Search

Dielectric susceptibilities of the following semiconductors for electromagnetic field of zero frequency are investigated: InN, GaN, AlN, In x Ga1-x N, In x Al1-x N, InN containing oxygen, and non-stoichiometric InN. The real part of certain dielectric susceptibility is investigated by the electron band structure of the corresponding semiconductor. The matrix elements are calculated as terms between local wave functions describing the highest electron state of the valence band and the lowest electron state of the conduction band. The dielectric susceptibility and the corresponding dielectric permittivity are determined for semiconductors given above. The obtained results are compared with existing experimental data and good agreement is found.

Within the framework of the effective-mass approximation and variational approach, we present calculations of the bound exciton binding energy, due to an ionized donor, in wurtzite In x Ga 1− x N/GaN strained quantum dots (QDs), considering three-dimensional confinement of the electron and hole in the QDs and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Our results show that the position of the ionized donor, the strong built-in electric field, and the structural parameters of the QDs have a strong influence on the donor binding energy. The variation of this energy versus position of the donor ion is in double figures of milli-electron volt. Realistic cases, including the donor in the QD and in the surrounding barriers, are considered.

The influence of antisite defects in InN is analyzed theoretically using a Linear Combination of Atomic Orbitals approach. The procedure used is validated by confirming the band gaps of common binary alloy semiconductor materials. InN with N In and In N antisite defects are then analyzed and it is found that in the case of InN:N In , the excess nitrogen acts as a donor species with its level resident in the conduction band. For InN:In N , it is shown that when there is a significant density of the excess indium present as the antisite defect, tunnel optical absorption should occur in the infrared at 0.2 eV.

We have observed an “above band-gap” Urbach like characteristic for gallium nitride films (at the high energy side of the band-edge). A combination of X-ray diffraction, secondary ion mass spectroscopy and optical transmission measurements were taken for gallium nitride samples of different thickness. From this data we demonstrate that the high energy Urbach like characteristic is related to the presence of an amorphous surface oxide. It is shown to dominate the absorption spectra of thin gallium nitride samples, for which the influence of surface oxidation is strongest.

New metric system called electron metric system having basic metric constant is introduced. The connection between the electron metric system and the external metric system is defined. The symmetry relationships of the multinary semiconductor compound alloys are defined according to the electron metric system. The basic metric constant is found on the basis of diatomic tetrahedral cell. The electron wave vector in the new system is found and the electron energy states are determined. It is shown correlation between them in the multinary crystal. LCAO electron band structures of In x Al 1-x N and of In x Ga 1-x N are presented. The phenomenon tunnel optical absorption is investigated in In x Al 1-x N, in In x Ga 1-x N, in InN containing oxygen and in non-stoichiometric InN. It is found the optical absorption edges begin in energies 0.2-1.62 eV that are lower than the energy band gaps due to this phenomenon. Existence of excitons of the structure is shown in these semiconductors and it is found that the peaks of the PL spectra correspond to annihilation energies of these excitons that change in interval 0.5-1.01 eV.

Following a short history of its development, the latest advances in the physics of InN and the arguments surrounding the band-gap controversy are critically reviewed. The role of oxygen in the material is examined, with new absorption data presented for the amorphous oxynitride contribution. Assumptions regarding oxygen alloying are dispelled. The recent evidence for a 1.1–1.5 eV band-gap is examined, as well as evidence for the possibility of a ∼0.7 eV trapping centre. Data for the newly discovered nitrogen:InN alloy system, extending out to levels of 33% excess nitrogen, are discussed, as are current techniques for stoichiometry analysis. Finally it is concluded that the current analysis of InN is not yet sufficient to ascribe a known band-gap to the material.

An interferometric method has been used to measure the piezoelectric coefficient d 33 in thin, InN layers grown by remote plasma-enhanced chemical vapour deposition. The measured value of the coefficient varies from around 3.1 to 4.7 pm V −1 . Theoretical predictions for these coefficients are scarce, but these values are significantly lower than those which are available. The discrepancy arises largely from the clamping of the film by the substrate, but may also be due to defects within the material.

