Muhammad Zafar Iqbal

COMSATS University Islamabad | CUI · Department of Physics

Study of the Cosmos, at best, is considered a semi-scientific discipline, primarily because the laboratory for carrying out measurements and tests of theories (the Cosmos) has been largely inaccessible for centuries. The cosmic vista into the yonder, however, continued to fascinate humankind due to its inherent beauty and sheer curiosity. The inven...

The study of incompressible Newtonian fluid flow in a small diameter permeable tube by considering slip effects on walls is presented with an application to the flow in renal tubule. The fluid absorption at the tube walls is taken as a function of wall permeability and trans-boundary pressure drop. Using suitable physical approximations, the Navier...

This paper reports a detailed study of the effects of irradiation and thermal annealing on deep levels in Rh-doped p-type GaAs grown by low-pressure metal-organic chemical-vapor deposition, using deep level transient spectroscopy (DLTS) technique. It is found upon irradiation with alpha particles that, in addition to the radiation-induced defect pe...

Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental e...

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to characterize Cu2+ ions substituting for Ti4+ ions in nominally undoped TiO2 crystals having the rutile structure. Illumination at 25 K with 442 nm laser light reduces the concentration of Cu2+ ions by more than a factor of 2. The laser light also reduces...

While zinc oxide is a promising material for blue and UV solid-state lighting devices, the lack of p-type doping has prevented ZnO from becoming a dominant material for optoelectronic applications. Over the past decade, numerous reports have claimed that nitrogen is a viable p-type dopant in ZnO. However, recent calculations by Lyons, Janotti, and...

Photoluminescence (PL) from GaN epilayers is found to be sensitive to the ambient atmosphere and length of UV exposure. We studied the effect of UV illumination in different ambients including air, oxygen, nitrogen and hydrogen gases on room-temperature PL of GaN grown on sapphire by molecular beam epitaxy. In some samples the PL intensity increase...

Epitaxial layers of p -GaAs grown on p⁺ -GaAs substrates by low-pressure metal organic chemical vapor deposition have been investigated using deep level transient spectroscopy (DLTS). One dominant peak and other relatively small peak, corresponding to deep levels at E_v+0.55 eV and E_v+0.96 (low field energies), respe...

Thermal annealing behaviour of deep levels in p-GaAs grown by metal-organic chemical-vapor deposition (MOCVD) has been studied by deep level transient spectroscopy (DLTS) technique. Thermal annealing is found to introduce at least six new defects, four majority-carrier emitting deep levels, situated at Ev+0.11, Ev+0.27, Ev+0.44 and Ev+0.89 eV in th...

We report the results of isochronal annealing study of deep levels in Rh-doped n-type GaAs grown by metal-organic vapor phase epitaxy (MOVPE). Deep level transient spectroscopy (DLTS) technique has been employed to study the effects of annealing on deep levels in Rh-doped p+nn+ junction samples. A majority-carrier emitting band of deep levels along...

Deep-level transient spectroscopy (DLTS) has been used to investigate deep levels in n-type Ru-doped GaAs grown by low-pressure metal-organic chemical-vapor deposition (LP-MOCVD). DLTS scans over a wide temperature range (12-470 K) reveal two prominent deep-level peaks associated with Ru, when compared with control samples with no deliberate Ru-dop...

We report a comprehensive study of deep levels in the entire band gap of alpha-irradiated n-type GaAs, grown by metal-organic vapor phase epitaxy (MOVPE), over a wide temperature scan range of 12-475 K. Deep level transient spectroscopy (DLTS) reveals the presence of, at least, seven radiation-induced deep level defects at energy positions Ec-0.05...

Deep levels associated with 4d transition-metal impurity rhodium have been investigated in GaAs epitaxial layers grown by metal-organic chemical vapor deposition, using deep level transient spectroscopy (DLTS) technique. A comprehensive study of deep levels in the entire bandgap of GaAs has been carried out using both n - and p -type GaAs crystals...

A preliminary study of growth interruption effects on the GaAs/AlGaAs layers is reported. Deep level transient spectroscopy (DLTS) is performed on Al Schottky barrier devices fabricated on Si-doped, isotype GaAs/Al 0.78 Ga 0.22 As heterostructures grown by molecular beam epitaxy (MBE), both with (200 s) and without, growth interruption. Our spectra...

