I. Hernández‐Calderón

Crystal structure and energy gap of CdTe thin films grown by radio frequency sputtering

We have investigated the influence of structural characteristics on the band gap of rf sputtered CdTe thin films grown at substrate temperatures in the 69–232u2009°C range. The results of scanning electron microscopy and x‐ray diffraction studies indicated that the films are a polycrystalline mixture of cubic and hexagonal phases with preferential growth of columnar type parallel to the cubic [111] direction. The band gap of the films was obtained from photoreflectance spectroscopy experiments carried out at room temperature. It was found that the films had a band gap larger than that of CdTe single crystals. This result has been correlated with the existence of lattice strain, quantum size effects, and hexagonal phase regions. By using theoretical models it was possible to estimate the contribution to the band gap shift due to strain and quantum size effects obtaining results in good agreement with the experiment. The study of annealed samples indicated that the effects of thermal treatments were to promote ...

Hillocks formation during the molecular beam epitaxial growth of ZnSe on GaAs substrates

Abstract We report on the peculiar formation of hillocks during the molecular beam epitaxial (MBE) growth of ZnSe on GaAs(1xa00xa00) substrates. By heating the substrates up to ∼550°C prior to the growth, a clean Ga-rich surface was obtained as confirmed by high-energy electron diffraction and Auger spectroscopy. During the MBE growth of ZnSe on this surface, characteristic hilly-like features were formed with a density of the order of 10 8 xa0cm −2 , as observed by atomic force microscopy. The hillocks propagate along the growth direction following the vapor–liquid–solid (VLS) growth mechanism favored by the remanent Ga on the GaAs surface after the heating process. These hillocks could be used, after an appropriate control of their physical dimensions, to synthesize nanostructures such as quantum wires and quantum dots.

Strain relaxation during the layer by layer growth of cubic CdSe onto ZnSe

A detailed reflection high-energy electron diffraction analysis shows relevant features of the lattice parameter relaxation of CdSe thin films grown in a layer-by-layer mode onto ZnSe. In situ investigations of different azimuths show a clear lattice parameter oscillation in the [110] azimuth. The lattice parameter has a minimum value (similar to that of ZnSe) during Se exposure steps, and a higher and increasing lattice parameter during Cd exposure steps. The behavior is ascribed to the formation of CdSe islands during Cd exposure steps. The cumulative effect in CdSe exposure steps is considered to be a consequence of a decrease in the island size with the number of cycles. Actual plastic deformation does occur after 5 ML.

Investigation of deep levels in ZnSe:Cl films grown by molecular beam epitaxy

Using ZnCl2 as a doping source, we have grown epitaxial layers of Cl‐doped ZnSe onto GaAs(100) substrates at a temperature of 285u2009°C by molecular beam epitaxy. The carrier concentration was controlled by the ZnCl2 source temperature. The maximum carrier concentration obtained was 1.2×1019 cm−3 with a resistivity of 2.7×10−3 Ωu2009cm. Higher doses of Cl atoms tend to decrease free carrier concentration and introduce additional defects; this effect was consistent with an increase of deep‐level photoluminescence emission. From deep‐level transient spectroscopy measurements we determined the presence of four traps with energy levels Ec around 0.50, 0.68, 0.74, and 1.20 eV. The last three traps have not been previously reported. One of them, Ec∼0.74 eV, presents a clear dependence with Cl concentration and thus can affect the performance of ZnSe:Cl‐based devices. The microscopic nature of these traps is discussed in terms of Zn vacancies (VZn) and VZn–Cl complexes.

Band gap and optical constants of microcrystalline CdTe thin films

We report the results obtained from optical characterization studies of CdTe thin films grown by the standard rf sputtering technique. Films were grown at different substrate temperatures (Ts) in the 70–200u2009°C range. From the analysis of absorption spectra we determined the index of refraction and absorption coefficient. Employing a model of direct transitions, the fundamental absorption edge E0 was calculated. The same experiments and analyses were performed for annealed films at 400u2009°C in N2 atmosphere. The optical constants do not show a clear correlation with substrate temperature for Ts<200u2009°C. Higher substrate temperatures and/or thermal annealing does bring the optical parameters closer to the single‐crystal values. An important and consistent reduction of E0 was observed for all films after thermal annealing. Defect induced absorption, transformation of the metastable hexagonal to the cubic phase of CdTe, and quantum size effects caused by the microcrystalline structure of the films are discussed ...

