Internal photoemission (IP) correlates with processes in which carriers are photoexcited and transferred from one material to another. This characteristic allows characterizing the properties of the heterostructure, for example, the band parameters of a material and the interface between two materials. IP also involves the generation and collection of photocarriers, which leads to applications in the photodetectors. This review discusses the generic IP processes based on heterojunction structures, characterizing -type band structure and the band offset at the heterointerface, and infrared photodetection including a novel concept of photoresponse extension based on an energy transfer mechanism between hot and cold carriers.
We report a wavelength threshold extension, from the designed value of 3.1 to 8.9 μm, in a p-type heterostructure photodetector. This is associated with the use of a graded barrier and barrier offset, and arises from hole–hole interactions in the detector absorber. Experiments show that using long-pass filters to tune the energies of incident photons gives rise to changes in the intensity of the response. This demonstrates an alternative approach to achieving tuning of the photodetector response without the need to adjust the characteristic energy that is determined by the band structure.
We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.
The use of GaMnAs as the emitter of an internal-photoemission (IPE) photodetector is studied. As a result of significantly high concentration of holes, GaMnAs-based IPE detector resembles a Schottky-barrier detector, which has the higher absorption and thus enhanced spectral response by comparing with the previously reported p-type GaAs/AlGaAs detectors. A GaMnAs/AlGaAs detector is expected to fully cover the 3 – 5-µm range of detection. The theoretical value of the responsibility is 0.0133 A/W at 3.5 µm.
The band offset at the interface of a heterojunction is one of the most important parameters determining the characteristics of devices constructed from heterojunction. Accurate knowledge of band offsets and their temperature dependence will allow one to simulate and predict the device performances. We present a temperature-dependent internal-photoemission spectroscopy (TDIPS) for studying the band offsets. Applications of the TDIPS into III–V and II–VI materials are discussed.
Free-carrier effects in a p-type semiconductor including the intra-valence-band and inter-valence-band optical transitions are primarily responsible for its optical characteristics in infrared. Attention has been paid to the inter-valence-band transitions for the development of internal photoemission (IPE) mid-wave infrared (MWIR) photodetectors. The hole transition from the heavy-hole (HH) band to the spin-orbit split-off (SO) band has demonstrated potential applications for 3–5 um detection without the need of cooling. However, the forbidden SO-HH transition at the C point (corresponding to a transition energy Δ0, which is the split-off gap between the HH and SO bands) creates a sharp drop around 3.6 um in the spectral response of p-type GaAs/AlGaAs detectors. Here, we report a study on the optical characteristics of p-type GaAs-based semiconductors, including compressively strained InGaAs and GaAsSb, and a dilute magnetic semiconductor, GaMnAs. A model-independent fitting algorithm was used to derive the dielectric function from experimental reflection and transmission spectra. Results show that distinct absorption dip at Δ0 is observable in p-type InGaAs and GaAsSb, while GaMnAs displays enhanced absorption without degradation around Δ0. This implies the promise of using GaMnAs to develop MWIR IPE detectors. Discussions on the optical characteristics correlating with the valence-band structure and free-hole effects are presented.
InAs/GaAs quantum dot (QD) and dots-in-well (DWELL) infrared photodetector (QDIP) based on p-type - valence-band intersublevel hole transitions are reported. Two response bands observed at 1.5–3 and 3– 10 lm are due to optical transitions from the heavy-hole to spin–orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. The p-type hole response displays a well-preserved spectral profile (independent of the applied bias) observed in both QD and DWELL detectors. At elevated temperatures between 100 and 130 K, the DWELL detector exhibits a strong far-infrared responses up to 70 lm. An external quantum efficiency of 17% is demonstrated. The studies show the promise of p-type QDs for developing infrared photodetectors.
The GaAs/AlGaAs material system is believed to have a band offset without remarkable influence from the interface. We report here probing a slightly higher (5–10 meV) valence-band offset at the GaAs-on-Al0.57Ga0.43As interface compared to that of the Al0.57Ga0.43As-on-GaAs interface, by using internal photoemission spectroscopy. This indicates the non-commutativity of band offset for GaAs/AlGaAs, i.e., the dependence on the order of the growth of the layers. This result is consistently confirmed by observations at various experimental conditions including different applied biases and temperatures.
Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DWELL) photodetector is reported. This detector displays expected mid-infrared response (from 3 to 10 um) at temperatures below 100 K, while strong THz responses up to 4.28 THz is observed at higher temperatures (100–130 K). Responsivity and specific detectivity at 9.2 THz (32.6 um) under applied bias of 0.4 V at 130 K are 0.3 mA/W and 1.4 x 106 Jones, respectively. Our results demonstrate the potential use of p-type DWELL in developing high operating temperature THz devices.
The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a wellpreserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The biasindependent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.
The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit λc that is related to the activation energy (or bandgap) of the semiconductor structure (or material) (Δ) through the relationship λ = hc/Δ. This spectral rule dominates device design and intrinsically limits the long-wavelength response of a semiconductor photodetector. Here, we report a new, long-wavelength photodetection principle based on a hot–cold hole energy transfer mechanism that overcomes this spectral limit. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. This enables a very long-wavelength infrared response. In our experiments, we observe a response up to 55 µm, which is tunable by varying the degree of hot-hole injection, for a GaAs/AlGaAs sample with Δ = 0.32 eV (equivalent to 3.9 µm in wavelength).
We report a study of internal photoemission spectroscopy (IPE) applied to a n-type Hg1xCdxTe/Hg1yCdyTe heterojunction. An exponential line-shape of the absorption tail in HgCdTe is identified by IPE fittings of the near-threshold quantum yield spectra. The reduction of quantum yield (at higher photon energy) below the fitting value is explained as a result of carrier-phonon scatterings. In addition, the obtained bias independence of the IPE threshold indicates a negligible electron barrier at the heterojunction interface.
We report the incorporation of a long-wavelength photovoltaic response (up to 8 lm) in a short-wavelength p-type GaAs heterojunction detector (with the activation energy of EA 0:40 eV), operating at 80 K. This wavelength-extended photovoltaic response is enabled by employing a non-symmetrical band alignment. The specific detectivity at 5 lm is obtained to be 3.5 1012 cm Hz1=2 /W, an improvement by a factor of 105 over the detector without the wavelength extension.
Uncooled split-off band infrared detectors have been demonstrated with an operational device response in the 3–5 um range. We have shown that it is possible to enhance this device response through reducing the recapture rate by replacing one of the commonly used flat barriers in the device with a graded barrier, which was grown using a “digital alloying” approach. Responsivity of approximately 80 lA/W (D* ¼ 1.4 108 Jones) were observed at 78 K under a 1 V applied bias, with a peak response at 2.8 um. This is an improvement by a factor of 25 times compared to an equivalent device with a flat barrier. This enhancement is due to improved carrier transport resulting from the superlattice structure, and a low recapture rate enabled by a reduced distance to the image force potential peak in the graded barrier. The device performance can be further improved by growing a structure with repeats of the single emitter layer reported here.
Band gaps of semiconductor materials are reduced due to band gap narrowing (BGN). Photoluminescence measurements on GaAs and AlGaAs thin films revealed a dependency of incident light intensity, and temperature in BGN in addition to the doping density. As a result, the valence band offset of p-doped GaAs/ AlGaAs heterojunctions were reduced under illumination and high temperatures. We present evidence of incident-light-intensity causing barrier reduction at temperature >50 K causing zero valence band offsets in low-barrier heterostructures such as p-GaAs/Al0.01Ga0.99As, in addition to the dark-current increase by thermal excitations, causing the device failure at high temperatures.
An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 lm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1:8 109 cm Hz1=2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.
We employ internal photoemission spectroscopy to directly measure the valence-band Van Hove singularity, and identify phonons participating in indirect intervalence-band optical transitions. Photoemission of holes photoexcited through transitions between valence bands displays a clear and resolvable threshold, unlike previous reports of interband critical points which become obscure in doped materials. We also demonstrate the enhancement of optical phonon-assisted features primarily contributing to the photoemission yield. This result is evidence of the relaxation of photoexcited hot holes through intravalence-band scatterings in heterojunctions, which contrast with intervalence-band scatterings in bulks.