Results of isochronal annealing up to a temperature of 660 K of n-type GaAs grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) are reported. Deep-level transient spectroscopy (DLTS) reveals EL2 as the only electron-emitting deep-level defect in our as-grown material, whereas three hole-emitting levels at E-v + 0.09 eV, E-v + 0...

Irradiation of p-GaAs grown by low-pressure metal-organic chemical-vapor deposition (LP-MOCVD) with α-particles from the 241Am source has been performed at room temperature. At least seven radiation-induced deep-level defects at energy positions Ev+0.09eV, Ev+0.11eV, Ev+0.16eV, Ev+0.24eV, Ev+0.32eV, Ev+0.42eV, and Ev+0.77eV were detected in the low...

EL2 is the ubiquitous native defect in crystalline GaAs grown by a variety of different techniques. It has been proposed to be a doubly charged deep-level center with two states having distinct energy levels in the band gap. While the singly charged state has been the subject of many experimental studies and is, therefore, well established, the dou...

Investigations of the alpha particle irradiation-induced defects in low-pressure metal-organic chemical-vapor deposition grown p -GaAs have been carried out. By employing deep-level transient spectroscopy, at least seven radiation-induced deep-level defects have been observed in the lower half of the band gap in the temperature range of 12-475 K ....

Deep-level defects related with 5d transition metal, osmium (Os) have been studied in ntype GaAs. Os has been incorporated in epitaxial n-GaAs layers in situ, during growth by lowpressure metal-organic chemical vapour phase epitaxy (MOVPE) technique. Mesa p+nn+ junction diodes are fabricated for investigations by deep level transient spectroscopy (...

The 5d transition-metal impurity, osmium, has been incorporated during the growth of n-type GaAs epitaxial layers using low-pressure metal-organic chemical-vapor deposition to characterize defect states associated with this heavy and, therefore, thermally stable dopant impurity. Deep-level transient spectroscopy has been employed to investigate the...

Results of a preliminary study on deep level transient spectroscopy (DLTS) investigations of osmium (Os) impurity in p-type GaAs, introduced in situ during MOCVD crystal growth, are reported for the first time. Os is clearly shown to introduce two prominent deep levels in the lower half-bandgap of GaAs at energy positions Ev + 0.42 eV (OsA) and Ev...

Ruthenium is introduced into GaAs during epitaxial growth by MOCVD. Preliminary results of DLTS investigation of the defect states associated with this 4d transition-metal impurity are reported for the first time. At least three deep levels are identified with Ru in the lower half-bandgap of GaAs at energy positions Ev + 0.38 eV, Ev + 0.52 eV and E...

Deep levels due to 5d transition metal, osmium (Os), in n-type GaAs epitaxial layers grown by metal-organic vapour-phase epitaxy are investigated using deep level transient spectroscopy (DLTS). Two clear and prominent peaks are observed in majority carrier (electron) emission spectra in GaAs:Os samples in addition to a weaker, broad feature at high...

p-type GaAs epilayers grown by metal-organic chemical vapour deposition (MOCVD) technique, doped in situ with rhodium (Rh) impurity, have been studied by deep level transient spectroscopy (DLTS). A composite peak consisting of emission signals from at least two deep levels in the lower-half band gap is identified with Rh. This peak is resolved usin...

High-purity GaN grown by hydride vapor-phase epitaxy was studied by time-resolved photoluminescence (PL). Two types of samples, 6–30-μm-thick GaN layers on sapphire substrate and a 200-μm-thick freestanding template, exhibited similar PL behavior. At low temperatures, the luminescence signal from the red, yellow, blue, and ultraviolet luminescence...

Deep levels have been characterized in n-type GaAs crystalline films grown by metalorganic chemical vapor deposition, doped in situ with 4d transition metal, rhodium, using the deep level transient spectroscopy (DLTS) technique. Two prominent broad bands of deep levels are found to be associated with Rh impurity, one in the upper half and the other...