K.S.A.B. would like to acknowledge the support of an Australian Research Council Fellowship. We would also like to acknowledge the support of the Australian Research Council through a Large grant and a Discovery grant; the support of a Macquarie University Research Development Grant, and the Australian Institute of Nuclear Science and Engineering for SIMS access.

Metallic gallium was observed on the surfaces of GaN commercial samples following argon ion milling. SIMS measurements confirmed that the commercial GaN had approximately 0.02% bulk oxygen present. The SIMS signal was standardized using a specimen of known oxygen content, as determined by elastic recoil detection analysis using 200 MeV heavy ions of 197 Au . Despite this 2–5% oxygen was observed by XPS in the bulk of the GaN after the argon ion milling. This oxygen is believed to be from the original surface oxide that re-cycles on the GaN surface during the ion milling.

Surface oxidation and growth-derived oxygen contamination for Al0.05Ga0.95Sb films, grown by metalorganic chemical vapour deposition (MOCVD), were systematically investigated using an X-ray photoelectron spectroscopy (XPS) system with high energy resolution. The Sb 3d5/2 and O 1s peaks were well resolved, as were the Ga 3d peaks. All samples investigated show oxide layers (Al2O3, Sb2O3 and Ga2O5) on their surfaces. In particular, the percentage of aluminium oxide was very high at the sample surface compared to AlSb. Carbon incorporation was also examined. Adventitious surface carbon was high; however, in the bulk material carbon was below the detection limit of XPS and secondary ion mass spectroscopy (SIMS). These results indicate extremely low carbon content for the MOCVD growth of Al0.05Ga0.95Sb epilayers.

Cathodoluminesence (CL) studies were performed for GaSb self-assembled quantum dots grown by atmospheric pressure metalorganic chemical vapour deposition on GaAs substrates. The evolution of quantum dot size and density was examined for samples grown for different periods. The CL peaks shifted to higher energies from 0.95 to 1.05 eV as the dot growth time increased from 3 to 7 s. This trend indicates a significant size quantisation effect for partially relaxed structures.

High quality polycrystalline gallium nitride (GaN) films have been grown by remote plasma enhanced laser induced chemical vapour deposition (RPE-LICVD) on sapphire, silicon, and quartz substrates at temperatures below 600 °C. Transmission spectra of the films indicate excellent properties with band gap 3.38 ± 0.02 eV. A yellow band-to-band transition at 2.2 eV is observed. X-ray diffraction patterns reveal the (0002) wurtzite reflection at 2θ = 34.6° is dominant. Raman spectra of the films are discussed with respect to the phonon frequencies and strain-related phenomena. Compositional analysis with heavy ion Elastic Recoil Detection shows stoichiometric nitrogen to gallium ratios and relatively small amounts of incorporated oxygen.

Nitrogen and ammonia plasma containment tubes of quartz and fused alumina (Al2O3) are evaluated in terms of the oxygen contamination contributed during gallium nitride film growth by RPE-CVD. In-situ gas and plasma monitoring, and material characterisation (UV–Vis transmission measurements, X-ray diffraction and SIMS) were used to determine the contaminant chemistries for the plasmas maintained in these containment tubes. Oxygen contamination using a quartz tube is shown to be unavoidable. Al2O3 based tubes used with an ammonia plasma also supply unacceptably high levels of oxygen for GaN growth. However, we show that alumina containment tubes may be conditioned in a nitrogen plasma, so that an aluminium nitride protective layer limits the oxygen contamination.

UV Raman and absorption measurements plots are used to demonstrate the improvement for InN samples following removal of a surface oxide by 1.0 M HCl etching. An increase in Raman spectra signal strength and a reduction of the apparent band-gap by up to 50 meV is observed. The thick surface oxide is believed to have formed as a result of ex-situ exposure of the samples on removal from the growth system. The importance of target nitridation for RF sputtered material is also demonstrated.

