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The chemical nature of the GaN surface combined with the bulk and surface electronic structure opens new potential application areas for this material. The nature of specific organic-GaN was developed for two cases in which the surface electronic structure of such interfaces was developed and utilized. The band alignment between n-GaN and pentacene was determined for surfaces prepared through reaction with HCl. The energy band offset was estimated through combined X-ray photoelectron spectroscopy and ultraviolet photoemission measurements. XPS measurements indicated that there was no interfacial chemical reaction. The measured valence band offset between the n-GaN and the vapor-deposited pentacene was estimated to be greater than 2 eV providing a favorable band-offset for hole-injection from the GaN layer into pentacene. The surface of a AlGaN/GaN heterojunction field effect transistor (HFET) structures was also functionalized by an adsorbed hemin layer and was shown to be sensitive to the presence of reactive species such as NO. The HFET structure provides enhanced sensitivity to changes in the surface electric field, altered through the adsorption of chemically active species either directly on the surface or through the interactions with surface functionalization.

Indium-arsenide (InAs)-based devices are promising for next generation electronic and optoelectronic applications. Improving these devices requires greater control of the InAs quantum well properties, which in part, are related to the strain induced from the substrate material, buffer layer thickness, barrier composition, doping, and the heterointerface. This report focuses on the latter and includes X-ray diffraction and Hall results from a statistical experimental design (SED), which focused on the growth sequence at the heterointerface and growth throughout the channel, and illuminates correlations between structural variations and the electronic properties in strained InAs quantum well structures produced by molecular beam epitaxy (MBE). This data suggest that as more antimony from the sublayer is incorporated in the InAs layer, a product of anion exchange between antimony surface atoms and ensuing arsenic flux atoms, the strain states vary along with InAs channel mobility.

Intrinsic sheet charge levels in unintentionally doped AlSb/InAs/AlSb quantum wells are reproducibly ∼10 12 /cm 2 . While these levels are suitable for depletion-mode operation, realizing enhancement-mode devices ultimately depends on the ability to significantly reduce sheet charge. In an effort to limit carrier accumulation in the InAs quantum well, we have studied the impact of the addition of a p-type, Be, doping plane between the channel and the cap and the modification of defect densities present in the films. Molecular beam epitaxy (MBE) process conditions including substrate temperature, Be-doping density, and doping plane spacing above the quantum well were varied during growth of InAs–Al x Ga 1− x Sb high electron mobility transistors (HEMTs) structures. Utilizing a 2 3 -full factorial statistical experimental design plus eight additional runs, the ranges for Be-doping (0–2e12/cm 2 ), substrate temperature (350–420°C), and separation above the quantum well (60–80 A) were examined. Relationships between MBE process conditions and transport properties in Be-doped InAs–Al x Ga 1− x Sb HEMT structures were established. Among the process parameters investigated, substrate temperature during Be-doping had the most significant impact on 2DEG concentration. Substrate temperature and sheet charge exhibited an inversely proportional relationship, where increasing substrate temperature resulted in decreasing sheet charge values. The lowest charge with corresponding mobility was 6.6×10 10 at 6000 cm 2 /V/s.

The chemistry and kinetics of lithium gallate (LGO) substrates during nitridation are investigated. Nitridation experiments have been carried out using two remote nitrogen RF plasma sources: in an MBE system and in a remote plasma MOCVD system. The difference between the two nitrogen sources is the pressure. The experiments were run in parallel to demonstrate that the same heterogeneous chemistry applies during LGO nitridation in both MBE and MOCVD environments, provided that the same species is produced in the gas phase. Surface analysis techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE), show that an optimal temperature of about 600degreesC and an optimal time that depends on the incident nitrogen density exists that results in the formation of similar to 5 Angstrom of GaN on LGO. The nitridation process competes with lattice damage that is enhanced by the presence of hydrogen.

