CdGeAs2 Single Crystals Research Articles

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.

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.

Correlation between dislocation etch pits, carrier concentration and optical absorption in CdGeAs2 grown by modified Vertical Bridgman method

As a promising material for second harmonic generation (SHG) and optical parametric oscillator (OPO), CdGeAs2 single crystals are difficult to reproducibly grow with low loss optical, particularly in the region of 5.5 μm. In addition, a correlation between processing conditions and absorption has attracted various researchers to focus on this problem. This paper describes a series of etching experiments and Hall measurements which were undertaken to see if there might be a relationship between dislocation density, carrier concentration and optical absorption. In this study, a single crystal was grown by the modified Vertical Bridgman method. The spontaneous nucleation growth direction has been determined as [102]. Etch pit densities (EPD) about 105 cm−2 were observed at the edges of the boules, whereas etch pits were almost not found in the center. The distribution of dislocation along the radial direction affected optical homogeneity, and the high EPD caused the low transmittance. Meanwhile, the samples are p type at room temperature with hole concentrations varying from 1014 to 1016 cm−3. A linear correlation between absorption at 5.5 μm and hole concentration is established. The results of these experiments are very useful for improving radial and axial optical uniformity of the crystal grown by modified Vertical Bridgman method.

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.

Synthesis and Structure of Mn-Doped CdGeAs2 Single Crystals

Large, perfect CdGeAs2 single crystals doped with 0.006, 0.49, and 0.89 wt % Mn are grown by the Bridgman method. The Mn concentration in the crystals is determined by atomic absorption spectrophotometry. X-ray diffraction results indicate that Mn doping influences the bond distances in CdGeAs2 and its lattice parameters. Based on the observed structural changes, a model is proposed for the Mn incorporation into the structure of CdGeAs2.

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.

Analysis of the temperature dependence of electron mobility in CdGeAs2 single crystals

We have performed an analysis of the experimental temperature dependences of the Hall mobility in CdGeAs2 single crystals with degenerate electron gas. The Boltzmann equation has been solved numerically in the effective-mass and isotropic-continuum approximations taking into account the complicated character of the polar phonon spectrum. A good agreement with the experimental data on drift mobility at T = 77 K has been obtained with the electron scattering, taking into account intrinsic singly charged Coulomb defects. At room temperature, agreement between theory and experiment cannot be achieved without considering the electron–plasmon scattering.

Analysis of the temperature dependence of electron concentration in CdGeAs2 single crystals

The temperature dependence of electron concentration in CdGeAs2 single crystals grown by a new method is analyzed. The concentration of native defects and the activation energy are calculated. It is shown that the activation energy has a resonance character, and the concentration of native defects in the temperature range studied (10–500 K) far exceeds the electron concentration.

Defect segregation in CdGeAs2

Increased axial temperature gradients and growth rates have resulted in segregation of unwanted absorbing defects to the edges of CdGeAs2 single crystals produced by the horizontal gradient freeze technique. Long-wavelength infrared imaging of polished boules revealed a “clear” central core with absorption losses 26 times lower than in the darker edge regions. This pronounced segregation is attributed to the preferred incorporation of native defects at facets that form near the side walls of the horizontal boat. EPR, GDMS, and Hall effect analysis were used to characterize the nature of these defects.

Observation of Record Electron Hall Mobility in CdGeAs2 Single Crystals

CdGeAs2 single crystals were prepared by low temperature crystallization from the nonstoichiometric melt. The temperature dependences of the electrical conductivity and the Hall coefficient were examined for CdGeAs2 single crystals. All the as-grown crystals show n-type conductivity with free electron concentration of 1018 cm-3 and Hall mobility of around 10000 cm2/(Vs) at room temperature. The Hall mobility increases with the temperature decrease and reaches 36000 cm2/(Vs) at 77 K. This is the first evidence of the high electron mobility for CdGeAs2 single crystals.

Evidence of High Electron Mobility in CdGeAs2 Single Crystals

AbstractCdGeAs2 single crystals were prepared by low temperature crystallization from a nonstoichiometric melt. The results of the first study of the temperature dependencies of the electrical conductivity and the Hall coefficient of n-type CdGeAs2 single crystals prepared by this new technique are reported. All the as-grown crystals show n-type conductivity. These single crystals had free electron concentrations of (1 - 2) 1018 cm−3 and Hall mobilities of more than 10000 cm2/(Vs) at T=300 K. These room temperature electron mobility values are four times larger than previously reported. In the high temperature region (T> 150K), the temperature dependence of the Hall mobility exhibited a behavior characteristic of phonon scattering. Below 150 K, a deviation from this behavior indicated an increasing contribution of static lattice defects to scattering. The Hall mobility in these crystals was found to reach ∼ 36000 cm2/( V· s ) at 77 K. The first photosensitive n-type CdGeAs2/n-type InSe heterostructures were constructed by contact methods. Photosensitivities of 70-100 V/were observed. This is substantially better than for previous structures using conventionally grown CdGeAs2 and indicates the high quality of the crystals. It is concluded that the low temperature growth technique is promising for the preparation of more perfect CdGeAs2crystals.