Remote plasma enhanced-laser-induced chemical vapor deposition was used to grow gallium nitride films on zinc oxide buffer layers deposited by atomic layer epitaxy on soda lime glass. Freestanding layers of gallium nitride were processed by etching away the substrate and ZnO buffer layer. The n-type carrier mobility for the GaN on ZnO/soda lime glass was found to be similar to the highest values achieved on pure silica, and was accompanied by high carrier concentration. As-grown polycrystalline materials were recrystallized at low temperature (below the 570°C gallium nitride growth temperature). This recrystallization process greatly improved the film structure with a self-assembled multilayer structure evident in the oxygen-rich surface layer of the films that had undergone the process.

The structure and composition of indium nitride (InN) films grown by radio frequency reactive sputtering as a function of target nitridation have been investigated. X-ray diffraction shows that the films are primarily polycrystalline with preferred (0 0 2) orientation indicating the c-axis of the hexagonal InN structure perpendicular to the substrate. Scanning electron microscopy shows that films grown from non-nitrided targets are characterised by smaller grain size and rougher surfaces, with no observable structure. Films grown with pre-nitrided targets have a continuous columnar morphology with relatively even surfaces. X-ray photoelectron spectroscopy and Rutherford backscattering techniques were used to quantify the amounts of In, N and O contents.

The authors would like to acknowledge the support of a U. S. NICOP Contract, No. N00014-99-1-GO17 sponsored through the U. S. Office of Naval Research. One of the authors (K.S.A.B.) would like to further acknowledge the support of a Macquarie University Research Fellowship.

We report on optical and electrical characterisation of InN thin films prepared by RF reactive sputtering of In target with pure nitrogen gas. The resistivity of the films is in the range of 10 −3 –10 −2 Ω cm, the mobility as high as 306 cm −2 V −1 s −1 and the carrier concentration is typically in the order of 10 19 cm −3 . Optical measurements show that films are highly degenerate with band gap values within 2.0–2.1 eV. Aging and annealing treatments indicate that physisorbed oxygen is eventually chemisorbed into the InN films, converting them into an oxynitride (InON) phase. This is observed through the increase of the band gap energy by about 0.2 eV in the samples aged for 6 months and an even greater increase of 0.8 eV in the samples annealed at 400°C.

An efficient combined numerical-analytical technique is developed for calculating states of the continuum spectrum in systems with quantum wells (QWs) with an arbitrary potential shape, described by a system of coupled Schrodinger equations, e.g., hole states in semiconductor QWs. Continuum-spectrum states are found exactly using the approach similar to the scattering theory. Scattering states (the in/out-solutions) and the S-matrix for the case of multichannel scattering in one-dimensional systems with QWs are constructed, and their symmetry is determined and analyzed. The method is applied to studying the hole scattering by GaInAs-InGaAsP QWs with strained layers. The hole transmission and reflection coefficients and the delay-time energy dependence are calculated in relation to parameters of the structures and values of the transversal momentum components. In the energy range in which the channel with heavy hole conversion into a propagating light hole is closed, scattering of the heavy hole on a QW has a resonant nature.

Metalorganic chemical vapour deposition growth of GaSb self-assembled quantum dots (QDs) on GaAs substrates was optimized with respect to their geometry. The results show that the size and density of the dots can be controlled by growth temperature, growth time and the dilution of the growth precursors. The dot width of 40 nm, the height of 4 nm and the density in the order of 10 10 cm −2 can be achieved. These results are comparable with GaSb/GaAs QDs grown by molecular beam epitaxy.

Aluminum nitride insulating layers have been grown at room temperature with a film resistivity of 3.3×1016 Ω cm on mercury cadmium telluride substrates. Insulator breakdown fields of 640 MV/m were reached. Capacitance–voltage measurements with Al/AlN/Hg0.76Cd0.24Te metalinsulator-semiconductor devices demonstrate band bending at the semiconductor surface indicating that damage to the substrate was minimized during film growth. A fixed interface charge density (given here as total charge per unit area divided by the electron charge=Qss/q) of +2×1011 cm−2 and a slow interface state density of 4×1010 cm−2 were measured. The procedures for achieving these high quality insulating layers are reported. Frequency dependent dielectric constant and dielectric loss tangent measurements, carried out at room temperature and 100 K, are also presented.

