in compensated p-type samples. Indium-doped n-typeCdGeAs2 crystals exhibited the more conventional thermal (i.e., phonon) behaviour.

Temperature dependence of polarized absorption bands in p-type CdGeAs2

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.

Donor-acceptor pair emission near 0.55 eV in CdGeAs2

A photoluminescence (PL) study has been performed from 5 to 200 K on a series of p-type bulk CdGeAs2 single crystals. Within our large set of samples, the peak position of an emission band at 5 K was observed to vary from about 0.54 to 0.58 eV when using 1064 nm excitation with a power density of 2.5 W/cm2. The variations of the PL peak position with excitation power and sample temperature have been measured and are consistent with donor-acceptor pair (DAP) recombination in the presence of potential fluctuations. We find that the position of the DAP emission in a particular crystal depends on the hole carrier concentration and the level of compensation. Thermal quenching activation energies of 14 and 120 meV are determined for the donor and acceptor states, respectively. The acceptor defect involved in this radiative recombination is the primary center responsible for the commonly observed room-temperature absorption band at 5.5 μm in p-type CdGeAs2 crystals.

Electron paramagnetic resonance of Cr2+ and Cr4+ ions in CdGeAs2 crystals

Electron paramagnetic resonance (EPR) has been used to investigate chromium ions in single crystals of CdGeAs2 grown by the horizontal gradient freeze technique. Signals from Cr2+ and Cr4+ ions were observed near 12 K. The Cr2+ ions have the 3d4 configuration with S=2 and substitute at the cadmium sites, while the Cr4+ ions have the 3d2 configuration with S=1 and substitute at the germanium sites. As a result, each impurity represents a neutral center. Both EPR spectra display the tetragonal symmetry of the lattice, which suggests that the centers most likely are not perturbed by nearby defects. The ratio of Cr2+ ions to Cr4+ ions in a particular CdGeAs2 crystal depends on the growth conditions and stoichiometry.

Temperature and pulse-duration dependence of second-harmonic generation in CdGeAs2.

A detailed investigation of frequency doubling of a transversely excited atmospheric CO2 laser operating at 9.55 microm with a CdGeAs2 crystal was carried out. The temperature of the crystal was varied between 80 and 295 K to maximize the frequency-doubled energy. The temporal shape of the generated beam at 4.775 microm was monitored to calculate its peak power. High values of midwave infrared pulse energy (16.65 mJ) and peak power (92 kW) were obtained, which can be of potential use in lidar systems.

Optical and EPR Study of Defects in Cadmium Germanium Arsenide

ABSTRACTBulk crystals of CdGeAs2 have been characterized using photoluminescence (PL), optical absorption, Hall effect, and electron paramagnetic resonance (EPR) techniques. An absorption band near 5.5 microns at room temperature is observed in all of the p-type samples we have studied. A correlation between the magnitude of this optical absorption and the excess hole concentration at room temperature is established. Also, an EPR signal is found to correlate with the strength of this absorption band. PL data are consistent with an increased concentration of shallow acceptors being present in high-absorption samples. From the EPR data, we suggest that a model for the paramagnetic defect associated with the absorption at 5.5 microns may be an acceptor on an anion site.

Radiation resistance of nonlinear crystals at a wavelength of 9.55 μm

The results of radiation resistance measurements for twelve nonlinear crystals are presented. The crystals include the well-known nonlinear CdGeAs2, ZnGeP2, AgGaSe2, GaSe, AgGaS2, and Ag3AsS3 crystals operating in the middle IR range, new mixed AgGaGeS4 and Cd0.35Hg0.65Ga2S4 crystals, two-phase (orange and yellow) HgGa2S4 crystal, and the doped GaSe:In crystal. The mixed crystals and the two-phase HgGa2S4 crystal are transparent in the range from 0.4 — 0.5 to 11.5 — 14.5 μm. The measurements were performed using a pulsed single-mode highly stability TEA CO2 laser with an output pulse duration of ~30 ns. The damage thresholds of new nonlinear AgGaGeS4 and Cd 0.35Hg0.65Ga2S4 crystals and of the HgGa2S4 crystal (the orange and yellow phases) were found to be 1.5 — 2.2 times higher than for the crystals operating in the middle IR range.

Anisotropy of Optoelectronic Properties and Atomic Ordering in CdGeAs2

Inelastic light scattering by optical phonons as well as temperature dependent electrical conductivity, Hall constant and photosensitivity were studied in ordered chalcopyrite structure CdGeAs2 crystals grown by ultra-low gradient freeze technique from near-stoich iometric melts.