Ultraviolet Materials and Devices

Differences in epitaxial-layer/substrate interface properties of hetero-junction field effect transistors fabricated by molecular beam epitaxy and metal organic chemical vapor deposition

In this paper, we demonstrate the fabrication of current aperture vertical electron transistors (CAVET) realized with two different epitaxial growth methods. Templates with a p-GaN current blocking layer (CBL) were deposited by metal organic chemical vapor deposition (MOCVD). Channel and barrier layers were then regrown by either molecular beam epitaxy (MBE) or MOCVD. Scanning electron microscope (SEM) images and atomic force microscope (AFM) height profiles are used to identify the different regrowth mechanisms. We show that an AlN interlayer below the channel layer was able to reduce Mg diffusion during the high temperature MOCVD regrowth process. For the low-temperature MBE regrowth, Mg diffusion was successfully suppressed. CAVET were realized on the various samples. The devices suffer from high leakage currents, thus further regrowth optimization is needed.

InP/InGaAsP/InGaAs avalanche photodiodes with sepa- rate absorption, grading, and multiplication regions (SAGM-APD's) have been fabricated from wafers grown by chemical beam epitaxy (CBE). These APD's exhibit low dark current ( 500-Mbit/s) lightwave sys- tems experiments have shown that avalanche photodiodes (APD's) can provide several decibels of improvement in receiver sensitivity compared to p-i-n photodiodes ( 11-(5). In long-wavelength (= 1.3- or 1.5-pm) lightwave receivers the highest sensitivities have been achieved with APD's having an InP multiplication region and an 1~,~~Ga,-,.~~As (subscripts deleted in following text) absorption region separated by a transition region consisting of one or more lattice-matched intermediate-bandgap In,Gal -,AsL.P1 -y layers, the separate absorption, grading, and multiplication regions (SAGM)- structure APD ( 11. To operate properly, the carrier concentra- tions and thicknesses of the different layers which comprise this structure must be controlled within very narrow tolerances (6). Although most of the SAGM-APD's that have been reported to date have been grown by liquid-phase epitaxy (LPE), the degree of precision and uniformity necessary to maintain the wafer parameters within the constraints required by this structure are difficult to achieve by this growth technique. Also, LPE is more difficult and more expensive to implement in large-scale production than vapor-phase tech- niques such as hydride vapor-phase epitaxy (VPE), metal- organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE). In the past, however, InP/In,- Gal-,As,P1-, photodetectors grown by MBE and MOCVD have not achieved performance characteristics as good as those grown by LPE due to the high background carrier concentra- tions of the In,Gal-,As,PI-, layers and the difficulty in achieving good InP/In,Gal -,As,P1 ->, interfaces. Recently, InPIInGaAsPiInGaAs SAGM-APD's grown by MOCVD (7) and VPE (8), (9) have been reported. In this paper we report on InP/InGaAsP/InGaAs SAGM-APD's grown by chemical beam epitaxy (CBE) (lo), a crystal growth technique that combines many of the advantages of MOCVD and MBE. A schematic cross section of the back-illuminated InP/

Chapter 6 - Chemical Beam Epitaxy

Preface

In recent years, wide band-gap semiconductor materials have attracted a great deal of attention for use in blue light-emitting and short-wavelength diodes. Additionally, due to its high conductance, chemical and thermal stability, and high piezoelectric coupling coefficient, ZnO is also used for piezoelectric devices, such as surface acoustic wave (SAW) devices [1] and bulk acoustic devices [2]. Various deposition techniques, including sputtering [3, 4], pulsed laser deposition (PLD) [5, 6], ion beam deposition [7], chemical vapor deposition (CVD) [8, 9], atomic layer deposition (ALD) [10], metal-organic chemical vapor deposition (MOCVD) [11, 12], and molecular beam epitaxy (MBE) [13] have been employed for the growth of ZnO films. However, MOCVD has an advantage in achieving devices in commercial level since high deposition rate and highquality film is attainable, especially at low pressure. High quality ZnO films grown on Si substrate pave the way from integration of devices with Si IC technology. Also, amorphous substrates such as SiO2 and glass substrate have obvious technological advantages and potential applications [14]. Although most researchers have grown ZnO films on sapphire substrates, there are not many reports on growing ZnO thin films on Si or Sibased materials by MOCVD technique. Furthermore, there are rare reports on the MOCVD growth of ZnO at low temperatures below 300 ◦C. In this work, we investigate the growth of ZnO thin films on silicon substrate at the temperature range of 100–200 ◦C by employing the MOCVD technique. Since the material properties strongly affect the performance of a device, the influence of substrate temperature on the structural properties of the samples are presented. The ZnO films were deposited on p-type Si(100) substrates by MOCVD system using Zn(C2H5)2 (99.9999% purity DEZn (diethylzinc)) and O2 (99.999% purity). Fig. 1 shows a schematic diagram of the MOCVD reactor used in our experiments. Before loading into the reactor, the substrate was cleaned in acetone for 10 min, HF (20:1) for 1 min and then rinsed by deionized water for 1 min. High-purity Ar was passed through the DEZn bubbler and saturated with DEZn vapor to the reactor. For MOCVD growth of ZnO films, the gas phase reaction will result in particle formation, which will degrade the ZnO film quality. In order to minimize the gas phase reaction, Zn(C2H5)2