GaN and AlN thin films have been grown by remote plasma enhanced chemical vapour deposition (RPE-CVD) with the assistance of ultraviolet (UV) irradiation during growth. High quality AlN insulating layers have been grown at room temperature for MIS devices. Film resistivities of up to 2.8 × 10 16 Ωcm and breakdown fields of over 1.8 MV/cm have been achieved. Preliminary results for GaN indicate severe nitrogen loss when using UV desorption with an ammonia plasma, however no nitrogen deficit is seen when using a nitrogen plasma. Optical absorption data show substantial improvement in material quality when using a nitrogen plasma in preference to an ammonia plasma for GaN RPE-CVD growth.

AlxGa1−xSb films in the regime 0×0.25 have been grown by metalorganic chemical vapor deposition on GaAs and GaSb substrates using TMAl, TMGa, and TMSb precursors. We report growth conditions and film properties, including the effect of V/III ratio and growth temperature on electrical and optical properties. Growth temperatures in the range of 520°C and 680°C and V/III ratios from 1 to 5 have been investigated. All epilayers grown exhibit p-type behavior. The mobility decreases and the carrier concentration increases sharply when a small amount of Al is incorporated into GaSb. The sharp cutoff and Fabry-Perot oscillations of the transmission spectra of the AlGaSb layers confirm the high quality of the films. The principle photoluminescence features observed are attributed to bound exciton and donor-acceptor transitions with FWHM comparable to the best values reported elsewhere.

Poor film adhesion to mercury cadmium telluride is a problem of general concern because of the low film deposition temperatures (

Metalorganic chemical vapour deposition grown GaSb films on GaAs have been optimised for combined morphology, electrical and optical properties by variations in substrate temperature and V/III ratio. The transport properties of the films grown at 540°C depend on precursor ratios with a V/III ratio of 0.72 providing hole mobilities of around 500 cm 2 V −1 s −1 and hole concentration about 3×10 16 cm −3 . For higher temperatures, the mobility decreases and the carrier concentration increases. Changing the V/III ratio for 540°C growth results in slight mobility loss in Sb-rich conditions and severe degradation in excess Ga conditions. Comparison of experimental and theoretical temperature dependencies of mobility shows good agreement, while differences between measured hole and inferred total impurity concentrations suggest the contribution of deeper compensating impurities. Films grown at lower temperatures were found to have very good optical quality and bandgaps equal to the bulk value of of 0.72 eV. Above 500°C, the band tailing starts to develop, when substantial disorder becomes apparent at 560°C and above. This disorder is also observed in the Raman spectra of films grown at these temperatures. For the V/III ratio the highest optical quality was obtained for films grown with V/III=2.0. The excitonic features in absorption are observed for the first time in GaSb.

The polymer polyvinylidene fluoride and its copolymers with trifluoroethylene are ferroelectric; they have switchable polarisation and display a Curie transition. These polymers are readily produced in the form of films with thickness down to 50 nm by conventional spin-casting. The properties of these very thin films are remarkably similar to those of commercial films with thickness of 10 μm or more. The dielectric constant is virtually unchanged down to 100 nm. Hysteresis loops are readily obtained, which indicate a coercive field of around 50 MVm−1, meaning that the films can be poled with voltages of the order of 10 V. The pyro-electric and piezoelectric coefficients are also similar in magnitude to those in thicker films and, when measured with a varying bias field, display a field dependence which reflects the underlying ferroelectric switching. The films show signs of fatigue after repetitive switching, with the polarisation dropping to half its normal value after a few thousand cycles. The...