We use internal photoemission spectroscopy to determine the conduction band offset of a type-II InAs/GaSb superlattice (T2SL) pBp photodetector to be 0:004 ð60:004Þ eV at 78 K, confirming its unipolar operation. It is also found that phonon-assisted hole transport through the B-region disables its two-color detection mode around 140 K. In addition, photoemission yield shows a reduction at about an energy of longitudinal-optical phonon above the threshold, confirming carrier-phonon scattering degradation on the photoresponse. These results may indicate a pathway for optimizing T2SL detectors in addition to current efforts in material growth, processing, substrate preparation, and device passivation.
The optical response of pristine and FeCl3-intercalated epitaxial graphene has been studied over the temperature range from 11 K to 296 K. The far-infrared (FIR) Drude conductivity of pristine graphene rises with increasing temperature, opposite to the behavior of intercalated graphene. This is a result of intercalation-induced p-type doping compensating the intrinsic n-doping in epitaxial graphene. Temperature-dependent Drude parameters are obtained by fitting the FIR response. This study demonstrates the influence of temperature variation on the optical properties of graphene, which should be a vital factor to be considered for graphene-based device applications.
The temperature-dependent characteristic of band offsets at the heterojunction interface was studied by an internal photoemission (IPE) method. In contrast to the traditional Fowler method independent of the temperature (T), this method takes into account carrier thermalization and carrier/dopant-induced band-renormalization and band-tailing effects, and thus measures the band-offset parameter at different temperatures. Despite intensive studies in the past few decades, the T dependence of this key band parameter is still not well understood. Re-examining a p-type doped GaAs emitter/undoped AlxGa1−xAs barrier heterojunction system disclosed its previously ignored T dependency in the valence-band offset, with a variation up to ∼−10−4 eV/K in order to accommodate the difference in the T -dependent band gaps between GaAs and AlGaAs. Through determining the Fermi energy level (Ef), IPE is able to distinguish the impurity (IB) and valence bands (VB) of extrinsic semiconductors. One important example is to determine Ef of dilute magnetic semiconductors such as GaMnAs, and to understand whether it is in the IB or VB.
The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 µm. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.
A theoretical analysis to improve the quantum efficiency of detectors sensing in multiple spectral bands is presented. The effective coupling between the incoming light and multiple absorbing regions for simultaneously improving the multi-band absorption efficiency is obtained by using resonant-cavity structures. An optimized cavity with only a Au bottom reflector gives rise to an enhancement factor of 11 in absorption compared to the conventional detector without the cavity. Further improvement, by a factor of 26, can be attained with the aid of a dual-band Bragg reflector placed at the top. The resulting multi-band resonant-cavity detector increases the response in three out of four detection bands contributing to the spectral range from visible to long-wave infrared (IR). The optimized detector is capable of serving multiple purposes, such as regular IR detection for atmospheric windows, gas sensing, and for optical communications.
The contributions of inter-valence band (IVB) transitions to the dielectric function (DF) by free holes among the split-off (so), light-hole (lh) and heavy-hole (hh) bands were investigated. A model was developed to determine the DF of two p-type semiconductors, GaAs and Ge1ySny with the Zinc-blend and Diamond crystal structures, respectively. The IVB transitions dominate the spectral range between 0.1–1eV with respect to the spin-orbit splittings between so-hh and lh-hh bands. In conjunction with inter-band transitions, free-carrier and lattice absorption, a complete DF model allows the determination of optical constants with improved accuracy in the spectral range covering both ultraviolet and infrared regions. The model should be applicable to most of the group III-V and IV materials since their valence band structures resemble the ones under investigation.
We report on the InAs quantum dot lasers grown by gas source molecular-beam epitaxy, respectively, on GaAs and InP substrates. Room temperature continuous-wave operation was achieved for both InAs/GaAs and InAs/InP quantum dot lasers, respectively, at 1:10 mm and 1:5421:70 mm wavelength region. More than 50 mW optical power was collected from one facet of the InAs/GaAs quantum dot lasers at 20 1C, while for InAs/InP quantum dot lasers the maximum output power was measured as 30 mW. For InAs/InP material system, by increasing the layer thickness of deposited InAs from 3.0 to 3.5 monolayers, the lasing wavelength can be extended from 1:521:6 mm to 1:621:7 mm. Moreover, a tunable quantum dot external cavity laser was demonstrated, utilizing the broad gain profile of InAs quantum dots.
