Minority carrier traps in epitaxial gallium arsenide phosphide

The optical injection of minority carriers in the depletion region of a reverse biased Schottky barrier has permitted the study of deep levels in vapour phase epitaxial n-type GaAsP (Te) structures. The deep level transient spectroscopy (DLTS) experiments have yielded two hole traps at (E_v + 0.60)eV and (E_v + O.36)eV. Additional subsidiary transient capacitance, photocurrent and capture experiments are analysed, the details of which give electron and hole capture cross-sections, trap densities and activation energies. Derived values of minority carrier lifetime (~1 ns) agree with experimental values and with injection-level dependent LED efficiency.     

Carrier recombination in gallium arsenide

This paper is concerned with an investigation of the mechanisms responsible for the recombination of excess carriers in gallium arsenide. The photoelectromagnetic and photoconductive techniques are used to measure the carrier lifetimes of n- and p-type samples with carrier concentrations ranging from 10⁷ cm⁻³ to 3 × 10¹⁸ cm⁻³ over temperatures from 80 to 400°K. It is shown that the highest measured minority carrier lifetimes of the more heavily doped samples are controlled by band-to-band radiative recombination. For higher resistivity samples the minority carrier lifetimes are determined by recombination centres lying at the centre of the band gap. Minority carrier trapping at a single level is observed in n-type samples but a more complex trapping model is needed to explain the behaviour of p-type samples.     

On the growth of gallium phosphide layers on gallium phosphide substrates by MOVPE

We have investigated the growth and luminescence of gallium phosphide layers grown by MOVPE at atmospheric pressure using trimethylgallium and phosphine in a hydrogen ambient. Both the morphology and the type of luminescence were found to be strongly dependent on growth conditions. Hexagonal defects are observed on the epitaxial surfaces, changing to rhombic defects when the growth takes place with a low V/III ratio. The origin of these defects is stacking faults. The density of these defects is dependent on the growth temperature. Photoluminescence studies at 2 K were also carried out. In the bound exciton spectrum of the gallium phosphide layers, the silicon impurity appears on the gallium site with sufficient phosphine excess and at sufficiently high growth temperatures. A deep impurity level, related to silicon on a phosphorus site, is present when growth takes place with a low phosphine excess. We explain all observations by a model that implies that the growing surface can be either gallium-rich (resulting in phosphorus vacancies) or phosphorus-rich, resulting in gallium vacancies. The phosphorus-rich surface enables smooth epitaxial layers to be grown. With growth taking place at a phosphine to trimethylgallium ratio greater than 2.5, the surface is in a configuration stabilized by phosphorus reconstructions.     

Gettering of epitaxial gallium phosphide using phosphosilicate glass

Chemically vapour deposited phosphosilicate glass films were used to getter epitaxial gallium-phosphide layers. Significant improvements in the electroluminescent efficiency of yellow light-emitting diodes fabricated from the gettered material were observed. Results suggest that the gettering removes compensating impurities as well defects which influence minority carrier lifetime.     

Gallium phosphide epitaxial layers with high mobility

Measurements on epitaxial layers of GaP, containing negligible amounts of GaAs, have yielded mobilities up to 200cm2/Vs at room temperature, and a peak value of 1480cm2/Vs. The residual impurity appears to be Si.     

Preferential etching of dislocations and stacking faults in gallium phosphide

A combination of optical microscopy and transmission electron microscopy has been used to provide direct evidence that pits formed by the action of a solution of 3 HF:1 H₂O₂ on epitaxial and substrate GaP material close to the (001 ) orientation are associated with dislocations. The formation of different pit shapes can be correlated with previous work on etch pits in GaP, namely the formation of D and S pits. Grooves have been produced on certain crystallographic orientations which are associated with certain types of stacking faults. It is now possible to use this etch for a direct count of the dislocation density in (001) GaP and to indicate the extent of stacking faults.     