We studied time-resolved photoluminescence (PL) over a temporal range 10−6–103 s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor–acceptor emission bands can be resolved in the PL spectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent P...

Doping crystalline silicon with silver results in a photoluminescence center with multiplet zero-phonon structure near 778.9 meV. We show that the published assignments of the vibronic sidebands are wrong, with severe implications for the relative transition probabilities of the luminescence transitions from the excited states. At low temperature,...

We report the investigation of reversible-transient effects in photoluminescence (PL) of GaN layers in different measurement ambients including air, oxygen, nitrogen, and hydrogen. While the presence of air and oxygen led to similar amounts of significant decrease in overall luminescence, nitrogen and hydrogen produced a much smaller effect. Data o...

AlxGa1-xN samples covering the entire range of alloy compositions, 0≤x≤1, were grown by plasma-assisted MBE on c-plane sapphire substrates. The aluminum mole fraction was determined, by three different techniques, namely, x-ray diffraction, secondary ion mass spectroscopy, and Rutherford backscattering spectrometry. The energy bandgaps of the alloy...

Preliminary results on the study of deep levels associated with 4d-transition metal, rhodium, in crystalline GaAs grown by metal-organic chemical vapour deposition (MOCVD) technique are reported for the first time. Deep level transient spectroscopy on n-type GaAs doped in situ with Rh during MOCVD growth reveals a broad majority carrier emission pe...

Results on the optical characterisation of copper deep acceptors throughout the alloy composition range (0<or=x<or=1) of GaAs1-xPx are reported. A comparison of the hole photoionisation spectra for different compositions, measured by the transient photocapacitance technique, reveals a broadening effect near the threshold which is interpreted in ter...

Deep level transient spectroscopy has been used to observe the effect of alpha particle irradiation on n-type InP grown by metal organic chemical vapour deposition (MOCVD). Eight majority carrier emitting levels E alpha 1, E alpha 2,..., E alpha 8 are found to be produced as a result of this irradiation. At least two of the observed levels (E alpha...

Deep energy levels arising from doping of n- and p-type InP with the 5d transition metal Os are studied, using electrical and optical deep level transient spectroscopy. Electrically active Os-related level concentrations up to 8.2 x 10(15) cm(-3) are observed representing up to 20% of the chemical concentration. Three prominent levels with low-elec...

Iron-induced deep levels have been studied in p-type silicon quenched from high temperatures, using deep-level transient spectroscopy. The interstitial iron donor and the iron - boron pair defect are observed and identified by a number of their characteristics including their mutual transformation. Our recent work revealed an interesting new proper...

Deep-level transient spectroscopy (DLTS) has been applied to study the interstitial iron donor (Fei) in thermally quenched p-type silicon and its interaction with some 4d and 5d transition metals. This centre is revealed by minority carrier injection in our samples with no deliberate transition metal doping. A number of its characteristics, includi...

The results of a study carried out on Pd-doped p+n Si junctions using deep-level transient spectroscopy (DLTS) are reported. Four deep levels associated with Pd at energy positions Ec-0.18 eV, Ec-0.22 eV, Ec -0.37 eV and Ec-0.59 eV are observed. These deep levels have been observed simultaneously in the same as-diffused Si:Pd samples for the first...

Magnetic anisotropy measurements have been performed in both zero-field-cooled and field-cooled states of two melt texture grown (MTG) superconductors. Hysteresis loops taken at various orientations ( theta ) of the c axis with respect to field H, indicate a dominant contribution of the c-axis magnetization for theta

The 4d-transition metal ruthenium presents a new dopant to fabricate thermally stable semi-insulating InP layers for both electron and hole injection. The layers are grown by low-pressure metalorganic chemical vapor deposition using tertiarybutylphosphine and trimethylindium as source materials for InP growth. Using bis(eta(5)-2,4-dimethylpentadien...

Low temperature photoluminescence (PL) measurements of Si+ ion-implanted GaAs samples were investigated by varying the ion beam current. Electrical measurements proved that higher activation rates can be obtained by using low ion beam currents (3 nA cm−2) compared with higher ones (60 nA cm−2). In the PL spectra, broad emissions at 840–850 nm can b...

