Metalorganic Vapor Phase Deposition Research Articles

Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD

We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 µm. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson–Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8 K show that a strong band edge photoluminescence of GaN on Si (1 1 1) emits light at an energy of 3.449 eV with a full width at half maximum (FWHM) of approximately 16 meV. At room temperature, the peak position and FWHM of this emission become 3.390 eV and 58 meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1 1 1) epitaxial layers.

AlGaN-InGaN-GaN Near Ultraviolet Light Emitting Diode

AlGaN-InGaN-GaN Near Ultraviolet Light Emitting DiodeA 382-nm InGaN/AlGaN light-emitting diode (LED) was made on a sapphire substrate by metal-organic vapour phase deposition (MOCVD) technique. Growing of the undoped and Si-doped GaN and AlxGa1-xN monocrystalline layers with a surface roughness of < 1 nm required for making light emitting devices has been carried out. To enhance the LED emission efficiency, a modified symmetric composition of an active single quantum well (SQW) structure was proposed. In addition to the conventional p-doped AlGaN:Mg electron overflow blocking barrier, ann-doped AlGaN:Si SQW barrier layer in the structure was formed that was meant to act as an additional electron tunneling barrier.

Uncooled infrared photodetectors in Poland

AbstractThe history and present status of the middle and long wavelength Hg1-xCdxTe infrared detectors in Poland are reviewed. Research and development efforts in Poland were concentrated mostly on uncooled market niche.Technology of the infrared photodetectors has been developed by several research groups. The devices are based on mercury-based variable band gap semiconductor alloys. Modified isothermal vapour phase epitaxy (ISOVPE) has been used for many years for research and commercial fabrication of photoconductive, photoelectromagnetic and other devices. Bulk growth and liquid phase epitaxy was also used. At present, the fabrication of IR devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition (MOCVD), frequently in combination with the ISOVPE.Photoconductive and photoelectromagnetic detectors are still in production. The devices are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, the PV devices could offer high performance and very fast response. At present, the uncooled long wavelength devices of conventional design suffer from two issues; namely low quantum efficiency and very low junction resistance. It makes them useless for practical applications. The problems have been solved with advanced 3D band gap engineered architecture, multiple cell heterojunction devices connected in series, monolithic integration of the detectors with microoptics and other improvements. Present fabrication program includes devices which are optimized for operation at any wavelength within a wide spectral range 1–15 μm and 200–300 K temperature range. Special solutions have been applied to improve speed of response. Some devices show picoseconds range response time. The devices have found numerous civilian and military applications.

Impact of Manganese incorporation on the structural and magnetic properties of MOCVD-grown Ga1−xMnxN

ABSTRACTThis paper reports the impact of the Mn incorporation on the structural and magnetic properties of Ga1−xMnxN on the metal-organic vapor phase deposition (MOCVD). Crystalline quality and phase purity were determined by high-resolution X-ray diffraction and indicated that no macroscopic second phases are formed during growth. Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the growth. SQUID measurements showed an apparent ferromagnetic hysteresis behavior. However, none of the requirements for room temperature ferromagnetism in the prevailing mean field DMS theories were found. Therefore, different origins of the ferromagnetic signal are discussed.

Real time monitoring of layer growth on planetary reactor ® by reflectance-anisotropy-spectroscopy

Reflectance anisotropy spectroscopy (RAS/RDS) has been used for basic growth studies in metal–organic vapor-phase deposition epitaxy (MOVPE). Because of its sensitivity for the uppermost atomic monolayers RAS is a suitable tool for the investigation of a variety of physical properties, such as surface stiochiometry and reconstruction. The enhanced performance of the EpiRAS measurement tool, made by LayTec, is suitable for MOVPE device growth monitoring and control in an industrial AIX 2600 G3 MOVPE reactor in the 8×4 inch configuration with gas foil rotation ®. A brief introduction will be given to the basic surface physics and surface chemistry causing the optical signatures. Examples will be given concerning the optical, real time measurement of the growth of a GaAs/AlAs distributed Bragg reflector (DBR) and (Al x Ga 1− x ) 0.5In 0.5P multi quantum well (MQW) laser structures. The results prove that in-situ monitoring of multi layer structures can be investigated in detail by RAS. The DBR was chosen to optimize the growth rate and thickness which was checked via X-ray data.

Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of selfassembled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of the photoluminescence peak with temperature increases. Cathodoluminescence studies of a small number of quantum dots showed that the broad peak observed in the photoluminescence spectra is, in fact, made up of a series of narrower peaks, coming from quantum dots of different sizes. While the intensity of luminescence from small dots drops monotonously with temperature rises, that from the large dots displays a peculiar behavior. It actually increases within the temperature range of 140–170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of luminescence from some quantum dots and decrease from others are believed to be responsible for the red shift and narrowing of the observed photoluminescence peak. A simple analytically solvable rate equation model was used to understand the spectral data. We suggest that the unusual behaviors observed can be understood as resulting from a transfer of thermally activated carriers from small to large quantum dots.

Atomic equilibrium concentrations in (InGa)As quantum dots

In InxGa1−xAs quantum dots (QDs), the plot of peak emission energies versus the total amount of indium exhibits a well defined pattern, which is independent of the indium concentration. Moreover, photoluminescence spectra of InAs QDs grown by atomic layer molecular beam epitaxy (ALMBE) roughly coincide with those of In0.5Ga0.5As QDs grown by metalorganic vapor phase deposition. We suggest that the total amount of In rather than the nominal In concentration determines the emission energy of these two sets of QDs, and that In interdiffusion is rather strong in ALMBE growth.

State Filling in Type II Quantum Dots

Many-body states in self-organized type II GaSbAs/GaAs quantum dots grown by metal organic chemical vapor phase deposition are studied. With increasing exciton occupation the photoluminescence shows a blue shift and becomes significantly broader developing a saturated plateau structure. The results reveal the properties of many-hole states in GaSbAs quantum dots determined by Coulomb charging and state filling.

State Filling in Type II Quantum Dots

Many-body states in self-organized type II GaSbAs/GaAs quantum dots grown by metal organic chemical vapor phase deposition are studied. With increasing exciton occupation the photoluminescence shows a blue shift and becomes significantly broader developing a saturated plateau structure. The results reveal the properties of many-hole states in GaSbAs quantum dots determined by Coulomb charging and state filling.

InAs/InAs(P,Sb) quantum-well laser structure for the midwavelength infrared region

Strained InAs1−xSbx(0<x<0.22) quantum wells with InAs, InAsP, or InAsPSb barrier layers have been grown by metal–organic vapour-phase deposition on InAs substrates. Their photoluminescence and electroluminescence were studied at 20 K. The photoluminescence peak-wavelength evolution with the antimony composition x and the nature of the barrier were theoretically predicted in a satisfactory manner under the assumption of a type-II band alignment for the InAsSb/InAs system with a valence band ratio Qv=−1.30. This hypothesis leads to a type-IIa band alignment for the arsenic-rich InAsSb/InAsP system. Starting from this result, a `W' laser structure, consisting of ten periods of InAsSb/InAsP/InAsSb/InAsPSb multiquantum wells in the active region, a broadened InAsPSb waveguide, and AlAsSb cladding layers, is proposed for room-temperature emission near 3.3 μm.

Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be εr=0.3±0.6% by Fourier filtering of HRTEM images and a moiré fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the SiNx layer. Weak beam imaging and plan view TEM show typical threading dislocations in the epilayer with a density of 3×109 cm−2 extending along 〈0001〉 which sometimes form grain boundaries. An AlxGa1−xN/GaN interface roughness of 3 monolayers is estimated and a small AlxGa1−xN surface roughness of 1.5 nm is obtained by HRTEM and atomic force microscopy investigations which correspond to two-dimensional growth. C–V and Hall measurements reveal two-dimensional electron gas at the Al32Ga68N/GaN interface that has a sheet carrier concentration of 4×1012 cm−2. The electron mobility of 820 cm2/Vs measured at room temperature is applicable for a HEMT grown on Si(111).