Relaxation oscillations and recombination in epitaxial N-type gallium arsenide

Relaxation oscillations produced by impact ionization within high-field domains were studied in epitaxial n-type gallium arsenide at room temperature and at 77°K. A study was also made of the behaviour at room temperature of excess carriers produced by hole-injection at the anode.From these measurements the majority and minority carrier lifetimes were obtained.Results from both vapour-grown and solution-grown epitaxial layers are presented. The lifetimes for these two types of material were found to be not very different. In some layers appreciable numbers of centres acting as hole traps (∼ 10¹⁴−10¹⁵ cm⁻³) were found.     

Deep level profiles at substrate-epitaxial interfaces in gallium phosphide

The spatial distribution of several species of deep level defects have been measured in the substrate-epitaxy interface region for both liquid phase and vapour phase gallium phosphide. It is found that certain deep levels, measured by photo-ionisation, and photo-capture methods, have greatly enhanced concentrations at the interface and in the last few microns of the substrate material. This pile up of defects is particularly severe in the vapour phase epitaxial layers. The deep level profiles have been mapped in detail for substitutional oxygen and for a transition metal associated defect. A significant difference in their behaviour has been observed. For the transition metal defect, additional in-diffusion experiments have been undertaken which confirm that the interfacial region provides preferential sitting conditions which result in a concentration enhancement. This observation is important for thin layer devices where the interfacial regions form part or are close to the active device volume. The measured capture cross-sections suggest that the large concentrations in the interface region would produce high levels of recombination, and may contribute to the large recombination velocities observed at interfaces.     

Carrier lifetimes in silicon epitaxial layers deposited on oxygen-implanted substrates

Carrier lifetimes in silicon epitaxial layers deposited on high-dose oxygen-implanted wafers have been obtained from measurements of diode storage times. A figure of 1.25 ?S was obtained for diodes in the implanted area, compared with 1.75 ?S for diodes outside the implanted area on the same wafer. This marginal degradation of lifetime indicates that the dielectrically isolated structure should be able to support bipolar and dynamic logic devices.     

Spin dependent recombination at dislocations in silicon

The unpaired electrons of dislocations with an edge component in Si give rise to an EPR signal. Under the resonant condition, a decrease of photoconductivity is observed. We have observed the derivative of the resonant signal at a microwave frequency of 9.2 GHz by modulating the sweeping magnetic field. The relaxation time of the spin system was studied by varying the modulation frequency. The amplitude of the resonant signal is discussed with respect to dependence of the background signal on the sweeping magnetic field. The signal consists of more than one absorption line and has been resolved into its components by computer evaluation. The energy level structure of the dislocations which produce the signal has been derived from Hall effect measurements and conventional optical methods. The influence of deformation temperature and the role of other heat treatment produced defects as a possible source of the signal are discussed.The strength of the effect is related to the strong polarization of exchange coupled paramagnetic centers. The experimental results suggest that the recombination process is of the triplet type. The anomalous phase of the resonant signal is explained by resonant emission of microwave power.     

High-efficiency Bragg cells in gallium phosphide

The design, fabrication and evaluation of efficient wideband Bragg cells in gallium phosphide is reported. The devices were fabricated using the <110> and <111> longitudinal modes of acoustic propagation with design bandwidths of 500 MHz centred on 1.2 GHz. State-of-the-art efficiencies of up to 115%/W and 220%/W, respectively, were obtained.     

Photoconductivity of copper-compensated gallium phosphide

Autocompensation effects in GaP after diffusive copper doping, amphotericity of electrical activity of the copper impurity, and the kinetics of interband photoconductivity and infrared quenching are explained in terms of a model of reconstruction of the bonds between the copper impurity and its immediate environment. In this picture, the acceptor state is associated with substitutional Cu atoms with tetrahedral coordination, while the donor state is associated with Cu atoms bonded to only two of the four phosphorus atoms. Fiz. Tekh. Poluprovodn. 32, 1165–1169     

Device assessment of the electrical activity of threading dislocations in strained Ge epitaxial layers

It is shown that the high density of threading dislocations (TDs) and, more specifically, the high density of point defects associated with it and present in our strained Ge epitaxial layers on a Si_0.2Ge_0.8 relaxed buffer layer degrades the mobility and the leakage current of pMOSFETs and p⁺n junctions fabricated therein. Annealing in the range 550–650 °C prior to gate stack deposition improves the device performance, although there is no marked change in the TD density. From this, it is concluded that the annealing may reduce the density of point defects grown in during the epitaxial deposition.     