Metalorganic chemical vapour deposition was used to compare initial growth and film evolution of GaSb on GaAs, Ge, InAs and GaSb under identical conditions. These substrate-layer pairs allow the effects of lattice mismatch and chemical compatibility to be differentiated. GaSb forms self-organised islands on GaAs and Ge, where mismatches are each about 8%, but not on closely lattice matched InAs and GaSb. Substantial differences were observed, however, between islands grown on Ge and GaAs, the former yielding fewer but larger islands than the latter. For example growth for 30 s under optimised conditions gave typical island lateral dimensions of 150 nm with heights around 35 nm on GaAs, but respective values of 250 nm and 70 nm on Ge. The volumetric growth rates suggest that nucleation in the GaSb/Ge system is slower than previously reported island growth rates in the GaSb/GaAs system. Thicker layers grown on GaAs, GaSb and InAs show similar elongated pyramidal surface features with clearly visible facets, whereas growth on Ge results in a much smoother but irregular surface. Both of these characteristics are attributed to a closer chemical similarity in GaSb/GaAs than in the GaSb/Ge system.

Laser-assisted chemical vapour deposition (LCVD) has become an attractive technology in the electronic industry over the past decade. One of the key benefits of LCVD is that instead of using the thermal energy, LCVD takes advantage of photon energy to induce chemical reactions between reactants and deposit fllms. In this work we are using the photolytic process in which the ArF 193 nm excimer laser beam is absorbed by the precursor gases. The deposition of GaN was carried out at temperatures around 600°C. Low growth temperatures, between 470°C and 680°C, make possible the use of a range of thermally unstable substrates, such as silicon, gallium arsenide, quartz glass, in addition to sapphire and SiC. GaN films grown by LCVD with a range of ammonia flow rates show a systematic variation in electrical parameters. The Hall mobility increases proportionally with flow rate, from 55 cm 2 /(V s) to 90 cm 2 /(V s), saturating at 95 cm 2 /(V s) at flow rates above about 100 ml/min. The electron carrier concentration decreases monotonically and reproducibly from 10 17 cm-3 to 6 x 10 14 cm-3 over the flow rate range 0 to 100 ml/min. The sharp increase in the Hall mobility indicates that the electron scattering inside the film has been reduced. Nitrogen vacancies are therefore assumed to be reduced, as growth conditions are not changed other than by the addition of excess nitrogen via the ammonia plasma. Electron concentration continues to fall beyond mobility saturation, suggesting that the reduction in nitrogen vacancy concentration thought to be re8ponsible for large n-type autodoping, is accompanied by compensation, possibly by increasing the density of other defects caused by ion damage.

The effective d33 of sub-micron piezoelectric polymer films, deposited on glass substrates, has been measured using an optical interferometer. The polymers used were polyvinylidene fluoride (PVDF) and copolymers of PVDF with trifluoroethylene (TrFE). The interferometer measures movement of the polymer surface in response to an applied electric field, and is easily able to detect motion of a picometre. In this situation, the rigid glass substrate effectively clamps the soft polymer film in the lateral direction. Nevertheless, effective d33 values approaching 10 pm Vhave been observed. One advantage of such thin films, is the ability to pole them with quite low voltages. For example, a 100 nm film can be poled by 10 volts. The low voltage required to achieve poling in such thin films means that the poling voltage can be left permanently connected, thus maintaining the piezoelectric activity. By adding a periodic bias voltage to the driving signal, it is possible to observe hysteresis in the piezoelectric coefficient corresponding to the polarization hysteresis loop. The piezoelectric coefficient shows an abrupt change in sign at the coercive field. corresponding to the reversal of the net polarization.

We present the results of optical, steady-state and time-resolved studies of photoluminescence and photoluminescence excitation in high-quality Al 0.3 Ga 0.7 As/GaAs quantum wells in which the presence of large (larger than the exciton radius) atomically flat islands can be inferred, identical to the case of interrupted MBE growth. Migration of excitons towards lower-lying energy states induced by local potential fluctuations and/or progressive localisation has been revealed and the transition rate between quantum well regions 24 to 25 monolayers thick has been derived to be 290 ps −1 .

