J. F. Schetzina

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O2 into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 1018 cm−3 with an electron mobility of 260 cm2/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.

Molecular beam epitaxy growth and properties of GaN films on GaN/SiC substrates

Two types of nitrogen plasma sources, an electron cyclotron resonance (ECR) plasma source and a radio frequency (rf) plasma source, were used for the growth of GaN by molecular beam epitaxy (MBE). GaN film quality was correlated with the optical emission characteristics of each type of nitrogen plasma source employed. The best quality GaN films were those grown using the rf nitrogen plasma source. This source was found to emit a much larger fraction of atomic nitrogen and 1st‐positive series excited molecular nitrogen in contrast to the ECR plasma source which mainly produced 2nd‐positive series excited molecular nitrogen and nitrogen molecular ions when operated under the same conditions. The benefit of homoepitaxial growth of GaN, using metalorganic vapor phase epitaxy grown GaN layers on basal plane 6H‐SiC, was seen by the observation of surface reconstructions before, during, and after GaN film growth by MBE. In addition, the MBE‐grown GaN films exhibited remarkably intense photoluminescence dominated...

High resolution electron microscope study of epitaxial CdTe‐GaAs interfaces

CdTe films have been grown on (100) GaAs substrates with two different epitaxial relations: (111)CdTe∥(100)GaAs and (100)CdTe∥(100)GaAs. High resolution electron microscope observation of these two types of interfaces was carried out in order to investigate the role of the substrate surface microstructure in determining which type of epitaxy occurs. The interface of the former type shows a direct contact between the CdTe and GaAs crystals, while the interface of the latter type has a very thin layer (∼10 A in thickness), which is most likely an oxide, between the two crystals. These observations suggest that the GaAs substrate preheating cycle prior to CdTe film growth is crucial in determining which type of epitaxy occurs in this system.

ZnSe light‐emitting diodes

We report the successful fabrication of ZnSe p‐n junction light‐emitting diodes in which Li and Cl are used as p‐type and n‐type dopants, respectively.

Growth of high mobility n‐type CdTe by photoassisted molecular beam epitaxy

We report details of the successful controlled substitutional doping of CdTe films with indium. These n‐type films were prepared using a new technique, photoassisted molecular beam epitaxy, in which the substrate is illuminated during the deposition process. In the present work, an argon ion laser was used as an illumination source. The incident light was found to produce immediate and significant changes in the electrical properties of the films. In particular, highly activated n‐type CdTe:In layers resulted.

Cd1−xMnxTe‐CdTe multilayers grown by molecular beam epitaxy

Single‐crystal multilayers of the dilute magnetic semiconductor Cd1−x Mnx Te (x∼0.2) alternating with CdTe have been successfully grown for the first time using the molecular beam epitaxy technique. Four sets of superlattices have been prepared consisting of 14, 60, 90, and 240 double layers of average thickness 460, 140, 75, and 37 A, respectively. Each set consists of two samples grown simultaneously using 7×15×1‐mm thick (0001) sapphire substrates onto which 5.0‐μm‐thick CdTe buffer layers were first deposited. X‐ray diffraction techniques were employed to verify that epitaxy had been achieved and to obtain the average lattice constant of each of the multilayer structures. X‐ray diffraction satellites were observed on both sides of the (111) diffraction peak of the superlattices composed of 14 and 60 alternating layers, respectively, which allowed an accurate estimate of the superlattice period, or double‐layer thickness, for these samples. Results of UV reflectance studies and photoluminescence experi...

High‐brightness blue and green light‐emitting diodes

We report high‐brightness blue and green light‐emitting diodes (LEDs) based on II–VI heterostructures grown by molecular beam epitaxy on ZnSe substrates. The devices consist of a 2–3 μm thick layer of n‐type ZnSe:Cl, a ∼0.1 μm thick active region of Zn0.9Cd0.1Se (blue) or ZnTe0.1Se0.9 (green), and a 1.0 μm thick p‐type ZnSe:N layer. The blue LEDs produce 327 μW (10 mA, 3.2 V), with the light output sharply peaked at 489 nm, and exhibit an external quantum efficiency of 1.3%. The green LEDs produce 1.3 mW (10 mA, 3.2 V) peaked at 512 nm, corresponding to an external quantum efficiency of 5.3%. In terms of photometric units, the luminous performance (luminous efficiency) of the devices is 1.6 lm/W (blue) and 17 lm/W (green), respectively, when operated at 10 mA.

Optical properties of polycrystalline CdTe films

The optical properties of polycrystalline CdTe films prepared using vacuum sublimation techniques are discussed. Refractive index measurements are reported for the wavelength region 450–2000 nm and compared with single‐crystal values. Extinction and absorption coefficient measurements are reported for the strong absorption regime (450–900 nm). uv reflectance measurements are also discussed. New structure in the uv reflectance of many of the film samples is attributed to stacking sequence changes which introduce elements of the hexagonal phase. X‐ray diffraction results support this conclusion. The reflectance data are analyzed using Birman’s double‐zone scheme and yield some information concerning the band structure of hexagonal CdTe.

Photoluminescence of CdTe: A comparison of bulk and epitaxial material

Photoluminescence (PL) studies of bulk and epitaxial CdTe samples obtained from several sources are discussed. Steady state PL measurements were carried out at temperatures ranging from 16–300 K. The effects of surface preparation, substrate temperature, and film thickness were studied in detail for homoepitaxial films grown on the (111)A and (100) planes of CdTe. PL studies of epitaxial CdTe films grown on (0001) sapphire by molecular beam epitaxy (MBE), by hot wall MBE, and by metal‐organic chemical vapor deposition (MOCVD), and on the (111)B and (100) planes of GaAs by MBE have also been completed. The CdTe epilayers on sapphire and GaAs substrates typically display a bright PL spectrum dominated by the near edge peak at 1.58 eV (77 K). In addition, a number of films exhibit a near edge peak at 1.503 eV at 300 K, which is indicative of high quality epitaxy and which allowed direct measurement of the room temperature band gap of CdTe. PL studies of epitaxial Cd1−x MnxTe films grown by MBE on 5.0 μm thic...

Growth of (100)CdTe films of high structural perfection on (100)GaAs substrates by molecular beam epitaxy

Growth of epitaxial (100) CdTe films on (100) GaAs substrates by molecular beam epitaxy is discussed. X‐ray diffraction, UV reflectance, photoluminescence, and transmission electron microscopy techniques were employed to characterize the film specimens. The high structural perfection of the layers was evidenced by line dislocation densities of ≤104/cm2 at the free surface of films ≂6.6 μm thick and by measurable excitonic photoluminescence (∼1.504 eV) at room temperature. The CdTe epilayers were smooth and mirrorlike in appearance.