Growth and characterization of GaSb bulk crystals with low acceptor concentration

GaSb bulk single crystals with low acceptor concentration were grown from a bismuth solution by the traveling heater method. The result is isoelectronic doping by Bi which gives a variation of the opto-electronic properties as a function of grown length as well as a pronounced microscopic segregation. Photoluminescence spectra at 4K show a decrease of the natural acceptor during growth, which is confirmed by Hall measurements. The electrical properties of this isoelectronic doped GaSb are hole concentrations and mobilities of N_A − N_D = 1.7 × 10¹⁶ cm⁻³ and μ = 870 cm²Vs at room temperature and N_A-N_D = 1 × 10¹⁶ cm⁻³ and μ = 4900 cm²/Vs at 77K, respectively. The lowest p-type carrier concentration measured at 300K is N_A − N_D = 3.3 × 10¹⁵ cm⁻³     

Characterization of very high purity InAs grown using trimethylindium and tertiarybutylarsine

The growth of high purity InAs by metalorganic chemical vapor deposition is reported using tertiarybutylarsine and trimethylindiμm. Specular surfaces were obtained for bulk 5-10 μm thick InAs growth on GaAs substrates over a wide range of growth conditions by using a two-step growth method involving a low temperature nucleation layer of InAs. Structural characterization was performed using atomic force microscopy and x-ray diffractometry. The transport data are complicated by a competition between bulk conduction and conduction due to a surface accumulation layer with roughly 2–4 × 10¹² cm⁻² carriers. This is clearly demonstrated by the temperature dependent Hall data. Average Hall mobilities as high as 1.2 x 10⁵ cm²/Vs at 50K are observed in a 10 μm sample grown at 540°C. Field-dependent Hall measurements indicate that the fitted bulk mobility is much higher for this sample, approximately 1.8 × 10⁵ cm²/Vs. Samples grown on InAs substrates were measured using high resolution Fourier transform photoluminescence spectroscopy and reveal new excitonic and impurity band emissions in InAs including acceptor bound exciton “two hole transitions.” Two distinct shallow acceptor species of unknown chemical identity have been observed.     

Investigation of iodine as a donor in MBE grown Hg1−xCdxTe

N-type Hg_1−xCd_xTe layers with x values of 0.3 and 0.7 have been grown by molecular beam epitaxy using iodine in the form of CdI₂ as a dopant. Carrier concentrations up to 1.1 × 10¹⁸ cm⁻³ have been achieved for x = 0.7 and up to 7.6 × 10¹⁷ cm⁻³ for x=0.3. The best low temperature mobilities are 460 cm²/(Vs) and 1.2 × 10⁵ cm²/(Vs) for x=0.7 and x=0.3, respectively. Using CdI₂ as the dopant modulation doped HgTe quantum well structures have been grown. These structures display very pronounced Shubnikov-de Haas oscillations and quantum Hall plateaus. Electron densities in the 2D electron gas in the HgTe quantum well could be varied from 1.9 × 10¹¹ cm⁻² up to 1.4 × 10¹² cm⁻² by adjusting the thicknesses of the spacer and doped layer. Typical mobilities of the 2D electron gas are of the order of 5.0 × 10⁴ cm²/(Vs) with the highest value being 7.8 × 10⁴ cm²/(Vs).     

Characterization of GaxIn1−xAs grown with TMIn

High quality Ga_xIn_1−xAs, lattice matched to InP, has been reproducibly grown by organometallic vapor phase epitaxy using trimethylgallium (TMGa), trimethylindium (TMIn), and AsH₃ in an atmospheric pressure reactor with no observable adduct formation. For the first time, using TMIn, room temperature electron mobilities of 10⁴ cm²/Vs and 77 K mobilities greater than 4 × 10⁴ cm²/Vs have beep obtained. Residual donor doping densities in the low 10¹⁵ cm⁻³ range have been routinely obtained. Material with excellent morphology has been grown from 540 to 670 C with the highest quality material being obtained near 650 C. The 4 K photoluminescence (PL) peak due to carbon is not seen in the material grown at higher temperatures; however, it increases dramatically as the growth temperature is lowered. This increased carbon incorporation leads to a sharp drop in the electron mobility, which exhibits a T^−0.5 behavior between 77 and 300 K. With optimum growth conditions, 4 K PL halfwidths of 4–5 meV are commonly observed. This high quality material is characterized by x-ray diffraction, PL, and Hall mobility measurements. Carbon and other impurity incorporation as a function of the growth parameters will be described.     