In this paper, we report microstructural characterization of InAs/GaAs quantum dots grown by molecular beam epitaxy. The InAs islands were annealed in a range of temperatures under Pa flux. Prior to annealing, it is shown that the InAs islands are mainly lens-shaped, mostly vertically self-aligned and coherent to GaAs (1 0 0) substrate. Relaxation by misfit dislocations is occasionally found. Abnormal relaxation is observed in larger islands. Stacking faults in a V-shape are frequently formed in the lateral sides of these regions. No obvious change in island density and self-alignment occurred when the dots are annealed under Pa flux at 300 � C. In contrast, remarkable changes took place upon annealingat 350 � C. The vertically self-aligned islands disappear, and smooth interfaces were obtained. This morphological transition is argued to result from the replacement of arsenic by phosphorus in the InAs dots. The damage to the crystalinity of the abnormally relaxed regions cannot be removed by the post-growth annealing. The remarkable morphological transition by post-growth annealing under dissimilar anion atmosphere offers another possibility for tuningthe dot morpholog y and hence their electronic properties. r 2003 Elsevier Science B.V. All rights reserved.

A thermodynamic approach is presented to assess the extent of anion exchange reactions during the heteroepitaxy (molecular beam epitaxy) of dissimilar anion III–V compound semiconductor structures. It is shown that the extent of anion exchange can be predicted by the change in the Gibbs free energy. Bond strength changes can only be used as a guide in comparing the relative tendency for exchange, rather than as a criterion. The driving force for anion exchange strongly depends on the conditions during interface formation. A number of important factors, including bond strength, misfit strain energy, surface structure and energy, the equilibrium between dimers V2 and tetramers V4, and segregation are discussed in terms of their contributions to the thermodynamics. r 2002 Elsevier Science B.V. All rights reserved.

We present a comprehensive study of the electrical, optical, and structural properties of wurtzite GaN films grown under various initial growth conditions The GaN films were grown directly on sapphire substrates using GaN nucleation layers by a Riber 3200 system with a radio-frequency plasma source. In situ reflection high-energy electron diffraction (RHEED) reveals a strong correlation between nucleation conditions, including the nitridation step, and the final surface reconstruction of the GaN thin film. Well-defined reconstruction patterns, primarily (2 � 2) and(4 � 4), are observed for some of the nucleation conditions. Hall mobility, photoluminescence (PL), X-ray rocking curve data, and transmission electron microscopy (TEM) measurements are usedto interpret the observedrelationship. The results show that for the conditions investigated, an unreconstructed (1 � 1) surface after growth correlates with improved electrical, optical, andstructural properties. The surface reconstructedthin film exhibits a strong columnar growth with inversion domains (IDs). We attribute the degraded characteristics to the presence of a mixture of both polarities in the films with reconstruction. r 2001 Publishedby Elsevier Science B.V.

Two series of strained InAsP/InP multiquantum wells (MQWs) were grown by solid source molecular beam epitaxy (SSMBE) simultaneously on Fe-doped InP substrates with a growth orientation of either (1 0 0), exact (1 1 1)B, and (1 1 1)B misoriented 1° toward 〈−2 1 1〉. The MQWs were grown using either dimer or tetramer arsenic (As 2 or As 4 ) over a substrate temperature range of 420–535°C. The θ –2 θ X-ray diffraction measurements, the atomic force microscopy (AFM) images of the surfaces, and the 8 K photoluminescence (PL) full-width at half-maximum (FWHM) values of the (1 1 1)B samples showed general improvement in the samples' material properties with increasing substrate temperature. While the X-ray diffraction measurements and the AFM images showed little difference between As 2 and As 4 , the 8 K PL FWHM values varied greatly with respect to the arsenic species used. The exact (1 1 1)B samples had narrower 8 K PL FWHM values when grown with As 2 . For growth on misoriented (1 1 1)B InP, the 8 K FWHM values narrowed with As 4 . The misoriented (1 1 1)B samples had the narrowest 8 K PL FWHM when grown with As 4 at a temperature of 520°C; the exact (1 1 1)B at 495°C when grown with As 2 .

