Conductivity and persistent photoconductivity in GaAs epitaxial films containing single and double delta-doped layers

Electrophysical parameters of single and double delta-doped layers in GaAs epitaxial films grown by the metal-organic chemical vapor deposition have been systematically investigated in the temperature range of 4.2 to 300 K. The 2D electron gas density distribution is affected by the overlap of wave functions in neighboring quantum wells, as a result of which the peak on the curve of the Hall mobility in the 2D electron gas versus the separation between the quantum wells shifts. The persistent photoconductivity in delta-doped layers is due to the change in the surface potential caused by the neutralization of the negative charge of surface states by photoexcited holes. A method for comparing delta-doped layers grown under different conditions at different depths from the sample surface has been suggested. Zh. éksp. Teor. Fiz. 113, 693–702 (February 1998)     

Photoconductive response of GaAs epitaxial layers

In the present work the photoconductive response of low resistivity, n-type GaAs epitaxial layers is studied by experimentally monitoring the dependence of the photoconductive gain (PG) optoelectronic parameter upon incident photon flux and temperature. The characterized samples fall into three major categories: ion implanted (II) GaAs epilayers formed within undoped, semi-insulating GaAs substrates; GaAs epitaxial layers grown by liquid phase epitaxy (LPE) on Cr-doped, semi-insulating GaAs substrates; and ungated GaAs MESFETs.     

Ohmic contacts to GaAs epitaxial layers

Formation of ohmic contacts onto GaAs epitaxial layers was reviewed. Because the Fermi energy of GaAs is pinned at the surface near the middle of the bandgap, it is impossible to choose a metal with the proper work function to make an ohmic contact. Instead, it is necessary to create a heavily doped surface layer and using field emission or thermionic field emission to achieve an ohmic contact. The oldest known contact metallization for GaAs is sequentially deposited thin films of Au, Ge, and Ni. It was shown that the ‘dopant diffusion model’, widely accepted to date to explain the formation of an n⁺ layer on GaAs to form the ohmic contact, is incorrect. Instead, the “solid phase regrowth model” was discussed in detail and shown to describe formation of ohmic contacts in this system. Based on this result, general rules for forming ohmic contacts to compound semiconductors with pinned Fermi levels or large values of electron affinities plus bandgap were expressed.     

Ultrafast photoconductors from low-temperature MOCVD-grown GaAs and InGaAs epitaxial layers

GaAs and InGaAs epitaxial layers were grown by Metal-Organic Chemical Vapor Deposition at Low substrate Temperatures (LT-MOCVD). The layers, grown at temperatures below 430° C were semi-insulating. Transmission electron microscopy images reveal uniformly distributed 3–10nm size clusters which consist, most probably, of zinc. Photodetectors fabricated from those layers feature nonlinear photocurrent versus optical power dependence. The nonlinearities are explained in terms of electric field redistribution. The nonlinear photoconductive correlation measurements show that excess carrier lifetimes are in the range of 20–50 ps.Deceased     

Nitrogen solubility and induced defect complexes in epitaxial GaAs:N

Thermodynamic calculation suggests that the formation of bulk GaN pins N chemical potential mu(N)< or =mu(max)(N), resulting in low equilibrium N solubility [N] in bulk GaAs:N. In epitaxial growth, however, a fully relaxed GaN phase cannot form prior to the spontaneous formation of a N-rich layer on the surface. First-principles total-energy calculations show that in the epitaxial regime one can increase mu(max)(N) considerably from equilibrium mu(max)(N) without triggering the spontaneous formation of such a N-rich layer. This enhances [N] by 8 orders of magnitude to about 4% at T = 650 degrees C in agreement with experiments. The dominant defects at high N concentration are qualitatively different from those at low [N].     

Photoluminescence spectra of Sn doped GaAs epitaxial layers: Influence of epitaxial process parameters

Photoluminescence measurements are performed with Sn doped GaAs epitaxial layers $\text{(}\text{n}\text{?}\text{= 4.10}{}^{18}$ to 1.1.10¹⁹ cm^?3) grown from a Sn solution. These samples exhibit the near-bandgap band and four further bands at 1.47, 1.33, 1.2 and 1.08 eV (77K). Their dependence on several parameters of the epitaxial process is studied systematically. The ratio of the peak intensities at 1.33 and 1.47 eV is observed to be influenced by the end temperature, by the growth rate of the layers, and by annealing processes. Comparison is made between the photoluminescence data and the doping concentrations which depend on the same parameters.     