Time of flight experimental studies of CdZnTe radiation detectors

A time of flight technique was used to study the carrier trapping time, τ, and mobility, μ, in CdZnTe (CZT) and CdTe radiation detectors. Carriers were generated near the surface of the detector by a nitrogen-pumped pulsed dye laser with wavelength ∼500 nm. Signals from generated electrons or holes were measured by a fast oscilloscope and analyzed to determine the trapping time and mobility of carriers. Electron mobility was observed to change with temperature from 1200 cm²/Vs to 2400 cm²/Vs between 293 K and 138 K, respectively. Electron mobilities were observed between 900 cm²/Vs and 1350 cm²/Vs at room temperature for various CZT detectors. Electron mobilities in various CdTe detectors at room temperature were observed between 740 cm²/Vs and 1260 cm²/Vs. Average electron mobility was calculated to be 1120 cm²/Vs and 945 cm²/Vs for CZT and CdTe, respectively. Hole mobilities in both CZT and CdTe were found to vary between 27 cm²/Vs and 66 cm²/Vs. Electron trapping times in CZT at room temperature varied from 1.60 μs to 4.18 μs with an average value of about 2.5 μs. Electron trapping time in CdTe at room temperature varied between 1.7 μs and 4.15 μs with an average value of about 3.1 μs.     

Very high purity In0.53Ga0.47As grown by molecular beam epitaxy

Very high purity In0_0.53Ga_0.47As layers were grown by molecular beam epitaxy (MBE). Origins ofn-type impurities in undoped In_0.53Ga_0.47As grown on an InP:Fe substrate were systematically examined. The most possible origins were impurities diffusing from the InP:Fe substrate and those contained in As molecular beam. These impurities were dramatically reduced by using an InAlAs buffer layer and a growth condition of high substrate temperature and low As pressure. The lowest electron concentration of the In0_0.53Ga_0.47As layer wasn = 1.8 × 10¹³ cm_-3 with mobilitiesμ = 15200 cm²/Vs at 300 K andμ = 104000 cm²/Vs at 77 K.     

High-Mobility Ga0.47ln0.53As/lnP heterostructure by atmospheric-pressure MOVPE using cyclopentadienyl indium

Lattice-matched Ga_0.47ln_0.53As/InP heterostructure was grown by atmosphericpressure metalorganic vapor phase epitaxy reaction system using monovalent cyclopentadienyl indium. The lattice-matched heterostructure showed electron mobilities ofμ_300K= 12700 cm²/Vs at n₈= 4.2 x 10¹¹ cm^-2 and μ_77K= 108000 cm²/Vs at n₈ = 3.9 x 10¹¹ cm^-2. The uniformity in electrical properties was measured by Hall element array with 400 μm pitch. Coefficient of variation in electron mobility was 0.18%.     

Transport properties of undoped Cd0.9Zn0.1Te grown by high pressure bridgman technique

The electron drift mobility of undoped Cd_0.9Zn_0.1Te grown by high-pressure Bridgman method is measured by a time-of-flight technique. The sample shows a room temperature mobility and mobility lifetime product of 950 cm²/Vs and 1.6 × 10⁻⁴cm²/V, respectively. The mobility increases monotonically with decreasing temperature to 3000 cm²/Vs at 100 K. The dominant scattering mechanism for the electron transport is discussed by comparing with the theoretical mobility obtained by iterative solution of the Boltzmann equation.     

Growth and characterization of undoped and N-type (Te) doped MOVPE grown gallium antimonide

The growth of GaSb by MOVPE and itsn-type doping using a dimethyltellurium dopant source are investigated. The results of growth rate, morphology and Te incorporation as a function of growth parameters are given. Increasing growth temperature and V/III reactant ratio were found to reduce the Te incorporation. The lowest Hall carrier concentrations obtained at room-temperature, onp-type andn-type MOVPE GaSb are respectively:p _H= 2.2 × 10¹⁶cm⁻³ with a Hall mobility ofμ _H= 860 cm²/V.s andn _H= 8.5 × 10¹⁵cm⁻³ withμ _H= 3860 cm²/V.s. Furthermore, Hall mobilities as high as 5000 cm²/V.s were measured onn-type GaSb samples.     