Increasing the operating temperature of infrared detectors is a prime importance for practical applications. The use of split-off band transitions has been proposed for high operating temperature infrared detectors. Initial results showed increasing the potential barrier for free carrier emission has led to increases in operating temperature from 150 K for a detector with an 8 lm threshold to room temperature for detector with a 4 lm threshold. However, these detectors showed a low responsivity due to the capture of carriers in each emitter. A proposal was made to use graded barriers with an offset between the barriers on the two sides of an emitter as a method of reducing the capture in the emitters. Two GaAs/AlGaAs samples with a single graded barrier (Al fraction x = 0.57 to 1 and 0.45 to 0.75, respectively) were used to test the effects. The sample with the lower barrier show responsivity increased by a factor of 10 or more compared to the higher graded barrier sample and detectors without the graded barrier. The higher graded barrier sample, space charge build up causes almost all potential drop across the first barrier, and hence reduces the response. Based on the modeling it is believed that this effect will be greatly reduced in detectors with multiple periods of graded barriers and emitters, allowing the full gain effects of the graded barriers to be realized.
Hole transitions from the heavy-hole hh to the light-hole lh band contributing to the 4–10 m response range are reported on p-GaAs/AlGaAs detectors. The detectors show a spectral response up to 16.5 m, operating up to a temperature of 330 K where the lh-hh response is superimposed on the free-carrier response. Two characteristic peaks observed between 5–7 m are in good agreement with corresponding energy separations of the lh and hh bands and thus originated from lh-hh transitions. Results will be useful for designing multi-spectral detection which could be realized on a single p-GaAs structure.
An analysis of dark current mechanisms has been performed on high-operating-temperature (up to 330 K) split-off (SO) band p+-GaAs/AlGaAs heterojunction infrared detectors (3–5 µm). In contrast to conventional 1-D current models due to carrier transport based on tunneling and/or thermionic emission mechanisms, a 2-D electrical model is used to explain nonuniformity degradation of zero-bias differential resistance (R0A) with temperatures as measured on SO detectors. The 2-D characteristic of carrier transport could have the limitation on high-temperature performances of detectors and, hence, needs optimizing. A theoretical model shows that this 2-D effect can be reduced by structural modifications such as using smaller mesa sizes, higher doping of the p+-GaAs layer, and a higher potential barrier that prospectively provides better electrical uniformity for SO detectors working at high temperatures.
InAs/InP(100) quantum dot lasers with lasing wavelength insensitive to operation temperature are demonstrated. Very high wavelength stability of 0.088 nm/K in the temperature range 80– 310 K is obtained, which is 6.2 times lower than that of the reference quantum well laser.
The temperature and laser pumping power dependent photoluminescence (PL) has been investigated on strained single quantum wells (SSQWs) using inductively coupled-plasma (ICP) etching method. Significant improvement of PL performances was observed in the plasma-etched SSQW in comparison with the as-grown samples. The enhancement factor increases with the increasing of temperature, but decreases with the increasing of pump power. At lower temperatures, the larger full width at half maximum (FWHM) and the blue-shift of PL peak positions for the plasma-etched SSQW may be attributed to the damage and reduced composition fluctuation within the quantum well structure aroused during plasma etching.
We report on two-color InAs/InP100 quantum dot lasers with tunable wavelength gap. Two peaks of lasing emission were observed simultaneously, while the high energy peak undergoes continuous blueshift with the increase in the injection current, and the low energy peak is somewhat fixed. Sophisticated studies of the wavelength gap as a function of the laser power prove that the two-peak lasing and shifting is not caused by the effect of Rabi oscillation. Moreover, comparison of electroluminescence and lasing spectra under different injection currents reveal the blueshift of the high energy peak is most likely related to the state-filling effect.