Negative electron affinity (NEA) GaAs photocathodes have attracted a wide scope of interest because of their high quantum efficiency and low dark emission. Traditionally, fabrication of GaAs photocathodes has taken two approaches: molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). Understanding the difference between these two methods in terms of device performance can help guide future device development. While past research has indicated that photocathodes grown by MOCVD generally have better spectral response and quantum efficiency, these reports are all based on steady-state analysis and measurement. There has been little prior work comparing the dynamic response of devices fabricated with different technologies. In this presentation, we report a comparative study of the ultrafast response of two gradient-doped GaAs photocathodes fabricated using two different methods, viz. MBE and MOCVD. Our approach is based on femtosecond pump-probe reflectometry (PPR), which measures the transient reflectivity of these devices upon optical excitation by femtosecond pulses. Preliminary PPR result shows that carrier build-up near photocathode surface in the MOCVD device is more efficient compared to the MBE device. A carrier-diffusion model is used to analyze photoelectron transport, accumulation, and decay in the active layer. Experiment-theory comparisons indicate a bi-exponential nature of free-electron population decay near device surface. Excellent agreement between theoretical predictions and measured data not only validates the numerical model but also allows various device parameters to be evaluated quantitatively.

1D topological systems for next-generation electronics

Two step growth of high quality long n-GaN:Si nanowires using μ-GaN seed on Si(111) by metalorganic chemical vapor deposition

: By the onset of the project many aspects of optical properties of InGaN semiconductors were unknown. From the literature publications, it was known that they strongly depend on the micro-and nanostructure of the semiconductor. S This need of further investigation was spurred by the across-the-board advances in the preparation of the high-brightness light-emitting devices in the blue region of the spectrum, which was difficult to obtain by other means. Over 3 years of the project we made new advances in the investigation of the mechanism of the light emission in InGaN. As expected, it was identified that the emission mechanism in InGaN is the recombination of electrons and holes. InGaN was made by two methods: molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Substantial problem was found in the correlation of the emission parameters of InGaN and method of their preparation, however eventually it was largely sorted out with the help of atomic force microscopy. It was firmly established that the traps on inhomogeneities act as centers for light emission and the LED activity should be attributed to this feature of the semiconductor. Strong evidence of the participation of nanoscale In quantum dots in the light-emitting process was the major objective of this 3-year project.

Environmental safety issues for molecular beam epitaxy platform growth technology

Solid solutions CdxHg1—xTe (CMT) are one of the principal IR microphotoelectronics materials. Quantum-sized multilayered CdTe—HgTe structures are viewed as the most promising for fabricating photodetectors using “band engineering.” With respect to the structural and electrophysical parameters of the CMT films, all common epitaxial methods give the same excellent results. The carrier mobility in n-type Cd0.2Hg0.8Te reaches µ 77 ≥ 2 ·105 cm2 •V/sec at a concentration n ≤ 1015 cm-3. Therefore, liquid-phase epitaxy, metalorganic chemical vapor deposition (MOCVD), sublimational—diffusional gas-phase epitaxy, and molecular-beam epitaxy (MBE) are equally capable of satisfying the demands of traditional photoelectronics in high-quality CMT layers. The single method of preparing quantum-sized structures and superlattices (SL) until very recently was MBE. The advantage of MBE for growing SL based on CMT is the low growth temperature compared to other epitaxy methods. The interdiffusion coefficients of HgTe and CdTe are relatively large. Interdiffusion of the layer components can destroy the SL. If the interdiffusion coefficients of HgTe and CdTe given in the literature are used [1], the calculated thickness of the diffusion layers at 200°C is 0.1-1.0 nm. Superlattices based on CMT can be grown by MBE at 200°C. This growth temperature cannot be exceeded substantially without destroying the sharpness of the composition change at the boundaries of the SL layers. In MOCVD, use of photoactivation [2], precracking [3], flame excitation of the reaction mixture [4], and nontraditional organometallic compounds [5] enabled the CdTe growth temperature to be lowered to 250-200°C (and even to 85°C for HgTe). Thus, ultrathin layers could be synthesized [6]. The development of a hydride MBE method using organometallic compounds, like that already used for A3B5, seems logical.

