Epitaxy of Solid Solutions and Multilayered Structures in the System Cd—Hg—Te

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/

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 study, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed. It is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles. Differences in the absolute stress values are mainly due to the difference in growth temperature. It is argued that in the case of MOCVD growth the onset of plasticity for higher growth temperatures is responsible for a larger stress relaxation in the buffer layer. It is further found that either process can result in highly off-stoichiometric GaN layers, as manifested by the large variations in the a and c lattice parameters caused by intrinsic point defects.

Chapter 1 - Heterostructure and Quantum Well Physics

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

Chapter 6 - Chemical Beam Epitaxy

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.

Chapter 1 - HgCdTe Infrared Detectors

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.

MBE and MOCVD growth and properties of self-assembling quantum dot arrays in III-V semiconductor structures

This paper presents a comparison of the dark currents and differential resistance of photodiodes, obtained using p-type cadmium-mercury telluride (CdHgTe) layers doped with boron ions, grown by molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE). The volt-ampere responses for diodes on CdHgTe layers with composition x=0.215, grown by MBE and LPE (x≈0.222) are characterized by various saturation currents (reverse bias −0.25V) of 1–2 and 5–10nA, even though the band gap is greater for the latter. In this case, the maximum differential resistance is 4×109 and 5×107Ω for diodes based on MBE and LPE layers, respectively. The experimental data are compared with the calculated values. Numerical modelling showed that, for MBE structures with low biases, the dark current is limited by the diffusion current and by the Shockley-Read-Hall current outside the n-p junction, whereas, in LPE structures, there is a substantial contribution of currents through traps in the depletion region.

Molecular beam epitaxy (MBE) has been employed almost entirely for the growth of arsenic compounds due to the lack of a suitable suitable solid phosphorous source. Advanced phosphide epitaxy has only been performed by metal organic chemical vapor deposition and phosphide based gas-source MBE. Solid source MBE (SSMBE), however, is capable of growing GaInAsP and AlGaInP layer structures without using toxic gases as source material. The key of SSMBE in producing high quality phosphides is a valved cracker cell for elemental phosphorus and arsenic, as well as the optimization of growth conditions such as growth temperature, P cracking zone temperature, P∕As ratio, ΔT between inner and outer heaters, and doping profiles. The 830nm material was grown in GEN III reactor on 2in. GaAs wafers doped with Si. Substrate growth temperature by a pyrometer was 500°C, the cracker temperature was kept at 850°C, and ΔT at 12°C. To verify the material quality, wafers were processed to make lasers with different cavity lengths and 56μm emitter widths. Internal quantum efficiency (ηi) of 95.5% and internal optical losses (αi) of 0.47cm−1 at 25°C were obtained. Devices were burn in for more than 5000h at cw mode, indicating high reliability.

The objective of this work is to establish molecular beam epitaxy (MBE) growth processes that can produce high quality InAs/GaInSb superlattice (SL) materials specifically tailored for very long wavelength infrared (VLWIR) detection. To accomplish this goal, several series of MBE growth optimization studies, using a SL structure of 47.0 A InAs/21.5 A Ga 0.75 In 0.25 Sb, were performed to refine the MBE growth process and optimize growth parameters. Experimental results demonstrated that our “slow” MBE growth process can consistently produce an energy gap near 50 meV. This is an important factor in narrow band gap SLs. However, there are other growth factors that also impact the electrical and optical properties of the SL materials. The SL layers are particularly sensitive to the anion incorporation condition formed during the surface reconstruction process. Since antisite defects are potentially responsible for the inherent residual carrier concentrations and short carrier lifetimes, the optimization of anion incorporation conditions, by manipulating anion fluxes, anion species, and deposition temperature, was systematically studied. Optimization results are reported in the context of comparative studies on the influence of the growth temperature on the crystal structural quality and surface roughness performed under a designed set of deposition conditions. The optimized SL samples produced an overall strong photoresponse signal with a relatively sharp band edge that is essential for developing VLWIR detectors. A quantitative analysis of the lattice strain, performed at the atomic scale by aberration corrected transmission electron microscopy, provided valuable information about the strain distribution at the GaInSb-on-InAs interface and in the InAs layers, which was important for optimizing the anion conditions.

Tensilely strained germanium has been considered a suitable material platform for the realization of a monolithically integrated infrared laser that could allow the development of miniaturized photonic integrated circuits. The crystalline quality of germanium is one of the concerns in this regard since it has to be in the high-quality single-crystal form to endure the required amounts of tensile strain so that the material turns into a gain medium. For that purpose, various researchers have developed tensilely strained Ge nano/microstructures fabricated from a high-crystalline-quality germanium-on-insulator substrate or an epitaxially grown germanium film on silicon, where the fabrication of germanium relies on costly processes (i.e., molecular beam epitaxy, metal–organic chemical vapor deposition). Here, we introduce a methodology to fabricate tensilely strained single-crystalline suspended Ge microstructures through a room-temperature-operated, easy-to-use, environmentally friendly physical vapor deposition technique, sputtering. A single rapid thermal annealing process allows both the crystallization of the sputtered Ge microstructures via liquid phase epitaxy and transforms the capping layer into a stressor. The dimensions of the microstructures, as well as the amount of strain transferred from the stressor, can be easily adjusted by varying the duration of the corresponding wet etching processes. Suspended germanium microstructures with lengths varying between 2.5 and 20 μm are fabricated, and uniaxial strain levels as high as 2.4% are transferred to microstructures along the [110] direction as demonstrated via Raman spectroscopy. The fabricated microstructures demonstrate room-temperature light emission in agreement with the strain profile calculated via finite element method simulations. The methods introduced in this work are suitable to fabricate moderately doped Ge, as well, with nanoscale dimensions for high strain transfer, which could enhance the gain coefficient and enable Ge to serve as the gain medium of a fully integrated CMOS-compatible laser.

Exploring the optimum growth conditions for InAs/GaSb and GaAs/GaSb superlattices on InAs substrates by metalorganic chemical vapor deposition