Low-temperature excitonic effects in a high-quality AlGaAs/GaAs single quantum well (25 monolayers wide) structure grown without growth interruptions at the interface were studied using steady-state, time-resolved and photoluminescence excitation spectroscopy. Heavy-hole free exciton and impurity (neutral donor) bound exciton emissions, associated with quantum well regions (islands) 24 and 25 monolayers wide, have been resolved and identified using excitation energy resonant with the heavy-hole exciton energy. Interisland exciton migration, until now reported only for growth interrupted structures, is observed, indicating that islands formation is not a specific property of growth interrupted structures. We estimate the rate of transfer of free excitons between regions of width 24 monolayers and 25 monolayers to be 290 . We show that the relative intensity of free-to-bound excitonic emissions strongly depends on interface morphology and that enhanced bound exciton recombination is observed under some conditions. The relevant mechanisms of such enhanced bound exciton recombination are proposed.

We present results of photoluminescence and cathodoluminescence measurements of strained undoped In0.15Ga0.85As/GaAs and In0.15Ga0.85As/Al0.15Ga0.85As quantum well structures, designed to throw light on the current controversy over light-hole band alignment at low In content. We compare these data with theoretical calculations of the confined state energies within the eight band effective mass approximation. Our analysis shows that for In0.15Ga0.85As/GaAs, the observed two transitions are consistent with either type I or type II alignment of the light hole band for band offset ratios within the accepted range. In the case of In0.15Ga0.85As/Al0.15Ga0.85As, however, our results clearly indicate type II alignment for the light hole band. We derive the band offset ratio Q, defined here as Q = δEc/δEg where δEc is the conduction band offset and δEg is the bandgap difference between the quantum well and the barrier in the presence of strain, for the In0.15Ga0.85As/Al0.15Ga0.85As system to be Q = 0.83 and discuss it in the context of the common anion rule.

We report results of electrical characterisation of MIS diode structures with Al metallisation, an insulating layer of low temperature grown (LT) GaAs on a n+ GaAs substrate. The I–V characteristics at varying temperatures between 120 K and 285 K and a.c. conductivity measurements between 1 kHz and 13 MHz at room temperature were examined. The I–V characteristics were analyzed through the dependence of the diode ideality factor on temperature. Values between 2.8 and 4 were obtained, typical of tunneling through the depletion potential barrier in the n+ GaAs at the interface. The high voltage part of the forward I–V characteristics indicates a low and weakly temperature dependent series resistance of the diode, associated with carrier transport through LT-GaAs, characterised by an activation energy of 32 meV. The a.c. dynamic resistance in the high voltage part of the I–V characteristics decreases as a function of frequency and between 2 MHz and 13 MHz approximately follows ωs dependence with s = 0.65. These results are consistently interpreted as the result of the transport of injected carriers through the LT GaAs layer via a hopping mechanism involving defects.

Results of optical measurements on a multiple asymmetric quantum-well device in the n-i-n configuration that demonstrate for the first time its applicability as a light modulator are presented. The contrast ratio at the fundamental heavy hole exciton resonance is close to 2 for small operating voltage of around 3.5 V. Dual wavelength operation was proved possible with a weaker contrast ratio at 1.68 eV. The energy levels and the absorption coefficient are calculated in the envelope function approximation within the eight-band k/spl times/p model with electric field-dependent transition energies in agreement with theory. Weak deviations from the step-like character of the calculated absorption spectrum are explained by the wavevector dependence of the optical matrix element.

Filtered ultraviolet (UV) light was used to desorb impurities present at the growth front of aluminium nitride prepared by remote plasma enhanced chemical vapour deposition at room temperature. Trimethylaluminium and trimethylamine alane were used as metalorganics. Unfiltered ultraviolet light from a mercury arc lamp was found to inhibit film growth entirely; a glass plate with a short wavelength 10% transmission cutoff of 330 nm was therefore used as a UV filter. Substantial improvements in the insulating properties of the AlN films were observed for films grown with the filtered UV light present. Improvements in film morphology and an increase in breakdown fields were also observed. Further experiments with pyrex filtered UV light with a short wavelength 10% transmission cutoff of 283 nm were carried out, though better results were achieved with glass filtering.

