N. Briot

Low pressure metalorganic vapour-phase epitaxy growth of ZnTe using triethylamine dimethyl zinc adduct

Abstract We present for the first time a detailed investigation of the growth of ZnTe layers deposited on GaAs substrates by low pressure metalorganic vapour-phase epitaxy using the new triethylamine dimethyl zinc adduct in combination with diisopropyl telluride as zinc and tellurium precursors respectively. The influence of the growth temperature ( T g ), the VI/II molar ratio, and the overall growth pressure ( P ) are studied. The successful growth of ZnTe is demonstrated at temperatures as low as 300°C. For growth temperatures ranging between 300 and 450°C, the growth rate is limited by the kinetics at the growing interface, and we measure an activation energy of 28 kcal/mol. At T g = 375°C, we found a linear variation of the growth rate with the overall growth pressure on the one hand, and with both zinc and telluride molar flows on the other hand. Low temperature reflectance and photoluminescence measurements were performed to characterize all the samples and are used to determine the optimal growth conditions: T g = 350°C, VI/II = 2, with P = 40 Torr. The stress experienced by the epilayers is investigated using the reflectance measurements. The relative intensity of the arsenic related bound exciton emission at 2.367 eV decreases as T g decreases. On the other hand, the I 1 b unknown acceptor related bound exciton emission at 2.356 eV decreases linearly with the growth rate.

Optical properties and thermal transport of carriers in (Zn,Cd)Se-ZnSe heterostructures

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn0.79Cd0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (Eexc = 2.661 eV) shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (Eexc=3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.

Low pressure MOVPE growth of ZnSe, ZnTe, and ZnSe/ZnTe strained-layer superlattices

ZnSe and ZnTe single-crystal layers have been grown onto (100) GaAs substrates by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) using the triethylamine-dimethylzinc adduct [DMZn(NEt3)] as the zinc precursor. The selenium and tellurium precursors were H2Se (5% in H2) and di-isopropyltellurium (DiPTe), respectively. These two semiconductors have been grown with different VI/II molar ratios, at different growth temperatures, and with an overall growth pressure ranging from 40 to 400 Torr. Optimal growth parameters have been determined by optical means for the two materials. This information was then used to grow ZnTe/ZnSe strained-layer superlattices. We have studied structures grown on both ZnSe and ZnTe relaxed buffer layers which display a drastic dependence of the Stokes shift between photoluminescence and the optical bandgap on the nature of the buffer layer. Growth interruptions have been used to optimize the optical properties of the superlattices. Theoretical modeling of superlattice band structures has been performed using results of optical and structural characterizations. Observations of zone center transitions as well as excitons associated with the miniband dispersion of the superlattices are reported, in agreement with the theoretical calculation.

Optical characterization of extremely high purity ZnSe grown by metal-organic vapour phase epitaxy using dimethylzinc-triethylamine adduct

Abstract Zinc selenide (ZnSe) is now a material of established interest for the design of optoelectronic devices, following the achievement of both types of conductivity by M. Haase et al . ( Appl. Phys. Lett., 59 (1991) 1272). The p-type doping of this material is now well established in molecular beam epitaxy, and work is in progress in many laboratories to transpose this to metal-organic vapour phase epitaxy (MOVPE). One of the problems to be solved in MOVPE, for the successful achievement of p-type doping, is that of obtaining a material with very low electron background concentrations, to avoid compensation effects. It has previously been demonstrated, by Jones, Wright and co-workers ( Semicond. Sci. Technol., 6 (1991) A 36; J. Cryst. Growth, 94 (1989) 441; J. Cryst. Growth, 104 (1991) 297) that the classical zinc precursor leads to prereactions and is invariably contaminated by trace amounts of iodine. To get rid of these problems, they employ a new precursor for zinc, that is dimethylzinc triethylamine adduct (Me 2 ZnEt 3 ). Their results demonstrate that the material purity can be increased. Following Wright et al. ( J. Cryst. Growth, 94 (1989) 441; J. Cryst. Growth, 104 (1991) 297), we have employed this original precursor to grow extremely high purity ZnSe. We investigated the growth conditions and determined the optimum growth parameters to be a growth temperature of 280 °C, a VI/II molar ratio of 5 and a reactor pressure of 40 Torr. The samples were characterised by low temperature photoluminescence and reflectivity experiments. We observed that the use of Me 2 ZnEt 3 led to a dramatic decrease of the donor bound exciton intensity in the 2 K photoluminescence spectrum, along with a correlated increase of the free exciton intensity. For the first time, free excitons and excited states up to the second (3s) were observed in reflectivity spectra. The spectra were theoretically fitted using a model similar to the one of Zheng et al. ( Appl. Phys. Lett., 52 (1988) 287), in the local approximation, including a “dead layer” as prescribed by Hopfield and Thomas ( Phys. Rev., 132 (1988) 563). Using this model, we accurately deduced the transition energy positions and oscillator strengths of the observed transitions. Moreover, we deduced that the damping parameter of the HH (1s) transition is 1.4 meV. Such a sharpness clearly indicates the high crystalline quality of the material.

