Hae-Seok Lee

Chemical Beam Epitaxy of GaAsN Thin Films with Monomethylhydrazine as N Source

GaAsN thin films were grown by chemical beam epitaxy (CBE) with monomethylhydrazine (MMHy) as a N source. Processes that determine the N composition in the GaAsN thin films were investigated in the growth temperature range from 340 to 480 °C. When growth temperature is low (340–390 °C), N composition is mainly determined by the supply of N species generated from MMHy and growth rate, since the desorption rate of N species from the growing surface is low. The effect of the desorption of N species on N composition is enhanced by increasing growth temperature (390–445 °C). When growth temperature is high (445–480 °C), the degree of N atom segregation from the grown layer increases, resulting in a marked decrease in N composition. Thus, N composition is determined by the balance of supply and desorption of N species, and growth rate.

Radiation response analysis of wide-gap p-AlInGaP for superhigh-efficiency space photovoltaics

We present here the direct observation of the majority and minority carrier defects generation from wide-band-gap (2.04eV) and thick (2μm) p-AlInGaP diodes and solar cells structures before and after 1MeV electron irradiation by deep level transient spectroscopy (DLTS). One dominant hole-emitting trap H1 (EV+0.37±0.05eV) and two electron-emitting traps, E1 (EC−0.22±0.04eV) and E3 (EC−0.78±0.05eV) have been observed in the temperature range, which we could scan by DLTS. Detailed analysis of the minority carrier injection annealing experiment reveals that the H1 center has shown the same annealing characteristics, which has been previously observed in all phosphide-based materials such as InP, InGaP, and InGaAsP. The annealing property of the radiation-induced defects in p-AlInGaP reveals that multijunction solar cells and other optoelectronic devices such as light-emitting diodes based on this material could be considerably better to Si and GaAs in a radiation environment.

17.6% Conversion Efficiency Multicrystalline Silicon Solar Cells Using the Reactive Ion Etching with the Damage Removal Etching

For lower reflectance, we applied a maskless plasma texturing technique using reactive ion etching (RIE) on acidic-textured multicrystalline silicon (mc-Si) wafer. RIE texturing had a deep and narrow textured surface and showed excellent low reflectance. Due to plasma-induced damage, unless the RIE-textured surfaces have the proper damage removal etching (DRE), they have a drop in and FF. RIE texturing with a proper DRE had sufficiently higher short circuit current than acidic-textured samples without a drop in open circuit voltage . And in order to improve efficiency of mc-Si solar cell, we applied RIE texturing with optimized DRE condition to selective emitter structure. In comparison with the acidic-textured solar cells, RIE-textured solar cells have above 200 mA absolute gain in Isc. And optimized RIE samples with a DRE by HNO3/HF mixture showed 17.6% conversion efficiency, which were made using an industrial screen printing process with selective emitter structure.

Photoelectromagnetic effects on electron and proton irradiated CuInSe2 thin films

Photoelectromagnetic effects in CuInSe2 (CIS) thin films irradiated with 3 MeV electrons or 380 keV protons have been investigated to evaluate the diffusion length. CIS thin films were deposited by radio frequency sputtering. The diffusion length was decreased due to irradiation-induced defects, as the electron fluence exceeded 1×1016 cm−2. The damage constant was estimated from the change in diffusion length before and after irradiation, and was about 4×10−9. After proton irradiation, the diffusion length decreased significantly as the proton fluence exceeded 1×1013 cm−2. The damage constant in this case was estimated to be about 1×10−4–4×10−5.

Growth of AlGaN epilayers related gas-phase reactions using TPIS-MOCVD

Abstract AlGaN epilayers on GaN/sapphire were successfully grown under various growth conditions using a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition. The Al composition in the solid was affected by the gas-phase parasitic reaction between NH 3 and trimethylaluminum (TMAl). As the operating pressure decreased, the Al composition in the solid increased over the ideal incorporation efficiency. This is due to a scavenging effect and a site-blocking effect. As the TMAl flow rate increased with fixed flow rates of NH 3 and trimethylgallium (TMGa), the Al concentration in the solid increased but started to saturate. As the TMGa flow rate decreased, the solid Al composition increased linearly, which means different parasitic reactions between TMGa:NH 3 and TMAl:NH 3 . In addition, we found that the separating plate that was inserted to the reactor in front of the heated susceptor to separate ammonia gas flow from MO source input played an important role in the AlGaN growth. Particularly, the separating plate was more attractive under high operating pressure. When it was inserted, a white crystalline solid formed by the adduct (TMAl:NH 3 ) of parasitic reaction in the gas phase disappeared. It also increased the Al concentration in the solid. SEM images of AlGaN epilayer’s surface showed many small islands due to the lack of surface mobility of adatoms.