The growth dynamics of the InGaAs/GaAs system have been investigated by desorption mass spectrometry (DMS). Indium desorption spectra indicate the presence of one or two desorption mechanisms depending on the V/III beam equivalent pressure ratio. The activation energy associated with one of the desorption processes is found to be 1.3 eV and independent of V/III ratio and arsenic species. Analysis of the decay curve allows the calculation of the indium surface population during growth. This population is compared for the different growth conditions investigated. Indium incorporation coefficient curves as a function of substrate temperature are presented. Indium incorporation is found to be enhanced using high V/III ratio and the arsenic dimer, As2.

The traditional approach to determine relationships between growth conditions and material properties has rested on the standard experimental approach - varying one parameter while holding all others constant. This technique does not effectively allow the observation of important interactions in complicated multilayer structures. Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy (MBE). It is shown that statistical experimental design is an effective method for optimizing complex multilayer structures quickly. This technique is very useful for the optimization of processes with a large number of interdependent parameters and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show for the first time the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures. The choice of an optimized oxide desorption process can lead to a decrease in the interfacial oxygen by almost two orders of magnitude.

A RHEED system has been developed that allows real-time monitoring of RHEED information throughout a multilayer growth run with rotation. The machine vision system consists of high-speed image capture hardware coupled with digital signal processing software that allows the real-time extraction/analysis of the RHEED intensity and width signals from the noise induced by substrate rotation. This system has been used to investigate the oxide desorption process on GaAs substrates, along with the specular spot intensity and width variation during the growth of a set of InGaAs AlGaAs single quantum well structures with systematically varied process parameters. A strong correlation of the specular spot intensity with growth parameters has been observed. It is also shown that the observed specular spot intensity can be used to predict the quality of the InGaAs quantum well structures.

Al0.48In0.52As lattice matched to InP and grown by MBE over a temperature range of 250 to 100°C and under an As4 pressure of 1x10−6 to 2x10−5 Torr has been investigated. Over this temperature range of 250 to 100°C, resistivity decreases from 2x107 to 3x106 Ω cm while photoluminescence intensity decreases by two orders of magnitude. Resistivity showed little sensitivity to change in As4 overpressure over the range investigated. Single crystal samples grown in the range of 100 to 150°C showed nonstochiometric excess As of up to 1.4% as determined by secondary ion mass spectrometry, and lattice expansion of 0.1% as determined by X-ray diffraction. Samples grown at temperatures greater than 200°C showed no excess As or lattice expansion.

Defect free strained layer epitaxy opens possibilities for further improvement on the quantum well two-dimensional electron gas FET (TEGFET) structures grown using the GaInAs/AlInAs on InP materials system. Increased freedom with composition allows for optimizing certain properties of the structure, such as, the conduction edge discontinuity which controls the maximum sheet concentration (ns); and the electron effective mass which influences the speed of the structure. These enhancements can be made respectively by increasing the Al concentration in the AlInAs and/or by decreasing the Ga concentration in the GaInAs. The maximum amount of strain which can be incorporated into the unrelaxed material sets an upper limit on the compositional tolerances. The tolerances will be shown to be large for the AlInAs and the active TEG GaInAs region. The compositions are obtained using the intensity oscillations observed in the reflection high energy electron diffraction (RHEED) specular beam during growth of GaAs, AlAs, and subsequently GaInAs on GaAs. X-ray rocking curves and photoluminescence (PL) are used to verify the calibrations for growths on InP. The dependency of the mobility on strain is shown. The maximum 300 K mobility obtained was 12,360 cm2/V·s with ns of 1.25×1012/cm2. The well thickness was 84 nm. The AlInAs and GaInAs were estimated to be +0.04 and +0.17 In fraction from lattice match respectively. A mobility of 11,550 cm2/V·s was obtained using a 34 nm well. Strong PL (10 K) peaks will be shown from the quantum well TEGFET structures having a full width at half maximum intensity as low as 12.5 meV. The energies are below the GaInAs bandgap energy ranging from 0.69 to 0.80 eV.