Mikołaj Badura

Influence of the AP MOVPE process parameters on properties of (In, Ga)(As, N)/ GaAs heterostructures for photovoltaic applications

GaAsN and InGaAsN semiconductor alloys with a small amount of nitrogen, so called dilute nitrides, are especially attractive for telecom lasers and very efficient multijunction solar cells applications. The epitaxial growth of these materials using MBE and MOVPE is a big challenge for technologists due to the large miscibility gap between GaAs and GaN. Additionally, elaboration of the growth process of quaternary alloys InGaAsN is more complicated than GaAsN epitaxy because a precise determination of their composition requires applying different examination methods and comparison of the obtained results. This work presents the influence of the growth parameters on the properties and alloy composition of the triple quantum wells 3×InGaAsN/GaAs grown by atmospheric pressure metal organic vapour phase epitaxy AP MOVPE. Dependence of the structural and optical parameters of the investigated heterostructures on the growth temperature and the nitrogen source concentration in the reactor atmosphere was analyzed. Material quality of the obtained InGaAsN quantum wells was studied using high resolution X-Ray diffraction HRXRD, contactless electro-reflectance spectroscopy CER, photoluminescence PL, secondary ion mass spectrometry SIMS, photocurrent PC and Raman RS spectroscopies, deep level transient spectroscopy DLTS and transmission electron microscopy TEM.

Influence of rapid thermal annealing on optical properties of (In, Ga)(As, N)/GaAs quantum wells

Dilute nitride (In, Ga)(As, N) alloys grown on GaAs substrate are a very attractive materials for optoelectronics. In this work we compare the optical properties of (In, Ga)(As, N)/GaAs triple quantum well grown by atmospheric pressure metal organic vapour phase epitaxy. As grown and annealed structures were investigated by means of photoluminescence and contactless electroreflectance spectroscopies. Energies of fundamental transition from each measurement were determined and compared, moreover the value of Stokes shift was assigned and discussed.

LP-MOVPE growth and properties of high Si-doped InGaAs contact layer for quantum cascade laser applications

Abstract The work presents doping characteristics and properties of high Si−doped InGaAs epilayers lattice−matched to InP grown by low pressure metal−organic vapour phase epitaxy. Silane and disilane were used as dopant sources. The main task of investigations was to obtain heavily doped InGaAs epilayers suitable for usage as plasmon−confinement layers in the construction of mid−infrared InAlAs/InGaAs/InP quantum−cascade lasers (QCLs). It requires the doping concentration of 1×1019 cm–3 and 1×1020 cm–3 for lasers working at 9 μm and 5 μm, respectively. The electron concentration increases linearly with the ratio of gas−phase molar fraction of the dopant to III group sources (IV/III). The highest electron concentrations suitable for InGaAs plasmon−contact layers of QCL was achieved only for disilane. We also observed a slight influence of the ratio of gas−phase molar fraction of V to III group sources (V/III) on the doping efficiency. Structural measurements using high−resolution X−ray diffraction revealed a distinct influence of the doping concentration on InGaAs composition what caused a lattice mismatch in the range of –240 ÷ –780 ppm for the samples doped by silane and disilane. It has to be taken into account during the growth of InGaAs contact layers to avoid internal stresses in QCL epitaxial structures.

Epitaxial regrowth of InP/InGaAs heterostructure on patterned, nonplanar substrates