Carrier removal in lattice-mismatched InGaP solar cells under 1-MeV-electron irradiation

Radiation-induced majority carrier removal is investigated from n+∕p− lattice-mismatched In0.56Ga0.44P solar cells under 1-MeV-electron irradiation. The change in carrier concentration in the 1×1017cm−3p− base layer is determined using standard capacitance–voltage techniques and found to proceed at a rate Rc=1.3cm−1, in agreement with that observed in lattice-matched InGaP. However, the observation of an increased short-circuit current and short-wavelength quantum efficiency over the unirradiated values at electron fluence levels in excess of 3×1015cm−2, allows the carrier concentration from the n+ emitter layer to be measured. By modeling the quantum efficiency of these solar cells, it is shown that the main photoresponse from these lattice-mismatched solar cells is due to drift transport, making the spectral response highly sensitive to changes in the width of the depletion region. Using this technique, the carrier concentration in the 2×1018cm−3 n+ emitter layer is found to be reduced to 1×1018cm−3 aft...

Deep-level defects introduced by 1 MeV electron radiation in AlInGaP for multijunction space solar cells

Presented in this paper are 1 MeV electron irradiation effects on wide-band-gap (1.97 eV) (Al0.08Ga0.92)0.52In0.48P diodes and solar cells. The carrier removal rate estimated in p-AlInGaP with electron fluence is about 1cm−1, which is lower than that in InP and GaAs. From high-temperature deep-level transient spectroscopy measurements, a deep-level defect center such as majority-carrier (hole) trap H2 (Eν+0.90±0.05eV) was observed. The changes in carrier concentrations (Δp) and trap densities as a function of electron fluence were compared, and as a result the total introduction rate, 0.39cm−1, of majority-carrier trap centers (H1 and H2) is different from the carrier removal rate, 1cm−1, in p-AlInGaP. From the minority-carrier injection annealing (100mA∕cm2), the annealing activation energy of H2 defect is ΔE=0.60eV, which is likely to be associated with a vacancy-phosphorus Frenkel pair (Vp‐Pi). The recovery of defect concentration and carrier concentration in the irradiated p-AlInGaP by injection relat...

Preparation of CuIn(S,Se)2 thin films by thermal crystallization in sulfur and selenium atmosphere

Abstract Thin films of CuIn(S,Se) 2 were prepared by thermal crystallization process for photovoltaic device applications and their properties were investigated. From EPMA analysis, S/(S+Se) atomic ratio in the thin films was changed from 0.18 to 1.0 with increasing the S/(S+Se) vapor ratio in the quartz ampoule. X-ray diffraction studies revealed that the thin films had a chalcopyrite CuIn(S,Se) 2 structure and the preferred orientation to the 112 plane. The SEM images showed that the grain sizes in CuIn(S,Se) 2 thin films decreased with the increase in the S/(S+Se) atomic ratio.

The influence of ammonia pre-heating to InGaN films grown by TPIS-MOCVD

Abstract The InGaN films on GaN layers using a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated by a high-resolution X-ray diffraction and the thermodynamic analysis was performed for an NH3 pre-heater by using the computational fluid-dynamic simulations. As the flow rates of NH3 increased, the In composition and the thickness of the InGaN films increased, which meant that the relative indium incorporation efficiency was dependent on the NH3 flow rate and affected by the growth rate. In a low NH3 flow condition, indium metal droplets appeared on the surface of the InGaN layer in a conventional MOCVD system and decreased the indium incorporation efficiency of the InGaN films. The thermodynamic model of the ammonia dissociation did not follow equilibrium conditions and adduct-driven species seemed the actual growth precursors in nitride. The catalyst effect of the NH3 pre-heater plays an important role in the mixing region and growing surface, resulting in the higher indium incorporation and droplet-free surface in a TPIS-MOCVD system.

Record Efficiency on Large Area P-Type Czochralski Silicon Substrates

In this work we report a world record independently confirmed efficiency of 19.4% for a large area p-type Czochralski grown solar cell fabricated with a full area aluminium back surface field. This is achieved using the laser doped selective emitter solar cell technology on an industrial screen print production line with the addition of laser doping and light induced plating equipment. The use of a modified diffusion process is explored in which the emitter is diffused to a sheet resistance of 90 Ω/ and subsequent etch back of the emitter to 120 Ω/. This results in a lower surface concentration of phosphorus compared to that of emitters diffused directly to 120 Ω/. This modified diffusion process subsequently reduces the conductivity of the surface in relation to that of the heavily diffused laser doped contacts and avoids parasitic plating, resulting an average absolute increase in efficiency of 0.4% compared to cells fabricated without an emitter etch back process.