Electrical and optical properties of CdGeAs2 single crystals irradiated with fast electrons

The effects of 10 MeV fast electron irradiation on conductivity type, resistivity, and optical absorption of CdGeAs2 single crystals were studied. Irradiation of as-grown p-type crystals with a fluence of 1×1017 cm−2 caused compensation of the semiconductor and a marked improvement in the optical transmission. A level of absorption below 0.1 cm−1 at 5⩽λ⩽11 μm was registered. Increase in the fluence to 2×1017 cm−2 led to conversion to n-type, a drop in the resistivity and deterioration of the transmission. Analysis of the absorption in p-type crystals showed that transitions from the V2 and V3 split-off bands to the V1 valence band represent the major absorption mechanism. Splitting energies ΔE2−1=0.15 eV and ΔE3−1=0.35 eV have been calculated. Absorption in n-type crystals increased as λ2.4 indicating absorption by free electrons and their scattering by optical phonons. The effect of irradiation on the optical absorption of CdGeAs2 is assigned to the radiation-induced changes in the compensation level.

Properties of Dopants in ZnGeP2, CdGeAs2, AgGaS2 and AgGaSe2

Ternary-chalcopyrite structure ZnGeP2, CdGeAs2 (II-IV-V2) and AgGaS2, AgGaSe2 (I-III-VI2) compounds are currently of technological interest. They show the most promise for practical nonlinear optical applications in the areas of high-efficiency optical parametric oscillators and frequency up-converters for the infrared (ir) range as well as for widespectral-range optoelectronic devices. (See also the article by Schunemann, Schepler, and Budni in this issue.) However extensive realization of their potential has still not been achieved. One of the principal difficulties in the way to obtaining high-device-quality ZnGeP2, CdGeAs2, AgGaS2, and AgGaSe2 single crystals is undesired optical absorption in their transparency range near the fundamental band edge induced by lattice-related defects. This article summarizes selected aspects of dopant-incorporation techniques of these crystals including dopant choice of dopant material and monitoring of dopant incorporation as done in our laboratory.In general for the ternary chalcopyrite compounds, doping-incorporation processes are more complicated in comparison to those of binary zinc-blende III-V compounds. The most common sources of dominant incorporation of acceptors and donors in as-grown chalcopyrites are believed to appear from (1) nonstoichiometric melts as well as by doping with different elements during the growth process and (2) incomplete removal of disorder on the cation sublattice during subsequent cooling. Furthermore the chalcopyrite structure II-IV-V2 undergoes a disorder-order phase transition upon cooling through approximately 1220 K for ZnGeP2 and 900 K for CdGeAs2. At these transition temperatures, solidification can be complicated also by supercooling phenomena, and the crystals transform from the cubic zinc-blende structure (where Zn atoms randomly fill cation sites) to the ordered chalcopyrite structure (e.g., when Zn and Ge occupy alternating cation sites in ZnGeP2).

Optoelectronic effects in p-CdGeAs2 single crystals and structures based on them

Spectra of inelastic light scattering by optical phonons in p-CdGeAs2 single crystals were obtained for the first time. The observed clear polarization dependence and the absence of any appreciable dependence of the intensity and frequency of the observed lines when the sample is swept in ≈300 µm steps indicates these CdGeAs2 single crystals grown by directional crystallization from a near-stoichiometric flux, are of high quality and homogeneous. The type of symmetry of the observed phonon lines is interpreted and it is shown that the force constants in CdGeAs2 and CdSnP2 crystals differ slightly. Temperature dependences of the electrical conductivity and the Hall constant were studied in oriented homogeneous p-CdGeAs2 single crystals. It was established that the conductivity of these crystals is determined by the deep acceptor level EA=0.175 eV and has the degree of compensation 0.5–0.6. The temperature dependence of the Hall mobility reflects the competition between impurity and lattice mechanisms of hole scattering. The photosensitivity of In/CdGeAs2 surface barrier structures reaches 20 µA/W at T=300 K and remains at this level within the fundamental absorption of CdGeAs2. It is concluded that these structures may be used as wide-band photoconverters for natural light and as selective photoanalyzers for linearly polarized radiation.

Anisotropy of the Charge Carrier Transport in II–IV–V2 Single Crystals

The temperature dependences of the electrical conductivity and of the Hall coefficient were determined for oriented II-IV-V2 single crystals. From the measurements of temperature dependence of the Hall constant, the Hall effect of II-IV-V2 single crystals is isotropic. An anisotropy of the Hall mobility of hole was detected experimentally in the CdGeAs2 and CdSiAs2 single crystals. The hole mobility anisotropy in ZnGeP2 was not determined. The knowledge of the anisotropy of the Hall mobility should help to improve the efficiency of semiconductor electronic devices made of II-IV-V2 materials.

Growth of CdGeAs2 single crystals by the chemical vapor transport method

We have grown CdGeAs2 single crystals by chemical vapor transport (CVT), a method not previously applied for this compound. The crystallographic data of this chalcopyrite (cell parametersa 0 = 5.9456 ± 0.0001A, c0 = 11.2131 ± 0.0007A) and its electrical transport properties are reported. Predominantly n-type crystals are obtained (at RTn = 1 · 1017cm−3, μn = 2000 cm2(Vs)−1). Vacuum heat treatment at 500° C yields a type conversion fromn- to p-type. In all p-type samples the minority carrier mobility is calculated to be larger than 10000 cm2(Vs)−1.

Efficient generation of the second harmonic of NH3laser radiation in CdGeAs2

The second harmonic of radiation from a molecular NH3 laser was generated for the first time in CdGeAs2 single crystals. The internal efficiency of the frequency doubling reached 5% for radiation at the wavelength of 11.7 μ, when the efficiency of conversion of CO2 laser radiation power (used for direct optical pumping of the NH3 laser) into 6-μ radiation was highest.