Room temperature storage and/or minority carrier injection behaviors of three prominent majority carrier levels H3 (Ev+0.34 eV), H4 (Ev+0.39 eV) and H5 (Ev+0.58 eV) and three minority carrier levels, 0.22, 0.29 and 0.35 eV below the conduction band edge in metal-organic chemical vapor deposition (MOCVD) grown p-type InP crystals produced by alph...

Deep-level transient spectroscopy (DLTS) has been used to investigate and characterize deep-level defects in nitrogen-doped p-type ZnSe layers grown by molecular beam epitaxy on GaAs substrates. At least three prominent levels, H1, H3 and H4, are found in the lower-half bandgap with thermal activation energies 0.31 eV, 0.63 eV and 0.94 - 0.99 eV re...

We report new data on the 6282 cm -1 photoluminescence band of a silver-related defect in silicon. We show that the main structure of the luminescence bandshape is caused by the vibronic interaction between the excited states of the centre, so that at low temperatures the phonon sideband originates from a forbidden zero-phonon line. The excited sta...

We report results of a combined electrical and optical DLTS study of Os-doped p-type InP grown by low pressure metalorganic chemical vapour deposition (LP-MOCVD). Two prominent deep levels in the lower half band-gap, labelled OsA and OsB, and one in the upper half (Os1) have been attributed to osmium. Detailed comparison of reduced-reverse bias, el...

Deep level defects induced by alpha radiation in p-type InP crystals, grown by low pressure metalorganic chemical vapour deposition (LP-MOCVD) have been studied by deep-level transient spectroscopy (DLTS). Two prominent hole-emitting levels with thermal activation energies 0.34 eV and 0.39 eV to the valence band and two electron-emitting levels wit...

Photoluminescence spectroscopy is used to investigate some of the recently reported effects of implantation dose and dose rate on the electrical activation of Si dopant in GaAs. Two new luminescence bands are observed to emerge in our spectra with the increasing Si dose at doses ( ∼ 2×1013 cm−2) where the carrier concentration is known to saturate....

Deep level defects induced by alpha radiation in p-type InP crystals, grown by low pressure metalorganic chemical vapour deposition (LP-MOCVD) have been studied by deep-level transient spectroscopy (DLTS). Two prominent hole-emitting levels with thermal activation energies 0.34 eV and 0.39 eV to the valence band and two electron-emitting levels wit...

We report results of a combined electrical and optical DLTS study of Os-doped p-type InP grown by low pressure metalorganic chemical vapour deposition (LP-MOCVD). Two prominent deep levels in the lower half band-gap, labelled OsA and OsB, and one in the upper half (Osl) have been attributed to osmium. Detailed comparison of reduced-reverse bias, el...

A detailed photoluminescence investigation of the effects of Al doping on GaAs grown by molecular‐beam epitaxy is reported. Materials with 0.1%, 1%, and 3% Al doping have been studied. Viewed as an Al x Ga 1-x As alloy semiconductor, our study on these materials presents results on the optical characteristics of the lowest Al composition material r...

The effects of Al as an isoelectronic dopant have been investigated on the deep level defects in GaAs grown by molecular‐beam epitaxy (MBE), using deep level transient spectroscopy. Two different compositions containing 0.1% Al (Al 0.001 Ga 0.999 As) and 1% Al (Al 0.01 Ga 0.99 As) have been studied. At least nine different deep levels have been det...

The influence of Al as an isoelectronic dopant on the deep level defects in GaAs grown by Molecular Beam Epitaxy (MBE) has been investigated using deep level transient spectroscopy (DLTS). Three different compositions of AlxGa1-xAs with x=0.001, 0.01 and 0.03 have been studied. At least nine different deep levels have been detected. They have been...

Interstitial iron donor has been studied in thermally quenched p-silicon using deep level transient spectroscopy. It is identified by measurements of a number of its fingerprints including the emission rate signature, its injection-induced production, its enhancement with thermal quenching temperature and as isothermal and isochronal annealing char...

Deep level transient spectroscopy has been used to investigate the effects of alpha irradiation and thermal annealing on deep levels in gold‐doped n‐type silicon. Data on thermal annealing characteristics of the well‐known gold acceptor up to temperatures as high as about 600 °C are reported. This level is found to be very stable, supporting the su...

