Madhu Sudan Kayastha

Direct growth of nanographene films by surface wave plasma chemical vapor deposition and their application in photovoltaic devices

Here, we report direct synthesis of nanographene films on silicon (n-Si) and glass (SiO2) substrates by microwave assisted surface wave plasma (MW-SWP) chemical vapor deposition (CVD) and their application in photovoltaic devices. The technique is a metal catalyst free, rapid growth process and the film can be deposited on different substrates; thus simplifying the synthesis process for various device applications. The directly grown graphene film consists of triangular shaped nanographene domains with sizes of 80–100 nm in length. The nanographene domains interconnect to form a continuous film which shows metallic characteristics. A Schottky junction based photovoltaic device is fabricated with directly grown nanographene film on n-Si and a conversion efficiency of 2.1% is achieved. This finding shows that a transparent nanographene film can be deposited on different substrates and can be integrated for various devices.

Ultrahigh-Purity Undoped GaAs Epitaxial Layers Prepared by Liquid Phase Epitaxy

Ultrahigh-purity (UHP) GaAs epilayers have been grown reproducibly by liquid phase epitaxy (LPE) with conventional solution baking in a H2 environment. We have succeeded in achieving a high-purity (HP) n-type GaAs epilayer with a low electron carrier concentration of 5.84×1012 cm-3 and a high mobility of 312,000 cm2 V-1 s-1 at 77 K, which is the highest value ever reported. Photoluminescence (PL) analyses at 15 K for both UHP n-type and p-type GaAs epilayers were also performed. We have clarified that the PL intensity of an HP GaAs epilayer has a strong correlation with the mobility measured by Hall measurements at 77 K. The electron carrier concentration of the UHP GaAs epilayer also agrees very well with that obtained from capacitance–voltage (C–V) measurements. We also showed that the recyclic growth process used in this study effectively improves the purity of the epilayers because the mobility at 77 K is enhanced approximately threefold and the ionized carrier concentration decreases threefold compared with their initial values. We also demonstrated that the recyclic growth process is effective in eliminating impurities such as carbon and silicon by analyzing the PL spectra.

High-Extinction Ratio and Low-Driving-Voltage Spatial Light Modulator by Use of Ultrahigh-Purity GaAs

Highly efficient surface normal spatial light modulators (SLMs) using ultrahigh-purity GaAs layers (30 µm thick) grown on (100)-oriented n+-GaAs substrate by liquid phase epitaxy (LPE) method have been realized. Clear exciton absorption has been observed even at room temperature. The extinction ratio of 25 dB has been demonstrated with a low-driving voltage (32 V) at 895 nm, based on electroabsorption (EA) effect. Very large depletion length over 150 µm is confirmed by analyzing the extinction ratio and capacitance–voltage (C–V) measurements. The calculated value of impurity concentration is very low (≈1012 cm-3), which indicates that the concentration of donor and acceptor in the device are highly compensated.

Experimental and theoretical study of excitonic electroabsorption in high-purity GaAs at room temperature

The absorption spectrum in high-purity GaAs has been theoretically studied in the presence of electric field taking into account of excitonic transition and continuum band transition at room temperature. We have calculated the Stark shift, height of exciton peak, linewidth broadening of exciton, extinction ratio as a function of electric field. We also have compared these results with the experimental results and found the close agreement with experimental.

Comparison of linewidth enhancement factor for compressively strained AlGaInAs and InGaAsP quantum well lasers

We have compared and analyzed the theoretical possibility for the extreme reduction in the linewidth enhancement (α- factor) in strained layer quantum-well (QW) lasers for AlGaInAs and InGaAsP material. Valence band structure and optical gain in both types of QWs under compressive strain have been calculated using 4×4 Luttinger-Kohn Hamiltonian. The Luttinger parameters of these quaternary materials were determined from the linear interpolation between the values of their respective binaries. The α-factor has been calculated as the ratio of the carrier induced change in real component of the complex refractive index to that in imaginary component of the refractive index. We have used Kramers-Kronig relations to calculate the refractive index change due to carrier induced. The α-factor was up to 1.5 times smaller in AlGaInAs QW than in InGaAsP QW lasers. The material modal differential gain is found to be approximately 1.38 times larger and material carrier induced refractive index change is 1.16 times smaller in the former material than the latter, respectively.

Dependence of threshold current density on quantum well composition for compressive strained-layer Al x Ga y In 1−x−y As lasers

Separate confinement AlGaInAs/InP compressive strained quantum well lasers especially in the wavelength range of 1.55 μm and 1.3 μm with 7.6 nm well thick have been studied. Threshold current density and emission wavelengths have been calculated as a function of well composition. A minimum of threshold current density of 146 Acm⁻² was obtained for the devices with Al<inf>0.1</inf>Ga<inf>0.16</inf>In<inf>0.74</inf>As well which is the lowest value ever reported at this well composition.

Low driving voltage spatial light modulator fabricated by ultrahigh-purity GaAs

Highly efficient surface normal spatial light modulators (SLMs) using ultrahigh purity GaAs layers (30 μm thick) grown on (100) — oriented n⁺ — GaAs substrate by liquid phase epitaxy (LPE) method have been realized. The extinction ratio of 25 dB has been demonstrated with a low — driving voltage (32 V) at 895 nm, based on electroabsorption (EA) effect. Very large depletion length over 150 μm is confirmed by analyzing the extinction ratio and capacitance- voltage (C-V) measurements. The calculated value of impurity concentration is low (≈10¹² cm⁻³) and indicates donor and accepter in the device are highly compensated.