利用MoO3为电极修饰层的F16CuPc/ CuPc异质结的双极性有机场效应晶体管

在本文中,我们利用钛青铜(CuPc)和氟化钛青铜(F16CuPc)作为空穴传输层和电子传输层的制备了具有异质结结构的有机场效应晶体管(OFETs)。与单层的F16CuPc晶体管相比,异质结结构的晶体管的电子迁移率从3.1×10-3cm2/Vs提高至8.7×10-3cm2/vs,然而,空穴的传输行为却没有被观测到。为了提高空穴的注入能力,我们利用MoO3对源-漏电极进行了修饰,有效地改善了空穴注入。并进一步证实了MoO3的引入使得器件的接触电阻变小,平衡了电子和空穴的注入,从而最终实现了器件的双极性传输。     

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C₆₀ as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4 cm²/Vs. Devices with pentacene showed a mobility of 0.16 cm²/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ～0.3 cm²/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.     

Si-implantation activation annealing of GaN up to 1400°C

Implant activation annealing of Si-implanted GaN is reported for temperatures from 1100 to 1400°C. Free electron concentrations up to 3.5×10²⁰ cm⁻³ are estimated at the peak of the implanted profile with Hall mobilities of ∼60 cm²/Vs for annealing at 1300°C for 30 s with an AIN encapsulant layer. This mobility is comparable to epitaxial GaN doped at a similarly high level. For annealing at ≥1300°C, the sample must be encapsulated with AIN to prevent decomposition of the GaN layer. Channeling Rutherford backscattering demonstrates the partial removal of the implant damage after a 1400°C anneal with a minimum channeling yield of 12.6% compared to 38.6% for the as-implanted spectrum. Scanning electron microscope images show evidence of decomposition of unencapsulated GaN after a 1300°C anneal and complete sublimation after 1400°C. The use of AIN encapsulation and annealing at temperatures of ∼1300°C will allow the formation of selective areas of highly doped GaN to reduce the contact and access resistance in GaN-based transistors and thyristors.     

The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm²/Vs with electron concentrations of 3 × 10¹⁷ cm⁻³, and defect densities less than 10⁸ cm⁻² is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.     

(AlGa)As grown by low pressure metalorganic vapor phase epitaxy using a N2 carrier

We have studied the growth of Al_xGa_1−xAs (0.24<x<0.34) using a N₂ carrier in low pressure metalorganic vapor phase epitaxy. Growth temperature, gas velocity, and V/III ratio were varied to achieve optimum growth conditions. Layers with excellent morphology and electrical and optical properties comparable to samples grown using standard conditions (with a H₂ carrier) can be deposited in a nitrogen ambient. Al_0.24Ga_0.76As bulk material grown on an AlAs buffer layer with a background doping of 1.3×10¹⁶ cm⁻³ showed Hall mobilities of 4500 and 2300 cm²/Vs at 77 and 300K. Photoluminescence studies at 2K revealed strong bound exciton transitions with a full width at half maximum of 5.2 meV for Al_0.29Ga_0.71AS.     

N- type doping of gallium antimonide and aluminum antimonide grown by molecular beam epitaxy using lead telluride as a tellurium dopant source

Lead telluride was used as a “captive” source of tellurium (Te) for the n-type doping of gallium antimonide (GaSb) and aluminum antimonide (AlSb)grown by molecular beam epitaxy. Controllable carrier concentrations from 1.2 x 10¹⁶ to 1.6 x 10¹⁸ cm^-3 were obtained. High room-temperature Hall mobilities of 4200 cm²/V · s were measured for the low-doped GaSb samples. In the growth temperature range of interest, doping ef-ficiencies are approximately 50% of those in GaAs. For GaSb, SIMS data show that the Te incorporation decreases significantly at growth temperatures above 500° C. How-ever, in AlSb, there is no significant reduction in the incorporation of Te up to at least 650° C. In contrast, the Te incorporation into AlSb decreases at low temperatures. There is also some evidence of surface segregation in AlSb. Contrary to other doping studies, increasing the Sb : Ga flux ratio was found to reduce the Te incorporation.     