INTERACTION BETWEEN Zn AND Cd ATOMS DURING THE ATOMIC LAYER EPITAXY GROWTH OF CdZnTe/ZnTe QUANTUM WELLS

Layers of 6 and 16 Cd–Te–Zn–Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin films. Different samples were grown at substrate temperatures of 260 and 290°C. Information about the kinetics of growth and surface reconstruction during the ALE growth of CdTe and ZnTe films, and Cd–Te–Zn–Te periods was obtained by means of reflection high-energy electron diffraction (RHEED) experiments and through the analysis of the temporal behavior of the intensities of several features of the RHEED patterns. The photoluminescence of the sample grown at 260°C presents two narrow and intense peaks corresponding to emission from quantum wells (QWs). However, the spectrum of the samples grown at 290°C does not show any feature associated with QWs, the spectrum resembling that of a ZnTe film. Cd replacement by Zn atoms explains the absence of the CdZnTe QWs at 290°C and a lower Cd content than expected at 260°C. The replacement of Cd atoms by Zn atoms in the CdTe surface was clearly demonstrated by Auger experiments.

Photoluminescence Study of Ultra-Thin CdSe Quantum Wells

CdSe ultra-thin quantum wells (UTQWs) with thickness in the 0.5 to 4 monolayers (ML) range were grown by pulsed beam epitaxy (PBE) between barriers of ZnSe at substrate temperatures of 260 and 290 °C. In each sample, five similar QWs separated by barriers around 200 A thick were deposited onto a 5000 A buffer layer of ZnSe. During the growth process, the surface was monitored in-situ by reflection high energy electron diffraction (RHEED); a 2D growth mode was observed in all the UTQWs. The analysis of the streak positions in the RHEED patterns indicated a critical thickness around 1.5 ML. The photoluminescence (PL) experiments exhibited intense and narrow peaks in the blue-green region of the spectra. The energy of the PL peaks presented a clear dependence on growth temperature and QW thickness. The emission of the UTQWs grown at 290 °C was blue-shifted in comparison to those grown at 260 °C. The PL peaks of the samples show the presence of different transitions, which cannot be explained in terms of thickness fluctuations.

Investigation of the structural properties of MBE grown ZnSe/GaAs heterostructures

The results of photoluminescence and Raman spectroscopies, high resolution X-ray diffraction, and Auger electron spectroscopy are analyzed in terms of the structural properties of the ZnSe/GaAs(l 0 0) system as a function of film thickness and substrate temperature. The results of Raman spectroscopy and X-ray diffraction clearly show that the strain in the film is inhomogeneous and depends only on film thickness and not on growth temperature in the 285-325°C range. From these experiments a value of N 0.17 pm is inferred for the critical thickness of ZnSe on GaAs. Photoluminescence experiments sensitive to the ZnSe/GaAs interface reveal the presence of strain in the GaAs substrate. Analysis of the intensities of the LMM Auger transitions of Zn and Se indicate the formation of an interfacial layer with excess of Se, suggesting the formation of a pseudomorphic Ga-Se compound mixed with ZnSe at the interfacial region.

NEAR BAND-EDGE OPTICAL PROPERTIES OF GAAS AT INTERFACES OF ZNSE/GAAS/GAAS BY PHASE SELECTION IN PHOTOREFLECTANCE

GaAs at the ZnSe/GaAs and GaAs/GaAs interfaces of ZnSe/GaAs/GaAs heterostructures is studied by phase selective photoreflectance (PR) spectroscopy. Four samples with ZnSe layers of various thickness were examined. We unambiguously determined the origin of two different features observed in the PR spectra by combining in phase and out of phase measurements, with PR measurements employing excitation lasers with different wavelengths. These two features are found to originate at different regions of the heterostructure. One contributing transition is a bulk-like signal, resembling that of bare GaAs, which originates in a region that encompasses the buffer layer/substrate GaAs homointerface. A second contributing signal is attributed to a strained region adjacent to the ZnSe/GaAs heterointerface. Both this second signal and the bulk-like signal show Franz–Keldysh oscillations that allow us to determine the electric field strength at the ZnSe/GaAs and GaAs/GaAs interfaces. It is found that the electric field s...

Stress in GaAs at the hetero-interface of ZnSe/GaAs/GaAs: a possible effect of pit filling and difference in thermal expansion coefficients

We report on the observation of stress effects on GaAs at the ZnSe–GaAs hetero-interface. Samples with the structure ZnSe/GaAs/GaAs(100) were grown by molecular beam epitaxy (MBE). ZnSe epilayers thickness ranged from 0.08 to 0.6 μm. Hetero-interfacial stress effects were investigated by photoreflectance (PR) and reflectance-difference spectroscopy (RDS). From a comparison between PR spectra and the second energy-derivative of the RDS spectra (SDRD) we conclude that both PR and RDS spectra have two components: (1) a bulk-like signal as for bare GaAs and (2) a signal coming from a strained region near the ZnSe–GaAs hetero-interface. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value e≅−0.0010±0.0004, independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. These have irregular cross-section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that there has to be present a strain in the upper atomic layers of the GaAs due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of same magnitude as that observed by PR.