Aluminium nitride thin films grown at room temperature on degenerate silicon (conducting) substrates have been studied using XPS. The hydrolysis layer at the surface of the AIN was examined using valence band measurements, and the effect of 5 kV argon ion milling used to remove the hydrolysis layer was scrutinized using angle-resolved XPS. The N/Al ratios found from the angle-resolved measurements indicate nitrogen depletion from the surface of the milled samples, whereas O/Al ratios indicate no such depletion of oxygen. After argon ion milling, carbon uptake from the ultrahigh vacuum analysis chamber was found to be significant.

The effect of the \ensuremath{\Gamma}- and X-band electrons in the ${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As/AlAs/GaAs double-barrier quantum well (DBQW) is investigated by a microscopic empirical pseudopotential calculation. The DBQW structure used in the calculation is designed as a 3--5-\ensuremath{\mu}m quantum-well infrared photodetector with an associated transition energy of 313 meV. DBQW tunneling transmission via \ensuremath{\Gamma}- and X-like states as a function of electron energy and applied voltage are described and compared to that in a single-barrier AlAs/GaAs quantum well. The dark current is simulated by the confined ground-state electron tunneling out of the well. We find that, at high-bias voltage, tunneling via X-like states increases the current by a few orders of magnitude. We have also varied the additional barrier thickness and found that for a very thin (20 \AA{}) additional barrier DBQW, the excited-state electrons are not blocked by the \ensuremath{\Gamma}-band barrier, and may give a high photocurrent without the assistance of the X band, although the dark current also increases. \textcopyright{} 1996 The American Physical Society.

Gallium nitride films have been grown by microwave plasma assisted laser induced chemical vapor deposition at about 550°C. Trimethylgallium and ammonia served as group II and group V sources, respectively. Ammonia was introduced into the reaction chamber by two separate lines. In one of the lines, ammonia first passed through a microwave cavity where it could be ionized into a plasma, before being delivered to the chamber. An ArF excimer laser (193 nm) was used to photodissociate the ammonia and trimethylgallium introduced through the other line. The room temperature electron concentration and Hall mobility of the films were measured to be in the range 10 15 –10 16 cm −3 and up to 200 cm 2 /Vs, respectively. Strong room temperature near-band photoluminescence detected from the films indicated their good optical quality. The photoconductive decay of photocarriers was also investigated.

Multivariable screening and response surface experiments have been performed to model ohmic contact resistance (Rc) of a Ni–Ge–Au ohmic metal process for n+ GaAs-based high electron mobility transistors (HEMTs). Seven variables were examined via a fractional factorial screening experiment to rank the effects of each process variable. The results of the screening experiment indicated that the most significant variables were total Ge and Au evaporated thickness, Ge-to-Au ratio, and the post-alloy cooling time. Response surface experiments were designed around these three variables to examine the first- and second-order effects. The results enabled the development of an empirical model of ohmic contact resistance from which a new low value of 0.03 ± 0.03 Ω · mm (one-sigma) was predicted. Twenty confirmation runs on the new process indicated an average Rc of 0.06 ± 0.02 Ω · mm (one-sigma), with a range of 0.02 Ω · mm to 0.11 Ω · mm, a reduction from the previous average process value of 0.14 ± 0.07 Ω · mm (one-sigma).

We report on the effects of hydrogenation of InSb GaAs epilayers. The as-grown MOCVD InSb is p-type with an acceptor activation energy of 16 meV. The 16 meV impurity, however, is fully passivated during protonation in a hydrogen microwave plasma, revealing a shallower, 2 meV acceptor which appears unaffected by further processing. The material remains p-type after hydrogenation and the Fermi level calculations reveal that the majority carriers over the entire temperature range are holes. The electron to hole mobility ratio is sufficiently large however, that the Hall data shows evidence for an apparent type conversion.

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH3) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH3 and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH 3 TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH 3 TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH3.