Deep‐level transient spectroscopy has been employed to study the defects in silver‐doped p‐type Si, their interaction with radiation‐induced defects, and the thermal annealing characteristics of both types of defects. The linearly graded nature of the n⁺p junctions used allows the acceptor (E c -0.54 eV) and donor (E v +0.35 eV) levels a...

Deep‐level transient spectroscopy has been used to study the deep levels introduced by silver impurity in silicon. New levels in addition to the well‐known silver‐related deep donor and acceptor levels have been found. Interaction of silver‐related defects with radiation‐induced defects has been studied using α irradiation. Data on the annealing ch...

We show that an exciton is self-trapped at the well-known ABC optical center in silicon by a lattice relaxation of 35+/-3 meV. The self-trapping arises from the electron component of the exciton, which is highly localized within about one atomic spacing of the core of the optical center. The effects of compressive stresses on the photon energy and...

Semiconductivity in YBa2Cu3O6+x has been characterized over nearly the complete range of oxygen concentrations. Resistivity vs temperature data reveal a thermally activated conductivity with systematic variation of the activation energy with oxygen content, hitherto unreported. This suggests the existence of some acceptor-like defects which correla...

The deep level transient spectroscopy technique has been used to study the interaction of 5.48 MeV α particles with deep levels in Pt‐doped n‐type silicon. Production rates and annealing behaviors of alpha‐radiation‐induced levels in the presence of platinum have been investigated. Isochronal annealing characteristics of Pt‐related levels before an...

Results of a detailed study to establish the optimum conditions for the normal-pressure synthesis of the YBa2Cu4O8 (124) superconductor, using Cu2(CN)2 as the starting Cu compound, are presented. The bulk 124 superconductor was synthesized directly, without the use of any wet-chemical methods or extrinsic reaction-rate enhancers; Cu2(CN)2 itself se...

Results of a detailed deep level transient spectroscopy study of Pd‐doped p⁺n Si diodes irradiated with 5.48 MeV α particles are presented, which also include investigations of isochronal annealing behavior of the deep level spectra up to a temperature of 350–400 °C. An extended comparison with results obtained on reference samples as we...

We report data on photoluminescence from the ''ABC'' isoelectronic centre under uniaxial stresses, using much larger stresses than hitherto. The vibronic band shape suggests that the excited state of the centre is a self-trapped exciton. This model correctly predicts the effects of uniaxial-stress perturbations and explains the loss of luminescence...

Photoluminescence spectra from silver doped silicon reveal new informationon the electronic structure of this defect. A `forbidden' line at 778.7 meV and a line at 784.2 meV are reported for the first time. Temperature dependence and uniaxial stress measurements are used to obtain further information on the nature of the defect.

The thermal decomposition behavior of cuprous cyanide, which has been used to synthesize ceramic superconductors, is studied using Fourier transform infrared spectroscopy, thermogravimetry, and differential thermal analysis. The results are best understood by assuming that cuprous cyanide polymerizes to various cyclized products upon heating which...

A simple method for the synthesis of YBa 2 Cu 4 O 8 superconductor at oxygen pressure of 1 atm is reported. The starting copper compound Cu 2 (CN) 2 makes it possible to synthesize near single phase material directly, without the use of reaction rate enhancers or wet chemical methods previously considered necessary. Resistivity and magnetic suscept...

Deep levels introduced by 5.48 MeV alpha particles in p‐type silicon have been studied using deep‐level transient spectroscopy. The generation rates of these defects have been obtained up to a dose of 1.2×10¹¹ α particles/cm². Detailed data have been obtained on the electrical characteristics of the two deep levels in the lowe...

A detailed deep‐level transient spectroscopy study of the characteristics of deep‐level defects introduced by 5.48 MeV alpha particles in low‐doped n‐Si is reported. The deep‐level characteristics studied include emission rate signatures, activation energies, capture cross sections and their temperature dependence, and defect concentrations and the...

Deep-level transient spectroscopy (DLTS) is shown to reveal a strong field dependence of the thermal emission of electrons from the singly occupied state (O1) of the deep-level oxygen defect in GaP. Detailed data on this field dependence and its variation with temperature over the range 350-470 K are reported. The experimental results are analyzed...

