II–IV–V2 materials offer the promise of enhanced functionality in optoelectronic devices due to their rich ternary chemistry. In this review, we consider the potential for new optoelectronic devices based on nitride, phosphide, and arsenide II–IV–V2 materials. As ternary analogs to the III–V materials, these compounds share many of the attractive features that have made the III–Vs the basis of modern optoelectronic devices (e.g. high mobility, strong optical absorption). Control of cation order parameter in the II–IV–V2 materials can produce significant changes in optoelectronic properties at fixed chemical composition, including decoupling band gap from lattice parameter. Recent progress has begun to resolve outstanding questions concerning the structure, dopability, and optical properties of the II–IV–V2 materials. Remaining research challenges include growth optimization and integration into heterostructures and devices.

Synthesis, structure, and optoelectronic properties of II–IV–V2 materials

ZnGeP2 and CdGeAs2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients (d36 equals 75 pm/V and 236 pm/V for ZnGeP2 and CdGeAs2, respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP2's transparency range (0.62−13 μm) makes it the optimum material for shifting the wavelength of 2-μm pump lasers into the 3–5-μm range via optical parametric oscillation (OPO), whereas that of CdGeAs2 (2.3–18 μm) is better suited for doubling the frequency of CO2 lasers (9–11 μm) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses.The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a-axis and c-axis thermal-expansion coefficients of ZnGeP2 differ by a factor of two, whereas those of CdGeAs2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.

Ultralow Gradient HGF-Grown ZnGeP2 and CdGeAs2 and Their Optical Properties

Large-grained ingots of CdGeAs2 have been grown from near-stoichiometric melts. Resistivity and Hall coefficient (RH) measurements were made on a large number of samples at 77 and 300 K and in some cases up to 450 K. Good fits to the log RH vs 1/T plots are obtained by using a model that assumes three kinds of electronic levels within the energy gap: donors, shallow acceptors, and acceptors with an ionization energy of 0. 30 eV. The deep acceptors are probably native defects, since their concentration varies by nearly four orders of magnitude from ingot to ingot with little change in impurity concentration. Between the intrinsic absorption edge at about 2. 5 Μm and the two-phonon absorption band at 18 Μm, the optical absorption increases with increasing deep acceptor concentration. By using oriented single-crystal samples ∼ 1 cm on a side, conversion efficiencies as high as 27% have been achieved for second-harmonic generation with single-mode pulses from a CO2 TEA laser.

Optical and electrical properties of CdGeAs2

Chapter 7 – electrical properties

Structural distortions and polymorphic behaviour in ABC2 and AB2C4 tetrahedral compounds

The phase relations in the ZnP2-Ge join in the Zn-Ge-P system were determined. ZnGeP2 melts congruently at 1027 ± 3°C and undergoes a solid state transformation at 950 ± 5°C, in agreement with previous work. The low temperature phase, designated as αZnGeP2, has the tetragonal chalcopyrite structure, while the structure of the high temperature phase, designated as βZnGeP2, is unknown. Two eutectics are present: one between αZnGeP2 and Ge at 865 ± 4°C and 95 mol % Ge, and the other, between βZnGeP2 and ZnP2 at 970°C ± 4°C and 15 mol % Ge. Large crack- and bubble-free single crystals of ZnGeP2 can be grown by directional solidification without seeding, and the phase transition appears to be no impediment to the growth of large crystals. The extra absorption of a few cm-1 which exists between the band edge and 2μ has been shown to depend on crystal growth conditions. The source of the absorption, however, has not been identified. The absorption spectrum of a crystal grown from a liquid Bi-Pb alloy is compared with the spectra of crystals grown directly from ZnGeP2 melts.

The ZnP 2 -Ge system and growth of single crystals of ZnGeP 2

Crystals of the ternary chalcopyrite CdGeP2 have been grown from doped and undoped melts by directional solidification. We find that the stoichiometry and temperature gradient in the melt are critical factors in determining crystal quality. The effect of excess phosphorous is particularly dramatic, causing a rapid deterioration of crystal quality when the excess is greater than 3 at.% in the charge. This empirical observation led us to perform measurements of the total vapor pressure over CdGeP2 as a function of temperature. The vapor species was found to consist almost entirely of phosphorous and the measured pressure at 790°C was 1.92±.15 atm. An extensive search for dopants suitable for the production of low-resistivity CdGeP2 was unsuccessful. It was found that doping heavily with In and Ga produced high-resistivity n- and p-type crystals, respectively, but no shallow donors or acceptors were found.

J. D. Wiley

Papers

The ZnP 2 -Ge system and growth of single crystals of ZnGeP 2

The melt growth and doping of CdGeP2