The values of minority carrier diffusion lengths and lifetimes in GaN and their implications for bipolar devices

The wide bandgap semiconductors GaN and AlGaN show promise as the high voltage standoff layers in high power heterostructure bipolar transistors and thyristors due to their electric breakdown characteristics. Material properties which significantly influence the design and performance of these devices are electron and hole diffusion lengths and recombination lifetimes. We report direct measurements of minority carrier diffusion lengths for both holes and electrons by electron beam induced current. For planar Schottky diodes on unintentionally doped n-type and p-type GaN grown by metal organic vapor phase deposition (MOCVD), the diffusion lengths were found to be (0.28±0.03) μm for holes and (0.2±0.05) μm for electrons. Minority carrier lifetimes of approximately 7 ns for holes and 0.1 ns for electrons were estimated from these measured diffusion lengths and mobilities. In the case of GaN grown by halide vapor phase epitaxy (HVPE) diffusion lengths in the 1–2 μm range were found. We attempt to correlate the measured diffusion lengths and lifetimes with the structural properties of GaN and to explain why linear dislocations might act as a recombination centers. We calculate the performance of nitride based bipolar devices, in particular thyristor switches. The forward voltage drop across standoff layer of the nitride based thyristor switch is shown to significantly depend on the minority carrier (hole) lifetime.

A density functional theory study of surface and gas phase processes occurring during the MOCVD of ZnS

Quantum chemistry was adopted to study thermo-chemical properties and reactivity of different gas and surface species of interest for the epitaxial metalorganic vapour phase deposition of ZnS. All calculations were performed using density functional theory methods such as the three parameters Becke-Perdew Wang hybrid DFT (B3PW91) method and different basis sets. The results of these studies enabled us to have a better understanding of the fundamental chemical steps that occur in the formation of the crystalline ZnS films and to discuss the impact of gas reactions on the overall deposition chemistry. Among the different metalorganic precursors considered were Me 2 Zn:Et 3 N, H 2 S and t BuSH. It was found, in agreement with previous experimental studies, that the bond energy between Me 2 Zn and Et 3 N is very low. The kinetic constant for the reaction between H 2 S and Me 2 Zn to yield HSZnMe and methane was determined by locating the transition state and it was found to have an activation energy of 17.4 kcal/mol. The kinetic rate is expected to be similar for the reaction between t BuSH and Me 2 Zn, yielding BuSZnMe and methane. Finally, by comparison between experimental and calculated data the rate-determining step for the growth of ZnS by Me 2 Zn:Et 3 N and t BuSH at low pressures and in the absence of gas phase pre-reactions was identified in the one site dissociative adsorption of Me 2 Zn or t BuSZnMe on S suface sites.

On the formation and nature of nanometer size clusters on the surface of ZnSe epilayers

ZnSe epilayers were grown on GaAs (001) substrates by metal organic chemical vapor phase deposition, using different group VI–II precursor flow ratios. Atomic force microscopy (AFM) examinations of their surface show that epilayers grown with a high VI/II ratio are not as stable as those grown with a low ratio. When exposed to air, Se clusters would appear and grow on the surface of the unstable epilayers. The ripening process could take as long as 50 days at room temperature. Secondary electron and cathodoluminescence images indicate that the clusters are more likely to emerge from areas of high defect density. Moreover, AFM topographic images of epilayers at an intermediate stage of ripening suggest that clusters near surface depressions would grow in size at the expense of others. By using a low accelerating voltage and allowing the clusters to grow to a size larger than the electron interaction volume, we used energy dispersive x-ray analysis to show that the clusters are made up entirely of Se.

Self-assembled InSb quantum dots grown on GaSb: A photoluminescence, magnetoluminescence, and atomic force microscopy study

We report a study of self-assembled quantum dots (QDs) of InSb embedded in a GaSb matrix grown by metalorganic vapor phase deposition. Growth temperatures and deposition times have been optimized for maximal photoluminescence peak intensities. Photoluminescence (PL), magneto-PL, and atomic force microscopy (AFM) have been performed to estimate the size of the QDs. The quantum dots luminesce in the midinfrared at around 0.73 eV. The application of magnetic fields up to 15 T both parallel and perpendicular to the growth direction enhanced the wetting layer and bulk PL intensity and enabled an estimate to be made of the QD height and widths as 2–4 and 20–30 nm, respectively. These sizes were confirmed by AFM.