Ambipolar organic transistors based on isoindigo derivatives

Structural and transistor properties of isoindigo derivatives are investigated. The unsubstituted isoindigo affords two polymorphs in addition to the reported brickwork structure; one has a stacking structure analogous to indigo, and another consists of nonplanar molecules. The unsubstituted isoindigo exhibits ambipolar transistor properties with the hole and electron mobilities more than 0.01 cm²/Vs, and 6.6′-diphenylisoindigo shows ambipolar transistor properties with the hole/electron mobilities of 0.037/0.027 cm²/Vs. Isoindigo derivatives with electron withdrawing groups show only electron transport, indicating that the lower limit of the HOMO level showing the hole transport is −5.7 eV.     

Sputtered growth of high mobility InN thin films on different substrates using Cu-ZnO buffer layer

This work reports the growth of c-plane textured InN thin films on Cu-ZnO buffered silicon, c-sapphire, bulk GaN and quartz substrates. A Cu-ZnO buffer layer was deposited on all the substrates before the growth of InN film. A highly c textured film was obtained on sapphire and quartz substrates. Structural properties were calculated using XRD and Raman analysis. It was observed that, induction of Cu-ZnO buffer layer reduced the lattice mismatch between Si/GaN substrates and InN film. The bandgap of the films was obtained using UV visible reflectance spectroscopy. Hall measurements show high mobility films in the range of 119–223 cm²/Vs and an electron concentration of 10¹⁹. These results are in good agreement with previous results but are first time recorded using RF magnetron sputtering. Surface topography of the films showed smooth surfaces, which are due to reduced lattice mismatch between film and the substrate.     

MOCVD Growth of InN on Si(111) with Various Buffer Layers

We report the growth of InN by metalorganic chemical vapor deposition on Si(111) substrates. It was found that the sharpest InN(002) x-ray diffraction peak could be achieved from the sample prepared on a complex buffer layer that consists of a low-temperature AlN, a graded Al _x Ga_1−x N (x = 1 → 0), and a high-temperature GaN. The resultant mobility of 275 cm²/V s thus obtained was 75% larger than that of the InN prepared on a single LT-AlN buffer layer only.     

Determination of Critical Thickness for Epitaxial ZnTe Layers Grown by Molecular Beam Epitaxy on (211)B and (100) GaSb Substrates

Cross-section electron micrographs, cathodoluminescence images, and confocal photoluminescence (cPL) images have been acquired for ZnTe layers deposited to various thicknesses on GaSb substrates with (211)B and (100) orientations. The critical thickness of ZnTe on GaSb is predicted to range between 115 nm and 329 nm, depending on the theoretical approach chosen. For ZnTe layers grown on (211)B GaSb with thickness exceeding 150 nm, dark spots and lines are present in all images. We associate these with dislocations generated at the ZnTe/GaSb interface. The discrepancy between this thickness value and a critical thickness value (350 nm to 375 nm) obtained for the (211)B orientation in a previous study is related to the distinction between the onset of misfit dislocations and the onset of significant plastic deformation. The former requires a direct imaging technique, as strain-related measurements such as x-ray diffraction do not have the resolution to detect the effects of small numbers of dislocations. For ZnTe layers on (100) GaSb, x-ray diffraction measurements indicate an abrupt change characteristic of dislocation multiplication at a thickness value in the range from 250 nm to 275 nm. High-resolution electron micrographs of the ZnTe/GaSb interface indicate that deoxidation using atomic hydrogen produces GaSb surfaces suitable for ZnTe epitaxy. cPL images of a 1.2-μm-thick lattice-matched ZnTe_0.99Se _0.01 layer grown on a 150-nm-thick ZnTe buffer layer on a (211)B GaSb substrate yield a threading dislocation density of ～7 × 10⁴ cm^?2.     

AIGaAsSb Buffer/Barrier on GaAs substrate for InAs channel devices with high electron mobility and practical reliability

InAs/AlGaAsSb deep quantum well was successfully formed on GaAs substrate and examined for two electron devices, Hall elements (HEs), and field-effect transistors (FETs). With a thin buffer layer of 600 nm AIGaAsSb on GaAs substrate, we observed high electron mobility more than 23000 cm²/Vs and extrinsic effective electron velocity of 2.2 x 10⁷ cm/s for a 15 nm thick InAs channel at room temperature. AIGaAsSb lattice matched to InAs was discussed from the view points of insulating property, carrier confinement, and oxidization rate. Reliability data good enough for practical use were also obtained for HEs. We demonstrated AIGaAsSb as a promising buffer/barrier layers for InAs channel devices on GaAs substrate, and we discussed the possible advantages of AIGaAsSb also for InGaAs FETs.