1.3 µm vertical-cavity surface-emitting lasers based on a novel gain media consisting of InAsP/InGaAsP strain-compensated multiple quantum wells are reported. SiO2/TiO2 dielectric thin-film pairs and wafer-bonded GaAs/Al(Ga)As distributed Bragg reflectors are used as the top and bottom cavity mirrors, respectively. The device with a 5 µm-diameter selectively etched tunnel-junction aperture exhibits submilliampere threshold current as low as 0.54 mA and single-transverse mode emission. Maximum output optical power of 1.9 mW was observed in multimode lasing devices.
The optical characteristics of directly wafer-bonded InP–GaAs heterojunctions have been investigated. By designing the bonding interface at standing-wave antinode, its influence on optical performances of bonded structures is magnified, which facilitates experimental detection using optical methods. Wavelength blueshift and reflectivity falling at the resonance mode were observed in wafer-bonded InP–GaAs heterostructures. Numerical analysis suggests that two effects involving thickness change of interfacial bonding layers and extra optical loss introduced by bonded junctions are responsible for the experimental observations, and these effects can be attenuated by lowering anneal temperatures and incorporating an InP/InGaAsP superlattice into the surface of InP-based materials. The results are useful for designing effective optical characteristics of wafer-bonded device structures. © 2009 The Electrochemical Society.
The intermixing effect on InAs0.45P0.55/InP strained multiple quantum wells (SMQWs) by inductively coupled plasma (ICP) etching and rapid thermal annealing (RTA) is investigated. Experiments show that the process of ICP etching followed RTA induces the blue shift of low temperature photoluminescence (PL) peaks of QWs. With increasing etching depth, the PL intensities are firstly enhanced and then diminished. This phenomenon is attributed to the variation of surface roughness and microstructure transformation inside the QW structure during ICP processing.
We report on the InAs quantum dots QDs laser in the 1.55 m wavelength region grown by gas source molecular-beam epitaxy. The active region of the laser structure consists of fivefold-stacked InAs QD layers embedded in the InGaAsP layer. Ridge waveguide lasers were processed and continuous-wave mode operation was achieved between 20 and 70 °C, with characteristic temperature of 69 K. High internal quantum efficiency 56% and low infinite length threshold current density (128 A/cm2 per QD layer) was obtained for the as-cleaved devices at room temperature. The lasing wavelength range between 1.556 and 1.605 m can be covered by varying the laser cavity length.
Luminescence enhancement effects are observed in the plasma-etched InAs0.45P0.55/ In0.68Ga0.32As0.45P0.55 quantum wells QWs. Characterizations of photoluminescence, atomic force microscopy, and secondary-ion mass spectroscopy reveal that surface roughening due to ion bombardment onto surface and microstructure changes resulted from Ar+ ions tunneling into the material in the plasma etching process account for the PL enhancement phenomenon. The combination of plasma etching and selective lift-off of the InP cap layer of the InAs0.45P0.55 /In0.68Ga0.32As0.45P0.55 QW structures allows us to separate the two enhancement factors, which indicates the Ar+ ions tunneling into the crystal is the dominant factor that enhances the luminescence emission of InAs0.45P0.55 /In0.68Ga0.32As0.45P0.55 quantum wells.
—An enhancement-mode ZnMgO transparent thin– film transistor (TFT) is fabricated, in which cubic-phase ZnMgO (C-ZnMgO) is used as gate insulator and hexagonal-phase ZnMgO (H-ZnMgO) is used as channel. The multilayers of C-ZnMgO and H-ZnMgO are grown on patterned indium-tin-oxide-coated glass in successive fashion at low temperature. Capacitor–voltage characteristics measured across the gate show that the H-ZnMgO channel is n-type. The C-ZnMgO isolating layer demonstrates low leakage current characteristics, i.e., 4 × 10−7 A/cm2, at a bias of 10 V. The transparent TFTs display a typical channel mobility of 1.5 cm2 V−1 s−1 and an on/off ratio of 104.
Cubic MgxZn1-xO thin films were deposited on amorphous silicon dioxide substrate by reactive electron beam evaporation (REBE) at low temperature (250 C). The characterizations of crystalline structure and morphology of the ternary films demonstrated that the cubic MgxZn1-xO films are of highly (0 0 1) orientation and have uniform surface. The cubic Mg0.83Zn0.17O film deposited on quartz demonstrates wide band gap (6.45 eV) and has very high transparency (>95%) in broad wavelength range from ultraviolet (0.3 um) to mid-infrared light (5.5 um). The refractive indices for the cubic MgxZn1-xO decrease as Mg fraction increases. The characters of low optical absorption in broad wavelength range and feasibility of changing refractive index by Mg fraction variation in the ternary MgxZn1-xO films could render potential applications in integrated optical devices.
InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) were grown using gas source molecular beam epitaxy. The luminescent properties of the SC-MQWs after thermal annealing and direct wafer-bonding onto a GaAs substrate were studied. It is shown that photoluminescence (PL) intensities of the samples are improved by a factor of 4.1 upon annealing at 620 ◦C. The luminescence intensities of the samples bonded at 580 and 650 ◦C under ∼5 MPa pressure and 35 min annealing process are comparable with that of the as-grown ones. A proposed mechanism of defects annihilation relating to the evolution of nanoscale As-rich and P-rich clusters is then used to account for the dependence of PL intensities and the full width at half-maximum of InAsP/InGaAsP SC-MQWs on annealing temperatures.
Direct wafer-bonded InP–GaAs structures were studied by cross-sectional observations using a field-emission scanning-electron microscope (FESEM) and by infrared-absorbance spectra using a Fourier-transform infrared spectroscopy. FESEM observations demonstrate that the interfaces of 560 and 580 °C bonded InP–GaAs structures are smooth and uninterrupted, while interfacial gaps appear for the samples bonded at 620 and 680 °C. However, large dimensional areas of bonding interfaces cannot be observed by FESEM because its inspection size is limited to microregions. Experimental results show that infrared-absorbance measurements can be an effective method for quality examination of bonded InP–GaAs structures. By soaking wax into poorly bonded interfaces and using its absorption characteristics at 3.383, 3.426, and 3.509 um, interfacial gaps are indirectly measured by infrared spectra. Absorbance-intensity mappings at absorption peaks were used to image poorly bonded areas. Thus the interface quality of the whole wafer-bonded sample can be seen clearly. Nonuniform pressure applied over the sample during annealing step accounts for poorly bonded interfaces. Using the improved fixture, uniformly bonded InP–GaAs structures that do not have interfacial gaps were obtained.
The microstructural properties of the MgxZn1-xO/Si(1 0 0) interface were investigated using transmission electron microscopy (TEM) and chemical states of the heterostructure were studied by high resolution X-ray photoelectron spectroscopy (XPS). By analyzing the valence band spectra of thin MgxZn1-xO/Si(1 0 0) heterostructures, the valence band offset between such Mg0.55Zn0.45O and Si(1 0 0) was obtained to be 2.3 eV. Using the cubic ternary thin films as insulators, metal–insulator– semiconductor (MIS) capacitors have been fabricated. Leakage current density lower than 3 10 x 7 A/cm2 is obtained under the electrical field of 600 kV/cm by current–voltage (I–V) measurement. Frenkel-Poole conduction mechanism is the main cause of current leakage under high electrical field.
We have grown long-period AlInP/InGaAsP strain-compensated multiple-layer heterostructures (SCMLHs) and SCMLHs combined with InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) by gas source molecular beam epitaxy. Etch pit density (EPD) for both structures are in magnitude of 105 cm2 . The increment of EPD with increase of period number is small, indicating low sensitivity of the dislocation density to the increase of period number of SCMLHs. High resolution X-ray diffraction and photoluminescence (PL) characterizations of the two structures demonstrate that crystal quality remains high due to strain compensation. Diffusion and segregation of indium were clearly observed in both InGaAsP and AlInP layers by secondary ion mass spectroscopy. As the thickness of epilayers increases In content increases, while Al decreases. PL for the structure of 20-pair AlInP/InGaAsP SCMLHs+InAsP/InGaAsP SC-MQWs shows strong luminescence and narrow line width. The strain-compensated technique can effectively suppress the formation of misfit dislocations in AlInP/InGaAsP SCMLHs although the thicknesses of epilayers are above the critical thickness of consisting materials, which may render its potential application in optoelectronic devices.