Epitaxial film growth and characterization

Far-infrared photothermal ionisation spectroscopy is used to investigate the nature of the donors present in high-purity epitaxial films of GaAs grown by metal-organic chemical vapour deposition (MOCVD), metal-organic gas source molecular beam epitaxy (MOMBE) and solid source molecular beam epitaxy (MBE). Except for one lower mobility sample, the MOCVD samples showed the donor line X1 to be dominant with >50% relative intensity. The higher mobility samples showed the presence of two relatively closely spaced components to X1. Hydrostatic pressures up to 20 kbar were used to increase the chemical shifts and to vary the absorption strengths of the lines. By this means, and by the simultaneous observation of the same components on the 1s-2p and 1s-3p transitions, it was possible to show that these components were not artefacts of over-absorption but arose from two different donor species of very similar chemical shifts. Consequently, any identification of donor X1 as being solely due to silicon must be treated with caution. Germanium and sulphur accounted for the remaining donor contamination in the MOCVD material, with germanium usually being present at higher levels than sulphur. In contrast, earlier far-infrared and photoluminescence data with samples of different origin have shown germanium to be dominant in MOCVD material. MOMBE growth produced very similar proportions of X1, germanium and sulphur, indicating that the gas sources are responsible for the contamination. The MBE samples grown in reactors of different manufacture showed levels of sulphur donors comparable with the background carbon acceptors as well as traces of germanium, in addition to the silicon deliberately introduced. The results for the highest mobility MBE sample with deliberate silicon doping suggest that the lower energy component of the 'X1' doublet is due to silicon. The most likely candidate for the higher energy component is selenium. An even higher mobility MBE sample which was n-type without deliberate silicon doping showed the higher energy component attributed to selenium.

In this thesis we investigate atomic scale imperfections and fluctuations in the quantum transport properties of novel semiconductor nanostructures. For this purpose, we have developed a numerically efficient supercell model of quantum transport capable of representing potential variations in three dimensions. This flexibility allows us to examine new quantum device structures made possible through state-of-the-art semiconductor fabrication techniques such as molecular beam epitaxy and nanolithography. These structures, with characteristic dimensions on the order of a few nanometers, hold promise for much smaller, faster and more efficient devices than those in present operation, yet they are highly sensitive to structural and compositional variations such as defect impurities, interface roughness and alloy disorder. If these quantum structures are to serve as components of reliable, mass-produced devices, these issues must be addressed. In Chapter 1 we discuss some of the important issues in resonant tunneling devices and mention some of thier applications. In Chapters 2 and 3, we describe our supercell model of quantum transport and an efficient numerical implementation. In the remaining chapters, we present applications. In Chapter 4, we examine transport in single and double barrier tunneling structures with neutral impurities. We find that an isolated attractive impurity in a single barrier can produce a transmission resonance whose position and strength are sensitive to the location of the impurity within the barrier. Multiple impurities can lead to a complex resonance structure that fluctuates widely with impurity configuration. In addition, impurity resonances can give rise to negative differential resistance. In Chapter 5, we study interface roughness and alloy disorder in double barrier structures. We find that interface roughness and alloy disorder can shift and broaden the n = 1 transmission resonance and give rise to new resonance peaks, especially in the presence of clusters comparable in size to the electron deBroglie wavelength. In Chapter 6 we examine the effects of interface roughness and impurities on transmission in a quantum dot electron waveguide. We find that variation in the configuration and stoichiometry of the interface roughness leads to substantial fluctuations in the transmission properties. These fluctuations are reduced by an attractive impurity placed near the center of the dot.