Incorporation of hydrogen in CdTe and HgTe epitaxial layers grown by MOCVD

The incorporation of hydrogen into MOCVD-grown layers of CdTe, HgTe and CdHgTe using H₂ as the vector gas has been studied. Concentrations of incorporated H going from 6.5 × 10¹⁷ cm^-3 to 5 × 10¹⁸ cm^-3 have been found by SIMS in CdTe layers. This concentration decreases with increasing growth temperature and decreasing bond strength of the host material.     

Influence of thermal processing on the properties of Sn-doped GaAs epitaxial layers

The influence of annealing at a temperature of 750–830°C on the electrophysical, luminescent, and structural characteristics of GaAs layers doped with various concentrations of tin is studied. It is shown that, for low doping levels, the layers possess properties with high thermal stability. During annealing, one observes a lowering of the concentration of electrons, a reduction of the lattice periodicity, and a change in the photoluminescence spectra of strongly-doped layers, which is explained by the process of the formation of complexes and by the decomposition of supersaturated solid solutions of impurity dopants.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 54–59, January, 1989.The authors express gratitude to M. P. Yakuben for x-ray topographical studies.     

Optimization of carbon incorporation in GaAs during molecular beam epitaxial growth

Carbon-doped GaAs with dopant concentrations up to about 10²⁰ cm^–3 has been grown by molecular beam epitaxy. Above a critical carbon concentration, which depends on the deposition parameters, the surface deteriorates and loses its mirror-like appearance. From X-ray diffractometry and scanning electron microscopy, a diagram is established separating two areas with rough and mirror-like surface morphologies. The electrical properties as well as the morphology of GaAs : C can be simultaneously improved by a careful adjustment of the deposition parameters according to this diagram. On leave at: Tokyo Institute of Technology, Research Center for Quantum Effect Electronics, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan     

Ambient and low temperature photoluminescence topography of GaAs subtrates, epitaxial and implanted layers

High-resolution photoluminescence topography of GaAs at 300 and 2 K is reported. Topics include lateral homogeneity of semi-insulating substrates, quality of surface polishing, evaluation of sheet carrier concentrations and optimization of implant activations. Spectrally resolved low temperature topography allows images to be generated of carrier temperature and lifetime in substrates and of quantum well thickness variations in heterostructures.     

Use of ab etchant for clarifying the structural-impurity inhomogeneties in epitaxial layers of GaAs

In this paper we demonstrate the possibility of using Abrahams-Buiocchi etchant, widely used for revealing the dislocation structure of GaAs monocrystals, for studying submicroscopic impurity micro-inhomogeneities in epitaxial layers of GaAs. The required information is achieved by using etching patterns of electron microscopy replicas for the study. The presence of the memory effect during etching is demonstrated as an example of growth stacking faults. Impurity micro-inhomogeneities are visualized in the bulk of the epitaxial layers, caused by the presence of centers of step obstructions on the growth surface.V. D. Kuznetsov Siberian Physicotechnical Institute affiliated with Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 111–114, May, 1992.     

Optical functions of InGaP/GaAs epitaxial layers from 0.01 to 5.5 eV

In_0.49Ga_0.51P films, both undoped and doped n- and p-type (up to 10¹⁸ cm^-3), were grown lattice matched on GaAs substrates, with different miscut angles, by Metal-Organic Vapour Phase Epitaxy (MOVPE) at different temperatures. The shift of the fundamental gap E₀, caused by “ordering effect” was measured as a function of temperature by photoluminescence. The complex refractive index = n + ik and the dielectric function = ɛ ₁ + iɛ ₂ at room temperature were determined from 0.01 to 5.5 eV by using complementary data from fast-Fourier-transform far-infrared (FFT-FIR), dispersive, and ellipsometric spectroscopies. The effect of the native oxide was accounted for and the self-consistency of the optical functions was checked in the framework of the Kramers-Kronig causality relations. In the restrahlen region the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below E₀, the refractive index was modelled by a Sellmeier dispersion relation; in the interband region the dielectric function was well reproduced by analytical lineshapes associated to seven critical points. Thus parametrized analytical expressions were obtained for the optical functions all over the spectral range, without discontinuities, to be used in the modelling and characterization of multi-layer structures, also on opaque substrates. Received 13 December 2001 Published online 25 June 2002     