Optimization and optical studies of ZnCdSeZnSe heterostructures grown by MOVPE

Graded index-separate confinement heterostructures (GRIN-SCH) based on ZnCdSe and ZnSe wide band gap semiconductors have been grown by low pressure MOVPE on (100) GaAs substrates. First, we have paid attention to the growth of thick ZnCdSe layers. A first study is carried out using selenium hydride, dimethyl cadmium and two dimethylzinc adducts as Se, Cd and Zn precursor respectively. This combination leads to intense prereactions and poor layer morphology. To limit this problem we have then used the tetrahydrothiophene:dimethylcadmium adduct. GRIN-SCH structures grown using the two Cd metalorganics are studied using photoluminescence. We have made a detailed study on the influence of the temperature on the carrier trapping and detrapping in the structures.

Material qualify assessment of epitaxial layers of ZnSe and related compounds

Abstract In this work, we introduce the etching technique on chemically angle polished samples (ETOCAPS) for II–VI material assessment. ETOCAPS can be used to examine simultaneously the general picture and crystal quality problems in a three-dimensional distribution via optical and scanning electron microscopes. Experimental ETOCAPS results are presented for ZnSe layers and ZnSe/ZnTe super-lattices (SLs) grown on GaAs. Systematic observation of the epitaxial layer by ETOCAPS may accelerate the optimization process which is necessary for the growth of reliable laser material.

Optical properties, electronic structure, and exciton binding energies in short period ZnS-ZnSe superlattice

We present a detailed examination of the optical properties and electronic structure taken from photoreflectance and photoluminescence data collected on a series of short-period ZnS-ZnSe superlattices grown by low pressure metalorganic vapor phase epitaxy. We studied the band offset problem and calculated the exciton binding energy using several variational models. The temperature dependence of the photoluminescence properties of these superlattices was analyzed in the context of a model which includes the influence of the interfacial disorder.

Contamination effects from tellurium in ZnS-ZnSe superlattices

Abstract We have studied the temperature dependence of the photoluminiscence of tellurium-doped ZnS-ZnSe superlattices. By comparing the reflectivity and photoluminescence data, we were able to identify both free exciton recombination and self-trapped exciton band energy split by 90 meV. The behaviour of the photoluminescence with temperature results from trapping versus detrapping effects which are ruled by an activation energy of 80 meV. This behaviour is analysed in the context of a configuration coordinate diagram.

Optical characterization of MOVPE-grown ZnSZnSe short period superlattices

We report on the growth of ZnS-ZnSe superlattices by MOVPE. These superlattices were characterized by optical spectroscopy. We have calculated the exciton binding energy in ZnSe-ZnS quantum wells in the context of the variational approach using models of varying degree of sophistication.

Photo-induced screening of the excitonic interaction in ZnSe-ZnTe type II strained-layer superlattices

Abstract We report the observation of nonlinear optical properties of ZnSe-ZnTe superlattices under photo-injected carriers, at pumped liquid helium temperature. Identification of several transitions measured by transmission on samples grown by metalorganic vapour phase epitaxy and etched away from the GaAs substrate is made in the context of the envelope function approach. This reveals that the conduction to valence band line-ups are type II in these samples. This situation is invoked in order to interpret the efficiency of the exciton screening.