Depth-resolved cathodoluminescence study of ZnxCd1−xSe epilayer grown on (001) InP by metal organic chemical vapor phase deposition

Optical properties of zincblende structured ZnxCd1−xSe epilayer grown on InP by metal organic chemical vapor phase deposition at temperatures of 360, 400, and 440 °C are investigated with low temperature cathodoluminescence spectroscopy (CL). Both near band gap and deep level emissions are found for the samples grown at 400 °C and above, but deep level emissions are absent for the sample grown at 360 °C. We conclude that the growth temperature should be kept below the temperature at which InP begins to decompose and diffusion of III–V constituents into the epilayer occurs. Evidence of this diffusion comes from an analysis of depth resolved CL studies, which shows that the deep level emissions occur mainly at the epilayer/substrate interface. By monitoring the ratio of the intensity of the deep level emissions to that of the near band emissions, we find that this ratio is larger for samples grown at high temperatures than those at low temperatures. Indium diffusion from the substrate into the epilayer is most likely the source of these deep levels.

The properties of ZnSc layers grown on GaAs and Si substrates by atomic layer epitaxy

ZnSe layers have been grown by atomic layer epitaxy using metal organic vapor-phase deposition system on to (100) GaAs and (100) Si substrates. The growth rate per cycle is strongly dependent on the substrate temperature. The self-limiting growth of one monolayer per cycle is obtained in the wide substrate temperature range of 100–200°C for GaAs substrates and 150–250°C for Si substrates. The X-ray patterns are similar on both substrates. The surface morphologies are smooth and mirror-like. The photoluminescence spectra show an emission at 443 nm at 77 K on both substates.

Free and bound excitons in thin wurtzite GaN layers on sapphire

Free and bound excitons have been studied by photoluminescence in thin () wurtzite-undoped GaN, n-type GaN:Si as well as p-type GaN:Mg and GaN:Zn layers grown by metal-organic chemical vapour phase deposition (MOCVD). An accurate value for the free A exciton binding energy and an estimate for the isotropically averaged hole mass of the uppermost valence band are deduced from the data on undoped samples. The acceptor-doped samples reveal recombination lines which are attributed to excitons bound to and respectively. These lines are spectrally clearly separated and the exciton localization energies are in line with Haynes' rule. Whenever a comparison is possible, it is found that the exciton lines in these thin MOCVD layers are ultraviolet-shifted by 20 to 25 meV as compared to quasi-bulk () samples. This effect is interpreted in terms of the compressive hydrostatic stress component which thin GaN layers experience when grown on sapphire with an AlN buffer layer.

Photoluminescence-excitation analysis of Er-doped GaAs grown by metalorganic vapor phase deposition

Er-doped GaAs grown by metalorganic chemical vapor deposition was studied by photoluminescence and photoluminescence excitation (PLE) measurements using a color center laser that provides tunable light in the 1450–1550 nm range. The light in this wavelength region can directly induce the 4I15/2→4I13/2 intra-4f-shell transition of Er3+ ions. The PLE spectra of a specific Er center formed by oxygen codoping under suitable growth conditions were conclusively assigned by site-selective spectroscopy. Non-site-selective PLE spectroscopy, on the other hand, revealed that the atomic configurations of Er3+ ions change according to the Er concentration, growth temperature, and oxygen codoping. Sharp lines related to Er3+ ions with well-defined atomic configurations, and inhomogeneously broadened structures related to aggregates of Er3+ ions, are evident in the PLE spectra. The PLE spectra due to the direct intra-4f-shell excitation also revealed semiquantitatively that there are Er centers that do not show luminescence under above-band gap photoexcitation, because of low energy transfer efficiency from the host.

Visible InGaP/InGaAlP quantum well top surface emitting laser diodes

Vertical cavity top surface emitting lasers emitting near 0.67 μm visible spectral region were fabricated by one-step low pressure metalorganic vapor phase deposition technique. The lasers used four 80-Å-thick InGaP/InGaAlP quantum wells active region, sandwiched between two Al0.5Ga0.5As/AlAs distributed Bragg reflectors. Room temperature pulsed threshold current was 10 mA for 15 μm diam devices.