Abstract The main goal of the studies on epitaxial regrowth process of InP on patterned substrates is to gain knowledge about growth rates and interface quality on various areas to improve the fabrication technology for future applications. Prepared samples were measured at every step of the process by scanning electron microscope (SEM), optical microscope with dark field and phase contrast modes, atomic force microscope (AFM) and also using optical profilometer WLI (White Light Interferometer). Fabrication steps were divided into three main groups. First was the epitaxial growth of 5 µm thick InP layer. Next was patterning, which was made by applying a mask film on the epilayer. Shapes of the mesas after wet chemical etching with photoresist as a mask as well as the shapes of mesas slopes were irregular on the whole substrate area. These problems were solved by the use of silicon nitride mask. The mesas shapes and their slopes became then regular, independently of etching depth. Second fabrication step was etching of selected area. Couple of solutions were examined, but in details HCl:H3PO4 mixture in various proportions, which gave the best results in mesas shapes and orientations relative to the substrate. After that, the etching mask material was removed from the epilayer using a buffered hydrofluoric acid (BHF). The last step was epitaxial regrowth. To see how the epitaxial growth process was performed on different areas of patterned substrate it was suggested using a “sandwich”, which consisted of 50 layers of indium phosphide and indium gallium arsenide. This idea helped to understand the phenomena occurring during the epitaxial growth on that kind of substrate. The highest growth rate occurred on the top of the mesas and the lowest on their slopes. Described experiments are introduction to the studies on epitaxial growth of buried heterostructure (BH).

GaInNas/GaAs QW Based Structures to Compensate Parasitic Effect of Quantum-Confined Stark Effect in Photodetector Applications

The inhomogeneities of multicomponent semiconductor alloys, as well as phase segregation, can be utilized for enhancement of photodetector absorption properties and thus its efficiency. In this paper, the influence of external electric field on the probability of light absorption in the GaInNAs quantum well is discussed. Both phenomenon: indium and nitrogen composition gradient as well as step-like quantum well are applied to design the QW with compensation of the Quantum-Confined Stark Effect (QCSE) Parasitic effect of QCSE results from decrease of the wave functions overlapping in the QWs placed in reverse biased junction, which finally decrease the efficiency of the photodetector.

Characterisation of AP-MOVPE grown (Ga, In)(N, As) structures by Raman spectroscopy

Over the last few years, ternary and quaternary semiconductor compounds containing (Ga, In) and (N, As) elements become subject of many studies. Both, indium and nitrogen, lowers the band gap of gallium arsenide, but their influence on lattice parameter is compensated. As a result it is possible to deposit epitaxial layers of 1 eV , or less, material which is matched to GaAs substrate. GaAs technology is well known and much cheaper than more sophisticated phosphorus alloys. Optoelectronic devices composed of dilute nitrides materials can be widely used as a telecommunication lasers, photodetectors or even photovoltaic cells. Investigated samples were performed using atmospheric-pressure MOVPE system with AIXTRON AIX200 R-and-D reactor. GaNAs layers were deposited as bulk layers, while GaInNAs material grown as bulk and additionally as quantum wells with GaAs barriers. Gallium arsenide substrates were utilized. Studies were performed utilizing Raman spectroscopy at room temperature. Phonons were excited using 514 nm Ar+ laser. Characteristic for such structures GaN-like local vibrational mode was observed to change its position with changing nitrogen concentration. GaAs-like longitudinal optic phonon also was investigated. As a result an attempt to measure nitrogen concentration in mentioned materials using Raman spectroscopy was performed.

Characterizations of GaInNAs/GaAs quantum wells

Heterostructures of GaInNAs/GaAs multiple quantum wells were characterized by high resolution x-ray diffraction. Complexity and stress compensating effect of such quaternary alloys cause many characterization problems. One of the most important issue is determination of composition of the material, which cannot be performed utilizing only one characterization method. That is why structural analysis had to be related with optical measurements which give different information correlated with composition. A comparison of theoretical calculations of energy band gap with energy of transitions in GaInNAs QWs from photoluminescence or contactless electro-reflectance measurements supplement the results of HRXRD and gives complete information about the structure required as a feedback to develop technology of heterostructures epitaxial growth.

Tunnel junction technology for multijunction solar cell applications

Solar cells are very promising renewable resource. High efficiency of AIIIBV based solar cells are achieved in multijunction constructions. The technology of these devices is very difficult due to the complex electrical and optical interactions between the different semiconductor layers. Usually, the individual subcells are interconnected via Esaki tunnel diodes. This work presents the technology and computer modelling of GaAs based tunnel diodes which will be applied in tandem GaAs/InGaAs solar cells. Designed structures were grown by atmospheric pressure metal organic vapour phase epitaxy. Simulation results as well as dopant profiles and measured DC I-U characteristics of the obtained tunnel diodes are presented and discussed.