The preparation of a high-temperature superconductor based on the Y-Ba-Cu-based system without the use of any extraneous oxygen is reported. Using starting materials with a maximum possible oxygen content of 3.5 oxygen atoms per formula unit of the end product and ambient nitrogen during synthesis, we observe ~90-K superconductivity. This challenge...

Direct measurements of the electron capture cross section of the Ev+0.32 eV platinum donor level are reported. Deep level transient spectroscopy (DLTS) has been employed for the measurements. The cross section has been found to be temperature independent in the range 140-190 K with a value of 2.5+or-1.5*10-17 cm2.

Observations on deep levels introduced in silicon by α‐particle irradiation are reported. Low‐doped n‐type samples are used and deep level transient spectroscopy is applied to detect the deep levels. Preliminary results provide evidence for some new defect states in addition to those previously reported. The study also reveals interesting metastabi...

The problem of identification of the dominant recombination center in platinum‐doped silicon is investigated using deep‐level transient spectroscopy (DLTS). Measurements using electrical and optical injection on platinum‐doped n type silicon are reported. Results of measurements show that the recently observed platinum‐related midgap level is an ef...

Results of an electrical characterization study of two hitherto unobserved midgap deep levels in GaP using deep‐level transient spectroscopy and single‐shot dark capacitance transient techniques are reported. These are the dominant majority‐carrier (electron) levels in the green‐light‐emitting diodes studied. Detailed electron emission rate and cap...

Transition rates for multiphoton absorption via direct band-to-band excitation have been calculated using a non-perturbative approach due to Jones and Reiss [3], based on the Volkov type final state wave functions. Both cases of parabolic and non-parabolic energy bands have been included in our calculations. Absorption coefficients have been obtain...

Results on the measurements of electric field dependence of thermal emission of holes from the 0.85‐eV level in liquid‐phase‐epitaxial GaP, using deep‐level transient spectroscopy, are reported for various temperatures. The observed field dependence is analyzed in terms of Poole–Frenkel emission from the three‐dimensional Coulomb and square‐well po...

Results from a comprehensive study of silver‐related deep levels in silicon are presented. Thermal emission rates and capture cross sections (along with their temperature dependence) for the two dominating Ag‐related levels have been measured using deep level transient spectroscopy. For both levels the measured capture cross sections are temperatur...

Red-emitting GaP LEDs are shown to provide highly sensitive temperature sensors. This investigation is based on the exploitation of capacitance variation rather than the usually employed forward voltage variation. Detailed results on thermometric characteristics are reported. A temperature variation of as low as 0.02 degree can be reliably sensed u...

Thermal emission rate data have been obtained on a 0.85‐eV deep hole level in liquid‐phase‐epitaxial GaP light‐emitting diodes using dark capacitance transients. Published emission rate data on this level exist for vapor‐phase‐epitaxial GaP. Detailed comparison with available data reveals a wide spread in emission rates casting serious doubts on th...

Observations of novel oscillations in the photoconductivity of Si:Au are presented. A possible interpretation in terms of nonlinear instability leading to chaos is offered.

The fundamental role of deep levels in semiconductors is well established. A number of different experimental techniques have emerged over the years to characterize the deep levels, each with its own specific advantages and emphasis. We report here preliminary investigations on such a deep level inadvertently incorporated in green GaP light-emittin...

The origin of the non-exponential behaviour of the previously reported [2] dark capacitance transients from red-emitting GaP LED's has been investigated in detail. In particular, the effects of the electric field and the junction edge-region on hole-emission from the 0.75 eV level has been studied. A small apparent electric-field enhancement of the...

Dark transient capacitance measurements have been used to investigate the deeplevel content of red GaP LEDs. Thermal emission rate data is obtained and the 0.75 eV killer centre is identified from the signature.

Thermally stimulated current measurements on Schottky barriers evaporated on bent GaP:N epitaxial layers are reported. A pronounced peak corresponding to a deep level with a binding energy of ∼0.57 eV is attributed to dislocations produced during plastic deformation.