The structure of porous gallium phosphide

Free films of porous GaP have been obtained for the first time and their microstructure has been investigated by transmission electron microscopy (TEM) and Raman scattering (RS). The TEM results showed that the microstructure of the porous-GaP films has a complicated spatial structure, but the local crystallographic orientation is preserved and corresponds to the initial substrate. Narrowing of the half-width and a simultaneous low-frequency shift of the LO-phonon peak were observed in the RS spectrum. This behavior can be satisfactorily explained in terms of the change in the plasmon-phonon interaction as a result of a decrease in the carrier density. A shift in the frequency of surface phonons of porous GaP as a function of the local void formation conditions was also observed. It is shown that in principle porous layers can be obtained without the void surfaces being covered appreciably with oxides or other products of an electrochemical reaction. Fiz. Tekh. Poluprovodn. 32, 235–240 (February 1998)     

Ion implantation of porous gallium phosphide

The effect of irradiation by Ar ions and thermal annealing on the properties of porous gallium phosphide (por-GaP) obtained by electrolytic methods is investigated. It is shown on the basis of Raman scattering and photoluminescence data that, in contrast with porous silicon, por-GaP does not have high radiation hardness, and that thermal annealing of defects in layers amorphized by ion implantation is impeded by the absence of a good crystal base for solid-state epitaxial recrystallization processes. Data on radiation-induced defect formation and from probing of the material with a rare-earth “luminescence probe” are consistent with a mesoporous structure of the material. Fiz. Tekh. Poluprovodn. 32, 990–994 (August 1998)     

Stimulated photoconductivity of copper-compensated gallium phosphide

The effect of the intrinsic excitation on the photoconductivity spectrum for the GaP:Cu samples is investigated. The specific features emerging as the additional peak at the photon energy ～1.8 eV are attributed to the effects of redistribution of nonequilibrium charge carriers generated from the first two energy levels of Cu in the surface potential field. The nature of the impurity photoconductivity band peaked at 1.05 eV with the edge at about 0.6 eV is discussed.     

Cathodoluminescence of oxygen-implanted zinc-doped gallium phosphide

Oxygen has been introduced by ion implantation into zinc-doped epitaxial gallium phosphide grown from the vapour phase. After annealing, cathodoluminescence from zinc-oxygen pairs is observed, having an efficiency approaching that of bulk zinc-oxygen doped solution-grown material. For a zinc concentration of 2 × 1017cm?3, an optimum oxygen concentration for cathodoluminescence of 3 × 1018cm?3 is exhibited.     

Intrinsic photoconductivity of copper-doped gallium phosphide

Studies of the intrinsic photoconductivity of copper-doped gallium phosphide reveal that increasing the light intensity incident on the sample leads to the appearance of an additional photoconductivity peak in the fundamental absorption band, which can be viewed as anomalous. It is found that the line shape of this new peak in the photoconductivity spectrum depends on the photon energy of the additional illumination. These results are explained by variations in the lifetime of minority carriers over the sample bulk due to reconstruction of copper centers. An expression, which qualitatively describes the experiment, is derived for the photoconductivity spectrum. Fiz. Tekh. Poluprovodn. 33, 916–920 (August 1999)     

Low-frequency noise in gallium nitride epitaxial layers with different degrees of order of mosaic structure

The correlation between the noise level 1/f and the degree of mosaic-structure order in gallium nitride epitaxial layers was studied for the first time. Samples with a doping level of N _d ?N _a ≈8×10¹⁶ cm^?3 and a relatively high degree of order were characterized by the Hooge parameter α≈1.5×10^?3. This value is unprecedently low for thin GaN epitaxial films. The Hooge parameter was significantly higher for samples with N _d ?N _a ≈1.1×10¹⁸ cm^?3 and a low degree of order despite the fact that α generally decreases with increasing doping level at the same degree of order. Thus, the degree of mosaic-structure order affects not only the optical and electrical characteristics but also the fluctuation parameters of GaN epitaxial layers.