We report the specific contact resistance at zero bias, Rc, and the barrier heights of rapidly thermally alloyed AuGe contacts on 1017 cm−3 n-GaSb. An alloying step of 1 min at about 300°C provides a minimum contact resistance of about 0.4 Ωcm2. Addition of Ni in the form of a AuNiGe process has little effect on Rc, but reduces the minimum alloying temperature to about 250°C. The further step of an added gold overlayer in the Au AuNiGe process further reduces the minimum alloying temperature to about 200°C. For contacts on 1017 cm−3 p-GaSb, TiAu proved superior in conductance compared to Ti and Cr. Alloying at 300°C slightly improved the quality of the contacts. The optimized processes provide compatible bipolar ohmic metallization for minimum thermal budget.

A number of authors make reference to “negative capacitances” observed during impedance measurements of metal-semiconductor and other semiconductor device structures at sufficiently low frequencies for parasitic inductances to be assumed negligible. Often, these negative capacitances are attributed to physical phenomena associated with the devices being measured. It is demonstrated in this paper that many such interpretations incorrectly neglect the importance of parasitic series inductances at low frequencies when device conductance is large, as in a forward biased Schottky barrier, or when large device leakage currents are present. Simulations of experimental data for a Schottky diode show that typical values of probe lead and other instrumental inductance may be sufficient to provide an instrumental explanation for the apparent effect.

Photoreflectance measurements of multilayered Al x Ga 1−x As GaAs semiconductor structure in the regime of high modulating laser powers are reported. The main photoreflectance feature observed at low powers at about 1.4 eV is related to Franz-Keldysh oscillations caused by electric field at the Al x Ga 1−x As GaAs interface. The value of electric field deduced from these oscillations is in agreement with that determined from calculation using the accepted value of the Al x Ga 1−x As GaAs band offset and the doping level in the GaAs layer. On increasing the power of the modulating laser, the spectrum changes its character and after decomposition reveals a feature at the energy corresponding to GaAs bandgap. The intensity of this line increases with the modulating laser power, moreover the line is in phase with the Franz-Keldysh oscillations. These observations are consistent with the assignment of the line to the photoreflectance at the underlying GaAs buffer/GaAs substrate interface. This finding underlines the altered character of the photoreflectance spectra at high light intensities, compared to the standard low intensity regime.

We investigated theoretically the electron mobility in low temperature grown GaAs (LT GaAs), based on the ‘internal Schottky barrier’ model, utilizing a Monte Carlo method. We used a novel approach to overcome the problem of inhomogeneity of the internal electric field due to the presence of precipitates. The behaviour of mobility as a function of temperature in LT GaAs for precipitate concentrations in the range 1015–1017 cm−3 is presented. This is the first result of mobility calculation in LT GaAs, based on the Monte Carlo method, reported to date. The dependence of mobility on the doping concentration is also presented.

We have studied both the molecular-beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) grown AlGaAsAlAsGaAs double barrier multiple quantum well structures for 3–5 μm infrared (IR) photodetector application. The intersubband absorptions of these samples were measured at the Brewster angle as well as through a multipass 45° wedge waveguide. In the former case, we have also studied the polarisation dependence of the IR transmission. The MBE grown sample has an absorption at about 3.1 μm, while the absorption peak for the MOCVD sample is shifted slightly, depending on the wafer location. The low temperature photoluminescence peaks show a significant shift when the probe position is moved across the MOCVD wafer along the gas flow direction, owing to thickness non-uniformity. This variation was also confirmed by double crystal X-ray diffraction data. Theoretically, we have carried out a bound and quasibound state energy calculation for these structures as a function of well width and related them to the experimental results. In addition, the IR photoresponse for the MBE grown sample at about 80 K has been measured for IR photodetector application.

Liquid phase epitaxial gallium arsenide layers, greater than 200 μm thickness and with a low net carrier concentration (NA,D ≈ 1013 cm−3) have been grown in a silicia growth system with silica crucibles. Analysis of electrical and chemical defects was carried out using capacitance-voltage (CV) measurements, deep level transient spectroscopy (DLTS) and secondary ion mass spectroscopy (SIMS). Details of the growth procedure are given and it is shown that silicon incorporation in the growth layer is not suppressed by the addition of ppm levels of oxygen to the main hydrogen flow.