Ni-doped ZnO (ZnO:Ni) films were fabricated on Si(0 0 1) substrates by reactive electron beam evaporation at low substrate temperature. The as-grown films were then annealed in oxygen ambient at higher temperatures. X-ray diffraction (XRD) results indicated that 5 at.% Ni-doped samples are still of single phase with the ZnO-like wurtzite structure. Photoluminescence (PL) measurements of Ni-doped samples illustrated the UV-PL emission centered at about 384 nm, which is ascribed to the nearband-edge (NBE) emissions of ZnO-like band structures. The UV-PL intensity becomes stronger along with the increase of annealing temperatures and reaches a maximum magnitude after annealed at 450 8C. However, along with the further increase of annealing temperatures, UV-PL intensity diminishes again. The UV-PL intensity of 450 8C-annealed samples is 213 times stronger than that of as-grown (doped) samples, which may render potential applications in optoelectronic devices, such as UV luminescent devices.
Strained InAs0.43P0.57/InP and strain-compensated InAs0.43P0.57/In0.7Ga0.3As0.43P0.57 multiple quantum well (MQW) structures were grown by gas source molecular beam epitaxy. The observations of up to 5 satellite-diffraction peaks from the high-resolution X-ray diffraction measurements demonstrate good crystalline quality for both structures. The temperature dependence of the 1e–1hh transition energies, the line width of photoluminescence (PL) spectra and emission efficiency Z of the two quantum well structures are compared by low-temperature PL measurements. The temperature dependence of the 1e–1hh transitions of the two quantum well structures is similar to that of InAs0.43P0.57 bulk material. The thermal activation energies obtained for strain-compensated MQW are larger than those obtained for the strained one. Consequently, the PL emission efficiency decays much slower for the strain-compensated MQW than that for the strained one when temperature increases, indicating the superior temperature stability of luminescent efficiency for the strain-compensated MQW. The obtained results can be used as references to the design and fabrication of optoelectronic devices.
A gold thin film percolation system, deposited on a glass surface by the vapor deposition method, has been fabricated. By using the expansive and mobile properties of the silicone oil drop, a characteristic wedge-shaped film system with a slope of ;1025 naturally forms during deposition. The electrical conductivity of the bandlike film, i.e., the uniform part of the wedge-shaped film with a fixed thickness, is measured with the four-probe method. It is found that the hopping and tunneling effects of the films are stronger than those of the other films. The dependence between the dc sheet resistance R0 and temperature T shows that the samples exhibit a negative coefficient dR0 /dT below the temperature T*. According to our experiment, it is suggested that all the anomalous behaviors of the system should be related to the characteristic microstructure of the samples, which results from the immediate quench processes by the oil drop during deposition. The experiment indicates that the relaxation period of the microstructure of the samples may be longer than 30 min.
An Au thin film system, deposited on silicone oil surfaces by the thermal deposition method, has been fabricated and its structure as well as electrical properties has been studied. A web-shaped characteristic surface morphology of the films is observed. The dc sheet resistance R of the metal films on the liquid surfaces is measured during and after deposition in situ by the fourprobe method. The time dependence of the sheet resistance can be explained in terms of the film growth mechanism on the oil surface. The anomalous I–V characteristics of the film system can be interpreted as a competition among the local Joule heating, hopping and tunnelling effects. It is found that the dc thirdharmonic coefficient B0 and the zero-power resistance R0 satisfy the power-law relation B0 ∝ R2+w 0 and the exponent w is close to zero. This result indicates that the hopping and tunnelling effects in the samples are much stronger than those of the other film systems. We also find Im ∝ R−β 0 with β = 0.79 ± 0.08 and Im ∝ B−γ 0 with γ = 0.49 ± 0.07, where Im is the critical current. The physical origins of the phenomena are discussed.
We report the formation of large ramified gold aggregates after deposition of Au on melting glass surfaces. The growth mechanism of the aggregates is traced to a two-stage process, the first of which involves nucleation and growth of compact Au clusters. The average diameter of the clusters is around 1.2 mm, which is approximately independent of the deposition rate. In the subsequent stage, the clusters diffuse randomly on the colloid surface and the diffusion coefficient is in the range between 1027 and 1028 cm2 /s. Finally they aggregate. It is found that the structure of the aggregates strongly depends on the aggregation time.