Characterisation of the interface between GaAs: Cr substrates andn-type epitaxial GaAs layers by infrared multiple interference analysis

Infrared reflectivity measurements on vapour phase grownn-GaAs epitaxial layers (n=7×10¹⁶...5×10¹⁸ cm^–3) deposited on semi-insulating GaAs:Cr substrates show interference structures whose strength cannot be explained by the interference pattern of a simple two layer system. Assuming a third very thin (0.4 m) interfacial layer it is possible to describe the experimental results. For Te doping the carrier concentration in the interfacial film is higher than in the volume of the epitaxial layer; it is lower for Sn doping. The results of this nondestructive optical method were confirmed by conductivity measurements while etching the sample. The origin of the interfacial layer is discussed in terms of non-steady state conditions at the beginning of the epitaxial growth.     

Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films

Magnetic shape memory nanostructures have a great potential in the field of the nanoactuators. The relationship between dimensionality, microstructure and magnetism characterizes the materials performance. Here, we study the martensitic transformation in supported and free-standing epitaxial Ni₄₇Mn₂₄Ga₂₉ films grown by sputtering on (0?0?1) MgO using a stoichiometric Ni₂MnGa target. The films have a Curie temperature of ~390 K and a martensitic transition temperature of ~120 K. Similar transition temperatures have been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films (with longer deposition time) present a wider martensitic transformation range that can be associated with small gradients in their chemical concentration due to the high vapour pressure of Mn and Ga. The magnetic anisotropy of the films shows a strong change below the martensitic transformation temperature. No features associated with variant reorientation induced by magnetic field have been observed. Annealed films in the presence of a Ni₂MnGa bulk reference change their chemical composition to Ni₄₉Mn₂₆Ga₂₅. The change in the chemical composition increases the martensitic transformation temperature, being closer to the stoichiometric compound, and reduces the transformation hysteresis. In addition, sharper transformations are obtained, which indicate that chemical inhomogeneities and defects are removed. Our results indicate that the properties of Ni–Mn–Ga thin films grown by sputtering can be optimized (fixing the chemical concentration and removing crystalline defects) by the annealing process, which is promising for the development of micromagnetic shape memory devices.     

Investigation of X-ray diffraction limitations upon the analysis of tellurium-atom injection into GaAs epitaxial layers

GaAs lattice “superdilation” caused by an introduced tellurium impurity, which is well known in publications, is experimentally studied. This phenomenon consists in the fact that the GaAs-lattice dilation can be more than 10 times greater than expansion that would appear upon the replacement of arsenic atoms with tellurium atoms if calculations are performed using the current-carrier concentration and Vegard’s law. The given phenomenon has already been observed at n _Te > 3 × 10¹⁸ cm^–3. A series of GaAs epitaxial layers heavily doped with tellurium and grown via metal-organic chemical vapor deposition are investigated using high-resolution X-ray diffractometry (HRXRD), secondary-ion mass spectrometry (SIMS), and the Hall effect. It is demonstrated that, despite a high Te concentration (10²⁰?10²¹ cm^–3) in the layer and variations in the growth conditions, the concentration estimates based on HRXRD data depend linearly on the results of elemental analysis performed by means of SIMS. The GaAs lattice expands even somewhat slighter as compared to the case where arsenic atoms are replaced with all Te atoms injected into the layer. At the same time, the Hall carrier concentration decreases sharply beginning at 2 × 10²⁰ cm^–3. In accordance with the obtained results, the examined phenomenon can be interpreted as the strong compensation of donor and acceptor carriers rather than as superdilation.